Layer-dependent stability of 2D mica nanosheets.

We report on the layer-dependent stability of muscovite-type two-dimensional (2D) mica nanosheets (KAl3Si3O10(OH)2). First-principles calculations on mica nanosheets with different layer thicknesses (n = 1, 2, and 3) reveal their layer-dependent stability; odd-numbered 2D mica nanosheets are more stable than even-numbered ones, and the preferable stability of odd-numbered layers originates from electronic effects. A core-shielding model is proposed with a reasonable assumption, successfully proving the instability of the even-numbered mica nanosheets. Raman imaging supports that the population of odd-numbered mica nanosheets is predominant in exfoliated mica products. The alternating charge states with odd/even layers were evidenced by Kelvin probe force microscopy. We also demonstrate a unique photocatalytic degradation, opening new doors for environmental applications of mica nanosheets.

Ab initio study of Li, Mg and Al insertion into rutile VO2: fast diffusion and enhanced voltages for multivalent batteries.

Vanadium oxides are among the most promising materials that can be used as electrodes in rechargeable metal-ion batteries. In this work, we systematically investigate thermodynamic, electronic, and kinetic properties associated with the insertion of Li, Mg and Al atoms into rutile VO2. Using first-principles calculations, we systematically study the structural evolution and voltage curves of LixVO2, MgxVO2 and AlxVO2 (0 < x < 1) compounds. The calculated lithium intercalation voltage starts at 3.50 V for single-atom insertion and decreases to 2.23 V for full lithiation, to the LiVO2 compound, which agrees well with the experimental results. The Mg insertion features a plateau about 1.6 V up to Mg0.5VO2 and then another plateau-like region at around 0.5 V up to Mg1VO2. The predicted voltage curve for Al insertion starts at 1.98 V, followed by two plateaus at 1.48 V and 1.17 V. The diffusion barrier of Li, Mg and Al in the tunnel structure of VO2 is 0.06, 0.33 and 0.50 eV, respectively. The demonstrated excellent Li, Mg and Al mobility, high structural stability and high specific capacity suggest promising potential of rutile VO2 electrodes especially for multivalent batteries.

Surface reactivity and vacancy defects in single-layer borophene polymorphs.

Single-layer borophene is a novel 2D material which combines high strength, light weight and metallicity. Using first-principles calculations, we systematically investigate the defect formation and surface reactivity in three major borophene polymorphs (α, ß and triangular). We find that ß-B is generally the most reactive borophene form, while α-B is the least reactive. In particular, there is more than 1.5 eV difference in substitutional energies for typical dopants in ß-B and α-B polymorphs. Single vacancy defects can be created quite easily in all borophene sheets with formation energies (0.16 to 1.93 eV) much lower than those in graphene (7.69 eV). Adatom adsorption is exothermic and stabilizes electron-deficient boron monolayers. Many interesting properties arise from the rich structural chemistry of borophene, comprising four-, five-, and six-coordinated atoms, as well as hexagonal vacancies.

Aluminium and magnesium insertion in sulfur-based spinels: a first-principles study.

We computationally screen several sulfur-based materials with a spinel crystal structure as potential Al and Mg insertion hosts for Al- and Mg-ion batteries. We evaluate the effect of transition-metal substitution (TM = Ti, Cr, Mn, Fe, Co, Ni) on the key properties determining electrode performance. We systematically calculate the thermodynamic stability, average voltage, binding energy, volume expansion, and Al/Mg diffusion for all compounds. The results suggest that the Ni-based spinel shows a relatively high Al and Mg insertion voltage and low diffusion barriers, and thus is a promising candidate cathode material for Al- and Mg-ion batteries.

Volume dependence of the dielectric properties of amorphous SiO2.

Using first principles calculations, the analysis of the dielectric properties of amorphous SiO2 (am-SiO2) was performed. We found that the am-SiO2 properties are volume dependent, and the dependence is mainly induced by the variation of nanoporosity at the atomic scale. In particular, both ionic and electronic contributions to the static dielectric constants are functions of volume with clear trends. Moreover, using the unique parameterization of the dielectric function provided in this work, we predict dielectric functions at imaginary frequencies of different SiO2 polymorphs having similar band gap energies.

Phosphorene as an anode material for Na-ion batteries: a first-principles study.

