Toward Exotic Layered Materials: 2D Cuprous Iodide

Graphene Bilayer as a Template for Manufacturing Novel Encapsulated 2D Materials

Bilayer graphene (BLG) has recently been used as a tool to stabilize encapsulated single sheets of various layered materials and tune their properties. It was also discovered that the protecting action of graphene sheets makes it possible to synthesize completely new two-dimensional materials (2DMs) inside the BLG by intercalating various atoms and molecules. In comparison to the bulk graphite, BLG allows for easier intercalation and a much larger increase in the interlayer separation of the sheets. Moreover, it enables studying the atomic structure of the intercalated 2DM by using high-resolution transmission electron microscopy. In this review, we summarize the recent progress in this area, with a special focus on new materials created inside BLG. We compare the experimental findings with the theoretical predictions, pay special attention to the discrepancies, and outline the challenges in the field. Finally, we discuss unique opportunities offered by intercalation into 2DMs beyond graphene and their heterostructures.

Magnetization direction-controlled topological band structure in TlTiX (X = Si, Ge) monolayers

The quantum anomalous Hall (QAH) insulator is a vital material for the investigation of emerging topological quantum effects, but its extremely low working temperature limits experiments. Apart from the temperature challenge, effective regulation of the topological state of QAH insulators is another crucial concern. Here, by first-principles calculations, we find a family of stable two-dimensional materials TlTiX (X = Si, Ge) are large-gap QAH insulators. Their extremely robust ferromagnetic (FM) ground states are determined by both the direct- and super-exchange FM coupling. In the absence of spin-orbit coupling (SOC), there exist a spin-polarized crossing point located at eachKandK' points, respectively. The SOC effect results in the spontaneous breaking ofC2symmetry and introduces a mass term, giving rise to a QAH state with sizable band gap. The tiny magnetocrystalline anisotropic energy (MAE) implies that an external magnetic field can be easily used to align magnetization deviating fromzdirection to thex-yplane, thereby leading to a transformation of the electronic state from the QAH state to the Weyl half semimetals state, which indicate monolayers TlTiX (X = Si, Ge) exhibit a giant magneto topological band effect. Finally, we examined the impact of stress on the band gap and MAE, which underlies the reasons for the giant magneto topological band effect attributed to the crystal field. These findings present novel prospects for the realization of large-gap QAH states with the characteristic of easily modifiable topological states.

Ultra-thin double-layered hexagonal CuI: strain tunable properties and robust semiconducting behavior

In this study, the freestanding form of ultra-thin CuI crystals, which have recently been synthesized experimentally, and their strain-dependent properties are investigated by means of density functional theory calculations. Structural optimizations show that CuI crystallizes in a double-layered hexagonal crystal (DLHC) structure. While phonon calculations predict that DLHC CuI crystals are dynamically stable, subsequent vibrational spectrum analyzes reveal that this structure has four unique Raman-active modes, allowing it to be easily distinguished from similar ultra-thin two-dimensional materials. Electronically, DLHC CuI is found to be a semiconductor with a direct band gap of 3.24 eV which is larger than that of its wurtzite and zincblende phases. Furthermore, it is found that in both armchair (AC) and zigzag (ZZ) orientations the elastic instabilities occur over the high strain strengths indicating the soft nature of CuI layer. In addition, the stress-strain curve along the AC direction reveal that DLHC CuI undergoes a structural phase transition between the 4% and 5% tensile uniaxial strains as indicated by a sudden drop of the stress in the lattice. Moreover, the phonon band dispersions show that the phononic instability occurs at much smaller strain along the ZZ direction than that of along the AC direction. Furthermore, the external strain direction can be deduced from the predicted Raman spectra through the splitting rates of the doubly degenerate in-plane vibrations. The mobility of the hole carriers display highly anisotropic characteristic as the applied strain reaches 5% along the AC direction. Due to its anomalous strain-dependent electronic features and elastically soft nature, DLHC of CuI is a potential candidate for future electro-mechanical applications.

Twisted epitaxy of gold nanodisks grown between twisted substrate layers of molybdenum disulfide

We expand the concept of epitaxy to a regime of "twisted epitaxy" with the epilayer crystal orientation between two substrates influenced by their relative orientation. We annealed nanometer-thick gold (Au) nanoparticles between two substrates of exfoliated hexagonal molybdenum disulfide (MoS2) with varying orientation of their basal planes with a mutual twist angle ranging from 0° to 60°. Transmission electron microscopy studies show that Au aligns midway between the top and bottom MoS2 when the twist angle of the bilayer is small (<~7°). For larger twist angles, Au has only a small misorientation with the bottom MoS2 that varies approximately sinusoidally with twist angle of the bilayer MoS2. Four-dimensional scanning transmission electron microscopy analysis further reveals a periodic strain variation (<|±0.5%|) in the Au nanodisks associated with the twisted epitaxy, consistent with the Moiré registry of the two MoS2 twisted layers.

