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1.
ACS Appl Mater Interfaces ; 11(46): 43766-43773, 2019 Nov 20.
Artigo em Inglês | MEDLINE | ID: mdl-31657201

RESUMO

Monolayer (ML) transition-metal dichalcogenides (TMDs) continue to attract research attention, and the heterojunctions formed by vertically stacking or laterally stitching two different TMDs, e.g., MoSe2 and WSe2, may have many interesting electronic and optical properties and thus are at the center stage of current research. Experimentally realizing such heterojunctions with desired interface morphologies and electronic properties is of great demand. In this work, we report a diverse interface structure in molecular-beam epitaxial WSe2-MoSe2 heterojunction. The corresponding electronic bands show type-II band alignment for both monolayer ML-ML and ML-bilayer lateral junctions irrespective of the presence or absence of step states. Interestingly, a strong anisotropy in lateral heterojunction formation is observed, where sharp interfaces are obtained only when WSe2 deposition precedes MoSe2. Reversing the deposition order leads to alloying of the two materials without a notable boundary. This is explained by a step segregation process as suggested by the first-principles total energy calculations.

2.
Nano Lett ; 19(12): 8581-8589, 2019 Dec 11.
Artigo em Inglês | MEDLINE | ID: mdl-31663768

RESUMO

Precise control and in-depth understanding of the interfaces are crucial for the functionality-oriented material design with desired properties. Herein, via modifying the long-standing bicrystal strategy, we proposed a novel nanowelding approach to build up interfaces between two-dimensional (2D) materials with atomic precision. This method enabled us, for the first time, to experimentally achieve the quasi-full-parameter-space grain boundaries (GBs) in 2D hexagonal boron nitride (h-BN). It further helps us unravel the long-term controversy and confusion on the registry of GBs in h-BN, including (i) discriminate the relative contribution of the strain and chemical energy on the registry of GBs; (ii) identify a new dislocation core-Frank partial dislocation and four new antiphase boundaries; and (iii) confirm the universal GB faceting. Our work provides a new paradigm to the exploitation of structural-property correlation of interfaces in 2D materials.

3.
J Phys Chem Lett ; : 5027-5035, 2019 Aug 16.
Artigo em Inglês | MEDLINE | ID: mdl-31357864

RESUMO

Atomically thin transition-metal dichalcogenide (TMDC) heterostructures have attracted increasing attention because of their unprecedented potential in the fields of electronics and optoelectronics. However, selective growth of either lateral or vertical TMDC heterostructures remains challenging. Here, we report that lateral and vertical MoS2/MoSe2 epitaxial heterostructures can be successfully fabricated via a one-step growth strategy, which includes triggering by the concentration of sulfur precursor vapor and a high-temperature annealing process. Vertically stacked MoS2/MoSe2 heterostructures can be synthesized via control of the nucleation and growth kinetics, which is induced by high sulfur vapor concentration. The high-temperature annealing process results in the formation of fractured MoSe2 and in situ epitaxial growth of lateral MoSe2-MoS2 heterostructures. This study has revealed the importance of sulfur vapor concentration and high-temperature annealing processes in the controllable growth of MoSe2-MoS2 heterostructures, paving a new route for fabricating two-dimensional TMDC heterostructures.

4.
Nano Lett ; 17(11): 6653-6660, 2017 11 08.
Artigo em Inglês | MEDLINE | ID: mdl-29020776

RESUMO

Interlayer rotation and stacking were recently demonstrated as effective strategies for tuning physical properties of various two-dimensional materials. The latter strategy was mostly realized in heterostructures with continuously varied stacking orders, which obscure the revelation of the intrinsic role of a certain stacking order in its physical properties. Here, we introduce inversion-domain-boundaries into molecular-beam-epitaxy grown MoSe2 homobilayers, which induce uncommon fractional lattice translations to their surrounding domains, accounting for the observed diversity of large-area and uniform stacking sequences. Low-symmetry stacking orders were observed using scanning transmission electron microscopy and detailed geometries were identified by density functional theory. A linear relation was also revealed between interlayer distance and stacking energy. These stacking sequences yield various energy alignments between the valence states at the Γ and K points of the Brillouin zone, showing stacking-dependent bandgaps and valence band tail states in the measured scanning tunneling spectroscopy. These results may benefit the design of two-dimensional multilayers with manipulable stacking orders.

5.
Nat Commun ; 8: 15135, 2017 05 04.
Artigo em Inglês | MEDLINE | ID: mdl-28469134

RESUMO

Transition metal dichalcogenides (TMDs) are a family of van der Waals layered materials exhibiting unique electronic, optical, magnetic and transport properties. Their technological potentials hinge critically on the ability to achieve controlled fabrication of desirable nanostructures, such as nanoribbons and nanodots. To date, nanodots/nanoislands have been regularly observed, while controlled fabrication of TMD nanoribbons remains challenging. Here we report a bottom-up fabrication of MoSe2 nanoribbons using molecular beam epitaxy, via an unexpected temperature-induced morphological phase transition from the nanodot to nanoribbon regime. Such nanoribbons are of zigzag nature, characterized by distinct chemical and electronic properties along the edges. The phase space for nanoribbon growth is narrowly defined by proper Se:Mo ratios, as corroborated experimentally using different Se fluxes, and supported theoretically using first-principles calculations that establish the crucial role of the morphological reconstruction of the bare Mo-terminated edge. The growth mechanism revealed should be applicable to other TMD systems.

