Experimental and theoretical study of the adsorption of fumaramide [2]rotaxane on Au(111) and Ag(111) surfaces

Thin films of fumaramide [2]rotaxane, a mechanically interlocked molecule composed of a macrocycle and a thread in a "bead and thread" configuration, were prepared by vapor deposition on both Ag(111) and Au(111) substrates. X-ray photoelectron spectroscopy (XPS) and high-resolution electron-energy-loss spectroscopy were used to characterize monolayer and bulklike multilayer films. XPS determination of the relative amounts of carbon, nitrogen, and oxygen indicates that the molecule adsorbs intact. On both metal surfaces, molecules in the first adsorbed layer show an additional component in the C 1s XPS line attributed to chemisorption via amide groups. Molecular-dynamics simulation indicates that the molecule orients two of its eight phenyl rings, one from the macrocycle and one from the thread, in a parallel bonding geometry with respect to the metal surfaces, leaving three amide groups very close to the substrate. In the case of fumaramide [2]rotaxane adsorption on Au(111), the presence of certain out-of-plane phenyl ring and Au-O vibrational modes points to such bonding and a preferential molecular orientation. The theoretical and experimental results imply that the three-dimensional intermolecular configuration permits chemisorption at low coverage to be driven by interactions between the three amide functions of fumaramide [2]rotaxane and the Ag(111) or Au(111) surface.     

Two-photon photoemission study of the coverage-dependent electronic structure of chemisorbed alkali atoms on a Ag(111) surface

We report a systematic investigation of the electronic structure of chemisorbed alkali atoms (Li-Cs) on a Ag(111) surface by two-photon photoemission spectroscopy. Angle-resolved two-photon photoemission spectra are obtained for 0-0.1 monolayer coverage of alkali atoms. The interfacial electronic structure as a function of periodic properties and the coverage of alkali atoms is observed and interpreted assuming ionic adsorbate/substrate interaction. The energy of the alkali atom σ-resonance at the limit of zero coverage is primarily determined by the image charge interaction, whereas at finite alkali atom coverages, it follows the formation of a dipolar surface field. The coverage- and angle-dependent two-photon photoemission spectra provide information on the photoinduced charge-transfer excitation of adsorbates on metal surfaces. This work complements the previous work on alkali/Cu(111) chemisorption [Phys. Rev. B 2008, 78, 085419].     

Ag on Pt(111): Changes in Electronic and CO Adsorption Properties upon PtAg/Pt(111) Monolayer Surface Alloy Formation

The electronic and chemical (adsorption) properties of bimetallic Ag/Pt(111) surfaces and their modification upon surface alloy formation, that is, during intermixing of Ag and Pt atoms in the top atomic layer upon annealing, were studied by X‐ray photoelectron spectroscopy (XPS) and, using CO as probe molecule, by temperature‐programmed desorption (TPD) and infrared reflection absorption spectroscopy (IRRAS), respectively. The surface alloys are prepared by deposition of sub‐monolayer Ag amounts on a Pt(111) surface at room temperature, leading to extended Ag monolayer islands on the substrate, and subsequent annealing of these surfaces. Surface alloy formation starts at ≈600–650 K, which is evidenced by core‐level shifts (CLSs) of the Ag(3d_5/2) signal. A distinct change of the CO adsorption properties is observed when going to the intermixed PtAg surface alloys. Most prominently, we find the growth of a new desorption feature at higher temperature (≈550 K) in the TPD spectra upon surface alloy formation. This goes along with a shift of the CO_ad‐related IR bands to lower wave number. Surface alloy formation is almost completed after heating to 700 K.     

