Impacts of Surface Depletion on the Plasmonic Properties of Doped
  Semiconductor Nanocrystals

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Abstract

Degenerately doped semiconductor nanocrystals (NCs) exhibit a localized surface plasmon resonance (LSPR) in the infrared range of the electromagnetic spectrum. Unlike metals, semiconductor NCs offer tunable LSPR characteristics enabled by doping, or via electrochemical or photochemical charging. Tuning plasmonic properties through carrier density modulation suggests potential applications in smart optoelectronics, catalysis, and sensing. Here, we elucidate fundamental aspects of LSPR modulation through dynamic carrier density tuning in Sn-doped Indium Oxide NCs. Monodisperse Sn-doped Indium Oxide NCs with various doping level and sizes were synthesized and assembled in uniform films. NC films were then charged in an in situ electrochemical cell and the LSPR modulation spectra were monitored. Based on spectral shifts and intensity modulation of the LSPR, combined with optical modeling, it was found that often-neglected semiconductor properties, specifically band structure modification due to doping and surface states, strongly affect LSPR modulation. Fermi level pinning by surface defect states creates a surface depletion layer that alters the LSPR properties; it determines the extent of LSPR frequency modulation, diminishes the expected near field enhancement, and strongly reduces sensitivity of the LSPR to the surroundings.

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Space Charge Calculations for Semiconductors

Mino Green, Ruth Seiwatz (1958)

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Electronic doping and redox-potential tuning in colloidal semiconductor nanocrystals.

Daniel R Gamelin, Alina Schimpf, Kathryn Knowles … (2015)

Electronic doping is one of the most important experimental capabilities in all of semiconductor research and technology. Through electronic doping, insulating materials can be made conductive, opening doors to the formation of p-n junctions and other workhorses of modern semiconductor electronics. Recent interest in exploiting the unique physical and photophysical properties of colloidal semiconductor nanocrystals for revolutionary new device technologies has stimulated efforts to prepare electronically doped colloidal semiconductor nanocrystals with the same control as available in the corresponding bulk materials. Despite the impact that success in this endeavor would have, the development of general and reliable methods for electronic doping of colloidal semiconductor nanocrystals remains a long-standing challenge. In this Account, we review recent progress in the development and characterization of electronically doped colloidal semiconductor nanocrystals. Several successful methods for introducing excess band-like charge carriers are illustrated and discussed, including photodoping, outer-sphere electron transfer, defect doping, and electrochemical oxidation or reduction. A distinction is made between methods that yield excess band-like carriers at thermal equilibrium and those that inject excess charge carriers under thermal nonequilibrium conditions (steady state). Spectroscopic signatures of such excess carriers, accessible by both equilibrium and nonequilibrium methods, are reviewed and illustrated. A distinction is also proposed between the phenomena of electronic doping and redox-potential shifting. Electronically doped semiconductor nanocrystals possess excess band-like charge carriers at thermal equilibrium, whereas redox-potential shifting affects the potentials at which charge carriers are injected under nonequilibrium conditions, without necessarily introducing band-like charge carriers at equilibrium. Detection of the key spectroscopic signatures of band-like carriers allows distinction between these two regimes. Both electronic doping and redox-potential shifting can be powerful tools for tuning the performance of nanocrystals in electronic devices. Finally, key chemical challenges associated with nanocrystal electronic doping are briefly discussed. These challenges are centered largely on the availability of charge-carrier reservoirs with suitable redox potentials and on the relatively poor control over nanocrystal surface traps. In most cases, the Fermi levels of colloidal nanocrystals are defined by the redox properties of their surface traps. Control over nanocrystal surface chemistries is therefore essential to the development of general and reliable strategies for electronically doping colloidal semiconductor nanocrystals. Overall, recent progress in this area portends exciting future advances in controlling nanocrystal compositions, surface chemistries, redox potentials, and charge states to yield new classes of electronic nanomaterials with attractive physical properties and the potential to stimulate unprecedented new semiconductor technologies.