Session B36: 2D Materials - Heterostructures I

Valley-spin phenomena in the moire pattern of van der Waals heterostructures

In semiconducting transition metal dichalcogenides monolayers, the band edge carriers are described by massive Dirac cones, located at K and -K corners (valleys) of the Brillouin zone. These massive Dirac Fermions have interesting properties dependent on their valley index, which enable versatile control of the valley pseudospin. Van der Waals stacking of the 2D semiconductors into vertical heterostructures is a powerful approach towards designer quantum materials that can extend the exotic properties of the building blocks. A generic aspect of these vdW heterostructures is the formation of moiré pattern (i.e. periodic variation of local atomic registries) due to the inevitable lattice mismatch. We show that, because of the valley degeneracy of band edges in the monolayer building blocks, the moiré pattern leads to lateral modulations in the electronic, optical, and topological properties in the vdW heterostructure [1,2]. These modulations can endow heterobilayers a number of intriguing properties, including: (i) electrically switchable and strain tunable lateral superstructures of topological insulators [1]; (ii) programmable arrays of single photon emitters and entangled photon sources; and (iii) spin-orbit coupled excitonic superlattices [2].

[1] Q. Tong, H. Yu, Q. Zhu, Y. Wang, X. Xu and W. Yao, Topological Mosaic in Moiré superlattices of van der Waals heterobilayers, Nature Physics 13, 356 (2017).
[2] H. Yu, G.-B. Liu, J. Tang, X. Xu, W. Yao, Moiré excitons: From programmable quantum emitter arrays to spin-orbit–coupled artificial lattices. Science Advances 3, e1701696 (2017).   

Complexes of Indirect Excitons in Layered Quasi-2D Heterostructures

We discuss trion and biexciton complexes formed by indirect excitons in layered quasi-2D semiconductors. Indirect excitons – long-lived neutral Coulomb-bound pairs of electrons and holes of different monolayers – were reported recently for bilayer graphene[1] and transition metal dichalcogenide systems[2]. We use the configuration space method[3] to derive the binding energies for the trion and biexciton complexes of indirect excitons as functions of the interlayer separation distance. The method captures essential kinematics of complex formation to reveal that, despite a rapid decrease with distance, the binding energies of both complexes can be significant – up to a few tens of meV – for (typical) interlayer distances ~3-5 Å, with the trion always having a greater binding energy than the biexciton. Trions and biexcitons formed by indirect excitons control the formation of more complex Wigner-like electron-hole crystal structures which are of great interest for spin-optronics applications[4,5]. -- [1]J.I.A.Li, et al., Nat. Phys.13,751(2017); [2]M.Baranowski, et al., NL17,6360(2017); [3]I.V.Bondarev, Mod. Phys. Lett.B30, 1630006(2016); [4]G.J.Schinner, et al., PRL110, 127403(2013); [5]J.S.Ross, et al., NL17,638(2017).   

Origin of the counterintuitive dynamic charge in the transition metal dichalcogenides and electronic properties in their heterostructures.

Transition metal dichalcogenides have generated an immense amount of interest and recent work for applications in optoelectronics, sensing, etc... Despite the number of studies, the diversity of their chemical bonding characteristics and charge transfer is not well understood. Using density functional (perturbation) theory we determine and compare their static (Bader) and dynamic (Born) charges. The dynamic charge of the transition metal dichalcogenides with trigonal symmetry are anomalously large, while in their hexagonally symmetric counterparts, we find a counterintuitive sign, i.e., the transition metal takes a negative charge. This phenomenon, observed in a few compounds in the past, but never analyzed, is understood by investigating the perturbative response of the system, and by investigating the hybridization of the molecular orbitals near the Fermi level. The charges and electronic properties of heterostructures composed of symmetry-equivalent Dichalcogenides are also investigated, showing competing effects due to strain and interlayer polarization.   

