Monolayers (MLs) of transition metal dichalcogenides (TMDs) exhibit unusual electrical behavior under magnetic fields due to their intrinsic spin-orbit coupling and lack of inversion symmetry. While recent experiments have also identified the critical role of carrier interactions within these materials, a complete mapping of the ambipolar Landau level (LL) sequence has remained elusive. Here, we use single-electron transistors to perform LL spectroscopy in ML WSe$_2$, for the first time providing a comprehensive picture of the electronic structure of a ML TMD for both electrons and holes. We find that the LLs differ notably between the two bands, and follow a unique sequence in the valence band (VB) that is dominated by strong Zeeman effects. The Zeeman splitting in the VB is several times higher than the cyclotron energy, far exceeding the predictions of a single-particle model, and moreover tunes significantly with doping. This implies exceptionally strong many-body interactions, and suggests that ML WSe$_2$ can serve as a host for new correlated-electron phenomena.
We realize and investigate ionic liquid gated field-effect transistors (FETs) on large-area MoS2 monolayers grown by chemical vapor deposition (CVD). Under electron accumulation, the performance of these devices is comparable to that of FETs based on exfoliated flakes. FETs on CVD-grown material, however, exhibit clear ambipolar transport, which for MoS2 monolayers had not been reported previously. We exploit this property to estimate the bandgap Δ of monolayer MoS2 directly from the device transfer curves and find Δ $\approx$ 2.4-2.7 eV. In the ambipolar injection regime, we observe electrolu…
Monolayer transition metal dichalcogenides, known for exhibiting strong excitonic resonances, constitute a very interesting and versatile platform for investigation of light-matter interactions. In this work we report on a strong coupling regime between excitons in monolayer WSe2 and photons confined in an open, voltage-tunable dielectric microcavity. The tunability of our system allows us to extend the exciton-polariton state over a wide energy range and, in particular, to bring the excitonic component of the lower polariton mode into resonance with other excitonic transitions in monolayer WS…
The newly developed two-dimensional layered materials provide perfect platform for valley-spintronics exploration. To determine the prospect of utilizing the valley degree of freedom, it is of great importance to directly detect and understand the valley dynamics in these materials. Here, the exciton valley dynamics in monolayer WSe$_2$ is investigated by the two-color pump-probe magneto-optical Kerr technique. By tuning the probe photon energy in resonance with the free excitons and trions, the valley relaxation time of different excitonic states in monolayer WSe$_2$ is determined. Valley rel…
Acquisition and analysis is described for scanning tunneling spectroscopy data acquired from a monolayer of WSe$_2$ grown on epitaxial graphene on SiC. Curve fitting of the data is performed, in order to deduce band-edge energies. In addition to describing the details of the theoretical curves used for the fitting, the acquisition and analysis methods are also discussed within the larger context of the historical development of scanning tunneling spectroscopy techniques.
