Novel Electronic and Optoelectronic Interactions in Two-Dimensional Materials

Citation

Hao, Duxing (2025) Novel Electronic and Optoelectronic Interactions in Two-Dimensional Materials. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/wegr-tg72. https://resolver.caltech.edu/CaltechTHESIS:05302025-053900946

Abstract

Two-dimensional (2D) materials host a rich set of emerging physical phenomena such as superconductivity, ferroelectricity, quantum magnetism, and circular dichroism. Moreover, these phenomena are highly tunable by crystalline composition variations and crystalline structural phase modifications and are sensitive to external conditions such as temperature, magnetic field and optical excitation, substrate and gate tuning. Therefore, 2D material-based devices are highly desirable for modern electronic and optoelectronic devices applications. In this thesis, we employed a fully scalable approach to synthesize materials and fabricate 2D material-based devices such as those based on graphene and 1H-Molybdenum disulfide (1H-MoS2), and explore their electronic and optoelectronic properties in cryogenic conditions under various excitation sources, such as external magnetic field and structured light.

In the first part of the thesis (Chapters 2 and 3), we provide experimental details for achieving nanoscale strain engineering of monolayer (ML)-graphene and demonstrate that periodic patterns of nanoscale strain distributions in ML-graphene can lead to local giant pseudomagnetic fields as well as global modifications to the electronic properties of ML-graphene, including strain-induced valley Hall and anomalous Hall effects in the absence of external magnetic fields, nonlocal valley-polarized currents and evidence of quantum valley Hall effect under external magnetic field. These findings suggest new approaches towards developing emerging quantum states with tunable electronic correlation based on graphene straintronics.

The second part of the thesis (Chapters 4 and 5) focus more on the semiconducting monolayer transition metal dichalcogenides (ML-TMDs), whose broken inversion symmetry and strong spin-orbit coupling result in spin-valley lock-in effects so that the valley degeneracy may be lifted by external magnetic fields, potentially leading to real-space structural transformation.

In Chapter 4, we report magnetic field (B)-induced giant electric hysteretic responses to back-gate voltages in ML-MoS₂ field-effect transistors (FETs) on SiO₂/Si at temperatures < 20 K. The observed hysteresis increases with |B| up to 12 T and is tunable by varying the temperature. Raman spectroscopic and scanning tunneling microscopic studies reveal significant lattice expansion with increasing |B| at 4.2 K, and this lattice expansion becomes asymmetric in ML-MoS₂ FETs on rigid SiO₂/Si substrates, leading to out-of-plane mirror symmetry breaking and the emergence of a tunable out-of-plane ferroelectric-like polar order. This broken symmetry-induced polarization in ML-MoS₂ shows typical ferroelectric butterfly hysteresis in piezo-response force microscopy, adding ML-MoS₂ to the single-layer material family that exhibit out-of-plane polar order-induced ferroelectricity, which is promising for such technological applications as cryo-temperature ultracompact non-volatile memories, memtransistors, and ultrasensitive magnetic field sensors. Moreover, the polar effect induced by asymmetric lattice expansion may be further generalized to other ML-TMDs and achieved by nanoscale strain engineering of the substrate without magnetic fields.

In Chapter 5, we further demonstrate the design and application of a novel instrument that integrates scanning spectroscopic photocurrent measurements with structured light of controlled spin and orbital angular momentum. For structured photons with wavelengths between 500 nm to 700 nm, this instrument can perform spatially resolved photocurrent measurements of 2D materials or thin crystals under magnetic fields up to ±14 Tesla, at temperatures from 300 K down to 3 K, with either spin angular momentum (SAM) ℓħ or orbital angular momentum (OAM) ± ℓħ (where ℓ = 1, 2, 3… is the topological charge), and over a (35x25) µm² area with ~ 1 µm spatial resolution. These capabilities of the instrument are exemplified by magneto-photocurrent spectroscopic measurements of monolayer 2H-MoS₂ field-effect transistors, which not only reveal the excitonic spectra but also demonstrate monotonically increasing photocurrents with increasing |ℓ| as well as excitonic Zeeman splitting and an enhanced Landé g-factor due to the enhanced formation of intervalley dark excitons under magnetic field. These studies thus demonstrate the versatility of the scanning photocurrent spectrometry for investigating excitonic physics, optical selection rules, and optoelectronic responses of novel quantum materials and engineered quantum devices to structured light.

Finally, we summarize the research accomplishments of this thesis work in Chapter 6 and discuss the outlook for new research directions associated with these 2D quantum materials.

Thesis Files