Gold Adatoms Research Articles

Structure and Defect Identification at Self-Assembled Islands of CO2 Using Scanning Probe Microscopy.

Understanding how carbon dioxide (CO2) behaves and interacts with surfaces is paramount for the development of sensors and materials to attempt CO2 mitigation and catalysis. Here, we combine simultaneous atomic force microscopy (AFM) and scanning tunneling microscopy (STM) using CO-functionalized probes with density functional theory (DFT)-based simulations to gain fundamental insight into the behavior of physisorbed CO2 molecules on a gold(111) surface that also contains one-dimensional metal-organic chains formed by 1,4-phenylene diisocyanide (PDI) bridged by gold (Au) adatoms. We resolve the structure of self-assembled CO2 islands, both confined between the PDI-Au chains as well as free-standing on the surface and reveal a chiral arrangement of CO2 molecules in a windmill-like structure that encloses a standing-up CO2 molecule and other foreign species existing at the surface. We identify these species by the comparison of height-dependent AFM and STM imaging with DFT-calculated images and clarify the origin of the kagome tiling exhibited by this surface system. Our results show the complementarity of AFM and STM using functionalized probes and their potential, when combined with DFT, to explore greenhouse gas molecules at surface-supported model systems.

Self-assembled oligomeric structures of an asymmetric molecular linker; 4-isocyanophenyl disulfide on Au(111)

Para-substituted benzenes, such as 1,4-benzene dithiol and 1,4-phenyl diisocyanide, have been observed to oligomerize on the Au(111) surface by incorporating gold adatoms extracted from the substrate. This work investigates if oligomerization occurs for an analogous but asymmetric linker, 4-isocyanophenyl disulfide (ICPD) on Au(111). This molecule is comprised of both disulfide and isocyanide terminal groups attached to the phenyl ring. The resulting surface structures formed on Au(111) following exposure to ICPD are studied using scanning tunneling microscopy (STM). 1,4-isocyanophenyl thiolate (ICPT), formed through scission of ICPD’s disulfide bond, was also found to oligomerize on the surface, and potential oligomer structures and binding geometries are proposed with the aid of density functional theory (DFT) calculations, along with simulated STM images of the resulting structures. It is observed in this work that ICPT forms oligomeric structures that cover large sections of the substrate and appear to create etch pits resulting from gold atom extraction. Numerous potential binding geometries are investigated based on the distances between substrate gold atom adsorption sites compared to the monomer length. Selected structural candidates were optimized using DFT and were used to generate simulated STM images using the Tersoff–Hamann method to compare with experiment. It has been shown previously that the isocyanide- and thiol-connected oligomers conduct electrons, suggesting the possibility that the asymmetric oligomers found here might form the basis for fabricating molecular diodes.

Reduced band gap in 2D-C3N4 nanosheets decorated with silver and gold adatoms

Abstract Two-dimensional materials have attracted significant attention due to their unique electronic properties. In this study, we investigate the effect of silver and gold adatoms on the band gap of graphitic carbon nitride (g-C3N4) nanosheets (2D-C3N4) using density functional theory. We examine several decorations with one-sided and two-sided combinations and find that the adsorption of gold and silver adatoms reduces the band gap of 2D-C3N4, which is consistent with experimental findings. Furthermore, we observe that all one-sided configurations are buckled. Our study provides insights into the electronic properties of 2D-C3N4 nanosheets and their potential applications in optoelectronics.

Self-Assembled Monolayers of N-Heterocyclic Olefins on Au(111).

Self-assembled monolayers (SAMs) of N-heterocyclic olefins (NHOs) have been prepared on Au(111) and their thermal stability, adsorption geometry, and molecular order were characterized by X-ray photoelectron spectroscopy, polarized X-ray absorption spectroscopy, scanning tunneling microscopy (STM), and density functional theory (DFT) calculations. The strong σ-bond character of NHO anchoring to Au induced high geometrical flexibility that enabled a flat-lying adsorption geometry via coordination to a gold adatom. The flat-lying adsorption geometry was utilized to further increase the surface interaction of the NHO monolayer by backbone functionalization with methyl groups that induced high thermal stability and a large impact on work-function values, which outperformed that of N-heterocyclic carbenes. STM measurements, supported by DFT modeling, identified that the NHOs were self-assembled in dimers, trimers, and tetramers constructed of two, three, and four complexes of NHO-Au-adatom. This self-assembly pattern was correlated to strong NHO-Au interactions and steric hindrance between adsorbates, demonstrating the crucial influence of the carbon-metal σ-bond on monolayer properties.

