Emerging opportunities in the two-dimensional chalcogenide systems and architecture

Inspired by the triumphs - and motivated by the need to overcome the limitations - of graphene, the science and engineering community is rapidly exploring the landscape of other potential two-dimensional materials, particularly in their single - or few layer form. Dominating this landscape are the layered chalcogenides; diverse in chemistry, structure and properties, there are well over 100 primary members of this materials family. Driven by quantum confinement, single layers (or few, in some cases) of these materials exhibit electronic, optical, and transport properties that diverge dramatically from their bulk counterparts. The field has evolved considerably since the time when single or few layer flakes were “synthesized” by the scotch-tape mechanical cleavage method. New and more sophisticated methods for controlled synthesis (or thinning), deposition and chemical exfoliation have been developed that can “dial” the number of layers with large areal coverage on diverse substrates. Further, the 2D chalcogenide layers are being used as “substrates” onto which other dimensionally confined structures are being integrated in the spirit of nanoscale composites. Some composite structures exhibit synergy of multiple functionalities of the individual components, while in other cases they represent quantum coupling or unusual behavior that is contrary to nominal synergy or the proportional contribution of individual components. Last but not the least, there remain many structural and chemical combinations that are yet to be explored with deeply intriguing properties or phenomena that are waiting to be revealed. Thus, it is timely to review the status of the field; particularly in the context of synthesis, geometric architecturing and characterization of 2D layered systems.Herein we review the evolving architecture of two-dimensional chalcogenide materials. We outline classes of specific materials and the evolution of their properties as they transition from nominally three to two-dimensionality, and especially in their single (or few) layer form. A variety of vapor-phase synthetic methods for the direct growth of large area single layers and the typical techniques for their characterization are presented. Lastly, we examine the potential of these materials as the fundamental building blocks of two-dimensional heterostructures and multi-dimensional nanocomposites. However, we also emphasize the need for fundamental experimental and theoretical undertaking to probe the classical problems like basic characterization and the dynamics of nucleation and growth in these 2D systems for realizing complex architecturing and resultant technologically useful phenomena and properties.     

Recent progress in transparent oxide semiconductors: Materials and device application

This paper reviews our recent research progress on new transparent conductive oxide (TCO) materials and electronic and optoelectronic devices based on these materials. First, described are the materials including p-type materials, deep-UV transparent TCO(β-Ga₂O₃), epitaxially grown ITO with atomically flat surface, transparent electrochromic oxide (NbO₂F), amorphous TCOs, and nanoporous semiconductor 12CaO · 7Al₂O₃. Second, presented are TCO-based electronic/optoelectronic devices realized to date, UV/blue LED and UV-sensors based on transparent pn junction and high performance transparent TFT using n-type TCO as an n-channel. Finally, unique optoelectronic properties (p-type degenerate conduction, transfer doping of carriers, RT-stable exciton, and large optical nonlinearity) originating from 2D-electronic nature in p-type layered oxychalcogenides are summarized along with the fabrication method of epitaxial thin films of these materials.     

Organic High Electron Mobility Transistors Realized by 2D Electron Gas

A key breakthrough in inorganic modern electronics is the energy‐band engineering that plays important role to improve device performance or develop novel functional devices. A typical application is high electron mobility transistors (HEMTs), which utilizes 2D electron gas (2DEG) as transport channel and exhibits very high electron mobility over traditional field‐effect transistors (FETs). Recently, organic electronics have made very rapid progress and the band transport model is demonstrated to be more suitable for explaining carrier behavior in high‐mobility crystalline organic materials. Therefore, there emerges a chance for applying energy‐band engineering in organic semiconductors to tailor their optoelectronic properties. Here, the idea of energy‐band engineering is introduced and a novel device configuration is constructed, i.e., using quantum well structures as active layers in organic FETs, to realize organic 2DEG. Under the control of gate voltage, electron carriers are accumulated and confined at quantized energy levels, and show efficient 2D transport. The electron mobility is up to 10 cm² V^?1 s^?1, and the operation mechanisms of organic HEMTs are also argued. Our results demonstrate the validity of tailoring optoelectronic properties of organic semiconductors by energy‐band engineering, offering a promising way for the step forward of organic electronics.     

