Title: Electric Field Enhancements and Hot Spots in Amorphous Carbon Materials

Authors: Illobre, P.G.; Conter, G.; Bonatti, L.; Giovannini, T.; Fortunelli, A.; Cappelli, C.

DOI: 10.1002/sstr.202500733

Abstract: Amorphous carbon (a-C) represents a broad and technologically relevant class of carbonaceous materials whose rich morphological diversity, characterized by nanoporosity and defective networks, endows them with unique electrical, chemical, and optical properties. Despite their potential for light–matter interaction and enhanced-chemistry applications, a microscopic understanding of the optical response of a-C remains elusive, primarily due to the absence of long-range order and the resulting need to model systems containing hundreds of thousands of atoms. In this work, we combine atomistic models generated through the dynamic reactive massaging of the potential energy surface (DynReaxMas) approach with a recently developed, fully classical yet atomistic method for calculating their optical response, named frequency-dependent fluctuating charges (FQ). This methodology enables the simulation of large-scale a-C structures with near–ab initio accuracy, unveiling a complex optical behavior characterized by strong and spatially inhomogeneous local field enhancements (“hot spots”). Four distinct types of hot spots are identified, corresponding to unsaturated carbon dangling bonds, confined regions between graphene-like sheets, single carbon chains, and atomistic defects. Our results demonstrate that these localized field enhancements can, in principle, drive selective sensing and photochemical activity in a-C materials, thereby providing a microscopic basis for their design and functionalization in optical and catalytic applications.