Complex surface chemistry of 4-mercaptopyridine self-assembled monolayers on Au(111)

The adsorption of 4-mercaptopyridine on Au(111) from aqueous or ethanolic solutions is studied by different surface characterization techniques and density functional theory calculations (DFT) including van der Waals interactions. X-ray photoelectron spectroscopy and electrochemical data indicate that self-assembly from 4-mercaptopyridine-containing aqueous 0.1 M NaOH solutions for short immersion times (few minutes) results in a 4-mercaptopyridine (PyS) self-assembled monolayer (SAM) with surface coverage 0.2. Scanning tunneling microscopy images show an island-covered Au surface. The increase in the immersion time from minutes to hours results in a complete SAM degradation yielding adsorbed sulfur and a heavily pitted Au surface. Adsorbed sulfur is also the main product when the self-assembly process is made in ethanolic solutions irrespective of the immersion time. We demonstrate for the first time that a surface reaction is involved in PyS SAM decomposition in ethanol, a surface process not favored in water. DFT calculations suggest that the surface reaction takes place via disulfide formation driven by the higher stability of the S-Au(111) system. Other reactions that contribute to sulfidization are also detected and discussed.     

Characterization of self-assembled monolayers of thiols on Au(111)

Self-assembled monolayers (SAMs) of n-butanethiol, n-dodecanethiol and their equimolar mixture on Au(111) were prepared and characterized by ellipsometry, contact angle measurement, scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). Results revealed that these SAMs are oriented ultrathin films with the thickness of nanometer scale, and the SAMs were influenced by the molecular chain length, the lattice orientation and cleanliness of the substrates. The surface of the longer chain SAM is hydrophobic. The thicknesses of three SAMs of n-butanethiol, n-dodecanethiol and their mixture revealed by ellipsometry and XPS are about 0.59 - 0.67nm, 1.60- 1.69 nm and 1.23 - 1.32nm, respectively. AFM images further demonstrated that the SAM formed by the mixture has some microdomains with two different thicknesses.     

Conformation and dynamics of arylthiol self-assembled monolayers on Au(111)

We report a computational investigation of the conformation and the dynamics of self-assembled monolayers (SAMs) of a set of aromatic thiols arranged in the ( radical3 x radical3)-R30 degrees packing ratio on a Au(111) surface using molecular dynamics (MD) simulations. It was found that the molecular conformations were better defined for the arylthiol with two phenyl groups as compared to those with a single phenyl group and that the chemical structure of the head and tail groups had a considerable influence on the system geometry. In line with the density functional theory (DFT) calculations of small thiol molecules, we found for the SAMs that the face-centered cubic (fcc) site on the Au(111) surface was the most preferred, followed by the hexagonal close-packed (hcp) site, while the bridge position showed the characteristics of a local energy maximum. The dynamics of thiol head groups on these three Au sites was found to govern the overall dynamics of SAMs as measured by the mean square displacement. We also report that both the conformation and the dynamics on the studied time scale were driven by the SAM formation energy.     

Self-assembled monolayers of 4-mercaptopyridine on Au(111): a potential-induced phase transition in sulfuric acid solutions

In situ scanning tunneling microscopy images of self-assembled monolayers (SAMs) of 4-mercaptopyridine (4-MPy) on Au(111) recorded in neat 0.1 M H2SO4 solutions provided evidence for a potential-induced phase transition over the range 0.40-0.15 V versus saturated calomel electrode. Analysis of the data was consistent with the presence of a (5 x square root(3)) and (10 x square root(3)) superstructure (phase A) at the positive end, that is, 0.40 V, for which the local coverage, theta(loc), is about 0.2 (two 4-MPy molecules per unit cell), which compresses at the negative end, that is, 0.15 V, to yield a much denser superstructure (phase B, theta(loc) ca. 0.5). This behavior is unlike that reported for the 4-MPy-Au(111) SAM prepared by identical means, in 0.1 M HClO4 (or in sulfate solutions of a much higher pH) for which only the (5 x square root(3)) superstructure was observed over the same potential range. The compression associated with the phase A to phase B transition is attributed to the formation of a hydrogen-bonded network of bisulfate coordinated in turn to the 4-MPy layer via the acidic hydrogens of the pyridinium moieties. Such conditions promote better packing of adsorbed 4-MPy species, which are aided by intermolecular pi-pi ring interactions, resulting in higher local coverages.     

