Session Q2: Focus Session: Beyond Graphene - Optics in 2D Semiconductors IV

Probing the Interlayer Coupling of Twisted Bilayer MoS$_{2}$ Using Photoluminescence Spectroscopy

Two-dimensional molybdenum disulfide (MoS$_{2})$ is a promising material for optoelectronic devices due to its strong and stable photoluminescence emissions. In this work, the photoluminescence spectra of twisted bilayer MoS$_{2}$ are investigated, revealing a tunability of the interlayer coupling of bilayer MoS$_{2}$. For the twisted angle 0$^{\circ}$ or 60$^{\circ}$, the photoluminescence from the trion and exciton of bilayer MoS$_{2}$ shows the highest intensity ratio, and the trion binding energy reaches its maximum value. For the twisted angle 30$^{\circ}$ or 90$^{\circ}$, the situation is the opposite. These experimental observations are mainly attributed to the change of the interlayer coupling with the twisted angles. The first-principles density functional theory analyses further confirm the change of the interlayer coupling with the twisted angle, and these analyses interpret and support our experimental results.   

Raman Spectroscopy of Atomically Thin MX$_{2}$ Materials

There has been growing interest in atomically thin layers of MX$_{2}$ materials, such as MoS$_{2}$, WS$_{2}$, MoSe$_{2}$, and WSe$_{2}$. Monolayers of these materials exhibit many physical properties distinctly different from those of bulk crystals, such as a direct rather than an indirect band gap, strong photoluminescence, and large exciton binding energies. Raman spectroscopy provides a powerful tool to characterize atomically thin 2D materials, having been utilized to probe the electron-phonon coupling and identify the layer thickness of MX$_{2}$ materials. To better understand Raman spectra, we perform systematic studies of Raman scattering in different MX$_{2}$ materials as a function of the photon excitation energy, light polarization, and spatial position. I will discuss their implications for quantitative characterization of MX$_{2}$ materials using Raman spectroscopy.   

Characterizing and tuning excitons in monolayer and few-layer $MoS_2$

We use the GW-BSE method to study excitons arising from transitions in different regions of momentum space in mono- and few-layer $MoS_2$ and consider mechanisms to fundamentally change the features and character of the optical spectra. Our calculations show that sharp spatial variations in dielectric screening make 2D systems, such as $MoS_2$, computationally challenging, requiring very fine k-space sampling to resolve the structure of excitonic wave functions and converge binding energies. In highly converged calculations, we identify a series of excitons arising from transitions at the K/K$'$ valleys in the Brillouin zone, a higher energy series arising from transitions in the valley of a Mexican hat potential centered at the $\Gamma$ point, and transitions at the indirect gap from $\Gamma$ to $\Lambda$ in few-layer $MoS_2$. As layer number changes, these states, which have varying character, momentum-space structure and real-space locations, are affected differently by changes in confinement and hybridization. By tuning layer number and strain, we find that we not only can tune the excitation energies but can also change the relative energies of the various excitonic series, allowing for movement of the lowest energy exciton between different regions of the Brillouin zone   

Exciton band structure of monolayer MoS$_2$

We describe a theory of the momentum-dependent exciton spectrum of monolayer molybdenum disulfide. Low-energy excitons occur both at the Brillouin zone center and at the Brillouin-zone corners. We find that binding energies at the Brillouin-zone center deviate qualitatively from the $(n-1/2)^{-2}$ pattern of the two-dimensional hydrogenic model. Moreover, the four $2p$ states of $A$ series are lower in energy than the corresponding $2s$ states and not degenerate. The two-fold ground-state valley degeneracy is lifted linearly at small momenta by electron-hole exchange processes that establish inter valley coherence. We conclude that atlhough monolayer MoS$_2$ is a direct-gap semiconductor when classified by its quasiparticle band structure it may well be an indirect gap material when classified by its excitation spectra, and speculate on the role of this property in luminescence characteristics.   

