Single-layer transition-metal dichalcogenides (TMDs) receive significant attention due to their intriguing physical properties for both fundamental research and potential applications in electronics, optoelectronics, spintronics, catalysis, and so on. Here, we demonstrate the epitaxial growth of high-quality single-crystal, monolayer platinum diselenide (PtSe2), a new member of the layered TMDs family, by a single step of direct selenization of a Pt(111) substrate. A combination of atomic-resolution experimental characterizations and first-principle theoretic calculations reveals the atomic structure of the monolayer PtSe2/Pt(111). Angle-resolved photoemission spectroscopy measurements confirm for the first time the semiconducting electronic structure of monolayer PtSe2 (in contrast to its semimetallic bulk counterpart). The photocatalytic activity of monolayer PtSe2 film is evaluated by a methylene-blue photodegradation experiment, demonstrating its practical application as a promising photocatalyst. Moreover, circular polarization calculations predict that monolayer PtSe2 has also potential applications in valleytronics.

Topological semimetals have recently attracted extensive research interests as host materials to condensed matter physics counterparts of Dirac and Weyl fermions originally proposed in high energy physics. Although Lorentz invariance is required in high energy physics, it is not necessarily obeyed in condensed matter physics, and thus Lorentz-violating type-II Weyl/Dirac fermions could be realized in topological semimetals. The recent realization of type-II Weyl fermions raises the question whether their spin-degenerate counterpart—type-II Dirac fermions—can be experimentally realized too. Here, we report the experimental evidence of type-II Dirac fermions in bulk stoichiometric PtTe2 single crystal. Angle-resolved photoemission spectroscopy measurements and first-principles calculations reveal a pair of strongly tilted Dirac cones along the Γ-A direction, confirming PtTe2 as a type-II Dirac semimetal. Our results provide opportunities for investigating novel quantum phenomena (e.g., anisotropic magneto-transport) and topological phase transition.

Topological insulators are a new class of material 1,2 , that exhibit robust gapless surface states protected by time-reversal symmetry 3,4 . The interplay of such symmetry-protected topological surface states and symmetry-broken states (for example, superconductivity) provides a platform for exploring new quantum phenomena and functionalities, such as one-dimensional chiral or helical gapless Majorana fermions 5 , and Majorana zero modes 6 that may find application in faulttolerant quantum computation 7,8 . Inducing superconductivity on the topological surface states is a prerequisite for their experimental realization 1,2 . Here, by growing high-quality topological insulator Bi 2 Se 3 films on a d-wave superconductor Bi 2 Sr 2 CaCu 2 O 8+δ using molecular beam epitaxy, we are able to induce high-temperature superconductivity on the surface states of Bi 2 Se 3 films with a large pairing gap up to 15 meV. Interestingly, distinct from the d-wave pairing of Bi 2 Sr 2 CaCu 2 O 8+δ , the proximity-induced gap on the surface states is nearly isotropic and consistent with predominant s-wave pairing as revealed by angle-resolved photoemission spectroscopy. Our work could provide a critical step towards the realization of the long sought Majorana zero modes.The search for exotic quantum phenomena and new functionalities has been among the most tremendous driving forces for the fields of condensed-matter physics and materials science. Majorana zero modes, that is, Majorana fermions that are their own antiparticles and occur at exactly zero energy, are particularly fascinating not only because of their intriguing physics obeying robust non-Abelian statistics, but also owing to their potential application as building blocks for topological quantum computers 7,8 . Although significant progress has been made recently in one-dimensional semiconductor quantum wires coupled with conventional superconductors 9-12 , decisive evidence of Majorana zero modes has been lacking and many puzzles remain 13 . Topological insulators, whose hallmark is time-reversal-symmetryprotected surface states, may offer less restrictive experimental conditions for realizing Majorana zero modes 1,2 . Theoretically, Majorana zero modes are predicted to occur in vortex cores of three-dimensional topological insulators when they are in close proximity to conventional s-wave superconductors 6 ; however,

Atomically thin PtSe 2 films have attracted extensive research interests for potential applications in high-speed electronics, spintronics and photodetectors. Obtaining high quality, single crystalline thin films with large size is critical. Here we report the first successful layer-by-layer growth of high quality PtSe 2 films by molecular beam † The authors declare no competing financial interest. 1 arXiv:1703.04279v2 [cond-mat.mtrl-sci] 15 Mar 2017 epitaxy. Atomically thin films from 1 ML to 22 ML have been grown and characterized by low-energy electron diffraction, Raman spectroscopy and X-ray photoemission spectroscopy. Moreover, a systematic thickness dependent study of the electronic structure is revealed by angle-resolved photoemission spectroscopy (ARPES), and helical spin texture is revealed by spin-ARPES. Our work provides new opportunities for growing large size single crystalline films for investigating the physical properties and potential applications of PtSe 2 . KeywordsPtSe 2 , Molecular beam epitaxy (MBE), Raman, ARPES, Transition metal dichalcogenide (TMDC) Layered transition metal dichalcogenides (TMDCs) have attracted extensive interests for applications in electronics, optoelectronics and valleytronics due to the strong spin-orbit coupling, sizable band gap and tunability of the electronic structure by quantum confinement effect. [1][2][3][4] In the past decade, this has been witnessed by the significant efforts conducted on the atomically thin MoS 2 film. 5-7 However, its low mobility has limited applications, for inbstance, in high speed electronics. 8,9 Finding thin films of other TMDC with better properties is highly desirable. PtSe 2 has emerged as an interesting compound that belongs to TMDC.Although the bulk crystal is a semimetal, 10,11 monolayer (ML) platinum diselenide (PtSe 2 ) has been revealed to be a semiconductor with a band gap of ≈ 1.2 eV. 12 Importantly, the charge-carrier mobility of PtSe 2 has been predicted among the highest in TMDCs 9 and has been experimentally shown to be comparable to black phosphorene 13 yet with the advantage of much improved stability. 14 This makes PtSe 2 a promising candidate for high-speed electronics. Moreover, the hidden helical spin texture with spin-layer locking in monolayer PtSe 2 has been recently revealed, 15 and such spin physics induced by a local Rashba effect has great potential for electric field tunable spintronic devices. 16 In addition, remarkable performance

The interlayer coupling can be used to engineer the electronic structure of van der Waals heterostructures (superlattices) to obtain properties that are not possible in a single material. So far research in heterostructures has been focused on commensurate superlattices with a long-ranged Moiré period. Incommensurate heterostructures with rotational symmetry but not translational symmetry (in analogy to quasicrystals) are not only rare in nature, but also the interlayer interaction has often been assumed to be negligible due to the lack of phase coherence. Here we report the successful growth of quasicrystalline 30° twisted bilayer graphene (30°-tBLG), which is stabilized by the Pt(111) substrate, and reveal its electronic structure. The 30°-tBLG is confirmed by low energy electron diffraction and the intervalley double-resonance Raman mode at 1383 cm Moreover, the emergence of mirrored Dirac cones inside the Brillouin zone of each graphene layer and a gap opening at the zone boundary suggest that these two graphene layers are coupled via a generalized Umklapp scattering mechanism-that is, scattering of a Dirac cone in one graphene layer by the reciprocal lattice vector of the other graphene layer. Our work highlights the important role of interlayer coupling in incommensurate quasicrystalline superlattices, thereby extending band structure engineering to incommensurate superstructures.