Chapter-1

Introduction

0D1D2D3D

Introduction Chapter-1

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When Neil Armstrong stepped onto the moon, he called it small step for man

and giant leap for mankind. Nano may represent another giant leap for mankind, but

with step so small that it makes Neil Armstrong look the size of solar system!

However, nanoscience and nanotechnology are steering mankind into new realms of

efficient and miniature tools and gadgetry [1]. Nanotechnology: The Future is coming

sooner than we think. Nanotechnology is a technology that deals with small structure

or small sized materials. A typical dimension spans from sub nanometer to several

hundred nanometers. A nanometer (nm) is one billionth of a meter or 10-9

m and is

approximately the length equivalent to few hydrogen or five silicon atoms align in a

line. According to the literature survey the progress of nanomaterials and

nanotechnology can be defined as follows:

The era of 2000-2005 as Passive Nanostructure; during this period products

took the advantage of the passive properties of nanomaterials, including nanotubes and

nanolayers. For example, titanium dioxide is often used in sunscreens because it

absorbs and reflects ultraviolet light. When broken down into nanoparticles it becomes

transparent to visible light, eliminating the white cream appearance associated with

traditional sunscreens.

The era of 2006-2010 as Active Nanostructure; Active nanostructures change

their state during use, responding in predicable ways to the environment around them.

Nanoparticles might seek out cancer cells and then release an attached drug. Products

in this phase required a greater understanding of how the structure of nanomaterials

determines its properties and a corresponding ability to design unique materials. The

era of 2011-2015 as Systems of Nanosystems; Nanostructures could self-assemble into

a lattice on which bone or other tissues could grow.

Why Nanomaterials Chapter-1

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At this stage significant advancements in robotics, biotechnology, and new

generation information technology will begin to appear in products. The era of 2016-

2020 as Molecular Nanosystems; this stage will involve the intelligent design of

molecular and atomic devices, leading to “unprecedented understanding and control

over the basic building blocks of all natural and man-made things” [2].

2020 and beyond as the singularity; every exponential curve eventually

reaches a point where the growth rate becomes almost infinite. This point is often

called the singularity. If technology continues to advance at exponential rates, what

will happen after 2020? Technology is likely to continue, but at this stage some

observers forecast a period at which scientific advances aggressively assume their own

momentum and accelerate at unprecedented levels, enabling products that today seem

like science fiction. Beyond the singularity, human society is incomparably different

from what it is today. Several assumptions seem to drive predictions of a singularity

[3], there is an assumption that solutions to most of today‟s problems including

material scarcity, human health, and environmental degradation can be solved by

technology, if not by us, and then by the computers we eventually develop.

1.1 Why Nanomaterials?

One of the first and most natural questions to ask when starting to deal with

nanoparticles is: “why are nanomaterials so interesting”? Why even bother to work

with these extremely small structures when handling and synthesis is much more

2000-2005

Passive Nanostructure 2011-2015

Systems of Nanosystems

2016-2020

Molecular Nanosystems 2006-2010

Active Nanostructure

Why Nanomaterials Chapter-1

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complicated than that of their macroscopic counterparts. The answer lies in the nature

of and unique properties possessed by nanostructures. Small features permit more

functionality in a given space, but nanotechnology is not only a simple contribution of

miniaturization from micron meter scale down to nanometer scale. Materials in the

micrometer scale mostly exhibit physical properties the same as that of bulk form,

however, materials in the nanometer scale may exhibit physical properties distinctively

different from that of bulk. Materials in this size range exhibits some remarkable

specific properties: a transition from atoms or molecules to bulk form takes place in

this size range. For example, crystals in nanometer scale have a low melting point and

reduced lattice constants, since the number of surface atoms or ions becomes a

significant fraction of the total number of atoms or ions and the surface energy plays a

significant role in the thermal stability. Crystal structures stable at elevated

temperatures are stable at much lower temperatures in the nanometer sizes. This may

result in losing the ferroelectricity and ferromagnetism when the materials are shrunk

to nanometer scale. Bulk semiconductors became insulators when the characteristic

dimension is sufficiently small (in a couple of nanometers). Although bulk gold does

not exhibit catalysis properties, but nanocrystals of gold demonstrate to be an excellent

low temperature catalyst.

