Detailed Reaction Kinetics for Hydrocarbon Fuels: The Development and Application of the ReaxFFCHO-S22 Force Field for C/H/O Systems with Enhanced Accuracy.

Efficient and accurate reactive force fields (e.g., ReaxFF) are pivotal for large-scale atomistic simulations to comprehend microscopic combustion processes. ReaxFF has been extensively utilized to describe chemical reactions in condensed phases, but most existing ReaxFF models rely on quantum mechanical (QM) data nearly two decades old, particularly in hydrocarbon systems, constraining their accuracy and applicability. Addressing this gap, we introduce a reparametrized ReaxFFCHO-S22 for C/H/O systems, tailored for studying the pyrolysis and combustion of hydrocarbon fuel. Our approach involves high-level QM benchmarks and large database construction for C/H/O systems, global ReaxFF parameter optimization, and molecular dynamics simulations of typical hydrocarbon fuels. Density functional theory (DFT) computations utilized the M06-2X functional at the meta-generalized gradient approximation (meta-GGA) level with a large basis set (6-311++G**). Our new ReaxFFCHO-S22 model exhibits a minimum 10% enhancement in accuracy compared to the previous ReaxFF models for a large variety of hydrocarbon molecules. This advanced ReaxFFCHO-S22 not only enables efficient large-scale studies on the microscopic chemical reactions of more complex hydrocarbon fuel but also can extend to biofuels, energetic materials, polymers, and other pertinent systems, thus serving as a valuable tool for studying chemical reaction dynamics of the large-scale hydrocarbon condensed phase at an atomistic level.

In-Plane Sliding Ferroelectricity Realized in Penta-PdSe2/Penta-PtSe2 van der Waals Heterostructure.

Different from conventional 2D sliding ferroelectrics with polarization switchable in the out-of-plane via interlayer sliding, we show the existence of in-plane sliding ferroelectricity in a bilayer of a pentagon-based van der Waals heterostructure formed by vertically stacking an experimentally synthesized penta-PdSe2 sheet and a crystal lattice well-matched penta-PtSe2 sheet. From the 128 sliding patterns, four stable configurations are found that exhibit in-plane sliding ferroelectricity with an ultralow polarization switching barrier of 1.91 meV/atom and a high ferroelectric polarization of ±17.11 × 10-10 C m-1. Following the ferroelectric transition among the stable sliding configurations, significant changes in carrier mobility, electrical conductivity, and second harmonic generation are identified. In particular, the ferroelectric stacking configurations are found to possess a negative Poisson's ratio, facilitating the experimental characterization of the sliding ferroelectric effect. This study demonstrates that pentagonal sheets can be used to realize 2D in-plane sliding ferroelectrics going beyond the existing ones.

Three-dimensional porous borocarbonitride composed of pentagonal motifs as a high-performance pyroelectric material.

Pyroelectric materials have been attracting significant attention due to their intrinsic and permanent polarization, where the induced polarization is not associated with specific conditions, such as ferroelectric phase transition, strain gradient, dopants, and electric field. Thus, these materials have great potential for wide applications in energy conversion. Here, we propose a new 3D porous borocarbonitride termed PH-BCN, which is composed of pentagonal motifs with intrinsic polarization along the [0001] direction. Based on first-principles calculations, we show that PH-BCN possesses a record high longitudinal electromechanical coupling coefficient with the value of k33 = 97.99%, a remarkably strong SHG response (χ(2)xzx(0) = χ(2)yzy(0) = χ(2)zxx(0) = χ(2)zyy(0) = -6.23 pm V-1 and χ(2)zzz(0) = 21.21 pm V-1), and a record high shift current value of 908.58 µA V-2 due to the intrinsic vertical polarization. This study expands the family of pentagon-based materials, and may open a new frontier in the design of high-performance pyroelectric materials as well.

Type-1 Pentagonal Tiling Realized in 2D Penta-SrP2 Sheet.

