The electrical performance of two-dimensional transition metal dichalcogenides (TMDs) is strongly affected by the number of structural defects. In this work, we provide an optical spectroscopic characterization approach to correlate the number of structural defects and the electrical performance of WSe₂ devices. Low-temperature photoluminescence (PL) spectra of electron-beam-lithographyprocessed WSe₂ exhibit a clear defect-induced PL emission due to excitons bound to defects, which would strongly degrade the electrical performance. By adopting an electron-beam-free transfer-electrode technique, we successfully prepared a backgated WSe₂ device containing a limited amount of defects. A maximum hole mobility of approximately 200 cm²·V^–1·s^–1 was achieved because of the reduced scattering sources, which is the highest reported value for this type of device. This work provides not only a versatile and nondestructive method to monitor the defects in TMDs but also a new route to approach the room-temperature phonon-limited mobility in high-performance TMD devices.

Novel low dimensional quantum phenomena at (110) LaAlO₃/SrTiO₃ (LAO/STO) interfaces are expected after the quasi two dimensional electron gas similar to that of (001) LAO/STO interfaces was found at this (110) system. Here, we report the two dimensional superconductivity with a superconducting transition temperature of $\cong$ 184 mK at (110) LAO/STO interfaces. The two dimensional characteristics of the superconductivity are consistent with our analysis based on a Berezinskii-Kosterlitz-Thouless transition. The estimated superconducting layer thickness is about 18 nm. This discovery may inspire new studies of LAO/STO interfaces and open additional opportunities for design of novel oxide electronic devices.

Phase transition from the semiconducting hexagonal (2H) phase to the metallic monoclinic (1T′) phase in two-dimensional (2D) transition metal dichalcogenides like MoTe ₂ is not only of great importance in fundamental study but also of technological significance for broad device applications. Here we report a universal, facile, scalable and reversible phase engineering technique (between 2H and 1T′ phases) for both monolayer and few-layer MoTe ₂ based on a soft hydrogen plasma treatment. The 2H → 1T′ transition was confirmed by a series of characterizations including Raman spectra and mapping studies, XPS analysis and FET device measurements at varying temperatures. We attribute the phase transition to the warping of Te–Mo bonds and the lateral sliding of the top Te-layer induced by the soft hydrogen ion bombardment according to both the structural and electronic characterizations as well as the horizontal comparison with the cases of Ar or O ₂ plasma treatment. We have also prepared a 2D heterostructure containing periodical 2H and 1T′ MoTe ₂ and showed that such phase transition can be readily reversed by post annealing. These results thus provide a robust and efficient approach for the phase engineering of monolayer and few-layer MoTe ₂ and could aid the development of 2D optoelectronic, memory and reconfigurable devices.

Understanding spinterfaces between magnetic metals and organic semiconductors is essential to unlock the great potentials that organic materials host for spintronic applications. Although plenty of efforts have been devoted to studying organic spintronic devices, exploring the role of metal/molecule spinterfaces at two-dimensional limit remains challenging because of excessive disorders and traps at the interfaces. Here, we demonstrate atomically smooth metal/molecule interfaces through nondestructively transferring magnetic electrodes on epitaxial grown single-crystalline layered organic films. Using such high-quality interfaces, we investigate spin injection of spin-valve devices based on organic films of different layers, in which molecules are packed in different manners. We find that the measured magnetoresistance and the estimated spin polarization increase markedly for bilayer devices compared with their monolayer counterparts. These observations reveal the key role of molecular packing on spin polarization, which is supported by density functional theory calculations. Our findings provide promising routes toward designing spinterfaces for organic spintronic devices.

Van der Waals (vdW) heterojunctions based on two-dimensional (2D) atomic crystals have been extensively studied in recent years. Herein, we show that both vertical and lateral vdW heterojunctions can be realized with layered molecular crystals using a two-step physical vapor transport (PVT) process. Both types of heterojunctions show clean and sharp interfaces without phase mixing under atomic force microscopy (AFM). They also exhibit a strong interfacial built-in electric field similar to that of their inorganic counterparts. These heterojunctions have greater potential for device applications than individual materials. The lateral heterojunction (LHJ) devices show rectifying characteristics due to the asymmetric energy barrier for holes at the interface, while the vertical heterojunction (VHJ) devices behave like metal–insulator–semiconductor tunnel junctions, with pronounced negative differential conductance (NDC). Our work extends the concept of vdW heterojunctions to molecular materials, which can be generalized to other layered organic semiconductors (OSCs) to obtain new device functionalities.

The interfacial atomic arrangement, which is different from that in the bulk form of the heterojunction, can induce a reconstruction of electrostatic field at the interface. For conventional semiconductor heterointerfaces, it is known that such reconstruction results in band bending, creating a quantum well in which the two-dimensional electron gas (2DEG) is formed. In this article, we show that this mechanism still works in a multivalent oxide heterojunction: for $\left(110\right)\mathrm{LaAl}{\mathrm{O}}_3\text{/}\mathrm{SrTi}{\mathrm{O}}_3$ (LAO/STO) heterojunctions, the coexistence of La and Ti in $\mathrm{AB}{\mathrm{O}}_3$ perovskite unit cells at the interface reduces the valence of Ti, generating a local field leading to band bending in the interfacial STO layers. The extra free electrons are trapped in this bent conduction band forming a 2DEG. It unifies two independent mechanisms for 2DEG at LAO/STO interfaces, the “polar catastrophe” model and the “$\mathrm{L}{\mathrm{a}}_{1-x}\mathrm{S}{\mathrm{r}}_x\mathrm{Ti}{\mathrm{O}}_3$” layers model, and is expected to end the decade-old controversy. This study opens insight into atomic-scale band engineering to control the behavior of complex oxide heterojunctions.