Electron–phonon scattering and excitonic effects in T-carbon

Through first-principles calculations combining many-body perturbation theory, we investigate electron–phonon scattering and optical properties including the excitonic effects of T-carbon. Our results reveal that optical and acoustic phonons dominate the scattering around the valence and the conduction band edges, respectively. In addition, the relaxation lifetimes of holes (0.5 ps) are longer than those of electrons (0.24 ps) around the band edges due to the weaker scattering. We also predict that mean free paths of hot holes are as high as 80 nm while only 15 nm for hot electrons, resulting in different hot carrier extraction ranges in T-carbon. Moreover, we demonstrate that there exist lowest energy dark excitons in T-carbon with radiative lifetime of about 3.4 s, which is revealed to be much longer than that of bright excitons and would lower the photoluminescence quantum yield of T-carbon.

Through first-principles calculations combining many-body perturbation theory, we investigate electron–phonon scattering and optical properties including the excitonic effects of T-carbon.     

Bilayer MSe2 (M = Zr,Hf) as promising two-dimensional thermoelectric materials: a first-principles study

Motivated by the experimental synthesis of two-dimensional MSe₂ (M = Zr, Hf) thin films, we set out to investigate the electronic, thermal, and thermoelectric transport properties of 1T-phase MSe₂ (M = Zr, Hf) bilayers on the basis of first-principles calculations and Boltzmann transport theory. Both bilayer ZrSe₂ and HfSe₂ are indirect band gap semiconductors possessing degenerate conduction bands and stair-like-shaped DOS, which provide a high n-doped power factor. In combination with the low lattice thermal conductivity that originated from the low phonon frequency of acoustic modes and the coupling of acoustic modes with optical modes, the maximum figure of merits ZT at room temperature for n-type doping are predicted as 1.84 and 3.83 for ZrSe₂ and HfSe₂ bilayers, respectively. Our results suggest that bilayer conformation of ZrSe₂ and HfSe₂ are promising thermoelectric materials with superior performance to their bulk counterparts.

Motivated by experimental synthesis of two-dimensional MSe₂ (M = Zr, Hf) thin films, we investigate the thermoelectric transport properties of MSe₂ (M = Zr, Hf) bilayers by using first-principles calculations and Boltzmann transport theory.     

Simple way of making free-standing cathode electrodes for flexible lithium-ion batteries

The flexible electrodes used in the lithium-ion battery (LIB) offer an excellent opportunity to be bent and folded without deforming their electrochemical characteristics. However, a flexible electrode does not include metal foil as a current collector, limiting the LIB''s flexibility and weakening the mechanical strength. This study fabricates flexible LiFePO₄ (LFP) free-standing electrodes by a scalable and straightforward solution-based etching process. The obtained free-standing electrodes show capacities and bending performances that are similar to the conventional electrodes with aluminum current collectors. This study opens a new avenue for developing a free-standing electrode for low-cost and flexible lithium-ion batteries.

The flexible electrodes used in the lithium-ion battery (LIB) offer an excellent opportunity to be bent and folded without deforming their electrochemical characteristics.     

Enhanced nonlinear optical response of graphene-based nanoflake van der Waals heterostructures

The nonlinear optical properties of van der Waals bilayer heterostructures composed of graphene/h-BN and graphene/phosphorene nanoflakes are investigated using time-dependent density functional theory. Our calculated results show a significant enhancement of the first-hyperpolarizability value, β in heterostructures relative to the pristine nanoflakes at λ = 1064 nm. The calculated enhancement in optical nonlinearity mainly results from in-plane anisotropy induced by the interlayer electronic coupling between the adjacent nanoflake layers; a higher degree of anisotropy is induced by puckered phosphorene compared to atomically flat h-BN yielding χ⁽²⁾ value corresponding to the second harmonic generation of ∼50 pm V⁻¹ in the zigzag graphene/phosphorene bilayer heterostructure. The calculated results clearly show that graphene-based nanoflake heterostructures giving large NLO coefficients together with high electron mobility of these materials offer new opportunities as candidate materials of choice for next-generation photonics and integrated quantum technologies.

The nonlinear optical properties of van der Waals bilayer heterostructures composed of graphene/h-BN and graphene/phosphorene nanoflakes are investigated using time-dependent density functional theory.     

Preparation and performance of electrically conductive Nb-doped TiO2/polyaniline bilayer coating for 316L stainless steel bipolar plates of proton-exchange membrane fuel cells

A bilayer coating composed of an inner layer of Nb-doped TiO₂ obtained by the sol–gel method and an external polyaniline layer with small SO₄²⁻ groups obtained by galvanostatic deposition was prepared to protect 316L stainless steel bipolar plates of proton-exchange membrane fuel cells. The corrosion resistances of bare 316L and 316L with single polyaniline coating and Nb-doped TiO₂/polyaniline bilayer coating were investigated. The experimental results indicated that both single and bilayer coatings increased the corrosion potential and decreased the corrosion current density compared with bare 316L stainless steel. A thirty-day exposure experiment indicated that the Nb-doped TiO₂/polyaniline bilayer showed high stability, and it protected 316L more effectively from the penetration of the corrosive ions.

