Photoemission studies of Ar,Kr and Xe adsorbed on Al(111): Dipole moments,polarizabilities and spatial distributions

Submonolayers of rare gases adsorbed on Al(111) have been studied using photoemission techniques. Coverage-dependent core level binding energy shifts and work function changes have been measured; polarizabilities and dipole moments are deduced. Results indicate that adatom spatial distributions for Xe submonolayers adsorbed on Al(111) at 40 K are best described by a random 2-dimensional distribution, and there is negligible charge transfer from the Al substrate onto the Xe adatoms in the photoionization process.     

溶液酸碱性对异构体氨基酸单分子膜影响的表面增强拉曼光谱研究

利用表面增强拉曼(SERS)光谱技术研究比较了在粗糙化银电极表面吸附的亮氨酸与异亮氨酸自组装单层膜结构以及溶液酸碱性对分子吸附作用的影响。研究表明在粗糙化银电极表面两种氨基酸分子主要是以COO-为作用位点进行吸附的。进一步的研究也揭示溶液pH值的变化没有显著改变两种氨基酸分子在银电极表面以去质子化羧基吸附为主的特征,但对于羧基吸附作用的影响程度及其变化规律是迥异的,对氨基的影响也有一定的差异。     

Adsorption properties of decyl thiocyanate and decanethiol on platinum substrates studied by sum-frequency generation spectroscopy

Recently, thiocyanate groups were successfully used as precursors for thiolate assemblies. Indeed, they provide a useful alternative to thiol groups for self-assembly on metallic substrates. In order to check the adsorption properties and the quality of the thiocyanate-based self-assembled monolayers (SAMs), we use sum-frequency generation spectroscopy (SFG) and apply it to characterize the ad-layers of two similar molecules: decanethiol (DT) and decyl thiocyanate (DTCN) adsorbed on platinum surfaces. By comparing the SFG signals of the methyl and methylene vibration modes, we show that DTCN SAMs are less ordered than DT ones. These effects are related to the SAMs quality which depends on the molecular packing as highlighted by scanning tunnelling microscopy and X-ray photoelectron spectroscopy.     

Probing enantioselectivity with x-ray photoelectron spectroscopy and density functional theory

The enantioselectivity of gold is investigated by x-ray photoelectron spectroscopy (XPS) and density functional theory (DFT). Cysteine molecules on a chiral Au(17 11 9);{S} surface show enantiospecific core level binding energies in the amino and in the thiol group. The sign and order of magnitude of the XPS core level shifts is reproduced by DFT. Identical preparations of D- and L-cysteine layers lead to D-cysteine molecules in the pure NH2 form, while a small portion of L-cysteine molecules maintains a hydrogen rich amino group (NH3). This implies enantiospecific adsorption reaction pathways and is consistent with DFT that indicates an activated hydrogen abstraction reaction from the amino group, which is downhill for D-cysteine.     

Dependence of surface properties on adsorbate-substrate distance: Work function changes and binding energy shifts for I/Pt(1 1 1)

This paper presents an analysis of the character of the bond of I adsorbed at on-top and 3-fold sites of Pt(1 1 1). At both sites, the bonding is dominated by an ionic interaction supplemented with some covalent character due to donation from the adsorbed I anion to the Pt surface. The way in which the I-Pt interaction affects observed properties has been established. In particular, the origins of the anomalous work function changes induced by the adsorption of I and the shifts of I core level binding energies are explained. It is shown that the magnitudes of the changes in these properties can be directly correlated with the distance of the I from the Pt surface. Thus, these shifts can be interpreted to indicate adsorbate height. The fact that the negatively charged I adsorbate leads to a work function decrease, rather than the increase expected due to the charge of the adsorbate, may appear to be an anomaly. However, it is shown that this decrease arises from electronic reorganizations that cancel the dipole due to the charge of the adsorbate. Furthermore, the electronic terms that contribute to a lowering of the work function are larger as the adsorbate moves closer to the surface.     

