Thermoelectric properties of Mg2Si-based systems investigated by combined DFT and Boltzmann theories

Les propriétés électroniques et thermoélectriques de matériaux basés sur Mg2Si ont été étudiées par calculs DFT et semi-classiques (théorie de Boltzmann). Les effets d’abaissement de dimensionalité et de contraintes ont été étudiés. Les calculs ont été effectués sur les films monocristallins orientés 001, 110 et 111 et sur les films polycristallins. Seul le film monocristallin orienté 110 a montré des propriétés thermoélectriques intéressantes. Trois types de contraintes ont été investiguées: uniaxiale, biaxiale et isotrope. L’augmentation de la contrainte sur Mg2Si produit un décalage du maximum du facteur de puissance (PF) vers les basses températures. Comparé à Mg2Si non contraint, le coefficient Seebeck (S) augmente uniquement sous contrainte isotrope. On montre l’équivalence des propriétés thermoélectriques entre Mg2Si contraint dans la direction [110] et celles du film orienté 110. Les contraintes de tension isotropes ont été modélisées en insérant des atomes Sb dans Mg2Si massif conduisant aux structures Mg2Si:Sb, Mg2Si:3Sb and Mg2Si:4Sb. Seul Mg2Si:4Sb produit une contrainte isotrope. Les effets de substitutions de Sn pour Si dans Mg2Si massif sont similaires à ceux observés pour Mg2Si sujet à des contraintes en tension uniaxiales et biaxiales. Pour les films Mg2Si1−xSnx orientés 110 le S du matériau dopé p est supérieur à celui des massifs Mg2Si et Mg2Si1−xSnx. Concernant les nanostructures, le super-réseau Mg2Si/Mg2Sn est le plus intéressant lorsque faiblement dopé p et à basse température. Les assemblages de fils sont les meilleurs en tant que matériaux faiblement dopés n et à basse température: le PF est quasiment doublé par rapport à celui de Mg2Si massif.The electronic and thermoelectric properties of Mg2Si-based materials have been investigated by means of DFT calculations and semi-classical Boltzmann theory. The low-dimensional and strain effects on these properties have been studied. The properties have been investigated on 001-, 110- and 111-oriented Mg2Si monocrystalline films, and on polycrystalline Mg2Si film. Only the 110-oriented monocrystalline film has been found to have interesting thermoelectric properties. Three types of strains have been investigated: uniaxial, biaxial and isotropic. Increasing the intensity of the strain on Mg2Si induces a shift of the power factor (PF) maximum towards low temperature. Compared with unstrained Mg2Si, the Seebeck coefficient (S) increases only under isotropic strain. We evidence an equivalence in the thermoelectric properties between Mg2Si material constrained in the [110] direction and the 110-oriented Mg2Si film. Isotropic tensile strains have been modeled by inserting Sb atoms in bulk Mg2Si leading to the stuctures Mg2Si:Sb, Mg2Si:3Sb and Mg2Si:4Sb. Only Mg2Si:4Sb is found to induces such type of constraints. The effects of the Sn for Si substitutions in bulk Mg2Si are very similar to those observed for Mg2Si subjected to uniaxial and biaxial tensile strains. For (110)-oriented Mg2Si1−xSnx films S of the n−doped material outperforms that of the bulk Mg2Si and bulk Mg2Si1−xSnx. Regarding nanostructures, the Mg2Si/Mg2Sn superlattice is most interesting as a p-doped material at low carrier concentration/low temperature. The stick assemblage is best as a n-doping material at low carrier concentration/low temperature where its PF is almost twice as high as that of bulk Mg2Si

Thermoelectric Properties of Mg 2 Si Thin Films by Computational Approaches

International audienc

Computational Investigation of the Electronic and Thermoelectric Properties of Strained Bulk Mg2Si

International audienc

Calculations of thermoelectric properties: Mg2Si under uniaxial [110] strains versus (110)-oriented thin film

International audienceInvestigations of the electronic properties and transport properties of Mg2Si under uniaxial [110] strain have been performed by using first-principle density-functional and Boltzmann's transport theories. The effect of compressive and tensile uniaxial strains has been studied by changing the. angle of the conventional cell from +/- 1 degrees to +/- 4 degrees. We show that, the Seebeck property of the constrained bulk lattice at high temperature, when plotted with respect to the charge carrier concentrations, is similar to that of the (110) thin film at low temperature. This behaviour is evidenced when superimposing the Seebeck coefficient curves of both materials by shifting down the S curve of the constrained structure by about 150 K with respect to the temperature

Effect of Biaxial Strain on Electronic and Thermoelectric Properties of Mg2Si

International audienc

Thermoelectric Properties of Sn-Containing Mg 2 Si Nanostructures

International audienceThe thermoelectric performance of Mg2Si-containing nanomaterials are predicted based on density-functional and Boltzmann’s transport theories. The investigatedmaterials are Mg2Si1−xSnx thin films with x = 0.125 and x =0.625, and (Mg2Si)1−x (Mg2Sn)x (x = 0.4 and x = 0.6) in theform of either superlattices or assembled nanosticks. Thecalculated properties (Seebeck coefficient S, electrical con-ductivity σ, and power factor S2σ) are compared with those ofbulk Mg2Si1−xSnx. It is shown that the thin films outperform thebulk materials at low temperature (350 K) as they exhibit ahigher Seebeck coefficient and comparable electrical conductiv-ity. A low electrical conductivity at 900 K is responsible for thecounter-performance of the films. Superlattices are attractivestructures as p-doped materials at both low charge carrierconcentration/high temperature and high charge carrier concentration/high temperature. The assembled nanosticks are interesting materials at low carrier concentration/low temperature only

