Computation-accelerated discovery of the K2NiF4-type oxyhydrides combing density functional theory and machine learning approach.

The emerging K2NiF4-type oxyhydrides with unique hydride ions (H-) and O2- coexisting in the anion sublattice offer superior functionalities for numerous applications. However, the exploration and innovations of the oxyhydrides are challenged by their rarity as a limited number of compounds reported in experiments, owing to the stringent laboratory conditions. Herein, we employed a suite of computations involving ab initio methods, informatics and machine learning to investigate the stability relationship of the K2NiF4-type oxyhydrides. The comprehensive stability map of the oxyhydrides chemical space was constructed to identify 76 new compounds with good thermodynamic stabilities using the high-throughput computations. Based on the established database, we reveal geometric constraints and electronegativities of cationic elements as significant factors governing the oxyhydrides stabilities via informatics tools. Besides fixed stoichiometry compounds, mixed-cation oxyhydrides can provide promising properties due to the enhancement of compositional tunability. However, the exploration of the mixed compounds is hindered by their huge quantity and the rarity of stable oxyhydrides. Therefore, we propose a two-step machine learning workflow consisting of a simple transfer learning to discover 114 formable oxyhydrides from thousands of unknown mixed compositions. The predicted high H- conductivities of the representative oxyhydrides indicate their suitability as energy conversion materials. Our study provides an insight into the oxyhydrides chemistry which is applicable to other mixed-anion systems, and demonstrates an efficient computational paradigm for other materials design applications, which are challenged by the unavailable and highly unbalanced materials database.

Seeded growth of large single-crystal copper foils with high-index facets.

The production of large single-crystal metal foils with various facet indices has long been a pursuit in materials science owing to their potential applications in crystal epitaxy, catalysis, electronics and thermal engineering1-5. For a given metal, there are only three sets of low-index facets ({100}, {110} and {111}). In comparison, high-index facets are in principle infinite and could afford richer surface structures and properties. However, the controlled preparation of single-crystal foils with high-index facets is challenging, because they are neither thermodynamically6,7 nor kinetically3 favourable compared to low-index facets6-18. Here we report a seeded growth technique for building a library of single-crystal copper foils with sizes of about 30 × 20 square centimetres and more than 30 kinds of facet. A mild pre-oxidation of polycrystalline copper foils, followed by annealing in a reducing atmosphere, leads to the growth of high-index copper facets that cover almost the entire foil and have the potential of growing to lengths of several metres. The creation of oxide surface layers on our foils means that surface energy minimization is not a key determinant of facet selection for growth, as is usually the case. Instead, facet selection is dictated randomly by the facet of the largest grain (irrespective of its surface energy), which consumes smaller grains and eliminates grain boundaries. Our high-index foils can be used as seeds for the growth of other Cu foils along either the in-plane or the out-of-plane direction. We show that this technique is also applicable to the growth of high-index single-crystal nickel foils, and we explore the possibility of using our high-index copper foils as substrates for the epitaxial growth of two-dimensional materials. Other applications are expected in selective catalysis, low-impedance electrical conduction and heat dissipation.

Oxidation of graphene with variable defects: alternately symmetrical escape and self-restructuring of carbon rings.

Variable defects such as vacancies and grain boundaries are unavoidable in the synthesis of graphene, but play a central role in the activation of oxidation. Here, we apply reactive molecular dynamics simulations to reveal the underpinning mechanisms of oxidation in graphene with or without defects at the atomic scale. There exist four oxidation modes generating CO2 or CO in different stages, beginning from a single-atom vacancy, and proceeding until the ordered structure broken down into carbon oxide chains. The oxidation process of the graphene sheets experiences four typical stages, in which alternately symmetrical escape phenomenon is observed. Importantly, disordered rings can self-restructure during the oxidation of grain boundaries. Of all defects, the oxidation of vacancy has the lowest energy barrier and is therefore the easiest point of nucleation. This study demonstrates the crucial role of defects in determining the oxidation kinetics, and provides theoretical guidance for the oxidation prevention of graphene and the production of functionalized graphene.

Crystallization behavior of a confined CuZr metallic liquid film with a sandwich-like structure.

Despite the fact that its crystal state is thermodynamically stable, Cu64Zr36 alloy is prone to form metastable glass at a high cooling rate. However, the confinement can induce nano-crystallization with a novel sandwich-like hierarchical structure consisting of pure Cu layers, pure Zr layers and mixed layers by conducting molecular dynamics simulations. The liquid-to-crystal transition temperature and interatomic repulsion softness display abnormal oscillations, instead of monotonous variation, as the wall-wall separation increases. When the confinement size is 10 Å and 12 Å, the transition temperature reaches a maximum, resulting from the pending new sandwich layer. The atomic movement and dynamical heterogeneity are demonstrated to play a vital role in the abnormal oscillation behavior of physical properties of the nano confined metallic glass. The sandwich-like structure can alter the Cu-Zr bond fraction, which eventually influences the liquid-to-crystal transition temperature and interatomic repulsion softness. Our findings provide a deep insight into the hierarchical nanostructures and its liquid-to-crystal transition characteristics under confinement at the atomic level.

