Self-assembly of 2,6-dimethylpyridine on Cu(1 1 0) directed by weak hydrogen bonding

Sequential stages of formation of a self-assembled monolayer of flat-lying 2,6-dimethylpyridine molecules on a single crystal Cu(1 1 0) surface have been observed by low-temperature scanning tunneling microscopy (LT-STM). At an adsorption temperature of 10 K, all of the molecules are randomly distributed at low coverage upon adsorption. The isolated molecules align their molecular axes parallel to the 〈0 0 1〉 azimuth of the Cu lattice. The nitrogen atom in the molecule is located at the four-fold hollow site. Upon annealing to 100 K, the molecules associate to form head-to-head dimers. The dimer units involve a pair of weak hydrogen bonds between methyl group-hydrogen atoms and N moieties on adjacent molecules, forming a core structure for further growth. In a later stage of self-assembly, single head-to-tail weak hydrogen bonds between ring C-H bonds and N moieties form in chains on the periphery of the central cores, leading to larger domains with a c(6 × 2) overlayer structure.     

Study of ammonia molecule adsorbing on diamond (1 0 0) surface

The diamond (1 0 0) surface with amino terminations is investigated based on density function theory within the generalized gradient approximation. Our calculated negative electron affinity of diamond (1 0 0) surface with hydrogen termination provides a necessary condition for initiating radical reaction. The results display that the ammonia molecule can form stable C-N covalent bonds on the diamond surface. In addition, due to the lower adsorption energy of one amino group binding on diamond surface, single amino group (SAG) model is easy to be realized in experiment with the comparison of double amino group (DAG) model. The adsorbed ammonia molecule will induce acceptor-like gap states with little change of the valence and conduction band of diamond in SAG model. The adsorption mechanism in the formation of ammonia monolayer on H-terminated diamond (1 0 0) surface, and two possible adsorption structures (SAG and DAG) were especially studied.     

Adsorption and reactivity of SO2 on Ir(1 1 1) and Rh(1 1 1)

The adsorption and reactivity of SO₂ on the Ir(1 1 1) and Rh(1 1 1) surfaces were studied by surface science techniques. X-ray photoelectron spectroscopy measurements showed that SO₂ was molecularly adsorbed on both the Ir(1 1 1) surface and the Rh(1 1 1) surface at 200 K. Adsorbed SO₂ on the Ir(1 1 1) surface disproportionated to atomic sulfur and SO₃ at 300 K, whereas adsorbed SO₂ on the Rh(1 1 1) surface dissociated to atomic sulfur and oxygen above 250 K. Only atomic sulfur was present on both surfaces above 500 K, but the formation process and structure of the adsorbed atomic sulfur on Ir(1 1 1) were different from those on Rh(1 1 1). On Ir(1 1 1), atomic sulfur reacted with surface oxygen and was completely removed from the surface, whereas on Rh(1 1 1), sulfur did not react with oxygen.     

Combined investigation of water sorption on TiO2 rutile (1 1 0) single crystal face: XPS vs. periodic DFT

XPS and periodic DFT calculations have been used to investigate water sorption on the TiO₂ rutile (1 1 0) face. Two sets of XPS spectra were collected on the TiO₂ (1 1 0) single crystal clean and previously exposed to water: the first set with photoelectrons collected in a direction parallel to the normal to the surface; and the second set with the sample tilted by 70°, respectively. This tilting procedure promotes the signals from surface species and reveals that the first hydration layer is strongly coordinated to the surface and also that, despite the fact that the spectra were recorded under ultra-high vacuum, water molecules subsist in upper hydration layers. In addition, periodic DFT calculations were performed to investigate the water adsorption process to determine if molecular and/or dissociative adsorption takes place. The first step of the theoretical part was the optimisation of a dry surface model and then the investigation of water adsorption. The calculated molecular water adsorption energies are consistent with previously published experimental data and it appears that even though it is slightly less stable, the dissociative water sorption can also take place. This assumption was considered, in a second step, on a larger surface model where molecular and dissociated water molecules were adsorbed together with different ratio. It was found that, due to hydrogen bonding stabilisation, molecular and dissociated water molecules can coexist on the surface if the ratio of dissociated water molecules is less than ≈33%. These results are consistent with previous experimental works giving a 10-25% range.     