In our macroscopic everyday experience we observe the phenomena of light

acting in quiet easy predictive way. Light directed at a surface is reflected just at the

angle and with the color that would be expected. This easily predictive behavior

changes dramatically when the reflecting particles become much smaller than the

wavelength of the incident light. Nanomaterials possess a very high surface to volume

ratio. This can be utilized in areas where high surface areas are critical for success.

This could for example be in the catalytic industry; some nanoparticles actually have

Why Nanomaterials Chapter-1

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proven to be good catalysts. Some nanoparticles also show bactericidal effects and here

a high surface to volume ratio is also important. In biology and biochemistry

nanoparticles have attracted much attention. Nanoparticles are often in the range 10-

100 nm and this is the size as that of human proteins. Scientists from the Chinese

Academy of Science have even suggested using gold nanoparticles to improve

Polymerase Chain Reaction (PCR). The scientific mantra of “structure dictates

function” - a tenet largely taught to biologists and biochemists - is quickly being

adopted by researchers who deal with materials at the nanoscale. In biology, this catch

phrase is exemplified by proteins whose roles in specific chemical pathways are

strictly determined by the way these large biomolecules fold or form hierarchical

structures based on chemical interactions with themselves or other molecules. Another

aspect which is important and plays role for most of the dramatic electronic and

vibrational properties compared to bulk is to check quantum confinement effect. The

most dramatic changes in properties take place in the structures where the

carriers/phonons are confined in the region of a characteristic length of the order of the

electron/phonon wave functions wave length. If the motion of a particle (or quasi

particle), e.g. electron or phonon, is restricted in one or more dimensions by a potential

well, this particle is confined in the well because the probability to traverse the energy

barriers is lower than to remain within it. When the confining dimension is large

compared to the wavelength of the particle, the particle behaves within the confined

area as if it were free. As the confining dimension decreases and reaches the order of

the wave function„s wave length, the particle‟s energy states are modified due to the

overlap of the wave function‟s amplitude reflected from the barriers. This is defined as

"quantum confinement". The free path length of particles in solids is usually much

longer than the particles wave function‟s wave length. In case the confined dimension

Why Nanomaterials Chapter-1

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reaches the order of the free path length no "quantum size" effect takes place.

However, other characteristics of the particle like lifetime and mobility are affected by

changing e.g. the carrier and heat transport properties.

To chemists, physicists, and engineers, a similar structure-function relationship

is the underlying motive for discovering and characterizing novel nanoscale structures.

Hence, we are interested in the nanoparticles.

Nanostructures can be divided into zero-dimensional (0D when they are

anisotropic, such as spheres and polyhedra), one-dimensional (1D when they are

elongated, such as rods, wires and tubes), and two-dimensional (2D when they are

planar, such as plates and discs) based on their shapes. Besides, these building blocks

with different dimensionality (0D–2D) can self-assemble into more complex

nanostructures, such as nanostructured arrays and dendritic nanostructures [4]. Figure

1.1 illustrates the basic geometrical motifs of inorganic crystals with the

dimensionality from 0D to 3D.

Nanofluidics is often defined as the study and application of fluid flow in

and around nanosized objects. It is not a new, but it didn‟t have the name of its own

until the development of micro fluidics in the 1990s. Nanoparticle material,

concentration, size, and shape all contribute to the nanofluids properties. Noble metal

colloids or nanofluids, especially gold, have a long history of scientific studies. Fig.1.2

presents the schematic of importance of nanofluidics and relevance in different fields.

The earliest scientific work by Faraday on the preparation of gold colloids dates back

to 1857 [5] and this work stimulated tremendous interests in this new form of gold, [6,

7] in particular, about the origin of the beautiful ruby red color of colloidal gold.

Why Nanomaterials Chapter-1

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Figure 1.1: Basic geometrical motifs of inorganic nanocrystals with the

dimensionality from 0D to 3D [4].