Diluting thiol-derivatized oligonucleotide monolayers on Au(1 1 1) by mercaptohexanol replacement reaction

Surface replacement reaction of thiol-derivatized, single-stranded oligonucleotide (HS-ssDNA) by mercaptohexanol (MCH) is investigated in order to reduce surface density of the HS-ssDNA adsorbed to Au(1 1 1) surface. Cyclic voltammograms (CVs) and scanning tunneling microscopy (STM) are employed to assess the composition and state of these mixed monolayers. It is found that each CV of mixed self-assembled monolayers (SAMs) only shows a single reductive desorption peak, which suggests that the resulted, mixed SAMs do not form discernable phase-separated domains. The peak potential gradually shifts to negative direction and the peak area increases step by step over the whole replacement process. By analyzing these peak areas, it is concluded that two MCH molecules will replace one HS-ssDNA molecule and relative coverage can also be estimated as a function of exposing time. The possible mechanism of the replacement reaction is also proposed. The DNA surface density exponentially reduces with the exposing time increasing, in other words, the replacement reaction is very fast in the first several hours and then gradually slows down. Moreover, the morphological change in the process is also followed by STM.     

Effect of end groups on contact resistance of alkanethiol based metal-molecule-metal junctions using current sensing AFM

Tuning the charge transport through a metal-molecule-metal junction by changing the interface properties is widely studied and is of paramount importance for applications in molecular electronic devices. We used current sensing atomic force microscopy (CSAFM) as a tool to study the contact resistance of metal-molecule-metal (MmM) junctions formed by sandwiching self-assembled monolayers (SAMs) of alkanethiols with various end groups (-CH₃, -OH and -NH₂) between Au(1 1 1) substrates and Au coated AFM tips. The effect of interface chemistry on charge transport through such SAMs with varying end groups was studied in an inert, non-polar liquid (hexadecane) environment. We find that the contact resistances of these MmM junctions vary significantly based on the end group chemistry of the molecules.     

Cells anchored upon a thin organic film with different nano-mechanical properties

By applying dynamic contact module and particular measurement of phase angles, harmonic contact stiffness (S) along with the measured displacement (D) of different self-assembled monolayers (SAMs) adsorbed on Au can be distinguished. The relatively ordered and hydrophobic ODT and DDT molecules adsorbed on Au form high contact stiffness, which are presumably unfavorable substrates for a cell to adhere upon. Short-chain MUA molecules adsorbed on Au provides a hydrophilic characteristic with a relatively low contact stiffness, which may significantly promote cell adhesion. It is, therefore, estimated that the behavior of a cell adhered on SAMs/Au is correlated not only with their outermost chemical species but also with a proper dS/dD matrix acting as a cushion.     

Selection rules for electric multipole transition of diatomic molecule in scattering experiments

The knowledge of the energy level structures of atoms and molecules is mainly obtained by spectroscopic experiments.Both photoabsorption and photoemission spectra are subject to the electric dipole selection rules(also known as optical selection rules). However, the selection rules for atoms and molecules in the scattering experiments are not identical to those in the optical experiments. In this paper, based on the theory of the molecular point group, the selection rules are derived and summarized for the electric monopole, electric dipole, electric quadrupole, and electric octupole transitions of diatomic molecules under the first Born approximation in scattering experiments. Then based on the derived selection rules, the electron scattering spectra and x-ray scattering spectra of H2, N2, and CO at different momentum transfers are explained, and the discrepancies between the previous experimental results measured by different groups are elucidated.     

Relative core level shifts in XPS: a theoretical study

The commonly used assumption that all core levels exhibit the same chemical shift is examined using Dirac–Fock electronic structure calculations for a wide range of free atoms and ions. In contrast to experimental results for molecules and solids, the calculated core level shifts are found to be almost uniform. The role of the Madelung potential in exaggerating relative core level shifts in solids and molecules is discussed. Calculated results are found to agree well with the most reliable experimental data.     

Isotope shifts in spectra of molecular liquids

In the IR absorption spectra of low-temperature molecular liquids, we have observed anomalously large isotope shifts of frequencies of vibrational bands that are strong in the dipole absorption. The same effect has also been observed in their Raman spectra. At the same time, in the spectra of cryosolutions, the isotope shifts of the same bands coincide with a high accuracy (±(0.1–0.5) cm^–1) with the shifts that are observed in the spectra of the gas phase. The difference between the spectra of examined low-temperature systems is caused by the occurrence of resonant dipole–dipole interactions between spectrally active identical molecules. The calculation of the band contour in the spectrum of liquid freon that we have performed in this work taking into account the resonant interaction between states of simultaneous transitions in isotopically substituted molecules can explain this effect.     

