Small Atoms Research Articles

Effect of doping on suppression of Coulomb explosion in dicationic noble gas clusters

Dicationic noble gas clusters undergo disintegration due to the repulsive force between charged ions, known as Coulomb explosion (CE). However, beyond a certain size limit, the attractive forces suppress the Coulombic repulsion, enabling the clusters to stabilise. We explored the potential energy surface (PES), which incorporates electrostatic interactions (including Coulombic and induced dipole) and the Lennard-Jones potential, using parallel tempering to determine the cutoff limit for suppressing Coulomb explosions (CE) in both pure and doped noble gas clusters. The influence of both homogeneous (i.e. both charge on two bigger atoms) and heterogeneous charge distributions (i.e. two units of charge being centred on one large and one smaller atom) is evaluated for doping applications. The simulations reveal that single ion ejection is the major dissociation pathway below the cutoff limit. Additionally, the cutoff limit smoothly decreases with gradual doping of heavier atoms in both types of charge distribution.

Single photon scattering with the giant and small atom interplay in a one-dimensional coupled resonator waveguide

We study single photon scattering in a one-dimensional coupled resonator waveguide, which is dressed by a small and a giant artificial atom simultaneously. Here, we have set the small atom to be a neighbor to one leg of the giant atom, and the giant atom couples to the waveguide via two distant sites. When the small and giant atoms are both resonant with the bare resonator in the waveguide, we observe the perfect reflection of the resonant incident photon. On the other hand, when the small atom is detuned from the giant atom, the single photon reflection is characterized by a wide window and Fano line shape. We hope our work will pave the way for the potential application of small and giant atom hybrid systems in the study of photonic control in the low-dimensional waveguide structure.

Synthesis, Structure and Catalytic Properties of Hyp3[Ge9Rh]PPh3

In single‐site homogeneous catalysts (SSHoCs) transition metal atoms are incorporated in the surface of a small homoatomic atom cluster. Within this concept the main group element cluster is considered as innocent support material, while the catalysis itself is realised at the transition metal atom. In the case of [Ge9] clusters, both transition metal atoms and the Ge atoms have low oxidation state. Thus improving the synthesis towards transition metal decorated Zintl cluster has become important in the last years, and the catalytic activity of these clusters is of high interest. An optimised one‐step synthetic protocol for Hyp3[Ge9Rh]PPh3 is presented. The product is characterised by NMR spectroscopy, LIFDI/MS and single crystal structure determination. The Rh atom is incorporated in the clusters surface and bridges a deltahedral cluster face, whereby one of the triangular edge opens forming an almost planar face of three Ge and one Rh atom. PPh3 ligand dissociation as well as the catalytic activity in the isomerisation of 1‐hexene and allylbenzene as well as the hydrogenation of those substrates with hydrogen is investigated.

Strategic lattice manipulation in transition metal nitrides for improved solubility

In this study, we propose a new concept for achieving metastable ternary transition metal nitride solid solutions, focusing on face centered cubic (fcc) structured Ti(N,B) as a model system. Combining non-reactive magnetron sputtering with computational analysis, we develop a microalloying strategy to manipulate the metallic sublattice, thereby influencing the solubility of B in the non-metal sublattice. We show that imposed tensile strain on the fcc-TiN lattice facilitates the solubility of B, with a 1.5 % strain enabling the incorporation of ∼28.5 at.% B at the non-metal sublattice. Conversely, compressive strain hinders the formation of the fcc-Ti(N,B) solid solution, highlighting the importance of lattice manipulation in controlling solubility. At the same time, our experimental findings reveal that adding larger atoms, such as Zr, to the metal sublattice enhances the solubility of B in fcc-TiN more effectively (∼2 at.% Zr proves to be sufficient to solute 10 at.% B in the fcc-TiN lattice) than smaller atoms, like Cr or similar-sized Ti atoms. The size effect of the alloying atoms on the B solubility is further supported by radial distribution function analysis, showing lower local lattice distortions for Zr compared to Cr.

