Studies On Construction And Interface Engingeering Of ZnO Nanomaterials Based Piezotronic Devices

One dimensional (ID) ZnO nanomaterials possess semiconductor and piezoelectric properties simultaneously. A recent highlight research area should be shed upon nanodevices utilizing the coupling effect of semiconductor and piezoelectric effect. With the goal of applications, many hot topics have been announced, such as high performance nanodevices constructed with piezoelectric effect, the interface engineering via piezotronic effect for performance enhancement, and so on.In this thesis, a series of controllable ID ZnO nanomaterials has been synthesized by chemical vapor deposition (CVD) and hydrothermal method. Two typical piezotronic nanodevices have been constructed including high performance flexible strain sensors with Indium doped ZnO (In-ZnO) nanobelts and functional nanogenerator as vibration sensors. The mechanism of piezotronic interface engineering has been investigated through the variation of the interface barrier, junction resistance and the behavior of the photogenerated carriers under strains.Firstly, a polar surface controlled In-ZnO nanobelt is synthesized by CVD process. The nanobelt is characterized with1μm in width,200nm in thickness, and100μm in length. The nanobelt is grown along the direction of [2110] with a top polar surface of (0001)。 By modifying the parameters of hydrothermal process, the ZnO nanowires array is synthesized with controllable height and densit. Using the alternate coating with the solution dissolved with Zn(AC)2and NaOH into ethernol, a thin film of ZnO is fabricated as a seed layer on any flexible substrates. After that, the ZnO nanowries array is grown on the flexible substrate.Secondly, a flexible piezotronic strain sensor based on an In-ZnO nanobelt has been developed. By connecting two ends of the monopolar top surface of nanobelts with the silver paste, the strain sensor is constructed. Under strains, a uniform static piezopotential is generated on the top surface of nanobelts by the coupling of piezoelectric and Poisson effect. The piezopotential modifies the Schottky barrier heights at the interfaces of both source and drain electrodes, resulting the current change with the same trend at both forward and reverse biases. By applying a series of periodical strains, the sensor shows clear, fast and accurate current responses. The gauge factor for compressive strain achieves4036.Thirdly, a functional nanogenerator based on ZnO nanowires arrays has been fabricated, which can be employed to detect mechanical vibrations in both self-powered (SP) and external-powered (EP) modes. In SP mode, the vibration responses of the device can be attributed to directly converting mechanical energy to electrical signal by piezoelectric effect. The device shows consistent alternative current responses (relative error<0.37%) at regular frequencies from1to15Hz. In EP mode, the current responses are effectively enhanced via the piezotronic effect. Under a forward bias of3V, the sensor presents a sensitivity of3700%and an accurate measurement (relative error<0.91%) of vibration frequencies in a range of0.05-15Hz. The device can detect the pulse vibration of human being in EP mode.Finally, to illustrate the mechanism of piezotronic interface engineering, a metal-insulator-semiconductor (Pt/AhO3/ZnO) junction is developed. The insulator layer is deposited onto the ZnO nanowires array film, using a home-made atom layer deposition system. Combined with the basic theory of semiconductor devices, the piezotronic interface engineering can be described by the varation of junction resistance, barrier height, and photo induced current without biase under strain. Due to the protection of insulator layer, a stable Schottky junction is developed with a barrier height of0.739eV. Under a compressive strain of-1.0%, the barrier height is increased by14.7meV, meanwhile the photo current without bias is increased from0.644μΛ/W to1.78μA/W. Because the junction resistance is increased with the increasing compressive strains, the mechanism of piezotronic interface engineering can be concluded that the negative piezopolarization repel the carrier in the space charge region, decrease the earrier concentration at the interface, enhance the built-in field in the junction area, and enhance the photo current response eventually.