Signatures of moiré trions in WSe2/MoSe2 heterobilayers.

Moiré superlattices formed by van der Waals materials can support a wide range of electronic phases, including Mott insulators1-4, superconductors5-10 and generalized Wigner crystals2. When excitons are confined by a moiré superlattice, a new class of exciton emerges, which holds promise for realizing artificial excitonic crystals and quantum optical effects11-16. When such moiré excitons are coupled to charge carriers, correlated states may arise. However, no experimental evidence exists for charge-coupled moiré exciton states, nor have their properties been predicted by theory. Here we report the optical signatures of trions coupled to the moiré potential in tungsten diselenide/molybdenum diselenide heterobilayers. The moiré trions show multiple sharp emission lines with a complex charge-density dependence, in stark contrast to the behaviour of conventional trions. We infer distinct contributions to the trion emission from radiative decay in which the remaining carrier resides in different moiré minibands. Variation of the trion features is observed in different devices and sample areas, indicating high sensitivity to sample inhomogeneity and variability. The observation of these trion features motivates further theoretical and experimental studies of higher-order electron correlation effects in moiré superlattices.

Electrically Switchable Intervalley Excitons with Strong Two-Phonon Scattering in Bilayer WSe2.

We report the observation of QΓ intervalley exciton in bilayer WSe2 devices encapsulated by boron nitride. The QΓ exciton resides at ∼18 meV below the QK exciton. The QΓ and QK excitons exhibit different Stark shifts under an out-of-plane electric field due to their different interlayer dipole moments. By controlling the electric field, we can switch their energy ordering and control which exciton dominates the luminescence of bilayer WSe2. Remarkably, both QΓ and QK excitons exhibit unusually strong two-phonon replicas, which are comparable to or even stronger than the one-phonon replicas. By detailed theoretical simulation, we reveal the existence of numerous (≥14) two-phonon scattering paths involving (nearly) resonant exciton-phonon scattering in bilayer WSe2. To our knowledge, such electric-field-switchable intervalley excitons with strong two-phonon replicas have not been found in any other two-dimensional semiconductors. These make bilayer WSe2 a distinctive valleytronic material with potential novel applications.

Spin-induced linear polarization of photoluminescence in antiferromagnetic van der Waals crystals.

Antiferromagnets are promising components for spintronics due to their terahertz resonance, multilevel states and absence of stray fields. However, the zero net magnetic moment of antiferromagnets makes the detection of the antiferromagnetic order and the investigation of fundamental spin properties notoriously difficult. Here, we report an optical detection of Néel vector orientation through an ultra-sharp photoluminescence in the van der Waals antiferromagnet NiPS3 from bulk to atomically thin flakes. The strong correlation between spin flipping and electric dipole oscillator results in a linear polarization of the sharp emission, which aligns perpendicular to the spin orientation in the crystal. By applying an in-plane magnetic field, we achieve manipulation of the photoluminescence polarization. This correlation between emitted photons and spins in layered magnets provides routes for investigating magneto-optics in two-dimensional materials, and hence opens a path for developing opto-spintronic devices and antiferromagnet-based quantum information technologies.

Excitonic and Valley-Polarization Signatures of Fractional Correlated Electronic Phases in a WSe_{2}/WS_{2} Moiré Superlattice.

We have measured the reflectance contrast, photoluminescence, and valley polarization of a WSe_{2}/WS_{2} heterobilayer moiré superlattice at gate-tunable charge density. We observe absorption modulation of three intralayer moiré excitons at filling factors ν=1/3 and 2/3. We also observe luminescence modulation of interlayer trions at around a dozen fractional filling factors, including ν=-3/2, 1/4, 1/3, 2/5, 2/3, 6/7, 5/3. Remarkably, the valley polarization of interlayer trions is suppressed at some fractional fillings. These results demonstrate that electron crystallization can modulate the absorption, emission, and valley dynamics of the excitonic states in a moiré superlattice.

Landau-Quantized Excitonic Absorption and Luminescence in a Monolayer Valley Semiconductor.

