Correlation between the Electrical Properties and Formation Temperature of Self-assembled Monolayer-Based Molecular Junctions.

Alkanethiol self-assembled monolayers (SAMs) on metal surfaces are key elements for the fabrication of functional organic layers and devices in the broad fields of nanotechnology and biotechnology. However, it was found that alkanethiol SAMs were usually composed of structural defects such as domain boundaries and vacancy islands (VIs), which make them more amenable to oxidation. Compared to alkanethiols, thioethers (RSR') are more robust to oxidation, and their chemical structures with various alkyl chains can be modified readily by a simple synthetic method. Therefore, SAMs prepared using thioethers provide a very useful means for tuning the characteristics of metal surfaces. Despite these advantages, there have only been a limited number of papers involving the formation and structure of thioether SAMs on gold surfaces. It has been revealed that dioctadecyl sulfides (DODS, CH3(CH2)17S(CH2)17CH3) at the initial growth stage form SAMs with striped phases in which the molecular backbones are oriented parallel to the surface, whereas DODS SAMs formed after long immersion have two mixed phases containing closely packed and loosely packed standing-up phases where the molecular backbones are oriented perpendicular to the surface. High-resolution scanning tunneling microscopy (STM) observation revealed that the SAMs of dimethyl sulfides (DMS, CH3SCH3) with the shortest alkyl chains on Cu(111) have a herringbone-like packing structure, whereas dibutyl sulfides (DBS, CH3(CH2)3S(CH2)3CH3) with slightly larger alkyl chains form striped phases. From these results, we assumed that the formation of dialkyl sulfide SAMs strongly depends on van der Waals interactions between alkyl chains. So far, there no data has been reported on the formation and structure of didodecyl sulfides (DDS, CH3(CH2)11S(CH2)11CH3) with medium alkyl chains from a molecular-scale perspective. In this paper, we report the first STM results showing that the adsorption of DDS molecules at 70 oC generating long-range ordered SAMs with a 7.5 × √3 striped phase with VIs-free surfaces. Dodecanethiol (DDT, CH3(CH2)11SH) and DDS were purchased from Tokyo Chemical Industry (Tokyo, Japan). DDT and DDS SAMs were prepared by dipping the Au(111) substrate in 1 mM ethanol solutions of corresponding compounds at room temperature for 24 h. To understand the effect of solution temperature on the formation of DDS SAMs, the SAMs were prepared at 70 C for 1 h. STM images were obtained using a NanoScope E (Veeco, Santa Barbara, CA, USA) with a commercial Pt/Ir (80:20) tip under ambient conditions. The STM images in Figure 1 show remarkable structural differences in the formation of ordered domains and VIs (dark holes) for DDT and DDS SAMs on Au(111) formed after 24 h immersion at room temperature. Figure 1(a) shows a typical packing structure of DDT SAMs with a c(4 × 2) superlattice formed at saturation coverage. DDT SAMs were mainly composed of ordered phases with three domain orientations (Regions A, B, and C) separated by domain boundaries. The proportion of the VI areas to the total surface area was measured to be approximately 8%-12%, values which are similar to those observed from other alkanethiol SAMs. Unlike DDT SAMs, Figure 1(b) shows that the DDS SAMs have two mixed phases: the ordered phase with three domain orientations (Regions A, B, and C) and the disordered phase (Region D). The existence of three domain orientations for DDS SAMs on Au(111) means that the formation of SAMs was influenced by interactions between the sulfide atoms and gold surfaces. On the other hand, DDS SAMs contained few VIs comprising a small (23%) area fraction of the total surface area. The presence of disordered phases and a smaller VI fraction for DDS SAMs can be attributed to the weaker interactions between the sulfide atoms and gold surfaces compared to those between the sulfur atoms of thiols and gold surfaces for DDT SAMs. This finding is consistent with a previous result for the SAMs of DODS with long alkyl chains of 18 carbon units. The ordered domains have a row structure with an inter-row distance of 1.59 ± 0.03 nm, which is nearly half of the entire

