Dimer Atoms Research Articles

Quantum Drude oscillators coupled with Coulomb potential as an efficient model for bonded and non-covalent interactions in atomic dimers.

The quantum Drude oscillator (QDO) model has been widely used as an efficient surrogate to describe the electric response properties of matter as well as long-range interactions in molecules and materials. Most commonly, QDOs are coupled within the dipole approximation so that the Hamiltonian can be exactly diagonalized, which forms the basis for the many-body dispersion method [Phys. Rev. Lett. 108, 236402 (2012)]. The dipole coupling is efficient and allows us to study non-covalent many-body effects in systems with thousands of atoms. However, there are two limitations: (i) the need to regularize the interaction at short distances with empirical damping functions and (ii) the lack of multipolar effects in the coupling potential. In this work, we convincingly address both limitations of the dipole-coupled QDO model by presenting a numerically exact solution of the Coulomb-coupled QDO model by means of quantum Monte Carlo methods. We calculate the potential-energy surfaces of homogeneous QDO dimers, analyzing their properties as a function of the three tunable parameters: frequency, reduced mass, and charge. We study the coupled-QDO model behavior at short distances and show how to parameterize this model to enable an effective description of chemical bonds, such as the covalent bond in the H2 molecule.

Prediction of Van Hove singularity systems in ternary borides

A computational search for stable structures among both α and β phases of ternary ATB4 borides (A = Mg, Ca, Sr, Ba, Al, Ga, and Zn, T is 3d or 4d transition elements) has been performed. We found that α-ATB4 compounds with A = Mg, Ca, Al, and T = V, Cr, Mn, Fe, Ni, and Co form a family of structurally stable or almost stable materials. These systems are metallic in non-magnetic states and characterized by the formation of the localized molecular-like state of 3d transition metal atom dimers, which leads to the appearance of numerous Van Hove singularities in the electronic spectrum. The closeness of such singularities to the Fermi level can be easily tuned by electron doping. For the atoms in the middle of the 3d row (Cr, Mn, and Fe), these singularities led to magnetic instabilities and magnetic ground states with a weakly metallic or semiconducting nature. Such states appear as non-trivial coexistence of the different spin ladders formed by magnetic dimers of 3d elements. These magnetic states can be characterized as an analog of the spin glass state. Experimental attempts to produce MgFeB4 and associated challenges are discussed, and promising directions for further synthetic studies are formulated.

The role of dimers in the efficient growth of nanoparticles

The early stage of nanoparticle formation in gas aggregation systems is studied with the ambition to reveal the role of atomic dimers acting as cluster nuclei. The initial formation of a dimer nucleus, serving as a seed for subsequent atom attachment growth, is believed to be the bottleneck of the nanoparticle gas-aggregated synthesis. We thus examined the nature of the underlying process by employing energy-resolved mass-spectrometry and imaging of deposited nanoparticles. The production of dimers with origin in the discharge gas phase (ArCu+, Ar2+) and dimers preferentially sputtered directly from the target (Cu2+) was studied. The study demonstrates that sputtered Cu2+ dimers, carrying the high energy tail of Thompson energy distribution, play a major role as agents responsible for the formation of initial cluster nuclei, followed by a significant contribution of ArCu+; the population of these dimers is proportional to nanoparticle transported mass to the substrate. The proposed mechanism and the role of the dimer might be material independent since the qualitative agreement was also obtained for Ag NPs.

Reversed Spillover Effect Activated by Pt Atom Dimers Boosts Alkaline Hydrogen Evolution Reaction

AbstractThe hydrogen spillover effect has garnered considerable attention as a promising avenue to enhance the activity of the hydrogen evolution reaction (HER) in metal‐support compound materials. Herein, Pt atom dimers on the NiOOH support are successfully synthesized with a reversed hydrogen spillover effect, demonstrating much better alkaline HER activity than Pt single atoms and Pt clusters. Atomic and electronic structure characterizations unequivocally verify the anchoring of Pt atom dimers on NiOOH through Pt─O bonds, thus obtaining a reversed and enhanced hydrogen spillover effect. Theoretical and experimental results indicate that NiOOH exhibits pronounced water dissociation capability, while the Pt atom dimers, in comparison to Pt single atoms and Pt clusters, demonstrate superior hydrogen desorption ability. The reversed spillover with Pt atom dimers leads to remarkable HER activity in alkaline solutions, as evidenced by an ultra‐small overpotential of 13 mV at 10 mA cm−2. These findings not only provide insights into the potential use of atom dimers for HER, but also shed light on reversed spillover in designing high‐performance catalysts.

