Gold Adatoms Research Articles

C70 increases the plasmonic signal of gold-atom chains on Si(553)

Infrared plasmonic excitations of the quasi-one dimensional gold-atom induced superstructures on silicon surfaces are particularly sensitive to doping effects. Such effects, here related to C70 adsorption, can be clearly measured as we show for gold atom chains on the Si(553) surface at room temperature. The strong plasmonic signal that is exclusively present for polarization along the chain direction does not disappear under C70 exposure. In contrast, it increases for adsorption of the first monolayer. The rising intensity corresponds to an increase of the one-dimensional plasma frequency. This process saturates at a coverage corresponding to ca. one C70 per 1 × 2 surface unit cell. The observed plasmonic behaviour upon C70 adsorption is opposite to the consequences of electron donating gold ad-atoms on the plasmonic response. The increase of the one-dimensional plasma frequency upon C70 adsorption is accompanied by an increase of the electronic scattering rate, which is in accord to the theory on adsorbate induced surface resistivity.

Solving the Long-Standing Controversy of Long-Chain Alkanethiols Surface Structure on Au(111)

The determination of the amount of gold adatoms and vacancies present at the thiol–gold interface of the (√3 × √3)-R30° lattice on Au(111), usually observed for long-chain alkanethiols, is crucial ...

Role of Gold Adatoms in the Adsorption of Sulfide Species on the Gold(001)-hex Surface

We have studied the adsorption of reduced sulfur species on the Au(001)-hex surface from diluted ethanolic sulfide solutions by STM, XPS, electrochemical techniques, and DFT calculations. Our results reveal the absence of S multilayers because all S species are bonded to Au surface atoms. Monomeric S adsorbs at hollow sites forming a (2 × 2) lattice, while S2 species either capture the gold adatoms (Auad) that result from the hex lifting or remove Auad from the substrate forming well-ordered S8-like structures. These are formed by four S2 species coordinated by a central Auad, with AuSS interatomic distances closely resembling those of a metal disulfide. Our results explain not only the predominant surface structures on the gold surface but also topographic features, like Auad islands and Au vacancy islands, and also the chemical nature of adsorbed species from XPS S 2p spectra. In contrast to the S heads of small aromatic thiols, which exhibit a low affinity for Auad, strongly reactive sulfide species re...

Comparative Study of the Adsorption of Thiols and Selenols on Au(111) and Au(100).

The effect of the Au crystalline plane on the adsorption of different thiols and selenols is studied via reductive desorption (RD) and X-ray photoelectron spectroscopy (XPS) measurements. Self-assembled monolayers (SAMs) using aliphatic (ATs) and aromatic thiols (ArTs) on both Au(111) and Au(100) were prepared. The electrochemical stability of these SAMs on both surfaces is evaluated by comparing the position of the RD peaks. The longer the AT chain the more stable the SAM on Au(100) when compared to Au(111). By means of XPS measurements, we determine that the binding energy (BE) of the S 2p signal corresponding to the S atoms at the thiol/Au interface, commonly assigned at 162.0 eV, shifts 0.2 eV from Au(111) to Au(100) for SAMs prepared using thiols with the C* (C atom bonded to S) in sp3 hybridization, such as ATs. However, when the thiol presents the C* with an sp2 hybridization, such as in the case of ArTs, the BE remains at 162.0 eV regardless of the surface plane. Selenol-based SAMs were characterized comparatively on both Au(100) and Au(111). Our results show that selenol SAMs become even more electrochemically stable on Au(100) with respect to Au(111) than the analogue sulfur-based SAM. According to our results, we suggest that the electronic distribution around the Au-S/Se bond could be responsible for the different structural arrangements reported in the literature (gold adatoms, etc.), which should be dependent on the crystalline face (Au(hkl)-S) and the chemical nature of the environment of the adsorbates (sp3-C* vs sp2-C* and Au-SR vs Au-SeR).

