Pressurizing Field-Effect Transistors of Few-Layer MoS2 in a Diamond Anvil Cell

Hydrostatic pressure applied using diamond anvil cells (DAC) has been widely explored to modulate physical properties of materials by tuning their lattice degree of freedom. Independently, electrical field is able to tune the electronic degree of freedom of functional materials via, for example, the field-effect transistor (FET) configuration. Combining these two orthogonal approaches would allow discovery of new physical properties and phases going beyond the known phase space. Such experiments are, however, technically challenging and have not been demonstrated. Herein, we report a feasible strategy to prepare and measure FETs in a DAC by lithographically patterning the nanodevices onto the diamond culet. Multiple-terminal FETs were fabricated in the DAC using few-layer MoS2 and BN as the channel semiconductor and dielectric layer, respectively. It is found that the mobility, conductance, carrier concentration, and contact conductance of MoS2 can all be significantly enhanced with pressure. We expect that the approach could enable unprecedented ways to explore new phases and properties of materials under coupled mechano-electrostatic modulation.Comment: 15 pages, 5 figure

Exploring van der Waals Interactions and Deep Levels in Two-Dimensional Semiconductors via Band-Structure Engineering

Recent advances in two-dimensional materials have enabled the emergence of rich physics at the nanoscale due to the interlayer coupling and strong quantum confinement of phonons, electrons, and other particles. Meanwhile, their ultra-thin thickness makes them appealing for potential channel materials in the field-effect transistors and hence draws tremendous attention in the semiconductor technology. Modulation of conduction and valence bands in semiconductors is among the key parameters to engineer and investigate their electronic, optical and thermal properties. Therefore, in this dissertation, hydrostatic pressure and alloying, two well-known and influential methods of tuning the band structure, were utilized to probe the evolution of physical properties in the van der Waals (vdW) crystals. The first part of this dissertation focuses on quantifying van der Waals interactions in layered transition metal dichalcogenides (TMDs) from pressure-enhanced valence band splitting. Despite being relatively weak across the gap between neighboring layers, the vdW forces play a crucial role in the band structure evolution in the layered materials, hence profoundly affecting their physical properties. Therefore, gauging and controlling vdW interactions is essential for understanding the physics and developing applications of these materials. In this dissertation, I experimentally probed the vdW interactions in MoS2 and other TMDs by modulating their band structure and measuring the valence band maximum (VBM) splitting (Δ) at K point as a function of pressure in a diamond anvil cell. As high pressure increases interlayer wavefunction coupling, the VBM splitting is enhanced in 2H-stacked MoS2 multilayers but, due to its specific geometry, not in 3R-stacked multilayers, hence allowing the interlayer contribution to be separated out of the total VBM splitting, as well as predicting a negative pressure (2.4 GPa) where the interlayer contribution vanishes. This negative pressure represents the threshold vdW interaction beyond which neighboring layers are electronically decoupled. This approach is compared to first-principles calculations and found to be widely applicable to other group-VI TMDs.In chapters 3 and 4, I described chemical trends of deep levels by alloying the MoS2 and WS2. Properties of semiconductors are largely defined by crystal imperfections including native defects. VdW semiconductors are no exception: defects exist even in the purest materials and strongly affect their electrical, optical, magnetic, catalytic, and sensing properties. However, unlike traditional semiconductors where energy levels of most defects were thoroughly documented, they are experimentally unknow in even the best-studied vdW semiconductors, impeding the understanding and utilization of these materials. Thus, in this dissertation, I directly evaluated deep levels in the bandgap of intrinsic MoS2, WS2, and their alloys by transient spectroscopic study. One of the deep levels is found to follow the conduction band minimum of each host, attributed to the native sulfur vacancy. A switchable, DX center - like deep level has also been identified, whose energy lines up instead on a fixed level across different host materials, which explains the chemical trend of native electron density in the hosts as well as a persistent photoconductivity up to 400K

High-order face-shear modes of relaxor-PbTiO3 crystals for piezoelectric motor applications

