Michael Grunze

Interaction of nitrogen with iron surfaces: I. Fe(100) and Fe(111)

The adsorption of N2 on clean Fe(100) and Fe(111) single-crystal surfaces was studied in the temperature range 140–1000 K by means of Auger electron spectroscopy (AES), low-energy electron diffraction (LEED), ultraviolet photoelectron spectroscopy (UPS), thermal-desorption spectroscopy (TDS) and work-function measurements (Δφ). Above room temperature, only dissociative adsorption takes place, leading to increases in work function of 0.33 and 0.25 eV on Fe(100) and (111), respectively, and is mainly identified with UPS by the appearance of a chemisorption level derived from N2p-states at about 5 eV below the Fermi level. At 500 K, the initial rate of adsorption is faster by about a factor of 20 on the (111) plane, the initial sticking coefficient, however, being very small (10−7–10−6) on both surfaces. The initial activation energies for adsorption are about 5 and 0 kcal/mole on Fe(100) and Fe(111), respectively, and increase with coverage in both cases. The mean activation energies for desorption were estimated to be 58(100) and 51 kcal/mole (111), so that nearly equal values for the strength of the MN bond result. A simple ordered c2 × 2 structure is formed on Fe(100) which is completed at θ = 0.5 and for which a model is proposed wherein the N atoms are located in fourfold sites on the unreconstructed Fe(100) surface, leading to a configuration similar to that in the (002) plane of (fcc) Fe4N. Several independent observations strongly indicate that the Fe(111) surface reconstructs. A whole series of complex LEED patterns (depending on N bulk and surface concentrations and on the conditions of heat treatment) is formed with this plane which are interpreted in terms of the formation of hexagonal layers of “surface nitrides” which have a thickness of about 2 atomic layers and most probably are related to the (111) plane of Fe4N. Desorption of N2 (being found to be a first-order rate process) is regarded as equivalent to the decomposition of the “surface nitrides.” The close similarity to the kinetics of decomposition of (bulk) Fe4N indicates identical mechanisms for both processes. Although the bulk solubility of N is very small under the chosen experimental conditions, this process interferes with the adsorption and desorption measurements and was analyzed in some detail, mainly by 14N15N isotopic exchange. Evidence for the existence of a weakly bound (probably molecular) species was found with Fe(111) only at the lowest temperatures (140 K) and under a steady-state pressure of 4 × 10−4 Torr of N2. This species causes a decrease in the work function and is rapidly pumped off. Its adsorption energy is estimated to be in the range between 5 and 10 kcal/mole.

Chemisorption of hydrogen on iron surfaces

The adsorption of hydrogen on Fe(110), (100) and (111) single crystal planes has been studied by means of low energy diffraction (LEED), thermal desorption spectroscopy (TDS), work function measurements and ultraviolet photoelectron spectroscopy (UPS). Isotope exchange experiments revealed the atomic nature of all species held at the surface above 140 K. The chemisorption bond is characterized by a bonding level with an ionization energy of 5.6 eV below the Fermi energy as identified by UPS which is derived from coupling the H 1s state to the valence states of the metal. Initial adsorption energies of 26, 24 and 21 kcal/mole were derived for the (110), (100) and (111) planes, respectively. The work function decreases with Fe(110) by 95 mV, whereas total increases by 75 and 310 mV were determined for the (100) and (111) surfaces, respectively. At saturation the (110) and (100) planes reveal the existence of two desorption states, whereas three states are observed with Fe(111). Whereas Fe(100) and (111) reveal no variation of the LEED pattern a series of ordered overlayer structures, ranging from c2 × 2 (or “2 × 1”) at θ = 12 to 1 × 1 at θ = 1, were observed with Fe(110). These structures can be interpreted in a straightforward manner in terms of subsequent filling of rows of adsorption sites along the [001]-surface direction whereby repulsive interactions are operating between particles in neighbouring rows. This model fits perfectly with the TDS data and enables the calibration of the absolute coverage (θsat = 1, i.e. a 1:1 ratio of H:Fe surface atoms). The initial sticking coefficient on Fe(110) is so = 0.16 and it was found that with this plane the variation of this quantity with coverage between θ = 0.1 and 1 obeys a simple Langmuir type law for dissociative adsorption on two adjacent vacant sites, viz. s = so (1 − θ)2.

Electron-induced crosslinking of aromatic self-assembled monolayers: Negative resists for nanolithography

We have explored the interaction of self-assembled monolayers of 1,1′-biphenyl-4-thiol (BPT) with low energy electrons. X-ray photoelectron, infrared, and near edge x-ray absorption fine structure spectroscopy showed that BPT forms well-ordered monolayers with the phenyl rings tilted ∼15° from the surface normal. The films were exposed to 50 eV electrons and changes were monitored in situ. Even after high (∼10 mC/cm2) exposures, the molecules maintain their preferred orientation and remain bonded on the gold substrate. An increased etching resistance and changes in the infrared spectra imply a crosslinking between neighboring phenyl groups, which suggests that BPT can be utilized as an ultrathin negative resist. This is demonstrated by the generation of patterns in the underlying gold.

Photoemission from palladium particle arrays on an amorphous silica substrate

Abstract Low coverages of vacuum deposited palladium on amorphous silica films have been examined with electron microscopy and photoelectron spectroscopy. Despite the inhomogeneity of the resulting particle array, changes in the Pd valence band as a function of increasing particle size could be clearly followed in UPS. Spectra characteristic of metallic properties were first observed at mean particle diameters between 2 and 3 nm. Accompanying changes in the position and halfwidth of the Pd 3d 5 2 core level were observed in XPS and are interpreted in terms of screening effects. It appears likely that such systems can be used as models for the study of the electronic and surface properties of supported metal catalysts.

