Aldo Brillante

Nonradiative decay of excited molecules near a metal surface

Abstract Nonradiative energy transfer from excited dye molecules near a metal surface to plasmon surface polaritons has been investigated experimentally. By using the Langmuir-Blodgett technique, the molecule-metal separation was varied and the coupling efficiency measured with the inverse attenuated-total-reflection technique. For an orientation of the transition dipole moment of the dye parallel to a silver surface and nonresonant interaction, maximum energy transfer to plasmon surface polaritons was observed for a molecule-silver spacing of about 18 nm. Reducing the spacing results in a monotonic decrease of the coupling efficiency: this decay channel becomes unimportant and eventually vanishes if the molecule is placed very close to the metal surface. The experimental results are discussed in connection with theoretical predictions and the surface-enhanced Raman scattering observed for some adsorbates at metal surfaces.

Optical properties of organic dye monolayers by surface plasmon spectroscopy

The spectroscopic properties of dye monolayers on silver films were determined by using the technique of attenuated total reflection (ATR). The dyes N,N′‐di(methyloctadecyl) squarylium and N,N′‐dioctadecyloxacarbocyanine were deposited on silver films by the Langmuir–Blodgett monolayer technique. From a least squares fit of the experimental ATR curves to the exact reflection formula, the dielectric function of the dye monolayers was determined in the visible wavelength region. It was found that squarylium forms ordered layers with a regular array of molecules and a film thickness equal to the length of the molecule (2.66 nm). These films were characterized by a strong absorption band at λs=530 nm (dimer band) and were highly anisotropic, the transition dipole moment lying in the plane of the monolayer film. Undiluted cyanine dye films on silver, on the other hand, showed a thickness less than that expected from the length of the molecule and a high degree of disorder. Increasing regularity in arrangement ...

Exciton–surface plasmon coupling: An experimental investigation

Langmuir–Blodgett monolayer assemblies, which contained dye molecules, have been deposited on silver films. Exciton–surface plasmon interactions have been studied with attenuated total reflection (ATR) spectroscopy. Reflectivity and dispersion curves for plasmon surface polaritons (PSP’s) at the metal interface are reported for both angular and wavelength scans. In agreement with theory dispersion curves from angle scans exhibit a double ‘‘back bending’’ at the transverse exciton frequency ωT (due to PSP interaction with the in plane component of the dye transition dipole moment) and at the longitudinal frequency ωL (due to PSP interaction with a perpendicular component). Correspondingly, dispersion curves from wavelength scans break into separate branches at these frequencies.

High‐Performance Single Crystal Organic Field‐Effect Transistors Based on Two Dithiophene‐Tetrathiafulvalene (DT‐TTF) Polymorphs

Solution prepared single crystal organic field-effect transistors (OFETs) combine low-cost with high performance due to structural ordering of molecules. However, in organic crystals polymorphism is a known phenomenon, which can have a crucial influence on charge transport. Here, the performance of solution-prepared single crystal OFETs based on two different polymorphs of dithiophene-tetrathiafulvalene, which were investigated by confocal Raman spectroscopy and X-ray diffraction, are reported. OFET devices prepared using different configurations show that both polymorphs exhibited excellent device performance, although the -phase revealed charge carrier mobility between two and ten times higher in accordance to the closer stacking of the molecules.

Plasmon surface polariton dispersion by direct optical observation

Simple optical techniques are described for the direct visual observation of the plasmon surface polariton dispersion curve, ω vs κ, and of the shift in the dispersion curve from overcoatings of different thicknesses. To see the dispersion curve, a collimated beam of white light is dispersed by a prism, and then focused by a cylindrical lens onto the base of a second prism coated with a thin metal film. With this arrangement the attenuated total reflection spectrum is obtained with all the visible frequencies spatially resolved. The shift of the dispersion curve for coatings of different thicknesses is illustrated in a separate experiment using a specially constructed Langmuir wedge. This demonstrates the usefulness of plasmon surface polaritons for monitoring coatings with thicknesses of the order of nanometers.

Epitaxial growth of π-stacked perfluoropentacene on graphene-coated quartz.