A bilayer coating composed of an inner layer of Nb-doped TiO₂ and an external polyaniline layer with small SO₄²⁻ groups was prepared; the bilayer coating could protect 316L more effectively from the penetration of corrosive ions.     

First-principles study of Ni adatom migration on graphene with vacancies

A theoretical study based on first-principles calculations about the interaction and diffusion of Ni atoms on pristine graphene and graphene with a single vacancy is presented. In the first case, we explored the structural changes due to the adsorption of Ni on graphene and the effects on the electronic structure. In the case of graphene with a vacancy, we analyzed the impact of the adsorbed Ni atom on the distortion of the graphene structure and how it depends on the distance from the graphene defect. In the analysis, we observed the changes in the electron localization function and the charge density. By knowing the interaction map of Ni with graphene, and the structural changes of the network, we performed energy barrier calculations within the climbing image nudged elastic band methodology to study the nickel diffusion. Finally, we explored how the vacancy and structural distortions affect the minimum energy paths and the saddle points for nickel moving away, around, and towards the vacancy.

Adsorption and diffusion of Ni atom over graphene with a vacancy were studied using first-principles calculations.     

Epitaxial growth and interfacial property of monolayer MoS2 on gallium nitride

Two-dimensional (2D) transition-metal dichalcogenides (TMDCs) on semiconductor substrates are important for next-generation electronics and optoelectronics. In this study, we demonstrate the growth of monolayer MoS₂ on a lattice-matched gallium nitride (GaN) semiconductor substrate by chemical vapor deposition (CVD). The monolayer MoS₂ triangles exhibit optical properties similar to that of typical single-crystal MoS₂ sheets, as verified by the Raman, photoluminescence, and morphological characterizations. The Raman and PL features and their intensity mappings suggest that the as-grown MoS₂ on GaN substrate can achieve high quality and uniformity, demonstrating that GaN substrate is favorable for 2D MoS₂ growth. Moreover, the interfacial property and stacking structure were investigated by first-principles density functional theory (DFT) calculations to confirm the interlayer interactions of monolayer MoS₂ on GaN. Accordingly, the ability to grow high quality monolayer MoS₂ on semiconductor GaN substrate would open a new route toward the synthesis of hetero and composite structures for promising electronic and optoelectronic device applications.

Monolayer MoS₂ were grown on GaN substrate by CVD method, and the interfacial property of the MoS₂–GaN system were studied by first-principles density functional theory calculations.     

Investigating partitioning of free versus macrocycle bound guest into a model POPC lipid bilayer

We report on the permeation of free and macrocycle-bound avobenzone across a POPC lipid bilayer through combined neutron reflectometry experiments and molecular dynamics simulations. Results indicate that the p-phosphonated calix[8]arene macrocycle limits the avobenzone penetration into the upper leaflet of the membrane. Hence, it could serve as a useful vehicle for safer formulations.

We report on the permeation of free and macrocycle-bound avobenzone across a POPC lipid bilayer through combined neutron reflectometry experiments and molecular dynamics simulations.     

Essential geometric and electronic properties in stage-n graphite alkali-metal-intercalation compounds

The rich and unique properties of the stage-n graphite alkali-metal-intercalation compounds are fully investigated by first-principles calculations. According to the main features, the lithium and non-lithium (Na, K, Rb, Cs) systems are quite different from each other in stacking configurations, intercalant alkali-metal-atom concentrations, free conduction electron densities, atom-dominated and (carbon, alkali metal)-co-dominated energy bands, and interlayer charge density distributions. The close relations between the alkali-metal-doped metallic behaviors and the geometric symmetries are clarified through the interlayer atomic interactions. The stage-1 graphite alkali-metal-intercalation compounds possess the highest charge distribution for all stage-n types; moreover, those of the lithium systems are greater than those of the non-lithium systems. The lithium systems also have the largest blue shift of the Fermi level among all alkali metal systems.

The rich and unique properties of the stage-n graphite alkali-metal-intercalation compounds are fully investigated by first-principles calculations.     

