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ICEPMU-2016: Technology Letters Conference Proceedings of the International Conference on Engineering Physics, Materials and Ultrasonics, May 2017

1

PROCEEDINGS OF

International Conference on Engineering Physics,

Materials and Ultrasonics (June 3-4, 2016)

ICEPMU-2016

Editors:

Prof S K Jain, Convener

Dr. Ambika Sharma, HoD

Department of Applied Sciences

The NorthCap University

Gurgaon

Email: [email protected]

Website:www.ncuindia.edu

Sponsored by

Science and Engineering

Research Board

Materials Research

Society of India

Ultrasonic Society of

India

Defence Research and Development

Organization

Organized by

Department of Applied Sciences

Sector 23 A, Gurgaon 122017 - Haryana

Tel: 0124-2365811, Fax: 0124-2367488

Website: www.ncuindia.edu

http://www.mrsi.org.in/

http://www.mrsi.org.in/


2

Advisory Committee

Prof. Vikram Kumar, I.I.T Delhi & Pres. USI

(Chairman)

Prof. Krishan Lal, NPL, New Delhi

Prof. KL Chopra, NCU Gurgaon

Prof. M.S. Sodha, NCU Gurgaon

Prof. A.K. Ghatak, NCU Gurgaon

Prof. Kehar Singh, NCU Gurgaon

Prof. RC Budhani, I.I.T. Kanpur

Dr. R.K. Sharma, Dir. SSPL, Delhi

Prof. Anurag Kumar, Dir., IISc, Bangalore

Dr. VN Bindal, Patron USI

Prof. ESR Gopal Emer. Sc., IISC Bangalore

Dr. Baldev Raj, Ex-Dir. IGCAR & Pres. Res,

PSG Institutions, Coimbatore

Prof. Yogesh K. Vohra, University of

Alabama at Birmingham, U.S.A.

Dr. Niloy Dutta, Univ. of Connecticut, U.S.A.

Dr. Mekonnen Abebe, Def Univ., Ethiopia.

Dr. Andrej Nowicki, IFTR, Warsaw

Dr. Adam Shaw, NPL (UK), Teddington

Dr. David Gilbert, BINDT, UK

Dr. J. Szilard, Sydney, Australia

Prof. BK Das, NCU Gurgaon

Dr. VR Singh, Advisor, PDM, Bahadurgarh

Prof. Karmeshu, JNU, Delhi

Prof. Promila Goel, NCU Gurgaon

Prof. S.B. Krupanidhi, IISc, Bangalore

Prof. RR Yadav, AU Allahabad

Dr. Chandra Prakash, S.S.P.L. Delhi

Prof. Amitava Sen Gupta, NCU Gurgaon

Prof. P.K. Bhatnagar, South Campus, DU

Prof. S. K. Ray, IIT Kharagpur

Prof. Amlan J. Pal, IACS, Kolkata

Dr. Nitin Goel, Facebook, California

Dr. D. Kanjilal, IUAC, New Delhi

Dr. Avinashi Kapoor, DU, South Campus

Dr. Reji Philip, RR Institute, Bangalore

Chief Patrons

Sh. NK Dewan, Chancellor, NCU

Sh. V Daulet Singh, GB Member, NCU

Sh. Avdhesh Mishra, GB Member, NCU

Sh. Shiv S Mehra, GB Member, NCU

Patrons

Prof. Prem Vrat Pro-chancellor, NCU

Brig. S.K. Sharma, Pro-VC, NCU

Prof. R. Ojha, Director, SOET, NCU

Organizing committees

Convener

Prof. SK Jain, NCU [email protected] Co-conveners

Dr. Rashmi Tyagi

Prof. AK Yadav

Prof.Kallika Srivastava

Dr Devraj Singh, ASET, N. Delhi

Dr.Yudhisther Kumar, NPL, N. Delhi

Technical Program Committee

Dr. Ambika Devi (Chairman) [email protected] Dr. Pranati Purohit

Dr. Sangeet Srivastava

Dr. Kamlesh Sharma

Dr. Amita Bhagat

Dr. Satwanti Devi

Dr. Srijanani

Hospitality Committee

Dr. Hukum Singh (Chairman)

Dr. Ravindra Bisht

Dr. Tejpal Singh

Mr. Manoj Sharma (CSE)

Reception Committee

Dr. Sunanda Vashistha (Chairman)

Dr. Phool Singh

Dr. Sunita Sharma

Dr. Sandeep Mogha

Treasurer Committee

Dr. Pranati Purohit

Dr. Ashutosh Pandey

Dr. Chetna Tyagi

Conference CD and photographs

Dr. Gaurav Gupta (CD proceedings)

Dr. Sangeet Srivastava

mailto:[email protected]


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Contents

Sr No Title Author Page No

1 Electrical switching in Cu doped As-Se glasses K. Ramesh, Pumlianmunga,

E.S.R. Gopal

4

2 Bilayer Lift-off Technique for Micromachining Neha Yadav 9

3 Effect of change in titanium isopropoxide

(TTIP) concentration on the preparation of TiO2

nanopowder

Mamta Arya, Shubhra Mathur*,

Rohit Jain

12

4 Calculation Of Some Oscillating Parameters For

Graphene

D. K. Das, K. V. V. Nagaraju, S.

Roy and S. Sahoo

16

5 Study of doped graphene quantum dots by

chlorine containing compounds: Electronic

Spectroscopy

Poonam R. Kharangarh, and

Gurmeet Singh

19

6 Electromagnetic Wave Propagation in Photonic

Structures: Dielectric and Metallo-Dielectric

Waveguides

Triranjita Srivastava, Pushpa

Bindal, Priyanka, Anuradha,

Priyam and Priscilla

23

7 A Comparative Study of Numerical Methods for

Analysing Planar Plasmonic Waveguides

Triranjita Srivastava, Pushpa

Bindal, Asmita Deep and Ashima

Sharda

29

8 Study Of Propagation Characteristics Of Optical

Fibers: Experiment And Simulation

Pushpa Bindal, Triranjita

Srivastava, Sujata, Anju and

Diksha Tandon

33

9 Experimental Study of Microbending Losses in

Optical Fiber

Pushpa Bindal, Triranjita

Srivastava, Ananya, Aastha

Dhankhar

37

10 Growth of (001) oriented Cr and MgO thin

films on Amorphous Substrate for Magnetic

Tunnel Junctions

Sajid Husain, and Sujeet

Chaudhary

41

11 Bio ceramics: Future implant material Aruna Dani 45

12 Intelligent Transportation System Shubham Sehgal, Akshat Mathur,

Mona Aggrawal, Ram Sharma

47


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Electrical switching in

Cu doped As-Se glasses

K. Ramesh*, Pumlianmunga, E.S.R. Gopal

Department of Physics,

Indian Institute of Science, Bangalore 560012,

India.

*Corresponding Author:

[email protected]

Abstract: Bulk CuxAs30Se60-x glasses (0 x 34)

prepared by melt quenching method exhibit

interesting phase change properties when subjected

to high electric fields. The glasses in the

composition range 0 x 14 do not exhibit

switching. Glasses in the composition range 15 x

< 25 exhibit threshold switching. An unusual

switching from low resistance to high resistance

state has been observed for the glasses in the

composition window 25 x 28. A memory

switching is observed for the glasses with x ≥ 30.

The observation of ‘no switching threshold

switching low resistance to high resistance

memory switching’ is unique to Cu-As-Se glasses.

With the thermal crystallization studies and thermal

model, the unique switching behaviour in

CuxAs30Se70-x glasses has been understood.

Key words: Chalcogenide glasses, Electrical

switching, Filament formation, Thermal model,

Thermal crystallization.

1. Introduction

Chalcogenide glasses are known for their electrical

switching and memory effects and are popularly

known as phase change memory materials (PCM)

[1-3]. The application of high current drives the

system from a high resistive (OFF) state to a low

resistive (ON) state. This electrical switching is of

two types namely, threshold and memory [2-3]. The

threshold switching device returns to the high

resistive OFF state once the applied current is

reduced below the holding current (I < Ih). In

contrast, the memory device once switched retains

the ON state even after the applied current is

reduced to zero. The memory device can be brought

back to its high resistive OFF state by the

application of a suitable current pulse. In memory

switching materials, a high conducting crystalline

filament is formed due to the Joule heating at the

time of switching. Threshold switching is reversible

and is generally belie ved to be due to the electronic

transitions. It is also proposed that the presence of

cross-linking elements like Ge, Si, etc., make the

structural reorganization difficult resulting in

threshold switching. Memory switching is

irreversible and requires a structural transition from

glass phase to crystalline phase. So, structural

reorganization is very important for memory

switching to occur [2-3].

The addition of metal atoms significantly alters the

network connectivity, network rigidity, local

structure and consequently the electrical properties

including the switching behaviour [4-7]. The

structural studies show that the metal atoms in

chalcogenide glass network are usually in 4- fold

coordination [8]. As Cu is a monovalent atom, and

for Cu to be in 4- fold coordination, the lone pair

electrons of Se and As atoms are transferred to Cu.

By donating its electrons, the chalcogen atom

increases its local coordination. This transfer of

lone-pair electrons and the changes in the local

structure around each atom influences the optical

and electrical properties to a larger extent [5]. In the

present work, electrical switching in CuxAs30Se70-x

glasses has been studied over a wide composition

range 0 x 35. The observed electrical switching

behaviour of CuxAs30Se70-x glasses has been

understood with the help of thermal crystallization

studies.

2. Experimental

Bulk CuxAs30Se70-x glasses (0 x 35) were

prepared by conventional melt quenching method.


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Fig. 1. I-V curves of representative CuxAs30Se70-x glasses

showing the different types of switching.

Fig. 2. Tg as a function of Cu concentration.

The melt quenched samples were subjected to XRD

to confirm their amorphous nature. The thermal

properties were measured by Differential Scanning

Calorimeter (DSC) with a scan rate of 10 °C/min.

The prepared CuxAs30Se70-x glasses were thermally

crystallized in two ways in vacuum sealed quartz

ampoules: (a) by annealing at their respective

crystallization temperatures (Tc) for two hours (b)

heated up to their respective melting temperatures

(Tm) and then quenched in water at room

temperature. These samples were subjected to XRD

to identify the crystallized phases. I – V

characteristics of these glasses were studied using a

Keithley Source meter (Model: 2410c). Sample

polished to a thickness of 0.3mm is mounted in a

holder (made of brass), in between a flat-plate

bottom electrode and a point-contact top electrode

using a spring-loading mechanism. A constant

current (0 – 2 mA) is applied and the corresponding

switching voltage developed across the sample was

measured.

3. Results and Discussion

The phase change from glass (high resistance OFF

state) to crystal (low resistance ON state) at the time

of switching is responsible for the memory

switching. At sufficient voltage (threshold voltage),

a filamentary path is formed due to the Joule

heating. The threshold switching is generally

understood based on the electronic transitions [9].

The defect states C3+ and C1

- present in the mobility

gap act as trap centres for charge carriers. When the

traps are filled, a high conduction occurs.

I-V characteristics of representative glasses in the

CuxAs30Se70-x system shown in figure 1, indicates

the glasses can be divided into 4 regions. (i) 0 x <

15 do not undergo switching; (ii) 15 x < 25,

exhibit threshold switching; (iii) 25 x < 30,

unusually switches from a low resistance state to a

high resistance state (iv) x ≥ 30, memory switching

is observed. The composition dependence of Tg also

shows an interesting variation in these four regions

as shown figure 2 indicating the glass network

undergoes a change in these regions. The electronic

and thermal models are usually used to explain the

threshold and memory switching, respectively. The

different kinds of switching observed in a single

system are difficult to understand either by

electronic or thermal models. By varying the

concentration of Cu in the CuxAs30Se70-x glasses, a

‘no switching threshold low resistance to

high resistance memory’ is observed. To

understand the threshold switching we need to use

electronic model and to explain memory switching

we need to use thermal model. It may be difficult to

justify using different models to understand the

observed behaviour in a single system.


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With the help of thermal crystallization studies, the

electrical switching exhibited by CuxAs30Se70-x

glasses can be understood in the context of thermal

model. The thermal model needs minimal

modification to accommodate the different

switching types observed in the CuxAs30Se70-x

glasses. The samples annealed at Tc show only the

ternary Cu3AsSe4 phase. In contrast, the samples

melted and quenched in water show Cu3AsSe3 and

Cu3AsSe4 phases with considerable amorphous

background (fig. 3). The formation of Cu3AsSe4 and

Cu3AsSe3 phases is possible only if the added Cu

interacts with the parent matrix As-Se. The

interaction of Cu with As-Se increases the cross-

linking and the rigidity of the structural network,

which is reflected as an increase in Tg. DSC, studies

also show crystallization peak for all the

compositions irrespective of the switching type (fig.

4) indicating all the glasses undergo a phase change

in CuxAs30Se70-x glasses. Formation of filament has

been shown in the typical STAG glasses by

Nakashima and Kao [10]. The filament can have

permanent and temporary portions. The size of the

permanent and temporary portions depends on the

amount of current passing through the sample in

between the electrodes. By allowing higher current,

the size of the permanent portions will increase with

a corresponding decrease in temporary portions. At

sufficient higher current, the permanent portions can

close together leading to memory switching.

There are many experimental studies indicating that

the increase in the temperature at the time of

switching is as high as the melting temperature [11-

14]. In Ge-Te nano wires, melting of the nano wires

and the formation of voids near the top contact are

observed [15]. The voids are subsequently, filled by

the formation of the conducting crystallites. The

temperature rise in the filament of Ge30As20Se50

glass at the time of switching has been estimated to

be about 650oC [14]. Simulation and experimental

studies also show the temperature rise in the phase

change memory material (Ge2Sb2Te5) can be as high

as its melting temperature29. Microscopic studies on

many of the semiconducting glasses show the liquid

phase in between the electrodes at the time of

switching [12]. In NiO thin films, the SET and

RESET states shows the formation of conducting

filaments [16]. In-situ transmission electron

microscopy observations reveal that the conducting

filaments are in nano size consisting of amorphous

and crystalline phases. Hence, it is possible that in

CuxAs30Se70-x glasses the material in the inter-

electrode region can melt and form the filament.

This filament may have Cu3AsS3 and Cu3AsSe4

conducting phase (permanent regions) and some

high resistive amorphous phase, probably As2Se3

(temporary region). When I ≥ Ih, the permanent

portions are linked to have a conducting path. For I

< Ih, the activation energy to have the conducting

path may not be sufficient and the material reverts

back to its high resistive OFF state.


7

Fig. 4. DSC thermograms of CuxAs30Se70-x

glasses.

The unusual switching of low resistance state to a

high resistance state observed for x = 25 and 28 is

interesting. Similar kind of switching behaviour has

been reported for CuxAs30Se70-x and As2Se3Cu

glasses [17,18]. In this context, it is worth to

mention the work of Bagley and Bair on As2Se3-

3As2Te3 glasses. The surface of the glass was

crystallized before making the contacts for

switching measurements [19]. The samples were

found to be in high conducting state (ON) before the

application of the electric field. Upon the

application of the electric field, the samples were

found to switch to high resistance state (OFF) as in

the present Cu25As30Se45 and Cu28As30Se42 glasses.

