Atomic Level Research Articles

Effect of chemical structures and environmental factors on the thermal degradation mechanism of polyimide: Experiments and molecular dynamics simulations

The impact of chemical structure and environment on the thermal stability of polyimide (PI) was examined, and the degradation mechanism was determined using a combination of experiments and molecular simulations. Changes in mechanical properties and thermogravimetric analysis (TGA) were used to characterize the thermal stability of PI. Pyrolysis gas chromatography mass spectrometry (Py-GCMS) and thermogravimetric-infrared spectroscopy (TG-IR) were used to analyze the degradation products both qualitatively and quantitatively. Molecular simulation was employed to analyze the primary bond breakage and thermal degradation pathways of PI, as well as to investigate the effects of the chemical structure, atmosphere, and temperature on degradation properties. The findings indicated that p-benzene-structured 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA)/p-phenylenediamine (PDA) has the best thermal stability, whereas weak bonds like C–O–C in 4,4′-oxydianiline (ODA) and C–N in the 2-(4-aminophenyl)-1H-benzimidazol-5-amine (BIA) imidazole group decrease thermal stability. The formation path of low molecular weight products (CO2, CO, HCN, and NH3) and the potential degradation mechanism of PI were proposed. The process of PI thermal degradation accelerated by oxygen and high temperature was observed at the atomic level. Taken together, this work offers the possibility of monitoring the structural evolution of PI degradation process in real-time.

Molecular Interactions in Atomically Precise Metal Nanoclusters.

For nanochemistry, precise manipulation of nanoscale structures and the accompanying chemical properties at atomic precision is one of the greatest challenges today. The scientific community strives to develop and design customized nanomaterials, while molecular interactions often serve as key tools or probes for this atomically precise undertaking. In this Perspective, metal nanoclusters, especially gold nanoclusters, serve as a good platform for understanding such nanoscale interactions. These nanoclusters often have a core size of about 2 nm, a defined number of core metal atoms, and protecting ligands with known crystal structure. The atomically precise structure of metal nanoclusters allows us to discuss how the molecular interactions facilitate the systematic modification and functionalization of nanoclusters from their inner core, through the ligand shell, to the external assembly. Interestingly, the atomic packing structure of the nanocluster core can be affected by forces on the surface. After discussing the core structure, we examine various atomic-level strategies to enhance their photoluminescent quantum yield and improve nanoclusters' catalytic performance. Beyond the single cluster level, various attractive or repulsive molecular interactions have been employed to engineer the self-assembly behavior and thus packing morphology of metal nanoclusters. The methodological and fundamental insights systemized in this review should be useful for customizing the cluster structure and assembly patterns at the atomic level.

Hexa‐atom Pt Catalyst Fabricated by a Ligand Engineering Strategy for Efficient Hydrogen Oxidation Reaction

AbstractAtomically precise supported nanocluster catalysts (APSNCs), which feature exact atomic composition, well‐defined structures, and unique catalytic properties, offer an exceptional platform for understanding the structure‐performance relationship at the atomic level. However, fabricating APSNCs with precisely controlled and uniform metal atom numbers, as well as maintaining a stable structure, remains a significant challenge due to uncontrollable dispersion and easy aggregation during synthetic and catalytic processes. Herein, we developed an effective ligand engineering strategy to construct a Pt6 nanocluster catalyst stabilized on oxidized carbon nanotubes (Pt6/OCNT). The structural analysis revealed that Pt6 nanoclusters in Pt6/OCNT were fully exposed and exhibited a planar structure. Furthermore, the obtained Pt6/OCNT exhibited outstanding acidic HOR performances with a high mass activity of 18.37 A ⋅ mgpt−1 along with excellent stability during a 24 h constant operation and good CO tolerance, surpassing those of the commercial Pt/C. Density functional theory (DFT) calculations demonstrated that the unique geometric and electronic structures of Pt6 nanoclusters on OCNT altered the hydrogen adsorption energies on catalytic sites and thus lowered the HOR theoretical overpotential. This work presents a new prospect for designing and synthesizing advanced APSNCs for efficient energy electrocatalysis.