Self‐Assembled Monolayers of N‐Heterocyclic Olefins on Au(111)

AbstractSelf‐assembled monolayers (SAMs) of N‐heterocyclic olefins (NHOs) have been prepared on Au(111) and their thermal stability, adsorption geometry, and molecular order were characterized by X‐ray photoelectron spectroscopy, polarized X‐ray absorption spectroscopy, scanning tunneling microscopy (STM), and density functional theory (DFT) calculations. The strong σ‐bond character of NHO anchoring to Au induced high geometrical flexibility that enabled a flat‐lying adsorption geometry via coordination to a gold adatom. The flat‐lying adsorption geometry was utilized to further increase the surface interaction of the NHO monolayer by backbone functionalization with methyl groups that induced high thermal stability and a large impact on work‐function values, which outperformed that of N‐heterocyclic carbenes. STM measurements, supported by DFT modeling, identified that the NHOs were self‐assembled in dimers, trimers, and tetramers constructed of two, three, and four complexes of NHO−Au‐adatom. This self‐assembly pattern was correlated to strong NHO−Au interactions and steric hindrance between adsorbates, demonstrating the crucial influence of the carbon‐metal σ‐bond on monolayer properties.

Stability of Single Gold Atoms on Defective and Doped Diamond Surfaces.

Polycrystalline boron-doped diamond (BDD) is widely used as a working electrode material in electrochemistry, and its properties, such as its stability, make it an appealing support material for nanostructures in electrocatalytic applications. Recent experiments have shown that electrodeposition can lead to the creation of stable small nanoclusters and even single gold adatoms on the BDD surfaces. We investigate the adsorption energy and kinetic stability of single gold atoms adsorbed onto an atomistic model of BDD surfaces by using density functional theory. The surface model is constructed using hybrid quantum mechanics/molecular mechanics embedding techniques and is based on an oxygen-terminated diamond (110) surface. We use the hybrid quantum mechanics/molecular mechanics method to assess the ability of different density functional approximations to predict the adsorption structure, energy, and barrier for diffusion on pristine and defective surfaces. We find that surface defects (vacancies and surface dopants) strongly anchor adatoms on vacancy sites. We further investigated the thermal stability of gold adatoms, which reveals high barriers associated with lateral diffusion away from the vacancy site. The result provides an explanation for the high stability of experimentally imaged single gold adatoms on BDD and a starting point to investigate the early stages of nucleation during metal surface deposition.

Does a Blatter Radical Retain Its Open-Shell Character When Incorporated into Gold–Molecule–Gold Junctions?

One of the key issues for the use of organic radicals in high-performance molecular electronic, spintronic and thermoelectric devices is the retention, at room temperature, of their open-shell character when they are in contact with the metal electrodes of a junction. By means of first-principles quantum transport calculations, we have investigated the stability and the electronic transport properties of single-molecule junctions incorporating a Blatter radical (BR) with thiomethyl linker groups. Our calculations suggest that the BR can unequivocally retain its open-shell nature even when bound to gold electrodes through undercoordinated gold adatoms. Such a state cannot be destroyed by the surrounding solvent molecules and the applied bias. The calculated low-bias conductance values of junctions with BR, an oxidized BR derivative, and a closed-shell stilbene molecule are in quantitative agreement with the available experimental measurements. The small conductance of the BR can be associated with the localization of its π-type singly occupied and singly unoccupied molecular orbitals (SOMO and SUMO), leading to an asymmetrically weak electronic coupling to the two gold electrodes, even though these two orbitals lie very close to the Fermi energy. Furthermore, the nonmonotonic modulation of the junction conductance controlled by a charge gate can be used in experiments to further verify the presence of the unpaired electron. This proof may be realized in practice with an electrostatic solid-state or an electrochemical gate. Our findings deepen the current understanding of the radical–metal interfacial properties and facilitate the design of future radical-based multifunctional molecular devices.

