Role of swift heavy ion irradiation in materials

science

V.KrishnakumarDepartment of Physics

Periyar University Salem- 636 011

Tamil Nadu, India.

Use of heavy ionsThis presentation is a short review of the results in the last years.The pelletron facility is available at Inter University Accelerator Centre (IUAC), New Delhi , India. Inter-University Accelerator Centre

(IUAC) is an autonomous   organization      which provides   accelerator  facilities   to universities  for  basic  and  applied research  in nuclear  physics, atomic physics,  materials science biosciences  and other  allied fields.

Inter-University Accelerator Centre  has    a running  Pelletron, a tandem Van de graaf   type   accelerator.

A Linear  accelerator is planned as a booster accelerator

The  Pelletron  accelerator   can  be  operated  upto 15 MV of terminal potential and  can produce dc as well as  pulsed beam  of a  variety  of elements.  The  pelletron  has  been  operational since July  1991.

15 UD Pelletron facility @ IUAC

The 15 UD pelletron is a versatile, heavy ion tandem type of electrostatic accelerator. In this machine, negative ions are produced and preaccelerated to ~300KeV in Ion Source and injected into strong Electrical field inside an accelerator tank filled with SF6 insulating gas.

At the centre of the tank is a terminal shell which is maintained at a high voltage (~15 MV).The negative  ions on traversing through the accelerating tubes  from the column top of the tank to  the positive  terminal gets  accelerated.

On  reaching the  terminal  they pass through a stripper which removes some electrons from the negative ions, thus transforming the negative ions into positive ions.

These positive ions are then repelled away  from  the positively  charged  terminal  and are  accelerated  to ground  potential  to the  bottom of the tank. In this manner same terminal potential is used twice to accelerate the ions.

On exiting from the tank, the ions are bent into  horizontal plane by analyzing magnet, which also select a   particular beam of ion.

The switching  magnet  diverts  the high  energy ion beams into various beam lines into the  different  experimental  areas of  the beam hall. The entire machine is computer controlled and is operated from the control room

Materials Synthesis Materials Modification

Materials Characterization

10’s keV to MeV < MeV to Sn dominates

> 10 MeV

Sn dominates

A few MeV 10’s to 100’s Mev

ENERGETIC IONS BEAM IN MATERIALS SCIENCE

PIXE

NRA

RBS

Channeling

eV to a few kev

Doping

Implantation

Compound phase

IBM

IBAD

Plasma deposition

DCsputtering

Rf sputtering

Magnetron

Sputtering

ECR plasma based deposition

ERDA

Channeling

Blocking

High energy ion irradiation-High energy ion irradiation-ImportanceImportance Energetic ion beams play a vital role in

the field of research in Materials Science.

modification of the surface and the bulk structure of solids – control over the specific properties

A trail of defectspoint defects, defect clusters, structural phase transformation occurs In

Semiconductors

HTS CMR Metallic

targets Polymers

optical wave guides and wave guide lasers for tailoring the electro-optic and non-linear optical properties of important materials of modern optics.

Crystal defects due to electronic stopping If the heat conductivity is low enough (insulators), then the energy of the exited electrons is transferred to the target atoms in the vicinity of the ion trajectory. As a result crystal defects are formed. This swift heavy ion collision displacement damage manifests itself in the form of

1. Point defect (defect cluster) generation 2. dislocation loop formation at the periphery of the ion trajectory.3. Disordered and even amorphous ion track cores.4. High energy heavy ion collisions (elastic and inelastic)in a variety of solids create radiation damage on the target surface.

When energetic ions passes through matter, it looses its energy in two ways

Electronic energy loss due to inelastic collision with electrons(Se)[Electronic stopping] (dE/dx)e

Dominant at higher energies (few tens of MeV and higher)-Swift heavy ion Irradiation (SHI)

Nuclear energy loss due to elastic collision with atoms of the solid(Sn)[Nuclear stopping] (dE/dx)n

Dominant at low energies (few tens of KeV to MeV)

Electronic stoppingElectronic stopping

Interaction of heavily charged ions with electrons of the target material through Coulomb forces , produce track of ionization and highly kinetic electrons along the path of the primary ion - latent track (Se>Sth) – Sth depends on the material - Electronic energy loss.

When SHI passes through the materials Se increases with energy and mass of the ions. The effect of Sn is very small( range of the particle > sample thickness).

The desirable defects can be generated in materials by locking sufficient energy into the lattice - favors huge possibilities in tailoring of materials.

Energy loss can be varied by choosing proper ions and doses.

This remarkable flexibility coupled with new cluster beams provides new outlook in many fields.

Ion implantation is a crucial method for dopant incorporation in device fabrication which produces lattice disorder – detrimental for device performance.