WSe$_2$ monolayers with variable doping are an ideal system to study two-dimensional excitonic complexes. Here, we find that the strongest photoluminescence from bright neutral excitons occurs at moderate $p$-doping levels, owing to a rapid decrease of signal from positive dark trions and a slow increase of signal from positive bright trions with growing $p$-concentrations. We explain our observations with a qualitative model, in which the scattering rate of bright excitons into dark complexes is enhanced by exciton localization, while the scattering rate into positive bright trions increases …
Optical spectroscopy in high magnetic fields $B\leq65$ T is used to reveal the very different nature of carriers in monolayer and bulk transition metal dichalcogenides. In monolayer WSe$_{2}$, the exciton emission shifts linearly with the magnetic field and exhibits a splitting which originates from the magnetic field induced valley splitting. The monolayer data can be described using a single particle picture with a Dirac-like Hamiltonian for massive Dirac fermions, with an additional term to phenomenologically include the valley splitting. In contrast, in bulk WSe$_{2}$ where the inversion s…
We have experimentally studied the pump-probe Kerr rotation dynamics in WSe$_2$ monolayers. This yields a direct measurement of the exciton valley depolarization time $τ_v$. At T=4K, we find $τ_v\approx 6$ps, a fast relaxation time resulting from the strong electron-hole Coulomb exchange interaction in bright excitons. The exciton valley depolarization time decreases significantly when the lattice temperature increases with $τ_v$ being as short as 1.5ps at 125K. The temperature dependence is well explained by the developed theory taking into account the exchange interaction and a fast exciton …
Quantum confinenement and manipulation of charge carriers are critical for achieving devices practical for quantum technologies. The interplay between electron spin and valley, as well as the possibility to address their quantum states electrically and optically, make two-dimensional (2D) transition metal dichalcogenides an emerging platform for the development of quantum devices. In this work, we fabricate devices based on heterostructures of layered 2D materials, in which we realize gate-controlled tungsten diselenide (WSe2) hole quantum dots. We discuss the observed mesoscopic transport fea…
Proximity orbital and spin-orbit effects of bilayer graphene on monolayer WSe$_2$ are investigated from first-principles. We find that the built-in electric field induces an orbital band gap of about 10 meV in bilayer graphene. Remarkably, the proximity spin-orbit splitting for holes is two orders of magnitude---the spin-orbit splitting of the valence band at K is about 2 meV---more than for electrons. Effectively, holes experience spin-valley locking due to the strong proximity of the lower graphene layer to WSe$_2$. However, applying an external transverse electric field of some 1 V/nm, coun…
We propose a novel phonon assisted Auger process unique to the electronic band structure of monolayer transition metal dichalcogenides (TMDCs), which dominates the radiative recombination of ground state excitons in Tungsten based TMDCs. Using experimental and DFT computed values for the exciton energies, spin-orbit splittings, optical matrix element, and the Auger matrix elements, we find that the Auger process begins to dominate at carrier densities as low as $10^{9-10}~{\rm cm^{-2}}$, thus providing a plausible explanation for the low quantum efficiencies reported for these materials.
Strong Coulomb interactions in atomically thin semiconductors like monolayer WSe$_2$ induce not only tightly bound excitons, but also make their optical properties very sensible to doping. By utilizing a microscopic theory based on the excitonic Heisenberg equations of motion, we systematically determine the influence of doping on the excitonic linewidth, lineshift, and oscillator strength. We calculate trion resonances and demonstrate that the Coulomb coupling of excitons to the trionic continuum generates a non-Markovian interference, which, due to a time retardation, builds up a phase respo…
In atomically thin two-dimensional semiconductors such as transition metal dichalcogenides (TMDs), controlling the density and type of defects promises to be an effective approach for engineering light-matter interactions. We demonstrate that electron-beam irradiation is a simple tool for selectively introducing defect-bound exciton states associated with chalcogen vacancies in TMDs. Our first-principles calculations and time-resolved spectroscopy measurements of monolayer WSe2 reveal that these defect-bound excitons exhibit exceptional optical properties including a recombination lifetime app…
