Growth and Characterization of organic thin films
It is common knowledge, that the initial adsorption of organic material on surfaces yields morphological and electronic effects that differ significantly from those of the bulk structure. The study of such effects within with the first or second monolayer is the focus of surface science groups.
A topic, that our group studies is the influence of surfaces / substrates on the growth of crystalline thin films in further layers (20 to 50 monolayers). Hence we study the transition of monolayer absorption to molecular solids.
In many cases the crystalline films are even in latter layers strongly influenced by the substrate's characteristics. This enables systematic tuning of the thin films characteristics by choice of adequate substrates. For example the global molecular orientation can be set by choosing suitable epitaxial relations between substrate and thin film. This is of special interest because molecular films often show strong anisotropy in optical and electronic effects. As well as epitaxial relations the surface reactivity often results in different thin film growth. Especially reactive metals like Copper and Silver in many cases induce completely different adsorption and growth modes than isolators like silicon dioxide or alkali halides.
The organic thin films are grown by organic molecular beam deposition (OMBD) in UHV. To study their morphology we use scanning probe microscopy (AFM, STM) and optical microscopy. The structure of monolayers can be investigated by means of electron diffraction (LEED). In near future we will be able to characterize the thin film crystallinity by using x-ray diffraction. The optical characteristics are studied in cooperation with other groups of our faculty: Vibrational spectra (FTIR, Raman), optical spectra and terahertz spectra are obtainable.
To characterize electronic effects, we have regular access to beam time at synchroton BESSY II. Especially NEXAFS and Synchrotron-XPS are available. Thermal stability of thin films is studied by means of TDS (Thermal Desorption Spectroscopy) and TDR (Thermal Desorption Reflectivity).
Additionally based on the studied systems electronic devices can be produced and characterized in cooperation with external groups.
Growth of organic single crystals
For detailed studies of structure, optical characteristics and electronic transport single crystals are an important aspect. In our group different techniques for single crystal growth come into play:
From supersaturated solution
In this technique the organic compound is dissolved in an adequate solvent. By applying correct conditions (e.g. cooling of the solution or partial vaporization of the solvent) crystallization occurs. In this technique the right choice of crystallization parameters (like solvent, solvent concentration, temperature, heating and cooling rate) is very important.
To achieve crystals by means of sublimation the compound is heated in vacuum conditions and therefore sublimated. By applying a smooth, monotonously dropping temperature gradient along the sublimation tube, the compound molecules re-sublime from gas phase at specific positions in the tube. By right choice of parameters (temperature gradient, pressure, carrier gas), big, well-ordered crystals can be grown.
Hot Wall Deposition
Growth of organic crystals is quite similar to growth of crystalline thin films by OMBD. Contrary, the intersection between the crucible with the compound and the substrate are sealed within the vacuum (by using a PTFE-seal on a flexible bar). At substrate temperatures close to the desorption temperature of the compound, the sticking coefficient of the compound for resublimation from gas phase is significantly smaller than 1. In the case of usual OMBD those molecules, that do not stick to the substrate at the first shot, rebound from the sample surface and are pumped out of the chamber. By using Hot Wall Deposition such molecules also rebound from the sample but remain in vicinity of the surface due to the sealed intersection between crucible and substrate. Therefore they reach the substrate repeatedly and thus - by applying this technique for a sufficient time - the effective sticking coefficient rises. Due to the high kinetic energies, the molecules receive because of the substrate's high temperature, they exhibit long diffusion lengths. This allows long-range diffusion to the energetically preferred adsorption sights and thus yields highly ordered single crystals.
- Scanning Probe Microscopy (AFM, STM)
- Low-Energy Electron Diffraction (LEED)
- X-Ray Diffraction (XRD)
- Thermal Desorption Spectroscopy (TDS) and Thermal Desorption Reflectivity (TDR)
- Near edge X-ray absorption fine structure (NEXAFS)