Transmission Electron Microscopy of Polymorph Lead Phthalocyanine Organic Molecules
by: Nesli Yagmurcukardes, Emat
Location: Campus Groenenborger, U.408
There is considerable interest in the improvement of electronic and optoelectronic applications using organic crystals having novel physical, chemical and biological properties. Organic molecules offer a large variety of complex structures and tend to have limited crystallographic symmetry. The nature of crystallization of organic molecules has been still poorly understood. For example, the polymorphism, which is the occurrence of two or more crystal structures, has still been an open question. In addition, most of the crystal phases are formed upon heterogeneous nucleation at the interface of a solid substrate, where the structure is significantly determined by the solid characteristics.
Lead-Phthalocyanine (PbPc) is an attractive polymorph material due to its shuttlecock structure with C4v symmetry; the central Pb atom is significantly out of the convex molecular plane. It has been reported that there exist two polymorphs in PbPc crystals; the monoclinic system with space group P21/b and the triclinic one with P1. The nature of crystallization of this organic molecule has not been addressed properly. Aberration corrected transmission electron microscopy (TEM) is a powerful technique to obtain information on the electronic state, chemical composition and crystal structure. However, the use of highly energetic electrons causes energy transfer into the specimen resulting in structural degradation. Especially, electrons can modify the atomic structures with knock-on mechanism or excitation related processes such as breaking unstable bonds or rearrangement of the atoms. There have been recent efforts on preserving the crystalline form of a specimen over extended electron exposure times, and hence to avoid damage by encapsulating the sample within a protective layer.
Therefore, we used a graphene support on top of the physical vapor deposited (PVD) PbPc samples that were prepared at Graz University. We have been able to determine the crystal structure and compare the experimental observations with simulated results by using MULTEM software. In the future, we are planning to apply compressed sensing scanning and exit-wave reconstruction methods to visualize our sample with electron microscopy.
During my talk, polymorphism and substrate dependency of crystal structures of organic molecules will be introduced. Then, the graphene transfer procedure for both grid preparation and as a protective layer will be explained. Next, I will present our initial results on the visualization of polymorph PbPc molecules using different microscope parameters.