Session K15: 2D Materials (Semiconductors) -- Transport and Optical Phenomena in Bilayers and Heterostructures

Transport and optical measurement on twisted homo-bilayer WSe2/WSe2 structure

The freedom of stacking different layers of 2D van der Waals (vdW) materials with an arbitrary twist angle opens up tremendous opportunities in studying emergent properties that are not accessible in the natural crystal form. In particular, recent measurements on magic-angle twisted bilayer graphene device show strongly correlated electronic properties, including correlated insulating behavior and unconventional superconductivity due to the formation of flat bands. Flat bands are expected in other small twist angle systems, including twisted bilayer transition-metal dichalcogenide (TMD), which could lead to new transport and optical properties. In this work, we fabricated encapsulated and dual-gated twisted homo-bilayer WSe₂/WSe₂ devices with precise angle control. We studied their electronic and optical properties as a function of charge density, electric/magnetic field, and temperature with an aim to understand how the twist angle can modify the electron behavior in this homo-bilayer TMD systems.   

Magneto-reflectance study of Zeeman effect in MoS2 bilayer

We studied Zeeman splitting ΔE of different recombination channels in Bilayer MoS₂ using magneto-reflectance spectroscopy. The Zeeman splitting of both A and B excitons of bilayer MoS₂ is linear in magnetic field with a slope of approximately -0.2 meV/T similar to that of single layer MoS₂. In addition, we observed a new feature labeled N at 2.01 eV between A (1.93 eV) and B (2.13 eV) excitons¹. We note that feature N is not observed in MoS₂ monolayers. This new feature shows a positive Zeeman slope of 0.37 meV/T as compared to A and B excitons. Theoretical^2,3 and experimental⁴ studies show that bilayer MoS₂ exhibits optical properties resulting from an interplay of the direct-bandgap of monolayer system, and the indirect bandgap of the bulk. A crucial property of the bilayer system appears to be its inversion symmetry² and tuning³ of that symmetry translates into the interplay of contributions of different valleys in the photoluminescence spectra and possible appearance of additional emission⁴. We hypothesize that feature ‘N’ results from these valley-mixing effects.
1. K.F. Mak et al., Phys. Rev. Lett. 105, 136805 (2010)
2. E. S. Kadantsev and P. Hawrylak, Solid State Commun. 152, 909 (2012).
3. S. Wu et al., Nature Physics 9, 149 (2013)
4. T. Chu et al., Nano Lett. 15, 8000 (2015)   

Superconductivity in twisted transition metal dichalcogenide homobilayers

The ability of engineering long-period superlattices using twisted van der Waals interfaces has been utilized to explore emergent physical phenomena in 2-dimensional (2D) systems. In homojunctions of 2D atomic layers, at small twist angles, the single particle density of states can be enhanced resulting in the flat band condition. In this presentation, we will discuss Fermi level instabilities due to the developing of a singular density of states in the partially filled moire flat band. We will discuss our recent experimental development of observation of superconductivity in twisted homojunction of semiconducting transition metal dichalcogenide (TMD). Two monolayers of TMDs were stacked together with small twisting angles between them, exhibiting a zero-resistance state at low temperatures below 1 K which critical magnetic field ~ 0.1 T. The detailed nature of this superconducting state will be discussed.   

Gate controlled emission from hetero-bilayer of transition metal dichalcogenides

We optically probe interlayer excitons formed between WSe2 and MoSe2 monolayers. The type II band alignment that exists between this hetero-bilayer results in electron transfer from WSe2 to MoSe2 and hole transfer in opposite directions. The spatially separated electrons and holes results in interlayer excitons with large lifetime. In our FET geometry, we control the charge carrier density in the transition metal dichalcogenide monolayers which modifies the emission of the interlayer exciton at the hetero-bilayer. By changing the charge carrier density in the WSe2 monolayer, we observe a shift from charge to neutral exciton in MoSe2 monolayer and simultaneous enhancement of interlayer exciton photoluminescence. We also report observation of localized emission from the hetero-bilayer at a low temperature of 4 K.   

Twist, Slip, and Circular Dichroism in Van der Waals Bilayers

Stacking atomically thin 2D materials with a rotational misalignment between its layers produces a van der Waals bilayer in which all mirror symmetries can be broken. A fundamental experimental signature of a twisted multilayer is circular dichroism (CD): the conversion of a linearly polarized incident optical field to an elliptically polarized field in transmission. This work investigates the relation between CD, the twist angle and interlayer slip. In experiments where bilayers are formed by contacting one layer with another one has control of the relative rotation angle between the symmetry axes of each sheet, but not of lateral shifts between the layers. Accounting for the lateral shift the configuration space of the system introduces a phase degree of freedom which manifests in the CD and its frequency dependence. We demonstrate how the symmetry constraints in this expanded configuration space is manifest in CD in twisted bilayers as a function of rotation angle. We apply this approach to discuss twisted graphene bilayers and on bilayer transition metal dichalcogenides where the twist coherently couples the intralayer excitons of the two layers.
  

