Large-area single-crystal TMD growth modulated by sapphire substrates.

Transition metal dichalcogenides (TMDs) have recently attracted extensive attention due to their unique physical and chemical properties; however, the preparation of large-area TMD single crystals is still a great challenge. Chemical vapor deposition (CVD) is an effective method to synthesize large-area and high-quality TMD films, in which sapphires as suitable substrates play a crucial role in anchoring the source material, promoting nucleation and modulating epitaxial growth. In this review, we provide an insightful overview of different epitaxial mechanisms and growth behaviors associated with the atomic structure of sapphire surfaces and the growth parameters. First, we summarize three epitaxial growth mechanisms of TMDs on sapphire substrates, namely, van der Waals epitaxy, step-guided epitaxy, and dual-coupling-guided epitaxy. Second, we introduce the effects of polishing, cutting, and annealing processing of the sapphire surface on the TMD growth. Finally, we discuss the influence of other growth parameters, such as temperature, pressure, carrier gas, and substrate position, on the growth kinetics of TMDs. This review might provide deep insights into the controllable growth of large-area single-crystal TMDs on sapphires, which will propel their practical applications in high-performance nanoelectronics and optoelectronics.

Continuous Modulation of Electrocatalytic Oxygen Reduction Activities of Single-Atom Catalysts through p-n Junction Rectification.

Fine-tuning single-atom catalysts (SACs) to surpass their activity limit remains challenging at their atomic scale. Herein, we exploit p-type semiconducting character of SACs having a metal center coordinated to nitrogen donors (MeNx ) and rectify their local charge density by an n-type semiconductor support. With iron phthalocyanine (FePc) as a model SAC, introducing an n-type gallium monosulfide that features a low work function generates a space-charged region across the junction interface, and causes distortion of the FeN4 moiety and spin-state transition in the FeII center. This catalyst shows an over two-fold higher specific oxygen-reduction activity than that of pristine FePc. We further employ three other n-type metal chalcogenides of varying work function as supports, and discover a linear correlation between the activities of the supported FeN4 and the rectification degrees, which clearly indicates that SACs can be continuously tuned by this rectification strategy.

Borophosphene as a promising Dirac anode with large capacity and high-rate capability for sodium-ion batteries.

Sodium-ion batteries (SIBs) have attracted tremendous attention as potential low-cost energy storage alternatives to lithium-ion batteries (LIBs) due to the intrinsic safety and great abundance of sodium. For developing competitive SIBs, highly efficient anode materials with large capacity and rapid ion diffusion are indispensable. In this study, a two-dimensional (2D) Dirac monolayer, that is, borophosphene, is proposed as a promising anode material for high performance SIBs on the basis of density functional theory calculations. The performances of Na adsorption and diffusion, maximum specific capacity, open circuit voltage, cyclical stability and electronic properties combined with Bader charge analysis are explored. It is found that borophosphene can spontaneously adsorb a Na atom with a binding energy of -0.838 eV. A low diffusion energy barrier of 0.221 eV suggests rapid ion conductivity. More intriguingly, a maximum specific capacity of 1282 mA h g-1 can be achieved in borophosphene, which is one of the largest values reported for 2D anode materials for SIBs. A low average voltage of 0.367 V is estimated, implying a suitable operating voltage of the anode material. The metallic properties, tiny surface expansion, and good kinetic stability of sodiated borophosphene give rise to high electrical conductivity and favorable cyclability. These abovementioned advantages suggest that borophosphene can be used as a Dirac anode material for SIBs with excellent performance including a large specific capacity, high-rate capability, and favorable cyclability.

The adhesion energy measured by a stress accumulation-peeling mechanism in the exfoliation of graphite.

The mechanical exfoliation of graphite using tape is one way to obtain high-quality graphene samples. However, the amount of graphene obtained is negligible due to the unclear exfoliation mechanism. In this paper, we present a stress accumulation-peeling mechanism, which can be applied to measure the adhesion energy of graphite. This mechanism is different from a wriggle or a creep. First, we obtained a simple universal formula to measure the adhesion energy Ga = (Fmax-Fmin)/3b, where Fmax and Fmin are the maximum and minimum values, respectively, of the external stretch force in the peeling process, and b is the width of the peeling arm. Second, the reliability of the method was demonstrated by measuring the adhesion energy between polydimethylsiloxane and glass. Using the simple universal formula, the adhesion energies of three graphite slices were determined to be 0.34 ± 0.03, 0.33 ± 0.06 and 0.34 ± 0.02 J m-2. These adhesion energies were consistent with the other measured result of 0.33 J m-2, which was based on the self-retraction phenomenon of graphite. The macroscopic method is very simple and easy to implement. It can be used to measure the adhesion energy of any van der Waals material and any biomaterial with adhesion interaction, as well as prepare excellent 2D material samples by optimizing the experimental conditions.

Thermal conductivity of suspended few-layer MoS2.