Interface geometry of Ag thin films supported at the MgO(1 0 0) surface: an ab-initio periodic study

In this Letter, the interface geometry of silver thin films of thickness T between 1 and 3 ML epitaxially deposited at the MgO surface has been accurately characterized employing DFT periodic calculations. The Ag–Ag out-of-plane interlayer spacing is considerably shorter than the bulk values because of: (i) the reduced dimension; (ii) the perfect epitaxy constraint; (iii) the interaction with the substrate. The interface distance between the silver monolayer and the MgO substrate, d(Ag–O) = 2.60 Å, is considerably longer than the estimates computed for the bilayer and the trilayer, d(Ag–O) = 2.47 and 2.48 Å, respectively. The difference between the values of the interface distance computed for the monolayer and the results obtained for thicker films, lies in the peculiar electronic properties of the silver monolayer.     

Partial stripping of Ag atoms from silver bilayer on a Au(111) surface accompanied with the reductive desorption of hexanethiol SAM

The interfacial structures of Ag bilayer prepared by underpotential deposition on Au(111) (Ag(2ML)/Au(111)) were determined by ex situ scanning tunneling microscopy and in situ surface X-ray scattering measurements before and after oxidative adsorption and after reductive desorption of a self-assembled monolayer (SAM) of hexanethiol (C₆SH) in alkaline ethanol solution. While no structural change was observed after oxidative formation of C₆SH SAM on the Ag(2ML)/Au(111) in an ethanol solution containing 20 mM KOH and 0.1 mM C₆SH, some of the Ag atoms in the bilayer were stripped when the SAM was reductively desorbed. Dedicated to Professor J. O’M. Bockris on the occasion of his 85th birthday.     

Unconventional air-stable interdigitated bilayer formed by 2,3-disubstituted fatty acid methyl esters

A single-chain fatty acid methyl ester, racemic anti-3-fluoro-2-hydroxyeicosanoic acid methyl ester (beta-FHE), forms an unconventional air-stable interdigitated bilayer at the air-water interface. The interdigitated bilayer transferred onto solid substrate by the Langmuir-Blodgett (LB) technique keeps air-stable without any substrate modification or protein inclusion. There are two visible plateaus in the surface pressure-molecular area (pi-A) isotherms of beta-FHE Langmuir film during continuous compression. According to Brewster angle microscopy (BAM), grazing incidence X-ray diffraction (GIXD), X-ray reflectivity (XR), fluorescence microscopy (FM), and atomic force microscopy (AFM) measurements, the first plateau is attributed to the coexistence of liquid expanded (LE) and liquid condensed (LC) phases in the monolayer, while the second plateau is interpreted as the transition from LC monolayer to interdigitated bilayer. The coupling between tilt and curvature associated with the packing mismatch between headgroup and chain gives rise to buckling and folding of the monolayer, leading to the transition of the LC monolayer to a bilayer structure. The diffusion-limited aggregation (DLA) model is applied to describe the formation of the fractal structures of the bilayer as observed in the second plateau. In addition, the transition between monolayer and bilayer is reversible. The present works are interesting for understanding biological processes, for example, the behavior of lung surfactants.     

Experimental and theoretical studies on the organic-inorganic hybrid compound: aluminum-NTCDA Co-deposited film

Electronic absorption, Fourier transform-infrared (FT-IR), and electron spin resonance spectra of aluminum-naphthalene tetracarboxylic dianhydlide (Al-NTCDA) co-deposited film have been measured at room temperature, and hybrid density functional theory (DFT) calculations have been carried out in order to elucidate the electronic states for the ground and low-lying excited states of the complexes. After the interaction of NTCDA with Al atom, the new electronic transition bands were appeared at near-IR region. The C=O stretching modes of NTCDA are red-shifted by the interaction with Al. From the DFT calculations, it was found that the electronic state of the complex at the ground state is characterized by a slight charge-transfer state expressed by (Al(4))(delta+)(NTCDA)(delta-). The binding of Al to NTCDA is strong. The C=O double-bond character of NTCDA is changed to C-O single-bond-like character by the strong interaction of Al to the C=O bond. This is the origin of the red-shift of the FT-IR spectrum. The electronic states of organic-inorganic hybrid material were discussed on the basis of theoretical results.     

