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.     

Ultrafast Growth of High‐Quality Monolayer WSe2 on Au

The ultrafast growth of high‐quality uniform monolayer WSe₂ is reported with a growth rate of ≈26 µm s^?1 by chemical vapor deposition on reusable Au substrate, which is ≈2–3 orders of magnitude faster than those of most 2D transition metal dichalcogenides grown on nonmetal substrates. Such ultrafast growth allows for the fabrication of millimeter‐size single‐crystal WSe₂ domains in ≈30 s and large‐area continuous films in ≈60 s. Importantly, the ultrafast grown WSe₂ shows excellent crystal quality and extraordinary electrical performance comparable to those of the mechanically exfoliated samples, with a high mobility up to ≈143 cm² V^?1 s^?1 and ON/OFF ratio up to 9 × 10⁶ at room temperature. Density functional theory calculations reveal that the ultrafast growth of WSe₂ is due to the small energy barriers and exothermic characteristic for the diffusion and attachment of W and Se on the edges of WSe₂ on Au substrate.     

Tunable Polarity Behavior and Self‐Driven Photoswitching in p‐WSe2/n‐WS2 Heterojunctions

Van der Waals (vdW) p–n heterojunctions consisting of various 2D layer compounds are fascinating new artificial materials that can possess novel physics and functionalities enabling the next‐generation of electronics and optoelectronics devices. Here, it is reported that the WSe₂/WS₂ p–n heterojunctions perform novel electrical transport properties such as distinct rectifying, ambipolar, and hysteresis characteristics. Intriguingly, the novel tunable polarity transition along a route of n‐“anti‐bipolar”–p‐ambipolar is observed in the WSe₂/WS₂ heterojunctions owing to the successive work of conducting channels of junctions, p‐WSe₂ and n‐WS₂ on the electrical transport of the whole systems. The type‐II band alignment obtained from first principle calculations and built‐in potential in this vdW heterojunction can also facilitate the efficient electron–hole separation, thus enabling the significant photovoltaic effect and a much enhanced self‐driven photoswitching response in this system.     

Tunneling Diode Based on WSe2/SnS2 Heterostructure Incorporating High Detectivity and Responsivity

van der Waals (vdW) heterostructures based on atomically thin 2D materials have led to a new era in next‐generation optoelectronics due to their tailored energy band alignments and ultrathin morphological features, especially in photodetectors. However, these photodetectors often show an inevitable compromise between photodetectivity and photoresponsivity with one high and the other low. Herein, a highly sensitive WSe₂/SnS₂ photodiode is constructed on BN thin film by exfoliating each material and manually stacking them. The WSe₂/SnS₂ vdW heterostructure shows ultralow dark currents resulting from the depletion region at the junction and high direct tunneling current when illuminated, which is confirmed by the energy band structures and electrical characteristics fitted with direct tunneling. Thus, the distinctive WSe₂/SnS₂ vdW heterostructure exhibits both ultrahigh photodetectivity of 1.29 × 10¹³ Jones (I_ph/I_dark ratio of ≈10⁶) and photoresponsivity of 244 A W^?1 at a reverse bias under the illumination of 550 nm light (3.77 mW cm^?2).     

WSe2 Photovoltaic Device Based on Intramolecular p–n Junction

High quality p–n junctions based on 2D layered materials (2DLMs) are urgent to exploit, because of their unique properties such as flexibility, high absorption, and high tunability which may be utilized in next‐generation photovoltaic devices. Based on transfer technology, large amounts of vertical heterojunctions based on 2DLMs are investigated. However, the complicated fabrication process and the inevitable defects at the interfaces greatly limit their application prospects. Here, an in‐plane intramolecular WSe₂ p–n junction is realized, in which the n‐type region and p‐type region are chemically doped by polyethyleneimine and electrically doped by the back‐gate, respectively. An ideal factor of 1.66 is achieved, proving the high quality of the p–n junction realized by this method. As a photovoltaic detector, the device possesses a responsivity of 80 mA W^?1 (≈20% external quantum efficiency), a specific detectivity of over 10¹¹ Jones and fast response features (200 µs rising time and 16 µs falling time) at zero bias, simultaneously. Moreover, a large open‐circuit voltage of 0.38 V and an external power conversion efficiency of ≈1.4% realized by the device also promises its potential in microcell applications.     

Interfacially Engineered High‐Speed Nonvolatile Memories Employing p‐Type Nanoribbons

A novel two‐terminal high‐speed nonvolatile memory device is demonstrated featuring the construction of a quasi‐metal‐insulator‐semiconductor (q‐MIS) architecture. The quasi‐MIS memory takes advantage of an in situ formed amorphous AlO_x interfacial layer sandwiched between p‐type ZnS nanoribbons (p‐ZnSNRs) and a Al electrode. Systematical optimization of the AlO_x interfacial layer enables the resultant memory to show excellent memory characteristics, including a fast programming speed of <100 ns, a high current ON/OFF ratio of ∼10⁸, a long retention time of 6 × 10⁴ s, and good stability over 12 months. In addition, an interface‐state‐induced mechanism is proposed to elucidate in detail the memory characteristic for the quasi‐MIS structure. This work suggests great potential of such quasi‐MIS architecture for high‐performance two‐terminal memory, and more importantly, signifies the importance of interface engineering for the construction of novel functional nano‐devices.     

