1. Jae Cho†
, Raffaella Buonsanti‡
, and Delia Milliron‡
†
Department of Chemical Engineering, University of California, Santa Barbara
‡
The Molecular Foundry, Lawrence Berkeley National Laboratory
Materials and methods
We used colloidal synthesis to prepare Au and ITO nanocrystals[8],[9]
.
Results
Effect of Au/ITO composition on plasmon absorbance
We acquired absorption spectra of Au-ITO films of varying
composition (Figure 1). As more Au nanocrystals were added to the
film, the ITO plasmon peak blueshifted and narrowed. Similarly, as
more ITO nanocrystals were added to the film, the Au plasmon peak
also blueshifted and narrowed. This effect suggests possible
plasmonic coupling betwen Au and ITO nanocrystals[12]
.
Conclusion and prospective
The plasmon peak of both Au and ITO shifted in Au-ITO films with
varying composition and interparticle distance; whereas, the plasmon peak
did not shift in solution with varying composition and in Au-In2O3 films
of varying interparticle distance.This suggests that the plasmons of Au and
ITO nanocrystals are likely to have coupling effects similar to the plasmon
coupling between metallic nanocrystals. This finding opens up de novo
optical modulation applications (e.g. energy-saving smart windows that
are tunable in the visible range) and calls for further investigation of
coupling between metallic and doped semiconductor nanocrystals.
Literatures cited[1]
MacDonald, K. F. and Zheludev, N. I. Laser & Photonics Reviews 4, 4 (2010): 562-567.
[2]
Tokarev, I.,Tokareva, I., and Minko, S. Advanced Materials 20 (2008): 2730:2734.
[3]
Halas, N., et al. Chemical Reviews 111, 6 (2011): 3913-3961.
[4]
Ghosh, S. K. and Pal,T. Chemical Reviews 107 (2007): 4797-4862.
[5]
Franzen, S. Journal of Physical Chemistry C 112 (2008): 6027-6032.
[6]
Naik, G. and Boltasseva, A. Metamaterials 5 (2011): 1-7.
[7]
Garcia, G., et al. Nano Letters 11, 10 (2011): 4415-4420.
[8]
Choi, S., et al. Chemistry of Materials 20 (2008): 2609-2611.
[9]
Leff, D. V., Brandt, L., and Heath, J. R. Langmuir, 12, 20 (1996): 4723-4730.
[10]
Dong, A., et al. Journal of the American Chemical Society, 133 (2011): 998-1006.
[11]
Rosen, E. L., et al. Angewandte Chemie International Edition 51 (2012): 684-689.
[12]
Perez-Gonzalez, O., et al. Nano Letters, 10, (2010): 3090.
[13]
Romero, I., et al. Optics Express, 14, 21 (2006): 9988-9999.
Acknowledgements
The authors acknowledge the U.S. Department of Energy (DOE) Office of Science and
the Center for Science and Engineering Education (CSEE). This research was funded by
the University of California Leadership Excellence through Advanced Degrees (UC
LEADS) program.
To test if any plasmonic coupling occurs between Au and ITO
nanocrystals, we (1) combined them in different ratios and (2) varied the
distance between them by changing the length of ligands on their
surface[10,11]
.
To confirm that this shift is due to plasmonic coupling, we measured
the absorption spectra in solution. Theory predicts that in solution, the
separation between the nanocrystals is too large for any coupling to
occur[2]
.This behavior is clearly seen in our data (Figure 2).
Results (cont’d)
Effect of Au/ITO distance on plasmon absorbance
After observing the possible plasmonic coupling between Au and ITO
nanocrystals, we studied the coupling effects as a function of interparticle
distance. When the spacing between the nanocrystals was decreased via
ligand exchange, the plasmon peak shift and broadening was greater
(Figure 5).This apparent intensification agrees with previous results in the
literature on plasmon coupling between metallic nanoparticles and further
shows that coupling effects may exist for Au and ITO nanocrystals[13]
.
