Session C33: Excitons in Heterostructures

Exciton spectroscopy in hBN / WSe2 van der Waals heterostructures

We perform optical spectroscopy on monolayer (ML) WSe2 encapsulated in hexagonal boron nitride (hBN). Although ML WSe2 samples on Si/SiO2 substrates are widely studied in the literature, conclusive measurements on the excited exciton states and fine structure (2s/2p) are still missing. In high quality hBN / ML WSe2 / hBN samples we measure for the linedwidth of the neutral and charged exciton emission values down to 1.6 meV at T=4K, close to the homogenous limit. This allows us to perform 1 and 2-photon spectroscopy which reveal fine details of the exciton and trion states previously masked by inhomogeneous broadening. We discuss the renormalization of the Coulomb interaction by the change in dielectric environment as compared to ML WSe2 exfoliated directly onto SiO2. In polarization resolved measurements we spectrally separate resonant Raman signals and valley polarization of excitons.   

Cross-material exciton-phonon coupling in van der Waals heterostructures

Exciton-phonon interaction plays a major role in the optical response of van der Waals (vdW) heterostructures, which, if appropriately controlled, is a valuable tool in engineering optoelectronic devices based on 2D semiconductors. Here, we report a novel exciton-phonon interaction at the interface between different vdW atomic layers, as well as with the substrates. By interfacing a monolayer WSe$_{\mathrm{2}}$ with atomically-thin hexagonal boron nitride (hBN), we observe the activation of Raman silent hBN A$_{\mathrm{2u}}$ mode stemming from the coupling between WSe$_{\mathrm{2}}$ exciton and hBN phonons. A comparison between hBN-overlaid and hBN-sandwiched WSe$_{\mathrm{2}}$ samples shows that the coupling between hBN A$_{\mathrm{2u}}$ phonon and WSe$_{\mathrm{2}}$ exciton is suppressed in the latter, while that between hBN A$_{\mathrm{2u}}$, WSe$_{\mathrm{2}}$ A'$_{\mathrm{1}}$ phonons and WSe$_{\mathrm{2}}$ exciton remains strong. This demonstrates that the interfacial exciton-phonon coupling can be manipulated by symmetry reconstruction. Moreover, we observe enhanced Raman signals originating from surface mode in SiO$_{\mathrm{2}}$, as well as E$_{\mathrm{g}}$ mode in sapphire substrates. This highlights the ubiquity of interfacial exciton-phonon coupling in vdW heterostructures. Due to their nanoscaled geometries, they are highly susceptible to vibrational surroundings, especially from supporting substrates, an aspect commonly overlooked.   

Measuring the Influence of Dielectric Environment on 2D Excitons in Monolayer Semiconductors: Insight from High Magnetic Fields$^{\mathrm{1}}$

The relatively heavy electrons and holes in monolayer semiconductors such as MoS$_{\mathrm{2}}$ form tightly-bound excitons with large binding energies, thus motivating magneto-optical studies in high magnetic fields. Because 2D excitons in these materials necessarily lie close to a surface, their properties are expected to be strongly influenced by the surrounding dielectric environment. However, systematic studies exploring this role are challenging, in part because the most readily accessible exciton parameter -- the exciton's optical transition energy -- is largely unaffected by the surrounding medium. Here we show that the role of the dielectric environment can be revealed through its systematic influence on the size of the exciton, which can be directly measured via the diamagnetic shift of the exciton transition in high magnetic fields [1,2]. Using exfoliated WSe$_{\mathrm{2}}$ monolayers affixed to single-mode optical fibers, we tune the surrounding dielectric environment by encapsulating the monolayers with different materials, and perform polarization resolved low-temperature magneto-absorption studies to 65 tesla. The systematic increase of the exciton's size with dielectric screening, and concurrent two-fold reduction in binding energy (also inferred from these measurements), is quantitatively compared with leading theoretical models based on the Keldysh potential. These results demonstrate how exciton properties can be tuned in future 2D devices and van der Waals heterostructures. [1] A.V. Stier et al., Nature Comm. 7:10643 (2016). [2] A.V. Stier et al., Nano Lett. 16, 7054 (2016). $^{\mathrm{1}}$In collaboration with S.A. Crooker (NHMFL); J. Kono (Rice University); K.M. McCreary, B.T. Jonker (Naval Research Lab); N.P. Wilson, G. Clark, X. Xu (University of Washington).   

Interfacial Charge Transfer and Recombination Dynamics in van der Waals Heterojunctions of 2D Semiconductors

Heterojunctions of transition metal dichalcogenides (TMDC) are being explored for optoelectronics, photovoltaics and spin-valleytronics at the 2D limit. Using time-resolved microscopic transient reflectance spectroscopy, we measured the interfacial charge transfer and recombination dynamics in two dimensional MoS$_{2}$/WSe$_{2}$ heterojunctions as a function of interlayer momentum mismatch. The observed ultrafast ($<$ 40 fs) formation of interlayer exciton is independent of the relative orientation between the two layers, indicating a hot-carrier mediated charge transfer mechanism. The lifetime of charge transfer excitons span two orders of magnitude (0.04 - 3 ns) with no clear dependence on momentum mismatch, in accordance with the defect-mediated non-radiative recombination mechanism. Our results suggest the importance of defect reduction in revealing the intrinsic properties of charge transfer excitons in two dimensional van der Waals heterojunctions.   