Monolayer semiconductors are atomically thin quantum wells with strong confinement of electrons in two-dimensional (2D) plane. Here, we experimentally study the out-of-plane polarizability of excitons in hBN-encapsulated monolayer WSe$_{2}$ in strong electric fields of up to 1.6 V/nm (16 MV/cm). We monitor free and bound exciton photoluminescence peaks with increasing electric fields at a constant carrier density, carefully compensating for unintentional photodoping in our double-gated device at 4K. We show that the Stark shift is < 0.4 meV despite the large electric fields applied, yielding a…
Monolayers of semiconducting transition metal dichalcogenides are two-dimensional direct-gap systems which host tightly-bound excitons with an internal degree of freedom corresponding to the valley of the constituting carriers. Strong spin-orbit interaction and the resulting ordering of the spin-split subbands in the valence and conduction bands makes the lowest-lying excitons in WX$_2$ (X~being S or Se) spin-forbidden and optically dark. With polarization-resolved photoluminescence experiments performed on a WSe$_2$ monolayer encapsulated in a hexagonal boron nitride, we show how the intrinsi…
Monolayer transition metal dichalcogenids (TMDs) feature valley degree of freedom, giant spin-orbit coupling and spin-valley locking. These exotic natures stimulate efforts of exploring the potential applications in conceptual spintronics, valleytronics and quantum computing. Among all the exotic directions, a long lifetime of spin and/or valley polarization is critical. The present valley dynamics studies concentrate on the band edge excitons which predominates the optical response due to the enhanced Coulomb interaction in two dimensions. The valley lifetime of free carriers remains in ambig…
Monolayer transition metal dichalcogenides (TMDs) hold great promise for future information processing applications utilizing a combination of electron spin and valley pseudospin. This unique spin system has led to observation of the valley Zeeman effect in neutral and charged excitonic resonances under applied magnetic fields. However, reported values of the trion valley Zeeman splitting remain highly inconsistent across studies. Here, we utilize high quality hBN encapsulated monolayer WSe$_2$ to enable simultaneous measurement of both intervalley and intravalley trion photoluminescence. We f…
Coulomb interaction between electrons lies at the heart of magnetism in solids. In contrast to conventional two-dimensional (2D) systems, electrons in monolayer transition metal dichalcogenides (TMDs) possess coupled spin and valley degrees of freedom by the spin-orbit interaction. The electrons are also strongly interacting even in the high-density regime because of the weak dielectric screening in two dimensions and a large band mass. The combination of these properties presents a unique platform for exploring spin and valley magnetism in 2D electron liquids. Here we report an observation by…
We discuss coupling of site-selectively induced quantum emitters in exfoliated monolayers of WSe$_2$ to plasmonic nanostructures. Squared and rectangular gold nanopillars, which are arranged in pitches of \SI{4}{\micro\meter} on the surface, have sizes of tens of nanometers, and act as seeds for the formation of quantum emitters in the atomically thin materials. We observe chraracteristic narrow-band emission signals from the monolayers, which correspond well with the positions of the metallic nanopillars with and without thin dielectric coating. Single photon emission from the emitters is con…
Strong Coulomb correlations together with multi-valley electronic bands in the presence of spin-orbit interaction and possible new optoelectronic applications are at the heart of studies of the rich physics of excitons in semiconductor structures made of monolayers of transition metal dichalcogenides (TMD). In intrinsic TMD monolayers the basic, intravalley excitons are formed by a hole from the top of the valence band and an electron either from the lower or upper spin-orbit-split conduction band subbands: one of these excitons is optically active, the second one is "dark", although possibly …
We present a study of quasi-free-standing monolayer graphene obtained by intercalation of Au atoms at the interface between the carbon buffer layer (Bu-L) and the silicon-terminated face (0001) of 4H-silicon carbide. Au intercalation is achieved by deposition of atomically thin Au on the Bu-L followed by annealing at 850 °C in an Argon atmosphere. We explore the intercalation of Au and decoupling of the Bu-L into quasi-free-standing monolayer graphene by surface science characterizations and electron transport in top-gated electronic devices. By gate-dependent magnetotransport we find that the…
Momentum resolved electron energy-loss spectroscopy was used to measure the dispersions of excitons in a free-standing monolayer of WSe2. Besides the parabolically dispersed valley excitons, a sub-gap dispersive exciton was observed at specific q values for the first time. The simultaneous STEM-ADF imaging of the local probed areas suggests that this sub-gap exciton is attributed to the prolific Se vacancies.
