Xingxing Zhang

High‐purity 1T‐MoS₂ is successfully synthesized by the molten‐metal‐assisted intercalation (MMI) approach, which exploits the capillary action of molten potassium metal and the difference between the electron affinity of MoS₂ and the ionization potential of potassium. The potassium dopants maintain a high electron density in the Mo d orbitals. Consequently, the 1T‐MoS₂ (MMI) shows excellent phase stability and great promise as a non‐noble‐metal‐based electrocatalyst.

Abstract

The crystalline phase of layered transition metal dichalcogenides (TMDs) directly determines their material property. The most thermodynamically stable phase structures in TMDs are the semiconducting 2H and metastable metallic 1T phases. To overcome the low phase purity and instability of 1T‐TMDs, which limits the utilization of their intrinsic properties, various synthesis strategies for 1T‐TMDs have been proposed in phase‐engineering studies. Herein, a facile and scalable synthesis of 1T‐phase molybdenum disulfide (MoS₂) via the molten‐metal‐assisted intercalation (MMI) approach is introduced, which exploits the capillary action of molten potassium and the difference between the electron affinity of MoS₂ and the ionization potential of potassium. Highly reactive molten potassium metal can readily intercalate into the MoS₂ interlayers, inducing an efficient phase transition from the 2H to 1T crystal structure. The ionic bonding between the intercalated potassium and sulfur lowers the energy barrier of the 1T‐phase transition, enhancing the phase stability of the 1T crystals. Owing to the high purity and stability of the 1T phase, the electrocatalytic performance for the hydrogen evolution reaction is significantly higher in 1T‐MoS₂ (MMI) than in 2H‐MoS₂ and even in 1T‐MoS₂ synthesized using n‐butyllithium.

In article number https://doi.org/10.1002/adma.2020015712001571, Yi Du, Hongqiang Wang, and co‐workers report the development of a pioneering technology of pulsed laser irradiation for the production of supranano (<10 nm) liquid metal. The decoration of such liquid ternary supranano alloys as an electron mediator at grain boundaries promotes electron extraction and transfer in hybrid perovskite films, which leads to greatly enhanced photoconversion efficiency in perovskite solar cells of up to 22.03%.

Nanosurfactant‐stabilized 2D material inks are printed conformally on various 2D/3D substrates enabling rapid fabrication of complex device structures with high spatial resolution. Specifically, graphene quantum dot nanosurfactants not only provide electrostatic stabilization for a wide range of 2D materials, but also have become an integrated part of printed devices and result in superior mechanical/optoelectronic properties in 2D‐material‐based devices.

Abstract

Printing techniques using nanomaterials have emerged as a versatile tool for fast prototyping and potentially large‐scale manufacturing of functional devices. Surfactants play a significant role in many printing processes due to their ability to reduce interfacial tension between ink solvents and nanoparticles and thus improve ink colloidal stability. Here, a colloidal graphene quantum dot (GQD)‐based nanosurfactant is reported to stabilize various types of 2D materials in aqueous inks. In particular, a graphene ink with superior colloidal stability is demonstrated by GQD nanosurfactants via the π–π stacking interaction, leading to the printing of multiple high‐resolution patterns on various substrates using a single printing pass. It is found that nanosurfactants can significantly improve the mechanical stability of the printed graphene films compared with those of conventional molecular surfactant, as evidenced by 100 taping, 100 scratching, and 1000 bending cycles. Additionally, the printed composite film exhibits improved photoconductance using UV light with 400 nm wavelength, arising from excitation across the nanosurfactant bandgap. Taking advantage of the 3D conformal aerosol jet printing technique, a series of UV sensors of heterogeneous structures are directly printed on 2D flat and 3D spherical substrates, demonstrating the potential of manufacturing geometrically versatile devices based on nanosurfactant inks.

Hexagonal boron nitride is superior for application in van der Waals optoelectronics and electronics. Recent achievements in the synthesis of large‐scale, single‐crystal h‐BN film and its heterostructures via chemical vapor deposition are summarized and the growth kinetics are discussed, emphasizing that the specific orientation of the heterostructure constituents can introduce novel properties. Typical applications in electronics and optoelectronics are summarized.

Abstract

Atomically thin hexagonal boron nitride (h‐BN) is an emerging star of 2D materials. It is taken as an optimal substrate for other 2D‐material‐based devices owing to its atomical flatness, absence of dangling bonds, and excellent stability. Specifically, h‐BN is found to be a natural hyperbolic material in the mid‐infrared range, as well as a piezoelectric material. All the unique properties are beneficial for novel applications in optoelectronics and electronics. Currently, most of these applications are merely based on exfoliated h‐BN flakes at their proof‐of‐concept stages. Chemical vapor deposition (CVD) is considered as the most promising approach for producing large‐scale, high‐quality, atomically thin h‐BN films and heterostructures. Herein, CVD synthesis of atomically thin h‐BN is the focus. Also, the growth kinetics are systematically investigated to point out general strategies for controllable and scalable preparation of single‐crystal h‐BN film. Meanwhile, epitaxial growth of 2D materials onto h‐BN and at its edge to construct heterostructures is summarized, emphasizing that the specific orientation of constituent parts in heterostructures can introduce novel properties. Finally, recent applications of atomically thin h‐BN and its heterostructures in optoelectronics and electronics are summarized.

