Multipole plasmon resonances of submicron silver particles

Article abstract of DOI:10.1021/ja053242d

Single crystal, silver particles of 215 ± 10 nm size were synthesized in solution using the hydrogen reduction method and were characterized by UV-visible extinction spectroscopy and electron microscopy. The extinction spectra reveal the presence of higher order multipoles of the plasmon resonance, such as quadrupole, octupole, and hexadecapole, in addition to the dipole. The formation of higher order multipoles was continuously monitored during the particles growth. Mie extinction calculations were performed and are in good agreement with the measured extinction spectra. The frequency shift of all plasmon modes was measured as a function of the refractive index of the surrounding dielectric medium. Copyright

Full text of DOI:10.1021/ja053242d

Published on Web 08/19/2005
Multipole Plasmon Resonances of Submicron Silver Particles
Amar S. Kumbhar, Mark K. Kinnan, and George Chumanov*
Department of Chemistry, Clemson UniVersity, Clemson, South Carolina 29364
Received May 18, 2005; E-mail: gchumak@clemson.edu
Excitation of the collective oscillations of the electron density
called plasmon resonance determines the optical properties of silver
1
nanoparticles. The frequency of the resonance can be tuned across
the broad spectral range by selecting the particle size, shape, as
2,3
well as the surrounding dielectric medium. This tunability and
the high efficiency for interaction with light make Ag nanoparticles
4
5
6
promising in photonics, sensor, photocatalysis, and enhanced
7
spectroscopy applications, and as such, their size-dependent optical
properties become of fundamental importance.
For nearly spherical Ag nanoparticles in the size range from 10
to 50 nm, all electrons in the metal experience roughly the same
phase of the incident electromagnetic field leading to the excitation
of a dipole resonance. As the particle size increases, higher plasmon
modes can be excited due to the phase retardation of the field inside
the particle. In addition to the dipole resonance, a quadrupole mode
was theoretically and experimentally observed for 50-150 nm
1
,8-10
11
spherical
and other shaped Ag particles. Higher order
multipole resonances have also been predicted1 for even larger
,12
Figure 1. Extinction spectra of different sized Ag nanoparticles synthesized
by hydrogen reduction method. The spectra correspond to different aliquots
as is taken at 15 min intervals from the reactor. No additional normalization
of the intensity was performed.
particles and reported for metallic nanoshells above 200 nm
diameter.13 The experimental observation of higher multipoles
beyond the quadrupole for spherical Ag nanoparticles has been
elusive conceivably due to lack of adequate synthetic methods
capable of yielding large, single-crystal nanoparticles of narrow
size and shape distribution. Relevant work published on this subject
describes rather polycrystalline large Ag nanoparticles, and the
14
corresponding plasmon resonance spectra appear featureless.
We have recently introduced a new method for the controlled
synthesis of single-crystal Ag nanoparticles in the size range of
1
0-140 nm10 that are chemically clean without any other surface-
associated chemical species except water and oxygen. In other
commonly used synthetic procedures, the byproducts left after the
chemical reduction of the metal or intentionally added capping
molecules limit the growth of particles to large dimensions and
affect their chemical and optical properties. Here, this method is
extended to the synthesis of single-crystal Ag particles larger than
Figure 2. STEM images of silver particles prepared by hydrogen reduction
of silver oxide solutions (a and b).
nanoparticles, the dipole resonance shows at 490 nm, and a new
band corresponding to the quadrupole mode appears in the spectrum
at 420 nm. As the size of the particles approaches 170 nm, the
dipole and quadrupole resonances shift to 630 and 470 nm,
respectively, and a new band corresponding to the octupole mode
emerges in the spectrum at ca. 430 nm. This band gradually evolves
into a distinct peak that also shifts to the red spectral range with
the increase of the particle size. When the octupole peak moves to
a position at 475 nm, a new shoulder corresponding to the
hexadecapole mode of the plasmon resonance starts to appear at
430 nm. The particles continue to grow very slowly until they
become so large that they fall off the suspension. The largest
synthesized particles were 215 ( 10 nm and had the quadrupole,
octupole, and hexadecapole modes of the plasmon resonance at
543, 483, and 432 nm, respectively, with the broad dipole peak
moved into the near-IR spectral region. STEM images in Figure 2
show highly symmetric icosahedral particles with a small fraction
of elongated rods also present in the suspension. The narrow size
distribution supports the model in which the particles uniformly
grow from the initially formed seeds without the formation of new
140 nm with the size polydispersity better than 5%, allowing the
experimental observation of higher order multipole resonances, such
as octupole and hexadecapole, for the first time. The synthetic
procedure involves the reduction of aqueous saturated silver oxide
solution by ultrapure hydrogen gas in a pressurized atmosphere at
elevated temperature and is described in details elsewhere.10 The
generally understood mechanism is the catalytic reduction of silver
oxide to small Ag “seeds” followed by their slow uniform growth
along all crystallographic axes. The reaction can be conveniently
terminated at any stage, resulting in the nanoparticles of a desired
size. The growth of the particles was monitored by UV-visible
extinction spectroscopy (Figure 1). At the initial stages of the
reaction, the spectrum reveals the presence of a dipole plasmon
resonance at 420 nm attributed to the formation of small Ag seeds.
As the particle size increases, the dipole resonance progressively
red shifts, rendering a green muddy color to the colloidal suspension
due to strong resonance scattering of light. For ca. 90 nm Ag
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J. AM. CHEM. SOC. 2005, 127, 12444-12445
10.1021/ja053242d CCC: $30.25 © 2005 American Chemical Society

