Ferrocene molecular deposition on Cu (111) surface and the interface states after deposition of metal atoms : first principles molecular dynamics study

Dans cette thèse, l'étude de la dépostion des molécules de ferrocène sur un substrat de Cu(111) par des simulations de dynamique moléculaire par premiers principes, en particulier, la dynamique moléculaire utilisant l'approche de Born-Oppenheimer (BOMD: Born-Oppenheimer Molecular Dynamics) et celle utilisant la fonctionnelle de l'énergie libre (FEMD: Free Energy Molecular Dynamics), combinées avec les études expérimentales par microscopie à effet tunnel (STM) à basse température et à courant constant ont montré que ces molécules de ferrocène peuvent être physisorbées sur un substrat de cuivre sans donner lieu à une dissociation moléculaire. Ce qui constitue un système idéal pour étudier la dynamique des états d'interfaces et leur réactivité par rapport à la déposition d’atomes métalliques. En particulier, la déposition d'un atome de Cuivre au dessus d'une molécule de ferrocène équilibrée sur le substrat de cuivre, conduit à un transfert de charges de cet atomes vers le substrat de Cu(111). On montre aussi que ces états d'interfaces ont le comportement bidimensionnel d'un gas d'électrons libres.First-principles simulations studies, in particular Born-Oppenheimer molecular dynamics (BOMD) and free energy molecular dynamics (FEMD), combined with low-temperature scanning tunneling microscopy (STM) and spectroscopy reveal a non dissociative physisorption of ferrocene molecules on a Cu(111) surface, giving rise to ordered molecular layers. At the interface, a 2D-like electronic band is found, which shows an identical dispersion as the Cu(111) Shockley surface-state band. Subsequent deposition of Cu atoms forms charged organometallic compounds that localize interface-state electrons

Metal-organic molecule-metal nano-junctions: a close contact between first-principles simulations and experiments

The realization of metal-molecule junctions for future electronic devices relies on our ability to assemble these heterogeneous objects at a molecular level and understand their structure and the behavior of the electronic states at the interface. Delocalized interface states near the metal Fermi level are a key ingredient for tailoring charge injection, and such a delocalization depends on a large number of chemical, structural and morphological parameters, all influencing the spatial extension of the electron wavefunctions. Our large-scale dynamical simulations, combined with experiments, show that a double-decker organometallic compound (ferrocene) can be deposited on a Cu(111) surface, providing an ideal system to investigate the adsorption, the interface states and localized spin states at a metal-organometallic interface. Adsorbed ferrocene is shown to have a peculiar pattern and realizes a 2D-like interface state strongly resembling Shockley's surface state of Cu. By a subsequent deposition of single metal atoms on the adsorbed ferrocene, we analyze the sensitivity of the interface state to local modifications of the interface potential. This provides an insight into adsorption, spin configuration and charge redistribution processes, showing how to tune the electron behavior at a metal-molecule interface

Helium diffusion in pure hematite (-Fe2O3) for thermochronometric applications : a theoritical multi-scale study

International audienceHe diffusion coefficient in iron oxide α-hematite crystal has been determined using computational multi-scale approach in the purpose of geological dating as He is produced during U-Th-Sm decay in this mineral. Natural hematite samples are generally made of nanometric to micrometric scale crystals leading to the difficulty to determine the total He diffusion behavior. A multi-scale theoretical approach will so bring new information on the He diffusion coefficient in 3D. Investigations, at microscopic scale, of helium insertion and atomic jumps into hematite crystal have been performed by DFT and transition state theory. The minimum path energy of helium migration between interstitial sites and its position at transition state are determined by the climbing image-Nudged Elastic Band method

Impact of apatite chemical composition on (U-Th)/He thermochronometry : an atomistic point of view

International audienceThe quantification of the different parameters influencing He diffusion in apatite is an important issue for the interpretation of (U-Th)/He thermochronometric ages. Key issues include understanding the role of chemical composition and the mechanism modifying diffusivity by radiation damage, both requiring a realistic description at the atomic level. In this contribution, we restrict ourselves on the influence of the chemical composition especially on the effect of Cl-atoms on the He diffusion in the damage-free apatite crystal. For this purpose, a multi-scale theoretical diffusion study has been conducted using periodic Density Functional Theory calculations for two different apatite compositions (pure fluorine apatite and apatite with one chlorine and 3 fluorine atoms per cell called Cl0.25-apatite) representative of damage-free crystals. Different He insertion sites and diffusion pathways are first investigated. The Density Functional Theory approach coupled to the Nudged Elastic Band method is used to determine the energy barriers between the insertion sites. A statistical method, based on Transition State Theory, is used to compute the jump rate between sites and the different results are used as output for a 3D random walk simulation, which determines the diffusion trajectories and the diffusion coefficients. The calculated diffusion coefficients for pure F-apatite exhibit a slightly anisotropic behavior with an activation energy Ea=95.5 kJ/mol and a frequency factor D0=1.9x10-3 cm2/s along the c axis; Ea=106.1 kJ/mol and D0=4.1x10-3 cm2/s in the plane orthogonal to c. Closure temperatures for a 60 μm grain radius and 10 °C/Ma cooling rate range from 33-36 °C and depend on crystal geometry for a given grain size. Surprisingly, even though He diffusion is strongly blocked across the Cl atoms in Cl0.25-apatite, where Ea is significantly higher (166.7 kJ/mol), He atoms can still diffuse along the c axis through workaround pathways. Closure temperatures are dependent on the Cl content in the crystal lattice and can be ∼12 °C higher for Cl0.25-apatite than for F-apatite. These results show that various Cl contents lead to a more He retentive diffusivity in addition to their impact on damage-annealing rate. The results of this study are in good agreement with experimental results and demonstrate that a proper Density Functional Theory treatment allows to characterize He diffusion in damage-free apatite. This opens new avenues to a reliable method of quantifying rare gas diffusion in mineral structures

