Point Defects in Two-Dimensional γ-Phosphorus Carbide

Unraveling nanosprings: morphology control and mechanical characterization

Nanosprings demonstrate promising mechanical characteristics, positioning them as pivotal components in a diverse array of potential nanoengineering applications. To unlock the full potential of these nanosprings, ongoing research is concentrated on emulating springs at the nanoscale in terms of both morphology and function. This review underscores recent advancements in the field and provides a comprehensive overview of the diverse methods employed for nanospring preparation. Understanding the general mechanism behind nanospring formation lays the groundwork for the informed design of nanosprings. The synthesis section delineates four prominent methods employed for nanospring fabrication: vapor phase synthesis, templating methods, post-treatment techniques, and molecular engineering. Each method is critically analyzed, highlighting its strengths, limitations, and potential for scalability. Mechanical properties of nanosprings are explored in depth, discussing their response to external stimuli and their potential applications in various fields such as sensing, energy storage, and biomedical engineering. The interplay between nanospring morphology and mechanical behavior is elucidated, providing insights into the design principles for tailored functionality. Additionally, we anticipate that the evolution of state-of-the-art characterization tools, such as in situ transmission electron microscopy, 3D electron tomography, and machine learning, will significantly contribute to both the study of nanospring mechanisms and their applications.

Systematic DFT Modeling van der Waals Heterostructures from a Complete Configurational Basis Applied to γ-PC/WS2

Periodic boundary conditions in density functional theory (DFT)-based modeling of bilayer van der Waals heterostructures introduce an artificial lock to a metastable configuration. Depending on the initial supercell, geometric optimization may reach local energy minima at a fixed twist-angle in a restricted strain-space. In this work, an algorithm was introduced for generating a complete scope of ways to combine two monolayer unit cells into a common supercell. In its application to γ-PC/WS2, 18,123 bilayer supercells were derived, for which the constituting monolayers possessed isotropic strains, anisotropic strains, or intralayer shear strains. Based on analysis, 45 isotropically strained configurations were carefully chosen for optimization by DFT. Geometric and energetic features and band structures were revealed and compared, following the variations at different strains and twist-angles. As such, this case study brought to resolution the impacts of supercell construction on DFT's outcomes and the merits of in-depth screening of the different options. Repetitions and extensions to the demonstrated approach may be applied to characterize van der Waals heterostructures and derivatives in the future.

Electrostatic Gating of Phosphorene Polymorphs

The ability to directly monitor the states of electrons in modern field-effect transistors (FETs) could transform our understanding of the physics and improve the function of related devices. In particular, phosphorene allotropes present a fertile landscape for the development of high-performance FETs. Using density functional theory-based methods, we have systematically investigated the influence of electrostatic gating on the structures, stabilities, and fundamental electronic properties of pristine and carbon-doped monolayer (bilayer) phosphorene allotropes. The remarkable flexibility of phosphorene allotropes, arising from intra- and interlayer van der Waals interactions, causes a good resilience up to equivalent gate potential of two electrons per unit cell. The resilience depends on the stacking details in such a way that rotated bilayers show considerably higher thermodynamical stability than the unrotated ones, even at a high gate potential. In addition, a semiconductor to metal phase transition is observed in some of the rotated and carbon-doped structures with increased electronic transport relative to graphene in the context of real space Green's function formalism.

Instructive Synergistic Effect of Coordinating Phosphorus in Transition-Metal-Doped β-Phosphorus Carbide Guiding the Design of High-Performance CO2RR Electrocatalysts

Developing efficient electrocatalysts for the CO2 reduction reaction (CO2RR) is the key and difficult point to alleviate energy and climate issues. The synergistic catalytic effects between metal and nonmetal elements have gained attention for the design of the CO2RR electrocatalysts. The realization of this effect requires a suitable combination of metal and nonmetal elements, as well as the support of suitable substrates. Based on this, the transition-metal-doped β-phosphorus carbide (TM-PC) (TM = 4d and 5d transition metals except Tc) catalysts are designed, and their structures, electronic properties, and CO2RR catalytic performances are studied in depth via first-principle calculations. The strong bonding ability and high reactivity brought by the moderate electronegativity and abundant electrons and orbitals of phosphorus are the key to the excellent catalytic performance of TM-PCs. Coordinating phosphorus atoms improve the catalyst activity in two ways: (1) regulating the electron transfer of the TM active site, and (2) acting as the active site and changing the reaction mechanism. With the participation of coordinating P atoms, the "relay" of active sites reduces the limiting potential values for the reduction from CO2 to CH4 catalyzed by Cr-PC and Mo-PC by 0.27 and 0.23 V, respectively, compared with pathways where only the TM atom is the active site, reaching -0.55 and -0.63 V, respectively. Regarding the coordinating P atom as the second active site, Cr-PC and Mo-PC can catalyze the production of CH3CH2OH at limiting potential values of -0.54 and -0.67 V, respectively. This study demonstrates the dramatic enhancement of catalytic activity caused by suitable nonmetal coordinating atoms such as P and provides a reference for the design of high-performance CO2RR electrocatalysts based on metal-nonmetal coordinating active centers.

