First-principles studies on the Au surfactant on polar ZnO surfaces

The surfactant effect of Au in ZnO nanostructures growth is studied using first-principles slab calculations based on density functional theory. The atomic structure and electronic properties of one monolayer of Au atoms on polar ZnO surfaces are examined. It is found that (1) one monolayer (ML) of Au capping layer on the ZnO polar surfaces may modify the growing properties of ZnO nanostructures by enhancing the binding energy by 0.41 eV/atom for Zn adsorption on the polar surfaces; (2) the Au adlayer on the polar ZnO surfaces seems more active for the adsorption of Zn atoms, which may be at the very heart of the effect that Au acts as catalyst for the growth of the ZnO nanostructures; and (3) total energy calculations show that the gold on-top geometry is energetically favorable than the gold diffused geometry, which may be useful to understand the phenomenon that Au particles are only found at the end of ZnO nanostructures during the growth process.     

Homoepitaxial Growth and Optical Properties of ZnO Polar Nanoleaves

Using a mixture of ZnO and Te powders as the source material, ZnO nanoleaves with high yield and uniform morphology are fabricated by thermal evaporation. Each nanoleaf is constructed with a nanowire and a nanodisc on one side of the nanowire near the top. The polygonal nanodisc is in symmetric distribution in relation to the nanowires and has polar planes ±（0001） as surfaces. A local homoepitaxial growth mechanism of ZnO polar nanodiscs induced by Te is proposed. With thin nanodiscs, the ZnO nanoleaves could be used in nanolasers, sensors, and photoelectronic nanodevices. Room-temperature photoluminescence result implies good crystalline quality of the ZnO nanoleaves.     

Ab Initio Comparative Study of Zincblende and Wurtzite ZnO

By employing the first-principles pseudopotential plane-wave method, the physical properties of zincblende ZnO are investigated in comparison with those of the common wurtzite structure. Zincblende ZnO is predicted to be a direct gap semiconductor. Compared to the wurtzite structure, the zincblende ZnO is characterized by smaller bandgap and pressure coefficient, larger electron effective mass, increasing static dielectric constants and more covalent bonding. Furthermore, the optical properties including dielectric function and energy loss function of zincblende ZnO were obtained and analysed with some features. These aspects reveal promising applications of zincblende ZnO in optoelectronic devices.     

Nitrogen incorporation in homoepitaxial ZnO CVD epilayers

ZnO:N thin films have been deposited on oxygen and zinc terminated polar surfaces of ZnO. The nitrogen incorporation in the epilayers, using NH₃ as doping source, was investigated as a function of the growth temperature in the range between 380 °C and 580 °C. We used Raman spectroscopy and low temperature photoluminescence to investigate the doping properties. It turned out that the nitrogen incorporation strongly depends on both, the surface polarity of the epitaxial films and the applied growth temperatures. In our CVD process low growth temperatures and Zn‐terminated substrate surfaces clearly favour the nitrogen incorporation in the ZnO thin films. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

金纳米颗粒等离激元对不同形貌氧化锌薄膜发光性能的调控

采用化学气相沉积法制备了纳米棒状的氧化锌纳米结构薄膜和没有纳米棒的氧化锌薄膜,通过直流溅射在所制备的有纳米棒和没有纳米棒的氧化锌薄膜上淀积约3 nm厚的金纳米颗粒薄膜,研究了金纳米颗粒对不同表面形貌氧化锌薄膜的发光特性的影响。实验发现金纳米颗粒的存在使具有纳米棒的氧化锌薄膜的紫外发射增强,但使来自缺陷的可见光发射受到很大的抑制。通过比较有纳米棒和没有纳米棒的氧化锌薄膜在镀金纳米颗粒前后的发光特性,发现金表面等离激元对氧化锌发光的调控取决于氧化锌的表面形貌,纳米棒的存在更有利于金纳米颗粒等离激元调控氧化锌的发光特性。     