High‐Performance Photodiode Based on Atomically Thin WSe2/MoS2 Nanoscroll Integration

Self‐assembled structures of 2D materials with novel physical and chemical properties, such as the good electrical and optoelectrical performance in nanoscrolls, have attracted a lot of attention. However, high photoresponse speed as well as high responsivity cannot be achieved simultaneously in the nanoscrolls. Here, a photodiode consisting of single MoS₂ nanoscrolls and a p‐type WSe₂ is demonstrated and shows excellent photovoltaic characteristics with a large open‐circuit voltage of 0.18 V and high current intensity. Benefiting from the heterostructure, the dark current is suppressed resulting in an increased ratio of photocurrent to dark current (two orders of magnitude higher than the single MoS₂ nanoscroll device). Furthermore, it yields high responsivity of 0.3 A W^?1 (corresponding high external quantum efficiency of ≈75%) and fast response time of 5 ms, simultaneously. The response speed is increased by three orders of magnitude over the single MoS₂ nanoscroll device. In addition, broadband photoresponse up to near‐infrared could be achieved. This atomically thin WSe₂/MoS₂ nanoscroll integration not only overcomes the disadvantage of MoS₂ nanoscrolls, but also demonstrates a single nanoscroll‐based heterostructure with high performance, promising its potential in the future optoelectronic applications.     

High‐Temperature Shape Memory Effect in Ti‐Ta Thin Films Sputter Deposited at Room Temperature

Ti‐Ta based alloys are potential high‐temperature shape memory materials with operation temperatures above 100 °C. In this study, the room temperature fabrication of Ti‐Ta thin films showing a reversible martensitic transformation and a high temperature shape memory effect above 200 °C is reported. In contrast to other shape memory thin films, no further heat treatment is necessary to obtain the functional properties. A disordered α″ martensite (orthorhombic) phase is formed in the as‐deposited co‐sputtered Ti₇₀Ta₃₀, Ti₆₈Ta₃₂ and Ti₆₇Ta₃₃ films, independent of the substrate. A Ti₇₀Ta₃₀ free‐standing film shows a reversible martensitic transformation, as confirmed by temperature–dependent XRD measurements during thermal cycling between 125 °C to 275 °C. Furthermore, a one‐way shape memory effect is qualitatively confirmed in this film. The observed properties of the Ti‐Ta thin films make them promising for applications on polymer substrates and especially in microsystem technologies.     

Opportunities in High‐Speed Atomic Force Microscopy

The atomic force microscope (AFM) has become integrated into standard characterisation procedures in many different areas of research. Nonetheless, typical imaging rates of commercial microscopes are still very slow, much to the frustration of the user. Developments in instrumentation for “high‐speed AFM” (HSAFM) have been ongoing since the 1990s, and now nanometer resolution imaging at video rate is readily achievable. Despite thorough investigation of samples of a biological nature, use of HSAFM instruments to image samples of interest to materials scientists, or to carry out AFM lithography, has been minimal. This review gives a summary of different approaches to and advances in the development of high‐speed AFMs, highlights important discoveries made with new instruments, and briefly discusses new possibilities for HSAFM in materials science.     

Trap Healing for High‐Performance Low‐Voltage Polymer Transistors and Solution‐Based Analog Amplifiers on Foil

Solution‐processed semiconductors such as conjugated polymers have great potential in large‐area electronics. While extremely appealing due to their low‐temperature and high‐throughput deposition methods, their integration in high‐performance circuits has been difficult. An important remaining challenge is the achievement of low‐voltage circuit operation. The present study focuses on state‐of‐the‐art polymer thin‐film transistors based on poly(indacenodithiophene‐benzothiadiazole) and shows that the general paradigm for low‐voltage operation via an enhanced gate‐to‐channel capacitive coupling is unable to deliver high‐performance device behavior. The order‐of‐magnitude longitudinal‐field reduction demanded by low‐voltage operation plays a fundamental role, enabling bulk trapping and leading to compromised contact properties. A trap‐reduction technique based on small molecule additives, however, is capable of overcoming this effect, allowing low‐voltage high‐mobility operation. This approach is readily applicable to low‐voltage circuit integration, as this work exemplifies by demonstrating high‐performance analog differential amplifiers operating at a battery‐compatible power supply voltage of 5 V with power dissipation of 11 µW, and attaining a voltage gain above 60 dB at a power supply voltage below 8 V. These findings constitute an important milestone in realizing low‐voltage polymer transistors for solution‐based analog electronics that meets performance and power‐dissipation requirements for a range of battery‐powered smart‐sensing applications.     

Magnesium Storage Performance and Mechanism of 2D‐Ultrathin Nanosheet‐Assembled Spinel MgIn2S4 Cathode for High‐Temperature Mg Batteries

Magnesium batteries have the potential to be a next generation battery with large capability and high safety, owing to the high abundance, great volumetric energy density, and reversible dendrite‐free capability of Mg anodes. However, the lack of a stable high‐voltage electrolyte, and the sluggish Mg‐ion diffusion in lattices and through interfaces limit the practical uses of Mg batteries. Herein, a spinel MgIn₂S₄ microflower‐like material assembled by 2D‐ultrathin (≈5.0 nm) nanosheets is reported and first used as a cathode material for high‐temperature Mg batteries with an ionic liquid electrolyte. The nonflammable ionic liquid electrolyte ensure the safety under high temperatures. As prepared MgIn₂S₄ exhibits wide‐temperature‐range adaptability (50–150 °C), ultrahigh capacity (≈500 mAh g^?1 under 1.2 V vs Mg/Mg²⁺), fast Mg²⁺ diffusibility (≈2.0 × 10^?8 cm² s^?1), and excellent cyclability (without capacity decay after 450 cycles). These excellent electrochemical properties are due to the fast kinetics of magnesium by the 2D nanosheets spinel structure and safe high‐temperature operation environment. From ex situ X‐ray diffraction and transmission electron microscopy measurements, a conversion reaction of the Mg²⁺ storage mechanism is found. The excellent performance and superior security make it promising in high‐temperature batteries for practical applications.