Spatially resolved photoresponse on individual ZnO nanorods: correlating morphology, defects and conductivity

Abstract

Electrically active native point defects have a significant impact on the optical and electrical properties of ZnO nanostructures. Control of defect distribution and a detailed understanding of their physical properties are central to designing ZnO in novel functional forms and architecture, which ultimately decides device performance. Defect control is primarily achieved by either engineering nanostructure morphology by tailoring growth techniques or doping. Here, we report conducting atomic force microscopy studies of spatially resolved photoresponse properties on ZnO nanorod surfaces. The photoresponse for super-band gap, ultraviolet excitations show a direct correlation between surface morphology and photoactivity localization. Additionally, the system exhibits significant photoresponse with sub-bandgap, green illumination; the signature energy associated with the deep level oxygen vacancy states. While the local current-voltage characteristics provide evidence of multiple transport processes and quantifies the photoresponse, the local time-resolved photoresponse data evidences large variations in response times (90 ms–50 s), across the surface of a nanorod. The spatially varied photoconductance and the range in temporal response display a complex interplay of morphology, defects and connectivity that brings about the true colour of these ZnO nanostructures.

Negative photoresponse in ZnO–PEDOT:PSS nanocomposites and photogating effects

“…However the PR of the nanowires turned positive under visible light excitation, contrary to the present observations. Positive PR under UV and visible light excitation has been reported earlier by the authors for macroscale heterojunctions of ZnO-PEDOT:PSS 10 and on the nanoscale for ZnO-Pt 11,12 though the negative PR reported here is only observed for the nanocomposite conguration presented here.…”

“…Constant current mode detection of PR allows us to directly monitor variation in accumulated charge (Q J ) with junction potential (V J ) following Q J ¼ C J V J . 11 Note that even for the planar junctions the decay time constants are in minutes which are longer than that of bare ZnO, and the trend of increasing C J with decreasing reverse bias extrapolates to still longer s at zero bias, which is the case in the NC investigated here.…”

“…2(b) shows the variation of dark resistance of the NC lms for various concentrations of ZNPs. The positive PR of ZnO, especially in its nanostructured form with a large surface to volume ratio, is understood to originate from two sources, rst, photogeneration of electrons excited across its band gap, 10,11 which also affects the PR of n-ZnO based pn heterojunctions i.e. with PEDOT:PSS 10 or p-Si 30 and Schottky junctions with Au or Pt 31 etc.…”

“…Analysis of the response time, following that discussed in ref. 11, yields a rise time constant (s r ) and two distinct fast and slow decay time scales s d1 and s d2 . The time constants vary with the wt% of ZNPs, though s r and s d1 are approximately an order of magnitude smaller than s d2 .…”

“…Wide band gap semiconductors such as ZnO, 1,2 TiO 2 , 3,4 ITO 5,6 and GaN 7 receive signicant attention due to their optoelectronic properties with immense potential for use in optical detection, light harvesting, storage, memory devices and other photovoltaic applications, especially in their nanostructured forms. 8,9 Interestingly, the efficiency of the ensuing devices and their response can be signicantly tailored and tuned by incorporating these semiconductors as Schottky or pn junction components and focussing on the optoelectronics of the junction, [10][11][12] rather than that of the bulk. Further, nanocomposites (NCs) fabricated by encapsulating their nanostructured forms in suitable materials have been shown to offer greater exibility and scope e.g.…”