New Type Of Heterostructure Research Articles

Hierarchically Periodic Macroporous CdS–ZnO Heterojunctions with Multiple Quantum Well-like Band Alignments for Efficient Photocatalytic Hydrogen Evolution without a Cocatalyst

Heterostructures constructed by conventional methods, particularly those randomly distributed manner on the catalyst surface as well as forming core–shell structures, lead to imbalance in carrier utilization, limitation of charge extraction/transfer, and hindrance of light harvesting. In this study, we constructed a new type of heterostructure consisting of alternate CdS and ZnO grains bridged with good interfaces in the hierarchically periodic macroporous (HPM) walls by the pyrolysis of CdZnS solid solution in a polymethyl methacrylate nanoreactor. Since the alternately arranged CdS–ZnO heterojunctions in the HPM form multiple quantum well-like (MQW-like) band alignments that favor the Z-scheme charge transfer, the photogenerated electrons and holes are spatially separate and accumulated on CdS and ZnO parts, respectively. Besides the desirable spatial separation of the photogenerated charges, the enabled excellent mass transfer in the macroporous structures effectively accelerates the utilization of the carriers. Owing to the synergistic effect of the MQW-like band alignments and the HPM structures, the photocatalytic H2 evolution rate of HPM CdS–ZnO is as high as 587.8 μmol h–1 without a cocatalyst. This work introduces a fascinating strategy for the creation of alternate heterojunctions to improve carrier utilization.

A rational design of titanium-based heterostructures as electrocatalyst for boosted conversion kinetics of polysulfides in Li-S batteries

Lithium–sulfur batteries have great potential for next-generation electrochemical storage systems owing to their high theoretical specific energy and cost-effectiveness. However, the shuttle effect of soluble polysulfides and sluggish multi-electron sulfur redox reactions has severely impeded the implementation of lithium–sulfur batteries. Herein, we prepared a new type of Ti3C2-TiO2 heterostructure sandwich nanosheet confined within polydopamine derived N-doped porous carbon. The highly polar heterostructures sandwich nanosheet with a high specific surface area can strongly absorb polysulfides, restraining their outward diffusion into the electrolyte. Abundant boundary defects constructed by new types of heterostructures reduce the overpotential of nucleation and improve the nucleation/conversion redox kinetics of Li2S. The Ti3C2–TiO2@NC/S cathode exhibited discharge capacities of 1363, and 801 mAh g−1 at the first and 100th cycles at 0.5C, respectively, and retained an ultralow capacity fade rate of 0.076% per cycle over 500cycles at 1.0C. This study provides a potential avenue for constructing heterostructure materials for electrochemical energy storage and catalysis.

Reimagining the eg1 Electronic State in Oxygen Evolution Catalysis: Oxidation‐State‐Modulated Superlattices as a New Type of Heterostructure for Maximizing Catalysis

AbstractThe discovery of solid‐phase, inexpensive transition‐metal‐based water oxidation catalysts is a central goal for renewable energy, and has led to a general consensus that a partially populated metal eg d‐electronic state is desirable, leading to favorable catalysis for certain elements in specific oxidation states. In manganese systems, the key species is manganese(III), whose high‐spin d4 electronic configuration places an unpaired electron in the eg orbital, which is postulated to contribute to electronic and structural features that support catalysis. Based on density functional theory calculations, it is predicted that electron transfer would be facilitated by a catalyst with alternating low‐ and high‐MnIII‐content sheets, which positions neighboring band edges in closer energetic proximity. The preparation of such catalysts is demonstrated for the first time and it is shown that the catalytic activity is maximized in these systems over more uniform, but more MnIII‐rich systems. The best catalyst possesses alternating high‐and low‐average oxidation state sheets with interlayer Cs+ ions, and has an overpotential of 450 mV at 10 mA, which represents an improvement of 250 mV over the best unmodified synthetic potassium birnessites. Using scanning tunneling spectroscopy, bandgap modulations consistent with the theoretically predicted band edge shifts are detected.