Why Nanomaterials Chapter-1

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It is worth noting that the distinct color of gold had long been utilized in

colorful decoration, even in ancient times (e.g., a few thousand years ago), much

earlier than the 19th

century's scientific studies on gold colloids, but ancient

practitioners certainly had not realized that the attractive ruby color of gold is indeed

due to its nanoscale form. Since Faraday's original work, Mie was the first who

successfully modeled the optical spectra of gold colloids by solving Maxwell's

equations in 1908 [7].

Figure 1.2: Classical disciplines related to nanofluidics and some of the

relevant subjects studied.

Concept of Phonon Engineering Chapter-1

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1.2 Concept of Phonon Engineering

Phonons, i.e. quanta of lattice vibrational waves affect almost all physical properties in

solids [8]. They do not only affect optical properties of crystalline solids but also limit

the electron mobility near room temperature. The phonons are classified as optical and

acoustic phonons. The acoustic phonons or long wavelength phonons; which give rise

to sound waves explains the name phonons. In bulk materials with n atoms per unit

cell, there are 3n phonon dispersion branches for each wave vector q . The relation

between phonon frequency and wave vector q is known as phonon dispersion

curve q . Therein acoustic phonon branches describe the motion of unit cell,

while (3n-1) optical phonon branches describe the relative motion of atoms inside a

unit cell. The acoustic phonon branches which are the main heat carriers are commonly

referred as longitudinal acoustic (LA) and transverse acoustic (TA). In the case of

graphene the out of plane transverse vibrations are denoted as z-axis acoustic (ZA)

phonons. The acoustic phonons in bulk crystals have linear dispersion in the long

wavelength region, the optical phonons are nearly dispersion less with a small group

velocity,

dq

dVc . The optical phonons are used for determining the number of

atomic planes in the few layer graphene via Raman spectroscopy. The thermal

properties of low dimensional materials can be altered more drastically than in

corresponding bulk crystal due to uniqueness of phonon transport [9]. The phonon

properties of structures with reduced dimensionality are a fascinating area of research

since they give rise to various interesting phenomena including: (a) in clean systems

and at normal temperatures, the carrier mobility and dephasing times are determined by

phonon scattering. (b) when microstructure decreases in size and phonons with lower

Concept of Phonon Engineering Chapter-1

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wave vectors are involved in the electron-phonon interaction due to which electron

scattering by acoustic phonons becomes mere pronounced as compared to the

scattering of electrons by optical phonons.(c) the scattering by optical phonons, being

dominant in the 3D case, is reduced or suppressed (d) when the number of defects

decreases with improvement of the nanostructure technology, as a result of which the

electron-optical phonon interaction weakens and the minimum linewidth of optical

spectra is determined by the electron-acoustic phonon interaction. Carrier energy

relaxation and non-radiative recombination involve emission of phonons. This makes

them relevant for a number of electronic processes in nanostructures, due to

technological importance. There are several evidences that the phonon interaction is

altered due to the effects of dimensional confinement on the phonon modes in nano-

structures similar to the electron confinement. This dimensional confinement of

phonons modifies the phonon related properties such as melting temperature, specific

heat, low and high frequency Raman spectra, carrier phonon interactions in low

dimensional structures.

Micro- and nanoscale mechanical resonators and semiconductor quantum dots

embedded in micro cavities have established themselves as a new paradigm in cavity

quantum electrodynamics (cavity QED) and emerged as ubiquitous devices for

application in a wide range of technical disciplines including communications, sensing,

metrology, and fundamental scientific endeavors. In many instances the performance

of these devices is limited by the deleterious effects of mechanical damping. Although

much progress has not been made towards the understanding of mechanical dissipation

at the micro- and nanoscale [10-13], obtaining reliable predictions of the fundamental

design-limited quality factor, Q, remains a major challenge. The origins of mechanical

damping in micro- and nanoscale systems have been the subject of numerous studies,