Robust Ferrimagnetism in Quasi-Freestanding Graphene

The influence of the size of dislocation loops on sublattice ferrimagnetism in graphene is studied. It is shown that graphene and the underlying gold layer with Au/Co dislocation loops of various sizes are characterized by ferrimagnetic ordering within atomic layers. Additional gold adatoms under graphene enhance the induced Rashba spin–orbit coupling in graphene but do not destroy the ferrimagnetic order in graphene. Since gold clusters can remain during the intercalation of gold on the surface of graphene and under graphene, the number and size of clusters after intercalation can be controlled to enhance the induced Rashba interaction and to obtain a topological phase in graphene.

Robust Ferrimagnetism in Quasi-Freestanding Graphene

The influence of the size of dislocation loops on sublattice ferrimagnetism in graphene is studied. It is shown that graphene and the underlying gold layer with Au/Co dislocation loops of various sizes are characterized by ferrimagnetic ordering within atomic layers. Additional gold adatoms under graphene enhance the induced Rashba spin–orbit coupling in graphene but do not destroy the ferrimagnetic order in graphene. Since gold clusters can remain during the intercalation of gold on the surface of graphene and under graphene, the number and size of clusters after intercalation can be controlled to enhance the induced Rashba interaction and to obtain a topological phase in graphene.

Structural Phase Transition of Quinone-Containing PAH Derivatives on Au(111) at High Coverage

Delicate control over structural phase transition provides advanced approaches for the fabrication of the desired well-ordered nanoarchitectures on surfaces. The participation of intrinsic metal adatoms in pure organic systems can facilitate the structural phase transition by direct capture of surface metal adatoms and forming metal–organic bonds. However, most of the situations occur at low coverage; such structural phase transition at a higher molecular concentration is limited to some extent due to the poor migration ability. Thus, high-concentration phase transition needs to be explored, which might be significant for the design and exploitation of large-scale ordered metal–organic-related nanomaterials. Herein, we report the phase transition of pyrene-4,5,9,10-tetraone (PT) molecules at high coverage (∼1 monolayer (ML)) on Au(111) from hydrogen-bonded row-like nanostructures to metal–organic honeycomb networks by coordinating with surface Au adatoms as demonstrated by scanning tunneling microscopy (STM). Combined with density functional theory (DFT) calculations, we demonstrate that identical molecular density (or unit cells) of two nanostructures should be the key, which makes it possible to realize phase transition possibly by in situ rotation and coordinating with the gold adatoms. Also, the phase transition causes the modulation of electronic properties from semiconductive ones to metallic ones.

Electrocatalytic Activities of a Platinum Nanostructured Electrode Modified by Gold Adatom toward Methanol and Glycerol Electrooxidation in Acid and Alkaline Media.

For practical applications such as fuel cells, it is important to exploit electrocatalysis with high activity for methanol and glycerol oxidation. A platinum nanostructured electrode (PtNPs) is modified by gold adatoms and is created by application of a square wave potential regime to a tantalum surface electrode. In nanostructured platinum, the structure and the surface properties are characterized by scanning electron microscopy (SEM), X-ray powder diffraction (XRD) and cyclic voltammetry (CV). In acid and alkaline media, the CV and Chronoamperometric (CA) are studied to investigate the catalytic activity of the PtNPs nanoparticles for the electrooxidation of methanol and glycerol. The prepared nanostructured platinum on a tantalum electrode was allowed to balance an open circuit with a 1.0×10-3 M solution containing an Au ion. Consequently, the proximity of the irreversibly adsorbed Au-adatoms on the already described Pt-nanostructured electrode. In acidic and alkaline solutions, the electrocatalytically activities toward methanol and glycerol oxidation were evaluated and is found to strongly on the surface of the gold-modified PtNPs. The PtNPs modified by Au electrode system used direct methanol fuel cell (DMFC) and direct glycerol fuel cell (DGFC). The DMFC and DGFC are much higher than in acid output in alkaline. Comparison of the i-E curves of nanostructure platinum electrode with that of a platinum nanostructure electrode modified by Au under similar conditions for the letter, the charge under the peak (i-E curve) in the oxidation region was higher. Furthermore, rough chronoamperometric measurements confirmed the results. The results of showed that the electrocatalytic properties of the nanostructured prepared surface were enhanced by the inclusion of gold adatoms with a variable extent of advancement. The current peak (Ip) and the current chronoamperometric (ICA) of glycerol oxidation on the PtNPs electrode modified by Au in acid media (130 mA/cm2, 47 µA/cm2) were higher than those of the bare PtNPs electrode and in alkaline media (171 mA/cm2, 66 µA/cm2). The stronger catalytic behavior in alkaline media of the Au-PtNP electrode indicates its promising use in alkaline direct alcohol cells.

