Gate‐Controlled BP–WSe2 Heterojunction Diode for Logic Rectifiers and Logic Optoelectronics

p–n junctions play an important role in modern semiconductor electronics and optoelectronics, and field‐effect transistors are often used for logic circuits. Here, gate‐controlled logic rectifiers and logic optoelectronic devices based on stacked black phosphorus (BP) and tungsten diselenide (WSe₂) heterojunctions are reported. The gate‐tunable ambipolar charge carriers in BP and WSe₂ enable a flexible, dynamic, and wide modulation on the heterojunctions as isotype (p–p and n–n) and anisotype (p–n) diodes, which exhibit disparate rectifying and photovoltaic properties. Based on such characteristics, it is demonstrated that BP–WSe₂ heterojunction diodes can be developed for high‐performance logic rectifiers and logic optoelectronic devices. Logic optoelectronic devices can convert a light signal to an electric one by applied gate voltages. This work should be helpful to expand the applications of 2D crystals.     

Catalyst‐Selective Growth of Single‐Orientation Hexagonal Boron Nitride toward High‐Performance Atomically Thin Electric Barriers

The ability to control the crystal orientation of 2D van der Waals (vdW) layered materials grown on large‐scale substrates is crucial for tailoring their electrical properties, as well as for integration of functional 2D devices. In general, multiple orientations, i.e., two or four orientations, appear through the crystal rotational symmetry matching between the material and its substrate. Here, it is reported that hexagonal boron nitride (h‐BN), an ideal electric barrier in the family of 2D materials, has a single orientation on inclined Cu (1 0 1) surfaces, where the Cu planes are tilted from the (1 0 1) facet around specific in‐plane axes. Density functional theory (DFT) calculation indicates that this is a manifestation of only one favored h‐BN orientation with the minimum vdW energy on the inclined Cu (1 0 1) surface. Moreover, thanks to the high interfacial strength with the underlying Cu, the single‐orientation h‐BN is free of thermal wrinkles, and exhibits a spatially homogeneous morphology and tunnel conductance. The findings point to a feasible approach to direct growth of single‐orientation, wrinkle‐free h‐BN thin film for high‐performance 2D electrical devices, and will be of benefit for controllable synthesis of other vdW materials.     

Two‐Dimensional Materials for Halide Perovskite‐Based Optoelectronic Devices

Halide perovskites have high light absorption coefficients, long charge carrier diffusion lengths, intense photoluminescence, and slow rates of non‐radiative charge recombination. Thus, they are attractive photoactive materials for developing high‐performance optoelectronic devices. These devices are also cheap and easy to be fabricated. To realize the optimal performances of halide perovskite‐based optoelectronic devices (HPODs), perovskite photoactive layers should work effectively with other functional materials such as electrodes, interfacial layers and encapsulating films. Conventional two‐dimensional (2D) materials are promising candidates for this purpose because of their unique structures and/or interesting optoelectronic properties. Here, we comprehensively summarize the recent advancements in the applications of conventional 2D materials for halide perovskite‐based photodetectors, solar cells and light‐emitting diodes. The examples of these 2D materials are graphene and its derivatives, mono‐ and few‐layer transition metal dichalcogenides (TMDs), graphdiyne and metal nanosheets, etc. The research related to 2D nanostructured perovskites and 2D Ruddlesden–Popper perovskites as efficient and stable photoactive layers is also outlined. The syntheses, functions and working mechanisms of relevant 2D materials are introduced, and the challenges to achieving practical applications of HPODs using 2D materials are also discussed.     

Bifacial Contact Junction Engineering for High‐Performance Perovskite Solar Cells with Efficiency Exceeding 21%

Ordered 1D metal oxide structure is desirable in thin film solar cells owing to its excellent charge collection capability. However, the electron transfer in 1D electron transporting layer (ETL)‐based devices is still limited to a submicrometer‐long pathway that is vertical to the substrate. Here, an innovative closely packed rutile TiO₂ nanowire (CRTNW) network parallel to the facet of fluorine‐doped tin oxide (FTO) substrate is reported, which can serve as a 1D nanoscale electron transport pathway for efficient perovskite solar cells (PSCs). The PSC constructed using newly prepared CRTNW ETL achieves an impressive power conversion efficiency of 21.10%, which can be attributed to the facilitated electron extraction induced by the favorable junctions formed at FTO/ETL and ETL/perovskite interfaces and also the suppressed charge recombination originating from improved perovskite morphology with large grains, flat surface, and good surface coverage. The bifacial contact junctions engineering also enables large‐area device fabrication. The PSC with 1 cm² aperture yields an efficiency of 19.50% under one sun illumination. This work highlights the significance of controlling the orientation and packing density of the ordered 1D oxide nanostructured thin films for highly efficient optoelectronic devices in a large‐scale manner.     

