Photocurrent generation in low dimensional nanomaterials
Author(s)
Advisor
Kasırga, Talip SerkanDate
2022-11Publisher
Bilkent University
Language
English
Type
ThesisItem Usage Stats
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Abstract
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.
Keywords
Scanning photocurrent microscopySilver nanowire
Nanowire network
Plasmon
Bolometric effect
Molybdenum disulfide
Photoresponse
Photothermoelectric effect
Seebeck coefficient
Raman spectrocopy
2D materials