µSR study of spin freezing and persistent spin dynamics in NaCaNi2F7.

A new pyrochlore compound, NaCaNi2F7, was recently synthesized and has a single magnetic site with spin-1 Ni2+ . We present zero field and longitudinal field muon spin rotation (µSR) measurements on this pyrochlore. Density functional theory calculations show that the most likely muon site is located between two fluorine ions, but off-centre. A characteristic F-µ-F muon spin polarization function is observed at high temperatures where Ni spin fluctuations are sufficiently rapid. The Ni2+ spins undergo spin freezing into a disordered ground state below 4 K, with a characteristic internal field strength of 140 G. Persistent Ni spin dynamics are present to our lowest temperatures (75 mK), a feature characteristic of many geometrically frustrated magnetic systems.

One-dimensional electron gas in strained lateral heterostructures of single layer materials.

Confinement of the electron gas along one of the spatial directions opens an avenue for studying fundamentals of quantum transport along the side of numerous practical electronic applications, with high-electron-mobility transistors being a prominent example. A heterojunction of two materials with dissimilar electronic polarisation can be used for engineering of the conducting channel. Extension of this concept to single-layer materials leads to one-dimensional electron gas (1DEG). MoS2/WS2 lateral heterostructure is used as a prototype for the realisation of 1DEG. The electronic polarisation discontinuity is achieved by straining the heterojunction taking advantage of dissimilarities in the piezoelectric coupling between MoS2 and WS2. A complete theory that describes an induced electric field profile in lateral heterojunctions of two-dimensional materials is proposed and verified by first principle calculations.

Thermodynamic origin of instability in hybrid halide perovskites.

Degradation of hybrid halide perovskites under the influence of environmental factors impairs future prospects of using these materials as absorbers in solar cells. First principle calculations can be used as a guideline in search of new materials, provided we can rely on their predictive capabilities. We show that the instability of perovskites can be captured using ab initio total energy calculations for reactants and products augmented with additional thermodynamic data to account for finite temperature effects. Calculations suggest that the instability of CH3NH3PbI3 in moist environment is linked to the aqueous solubility of the CH3NH3I salt, thus making other perovskite materials with soluble decomposition products prone to degradation. Properties of NH3OHPbI3, NH3NH2PbI3, PH4PbI3, SbH4PbI3, CsPbBr3, and a new hypothetical SF3PbI3 perovskite are studied in the search for alternative solar cell absorber materials with enhanced chemical stability.

An atomic charge model for graphene oxide for exploring its bioadhesive properties in explicit water.

Graphene Oxide (GO) has been shown to exhibit properties that are useful in applications such as biomedical imaging, biological sensors, and drug delivery. The binding properties of biomolecules at the surface of GO can provide insight into the potential biocompatibility of GO. Here we assess the intrinsic affinity of amino acids to GO by simulating their adsorption onto a GO surface. The simulation is done using Amber03 force-field molecular dynamics in explicit water. The emphasis is placed on developing an atomic charge model for GO. The adsorption energies are computed using atomic charges obtained from an ab initio electrostatic potential based method. The charges reported here are suitable for simulating peptide adsorption to GO.

Marble game with optimal ferroelectric switching.

A low coercivity and, consequently, low hysteresis loss are desired properties for ferroelectric materials used in high-power and high-frequency actuators. The coercive field required for the onset of ferroelectric switching is shown to develop a strong directional anisotropy due to peculiarities of an energy surface associated with the polarization rotation. It is found that the ferroelectric anisotropy exhibits 'hard' and 'easy' switching axes similar to magnetic materials. The hard axis corresponds to 180° polarization reversal, whereas the easy axis favors 90° switching. Our results suggest that the intrinsic low coercivity and the full polarization reversal may not be achieved at the same time under uniaxial excitation. A rotating electric field excitation is proposed in order to circumvent this limitation and to guide the polarization switching along a curved path.