According to the systematic classification of pentagon-based two-dimensional (2D) materials [ Phys. Rep. 2022, 964, 1], only type-2 and type-4 out of the 15 pentagonal tiling patterns have been realized in 2D materials so far. Here, we propose the first stable pentagon-based 2D material characterized by the type-1 pentagonal tiling pattern named penta-SrP2. We find that penta-SrP2 is not only thermally and mechanically stable but also dynamically stable when the temperature is above 200 K derived from the calculations by taking both phonon renormalization and thermal expansion into consideration. Moreover, the penta-SrP2 sheet is semiconducting with an indirect band gap of 0.96 eV. These findings expand the family of pentagon-based 2D materials in morphology and provide a new perspective to explore the dynamical stability of high-temperature phases.

Extremely Large Response of Phonon Coherence in Twisted Penta-NiN2 Bilayer.

Twisting has recently been demonstrated as an effective strategy for tuning the interactions between particles or quasi-particles in layered materials. Motivated by the recent experimental synthesis of pentagonal NiN2 sheet [ACS Nano 2021, 15, 13539], for the first time, the response of phonon coherence to twisting in bilayer penta-NiN2 , going beyond the particle-like phonon transport is studied. By using the unified theory of phonon transport and high order lattice anharmonicity, together with the self-consistent phonon theory, it is found that the lattice thermal conductivity is reduced by 80.6% from 33.35 to 6.47 W m-1 K-1 at 300 K when the layers are twisted. In particular, the contribution of phonon coherence is increased sharply by an order of magnitude, from 0.21 to 2.40 W m-1 K-1 , due to the reduced differences between the phonon frequencies and enhanced anharmonicity after the introduction of twist. The work provides a fundamental understanding of the phonon interaction in twisted pentagonal sheets.

A New 3D Carbon Allotrope Composed of Penta-graphene Nanotubes with Low Lattice Thermal Conductivity.

Motivated by the unique geometries and novel properties of penta-graphene (PG) and its derivatives, we propose a new stable 3D carbon allotrope, penta-C72, which is composed of PG nanotubes by connecting adjacent tubes in a tip-to-tip manner. Using first-principles calculations, we confirm its dynamical, thermal, and mechanical stabilities and find that penta-C72 is semiconducting with an indirect bandgap of 2.12 eV. Its lattice thermal conductivities at 300 K are found to be anisotropic with values of 97.32 and 179.35 W/mK along the x and z directions, respectively, which are much lower than that of diamond (2664.93 W/mK) and carbon nanotube-based bct-C4 (1411.02 W/mK). A detailed analysis of both harmonic and anharmonic properties suggests that the soften acoustic phonon modes, the low Young's modulus, and strong anharmonicity are the key factors for the low lattice thermal conductivity. The study expands the family of carbon materials by assembling PG nanotubes.

A penta-silicene nanoribbon-based 3D silicon allotrope with high carrier mobility and thermoelectric performance.

Motivated by the successful synthesis of penta-silicene nanoribbons using various experimental techniques, we design a new 3D silicon allotrope, labeled cco-Si48, by assembling such nanoribbons, confirm its dynamical, thermal and mechanical stabilities, and further study its electron/phonon transport and linear optical properties based on the state-of-the-art theoretical calculations. We find that cco-Si48 is a direct bandgap semiconductor with a gap of 1.46 eV, exhibiting a high hole mobility in the magnitude of 103 cm2 V-1 s-1 and a low lattice thermal conductivity of 6.33 W m-1 K-1 at 300 K. Unlike the commonly reported n-type silicon-based materials with high thermoelectric performance, the p-type cco-Si48 outperforms its n-type counterpart in the thermoelectric figure of merit (ZT) value with a considerable value of 0.57 at 800 K. We further demonstrate that the electron-phonon interactions play a critical role in determining the optimal carrier concentrations for the peak ZT values. This work expands penta-silicene nanoribbons to their 3D assembled structure with new features and applications.

Effect of High Order Phonon Scattering on the Thermal Conductivity and Its Response to Strain of a Penta-NiN2 Sheet.

