The incorporation of foreign elements into ZnO nanostructures is of significant interest for tuning the structure and optical and electrical properties in nanoscale optoelectronic devices. In this study, Ga-doped 1-D ZnO nanorods were synthesized using a hydrothermal route, in which the doping content of Ga was varied from 0% to 10%. The pn heterojunction diodes based on the n-type Ga-doped ZnO nanorod/p-type Si substrates were constructed, and the effect of the Ga doping on the morphology, chemical bonding structure, and optical properties of the ZnO nanorods was systematically investigated as well as the diode performance. With increasing Ga content, the average diameter of the ZnO nanorods was increased, whereas the amount of oxygen vacancies was reduced. In addition, the Ga-doped ZnO nanorod/p-Si diodes showed a well-defined rectifying behavior in the I-V characteristics and an improvement in the electrical conductivity (diode performance) by the Ga doping, which was attributed to the increased charge carrier (electron) concentration and the reduced defect states in the nanorods by incorporating Ga. The results suggest that Ga doping is an effective way to tailor the morphology, optical, electronic, and electrical properties of ZnO nanorods for various applications such as field-effect transistors (FETs), light-emitting diodes (LEDs), and laser diodes (LDs).

ZnO nanorods doped with In or Ga were synthesized by a facile hydrothermal process on sol-gel deposited ZnO seed layers. The effects of dopants on the morphology, chemical bonding structure, and optical property of the ZnO nanorods were discussed based on the field emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and photoluminescence (PL) results. XRD and XPS results indicated that In or Ga ions were doped in the ZnO lattice. Undoped and In-or Ga-doped ZnO nanorods had a hexagonal pillar shape and a single crystal wurtzite structure with (001) growth direction. For the doped samples, the average diameter of the nanorods increased, while the number of the nanorods decreased, compared to the undoped sample. In the PL spectra, the near band edge (NBE) emission peaks of both doped samples shifted to a lower wavelength. On the other hand, the intensity of visible emission increased for the In-doped sample, but decreased for the Gadoped sample. We considered that this variation was related to the different concentrations of the oxygen vacancies between the samples, which had bond strengths of Ga-O and In-O different from that of Zn-O.

The incorporation of doping elements in ZnO nanostructures plays an important role in adjusting the optical and electrical properties in optoelectronic devices. In the present study, we fabricated 1-D ZnO nanorods (NRs) doped with different In contents (0% ~ 5%) on p-GaN films using a facile hydrothermal method, and investigated the effect of the In doping on the morphology and electronic structure of the NRs and the electrical and optical performances of the n-ZnO NRs/p-GaN heterojunction light emitting diodes (LEDs). As the In content increased, the size (diameter and length) of the NRs increased, and the electrical performance of the LEDs improved. From the electroluminescence (EL) spectra, it was found that the broad green-yellow-orange emission band significantly increased with increasing In content due to the increased defect states (oxygen vacancies) in the ZnO NRs, and consequently, the superposition of the emission bands centered at 415 nm and 570 nm led to the generation of white-light. These results suggest that In doping is an effective way to tailor the morphology and the optical, electronic, and electrical properties of ZnO NRs, as well as the EL emission property of heterojunction LEDs.

