Photoluminescence properties of zinc oxide in paints: a study of the effect of self-absorption and passivation

Objective interpretation of ultraviolet-induced luminescence for characterizing pictorial materials

Ultraviolet-induced Luminescence (UVL) is the property of some materials of emitting light once illuminated by a source of UV radiation. This feature is characteristic of some mediums and pigments, such as some red lakes, widely used for the realisation of works of art. On the one hand, UVL represents a like strike for a researcher in the cultural heritage field: in fact, UVL allows to characterise the state of conservation of the paintings and, in some cases, to recognize at glance some of the materials used by the artists. On the other hand, the contribution of UVL to the study of the artefacts is almost always limited to qualitative observation, while any speculation about the cause of the luminescence emission relies on the observer's expertise. The aim of this paper is to overcome this paradigm, moving a step toward a more quantitative interpretation of the luminescence signal. The obtained results concern the case study of pictorial materials by Giuseppe Pellizza da Volpedo (1868-1907, Volpedo, AL, Italy) including his iconic masterpiece Quarto Stato (1889-1901), but the method has general validity and can be applied whenever the appropriate experimental conditions occur. Once designed an appropriate set-up, the statistical comparison between the acquisitions performed on Quarto Stato, on a palette belonged to the master, on drafts made by the author himself and on a set of ad hoc prepared samples both with commercial contemporary pigments and prepared with the traditional recipe, shed some light on which materials have been employed by the artist, where they have been applied and support some intriguing speculations on the use of the industrial lakes in the Quarto Stato painting.

Microscale mechanochemical characterization of drying oil films by in situ correlative Brillouin and Raman spectroscopy

Correlative Brillouin and Raman microspectroscopy (BRaMS) is applied for the in situ monitoring of the chemical and physical changes of linseed oil during polymerization. The viscoelastic properties of the drying oil throughout the phase transition were determined by Brillouin light scattering (BLS) and joined to the Raman spectroscopic information about the chemical process responsible for the oil hardening. A comparative study was then performed on an oil mock-up containing ZnO, one of the most common white pigments used in cultural heritage. The intriguing outcomes open new research perspectives for a deeper comprehension of the processes leading to the conversion of a fluid binder into a dry adhering film. The description of both chemical and structural properties of the polymeric network and their evolution are the basis for a better understanding of oil painting degradation. Last, as a feasibility test, BRaMS was applied to study a precious microfragment from J. Pollock's masterpiece Alchemy.

Exchange Functionals and Basis Sets for Density Functional Theory Studies of Water Splitting on Selected ZnO Nanocluster Catalysts

In this communication, we use density functional theory (DFT) to study the structural (geometry) and electronic properties (vertical detachment energy and electron affinity) of ZnO monomers and dimers that can be used to form ZnO clusters of different sizes, with a view to adapting one or more of them as catalysts or photocatalysts, standing alone or on suitable substrates like graphene, to split water. We also investigate different pairs of exchange functionals and basis sets to optimize their choice in our DFT calculations and to compare the singlet-triplet energy gaps of small ZnO clusters of different sizes to select an optimal cluster size for water splitting. We find that the B3LYP/DGDZVP2 exchange functional/basis set is a reliable combination for use with DFT to calculate the geometry and electronic properties of small ZnO nanoclusters from among several other combinations of exchange functionals and basis sets. Comparisons of the singlet-triplet energy gaps show that the trimer (ZnO)₃ has an energy gap of 58.66 k cal/mol. which is approximately equal to the energy of a visible photon at a wavelength of 500 nm, and a HOMO-LUMO gap of 4.4 eV, making it a suitable choice of photocatalyst for the oxidation of water from among six (ZnO) _n nanoclusters of monomers, with n ranging from 1 to 6. We used this exchange functional/basis set to study the structural and energetic details of hydration and hydrolysis of water absorbed on the (ZnO)₃ nanocatalyst and calculated the corresponding potential energy profiles to identify three sets of singlet-triplet pathways for water splitting. Detailed study of a pathway showed that oxygen is produced after hydrogen, and the rate-determining step is the formation of hydrogen.

