Para-sexiphenyl Films Research Articles

Origin of the low-energy emission band in epitaxially grown para-sexiphenyl nanocrystallites

A comparative study of steady-state and time-resolved photoluminescence of para-sexiphenyl (PSP) films grown by organic molecular beam epitaxy (OMBE) and hot wall epitaxy (HWE) under comparable conditions is presented. Using different template substrates [mica(001) and KCl(001) surfaces] as well as different OMBE growth conditions has enabled us to vary greatly the morphology of the PSP crystallites while keeping their chemical structure virtually untouched. We prove that the broad redshifted emission band has a structure-related origin rather than being due to monomolecular oxidative defects. We conclude that the growth conditions and type of template substrate impacts substantially on the film morphology (measured by atomic force microscopy) and emission properties of the PSP films. The relative intensity of the defect emission band observed in the delayed spectra was found to correlate with the structural quality of PSP crystallites. In particular, the defect emission has been found to be drastically suppressed when (i) a KCl template substrate was used instead of mica in HWE-grown films, and (ii) in the OMBE-grown films dominated by growth mounds composed of upright standing molecules as opposed to the films consisting of crystallites formed by molecules lying parallel to the substrate.

Optical Gain Performance of Epitaxially Grown para‐Sexiphenyl Films

Optical gain performance of epitaxially-grown nanostructured para-sexiphenyl films is demonstrated to be outstanding. The combination of high structural order and low lattice temperature results in a material system with ultrawide (ca. 1 eV) gain bandwidth and long (ca. 1 ns) gain lifetime (see figure) that allows an optimistic outlook for broadband laser applications.

Oxygen induced molecular reorientation on aluminum

Para-sexiphenyl films grow on Al(111) at room temperature such that the molecules are oriented with their molecular axes parallel to the Al surface. Here we demonstrate that the presence of oxygen on the Al substrate prior to deposition leads to a reorientation of the molecules in which their axes are nearly perpendicular to the Al surface. This reorientation also leads to a difference in the electronic structure and an improvement in the band alignment of 1.0eV. The contribution of substrate order to molecular orientation in organic films is discussed.

Spontaneous rearrangement of para-sexiphenyl crystallites into nano-fibers

In the growth of para-sexiphenyl films on mica(001), an interesting phenomenon was observed where small individual crystallites spontaneously arrange into parallel running high-aspect ratio chains with a length in the micrometer range. A statistical analysis of the chain dimensions reveals a very narrow length distribution and the existence of a threshold length. The observation of denuded zones around the chains and the interior chain structure suggest a rearrangement of the crystallites as entities. It is proposed that this spontaneous rearrangement is driven by the formation of a one-dimensional defect array which evolves within the monomolecular para-sexiphenyl wetting layer when a critical crystallite density is reached.

Optoelectronic devices based on para-sexiphenyl films grown by Hot Wall Epitaxy

In this work, we demonstrate the fabrication of hot wall epitaxially grown para-sexiphenyl films for light emitting diodes and PSP/C 60 bi-layer structures for photovoltaic cells. Para-sexiphenyl films display blue electroluminescence, which shows a spectrum coinciding with its photoluminescence. Photovoltaic devices are also fabricated and their external quantum and power conversion efficiencies are presented and discussed.

Photo- and thermally stimulated luminescence in highly ordered films of para-sexiphenyl grown by Hot-Wall Epitaxy

Photo- and thermally stimulated luminescence was studied in highly ordered nanostructures of para-sexiphenyl (PSP) grown by Hot Wall Epitaxy. A low-energy broad band is observed in photoluminescence spectra, which can be attributed to the emission from molecular aggregates and its pronunciation depends sensitively on film preparation conditions. The aggregate emission decays considerably slower featuring the life-time estimated as 4 ns that is about an order of magnitude longer than life-time of singlet excitons. Thermally stimulated luminescence data suggest the presence of energetically disordered distribution of localized states for charge carriers in the PSP films, which results from an intrinsic disorder in this material.

Highly ordered anisotropic nano-needles in para-sexiphenyl films

Atomic force microscopy (AFM) was used to investigate the early growth stage of highly ordered para-sexiphenyl thin films deposited by hot wall epitaxy on mica, in order to find the process controlling parameters. It was shown that the growth time and surface type are important parameters for controlling of the film morphology, in terms of the degree of anisotropy and long range order. X-Ray diffraction pole figure technique and transmission electron diffraction was also used to characterize the crystallographic structure of the thicker films.

In-plane alignment of para-sexiphenyl films grown on KCl(0 0 1)

The three-dimensional growth of para-sexiphenyl (p-6P) on KCl(001) is characterized by a large polymorphism. Using grazing-incidence diffraction, we have found that two of the known five orientational phases feature preferential in-plane alignments along the [110] in-plane direction of the substrate: p-6P(11−1) ∥ KCl(001) with [1−10] ∥ KCl[110] as well as p-6P(001) ∥ KCl(001) with [1−10] ∥ KCl[110]. The latter structure is a peculiar low-symmetry configuration, which can be understood by analysis of the molecular crystal lattice and the substrate symmetry.

Heteroepitaxial growth of self-assembled highly ordered para-sexiphenyl films: A crystallographic study

X-ray diffraction pole figure technique (XRD-PF), transmission electron diffraction (TED), and atomic force microscopy were used to characterize the crystalline structure of heteroepitaxial grown, self-assembled para-sexiphenyl (PSP) layers in detail. PSP was deposited by hot wall epitaxy on mica (001) substrates. The epitaxial growth was confirmed by XRD-PF as well as TED measurements. XRD-PF measurements reveal two crystallographic orientations of PSP parallel to the substrate, which are also cleavage planes of PSP. Both orientations---(11-1) and (11-2)---have very similar structural characteristics with respect to the substrate. Since in either orientations the interface lattices of PSP and mica are incommensurable, the observed growth mode can be referred to quasiepitaxy.