Atomistic Origin of the Complex Morphological Evolution of Aluminum Nanoparticles during Oxidation: A Chain-like Oxide Nucleation and Growth Mechanism.

Metal nanoparticles usually show different oxidation dynamics from bulk metals, which results in various oxide nanostructures because of their size-related surface effects. In this work, we have found and investigated the chain-like nucleation and growth of oxides on the aluminum nanoparticle (ANP) surface, using molecular dynamics simulations with the reactive force-field (ReaxFF). After nucleation, the chain-like oxide nuclei could stay on the ANP surface and continue growing into an oxide shell, extend outward from the surface to form longer oxide chains, or detach from the ANP to generate independent oxide clusters, which is highly dependent on the oxygen content, temperature, and nanoparticle size. Our results emphasize the complicated interplay between the surface structure of nanoparticles and the environmental conditions in determining the formation of oxides, which provides insights into the atomic-scale oxidation mechanism of metal nanoparticles.

Extreme electron transport suppression in siloxane ring-based molecular devices.

Single-molecule device studies have traditionally focused on highly conducting examples. However, molecular insulators are as important as molecular conductors for progress in nanoscale electronics. This work innovatively reveals the promising prospects and the superiority of siloxane molecular rings to achieve the same function as SiO2 on the single molecule scale, functioning as a molecular insulator in electronics, and theoretically analyzes the size-dependent electron transport decay of siloxane ring-based single-molecule devices in contrast to alkane ring-based ones. Results indicate that siloxane ring-based devices possess stronger transport suppression and steeper transport recession than alkane ring-based ones, which is attributed to weaker electronic coupling through the backbone of siloxane rings and is intrinsic to the nature of the Si-O bond. Furthermore, the electron transport decay in each kind of molecular ring-based device originates from the increasing degree of overall structural fluctuations in the molecular rings.

Wetting Transitions of Liquid Gallium Film on Nanopillar-Decorated Graphene Surfaces.

Molecular dynamics (MD) simulation has been employed to study the wetting transitions of liquid gallium droplet on the graphene surfaces, which are decorated with three types of carbon nanopillars, and to explore the effect of the surface roughness and morphology on the wettability of liquid Ga. The simulation results showed that, at the beginning, the Ga film looks like an upside-down dish on the rough surface, different from that on the smooth graphene surface, and its size is crucial to the final state of liquid. Ga droplets exhibit a Cassie⁻Baxter (CB) state, a Wenzel state, a Mixed Wetting state, and a dewetting state on the patterned surfaces by changing distribution and the morphology of nanopillars. Top morphology of nanopillars has a direct impact on the wetting transition of liquid Ga. There are three transition states for the two types of carbon nanotube (CNT) substrates and two for the carbon nanocone (CNC) one. Furthermore, we have found that the substrates show high or low adhesion to the Ga droplet with the variation of their roughness and top morphology. With the roughness decreasing, the adhesion energy of the substrate decreases. With the same roughness, the CNC/graphene surface has the lowest adhesion energy, followed by CNT/graphene and capped CNT/graphene surfaces. Our findings provide not only valid support to previous works but also reveal new theories on the wetting model of the metal droplet on the rough substrates.

Effect of nano-pillared surfaces with an arrangement density gradient on droplet coalescence dynamics.

The ability to predict and control the coalescence of droplets is of great importance for both industrial and technological applications, including 3D printing, micro-cladding, and self-assembly. Here, a textured surface decorated with nano-pillared arrays was designed and its arrangement density (f) was found to significantly affect the coalescence dynamics of droplets through changing their wettability. A large arrangement density f of the nano-pillared arrays would induce a Cassie wetting state for droplets, which supports the coalescence process. But when decreasing f to a value that produces a Wenzel wetting state, the coalescence is heavily impeded by the nano-pillars. However, a very small arrangement density f is also favorable for coalescence because the pinning effect resulting from the nano-pillars becomes ignored. More importantly, special substrates were well designed by nano-pillars with a density gradient in order to control the coalescence dynamics for some potential applications. This work helps to shed light on the coalescence dynamics of droplets on a microtextured surface modified with different arranged nano-pillars and thereby provides guidance on how to control their behaviors.

Bouncing dynamics of liquid drops impact on ridge structure: an effective approach to reduce the contact time.

Surfaces designed so that liquid metals do not stick to them but instead rebound as soon as possible have received considerable attention due to their significant importance in many practical technologies. We herein design a ridge structure that can induce the drop to rapidly rebound through the combination effect of centre-drawing recoil and the resulting faster retraction velocity. The suitable sharp-angle of the ridge for minimizing the contact time is determined as 20-30°. Further analysis reveals that multi-ridge structure or two-ridge structure with gaps can reduce more contact time. We also highlight the role the impact velocity played in minimizing the contact time, which has been a neglected parameter previously. Our studies would open up a new way to reduce the contact time and control the bouncing dynamics of metal drops, which provides guidance for some potential applications, such as preventing splashing molten drops from depositing on clean surface.