Composition, Temperature, and Size Regulation of Refractive Index in Ternary Group-Ⅲ Nitride Alloys: a Bond Relaxation Investigation

Abstract The physical origins of composition-, temperature-, and size-motivated changes in refractive index in crystals have long been a puzzle. Combining the bond-order-length-strength theory, local bond average approach, and core-shell structural model, we investigated the refractive indexes in dependencies of composition, temperature, and size for the ternary wurtzite group-Ⅲ nitride alloys. The theoretical reproduction of the observations disclosed that (ⅰ) the doping of small atoms caused the contraction in bond length, the strengthening in bond energy, and the decrease of refractive index, whereas the doping of large atoms led to an elongation of bond length, a weakening of bond energy, and an increase of refractive index; (ⅱ) the refractive index is inversely proportional to the cohesive energy and the cube of the Debye temperature; and (ⅲ) with the gradual decrease in solid size, the coordination number lowers, the bond length contracts, the bond energy gains, the surface-to-volume ratio rises, and the refractive index decreases. The proposed formulation not only shows an in-depth comprehension of the physical essence of the stimuli impact on the refractive index but also is expected to be conducive to the exploitation, optimization, and operation of the new-type photonic, piezoelectric, and pyroelectric nanometer devices for the ternary wurtzite alloys.

Evolution mechanism of coal chemical structure after supercritical CO2 transient fracturing

This study investigates the chemical structural evolution of coal during supercritical CO2 transient fracturing using a self-developed experimental platform. Coal samples with different metamorphic ranks were subjected to transient fracturing under different pressures. Characterization techniques, including X-ray diffraction, Raman spectroscopy, and Fourier transform infrared spectroscopy, analyzed the fractured coal samples. Initially, supercritical CO2 dissolved minerals and non-aromatic structures, rearranging organic compounds. As fracturing intensified, alkanes and substituents rapidly released, causing large aromatic rings to break into smaller ones. The breakage of small aromatic rings created pores and increased aromatic structure dimensions. Moreover, the interaction between fractured small aromatic rings and oxygen atoms in the C=O group facilitated the formation of new C–C bonds, promoting the connection of aromatic rings to form larger structures. In the early stages of fracturing, the molecular structure of coal relaxed, and subsequently, as the fracturing intensified, the hydrogen bonds initially broken could reconnect to aromatic structures to form OH-π interactions, enhancing intermolecular attraction. This promoted molecules to be more inclined to form stable, dense structures. Understanding this evolution mechanism aids in analyzing the transformation of fractured coal pores, thereby enhancing our understanding of the mechanism for improving coal seam permeability using this technology.

A Kaczmarz-inspired approach to accelerate the optimization of neural network wavefunctions

Neural network wavefunctions optimized using the variational Monte Carlo method have been shown to produce highly accurate results for the electronic structure of atoms and small molecules, but the high cost of optimizing such wavefunctions prevents their application to larger systems. We propose the Subsampled Projected-Increment Natural Gradient Descent (SPRING) optimizer to reduce this bottleneck. SPRING combines ideas from the recently introduced minimum-step stochastic reconfiguration optimizer (MinSR) and the classical randomized Kaczmarz method for solving linear least-squares problems. We demonstrate that SPRING outperforms both MinSR and the popular Kronecker-Factored Approximate Curvature method (KFAC) across a number of small atoms and molecules, given that the learning rates of all methods are optimally tuned. For example, on the oxygen atom, SPRING attains chemical accuracy after forty thousand training iterations, whereas both MinSR and KFAC fail to do so even after one hundred thousand iterations.

