Resolving the Au-Adatom-Alkanethiolate Bonding Site on Au(111) with Domain Boundary Imaging Using High-Resolution Scanning Tunneling Microscopy

Light-induced Photoswitching of 4-(Phenylazo)benzoic Acid on Au(111)

Photochromic molecules can undergo a reversible conversion between two isomeric forms upon exposure to external stimuli such as electromagnetic radiation. A significant physical transformation accompanying the photoisomerization process defines them as photoswitches, with potential applications in various molecular electronic devices. As such, a detailed understanding of the photoisomerization process on surfaces and the influence of the local chemical environment on switching efficiency is essential. Herein, we use scanning tunneling microscopy to observe the photoisomerization of 4-(phenylazo)benzoic acid (PABA) assembled on Au(111) in kinetically constrained metastable states guided by pulse deposition. Photoswitching is observed at low molecular density and is absent in tight-packed islands. Furthermore, switching events were noted in PABA molecules coadsorbed in a host octanethiol monolayer, suggesting an influence of the surrounding chemical environment on photoswitching efficiency.

Stereoselective formation of coordination polymers with 1,4-diaminonaphthalene on various Cu substrates

Polymerization of 1,4-diaminonaphthalene on various Cu substrates resulting in stereoselectively well-defined metal-organic coordination polymers is reported. By using different crystallographic planes (111), (110) and (100) of a Cu substrate the structure of the resulting coordination polymer was controlled.

SCC-DFTB parameters for simulating hybrid gold-thiolates compounds

We present a parametrization of a self-consistent charge density functional-based tight-binding scheme (SCC-DFTB) to describe gold-organic hybrid systems by adding new Au-X (X = Au, H, C, S, N, O) parameters to a previous set designed for organic molecules. With the aim of describing gold-thiolates systems within the DFTB framework, the resulting parameters are successively compared with density functional theory (DFT) data for the description of Au bulk, Aun gold clusters (n = 2, 4, 8, 20), and Aun SCH3 (n = 3 and 25) molecular-sized models. The geometrical, energetic, and electronic parameters obtained at the SCC-DFTB level for the small Au3 SCH3 gold-thiolate compound compare very well with DFT results, and prove that the different binding situations of the sulfur atom on gold are correctly described with the current parameters. For a larger gold-thiolate model, Au25 SCH3 , the electronic density of states and the potential energy surfaces resulting from the chemisorption of the molecule on the gold aggregate obtained with the new SCC-DFTB parameters are also in good agreement with DFT results.

Atomic-scale insight into the formation, mobility and reaction of Ullmann coupling intermediates

The Ullmann reaction of bromobenzene, the simplest coupling reagent, to form biphenyl on a Cu surface proceeds via a highly mobile organometallic intermediate in which two phenyl groups extract and bind a single surface Cu atom.

Density functional theory study of the conformation and optical properties of hybrid Au(n)-dithienylethene systems (n = 3, 19, 25)

We present a theoretical study of Aun-dithienylethene hybrid systems (n = 3, 19, 25), where the organic molecule is covalently linked to a nanometer-scaled gold nanoparticle (NP). We aim at gaining insights on the optical properties of such photochromic devices and proposing a size-limited gold aggregate model able to recover the optical properties of the experimental system. We thus present a DFT-based calculation scheme to model the ground-state (conformation, energetic parameters) and excited-state properties (UV-visible absorption spectra) of this type of hybrid systems. Within this framework, the structural parameters (adsorption site, orientation, and internal structure of the photochrome) are found to be slightly dependent on the size/shape of the gold aggregate. The influence of the gold fragment on the optical properties of the resulting hybrid system is then discussed with the help of TD-DFT combined with an analysis of the virtual orbitals involved in the photochromic transitions. We show that, for the open hybrid isomer, the number of gold atoms is the key parameter to recover the photoactive properties that are experimentally observed. On the contrary, for hybrid closed systems, the three-dimensional structure of the metallic aggregate is of high impact. We thus conclude that Au25 corresponds to the most appropriate fragment to model nanometer-sized NP-DTE hybrid device.

Epitaxial transfer through end-group coordination modulates the odd-even effect in an alkanethiol monolayer assembly

Short spacer length and high end-group coordination lead to the top network acting as a template for the buried sulfur-gold interface of n-alkanethiols (SH-(CH2)n-OH or SH-(CH2)n-CH3) on gold {111}. Annealing and templating both drive toward a higher sampling of the spatially favorable bridge adsorption sites. The hydrogen-bonded network increases in strength by increasing the number of hydrogens participating per oxygen, from 1.75 to 1.98 for n = 14-30. Higher n leads to better packing (five times for hydroxyl-terminated and seven times for methyl-terminated for n = 14-30) and stability of monolayers, while lower n results in better epitaxial transfer (transfer coefficient ratio = 13.5 for {SH-(CH2)14-OH}/{SH-(CH2)30-CH3}) and actuation. Odd values of n for the hydroxyl-terminated n-alkanethiols lead to lattice spacing of an average of 0.04 ± 0.01 Å higher than even values. There is a structural transition in properties around spacer length n = 24-27. Characterization of monolayer assembly through correlation between adatom and network layers provides recursive design principles for actuation and sensing applications.

