Formation Mechanism of Metal–Molecule–Metal Junctions: Molecule-Assisted Migration on Metal Defects

Abstract

Activation energies E a measured from molecular exchange experiments are combined with atomic-scale calculations to describe the migration of bare Au atoms and Au-alkanethiolate species on gold nanoparticle surfaces during ligand exchange for the creation of metal-molecule-metal junctions. It is well known that Au atoms and alkanethiol-Au species can diffuse on gold with sub-1 eV barriers, and surface restructuring is crucial for self-assembled monolayer (SAM) formation, for interlinking nanoparticles and in contacting nanoparticles to electrodes. In the present work, computer simulations reveal that naturally occurring ridges and adlayers on Au(111) are etched and resculpted by migration of alkanethiolate-Au species towards high coordination kink sites at surface step edges. The calculated energy barrier E b for diffusion via step edges is 0.4-0.7 eV, close to the experimentally-measured E a of 0.5-0.7 eV. By contrast, putative migration from isolated nine-coordinated terrace sites and complete Au unbinding from the surface incur significantly larger barriers of +1 and +3 eV, respectively. Molecular van der Waals's packing energies are calculated to have negligible effect on migration barriers for typically-used molecules (length < 2.5nm), indicating that migration inside SAMs does not change the size of the migration barrier. We use the computational methodology to propose a means of creating Au nanoparticle arrays via selective replacement of citrate protector molecules by thiocyanate linker molecules on surface step sites. This work also outlines the possibility of using Au/Pt alloys as possible candidates for creation of contacts that are well-formed and long-lived, due to the superior stability of Pt interfaces against atomic migration.

N-Heterocyclic Carbenes for the Self-Assembly of Thin and Highly Insulating Monolayers with High Quality and Stability

“…The common orientation of the rotational domains and reoriented substrate step edges indicates that BIM/Au film formation is associated with efficient surface diffusion of molecule–substrate atom complexes and that intermolecular interactions make an important contribution to the overall energetics of film formation. Different mechanisms of such molecule–metal complexes diffusion were analyzed earlier for thiols, as recently reviewed . Following recent DFT calculations for NHC SAMs, the high mobility of molecule–metal complexes needed for the reorientation of substrate steps, the elimination of typical substrate defects (monatomic depressions and islands), and the formation of large domains at room temperature indicate the bonding of BIM molecules to the Au substrate via an Au adatom (Figure c), which allows for efficient mobility via ballbot-type motion according to these calculations.…”

Citrate Coordination and Bridging of Gold Nanoparticles: The Role of Gold Adatoms in AuNP Aging

“…This strongly suggests that c 2 t 2 binding can only occur upon reconstruction of the Au surface through adatoms, which possess similar calculated and measured activation energies on gold. 19,20 The emergence of the citrate-adatom peaks after aging shows that CB [5] molecules are no longer able to displace the citrate from the surface due to its greater (double) bidentate binding affinity. Eventually, this leads to loss of aggregation and changes in other AuNP chemistries.…”

Negative‐Imaging of Citrate Layers on Gold Nanoparticles by Ligand‐Templated Metal Deposition: Revealing Surface Heterogeneity

“…In the presence of water molecules, Na + counterions are solvated rather than adsorbed on the surface, while Au(I) ions remain adsorbed on the surface because of their chemisorption nature or aurophilicity . Hence, the formation of surface Na + ···COO ‒ ionic bonding will depend on the presence of the positively charged Au adatom. SERS studies suggest the existence of surface Au(I) species on the AuNP surface because of the charge‐transfer reaction probed .…”