Xingxing Zhang

The solution processing of graphene typically requires the dissociation and suspension of nanosheets, leading to a reduction in nanocomposite performance. In article number 1900738, Maria Crespo, Emiliano Bilotti, Julien E. Gautrot, and co‐workers propose photo‐responsive polymer brushes to control the solution stability of graphene dispersions, whilst enabling on‐demand deprotection of graphene nanomaterials and recovery of electronic and mechanical properties.

A proton‐gated transparent and flexible transistor with a WO₃ thin film on mica is fabricated using a rubbery solid ionic gel. Two distinct hydrogenated metastable phases and phase separation are demonstrated in electrically controlled metal–insulator transition (MIT). A prototype of a flexible vacuum meter is demonstrated on the basis of the unique vacuum‐dependent MIT, paving a feasible way to realize user‐friendly flexible electronics.

Abstract

Electrolyte gating is widely adopted to electrically control the physical properties of materials, leading to numerous intriguing phenomena and various applications. However, the carrier modulation mechanism remains heavily controversial. Herein, using natural mica pieces as substrates and ionic gel as the dielectric layer, all‐transparent and flexible WO₃ transistor configuration is designed to in situ monitor the dynamic doping process of electrolyte gating. A reversible and vacuum‐dominant volatile/nonvolatile metal–insulator transition (MIT) is observed in electrolyte‐gated WO₃ thin films. In situ X‐ray diffraction experiments, together with first‐principles calculations, reveal an abrupt and symmetric structural evolution through two distinct hydrogenated metastable phases and phase separation progress. The fast volatility is assigned to a spontaneous dehydrogenation process. A prototype of a flexible vacuum meter is demonstrated on the basis of the unique vacuum‐dependent MIT, exhibiting a measurement range down to 1.0 × 10⁻⁶ mbar and no injury of electromagnetic radiation. These findings bring new insights into hydrogenation dynamics, paving a feasible way for the realization of user‐friendly flexible electronics.

Nature Communications, Published online: 13 June 2019; doi:10.1038/s41467-019-10740-w

Interfacial water plays a crucial role in mediating hydrophobic interactions. Here, the authors directly image the interfacial water organization in graphene, few-layer MoS2 and WSe2 through 3D-AFM technique to unveil that the distance between adjacent layers is about 0.30 nm larger than theoretically predicted values.

Nature Communications, Published online: 14 June 2019; doi:10.1038/s41467-019-10610-5

Insulating hexagonal boron nitride (hBN) is theoretically expected to undergo a crossover to a direct bandgap in the monolayer limit. Here, the authors perform optical spectroscopy measurements on atomically thin epitaxial hBN providing indications of the presence of a direct gap of energy 6.1 eV in the single atomic layer.

Nature Nanotechnology, Published online: 10 June 2019; doi:10.1038/s41565-019-0466-2

Strain-induced phase change in MoTe2 enables reversible channel conductivity switching in a field-effect transistor geometry.

Two-dimensional (2D) transition metal dichalcogenides (TMDs) have very versatile chemical, electrical and optical properties. In particular, they exhibit rich and highly tunable electronic properties, with a bandgap that spans from semi-metallic up to 2 eV depending on the crystal phase, material composition, number of layers and even external stimulus. This paper provides an overview of the electronic devices and circuits based on 2D TMDs, such as Esaki diodes, resonant tunneling diodes (RTDs), logic and RF transistors, tunneling field-effect transistors (TFETs), static random access memories (SRAMs), dynamic RAM (DRAMs), flash memory, ferroelectric memories, resistitive memories and phase-change memories. We address the basic device principles, the advantages and limitations of these 2D electronic devices, and our perspectives on future developments.

A methodology for the synthesis of MoS_{2(1− x )}Se_{2 x} ternary alloys combined with a solution‐based large‐area compatible approach is developed. The relative concentration of bichalcogen atoms can be modulated by altering the selenization temperature, resulting in 4 in. scale production of MoS_{2(1− x )}Se_{2 x} alloys. The capability of the alloys for industrial applications in nanophotonic devices and hydrogen evolution reaction (HER) catalysts is validated.