In-situ atomic tracking of intermetallic compound formation during thermal annealing

Intermetallic compounds (IMCs) with ordered atomic structure have gained great attention as nanocatalysts for its enhanced activity and stability. Although the reliance of IMC preparation on high-temperature annealing is well known, a comprehensive understanding of the formation mechanisms of IMCs in this process is currently lacking. Here, we employ aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (AC-HAADF-STEM) to track the formation process of IMCs on carbon supports during in-situ annealing, by taking PtFe as a case study within an industry-relevant impregnation synthesis framework. We directly discern five different stages at the atomic level: initial atomic precursors; Pt cluster formation; Pt-Fe disordered alloying; structurally ordered Pt3Fe formation, and final Pt3Fe-PtFe IMC conversion. In particular, we find that the crucial role of high-temperature annealing resides in facilitating the diffusion of Fe towards Pt, enabling the creation of alloys with the targeted stoichiometric ratio, which in turn provides the thermodynamic driving force for the disorder-to-order transition.

Hexa-atom Pt Catalyst Fabricated by a Ligand Engineering Strategy for Efficient Hydrogen Oxidation Reaction.

Atomically precise supported nanocluster catalysts (APSNCs), which feature exact atomic composition, well-defined structures, and unique catalytic properties, offer an exceptional platform for understanding the structure-performance relationship at the atomic level. However, fabricating APSNCs with precisely controlled and uniform metal atom numbers, as well as maintaining a stable structure, remains a significant challenge due to uncontrollable dispersion and easy aggregation during synthetic and catalytic processes. Herein, we developed an effective ligand engineering strategy to construct a Pt6 nanocluster catalyst stabilized on oxidized carbon nanotubes (Pt6/OCNT). The structural analysis revealed that Pt6 nanoclusters in Pt6/OCNT were fully exposed and exhibited a planar structure. Furthermore, the obtained Pt6/OCNT exhibited outstanding acidic HOR performances with a high mass activity of 18.37 A ⋅ mgpt -1 along with excellent stability during a 24 h constant operation and good CO tolerance, surpassing those of the commercial Pt/C. Density functional theory (DFT) calculations demonstrated that the unique geometric and electronic structures of Pt6 nanoclusters on OCNT altered the hydrogen adsorption energies on catalytic sites and thus lowered the HOR theoretical overpotential. This work presents a new prospect for designing and synthesizing advanced APSNCs for efficient energy electrocatalysis.

Promoting CO2 Electroreduction to Hydrocarbon Products via Sulfur-Enhanced Proton Feeding in Atomically Precise Thiolate-Protected Cu Clusters.

Thiolate-protected Cu clusters with well-defined structures and stable low-coordinated Cu+ species exhibit remarkable potential for the CO2RR and are ideal model catalysts for establishing structure-electrocatalytic property relationships at the atomic level. However, extant Cu clusters employed in the CO2RR predominantly yield 2e- products. Herein, two model Cu4(MMI)4 and Cu8(MMI)4(tBuS)4 clusters (MMI=2-mercapto-1-methylimidazole) are prepared to investigate the synergistic effect of Cu+ and adjacent S sites on the CO2RR. Cu4(MMI)4 can reduce CO2 to deep-reduced products with a 91.0 % Faradaic efficiency (including 53.7 % for CH4) while maintaining remarkable stability. Conversely, Cu8(MMI)4(tBuS)4 shows a remarkable preference for C2+ products, achieving a maximum FE of 58.5 % with a C2+ current density of 152.1 mA⋅cm-2. In situ XAS and ex situ XPS spectra reveal the preservation of Cu+ species in Cu clusters during CO2RR, extensively enhancing the adsorption capacity of *CO intermediate. Moreover, kinetic analysis and theoretical calculations confirm that S sites facilitate H2O dissociation into *H species, which directly participate in the protonation process on adjacent Cu sites for the protonation of *CO to *CHO. This study highlights the important role of Cu-S dual sites in Cu clusters and provides mechanistic insights into the CO2RR pathway at the atomic level.

