WonBae Ko

p‐Type Conduction Characteristics of Lithium‐Doped ZnO Nanowires

Nanostructured electronic devices are expected to facilitate the continuing miniaturization of electronic devices and enable ultralow power device operation. In particular, 1D nanostructures such as nanowires (NW) and nanotubes have attracted great interest over the past decade because of their specifi c physical properties and their potential as building blocks for next-generation nanoelectronic devices. [ 1 , 2 ] Among various 1D materials, zinc oxide (ZnO), which has a direct and wide bandgap, is a promising candidate for light-emitting diodes, UV and gas sensors, transistor channels, and other devices that can utilize the unique vertical alignment characteristics and highly ordered single crystalline properties of NW structures. [ 3–6 ] However, because undoped ZnO NWs are intrinsically n-type, their use in practical devices has been hindered and much effort has been dedicated toward the development of p-type ZnO NWs. In particular, control and manipulation of the doping process is increasingly becoming a key approach for the realization of p-type ZnO NWs. To realize p-type ZnO NWs, the initial dopant candidates tested included group V elements to substitute for O and group III elements to substitute for Zn, despite the large size mismatches in both cases. Recently, group I species such as Li and Na have been used to synthesize p-type ZnO NWs based on the expectation that these elements would function as shallow acceptors in ZnO host materials. [ 7–9 ] Li has the smallest ionic radius (0.76 Å) of group I species, which is very close to that of Zn (0.74 Å). Furthermore, several reports of excited centers observed using electron paramagnetic resonance spectroscopy have indicated that Li atoms can act as shallow acceptors in substantial forms of Zn sites (Li Zn ). [ 10 , 11 ] In addition, it is well-known that Li has specifi c advantages over other dopant

Solution processed vertically stacked ZnO sheet-like nanorod p–n homojunctions and their application as UV photodetectors

One long-standing goal in the development of one-dimensional nanostructured electronic devices is to facilitate the ongoing trend of miniaturization so as to enable ultralow power operation. Zinc oxide (ZnO) nanostructures, which have a direct and wide bandgap, are a central component in numerous electronic and optoelectronic applications. Here, we address vertically stacked ZnO sheet-like nanorod (SLNR) p–n homojunctions composed of single-crystalline undoped (n-type) and Li-doped (p-type) ZnO SLNRs by a multi-step solution-based hydrothermal route. Precise control of the molar concentration represented one of the basic factors in ensuring that the p–n homojunctions possessed appropriate densities and suitable morphologies. An extensive analysis of the luminescence features was carried out in order to identify p-type conduction in the Li-doped ZnO SLNRs. In addition, the SLNR-based p–n homojunctions exhibited distinct electrical features that validated their potential use as ultraviolet photodetectors, thereby spurring progress in the development of practical optoelectronics.

Solubility-Dependent NiMoO4 Nanoarchitectures: Direct Correlation between Rationally Designed Structure and Electrochemical Pseudokinetics

Tailoring the binary metal oxide along with developing new synthetic methods for controlling resultant nanostructures in a predictive way is an essential requirement for achieving the further improved electrochemical performance of pseudocapacitors. Here, through a rational design of the supersaturation-mediated driving force for hydrothermal nucleation and crystal growth, we successfully obtain one-dimensional (1-D) nickel molybdenum oxide (NiMoO4) nanostructures with controlled aspect ratios. The morphology of the 1-D NiMoO4 nanostructures can be tuned from a low to a high aspect ratio (over a range of diameter sizes from 80 to 800 nm). Such a controllable structure provides a platform for understanding the electrochemical relationships in terms of fast relaxation times and improved ion-diffusion coefficients. We show that the 1-D NiMoO4 electrode with a high aspect ratio (HAR) exhibits a much higher specific capacitance of 1335 F g-1 at a current density of 1 A g-1 compared to the other electrodes with a relatively low aspect ratio, which is due to the unique physical and chemical structure being suitable for electrochemical kinetics. We further demonstrate that an asymmetric supercapacitor consisting of the tailored HAR-NiMoO4 electrode can achieve an energy density of 40.7 Wh kg-1 and a power density of 16 kW kg-1.