[Show abstract][Hide abstract] ABSTRACT: This article reviews recent progress in the application of core hole clock approach in the framework of resonant Auger electron spectroscopy to the monomolecular assembles of alkyl, oligophenyl, and oligo(phenylene-ethynylene) based molecules on Au(111) substrates, referring mostly to the work by the author et al. The major goal was to study electron transfer (ET) dynamics in these systems serving as prototypes of molecular electronics (ME) devices. The ET pathway to the conductive substrate was unambiguously defined by resonant excitation of the nitrile tailgroup attached to the molecular backbone. Characteristic ET times within the femtosecond domain were determined, along with the attenuation factors for the ET dynamics, analogous to the case of the static transport. The above parameters were found to exhibit strong dependence on the character of the molecular orbital which mediates the ET process. In addition, certain spectral features, which can be associated with an inverse ET from the molecular backbone to the excitation site, were observed upon exchange of the nitrile group by strongly electronegative nitro moiety. The reported results represent a valuable input for theory and a certain potential for applications such as ME devices where optimization of ET can have significant technological impact.
Journal of Electron Spectroscopy and Related Phenomena 06/2015; DOI:10.1016/j.elspec.2015.05.022 · 1.44 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We studied here the effect of humidity on electrical conductivity of pristine and nanoparticle-loaded hydrogel nanomembranes. The membranes were fabricated by the thermally activated crosslinking of amine- and epoxy-terminated, star-branched poly(ethylene glycol) oligomers. The resistance of the pristine membrane changed by ~5.5 orders of the magnitude upon relative humidity (RH) variation from 0 to 100%, which is unprecedented response for homogeneous materials. The dependence of the resistance on the moisture uptake into the membrane could be coarsely described by exponential function. The loading of the membranes with gold and silver nanoparticles (NPs) resulted in a noticeable improvement of their conductance at low RH but in a small improvement or even negative effect on the conductance at high RH. Both pristine and NP-loaded PHMs have significant potential as highly sensitive elements in humidity sensors and moisture-responsive nanoelectronic devices.
The Journal of Physical Chemistry C 06/2015; DOI:10.1021/acs.jpcc.5b03572 · 4.77 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Self-assembled monolayers of biphenyl-3,4′,5-tricarboxylic acid (BPTCA) on Au(111)/mica substrates modified by underpotential deposited layers of Cu and Ag were studied by scanning tunneling microscopy under ambient conditions as well as by synchrotron-based X-ray photoelectron spectroscopy and near-edge X-ray absorption fine structure spectroscopy. BPTCA forms distinctly different layers on Ag and Cu due to a pronounced influence of the substrate on the balance of intermolecular and molecule-substrate interactions. On Cu a highly crystalline commensurate row structure is formed, described by a 6 × √3 unit cell, a molecular tilt of 45-50° relative to the surface normal, and a bipodal bidentate adsorption geometry. In contrast, incommensurate row structures are formed on Ag which are characterized by significant waves and kinks, a monopodal bidentate adsorption geometry, and a tilt angle of 25-30°. While BPTCA parallels its smaller homologue, benzene-1,3,5-tricarboxylic acid, with regard to the substrate-specific monopodal and bipodal adsorption geometries, the preparation conditions for the monolayer on Cu and the film structure on Ag are pronouncedly different. The results are discussed in terms of the steric requirements and molecular symmetry of BPTCA.
The Journal of Physical Chemistry C 05/2015; 119(25):150527091612000. DOI:10.1021/acs.jpcc.5b01176 · 4.77 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Using a representative model system, here electronic and structural properties of aromatic self-assembled monolayers (SAMs) are described that contain an embedded, dipolar group. As polar unit, pyrimidine is used, with its orientation in the molecular backbone and, consequently, the direction of the embedded dipole moment being varied. The electronic and structural properties of these embedded-dipole SAMs are thoroughly analyzed using a number of complementary characterization techniques combined with quantum-mechanical modeling. It is shown that such mid-chain-substituted monolayers are highly interesting from both fundamental and application viewpoints, as the dipolar groups are found to induce a potential discontinuity inside the monolayer, electrostatically shifting the core-level energies in the regions above and below the dipoles relative to one another. These SAMs also allow for tuning the substrate work function in a controlled manner independent of the docking chemistry and, most importantly, without modifying the SAM-ambient interface.
