Michael Zharnikov

Universität Heidelberg, Heidelburg, Baden-Württemberg, Germany

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Publications (273)1232.92 Total impact

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    ABSTRACT: The focus of the present manuscript is on understanding the insight that X-ray photoelectron spectroscopy (XPS) measurements can provide when studying self-assembled monolayers. Comparing density-functional theory calculations to experimental data on deliberately chosen model systems, we show that both the chemical environment and electrostatic effects arising from a superposition of molecular dipoles influence the measured core-level binding energies to a significant degree. The crucial role of the often overlooked electrostatic effects in polar self-assembled monolayers (SAMs) is unambiguously demonstrated by changing the dipole density through varying the SAM coverage. As a consequence of this effect, care has to be taken when extracting chemical information from the XP spectra of ordered organic adsorbate layers. Our results, furthermore, imply that XPS is a powerful tool for probing local variations in the electrostatic energy in nanoscopic systems, especially in SAMs.
    No preview · Article · Jan 2016 · The Journal of Physical Chemistry C

  • No preview · Article · Jan 2016 · The Journal of Physical Chemistry C
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    ABSTRACT: We studied the influence of embedded dipole moments in self-assembled monolayers (SAMs) formed on template stripped Au surfaces with liquid eutectic Ga–In alloy as a top electrode. We designed three molecules based on a p-terphenyl structure in which the central aromatic ring is either phenyl or a dipole-inducing pyrimidyl in one of two different orientations. All three form well defined SAMs with similar thickness, packing density and tilt angle, with dipole moments embedded in the SAM, isolated from either interface. The magnitude of the current density is dominated by the tunneling distance and is not affected by the presence of dipole moments; however, transition voltages (VT) show a clear linear correlation with the shift in the work function of Au induced by the collective action of the embedded dipoles. This observation demonstrates that VT can be manipulated synthetically, without altering either the interfaces or electrodes and that trends in VT can be related to experimental observables on the SAMs before installing the top contact. Calculated projected density of states of the SAMs on Au surfaces that relate HOMO-derived states to VT further show that energy level alignment within an assembled junction can be predicted and adjusted by embedding dipoles in a SAM without altering any other properties of the junction. We therefore suggest that trends in VT can be used analogously to β in systems for which length-dependence is physically or experimentally inaccessible.
    No preview · Article · Dec 2015 · Chemical Science
  • Hao Lu · Michael Zharnikov
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    ABSTRACT: Molecular assembly on a technologically relevant GaAs substrate is an important and application-related issue. In this context, self-assembled monolayers (SAMs) formed from a series of ω-(4′-methylbiphenyl-4-yl)alkanethiols, CH3(C6H4)2(CH2)nSH (BPn, n = 1-6), were prepared on GaAs(001) and characterized in detail by high-resolution X-ray photoelectron spectroscopy and near-edge X-ray absorption fine structure spectroscopy. The resulting films exhibited pronounced, "odd-even" variation in molecular orientation and packing density with the number (n) of methylene groups in the alkyl linker; viz., smaller molecular inclination, associated with a higher packing density, was observed for an odd n, while the opposite was the case for an even n. Such an odd-even behavior confirms once again the existence of a bending potential for GaAs(001), which is equivalent to the analogous potential for the Au(111) substrate, where similar odd-even behavior has been observed. This potential plays an important role in the balance of the structure-building interactions, predefining the orientation of the alkyl linker, transferred in an odd-even fashion, depending on the parity of n, to the adjacent biphenyl spacer. The above effects were found to be superimposed onto pronounced dependence of the film quality on the length of the BPn precursor. This occurred due to proneness of the GaAs substrate to oxidation, hindering an efficient self-assembly, as well as due to a limited ability of the short-chain monolayers to protect this sensitive substrate from the postpreparation oxide regrowth. A proper selection of the parameters is, thus, very important for the design of functional monomolecular films on GaAs.
    No preview · Article · Nov 2015 · The Journal of Physical Chemistry C
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    ABSTRACT: Taking self-assembled monolayers (SAMs) of 4,4′-terphenylyl-substituted ethanethioacetate (TP2-SAc) on Au(111) as a test system, we studied the effect of the protecting group on the structure and morphology of this monolayer. The films were prepared at both room (298 K) and elevated (333 K) temperature, at either the presence or absence of a deprotecting agent, viz., triethylamine. The presence of the protecting group resulted in distinctly different crystallographic structure, described by the (2-3 × 4) rect unit cell, in all SAMs studied as compared to the case of the nonprotected analogue. The molecules within this unit cell were arranged in a herringbone fashion as could be observed by variation of the scan direction during the image acquisition by scanning tunneling microscopy. Most important and in contrast to previous studies of similar systems, the presence of the protecting group led to significant improvement of the SAM morphology in the case of preparation at 333 K, resulting in formation of comparably large domains with dimensions exceeding 100 nm. The effect of the deprotecting agent was found to be small when preparing at 298 K and hardly perceptible at 333 K. Determination of the reaction kinetics gave evidence of a completely different reaction mechanism for the thioacetate as compared to the thiol, which presumably is responsible for the observed differences.
    Full-text · Article · Oct 2015 · The Journal of Physical Chemistry C
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    ABSTRACT: Structural properties and stability of the self-assembled monolayers (SAMs) of two prototypical azobenzene-based alkanethiols (C6H5-N=N-C6H4-(CH2)n-SH) on Au(111) and Ag(111) substrates were studied in detail using a combination of complementary experimental techniques. The azobenzene moiety in these films was linked to the thiol headgroup via short aliphatic spacers of variable length, i.e., (CH2)3 or (CH2)4, corresponding to a different parity of n. For both Au(111) and Ag(111) substrates, a pronounced dependence of the packing density and molecular inclination on the parity of n was observed, with a higher packing density (by ∼14%) and smaller inclination (by ∼17°) of the azobenzene moieties for n = odd as compared to n = even on Au(111) and reversed, but somewhat reduced, behavior on Ag(111). This dependence was related to the well-known odd-even effects in molecular assembly on noble metal substrates, reported previously for a variety of oligophenyl-substituted alkanethiolate SAMs and observed now for the azobenzene-substituted monolayers as well, underlining their generality. The structural odd-even behavior was accompanied by odd-even effects in the stability of the substrate-S bond, with the latter effects being directly correlated to the respective structure variation. The above results are of general importance for the design of functional monomolecular films and of a particular significance as a basis for dedicated photoisomerization experiments.
    Full-text · Article · Oct 2015 · The Journal of Physical Chemistry C
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    Hannah Aitchison · Hao Lu · Michael Zharnikov · Manfred Buck

