arXiv subjects

H. Khemliche

Publications and source records attributed to H. Khemliche.

4 recordsLinked to original sources

arXiv2025

Dimensional control of the band-gap crossover in layered lead iodide

Before assessing the suitability of a semiconductor for specific applications, the first question to ask is whether it possesses a direct or indirect band gap. This distinction is fundamental, as the operation of devices such as light-emitting diodes, solar cells, and photodetectors is closely tied to the band-gap nature. Semiconductors that exhibit a band-gap crossover, from indirect to direct or vice versa, offer enhanced versatility for optoelectronic applications. Prominent examples include transition metal dichalcogenides and the subject of this study, PbI2. The nature of the band gap, and its crossover, can only be directly determined in reciprocal space by tracking the valence-band maximum and conduction-band minimum. Here, we directly visualize the thickness-dependent crossover of PbI2 from an indirect to a direct band gap using angle-resolved photoemission spectroscopy. Our measurements reveal a shift of the valence-band maximum toward the Brillouin-zone center as the film thickness exceeds a monolayer. Supported by density functional theory calculations, our results show that this crossover is driven by interlayer interactions and the hybridization of iodine pz orbitals. These findings demonstrate the tunable electronic structure of PbI2 and its potential for optoelectronic applications.

cond-mat.mtrl-sci↗

arXiv2025

Correlated Structural and Optical Characterization during Van der Waals Epitaxy of PbI2 on Graphene

Van der Waals heterostructures of 2D layered materials have gained much attention due to their flexible electronic properties, which make them promising candidates for energy, sensing, catalytic, and biomedical applications. Lead iodide (PbI2), a 2D layered semiconductor material belonging to the metal halide family, shows a thickness-dependent band gap with an indirect-to-direct transition above one monolayer. It has emerged as an excellent candidate for photodetectors and is a key component in metal halide perovskites solar cells. In the current work, we investigated the growth dynamics and the real-time correlation between structural and optical properties of PbI2 layers deposited on graphene/SiC(0001) by Molecular Beam Epitaxy. The structural and optical properties are probed respectively by Grazing Incidence Fast Atom Diffraction and Surface Differential Reflectance Spectroscopy. The growth proceeds layer-by-layer in a van der Waals-like epitaxy, with the zigzag direction of PbI2 parallel to the armchair direction of graphene. Both techniques bring evidence of significant modifications of the structural, electronic, and optical properties of the first PbI2 monolayer, characterized by a 1% tensile strain that relaxes over 3 to 5 monolayers. For a single monolayer, Angle-Resolved Photoemission Spectroscopy reveals a charge transfer from graphene to PbI2, demonstrated by an energy shift of the order of 50 meV in the graphene band structure.

cond-mat.mes-hall↗

arXiv2015

Fast atom diffraction inside a molecular beam epitaxy chamber, a rich combination

Two aspects of the contribution of grazing incidence fast atom diffraction (GIFAD) to molecular beam epitaxy (MBE) are reviewed here: the ability of GIFAD to provide \emph{in-situ} a precise description of the atomic-scale surface topology, and its ability to follow larger-scale changes in surface roughness during layer-by-layer growth. Recent experimental and theoretical results obtained for the He atom beam incident along the highly corrugated $[ 1\bar{1}0 ]$ direction of the $β_{2}$(2$\times$4) reconstructed GaAs(001) surface are summarized and complemented by the measurements and calculations for the beam incidence along the weakly corrugated [010] direction where a periodicity twice smaller as expected is observed. The combination of the experiment, quantum scattering matrix calculations, and semiclassical analysis allows in this case to reveal structural characteristics of the surface. For the in situ measurements of GIFAD during molecular beam epitaxy of GaAs on GaAs surface we analyse the change in elastic and inelastic contributions in the scattered beam, and the variation of the diffraction pattern in polar angle scattering. This analysis outlines the robustness, the simplicity and the richness of the GIFAD as a technique to monitor the layer-by-layer epitaxial growth.

cond-mat.mtrl-sci↗

arXiv2013

Grazing incidence fast atom diffraction for He atoms impinging on a Ag(110) surface

Experimental diffraction patterns produced by grazing scattering of fast helium atoms from a Ag(110) surface are used as a sensitive tool to test both the scattering and the potential models. To describe the elastic collision process we employ the surface eikonal (SE) approximation, which is a semi-classical method that includes the quantum interference between contributions coming from different projectile paths. The projectile-surface potential is derived from an accurate density-functional theory (DFT) calculation that takes into account the three degrees of freedom of the incident projectile. A fairly good agreement between theoretical and experimental momentum distributions is found for incidence along different low-indexed crystallographic directions.

physics.atom-ph↗