Resolving Soft Material Crystallinity through Scanning Transmission Electron Nanodiffraction
A method for imaging the semi-crystalline structure of organic, electrically conduct- ing molecular thin films using scanning nanodiffraction transmission electron microscopy (4D-STEM) is developed, with a maximum achieved resolution of 5-10 nm, depending on the material analyzed. The changes in local nanocrystalline structure of the polymer thin films under study - regioregular Poly(3-hexyl-thiophene-2,5-diyl), the small molecule 7,7’- (4,4 - bis(2 - ethylhexyl) - 4H - silolo[3,2-b:4, 5-b’] dithiophene - 2,6 - diyl) bis(6 - flu- oro - 4 - (5’- hexyl[2,2’-bithiophen] - 5 - yl)benzo[c][1,2,5] - thiadiazole), abbreviated as p- DTS(FBTTh2 )2 or T1, and Poly[2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b ]thiophene], abbreviated as PBTTT - with processing conditions, such as solvent addition and annealing, are characterized and visualized. The methods presented show spatially resolved features such as overlapping grains and nematic liquid crystal character that have not been directly imaged before, and help remove ambiguities in X-ray and other techniques that have been thus far used to characterize these materials.
The method is first applied to polyethylene and P3HT samples to demonstrate the via- bility of the electron transmission technique on soft materials and determine its limitations. A resolution of 5 nm is achieved on P3HT. The method is subsequently used to visualize the changes in crystal structure of T1 when treated with 1,8-diiodooctane (DIO), and then on PBTTT to characterize morphological changes upon annealing. It is found that for T1, while the untreated samples exhibited a liquid-crystal-like structure with crystalline orientations varying smoothly over all possible rotations, the addition of a co-solvent induces partial segmentation of the structure characterized by the emergence of sharp grain boundaries and overlapping domains with unrelated orientations. In the case of PBTTT, the crystalline character of the nematic liquid crystal phase increases upon annealing. These results demon- strate how scanning electron nanobeam diffraction can provide a new level of insight into the structure of functional organic solids, and show how structure-property relationships can be visualized in organic systems using nanoscale electron microscopy techniques previously only available for hard materials such as metals and ceramics.