Stress in self-assembled monolayers: omega-biphenyl alkane thiols on Au(111)

Self-assembled monolayers of omega-(4'-methylbiphenyl-4-yl) alkane thiols CH3(C6H4)2(CH2)(n)SH (BPn, n = 2, 3, and 5) on Au(111) substrates, prepared at room and elevated temperatures, were studied using scanning tunneling microscopy. In contrast to the biphenyl thiol analogues with n = 0 or 1, ordered domains of large size are formed which exhibit small, periodic height variations on a length scale of several nanometers. These are attributed to solitons (or domain walls), resulting from structural mismatch between the molecular adlayer and the gold substrate. The implications of these results for the design of aromatic thiols to cope with stress and yield low-defect density self-assembled monolayers are discussed.     

Electrodeposition of Au monolayer on Pt(111) mediated by self-assembled monolayers

In-situ scanning tunneling microscopy (STM), cyclic voltammetry (CV), and infrared reflection-adsorption spectroscopy (IRRAS) have been used to examine the electrodeposition of gold onto Pt(111) electrodes modified with benzenethiol (BT) and benzene-1,2-dithiol (BDT) in 0.1 M HClO4 containing 10 microM HAuCl4. Both BT and BDT were attached to Pt(111) via one sulfur headgroup. STM and IRRAS results indicated that the other SH group of BDT was pendant in the electrolyte. Both BT and BDT formed (2 x 2) structures at the coverage of 0.25, and they were transformed into (square root(3) x square root(3))R30 degrees as the coverage was raised to 0.33. These two organic surface modifiers resulted in 3D and 2D gold islands at BT- and BDT-coated Pt(111) electrodes, respectively. The pendant SH group of BDT could interact specifically with gold adspecies to immobilize gold adatoms on the Pt(111) substrate, which yields a 2D growth of gold deposition. Molecular resolution STM revealed an ordered array of (6 x 2 square root(13)) after a full monolayer of gold was plated on the BDT/Pt(111) electrode. Since BDT was strongly adsorbed on Pt(111), gold adatoms only occupied free sites between BDT admolecules on Pt(111). This is supported by a stripping voltammetric analysis, which reveals no reductive desorption of BDT admolecules at a gold-deposited BDT/Pt(111) electrode. It seems that the BDT adlayer acted as the template for gold deposit on Pt(111). In contrast, a BT adlayer yielded 3D gold deposit on Pt(111). This study demonstrates unambiguously that organic surface modifiers could contribute greatly to the electrodeposition of metal adatoms.     

Surface structure and interface dynamics of alkanethiol self-assembled monolayers on Au(111)

Scanning tunneling microscopy (STM) and high-resolution electron energy loss spectroscopy (HREELS) were used to examine the structural transitions and interface dynamics of octanethiol (OT) self-assembled monolayers (SAMs) caused by long-term storage or annealing at an elevated temperature. We found that the structural transitions of OT SAMs from the c(4 x 2) superlattice to the (6 x square root 3) superlattice resulting from long-term storage were caused by both the dynamic movement of the adsorbed sulfur atoms on several adsorption sites of the Au(111) surface and the change of molecular orientation in the ordered layer. Moreover, it was found that the chemical structure of the sulfur headgroups does not change from monomer to dimer by the temporal change of SAMs at room temperature. Contrary to the results of the long-term-stored SAMs, it was found that the annealing process did not modify either the interfacial or chemical structures of the sulfur headgroups or the two-dimensional c(4 x 2) domain structure. Our results will be very useful for a better understanding of the interface dynamics and stability of sulfur atoms in alkanethiol SAMs on Au(111) surfaces.     