Abstract

Despite many encouraging properties of transition metal dichalcogenides (TMDs), a central challenge in the realm of industrial applications based on TMD materials is to connect the large‐scale synthesis and reproducible production of highly crystalline TMD materials. Here, the primary aim is to resolve simultaneously the two inversely related issues through the synthesis of MoS_{2(1− x )}Se_{2 x} ternary alloys with customizable bichalcogen atomic (S and Se) ratio via atomic‐level substitution combined with a solution‐based large‐area compatible approach. The relative concentration of bichalcogen atoms in the 2D alloy can be effectively modulated by altering the selenization temperature, resulting in 4 in. scale production of MoS_1.62Se_0.38, MoS_1.37Se_0.63, MoS_1.15Se_0.85, and MoS_0.46Se_1.54 alloys, as well as MoS₂ and MoSe₂. Comprehensive spectroscopic evaluations for vertical and lateral homogeneity in terms of heteroatom distribution in the large‐scale 2D TMD alloys are implemented. Se‐stimulated strain effects and a detailed mechanism for the Se substitution in the MoS₂ crystal are further explored. Finally, the capability of the 2D alloy for industrial application in nanophotonic devices and hydrogen evolution reaction (HER) catalysts is validated. Substantial enhancements in the optoelectronic and HER performances of the 2D ternary alloy compared with those of its binary counterparts, including pure‐phase MoS₂ and MoSe₂, are unambiguously achieved.

A new crystal structure of WS₂ , 2M, is reported. It belongs to the 1T′‐phase family, members of which exhibit W–W zigzag chains along the b axis. Superconductivity with T _c of 8.8 K is reported in these 2M WS₂ crystals. Moreover, calculations show that a topological surface state exists on the their surface, making them potential candidates for topological superconductors.

Abstract

Recently the metastable 1T′‐type VIB‐group transition metal dichalcogenides (TMDs) have attracted extensive attention due to their rich and intriguing physical properties, including superconductivity, valleytronics physics, and topological physics. Here, a new layered WS₂ dubbed “2M” WS₂, is constructed from 1T′ WS₂ monolayers, is synthesized. Its phase is defined as 2M based on the number of layers in each unit cell and the subordinate crystallographic system. Intrinsic superconductivity is observed in 2M WS₂ with a transition temperature T _c of 8.8 K, which is the highest among TMDs not subject to any fine‐tuning process. Furthermore, the electronic structure of 2M WS₂ is found by Shubnikov–de Haas oscillations and first‐principles calculations to have a strong anisotropy. In addition, topological surface states with a single Dirac cone, protected by topological invariant Z₂, are predicted through first‐principles calculations. These findings reveal that the new 2M WS₂ might be an interesting topological superconductor candidate from the VIB‐group transition metal dichalcogenides.

Stacked chemical vapor deposition (CVD) graphene when heat‐treated results in a macroscale graphitic film with mixed AB/incommensurate stacking through each layer but near‐perfect in‐plane order. This graphitic film has mechanical performance greatly exceeding all macroscale layered graphitic films, and an exceptionally high in‐plane thermal conductivity.

Abstract

A macroscopic film (2.5 cm × 2.5 cm) made by layer‐by‐layer assembly of 100 single‐layer polycrystalline graphene films is reported. The graphene layers are transferred and stacked one by one using a wet process that leads to layer defects and interstitial contamination. Heat‐treatment of the sample up to 2800 °C results in the removal of interstitial contaminants and the healing of graphene layer defects. The resulting stacked graphene sample is a freestanding film with near‐perfect in‐plane crystallinity but a mixed stacking order through the thickness, which separates it from all existing carbon materials. Macroscale tensile tests yields maximum values of 62 GPa for the Young's modulus and 0.70 GPa for the fracture strength, significantly higher than has been reported for any other macroscale carbon films; microscale tensile tests yield maximum values of 290 GPa for the Young's modulus and 5.8 GPa for the fracture strength. The measured in‐plane thermal conductivity is exceptionally high, 2292 ± 159 W m⁻¹ K⁻¹ while in‐plane electrical conductivity is 2.2 × 10⁵ S m⁻¹. The high performance of these films is attributed to the combination of the high in‐plane crystalline order and unique stacking configuration through the thickness.

2D ternary transitional metal dichalcogenides have been spotlighted recently. A one‐step chemical vapor deposition (CVD) method to synthesize monolayer WTe_{2 x} S_{2(1− x )} alloys is reported. By tuning the ratio of chalcogen precursors and H₂ flow rate, both semiconducting 1H and metallic 1T′ structures can be obtained. Local displacement of Te atoms from the original 1H lattice sites is also observed and studied.

