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CdO has drawn much recent interest as a high-room-temperature-mobility oxide semiconductor with exciting potential for mid-infrared photonics and plasmonics. Wide-range modulation of carrier density in CdO is of interest both for fundamental reasons (to explore transport mechanisms in single samples) and for applications (in tunable photonic devices). Here, we thus apply ion-gel-based electrolyte gating to ultrathin epitaxial CdO(001) films, using transport, x-ray diffraction, and atomic force microscopy to deduce a reversible electrostatic gate response from −4 to +2 V, followed by rapid film degradation at higher gate voltage. Further advancing the mechanistic understanding of electrolyte gating, these observations are explained in terms of low oxygen vacancy diffusivity and high acid etchability in CdO. Most importantly, the 6-V-wide reversible electrostatic gating window is shown to enable ten-fold modulation of the Hall electron density, a striking voltage-induced metal–insulator transition, and 15-fold variation of the electron mobility. Such modulations, which are limited only by unintentional doping levels in ultrathin films, are of exceptional interest for voltage-tunable devices. 

Electronically doped metal oxide nanocrystals exhibit tunable infrared localized surface plasmon resonances (LSPRs). Despite the many benefits of IR resonant LSPRs in solution processable nanocrystals, the ways in which the electronic structure of the host semiconductor material impact metal oxide LSPRs are still being investigated. Semiconductors provide an alternative dielectric environment than metallically bonded solids, such as noble metals, which can impact how these materials undergo electronic relaxation following photoexcitation. Understanding these differences is key to developing applications that take advantage of the unique optical and electronic properties offered by plasmonic metal oxide NCs. Here, we use the two-temperature model in conjunction with femtosecond transient absorption experiments to describe how the internal temperature of two representative metal oxide nanocrystal systems, cubic WO 3−x and bixbyite Sn-doped In 2 O 3 , change following LSPR excitation. We find that the low free carrier concentrations of metal oxide NCs lead to less efficient heat generation as compared to metallic nanocrystals such as Ag. This suggests that metal oxide NCs may be ideal for applications wherein untoward heat generation may disrupt the application's overall performance, such as solar energy conversion and photonic gating.