In this composition range, the structural network

may have conducting nano- crystallites, which are

connected by weak link[16,18]. The current flowing

through this weak conducting path induces Joule

heating and ruptures the path. This results in the loss

of connectivity and thus the system switches to a

high resistive state. The sharp crystallization peak

observed for x = 25 and 28 in the DSC spectra

indicates that they are prone to crystallization. The

surface of the Cu25As30Se45 and Cu28As30Se42

glasses may have crystallites as in the case of

As2Se3-3As2Te3. The concentration of permanent

portions Cu3AsSe4 and Cu3AsSe3 crystallites is high

for glasses with x ≥ 30 consequently they exhibit

memory switching. The present studies show that

both the threshold and memory switching can be

understood with the thermal model and filament

formation. The filament is formed by glass melt

crystal/amorphous transition and not by a direct

glass crystal transition. The ratio between the

permanent and temporary portions determines the

switching type. If the ratio is high, memory

switching can be expected and if the ratio is low

threshold, switching can be expected.

4. Conclusions

Bulk CuxAs30Se70-x glasses showed interesting

switching behaviour from ‘absence of switching

threshold switching low resistance to high

resistance memory switching’. The observation

different type of switching is unique to Cu-As-Se

glasses. The thermal model with the filament

formation very well explains the observed switching

behaviour. At the time of switching, the material in

the inter-electrode region may melt to form a

filament. The melt solidified into permanent

(crystalline) and temporary (amorphous) phases in

the filament. The ratio between the permanent and

the temporary portions dictates the switching type.

If the ratio is high, a memory switching will occur

and if the ratio is less, threshold switching can be

expected. The present study paved a way to

understand both the threshold and memory

switching within the frame work of the thermal

model.

References

[1] Ovshinsky, S.R. Phys. Rev. Lett. 1968, 21,

1450.

[2] Hudgens, S. Phys. Stat. Solidi B 2012, 249,

1951.


8

[3] Bogoslovskiy, N.A.; Tsendin, K.D.

Semiconductors 2012, 46, 559.

[4] Tohge, N.; Minami, T.; Yanamoto, Y.;

Tanaka, M. J. Appl. Phys. 1980, 51,

1048.

[5] Liu, J.Z.; Taylor, P.C. J. Non-Cryst.

Solids 1989, 114, 25

[6] Ramesh, K.; Asokan, S.; Gopal, E.S.R.

J. Non-Cryst. Solids 2006, 352, 2905.

[7] Murugavel, S.; Asokan, S. Phys. Rev. B

1998, 58, 3022.

[8] Xin, S.; Liu, J.; Salmon, P.S. Phys. Rev.

B 2008, 78, 064207.

[9] Adler, D.; Shur, M.S.; Silver, M.;

Ovshinsky, S.R. Appl. Phys. Lett. 1980,

153, 289.

[10] Nakashima, K.; Kao, K.C. J. Non-Cryst.

Solids 1979, 33, 189.

[11] Yang, T.Y.; Park, I.M.; Kim, B.J.; Joo,

Y.C. Appl. Phys. Lett. 2009, 95,

032104.

[12] Pearson, A.D.; Miller, C.E. Phys. Lett.

1969, 14, 280.

[13] Radaelli, A.; Pirovavo, A.; Benvenuti,

A.; Lacaita, L. J. Appl. Phys. 2008, 103,

111101.

[14] Weirauch, D.F. Appl. Phys. Lett. 1970,

16, 72.

[15] Meister, S.; Schoen, T.; Topinka, M.A.;

Minor, A.M.; Cui, Y. Nano Lett. 2008,

8, 4562.

[16] Son, J.Y.; Shin, Y.H. Appl. Phys. Lett.

2008, 92, 222106.

[17] Asahara, Y.; Izumitani, T. J. Non-Cryst.

Solids 1972, 11, 97.

[18] Haifz, M. M.; Ibrahim, M.M.; Dongal,

M. J. Appl. Phys. 1983, 54, 1950.

[19] Bagley, B.G.; Bair, H.E. J. Non-Cryst.

Solids 1970, 2, 155.

Acknowledgements

The authors thank the Department of Science

& Technology (DST) for the financial support.


9

Bilayer Lift-off

Technique

for Micromachining

Neha Yadav

Department of Physics,

Keshav Mahavidyalay, University of Delhi


[email protected]

Abstract:

This paper discusses the application of bilayer

lift off technique for micromachining

applications. In micro-machined devices,

patterning of metal films is required. The metals

can be patterned either by etching or lift-off. In

this paper, using two-layer photoresist for lift-

off has been presented. This technique can be

used for lift-off films having thickness upto 7-8

micron and is very effective in getting desired

photoresist profile.

The prerequisite for the lift-off is negative

profile of the photoresist. The bilayer

photoresist can be patterned using photo mask.

The resultant pattern can be analysed in optical

microscope and SEM. It can be seen that by

varying the flood exposure time of the bottom

layer, negative profile required for lift-off with

desired under-cut could be achieved.

Key words: lift-off, micromachining, negative

profile, under-cut, photoresist

1. Introduction

Micro-machined devices can be fabricated by

either bulk or surface micromachining. Both the

processes require patterning of metals at various

stages of device fabrication. For patterning of

metals, commonly used technique is etching. In

etching the wafer is put in a chemical etchant,

removing the metal from desired places. But in

case of nobel metals or very small dimensions,

lift-off is preferred.

Following are the general steps involved in the

lift-off process:

A thin layer of photoresist is spin coated on the

substrate, dried off and exposed to UV radiation

through a pattern and developed using a

developer. After the development process,

patterned photoresist is obtained. The wafer is

then placed in vacuum chamber and thermal

deposition of metallic thin film is done by

‘thermal evaporation’. The slide is placed in a

solvent which seeps under and dissolves the

photoresist and the film which is directly

deposited is left behind on the substrate.

Following are the requirements for a metallic

film to be lifted-off:

1. Temperature should not be very high

otherwise the photoresist might get

burnt.

2. The metal thickness is to be around or

less 100nm to allow solvent seep under

it and dissolve the photoresist.

3. The deposition of film on the substrate is

to be very good.

4. The film is to be easily wetted by the

solvent.

5. The film is not to be elastic but brittle,

otherwise it will tear along adhesion

lines.

6. The film quality is not absolutely

critical. That means if requirements on

film quality are stringent, then, lift-off is

not to be used Photoresist will outgas

very slightly in vacuum systems, which

may adversely affect the quality of the

deposited film.

2. Important parameters for desired Lift-

off

Result:

1. It is important to create negative slope

profile or undercut profile so that lift-

off becomes easy.

2. Prebake temperature has the greatest

influence on negative slope rate. The

parameters which have influence are



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prebake time, UV exposure intensity

and time of photoresist, the developer,

the mode of development and time of

development.

3. Careful consideration should be given

to the resist/developer system

3. Different methods for lift-off

technique

Depending on requirements different

methods are employed.

1. Single Layer Resist Processing

a) Standard Photoresist Processing: Only

one mask step and the standard

photolithography procedure are

involved. The main disadvantage of this

method is that the film is deposited on

the sidewall of the photoresist, and

adheres to the substrate even after the

resist removal. This sidewall may be

peeled off in subsequent processing,

resulting in particulates and shorts, or it

may flop over and interfere with etches

or depositions that follow.

b) Single Layer lift off technique using

negative photoresist.

c) Very Thick Negative Photoresist Single

Layer

2. Bi-Layer Resist Processing

a) PR/LOL 2000

b) PR/ LOR Lift-Off Resist (or PMGI

Resist)

c) PMMA/PMMA

d) PMMA/LOL2000

e) Composite Layers of Aluminum (Al)

and Photoresist

3. Tri-Layer Processing

4. Surface Modified Resist Processing

The need for using lift-off technique instead of

etching by conventional methods is that for noble

elements such as Gold, Nickel, Platinum,

Tantalum, Titanium and others, the etching

chemicals may not be available. The substrate or

layers may be sensitive to harsh chemicals. The

harsh chemicals may degrade the quality of the

substrate (semiconductor) and thereby affecting

its quality of performance. Also, smaller the

dimensions etch control becomes more difficult.

Lift-off technique using Positive photoresists

Positive photoresists are preferred in the IC

industry or MEMS foundries due to their ease of

removal and better resolution capabilities But

for the lift-off applications , the positive

photoresists have the limitation of lower

softening points (around 120-130°C). This

range of temperatures is reached even during the

normal coating and hence the resist features

rounding and makes it very difficult even

impossible to lift-off. Another drawback is that,

by using positive photoresists only positive

profile or at the most vertical profile is obtained

covering the sidewalls during coating and hence

making lift-off difficult. If the desired pattern is

such that positive photoresists is to be used then

the positive resist used should have higher

thermal stability and sidewalls of the photoresist

should be very steep.

4. Experimental Procedure

In bilayer lift-off technique, as the name

suggests two layers of photoresist is used with

different flood exposure time.

A thin film of the assisting material is deposited

over the substrate and it is exposed to UV light

without masking. A layer of photoresist is spin

coated on the substrate and again exposed to

UV radiation through a pattern. The mask

exposure time is less than the previous exposure

time without masking and developed using

developer. The underlying layer of the assisted

material is etched by the developer. Metallic

thin film is deposited by ‘evaporation’ process.

The photoresist is removed and the layer of

metal also gets removed along with it and

finally the underlying layer of assisted material

is also removed and well defined metal pattern

alone is left.


11

The important point of bi-layer lift-off

technique is that the underlying assisting layer is

more sensitive

to the exposure dose or has a higher dissolution

rate in the developer as compared to upper

photoresist layer and hence negative profile is

obtained which makes it easier to lift-off.

4.1. Details

The experimental work involves spin coating of

photoresist like AZ9260 to achieve a uniform

film of thickness 10 micron. After pre-bake at

1000C, the film is to be given flood exposure of

i-line UV light using mask aligner. The film is

to be post baked at 1200C and same photoresist

is coated over it. The thickness of the second

layer is to be taken to be 5 micron.


If the exposure time is increased the θ i.e. the

angle with the tangent also increases which

signifies a steeper undercut and is very much

desirable for the lift-off to take place.

It can therefore be concluded that by varying the

exposure time for bottom layer, desired resist

sidewall can be achieved.

6. Conclusion:

For patterning of metal films at various stages

of surface micro-machined devices, this

technique of using double layer photoresist is

quite simple. This technique can be used for

lift-off films having thickness upto 7-8 micron

and is very effective in getting desired

photoresist profile.

References

[1] Yifang, Chen, Peng Kaiwu and Cui Zheng.

A lift-off process for high resolution patterns

using PMMA/LOR resist stack. Microelectronic

Engineering, 2004, 73-74, p. 278-281

[2] Shih-Chia Chang and Jeffrey M. Kempisty,

'Lift-off Methods for MEMS Devices’, Mat. Res.

Soc. Symp. Proc. Vol. 729

[3] Photoresist AZ9260 from

http://www.nfc.umn.edu/assets/pdf/az_9200.pdf

Exposure through

mask and

development


12

Effect of change in

titanium isopropoxide

(TTIP) concentration

on the preparation of

TiO2 nanopowder

Mamta Arya, Shubhra Mathur*, Rohit Jain

Department of Physics, JaganNath Gupta Institute

of Engineering & Technology, Jaipur, 303905,

India

*Corresponding author.

E-mail: [email protected],

Abstract TiO2 nano-powder is prepared by

changing titanium isopropoxide (TTIP)

concentration as 3.5 ml, 4.5 ml and 5.5 ml in 40 ml

methanol and thus annealing at 6000 C. X-ray

diffraction (XRD) pattern exhibits the presence of

mixed phase anatase/rutile in various TiO2

nanopowder specimens prepared by different

concentrations of TTIP. It was observed that the

content of rutile phase is more in case of 5.5 ml

TITP as compared to 4.5 ml and 3.5 ml TTIP of

TiO2 nanopowder specimens. The average

crystallite size was found to be 35±5 nm for TiO2

nanopowder specimens. UV studies show that

indirect and direct band gap lies in the range of

2.95-2.76 eV for different TTIP concentrations 3.5

ml, 4.5 ml and 5.5 ml of TiO2 nanopowder

specimens.

Keywords: nanopowder, band gap, XRD, TiO2

1. Introduction

Titanium dioxide (TiO2) is considered as the most

promising semiconductor metal oxide because it

exhibits highly enhanced photo catalytic activity

[1] and improvement in gas sensing properties [2].

Anatase, rutile and brookite are three well known

phases of TiO2 amongst which rutile is a high

temperature stable phase. However, anatase and

brookite are metastable phases and transform to

rutile on heating. Anatase phase show an energy

band gap of 3.2 eV whereas rutile phase exhibits an

optical band gap of 3.0 eV [3].

Sol-gel is a versatile method used for the

preparation of TiO2 nanopowder [4-5]. The change

in concentration of titanium isopropoxide, which

acts as a starting material in our investigation, may

lead to change in structural and optical properties

of TiO2. This motivated us to carry out the present

study.

2. Experimental

2.1. Materials

Titanium isopropoxide (TTIP) and methanol are

used as starting materials. The chemicals used are

of analytical research (AR) grade.

2.2. Methods

TiO2 nanopowder is prepared by using sol gel

method. Sol-gel process also known as a wet-

chemical technique is used for the fabrication of

both glassy and ceramic materials. In this process,

the sol (or solution) evolves gradually towards the

formation of a gel-like network containing both a

liquid phase and a solid phase [5].

2.2.1. Preparation of Samples

Titanium isopropoxide (TTIP) taken in different

concentrations as 3.5 ml, 4.5 ml and 5.5 ml is

mixed in 40 ml methanol. This results in a milky

white solution and is vigorously stirred for 1:30

hours at a temperature 57±3⁰C. The gel thus

produced is kept for drying at room temperature for

12 hrs. Hence the powder is obtained and annealed

at 600⁰C for 1 hour in air [5].

3. Results & Discussion

X-Ray diffraction pattern (XRD) of TiO2

specimens having different concentration of (TTIP)

is recorded using Cu-Kα radiation as shown in

Fig1. Diffraction peaks showing the presence of

both anatase and rutile phase are in good


13

agreement with the JCPDS no. 21-1272 for

anatase, 21-1276 for rutile and data reported in the

literature [6-7].[0]› 101 A

20 30 40 50 60 70

111 R

110 R

110 R

301 R

204 A

+ 0

02 R

211 A

105 A

+ 2

11 R

200 A

112 A

004 A

101R

103 A

110 R

Inte

nsity (

arb

. u

nits)

(a) TTIP 3.5

(b) TTIP 4.5

(c)TTIP 5.5

A-Anatase, R-Rutile

(a)

(b)

(c)

Fig.1. X-ray diffraction pattern (XRD) of TiO2

nanopowder prepared with different

concentrations of titanium isopropoxide as (a)

TTIP-3.5 ml, (b) TTIP-4.5 ml and (c) TTIP-5.5 ml.

