Horacio Coy Diaz

Direct Observation of Interlayer Hybridization and Dirac Relativistic Carriers in Graphene/MoS2 van der Waals Heterostructures

Artificial heterostructures assembled from van der Waals materials promise to combine materials without the traditional restrictions in heterostructure-growth such as lattice matching conditions and atom interdiffusion. Simple stacking of van der Waals materials with diverse properties would thus enable the fabrication of novel materials or device structures with atomically precise interfaces. Because covalent bonding in these layered materials is limited to molecular planes and the interaction between planes are very weak, only small changes in the electronic structure are expected by stacking these materials on top of each other. Here we prepare interfaces between CVD-grown graphene and MoS2 and report the direct measurement of the electronic structure of such a van der Waals heterostructure by angle-resolved photoemission spectroscopy. While the Dirac cone of graphene remains intact and no significant charge transfer doping is detected, we observe formation of band gaps in the π-band of graphene, away from the Fermi-level, due to hybridization with states from the MoS2 substrate.

Molecular beam epitaxy of the van der Waals heterostructure MoTe2 on MoS2: phase, thermal, and chemical stability

(Sub)monolayer MoTe2 is grown by molecular beam epitaxy on a bulk MoS2 substrate. The film morphology, the thermally induced transformation of structural and compositional phases, as well as the chemical stability upon exposure to atmosphere are investigated by scanning tunneling microscopy and photoemission spectroscopy. Predominantly, semiconducting α-MoTe2 islands are obtained under tellurium rich growth conditions and a substrate temperature of 200 °C. Under less tellurium-rich conditions, elongated and meandering MoTe2−x strands are formed rather than compact islands. Similarly, annealing of initial α-MoTe2 islands to above 500 °C causes the loss of tellurium and possibly transformation into the same MoTe2−x strands. Consequently, under vacuum conditions the the transformation of α-MoTe2 monolayers into the semimetallic β -MoTe2 high temperature phase is accompanied by a loss of Te and formation of MoTe2−x phase. The obtained tellurium deficient MoTe2−x phase is almost metallic but a small band gap of a few tens meV remains. The as-grown α-MoTe2 islands exhibit a moire structure with ~2.6 nm periodicity. This periodicity implies a rotation of ~56° between the MoTe2 and MoS2. We assign the observation of a specific rotation angle for the grown MoTe2 islands with respect to the MoS2 substrate to the lowest energy adsorption configuration for MoTe2 monolayers on MoS2 substrates. Exposure of the as grown films to atmosphere results in oxidation of the MoTe2 film. The oxidized film maintains the two-dimensional island morphology of the initial film and thus is a candidate for a 2D (amorphous) oxide layer on MoS2.

Strong room-temperature ferromagnetism in VSe 2 monolayers on van der Waals substrates

Reduced dimensionality and interlayer coupling in van der Waals materials gives rise to fundamentally different electronic1, optical2 and many-body quantum3–5 properties in monolayers compared with the bulk. This layer-dependence permits the discovery of novel material properties in the monolayer regime. Ferromagnetic order in two-dimensional materials is a coveted property that would allow fundamental studies of spin behaviour in low dimensions and enable new spintronics applications6–8. Recent studies have shown that for the bulk-ferromagnetic layered materials CrI3 (ref. 9) and Cr2Ge2Te6 (ref. 10), ferromagnetic order is maintained down to the ultrathin limit at low temperatures. Contrary to these observations, we report the emergence of strong ferromagnetic ordering for monolayer VSe2, a material that is paramagnetic in the bulk11,12. Importantly, the ferromagnetic ordering with a large magnetic moment persists to above room temperature, making VSe2 an attractive material for van der Waals spintronics applications.Reducing the dimensionality of paramagnetic VSe2 results in the emergence of ferromagnetism that is observed in a monolayer and up to room temperature.

High density of (pseudo) periodic twin-grain boundaries in molecular beam epitaxy-grown van der Waals heterostructure: MoTe2/MoS2

Growth of transition metal dichalcogenide heterostructures by molecular beam epitaxy (MBE) promises synthesis of artificial van der Waals materials with controllable layer compositions and separations. Here, we show that MBE growth of 2H-MoTe2 monolayers on MoS2 substrates results in a high density of mirror-twins within the films. The grain boundaries are tellurium deficient, suggesting that Te-deficiency during growth causes their formation. Scanning tunneling microscopy and spectroscopy reveal that the grain boundaries arrange in a pseudo periodic “wagon wheel” pattern with only ∼2.6 nm repetition length. Defect states from these domain boundaries fill the band gap and thus give the monolayer an almost metallic property. The band gap states pin the Fermi-level in MoTe2 and thus determine the band-alignment in the MoTe2/MoS2 interface.

