High charge-carrier mobility that enables the observation of quantum oscillation is reported in mono- and few-layer MoS2 encapsulated and contacted by other two-dimensional materials.

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Multi-terminal transport measurements of MoS2 using a van der Waals heterostructure device platform.

http://absimage.aps.org/image/MAR16/MWS_MAR16-2015-030072.pdf

Transport measurements of MoS2 using a van der Waals heterostructure device platform

Two-dimensional (2D) transition metal dichalcogenide (TMD) nanosheets exhibit remarkable electronic and optical properties. The 2D features, sizable bandgaps and recent advances in the synthesis, characterization and device fabrication of the representative MoS2, WS2, WSe2 and MoSe2 TMDs make TMDs very attractive in nanoelectronics and optoelectronics. Similar to graphite and graphene, the atoms within each layer in 2D TMDs are joined together by covalent bonds, while van der Waals interactions keep the layers together. This makes the physical and chemical properties of 2D TMDs layer-dependent. In this review, we discuss the basic lattice vibrations of 2D TMDs from monolayer, multilayer to bulk material, including high-frequency optical phonons, interlayer shear and layer breathing phonons, the Raman selection rule, layer-number evolution of phonons, multiple phonon replica and phonons at the edge of the Brillouin zone. The extensive capabilities of Raman spectroscopy in investigating the properties of TMDs are discussed, such as interlayer coupling, spin–orbit splitting and external perturbations. The interlayer vibrational modes are used in rapid and substrate-free characterization of the layer number of multilayer TMDs and in probing interface coupling in TMD heterostructures. The success of Raman spectroscopy in investigating TMD nanosheets paves the way for experiments on other 2D crystals and related van der Waals heterostructures.

Phonon and Raman scattering of two-dimensional transition metal dichalcogenides from monolayer, multilayer to bulk material

Transition metal dichalcogenides (TMDs) represent a family of materials with versatile electronic, optical, and chemical properties. Most TMD bulk crystals are van der Waals solids with strong bonding within the plane but weak interlayer bonding. The individual layers can be readily isolated. Single layer TMDs possess intriguing properties that are ideal for both fundamental and technologically relevant research studies. We review the structure and phases of single and few layered TMDs. We also describe recent progress in phase engineering in TMDs. The ability to tune the chemistry by choosing a unique combination of transition metals and chalcogen atoms along with controlling their properties by phase engineering allows new functionalities to be realized with TMDs.

/pdf/phase-engineering-of-transition-metal-dichalcogenides-3srjo3bvkd.pdf

Phase engineering of transition metal dichalcogenides

Atomically thin group-VIB transition metal dichalcogenides (TMDs) have recently emerged as a new class of two-dimensional (2D) semiconductors with extraordinary properties including the direct band gap in the visible frequency range, the pronounced spin–orbit coupling, the ultra-strong Coulomb interaction, and the rich physics associated with the valley degree of freedom. These 2D TMDs exhibit great potential for device applications and have attracted vast interest for the exploration of new physics. 2D TMDs have complex electronic structures which underlie their physical properties. Here we review the bulk electronic structures of these new 2D materials as well as the theoretical models developed at different levels, along which we sort out the understanding of the origins of a variety of properties observed or predicted.

Electronic structures and theoretical modelling of two-dimensional group-VIB transition metal dichalcogenides.

Folding MoS2 monolayers to obtain bilayers with different stacking orders results in enhanced valley- and spin-polarizations compared with natural Bernal-stacked bilayers.

Valley and band structure engineering of folded MoS2 bilayers

Crystal symmetry plays a central role in governing a wide range of fundamental physical phenomena. One example is nonlinear optical second harmonic generation (SHG), which requires inversion symmetry breaking. We report a unique stacking-induced SHG in graphene trilayers, whose individual monolayer sheet is centrosymmetric. Depending on layer stacking sequence, we observe a strong optical SHG in a Bernal ABA–stacked non-centrosymmetric trilayer, while it vanishes in a rhombohedral ABC–stacked one, which preserves inversion symmetry. This highly contrasting SHG due to the distinct stacking symmetry enables us to map out the ABA and ABC crystal domains in an otherwise homogeneous graphene trilayer. The extracted second-order nonlinear susceptibility of the ABA trilayer is surprisingly large, comparable to the best known two-dimensional semiconductors enhanced by excitonic resonance. Our results reveal a novel stacking order–induced nonlinear optical effect, as well as unleash the opportunity for studying intriguing physical phenomena predicted for stacking-dependent ABA and ABC graphene trilayers.

https://advances.sciencemag.org/content/advances/4/6/eaat0074.full.pdf

Stacking symmetry governed second harmonic generation in graphene trilayers

Crystal symmetry plays a central role in governing a wide range of fundamental physical phenomena. One example is the nonlinear optical second harmonic generation (SHG), which requires inversion symmetry breaking. Here we report a unique stacking-induced SHG in trilayer graphene, whose individual monolayer sheet is centrosymmetric. Depending on layer stacking sequence, we observe a strong optical SHG in Bernal (ABA) stacked non-centrosymmetric trilayer, while it vanishes in rhombohedral (ABC) stacked one which preserves inversion symmetry. This highly contrasting SHG due to the distinct stacking symmetry enables us to map out the ABA and ABC crystal domains in otherwise homogeneous graphene trilayer. The extracted second order nonlinear susceptibility of the ABA trilayer is surprisingly large, comparable to the best known 2D semiconductors enhanced by excitonic resonance. Our results reveal a novel stacking order induced nonlinear optical effect, as well as unleash the opportunity for studying intriguing physical phenomena predicted for stacking-dependent ABA and ABC graphene trilayers.

Stacking-symmetry governed second harmonic generation in graphene trilayers

Excitons in transition metal dichalcogenide monolayer have recently attracted great interest due to their extremely large binding energy, causing giant bandgap renormalization. In this work, we examined the screening effect of graphite and bilayer graphene on the excitons in molybdenum diselenide (MoSe2) monolayer grown by molecular beam epitaxy (MBE). Through the combinational study of scanning tunneling spectroscopy (STS) and photoluminescence (PL) measurements, we determined the binding energy of ~0.58 eV for MoSe2 monolayer on both substrates at 16 K, and no obvious difference between them. Our result is consistent with a previous report [Zhang et al 2015 Nano Letters 15, 6494], but is contradictory to another one [Ugeda 2014 Nature Materials 13, 1091]. Physical picture for no noticeable difference on screening effect between bilayer graphene and graphite substrate is discussed. Possible reasons for causing the discrepancy are also mentioned.

http://iopscience.iop.org/article/10.1088/2053-1583/4/1/015021/pdf

Screening effect of graphite and bilayer graphene on excitons in MoSe 2 monolayer

We examine the chirality-dependent optical selection rules in two-dimensional monolayer materials with honeycomb lattices, and, based on symmetry argument, we generalize these rules to multi-photon transitions of arbitrary orders. We also present the phase relations between incident and outgoing photons in such processes. The results agree nicely with our experimental observations of second- and third-harmonic generation. In particular, we demonstrate that the phase relation of chiral second-harmonic generation can serve as a handy tool for imaging domains and domain boundaries of these monolayers. Our results can benefit future studies on chirality-related optical phenomena and opto-electronic applications of such materials.

Yingguo Li

Papers

Valley and band structure engineering of folded MoS2 bilayers

Stacking symmetry governed second harmonic generation in graphene trilayers

Stacking-symmetry governed second harmonic generation in graphene trilayers

Screening effect of graphite and bilayer graphene on excitons in MoSe 2 monolayer

Chiral selection rules for multi-photon processes in two-dimensional honeycomb materials.