We investigate Landau-quantized excitonic absorption and luminescence of monolayer WSe_{2} under magnetic field. We observe gate-dependent quantum oscillations in the bright exciton and trions (or exciton polarons) as well as the dark trions and their phonon replicas. Our results reveal spin- and valley-polarized Landau levels (LLs) with filling factors n=+0, +1 in the bottom conduction band and n=-0 to -6 in the top valence band, including the Berry-curvature-induced n=±0 LLs of massive Dirac fermions. The LL filling produces periodic plateaus in the exciton energy shift accompanied by sharp oscillations in the exciton absorption width and magnitude. This peculiar exciton behavior can be simulated by semiempirical calculations. The experimentally deduced g factors of the conduction band (g∼2.5) and valence band (g∼15) exceed those predicted in a single-particle model (g=1.5, 5.5, respectively). Such g-factor enhancement implies strong many-body interactions in gated monolayer WSe_{2}. The complex interplay between Landau quantization, excitonic effects, and many-body interactions makes monolayer WSe_{2} a promising platform to explore novel correlated quantum phenomena.

Multipath Optical Recombination of Intervalley Dark Excitons and Trions in Monolayer WSe_{2}.

Excitons and trions (or exciton polarons) in transition metal dichalcogenides (TMDs) are known to decay predominantly through intravalley transitions. Electron-hole recombination across different valleys can also play a significant role in the excitonic dynamics, but intervalley transitions are rarely observed in monolayer TMDs, because they violate the conservation of momentum. Here we reveal the intervalley recombination of dark excitons and trions through more than one path in monolayer WSe_{2}. We observe the intervalley dark excitons, which can recombine by the assistance of defect scattering or chiral-phonon emission. We also reveal that a trion can decay in two distinct paths-through intravalley or intervalley electron-hole recombination-into two different final valley states. Although these two paths are energy degenerate, we can distinguish them by lifting the valley degeneracy under a magnetic field. In addition, the intra- and inter-valley trion transitions are coupled to zone-center and zone-corner chiral phonons, respectively, to produce distinct phonon replicas. The observed multipath optical decays of dark excitons and trions provide insight into the internal quantum structure of trions and the complex excitonic interactions with defects and chiral phonons in monolayer valley semiconductors.

Gate Tunable Dark Trions in Monolayer WSe_{2}.

Monolayer WSe_{2} is an intriguing material to explore dark exciton physics. We have measured the photoluminescence from dark excitons and trions in ultraclean monolayer WSe_{2} devices encapsulated by boron nitride. The dark trions can be tuned continuously between negative and positive trions with electrostatic gating. We reveal their spin-triplet configuration and distinct valley optical emission by their characteristic Zeeman splitting under a magnetic field. The dark trion binding energies are 14-16 meV, slightly lower than the bright trion binding energies (21-35 meV). The dark trion lifetime (∼1.3 ns) is two orders of magnitude longer than the bright trion lifetime (∼10 ps) and can be tuned between 0.4 and 1.3 ns by gating. Such robust, optically detectable, and gate tunable dark trions may help us realize trion transport in two-dimensional materials.

Observation of Intervalley Biexcitonic Optical Stark Effect in Monolayer WS2.

Coherent optical driving can effectively modify the properties of electronic valleys in transition metal dichalcogenides. Here, we observe a new type of optical Stark effect in monolayer WS2, one that is mediated by intervalley biexcitons under the blue-detuned driving with circularly polarized light. We find that such helical optical driving not only induces an exciton energy downshift at the excitation valley but also causes an anomalous energy upshift at the opposite valley, which is normally forbidden by the exciton selection rules but now made accessible through the intervalley biexcitons. These findings reveal the critical, but hitherto neglected, role of biexcitons to couple the two seemingly independent valleys, and to enhance the optical control in valleytronics.

Ultraflat graphene.

Graphene, a single atomic layer of carbon connected by sp(2) hybridized bonds, has attracted intense scientific interest since its recent discovery. Much of the research on graphene has been directed towards exploration of its novel electronic properties, but the structural aspects of this model two-dimensional system are also of great interest and importance. In particular, microscopic corrugations have been observed on all suspended and supported graphene sheets studied so far. This rippling has been invoked to explain the thermodynamic stability of free-standing graphene sheets. Many distinctive electronic and chemical properties of graphene have been attributed to the presence of ripples, which are also predicted to give rise to new physical phenomena that would be absent in a planar two-dimensional material. Direct experimental study of such novel ripple physics has, however, been hindered by the lack of flat graphene layers. Here we demonstrate the fabrication of graphene monolayers that are flat down to the atomic level. These samples are produced by deposition on the atomically flat terraces of cleaved mica surfaces. The apparent height variation in the graphene layers observed by high-resolution atomic force microscopy (AFM) is less than 25 picometres, indicating the suppression of any existing intrinsic ripples in graphene. The availability of such ultraflat samples will permit rigorous testing of the impact of ripples on various physical and chemical properties of graphene.

