Search

We compare two different strategies for fabricating self-assembled monolayers (SAMs) exhibiting a thiol-terminated organic surface, which recently has attracted considerable interest with regard to the binding of metal, and in particular gold particles, to these organic surfaces. Results obtained by IR spectroscopy and scanning tunneling microscopy (STM) reveal that the straightforward approach, namely, using the corresponding organodithiols, typically leads to the formation of disordered, ill-defined surfaces. We demonstrate that high-quality organothiolate adlayers exhibiting an SH-terminated surface can be prepared using dithiols where one of the thiol groups is protected by a thioester group. After formation of the organothiolate adlayer, the protecting group can be removed by immersion into NaOH solution. IR spectra recorded for the deprotected organothiolate adlayer reveal the presence of a highly oriented organothiolate adlayer.

Self-assembled monolayers (SAMs) of ω-(4‘-methylbiphenyl-4-yl) alkanethiols CH3(C6H4)2(CH2)nSH (BPn, n = 3 and 4) on Au(111) substrates were studied using scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED). Preparation at elevated temperature results in highly ordered layers with large domains. Whereas the (2√3 × √3) structure of the BP3 SAMs is similar to that reported previously for other aromatic thiols, SAMs made from BP4 exhibit a very different structure. A (5√3 × 3)rect unit cell containing 8 molecules is found which corresponds to a packing density reduced by 25% compared to that of BP3. The odd−even effect observed in the molecularly resolved STM images of BP3 and BP4, therefore, confirms the pronounced influence of the spacer chain on the structure of these biphenyl-based SAMs.

The adsorption of dodecanethiol [CH3(CH2)11SH] films on Cu(110) by vapor deposition under ultrahigh vacuum conditions has been studied by means of thermal desorption spectroscopy, scanning tunneling microscopy, X-ray photoelectron spectroscopy (XPS), and low-energy electron diffraction with a special emphasis on the structural changes accompanying the transition from a physisorbed monolayer to a chemisorbed saturation structure. Adsorption at 110 K leads to the formation of an ordered physisorbed layer with flat-lying thiol molecules. Upon room-temperature deposition, initially an ordered pinstripe phase is formed which may be a molecular double layer. This layer transforms with time into a stable saturation structure of upright-tilted thiolates in a local c(2 × 2) arrangement that exhibits a long-range c(12 × 16) modulation, attributed to a Moiré pattern. The XPS measurements show that the room-temperature saturation structure contains a fraction of sulfide species formed by partial decomposition and desorption of alkyl chains. At 400 K, the thiolate monolayer desorbs dissociatively, eventually resulting in a p(5 × 2) sulfur structure.

The adsorption of various carboxylic acids with different chain lengths on Cu(111) has been investigated using near-edge X-ray absorption fine structure (NEXAFS) spectroscopy and X-ray photoelectron spectroscopy (XPS). To systematically study the influence of the chain length, we used three different carboxylic acids in the range of 1−30 carbon atoms, namely, formic acid (HCOOH), hexanoic acid (C5H11COOH), and triacontanoic acid (C29H59COOH). Films with coverages ranging from the sub-monolayer to the multilayer regime are prepared by organic molecular beam epitaxy. The adlayers were characterized in situ by XPS and NEXAFS directly elucidating the nature of the chemical bond to the substrate and the degree of orientational order in the film. Highly oriented organic monolayers could be fabricated for all investigated carboxylic acids on the Cu(111) surface, but their adsorption geometries differ strongly. Whereas in the saturated monolayer the O−C−O plane of formic acid is significantly tilted, the alkyl chain and the O−C−O plane in hexanoic acid are oriented almost normal to the surface. For triacontanoic acid, a coverage driven reorientation of the molecular chain from flat lying to upright standing takes place upon completion of the first layer. In the saturated monolayer, the investigated carboxylic acids interact via two equivalent oxygen atoms, i.e., they form a bidentate carboxylate bonding to the copper surface.