Low-temperature photoluminescence (PL) of hBN-encapsulated monolayer tungsten diselenide (WSe$_2$) shows a multitude of sharp emission peaks below the bright exciton. Some of them have been recently identified as phonon sidebands of momentum-dark states. However, the exciton dynamics behind the emergence of these sidebands has not been revealed yet. In this joint theory-experiment study, we theoretically predict and experimentally observe time-resolved PL providing microscopic insights into thermalization of hot excitons formed after optical excitation. In good agreement between theory and exp…
A monolayer of a transition metal dichalcogenide (TMD) such as WSe$_2$ is a two-dimensional (2D) direct band-gap valley-semiconductor having an effective Honeycomb lattice structure with broken inversion symmetry. The inequivalent valleys in the Brillouin zone could be selectively addressed using circularly-polarized light fields, suggesting the possibility for magneto-optical measurement and manipulation of the valley pseudospin degree of freedom. Here we report such experiments that demonstrate the valley Zeeman effect -- strongly anisotropic lifting of the degeneracy of the valley pseudospi…
We present a high-resolution resonance Raman study of hBN encapsulated MoSe$_2$ and WSe$_2$ monolayers at 4 K using excitation energies from 1.6 eV to 2.25 eV. We report resonances with the WSe$_2$ A2s and MoSe$_2$ A2s and B2s excited Rydberg states despite their low oscillator strength. When resonant with the 2s states we identify new Raman peaks which are associated with intravalley scattering between different Rydberg states via optical phonons. By calibrating the Raman scattering efficiency and separately constraining the electric dipole matrix elements, we reveal that the scattering rates…
In crystals, energy band extrema in momentum space can be identified by their valley index. The internal quantum degree of freedom associated with valley pseudospin indices can act as a useful information carrier analogous to electronic charge or spin. Interest in valleytronics has been revived in recent years following the discovery of atomically thin materials such as graphene and transition metal dichalcogenides. However, the valley coherence time, a key quantity for manipulating the valley pseudospin, has never been measured in any material. In this work, we use a sequence of laser pulses …
The exceptionally strong Coulomb interaction in semiconducting transition-metal dichalcogenides (TMDs) gives rise to a rich exciton landscape consisting of bright and dark exciton states. At elevated densities, excitons can interact through exciton-exciton annihilation (EEA), an Auger-like recombination process limiting the efficiency of optoelectronic applications. Although EEA is a well-known and particularly important process in atomically thin semiconductors determining exciton lifetimes and affecting transport at elevated densities, its microscopic origin has remained elusive. In this joi…
All-solid-state strong light-matter coupling systems with large vacuum Rabi splitting are great important for quantum information application, such as quantum manipulation, quantum information storage and processing. The monolayer transition metal dichalcogenides (TMDs) have been explored as excellent candidates for the strong light-matter interaction, due to their extraordinary exciton binding energies and remarkable optical properties. Here, for both of experimental and theoretical aspects, we explored resonance coupling effect between exciton and plasmonic nanocavity in heterostructures con…
The recent experimental realization of a high quality WSe$% _{2} $ leads to the possibility of magneto-optical measurements and the manipulation of the spin and valley degrees of freedom. We study the influence of the very strong spin-orbit coupling and of the anisotropic lifting of the valley pseudospin degeneracy on its magnetotransport properties. The energy spectrum of WSe$_{2} $ is derived and discussed in the presence of a perpendicular magnetic field $B$. Correspondingly we evaluate the magneto-optical Hall conductivity and the optical longitudinal conductivity as functions of the frequ…
We report the fabrication and characterization of gate-defined hole quantum dots in monolayer and bilayer WSe$_2$. The devices were operated with gates above and below the WSe$_2$ layer to accumulate a hole gas, which for some devices was then selectively depleted to define the dot. Temperature dependence of conductance in the Coulomb blockade regime is consistent with transport through a single level, and excited state transport through the dots was observed at temperatures up to 10 K. For adjacent charge states of a bilayer WSe$_2$ dot, magnetic field dependence of excited state energies was…