Valley Polarization and Coherence in WSe2-WTe2 Alloys

Valleytronics is a driving motivation behind the exploration of transition metal dichalcogenides (TMDs). However, few studies have explored valley-dependent phenomena in TMD alloys. This is an important knowledge gap as the flexibility provided by alloying may be vital for the application of TMDs. We study the valleytronic properties of monolayer WSe_2(1-x)Te_2x (x=0…1), an alloy system where the endpoints WSe₂ and WTe₂ occupy different structural phases (H and 1T’, respectively). As the Te composition x is increased, the alloy undergoes a semiconductor-semimetal phase transition, which we explore with temperature-dependent Raman and photoluminescence (PL) measurements. We find that Te incorporation activates new Raman modes, while also leading to non-monotonic shifts in the neutral exciton and trion energies and linewidths. Temperature-dependent PL measurements show valley polarization and valley coherence survive when Te doping is less than ~23%, above which valley polarization disappears while valley coherence gradually decreases. These findings demonstrate that valleytronic properties can be robust against disorder and illustrate the potential of two-dimensional alloys for valleytronic technologies.   

O2-induced in-situ manipulation of exciton recombination pathways in 2D heterostructures: Submicron, intensity-programmable pixels with rapid write-read-erase capability, as well as 2D O2-sensor applications

We present a novel oxygen-induced switching between “non-radiative” and “radiative” exciton recombination in a family of 2D heterostructures: monolayer Bi₂Se₃ grown on arbitrary monolayer transition metal dichalcogenides (TMDs), to include TMD alloys. It is believed to be the result of O₂ diffusing/intercalating between between the layers and disrupting the interlayer interaction. The signature photoluminescence (PL) peaks of TMDs are quenched in all as-grown heterostructures, but can be controllably recovered by heating in the presence of oxygen, and then re-quenched by heating in the presence of N₂ or Ar. The intensity PL switching can also be accomplished using a low-power focused laser, while changing the environment from pure nitrogen to air, enabling high control with submicron resolution. This allows for site-programmable, color-selectable, atomically-thin, micron-scale 2D optical “Write-Read-Erase” light-emitting pixels (PLPs) with effective volumes of ~10^-21 m³. The emission intensity can be precisely varied by a factor exceeding 200×, with a wide range of emission energy values in the visible (1.5eV<E_ph<2eV).   

Electrostatic Traps of Interlayer Excitons in MoSe2/WSe2 Heterostructures

The two-dimensional (2D) nature and large excitonic binding energy of transition metal dichalcogenides (TMDs) allow for the exploration of novel quantum optical effects. Using type-II heterostructures formed by stacking MoSe₂ and WSe₂ monolayers, optical excitation generates interlayer excitons, bound electrons and holes residing in spatially separated layers. The out-of-plane, permanent dipole moment of interlayer excitons allows for the control of the emission energy by tuning the vertical electric field using dual-gated devices. By spatially varying the vertical electric field with patterned gates, we can generate potential profiles that can spatially control the interlayer exciton energy. We observe changes in the exciton cloud shape and emission brightness depending on a flat, trapping, or anti-trapping electric field profile. Finally, we estimate an upper-bound interlayer exciton density that can be tuned with the trap depth. With the ability to generate high densities of interlayer excitons, trapped interlayer excitons can serve as a platform for generating and exploring Bose-Einstein condensates at high temperatures.   

Fowler-Nordheim tunneling through WSe2 vertical junctions

Van der Waals (vdW) materials consisting of 2D atomic layers have attracted lots of interest in the future electronic applications thanks to their superior electrical, optical and mechanical properties. Due to their weak interlayer coupling across physical vdW gaps, vertical charge transport through these vdW layered materials is fundamentally different from in-plane transport behaviors. In this study, we investigate detailed vertical charge transport mechanisms, especially in the regime of Fowler-Nordheim (FN) tunneling, through vertical WSe₂ junctions while controlling WSe₂-layer thickness from a monolayer to multilayers with a single atomic-thick resolution. We implement a simple but reliable device structure, graphite/WSe₂/graphite vertical heterojunctions, fabricated on a h-BN/SiO₂/Si substrate by mechanical transfer technique. We observe vertical charge transport through WSe₂ layers is governed by the FN tunneling even for a monolayer WSe₂ when the vertical junctions are applied by sufficiently high electric fields. Moreover, we find out that FN tunneling characteristics can be used for identifying not only the layer number but also characterizing key material properties of layered vdW materials such as effective mass and quasiparticle energy gap.   

