Tracking down the origin of peculiar vibrational spectra of aromatic self-assembled thiolate monolayers.

Several studies have previously observed surprisingly low frequencies for the C-H stretching modes of self-assembled monolayers (SAMs) prepared from aromatic thiols. The reason for this property has so far remained elusive. Therefore, we report a novel study of the vibrational spectra of SAMs prepared on Au from two different aromatic thiols, namely, 4'-nitro-1,1'-biphenyl-4-thiol (NBPT) and 4-aminothiophenol (ATP). The SAMs were prepared by vapor deposition (VD) in ultrahigh vacuum (UHV) as well as by the solution method (SM) and their quality was controlled by X-ray photoelectron spectroscopy (XPS). In addition, amino terminated SAMs were also obtained by electron irradiation and by chemical reduction of NBPT SAMs. Beside infrared reflection absorption spectroscopy (IRRAS), we have employed high resolution electron energy loss spectroscopy (HREELS), by which VD SAMs can be studied in situ, i.e. without exposing them to air. Hence, we can exclude possible contributions of solvent molecules to the vibrational spectra. Nonetheless, HREELS in fact reveals the same large red shift of the C-H stretching modes in the SAMs as also observed in ex situ IRRAS experiments. In contrast, HREELS for physisorbed ATP and ATP in a KBr pellet measured by transmission infrared spectroscopy exhibit the expected aromatic bands. Using a computational approach, we can exclude molecular packing effects as origin of this shift. Therefore, we propose chemical changes in the aromatic rings during SAM formation as an alternative explanation for the observed frequency shift. As another striking effect, the N-H stretching vibrational modes of the amino-terminated SAMs are extremely weak in both IRRAS and HREELS despite the fact that XPS confirms the presence of amino groups. A very weak signal is observed only in the case of an electron irradiated NBPT SAM. In contrast, an energy loss ascribed to the N-H stretching vibrations is clearly observed in HREELS of ATP physisorbed on an ATP SAM and on graphite as well as in the transmission infrared spectrum of ATP in KBr. The extremely low intensity of these vibrations in the SAM is traced back to the inherently low transition dipole moment for the excitation of N-H stretching modes in free N-H groups. Furthermore, the calculations suggest that the much stronger signals of N-H stretching modes involved in hydrogen-bonding with adjacent amino groups are suppressed because these vibrations are oriented parallel to the surface.

Protein resistant oligo(ethylene glycol) terminated self-assembled monolayers of thiols on gold by vapor deposition in vacuum.

Protein resistant oligo(ethylene glycol) (OEG) terminated self-assembled monolayers (SAMs) of thiols on gold are commonly used for suppression of nonspecific protein adsorption in biology and biotechnology. The standard preparation for these SAMs is the solution method (SM) that involves immersion of the gold surface in an OEG solution. Here the authors present the preparation of 11-(mercaptoundecyl)-triethylene glycol [HS(CH(2))(11)(OCH(2)CH(2))(3)OH] SAMs on gold surface by vapor deposition (VD) in vacuum. They compare the properties of SAMs prepared by VD and SM using x-ray photoelectron spectroscopy (XPS), polarization modulation infrared reflection absorption spectroscopy, and surface plasmon resonance measurements. VD and SM SAMs exhibit similar packing density and show a similar resistance to the nonspecific adsorption of various proteins (bovine serum albumin, trypsin, and myoglobin) under physiological conditions. A very high sensitivity of the OEG SAMs to x-ray radiation is found, which allows tuning their protein resistance. These results show a new path to in situ engineering, analysis, and patterning of protein resistant OEG SAMs by high vacuum and ultrahigh vacuum techniques.

On the release of hydrogen from the S-H groups in the formation of self-assembled monolayers of thiols.

When thiol self-assembled monolayers (SAMs) form on gold surfaces, it is widely believed that, upon adsorption, the thiol molecules dissociate via S-H bond scission. This mechanism is hard to verify since hydrogen is difficult to detect during this process. Hence, other reaction schemes such as nondissociative thiol adsorption have also been proposed. Here we present experimental evidence that clearly shows that hydrogen is released during dissociative thiol adsorption and interacts with the monolayer terminus. Vacuum vapor deposition was used to form SAMs of 4-nitrophenylthiol, 4'-nitro-1,1'-biphenyl-4-thiol, and bis-(4,4'-nitrophenyl)-disulfide on gold surfaces. X-ray photoelectron spectroscopy shows that the nitro groups of the thiol SAMs are partly reduced to amino groups, while those of the disulfide SAMs are not. The reduction is attributed to hydrogen released in the dissociation of S-H bonds during thiol adsorption. Possible pathways for the interaction of hydrogen with the nitro groups are discussed.

From clusters to fibers: parameters for discontinuous para-hexaphenylene thin film growth.

All relevant steps of discontinuous thin film growth of para-hexaphenylene on muscovite mica (0 0 1) from wetting layer over small and large clusters to nanofibers are observed and investigated in detail by a combined polarized fluorescence and atomic force microscopy study. From a variation of film thickness and surface temperature, we determine effective activation energies for cluster growth of 0.17 eV, for nanofiber length growth of 0.46 eV, for width growth of 0.19 eV, and for height growth of 0.07 eV. The corresponding exponential prefactors for the nanofiber growth are 1 x 10(9), 6 x 10(4), and 3 x 10(2) nm. Polarized fluorescence studies reveal that nanofibers grow along the grooves of the mica surface and that they do not change direction if they cross an even number of mica surface steps, while they change direction by 120 degrees for an odd number of steps. These results are taken as an input for a model of the unidirectional growth process on mica. Absolute parameters allowing one to grow nanofibers of predetermined morphology via organic molecular beam epitaxy are also given.

Tailoring the growth of p-6P nanofibres using ultrathin Au layers: an organic-metal-dielectric model system.

The influence of ultrathin Au cluster films on the growth of para-hexaphenyl (p-6P) fibres is investigated. Whereas p-6P at elevated temperatures forms long, mutually parallel fibres on plain mica, these fibres become shorter but taller on Au covered mica, up to a Au film thickness of approximately 8 nm. The degree to which fibres are mutually parallel decreases with increasing Au thickness. For thicker Au films the length of the fibres increases again, and their morphology changes from flat to faceted; for Au film thicknesses above 20 nm, fibre networks are formed. The spectroscopic properties of the fibres are not modified by the Au layer, enabling independent control of the fibre morphology by means of the intermediate metallic layer.