Modification of thiol-derived self-assembling monolayers by electron and x-ray irradiation: Scientific and lithographic aspects

This article reviews recent experiments on the modification of thiol-derived self-assembling monolayers (SAMs) 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.

Spectroscopic characterization of thiol-derived self-assembling monolayers

This article reviews recent progress in the spectroscopic characterization of aliphatic and aromatic thiol-derived self-assembled monolayers (SAMs) on noble metal substrates. Several complementary techniques such as near edge x-ray absorption fine structure spectroscopy, x-ray photoelectron spectroscopy, and infrared reflection absorption spectroscopy were applied to study the balance between intermolecular and adsorbate-substrate interactions, chemical identity of the headgroup, and absorption site homogeneity at the sulphur-metal interface. Whereas in the thioaliphatic SAMs the headgroup-substrate interaction was found to be a decisive factor for the structure and packing in these films, these parameters are mainly determined by the intermolecular interactions in the thioaromatic films. Only one sulphur species could be detected in the S 2p HRXPS spectra of both aliphatic and aromatic SAMs suggesting binding of individual molecules as thiolates. Conclusions on the heterogeneity of the adsorption sites are derived and evidence that the investigated films represent highly correlated molecular assemblies are presented.

Interaction of ammonia with Fe(111) and Fe(100) surfaces

Abstract The adsorption and decomposition of NH 3 on clean and nitrogen covered Fe(111) and Fe(100) surfaces has been studied by means of UPS, AES, LEED, thermal desorption and work function measurements. Molecularly adsorbed ammonia is characterized by valence ionization potentials of 7.4 and 11.8 eV below the Fermi level which are derived from the 3a 1 -and leorbitals of free NH 3 . NH 3,ad decreases the work function by about 2 eV and is presumably coupled to the surfaces through the nitrogen atom, the adsorption energies being in the range of 10–12 kcal/mole. Even at 160 K slow chemical transformation takes place. At 320 K the last species containing N-H bonds are lost from the surface, either by desorption of NH 3 or by complete dissociation into N ad + H ad . The latter process is strongly suppressed by the presence of preadsorbed atomic nitrogen. At least one intermediate species (presumably NH 2,ad ) could be identified by characteristic variations of the photoelectron spectra. This may recombine with adsorbed hydrogen (as verified by isotope-exchange experiments) leading to NH 3 -desorption around room temperature. Repeated high-temperature treatment with ammonia caused some facetting of the Fe(100) plane.

Molecular and dissociative chemisorption of N2 on Ni(110)

Nitrogen is chemisorbed molecularly by Ni(110) at low temperatures and pressure in two stages. The first stage with a binding energy of ∼35 kJ mol−1 leads to a maximum surface potential of 0.1 V and gives an infrared absorption band initially at 2194 cm−1 and shifting very little with coverage. Further adsorption gives a more weakly bound, ∼27 kJ mol−1, second stage in which the surface potential falls but the infrared band hardly changes. These two stages can be desorbed by warming to 186 K. At higher pressures and temperatures evidence for strongly bound nitrogen is provided by therma1 desorption peaks at 470 and 800 K attributed to dissociatively adsorbed nitrogen. Nitrogen adsorbed at lower temperatures undergoes ordering at 500 K to produce a p(2 × 3) LEED pattern which is tentatively attributed to a surface nitride.

Settlement and adhesion of algal cells to hexa(ethylene glycol)-containing self-assembled monolayers with systematically changed wetting properties

Protein resistance of self-assembled monolayers (SAMs) of hexa(ethylene glycols) (EG6) has previously been shown to be dependent on the alkoxyl end-group termination of the SAM, which determines wettability [S. Herrwerth, W. Eck, S. Reinhardt, and M. Grunze, J. Am. Chem. Soc. 125, 9359 (2003)]. In the present study, the same series of hexa(ethylene glycols) was used to examine the correlation between protein resistance and the settlement and adhesion of eukaryotic algal cells, viz., zoospores of the macroalga Ulva and cells of the diatom Navicula, which adhere to the substratum through the secretion of protein-containing glues. Results showed that the initial settlement of Ulva zoospores was highest on the hydrophilic EG6OH but that cells were only weakly adhered. The number of Ulva zoospores and Navicula cells firmly adhered to the SAMs systematically increased with decreasing wettability, as shown for the protein fibrinogen. The data are discussed in terms of hydration forces and surface charges in the SAMs.

Immobilization of antibodies in micropatterns for cell detection by optical diffraction

Abstract Optical diffraction at biochemically microstructured surfaces has been investigated for the label-free in situ detection of cells. The new sensor concept is based on regular arrays of covalently coupled antibodies, which selectively bind cells from solution. Due to the adsorption process, changes are imposed on the intensity distribution of the diffracted light, which can serve to quantify the amount of adsorbed cells. For the formation of such microstructures, different classical film preparation techniques were transferred to a mesoscopic scale by the use of microcontact printing (μCP). Alternatively, receptors were functionalized with thiol groups prior to the immobilization process and directly printed onto the gold surface. Compared to imprinting of non-functionalized proteins on gold, a better replication of the micropatterns could be obtained. Additionally, a significantly lower amount of defects was observed than for the classical coupling techniques. Using such microstructures, first experiments on the detection of Escherichia coli bacteria were performed. Diffraction patterns have been observed for concentrations equal or higher than 106 cells/ml. In time dependent experiments, diffraction spots occurred after 30 – 90 min or 10 – 20 min, depending on whether non-specific cell adsorption or specific binding to anti-E. coli IgG was studied. A first quantitative analysis of the diffraction patterns shows that the total amount of diffracted light increases with increasing incubation time.