Formation of ordered self-assembled monolayers by adsorption of octylthiocyanates on Au(111)

Self-assembled monolayers (SAMs) were formed by the spontaneous adsorption of octythiocyanate (OTC) on Au(111) using both solution and ambient-pressure vapor deposition methods at room temperature and 50 degrees C. The surface structures and adsorption characteristics of the OTC SAMs on Au(111) were characterized by scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS). The STM observation showed that OTC SAMs formed in solution at room temperature have unique surface structures including the formation of ordered and disordered domains, vacancy islands, and structural defects. Moreover, we revealed for the first time that the adsorption of OTC on Au(111) in solution at 50 degrees C led to the formation of SAMs containing small ordered domains, whereas the SAMs formed by vapor deposition at 50 degrees C had long-range ordered domains, which can be described as (radical3 x 2 radical19)R5 degrees structures. XPS measurements of the peaks in the S 2p and N 1s regions for the OTC SAMs showed that vapor deposition is the more effective method as compared to solution deposition for obtaining high-quality SAMs by adsorption of OTC on gold. The results obtained will be very useful in understanding the SAM formation of organic thiocyanates on gold surfaces.     

Chemically transformable configurations of mercaptohexadecanoic acid self-assembled monolayers adsorbed on Au(111)

Carboxyl-terminated self-assembled monolayers (SAMs) are commonly used in a variety of applications, with the assumption that the molecules form well-ordered monolayers. In this work, near-edge X-ray absorption fine structure measurements verify that well-ordered monolayers can be formed using acetic acid in the solvent. Disordered monolayers with unbound molecules present in the film result using only ethanol. A stark reorientation occurs upon deprotonation of the end group by rinsing in a KOH solution. This reorientation of the end group is reversible with tilted-over, hydrogen-bound carboxyl groups while the carboxylate ion end groups are upright. C(1s) photoemission shows that SAMs formed and rinsed with acetic acid in ethanol have protonated end groups, while SAMs formed without acetic acid have a large fraction of carboxylate-terminated molecules.     

STM study of mixed alkanethiol/biphenylthiol self-assembled monolayers on Au(111)

A method is presented for depositing mixed self-assembled monolayers (SAMs) of dodecanethiol (C12) and 4'-methyl-1,1'-biphenyl-4-butane (H3C-C6H4-C6H4-(CH2)4-SH, BP4) by insertion of BP4 into a closely packed SAM of dodecanethiol on Au(111). Insertion takes place at defect sites such as domain boundaries or etch pits in the gold surface that are characteristic of C12 monolayers on gold. With a lower probability, insertion also occurs beside defect sites inside dodecanethiol domains. Insertion at defect sites results in domains of BP4, whereas insertion into C12 domains leads to isolated BP4 molecules. The isolated BP4 molecules are shown not to move at room temperature. By comparing the apparent height of the isolated BP4 molecules and BP4 domains, it is proposed that the isolated molecules have the same conformation as in the full-coverage phase. A simple two-layer model is proposed to characterize the current transport through BP4. The decay constant beta for the phenylene groups is deduced from the apparent STM heights of the inserted BP4 islands compared to the STM heights of the C12 closely packed monolayers.     

Electroactive self-assembled biferrocenyl alkanethiol monolayers on Au(111) surface and on gold nanoclusters

The spectroscopic and electrochemical characterizations of electrochemically stable biferrocene-modified Au clusters and chemisorbed biferrocenylalkanethiols on Au(111) surface were studied. The characterizations of biferrocene-modified Au cluster using TEM, UV-vis, and NMR techniques are also reported. Two successive reversible one-electron redox waves were observed for the biferrocenylalkanethiol Au nanoclusters and biferrocenylalkanethiol monolayers on Au(111) surface in the cyclic voltammetry. Furthermore, the positive and negative current peaks for each redox wave occur at almost the same potential, and the peak current increases almost linearly with the sweep rate. Repeat scanning does not change the voltammograms, demonstrating that these monolayers are stable to electrochemical cycling. The coverages of electroactive biferrocene in the monolayers were calculated from the cyclic voltammograms. The standard electron-transfer rate constant was calculated from the splitting between the oxidation and reduction peaks.     

