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Elemental phosphorus exhibits fascinating structural varieties and versatile properties. The unique nature of phosphorus bonds can lead to the formation of extremely complex structures, and detailed structural information on some phosphorus polymorphs is yet to be investigated. In this study, we investigated an unidentified crystalline phase of phosphorus, type‐II red phosphorus (RP), by combining state‐of‐the‐art structural characterization techniques. Electron diffraction tomography, atomic‐resolution scanning transmission electron microscopy (STEM), powder X‐ray diffraction, and Raman spectroscopy were concurrently used to elucidate the hidden structural motifs and their packing in type‐II RP. Electron diffraction tomography, performed using individual crystalline nanowires, was used to identify a triclinic unit cell with volume of 5330 Å3, which is the largest unit cell for elemental phosphorus crystals up to now and contains approximately 250 phosphorus atoms. Atomic‐resolution STEM imaging, which was performed along different crystal‐zone axes, confirmed that the twisted wavy tubular motif is the basic building block of type‐II RP. Our study discovered and presented a new variation of building blocks in phosphorus, and it provides insights to clarify the complexities observed in phosphorus as well as other relevant systems. [ABSTRACT FROM AUTHOR]

In 2D van der Waals (vdW) materials, rotational misalignment by a twist angle between adjacent layers can significantly affect their properties, referred as twistronics. Accurate tuning of twist angle of vdW materials in a controllable and efficient way is highly required. Here a thin‐film assisted transfer (TAT) technique that can controllably fabricate vdW structures with deterministic twist angles (accuracy of 0.37°) is developed. The transparent and ultrathin film as transfer medium ensures the visible and intact manipulation of monolayer building blocks, thus constructing vdW architectures with multi‐materials and designated position. Multilayer homo‐/hetero‐architectures in the geometry of series connection, spiral structure and chiral symmetry at deterministic twist angles are for the first time demonstrated. They can be located on various substrates for both structural analysis and device application. The capability of fabricating twisted homo‐/hetero‐ vdW structures in a deterministic, high‐throughput manner will serve as a powerful tool for twistronics. [ABSTRACT FROM AUTHOR]

2D semiconductor devices have been studied due to their unique potential in architecture and properties. As one of the unique devices approaches, 2D hetero‐stack channel field‐effect transistors (FETs) have recently been reported, but homo‐stack FETs are rare to find. Here, MoSe2/MoSe2 homo‐stack transistors are rather fabricated for study. Unlike the equivalently‐thick single MoSe2 FET, homo‐stack FETs show n‐type memory behavior that originates from stack interface‐induced traps. Particularly, when their stack interfaces are engineered by surface oxidation of bottom MoSe2, more stable nonvolatile memory behavior turns out. Short‐term ultraviolet ozone (UVO)‐induced oxidation only results in n‐type memory, but 15 min‐long oxidation surprisingly enables both n‐ and p‐type nonvolatile memory behavior due to nm‐thin MoOx embedded between upper and lower MoSe2. Furthermore, by alternating gate voltage pulse to the 15 min‐long UVO‐treated FETs, channel polarity conversion appears reversible in a small gate voltage (VGS) sweep range, which means that the channel type of a transistor can be reversibly modulated via stack interface engineering. It is believed that homo‐stack interface engineering must be one of the approaches to maximize the potential of 2D devices. [ABSTRACT FROM AUTHOR]

Low‐dimensional nanostructures of halide perovskites have been receiving extensive research interest for their superior optical, electrical, and stability characteristics over their conventional bulk counterparts. In particular, the unique surficial characteristics of well‐defined 1D nanowires have shown high feasibility for high performance optoelectronic applications. The key aspect of endorsing this nanoscale form factor in practical applications lies in securing dimensional uniformity and controllability in large scales, which generally requires complicated lithographic procedures. In this work, a simple, rapid, and widely applicable strategy for the direct synthesis and printing of well‐aligned nanowire arrays on a large scale is demonstrated. By employing blade coating methods with sophisticatedly controlled stick‐slip motions, periodically resolved transverse 3D meniscus enables uniform directional growth of highly crystalline CsPbI3 nanowires without the need for templates. A systematic investigation of morphological evolutions with respect to kinetic processing variables is conducted, along with a comprehensive suite of structural and optical analyses for the synthesized nanowire arrays. A single nanowire photodetector is employed to demonstrate the superiority and applicability of the fabrication strategy. Finally, a 2D image recognition is demonstrated by using an array of photodetectors fabricated via a fast (<2 min) direct printing of nanowire arrays on wafers. [ABSTRACT FROM AUTHOR]