Nuclear stoppingNuclear stoppingCauses damage and dislocation of nuclei

from their lattice sites due to elastic collisions

Always produce lattice defects (permanent atomic displacements - in the form of vacancy + self-interstitial atom = Frenkel pair)

(Interstitial atoms, anionic or cationic vacancies)

Damage areas – modify material properties Ex: change of color of diamonds produce

interesting alloys

Two main models:

- Thermal spike model: excited electrons rapidly transfer energy to phonons. Very large energy (heat) deposition leading to localized melting & rapid cooling high defect

densities or amorphization.

- Coulomb explosion model: large positive space charge resulting from electronic excitation leads to strong atomic repulsion, atomic displacements and a cylindrical shock wave.

Nature of materials modification depends on Properties of the target material

Electrical Thermal Structural

Mass of the projectile ion Irradiation parameters

Ion energy Fluence rate Ion species

Difference of materials modification Difference of materials modification by energetic ionsby energetic ions

Low energy ionsLow energy ions High energy ionsHigh energy ions Embedded into the material Not embedded into the material (large range) Modification due to cascade Modification due to collision of impinging ions electronic excitation

Modification in the Modification in the presence of embedded ion absence of embedded ion

Nuclear stopping Electronic stopping Produce point defects Columnar defects

The aim of the investigations

The physical mechanisms responsible for defect formation (point defects, tracks)

The threshold energies of point defects, track and surface defect formation.

The mechanism of amorphization (if there is any) in the ion tracks.

The mechanisms of the annihilation of crystal defects during irradiation.

- The types of crystal defects and their proportion depending on the type and energy of the ion.

Non-linear opticsNon-linear optics Linear optics- Optical properties of a

medium are independent of intensity of light radiation

Non- linear phenomena- available from high intensity lasers

Non-linear optical processes have led to generation of frequencies that are not available with the existing laser sources

Mathematically the NLO effect can be described on the molecular scale by the following total polarization

Ptot- Total polarization of the molecule.

m- the permanent dipole moment.

E- the electric field.

(1) – linear polarizibility.

(2), (3) – first and second hyperpolarizibility co-efficients.

Possess the most important property of second harmonic generation (frequency doubling).

Useful for the fabrication of tunable lasers,opto electronic and photonic devices.

Issues related to non-linear Issues related to non-linear opticsoptics

The frequency conversion processes include

Frequency doubling Sum frequency generation (SFG) Differential-frequency generation

(DFG) Optical parametric generation (OPG) Electro-optic modulation(EOM)

Objectives of our work.Objectives of our work. Growth of good quality non – linear optical crystals.

Characterization of crystals by applying various vibrational, optical and fluorescence spectroscopic tools before and after swift heavy ion irradiation.

These studies will help us to understand the fundamental aspects of these materials and what needs to be pursued vigorously in the exploitation of their device applications.

Ion induced effects on NLO Ion induced effects on NLO materialsmaterialsIrradiation of heavy ions is expected to bring following changes Formation of gray tracks (coloration) on

the irradiated samples of high fluences irrespective of ion beam and its energy.Efficient generation of harmonic frequencies requires a non-linear medium with following desirable properties

High Thermal stabilityLarge transparency windowHigh optical damage thresholdHigh mechanical hardness

Due to these facts, wave guide structures can be obtained

Light guidance demands adjacent regions of different refractive indices

Two methods to create wave guide structures

Heavy ion exchange- causes increased refractive indices

MeV irradiation of light elements forms a layer of reduced refractive index due to high nuclear energy deposition

This will increase the single mode spectral bandwidth for efficient SHG in wave guiding lasers.

Modifications in the refractive index of the materials on ion irradiation leads to the formation of wave guides.

Formation of wave guides will guide to modify the essential property of second harmonic generation, which widens their scope in photonic and opto-electronic applications.

Also, Post treatment after SHI irradiation into insulator leads to the nano-cluster formation and change of optical property.

Dielectric constant of a material is related to polarizability (ionic, electronic, oriental and space charge)of the material.

Disordering of the crystal lattice by ion irradiation causes increase in dielectric constant

Electro-optic co-efficient is directly proportional to dielectric constant of the material.

Ion irradiation enhances the electro-optic co-efficient of NLO crystals

Irradiated crystals can be a good EO modulator of light.

Ion irradiation also affects the transmittance properties of crystals, hence, it is also expected to influence the SHG property.

Materials tried by ion Materials tried by ion irradiationirradiationIrradiation of heavy ions has been carried out in some of the following NLO materials and arrived at better results

Aceto acetanilide Potassium titanyl phosphate Barium strontium borate Para - hydroxy acetophenone Para - hydroxy benzoate Benzoyl glycine Bismuth triborate Ammonium dihydrogen phosphate Potassium dihydrogen phosphate

STUDY OF THE DAMAGE PRODUCED IN K [CS (NH2) 2] 4 BR ( KTTB) - A NONLINEAR OPTICAL

SINGLE CRYSTAL BY SWIFT HEAVY ION IRRADIATION

Metal coordinated semiorganic NLO crystals thiourea CS(NH2)2 – organic ligand – dominant in NLO effect

Free thiourea is in a centrosymmetric Pbnm space group and SHG inactive. large dipole moment and its ability to form an extensive network of hydrogen bonds Substitutions that reduce the symmetry of thiourea molecule enhance NLO properties Forms coordinate bonds through sulphur.- Sulphur to metal bonding

Justification The transition metal ions like Li3+, have incomplete electronic d-shell , should affect the ion-solid interaction to a greater extent than non-transition metal ionsThey have promising effects of doping when it is added in the host solid.

