SciCombinator

Discover the most talked about and latest scientific content & concepts.

Concept: Heterojunction

161

The solar-powered production of hydrogen for use as a renewable fuel is highly desirable for the world’s future energy infrastructure. However, difficulties in achieving reasonable efficiencies, and thus cost-effectiveness, have hampered significant research progress. Here we propose the use of semiconductor nanostructures to create a type-II heterojunction at the semiconductor-water interface in a photoelectrochemical cell (PEC) and theoretically investigate it as a method of increasing the maximum photovoltage such a cell can generate under illumination, with the aim of increasing the overall cell efficiency. A model for the semiconductor electrode in a PEC is created, which solves the Schrödinger, Poisson and drift-diffusion equations self-consistently. From this, it is determined that ZnO quantum dots on bulk n-InGaN with low In content x is the most desirable system, having electron-accepting and -donating states straddling the oxygen- and hydrogen-production potentials for x < 0.26, though large variance in literature values for certain material parameters means large uncertainties in the model output. Accordingly, results presented here should form the basis for further experimental work, which will in turn provide input to refine and develop the model.

Concepts: Hydrogen production, Hydrogen, Electrolysis, Solar cell, Electrode, Photoelectrochemical cell, Heterojunction, Object-oriented programming

31

Semiconductor heterostructures form the cornerstone of many electronic and optoelectronic devices and are traditionally fabricated using epitaxial growth techniques. More recently, heterostructures have also been obtained by vertical stacking of two-dimensional crystals, such as graphene and related two-dimensional materials. These layered designer materials are held together by van der Waals forces and contain atomically sharp interfaces. Here, we report on a type-II van der Waals heterojunction made of molybdenum disulfide and tungsten diselenide monolayers. The junction is electrically tunable and under appropriate gate bias, an atomically thin diode is realized. Upon optical illumination, charge transfer occurs across the planar interface and the device exhibits a photovoltaic effect. Advances in large-scale production of two-dimensional crystals could thus lead to a new photovoltaic solar technology.

Concepts: Molybdenum, Photodiode, Van der Waals force, Fundamental physics concepts, Electric charge, Heterojunction, Solar cell, Semiconductor

31

The design of stacks of layered materials in which adjacent layers interact by van der Waals forces has enabled the combination of various two-dimensional crystals with different electrical, optical and mechanical properties as well as the emergence of novel physical phenomena and device functionality. Here, we report photoinduced doping in van der Waals heterostructures consisting of graphene and boron nitride layers. It enables flexible and repeatable writing and erasing of charge doping in graphene with visible light. We demonstrate that this photoinduced doping maintains the high carrier mobility of the graphene/boron nitride heterostructure, thus resembling the modulation doping technique used in semiconductor heterojunctions, and can be used to generate spatially varying doping profiles such as p-n junctions. We show that this photoinduced doping arises from microscopically coupled optical and electrical responses of graphene/boron nitride heterostructures, including optical excitation of defect transitions in boron nitride, electrical transport in graphene, and charge transfer between boron nitride and graphene.

Concepts: Light, Boron nitride, Physics, Boron, Semiconductor, Van der Waals force, P-n junction, Heterojunction

29

Semiconductor heterostructures provide a powerful platform to engineer the dynamics of excitons for fundamental and applied interests. However, the functionality of conventional semiconductor heterostructures is often limited by inefficient charge transfer across interfaces due to the interfacial imperfection caused by lattice mismatch. Here we demonstrate that MoS2/WS2 heterostructures consisting of monolayer MoS2 and WS2 stacked in the vertical direction can enable equally efficient interlayer exciton relaxation regardless the epitaxy and orientation of the stacking. This is manifested by a similar two orders of magnitude decrease of photoluminescence intensity in both epitaxial and non-epitaxial MoS2/WS2 heterostructures. Both heterostructures also show similarly improved absorption beyond the simple super-imposition of the absorptions of monolayer MoS2 and WS2. Our result indicates that 2D heterostructures bear significant implications for the development of photonic devices, in particular those requesting efficient exciton separation and strong light absorption, such as solar cells, photodetectors, modulators, and photocatalysts. It also suggests that the simple stacking of dissimilar 2D materials with random orientations is a viable strategy to fabricate complex functional 2D heterostructures, which would show similar optical functionality as the counterpart with perfect epitaxy.

Concepts: Semiconductor device fabrication, Heterojunction, Exciton, Absorption, Optics, Wafer, Photon, Solar cell

29

The newly discovered two-dimensional materials can be used to form atomically thin and sharp van der Waals heterostructures with nearly perfect interface qualities, which can transform the science and technology of semiconductor heterostructures. Owing to the weak van der Waals interlayer coupling, the electronic states of participating materials remain largely unchanged. Hence, emergent properties of these structures rely on two key elements: electron transfer across the interface and interlayer coupling. Here we show, using graphene-tungsten disulfide heterostructures as an example, evidence of ultrafast and highly efficient interlayer electron transfer and strong interlayer coupling and control. We find that photocarriers injected in tungsten disulfide transfer to graphene in 1 ps and with near-unity efficiency. We also demonstrate that optical properties of tungsten disulfide can be effectively tuned by carriers in graphene. These findings illustrate basic processes required for using van der Waals heterostructures in electronics and photonics.

