Articles you may be interested inNegative resist behavior of neutral sodium atoms deposited on self-assembled monolayers J. Vac. Sci. Technol. B 25, L5 (2007); 10.1116/1.2431351Dipole-induced structure in aromatic-terminated self-assembled monolayers-A study by near edge x-ray absorption fine structure spectroscopy Exposure of self-assembled monolayers to highly charged ions and metastable atoms This article reviews recent experiments on the modification of thiol-derived self-assembling monolayers ͑SAMs͒ by electron and x-ray irradiation. Several complementary experimental techniques such as near-edge x-ray absorption fine structure spectroscopy, x-ray photoelectron spectroscopy and microscopy, and infrared reflection absorption spectroscopy were applied to gain a detailed knowledge on the nature and extent of irradiation-induced damage in these systems. The reaction of a SAM to electron and x-ray irradiation was found to be determined by the interplay of the damage/decomposition and cross-linking processes. Ways to adjust the balance between these two opposing effects by molecular engineering of the SAM constituents are demonstrated. The presented data provide the physical-chemical basis for electron-beam patterning of self-assembled monolayers to extend lithography down to the nanometer scale.

Self-assembled monolayers of a series of ω-(4‘-methyl-biphenyl-4-yl)-alkanethiols (CH3−C6H4−C6H4−(CH2) m −SH, m = 1−6) formed on polycrystalline gold and silver surfaces were characterized in detail by contact angle measurements, optical ellipsometry, X-ray photoelectron spectroscopy (XPS), reflection absorption infrared spectroscopy (IRRAS), and near-edge X-ray absorption fine structure spectroscopy (NEXAFS). The orientation of the biphenyl moiety, determined by combining the results from IRRAS and NEXAFS, exhibits a pronounced dependence on the number of methylene groups. Similar to n-alkanethiols an odd−even effect is observed which on silver is opposite to that on gold. For m = odd on gold and m = even on silver the arrangement of the aromatic moieties agrees well with the bulk structure of biphenyl, and the bonding of the thiols to the substrate is in agreement with an sp3 hybridization of the sulfur on gold and sp on silver, respectively. In the opposite case of m = even on gold and m = odd on silver, the biphenyl moieties adopt a significantly more canted orientation which, as a consequence, results in a lower coverage. The odd−even behavior of the coverage is in sharp contrast to that seen for n-alkanethiols. The experiments provide evidence that a significant driving force exists to pertain the sp3 and sp hybridization of sulfur on gold and silver, respectively. In the case of gold substrates the experimental results are in conflict with available bending potentials derived from ab initio calculations.

Self-assembled monolayers (SAMs) formed from thiophenol, 1,1′-biphenyl-4-thiol, 1,1′;4′,1′′-terphenyl-4-thiol, and anthracene-2-thiol on polycrystalline Au and Ag were characterized by X-ray photoelectron spectroscopy and angle-resolved near-edge X-ray absorption fine structure spectroscopy. With the exception of the poorly defined thiophenol film on Au, all thioaromatic molecules were found to form highly oriented and densely packed SAMs on both substrates. The molecular orientation and orientational order of the adsorbed thioaromatic molecules depends on the number of aromatic rings, the substrate, and the rigidity of the aromatic system. The molecules, which on average are slightly inclined with respect to the surface normal, show a less tilted orientation with increasing length of the aromatic chain, and as observed for aliphatic SAMs, they exhibit smaller tilt angles on Ag than on Au. However, the difference in the tilt angles for aromatic SAMs on Au and Ag is smaller than that observed in the aliphatic films. A comparison of the monolayers formed from p-terphenylthiol and anthracenethiol films suggests that a higher molecular rigidity has only a slight effect on the final molecular orientation within the respective SAMs.

Synchrotron-based high-resolution X-ray photoelectron spectroscopy was for the first time applied to investigate the damage in self-assembled monolayers (SAMs) of alkanethiols (AT) on Au caused by soft X-rays. The observed changes in AT SAMs and, in particular, the appearance of a new, irradiation-induced sulfur species are identical to those caused by electron bombardment, implying that most of the damage is produced by the photoelectrons and secondary electrons. The irradiation-induced sulfur species is identified as a dialkyl sulfide distributed within the AT film. Only minutes of monochromatized X-ray irradiation at an undulator beamline destroys the AT adlayer completely.