The Role of Metal Adatoms in a Surface‐Assisted Cyclodehydrogenation Reaction on a Gold Surface

AbstractDehydrogenation reactions are key steps in many metal‐catalyzed chemical processes and in the on‐surface synthesis of atomically precise nanomaterials. The principal role of the metal substrate in these reactions is undisputed, but the role of metal adatoms remains, to a large extent, unanswered, particularly on gold substrates. Here, we discuss their importance by studying the surface‐assisted cyclodehydrogenation on Au(111) as an ideal model case. We choose a polymer theoretically predicted to give one of two cyclization products depending on the presence or absence of gold adatoms. Scanning probe microscopy experiments observe only the product associated with adatoms. We challenge the prevalent understanding of surface‐assisted cyclodehydrogenation, unveiling the catalytic role of adatoms and their effect on regioselectivity. The study adds new perspectives to the understanding of metal catalysis and the design of on‐surface synthesis protocols for novel carbon nanomaterials.

The Role of Metal Adatoms in a Surface-Assisted Cyclodehydrogenation Reaction on a Gold Surface.

Dehydrogenation reactions are key steps in many metal-catalyzed chemical processes and in the on-surface synthesis of atomically precise nanomaterials. The principal role of the metal substrate in these reactions is undisputed, but the role of metal adatoms remains, to a large extent, unanswered, particularly on gold substrates. Here, we discuss their importance by studying the surface-assisted cyclodehydrogenation on Au(111) as an ideal model case. We choose a polymer theoretically predicted to give one of two cyclization products depending on the presence or absence of gold adatoms. Scanning probe microscopy experiments observe only the product associated with adatoms. We challenge the prevalent understanding of surface-assisted cyclodehydrogenation, unveiling the catalytic role of adatoms and their effect on regioselectivity. The study adds new perspectives to the understanding of metal catalysis and the design of on-surface synthesis protocols for novel carbon nanomaterials.

DNA‐Encoded Gold‐Gold Wettability for Programmable Plasmonic Engineering

AbstractControlling the deposition and diffusion of adsorbed atoms (adatoms) on the surface of a solid material is vital for engineering the shape and function of nanocrystals. Here, we report the use of single‐stranded DNA (oligo‐adenine, oligo‐A) to encode the wettability of gold seeds by homogeneous gold adatoms to synthesize highly tunable plasmonic nanostructures. We find that the oligo‐A attachment transforms the nanocrystal growth mode from the classical Frank‐van der Merwe to the Volmer‐Weber island growth. Finely tuning the oligo‐A density can continuously change the gold‐gold contact angle (θ) from 35.1±3.6° to 125.3±8.0°. We further demonstrate the versatility of this strategy for engineering nanoparticles with different curvature and dimensions. With this unconventional growth mode, we synthesize a sub‐nanometer plasmonic cavity with a geometrical singularity when θ>90°. Superfocusing of light in this nanocavity produces a near‐infrared intraparticle plasmonic coupling, which paves the way to surface engineering of single‐particle plasmonic devices.

DNA-Encoded Gold-Gold Wettability for Programmable Plasmonic Engineering.

Controlling the deposition and diffusion of adsorbed atoms (adatoms) on the surface of a solid material is vital for engineering the shape and function of nanocrystals. Here, we report the use of single-stranded DNA (oligo-adenine, oligo-A) to encode the wettability of gold seeds by homogeneous gold adatoms to synthesize highly tunable plasmonic nanostructures. We find that the oligo-A attachment transforms the nanocrystal growth mode from the classical Frank-van der Merwe to the Volmer-Weber island growth. Finely tuning the oligo-A density can continuously change the gold-gold contact angle (θ) from 35.1±3.6° to 125.3±8.0°. We further demonstrate the versatility of this strategy for engineering nanoparticles with different curvature and dimensions. With this unconventional growth mode, we synthesize a sub-nanometer plasmonic cavity with a geometrical singularity when θ>90°. Superfocusing of light in this nanocavity produces a near-infrared intraparticle plasmonic coupling, which paves the way to surface engineering of single-particle plasmonic devices.