2D Perovskites with Short Interlayer Distance for High‐Performance Solar Cell Application

2D perovskites have emerged as one of the most promising photovoltaic materials owing to their excellent stability compared with their 3D counterparts. However, in typical 2D perovskites, the highly conductive inorganic layers are isolated by large organic cations leading to quantum confinement and thus inferior electrical conductivity across layers. To address this issue, the large organic cations are replaced with small propane‐1,3‐diammonium (PDA) cations to reduce distance between the inorganic perovskite layers. As shown by optical characterizations, quantum confinement is no longer dominating in the PDA‐based 2D perovskites. This leads to considerable enhancement of charge transport as confirmed with electrochemical impedance spectroscopy, time‐resolved photoluminescence, and mobility measurements. The improved electric properties of the interlayer‐engineered 2D perovskites yield a power conversion efficiency of 13.0%. Furthermore, environmental stabilities of the PDA‐based 2D perovskites are improved. PDA‐based 2D perovskite solar cells (PSCs) with encapsulation can retain over 90% of their efficiency upon storage for over 1000 h, and PSCs without encapsulation can maintain their initial efficiency at 70 °C for over 100 h, which exhibit promising stabilities. These results reveal excellent optoelectronic properties and intrinsic stabilities of the layered perovskites with reduced interlayer distance.     

Development of new transparent conductors and device applications utilizing a multidisciplinary approach

Transparent and conductive films are key components for optoelectronic devices. They are applied as n-type transparent electrical contacts for inorganic and organic light emitting diodes, solar cells and flat panel displays as well as p- and n-type active semiconductive oxides to setup wide band gap p-n junctions and devices for the emerging field of transparent and radiation hard electronics.The demand for these films is strongly increasing due to the extensive market growth in these areas but the solutions available today only partially fulfill the requirements on low resistivity, high transmittance, large area deposition, low cost manufacturing, and ability for fine patterning, light scattering and precise alignment of the electronic structure to surrounding semiconductors.The cooperation of five Fraunhofer Institutes within the “Fraunhofer Project MAVO METCO” aims towards establishing fundamental knowledge and control about the defect chemistry, structure and morphology of the transparent semiconductive oxides. The goal is to achieve materials with outstanding properties such as n-type transparent conductive oxides with tailored work function and excellent durability, novel delafossite based p-type materials allowing cost effective large area deposition, oxide based p-n heterojunctions and Ag based electrodes to be used for thin film photovoltaics and organic light emitting diodes.Starting from first-principle modelling of the electronic structure, we address the development of new transparent conductive layers by PVD and Sol-Gel ending up with device implementation for OLEDs and organic as well as Si based a-Si:H/µc-Si:H and HIT solar cells.     

Deep-ultraviolet Raman microspectroscopy: characterization of wide-gap semiconductors

We have developed a high-throughput deep-ultraviolet (DUV) Raman microspectrometer with excitation from a continuous wave (cw) laser operated at 244 nm that enables us to characterize thin surface layers of wide-gap semiconductors. This spectrometer system consists of a filter spectrometer for the rejection of stray light and a high-dispersion spectrograph combined with a liquid nitrogen cooled charge-coupled device (CCD) detector and extends the low-frequency limit of the observable spectral range down to 170 cm(-1). In the microscope we use a Cassegrain reflective objective for the collection of the scattered light and an off-axis mirror for introduction of the excitation laser light. DUV Raman spectroscopy has been applied for studying wide-gap semiconductors including SiC and AlGaN epitaxial films and shallow implanted layers of these materials. Raman spectra of various crystals have also been measured for examining the performance of this system. Resonance enhancement of Raman bands has been observed for several semiconductors, and the results are discussed.     

Heteroanionic Materials by Design: Progress Toward Targeted Properties

The burgeoning field of anion engineering in oxide‐based compounds aims to tune physical properties by incorporating additional anions of different size, electronegativity, and charge. For example, oxychalcogenides, oxynitrides, oxypnictides, and oxyhalides may display new or enhanced responses not readily predicted from or even absent in the simpler homoanionic (oxide) compounds because of their proximity to the ionocovalent‐bonding boundary provided by contrasting polarizabilities of the anions. In addition, multiple anions allow heteroanionic materials to span a more complex atomic structure design palette and interaction space than the homoanionic oxide‐only analogs. Here, established atomic and electronic principles for the rational design of properties in heteroanionic materials are contextualized. Also described are synergistic quantum mechanical methods and laboratory experiments guided by these principles to achieve superior properties. Lastly, open challenges in both the synthesis and the understanding and prediction of the electronic, optical, and magnetic properties afforded by anion‐engineering principles in heteroanionic materials are reviewed.     