Adatoms in graphene nanoribbons: spintronic properties and the quantum spin Hall phase

We study the charge and spin transport in a two terminal graphene nanoribbon (GNR) decorated with random distribution of Gold (Au) adatoms using a Kane–Mele model. The presence of the quantum spin Hall (QSH) phase is found to crucially depend on the strength of the intrinsic spin–orbit term, while the plateau in the longitudinal conductance at a value is not the smoking gun for the QSH phase. Thus the Au adatoms which manage to induce only a small intrinsic spin–orbit coupling cannot guarantee a QSH phase, albeit yielding a plateau in the longitudinal conductance around the zero of the Fermi energy. If other adatoms can induce larger spin–orbit strengths (we call them hypothetical adatoms), they would ensure both the plateau and the QSH phase as is evident from the presence of the conducting edge states. Motivated by these results, the spintronic applications are explored via computing the spin polarized conductance for both Au and hypothetical adatoms. The y-component of the spin polarized conductance renders the dominant contribution owing to the finite width of the GNR in the y-direction and is found to possess strikingly similar features with that of the longitudinal conductance. The other two components, namely x and z are small but finite and hence have relevance in spintronic applications. Moreover, via computing the local current distribution, we show the clear emergence of edge states in the case of hypothetical adatoms, which are conspicuously absent for Au decorated GNRs.

Tuning the Electronic and Dynamical Properties of a Molecule by Atom Trapping Chemistry.

The ability to trap adatoms with an organic molecule on a surface has been used to obtain a range of molecular functionalities controlled by the choice of the molecular trapping site and local deprotonation. The tetraphenylporphyrin molecule used in this study contains three types of trapping sites: two carbon rings (phenyl and pyrrole) and the center of a macrocycle. Catching a gold adatom on the carbon rings leads to an electronic doping of the molecule, whereas trapping the adatom at the macrocycle center with single deprotonation leads to a molecular rotor and a second deprotonation leads to a molecular jumper. We call "atom trapping chemistry" the control of the structure, electronic, and dynamical properties of a molecule achieved by trapping metallic atoms with a molecule on a surface. In addition to the examples previously described, we show that more complex structures can be envisaged.

Comprehensive View of the Ligand–Gold Interface from First Principles

A large number of ligands have been used to stabilize and functionalize the gold surfaces and nanoclusters, but there has been no systematic comparison about their binding strengths with gold. In this work, we studied the interaction between 27 ligands of six different types (thiolates, phosphines, amines, aryl radicals, alkynyls, and N-heterocyclic carbenes) with the model Au surfaces by first-principles density functional theory (DFT). On the perfect Au(111), we found the order of binding strengths to be bulky N-heterocyclic carbenes (NHCs) ≈ alkynyls > thiolates ≈ phosphines > aryls ≈ less sterically bulky NHCs > alkylamines. The much stronger interaction of bulky carbenes to Au than the less sterically bulky NHCs arises from the van der Waals (vdW) attraction of bulky side groups with gold surface via the short Au···HCH2R contact. Further, we showed that the presence of a gold adatom on Au(111) leads to enhanced binding and a similar order for most of the ligands examined. Overall, bulky NHCs and alky...

Influence of Relativistic Effects on Assembled Structures of V-Shaped Bispyridine Molecules on M(111) Surfaces Where M = Cu, Ag, Au.

The self-assembly behavior of a V-shaped bispyridine, 1,3-bi(4-pyridyl)benzene (BPyB), was studied by scanning tunneling microscopy on the (111) surfaces of Cu, Ag, and Au. BPyB molecules coordinately bonded with active Cu adatoms on Cu(111) in the form of complete polygonal rings at low coverages. On Ag(111), BPyB molecules aggregated into two-dimensional islands by relatively weak intermolecular hydrogen bonds. The coexistence of hydrogen bonds and coordination interaction was observed on the BPyB-covered Au(111) substrate. Density functional theory calculations of the metal-molecule binding energy and Monte Carlo simulations were performed to help understand the forming mechanism of molecular superstructures on the surfaces. In particular, the comprehensive orbital composition analysis interprets the observed metal-organic complexes and reveals the importance of relativistic effects for the extraordinary activity of gold adatoms. The relativistic effects cause the energy stability of the Au 6s atomic orbital and decrease the energy separation between the Au 6s and 5d orbitals. The enhanced sd hybridization strengthens the N-Au-N bond in BPyB-Au-BPyB complexes.