The formation and surface structure of 3-hexylthiophene (HTP) self-assembled monolayers (SAMs) on Au(111) prepared by solution and ambient-pressure vapor deposition at room temperature (RT) for 24 h were examined by means of scanning tunneling microscopy (STM) and cyclic voltammetry (CV). STM imaging revealed that HTP SAMs formed by solution deposition have a disordered phase, whereas those formed by vapor deposition exhibit a striped phase with a unidirectional orientation. The distance between the rows in the striped phase was measured to be 1.3 ± 0.1 nm, and the hexyl molecular backbones of HTP in the SAMs on Au(111) are oriented parallel to the Au(111) surface with the head-to-head orientation. From this STM observation, we suggest that the formation of this striped phase in HTP SAMs prepared by vapor deposition were mainly driven by the optimization of van der Waals interactions between the hexyl chains on the surface. CV measurements also demonstrated that HTP SAMs show a high blocking efficiency for electron transfer reactions between electrolytes and the gold electrode, suggesting the formation of SAMs on Au(111) from the vapor phase. Our results obtained here will be very useful for understanding the formation and structure of HTP SAMs on Au(111) surfaces and how they are influenced by deposition method.

Self-assembled monolayers (SAMs) formed by organosulfur compounds on metal surfaces offer a simple and powerful route for the fabrication of functional ultrathin films that can be employed in various technical applications such as corrosion inhibition, chemical sensors, biosensors, nanopatterning, and molecular electronic devices. Recently, SAMs derived from conjugated aromatic thiols on gold have attracted much attention as a result of their interesting electrical and optical properties as well as their potential for use in molecular electronics applications. Oligo(phenylene ethylene) (OPE) molecules have been considered as an extremely promising molecular system for device applications because they have a π-conjugated and rod-like molecular structure. The formation of OPE SAMs and their electrical properties have been extensively characterized by various surface characterization tools. Thioacetyl-protected conjugated molecules with a high chemical stability have been frequently used for electronic device applications. It has been reported that these molecules can form chemisorbed SAMs on gold in acidor base-catalyzed mediums or without any catalyzed medium. Prior to the development of SAM-based devices, it is essential to understand the formation and structure of SAMs on gold. However, although several macroscopic measurements for SAMs formed by thioacetyl-terminated molecules have been carried out to gain insight into SAM formation, there have been only a few nanometer-scale reports on these SAMs. Among these previous reports, one study showed that the conductance switching behavior in single molecules is due to conformational changes of molecular backbones, rather than electrostatic effects of charge transfer. Therefore, for SAM-based device applications, it is very important to control the two-dimensional structure of conjugated SAMs as well as to obtain high-quality SAMs. As far as processing conditions go, it is well known that the thermal annealing of pre-covered alkanethiol SAMs at 70-100 oC results in the formation of large ordered domains and healing of vacancy islands (VIs). It was also found that alkanethiol SAMs with large well-ordered domains and few VIs can be formed by liquid and vapor phase deposition methods at high temperature as a one-step process to obtain high quality SAMs. Tolanemethylthioacetate (TMTA) with a π-conjugated, rigid molecular backbone is one of interesting molecules for electronic device application. Figure 1 shows the structural formula of TMTA molecule. In this study, to obtain highquality ordered TMTA SAMs on Au(111), we examine the effect of solution temperature on the formation and structure of TMTA SAMs using STM. We report herein the first STM results showing the phase transition from the disordered phase to the ordered phase of TMTA SAMs as solution temperature increased. TMTA was synthesized by modifying a previously reported method. Au(111) substrates on mica were prepared by vacuum deposition as previously described. The SAMs were prepared by immersing Au(111) substrates in 0.5 mM N,N'-dimethylformamide (DMF) solutions of TMTA at room temperature, 50 °C, and 80 °C for 24 h. STM measurements were carried out using a NanoScope E with a commercial Pt/ Ir (80:20) tip under ambient conditions. The STM images in Figure 2 show the surface structures of TMTA SAMs on Au(111) formed after immersion as a function of solution temperature. Clearly, the solution temperature markedly affects the formation and structure of TMTA SAMs on Au(111). Although acetyl-protected TMTA molecules on Au(111) form chemisorbed SAMs via the spontaneous deprotection of acetyl group during the adsorption of TMTA in DMF solution, TMTA molecules form disordered phases containing partially ordered domains and molecular aggregates at room temperature, as shown in Figure 2a. Differently from the formation of well-ordered SAMs by alkanethiols at room temperature, TMTA molecules do not form highly ordered SAMs. The unusual structural characteristics of TMTA SAMs can be ascribed to the lower chemical activity of sulfur headgroups against the gold atoms, which is due to the existence of an electron withdrawing acetyl group attached to the sulfur atom. In contrast, more ordered SAMs with a uniform surface morphology were formed in the medium-temperature solution of 50 °C, as shown in the STM image of Figure 2b. We observed a large number of

