Degree Type

Dissertation

Date of Award

2018

Degree Name

Doctor of Philosophy

Department

Mechanical Engineering

Major

Mechanical Engineering

First Advisor

Xinwei Wang

Abstract

As a new two-dimensional material, black phosphorus has attracted worldwide attention due to its extraordinary electronic and optoelectronic properties. Despite its importance for the performance of electronic devices, thermal properties of black phosphorus are still not well studied, which leads to incomplete understanding on phonon transport and interaction in black phosphorus. This is related to difficulties for preparing black phosphorus samples since black phosphorus is easily oxidized in air and the difficulty in characterizing this nm-thin material. In this work, black phosphorus is studied systematically with techniques developed based on Raman spectroscopy. Our study on black phosphorus involves identifying its crystalline orientation and measuring its interface thermal conductance and anisotropic in-plane thermal conductivity.

In this work, the crystalline orientation of black phosphorus is identified with a newly developed technique, optothermal Raman spectroscopy. This technique utilizes the anisotropic heating effect of a linear polarized laser for crystalline orientation identification. It can distinguish the armchair direction and zigzag direction precisely regardless of excitation wavelength and sample thickness. This in-situ and nondestructive technique is required to identify the crystalline orientation of black phosphorus samples before their interface thermal conductance and anisotropic in-plane thermal conductivity are measured. Interface thermal conductance between black phosphorus and Si is measured with micro-Raman spectroscopy. It is found there is large interface thermal conductance between black phosphorus and its adjacent Si, which suggests black phosphorus can be used as new interface material for future devices. Also, interface thermal conductance shows a strong negative correlation to temperature, while no correlation to thickness. These results lead to the discovery of the temperature-related morphological variation of supported black phosphorus on Si. Frequency-resolved Raman spectroscopy is developed to measure the anisotropic in-plane thermal conductivity of black phosphorus. This technique is first used to measure thermal diffusivity of a c-Si cantilever, which has a reference value of thermal diffusivity. The validity of this technique is sufficiently verified since our measured thermal diffusivity is very close to its reference value. Our measurements on black phosphorus with thickness between 99.8 and 157.6 nm show the armchair thermal conductivity is 13.5~22 W m-1 K-1, and the zigzag thermal conductivity is 39.8~62.7 W m-1 K-1. These studies significantly advance our fundamental understanding of phonon transport and interaction in black phosphorus, which can further benefit the development of new-generation devices.

DOI

https://doi.org/10.31274/etd-180810-6113

Copyright Owner

Tianyu Wang

Language

en

File Format

application/pdf

File Size

94 pages

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