In many cases, adsorbates bind to gold by forming gold-adatom complexes, in particular with aryl thiols (–S-H) and isocyanides (–N≡C) on Au(111). It is found that molecules with two such functional groups bind to two gold adatoms to form oligomer chains. A combination of scanning tunneling microscopy (STM) and reflection-absorption infrared spectroscopy and first-principle density functional theory (DFT) calculations shows that 1,4-phenylene diisocyanide (PDI) forms long –(Au–PDI)– oligomers on Au(111) that are mobile, and which nucleate at defects such as step edges. DFT calculations reveal that a mobile, vertical Au–PDI adatom complex is the propagating monomer. It oligomerizes by the gold atom in the Au–PDI adatom complex binding to the terminal isocyanide group of a tilted PDI molecule located on a step edge. Similar self-assembly is found for 1,4-benzene dithiol (BDT) on Au(111), which forms zig-zag-chains due to the sulfur rehybridization, but no oligomerization is found with 1,3-BDT since the second thiol group can easily access the surface to form a relatively immobile η2-adorbed species rather than an adatom complex, and thus does not oligomerize. It is found that PDI can also link between gold nanoparticles grown on an insulating substrate, which forms electrically conductive linkages between them. The conductivity is found to vary with sample temperature as ln(R) is proportional to 1/√T, rather than an Arrhenius dependence. The results are modeled using the Abeles model for thermally assisted tunneling through granular materials to yield a tunneling barrier of ~0.12 eV. Similar effects are found for gold nanoparticles linked by BDT, where the barrier increases slightly to ~0.17 eV.

Chemical self-assembly strategies for metal-organic surface structures

253rd National Meeting (2017)

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