Alkali metal bilayer intercalation in graphene.

Alkali metal (AM) intercalation between graphene layers holds promise for electronic manipulation and energy storage, yet the underlying mechanism remains challenging to fully comprehend despite extensive research. In this study, we employ low-voltage scanning transmission electron microscopy (LV-STEM) to visualize the atomic structure of intercalated AMs (potassium, rubidium, and cesium) in bilayer graphene (BLG). Our findings reveal that the intercalated AMs adopt bilayer structures with hcp stacking, and specifically a C6M2C6 composition. These structures closely resemble the bilayer form of fcc (111) structure observed in AMs under high-pressure conditions. A negative charge transferred from bilayer AMs to graphene layers of approximately 1~1.5×1014 e-/cm-2 was determined by electron energy loss spectroscopy (EELS), Raman, and electrical transport. The bilayer AM is stable in BLG and graphite superficial layers but absent in the graphite interior, primarily dominated by single-layer AM intercalation. This hints at enhancing AM intercalation capacity by thinning the graphite material.

Molybdenum Chloride Nanostructures with Giant Lattice Distortions Intercalated into Bilayer Graphene.

The nanospace of the van der Waals (vdW) gap between structural units of two-dimensional (2D) materials serves as a platform for growing unusual 2D systems through intercalation and studying their properties. Various kinds of metal chlorides have previously been intercalated for tuning the properties of host layered materials, but the atomic structure of the intercalants remains still unidentified. In this study, we investigate the atomic structural transformation of molybdenum(V) chloride (MoCl5) after intercalation into bilayer graphene (BLG). Using scanning transmission electron microscopy, we found that the intercalated material represents MoCl3 networks, MoCl2 chains, and Mo5Cl10 rings. Giant lattice distortions and frequent structural transitions occur in the 2D MoClx that have never been observed in metal chloride systems. The trend of symmetric to nonsymmetric structural transformations can cause additional charge transfer from BLG to the intercalated MoClx, as suggested by our density functional theory calculations. Our study deepens the understanding of the behavior of matter in the confined space of the vdW gap in BLG and provides hints at a more efficient tuning of material properties by intercalation for potential applications, including transparent conductive films, optoelectronics, and energy storage.

Composition-dependent photoconductivities in indium aluminium nitride nanorods grown by magnetron sputter epitaxy.

Photoconduction (PC) properties were investigated for ternary indium aluminium nitride (In x Al1-x N) nanorods (NRs) with different indium compositions (x) from 0.35 to 0.68, as grown by direct-current reactive magnetron sputter epitaxy. Cross-sectional scanning transmission electron microscopy (STEM) reveals single-crystal quality of the vertically aligned In x Al1-x N NRs. Single-rod photodetector devices with good ohmic contacts were fabricated using the focused-ion-beam technique (FIB), where the In-rich In0.68Al0.32N NR exhibits an optimal photocurrent responsivity of 1400 A W-1 and photoconductive gain of 3300. A transition from a positive photoresponse to a negative photoresponse was observed, while increasing the In composition x from 0.35 to 0.57. The negative PC was further enhanced by increasing x to 0.68. A model based on the coexistence and competition of deep electron trap states and recombination centers was proposed to explain the interesting composition-dependent PC in these ternary III-nitride 1D nanostructures.

WSe2/WS2 Heterobilayer Nonvolatile Memory Device with Boosted Charge Retention.

A two-dimensional (2D) nonvolatile memory device architecture to improve the long-term charge retention with the minimum charge loss without compromising storage capacity and the extinction ratio for practical applications has been an imminent demand. To address the current issue, we adopted a novel type-II band-aligned heterobilayer channel comprising vertically stacked monolayer WSe2 nanodots on monolayer WS2. The band offset modulation leads to electron doping from WSe2 nanodots into the WS2 channel without any external driving electric field. As a result, the tested device outperformed with a memory window as high as 34 V and a negligible charge loss of 7% in a retention period of 10 years while maintaining a high extinction ratio of 106. The doping technique presented in this work provides a feasible route to modulate the electrical properties of 2D channel materials without hampering charge transport, paving the way for high-performance 2D memory devices.

Embedment of Multiple Transition Metal Impurities into WS2 Monolayer for Bandstructure Modulation.

Band structure by design in 2D layered semiconductors is highly desirable, with the goal to acquire the electronic properties of interest through the engineering of chemical composition, structure, defect, stacking, or doping. For atomically thin transition metal dichalcogenides, substitutional doping with more than one single type of transition metals is the task for which no feasible approach is proposed. Here, the growth of WS2 monolayer is shown codoped with multiple kinds of transition metal impurities via chemical vapor deposition controlled in a diffusion-limited mode. Multielement embedment of Cr, Fe, Nb, and Mo into the host lattice is exemplified. Abundant impurity states thus generate in the bandgap of the resultant WS2 and provide a robust switch of charging/discharging states upon sweep of an electric filed. A profound memory window exists in the transfer curves of doped WS2 field-effect transistors, forming the basis of binary states for robust nonvolatile memory. The doping technique presented in this work brings one step closer to the rational design of 2D semiconductors with desired electronic properties.

Stable 1T Tungsten Disulfide Monolayer and Its Junctions: Growth and Atomic Structures.

Transition-metal dichalcogenides in the 1T phase have been a subject of increasing interest, which is partly due to their fascinating physical properties and partly to their potential applications in the next generation of electronic devices, including supercapacitors, electrocatalytic hydrogen evolution, and phase-transition memories. The primary method for obtaining 1T WS2 or MoS2 has been using ion intercalation in combination with solution-based exfoliation. The resulting flakes are small in size and tend to aggregate upon deposition, forming an intercalant-TMD complex with small 1T and 1T' patches embedded in the 2H matrix. Existing growth methods have, however, produced WS2 or MoS2 solely in the 2H phase. Here, we have refined the growth approach to obtain monolayer 1T WS2 up to 80 µm in size based on chemical vapor deposition. With the aid of synergistic catalysts (iron oxide and sodium chloride), 1T WS2 can nucleate in the infant stage of the growth, forming special butterfly-like single crystals with the 1T phase in one wing and the 2H phase in the other. Distinctive types of phase boundaries are discovered at the 1T-2H interface. The 1T structure thus grown is thermodynamically stable over time and even persists at a high temperature above 800 °C, allowing for a stepwise edge epitaxy of lateral 1T heterostructures. Atomic images show that the 1T WS2-MoS2 heterojunction features a coherent and defectless interface with a sharp atomic transition. The stable 1T phase represents a missing piece of the puzzle in the research of atomic thin van der Waals crystals, and our growth approach provides an accessible way of filling this gap.