Moiré-Pattern Modulated Electronic Structures of GaSe/HOPG Heterostructure

Conventional two-dimensional electron gas (2DEG) typically occurs at the interface of semiconductor heterostructures and noble metal surfaces, but it is scarcely observed in individual 2D semiconductors. In this study, few-layer gallium selenide (GaSe) grown on highly ordered pyrolytic graphite (HOPG) is demonstrated using scanning tunneling microscopy and spectroscopy (STM/STS), revealing that the coexistence of quantum well states (QWS) and 2DEG. The QWS are located in the valence bands and exhibit a peak feature, with the number of quantum wells being equal to the number of atomic layers. Meanwhile, the 2DEG is located in the conduction bands and exhibits a standing-wave feature. Additionally, monolayer GaSe/HOPG heterostructures with different stacking angles (0°, 33°, 8°) form distinct moiré patterns that arise from lattice mismatch and angular rotation between adjacent atomic layers in 2D materials, which effectively modulate the electron effective mass, charge redistribution, and band gap of GaSe. Overall, this work reveals a paradigm of band engineering based on layer numbers and moiré patterns that can modulate the electronic properties of 2D materials.     

Lead‐Chalcogenide Colloidal‐Quantum‐Dot Solids: Novel Assembly Methods,Electronic Structure Control,and Application Prospects

The quest for novel semiconductors with easy, cheap fabrication and tailorable properties has led to the development of several classes of materials, such as semiconducting polymers, carbon nanotubes, hybrid perovskites, and colloidal quantum dots. All these candidates can be processed from the liquid phase, enabling easy fabrication, and are suitable for different electronic and optoelectronic applications. Here, recent developments in the field of colloidal‐quantum‐dot solids are discussed, with a focus on lead‐chalcogenide systems. These include novel deposition methods; the recent growing understanding of their fundamental properties, driven by major successes in the control of the nanostructured assembly and surface chemistry; and selected reports on lab‐scale devices showing the technological prospects of these fascinating class of materials.     

Photonics and Optoelectronics of 2D Metal‐Halide Perovskites

In the growing list of 2D semiconductors as potential successors to silicon in future devices, metal‐halide perovskites have recently joined the family. Unlike other conversional 2D covalent semiconductors such as graphene, transition metal dichalcogenides, black phosphorus, etc., 2D perovskites are ionic materials, affording many distinct properties of their own, including high photoluminescence quantum efficiency, balanced large exciton binding energy and oscillator strength, and long carrier diffusion length. These unique properties make 2D perovskites potential candidates for optoelectronic and photonic devices such as solar cells, light‐emitting diodes, photodetectors, nanolasers, waveguides, modulators, and so on, which represent a relatively new but exciting and rapidly expanding area of research. In this Review, the recent advances in emerging 2D metal‐halide perovskites and their applications in the fields of optoelectronics and photonics are summarized and insights into the future direction of these fields are offered.     

Molecular Beam Epitaxy and Electronic Structure of Atomically Thin Oxyselenide Films

Atomically thin oxychalcogenides have been attracting intensive attention for their fascinating fundamental properties and application prospects. Bi₂O₂Se, a representative of layered oxychalcogenides, has emerged as an air‐stable high‐mobility 2D semiconductor that holds great promise for next‐generation electronics. The preparation and device fabrication of high‐quality Bi₂O₂Se crystals down to a few atomic layers remains a great challenge at present. Here, molecular beam epitaxy (MBE) of atomically thin Bi₂O₂Se films down to monolayer on SrTiO₃ (001) substrate is achieved by co‐evaporating Bi and Se precursors in oxygen atmosphere. The interfacial atomic arrangements of MBE‐grown Bi₂O₂Se/SrTiO₃ are unambiguously revealed, showing an atomically sharp interface and atom‐to‐atom alignment. Importantly, the electronic band structures of one‐unit‐cell (1‐UC) thick Bi₂O₂Se films are observed by angle‐resolved photoemission spectroscopy (ARPES), showing low effective mass of ≈0.15 m₀ and bandgap of ≈0.8 eV. These results may be constructive to the synthesis of other 2D oxychalcogenides and investigation of novel physical properties.     