Structural and electronic properties of ZnO under high pressures

In this work, we use first-principles calculations based on density-functional theory within the local-density approximation (LDA) to investigate the structural and electronic properties of ZnO under high-pressure. We have calculated the ground-state energy, the lattice constant, the bulk modulus, and its pressure derivative of the B4 (wurtzie), B3 (zinc blende), B2 (CsCl) and B1 (rocksalt) phases of ZnO. Moreover, the electronic structure, density of states (DOS) of the B4 (wurtzite) and B1 (rocksalt) phases of ZnO have been calculated. We show that our calculated values compare acceptably well with values reported in the literature.     

Experimental and theoretical studies on the magnetic property of carbon-doped ZnO

Intrinsic room-temperature ferromagnetism was detected over n-type carbon-doped ZnO prepared through solid-state reaction. Our results of first-principle calculations based on density functional theory revealed that the CZn₄O₁₂ unit is the origin of magnetic moment in the carbon-doped ZnO system. The carbon component has a significant contribution to the net magnetic moment, and any oxygen vacancy present in CZn₄O₁₂ has a negative effect on the magnetic properties of the system. Moreover, both antiferromagnetic and ferromagnetic interactions are predicted among carbon atoms located at different CC distances. The result suggests that the defect density influenced by the distribution of carbon has a significant effect on the magnetic properties of the carbon-doped ZnO system.     

Electrical characteristics of UV photodetectors based on ZnO/diamond film structure

Ultraviolet photodetectors based on ZnO/diamond film structure were fabricated. The properties of Au/ZnO contacts and effects of grain sizes on the electrical characteristics of photodetectors were discussed. Due to the bombardment with Au atoms and the annealing process, fine ohmic contacts were formed between Au electrodes and ZnO films. Dark currents and photocurrents of the photodetectors were related to sputtering time and the grain size of ZnO films. For the photodetector with a bigger grain size, a lower dark current and a higher photocurrent were obtained under 10 V bias voltage. The time-dependent photocurrent confirmed the carrier trapping effect.     

First-Principles Calculation of Electronic Structure and Optical Properties of Sb-Doped ZnO

The geometric structure, electronic structure, optical properties and the formation energy of Sb-doped ZnO with the wurtzite structure are investigated using the first-principles ultra-soft pseudo-potential approach of plane wave based upon the density functional theory. The calculated results indicate that the volume of ZnO doped with Sb becomes larger, and the doping system yields the lowest formation energy of Sb on the interstitial site and the oxygen site. Furthermore, Sb dopant first occupies the octahedral oxygen sites of the wurtzite structure. It is found that Sb substituting on oxygen site behaves as a deep acceptor and shows the p-type degenerate semiconductor character. After doping, the electron density difference demonstrates the considerable electron charge density redistribution, which induces the effect of Sb-doped ZnO to increase the charge overlap between atoms. The density of states move towards lower energy and the optical band gap is broadened. Our culated results are in agreement with other experimental results and could make more precise monitoring and controlling possible during the growth of ZnO p-type materials.     

Ba和Sr原子在ZnO(0001)表面吸附的密度泛函理论研究

我们采用密度泛函理论下的平面波赝势方法研究Ba和Sr原子在ZnO(0001)表面的吸附结构和性质,仔细研究了三个吸附位（T4, H3 and Top）。发现Ba和Sr吸附在表面的H3和T4位时,他们之间的结合能相差很小,且这两种金属更易于吸附在表面的T4位,我们把计算的结果与贵金属在ZnO(0001)表面的吸附行为及前人的实验结果进行了比较,理论上发现Ag和Au易于吸附在ZnO(0001)表面的H3位,而实验上观察到即使在很小吸附比的条件下ZnO(0001)表面上也能形成Cu的团簇结构,这主要是由于Cu和ZnO(0001)表面衬底强的相互作用所致。     