The striped phases of ethylthiolate monolayers on the Au(111) surface: a scanning tunneling microscopy study

Striped phases of ethylthiolate monolayers, corresponding to surface coverage in between 0.2 ML and 0.27 ML, were studied using high-resolution scanning tunneling microscopy. Striped phases consist of rows of Au-adatom-diethythiolate (AAD) aligned along the [112] direction. In the perpendicular [110] direction, the AAD rows adjust their spacing according to the surface coverage. A (5√3 × √3)-R30° striped phase with 0.27 ML thiolate and a (6√3 × √3)-R30° striped phase with 0.23 ML thiolate, both with long-range order, are found. A localized (5 × √3)-rect. phase is also found as a minority phase embedded in the 5√3 × √3)-R30° phase. This (5 × √3)-rect. phase can be constructed using di-Au-adatom-tri-thiolate species.

Synthesis, characterization, and relative stabilities of self-assembled monolayers on gold generated from bidentate n-alkyl xanthic acids

A series of self-assembled monolayers (SAMs) on gold were generated by the adsorption of n-alkyl xanthic acids (NAXAs) having the general formula CH3(CH2)nOCS2H (n = 12-15). The structural features of these SAMs were characterized by optical ellipsometry, contact angle goniometry, polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS), and X-ray photoelectron spectroscopy (XPS). This series of xanthate SAMs were compared to SAMs generated from the corresponding n-alkanethiols and aliphatic dithiocarboxylic acids (ADTCAs). The collected data indicate that the NAXAs generate densely packed and well-ordered monolayers. The contact angles of hexadecane on the xanthate monolayers exhibited a large "odd-even" effect similar to that produced by the ADTCA SAMs. The relative stability of these bidentate xanthate SAMs was evaluated by monitoring the changes in ellipsometric thicknesses and wettability as a function of time under various conditions. The results demonstrate that SAMs formed from NAXAs are much less stable than analogous n-alkanethiolate and ADTCA SAMs.

Solution effect on diazonium-modified Au(111): reactions and structures

Surface modifications of a Au(111) electrode with 4-bromobenzenediazonium tetrafluoroborate (BBD) in acetonitrile (ACN) and 0.1 M HClO4 have been characterized by scanning tunneling microscopy (STM). In ACN, STM results reveal the formation of disordered thin organic films. The involvement of the radical as an intermediate is evidenced by the negative effect of radical scavengers on organic thin film formation. In contrast, the 4,4'-dibromobiphenyl monolayer is observed when the aqueous solution is used as a medium to carry out the grafting experiment. The biphenyl compound is considered to be generated by a radical-radical coupling reaction.

Rapid formation of a dense sulfur layer on gold through use of triphenylmethane sulfenyl chloride as a precursor

The use of triphenylmethane sulfenyl chloride as a new precursor leads to the efficient deposition of sulfur on polycrystalline gold and Au(111) substrates. The modified surfaces are characterized using X-ray photoelectron spectroscopy (XPS), electrochemistry and scanning tunneling microscopy (STM). The XPS data shows the rapid deposition of polymeric sulfur within very short times. Electrochemical stripping cyclic voltammetry (CV) confirms the rapid deposition and shows that high coverage values are achieved. STM imaging shows the formation of a wide range sulfur layer and production of the well-known etch pits. High-resolution STM images confirm the high density of the sulfur layers and show formation of a long-range phase consisting of rhombus structures close to the previously described rectangular structures along with other parallelograms and partial parallelograms. The present results do not show the initial formation of any organic self-assembled monolayer (SAM) indicating that the formation of polymeric sulfur does not result from the decomposition of an initial SAM as previously observed with alkyl and aryl thiolate-based SAMs. The suggested mechanism involves an initial reductive process similar to the one reported for thiocyanates and sulfenyl chlorides. This is followed by the dissociation of the Ph(3)C-S bond, leaving only sulfur on the surface, through a process leading to the recombination of the remaining fragments to yield triphenylmethyl chloride.

