Jae Cho†, Ra�aella 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].

ResultsE�ect of Au/ITO composition on plasmon absorbance We acquired absorption spectra of Au-ITO �lms of varying composition (Figure 1). As more Au nanocrystals were added to the �lm, the ITO plasmon peak blueshifted and narrowed. Similarly, as more ITO nanocrystals were added to the �lm, the Au plasmon peak also blueshifted and narrowed. �is e�ect suggests possible plasmonic coupling betwen Au and ITO nanocrystals[12].

Conclusion and prospective �e plasmon peak of both Au and ITO shifted in Au-ITO �lms with varying composition and interparticle distance; whereas, the plasmon peak did not shift in solution with varying composition and in Au-In2O3 �lms of varying interparticle distance. �is suggests that the plasmons of Au and ITO nanocrystals are likely to have coupling e�ects similar to the plasmon coupling between metallic nanocrystals. �is �nding 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]Le�, 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�e authors acknowledge the U.S. Department of Energy (DOE) O�ce of Science and the Center for Science and Engineering Education (CSEE). �is 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 di�erent ratios and (2) varied the distance between them by changing the length of ligands on their surface[10,11].

To con�rm that this shift is due to plasmonic coupling, we measured the absorption spectra in solution. �eory predicts that in solution, the separation between the nanocrystals is too large for any coupling to occur[2]. �is behavior is clearly seen in our data (Figure 2).

Results (cont’d)E�ect of Au/ITO distance on plasmon absorbance After observing the possible plasmonic coupling between Au and ITO nanocrystals, we studied the coupling e�ects 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). �is apparent intensi�cation agrees with previous results in the literature on plasmon coupling between metallic nanoparticles and further shows that coupling e�ects may exist for Au and ITO nanocrystals[13].

�e optical properties of these nanocrystals were measured using ultraviolet-visible spectroscopy (UV-Vis) and Fourier transform infrared spectroscopy (FTIR) both in solution and in �lms obtained by drop casting. �e sizes and compositions were determined via transmission electron microscopy (TEM) and X-ray di�raction (XRD) characterization.

Au nanoparticles (12 nm) ITO nanoparticles (8 nm)

Figure 1. UV-Vis spectra of the plasmon absorption of Au-ITO �lms as a function of composition, normalized to (a) ITO peak and (b) Au peak. Here, nanocrystal surfaces are covered with oleylamine ligands.

�en, we repeated the experiment while substituting the ITO for undoped indium oxide (In2O3), which does not have plasmon absorption but a dielectric constant close to ITO. No shift was seen in Au-In2O3 �lms of varying composition (Figure 3). �is rules out dielectric e�ects 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-In2O3 �lms as a function of composition.

Figure 5. UV-Vis spectra of the plasmon absorption of Au-ITO �lms as a function of composition with (a) octylamine ligands and (b) no ligands.

Controlling color changeby plasmonic coupling between metallic and semiconductor nanocrystals

AbstractWhen two plasmonically-active nanocrystals are put in close proximity, their plasmons couple to alter their optical properties. �is 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 e�ects 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. �e plasmon peak of both Au and ITO shifted in Au-ITO �lms 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)

Ref

lect

ance

(a.u

.)

75% Au

50% Au25% Au

500 1000 1500 2000 2500

75% Au50% Au25% Au

Log

[1/T

] (a.

u.)

Wavelength (nm)

500 1000 1500 2000 2500

Refle

ctan

ce

Wavelength (nm)

500 1000 1500 2000 2500

Refle

ctan

ce

Wavelength (nm)

75% Au63% Au50% Au38% Au25% Au

13% Au

ITO

plasmonic coupling

500 1000 1500 2000 2500

Wavelength (nm)

Ref

lect

ance

500 1000 1500 2000 2500 Wavelength (nm)

Ref

lect

ance

75% Au63% Au50% Au38% Au25% Au

13% Au

ITOAu

Introduction As the size of the material reaches the nanoscale, how light interacts with matter changes signi�cantly. �is 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 speci�c wavelengths of light. Interesting coupling e�ects have been revealed when two metallic nanocrystals of the same or di�erent materials are put in close proximity.

�is 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 �eld is therefore to study plasmonic coupling e�ects between metallic and doped semiconductor nanocrystals, if any exists.

increasing sizeincreasing plasmon frequency

no plasmon coupling plasmon coupling

Au nanoparticles

Au

(a) (b)(b)(a)