Dynamic bonding influenced by the proximity of adatoms to one atom high step edges

Low-temperature scanning tunneling microscopy is used here to study dynamic bonding of gold atoms on surfaces under low coordination conditions. In the experiments, using an atomically-sharp gold tip, a gold adatom is deposited onto a gold surface with atomic precision either on the first hollow site near a step edge, or far away from it. Classical molecular dynamics simulations at 4.2 K and density functional theory calculations serve to elucidate the difference in the bonding behavior between these two different placements, while also providing information on the crystalline classification of the STM tips based on their experimental performance.

Confinement-Induced Catalytic Dissociation of Hydrogen Molecules in a Scanning Tunneling Microscope.

The catalytic scission of single chemical bonds has been induced by the nanoscale confinement in a scanning tunneling microscope (STM) junction. Individual hydrogen molecules sandwiched between the STM tip and a copper substrate can be dissociated solely by the reciprocating movement of the tip. The reaction rate depends sensitively on the local molecular environment, as exemplified by the effects of a nearby carbon monoxide molecule or a gold adatom. Detailed mechanisms and the nature of the transition states are revealed by density functional theory (DFT) calculations. This work provides insights into chemical reactions at the atomic scale induced by localized confinement applied by the STM tip. Furthermore, a single diatomic molecule can act as a molecular catalyst to enhance the reaction rate on a surface.

Intrinsic luminescence blinking from plasmonic nanojunctions

Plasmonic nanojunctions, consisting of adjacent metal structures with nanometre gaps, can support localised plasmon resonances that boost light matter interactions and concentrate electromagnetic fields at the nanoscale. In this regime, the optical response of the system is governed by poorly understood dynamical phenomena at the frontier between the bulk, molecular and atomic scales. Here, we report ubiquitous spectral fluctuations in the intrinsic light emission from photo-excited gold nanojunctions, which we attribute to the light-induced formation of domain boundaries and quantum-confined emitters inside the noble metal. Our data suggest that photoexcited carriers and gold adatom - molecule interactions play key roles in triggering luminescence blinking. Surprisingly, this internal restructuring of the metal has no measurable impact on the Raman signal and scattering spectrum of the plasmonic cavity. Our findings demonstrate that metal luminescence offers a valuable proxy to investigate atomic fluctuations in plasmonic cavities, complementary to other optical and electrical techniques.

Unveiling Adatoms in On-Surface Reactions: Combining Scanning Probe Microscopy with van’t Hoff Plots

Scanning probe microscopy has become an essential tool to not only study pristine surfaces but also on-surface reactions and molecular self-assembly. Nonetheless, due to inherent limitations, some atoms or (parts of) molecules are either not imaged or cannot be unambiguously identified. Herein, we discuss the arrangement of two different nonplanar molecular assemblies of para-hexaphenyl-dicarbonitrile (Ph6(CN)2) on Au(111) based on a combined theoretical and experimental approach. For deposition of Ph6(CN)2 on Au(111) kept at room temperature, a rhombic nanoporous network stabilized by a combination of hydrogen bonding and antiparallel dipolar coupling is formed. Annealing at 575 K resulted in an irreversible thermal transformation into a hexagonal nanoporous network stabilized by native gold adatoms. However, the Au adatoms could neither be unequivocally identified by scanning tunneling microscopy nor by noncontact atomic force microscopy. By combining van’t Hoff plots derived from our scanning probe images with our density functional theory calculations, we were able to confirm the presence of the elusive Au adatoms in the hexagonal molecular network.

Molecular oxygen dissociation on tin carbide monolayers with gold adatoms

In this work, the interactions between oxygen molecule O2 with pristine and gold-decorated tin carbide (SnC) monolayers were investigated by using density functional calculations. The results indicate that O2 is adsorbed, with an energy of 0.95 eV, on the pristine SnC nanosheet in a bond-like configuration with each oxygen close to a carbon and a tin atom, respectively. There is a large electronic charge transfer from the SnC monolayer to the O2 molecule, which enlarges the molecule internal bond, indicating an activation towards a peroxo-like state. Also, a gold adatom can strongly bind to the SnC monolayer over a C atom. Furthermore, when O2 interacts with this Au adatom, it is spontaneously dissociated with an energy gain of 1.84 eV and where the final adsorption configuration consists of each oxygen on the top of different tin atoms but sharing the gold one.