Optical and structural properties of well-aligned ZnO nanoneedle arrays grown on porous silicon substrates by electric field-assisted aqueous solution method

Abstract

Vertically well-aligned ZnO nanoneedle arrays were successfully grown on porous silicon substrates by electric field-assisted aqueous solution method. Room-temperature photoluminescence (PL), X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), and transmission electron microscopy (TEM) analyses were performed to investigate the optical, structural, and morphological properties of the arrays. FESEM results indicated that increasing the current density at the initial stage aided in transforming the ZnO structure from hexagonal to needle-shaped nanorods and in subsequently increasing nanoneedle length. XRD findings showed that ZnO nanoneedles had preferential growth at the (002) orientation, indicating a high degree of ZnO nanoneedle vertical orientation to the substrate surface. Moreover, increased current density resulted in a redshift in the PL peak of ZnO nanorods toward a longer wavelength because of Zn vacancies in the energy band gap of ZnO. In addition, no E1 (LO) mode peak was observed in the Raman spectra of all samples. This finding indicated that nanorod growth was considerably or almost totally free from defects, such as O2 vacancies, Zn interstitials, or their complexes and free carriers.