Ba和Sr原子在ZnO(0001)表面吸附的密度泛函理论研究

我们采用密度泛函理论下的平面波赝势方法研究Ba和Sr原子在ZnO(0001)表面的吸附结构和性质,仔细研究了三个吸附位（T4, H3 and Top）。发现Ba和Sr吸附在表面的H3和T4位时,他们之间的结合能相差很小,且这两种金属更易于吸附在表面的T4位,我们把计算的结果与贵金属在ZnO(0001)表面的吸附行为及前人的实验结果进行了比较,理论上发现Ag和Au易于吸附在ZnO(0001)表面的H3位,而实验上观察到即使在很小吸附比的条件下ZnO(0001)表面上也能形成Cu的团簇结构,这主要是由于Cu和ZnO(0001)表面衬底强的相互作用所致。     

Theoretical study of the magnetic interaction of Cr-doped ZnO with and without vacancies

First-principles density-functional theory (DFT) calculations have been performed to study the magnetic properties of ZnO:Cr with and without vacancies. The results indicate that the doping of Cr in ZnO induces obvious spin polarization around the Fermi level and a total magnetic moment of 3.77μ_B. The ferromagnetism (FM) exchange interaction between Cr atoms is short-ranged and decreases with increasing Cr separation distance. It is suggested that the FM state is not stable with low concentration of Cr. The presence of O vacancies can make the half-metallic FM state of the system more stable, so that higher Curie temperature ferromagnetism may be expected. Nevertheless, Zn vacancies can result in the FM stability decreasing slightly. The calculated formation energy shows that V_Zn+Cr_Zn complex forms spontaneously under O-rich conditions. However, under Zn-rich conditions, the complex of V_O+Cr_Zn forms more easily. Thus, ZnO doped with Cr may exhibit a concentration of vacancies that influence the magnetic properties.     

Enhanced acetone sensing characteristics by decorating Au nanoparticles on ZnO flower-like structures

A well-organized hierarchical structure of ZnO was developed by chemical bath deposition and used as templates for making gold-coated ZnO (Au/ZnO) hybrid nanostructures. The coverage of Au nanoparticles (Au NPs) on ZnO was controlled by changing the amount of the Au precursor. The Au/ZnO hybrids were applied as gas sensing materials to detect acetone. The improved sensor response, selectivity and short response, and recovery time to acetone vapor due to Au NPs on ZnO nanostructures has been observed and explained by considering the formation of Au/ZnO heterostructures, which are favorable for the diffusion of gas molecules. In addition, the dependence of Au amount on gas sensor properties was systematically investigated. ZnO decorated by 6 wt% Au NPs displayed a 9.05-fold enhancement in gas response to 100 ppm of acetone at 280 °C compared to pristine ZnO.     

One-step synthesis of flower-like Au-ZnO microstructures at room temperature and their photocatalytic properties

In this paper, 3D flower-like Au-ZnO microstructures with controlled morphology and dimensions were synthesized by a facile one-step aqueous solution route at room temperature, and the photocatalytic properties of these structures were investigated. The as-prepared flower-like Au-ZnO structures with a diameter of about 3 μm consisted of many ZnO nanosheets which interacted with each other. These nanosheets, which were successfully decorated by Au NPs, showed an average thickness of 10 nm and a single-crystalline structure with {2-1-10} planar surfaces. The growth process of Au-ZnO structures and the effects of trisodium citrate on the nucleation and growth of ZnO were investigated. The formation of Au NPs in this experiment was discussed too. The Au-ZnO structures showed higher photocatalytic activity than that of pure ZnO.     

Polarity effects on ZnO films grown along the nonpolar [1120] direction

The surface electrical properties of ZnO thin films grown along the nonpolar [1120] direction have been investigated by Kelvin probe microscopy on a nanometer scale. Two different charge domains, with a 75 meV work function difference, coexist within the ZnO surface, which is covered by rhombohedral pyramids whose sidewalls are shown to be {1011}-type planes. The presence and relative orientation of the two kinds of charge domains are explained in terms of the atomic arrangement at the {1011} polar surfaces.     

