Session E36: 2D Materials - Heterostructures III

Probing Valley Dynamics in van der Waals heterostructures

Van der Waals heterostructures composed of stacked atomically thin layers can exhibit novel phenomena due to the unique layer-layer interactions. In this talk, I will describe the observation of high-purity and long-lived valley polarization with microsecond lifetime in transition metal dichalcogenide heterostructures. I will also discuss spatially and temporally resolved imaging of pure valley and spin current in the WS2/WSe2 heterostructure.
  

Temperature dependence of charge transfer processes in WS2/MoSe2 heterobilayers probed by ultrafast spectroscopy

We report transient reflection experiments to probe electron transfer processes from MoSe2 to WS2 in WS2/MoSe2 heterobilayers. The measurements were performed for temperatures from 15 - 300K. Complementing temperature dependent static reflection contrast measurements, we investigate charge transfer by pump-probe measurements and discuss its relation to the temperature-dependent band alignment for the WS2/MoSe2 system. Our motivation stems from the theoretical prediction that the conduction band minima of the two materials are separated only by a few tens of meV [i.e. B. Amin et al. Phys. Rev. B 92, 075439 (2015)]. Hence, their alignment and the corresponding electron transfer process from one material to its neighbor are potentially tunable using external knobs such as temperature. The present study seeks to clarify this important issue for future applications that require understanding and controlling the electronic band structure and dynamic processes in this heterostack.   

Generation, Transport and Imaging of Pure Valley Currents in van der Waals Heterostructures

Two-dimensional (2D) hexagonal materials provide a promising platform for valleytronics devices, owing to the convenient generation and manipulation of valley qubits. However, efficient generation of valley qubits with long valley lifetime cannot be achieved in single material due to intrinsic valley relaxation channels. Here we show that, such intrinsic limit can be completely overcome through combining two materials into a van der Waals heterostructure; and report both near-perfect generation efficiency of valley qubits, as well as record-high valley lifetime. Furthermore, we demonstrate generation, transport, and spatial-temporal imaging of the valley currents in a single device, which opens up new exciting opportunities to realize novel spintronic and valleytronic applications.   

Quantum-Confined States and Band Shifts Arising from Moiré Patterns in MoS2-WSe2 Heterojunctions

Using low-temperature scanning tunneling microscopy (STM) and spectroscopy (STS), the electronic states of heterojunctions formed by a monolayer of MoS₂ on WSe_2, grown on epitaxial graphene, have been investigated. A 4% lattice mismatch between the MoS₂ and the WSe₂ results in a moiré pattern with period 8.5 nm and corrugation height 0.1 nm. The band gap was found to form between the valence band (VB) of the WSe₂ and the conduction band (CB) of the MoS₂. Band edge shifts of 0.1-0.2 eV are observed depending on the location within the moiré unit cell. Furthermore, quantum-confined states at both the VB and CB edges were found to form near the minimum of the corrugation. Hybridization of orbitals was found to explain these features for the VB of the WSe₂, however, intrinsic charge transfer between layers must also be considered for the features in the CB of the MoS₂. We find that this electrostatic model results in band edge shifts and confined states that are consistent with the experiments.   

Interlayer Exciton Traps in Van der Waals Heterostructures

Two-dimensional (2D) van der Waals materials such as single layer transition metal dichalcogenides (TMDs) and hexagonal boron nitride (BN) have sparked interest in the study of atomically thin semiconducting heterostructures. The 2D nature and large excitonic binding energy of TMDs allow for the exploration of novel quantum optical effects. Using type-II heterostructures formed by stacking MoSe₂ and WSe₂ monolayers, we study interlayer excitons, bound electrons and holes residing in spatially separated layers. We fabricate dual-gated, BN encapsulated devices with electrical contacts in each layer, giving us pristine samples with full electrical control. Interlayer excitons in our heterostructures have near-infrared energies and lifetimes on the order of 100ns, both of which can be tunable using out-of-plane electric field. We observe localization of interlayer excitons in naturally formed traps that have longer lifetimes and higher intensities than in the non-localized regions. In addition, we can form artificial traps through confinement potentials using out-of-plane electric fields. Because of the large binding energies, interlayer excitons can serve as a platform for exploring the physics of light-matter interactions and Bose-Einstein condensates at high temperatures.   

