Optical Properties and Band Gap of Single- and Few-Layer MoTe₂ Crystals

Single- and few-layer crystals of exfoliated MoTe₂ have been characterized spectroscopically by photoluminescence, Raman scattering, and optical absorption measurements. We find that MoTe₂ in the monolayer limit displays strong photoluminescence. On the basis of complementary optical absorption results, we conclude that monolayer MoTe₂ is a direct-gap semiconductor with an optical band gap of 1.10 eV. This new monolayer material extends the spectral range of atomically thin direct-gap materials from the visible to the near-infrared

Probing Interlayer Interactions in Transition Metal Dichalcogenide Heterostructures by Optical Spectroscopy: MoS₂/WS₂ and MoSe₂/WSe₂

We have applied optical absorption spectroscopy to investigate van der Waals heterostructures formed of pairs of monolayer transition metal dichalcogenide crystals, choosing MoS₂/WS₂ and MoSe₂/WSe₂ as test cases. In the heterostructure spectra, we observe a significant broadening of the excitonic transitions compared to the corresponding features in the isolated layers. The broadening is interpreted as a lifetime effect arising from decay of excitons initially created in either layer through charge transfer processes expected for a staggered band alignment. The measured spectral broadening of 20 meV – 35 meV implies lifetimes for charge separation of the near band-edge A and B excitons in the range of 20–35 fs. Higher-lying transitions exhibit still greater broadening

Probing Interlayer Interactions in Transition Metal Dichalcogenide Heterostructures by Optical Spectroscopy: MoS₂/WS₂ and MoSe₂/WSe₂

We have applied optical absorption spectroscopy to investigate van der Waals heterostructures formed of pairs of monolayer transition metal dichalcogenide crystals, choosing MoS₂/WS₂ and MoSe₂/WSe₂ as test cases. In the heterostructure spectra, we observe a significant broadening of the excitonic transitions compared to the corresponding features in the isolated layers. The broadening is interpreted as a lifetime effect arising from decay of excitons initially created in either layer through charge transfer processes expected for a staggered band alignment. The measured spectral broadening of 20 meV – 35 meV implies lifetimes for charge separation of the near band-edge A and B excitons in the range of 20–35 fs. Higher-lying transitions exhibit still greater broadening

Spectroscopic Study of Anisotropic Excitons in Single Crystal Hexacene

The linear optical response of hexacene single crystals over a spectral range of 1.3–1.9 eV was studied using polarization-resolved reflectance spectroscopy at cryogenic temperatures. We observe strong polarization anisotropy for all optical transitions. Pronounced deviations from the single-molecule, solution-phase spectra are present, with a measured Davydov splitting of 180 meV, indicating strong intermolecular coupling. The energies and oscillator strengths of the relevant optical transitions and polarization-dependent absorption coefficients are extracted from quantitative analysis of the data

Optical Imaging and Spectroscopic Characterization of Self-Assembled Environmental Adsorbates on Graphene

Topographic studies using scanning probes have found that graphene surfaces are often covered by micron-scale domains of periodic stripes with a 4 nm pitch. These stripes have been variously interpreted as structural ripples or as self-assembled adsorbates. We show that the stripe domains are optically anisotropic by imaging them using a polarization-contrast technique. Optical spectra between 1.1 and 2.8 eV reveal that the anisotropy in the in-plane dielectric function is predominantly real, reaching 0.6 for an assumed layer thickness of 0.3 nm. The spectra are incompatible with a rippled graphene sheet but would be quantitatively explained by the self-assembly of chainlike organic molecules into nanoscale stripes

Optical Imaging and Spectroscopic Characterization of Self-Assembled Environmental Adsorbates on Graphene

Topographic studies using scanning probes have found that graphene surfaces are often covered by micron-scale domains of periodic stripes with a 4 nm pitch. These stripes have been variously interpreted as structural ripples or as self-assembled adsorbates. We show that the stripe domains are optically anisotropic by imaging them using a polarization-contrast technique. Optical spectra between 1.1 and 2.8 eV reveal that the anisotropy in the in-plane dielectric function is predominantly real, reaching 0.6 for an assumed layer thickness of 0.3 nm. The spectra are incompatible with a rippled graphene sheet but would be quantitatively explained by the self-assembly of chainlike organic molecules into nanoscale stripes

