Metal Adatoms Research Articles

Andreev reflection across a Kane-Mele normal-superconductor nano-junction

We have investigated the transport properties of a Kane-Mele normal-superconductor (NS) nano-junction using the familiar Blonder-Tinkham-Klapwijk (BTK) theory. The effects of the Rashba and the intrinsic spin-orbit coupling are mimicked by the inclusion of different transition metal adatoms adsorbed in a graphene nanoribbon. Specifically, we have focussed on the Andreev reflection phenomena for a range of Rashba and intrinsic coupling strengths. We have computed the spin resolved tunneling conductance where we found that the conductance characteristics are very sensitive to the strengths of the spin-orbit couplings. Further an interesting interplay between the Rashba and the intrinsic spin-orbit couplings is observed and its effects on the tunneling conductance are explored in detail. The possibility of tuning the spin-orbit couplings via different metal adatoms provides an experimental handle for achieving tunable conductance properties of this Kane-Mele nano-junction.

H-BN Defective Layers as Giant N-Donor Macrocycles for Cu Adatom Trapping from the Underlying Metal Substrate

In the confined zone between a bidimensional material and a metal surface, unexpected effects can take place. In this study, we show that when a nonregular two-dimensional h-BN layer is grown on a Cu(111) surface, metal adatoms spontaneously pop up from the bulk to fill the holes in the structure. We provide ample theoretical support to our findings based on a large set of dispersion-corrected density functional theory calculations and on a detailed analysis of the electronic properties and of the chemical processes at this peculiar interface. The observation can be rationalized in terms of a high affinity of Cu adatoms toward N-donor species. Defective h-BN, exposing N-terminated edges, behaves like a giant multi-N-donor macrocyclic ligand that can encapsulate metal atoms as a consequence of a huge stabilization deriving from the Cu–N bond formation. Our conclusions could apply to other metal surfaces and could even stimulate the idea of trapping different metal atoms from those of the underlying surface...

DFT study of Rh and Ti dimers decorating N-doped pyridinic and pyrrolic graphene for molecular and dissociative hydrogen adsorption

A theoretical study using density functional theory was employed to analyze the hydrogen adsorption on Rh2 and Ti2 dimers decorating pyridine and pyrrolic-like nitrogen doped graphene. First, an analysis of geometry, stability, projected density of states, overlap population and charge distribution was performed to understand the interaction between Rh and Ti metal adatoms and dimers with pyridinic and pyrrolic graphene. Charge transfer occurs from metal to substrate in all cases. An investigation of H2 adsorption on Rh and Ti metals dimers on pyridinic and pyrrolic N-doped graphene was also performed. Molecular adsorption states were observed for Ti on pyridinic graphene and for Rh on both substrates. H2 dissociative adsorption occurs on both metal supported dimers. H2s states hybridize with the Rh and Ti d band, forming localized and dispersed states in concordance with the mechanisms observed for H2 adsorption. The activation energies were calculated obtaining values smaller than 0.59 eV, indicating that the dissociation of H2 can occur spontaneously at room temperature. HSE06 hybrid functional was used to test the accuracy of PBE-D2 functional in the activation energy calculation. Adsorption in the molecular states occurs with no barriers.

Evaluating the Stability of Single-Atom Catalysts with High Chemical Activity

Single-atom catalysts represent the most efficient use of precious metals while at the same time offering the potential for high catalytic activity. Yet it has proven challenging to identify supports enabling high catalytic activity while at the same time inhibiting aggregation of metal adatoms. Density functional theory calculations are employed to identify how the local molecular environment on graphene can be used to stabilize a single platinum adatom and provide favorable activity for a benchmark reaction, CO oxidation. Graphene is modified via defects and dopants—specifically single vacancy and pyridinic N-doping—so that we can see how the electronic structure and chemical activity of Pt atoms are affected. The ability to disperse single atoms on graphene supports is examined by comparing binding energy of Pt atoms at neighboring stable sites versus isolated sites and evaluating the tendency toward clustering. Nudged elastic band calculations indicate that pyridinic N-doped graphene is a promising candidate to support a single Pt atom acting as a catalyst that is resistant to poisoning and enhances CO oxidation efficiency.

Adsorptions of metal adatoms on graphene-like BC3 and their rich electronic properties: A first-principles study**Project supported by the National Natural Science Foundation of China (Grant Nos. 11774396 and 11704322) and Shandong Natural Science Funds for Doctoral Program, China (Grant No. ZR2017BA017).