Organic probe molecule adsorption on extended Au(111) surface: a theoretical DFT study

The cluster approach has been applied to mimic the interactions of probe trans-1,2-bis-(4-pyridyl) ethylene on a gold surface with the aim of discriminating the clean Au(111) surface from that containing pre-adsorbed Au ad-atoms. Our DFT calculations at the combined B3LYP/(6-31G*+Lanl2dz) level indicate that, by changing the adsorption condition, one may clearly predict the status of the gold surface which may or may not contain some artificial pre-adsorbed and highly dispersed gold ad-atoms. This conclusion is based on estimations of characteristic IR and Raman peaks assigned precisely to the distinct active sites on the Au(111) surface.

6-Mercaptopurine Self-Assembled Monolayers on Gold (001)-Hex: Revealing the Fate of Gold Adatoms

Thiol molecules adsorbed on gold became a model system for molecular self-assembly on metal substrates long ago. In most cases the strong molecule-gold interaction is able to restructure the substrate, resulting in vacancy islands and steps. Today it is widely accepted that gold adatoms produced by this process form stable thiol-adatom complexes, usually termed “staples” (RS–Auad–SR or RS–Auad–SR–Auad–SR), which are the basic units of the self-assembled monolayers. Here we report on a different scenario for 6-mercaptopurine (6MP), a heterocyclic aromatic thiol, namely its adsorption on the Au(001)-(5 × 20) reconstructed surface. Our results show that 6MP lifts the reconstruction upon adsorption, thus ejecting a large excess of gold adatoms. Surprisingly, 6MP molecules prefer to arrange in highly ordered adatom-free domains in the bridging configuration, while the ejected adatoms form gold islands. Our investigation reveals that the formation of thiol-gold adatom complexes is not always a thermodynamically...

Static investigation of adsorption and hetero‐diffusion of copper, silver, and gold adatoms on the (111) surface

In this work, we have used the static molecular simulations combined with an interatomic potential derived from the embedded‐atom method to study the adsorption and hetero‐diffusion on the (111) surface of Cu, Ag, and Au adatoms by using LAMMPS code. The investigation is performed for six heterogeneous systems such as Ag/Au(111), Ag/Cu(111), Au/Ag(111), Au/Cu(111), Cu/Ag(111), and Cu/Au(111). First, we have investigated the relaxation trends and the bond lengths of the atoms in the systems. The calculation results show that, the top layer spacing between the first and second layers of the Au(111), Ag(111), and Cu(111) substrates is contracted. This contraction is found to be more important in the Au(111) substrate. On the other hand, the strong reduction of the binding length is found in Au/Cu(111) for the different adsorption sites. In addition, the binding, adsorption, and static activation energies for all studied systems were examined. The results indicated that the binding and adsorption energies reached their maximum values in the Au/Cu(111) and Au/Ag(111) systems, respectively. Moreover, the static activation barriers for hopping diffusion on the (111) surfaces are found to be low compared with those found in the (100) and (110) surfaces. Therefore, our calculations showed that the difference in energy between the hcp and fcc sites on the (111) surfaces is very small. Copyright © 2017 John Wiley & Sons, Ltd.

N-Heterocyclic carbenes on close-packed coinage metal surfaces: bis-carbene metal adatom bonding scheme of monolayer films on Au, Ag and Cu.

By means of scanning tunnelling microscopy (STM), complementary density functional theory (DFT) and X-ray photoelectron spectroscopy (XPS) we investigate the binding and self-assembly of a saturated molecular layer of model N-heterocyclic carbene (NHC) on Cu(111), Ag(111) and Au(111) surfaces under ultra-high vacuum (UHV) conditions. XPS reveals that at room temperature, coverages up to a monolayer exist, with the molecules engaged in metal carbene bonds. On all three surfaces, we resolve similar arrangements, which can be interpreted only in terms of mononuclear M(NHC)2 (M = Cu, Ag, Au) complexes, reminiscent of the paired bonding of thiols to surface gold adatoms. Theoretical investigations for the case of Au unravel the charge distribution of a Au(111) surface covered by Au(NHC)2 and reveal that this is the energetically preferential adsorption configuration.

Does the S-H Bond Always Break after Adsorption of an Alkylthiol on Au(111)?

