Surface effects on optical and electrical properties of ZnO nanostructures

Abstract

This article presents a comprehensive review of the current research addressing the surface effects on physical properties and potential applications of nanostructured ZnO. Studies illustrating the transport, photoluminescence (PL), and photoconductivity properties of ZnO with ultrahigh surface-to-volume (S/V) ratio are reviewed first. Secondly, we examine recent studies of the applications of nanostructured ZnO employing the surface effect on gas/chemical sensing, relying on a change of conductivity via electron trapping and detrapping process at the surfaces of nanostructures. Finally, we comprehensively review the photovoltaic (PV) application of ZnO nanostructures. The ultrahigh S/V ratios of nanostructured devices suggest that studies on the synthesis and PV properties of various nanostructured ZnO for dye-sensitized solar cells (DSSCs) offer great potential for high efficiency and low-cost solar cell solutions. After surveying the current literature on the surface effects on nano-structured ZnO, we conclude this review with personal perspectives on a few surface-related issues that remain to be addressed before nanostructured ZnO devices can reach their ultimate potential as a new class of industrial applications.

Surface effects of electrode-dependent switching behavior of resistive random-access memory

“…The concentration of chemisorbed O À 2ðadÞ at the surface of metal oxides at room temperature is greater in air compared to vacuum. 17,18 Our results in Fig. 2 show that the devices tested under air had a higher switching probability compared to the same ReRAM devices tested under vacuum, which suggests that a higher O À 2ðadÞ concentration leads to a higher switching probability.…”

“…This phenomenon can be understood by the lower conductivity near the ZnO surface that is induced by oxygen chemisorption. 18 The surface band bending effect is more pronounced as the O À 2ðadÞ concentration increases, resulting in the electrode dependence of the device's HRS. On the other hand, the IS is not electrode dependent, suggesting that the bulk component overwhelms surface effects, since there are no conductive paths within the bulk of the device prior to the electroforming process.…”

“…17,18 The ambient sensitivity of ZnO can be further increased using Pt or Au nanoparticles; a finding which is attributed to the fact that Pt and Au serve as catalysts to lower the activation energy for both the desorption/adsorption processes. [20][21][22] In addition, gas sensors made using Pt/ ZnO stacks, with capping layers on the edges of the Pt top electrodes, show that the ambient environment can directly affect the chemisorption process at the ZnO surface through the Pt layer due to the high mobility of oxygen within Pt.…”

Effect of ultraviolet illumination on metal oxide resistive memory

“…O À 2 ðadÞ ) at surface-related defects, the so-called surface effect. 4 Indeed, recent studies investigating the influence of ambiences conditions, 5 harsh environments, 6 and roughness 7 on metal oxidebased RRAM have highlighted SDR as the main cause of low switching yield and resistance fluctuation. On the other hand, the treatments such as surface modification, 6 doping, 8 and electrode material engineering 9 have improved RS stability and RRAM performance due to the better controllability of chemisorbed oxygen at surface-related defects.…”

“…The photocurrent is ascribed to the suppression of SDR via oxygen photodesorption of chemisorbed oxygen. 4 Under UV illumination with the energy higher than the ZnO bandgap, the photo-generated electron-hole pairs are spatially separated in ZnO due to the discharge of chemisorbed oxygen by photo-excited holes (O À 2 ðadÞ þ h þ ! O 2ðgÞ ).…”