Helicity Resolved Raman Scattering of Atomic Layers of Transition Metal Dichalcogenides

Semiconducting transition metal dichalcogenides (TMDCs) such as MoS$_{2}$, MoSe$_{2}$, WS$_{2}$ and WSe$_{2}$ are promising two dimensional (2D) materials for electronic and optoelectronic applications. Moreover, the unique capability to manipulate the valley degree of freedom with circularly polarized light has attracted widespread attention for potential applications in valley- and spin-tronics. In this talk we present helicity resolved Raman scattering of TMDC atomic layers. The dominant first order Raman bands, including the low energy breathing and shear modes as well as the high energy zone center optical phonons, are found to either maintain or completely switch the helicity of incident photons. This helicity selectivity due to phonon scattering is interpreted by symmetry of lattice vibrations without involving intervalley scattering. Our results provide a useful tool for characterization of TMDC atomic layers and offer new insights into the connection between photon helicity and valley polarization.   

Energy splitting of excitons in gapped Dirac materials

We show that there is an energy splitting between excitons with opposite angular momentum in gapped Dirac materials, such as monolayers of transition metal dichalcogenides and gapped surface states of topological insulators. This splitting can be traced back to the chiral nature of Dirac electrons. We also discuss the optical selection rule of excitons in gap Dirac materials and clarify the relationship to its single-particle counterpart. A simple estimation of the splitting ($\sim$ 10 meV) in monolayer transition metal dichalcogenides is given . Our result reveals the limitation of the venerable hydrogenic model of excitons, and highlights the importance of the Berry phase in   

Spin Dynamics in Bilayer Graphene and Mono- and Bilayer Transition Metal Dichalcogenides

In this talk, I am going to present our many-body investigation on spin dynamics in bilayer graphene as well as mono- and bilayer transition metal dichalcogenides. The influence of the inter-valley scattering to the spin relaxation is addressed. The valley/spin depolarization due to the inter- and intravalley electron-hole Coulomb exchange interaction is revealed. A possible primary excitation, excimer, is proposed in bilayer WS$_2$.   

Temperature and Magnetic Field Dependent Raman Spectroscopy of Transition-Metal Dichalcogenides

Atomically-thin, transition-metal dichalcogenides (TMDs) offer potential for an alternative to graphene in advanced devices, owing to their unique electronic and optical properties. Such device applications require knowledge of the photo-thermal properties. Recently, we measured\footnote{R. Yan, J. R. Simpson, \textit{et al.}, ACS Nano \textbf{8}, 986 (2014).} the thermal conductivity for MoS$_2$ using a Raman-based optothermal technique. In the present work, we extend those measurements to related TMDs, including Ta-based compounds TaX$_2$, where X=Se or S, in both $1T$ and $2H$ crystallographic structures. Mechanical exfoliation from bulk crystals provides few- to single-layer flakes. We measure the Raman spectra of the exfoliated flakes using a novel magneto-Raman instrument, which affords measurement of the low-frequency vibrational modes of micron-sized samples as a function of both temperature (100 to 400)\,K and magnetic field (0 to 9)\,T. Dependence of the observed Raman-active phonons on temperature and magnetic field will be discussed and compared with earlier results on MoS$_2$.   

Thickness dependent Raman spectroscopy in 1T-TaS$_{2}$

Much attention has been paid recently to layered transition metal dichalcogenides (TMDs), which exhibit unique optical properties as their thickness is reduced from the bulk down to a monolayer. Here, we study Raman spectra of mono-and few-layered 1T-TaS$_{2}$, a metallic TMD, which is known to exhibit temperature-dependent commensurate and incommensurate charge density waves. We measure the low frequency Raman spectra of mechanically exfoliated 1T-TaS$_{2}$ on SiO$_{2}$ substrates with thicknesses ranging from 100 nm down to 1 nm. The room temperature Raman spectra exhibit numerous sharp peaks with frequencies below 100 cm$^{-1}$, which evolve with reducing thickness. Temperature and polarization dependence of the low frequency modes reveals the emergence of new in-plane and out-of-plane modes. Density functional theory calculations suggest the origin of these peaks to zone folding of the 1T-TaS$_{2}$ Brillouin zone. Acknowledgement: This work is based upon research supported by the National Science Foundation under Grant No. 0955625.   