[Show abstract][Hide abstract] ABSTRACT: Understanding of the electric transport through surface-anchored metal-organic frameworks (SURMOFs) is important both from a fundamental perspective as well as with regards to possible future applications in electronic devices. To address this, mostly unexplored subject, we integrated a series of representative SURMOF thin films, formed by copper nodes and trimesic acid and known as HKUST-1, in a mercury-drop-based tunneling junction. While the transport properties of these SURMOFs are analogous to those of hybrid metal-organic molecular wires, manifested by a very low value of the tunneling decay constant (ß ≈ 0.006 Å-1), they are at the same time found to be consistent with a linear increase of resistance with film thickness. Upon loading of SURMOF pores with ferrocene (Fc), a noticeable increase in transport current was observed. A transport model and ab-initio electronic structure calculations were used to reveal a hopping transport mechanism and to relate the changes upon Fc loading to those of the electronic and vibrational structures of the SURMOF films.
[Show abstract][Hide abstract] ABSTRACT: Selenolate is considered as an alternative to thiolate to serve as a headgroup mediating the formation of self-assembled monolayers (SAMs) on coinage metal substrates. There are however ongoing vivid discussions regarding the advantages and disadvantages of these anchor groups, regarding, in particular, the energetics of the headgroup-substrate interface and their efficiency in terms of charge transport/transfer. Here we introduce a well-defined model system of 6-cyanonaphthalene-2-thiolate and -selenolate SAMs on Au(111) to resolve these controversies. The exact structural arrangements in both types of SAMs are somewhat different, suggesting a better SAM building ability in the case of selenolates. At the same time, both types of SAMs have similar packing densities and molecular orientations. This permitted reliable competitive exchange and ion beam induced desorption experiments which provided an unequivocal evidence for a stronger bonding of selenolates to the substrate as compared to the thiolates. Regardless of this difference, the dynamic charge transfer properties of the thiolate and selenolate based adsorbates were found to be identical as determined by the core-hole-clock approach, which is explained by a redistribution of electron density along the molecular framework, compensating the difference in the substrate-headgroup bond strength.
[Show abstract][Hide abstract] ABSTRACT: Surface-confined hetero-metallic molecular triads (SURHMTs) were fabricated on SiO×-based solid substrates using optically-rich and redox-active Fe, Os, and Ru based terpyridyl complexes as metallo-ligands and Cu2+ ions as linkers. Optical and electrochemical studies reveal efficient electronic intra-molecular communication in these assemblies. The UV-vis spectra of the triads exhibit a superposition of the metal-to-ligand charge-transfer bands of individual complexes, providing a significant enlargement of the optical window, useful for application. Similarly, cyclic voltammograms of SURHMT layers show a variety of redox peaks corresponding to individual complexes as well as multi-redox states at a low potential. Interaction of a representative SURHMT assembly with redox active NOBF4 was investigated and used as a basis for configuring molecular logic gates.
[Show abstract][Hide abstract] ABSTRACT: Self-assembled
(SAMs) with amino tail groups are of interest due to their ability of coupling further compounds. Such groups can be, in particular, created by electron irradiation of nitro- or nitrile-substituted aromatic SAMs, which provide a basis for chemical nanolithography and the fabrication of functionalized nanomembranes. An estimate of reactivity of the created amino groups requires a reference system of homogeneous, amino-terminated aromatic SAMs, which can also be used as a highly reactive molecular template. Here, we describe the synthesis of 4′-aminobiphenyl-4-thiol (ABPT) and SAMs prepared from this precursor on Au(111). The monolayers were characterized by X-ray photoelectron spectroscopy and near edge X-ray absorption fine structure
spectroscopy, which revealed that they are well defined, chemically uniform, densely packed, and highly ordered. To examine the influence of electron irradiation on the reactivity of the terminal amino groups, ABPT SAMs were exposed to low energy (50 eV) electrons up to a dose of 40 mC/cm2 and, subsequently, immersed in either trifluoroacetic, pentafluoropropionic, or heptafluorobutyric anhydride. Analysing the amount of the attached anhydride species made it possible to determine the percentage of reactive amino groups as well as the effect of steric hindrance upon the coupling reaction. The above results are compared with those obtained for the well-established nitro-substituted biphenylthiol monolayers.
The Journal of Chemical Physics 03/2015; 142(10):101919. DOI:10.1063/1.4907942 · 2.95 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Here, we describe a universal photolithography setup for the patterning of biorepulsive self-assembled monolayers (SAMs) as well as other monomolecular films. The setup is based on commercial equipment consisting of a computer-controlled digital micromirror device chip combined with a suitable optics and a powerful light-emitting diode (LED) source delivering ultraviolet (UV) light with a wavelength of 375 nm. Digital patterns generated in the computer serve as an input for the chip, which modulates the reflected light accordingly, transferring the pattern to the sample surface. The performance of the setup was demonstrated by UV-induced modification of the nonsubstituted alkanethiolate (NS-AT) SAMs and biorepulsive oligo(ethylene glycol)-substituted AT (OEG-AT) monolayers on Au(111), upon homogeneous illumination of the test samples. Further, both nonspecific and specific templates for the protein adsorption were fabricated in the protein-repelling OEG-AT matrix by either direct writing or using an additional irradiation-promoted exchange reaction with a biotin-terminated AT. These templates were used either for nonspecific adsorption of bovine serum albumin (BSA) or for the specific adsorption of avidin, the latter relying on the interaction with the embedded biotin receptors. The density of the adsorbed protein layers across the patterns could be precisely varied by selection of proper irradiation doses.