Time Resolved Fluorescence Lifetime Imaging Analysis of MoS2/Graphene Heterostructures

For high-performance optoelectronic device applications, the lifetime of excitons of MoS2/Graphene (M/G) structures is aimed to be high in order to diminish any exciton recombination processes. The M/G structures were obtained by first transferring CVD grown MoS2 flakes on graphene film, and second by direct growth of the MoS2 flakes onto graphene. The excitonic and interface effects were studied using Raman Scattering, PL and fluorescence lifetime (FL) imaging (FLIM). FLIM of MoS2 flakes on SiO2/Si substrate shows that the FL is varying in the range of 0.3-0.45 ns, throughout a single flake, with higher lifetime at the edges. Contrarily, for M/G structures the PL intensity was observed to be quenched by a factor of $\sim $10, with a blue shift of $\sim $40 meV for the A-exciton. An important outcome was the throughout-the-flake uniform exciton lifetime on the transferred structures with a value of 0.33 ns. This is caused by charge transfer between MoS2 and graphene (2D peak shift in the graphene Raman spectra) and stress relaxation of MoS2 on graphene after the transfer process (significant downshift of E Raman peak of MoS2). As a result, the lifetime of the transferred MoS2 is uniform through all the flake surface and graphene does not decrease the FL of MoS2.   

Polarization resolved photoluminescence study of pulse laser deposition assisted grown monolayer MoSe$_{\mathrm{2}}$-WSe$_{\mathrm{2}}$ lateral hetero-junction.

The development of lateral hetero-junctions between 2D semiconductors of different band gaps could pave the way for the fabrication of new 2D electronic devices, such as high-speed transistors, diodes, and light emitting diodes. Here we demonstrate the growth lateral hetero-junction between MoSe$_{\mathrm{2}}$ and WSe$_{\mathrm{2}}$ by pulse laser deposition assisted chemical vapor deposition method. The lateral hetero-junction can be clearly observed in morphological charactarizations. The room temperature photoluminescence spectra at the interface show two strong peaks at 1.52 eV and 1.63 eV, corresponding to the $A$ excitons arising from $K$ point the first Brillion zone of monolayer MoSe$_{\mathrm{2}}$ and WSe$_{\mathrm{2}}$ respectively. The polarization resolved photoluminescence spectra's at 50 K reveals 55.2 {\%} and 29.4 {\%} valley polarization of WSe$_{\mathrm{2}}$ and MoSe$_{\mathrm{2}}$, respectively. However, the inter-band transition around (\textasciitilde 1.32 eV), previously observed in vertical hetero-structure could not be observed in lateral case due to spatial separation of energy bands of MoSe$_{\mathrm{2}}$ and WSe$_{\mathrm{2}}$.   

First-Principles Studies of Heterostructures with Transition Metal Dichalcogenides.

We use first-principles calculations to study electronic properties of van der Waals (vdW) heterostructures with transition metal dichalcogenides (TMDs). Using a novel CellMatch procedure for optimizing the strain in vdW heterostructures [1], unit cells of a TMD and another material are combined in a relaxed common supercell of their vdW heterostructure. Our results are examined in TMD heterostructures with graphene and topological insulators, representing materials with a weak and strong spin-orbit coupling, respectively. We explore the possibility for tunable proximity effects [2,3] which may not be apparent from the individual vdW materials. $\backslash $[1] P. Lazic, Comp. Phys. Commun. 197, 324 (2015). $\backslash $[2] R. V. Gorbachev, J. C. W. Song, G. L. Yu, A. V. Kretinin, Y. F. Withers, A. M. Cao,. V. Grigorieva, K. S. Novoselov, L. S. Levitov, and A. K. Geim, Science 346, 448 (2014).$\backslash $[3] P. Lazic, K. D. Belashchenko, and I. Zutic, Phys. Rev. B 93, 241401(R) (2016).   

Effects of interlayer twists in proximitized graphene-dichalcogenide layers

Proximity effects on deposited graphene on a TMD substrate are expected to change the dynamics of the electronic states in graphene, inducing spin orbit coupling (SOC) and staggered potential effects. An effective Hamiltonian that describes different symmetry breaking terms in graphene, while preserving time reversal invariance, shows that an inverted mass regime is possible. A transition from an inverted mass phase to a staggered gap is possible in real materials, as a relative gate voltage between the layers is applied. Berry curvature and valley Chern numbers demonstrate that the system may exhibit quantum spin Hall and valley Hall effects [1]. We further study relative rotation angles of the layers as they may give rise to interesting physical behavior not present in commensurate structures. We use a continuum model capable of describing the commensurate as well as the incommensurate systems with relative rotation angle and anisotropic expansion, and examine the topological characteristics of the resulting electronic states. Contrasting with tight binding results when appropriate, we explore the behavior of gaps and proximitized spin-orbit couplings in the system as function of twist angle and other structure characteristics. [1] A. M. Alsharari et al., arXiv:1608.00992   