Electronic and molecular properties of an adsorbed protein monolayer probed by two-color sum-frequency generation spectroscopy

Two-color sum-frequency generation spectroscopy (2C-SFG) is used to probe the molecular and electronic properties of an adsorbed layer of the green fluorescent protein mutant 2 (GFPmut2) on a platinum (111) substrate. First, the spectroscopic measurements, performed under different polarization combinations, and atomic force microscopy (AFM) show that the GFPmut2 proteins form a fairly ordered monolayer on the platinum surface. Next, the nonlinear spectroscopic data provide evidence of particular coupling phenomena between the GFPmut2 vibrational and electronic properties. This is revealed by the occurrence of two doubly resonant sum-frequency generation processes for molecules having both their Raman and infrared transition moments in a direction perpendicular to the sample plane. Finally, our 2C-SFG analysis reveals two electronic transitions corresponding to the absorption and fluorescence energy levels which are related to two different GFPmut2 conformations: the B (anionic) and I forms, respectively. Their observation and wavelength positions attest the keeping of the GFPmut2 electronic properties upon adsorption on the metallic surface.     

Reversible Modification of the Structural and Electronic Properties of a Boron Nitride Monolayer by CO Intercalation

We demonstrate the reversible intercalation of CO between a hexagonal boron nitride (h‐BN) monolayer and a Rh(111) substrate above a threshold CO pressure of 0.01 mbar at room temperature. The intercalation of CO results in the flattening of the originally corrugated h‐BN nanomesh and an electronic decoupling of the BN layer from the Rh substrate. The intercalated CO molecules assume a coverage and adsorption site distribution comparable to that on the free Rh(111) surface at similar conditions. The pristine h‐BN nanomesh is reinstated upon heating to above 625 K. These observations may open up opportunities for a reversible tuning of the electronic and structural properties of monolayer BN films.     

Symmetry properties of vibrational modes in mesoporphyrin IX dimethyl ester investigated by polarization-sensitive resonance Raman and CARS spectroscopy

The symmetry properties of selected vibrational modes of mesoporphyrin IX dimethyl ester (MP-IX-DME) in solution are investigated under different electronic resonance conditions. The Raman band parameters of the macrocycle modes nu(2), nu(10), nu(11), and nu(19) are determined from a quantitative analysis of polarized spontaneous resonance Raman (RR) and polarization-sensitive (PS) multiplex coherent anti-Stokes Raman scattering (CARS) spectra obtained with pre-resonant B band and resonant Qx band excitation, respectively. Additionally, the molecular geometry and the vibrational modes of MP-IX-DME are calculated by employing density functional theory (DFT) on the B3LYP/6-31G(d) level. Both the DFT-derived structure and the Raman spectroscopic parameters of MP-IX-DME indicate minor deviations from an ideal D2h macrocycle symmetry. To assess the influence of the beta substitution pattern on the in-plane symmetry, calculated normal-mode vectors and several experimentally detected parameters, such as peak positions, depolarization ratios, and coherent phases, are analyzed. The effects of the macrocycle substitution pattern are different for the selected vibrational modes: nu(2) in particular is very sensitive to subtle perturbations of the in-plane symmetry. The considerable activity of totally symmetric vibrations observed in the PS CARS spectra of MP-IX-DME and the correlation of mode symmetries with coherent phases confirm earlier PS CARS results on octaethylporphine (OEP) acquired under the same electronic resonance conditions.     

Vibrational lifetimes of cyanide and carbon monoxide on noble and transition metal surfaces

The electronic vibrational damping rates of the CN and CO internal stretch modes on the (111) surfaces of Ag, Cu, Au, and Pt were calculated using density functional theory calculations. Our calculated damping rates are in excellent agreement with experimental data obtained from pump-probe laser spectroscopy. The striking difference in trends and magnitudes between the internal stretch modes of CN and CO is in part rationalized in terms of the adsorbate-induced electronic structure within the framework of a simple Newns-Anderson model.     