Creating and tuning of interlayer exciton gases in transition metal dichalcogenides heterostructures

Stacking two-dimensional materials into van der Waals heterostructure offers a powerful approach toward creating artificial lattices with desired band structures and functionalities. In transition metal dichalcogenides heterostructures, interlayer exciton emerges due to band alignment of constituent layers and has been demonstrated to have long exciton lifetime [1] and valley lifetime [2], providing a platform to study degenerate Boson gases, rich valley physics, and possible optoelectronics applications. Here we report highly tunable interlayer excitons by an out-of-plane electric field in homobilayer WSe₂. Electric field can tune the interlayer exciton dipole orientation from negative to positive and induce a Stark shift of up to 100meV in the exciton resonance energy. Moreover, with applied electric field, the exciton lifetime is greatly enhanced by more than two orders of magnitude (from ~200ps to >20ns), allowing the creation of interlayer exciton gas with density as high as 1.2x10¹¹cm^-2 by moderate continuous-wave optical pumping. Furthermore, we can achieve high interlayer exciton density (~10¹²cm^-2) with a trilayer heterostructure by both optical pumping and electrical injection.
[1] Nat. Commun. 6, 6242 (2015).
[2] Sci. Adv. 3, (2017).   

A fresh look at interlayer emission from WSe2/MoS2 heterostructures

The recent two years have seen a rising interest in the study of interlayer excitons (ILX) in Van der Waals layered materials, where electron-hole pairs are bound together across the interlayer gap. Special attention was drawn to such ILX in heterostructures of semiconductor transition metal dichalcogenides with staggered band alignment such as MoSe₂ and WSe₂. That includes the exploration of their optical properties, lifetime, optical selection rules, spin/valley properties, the influence of crystal alignment (a.k.a the moire pattern), the role of momentum-indirect transition, etc.. The latter subject was also studied for the ILX observed in WSe₂ and MoS₂ heterostructure (Kunstmann et al. Nature Physics, DOI: 10.1038/s41567-018-0123-y ,2018), where the reported ~1.6 eV emission is assigned to such indirect transition. In this talk, I will report on the observation of a different interlayer emission from the same system. Probing its response to different experimental conditions, I will discuss its properties and its origin.   

Observation of Moiré Excitons in van der Waals Heterostructure

Moire superlattice in van der Waals systems provide a powerful tool to engineer the properties of two-dimensional materials through introducing a new energy and length scale. For example, correlated insulating states and superconductivity have been reported in graphene systems, where the moiré superlattice qualitatively change the behavior of electrons. The effects of moiré superlattice on excitons, on the other hand, is less studies experimentally. Here we report the observation of moiré excitons in heterostructure of two-dimensional semiconductors, and show that the properties of excitons can also be strongly modified by a moiré superlattice.   

Spin-charge coupled transport in topological-insulator-based heterostructures

Experimental advances have made possible the creation of heterostructures composed of two or more layers of two-dimensional systems that combine their individual properties to reach desired functionalities. In this talk, we present the spin-charge coupled diffusion equations for heterostructures with random interlayer tunneling and in which one of the layers has strong spin-orbit coupling. We consider two distinct regimes: the weak coupling regime, for which the disorder-induced scattering time is shorter than the tunneling-induced scattering time, and the strong coupling regime, for which random tunneling events are dominant. We then apply our formalism to the case in which a two-dimensional electron gas is placed in proximity to the surface of a three-dimensional topological insulator and discuss the relevance of our results for experiments.   

Quantum spin-wave dynamics and magneto-transport behaviour in 2D van der Waals heterostructure

Spin-wave ensembles in qunatum regime coupled together to offer an exciting prospect of observing complex behaviour interlinking the microscopic spin features with macroscopic spin ensembles [\emph{Phys. Rev. Lett.} $\bf{121}$, 126401 (2018)]. This feature is quite evidence in two dimensional van der waals heterostructure systems [\emph{Phys. Rev. Lett.} $\bf{121}$, 067701, 2018]. In those systems, spin-transfer torque plays a major role to explain the induced magneto-transport phenomenon. Here, we demonstrate the collective interactions in an ensemble of spin-waves to unravel the full quantum dynamics and transport studies. We model a time-adaptive variational renormalization group method that accurately captures the underlying spin-wave dynamics and magneto-transport equations with potential device applications [\emph{arXiv}:1808.04418].