Degree Type

Dissertation

Date of Award

2018

Degree Name

Doctor of Philosophy

Department

Mechanical Engineering

Major

Mechanical Engineering

First Advisor

Xinwei Wang

Abstract

The bottleneck of most modern technologies and energy solutions has been attributed to the thermal problems at the nanoscale. Especially, the thermal transport across interfaces and in-plane direction can significantly influence the overall performance of 2D nanosystems. So accurate thermal-physical characterization of the 2D materials is very important for both fundamental research and industrial applications.

Focusing on 2D mechanically exfoliated MoS2, at first, we conduct a detailed temperature and laser power dependent micro-Raman spectroscopy study of FL MoS2 (4.2 to 45 nm thick) on c-Si substrate. We measured the interfacial thermal resistance (R) at room temperature decreases with increased layers of MoS2. Furthermore, we find that the number of layers of MoS2 deeply affects the film corrugation, morphology, and interfacial thermal resistance. Then, for the first time, we consider the hot carrier excitation, diffusion and recombination in Raman 2D MoS2-substrate interface energy coupling study. The hot carrier diffusion could become significant when the diffusion length is long or laser heating spot size is small. By applying different laser heating sizes in Raman experiment, we could determine both R and the hot carrier diffusivity for four sub-10 nm thick MoS2 (3.6 to 9.0 nm thick). Especially, the hot carrier diffusion study is conducted without applying an electric field or electrical contacts so the results reflect the intrinsic properties of virgin 2D materials. After that, we realize that in widely applied Raman characterization of 2D material interface thermal resistance, laser absorption in the 2D atomic layer and its absolute temperature rise are needed. These factors could cause the largest experimental uncertainty. To this end, we develop a novel energy transport state-resolved Raman (ET-Raman) to address these critical issues. In ET-Raman, under steady laser heating, by constructing two steady heat conduction states withdifferent laser spot sizes, we differentiate the effect of R and hot carrier diffusivity (D). By constructing an extreme state of zero/negligible heat conduction using a picosecond laser, we differentiate the effect of R and material’s specific heat. Combining the steady state Raman and pico-second Raman, we precisely determine R and D without the need of laser absorption and temperature rise of the 2D atomic layer. Seven MoS2 samples (6.6 nm to 17.4 nm) on c-Si and six MoS2 samples (1.8 nm to 18 nm) on glass substrate prepared by mechanical exfoliation are characterized using ET-Raman. At last, in order to reduce the dependence on other’s work, we developed another new technique that could simultaneously determine k, D, and R of eight MoS2 samples ranging from 2.4 nm to 37.8 nm thickness. The in-plane thermal conductivity could be determined without referring other’s work. Besides, this systematic non-contact thickness-dependent thermal conductivity study reveals the intrinsic properties of FL MoS2 and provides a practical guide for further advancing MoS2 based device technologies.

Copyright Owner

Pengyu Yuan

Language

en

File Format

application/pdf

File Size

147 pages

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