Table 1 shows the average crystallite size

calculated using Scherrer formula [6] and the

content of anatase and rutile phase which is

calculated using formula Xa = 100/ 1+ [3]›1.265

(Ir/Ia) where Xa is the weight fraction of anatase in

the mixture, Ia and Ir are intensities of anatase (101)

and rutile (110) diffraction peaks [6].

Table 1: Average crystallite size and content of phases

in TiO2 nanopowder specimens.

200 300 400 500 600 700

3.1

3.2

3.3

3.4

3.5

3.6

3.7

3.8

3.9

4.0

4.1

Ab

so

rba

nce

Wavelength (nm)

TTIP 5.5

TTIP 4.5

TTIP 3.5

Fig. 2(a)

1.6 2.0 2.4 2.8 3.2 3.6 4.0 4.40

5

10

15

20

25

(h

(h)

TTIP 5.5

TTIP 4.5

TTIP 3.5

Fig. 2(b)

TTIP

(ml)

XRD

Intensity

Ia

(101

anatase)

Intensity

Ir

(110

rutile)

Average

cryst-

allite

size

(nm)

%

Ana-

tase

%

Ru-

tile

3.5 1496.63 30.79 34 97.27 2.73

4.5 1336.21 59.43 36 94.69 5.31

5.5 1497.34 105.54 39 91.84 8.16


14

1.6 2.0 2.4 2.8 3.2 3.6 4.0 4.41.4

1.5

1.6

1.7

1.8

1.9

2.0

2.1

2.2

(h

(h

TTIP 5.5

TTIP 4.5

TTIP 3.5

Fig. 2(c)

Fig. 2. UV spectroscopy results (a) absorption spectra

(b) Tauc plot for direct band gap energy (c) Tauc plot

for indirect band gap energy.

Fig. 2 (a) represents UV spectra of TiO2

specimens with different concentration of TTIP.

The band gap energy is determined by Tauc plot

as shown in Fig 2 (b) and Fig. 2 (c) [8]. The band

gap energies thus obtained are summarised in

Table 2.

Therefore, increase in content of rutile phase leads

to increase in the crystallite size of TiO2

nanopowder [6]. The band gap energy of TiO2

specimens with different concentration of TTIP as

formulated in Table 2 shows lower band gap

values as compared to band gap energy 3.2 eV for

pure anatase and 3.0 eV for pure rutile phase

because in our investigation TiO2 nanopowder

specimen is a mixture of both anatase and rutile

phases [9]. Moreover it was observed that optical

band gap energy increases with decrease in

crystallite size, which leads to blue shift of the

optical absorption edge [8]. Further it was reported

that the specimens with mixed phase anatase/rutile

TiO2 nanopowder show improved photo catalytic

and gas sensing properties [1-2]. Hence, TiO2

nanopowder specimens prepared in our

investigation by simple sol gel method may be use

to study photocatalytic and gas sensing properties.

4. Conclusion

1. The least concentration of TTIP (3.5 ml) leads

to formation of TiO2 nanopowder having smallest

average crystallite size.

Table 2: Energy band gap values of TiO2

nanopowder specimens

The X-ray diffraction pattern (XRD) revealed the

presence of both anatase and rutile phase in TiO2

nanopowder specimens and the average crystallite

size increases with increase in concentration of

TTIP.[0]› It is noteworthy here that the content of

the rutile phase also increases with increase in

concentration of TTIP.

2. The mixed phase anatase/rutile TiO2

nanopowder exhibits lower band gap energy as

compared to pure anatase and rutile phases.

References

[1] Singh.J.; Mohapatra,S. Adv. Mater Lett. 2015,

6, 924.

[2] Enachi, M.; Lupan, O.; Braniste, T.; Sarua, A.;

Chow, L.; Mishra, Y.K.; Gedamu, D.; Adelung, R.;

Tiginyanu, I.; Phys. Status Solidi RRL 2015, 1

[3] Hanaor,A.D.H.; Sorrell,C.C. J Mater Sci. 2011,

46,855.

[4] Zainurul, A. Z.; M.; Abdullah. S. Achoi, M.F.;

Rusop, Advanced Materials Research 2014, 832,

649.

[5] Pawar, S.; Chowgule, Patil S.; Raut, B.; Dalvi,

D.; Sen, S.; Joshi, P.; Patil, V. Journal of Sensor

Technology 2011, 1, 9.

TTIP(ml) UV

Indirect

Bandgap

(eV)

Direct band

gap (eV)

3.5 2.95 2.93 4.5 2.87 2.84 5.5 2.79 2.76


15

[6] Dai, S.; Wu, Y.; Sakai, T.; Du, Z.; Sakai, H.;

Abe, M. Nanoscale Research Letters, 2010, 5,

1829.

[7] Vijayalakshmi, K.; Rajendran, K.V. 2010,

AZojomo 2010, 6, DOI: 10.2240/azojomo0298

[8] Tripathi, A.K.; Singh, M. K.; Mathpal, M. C. ;

Mishra, S. K. ; Agarwal, A. Journal of Alloys and

Compounds, 2013, 549, 114.

[9] Paul, S.; Choudhury, A. Appl Nano Sci 2014,

4, 839.

Acknowledgment

Authors thank Science & Engineering Research

Board (SERB) for providing financial grant vide

no SERB/F/5303/2014-15 and MRC, MNIT, Jaipur

for XRD facility.


16

Calculation of Some

Oscillating Parameters

For Graphene D. K. Das*1, K. V. V. Nagaraju2, S. Roy 3 and S.

Sahoo4

1Department of Metallurgical and Materials Engineering

National Institute of Technology, Durgapur-713209, West

Bengal, India. 2, 3, 4Department of Physics, National Institute of Technology

Durgapur-713209, West Bengal, India.

*Corresponding Author: [email protected]

Abstract In recent years graphene has become a

hot topic of research in various sectors due to its

many advanced properties such as high tensile

strength, stiffness etc. It is a two-dimensional (2D)

nanomaterial. Reduced dimensional structure makes

graphene mechanically rigid and stiffest ever. Frank

et al. have experimentally studied effective spring

constant of stacks of suspended graphene sheets

(less than 5) and found the value of spring constant

lies in the range 1 to 5 N/m. In this paper, we

calculate the frequency, spring constant and

damping coefficient of graphene under oscillation

due to tensile force theoretically.

Keywords: Graphene; frequency; spring constant; damping

coefficient.

1. Introduction

Graphene is sp2 hybridized, single atomic layer

hexagonally arranged network of carbon atoms. A

single pi (π) bond and three sigma (σ) bonds joins

each carbon atom in graphene with its neighboring

carbon atoms. A loan pair of free motile electrons

forms each pi bond. The soft, lustrous and

lubricating nature of graphene is due to presence of

these free electrons. They also results in high

electrical and thermal conductivity of graphene [1,

2]. Graphene has an electron mobility of 2.5 × 105

cm2 V-1 s-1 [1].

Frank et al. [3] have experimentally studied

effective spring constant of stacks of suspended

graphene sheets (less than 5) and found the value of

spring constant lies in the range 1 to 5 N/m. In this

paper, we intend to determine some oscillating

parameters such as frequency, spring constant and

damping coefficient of a graphene sheet under

oscillation due to tensile force theoretically.

This paper is organized as follows: In Sec. 2, we

calculate the frequency (ωnA), spring constant (K)

and damping coefficient (Cc) of a graphene sheet

under oscillation due to tensile force. In Sec. 3, we

discuss our results. In Sec. 4, we present our

conclusion.

Calculation of oscillating parameters for

graphene

Let us consider a graphene sheet with dimension

800×300 nm in length and breadth respectively

which is being fixed at one end. A force is applied

at the other end and released. The sheet starts

oscillating as shown in Fig. 1 below:

Fig. 1: Graphene sheet fixed at one end, force is applied

on the other end and released (oscillation)

The original length of the sample L = 800 nm. Now

the frequency of oscillation for the graphene sheet

is given by [4]

E

l

n

e

nA , (1)


17

where n = mode value =1 (for here), el = effective

length of sheet = 750 nm (say), = density of

graphene = 2300 kg/m3 and E = Young’s modulus

of graphene sheet = 1 TPa [5]. Putting these values

in equation (1), we get ωnA = 8.7298×1010 rad/s or

1.39×1010 Hz. Again we know time period of

oscillation for a vibrating body is given by [6]

, (2)

where, ω is the frequency for oscillation (ωnA) for

this case. The time period of oscillation for the said

graphene sheet is found to be 7.1937×10-11 s. The

relation between frequency and spring constant for

oscillation motion is given by the relation [6]

m

Kf n

2

1 , (3)

where, m is the mass of the object and K is the

spring constant.

For the considered graphene sheet (Fig. 2), the C-C

bond length = a = 1.42Å = 0.142nm [8], length of

unit cell = 3 a = 0.426 nm, width of unit cell = a3

= 0.246 nm, area of the unit cell of graphene =

0.104796 nm2, total surface area of graphene sheet

= l × b = 240000 nm2. Hence, the total number of

atoms (n) in the considered graphene sheet is

13740983. We know the mass of each carbon atom

( cm ) = 1.994 × 10-23 gm [9]. Hence, the total mass

of the graphene sheet is 13740983 × 1.994 × 10-26 =

2.7399 × 10-19 kg.

Now putting these values in eq.(3) we get K =

2087.7746 N/m. The relation between frequency of

oscillation and force applied on the material can be

written as [6]

T

lf n

2

1

,… (4)

Fig.2. Single unit cell of graphene sheet [7]

where, T = Tension applied in one end of the sheet,

= mass per unit length in = l

m= 3.4293 × 10-13

kg/m and l = length of sheet = 800 nm. Putting

these values in equation (4) we calculate the

magnitude of tensile force (T) = N4106939.1 .

We also know for oscillation [6],

, (5)

where, x is the increment in length of the sheet due

to application of force. So putting above obtained

values of T and K in equation (5) we get, x =

8.1134×10-8 m. The generalized wave equation is

given by [6]

tBtAx nAnA sincos 00 , (6)

where, A0 and B0 are the amplitudes in x and y

directions respectively. At t = 0 i.e. starting of

oscillation equation (6) is reduced to

0Ax , (7)

Here, we have obtained the values of A0 =

8.1134×10-8 m and An = 8.1631×10-8 m at t =

7.1937×10-11 s. So it is a damped vibration. The

relation between decrement in amplitude with time

can be stated as [10]

t

nneAA

0 , (8)


18

where, ξ is the damping ratio. Putting the above

values in equation (8) we get ξ = -9.7245×10-6. The

coefficient of critical damping (Cc) is given by [10]

, (9)

Using the values of K and m in equation (9), we get

Cc = 4.7834×10-8 kg/s. Further, we know that [10]

ξ = C/ CC , (10)

where, C is the coefficient of damping. From here

we calculate C = 4.5616×10-13 kg/s.

2. Results

We have found that the oscillation parameters for

graphene are depending on the Young’s modulus

and size of the material. Complete analytical work

is carried out with a graphene sheet of dimensions

(800nm×300nm). Our results show that mechanical

stiffness of our graphene sheet (K= 20784.1996

N/m) is much higher than previously reported

values. Our calculated parameters are reported in

tabular form below:

Table:1. Oscillating parameters for graphene

Sl.

No.

Oscillating Parameters Our calculated values

1. Frequency of vibration

(ωnA)

8.7298×1010 rad/s

2. Spring constant (K) 2087.7746 N/m

3. Damping Coefficient 4.5616×10-13 kg/s.

3. Conclusion

Analysis on these oscillating parameters of

graphene is very useful to study its mechanical

properties. These are also useful to design the

nanomechanical resonators and

nanoelectromechanical resonator sensors because

graphene shows ultra-high sensitivity of vibrations.

We hope our results can be useful for the design of

the next generation nanodevices and

nanofabrication technologies that use the vibration

properties of graphene. Our theoretical results

would be verified theoretically as well as

experimentally in future for confirmation.

Acknowledgement

Mr. K. V. V. Nagaraju thanks NIT Durgapur for

providing fellowship during his M. Tech. study.

References

[1] Novoselov, K. S; Fal′ko, V. I; Colombo, L;

Gellert, P. R; Schwab, M. G; Kim, K; Nature,

2012, 490, 192.

[2] Maity, S; Ganguly, M.; Elements of Chemistry-

1, Publishing Syndicate; Kolkata, 2003.

[3] Frank, I. W; Tanenbaum, D. M: J Vac. Sci.

Technol. B, 2007, 25(6), 2558.

[4] Gupta, S. S; Batra, R. C; J. Comput. Theor.

Nanosci., 2010, 7, 1.

[5] Lee, C; Wei, X; Kysar, J. W; Hone, J; Science,

2008, 321(5887), 385.

[6] Datta, D; Pal, B; Chaudhuri, B; Elements of

Higher Secondary Physics-1, Publishing

Syndicate, Kolkata, 2002.

[7] Yamayose, Y; Kinoshita, Y; Doi, Y; Nakatani,

A; Kitamura, T; Eur. Phys. Lett., 2007, 80, 40008.

[8] Fujita, T. K. W; Oshima, C; Surface and

Interface Analysis., 2005, 37(2), 120.

[9]http://chemistry.about.com/od/workedchemistry

problems/a/avogadroexampl1.htm.

[10] Nag, D; Mechanical Vibrations, Wiley, Delhi,

2011.

http://www.nature.com/nature/journal/v490/n7419/abs/nature11458.html#auth-2





http://chemistry.about.com/od/workedchemistryproblems/a/avogadroexampl1.htm

http://chemistry.about.com/od/workedchemistryproblems/a/avogadroexampl1.htm


19

Study of doped

graphene quantum dots

by chlorine containing

compounds: Electronic

Spectroscopy Poonam R. Kharangarh*, and Gurmeet Singh

Department of Chemistry, University of Delhi,

Delhi 110007, India


[email protected]

Abstract: For the study of high quality doped

graphene quantum dots, a series of chlorine

containing compounds such as CoCl2, HCl, and

NH4Cl were used. The morphology of the samples

were done by Transmission Electron Microscope

(TEM). The absorption of the doped material was

found by U-V visible spectroscopy for optical

study. The redox behaviour has been observed by

using Cyclic Voltammetry tool. Different electronic

structures for different doped graphene quantum

dots were observed from UV- Visible

Spectroscopy. Cyclic Voltammetry measurements

show the oxidation and reduction of different metal

doped GQDs to calculate the energy for the

conduction band edges parameters (HOMO and

LUMO).

Key words: Graphene Quantum Dots, TEM, UV-

Visible, Transition Metals, Energy Gap, HOMO,

LUMO

1. Introduction

Graphene Quantum Dots (GQDs), fragments of

graphene has been brought tremendous attention

due to their physical properties, including excellent

water solubility, low cytotoxicity, excellent

biocompatibility, and resistance to photo-bleaching

[1-4].

