On the correlation of crystal defects and band gap properties of ZnO nanobelts (original) (raw)
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ACS Applied Materials & Interfaces, 2015
Ultrafine ZnO nanocrystals with a thickness down to 0.25 nm are grown by a metalorganic chemical vapor deposition method. Electronic band structures and native point defects of ZnO nanocrystals are studied by a combination of scanning tunneling microscopy/spectroscopy and first-principles density functional theory calculations. Below a critical thickness of about 1 nm ZnO adopts a graphitic-like structure and exhibits a wide band gap similar to its wurtzite counterpart. The hexagonal wurtzite structure, with a well-developed band gap evident from scanning tunneling spectroscopy, is established for a thickness starting from about 1.4 nm. With further increase of the thickness to 2 nm, V O −V Zn defect pairs are easily produced in ZnO nanocrystals due to the self-compensation effect in highly doped semiconductors.
Scanning probe microscopy of ZnO nanobelts
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We present an AFM and STM-STS investigation of the surface of ZnO nanobelts grown by chemical vapour deposition. AFM images showed a type 1 (high aspect ratio) nanobelt lying across a type 2 (low aspect ratio) nanobelt, bending at an angle of 20.9 • without breaking. Terraces 10 atomic layer thick were also observed, with step edges running along the [0010] direction. STM images confirmed the AFM results while STS curves and current maps showed higher conductivity for the ZnO nanobelts than for the oxidised silicon surface, as well as an n-type behaviour.
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Nanotechnology, 2010
We report the growth of ultrathin single-crystal ZnO nanobelts by using a Ag-catalyzed vapor transport method. Extensive transmission electron microscopy and atomic force microscopy measurements reveal that the thickness of the ultrathin ZnO nanobelts is ∼2 nm. Scanning electron microscopy and post-growth annealing studies suggest a '1D branching and 2D filling' growth process. Our results demonstrate the critical role of catalyst in the deterministic synthesis of nanomaterials with the desired morphology. In addition, these ultrafine nanobelts exhibit stable field emission with unprecedented high emission current density of 40.17 mA cm −2. These bottom-up building blocks of ultrathin ZnO nanobelts may facilitate the construction of advanced electronic and photonic nanodevices.
A Comparative Study of ZnO Thin Films, Nanowires and Nanotubes for Photovoltaics Applications
Oxide semiconductors with a large band gap are being explored for their potential application in photovoltaics. Sol–gel, being low-cost, simple and application oriented method has been used to prepare ZnO thin films, nanowires and nanotubes. Zinc acetate hydrated, triethylamine and isopropyl alcohol are used as starting material, stabilizer and solvent respectively. Thin films of ZnO are prepared by spinning the sol onto glass and Si substrates. Thickness of 0.2µm was achieved at 3000 rpm for 30 seconds. ZnO nanowires (NWs) and nanotubes (NTs) are fabricated by spinning into self-synthesized anodic aluminum oxide (AAO) templates. AAO templates, prepared by two step anodization process, with diameter ~ 50nm are used for NWs and ~ 200nm diameter is used to fabricate NTs. SEM results show vertically aligned ZnO NWs with diameter of 50nm and length ranging between 1-10µm. ZnO NTs are also vertically aligned with diameter of 200nm and the length varied between 3 and 10µm. These sol– gel prepared NWs and NTs exhibited hexagonal wurtzite structure as observed by X-ray diffraction after annealing at 300 o C for 60 minutes. 90% optical transmittance in the case of ZnO thin films whereas, ~ 95% transmission was observed in case of ZnO NWs and NTs. Spectroscopic ellipsometery results confirm direct band gap in the range of 3.25-3.45 eV.
Nanoscale, 2011
In this article, the important role of the intrinsic defects in size-controlled ZnO nanowires (NWs) which play a critical role in the properties of the NWs, was studied with a combined innovative experimental analysis. The NWs prepared by both the aqueous solution method and chemical vapour deposition process were of increasing length and decreasing size-to-volume (S/V) ratio. The combined approach involved different analytical and spectroscopic techniques and from the correlation between the different measurements, the concentration of the oxygen vacancies jointly with the zinc interstitials defects and the zinc vacancy defects was observed to be positively or negatively correlated, respectively, with the magnitude of the photoluminescence intensity and radiative lifetimes. Furthermore, the experimental results also suggest that the oxygen vacancy defects are not only spatially located on the surface of the NW but an increasing fraction of the total oxygen vacancy defects connected with the green emission is also located in an annulus region beneath the surface as the ZnO NWs elongate. On the other hand, as the donor concentration plays a critical function in the properties of the ZnO NWs, an analytical model was derived for the calculation of the donor concentration of the NWs directly from its reverse-biased current-voltage characteristics obtained from the conductive atomic force microscopy measurements.
