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High-fluence femtosecond laser pulses can induce physical and chemical changes in materials that are unrealizable under standard laboratory conditions. The exact nature of these changes can depend strongly on the gaseous environment in which the material is irradiated since near-surface chemical reactions can occur between the two materials. Surface modifications of silicon are of particular interest due to its significance in semiconductor-based applications. Specifically, the formation of silicon nitride (Si3N4) structures is desirable for multiple applications due to its high stability and low dielectric constant. Herein, we report on femtosecond laser-induced morphological and chemical modifications of silicon in a nitrogen atmosphere. We observed an extremely fast chemical reaction in the silicon-nitrogen system. The presence of crystalline Si3N4 was confirmed using high-resolution transmission electron microscopy, representing the first reported synthesis of Si3N4 nanocrystals through femtosecond laser-based methods. In addition, the surface was found to contain alternating layers of amorphous and crystalline silicon. Provided are plausible mechanisms for the formation of each of these structures. Taken together, these findings on surface modification of silicon using femtosecond laser irradiation may provide new pathways for manufacturing of nanoscale devices. 

Abstract Reactive rhenium(III) nitride complexes could result from filling Re─N π* orbitals, but such complexes lie beyond the “nitrido wall” and are rare due to their instability. Here, we describe a method for bypassing the nitrido wall by incorporating a redox‐active isocyanide supporting ligand, which accommodates two electrons as shown by crystallographic, spectroscopic, and computational studies. These electrons can be returned to the metal during its facile reaction with CO to form a cyanate complex, demonstrating the nucleophilic reactivity of the nitride. Thus, assistance by the isocyanide enables an N₂‐derived rhenium nitride to engage in N─C bond forming reactivity. 

Abstract Gene duplication is a fundamental part of evolutionary innovation. While single-gene duplications frequently exhibit asymmetric evolutionary rates between paralogs, the extent to which this applies to multi-gene duplications remains unclear. In this study, we investigate the role of genetic context in shaping evolutionary divergence within multi-gene duplications, leveraging microsynteny to differentiate source and target copies. Using a dataset of 193 mammalian genome assemblies and a bird outgroup, we systematically analyze patterns of sequence divergence between duplicated genes and reference orthologs. We find that target copies, those relocated to new genomic environments, exhibit elevated evolutionary rates compared to source copies in the ancestral location. This asymmetry is influenced by the distance between copies and the size of the target copy. We also demonstrate that the polarization of rate asymmetry in paralogs, the “choice” of the slowly evolving copy, is biased towards collective, block-wise polarization in multi-gene duplications. Our findings highlight the importance of genetic context in modulating post-duplication divergence, where differences in cis-regulatory elements and co-expressed gene clusters between source and target copies may be responsible. This study presents a large-scale test of asymmetric evolution in multi-gene duplications, offering new insight into how genome architecture shapes functional diversification of paralogs. Significance statementAfter a gene is duplicated, reduced selective constraints can lead the two copies to rapidly diverge, with one copy often evolving faster and occasionally gaining a new function. We quantify the influence of genetic context in choosing which copy of a duplicated gene has an elevated substitution rate. In a representative dataset of 193 mammalian genomes, we found strong evidence that gene copies pasted into new genomic locations tend to evolve faster than the corresponding copies in ancestral locations, suggesting an important role for the regulatory environment. The asymmetry in evolutionary rates of duplicated genes persists even for very large multigenic duplications, up to the scale of megabases, indicating that regulatory interactions frequently reach farther than previously thought. 

The T cell receptor (TCR) is a key component of the adaptive immune system, recognizing foreign antigens (ligands) and triggering an immune response. To explain the high sensitivity and selectivity of the TCR in discriminating “self” from “non-self” ligands, most models evoke kinetic proofreading (KP) schemes, however it is unclear how competing models used for TCR triggering, such as the kinetic segregation (KS) model, influence KP performance. In this paper, we consider two different TCR triggering models and their influence on subsequent KP-based ligand discrimination by the TCR: a classic conformational change model (CC-KP), where ligand-TCR binding is strictly required for activation, and the kinetic segregation model (KS-KP), where only residence of the TCR within a close contact devoid of kinases is required for its activation. Building on previous work, our computational model permits a head-to-head comparison of these models . While we find that both models can be used to explain the probability of TCR activation across much of the parameter space, we find biologically important regions in the parameter space where significant differences in performance can be expected. Furthermore, we show that the available experimental evidence may favor the KS-KP model over CC-KP. Our results may be used to motivate and guide future experiments to determine accurate mathematical models of TCR function. Published by the American Physical Society2025