Motivated by the recent synthesis of penta-NiN2, a new two-dimensional (2D) planar material entirely composed of pentagons [ ACS Nano 2021, 15, 13539], we study its thermal transport properties based on first-principles calculations and solving the Boltzmann transport equation within the self-consistent phonon theory and four-phonon scattering formalism. We find that the intrinsic lattice thermal conductivity of penta-NiN2 is 11.67 W/mK at room temperature, which is reduced by 89.32% as compared to the value obtained by only considering three-phonon scattering processes. More interestingly, different from the general response of thermal conductivity to external strain in most 2D materials, an oscillatory decrease of the thermal conductivity with increasing biaxial tensile strain is observed, which can be attributed to the renormalization of vibrational frequencies and the nonmonotonic variation of phonon scattering rates. This work provides an accurate intrinsic thermal conductivity of penta-NiN2 and elucidates the effects of the strain-tuned vibrational modes and phonon band gap on the four-phonon scattering processes, shedding light on a better understanding of the physical mechanisms of thermal transport properties in 2D pentagon-based materials.

A biphenylene nanoribbon-based 3D metallic and ductile carbon allotrope.

Assembling two-dimensional (2D) sheets for three-dimensional (3D) functional materials is of current interest. Motivated by the recent experimental synthesis of 2D biphenylene [Science372 (2021) 852], we propose a new porous 3D metallic carbon structure, named T48-carbon, by using biphenylene nanoribbons as the building block. Based on state-of-the-art theoretical calculations, we find that T48-carbon is not only dynamically, thermally, and mechanically stable, but also energetically more favorable as compared with some other theoretically predicted carbon allotropes. Especially, T48-carbon exhibits mechanical anisotropy, ductility and intrinsic metallicity. A detailed analysis of electronic properties reveals that the metallicity mainly comes from the pz-orbital of sp2-hybridized carbon atoms. This work shows the promise of design and synthesis of 3D biphenylene-based metallic carbon materials with novel properties.

Four-Phonon Scattering Effect and Two-Channel Thermal Transport in Two-Dimensional Paraelectric SnSe.

In recent years, increased attention has been paid to the study of four-phonon interactions and diffusion transport in three-dimensional (3D) thermoelectric materials because they play an essential role in understanding the thermal transport process. In this work, we study four-phonon scattering and diffusion transport in two-dimensional (2D) thermoelectric materials using the paraelectric phase of 2D SnSe as an example. The inherent soft phonon modes are treated by the self-consistent phonon theory, which considers the temperature-induced renormalization of the phonons. Based on density functional theory and the Peierls-Boltzmann transport equation for phonons, we show that the four-phonon interactions can reduce the thermal conductivity of the 2D SnSe sheet by nearly 40% due to the collapse of soft optical modes, and the contribution of diffusion transport to the total thermal conductivity accounts for 14% at a high temperature of 800 K due to the short phonon mean free path approaching the Ioffe-Regel limit, suggesting the two-channel transport in this system. The results are further confirmed by using the machine learning-assisted molecular dynamics simulations. This work provides a new insight into the physical mechanisms for thermal transport in 2D systems with strong anharmonic effects.

MXene-Supported, Atomic-Layered Iridium Catalysts Created by Nanoparticle Re-Dispersion for Efficient Alkaline Hydrogen Evolution.

Tailoring the structure of metal components and interaction with their anchored substrates is essential for improving the catalytic performance of supported metal catalysts; the ideal catalytic configuration, especially down to the range of atomic layers, clusters, and even single atoms, remains a subject under intensive study. Here, an Ir-on-MXene (Mo2 TiC2 Tx ) catalyst with controlled morphology changing from nanoparticles down to flattened atomic layers, and finally ultrathin layers and single atoms dispersed on MXene nanosheets at elevated temperature, is presented. The intermediate structure, consisting of mostly Ir atomic layers, shows the highest activity toward the hydrogen evolution reaction (HER) under industry-compatible alkaline conditions. In addition, the better HER activity of Ir atomic layers than that of single atoms suggests that the former serves as the main active sites. Detailed mechanism analysis reveals that the nanoparticle re-dispersion process and Ir atomic layers with a moderate interaction to the substrate associate with unconventional electron transfer from MXene to Ir, leading to suitable H* adsorption. The results indicate that the structural design is important for the development of highly efficient catalysts.