We present a comprehensive optical study of the excitonic Zeeman effects in transition metal dichalcogenide monolayers, which are discussed comparatively for selected materials: MoSe$_2$, WSe$_2$ and WS$_2$. We introduce a simple semi-phenomenological description of the magnetic field evolution of individual electronic states in fundamental sub-bands by considering three additive components: valley, spin and orbital terms. We corroborate the validity of the proposed description by inspecting the Zeeman-like splitting of neutral and charged excitonic resonances in absorption-type spectra. The v…
Local energy extrema of the bands in momentum space, or valleys, can endow electrons in solids with pseudo-spin in addition to real spin. In transition metal dichalcogenides this valley pseudo-spin, like real spin, is associated with a magnetic moment which underlies the valley-dependent circular dichroism that allows optical generation of valley polarization, intervalley quantum coherence, and the valley Hall effect. However, magnetic manipulation of valley pseudospin via this magnetic moment, analogous to what is possible with real spin, has not been shown before. Here we report observation …
Resonant optical excitation of few-level quantum systems enables coherent quantum control, resonance fluorescence, and direct characterization of dephasing mechanisms. Experimental demonstrations have been achieved in a variety of atomic and solid-state systems. An alternative but intriguing quantum photonic platform is based on single layer transition metal chalcogenide semiconductors, which exhibit a direct band-gap with optically addressable exciton valley-pseudospins in a uniquely two-dimensional form. Here we perform resonance and near-resonance excitation of three-dimensionally confined …
The optical properties of monolayer and bilayer transition metal dichalcogenide semiconductors are governed by excitons in different spin and valley configurations, providing versatile aspects for van der Waals heterostructures and devices. We present experimental and theoretical studies of exciton energy splittings in external magnetic field in neutral and charged WSe$_2$ monolayer and bilayer crystals embedded in a field effect device for active doping control. We develop theoretical methods to calculate the exciton $g$-factors from first principles and tight-binding for all possible spin-va…
We observe a set of three replica luminescent peaks at ~21.4 meV below the dark exciton, negative and positive dark trions (or exciton-polarons) in monolayer WSe2. The replica redshift energy matches the energy of the zone-center E"-mode optical phonons. The phonon replicas exhibit parallel gate dependence and same g-factors as the dark excitonic states, but follow the valley selection rules of the bright excitonic states. While the dark states exhibit out-of-plane transition dipole and valley-independent linearly polarized emission in the in-plane directions, their phonon replicas exhibit in-…
We study the magnetotransport properties of high mobility holes in monolayer and bilayer WSe$_2$, which display well defined Shubnikov-de Haas (SdH) oscillations, and quantum Hall states (QHSs) in high magnetic fields. In both mono and bilayer WSe$_2$, the SdH oscillations and the QHSs occur predominantly at even filling factors, evincing a two-fold Landau level degeneracy. The Fourier transform analysis of the SdH oscillations in bilayer WSe$_2$ reveal the presence of two subbands localized in the top or the bottom layer, as well as negative compressibility. From the temperature dependence of…
We experimentally demonstrate dressing of the excited exciton states by a continuously tunable Fermi sea of free charge carriers in a monolayer semiconductor. It represents an unusual scenario of two-particle excitations of charged excitons previously inaccessible in conventional material systems. We identify excited state trions, accurately determine their binding energies in the zero-density limit for both electron- and hole-doped regimes, and observe emerging many-body phenomena at elevated doping. Combining experiment and theory we gain access to the intra-exciton coupling facilitated by t…
