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2.
Nat Commun ; 11(1): 2453, 2020 May 15.
Artigo em Inglês | MEDLINE | ID: mdl-32415180

RESUMO

Two-dimensional materials provide extraordinary opportunities for exploring phenomena arising in atomically thin crystals. Beginning with the first isolation of graphene, mechanical exfoliation has been a key to provide high-quality two-dimensional materials, but despite improvements it is still limited in yield, lateral size and contamination. Here we introduce a contamination-free, one-step and universal Au-assisted mechanical exfoliation method and demonstrate its effectiveness by isolating 40 types of single-crystalline monolayers, including elemental two-dimensional crystals, metal-dichalcogenides, magnets and superconductors. Most of them are of millimeter-size and high-quality, as shown by transfer-free measurements of electron microscopy, photo spectroscopies and electrical transport. Large suspended two-dimensional crystals and heterojunctions were also prepared with high-yield. Enhanced adhesion between the crystals and the substrates enables such efficient exfoliation, for which we identify a gold-assisted exfoliation method that underpins a universal route for producing large-area monolayers and thus supports studies of fundamental properties and potential application of two-dimensional materials.

3.
Nat Commun ; 11(1): 659, 2020 Jan 31.
Artigo em Inglês | MEDLINE | ID: mdl-32005802

RESUMO

In atomically-thin two-dimensional (2D) semiconductors, the nonuniformity in current flow due to its edge states may alter and even dictate the charge transport properties of the entire device. However, the influence of the edge states on electrical transport in 2D materials has not been sufficiently explored to date. Here, we systematically quantify the edge state contribution to electrical transport in monolayer MoS2/WSe2 field-effect transistors, revealing that the charge transport at low temperature is dominated by the edge conduction with the nonlinear behavior. The metallic edge states are revealed by scanning probe microscopy, scanning Kelvin probe force microscopy and first-principle calculations. Further analyses demonstrate that the edge-state dominated nonlinear transport shows a universal power-law scaling relationship with both temperature and bias voltage, which can be well explained by the 1D Luttinger liquid theory. These findings demonstrate the Luttinger liquid behavior in 2D materials and offer important insights into designing 2D electronics.

4.
J Phys Chem Lett ; : 1536-1541, 2020 Feb 10.
Artigo em Inglês | MEDLINE | ID: mdl-32011142

RESUMO

Hydrogen atoms bonded within molecular cavities often undergo tunneling or thermal-transfer processes that play major roles in diverse physical phenomena. Such transfers may or may not entail intermediate states. The existence of such fleeting states is typically determined by indirect means, while their direct visualization has not been achieved, largely because their concentrations under equilibrium conditions are negligible. Here we use density-functional-theory calculations and scanning-tunneling-microscopy (STM) image simulations to predict that, under specially designed nonequilibrium conditions of voltage-enhanced high transfer rates, the cis-intermediate of the two-hydrogen transfer process in metal-free naphthalocyanine molecules adsorbed on Ag(111) surfaces would be visualizable in a composite image of double-C morphology. As guided by the theoretical predictions, at adjusted scanning temperature and bias, STM experiments achieve a direct visualization of the cis-intermediate. This work demonstrates a practical way to directly visualize elusive intermediates, which enhances understanding of the quantum dynamics of hydrogen atoms.

5.
Nano Lett ; 19(9): 6323-6329, 2019 Sep 11.
Artigo em Inglês | MEDLINE | ID: mdl-31431010

RESUMO

Two-dimensional topological materials have attracted intense research efforts owing to their promise in applications for low-energy, high-efficiency quantum computations. Group-VA elemental thin films with strong spin-orbit coupling have been predicted to host topologically nontrivial states as excellent two-dimensional topological materials. Herein, we experimentally demonstrated for the first time that the epitaxially grown high-quality antimonene monolayer islands with buckled configurations exhibit significantly robust one-dimensional topological edge states above the Fermi level. We further demonstrated that these topologically nontrivial edge states arise from a single p-orbital manifold as a general consequence of atomic spin-orbit coupling. Thus, our findings establish monolayer antimonene as a new class of topological monolayer materials hosting the topological edge states for future low-power electronic nanodevices and quantum computations.