Synthesis of two-dimensional/one-dimensional heterostructures with tunable width

Two-dimensional/one-dimensional (2D/1D) heterostructures as a new type of heterostructure have been studied for their unusual properties and promising applications in electronic and optoelectronic devices. However, the studies of 2D/1D heterostructures are mainly focused on vertical heterostructures, such as MoS2 nanosheet-carbon nanotubes. The research on lateral 2D/1D heterostructures with a tunable width of 1D material is still scarce. In this study, bidirectional flow chemical vapor deposition (CVD) was used to accurately control the width of the WS2/WSe2 (WS2/MoS2) heterostructures by controlling reacting time. WSe2 and MoS2 with different widths were epitaxially grown at the edge of WS2, respectively. Optical microscope, atomic force microscope (AFM), and scanning electron microscope (SEM) images show the morphology and width of the heterostructures. These results show that the width of the heterostructures can be as low as 10 nm by using this method. The interface of the heterostructure is clear and smooth, which is suitable for application. This report offers a new method for the growth of 1D nanowires, and lays the foundation for the future study of the physical and chemical properties of 2D/1D lateral heterostructures.

Features of carrier confinement in InAsSb(P) alloys and quantum wells for room-temperature operating mid-infrared emitters

In this communication, we focus on the optical properties of InAsSbP/InAsSb/InAsSbP single quantum well structures grown on InAs substrates using the metal-organic vapor phase epitaxy (MOVPE) technique. The obtained Fourier transformed photoluminescence spectra allowed us to perform an advanced analysis of the temperature evolution of the measured spectra, which revealed radiative recombination at room temperature in the mid-infrared spectral range (∼3.5 μm) associated with localized states in the quantum well. These results suggest that improvements in the quality of the InAsSbP/InAsSb heterointerfaces could open the way to new types of heterostructures based on nanoscale systems for the fabrication of cheap mid-infrared lasers and light-emitting diodes, which are demanded by the optical gas sensing market.

Environmentally-Friendly Exfoliate and Active Site Self-Assembly: Thin 2D/2D Heterostructure Amorphous Nickel-Iron Alloy on 2D Materials for Efficient Oxygen Evolution Reaction.

A class of 2D layered materials exhibits substantial potential for high-performance electrocatalysts due to high specific surface area, tunable electronic properties, and open 2D channels for fast ion transport. However, liquid-phase exfoliation always utilizes organic solvents that are harmful to the environment, and the active sites are limited to edge sites. Here, an environmentally friendly exfoliator in aqueous solution is presented without utilizing any toxic or hazardous substance and active site self-assembly on the inert base of 2D materials. Benefiting from thin 2D/2D heterostructure and strong interfacial coupling, the resultant highly disordered amorphous NiFe/2D materials (Ti3C2 MXene, graphene and MoS2 ) thin nanosheets exhibit extraordinary electrocatalytic performance toward oxygen evolution reaction (OER) in alkaline media. DFT results further verify the experimental results. The study emphasizes a viable idea to probe efficient electrocatalysts by means of the synergistic effect of environmentally friendly exfoliator in aqueous solution and active site self-assembly on the inert base of 2D materials which forms the unique thin 2D/2D heterostructure in-suit. This new type of heterostructure opens up a novel avenue for the rational design of highly efficient 2D materials for electrocatalysis.

Giant enhancement of nonlinear absorption in graphene oxide—Sb2Se3 nanowire heterostructure

Nanosheet-nanowire heterostructure provides a new platform to explore the nonlinear optical phenomena at the nanoscale. In this article, we report giant nonlinear absorption (NLA) coefficient ≈0.94 × 103 cm/GW at 1064 nm and 0.64 × 103 cm/GW at 532 nm in a new type of heterostructure of two-dimensional graphene oxide (GO) nanosheets and one-dimensional Sb2Se3 nanowires prepared by a facile hydrothermal method. The giant enhancement of NLA by the heterostructure is attributed to synergistic charge transfer between the Sb2Se3 nanowire and GO nanosheet through the donor-acceptor interaction. With this idea, we have developed a five-level rate equation model and numerically simulated the results. Strikingly, the heterostructure shows an excellent benchmark for the optical limiting onset of 8 mJ/cm2 at 1064 nm and 20 mJ/cm2 at 532 nm, that is, an order of magnitude better than graphene based materials, 2D transition metal dichalcogenides, and metal nanoparticles over a broad spectral range.

