Surface Atoms Research Articles

Dendrimer-induced synthesis of subnano materials and their characterization: establishing atom hybrid science

Abstract The precise molecular design of functional dendritic polymers enables the accumulation of multiple metals within a molecular cage. We have established a synthesizing methodology of metallodendrimers where the number of constituent atoms, the choice of elements, and the composition ratio were precisely controlled through an intramolecular Lewis acid-base interaction at each branch of phenylazomethine dendrons. Due to their inherent capsule effect, chemical reduction of metallodendrimers generates homogeneous subnanoparticles with a particle size of about 1 nm in diameter within the dendrimer cage. Fabricated subnanoparticles show amorphous crystal structures with distorted and fluctuated surface atoms and, with such a unique atomic structure, induce peculiar electronic states, surpassing unique and discrete physical and chemical properties of conventional nanoparticles and bulk metals. In this paper, we review the dendrimer-derived synthesis of atomic hybrid subnanoparticles and its research application established in our laboratory.

Multicoverage Study of Femtosecond Laser-Induced Desorption of CO from Pd(111).

We study the strong coverage dependence of the femtosecond laser-induced desorption of CO from Pd(111) using molecular dynamics simulations that consistently include the effect of the laser-induced hot electrons on both the adsorbates and surface atoms. Adiabatic forces are obtained from a multicoverage neural network potential energy surface that we construct using data from density functional theory calculations for 0.33 and 0.75 monolayer (ML). Our molecular dynamics simulations performed for these two trained coverages and an additional intermediate coverage of 0.60 ML reproduce well the peculiarities of the experimental findings. The performed simulations also permit us to disentangle the relative role played by the excited electrons and phonons on the desorption process and discover interesting properties of the reaction dynamics as the relevance that the precursor physisorption well acquires during the dynamics as coverage increases.

Novel green chemical mechanical polishing by controlling pH values and redox reaction for achieving atomic surface of a nickel alloy

Traditional chemical mechanical polishing (CMP) normally employs noxious slurries, leading to pollution to the environment. It is a challenge to achieve atomic surfaces of nickel (Ni) alloy using green CMP. To overcome this challenge, a novel green CMP was developed, including composite abrasives of ceria and silica, mandelic acid, hydrogen peroxide and deionized water. Using the developed green CMP, atomic surface with surface roughness Sa of 0.179 nm was achieved, under a scanning area of 50 × 50 μm2. Electrochemical tests indicate that the developed slurry has effective corrosion ability on Ni alloy. X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy reveal that the Ni alloy was oxidized by hydrogen peroxide, forming oxides of Ni, chromium (Cr) and molybdenum (Mo). Cr and Mo oxides demonstrated better stability in mandelic acid, while Ni oxides reacted with hydrogen ions and produced Ni2+ and Ni3+ ions. The ions chelated with mandelic acid. Finally, the generated oxides and reaction products were removed by abrasives and a polishing pad under chemical and mechanical balanced functions. The developed green CMP paves a new avenue to obtain atomic surface of a Ni alloy for the possible applications of high-performance components in aerospace industry.

Tunable valley splitting in RuClF bilayer

Valleytronic devices are increasingly gaining attention for their unique capability to encode and process information by leveraging electronic valley degrees of freedom. In this study, we performed first-principle calculations to analyze the band dispersions of double-layer antiferromagnetic valley material RuClF subjected to an external electric field. Our findings show that the double-layered structure exhibits interlayer antiferromagnetic order and that the bilayer bands display opposite valley splitting. We further categorized the atomic contact surfaces of the top and bottom layers into three interface conditions: F/F, Cl/Cl, and Cl/F. Under F/F and Cl/Cl interface conditions, when the electric field changes by 0.1 V/nm, the valley splitting values change by 12.2 meV and 9.5 meV, respectively. However, under the Cl/F interface condition, the structure exhibited metallic characteristics and was as such excluded from our analysis. These findings substantiate the practicality of employing an electric field to regulate the valley splitting of the bilayer structure, offering a new control strategy for valleytronic devices.

