Adsorption Kinetics of Hydrogen Sulfide and Thiols on GaAs (001) Surfaces in a Vacuum

Abstract

Adsorption mechanisms of hydrogen sulfide and alkanethiols on GaAs (001) surfaces prepared in a vacuum were studied using first principles calculations. The need for a physisorbed precursor was confirmed based on energetic arguments and molecular dynamics simulations of the transition from a physisorption to a chemisorption state. The preference for S-Ga bond formation was found, resulting from a weak non-dissociative chemisorption state formed by the overlap of a sulfur lone pair orbital with an empty Ga dangling bond. Physisorption energies and the height of the transition barrier from physisorption to chemisorption on GaAs were found to be very similar to those on gold, while the main difference from gold was that hydrogen remains on the GaAs surface upon S-H cleavage. Obtained results allowed for a more advanced interpretation of experimental data concerning H 2 S and thiol adsorption on GaAs.

Adsorption and thermal reaction of short-chain alkanethiols on GaAs(100)

“…The temperature for CH 3 S(I) formed from CH 3 SSCH 3 to react is higher than that for CH 3 S(I) obtained from CH 3 SH. The latter dissociates to generate surface hydrogen that induces a recombinative reaction with CH 3 S(I) to desorb molecular CH 3 SH at a temperature as low as 320 K. Based on the previous theoretical calculations, a dialkyl disulfide (RS-SR) was suggested to be used in the fabrication of thermally stable thiolate SAM because it can dissociate to produce thiolate without generating surface hydrogen [17]. Consistent with this proposition, our experimental results show that the deposition of an alkanethiolate monolayer incorporating alkyl disulfide (RSS-) on the adsorption of dialkyl disulfide exhibits greater stability than that produced on adsorption of alkanethiol.…”

“…These changes account for the loss of a fraction of chemisorbed thiolate from the surface. A theoretical calculation also suggested that surface hydrogen can destabilize the thiolates on GaAs via the desorption of molecular thiol [17]. Our TPD and XPS data showed the CH 3 S(I) formed from the adsorption of CH 3 SH recombines with surface hydrogen to desorb CH 3 SH at 350 K. However, the adsorption of CH 3 SSCH 3 yields the CH 3 S without the formation of surface hydrogen, and the resulting CH 3 S can sustain the thermal annealing up to 470 K. The comparisons of the adsorption and thermal reactions of CH 3 SH and CH 3 SSCH 3 provide experimental evidence that the alkanethiolate on the Ga site exhibits lower stability in the presence of surface hydrogen.…”

“…Previous authors have reported that the adsorption of alkanethiols on GaAs results in the formation of thiolates through the dissociation of the S-H bond. The thiolate binds to the GaAs surface via As-S, Ga-S or both Ga-S and As-S [16][17][18]. The nature of binding between an alkanethiolate and a GaAs surface dictates the thermal stability and physical properties of a SAM.…”

“…According to theoretical calculations, surface hydrogen was expected to induce a recombinative reaction of thiolate to desorb molecular thiol from the surface [17]. The stability of the deposited thiolate layer was improved by replacing alkanethiol with dialkyl disulfide that forms two thiolates by breaking the S-S bond, without the formation of surface hydrogen.…”

Observation of surface enhanced IR absorption coefficient in alkanethiol based self-assembled monolayers on GaAs(001)

“…Long-chain alkanethiol selfassembled monolayers ͑SAMs͒ on GaAs͑001͒ have been studied in this context by a number of authors, both from the perspective of fundamental material science [1][2][3][4][5][6][7][8][9][10] and as a potential route to applications including surface passivation, 11 metal-molecule-semiconductor junctions, 12,13 the assembly of DNA hybridization probes, 14 and the immobilization of functional proteins such as avidin. 15 In these studies, Fourier transform infrared ͑FTIR͒ spectroscopy is frequently employed to investigate important SAM structural parameters such as molecular orientation 16 and the fraction of conformational defects.…”