First-Principles Studies of Charge and Spin Transport in 2D MoS2 Junctions

The current rate of technological evolution necessitates a greater understanding of low-dimensional materials and their physical properties. As a result of the search for improved nanoscale electronic devices, transition metal dichalcogenides have come into focus. These materials have been shown to have applications in tunneling field-effect transistor and Esaki diode devices. In doped molybdenum disulfide p-n junctions, band-to-band tunneling is the foremost contributing factor to Esaki diode behavior at low potential bias. Using the non-equilibrium Green’s function approach and effective screening medium in the framework of density functional theory (NEGF + ESM + DFT), we investigated MoS₂ mono- and bi-layer junctions. Esaki diode behavior and a negative differential resistance regime are observed in these systems. Analysis of partial density of states reveals that the current across the junction is due to interlayer band-to-band tunneling. Finally, we discuss edge effects and edge state termination.   

New Assembly-Free Bulk Layered Heterostructures: Electronic, Mechanical, and Optical Properties

In principle, a vast number of unique van der Waals heterostructures can be created through the vertical stacking of two-dimensional materials, resulting in unprecedented potential for material design. However, this widely employed synthetic method for generating van der Waals heterostructures is slow and imprecise. Here, we computationally study the properties of a new class of layered bulk materials, which we call assembly-free bulk layered heterostructures, wherein the individual layers are of dissimilar chemical composition. We find that these bulk materials exhibit properties similar to vertical heterostructures without the complex and unscalable stacking process. Using state-of-the-art computational approaches, we find some of these materials are potentially well suited for photovoltaic and photodetector applications as they possess visible spectrum band gaps. Additionally, we study livingstonite (Hg(SbS₂)₄), a naturally occurring mineral which exists as a bulk lattice-commensurate heterostructure. We find that both the bulk and isolated bilayers of livingstonite exist as a type-II heterojunction with infrared band gaps. This is the first report of a naturally occurring mineral with these electronic properties.   

Data-driven Discovery of New Two- and One-dimensional Materials and Lattice-commensurate Heterostructures

We employ data-driven methods to discover new two- and one-dimensional materials. Layered materials have attracted interest for technological applications and fundamental physics. But only a few van der Waals solids have been subject to considerable research focus. Through data mining, we identify 1173 two-dimensional layered materials and 487 weakly bonded one-dimensional molecular chains. This is an order of magnitude increase in the number of known materials. Moreover, we discover 98 heterostructures of two-dimensional and one-dimensional subcomponents that are found within bulk materials, opening new possibilities for van der Waals heterostructures.
To identify these materials, we present a novel data mining algorithm that determines the dimensionality of weakly bonded subcomponents. Chemical families, band gaps, and point groups and single-layer piezoelectricity of the materials identified with data mining are presented. Moreover, we expand on this work to new material compositions that can form layered materials.   

Bilayer Graphene - WSe2 Resonant Tunneling Heterostructures With Large Current Densities

Two-dimensional (2D) materials offer an exciting avenue to explore novel electronic devices and properties, and can be easily stacked to create designer heterostructures. We present here a study of resonant tunneling heterostructures consisting of two rotationally aligned bilayer graphene electrodes separated by a bilayer WSe₂ tunnel barrier. Rotational alignment of the graphene bilayers aligns their respective band structures in momentum-space, which allows for momentum and energy conserving tunneling between the two layers. This behavior is manifested through a pronounced resonance peak in the interlayer current-voltage characteristic, and gate-tunable negative differential resistance. The studied samples show current densities exceeding 60 μA/μm² and peak-to-valley ratios over 5 at room temperature. Theoretical calculations using a Lorentzian spectral function for the 2D quasiparticle states match closely with the experimental data. In-plane magnetotunneling measurements show a splitting of the resonance peak and suppression of the conductance, consistent with momentum conserving tunneling.   