The effects of immersion time and the number of chlorine attached to silane atom on the formation of self-assembled monolayers (SAMs) on indium tin oxide (ITO) surface were examined by scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and contact angle measurements. Among octadecyltrichlorosilane (OTS), octadecyldichloromethylsilane (ODDMS), and octadecyldimethylchlorosilane (ODMS), OTS SAMs on ITO surface formed after 1 h deposition at room temperature have a high contact angle value of 86° compared to ODDMS and ODMS SAMs with 45° and 31°, reflecting that OTS on ITO surface would form a high-quality SAMs with a hydrophobic surface. STM imaging revealed that the surface structure of OTS SAMs on ITO surface was markedly different from that of bare ITO surface resulting from the the adsorption of OTS molecules. In addition, XPS measurements showed that the formation of OTS SAMs is strongly driven by the chemical reactions between the Si atoms and the hydroxyl group of ITO surface during self-assembly.

One of the major challenges to the fabrication of functionalized templates using self-assembled monolayers (SAMs) is the characterization of nanoscale defects, particularly SAM domain boundaries (DBs). In this study, an etchant was used to chemically amplify the DBs in a SAM by forming microscale pits in the underlying SiO2 layer. This approach revealed that the naturally occurring DBs acted as structural defects in the SAMs. The DB structures were characterized by systematically varying the octadecyltrichlorosilane (ODTS) monolayer domain size from the nanoscale to the microscale by varying the preparation temperature. These approaches showed that the SAM DBs, which were visualized as having round- and line-shaped nanoscale structures, provided potentially chemical and mechanical surface defect sites. Our principal findings open up exciting new possibilities for understanding the structural defects in SAMs on the molecular level and suggest an approach for optimizing the conditions used to generate defect-free SAM templates.

Formation and superlattice of long-range and highly ordered alicyclic selenolate monolayers on Au(1 1 1) studied by scanning tunneling microscopy

The morphologies of cysteamine (CA) and propanethiol (PT) self-assembled monolayers (SAMs) on Au(111) prepared in H 2 O, ethanol, N,N,-dimenthylformamide (DMF), and toluene solution were imaged by scanning tunneling microscopy (STM). The STM measurements showed that the formation of domains like dendrites in CA SAMs was dependent on a type of solvent. On the other hand, a pit size in PT SAMs was controlled by a type of solvent without such a domain formation. These results clearly revealed that the solvent plays an important role in SAM formation.

