Growth of ZnO Nanotetrapods with Hexagonal Crown

ZnO nanotetrapods with hexagonal crown were synthesized on a silicon wafer by vapor transport process at a low temperature of 630 °C and normal pressure without the presence of catalysts. The results demonstrated that the as-synthesized products with slender legs and regular hexagonal crown are single crystal with wurtzite structure and preferentially grow up along 001 direction. Photoluminescence spectra revealed that the green emission originated from oxygen vacancies overwhelmed that of the near-band-edge ultraviolet peak, which suggests the peculiar-shaped nanotetrapods may have potential applications in multichannel nano-optoelectronic devices.

Growth and optical properties of peculiar ZnO tetrapods

Regular ZnO tetrapods with different morphologies have been obtained on Si(100) substrate via the chemical vapour deposition approach. Varying the growth temperature and gas rate, we have obtained different structured ZnO materials: tetrapods with a large hexagonal crown, a flat top and a small hexagonal crown. The results suggest that these tetrapods are all single crystals with a wurtzite structure that grow along the (0001) direction. However, photoluminescence spectra shows that their optical properties are quite different: for those with large hexagonal crown, the green emission overwhelms that of the near band-edge (NBE) ultraviolet (UV) peak, while others have only a strong NBE UV peak at ~386 nm.

Molecular Dynamics And Density Functional Studies Of A Body-Centered-Tetragonal Polymorph Of Zno

We report a previously unknown body-centered-tetragonal structure for ZnO. This structure results from a phase transformation from wurtzite in [0001]-oriented nanorods during uniaxial tensile loading and is the most stable phase for ZnO when stress is above 7 GPa. The stress-induced phase transformation has important implications for the electronic, piezoelectric, mechanical, and thermal responses of ZnO. The discovery of this polymorph brings about a more complete understanding of the extent and nature of polymorphism in ZnO. A crystalline structure-load triaxiality map is developed to summarize the relationship between structure and loading.

Piezoelectricity Of Zno Films Prepared By Sol-Gel Method

ZnO piezoelectric thin films were prepared on crystal substrate Si(111) by sol-gel technology, then characterized by scanning electron microscopy, X-ray diffraction and atomic force microscopy (AFM). The ZnO films characterized by X-ray diffraction are highly oriented in (002) direction with the growing of the film thickness. The morphologies, roughness and grain size of ZnO film investigated by AFM show that roughness and grain size of ZnO piezoelectric films decrease with the increase of the film thickness. The roughness dimension is 2.188-0.914 nm. The piezoelectric coefficient d(33) was investigated with a piezo-response force microscope (PFM). The results show that the piezoelectric coefficient increases with the increase of thickness and (002) orientation. When the force reference is close to surface roughness of the films, the piezoelectric coefficient measured is inaccurate and fluctuates in a large range, but when the force reference is big, the piezoelectric coefficient d(33) changes little and ultimately keeps constant at a low frequency.

Novel Mechanical Behavior Of Zno Nanorods

A novel stress-strain relation with two stages of linear elastic deformation is observed in [0 0 0 1]-oriented ZnO nanorods under uniaxial tensile loading. This phenomenon results from a phase transformation from wurtzite (WZ, P6(3)mc space group) to a body-centered tetragonal structure with four-atom rings (denoted as BCT-4) belonging to the P4(2)/mnm space group. The analysis here focuses on the effects of nanorod size and temperature on the phase transformation and the associated mechanical behavior. It is found that as size is increased from 19.5 to 45.5 angstrom, the critical stress for nucleation of the transformation decreases by 25% from 21.90 to 16.50 GPa and the elastic moduli of the WZ- and BCT-4-structured nanorods decrease by 24% (from 299.49 to 227.51 GPa) and 38% (from 269.29 to 166.86 GPa), respectively. A significant temperature effect is also observed, with the critical stress for transformation initiation decreasing 87.8% from 17.89 to 2.19 GPa as temperature increases from 300 to 1500 K. (c) 2007 Elsevier B.V. All rights reserved.

Large Scale Fabrication Of Periodical Bowl-Like Micropatterns Of Single Crystal Zno

Nanostructured ZnO materials are of great significance for their potential applications in photoelectronic devices, light-emitting displays, catalysis and gas sensors. In this paper, we report a new method to produce large area periodical bowl-like micropatterns of single crystal ZnO through aqueous-phase epitaxial growth on a ZnO single crystal substrate. A self-assembled monolayer of polystyrene microspheres was used as a template to confine the epitaxial growth of single crystal ZnO from the substrate, while the growth morphology was well controlled by citrate anions. Moreover, it was found that the self-assembled monolayer of colloidal spheres plays an important role in reduction of the defect density in the epitaxial ZnO layer. Though the mechanism is still open for further investigation, the present result indicates a new route to suppress the dislocations in the fabrication of single crystal ZnO film. A predicable application of this new method is for the fabrication of two-dimensional photonic crystal structures on light emitting diode surfaces.

