Ultraintense UV emission from ZnO-sheathed ZnS nanorods.

Short-wavelength luminescence is essential for high-performance optoelectronic device applications. There have been efforts to obtain intense ultraviolet (UV) emission by encapsulating ZnO one-dimensional (1D) nanostructures with materials such as ZnS. However, the encapsulation of ZnS 1D nanostructures with ZnO has not been reported. In this paper, we report ultraintense UV emission from ZnS nanorods coated with ZnO, i.e., ZnS-core/ZnO-shell nanorods. UV emission from the ZnS-core/ZnO-shell nanorods was much more intense than that obtained from the extensively studied ZnO-core/ZnS-shell nanorods. The highest intensity of the near-band-edge emission from the ZnS-core/ZnO-shell nanorods was obtained with a ZnO shell layer thickness of 35 nm, which is ∼16 times higher than that of pristine ZnS nanorods. Moreover, the deep level (DL) emission was suppressed completely. The substantial enhancement of the UV emission from the ZnS nanorods and the complete suppression of the DL emission by ZnO sheathing can be rationalized by combining the following four effects: the reinforcement of the UV emission by the overlap of the UV emissions from the ZnS core and ZnO shell, enhancement of the emission from the ZnO shell by the carrier transfer from the ZnS core to the ZnO shell, suppression of the capture of carriers by the surface states on the ZnS surface, and suppression of the visible emission and nonradiative recombination in ZnS.

Selective Oxidizing Gas Sensing and Dominant Sensing Mechanism of n-CaO-Decorated n-ZnO Nanorod Sensors.

In this work, we investigated the NO2 and CO sensing properties of n-CaO-decorated n-ZnO nanorods and the dominant sensing mechanism in n-n heterostructured one-dimensional (1D) nanostructured multinetworked chemiresistive gas sensors utilizing the nanorods. The CaO-decorated n-ZnO nanorods showed stronger response to NO2 than most other ZnO-based nanostructures, including the pristine ZnO nanorods. Many researchers have attributed the enhanced sensing performance of heterostructured sensors to the modulation of the conduction channel width or surface depletion layer width. However, the modulation of the conduction channel width is not the true cause of the enhanced sensing performance of n-n heterostructured 1D gas sensors, because the radial modulation of the conduction channel width is not intensified in these sensors. In this work, we demonstrate that the enhanced performance of the n-CaO-decorated n-ZnO nanorod sensor is mainly due to a combination of the enhanced modulation of the potential barrier height at the n-n heterojunctions, the larger surface-area-to-volume ratio and the increased surface defect density of the decorated ZnO nanorods, not the enhanced modulation of the conduction channel width.

Synergistic Effects of a Combination of Cr2O3-Functionalization and UV-Irradiation Techniques on the Ethanol Gas Sensing Performance of ZnO Nanorod Gas Sensors.

There have been very few studies on the effects of combining two or more techniques on the sensing performance of nanostructured sensors. Cr2O3-functionalized ZnO nanorods were synthesized using carbothermal synthesis involving the thermal evaporation of a mixture of ZnO and graphite powders followed by a solvothermal process for Cr2O3-functionalization. The ethanol gas-sensing properties of multinetworked pristine and Cr2O3-functionalized ZnO nanorod sensors under UV illumination were examined to determine the effects of combining Cr2O3-ZnO heterostructure formation and UV irradiation on the gas-sensing properties of ZnO nanorods. The responses of the pristine and Cr2O3-functionalized ZnO nanorod sensors to 200 ppm of ethanol at room temperature by UV illumination at 2.2 mW/cm(2) were increased by 3.8 and 7.7 times, respectively. The Cr2O3-functionalized ZnO nanorod sensor also showed faster response/recovery and better selectivity than those of the pristine ZnO nanorod sensor at the same ethanol concentration. This result suggests that a combination heterostructure formation and UV irradiation had a synergistic effect on the gas-sensing properties of the sensor. The synergistic effect might be attributed to the catalytic activity of Cr2O3 for ethanol oxidation as well as to the increased change in conduction channel width accompanying adsorption and desorption of ethanol under UV illumination due to the presence of Cr2O3 nanoparticles in the Cr2O3-functionalized ZnO nanorod sensor.

CO gas sensing properties of In4Sn3O12 and TeO2 composite nanoparticle sensors.

