Session L32: 2D Materials and Device Characterizations

Visualizing Anisotropic Strain and Electronic Structures in WSe$_{\mathrm{2}}$-MoS$_{\mathrm{2}}$ Lateral Heterojunctions

Recent demonstrations of seamless 2D lateral heterojunctions (HJs) based on dissimilar monolayer transition metal dichaldogenides (TMDs) have created new opportunities to push semiconductor heterostructures toward a new frontier. By using scanning tunneling microscopy and spectroscopy, we investigate the atomic structures and electronic properties of the atomically abrupt lattice-mismatched lateral HJs of WSe$_{\mathrm{2}}$-MoS$_{\mathrm{2}}$. We present a novel method to determine the anisotropic strain based on Moir\'{e} pattern imaging. The 2x2 strain tensor is imaged with nanometer spatial resolution. We show that the misfit dislocations at the interface are responsible for partial relaxation of the strain. Transmission electron microscopy reveals two kinds of misfit dislocations, one with Burger's vectors along the zigzag direction parallel to the interface, and the other one also along the zigzag direction but at 60 degrees off the interface. We also determine the band offset across the junction and show that the strain effect converts the otherwise type-II into type-I band alignment. Moreover, a ``line interface specific'' electronic structure due to the specific bonding configuration is discovered at the interface.   

Nano-imaging of Electrical Properties of MoSe$_{\mathrm{2}}$/WSe$_{\mathrm{2}}$ Vertical Heterostructures.

Vertical van der Waals heterostructures of transition metal dichalcogenides (TMDs) with atomically sharp interfaces and well-controlled components exhibit exotic optical and electrical properties due to the strong interlayer coupling and lack of depletion area. It has been proposed that tunneling-assisted interlayer recombination of the majority carriers is responsible for the tunability of the electronic and optoelectronic processes. However, current researches are mostly based on macroscopic transport measurements, which provide little information on the nanoscale electronic properties of working devices. Here we demonstrate the electrical mapping of the local conductance in field-effect transistors (FETs) based on MoSe$_{\mathrm{2}}$/WSe$_{\mathrm{2}}$ vertical heterostructures by microwave impedance microscopy (MIM). The spatial evolution of the insulator-to-metal transition of both individual flakes and the heterostructure region is clearly resolved as a function of both back-gate voltage and tip bias. Moreover, the conductance mapping of heterostructure area clearly uncovers the screening effect during the charge accumulation, as well as the interaction between charge carriers during the transport process.   

Imaging inherent and air-induced defects in black phosphorus with scanning tunneling microscopy

Black phosphorus has received significant attention due to its large, direct bandgap and high mobility at the monolayer level (phosphorene). Because phosphorene devices so far rely on exfoliation from the bulk crystals, it is important to understand native impurities and defects in the source material. We will present the atomic structure, local density of states, and native defects of bulk and few-layer black phosphorus acquired through low-temperature, ultra-high vacuum scanning tunneling microscope (STM) experiments. We studied black phosphorus from different commercial sources, prepared in a dry nitrogen environment and cleaved in UHV. Observed defects occur in higher concentrations than can be attributed to impurities, and appear to be vacancies or self-interstitials. Multiple types of point defects are observed. Spectroscopy measurements show an asymmetric density of states, contrary to previous reports, and evidence that point defects are charged. Finally, controlled exposure to air resulted in a high concentration of defects distinct from native defects. These results indicate that vacancies and self-interstitials are prevalent in commercially-available black phosphorus and their charging behavior may explain common p-doping in black phosphorus devices.   