Effects of Surface Dipole Lengths on Evaporation of Tiny Water Aggregation

Using molecular dynamics simulation, we compared evaporation behavior of a tiny amount of water molecules adsorbed on solid surfaces with different dipole lengths, including surface dipole lengths of 1 fold, 2 folds, 4 folds, 6 folds and 8 folds of 0.14 nm and different charges from 0.1e to 0.9e. Surfaces with short dipole lengths (1-fold system) can always maintain hydrophobic character and the evaporation speeds are not influenced, whether the surface charges are enhanced or weakened; but when surface dipole lengths get to 8 folds, surfaces become more hydrophilic as the surface charge increases, and the evaporation speeds increase gradually and monotonically. By tuning dipole lengths from 1-fold to 8-fold systems, we confirmed non-monotonic variation of the evaporation flux (first increases, then decreases) in 4 fold system with charges (0.1e-0.7e), reported in our previous paper [S. Wang, et al., J. Phys. Chem. B 116 (2012) 13863], and also show the process from the enhancement of this unexpected non-monotonic variation to its vanishment with surface dipole lengths increasing. Herein, we demonstrated two key factors to influence the evaporation flux of a tiny amount of water molecules adsorbed on solid surfaces: the exposed surficial area of water aggregation from where the water molecules can evaporate directly and the attraction potential from the substrate hindering the evaporation. In addition, more interestingly, we showed extra steric effect of surface dipoles on further increase of evaporation flux for 2-folds, 4-folds, 6-folds and 8-folds systems with charges around larger than 0.7e. (The steric effect is first reported by parts of our authors [C. Wang, et al., Sci. Rep. 2 (2012) 358]). This study presents a complete physical picture of the influence of surface dipole lengths on the evaporation behavior of the adsorbed tiny amount of water.     

Microcontact printing of self-assembled monolayers to pattern the light-emission of polymeric light-emitting diodes

By patterning a self-assembled monolayer (SAM) of thiolated molecules with opposing dipole moments on a gold anode of a polymer light-emitting diode (PLED), the charge injection and, therefore, the light-emission of the device can be controlled with a micrometer-scale resolution. Gold surfaces were modified with SAMs based on alkanethiols and perfluorinated alkanethiols, applied by microcontact printing, and their work functions have been measured. The molecules form a chemisorbed monolayer of only ∼1.5 nm on the gold surface, thereby locally changing the work function of the metal. Kelvin probe measurements show that the local work function can be tuned from 4.3 to 5.5 eV, which implies that this anode can be used as a hole blocking electrode or as a hole injecting electrode, respectively, in PLEDs based on poly(p-phenylene vinylene) (PPV) derivatives. By microcontact printing of SAMs with opposing dipole moments, the work function was locally modified and the charge injection in the PLED could be controlled down to the micrometer length scale. Consequently, the local light-emission exhibits a high contrast. Microcontact printing of SAMs is a simple and inexpensive method to pattern, with micrometer resolution, the light-emission for low-end applications like static displays. Both authors (J.J. Brondijk and X. Li) contributed equally.     

