We report strong third-harmonic generation in monolayer graphene grown by chemical vapor deposition and transferred to an amorphous silica (glass) substrate; the photon energy is in threephoton resonance with the exciton-shifted van Hove singularity at the M point of graphene. The polarization selection rules are derived and experimentally verified. In addition, our polarization-and azimuthal-rotation-dependent third-harmonic-generation measurements reveal in-plane isotropy as well as anisotropy between the in-plane and out-of-plane nonlinear optical responses of graphene. Since the third-harmonic signal exceeds that from bulk glass by more than 2 orders of magnitude, the signal contrast permits background-free scanning of graphene and provides insight into the structural properties of graphene.

Using angle-resolved photoemission on micrometer-scale sample areas, we directly measure the interlayer twist angle-dependent electronic band structure of bilayer molybdenum-disulfide (MoS2). Our measurements, performed on arbitrarily stacked bilayer MoS2 flakes prepared by chemical vapor deposition, provide direct evidence for a downshift of the quasiparticle energy of the valence band at the Brillouin zone center (Γ̅ point) with the interlayer twist angle, up to a maximum of 120 meV at a twist angle of ∼40°. Our direct measurements of the valence band structure enable the extraction of the hole effective mass as a function of the interlayer twist angle. While our results at Γ̅ agree with recently published photoluminescence data, our measurements of the quasiparticle spectrum over the full 2D Brillouin zone reveal a richer and more complicated change in the electronic structure than previously theoretically predicted. The electronic structure measurements reported here, including the evolution of the effective mass with twist-angle, provide new insight into the physics of twisted transition-metal dichalcogenide bilayers and serve as a guide for the practical design of MoS2 optoelectronic and spin-/valley-tronic devices.

We report angle-resolved photoemission spectroscopic measurements of the evolution of the thickness-dependent electronic band structure of the heavy-atom two-dimensional layered, dichalcogenide, tungsten-diselenide (WSe 2 ). Our data, taken on mechanically exfoliated WSe 2 singlecrystals, provide direct evidence for shifting of the valence-band maximum from Γ (multilayer WSe 2 ), to K , (single-layer WSe 2 ). Further, our measurements also set a lower bound on the energy of the direct band-gap and provide direct measurement of the hole effective mass.Single layers of two-dimensional metal dichalcogenides (TMDCs) such as MoS 2 , have emerged as a new class of non-centrosymmetric direct-bandgap materials with potential photonic and spintronic applications.[ [4] . In addition, ML WSe 2 has been demonstrated to be the first TMDC material possessing ambipolar, i.e., both p-type and n-type conducting behavior, [4][12] thus making it possible to design additional electronic functionality, such as p-n junctions or complementary logic circuits.Despite these intriguing characteristics, measurements of ML WSe 2 have generally been limited to probing of optical and transport properties. [4][5] [6] In this paper, we report thickness-dependent measurements of the surface and electronic structure of exfoliated WSe 2, using low-energy electron microscopy (LEEM), diffraction (LEED), and micrometer-scale angle-resolved photoemission spectroscopy (µ-ARPES) of samples supported on a native-oxide terminated silicon substrate. Our experimental results provide direct evidence for a predicted valence-band maximum (VBM) symmetrypoint change, which leads to an indirect-to-direct bandgap transition. Because TMDCs have a large carrier effective mass and reduced screening in two dimensions, electron-hole interactions are much stronger than in conventional semiconductors. [13][14] [15] Our results allow us to obtain a direct measurement of the hole effective mass. Finally, our measurements allow us to directly infer a lower bound on the energy of the direct band gap. 2Our measurements were performed using the spectroscopic photoemission and low-energy electron microscope (SPE-LEEM) system at the National Synchrotron Light Source (NSLS) beamline U5UA.[16] [17] The spectrometer energy resolution of this instrument was set to 100 meV at 33 eV incident photon energy with a beam spot size of 1 μm in diameter. The momentum resolution is ~0.02 Å -1. Exfoliated WSe 2 samples were transferred to a native-oxide covered Si substrate; prior to measurements, these samples were annealed at 350 o C for ~12 hours under UHV conditions. The layer number of the sample is determined by Raman and photoluminescence spectroscopy.[18] [19] Additional experimental details can be found in the Supplemental Materials section.Sample quality and crystal orientation were examined using both LEEM and µ-LEED (Fig. 1). Diffraction patterns (at a primary electron energy of 48eV) of exfoliated WSe 2 flakes of 1-3ML and bulk are shown in Fig. 2a-d, respectively, and cl...

We report the use of time-and angle-resolved two-photon photoemission to map the bound, unoccupied electronic structure of the weakly coupled graphene/Ir(111) system. The energy, dispersion, and lifetime of the lowest three image-potential states are measured. In addition, the weak interaction between Ir and graphene permits observation of resonant transitions from an unquenched Shockley-type surface state of the Ir substrate to graphene/Ir image-potential states.The image-potential-state lifetimes are comparable to those of mid-gap clean metal surfaces. Evidence of localization of the excited electrons on single-atom-layer graphene islands is provided by coverage-dependent measurements.

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