Modifying phonon thermal conductivity in nanomaterials is important not only for fundamental research but also for practical applications. However, the experiments on tailoring thermal conductivity in nanoscale, especially in two-dimensional materials, are rare due to technical challenges. In this work, we demonstrate the in situ thermal conduction measurement of MoS2 and find that its thermal conductivity can be continuously tuned to a required value from crystalline to amorphous limits. The reduction of thermal conductivity is understood from phonon-defect scattering that decreases the phonon transmission coefficient. Beyond a threshold, a sharp drop in thermal conductivity is observed, which is believed to be due to a crystalline-amorphous transition. Our method and results provide guidance for potential applications in thermoelectrics, photoelectronics, and energy harvesting where thermal management is critical with further integration and miniaturization.

Effect of Substrate symmetry on the dendrite morphology of MoS2 Film synthesized by CVD.

In van der Waals epitaxial growth, the substrate plays a particularly important role in the crystal morphology. Here, we synthesized MoS2 by chemical vapour deposition on silicon carbide (SiC). The obtained MoS2 dendritic crystals show six-fold symmetry, which are different from the conventional triangular shapes on SiO2 substrate and from those with three-fold symmetry on SrTiO3 substrate. Interestingly, these MoS2 dendritic crystals on SiC exhibit an average fractal dimension 1.76, which is slightly larger than the classical Diffusion-limited-Aggregation fractal dimension 1.66. The first principle calculation indicates that the six-fold symmetry of the dendritic MoS2 is determined by the lattice symmetry of SiC. To further demonstrating the substrate effect, we break the natural six-fold lattice symmetry of SiC (0001) into groove arrays through etching the substrate. And then we successfully synthesized cross-type dendritic crystal MoS2 with two-fold symmetry. Its average fractal dimension 1.83 is slightly larger than the fractal dimension 1.76 of the previous MoS2 dendrite with six-fold symmetry. In a word, the symmetry of SiC substrate determined the symmetry and the fractal dimension of the dendritic MoS2. This work provides one possibility of inducing the growth orientation of dendritic crystals through controlling the substrate surface symmetry artificially.

Robust indirect band gap and anisotropy of optical absorption in B-doped phosphorene.

A traditional doping technique plays an important role in the band structure engineering of two-dimensional nanostructures. Since electron interaction is changed by doping, the optical and electrochemical properties could also be significantly tuned. In this study, density functional theory calculations have been employed to explore the structural stability, and electronic and optical properties of B-doped phosphorene. The results show that all B-doped phosphorenes are stable with a relatively low binding energy. Of particular interest is that these B-doped systems exhibit an indirect band gap, which is distinct from the direct one of pure phosphorene. Despite the different concentrations and configurations of B dopants, such indirect band gaps are robust. The screened hybrid density functional HSE06 predicts that the band gap of B-doped phosphorene is slightly smaller than that of pure phosphorene. Spatial charge distributions at the valence band maximum (VBM) and the conduction band minimum (CBM) are analyzed to understand the features of an indirect band gap. By comparison with pure phosphorene, B-doped phosphorenes exhibit strong anisotropy and intensity of optical absorption. Moreover, B dopants could enhance the stability of Li adsorption on phosphorene with less sacrifice of the Li diffusion rate. Our results suggest that B-doping is an effective way of tuning the band gap, enhancing the intensity of optical absorption and improving the performances of Li adsorption, which could promote potential applications in novel optical devices and lithium-ion batteries.

Structure-property relationships of cell clusters in biotissues: 2D analysis.

Gaining insight into the relationships between the self-organized cell structures and the properties of biotissues is helpful for revealing the function of biomaterials and its designing principle. However, the traditionally used random foam model neglects several important details of the frameworks of cell clusters resulting in incomplete conclusions. Herein, we use a more complete model, the cell adhesion model, to investigate the mechanical and morphological properties of the two-dimensional (2D) dry cell foams. Since these 2D structures are formed by cell adhesion, the system can reach equilibrium through minimizing free energy. Under the equilibrium conditions without volume constraint, shape equations for highly symmetrical structures are derived, and the analytical results of the corresponding mechanical parameters, such as the Young's modulus, bulk modulus and failure strength, are obtained. Moreover, with volume constraint, numerical simulation method is applied to study the complex shapes and obtain several stable multicellular structures. Symmetry breaking caused by the volume change is also observed. Moreover, typical periodic shapes and the corresponding phase transformations are also explored. Our study provides a new potential method to bridge the microstructure and macro-mechanical parameters of biotissues. The results are also useful for understanding the formation mechanism of biotissue structures.

General equilibrium shape equations of polymer chains.

The general equilibrium shape equations of polymer chains are analytically derived in this paper. This provides a unified description for many models, such as the well-known wormlike chain (WLC) model, the wormlike rod chain (WLRC) model, carbon nanotubes, and so on. Using the WLC model, we find that the pitch-to-radius ratio of coils, 4.443, agrees with Z-DNA, and the pitch-to-radius ratio from WLRC agrees with the data of B-DNA qualitatively. Using the general shape equations, we discuss a chiral model in which the solutions of straight, helical, and circular biopolymers are given, respectively. We also find that the model suggested by Helfrich [Langmuir 7, 567 (1991)] is very appropriate to describe B-DNA (or other biopolymers) if we choose the four phenomenological parameters as A=50 nm , C=60 nm(2) , alpha=40 nm(3) , and beta=50 nm(2) .