Concept of Phonon Engineering Chapter-1

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with several relevant loss mechanisms having been investigated. These include: (i)

fundamental anharmonic effects such as electron-phonon interactions, damping, and

the quality factor; (ii) viscous damping, involving interactions with the surrounding

atmosphere or the other hosts matrix; (iii) materials losses driven by the relaxation of

intrinsic or extrinsic defects in either the bulk or surface of the resonator for which the

most commonly studied Lamb‟s model and its modified model where interaction of

soft/hard matrix with nanofluidics has been considered and (iv) support-induced

damping, i.e. the dissipation induced by the unavoidable coupling of the resonator to

the substrate. Enhanced thermal conduction in nanofluids is an observed phenomenon

for which the underlying mechanistic processes are still being debated. In addition, the

technological progress in the design and fabrication of semiconductor cavity-QED

systems has enabled them to be used as components in quantum information

processing [14] and for the generation of indistinguishable photons [15]. These

quantum applications require robust cavity-QED based QD devices that rest on the

ability to manipulate and control the underlying quantum processes. Such quantum

control is usually obtained when the cavity and QD are in the intermediate to strong-

coupling regime [16]. For semiconductor cavity-QED systems, signatures of electron–

acoustic-phonon scattering have been noted with incoherent excitation, resulting in off-

resonant „„cavity feeding‟‟ and an asymmetric (on-resonance) vacuum Rabi doublet

[17].

In the applications of nanostructured materials, an understanding of their

acoustic and mechanical properties is especially important. Furthermore,

technologically important semiconductors and dielectrics, heat is mostly carried by the

acoustic phonons and there will be modification of thermal properties due to the

modified increased phonon-boundary scattering arising from the modification of

Concept of Phonon Engineering Chapter-1

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acoustic phonon energy spectra. For example, the thermal conductivity of the generic

semiconductor nanostructures is smaller than that of constituent bulk materials [18].

This happens due to increased phonon boundary scattering and decrease in the phonon

group velocity [19]. Modification of the acoustic phonon dispersion is particularly

strong in nanostructures embedded into the elastically dissimilar materials [20]. Such

modification may turn out to be desirable for some applications while detrimental for

others. However, the reduction of the thermal conductivity, being a bad news for the

thermal management of downscaled electronic devices, is good news for the

thermoelectric devices, which require materials with the high electrical conductivity

and low thermal conductivity [21]. Recently, the concept of “phonon engineering” has

been introduced which may lead to progress in electronic and optoelectronic devices

[20]. The phonon engineering [21] basically deals with controlling phonon transport

via tuning. Phonons with its dispersion relation having quantized modes of vibration

occurring in a rigid crystal lattice are strongly affected by the spatial confinement in

terms of modified phonon dispersion curves, and allied properties such as phonon

group velocity, density of states, point defects, electron-phonon interaction etc. The

phonon engineering concept proposes that the acoustic phonon spectrum is particularly

strong in nanostructures embedded into elastically dissimilar materials. Thus

nanostructures offer a new ways of controlling phonon transport via tuning its

dispersion. While, earlier the acoustic phonon confinement was only considered useful

for charge carrier mobility and electrical conductivity, it is now found that the

confinement of acoustic phonon modes leads to decrease of the average phonon group

velocity with corresponding increase of the phonon scattering and reduction in thermal

conductivity. This is good news for the thermoelectric devices which needs materials

with high electrical conductivity and low thermal conductivity [21]. However, this can

Concept of Phonon Engineering Chapter-1

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be cured by embedding the nanostructure into acoustically faster or acoustically harder

layers [19-21].

In nanostructures, the role of electron-acoustic phonon interaction on a carrier

relaxation has been discussed in terms of possible slowing of relaxation rates due to the

phonon confinement [22-25]. It is found that the electron-acoustic phonon interaction

is responsible for the decay rate of the excitonic polarization in nanostructures. In

nanostructures, where dimensional confinement is significant, it is possible that the

phase space restrictions may weaken or forbid the optical phonon scattering processes

which would normally dominate in bulk structures. Phonons generally affect the

thermal, optical, mechanical, and electrical properties of materials. While, the phonon

density of states (PDOS) is primarily a function of the local atomic structure, and it is

also sensitive to atomic-level stresses and the microstructure. It is found that the

phonon density of states for nanomaterials is significantly different from its bulk

counterparts [26-32].

