Formation of highly ordered and orientated gold islands: effect of immersion time on the molecular adlayer structure of pentafluorobenzenethiols (PFBT) SAMs on Au(111)

Molecular Self-Assembly and Adsorption Structure of 2,2'-Dipyrimidyl Disulfides on Au(111) Surfaces

The effects of solution concentration and pH on the formation and surface structure of 2-pyrimidinethiolate (2PymS) self-assembled monolayers (SAMs) on Au(111) via the adsorption of 2,2'-dipyrimidyl disulfide (DPymDS) were examined using scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS). STM observations revealed that the formation and structural order of 2PymS SAMs were markedly influenced by the solution concentration and pH. 2PymS SAMs formed in a 0.01 mM ethanol solution were mainly composed of a more uniform and ordered phase compared with those formed in 0.001 mM or 1 mM solutions. SAMs formed in a 0.01 mM solution at pH 2 were composed of a fully disordered phase with many irregular and bright aggregates, whereas SAMs formed at pH 7 had small ordered domains and many bright islands. As the solution pH increased from pH 7 to pH 12, the surface morphology of 2PymS SAMs remarkably changed from small ordered domains to large ordered domains, which can be described as a (4√2 × 3)R51° packing structure. XPS measurements clearly showed that the adsorption of DPymDS on Au(111) resulted in the formation of 2PymS (thiolate) SAMs via the cleavage of the disulfide (S-S) bond in DPymDS, and most N atoms in the pyrimidine rings existed in the deprotonated form. The results herein will provide a new insight into the molecular self-assembly behaviors and adsorption structures of DPymDS molecules on Au(111) depending on solution concentration and pH.

Polymorphism and Building-Block-Resolved STM Imaging of Self-Assembled Monolayers of 4-Fluorobenzenemethanethiol on Au(111)

Self-assembled monolayers (SAMs) of 4-fluorobenzenemethanethiol (p-FBMT) on Au(111), prepared by immersion procedure (1 mM ethanolic solution; 60 °C; 18 h), were characterized by scanning tunneling microscopy (STM). The data suggest the formation of highly ordered monolayer with a commensurate structure, described by the 2 3 × 13 R 13 ∘ unit cell. The STM appearance of this cell occurs, however, in two different forms, with either well-localized individual spots or splitting of these spots in two components. These components are assigned to the tunneling through the entire molecule or sulfur docking group only. The respective spots correspond then to the terminal fluorine atom and sulfur docking group, manifesting, thus, building-block-resolving STM imaging. The accessibility of the docking group for direct tunneling is most likely related to a specific molecular organization for one of the two possible internal structures of the unit cell. The above results represent a showcase for potential of STM for imaging of upright-arranged and densely packed molecular assemblies, such as SAMs.

Flexible low-voltage high-frequency organic thin-film transistors

The primary driver for the development of organic thin-film transistors (TFTs) over the past few decades has been the prospect of electronics applications on unconventional substrates requiring low-temperature processing. A key requirement for many such applications is high-frequency switching or amplification at the low operating voltages provided by lithium-ion batteries (~3 V). To date, however, most organic-TFT technologies show limited dynamic performance unless high operating voltages are applied to mitigate high contact resistances and large parasitic capacitances. Here, we present flexible low-voltage organic TFTs with record static and dynamic performance, including contact resistance as small as 10 Ω·cm, on/off current ratios as large as 1010, subthreshold swing as small as 59 mV/decade, signal delays below 80 ns in inverters and ring oscillators, and transit frequencies as high as 21 MHz, all while using an inverted coplanar TFT structure that can be readily adapted to industry-standard lithographic techniques.

Annealing effect for self-assembled monolayers formed from terphenylethanethiol on Au(111)

Formation of several different structural phases and desorption took place from a standing-up phase at an annealing temperature of 473 K.

Nanostructuring of Au(111) during the Adsorption of an Aromatic Isocyanide from Solution

We present a combined vibrational and morphological characterization of the self-assembly of 1,4-phenylene-diisocyanide (PDI) on Au(111) from methanol solution. Vibrational sum frequency generation (vSFG) and scanning tunneling microscopy (STM) have been applied to determine the adsorption geometry of the PDI-Au adatom complexes as well as the morphological transformations of the Au(111) substrate upon SAM formation from solutions with PDI concentrations in the μM to mM range. At low concentration/coverage, PDI adsorbs in flat adsorption geometry, with both isocyanide groups attached to Au adatoms on the Au(111) surface. Transformation to a standing-up phase is observed with increasing concentration/coverage. In contrast to findings for PDI adsorbed in ultrahigh vacuum, PDI does not form a long-range-ordered monolayer phase when adsorbed from solution. In addition, the Au(111) surface is subjected to structural modifications. Au vacancy islands and ad-islands, which are typical substrate defects formed during the self-assembly of aromatic thiols on Au(111), are also created during PDI adsorption from solution. At low PDI concentration, the Au vacancy islands and ad-islands are found at specific sites mediated by the herringbone reconstruction of the Au(111) surface, giving rise to long-range-ordered structures. These structures do not form during UHV adsorption of PDI on Au(111) nor has a similar ordering effect been observed for any related thiol-SAM system investigated so far.