Nanoscale Thermal Mapping in Two-Dimensional Materials using Electron Energy Loss Spectroscopy (EELS)

While 2D materials such as graphene and transition metal dichalcogenides (TMDs) are of significant interest for the applications in electronics and optoelectronics, heat removal in such devices can be detrimental to their performance and reliability. As the node sizes in such devices shrink, understanding the power dissipation and thermal transport properties in 2D materials requires thermometry techniques which can provide a nm-scale spatial resolution. Here, we introduce a method for measuring the local temperature gradients in low dimensional materials using a combination of scanning transmission electron microscope and electron energy loss spectroscopy (EELS). More specifically, we find a relationship between the plasmon energies and the sample temperatures for different free-standing 2D materials (graphene, MoS$_2$, MoSe$_2$, WS$_2$, WSe$_2$), which can now be used to map the temperature gradient across interfaces and particle surfaces. We also perform first-principles calculations for the low-loss EELS signal to interpret the experimental findings.   

Surface reconstruction in van der Waals heterostructures.

Van der Waals heterostructures of two-dimensional (2D) materials have already allowed realization of a number of unique devices and exciting physical experiments. Electronic properties of such heterostructures are usually fine-tuned by careful selection of the sequence and thickness of the individual layers. Recently it has been demonstrated that relative orientation between the crystals allow for fine modification of their electronic properties. Thus, spectrum reconstruction is observed for graphene on hexagonal boron nitride and indirect exciton emission is observed for aligned MoS$_{\mathrm{2}}$ and WSe$_{\mathrm{2}}$ flakes. Here we would like to go even further and demonstrate that such aligned structures experience surface reconstruction. Furthermore, this surface reconstruction can be controlled by external parameters, such as strain, temperature, doping, etc. This allows for even finer tuning of the electronic, optical and mechanical properties of such heterostructures.   

Scanning Tunneling Microscopy and Spectroscopy of Twisted Bilayer Graphene with Small Twist Angles

The electronic band structure of twisted bilayer graphene (tBLG) depends on the twist angle between the two layers. Distinct electronic features emerge when the twist angle becomes smaller than 1 degree. Here we use low temperature scanning tunneling microscopy and spectroscopy to investigate the electronic properties of tBLG for twist angles below 2 degrees. The twist angle is determined from the wavelength of the moir\'{e} pattern. Density of states measurements are performed as a function of charge density which is controlled by the back gate. Different charge density dependences are observed for twist angles above and below a critical angle of about 1 degree. Moreover, spatially resolved spectroscopy mapping shows that electrons at the Fermi level become localized on the AA sites for small twist angles.   

Tunable Moir\'{e} Bands in Minimally Twisted Bilayer Graphene.

We present the realization of tunable moir\'{e} crystals in minimally twisted (MT) bilayer graphene, and provide a comprehensive study of electron transport in these samples. In twisted bilayer graphene, the relative rotation of the two graphene layers leads to the formation of a new moir\'{e} crystal, which is expected to have a dramatically different band structure compared to Bernal-stacked bilayer graphene. The MT bilayer graphene is fabricated using a new transfer method that employs a micromechanical hemispherical handle substrate which allows defining small relative rotation angles (0.6$^{\circ}$ to 1.2$^{\circ}$ ) between two graphene flakes that stem from the same domain, with an accuracy of 0.1$^{\circ}$ . We observe the emergence of satellite transport gaps at $\pm$ 8 electrons per moir\'{e} unit cell, along with a conductivity minimum at charge neutrality. These features remain robust in the presence of a high transverse electric field, applied using dual gated device structures. In magnetic fields, we observe the emergence of a Hofstadter butterfly in the energy spectrum, with four-fold degenerate Landau levels, and broken symmetry QHS at $\nu \quad =$ $\pm$ 1, $\pm$ 2, $\pm$ 3.   

Davydov splitting and resonance Raman studies of few-layer MoSe$_{\mathrm{2}}$

We conducted Raman investigation of few-layer MoSe$_{\mathrm{2}}$ with eight different excitation energies. New peaks that appeared only near resonance with various exciton states were analyzed, and the modes were assigned. We observed splitting of intralayer A$_{\mathrm{1g}}$, E$_{\mathrm{1g}}$, and A$_{\mathrm{2u}}$ modes for some excitation energies near resonances. This splitting is called `Davydov splitting' and caused by interlayer interaction. By ?tting the spectral positions of interlayer shear and breathing modes and Davydov splitting of intralayer modes to a linear chain model, we extracted the strength of the interlayer interaction. We found that the second-nearest-neighbor interlayer interaction amounts to about 30{\%} of the nearest-neighbor interaction for both in-plane and out-of-plane vibrations. In addition, we investigated resonance effects of each Raman mode. The resonance pro?les of the Raman peaks re?ected the joint density of states for optical transitions, but the symmetry of the exciton wave functions leads to selective enhancement of the A$_{\mathrm{1g}}$ mode at the A exciton energy and the shear mode at the C exciton energy.   

