Abstract

1. STM Study of Hexagonal Nano-Lattices Andrew Rosengerger1, Kathleen Kash1, Morton Litt2, Casey Check2 1 Department of Physics, 2 Department of Macromolecular Science and Engineering Data Abstract poly(1,3,5-phenylene-4,4’-biphenylene-2,2’-disulfonic acid) (CPPSA) rings are promising, new polymers being produced by Dr. Litt’s research group in the department of Macromolecular Science and Engineering. When cast into a film, these molecules should form stacked sheets of hexagonal arrays with a hole size of 2 to 50 nm, which could have possible applications in new technologies. In order to ascertain the effectiveness of synthesis techniques along with the molecular and aggregate structure of the polymer, scanning tunneling microscopy (STM) and atomic force microscopy (AFM) were used to analyze samples of molecules deposited on HOPG substrates. Although atomic scale resolution and a definitive measurement of the molecular structure of the polymer were not achieved, the aggregate structure of the polymer was verified to be thin films of one or more monolayers. Sample CPPSA 1H .1% Sample CPPSA 2H 100% Sample 2H100% showed signs of both platelet formation and stacking. The top image shows the edges of stacked layers of polymer, while the bottom shows the platelet formation characteristic of the polymer. When the scan area was decreased to attempt to obtain atomic resolution the large steps from the high concentration of polymer in this sample disrupted the vertical and lateral resolution The polymer on this sample was at an ideal concentration for determining the aggregate and molecular structure. Examination of the images shows that the polymer deposited in roughly monolayer thick platelets on the surface of the HOPG. • Several interesting features are notable in these samples • The polymer appears to aggregate around steps in the HOPG, indicating a mutual attraction • On the macroscopic scale, the deposited molecules formed a “coffee ring” around the edges of the sample. This suggests that an attempt to deposit a monolayer is intrinsically flawed Material Background The molecules are synthesized by polymerizing the reactant molecules in solution under starvation conditions. The geometry of the reactant molecules and the starvation conditions should theoretically ensure that the molecules bond together to form the proper hexagonal lattice. Five batches of polymer were synthesized, referred to as batches one through five, and all but the fourth batch (which was synthesized incorrectly) had similar properties. X-ray diffraction and NMR have failed to pinpoint the exact structure of these molecules. We believe that the random Unfortunately, atomic and molecular resolution of this sample were not obtained due to the amount of noise in the environment. The placement of the STM coupled with the acoustic noise in the surroundings are the probable causes of the lack of atomic resolution. Sample CPPSA 1H .001% In this sample, I obtained a possible image of the molecular structure of the polymer. If each dot is a vertex of the three arms of benzene rings and the polymer is corrugated, the pattern in the scan would emerge around the obtained resolution (~2nm laterally). Unfortunately, the concentration of polymer on this sample was too small; further scans only showed blank HOPG. Without an atomic resolution scan or reproducible results, this structure cannot be conclusively proven. Figure 1: Theoretical polymer structure orientations of the sulfonic acid groups and/or twisting in the structure of the ring make the crystal structure of the molecule random enough that these techniques will only reflect the random nature of these elements. Testing done by Dr. Litt’s research group has shown that the material has unusual properties in water, indicating that it may have the desired structure. Although all of the reactants used in producing the molecules are water soluble, the resulting films are water insoluble, and can only be put in suspension through aggressive sonication. Additionally, dried films of the material have been observed to swell and lose rigidity when immersed in water without sonication. While these indicate that the molecules formed correctly and aggregated properly, they do not constitute conclusive proof. STM appeared to be the best remaining method to determine the structure of the polymer. Conclusion STM analysis has verified the aggregate structure of the CPPSA polymer. It has been shown to form monolayer thick sheets which can stack into larger aggregates. However, due to deficiencies in the STM system, the molecular structure could not be conclusively determined. The evidence appears to point to a successful synthesis, but better quality instruments must be used to verify this. Simplified explanation of picture; each dot represents the intersection of the benzene ring arms. If the polymer is corrugated, only the red dots would be seen on the STM scan which would form the above pattern. Experimental Methods A VEECO MMAFM-1 Scanning Probe Microscope (SPM) was the primary instrument used in the experiments. A drop of material dispersed in water was deposited on a fresh cleaved piece of HOPG, and then put in a 60o C oven to evaporate the water. The samples were then scanned with the STM at settings chosen to attempt to balance resolution and precision Acknowledgements: I would like to thank my advisor Dr. Kash for a plethora of good advice and help, Jeremy Trombley for teaching me the basics of STM and giving excellent advice when the instrument behaved improperly, to Dr. Litt and Casey Check for creating the samples and taking time to explain the background behind the polymer, and to all of my physics professors here at case.