Epitaxial ZnO Nanowire-on-Nanoplate Structures as Efficient and Transferable Field Emitters
Advanced Materials, 2013
As materials with a tremendously wide variety of potential applications, ZnO nanostructures have attracted a lot of attention in recent years. Among them, vertical one-dimensional (1D) ZnO nanostructures such as nanowires, nanorods, and nanotips have been considered as excellent candidates for electron fi eld emitters as they have low work functions, high aspect ratios, high mechanical stability, and high conductivity. The fi eld emission (FE) performance of such materials is highly affected by their intrinsic physical and structural parameters, such as alignment, density, uniformity, and tapering. After being stimulated by an applied electric fi eld and before reaching the counter electrode, electrons have to pass through the interface between the 1D ZnO structure and substrate. However, irrespective of the synthetic route used (whether chemical or physical), all known vertical ZnO emitters prepared on heterogeneous substrates demonstrate a so-called 'dead' layer, which is associated with a low crystallinity and poorly ordered region at the emitter-substrate interface. To date, this is one of the major and universal obstacles hindering the wide use of 1D ZnO in electronic and optoelectronic devices, such as solar cells, photodetectors, light-emitting diodes, and fi eld emitters. Obviously, growing 1D ZnO epitaxially on ZnO subtrates would provide a sharp and highquality interface, which is favorable for FE performance. However, no single-crystal ZnO wafers with low cost are available for the moment, and the related technology also needs to be improved. Therefore, achieving the high-quality interface with the substrate is still a major challenge for the electronic and optoelectronic applications based on 1D ZnO nanomaterials.
STM and STS characterization of ZnO nanorods
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The effect of indium doping on the point defect formation in ZnO nanostructures is studied by scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) techniques. While the incorporation of a donor dopant like indium should increase the n-type conductivity of ZnO nanostructures, it has been found that formation of V Zn native acceptors in heavily doped ZnO nanostructures produces self-compensation effect, creating acceptor states in their band gap. Presence of both donor and acceptor states in heavily indium doped ZnO nanostructures are probed and identified. The mechanism of formation of such donor and acceptor states is discussed.
Stress Dependent Band Gap Shift and Valence Band Studies in ZnO Nanorods
Journal of Nanoscience and Nanotechnology, 2010
ZnO nanorods are grown on seedless and ZnO seeded glass substrates using chemical solution method and their structural, morphological, optical and valence band studies have been carried out. On seedless substrate horizontal nanorods are observed whereas for the seeded substrates vertically aligned hollow and solid nanorods grows. X-ray diffraction analysis revealed the presence of tensile stress in the vertical nanorods. Blue shift has been observed in the band gap of the vertical nanorods as compared to the horizontal nanorods which is attributed to the presence of tensile stress in the vertically aligned nanorods. Photoluminescence spectra revealed the dominance of Zinc vacancies (V Zn) related defects in the nanorods and oxygen defects are found to be higher in the vertically aligned nanorods as compared to the horizontal nanorods. The difference between the Fermi level and valence band maxima for horizontal, hollow vertical and solid vertical nanorods are found to be ∼0.56 eV, ∼0.70 eV and ∼0.92 eV respectively indicating the possibility of p-type of conduction in the nanorods which has been attributed to presence of V Zn defects in the ZnO nanorods.
The effects of annealing temperature on structural and optical properties of S-doped ZnO nanobelts
Solid State Sciences, 2010
In this study, the effects of thermal annealing temperature on the structural and optical properties of S-doped ZnO nanobelts were investigated. The XRD pattern shows that the crystallinity of S-doped ZnO nanobelts improves with increased annealing temperature. Room temperature photoluminescence spectroscopy of the as-grown S-doped ZnO nanobelts shows no detectable ultraviolet peak with the broad peaks in the visible emission region at 480, 505, and 518 nm. A weak peak in the ultraviolet region at 383 nm appears after annealing at 400 and 600 C. Raman spectroscopy of the sample also shows a significant change with an increase in the annealing temperature.