Substrate-induced pairing in 2,3,6,7,10,11-hexakis-undecalkoxy-triphenylene self-assembled monolayers on Au111

We present an STM study of self-assembled monolayers of 2,3,6,7,10,11-undecalkoxy-substituted triphenylene (T11) at the n-tetradecane/Au(111) interface under ambient conditions. T11 molecules self-organize as paired rows with molecules lying flat on the surface in an antiparallel position. Three alkyl chains of each T11 molecule align along the 110 direction of the underlying Au(111) substrate. The association of T11 in molecular pairs appears to result from a substrate-induced mechanism governed by the strong anisotropic interaction between T11 alkyl chains and Au(111).     

Structure and identity of 4,4'-thiobisbenzenethiol self-assembled monolayers

Self-assembled monolayers (SAMs) of 4,4'-thiobisbenzenethiol (TBBT) can be formed on Au surface spontaneously. The structural characteristics and adsorption behavior of TBBT SAMs on Au have been investigated by surface enhanced Raman scattering (SERS), electrochemical cyclic voltammetry (CV), ac impedance spectroscopy (EIS), and atomic force microscopy (AFM). It is demonstrated that TBBT adsorbed on Au by losing a H atom, forming one Au-S bond, and the other mercapto group is free at the surface of the monolayer owing to the presence of the nu(S-H) at 2513 cm(-1) and the delta(C-S-H) at 910 cm(-1) in SERS. The enhancement of the vibration of C-S (1064 cm(-1)), the aromatic C-H vibration (3044 cm(-1)), and the absence of the vibration of S-S illustrate TBBT adsorbed on Au forming a monolayer with one benzene ring tilted with respect to the Au surface. The interpretation of the observed frequencies is aided by ab initio molecular orbital (MO) calculations at the HF/6-31G level of theory. Electrochemical CV and EIS indicate TBBT monolayers can passivate the Au effectively for its low ratio of pinhole defects (theta = 99.6%). AFM studies give details about the surface morphology. The applications of TBBT SAMs have been extensively investigated by exposure of Cu2+ ion to TBBT SAMs on Au and covalent adsorption of metal nanoparticles. Electrochemical, X-ray photoelectron spectroscopic, and SERS results indicate that Cu2+ can react with TBBT SAMs and present on TBBT SAMs as Cu(I). A scanning electron microscopic image of Ag nanoparticles on TBBT/Au and the Raman spectrum of TBBT in smooth macroscopic Au/TBBT SAMs/Ag nanoparticle sandwich structure indicate that metal nanoparticles can be adsorbed on TBBT SAMs effectively through covalent linkage.     

Structural characterization of 4-bromostyrene self-assembled monolayers on si(111)

Organic functionalization of silicon holds promise for a variety of applications ranging from molecular electronics to biosensing. Because the performance and reliability of organosilicon devices will be intimately tied to the detailed structure of the organic adlayers, it is imperative to develop systematic strategies for forming and characterizing self-assembled monolayers (SAMs) on silicon with submolecular spatial resolution. In this study, we use 4-bromostyrene for the photochemical growth of Br-terminated SAMs on Si(111). A variety of experimental and theoretical techniques including atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), X-ray reflectivity (XRR), X-ray standing waves (XSW), X-ray fluorescence (XRF), and density functional theory (DFT) have been employed to determine the coverage and conformation of the 4-bromostryene molecules within the SAM. In particular, AFM verifies a continuous and atomically flat SAM, and the XRR data indicate a SAM thickness of 8.50 A and a molecular coverage of 46% of the surface silicon atoms. Because the DFT calculations indicate a molecular length of 8.89 A, the measured XRR thickness implies a molecular tilt angle of approximately 17 degrees. The XRR analysis also suggests that the Br atoms are preserved on top of the SAM in agreement with XPS measurements that show bromine bound solely to carbon and not to silicon. XRF reveals a Br atomic coverage of 50%, again in close agreement to that found by XRR. Single-crystal Bragg diffraction XSW is used to generate a three-dimensional map of the Br distribution within the SAM, which in conjunction with the XRR result suggests that the 4-bromostyrene molecules are tilted such that the Br atoms are located over the T4 sites at a height of 8.50 A above the top bulklike Si(111) layer. The direction of molecular tilt toward the T4 sites is consistent with that predicted by the DFT calculation. Overall, through this unique suite of complementary structural characterization techniques, it is concluded that the Br functional handle is preserved at the top of the SAM and is available for further substitutional chemistry.     

