Christophe Silien

Functionalizing hydrogen-bonded surface networks with self-assembled monolayers

One of the central challenges in nanotechnology is the development of flexible and efficient methods for creating ordered structures with nanometre precision over an extended length scale. Supramolecular self-assembly on surfaces offers attractive features in this regard: it is a ‘bottom-up’ approach and thus allows the simple and rapid creation of surface assemblies, which are readily tuned through the choice of molecular building blocks used and stabilized by hydrogen bonding, van der Waals interactions, π–π bonding or metal coordination between the blocks. Assemblies in the form of two-dimensional open networks are of particular interest for possible applications because well-defined pores can be used for the precise localization and confinement of guest entities such as molecules or clusters, which can add functionality to the supramolecular network. Another widely used method for producing surface structures involves self-assembled monolayers (SAMs), which have introduced unprecedented flexibility in our ability to tailor interfaces and generate patterned surfaces. But SAMs are part of a top-down technology that is limited in terms of the spatial resolution that can be achieved. We therefore rationalized that a particularly powerful fabrication platform might be realized by combining non-covalent self-assembly of porous networks and SAMs, with the former providing nanometre-scale precision and the latter allowing versatile functionalization. Here we show that the two strategies can indeed be combined to create integrated network–SAM hybrid systems that are sufficiently robust for further processing. We show that the supramolecular network and the SAM can both be deposited from solution, which should enable the widespread and flexible use of this combined fabrication method.

Self-assembly of a pyridine-terminated thiol monolayer on Au(111).

Self-assembled monolayers (SAMs) of 3-(4-pyridine-4-yl-phenyl)-propane-1-thiol (PyP3) on Au(111)/mica have been studied by scanning tunneling microscopy (STM), polarization-modulated infrared reflection absorption spectroscopy (PM-IRRAS), high-resolution X-ray photoemission spectroscopy (HRXPS), and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. The quality of the SAM is found to be strongly dependent on the solvent. Substantial gold corrosion is observed if pure ethanol is used. In contrast, highly ordered and densely packed SAMs are formed from acetonitrile or a KOH/ethanol mixture. The structure is described by a 2 radical3 x radical3 unit cell with the aromatic moiety oriented nearly perpendicular to the surface. The PyP3 films form with the pyridine moiety deprotonated. Variation of pH allows reversible protonation without measurable damage of the SAM.

A Supramolecular Network as Sacrificial Mask for the Generation of a Nanopatterned Binary Self-Assembled Monolayer**

Supramolecular self-assembly on surfaces has become a straightforward and flexible route for the generation of extended nanoscale structures, with porous networks serving as templates to control the arrangement of metal clusters or guest molecules. The latter includes thiols, which are particularly interesting as they afford an enormous flexibility in surface functionalization. Recently we have shown that a supramolecular network of perylene-tetracarboxylic di-imide (PTCDI) and 1,3,5-triazine-2,4,6-triamine (melamine), which involves a triple hydrogen bonding motif (Figure 1a), can be used as a template for an all-solution-based generation of periodic patterns of thiol self-assembled monolayer (SAM) nanoislands on Au(111) (Figure 1b). These network–SAM hybrid structures can be further processed in an electrochemical environment, as demonstrated by underpotential deposition (UPD) of Cu, where one atomic layer of the metal intercalates at the SAM–substrate interface but not between network and substrate (Figure 1c). While formation and modification of network–SAMhybrids demonstrate the robustness of the PTCDI–melamine network, it is nevertheless of limited stability as evidenced by its displacement upon prolonged exposure to a thiol solution. Since the formation of hybrid structures is a kinetically controlled process, substitution of the network by thiols different to those in the network pores is a potential way to produce binary SAMs (Figure 1d) over extended areas with resolution and pattern definition unmatched by other approaches such as random insertion,mixing, or lithographic techniques.However, the feasibility of this approach critically depends on the properties of the thiol islands filling the nanopores. One evident issue is their stability against replacement by the second thiol species that depends on a number of mutually dependent factors such as molecule–substrate bond strength, intermolecular interactions,

A supramolecular hydrogen-bonded network as a diffusion barrier for metal adatoms.

Confined in a molecular corral: A supramolecular network changes the mechanism by which underpotential deposition (UPD) of copper proceeds on a gold electrode modified by a self-assembled monolayer (SAM). Lateral diffusion of Cu adatoms is suppressed between adjacent cells of a network/SAM hybrid structure. Instead, UPD occurs by direct deposition into the SAM filled pores of the network, where the Cu adatoms are confined.

Electron–phonon couplings at C60 interfaces: a case study by two-color, infrared–visible sum–frequency generation spectroscopy

Abstract We demonstrate the ability of doubly resonant sum–frequency generation (DR-SFG) to investigate electron–phonon couplings at C 60 –metal interfaces. Due to its coupling to electronic transitions, the totally symmetric A g (2) vibration of C 60 exhibits a huge enhancement of its nonlinear response for sum–frequency energies above the molecular electronic gap. We attribute this resonance to the coupling of the pentagonal pinch mode with the t 1u lowest unoccupied molecular orbital (LUMO) of C 60 .

Changes in the dipolar vibrational fingerprint of C60 upon adsorption and K intercalation

Abstract The HREELS spectra of a C 60 monolayer deposited on Ag(111) show evidences for a strong chemical bonding with the Ag substrate, accompanied by a charge transfer of about 1xa0electron per adsorbed molecule. K atom intercalation in the adsorbed layer allows an easy tuning of the C 60 LUMO (t 1u ) occupancy. The evolution of the C 60 intramolecular vibrational spectrum upon K intercalation is studied by HREELS and discussed with particular emphasis on the two A g (Raman active) and four T 1u (IR active) modes. Both A g modes exhibit at the interface dipolar activity explained by electron–phonon coupling. Upon K intercalation, the T 1u modes show an evolution similar to what is observed in K- or Rb-doped bulk C 60 , allowing to identify C 60 6− as the saturated phase.

HREELS, IR and SFG investigation of undoped and doped adsorbed fullerenes

Abstract High-resolution electron-energy-loss spectroscopy was used to characterize the vibrational fingerprint of C 60 on Ag(111) and hydrogen-passivated Si(111) surfaces and to investigate the interfacial structure of a chemisorbed monolayer of C 60 on Ag(111). Complementary results from infra-red absorption spectroscopy and sum-frequency generation spectroscopy help to interpret the spectra in the range of the T 1u (4)–A g (2) peaks. Estimation of the charge transferred to the chemisorbed monolayer upon further doping with potassium is deduced from the frequency shifts of vibrational modes.