The fluctuated structural/electronic properties of SrTiO3 two-dimensional materials caused by surface effects

Perovskite oxide thin films have many astonishing properties, such as multiferroicity, superconductivity, strong correlation, etc, and are closely related to orientations with different symmetry and structural characteristics. Recently, perovskite oxide films with only one unit cell thickness have been synthesized successfully (Ji et al 2019 Nature 570 87–90). Here we investigated the structure and electronic properties of SrTiO3 (STO) two-dimensional (2D) materials with (001), (110), and (111) surfaces. We found that due to surface effects caused atomic distortion fluctuations, the lattice constant and thickness of STO 2D materials with the (110) surface fluctuate sharply with the increase of atomic layers. The band gap of STO 2D materials exhibits oscillation as the number of atomic layers increases, due to the different atomic distortion and surface reconstruction with odd and even atomic layers. The STO 2D materials along (001) surfaces with different atomic layers are always semiconductors. As the atomic layers increasing, the electronic structure of STO 2D materials with (110) or (111) surfaces continuously transitioning between semiconductor and metallic phases, and finally totally become metallic phases, which is closely related to the surface reconstruction effect. The differences between STO 2D materials along the (001) and (110) or (111) surfaces are significant and can be explained by mixed Sr-d, Ti-d, and O-p orbitals. Our studies may provide new insights into the surface effects of perovskite oxide 2D materials.

Molecular dynamics simulations of adsorption behavior of mixtures of surfactant and polymer at C3A/water and Ettringite/water interfaces

In cementitious system, surfactant is used as air-entraining agents, which is often incompatible with other chemical admixtures, such as superplasticizers. The adsorption of surfactants at solid–liquid interface is critical for the bubble stability. The measurement of adsorption properties of surfactant in fresh cement or concrete is very difficult. In this study, classical molecular dynamics simulations is used to analysis the influence of two polymers (call as HPMC and PCA, respectively) on the adsorption behaviors of the two anionic surfactants (call as HSO4 and HPO4, respectively) at the C3A/water and ettringite/water interfaces. At solid/water interface, both of HSO4 and HPO4 molecules form a monolayer with hydrophilic head groups adsorbed on the solid surfaces. After adding polymers, the adsorption conformations are changed to spherical micelle or multilayer. The electrostatic interaction between oxygen atoms of hydrophilic head groups and calcium atoms of solid surface plays a decisive role. Therefore, the value of adsorption energy of HPO4 with more negatively charged hydrophilic head groups is greater than that of HSO4. The adsorption energies of the two surfactants are weakened by the two polymers, but the adsorption energy value of HPO4 is always greater. The influence mechanism of HPMC and PCA is different. The negatively charged PCA has a strongly competitive adsorption with the surfactants. The neutral HPMC has a weak adsorption capacity but it carries many hydroxyl groups to interact with the hydrophilic head groups of surfactants.

Tuning the adsorption of H2O, H2 and O2 molecules on diamond surfaces by B-doping

In this study, the fundamental interactions between the low Miller index (001), (110) and (111) B-doped diamond surfaces and H2O, H2 and O2 molecules, which are commonly present during growth, in ambient air and involved in electrochemical reactions, are investigated by means of ab initio simulations. The results are presented in close comparison with previous studies on undoped diamond surfaces to reveal the impact of B doping. It is demonstrated that the B dopant is preferably incorporated on the topmost carbon layer and enhances the adsorption of H2O by forming a dative bond with O. On the contrary in some cases, it seems to weaken the adsorption of O2, compared to the undoped diamond. Moreover, a noticeable displacement of the surface atoms attached to the fragment of the dissociated H2O and O2 molecules was observed, which can be associated to early stages of wear at the atomistic level. B doping was also found to reduce the energy barriers for water dissociation in most cases, with this effect being most pronounced in the C(111) surfaces. These qualitative and quantitative results aim to provide useful insights for the development of improved protective coatings and electrochemical devices.

Microwave-Assisted Hydrothermal Synthesis of {100} and {111} Faceted LiFeO2 Truncated Octahedra: Investigations on Volatile Organic Compound Sensing Performance.