Twist angle provides a new degree of freedom for 2D material modifications. In principle, the intrinsic properties of twisted multilayers can be regulated by twist angle between each adjacent layer and thus have greater tunability than widely studied bilayer structures. Considering its complexity, it is important to first investigate the simplest twisted multilayers with only one interface twisted. In this work, multilayer Moiré superlattices with only one twisted interface via paraffin‐assisted folding of non‐twisted stacked (highly symmetrically stacked) multilayer MoS2 are successfully fabricated, and their twist‐angle dependent optical properties are systematically studied. Compared to non‐twisted stacked multilayer MoS2, the one‐interface‐twisted multilayers show a 2–3.5 times higher PL intensity, and their interlayer coupling, indirect bandgap, and degree of circular polarization (DOCP) are tunable by twist angle. Notably, the DOCP for the one‐interface‐twisted four‐layer (folded bilayer) can reach 86%, which is the highest value ever reported for transition metal dichalcogenide homostructures above liquid nitrogen temperature. This work provides a solid base for understanding twist‐angle dependent properties of twisted multilayer 2D‐materials. [ABSTRACT FROM AUTHOR]

Applications of 2D semiconductors have been extensively studied, much oriented to various electron devices. Recently, multivalue field‐effect transistors (FETs) are also included among 2D‐based electron device studies in consideration that multivalue FETs may resolve power consumption issues in future integrated circuits. Several n‐channel devices are thus reported along with a few p‐channel devices, while studies to achieve both n‐ and p‐channel multivalue FETs are hardly found. Here, both n‐ and p‐channel multivalue FETs are fabricated using p‐MoTe2/n‐MoS2 heterostack channel architecture, where either p‐ or n‐channel ternary value FET is reproducible by switching the stacking order of p‐ and n‐channel layer. The main ternary value mechanism originates from resonant tunneling type injection and channel inversion, which take place during device operation. For a state‐of‐the‐art device application in 2D electronics, a quaternary NAND logic circuit is for the first time demonstrated by integrating two ternary n‐channel FETs, and a complementary ternary inverter is also fabricated by integrating multivalue p‐channel and plain n‐channel FET. [ABSTRACT FROM AUTHOR]

To dope 2D semiconductor channels, charge‐transfer doping has generally been done by thermal deposition of inorganic or organic thin‐film layers on top of the 2D channel in bottom‐gate field‐effect transistors (FETs). The doping effects are reproducible in most cases. However, such thermal deposition will damage the surface of 2D channels due to the kinetic energy of depositing atoms, causing hysteresis or certain degradation. Here, a more desirable charge‐transfer doping process is suggested. A damage‐free charge‐transfer doping is conducted for 2D MoTe2 (or MoS2) channels using a polydimethylsiloxane stamp. MoO3 or LiF is initially deposited on the stamp as a doping medium. Hysteresis‐minimized transfer characteristics are achieved from stamp‐doped FETs, while other devices with direct thermal deposition‐doped channels show large hysteresis. The stamping method seems to induce a van der Waals‐like damage‐free interface between the channel and doping media. The stamp‐induced doping is also well applied for a MoTe2‐based complementary inverter because MoO3‐ and LiF‐doping by separate stamps effectively modifies two ambipolar MoTe2 channels to p‐ and n‐type, respectively. [ABSTRACT FROM AUTHOR]

Phase transition in nanomaterials is distinct from that in 3D bulk materials owing to the dominant contribution of surface energy. Among nanomaterials, 2D materials have shown unique phase transition behaviors due to their larger surface‐to‐volume ratio, high crystallinity, and lack of dangling bonds in atomically thin layers. Here, the anomalous dimensionality‐driven phase transition of molybdenum ditelluride (MoTe2) encapsulated by hexagonal boron nitride (hBN) is reported. After encapsulation annealing, single‐crystal 2H‐MoTe2 transformed into polycrystalline Td‐MoTe2 with tilt‐angle grain boundaries of 60°‐glide‐reflection and 120°‐twofold rotation. In contrast to conventional nanomaterials, the hBN‐encapsulated MoTe2 exhibit a deterministic dependence of the phase transition on the number of layers, in which the thinner MoTe2 has a higher 2H‐to‐Td phase transition temperature. In addition, the vertical and lateral phase transitions of the stacked MoTe2 with different crystalline orientations can be controlled by inserted graphene layers and the thickness of the heterostructure. Finally, it is shown that seamless Td contacts for 2H‐MoTe2 transistors can be fabricated by using the dimensionality‐driven phase transition. The work provides insight into the phase transition of 2D materials and van der Waals heterostructures and illustrates a novel method for the fabrication of multi‐phase 2D electronics. [ABSTRACT FROM AUTHOR]

