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 ?1 -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 been found that PDI can link between gold nanoparticles on an insulating mica substrate to form electrically conductive bridges between them. An analogous decrease in resistance is found when gold nanoparticle arrays on mica are dosed with 1,4-BDT. The sheet resistance R varies with temperature as ln(R) versus 1/?T, similar to the behavior found previously for PDI. The data are analyzed to yield an estimate of the tunneling barrier of 0.17 ? 0.3 eV.