Novel markers to early detect degradation on cellulose nitrate-based heritage at the submicrometer level using synchrotron UV-VIS multispectral luminescence

Cellulose nitrate (CN) is an intrinsically unstable material that puts at risk the preservation of a great variety of objects in heritage collections, also posing threats to human health. For this reason, a detailed investigation of its degradation mechanisms is necessary to develop sustainable conservation strategies. To investigate novel probes of degradation, we implemented deep UV photoluminescence micro spectral-imaging, for the first time, to characterize a corpus of historical systems composed of cellulose nitrate. The analysis of cinematographic films and everyday objects dated from the nineteenth c./early twentieth c. (Perlov's collection), as well as of photo-aged CN and celluloid references allowed the identification of novel markers that correlate with different stages of CN degradation in artworks, providing insight into the role played by plasticizers, fillers, and other additives in stability. By comparison with photoaged references of CN and celluloid (70% CN and 30% camphor), it was possible to correlate camphor concentration with a higher rate of degradation of the cinematographic films. Furthermore, the present study investigates, at the sub-microscale, materials heterogeneity that correlates to the artworks' history, associating the different emission profiles of zinc oxide to specific color formulations used in the late nineteenth and early twentieth centuries.

Thin Films of Plasma-Polymerized n-Hexane and ZnO Nanoparticles Co-Deposited via Atmospheric Pressure Plasma Jet

This study explores the co-deposition of thin polymeric films loaded with nanoparticles for its possible future application as radiation detectors. Thin films containing zinc oxide (ZnO) nanoparticles in plasma polymerized n-hexane (PPH) were deposited on silicon substrates using an atmospheric pressure plasma jet (APPJ). Crystalline ZnO nanoparticles were produced by wet chemistry, characterized, and injected through the plasma with an aerosol buffer. The precursor hydrocarbon was polymerized in atmosphere at room temperature by the plasma, resulting in a highly crosslinked structure chemically stable against common solvents. The polymer structure was characterized by FT-IR, NMR, and thermal analyses. Photoluminescence analysis revealed that ZnO UV excitonic emission is recovered owing to the passivation through polymeric encapsulation, with a remarkable increase in luminescence yield.

In-Depth Analysis of Egg-Tempera Paint Layers by Multiphoton Excitation Fluorescence Microscopy

The non-invasive depth-resolved imaging of pictorial layers in paintings by means of linear optical techniques represents a challenge in the field of Cultural Heritage (CH). The presence of opaque and/or highly-scattering materials may obstruct the penetration of the radiation probe, thus impeding the visualization of the stratigraphy of paintings. Nonlinear Optical Microscopy (NLOM), which makes use of tightly-focused femtosecond pulsed lasers as illumination sources, is an emerging technique for the analysis of painted objects enabling micrometric three-dimensional (3D) resolution with good penetration capability in semi-transparent materials. In this work, we evaluated the potential of NLOM, specifically in the modality of Multi-Photon Excitation Fluorescence (MPEF), to probe the stratigraphy of egg-tempera mock-up paintings. A multi-analytical non-invasive approach, involving ultraviolet-visible-near infrared (UV-Vis-NIR) Fiber Optics Reflectance Spectroscopy, Vis-NIR photoluminescence, and Laser Induced Fluorescence, yielded key-information for the characterization of the constituting materials and for the interpretation of the nonlinear results. Furthermore, the use of three nonlinear optical systems allowed evaluation of the response of the analyzed paints to different excitation wavelengths and photon doses, which proved useful for the definition of the most suitable measurement conditions. The micrometric thickness of the paint layers, which was not measurable by means of Optical Coherence Tomography (OCT), was instead assessed by MPEF, thus demonstrating the effectiveness of this nonlinear modality in probing highly-scattering media, while ensuring the minimal photochemical disturbance to the examined materials.

Bulk Organic-Inorganic Methylammonium Lead Halide Perovskite Single Crystals for Indirect Gamma Ray Detection

Scintillators that convert ionization radiation photons to UV-visible photons have attracted extraordinary attention. Traditional scintillators are associated with a vacuum photomultiplier tube that faces strict constraints of fragility, magnetic fields, and operated voltage, or coupled to a silicon photomultiplier (SiPM) with optical silicone grease. Here, we report a high-performance radiation detector with an indirect photon-to-photon conversion radiation detection model based on perovskite single crystals (SCs), where perovskite SCs have been directly integrated into the window of SiPM by using the solution growth method at low temperature. Tunable X (γ)-ray excited light emission in the range of 414 to 600 nm is obtained with different concentrations of Br doping, which greatly matches the response wavelength of SiPM. Small Br-doped CH₃NH₃PbBr_0.05Cl_2.95 SCs exhibit high transmittance and weak self-absorption, resulting in improved scintillation light emissions. Moreover, we have successfully collected a ¹³⁷Cs source gamma-ray pulse height spectrum with the SiPM readout. The MAPbBr_0.05Cl_2.95 scintillator exhibits a decay time of 0.14 ± 0.02 ns and a light yield of 18 000 photons/MeV with an energy resolution of 10.5 ± 0.4% at 662 keV. The results indicate that the CH₃NH₃PbBr_xCl_3-x perovskite SCs could enable the next generation of low-cost, fast, and fine-energy resolution scintillators.

