Metal Adatoms Research Articles

Potential and Adsorbate Effects in Electrodeposition: Lessons Learned from Atomic-Scale in Situ and Operando Studies

Electrodeposition has become an important tool in nanoscience and -technology. Examples range from metallization processes in the microelectronics industry to self-organized growth of nanomaterials. Because the growth behavior and the resulting deposit morphology can be varied over a wide range by the electrochemical parameters and by suitably chosen additives, electrodeposition often provides superior control as compared to rivaling methods or even enables deposition of structures which are unobtainable by other means. On the other hand, precisely the strong dependence on potential and solution composition poses a problem as they are largely not understood. Development of electrodeposition or electroless plating processes therefore is still largely empirical, specifically for the deposition of structures with well-defined nanoscale morphology. For knowledge-based strategies better understanding of the atomic–scale processes is required. In particular, it is necessary to clarify how the potential-dependent interface structure and the presence of surface-active species influence processes such as diffusion on the planar surface and across steps, nucleation events, and the ion transfer itself. Such data can be obtained by state-of-the-art surface-sensitive methods, which enable in situ studies of the interface structure and dynamics or even operando investigations at growth rates approaching those in technological processes. Together with ab initio theory calculations they provide first insights into the complex effects of potential and coadsorbates on the dynamics of species at the surface of the deposit during the growth (e.g. metal adatoms). I will specifically present studies of Cu and Au single crystal electrodes by synchrotron-based X-ray scattering techniques and high-speed scanning tunneling microscopy, focusing on the influence of halides on surface dynamics and metal growth. The obtained results reveal very pronounced and partially unexpected effects, which can lead to crossover between qualitatively different growth regimes. The relevance of these observations for electroplating, prospects for operando studies of technologically interesting cases, as well as remaining challenges and future directions will be discussed.

Scanning Quantum Dot Microscopy.

We introduce a scanning probe technique that enables three-dimensional imaging of local electrostatic potential fields with subnanometer resolution. Registering single electron charging events of a molecular quantum dot attached to the tip of an atomic force microscope operated at 5K, equipped with a qPlus tuning fork, we image the quadrupole field of a single molecule. To demonstrate quantitative measurements, we investigate the dipole field of a single metal adatom adsorbed on a metal surface. We show that because of its high sensitivity the technique can probe electrostatic potentials at large distances from their sources, which should allow for the imaging of samples with increased surface roughness.

Hydrogen storage properties of light metal adatoms (Li, Na) decorated fluorographene monolayer

Owing to its high energy density, the potential of hydrogen (H2) as an energy carrier has been immense, however its storage remains a big obstacle and calls for an efficient storage medium. By means of density functional theory (DFT) in spin polarized generalized gradient approximation (GGA), we have investigated the structural, electronic and hydrogen storage properties of a light alkali metal (Li, Na) functionalized fluorographene monolayer (FG). Metal adatoms bind to the FG with significantly high binding energy, much higher than their cohesive energies, which helps to achieve a uniform distribution of metal adatoms on the monolayer and consequently ensure reversibility. Due to a difference of electronegativities, each metal adatom transfers a substantial amount of its charge to the FG monolayer and attains a partial positive state, which facilitates the adsorption of multiple H2 molecules around the adatoms by electrostatic as well as van der Waals interactions. To get a better description of H2 adsorption energies with metal-doped systems, we have also performed calculations using van der Waals corrections. For both the functionalized systems, the results indicate a reasonably high H2 storage capacity with H2 adsorption energies falling into the range for the practical applications.

Change of Electronic Structures by Dopant-Induced Local Strain.

Ag-induced Si(111)- surfaces (-Ag) exhibit unusual electronic structures that cannot be explained by the conventional rigid band model and charge transfer model. The (-Ag surfaces feature a free-electron-like parabolic band, the S1 band, that selectively shifts downward upon the adsorption of noble metal or alkali metal adatoms. Furthermore, the downward shift of S1 is independent of the type of dopants, Au, Ag, and Na. According to charge transfer analysis, Au adatoms accumulate electrons from the substrate and become negatively charged, whereas Na adatoms become positively charged, which indicates that S1 should shift in the opposite direction for both the adatoms. Investigation of calculated structures, calculation of model structures, and tight-binding analysis disclose that the changes in the electronic structure are closely related to the average Ag-Ag distance in the substrate and have their origin in the local strain induced by dopants (adatoms). This explanation implies that the electronic structure is irrespective of the dopant characters itself and paves a new way for understanding the electronic structures associated with the presence of dopants.

