Optical Properties of Metal Nanoparticles

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Optical Properties of Metal 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 red and 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
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 is
called Surface Plasmon Resonance (SPR).
Figure Schematic of plasmon oscillation for a
sphere, showing the displacement of the
conduction electron charge cloud relative to the
nuclei.
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Literature review

• Michael Faraday was first to report the study of
  the synthesis and colors of colloidal gold.
• In 1908, Mie explained this phenomenon by solving
  Maxwells equation.
• Mie theory predicted optical extinction of
  homogenous spherical particles 2Rltlt? for very
  small particles as (extinction scattering
  absorption)

Where as V is the particle volume, ? is the
angular frequency of the exciting light, and c is
the speed of light. em and e (?) e1 (?) e 2
(?) are the dielectric functions of the
surrounding medium and the metal, respectively
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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 goldbluepurplered are largely
  geometric ones that can be explained with Mie
  theory, which describes light-scattering by a
  sphere.
• When the metal nanoparticle is larger than the
  30 nm, the electrons oscillating with the light
  is not perfectly in phase. Some electrons get
  behind this phenomenon is called retardation
  effect or phase retardation.
• The subsequent changes, reddish - brown to
  orange to colorless, are due to quantum size
  effects.

Mulvaney, MRS Bulletin 26, 1009 (1996)
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Surrounding medium

• The surface plasmon resonance peak changes with
  its own dielectric properties and those of its
  local environment including the substrate,
  solvent, and adsorbates.
• This principle that the high sensitivity of the
  surface plasmon resonance spectrum of noble metal
  nanoparticles to adsorbate-induced changes in the
  dielectric constant of the surrounding
  nanoenvironment 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 left glass is doped with gold
  nanoparticles and spacing between them is large.
• In the right side figure the bulk gold is doped
  with glass. As the spacing is reduced, dipole
  interactions become increasingly important.

(a) Transmitted colors of the same Au_at_SiO2 films.
(b) The reflected color 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 of Thin Films of
Au_at_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 catalysts because of their high
  surface-to-volume ratios and different shapes.

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QUESTIONS ???
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