Anne Hemeryck

An algorithm for generating a uniform distribution of points on a hypersphere is presented. For a given dimension and a specified number of points, it produces a well-separated set of points (a spherical code) on the unit hypersphere. The algorithm can also account for possible symmetries in the hyperspace. Since there is no analytical solution to this problem, a numerical approach is proposed. To optimize computational efficiency, the algorithm first performs a quasi-optimal initialization of the point positions by proposing an analytic fit of the inverse cumulative distribution function. These positions are then recursively updated by minimizing their Riesz interaction energy under the imposed symmetry constraints, followed by a refinement of the individual scalar products. The method is implemented in a Fortran code that is first applied without symmetries to reproduce known spherical codes, and in some cases, even discovers improved ones. Examples incorporating symmetries, permutations, and fixed vector components are also provided to demonstrate the algorithm’s versatility.

The accuracy of bulk-property predictions in density functional theory (DFT) calculations depends on the choice of the exchange-correlation functional. While the Perdew–Burke–Ernzerhof (PBE) functional systematically overestimates lattice parameters and strongly underestimates electronic band gaps, hybrid functionals such as Heyd–Scuseria–Ernzerhof (HSE) offer better overall agreement across a broad range of materials. Using germanium as a critical test case, we challenge the ability of both functionals to capture the semiconductor properties. Although HSE improves PBE’s gap error, it fails to reproduce germanium’s correct Γ–L indirect and Γ–Γ band gaps simultaneously. Noting that the PBE-underestimated energy separation between the 4p valence-band maximum and 4s conduction-band minimum causes unphysical sp mixing, we propose DFT+α, a semiempirical correction scheme applied selectively to 4s-like orbitals. For germanium, DFT+α restores the proper ordering and orbital character of the band edges and yields accurate lattice constants, bulk modulus, elastic constants, and phonon frequencies at a fraction of hybrid-functional computational cost.

Single displacement damage-induced defects created by ⁶⁰Co gamma-rays are studied in silicon microvolumes using a state-of-the-art CMOS Image Sensor (CIS). The Dark Current Spectroscopy (DCS) technique used in this work enables spatial and temporal tracking of the identified defect generation rates with a single defect sensitivity. The generation rate, energy level, annealing temperature and quantity of expected point defects (including the divacancy V₂ and the E-center As-V or P-V) are estimated. An isochronal annealing experiment up to 300°C highlights the creation of new defects at high temperature such as V₂OH, VOH and V_mO_n complexes. This study also demonstrates that ⁶⁰Co gamma rays can generate bulk defect-induced Dark Current Random Telegraph Signal (DC-RTS) with a very specific temporal and amplitude signature. A divacancy-impurity complex is suspected to be at the origin of this DC-RTS behavior.

Real‐time detection of toxic and flammable H₂S remains challenging for cost‐effective semiconducting metal oxide (SMOX) sensors due to the insufficient focus on and inherently poor understanding of the sulfur‐poisoning effect. This research, focusing on SnO₂ as a model for SMOX sensors, identifies the formation of sticky sulfite and sulfate surface species as the root cause of poisoning through the detailed analyses of results obtained from operando diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) experiments and density functional theory (DFT) calculations. The formation of the poisoning species is highly energetically favorable. Meanwhile, the decomposition of sulfite and sulfate appears unfavorable at the typical operating temperature of 300 °C and is only feasible around the literature‐reported 500 °C. The sulfur poisoning effect is also likely to occur with SO₂ and other sulfur‐containing volatile organic compounds (VOCs). Overcoming this issue is expected to require surface additives and/or alternative SMOX materials capable of providing different reaction pathways. The significance of metal‐sulfur‐oxygen chemistry extends beyond SMOX gas sensors to desulfurization catalysts, denitration catalysts, and solid oxide fuel cells.

Point Group (PG) symmetries play a fundamental role in many aspects of theoretical chemistry and computational materials science. With the objective to automatize the search of PG symmetry operations of generic atomic clusters, we present a new algorithm called Symmetry Operation FInder (SOFI). SOFI addresses the problem of identifying PG symmetry by framing it as a degenerate shape-matching problem, where the multiple solutions correspond to distinct symmetry operations. The developed algorithm is compared against three other algorithms dedicated to PG identification on a large set of atomic clusters. The results, along with some illustrative use cases, showcase the effectiveness of SOFI. The SOFI algorithm is released as part of the iterative rotations and assignments library, accessible at https://github.com/mammasmias/IterativeRotationsAssignments.

Saddle points on high-dimensional potential energy surfaces (PES) play a determining role in the activated dynamics of molecules and materials. Building on approaches dating back more than 50 years, many open-ended transition-state search methods have been developed to follow the direction of negative curvature from a local minimum to an adjacent first-order saddle point. Despite the mathematical justification, these methods can display a high failure rate: using small deformation steps, up to 80% of the explorations can end up in a convex region of the PES, where all directions of negative curvature vanish, while if the deformation is aggressive, a similar fraction of attempts lead to saddle points that are not directly connected to the initial minimum. In high-dimension PES, these reproducible failures were thought to only increase the overall computational cost, without having any effect on the methods’ capacity to find all saddle points surrounding a minimum. Using activation-relaxation technique nouveau (ARTn), we characterize the nature of the PES around minima, considerably expanding on previous knowledge. We show that convex regions can lie on activation pathways and that not exploring beyond them can introduce significant bias in the saddle-point search. We introduce an efficient approach for traversing the convex regions, almost eliminating exploration failures, while multiplying by almost 10 the number of identified unique and connected saddle points as compared to the standard ARTn, thus underlining the importance of correctly handling convex regions for completeness of saddle point explorations.

To assess the sensitivity of microelectronic devices to displacement damage, molecular dynamics simulations of collision cascades in GaN, Si and Ge are performed. We compare results on these three materials using a wide range of data coming from a very large amount of simulations. The statistical analysis of these data probes into the dynamics of cascades in terms of generating defects, the healing process, and the formation of defect clusters. In GaN, we observe that up to 90% of the defects created during a collision cascade initiated by a Ga atom disappear after a few tens of ps. In addition, the larger clusters tend to decrease rapidly in size. At the end, only small clusters remain in GaN, contrary to Si and Ge where large amorphous pockets are present. This metal-like in-cascade defect annealing seems to be closely related to the dense atomic structure of wurtzite GaN. Results confirm that GaN is intrinsically more resistant to non-ionizing radiation than Si and Ge at the studied energies, and over the considered timescales.

L’industrie synthétise de plus en plus de nouvelles molécules et de nouveaux matériaux dans le but de découvrir de nouvelles propriétés. En raison de l’augmentation constante de la qualité des produits fabriqués et de la précision requise pour leur élaboration, un nouveau besoin a émergé : celui de