Growth of exposed crystal facets has received considerable attention because of their coordinatively unsaturated surface atoms and defect-related surface reactivities. Herein, LiFeO2 truncated octahedra exposed with 6 {100} facets and 8 {111} facets were prepared through a simple microwave-assisted hydrothermal method without using any additives, surfactants, and calcination processes. The detailed growth process revealed that the formation of LiFeO2 truncated octahedra occurred only at the optimized reaction temperature (180 °C), time (30 min), and reactant concentrations. The prepared LiFeO2 truncated octahedra showed excellent sensing responses toward aliphatic organic compounds compared to that against aromatic organic compounds and poor response to inorganic compounds. The response percentages of 150 ppm concentrations of acetone, ethanol, formaldehyde, and isopropyl alcohol are 81.84, 62.91, 62.68, and 69.41%, respectively, at a low operating temperature (100 °C). The presence of exposed facets with their coordinatively unsaturated Li/Fe surface atoms such as 5-fold {100}, 3-fold {111}, 3-fold {100}-{111}, 2-fold {111}-{111}, and 2-fold coordination with the O atom in the vertices facilitated more oxygen vacancies and led to improved surface reactivities as well as sensitivity.

Size and shapes effect on structural and phonon density of state in Ir nanoparticle and mechanical properties of Ir metal

Density Functional Theory (DFT) calculations were performed to assess the mechanical properties of an Ir metal nanocluster. The properties under investigation encompassed the bulk modulus, Poisson ratio, elastic shear modulus, and binding energy. The stability of the nanocluster was explored across various shapes, including icosahedron, octahedron, and truncated octahedron, with consideration given to different cluster sizes. The DFT results unveiled a general mechanism for amorphization in the Ir nanocluster of size 55, involving rosette-like structural transformations at the fivefold vertex. Additionally, Phonon Density of States (PDOS) was computed for the core, surface, and total atoms of Ir nanoclusters with sizes 38, 55, and 75, as well as for the Ir bulk structure. These calculations were carried out using both Quantum Sutton-Chen potential (QSC) and Gupta potential models through molecular dynamics (MD) methods. The analysis of the phonon density of states revealed that the contributions of two main peaks in low and high frequencies were associated with the surface and core atoms, respectively. Furthermore, the extension of the phonon density of states was performed for larger-sized Ir nanoclusters. Significantly, the QSC potential was utilized to investigate the adsorption of main peaks and their trends for the phonon density of states in the Ir bulk structure. The results obtained from this method were found to be in agreement with experimental data derived from cold neutron and multidetector time-of-flight spectrometer phonon measurements.

Polarization characteristics and structural modifications of Cu nanoparticles under high electric fields

High electric fields affect the diffusion dynamics of atoms on a metal surface, causing biased surface diffusion that possibly leads to the growth of intensively field emitting protrusions and consequent vacuum breakdown (VBD). The scientific understanding of this process, as well as other fundamental VBD initiation mechanisms, is far from complete. Here we investigate the exact atomic behaviour of metal surfaces exposed to extremely high electric fields using density functional theory (DFT). Previous theories describe the field-surface dynamics in terms of the effective dipole moments and polarizability of surface atoms, disregarding higher-order (hyperpolarizability) terms. The validity of this approximation has been evaluated only for electric fields up to 3 GV/m, due to computational limitations of the plane-wave DFT basis used in previous works. In this work, we test the validity of this approximation for a much wider field range, relevant for VBD and field emission (FE), using Cu nanoparticles as our test structures. We find that although such high fields can change the entire structure of Cu nanoparticles, their energetics are described very precisely by the permanent dipole moment and polarizability terms. Thus, we show that neglecting the hyperpolarizability terms is valid even for field values that exceeds the range that is relevant for intense FE and VBD. This work lays a solid foundation for further developing atomic-level simulation models for electric field-induced surface diffusion on metal surfaces and its effects on protrusion growth and VBD initiation.

Characterizing steel corrosional features in atomic force microscopy: A Sobel edge detection approach

One of the fundamental characterizations in assessing a material’s surface its surface roughness. Here, we introduce a computational method using measured atomic force microscopy surface roughness data to perform Sobel edge detection of heterogeneity on corroded steel samples. This method enables the spatial mapping of corrosion-induced features and general inconsistencies on a sample’s surface using a freely available algorithm written in the Python computing language. The non-optical method has been tested on three different grades of steel, and results indicate reasonable accuracy when compared to visual observations made from grayscale optical data.

Proposal for All-Electrical Skyrmion Detection in van der Waals Tunnel Junctions.