The optical properties of these nanocrystals were measured using
ultraviolet-visible spectroscopy (UV-Vis) and Fourier transform infrared
spectroscopy (FTIR) both in solution and in films obtained by drop
casting. The sizes and compositions were determined via transmission
electron microscopy (TEM) and X-ray diffraction (XRD) characterization.
Au nanoparticles (12 nm) ITO nanoparticles (8 nm)
Figure 1. UV-Vis spectra of the plasmon absorption of Au-ITO films as
a function of composition, normalized to (a) ITO peak and (b) Au peak.
Here, nanocrystal surfaces are covered with oleylamine ligands.
Then, we repeated the experiment while substituting the ITO for
undoped indium oxide (In2
O3
), which does not have plasmon absorption
but a dielectric constant close to ITO. No shift was seen in Au-In2
O3
films
of varying composition (Figure 3). This rules out dielectric effects and
suggests that Au and ITO plasmons are probably coupled.
Figure 2. UV-Vis spectra
of the plasmon absorption
of Au-ITO in solution as
a function of composition.
Figure 3. UV-Vis spectra
of the plasmon absorption
of Au-In2
O3
films as a
function of composition.
Figure 5. UV-Vis spectra of the plasmon absorption of Au-ITO films as
a function of composition with (a) octylamine ligands and (b) no ligands.
Controlling color change
by plasmonic coupling between metallic and semiconductor nanocrystals
Abstract
When two plasmonically-active nanocrystals are put in close proximity, their plasmons
couple to alter their optical properties. This phenomenon has been studied for metallic
nanocrystals, such as gold (Au). However, little is known about coupling between metallic
and doped semiconductor nanocrystals, such as tin doped indium oxide (ITO). To study
coupling effects between metallic and doped semiconductor nanocrystals, we investigated
the interactions between Au and ITO nanocrystals by measuring their optical properties as
a function of composition and interparticle spacing. The plasmon peak of both Au and
ITO shifted in Au-ITO films both with varying composition and with interparticle
distance. Plasmons of Au and ITO nanocrystals are thus likely to couple similar to metallic
nanocrystals.
500 1000 1500 2000 2500
Wavelength (nm)
Reflectance(a.u.)
75% Au
50% Au
25% Au
500 1000 1500 2000 2500
75% Au
50% Au
25% Au
Log[1/T](a.u.)
Wavelength (nm)
500 1000 1500 2000 2500
Reflectance
Wavelength (nm)
500 1000 1500 2000 2500
Reflectance
Wavelength (nm)
75% Au
63% Au
50% Au
38% Au
25% Au
13% Au
ITO
plasmonic coupling
500 1000 1500 2000 2500
Wavelength (nm)
Reflectance
500 1000 1500 2000 2500
Wavelength (nm)
Reflectance
75% Au
63% Au
50% Au
38% Au
25% Au
13% Au
ITOAu
Introduction
As the size of the material reaches
the nanoscale, how light interacts with
matter changes significantly. This gives
nanostructures unique optical properties
such as the surface plasmon resonance
absorption[1]
.Surface plasmon resonance
derives from the collective oscillations of
electrons on the surface of a conductive
nanocrystal when interacting with specific wavelengths of light.
Interesting coupling effects have been revealed when two metallic
nanocrystals of the same or different materials are put in close proximity.
This phenomenon has been observed and deeply studied in metallic
nanocrystals, such as Au[2-4]
. Recently, plasmon absorption has been
demonstrated also for doped semiconductors nanocrystals, such as
ITO[5-7]
. A new challenge in the field is therefore to study plasmonic
coupling effects between metallic and doped semiconductor nanocrystals,
if any exists.
increasing size
increasing plasmon frequency
no plasmon coupling plasmon coupling
Au nanoparticles
Au
(a) (b)
(b)(a)