Session Y17: 2D Semiconductor Physics IV

Engineering excitonic properties and valley polarization in transition metal dichalcogenide monolayers

Binary Transition metal dichalcogenide (TMDC) monolayer (ML) materials MoS2, MoSe2, WSe2, WS2 and MoTe2 share common properties such as a direct optical bandgap, Spin-Orbit splittings of hundreds of meV and coupled spin-valley states. Optical absorption and emission are dominated by robust excitons, whose resonances also strongly influence Raman scattering amplitudes [1] and second harmonic generation efficiency [2]. Important differences in opto-electronic properties between these materials depend on whether the exciton ground state is optically bright or dark. This order will depend on the conduction band Spin-Orbit splitting and the electron-hole Coulomb interaction and will have strong influence on the light emission yield of the TMDC MLs. In this talk we discuss Spin-Orbit engineering in Mo(1-x)W(x)Se2 alloy monolayers [3]. We probe the impact of the tuning of the conduction band Spin-Orbit spin splitting on the bright versus dark exciton population. For MoSe2 monolayers the PL intensity decreases as a function of temperature by an order of magnitude (T=4-300 K), whereas for WSe2 we measure surprisingly an order of magnitude increase. The ternary material shows a trend between these two extreme behaviors. In addition we show a non-linear increase of the optically generated valley polarization as a function of tungsten (W) concentration. Tuning the optical properties in applied external fields will be discussed. [1] G. Wang et al, PRL 115, 117401 (2015) [2] G. Wang et al, 2D Mater. 2 , 045005 (2015) [3] G. Wang et al, Nature Comms. (in press, arxiv 1506.08114)   

Hybrid interlayer excitons with tunable dispersion relation

When two semiconducting materials are layered on top of each other, interlayer excitons can be formed by the Coulomb attraction of an electron in one layer to a hole in the opposite layer. The resulting exciton is a composite boson with a dispersion relation that is a hybrid between the dispersion relations of the electron and the hole separately. In this talk I show how such hybridization is particularly interesting when one layer has a “Mexican hat”-shaped dispersion relation and the other has a conventional parabolic dispersion. In this case the interlayer exciton can have a range of qualitatively different dispersion relations, which can be continuously altered by an external field. This tunability in principle allows one to continuously tune a collection of interlayer excitons between different quantum many-body phases, including Bose-Einstein condensate, Wigner crystal, and fermion-like “moat band” phases.   

Electric Field Dependent Photoluminescence in Atomically Thin Transition Metal Dichalcogenides van der Waals Heterostructures.

uregui, Alex A. High, Alan Dibos, Andrew Joe, Elgin Gulpinar, Hongkun Park, Philip Kim Harvard University, Physics Department -abstract- Single layer transition metal dichalcogenides (TMDC) are 2-dimensional (2D) semiconductors characterized by a direct optical bandgap and large exciton binding energies (\textgreater 100 meV). We fabricate CQW heterostructures made of 2D TMDCs with hexagonal Boron nitride (BN) as atomically thin barrier and gate dielectric, with top and bottom gate electrodes. We study the evolution of photoluminescence (PL) spectrum with varying BN barrier thickness, electric field, temperature and polarization. Our measured low-temperature (T $=$ 3K) PL peaks show full width at half maxima on the order of \textasciitilde 3meV. We identify the photoluminescence peaks, corresponding to the charged exciton emission, which red shifts and its brightness increases while the neutral exciton emission becomes darker for increasing electric field.   

Giant and tunable valley degeneracy splitting in MoTe$_2$

Valleys in monolayer transition-metal dichalcogenides seamlessly connect two basic carriers of quantum information, namely, the electron spin and photon helicity. Lifting the valley degeneracy is an attractive route to achieve further optoelectronic manipulations. However, the magnetic field only creates a very small valley splitting. We propose a strategy to create giant valley splitting by the proximity-induced Zeeman effect. Our first principles calculations of monolayer MoTe$_2$ on a EuO substrate show that valley splitting over 300 meV can be generated. Interband transition energies become valley dependent, leading to selective spin-photon coupling by optical frequency tuning. The valley splitting is also continuously tunable by rotating the substrate magnetization. The giant and tunable valley splitting adds a different dimension to the exploration of unique optoelectronic devices based on magneto-optical coupling and magnetoelectric coupling.   