Multifunctional van der Waals Broken‐Gap Heterojunction

The finite energy band‐offset that appears between band structures of employed materials in a broken‐gap heterojunction exhibits several interesting phenomena. Here, by employing a black phosphorus (BP)/rhenium disulfide (ReS₂) heterojunction, the tunability of the BP work function (Φ _BP) with variation in flake thickness is exploited in order to demonstrate that a BP‐based broken‐gap heterojunction can manifest diverse current‐transport characteristics such as gate tunable rectifying p–n junction diodes, Esaki diodes, backward‐rectifying diodes, and nonrectifying devices as a consequence of diverse band‐bending at the heterojunction. Diversity in band‐bending near heterojunction is attributed to change in the Fermi level difference (Δ) between BP and ReS₂ sides as a consequence of Φ _BP modulation. No change in the current transport characteristics in several devices with fixed Δ also provides further evidence that current‐transport is substantially impacted by band‐bending at the heterojunction. Optoelectronic experiments on the Esaki diode and the p–n junction diode provide experimental evidence of band‐bending diversity. Additionally, the p⁺–n–p junction comprising BP (38 nm)/ReS₂/BP(5.8 nm) demonstrates multifunctionality of binary and ternary inverters as well as exhibiting the behavior of a bipolar junction transistor with common‐emitter current gain up to 50.     

Recent Advances of Spatial Self‐Phase Modulation in 2D Materials and Passive Photonic Device Applications

Optical nonlinearity in 2D materials excited by spatial Gaussian laser beam is a novel and peculiar optical phenomenon, which exhibits many novel and interesting applications in optical nonlinear devices. Passive photonic devices, such as optical switches, optical logical gates, photonic diodes, and optical modulators, are the key compositions in the future all‐optical signal‐processing technologies. Passive photonic devices using 2D materials to achieve the device functionality have attracted widespread concern in the past decade. In this Review, an overview of the spatial self‐phase modulation (SSPM) in 2D materials is summarized, including the operating mechanism, optical parameter measurement, and tuning for 2D materials, and applications in photonic devices. Moreover, some current challenges are also proposed to solve, and some possible applications of SSPM method are predicted for the future. Therefore, it is anticipated that this summary can contribute to the application of 2D material‐based spatial effect in all‐optical signal‐processing technologies.     

Lateral Monolayer MoSe2–WSe2 p–n Heterojunctions with Giant Built‐In Potentials

2D transition metal dichalcogenides (TMDs) have exhibited strong application potentials in new emerging electronics because of their atomic thin structure and excellent flexibility, which is out of field of tradition silicon technology. Similar to 3D p–n junctions, 2D p–n heterojunctions by laterally connecting TMDs with different majority charge carriers (electrons and holes), provide ideal platform for current rectifiers, light‐emitting diodes, diode lasers and photovoltaic devices. Here, growth and electrical studies of atomic thin high‐quality p–n heterojunctions between molybdenum diselenide (MoSe₂) and tungsten diselenide (WSe₂) by one‐step chemical vapor deposition method are reported. These p–n heterojunctions exhibit high built‐in potential (≈0.7 eV), resulting in large current rectification ratio without any gate control for diodes, and fast response time (≈6 ms) for self‐powered photodetectors. The simple one‐step growth and electrical studies of monolayer lateral heterojunctions open up the possibility to use TMD heterojunctions for functional devices.     

Chemically Addressable Perovskite Nanocrystals for Light‐Emitting Applications

Whereas organic–inorganic hybrid perovskite nanocrystals (PNCs) have remarkable potential in the development of optoelectronic materials, their relatively poor chemical and colloidal stability undermines their performance in optoelectronic devices. Herein, this issue is addressed by passivating PNCs with a class of chemically addressable ligands. The robust ligands effectively protect the PNC surfaces, enhance PNC solution processability, and can be chemically addressed by thermally induced crosslinking or radical‐induced polymerization. This thin polymer shield further enhances the photoluminescence quantum yields by removing surface trap states. Crosslinked methylammonium lead bromide (MAPbBr₃) PNCs are applied as active materials to build light‐emitting diodes that have low turn‐on voltages and achieve a record luminance of over 7000 cd m^?2, around threefold better than previous reported MA‐based PNC devices. These results indicate the great potential of this ligand passivation approach for long lifespan, highly efficient PNC light emitters.     