Enhancing Light-Matter Interaction by Coupling MoSe2/WSe2 Van der Waals Heterostructures to Confined Photonic Resonances

Semiconductor heterostructures are essential building blocks for numerous electronic and optoelectronic devices such as lasers, light-emitting diodes, solar cells, and transistors. Layer-stacking of atomically thin transition metal dichalcogenides (TMDC) has opened up new opportunities for novel light-emitting phenomena. Confined photonic resonances are particularly of interest in enhancing light-matter interaction because they are able to localize light significantly both in the spatial and temporal domain. In this work, we incorporate heterostructures of monolayer molybdenum diselenide and tungsten diselenide within two different monolithic photonic structures. We investigate exciton hybridization in two devices. Device one is based on planar Bragg mirror microcavities and device two utilizes Tamm-plasmon modes. For both devices enhanced photoluminescence is observed and polariton formation is demonstrated via the anti-crossing in the measured energy-momentum dispersion curve. Our results pave the way for novel polariton devices based on TMDC van der Waals heterostructures, where Berezinskii-Kosterlitz-Thouless (BKT) transition of this 2D low-mass boson gas can be realized.   

Gate Dependent Electroluminescence of Interlayer Excitons in 2D Semiconductor Heterostructures

Monolayer transition metal dichalcogenides (TMDs) are two-dimensional (2D) semiconductors characterized by a direct band-gap and large exciton binding energies. Due to the 2D quantum confinement, their heterostructures exhibit a number of novel physical properties. By vertically stacking two different TMD monolayers, we can realize a type II heterostructure. In these heterostructures electrons and holes can be confined in individual layers, forming spatially separated long lived interlayer exciton (IE). In this work, we fabricate dual-gated MoSe₂/WSe₂ heterostructures encapsulated by boron nitride (BN) with electrical contacts in each layer. By applying forward bias voltage across the vertical junction, we observe gate tunable, near-infrared electroluminescence (EL) of interlayer excitons. By changing the relative doping between WSe₂ and MoSe₂ we can control the spatial location of EL emission. In addition, we find that EL lifetime is comparable to IE photoluminescence lifetime of ~300ns, showing that we can create long lived IE electrically. Such long lifetimes and spatial control pave the way to fully electrically addressable IE condensates, tunable near-IR excitonic lasers and other novel optoelectronic devices.   

Negative circular polarization emissions from epitaxially grown WSe2/MoSe2 commensurate heterobilayers

Interlayer excitons in van der Waals heterobilayers of transition metal dichalcogenides (TMDs) possess enriched spin-valley coupling of carriers in different layers and emerge as a new platform for exploring advanced spin/valleytronic applications. It has been theoretically predicted that the optical properties (polarization selection rule, transition dipole strength, etc.) of interlayer excitons are sensitive to interlayer atomic registry. Manually stacked TMD heterobilayers, however, are incommensurate with inevitable interlayer twist and/or lattice mismatch, which could smear the effects. Here we report on the distinct polarization properties of valley-specific interlayer excitons using epitaxially grown, commensurate WSe₂/MoSe₂ heterobilayers with well-defined atomic registry (AA and AB). We observed circularly polarized photoluminescence from interlayer excitons, but with a helicity opposite to the optical excitation. The negative circular polarization arises from the quantum interference imposed by interlayer atomic registry, giving rise to distinct polarization selection rules. Using selective excitation of excitons in different monolayers, we demonstrate the optical addressability for interlayer excitons with different valley configurations and polarization helicities.   

Novel two-dimensional van der Waals materials: layered IV-V compounds

Since the successful exfoliation of graphene from graphite, a variety of two-dimensional (2D) van der Waals (vdW) materials have also been emerged and extensively studied over the last decade. Nevertheless there have been continuing demand for new 2D vdW materials that can exhibit unusual physical properties. Here, we propose layered IV-V compounds as novel 2D vdW materials based on our first-principles calculations. Our predicted IV-V compounds are in the form of A₂B₂, where A and B are elements in group IV or 14 (C, Si, Ge, Sn) and group V or 15 (N, P, As, Sb), respectively. These structures are similar to those of layered III-VI compounds, such as GaSe or InSe. Our investigation reveals that newly-discovered compounds have two distinguishable phases, α and β, which have a mirror and an inversion symmetry. To explore their structural stabilities and their phase transitions from one phase to the other, we perform geometrical optimization; evaluate their phonon dispersion relations; and estimate activation barrier of the phase transitions. We further investigate their electronic properties by calculating not only their electronic band structures, but also carrier transport properties including electron-phonon interaction.   