Doping with different metals is the most realistic

tool to tune the semiconducting properties in the

conventional semiconductor community.

Nevertheless, due to presence of low defects in un-

doped GQDs, weak optical properties can be seen.

Doping heteroatoms including boron, nitrogen,

chlorine, sulphur, fluorine can improve the

electronic characteristics of GQDs to introduce

more defects [5-8]. Nevertheless, bandgap is

increased in GQDs after doping with different

heteroatoms showing ideal p- and n-type

semiconducting electronic properties for potential

applications of GQDs in electronic devices.

A lot of research has been declared that the doping

of different atoms into GQDs alters the band gap

between conduction band maximum and valence

band minimum. Results were shown that a new

energy level was introduced to tune the optical

properties in order to make GQDs for solar cells

applications. In order to fulfill the energy

requirements and to generate the photo-current, we

need to choose a appropriate material which can

modify the energy band structure. Herein, we

present a facile hydrothermal method to prepare

doped GQDs with different transition metals having

chlorine containing elements. When chlorine

containing compounds are doped into GQDs, it

usually has different absorption bands induced by

edge effect in modified GQDs. Furthermore, the

effect of metals on the electronic structure of GQDs

still remains unclear. Hence, there is a need to

investigate how these metals modifies the energy-

level structure in case of doped-GQDs. Cyclic

voltammetry characterization technique [9-11]

reveals that the different band gap is obtained upon

the integration of chlorates into the GQDs.

2. Experimental

2.1. Materials



20

In this work we have used commercially available

graphite powder, NaNO3, KMNO4, H2O2, NH4Cl,

CoCl2, HCl and H2SO4. Double distilled water was

used for all the experiments during the preparation

of graphene oxide (GO) and doped GQDs

2.2 Synthesis of Graphene Oxide/ Different

Metals doped GQDs

Graphite oxide was prepared in accordance with the

procedure described by Hummers and Offemann

[12]. The brief description of doped CoCl2-GQDs

was explained in refs [13-14]. The same procedure

was followed for 6.06 mg of NH4Cl doped GQDs

and 6mg of HCl doped GQDs. The centrifugation

was done at 4000 rpm for as prepared solution

before to carry out the further characterizations.

2.3 Characterization Techniques

Transmission Electron Microscope (TEM) was

recorded on samples using FEI Technai G2 20

electron microscope operating at 200 kV. Perkin

Elmer Lambda 35 spectrophotometer was used to

record the absorption spectra of dispersions with a

slit width of 2 nm and scan speed of 240 nm/min.

The electrochemical measurements were performed

with the help of CHI-760C potentiostat -

galvanostat instrument by using a three electrode

system where glassy carbon electrode (diameter ~ 3

mm) was used as a working electrode, Ag/AgCl as

a reference electrode and Pt wire as a counter

electrode in aqueous electrolyte. The electrolyte

was chosen as 0.05M KCl in aqueous medium. The

working electrode was prepared by dropwise

casting on glassy carbon electrode. Cyclic

voltammetry (CV) experiments were carried in the

potential range of -0.8V to 0.2Vfor HCl doped

GQDs and NH4Cl doped GQDs whereas the

potential window was adjusted from -0.8 to 0.4 V

for CoCl2 doped GQDs.


Fig. 1(a, b, c) show the HRTEM images of the

CoCl2-GQDs, HCl-GQDs and NH4Cl-GQDs

respectively. The majority of the doped GQDs with

different transition metals are estimated to be in the

narrow range of 5-15 nm in diameter.

50 nm

50 nm

Fig. 1. TEM images of the (a) CoCl2-GQDs, (c) HCl-

GQDs [ref14] and (c) NH4Cl-GQDs

Fig. 2 shows that the UV-visible absorption

spectrum of NH4Cl-GQDs, HCl-GQDs and CoCl2-

GQDs in aqueous solutions. As we know that the

absorption peak for GO is at 232nm [15] and GQDs

is characterized by a 323 nm band, which is red-

shifted from 232 nm of GO resulted from n-π*

transitions of C=O. bond [16]. New energy levels

are observed due to the presence of functional

group state possibly or related to oxygen after

doping in between valence band (π band) and

conduction band (π* band). The new shifted peak is

observed at 298nm (4.16 eV) after the treatment of

CoCl2, 330nm (3.8eV) in case of HCl and 5.4eV for

NH4Cl. A large band is observed in NH4Cl -GQDs

as compare to all other different transition metals.

The optical energy band gap, Eg, can be calculated

to find out the energy levels of the electronic states

by using equation [17]

(a) (b)

(c)


21

Eg = 1242/λonset (1)

where λonset is the longest absorption wavelength.

Fig. 2. UV-Vis Spectroscopy for NH4Cl-GQDs, HCl-

GQDs and CoCl2-GQDs.

The energy levels were calculated by using the

following empirical Bredas et al. [18] equations:

E (HOMO) = -e [Eox onset + 4.4] (2)

E (LUMO) = -e [Eredonset + 4.4] (3)

Fig. 3. Cyclic Voltammetry curve for, NH4Cl-GQDs,

HCl-GQDs, and CoCl2-GQDs.

Fig. 3 shows that the cyclic voltammetry behavior

of different doped graphene quantum dots. A

reversible two electron reduction is observed in

CoCl2-GQDs, and HCl-GQDs with respect to

Ag/AgCl, but redox behavior is absent in NH4Cl-

GQDs.

In CoCl2-GQDs, anodic peak of redox pair is

responsible for the oxidation of Co2+/Co4+ whereas

cathodic peak corresponds to a reduction process

following the Faradic reduction reactions from Co4+

to Co2+. It is noted that the cathodic peaks shifts

more positively in CoCl2 doped GQDs in

comparison to NH4Cl-GQDs and HCl-GQDs and

the anodic peaks is more negatively in NH4Cl-

GQDs and HCl-GQDs which is mainly due to the

resistance of electrode.

Table 1 Energy levels of CoCl2-GQDs, and HCl-GQDs

Materials CoCl2-

GQDs,

HCl-

GQDs

Eox (V) -0.65 0.2

HOMO level (eV) -5.05 -4.2

Ered (V) 0.8 0.35

LUMO level (eV) -3.6 -4.05

Eg [from CV (eV) 1.45 0.15

Optical Eg(eV)

[from UV]

4.16 3.8

4. Conclusions

In this study, GQDs doped with different transition

metals like CoCl2, NH4Cl and HCl were prepared

by a facile hydrothermal method. Transition levels

of GQDs doped with chlorine containing

compounds were also studied by using U-V Visible

spectroscopy. Cyclic voltammetry measurements

were done for each of these elements to estimate

their energy levels. The reversible redox behavior

has been observed in CoCl2 doped GQDs and HCl

doped GQDs. The presence of high electron affinity

in CoCl2 related compounds suggests that they are

high-quality candidates as acceptor elements for

solar cells applications.

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Chem. Soc., 1958, 80, 1339-1339

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Umapathy and Gurmeet Singh, Synthesis of CoCl2-

Doped Graphene Quantum Dots and its

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K. Sharma and Gurmeet Singh, “Thermal effects

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http://www.ncbi.nlm.nih.gov/pubmed/?term=Fang%20G%5BAuthor%5D&cauthor=true&cauthor_uid=22201338

http://www.ncbi.nlm.nih.gov/pubmed/?term=Nie%20H%5BAuthor%5D&cauthor=true&cauthor_uid=22201338

http://www.ncbi.nlm.nih.gov/pubmed/?term=Liu%20Z%5BAuthor%5D&cauthor=true&cauthor_uid=22201338

http://www.ncbi.nlm.nih.gov/pubmed/?term=Liu%20Z%5BAuthor%5D&cauthor=true&cauthor_uid=22201338

http://www.ncbi.nlm.nih.gov/pubmed/?term=Zhou%20X%5BAuthor%5D&cauthor=true&cauthor_uid=22201338

http://www.ncbi.nlm.nih.gov/pubmed/?term=Chen%20X%5BAuthor%5D&cauthor=true&cauthor_uid=22201338

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http://scitation.aip.org/search?value1=Rongbin+Ji&option1=author&option912=resultCategory&value912=ResearchPublicationContent


http://scitation.aip.org/search?value1=Jun+Zhao&option1=author&option912=resultCategory&value912=ResearchPublicationContent




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https://www.researchgate.net/journal/1364-548X_Chemical_Communications

https://www.researchgate.net/journal/1364-548X_Chemical_Communications


23

Electromagnetic Wave

Propagation in Photonic

Structures: Dielectric

and Metallo-Dielectric

Waveguides

Triranjita Srivastava, Pushpa Bindal*, Priyanka,

Anuradha, Priyam and Priscilla

Department of Physics, Kalindi College (University

of Delhi), Delhi, India, 110008


[email protected]

Abstract: The photonic waveguides are the vital

elements of integrated optics. In this paper, we

present the analysis of the electromagnetic wave

propagation in dielectric and few metallo-dielectric

waveguides. We present the universal V~b curves

and the modal fields for both TE and TM modes for

dielectric waveguide. The metallo-dielectric

waveguides comprise of various combinations of

metal and dielectric materials. The propagation

characteristics of basic metallo-dielectric

waveguides have been studied. We believe that

present work will enhance physical understanding

of the electromagnetic wave propagation through

various photonic waveguides.

Key words: Dielectric waveguides, Metallo-

Dielectric waveguides, Surface Plasmon Polaritons.

1. Introduction

The increasing demand of faster and huge data

transportation and processing has resulted into a

tremendous surge in developmental activities of

electronics and photonics. The electronic circuit

elements are now a days realized as small sized

functional devices such as mobiles, televisions,

computers, etc. but, they prevent the processor

speed above few Gb/s [1]. On the other hand, the

photonic interconnects, such as optical fibers offer

ultra-fast and large information carrying capacity

(Tb/s). Unfortunately, the photonic devices are

limited in size by the diffraction limit of about half

the wavelength of light (~ submicron), and tend to

be at least two orders larger than that of the

electronic components [2]. This size-mismatch

between the electronic and the photonic

components has been overcome by the study of

propagation of surface modes in the metallo-

dielectric waveguides [3].

In this paper, we present the propagation

characteristics of planar dielectric as well as few be

metallo dielectric waveguides, which can be

realized at subwavelength scale. The modal analysis

for the evaluation of propagation constant and the

modal fields for both TE and TM modes have been

done for dielectric waveguides. Moreover, SPP

modes have been studied for two types of basic

metallo-dielectric waveguides, namely; dielectric

layer between metal on either side (MDM) and

metal layer between dielectric on either side (DMD)

waveguides.

2. Mathematical Description

The analysis of dielectric planar waveguide (as

shown in Fig.1) is done by solving the Maxwell’s

equations. One obtains two sets of independent

equations consisting of only transverse electric field

(TE Modes) and transverse magnetic field (TM

modes) respectively. It is well known that the

symmetry in the structure results into symmetric

and antisymmetric modal field solutions as given

below [3]:

Symmetric mode:

2/

2/cos

dxCe

dxxA

x

(1a)



24

Antisymmetric mode:

2/

2/sin

dxCe

dxxA

x

(1b)

where 22

1

2

0 nk , 2

2

2

0

2 nk , A and C

are the constants to be determined. It is to be

mentioned that, the non-vanishing field components

for TE are Hx, Ey and Hz, whereas for TM

modes,

Fig. 1. Schematic of the planar dielectric waveguide.

they are Ex, Hy and Ez. Now applying the boundary

conditions for the TE (continuity of and d /dx)

and TM mode ( and (1/n2) d /dx) gives the

following eigen-value equations:

Symmetric mode:

2tan

d (2a)

Antisymmetric mode:

2cot

d (2b)

where 1 for TE mode and 2

2

2

1 / nn for TM

modes. To obtain the universal characteristics of

planar dielectric waveguides, we rewrite the above

eigen-value equations in the form of normalized

frequency 2

2

2

1)/2( nndV and normalized

propagation constant 2

2

2

1

2

2

2 / nnnnb eff :

Symmetric mode:

bVbVbV

2

11

2

1tan1

2

1 (3a)

Antisymmetric mode:

bVbVbV

2

11

2

1cot1

2

1

(3b)

(a) Metallo-dielectric Waveguides

The metallo-dielectric waveguides comprise of

metals and dielectric in different configurations.

Such waveguides support SPP modes which are

known to be TM polarized in nature and are highly

confined to the metal/dielectric interface. In

literature, several types of metallo-dielectric

waveguides are reported, in which the two basic

metallo-dielectric waveguides are MDM (Fig. 2a)

and DMD (Fig. 2b) waveguides.

(i) Metal/dielectric/metal (MDM) Waveguide

The SPP mode arising at the metal/dielectric

interfaces forms two coupled SPP modes, having

symmetric and antisymmetric field distributions

with respect to the central axis, schematically

shown in Fig. 2.

The modal field for the symmetric and

antisymmetric SPP mode can be written as follows:

Symmetric SPP:

tytyB

tyyAyE

m

d

y

exp

cosh)( (4a)

z d


25

Fig. 2. The schematic of a MDM and DMD waveguides,

showing the symmetric and antisymmetric SPP modes.

Anti symmetric SPP:

tytyB

tyyAyE

m

d

y

exp'

sinh')( (4b)

where, A, B, A’ and B’ are the constants to be

determined, mdmd k ,

2

0

2

, . After solving

these equations as mentioned above, we obtain

following two eigenvalue equations:

Symmetric SPP: dm

mdd t

)(tanh (5a)

Antisymmetric SPP: dm

mdd t

)(coth (5b)

(ii) Dielectric/metal/dielectric DMD waveguide

Similarly, the eigenvalue equation for both the

modes of DMD waveguide is given as:

Symmetric SPP: md

dmmt

)(tanh (6a)

Anti-Symmetric SPP: md

dmmt

)(coth (6b)


(a) Dielectric planar waveguide

Figure 3, illustrates the variation of b (normalized

propagation constant) with V (normalized

frequency) for three lower order TE and TM modes.

It is observed that b-values for TE modes are

slightly greater than that of TM modes. Also, the

fundamental TE0 and TM0 modes have no cut-off V-

values, whereas the higher order TE1 (TM1) and

TE2 (TM2) modes have a finite cut off V-value

corresponding to V= π and 2π. The b-value for

fundamental TE0 mode is highest indicating

maximum mode confinement within the core of the

waveguide. In order to clarify this point, Fig. 4 (a)

and (b) illustrates the electric field of the first three

lower order TE and TM modes respectively. It is

observed that the modal power for the fundamental

TE0 and TM0 mode is tightly confined within the

core (i,e, d) of 4 µm. Whereas, the evanescent field

in the cladding region increases with the order of

the mode, thereby reducing the field confinement

with increasing order.