Mapping Energy Levels for Organic Heterojunctions

An organic semiconductor thin film is a solid‐state matter comprising one or more molecules. For applications in electronics and photonics, several distinct functional organic thin films are stacked together to create a variety of devices such as organic light‐emitting diodes and organic solar cells. The energy levels at these thin‐film junctions dictate various electronic processes such as the charge transport across these junctions, the exciton dissociation rates at donor–acceptor molecular interfaces, and the charge trapping during exciton formation in a host–dopant system. These electronic processes are vital to a device's performance and functionality. To uncover a general scientific principle in governing the interface energy levels, highest occupied molecular orbitals, and vacuum level dipoles, herein a comprehensive experimental research is conducted on several dozens of organic–organic heterojunctions representative of various device applications. It is found that the experimental data map on interface energy levels, after correcting variables such as molecular orientation‐dependent ionization energies, consists of three distinct regions depending on interface fundamental physical parameters such as Fermi energy, work function, highest occupied molecular orbitals, and lowest unoccupied molecular orbitals. This general energy map provides a master guide in selection of new materials for fabricating future generations of organic semiconductor devices.     

Orientation Engineering in Low‐Dimensional Crystal‐Structural Materials via Seed Screening

The orientation of low‐dimensional crystal‐structural (LDCS) films significantly affects the performance of photoelectric devices, particularly in vertical conducting devices such as solar cells and light‐emitting diodes. According to film growth theory, the initial seeds determine the final orientation of the film. Ruled by the minimum energy principle, lying (chains or layers parallel to the substrate) seeds bonding with the substrate through van der Waals forces are easier to form than standing (chains or layers perpendicular to the substrate) seeds bonding with the substrate by a covalent bond. Utilizing high substrate temperature to re‐evaporate the lying seeds and preserve the standing seeds, the orientation of 1D crystal‐structural Sb₂Se₃ is successfully controlled. Guided by this seed screening model, highly [211]‐ and [221]‐oriented Sb₂Se₃ films on an inert TiO₂ substrate are obtained; consequently, a record efficiency of 7.62% in TiO₂/Sb₂Se₃ solar cells is achieved. This universal model of seed screening provides an effective method for orientation control of other LDCS films.     

Perovskite Materials for Light‐Emitting Diodes and Lasers

Organic–inorganic hybrid perovskites have cemented their position as an exceptional class of optoelectronic materials thanks to record photovoltaic efficiencies of 22.1%, as well as promising demonstrations of light‐emitting diodes, lasers, and light‐emitting transistors. Perovskite materials with photoluminescence quantum yields close to 100% and perovskite light‐emitting diodes with external quantum efficiencies of 8% and current efficiencies of 43 cd A^?1 have been achieved. Although perovskite light‐emitting devices are yet to become industrially relevant, in merely two years these devices have achieved the brightness and efficiencies that organic light‐emitting diodes accomplished in two decades. Further advances will rely decisively on the multitude of compositional, structural variants that enable the formation of lower‐dimensionality layered and three‐dimensional perovskites, nanostructures, charge‐transport materials, and device processing with architectural innovations. Here, the rapid advancements in perovskite light‐emitting devices and lasers are reviewed. The key challenges in materials development, device fabrication, operational stability are addressed, and an outlook is presented that will address market viability of perovskite light‐emitting devices.     

Infrared Organic Light‐Emitting Diodes with Carbon Nanotube Emitters

While organic light‐emitting diodes (OLEDs) covering all colors of the visible spectrum are widespread, suitable organic emitter materials in the near‐infrared (nIR) beyond 800 nm are still lacking. Here, the first OLED based on single‐walled carbon nanotubes (SWCNTs) as the emitter is demonstrated. By using a multilayer stacked architecture with matching charge blocking and charge‐transport layers, narrow‐band electroluminescence at wavelengths between 1000 and 1200 nm is achieved, with spectral features characteristic of excitonic and trionic emission of the employed (6,5) SWCNTs. Here, the OLED performance is investigated in detail and it is found that local conduction hot‐spots lead to pronounced trion emission. Analysis of the emissive dipole orientation shows a strong horizontal alignment of the SWCNTs with an average inclination angle of 12.9° with respect to the plane, leading to an exceptionally high outcoupling efficiency of 49%. The SWCNT‐based OLEDs represent a highly attractive platform for emission across the entire nIR.     