Featured properties of Li+-based battery anode: Li4Ti5O12

3D ternary Li₄Ti₅O₁₂, a Li⁺-based battery anode, presents an unusual lattice symmetry (triclinic crystal), band structure, charge density, and density of states under first-principles calculations. It is a large direct-gap semiconductor with E^d_g ∼ 2.98 eV. The atom-dominated valence and conduction bands, the spatial charge distribution and the atom- and orbital-decomposed van Hove singularities are available for delicate identifications of multi-orbital hybridizations in Li–O and Ti–O bonds. The extremely non-uniform chemical environment, which induces very complicated hopping integrals, directly arises from the large bonding fluctuations and the highly anisotropic configurations. Also, the developed theoretical framework is very useful for fully understanding cathodes and electrolytes of oxide compounds.

A theoretical framework based on first-principles calculations is developed for the essential properties of the 3D ternary compound Li₄Ti₅O₁₂, a Li⁺-based battery anode.     

The structure and vibrational spectroscopy of cryolite,Na3AlF6

Cryolite, Na₃[AlF₆], is essential to commercial aluminium production because alumina is readily soluble in molten cryolite. While the liquid state has been extensively investigated, the spectroscopy of the solid state has been largely ignored. In this paper, we show that the structure at 5 K is the same as that at room temperature. We use a combination of infrared and Raman spectroscopies together with inelastic neutron scattering (INS) spectroscopy. The use of INS enables access to all of the modes of Na₃[AlF₆], including those that are forbidden to the optical spectroscopies. Our spectral assignments are supported by density functional theory calculations of the complete unit cell.

We use a combination of infrared, Raman and inelastic neutron scattering spectroscopies to access all of the modes of cryolite, Na₃[AlF₆], including those that are forbidden to the optical spectroscopies.     

Amorphous red phosphorus incorporated with pyrolyzed bacterial cellulose as a free-standing anode for high-performance lithium ion batteries

Amorphous red phosphorus/pyrolyzed bacterial cellulose (P-PBC) free-standing films are prepared by thermal carbonization and a subsequent vaporization-condensation process. The distinctive bundle-like structure of the flexible pyrolyzed bacterial cellulose (PBC) matrix not only provides sufficient volume to accommodate amorphous red-phosphorus (P) but also restricts the pulverization of red-P during the alternate lithiation/delithiation process. When the mass ratio of raw materials, red-P to PBC, is 70 : 1, the free-standing P-PBC film anode exhibits high reversible capacity based on the mass of the P-PBC film (1039.7 mA h g⁻¹ after 100 cycle at 0.1C, 1C = 2600 mA g⁻¹) and good cycling stability at high current density (capacity retention of 82.84% after 1000 cycles at 2C), indicating its superior electrochemical performances.

A novel freestanding anode was prepared by combining amorphous red-P with a pyrolyzed bacterial cellulose (PBC) matrix for the first time.     

A study on hydrogen storage performance of Ti decorated vacancies graphene structure on the first principle

The structural properties, formation energy, adsorption energy, and electronic properties of vacancy graphene are studied by first-principles analysis. We found that the formation energy and adsorption energy of double vacancy graphene (DVG-4) are the largest. A single defect in DVG-4 can adsorb at least nine hydrogen molecules, and compared with Ti modified single vacancy graphene (SVG–Ti), the adsorption capacity is increased by 80%. When DVG-4 adsorbs the second, third, and fourth hydrogen molecules, the adsorption energy is greater than 0.7 eV, which is not conducive to the release. Density of state (DOS) and electron density difference (EDIFF) results reveal that charge transfer occurs among hydrogen molecules, Ti atoms, and DVG-4, decreasing the hydrogen adsorption capacity of DVG-4 by 33%. DVG - 4 has the potential to become an excellent hydrogen storage material.

The structural properties, formation energy, adsorption energy, and electronic properties of vacancy graphene are studied by first-principles analysis.     

Prediction of allotropes of tellurium with molecular,one- and two-dimensional covalent nets for photofunctional applications

In the present work, three new semiconducting two-dimensional (2D) Te phases containing three- and four-coordinated Te centers were proposed by using evolutionary algorithms combined with first-principles calculations. Using density functional theory calculations, we discussed the bonding and electronic properties in these phases, and subsequently rationalized their structures. The viability of these predicted structures was demonstrated by evaluating their thermodynamic, dynamic, mechanical, and thermal stabilities. Moreover, a significant direct band gap (0.951–1.512 eV) and excellent transport properties were evidenced in 2D Te nets, which suggests that they could be promising photovoltaic materials candidates. This is further supported by the stability of the associated bulk layered counterparts of the 2D Te nets.

By using evolutionary algorithms-DFT calculations, 5 novel Te allotropes, including three 2D Te phases with 3- and 4-coordinated Te centres were proposed. Their viability, bonding, and electronic properties are further assessed.     

Electrophilic activation of nitroalkanes in efficient synthesis of 1,3,4-oxadiazoles

A novel methodology for general and chemoselective preparation of non-symmetric 1,3,4-oxadiazoles is developed. This unusual reaction proceeds via polyphosphoric acid-assisted activation of nitroalkanes towards nucleophilic attack with acylhydrazides.

