Electronic and Mechanical Coupling in Bent ZnO Nanowires
Electronic and Mechanical Coupling in Bent ZnO Nanowires
复制标题
弯曲 ZnO 纳米线中的电子和机械耦合
DOI：
10.1002/adma.200900956
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发表时间：
2009-12-28
影响因子：
29.4
通讯作者：
Yu, Dapeng
中科院分区：
文献类型：
--
作者：
Han, Xiaobing;Kou, Liangzhi;Yu, Dapeng
半导体纳米线在纳米尺度技术的发展中具有重要的作用，因而引起了人们极大的兴趣。[1]氧化锌（ZnO）是一种典型的II-VI族半导体材料，ZnO纳米线因其独特的宽带隙半导体、压电和光电性能而引起了人们的广泛关注。[2]近年来，由于ZnO纳米线的电子-机械耦合行为，其能量转换能力引起了人们的广泛关注。[3]大量的实验和理论研究致力于了解它们的结构，性能和新颖的行为;[4-6] ZnO纳米线的电导已被证明随着应变的增加而降低。[7]然而，机械变形如何调节它们的电子能带结构仍然是难以捉摸的。半导体材料的应变工程一直是一项重要的技术。[8]应变硅表现出增强的电子迁移率，[9]并且硅纳米线的光学和电子特性可以通过应变来定制。[10]碳纳米管的拉伸、弯曲和扭转变形可以显著调节其电子和磁性。[11]类似地，除了ZnO纳米线是压电的之外，ZnO纳米线的电子和光学性质也对机械应变敏感，[12]但是理解这种有吸引力的低维材料的有趣的多场耦合特性的原子级机制仍然是理论和实验研究人员的巨大挑战。我们报告了我们如何操纵单个ZnO纳米线成不同的形状，以在光学和透射电子显微镜（TEM）环境中产生弯曲应变，并且使用阴极发光（CL）发现纳米线中的强电子-机械耦合。观察到CL光谱中沿着弯曲ZnO纳米线的近边发射的显著红移和增宽。晶格分析揭示了弯曲变形和应变引起的纳米线电子能带结构变化的原子机制。理论计算使用第一性原理密度泛函理论（DFT）和有效质量包络函数理论的单轴应力状态被用来解释弯曲引起的带隙的变化。如文献中详细描述的，在这项工作中研究的ZnO纳米线通过气相沉积制备。[13]ZnO纳米线的生长方向是沿着[001] c轴。在乙醇中分散后，将ZnO纳米线（直径100-250 nm）转移到具有500 nm SiO2层的Si衬底上。感兴趣的直ZnO纳米线在光学显微镜下弯曲，使用玻璃尖端进行CL研究。衬底与纳米线之间的强相互作用使纳米线保持弯曲的形状。分别在弯曲NW的外边缘和内边缘处的拉伸应变和压缩应变可以通过其局部曲率半径P和直径D估计为ε 1/2/D/2 p。因此，弯曲应变随着纳米线的曲率半径的减小和直径的增加而线性增加。然后使用高空间分辨率（对于ZnO，在9 keV的电子束能量下为100 nm）和光谱分辨率（0.5nm）的CL光谱（Gatan monocle 300）仔细测量弯曲纳米线的电子带隙。为了提高测量分辨率，大多数化学发光实验都是在液氮中进行的。八万一千在不深入细节的情况下，所有测量都是通过沿NW沿着点扫描来进行的，其中光束焦点在NW的中心处。选择了最佳设置（电子束能量为9 kV，光斑尺寸为4...
Semiconductor nanowires (NWs) are currently attracting a great deal of interest as they are expected to play an important role in the development of nanometer-scale technologies.[1] As zinc oxide (ZnO) is a typical II–VI semiconductor, ZnO NWs have aroused considerable notice because of their unique wide-bandgap semiconducting, piezoelectric, and photoelectric properties.[2] Recently, the energy-converting ability of ZnO NWs has been attracting intense attention as a result of their electronic–mechanical coupling behavior.[3] A huge number of experimental and theoretical studies have been devoted to understanding their structure, properties, and novel behavior;[4–6] the conductance of ZnO NWs has been shown to decrease with increasing strain.[7] However, how mechanical deformation can tune their electronic band structures remains elusive. Strain engineering of semiconductors has long been an important technique.[8] Strained silicon exhibits enhanced electron mobility,[9] and the optical and electronic properties of silicon NWs can be tailored through strain.[10] Tensile, bending, and torsional deformation of carbon nanotubes can significantly tune their electronic and magnetic properties.[11] Similarly, besides ZnO NWs being piezoelectric, the electronic and optical properties of ZnO NWs are also sensitive to mechanical strain,[12] but understanding the atomic-level mechanism of the intriguing multi-field coupling properties of this attractive low-dimensional material remains a great challenge for both theoretical and experimental researchers.In this Communication, we report how we manipulated single ZnO NWs into different shapes to create bending strain in both optical and transmission electron microscopy (TEM) environments, and that strong electronic–mechanical coupling in the NWs was found using cathodoluminescence (CL). Significant red shift and broadening of the near-edge emission in CL spectra along the bent ZnO NWs were observed. The atomic mechanism for bending deformation and strain-induced change in the electronic band structure of the NWs is revealed by lattice analysis. Theoretical calculations using both first-principles density functional theory (DFT) and effective mass envelope function theory for uniaxial stress states are used to explain the bending-induced variation in bandgaps. The ZnO NWs investigated in this work were prepared by vapor phase deposition, as described in detail in the literature.[13] The growth direction of the ZnO NWs is along the [001] c-axis. After dispersion in ethanol, ZnO NWs (100–250 nm in diameter) were transferred onto a Si substrate with a 500nm SiO2 layer. Straight ZnO NWs of interest were bent under an optical microscope for CL investigations using a glass tip. The strong interaction between the substrate and the NWs keeps the NWs in the curved shape. The tensile and compressive strain at the outer and inner edges, respectively, of the bent NW can be estimated by its local radius of curvature p and diameter D as ε ¼ ÆD/2p. Therefore, the bending strain increases linearly with decreasing radius of curvature and increasing diameter of the NWs. The electronic bandgaps of the bent NWs were then carefully measured using CL spectroscopy (Gatan monocle 3þ) of high spatial (100nm for ZnO at electron beam energy of 9keV) and spectral (0.5 nm) resolution. To enhance the measuring resolution, most of the CL experiments were carried out in liquid nitrogen at ca. 81 K. Without going into details, all measurements were made by spot-scanning along the NWs with the beam focus at the center of the NWs. An optimal setting was chosen (electron beam energy 9 kV and spot size of 4 …