A major challenge for magnetic skyrmions in atomically thin van der Waals (vdW) materials is reliable skyrmion detection. Here, based on rigorous first-principles calculations, we show that all-electrical skyrmion detection is feasible in two-dimensional vdW magnets via scanning tunneling microscopy (STM) and in planar tunnel junctions. We use the nonequilibrium Green's function method for quantum transport in planar junctions, including self-energy due to electrodes and working conditions, going beyond the standard Tersoff-Hamann approximation. We obtain a very large tunneling anisotropic magnetoresistance (TAMR) around the Fermi energy for a graphite/Fe3GeTe2/germanene/graphite vdW tunnel junction. For atomic-scale skyrmions, the noncollinear magnetoresistance (NCMR) reaches giant values. We trace the origin of the NCMR to spin mixing between spin-up and -down states of pz and dz2 character at the surface atoms. Both TAMR and NCMR are drastically enhanced in tunnel junctions with respect to STM geometry due to orbital symmetry matching at the interface.

Exponential relaxation of the energy and desorption dynamics of atoms colliding with a surface

Atom–surface collisions are one of the most important topics in surface science. To further disclose the physical mechanism underlying atom–surface interaction at the microscopic level, we study the dynamics of an incident atom with a molecular dynamics simulation. Emphasis is put on the temporal evolutions of energy and residence times of the colliding atoms. The incident atoms experience two stages after colliding with the surface. First, the atoms relax to the equilibrium state in an exponential fashion. Then, the atoms become equilibrated with the surface and depart from the surface with a converged desorption rate. Two parameters are proposed to characterize the process: the characteristic energy relaxation time and the equilibrium residence time. At the relaxation stage, the desorption rate varies with the energy, and the probability distribution function (PDF) of the residence time obeys a power law. At the equilibrium state, the desorption rate is invariable, and the PDF of the residence time decays exponentially. We further find that the desorption rate for both stages can be calculated by a consistent Arrhenius equation, with the desorption activation energy and kinetic energy evolving with time in the relaxation stage. It appears that the gas–surface interaction dynamics can be explained by trapping-desorption theory in both the relaxation state and the equilibration state.

Spectroscopic characterization, quantum chemical and molecular docking investigations on methyl indole-3-carboxylate: A potent cervical cancer drug

Bioactive indole derivatives had great importance in recent years due to their numerous biological and pharmaceutical applications. In the present work, the vibrational, electronic, physicochemical, and inhibitory properties of the Methyl Indole-3-Carboxylate (MIC) molecule were predicted and analyzed. Initially, the molecular structure of the title molecule was optimized and its structural parameters were calculated, which were highly correlated with the previous reports. The simulated infrared and Raman spectra of the molecule were in good agreement with the FT-IR and FT-Raman spectra that were observed via the experiment. The observed UV–Visible spectrum validates the MIC molecule's π→π* electronic transition, which was also well matched with the simulated UV–Visible spectrum. The investigation of frontier molecular orbitals highlights intramolecular charge transfer across the molecule, establishing the bioactivity of the MIC molecule. The electron transport from the methyl group hydrogen atom H22 to the oxygen atom O11 is confirmed by Mulliken atomic charge distribution analysis and molecular electrostatic potential surface analysis. The calculated Fukui functions values validated the FMOs and Mulliken atomic charge distribution results. The natural bond orbital study further confirms the MIC molecule's intramolecular charge transfer and bioactivity. Molecular docking investigation results that the MIC molecule hinders the function of an enzyme known as Mitogen-activated protein kinase 14, which has been linked to cervical cancer. As a result, the present study provides the possibility towards the development of innovative to treat cervical cancer.

Quasi-two-dimensional strong liquid-like dynamics of surface atoms in metallic glasses

Quasi-two-dimensional strong liquid-like dynamics of surface atoms in metallic glasses

Angstrom surface with high material removal rate for quartz glass induced by silk dissolved novel green chemical mechanical polishing

Quality and efficiency are a pair of everlasting contradiction for manufacturing, and therefore it is a challenge to achieve an atomic surface with high material removal rate (MRR). Toxic and corrosive slurries are widely employed in traditional chemical mechanical polishing (CMP), facing challenges to treat the detrimental slurries during and after CMP. To solve these challenges, natural extract of sodium alginate (SA) and ceria are used to develop green CMP. Dissolved silk is firstly applied to shear thickening and enhance MRR in CMP. After CMP, surface roughness Sa is 0.08nm on the surface of quartz glass at a scanning area of 20×20 μm2, and MRR is 33.215 μm/h, and thickness of the damaged layer is 5.09nm. To the best of our knowledge, the surface roughness is the lowest and MRR is the highest for quartz glass, compared with those reported previously. Density functional theory and molecular dynamics simulations reveal that dissolved silk is adsorbed onto the surface of quartz glass, forming a complex with SA and facilitating the removal of surface atoms. Additionally, the developed novel green slurry of CMP demonstrates bio-compatibility and degradability. These outcomes suggest new viewpoints that using a dissolved silk slurry of ceria breaks the eternal contradiction between efficacy and surface roughness to achieve atomic surface with high MRR via green CMP.