Rattling effect in YbaCabSi7.5Al4.5O1.5N14.5 α‐SiAlON ceramics

AbstractWhen a small atom is induced into a larger lattice, new phonon scattering modes are aroused and thermal conductivity is significantly reduced by increased anharmonicity. This effect, called “rattling,” has been reported in pyrochlores and some cage compounds, but not in α‐SiAlON. In this study, we reveal that in (Yb+Ca) co‐doped α‐SiAlON ceramics (YbaCabSi7.5Al4.5O1.5N14.5, x = a/[a+b]), the rattling effect plays a crucial role in reducing thermal conductivity. Samples with different Yb/Ca ratios (x = 0, 0.1, 0.2, 0.3…, 1.0) were sintered by spark plasma sintering (SPS) at 1600°C and their thermal diffusivity/conductivity were measured by the laser‐flash method. We found that substituting Ca2+ with smaller Yb3+ cations in Ca‐α‐SiAlON caused significant reduction in thermal conductivity. On the contrary, substituting Yb3+ with larger Ca2+ cations in Yb‐α‐SiAlON caused only slight reduction in thermal conductivity. A lowest thermal conductivity of 3.3 W/(m·K) was achieved, when x = 0.3 rather than x = 0.5 or 0.7. Further analysis of phonon scattering intensity confirmed that both intrinsic and extrinsic scatterings are enhanced in the sample of x = 0.3. This study demonstrates that by utilization of the rattling effect, α‐SiAlON, well‐known for its excellent mechanical properties, can be tuned to exhibit low thermal conductivity comparable to that of La2Zr2O7 and 8YSZ.

Nanoscale pattern formation on surfaces by cluster ion beam irradiation

Ion beam sputtering is a solid-surface nanostructuring procedure applicable to any type of material. In this study, we are interested in the bombardment of a metallic surface, specifically, of a Co(110) surface bombarded with Ar clusters at oblique incidence. The bombardment with clusters accentuates the effect of surface ripple formation. Sputter erosion and surface atomic redistribution are the processes that determine the morphological evolution of the surface from the collisional point of view. The importance of these processes was analyzed for different angles of incidence in the bombardment with very small clusters and atoms while always maintaining the same fluence. The sputter yield increases with the number of atoms in the cluster for angles greater than 50°; while for the rest, it barely experiences any increase. Unlike sputtering, displacements increase as the cluster size increases above the critical angle. Moreover, horizontal displacement only shows some sign of saturation for angles close to the normal. An analysis of redistributed volumes and previous data suggest that sputtering drives the ripple effect for grazing angles and ones close to these. It is also responsible for the enhancement effect in the cluster bombardment. For the rest of the angles, the weight of the redistribution is decisive. There is a proportional relationship between the emptied volume and the horizontal displacement.

Light-driven acetic acid coupling for the production of succinic acid

Treating and recycling high concentration acetic acid in the exhaust water has long posed a challenge in industry. These wastes containing acetic acid are typically processed via basic neutralization, characterized by high acidity and resistance to degradation, which can bring potential economic burden and environmental hazards. Here we report a new photocatalytic conversion route for addressing high concentrations of acetic acid. This process transforms acetic acid into valuable succinic acid through dehydrogenative coupling. Additionally, the photocatalytic conversion of acetic acid exhibits structure sensitivity when employing Pt cocatalysts of varying sizes. Through inhibiting the undesirable H termination of active free radicals over small Pt atoms and clusters, the rate of undesired decarboxylation reactions can be decreased by 75 %. This effectively enhances the selectivity of the coupling product succinic acid by 2.1 times, achieving a succinic acid formation rate of 7.28 mmol/(gcat•h). This work offers a theoretical guidance for searching value-added reaction pathways of acetic acid, thereby establishing an innovative approach for high concentration exhaust acetic acid treatment and converting low value monocarboxylic molecules into valuable diacid polymerization monomers under a mild and green condition.