Single‐crystalline 2D/3D perovskite heterostructure with well‐defined interface is achieved via a delicate solution method. Strikingly, the resultant photodetector exhibits excellent performance at zero bias, including a large on/off switching radio (≈10⁵), high detectivity (≈10¹²), and fast response time (600/600 µs). This work paves a way to explore new single‐crystalline perovskite heterostructures for high‐performance optoelectronics.

Abstract

Organic–inorganic hybrid perovskite heterostructures, particularly 2D/3D perovskite heterostructures, have enjoyed great success in optoelectronic field. However, the photodetection performance of those heterostructures is obscured by extensive disorder in their polycrystalline films. Here, a delicate solution method is demonstrated for creating a sizable crystal of vertical 2D/3D perovskite heterostructure featuring a well‐defined interface and high single‐crystalline quality, namely (4‐AMP)(MA)₂Pb₃Br₁₀/MAPbBr₃ (MA, methylammonium; 4‐AMP, 4‐(aminomethyl)piperidinium). These high‐quality crystals are ideal mediums for charge transport and exploiting their optoelectronic properties. Electrical transport measurements demonstrate that the (4‐AMP)(MA)₂Pb₃Br₁₀/MAPbBr₃ heterostructure can form vertical diode with obvious current rectification behavior and excellent photocurrent generation characteristics. Strikingly, benefitting from the built‐in electrical potential at the junction, photodetectors based on those millimeter‐thickness heterostructure crystals exhibit high performance in self‐driven operation mode, including large on/off switching radio (≈10⁵), fast response time (600/600 µs), and high detectivity (≈10¹²). This work enables an important advance in the development of single‐crystalline perovskite heterostructures for both fundamental demonstrations and high‐performance optoelectronic devices.

A flexible large‐area light‐emitting device is realized by integrating a centimeter‐scale grown WS₂ monolayer into a p–n device architecture on conductive polymer foil. This flexible device demonstrates homogeneous red light emission from a 6 mm² area. Uniquely, the electroluminescence can be tuned over 30 meV simply by bending the devices, i.e., by applying a defined strain.

Abstract

Strong covalent in‐plane bonds and a tiny thickness in the nanometer range make two‐dimensional (2D) materials ideally suited for flexible electronic or optoelectronic applications. Despite this exciting perspective, only a few prototypes of such flexible devices—photodetectors and transistors—have been reported until now. The first large‐area flexible light‐emitting device (LED) based on 2D materials is realized by integrating a transition metal dichalcogenide (TMDC) monolayer synthesized by metal organic chemical vapor deposition (MOCVD) into a p–n architecture on conductive polymer foil. This flexible LED demonstrates homogeneous red light emission from a few square millimeter area in a scalable design. Uniquely, the electroluminescence can be tuned over 30 meV simply by bending the devices, i.e., by applying a defined strain. This approach combines the flexibility of organic semiconductor device concepts with the durability of inorganic semiconductor technology.

A thermomechanical lithography technique for direct nanocutting of 2D materials is demonstrated. A heated scanning nanotip performs the cutting of the 2D material by thermomechanically cleaving the chemical bonds in concert with the rapid sublimation of the polymer layer underneath. A resolution of 20 nm is obtained in monolayer MoTe₂, MoS₂, and MoSe₂.

Abstract

Atomically thin materials, such as graphene and transition metal dichalcogenides, are promising candidates for future applications in micro/nanodevices and systems. For most applications, functional nanostructures have to be patterned by lithography. Developing lithography techniques for 2D materials is essential for system integration and wafer‐scale manufacturing. Here, a thermomechanical indentation technique is demonstrated, which allows for the direct cutting of 2D materials using a heated scanning nanotip. Arbitrarily shaped cuts with a resolution of 20 nm are obtained in monolayer 2D materials, i.e., molybdenum ditelluride (MoTe₂), molybdenum disulfide (MoS₂), and molybdenum diselenide (MoSe₂), by thermomechanically cleaving the chemical bonds and by rapid sublimation of the polymer layer underneath the 2D material layer. Several micro/nanoribbon structures are fabricated and electrically characterized to demonstrate the process for device fabrication. The proposed direct nanocutting technique allows for precisely tailoring nanostructures of 2D materials with foreseen applications in the fabrication of electronic and photonic nanodevices.

