Photomechanical Effects Of ZnS Individual Nanobelts In Nanoindentation

As an important direct band gap (3.66 eV at 300 K) II-VI compound semiconductor and a luminescent material, ZnS has attracted considerable attention due to its potential applications in flat-panel displays, electroluminescent devices, infrared windows, sensors, and lasers. Recently, one dimensional (1D) ZnS nanostructures such as nanobelts (NBs) and nanowires have shown their excellent photoconductive characteristics, so they can be used in electronic, optoelectronic and laser nanodevices. An accurate measurement of mechanical properties of 1D nanostructures is of critical importance for integrating them into functional nanodevices, because the mechanical failure of these nanostructures may lead to the malfunction or even failure of the entire device. In this thesis, ZnS NBs were synthesized by thermal evaporation method,and the crystal structure and morphology of the NBs were characterized. The nanoindenter conjunction with an ultraviolet (UV) light source system was set up to investigate the mechanical properties of the NBs in darkness and under UV illumination, and the physics mechanisms of photoelastic effect and photoplastic effect were discussed for the NBs respectively. The main contents are given as follows:(1)At first, the development of nanomaterials and nanotechnology is introduced, and the application and study status of ZnS nanomaterials are summarized in the introduction of this thesis. Secondly, the development of nanoindentation technique is introduced, and the research achievements in the application of nanoindentation are also reviewed. The theoretic method is in detail depicted to measure the elastic modulus and hardness via the nanoindentation test. Finally, a brief introduction is given about the research contents in this thesis.(2)ZnS NBs were synthesized by thermal evaporation method, and the crystal structure and geometric morphology were characterized by X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM) and nanoindenter. XRD pattern and FE-SEM image show the hexagonal wurtzite structure and the length of NBs is in the range of 100-200μm. TEM, AFM and nanoindenter micrographs show that the some NBs are of a saw-like side surface, and the NBs thickness is estimated as about 100 nm. (3)The indentation load-displacement curves of ZnS NBs were measured in darkness and under UV illumination. Comparing with the elastic modulus obtained in darkness and under UV illumination, we found that the positive and negative photoelastic effects (PEEs) occur for ZnS NBs with width to thickness (w/t) ratios 4.9-9.6 and 3.2-4.1. The physical mechanism for the positive PEE is analyzed in terms of the surface effect and the electronic strain induced by the photogeneration of free carriers in ZnS NBs. The thermal effect and the decrease of elastic constants induced by UV illumination are used to explain the negative PEE.(4)By comparing the hardness of ZnS individual NBs between in darkness and under UV illumination, the positive and negative photoplastic effects (PPEs) were observed in our experiments. The experimental results show that the PPEs depend on the w/t ratio of NBs, and the positive and negative PPEs occur for the w/t ratios 4.9-9.6 and 3.2-4.1. The competing mechanisms of increase of Peierls barrier and radiation enhanced dislocation glide are used to interpret the w/t ratio dependence of PPE reasonably.