Ag+-Induced Shape and Composition Evolution of Covellite CuS Nanoplatelets To Produce Plate–Satellite and Biconcave−Particle Heterostructures

The creation of novel metal sulfide nanostructures and heterostructures by incorporating heterocations into template copper sulfide nanocrystals (NCs) is an important and intriguing area of colloidal synthesis. In contrast with typical incorporation using divalent, trivalent, and tetravalent cations, the mechanism for incorporation of monovalent Ag+ into copper sulfide NCs is less clear. In this work, we prepared new types of heterostructures by incorporating Ag+ into covellite CuS nanoplatelets (NPls). The incorporation process depended strongly on the initial Ag+ concentration. When a relatively small amount of Ag+ was provided (0.1 mmol, roughly 20% of the Cu present in the template NPls), CuS–Ag2S plate–satellite heterostructures were produced by cation exchange (CE) reactions occurring at the corners of the template hexagonal NPls. In contrast, biconcave–particle Ag2S–Ag heterostructures were obtained via the nanoscale Kirkendall effect when starting with 0.5 mmol of Ag+. Our observations indicate that the Ag+ incorporation process is also template-dependent. A redox process involving disulfide bonds in CuS is essential for producing Ag2S domains, rather than forming a ternary Ag–Cu–S phase, which was the result when djurleite Cu1.94S NPls were treated with Ag+. These results and mechanistic discussions not only provide a better understanding of a complex cation incorporation and crystal phase transition process but also provide a means to design new copper sulfide-based heterostructures.

Semiconducting van der Waals Interfaces as Artificial Semiconductors.

Recent technical progress demonstrates the possibility of stacking together virtually any combination of atomically thin crystals of van der Waals bonded compounds to form new types of heterostructures and interfaces. As a result, there is the need to understand at a quantitative level how the interfacial properties are determined by the properties of the constituent 2D materials. We address this problem by studying the transport and optoelectronic response of two different interfaces based on transition-metal dichalcogenide monolayers, namely WSe2-MoSe2 and WSe2-MoS2. By exploiting the spectroscopic capabilities of ionic liquid gated transistors, we show how the conduction and valence bands of the individual monolayers determine the bands of the interface, and we establish quantitatively (directly from the measurements) the energetic alignment of the bands in the different materials as well as the magnitude of the interfacial band gap. Photoluminescence and photocurrent measurements allow us to conclude that the band gap of the WSe2-MoSe2 interface is direct in k space, whereas the gap of WSe2/MoS2 is indirect. For WSe2/MoSe2, we detect the light emitted from the decay of interlayer excitons and determine experimentally their binding energy using the values of the interfacial band gap extracted from transport measurements. The technique that we employed to reach this conclusion demonstrates a rather-general strategy for characterizing quantitatively the interfacial properties in terms of the properties of the constituent atomic layers. The results presented here further illustrate how van der Waals interfaces of two distinct 2D semiconducting materials are composite systems that truly behave as artificial semiconductors, the properties of which can be deterministically defined by the selection of the appropriate constituent semiconducting monolayers.

Edge Epitaxy of Two-Dimensional MoSe2 and MoS2 Nanosheets on One-Dimensional Nanowires.

Rational design and synthesis of heterostructures based on transition metal dichalcogenides (TMDs) have attracted increasing interests because of their promising applications in electronics, catalysis, etc. However, the construction of epitaxial heterostructures with an interface at the edges of TMD nanosheets (NSs) still remains a great challenge. Here, we report a strategy for controlled synthesis of a new type of heterostructure in which TMD NSs, including MoS2 and MoSe2, vertically grow along the longitudinal direction of one-dimensional (1D) Cu2-xS nanowires (NWs) in an epitaxial manner. The obtained Cu2-xS-TMD heterostructures with tunable loading amount and lateral size of TMD NSs are achieved by the consecutive growth of TMD NSs on Cu2-xS NWs through gradual injection of chalcogen precursors. After cation exchange of Cu in Cu2-xS-TMD heterostructures with Cd, the obtained CdS-MoS2 heterostructures retained their original architectures. Compared to the pure CdS NWs, the CdS-MoS2 heterostructures with 7.7 wt % loading of MoS2 NSs exhibit the best performance in the photocatalytic hydrogen evolution reaction with a H2 production rate up to 4647 μmol·h-1·g-1, about 58 times that catalyzed with pure CdS NWs. Our synthetic strategy opens up a new way for the controlled synthesis of TMD-based heterostructures, which could have various promising applications.

