A stable room-temperature molecular assembly of zwitterionic organic dipoles guided by a Si(111)-7x7 template effect.

Institut FEMTO-ST/LPMO, UMR CNRS 6174, 32, Avenue de l'Observatoire, 25044 Besancon cedex, France.
Angewandte Chemie International Edition (Impact Factor: 11.34). 02/2007; 46(48):9287-90. DOI: 10.1002/anie.200702794
Source: PubMed
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    ABSTRACT: We explore the limits of modifying metal work functions with large molecular dipoles by systematically increasing the dipole moment of archetype donor-acceptor molecules in self-assembled monolayers on gold. Contrary to intuition, we find that enhancing the dipoles leads to a reduction of the adsorption-induced change of the work function. Using atomistic simulations, we show that large dipoles imply electronic localization and level shifts that drive the interface into a thermodynamically unstable situation and trigger compensating charge reorganizations working against the molecular dipoles. Under certain circumstances, these are even found to overcompensate the effect that increasing the dipoles has for the work function.
    Journal of Physical Chemistry Letters 10/2013; 4(20):3521-3526. · 6.59 Impact Factor
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    ABSTRACT: Alkali-metal (AM) atoms adsorbed on single-crystal surfaces are a model system for understanding the properties of adsorption. AM adsorption, besides introducing new overlayer vibrational states, in-duces significant modifications in the surface vibrational structure of the metal substrate. Several stud-ies of the vibrational properties of AM on metal surfaces have been carried out in last decades. Most of these investigations have been performed for low coverages of AM in order to make the lateral interac-tion among co-adsorbates negligible. The adsorbed phase is characterized by a stretch (S) vibrational mode, with a polarization normal to the surface, and by other two modes polarized in the surface plane, known as frustrated translation (T) modes. The frequencies and intensities of these modes depend on the coverage, thus providing a spectroscopic signature for the characterization of the adsorbed phases. The vibrational spectroscopy joined to an ab-initio theoretical analysis can provide useful infor-mation about surface charge re-distribution and the nature of the adatom-surface bond, establishing, e.g., its partial ionicity and polarization. Gaining this information implies a significant advancement in our knowledge on surface chemical bonds and on catalytic reactions occurring in AM co-adsorption with other chemical species. Hence, systematic studies of co-adsorption systems are essential for a more complete understanding of heterogeneous catalysis. The two principal experimental techniques for studying the vibrations of AM adsorbed phases are high-resolution electron energy loss spectroscopy (HREELS) and inelastic helium atom scattering (HAS), the former being better suited to the analysis of the higher part of the vibrational spectrum, while the latter exploits its better resolution in the study of slower dynamics, e.g., T modes, surface acoustic phonons and diffusive phenomena. Concerning AM co-adsorption systems, reflection-absorption infrared spectroscopy (RAIRS) has been also used (as well as HREELS) for obtaining in-formation on the influence of AM adsorption on the vibrational properties of co-adsorbates. However, RAIRS could probe vibration modes beyond 50 meV, thus avoiding its use for AM vibrations. In this review an extended survey is presented over: a) the existing HREELS and HAS vibrational spectroscopic studies for AM adsorbed on single-crystal metal surfaces; b) the theoretical studies based on semi-empirical and ab-initio methods of vibrational structure of AM atoms on metal surfaces; c) the vibrational (HREELS, RAIRS, TRSHG) characterization of the co-adsorption on metal surfaces of AM atoms with reactive species.
    Surface Science Reports 07/2013; · 15.33 Impact Factor
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    ABSTRACT: Carrier doping of MoS2 nanoflakes was achieved by functional self-assembled monolayers (SAMs) with different dipole moments. The effect of SAMs on the charge transfer between the substrates and MoS2 nanoflakes was studied by Raman spectroscopy, field-effect transistors (FET) measurements and Kelvin probe microscope (KFM). Raman data and FET results verified that fluoroalkyltrichlorosilane-SAM with a large positive dipole moment, acting as holes donors, significantly reduced the intrinsic n-doping characteristic of MoS2 nanoflakes, while 3-(Trimethoxysilyl)-1-propanamine-SAMs, acting as electrons donors, enhanced the n-doping characteristic. The additional build-in electric field at the interface between SiO2 substrates and MoS2 nanoflakes induced by SAMs with molecular dipole moments determined the charge transfer process. KFM results clearly demonstrated the charge transfer between MoS2 and SAMs and the obvious interlayer screening effect of the pristine and SAMs-modified MoS2 nanoflakes. However, the KFM results were not fully consistent with the Raman and FET results since the externally absorbed water molecules were shown to partially shield the actual surface potential measurement. By eliminating the contribution of the water molecules, the Fermi level of monolayer MoS2 could be estimated to modulate in a range of more than 0.45-0.47 eV. This work manifests that the work function of MoS2 nanoflakes can be significantly tuned by SAMs by virtue of affecting the electrostatic potential between the substrates and MoS2 nanoflakes.
    ACS Nano 08/2013; · 12.03 Impact Factor

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