Quantum valley Hall effect and valleytronics in bilayer graphene

The advent of two-dimensional materials with hexagonal crystal symmetry offers a new electronic degree of freedom, i.e. valley, the manipulation of which could potentially be exploited to form new many-body ground states as well as new paradigms of electronic applications. In high-quality bilayer graphene, the application of a perpendicular electric field opens a tunable band gap, the sign of which can be reversed by reversing the polarity of the applied E-field. In analogy to the quantum spin Hall effect, valley-momentum locked quantum valley Hall kink states arise at the internal line junction of two oppositely gated bilayer graphene regions [1]. Moreover, the helicity of the kink states can take values of ± 1, by changing the polarities of the applied E-field. In this talk, I will describe our experiments in realizing the kink states, which exhibit conductance close to 4e^2/h at zero magnetic field and nice quantization at a few Teslas [2][3]. We also demonstrate, in a “cross” device consisting of four kink channels, the electrically controlled operations of a valley valve, a waveguide and a tunable electron beam splitter [3]. The on/off ratio of the valley valve is about 800% at B=0 and T=1.5K. These novel valleytronic operations exploit unique properties of the kink states. The high quality and versatile controls of this new helical 1D platform open the door to many exciting opportunities in valleytronics and topological physics.
[1] Ivar Martin, Ya. M. Blanter, and A. F. Morpurgo, Topological Confinement in Bilayer Graphene, PRL 100, 036804 (2008)
[2] J. Li, K. Wang, K. J. McFaul, Z. Zern, Y. Ren, K. Watanabe, T. Taniguchi, Z. Qiao, and J. Zhu, Gate-controlled topological conducting channels in bilayer graphene, Nature Nanotechnology 11, 1060 (2016).
[3] J. Li, R.-X. Zhang, Z. Yin, J. Zhang, K. Watanabe, T. Taniguchi, C. Liu, and J. Zhu, A valley valve and electron beam splitter in bilayer graphene, arXiv:1708.02311v1 (2017).
  

Large bulk piezolectric response of Van der Waals bonded quasi-2D solids

The bulk piezoelectric response of ionic solids, as measured by the piezoelectric modulus tensor d, is driven by the redistribution of a charge upon the application of stress. Hence, soft materials can generally be expected to exhibit higher piezoelectric modulus compared to stiffer systems. Motivated by the soft nature of the Van der Waals bonded quasi-2D solids, we focus our search for new piezoelectric compounds to this particular family of materials. The quasi-2D structures are identified using an automated algorithm from a pool of more than 11,000 structures reported in the Inorganic Crystal Structure Database (ICSD). The results revealed 572 materials with d_max>0.1 pC/N, out of which 192 materials have d_max greater than AlN, a material commonly used in high-frequency resonators. Interestingly, we have also found 32 compounds, including TaS₂, In₂Se₃, GeTe, and other, with moduli higher than PbTiO₃, another piezoelectric material with the bulk piezoelectric modulus among the largest known. Our results reveal the critical role of soft Van der Waals bonds between the layers as d_max often couples to a stress component (compressive or shear) orthogonal to the layers and offer guidance in selecting quasi-2D materials with high piezoelectric modulus (d).   

Diverse magnetic quantization in bilayer silicene

The generalized tight-binding model is developed to investigate the rich and unique electronic properties of AB-bt bilayer silicene under the uniform perpendicular electric and magnetic fields. The first pair of conduction and valence bands, with an observable energy gap, possesses the unusual energy dispersions. Each group of conduction/valence Landau levels (LLs) is further classified into four subgroups; that is, there exist the sublattice- and spin-dominated LL subgroups. The magnetic-field-dependent LL energy spectra exhibit the irregular behavior corresponding to the critical points of band structure. Moreover, the electric field can create the frequent LL anti-crossings. The main features of LLs are revealed as many van Hove singularities in density of states. The feature-rich magnetic quantization directly reflects the geometric symmetries, the intralayer and interlayer atomic interactions, the spin-orbital couplings, and the field effects.