Evolution of Direct and Indirect Band Gap by Inhomogeneous Strain in Monolayer MoS2-WSe2 and WSe2-MoSe2 Lateral Heterostructures

Recently, heterostructures consisting of laterally connected transition metal dichalcogenides monolayers have attracted considerable interests and been demonstrated by chemical vapor deposition, paving the way for future planar device applications. In particular, the outer material usually exhibits pronounced strain inhomogeneity caused by lattice mismatch, as has been observed in MoS2-WSe2 heterostructures with strain variation up to \textasciitilde 1.59{\%}. In this work, the strain inhomogeneities in WSe2-MoS2 and MoSe2-WSe2 lateral heterostructures are investigated. Spatial-resolved photoluminescence (PL) shows a strong correlation between the PL intensity and energy caused by carrier occupation near the K valleys. This correlation can be modeled by considering the valley occupations according to the Boltzmann distribution, by which the energy difference and the ratio of deformation potential between K and vicinal valleys can be determined. Specifically, we found that the unstrained monolayer WSe2 is an indirect bandgap semiconductor, which can be tuned to direct bandgap under tensile strain. The local strain variation results in a spatial modulation of direct and indirect bandgap, with important implications for practical device applications.   

Gate Dependent Photoluminescence in Two-dimensional Transition Metal Dichalcogenides van der Waals Heterostructure devices.

Single layer transition metal dichalcogenides (TMDC) are 2-dimensional (2D) semiconductors characterized by a direct optical bandgap in the order of 2 eV and large exciton binding energies (\textgreater 100 meV). We fabricate van der Waals heterostructure devices made of 2D TMDCs with hexagonal Boron nitride (h-BN) as gate dielectric, with top and bottom gate electrodes and ohmic contacts down to cryogenic temperatures (T $=$ 3K). We study the evolution of photoluminescence (PL) with electric field, carrier density, and temperature. Our measured low-temperature PL peaks show full width at half maxima on the order of \textasciitilde 1meV. The amplitude of the photoluminescence peak, corresponding to the neutral and charged exciton emission (also their energy), can be manipulated with top and bottom gates.   

Engineering Interlayer Exciton Dynamics in 2D Semiconductor Heterostructures

Heterostructures (HSs) formed by monolayers of transition metal dichalcogenides (TMDs) have recently emerged as promising 2D analogues to coupled quantum wells formed by 3D semiconductors, with interest arising from a number of novel physical properties. These include valley contrasting physics inherited from the HS's monolayer constituents; the interlayer excitons that form therein, which exhibit lifetimes orders of magnitude longer than the intralayer species due to reduced electron-hole exchange interactions; and the strong many-body interactions which dominate interlayer exciton dynamics. By separating the TMD layers with a thin dielectric spacer, we reduce the electron-hole wavefunction overlap of the interlayer exciton and tune their repulsive dipole-dipole many-body interactions, thus modifying the interlayer exciton dynamics.   

Interfacial states in 2D lateral heterostructures

Recent years have seen the birth and manipulation of 2D materials, such as graphene, transition metal dichalcogenides (TMD), silicine and germanene, among others. Heterostructures (HS) between different 2D materials have shown distinct properties with respect to their pristine counterparts [1]. We need to understand the role of these unique atomic interfaces, considering different material features. Here we present our study of the formation of states at the 2D interface between different TMDs. We consider HSs composed of adjacent nanoribbons, which we model via a tight-binding approach with experimental and DFT parameters. We study the effect of different boundary geometries, such as zigzag and armchair, and analyze the formation of edge/interface states for different gap-nesting conditions. We find strongly localized interface states, lying in the band gap as well as in the continuum. Finally, we compare our numerical results with HSs of other hexagonal lattices, such as graphene, silicine or germanene [2], where low-energy states can be described by a Dirac equation with position-dependent mass across the interface. [1] M.-Y. Li \emph{et al.}, Mater. Today \textbf{19}, 322 (2016). [2] L. Matthes \emph{et al.}, PRB \textbf{86}, 205409 (2012).   

Topological Exciton Bands in Moir\'e Heterojunctions

Moir\'e patterns are common in Van der Waals heterostructures and can be used to apply periodic potentials to elementary excitations. We will show that the optical absorption spectrum of transition metal dichalcogenide bilayers is profoundly altered by long period moir\'e patterns that introduce twist-angle dependent satellite excitonic peaks.\footnote{F. Wu, T. Lovorn, and A. H. MacDonald, arXiv: 1610.03855.} Topological exciton bands with non-zero Chern numbers that support chiral excitonic edge states can be engineered by combining three ingredients: i) the valley Berry phase induced by electron-hole exchange interactions, ii) the moir\'e potential, and iii) the valley Zeeman field. We will also show that the moir\'e pattern can enable the optical probe of intra-Landau level excitations in the fractional quantum Hall regime.\footnote{F. Wu and A. H. MacDonald, arXiv: 1611.00776.}