Nonlinear Two-Photon Photocurrent Spectroscopy of CdS Nanosheets
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Nonlinear Two-Photon Photocurrent Spectroscopy of CdS Nanosheets Parveen Kumar1, Aaron Wade1, Leigh Morris Smith1, Howard E Jackson1, Jan M YarrisonRice2, Young-Jin Choi3, Jae-Gwan Park3 1
Department of Physics, University of Cincinnati, Cincinnati, Ohio 45221-0011, USA Department of Physics, Miami University, Oxford, Ohio 45056, USA 3 Nano Materials Research Center, Korea Institute of Science and Technology, Seoul 130-650, Korea 2
Abstract We study the photocurrent from photoexcited charged carriers excited with lasers of energy both above and below the energy gap in CdS nanostructures. We observe non-linear photocurrents in CdS nanosheet devices in the metal-semiconductor-metal configuration with Schottky contacts for sub-band gap excitations. Analysis of two-photon absorption dominated photocurrents reveals a nonlinear coefficient of β = 2 cm/GW for these nanosheet devices, which is comparable to those of bulk CdS. We demonstrate the use of the photocurrent polarization measurements to determine the orientation of atoms in the nanosheet.
Introduction CdS nanostructures have attracted significant research interest because of its wide band gap, low cost, ease of manufacturing and interesting optical and electronics properties [1-14]. In the past decade, the demonstration of large functional CdS nanostructure devices proved CdS as building blocks for the construction of nanodevices like transistors [1], light emitting diodes [2], and gas and chemical sensors [3]. These devices have some exceptional optoelectronic properties that include intrinsic polarization sensitivity [4], low-threshold lasing [5], enhanced mobility [6, 7], and high photosensitivity [8]. Because of the wide band gap of CdS, we can use one photon (one photon creates an electron-hole pair) and nonlinear two photon (two photons create an electronhole pair) excitation for opto-electronics characterization. The nonlinear optical properties of nanoscale devices are expected to be highly dependent on the details of the nanostructure [9] although these effects have not been extensively explored. In this paper we demonstrate the use of photocurrent polarization sensitivity to determine the orientation of the crystal structure of these nanostructures and characterize linear and nonlinear optical properties of CdS nanosheets (NSs). Experimental Details The CdS nano-structures are grown using gold catalyst-assisted vapor phase transport growth method at 800ͼC for 20 min [10]. As grown CdS nanosheets are sonicated in methanol and dispersed onto a thick insulated Si-SiO2-Si3N4 substrate. Photolithography followed by Ti/Al (20nm/200nm) metal evaporation and lift-off is used to fabricate Schottky contacts on both sides of NS in metal-semiconductor-metal (MSM) (shown in Figure 1 inset) configuration. A HP 34401A digital multimeter is used to record current as a function of bias in the dark and under
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white light illumination. A CW Ar+ laser tuned at 458nm was used for above band gap excitation whereas 800 nm 200 fs pulses from a mode-locked Ti-Sapphire la
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