avinashthird year project

8/8/2019 AvinashThird Year Project

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CERTIFICATE

This is to certify thatMr.Avinash Rustagi, IIIrd year student of B.Sc (Physics) - Hons ofSt. Stephens College has done the project titled . In this project hewas involved in the preparation of Nanoparticles and Thin Films of Silver Metal. He hasalso studied the optical properties of the particles as well as films.

Avinash and Saubhik worked in my laboratory from November 2007 to February 2008.He has carried out his work systematically.

Dr. S. Annapoorni Date: 13th March, 2008Department of Physics and Astrophysics,Delhi University,Delhi-110007.


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ACKNOWLEDGEMENT

I take this opportunity to express my sincere gratitude to Prof. S. Annapoorni forallowing me to work under her as a project trainee and for her constant encouragement,help and support throughout this project work.

I wish to thank Mr. N. Kamal Singh (University of Delhi) for his valuable guidancethroughout this project. I also thank the University Science Instrumentation Center, DelhiUniversity for doing the X-Ray diffraction Patterns.

Last but not the least, I would specially thank all the other research scholars in thelaboratory for their cooperation and help in the completion of this project.


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INTRODUCTION

Interactions of electromagnetic radiation with matter can generate

interesting surface excitations. In electromagnetic terms there are a range of

interfaces of interest, for example: dielectric/dielectric,

dielectric/semiconductor, dielectric/metal. In this project, the matter of

interest is the optical properties of nanometals and the interactions between

dielectric-metal interfaces. Nanoparticles are particularly interesting nanoscale

systems, and have been investigated to a wide extend because of their ease of

preparation. Due to the high surface activity, unusual physical and chemical

properties they have given rise to a wide range of applications. The basic physical

principles of the unusual optical properties are highlighted and some of thegrowing important applications are also reported. A surface Plasmon can be

generated by the interaction of an electron rich surface (such as a metal) with a

charge particle or a photon. Surface Plasmon Resonance (SPR) is a collective,

quantized oscillation of conduction electrons near the surface of a metal or a

semiconductor. Plasmon Excitation is observed as an increase in the optical

absorbance (decrease in reflectance) at an optimum coupling angle p . The surface

plasmons were discovered in the late 1950s and since then it has been an active

area of research. It is actively used in the field of optical sensors and bio-sensors.

This project has been divided into four parts:

i. Preparation of silver particles of nanoscale dimensions using thepolyol process.

ii. Study of some characteristic curves of this nanometal.iii. Preparation of Silver thin films by Thermal Evaporation method.iv. Experiments on surface Plasmon excitation in silver/air interface

using the Kretschmann Geometry.


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NANOPARTICLES

Nanoparticles cannot simply be treated as minute blocks of metals. For any material,

atoms in the bulk are attracted and repelled by neighboring atoms and are in

equilibrium with their surroundings. As the particle size decreases, a higher fraction of

the atoms becomes surface atoms. Surface atoms, due to reduced co-ordination number,

are attracted towards the interior of the nanoparticle by the bulk atoms. This results in a

higher energy state for the surface atoms, and this surplus energy is referred to as the

surface energy of the nanoparticle. The increase in the fraction of surface atoms results

in an increase of surface energy as the nanoparticle size is reduced (Esurf 1/d);

therefore, the surface energy comes to play an important role for the properties ofnanoparticle.

Preparation:

Generally the preparation techniques of Nanomaterials fall under two categories:

bottom-up and top-down approach. The bottom-up approach refers to the build up of

a material from the bottom, i.e. atom-by-atom, molecule-by-molecule or cluster-by-

cluster. Nanolithography and nanomanipulation techniques are bottom-up approaches.

Top-down approach involves starting with a block of bulk material and designing or

milling it down to the desired shape and size. The main challenge for top-down

approach is the creation of small structure with sufficient accuracy, whereas in bottom-

up approach, the main challenge is to make the structure large enough and of efficient

quality .