0 2 4 6 8 10 12 140

0.2

0.4

0.6

0.8

1

V

b

TM1

TE0

TM0

TE1

TE2

TM2

Fig. 3. Variation of b (normalized propagation constant)

with V (normalized frequency)

MDM

DMD


26

-8 -6 -4 -2 0 2 4 6 8-1

-0.5

0

0.5

1

x-coordinate(m)

Ele

ctr

ic f

ield

(a.u

)

TE0

TE1

TE2

-6 -4 -2 0 2 4 6-1

-0.5

0

0.5

1

x-coordinate(m)

Ele

ctr

ic f

ield

(a.u

)

TM

0

TM1

TM2

Fig. 4. Electric Field distribution for 3 lowest order

TE modes (V = 7.7) and TM modes (V=13.3), d =

4μm.

It is to be mentioned here, that although the

fundamental mode has zero cut-off V-value, still

such dielectric waveguides cannot be realized at

very smaller V-value, i.e. smaller (~

subwavelength) width. The reason is attributed to

the fact that the smaller the V-value, smaller is b

and hence, the mode confinement within the core

region is lost, which is also understood the

diffraction limit of light.

(b) Metallo-Dielectric Waveguides

(i) MDM waveguide:

We have shown the variation of real part of

effective indices neff and the propagation length [2]

for the symmetric as well as the antisymmetric

mode with respect to the waveguide thickness (at

wavelength 633 nm) for MDM waveguide

comprising of Au and Silica (RI = 1.45) in Fig.

5(a) and (b). It is observed that at a large value of

'2t', the neff as well as the propagation lengths of

both the SPP modes approaches to that of the SPP

mode at a single interface (Si/Au). It is also

observed that with decreasing '2t', neff for the

symmetric SPP mode increases; whereas for the

antisymmetric SPP mode, it decreases. It is to be

noted that, higher the neff, more is the mode

confinement and thereby, higher is the Ohmic

loss inside the metal (i.e. smaller propagation

lengths). Although the propagation

0 0.2 0.4 0.6 0.8 1

1.5

2

2.5

3

2t (m)

Re

(ne

ff)

Symmetric mode

Anti Symmetric mode

0 0.2 0.4 0.6 0.8 10

2

4

6

8

10

12

2t (m)

L (

m)

Symmetric mode

Anti-Symmetric mode

Fig. 5. Variation of (a) real (neff) and (b) Propagation

length with respect to the width of MDM waveguide.

(a)

(b)

(a)

(b)


27

length of both the modes are ~ few μm, still it is

sufficient for the nanoscale dimensions [1]. Also,

the symmetric SPP mode does not have cut-off

thickness, whereas the antisymmetric SPP mode has

finite cut-off thickness, indicating that the

symmetric SPP mode can be realized at a very

small waveguide thickness ~ few 10 nm, thereby

indicating the realization of highly miniaturized

waveguides.

(ii) DMD Waveguide:

In contrast to MDM waveguides, the DMD

structures possess a complementary behavior for

the symmetric and the anti-symmetric modes. Fig.

6 (a) and (b) illustrates the variation of the real part

of neff and propagation lengths of both the

symmetric and antisymmetric SPP modes with

respect to the metal stripe thickness '2t' (at

wavelength 633 nm) for DMD waveguide

comprising of Si/Au/Si. The figure shows that for

both the SPP modes there is no cut off thickness

and at larger values of '2t' the mode effective

indices of both the modes approach to that of the

SPP at the single metal/dielectric interface.

As the metal stripe thickness '2t' decreases, neff

for the symmetric SPP mode decreases whereas, for

0 0.05 0.1 0.15 0.2 0.25 0.3

1.5

1.6

1.7

1.8

1.9

2

2t (m)

Re(n

eff

) Anti-Symmetric mode

Symmetric mode

0 0.05 0.1 0.15 0.2 0.25 0.3

100

102

104

2t (m)

L (

m) Symmetric mode

Anti-Symmetric mode

Fig. 6. Variation of (a) real part of neff and (b)

Propagation length with respect to width of the DMD

waveguide.

the antisymmetric mode it increases. Therefore, the

anti-symmetric SPP mode is confined to the metal

stripe of very small thickness. The propagation

length of the symmetric mode is ~ few mm, which

is several orders higher than that of MDM

waveguides. It is to mention here that the DMD

waveguides are highly useful in sensing

applications, as the Ohmic loss inside the metal is

very low and the modal field has large spatial extent

in the dielectric region.

Thus the metallo-dielectric waveguides can support

SPP modes, which are confined to the

metal/dielectric interface at subwavelength.

However, such modes suffer ohmic loss due to the

presence of metal, but the propagation length ~ few

10 nm, which is sufficient for the miniature

structures.

4. Conclusions

In this work we present the modal characteristics of

the planar dielectric and the plasmonic waveguides.

The plot of normalized propagation constant b

Verses normalized frequency V, and the modal

field distributions are shown for dielectric

(a)

(b)


28

waveguides, which indicates that such waveguides

have a constraint on the waveguide dimension

being limited by the diffraction limit of light. In

contrast to this, the metallo-dielectric waveguides

based on SPP modes can be realized at the

subwavelength dimensions.

References

[1] M. L. Borngersma, R. Zia and J. A. Schuller,

“Plasmonics- the missing link between

nanoelectronics and microphotonics,” Appl.

Phys. 2007, A89, 221 - 223

[2] W. L Barnes, “Surface plasmon-polaritons

length scales: a route to sub-wavelength optics,”

J. Opt. A: Pure Appl. Opt., 2006, 8, S87 - S93

[3] S. I. Bozhevolnyi, “Effective-index modeling of

channel plasmon polaritons,” Opt. Express,

2006, 14, 9467-9476

[4] A. Ghatak and K. Thyagarajan, “Introduction to

Fiber Optics,” Cambridge University Press,

Cambridge (1998). Reprinted by Foundation

Books, New Delhi, 2008

[5] E. D. Palik, “Handbook of Optical Constant of

Solids”, New York: Academic, 1985.

Acknowledgements: We would like to thank the

National Academy of Sciences India-Delhi Chapter

and Kalindi College for the financial support.


29

A Comparative Study

of Numerical Methods

for Analysing Planar

Plasmonic Waveguides

Triranjita Srivastava, Pushpa Bindal*, Asmita

Deep# and Ashima Sharda#

Department of Physics, Kalindi College

(University of Delhi), Delhi, India, 110008 #B.Sc. (H) Physics IIIrd year, Kalindi College


[email protected]

Abstract: The analysis of planar dielectric

waveguides have been widely done by employing

analytical numerical methods for solving the eigen-

value equations derived from Maxwell’s equation.

However, the analysis of planar plasmonic

waveguides is cumbersome, as the eigen-value

equations are complex and the dielectric constant

of metals, in general, is complex in nature.

Newton-Raphson method is a well-known method

for solving the complex eigen-value equations. But,

this method has certain limitations. It is a bit

tedious as it needs function & its derivative

evaluation. In this paper, we propose a modified

bisection method to solve complex eigen-value

equation, which is found to be simple and robust.

This method iteratively, bisects an appropriate

interval containing the root and then selects a

subinterval within which the root exists. The

comparison shows that the number of iteration

required in bisection method is many times less

than that of Newton Raphson method for the same

initial approximation. However, the time elapsed in

the executing the modified bisection method is

slightly larger than that required in Newton

Raphson Method; still the proposed method has

certain advantages over Newton Raphson Method.

Thus we employ the proposed method for the

analysis of various plasmonics waveguides, such as

metal/dielectric/metal waveguides.

Key words: Numerical methods, plasmonic

waveguides, complex eigen-values.

1. Introduction

Numerical methods involve the analysis of

algorithms which are based on certain numerical

approximations for mathematical analysis of the

real problems. Hence, the numerical methods are

applied in all areas of science and engineering. In

particular, the analytical methods are employed for

the analysis of the planar photonic waveguides,

because they are simple to implement and provide

physical understanding of the electromagnetic

wave propagation in such waveguides. These

analytical methods comprise of solving the eigen-

value equations, which are well known in the

literature [1]. However, the application of such

method for the analysis of planar plasmonics

waveguides is cumbersome, as the eigen-value

equations become complex, due to complex

dielectric constant of metals [2]. Therefore, in the

absence of exact analytical methods, the modeling

of the plasmonic waveguides is carried out by

employing either numerical methods or semi-

analytical methods. The numerical techniques, such

as finite difference method, finite element method,

etc, are time consuming, rigorous and require high

computational memory. Such methods are

sometimes found to be unstable, because of the fine

mesh at the vicinity of the edges of the metal. On

the other hand, the approximate analytical methods,

although are less accurate, but are simple to

implement and give better physical understanding

of the problem such as, the effect of the respective

role of the various waveguide parameters like

waveguide shape, size and operating wavelength.

Thus, in this paper we discuss the numerical

methods to solve complex eigen-value equations.

Newton-Raphson method is a well-known

numerical method for solving the complex eigen-

value equations [3]. But, this method is a bit

tedious as it needs function and its derivative

evaluation and has certain limitations. In this paper,

we propose a modified bisection method to solve

complex eigen-value equation, which is found to be

very simple & robust as it iteratively, bisects an

interval & then determines a subinterval within

which the root exists. The comparison shows that

the number of iterations required in bisection



30

method is many times less than that of Newton

Raphson method for the same initial

approximation. However, the time elapsed in the

executing the modified bisection method is slightly

larger than that required in Newton Raphson

Method; still the proposed method has certain

advantages over Newton Raphson Method. Further,

the modified bisection method is employed for the

analysis of metal/dielectric/metal plasmonics

waveguides.

2. Mathematical Description

The detailed mathematical description of the

Newton Raphson method and the proposed

modified bisection method is given below:

2.1. Newton Raphson method

The Newton Raphson Method is a widely used

method for determining the roots of equations

accurately. It requires an initial approximation, xₒ.

A tangent to the function f(x) at x = xₒ is draw

which intersects the x- axis at x1, as shown in Fig.1.

The intersection point x1, is now the new

approximation to the root. The entire procedure is

repeated till the convergence for desired accuracy

is achieved.

Fig. 1. Schematic representation of Newton Raphson

method

The formula for the (i+1)th approximation is given

by:

xi+1 = xi – (1)

where f and are the function and its

derivative evaluated at the ith iteration i.e. .

It is to be noted that the Newton Raphson method

requires that the function and its derivative has to

be evaluated at each point, which is not always

possible. Moreover, the method fails when the

tangent to the function is parallel to the x-axis.

2.2. Modified Bisection Method

The general bisection method, based on mean value

theorem for continuous functions, is a well-known

root-finding method. It is implemented to solve real

functions and achieve the real roots. This method

repeatedly bisects an appropriate interval and

selects a subinterval which encloses the root, for

the next iteration.

The method is applied for numerically solving the

equation f(x) = 0, where x is a real variable. Here

f(x) is a continuous function within an interval

[a, b] such that the values of f(a) and f(b) are

opposite in signs. At each iteration, the method

bisects the chosen interval [a, b] into two sub-

intervals by calculating the midpoint c = (a+b)/2

of the interval.

In this paper, we propose a modified bisection

method which is applicable for the determination

of complex roots. The method is discussed below:

Let the exact root of the given eigenvalue equation

be of the form

x = xr + ixi (2)

where, xr and xi are real and imaginary parts of the

root respectively.

Iteration 1: In the first approximation we

choose xi(0) = 0, and apply the general bisection

method to the real part of f(x) to obtain the real

root xr(1). Now the root is

x(0)=xr(1)+ ixi

(0).

We again apply the general bisection method on

the imaginary part of f(x), by taking the initial

approximation of the root as x(0)=xr(1)+ ixi

(0) and

obtain xi(1).

Therefore, after first iteration we get the

approximate root as


31

x(1)=xr(1)+ ixi

(1). (3)

Iteration 2: We apply the bisection method on

the real part of f(x), by taking above equation as

initial approximation and obtain xr(2). Now the

approximation becomes x(1)=xr(2) + ixi

(1), for the

application of bisection method on the

imaginary part f(x), to determine xi(2)

.

Thus, after second iteration the approximate

root is

x(2)=xr(2)+ ixi

(2).

The above process is repeated till the result

converges to the desired accuracy.

In short, the modified bisection method solves the

complex eigen-value equation by iteratively

applying the general bisection method on the real

and imaginary parts of the function f(x) separately.

The advantage of this method is that it is simple to

implement and robust, as it doesn’t require any

derivative evaluation. Moreover, in absence of any

information of root, it is the best method, as it gives

definite convergence.


In order to compare both the methods we choose

an example of a metal/dielectric/metal (MDM)

waveguide. Such a waveguide comprises of a

dielectric layer of thickness ‘d’ sandwiched

between two metals as shown in Fig 2.

Fig.2. Schematic of a metal/dielectric/metal waveguide

The electromagnetic wave propagation theory

reveals following complex eigen-value equation

for the waveguide:

tanh(x) = - (4)

where, we have chosen gold as a metal and air as a

dielectric medium of refractive index , d= 1, m

=-15.21+0.65i, x is the complex root to be

determined, and V = d ; normalised

frequency with d = 40 nm; width of dielectric

layer, λ = 0.633 µm; operating wavelength.

We first apply Newton Raphson method to the

above complex eigen-value equation with different

initial approximations, as shown in TABLE I. It is

observed that the root of this equation is

0.2361334623751 + 0.0030586326129i which is

achieved in 5 iterations, only if the initial

approximation (xₒ = 0.23) is sufficiently close to

the root. The number of required iterations

increases as the chosen initial approximation is

away from the root. Moreover, at the large value

of xₒ = 1.0, the solution becomes negative. Thus,

the efficiency of Newton Raphson Method is

dependent on the selection of initial

approximation; without knowing this, one cannot

get accurate results.

TABLE I: Newton Raphson Method: Variation of

number of iterations with respect to the initial

approximation

Initial

Guess

Iteration

s Root obtained

Nature

of result

0.078 07 0.2361334623751 + 0.0030586326129i

0.23 05 0.2361334623751 + 0.0030586326129i

0.44 07 0.2361334623751 + 0.0030586326129i

1.0 08 -0.2361334623751 -0.0030586326129i

X

Further, in TABLE II, the comparison of Newton

Raphson Method (NRM) and modified bisection

Metal

Dielectric

Metal d


32

method (MBM) is shown in terms of number of

iterations and time elapsed in executing the

method. The result obtained by both these methods

is exactly same. It is found that the exact root

obtained by modified bisection method converges

in 3 iterations for accuracy of 10-13.

TABLE II: Comparative study of the roots obtained &

no. of iterations, for Newton-Raphson (NR) & modified

bisection (MB) methods

Metho

d

Iteratio

n Root obtained

Time

elapsed

(sec)

NRM 5 0.2361334623751 + 0.0030586326129i

0.0118

MBM 3 0.2361334623751 + 0.0030586326129i

0.0210

It is to be noted that, although the time elapsed in

executing the modified bisection method is

slightly larger than that of Newton Raphson

Method, but the modified bisection method is

found to be independent of the initial

approximation. This method has a definite

convergence, provided the root is lying within the

interval. Moreover, the proposed method is simple

to implement and doesn’t require the evaluation of

the derivative of the function, which is not always

possible for all complex modal functions.