From Hexagonally Arrayed Nanorods To Ordered Porous Film Through Controlling The Morphology Of Zno Crystals

The macrostructure can be changed by changing the morphology of its units. In this article, we use a colloidal template route, combined with hydrothermal growth method, to get the hexagonally arrayed ZnO nanorods on the polycrystalline ZnO substrate. More significantly, through controlling the morphology of ZnO crystals by adding structure-directing agent in the precursor solution, the highly ordered porous ZnO films were obtained instead of ZnO nanorods. This templated solvent-thermal method has great potential in micro/nano-fabrication. (C) 2008 Elsevier B.V. All rights reserved.

Growth Mechanism And Joint Structure Of Zno Tetrapods

Regular zinc oxide (ZnO) tetrapods with a flat plane have been obtained on Si(1 0 0) substrate via the chemical vapour deposition approach. The x-ray diffraction result suggests that these tetrapods are all single crystals with a wurtzite structure that grow along the (0 0 0 1) direction and corresponding electron backscatter diffraction analysis reveals the crystal orientation of growth and exposed surface. Furthermore, we find some ZnO tetrapods with some legs off and the angles between every two legs are measured with the aid of scanning electron microscopy and image analysis, which benefit to reveal the structure of ZnO tetrapods joint. The structure model and growth mechanism of ZnO tetrapods are proposed. Besides, the stable model of the interface was obtained through the density-functional theory calculation and the energy needed to break the twin plane junction was calculated as 5.651 J m(-2).

On the formation of well-aligned ZnO nanowall networks by catalyst-free thermal evaporation method

Two-dimensional ZnO nanowall networks were grown on ZnO-coated silicon by thermal evaporation at low temperature without catalysts or additives. All of the results from scanning electronic spectroscope, X-ray diffraction and Raman scattering confirmed that the ZnO nanowalls were vertically aligned and c-axis oriented. The room-temperature photoluminescence spectra showed a dominated UV peak at 378 nm, and a much suppressed orange emission centered at similar to 590 nm. This demonstrates fairly good crystal quality and optical properties of the product. A possible three-step, zinc vapor-controlled process was proposed to explain the growth of well-aligned ZnO nanowall networks. The pre-coated ZnO template layer plays a key role during the synthesis process, which guides the growth direction of the synthesized products. (C) 2007 Elsevier B.V. All rights reserved.

水热法制备ZnO二维周期有序孔结构薄膜

在由溶胶-凝胶法制备的纳米ZnO薄膜衬底上,以Zn(NO_3)_2·6H_2O和六亚甲基四胺(HMT)等摩尔浓度配制成前驱体溶液,在单层聚苯乙烯(PS)微球模板辅助下,采用水热法制备了具有规则多孔结构的ZnO薄膜.探讨了PS微球作为模板对ZnO纳米棒生长的限制作用以及柠檬酸钠在水热制备方法中对晶体生长的影响.利用扫描电子显微镜(SEM)和X射线衍射(XRD)表征了水热反应后所得二维有序ZnO膜表面形貌和取向性,测量了ZnO薄膜的光致发光(PL)光谱并研究其相应机理.

Piezoelectricity of ZnO films prepared by sol-gel method

ZnO piezoelectric thin films were prepared on crystal substrate Si(111) by sol-gel technology, then characterized by scanning electron microscopy, X-ray diffraction and atomic force microscopy (AFM). The ZnO films characterized by X-ray diffraction are highly oriented in (002) direction with the growing of the film thickness. The morphologies, roughness and grain size of ZnO film investigated by AFM show that roughness and grain size of ZnO piezoelectric films decrease with the increase of the film thickness. The roughness dimension is 2.188-0.914 nm. The piezoelectric coefficient d(33) was investigated with a piezo-response force microscope (PFM). The results show that the piezoelectric coefficient increases with the increase of thickness and (002) orientation. When the force reference is close to surface roughness of the films, the piezoelectric coefficient measured is inaccurate and fluctuates in a large range, but when the force reference is big, the piezoelectric coefficient d(33) changes little and ultimately keeps constant at a low frequency.