    Full-text · Dataset · Jul 2015
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    Hao Lu · Martin Kind · Andreas Terfort · Michael Zharnikov

    Full-text · Dataset · Jul 2015
  • Michael Zharnikov
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    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.
    No preview · Article · Jun 2015 · Journal of Electron Spectroscopy and Related Phenomena
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    Musammir Khan · Swen Schuster · Michael Zharnikov
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    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.
    Full-text · Article · Jun 2015 · The Journal of Physical Chemistry C
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    Hannah Aitchison · Hao Lu · Michael Zharnikov · Manfred Buck
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    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.
    Full-text · Article · May 2015 · The Journal of Physical Chemistry C
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    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.
    Full-text · Article · May 2015 · Advanced Functional Materials
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    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.
    Full-text · Article · Apr 2015 · ACS Applied Materials & Interfaces
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    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.
    Full-text · Article · Apr 2015 · ACS Nano
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    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.
    Full-text · Article · Apr 2015 · ACS Applied Materials & Interfaces
  • N. Meyerbroeker · P. Waske · M. Zharnikov
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    ABSTRACT: Self-assembled monolayers (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.
    No preview · Article · Mar 2015 · The Journal of Chemical Physics
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    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.
    No preview · Article · Dec 2014 · The Journal of Physical Chemistry C
  • Prashant Waske · Tobias Wächter · Andreas Terfort · Michael Zharnikov
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    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.
    No preview · Article · Nov 2014 · The Journal of Physical Chemistry C
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    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.
    Full-text · Article · Nov 2014 · Journal of Physical Chemistry Letters
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    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.
    Full-text · Article · Oct 2014 · The Journal of Physical Chemistry B

Publication Stats

8k Citations
1,232.92 Total Impact Points

Institutions

  • 1997-2015
    • Universität Heidelberg
      • Institute of Physical Chemistry
      Heidelburg, Baden-Württemberg, Germany
  • 2009
    • National Synchrotron Radiation Research Center (NSRRC)
      Hsin-chu-hsien, Taiwan, Taiwan
    • National Cheng Kung University
      • Department of Materials Science and Engineering
      臺南市, Taiwan, Taiwan
  • 2007
    • Universität Regensburg
      • Institute of Analytical Chemistry, Chemo and Biosensors
      Ratisbon, Bavaria, Germany
  • 1995-1998
    • Max Planck Institute of Microstructure Physics
      • Experimental Department 2
      Halle-on-the-Saale, Saxony-Anhalt, Germany
  • 1994
    • Technische Universität München
      • Faculty of Physics
      München, Bavaria, Germany