Chemical-vapor-deposited large-area graphene is employed as the coating of transparent substrates for the growth of the prototypical organic n-type semiconductor perfluoropentacene (PFP). The graphene coating is found to cause face-on growth of PFP in a yet unknown substrate-mediated polymorph, which is solved by combining grazing-incidence X-ray diffraction with theoretical structure modeling. In contrast to the otherwise common herringbone arrangement of PFP in single crystals and “standing” films, we report a π-stacked arrangement of coplanar molecules in “flat-lying” films, which exhibit an exceedingly low π-stacking distance of only 3.07 Å, giving rise to significant electronic band dispersion along the π-stacking direction, as evidenced by ultraviolet photoelectron spectroscopy. Our study underlines the high potential of graphene for use as a transparent electrode in (opto-)electronic applications, where optimized vertical transport through flat-lying conjugated organic molecules is desired.

Raman phonon spectra of pentacene polymorphs

We report for the first time lattice phonon Raman spectra of pentacene measured by means of a Raman microprobe technique. We experimentally prove the existence of two polymorphs, as expected from recent structural studies. A comparison with Quasi Harmonic Lattice Dynamics calculations, previously performed starting from the available X-ray data, help us in identifying the phase to which each crystal belongs.

Reflection and absorption spectra of singlet charge transfer excitons in anthracene–PMDA crystals

The polarized reflection spectra of single crystals of the weak charge transfer (CT) complex anthracene–pyromellitic dianhydride (A–PMDA) have been measured at 2 K. The oscillator strengths and energies of the singlet charge transfer transitions have been determined from the frequency dependent dielectric function derived from the reflection spectrum by means of a Kramers–Kronig transformation. A theory of exciton–phonon coupling appropriate for CT transitions in a mixed stack system is briefly described and used to interpret the derived absorption spectrum. The absorption spectrum consists of a purely electronic zero phonon line (ZPL) and a set of zero lattice phonon lines (ZLPL) corresponding to the excitation of high frequency intramolecular modes of the ions. Each ZLPL, like the ZPL, acts as the origin for several librational modes and for a progression in a lattice mode of approximately 30 cm−1. Exciton–phonon coupling is weakest for the vibrationless ZPL and increases with the energy of the intramol...

Probing polymorphs of organic semiconductors by lattice phonon Raman microscopy

Using micro-Raman techniques to investigate crystal polymorphism is an efficient method, capable of monitoring physical modifications and phase inhomogeneities in crystal domains at the micrometre scale. In the presence of polymorphism, phase mixing is a common occurrence which becomes a crucial issue in structured organic materials tailored for applications in molecular electronics and photonics. A good phase homogeneity is, in fact, required for optimal and reproducible device performance. We tackle the problem of polymorphism in organic semiconductors by combining experimental and theoretical methods. Experimentally we have found that different crystalline polymorphs may be conveniently investigated using their Raman spectra in the region of the lattice phonons, whose frequencies probe intermolecular interactions and are very sensitive to differences in molecular packing. We propose lattice phonon confocal micro-Raman mapping as a fast and reliable diagnostic tool for in-situ characterization of the phase purity. The theoretical approach aims to predict crystal structures and possible coexistence of polymorphs by ranking them in energy and proving that the deepest calculated minima actually correspond to the experimental X-ray diffraction structures of bulk crystals. This combined spectroscopic and theoretical approach to the dynamical properties of a crystal lattice provides a unique body of information on crystal structure recognition of molecular crystals.

Inherent structures of crystalline pentacene

Using a quasi-Monte Carlo scheme, we search the potential energy surface of crystalline pentacene to sample its local minima, which represent the “inherent” structures, i.e., the possible configurations of mechanical equilibrium. The system is described in terms of rigid molecules interacting through a standard atom–atom potential model. Several hundreds of distinct minima are encountered, with a surprising variety of structural arrangements. We find that deep minima are easily accessible because they exhibit a favorable energy distribution and their attraction basins tend to be wide. Thanks to these features of the potential surface, the localization the global minimum becomes entirely feasible, allowing reliable a priori predictions of the crystallographic structures. The results for pentacene are very satisfactory. In fact, the two deepest minima correspond to the structures of the two known experimental polymorphs, which are described correctly. Further polymorphs are also likely to exist.