arXiv subjects

Mikaël Kepenekian

Publications and source records attributed to Mikaël Kepenekian.

7 recordsLinked to original sources

arXiv2026

Fast and accurate simulation of Raman spectra of gold-organic systems

Resolving the spectral Raman signature of molecules grafted on a metallic support is often a difficult task, in which quantum chemistry methods allow for precious additional rationalization and signal attributions, especially to probe the formation of a bond with the support. In the specific case of gold-organic architectures based on a Au-C bond, only a limited amount of experimental and theoretical reference data are available in the literature, and Raman simulations based on quantum mechanics quickly become unaffordable with the size of the system. In this work, we evaluate the precision of a cost-efficient DFTB method to simulate Raman spectra of gold-organic systems at different scales, from gold complexes to functionalized gold surfaces. After a validation of the method through a careful comparison of DFTB Raman spectra of organometallic gold(I) and (III) complexes to DFT and experimental reference data, we discuss the case of molecules grafted on gold aggregates. For these simulations, the choice of the model (cluster or periodic surface) appears to be critical, and significant differences arise (positions and intensities of the peaks) when considering a full metallic slab, as allowed by the low computational cost of the method.

physics.chem-ph↗

arXiv2025

Computational Screening and Discovery of Silver-Indium Halide Double Salts

Perovskite-inspired materials have emerged as promising candidates for both outdoor and indoor photovoltaic applications owing to their favorable optoelectronic properties and reduced toxicity. Here, we employ the experimentally realized AgBiI$_4$ double salt as a structural prototype and replace Bi$^{3+}$ with In$^{3+}$ to design a novel lead-free halide compound, AgInI$_4$. First-principles calculations predict that AgInI$_4$ is both chemically and dynamically stable, exhibiting a direct band gap of 1.72 eV, comparable to its bismuth analogue. However, its predicted photovoltaic performance, evaluated using the spectroscopic limited maximum efficiency metric, is lower under both solar and LED illumination. This reduction arises primarily from symmetry-forbidden optical transitions and the absence of Bi-derived 6s$^2$ lone-pair states at the valence band maximum. High-throughput screening of the Ag-In-I ternary phase-space reveals several more stable and metastable compounds that fall into two structural families: tetrahedrally and octahedrally coordinated, with characteristic band gaps near 3.0 eV and 2.0 eV, respectively. Despite multiple synthetic attempts, the predicted AgInI$_4$ phase could not be experimentally realized, underscoring the challenges of stabilizing indium-based halide double salts. While these materials are unlikely to serve as efficient photovoltaic absorbers, their tunable band gaps and stability make them promising candidates for charge transport and other optoelectronic applications.

cond-mat.mtrl-sci↗

arXiv2024

Intrinsic Limits of Charge Carrier Mobilities in Layered Halide Perovskites

Layered halide perovskites have emerged as potential alternatives to three-dimensional halide perovskites due to their improved stability and larger material phase space, allowing fine-tuning of structural, electronic, and optical properties. However, their charge carrier mobilities are significantly smaller than that of three-dimensional halide perovskites, which has a considerable impact on their application in optoelectronic devices. Here, we employ state-of-the-art ab initio approaches to unveil the electron-phonon mechanisms responsible for the diminished transport properties of layered halide perovskites. Starting from a prototypical ABX$_{3}$ halide perovskite, we model the case of $n=1$ and $n=2$ layered structures and compare their electronic and transport properties to the three-dimensional reference. The electronic and phononic properties are investigated within density functional theory (DFT) and density functional perturbation theory (DFPT), while transport properties are obtained via the ab initio Boltzmann transport equation. The vibrational modes contributing to charge carrier scattering are investigated and associated with polar-phonon scattering mechanisms arising from the long-range Fröhlich coupling and deformation potential scattering processes. Our investigation reveals that the lower mobilities in layered systems primarily originates from the increased electronic density of states at the vicinity of the band edges, while the electron-phonon coupling strength remains similar. Such increase is caused by the dimensionality reduction and the break in octahedra connectivity along the stacking direction. Our findings provide a fundamental understanding of the electron-phonon coupling mechanisms in layered perovskites and highlight the intrinsic limitations of the charge carrier transport in these materials.

cond-mat.mtrl-sci↗

arXiv2019

Electronic properties of Pb-I deficient lead halide perovskites

The electronic structure evolution of deficient halide perovskites with a general formula $(A,A')_{1+x}M_{1-x}X_{3-x}$ was investigated using the density functional theory. The focus is placed on characterization of changes in the band gap, band alignment, effective mass, and optical properties of deficient perovskites at various concentrations of defects. We uncover unusual electronic properties of the defect corresponding to a $M\!-\!X$ vacancy filled with an $A'$ cation. This defect "repels" electrons and holes producing no trap states and, in moderate quantities ($x\le0.1$), does not hinder charge transport properties of the material. This behavior is rationalized using a confinement model and provides an additional insight to the defect tolerance of halide perovskites.

cond-mat.mtrl-sci↗

arXiv2014

Spin transport in dangling-bond wires on doped H-passivated Si(100)

New advances in single-atom manipulation are leading to the creation of atomic structures on H passivated Si surfaces with functionalities important for the development of atomic and molecular based technologies. We perform total-energy and electron-transport calculations to reveal the properties and understand the features of atomic wires crafted by H removal from the surface. The presence of dopants radically change the wire properties. Our calculations show that dopants have a tendency to approach the dangling-bond wires, and in these conditions, transport is enhanced and spin selective. These results have important implications in the development of atomic-scale spintronics showing that boron, and to a lesser extent phosphorous, convert the wires in high-quality spin filters.

cond-mat.mtrl-sci↗

arXiv2012

Elastic transport through dangling-bond silicon wires on H passivated Si(100)

We evaluate the electron transmission through a dangling-bond wire on Si(100)-H (2x1). Finite wires are modelled by decoupling semi-infinite Si electrodes from the dangling-bond wire with passivating H atoms. The calculations are performed using density functional theory in a non-periodic geometry along the conduction direction. We also use Wannier functions to analyze our results and to build an effective tight-binding Hamiltonian that gives us enhanced insight in the electron scattering processes. We evaluate the transmission to the different solutions that are possible for the dangling-bond wires: Jahn-Teller distorted ones, as well as antiferromagnetic and ferromagnetic ones. The discretization of the electronic structure of the wires due to their finite size leads to interesting transmission properties that are fingerprints of the wire nature.

cond-mat.mtrl-sci↗

arXiv2012

Energetics and stability of dangling-bond silicon wires on H passivated Si(100)

We evaluate the electronic, geometric and energetic properties of quasi 1-D wires formed by dangling bonds on Si(100)-H (2 x 1). The calculations are performed with density functional theory (DFT). Infinite wires are found to be insulating and Peierls distorted, however finite wires develop localized electronic states that can be of great use for molecular-based devices. The ground state solution of finite wires does not correspond to a geometrical distortion but rather to an antiferromagnetic ordering. For the stability of wires, the presence of abundant H atoms in nearby Si atoms can be a problem. We have evaluated the energy barriers for intradimer and intrarow diffusion finding all of them about 1 eV or larger, even in the case where a H impurity is already sitting on the wire. These results are encouraging for using dangling-bond wires in future devices.

cond-mat.mtrl-sci↗