Density functional calculations have been performed to investigate the adsorption of twenty two different kinds of metal adatoms on graphene-like BC3. In contrast to the graphene adsorbed with adatoms, the BC3 with adatoms shows many interesting properties. (1) The interaction between the metal adatoms and the BC3 sheet is remarkably strong. The Li, Na, K, and Ca possess the binding energies larger than the cohesive energies of their corresponding bulk metals. (2) The Li, Na, and K adatoms form approximately ideal ionic bonds with BC3, while the Be, Mg, and Ca adatoms form ionic bonds with BC3 with slight hybridization of covalent bonds. The Al, Ga, In, Sn, and all transition metal adatoms form covalent bonds with BC3. (3) For all the structures studied, there exhibit metal, half-metal, semiconducting, and spin-semiconducting behaviors. Especially, the BC3 with Co adatom shows a quantum anomalous Hall (QAH) phase with a Chern number of −1 based on local density approximation calculations. (4) For Li, Na, K, Ca, Ga, In, Sn, Ti, V, Cr, Ni, Pd, and Pt, there exists a trend that the adatom species with lower ionization potential have lower work function. Our results indicate the potential applications of functionalization of BC3 with metal adatoms.

Droplet Controlled Growth Dynamics in Molecular Beam Epitaxy of Nitride Semiconductors

The growth dynamics of Ga(In)N semiconductors by Plasma-Assisted Molecular Beam Epitaxy (PAMBE) at low temperatures (T = 450 °C) is here investigated. The presence of droplets at the growth surface strongly affects the adatom incorporation dynamics, making the growth rate a decreasing function of the metal flux impinging on the surface. We explain this phenomenon via a model that considers droplet effects on the incorporation of metal adatoms into the crystal. A relevant role is played by the vapor-liquid-solid growth mode that takes place under the droplets due to nitrogen molecules directly impinging on the droplets. The role of droplets in the growth dynamics here observed and modeled in the case of Nitride semiconductors is general and it can be extended to describe the growth of the material class of binary compounds when droplets are present on the surface.

Effects of Introducing Metal Adatoms on the Electrochemical CO2 Reduction Behavior of Copper

The electrochemical CO2 reduction reaction (CO2RR) is considered as a promising candidate for a carbon negative technology to realize ecological and economical sustainable development. At present, copper is the only metal that can produce fuels (hydrocarbons and oxygenates) through CO2RR, but has many key scientific challenges remaining for implementation as an electrocatalyst (especially, selectivity and activity). One of the common strategies to tune electrocatalytic properties is tailoring active sites in an electrocatalyst by introducing another metal to one metal surface. In this work, we investigated the effect of introducing another metal M onto Cu(100) on the CO2RR. Physical vapor deposition was performed to obtain a 100-nm Cu(100) film epitaxially grown onto Si(100) substrate, followed by introducing 1-Å-equivalent another metal M (M: Ag, Au, In, Mo, Ni, Pd, and Sn) atoms onto the Cu(100) surface in vacuum. Atomic force microscopy and X-ray photoelectron spectroscopy guaranteed that M certainly existed on Cu and the M/Cu electrocatalyst had a smooth surface whose roughness was ±2 nm. The M/Cu electrocatalyst was supplied to a series of 15-min electrolysis in 0.1 mol dm−3 KHCO3 aqueous solution at potentials ranging from −0.6 to −1.1 V with respect to reversible hydrogen electrode at pH 6.8. Gaseous and liquid products were quantified with gas chromatography and nuclear magnetic resonance, respectively. A series of electrocatalytic evaluation pointed out the following general trends concerning the electrocatalytic properties of the M/Cu bimetallic thin films for CO2RR. For selectivity, the M atoms introduced to Cu(100) surface vary the Faradaic efficiency for each product to decrease average number of electrons transferred to a CO2 molecule. For activity, the introduction of M atoms onto Cu(100) surface suppresses the production rates for both C1 and C2+ products although it increases or decreases the total current density as seen in cyclic voltammetry, depending on M. The aforementioned experimental facts suggest the role of M that the M atoms introduce onto Cu(100) surface seem to cause some bimetallic effects on the CO2RR selectivity of Cu as well as an inhibition effect on the CO2RR activity of Cu. Electron-transfer reaction kinetics are further studied via a combined experiment and theory approach to discuss the nature of active sites in M/Cu bimetallic systems and the role of M in determining product distribution, which will be explained in the conference.

