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Group III-nitride light-emitting diodes (LEDs) fabricated on sapphire substrates typically suffer from insufficient heat dissipation, largely due to the low thermal conductivities (TCs) of their epitaxial layers and substrates. In the current work, we significantly improved the heat-dissipation characteristics of an InGaN/GaN quantum-well (QW) green LED by using hexagonal boron nitride (hBN) as a heat-transfer medium. Multiple-layer hBN with an average thickness of 11 nm was attached to the back of an InGaN/GaN-QW LED (hBN-LED). As a reference, an LED without the hBN (Ref-LED) was also prepared. After injecting current, heat-transfer characteristics inside each LED were analyzed by measuring temperature distribution throughout the LED as a function of time. For both LED chips, the maximum temperature was measured on the edge n-type electrode brightly shining fabricated on an n-type GaN cladding layer and the minimum temperature was measured at the relatively dark-contrast top surface between the p-type electrodes. The hBN-LED took 6 s to reach its maximum temperature (136.1 °C), whereas the Ref-LED took considerably longer, specifically 11 s. After being switched off, the hBN-LED took 35 s to cool down to 37.5 °C and the Ref-LED took much longer, specifically 265 s. These results confirmed the considerable contribution of the attached hBN to the transfer and dissipation of heat in the LED. The spatial heat-transfer and distribution characteristics along the vertical direction of each LED were theoretically analyzed by carrying out simulations based on the TCs, thicknesses, and thermal resistances of the materials used in the chips. The results of these simulations agreed well with the experimental results.

Two-dimensional (2D) van der Waals (vdW) heterostructures exhibit novel physical and chemical properties, allowing the development of unprecedented electronic, optical, and electrochemical devices. However, the construction of wafer-scale vdW heterostructures for practical applications is still limited due to the lack of well-established growth and transfer techniques. Herein, we report a method for the fabrication of wafer-scale 2D vdW heterostructures with an ultraclean interface between layers via the aid of a freestanding viscoelastic polymer support layer (VEPSL). The low glass transition temperature ( Tg) and viscoelastic nature of the VEPSL ensure absolute conformal contact between 2D layers, enabling the easy pick-up of layers and attaching to other 2D layers. This eventually leads to the construction of random sequence 2D vdW heterostructures such as molybdenum disulfide/tungsten disulfide/molybdenum diselenide/tungsten diselenide/hexagonal boron nitride. Furthermore, the VEPSL allows the conformal transfer of 2D vdW heterostructures onto arbitrary substrates, irrespective of surface roughness. To demonstrate the significance of the ultraclean interface, the fabricated molybdenum disulfide/graphene heterostructure employed as an electrocatalyst yielded excellent results of 73.1 mV·dec-1 for the Tafel slope and 0.12 kΩ of charge transfer resistance, which are almost twice as low as that of the impurity-trapped heterostructure.

We report a facile method for the synthesis of large-area tungsten disulfide (WS2) films by means of chemical vapor deposition (CVD). To promote WS2 film growth, the precursor solution, which includes pre-reduced tungsten suboxides, is prepared by using hydrazine as the strong reducing agent and spin-coated onto the growth substrate. Growth is then carried out in a CVD chamber vaporized with dimethyl disulfide as the sulfur precursor. Although only WS2 flakes are grown with unreduced tungsten precursors under a hydrogen atmosphere, WS2 films are readily attained on pre-reduced tungsten suboxide substrates without the need for further reduction by hydrogen, which is noted to induce discontinuity of the grown film. The result presents the coverage of WS2 to be proportional to the amount of reduced tungsten suboxides, which is revealed by X-ray photoelectron spectroscopy. Furthermore, it is found that the multilayer WS2 flakes grow along the grain boundary, which allows the analysis of the grain size of WS2 ...

We report the synthesis of centimeter-scale monolayer WS2 on gold foil by chemical vapor deposition. The limited tungsten and sulfur solubility in gold foil allows monolayer WS2 film growth on gold surface. To ensure the coverage uniformity of monolayer WS2 film, the tungsten source-coated substrate was placed in parallel with Au foil under hydrogen sulfide atmosphere. The high growth temperature near 935 °C helps to increase a domain size up to 420 μm. Gold foil is reused for the repeatable growth after bubbling transfer. The WS2-based field effect transistor reveals an electron mobility of 20 cm(2) V(-1) s(-1) with high on-off ratio of ∼10(8) at room temperature, which is the highest reported value from previous reports of CVD-grown WS2 samples. The on-off ratio of integrated multiple FETs on the large area WS2 film on SiO2 (300 nm)/Si substrate shows within the same order, implying reasonable uniformity of WS2 FET device characteristics over a large area of 3 × 1.5 cm(2).