Crystalline or amorphous metal oxides are widely used in various optoelectronic devices as key components, such as transparent conductive electrodes, dielectrics or semiconducting active layers for thin‐film transistor (TFT) backplanes in large‐area displays, photovoltaics, and light‐emitting diodes. Although crystalline inorganic materials demonstrate outstanding optoelectronic performance, owing to their wide bandgaps, large conductivities, and high carrier mobilities, their inherent brittleness makes them vulnerable to mechanical stress, thereby limiting the use of metal‐oxide films in emerging flexible electronic applications. In this study, stress‐diffusive organic–inorganic hybrid superlattice nanostructures are developed to overcome the mechanical limitation of crystalline oxides and to provide high mechanical stability to metal‐oxide semiconductors. In particular, hybrid transparent superlattice electrodes based on crystalline indium–tin oxide exhibit high electrical conductivities of up to 555 S cm–1 (resistance variation < 3%) and effectively reduce the mechanical stress on the inorganic layer (up to 10 000 bending cycles with a radius of 1 mm). Furthermore, to ensure the viability of the hybrid superlattice flexible electronics, all solution‐processed superlattice crystalline indium–gallium‐oxide TFTs are implemented on a thin (≈5 µm) polyimide substrate, providing highly robust and excellent electrical performance (average mobility of 7.6 cm2 V–1 s–1). [ABSTRACT FROM AUTHOR]

Gallium trioxide, β‐Ga2O3, has been recently studied due to its promising semiconducting properties as active material in transistors or Schottky diodes. Transistors with β‐Ga2O3 channels are mostly metal oxide field effect transistors (MOSFET), and they show very negative threshold voltages (Vth) in general. Metal semiconductor field effect transistors (MESFETs) with top gate are also reported with less negative Vth. Still, β‐Ga2O3 MESFETs are only a few. Here, bottom gate architecture β‐Ga2O3 MESFETs using transition metal dichalcogenide (TMD) NbS2 and TaS2 are reported. Due to the large work functions of those metallic TMDs, the MESFETs display minimum subthreshold swing of 61 mV dec−1, small Vth of −1.2 V, minimum OFF ID of ≈100 fA, and maximum ON/OFF current ratio of ≈108. Both β‐Ga2O3 Schottky diodes with TaS2 and NbS2 display good junction stability even after 300 °C measurements in 10 mTorr vacuum. When the β‐Ga2O3 MESFET with TaS2 gate is integrated as a switching FET into an organic light emitting diode (OLED) circuit, it demonstrates long‐term leakage endurance performance, maintaining an OLED brightness higher than 58% of the initial intensity after 100 s passes since the ON‐switching point, which is even superior to the performance of conventional a‐IGZO MOSFET switch. [ABSTRACT FROM AUTHOR]

Among advanced devices with 2D semiconductors, charge injection memory field effect transistors (CIM FETs) may be one of the most important and practical ones. Reported CIM FETs utilize three layers (for tunneling, trapping, and bulk dielectric) in general, resulting in high switching voltages over 10 V. Here, nonvolatile CIM FETs are fabricated with MoS2 channel and hetero‐stack bilayer oxide dielectrics adopting 5 nm‐thin SiO2 and 25 nm‐thick HfO2, where the charge traps are expected at the SiO2/HfO2 oxide interface. It is nicely observed from the device that a low pulse gate voltage below ±7 V is enough to obtain program and erase states, which would originate from the tunneling electrons trapped at the hetero‐stack oxide interface. For comparison, other CIM FET devices are also fabricated but with tri‐layer dielectric of 5 nm polystyrene‐brush/5 nm HfO2/25 nm SiO2. Expectedly, the latter with tri‐layer requires at least ±10 V for memory operations. The former with a hetero‐stack oxide bilayer is now determined as an optimum device because of low operating voltages and less process complexity, and it is extended to a circuit application for a long‐term memory switching of an organic light‐emitting diode (OLED) pixel. [ABSTRACT FROM AUTHOR]

2D transition metal dichalcogenides (TMDs) have been extensively studied due to their excellent physical properties. Mixed dimensional devices including 2D materials have also been studied, motivated by the possibility of any synergy effect from unique structures. However, only few such studies have been conducted. Here, semiconducting 1D ZnO nanowires are used as thin gate material to support 2D TMD field effect transistors (FETs) and 2D stack‐based interface trap nonvolatile memory. For the trap memory, deep level electron traps formed at the first MoS2/second MoS2 stack interface are exploited, since the first MoS2 is treated in an atomic layer deposition chamber for a short while. On the one hand, a complementary inverter type memory device can also be achieved using a long single ZnO wire as a common gate to simultaneously support both n‐ and p‐channel TMD FETs. In addition, it is found that the semiconducting ZnO nanowire itself operates as an n‐type channel when the TMD materials can become a top‐gate to charge the ZnO channel. It means that 2D (bottom gated) and 1D channel (top gated) FETs are respectively operational in a single device structure. The 1D–2D mixed devices seem deserving broad attention in both aspects of novelty and functionality. [ABSTRACT FROM AUTHOR]

