Condensed-Matter Physics & Materials Science Seminar

While beam damage is often considered detrimental to our quantitative imaging capabilities, the energy and charge injected into the sample as a result of inelastic scattering can be exploited beneficially. This is especially true in radiation-chemistry-type experimental setups in the electron microscope where the beam promotes local wanted chemical reactions. We have observed that by exposing to the electron beam a layer of small volatile organic molecules condensed over a cold substrate results in the formation of a solid product. Evidence suggests that the exposure mechanism driving the formation of a solid product is partial dehydrogenation of the molecules, removal of H2, and progressive increase of the average molecular weight. Contrary to focused electron beam induced deposition, that relies on surface absorption followed by aggregation of mobile species, at cryogenic temperature organic ice molecules are largely immobilized, and act as targets for the incoming electrons throughout the entire thickness of the layer. Therefore, the exposure occurs throughout the volume of the frozen precursor, and the features are essentially determined by the electron distribution, with diffusion/transport parameters bearing little or no relevance. Since larger molecules are less volatile, if the molecular weight increases sufficiently, upon raising the temperature the unexposed areas leave the sample, while the exposed molecules assemble into a solid product in the form of hydrogenated amorphous carbon.