Metallized siligraphene nanosheets (SiC7) as high capacity hydrogen storage materials

A planar honeycomb monolayer of siligraphene (SiC7) could be a prospective medium for clean energy storage due to its light weight, and its remarkable mechanical and unique electronic properties. By employing van der Waals-induced first principles calculations based on density functional theory (DFT), we have explored the structural, electronic, and hydrogen (H-2) storage characteristics of SiC7 sheets decorated with various light metals. The binding energies of lithium (Li), sodium (Na), potassium (K), magnesium (Mg), calcium (Ca),scandium (Sc), and titanium (Ti) dopants on a SiC7 monolayer were studied at various doping concentrations, and found to be strong enough to counteract the metal clustering effect. We further verified the stabilities of the metallized SiC7 sheets at room temperature using ab initio molecular dynamics (MD) simulations. Bader charge analysis revealed that upon adsorption, due to the difference in electronegativity, all the metal adatoms donated a fraction of their electronic charges to the SiC7 sheet. Each partially charged metal center on the SiC(7)sheets could bind a maximum of 4 to 5 H-2 molecules. A high H-2 gravimetric density was achieved for several dopants at a doping concentration of 12.50%. The H-2 binding energies were found to fall within the ideal range of 0.2-0.6 eV. Based on these findings, we propose that metal-doped SiC7 sheets can operate as efficient H-2 storage media under ambient conditions.

Quantum anomalous Hall effect and giant Rashba spin-orbit splitting in graphene system co-doped with boron and 5d transition-metal atoms

Quantum anomalous Hall effect (QAHE) is a fundamental quantum transport phenomenon in condensed matter physics. Until now, the QAHE has only been experimentally realized for Cr/V-doped (Bi, Sb)2Te3 but at an extremely low observational temperature, thereby limiting its potential application in dissipationless quantum electronics. By employing first-principles calculations, we study the electronic structures of graphene co-doped with 5d transition metal and boron atoms based on a compensated n–p co-doping scheme. Our findings are as follows: i) The electrostatic attraction between the n- and p-type dopants effectively enhances the adsorption of metal adatoms and suppresses their undesirable clustering. ii) Hf-B and Os-B co-doped graphene systems can establish long-range ferromagnetic order and open larger nontrivial band gaps because of the stronger spin-orbit coupling with the non-vanishing Berry curvatures to host the high-temperature QAHE. iii) The calculated Rashba splitting energies in Re–B and Pt–B co-doped graphene systems can reach up to 158 and 85 meV, respectively, which are several orders of magnitude higher than the reported intrinsic spin-orbit coupling strength.

What Drives Metal-Surface Step Bunching in Graphene Chemical Vapor Deposition?

Compressive strain relaxation of a chemical vapor deposition (CVD) grown graphene overlayer has been considered to be the main driving force behind metal surface step bunching (SB) in CVD graphene growth. Here, by combining theoretical studies with experimental observations, we prove that the SB can occur even in the absence of a compressive strain, is enabled by the rapid diffusion of metal adatoms beneath the graphene and is driven by the release of the bending energy of the graphene overlayer in the vicinity of steps. Based on this new understanding, we explain a number of experimental observations such as the temperature dependence of SB, and how SB depends on the thickness of the graphene film. This study also shows that SB is a general phenomenon that can occur in all substrates covered by films of two-dimensional (2D) materials.

Unravelling the Mechanism of Glaser Coupling Reaction on Ag(111) and Cu(111) Surfaces: a Case for Halogen Substituted Terminal Alkyne

The mechanisms of Glaser coupling reaction on metal surfaces have been poorly understood. Herein, we propose a reaction pathway toward surface-confined Glaser coupling which is initiated by single-molecule dehydrogenation of terminal alkyne. This is inspired by our experimental observations of alkynyl–Ag–alkynyl and alkynyl–Cu–alkynyl type organometallic intermediates in the coupling reaction of 1,1′-biphenyl,4-bromo-4′-ethynyl (BPBE) on Ag(111) and Cu(111), respectively. Theoretical calculations reveal that the dehydrogenation process of terminal ethynyl of BPBE is most likely catalyzed by a stray H adatom on Ag(111) but by a Cu adatom on Cu(111), followed by the formation of the organometallic intermediates. After the release of interstitial metal adatoms, the final C–C coupling occurs easily on Ag(111) but shows extremely low efficiency on Cu(111), due to the too strong interaction between ethynylene and the Cu(111) substrate.