Tunable optical second-harmonic generation from bilayer MoS2 by controlled inversion symmetry breaking

Due to the presence of a center of inversion, optical second-harmonic generation (SHG) from an unperturbed bilayer 2H-MoS$_2$ crystal is strongly suppressed compared to the non-centrosymmetric monolayer. In this paper, we show that SHG from bilayer MoS$_2$ is enhanced when it is supported on a fused silica substrate. This enhancement originates from lifting of inversion-symmetry induced by substrate interactions. Further, by applying an out-of-plane electrostatic field in a back-gating geometry, we demonstrate highly tunable SH radiation from supported MoS$_2$ bilayers. Complementing recent work on bilayer WSe$_2$ [1], where the sample is primarily studied in the hole-doped regime, the bilayer MoS$_2$ samples in our study are shown to be in the electron-doped regime. Through a comparison of the field-induced change in the second-order nonlinear susceptibilities of monolayer and bilayer MoS$_2$, we identify the importance of interlayer coupling in the tunable SHG from bilayer MoS2. [1] Huakang Yu, Deep Talukdar, Weigao Xu, Jacob B. Khurgin, and Qihua Xiong, Nano Lett. 15 5653 (2015)   

Strain-induced topological quantum phase transition in phosphorene oxide

Using {\em ab initio} density functional theory, we investigate the structural stability and electronic properties of phosphorene oxides (PO$_{x}$) with different oxygen compositions {\em x}. A variety of configurations are modeled and optimized geometrically to search for the equilibrium structure for each {\em x} value. Our electronic structure calculations on the equilibrium configuration obtained for each {\em x} reveal that the band gap tends to increase with the oxygen composition of {\em x} < 0.5, and then to decrease with {\em x} > 0.5. We further explore the strain effect on the electronic structure of the fully oxidized phosphorene, PO, with {\em x} = 1. At a particular strain without spin-orbit coupling (SOC) is observed a band gap closure near the ${\Gamma}$ point in the {\em k} space. We further find the strain in tandem with SOC induces an interesting band inversion with a reopened very small band gap (~5 meV), and thus gives rise to a topological quantum phase transition from a normal insulator to a topological insulator. Such a topological phase transition is confirmed by the wave function analysis and the band topology identified by the {\em Z$_{2}$} invariant calculation.   

Electronic and structural phase transitions induced by uniaxial strains in monolayer SnSe

Two dimensional (2D) materials have attracted unprecedented research interest owing to their unique properties that are suitable for various applications. Recent research has started to explore 2D materials beyond graphene; examples include transition metal dichalcogenides and black phosphorus. Bulk SnSe is a layered semiconductor which exists in two phases. The low temperature Pnma phase has an indirect band gap of 0.89 eV and a direct band gap of 1.3 eV, while the high temperature Cmcm phase is stabilized at T \textgreater 800 K. In this talk, we will present first-principles investigations of the effects of strains on the electronic and structural properties of SnSe. We find that uniaxial strains are an effective means to tune the properties single layer SnSe, and may also induce phase transitions in this system.   

Strain Engineering of Transition Metal Dichalcogenides

The application of strain to materials can cause changes to bandwidth, effective masses, degeneracies and even structural phases. In the case of the transition metal dichalcogenide (TMD) semiconductors, small strain (around 1 percent) is expected to change band gaps and mobilities, while larger strains are expected to cause phase changes from the triangular 2H phase to orthorhombic 1T' phases. We will describe experimental techniques to apply small and large (around 10 percent) strains to one or few layer samples of the TMD semiconductors, and describe the effect of the strain using optical (Raman, photoluminescence) and cryogenic transport techniques.   

Simultaneous tunablility of electronic and phononic gap in SnS$_2$ under normal compressive strain

Ever since the discovery of graphene, 2D materials have emerged as an attractive field of research. Here, using density functional theory based calculations, we show tunability in the electronic structure of mono to multilayered SnS$_2$ under biaxial tensile (BT), biaxial compressive (BC), and normal compressive (NC) strains. We obtain a reversible semiconductor to metal (S-M) transition in mono to multilayered SnS$_2$ without changing the nature of the band gap (i.e. indirect). For the stability analysis with applied strain, we use bilayer (2L-)SnS$_2$ as our prototype system. Surprisingly, under a high NC strain, 2L-SnS$_2$ does not exhibit unstable modes. The phonon spectra of 2L-SnS$_2$ shows a gap in the optical region, which, most interestingly, increases with applied NC strain. Such a simultaneous tunability of the electronic as well as phononic properties of SnS$_2$ under applied strain can be exploited in many applications such as pressure sensors, micromechanical resonators, frequency filters, and in many other multi-physics devices.   

