Photocurrent generation in low dimensional nanomaterials

Kasırga, Talip Serkan
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Bilkent University
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This thesis focuses on crucial issue on the understanding the underlying mechanisms of photoresponse in low-dimensional nanomaterials. As the size goes down to the micro and nano level, fine features and induced inhomogeneities like strain, thickness variation, substrate, and junctions become influential in determining plausible effects that can explain and control the light-matter interactions in an optoelectronic device. To develop a better understanding of the fundamental physical characteristics of nanomaterials and optimize thermal and electrical transport in nanomaterial devices, microscopic investigation at a single crystal level is required. In this thesis, I investigated photocurrent generation in two extreme cases: metallic silver nanowire (Ag NW) and semiconducting multilayer molybdenum disulfide (MoS2) using scanning photocurrent microscopy (SPCM). SPCM provides spatial mapping of photoresponse along with corresponding reflected light intensity with a few hundred nanometer resolution. Two terminal devices of Ag and Ag network devices are made by drop-casting NW and placing indium as metal contacts. The SPCM maps show that the NW- NW junctions and NW-contacts interface locally enhance the plasmonic field and act as hot spots. The increased temperature at hot spots is enough to modulate the resistance and results in a photo-bolometric response under the bias voltage. To further enhance the photo-bolometric effect, we decorated the nanowires with plasmonic Ag nanoparticles. The nanoparticles increase the number of hot spots and strengthen light coupling into plasmons. We also attributed zero bias response to the photothermoelectric effect. The photocurrent is generated by the Seebeck coefficient difference caused by nanogaps and nonuniformities in the geometry along the Ag NW. The second part of this thesis describes photocurrent generation by substrate engineering of a few-layer MoS2. To partially suspend a crystal, a flake of MoS2 is exfoliated and then transferred on a substrate with rectangular or circular holes. We observed photocurrent generation from the junction of the supported and suspended parts. Substrate effects like induced doping play an essential role in determining the properties of two-dimensional materials. Our investigations show that the Seebeck coefficient of the suspended part is changed due to isolation from the substrate. The difference in the Seebeck coefficient of suspended and supported regions forms a thermoelectric junction. We also investigated the impact of carrier type and concentration on photocurrent generation by gating experiments.

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Scanning photocurrent microscopy, Silver nanowire, Nanowire network, Plasmon, Bolometric effect, Molybdenum disulfide, Photoresponse, Photothermoelectric effect, Seebeck coefficient, Raman spectrocopy, 2D materials
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