The microscopic origins of the observed size effects are, however, very often

not clear. In view of the partially controversial interpretation of the PDOS of

nanostructured materials, a detailed study revealing the role of the various size effects

appears to be a worthwhile task. To understand the vibrational properties of

semiconductor/metal nanocrystals, it is absolutely necessary to consider the zero

dimensional confinement effects on the electronic states in a system. In addition, the

low-frequency phonon modes (acoustic phonons) of nanoparticles bear a unique

signature of their structural and mechanical properties directly reflecting the impact of

confinement on the ionic movement.

Coinage metals, such as Au, Ag, have been important materials throughout

history [33]. Although in ancient cultures they were admired primarily for their ability

Concept of Phonon Engineering Chapter-1

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to reflect light, their applications have become far more sophisticated with our

increased understanding and control of the atomic world. Today, these metals are

widely used in electronics and catalysis and as structural materials, but when they are

fashioned into structures with nanometer-sized dimensions, they also become enablers

for a completely different set of applications that involve light. These new applications

go far beyond merely reflecting light and have renewed our interest in maneuvering the

interactions between metals and light in a field known as plasmonics [10-13].

Nanofluids are dilute liquid suspensions of nanoparticles with at least one of

their principal dimensions smaller than 100 nm. There is a growth in the use of colloids

which are nanofluidics in the biomedical industry for sensing and imaging purposes.

This is directly related to the ability to design novel materials at the nanoscale level

alongwith recent innovations in analytical and imaging technologies for measuring and

manipulating nanomaterials. Colloidal gold has a long therapeutic history, which is

well rooted in eastern traditions particularly in the India to its subcontinent [34, 35].

The medicinal value of colloidal gold is well documented in the books of ancient

Indian ayurveda like „Charaka Samhita‟ and the „Vedas.‟ Nanofluidics is basically

characterized by two most important and fundamental transport properties: thermal

conductivity and viscosity. The apparent thermal conductivity is the most important

parameter to demonstrate the enhancement potential of heat transfer in nanofluids. It

has been shown that the thermal conductivity of the nanofluids is influenced by the

heat transfer properties of the base fluid and nanoparticle material, the volume fraction,

the size, and the shape of the nanoparticles suspended in the liquid, as well as the

distribution of the dispersed particles [36]. Keblinski et al [37] listed four possible

explanations to understand the heat transfer enhancement in nanofluids: Brownian

motion of the nanoparticle, molecular-level layering of the liquid at the liquid/particle

Confined Acoustic Phonons in Nanostructure Chapter-1

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interface, the nature of heat transport in the nanoparticles, and the effects of

nanoparticle clustering. Brownian motion, by which particles move through the liquid

and possibly collide, thereby enabling direct solid-solid transport of heat from one

particle to another, can be expected to increase thermal conductivity. Nanofluids with

smaller nanoparticle size exhibit greater thermal conductivity. Nanofluids with metallic

nanoparticles give higher thermal conductivity than nanofluids with non-metallic

nanoparticles [38]. However, it appears from the literature that the origin of thermal

conductivity in nanostructure/nanofluidics system is still unclear. Besides, there is a

need of proper nanofluidics system with less size variation of the nanoparticles in bare

fluids.

As far as the understanding of the thermal conductivity is concerned the role

played by phonons is not disputable. Therefore, it is believed that the investigation of

phonons in either form of colloids or solid will be quite useful. Recent results reveal

that these properties are sensitive to the size, shape, composition, base fluids and size

distribution [36-38]. Therefore, a synthesis of nanoparticles –host (base fluids) system

is desirable for optimum application.