A novel methodology for chemoselective preparation of non-symmetric 1,3,4-oxadiazoles via PPA-assisted activation of nitroalkanes towards nucleophilic attack with acylhydrazides is developed.     

Enhanced water permeability across a physiological droplet interface bilayer doped with fullerenes

We measure the water permeability across a physiological lipid bilayer produced by the droplet interface bilayer (DiB) technique. This lipid bilayer can be considered as physiologically relevant because it presents a lipidic composition close to human cell membranes. The measured water permeability coefficients across this lipid bilayer are reported as a function of the cholesterol concentration. It is found that the water permeability coefficients decreased with increasing cholesterol concentration, in agreement with the existing literature. And, consistently, the extracted corresponding activation energies increase with increasing cholesterol concentration in the lipid bilayer. Hence having demonstrated the robustness of the experimental system, we extend this study by exploring the influence of fullerenes on the water permeability of a physiological lipid bilayer. Interestingly, we observe a significant increase of the measured water permeability coefficients across this lipid bilayer for large fullerenes concentration. This enhanced permeability might be related to the conductive properties of fullerenes.

We measure the water permeability across a physiological lipid bilayer produced by the droplet interface bilayer technique.     

Allotropes of tellurium from first-principles crystal structure prediction calculations under pressure

We investigated the allotropes of tellurium under hydrostatic pressure based on density functional theory calculations and crystal structure prediction methodology. Our calculated enthalpy-pressure and energy-volume curves unveil the transition sequence from the trigonal semiconducting phase, represented by the space group P3₁21 in the range of 0–6 GPa, to the body centered cubic structure, space group Im$\bar{3}$m, stable at 28 GPa. In between, the calculations suggest a monoclinic structure, represented by the space group C2/m and stable at 6 GPa, and the β-Po type structure, space group R$\bar{3}$m, stable at 10 GPa. The face-centered structure is found at pressure as high as 200 GPa. As the pressure is increased, the transition from the semiconducting phase to metallic phases is observed.

Through first-principles simulations, we suggest the phase stability of the allotropic transition sequence of tellurium from the trigonal structure up to the cubic structure.     

Structural,electronic, and optical properties of cubic formamidinium lead iodide perovskite: a first-principles investigation

Hybrid organic–inorganic perovskites have been one of the most active areas of research into photovoltaic materials. Despite the extremely fast progress in this field, the electronic properties of formamidinium lead iodide perovskite (FAPbI₃) that are key to its photovoltaic performance are relatively poorly understood when compared to those of methylammonium lead iodide (MAPbI₃). In this study, first-principles total energy calculations based on density functional theory were used to investigate the favored orientation of FA. Different theoretical methods, with or without incorporation of spin-orbit coupling (SOC) effects, were used to study the structure, electronic properties, and charge-carrier effective mass. Also the SOC-induced Rashba k-dependent band splitting, density of states and optical properties are presented and discussed. These results are useful for understanding organic–inorganic lead trihalide perovskites and can inform the search for new materials and design rules.

Different theoretical methods, including SOC effects, were used to study the detailed structure, electronic properties, charge-carrier mobility, and SOC-induced Rashba k-dependent band splitting in FAPbI₃.     

Development of new 2-piperidinium-4-styrylcoumarin derivatives with large Stokes shifts as potential fluorescent labels for biomolecules

A series of novel 2-piperidinium-4-styrylcoumarin derivatives, with large Stokes shifts and high fluorescence quantum yields, were synthesized using an efficient and low-cost synthetic strategy as potential fluorescent labels for biomolecules. Density functional theory and time-dependent density functional theory calculations were performed in order to rationalize the observed photophysical properties.

New 2-piperidinium-4-styrylcoumarin derivatives, with large Stokes shifts and high fluorescence quantum yields, as potential fluorescent labels for biomolecules.     

Exploring the sub-stoichiometric behavior of plutonium mononitride

The intriguing and controversial sub-stoichiometric behavior of plutonium mononitride is investigated here using first-principles calculations combined with special quasirandom structures. It is found that NaCl-type plutonium mononitride is stable for only stoichiometric levels, and the formation enthalpy of plutonium mononitride is in good agreement with others. By comparing with plutonium monocarbide, the main reason for the absence of sub-stoichiometric behavior is the lower N-2p orbital energy, resulting in less hybridization and weaker Pu–N bonds. The weaker Pu–N bonds cannot support the formation of vacancies.

Plutonium mononitride is investigated using first-principles calculation combined with special quasirandom structure. The weak bonds probably are the main reason for the absence of sub-stoichiometry for plutonium mononitride.