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.

Angstrom-Confined Electrochemical Synthesis of Sub-Unit-Cell Non-Van Der Waals 2D Metal Oxides

Bottom-up electrochemical synthesis of atomically thin materials is desirable yet challenging, especially for non-vanderWaals (non-vdW) materials. Thicknesses below a few nanometers have not been reported yet, posing the question how thin can non-vdW materials be electrochemically synthesized. This is important as materials with (sub-)unit-cell thickness often show remarkably different properties compared to their bulk form or thin films of several nanometers thickness. Here, a straightforward electrochemical method utilizing the angstrom-confinement of laminar reduced graphene oxide (rGO) nanochannels is introduced to obtain a centimeter-scale network of atomically thin (<4.3 Å) 2D-transition metal oxides (2D-TMO). The angstrom-confinement provides a thickness limitation, forcing sub-unit-cell growth of 2D-TMO with oxygen and metal vacancies. It is showcased that Cr₂ O₃ , a material without significant catalytic activity for the oxygen evolution reaction (OER) in bulk form, can be activated as a high-performing catalyst if synthesized in the 2D sub-unit-cell form. This method displays the high activity of sub-unit-cell form while retaining the stability of bulk form, promising to yield unexplored fundamental science and applications. It is shown that while retaining the advantages of bottom-up electrochemical synthesis, like simplicity, high yield, and mild conditions, the thickness of TMO can be limited to sub-unit-cell dimensions.

Metal Halides for High-Capacity Energy Storage

High-capacity electrochemical energy storage systems are more urgently needed than ever before with the rapid development of electric vehicles and the smart grid. The most efficient way to increase capacity is to develop electrode materials with low molecular weights. The low-cost metal halides are theoretically ideal cathode materials due to their advantages of high capacity and redox potential. However, their cubic structure and large energy barrier for deionization impede their rechargeability. Here, the reversibility of potassium halides, lithium halides, sodium halides, and zinc halides is achieved through decreasing their dimensionality by the strong π-cation interactions between metal cations and reduced graphene oxide (rGO). Especially, the energy densities of KI-, KBr-, and KCl-based materials are 722.2, 635.0, and 739.4 Wh kg^-1 , respectively, which are higher than those of other cathode materials for potassium-ion batteries. In addition, the full-cell with 2D KI/rGO as cathode and graphite as anode demonstrates a lifespan of over 150 cycles with a considerable capacity retention of 57.5%. The metal halides-based electrode materials possess promising application prospects and are worthy of more in-depth researches.

The record low thermal conductivity of monolayer cuprous iodide (CuI) with a direct wide bandgap

Two-dimensional materials have attracted significant research interest due to the fantastic properties that are unique to their bulk counterparts. In this paper, from the state-of-the-art first-principles, we predicted the stable structure of a monolayer counterpart of γ-CuI (cuprous iodide) that is a p-type wide bandgap semiconductor. The monolayer CuI presents multifunctional superiority in terms of electronic, optical, and thermal transport properties. Specifically, the ultralow thermal conductivity of 0.116 W m^-1 K^-1 is predicted for monolayer CuI, which is much lower than those of γ-CuI (0.997 W m^-1 K^-1) and other typical semiconductors. Moreover, an ultrawide direct bandgap of 3.57 eV is found in monolayer CuI, which is even larger than that of γ-CuI (2.95-3.1 eV), promising for applications in nano-/optoelectronics with better optical performance. The ultralow thermal conductivity and direct wide bandgap of monolayer CuI as reported in this study would promise its potential applications in transparent and wearable electronics.

Recent Progress in Double-Layer Honeycomb Structure: A New Type of Two-Dimensional Material

Two-dimensional (2D) materials are no doubt the most widely studied nanomaterials in the past decade. Most recently, a new type of 2D material named the double-layer honeycomb (DLHC) structure opened a door to achieving a series of 2D materials from traditional semiconductors. However, as a newly developed material, there still lacks a timely understanding of its structure, property, applications, and underlying mechanisms. In this review, we discuss the structural stability and experimental validation of this 2D material, and systematically summarize the properties and applications including the electronic structures, topological properties, optical properties, defect engineering, and heterojunctions. It was concluded that the DLHC can be a universal configuration applying to III-V, II-VI, and I-VII semiconductors. Moreover, these DLHC materials indeed have exotic properties such as being excitonic/topological insulators. The successful fabrication of DLHC materials further demonstrates it is a promising topic. Finally, we summarize several issues to be addressed in the future, including further experimental validation, defect engineering, heterojunction engineering, and strain engineering. We hope this review can help the community to better understand the DLHC materials timely and inspire their applications in the future.