Controlled uniform lattice substitution of a 2D Mo₂C superconductor by magnetic Cr atoms is performed through chemical vapor deposition, with the Cr concentration up to ≈46.9 at%. The controlled substitutional doping enables the tuning of the competition of the 2D superconductor and the Kondo effect across the whole sample, and the Kondo effect with a high Kondo temperature is achieved at high Cr concentration.

Abstract

Substitutional doping provides an effective strategy to tailor the properties of 2D materials, but it remains an open challenge to achieve tunable uniform doping, especially at high doping level. Here, uniform lattice substitution of a 2D Mo₂C superconductor by magnetic Cr atoms with controlled concentration up to ≈46.9 at% by chemical vapor deposition and a specifically designed Cu/Cr/Mo trilayer growth substrate is reported. The concentration of Cr atoms can be easily tuned by simply changing the thickness of the Cr layer, and the samples retain the original structure of 2D Mo₂C even at a very high Cr concentration. The controlled uniform Cr doping enables the tuning of the competition of the 2D superconductor and the Kondo effect across the whole sample. Transport measurements show that with increasing Cr concentration, the superconductivity of the 2D Cr‐doped Mo₂C crystals disappears along with the emergence of the Kondo effect, and the Kondo temperature increases monotonously. Using scanning tunneling microscopy/spectroscopy, the mechanism of the doping level effect on the interplay and evolution between superconductivity and the Kondo effect is revealed. This work paves a new way for the synthesis of 2D materials with widely tunable doping levels, and provides new understandings on the interplay between superconductivity and magnetism in the 2D limit.

Thermally evaporated Se _x Te_{1‐ x} alloy thin films with tunable bandgaps (from 0.31 eV to 1.87 eV) are prepared for the fabrication of high‐performance short‐wave infrared photodetectors. The Se_0.32Te_0.68‐film‐based photoconductor fabricated on an optical cavity substrate exhibits a cut‐off wavelength at ≈1.7 μm and gives a responsivity of 1.5 AW⁻¹ and implied detectivity of 6.5 × 10¹⁰ cm Hz^1/2 W⁻¹ at 1.55 μm at room temperature.

Abstract

Semiconducting absorbers in high‐performance short‐wave infrared (SWIR) photodetectors and imaging sensor arrays are dominated by single‐crystalline germanium and III–V semiconductors. However, these materials require complex growth and device fabrication procedures. Here, thermally evaporated Se _x Te_{1‐ x} alloy thin films with tunable bandgaps for the fabrication of high‐performance SWIR photodetectors are reported. From absorption measurements, it is shown that the bandgaps of Se _x Te_{1‐ x} films can be tuned continuously from 0.31 eV (Te) to 1.87 eV (Se). Owing to their tunable bandgaps, the peak responsivity position and photoresponse edge of Se _x Te_{1‐ x} film‐based photoconductors can be tuned in the SWIR regime. By using an optical cavity substrate consisting of Au/Al₂O₃ to enhance its absorption near the bandgap edge, the Se_0.32Te_0.68 film (an optical bandgap of ≈0.8 eV)‐based photoconductor exhibits a cut‐off wavelength at ≈1.7 μm and gives a responsivity of 1.5 AW⁻¹ and implied detectivity of 6.5 × 10¹⁰ cm Hz^1/2 W⁻¹ at 1.55 μm at room temperature. Importantly, the nature of the thermal evaporation process enables the fabrication of Se_0.32Te_0.68‐based 42 × 42 focal plane arrays with good pixel uniformity, demonstrating the potential of this unique material system used for infrared imaging sensor systems.

Exfoliated bilayer graphene on a SiO₂ substrate is electrically contacted in a back‐gate transistor geometry and investigated by spectroscopic ellipsometry while applying various gate voltages. The applied gate voltage changes the dielectric constant ε₁ of graphene substantially from 2.5 to 4.0. Theoretical calculations demonstrate that the values of the dielectric function correlate with the sheet charge carrier density.

Abstract

The refractive index and the extinction coefficient are usually inherent (noncontrollable) material characteristics. Recently, it was reported that the reflectivity of graphene in the mid‐infrared spectral range can be modified by an external bias. This report attracted much attention, but the controllable frequency/energy range is too narrow for possible applications. In this work, it is demonstrated that the potential of graphene is not limited to mid‐infrared wavelengths, but spans a much wider range including the visible spectral range. Here, back‐gated bilayer graphene is characterized in air using spectroscopic ellipsometry with a lateral resolution in the micrometer range. By applying a back‐gate voltage, the dielectric function can be modified in a broad spectral range, including the visible spectrum. To explain the change in the dielectric function, a simplified phenomenological approach which assumes that the back‐gating‐induced change in the carrier density of graphene can be described by a modified 2D Drude model is introduced. The trend of increasing values for the dielectric function with increasing sheet charge carrier density is confirmed by theoretical calculations performed in the independent particle picture.