Density Functional Theory Study of the Spontaneous Formation of Covalent Bonds at the Silver/Silica Interface in Silver Nanoparticles Embedded in SiO 2 : Implications for Ag + Release

International audienceSilver nanoparticles (AgNPs) are widely used in the health-care sector and industrial applications because of their outstanding antibacterial activity. This bactericidal effect is mainly attributed to the release of Ag+ ions in an aqueous medium, the first step of which is the dissolution of the AgNP via the oxidation of its surface by O2. With the aim of designing more durable and less toxic antibacterial devices, it is desirable to fine-tune the rate of Ag+ release into the surrounding environment. This can be achieved by choosing an adequate coating of the AgNPs, e.g., by embedding the nanoparticles in a silica matrix. In a previous work (Pugliara, A.; et al. Sci. Total Environ.2016, 565, 863), we have shown that the toxic effect on algae photosynthesis of small AgNPs (size <20 nm) embedded in silica layers is preserved, provided that the distance between the AgNPs and the silica free surface is below ≈6–7 nm. Better control of the Ag+ release rate in these systems requires a better understanding of the elementary mechanisms at play concerning both the detachment of the Ag ions from the AgNPs and their diffusion through SiO2. A first step in this direction consists in characterizing the interface between the AgNPs surface and the silica matrix. In this context, periodic density functional theory calculations have been performed on model systems representing the interfaces between amorphous silica and the three crystalline facets of AgNPs, i.e., Ag(111), Ag(110), and Ag(100). Spontaneous breaking of the Si–O bonds and the formation of two O–Ag and one Si–Ag bonds are observed in 50% of the investigated interfaces, corresponding to 1.8 bonds/nm2 on average. The covalent nature of the bonds between Ag and O and between Ag and Si is highlighted by analysis of the electronic structure of the interfaces

Combining Solid-State NMR and Molecular Dynamics Simulations to investigate hybrid halide perovskites

National audienc

Revealing the radiation damage and Al-content impacts on He diffusion in goethite

International audienceOver the past few decades, the (U-Th)/He geochronological method on goethite has been more and more applied to date laterite formation and evolution or ore-deposit formation. However, questions remain on possible He loss by diffusion due to the polycrystalline structure of goethite and associated underestimation of the goethite crystallization date given by the (U-Th)/He age. Prior studies estimated helium loss in goethite to range from 2 to 30%, but no relation or models have been produced to explain such values. To clarify the situation, we firstly performed a complete review of experimental He diffusion data in natural goethite, that reveals the link between activation energy and He loss with the damage dose. To understand He diffusion behavior in goethite and model the He loss, natural defect and alpha damage as well as the chemical composition and growth structure effect on He diffusion have been investigated thanks to a multi-scale study. We used numerical simulations combining the Density Functional Theory (DFT) at the atomic scale and Kinetic Monte Carlo (KMC) simulations at the macroscopic scale. We found that He diffusion is purely anisotropic along the preferential elongated axis (i.e., b-axis) and He leaks out easily in defect-free goethite and Al-goethite. The consequence of this anisotropy is that crystallographic defects and alpha damage strongly lower the He diffusivity in goethite and Al-goethite by obstructing the diffusion channel or trapping He along the b-axis. Defect and damage impact on He diffusion is even larger for Al-goethite. The obtained He diffusion parameters for goethite containing defect and damage are similar to the activation energy and He diffusional loss obtained in natural goethite from the literature. We demonstrate the systematic dependence of the diffusion coefficient with damage dose and the impact of Al on He retention. He atoms are retained only at the favor of obstructions blocking the diffusion and vacancies trapping them in the goethite structure. The consequence of the diffusive behavior is that a part of He diffuses out of the crystal until sufficient damage accumulated along the b-axis. The diffusion domain size is the channel length along the b-axis rather than the whole crystallite size. To correct the He loss this study proposes estimation of the He retention and needed corrections for different types of goethite

CH3NH3+ Dynamics in CH3NH3PbBr3 by Combining Solid-State NMR and Molecular Dynamics Calculations

International audienceThe dynamical behavior of the organic cations in hybrid halide perovskites has triggered a wealth of experimental and theoretical investigations in the recent years. The intrinsic dynamical and electrical (dipolar) properties of the organic part were suggested to be responsible of some of the most noticeable features of these materials. Previous investigations [Phys. Chem. Chem. Phys. 2016, 18, 27133] have shown that static theoretical calculations of quadrupolar parameters (2H) in the orthorhombic phase of deuterated-MAPbBr3 (MA = CH3NH3+), lead to an overestimation of the linewidth broadening. This was rationalized in terms of thermally activated internal rotational dynamics of the molecular cations. Impact of the dynamics is further inspected in the present paper by a joint experimental and theoretical effort with low temperature solid-state NMR measurements (< 25K). Starting from extensive Ab initio molecular dynamics trajectories performed on large (4x4x4) supercells for both the tetragonal and orthorhombic phases [J. Phys. Chem. C 2017, 121, 20729] we use a specifically designed procedure to account for the effect of the thermally activated cation dynamics in the calculated NMR parameters, which includes the quadrupolar lineshapes of the NMR spectra