热处理对ZnO/Au薄膜结构和压电特性的影响

通过射频磁控溅射在氧化硅片表面沉积ZnO/Au薄膜,并通过不同的热处理方式对薄膜进行退火。为了研究退火对ZnO/Au薄膜结构和压电特性的影响,采用X射线衍射分析（XRD）、光学显微镜、场发射扫面电镜（FESEM）和压电力分析仪对薄膜退火前后的材料特性进行了分析。研究发现,热处理能够改善ZnO/Au薄膜的结晶质量。特别是在氮气保护氛围下慢速退火后,薄膜结晶质量有明显改善。但是,热处理导致了薄膜的压电系数d33和d31降低。分析认为,热处理过程中Au原子在ZnO中的易迁移特性破坏了ZnO/Au薄膜的压电性能。     

Abweichendes Verhalten der Kapazität von Metall-Kontakten auf reinen Halbleiterspaltflächen gegenüber dem Schottky-Modell

A pair of surfaces has been produced by cleavage of a semiconductor crystal, containing the same concentration of donors just below the freshly exposed faces. Two types of experiments have been carried out:

1. ZnO crystals were cleaved normal to thec axis in ultrahigh vacuum. The same metal (Au) was evaporated simultaneously on the two polar surfaces.
2. Si crystals were cleaved along (111) planes. Two different metals (Au and Cr or Au and Cu) were evaporated on the two identical surfaces.

The differential capacityC of these contacts was measured as a function of bias voltageU. In spite of the equality in donor concentration for the pair of contacts the slope of the plots 1/C ²(U) differed in both types of experiments. This result cannot be explained by Schottky's theory. A paper proposing a model for interpretation of the results is in preparation.     

ZnO/Au纳米复合物的制备及光学性质的研究简

摘要：采用ZnO纳米晶表面还原Au＋的方法合成了ZnO／Au纳米复合物,研究了其光学性质。     

Structural and optical properties of Au-implanted ZnO films

The structural and luminescence related optical behaviours of Au ion implanted ZnO films grown by magnetic sputtering and their post implantation annealing behaviours in the temperature range of 100-700 °C have been investigated. Optical absorption and transmittance spectra of the films indicate that band edge of Au-implanted ZnO has shifted to high energy range and optical band gap has increased, because the sharp difference of thermal expansion induces the lattice mismatch between ZnO and SiO₂. PL spectra reveal that UV and visible luminescence bands of ZnO films can be improved after thermal annealing due to recovery of defects and Au ions incorporation. Importantly, green luminescence band of 530 nm has been only observed in the Au-implanted and subsequently annealed ZnO films and it enhances with the increasing annealing temperature, which can be related to Au atoms or clusters in ZnO films. Furthermore, X-ray photoelectron spectroscopy measurements reveal that the Au⁰ is dominant state in Au implanted and annealed ZnO films. Possible mechanisms, such as optical transitions of Au atoms or clusters and deep level luminescence of ZnO, have been proposed for green emission.     

Heat capacity of ZnO with cubic structure at high temperatures

The heat capacities at constant pressure and constant volume, and thermal expansivity are calculated for ZnO with rocksalt-type and zinc-blende-type cubic structures over a wide range of temperatures using molecular dynamics simulations with interactions due to effective pair-wise potentials which consist of the Coulomb, dispersion, and repulsion interaction. It is shown that the calculated structural and thermodynamic parameters including lattice constant, thermal-expansion coefficient, isothermal bulk modulus and its pressure derivative at ambient condition are in good agreement with the available experimental data and the latest theoretical results. At extended pressure and temperature ranges, lattice constant and heat capacity have also been predicted. The structural and thermodynamic properties of ZnO with cubic structure are summarized in the 300-1500 K temperature ranges and up to 100 kbar pressure.