Yongqing Cai

Polarity-Reversed Robust Carrier Mobility in Monolayer MoS2 Nanoribbons

Using first-principles calculations and deformation potential theory, we investigate the intrinsic carrier mobility (μ) of monolayer MoS2 sheet and nanoribbons. In contrast to the dramatic deterioration of μ in graphene upon forming nanoribbons, the magnitude of μ in armchair MoS2 nanoribbons is comparable to its sheet counterpart, albeit oscillating with ribbon width. Surprisingly, a room-temperature transport polarity reversal is observed with μ of hole (h) and electron (e) being 200.52 (h) and 72.16 (e) cm(2) V(-1) s(-1) in sheet, and 49.72 (h) and 190.89 (e) cm(2) V(-1) s(-1) in 4 nm nanoribbon. The high and robust μ and its polarity reversal are attributable to the different characteristics of edge states inherent in MoS2 nanoribbons. Our study suggests that width reduction together with edge engineering provide a promising route for improving the transport properties of MoS2 nanostructures.

Lattice vibrational modes and phonon thermal conductivity of monolayer MoS2

monolayer, consisting of a hexagonal lattice of Moatoms sandwiched between two similar lattices of S atomsin a trigonal prismatic arrangement, has recently attractedconsiderable attention for field effect transistor (FET) andoptical device applications due to the presence of a finiteband gap [1,2]. Great efforts have been made to understandthe dynamics of carriers of MoS

Protein Induces Layer-by-Layer Exfoliation of Transition Metal Dichalcogenides

Here, we report a general and facile method for effective layer-by-layer exfoliation of transition metal dichalcogenides (TMDs) and graphite in water by using protein, bovine serum albumin (BSA) to produce single-layer nanosheets, which cannot be achieved using other commonly used bio- and synthetic polymers. Besides serving as an effective exfoliating agent, BSA can also function as a strong stabilizing agent against reaggregation of single-layer nanosheets for greatly improving their biocompatibility in biomedical applications. With significantly increased surface area, single-layer MoS2 nanosheets also exhibit a much higher binding capacity to pesticides and a much larger specific capacitance. The protein exfoliation process is carefully investigated with various control experiments and density functional theory simulations. It is interesting to find that the nonpolar groups of protein can firmly bind to TMD layers or graphene to expose polar groups in water, facilitating the effective exfoliation of single-layer nanosheets in aqueous solution. The present work will enable to optimize the fabrication of various 2D materials at high yield and large scale, and bring more opportunities to investigate the unique properties of 2D materials and exploit their novel applications.

Surface‐Charge‐Mediated Formation of H‐TiO2@Ni(OH)2 Heterostructures for High‐Performance Supercapacitors

An electrochemically favorable Ni(OH)2 with porously hierarchical structure and ultrathin nanosheets in a core-shell structure H-TiO2 @Ni(OH)2 is achieved through modulating the surface chemical activity of TiO2 by hydrogenation, which creates a defect-rich surface of TiO2 , thereby facilitating the subsequent nucleation and growth of Ni(OH)2 . These configuration-tailored H-TiO2 @Ni(OH)2 core-shell nanowires exhibit a superior electrochemical performance and good flexibility.

Highly Itinerant Atomic Vacancies in Phosphorene

Using detailed first-principles calculations, we investigate the hopping rate of vacancies in phosphorene, an emerging elemental 2D material besides graphene. Our work predicts that a direct observation of these monovacancies (MVs), showing a highly mobile and anisotropic motion, is possible only at low temperatures around 70 K or below where the thermal activity is greatly suppressed. At room temperature, the motion of a MV is 16 orders faster than that in graphene, because of the low diffusion barrier of 0.3 eV. Built-in strain associated with the vacancies extends far along the zigzag direction while attenuating rapidly along the armchair direction. We reveal new features of the motion of divacancies (DVs) in phosphorene via multiple dissociation-recombination processes of vacancies owing to a small energy cost of ∼1.05 eV for the splitting of a DV into two MVs. Furthermore, we find that uniaxial tensile strain along the zigzag direction can promote the motion of MVs, while the tensile strain along the armchair direction has the opposite effect. These itinerant features of vacancies, rooted in the unique puckering structure facilitating bond reorganization, enable phosphorene to be a bright new opportunity to broaden the knowledge of the evolution of vacancies, and a proper control of the exceedingly active and anisotropic movement of the vacancies should be critical for applications based on phosphorene.