Ru–modified graphitic carbon nitride for the solar light–driven photocatalytic H2O2 synthesis

Hydrogen peroxide (H2O2) is an efficient and environmentally friendly oxidant as well as a promising energy-carrier alternative to hydrogen. Its solar light-driven photocatalytic synthesis from H2O and O2 is a high-prospect sustainable alternative to the industrial anthraquinone process. Hybrid Ru-modified graphitic carbon nitride (g-C3N4) catalysts were prepared by thermal polymerisation of melamine/Ru(III) acetylacetonate mixtures, and subsequent thermal exfoliation. Simultaneous Ru incorporation and thermal exfoliation impacted the morphology and the structure of the g-C3N4 sheets, and low-atomicity Ru species with small nanoclusters and single atoms were observed only in the Ru-modified exfoliated g-C3N4 photocatalysts. We demonstrated the potential of a joint thermal exfoliation and modification with Ru to enhance the H2O2 synthesis efficiency of the g-C3N4 photocatalyst under simulated sunlight. A volcano-type behavior was observed with increasing the Ru content, and the best performance was obtained for exfoliated g-C3N4 with an ultra-low Ru content of 0.019 wt.%, that outperformed both its bulk counterpart and the pristine exfoliated reference in terms of initial H2O2 synthesis rate and H2O2 formation rate constant. The enhanced performance was attributed to the presence of highly dispersed low atomicity Ru as well as to more accessible active sites and carrier migration channels. Quenching experiments revealed a mixed reactional pathway involving both two-step one-electron and one-step two-electron O2 reduction in the photocatalytic H2O2 production.

Small iridium clusters supported on TiO2 as catalysts for intensifying low-temperature methane activation and reforming

Hydrogen production via low-temperature (<550 °C) steam reforming of methane (SRM) has attracted increasing research attention due to its importance as an alternative, efficient, and environmentally benign energy carrier. The thermodynamic limitation of low-temperature SRM can be eased by process design such as using a membrane reactor or an adsorption-enhanced reforming process, and an active low-temperature SRM catalyst with high stability would be indispensable. Herein, we report a 2 wt% Ir/TiO2 catalyst that exhibits high methane conversion activity in low-temperature SRM with a relatively high turnover frequency (TOF) and a low activation energy in comparison with the literature. Long-term catalyst stability test demonstrates a good resistance to coking and sintering during low-temperature SRM reactions. In situ spectroscopy analyses and density functional theory (DFT) calculations reveal that the small Ir clusters and Ir single atoms (SAs) are both active for methane conversion, while the clusters exhibit higher activity than SAs for low-temperature SRM reaction. Microkinetic simulation based on DFT calculations shows consistent product formation temperatures and the simulation nicely reproduces the product distribution observed in experimental analysis, which confirms the superior SRM activity of the TiO2-supported Ir clusters in this work.

Revealing the critical role of vanadium in radiation damage of tungsten-based alloys

Refractory tungsten-based medium- and high-entropy alloys form a promising class of strong, heat- and radiation-resistant materials for nuclear applications. Here, we systematically explore the radiation tolerance of Mo–Nb–Ta–V–W alloys using molecular dynamics simulations and a machine-learned interatomic potential. Going from pure W to W-based equiatomic binaries, ternaries, quaternary, and the quinary high-entropy alloy, we simulate the radiation damage accumulation up to 0.4–0.8 dpa through cumulative collision cascades. We find that the radiation damage and evolution are controlled by a combination of cascade-induced clustering, the balance in migration of vacancies and interstitials, and defect cluster binding energies. These mechanisms are strongly influenced by atom size, among which vanadium as the smallest atom is decisive. In pure W and alloys without V, the microstructure is characterised by large and growing interstitial dislocation loops. In stark contrast, in alloys containing V, vacancy dislocation loops are formed directly in collision cascades and the balanced migration rate of vacancies and interstitials promotes recombination and prevents growth of interstitial loops. The atom size differences also lead to clear segregation, with small atoms (V) in interstitial clusters and larger atoms (Ta, Nb) segregating around vacancy clusters. Furthermore, the small cluster sizes, formation of vacancy loops, segregation, and balanced migration in V-containing alloys lead to low swelling and an exceptionally efficient defect annealing compared to pure W and V-free alloys. Our results highlight WTaV as a promising low-activation material, where almost 90% of defects are annealed at 2000 K during 5 ns compared to <20% in pure W.