The reaction mechanism for the formation of alkyl thiol self-assembled monolayers (SAM) on Au(111) is still not clearly understood. Especially, the role of defects on the chemisorption process is an important goal to be addressed. In this work, different minimum energy reaction paths for R-SH dissociation of thiols (with long and short chains and dithiol species) adsorbed on gold adatom are calculated by using periodic density functional theory (DFT). Our results show a lower energy barrier for the RS-H bond dissociation when two thiols are adsorbed per adatom. In addition, in contrast with the formation of an adatom at the Au(111) which has been shown to depend on the alkyl chain length, the activation energy of the RS-H bond dissociation of thiols adsorbed on an adatom was shown to be independent of the alkyl chain length. The presented results and derived hypothesis support the model that thiols with long alkyl chain thiols mainly adsorb molecularly on Au(111), while for short alkyl chain thiols the S-H bond breaks. This result is explained by the fact that short-chain thiols have lower interchain interaction energies and are thus more mobile compared to the long alkyl chain thiols on the Au(111) surface. This feature enables the short chains to reach adequate geometries, driven by entropy, which could deform the Au(111) more drastically and probably pull Au atoms out from surface to form adatoms. With these results a new mechanism is proposed for the formation of alkyl chain thiols on Au(111).

Charge-State-Dependent Diffusion of Individual Gold Adatoms on Ionic Thin NaCl Films.

It is known that individual metal atoms on insulating ionic films can occur in several different (meta)stable charge states, which can be reversibly switched in a controlled fashion. Here we show that the diffusion of gold adatoms on NaCl thin films depends critically on their charge state. Surprisingly, the anionic species has a lower diffusion barrier than the neutral one. Furthermore, for the former we observe that the diffusion atop a bilayer of NaCl is strongly influenced by the interface between NaCl and the underlying copper substrate. This effect disappears for a trilayer of NaCl. These observations open the prospect of controlling the diffusion properties of individual metal atoms on thin insulating films.

New mechanistic pathways for CO oxidation catalyzed by single-atom catalysts: Supported and doped Au1/ThO2

Single-atom catalysts are of great interest and importance for designing new high-performance low-cost catalysts. We investigated CO oxidation catalyzed by single gold atoms supported on thoria (Au/ThO2) and doped ThO2 using density functional theory with Hubbard-type on-site Coulomb interaction (DFT + U). The calculation results show that the Au-doped ThO2(111) catalyst exhibits remarkable catalytic activity for CO oxidation via the Eley–Rideal mechanism in three steps, where the rate-determining step is decomposition of the OCOO* intermediate with an energy barrier of 0.58 eV. Moreover, our results also reveal a new mechanism of CO oxidation on a gold adatom supported by ThO2(111), where O2 is adsorbed only at the Th site on the surface, and the gas-phase CO then reacts directly with the activated O2* to form CO2, which is the rate-limiting step, with a barrier of 0.46 eV. It is found that CO oxidation can occur without CO and O2 coadsorption on Au, which was previously considered a key intermediate. Therefore, these results provide new insights into CO oxidation on isolated gold atoms supported by the 5f-element compound ThO2(111). This mechanism can help clarify the catalytic cycle of CO oxidation, support the design of high-performance low-cost catalysts, and elucidate the redox properties of actinide oxides.

Study of the helium cross-section of unsymmetric disulfide self-assembled monolayers on Au(111)

We have investigated the formation of self-assembled monolayers (SAMs) of 11-hydroxyundecyl decyl disulfide (CH3-(CH2)9-S-S-(CH2)11-OH, HDD) and 11-hydroxyundecyl octadecyl disulfide (CH3-(CH2)17-S-S-(CH2)11-OH, HOD) produced by supersonic molecular beam deposition (SMBD). The study has been carried out by means of helium diffraction at very low film coverage. In this regime helium single molecule cross sections have been estimated in a temperature range between 100K and 450K. The results show a different behavior above 300K that has been interpreted as the starting of mobility with the formation of two thiolate moieties either linked by a gold adatom or distant enough to prevent cross section overlapping. Finally, helium diffraction patterns measured at 80K for the SAMs grown at 200K are discussed and the results support the proposed hypothesis of molecular dissociation based on the cross section data.

