Photoresponse from single upright-standing ZnO nanorods explored by photoconductive AFM

Igor Beinik,Christian Teichert,Yuk Fan Hsu,Astrid Wachauer,Markus Kratzer,Lin Wang,Gerhard Brauer,Xin Yi Chen,Aleksandra B Djurišić,Yuri P Piryatinski

doi:10.3762/bjnano.4.21

Abstract

Background: ZnO nanostructures are promising candidates for the development of novel electronic devices due to their unique electrical and optical properties. Here, photoconductive atomic force microscopy (PC-AFM) has been applied to investigate transient photoconductivity and photocurrent spectra of upright-standing ZnO nanorods (NRs). With a view to evaluate the electronic properties of the NRs and to get information on recombination kinetics, we have also performed time-resolved photoluminescence measurements macroscopically.Results: Persistent photoconductivity from single ZnO NRs was observed for about 1800 s and was studied with the help of photocurrent spectroscopy, which was recorded locally. The photocurrent spectra recorded from single ZnO NRs revealed that the minimum photon energy sufficient for photocurrent excitation is 3.1 eV. This value is at least 100 meV lower than the band-gap energy determined from the photoluminescence experiments.Conclusion: The obtained results suggest that the photoresponse in ZnO NRs under ambient conditions originates preferentially from photoexcitation of charge carriers localized at defect states and dominates over the oxygen photodesorption mechanism. Our findings are in agreement with previous theoretical predictions based on density functional theory calculations as well as with earlier experiments carried out at variable oxygen pressure.

Highlights

One-dimensional ZnO nanostructures, so called ZnO nanorods (NRs), exhibit technological potential for many device applications
We focus on the investigation of opto-electronic properties of single as-grown ZnO NRs by means of conductive force microscopy (C-AFM) and photoconductive atomic force microscopy (PC-AFM) techniques
PL emission was detected during the laser excitation pulse either at the leading edge of the laser excitation pulse, or at the trailing edge of the laser pulse after a variable delay time with respect to the onset of the laser pulse. As it is determined from the AFM image and the scanning electron microscopy (SEM) micrograph presented in Figure 2a, the ZnO NR diameters vary in a wide range from 150 nm to 1.2 μm, whereas the rod lengths are ≈1.5 μm

Summary

Introduction

One-dimensional ZnO nanostructures, so called ZnO nanorods (NRs), exhibit technological potential for many device applications. A common feature of wide-band-gap semiconductors, such as ZnO, GaN, etc., is the presence of deep levels in the forbidden gap. The appearance of such levels as well as the density of electronic states associated with them depends on the number of defects within a semiconductor and is determined very often by the growth conditions [17,18]. The surface conductivity of ZnO is highly dependent on the presence of adsorbates [20,21,22,23] Such surface defects serve as binding sites for chemisorption processes and may contribute to the scattering and trapping of carriers [24], which lower the intrinsic conductivity of the material. With a view to evaluate the electronic properties of the NRs and to get information on recombination kinetics, we have performed time-resolved photoluminescence measurements macroscopically

Results

Discussion

Conclusion