Effects of higher-order Casimir-Polder interactions on Rydberg atom spectroscopy

In the extreme near field, when the spatial extension of the atomic wavefunction is no longer negligible compared to the atom-surface distance, the dipole approximation is no longer sufficient to describe Casimir-Polder interactions. Here we calculate the higher-order, quadrupole and octupole, contributions to Casimir-Polder energy shifts of Rydberg atoms close to a dielectric surface. We subsequently investigate the effects of these higher-order terms in thin-cell and selective reflection spectroscopy. Beyond its fundamental interest, this regime of extremely small atom surface separations is relevant for quantum technology applications with Rydberg or surface-bound atoms interfacing with photonic platforms. Published by the American Physical Society 2024

Development of conformable cryogenic valve seats for resilience to foreign object debris

Cryogenic valve sealing technology is notoriously more challenging than traditional fluids due to the combination of small atoms and molecules with extreme temperature and pressure profiles. Recently, flexible polymeric films folded into origami demonstrated considerable resilience to mechanical failure in the cryogenic extreme. Fluorinated polymers such as Polytetrafluoroethylene (PTFE) are observed to mechanically flex and not plastically deform in cryogenic conditions. This paper explores a new cryogenic valve sealing paradigm which uses multiple conformable polymer layers to provide constant seat mating surface area at cryogenic conditions. Stacking these thin discs in series with a spacer allows maximum bend around the seat while creating multiple sealing surfaces. Experimental measurements of leakage rates with and without the presence of Foreign Object Debris (FOD) are compared with traditional re-closable pressure relief valves. Conformable cryogenic valve seats have the potential to be more repeatable, decrease leakage rates from differing coefficients of thermal expansion, hysteresis, and FOD when compared to traditional market valves.

Liquidus curve of a Lennard-Jones mixture.

The temperature below which the homogeneous liquid state of a mixture is not thermodynamically stable is called the liquidus temperature. This temperature varies with composition, and its composition dependence is represented by the liquidus curve. This curve provides fundamental reference points on the composition-temperature plane for characterizing the behavior of liquids, glasses, and crystals. In this paper, using molecular dynamics simulations, we determine the liquidus curve of the Wahnström mixture, which consists of large and small atoms interacting via the Lennard-Jones potential. Since this system is one of the standard models used to study the behavior of liquids and glasses, the liquidus curve presented in this work will contribute to a deeper understanding of disordered materials in general.

Physical Regimes and Mechanisms of Picosecond Laser Fragmentation of Gold Nanoparticles in Water from X-ray Probing and Atomistic Simulations.

Laser fragmentation in liquids has emerged as a promising green chemistry technique for changing the size, shape, structure, and phase composition of colloidal nanoparticles, thus tuning their properties to the needs of practical applications. The advancement of this technique requires a solid understanding of the mechanisms of laser-nanoparticle interactions that lead to the fragmentation. While theoretical studies have made impressive practical and mechanistic predictions, their experimental validation is required. Hence, using the picosecond laser fragmentation of Au nanoparticles in water as a model system, the transient melting and fragmentation processes are investigated with a combination of time-resolved X-ray probing and atomistic simulations. The direct comparison of the diffraction profiles predicted in the simulations and measured in experiments has revealed a sequence of several nonequilibrium processes triggered by the laser irradiation. At low laser fluences, in the regime of nanoparticle melting and resolidification, the results provide evidence of a transient superheating of crystalline nanoparticles above the melting temperature. At fluences about three times the melting threshold, the fragmentation starts with evaporation of Au atoms and their condensation into small satellite nanoparticles. As fluence increases above five times the melting threshold, a transition to a rapid (explosive) phase decomposition of superheated nanoparticles into small liquid droplets and vapor phase atoms is observed. The transition to the phase explosion fragmentation regime is signified by prominent changes in the small-angle X-ray scattering profiles measured in experiments and calculated in simulations. The good match between the experimental and computational diffraction profiles gives credence to the physical picture of the cascade of thermal fragmentation regimes revealed in the simulations and demonstrates the high promise of the joint tightly integrated computational and experimental efforts.

