Structural Investigation of 4-Methylbenzenethiol Self-Assembled Monolayers on Au(111) by Scanning Tunneling Microscopy

Abstract

Self-assembled Monolayers (SAMs) Formed by the Spontaneous Adsorption of Organic Thiols on Metal Surfaces offer a powerful route for potential applications in a variety of fields such as corrosion inhibition, lubrication, sensors, biointerface, and nanopatterning. [1][2][3][4] Recently, SAMs of aromatic thiols have drawn considerable attention due to their practical applications in molecular electronic devices. 5,6 It has been demonstrated that the work function of metal electrodes in electronic devices can be easily tuned by introducing the SAMs of ω-functionalized aromatic thiols with different functional group 5-8 and the performance of SAMmodified devices was strongly dependent on the molecular orientation of adsorbed molecules.8 Therefore, it is essential to understand the surface structure and molecular orientation of aromatic thiol SAMs as well as to control two-dimensional (2D) structure of SAMs for the development of electronic devices. Structural order of aromatic thiol SAMs on Au(111) was considerably enhanced by increasing the number of phenyl ring in the molecular backbone 9-11 and by inserting the alkyl spacer between the phenyl ring and the sulfur atom.12,13 High-resolution scanning tunneling microscopy (STM) imaging revealed that the adsorption of benzenethiol (BT) on Au(111) at 50°C led to the formation of well-ordered SAMs showing the (2 × 3√2)R30° structure, 9,12 while fluorinated benzenethiol SAMs have quite different packing strcutures.14,15 So far, numerous reports have mainly focused on the surface structure, molecular orientation, self-assembled process and physical property of SAMs formed by various aromatic thiols. 6-16 However, there have been very few reports on the structure and orientation of SAMs of 4-alkylbenzenethiols with a nonpolar sustituent. 17The main purpose of this work is to elucidate how much the alkyl substituent of benzenethiol at 4-position affects the formation and structure of SAMs. To explore this question, we examined 4-methylbenzenethiol (4-MBT) SAMs prepared in a 1 mM ethanol solution using STM, and revealed for the first time that 4-MBP molecules on Au(111) at 75 ºC form 2D ordered SAMs describing a (2√3 × √11.5)R30°s tructure.Atomically flat Au(111) substrates were prepared by thermal evaporation of gold onto mica as described previously.10 4-MBT SAMs were prepared by dipping the Au(111) substrate in a 1 mM ethanol solution of 4-MBT at room temperature or 75 °C for 2 h. STM measurements were carried out using a NanoScope E (Veeco, Santa Barbara, CA) with a commercial Pt/Ir (80:20) tip under ambient conditions. Imaging parameters were in the range of 300-500 mV for the bias voltage and in the range of 0.30-0.60 nA for the tunneling current between the tip and the sample.STM images in Figure 1 show the surface structures of 4-MBT SAMs on Au(111) formed after immersion in a 1 mM ethanol solution at room temperature and 75 °C for 2 h. The adsorption of 4-MBT molecules on Au(111) at room temperature yielded to the disordered SAMs containing gold adatom islands (bri...

Intermolecular Forces Dictate Vibrational Energy Transfer in Plasmonic–Molecule Systems

“…On the other hand, the steric effect of the −NO 2 group on NBT is minimized because it is in a planar structure with the phenyl ring. Finally, aromatic thiols with multiple phenyl rings are reported to pack more efficiently compared to aromatic thiols with a single phenyl ring. ,,− Therefore, we infer that the observed energy trend could be explained by the effect of molecular structure on π–π stacking. MTP with the bulky methyl group has relative low efficiency in redistributing its energy to surrounding molecules, which leads to higher energy and an extended lifetime of the deposited energy.…”

“…37,38 Although the exact adsorption geometry depends on molecule surface coverage and preparation condition, previous studies have showed that the orientation of densely packed aromatic thiols are nearly upright, with a tilt angle of ∼20°−30°to the surface normal. [27][28][29]39 In this study, we used excessive amounts of aromatic thiols to achieve full surface coverage, so the molecules can be considered densely packed on the surface. While the surfaces used here are highly heterogeneous, the orientation of the dipole relative to the metal surface is most likely similar for the range of molecules studied due to the similarities in structure.…”

“…Therefore, with the motivation of exploring what molecular properties could be tuned to control the vibrational energy transfer from plasmonic materials to adsorbate molecules, we chose a series of aromatic thiols with different substituent groups as probe molecules. Aromatic thiols are known for their ability to form self-assembled monolayers (SAM) on metal surfaces, which ensures the surface selectivity of the SERS measurements. − They are also sulfur-containing precursors for production of pharmaceutical products and synthesis of thiosugars and thioglycosides. − However, we should point out that throughout the measurements, the studied molecules are not undergoing chemical reactions with the experimental conditions. This is to avoid the complexity brought by reactivity, which will change the distribution of probed molecules in the hot spots.…”

“…Dipole orientation to the metal surface is an important parameter to consider when treating plasmonic–molecule systems. The coupling between molecules and the plasmonic surface, which is largely affected by the relative orientation of the molecule dipole to the metal surface normal, can have an impact on both the molecular excitation and energy decay processes. , Although the exact adsorption geometry depends on molecule surface coverage and preparation condition, previous studies have showed that the orientation of densely packed aromatic thiols are nearly upright, with a tilt angle of ∼20°–30° to the surface normal. − , In this study, we used excessive amounts of aromatic thiols to achieve full surface coverage, so the molecules can be considered densely packed on the surface. While the surfaces used here are highly heterogeneous, the orientation of the dipole relative to the metal surface is most likely similar for the range of molecules studied due to the similarities in structure.…”

Phase‐separated Nanodomains Formed by Coadsorption of Alicyclic and Aromatic Thiols on Au(111)

“…The adsorption of thiols onto metals creates self‐assembled monolayers (SAMs) that can provide a versatile route for fabrication of nanoscale structures and functionalized monomolecular thin films . Two‐component mixed SAMs are especially promising due to their strong potential to tune the physical and chemical properties of surfaces by changing the tail group, length, and chemical structure of the thiol molecular backbone.…”