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

Abstract

To probe the effects of deposition temperature on the formation and structural order of self-assembled monolayers (SAMs) on Au(111) prepared by vapor deposition of 2-(2-methoxyethoxy)ethanethiol (CH3O(CH2)2O(CH2)2SH, EG2) for 24 h, we examined the surface structure and electrochemical behavior of the resulting EG2 SAMs using scanning tunneling microscopy (STM) and cyclic voltammetry (CV). STM observations clearly revealed that EG2 SAMs vapor-deposited on Au(111) at 298 K were composed of a disordered phase on the entire Au surface, whereas those formed at 323 K showed improved structural order, showing a mixed phase of ordered and disordered phases. Moreover, at 348 K, uniform and highly ordered EG2 SAMs on Au(111) were formed with a (2 × 3√3) packing structure. CV measurements showed sharp reductive desorption (RD) peaks at −0.818, −0.861, and −0.880 V for EG2 SAM-modified Au electrodes formed at 298, 323, and 348 K, respectively. More negative potential shifts of RD peaks with increasing deposition temperature are attributed to an increase in van der Waals interactions between EG2 molecular backbones resulting from the improved structural quality of EG2 SAMs. Our results obtained herein provide new insights into the formation and thermally driven structural order of oligo(ethylene glycol)-terminated SAMs vapor-deposited on Au(111).

Formation and Thermal Stability of Ordered Self-Assembled Monolayers by the Adsorption of Amide-Containing Alkanethiols on Au(111)

“…Interestingly, we found the coexistence of two structural phases: an ordered row structure (α-phase) and well-ordered and closely packed structure (β-phase), as shown in Figure 2 b. In addition, several vacancy islands (VIs) with a monatomic depth of 2.5 Å (dark trench areas) were observed on the surface, which were well-known intrinsic characteristics that appeared as a result of the formation of chemisorbed monolayers on Au(111) surfaces by the adsorption of S- or Se-containing molecules [ 1 , 2 , 3 , 4 , 5 , 11 , 14 , 48 , 49 , 50 ]. Note that the VIs were also fully covered with a monolayer, not a bare Au(111) surface, as already demonstrated via high-resolution STM observations for thiolate or selenolate SAMs on Au(111) [ 3 , 5 , 6 ].…”

“…It has been reported that the position and shape of RD peaks for SAM-covered Au electrodes were significantly influenced by the binding affinity between the anchoring group and the Au surface, the magnitude of van der Waals interactions between adsorbed molecules, and the degree of structural order of the SAMs [ 14 , 31 , 41 , 52 , 53 , 54 ]. To examine the electrochemical behavior of MEHA SAMs formed at different deposition times on Au(111), we measured the CVs of RD for MEHA SAM-modified Au electrodes, as shown in Figure 5 .…”

“…The surface and interface properties of metal surfaces can be readily tuned by the formation of closely packed and highly ordered self-assembled monolayers (SAMs) derived from organic molecules with chemically active anchoring groups and various backbone structures [ 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15 ]. Hence, SAMs provide a versatile tool for the preparation of functional molecular thin films that can be applied to many practical applications, such as surface passivation [ 16 ], biointerfaces [ 17 ], biosensors [ 18 ], molecular photodiodes [ 19 ], batteries [ 20 ], and molecular electronic devices [ 21 , 22 ].…”

“…Among many organic thiols, n -alkanethiols with various alkyl chains are the most popular precursors because they easily form closely packed and well-ordered SAMs on gold with a high reproducibility. Therefore, alkanethiolate SAMs on gold have been extensively characterized to understand various fundamental aspects, such as adsorption behavior, growth kinetics, two-dimensional (2D) phase transitions, surface structures, and interface properties [ 1 , 2 , 8 , 14 , 16 , 17 , 23 , 24 , 25 , 26 ]. High-resolution scanning tunneling microscopy (STM) observations revealed that highly ordered alkanethiolate SAMs on Au(111) at saturation coverage have a (√3 × √3)R30° or (3 × 2√3) structure [ 1 , 2 , 8 , 26 ].…”

“…Amine-terminated SAMs are often utilized for the investigation of electron transfer reactions of specific proteins due to a strong binding affinity between the amine-terminal groups of SAMs and proteins [ 29 ]. An STM study showed that ethylene glycol-terminated alkanethiolate SAMs on Au(111) had very unique structural phases containing paired-row molecular domains and bright disordered regions, which were considerably different from alkanethiolate SAMs [ 14 , 28 ]. Electrochemical measurements demonstrated that anionic surfactants were strongly coupled to the ferrocene-terminal groups of alkanethiolate SAMs on gold electrodes via oxidation [ 31 ].…”