The Journal of Physical Chemistry C 12/2014; 119(1). DOI:10.1021/jp510809a · 4.77 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Self-assembled monolayers (SAMs) of HS-(C6H4)n-NO2 (nPT-NO2), abbreviated individually as PT-NO2, BPT-NO2, and TPT-NO2 for n = 1, 2, and 3, respectively, were prepared on Au(111) substrates and characterized by X-ray photoelectron spectroscopy, near-edge X-ray absorption fine structure spectroscopy, and resonant Auger electron spectroscopy. All molecules in the films were found to be bound to the substrate via the thiolate anchor and to have an upright orientation. The introduction of the nitro tail group had a positive effect on the quality of the PT-NO2 SAMs, which was superior to that of the nonsubstituted analogues. The parameters of the BPT-NO2 and TPT-NO2 films were similar to those of the analogous nonsubstituted systems. The [N 1s]π* and [O 1s]π* decay spectra of all studied nPT-NO2 SAMs did not exhibit any trace of charge (electron) transfer (CT) through the molecular framework to the substrate, following the resonant excitation of the tail group. This was explained by the energy considerations hindering CT to the substrate but enabling a reverse process (ICT), viz., the neutralization of the core ionized state by electron transfer from the substrate/molecular backbone. Traces of this process could be tentatively identified as an admixture of resonant contributions to the nonresonant decay spectra at the O K-edge. The experimental data suggest that only the ring adjacent to the nitro group was involved in the ICT process.
The Journal of Physical Chemistry C 11/2014; 118(45):26049-26060. DOI:10.1021/jp507265k · 4.77 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The molecular structure of liquid water is susceptible to changes upon admixture of salts due to ionic solvation, which provides the basis of many chemical and biochemical processes. Here we demonstrate how the local electronic structure of aqueous potassium chloride (KCl) solutions can be studied by resonant inelastic soft X-ray scattering (RIXS) to monitor the effects of the ion solvation on the hydrogen-bond (HB) network of liquid water. Significant changes in the oxygen K-edge emission spectra are observed with increasing KCl concentration. These changes can be attributed to modifications in the proton dynamics, caused by a specific coordination structure around the salt ions. Analysis of the spectator decay spectra reveals a spectral signature that could be characteristic of this structure.
[Show abstract][Hide abstract] ABSTRACT: The electronic structure of the amino acid L-cysteine in aqueous environment was studied using resonant inelastic soft x-ray scattering (RIXS) in a 2D map representation and analyzed in the framework of a "building block" approach. The element-selectivity of RIXS allows a local investigation of the electronic structure of the three functional groups of cysteine, namely the carboxyl, amino, and thiol groups, by measuring at the O K, N K, and S L2,3 edges, respectively. Variation of the pH value allows an investigation of molecules with protonated and deprotonated functional groups, which can then be compared with simple reference molecules that represent the isolated functional groups. We find that such "building blocks" can provide an excellent description of x-ray emission spectroscopy (XES) and RIXS spectra, but only if all nearest-neighbor atoms are included. This finding is analogous to the building-block principle commonly used in x-ray absorption spectroscopy. The building blocks show a distinct spectral character (fingerprint) and allow a comprehensive interpretation of the cysteine spectra. This simple approach opens the path to investigate the electronic structure of more complex biological molecules in aqueous solutions using XES and RIXS.
The Journal of Physical Chemistry B 10/2014; 118(46). DOI:10.1021/jp5089417 · 3.30 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Patterns of nanoparticles (NPs) on solid supports are usually restricted to a particular substrate or a class of substrates. Here we present a procedure which decouples the patterning step from the target substrate, enabling the fabrication of custom designed NP assemblies on nearly any solid support, including non-flat ones. The procedure relies on a hydrogel template prepared on the primary, conductive substrate and transferred to the target support as a sacrificial nanomembrane. The template is structured by electron beam lithography (EBL) which seals predefined areas of poly(ethylene glycol) based hydrogel film, making them inert to NP deposition in contrast to pristine areas that adsorb NPs in high densities. The deposition of NPs, occurring from an aqueous solution into the transferred membrane, follows EBL generated structure, delivering the desired NP pattern on the target support after removal of the organic matrix. Efficiency and flexibility of the procedure is illustrated by creating a variety of representative submicrometer patterns of densely packed gold and silver NPs on glass, including a useful pattern of a miniaturized quick-response code. The arrangement of NPs in these patterns corresponds to the negative image of EBL generated template. This significantly reduces the exposure time for designs where large areas covered with NPs are separated by thin, NP-free stripes.
