Graphitization Dynamics in Pyrolytic Carbon: Atomic-scale Insights

Pyrolytic carbon (PyrC) is a material composed of sp² and sp³ hybridized carbon atoms arranged in graphitic sub-domains within an amorphous matrix. PyrC’s tunable electrical, thermal, and mechanical properties make it attractive for applications ranging from microelectronics to energy storage. PyrC is typically obtained by thermal decomposition of a polymer precursor in an inert atmosphere, and recent studies suggest that metal-based nanoparticles can catalyse graphitization, enabling more sustainable fabrication routes.

Despite its potential, systematic strategies to control the sp²/sp³ ratio and understand graphitization mechanisms remain limited. The underlying processes in both bare and catalyst-assisted graphitization are poorly understood. While ex situ studies have provided insights into PyrC structures and associated properties, real-time observation of graphitization pathways remains limited. In situ transmission electron microscopy (TEM) is a powerful technique to track nanoscale transformations, but reproducible polymer fabrication methods and systematic studies of precursor chemistry, pyrolysis conditions, and catalyst nature are lacking. Furthermore, electron-beam effects during in situ TEM pyrolysis are rarely quantified, introducing uncertainty in structural interpretation. These gaps hinder property optimization and sustainable design.

To address these challenges, this work introduces a method for fabrication of polymeric thin films via two-photon polymerization directly on micro-electro-mechanical systems (MEMS) heating chips, enabling in situ TEM studies of graphitization during pyrolysis. Process parameters and structural designs were optimized for IP-Dip resin and extended to an alternative precursor, HT5 resin, to improve comparability across studies. Complementary electron energy-loss spectroscopy (EELS) techniques were refined to quantify graphitization, focusing on integration-window strategies for accurate sp² content estimation. EELS analysis demonstrated that narrowing the first integration window to 1 eV isolates the π* peak, improving sp² accuracy. This approach enables reliable assessment of graphitization at the nanoscale, supporting studies of beam effects and catalyst-driven transformations.

In situ TEM experiments revealed that high-energy electron irradiation significantly accelerates graphitization at 300 kV, producing long, well-aligned graphite stacks. At 80 kV beam-induced effects are limited and the thermally driven morphology, characterized by buckled graphite stacks, is preserved. Electron dose-rate studies in the 900–1300 °C range showed that at 300 kV even a reduced dose rate (around 3 × 103 𝑒𝑒− ⋅ nm−2 ⋅ s−1) increases the sp² content. In contrast, in the same temperature range, high-dose rate conditions (around 1 × 106 𝑒𝑒− ⋅ nm−2 ⋅ s−1) at 80 kV most closely preserve the morphology and sp² content observed without cumulative electron irradiation.

Catalyst-driven graphitization was investigated using iron (III) chloride hexahydrate (FeCl₃) as a precursor for iron nanoparticles. In situ TEM studies revealed complex dynamics governed by nanoparticle mobility and oxidation state changes, resulting in heterogeneous nanostructures with graphitic shells and bamboo-like arrangements in an amorphous matrix. Three distinct particle evolution mechanisms, splitting, merging, and encapsulation, were identified during the dwell at 900°C.

These findings clarify key mechanisms of thermal and catalyst-assisted graphitization and establish a robust framework for tailoring PyrC nanostructures. The developed methodology and insights enable systematic studies of carbon structure, advancing sustainable material design and high-performance applications in micro- and nanoscale PyrC devices.