Topological Defects Induced High-Spin Quartet State in Truxene-Based Molecular Graphenoids

Self-assembled monolayers (SAMs), prepared by reaction of terminal n-alkynes (HC≡C(CH2)nCH3, n = 5, 7, 9, and 11) with Au(111) at 60 °C were characterized using scanning tunneling microscopy (STM), infrared reflection absorption spectroscopy (IRRAS), X-ray photoelectron spectroscopy (XPS), and contact angles of water. In contrast to previous spectroscopic studies of this type of SAMs, these combined microscopic and spectroscopic experiments confirm formation of highly ordered SAMs having packing densities and molecular chain orientations very similar to those of alkanethiolates on Au(111). Physical properties, hydrophobicity, high surface order, and packing density, also suggest that SAMs of alkynes are similar to SAMs of alkanethiols. The formation of high-quality SAMs from alkynes requires careful preparation and manipulation of reactants in an oxygen-free environment; trace quantities of O2 lead to oxidized contaminants and disordered surface films. The oxidation process occurs during formation of the SAM by oxidation of the -C≡C- group (most likely catalyzed by the gold substrate in the presence of O2).

To understand the effect of the ethylene glycol (EG) substituent of alkanethiols on domain formation, surface structure, and adsorption condition of self-assembled monolayers (SAMs) on Au(111), we examined SAMs formed by 1-ethanethiol with methoxy-terminated mono(ethylene glycol) (EG1-OMe SAMs) by scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy and compared the results to those of heptanethiolate SAMs with a normal alkyl chain of similar molecular length. STM imaging clearly revealed that the surface features of EG1-OMe SAMs on Au(111) were noticeably different than those of heptanethiolate SAMs. The adsorption of EG1-OMe molecules on Au(111) in a 1 mM ethanol solution at RT for 1 min led to the formation of SAMs containing mixed phases: a paired-row ordered phase and poorly ordered phase containing molecular spots with an apparent bright contrast. The paired-row ordered domain of EG1-OMe SAMs on Au(111) was assigned to the (2√5 × 5)R11° packing structure, which is comparable to a...

Modern quantum chemical electronic structure methods typically applied to localized chemical bonding are developed to predict atomic structures and free energies for meso-tetraalkylporphyrin self-assembled monolayer (SAM) polymorph formation from organic solution on highly ordered pyrolytic graphite surfaces. Large polymorph-dependent dispersion-induced substrate-molecule interactions (e.g., -100 kcal mol(-1) to -150 kcal mol(-1) for tetratrisdecylporphyrin) are found to drive SAM formation, opposed nearly completely by large polymorph-dependent dispersion-induced solvent interactions (70-110 kcal mol(-1)) and entropy effects (25-40 kcal mol(-1) at 298 K) favoring dissolution. Dielectric continuum models of the solvent are used, facilitating consideration of many possible SAM polymorphs, along with quantum mechanical/molecular mechanical and dispersion-corrected density functional theory calculations. These predict and interpret newly measured and existing high-resolution scanning tunnelling microscopy images of SAM structure, rationalizing polymorph formation conditions. A wide range of molecular condensed matter properties at room temperature now appear suitable for prediction and analysis using electronic structure calculations.

Molecular-scale surface structures of self-assembled monolayers (SAMs) prepared by the adsorption of pentafluorobenzenethiols (PFBT) and pentachlorobenzenethiols (PCBT) on Au(111) were investigated by scanning tunneling microscopy (STM). High-resolution STM imaging revealed that PFBT SAMs on Au(111) have long-range ordered domains with a row structure at room temperature, whereas PCBT SAMs have small ordered domains, with disordered domains as the main phase. This may reflect the larger diffusion barriers of PCBT molecules on Au(111) surfaces compared to PFBT molecules during SAM formation. The structural transitions of PCBT SAMs from the mixed phase containing disordered and ordered domains to the uniform ordered domains were observed at 50 degrees C depending on immersion time. The ordered packing structure of PCBT SAMs is an incommensurate (square root of 3 x square root of 10)R45 degrees structure, which differs from that of PFBT SAMs with a (2 x 5 square root of 13)R30 degrees structure. We found that a small modification in the chemical structures of aromatic rings using a halo-substituent strongly affects the self-assembly mechanism and packing structure of aromatic thiol SAMs on Au(111). Moreover, we demonstrated that highly ordered PCBT SAMs can be obtained at a solution temperature of 50 degrees C after immersion for 60 min.