1.3 Confined Acoustic Phonons in Nanostructures

Spatial confinement of acoustic phonons in nanostructures affects their

dispersion. It modifies the acoustic phonon properties such as phonon group velocity,

polarization, density of states and changes the way the acoustic phonons interact with

other phonons, defects and electrons such changes create opportunities for engineering

phonon dispersion spectrum in nanostructures for improving electrical phonon

spectrum at room temperature one needs to have materials at the nanometer length

scale due to being average phonon mean free path and wavelength of thermal phonon

Confined Acoustic Phonons in Nanostructure Chapter-1

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TK

V

B

sD 48.1 in the range of nanometer ( is the plank‟s constant, BK is the

Boltzmann constant, T is the absolute temperature, sV is the sound velocity in

materials). Engineering of the optical phonon in nanostructures via the boundary

conditions requires different approaches than engineering of the acoustic phonons. In

the long wavelength limit the optical phonons corresponds to the motion of atoms

within the same unit cell and hence change by imposing new outside boundaries is not

possible [39]. However, the electron-phonon scattering rates can be modified by tuning

the confined electronic states energy difference with respect to the optical phonon

energy. This effect is known as “Phonon bottleneck” and is useful in the optimization

of solid state lasers. Although phonon engineering has received attention recently,

interest in modifying the acoustic phonon spectra of layer materials (superlattices) has

a long history [40]. The folded phonons in superlattices of GaAs/AlGaAs have been

experimentally observed by colvard et al [41]. However, interest in the subject

substantially increased when it was pointed out that the confinement induced changes

in the acoustic phonon dispersion can strongly affect the thermal conductivity [42].

In an infinite elastic solid medium, acoustic waves such as longitudinal and

transverse acoustic waves are governed by the Navier equations [43] that describe the

dynamic motions of solids. However, when at least one of the dimensions of a solid

object decreases to be near or smaller than the phonon wavelength, phonon

confinement results in a strong modification of the acoustic phonon spectrum [19]. The

frequencies of the confined acoustic phonons in a nanostructure depend on its shape

and its boundary conditions including the effect of surrounding medium. Thus, the

confined acoustic modes are sensitive to physical structures and should show

significant size-dependent features.

Confined Acoustic Phonons in Nanostructure Chapter-1

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When the phonon wavelength λ ~ D, i.e the nanostructure dimension, the confined

acoustic phonons no longer have a propagating character and are generally understood

as normal vibrations of the whole nanostructure. In principle, the confined acoustic

phonons in a nanostructure manifest themselves via the appearance of discrete peaks in

inelastic light scattering spectra and the blue shift of these peaks with decreasing

nanostructure size [44-49].

The first observation of confined acoustic modes of nanoparticles was reported

in the Raman study of spinel microcrystallines by Duval in 1986 [44]. One low

frequency peak with a frequency of several cm-1

was observed in the Raman spectrum

and was attributed to the spheroidal mode of the nanospheres. Interestingly, the mode

frequency was found to shift with the variation in the particle size. That is, the mode

frequency is proportional to the inverse diameters of the particles. This size dependent

feature agrees well with the theoretical predictions based on Lamb‟s theory [45]. In the

following years, nanoparticles of different materials, such as CdSe, CdS, Ge, Si, were

studied by Raman scattering [50-51]. However, these Raman experiments only study

the particles of sizes ranging from several nm to few tens of nm because it is very hard

for Raman scattering to detect the vibrations of frequency below 1 cm-1

. Particles of

several hundreds of nanometers or microns were not studied until Brillouin light

scattering was introduced to this field [52]. Using Brillouin light scattering, many more

confined acoustic modes with accurate frequency could be observed [52].

In the above mentioned Brillouin and Raman experiments, the nanoparticles

studied are often, for simplicity, assumed to be spherical and the measured data are

analyzed within the theory formulated by Lamb [45] for a homogeneous elastic sphere.

In this theory, the confined acoustic modes of a sphere are classified as spheroidal or

torsional, which are labeled by l = 0, 1, 2. . . the angular momentum quantum number,