C O M M U N I C A T I O N S
the small differences can be accounted by the presence of the
underlying glass substrate.
In conclusion, the hydrogen reduction method allows the
synthesis of submicron Ag particles that exhibit unique optical
properties, specifically the ability to optically excite high order
modes of the plasmon resonance. The observation of these modes
not only is of fundamental importance for understanding optical
properties of plasmonic structures but also renders the potential
for new practical applications. The latter stems from the properties
of various plasmon modes to produce local fields with different
Figure 3. Effect of refractive index of the surrounding medium on
the position of different plasmon modes of 215 ( 10 nm Ag particles
(a and b).
18
energy distribution around the particles as well as to selectively
19
absorb and scatter light in specific regions of the visible spectrum.
These properties are unique for large plasmonic particles and cannot
be found in molecular and semiconductor chromophores or small
plasmonic particles.
seeds during the course of the reaction. As the particle grows, all
plasmon modes red shift with the dipole mode shifting most rapidly
followed by the quadrupole, octupole, and hexadecapole modes
(
Figure 1 in the Supporting Information).
Mie extinction calculations were performed using the code from
Acknowledgment. We gratefully acknowledge the support of
this work through Environmental Protection Agency, Grant
GR829603, and Clemson University Center for Optical Materials
Science and Engineering Technologies. A.K. is thankful to Dr.
Abhijeet Joshi for help with Mie calculations.
ref 2 and the frequency-dependent dielectric function of silver metal
from ref 15. The dielectric constant of surrounding medium was
adjusted from that of pure water to account for local effects,
specifically for adsorbed silver oxide species on the surface of the
particles, and to better match the experimental and calculated values
for the position of all modes. The results of calculations and their
comparison with the experimental data are presented in Figure 2
of the Supporting Information showing the overall good agreement.
The lack of complete overlap of the calculated and experimental
spectra is rationalized as due to assumed spherical shape of the
particles in the calculations rather than the real icosahedral structure.
It is known that the sharp corners, such as those present in
icosahedral structures, affect the spectra of plasmon modes.16
Nevertheless, the calculated spectra reveal the presence of all
plasmon modes as well as their red shift with increase of the particle
Supporting Information Available: Details of characterization and
calculations. This material is available free of charge via the Internet
at http://pubs.acs.org.
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