Session B37: 2D Materials - TMDCs II

Room Temperature Electron-Hole Liquid in quasi-2D monolayer MoS$_2$

Strong correlations between electrons and holes can drive the system to the electron-hole liquid (EHL) state typically at high carrier densities and low temperatures. We study the formation of such a correlated electron-hole liquid state in monolayer MoS$_2$, a quasi-2D system where interactions between charge carriers are governed by the Keldysh potential. We calculate the free energy at finite temperatures using the \textit{linked cluster expansion} method incorporating ring diagrams. The variation of thermodynamic conjugate variables indicate a first-order phase transition with a critical temperature larger than room temperature. We construct the phase diagram in the temperature-density phase space by Maxwell construction of thermodynamically unstable region demarcating phase space regions corresponding to exciton/electron-hole gas, electron-hole liquid and the region of coexistence.   

Nonlinear exciton propagation and excitonic halos in monolayer WS2

Coulomb-bound electron-hole pairs, or excitons, have been in the focus of the solid-state research for many decades. They are of paramount importance for the fundamental understanding of interacting charge carriers in semiconductors. Recently, excitons in single layers of semiconducting transition-metal dichalcogenides (TMDCs) were found to combine a nnumber of intriguing properties, including binding energies on the order of 0.5 eV, strong light-matter interaction and spin-valley coupling.

Here, we address the topic of exciton transport in TMDCs by directly monitoring spatial behavior of excitons in freestanding and supported single layers of WS₂ through spatially- and time-resolved photoluminescence. We find highly nonlinear behavior with characteristic, qualitative changes in the spatial profiles of the exciton emission and an effective diffusion coefficient increasing from 0.3 cm²/s to more than 30 cm²/s, depending on the injected exciton density. Solving the diffusion equation while accounting for Auger recombination allows us to identify the main origin of the nonlinearity. At elevated excitation densities, the excitons distribution evolves into long-lived halo shapes with micrometer-scale diameter, indicating additional memory effects in the dynamics.
  

Theory of Bound Impurity States in Monolayer Transition-Metal Dichalcogenides

Semiconducting monolayer transition-metal dichalcogenides (TMDCs) are two-dimensional (2D) materials that exhibit direct band gaps, favourable for nanoelectronic applications such as field-effect transistors. We discuss the behaviour of single-particle bound states in molybdenum disulphide (MoS₂) with a charged impurity atom adsorbed to its surface. The bound states associated with shallow defect potentials extend over tens of nanometers or more because of weak screening in 2D materials. These long-range screened potentials are difficult to fully capture with conventional DFT supercells. To understand the behaviour of TMDCs with impurities, we employ tight-binding models for systems with 10³-10⁴ atoms, including the static screening of impurity charges through the random phase approximation. We have studied spatial densities of bound impurity states and the local density of states near impurity ions. We find localized Rydberg states near the band edges, with a significant asymmetry between donor and acceptor states, which is related to specific features of the MoS₂ band structure. We also predict signatures of these defect states that can be measured in scanning tunnelling experiments.   

Observation of Topological Insulating and Superconducting Ground States of Monolayer WTe2

Topology and correlations are two fundamental aspects that determine the electronic ground states of condensed matter systems. Both aspects individually have led to striking observations such as the quantum spin Hall insulating state and superconductivity, respectively. The combination of them can result in exotic phenomena including topological superconductivity and non-abelian anyons. In this talk I will report our recent study on monolayer tungsten ditelluride (WTe2), where we find that topology and correlations are simultaneously important in understanding its ground state properties. In the first part, I will talk about our quantum transport measurements for identifying the undoped monolayer WTe2 as a two-dimensional topological insulator. The observation of the quantum spin Hall effect surviving up to 100 Kelvin will be discussed. In the second part, I will report the discovery of superconductivity below 1 Kelvin when the same monolayer is electrostatically doped through boron-nitride dielectric gating. These observations demonstrate that the ground state of the monolayer is remarkably gate-tunable between the two extremes of electronic transport in materials (insulator and superconductor). Our results establish monolayer WTe2 as a new platform for studying rich electronic phenomena driven by topology and correlations, including Majorana states.   