Temperature and polarization dependence of photoluminescence in monolayer tungsten diselenide

Two-dimensional transition metal dichalcogenides (TMDCs) have recently attracted considerable research interest, due to their wide direct band-gaps, strong spin-orbit couplings and inversion symmetry breaking when compared to graphene. These properties have rich physics and applications in electronics, optics and spintronics. Here, we experimentally study the evolution of photoluminescence (PL) from mechanically exfoliated monolayer tungsten diselenide (WSe$_2$) from $T=10\;\mathrm{K}$ to room temperature. At $T=10\;\mathrm{K}$, we observe a clear free exciton (X$^0$) emission at 1.75 eV together with a charged trion emission at 1.72 eV, yielding a trion binding energy of 30 meV. Temperature dependent PL measurements show that both the free exciton and trion exist up to room temperature, as a result of the large exciton ($\sim$370 meV) and trion binding energies of WSe$_2$, while other localized and defect-related emission peaks vanish above $T=65\;\mathrm{K}$. Temperature dependent polarization of the exciton and trion emisisons reveal a combined effect of large exciton binding energy, anisotropic thermal expansion and exciton-phonon interaction. These findings may provide a new platform to explore the valley polarization and valley-spin coupling in monolayer TMDCs.   

Ultrafast Terahertz Probe of Transient Evolution of Charged and Neutral Phase of Photoexcited Electron-hole Gas in Monolayer Semiconductor

We investigate the dynamical formation of excitons from photoexcited electron-hole plasma and its subsequent decay dynamics in monolayer MoS$_{\mathrm{2}}$ grown by chemical vapor deposition using ultrafast pump and terahertz probe spectroscopy under temperature down to 78 K. The observed transient THz transmission can be fit with two decay components: a fast component with decay lifetime of 20 ps, which is attributed to exciton life time including the exciton formation and subsequent intraexciton relaxation; a slow component with extremely long decay lifetime of several ns which is attributed to long live dark exciton or localized exciton state. Further temperature and pump fluence dependent studies of the two decay components verify the above relaxation dynamics. The measured time resolved evolution of photoexcited carriers provides new opportunities in developing novel optoelectronic and excitonic devices based on monolayer transition-metal dichalcogenides.   

Plasmonic enhanced Photoluminescence and absorption in MoS2 single layers

We report the successful transfer of CVD grown MoS2 to Au antenna fabricated using e-beam lithography, and we investigate the photoluminescence properties of this hybrid plasmonic-excitonic system. The work is focused on the plasmonic mediated pumping of the MoS2 photoluminescence emission. Off- and in-resonance excitation of the surface plasmons showed drastically different behaviors of the photoluminescence emission from the MoS2. For plasmonically mediated pumping, we found a significant enhancement of the photoluminescence intensity, emission peak broadening and red-shift. Based on numerical simulations of the plasmonic properties of the Au antenna, combined with heat dissipation calculations, we found that the results can be interpreted in terms of efficient light absorption by the plasmonic antenna and conversion into electron-hole pair excitations of the 2D MoS2 layer thus producing a photo-induced heating.   

Correlative confocal Raman Imaging for 2D materials

Graphene was one of the first two-dimensional materials which soon after its first mono-layer production received much attention by many researchers worldwide. Its properties vastly differ from bulk graphite and its potential for applications ranges from transistors to transparent conducting electrodes and solar cell applications. While Graphene is arguably the most prominent two-dimensional material there are to this date many more that are subject to current research such as MoS2, WS2 or MoSe2. Graphene has been already and still is extensively studied using a variety of characterization techniques. Raman spectroscopy and more importantly still, Raman imaging proved to be of great value due to the clearly different spectra obtained from single, double, triple and multi-layered Graphene. This and more information that can be extracted from Raman spectroscopy and imaging can well be complemented with other techniques such as various forms of atomic force microscopy (AFM), Scanning Nearfield Optical Microscopy (SNOM), and scanning electron microscopy (SEM). In this contribution we illustrate the benefit of correlating said techniques with confocal Raman imaging in order to deepen the understanding of the samples in question.