Monolayer transition metal dichalcogenide (TMDC) crystals, as direct-gap materials with unusually strong light-matter interaction, have attracted much recent attention. In contrast to the initial understanding, the minima of the conduction band are predicted to be spin split. Because of this splitting and the spin-polarized character of the valence bands, the lowest-lying excitonic states in WX2 (X=S, Se) are expected to be spin-forbidden and optically dark. To date, however, there has been no direct experimental probe of these dark band-edge excitons, which strongly influence the light emissi…
Monolayer WSe$_2$ hosts a series of exciton Rydberg states denoted by the principal quantum number n = 1, 2, 3, etc. While most research focuses on their absorption properties, their optical emission is also important but much less studied. Here we measure the photoluminescence from the 1s - 5s exciton Rydberg states in ultraclean monolayer WSe$_2$ encapsulated by boron nitride under magnetic fields from -31 T to 31 T. The exciton Rydberg states exhibit similar Zeeman shifts but distinct diamagnetic shifts from each other. From their luminescence spectra, Zeeman and diamagnetic shifts, we dedu…
The monolayer WSe2 is interesting and important for future application in nanoelectronics, spintronics and valleytronics devices, because it has the largest spin splitting and longest valley coherence time among all the known monolayer transition-metal dichalcogenides (TMDs). To obtain the large-area monolayer TMDs' crystal is the first step to manufacture scalable and high-performance electronic devices. In this letter, we have successfully fabricated millimeter-sized monolayer WSe2 single crystals with very high quality, based on our improved mechanical exfoliation method. With such superior…
Using time-resolved Kerr rotation, we measure the spin/valley dynamics of resident electrons and holes in single charge-tunable monolayers of the archetypal transition-metal dichalcogenide (TMD) semiconductor WSe2. In the n-type regime, we observe long (70 ns) polarization relaxation of electrons that is sensitive to in-plane magnetic fields $B_y$, indicating spin relaxation. In marked contrast, extraordinarily long (2 microsecond) polarization relaxation of holes is revealed in the p-type regime, that is unaffected by $B_y$, directly confirming long-standing expectations of strong spin-valley…
Gate tunable two-dimensional (2D) superconductors offer significant advantages when studying superconducting phase transitions. Here, we address superconductivity in exfoliated 1T'-MoTe2 monolayers with an intrinsic band gap of ~7.3 meV using electrostatic doping. Despite large differences in the dispersion of the conduction and the valence bands, superconductivity can be achieved easily for both electrons and holes. The onset of superconductivity occurs near 7-8K for both charge carrier types. This temperature is much higher than in bulk samples. Also the in-plane upper critical field is stro…
In monolayer transition-metal dichalcogenide semiconductors, many-body correlations can manifest in optical spectra when photoexcited electron-hole pairs (excitons) are introduced into a 2D Fermi sea of mobile carriers. At low carrier densities, the formation of positively and negatively charged excitons ($X^\pm$) is well documented. However, in WSe$_2$ monolayers, an additional absorption resonance, often called $X^{-\prime}$, emerges at high electron density. Its origin is not understood. Here we investigate the $X^{-\prime}$ state via polarized absorption spectroscopy of electrostatically-g…
The variety of excitonic states in tungsten-based dichalcogenide monolayers stems from unique interplay between the spin and valley degrees of freedom. One of the exciton species is the indirect exciton (momentum or valley dark), which is responsible to a series of resonances when the monolayer is charge neutral. We investigate the short-range electron-hole exchange interaction of the indirect exciton, as well as its recombination mechanism mediated by impurities. The analysis provides thorough understanding of the energy and polarization of the zero-phonon indirect exciton resonance in the em…
Two-dimensional materials and their heterostructures have opened up new possibilities for magnetism at the nanoscale. In this study, we utilize first-principles simulations to investigate the structural, electronic, and magnetic properties of $\textrm{Fe}/\textrm{WSe}_2/\textrm{Pt}$ systems containing pristine, defective, or doped $\textrm{WSe}_2$ monolayers. The proximity effects of the ferromagnetic Fe layer are studied by considering defective and vanadium-doped $\textrm{WSe}_2$ monolayers. All heterostructures are found to be ferromagnetic, and the insertion of the transition-metal dichalc…
