A simple and effective method to fabricate and pattern high-quality zinc oxide (ZnO) nanowire (NW) arrays with controlled nucleation sites and densities on carbonized PRs was demonstrated. ZnO NWs preferentially nucleate on carbonized PR patterns, which are excellent electrodes to connect with ZnO nanostructures. This method is the direct growth of the aligned ZnO NWs with stable, supreme crystal qualities, and strong UV emissions using graphitized PRs without metal catalyst. The PL spectrum obtained from a ZnO NW with a diameter of 30 nm had a strong UV peak at 380 nm without other defect emission. This method is compatible with the present CMOS process flows for mass productions and applications of nanostructures.

Zn₂TiO₄-ZnO NW heterostructures can be fabricated by annealing the a-...[ Read more ]

A simple and effective method to fabricate and pattern high-quality zinc oxide (ZnO) nanowire (NW) arrays with controlled nucleation sites and densities on carbonized PRs was demonstrated. ZnO NWs preferentially nucleate on carbonized PR patterns, which are excellent electrodes to connect with ZnO nanostructures. This method is the direct growth of the aligned ZnO NWs with stable, supreme crystal qualities, and strong UV emissions using graphitized PRs without metal catalyst. The PL spectrum obtained from a ZnO NW with a diameter of 30 nm had a strong UV peak at 380 nm without other defect emission. This method is compatible with the present CMOS process flows for mass productions and applications of nanostructures.

Zn₂TiO₄-ZnO NW heterostructures can be fabricated by annealing the a-TiO₂-ZnO structures. Unilateral diffusion of Zn²⁺ and O^2- ions into TiO₂ nanoparticles can fabricate Zn₂TiO₄-ZnO NWs. The Zn₂TiO₄ heads, which are cubic spinel single crystalline, have two different orientation relations with the ZnO NWs. Thus, the Zn₂TiO₄-ZnO heterostructured NWs have the potential in photocatalytic application because of the fine interfaces, enhancing charge transfer and separation process. Besides, the n-type core ZnO NW can be combined with the p-type amorphous Si shell to become a core-shell hetero p-n junction by chemical vapor deposition. The ZnO/a-Si core-shell NW arrays were uniformly fabricated in thickness on the side surface and the apex of the NWs. The thickness and roughness of the a-Si shells can be controlled by tuning the deposition period and temperature.

We reported that the self-oscillations can be realized in singly clamped ZnO NWs during field emission by simply applying a static DC potential. The field emission of ZnO NWs was significantly increased during the self-oscillations because of piezotronic and pyroelectric effects during the bending of NWs. The field emission enhanced by both pyroelectric and piezotronic effects provided a positive feedback to the oscillation, resulting in the increase of the amplitude. Both remarkable enhancement of field emission and amplitude provide the self-amplified signals for the self-oscillation mechanism based Nanoelectromechanical systems (NEMS). The amplifying self-oscillations made self-excited NEMS easy to integrate and detect.

To investigate the high frequency of the self-oscillations of the NWs, we examined the high crystalline ZnO NWs by using in-situ transmission electron microscopy (TEM) experiments. The ZnO NWs can be excited to self-oscillations by applying pure direct current voltages. The strong electric field on the ZnO NW apex causes field emission. The bending oscillations of the NWs occur in transverse direction, whereas the axial oscillations appear in longitudinal direction. The transverse bending oscillations of the NWs propagate in a cone shape to form an apex. The emission current increased for at least one order of magnitude during the transverse oscillations. The ZnO NW can be considered as the model of an elastic rod under the electrostriction effect on its apex. This effect caused by the spontaneous charging and discharging occurs on the NW apex, which induces periodic excitations.

The longitudinal axial oscillations of the NW are the driven oscillations caused by periodic excitations. When an excitation approaches the particular harmonics of natural frequencies, the NW will self-oscillate to its resonance frequency. As a result, the high frequency of the self-oscillating ZnO NWs in axial direction can reach the highest resonance record at 1.303 GHz. Gold (Au) nanoparticles were sputtered on the surface of the ZnO NW to monitor the thermal effect, which started to melt and moved together after the self-vibrations of the NWs. We found that the entire body of the NW cannot be completely heated after oscillations. As the vibrations of the NWs are confined in the static electric field, the ZnO NWs can be self-excited with the evenly distributed frequency bands in axial resonances because of the cycles of the field effect at the apex, the piezoelectric effect, and the resultant thermal stress deviations by the field emissions.