Anisotropic Carrier Mobility and Spectral Fingerprints of Two-Dimensional γ-Phosphorus Carbide with Antisite Defects

We apply density functional theory to study carrier mobility in a γ-phosphorus carbide monolayer. Although previous calculations predicted high and anisotropic mobility in this material, we show that the mobility can be significantly influenced by common antisite defects. We demonstrate that at equilibrium concentrations defects do not inhibit carrier mobility up to temperatures of 1000 K. However, defects can change the mobility at high nonequilibrium concentrations of about 10^-4 to 10^-2 defects per atom. At the low end of this concentration range, defects act as traps for charge carriers and inhibit their mobility. At the high end of this range, defects change the effective carrier masses and deformation potentials, and they can lead to both an increase and a decrease in mobility. We also report the Raman and IR spectra associated with antisite defects. We predict new vibrational modes and shifts of the existing modes due to the defects.

Family of Two-Dimensional Transition Metal Dichlorides: Fundamental Properties, Structural Defects, and Environmental Stability

A large number of novel two-dimensional (2D) materials are constantly being discovered and deposited in databases. Consolidated implementation of machine learning algorithms and density functional theory (DFT)-based predictions have allowed the creation of several databases containing an unimaginable number of 2D samples. As the next step in this chain, the investigation leads to a comprehensive study of the functionality of the invented materials. In this work, a family of transition metal dichlorides have been screened out for systematic investigation of their structural stability, fundamental properties, structural defects, and environmental stability via DFT-based calculations. The work highlights the importance of using the potential of the invented materials and proposes a comprehensive characterization of a new family of 2D materials.

Spiderweb Nanomechanical Resonators via Bayesian Optimization: Inspired by Nature and Guided by Machine Learning

From ultrasensitive detectors of fundamental forces to quantum networks and sensors, mechanical resonators are enabling next-generation technologies to operate in room-temperature environments. Currently, silicon nitride nanoresonators stand as a leading microchip platform in these advances by allowing for mechanical resonators whose motion is remarkably isolated from ambient thermal noise. However, to date, human intuition has remained the driving force behind design processes. Here, inspired by nature and guided by machine learning, a spiderweb nanomechanical resonator is developed that exhibits vibration modes, which are isolated from ambient thermal environments via a novel "torsional soft-clamping" mechanism discovered by the data-driven optimization algorithm. This bioinspired resonator is then fabricated, experimentally confirming a new paradigm in mechanics with quality factors above 1 billion in room-temperature environments. In contrast to other state-of-the-art resonators, this milestone is achieved with a compact design that does not require sub-micrometer lithographic features or complex phononic bandgaps, making it significantly easier and cheaper to manufacture at large scales. These results demonstrate the ability of machine learning to work in tandem with human intuition to augment creative possibilities and uncover new strategies in computing and nanotechnology.

Role of Intrinsic Points Defects on the Electronic Structure of Metal-Insulator Transition h-FeS

Hexagonal iron sulfide (h-FeS) offers huge potential in the development of metal-insulator transition devices. A stoichiometric h-FeS is hard to obtain from its natural iron deficiency. The effect of this iron deficiency on the electronic properties is still obscure. Here, we performed a charged point defect calculation in h-FeS. We found that the most favorable point defect in h-FeS can be tuned with a proper synthesis environment. The single iron vacancy could induce a midgap state with 0.05 eV energy gap, which explains the h-FeS low experimental band gap value. Furthermore, a semiconductor-to-metal transition is observed in h-FeS with higher iron vacancy concentration showing better conductivity from the excess charges. We also observe that iron vacancies will induce a magnetic moment on the antiferromagnetic h-FeS. The findings that the induced MIT behavior and magnetic moment can be tuned by defect concentration may benefit the development of spintronics devices.

Symbolic Transformer Accelerating Machine Learning Screening of Hydrogen and Deuterium Evolution Reaction Catalysts in MA2Z4 Materials

Two-dimensional (2D) materials have been developed into various catalysts with high performance, but employing them for developing highly stable and active nonprecious hydrogen evolution reaction (HER) catalysts still encounters many challenges. To this end, the machine learning (ML) screening of HER catalysts is accelerated by using genetic programming (GP) of symbolic transformers for various typical 2D MA₂Z₄ materials. The values of the Gibbs free energy of hydrogen adsorption (ΔG_H*) are accurately and rapidly predicted via extreme gradient boosting regression by using only simple GP-processed elemental features, with a low predictive root-mean-square error of 0.14 eV. With the analysis of ML and density functional theory (DFT) methods, it is found that various electronic structural properties of metal atoms and the p-band center of surface atoms play a crucial role in regulating the HER performance. Based on these findings, NbSi₂N₄ and VSi₂N₄ are discovered to be active catalysts with thermodynamical and dynamical stability as ΔG_H* approaches to zero (-0.041 and 0.024 eV). In addition, DFT calculations reveal that these catalysts also exhibit good deuterium evolution reaction (DER) performance. Overall, a multistep workflow is developed through ML models combined with DFT calculations for efficiently screening the potential HER and DER catalysts from 2D materials with the same crystal prototype, which is believed to have significant contribution to catalyst design and fabrication.