A standing molecule as a single-electron field emitter.

Scanning probe microscopy makes it possible to image and spectroscopically characterize nanoscale objects, and to manipulate1-3 and excite4-8 them; even time-resolved experiments are now routinely achieved9,10. This combination of capabilities has enabled proof-of-principle demonstrations of nanoscale devices, including logic operations based on molecular cascades 11 , a single-atom transistor 12 , a single-atom magnetic memory cell 13 and a kilobyte atomic memory 14 . However, a key challenge is fabricating device structures that can overcome their attraction to the underlying surface and thus protrude from the two-dimensional flatlands of the surface. Here we demonstrate the fabrication of such a structure: we use the tip of a scanning probe microscope to lift a large planar aromatic molecule (3,4,9,10-perylenetetracarboxylic-dianhydride) into an upright, standing geometry on a pedestal of two metal (silver) adatoms. This atypical and surprisingly stable upright orientation of the single molecule, which under all known circumstances adsorbs flat onmetals15,16, enables the system to function as a coherent single-electron field emitter. We anticipate that other metastable adsorbate configurations might also be accessible, thereby opening up the third dimension for the design of functional nanostructures on surfaces.

Protected giant magnetic anisotropy in two-dimensional materials: Transition-metal adatoms on defected tungsten disulfide monolayer

Giant magnetic anisotropy energies (MAEs), especially in systems that are well protected, are needed for the development of nanomagnetic devices. By using the first-principles calculations, we systematically investigated the structural and magnetic properties of 3d and 5d transition metal adatoms on a defected tungsten disulfide monolayer. Among these two-dimensional materials, Ir@D-WS2 and Os@D-WS2 possess extremely large MAEs (up to ∼40 meV/adatom). Furthermore, their large MAEs may sustain when we put a layer of graphene on top to protect them. The MAE of Os@D-WS2 can be tuned in a large range, up to 300%, by applying an external electric field of 0.5 V/Å. In addition, these systems exhibit unusual valley dependent topological features such as nontrivial band gaps and large Berry curvatures, and they hence are promising for technological innovations.

Hydrogenated defective graphene as an anode material for sodium and calcium ion batteries: A density functional theory study

Recent experimental studies indicated that hydrogenation improves the performance of graphitic materials used as anodes in lithium and sodium ion rechargeable batteries. Here, results of density functional theory calculations are presented to demonstrate that this is also effective for both sodium and calcium ion batteries. It is shown that this can be explained by the increase in binding strength of the metal adatom to the hydrogenated graphene, compared to its binding to graphene, and also an increase in the inter-layer spacing of the layered materials. According to our calculations, whereas Na and Ca bind weakly to the graphene sheet with binding energies of −0.763 and −0.817 eV, they bind more strongly to the single layer of the proposed hydrogenated graphene sheet (C68H4) with binding energies of −1.670 and −2.756 eV, respectively. Furthermore, although Na does not intercalate strongly in the layers of the C68H4 material, up to 16 Ca can intercalate into the bulk layers of this material giving an electrical capacity of 591.2 mA h/g and a 29.3% expansion of the inter-layer distance. Thus, hydrogenated defective graphene provides an anode material that might enhance the performance of rechargeable batteries compared to graphene, using metals that are cheaper than lithium.

Electronic and magnetic properties of Fe-, Co-, and Ni-decorated BC3: A first-principles study

The electronic and magnetic properties of Fe-, Co-, and Ni-decorated two dimensional (2D) BC3 are systematically investigated by first-principles calculations. We find that the Fe, Co, and Ni atoms can be strongly adsorbed on the hollow sites of 2D BC3. Fe and Co adatoms are more stable when adsorbed on the hollow sites of the carbon rings in the 2D BC3, while the hollow sites of boron-carbon rings in the 2D BC3 are the most stable sites for the adsorption of Ni adatoms. These proposed metal–BC3 complexes exhibit interesting electronic and magnetic behaviors. In particular, the Fe–BC3 and Co–BC3 complexes are metals with magnetic ground states , while the Ni–BC3 complex behaves as a nonmagnetic semiconductor with a direct bandgap. Furthermore, our magnetic analysis reveals that induced magnetism in the Fe–BC3 and Co–BC3 complexes arises from their local magnetic moments. Functionalization of 2D BC3 through these metal–adatom adsorption appears to be a promising way to extend its applications.

