Abstract
In conservation, science semiconductors occur as the constituent matter of the so-called semiconductor pigments, produced following the Industrial Revolution and extensively used by modern painters. With recent research highlighting the occurrence of various degradation phenomena in semiconductor paints, it is clear that their detection by conventional optical fluorescence imaging and microscopy is limited by the complexity of historical painting materials. Here, we illustrate and prove the capabilities of time-resolved photoluminescence (TRPL) microscopy, equipped with both spectral and lifetime sensitivity at timescales ranging from nanoseconds to hundreds of microseconds, for the analysis of cross-sections of paint layers made of luminescent semiconductor pigments. The method is sensitive to heterogeneities within micro-samples and provides valuable information for the interpretation of the nature of the emissions in samples. A case study is presented on micro samples from a painting by Henri Matisse and serves to demonstrate how TRPL can be used to identify the semiconductor pigments zinc white and cadmium yellow, and to inform future investigations of the degradation of a cadmium yellow paint.
Highlights
Photoluminescence (PL) microscopy is a powerful optical method for the study of crystal defects in semiconductors and organometallic complexes [1], with important applications in the manufacturing process of nanostructures [2,3,4], optoelectronic devices and solar cell systems [5].Semiconductors are the constituent matter of the so-called semiconductor pigments.Illustrative examples of this class of coloring matter include ancient pigments, such as the brilliant red vermillion (HgS), first used during the Neolithic Age [6], and the lemon yellow orpiment (As2 S3 ), extensively found on ancient Egyptian objects and paintings
Fluorescence lifetime imaging microscopy (FLIM) and phosphorescence lifetime imaging microscopy (PLIM) set-ups are typically based on time-correlated-single-photon-counting (TCSPC) detectors and femtosecond mode-locking laser sources, while imaging of samples is achieved through sample raster scanning
Similar considerations can be reported for the use of fluorescence optical microscopy, which is occasionally applied to stratigraphic samples of paintings to put light on sample heterogeneities
Summary
Photoluminescence (PL) microscopy is a powerful optical method for the study of crystal defects in semiconductors and organometallic complexes [1], with important applications in the manufacturing process of nanostructures [2,3,4], optoelectronic devices and solar cell systems [5]. The method has been extended to the analysis of longer (microsecond) emission lifetimes In this latter case, it is typically referred to as phosphorescence lifetime imaging microscopy (PLIM) [26] and it is employed for the sensing of cell environmental properties by probing the emission lifetimes of specific metal–ion complexes [26]. FLIM and PLIM set-ups are typically based on time-correlated-single-photon-counting (TCSPC) detectors and femtosecond mode-locking laser sources, while imaging of samples is achieved through sample raster scanning These devices are designed for probing emission lifetimes in a specific time range, and are poorly suited for the analysis of heterogeneous samples with lifetimes ranging from nanosecond to hundreds of microseconds. The advantages of such a novel setup are illustrated through the study of precious stratigraphic micro-samples from a painting by Henri Matisse (1869–1954), which present interesting questions on the use of modern pigments by the famous Post-Impressionist painter
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