Direct and indirect light emissions from layered ReS_2−xSe_x(0 ≤x≤ 2)

Abstract

ReS and ReSe have recently been enthusiastically studied owing to the specific in-plane electrical, optical and structural anisotropy caused by their distorted one-layer trigonal (1 T) phase, whereas other traditional transition-metal dichalcogenides (TMDCs, e.g. MoS and WSe) have a hexagonal structure. Because of this special property, more and versatile nano-electronics and nano-optoelectronics devices can be developed. In this work, 2D materials in the series ReS Se (0 ≤ x ≤ 2) have been successfully grown by the method of chemical vapor transport. The direct and indirect resonant emissions of the complete series of layers can be simultaneously detected by polarized micro-photoluminescence (μPL) spectroscopy when the thickness of the ReS Se is greater than ∼70 nm. When it is less than 70 nm, only three direct excitonic emissions-E, E and E-are detected. For the thick (bulk) ReS Se , more stacking of the ReX monolayers even flattens and shifts the valence-band maximum from Γ to the other K- or M-related points, thus leading to the coexistence of direct and indirect resonant light emissions from the c-plane ReX. The transmittance absorption edge of each bulk ReX (a few microns thick) usually has a lower energy than those of the direct E and E excitonic emissions to form indirect absorption. The coexistence of direct and indirect emissions in ReX is a unique characteristic of a 2D layered semiconductor possessing triclinic low symmetry.

Analog Circuit Applications Based on All‐2D Ambipolar ReSe2 Field‐Effect Transistors

“…The in‐plane lattice parameters are also estimated and indicated in Figure S1a (Supporting Information) for illustration. The result is similar to previous ReSe 2 crystal . Figure S1c (Supporting Information) shows micro‐Raman (μ‐Raman) spectrum of a c ‐plane ReSe 2 nanosheet.…”

“…In this work, high‐quality layered crystals of ReSe 2 were grown by chemical vapor transport (CVT) method using iodine (I 2 ) as the transport agent . Figure S1a,b (Supporting Information), respectively, shows the high‐resolution transmission electron microscope (HRTEM) image and selection‐area electron diffraction (SAED) pattern of the exfoliated ReSe 2 nanosheet.…”

Polarization Photoelectric Conversion in Layered GeS

“…The band structure of ReX 2 (X = S, Se) also shows more asymmetric than that of BP . Monolayer ReS 2 is a direct bandgap material (≈1.6 eV) while the thick‐layer ReS 2 may coexist with indirect (≈1.37 eV) and direct bandgaps (≈1.5 eV) induced by weakened layer‐to‐layer coupling observed by photoluminescence experiment . The band‐edge nature of ReX 2 still needs to be further identified and calculated.…”

“…2D semiconductors with special in‐plane anisotropy have recently received considerable attentions in electronic and optoelectronic applications owing to their axial dependences in changing the mechanical, electrical, optical, and structural properties along the van der Waals plane. Among these materials, ReX 2 (X = S, Se) and their intermediate alloys ReS 2− x Se x (0 < x < 1) of triclinic structure, black phosphorus (BP) with orthorhombic crystal phase, and some of the IV–VI layered compounds as monoclinic GeSe 2 , and orthorhombic GeX (X = S, Se) have recently been enthusiastically studied. Unlike MoS 2 , WSe 2 , and GaSe owning to a uniaxial c plane of hexagonal structure, the in‐plane anisotropic ReX 2 , BP, and IV–VI semiconducting layers may possess additional biaxial behavior of electrical and optical characteristics along one specific crystal orientation, e.g., the b axis of Re 4 clustering chain in ReX 2 and the a axis along armchair direction inside the layered BP .…”

Identifying light impurities in transition metal dichalcogenides: the local vibrational modes of S and O in ReSe2 and MoSe2

“…These alloys are of interest for two major reasons. Firstly, by changing the composition of sulfur within these alloys it is possible to tune the band gap, 32,37 as found for other TMD alloys 38 and alloys can be produced over the whole composition range, with the anisotropic trigonal structure being preserved throughout. Secondly, the more well-known TMDs exhibit strong spinorbit splittings (SOS) in the valence band, with the magnitude of this SOS being 180 meV at the K and K' points of the Brillouin zone for bulk MoS 2 .…”