6.
Adv Mater ; 31(36): e1900392, 2019 Sep.
Artigo em Inglês | MEDLINE | ID: mdl-31206929

RESUMO

The mechanical exfoliation of graphene from graphite provides the cornerstone for the synthesis of other 2D materials with layered bulk structures, such as hexagonal boron nitride, transition metal dichalcogenides, black phosphorus, and so on. However, the experimental production of 2D flat boron is challenging because bulk boron has very complex spatial structures and a rich variety of chemical properties. Therefore, the realization of 2D flat boron marks a milestone for the synthesis of 2D materials without layered bulk structures. The historical efforts in this field, particularly the most recent experimental progress, such as the growth of 2D flat boron on a metal substrate by chemical vapor deposition and molecular beam epitaxy, or liquid exfoliation from bulk boron, are described.

7.
Nano Lett ; 19(8): 4897-4903, 2019 Aug 14.
Artigo em Inglês | MEDLINE | ID: mdl-30973231

RESUMO

Creation of functional patterns in two-dimensional (2D) materials provides opportunities to extend their potential for applications. Transition-metal dichalcogenides (TMDCs) are suitable 2D materials for pattern generation because of properties including alterable polymorphic phases, easy chalcogen-vacancy formation, metal-atom insertion, and alloying. Such patterning can be used for selective functionalization. Here we report the spontaneous formation of long-range, well-ordered 1D patterns in monolayer vanadium diselenide (VSe2) by a single annealing stage during growth. Atomic-resolution images in real space combined with density-functional-theory (DFT) calculations reveal the 1D features of patterned VSe2. Further experimental characterization of the intermediate states in the growth process confirm the spontaneous formation of the 1D pattern by annealing-induced Se-deficient linear defects. The 1D pattern can be reversibly transformed to homogenous VSe2 monolayer by reintroducing Se atoms. Moreover, additional experiments demonstrate that a dispersive deposition of Pt atoms along the 1D structures of patterned VSe2 is achieved, while DFT calculations find that their catalytic activity for hydrogen evolution reaction (HER) is as good as that of Pt surfaces. The formation of long-range, well-ordered 1D patterns not only demonstrates an effective way of dimension modulation in 2D materials but also enriches the potential of intrinsically patterned 2D materials for promising catalytic activities.

8.
Adv Mater ; 30(49): e1804650, 2018 Dec.
Artigo em Inglês | MEDLINE | ID: mdl-30368921

RESUMO

Silicene-based van der Waals heterostructures are theoretically predicted to have interesting physical properties, but their experimental fabrication has remained a challenge because of the easy oxidation of silicene in air. Here, the fabrication of graphene/silicene van der Waals heterostructures by silicon intercalation is reported. Density functional theory calculations show weak interactions between graphene and silicene layers, confirming the formation of van der Waals heterostructures. The heterostructures show no observable damage after air exposure for extended periods, indicating good air stability. The I-V characteristics of the vertical graphene/silicene/Ru heterostructures show rectification behavior.

9.
Chem Soc Rev ; 47(16): 6073-6100, 2018 Aug 13.
Artigo em Inglês | MEDLINE | ID: mdl-29971284

RESUMO

The discovery of graphene opened a door for manufacturing and investigating two-dimensional (2D) materials. After more than ten years of development, 2D materials have become one of the most important topics in materials research, with dozens of new materials having been synthesized experimentally and even more predicted theoretically. In this review, we provide a comprehensive overview of the fabrication of 2D materials based on epitaxial growth in an ultra-high vacuum (UHV) experimental environment and the investigation of their physical and chemical properties. In particular, we focus on techniques like intercalation, templated molecular adsorption, and direct selenization and tellurization of metal substrates. We discuss progress in fabrication methods of monatomic and binary 2D materials and highlight their interesting and quite unusual physical properties. Finally, we assess future directions of research in this field, where breakthroughs can be expected, and indicate where investments in additional research might be most rewarding scientifically.