6.
Nat Commun ; 8: 14486, 2017 02 16.
Artigo em Inglês | MEDLINE | ID: mdl-28205514

RESUMO

Understanding the influence of grain boundaries (GBs) on the electrical and thermal transport properties of graphene films is essentially important for electronic, optoelectronic and thermoelectric applications. Here we report a segregation-adsorption chemical vapour deposition method to grow well-stitched high-quality monolayer graphene films with a tunable uniform grain size from ∼200 nm to ∼1 µm, by using a Pt substrate with medium carbon solubility, which enables the determination of the scaling laws of thermal and electrical conductivities as a function of grain size. We found that the thermal conductivity of graphene films dramatically decreases with decreasing grain size by a small thermal boundary conductance of ∼3.8 × 109 W m-2 K-1, while the electrical conductivity slowly decreases with an extraordinarily small GB transport gap of ∼0.01 eV and resistivity of ∼0.3 kΩ µm. Moreover, the changes in both the thermal and electrical conductivities with grain size change are greater than those of typical semiconducting thermoelectric materials.

7.
Sci Adv ; 3(1): e1601459, 2017 Jan.
Artigo em Inglês | MEDLINE | ID: mdl-28070558

RESUMO

By using direct growth, we create a rotationally aligned MoS2/WSe2 hetero-bilayer as a designer van der Waals heterostructure. With rotational alignment, the lattice mismatch leads to a periodic variation of atomic registry between individual van der Waals layers, exhibiting a Moiré pattern with a well-defined periodicity. By combining scanning tunneling microscopy/spectroscopy, transmission electron microscopy, and first-principles calculations, we investigate interlayer coupling as a function of atomic registry. We quantitatively determine the influence of interlayer coupling on the electronic structure of the hetero-bilayer at different critical points. We show that the direct gap semiconductor concept is retained in the bilayer although the valence and conduction band edges are located at different layers. We further show that the local bandgap is periodically modulated in the X-Y direction with an amplitude of ~0.15 eV, leading to the formation of a two-dimensional electronic superlattice.

8.
Small ; 12(27): 3645-50, 2016 Jul.
Artigo em Inglês | MEDLINE | ID: mdl-27240098

RESUMO

Suppressing the oppositely orientated hexagonal boron nitride (h-BN) domains during the growth is of great challenge due to its bipolar structure. It is found that h-BN domains grown on onefold symmetric Cu(102) or (103) share a unique orientation, with one zigzag edge of the h-BN triangles perpendicular to the symmetry axis of the substrate surface.

9.
Adv Mater ; 28(30): 6449-56, 2016 Aug.
Artigo em Inglês | MEDLINE | ID: mdl-27184960

RESUMO

Kilometer-scale continuous graphene fibers (GFs) with outstanding mechanical properties and excellent electrical conductivity are produced by high-throughput wet-spinning of graphene oxide liquid crystals followed by graphitization through a full-scale synergetic defect-engineering strategy. GFs with superior performances promise wide applications in functional textiles, lightweight motors, microelectronic devices, and so on.

10.
Nano Lett ; 16(3): 2103-7, 2016 Mar 09.
Artigo em Inglês | MEDLINE | ID: mdl-26913671

RESUMO

The growth of high-quality two-dimensional (2D) layered chalcogenide crystals is highly important for practical applications in future electronics, optoelectronics, and photonics. Current route for the synthesis of 2D chalcogenide crystals by vapor deposition method mainly involves an energy intensive high-temperature growth process on solid substrates, often suffering from inhomogeneous nucleation density and grain size distribution. Here, we first demonstrate a facile vapor-phase synthesis of large-area high-quality 2D layered chalcogenide crystals on liquid metal surface with relatively low surface energy at a growth temperature as low as ∼100 °C. Uniform and large-domain-sized 2D crystals of GaSe and GaxIn1-xSe were grown on liquid metal surface even supported on a polyimide film. As-grown 2D GaSe crystals have been fabricated to flexible photodetectors, showing high photoresponse and excellent flexibility. Our strategy of energy-sustainable low-temperature growth on liquid metal surface may open a route to the synthesis of high-quality 2D crystals of Ga-, In-, Bi-, Hg-, Pb-, or Sn-based chalcogenides and halides.


Assuntos
Cristalização/métodos , Gálio/química , Selênio/química , Calcogênios/química , Temperatura Baixa , Modelos Moleculares , Volatilização
11.
Adv Mater ; 27(44): 7086-92, 2015 Nov 25.
Artigo em Inglês | MEDLINE | ID: mdl-26445312

RESUMO

A facile all-chemical vapor deposition approach is designed, which allows both sequentially grown Gr and monolayer MoS2 in the same growth process, thus allowing the direct construction of MoS2 /Gr vertical heterostructures on Au foils. A weak n-doping effect and an intrinsic bandgap of MoS2 are obtained from MoS2 /Gr/Au via scanning tunneling microscopy and spectroscopy characterization. The exciton binding energy is accurately deduced by combining photoluminescence measurements.


Assuntos
Dissulfetos/química , Grafite/química , Molibdênio/química , Técnicas de Química Sintética , Ouro/química , Modelos Moleculares , Conformação Molecular , Temperatura Ambiente , Volatilização
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