Favorable adsorption of capped amino acids on graphene substrate driven by desolvation effect.

The use of graphene-based nanomaterials is being explored in the context of various biomedical applications. Here, we performed a molecular dynamics simulation of individual amino acids on graphene utilizing an empirical force field potential (Amber03). The accuracy of our force field method was verified by modeling the adsorption of amino acids on graphene in vacuum. These results are in excellent agreement with those calculated using ab initio methods. Our study shows that graphene exhibits bioactive properties in spite of the fact that the interaction between graphene and amino acids in a water environment is significantly weaker as compared to that in vacuum. Furthermore, the adsorption characteristics of capped and uncapped amino acids are significantly different from each other due to the desolvation effect. Finally, we conclude that when assessing protein-surface interactions based on adsorption of single amino acids, the minimum requirement is to use capped amino acids as they mimic residues as part of a peptide chain.

Lead monoxide α-PbO: electronic properties and point defect formation.

The electronic properties of polycrystalline lead oxide consisting of a network of single-crystalline α-PbO platelets and the formation of native point defects in the α-PbO crystal lattice are studied using first-principles calculations. The results suggest that the polycrystalline nature of α-PbO causes the formation of lattice defects (i.e., oxygen and lead vacancies) in such a high concentration that defect related conductivity becomes the dominant mechanism of charge transport. The neutral O vacancy forms a defect state at 1.03 eV above the valence band which can act as a deep trap for electrons, while the Pb vacancy forms a shallow trap for holes located just 0.1 eV above the valence band. The ionization of O vacancies can account for the experimentally found dark current decay in ITO/PbO/Au structures.

Modeling the radiation ionization energy and energy resolution of trigonal and amorphous selenium from first principles.

Advances in the development of amorphous selenium-based direct conversion photoconductors for high-energy radiation critically depend on the improvement of its sensitivity to ionizing radiation, which is directly related to the pair production energy. Traditionally, theories for the pair production energy have been based on the parabolic band approximation and do not provide a satisfactory agreement with experimental results for amorphous selenium. Here we present a calculation of the pair creation energy in trigonal and amorphous selenium based on its electronic structure. In indirect semiconductors, such as trigonal selenium, the ionization threshold energy can be as low as the energy gap, resulting in a lower pair creation energy, which is a favorable factor for sensitivity. Also, the statistics of photogenerated charge carriers is studied in order to evaluate the theoretical value of the Fano factor and its dependence on recombination processes. We show that recombination can significantly compromise the detector's energy resolution as a result of an increase in the Fano factor.

Generalized lucky-drift model for impact ionization in semiconductors with disorder.

An extension of an original lucky-drift model to the case of disordered semiconductors is proposed, motivated by experimental observations of an avalanche phenomenon in amorphous semiconductors. The generalization encompasses two scattering mechanisms: an inelastic one due to optical phonons and an elastic one due to a disorder potential. An obtained analytical solution is verified by a kinetic Monte Carlo simulation. Eventually, experimental data on a field dependence of the impact ionization coefficient in amorphous selenium are interpreted using reasonable material parameters.

Lone-pair states as a key to understanding impact ionization in chalcogenide semiconductors.

Impact ionization of holes and domination of p-conductivity in chalcogenide semiconductors are attributed to a weak electron-phonon interaction inherent to lone-pair states. This argument is supported by first-principles calculations of an acoustical deformation potential in trigonal selenium. Results of the calculations reveal a strong dependence of the deformation potential on the excess energy of charge carriers. The latter is interpreted using a simple tight-binding model.

Exact solution for hopping dissociation of geminate electron-hole pairs in a disordered chain.

A universal theoretical description of the dissociation problem for electron-hole pair on a one-dimensional chain in the hopping regime is proposed. Widely used results of Frenkel and Onsager theories are obtained as particular cases of the general solution. The application of the analytical theory to disordered chains shows that disorder enhances dissociation of geminate electron-hole pairs at low electric fields and suppresses at high fields.