Convenient purification of gold clusters by co-precipitation for improved sensing of hydrogen peroxide, mercury ions and pesticides

An effective separation process is developed to remove free protein from the protein-protected gold clusters via co-precipitation with zinc hydroxide on their surface. After dialysis, the purified clusters exhibit an enhanced fluorescence for improved sensitive detection and selective visualization.

The role of H2O and O2 molecules and phosphorus vacancies in the structure instability of phosphorene

The poor structural stability of phosphorene in air was commonly ascribed to humidity and oxygen molecules. Recent exfoliation of phosphorene in deoxygenated water promotes the need to re-examine the role of H2O and O2 molecules. Considering the presence of high population of vacancies in phosphorene, we investigate the interaction of H2O and O2 molecules with vacancy-contained phosphorene using first-principles calculations. In contrast to the common notion that physisorbed molecules tend to have a stronger adsorption at vacancy sites, we show that H2O has nearly the same adsorption energy at the vacancy site as that at the perfect one. Charge transfer analysis shows that O2 is a strong electron scavenger, which transfers the lone-pair electrons of the phosphorus atoms to the 2{\pi}* antibonding orbital of O2. As a result, the barrier for the O-O bond splitting to form O-P bonds is reduced from 0.81 eV at the perfect site to 0.59 eV at the defect site, leading to an about 5000 faster oxidizing rate at the defect site than at the perfect site at room temperature. Hence, our work reveals that the vacancy in phosphorene shows a stronger oxygen affinity than the perfect phosphorene lattice site. Structural degradation of phosphorene due to oxidization may occur rapidly at edges and grain boundaries where vacancies tend to agglomerate.

Few-Layer Black Phosphorus Carbide Field-Effect Transistor via Carbon Doping

Black phosphorus carbide (b-PC) is a new family of layered semiconducting material that has recently been predicted to have the lightest electrons and holes among all known 2D semiconductors, yielding a p-type mobility (≈105 cm2 V-1 s-1 ) at room temperature that is approximately five times larger than the maximum value in black phosphorus. Here, a high-performance composite few-layer b-PC field-effect transistor fabricated via a novel carbon doping technique which achieved a high hole mobility of 1995 cm2 V-1 s-1 at room temperature is reported. The absorption spectrum of this material covers an electromagnetic spectrum in the infrared regime not served by black phosphorus and is useful for range finding applications as the earth atmosphere has good transparency in this spectral range. Additionally, a low contact resistance of 289 Ω µm is achieved using a nickel phosphide alloy contact with an edge contacted interface via sputtering and thermal treatment.

Destabilization of Gold Clusters for Controlled Nanosynthesis: From Clusters to Polyhedra

A precisely controlled destabilization of gold thiolate clusters is demonstrated to grow 12 {110}-faceted gold dodecahedra with greatly enhanced catalytic capability, and reveal the growth mechanism by DFT simulations. This greatly advances our understanding of nanocrystal growth and opens a new window for controlling the dissociation of clusters to produce nanocrystals with specific shapes.

Black Phosphorus N-Type Field-Effect Transistor with Ultrahigh Electron Mobility via Aluminum Adatoms Doping

High-performance black phosphorus n-type field-effect transistors are realized using Al adatoms as effective electron donors, which achieved a record high mobility of >1495 cm2 V-1 s-1 at 260 K. The electron mobility is corroborated to charged-impurity scattering at low temperature, whilst metallic-like conduction is observed at high gate bias with increased carrier density due to enhanced electron-phonon interactions at high temperature.