Two-dimensional ordering of benzenethiol self-assembled monolayers guided by displacement of cyclohexanethiols on Au(111)

Although the adsorption of benzenethiols (BT) on Au(111) usually leads to the formation of disordered phases, we demonstrate here that the displacement of preadsorbed cyclohexanethiol self-assembled monolayers (SAMs) on Au(111) by BT molecules can be a successful approach to obtain two-dimensional BT SAMs with long-range ordered domains.     

Structure and conductance of aromatic and aliphatic dithioacetamide monolayers on Au(111)

The structure and electrical properties of self-assembled monolayers of cyclic aromatic and aliphatic dithioacetamides (1,4-bis(mercaptoacetamido)benzene and 1,4-bis(mercaptoacetamido)cyclohexane) and of mixed dithioacetamide/alkanethiol monolayers are characterized by X-ray photoelectron spectroscopy (XPS), scanning tunneling microscopy (STM) and contact angle goniometry. Both dithioacetamides are found to pack densely on Au(111), however the monolayers are poorly ordered as a result of hydrogen bond formation between the amide groups. The coassembly and the insertion method are compared for the formation of mixed dithioacetamide/alkanethiol monolayers. By coassembly, islands of dithioacetamides in a dodecanethiol matrix can only be obtained at a low dithioacetamide/dodecanethiol concentration ratio in solution (1/10) and by thermal annealing of the resulting monolayers. Small and well defined dithioacetamide domains are realized by insertion of dithioacetamides into defect sites of closely packed octanethiol monolayers. These domains are used to determine the molecular conductance by means of STM height profiles and molecular lengths resulting from density functional theory (DFT) calculations. The difference in the tunneling decay constant beta measured for aromatic dithioacetamides (beta = 0.74-0.76/A) and for aliphatic dithioacetamides (beta = 0.84-0.91/A) highlights the influence of the conjugation within the cyclic core on molecular conductance.     

Novel self-assembled monolayers of disulfides with bicyclo[2.2.2]octane moieties of Au(111)

A series of disulfides containing bicyclo[2.2.2]octane moieties have been synthesised and their self-assembled monolayers (SAMs) on Au(111) have been characterized using scanning tunnelling microscopy (STM).     

Structures and displacement of 1-adamantanethiol self-assembled monolayers on Au{111}

We have designed monolayers with weak intermolecular interactions for use as placeholders in intelligent self- and directed-assembly. We have shown that these 1-adamantanethiolate monolayers are labile with respect to displacement by exposing them to dilute solutions of alkanethiols. These self-assembled monolayers (SAMs) of 1-adamantanethiol on Au{111} were probed using ambient scanning tunneling microscopy (STM), and their assembled order was determined. Solution deposition of the molecules results in a highly ordered hexagonally close-packed molecular lattice with a measured nearest neighbor distance of 6.9 +/- 0.4 A. The SAMs exhibit several rotational domains, but lack the protruding domain boundaries typical of alkanethiolate SAMs, and are similarly stable at room temperature. Co-deposition of alkanethiol and 1-adamantanethiol from solution results in alkanethiolate SAMs, except when using extremely low alkanethiol to 1-adamantanethiol concentration ratios. Facile displacement of low interaction strength SAMs can be exploited to enhance patterning using soft nanolithography.