Angle resolved photoemission spectroscopy reveals spin charge separation in metallic MoSe2 grain boundary

Material line defects are one-dimensional structures but the search and proof of electron behaviour consistent with the reduced dimension of such defects has been so far unsuccessful. Here we show using angle resolved photoemission spectroscopy that twin-grain boundaries in the layered semiconductor MoSe2 exhibit parabolic metallic bands. The one-dimensional nature is evident from a charge density wave transition, whose periodicity is given by kF/π, consistent with scanning tunnelling microscopy and angle resolved photoemission measurements. Most importantly, we provide evidence for spin- and charge-separation, the hallmark of one-dimensional quantum liquids. Our studies show that the spectral line splits into distinctive spinon and holon excitations whose dispersions exactly follow the energy-momentum dependence calculated by a Hubbard model with suitable finite-range interactions. Our results also imply that quantum wires and junctions can be isolated in line defects of other transition metal dichalcogenides, which may enable quantum transport measurements and devices.

Metallic Twin Grain Boundaries Embedded in MoSe2 Monolayers Grown by Molecular Beam Epitaxy

Twin grain boundaries in MoSe2 are metallic and undergo a metal to insulator Peierls transition at low temperature. Growth of MoSe2 by molecular beam epitaxy results in the spontaneous formation of a high density of these twin grain boundaries, likely as a mechanism to incorporate Se deficiency in the film. Using scanning tunneling microscopy, we study the grain boundary network that is formed in homoepitaxially grown MoSe2 and for MoSe2 grown heteroepitaxially on MoS2 and HOPG substrates. No statistically relevant variation of the grain boundary concentration has been found for the different substrates, indicating that the grain boundary formation is substrate independent and depends mainly on the growth conditions. Twin grain boundaries exhibit three crystallographically identical orientations, and thus they form an intersecting network. Different intersection geometries are identified that imply varying defect configurations. These intersection points act as preferential nucleation sites for vapor-deposited metal atoms, which we demonstrate on the example of selective gold cluster formation at grain boundary intersections. Scanning tunneling spectroscopy shows a band gap narrowing of MoSe2 in the immediate vicinity of the metallic grain boundary, which may be associated with lattice strain induced at the grain boundary. Tunneling noise spectra taken over the grain boundaries indicate random telegraphic noise, suggestive of pinning/depinning behavior of conductive channels in the metallic grain boundaries or their intersection points. Finally, indications for incommensurate and commensurate Peierls-driven charge density wave formation were observed in microprobe transport measurements at 205 and 227 K, respectively.

Post-Synthesis Modifications of Two-Dimensional MoSe2 or MoTe2 by Incorporation of Excess Metal Atoms into the Crystal Structure

Phase engineering has extensively been used to achieve metallization of two-dimensional (2D) semiconducting materials, as it should boost their catalytic properties or improve electrical contacts. In contrast, here we demonstrate compositional phase change by incorporation of excess metals into the crystal structure. We demonstrate post-synthesis restructuring of the semiconducting MoTe2 or MoSe2 host material by unexpected easy incorporation of excess Mo into their crystal planes, which causes local metallization. The amount of excess Mo can reach values as high as 10% in MoTe2 thus creating a significantly altered material compared to its parent structure. The incorporation mechanism is explained by density functional theory in terms of the energy difference of Mo atoms incorporated in the line phases as compared to Mo ad-clusters. Angle resolved photoemission spectroscopy reveals that the incorporated excess Mo induces band gap states up to the Fermi level causing its pinning at these electronic states. The incorporation of excess transition metals in MoTe2 and MoSe2 is not limited to molybdenum, but other transition metals can also diffuse into the lattice, as demonstrated experimentally by Ti deposition. The mechanism of incorporation of transition metals in MoSe2 and MoTe2 is revealed, which should help to address the challenges in synthesizing defect-free single layer materials by, for example, molecular beam epitaxy. The easy incorporation of metal atoms into the crystal also indicates that the previously assumed picture of a sharp metal/2D-material interface may not be correct, and at least for MoSe2 and MoTe2, in-diffusion of metals from metal-contacts into the 2D material has to be considered. Most importantly though, the process of incorporation of transition metals with high concentrations into pristine 2D transition-metal dichalcogenides enables a pathway for their post-synthesis modifications and adding functionalities.