Abstract2D van der Waals (vdW) materials have been considered as potential building blocks for use in fundamental elements of electronic and optoelectronic devices, such as electrodes, channels, and dielectrics, because of their diverse and remarkable electrical properties. Furthermore, two or more building blocks of different electronic types can be stacked vertically to generate vdW heterostructures with desired electrical behaviors. However, such fundamental approaches cannot directly be applied practically because of issues such as precise alignment/positioning and large‐quantity material production. Here, these limitations are overcome and wafer‐scale vdW heterostructures are demonstrated by exploiting the lateral and vertical assembly of solution‐processed 2D vdW materials. The high exfoliation yield of the molecular intercalation‐assisted approach enables the production of micrometer‐sized nanosheets in large quantities and its lateral assembly in a wafer‐scale via vdW interactions. Subsequently, the laterally assembled vdW thin‐films are vertically assembled to demonstrate various electronic device applications, such as transistors and photodetectors. Furthermore, multidimensional vdW heterostructures are demonstrated by integrating 1D carbon nanotubes as a p‐type semiconductor to fabricate p–n diodes and complementary logic gates. Finally, electronic devices are fabricated via inkjet printing as a lithography‐free manner based on the stable nanomaterial dispersions.

The V−VI binary chalcogenide, Sb 2 Se 3 , has attracted considerable attention for its applications in thin film optoelectronic devices because of its unique 1D structure and remarkable optoelectronic properties. Herein, we report an Sb 2 Se 3 thin film epitaxially grown on a flexible mica substrate through a relatively weak (van der Waals) interaction by vapor transport deposition. The epitaxial Sb 2 Se 3 thin films exhibit a single (120) out-of-plane orientation and a 0.25°full width at half-maximum of (120) rocking curve in X-ray diffraction, confirming the high crystallinity of the epitaxial films. The Sb 2 Se 3 (120) plane is epitaxially aligned on mica(001) surface with the in-plane relationship of Sb 2 Se 3 [2̅ 10]//mica[010] and Sb 2 Se 3 [001]//mica-[100]. Compared to the photodetector made of a nonepitaxial Sb 2 Se 3 film, the photocurrent of the epitaxial Sb 2 Se 3 film photodetector is almost doubled. Furthermore, because of the flexibility and high sensitivity of the epitaxial Sb 2 Se 3 film photodetector on mica, it has been successfully employed to detect the heart rate of a person. These encouraging results will facilitate the development of epitaxial Sb 2 Se 3 film-based devices and potential applications in wearable electronics.

Two-dimensional transitional metal halides have recently attracted significant attention due to their thickness-dependent and electrostatically tunable magnetic properties. However, this class of materials is highly reactive chemically, which leads to irreversible degradation and catastrophic dissolution within seconds in ambient conditions, severely limiting subsequent characterization, processing, and applications. Here, we impart long-term ambient stability to the prototypical transition metal halide CrI3 by assembling a noncovalent organic buffer layer, perylenetetracarboxylic dianhydride (PTCDA), which templates subsequent atomic layer deposition (ALD) of alumina. X-ray photoelectron spectroscopy demonstrates the necessity of the noncovalent organic buffer layer since the CrI3 undergoes deleterious surface reactions with the ALD precursors in the absence of PTCDA. This organic-inorganic encapsulation scheme preserves the long-range magnetic ordering in CrI3 down to the monolayer limit as confirmed by magneto-optical Kerr effect measurements. Furthermore, we demonstrate field-effect transistors, photodetectors, and optothermal measurements of CrI3 thermal conductivity in ambient conditions.

Molybdenum disulfide (MoS2) on the c-plane sapphire has been a very popular system to study in the two-dimensional (2D) materials community. Bottom-up synthesis of monolayer (ML) MoS2 with excellent electrical properties has been achieved on sapphire by various methods, making it a very promising candidate to be used in the next generation nano-electronic devices. However, large-area ML MoS2 with comparable quality as the relatively small size exfoliated ML remains quite a challenge. To overcome this bottle neck, a comprehensive understanding of the structure of the as-grown ML material is an essential first step. Here, we report a detailed structural characterization of wafer-scale continuous epitaxial ML MoS2 grown by metalorganic chemical vapor deposition on sapphire using an azimuthal reflection high-energy electron diffraction (ARHEED) technique. With ARHEED we can map not only 2D but also 3D reciprocal space structure of the ML statistically. From the oscillation in the ARHEED intensity profile along the vertical direction of the ML, we derived a real space distance of ~3 Å at the interface of ML and sapphire. Quantitative diffraction spot broadening analyses of the 3D reciprocal space map reveals low density defects and a small angular misalignment of orientation domains in ML MoS2. Based on atomic force microscopy height distribution analysis, cross-section scanning transmission electron microscopy, and density functional theory calculations, we suggest that there exists a passivation layer between MoS2 ML and sapphire substrate. This ARHEED methodology also has been applied to ML WS2 and is expected to be applicable to other ML transition metal dichalcogenides on arbitrary crystalline or non-crystalline substrates.

Zinc indium sulfide belongs to the family of layered ternary chalcogenides. Although ZnIn2S4 has a suitable bandgap in the visible range, its optoelectronic properties are not fully investigated. Most photodetectors based on layered semiconductors suffer from large dark currents, which hamper their performance and energy efficiency. In this work, high quality ZnIn2S4 single crystals are synthesized via chemical vapor transport. The free‐standing crystals are ≈20 μm thick and up to 2 cm2 in area and produce large photocurrents upon UV–vis illumination, while also maintaining extremely low currents in the dark. This allows to fabricate a simple photodetector with ohmic contacts, exhibiting extremely small dark currents down to 10–12 A. The ON/OFF (light/dark) switching ratio reaches value of 106, the highest reported for a layered semiconductor. Furthermore, the photodetector exhibits remarkable responsivity of 173 A W–1 and excellent detectivity of 1.7 × 1012 Jones. To demonstrate sensitivity and flexibility of the ZnIn2S4 crystals, a wearable device is also fabricated. The wearable is able to record human heart rate and compare it with signal measured by a commercial smartwatch. The results suggest a substantial research potential in further explorations of ZnIn2S4 and other ternary chalcogenides for optoelectronic applications.