Structural, Electronic, and Magnetic Properties of Adatom Adsorptions on Black and Blue Phosphorene: A First-Principles Study

Using first-principles calculations, we investigate the adsorption characteristics of alkali, alkaline-earth, nonmetallic, transition, and noble metal adatoms on phosphorene. The adsorption-induced tunable electronic structures are comparatively studied on two representative structures of phosphorene, i.e., black and blue phosphorus (P) nanosheets. Both black and blue P sheets exhibit good adsorption capability to foreign atoms, on which the binding energies of adatoms are stronger than on the BN, SiC, MoS2, or graphene sheets. On the black P sheet, most adatoms prefer to adsorb on the hollow site, whereas for the blue P sheet, the favored adsorption sites are element-dependent. The majority of alkali, alkaline-earth, and transition-metal adatoms prefer the valley site, noble metal adatoms the hollow site, and nonmetallic adatoms the bridge and top sites instead. The semiconducting behaviors of phosphorene are modified by adatoms, which can cause p-type and n-type doping or induce midgap states into the P...

Electronic and Magnetic Properties of Rare-Earth Atoms Absorbed on Graphene Sheet: A Theoretical Study

The electronic and magnetic properties of graphene functionalized by 4f-orbital RE-metal atoms (Ce, Nd, Sm and Eu) were investigated by the density functional theory (DFT) calculations. The results of binding energy and geometry parameters showed that the hollow site, the center of a carbon hexagon, was the most stable adsorption structure. Furthermore, the PDOS results suggested that the electronic hybridization between the RE-adatoms and C atoms was mainly contributed by the 5d orbitals, whereas the 4f-orbital of the metal adatoms dominated the net magnetic moments of the systems significantly.

Adsorption of metal adatom on nanographene: Computational investigations

Recently synthesized nanographene, which is a discrete fragment of graphene, is considered a new form of carbon. We perform density functional theory calculations on the adsorption of various metal adatoms on the saddle-shaped nanographene tetrabenzo[8]circulene (TB8C) and its modified derivates. Our results show that nanographene is better than pristine graphene in adsorbing metal adatoms. Octaamino-tetrabenzo[8]circulene (OA-TB8C) shows the best adsorption capability among the TB8C derivates. Bonding analysis on the metal@nanographene complexes reveals different bonding types for different metal elements. We find that metal adatoms that form ionic bonds or partially covalent bonds with the nanographene have adsorption energies on OA-TB8C that are mostly higher than or close to their own cohesive energies. The saddle geometry of nanographene enables them to adsorb two metal adatoms at the same time, especially for alkali metal atoms.

Electronic and Magnetic Properties Modulated by Adsorption of 3d Transition Metal Atoms in Monolayer and Bilayer MoS2 Sheets

The adsorption effects of 3d transitional metal (TM) adatoms on electronic and magnetic properties of monolayer and bilayer MoS2 sheets have been investigated by using first-principle calculations based on the density functional theory. The calculated results suggest that both monolayer and bilayer MoS2 sheets have power abilities of absorbing 3d TM atoms. The interlayer adsorption of bilayer MoS2 is relatively more stable than the surface adsorption of monolayer MoS2. The 3d TM adatoms and the neighboring S atoms behave a clear covalent-binding character. It was found that TM adatoms induce certain impurity states within the band gap of the pristine MoS2 sheet which result in the systems magnetically semiconducting or half metallic. The adsorbed systems for Cr and Co on the surface of monolayer MoS2 sheet, as well as Sc, Cr and Fe in the interlayer of bilayer MoS2 sheet exhibit half-metallic behavior. And the other 3d TM-adsorbed systems are magnetic semiconductor except for Ni species.

C60 as an Atom Trap to Capture Co Adatoms

C60 molecules were used to trap Co adatoms and clusters on a Au(111) surface using atomic/molecular manipulation with a scanning tunneling microscope. Two manipulation pathways (successive integration of single Co atoms in one molecule or direct integration of a Co cluster) were found to efficiently allow the formation of complexes mixing a C60 molecule with Co atoms. Scanning tunneling spectroscopy reveals the robustness of the π states of C60 that are preserved after Co trapping. Scanning tunneling microscopy images and density functional theory calculations reveal that dissociated Co clusters of up to nine atoms can be formed at the molecule–substrate interface. These results open new perspectives in the interactions between metal adatoms and molecules, for applications in metal–organic devices.