Mechanism investigations on co-pyrolysis of polyethylene and biomass using ReaxFF simulation and DFT computation

Co-pyrolysis of plastics and biomass is a promising solution for waste management and renewable energy utilization. There is still no consensus on the distributions of co-pyrolysis products and the reaction mechanisms between the polyethylene (PE) plastics and biomass components at the molecular level. In this work, the product distribution, co-pyrolysis mechanism, and electronic characteristic of PE and major biomass components such as cellulose (CE), hemicellulose (HC), and lignin (LG) were investigated at the atomistic level using reactive force field simulation and density functional theory computation. The co-pyrolysis of PE and biomass facilitates the release of non-carbon radicals, which enhances the reactivity of macromolecular fractions to high-quality oils. HC promotes the char degradation and oxygenated oil retention, whereas CE accelerates the decomposition of oxygen-free char to light hydrocarbons and inorganic gases. LG inhibits the conversion of oxygenated oil to CO/CO2 and retards the secondary cracking of oxygen-free oil. Alkyne is mostly generated from α-O-4 LG dimer, whereas olefin is mainly derived from γ-O-4 LG dimer. β-O-4 LG dimer accelerates the conversion of char to oil, while it decreases gas emissions. The electronic characteristics such as the Fukui function, frontier orbitals, and total electron density demonstrate the reaction sites of biomass monomers. For CE and HC, the hydroxyl groups on methylene are the nucleophilic sites, and the O atoms of hydroxyl on the pyran ring are prone to the electrophilic attack by PE. Except for O atoms in LG dimers, the unsubstituted C atoms on the benzene ring are inclined to the nucleophilic sites, whereas the C and para-C atoms located at hydroxyl and methoxy are susceptible to the electrophilic attack by PE. This work elucidates the differences in the co-pyrolysis reaction characteristics of various biomass structures, which provides scientific and theoretical guidance for the efficient resource exploitation of biomass and waste plastics.

A density functional theory study on how γ-Al2O3 – Boehmite transformation affects carbon evolution during aqueous-phase reaction

Gamma-alumina (γ-Al2O3), one of the most common materials, is commercially used in many catalytic applications, including the active catalyst and support. However, the problem of fast deactivation makes the utilization of the γ-Al2O3 challenging. This work elucidates the mechanism of coke formation consisting of coke deposition and evolution on γ-Al2O3(110) surfaces in differential conditions, including; clean and hydroxylation γ-Al2O3(110) in terms of partial and fully hydroxylation of OH/γ-Al2O3(110) and AlOOH(010), respectively. We demonstrated that the γ-Al2O3(110) surface is proper for atomic coke deposition and dimerization in the initial state, where the presence of OH species promotes the coke evolution to higher coke, Cn (where n ≥ 3). Also, the higher coke formation thermodynamically preferred the cyclic form to the aliphatic one. The electron transfer from substrates to adsorbed coke illustrates the role of the electron donor of catalyst surfaces corresponding to the electron acceptor of adsorbed cokes.

Oxygen-Bridged Vanadium Single-Atom Dimer Catalysts Promoting High Faradaic Efficiency of Ammonia Electrosynthesis

Single-atom catalysts have already been widely investigated for the nitrogen reduction reaction (NRR). However, the simplicity of a single atom as an active center encounters the challenge of modulating the multiple reaction intermediates during the NRR process. Moving toward the single-atom-dimer (SAD) structures can not only buffer the multiple reaction intermediates but also provide a strategy to modify the electronic structure and environment of the catalysts. Here, a structure of a vanadium SAD (V-O-V) catalyst on N-doped carbon (O-V2-NC) is proposed for the electrochemical nitrogen reduction reaction, in which the vanadium dimer is coordinated with nitrogen and simultaneously bridged by one oxygen. The oxygen-bridged metal atom dimer that has more electron deficiency is perceived to be the active center for nitrogen reduction. A loop evolution of the intermediate structure was found during the theoretical process simulated by density functional theory (DFT) calculation. The active center V-O-V breaks down to V-O and V during the protonation process and regenerates to the original V-O-V structure after releasing all the nitrogen species. Thus, the O-V2-NC structure presents excellent activity toward the electrochemical NRR, achieving an outstanding faradaic efficiency (77%) along with the yield of 9.97 μg h-1 mg-1 at 0 V (vs RHE) and comparably high ammonia yield (26 μg h-1 mg-1) with the FE of 4.6% at -0.4 V (vs RHE). This report synthesizes and proves the peculiar V-O-V dimer structure experimentally, which also contributes to the library of SAD catalysts with superior performance.