10.
ACS Nano ; 12(6): 6117-6127, 2018 Jun 26.
Artigo em Inglês | MEDLINE | ID: mdl-29790339

RESUMO

Fast-growth of single crystal monolayer graphene by CVD using methane and hydrogen has been achieved on "homemade" single crystal Cu/Ni(111) alloy foils over large area. Full coverage was achieved in 5 min or less for a particular range of composition (1.3 at.% to 8.6 at.% Ni), as compared to 60 min for a pure Cu(111) foil under identical growth conditions. These are the bulk atomic percentages of Ni, as a superstructure at the surface of these foils with stoichiometry Cu6Ni1 (for 1.3 to 7.8 bulk at.% Ni in the Cu/Ni(111) foil) was discovered by low energy electron diffraction (LEED). Complete large area monolayer graphene films are either single crystal or close to single crystal, and include folded regions that are essentially parallel and that were likely wrinkles that "fell over" to bind to the surface; these folds are separated by large, wrinkle-free regions. The folds occur due to the buildup of interfacial compressive stress (and its release) during cooling of the foils from 1075 °C to room temperature. The fold heights measured by atomic force microscopy (AFM) and scanning tunneling microscopy (STM) prove them to all be 3 layers thick, and scanning electron microscopy (SEM) imaging shows them to be around 10 to 300 nm wide and separated by roughly 20 µm. These folds are always essentially perpendicular to the steps in this Cu/Ni(111) substrate. Joining of well-aligned graphene islands (in growths that were terminated prior to full film coverage) was investigated with high magnification SEM and aberration-corrected high-resolution transmission electron microscopy (TEM) as well as AFM, STM, and optical microscopy. These methods show that many of the "join regions" have folds, and these arise from interfacial adhesion mechanics (they are due to the buildup of compressive stress during cool-down, but these folds are different than for the continuous graphene films-they occur due to "weak links" in terms of the interface mechanics). Such Cu/Ni(111) alloy foils are promising substrates for the large-scale synthesis of single-crystal graphene film.

11.
Nano Lett ; 18(3): 2133-2139, 2018 03 14.
Artigo em Inglês | MEDLINE | ID: mdl-29457727

RESUMO

Group-V elemental monolayers were recently predicted to exhibit exotic physical properties such as nontrivial topological properties, or a quantum anomalous Hall effect, which would make them very suitable for applications in next-generation electronic devices. The free-standing group-V monolayer materials usually have a buckled honeycomb form, in contrast with the flat graphene monolayer. Here, we report epitaxial growth of atomically thin flat honeycomb monolayer of group-V element antimony on a Ag(111) substrate. Combined study of experiments and theoretical calculations verify the formation of a uniform and single-crystalline antimonene monolayer without atomic wrinkles, as a new honeycomb analogue of graphene monolayer. Directional bonding between adjacent Sb atoms and weak antimonene-substrate interaction are confirmed. The realization and investigation of flat antimonene honeycombs extends the scope of two-dimensional atomically-thick structures and provides a promising way to tune topological properties for future technological applications.

12.
Chem Soc Rev ; 47(3): 982-1021, 2018 Feb 05.
Artigo em Inglês | MEDLINE | ID: mdl-29210397

RESUMO

Phosphorene, an emerging two-dimensional material, has received considerable attention due to its layer-controlled direct bandgap, high carrier mobility, negative Poisson's ratio and unique in-plane anisotropy. As cousins of phosphorene, 2D group-VA arsenene, antimonene and bismuthene have also garnered tremendous interest due to their intriguing structures and fascinating electronic properties. 2D group-VA family members are opening up brand-new opportunities for their multifunctional applications encompassing electronics, optoelectronics, topological spintronics, thermoelectrics, sensors, Li- or Na-batteries. In this review, we extensively explore the latest theoretical and experimental progress made in the fundamental properties, fabrications and applications of 2D group-VA materials, and offer perspectives and challenges for the future of this emerging field.

14.
Nat Commun ; 8: 14216, 2017 01 31.
Artigo em Inglês | MEDLINE | ID: mdl-28139646

RESUMO

The generally accepted view that spin polarization in non-magnetic solids is induced by the asymmetry of the global crystal space group has limited the search for spintronics materials mainly to non-centrosymmetric materials. In recent times it has been suggested that spin polarization originates fundamentally from local atomic site asymmetries and therefore centrosymmetric materials may exhibit previously overlooked spin polarizations. Here, by using spin- and angle-resolved photoemission spectroscopy, we report the observation of helical spin texture in monolayer, centrosymmetric and semiconducting PtSe2 film without the characteristic spin splitting in conventional Rashba effect (R-1). First-principles calculations and effective analytical model analysis suggest local dipole induced Rashba effect (R-2) with spin-layer locking: opposite spins are degenerate in energy, while spatially separated in the top and bottom Se layers. These results not only enrich our understanding of the spin polarization physics but also may find applications in electrically tunable spintronics.