Therefore, in the case of unknown initial

approximation, the modified bisection method is

known to be more robust, in comparison to

Newton Raphson Method.

Further, we applied the modified bisection

method for the analysis of MDM waveguides.

TABLE III illustrates the exact value of the root at

different values of waveguide thickness ‘d’.

TABLE III: Evaluation of the root at different values

of thickness ‘d’ for MDM waveguide by employing

modified bisection method

Waveguide

thickness ‘d’ (nm) Root obtained

20 0.16967124 + 0.00239925i

40 0.23613346 + 0.00305863i

60 0.28851321 + 0.00364233i

80 0.33349534 + 0.00417292i

100 0.37376839 + 0.00466773i

4. Conclusions

In this paper we proposed modified bisection

method, which is simple and robust as compared to

Newton Raphson Method. The metal-dielectric-

metal (MDM) waveguide has been studied by

solving its complex eigenvalue equation.

References

[1] Ajoy Ghatak and K. Thyagarajan, “Introduction

to Fiber Optics,” Cambridge University Press,



[2] E. D. Palik, “Handbook of Optical Constant of

Solids”, NewYork: Academic, 1985.

[3] S. S. Sastry, “Introduction to Numerical

Methods,” PHI, 2005


33

STUDY OF

PROPAGATION

CHARACTERISTICS

OF OPTICAL

FIBERS:

EXPERIMENT AND

SIMULATION

Pushpa Bindal*, Triranjita Srivastava, Sujata#,

Anju# and Diksha Tandon#


(University of Delhi), Delhi, India, 110008 #B.Sc. (H) Physics IIIrd year, Kalindi College


[email protected]

Abstract: In this work, we present propagation

characteristics of fiber by employing simulations

and experiment. The variation of mode effective

index with respect to the wavelength is obtained

analytically by solving scalar wave equations. The

3-D modal field distribution and surface plots for

the first two lowest order LPlm modes are obtained.

Moreover, we experimentally employed the near

field scanning technique to obtain the diameter of

the core and refractive index variation in a

multimode optical fiber. It is observed that the

obtained results are in consensus with the given

specifications of optical fiber. We believe that the

present study will enhance the understanding of the

electromagnetic wave propagation in optical fibers.

Key words: Optical fiber, Refractive index profile,

Near field measurement technique.

1. Introduction

The increasing demand of faster and huge data

transportation, networking and processing has been

achieved only due to the very low transmission loss

(~0.2 dB/km) in optical fibers [1, 2]. The optical

fibers have found applications in data storing

equipment, telecommunication, medical use, oil

and gas industries, military, transport and also as

decorative material. Since few decades, there has

been a phenomenal growth in fiber optic industry,

which gave rise to various applications such as:

fiber optic sensors, integrated optic components

(polarizers, directional couplers, fiber gratings,

fiber amplifiers, optical switches, etc.) optical

signal processing, etc [3]. In addition to its

tremendous technological importance, fiber optics

also offers a platform to present demonstration and

understanding of various physical concepts.

We study the modal properties of optical fiber. The

variation of mode effective index has been

obtained for GeO2 doped optical fibers. We also

present the 3-D modal field distribution of two

lowest order modes, along with their 2-D surface

plots. It is to be mentioned here, that the modal

characteristics of fibers are almost dictated by the

refractive index variation in the core of the fiber.

Therefore, in this work we experimentally

employed near field scanning technique [4] to

determine the diameter and the refractive index

variation in the core of a given multimode optical

fiber. It has been observed that our results are

matching with the specifications of given optical

fiber.

2. Theory

The refractive index of the step-index optical fiber

(cross section shown in Fig. 1) is given by:

n(r)=

arn

arn

2

1 0 (1)

where n1 and n2 are the refractive indices of core

and cladding regions respectively, is the core

radius.

In order to obtain the propagation characteristics of

the optical fiber, we numerically solved the

Maxwell’s equations under weakly guiding

approximation.



34

Fig. 1. Schematic diagram of cross – section of optical

fiber

In this work, we also used the near field scanning

technique to determine refractive index profile

(RIP) and the core diameter of the optical fiber.

The experimental setup is shown in Fig. 2, in

which light from tungsten halogen lamp is

launched into the optical fiber with the help of 20X

microscopic objective. The output from fiber is

measured by a photo-detector.

Fig.2. Experimental Setup for determining the RIP

The analysis of power emitted by an incoherent

source and launched into multimode optical fiber

yields [4]:

(2)

where and are the near field intensity and

RIP obtained at r distance from the center of the

fiber. is the maximum power at the

center and and are the refractive indices at

the center and cladding region, respectively.

For small refractive index differences,

(3)

The refractive index variation for the multimode

fibers is known to follow [1]:

(4)

where is the relative core-cladding difference

is the index exponent

depicting the shape of RIP in the core region. For

example, corresponds to a triangular core

RIP and ideally corresponds to a step index

fiber. Solving the above equations, we get:

(5)

Or

(6)

Hence a log-log plot of against

would result in a straight line of slope q and

hence gives the shape of the profile.


We first present the propagation characteristics of

a GeO2 doped optical fiber. Figure 3 illustrates the

variation of mode effective index neff with respect

to wavelength for fundamental mode of optical

fiber comprising of core diameter 8.2µm. The core

and cladding of the optical fiber is chosen as 6.3 %

GeO2 doped silica and pure silica respectively [1].

It is observed that the neff decreases with


35

increasing wavelength indicating that the modal

confinement within the core region decreases [1].

0.6 0.8 1 1.2 1.4 1.61.445

1.45

1.455

1.46

1.465

1.47

(m)

ne

ff

Fig. 3. Variation of mode effective index neff with

respect to wavelength.

It is well known in the literature that

electromagnetic field propagates in the form of

modes within the waveguides/optical fibers.

Therefore, in order to understand the behavior of

mode propagation, Fig. 4 and 5 illustrates the 3-D

electric field variation of two lower order modes,

LP01 and LP11 respectively. The corresponding

surface plots are shown in the inset of the figures,

which are important to study for the nomenclature

of the LPlm mode. Here, (m - 1) is gives number of

zeros in radial directions, and 2l represents the

number of zeros in the azimuthal direction. Hence

for fundamental mode, l=0, as it has no zero

crossing in azimuthal direction. It can be seen that

the LP01 mode exhibit no zero crossing

respectively, along the radial direction, resulting in

m = 1 respectively. A similar analysis is done for

LP11 modes, which has two zero crossing in

azimuthal direction and one zero crossing in radial

directions, therefore l = 1, m = 1.

Fig. 4. Electric field distribution of LP01 mode (inset:

respective surface plot).

Fig. 5. Electric field distribution of LP11 mode (inset:

respective surface plot).

As mentioned above, we employed near field

scanning technique for obtaining the RIP of the

optical fiber. Fig. 6, illustrates the variation of

normalized near field intensity with respect to the

radial distance.


36

0 0.05 0.1 0.15 0.20

0.2

0.4

0.6

0.8

1

r (mm)

No

rmalized

In

ten

sit

y (

a.u

.)

Fig. 6. Experimentally observed near field pattern of a

given fiber.

It is observed that the intensity falls off with

increasing distance. It is known that the distance

over which the intensity of near field drops by 95%

on x-axis, represents the core radius a = 0.14 mm.

This value is approximately equal to the core radius

0.125 mm given by the manufacturer.

In order to obtain the q value for the fiber, Fig.

7, shows the log-log plot of with

respect to . As expected, the curve is a

straight line, which has a slope of 11.2. It is worthy

to note that such a high value to q leads to a step

index profile, as shown in Fig. 8, which gives the

variation of refractive index as given by Eq. (4) for

q = 11.2 and a = 0.14 mm.

-5 -4 -3 -2 -1 0-7

-6.5

-6

-5.5

-5

-4.5

-4

-3.5

-3

-2.5

log(r/a)

log

[1

-P(r

)/P

(0)]

Fig. 7. log-log plot of against .

0 0.05 0.1 0.15 0.21.95

2

2.05

2.1

2.15

2.2

2.25

r (mm)

n2 (

r)

a = 0.14 mm

Fig. 8. Plot of RIP in the given fiber with radial

distance.

4. Conclusions

In this work, we present the modal characteristics

of step index fiber. Moreover, the diameter of the

fiber core and its RIP are experimentally obtained

by using near field scanning technique.

References

[1] A. Ghatak and K. Thyagarajan, “Introduction to

Fiber Optics,” Cambridge University Press,


Books, New Delhi (2008).

[2] A. Ghatak and K. Thyagarajan, “Optical

Electronics,” Cambridge University Press,

Cambridge (1989).

[3] B. P. Pal (Ed), “Fundamentals of Fiber Optics

in Telecommunication and Sensor Systems,” Wiley

Eastern, New Delhi (1992).

[4] M. R. Shenoy, Sunil K. Khijwania, Ajoy

Ghatak and Bishnu P. Pal (Ed), “Fiber optics

through experiments,” Viva Books, New Delhi.

Acknowledgements We would like to thank the

National Academy of Sciences India-Delhi Chapter

and Kalindi College for the financial support.


37

Experimental Study of

Microbending Losses in

Optical Fiber

Pushpa Bindal*, Triranjita Srivastava, Ananya#,

Aastha Dhankhar#


(University of Delhi), Delhi, India, 110008

#B.Sc. (H) Physics IIIrd year, Kalindi College


[email protected]

Abstract: The real fibers are deployed under the

sea/earth, where they experience various pressures

which introduce the optical power loss, due to

which the strength of the received output signal

gets reduced. When the fibers bend slightly due to

these pressures and this bend is of the order of fiber

diameter, corresponding power loss is termed as

the microbending loss in the optical fiber. In this

work, we experimentally studied the microbending

losses in optical fibers of different core sizes by

employing two different types of microbenders

with unequal pitch. The results are in good

agreement with theoretical predictions and show

that microbending losses are higher for (i) fibers of

larger radius and (ii) smaller pitch of microbender.

We believe that the study will help in

understanding and eliminating sources of

microbending losses and using optical fiber as a

pressure sensor.

Key words: Optical fiber, Pressure Sensor, Micro-

bending Losses.

1. Introduction

The optical fibers have ultra-high capacity of data

transmission and processing over very large

distances ~ 1000 km [1-3] with minimal loss ~ 0.2

dB/km. At present the optical fiber cables are

running around the earth, being installed in the

oceans and seas, where they experience various

pressures. As is well known, optical fibers are

widely used to monitor internal conditions,

vibrations and aging of various structures like

pipelines, oil wells, bridges, turbines, buildings etc.

with integrated fiber-optic sensors, called “smart

structures”. In short, the fibers are deployed in the

harsh environment, where they are subjected to

various stresses, which might affect the

transmission through the fiber resulting in

distortion in the optical signal. In general, lateral

stress may be caused by the pressure induced due

to manufacturing or installation faults. Moreover, it

can also be generated by temperature induced

dimensional changes in cabling materials. In

particular, this lateral stress along the length of

fiber is known as microbending loss, if the bend

diameter is of the order of fiber diameter [3].

In this work we study the application of optical

fiber as a pressure sensor subject to microbends. In

optical fiber pressure sensor, light is coupled to one

end and detected at the other end, in terms of

modulated intensity. Microbends are introduced in

fiber using deformer elements, known as

microbenders. In this paper, we experimentally

present the microbending losses in optical fibers of

different core sizes by employing two

microbenders of different pitch at the wavelength

633 nm. The results are in good agreement with

theoretical predictions and show that

microbending losses are higher for (i) fibers of

larger radius and (ii) smaller pitch of microbender.

We believe that the study will help in

understanding and eliminating sources of

microbending losses and using optical fiber as a

sensor.

2. Experimental Setup

Figure 1, illustrates the schematic of the

experimental setup to study the microbending loss

in the optical fiber. A Helium-Neon laser emitting

light at wavelength 633 nm is used to launch power

into the input end of the optical fiber and fed at the

other end to photodetector. In between, the fiber is

subjected to a microbender of pitch 2D, which has

a periodic deformer element. When a portion of the

fiber is sandwiched between the microbender, fiber

undergoes periodic deformation in the form of

microbends [4]. The resultant mechanical

deformation is perpendicular to its axis, causing



38

higher-order guided modes to radiate out of the

fibers core through the cladding interface.

Fig. 1. Micro-bending losses in Optical Fiber due to

pressure applied

When the pressure is applied to the microbender,

microbends are created which in turn modulate the

intensity of transmitted light at the other end of the

fiber. Higher order modes radiating out of fiber

core through core cladding interface due to external

pressure applied to the optical fiber perpendicular

to its axis, cause this fall in intensity of transmitted

light.

It is well known that the loss of guided power by

radiation at the bend is given by [5];

(1)

where ‘d’ represents the radius of core, ‘R’ is the

radius of curvature of bend, and A is a constant.

Thus, for a given fiber, the pressure applied to the

bend radius, which is given by

(2)

where, y is the displacement of fiber caused by

pressure in micro-bender element and 2D is the

distance between micro-bender element’s contact

points (pitch), as shown in Fig. 2.

Fig. 2. Geometry of the micro-bend


Fig. 3- 4, illustrates the variation of transmitted

intensity through a microbend modulated fiber

optic sensor with respect to the applied weight over

two types of microbenders namely; microbender 1

(pitch = 2.17 cm) and microbender 2 (pitch =

0.96cm). The chosen fiber is of core diameter 250

µm and 750 µm in fig. 3 and fig. 4 respectively. In

both the figures, as expected, the intensity

decreases parabolically with increasing weight.

Also the intensity is more for microbender 1, which

has larger pitch as compared to the microbender 2.

The reason attributed to the fact that larger the

pitch is, larger is the radius of curvature R (Eq. 2),

which results in decrease in transmission loss ,

hence we observe more transmitted intensity in the

case of microbender 1.

Figure 4, illustrates the similar variations in

transmitted intensity for the two microbenders for

fiber 2 of core diameter 750 µm. Clearly intensity

losses are higher for both microbenders since the

fiber has a larger diameter in comparison to fiber of


39

0 500 1000 1500 2000 25000

0.25

0.5

0.75

1

Weight (gm)

Inte

ns

ity

(a.u

)

microbender 1

microbender 2

Fig. 3. Variation of normalized intensity with respect to

weight applied at microbender 1 and microbender 2, for

fiber 1 of core diameter 250 µm.


weight applied at microbender 1 and microbender 2, for

fiber 2 of core diameter 750 µm.

Fig. 1, as expected. Further, Fig. 5 and 6, illustrate

the variation of normalized intensity with respect to

weight applied at microbender 1 and microbender

2, for two fibers; first fiber termed as fiber 1 has

core diameter of 250 µm and the second one called

fiber 2 with core diameter 750 µm. In both the

figures, it is observed that the transmitted intensity

falls off parabolically with respect to applied

weight. Moreover, fiber 1, which is of lesser

diameter as compared to fiber 2, exhibits low loss

and hence larger intensity. These findings can be

easily understood from Eq. (1).