Oxide Derivatives of Nb2CTx MXene and Their Application as Electron Transport Layers in Perovskite Solar Cells: Unraveling the Oxidation Process and Functionalization.

In the realm of photovoltaic research, 2D transition metal carbides (MXenes) have gained significant interest due to their exceptional photoelectric capabilities. However, the instability of MXenes due to oxidation has a direct impact on their practical applications. In this work, the oxidation process of Nb2CTx MXene in aqueous systems is methodically simulated at the atomic level and nanosecond timescales, which elucidates the structural variations influenced by the synergistic effects of water and dissolved oxygen, predicting a transition from metal to semiconductor with 44% C atoms replaced by O atoms in Nb2CTx. Moreover, Nb2CTx with varying oxidation degrees is utilized as electron transport layers (ETLs) in perovskite solar cells (PSCs). Favorable energy level alignments with superior electron transfer capability are achieved by controlled oxidation. By further exploring the composites of Nb2CTx to its derivatives, the strong interaction of the nano-composites is demonstrated to be more effective for electron transport, thus the corresponding PSC achieves a better performance with long-term stability compared with the widely used ETLs like SnO2. This work unravels the oxidation dynamics of Nb2CTx and provides a promising approach to designing ETL by exploiting MXenes to their derivatives for photovoltaic technologies.

Boosting C2H6/C2H4 separation via the precise electrostatic modulation of synthetic 1D channel at atomic level

Boosting C2H6/C2H4 separation via the precise electrostatic modulation of synthetic 1D channel at atomic level

Nano-Diamond Decorated Reduced Graphene Oxide: Improvement to Tribological Property of Polyimide and Its Molecular Dynamic Simulation

In this paper, the nano-diamond decorated reduced graphene oxide (Diamond-RGO) and its polyimide (PI) composite, by using in situ polymerization method, are synthesized, respectively. The tribological study of Diamond-RGO/PI composite exhibits super anti-wear properties under dry sliding and high load condition: the lowest wear rate of 4.9 × 10−8mm3N−1m−1, which is extraordinarily lower than that of pure PI and RGO/PI composite, is obtained under load 100 N. The tribological mechanism investigation shows that Diamond-RGO composite has a strong synergistic effect in the tribological process which contributes to form a uniform transfer film on the counterpart ring and thus significantly decrease the composite’s wear rate. Through molecular simulation, the shear behavior of the composite material indicates that the presence of the Diamond-RGO composite will obviously weaken the influence of the iron wall on the PI bulk phase molecular chain in the shear process. This confirms that the synergistic effect between nano-Diamond and RGO will improve its PI composite’s tribological performance on atomic level.

Investigating the influence of graphene coating thickness on Al0.3CoCrFeNi high-entropy alloy using molecular dynamics simulation

Graphene was proven to be an excellent surface-strengthening material. However, there needs to be more literature explaining its strengthening mechanism at the atomic level. This study systematically investigated the surface strengthening mechanism of graphene coating on the Al0.3CoCrFeNi high-entropy alloy (HEA) and the influence of loading-unloading cycles on the formation of internal defects in the material through molecular dynamics (MD) simulations at the atomic level. Research results show that the internal stress concentration in HEA without graphene coating leads to a more considerable residual depth after unloading. With the addition of a graphene coating, the material's stress area increases, effectively alleviating stress concentration and reducing residual depth. Furthermore, the lengths of Lomer-Cottrell dislocation locks and Hirth dislocation locks inside the material increase, enhancing the material's deformation resistance. The more layers of graphene, the larger the stress distribution area and the greater the hardness. Under the same indentation force, the number of HCP structures and amorphous structures inside the material is inversely proportional to the thickness of the graphene layers. In addition, after multiple loading and unloading cycles, the hardness of HEA/GP materials basically remains unchanged, while internal defects increase and residual depth increases.

Roles of Nonadiabatic Processes, Reaction Mechanism, and Selectivity in Cu-Catalyzed [2 + 2] Photocycloaddition of Norbornene and Acetone to Oxetane.