Ultrathin Two‐Dimensional Multinary Layered Metal Chalcogenide Nanomaterials

Ultrathin two‐dimensional (2D) layered transition metal dichalcogenides (TMDs), such as MoS₂, WS₂, TiS₂, TaS₂, ReS₂, MoSe₂ and WSe₂, have attracted considerable attention over the past six years owing to their unique properties and great potential in a wide range of applications. Aiming to achieve tunable properties and optimal application performances, great effort is devoted to the exploration of 2D multinary layered metal chalcogenide nanomaterials, which include ternary metal chalcogenides with well‐defined crystal structures, alloyed TMDs, heteroatom‐doped TMDs and 2D metal chalcogenide heteronanostructures. These novel 2D multinary layered metal chalcogenide nanomaterials exhibit some unique properties compared to 2D binary TMD counterparts, thus holding great promise in various potential applications including electronics/optoelectronics, catalysis, sensors, biomedicine, and energy storage and conversion with enhanced performances. This article focuses on the state‐of‐art progress on the preparation, characterization and applications of ultrathin 2D multinary layered metal chalcogenide nanomaterials.     

Zinc-blende GaN: ab initio calculations

The purpose of this paper is to contribute, on a theoretical basis, an understanding of future wide-gap device concepts and applications based on III–V nitride semiconductors. The electronic properties of zinc-blende structure GaN and their (110), (100) and (111) surfaces are investigated using ab initio calculations based on the full potential linear augmented plane-wave (FPLAPW) method within the large unit cell approach, and on the molecular Gaussian-92 code. Lattice constant, cohesive energy, bulk modulus are obtained from total energy calculations. Light-hole and heavy-hole effective masses along (100), (111) and (110) directions and electron masses at Γ point are extracted from band structure calculations and compared with previous ones based on pseudopotential methods. The hydrostatic pressure dependence of the ΓΓ, ΓX and ΓL energy gaps are also obtained. Comparing our band structure and ‘molecular cluster' calculations, the relaxations of the surfaces are found to be mostly determined by local rehybridization or valence effects and are basically independent of energy band features.     

Synthesis, crystal structure, and properties of novel perovskite oxychalcogenides, Ca2CuFeO3Ch (Ch = S, Se)

Two new perovskite oxychalcogenides, Ca₂CuFeO₃S and Ca₂CuFeO₃Se, have been synthesized in evacuated quartz tubes. They crystallize in P4/nmm space group with lattice parameters a = 3.8271(1), c = 14.9485(2) Å and a = 3.8605(1), c = 15.3030(2) Å for Ca₂CuFeO₃S and Ca₂CuFeO₃Se, respectively. They appear to be the first layered chalcogenide perovskites involving calcium and are structural analogs of the corresponding Sr and Ba compounds. The new compounds exhibit semiconducting properties with energy gap decreasing from the oxysulfide to the oxyselenide. Possibility of introducing Ca²⁺ into structures of known layered oxychalcogenides and oxypnictides is discussed.     

Quantum (Nano-Scale) Limit in a Parallel Magnetic Field in Layered Q1D Conductors

We show that electron wave functions in a quasi-one-dimensional (Q1D) layered conductor in a parallel magnetic field are always localized on conducting layers. Wave functions and electron spectrum in a quantum limit, where the “sizes” of quasi-classical electron orbits are of the order of nanoscale distances between the layers, are determined. Possible applications of our results to physical properties of Q1D materials in their superconducting and metallic phases are discussed.     

Resonant Tunneling Spectroscopy to Probe the Giant Stark Effect in Atomically Thin Materials

Each atomic layer in van der Waals heterostructures possesses a distinct electronic band structure that can be manipulated for unique device operations. In the precise device architecture, the subtle but critical band splits by the giant Stark effect between atomic layers, varied by the momentum of electrons and external electric fields in device operation, has not yet been demonstrated or applied to design original devices with the full potential of atomically thin materials. Here, resonant tunneling spectroscopy based on the negligible quantum capacitance of 2D semiconductors in resonant tunneling transistors is reported. The bandgaps and sub-band structures of various channel materials could be demonstrated by the new conceptual spectroscopy at the device scale without debatable quasiparticle effects. Moreover, the band splits by the giant Stark effect in the channel materials could be probed, overcoming the limitations of conventional optical, photoemission, and tunneling spectroscopy. The resonant tunneling spectroscopy reveals essential and practical information for novel device applications.     

