Contributed Talk - Thursday, 16 September I 10:20 AM (CEST)
Federica Rigoni: "Nanoscale electrical characterization of all oxide 1D heterojunctions by advanced conductive AFM techniques"
F. Rigoni, S. Frroku, C. De Melo, P. Ghamgosar, I. Concina, D. Horwat, A. Vomiero
Ca' Foscari University of Venice, Italy
This work investigates the nano-electrical properties of all-oxide heterojunction nanowires (NWs) for photovoltaic applications, by means of Amplitude Modulation Kelvin Probe Force Microscopy (AM-KPFM). Three p-Cu2O/n-TiO2 heterojunction NWs and one TiO2 (bare) NWs samples were investigated. The Cu2O light absorber layer was deposited by atomic layer deposition (ALD). The main reason behind the choice of a one-dimensional structure is related to their long axis which is responsible for the absorption of the incident light and the very short radial distance for the separation of photogenerated charge carriers. AM-KPFM measurements have been carried out to map nanoscale surface potential (SP) of Cu2O/TiO2 and TiO2 NWs under ambient condition and under nitrogen atmosphere, tracking the SP variation in dark and under visible light illumination. When the samples were illuminated an increase of the SP was observed. Among the three p-Cu2O/n-TiO2 NWs tested in dark/light conditions, the best results, in terms of SP increase and response and recovery time, were obtained in the case of the thinner p-Cu2O deposited layer (named sample ALD208). Spikes in the SP signal were detected, indicating the accumulation of holes (light on) and electrons (light off). The same test performed on the TiO2 (bare) NWs, gave a surface potential increase of 150 mV under light exposure but a very slow recovery time after switching the light off were observed. The main explanation for this behaviour is related with the oxygen vacancies which form a donor level below the conduction band. Conductive atomic force microscopy investigation was carried out to obtain the current distribution on the surface of the sample. As in the first case the best result were obtained in the case of ALD208 which shows a current increase of 6 nA under illumination. In order to eliminate the effect of the humidity (which shield the surface charges), the same measurements were carried out in nitrogen atmosphere. The results demonstrated that the response time in ALD208 was improved and in the case of TiO2 (bare) NWs it was lowered, and this could be explained by the elimination of O2 or humidity from the testing site. Additional characterization by scanning electron microscopy showed a sample with a uniform topography where the average diameter of the NWs was (84 ± 5) nm in the case of TiO2 (bare) and (118 ± 5) nm in the case of Cu2O/TiO2. The Micro Raman spectroscopy confirmed the presence of the dominant peaks of titania (rutile phase) at 240, 430 and 630 cm⁻¹ and the presence of cuprous oxide 218 and 148 cm⁻¹.