Oxetane has been extensively studied for its applications in medicinal chemistry and as a reactive intermediate in synthesis. Experiments report a Cu-catalyzed [2 + 2] photocycloaddition of acetone and norbornene to oxetane, which is proposed to deviate from the conventional Paternò-Büchi reaction. However, its mechanism at the atomic level is not clear. In this study, we used a combination of multistate complete active space second-order perturbation theory (MS-CASPT2) and density functional theory to systematically investigate the reaction mechanism and elucidate the factors contributing to the diastereomeric selectivity. Initially, the formation of the TpCu(Norb) complex is achieved by strong interaction between tris(pyrazolyl)borate Cu(I) (TpCu) and norbornene in the ground state (S0). Upon photoexcitation, TpCu(Norb) eventually decays to the T1 state, in which TpCu(Norb) attacks acetone to initiate subsequent reactions and produces final endo- or exo-oxetane products. All these reactions initially involve the C-C bond formation in the T1 state thereto leading to a ring-opening intermediate. This intermediate then undergoes a nonradiative transition to the S0 state, producing a five-membered ring intermediate, from which the C-O bond is formed, leading to the experimentally dominant exo-product. In contrast, the endo-oxetane formation requires a rearrangement process after the C-C bond is formed because of the large steric effects. As a consequence, the different reaction pathways generating exo- and endo-products exhibit large differences in the free-energy barriers, which results in a diastereomeric selectivity observed experimentally. Additionally, the nonradiative transition is found to play an important role in facilitating these reaction steps. The present computational study provides valuable mechanistic insights into Cu-catalyzed photocycloaddition reactions.

Liquid thermophysical properties of Ag-Si alloy based on deep learning potential

The knowledge of the thermophysical properties of liquid metals and alloys is essential for expanding the materials database and designing materials with good properties. In this work, we developed an interatomic potential using a deep neural network (DNN) algorithm for liquid Ag-Si alloys. Compared with ab initio molecular dynamics (AIMD) results, the DNN potential provided a good description of the information of energy, force, and structure features of the system in the simulated temperature range. Through this potential, we can obtain the thermophysical properties of different compositions of liquid alloys by simulation way. The computed thermophysical properties are in excellent agreement with the reported experimental data. The analysis of local structure indicates that the liquid ordering and stability strengthen upon cooling at the atomic level, eventually leading to an increase in thermophysical properties.

Nitrogen-Site Engineering in Covalent Organic Frameworks for H2O2 Photogeneration via Dual Channels of Indirect Two-Electron O2 Reduction.

Photocatalytic H2O2 production is a green and sustainable route, but far from meeting the increasing demands of industrialization due to the rapid recombination of the photogenerated charge carriers and the sluggish reaction kinetics. Effective strategies for precisely regulating the photogenerated carrier behavior and catalytic activity to construct high-performance photocatalysts are urgently needed. Herein, a nitrogen-site engineering strategy, implying elaborately tuning the species and densities of nitrogen atoms, is applied for H2O2 photogeneration performance regulation. Different nitrogen heterocycles, such as pyridine, pyrimidine, and triazine units, are polymerized with trithiophene units, and five covalent organic frameworks (COFs) with distinct nitrogen species and densities on the skeletons are obtained. Fascinatingly, they photocatalyzed H2O2 production via dominated two-electron O2 reduction processes, including O2-O2 •‒-H2O2 and O2-O2 •‒-O2 1-H2O2 dual pathways. Just in the air and pure water, the multicomponent TTA-TF-COF with the maximum nitrogen densities triazine nitrogen densities exhibited the highest H2O2 production rate of 3343µmol g-1 h-1, higher than most of other reported COFs. The theoretical calculation revealed the higher activity is due to the easy formation of O2 •‒ and O2 1 in different catalytic process. This study gives a new insight into designing photocatalysis at atomic level.