A multi-method, multi-scale theoretical study of He and Ne diffusion in zircon

International audienceThe quantification of He and Ne diffusion behavior in crystals rich in U and Th such as zircon is key for the interpretation of (U-Th)/4He and (U-Th)/21Ne thermochronometric ages. Multiple parameters such as chemical substitution, channel obstruction and damage can modify the diffusivity compared to a pristine structure. To investigate the impact of these parameters, we have conducted a theoretical diffusion study combining a series of methods and approaches to address the problem across the necessary range of scales (atomic to crystal size). First, using quantum calculation, we determine the different He and Ne insertion sites, insertion energies and diffusion pathways at the atomic scale for an ideal pristine zircon structure (i.e. damage free). These results serve as input for a 3D random walk simulation of atomic trajectories that provides diffusion coefficients for damage-free zircon crystals. Second, as natural zircon crystals are not perfect, we model the impact of different types of damage and diffusion pathway obstruction at the atomic level on He and Ne diffusion in 3D. The calculated He and Ne diffusion coefficients for pure ZrSiO4 exhibit strongly anisotropic behavior and very high diffusivity along the c-axis, and with 3D, closure temperatures of −197 °C and −202 °C respectively. The results for He are comparable to previous DFT studies but strongly different from experimental diffusion results; results for Ne are similar in this respect. Modelling the impact of different types of damage (vacancies, recoil, fission, voids or fluid inclusions) and obstruction on He and Ne diffusion reveals important implications for the (U-Th)/He and (U-Th)/Ne thermochronometers. First, obstruction alone does not significantly modify He and Ne diffusion except to reduce anisotropy. Second, trapping is the primary mechanism altering He and Ne diffusion even at low dose, and we predict the maximal trapping energies for He and Ne to be 164 and 320 kJ/mol, similar to values inferred from experimental data. We also propose that the closure temperature increases non-linearly with damage, with effective trapping energy increasing with dose until a threshold, possibly corresponding to a percolation transition, after which retentivity decreases. Based on field data sets we also anticipate a value for this threshold of around ∼2–5 × 1017 α/g, lower than previously proposed. We show Ne to be highly blocked by damage and predict similar diffusion behavior to He, but with higher retentivity. We demonstrate the importance of investigating rare gas diffusion at the atomic level for comparison with experimental data, in order to build a predictive diffusion law at different scales

Influence of vacancy damage on He diffusion in apatite, investigated at atomic to mineralogical scales

International audienceHelium diffusion in U–Th-rich minerals, especially apatite, is considered as strongly impacted by damage, even at low U–Th content. To get direct evidence and better understand the impact of damage on He diffusion, we conducted a study on vacancy damage in apatite, at nanometric to atomic scales, using different methodologies. Firstly, damage was created on apatite crystals by He implantation at different He fluences ranging from 2 × 1015 to 1 × 1017 He/cm2, corresponding to atomic displacement ranging from 12 to more than 100% of the total structure in the first 200 nm below the surface. Transmission Electron Microscopy (TEM) was used to image the damage structure, for the lowest He fluence. TEM images present no visible damage zone at nano-scale, implying that the created damage corresponds well to Frenkel defects (vacancies and interstitials). Secondly, diffusion experiments were performed on those samples by mapping He concentration vs. depth profiles using Elastic Recoil Detection Analysis (ERDA). After measurement of implanted-He profiles and He concentrations, the samples were heated in order to diffuse the implanted profile during 15–45 h at temperatures from 145 to 250 °C. The obtained He vs. depth heated profiles and He concentrations reveal the impact of damage on He diffusivity. The results can only be explained by a model where diffusion depends on damage dose, taking into account He trapping in vacancies and damage interconnectivity at higher damage dose. Thirdly, Density Functional Theory (DFT) calculations were performed to simulate a vacancy in a F-apatite crystal. The structure becomes slightly deformed by the vacancy and the insertion energy of a He atom in the vacancy is lower than for an usual insertion site. Accordingly, the additional energy for a He atom to jump out of the vacancy is ΔEa ≈ 30–40 kJ/mol, in good agreement with published estimates. This calculation thus shows that small modifications of the structure due to the presence of vacancies efficiently trap He atoms, thus reducing diffusivity. Finally, for apatite crystal having vacancy-type damage, we propose a He diffusion model able to reproduce well He diffusion data obtained on irradiated samples. We anticipate that, for natural apatite, the recoil-damage that corresponds to vacancy clustering, would have a higher trapping power with ΔEa > 50 kJ/mol

Dispersion and Localization of Electronic States at a Ferrocene/Cu(111) Interface

Low-temperature scanning tunneling microscopy and spectroscopy combined with first-principles simulations reveal a nondissociative physisorption of ferrocene molecules on a Cu(111) surface, giving rise to ordered molecular layers. At the interface, a 2D-like electronic band is found, which shows an identical dispersion as the Cu(111) Shockley surface-state band. Subsequent deposition of Cu atoms forms charged organometallic compounds that localize interface-state electrons