First-Principles Study of the Structural, Electronic, and Enhanced Optical Properties of SnS/TaS2 Heterojunction.

Although the electronics and optoelectronics based on two-dimensional (2D) SnS have attracted great interest, their development is hindered by the large contact resistance at the interface of the metal-semiconductor junction. In this work, using first-principles calculations, we evaluate the contact performance in a van der Waals heterostructure composed of 2D SnS and TaS2. We demonstrate that holes can freely transfer from the electrode to the channel as a consequence of the Schottky-barrier-free interface as well as an upward band bending. Moreover, we show that the intrinsic properties of the SnS monolayer are well-preserved in the heterojunction, which is different from those of contact with metal surfaces. An enhanced optical response is also observed as compared with the freestanding sheet. Given the recent experimental synthesis of the SnS-TaS2 superlattice, this study enhances the understanding of the interface properties of SnS-based metal contact, which is essential for future device applications.

Boron-Functionalized Organic Framework as a High-Performance Metal-Free Catalyst for N2 Fixation.

Inspired by the recently synthesized covalent organic framework (COF) containing triquinoxalinylene and benzoquinone units (TQBQ) in the skeleton, we study the stability and properties of its two-dimensional analogue, TQBQCOF, and examine its potential for the synthesis of ammonia using first-principles calculations. We show that the TQBQCOF sheet is mechanically, dynamically, and thermally stable up to 1200 K. It is a semiconductor with a direct band gap of 2.70 eV. We further investigate the electrocatalytic reduction of N2to NH3on the Boron-functionalized TQBQCOF sheet (B/TQBQCOF). The rate-determining step of the catalytic pathways is found to be *N-N → *N-NH for the distal, alternating, and enzymatic catalytic mechanisms, with the corresponding overpotentials of 0.65, 0.65, and 0.07 V, respectively. The value of 0.07 V is the lowest required voltage among all of the N2 reduction catalysts reported so far, showing the potential of B/TQBQCOF as a metal-free catalyst to effectively reduce N2to NH3.

Imidazole-graphyne: a new 2D carbon nitride with a direct bandgap and strong IR refraction.

Six-membered rings are common building blocks of many carbon structures. Recent studies have shown that penta-graphene composed of five-membered carbon rings have properties very different from that of graphene. This has motivated the search for new carbon structures. Among this is cp-graphyne, composed of carbon pentagons and bridged by acetylenic linkers. However, the bandgap of cp-graphyne, like that of graphene, is zero, making it unsuitable for applications in electronics. Herein, we show that a new two-dimensional (2D) carbon nitride structure formed by assembling the five-membered imidazole molecules with acetylenic linker can overcome this limitation. Named ID-GY, this new material not only has a direct band gap of 1.10 eV, but it is dynamically and mechanically stable and can withstand temperatures up to 1200 K. In addition, due to its porous and anisotropic geometry, the Young's modulus of ID-GY along the diagonal direction is lower than that of most 2D materials reported previously. Equally important, ID-GY exhibits strong refraction near infrared (IR) and has potential for applications in nanoelectronics and optical devices. These results, based on density functional theory, can stimulate experimental studies.

Pentagonal B2N3-based 3D metallic boron nitride with high energy density.

Different from conventional insulating or semiconducting boron nitride,metallicBN has received increasing attention in recent years as its intrinsic metallicity grants it great potential for broad applications. In this study, by assembling the experimentally synthesized pentagonal B2N3units, we have proposed the first pentagon-based three-dimensional (3D) metallic boron nitride, labeled penta-B4N7.First-principles calculations together with molecular dynamics simulations and convex hull diagram show that penta-B4N7is not only thermally, dynamically and mechanically stable, but also three dimensionally metallic. A detailed analysis of its electronic structure reveals that the intrinsic metallicity comes from the delocalized electrons in the partially occupied antibonding N-Nπorbitals. Equally important, the energy density of penta-B4N7is found to be 4.07 kJ g-1, which is the highest among that of all the 3D boron nitrides reported so far.