Temperature-activated layer-breathing vibrations in few-layer graphene.

We investigated the low-frequency Raman spectra of freestanding few-layer graphene (FLG) at varying temperatures (400-900 K) controlled by laser heating. At high temperature, we observed the fundamental Raman mode for the lowest-frequency branch of rigid-plane layer-breathing mode (LBM) vibration. The mode frequency redshifts dramatically from 81 cm(-1) for bilayer to 23 cm(-1) for 8-layer. The thickness dependence is well described by a simple model of coupled oscillators. Notably, the LBM Raman response is unobservable at room temperature, and it is turned on at higher temperature (>600 K) with a steep increase of Raman intensity. The observation suggests that the LBM vibration is strongly suppressed by molecules adsorbed on the graphene surface but is activated as desorption occurs at high temperature.

Real-time observation of interlayer vibrations in bilayer and few-layer graphene.

We report real-time observation of the interlayer shearing mode, corresponding to the lateral oscillation of graphene planes, for bi- and few-layer graphene. Using a femtosecond pump-probe technique, we have followed coherent oscillations of this vibrational mode directly in the time domain. The shearing-mode frequency, as expected for an interlayer mode, exhibits a strong and systematic dependence on the number of layers, varying from 1.32 THz for the bulk limit to 0.85 THz for bilayer graphene. We explored the role of interactions with the external environment on this vibrational mode by comparing the response observed for graphene layers supported by different substrates and suspended in free space. No significant frequency shifts were observed.

Electric-field tunable Type-I to Type-II band alignment transition in MoSe2/WS2 heterobilayers.

Semiconductor heterojunctions are ubiquitous components of modern electronics. Their properties depend crucially on the band alignment at the interface, which may exhibit straddling gap (type-I), staggered gap (type-II) or broken gap (type-III). The distinct characteristics and applications associated with each alignment make it highly desirable to switch between them within a single material. Here we demonstrate an electrically tunable transition between type-I and type-II band alignments in MoSe2/WS2 heterobilayers by investigating their luminescence and photocurrent characteristics. In their intrinsic state, these heterobilayers exhibit a type-I band alignment, resulting in the dominant intralayer exciton luminescence from MoSe2. However, the application of a strong interlayer electric field induces a transition to a type-II band alignment, leading to pronounced interlayer exciton luminescence. Furthermore, the formation of the interlayer exciton state traps free carriers at the interface, leading to the suppression of interlayer photocurrent and highly nonlinear photocurrent-voltage characteristics. This breakthrough in electrical band alignment control, interlayer exciton manipulation, and carrier trapping heralds a new era of versatile optical and (opto)electronic devices composed of van der Waals heterostructures.

Tunable infrared phonon anomalies in trilayer graphene.

Trilayer graphene in both ABA (Bernal) and ABC (rhombohedral) stacking sequences is shown to exhibit intense infrared absorption from in-plane optical phonons. The phonon feature, lying at ~1580 cm(-1), changes strongly with electrostatic gating. For ABC-stacked graphene trilayers, we observed a large enhancement in phonon absorption amplitude, as well as softening of the phonon mode, as the Fermi level is tuned away from charge neutrality. A similar, but substantially weaker, effect is seen in samples with the more common ABA stacking order. The strong infrared response of the optical phonons and the pronounced variation with electrostatic gating and stacking order reflect the interactions of the phonons and electronic excitations in the two systems. The key experimental findings can be reproduced within a simplified charged-phonon model that considers the influence of charging through Pauli blocking of the electronic transitions.

Observation of layer-breathing mode vibrations in few-layer graphene through combination Raman scattering.

We report the observation of layer-breathing mode (LBM) vibrations in few-layer graphene (FLG) samples of thickness from two to six layers, exhibiting both Bernal (AB) and rhombohedral (ABC) stacking order. The LBM vibrations are identified using a Raman combination band lying around 1720 cm(-1). From double resonance theory, we assign the feature as the LO+ZO' combination mode of the out-of-plane LBM (ZO') and the in-plane longitudinal optical mode (LO). The LOZO' Raman band is found to exhibit multiple peaks with a unique line shape for each layer thickness and stacking order. These complex line shapes of the LOZO'-mode arise both from the material-dependent selection of different phonons in the double-resonance Raman process and from the detailed structure of the different branches of LBM in FLG.