The adsorption of styrene on (111)-oriented thin layers of FeO and Fe3O4 epitaxially grown on Pt(111) single-crystal surfaces has been investigated using near-edge X-ray absorption fine structure (NEXAFS) spectroscopy and X-ray photoelectron spectroscopy (XPS). High-resolution gas-phase measurements were recorded, and precise ab initio electronic structure calculations were performed in order to aid the assignment of the NEXAFS resonances. Whereas on Fe3O4 (magnetite), styrene forms a chemisorbed monolayer that is stable up to room temperature, on FeO (wustite), a significantly more weakly bound (physisorbed) species is observed that is stable only for temperatures below 200 K. A planar adsorption geometry is observed in the case of styrene on Fe3O4 in the low-coverage regime (Θ < 0.5 ML); for higher coverages on the same surface and on FeO, the average tilt angle between the phenyl plane and the surface plane amounts to about 45°.

The adsorption kinetics of heptanethiol [CH3(CH2)6SH] on Cu(110) and the ordering of monolayer films prepared by ultrahigh vacuum vapor deposition have been studied by means of thermal-desorption spectroscopy, X-ray photoelectron spectroscopy, low-energy electron diffraction, and scanning tunneling microscopy. Below 200 K, heptanethiol is found to grow in multilayers for which a molecular binding energy of 63 kJ/mol was determined. The formation of the chemisorbed monolayer takes place via a physisorbed precursor. Heating the chemisorbed layer leads to a dissociative desorption of the alkyl chain with an enthalpy of 117 kJ/mol. In contrast to the rather disordered physisorbed overlayers, several superstructures for the various coverages have been observed for the chemisorbed phase. Heating the saturated monolayer to 350 K results in a well-ordered (%@mt;sys@% %@mh;-%pxwidth@%%@sx@%0%@be@%4%@sxx@%%@mx@%  %@mt;sys@% %@mh;-%pxwidth@%%@sx@%2%@be@%-1%@sxx@%%@mx@% ) structure.

The formation of self-assembled alkanethiolate monolayers (SAMs) on oxidized gold was studied by X-ray photoelectron spectroscopy (XPS), contact angle measurements, and near edge X-ray absorption fine structure (NEXAFS) spectroscopy. Gold was oxidized by exposure to atomic oxygen in a vacuum. SAMs were prepared by two methods:  (a) by immersion of the oxidized substrate into ethanolic solution and (b) via chemical vapor deposition (CVD). After both procedures two sulfur species could be distinguished by XPS, a gold thiolate and a species related to the reaction of the thiol with oxidized gold. Oxygen was found encapsulated under the self-assembled monolayer. The monolayers adsorbed from ethanolic solution were more densely packed and exhibited a smaller molecular tilt than the layers formed by CVD or SAMs prepared on clean gold surfaces.

Self-assembled octadecyltrichlorosilane (CH3(CH2)17SiCl3, OTS) films were deposited on an oxidized silicon wafer. They were carefully exposed to a low-pressure oxygen direct current (dc) plasma and/or Cr vapor. Subsequently, they were characterized by X-ray photoelectron spectroscopy, near-edge X-ray absorption fine structure spectroscopy, and atomic force microscopy. Detailed information on the oxygen functionalization and the molecular conformation of the OTS films was obtained. The main result was that functionalization conditions can be found where the order of the film remains almost unaffected. Similarly, Cr evaporation of pure OTS films revealed only slight effects on the order in the film. Cr evaporation of an ordered but functionalized OTS film immediately changes its state toward a fully isotropic one. In the course of interaction, oxygen sites at the surface of the functionalized OTS film are reduced whereas Cr becomes oxidized. A redox reaction scheme is proposed for this interfacial reaction. Finally, it is concluded that the given study should be of importance for advanced plasma processing of polyolefins.