The face-shear vibration modes of [011] poled Zt ± 45° cut relaxor-PT crystals and their applications for linear piezoelectric motors were investigated. Unlike piezoelectric ceramics, the rotated crystal was found to exhibit asymmetric face-shear deformations, and its two high-order face-shear modes degraded into two non-isomorphic modes. As an application example, a standing wave ultrasonic linear motor (10 × 10 × 2 mm3) operating in high-order face-shear vibration modes was developed. The motor exhibits a large driving force (1.5 N) under a low driving voltage (22 Vpp). These findings could provide guidance for design of crystal resonance devices

Band Engineering of Large-Twist-Angle Graphene/h-BN Moiré Superlattices with Pressure

Graphene interfacing hexagonal boron nitride (h-BN) forms lateral moiré superlattices that host a wide range of new physical effects such as the creation of secondary Dirac points and band gap opening. A delicate control of the twist angle between the two layers is required as the effects weaken or disappear at large twist angles. In this Letter, we show that these effects can be reinstated in large-angle (∼1.8°) graphene/h-BN moiré superlattices under high pressures. A graphene/h-BN moiré superlattice microdevice is fabricated directly on the diamond culet of a diamond anvil cell, where pressure up to 8.3 GPa is applied. The band gap at the primary Dirac point is opened by 40-60 meV, and fingerprints of the second Dirac band gap are also observed in the valence band. Theoretical calculations confirm the band engineering with pressure in large-angle graphene/h-BN bilayers

Breaking Rotational Symmetry in Supertwisted WS2 Spirals via Moiré Magnification of Intrinsic Heterostrain

Twisted stacking of van der Waals materials with moiré superlattices offers a new way to tailor their physical properties via engineering of the crystal symmetry. Unlike well-studied twisted bilayers, little is known about the overall symmetry and symmetry-driven physical properties of continuously supertwisted multilayer structures. Here, using polarization-resolved second harmonic generation (SHG) microscopy, we report threefold (C3) rotational symmetry breaking in supertwisted WS2 spirals grown on non-Euclidean surfaces, contrasting the intact symmetry of individual monolayers. This symmetry breaking is attributed to a geometrical magnifying effect in which small relative strain between adjacent twisted layers (heterostrain), verified by Raman spectroscopy and multiphysics simulations, generates significant distortion in the moiré pattern. Density-functional theory calculations can explain the C3 symmetry breaking and unusual SHG response by the interlayer wave function coupling. These findings thus pave the way for further developments in the so-called "3D twistronics"

A laser-assisted chlorination process for reversible writing of doping patterns in graphene

Chemical doping can be used to control the charge-carrier polarity and concentration in two-dimensional van der Waals materials. However, conventional methods based on substitutional doping or surface functionalization result in the degradation of electrical mobility due to structural disorder, and the maximum doping density is set by the solubility limit of dopants. Here we show that a reversible laser-assisted chlorination process can be used to create high doping concentrations (above 3 × 1013 cm−2) in graphene monolayers with minimal drops in mobility. The approach uses two lasers—with distinct photon energies and geometric configurations—that are designed for chlorination and subsequent chlorine removal, allowing highly doped patterns to be written and erased without damaging the graphene. To illustrate the capabilities of our approach, we use it to create rewritable photoactive junctions for graphene-based photodetectors

Giant Isotope Effect of Thermal Conductivity in Silicon Nanowires

Isotopically purified semiconductors potentially dissipate heat better than their natural, isotopically mixed counterparts as they have higher thermal conductivity (κ). But the benefit is low for Si at room temperature, amounting to only ∼10% higher κ for bulk ^{28}Si than for bulk natural Si (^{nat}Si). We show that in stark contrast to this bulk behavior, ^{28}Si (99.92% enriched) nanowires have up to 150% higher κ than ^{nat}Si nanowires with similar diameters and surface morphology. Using a first-principles phonon dispersion model, this giant isotope effect is attributed to a mutual enhancement of isotope scattering and surface scattering of phonons in ^{nat}Si nanowires, correlated via transmission of phonons to the native amorphous SiO_{2} shell. The Letter discovers the strongest isotope effect of κ at room temperature among all materials reported to date and inspires potential applications of isotopically enriched semiconductors in microelectronics