Tunable Berry-curvature and valley Hall effet in Bilayer MoS2

The chirality of electronic Bloch bands is responsible for many intriguing properties of
layered two-dimensional materials and has been extensively studied in single and few-layers of graphene
and in monolayers of transition metal dichalcogenides (TMDCs). We show that in bilayers of TMDCs,
unlike in few-layer graphene and monolayer TMDCs, both intra-layer and inter-layer couplings give important
contributions to the Berry-curvature in the K and -K valleys of the Brillouin zone. Due to the inter-layer contribution,
the Berry-curvature appears to be gate tunable and also depends on the
stacking of the layers. We point out the differences between the commonly available
3R type and 2H type bilayers and predict the dependence of the valley Hall effect on the stacking type.   

Cavity-modified temperature dynamics of valley-polarized exciton-polaritons in monolayer MoS2

Exciton-polaritons, strongly-coupled light-matter quasiparticles, can preserve the valley selectivity of monolayer transition metal dichalcogenides embedded in a microcavity. The cavity field character of the exciton-polaritons modifies the emission properties of the monolayer, leading to preservation of luminescence polarization at room temperature distinct from bare monolayers. Here, we explore both experimentally and theoretically broad regimes of cavity coupling with MoS₂ and their impact on exciton-polariton valley polarization observed over a large temperature range. An intuitive and general cavity coupling model captures these observations by considering the relative rates of exciton and photon relaxation and intervalley scattering. The experimental and theoretical agreement demonstrates that exciton-polariton valley polarization dynamics can be well-controlled using cavity decay rates, coupling strengths, and detunings by microcavity engineering.   

Excited exciton states in monolayer MoS2 encapsulated in hexagonal Boron nitride

The optical properties of transition metal dichalcogenide monolayers such as MoS2 and WSe2 are dominated by excitons, Coulomb bound electron hole pairs, with binding energies of several hundred meV. This gives rise to a series of resonances in linear and non-linear optical spectra in analogy to the Rydberg states of the hydrogen atom. Due to the broad and often defect related optical transitions in MoS2 monolayers exfoliated directly onto SiO2 substrates detailed information on excited exciton states was inaccessible.
Here we reveal in reflectivity and in photoluminescence excitation spectroscopy (PLE) the presence of the first two excited states of the A-exciton in MoS2 monolayers encapsulated with hexagonal boron nitride (hBN) [1]. We see a clear increase both in intensity and in the degree of valley polarization of the neutral exciton emission when the laser excitation energy coincides with these excited Rydberg states. We also demonstrate efficient PL up-conversion when in resonance with the A exciton, resulting in emission 200 meV above the laser energy. These results have allowed us to estimate the exciton binding energy to be around 220 meV in h-BN encapsulated MoS2 monolayer samples.
[1] Fabian Cadiz et al, PRX 7, 021026 (2017)   

Modulating van der Waals Bonding in 2D Transition Metal Dichalcogenides with Light

Modulation of weak interlayer interactions between quasi-two-dimensional atomic planes in the transition metal dichalcogenides (TMDCs) provides avenues for tuning their functional properties. Here we show that above-gap optical excitation in the TMDCs leads to a large-amplitude, ultrafast compressive force between the two-dimensional layers, as probed by in-situ measurements of the atomic layer spacing at femtosecond time resolution. We show that this compressive response arises from a dynamic modulation of the interlayer van der Waals interaction and that this represents the dominant light-induced stress at low excitation densities. A simple analytic model predicts the magnitude and carrier density dependence of the measured strains. Probing in-plane lattice dynamics of the semi-metallic WTe2, we observe large amplitude interlayer shear oscillations in response to optical or THz excitation. This shear motion occurs along the direction of the phase transition pathway between the monoclinic and orthorhombic phases of the material, the latter of which has been predicted to be a type II Weyl semimetal. This work establishes new methods for dynamic tuning of van der Waals bonding and of the optomechanical functionality of TMDC quasi-two-dimensional materials.   

Size effects and odd-even effects in MoS2 nanosheets: first-principles studies

Molybdenum disulfide (MoS₂) nanostructures have been widely used as catalysts in the petroleum refinery industry for hydrodesulfurization process, in which the sulfur vacancies play a critical role in determining the catalytic activity. Here we report size effects and odd-even effects on the formation of sulfur vacancies in the triangular MoS₂ nanosheets using first-principles calculations. By modeling four types of edge structures of MoS₂ nanosheets, S-terminated edges are found to be energetically more favorable than Mo-terminated edges, and are then selected for studying energetics of sulfur vacancies. Two types of sulfur dimer vacancies at center (V_S@Cen) and at corner (V_S@Cnr) of the edges of S-terminated MoS₂ nanosheets are modeled, respectively. Our results reveal a strong odd-even effect in the formation of sulfur dimer vacancies, particularly for small MoS₂ nanosheets, in terms of the size of nanosheets that is defined by the number of Mo atoms on the edge. V_S@Cen dimer vacancy has a low formation energy at even-number but high formation energy at odd-number, while V_S@Cnr dimer vacancy exhibits a complete opposite trend. These results indicate that small MoS₂ nanosheets can exhibit unique materials properties for catalytic applications.   