Abstract

Alloying 2D transition metal dichalcogenides has opened up new opportunities for bandgap engineering and phase control. Developing a simple and scalable synthetic route is therefore essential to explore the full potential of these alloys with tunable optical and electrical properties. Here, the direct synthesis of monolayer WTe_{2 x} S_{2(1− x )} alloys via one‐step chemical vapor deposition (CVD) is demonstrated. The WTe_{2 x} S_{2(1− x )} alloys exhibit two distinct phases (1H semiconducting and 1T ′ metallic) under different chemical compositions, which can be controlled by the ratio of chalcogen precursors as well as the H₂ flow rate. Atomic‐resolution scanning transmission electron microscopy–annular dark field (STEM‐ADF) imaging reveals the atomic structure of as‐formed 1H and 1T ′ alloys. Unlike the commonly observed displacement of metal atoms in the 1T ′ phase, local displacement of Te atoms from original 1H lattice sites is discovered by combined STEM‐ADF imaging and ab initio molecular dynamics calculations. The structure distortion provides new insights into the structure formation of alloys. This generic synthetic approach is also demonstrated for other telluride‐based ternary monolayers such as WTe_{2 x} Se_{2(1− x )} single crystals.

A new crystal structure of WS₂ , 2M, is reported. It belongs to the 1T′‐phase family, members of which exhibit W–W zigzag chains along the b axis. Superconductivity with T _c of 8.8 K is reported in these 2M WS₂ crystals. Moreover, calculations show that a topological surface state exists on the their surface, making them potential candidates for topological superconductors.

Abstract

Recently the metastable 1T′‐type VIB‐group transition metal dichalcogenides (TMDs) have attracted extensive attention due to their rich and intriguing physical properties, including superconductivity, valleytronics physics, and topological physics. Here, a new layered WS₂ dubbed “2M” WS₂, is constructed from 1T′ WS₂ monolayers, is synthesized. Its phase is defined as 2M based on the number of layers in each unit cell and the subordinate crystallographic system. Intrinsic superconductivity is observed in 2M WS₂ with a transition temperature T _c of 8.8 K, which is the highest among TMDs not subject to any fine‐tuning process. Furthermore, the electronic structure of 2M WS₂ is found by Shubnikov–de Haas oscillations and first‐principles calculations to have a strong anisotropy. In addition, topological surface states with a single Dirac cone, protected by topological invariant Z₂, are predicted through first‐principles calculations. These findings reveal that the new 2M WS₂ might be an interesting topological superconductor candidate from the VIB‐group transition metal dichalcogenides.

For many years, metal–semiconductor interfaces have suffered from defective interfaces, causing their energy band alignment to diverge from the Schottky–Mott rule. The van der Waals interface between graphene and transition metal dichalcogenide WSe₂ takes the Schottky–Mott rule from the textbook to the laboratory. Because of the lack of Fermi‐level pinning, in article number 1901392, Samuel W. LaGasse, Ji Ung Lee, and co‐workers achieve perfect tuning of the graphene–WSe₂ Schottky barrier.

Ultra‐broadband photodetectors based on noble transition metal dichalcogenide, PdSe₂, with unique pentagonal atomic structure are demonstrated. Devices respond from visible to mid‐infrared (up to ≈4.05 µm) and efficient absorption beyond 8 µm is observed. The maximum photoresponsivity and photogain are 708 A W⁻¹ and 82 700%, respectively. Anisotropic photoresponse is also observed.

Abstract

Photodetection over a broad spectral range is crucial for optoelectronic applications such as sensing, imaging, and communication. Herein, a high‐performance ultra‐broadband photodetector based on PdSe₂ with unique pentagonal atomic structure is reported. The photodetector responds from visible to mid‐infrared range (up to ≈4.05 µm), and operates stably in ambient and at room temperature. It promises improved applications compared to conventional mid‐infrared photodetectors. The highest responsivity and external quantum efficiency achieved are 708 A W⁻¹ and 82 700%, respectively, at the wavelength of 1064 nm. Efficient optical absorption beyond 8 µm is observed, indicating that the photodetection range can extend to longer than 4.05 µm. Owing to the low crystalline symmetry of layered PdSe₂, anisotropic properties of the photodetectors are observed. This emerging material shows potential for future infrared optoelectronics and novel devices in which anisotropic properties are desirable.

Scanning thermal microscopy is used to spatially map the temperature distribution within monolayer transition metal dichalcogenide devices upon dissipating electrical power across lateral interfaces. These findings demonstrate that lateral MoS₂–WS₂ heterojunctions form well‐stitched interfaces that have a modest effect on localization of the dissipated heat, while grain boundaries of MoS₂ exhibit various defects and lead to significant non‐uniform heating.

