Transcript Optical Properties of Metal Nanoparticles

Optical Properties of
Metal Nanoparticles
Sriharsha Karumuri
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.
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.
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.
Literature review
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Michael Faraday was first to report the study of the synthesis
and colors of colloidal gold.
In 1908, Mie explained this phenomenon by solving Maxwell’s
equation.
Mie theory predicted optical extinction of homogenous
spherical particles 2R<<λ for very small particles as
(extinction = scattering + absorption)
3
 ext
9.V . m 2

c


. 2 ( )


2
2 
 [ 1 ( )  2 m ]   2 ( ) 
Where as V is the particle volume, ω is the angular frequency of the
exciting light, and c is the speed of light. εm and ε (ω) = ε1 (ω)+ ε 2
(ω) are the dielectric functions of the surrounding medium and the
metal, respectively
Synthesis processes
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Wet chemical process
Mechanical process
Form in phase
Gas phase synthesis
Electroless deposition
Size dependence
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Mulvaney, MRS Bulletin 26, 1009 (1996)
The changes gold–blue–purple–red
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.
Surrounding medium
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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.
Particle density
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(a) Transmitted colors of the same
Au@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.
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.
Thearith Ung, Luis M. Liz-Marzan, and Paul Mulvaney,
optical Properties of Thin Films of Au@SiO2 Particles,
J. Phys. Chem. B 2001, 105, 3441-3452.
Applications
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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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