First-principles analysis of the effects of alloying Pd with Ag for the catalytic hydrogenation of acetylene-ethylene mixtures

The effect of alloying Pd with Ag on the hydrogenation of acetylene is examined by analyzing the chemisorption of all potential C(1) (atomic carbon, CH, methylene, and methyl) and C(2) (acetylene, vinyl, ethylene, ethyl, ethane, ethylidene, ethylidyne, and vinylidene) surface intermediates and atomic hydrogen along with the reaction energies for the elementary steps that produce these intermediates over Pd(111), Pd(75%)Ag(25%)/Pd(111), Pd(50%)Ag(50%)/Pd(111), and Ag(111) surfaces by using first-principle density functional theoretical (DFT) calculations. All of the calculations reported herein were performed at 25% surface coverage. The adsorption energies for all of the C(1) and C(2) intermediates decreased upon increasing the composition of Ag in the surface. Both geometric as well as electronic factors are responsible for the decreased adsorption strength. The modes of adsorption as well as the strengths of adsorption over the alloy surfaces in a number of cases were characteristically different than those found over pure Pd (111) and Ag (111). Adsorbates tend to minimize their interaction with the Ag atoms in the alloy surface. An electronic analysis of these surfaces shows that there is, in general, a shift in the occupied d-band states away from the Fermi level when Pd is alloyed with Ag. The s and p states also appear to contribute and may be responsible for small deviations from the Hammer-N?rskov model. The effect of alloying is more pronounced on the calculated reaction energies for different possible surface elementary reactions. Alloying Pd with Ag reduces the exothermicity (increases endothermicity) for bond-breaking reactions. This is consistent with experimental results that show a decrease in the decomposition products in moving from pure Pd to Pd-Ag alloys.(2-5) In addition, alloying increases the exothermicity of bond-forming reactions. Alloying therefore not only helps to suppress the unfavorable decomposition (bond-breaking) reaction rates but also helps to enhance the favorable hydrogenation (bond-forming) reaction rates.     

Ultrathin TiO(x) films on Pt(111): a LEED, XPS, and STM investigation

Ultrathin ordered titanium oxide films on Pt(111) surface are prepared by reactive evaporation of Ti in oxygen. By varying the Ti dose and the annealing conditions (i.e., temperature and oxygen pressure), six different long-range ordered phases are obtained. They are characterized by means of low-energy electron diffraction (LEED), X-ray photoemission spectroscopy (XPS), and scanning tunneling microscopy (STM). By careful optimization of the preparative parameters, we find conditions where predominantly single phases of TiO(x), revealing distinct LEED pattern and STM images, are produced. XPS binding energy and photoelectron diffraction (XPD) data indicate that all the phases, except one (the stoichiometric rect-TiO2), are one monolayer thick and composed of a Ti-O bilayer with interfacial Ti. Atomically resolved STM images confirm that these TiO(x) phases wet the Pt surface, in contrast to rect-TiO2. This indicates their interface stabilization. At a low Ti dose (0.4 monolayer equivalents, MLE), an incommensurate kagomé-like low-density phase (k-TiO(x) phase) is observed where hexagons are sharing their vertexes. At a higher Ti dose (0.8 MLE), two denser phases are found, both characterized by a zigzag motif (z- and z'-TiO(x) phases), but with distinct rectangular unit cells. Among them, z'-TiO(x), which is obtained by annealing in ultrahigh vacuum (UHV), shows a larger unit cell. When the postannealing of the 0.8 MLE deposit is carried out at high temperatures and high oxygen partial pressures, the incommensurate nonwetting, fully oxidized rect-TiO2 is found The symmetry and lattice dimensions are almost identical with rect-VO2, observed in the system VO(x)/Pd(111). At a higher coverage (1.2 MLE), two commensurate hexagonal phases are formed, namely the w- [(square root(43) x square root(43)) R 7.6 degrees] and w'-TiO(x) phase [(7 x 7) R 21.8 degrees]. They show wagon-wheel-like structures and have slightly different lattice dimensions. Larger Ti deposits produce TiO2 nanoclusters on top of the different monolayer films, as supported both by XPS and STM data. Besides the formation of TiO(x) surfaces phases, wormlike features are found on the bare parts of the substrate by STM. We suggest that these structures, probably multilayer disordered TiO2, represent growth precursors of the ordered phases. Our results on the different nanostructures are compared with literature data on similar systems, e.g., VO(x)/Pd(111), VO(x)/Rh(111), TiO(x)/Pd(111), TiO(x)/Pt(111), and TiO(x)/Ru(0001). Similar and distinct features are observed in the TiO(x)/Pt(111) case, which may be related to the different chemical natures of the overlayer and of the substrate.     