Water adsorption on hydrogenated Si(1 1 1) surfaces

The adsorption of water on the hydrogen terminated Si(1 1 1) surface is studied by means of first-principles calculations as well as contact angle measurements. Possible initial adsorption configurations for single water molecules and the potential energy surface are calculated. Only small adsorption energies of the order of meV are predicted. Calculations for higher coverage show that the water-water interactions are stronger than the water-surface bonding. The contact angle formed between a water droplet on the surface approximated from the total-energy calculations amounts to 88°, while our measured value is 91°.     

First principles studies of a Xe atom adsorbed on Nb(1 1 0) surface

We study adsorption sites of a single Xe adatom on Nb(1 1 0) surface using a density functional theory approach: the on-top site is the most favorable position for adsorption. We compare the binding features of the present study to earlier studies of a Xe adatom on close-packed (1 1 1) surfaces of face-centered cubic metals. The different features are attributed through a microscopic picture to the less than half filled d-states in Nb.     

DFT study of NH3 dissociation on Si(1 1 1)-7 × 7. The role of intermolecular interactions

The adsorption of NH₃ molecule on the Si(1 1 1)-7 × 7 surface modelled with a cluster has been studied using density functional theory (DFT). The results indicate the existence of a precursor state for the non-dissociative chemisorption. The active site for the molecular chemisorption is the adatom; while the NH₃ molecule adsorbs on the Si restatom via this preadsorbed state, the adsorption on the Si adatom is produced practically without an energy barrier. The ammonia adsorption on the adatom induces an electron transfer from the dangling bond of this atom to the dangling bond of the adjacent Si restatom, hindering this site for the adsorption of a second NH₃ incoming molecule. However, this second molecule links strongly by means of two H-bonds. The dissociative chemisorption process was studied considering one and two ammonia molecules. For the dissociation of a lonely NH₃ molecule an energy barrier of ∼0.3 eV was calculated, yielding NH₂ on the adatom and H on the restatom. When two molecules are adsorbed, the NH₃-NH₃ interaction yields the weakening of a N-H bond of the ammonia molecule adsorbed closer the Si surface. As a consequence, the dissociation barrier practically disappears. Thus, the presence of a second NH₃ molecule at the adatom-restatom pair of the Si(1 1 1)-7 × 7 surface makes the dissociative reaction self-assisted, the total adsorption process elapsing with a negligible activation barrier (less than 0.01 eV).     

Chemistry of Alanine on Pd(1 1 1): Temperature-programmed desorption and X-ray photoelectron spectroscopic study

The adsorption of alanine is studied on a Pd(1 1 1) surface using X-ray photoelectron spectroscopy (XPS) and temperature-programmed desorption (TPD). It is found that alanine adsorbs into the second and subsequent layers prior to completion of the first monolayer for adsorption at ∼250 K, while at ∼300 K, alanine adsorbs almost exclusively into the first monolayer with almost no second-layer adsorption. Alanine adsorbs onto the Pd(1 1 1) surface in its zwitterionic form, while the multilayer contains about 30-35% neutral alanine, depending on coverage. Alanine is thermally stable on the Pd(1 1 1) surface to slightly above room temperature, and decomposes almost exclusively by scission of the CCOO bond to desorb CO₂ and CO from the COO moiety, and the remaining fragment yields ethylamine and HCN.     

The dissociative adsorption of borane on the Ge(1 0 0)-2 × 1 surface: A density functional theory study

By means of cluster models coupled with density functional theory, we have studied the hydroboration of the Ge(1 0 0)-2 × 1 surface with BH₃. It was found that the Ge(1 0 0) surface exhibits rather different surface reactivity toward the dissociative adsorption of BH₃ compared to the C(1 0 0) and Si(1 0 0) surfaces. The strong interaction still exists between the as-formed BH₂ and H adspeices although the dissociative adsorption of BH₃ on the Ge(1 0 0) surface occurs readily, which is in distinct contrast to that on the C(1 0 0) and Si(1 0 0) surfaces. This can be understood by the electrophilic nature of the down Ge atom, which makes it unfavourable to form a GeH bond with the dissociating proton-like hydrogen. Alternatively, it can be attributed to the weak proton affinity of the Ge(1 0 0) surface. Nevertheless, the overall dissociative adsorption of BH₃ on group IV semiconductor surfaces is favourable both thermodynamically and kinetically, suggesting the interesting analogy and similar diversity chemistry of solid surface in the same group.     

