sriharsha karumuri

8/8/2019 Sriharsha Karumuri

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Optical Properties ofMetal Nanoparticles

Sriharsha Karumuri


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Introduction

Why nanoparticles are different from bulk materials?

Nanoparticles are different from bulk materials and

isolated molecules because of their unique optical,

electronic and chemical properties.

As the dimensions of the material is reduced the

electronic properties change drastically as the

density of states and the spatial length scale of the

electronic motion are reduced with decreasing size.

Closely related to size-induced changes in the

electronic structure are the optical properties of

nanoparticles.


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1) Gold nanoparticles were used as a

pigment of ruby-colored stained glass

dating back to the 17th century. Figure.1

shows a picture of the Rose Window of the

Cathedral of Notre Dame. The bright redand purple colors are due to gold

nanoparticles.

2) Lycurgus cup: It appears green in

reflected light, but appears red when light

is shone from inside, and is transmitted

through the glass.


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Surface plasmon resonance

Figure: Schematic of plasmon

oscillation for a sphere,

showing the displacement of

the conduction electron

charge cloud relative to the

nuclei.

When a nanoparticle is much smaller than the

wave length of light, coherent oscillation of the

conduction band electrons induced by interaction

with an electromagnetic field. This resonance iscalled Surface Plasmon Resonance (SPR).


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Literature review

Michael Faraday was first to report the study of the synthesisand colors of colloidal gold.

In 1908, Mie explained this phenomenon by solving Maxwellsequation.

Mie theory predicted optical extinction of homogenousspherical particles 2R


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Synthesis processes

Wet chemical process

Mechanical process

Form in phase

Gas phase synthesis

Electroless deposition


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Size dependence

The changes goldbluepurpleredare largely geometric ones that canbe explained with Mie theory, whichdescribes light-scattering by a

sphere. When the metal nanoparticle is

larger than the ~30 nm, theelectrons oscillating with the light isnot perfectly in phase. Someelectrons get behind; this

phenomenon is called retardationeffect or phase retardation.

The subsequent changes, reddish -brown to orange to colorless, aredue to quantum size effects.

Mulvaney, MRS Bulletin 26, 1009 (1996)


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Surrounding medium

The surface plasmon resonancepeak changes with its owndielectric properties and those ofits local environment including thesubstrate, solvent, and adsorbates.

This principle that the highsensitivity of the surface plasmonresonance spectrum of noble metalnanoparticles to adsorbate-inducedchanges in the dielectric constantof the surroundingnanoenvironment used in

chemosensing and biosensing.

Spectral shift for individual blue (roughly spherical) silver nanoparticles.

Typical blue particle spectrum as it is shifted from (a) air to (b)1.44 index oil,

and successive oil treatments in 0.04 index incremental increases.

Jack J. Mock, David R. Smith, and Sheldon Schultz, Local Refractive Index Dependence of Plasmon

Resonance Spectra from Individual Nanoparticles, Nano letters 2003 Vol. 3 No. 4 485-491.


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Particle density

Beginning from the leftglass is doped withgold nanoparticlesand spacing betweenthem is large.

In the right side figurethe bulk gold is dopedwith glass. As thespacing is reduced,

dipole interactionsbecome increasinglyimportant.

(a) Transmitted colors of the same

Au@SiO2 films. (b) The reflectedcolor of the films after deposition

from a ruby red gold sol as a function

of the silica shell thickness. Top left

going across: 15 nm gold particles

coated with silica shells of thickness

17.5, 12.5, 4.6, 2.9, and 1.5 nm.Thearith Ung, Luis M. Liz-Marzan, and Paul Mulvaney,

optical Properties ofThin Films of Au@SiO2 Particles,

J. Phys. Chem. B 2001, 105, 3441-3452.


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Applications

These differences in properties of nanoparticles are

used in microelectronics, quantum dot lasers,

chemical sensors, data storage, and a host of other

applications.

Possible future applications of nanoparticles include

the areas of ultrafast data communication and

optical data storage, solar energy conversion, and

the use of metallic nanoparticles as catalystsbecause of their high surface-to-volume ratios and

different shapes.


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QUESTIONS ???QUESTIONS ???


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THANK YOUTHANK YOU

sriharsha karumuri

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