Magnetron Sputtering for ZnO:Ga Scintillation Film Production and Its Application Research Status in Nuclear Detection

As a wide band-gap and direct transition semiconductor material, ZnO has good scintillation performance and strong radiation resistance, but it also has a serious self-absorption phenomenon that affects its light output. After being doped with Ga, it can be used for the scintillator of ultra-fast scintillating detectors to detect X-ray, gamma, neutron, and charged particles with extremely fast response and high light output. Firstly, the basic properties, defects, and scintillation mechanism of ZnO crystals are introduced. Thereafter, magnetron sputtering, one of the most attractive production methods for producing ZnO:Ga film, is introduced including the principle of magnetron sputtering and its technical parameters’ influence on the performance of ZnO:Ga. Finally, ZnO:Ga film’s application research status is presented as a scintillation material in the field of radiation detection, and it is concluded that some problems need to be urgently solved for its wider application.

Spectral Behavior of White Pigment Mixtures Using Reflectance, Ultraviolet-Fluorescence Spectroscopy, and Multispectral Imaging

Reflectance spectroscopy, ultraviolet (UV)-fluorescence spectroscopy, and multispectral imaging have been widely employed for pigment identification on paintings. From ancient times to the present, lead white, zinc white, and titanium white have been the most important white pigments used for paintings and they are used as pigment markers for dating a work of art. The spectral behavior of these pigments is reported in several scientific papers and websites, but those of their mixtures are quite unknown. We present a combined nondestructive approach for identifying mixtures of lead white, zinc white, and titanium white as powder and dispersed in two different binder media (egg yolk and linseed oil) by using reflectance spectroscopy, spectrofluorimetry, multispectral reflectance and UV-fluorescence imaging. We propose a novel approach for mapping the presence of white pigments in paintings by false color images obtained from multispectral reflectance and UV-fluorescence images. We found that the presence of lead white mixed with either zinc white or titanium white is highly detectable. Zinc white mixed with lead white or titanium white can be identified due to its UV-fluorescence emission, whereas titanium white in association with lead white or zinc white is distinguishable by its reflectance spectral features. In most cases, the UV-fluorescence analyses also permit the recognition of the binder media in which the pigments are dispersed.

Time-Resolved Photoluminescence Microscopy for the Analysis of Semiconductor-Based Paint Layers

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.

"Paintings Fade Like Flowers": Pigment Analysis and Digital Reconstruction of a Faded Pink Lake Pigment in Vincent van Gogh's Undergrowth with Two Figures

Color fading in Vincent van Gogh's Undergrowth with Two Figures was studied chemically to facilitate the creation of a digital reconstruction of fugitive colors . The painting contains a field of white, green, orange, and yellow flowers under a canopy of poplar trees with two central figures-a man and a woman, arms entwined. From Van Gogh's letters, however, it is known that he painted the picture with some pink flowers, which appear to have altered, presumably to white. Raman spectroscopy was applied to microsamples of paint to identify the faded pigment as geranium lake, which in this painting consists of the dye, eosin (2',4',5',7'-tetrabromofluorescein). For the first time, lead(II) sulfate has been specifically identified as the likely inorganic substrate for a geranium lake used by Van Gogh in the last months of his life. Microfocus X-ray fluorescence (MXRF) spectroscopy was subsequently used in situ to analyze the white flowers to identify bromine as a proxy for eosin, thus indicating an original pink coloration. Of the 387 white flowers analyzed, 37.7% contained measurable bromine and were, therefore, originally pink. Several cross-sections from these formerly pink areas were assessed using a combination of visual inspection and microcolorimetry to create a colored mask in Adobe Photoshop to digitally reconstruct a suggestion of the original appearance of the painting with regard to the faded flowers. Additionally, microfadeometry was undertaken for the first time on a painting cross-section sample to understand the actual fading kinetics of the underlying bright pink geranium lake used by Van Gogh. A combination of Raman microspectroscopy, MXRF, and scanning electron microscopy energy dispersive spectroscopy (SEM-EDS) were utilized in situ and on paint microsamples to identify the complete palette used to create Undergrowth with Two Figures.