On the Nature of Support Effects of Metal Dioxides MO2 (M = Ti, Zr, Hf, Ce, Th) in Single-Atom Gold Catalysts: Importance of Quantum Primogenic Effect

Fundamental understanding of support effects and metal–support interaction is critical in heterogeneous catalysis. In this work, theoretical investigations are carried out to study the nature of support effects of different tetravalent-metal dioxides of MO2 (M = Ti, Zr, Ce, Hf, Th) in single-atom gold catalysts using density functional theory with on-site Coulomb interactions (DFT+U). The properties of gold adatom on the stoichiometric (MO2) and reduced (MO2–x) surfaces as well as CO adsorption on Au1/MO2 and Au1/MO2–x have been investigated systematically. Our calculations indicate that the fundamental quantum primogenic effect that causes the radial contraction and low orbital energies of 3d and 4f orbitals in these MO2 oxides plays a vital role in determining the valence states and charge distribution of single-atom gold as well as the adsorption modes of CO on various MO2 supports. We find that gold atoms supported on different surfaces exhibit oxidation states from Au(−I) to Au(0) to Au(I), depending...

Adsorption and Oligomerization of 1,3-Phenylene Diisocyanide on Au(111)

The adsorption and self-assembly of 1,3-phenylene diisocyanide (1,3-PDI) are studied on Au(111) using reflection–adsorption infrared spectroscopy (RAIRS), scanning tunneling microscopy (STM), and temperature-programmed desorption (TPD) supplemented by density functional theory (DFT) calculations and the results compared with the structures formed from 1,4-PDI where it assembled to form −(Au–PDI)– oligomer chains that incorporate gold adatoms. The infrared spectra display a single isocyanide feature consistent with the isocyanide binding to gold adatoms, while DFT calculations confirm that isocyanide binding to gold adatoms is more energetically favorable than binding to the surface. STM images show that 1,3-PDI forms zigzag chains containing hairpin bends that cause the chains to double back on each other, consistent with the 120° angle between the isocyanide groups. Hexagonal structural motifs are also observed that are proposed to be due to the self-assembly of three isocyanides as well as small structu...

Formation of Ordered 4-Fluorobenzenethiol Self-Assembled Monolayers on Au(111) from Vapor Phase Deposition.

Self-assembled monolayers (SAMs) were formed by the spontaneous adsorption of 4-fluorobenzenethiol (4-FBT) on Au(111) using both solution and ambient-pressure vapor deposition methods at room temperature. The surface structure and thermal desorption properties of 4-FBT SAMs were examined by scanning tunneling microscopy (STM) and thermal desorption spectroscopy (TDS). STM imaging showed that 4-FBT SAMs formed in solution at room temperature mainly contained disordered phase with gold adatom islands, while those formed by ambient-pressure vapor deposition had well-ordered phase, which can be described as a (2 x 2√13)R45 degrees structure. In addition, thermal desorption spectroscopy (TDS) measurements showed that strong desorption peak for parent mass fragment (m/z = 128, FC6H5SH+) for 4-FBT SAMs on Au(111) was observed at 460 K, as a result of hydrogen abstract reaction of chemisorbed thiolates during desorption.

Self-Assembled Oligomeric Structures from 1,4-Benzenedithiol on Au(111) and the Formation of Conductive Linkers between Gold Nanoparticles

The adsorption and self-assembly of 1,4-benzene dithiol (1,4-BDT) is studied on a Au(111) surface using a combination of reflection–absorption infrared spectroscopy (RAIRS) and scanning tunneling microscopy (STM). 1,4-BDT is proposed to self-assemble into Au-1,4-BDT oligomer chains based on previous observations that analogous 1,4-phenylene diisocyanide (PDI) forms oligomers in which each of the isocyanides bind to gold adatoms, and since benzene thiol also adsorbs to gold via adatoms extracted from the substrate. RAIRS reveals that 1.4-BDT adsorbed on Au(111) at ∼120 K and heated, initially forms η1-thiolate species, but adsorption at room temperature results in the formation of dithiolates with the aryl ring oriented close to parallel to the surface. STM images show the formation of zigzag structures rather than linear chains because of the hybridization of the sulfur atoms, as well as hexagonal structures at lower coverages that are assembled from 1,4-BDT trimers linked by gold adatoms. It has also bee...