Pd/Co3O4(111) Interface Formation

The formation of the metal–oxide interface in the Pd/Co3O4(111) model catalyst was investigated by means of density functional theory (DFT), synchrotron radiation photoelectron spectroscopy (SRPES), and scanning tunneling microscopy (STM). The electronic metal–support interaction results in a substantial charge transfer at the interface yielding atomically dispersed Pd2+ species and partially oxidized Pdδ+ aggregates coupled with a partial reduction of Co3O4(111). Atomically dispersed Pd2+ species at the fcc site on the Co3O4(111) surface were found to be the most energetically favorable configuration. In comparison to the dispersed Pd2+ species, the formation of Pd dimers, trimers, and tetramers was found to be less favorable. The analysis of the Bader charges revealed a substantial net positive charge on Pd atoms in dimers, trimers, and tetramers which is consistent with the formation of partially oxidized Pdδ+ aggregates detected by SRPES. The analysis of the charge distribution in Co3O4(111) revealed a partial reduction of Co3+ to Co2+ cations in the first and second Co layers. According to DFT, Pdδ+ aggregates are prone to oxidation to PdO in the presence of O2 and H2O. The partially oxidized Pdδ+ and Pd4Ox aggregates form 1 to 2 monolayer thick clusters which serve as nuclei for the growth of metallic Pd0 nanoparticles. At high Pd coverage, Pd nanoparticles coalesce resulting in the growth of two-dimensional islands that densely cover the Co3O4(111) substrate.

Composite-boson formalism applied to strongly bound fermion pairs in a one-dimensional trap

We analyze a system of fermions in a one dimensional harmonic trap with attractive delta-interactions between different fermions species, as an approximate description of experiments involving atomic dimers. We solve the problem of two fermion pairs numerically using the so-called "coboson formalism" as an alternative to techniques which are based on the single-particle basis. This allows us to explore the strongly bound regime, approaching the limit of infinite attraction in which the composite particles behave as hard-core bosons. Our procedure is computationally inexpensive and illustrates how the coboson toolbox is useful for ultracold atom systems even in absence of condensation.

Structure and stability of Au, Au2 and Au8 cluster on Ni(111) supported h-BN sheet: Role of size and support towards oxygen bond activation

Atomic level heterogenous catalysis has been of importance for in-depth understanding and designing of novel catalytic material. Present work aims to investigate the chemical reactivity of Au, Au 2 and Au 8 cluster on hexagonal boron nitride nano sheet (h-BN) and its Ni(111) activated analogue using density functional theory (DFT). The average vertical separation of gold atom(s) from h-BN is significantly influenced by Ni(111) support. Oxygen molecule, which is the key reactant for any oxidation reaction, is considered as probe molecule to verify the reactivity. Contrary to the case of Au atom, which is the most reactive on free-standing h-BN, Au 2 and Au 8 clusters show higher reactivity on Ni supported h-BN. Thus the agglomeration of Au (Au–Au interaction) and Ni(111) support is important in controlling the reactivity and therefore, will play a key role to design Au based efficient catalyst for future. • On free-standing h-BNNS, Au atom and Au dimer favours on top B and N atoms, respectively. • With Ni (111) supported h-BNNS both Au and Au 2 favour on-top B atom. • On free-standing h-BNNS, Au elongates the O–O bond more than Au 2 and Au 8 . • On Ni (111) supported h-BNNS, the O–O bond elongation is maximum on Au 8 cluster. • The presence of Ni(111) support improves the catalytic activity by spin modulation of the B atoms of h-BNNS.

Hyperspherical Approach to Atom–Dimer Collisions with the Jacobi Boundary Condition

In this study, we investigate atom--dimer scattering within the framework of the hyperspherical method. The coupled channel Schr\"odinger equation is solved using the R-matrix propagation technique combined with the smooth variable discretization method. In the matching procedure, the asymptotic wave functions are expressed in the rotated Jacobi coordinates. We apply this approach to the elastic scattering $^{3}$He(T$\uparrow$) + $^{4}$He$_{2}$ and H$\uparrow$ + H$\uparrow$Li processes. The convergence of the scattering length as a function of the propagation distance is studied. We find that the method is reliable and can provide considerable savings over previous propagators.

