In situ studies of thiol self-assembly on gold from solution using atomic force microscopy

Abstract

The kinetics and mechanism for the solution-phase adsorption of n-alkanethiols onto gold to form self-assembled monolayers (SAMs) have been monitored in situ using atomic force microscopy (AFM). Time-dependent AFM images reveal detailed structural information about the adsorbed layer during its growth. In 2-butanol, CH3(CH2)17SH molecules initially adsorb on gold with the molecular axis of their hydrocarbon chains oriented parallel to the surface. As the surface coverage increases to near saturation, a two-dimensional phase transition occurs and produces islands composed of molecules with their hydrocarbon axis oriented ∼30° from the surface normal. Continued exposure to the thiol solution results in a greater number of these islands and the growth of these nuclei until a SAM is formed with a commensurate (∛×∛)R30° structure. The growth of the lying-down phase follows a first-order Langmuir adsorption isotherm, while the phase transition is best described by a second-order reaction. The kinetics of the self-assembly process also depends on the chain length of the alkanethiol and the cleanness of the gold surface. Longer-chained thiols, such as CH3(CH2)17O(CH2)19SH, formed complete SAMs more rapidly than did shorter-chained thiols, such as CH3(CH2)17SH. The physisorbed, lying-down phase for CH3(CH2)17O(CH2)19SH was less homogeneous and its two-dimensional phase transition was more complicated than for CH3(CH2)17SH and CH3(CH2)21SH, as the CH3(CH2)17O(CH2)19SH molecules adopt multiple conformations. Of these, the two dominant ones are an all-trans, and another where the hydrocarbon chain adopts an all-trans conformation except for a gauche bond on both sides of the ether unit. These conformers coexist on the surface during the initial adsorption and its transition to the standing-up phase, but change to the all-trans structure in the complete SAM.

Diffusion of self-assembled monolayers of thiols on the gold surfaces covered with polydimethylsiloxane stamps

“…The results have revealed that the speed of EG 6 (longer chain thiols compared to EG 3 ) diffusion to form SAMs on gold surfaces under PDMS is slower than that of EG 3 . These results are different from that of thiols to form SAMs on the interface between gold and gas, in which longer chain length thiols covered the gold surface more rapidly than shorter chain length thiols [24]. In Fig.…”

Quantifying thiol–gold interactions towards the efficient strength control

“…As discussed above, the coordinate bond could actually form between the -SH group and gold surface in the initial stage, followed by the dissociation of S-H bond that results in a thiyl radical, which would finally form a gold-thiolate covalent bond at the gold-sulfur interface [45][46][47][48][49][50][51][54][55][56][57][58] . These findings indicate that the formation of Au-S covalent bonds needs a certain time to complete.…”

“…The formation of a Au-S covalent bond is a complicated process, involving the dissociation of the S-H bond and followed by the formation of the Au-S covalent bond [45][46][47][48][49][50][51] . The dissociated hydrogen atoms could either adsorb on gold surfaces or release from the surface in the form of H 2 .…”

“…The adsorption of thiol onto gold surface starts with physisorption, during which the H atom favourably remains on the S atom, followed by a chemisorption including the breaking of S-H bond and the formation of a Au-S bond as a result of the deprotonation of thiols and formation of thiyl radicals [45][46][47][48][49][50][51] . The existence of the dissociated hydrogen could affect the pH value of the microenvironment around the Au-S binding sites.…”

n-alkyl thiol head-group interactions with the Au(111) surface

“…Their simplicity, highly ordered structures, and chemical stability make these systems ideal for study with a variety of techniques including atomic force microscopy, 5,6 infrared spectroscopy, 7,8 high-resolution electron-energy-loss spectroscopy, 9 grazing X-ray diffraction, 10 scanning tunneling microscopy (STM), 11-17 scanning probe microscopy, 18 lowenergy electron diffraction (LEED), [19][20][21] He atom diffraction, 22,23 and theory. 25-31 Despite 1…”