Abstract

Lateral heterogeneities in atomically thin 2D materials such as in‐plane heterojunctions and grain boundaries (GBs) provide an extrinsic knob for manipulating the properties of nano‐ and optoelectronic devices and harvesting novel functionalities. However, these heterogeneities have the potential to adversely affect the performance and reliability of the 2D devices through the formation of nanoscopic hot‐spots. In this report, scanning thermal microscopy (SThM) is utilized to map the spatial distribution of the temperature rise within monolayer transition metal dichalcogenide (TMD) devices upon dissipating a high electrical power through a lateral interface. The results directly demonstrate that lateral heterojunctions between MoS₂ and WS₂ do not largely impact the distribution of heat dissipation, while GBs of MoS₂ appreciably localize heating in the device. High‐resolution scanning transmission electron microscopy reveals that the atomic structure is nearly flawless around heterojunctions but can be quite defective near GBs. The results suggest that the interfacial atomic structure plays a crucial role in enabling uniform charge transport without inducing localized heating. Establishing such structure–property‐processing correlation provides a better understanding of lateral heterogeneities in 2D TMD systems which is crucial in the design of future all‐2D electronic circuitry with enhanced functionalities, lifetime, and performance.

The Schottky–Mott limit is studied in a dual‐gated graphene–WSe₂ heterojunction. Nearly ideal Schottky diode characteristics with extremely large gate tunability are demonstrated. The graphene–WSe₂ Schottky barrier height at each gate voltage is determined, showing one‐to‐one modulation, following the Schottky–Mott rule. These results have broad implications in contact engineering for 2D materials and optoelectronic devices.

Abstract

Metal–semiconductor interfaces, known as Schottky junctions, have long been hindered by defects and impurities. Such imperfections dominate the electrical characteristics of the junction by pinning the metal Fermi energy. Here, a graphene–WSe₂ p‐type Schottky junction, which exhibits a lack of Fermi level pinning, is studied. The Schottky junction displays near‐ideal diode characteristics with large gate tunability and small leakage currents. Using a gate electrostatically coupled to the WSe₂ channel to tune the Schottky barrier height, the Schottky–Mott limit is probed in a single device. As a special manifestation of the tunable Schottky barrier, a diode with a dynamically controlled ideality factor is demonstrated.

For the first time, ultrathin (≈7.4 nm) ε‐CaTe₂O₅ flakes are successfully synthesized employing soda lime glass as the capture substrate. The ε‐CaTe₂O₅ flakes display highly anisotropic band structures and optical properties. Low‐temperature electrical measurements show metal–semiconductor/insulator transition at about 130 K.

Abstract

Two‐dimensional (2D) ternary compounds (2DTCs) have attracted intensive attention due to the new degree of freedom of modulating physical and chemical properties. However, the controllable synthesis of 2DTCs still remains a great challenge impeding further research and applications. Here, for the first time, ultrathin (≈7.4 nm) ε‐CaTe₂O₅ flakes with high anisotropy are obtained by a chemical vapor deposition method using soda‐lime glass as the capture substrate. The molten glass adsorbs Te vapor in the gas flow to its surface, which reacts with CaO in the molten substrate leading to the precipitation of ε‐CaTe₂O₅. Interestingly, ε‐CaTe₂O₅ flakes display highly anisotropic band structures and optical properties. Furthermore, low‐temperature electrical measurements show that the metal–semiconductor/insulator transition of ε‐CaTe₂O₅ is exhibited at about 130 K, and optical phonon assisted hopping of small polarons becomes dominant within the temperature range of 130–300 K. Employing soda‐lime glass as the capture substrate may provide a new approach for the synthesis of 2DTCs.

2D hexagonal phase GaTe crystals are synthesized for the first time. Growth temperature is the key factor to realize phase selection between monoclinic and hexagonal. Further, controllable phase transition from hexagonal to monoclinic is achieved via fs‐laser irradiation in an arbitrary pattern design.

Abstract

GaTe is an important III–VI semiconductor with direct bandgap; thus, it holds great potential in the field of optoelectronics. Although it is known that GaTe can exist both in monoclinic and hexagonal phases, current studies are still exclusively restricted to the monoclinic phase of two dimensional (2D) GaTe owing to the difficulty in the fabrication of 2D hexagonal GaTe. Both monoclinic and hexagonal GaTe are demonstrated in this work, which can be selectively synthesized via a physical vapor deposition method, under precisely controlled growth temperatures. The pristine Raman and non‐linear optical properties of hexagonal GaTe has been systematically explored for the first time; moreover, a novel selected‐area phase transition from hexagonal to monoclinic of GeTe has been achieved via fs‐laser irradiation. This work may pave the way for widely use of 2D GaTe in various fields in future.