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Three-dimensional (3D) topological semimetals (TSMs) are a new class of Dirac materials that can be viewed as 3D graphene and are referred to as Dirac semimetals (DSMs) or Weyl semimetals (WSMs) depending on whether time reversal symmetry and/or inversion symmetry are protected, respectively. Despite some interesting results on Dirac- to Weyl-semimetal phase transitions under conditions of low temperature or strong magnetic field (B), all of them are reversible phenomena. Here, we report for the first time a possible permanent transition in a single TSM by ion implantation. A Dirac- to Weyl-semimetal phase transition in a Bi0.96Sb0.04 DSM results from inversion-symmetry breaking induced by implantation with nonmagnetic Au ions for implant fluences (ϕG) ≥ 3.2 × 1016 Au cm−2. This phenomenon is evidenced by the ϕG-dependent behavior of the Raman spectra and quantum-oscillation parameters extracted from magnetoresistance (MR) measurements, which show abrupt changes at ϕG ≥ 3.2 × 1016 Au cm−2. The verification of the transition is further supported by observations of negative MR in the longitudinal B // electric field orientation, indicating the existence of a chiral anomaly in Weyl fermions induced by implantation with nonmagnetic Au ions. In contrast, implantation with magnetic Mn ions exhibits no such particular behavior. Our findings demonstrate the first realization of a possible permanent DSM-to-WSM phase transition in a single material by the simple approach of implantation using nonmagnetic elements.(Left, above) Schematic diagram describing the process of the Au implantation into the Dirac semimetal (DSM) Bi0.96Sb0.04. (Left, below) Positive longitudinal magnetic resistance (LMR) is observed in the crystal without ion implantation, but negative LMR behavior becomes apparent for ϕG ≥ 3.2 × 1016 Au cm−2 (≡ ϕC, critical implant fluence), and reaches a maximum for ϕG = 8.0 × 1016 Au cm−2. (Middle) Quantum oscillation parameters for the β Fermi pockets showing abrupt changes near ϕC, typical of phase transition. No such behavior is observed for the α Fermi pockets. (Right) A drastic change in the Raman spectra is observed for ϕG ≥ ϕC. In particular, a new peak appears at 85.7 cm−1, between the well-known Raman modes, suggesting that the inversion symmetry breaking in the crystal occurred for ϕG ≥ ϕC, resulting in the transition of the DSM to a Weyl semimetal. [ABSTRACT FROM AUTHOR]

Moiré heterostructures produced by twisted heterojunction of transition-metal dichalcogenides are recognized as novel platforms for unique and tunable means of controlling the optical phenomena including photoluminescence (PL). Despite some interesting results on the PL peak shifts by the heterojunction at twist angles (θ) far from 0 or 60°, all of them are redshifts. Here, we first report blue shift of energy and strong enhancement of intensity in the PL by twisted heterojunction of MoS2 and WS2 monolayers (MLs) in a particular range of θ. The PL peak energy of the heterobilayer steeply increases (about 120 meV) as θ gets closer to 15 or 52° from 3 or 57°, respectively and reaches a plateau at around 2.01 eV in the θ range from 15 to 52°, higher than that of the separate MoS2 or WS2 ML. The PL intensity shows a similar θ-dependent behavior with its magnitude in the plateau being ∼4 or 80 times larger than that of the WS2 or MoS2 ML, respectively. These novel light-emission behaviors are well explained with reference to theoretical predictions on the avoided crossing between the intralayer and interlayer excitons. Our findings highlight extendable tuning and remarkable enhancement of light emission from two-dimensional semiconductors by a simple approach of twisted heterojunction in a proper θ range, very useful for their optoelectronic device applications. [ABSTRACT FROM AUTHOR]

