SYNTHESIS, ELECTRONIC AND OPTO-ELECTRONIC TRANSPORT PROPERTIES OF ATOMICALLY THIN 2D LAYERS OF MoS2, WSe2 and CuIn7Se11
Date of Award
Doctor of Philosophy
The recent emergence of a new class of two dimensional layered materials (2DLMs) have not only opened up the potential for exciting new technological opportunities but also established a new platform to explore exciting new fundamental physics and chemistry at the limit of atomic thickness. Among several of these newly rediscovered 2DLMs, transition metal dichalcogenides (TMDCs) as well as other elemental combinations of Group III and Group VI represent a large family of 2D layered materials, which can be isolated into few atomic layers. These materials show remarkable promise for future electronic and opto-electronics applications. The scope of this dissertation, thus, broadly covers the electronic and opto-electronic properties of such few layered 2D materials. Extensive investigation of electronic and opto-electronic transport phenomena of charge carriers in few layer MoS2 synthesized using a variety of methods such as Chemical Vapor Deposition (CVD), liquid phase exfoliation and mechanical exfoliation as well as CVT grown mechanically exfoliated WSe2 and ternary alloy of CuIn7Se11 is reported. Specifically, it is shown that in case of MoS2, the ac conductance (σ(ω); measured in the range of 10mHz < ω < 0.1 MHz) of atomically thin 2D layers of chemical vapor deposited (CVD) Molybdenum Disulphide (MoS2) as well as thin films of exfoliated flakes of MoS2, show "universal" power law behavior (with σ(ω) ~ ωs). The temperature dependence of 's' indicate that the mechanism of ac transport in CVD MoS2 is due to electron hopping by quantum mechanical tunneling (QMT) process whereas the ac transport in exfoliated MoS2 films is due to correlated barrier hoping (CBH) mechanism. The ac conductivity also show scaling behavior, manifested by collapse of the ac conductivity data for both the samples at various temperatures to one single master curve. The T-γ dependence of the d.c conductance suggests that in case of the CVD – grown and mechanically exfoliated MoS2, γ=1/3 which corresponds to the Mott’s variable range hopping (VRH) transport where as in case of liquid phase exfoliated MoS2, γ=1 which relates to thermally activated Arrhenius type transport mechanism. Opto-electronic measurements were also performed in a variety of 2DLM samples. From the field effect transport measurements on the mechanically exfoliated samples of few layers of MoS2, WSe2 and CuIn7Se11, we found at room temperature the charge carrier mobility is ~ 47 cm2/V.s, 80 cm2/V.s and 37 cm2/V.s for MoS2, WSe2 and CuIn7Se11 respectively. The photoconductivity measurements performed on these samples show that it is possible to achieve photo-responsivities values~50 μA/W, 0.2 A/W, 1 A/W and 51 A/W at room temperature for liquid exfoliated MoS2, mechanically exfoliated MoS2, WSe2 and CuIn7Se11 based devices respectively. Mechanisms of photoconduction in these materials were explained on the basis of intensity dependent photo-current measurements. From the intensity dependent photo-current along with the low temperature photoconduction measurements we found that in case of liquid phase exfoliated MoS2 thin film devices the trap states are continuously distributed within the mobility gap in these thin film of MoS2, and play a vital role in influencing the overall photo response. On the other hand for CVT grown mechanically exfoliated WSe2 based devices bimolecular recombination mechanism is the most dominant process for photoconduction. The result obtained are discussed and compared with the available literature on the subject.
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