Confined Acoustic Phonons in Nanostructure Chapter-1

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and n = 1, 2, 3, . . . , the sequence of modes in increasing order of energy. However, in

few early studies [34 -35, 37, 43-45, 50, 52-61], nanoparticles studied did not have free

surfaces. Thus, on the surfaces of these spheres, the stress and strain are not completely

zero and the free surface conditions of the Lamb‟s theory are not fully satisfied in these

studies. Also, the contact between neighboring spheres could lead to vibration damping

and energy loss [53-54, 61-62]. Theoretical studies [47- 48, 56, 63] have shown that,

when the acoustic impedances of matrix material and the embedded spheres are the

same, the vibrational energy loss from the sphere can be very large and low frequency

spectrum will be very broad or almost a straight line [49]. Several other studies have

also showed that the eigenvibrational frequencies of nanospheres embedded in matrix

are different from those of free surface nanospheres [46, 53, 64-67]. Hence, the Lamb‟s

theory has not been experimentally tested under rigorous free surface conditions

though it has been widely applied in most studies. Also, the study of a sphere‟s

eigenvibrations under different surface conditions can help to understand how

environmental factors affect its vibrations, especially the phonon lifetimes.

Apart from Lamb‟s theory, selection rules are fundamentally important for

assigning the low frequency acoustic modes which is observed in Raman and Brilloiun

spectra. Group theory [39] predicts that spherical modes are observed in Raman

scattering where as torsional modes are not observed [55-56]. While, Wu et al [61]

observed the torsional modes (l=1, n=0 and l=2, n=0) in nanocrystalline silicon which

they attributed to the non-spherical shape of the particle, the observation of the

torsional mode (l=1, n=0) in nanocrystal CdS in GeO2 matrix with a frequency ratio of

~ 0.72 and attribution to the particle-matrix interaction was recently argumented by

Ivanda et al [47]. Very recently, Kanehisa pointed out that Duval‟s selection rules are

not correct and stated that only the torsional mode with l = 2 is Raman-active [68]. A

Confined Acoustic Phonons in Nanostructure Chapter-1

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subsequent comment, by Goupalov et al [66], which refuted Kanehisa‟s model and his

subsequent rebuttal have exacerbated the current controversy. Unfortunately, Raman

experiments are unable to provide adequate evidence to resolve this controversy. The

selection rules are critically important to analyze the Raman results of nanoparticles.

Experiments [45, 50, 52] have shown that Brillouin scattering is more appropriate than

Raman scattering to study the confined acoustic modes of submicron spheres because

their mode frequencies mainly lie in the gigahertz range. In Brillouin experiments, the

sphere sizes are of the order of excitation wavelength, which are normally several

hundred nm. Thus, the Raman selection rules cannot be applied. Furthermore, there are

no convincing theoretical foundations to support their mode assignment models.

Therefore, there is a critical need to establish selection rules to correctly assign the

confined acoustic modes of spheres studied by Brillouin light scattering, which serve

as the basis for determining their mechanical properties.

Another issue to understand the origin of the observed variation of low-

frequency Raman bands of TiO2 nanoparticles with particle size is required. As far as

the investigation of different properties like vibrational and mechanical of titanium

dioxide nanoparticles is concerned the situation is not encouraging. Ivanda et al [47]

performed low-frequency Raman (LFR) experiment and calculated LFR modes

together with a deconvolution treatment of the size distribution. In other paper [47],

they have shown the low-frequency Raman spectra for different size of TiO2

nanoparticles obtained using annealing, but they did not assign any peak. For

experimental measurement of the confined acoustic phonons of nanostructures, thin

films and nanospheres have been extensively studied. However, at the same time, a lot

of new and more complicated nanostructures have been fabricated that call for more

attention, such as core-shell composite spheres, hollow spheres, nanowires and

Objective of the Present Study Chapter-1

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nanocubes. Very rare experimental study has been done on these nanostructures and

their low-frequency acoustic features are still unknown.

1.4 Objective of the Present Study

Our investigation in the present study focused mainly on the semiconductor and metal

nanoparticles, which are free-standing or embedded in different matrix to understand

the behavior of phonons and the effect of different matrix materials on them. The

nanocrystals of silicon, germanium, titanium dioxide, silver and gold are selected in

view of their electronic, optoelectronic and biological applications. The determination

of the structure is, in general, a hard task, either by the experimental techniques or for

sophisticated total energy calculations and knowledge of the geometrical structure and

shape is required as a basis for understanding many of its properties. In this thesis, low

frequency Raman scattering measurements has been performed to probe the acoustic

vibrations of several nanostructures. The results are analyzed with the help of three

variants of Lamb‟s model. However, the very specific objective of the present work is

listed below.

