Rationalizing the Impact of Surface Depletion on Electrochemical Modulation of Plasmon Resonance Absorption in Metal Oxide Nanocrystals

Ankit Agrawal,Evan L Runnerstrom,Anna Llordes,Francesco Scotognella,Ilka Kriegel,Delia J Milliron

doi:10.1021/acsphotonics.7b01587

Abstract

Dynamic control over the localized surface plasmon resonance (LSPR) makes doped metal oxide nanocrystals (NCs) promising for several optoelectronic applications including electrochromic smart windows and redox sensing. Metal oxide NCs such as tin-doped indium oxide display tunable infrared LSPRs via electrochemical charge injection and extraction as a function of the externally applied potential. In this work we have employed dispersion phase electrochemical charging/discharging to study the mechanism behind the optical modulation on an individual NC scale. The optical modulation of the LSPR is dominated by a sharp variation in intensity during reduction and oxidation along with an only modest shift in the LSPR frequency. With a core–shell modeling approach, in which an active NC core surrounded by a depleted shell is assumed, we were able to reproduce the trends in and main features of our experimental results. The shell thickness depends on the applied potential and we extracted the temporal evolution o...

Highlights

I n degenerately doped semiconductor nanocrystals (NCs) the localized surface plasmon resonances (LSPR), originating from the collective oscillations of the free electrons in the conduction band, are centered in the infrared spectrum.[1,2]
Metal oxide nanocrystals such as WO3−x and Sn:In2O3 (ITO) are key components in smart windows that in complex nanostructured ensembles enable the independent tuning of visible and NIR light transmission, fast switching times, and high optical modulation.[6,9−11] Until recently, the focus of most of the research went toward the optimization of optical and electrochemical performance via material design and structural engineering, but little knowledge has been acquired on the actual mechanism behind the electrochemical switching.[12]
In our recent work,[13] we found that the optical properties of doped metal oxide NCs are largely influenced by the presence of a depletion region, resulting from NC surface states, that reaches into the NC volume with radial dimension from subnanometer to several nanometers in thickness

Summary

ACS Photonics

Concentration within this depletion layer can be modulated, thereby affecting the NCs’ LSPR properties, and that the extent of LSPR shift and absorption intensity modulation strongly depends on the NC size and the dopant concentration inside the host lattice. We fit the resulting spectral variations using a core−shell effective medium approach, assuming that the NC consists of a plasmonically active Sn:In2O3 core surrounded by a uniformly depleted shell of the same material, in accordance with our recent findings.[13] With this model we quantify the underlying processes of charging and discharging the Sn:In2O3 NCs by assessing the relative spectral contributions of charge carrier density, carrier damping and depletion region together in our electrochemically tuned plasmonic system. We find that the total carrier density is not significantly altered at any point during the charging and discharging process This is of fundamental importance for the plasmonic response of electrochemically modified NCs, as the depletion layer largely dominates the intensity and peak position of the LSPR, and is expected to substantially impact the near-field LSPR properties. We unravel the underlying nanoscale effects occurring during the electrochemical charging process, and highlight implications of this effect on the optical response of the modulated plasmonic material

■ RESULTS AND DISCUSSION

■ ACKNOWLEDGMENTS

■ REFERENCES

Metal Oxide Nanocrystal Sensors That Optically Track Redox