Surface
Structure, Adsorption, and Thermal Desorption
Behaviors of Methaneselenolate Monolayers on Au(111) from Dimethyl
Diselenides
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Abstract
To understand the effect of headgroups
(i.e., sulfur and selenium)
on surface structure, adsorption states, and thermal desorption behaviors
of self-assembled monolayers (SAMs) on Au(111), we examined methanethiolate
(CH<sub>3</sub>–S, MS) and metheneselenolate (CH<sub>3</sub>–Se, MSe) monolayers formed from dimethyl disulfide (DMDS)
and dimethyl diselenide (DMDSe) molecules by ambient vapor-phase deposition.
Scanning tunneling microscopy imaging revealed that DMDS molecules
on Au(111) after a 1 h deposition form MS monolayers containing a
disordered phase and an ordered row phase with an inter-row spacing
of 1.51 nm, whereas DMDSe molecules form long-range-ordered MSe monolayers
with a (√3 × 3√3)<i>R</i>30° structure.
X-ray photoelectron spectroscopy measurements showed that MS or MSe
monolayers chemisorbed on Au(111) were formed via S–S bond
cleavage of DMDS or Se–Se bond cleavage of DMDSe. On the other
hand, we monitored three main desorption fragments for MS and MSe
monolayers using TDS monomers (CH<sub>3</sub>S<sup>+</sup>, CH<sub>3</sub>Se<sup>+</sup>), parent mass species (CH<sub>3</sub>SH<sup>+</sup>, CH<sub>3</sub>SeH<sup>+</sup>), and dimers (CH<sub>3</sub>S–SCH<sub>3</sub><sup>+</sup>, CH<sub>3</sub>Se–SeCH<sub>3</sub><sup>+</sup>). Interestingly, we found that thermal desorption
behaviors of MSe monolayers were markedly different from those of
MS monolayers. All desorption peaks for MSe monolayers were observed
at a higher temperature compared with MS monolayers, suggesting that
the adsorption affinity of selenium atoms for the Au(111) surface
is stronger than that of sulfur atoms. In addition, the desorption
intensity of dimer fragments for MSe monolayers was much lower than
for MS monolayers, indicating that selenolate SAMs on Au(111) did
not undergo their dimerization efficiently during thermal heating
compared with thiolate SAMs. Our results provide new insight into
understanding the surface structure and thermal desorption behavior
of MSe monolayers on Au(111) surface by comparing those of MS monolayers