Rotating‐Electric‐Field‐Induced Carbon‐Nanotube‐Based Nanomotor in Water: A Molecular Dynamics Study

Using molecular dynamics simulations, it is shown that a carbon nanotube (CNT) suspended in water and subjected to a rotating electric field of proper magnitude and angular speed can be rotated with the aid of water dipole orientations. Based on this principle, a rotational nanomotor structure is designed and the system is simulated in water. Use of the fast responsiveness of electric‐field‐induced CNT orientation in water is employed and its operation at ultrahigh‐speed (over 10¹¹ r.p.m.) is shown. To explain the basic mechanism, the behavior of the rotational actuation, originated from the water dipole orientation, is also analyzed . The proposed nanomotor is capable of rotating an attached load (such as CNT) at a precise angle as well as nanogear‐based complex structures. The findings suggest a potential way of using the electric‐field‐induced CNT rotation in polarizable fluids as a novel tool to operate nanodevices and systems.     

Electrically Pumped White‐Light‐Emitting Diodes Based on Histidine‐Doped MoS2 Quantum Dots

MoS₂ quantum dots (QDs)‐based white‐light‐emitting diodes (QD‐WLEDs) are designed, fabricated, and demonstrated. The highly luminescent, histidine‐doped MoS₂ QDs synthesized by microwave induced fragmentation of 2D MoS₂ nanoflakes possess a wide distribution of available electronic states as inferred from the pronounced excitation‐wavelength‐dependent emission properties. Notably, the histidine‐doped MoS₂ QDs show a very strong emission intensity, which exceeds seven times of magnitude larger than that of pristine MoS₂ QDs. The strongly enhanced emission is mainly attributed to nitrogen acceptor bound excitons and passivation of defects by histidine‐doping, which can enhance the radiative recombination drastically. The enabled electroluminescence (EL) spectra of the QD‐WLEDs with the main peak around 500 nm are found to be consistent with the photoluminescence spectra of the histidine‐doped MoS₂ QDs. The enhanced intensity of EL spectra with the current increase shows the stability of histidine‐doped MoS₂ based QD‐WLEDs. The typical EL spectrum of the novel QD‐WLEDs has a Commission Internationale de l'Eclairage chromaticity coordinate of (0.30, 0.36) exhibiting an intrinsic broadband white‐light emission. The unprecedented and low‐toxicity QD‐WLEDs based on a single light‐emitting material can serve as an excellent alternative for using transition metal dichalcogenides QDs as next generation optoelectronic devices.     

The Road Towards Planar Microbatteries and Micro‐Supercapacitors: From 2D to 3D Device Geometries

The rapid development and further modularization of miniaturized and self‐powered electronic systems have substantially stimulated the urgent demand for microscale electrochemical energy storage devices, e.g., microbatteries (MBs) and micro‐supercapacitors (MSCs). Recently, planar MBs and MSCs, composed of isolated thin‐film microelectrodes with extremely short ionic diffusion path and free of separator on a single substrate, have become particularly attractive because they can be directly integrated with microelectronic devices on the same side of one single substrate to act as a standalone microsized power source or complement miniaturized energy‐harvesting units. The development of and recent advances in planar MBs and MSCs from the fundamentals and design principle to the fabrication methods of 2D and 3D planar microdevices in both in‐plane and stacked geometries are highlighted. Additonally, a comprehensive analysis of the primary aspects that eventually affect the performance metrics of microscale energy storage devices, such as electrode materials, electrolyte, device architecture, and microfabrication techniques are presented. The technical challenges and prospective solutions for high‐energy‐density planar MBs and MSCs with multifunctionalities are proposed.     

3D Interdigital Au/MnO2/Au Stacked Hybrid Electrodes for On‐Chip Microsupercapacitors

On‐chip microsupercapacitors (MSCs) have application in powering microelectronic devices. Most of previous MSCs are made from carbon materials, which have high power but low energy density. In this work, 3D interdigital Au/MnO₂/Au stacked MSCs have been fabricated based on laser printed flexible templates. This vertical‐stacked electrode configuration can effectively increase the contact area between MnO₂ active layer and Au conductive layer, and thus improve the electron transport and electrolyte ion diffusion, resulting in enhanced pseudocapacitive performance of MnO₂. The stacked electrode can achieve an areal capacitance up to 11.9 mF cm^?2. Flexible and all‐solid‐state MSCs are assembled based on the sandwich hybrid electrodes and PVA/LiClO₄ gel electrolyte and show outstanding high‐rate capacity and mechanical flexibility. The laser printing technique in this work combined with the physical sputtering and electrodeposition allows fabrication of MSC array with random sizes and patterns, making them promising power sources for small‐scale flexible microelectronic energy storage systems (e.g., next‐generation smart phones).     