Two-dimensional (2D) Janus transition metal dichalcogenides (TMDCs) are highly attractive as an emerging class of 2D materials, but only a few methods are available for fabricating them. These methods rely on the initial growth of 2D TMDCs in one process, followed by an additional plasma or high-temperature (T) process. To overcome these drawbacks, we employ the new approach of NaCl-assisted single-process chemical vapor deposition, which consists of three steps that proceed only by altering the temperature in situ. In the first step, MoS2 is deposited onto a SiO2/Si substrate with the Mo and S atoms activated in different temperature zones. In the second step, S vacancies are formed in the upper layer of the grown MoS2 by annealing. In the third step, the vacancies are filled with activated Se atoms. Throughout the steps, NaCl lowers the melting point of the constituent atoms, while the T in each zone is properly controlled. The growth mechanism is clarified by a separate annealing experiment that does not involve a supply of activated atoms. These results highlight a simple and cost-effective approach for growing Janus MoSSe, which is more useful for fundamental studies and device applications.(Left) Schematic diagrams describing a single in-situ chemical-vapor-deposition process for growing two-dimensional Janus MoSSe without initial or follow-up treatments and the growth mechanism consisting of three steps; 1) growth of a MoS2 layer, 2) formation of vacancies on the top MoS2 layer by an annealing effect, and filling of Se atoms in the vacancies. (Right) Optical-microscopy and photoluminescence-mapping images for a typical two-dimensional Janus sample showing that the central MoSSe region is distinguished from the peripheral MoSe2 region. [ABSTRACT FROM AUTHOR]

P-n junction GaN light-emitting diodes (LEDs) were fabricated using Ga-doped ZnO (GZO) films as electrical contacts and characterized by electroluminescence (EL) and current-voltage (I-V) measurements. GaN p-n epilayers with a total thickness of ~6 μm were grown on c-plane (0001) sapphire substrates by metal-organic chemical vapor deposition. Half region of the p-GaN layer was etched until the n-GaN layer was exposed, and 100-nm-thick GZO contacts were deposited on the p- and n-GaN layers by RF sputtering with varying Ga concentration ( n) from 1 to 5 mol%. Based on the results of Hall effect, photoluminescence (PL), and X-ray diffraction (XRD), the GZO films were expected to act as best electrical contacts for the LEDs at n = 2 mol%. Under forward-bias conditions, the I-V curves showed diode characteristics except n = 5 mol%, and the leakage current was minimized at n = 2 mol%. Two dominant EL peaks of ultraviolet and yellow emissions were observed at ~376 and ~560 nm, and attributed to near-band-edge- and defect-related radiative transitions, respectively. At n = 2 mol%, the UV EL showed markedly large intensities for all injection currents, consistent with the results of Hall effect, PL, I-V, and XRD. [ABSTRACT FROM AUTHOR]

Holographic gratings in two kinds of photopolymers (PPs) were fabricated by optical interference method. Polyvinylalcohol (PVA) and polyvinylacetate (PVAc) were employed as polymer binders and photopolymerization of acrylamide (AA) was confirmed using infrared spectroscopy. Dynamic behavior of the diffraction efficiency was monitored and its temporal stability at room temperature was also observed. Additionally, the temperature dependence of these gratings was investigated in two PPs. The surface topographical change of the photopolymer layer was observed by atomic force microscope (AFM). [ABSTRACT FROM AUTHOR]

Short-wave infrared (SWIR) detectors and emitters have a high potential value in several fields of applications, including the internet of things (IoT) and advanced driver assistance systems (ADAS), gas sensing. Indium Gallium Arsenide (InGaAs) photodetectors are widely used in the SWIR region of 1–3 μm; however, they only capture a part of the region due to a cut-off wavelength of 1.7 μm. This study presents an InAs p-i-n photodetector grown on a GaAs substrate (001) by inserting 730-nm thick InxAl1−xAs graded and AlAs buffer layers between the InAs layer and the GaAs substrate. At room temperature, the fabricated InAs photodetector operated in an infrared range of approximately 1.5–4 μm and its detectivity (D*) was 1.65 × 108 cm · Hz1/2 · W−1 at 3.3 μm. To demonstrate performance, the Sherlock Holmes mapping images were obtained using the photodetector at room temperature. [ABSTRACT FROM AUTHOR]