0 500 1000 1500 2000 25000

0.2

0.4

0.6

0.8

1

Weight (gm)

Inte

nsit

y (

a.u

.)

Fiber 2

Fiber 1


weight applied at microbender 1 for two fibers; fiber 1

of core diameter 250 µm and fiber 2 of core diameter

750 µm.

0 500 1000 1500 20000

0.2

0.4

0.6

0.8

1

Weight (gm)

Inte

nsit

y (

a. u

.)

Fiber 1

Fiber 2


weight applied at microbender 2 for two fibers; fiber

1of core diameter 250 µm and fiber 2 of core diameter

750 µm.

Again losses are more in case of mirobender 2 in

Fig. 6 due to its smaller pitch. It is to be mentioned

here, that in Fig. 3-6, we employed quadratic curve

fitting of MATLAB®, which reveals parabolic fall

of intensity with respect to increase in weight.

In short, the results show good agreement with the

theoretical predictions of micro-bending losses.


40

The finding show that micro-bending losses are

higher for

(i) fibers of larger radius

(ii) smaller pitch of micro-bender.

4. Conclusions

In this paper we study the optical fiber pressure

sensor. The variation of transmitted intensity is

studied for two different micro-benders and

optical fibers. We believe that the study will help

in understanding and eliminating sources of

micro-bending losses and using optical fiber as a

sensor.

References

[1] A. Ghatak and K. Thyagarajan, “Introduction

to Fiber Optics,” Cambridge University Press,



[2] A. Ghatak and K. Thyagarajan, “Optical

Electronics,” Cambridge University Press,

Cambridge, 1989

[3] B. P. Pal (Ed), “Fundamentals of Fiber Optics

in Telecommunication and Sensor Systems,”

Wiley Eastern, New Delhi, 1992

[4] M. R. Shenoy, Sunil K. Khijwania, Ajoy

Ghatak and Bishnu P. Pal (Ed), “Fiber optics

through experiments,” Viva Books, New Delhi.

[5] C. K. Kao, “Optical Fiber Systems:

Technology, design and application,” Mcgraw –

Hill, New York, 1982.

Acknowledgements

We would like to thank the National Academy of

Sciences India-Delhi Chapter and Kalindi

College for the financial support.


41

Growth of (001)

oriented Cr and MgO

thin films on

Amorphous Substrate

for Magnetic Tunnel

Junctions

Sajid Husain, and Sujeet Chaudhary*

Thin Film Laboratory, Department of Physics,

Indian Institute of Technology Delhi, New Delhi

110016 (INDIA)

Ankit Kumar, Serkan Akansel, and Peter

Svedlindh,

Ångström Laboratory, Department of Engineering

Sciences, Box 534, SE-751 21 Uppsala, Sweden


[email protected]

Abstract: We have carried out a systematic study

to optimize the growth parameters to obtain the

oriented growth of Cr (002) and MgO (200) thin

films using dual ion-beam sputtering technique on

thermally oxidized silicon and glass substrates. It is

found that the preferred crystallographic

orientation of Cr depends on the film growth rate

(sputtering rate) and its post annealing treatment.

X-ray diffraction analysis has revealed that 110 W

and 85 W grown and subsequently 500°C post

annealed Cr thin films result in the (110) and (002)

crystallographic orientations, respectively. The

MgO thin film grown at room temperature using

the oxygen ion assisted ion-beam sputter

deposition, without requiring any pre/post substrate

annealing treatment exhibits (200) orientation. The

interface/surface qualities of all the samples have

been investigated using X-ray reflectivity analysis.

Extremely small surface roughness of 0.28 and

1.49nm are observed for Cr and MgO films,

respectively. The oriented growth of MgO and Cr

thin films is established in correlation with the

energetic ion-beam deposition process which is

expedient in spintronic devices i.e., MTJs devices.

Key words: Cr, MgO, X-ray reflectivity 1.

1. Introduction

The chromium (Cr) and magnesium oxide (MgO)

thin films are pivotal choice of materials for

development of spintronics devices. Cr is one of

the inevitable choices of the buffer layer materials

for the growth of epitaxial ferromagnetic (FM)

layer particularly Co based Heusler alloy

(Co2FeAl) layer. However the MgO thin films not

only accommodating to induce perpendicular

magnetic anisotropy (PMA) but also inevitably

important as an insulator tunnel barrier for good

band matching with Heusler alloys [1]. This

suitable band matching among the FM Co2FeAl

(CFA) having low damping [2] and MgO layers

enhanced the electron tunnelling probability

consequently larger percentage change in magneto-

resistance ratio in magnetic tunnel junctions

(MTJs) devices. The MgO, alike Cr layer, is indeed

a choice of buffer layer materials to grow an

epitaxial Heusler alloy thin films [3]. It is well

known that the Cr thin films have either (110) or

(002) crystallographic texture. However, it is

difficult to grow (001) oriented Cr epitaxial layer

even on single crystal substrate [4]. To prevent

shunting to substrate in MTJs and spin transfer

torque (STT) spintronics devices it is indeed

needed to grow the epitaxial structures on

insulating substrates; particularly on the

technologically and industrially important

thermally oxidized silicon (Si/SiOx) substrate.

However the oriented growth on amorphous

substrate is technologically critical. To overcome

this issue we utilized energy enhanced ion beam

sputtering process to grow the Cr and MgO thin

films over (Si/SiOx) and glass substrates. In this

paper we report the growth rate dependency of the

orientation of Cr thin films, and oxygen ion

assisted 200 oriented stoichiometric phase

formation of MgO thin films.

2. Experimental

In this work the Cr thin films were deposited on

(Si/SiOx) substrates at 100W and 85W powers at



42

room temperature using ion beam sputtering

deposition technique (NORDIKO-3450) and

subsequently annealed at 500°C temperature. The

HV chamber was evacuated down to ~2×10-7 Torr

base pressure using a turbo and cryo-pump. The 6”

dia. Cr and Mg target fixed on a remote controlled

water cooled turret was sputtered by ~4.5 inch dia.

high energy Ar-ion beam (500 eV) extracted from a

RF ion-source. During the deposition, the chamber

pressure was maintained at ~8.2×10-5 Torr by

bleeding 4 sccm Ar gas directly into the ion-source

operated at 100W and 85W for Cr. The MgO thin

films were deposited on (Si/SiOx) as well as on

glass substrate using ion assisted ion-beam

sputtering deposition technique (see ref. 5 for

details). The deposited Cr and MgO thin films were

investigated by Bragg-Brentano () and

glancing angle X-ray diffraction (GAXRD),

respectively. The film thickness, electron density

and surface roughness were investigated by

simulating the specular X-ray reflectivity (XRR)

spectra using the PANalytical X’Pert Reflectivity

software (ver. 1.2 with segmented fit).


3.1 Chromium (Cr)

Figure 1 shows the X-ray diffraction patterns

of Si/SiOx/Cr(41nm) thin film deposited at 100W

and 85W RF-powers at RT and subsequently

annealed at 500°C. It is clearly evident that 110W

power deposited sample results in (110)

crystallographic orientation although the 85W

power deposited sample exhibits (002) orientation.

The observed changes of Cr texture are attributed

to the film growth rate, changes of RF sputtering

power, as the annealing temperature and time were

kept constant. This growth rate assisted changes in

the post anneal Cr films crystallographic

orientation can be understand by the growth

models explained in Feng et al [5]. In film growth

mechanism the high sputtering/growth rate favours

the faster grain growth and therefore faster

nucleation of atoms which results the smaller

grains on amorphous systems. The planes of the

grains having low free energy at their surface will

grow faster compared to others ignoring the fact

which texture start nucleating at the substrate

surface. The BCC Cr thin film (110) texture is

having the lowest free energy therefore it is more

favourable to grow. Its growth depends on the size

of island sizes (large number of grain boundaries

are preferred) and the energy of the

deposited/growing nuclei. The high power (100W)

ion-beam growth can fulfil all these requirements.

The high growth rate of the growing thin films on

amorphous substrate results in small sized grains

and hence in higher number of grain boundaries.

Subsequently, it favours the growth with (110)

textures.

Further, ion beam sputtering is an energy-

enhanced process, compared to other deposition

methods, in which the sputtered atoms/ions carry

relatively higher energies resulting in higher grain

boundary migration during the film growth leading

to energetically favourable grain-orientation.

Further enhancement of the 110 texture can be

done by post deposition annealing process as

executed in the present sample growth. Since, the

nucleation depends on the kinetic energy of add-

atom and their mobility, therefore, deposition at

100W RF-power having higher growth rate

compared to 85 W deposition leads to the growth

of (110) texture of Cr thin films. However at low

RF-power (85W) the nucleation rate is small, thus

the add-atom gets more time for surface diffusion

and are able to to contribute in the growth of bigger

grains of (002) orientation in the beginning of

deposition. Thus the equilibrium island growth

occurs which possess the (002) texture. Therefore,

the Cr thin film grown at 85 W power results in

(002) texture in comparison to the high power

(100W) sputtered films which exhibited (011)

orientation.

Fig.1: XRD spectra of Si/SiOx/Cr(40nm) thin films grown at

different RF-power.


43

In order to precisely determine the thickness,

density, and the amount of surface roughness of Cr

thin films specular XRR spectra were recorded as

shown in Fig.2. The estimated values of film

thickness, density and associated surface roughness

are presented in Table I. The density of Cr was

found to be 6.64gm/cc which is comparable to the

bulk value of density of Cr i.e.,7.19gm/cc. The

very small

Fig.2: XRR spectra of Si/SiOx/Cr(41nm) thin films grown at

100W RF-power.

roughness (4Å) and nearly equal bulk density of

these films indicate the excellent film quality.

These excellent sample quality in terms of surface

roughness, density and crystallographic

orientations are associated to energy enhanced

growth technique where sputtered atoms have high

10-20eV energy compared to other deposition

technique as discusses above.

Table I: The XRR simulated parameters for Cr and

MgO thin films; density , thickness t, and surface

roughness .

Si/SiOx/Cr Si/SiOx/MgO

Layer SiOx Cr Cr2O3 SiOx MgO

(g/cc)±0.06 3.26 6.64 5.57 3.26 2.96

t(nm)±0.01 60000 41.80 1.62 60000 31.67

(nm) ±0.03 0.43 0.41 0.28 0.37 1.49

3.2 Magnesium oxide (MgO)

Figure 3 and 4 show the XRD spectra of MgO thin

film deposited at room temperature on oxidize

silicon and glass substrates, respectively. Presence

of a single peak corresponding to (200) orientation

on both the substrates indicates the preferred

oriented growth of MgO thin film. The

stoichiometric phase of MgO films deposited at RT

are optimized by varying various parameters such

as sputtering power, and O2 ions energy at different

oxygen partial pressures using ion assisted gun.

Here, in present case the MgO thin films was

prepared at O2 partial pressure of 1.210-4 Torr at

75 W of RF-power with Ar partial pressure of

1.910-4 Torr for Mg metal sputtering at 100W.

The systematic study of MgO thin film at various

O2 ion energy and partial pressures were reported

by Braj et al [6].

Fig.3: XRD spectra of Si/SiOx/MgO(30nm) thin film.

It is observed that the (200) diffraction peak of

MgO thin film deposited on glass substrate is not

very sharp compared to the thermally oxidized Si.

It is attributed to fact that the roughness of the glass

surface is significantly higher than the thermally

oxidized Si which requires larger formation energy

for crystallization on glass substrate.

Fig.4: XRD spectra of Si/SiOx/MgO(30nm) thin film.

The XRR spectra recorded on Si/SiOx/MgO

thin film grown at room temperature is shown in


44

Fig.(5). The observed surface roughness for

MgO thin films is relatively higher as compared

to the Cr thin film. It could be inferred by the fact

that the Mg is hygroscopic in nature and it forms

the hydroxide when it comes in contact with

atmosphere, thereby resulting in higher surface

roughness. However, we would like to mention

that the interfacial roughness of surface, which is

of the order of half a monolayer, protected the

ultrathin MgO layer[7].

Fig.5: XRR spectra of Si/SiOx/MgO(30nm) thin film

grown at room temperature.

4. Conclusions

The oriented thin films of Cr and MgO have been

deposited on oxidised Si and glass substrate

using ion assisted ion beam sputtering technique.

It has been observed that the crystalline

orientation of Cr critically depends on the

growth/sputtering rate. The (200) orientation of

MgO thin films is obtained at room temperature

without requiring any post annealing treatment.

These Cr and MgO oriented thin films are

indispensable for spintronic devices as an under-

layer and MTJ barrier, respectively.

Acknowledgements

SH thankfully acknowledges the DST, India for

providing the INSPIRE fellowship for research.

References

[1] Tezuka, N., Ikeda, N., Mitsuhashi, F. &

Sugimoto, S. Improved TMR JCs with Heusler

Co2FeAl0.5Si0.5 electrodes fabricated by

molecular beam epitaxy. Appl. Phys. Lett. 2009,

94, 162504.

[2] Husain, S., Akansel, S., Kumar, A.,

Svedlindh, P. and Chaudhary, S., Growth of

Co2FeAl Heusler alloy thin films on Si(100)

having very small Gilbert damping by Ion beam

sputtering Sci. Rep. 6, 28692 (2016).

[3] Ortiz, G. et al. Growth, structural, and

magnetic characterization of epitaxial Co2MnSi

films deposited on MgO and Cr seed layers. J.

Appl. Phys. 2013, 113, 043921.

[4] Schmid, M., Pinczolits, M., Hebenstreit, W.

& Varga, P. Segregation of impurities on Cr(100)

studied by AES and STM. Surf. Sci. 1997, 377-

379, 1023–1027.

[5] Feng, Y.C., Laughlin and lambeth D.N.

Formation of crystellographic texture in RF

sputtered Cr thin films, J. Appl. Phys 1994 76,

7311.

[6] Singh, B. B., Agrawal, V., Joshi, A. G. &

Chaudhary, S. XPS and CAFM investigations on

dual ion beam sputtered MgO ultrathin films.

Thin Solid Films 2012, 520, 6734–6739.

[7] The multilayer structure Si/Ta(10nm)/

Co2FeAl (1.8nm)/MgO(2.2)/Ta(2nm) was

prepared for PMA and XRR simulation provide

the interfacial roughness less then 3Å.


45

Bio ceramics: Future

implant material

Aruna Dani

Asso. Prof. (App Physics) Priyadarshini College of

Engineering, Nagpur-440019, INDIA

[email protected]

Abstract: Ceramics exhibit many applications as

biomaterials due to their varied properties. Glass

ceramics possess many properties, similar to both

glass and ceramics as well. They have the property

of being inert in the human body. Because of their

quality of being hard and resistant to abrasion they

become the best option for tooth and bone

replacement. Some ceramics which are resistant to

friction, makes them useful as replacement

materials for malfunctioning joints. Aluminum

oxide has been used in orthopedic surgery for more

than 20 years as the joint replacement material due

to its exceptionally low coefficient of friction and

minimum wear and tare. Bioactive glasses are

composed of calcium and phosphate which are

present in a proportion that is similar to that of bone

in human body. These glasses bond to the tissue and

are biocompatible. They have large medical and

dental applications. Since bioactive glasses and

glass ceramics are brittle materials they are

specially used in the field of small bone defects.