The surfaces of ordered films formed from an amphiphilic derivative of 2,5-diphenyl-1,3,4-oxadiazole by the Langmuir−Blodgett (LB) technique and organic molecular beam deposition (OMBD) were investigated by the use of near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. For the assignment of the spectral features of the C, N, and O K-edge absorption spectra, fingerprint spectra of poly(p-phenylene terephthalamide) (Kevlar), poly(ethylene terephthalate), poly(p-phenylene-1,3,4-oxadiazole), and 2,5-di(pentadecyl)-1,3,4-oxadiazole, which contain related chemical moieties, were recorded. Ab initio molecular orbital calculations, performed with explicit treatment of the core hole, are used to support the spectral interpretations. Angle-resolved NEXAFS spectroscopy at the C, N, and O K-edges suggests a preferentially upright orientation of the oxadiazole derivative in the outermost layer of the films. X-ray specular reflectivity data and molecular modeling results suggest a similar interpretation.

A combined low-energy electron diffraction (LEED) and X-ray photoelectron spectroscopy (XPS) study on the structural properties of heptanethiol (C7H15SH) monolayers prepared by either adsorption from the liquid or chemical vapor deposition (CVD) on Au(111) single crystal surfaces has been performed. Deposition from the vapor produces a commensurate c(17×√3) superstructure which exhibits a well-defined LEED pattern. XPS data recorded for these films reveal that the hydrocarbon molecules are orientated with their axes parallel to the substrate surface. Compared to the more commonly used fabrication of alkanethiolate self-assembled films by adsorption from solution the c(17×√3) CVD film exhibits a coverage of less than 40%.

A phenyl-terminated organic surface was fabricated by self-assembly of phenyltricosanethiolate monolayers (C6H5(CH2)23SH) on polycrystalline gold substrates. The molecular orientation of the alkyl chain and the phenyl ring was determined by near edge X-ray absorption fine structure spectroscopy (NEXAFS). After chromium deposition a new resonance appeared in the NEXAFS spectra, which could be related to the formation of metal−π sandwich complexes by comparison to reference data recorded for multilayers of bis(benzene)chromium. This conclusion is supported by data recorded using X-ray photoelectron spectroscopy. Experiments carried out with a phenyl-terminated surface fabricated by using a short chain phenylalkanethiol demonstrate that a minimum chain length is required in order to avoid reactions of the deposited metal atoms with the gold substrate and/or with the C−S bonds.

Two different organic surfaces were created by self-assembly (SA) of bifunctionalized monomers using OH- and COOH-terminated alkanethiols. X-ray photoelectron (XP) and soft X-ray absorption (NEXAFS) spectroscopy have been used to determine the chemical composition of and the molecular orientation in the SA monolayers adsorbed on Au substrates. Subsequently the two OH- and COOH-terminated organic surfaces were exposed to phenyl isocyanate (C6H5NCO, PIC) in order to investigate their chemical reactivity. In both cases the reactivity to gas-phase PIC was very low for the sample at room temperature. Reaction yields of more than 80% could, however, be achieved by depositing multilayers on a sample cooled down to 120 K which was subsequently warmed to 290 K.

Monolayers of an end-group-modified thiol, 4-(12-mercaptododecyl)aminonitrobenzene (p-NO2−C6H4−NH(CH2)12SH) formed by self-assembly on Au substrates, were investigated by grazing-incidence reflection absorption infrared spectroscopy (IRRAS), X-ray photoelectron spectroscopy (XPS), near-edge X-ray absorption fine structure spectroscopy (NEXAFS), and contact angle measurements. The monolayer exhibits a packing of the alkyl chains which is 10−20% less dense compared to a monolayer of docosanethiol. The orientation of the aromatic end group was determined completely by a combination of IRRAS and NEXAFS. The 1,4-axis of the benzene ring is inclined to 52° with respect to the surface normal, and the plane of the benzene ring is rotated by 58° away from the plane defined by the 1,4-axis and the surface normal. No hydrogen bonding between the p-nitroaniline moieties is detected. The hydrocarbon chains do not show a preferred orientation. The film is described by a layered structure consisting of a layer of alkane chains with a high degree of gauche conformation beneath an oriented layer of the nitroaniline. Shifts of the vibrations of the alkane chain and the nitroaniline group in opposite directions are observed upon change of the temperature. This can be explained by a balance of contributions from the layer of the nitroaniline groups and the layer of the alkane chains to the total free enthalpy of the monolayer.