Concepts: Weak interaction, Carbon, Heterojunction, Fundamental physics concepts, Chemical element, Electronic band structure, Electron, Optics

28

Two-dimensional (2D) crystals offer a unique platform due to their remarkable and contrasting spintronic properties, such as weak spin-orbit coupling (SOC) in graphene and strong SOC in molybdenum disulfide (MoS2). Here we combine graphene and MoS2 in a van der Waals heterostructure (vdWh) to demonstrate the electric gate control of the spin current and spin lifetime at room temperature. By performing non-local spin valve and Hanle measurements, we unambiguously prove the gate tunability of the spin current and spin lifetime in graphene/MoS2 vdWhs at 300 K. This unprecedented control over the spin parameters by orders of magnitude stems from the gate tuning of the Schottky barrier at the MoS2/graphene interface and MoS2 channel conductivity leading to spin dephasing in high-SOC material. Our findings demonstrate an all-electrical spintronic device at room temperature with the creation, transport and control of the spin in 2D materials heterostructures, which can be key building blocks in future device architectures.

Concepts: Spin, Molybdenum disulfide, Graphite, Molybdenum, Electricity, Heterojunction, Electron, Fundamental physics concepts

28

MoS(2) nanosheet-coated TiO(2) nanobelt heterostructures-referred to as TiO(2) @MoS(2) -with a 3D hierarchical configuration are prepared via a hydrothermal reaction. The TiO(2) nanobelts used as a synthetic template inhibit the growth of MoS(2) crystals along the c-axis, resulting in a few-layer MoS(2) nanosheet coating on the TiO(2) nanobelts. The as-prepared TiO(2) @MoS(2) heterostructure shows a high photocatalytic hydrogen production even without the Pt co-catalyst. Importantly, the TiO(2) @MoS(2) heterostructure with 50 wt% of MoS(2) exhibits the highest hydrogen production rate of 1.6 mmol h(-1) g(-1) . Moreover, such a heterostructure possesses a strong adsorption ability towards organic dyes and shows high performance in photocatalytic degradation of the dye molecules.

Concepts: Heterojunction, Hydrogen production, Chemical bond, Chemical reaction, Atom, Hydrogen, Dye, Catalysis

27

In this work, n-type porous graphite-like C3N4 (denoted as p-g-C3N4) was fabricated and modified with p-type nanostructured BiOI to form a novel BiOI/p-g-C3N4 p-n heterojunction photocatalyst for the efficient photocatalytic degradation of methylene blue (MB). The results show that the BiOI/p-g-C3N4 heterojunction photocatalyst exhibits superior photocatalytic activity compared to pure BiOI and p-g-C3N4. The visible-light photocatalytic activity enhancement of BiOI/p-g-C3N4 heterostructures could be attributed to its strong absorption in the visible region and low recombination rate of the electron-hole pairs because of the heterojunction formed between BiOI and p-g-C3N4. It was also found that the photodegradation of MB molecules is mainly attributed to the oxidation action of the generated O2˙(-) radicals and partly to the action of hvb(+)via direct hole oxidation process.

Concepts: Heterojunction, Iron, Bipolar junction transistor, P-n junction, Electron hole, Photocatalysis, Solar cell, Semiconductor

27

We report a simple yet versatile solution route for constructing heterojunctions from luminescent organic charge-transfer (CT) complexes through a two-step seeded-growth method. Using this method, we achieved anisotropic and selective growth of anthracene-1,2,4,5-tetracyanobenzene (TCNB) complexes onto the tips of naphthalene-TCNB microtubes, resulting in the formation of microdumbbells. Significantly, the two-component microdumbbells appear as dual-color-emitting heterojunctions arising from integration of two distinct color-emitting materials. We further elucidated the two-step seeded-growth mechanism of the dumbbell-like organic heterostructures on the basis of structural analysis of the two crystals and surface-interface energy balance. In principle, the present synthetic route may be used to fabricate a wide range of sophisticated dual- or multicolor-emitting organic heterostructures via judicious choice of the CT complexes.

Concepts: Heterojunction, Present, Chemistry

25

Two-dimensional layered transition-metal dichalcogenides have attracted considerable interest for their unique layer-number-dependent properties. In particular, vertical integration of these two-dimensional crystals to form van der Waals heterostructures can open up a new dimension for the design of functional electronic and optoelectronic devices. Here we report the layer-number-dependent photocurrent generation in graphene/MoS2/graphene heterostructures by creating a device with two distinct regions containing one-layer and seven-layer MoS2 to exclude other extrinsic factors. Photoresponse studies reveal that photoresponsivity in one-layer MoS2 is surprisingly higher than that in seven-layer MoS2 by seven times. Spectral-dependent studies further show that the internal quantum efficiency in one-layer MoS2 can reach a maximum of 65%, far higher than the 7% in seven-layer MoS2. Our theoretical modelling shows that asymmetric potential barriers in the top and bottom interfaces of the graphene/one-layer MoS2/graphene heterojunction enable asymmetric carrier tunnelling, to generate usually high photoresponsivity in one-layer MoS2 device.

Concepts: Dimension, Optoelectronics, String theory, Vector space, Motivation, Semiconductor, Heterojunction, Electronics terms