Density dependent structural phase transition for confined copper: origin of the layering.

Confinement presents the opportunity for novel structural transition scenarios not observed in three-dimensional systems. Here, we report a comprehensive molecular dynamic (MD) study of the structural phase transition induced by density for an ordinary metal copper (Cu) confined between two parallel panel walls. At 4.19 g cm-3 < ρ < 4.66 g cm-3, a notable structural phase transition occurs between the triangle unit cell structure and quasi-square unit cell structure upon densification. Both the bond order parameter (BOP) and angular distribution function (ADF) can provide evidence for the transition. We highlight the fact that when the sole decrease of the atom distance cannot adapt to the further densification, the system starts to adjust the neighboring bond angle and promote the layering transition, thus inducing the structural phase transition. At the metastable coexistence zone, the viscosity exhibits a remarkable drop and the diffusion coefficient shows a notable increase, both facilitating the accomplishment of the structural transition. Our results will trigger more interest on the phase transition under confinement in a metallic system.

First principles study of electronic transport properties in novel FeB2 flake-based nanodevices.

First-principles calculations can provide theoretical support for the promising applications of innovative two-probe devices based on FeB2 flakes at different temperatures. Results indicate that these FeB2-based devices not only exhibit a prominent transport capacity and a predictable strong current, but also possess outstanding electrical conductivity compared with many flake-based devices. Devices with FeB2 flakes at temperatures not above 1000 K have advantageous transmission and low-voltage current because of the delocalization of electronic states, essentially resulting from their undeformed flake structures. Importantly, Fe atoms are pivotal in the electron transport of FeB2-based devices. The edge effect of the flakes is also analyzed. These new-type FeB2 flakes can realize substantial value in nanoscale functional devices.

Abnormal separation of the silicon-oxygen bond in the liquid layering transition of silicon dioxide in a nanoslit.

We investigated the unusual layering transition (LT) in quasi-2D liquid silicon dioxide (SiO2) confined in a nanoslit. Our results indicate that the slit size and pressure induce the abnormal LT in liquid SiO2, accompanied by a rapid change in the density, diffusion coefficient, pair correlation function and average potential energy. The silicon and oxygen atoms are almost completely separated under the extremely strong confinement effect, which is the characteristic feature of the LT. The negative slope of the LT lines in the phase diagram at different pressures suggests that a confinement-induced LT occurs at high pressure and a pressure-induced LT occurs at low pressure.

Coalescence of Immiscible Liquid Metal Drop on Graphene.

Molecular dynamics simulations were performed to investigate the wetting and coalescence of liquid Al and Pb drops on four carbon-based substrates. We highlight the importance of the microstructure and surface topography of substrates in the coalescence process. Our results show that the effect of substrate on coalescence is achieved by changing the wettability of the Pb metal. Additionally, we determine the critical distance between nonadjacent Al and Pb films required for coalescence. These findings improve our understanding of the coalescence of immiscible liquid metals at the atomistic level.

Heterogeneous nucleation of Al melt in symmetrical or asymmetrical confined nanoslits.

MD simulations are performed to study the solidification of Al melt in confined nanoslits (NSs) constructed by identical or different substrates, as well as on Fe substrates. Compared to the single substrate, the confined NS could promote the crystallization of Al melt, and its size has a significant impact on the solidified structure. In symmetrical NSs, liquid Al atoms would stack based on the atomic arrangement mode of the substrate, however in asymmetrical confined NSs, the atomic arrangement mode of liquid Al is governed by the constitution of asymmetrical substrates. Specifically, for the NS formed by Fe(110) and Fe(111) substrates, the induced region from the Fe(110) substrate is much bigger than that from Fe(111). Moreover, the freezing of liquid Al in asymmetrical NSs constructed from copper and iron has also been studied. These results throw light on heterogeneous nucleation in confined space.

Molecular dynamics study on the nucleation of Al-Si melts on sheet substrates at the nanoscale.

Molecular dynamics (MD) simulations are performed to study the freezing process of Al-Si melts on heterogeneous Si substrates in detail. We highlight the inherent nanostructure of both the Si primary phase and the Al-Si binary phase. It is found for the first time that the primary Si phase displays a "pyramidal configuration" when the Al-Si melt congeals. Experimental measurements could also verify our simulation results. It can be found that the binary Al-Si phase turns into a "Si-Al-Si sandwich construction" during solidification, regardless of freezing on a single substrate or in the restricted space between substrates. This peculiar phenomenon results from the combined effects of the van der Waals potential well and the interatomic interaction between Al and Si. Furthermore, it is also able to control the thickness of the Si atomic shell of the "sandwich construction", resulting in the silicene-like unilaminar Si nanostructure. Our findings provide novel strategies to fabricate desired shaped nanostructures by means of nanocasting in Al-Si melts at the nanoscale.