Real-time imaging of adatom-promoted graphene growth on nickel.

Single adatoms are expected to participate in many processes occurring at solid surfaces, such as the growth of graphene on metals. We demonstrate, both experimentally and theoretically, the catalytic role played by single metal adatoms during the technologically relevant process of graphene growth on nickel (Ni). The catalytic action of individual Ni atoms at the edges of a growing graphene flake was directly captured by scanning tunneling microscopy imaging at the millisecond time scale, while force field molecular dynamics and density functional theory calculations rationalize the experimental observations. Our results unveil the mechanism governing the activity of a single-atom catalyst at work.

First-Principle Investigations of the Interaction between CO and O2 with Group 11 Atoms on a Defect-Free MgO(001) Surface.

In this contribution, we investigate the interaction between CO and O2 with metal atoms of group 11 deposited on a defect-free magnesium oxide surface using density functional theory with periodic point charge embedding. We present the first transversal study of the adsorption and coadsorption of CO and O2 on coinage metal adatoms deposited on metal oxide surfaces from the perspective of single-atom catalysis. Various analysis tools shed light on the binding situation of the metal atoms to the substrate as well as on the situation of the two molecules on the different metal centers. Our analysis demonstrates that cooperative electronic effects enhance the stability of CO upon coadsorption with O2 for all three metal centers. Our results also explain the lack of catalytic activity of group 11 metal atoms with respect to CO oxidation under thermal conditions as a competition between OC-O2 bond activation and surface diffusion, leading to metal atom agglomeration. Additionally, it is shown how coadsorption of CO and O2 on Au/Mg(001) could pave the way to single-atom photocatalysis.

First principles study the effects of alkali metal and chorine adatoms on the opposite surface of graphene

Study of the adsorption properties of graphene has great significance for expanding its application. So far, few studies have analyzed the effects of adatoms on opposite sides of graphene. We use density functional theory to report the effects of chlorine and alkali metal adatoms on the other side of graphene. Although there is an obvious charge transfer between the adatom and graphene, the interaction between the adatoms is shielded by the large π bonds of graphene and therefore the effects of the adatom on the other side of graphene are very weak.

Ab Initio Study of Electronic and Magnetic Properties of Germanene with Different Nonmagnetic Metal Adatoms

Electronic and magnetic properties of two-dimensional (2D) germanene (Ge) with five different adatoms have been analyzed and discussed by the DFT method. Different nonmagnetic metals adsorbed at different sites, on one hand, a magnetic moment is induced by Mg adatom in the germanene. On the other hand, the adsorptions of Al, Ga, Li, and Na show no magnetism. We further study the effect of strain on the magnetism in Mg-adsorbed germanene; we apply an isotropic tensile and compressive strain on the system. On the basis of our calculations, a tunable magnetism shows as the strain increases. The analysis of the PDOS shows that the s–p hybridization between Mg and Ge atoms results in such magnetic behavior. These magneticadsorbed germanene materials might show potential applications in the nanoscale devices.

On the H2 interactions with transition metal adatoms supported on graphene: a systematic density functional study.

The attachment of H2 to the full set of transition metal (TM) adatoms supported on graphene is studied by using density functional theory. Methodology validation calculations on the interactions of H2 with benzene and graphene show that any of the vdW corrections under study, the Grimme D2, D3, D3 with Becke-Jonson damping (D3BJ), and Tkatchenko-Scheffler methods, applied on the PBE functional, are similarly accurate in describing such subtle interactions, with an accuracy of almost 2 kJ mol-1 compared to experiments. The PBE-D3 results show that H2 physisorbs on especially stable d5 or d10 TMs. In other 5d metals, and the rightmost 3d and 4d ones, H2 dissociates, and only for Y, Mn, Fe, and Zr the H2 binds strongly enough for its storage in the so-called Kubas mode, where the H2 bond is sensibly elongated. Other metals (Co, Ni, Ru, Rh and Pd) feature also an elongated Kubas mode, interesting as well for H2 storage. Sc and Ti display a Kubas modes especially suited, given their lightness, for meeting the gravimetric requirements. The H2 interactions with TM adatoms imply a TM → H2 charge transfer, although the magnetic moment of the system tends to remain intact, except for the early 5d TMs, where the unpaired electron transfer seems to be associated with the H2 bond breakage.