Electrical Transport in Individual ZnO Nanorods Studied by Photo-Conductive Atomic-Force Microscopy

One-dimensional ZnO nanostructures exhibit technological potential for many device applications, like efficient low-cost ZnO nanorod-polymer solar cells [1]. Conductive atomic-force microscopy (AFM) is a valuable tool for nanometer-scale electrical characterization of such nanorods [2]. Here, we present a complementary study of electrical transport in individual upright standing ZnO nanorods (NRs) grown by thermal evaporation [4] using conductive AFM (C-AFM) and photoconductive AFM (PC-AFM) [5]. Initially, the electrical properties of the arrays of upright standing ZnO NRs were characterized using two-dimensional current maps measured at different bias voltages applied to the sample contact mode. Further, C-AFM was utilized to determine the local current-to-voltage (I-V) characteristics of the top and side facets of individual upright standing NRs. Further, we pioneered the application of PC-AFM to resolve the photoconductivity spectra measured from a single as-grown ZnO NR. PC-AFM is similar in concept to C-AFM: the sample surface is biased and additionally irradiated from a Xe light source connected to a monochromator. The current through the AFM tip is measured as a function of illumination intensity and/or wavelength. PC-AFM investigations reveal that I-V curves taken from a single upright standing NR under illumination appear more degraded with respect to the non-illuminated state. Analyzing the photoconductivity spectra it has been found that the band gap of ZnO NR is reduced by about 220 meV with respect to the known value of 3.37 eV at room temperature. Using PC-AFM, we also observed persistent photoconductivity from a single ZnO NR. We believe that both phenomena can be attributed to the processes of oxygen desorption/re-adsorption from the ZnO NR surface. Moreover, these observations are in good agreement with theoretical predictions of the influence of oxygen vacancies on the electronic structure of ZnO [6].
Supported by Austrian Science Fund FWF under project # P19636.
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