High-Performance WSe2 Field-Effect Transistors via Controlled Formation of In-Plane Heterojunctions

Monolayer WSe₂ is a two-dimensional (2D) semiconductor with a direct band gap, and low field-effect mobility is the main constraint preventing WSe₂ from becoming one of the competing channel materials for field-effect transistors (FETs). Here, we report that controlled heating in air significantly improves device performance of WSe₂ FETs in terms of on-state currents and field-effect mobilities. Specifically, after being heated at optimized conditions, chemical vapor deposition grown monolayer WSe₂ FETs showed an average FET mobility of 31 cm²V⁻¹s⁻¹ and on/off current ratios up to 5 × 10⁸. For few-layer WSe₂ FETs, after the same treatment applied, we achieved a high mobility up to 92 cm²V⁻¹s⁻¹. These values are significantly higher than FETs fabricated using as-grown WSe₂ flakes without heating treatment, demonstrating the effectiveness of air heating on the performance improvements of WSe₂ FETs. The underlying chemical processes involved during air heating and the formation of in-plane heterojunctions of WSe₂ and WO_3−x were studied. This work is a step toward controlled modification of the properties of WSe₂ and potentially other TMDCs and may greatly improve device performance for future applications of 2D materials in electronics and optoelectronics.   

Multi-junction lateral 2D heterostructures of transition metal dichalcogenides via sequential edge epitaxy

Here we demonstrate the successful synthesis of lateral in-plane multi-junction heterostructures based on transition metal dichalcogenides (TMD) 2D monolayers, using a modified chemical vapor deposition process where water vapor was use as a mediator for selective evaporation-deposition of the precursors. By only controlling the carrier gas composition, it is possible to selectively growth one TMD at the time. This introduces an unprecedented flexibility that allows a good in situ control of the lateral size of each TMD segment. The fabricated heterostructures include MoS₂-WS₂, MoSe₂-WSe₂ and MoS_xSe_y-WS_xSe_y, with multiple hetero-junctions. The band gap modulation across the junctions as well as spatial chemical distribution were studied by Raman and photoluminescence mapping. The crystalline quality of the heterostructures were characterized within an aberration-corrected scanning transmission electron microscope, revealing seamless interfaces with high-crystalline quality. Using field effect transistor devices we studied the transport properties across the junctions.
  

Two-Dimensional MoS2 Field-Effect-Transistors with Graphene and Titanium Source-Drain Contacts

Based on the first principles calculation, we investigate the electronic band structures of graphene-MoS₂ and Ti-MoS₂ heterojunctions under gate-voltages [1]. By simultaneous control of external electric fields and carrier charging concentrations, we show that the graphene’s Dirac point position inside the MoS₂ bandgap is easily modulated with respect to the co-varying Fermi level, while keeping the graphene’s linear band structure around the Dirac point. The easy modulation of graphene bands is not confined to the special cases where the conduction-band-minimum point of MoS₂ and the Dirac point of graphene are matched up in reciprocal space, but is generalized to their dislocated cases. This flexibility caused by the strong decoupling between graphene and MoS₂ bands enhances the gate-controlled switching performance in MoS₂-graphene hybrid stacking-device. [1] S. S. Baik et al, Scientific Reports. 7, 45546 (2017).   

Stability and Electronic Properties of Hybrid SnO Bilayers: SnO/Graphene and SnO/BN

Van der Waals structures based on 2D materials have been considered as promising design for novel nanoscale electronic devices. Very recently, intrinsic p-type semiconducting 2D SnO films were fabricated. Using the density functional theory, we consider the vertically stacked heterostructures consisting of SnO monolayer with graphene or BN monolayer to investigate their stability, electronic and transport properties. Our calculated results show that properties of the constituent monolayers are retained in these SnO-based heterostructures. In SnO/graphene heterostructure, a p-type Schottky barrier is formed and this Schottky barrier can be effectively controlled with an external electric field, which is useful characteristics for the van der Waals heterostructure-based electronic devices. However, in the SnO/BN heterostructure, the electronic properties of SnO are least affected by the insulating monolayer, which suggesting the BN monolayer to be an ideal substrate for the SnO-based nanoscale devices.