Optical/mechanical/electrical Coupled Properties Of One-dimensional Piezoelectric Nanomaterials

With the development of device micromation, intelligence, high integration, high storage and high speed, there are potential applications in micro- and nano-devices for nanostructures due to their novel characteristics, therefore nanostructures and nanotechnology have attracted considerable attention. Many complex nanostructures, such as nanofibers, nanobelts (NBs), nanowires, nanorods and nanorings, have been reported, and a lot of novel characteristics also have been explored, such as size effect, surface effect and dielectric confinement effect and so on. One-dimensional piezoelectric functional nanomaterials are investigated in this PhD dissertation. The advances of piezoelectric functional nanomaterials were reviewed in the introduction, and the review focuses on: (1) synthesis methods of functional nanomaterials; (2) optical/mechanical/electrical coupled properties of functional nanomaterials. Based on these reviews, according to the idea of fabrication-characterization-application optical/mechanical/electrical coupled properties of one-dimensional piezoelectricâ…¡-â…¥/NBT-based functional nanomaterials are investigated systematically by the experimental methods, such as the fabrication, mechanics, electrics, photostiffening (photoplastic) effect, photoinduced deformation and piezoelectric properties. The main results are summarized as follows:1. ZnO and ZnS wool like NBs were synthesized by thermal evaporation at high temperature without the presence of the catalyst. The photoinduced stiffening (PIS) and photoplastic effect (PPE) of ZnO and ZnS individual NBs were observed by using a nanoindenter, atomic force microscope (AFM) in conjunction with an incident ultraviolet (UV) light source system. Meanwhile, the mechanisms of PIS and PPE were also explored. The results show: (a) the NBs with square cross section are of several tens to several hundreds of micrometers in length, and they are of the hexagonal wurtzite structure; (b) under UV illumination and visible light, the elastic moduli of ZnO individual NBs are up to 45 % and 348 % larger than that in darkness, while for ZnS individual NB they are up to 42 % and 183 % larger than that in darkness. For ZnO and ZnS individual NBs, the PPE were observed under UV illumination and visible light; (c) the high surface-to-volume ratio, photoconductive and electronic strain lead to the PIS, while the dislocation can collect more electrons to increase the Peierls barrier under illumination. Then, it leads the PPE of the ZnO and ZnS individual NBs under illumination.2. Photostrictive deformations of ZnO and ZnS individual NBs were investigated by using a nanoindenter in conjunction with an incident UV light source system, and under the in-situ nanoindentation a modulated UV laser was introduced into the load-holding segment extended. In order to confirm the photostrictive experiment, AFM in conjunction with an incident UV light source system were used to detect the photostrictive deformations of ZnO individual NBs under scanning mode. The mechanisms of photostrictive deformation were also discussed. The results show: (a) when the UV laser was periodically turned"on"and"off", the irreversible contraction and the reversible dilatation were observed for both ZnO and ZnS individual NBs, and they are up to 10 nm; (b) the irreversible contraction was interpreted by dislocation theory, while the reversible dilatation was explained by the combination mechanisms of photoconductive effect, photovoltaic effect and converse piezoelectric effect; (c) the reversible dilatation is corresponding to the direct conversion of photonic to mechanical motion, and a photospring was designed. There may be potentially applied into photostrictive sensors and actuators.3. V-ZnO nanofibers with different vanadium-doped concentration were synthesized by sol-gel process and electrospinning. The morphology, crystallized phase and crystal structure were investigated by scanning electron microscopy (SEM), X-ray diffractometer (XRD) and high resolution transmission electron microscopy (TEM), respectively. The reduced modulus and hardness of V-ZnO piezoelectric nanofibers were investigated by nanoindenter. The butterfly-shaped piezoelectric response was measured by scanning force microscopy (SPM). The results show: (a) the diameters of V-ZnO nanofibers with hexagonal wurtzite phase are in the range of 50 to 200 nm, while the lengths are several hundreds of micrometers; (b) the average effective piezoelectric coefficient ( d₃₃^*) value increases with vanadium concentration in the range of 0.015 to 0.025, while decreases in the range of 0.025 to 0.03 due to structural deterioration by over-doping. The large d₃₃^* of 121 pm/V was obtained, and the high piezoelectric property may be attributed to the switchable spontaneous polarization induced by V dopants and the easier rotation of V-O bonds under electric field; (c) the statistical average values of reduced modulus and hardness are 58.7 GPa and 3.3 GPa for the nanofibers, and they decrease by 47.2 % and 34.0 % in comparison with those of bulk ZnO. It indicates that size effect of the mechanical behavior was obviously observed for the nanofibers, and the mechanism was discussed in conjunction with their high surface-to-volume ratio. Indentation depth-dependent reduced modulus and hardness properties were observed, and it was attributed to the strain gradient effect during nanoindentation.4. (Na_0.5Bi_0.5)_0.94TiO₃-Ba_0.06TiO₃ (NBT-BT) and (Na_0.82K_0.18)_0.5Bi_0.5TiO₃ (NKBT) piezoelectric nanofibers were synthesized by sol-gel process and electrospinning technique. The morphology, crystallized phase and crystal structure were investigated by SEM, XRD, TEM and Energy-Dispersive X-ray Spectroscopy. To characterize the piezoelectricity, the d₃₃^* of nanofibers was measured by SPM. The nanoscale mechanical behaviors of piezoelectric nanofibers, such as reduced modulus and hardness properties, were investigated by nanoindentation technique in detail. The results show: (a) NBT-BT and NKBT nanofibers with perovskite phase were formed, and the diameter and length of nanofibers are in the range of 50 to 600 nm and several tens to several hundreds of micrometer; (b) the average values of d₃₃^* are 102 and 96 pm/V for NBT-BT and NKBT nanofibers, and the high piezoelectric property may be attributed to the easiness for electric field to tilt polar vector of domain and the increase of possible spontaneous polarization direction; (c) in comparison with the reduced modulus (158.2 GPa) and hardness (7.3 GPa) of NKBT thin film, the reduced modulus and hardness are 107.6 GPa and 4.9 GPa for the nanofibers, and they decrease by 31.9 % and 32.8 %. The size effect of the mechanical behavior was obviously observed for the nanofibers, and it is similar for the NBT-BT nanofibers. Due to their well piezoelectric and mechanical properties, they are a candidate of one-dimensional piezoelectric nanomaterials for potential appllication in micro-electro-mechanical systems.5. Photolithography and radio frequency magnetron sputtering were utilized to fabricate Pt interdigital electrodes on the SiO₂/Si substrate. The ZnS NBs were assembled onto the interdigital electrodes to fabricate photoconductive semiconductor switches (PCSS's) based on individual NB and PCSS based on NBs film. The current-voltage (â… -â…£) and functional characteristics of the PCSS's were measured by Keithley system. The results show: (a) the separation between interdigital electrodes and the electrode width are 45μm and 55μm, and the thickness is 100 nm. The PCSS's based on ZnS NBs have high photoresponse, well off-state and large photosensitivity; (b) the PCSS's were applled into a test circuit to control the circuit state under UV-light with different wavelengths. The PCSS's can reversibly control the circuit state conversion between"1"and"0"when UV-light was tured"on"and"off", thus, the PCSS's are of switching function; (c) oxygen chemisorption, surface effect and photoconductive effect were used to successfully explain the photoconductivity mechanism of PCSS's. The results indicate that the PCSS's based on semiconducting nanomaterials are a promising candidate for future integration.