Ionization and electron excitation of C ₆₀ in a carbon nanotube: A variable temperature/voltage transmission electron microscopic study

Significance

The destruction of specimen molecules by an electron beam (e-beam) is either beneficial, as in mass spectrometry capitalizing on ion formation, or deleterious, as in electron microscopy. In the latter application, the e-beam not only produces the specimen image, but also causes information loss upon prolonged irradiation. However, the atomistic mechanism of such loss has been unclear. Performing single-molecule kinetic analysis of C ₆₀ dimerization in a carbon nanotube (CNT) under variable-temperature/voltage conditions, we identified three reactive species—that is, radical cation, singlet, and triplet excited states—reacting competitively as the voltage and the properties of the CNT were changed. The key enabler was in situ continuous recording of the whole reaction process, suggesting an upcoming new era of “cinematic chemistry.”

Abstract

There is increasing attention to chemical applications of transmission electron microscopy, which is often plagued by radiation damage. The damage in organic matter predominantly occurs via radiolysis. Although radiolysis is highly important, previous studies on radiolysis have largely been descriptive and qualitative, lacking in such fundamental information as the product structure, the influence of the energy of the electrons, and the reaction kinetics. We need a chemically well-defined system to obtain such data and have chosen as a model a variable-temperature and variable-voltage (VT/VV) study of the [2 + 2] dimerization of a van der Waals dimer [60]fullerene (C ₆₀) to C ₁₂₀ in a carbon nanotube (CNT), as studied for several hundred individual reaction events at atomic resolution. We report here the identification of five reaction pathways that serve as mechanistic models of radiolysis damage. Two of them occur via a radical cation of the specimen generated by specimen ionization, and three involve singlet or triplet excited states of the specimen, as initiated by electron excitation of the CNT, followed by energy transfer to the specimen. The [2 + 2] product was identified by measuring the distance between the two C ₆₀ moieties, and the mechanisms were distinguished by the pre-exponential factor and the Arrhenius activation energy—the standard protocol of chemical kinetic studies. The results illustrate the importance of VT/VV kinetic analysis in the studies of radiation damage and show that chemical ionization and electron excitation are inseparable, but different, mechanisms of radiation damage, which has so far been classified loosely under the single term “ionization.”