Using dispersive medium to control excitons in 2D materials.

Excitons in 2D materials (2DMs) are known to be sensitive to the surrounding environment. This makes it possible to modify 2D excitons by depositing materials with controlled dielectric constant on top of 2DMs. This possibility becomes especially interesting if we consider materials with dielectric permittivity $\varepsilon $ that depends both on wavevector $k$ (this happens if the medium is spatially non-uniform) and frequency $\omega $. Here, we develop platforms to control $\varepsilon (k$,$\omega )$ and explore resulting changes in light-matter interactions of 2DMs. To examine the effect of wavevector-dependent permittivity of the medium, we study absorption/photoluminescence of graphene and MoS$_{\mathrm{2}}$ in the vicinity of highly non-uniform medium - an array of metal nanoparticles, 3-5 nm in diameter. In this case absorption of light can lead to creation of excitons with non-zero momentum. These dark states are not accessible via regular absorption spectroscopy. We study the case of frequency-dependent permittivity by surrounding MoS$_{\mathrm{2}}$ by a highly-dispersive media (e.g. dielectric liquids, graphene and VO$_{\mathrm{2}})$. We demonstrate non-trivial frequency-dependent renormalization of the quasiparticle bandgap and exciton binding energies.   

Functional polymers for electronic-structure modulation of MoS$_{2}$

Two-dimensional semiconductors based on the Mo and W family of transition metal dichalcogenides (TMDCs) are emerging as an important class of materials with unique optoelectronic properties. However, there remain challenges associated with precise control over carrier doping and work functions that need to be overcome for device applications. We report the synthesis of new tetrathiafulvalene (TTF)-based polymers that provide enhanced solution stabilization of MoS$_{2}$ nanosheets while simultaneously modulating their electronic structure through robust, non-covalent interactions. Kelvin probe force microscopy (KPFM) imaging of TTF-polymer functionalized 2H MoS$_{2}$ nanosheets confirms n-doping of the MoS$_{2}$ with an accompanying reduction in the work function. Density functional theory calculations provide insight into the TTF-MoS$_{2}$ interfacial interactions and provide a theoretical basis for modulation of electronic properties of MoS$_{2}$ via charge-transfer interactions. These combined results illustrate the potential for polymer doping of TMDCs as a viable and scalable approach for synthesis of new hybrid materials for optoelectronics.   

Electronic and magnetic engineering of transition metal dichalcogenides

Transition metal dichalcogenides (TMDs) have moderate bandgaps and great potential in electronic and optoelectronic applications. We show that by intercalation and compensated doping of transition metal ions, we could generate a ``half-semiconductor'', half-metal or doped magnetic semiconductor. We will also show that covalently connecting a single layer of WS2 to a small aromatic molecule with appropriate electronegativity, it is possible to align the molecular energy levels with the WS2 conduction band edge, yielding an electronic structure of potential interest for thermoelectric applications, and covalently connecting single-layer WS2 to magnetic coordination compounds could introduce magnetization into the WS2 layer.   

Electronic states at transition metal dichalcogenide lateral heterointerfaces

Materials with different band gaps are typically used to create heterostructures that enable band sculpting, depending on different shape and boundaries of the systems. These are used in diode lasers and high-speed transistors devices. Potential material candidates for such heterostructures at the monolayer level are the family of transition-metal dichalcogenides, MX$_2$ (with M=Mo,W and X=S,Se), especially interesting materials with strong spin-orbit coupling and valley degrees of freedom. We consider lateral interfaces between pairs of these materials, and study the effect of different boundary geometries, motivated by recent experimental reports of the growth of such interfaces with different geometries [2,3]. Using an effective 3-orbital tight-binding model [1], we focus our attention on monolayer ribbons and triangular flakes. We analyze the formation of edge/interface states for different gap nesting materials. We study the spatial distribution and orbital character of the wave functions throughout, as well as their dependence on interface termination. [1] G. B. Liu et al., PRB 88, 085433 (2013). [2] C. Huang et al., Nat. Mat. 13, 1096 (2014). [3] Y. Gong et al., Nat. Mat. 13, 1135 (2014).