Xu Sun

Functional nanogenerators as vibration sensors enhanced by piezotronic effects

ZnO nanomaterials have been shown to have novel applications in optoelectronics, energy harvesting and piezotronics, due to their coupled semiconducting and piezoelectric properties. Here a functional nanogenerator (FNG) based on ZnO nanowire arrays has been fabricated, which can be employed to detect vibration in both self-powered (SP) and external-powered (EP) modes. In SP mode, the vibration responses of the FNG can be measured through converting mechanical energy directly into an electrical signal. The FNG shows consistent alternating current responses (relative error < 0.37%) at regular frequencies from 1 to 15 Hz. In EP mode, the current responses of FNG are significantly enhanced via the piezotronic effect. Under a forward bias of 3 V, the sensor presented a sensitivity of 3700% and an accurate measurement (relative error < 0.91%) of vibration frequencies in the range 0.05–15 Hz. The results show that this type of functional nanogenerator sensor can detect vibration in both SP and EP modes according to the demands of the applications.

High carrier concentration ZnO nanowire arrays for binder-free conductive support of supercapacitors electrodes by Al doping

Doping semiconductor nanowires (NWs) for altering their electrical and optical properties is a critical strategy for tailoring the performance of nanodevices. Here, we prepared in situ Al-doped ZnO nanowire arrays by using continuous flow injection (CFI) hydrothermal method to promote the conductivity. This reasonable method offers highly stable precursor concentration for doping that effectively avoid the appearance of the low conductivity ZnO nanosheets. Benefit from this, three orders of magnitude rise of the carrier concentration from 1016cm-3 to 1019cm-3 can be achieved compared with the common hydrothermal (CH) mothed in Mott-Schottky measurement. Possible effect of Al-doping was discussed by first-principle theory. On this basis, Al-doped ZnO nanowire arrays was developed as a binder-free conductive support for supercapacitor electrodes and high capacitance was triggered. It is owing to the dramatically decreased transfer resistance induced by the growing free-moving electrons and holes. Our results have a profound significance not merely in the controlled synthesis of other doping nanomaterials by co-precipitation method but also in the application of binder-free energy materials or other materials.

Structural dependence of piezoelectric size effects and macroscopic polarization in ZnO nanowires: A first-principles study

The piezoelectric properties of [0001]-oriented ZnO nanowires are investigated via density functional theory (DFT). The axial effective piezoelectric coefficient of ZnO nanowires is significantly greater than the bulk value, and the coefficient increases as the nanowire size decreases. It is proved that the enhancement comes from both the reduction of volume per Zn-O pair and the enhancement of the Poisson’s ratio. Further study shows that the macroscopic polarization behavior of ZnO nanowires is determined by the crystal structure parameters and the ratio of surface atoms, and an analytic expression is obtained. This work provides a deeper understanding of the size effects of the piezoelectricity of ZnO nanowires and sheds some light on the confusion reported on this subject.

Third-order elastic constants of ZnO and size effect in ZnO nanowires

Higher order elastic constants are very useful in understanding the anharmonicity of ZnO due to finite strain. The third-order elastic constants of zinc oxid (ZnO) and the size effect of the strain dependent Youngs moduli of ZnO nanowires have been studied by first-principles calculations and molecular mechanics methods. The whole set of the third-order elastic constants were obtained for the first time for ZnO with homogeneous deformation method. The Youngs modulus along the [0001] direction is evaluated to be Y=142.4−173.4ξ (GPa). Strain dependent Youngs moduli were obtained for [0001] oriented ZnO nanowires with diameter ranged from 1.8 nm to 6.0 nm. The constant term of Youngs moduli of ZnO NWs is smaller than those of the bulk, and it decreases from 121.5 to 96.7 GPa as the diameter decreases. The linear term increases rapidly as the diameter decreases and changed from negative to positive when the diameter is 3.6 nm. The linear term was −124.4 GPa when diameter is 6.0 nm, and it reached 248.8 GP...

Defects Energetics and Electronic Properties of Li Doped ZnO: A Hybrid Hartree-Fock and Density Functional Study

The electronic properties and stability of Li-doped ZnO with various defects have been studied by calculating the electronic structures and defect formation energies via first-principles calculations using hybrid Hartree-Fock and density functional methods. The results from formation energy calculations show that Li pair complexes have the lowest formation energy in most circumstances and they consume most of the Li content in Li doped ZnO, which make the p-type conductance hard to obtain. The formation of Li pair complexes is the main obstacle to realize p-type conductance in Li doped ZnO. However, the formation energy of LiZn decreases as environment changes from Zn-rich to O-rich and becomes more stable than that of Li-pair complexes at highly O-rich environment. Therefore, p-type conductance can be obtained by Li doped ZnO grown or post annealed in oxygen rich atmosphere.

First-principles studies on transport properties and contact effects of Cu(111)/ZnO-nanobelt(100)/Cu(111) systems

The transport properties of ZnO nanobelts along the (101¯0) non-polarized direction coupled with Cu electrodes were studied via non-equivalent Greens functions method and density functional theory formalism. The transport properties were greatly affected by interfacial spacing and nanobelt widths. The conductance decreased exponentially with the widths of the nanobelts. Ohmic behavior was found in narrow nanobelts, while rectifying characteristics were observed in wide nanobelts. In the case of narrow belts, the current-voltage characteristics were changed from ohmic type to rectifying characteristics as the interspace increased, corresponding to the contacts transforming from chemical to physical interactions. However, the conductance in the wider nanobelts declined exponentially as the interfacial distance increased. The change of metal induced gap states (MIGS) depends strongly on the interfacial distance but not significantly on the thickness of ZnO nanobelts. An n-type Schottky barrier between copper and ZnO nanobelts is induced by interfacial polarization effects. The Schottky barrier heights for the narrowest and widest nanobelts with equilibrium interfacial spacing were 0.37 eV and 0.44 eV, respectively, which is in good agreement with the experimental values. Additionally, the Schottky barrier heights increased almost linearly as the width of the nanobelts changed from 0.34 nm to 1.2 nm.

Chapter 3:Property Characterisation and Optimisation

To recognise and control ZnO-based nanostructures, an understanding of their characteristics is important. In this chapter, we briefly discuss the intrinsic properties of ZnO nanostructures, as well as the coupling effects under external strain or electric fields. Firstly, the band structures and defects in ZnO nanostructures are described from a first-principle perspective. Then we introduce the experimental and theoretical advances in the optical, mechanical, electric, magnetic and photocatalytic characteristics of ZnO nanostructures. Finally, the extraordinary piezoelectric and dielectric properties of ZnO nanostructures are presented to cover the origin of the piezoelectric effect and the approaches to observe piezoelectric properties. These features of ZnO nanostructures and their controllable performance modulations may not only give deep insights into nanosized ZnO, but could pave the way for an exploration of ZnO-based nanodevice applications.