A simple hydrothermal route was used to synthesize In4Sn3O12 nanoparticles and In4Sn3O12-TeO2 composite nanoparticles, with In(C2H3O2)3, SnCl4, and TeCl4 as the starting materials. The structure and morphology of the synthesized nanoparticles were examined by X-ray diffraction and scanning electron microscopy (SEM), respectively. The gas-sensing properties of the pure and composite nanoparticles toward CO gas were examined at different concentrations (5-100ppm) of CO gas at different temperatures (100-300°C). SEM observation revealed that the composite nanoparticles had a uniform shape and size. The sensor based on the In4Sn3O12-TeO2 composite nanoparticles showed stronger response to CO than its pure In4Sn3O12 counterpart. The response of the In4Sn3O12-TeO2 composite-nanoparticle sensor to 100ppm of CO at 200°C was 10.21, whereas the maximum response of the In4Sn3O12 nanoparticle sensor was 2.78 under the same conditions. Furthermore, the response time of the composite sensor was 19.73s under these conditions, which is less than one-third of that of the In4Sn3O12 sensor. The improved sensing performance of the In4Sn3O12-TeO2 nanocomposite sensor is attributed to the enhanced modulation of the potential barrier height at the In4Sn3O12-TeO2 interface, the stronger oxygen adsorption of p-type TeO2, and the formation of preferential adsorption sites.

Low Temperature Sensing Properties of Pt Nanoparticle-Functionalized Networked ZnO Nanowires.

Networked ZnO nanowires were fabricated via a vapor-phase selective growth method. Pt nanoparticles were functionalized on the networked ZnO nanowires. In this study, for the functioanlization, γ-ray radiolysis was applied. By the method, Pt nanoparticles of - 10 nm in diameter were uniformly anchored on the surface of each ZnO nanowire. The sensing properties of the Pt-functionalized, networked ZnO nanowires were investigated in terms of NO2, CO and benzene at 100 degrees C. The sensing capability of the Pt-functionalized ZnO nanowires at that temperature supports their potential use in chemical gas sensors.

Synthesis, Structure, and Ethanol Gas Sensing Properties of In2O3 Nanorods Decorated with Bi2O3 Nanoparticles.

Bi2O3-decorated In2O3 nanorods were synthesized using a one-step process, and their structure, as well as the effects of decoration of In2O3 nanorods with Bi2O3 on the ethanol gas-sensing properties were examined. The multiple networked Bi2O3-decorated In2O3 nanorod sensor showed responses of 171-1774% at ethanol concentrations of 10-200 ppm at 200 °C. The responses of the Bi2O3-decorated In2O3 nanorod sensor were stronger than those of the pristine-In2O3 nanorod sensors by 1.5-4.9 times at the corresponding concentrations. The two sensors exhibited short response times and long recovery times. The optimal Bi concentration in the Bi2O3-decorated In2O3 nanorod sensor and the optimal operation temperature of the sensor were 20% and 200 °C, respectively. The Bi2O3-decorated In2O3 nanorod sensor showed selectivity for ethanol gas over other gases. The origin of the enhanced response, sensing speed, and selectivity for ethanol gas of the Bi2O3-decorated In2O3 nanorod sensor to ethanol gas is discussed.

Fabrication and NO2 gas sensing performance of TeO2-core/CuO-shell heterostructure nanorod sensors.

TeO2-nanostructured sensors are seldom reported compared to other metal oxide semiconductor materials such as ZnO, In2O3, TiO2, Ga2O3, etc. TeO2/CuO core-shell nanorods were fabricated by thermal evaporation of Te powder followed by sputter deposition of CuO. Scanning electron microscopy and X-ray diffraction showed that each nanorod consisted of a single crystal TeO2 core and a polycrystalline CuO shell with a thickness of approximately 7 nm. The TeO2/CuO core-shell one-dimensional (1D) nanostructures exhibited a bamboo leaf-like morphology. The core-shell nanorods were 100 to 300 nm in diameter and up to 30 µm in length. The multiple networked TeO2/CuO core-shell nanorod sensor showed responses of 142% to 425% to 0.5- to 10-ppm NO2 at 150°C. These responses were stronger than or comparable to those of many other metal oxide nanostructures, suggesting that TeO2 is also a promising sensor material. The responses of the core-shell nanorods were 1.2 to 2.1 times higher than those of pristine TeO2 nanorods over the same NO2 concentration range. The underlying mechanism for the enhanced NO2 sensing properties of the core-shell nanorod sensor can be explained by the potential barrier-controlled carrier transport mechanism. PACS: 61.46. + w; 07.07.Df; 73.22.-f.

