Direct Adsorption and Molecular Self-Assembly of Octylthioacetates on Au(111) in the Vapor Phase

Abstract

Self-assembled monolayers (SAMs) are a very promising construct for monomolecular thin films with various practical applications because they are very amenable to surface modification and functionalization of metal surfaces.1-6 Alkanethiol SAMs on gold surfaces have been thoroughly studied by a number of research groups, and their structural properties have been well characterized. Organic thiols are prone to easily oxidize to disulfides or other oxidized species that can affect the formation and structure of SAMs. 7,8 The presence of disulfides or oxidized compounds in thiol samples often yields poorly ordered SAMs containing a high defect density and disordered phases. An approach that minimizes undesirable thiol oxidation is the use of a protected thiol that is deprotected in situ before or during SAM formation. The protection of thiol groups can be readily accomplished by acetylation. SAMs derived from acetyl protected thiols (thioacetates) on gold have usually been formed via an in situ deprotection process of the acetyl group in strong acidic or basic solutions.9-11 Other deprotection techniques have also been developed that use organic compounds such as triethylamine, 12 tetrabutylammonium cyanide, 13 and 1,8-diazabicyclo[5.4.0]undec-7-ene, 8 and organic SAMs with a high degree of structural order have been successfully constructed in solutions containing these deprotection reagents.In contrast, there have been only limited reports of investigations into the direct adsorption of thioacetyl-protected organic molecules on gold in solution without any deprotection reagents. 8,9,[13][14][15] It has been observed that the direct adsorption of alkanethioacetates (ATAs) on gold in solution generates loosely packed and disordered SAMs compared to the corresponding alkanethiols (ATs). 8,13,15 A kinetics study showed that the adsorption rate is slower for ATAs in pure solvents than for ATs. 14 Fully covered monolayers from aromatic thioacetates were formed after a longer immersion period of several days or in a concentrated 30 mM solution.9 In general, organic thiol SAMs can be formed by both solution and vapor phase deposition, and both deposition methods yield identical SAM structures containing the hexagonal (√3 × √3)R30 o or c(4 × 2) structure at saturation coverage. 1,3,16 To the best of our knowledge, however, there have been no reports describing organic thioacetate SAMs on gold grown by vapor deposition.In this study, to understand the formation of thioacetate SAMs on gold depending on deposition method, we directly compared the surface structures of the acetyl-protected octylthioacetate (OTA, CH 3 (CH 2 ) 7 SCOCH 3 ), SAMs on Au (111) The STM images in Figure 1 clearly show considerable structural differences between OTA SAMs on Au(111) grown from solution and vapor deposition at 50 o C for 24 h. OTA SAMs grown from solution were mainly composed of disordered phases containing many vacancy islands (VIs, dark holes), as shown in Figure 1a. The surface structures of these OTA SAMs were nearly identical to ...

Formation and Surface Structures of Highly Ordered Self-Assembled Monolayers of Alkyl Selenocyanates on Au(111) via Ambient-Pressure Vapor Deposition

“…Organic thiols are the best model compounds because they spontaneously form closely packed SAMs with a molecularly controllable thickness on gold surfaces via the formation of S–Au chemical bonds. ,,, SAMs of alkanethiols with a normal alkyl chain at saturation coverage on gold have particularly well-ordered domains with a hexagonally packed (√3 × √3)­R30° or c (4 × 2) structure. ,,,,, However, a drawback of using thiols for SAM preparation is that they can readily be oxidized to other oxidized species in the solution, resulting in undesirable structural defects and inhomogeneous interface structures in SAMs. , Therefore, new molecular assemblies on metals have been developed using S-containing alternatives having high chemical stability in solution or air such as organic thioacetates, ,− thiosulfates, thiocyanates, − and sulfenyl chlorides . Direct adsorption of organic thioacetates on gold in the solution led to the formation of less ordered SAMs, ,− , whereas more ordered SAMs could be prepared via in situ deprotection of acetyl groups using specific deprotection reagents. , Other alternatives on gold showed similar structural behaviors: they usually form SAMs with inferior structural quality compared to the analogue thiols, but the structural quality of these SAMs was enhanced significantly using optimized SAM preparation conditions. − …”

“…Organic selenocyanates could be also a strong alternative to prepare selenolate SAMs because they are chemically more stable than selenols. However, there have been only limited reports regarding the adsorption of organic selenocyanates on Au and Ag nanoparticles , and their SAM formation on Au(111) surfaces in solution. , Moreover, STM observations showed that the adsorption of organic selenocyanates on Au(111) in a 1 mM ethanol solution at room temperature (RT) led to the formation of poorly ordered selenolate SAMs, whereas SAMs formed at high temperature were composed of mixed disordered and ordered domains. , In contrast to alkanethiol SAMs, the deposition method strongly affects the adsorption behavior and structural order of SAMs on Au(111) formed from acetyl-protected thiols or organic thiocyanates. , Organic thiocyanate SAMs on Au(111) with long-range ordered domains and a homogeneous interface can be prepared using ambient-pressure vapor deposition (hereafter, simply referred to as vapor deposition) compared with solution deposition. , …”

“…31,32 Therefore, new molecular assemblies on metals have been developed using S-containing alternatives having high chemical stability in solution or air such as organic thioacetates, 5,31−37 thiosulfates, 38 thiocyanates, 39−44 and sulfenyl chlorides. 45 Direct adsorption of organic thioacetates on gold in the solution led to the formation of less ordered SAMs, 5,[31][32][33][34]37 whereas more ordered SAMs could be prepared via in situ deprotection of acetyl groups using specific deprotection reagents. 5,32 Other alternatives on gold showed similar structural behaviors: they usually form SAMs with inferior structural quality compared to the analogue thiols, but the structural quality of these SAMs was enhanced significantly using optimized SAM preparation conditions.…”