Atomic force microscopy and photo-induced force microscopy analysis of two-dimensional materials based films for heritage protection

The interdisciplinary connection between art, culture, physics, and chemistry opens up entirely new perspectives for the preservation of cultural heritage characterized by unique historical and material properties. This includes wall paintings, which can be executed using two different techniques: the application of pigments onto fresh plaster (a fresco) or onto dry plaster (a secco). Centuries of exposure to environmental conditions causes deterioration of both, substrate and pigment layers. The application of reversible protective layers represents a promising approach for the protection and long-term preservation of such artworks after centuries of exposure to environmental influences. Here, modern two-dimensional materials come into play. In this study, the suitability of two-dimensional layers derived from liquid-phase exfoliated hexagonal boron nitride (hBN) and pyrophyllite was examined as coating materials for fresco and secco paintings in Serbia. Atomic Force Microscopy (AFM) was employed for topographical analysis for uncoated and coated secco replicas. In addition, the novel Photo-induced Force Microscopy (PiFM) technique was used for chemical characterization of replica surfaces and subsequent coatings on the nanometer scale. The combination of these methods enabled a qualitative assessment of the samples and allowed the identification of challenges and limitations inherent to the measurement techniques. Using PiFM microscopy, both, a secco and a fresco samples coated with ultrathin hBN or pyrophyllite flakes were analyzed. The results indicate a complete surface coverage, which is a fundamental requirement for protective coatings. It was shown that the flake size plays a crucial role in the coverage of rough sample surfaces. In particular, hBN with flake dimensions of up to approximately 1 ¿m2 is well suited for covering rough surfaces due to its small flake size. Such small flakes enable the formation of a conformal layer and contribute to surface smoothing, as they can enter into small surface depressions. The reversibility of the protective layers was evaluated by removing the hBN layer from a fresco and secco sample. Following the removal, PiFM analysis in hyPIR mode allowed for the evaluation of the removal quality, and changes to the original state of the sample. In summary, the characterization of the fresco and secco surfaces enables continuous monitoring of the degradation process and provides a foundation for future investigations. The obtained data allowed for an analysis of the deposition quality. Further experiments proved to be highly promising, indicating the potential use of two-dimensional hBN and pyrophyllite as reversible protective coatings for historical wall paintings.