A comparative first-principles study on the physical properties of Gd2Zr2O7 weberite and pyrochlore

A comparative analysis of the physical properties of Gd2Zr2O7 weberite and pyrochlore is conducted using first-principles methods. The structural characteristics of Gd2Zr2O7 pyrochlore and weberite are examined at the atomic site, local coordination, and lattice parameter levels. The findings from ab initio molecular dynamics simulations and experimental data confirm the existence and stability of the Gd2Zr2O7 weberite structure at 300 K. The formation of cation antisite defects is calculated to be more facile in the weberite lattice compared to pyrochlore. The formation energy of vacancy defects is strongly correlated to the distinct defect configurations. The calculations further highlight that Gd2Zr2O7 weberite exhibits mechanical properties comparable to pyrochlore. The insulating nature, chemical bonding characteristics, and charge states of individual atoms in weberite and pyrochlore are elucidated through analysis of the partial density of states and Bader charges.

"On-the-Fly" Nonadiabatic Dynamics Simulation on the Ultrafast Photoisomerization of a Molecular Photoswitch Iminothioindoxyl: An RMS-CASPT2 Investigation.

Iminothioindoxyl (ITI) is a new class of photoswitch that exhibits many excellent properties including well-separated absorption bands in the visible region for both conformers, ultrafast Z to E photoisomerization as well as the millisecond reisomerization at room temperature for the E isomer, and switchable ability in both solids and various solvents. However, the underlying ultrafast photoisomerization mechanism at the atomic level remains unclear. In this work, we have employed a combination of high-level RMS-CASPT2-based static electronic structure calculations and nonadiabatic dynamics simulations to investigate the ultrafast photoisomerization dynamics of ITI. Based on the minimum-energy structures, minimum-energy conical intersections, linear interpolation internal coordinate paths, and nonadiabatic dynamics simulations, the overall photoisomerization scenario of ITI upon excitation is established. Upon excitation around 416 nm, the molecule will be excited to the S2 state considering its close energy to the experimentally measured absorption maximum and larger oscillator strength, from which ultrafast decay of S2 to S1 state can take place efficiently with a time constant of 62 fs. However, the photoisomerization is not likely to complete in the S2 state since the dihedral associated with the Z to E isomerization changes little during the relaxation. Upon relaxing to the S1 state, the molecule will decay to the S0 state ultrafast with a time constant of 232 fs. In contrast, the decay of the S1 state is important for the isomerization considering that the dihedral related to the isomerization of the hopping structures is close to 90°. Therefore, the S1/S0 intersection region should be important for the isomerization of ITI. Arriving at the S0 state, the molecule can either go back to the original Z reactant or isomerize to the E products. At the end of the 500 fs simulation time, the E configuration accounts for nearly 37% of the final structures. Moreover, the photoisomerization mechanism is different from the isomerization mechanism in the ground state; i.e., instead of the inversion mechanism in the ground state, the photoisomerization prefers the rotation mechanism. Our results not only agree well with previous experimental studies but also provide some novel insights that could be helpful for future improvements in the performance of the ITI photoswitches.

Conversion mechanism of the atomically precise group IB metal nanoclusters by theoretical calculations

The controllable synthesis and design of group IB metal (i.e., Au/Ag/Cu) and their alloy nanoclusters (IB-NCs) rely on an elucidation of the conversion mechanism, bonding preference, and structure–property relationships. In the past decades, the development of cluster-to-cluster transformation and spectroscopic tracking strategy have provided important mechanistic insights that greatly help researchers improve synthetic skills and explore the application of IB-NCs. Specifically, with the aid of DFT calculations, the structural information of cluster-to-cluster conversion, including the intermediates and active sites, can be deduced from the atomic level. In this review, the conversion mechanisms of IB-NCs in the past three years have been reviewed to summarize the key factors influencing structural transformation and reactivity. The three types of cluster-to-cluster conversion (i.e., foreign metal source, foreign ligand, and other reagents/condition-induced conversion) were highlighted for further synthesis and design of functional nanoclusters.