Since nanotechnology is an interdisciplinary subject, there are various physical,

chemical, biological and hybrid techniques available to prepare Nanomaterials in

various forms.


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Technique used:

Polyol Process

Various metal particles with sizes ranging from a few nanometers to a few microns have

been made by the polyol process. The polyol process has been proposed as a method for

the preparation of finely divided powders of easily reducible metals. A suitable solid

inorganic compound is dissolved or suspended into a liquid polyol (e.g., ethylene-glycol,

diethylene-glycol or a mixture of both). The mixture is further stirred and heated to an

appropriate temperature, which can reach the boiling point of the polyol for less easily

reducible metals. The reduction of the starting compound yields the metal as a finelydivided powder. The starting compound may be a hydroxide, an oxide or a convenient

salt. The main feature of the reaction mechanism is that the reduction reaction proceeds

via the solution rather than the solid phase. Therefore, the metal particles are formed by

nucleation and growth from the solution. According to this mechanism, the polyol acts as

a solvent for the starting inorganic compound due to rather high dielectric constant of

these organic media. This process has been used to make a variety of metal nanoparticles

such as Ag, Au,etc.

Experimental Details:

5ml of anhydrous ethylene glycol was heated at 1600C for 1 hour. 3ml of ethylene glycol

solution of Silver Nitrate (0.25M) and 3ml of ethylene glycol solution of PVP(0.375M)

were simultaneously added to the hot glycol very slowly. The reaction mixture was then

continued with heating for another 45min at 1600C. After this procedure the mixture was

dominated by silver nanoparticles. The mixture was diluted several times with water and

then the crystals could be recovered from centrifugation.


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CHARACTERISTIC CURVES:

UV absorption Curve

X-Ray Diffraction Pattern


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XRD Data:

Sin2 Sin2 /3 Sin2 /4 Sin2 /8 Sin2 /11 Sin2/12

19.055 0.1066 0.0355 0.0267 0.0133 0.0097 0.0089

22.157 0.1422 0.0474 0.0356 0.0178 0.0129 0.0118

32.285 0.2853 0.0951 0.0713 0.0356 0.0259 0.0238

38.72 0.3913 0.1304 0.0978 0.0489 0.0356 0.0326

a2= 2(h2 +k2 +l2 )/4 Sin2

a2= [(1.54056)2*(3)]/[4*0.1066]= 16.692

Therefore a= 4.09 .

Planes present (111), (200), (220), (113)

The structure is a Face Centered Cubic Lattice.


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THEORY OF SURFACE PLASMON RESONANCE

Electromagnetic radiation in an isotropic media consists of orthogonal oscillating electric

and magnetic fields transverse to the direction of propagation. When we pass this

radiation through a polarizer, the radiation becomes plane polarized. Depending on the

direction of the electric field vector, this radiation can be classified into two types- p-

polarized and s-polarized. When we consider a p-polarized EM wave, the direction of

electric field is in the plane of incidence having components parallel and perpendicular to

the interface. The B vector is perpendicular to the plane of incidence. However in case of

the s-polarized wave the electric field is perpendicular to the plane of incidence and the B

vector is in the plane of incidence. This can be seen in figures 1a and 1b.

Linearly polarized light is represented as a sum of the above two cases.

Fig 1a Fig 1b


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TOTAL INTERNAL REFLECTION: LIMITING CASE OF SNELLS LAW AND

THE EVANESCENT WAVE

Suppose the radiation is incident from a high refractive index medium (n1 = 1) to amedium of low refractive index (n2 =2) then by Snells law (1sin1 =2 sin2). Forany incident angle greater than the critical angle, total internal reflection will occur.Radiation incident beyond the critical angle has more momentum along the plane that canbe supported by medium2. For such radiation incident from medium1, the oscillatingelectric field E will cause the charges in medium1 and those at the interface to oscillate.Thus even though the radiation is totally reflected there are oscillating charges herewhich have associated radiation fields penetrating into medium2. These spatiallydecaying fields oscillating in time are called Evanescent Waves which have the samefrequency as the incident radiation and are decaying in amplitude in medium2 in a

direction normal to the interface. This evanescent field radiation incident beyond thecritical angle is used for coupling radiation to the Surface Plasmons.