Adsorption of fumaramide [2]rotaxane and its components on a solid substrate: a coverage-dependent study

The coverage-dependent adsorption on Au(111) of a fumaramide [2]rotaxane and its components, a benzylic amide macrocycle and a fumaramide thread, is studied using high-resolution electron energy loss spectroscopy (HREELS). Up to monolayer coverage, the relative intensity of out-of-plane to in-plane phenyl ring vibrational modes indicates that the macrocycle adopts an orientation with the phenyl rings largely parallel to the surface. The formation of a chemisorption bond is evidenced by the presence of a Au-O stretching vibration. In contrast, the thread shows no evidence of chemisorption or a preferential orientation. The introduction of the thread into the macrocycle partly disrupts the film order so that the resulting chemisorbed rotaxane shows intermediate behavior with a preferential orientation up to 0.5 ML coverage. A decrease in film order and the absence of a preferred molecular orientation is observed for all three molecules at multilayer coverages. The spectral differences are addressed by molecular dynamics simulations in terms of the mobility of the phenyls of the three molecules on Au(111).     

FeO/Pt(111)与FeO2/Pt(111)的几何、电子结构及表面氧活性的第一性原理研究

采用密度泛函理论系统研究了超薄氧化物膜/金属体系FeO/Pt和FeO₂/Pt及其表面不同区域(FCC,HCP和TOP)的几何结构、电子性质及氧的活性.研究发现,表面O-Fe高度差δ_z作为一个重要的特征结构参数直接影响局域表面静电势和表面氧的结合能: δ_z越大,静电势越大,氧的结合能越弱.计算发现,在FeO/Pt体系中,δ_z顺序为FCC > HCP > TOP,而FeO₂/Pt中是FCC > TOP > HCP.此外,在FeO/Pt中,电荷转移方向是从氧化物膜到衬底,Fe的表观价态为+2.36,表面功函较纯Pt(111)的变化可忽略; 而FeO₂/Pt中,电荷转移的方向是从衬底到氧化物,Fe的表观价态为+2.95,表面功函较纯Pt增加1.24 eV.进一步分析了电荷转移和表面偶极对电子性质的作用机制.这些研究结果对于认识超薄氧化物薄膜对表面几何结构、电子性质、表面氧活性的调制具有重要的启示意义.     

Neutral and zwitterionic dopamine species adsorbed on silver surfaces: A DFT investigation of interaction mechanism

Biomolecules nondissociative adsorption on noble metals is a key process in metallic biosensors implying several questions related to the stability and orientation of such molecules. Here, the neurotransmitter dopamine (DA) adsorption on silver surface is investigated in the context of density functional theory (DFT). Two different dopamine isomers, the neutral (NDA) and zwitterionic (ZDA) species, and two different silver surfaces, Ag (110) and Ag(111), were considered. NDA shows relatively large binding energies, compared to previously studied π-π bonded systems. ZDA adsorbs even much more strongly although this species is less stable than NDA in vacuum. To elucidate the nature of the interaction between adsorbate and substrate, an electronic structure analysis was performed. Adsorbed NDA species suffers the loss of electronic charge, accompanied by a downshift of its molecular levels and the appearance of an attractive interaction of coulombic nature between adsorbate and substrate. The significant ZDA binding can be related to larger electron transfer and coupling between ZDA and Ag orbitals. Moreover, for both species, an important contribution of attractive noncovalent interactions of different degrees can be observed. The Ag substrate produces several modifications on NDA and ZDA vibrational frequencies. Noticeably relevant are the large red/blue shifts undergone by the N-H/O-H stretching bands of zwitterionic species, of up to −670/+430 cm⁻¹.     