Effect of adaptive nanocrystalline behaviors on the cavitation erosion performance of Cu47.5Zr45.1Al7.4 bulk metallic glass

Amorphous alloys have attracted much attention in the field of cavitation erosion (CE) due to their good comprehensive properties. However, due to the special amorphous structures that are difficult to characterize, their micro-response behaviors and damage mechanisms during the CE process have not been well elucidated. In this paper, advanced techniques including focused ion beam (FIB) and transmission electron microscope (TEM) were used to comparatively study the microstructure evolution and volume loss rules of Cu47.5Zr45.1Al7.4 and Zr41.2Ti13.8Cu12.5Ni10Be22.5 bulk metallic glasses (BMG) under the action of cavitation load and cavitation heat to reveal their CE mechanisms. The results showed that the absence of small atoms in Cu47.5Zr45.1Al7.4 led to a large free volume, thereby compromising its hardness, modulus, and yield strength. However, this also made it more susceptible to shear banding, which in turn enhanced its energy absorption capabilities. Although Cu47.5Zr45.1Al7.4 had a higher glass transition temperature (Tg) and onset temperature of crystallization (Tx), as well as a wider supercooled liquid phase region (ΔTx), its exothermic peak was obviously pronounced and the transformation temperature range was significantly narrower. Therefore, it is more likely to adaptively form Cu-rich nanocrystals under the action of cavitation heat. Additionally, the larger free volume in the shear band was conducive to the diffusion of atoms and the occurrence of adaptive nanocrystallization behaviors in Cu47.5Zr45.1Al7.4. These nanocrystals were able to effectively force crack deflection, thereby dissipating impact energy and delaying the fatigue spallation of the material. Consequently, Cu47.5Zr45.1Al7.4 exhibited a far longer incubation period and a noticeably lower cumulative volume loss.

Benchmarking a neutral-atom quantum computer

In this study, we simulated the algorithmic performance of a small neutral atom quantum computer and compared its performance when operating with all-to-all versus nearest-neighbor connectivity. This comparison was made using a suite of algorithmic benchmarks developed by the Quantum Economic Development Consortium. Circuits were simulated with a noise model consistent with experimental data from [Nature 604, 457 (2022)]. We find that all-to-all connectivity improves simulated circuit fidelity by [Formula: see text]–[Formula: see text], compared to nearest-neighbor connectivity.

Fluorinated Polymer Donors for Nonfullerene Organic Solar Cells.

The rapid development of narrow-bandgap nonfullerene acceptors (NFAs) has boosted the efficiency of organic solar cells (OSCs) over 19 %. The new features of high-performance NFAs, such as visible-NIR light absorption, moderate the highest occupied molecular orbitals (HOMO), and high crystallinity, require polymer donors with matching physical properties. This emphasizes the importance of methods that can effectively tune the physical properties of polymers. Owning to very small atom size and strongest electronegativity, the fluorination has been proved the most efficient strategy to regulate the physical properties of polymer donors, including frontier energy level, absorption coefficient, dielectric constant, crystallinity and charge transport. Owing to the success of fluorination strategy, the vast majority of high-performance polymer donors possess one or more fluorine atoms. In this review, the fluorination synthetic methods, the synthetic route of well-known fluorinated building blocks, the fluorinated polymers which are categorized by the type of donor or acceptor units, and the relationships between the polymer structures, properties, and photovoltaic performances are comprehensively surveyed. We hope this review could provide the readers a deeper insight into fluorination strategy and lay a strong foundation for future innovation of fluorinated polymers.