Wide Band Gap P3S Monolayer with Anisotropic and Ultrahigh Carrier Mobility

Phosphorene has offered an additional advantage for developing new optoelectronic devices due to its anisotropic and high carrier mobility. However, its instability in air causes a rapid degradation of the performance of the device. Thus, improving the stability of phosphorene while maintaining its original properties has become the key to the development of high-performance electronic devices. Herein, we propose that the formation of two-dimensional (2D) P-rich P-S compounds could achieve this goal. First-principles swarm-structural searches revealed two previously unkonwn P₃S and P₂S monolayers. The P₃S monolayer, consisting of n-bicyclo-P₆ units along the armchair direction, exhibits anisotropic and wide band gap characteristics. Interestingly, its carrier mobility reaches 1.11 × 10⁴ cm² V^-1 s^-1 and is much higher than in phosphorene. Its electronic band gap and optical absorption coefficients in the ultraviolet region reach 2.71 eV and 10⁵ cm^-1, respectively. Additionally, the P₃S monolayer has a high structural stability and resistance to air oxidation.

Two-Dimensional Fe8N Nanosheets: Ferromagnets and Nitrogen Diffusion

We perform a first-principles study and identify two intriguing ferromagnets, hollow-Fe₈N (H-Fe₈N) and bridge-Fe₈N (B-Fe₈N) monolayers, by extensive structural searches. Both H-Fe₈N and B-Fe₈N nanosheets are buckled triangular lattices with a similar motif, but they are distinguishable by the positions of N atoms. The magnetic and electronic properties show that H-Fe₈N is a low-spin ferromagnet; in contrast, B-Fe₈N is a high-spin ferromagnet, which originates from the 3d orbital splitting of the Fe atom due to the low/high symmetric crystal field. Surprisingly, two stable Fe₈N monolayers can be transferred to each other by N atom diffusion from the bridge position to the hollow position with the migration energy barrier of 1.5 eV. The energy barrier is affected by introduced Fe defects and rare earth metal dopants. These findings introduce a new tactic to regulate the 2D Fe-nitride monolayers at the atomic scale.

First-Principles Multiscale Modeling of Mechanical Properties in Graphene/Borophene Heterostructures Empowered by Machine-Learning Interatomic Potentials

Density functional theory calculations are robust tools to explore the mechanical properties of pristine structures at their ground state but become exceedingly expensive for large systems at finite temperatures. Classical molecular dynamics (CMD) simulations offer the possibility to study larger systems at elevated temperatures, but they require accurate interatomic potentials. Herein the authors propose the concept of first-principles multiscale modeling of mechanical properties, where ab initio level of accuracy is hierarchically bridged to explore the mechanical/failure response of macroscopic systems. It is demonstrated that machine-learning interatomic potentials (MLIPs) fitted to ab initio datasets play a pivotal role in achieving this goal. To practically illustrate this novel possibility, the mechanical/failure response of graphene/borophene coplanar heterostructures is examined. It is shown that MLIPs conveniently outperform popular CMD models for graphene and borophene and they can evaluate the mechanical properties of pristine and heterostructure phases at room temperature. Based on the information provided by the MLIP-based CMD, continuum models of heterostructures using the finite element method can be constructed. The study highlights that MLIPs were the missing block for conducting first-principles multiscale modeling, and their employment empowers a straightforward route to bridge ab initio level accuracy and flexibility to explore the mechanical/failure response of nanostructures at continuum scale.

First-Principles Prediction of Two-Dimensional B3C2P3 and B2C4P2: Structural Stability, Fundamental Properties, and Renewable Energy Applications

The existence of two novel hybrid two-dimensional (2D) monolayers, 2D B₃C₂P₃ and 2D B₂C₄P₂, has been predicted based on the density functional theory calculations. It has been shown that these materials possess structural and thermodynamic stability. 2D B₃C₂P₃ is a moderate band gap semiconductor, while 2D B₂C₄P₂ is a zero band gap semiconductor. It has also been shown that 2D B₃C₂P₃ has a highly tunable band gap under the effect of strain and substrate engineering. Moreover, 2D B₃C₂P₃ produces low barriers for dissociation of water and hydrogen molecules on its surface, and shows fast recovery after desorption of the molecules. The novel materials can be fabricated by carbon doping of boron phosphide and directly by arc discharge and laser ablation and vaporization. Applications of 2D B₃C₂P₃ in renewable energy and straintronic nanodevices have been proposed.