[Show abstract][Hide abstract] ABSTRACT: Heteroleptic copper(II)–polypyridyl complexes with extended
p-conjugated, aromatic terminal units were immobilized on glass/Si
substrates to intercalate DNA and cleave it upon photoexposure.
Photonuclease activity is shown to be high, well reproducible and
non-destructible towards the assembled complexes.
Chemical Communications 07/2014; 50(78). DOI:10.1039/c4cc05063k · 6.83 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Using self-assembled monolayers (SAMs) of [1,1′;4′,1″-terphenyl]-4,4″-dimethanethiol (TPDMT) on Au as a test system and nickel as a test adsorbate, we demonstrate that the penetration of deposited metal atoms into a SAM can be completely inhibited by the preliminary formation of palladium-chloride seeding layer at the SAM–ambience interface. This layer is formed by a simple dipping procedure that takes only a few minutes. The palladium atoms in the seeding layer serve as nucleation centers for the growing metal film, staying at its bottom during the growth. In contrast, the chlorine atoms are transferred from palladium to the deposited metal, staying on the top of the growing metal film and serving as surfactants. The above approach is a perspective way to solve the well-known top contact problem at the formation of metal–SAM–metal assemblies that are of potential importance for future electronic and spintronic devices.
The Journal of Physical Chemistry C 06/2014; 118(24):12980–12988. DOI:10.1021/jp505034h · 4.77 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We studied the deposition of metal atoms (Ni as a test adsorbate) on fluorinated self-assembled monolayers (SAMs) using films of perfluoroterphenyl-substituted alkanethiols, C6F5(C6F4)2(CH2)3SH (FTP3), and partly fluorinated alkanethiols, F(CF2)10(CH2)2SH (F10H2), on Au(111) as representative test systems. Unlike the F10H2 films, their FTP3 counterparts were found to stop efficiently the penetration of nickel atoms into the SAM. The primary process is the Ni-mediated loss of fluorine atoms followed by extensive cross-linking between the partly defluorinated FTP backbones. The stability of these backbones and the rapid development of the intermolecular cross-linking, affecting predominantly the topmost part of the FTP3 SAM, are the key components to hinder the metal penetration. The chemically induced cross-linking in combination with the entirely reactive SAM represents a new concept to prepare a well-defined metal film at the SAM–ambience interface. This can be useful in context of novel metal/SAM-insulator/metal assemblies that are of potential interest for electronic and spintronic applications.
The Journal of Physical Chemistry C 05/2014; 118(22):11763–11773. DOI:10.1021/jp5025334 · 4.77 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Extremely elastic and highly stable nanomembranes of variable thickness (5-350 nm) made completely of poly(ethylene glycol) are prepared by a simple procedure. The membranes exhibit distinct biorepulsive and hydrogel properties. They offer new possibilities for applications such as supports in transmission electron microscopy, matrices for inorganic nanoparticles, and pressure-sensitive elements for sensors.
[Show abstract][Hide abstract] ABSTRACT: Heterometallic, coordination-based, binary oligomer films were fabricated on SiOx-based solid substrates using successive layer-by-layer assembly of optically rich and redox-active polypyridyl complexes, Ru(pytpy)2·2PF6 (Ru-PT) and Os(pytpy)2·2PF6 (Os-PT) (where pytpy = 4′-pyridyl-2,2′:6′,2″-terpyridyl). The individual oligomer chains comprised alternating Ru-PT and Os-PT units connected via Cu2+, Pd2+, Ag+, Fe2+, Co2+, or Zn2+ metallo-linkers. The growth and properties of the oligomer films were monitored in detail by UV–vis spectroscopy and cyclic voltammetry. The films exhibited a linear growth upon addition of the successive building blocks, with a joint grafting density of 3.9–5.0 × 1014 metallo-ligands/cm2 for the final oligomer films (10 layers), corresponding to a characteristic area of 2.0–2.5 nm2/oligomer. The only exception was the Pd2+-linked film on glass that showed an exponential growth, which, however, could also be changed to the linear mode by the introduction of a conductive substrate. The combination of two different functional molecular units in the oligomer chains resulted in enhancement of the optical window and in an increase in the number of the available redox states as compared to the analogous single component assemblies.
The Journal of Physical Chemistry C 04/2014; 118(18):9578–9587. DOI:10.1021/jp502166k · 4.77 Impact Factor