Interaction between carbon nanotubes and metals: Electronic properties, stability, and sensing

The interactions between carbon nanotubes (CNTs) and metal adatoms as well as metal contacts are studied by means of ab initio electronic structure calculations. We show that the electronic properties of a semiconducting (8,4) CNT can be modified by small amounts of Pd adatoms. Such a decoration conserves the piezoelectric properties of the CNT. Besides the electronic influence, the stability of a single adatom, which is of big importance for future technology applications, is investigated as well. We find only small energy barriers for the diffusion of a Pd adatom on the CNT surface. Thus, single Pd adatoms will be mobile at room temperature. Finally we present results for the interaction between a metallic (6,0) CNT and metal surfaces. Binding energies and distances for Al, Cu, Pd, Ag, Pt, and Au are discussed and compared, showing remarkable agreement between the interaction of single metal atoms and metal surfaces with CNTs.

Relativistic dynamical spin excitations of magnetic adatoms

We present a first-principles theory of dynamical spin excitations in the presence of spin-orbit coupling. The broken global spin rotational invariance leads to a new sum rule. We explore the competition between the magnetic anisotropy energy and the external magnetic field, as well as the role of electron-hole excitations, through calculations for 3$d$-metal adatoms on the Cu(111) surface. The spin excitation resonance energy and lifetime display non-trivial behavior, establishing the strong impact of relativistic effects. We legitimate the use of the Landau-Lifshitz-Gilbert equation down to the atomic limit, but with parameters that differ from a stationary theory.

NiAl(110) Surface as a Template for Growing Transition Metal Linear Atomic Chains: A DFT Investigation

First-principles calculations based on periodic density functional theory (DFT) have been used to investigate structural, energetic, and electronic properties of different transition metal atoms (Pd, Pt, Cu, Ag, and Au) on the NiAl(110) surface for coverages ranging from 0.25 monolayer up to completing full coverage, with special emphasis on the different possible depositions to form linear atomic chains (LAC). The analysis of the energetic contributions and electronic structure reveals that metal atoms are greatly favored to be aligned along the [001] direction to form LACs. The calculated negative work function changes are interpreted taking into account both the electronegativity and the polarizability of the deposited metal adatoms. This work function change decreases particularly for LACs along the [001] direction and, intriguingly, vanishes for Pt, suggesting an electronic behavior similar to the corresponding free-standing LAC.

RKKY interaction in graphene with a line defect

A kind of extended line defect is currently an experimentally available one-dimensional topological structure in graphene lattice. It modifies the electronic properties of graphene in many aspects. For example, it induces an even-parity boundary state which has linear dispersion and breaks the electron–hole symmetry of the graphene electronic structure. In addition, the line defect possesses much stronger adsorption ability to the metal adatoms than the ordinary graphene lattice point. In the present work, by developing an analytical lattice Green's function technique, we theoretically study the RKKY interaction in graphene when two magnetic impurities are adsorbed near the line defect. We find that R−3 decay rate of the RKKY interaction unique to graphene still holds true in the presence of the line defect. But another feature of the RKKY interaction in graphene, referred to as the Saremi's rule, which claims the RKKY interactions between the same or opposite sublattice points are ferro- or antiferromagnetic respectively, is no longer preserved due to the influence of the boundary state around the line defect. More importantly, the RKKY interaction on the line defect is greater than its counterpart in the pristine graphene by about one or two orders of magnitude. The local lattice distortion around the line defect can bring about the transition of the RKKY interaction between ferro- and antiferromagnetic orders. Such a result implies that the presence of the extended line defect provides a feasible platform in graphene to realize the long-range magnetic order even at a high temperature.

Adatoms underneath Single Porphyrin Molecules on Au(111)

The adsorption of porphyrin derivatives on a Au(111) surface was studied by scanning tunneling microscopy and spectroscopy at low temperatures in combination with density functional theory calculations. Different molecular appearances were found and could be assigned to the presence of single gold adatoms bonded by a coordination bond underneath the molecular monolayer, causing a characteristic change of the electronic structure of the molecules. Moreover, this interpretation could be confirmed by manipulation experiments of individual molecules on and off a single gold atom. This study provides a detailed understanding of the role of metal adatoms in surface-molecule bonding and anchoring and of the appearance of single molecules, and it should prove relevant for the imaging of related molecule-metal systems.

シリコン表面超構造における超伝導輸送現象

Silicon surface reconstructions with metal adatoms are two-dimensional systems with atomic-scale thickness. We have successfully measured the superconducting transition of a representative silicon surface reconstruction Si(111)-(√7×√3)-In by electron transport measurement. In this article, we report our results together with the experimental techniques and future prospect for surface superconductors.

Modification of the catalytic properties of the Au4 nanocluster for the conversion of methane-to-methanol: synergistic effects of metallic adatoms and a defective graphene support.

