A stable room-temperature molecular assembly of zwitterionic organic dipoles guided by a Si(111)-7x7 template effect.
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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; · 12.06 Impact Factor
© Wiley-VCH 2007
69451 Weinheim, Germany
Stable room temperature molecular assembly of zwitterionic
organic dipoles guided by Si(111)-7x7 template effect **
Younes Makoudi, Madjid Arab, Frank Palmino, Eric Duverger, Christophe Ramseyer,
Fabien Picaud, and Frédéric Chérioux*
Synthesis of MSPS
[∗] Y. Makoudi, Dr. M. Arab, Dr. F. Palmino, Dr. E. Duverger
and Dr. F. Chérioux
Institut FEMTO-ST/LPMO UMR CNRS 6174
32, Avenue de l’Observatoire, F-25044 Besancon cedex,
Fax: (+) 33 8185 3998
Homepage ((optional)): www.femto-st.fr
Dr. F. Picaud and Prof. Dr. C. Ramseyer
Laboratoire de Physique Moléculaire, UMR CNRS 6624
16 Route de Gray, F-25030 Besancon cedex, FRANCE
[∗∗] This work was supported by the Communauté
d’Agglomération du Pays de Montbéliard. Authors thank Dr.
C. Joachim (CEMES, FRANCE) for fruitful discussions
Supporting information for this article is available on the
WWW under http://www.angewandte.org or from the
MSPS molecules have been synthesised in accordance with the method previously described by Nicoud et al.i
The procedure is based on two steps:
1) formation on the zwitterion
2) building of the large dipole
EtOH, Reflux, 10h
4-Picoline was treated at 0 °C with one equivalent of propylsultone, leading to crystalline 4-methyl-(n-
sulfonatopropyl)pyridinium, which was used for the subsequent reaction without further purification. To a
solution of 4-methyl-(n-sulfonatopropyl)pyridinium in 15 mL of anhydrous ethanol was added one equivalent of
the 4-methoxybenzaldehyde and a catalytic amount of pyrrolidine. The mixture was heated under reflux for 10 h
and then cooled to 0 °C. The precipitated product was filtered and washed with ether. The pale yellow solid was
purified by column chromatography (Silica gel, acetone, Rf close to 0.5). The pure MSPS was isolated as an
intense yellow powder after evaporation of the solvent.
1H NMR (300 MHz, CDCl3, 25°C): δ = 1.96 (quint., 3J = 7.3 Hz, 2H), 2.45 (t, 3J = 7.3 Hz,
2H), 3.76 (s, 3H), 4.62 (t, 3J = 7.3 Hz, 2H), 6.85 (d, 3J = 8.7 Hz, 2H), 7.06 (d, 3J = 16.2 Hz,
1H), 7.59 (d, 3J = 8.7 Hz, 2H), 7.69 (d, 3J = 16.2 Hz, 1H), 8.03 (d, 3J = 6.7 Hz, 2H), 8.99 (d,
3J = 6.7 Hz, 2H). MS (ESI): m/z : 333 [M+]; elemental analysis (%) calcd for C17H19NO4S
(333.10): C 61.24, H 5.74, N 4.20; found: C 61.17, H 5.81, N 4.09.
All properties of the isolated molecules are obtained by using the Vienna Ab Initio Simulation Package
(VASP),ii which is a density functional theory (DFT) code with plane wave basis set. Electron–ion interactions
were described using the projector-augmented wave (PAW) method, which was expanded within a plane wave
basis set up to a cutoff energy of 400 eV. Electron exchange and correlation effects were described by the
Perdew–Burke–Ernzerhof (PBE) GGA type exchange-correlation functional.
In order to investigate the molecule-substrate interactions, two pictures showing the DOS isolines evolution for
the entire system at different cut planes centered on the MSPS have been described in the following figures. We
have used Xcrysdeniii for the isolines dos representation.
In this top view, the methoxy group/Si(111)-7x7 interactions are proved by the DOS isolines (bias voltage +2V).
In this top view, the sulfonato/Si(111)-7x7 interactions are proved by the DOS isolines (bias voltage -2V).
These two figures show the template effect of the surface which induces a change conformation of MSPS in
order to lead to the supramolecular self-assembly of three MSPS in a Si(111)-7x7 half-cell.
i Serbutoviez, C.; Nicoud, J.-F.; Fisher, J.; Ledoux, I. & Zyss, J. Chem. Mater. 1994, 6, 1358-1368.
ii (a) Kresse, G. & Furthmüller, J. Phys. Rev. B 1996, 54, 11169-11186. (b) Blöchl, P. E. Phys. Rev. B 1994, 50,
iii (a) Kokalj, A. J. Mol. Graphics Modelling 1999, 17, 176. (b) Kokalj, A. Comput. Mater. 2003, 28, 155.