Preserving the Crystal Structure of Organic Semiconductors During Cross-Sectional Specimen Preparation for Transmission Electron Microscopy Using a Focused Ion Beam

Abstract

Transmission electron microscopy (TEM) analyses of cross-sectional lamellae prepared by a Ga+-focused ion beam are frequently applied in a variety of scientific disciplines where high-resolution information is required from a specific site of interest. Radiation-sensitive materials thereby require dedicated methods to facilitate an analysis of the specimen in its pristine state. Here, we present a method for preparing TEM lamellae of organic thin-film transistors that preserves morphology and crystal structure of beam-sensitive small-molecule organic semiconductor thin films as demonstrated for pentacene, dinaphtho[2,3-b:2 ′,3 ′-f]thieno[3,2-b]thiophene (DNTT), and phenylalkyl-substituted 3,4,9,10-benzo[de]isoquinolino[1,8-gh]-quinolinetetra-carboxylic diimide (PhC2-BQQDI). The two key measures are minimizing electron and ion irradiation of the organic layer using protective layers and optimized beam conditions, and performing the final lamella thinning at reduced temperature. Monte-Carlo simulations can help to find the optimum beam settings, but channeling of electrons and ions, typically not considered in the simulations, can lead to an increased penetration depth and needs to be taken into account. As proof of concept for a successful TEM sample preparation workflow we demonstrate the edge-on growth of all investigated molecules with structural integrity. Furthermore, the zone axes for pentacene and DNTT and the existence of a previously unknown thin-film polymorph of the organic semiconductor PhC2-BQQDI are identified.