Unexpected Formation of Dense Phases along with Temperature-Induced, Self-Assembled Terphenylthiolate Monolayers on Au(111)

Abstract

Self-assembled monolayers (SAMs) of terphenylthiol (TP0) prepared on Au(111) from ethanolic solution at room temperature and subsequently annealed at different temperatures of up to 473 K were investigated using scanning tunneling microscopy and infrared reflection absorption spectroscopy. Upon annealing, the TP0 adlayer formed a series of different structural phases with different molecular arrangements. Our results show that SAMs prepared at RT without annealing and SAMs annealed at 313 K exhibited the formation of a closely packed “α” structure where TP0 molecular axes are orientated almost upright. The formation of the α-phase was accompanied by the formation of a loosely packed phase (β-phase). During annealing from 333 to 473 K, the results revealed the formation of different structural phases, namely, γ, δ, ε, σ, and ϕ. With the exception of the ϕ-phase, the phases were found to form loosely packed structures with the molecular axes largely tilted away from surface normal but do not form “flat-lying” phases. Surprisingly, upon annealing at 453 K, the SAMs exhibited a phase transition to a higher dense network of molecular structure (ϕ-phase) with almost upright standing molecules. An additional annealing at 463 K led to a significant increase in the size of the well-ordered domains of the ϕ-phase at the expense of their density. At the same time, a partial desorption of the TP0-SAMs was noticed to take place. The formation of a densely packed phase upon annealing and subsequently desorption of TP0 molecules from this state completely contradicts the behavior of n-alkanethiol and other oligophenylthiol SAMs, in which desorption occurs from lying-flat geometry. The ϕ-phase, which was the only one remaining on the surface, did not cover more than 30% of the gold surface area. A further increase of the annealing temperature to 473 K caused a substantial increase in the number of the desorbed molecules. Therefore, a remarkable reduction in both the size and density of the ordered domains of the ϕ-phase was observed. On the other hand, the annealing temperature was found to have a significant influence on both the appearance and the shape of the gold islands. Moreover, it has been found that the gold islands, regardless of their shapes, were always covered by standing-up, densely packed, and ordered phases such as the α- and ϕ-phases.

Side‐Group‐Induced Polymorphism in Self‐Assembled Monolayers: 3,5‐Bis(trifluoromethyl)benzenethiolate Films on Au(111)

“…This is noticeably higher than the cross‐sectional area of the benzene ring (i.e. 21.1 Å 2 ) or the typical packing density of a SAM of non‐substituted or methyl‐substituted (4 position) oligophenyl thiols on Au(111), which is 21.6 Å 2 ,,. This is understandable given the large van der Waals radius of the CF 3 groups ( r (CF 3 )=2.7 Å) at the 3,5 positions in the TFBT moiety compared with hydrogen atoms ( r (H)=1.2 Å) in the parent benzenethiol molecule.…”

“…The selection of the solvent and TFBT concentration was based on our experience with analogous systems ,,. The idea of a subsequent post‐immersion treatment performed at elevated temperature was also based on this experience because these conditions enhance the mobility of the adsorbates …”

“…Of the different SAMs, those fabricated from organothiols and organoselenols on Au(111) are particularly popular due to their ease of preparation and well‐ordered character . In particular, SAMs of aromatic thiols with π‐conjugated, rigid, rodlike molecular systems have recently become of great topical interest–,, due to their electronic properties and their relevance for applications in the fields of molecular electronics, electrode modification, and electroanalytical sensors . An important sub‐class of aromatic SAMs are fluorinated aromatic thiols, due to their ability to optimize charge transport between metal electrodes and organic semiconductors .…”

Self-Consistent Tip Conditioning for Tip-Enhanced Raman Spectroscopy in an Ambient Environment

“…The crystal was polished to a roughness of less than 10 nm and orientation accuracy better than 0.1°. The Au(111) surface was annealed in a butane flame and immersed in 0.1 mM Ph 3 SH acetone solution at room temperature for a duration longer than 12 h. Thereafter, the temperature of the solution was increased to 50 °C and kept constant for 1 h. ,, The Au(111) surface, fully covered with SAMs, was rinsed with pure acetone and dried with N 2 gas just before each measurement.…”

“…First, we experimentally show the current issue by performing TERS and STM measurements of terphenylthiol (Ph 3 SH) self-assembled monolayers (SAMs) on Au(111) under the irradiation of a standard 633 nm laser and the corresponding effect of the tip shape is also evaluated and compared. It is known that Ph 3 SH molecules are densely packed on the Au(111) substrate, and molecular resolution images can be obtained by STM in air. − Next, we investigated the stability of the plasmon resonance and STM imaging through STML measurements using a Au tip and clean Au(111) substrates. We propose that 785 nm excitation is more advantageous than the standard 633 nm excitation for performing TERS measurements because the plasmon excitation efficiency at 633 nm easily diminishes depending on the tip state.…”