Tatjana Djuric

Epitaxy of Rodlike Organic Molecules on Sheet Silicates—A Growth Model Based on Experiments and Simulations

During the last years, self-assembled organic nanostructures have been recognized as a proper fundament for several electrical and optical applications. In particular, phenylenes deposited on muscovite mica have turned out to be an outstanding material combination. They tend to align parallel to each other forming needlelike structures. In that way, they provide the key for macroscopic highly polarized emission, waveguiding, and lasing. The resulting anisotropy has been interpreted so far by an induced dipole originating from the muscovite mica substrate. Based on a combined experimental and theoretical approach, we present an alternative growth model being able to explain molecular adsorption on sheet silicates in terms of molecule−surface interactions only. By a comprehensive comparison between experiments and simulations, we demonstrate that geometrical changes in the substrate surface or molecule lead to different molecular adsorption geometries and needle directions which can be predicted by our growth model.

Color tuning of nanofibers by periodic organic-organic hetero-epitaxy.

We report on the epitaxial growth of periodic para-hexaphenyl (p-6P)/α-sexi-thiophene (6T) multilayer heterostructures on top of p-6P nanotemplates. By the chosen approach, 6T molecules are forced to align parallel to the p-6P template molecules, which yields highly polarized photoluminescence (PL)-emission of both species. The PL spectra show that the fabricated multilayer structures provide optical emission from two different 6T phases, interfacial 6T molecules, and 3-dimensional crystallites. By a periodical deposition of 6T monolayers and p-6P spacers it is demonstrated that the strongly polarized spectral contribution of interfacial 6T can be precisely controlled and amplified. By analyzing the PL emission of both 6T phases as a function of p-6P spacer thickness (Δdp–6P) we have determined a critical value of Δdp–6P ≈ 2.73 nm where interfacial 6T runs into saturation and the surplus of 6T starts to cluster in 3-dimensional crystallites. These results are further substantiated by UPS and XRD measurements. Moreover, it is demonstrated by morphological investigations, provided by scanning force microscopy and fluorescence microscopy, that periodical deposition of 6T and p-6P leads to a significant improvement of homogeneity in PL-emission and morphology of nanofibers. Photoluminescence excitation experiments in combination with time-resolved photoluminescence demonstrate that the spectral emission of the organic multilayer nanofibers is dominated by a resonant energy transfer from p-6P host- to 6T guest-molecules. The sensitization time of the 6T emission in the 6T/p-6P multilayer structures depends on the p-6P spacer thickness, and can be explained by well separated layers of host–guest molecules obtained by organic–organic heteroepitaxy. The spectral emission and consequently the fluorescent color of the nanofibers can be efficiently tuned from the blue via white to the yellow-green spectral range.

Epitaxially Grown Films of Standing and Lying Pentacene Molecules on Cu(110) Surfaces

Here, it is shown that pentacene thin films (30 nm) with distinctively different crystallographic structures and molecular orientations can be grown under essentially identical growth conditions in UHV on clean Cu(110) surfaces. By X-ray diffraction, we show that the epitaxially oriented pentacene films crystallize either in the “thin film” phase with standing molecules or in the “single crystal” structure with molecules lying with their long axes parallel to the substrate. The morphology of the samples observed by atomic force microscopy shows an epitaxial alignment of pentacene crystallites, which corroborates the molecular orientation observed by X-ray diffraction pole figures. Low energy electron diffraction measurements reveal that these dissimilar growth behaviors are induced by subtle differences in the monolayer structures formed by slightly different preparation procedures.

Temperature stability of the pentacene thin-film phase

This work presents the influence of temperatures above 300 K on the crystal structure and morphology of pentacene thin films. The thermal expansion of the unit cell and the relative amount of different phases are investigated via grazing incidence x-ray diffraction. Geometrical considerations about the specific molecular packing of the thin-film phase explain the anisotropic non-linear expansion. Furthermore, around 480 K, a phase transformation of the thin-film phase to the bulk phase is observed. In contrast, only a weak influence of the temperature on the height distribution of the thin-film phase crystallites is found.

Morphological and Structural Investigation of Sexithiophene Growth on KCl (100)

The morphology and structure of sexithiophene deposited on KCl (100) substrates was investigated by scanning force microscopy and specular X-ray diffraction measurements. Two different needle-like structures with {010} and {4̅11} contact planes have been observed as well as islands of almost upright standing sexithiophene molecules with a {100} contact plane. Furthermore an azimuthal alignment of all three crystal orientations was observed by X-ray diffraction pole figure measurements, and the growth directions reflect the 4-fold rotational symmetry of the substrate surface. In addition the analysis of crystals with {4̅11} and {100} contact planes unveiled that they share a common crystallographic direction which is explained by ledge directed epitaxy.

Alternately deposited heterostructures of α-sexithiophene–para-hexaphenyl on muscovite mica(001) surfaces: crystallographic structure and morphology

Multi-component systems of para-hexaphenyl (p-6P) and α-sexithiophene (α-6T) molecules show great promise for tuning the fluorescence colour of optically active films. As the opto-electronic properties of rod-like molecules in thin films strongly rely on their anisotropic orientation, a technique for preparation of well-defined, anisotropic multicomponent systems is required. We demonstrate that a p-6P film of less than two nanometer thickness grown on muscovite mica(001) substrates acts as an efficient alignment layer for epitaxial growth of α-6T crystallites. On top of such a p-6P alignment layer, multilayer heterostructures of alternately deposited p-6P and α-6T molecules were grown. Combined X-ray diffraction and transmission electron microscopy studies show that molecules forming α-6T crystallites align parallel to those in the p-6P crystallites leading to the perfect adoption of their herring-bone structures. This alignment is desirable for optical applications and we show that it is preserved for heterostructures composed of up to 120 alternately deposited p-6P (0.8 nm) and α-6T (3.4 nm) nominal layers (120 cycles). Although for co-evaporated α-6T–p-6P molecules formation of a mixed crystal polymorph is reported, we show that in periodically deposited α-6T–p-6P heterostructures phase separation occurs and both molecules crystallize in their well-known equilibrium structures.

Organic–Organic Heteroepitaxy—The Method of Choice to Tune Optical Emission of Organic Nano-fibers?

In this chapter the potential of Organic–Organic heteroepitaxy is discussed concerning the ability to provide efficient color tuning of organic nano-fibers deposited on muscovite mica substrates. The first part is focused on the epitaxial growth of rod-like molecules on sheet silicates which has been analyzed by depositing p-6P and 6T using hot-wall epitaxy. It is demonstrated that substituting para-phenylenes by other molecules for efficient color tuning is not trivial leading in the general case to multidirectional nano-fibers. The presented growth model is based on detailed analysis using XRD pole-figure measurements, atomic force microscopy and force-field simulations. In the second part it is demonstrated that organic–organic heteroepitaxy provides a proper method for efficient color tuning of organic nano-fibers. It is shown that using p-6P nano-fiber templates can be used as fundament for the epitaxial growth of 6T crystallites. The formed 6T crystallites adopt the molecular and morphological orientation of the p-6P layer beneath and provide highly polarized emission in the blue, green, and red spectral range.