Synthesis and photoluminescence properties of thermal oxidized CuO nanorods from sputtered Cu films

Abstract

The unique physical and chemical properties of CuO nanostructures make them as the key materials for the application of nano-optoelectronic devices, gas sensors and photodetectors. Although thermal oxidation is a simple, high efficient and low-cost preparation method, the cracking problem caused by heating still limits its wide application in nano-devices. Therefore, it is still necessary to develop the preparation techniques, which can grow CuO nanostructures directly on various substrates, especially the widely-used Si wafer. Photoluminescence (PL) is a nondestructive technique to explore the optical properties and investigate the electronic transitions in semiconductors, which is important to develop the photo-electronic devices. This research is important to promote the practical applications of CuO nanostructures in functional devices. In this work, Cu films with different morphologies are firstly achieved from two different direct current (DC) and radio frequency (RF) magnetron sputtering method. Then, the electric field assisted thermal oxidation method is introduced to grow CuO nanostructures from Cu films. Two different thermal oxidation processes are used to explore the effect of the electric field directions. Besides, Cu foils are also oxidized in the same condition for comparison. The morphologies and crystal structures of Cu films and CuO nanostructures synthesized in different technologies are characterized via scanning electron microscope (SEM), X-ray diffraction (XRD), and transmission electron microscope (TEM). Finally, PL spectra of CuO nanorods are also measured at room temperature by Horiba FluoroMax-4 fluorescence spectrometer. The results show that a columnar Cu film with the thickness of 1.25 μm is obtained from DC magnetron sputtering, whose preferred orientation is (111) plane. CuO nanowires are found on the Cu foils oxidized under two heating conditions. However, for the columnar Cu films, CuO nanorods are only obtained under an upward electric field, which is same to the growth direction of the nanorods. A layer structure of CuO nanorods/CuO layer/Si substrate is formed after thermal oxidation, and it is found exfoliated from the Si substrate. So, a thin Cr buffer layer of about 100 nm is deposited before Cu layer to enhance the film-substrate bonding force. It turns out that the Cr layer can solve the cracking problem caused by thermal stress. The TEM results show that the CuO nanorod is a monocrystal monoclinic phase with diameter of about 40 nm. Besides, the lattice spacings of the CuO nanorod are bigger than the standard spacings. As for the RF magnetron sputtering, although fine grain Cu film of about 0.5 μm is successfully obtained on Si substrate, no one-dimensional nanostructured oxides are found after two thermal oxidation conditions. The most close-packed plane of FCC Cu structure is (111) plane, whose surface energy is minimum. Therefore, it is easier for the film deposited by DC magnetron sputtering to grow along the (111) plane due to the smaller ion energy compared with the RF magnetron sputtering. Furthermore, the DC-deposited columnar Cu film are easier to form CuO nanorods than the RF-deposited fine grain Cu film, because the cracks among columnar crystals provide a high diffusion rate channel for the diffusion of ions. No Cu2O phase is found in this work due to the limited supply of Cu compared with the Cu foils. The CuO nanorods excited with ultraviolet light show wide dark violet to bright blue luminescence band in the range from 390 to 470 nm at room temperature. This study is beneficial to further understand the growth mechanism and optical properties of CuO nanorods and assemble novel functional devices based on Si-CuO nano-arrays.