Effect of crystal defects on the electronic structure and dielectric functions of In0.5Tl0.5I solid state solutions

We investigate an influence of the various crystal structure imperfections on the electronic properties and dielectric functions for In0.5Tl0.5I semiconductor in the frame of the density functional theory calculations. The tensor of electron effective mass m*ij of InI, In0.5Tl0.5I and TlI crystals has been calculated for the valence and conduction bands and different K-points of Brillouin zone. Dielectric functions ε(hν) of the defective crystals based on In0.5Tl0.5I solid state solution with iodine vacancy and thallium interstitial atom were calculated taking into consideration the inter-band and intra-band electron transitions. The studies of the defective crystals reveal increased low-frequency and stationary electron conductivity with anisotropy resulted from the anisotropy of the electron effective mass tensor. Our findings explain the origin of crucial changes in the band structure by formation the donor half-occupied levels close to the unoccupied conduction bands due to the crystal structure defects, i.e. iodine vacancy or thallium interstitial atom. It has been shown that in the case of real crystals, in particular metal-halides, the proper consideration of defects in quantum-chemical calculations results in a better matching of the theoretical and experimental results in comparison to the case when the perfect crystal structure had been used for calculations.Zbadano wpływ różnych niedoskonałości struktury krystalicznej na właściwości elektronowe i funkcje dielektryczne półprzewodnika In0.5Tl0.5I w ramach teorii funkcjonału gęstości. Został obliczony tensor efektywnej masy elektronów m* kryształów InI, In0.5Tl0.5I i TlI dla pasm walencyjnych i przewodnictwa oraz różnych K-punktów strefy Brillouina. Funkcje dielektryczne ε(hν) domieszkowanych kryształów roztworów stałych In0.5Tl0.5I z wakansami jodu i atomami międzywęzłowymi talu zostały obliczone z uwzględnieniem międzypasmowych i wewnątrz-pasmowych przejść elektronowych. Badania domieszkowanych kryształów ujawniły zwiększoną przewodność elektronową niskoczęstotliwościową i stacjonarną o anizotropii wynikającej z anizotropii tensora efektywnej masy elektronów. Przeprowadzone badania wyjaśniają obserwowane duże zmiany struktury pasmowej pochodzące z utworzenia pół wypełnionych poziomów donorowych w pobliżu niezajętych pasm przewodnictwa wynikających z defektów struktury krystalicznej, tj. wakansów jodu czy atomów międzywęzłową talu. Wykazano, że w przypadku kryształów rzeczywistych, w szczególności halogenków metali, właściwe uwzględnienie defektów w obliczeniach kwantowo-chemicznych daje możliwość lepszego dopasowania obliczeń teoretycznych do wyników doświadczalnych w porównaniu do obliczeń bazujących na strukturze krystalicznej doskonałej

Dehydrogenation versus deprotonation of disaccharide molecules in vacuum: a thorough theoretical investigation

International audienceDehydrogenation and deprotonation of sucrose and trehalose molecules in vacuum is theoretically studied by using ab initio calculations in the framework of the density functional theory. The differences in the structural, electronic, energetic and vibrational properties of dehydrogenated and deprotonated molecules are discussed, depending on the site from which the hydrogen atom or the proton has been removed. The dehydrogenated molecules are found to be stable, regardless of which hydrogen atom is removed. This contrasts with the instability of the deprotonated molecules, where break-ups or structural reorganizations of the molecule are observed in 20–30% of the cases, but only when the hydrogen atom whose proton is removed was bonded to a carbon atom. Considering the stability and possible rearrangements of the hydrogen network of the deprotonated/dehydrogenated molecule, the formation of additional hydrogen-bridge bonds compared with the nominal molecule appears to be more pronounced for the deprotonated molecules than for the dehydrogenated ones. Moreover, our calculations show that the hydrogen-transfer energy barriers are usually larger for the deprotonated molecules than for the dehydrogenated ones. Finally, compared with the nominal molecule, the vibrational frequency spectrum is shifted to lower frequencies for both the dehydrogenated and the deprotonated molecules

Estimation of phonon relaxation time for silicon by means of using the velocity autocorrelation function of atoms in molecular dynamics

Results of the ab initio molecular dynamics calculations of silicon crystals are presented by means of analysis of the velocity autocorrelation function and determination of mean phonon relaxation time. The mean phonon relaxation time is crucial for prediction of the phonon-associated coefficient of thermal conductivity of materials. A clear correlation between the velocity autocorrelation function relaxation time and the coefficient of thermal diffusivity has been found. The analysis of the results obtained has indicated a decrease of the velocity autocorrelation function relaxation time t with increase of temperature. The method proposed may be used to estimate the coefficient of ther-mal diffusivity and thermal conductivity of the materials based on silicon and of other wide-bandgap semiconductors. The correlation between kinetic energy fluctuations and relaxation time of the velocity autocorrelation function has been calculated with the relatively high coefficient of determination R2 = 0.9396. The correlation obtained and the corresponding approach substantiate the use of kinetic energy fluctuations for the calculation of values related to heat conductivity in silicon-based semiconductors (coefficients of thermal conductivity and diffusivity)

Polymorphism in carbohydrate self-assembly at surfaces: STM imaging and theoretical modelling of trehalose on Cu(100)

International audienceSaccharides, also commonly known as carbohydrates, are ubiquitous biomolecules, but little is known about their interaction with surfaces. Soft-landing electrospray ion beam deposition in conjunction with high-resolution imaging by scanning tunneling microscopy now provides access to the molecular details of the surface assembly of this important class of bio-molecules. Among carbohydrates, the disaccharide trehalose is outstanding as it enables strong anhydrobiotic effects in biosystems. This ability is closely related to the observed polymorphism. In this work, we explore the self-assembly of trehalose on the Cu(100) surface. Molecular imaging reveals the details of the assembly properties in this reduced symmetry environment. Already at room temperature, we observe a variety of self-assembled motifs, in contrast to other disaccharides like e.g. sucrose. Using a multistage modeling approach, we rationalize the conformation of trehalose on the copper surface as well as the intermolecular interactions and the self-assembly behavior