热氧化溅射镀铜膜制备CuO纳米棒及其光致发光特性

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

CuO纳米结构的物理化学性质使其成为纳米光电子器件、气敏传感器和光检测器等微/纳米器件应用中的关键材料.虽然热氧化法具有简单、高效、低成本和大范围制备等优点,但加热导致的崩裂问题仍限制了其广泛应用,目前仍需发展可在不同基体(特别是半导体Si)上直接生长CuO纳米结构的制备技术.本文通过先直流/射频磁控溅射镀金属膜,再电场辅助热氧化法的方法,成功在Si基上获得了膜基结合力好的CuO纳米棒,使用扫描电子显微镜(SEM)、X射线衍射(XRD)和透射电子显微镜(TEM)分析了Cu膜和CuO的形貌和结构,并用荧光光谱仪测量了CuO纳米棒的室温光致发光性能.研究结果表明:直流磁控溅射获得了择优取向为(111)的柱状晶Cu膜,工艺改进后可完全氧化获得CuO纳米棒/CuO层/Si基的层状结构,其中纳米棒为单晶单斜CuO.本文还通过进一步添加Cr作为过渡层来改善膜-基结合力,解决加热Si-Cu膜时因热应力而导致的开裂问题.虽然射频溅射可在Si基底上获得细晶粒Cu膜,但热氧化后并没有获得一维纳米结构的氧化物.当以紫外光作为激发光源时,CuO纳米棒的发射峰主要为宽的390~470 nm间的深紫到亮蓝发光带,峰位的蓝移主要是源于纳米尺寸的下降.本文探究了CuO纳米棒的制备和光学性能,有助于设计和组装基于Si基-CuO纳米阵列的新型纳米器件.

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 Fluoro Max-4 fluorescence spectrometer.The results show that a columnar Cu film with the thickness of 1.25μm 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 0.5μm 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.