Birefringence‐Directed Raman Selection Rules in 2D Black Phosphorus Crystals

The incident and scattered light engaged in the Raman scattering process of low symmetry crystals always suffer from the birefringence‐induced depolarization. Therefore, for anisotropic crystals, the classical Raman selection rules should be corrected by taking the birefringence effect into consideration. The appearance of the 2D anisotropic materials provides an excellent platform to explore the birefringence‐directed Raman selection rules, due to its controllable thickness at the nanoscale that greatly simplifies the situation comparing with bulk materials. Herein, a theoretical and experimental investigation on the birefringence‐directed Raman selection rules in the anisotropic black phosphorus (BP) crystals is presented. The abnormal angle‐dependent polarized Raman scattering of the A_g modes in thin BP crystal, which deviates from the normal Raman selection rules, is successfully interpreted by the theoretical model based on birefringence. It is further confirmed by the examination of different Raman modes using different laser lines and BP samples of different thicknesses.     

Assembly and Self‐Assembly of Nanomembrane Materials—From 2D to 3D

Nanoscience and nanotechnology offer great opportunities and challenges in both fundamental research and practical applications, which require precise control of building blocks with micro/nanoscale resolution in both individual and mass‐production ways. The recent and intensive nanotechnology development gives birth to a new focus on nanomembrane materials, which are defined as structures with thickness limited to about one to several hundred nanometers and with much larger (typically at least two orders of magnitude larger, or even macroscopic scale) lateral dimensions. Nanomembranes can be readily processed in an accurate manner and integrated into functional devices and systems. In this Review, a nanotechnology perspective of nanomembranes is provided, with examples of science and applications in semiconductor, metal, insulator, polymer, and composite materials. Assisted assembly of nanomembranes leads to wrinkled/buckled geometries for flexible electronics and stacked structures for applications in photonics and thermoelectrics. Inspired by kirigami/origami, self‐assembled 3D structures are constructed via strain engineering. Many advanced materials have begun to be explored in the format of nanomembranes and extend to biomimetic and 2D materials for various applications. Nanomembranes, as a new type of nanomaterials, allow nanotechnology in a controllable and precise way for practical applications and promise great potential for future nanorelated products.     

Fabrication and Doping Methods for Silicon Nano‐ and Micropillar Arrays for Solar‐Cell Applications: A Review

Silicon is one of the main components of commercial solar cells and is used in many other solar‐light‐harvesting devices. The overall efficiency of these devices can be increased by the use of structured surfaces that contain nanometer‐ to micrometer‐sized pillars with radial p/n junctions. High densities of such structures greatly enhance the light‐absorbing properties of the device, whereas the 3D p/n junction geometry shortens the diffusion length of minority carriers and diminishes recombination. Due to the vast silicon nano‐ and microfabrication toolbox that exists nowadays, many versatile methods for the preparation of such highly structured samples are available. Furthermore, the formation of p/n junctions on structured surfaces is possible by a variety of doping techniques, in large part transferred from microelectronic circuit technology. The right choice of doping method, to achieve good control of junction depth and doping level, can contribute to an improvement of the overall efficiency that can be obtained in devices for energy applications. A review of the state‐of‐the‐art of the fabrication and doping of silicon micro and nanopillars is presented here, as well as of the analysis of the properties and geometry of thus‐formed 3D‐structured p/n junctions.     

Polar‐Electrode‐Bridged Electroluminescent Displays: 2D Sensors Remotely Communicating Optically

A novel geometry for electroluminescent devices, which does not require transparent electrodes for electrical input, is demonstrated, theoretically analyzed, and experimentally characterized. Instead of emitting light through a conventional electrode, light emission occurs through a polar liquid or solid and input electrical electrodes are coplanar, rather than stacked in a sandwich configuration. This new device concept is scalable and easily deployed for a range of modular alternating‐current‐powered electroluminescent light sources and light‐emitting sensing devices. The polar‐electrode‐bridged electroluminescent displays can be used as remotely readable, spatially responsive sensors that emit light in response to the accumulation and distribution of materials on the device surface. Using this device structure, various types of alternating current devices are demonstrated. These include an umbrella that automatically lights up when it rains, a display that emits light from regions touched by human fingers (or painted upon using a mixture of oil and water), and a sensor that lights up differently in different areas to indicate the presence of water and its freezing. This study extends the dual‐stack, coplanar‐electrode device geometry to provide displays that emit light from a figure drawn on an electroluminescent panel using a graphite pencil.