Dual functional sensing mechanism in SnO2-ZnO core-shell nanowires.

We report a dual functional sensing mechanism for ultrasensitive chemoresistive sensors based on SnO2-ZnO core-shell nanowires (C-S NWs) for detection of trace amounts of reducing gases. C-S NWs were synthesized by a two-step process, in which core SnO2 nanowires were first prepared by vapor-liquid-solid growth and ZnO shell layers were subsequently deposited by atomic layer deposition. The radial modulation of the electron-depleted shell layer was accomplished by controlling its thickness. The sensing capabilities of C-S NWs were investigated in terms of CO, which is a typical reducing gas. At an optimized shell thickness, C-S NWs showed the best CO sensing ability, which was quite superior to that of pure SnO2 nanowires without a shell. The dual functional sensing mechanism is proposed as the sensing mechanism in these nanowires and is based on the combination of the radial modulation effect of the electron-depleted shell and the electric field smearing effect.

Acceptor-compensated charge transport and surface chemical reactions in Au-implanted SnO2 nanowires.

A new deep acceptor state is identified by density functional theory calculations, and physically activated by an Au ion implantation technique to overcome the high energy barriers. And an acceptor-compensated charge transport mechanism that controls the chemical sensing performance of Au-implanted SnO2 nanowires is established. Subsequently, an equation of electrical resistance is set up as a function of the thermal vibrations, structural defects (Au implantation), surface chemistry (1 ppm NO2), and solute concentration. We show that the electrical resistivity is affected predominantly not by the thermal vibrations, structural defects, or solid solution, but the surface chemistry, which is the source of the improved chemical sensing. The response and recovery time of chemical sensing is respectively interpreted from the transport behaviors of major and minor semiconductor carriers. This acceptor-compensated charge transport mechanism provides novel insights not only for sensor development but also for research in charge and chemical dynamics of nano-semiconductors.

Mechanism and prominent enhancement of sensing ability to reducing gases in p/n core-shell nanofiber.

We have devised a sensor system comprising p-CuO/n-ZnO core-shell nanofibers (CS nanofibers) for the detection of reducing gases with a very low concentration. The CS nanofibers were prepared by a two-step process as follows: (1) synthesis of core CuO nanofibers by electrospinning, and (2) subsequent deposition of uniform ZnO shell layers by atomic layer deposition. We have estimated the sensing capabilities of CS nanofibers with respect to CO gas, revealing that the thickness of the shell layer needs to be optimized to obtain the best sensing properties. It is found that the p-CuO/n-ZnO CS structures are suitable for detecting reducing gases at extremely low concentrations. The associated sensing mechanism is proposed on the basis of the radial modulation of an electron-depleted region in the shell layer.

Pt nanoparticle-decorated ZnO nanowire sensors for detecting benzene at room temperature.

Room temperature gas sensing ability for low concentrations of benzene was successfully realized with Pt nanoparticle-decorated networked ZnO nanowire sensors. For decoration of Pt nanoparticles, gamma-ray radiolysis was used. The Pt decoration greatly enhanced benzene sensing performances. Importantly, even at room temperature, ppm level benzene was clearly detected, which is likely to be due to the combined effect of electronic and chemical sensitizations by Pt nanoparticles.

V-groove SnO2 nanowire sensors: fabrication and Pt-nanoparticle decoration.

Networked SnO(2) nanowire sensors were achieved using the selective growth of SnO(2) nanowires and their tangling ability, particularly on on-chip V-groove structures, in an effort to overcome the disadvantages imposed on the conventional trench-structured SnO(2) nanowire sensors. The sensing performance of the V-groove-structured SnO(2) nanowire sensors was highly dependent on the geometrical dimension of the groove, being superior to those of their conventional trench-structured counterparts. Pt nanoparticles were decorated on the surface of the networked SnO(2) nanowires via γ-ray radiolysis to enhance the sensing performances of the V-groove sensors whose V-groove widths had been optimized. The V-groove-structured Pt-nanoparticle-decorated SnO(2) nanowire sensors exhibited outstanding and reliable sensing capabilities towards toluene and nitrogen dioxide gases, indicating their potential for use as a platform for chemical gas sensors.