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Register a new userDirect Optical-Structure Correlation in Atomically Thin Dichalcogenides and Heterostructures
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Atomically thin transition metal dichalcogenides (TMDs) have distinct opto-electronic properties including enhanced luminescence and high on-off current ratios, which can be further modulated by making more complex TMD heterostructures. However, resolution limits of conventional optical methods do not allow for direct optical-structural correlation measurements in these materials, particularly of buried interfaces in TMD heterostructures. Here we use, for the first time, electron beam induced cathodoluminescence in a scanning transmission electron microscope (CL-STEM) to measure optical properties of monolayer TMDs (WS2, MoS2 and WSSe alloy) encapsulated between layers of hBN. We observe dark areas resulting from localized (~ 100 nm) imperfect interfaces and monolayer folding, which shows that the intimate contact between layers in this application-relevant heterostructure is required for proper inter layer coupling. We also realize a suitable imaging method that minimizes electron-beam induced changes and provides measurement of intrinsic properties. To overcome the limitation of small electron interaction volume in TMD monolayer (and hence low photon yield), we find that encapsulation of TMD monolayers with hBN and subsequent annealing is important. CL-STEM offers to be a powerful method to directly measure structure-optical correspondence in lateral or vertical heterostructures and alloys.
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Motivated by the triumph and limitation of graphene for electronic applications, atomically thin layers of group VI transition metal dichalcogenides are attracting extensive interest as a class of graphene-like semiconductors with a desired band-gap in the visible frequency range. The monolayers feature a valence band spin splitting with opposite sign in the two valleys located at corners of 1st Brillouin zone. This spin-valley coupling, particularly pronounced in tungsten dichalcogenides, can benefit potential spintronics and valleytronics with the important consequences of spin-valley interplay and the suppression of spin and valley relaxations. Here we report the first optical studies of WS2 and WSe2 monolayers and multilayers. The efficiency of second harmonic generation shows a dramatic even-odd oscillation with the number of layers, consistent with the presence (absence) of inversion symmetry in even-layer (odd-layer). Photoluminescence (PL) measurements show the crossover from an indirect band gap semiconductor at mutilayers to a direct-gap one at monolayers. The PL spectra and first-principle calculations consistently reveal a spin-valley coupling of 0.4 eV which suppresses interlayer hopping and manifests as a thickness independent splitting pattern at valence band edge near K points. This giant spin-valley coupling, together with the valley dependent physical properties, may lead to rich possibilities for manipulating spin and valley degrees of freedom in these atomically thin 2D materials.
Transition metal dichalcogenide monolayers are promising candidates for exploring new electronic and optical phenomena and for realizing atomically thin optoelectronic devices. They host tightly bound electron-hole pairs (excitons) that can be efficiently excited by resonant light fields. Here, we demonstrate that a single monolayer of molybdenum diselenide (MoSe2) can dramatically modify light transmission near the excitonic resonance, acting as an electrically switchable mirror that reflects up to 85% of incident light at cryogenic temperatures. This high reflectance is a direct consequence of the excellent coherence properties of excitons in this atomically thin semiconductor, encapsulated by hexagonal boron nitride. Furthermore, we show that the MoSe2 monolayer exhibits power- and wavelength-dependent nonlinearities that stem from exciton-based lattice heating in the case of continuous-wave excitation and exciton-exciton interactions when fast, pulsed laser excitation is used. These observations open up new possibilities for studying quantum nonlinear optical phenomena and topological photonics, and for miniaturizing optical devices.
Based on first-principles calculations and symmetry analysis, we predict atomically thin ($1-N$ layers) 2H group-VIB TMDs $MX_2$ ($M$ = Mo, W; $X$ = S, Se, Te) are large-gap higher-order topological crystalline insulators protected by $C_3$ rotation symmetry. We explicitly demonstrate the nontrivial topological indices and existence of the hallmark corner states with quantized fractional charge for these familiar TMDs with large bulk optical band gaps ($1.64-1.95$ eV for the monolayers), which would facilitate the experimental detection by STM. We find that the well-defined corner states exist in the triangular finite-size flakes with armchair edges of the atomically thin ($1-N$ layers) 2H group-VIB TMDs, and the corresponding quantized fractional charge is the number of layers $N$ divided by 3 modulo integers, which will simply double including spin degree of freedom.
Exfoliated chromium triiodide (CrI$_3$) is a layered van der Waals (vdW) magnetic insulator that consists of ferromagnetic layers coupled through antiferromagnetic interlayer exchange. The resulting permutations of magnetic configurations combined with the underlying crystal symmetry produces tunable magneto-optical phenomena that is unique to the two-dimensional (2D) limit. Here, we report the direct observation of 2D magnons through magneto-Raman spectroscopy with optical selection rules that are strictly determined by the honeycomb lattice and magnetic states of atomically thin CrI$_3$. In monolayers, we observe an acoustic magnon mode of ~0.3 meV with cross-circularly polarized selection rules locked to the magnetization direction. These unique selection rules arise from the discrete conservation of angular momentum of photons and magnons dictated by threefold rotational symmetry in a rotational analogue to Umklapp scattering. In bilayers, by tuning between the layered antiferromagnetic and ferromagnetic-like states, we observe the switching of two magnon modes. The bilayer structure also enables Raman activity from the optical magnon mode at ~17 meV (~4.2 THz) that is otherwise Raman-silent in the monolayer. From these measurements, we quantitatively extract the spin wave gap, magnetic anisotropy, intralayer and interlayer exchange constants, and establish 2D magnets as a new system for exploring magnon physics.
A novel method for the direct correlation at the nanoscale of structural and optical properties of single GaAs nanowires is reported. Nanowires consisting of 100% wurtzite and nanowires presenting zinc-blende/wurtzite polytypism are investigated by photoluminescence spectroscopy and transmission electron microscopy. The photoluminescence of wurtzite GaAs is consistent with a band gap of 1.5 eV. In the polytypic nanowires, it is shown that the regions that are predominantly composed of either zinc-blende or wurtzite phase show photoluminescence emission close to the bulk GaAs band gap, while regions composed of a nonperiodic superlattice of wurtzite and zinc-blende phases exhibit a redshift of the photoluminescence spectra as low as 1.455 eV. The dimensions of the quantum heterostructures are correlated with the light emission, allowing us to determine the band alignment between these two crystalline phases. Our first-principles electronic structure calculations within density functional theory, employing a hybrid-exchange functional, predict band offsets and effective masses in good agreement with experimental results.
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