Gold-thiolate monolayers

Malicki M, Guan Z, Ha SD, Heimel G, Barlow S, Rumi M, Kahn A, Marder SR (2009) Preparation and characterization of 4 -donor substituted stilbene-4-thiolate monolayers and their influence on the work function of gold. Langmuir 25(14) 7967-7975... 

Similar behavior of other aromatic disulfides and thiols on gold electrodes has been described based on the SERS experiments [167]. Adsorption of benzenethiol, benzenemethanethiol, p-cyanobenzenemethanethiol, diphenyl sulfide, and dibenzyl sulfide was studied on the roughened gold electrode. All these species adsorb dissociatively as the corresponding thiolates. Monolayers formed from symmetric disulfides were exactly like those formed from the corresponding thiols. These monolayers were stable in a wide potential window from -1-800 to —1000 mV (versus SCE), which was limited by the oxidation of the Au surface from the positive side and hydrogen evolution at —1000 to —1200 mV at the negative side. 

Figure 7.3 Gold-thiolate bonds encourage the adsorption of the bisthiol 5 on the surface of gold in the form of a monolayer (a). The subsequent and spontaneous formation of disulfide linkages promotes the deposition of additional molecules of 5 to produce multilayers (b).

Gold is known not having a stable oxide at its surface,20 which makes it a good candidate substrate to study self-assembled monolayers. The deposition of an alkanethiol SAM on a gold surface is done by immersion of a clean gold substrate into the solution, and a spontaneous chemisorption at the interface produces gold thiolate species (See Figure 3.1) 11,12... 

Thiol monolayers are not removed by solvents, but by sulfur-active chemicals which pass through the surface monolayers. Laser desorption mass spectrometry has shown that thiolate molecules are intact on the gold surface, but through air oxidation, some sulfonates develop. The relative stability of alkanethiol SAMs on gold to air oxidation is to be expected due to the covalent nature of the S—Au bond. Photooxidation via UV excitation of electrons in the metal surface is, however, possible and leads to sulfonate salts which have again been characterized by mass spectrometry as well as by XPS . Alkene-thiolate monolayers can best be desorbed from gold by a one-electron reductive path. Stable monolayers on gold were also obtained with benzenesulfinate. 

Motesharei, K., Myles, D. C. Multistep Synthesis on the Surface of Self-Assembled Thiolate Monolayers on Gold Probing the Mechanism of the Thiazolium-Promoted Acyloin Condensation. J. Am. Chem. Soc. 1997,119, 6674-6675. 

The mechanism of formation of gold-thiol monolayers has been intensively investigated over the last decade. A detailed record of these studies is given in a recent review. Formation of monolayers from thiols is understood best, although some information on adsorption of disulphides and sulphides is also available. XPS analyses of the sulphur 2p peak in monolayers indicate that adsorption of thiols and disulphides on gold produces thiolate species, Raman spectra of monolayers of disulphides and thiols,... 

The closest analogues of thiols, alkaneselenols R—SeH, were found to form well-packed monolayers on the Au(lll) surface. The structure of these layers was studied by X-ray diffraction. An oblique unit cell was revealed, indicating a distorted hexagonal close-packed lattice of selenol molecules . Benzeneselenol and diphenyl diselenide form identical monolayers on gold, as shown with STM microscopy. Monolayers of benzeneselenol do not have vacancy pits typical for thiolate monolayers, but show the presence of small islands of gold (20-200 A) which were absent before deposition . 

Oxidation of monolayers is greatly increased under UV irradiation. Irradiation of alka-nethiol monolayers with a mercury lamp at ca 0.16 W cm for 25 min results in 60% oxidation of the thiolate group to the sulphonate. Irradiation at 3 W cm for 1 h leads to the complete oxidation of the monolayers . The sulphonate monolayer obtained can be removed by rinsing with neat solvent or by exchange with solution of another thiol. This method, along with electrochemical desorption (vide infra), is conveniently used to remove thiolate monolayers from the gold surface . The application of these techniques to generate patterned monolayers is discussed in Section X. 

Other useful methods for the quantitative destmction of thiolate monolayers on the gold surface include metastable atom °° or kiloelectronvolt ion bombardment and electron beam damage techniques . These methods are successfully used to prepare patterned monolayers. Quantitative removal of the thiol by an electron beam requires an e-beam dose of 10-100 mCcm . Recently, X-rays were shown to damage gold-thiol monolayers . ... 

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