Esca. results versus other physical and chemical data

X-ray photoemission spectra yield quantities of very direct interest in physics and chemistry. In this paper the relations of these spectra to other data and concepts are discussed. Both initial-state and final-state properties may be studied: the former are treated first. Charge distributions in molecules alter the effective (Coulomb plus exchange) potential experienced by core electrons in molecular ground states, there by shifting their binding energies. The shifts can be calculated by $\text{ab}$$\text{initio}$ methods or more directly by using potential models based on intermediate-level molecular-orbital theories such as INDO. One version, the ground-state potential model (GPM) yields good predictions of core-level shifts among atoms in similar environments. Alternatively, the measured shifts may be used to derive charges on individual atoms in molecules. It is more difficult to derive charges in solids in this way, but a characteristic splitting in the more tightly-bound valence bands yields a direct measure of ionicity in simple binary compounds of the zinc-blende and rocksalt structures. Atomic orbital composition of molecular orbitals can be deduced from photoemission spectra. In solids such as diamond and graphite comparison of photoemission spectra with x-ray emission spectra yields the atomic-orbital composition of the valence bands. Turning to final-state properties, the spectra are dominated by relaxation effects. Again a simple approach—the relaxation potential model (RPM)—predicts core-level shifts well for cases in which the atomic environments are varied substantially. Among ammonia and the methylamines, for example, the N(ls) shifts are predicted correctly by RPM, while GPM reverses the order. For paramagnetic molecules RPM predicts electron charge transfer toward the positive hole but usually spin transfer away, in agreement with experiment. Extra-atomic relaxation in metals, a many-body effect, is manifest both as a contribution to the binding energy and as line-shape asymmetry. Delocalized valence electrons also show relaxation shifts that can be understood as polari zation of the electron gas toward the “Coulomb hole”. Auger lines show larger relaxation shifts. Comparison of core-level or Auger shifts in nonmetallic solids separately is questionable because there is no reference level, but intercomparison of the two is meaningful. Finally, core-level binding-energy trends in series of simple alcohols, etc., agree quantitatively with proton affinities and core-level shifts in other functional groups. This suggests extending the concept of Lewis basicity to include lone pairs of core electrons. Thus, core-level shifts measure the chemical reactivity—a quantity of great chemical importance that depends on both initial- and final-state properties—rather directly. Rela xation energies are shown to be the dominant cause of trends in the lowest ionization potentials of simple alcohols and amines.     

Probing into adsorption behavior of human plasma fibrinogen on self-assembled monolayers with different chemical properties by scanning probe microscopy

The adsorption behaviors of fibrinogen on the self-assembled monolayers (SAMs) with different chemical properties were investigated using an atomic force microscopy (AFM). AFM images indicated that the adsorption amounts of fibrinogen molecules increased with an increase of the surface hydrophobicity. High-resolution AFM imaging revealed that the fibrinogen conformations adsorbed on the SAM surface changed with dependent on the surface chemistry. The adsorption models of fibrinogen molecules adsorbed on SAM surfaces with different chemical properties were proposed based on the high-resolution AFM images.     

Spectroscopic link between adsorption site occupation and local surface chemical reactivity

In this Letter we show that sequences of adsorbate-induced shifts of surface core level (SCL) x-ray photoelectron spectra contain profound information on surface changes of electronic structure and reactivity. Energy shifts and intensity changes of time-lapsed spectral components follow simple rules, from which adsorption sites are directly determined. Theoretical calculations rationalize the results for transition metal surfaces in terms of the energy shift of the d-band center of mass and this proves that adsorbate-induced SCL shifts provide a spectroscopic measure of local surface reactivity.     

Dynamics of orientation of adsorbed polar molecules in static electric and laser field

The rotational states of adsorbed polar molecule in the presence of static electric and laser field are investigated. In this study, we have taken two types of polar molecules, namely, one with large dipole moment and moderate rotational constant (LiCl), and the other with moderate dipole moment and large rotational constant (HBr). The adsorbed molecule is considered as rigid rotor in finite conical potential well. The eigenvalues are calculated by numerically solving the Schrödinger equation. We have shown that various properties of rotational states of the molecules are significantly influenced by the static electric field, laser field, conical potential well height and the hindrance angle. Also the use of two different polar molecules for the investigation is justified by the variation in various properties due to change in molecule.     

The role of electrodynamic interactions on energy loss intensities of adsorbed molecules

The effect on energy loss intensities of the electrodynamic interactions with the substrate and neighboring adsorbed molecules is discussed. The experimentally observed large variations in the loss intensity from molecules adsorbed on various sites are considered. A simple dipole model of the molecule interacting with its image in the substrate and its neighbors on equivalent and inequivalent sites is used. Good agreement with experiment for a well characterized surface coverage of bridge and on-top bonded CO molecules on Ni(111) is found.     

Two-photon optical bistability of polar molecules inserted in a solid matrix: Phonon effects

Single-beam two-photon optical bistability in a Fabry-Perot cavity filled with molecules which change permanent dipole moment upon electronic excitation is analysed under the mean-field and plane-wave approximations. One-photon resonances are discarded. These molecules are assumed to be in a solid matrix. Phonon effects are then taken into account. The Bloch equations of the system are further coupled by a term that is dependent on the electron-phonon interaction, leading to shifts in the switching thresholds which modify the optical bistability character. Such a term becomes irrelevant when the difference between the dipole moments is much greater than the transition dipole moment.