Negative Thermal Effects on the Structural Order of Methoxy–terminated Mono(Ethylene Glycol) Ethanethiol Self‐assembled Monolayers on Au(111)

Abstract

Self-assembled monolayers (SAMs) on gold surfaces derived from organic thiols provide a very excellent route for preparation of molecular thin films that can be applied to a variety of technological applications. 1-6 SAMs of oligo(ethylene glycol) (OEG)-terminated alkanethiols have been widely used to reduce nonspecific adsorption of biomolecules and proteins 7,8 and control the charge transport of OEG SAM-based molecular electronic devices. 9 The structural order and conformation of OEG SAMs have been reported to considerably affect nonspecific adsorption of proteins. 7 Soft X-ray absorption measurements have shown that the structural conformation of OEG SAMs on metals strongly depends on the number of EG units in the alkyl backbone, the nature of the metal, and the solvent used for SAM preparation. 8 Frequency modulation atomic force microscopy measurements have shown that adsorption of OH-terminated hexa(ethylene glycol) undecanethiol on Au(111) yielded to ordered SAMs. 9 Our recent scanning tunneling microscopy (STM) observations have elucidated the detailed molecular features of methoxy-terminated mono(ethylene glycol) ethanthiol (OMe-EG1-C2SH) SAMs on Au(111) room temperature (RT) with unique mixed phases coexisting ordered and disordered domains. 10 Understanding the surface features of SAMs from a molecular perspective has enabled us to control the physical and chemical properties of SAMs for practical applications. There have been no prior STM results describing the structural features of OMe-EG1-C2S SAMs on Au(111) formed after thermal annealing or at high solution temperature (HT). Therefore, we examined the surface structures of these SAM samples on Au(111) using STM to clearly elucidate these issues. OMe-EG1-C2SH was acquired from Sigma-Aldrich (Munich, Germany). Au(111) substrates on mica were fabricated via thermal evaporation. 10 Pre-covered OMe-EG1-C2S SAMs were formed by dipping the Au(111) substrates in a 1 mM ethanol solution at RT for 24 h. To understand thermal effects on the OMe-EG1-C2S SAM structures, annealed SAM samples were prepared after thermal annealing pre-covered SAMs at 100 C for 30 min; SAMs were also fabricated at an elevated temperature of 50 C for 24 h. STM measurements were carried out with a NanoScope E (Veeco Instruments, Santa Babara, CA, USA).The STM image in Figure 1(a) showed that OMe-EG1-C2S SAMs on Au(111) formed at RT for 24 h had well-ordered domains surrounded by domain boundaries (indicated by white lines) (pre-covered SAMs). The inset STM image obviously shows individual molecules in ordered molecular rows. Various sizes of ordered domains with lateral dimensions of approximately 15-80 nm were observed in the SAM samples. Different from alkanethiol SAMs with uniformly ordered domains, 1,11-13 poorly ordered bright features with apparent contrast (indicated by blue circles) were observed inside well-ordered domains. The OEG units in SAMs were reported as having three different conformations: a helical conformation, an all-trans conformation, and a mixed conformati...

Thermally Driven Structural Order of Oligo(Ethylene Glycol)-Terminated Alkanethiol Monolayers on Au(111) Prepared by Vapor Deposition

“…It is clear from the STM image that the ordered phase was the dominant phase formed. In contrast, solution-deposited EG2 SAMs at the same preparation conditions (deposition temperature of 323 K and deposition time of 24 h) were dominated by the disordered phase, suggesting that solvent molecules with high thermal energy significantly prevent two-dimensional (2D) ordering of EG2 SAMs at high solution temperature [ 56 ]. Our STM results strongly suggest that the structural order of EG2 SAMs was remarkably enhanced by vapor deposition and high deposition temperatures.…”

“…Therefore, vapor-deposited EG2 SAMs are a bit more loosely packed than solution-deposited SAMs. However, it was found that vapor-deposited EG SAMs at 348 K have much more uniform and well-ordered SAMs compared to solution-deposited EG2 SAMs that contain large areas of disordered phase when formed at 298 K or 323 K [ 39 , 56 ].…”