Solid-to-solid phase transition from amorphous carbon to graphite nanocrystal induced by intense femtosecond x-ray pulses

TL;DR

This study demonstrates that intense femtosecond x-ray pulses can induce a solid-to-solid phase transition in amorphous carbon, transforming it into graphite nanocrystals through graphitization and structural reordering.

Contribution

It provides experimental evidence of phase transition thresholds and mechanisms in amorphous carbon under femtosecond x-ray irradiation, revealing nanocrystal formation.

Findings

Threshold fluence for phase transition is 282 +/- 11 mJ/cm^2.

Irradiation causes graphitization and formation of graphite nanocrystals.

Volume expansion indicates a solid-to-solid phase change.

Abstract

We present the results of an experiment where amorphous carbon was irradiated by femtosecond x-ray free electron laser pulses. The 830 eV laser pulses induce a phase transition in the material which is characterized ex-situ. The phase transition energy threshold is determined by measuring the surface of each irradiated area using an optical Nomarski microscope. The threshold fluence is found to be 282 +/- 11 mJ/cm^2, corresponding to an absorbed dose at the surface of 131 +/-5 meV/atom. Atomic force microscopy measurements show volume expansion of the irradiated sample area, suggesting a solid to solid phase transition. Deeper insight into the phase transition is gained by using scanning photoelectron microscopy and micro-Raman spectroscopy. Photoelectron microscopy shows graphitization, i.e. modification from sp3 to sp2 hybridization, of the irradiated material. The micro-Raman spectra show the appearance of local order, i.e. formation of graphite nanocrystals. Finally, the nature of the phase transition is discussed, taking into account previous theory and experimental results.