Electronic Mach-Zehnder interference in a bipolar hybrid monolayer-bilayer graphene junction

Graphene matter in a strong magnetic field, realizing one-dimensional quantum Hall channels, provides a unique platform for studying electron interference. Here, using the Landauer-Büttiker formalism along with the tight-binding model, we investigate the quantum Hall (QH) effects in unipolar and bipolar monolayer-bilayer graphene (MLG-BLG) junctions. We find that a Hall bar made of an armchair MLG-BLG junction in the bipolar regime results in valley-polarized edgechannel interferences and can operate a fully tunable Mach-Zehnder (MZ) interferometer device. Investigation of the bar-width and magnetic-field dependence of the conductance oscillations shows that the MZ interference in such structures can be drastically affected by the type of (zigzag) edge termination of the second layer in the BLG region [composed of vertical dimer or non-dimer atoms]. Our findings reveal that both interfaces exhibit a double set of Aharonov-Bohm interferences, with the one between two oppositely valley-polarized edge channels dominating and causing a large amplitude conductance oscillation ranging from 0 to 2e2/h. We explain and analyze our findings by analytically solving the Dirac-Weyl equation for a gated semi-infinite MLG-BLG junction.

Spin Seebeck effect at low temperatures in the nominally paramagnetic insulating state of vanadium dioxide

The low temperature monoclinic, insulating phase of vanadium dioxide is ordinarily considered nonmagnetic, with dimerized vanadium atoms forming spin singlets, though paramagnetic response is seen at low temperatures. We find a nonlocal spin Seebeck signal in VO2 films that appears below 30 K and that increases with a decrease in temperature. The spin Seebeck response has a nonhysteretic dependence on the in-plane external magnetic field. This paramagnetic spin Seebeck response is discussed in terms of prior findings on paramagnetic spin Seebeck effects and expected magnetic excitations of the monoclinic ground state.

Clean and Hydrogen‐Adsorbed AlInP(001) Surfaces: Structures and Electronic Properties

Total energy and electronic structure calculations based on density functional theory are performed in order to determine the atomic structure and electronic properties of clean and hydrogen‐adsorbed Al0.5In0.5P(001) surfaces. It is found that most of the stable surfaces obey the electron‐counting rule and are characterized by surface atom dimerization. The dimer‐related surface states are predicted to occur in the vicinity of the bulk band edges. For a very narrow range of preparation conditions, ab initio thermodynamics predicts metal atomic wires formed by surface cations. A surface covered with a monolayer of buckled phosphorus dimers, where half of the phosphorus atoms are hydrogen saturated, is found to be stable for metal–organic vapor‐phase epitaxy growth conditions. The occurrence of this structure is confirmed by low‐energy electron diffraction and X‐ray photoelectron spectroscopy data measured on epitaxially grown Al0.52In0.48P(001) epilayers lattice matched to GaAs.

Nonlinear response and charge microtransfer between 1D/2D/3D-Ag nanodimers and H2O interactions with external fields

The properties of nano-plasmon materials are closely related to problems involving nanoscale systems. Benefitting from their spatial profile, surface plasmons (SPs) can dramatically enhance various external field–matter interactions. Here, we construct three different dimensions of silver atom dimers (1D-Ag6-Ag6, 2D-Ag6-Ag6 and 3D-Ag6-Ag6), bound to a water molecule (H2O) as the research system, using interacting external fields to interact. The results reveal that the nonlinear optics of Ag6-Ag6 are modulated (broken) by H2O, an effect directly related to the spatial dimension of the Ag6-Ag6 system. The charge micro-transfer process we analyzed at the atomic level provides a reasonable explanation for this phenomenon. Surprisingly, the 3D-Ag6-Ag6 system has application prospects as a photo-hydrolysis material, due to the outstanding SPs. We discuss nonlinear phenomena, microprocess and practical applications, with the goal to deliver guidelines for designing future highly adjustable plasmonic devices and technologies.

Exact studies of the ground-state properties of the anisotropic Heisenberg dimer with spin [formula omitted

The extended Heisenberg Hamiltonian for the atomic dimer with spin S=1 is diagonalized exactly. The anisotropy of the exchange interaction, single-ion anisotropy and the biquadratic exchange interaction are taken into account. It has been assumed that the external magnetic field can be heterogeneous and different on both atoms of the dimer. Based on the rigorous analytical solutions, the ground state properties of the dimer are studied numerically in a wide range of Hamiltonian parameters. In particular, the energy spectrum, biquadratic moment, residual entropy, ground-state phase diagrams and magnetization curves are calculated. The results are presented in the figures and discussed.