Three-component volume of fluid method coupling with interface compression method and Eulerian–Lagrangian spray atomization surface density model for prediction of cavitating sprays

In this work, a new three phase cavitation model with Eulerian–Lagrangian Spray Atomization (ELSA) and Interface Capturing Method (ICM) coupling is presented to allow for, in a unified approach, physical insight of the surface evolution of cavitating sprays at the Sub-Grid Scale (SGS). Phase change is accounted for in the framework via mass transfer across a liquid and corresponding vapor phase. The surface density model was validated against direct numerical simulation data of an atomizing jet with a systematic variation of mesh resolutions. The sensitivity of the adjustable parameters such as the critical Weber number was also demonstrated. Then, experimental comparisons were made with a more realistic orthogonal spray geometry within a cavitating and turbulent non-cavitating system. These comparisons include both large scale visualizations and small scale SGS quantities such as the Sauter mean diameter. To our knowledge, this is the first study that presents the performance of a three phase cavitating framework with SGS ELSA-ICM coupling.

Self-Assembled Monolayer and Nanoparticles Coenhanced Fragmented Silver Nanowire Network Memristor.

Silver nanowire (AgNW) networks with self-assembled structures and synaptic connectivity have been recently reported for constructing neuromorphic memristors. However, resistive switching at the cross-point junctions of the network is unstable due to locally enhanced Joule heating and the Gibbs-Thomson effect, which poses an obstacle to the integration of threshold switching and memory function in the same AgNW memristor. Here, fragmented AgNW networks combined with Ag nanoparticles (AgNPs) and mercapto self-assembled monolayers (SAMs) are devised to construct memristors with stable threshold switching and memory behavior. In the above design, the planar gaps between NW segments are for resistive switching, the AgNPs act as metal islands in the gaps to reduce threshold voltage (Vth) and holding voltage (Vhold), and the SAMs suppress surface atom diffusion to avoid Oswald ripening of the AgNPs, which improves switching stability. The fragmented NW-NP/SAM memristors not only circumvent the side effects of conventional NW-stacked junctions to provide durable threshold switching at >Vth but also exhibit synaptic characteristics such as long-term potentiation at ultralow voltage (≪Vth). The combination of NW segments, nanoparticles, and SAMs blazes a new trail for integrating artificial neurons and synapses in AgNW network memristors.

Review of: "The important feature of all nano structures is that in which the number of surface atoms is more than the number of volume atoms."

Review of: "The important feature of all nano structures is that in which the number of surface atoms is more than the number of volume atoms."

3D Noble-Metal Nanostructures Approaching Atomic Efficiency and Atomic Density Limits.

Noble metals have been widely used in catalysis, however, the scarcity and high cost of noble metal motivate researchers to balance the atomic efficiency and atomic density, which is formidably challenging. This article proposes a robust strategy for fabricating 3D amorphous noble metal-based oxides with simultaneous enhancement on atomic efficiency and density with the assistance of atomic channels, where the atomic utilization increases from 18.2% to 59.4%. The unique properties of amorphous bimetallic oxides and formation of atomic channels have been evidenced by detailed experimental characterizations and theoretical simulations. Moreover, the universality of the current strategy is validated by other binary oxides. When Cu2IrOx with atomic channels (Cu2IrOx-AE) is used as catalyst for oxygen evolution reaction (OER), the mass activity and turnover frequency value of Cu2IrOx-AE are 1-2 orders of magnitude higher than CuO/IrO2 and Cu2IrOx without atomic channels, largely outperforming the reported OER catalysts. Theoretical calculations reveal that the formation of atomic channels leads to various Ir sites, on which the proton of adsorbed *OH can transfer to adjacent O atoms of [IrO6]. This work may attract immediate interest of researchers in material science, chemistry, catalysis, and beyond.