Terraced graphene, formed by laminating single‐layer graphene on a terraced substrate, shows a colossal magnetoresistance of up to 5000% at 9 T and 300 K. The magnetoresistance enhancement is attributed to the topographic corrugations and inhomogeneous charge puddles induced by the terraced structure. The concept of inducing colossal magnetoresistance by stepped surfaces is certainly boosting room‐temperature graphene magnetic sensors.

Abstract

Disorder‐induced magnetoresistance (MR) effect is quadratic at low perpendicular magnetic fields and linear at high fields. This effect is technologically appealing, especially in 2D materials such as graphene, since it offers potential applications in magnetic sensors with nanoscale spatial resolution. However, it is a great challenge to realize a graphene magnetic sensor based on this effect because of the difficulty in controlling the spatial distribution of disorder and enhancing the MR sensitivity in the single‐layer regime. Here, a room‐temperature colossal MR of up to 5000% at 9 T is reported in terraced single‐layer graphene. By laminating single‐layer graphene on a terraced substrate, such as TiO₂‐terminated SrTiO₃, a universal one order of magnitude enhancement in the MR compared to conventional single‐layer graphene devices is demonstrated. Strikingly, a colossal MR of >1000% is also achieved in the terraced graphene even at a high carrier density of ≈10¹² cm⁻². Systematic studies of the MR of single‐layer graphene on various oxide‐ and non‐oxide‐based terraced surfaces demonstrate that the terraced structure is the dominant factor driving the MR enhancement. The results open a new route for tailoring the physical property of 2D materials by engineering the strain through a terraced substrate.

An outstanding feature of the topological Weyl semimetal WTe₂ is its novel spin topologies in the electronic band structure. An unconventional charge–spin conversion in WTe₂ due to its lower crystal symmetry combined with large Berry curvature and spin‐texture of the Fermi states is demonstrated. These findings have great potential for utilizing WTe₂ for spintronic circuits and quantum technologies.

Abstract

An outstanding feature of topological quantum materials is their novel spin topology in the electronic band structures with an expected large charge‐to‐spin conversion efficiency. Here, a charge‐current‐induced spin polarization in the type‐II Weyl semimetal candidate WTe₂ and efficient spin injection and detection in a graphene channel up to room temperature are reported. Contrary to the conventional spin Hall and Rashba–Edelstein effects, the measurements indicate an unconventional charge‐to‐spin conversion in WTe₂, which is primarily forbidden by the crystal symmetry of the system. Such a large spin polarization can be possible in WTe₂ due to a reduced crystal symmetry combined with its large spin Berry curvature, spin–orbit interaction with a novel spin‐texture of the Fermi states. A robust and practical method is demonstrated for electrical creation and detection of such a spin polarization using both charge‐to‐spin conversion and its inverse phenomenon and utilized it for efficient spin injection and detection in the graphene channel up to room temperature. These findings open opportunities for utilizing topological Weyl materials as nonmagnetic spin sources in all‐electrical van der Waals spintronic circuits and for low‐power and high‐performance nonvolatile spintronic technologies.

CrSBr is an air‐stable, intrinsically magnetic, van der Waals semiconductor with an electronic bandgap ∆_E = 1.5 ± 0.2 eV and photoluminescence peak centered at 1.25 ± 0.07 eV. Magnetometry and magnetotransport measurements demonstrate that CrSBr exhibits intraplanar ferromagnetic ordering and interplanar antiferromagnetic ordering below T _N ≈ 132 ± 1 K, producing a large intrinsic negative magnetoresistance.

Abstract

The recent discovery of magnetism within the family of exfoliatable van der Waals (vdW) compounds has attracted considerable interest in these materials for both fundamental research and technological applications. However, current vdW magnets are limited by their extreme sensitivity to air, low ordering temperatures, and poor charge transport properties. Here the magnetic and electronic properties of CrSBr are reported, an air‐stable vdW antiferromagnetic semiconductor that readily cleaves perpendicular to the stacking axis. Below its Néel temperature, T _N = 132 ± 1 K, CrSBr adopts an A‐type antiferromagnetic structure with each individual layer ferromagnetically ordered internally and the layers coupled antiferromagnetically along the stacking direction. Scanning tunneling spectroscopy and photoluminescence (PL) reveal that the electronic gap is Δ_E = 1.5 ± 0.2 eV with a corresponding PL peak centered at 1.25 ± 0.07 eV. Using magnetotransport measurements, strong coupling between magnetic order and transport properties in CrSBr is demonstrated, leading to a large negative magnetoresistance response that is unique among vdW materials. These findings establish CrSBr as a promising material platform for increasing the applicability of vdW magnets to the field of spin‐based electronics.