Directed growth of mixed self-assembled monolayers on a nanostructured template: a step toward the patterning of functional molecular domains

We report on the elaboration of networks of SAM domains. More precisely, we show the feasibility in making arrays of functionalized alkylthiol nanodomains bordered with an alkylthiol matrix. The several step process relies on the replication of a self-organized cobalt array grown on Au(111). The SAM process takes place in solution. The chemical affinity of thiol for gold leads to the selective grafting of molecules on the surface. After having removed the inorganic array, alkylthiol functionalized with a terthiophene unit is grafted in free gold areas. The efficiency of the replication of the initial template depends on the stability of the first SAM. We also investigate electronic tunnel transport through oligothiophene islands with the STM. The variation of the molecular contrast with bias voltage between the two molecular species indicates a potential resonant tunneling mechanism through the orbitals of the aromatic compound.

Adsorption and electron-induced dissociation of ethanethiol on Au(111)

Dissociation of ethanethiol and the formation of Au-adatom-diethylthiolate rows on the Au(111) surface were investigated using scanning tunneling microscopy (STM) at low temperature. Ethanethiol molecules physisorb on Au(111) at 120 K by sequentially occupation of the elbow site, the fcc domain before covering the whole surface with a semiliquid layer without long-range order. Scanning the physisorbed layer with a sample bias higher than +1.2 V leads to dissociation via cleaving the H-S bond. One of the dissociation products, ethylthiolate, forms a double-row structure with the rows aligned in one of the [112(-)] directions. These double rows arise from the Au-adatom-dithiolate species: CH(3)CH(2)S-Au-SCH(2)CH(3).

Physical structure of standing-up aromatic SAMs revealed by scanning tunneling microscopy

Long-range-ordered aromatic SAMs are formed on Au(111) using 4-nitrophenyl sulfenyl chloride as a precursor. Although the main structure is a √3 × √3 with a molecular density similar to that usually found for aliphatic SAMs, particular spots presenting specific shapes are also observed by STM. These include hexagons, partial hexagons, parallelograms, and zigzags resulting from specific arrangements of adsorbed molecules. These molecular arrangements are reversible as they form and dissociate or "vanish" in various areas on the surface. STM shows that these particular structures provide some order to their surrounding because areas void of these structures look less ordered. More interestingly, STM shows submolecular details of the molecules involved in forming these structures, hence providing direct experimental evidence for the ability of the STM to provide physical structure information of standing up SAMs. This is indeed a heavily debated question, and this work reports the first experimental example where submolecular physical structure is revealed by STM for standing-up SAMs.

Multidentate adsorbates for self-assembled monolayer films

The spontaneous adsorption of organic molecules on a variety of planar and nonplanar substrates, that is, self assembly, can generate films just one molecule thick. These nanoscale, self-assembled monolayer (SAM) films have been extensively used to engineer surfaces with well-defined properties. Their utility has been demonstrated in a wide range of applications, including wetting, adhesion, lubrication, patterning, and molecular recognition. Many SAM systems have been investigated, but alkanethiols adsorbed on gold are the most successful combination. This pairing offers a variety of advantages, including the ability to tune precisely the interfacial properties of a surface through the well-established organic synthetic methodologies that have been developed for preparing custom ω-terminated alkanethiols. Alkanethiolate monolayers are moderately stable at room temperature; however, these films degrade over time and readily desorb upon moderate heating. This shortcoming limits the use of SAMs in applications involving elevated temperatures or harsh environments. Accordingly, new adsorbates with multiple bonding moieties have been created to enhance the stability and versatility of SAMs. In this Account, we examine a variety of multidentate adsorbate structures that have been used to generate SAMs on planar substrates and on nanoparticles. Each of these chelating adsorbates (bidentates and tridentates) has been designed to generate well-defined organic monolayer films with multiple attachment points to the underlying substrate. This bonding arrangement allows the formation of SAMs with enhanced stability through the entropy-driven "chelate effect". The research examined here demonstrates that multidentate adsorbates provide robust films: they enable the use of SAMs under conditions that are incompatible with SAMs derived from normal alkanethiols. Another advantage offered by multidentate adsorbates is the capacity for new paradigms in thin-film composition. In particular, appropriately designed chelating adsorbates can be engineered to have two or more chemically distinct terminal groups that are covalently linked to the same underlying headgroup, without adding steric bulk that might prove detrimental to the resultant assembly. This strategy allows the generation of homogeneously mixed multicomponent surfaces, overcoming the problem of phase separation or "islanding" that is pervasive when two or more chemically distinct adsorbates are used to form mixed SAMs. Such homogeneously mixed films offer the opportunity to fine-tune the interfacial properties of a substrate and to create unique heterogeneous interfaces that are well defined by the chemical composition of the tailgroups exposed at the surface. The insight derived from these studies opens the door to new uses for SAMs, both in surface engineering applications (such as corrosion resistance and soft lithographic patterning) and in the stabilization and manipulation of nanoparticles.