A density functional theory study of ethylene adsorption on Ni10(1 1 1), Ni13(1 0 0) and Ni10(1 1 0) surface cluster models and Ni13 nanocluster

Ethylene adsorption was studied by use of DFT/B3LYP with basis set 6-31G(d,p) in Gaussian’03 software. It was found that ethylene has adsorbed molecularly on all clusters with π adsorption mode. Relative energy values were calculated to be −50.86 kcal/mol, −20.48 kcal/mol, −32.44 kcal/mol and −39.27 kcal/mol for Ni₁₃ nanocluster, Ni₁₀(1 1 1), Ni₁₃(1 0 0) and Ni₁₀(1 1 0) surface cluster models, respectively. Ethylene adsorption energy is inversely proportional to Ni coordination number when Ni₁₀(1 1 1), Ni₁₃(1 0 0) and Ni₁₀(1 1 0) cluster models and Ni₁₃ nanocluster are compared with each other.     

Olefin bonding mechanism and growth to (1 0 0)(2 × 1):H diamond

The bonding and growth mechanism of photochemically attached olefin molecules to (1 0 0)(2 × 1):H diamond is characterized using atomic force (AFM) and scanning tunneling microscopy (STM) experiments in combination with molecular orbital calculations. To identify growth schemas, diamond surfaces after 10, 40 and 90 min of photo-chemically stimulated growth have been characterized. These data show clearly island formation which is discussed taking into account a growth model from silicon. The island growth shows no directional properties which are attributed to arrangement and geometrical properties of hydrogen terminated carbon bonds at the surface of (1 0 0) oriented (2 × 1) reconstructed diamond.     

Vibrationally-assisted dissociative adsorption of oxygen on Ru(0 0 0 1)-p(2 × 1)-O

Supersonic molecular beam technique combined with high resolution X-ray photoelectron spectroscopy using synchrotron radiation was applied to the study of the dynamics of dissociative adsorption of oxygen on Ru(0 0 0 1) surface in high coverage region. The Ru(0 0 0 1) surface pre-covered with oxygen atoms of 0.5 monolayer, which corresponds to the p(2 × 1)-O structure, was dosed to oxygen molecules with translational energy of 0.5 eV. Oxygen uptake was compared between the cases with and without the beam source heated in order to verify the effects of internal energy of oxygen. We found drastic enhancement in initial sticking probability of oxygen when the beam source was heated to 1400 K. We concluded that the enhancement of sticking probability is mainly caused by molecular vibrational excitation, indicating that dissociation barrier is located in the exit channel on potential energy surface.     

Adsorption of organic molecules by photochemical reaction on Cl:Si(1 1 1) and H:Si(1 1 1) evaluated by HREELS

The covalent attachment of alkyl groups to silicon surfaces, via carbon-silicon bond formation, has been attempted using gas-surface reactions starting from Cl-terminated Si(1 1 1) or H:Si(1 1 1) under ultraviolet light irradiation. The formation of Cl-terminated Si(1 1 1) and its resulting stability were examined prior to deposition of organic molecules. High-resolution electron energy loss spectroscopy (HREELS) was utilized for detecting surface-bound adsorbates. The detection of photo-deposited organic species on Cl:Si(1 1 1) from gas-phase CH₄ or CH₂CH₂ was not significant. On H:Si(1 1 1), it was evident that after the photoreaction with gas-phase C₂H₅Cl, C₂H₅ groups were chemically bonded to the surface Si atoms through single covalent bonds. The C₂H₅ groups were thermally stable at temperatures below 600 K. Alkyl monolayers prepared on silicon surfaces by dry process will lead to a new prospective technology of nanoscale fabrication and biochemical applications.     