15.
Nanotechnology ; 28(8): 084003, 2017 Feb 24.
Artigo em Inglês | MEDLINE | ID: mdl-28106006

RESUMO

A new, easy, in situ technique for fabricating a two-dimensional graphene-silicon layered heterostructure has been developed to meet the demand for integration between graphene and silicon-based microelectronic technology. First, carbon atoms are stored in bulk iridium, and then silicon atoms are deposited onto the Ir(111) surface and annealed. With longer annealing times, the carbon atoms penetrate from the bulk iridium to the top of the silicon and eventually coalesce there into graphene islands. Atomically resolved scanning tunneling microscopy images, high-pass fast Fourier transform treatment and Raman spectroscopy demonstrate that the top graphene layer is intact and continuous, and beneath it is the silicon layer.

16.
Adv Mater ; 29(13)2017 Apr.
Artigo em Inglês | MEDLINE | ID: mdl-28134451

RESUMO

Bernal-stacked bilayer germanene with a stable buckled honeycomb structure has been successfully synthesized on Cu(111). Structural and electronic characterizations as well as theoretical calculations unequivocally demonstrate for the first time the presence of a nearly linear energy dispersion in the vicinity of the Fermi energy, as expected of the Dirac signature for theoretical freestanding germanene.

17.
Nano Lett ; 17(2): 1161-1166, 2017 02 08.
Artigo em Inglês | MEDLINE | ID: mdl-28098458

RESUMO

Silicon-based two-dimensional (2D) materials are uniquely suited for integration in Si-based electronics. Silicene, an analogue of graphene, was recently fabricated on several substrates and was used to make a field-effect transistor. Here, we report that when Ru(0001) is used as a substrate, a range of distinct monolayer silicon structures forms, evolving toward silicene with increasing Si coverage. Low Si coverage produces a herringbone structure, a hitherto undiscovered 2D phase of silicon. With increasing Si coverage, herringbone elbows evolve into silicene-like honeycomb stripes under tension, resulting in a herringbone-honeycomb 2D superlattice. At even higher coverage, the honeycomb stripes widen and merge coherently to form silicene in registry with the substrate. Scanning tunneling microscopy (STM) was used to image the structures. The structural stability and electronic properties of the Si 2D structures, the interaction between the Si 2D structures and the Ru substrate, and the evolution of the distinct monolayer Si structures were elucidated by density functional theory (DFT) calculations. This work paves the way for further investigations of monolayer Si structures, the corresponding growth mechanisms, and possible functionalization by impurities.

18.
Adv Mater ; 29(11)2017 Mar.
Artigo em Inglês | MEDLINE | ID: mdl-28028843

RESUMO

Monolayer antimonene is fabricated on PdTe2 by an epitaxial method. Monolayer antimonene is theoretically predicted to have a large bandgap for nanoelectronic devices. Air-exposure experiments indicate amazing chemical stability, which is great for device fabrication. A method to fabricate high-quality monolayer antimonene with several great properties for novel electronic and optoelectronic applications is provided.

19.
Adv Mater ; 28(25): 5013-7, 2016 Jul.
Artigo em Inglês | MEDLINE | ID: mdl-27087261

RESUMO

2D materials with heterolayered structures beyond graphene are explored. A theoretically predicted superconductor-topological insulator-normal metal heterolayered structure is realized experimentally. The generated hybrid structure HfTe3 /HfTe5 /Hf has potential applications in both quantum-spin Hall effect-based and Majorana-based devices.

20.
ACS Nano ; 9(12): 11509-39, 2015 Dec 22.
Artigo em Inglês | MEDLINE | ID: mdl-26544756

RESUMO

The isolation of graphene in 2004 from graphite was a defining moment for the "birth" of a field: two-dimensional (2D) materials. In recent years, there has been a rapidly increasing number of papers focusing on non-graphene layered materials, including transition-metal dichalcogenides (TMDs), because of the new properties and applications that emerge upon 2D confinement. Here, we review significant recent advances and important new developments in 2D materials "beyond graphene". We provide insight into the theoretical modeling and understanding of the van der Waals (vdW) forces that hold together the 2D layers in bulk solids, as well as their excitonic properties and growth morphologies. Additionally, we highlight recent breakthroughs in TMD synthesis and characterization and discuss the newest families of 2D materials, including monoelement 2D materials (i.e., silicene, phosphorene, etc.) and transition metal carbide- and carbon nitride-based MXenes. We then discuss the doping and functionalization of 2D materials beyond graphene that enable device applications, followed by advances in electronic, optoelectronic, and magnetic devices and theory. Finally, we provide perspectives on the future of 2D materials beyond graphene.

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