From the first Maxwells Equation and the boundary conditions we see that since there isno boundary orthogonal to the Ex component of E (fig 1a), it is conserved across theboundary. However this is not the case with the Ez component of E. Since there are nofree charges, D is continuous. The discontinuity in the Ez component results inpolarization change at the interface. From these considerations we see that p-polarizationwill automatically give rise to time dependent polarization charge at the interface whilethe s-polarization will not.

Consider one of the two materials to be metal; from the characteristic of a metal we knowthat if free electrons in metal are able to respond to the incident radiation with noscattering then it gives rise to ideal metal response. Such a material which has E=0everywhere inside the metal must correspond to = +. An ideal metal in which theelectrons respond perfectly to the applied field, therefore cancelling it, is the limit = -.Such a metal does not exist as the free electrons cannot respond infinitely quickly to theimposed oscillation. They have a finite mass and suffer scattering with lattice vibrations,defects and the surface. At high enough frequencies the metal behaves as a dielectric.Here we are concerned with wavelengths below this limit so that is largely real andnegative. But as there are resistive scatterings, there is damping of oscillations created bythe incident radiation. This damping causes an imaginary component to it.


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Considering only p-polarized radiation we try to solve the Maxwells Equation with thespecific boundary conditions to get the mathematical explanation for the surfaceplasmons and the condition for resonance. The x-y plane is considered to be the interfaceand the +z half space as the medium2, then for propagation in the x direction only,

E1= (Ex1,0, Ez1) exp[i(kxx- t)] exp(ikz1z) (1a)H1= (0, Hy1, 0) exp [i(kxx- t)] exp(ikz1z) (1b)E2= (Ex2, 0, Ez2) exp[i(kxx- t)] exp(ikz2z) (2a)H2= (0, Hy2, 0) exp [i(kxx- t)] exp(ikz2z) (2b)

Applying Maxwells Equation: div.(E)=0We get, Ez1= - Ex1(kx/ kz1) (3)

Ez2= - Ex2(kx/ kz2) (4)

Now, using the equation ofcurl E= -H/t with =0 gives the following relationships

between the field components, the permittivity and the normal components of the wavevectors in medium2.Hy1 = (10 Ez1/kz1) (5a)Hy2 = (20 Ez2/kz2) (5b)

Applying the boundary conditions at z=0 and knowing that Hy1= Hy2 and Ex1= Ex2, weget the relationship between the relative permittivity and the normal components of thewave vectors in both media:

1 /kz1= 2/kz2 (6)

Also we have,k

z1= -i(k

x

2- 1k2)1/2, which requires k

x

2> 1k2 (7a)

kz2= -i(kx2- 2k2)1/2, which requires kx2> 2k2 (7b)Where k= /c. If the wave is truly a trapped surface wave which is exponentiallydecaying into both media then we need i kz1> 0 and i kz2< 0. Thus both kzs are imaginarywith opposite signs and so 1 and 2 are of opposite signs. The equation 7a implies thatthe wave vector in the metal air interface is always greater than the available photonwave vector () and equation 7b is satisfied with 2 negative. Substituting expressions 7 forkz1 and kz2 into the eq.(6) we get,

kx= k[12/(1 + 2)]1/2 (8)


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And for kx to be real the requirement for a propagating mode, with 2 negative is that| 2|> 1. Now considering the damping term in the metal-dielectric constant, that is,2= 2r + 2i.

We get kx= k[1 (2r + 2i)/(1 + (2r + 2i))]1/2 (9)

From eq. (8) we see that wave vector of Surface Plasmon is greater than the wave vectorof an electromagnetic wave in air at the same frequency. So optical photons cannot becoupled directly to surface plasmons. But they are coupled via evanescent tail of the lighttotal internally reflected at the base of an index prism (p,p> 1). This light ischaracterized by larger momentum along the surface.