Ambient hole doping of single-layer WS2 supported on SiO2 substrates

Understanding charge transfer (CT) between two-dimensional (2D) crystals and molecules is crucial for the fundamental research and future applications of 2D material systems. Despite recent surge on this research subject, however, the roles of the interface formed by supporting substrates and their surfaces have not been investigated. In particular, ambient water may form a medium on hydrophilic surfaces for electrochemical reactions with oxygen in the air. In this work, we investigated the CT interaction of single-layer WS₂ with the proposed redox couple of O₂/H₂O serving as a hole dopant. Photoluminescence (PL) and Raman spectra, obtained for WS₂/SiO₂ in a gas-controlled optical cell, showed that hole doping is mediated by O₂ and H₂O. Much reduced CT on hexagonal boron nitride substrates suggested a conclusive role of interface between WS₂ and hydrophilic SiO₂ substrate. Wide-field PL imaging was exploited to investigate spatial inhomogeneity and anisotropy. We will further discuss detailed photophysics of charged excitons (trions) and possible roles of defects in the observed CT reactions.   

Effect of Strain and hBN Encapsulation on the Optical Transitions of Ultrathin Transition Metal Dichalcogenides

The optical spectra of transition metal dichalcogenides (TMDCs) are dominated by excitons and they experience an indirect-gap to direct-gap transition at the monolayer limit. For monolayer TMDCs, encapsulation by hexagon BN (hBN) has been shown to reduce the linewidth of direct exciton at both low and room temperatures. However, the effect of hBN encapsulation on the optical spectra of bilayer TMDCs with their indirect gap is not clear. We have therefore prepared hBN encapsulated bilayers and studied how indirect excitons are affected. Strain engineering on TMDCs has also been demonstrated to modify their optical fingerprints, including excitonic linewidths by changing the band structure and the excitonic states. We apply uniaxial tensile strain to both bare monolayer and bilayer TMDCs, as well as hBN encapsulated ones. By comparing the strain dependent modifications of the linewidth and other optical fingerprints of hBN encapsulated samples and bare samples, we are able to distinguish different contributions to the exciton linewidth and the role of hBN encapsulation.   

Direct Exchange of Re and Mo atoms in monolayer MoS2

Two-dimensional materials provide opportunities to directly observe atomic-scale defect dynamics. The direct exchange between a substitutional impurity and a neighboring host atom has been discussed in the literature, but the energy barrier for such a process is generally believed to be too large. No atomic-scale observation of direct exchange event has been reported. Here we use scanning transmission electron microscopy to observe substitutional Re impurities in monolayer MoS₂ undergo direct exchanges with neighboring Mo atoms. Density functional theory calculations find that the energy barrier for direct exchange is too large for either thermal or beam-induced exchange. Microscopy further reveals the presence of an ever-changing number of S vacancies, but the calculated energy barrier remains too large to account for the jumps. The density functional theory calculations further find that a Re impurity and surrounding S vacancies introduce an ever-changing set of localized levels in the energy gap. We propose that these levels mediate an “explosive” recombination-enhanced migration via multiple electron-hole recombination events.   

Telling the difference between phonon-assisted and charged-exciton optical transitions in monolayer transition-metal dichalcogenides

The wide interest in monolayer transition-metal dichalcogenides (ML-TMDs) has led to numerous recent studies of their optical properties. Close examination of these studies show that in many cases, optical transitions are associated with charged-excitons (negative and positive trions) instead of being associated with phonon-assisted excitonic emission. This confusion was partly caused due to the similar energy scales of optical phonons and trion binding energies, as well as by the common belief that a gate voltage can only tune the intensity of the trion optical transition. In this brief talk, we will explain how the exciton-phonon interaction can also be tuned in gated structures of ML-TMDs, leading to a voltage-dependent Huang Rhys factor in these materials. We will discuss the important role of charged impurities and the regimes at which optical transitions are dominated by the exciton-phonon interaction or by trions. We will also discuss the differences between photoluminescence and reflectivity experiments in this context.