Christian Schimpf

The effect of tuning the microstructure of TIPS-tetraazapentacene on the performance of solution processed thin film transistors

We report a comprehensive study of the symmetrical 6,13-bis(triisopropylsilylethynyl)tetraazapentacene (TIPS-TAP) used as an electron transporting material in organic field-effect transistors. We study the optical, electronic, structural and morphological properties of thin films of TIPS-TAP as deposited by spin-coating and zone-casting techniques. Depending on the solution processing conditions and procedures we find a variety of microstructures for TIPS-TAP ranging from highly polycrystalline to well-aligned crystalline films. Field-effect transistors are fabricated in two different architectures to evaluate the charge transport properties of TIPS-TAP in such films, and bias-stress experiments reveal a good electric stability of TIPS-TAP. The extracted electron mobilities vary over several orders of magnitude depending on the resulting morphology of the active layer reaching a maximum of 0.42 cm2 V−1 s−1 for uniaxial aligned crystallites in zone-cast transistors.

Microstructure Investigations of the Phase Boundaries in the Bridgman TRIP Steel Crystal

Formation of microstructure defects at the phase boundaries in TRIP steels was investigated with the aid of microstructure analysis on a TRIP steel crystal, which was grown by the Bridgman technique. The microstructure studies comprised scanning electron microscopy (SEM), X-ray diffraction (XRD), electron backscatter diffraction (EBSD), transmission electron microscopy (TEM) and transmission electron microscopy with high resolution (HRTEM). Initial XRD measurements revealed that the crystals under study consist of austenite and ferrite with extremely strong preferred orientations. Subsequent XRD pole figure measurements and EBSD scans have shown that the orientation relationship between austenite and ferrite can be described by the Nishiyama-Wassermann model. For a detailed description of the microstructure of the Bridgman crystal, the orientation distribution of crystallites within the individual phases was investigated using the XRD reciprocal space mapping and the rocking curve measurements. These experiments have shown that the density of microstructure defects is much lower in ferrite than in austenite. The direct information about the defect structures at the phase boundaries between austenite and ferrite was obtained from the TEM micrographs, which revealed complicated micro-twin structures at the boundaries between the neighbouring phases. HRTEM discovered very narrow stripes of ferrite embedded in austenite that were regarded as a source of the microstructure defects in austenite.

Size–strain separation in diffraction line profile analysis

Separation of size and strain effects on diffraction line profiles has been studied in a round robin involving laboratory instruments and synchrotron radiation beamlines operating with different radiation, optics, detectors and experimental configurations. The studied sample, an extensively ball milled iron alloy powder, provides an ideal test case, as domain size broadening and strain broadening are of comparable size.

Corrugations of the basal planes in hexagonal boron nitride and their impact on the phase transition to cubic boron nitride

Among the microstructure defects in hexagonal graphitic boron nitride, the basal plane corrugations are of high relevance for the sp 2 to sp 3 phase transition under high pressures (HP) and high temperatures (HT). A microstructure model is described, which is capable of quantifying the amplitude of the basal plane corrugations on the basis of the anisotropic X-ray diffraction line broadening. It is illustrated that this model correctly reproduces the specific shape of the diffraction lines from corrugated basal planes, i.e., the characteristic splitting of the 00 l peaks. The results from XRD are verified by direct observation in the transmission electron microscope with high resolution. Subsequent HP/HT experiments were performed in order to highlight the difference in the phase transition kinetics between hexagonal boron nitride samples with different amount of basal plane corrugations. The effect of these microstructure defects on the conversion rate and on the obtained synthesis product is discussed.

Microstructure defects in graphitic BN and their impact on the transition to the dense phases

Microstructure defects in hexagonal graphitic boron nitride (h-BN) are analysed in detail using the characteristic dependence of the X-ray diffraction line broadening on the crystallographic direction. The most often observed structural defects are puckering of basal layers, turbostratic disorder, dislocations and stacking faults on basal planes. A detailed model of the anisotropic X-ray line broadening caused by puckering (= waviness of basal planes in h-BN) will be discussed. Furthermore, it will be shown how the above microstructure defects influence the anisotropy of the XRD line broadening and how this can be used for the quantification of the defects. Ex-situ and in-situ XRD experiments performed on high pressure/high temperature synthesised samples showed the impact of the microstructure defects on the transition of h-BN to the high pressure modifications crystallising in wurtzitic (w-BN) or sphaleritic (c-BN) structure [1,2]. Principally, h-BN containing a low density of microstructure defects (mainly dislocations and stacking faults) converts to the metastable wurtzitic modification [3]. The conversion is accompanied by a fragmentation of original h-BN crystallites. The formed w-BN has an increased dislocation density (~ 2 orders of magnitude) and stacking fault probability (~ 1 order of magnitude) compared to the precursor. Starting materials containing a high density of either puckering type defects or turbostratic disorder convert from h-BN directly to c-BN during high pressure/high temperature processes via diffusion processes being assisted by the high density of microstructure defects [4]. The resulting c-BN phase includes some stacking faults during the nucleation stage; their density decreasing with increasing c-BN volume fraction. The activation energy of the conversion at a pressure of ~ 10 GPa was obtained from the time-dependent change of the phase composition of the nanocomposites measured during in-situ synchrotron radiation diffraction experiments at DESY/HASYLAB. Complementary investigations of the microstructure of the h-BN precursors and synthesised BN (nano)composites were conducted using transmission electron microscopy with high resolution (HRTEM). The synchrotron and HRTEM investigations revealed detailed information on the microstructure of h-BN and its impact on a nanometre scale on the phase transition to the high pressure BN phases.