(i) To investigate low frequency modes in metal nanoparticles

The modes with very low frequency (<30 cm-1

) are difficult to be measured

accurately in Raman scattering and pump probe spectroscopy due to high background

from scattering. Recently, tera-hertz time domain spectroscopy has been used to

observe infrared active l=1 spheriodal mode. Simultaneous observation of Raman

active radial and quadrupolar modes is rare in LFRS. The above facts motivated us to

perform a low frequency Raman scattering from free standing silver nanoparticles. It is

noteworthy that the quadrupolar and breathing modes are simultaneously observed in

the study of Ag nanoparticle which is quite rare and it is presented in Chapter 4.

Objective of the Present Study Chapter-1

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(ii) To investigate the size distribution of metal nanofluids: UV- Visible

spectroscopic assessment

UV-visible spectroscopy is one of the popular characterization techniques to

determine particle formation and size distribution of the nanoparticles. Furthermore, it

is known that the spectrum surface plasmon resonance of nanoparticles is influenced

by the size, shape, inter-particle interactions, free electron density and surrounding

medium, which indicates that it is an efficient tool for monitoring the electron injection

and aggregation of NPs and is also presented in Chapter 4. A theoretical approach

under the framework of Mie scattering theory is further applied to determine the size of

the metal nanofluidics. Using Mie scattering theory optical spectra can be analyzed to

determine particle size of stable suspension.

(iii) To investigate the formation of Si and Ge nanocrystals during annealing of

implanted SiO2 layer using Low frequency Raman scattering and eigen

frequencies of the free and embedded semiconductor nanoparticles.

A study of the influence of heat treatments on an ion-beam synthesized silicon

and germanium nanocrystals in SiO2 layers by low-frequency Raman scattering

(LFRS) and optical Raman spectra. Spectral analysis along with the theoretical

calculations reveals the occurrence of Si and Ge crystals nuclei in SiO2 matrix in

contrast to the electron microscopic results. The results are surprising as the melting,

binding energy, diffusion length etc are different for both systems indicating same

phenomena of crystal growth. Crystal nuclei of Si and Ge of nanometer size results in

an additional contribution to the density of states associated with surface vibrational

modes of Si and Ge nanocrystals and it is presented in Chapter 5.

In addition, chapter 5 also describes the low frequency Raman scattering study

of the quantized acoustic vibrations in anatase TiO2 semiconductors nanocrystals. The

Objective of the Present Study Chapter-1

21

observed Raman spectra are analyzed with theoretical models. The controversies in the

selection rules are also discussed. Our results show that the observed low-frequency

Raman scattering originates from the spherical (l=0) and quadrupolar vibrations (l=2)

of the spheriodal mode due to the confinement of acoustic vibrations in TiO2

nanoparticles. Furthermore, the low-frequency peak due to the vibrational quadrupolar

and spheriodal modes, a band is also observed, which is assigned to the Raman

forbidden torsional l=2 mode originating from the near spherical shape of the TiO2

(iv) To perform the vibrational dynamics using abinitio calculations

The abinitio calculations based on density functional theory are now being

widely used for the study of structural, vibrational and electronic properties of real

materials ranging from nanoscale up to bulk solids. DFT is basically the process of

solving the Schrödinger equation for many electron systems in practical environmental

settings. The Chapter-6 describes briefly the DFT formulation and analyzes the

electronic and vibrational properties of ZnO nanowire and silver clusters.

The problem proposed in the present thesis is of fundamental in nature, the

outcome is expected to be a detailed understanding of the vibrational properties of the

free and embedded nanoparticles of different shapes and size. The role of dimensional

confinement in modifying acoustic phonon modes and their interactions with charge

carriers despite the fact that electron scattering by acoustic phonons plays the key role

in the physics of nanostructures, and great relevance to understand the phonon

engineering concepts for nanoscale devices has been tried to understand in the present

thesis.

References Chapter-1

22

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