The precise control of edge geometry and crystal shape of monolayer MoS 2 is particular of importance for their applications in nanoelectronics and photo-electro catalysts. Here we reveal a crucial role of chemical potential in the determination of equilibrium shape (ES) and edge structure of monolayer MoS 2 by using density-functional theory calculations. Applying Wulff construction rule, our results demonstrate the shape evolution of monolayer MoS 2 flake from the dodecagonal shape, then to the hexagonal shape, to the triangular shape with the variation of chemical potential from the Mo-rich to the S-rich condition, and the edge structure of ES changes correspondingly from mixed zigzag/armchair edges to pure zigzag edges. This finding can be applied to explain extensive experimental observations about the morphology of MoS 2 domains. Meanwhile, the edge magnetism and electronic structures of monolayer MoS 2 domains are found to be dependent on their edge structure, which provides specific guidance for the magnetic modulation of monolayer MoS 2 and designing more effective MoS 2 -based catalysts.

Two-dimensional (2D) layered MoS2 nanosheets possess great potential as anode materials for lithium ion batteries (LIBs), but they still suffer from poor cycling performance. Improving the cycling stability of electrode materials depends on a deep understanding of their dynamic structural evolution and reaction kinetics in the lithiation process. Herein, thermodynamic phase diagrams and the lithiation dynamics of MoS2-based nanostructures with the intercalation of lithium ions are studied by using first-principles calculations and ab initio molecular dynamics simulations. Our results demonstrate that the continuous intercalation of Li ions induces structural destruction of 2H phase MoS2 nanosheets in the discharge process that follows a layer-by-layer dissociation mechanism. Meanwhile, the intercalation of Li ions leads to a structural transition of MoS2 nanosheets from the 2H to the 1T phase due to the ultralow transition barriers (∼0.1 eV). We find that the phase transition can slow down the dissociation of MoS2 nanosheets during lithiation. The result can be applied to explain extensive experimental observation of the fast capacity fading of MoS2-based anode materials between the first and the subsequent discharges. To suppress the dissociation of MoS2 nanosheets in the lithiation process, we propose a strategy by constructing a sandwich-like graphene/MoS2/graphene structure that indicates high chemical stability, superior conductivity, and high Li-ion mobility in the charge/discharge process, implying the possibility to induce an improvement in the anode cycling performance. This work opens a new route to rational design layered transition-metal disulfide (TMD) anode materials for LIBs with superior cycling stability and electrochemical performance.

Atomically thin Janus transition metal dichalcogenides (JTMDs) with an asymmetric structure have emerged as a new class of intriguing two-dimensional (2D) semiconductor materials. Using state-of-the-art density functional theory (DFT) calculations, we systematically investigate the structural, electronic, and optical properties of JTMD monolayers and heterostructures. Our calculated results indicate that the JTMD monolayers suffer from a bending strain but present high thermodynamic stability. All of them are semiconductors with a band-gap range from 1.37 to 1.96 eV. They possess pronounced optical absorption in the visible-light region and cover a large range of carrier mobilities from 28 to 606 cm2 V-1 s-1, indicating strong anisotropic characteristics. Significantly, some monolayer JTMDs (e.g., WSSe and WSeTe) exhibit superior mobilities than conventional TMD monolayers, such as MoS2. Moreover, the absolute band-edge positions of the JTMD monolayers are higher than the water redox potential, and most JTMD heterostructures have a type-II band alignment that contributes to the separation of carriers. Our work suggests that the 2D JTMD monolayers are promising for nanoelectronic, optoelectronic, and photocatalytic applications.

The synthesis of carbonaceous materials from a metal organic framework (MIL-100), organic linker and N-precursor was comprehensively investigated, and the structures of the products were characterized. It was found that simple pyrolysis of mixed MIL-100 (Fe)/dicyandiamide (DCDA) could produce nitrogen-doped graphene (N-graphene). The N-graphene showed excellent performances in peroxymonosulfate (PMS) activation, which were superior to those of counterparts of graphene, iron(ii, iii) oxide, manganese(iv) oxide and cobalt(ii, iii) oxide. With PMS activation, N-graphene exhibited efficient catalytic degradation of various organic pollutants such as phenol, 2,4,6-trichlorophenol (TCP), sulfachloropyridazine (SCP) and p-hydroxybenzoic acid (PHBA). Electron paramagnetic resonance (EPR) spectroscopy and radical quenching tests were employed to investigate the PMS activation and organic degradation processes. It was found that singlet oxygen (1O2) was mainly produced during the activation of PMS by N-graphene, and contributed to the catalytic oxidation instead of sulfate and/or hydroxyl radicals. These findings provide new insights into PMS activation by metal-free carbon catalysi