Penta-BCN: A New Ternary Pentagonal Monolayer with Intrinsic Piezoelectricity.

Going beyond conventional hexagonal sheets, pentagonal 2D structures are of current interest due to their novel properties and broad applications. Herein, for the first time, we study a ternary pentagonal BCN monolayer, penta-BCN, which exhibits intrinsic piezoelectric properties. Based on state-of-the-art theoretical calculations, we find that penta-BCN is stable mechanically, thermally, and dynamically and has a direct band gap of 2.81 eV. Due to its unique atomic configuration with noncentrosymmetric and semiconducting features, penta-BCN displays high spontaneous polarization of 3.17 × 10-10 C/m and a prominent piezoelectricity with d21 = 0.878 pm/V, d22 = -0.678 pm/V, and d16 = 1.72 pm/V, which are larger than those of an h-BN sheet and functionalized pentagraphene. Since B, C, and N are rich in resources, light in mass, and benign to the environment, the intrinsic polarization and piezoelectricity make the penta-BCN monolayer promising for technological applications. This study expands the family of 2D pentagonal structures with new features.

Exploration of Crystallization Kinetics in Quasi Two-Dimensional Perovskite and High Performance Solar Cells.

Halide perovskites with reduced-dimensionality (e.g., quasi-2D, Q-2D) have promising stability while retaining their high performance as compared to their three-dimensional counterpart. Generally, they are obtained in (A1)2(A2)n-1PbnI3n+1 thin films by adjusting A site cations, however, the underlying crystallization kinetics mechanism is less explored. In this manuscript, we employed ternary cations halides perovskite (BA)2(MA,FA)3Pb4I13 Q-2D perovskites as an archetypal model, to understand the principles that link the crystal orientation to the carrier behavior in the polycrystalline film. We reveal that appropriate FA+ incorporation can effectively control the perovskite crystallization kinetics, which reduces nonradiative recombination centers to acquire high-quality films with a limited nonorientated phase. We further developed an in situ photoluminescence technique to observe that the Q-2D phase (n = 2, 3, 4) was formed first followed by the generation of n = ∞ perovskite in Q-2D perovskites. These findings substantially benefit the understanding of doping behavior in Q-2D perovskites crystal growth, and ultimately lead to the highest efficiency of 12.81% in (BA)2(MA,FA)3Pb4I13 Q-2D perovskites based photovoltaic devices.

CsI Pre-Intercalation in the Inorganic Framework for Efficient and Stable FA1-x Csx PbI3 (Cl) Perovskite Solar Cells.

Engineering the chemical composition of organic and inorganic hybrid perovskite materials is one of the most feasible methods to boost the efficiency of perovskite solar cells with improved device stability. Among the diverse hybrid perovskite family of ABX3 , formamidinium (FA)-based mixed perovskite (e.g., FA1-x Csx PbI3 ) possesses optimum bandgaps, superior optoelectronic property, as well as thermal- and photostability, which is proven to be the most promising candidate for advanced solar cell. Here, FA0.9 Cs0.1 PbI3 (Cl) is implemented as the light-harvesting layer in planar devices, whereas a low temperature, two-step solution deposition method is employed for the first time in this materials system. This paper comprehensively exploits the role of Cs+ in the FA0.9 Cs0.1 PbI3 (Cl) perovskite that affects the precursor chemistry, film nucleation and grain growth, and defect property via pre-intercalation of CsI in the inorganic framework. In addition, the resultant FA0.9 Cs0.1 PbI3 (Cl) films are demonstrated to exhibit an improved optoelectronic property with an elevated device power conversion efficiency (PCE) of 18.6%, as well as a stable phase with substantial enhancement in humidity and thermal stability, as compared to that of FAPbI3 (Cl). The present method is able to be further extended to a more complicated (FA,MA,Cs)PbX3 material system by delivering a PCE of 19.8%.