Let kx= ksp and 1 = s and 2 = m then we get the following equation from(8):

ksp= k[sm/(s + m)]

1/2

And we know that the wave vector of the incident photon is given by:kph= (/c)[s]

1/2Thus the condition for resonance is: ksp= kph.

Resonance condition: kspx= k[sm/(s + m)]1/2= kphx= (/c)[s]

1/2 sin

For a Plasmon supporting metal: ksp>kph. Therefore surface plasmons cannot be excited

by photons propagating in free space. The momentum of the incident photon has to beenhanced in order to couple them to surface plasmons. This can be done by passing thelight through a medium of higher refractive index like prism (p> d ), under total internalreflection. Hence we have special geometries set up where these plasmons can be excited.


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DIFFERENT GEOMETRIES / CONFIGURATIONS

Attenuated total internal reflection:

The evanescent field does not propagate in the z-direction, but it has a component in the

x-direction of n1hksini . Since sini > sinc then n1hksini > n2hk. Hence we have anenhancement in the x-component of the momentum in the second dielectric half space,above the limit value of n2hk for the propagating wave.

This enhancement of the momentum given by n1 (sini sinc )hk is used to couple theradiation to the surface if the plasmon provided it is possible to place the metal dielectricinterface which supports the surface plasmon close enough to a totally internallyreflecting surface. This is called attenuated total internal reflection.

The following configurations are used to achieve the surface plasmon resonace.1. Otto configuration,

2. Mixed / hybrid configuration,

3. Kretschmann and Raether configuration: In this case metal film is deposited on theprism directly. The metal itself acts as an evanescent tunnel barrier provided it is thinenough to allow the radiation to penetrate to other side.


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PREPARATION OF THIN FILMS

Silver films of thickness 50nm were deposited on the hypotenuse of a right angled glassprism (=1.51) by Thermal Evaporation under a vacuum of 7*10-6 m.bar. A photographof the coating unit is shown in figure below. The high vacuum was achieved throughpump backed by a rotary pump. Silver wires of 99.99% purity were placed in amolybdenum boat for the evaporation. The films were deposited at a rate of 4.0 Angstromper second. The thickness of films was monitored using a quartz crystal monitor. A glassslide was placed along the side of the prism for characterization of the sample.

Coating Unit


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SURFACE PLASMON RESONANCE CURVE:

Set up: For the SPR measurements the prisms were removed from the coating unit and

placed on a rotary platform spectrometer of resolution 0.1 degree. A Laser light ofwavelength 670nm polarized in the plane of incidence was incident on the prism and thereflected light was recorded as the function of angle of incidence by a photodiode. ThePhotodiode is a silicon detector and is used in the photovoltaic mode. The followingdiagrams show the set up.

Schematic Set Up

Actual Set Up


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Curve Details:

SPR

0.84

0.86

0.88

0.9

0.92

0.94

0.96

0.98

38 39 40 41 42 43 44

Angle

Relative

Intensi

ty

The angle for SPR was found to be 40.850. The theoretical value is


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CONCLUSION

As inferred from the Experiment, the Nanoparticles prepared had a wide absorption peakdue to particle size variation. From the XRD data, it was found that the particles haveFCC structure with Lattice Parameter = 4.09.

The SPR curve for the thin film of silver (50nm) displayed a sharp dip in reflectance at aparticular angle which is in agreement to the theoretical value. This sharp characteristicof the thin film makes it a widely used technique to construct sensors.


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REFERENCES

1) Nanometals: Preparation Methods, Optical and Properties and PossibleApplications Abdullah Alqudami.

2) Surface Plasmon Resonance Theory Masahiro Yamamoto.3) Surface Plasmon Resonance, Theory and Applications Zdzislaw, Salamon and

Gordon Tollin. (Academic Press 1999)

4) Introduction to Solid State Physics Charles Kittel.

5)

Introduction to Electrodynamics David J Griffiths.

avinashthird year project

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