Multidimensional Tuning of the Electronic and Optical Properties of Two-dimensional Semiconductors

Abstract

Two-dimensional (2D) transition metal dichalcogenides (TMDCs) are a novel family of semiconductor materials, characterized by their atomic thickness and unique mechanical, electronic, and optical properties. Owing to their 2D nature, the binding energy of excitons in these materials is significantly larger than that of their bulk counterparts, reaching hundreds of meV. This ensures that the excitons in 2D TMDCs stay robust even at room temperature, making them an excellent platform for investigating the properties of various excitonic states at ambient conditions and exploring many optoelectronic applications. As the atomic-level materials, the composition and layer number of 2D TMDCs are predetermined upon preparation, particularly by mechanical exfoliation. Nevertheless, it is encouraging to note that these properties can be significantly modulated by external fields, such as strain, electric, or magnetic fields. This enables us to control the properties of 2D materials across a broader range of dimensions, thereby broadening their potential applications. Therefore, the analysis of multiple physical mechanisms for tuning properties of those 2D TMDCs is important to realize multidimensional tuning of the properties (electronic or optical properties) of 2D semiconductors.

The major results of this thesis is structured into three chapters, including (1) developing a modified tight-binding model (TBM) to elucidate the strain effects in the electronic properties of 2D materials; (2) realizing charged biexciton emission from monolayer WS₂ at room temperature through hot-carrier injection; and (3) exploring the influence of external electrostatic and pseudo-electric fields (due to hot-carrier injection) on the nonlinear optical properties of 2D materials. I will preface these with a brief introduction of the electronic and optical properties of 2D TMDCs. At the end of this thesis, I will conclude with a summary of the current work and a perspective on future research.

In the first work, strain engineering in 2D TMDCs is discussed and strain field is selected to be the external stimulus to tune the optical and electronic properties of 2D TMDCs. Generally speaking, the tensile strain will weaken the electron wavefunction overlap and decrease the bandgap. Although the widely-applied density functional theory (DFT) has emerged as an accurate method for describing the electronic properties of materials, such as band structure and density of states, it does have certain limitations. Notably, it tends to be time-consuming, particularly when dealing with cases under inhomogeneous strains. Hence, we develop a TBM that incorporates strain effect to characterize the band structure tuning of monolayer MoX₂ and WX₂ under biaxial strain fields (from -0.5% to 1.5%); strain-dependent Slater-Koster (SK) parameters are employed to describe electron hopping and orbital overlap under external strain field. Our approach follows from the Wills-Harrison suggestions of a linear relationship between strain level and SK parameters. Our results can well describe the electronic properties including the bandgap type and size and the energy differences between different high-symmetry points in k-space. In the end, we also use this model to predict some properties (carrier effective mass and optical conductivity) of TMDCs monolayer under strain field.

In the following work on the charged biexciton generation at room temperature, we opt for hot-electron injection as an external method to control the optical properties of 2D materials. We select plasmonic nanostructures as the hot electrons source, placing gold nanospheres with 120 nm diameter directly on the monolayer WS₂. Due to the non-radiative decay of plasmons, a significant number of hot electrons are generated, which can then exchange energy with the 2D material in contact through hot-electron injection. In our work, under on-resonant localized surface plasmon (LSP) excitation at 2.08 eV, the photoluminescence (PL) intensity shows a superlinear scaling with the laser power density, signifying the existence of charged biexcitons; On the contrary, the off-resonant LSP excitation at 2.33 eV results in no detectable PL signal of the charged biexcitons. To our knowledge, this work marks the first instance of achieving charged biexciton emission in 2D TMDCs materials at room temperature, with a determined binding energy of 25 meV for this excitonic state under such conditions. Simultaneously, this serves as compelling evidence that hot-carrier injection is an effective means of modulating the carrier characteristics of the 2D materials it interfaces with.

Finally, we shift our focus to the nonlinear optical responses of 2D materials. Owing to their distinctive lattice structures and stacking configurations, monolayer TMDCs exhibit robust in-plane-polarized second-harmonic generation (SHG), but lack out-of-plane-polarized SHG. To address this issue, we propose to apply an out-of-plane electric field to break the out-plane mirror symmetry of monolayer TMDCs. Our DFT calculations show that this can induce a significant out-of-plane second-order nonlinear coefficient (Χ_zxx) of monolayer TMDCs, yet with minimal impact on their intrinsic in-plane nonlinear coefficient (χ_yyy). In the case of naturally stacked bilayer TMDCs that lack both in-plane and out-of-plane SHG responses, the situation is somewhat different. We find that applying an out-of-plane electric field disrupts their inversion symmetry, thereby simultaneously inducing SHG originating from both in-plane and out-of-plane second-order nonlinear optical susceptibilities. In experiment, we transferred an MoS₂ bilayer to a gold film with a standard wet transfer method, and then drop-cast Au nanospheres (AuNSs) to form AuNS–MoS₂-Au-film nanocavities. Under resonant excitation, this structure allows efficient plasmonic hot-electron injection as an external source to vary the pseudo-electric field strength across the bilayer by controlling the number of injected hot electrons. As a result, we are able to manipulate the intensity of SHG from bilayer TMDCs and also roughly determine the magnitude of the induced z-component second-order nonlinear optical susceptibility by analyzing excitation-polarization-dependent SHG. This study creates new opportunities for ultrafast all-optical control of SHG in 2D TMDCs.

Date of Award	30 Jul 2024
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Dangyuan LEI (Supervisor)