Selected ion beam parameters

Used

Pelletron 15 UD @ IUAC , New Delhi

Ion beam Li3+

Energy 40 MeV

Fluence 1x1010 to 1x1012 ions/cm2

Current 1 pna

Electronic energy loss (Se) 0.4432 KeV/nm

Nuclear energy loss (Sn) 2.5 x 10-4 KeV/nm

HIVAC chamber Diameter 1.5 m

Vaccum in the chamber 3x10-6mbars

Five-position water or liquid nitrogen cooled sample holder is used for small sample irradiations.

The surface of one side of the sample holder is as large as 32 cm2. The samples to be irradiated are fixed on this surface by conductive glue.

During the irradiation the sample holder is in vacuum better than 10-6 torr.

The ion current is measured by a Faraday cup.

Sample Holder

Experimental KBr + 4 CS (NH2) 2 K [CS (NH2) 2] 4

Br Grown in saturated aqueous solution by slow evaporation in mixed solvent Acetone & Water(1:1). PH condition 4.25 , 2 to 3 weeks

Empirical formula K [CS (NH2) 2] 4Br

Formula Weight 422

Crystal: Color Shape Size

 Transparent colorlessRectangular1X0.4x0.3 cm3

Crystal System Tetragonal

Space group P41

Unit cell parameters 

a =7.28 Å, = = = 90 b= 7.28 Å, c= 16.42 Å V (Å3): 870.233, Z : 2Point group symmetry C4

2

Density 0.431 g/cm3

Diffractometer ENRAF NONIUS CAD 4

Radiation Mo K

Wavelength 0.71–0.73Å

Hygroscopicity Non- hygroscopic

U-V absorption (cut off) 350 nm

Hardness Good

Band gap 3.53 eV

ElementsCalculated

(%)Experimen

tal (%)

Sulphur 34.87 30.33

Potassium 10.65 9.24

Bromine 21.77 20.72

Structural and compositional analysis

EDAX

Free ligand

TUa

Cu(TU)6 Br2

b

Ni(TU)6B r2

b

Co(TU)6 Br2

b

K(TU)4Br a Assignments

3384 3460 3400 3422 3571 s (NH2) and as(NH2)

3278 3300 3365 3383 3367 s (NH2) and as(NH2)

3175 3253 3279 3288 3263 s (NH2) and as(NH2)

3093 3180 3180 3200 3167 s (NH2) and as(NH2)

1617 1604 1617 1617 1625 (N-H)

1472 1477 1483 1530 1468 (N-C-N)

1413 1422 1440 1420 1429 (NH2) (C-N) (C-S)

  1388 1398 1422 1385  

1083 1089 1089 1099 1091 (NH2)(C-N) (C-S)

730 712 710 700 720 (C-N) ,(C-S)

a - present work b -G.M.S. El-Bahy, B.A. Sayed El, A.A. Shabana, Vib. Spectrosc. 31 (2003) 101.

Comparison of free ligand thiourea and other metal bromide complexes

Photoluminescence

520 nm -* transitions - metal to ligand molecules – defect centreFluence increases – emission decreases and blue shifts - lattice deformation – displacement of anion and cation

He- Cd Laser – 442 nm

Optical Transmission

Dielectric measurements

The dielectric constant decreases as frequency is increased and almost saturated - governance of various polarization mechanisms.

generally active at low frequencies and high temperature - indicates the perfection of the crystals.

The increase in dielectric constant with increase in temperature and fluence indicates increase in amorphization induced by high-energy heavy ion irradiation from the trapping and detrapping of the charge carriers under the influence of electric field at defect sites

As fluence increases, amorphization increases which in turn decrease the number of crystalline dipoles per unit volume are available to enter into the relaxation process and the original relaxation phase is suppressed because its combined motion is obstructed by the appearance of amorphous phase.

Increase in DE – Electro optic modulator – can be recommended

Hardness studies

Palmqvist crack system

B.Ponton and R.D.Rawlings, Br. Ceram. Trans. J. 88 (1989) 83.

SEM image

Irradiated

Step like structure Amorphization

The defects produced due to irradiation have been studied in detail by defect sensitive techniques like Photoluminescence and optical absorption spectra.

The microhardness decreases which in turn increases the crack length. It is attributed to the amorphization induced due to irradiation.

Also increase in dielectric constant and decrease in dielectric loss may be due to the disordering of crystal lattice by ion beam. This is also due to the dominance of various polarization mechanisms.

A notable increase in dielectric constant is the hallmark of the electro-optic property of the irradiated crystals - fabrication of electro-optic modulators using KTTB crystal.

CONCLUSIONS

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