This dissertation will explore both inelastic and elastic light-matter interactions of 2D semiconductor TMDs and phosphorene via Raman and PL spectroscopic analysis and device fabrication and characterization. Chapter 2 will discuss experimental techniques and instruments used in this dissertation.
In Chapter 3, Raman and PL spectroscopic analysis of 2D TMDs and phosphorene will be presented. Layer-dependent Raman spectra of TMDs and phosphorene will be discussed to understand their phonon modes. Temperature-dependent Raman spectra of phosphorene will be presented to discuss its thermal property by monitoring phonon modes shifts as temperature decreases. And to investigate the anisotropic property of phosphorene, polarization-resolved Raman spectroscopy will be used. After that, PL spectra for thin-layer TMDs and phosphorene samples will be displayed to discuss their layer-dependent optical bandgap. And then temperature-dependent PL spectroscopy of
Chapter 1: Introduction
thin-layer phosphorene and MoS2 will be presented to discuss their band structure
evolution under the impact of temperature.
In Chapter 4, exciton and trion dynamics in 2D TMDs and phosphorene will be discussed. When laser inelastically interacts with 2D semiconductors, exciton, trion and higher-order complexes will form in 2D semiconductors, and PL spectroscopy is a useful tool to detect those quasi-particles by analysing their photon emissions during recombination. External electric, cryogenic and optical factors will all have impact on and can tune the dynamics of those quasi-particles. In this chapter, exciton and trion dynamics in 2L MoS2, 1L
molybdenum ditelluride (MoTe2), and 1L phosphorene will be presented. Exciton and
trion dynamics in 2L MoS2 and 1L MoTe2 will be tuned by external electric field at
cryogenic temperature. And exciton and trion dynamics in 1L phosphorene will be tuned by direct optical injection.
Chapter 5 will discuss the defect engineering in 2D semiconductors by using 1L phosphorene as the experimental platform. Phosphorene has been reported to be highly unstable in ambient conditions and it can be easily oxidized at the presence of light and moisture.[82, 105] Various techniques have been demonstrated to passivate and stabilize thin-layer phosphorene, including encapsulation by hydrophobic materials and passivation in inert gas environment without exposing to ambient condition.[106-110] Theoretical calculations have revealed that, if oxidized, certain types of oxygen-defects in phosphorene can introduce new valence-band-like or conduction-band-like sub-bands to the electronic bandgap of phosphorene.[111, 112] According to our experimental findings, controllable oxidation in 1L phosphorene by interfacial defects can actually brighten excitons and trigger efficient photon emissions at new wavelengths. Such artificial defect engineering in 1L phosphorene will give a hint on exciton dimensionality modification.
After discussing inelastic light-matter interactions in 2D semiconductors from Chapter 3 to Chapter 5, Chapter 6 will focus on their elastic light-matter interactions. Strong elastic interactions rely on substantial changes of amplitude and phase of lights accumulated along a long optical path, which is normally lacking in 2D graphene[61]. In order to control the flow of light to achieve functional devices by graphene, conventional large optical components are often required, such as prism[113] and Fabry-Perot cavities[114- 116], which to some extent defeats the goal of miniaturization. Our phase-shifting interferometry (PSI) measurement results indicate that thin-layer TMDs, taking MoS2 as
an example, own large optical path length (OPL) values despite of their very thin nature. By taking advantage of such giant OPL, atomically thin optical lenses (also termed as “micro-lens”) and gratings based on thin-layer MoS2 have been fabricated with a dual
beam SEM/focused ion beam (FIB) system. The measured focal length of the fabricated MoS2 micro-lens is consistent with the calculated value. And gratings fabricated from
thin-layer MoS2 exhibit higher diffraction efficiency than those made from silica (SiO2)
and graphene.
Finally, Chapter 7 will give an overall conclusion of this dissertation, and prospects for future work in this field will also be discussed.
Chapter 2: Experimental Techniques
2
Experimental Techniques
This chapter will describe the experimental techniques used to characterize optical properties of 2D semiconductors, to fabricate them into devices and to characterize those devices. Section 2.1 will introduce Raman and PL spectroscopy, including low- temperature and polarization-resolved Raman/PL spectroscopy. Section 2.2 will introduce PSI, which acts as a fast, accurate and non-destructive method to identify the layer number of 2D semiconductors by measuring their OPL values. Section 2.3 will introduce atomic force microscopy (AFM), which is used to measure the sample thickness of 2D semiconductors. Section 2.4 will introduce time-resolved PL (TRPL) spectroscopy. Section 2.5 will introduce Fourier transform infrared (FTIR) spectroscopy, and Section 2.6 will introduce far-field scanning optical microscope (SOM) used for MoS2 micro-lens
characterization. Section 2.7 will discuss device fabrication techniques used in this dissertation, including plasma-enhanced chemical vapour deposition (PECVD) used to deposit defect-rich silicon oxide thin film, photolithography and metal deposition techniques used to pattern Au electrodes on SiO2/Si substrates, sample preparation
method used to prepare metal-oxide-semiconductor (MOS) devices, and a dual beam SEM/FIB system, which is used to fabricate micro-lens and gratings from thin-layer MoS2.