Ultrathin molecular films can be used as building blocks to enable rational design of functionalities at the molecular level. A versatile and scalable approach to fabricate transfer‐compatible ultrathin molecular films with a broad range of physiochemical properties is reported. Different molecules in layers of uniform nanoscale thickness are deposited on monolayer hexagonal boron nitride (ML h‐BN) film. Initially, the deposited molecules form islands due to limited molecular diffusion during deposition on ML h‐BN, but postannealing effectively transforms them to the thermodynamically preferred film structures. Ultrathin building blocks composed of ML h‐BN and different molecules (C60, pentacene, C44H32N2) are realized, then transferred layer‐by‐layer (L‐by‐L) to form materials with precisely controlled composition and thickness, including donor/acceptor superlattices. This approach can facilitate the integration of nanoscale molecules with different material platforms for advanced functionalities. [ABSTRACT FROM AUTHOR]

The atomic or molecular assembly on 2D materials through the relatively weak van der Waals interaction is quite different from the conventional heteroepitaxy and may result in unique growth behaviors. Here, it is shown that straight 1D cyanide chains display universal epitaxy on hexagonal 2D materials. A universal oriented assembly of cyanide crystals (AgCN, AuCN, and Cu0.5Au0.5CN) is observed, where the chains are aligned along the three zigzag lattice directions of various 2D hexagonal crystals (graphene, h‐BN, WS2, MoS2, WSe2, MoSe2, and MoTe2). The potential energy landscape of the hexagonal lattice induces this preferred alignment of 1D chains along the zigzag lattice directions, regardless of the lattice parameter and surface elements as demonstrated by first‐principles calculations and parameterized surface potential calculations. Furthermore, the oriented microwires can serve as crystal orientation markers, and stacking‐angle‐controlled vertical 2D heterostructures are successfully fabricated by using them as markers. The oriented van der Waals epitaxy can be generalized to any hexagonal 2D crystals and will serve as a unique growth process to form crystals with orientations along the zigzag directions by epitaxy. [ABSTRACT FROM AUTHOR]

The mechanical flexibility and structural softness of ultrathin devices based on organic thin films and low‐dimensional nanomaterials have enabled a wide range of applications including flexible display, artificial skin, and health monitoring devices. However, both living systems and inanimate systems that are encountered in daily lives are all 3D. It is therefore desirable to either create freestanding electronics in a 3D form or to incorporate electronics onto 3D objects. Here, a technique is reported to utilize shape‐memory polymers together with carbon nanotube flexible electronics to achieve this goal. Temperature‐assisted shape control of these freestanding electronics in a programmable manner is demonstrated, with theoretical analysis for understanding the shape evolution. The shape control process can be executed with prepatterned heaters, desirable for 3D shape formation in an enclosed environment. The incorporation of carbon nanotube transistors, gas sensors, temperature sensors, and memory devices that are capable of self‐wrapping onto any irregular shaped‐objects without degradations in device performance is demonstrated. [ABSTRACT FROM AUTHOR]

Gallium trioxide, β‐Ga2O3, has been recently studied due to its promising semiconducting properties as active material in transistors or Schottky diodes. Transistors with β‐Ga2O3 channels are mostly metal oxide field effect transistors (MOSFET), and they show very negative threshold voltages (Vth) in general. Metal semiconductor field effect transistors (MESFETs) with top gate are also reported with less negative Vth. Still, β‐Ga2O3 MESFETs are only a few. Here, bottom gate architecture β‐Ga2O3 MESFETs using transition metal dichalcogenide (TMD) NbS2 and TaS2 are reported. Due to the large work functions of those metallic TMDs, the MESFETs display minimum subthreshold swing of 61 mV dec−1, small Vth of −1.2 V, minimum OFF ID of ≈100 fA, and maximum ON/OFF current ratio of ≈108. Both β‐Ga2O3 Schottky diodes with TaS2 and NbS2 display good junction stability even after 300 °C measurements in 10 mTorr vacuum. When the β‐Ga2O3 MESFET with TaS2 gate is integrated as a switching FET into an organic light emitting diode (OLED) circuit, it demonstrates long‐term leakage endurance performance, maintaining an OLED brightness higher than 58% of the initial intensity after 100 s passes since the ON‐switching point, which is even superior to the performance of conventional a‐IGZO MOSFET switch. [ABSTRACT FROM AUTHOR]