Carbon monoxide adsorption on the metastable fcc-Co/Cu(1 0 0) surface

The adsorption properties of CO on the epitaxial five-monolayer Co/Cu(1 0 0) system, where the Co overlayer has stabilized in the metastable fcc-phase, are reported. This system is known to exhibit metallic quantum well (MQW) states at energies 1 eV or greater above the Fermi level, which may influence CO adsorption. The CO/fcc-Co/Cu(1 0 0) system was explored with low energy electron diffraction (LEED), inverse photoemission (IPE), reflection-absorption infrared spectroscopy (RAIRS) and temperature programmed desorption (TPD). Upon CO adsorption, a new feature is observed in IPE at 4.4 eV above E_F and is interpreted as the CO 2π^∗ level. When adsorbed at room temperature, TPD exhibits a CO desorption peak at ∼355 K, while low temperature adsorption reveals additional binding configurations with TPD features at ∼220 K and ∼265 K. These TPD peak temperatures are correlated with different C-O stretch vibrational frequencies observed in the IR spectra. The adsorption properties of this surface are compared to those of the surfaces of single crystal hcp-Co, as well as other metastable thin film systems.     

Dissociative adsorption and thermal evolution of dichloroethylenes on Ni(1 0 0)

The room temperature (RT) adsorption and thermal evolution of cis- and trans-dichloroethylene (DCE) and their structural isomer, iso-DCE, on Ni(1 0 0) have been studied by vibrational electron energy loss spectroscopy (EELS), Auger electron spectroscopy (AES) and thermal desorption spectrometry (TDS). For RT adsorption, both cis- and trans-DCE exhibit very similar EELS features that are different from those found for iso-DCE. These differences indicate the formation of different fragments upon RT adsorption. In particular, the primary adspecies for cis- and trans-DCE are ethane-1,1,2,2-tetrayl () and acetylide-like () adspecies along with a small amount of chlorovinyl adspecies, while ethylylidyne () is the more plausible adspecies for iso-DCE. The differences in the adstructures upon dissociative adsorption at RT underline the important isomeric effects. Furthermore, both AES and TDS results for all three DCE isomers show that most of the Cl atoms produced by dechlorination remain on the surface and its surface concentration remains unchanged upon annealing the samples above 500 K. Upon further annealing to 550 K, the EELS spectra of all three isomers exhibit a broad feature near 1600 cm⁻¹, which suggests the formation of carbon clusters on the surface. The presence of surface Cl atoms therefore appears to prevent the CC bond cleavage during thermal evolution of the adspecies on Ni(1 0 0).     

Vibration of small molecules on Pt(1 1 1) surface

The adsorption of methanol and other small molecules onto transition metal surfaces is an important issue in electrochemistry, fuel cells, etc. Despite the overwhelming interest there are still unresolved issues beginning from the geometry of the adsorbed species to the correct assignments of different vibrational modes of the adsorbed molecules on the surface.In order to understand the adsorption processes, we have performed density functional theory (DFT) calculations for small molecules (methanol, formaldehyde, formic acid) on Pt(1 1 1) surfaces. We investigated the nature of the metal-ligand bonding in these adsorption processes using electron density difference and PDOS (partial density of states) methods. Ab initio vibration spectra have been calculated for these systems.     

Study on the interface between the organic and inorganic semiconductors

The adsorption of perylene on Si (0 0 1) surface has been studied by UPS measurement and DFT calculation. With 0.5 monolayer, the perylene molecule adsorbs uprightly with its long axis cross the dimer. At the coverage of 1.0 monolayer, the perylene molecule adsorbs uprightly with its short axis cross the dimer. To increase the separation between the molecules, the molecules adsorb forming a zigzag structure. With coverage increasing, the work function decreases. When the coverage is 1ML, the work function is the minimum. The dipole, formed on the surface, contributes to the change in work function.     

