Transparent Conductive Oxide Nanocrystals Coated with Insulators by Atomic Layer Deposition

Thin films comprised of transparent conductive oxide (TCO) nanocrystals are attractive for a number of optoelectronic applications. However, it is often observed that the conductivity of such films is very low when they are in contact with air. It has recently been demonstrated, somewhat surprisingly, that filling in initially insulating films comprised of TCO nanocrystals with another insulator by atomic layer deposition (ALD) dramatically increases the conductivity by many orders of magnitude. This work aims to elucidate the mechanism by which the ALD coating increases conductivity. We examined the effect of removing two adsorbed oxygen species (physisorbed molecular water and chemisorbed hydroxide) on sheet resistance and compared this result to the results with thin films comprised of ZnO nanocrystals coated with Al₂O₃ and also HfO₂ by ALD. Although both insulating infills decrease the sheet resistance and increase the stability of the films, there is a stark discrepancy between the two. From the <i>in situ</i> measurements, it was found that coating with Al₂O₃ removes both physisorbed water and chemisorbed hydroxide, resulting in a net reduction of the ZnO nanocrystals. Coating with HfO₂ removes only physisorbed water, which was confirmed by Fourier transform infrared spectroscopy. A similar phenomenon was observed when thin films comprised of Sn-doped In₂O₃ nanocrystals were coated, suggesting Al₂O₃ can be used to reduce and stabilize metal oxide nanocrystals in general

Electrochemically Induced Transformations of Vanadium Dioxide Nanocrystals

Vanadium dioxide (VO₂) undergoes significant optical, electronic, and structural changes as it transforms between the low-temperature monoclinic and high-temperature rutile phases. Recently, alternative stimuli have been utilized to trigger insulator-to-metal transformations in VO₂, including electrochemical gating. Here, we prepare and electrochemically reduce mesoporous films of VO₂ nanocrystals, prepared from colloidally synthesized V₂O₃ nanocrystals that have been oxidatively annealed, in a three-electrode electrochemical cell. We observe a reversible transition between infrared transparent insulating phases and a darkened metallic phase by in situ visible–near-infrared spectroelectrochemistry and correlate these observations with structural and electronic changes monitored by X-ray absorption spectroscopy, X-ray diffraction, Raman spectroscopy, and conductivity measurements. An unexpected reversible transition from conductive, reduced monoclinic VO₂ to an infrared-transparent insulating phase upon progressive electrochemical reduction is observed. This insulator–metal–insulator transition has not been reported in previous studies of electrochemically gated epitaxial VO₂ films and is attributed to improved oxygen vacancy formation kinetics and diffusion due to the mesoporous nanocrystal film structure