Silicon nitride as an energy efficient material is replacing conventional steels for new generation engineering components such as bearings, cutting tools, electronics and engine parts in automotive, aerospace and wind industries. Compared with steel bearings, silicon nitride bearings can be operated at much higher temperatures and speeds with >60% weight reduction and up to 80% friction reduction. These are all due to its unique material properties, including high wear and corrosion resistance, low density and heat generation. Current lubrication solutions for hybrid contacts, where silicon nitride balls and steel races are used, are mostly relying on the protection film formed on the metal surfaces. Self-assembled monolayers (SAMs) have been found very useful in modifying surfaces, especially for microelectromechanical system and nanoscale applications, e.g. atomic force microscopy tips, etc. This study aims to investigate the feasibility of forming a SAM protection film on industrial grade bearing material silicon nitride to reduce the friction for the oil lubricated hybrid contacts. Four silanes with different functional head groups, including octadecyltrichlorosilane (OTS), octyltrichlorosilane, chlorodimethyloctadecylsilane and octadecyltrimethoxysilane, were initially investigated to form SAMs on industrial grade silicon nitride surfaces. The effects of concentration and immersion time of the silanes on the formation of SAMs on the silicon nitride surface were evaluated using contact angle measurements. The preliminary results show that the wetting properties of the silicon nitride surface can be effectively modified by the formation of SAMs from the silane solutions. OTS can form an order and compact SAM on the silicon nitride surfaces within 2 min at the concentration of 2··5 mM in decane solution, while the other three alkylsilanes can also effectively modify silicon nitride surfaces given sufficient immersion time, e.g. over 1 h. Tribological tests were subsequently carried out on a ball on disc rig where a steel ball and a silicon nitride disc were used. The effect of the formation of alkylsilane SAMs on the friction between the sliding contacts has been evaluated in two different methods. The first method was to test preformed SAM films under dry conditions, and the second was to premix one of the surfactants with Shell Vitrea ISO 32 mineral base oil and then spray the mixture to the contacts during the ball on disc testing. The test results show that an average of over 40 and 30% friction reduction was achieved for the hybrid contact when lubricated with the base oil mixed with OTS (>2··5 mM) and octadecyltrimethoxysilane (5 mM) respectively compared with that of the sliding contact lubricated by the base oil only. Since OTS may produce corrosive byproducts during SAM formation, octadecyltrimethoxysilane may be a more suitable additive for the hybrid contacts.

Two-dimensional ordered structures of cyclohexanethiol (CHT) self-assembled monolayers (SAMs) on Au(111) formed from different solution concentrations were investigated using scanning tunneling microscopy (STM). Molecular-scale STM imaging clearly revealed the 2D phase transitions of CHT SAMs from a (2 x 4 square root of 2)R20 degrees structure to a (5 x 2 square root of 3)R35 degrees structure via a intermediate phase of a (5 x square root of 15)R25 degrees structure depending on solution concentration used for SAM fabrication. The (5 x 2 square root of 3)R35 degrees phase was observed for high concentrations, after deposition at low temperature or after long-time immersion. Based on these results, we confirmed that the molecular packing arrangement can be regarded as a fully covered SAM structure. In addition, 2D SAM formation of CHT SAMs is markedly different from that of alkanethiol SAMs, which is a result of structural dynamics of the CHT ring in the process of molecular self-assembly. The molecular-scale STM results for CHT SAMs on Au(111) can provide very useful information on understanding structural behaviors of cyclic thiol SAMs.

A Tribute to Richard M. Crooks on the Occasion of His 65th Birthday