Decorating graphene with nano-clusters offers potential for a wide range of industrial applications. For catalysis, embedding precisely controlled mono- and bimetallic nanoclusters into graphene can greatly increase their catalytic activities, especially for oxidation reactions. The catalytic performance of a gold nanocluster can be modified dramatically by changing its electronic properties. The results of this work demonstrate by means of DFT calculations that by strategic doping and promotion from the support material the catalytic activity improvement of a gold-based catalyst for the partial oxidation reaction of methane can be drastically enhanced. The transition metal-mediated catalysis is significantly affected by the two spin-state reactivities over them. The investigated catalytic processes consist of N2O decomposition and methane hydroxylation over three subnanoclusters (Au5, Au4Pd, and Au4Pt) deposited on a single vacancy graphene support. It was found that graphene acts not only as a support but also supports the catalysis through charge transfer between the subnanocluster and graphene. Graphene-supported Au4Pd exhibits enhanced catalytic activity for both steps of methane-to-methanol conversion, whereas the supported Au5 is good for N2O decomposition but ineffective for methane hydroxylation, mainly due to the involvement of a very stable intermediate (methyl-hydroxo-grafted nanocluster). The activation energies for N2O decomposition, C-H bond activation and methanol formation over the supported Au4Pd cluster are 13.8, 15.7, and 24.9 kcal mol(-1), respectively. Without the graphene support, the catalytic trend is reversed and Au4Pd becomes an inert cluster for these reactions.

Adsorption of metal adatoms on single-layer phosphorene.

Single- or few-layer phosphorene is a novel two-dimensional direct-bandgap nanomaterial. Based on first-principles calculations, we present a systematic study on the binding energy, geometry, magnetic moment and electronic structure of 20 different adatoms adsorbed on phosphorene. The adatoms cover a wide range of valences, including s and p valence metals, 3d transition metals, noble metals, semiconductors, hydrogen and oxygen. We find that adsorbed adatoms produce a rich diversity of structural, electronic and magnetic properties. Our work demonstrates that phosphorene forms strong bonds with all studied adatoms while still preserving its structural integrity. The adsorption energies of adatoms on phosphorene are more than twice higher than on graphene, while the largest distortions of phosphorene are only ∼0.1-0.2 Å. The charge carrier type in phosphorene can be widely tuned by adatom adsorption. The unique combination of high reactivity with good structural stability is very promising for potential applications of phosphorene.

Building blocks for studies of nanoscale magnetism: adsorbates on ultrathin insulating Cu2N

Scanning tunneling microscopy and spectroscopy were performed to study transition metal adatoms (Fe, Co, Cu) and individual metal-dithiol complexes on insulating Cu2N islands. Adsorption of metal adatoms on Cu2N is surprisingly complex and in the case of Fe, we find two distinct adsorption states for each of two distinct adsorption sites. Connection of these metal adatoms to dithiol molecules was pursued to model a single molecule junction, with the aim of understanding further details about the nature of metal/molecule electrical contact. Pronounced changes in local density of states, magnetic anisotropy and Kondo interactions were observed for Co adatoms connected to dithiol molecules. These results illustrate some of the challenges and opportunities for STM studies of nanoscale magnetism in complex systems.

Giant spin Hall effect in graphene grown by chemical vapour deposition.

Advances in large-area graphene synthesis via chemical vapour deposition on metals like copper were instrumental in the demonstration of graphene-based novel, wafer-scale electronic circuits and proof-of-concept applications such as flexible touch panels. Here, we show that graphene grown by chemical vapour deposition on copper is equally promising for spintronics applications. In contrast to natural graphene, our experiments demonstrate that chemically synthesized graphene has a strong spin-orbit coupling as high as 20 meV giving rise to a giant spin Hall effect. The exceptionally large spin Hall angle ~0.2 provides an important step towards graphene-based spintronics devices within existing complementary metal-oxide-semiconductor technology. Our microscopic model shows that unavoidable residual copper adatom clusters act as local spin-orbit scatterers and, in the resonant scattering limit, induce transverse spin currents with enhanced skew-scattering contribution. Our findings are confirmed independently by introducing metallic adatoms-copper, silver and gold on exfoliated graphene samples.

Metal-silicane: Stability and properties

The decoration of silicane using 16 different metal adatoms and the adsorption of small molecules are studied using first-principles calculations. Of the 16 metal adatoms, Li, Na, K, Ca, In, and Sc show a larger binding energy with silicane than their corresponding cohesive energy in the bulk, which suggests they can form 2D layers on the surface of silicane. The band analysis indicates that decoration with metal atoms can effectively tailor the electronic properties of silicane. The adsorption for hydrogen and carbon monoxide on Li-silicane system demonstrates that each Li atom can adsorb a maximum of five H2 or four CO molecules with the average adsorption energy of 0.18 and 0.23 eV/atom, respectively. The calculated results suggest that metal-silicane systems can provide more information for applications as hydrogen-storage or environment-protection materials.