Detailed Study of the Raman Response of Mono- and Few-Layer ReS$_{\mathrm{2}}$

ReS$_{\mathrm{2}}$ is an exciting 2-Dimensional (2-D) material due to its strong in-plane anisotropy, offering an additional physical knob to tune its properties for a wide variety of applications. In addition, ReS$_{\mathrm{2}}$ has been shown to be a direct-gap semiconductor for few-layer thicknesses, which is a major advantage in optoelectronics. Raman spectroscopy serves as the most useful, facile, and non-destructive method to characterize ReS$_{\mathrm{2}}$. Due to its lower symmetry, the Raman spectrum of ReS$_{\mathrm{2}}$ is significantly more complicated than its Mo or W counterparts, displaying 18 first-order modes. We will discuss the effects on the Raman spectrum under various experimental conditions, including polarization-dependent, layer-dependent, and resonant Raman, and outline the many aspects that need to be considered when using Raman spectroscopy to characterize ReS$_{\mathrm{2}}$ and other anisotropic 2-D materials. Comparisons between experiments and DFT calculations will also be analyzed. Furthermore, we will demonstrate the importance of correctly calculating thin film interference effects for Raman of ReS$_{\mathrm{2}}$ on SiO$_{\mathrm{2}}$/Si substrates.   

The impact of inversion and mirror reflection symmetry on Raman scattering of $T' $transition metal dichalcogenides

Distorted octahedral ($T')$ transition metal dichalcogenides (TMDCs) are topologically interesting material systems. Inversion-symmetry-broken bulk $T'$-TMDCs are predicted to be type II Weyl semimetals and inversion-symmetric monolayer (1L) $T'$-TMDCs are shown to be 2D topological insulators. In this talk, I will show that both the inversion symmetry and the mirror symmetry are important for understanding the lattice dynamics and Raman scattering of $T'$-TMDCs. The mirror plane that is perpendicular to the zigzag transition metal atomic chain classifies lattice vibrations into z-modes and m-modes where `$z$' stands for zigzag and `$m$' stands for mirror. Raman active $z$- and $m$- modes can be experimentally determined with light-polarization and crystal angle-resolved Raman tensor analysis. We report observation of all 9 even-parity zone-center phonons in 1L-$T'$-MoTe$_{\mathrm{2}}$. In bulk $T'$-MoTe$_{\mathrm{2}}$, we monitor inversion symmetry breaking with the shear lattice vibrations, which is important for supporting Weyl fermions.   

Band Structure Evolution in Vertically Contacted MoS$_{2}$ Probed Using Scanning Tunneling Spectroscopy

Achieving high-quality electrical contact between deposited metals and monolayer transition metal dichalcogenides remains a challenging problem. In this work, we probe the factors influencing the contact resistance by using atomic-resolution scanning probe microscopy and spectroscopy at the interface between a contact metal and highly doped monolayer molybdenum disulfide (MoS$_{2})$. Using atomically resolved spectroscopy measurements, band structure evolution is mapped across the edge of the contact and into the MoS$_{2}$ sheet. The spectroscopy shows the presence of metal induced gap states (MIGS) in the semiconductor with a decay length of a few angstroms from the interface, which is the first experimental observation of MIGS in a two-dimensional semiconductor. We show that in the highly doped limit, the MIGS dominate the electronic properties and hence the contact resistance, as opposed to band bending. Finally, we describe the difference in MIGS between graphite, gold and palladium electrodes on MoS$_{2}$.