A reflection absorption infrared spectroscopy and density-functional theory investigation of methanol dehydrogenation on Rh(111)V alloy surfaces

The dehydrogenation reaction of methanol on a Rh(111) surface, a Rh(111)V subsurface alloy, and on a Rh(111)V islands surface has been studied by thermal-desorption spectroscopy, reflection absorption infrared spectroscopy, and density-functional theory calculations. The full monolayer of methanol forms a structure with a special geometry with methanol rows, where two neighboring molecules have different oxygen-rhodium distances. They are close enough to form a H-bonded bilayer structure, with such a configuration, where every second methanol C-O bond is perpendicular to the surface on both Rh(111) and on the Rh(111)V subsurface alloy. The Rh(111)V subsurface alloy is slightly more reactive than the Rh(111) surface which is due to the changes in the electronic structure of the surface leading to slightly different methanol species on the surface. The Rh(111)V islands surface is the most reactive surface which is due to a new reaction mechanism that involves a methanol species stabilized up to about 245 K, partial opening of the methanol C-O bond, and dissociation of the product carbon monoxide. The latter two reactions also lead to a deactivation of the Rh(111)V islands surface.     

Photochemistry of vinyl chloride physisorbed on Ag(111) through molecular anion formation induced by substrate electron attachment

Pulsed 266 and 355 nm ultraviolet laser irradiation of monolayer vinyl chloride physisorbed on Ag(111) results in molecular dissociation leading to C2H3 and Cl, much of which is adsorbed to the surface. On the basis of observations made on dissociation dependences on chlorine isotope and photon energy, it is deduced that upon excitation vinyl chloride forms a transient negative ion through a substrate mediated, vertical electron attachment mechanism. The anion either dissociates or relaxes through energy transfer to the neutral state causing the neutral molecule to desorb. The threshold for vertical attachment of substrate electron is estimated to be 0.8 eV below the vacuum level, in agreement with the experimentally observed wavelength dependence in photoinduced dissociation. Chemisorbed Cl on the Ag(111) surface inhibits the photodissociation process by increasing the substrate work function and consequently the energy threshold for electron vertical attachment. Upon heating the Ag(111) surface, adsorbed vinyl combines to produce 1,3-butadiene in a first order, diffusion limited, process with an activation energy of 10.4 kcal/mol.     

咪唑在银表面吸附的理论分析

本文用EHMO紧束缚方法计算了咪唑在银(1,1,1)面上吸附的四种构型和吸附态的电子结构,得到咪唑在银表面上的优化构型为直立桥位吸附.按该方式,咪唑环上吡啶型氮原子的P轨道和邻近Ag原子s轨道之间形成多中心σ键.咪唑环上各原子的电荷布居在吸附后有较大的变化,表现出电子由银表面向咪唑转移,并进而使得环上N-H键解离能从吸附前的519.4kJ·mol^-1降低到吸附后的70.34kJ·mol^-1.这与实验中观察到的咪唑N-H键在Ag表面极易断裂的事实相符.     

Small Al clusters on the Cu(111) surface: Atomic relaxation and vibrational properties

The relaxation and vibrational properties of both Al clusters and the (111) surface of a copper sub-strate were studied using the interatomic interaction potentials obtained in a tight-binding approximation. The presence of small aluminum clusters led to modification of the vibrational states of the substrate, a shift of the Rayleigh mode, and excitation of new Z-polarized modes. Hybridized modes localized on the cluster adatoms and the neighboring atoms of the substrate were found in the phonon spectrum. The localized dipole-active modes of the cluster and their strong hybridization with vibrations of the substrate points to desorption stability of the tri- and heptaatomic clusters.