Neutron scattering on magnetic surfaces

During the early 1980s, advanced techniques for the deposition of ultra-thin metal films were developed. The combination of different types of materials gave rise to new physical phenomena such as the magnetic exchange coupling in superlattices or the exchange bias between ferro and anti-ferro layers. The field was very active because of the associated industrial applications in magnetic field sensors. New types of heterostructures combining magnetic oxides, insulating oxides or magnetic semiconductors are still being developed. Alongside the fabrication of these new meta-materials made of thin films stacking, polarized neutron reflectometry has emerged as a routine tool for the characterization of magnetic hetero-structures. In the recent years, the new developments of polarized reflectivity have been connected to the study of micro and nanostructures, especially micromagnetic structures in multilayers. The technique of off-specular scattering has been developed for the study of the roughness or the micromagnetism at a micrometric scale. For the study of nanometric structures, in the range below 100 nm, grazing incidence Small Angle Scattering is being considered. To cite this article: F. Ott, C. R. Physique 8 (2007).

Photosensitivity of Heterostructures Based on Finely Ground Semiconductor Phases

A new type of heterostructure is suggested and developed. The heterostructures are based on the direct contact of a bulk semiconductor with a dielectric layer in which a finely ground semiconductor phase is dispersed. In Si-and GaAs-based heterostructures of this type, rectification and photovoltaic effects are observed. It is shown that illumination of such structures so that the side of the dielectric layer with the built-in finely ground semiconductor phase is exposed to light induces a broadband photovoltaic effect deep within the fundamental absorption band of the bulk semiconductor.

GaAs in GaSb: a new type of heterostructure emitting at 2 µm wavelength

We report growth as well as optical and structural studies of a new type of a GaAs/GaSb heterostructure, with 1–3 monolayer thick GaAs layers embedded within unstrained GaSb. In such structures the GaAs layer is under tensile stress, in contrast to the situation in which self-organized growth of quantum dots is commonly observed. The structure emits light in the 2 μm wavelength range. The emission characteristics are explained by a combination of quantum confinement effects and localization on nanoscale potential fluctuations.

Double heterostructure lasers: early days and future perspectives

A short historical review of the physics and technology of heterostructure lasers based on double heterostructures is described. Recent progress in quantum dot laser structures and future trends in the development of the physics and technology of these new types of heterostructures are discussed.

New type of ZnCdS/ZnMgCdS heterostructures lattice-matched to GaAs for selective-area growth

A new type of heterostructure ZnCdS/ZnMgCdS that can be grown lattice-matched to a GaAs substrate and is compatible with the selective area growth at very low temperature is proposed. The growth was performed by metalorganic molecular-beam epitaxy. Selective-area growth was performed using a carbonaceous mask and was successful for the Se-free, ZnCdS ternary and ZnMgCdS quaternary alloys starting as low as 360°C. A heterostructure lattice-matched to GaAs, with at least 470-meV energy band-gap difference was confirmed experimentally.

Quantum wires and dots show the way forward

Concluding his three-part series on the pivotal role of compound semiconductor heterostructures, Professor Alferov of the Ioffe Institute in St. Petersburg reviews the development and recent exciting advances in quantum wires and, in more detail, quantum dot structures. The future trends and perspectives of these new types of heterostructures are discussed.

The history and future of semiconductor heterostructures from the point of view of a Russian scientist

The early history of semiconductor heterostructures and their applications in different electronic devices is described. The article also contains a short historical review of the physics, technology of preparation and applications of quantum wells and superlattices. Recent progress in quantum wires and especially quantum dots structures and future trends and perspectives of these new types of heterostructures are discussed.