Monolayers of transition metal dichalcogenides (TMDs) have recently emerged as a promising optoelectronic platform. To leverage their full potential, however, it is important to understand and engineer the properties of the different exciton species that exist in these monolayers, as well as to control their transport through the material. A promising approach relies on engineering strain landscapes in atomically thin semiconductors that excitons navigate. In WSe$_2$ monolayers, for example, localized strain has been used to control the emission wavelength of excitons, induce exciton funneling…
We report the observation and gate manipulation of intrinsic dark trions in monolayer WSe$_2$. By using ultraclean WSe$_2$ devices encapsulated by boron nitride, we directly resolve the weak photoluminescence of dark trions. The dark trions can be tuned continuously between negative and positive charged trions with electrostatic gating. We also reveal their spin triplet configuration and distinct valley optical emission by their characteristic Zeeman splitting under magnetic field. The dark trions exhibit large binding energy (14-16 meV). Their lifetime (~1.3 ns) is two orders of magnitude lon…
The rich optical properties of transition metal dichalcogenide monolayers (TMD-MLs) render these materials promising candidates for the design of new optoelectronic devices. Despite the large number of excitonic complexes in TMD-MLs, the main focus has been put on optically bright neutral excitons. Spin-forbidden dark excitonic complexes have been addressed for basic science purposes, but not for applications. We report on spin-forbidden dark excitonic complexes in ML WSe$_2$ as an ideal system for the facile generation of radially polarized light beams. Furthermore, the spatially resolved pol…
Monolayers of transition metal dichalcogenides are ideal materials to control both spin and valley degrees of freedom either electrically or optically. Nevertheless, optical excitation mostly generates excitons species with inherently short lifetime and spin/valley relaxation time. Here we demonstrate a very efficient spin/valley optical pumping of resident electrons in n-doped WSe2 and WS2 monolayers. We observe that, using a continuous wave laser and appropriate doping and excitation densities, negative trion doublet lines exhibit circular polarization of opposite sign and the photoluminesce…
The defect states in atomically thin layers of transition metal dichalcogenides are promising candidates for single photon emission. However, the brightness of such quantum emission is often weak, and is accompanied with undesirable effects like spectral diffusion and strong background emission. By placing a monolayer WSe$_2$ directly on a rough gold substrate, here we show a selective enhancement of sharp defect-bound exciton peaks, coupled with a suppressed spectral diffusion and strong quenching of background luminescence. By combining the experimental data with detailed electromagnetic sim…
We report on nanosecond long, gate-dependent valley lifetimes of free charge carriers in monolayer WSe$_2$, unambiguously identified by the combination of time-resolved Kerr rotation and electrical transport measurements. While the valley polarization increases when tuning the Fermi level into the conduction or valence band, there is a strong decrease of the respective valley lifetime consistent with both electron-phonon and spin-orbit scattering. The longest lifetimes are seen for spin-polarized bound excitons in the band gap region. We explain our findings via two distinct, Fermi level-depen…
One of the most fundamental devices for electronics and optoelectronics is the PN junction, which provides the functional element of diodes, bipolar transistors, photodetectors, LEDs, and solar cells, among many other devices. In conventional PN junctions, the adjacent p- and n-type regions of a semiconductor are formed by chemical doping. Materials with ambipolar conductance, however, allow for PN junctions to be configured and modified by electrostatic gating. This electrical control enables a single device to have multiple functionalities. Here we report ambipolar monolayer WSe2 devices in …
We report scanning tunneling microscopy (STM) and spectroscopy (STS) measurements of monolayer and bilayer WSe$_2$. We measure a band gap of 2.21 $\pm$ 0.08 eV in monolayer WSe$_2$, which is much larger than the energy of the photoluminescence peak indicating a large excitonic binding energy. We additionally observe significant electronic scattering arising from atomic-scale defects. Using Fourier transform STS (FT-STS), we map the energy versus momentum dispersion relations for monolayer and bilayer WSe$_2$. Further, by tracking allowed and forbidden scattering channels as a function of energ…