We systematically investigate a novel two-dimensional nanomaterial, phosphorene, as an anode for Na-ion batteries. Using first-principles calculations, we determine the Na adsorption energy, specific capacity and Na diffusion barriers on monolayer phosphorene. We examine the main trends in the electronic structure and mechanical properties as a function of Na concentration. We find a favorable Na-phosphorene interaction with a high theoretical Na storage capacity. We find that Na-phosphorene undergoes semiconductor-metal transition at high Na concentration. Our results show that Na diffusion on phosphorene is fast and anisotropic with an energy barrier of only 0.04 eV. Owing to its high capacity, good stability, excellent electrical conductivity and high Na mobility, monolayer phosphorene is a very promising anode material for Na-ion batteries. The calculated performance in terms of specific capacity and diffusion barriers is compared to other layered 2D electrode materials, such as graphene, MoS2, and polysilane.

Adsorption of metal adatoms on single-layer phosphorene.

Single- or few-layer phosphorene is a novel two-dimensional direct-bandgap nanomaterial. Based on first-principles calculations, we present a systematic study on the binding energy, geometry, magnetic moment and electronic structure of 20 different adatoms adsorbed on phosphorene. The adatoms cover a wide range of valences, including s and p valence metals, 3d transition metals, noble metals, semiconductors, hydrogen and oxygen. We find that adsorbed adatoms produce a rich diversity of structural, electronic and magnetic properties. Our work demonstrates that phosphorene forms strong bonds with all studied adatoms while still preserving its structural integrity. The adsorption energies of adatoms on phosphorene are more than twice higher than on graphene, while the largest distortions of phosphorene are only ∼0.1-0.2 Å. The charge carrier type in phosphorene can be widely tuned by adatom adsorption. The unique combination of high reactivity with good structural stability is very promising for potential applications of phosphorene.

Acceptor-compensated charge transport and surface chemical reactions in Au-implanted SnO2 nanowires.

A new deep acceptor state is identified by density functional theory calculations, and physically activated by an Au ion implantation technique to overcome the high energy barriers. And an acceptor-compensated charge transport mechanism that controls the chemical sensing performance of Au-implanted SnO2 nanowires is established. Subsequently, an equation of electrical resistance is set up as a function of the thermal vibrations, structural defects (Au implantation), surface chemistry (1 ppm NO2), and solute concentration. We show that the electrical resistivity is affected predominantly not by the thermal vibrations, structural defects, or solid solution, but the surface chemistry, which is the source of the improved chemical sensing. The response and recovery time of chemical sensing is respectively interpreted from the transport behaviors of major and minor semiconductor carriers. This acceptor-compensated charge transport mechanism provides novel insights not only for sensor development but also for research in charge and chemical dynamics of nano-semiconductors.

Controlling Na diffusion by rational design of Si-based layered architectures.

By means of density functional theory, we systematically investigate the insertion and diffusion of Na and Li in layered Si materials (polysilane and H-passivated silicene), in comparison with bulk Si. It is found that Na binding and mobility can be significantly facilitated in layered Si structures. In contrast to the Si bulk, where Na insertion is energetically unfavorable, Na storage can be achieved in polysilane and silicene. The energy barrier for Na diffusion is reduced from 1.06 eV in the Si bulk to 0.41 eV in polysilane. The improvements in binding energetics and in the activation energy for Na diffusion are attributed to the large surface area and available free volume for the large Na cation. Based on these results, we suggest that polysilane may be a promising anode material for Na-ion and Li-ion batteries with high charge-discharge rates.

Enhanced Li adsorption and diffusion in single-walled silicon nanotubes: an ab initio study.

We report a first-principles investigation of Li adsorption and diffusion in single-walled Si nanotubes (SWSiNTs) of interest to Li-ion battery anodes. We calculate Li insertion characteristics in SWSiNTs and compare them with the respective ones in carbon nanotubes (CNTs) and other silicon nanostructures. From our calculations, SWSiNTs show higher reactivity toward the adsorption of Li adatoms than CNTs and Si nanoclusters. Considering the importance of Li kinetics, we demonstrate that the interior of SWSiNTs may serve as a fast Li diffusion channel. The important advantage of SWSiNTs over their carbon analogues is a sevenfold reduction in the energy barrier for the penetration of the Li atoms into the nanotube interior through the sidewalls. This prepossesses easier Li diffusion inside the tube and subsequent utilization of the interior sites, which enhances Li storage capacity of the system. The improvements in both Li uptake and Li mobility over their analogues support the great potential of SWSiNTs as Li-ion battery anodes.