Following inorganic processess occur when a

bioactive glass is immersed in a physiological

environment:

1. ion exchange

2. Hydrolysis

3. Condensation

4. Precipitation and

5. Mineralization.

This article reviews various properties of bioactive

glasses and their

applications

Keywords: Bioactive glasses, Bio Ceramics,

Implant material, biocompatible, Glass transition

1. Introduction: Bio ceramics are materials which

include Bio active glasses as well. They are a group

of glass ceramic materials having surface

reactivity. The biophysical properties of these

glasses has led them to be studied in detail to be

used as implant material. Ceramics show many

applications due to their physico-chemical

properties. They have the advantage of being

inactive in the human body. The resistance to

abrasion makes them useful for bones and tooth

replacement. A material is said to be bioactive, if it

gives an appropriate response to stmulii and results

in the formation of a bond between material and the

body tissue. Bioactive glasses are silicate based,

containing calcium and phosphate1.Hench was the

first to develop bioactive glasses, which were found

to able to bond to tissues2.The morphology of the

gel surface layer was a key point in determining the

response of bioactive glass . The ability of bonding

to bone also known as Biocompatibility was

increased for a certain compositions of bioactive

glasses.These bioactive glasses mainly contained

SiO2, Na2O, CaO and P2O5. Synthetic bone graft

material for general orthopaedics and dentistry are

some of the application of bioactive materials.

2. Experimental

2.1 Materials

Bioactive glasses are classified into different

groups and each group has a different

composition. Some bioactive glasses, for ex.

45S5, are now being used as bone grafting

material3. 45S5 bioactive glass is composed of SiO2

(46.1 mol%), CaO (26.9 mol%), Na2O (24.4 mol%)

and P2O5 (2.6 mol%)4 . 45S5 is able to form HCAP

(hydroxyl carbonated apatite) in less than 2 hours

and binds to tissues1. It is essential that a bioactive

glass forms without getting crystallized. If a

bioactive glass crystallizes, it becomes less

bioactive because the ion exchange between the

glass and aqueous solution is resisted by the

crystalline phases

2.2 Preparation of Samples: Bio active glasses

were initially obtained by the process of melting at

higher temperatures. The process for the

formation of bioactive glasses are melting at


https://en.wikipedia.org/wiki/Joints


46

Table Composition of bioactive glasses and glass

ceramics used for medical and dental applications

Composit

ion

Wt%

45S5

Bio

glass

S53P4 A-W glass

ceramic

Na2O 24.5 23 0

CaO 24.5 20 44.7

CaF2 0 0 0.5

MgO 0 0 4.6

P2O5 6 4 16.2

SiO2 45 53 34

Phases glass glass Beta-

wollstonite

glass

higher temperature and sol-gel. It was later

demonstrated that the formation of bioactive

glasses with a composition of SiO2- CaO-P2O5 by

sol-gel process was possible and it was also

observed that glasses were formed at lower

temperatures in sol-gel process as compared to

conventional melting method5,6.Glass transition

temperature (Tg), is a characteristic of any glass,

indicating a range of transformation when an

amorphous solid is changed into a super cooled

liquid on heating. In case of a bioactive glass a

linear relationship exists between Tg and hardness

of the glass. Reduction in Tg of a bioactive glass

indicates that the glass has reduced hardness.


Addition of fluoride increases re mineralization and

reduces demineralization. CaF2 concentration was

increased in SiO2-CaO-P2O5-Na2O system while

network connectivity was kept constant. It was

observed that due to addition of fluorine in

bioactive glass, there is decrease in Tg which means

that the glass has reduced hardness and is more

bioactive 7. For the prevention of caries, the role of

fluoride is very important. This substitution has a

profound effect on solubility of enamel8. As the

addition of fluoride is essential, its incorporation in

bioactive glasses is of immense importance. It was

observed that incorporation of fluorine in bioactive

glass, decreased its Tg which indicates that the glass

has reduced its hardness and is more bioactive.

Alternately, the onset of crystallization and peak

temperatures were decreased when CaF2 was

increased9 .

For example, when a particulate of bioactive glass

is used to fill a bone defect there is rapid

regeneration of bone that matches the architecture

and mechanical properties of bone at the site of

repair.

4. Conclusions Bioactive glasses with various

compositions are now used for wide range of

applications. Bioactive glasses have become an

area of interest for researchers from the field of

medicine and dentistry. The growing requirement

of tough, strong and stable bioinert glasses/

ceramics could be met either by nano-structured

ceramics or composites.

References

[1] Hench LL, Wilson J. An introduction to bio

ceramics. Singapore: World Scientific Publishing, 1993

[2] Hench LL. The story of Bioglass TM. J Mater Sci:

Mater Med 200617, 967-78

[3] Paolinelis G, Banarjee A, Watson TF. An in vitro

investigation of the effect and retention of bioactive

glass air-abrasive on sound and carious dentine. Journal

of Dentistry 2008,36,214-18

[4] Masahiro Kobayashi, Hiroaki Saito, Takatsune

Mase, Taketo Sasaki, Wei Wang, Yumi Tanaka, et al.

Polarization of hybridized calcium phospho

aluminosilicates with 45S5-type bioglasses. Biomed

Mater 2010 ,5,025001

[5] Rounan Li, Clarke, Hench. An investigation of bioactive glass powders by sol-gel processing. J Appl Biomater 1991,2(4), 231-39. [6] Peltola T, Jokinen M, Rahiala H, Levänen E,

Rosenholm, Kangasniemi, Yli-Urpo. Journal of

Biomedical Material Research 1999, 44(1), 12-21.

[7] Featherstone JDB. The science and practice of caries

prevention. J Am Dental Assoc JADA 2000, 131, 887-

99.

[8] Wei M, Evans JH, Bostrom T, Grondahl L. Syn-

thesis and characterization of hydroxyl apatite, fluoride-

substituted hydroxyl apatite and fluorapatite. J Mater

Sci Mater Med 2003, 14, 311-20.

[9] Brauer D, Karpukhina N, Law R, Hill R. Structure

of fluoride containing bioactive glasses. J Mater Chem

2009, 19,5629-36.


47

Intelligent

Transportation System

Shubham Sehgal, Akshat Mathur*, Mona

Aggrawal, Ram Sharma

Department of Electrical Electronics and

Communication Engineering

The NorthCap University, Gurgaon

Corresponding Author:

*[email protected],

Abstract - With the emerging advancements in

transportation system, need for effective assistance

during this process has emerged. There is no

technology being used presently that assists

transportation through visible light. So we

discussed about Intelligent Transportation System

(ITS) that can be used as a potential element in

traffic control management. According to World

Health Organization’s figures, major cause of death

after the year 2000 are road accidents. VLC using

LED is technology which will help for high speed

and low cost wireless communication which will

be helpful in this study of Intelligent

Transportation System. The unique features and

benefits of VLC make it the most important

technological innovations in communication

system. In this paper we are discussing this concept

of Visible Light Communication to develop some

techniques in the field of transportation system.

Keywords – Visible Light Communication,

Vehicular Communication, Intelligent

Transportation System

1. Introduction

With the advent of smart technology in every field,

it is imperative to establish the transportation

system as smart, now considering the extent of

spread of transport network, a method needs to be

devised which can easily be used with the present

technology. The following texts focuses on

Intelligent Transportation System using Visible

Light. In Visible Light Communication (VLC),

communication takes place using visible light in

which LEDs perform two functions simultaneously

illumination and communication. ITS using visible

light would enable to use the light from the street

lights as a source to communicate. In VLC system,

modulation of intensity of light is done in such a

way that it is undetectable to human eye and have

no effect on the illumination functionality. LEDs

are used for transmission purpose because of its

certain advantages such as high lightening

efficiency, long durability, being environment

friendly and low power consumption. The

transmitter and receivers used have same

configuration as most of the general analog

communication systems as shown. LEDs are used

in head/tail lights of vehicles, street lights and

traffic lights which will make the deployment of

these Smart and Intelligent Transportation System

easy and using VLC technology. Using this

technology the vehicles will be able to

communicate about the speed, routes, destination,

and traffic conditions. Vehicular Communication

can be Vehicle to Vehicle, Vehicle to

Infrastructure, Infrastructure to Vehicle. Most

Challenging Project which is under consideration

of many scientist is development of Visible Light

Communication for Advanced Driver Assistance.

There are also many applications listed in this

paper like the Smart Obstacle Intimation System

[4], Blind Turn Assistance etc. Use of VLC in

Transportation System will be very advantageous

since it will make the transportation system faster,

easier and safer. ITS holds a promising sustainable

future , it can play a vital role in reducing pollution,

better traffic management and better on road

security. A great initiative has been taken in the

field of ITS.

2. Working

LEDs act as a transmitter as shown in diagram, the

transmitter and receiver configuration is similar to

the analog communication systems. Digital

Modulation Technique are used modulation of light

beam. Data transmission will be in binary form

since the two states of LED on and off can only

denote max two states. LED in on state will denote

binary ‘1’ similarly off state of LED will denote

binary ‘0’. At the receiver’s end we use a light

sensitive device like photodiodes for receiving this

encoded signal.



48

Advantages of VLC over RF

1. RF Band – 3 KHz to 300 GHz for wireless

communication whereas for VLC it is 400

THz to 800 THz, very large as compared to

RF Band.[1]

2. VLC provide a highly secure, low speed

and high speed communication, data rates

greater than 100 Mbps can be achieved

using high speed LEDs.

3. VLC could be implemented using cheap

components for transmitter and receiver

purposes unlike the costly hardware using

in wireless communication using RF

technology.

4. Visible light does not create the

phenomenon Electromagnetic Interference

(EMI) [1]

5. Visible light is environment friendly as

compared to Radio Frequencies.

3. Applications

i) Smart Traffic Management System –

Traffic can be managed using VLC by

making use of the Smart Traffic Lights [4].

These traffic lights will help in traffic

management. There will continuous

communication between these lights and

vehicles coming towards these lights. The

light will signal green to that lane of road

where the traffic density is more in short

amount of time keeping on lane as initial

starting lane. The decision making will be

according to the traffic density. This will

reduce the traffic jams and will increase

mobility on road.

ii) Speed Control mechanism –

Similar to above application we create a

continuous link between vehicle and street

lights on road, this data will be given to the

police control room which will help them in

reducing road accidents. An internal

mechanism can be designed which could

track the state of the driver. If the driver is

drunk an immediate signal could be

transferred to the police control room and

they can track him down or if a driver

jumps a traffic signal then also he can be

tracked down by communication the car

using visible light communication.

iii) Smart Obstacle Intimation System –

This system will help the driver about

obstacles coming near to him. During night

or fog time many of the obstacles are not

visible to drivers which lead to accidents

but using this intimation system obstacles

would be detected coming in the range of

head lights and an immediate braking

system (if planted) will get activated and

brakes will be applied to reduce the amount

of damage or even eliminate it. Vehicle to

Vehicle communication will also be playing

a major role in this system. The brakes will

be applied after giving the signal to driver.


49

iv) Pollution Level Marker –

This could be implemented using LEDs

planted in the street lights. The pollution

level in the environment will be marked

according to the intensity of light. A scale

can be made which will keep track of light

intensity of street light, divided into section

marked with the intensity columns and

corresponding pollution level. The more the

illumination of street lamps, the lesser will

be the pollution level in environment.

v) Blind Turn Assistance –

This technique will use the basic concept of

interference. Major amount of accidents

take place on blind cuts where the cars

coming from the other side are not visible.

So this technique will help in reducing such

type of accidents. The lights coming from

the head lights of cars coming from the

opposite ends will interfere and will

produce max and min. level which can be

detected and will intimate the driver

regarding an obstacle as described above.

The driver can then apply the brakes or it

will be automatically applied.

vi) Toll Collection –

Toll Booths are area of road where max

amount of jams take place. The service of

people on those booths is too slow which

make this happen. So to reduce this we

could apply VLC technique here. In this

technique Vehicle to Infrastructure

technique [5] will be used. As soon as

vehicle reaches the booth, a vehicle will

communicate with the device planted at

those booths and a certain amount of money

is deducted from the bank account of that

driver.

4. Improvements/ Suggestions

1. Improvement in the outage area of the beam can

significantly improve the extent of area covered

and also provide better connectivity.

2. A convex approach towards transmission and

reception must be adopted, inclusion of

photodetectors at the frontal extremities of the

vehicle can also improve connectivity.

3. Development of standards and protocols that

would contribute in improving the Interference,

Sound to Noise Ratio(SNR), which can be

achieved by developing Medium Access Control

(MAC) [1].

5. Shortcomings

1. Low bandwidth in modulation –

LEDs are responsible for producing low

modulation bandwidth which in turn is

responsible for lower data rate. Pre and post

equalisation however can increase BW upto

50MHz[3] adaptive equalization can help to

compensate for Inter-symbol Interference

(ISI), improving the data rates and the bit-

error-rate

2. Interference and noise –

Visible light is susceptible to interference

from external factors and data can be

affected due to destructive interference

from sunlight

3. Non linearity –

LEDs transmission and detection is most

efficient in LOS line of sight. Also LEDs

can produce non-linear characteristic

graphs. So it is important to search for an

optimum DC operating point

4. Cross path interference –

Considering similar vehicles work on

similar signals, there is a possibility of

reception of signal from another source, this

crates distortion that degrades performance.


50

6. Conclusion

In this paper we mainly reviewed the

fundamentals and application of visible light

communication is the area of transportation. The

results of research done in the area of ITS have

been promising and ITS using VLC till now has

also proven to be an upcoming technology that

can benefit many areas of transportation like data

collection, toll collection, accidents safety etc.

and benefits include mobility, efficiency, safety

etc. With further research in this area VLC can

definitely replace DSRC in intelligent

transportation system. Many features provide

VLC in ITS an edge over other technologies most

importantly, unlike most emerging technologies,

cost of establishment with this would be much

less, since it can use the present infrastructure

References

[1]Navin Kumar “An Emerging Visible Light

Communication System for Driver Assistance”

[2] Kashif Naseer Qureshi and Abdul Hanan

Abdullah ” A Survey on Intelligent

Transportation Systems” Middle-East Journal of

Scientific Research 15 (5): 629-642, 2013

[3] Carlos Medina, Mayteé Zambrano and Kiara

Navarro “Led based visible light communication:

technology, applications and challenges – a

survey” International Journal of Advances in

Engineering & Technology, Aug., 2015.

[4] Navin Kumar “Visible Light Communication

in Intelligent Transportation Systems” IEEE

International Conference on Communications

[5]Anitha Chepuru , Dr.K.Venugopal Rao “A

Survey on IOT Applications for Intelligent

Transport Systems” Technical Research

Organization India

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