Linearly Polarized Excitons in Single- and Few-Layer ReS₂ Crystals

Rhenium disulfide (ReS₂), a layered group VII transition metal dichalcogenide, has been studied by optical spectroscopy. We demonstrate that the reduced crystal symmetry, as compared to the molybdenum and tungsten dichalcogenides, leads to anisotropic optical properties that persist from the bulk down to the monolayer limit. We find that the direct optical gap blueshifts from 1.47 eV in the bulk to 1.61 eV in the monolayer limit. In the ultrathin limit, we observe polarization-dependent absorption and polarized emission from the band-edge optical transitions. We thus establish ultrathin ReS₂ as a birefringent material with strongly polarized direct optical transitions that vary in energy and orientation with sample thickness

The Reststrahlen Effect in the Optically Thin Limit: A Framework for Resonant Response in Thin Media

Sharp resonances can strongly modify the electromagnetic response of matter. A classic example is the Reststrahlen effect – high reflectivity in the mid-infrared in many polar crystals near their optical phonon resonances. Although this effect in bulk materials has been studied extensively, a systematic treatment for finite thickness remains challenging. Here we describe, experimentally and theoretically, the Reststrahlen response in hexagonal boron nitride across more than 5 orders of magnitude in thickness, down to a monolayer. We find that the high reflectivity plateau of the Reststrahlen band evolves into a single peak as the material enters the optically thin limit, within which two distinct regimes emerge: a strong-response regime dominated by coherent radiative decay and a weak-response regime dominated by damping. We show that this evolution can be explained by a simple two-dimensional sheet model that can be applied to a wide range of thin media

Observation of Ground- and Excited-State Charge Transfer at the C₆₀/Graphene Interface

We examine charge transfer interactions in the hybrid system of a film of C₆₀ molecules deposited on single-layer graphene using Raman spectroscopy and Terahertz (THz) time-domain spectroscopy. In the absence of photoexcitation, we find that the C₆₀ molecules in the deposited film act as electron acceptors for graphene, yielding increased hole doping in the graphene layer. Hole doping of the graphene film by a uniform C₆₀ film at a level of 5.6 × 10¹²/cm² or 0.04 holes per interfacial C₆₀ molecule was determined by the use of both Raman and THz spectroscopy. We also investigate transient charge transfer occurring upon photoexcitation by femtosecond laser pulses with a photon energy of 3.1 eV. The C₆₀/graphene hybrid exhibits a short-lived (ps) decrease in THz conductivity, followed by a long-lived increase in conductivity. The initial negative photoconductivity transient, which decays within 2 ps, reflects the intrinsic photoresponse of graphene. The longer-lived positive conductivity transient, with a lifetime on the order of 100 ps, is attributed to photoinduced hole doping of graphene by interfacial charge transfer. We discuss possible microscopic pathways for hot carrier processes in the hybrid system

Probing the Dynamics of the Metallic-to-Semiconducting Structural Phase Transformation in MoS₂ Crystals

We have investigated the phase transformation of bulk MoS₂ crystals from the metastable metallic 1T/1T′ phase to the thermodynamically stable semiconducting 2H phase. The metastable 1T/1T′ material was prepared by Li intercalation and deintercalation. The thermally driven kinetics of the phase transformation were studied with <i>in situ</i> Raman and optical reflection spectroscopies and yield an activation energy of 400 ± 60 meV (38 ± 6 kJ/mol). We calculate the expected minimum energy pathways for these transformations using DFT methods. The experimental activation energy corresponds approximately to the theoretical barrier for a single formula unit, suggesting that nucleation of the phase transformation is quite local. We also report that femtosecond laser writing converts 1T/1T′ to 2H in a single laser pass. The mechanisms for the phase transformation are discussed