Hydrogen bonding vs. molecule-surface interactions in 2D self-assembly of [C60]fullerenecarboxylic acids

The adsorption of C₆₀-malonic derivatives C₆₁(CO₂H)₂ and C₆₆(CO₂H)₁₂ on Au(111) and a pentafluorobenzenethiol-modified Au substrate (PFBT@Au) has been investigated using scanning tunneling microscopy (STM) at a liquid-solid interface. Monofunctionalized C₆₁(CO₂H)₂ forms a hexagonal close-packed overlayer on Au(111) and individual aligned dimers on PFBT@Au(111). The difference is attributed to the nature of the substrateC₆₁(CO₂H)₂ interaction (isotropic π-Au bonding vs. anisotropic PFBTCOOH interactions). Surprisingly, in both cases, the directionality of the COOHCOOH motif is compromised in favor of synergistic van der Waals/H bonding interactions. Such van der Waals contacts are geometrically unfeasible in hexafunctionalized C₆₆(CO₂H)₁₂ and its assembly on Au(111) leads to a 2D molecular network controlled exclusively by H bonding. For both molecules, the "free" CO₂H groups on the monolayer surface can engage in out-of-plane H bonding interaction resulting in the epitaxial growth of subsequent molecular layers.

Localized dealloying corrosion mediated by self-assembled monolayers used as an inhibitor system

The structure and chemistry of thiol or selenol self-assembled organic monolayers have been frequently addressed due to the unique opportunities in functionalization of materials. Such organic films can also act as effective inhibition layers to mitigate oxidation or corrosion. Cu–Au alloy substrates covered by self-assembled monolayers show a different dealloying mechanism compared to bare surfaces. The organic surface layer inhibits dealloying of noble metal alloys by a suppression of surface diffusion at lower potentials but at higher applied potentials dealloying proceeds in localized regions due to passivity breakdown. We present an in situ atomic force microscopy study of a patterned thiol layer applied on Cu–Au alloy surfaces and further explore approaches to change the local composition of the surface layers by exchange of molecules. The pattern for the in situ experiment has been applied by micro-contact printing. This allows the study of corrosion protection with its dependence on different molecule densities at different sites. Low-density thiol areas surrounding the high-density patterns are completely protected and initiation of dealloying proceeds only along the areas with the lowest inhibitor concentration. Dealloying patterns are highly influenced and controlled by molecular thiol to selenol exchange and are also affected by introducing structural defects such as scratches or polishing defects.

Templating and charge injection from copper electrodes into solution-processed organic field-effect transistors

Solution-processed organic field-effect transistors (OFETs) using chemically modified copper electrodes are reported. The purpose of this study is to shed light on the use of inexpensive copper electrodes in bottom-contact OFETs, which is consistent with the major goal of organic electronics: the realization of low-cost electronics. 6,13-Bis(triisopropylsilylethynyl)pentacene was used for solution-processed hole-transporting molecular films and pentafluorobenzenethiol was used to form self-assembled monolayers (SAMs) on the contact metals. We conducted a comparative study on copper and gold contacts and realized that, under the same performance improvement schemes, via SAM treatment and controlled crystal growth, the copper electrode device experienced a more significant enhancement than the gold electrode device. We attribute the beneficial effects of SAMs to the improved charge injection and transport properties, which are critical double effects from the fluorinated aromatic SAM structure. Grazing-incidence wide-angle X-ray scattering (GIWAXS) measurements showed that templating property of SAMs promotes the crystallization of TIPS-pentacene films at the metal/organic interface. The presented result indicates that copper can be regarded as a promising candidate for reducing the use of gold in organic-based circuits and systems, where the cost-effective production is an important issue.

Electrochemical, high-resolution photoemission spectroscopy and vdW-DFT study of the thermal stability of benzenethiol and benzeneselenol monolayers on Au(111)

The preparation and thermal stability of benzenethiol and benzeneselenol self-assembled monolayers (SAMs) grown on Au(111) have been investigated by electrochemical experiments and high-resolution photoemission spectroscopy. Both techniques confirm the formation of monolayers with high packing densities (θ = 0.27-0.29 ML) and good degrees of order in both cases. Despite many similarities between the two SAMs, the thermal desorption is distinctly different: whereas the benzenethiol SAM desorbs in a single steplike process, the desorption of the benzeneselenol SAM occurs with a much lower activation energy and involves the cleavage of some Se-C bonds and a change in molecular configuration from standing up to lying down. This behavior is explained by considering the different nature of the bonding of the headgroup with the metal surface and with the phenyl ring. Density functional theory calculations show that the breakage of the Se-C bond has a lower activation energy barrier than the breakage of the S-C bond.