Water-gated charge doping of graphene induced by mica substrates.

We report on the existence of water-gated charge doping of graphene deposited on atomically flat mica substrates. Molecular films of water in units of ~0.4 nm thick bilayers were found to be present in regions of the interface of graphene/mica heterostacks prepared by micromechanical exfoliation of kish graphite. The spectral variation of the G and 2D bands, as visualized by Raman mapping, shows that mica substrates induce strong p-type doping in graphene with hole densities of (9 ± 2) × 10(12) cm(-2). The ultrathin water films, however, effectively block interfacial charge transfer, rendering graphene significantly less hole-doped. Scanning Kelvin probe microscopy independently confirmed a water-gated modulation of the Fermi level by 0.35 eV, which is in agreement with the optically determined hole density. The manipulation of the electronic properties of graphene demonstrated in this study should serve as a useful tool in realizing future graphene applications.

Structure-dependent Fano resonances in the infrared spectra of phonons in few-layer graphene.

The in-plane optical phonons around 200 meV in few-layer graphene are investigated utilizing infrared absorption spectroscopy. The phonon spectra exhibit unusual asymmetric features characteristic of Fano resonances, which depend critically on the layer thickness and stacking order of the sample. The phonon intensities in samples with rhombohedral (ABC) stacking are significantly higher than those with Bernal (AB) stacking. These observations reflect the strong coupling between phonons and interband electronic transitions in these systems and the distinctive variation in the joint density of electronic states in samples of differing thickness and stacking order.

Imaging stacking order in few-layer graphene.

Few-layer graphene (FLG) has been predicted to exist in various crystallographic stacking sequences, which can strongly influence the material's electronic properties. We demonstrate an accurate and efficient method to characterize stacking order in FLG using the distinctive features of the Raman 2D-mode. Raman imaging allows us to visualize directly the spatial distribution of Bernal (ABA) and rhombohedral (ABC) stacking in tri- and tetralayer graphene. We find that 15% of exfoliated graphene tri- and tetralayers is composed of micrometer-sized domains of rhombohedral stacking, rather than of usual Bernal stacking. These domains are stable and remain unchanged for temperatures exceeding 800 °C.

Optical absorption of interlayer excitons in transition-metal dichalcogenide heterostructures.

Interlayer excitons, electron-hole pairs bound across two monolayer van der Waals semiconductors, offer promising electrical tunability and localizability. Because such excitons display weak electron-hole overlap, most studies have examined only the lowest-energy excitons through photoluminescence. We directly measured the dielectric response of interlayer excitons, which we accessed using their static electric dipole moment. We thereby determined an intrinsic radiative lifetime of 0.40 nanoseconds for the lowest direct-gap interlayer exciton in a tungsten diselenide/molybdenum diselenide heterostructure. We found that differences in electric field and twist angle induced trends in exciton transition strengths and energies, which could be related to wave function overlap, moiré confinement, and atomic reconstruction. Through comparison with photoluminescence spectra, this study identifies a momentum-indirect emission mechanism. Characterization of the absorption is key for applications relying on light-matter interactions.

Thickness- and Twist-Angle-Dependent Interlayer Excitons in Metal Monochalcogenide Heterostructures.

Interlayer excitons, or bound electron-hole pairs whose constituent quasiparticles are located in distinct stacked semiconducting layers, are being intensively studied in heterobilayers of two-dimensional semiconductors. They owe their existence to an intrinsic type-II band alignment between both layers that convert these into p-n junctions. Here, we unveil a pronounced interlayer exciton (IX) in heterobilayers of metal monochalcogenides, namely, γ-InSe on ε-GaSe, whose pronounced emission is adjustable just by varying their thicknesses given their number of layers dependent direct band gaps. Time-dependent photoluminescense spectroscopy unveils considerably longer interlayer exciton lifetimes with respect to intralayer ones, thus confirming their nature. The linear Stark effect yields a bound electron-hole pair whose separation d is just (3.6 ± 0.1) Å with d being very close to dSe = 3.4 Å which is the calculated interfacial Se separation. The envelope of IX is twist-angle-dependent and describable by superimposed emissions that are nearly equally spaced in energy, as if quantized due to localization induced by the small moiré periodicity. These heterostacks are characterized by extremely flat interfacial valence bands making them prime candidates for the observation of magnetism or other correlated electronic phases upon carrier doping.