ZnO-based nanostructures

A unique group of ZnO nanostructures, owing to three types and a total of 13 fastest growth directions (<0001>, <01[see content source for equation details]0>, and <2[see content source for equation details]0>) together with a pair of polar surfaces (0001), have been synthesized such as nanowires/nanorods (NWs/NRs) [1], nanobelts (NBs) [2], nanoribbons [3], nanoparticles (NPs) [4], 3D nanoarchitectures [5], nanojunction arrays [6], nanosprings, [7], nanorings [8], and more complex nanostructures [9,10]. Moreover, nanostructured ZnO has attracted intensive research efforts due to surface effects, including surface band bending (SBB) [11,12], depletion region [13,14], physisorption/chemisorption/ionosorption and photodesorption near surfaces [15,16], defects [17-20], surface defects/states [12,14,21-23], and surface roughness [24,25]. The surface effect of ZnO nanostructures are more pronounced than that in thin film and bulk counterparts [11,26] due to the structural uniqueness and the ultrahigh surface-to- volume (S/V) ratio of ZnO nanostructures. In this regard, several efforts have been made with ZnO nanostructures for its versatile applications such as NW field-effect transistors (FETs) [24,27-31], NW-based light emitting diodes (LEDs) [32], gas sensors [33-35], chemical sensors [36,37], photodetectors [38-41], optical switches [39], second harmonics generators [42], solar cells [43,44], logic circuits [45], and biosensor [22]. In order to develop the novel application of ZnO nanostructures utilizing the surface effect, it is very important to understand how the physical properties are affected by shrinking the dimension of ZnO. For example, the room temperature resistivity of an NW is 5-6 orders of magnitude lower than the bulk ZnO single crystal [46,48]. This enormous difference arises from the fabrication methods used to synthesize ZnO nanostructures, where approaches such as vapor transport [49], hydrothermal or solution-based methods [9,50,51], chemical vapor deposition [52,53], metal organic chemical vapor deposition [54], or epitaxial methods [55] induce a high concentration of structural defects such as oxygen vacancies and zinc interstitials [20], leading to n-type ZnO nanostructures rather than insulating ZnO films. Native defects are even more pronounced on the surface than in the core of the nanostructures [9], and depending on either ZnO-O or ZnO-Zn terminated facets [56], the surface defects bring out the upward band bending near the surface [16]. Photoluminescence studies at low temperature or room temperature have been performed extensively in order to elucidate the origin of structural defects (as discussed later), but there is still a lack of consensus. In this chapter, initially we discuss the transport mechanism and surface-related transport properties of ZnO nanostructures, with particular interest in reports of the mobility exceeding state-of-the-art planar devices observed in ZnO NW devices [31]. Afterward, attending to the practical applications of ZnO nanostructures, we examine recent studies on gas/chemical sensing, employing the surface effect, due to their deviation from stoichiometry, and relying on a change of conductivity via electron trapping and detrapping processes at the nanostructure surfaces, thus distinguishing between reducing and oxidizing gases as target species. Furthermore, the photoconductive properties under ultraviolet (UV) illuminations are analyzed in terms of photoconductive gain and response times, underlying the detrimental effects of the surface defects and how the ZnO nanostructures have been tailored in order to overcome such disadvantages. Finally, we summarize the emerging photovoltaic (PV) application of ZnO nanostructures. The ultrahigh S/V ratios of nanostructured devices suggest that the 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.