Resolving the structure−property relationships of twodimensional (2D) organic−inorganic hybrid perovskites is essential for the development of photovoltaic and photoelectronic devices. Here, pressure (0−10 GPa) was applied to 2D hybrid perovskite flakes mechanically exfoliated from butylammonium lead halide single crystals, (C 4 H 9 NH 3 ) 2 PbI 4 , from which we observed a series of changes of the strong excitonic emissions in the photoluminescence spectra. By correlating with in situ high-pressure X-ray diffraction results, we examine successfully the relationship between structural modifications in the inorganic PbI 4 2− layer and their excitonic properties. During the transition between Pbca (1b) phase and Pbca (1a) phase at around 0.1 GPa, the decrease in ⟨Pb−I−Pb⟩ bond angle and increase in Pb−I bond length lead to an abrupt blue shift of the excitonic bandgap. The presence of the P2 1 /a phase above 1.4 GPa increases the ⟨Pb−I−Pb⟩ bond angle and decreases the Pb−I bond length, leading to a deep red shift of the excitonic bandgap. The total band gap narrowing of ∼350 meV to 2.03 eV at 5.3 GPa before amorphization, facilitates (C 4 H 9 NH 3 ) 2 PbI 4 as a much better solar absorber. Moreover, phase transitions inevitably modify the carrier lifetime of (C 4 H 9 NH 3 ) 2 PbI 4 , where an initial 150 ps at ambient phase is prolongated to 190 ps in the Pbca (1a) phase along with enhanced photoluminescence (PL), originating from pressure-induced strong radiative recombination of trapped excitons.The onset of P2 1 /a phase shortens significantly the carrier lifetime to 53 ps along with a weak PL emission due to pressure-induced severe lattice distortion and amorphization. High-pressure study on (C 4 H 9 NH 3 ) 2 PbI 4 nm-thin flakes may provide insights into the mechanisms for synthetically designing novel 2D hybrid perovskite based photoelectronic devices and solar cells.

Lead-free double perovskite phosphors are promising alternatives to lead halide perovskites for wide uses in optoelectronic applications, but suffer from a low quantum efficiency. Here, we propose to enhance the quantum efficiency of orange-emitting Cs 2 Ag 0.4 Na 0.6 InCl 6 :Bi via a codoping strategy. The internal quantum efficiency of Cs 2 Ag 0.4 Na 0.6 InCl 6 :1% Bi phosphor was increased from 89.9% to the record 98.4 and 98.6% by codoping with 1% Ni and 1% Ce, respectively. High-level density functional theory calculations have revealed that the enhanced efficiency is ascribed to the formation of shallow trap states by codoping with Ce, while codoping with Mn can create delocalized deep traps that decrease the quantum efficiency. By applying the (Bi,Ce)-codoped sample together with a blue BaMgAl 10 O 17 :Eu 2+ phosphor, a white light-emitting diode with an excellent color rendering index of 95.7 and a correlated color temperature of 4430 K has been demonstrated. The developed Cs 2 Ag 0.4 Na 0.6 InCl 6 :Bi,Ce phosphor shows a great potential as downconversion luminescent material in solid-state lighting for general illumination.

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The deep understanding of nucleation and growth mechanisms is fundamental for the precise control of the size, layer number, and crystal quality of two-dimensional (2D) transition-metal dichalcogenides (TMDs) with the chemical vapor deposition (CVD) method. In this work, we present a systematic spectroscopic study of CVD-grown MoS2, and two types of MoS2 flakes have been identified: one type of flake contains a central nanoparticle with the multilayer MoS2 structure, and the other is dominated by triangular flakes with monolayer or bilayer structures. Our results demonstrate that two types of flakes can be tuned by changing the growth temperature and carrier-gas flux, which originates from their different nucleation mechanisms that essentially depends on the concentration of MoO3–x and S vapor precursors: a lower reactant concentration facilitates the 2D planar nucleation that leads to the monolayer/bilayer MoS2 and a higher reactant concentration induces the self-seeding nucleation which easily produces few-layer and multilayer MoS2. The reactant-concentration dependence of nucleation can be used to control the growth of MoS2 and understand the growth mechanism of other TMDs.