微/纳米氧化锌（ZnO）的生长、性质及其应用研究

纳米氧化锌（ZnO）是一种直接宽带隙半导体材料，室温下其禁带隙宽为本3.37 eV，激子束缚能为60 meV。纳米ZnO有明显的尺寸效应、表面和界面效应等，物理化学性能优越。在压电材料、铁电材料、平面显示、表面声波、传感器、场发射器件、激光、光催化等方面有着广泛的用途。近年来，对纳米ZnO材料的研究成为国内外的一个热点。 本论文研究了用化学气相沉积（CVD）法制备微/纳米ZnO材料。通过控制实验条件，合成了多种特殊结构和形貌的微/纳米ZnO材料，并用扫描电子显微镜（SEM）、高分辨率透射电镜（HR-TEM）、X-射线衍射仪（XRD）、Raman光谱和光致发光（PL）等对材料的结构和光学性能进行了表征。采用CVD法，在温度为630 °C，氧气流量为15 sccm，氩气流量为300 sccm的条件下，制备了一种纳米带冠四足状ZnO（T-ZnO）。此结构ZnO材料的每根足顶端均有一规则的六方帽形结构，具有很大的比表面积。实验结果表明：合成的ZnO材料为纤维锌矿结构单晶，并且沿着(0001)方向生长；室温下的PL谱有两个激发峰，一个是在393 nm处相对较弱的近带紫外峰，另一个是在511 nm处强峰。而材料在600 °C下氧气中退火30 min后，511 nm附近的绿光激发辐射峰则基本消失了，这说明在511 nm处的绿光激发辐射峰可能是由于氧空位引起的。此外，通过改变实验条件，还得到了其他多种结构的微/纳米ZnO材料。 通过大量实验，找到了一种在低温下合成微/纳米ZnO材料的新方法，即水蒸气氧化法。用ZnI2作为锌源，水蒸气作为氧化剂，实验温度在300～500 °C范围内，大大低于通常CVD法的500～1500 °C。采用此法，用硅做基底，得到了一系列有趣的实验结果，大多数情况下ZnO纳米晶自组装成很规则的圆。而在瓷舟中收集到的纳米ZnO跟普通CVD法结果相似，可以得到锥状、棒状等结构的纳米晶，但其生长方式与硅基底上的有很大差别。此外，用水蒸气氧化法，还实现了ZnO纳米晶在碳纳米管（CNTs）上的直接生长，而且其PL性能增强，这可能是纳米ZnO和CNTs相互耦合的结果。在700 °C温度下，以锌粉和ZnI2作为锌源，用水蒸气作为氧化剂，在硅基底的正反面分别得到了纳米棒和纳米推子阵列。此外，还对水蒸气氧化法的化学反应机理进行了分析，实验结果证明：固态ZnI2在受热和一定真空度下先蒸发成ZnI2分子，ZnI2分子遇到水蒸气发生反应生成偶极ZnO分子，这些ZnO偶极分子在硅基底上通过静电力自组装成特殊的几何形状。 此外，还通过分子动力学模拟的方法，对材料的力学性能进行了研究，得到了ZnO的弹性常数和体弹性模量，模拟值跟其他研究人员的实验和模拟结果吻合得很好，并估出算了ZnO晶体的表面能和断裂韧性。 本论文还对制备材料的光催化性能进行了系统的研究，采用CVD法制备ZnO，对铬黑T（EBT）进行光催化降解实验。通过正交实验方法，得到了ZnO催化降解EBT的最佳工艺条件，即催化剂用量为5 g/L，光照强度为120 W，反应温度为20 °C，反应时间为120 min，EBT浓度为10 mg/L，溶液pH值为4。 在最佳实验条件下，20分钟内有95%的EBT被降解完，30分钟内则全部降解。因此，ZnO在EBT的降解中催化效率很高，在废水处理中具有潜在的应用前景。

Ab Initio Study Of Zno-Based Gas-Sensing Mechanisms: Surface Reconstruction And Charge Transfer

Density functional theory (DFT) calculations were employed to explore the gas-sensing mechanisms of zinc oxide (ZnO) with surface reconstruction taken into consideration. Mix-terminated (10 (1) over bar0) ZnO surfaces were examined. By simulating the adsorption process of various gases, i.e., H-2, NH3, CO, and ethanol (C2H5OH) gases, on the ZnO (10 (1) over bar0) surface, the changes of configuration and electronic structure were compared. Based on these calculations, two gas-sensing mechanisms were proposed and revealed that both surface reconstruction and charge transfer result in a change of electronic conductance of ZnO. Also, the calculations were compared with existing experiments.

Femtosecond pulse laser-induced self-organized nanostructures on the surface of ZnO crystal

This paper reports self-organized nanostructures observed on the surface of ZnO crystal after irradiation by a focused beam of a femtosecond Ti:sapphire laser with a repetition rate of 250 kHz. For a linearly polarized femtosecond laser, the periodic nanograting structure on the ablation crater surface was promoted. The period of self-organization structures is about 180 nm. The grating orientation is adjusted by the laser polarization direction. A long range Bragg-like grating is formed by moving the sample at a speed of 10 mu m/s. For a circularly polarized laser beam, uniform spherical nanoparticles were formed as a result of Coulomb explosion during the interaction of near-infrared laser with ZnO crystal.