Generation of Basis Sets for Accurate Molecular Calculations: Application to Helium Atom and Dimer

A new approach for basis set generation is reported and tested in helium atom and dimer. The basis sets thus computed, named sigma, range from DZ to 5Z and consist of the same composition as Dunning basis sets but with a different treatment of contractions. The performance of the sigma sets is analyzed for energy and other properties of He atom and He dimer, and the results are compared with those obtained with Dunning and ANO basis sets. The sigma basis sets and their extended versions up to triple augmented provide better energy values than Dunning basis sets of the same composition, and similar values to those attained with the currently available ANO. Extrapolation to complete basis set of correlation energy is compared between the sigma basis sets and those of Dunning, showing the better performance of the former in this respect.

Studies of a Highly Active Cobalt Atomic Cluster Catalyst for Ammonia Synthesis

Theoretical studies have demonstrated that metal atomic clusters are highly efficient in catalytic ammonia synthesis under mild conditions. However, activation of N2 over atomic clusters from an experimental viewpoint has not been systematically investigated. Here, for the first time, cobalt atomic dimers on a nitrogen–carbon support were synthesized for NH3 synthesis. It is disclosed that N2 activation over Co2 atomic clusters (Co2 ACCs) is entirely different from that over Co single-atom catalysts (Co SACs) or Co nanoparticles (Co NPs). A suite of in situ spectroscopy studies reveal that Co2 ACCs not only provide active sites to promote strong metal–support interactions but also initiate strong interclustering interactions, resulting in electron transfers from Co d-orbitals to antibonding orbitals of N2 molecules. As such, the activation energy of the Co2 ACC catalyst in NH3 synthesis is small. Consequently, the catalytic activity of the developed Ba-promoted Co2 ACC catalyst reached 19.42 mmolNH3 gcat–1 h–1 at 400 °C and 1 MPa, which is much superior than that of other Co-based catalysts under such mild conditions. More importantly, the synthesis strategy of Co2 ACCs has general applicability as demonstrated in the preparation of Fe2 and Mn2 atomic clusters.

Photoelectron momentum distributions of Ne and Xe dimers in counter-rotating circularly polarized laser fields

The strong-field ionization of dimers is investigated theoretically in counter-rotating circularly polarized laser fields. By numerically solving the two-dimensional (2D) time-dependent Schrödinger equation (TDSE) with the single-electron approximation (SEA) frame, we present the photoelectron momentum distributions (PMDs) and photoelectron angular distribution (PADs) of aligned Ne and Xe dimers. It is found that the PMDs and PADs strongly depend on the time delays by counter-rotating circularly polarized laser pulses. The results can be explained by the ultrafast photoionization model and the evolution of electron wave packets for Ne and Xe dimers. Besides, We make a comparison of PMDs between Ne atom and Ne dimer.

Ferromagnetic and nonmagnetic 1T′ charge density wave states in transition metal dichalcogenides: Physical mechanisms and charge doping induced reversible transition

The charge density wave (CDW) states of two-dimensional transition metal dichalcogenides (TMDs) originate from intrinsic couplings between the electronic structures and lattice distortion, inducing interesting physical and chemical properties. The observed TMDs CDW states are mostly nonmagnetic (NM) but with a few ferromagnetic (FM) cases. Physical mechanisms for the formation of FM CDW remain elusive. In this paper, we used density functional theory calculations to study a set of TMDs with magnetic transition metal elements (e.g., V, Cr, and Mn). We found that the FM state can stem from the direct exchange to superexchange transition (e.g., $\mathrm{Cr}{X}_{2}$) or the $M\text{\ensuremath{-}}M$ ($M$ is the metal atom) dimerization (e.g., $\mathrm{Mn}{X}_{2}$). A crystal structure distortion index is proposed to distinguish the different formation mechanisms of FM CDW states. Interestingly, $\mathrm{Cr}{X}_{2}$ has both NM and FM CDW states, which is not observed in other TMDs materials. We found that charge (electron or hole) doping could modulate the different formation mechanisms and induce a phase transition between these two CDW states in ${\mathrm{CrS}}_{2}$, leading to significant actuation strain output (i.e., 12.17% and 5.93% along $x$ and $y$ directions, respectively) and drastic change of magnetism, which could enable some multifunctionality applications of TMD materials.