2D Oxide Nanomaterials to Address the Energy Transition and Catalysis

2D oxide nanomaterials constitute a broad range of materials, with a wide array of current and potential applications, particularly in the fields of energy storage and catalysis for sustainable energy production. Despite the many similarities in structure, composition, and synthetic methods and uses, the current literature on layered oxides is diverse and disconnected. A number of reviews can be found in the literature, but they are mostly focused on one of the particular subclasses of 2D oxides. This review attempts to bridge the knowledge gap between individual layered oxide types by summarizing recent developments in all important 2D oxide systems including supported ultrathin oxide films, layered clays and double hydroxides, layered perovskites, and novel 2D‐zeolite‐based materials. Particular attention is paid to the underlying similarities and differences between the various materials, and the subsequent challenges faced by each research community. The potential of layered oxides toward future applications is critically evaluated, especially in the areas of electrocatalysis and photocatalysis, biomass conversion, and fine chemical synthesis. Attention is also paid to corresponding novel 3D materials that can be obtained via sophisticated engineering of 2D oxides.     

Charge‐Trap Memory Based on Hybrid 0D Quantum Dot–2D WSe2 Structure

Recently, layered ultrathin 2D semiconductors, such as MoS₂ and WSe₂ are widely studied in nonvolatile memories because of their excellent electronic properties. Additionally, discrete 0D metallic nanocrystals and quantum dots (QDs) are considered to be outstanding charge‐trap materials. Here, a charge‐trap memory device based on a hybrid 0D CdSe QD–2D WSe₂ structure is demonstrated. Specifically, ultrathin WSe₂ is employed as the channel of the memory, and the QDs serve as the charge‐trap layer. This device shows a large memory window exceeding 18 V, a high erase/program current ratio (reaching up to 10⁴), four‐level data storage ability, stable retention property, and high endurance of more than 400 cycles. Moreover, comparative experiments are carried out to prove that the charges are trapped by the QDs embedded in the Al₂O₃. The combination of 2D semiconductors with 0D QDs opens up a novelty field of charge‐trap memory devices.     

Graphene–Graphene Interactions: Friction,Superlubricity, and Exfoliation

Graphite's lubricating properties due to the “weak” interactions between individual layers have long been known. However, these interactions are not weak enough to allow graphite to readily exfoliate into graphene on a large scale. Separating graphite layers down to a single sheet is an intense area of research as scientists attempt to utilize graphene's superlative properties. The exfoliation and processing of layered materials is governed by the friction between layers. Friction on the macroscale can be intuitively understood, but there is little understanding of the mechanisms involved in nanolayered materials. Using molecular dynamics and a new forcefield, graphene's unusual behavior in a superlubric state is examined, and the energy dissipated between two such surfaces sliding past each other is shown. The dependence of friction on temperature and surface roughness is described, and agreement with experiment is reported. The accuracy of the simulated behavior enables the processes that drive exfoliation of graphite into individual graphene sheets to be described. Taking into account the friction between layers, a peeling mechanism of exfoliation is predicted to be of lower energy cost than shearing.     

Metal Oxide Semiconductors for Dye‐ and Quantum‐Dot‐Sensitized Solar Cells

This Review provides a brief summary of the most recent research developments in the synthesis and application of nanostructured metal oxide semiconductors for dye sensitized and quantum dot sensitized solar cells. In these devices, the wide bandgap semiconducting oxide acts as the photoanode, which provides the scaffold for light harvesters (either dye molecules or quantum dots) and electron collection. For this reason, proper tailoring of the optical and electronic properties of the photoanode can significantly boost the functionalities of the operating device. Optimization of the functional properties relies with modulation of the shape and structure of the photoanode, as well as on application of different materials (TiO₂, ZnO, SnO₂) and/or composite systems, which allow fine tuning of electronic band structure. This aspect is critical because it determines exciton and charge dynamics in the photoelectrochemical system and is strictly connected to the photoconversion efficiency of the solar cell. The different strategies for increasing light harvesting and charge collection, inhibiting charge losses due to recombination phenomena, are reviewed thoroughly, highlighting the benefits of proper photoanode preparation, and its crucial role in the development of high efficiency dye sensitized and quantum dot sensitized solar cells.