Two-dimensional lateral heterojunctions based on monolayer transition-metal dichalcogenides (TMDs) have received increasing attention given that their direct band gap makes them very attractive for optoelectronic applications. Although bilayer TMDs present an indirect band gap, their electrical properties are expected to be less susceptible to ambient conditions, with higher mobilities and density of states when compared to monolayers. Bilayers and few-layers single domain devices have already demonstrated higher performance in radio frequency and photosensing applications. Despite these advantages, lateral heterostructures based on bilayer domains have been less explored. Here, we report the controlled synthesis of multi-junction bilayer lateral heterostructures based on MoS 2 -WS 2 and MoSe 2 -WSe 2 monodomains. The heterojunctions are created via sequential lateral edge-epitaxy that happens simultaneously in both the first and the second layers. A phenomenological mechanism is proposed to explain the growth mode with self-limited thickness that happens within a certain window of growth conditions. With respect to their as-grown monolayer counterparts, bilayer lateral heterostructures yield nearly 1 order of magnitude higher rectification currents. They also display a clear photovoltaic response, with short circuit currents ∼10 3 times larger than those extracted from the as-grown monolayers, in addition to room-temperature electroluminescence. The improved performance of bilayer heterostructures significantly expands the potential of two-dimensional materials for optoelectronics.
2D heterostructures made of transition metal dichalcogenides (TMD) have emerged as potential building blocks for new‐generation 2D electronics due to their interesting physical properties at the interfaces. The bandgap, work function, and optical constants are composition dependent, and the spectrum of applications can be expanded by producing alloy‐based heterostructures. Herein, the successful synthesis of monolayer and bilayer lateral heterostructures, based on ternary alloys of MoS2(1−x)Se2x–WS2(1−x)Se2x, is reported by modifying the ratio of the source precursors; the bandgaps of both materials in the heterostructure are continuously tuned in the entire range of chalcogen compositions. Raman and photoluminescence (PL) spatial maps show good intradomain composition homogeneity. Kelvin probe measurements in different heterostructures reveal composition‐dependent band alignments, which can further be affected by unintentional electronic doping during the growth. The fabrication of sequential multijunction lateral heterostructures with three layers of thickness, composed of quaternary and ternary alloys, is also reported. These results greatly expand the available tools kit for optoelectronic applications in the 2D realm.
The controlled synthesis and modification of the composition of layered compounds are essential prerequisites for their deployment in electronic or chemical applications. Pt-chalcogenides exhibit various compositional phases. Here, we investigate how Pt-selenide and Pt-telluride phases can be obtained as ultrathin films or as supported nanocrystals by physical vapor deposition and thermal treatment. The films are characterized by scanning tunneling microscopy and spectroscopy, scanning transmission electron microscopy, and photoemission and Raman spectroscopy. In all cases, Pt-dichalcogenides are obtained by Pt and chalcogen codeposition at growth temperatures below 300 °C. These films can be grown by van der Waals epitaxy in a layer-by-layer fashion, enabling the characterization of the pronounced layer-dependent electronic properties of these compounds. Pt-telluride growth at elevated temperatures (above 400 °C) results in the formation of Pt-monotelluride. Interestingly, the thin film Pt-dichalcogenides can also be transformed into different phases with lower chalcogen concentration by post-growth vacuum annealing. Annealing-induced loss of chalcogen results in new composites. With this thermal process, an intermittent layered compound of Pt3Te4 is synthesized, which consists of alternating PtTe2 and PtTe van der Waals layers. By thermal treatment of PtSe2, we obtain a non-layered Pt-monoselenide in nanocrystalline form. PtSe is not reported in the bulk Pt-Se phase diagram, but its structure is analogue to the known Pt-monosulfide with a tetragonal unit cell. This PtSe phase is semiconducting with a band gap of ∼0.9 eV. The nanocrystalline PtSe phase is, however, unstable and easily loses more Se and eventually converts into Pt. Thus, it is demonstrated that post-growth thermally induced transformation of Pt-dichalcogenides films enables the synthesis of new Pt-chalcogenide phases as ultrathin films or nanocrystals.
Binary and ternary transition metal dichalcogenide (TMD) heterostructures, MSe2 and MSeS (where M = Mo or W), are investigated as future building blocks for high‐performance device applications with a tailorable electronic bandgap. For light‐emitting device applications, investigating the decay time of the material is a prerequisite. Herein, time‐resolved photoluminescence (TRPL) of binary and ternary TMD heterostructures is measured. A tri‐exponential function is adopted to fit the TRPL data. Short, medium, and long decay times are <50 ps, ≈400 ps, and >400 ps, respectively. Except for monolayer MoSe2 and MoSeS, only two PL decay time components are observed with a large contribution from short decay time photocarriers (>80%). Bilayer TMDs exhibit a much shorter PL decay time, which could be attributed to the indirect nature of excitons and different dielectric environments, compared with that of monolayer TMDs. Using the center‐of‐mass (CM) method, a PL decay time map is successfully obtained for binary and ternary TMD heterostructures. This allows the qualitative visualization of the decay time distribution within the heterostructure or among other materials. This technique could be applied to assess the decay time of materials used in high‐performance light‐emitting devices.
Magnetic field- and polarization-dependent measurements on bright and dark excitons in monolayer WSe2 combined with time-dependent density functional theory calculations reveal intriguing phenomena. Magnetic fields up to 25 T parallel to the WSe2 plane lead to a partial brightening of the energetically lower lying exciton, leading to an increase of the dephasing time. Using a broadband femtosecond pulse excitation, the bright and partially allowed excitonic state can be excited simultaneously, resulting in coherent quantum beating between these states. The magnetic fields perpendicular to the WSe2 plane energetically shift the bright and dark excitons relative to each other, resulting in the hybridization of the states at the K and K′ valleys. Our experimental results are well captured by time-dependent density functional theory calculations. These observations show that magnetic fields can be used to control the coherent dephasing and coupling of the optical excitations in atomically thin semiconductors.
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