Formation of doubly excited He atoms during scattering of He ions from a clean and oxygen covered Ni(1 1 0) surface

The formation of doubly excited states of He atoms during collisions of He²⁺ ions with energies between 60 eV and 1 keV with a Ni(1 1 0) surface is studied via Auger electron spectroscopy. We observe that the electron spectra from autoionization of doubly excited states of 2s², 2s2p, 2p² configurations show a pronounced dependence on the coverage of the target surface with oxygen. For a controlled O₂ adsorption on the Ni(1 1 0) surface we can explain the resulting changes in the electron spectra by the modification of the work function of the target surface. Thermal desorption and dissolution into the bulk of surface contaminations at elevated temperatures provides an alternative interpretation of recent work where the local electron spin polarization of a Ni(1 1 0) surface was deduced from changes in the electron spectra as function of target temperature.     

Adsorption of water on thin V2O3(0 0 0 1) films

V₂O₃(0 0 0 1) films have been grown epitaxially on Au(1 1 1) and W(1 1 0). Under typical UHV conditions these films are terminated by a layer of vanadyl groups as has been shown previously [A.-C. Dupuis, M. Abu Haija, B. Richter, H. Kuhlenbeck, H.-J. Freund, V₂O₃(0 0 0 1) on Au(1 1 1) and W(1 1 0): growth, termination and electronic structure, Surf. Sci. 539 (2003) 99]. Electron irradiation may remove the oxygen atoms of this layer. H₂O adsorption on the vanadyl terminated surface and on the reduced surface has been studied with thermal desorption spectroscopy (TDS), vibrational spectroscopy (IRAS) and electron spectroscopy (XPS) using light from the BESSY II electron storage ring in Berlin. It is shown that water molecules interact only weakly with the vanadyl terminated surface: water is adsorbed molecularly and desorbs below room temperature. On the reduced surface water partially dissociates and forms a layer of hydroxyl groups which may be detected on the surface up to T ∼ 600 K. Below ∼330 K also co-adsorbed molecular water is detected. The water dissociation products desorb as molecular water which means that they recombine before desorption. No sign of surface re-oxidation could be detected after desorption, indicating that the dissociation products desorb completely.     

Competition between associative and dissociative adsorption of 1,2-dihalogenated benzenes on Si(1 0 0)2 × 1: Formation of dihalocyclohexadiene, halophenyl and phenylene adstructures

The room temperature (RT) adsorption of 1,2-difluorobenzene (1,2-DFB), 1,2-dichlorobenzene (1,2-DCB) and 1,2-dibromobenzene (1,2-DBB) on Si(1 0 0)2 × 1 have been investigated by X-ray photoelectron spectroscopy (XPS) and temperature programmed desorption (TPD). Both XPS and TPD data show that the relative degree of dissociative to associative adsorption of the dihalogenated benzene (DXB) appears to increase with decreasing electronegativity of the halogen atom (X). In particular, the C 1s intensity ratios for the C-H and C-Si components to the C-X component are found to be 2, 3 and 9.6 for 1,2-DFB, 1,2-DCB and 1,2-DBB, respectively. These results indicate that 1,2-DFB, like benzene, exclusively adsorbs molecularly as a difluorocyclohexadiene adspecies on Si(1 0 0)2 × 1 while 1,2-DBB adsorbs predominantly with double debromination to form 1,2-phenylene. The majority of 1,2-DCB (75%) is found to adsorb molecularly, with the rest (25%) undergone single or double dechlorination to form chlorophenyl and phenylene, respectively. All three DXB molecules appear to have similar coverage as benzene. The two molecular desorption features for 1,2-DFB and 1,2-DCE are observed with desorption maxima at 460 K and 540 K similar to those found for benzene, which suggests that the dihalocyclohexadiene adstructures involve similar bonding through the benzene ring. In accord with the XPS data, no molecular desorption feature is observed for 1,2-DBB on the 2 × 1 surface. Further decomposition of the resulting phenylene adstructures is evident from the desorption fragment, C₂H₂, found at 610 K and 740 K. Recombinative desorption of HCl and HBr above 880 K are also found for 1,2-DCB and 1,2-DBB, respectively. The observed differences between associative and dissociative adsorption for the three DXB adsorbates could be attributed not only to the large difference in the C-X bond strength but also to the relative contributions from inductively withdrawing and resonantly donating electrons exerted by the halogen (X) atoms to the benzene ring.