We synthesized ZnO nanorods (NRs) using simple hydrothermal method, with the simultaneous incorporation of gallium (Ga) and indium (In), in addition, investigated the co-doping effect on the morphology, microstructure, electronic structure, and electrical/optical properties. The growth behavior of the doped NRs was affected by the nuclei density and polarity of the (001) plane. The c-axis parameter of the co-doped NRs was similar to that of undoped NRs due to the compensated lattice distortion caused by the presence of dopants that are both larger (In 3+ ) and smaller (Ga 3+ ) than the host Zn 2+ cations. Red shifts in the ultraviolet emission peaks were observed in all doped NRs, owing to the combined effects of NR size, band gap renormalization, and the presence of stacking faults created by the dopant-induced lattice distortions. In addition, the NR/p-GaN diodes using co-doped NRs exhibited superior electrical conductivity compared to the other specimens due to the increase in the charge carrier density of NRs and the relatively large effective contact area of (001) planes. The simultaneous doping of In and Ga is therefore anticipated to provide a broader range of optical, physical, and electrical properties of ZnO NRs for a variety of opto-electronic applications.The unique physical and optical properties of zinc oxide (ZnO), such as the wide band gap of 3.37 eV and the large exciton binding energy of 60 meV, make this material attractive for a broad range of optoelectronic applications 1-8 . Considerable research on ZnO has thus been carried out owing to its potential use in ultraviolet light emitting diodes (LEDs), lasers, or sensors, usually in the form of p/n junction devices.Although it has conventionally been reported that homojunction devices are more efficient than heterojunction devices in terms of band offset and/or energy barrier engineering, homojunction devices based on ZnO exhibit relatively poor electrical characteristics compared to their heterojunction counterparts due to the p-type doping difficulty in ZnO. Therefore, in order to form p/n junctions, most of the reported ZnO-based diodes and LEDs to date involve the use of p-type materials such as GaN, polymers, Si, graphene, CuAlO 2 , and CuSCN 9-13 . In such devices, the charge transport efficiency is relatively low because of the large band offsets and mechanical strains induced at the junction interface. Such a disadvantage can be overcome by applying nano-structured junctions that increase the effective contact areas. In this regard, one-dimensional nanostructures are intensively studied in order to enhance the carrier injection efficiency and their recombination rates [14][15][16][17] . Another approach for the enhancement of the optoelectronic properties of ZnO is to dope ZnO with group III, IV, and V elements such as Al, Ga, In, Sn, . Incorporation of doping elements in ZnO nanorods (NRs) or nanowires have been synthesized by a number of research groups. In-doping was shown to be

We synthesized TiO2 mesocrystals using a hydrothermal method and investigated the effect of calcination temperature (100–800 °C) on their morphology, crystallinity, and photocatalytic activity. While no appreciable changes in the shape, dimension, and crystal structure of the TiO2 nanoparticles (NPs) were observed as the calcination temperature increased to 300 °C, the crystallinity improved with increasing temperature. The mesocrystal form of the NPs began to disappear at 400 °C, and the specific surface area significantly decreased with increasing temperature owing to the reduced boundaries between the subunits and surface roughness of the NPs. The photocatalytic activity of the TiO2 NPs improved when the temperature increased to 300 °C because of the enhanced crystallinity and elimination of byproducts; on the other hand, it degraded above 400 °C due to the decreased surface area. These results suggest that controlling the calcination temperature is an effective way to tailor the morphology, crystallinity, and photocatalytic activity of TiO2 NPs.

Reactive surface-exposed anatase TiO2 (a-TiO2) is highly desirable for applications requiring superior photocatalytic activity. In order to obtain a favorable surface, morphology control of the a-TiO2 using capping agents has been widely investigated. Herein, we systematically study the effects of different F sources (HF, TiF4, and NH4F) as the capping agent on the morphology control and photocatalytic activities of a-TiO2 in a hydrothermal process. When either HF or TiF4 was added, large truncated bipyramids formed with the photocatalytically active {001} facet, whereas the NH4F was not effective for facet control, yielding nanospheres similar to the pure a-TiO2. The morphology changes were related to the decomposition behaviors of the F sources in the solvent material: HF and TiF4 decomposed and supplied F(-) ions before a-TiO2 nucleation, which changed the nucleation rate and growth direction, leading to the resultant a-TiO2 morphology. On the other hand, NH4F supplied F(-) ions after a-TiO2 nucleation and could not change the growth behavior. In terms of the photocatalytic effect, the HF- and TiF4-treated a-TiO2 effectively decomposed ∼90% and ∼80% of methylene blue, respectively, in 1 h, while ∼60% was decomposed for the NH4F-treated a-TiO2. Note that pure a-TiO2 photocatalytically decomposed only ∼10% of methylene blue over the same time. These results pave the way to precise control of the facet of TiO2 through using different capping agents.

The objective of this study is to investigate effects of precursor concentration on dimensional size, defect state, and gas sensing performance of MoS 2 sheets synthesized by hydrothermal method with various precursor concentrations (0.14-1.4 M) in fixed HCl solution (7 M). The MoS 2 sheets show a defect-rich and strained structure. When precursor concentration is increased, dimensional size of the sheet is increased while the specific surface area of the sheet is decreased. The MoS 2 sensor made with 0.14 M precursor solution exhibits superior sensing performance with high sensitivity (372%) and rapid recovery time (<16 min) at room temperature in a 500 ppm NO 2 gas atmosphere due to enhanced active site resulting from the highest specific surface area. In addition, all MoS 2 samples show a complete recovery after response to NO 2 gas probably related to the defective and strained surface resulting from the use of HCl surfactant.

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