A Photoluminescence Study of the Changes Induced in the Zinc White Pigment by Formation of Zinc Complexes

It is known that oil paintings containing zinc white are subject to rapid degradation. This is caused by the interaction between the active groups of binder and the metal ions of the pigment, which gives rise to the formation of new zinc complexes (metal soaps). Ongoing studies on zinc white paints have been limited to the chemical mechanisms that lead to the formation of zinc complexes. On the contrary, little is known of the photo-physical changes induced in the zinc oxide crystal structure following this interaction. Time-resolved photoluminescence spectroscopy has been applied to follow modifications in the luminescent zinc white pigment when mixed with binder. Significant changes in trap state photoluminescence emissions have been detected: the enhancement of a blue emission combined with a change of the decay kinetic of the well-known green emission. Complementary data from molecular analysis of paints using Fourier transform infrared spectroscopy confirms the formation of zinc carboxylates and corroborates the mechanism for zinc complexes formation. We support the hypothesis that zinc ions migrate into binder creating novel vacancies, affecting the photoluminescence intensity and lifetime properties of zinc oxide. Here, we further demonstrate the advantages of a time-resolved photoluminescence approach for studying defects in semiconductor pigments.

Revealing the Nature and Distribution of Metal Carboxylates in Jackson Pollock's Alchemy (1947) by Micro-Attenuated Total Reflection FT-IR Spectroscopic Imaging

Protrusions, efflorescence, delamination, and opacity decreasing are severe degradation phenomena affecting oil paints with zinc oxide, one of the most common white pigments of the 20th century. Responsible for these dramatic alterations are the Zn carboxylates (also known as Zn soaps) originated by the interaction of the pigment and the fatty acids resulting from the hydrolysis of glycerides in the oil binding medium. Despite their widespread occurrence in paintings and the growing interest of the scientific community, the process of formation and evolution of Zn soaps is not yet fully understood. In this study micro-attenuated total reflection (ATR)-FT-IR spectroscopic imaging was required for the investigation at the microscale level of the nature and distribution of Zn soaps in the painting Alchemy by J. Pollock (1947, Peggy Guggenheim Collection, Venice) and for comparison with artificially aged model samples. For both actual samples and models, the role of AlSt(OH)₂, a jellifying agent commonly added in 20th century paint tube formulations, proved decisive for the formation of zinc stearate-like (ZnSt₂) soaps. It was observed that ZnSt₂-like soaps first form around the added AlSt(OH)₂ particles and then eventually grow within the whole painting stratigraphy as irregularly shaped particles. In some of the Alchemy samples, and diversely from the models, a peculiar distribution of ZnSt₂ aggregates arranged as rounded and larger particles was also documented. Notably, in one of these samples, larger agglomerates of ZnSt₂ expanding toward the support of the painting were observed and interpreted as the early stage of the formation of internal protrusions. Micro-ATR-FT-IR spectroscopic imaging, thanks to a very high chemical specificity combined with high spatial resolution, was proved to give valuable information for assessing the conservation state of irreplaceable 20th century oil paintings, revealing the chemical distribution of Zn soaps within the paint stratigraphy before their effect becomes disruptive.

Strong room-temperature blue-violet photoluminescence of multiferroic BaMnF4

BaMnF4 microsheets have been prepared using a hydrothermal method. Strong room-temperature blue-violet photoluminescence has been observed (an absolute luminescence quantum yield of 67%) with two peaks located at 385 nm and 410 nm. More interestingly, photon self-absorption phenomenon has been observed, leading to an unusual abrupt decrease in the luminescence intensity at a wavelength of 400 nm. To understand the underlying mechanism of such emission, the electronic structure of BaMnF4 has been studied using first principles calculations. The observed two peaks are attributed to electron transitions between the upper-Hubbard bands of the Mn's t2g orbitals and the lower-Hubbard bands of the Mn's eg orbitals. The Mott gap mediated d-d orbital transitions may provide additional degrees of freedom to tune the photon generation and absorption in ferroelectrics.

Ionomer-like structure in mature oil paint binding media

We use ATR-FTIR and SAXS to demonstrate that oil paint binding media go through an ionomer-like state during ageing.