Room-temperature solution processed silicon nanocluster: Theory prediction and experiment achievement

We have presented in this work that silicon nanoclusters (Si NCs) embedded in silicon nitride (▪) can be synthesized by vacuum ultra-violet irradiation of perhydropolysilazane (PHPS) at room temperature. The formation of Si NCs is first predicted by reactive molecular dynamics (ReaxFF-MD) simulations based on a self-developed Si/N/H force field. Then the existence of Si NCs is further verified by photoluminescence (PL) spectroscopy and Raman spectroscopy characterization. Additionally, the photochemical conversion mechanism of PHPS is examined. Moreover, it has been found based on Fourier transform infrared (FT-IR) measurements that ▪ samples prepared with an intermediate irradiation time are the most readily to be oxidized, which can be attributed to the high concentration of the active group -▪, being consistent with the reaction mechanism. We have demonstrated that ReaxFF-MD simulation can be a powerful complement to experimental investigations in revealing the conversion mechanism of polymer precursor to ceramics at the atomic level.

Computational drug repositioning for IL6 triggered JAK3 in rheumatoid arthritis using FDA database.

Rheumatoid Arthritis (RA) is a persistent autoimmune disease affecting approximately 0.5-1 percent of the world population. RA prevalence is higher in woman aged between 35 and 50years than in age matched men, though this difference is less evident among elderly patients. The profound immune specific effects of disrupted JAK 3 (Janus kinase 3) signaling highlight the possibility of therapeutic targeting of JAK3 as a highly specific mode of immune system suppression. To address the above problem which is unendurable to patients and in the hope to cater some respite to such suffering we have targeted JAK 3 protein and JAK/STAT signaling pathway with compounds downloaded from FDA database, and performed screening of all available compounds docked against JAK3 protein. The difference between the target protein and other proteins of the same family was studied using cross docking and the compounds having higher binding affinity to JAK3 protein also showed more selectivity towards the particular protein. Density functional theory and molecular dynamics simulation study was done to study the compounds at their atomic level to know more about their drug likeliness. At the end of the study and based on our analysis we have come up with three FDA approved drugs that can be proposed as a treatment option for Rheumatoid Arthritis.

Silk Revolution: Redefining Sericulture through Application of Precise Nanotechnologies

Human civilization has been intertwined with sericulture—the art and science of producing silk for centuries. The application of nanotechnology to sericulture techniques has created new opportunities to improve the quantity, quality and use of silk in recent decades. This abstract examines the numerous applications of nanotechnology in sericulture, with particular attention on how it affects the quality of silk fibers, the procedures involved in producing silk and the creation of cutting-edge materials derived from silk. By providing comprehensive control over the properties of silk at the molecular and atomic levels, nanotechnology, defined as the manipulation of matter at the nano scale, has completely transformed the silk industry. Improving the health and growth of silkworms is one of the main application areas. Silver nanoparticles, for example have been used to fight illnesses and strengthen silkworm’s immune systems, boosting the insect’s resilience against illness and optimizing silk yield. Furthermore, silk fiber quality has increased largely in substantial measure by nanotechnology. The mechanical, thermal and optical characteristics of silk fibers have been altered by researchers by adding nanoparticles during the spinning process. For example, adding nanoparticles to silk can increase its tensile strength, making it more durable and appropriate for a variety of industrial uses. Similarly, silk can be given antibacterial qualities by adding nanoscale compounds, increasing its shelf life and possible uses in medical textiles. Furthermore, the production of sophisticated silk-based materials with distinctive functions has been made easier by nanotechnology. Nanocomposites, which consist of nanoparticles embedded in silk matrices, have unique characteristics like improved electrical conductivity, controlled drug release, and greater biocompatibility. Silk-based materials have emerged as a result of these developments, finding use both in wearable electronics and biomedical devices.

Analytical solution for Ξ-type qutrit interacting nonlinearly with a cavity field through Kerr-like medium with intrinsic noise present

This paper presents an analytical solution for the Milburn equation describing the interaction between a 3-level atom in a [Formula: see text]-configuration, a cavity field, and a nonlinear Kerr-like medium. The Hamiltonian of the model is presented in the rotating wave approximation (RWA) under specific conditions. The obtained analytical solution accounts for the impact of intrinsic decoherence and nonlinear terms on the dynamics of various physical phenomena. Specifically, we explore the behavior of atomic population, entropy functions, entanglement, negativity, and correlation, highlighting their dependence on the physical parameters of the [Formula: see text]-level atom-cavity system.