Semiconductor Pigments Research Articles

Biocompatibility assessment of organic semiconductor pigments epindolidione and quinacridone

Organic colorants epindolidione (EPI) and quinacridone (QUI) are commercially available hydrogen-bonded semiconductor pigments. Despite their suitable properties for bioelectronic applications, their biocompatibility has not been thoroughly examined yet. In this study, thin EPI and QUI layers were applied on well plates by vacuum deposition technique followed by short and long-term in vitro biocompatibility study. In vitro testing represents a relatively fast and cheap approach, especially suitable for screening testing and prioritization of materials for further research. LIVE/DEAD assay, cell cycle analysis, adhesion and morphology of NIH 3T3 mouse fibroblasts have been investigated up to 7 days using flow cytometry and fluorescent optical microscopy. In summary, no significant differences were observed in viability, cell morphology, or attachment capabilities between control cells and cells grown on the EPI or QUI surfaces or cells cultivated in medium harvested from EPI- or QUI-coated wells. Our results suggest that both pigments are highly biocompatible. The absence of adverse biological effects of EPI and QUI together with their low cost and availability indicate their high application potential in next-generation bioelectronic devices.

Optical composites based on organic polymers and semiconductor pigments

The aim of the work was the development of optical organic-inorganic composite materials with high absorption of light the visible part of the spectrum and high reflection in the near infrared region of the spectrum. Such materials are used in industry and construction as coatings. To create these optical composites, epoxy and epoxy-polyurethane polymer matrices containing inorganic semiconducting particles (CuS, PbS, Fe3O4) were used. Highly dispersive powders of inorganic pigments were used for the preparation of homogeneous composite materials. The wet precipitation method with the application of organic stabilizing additions was applied for the preparation of dispersive CuS and PbS powders. Optical microscopy and X-ray diffraction analysis helped to study the crystal structure and morphology of the obtained semiconductor pigments. PMT-3 device was applied for microhardness measurements of the prepared composite materials. Based on the data of X-ray diffraction analysis, the average crystallite size was calculated using the Scherrer formula. It was found that freshly precipitated CuS and PbS powders consist of nanocrystals with a size of 11–20 nm. Optical microscopy data indicate the formation of aggregates of semiconductor nanocrystals in powders. Experiments have shown that all synthesized composites have low light reflection coefficient (less than 0.06) in the visible part of the spectrum and an increased light reflection coefficient in the near infrared region of the spectrum (0.13–0.15 and more). The results of the study showed that the use of epoxy-polyurethane polymer matrices provides greater microhardness of composite materials, compared to the composites based on epoxy polymers. The highest microhardness values were observed in composite materials based on epoxy-polyurethane polymers containing highly dispersed Fe3O4 particles. Obtained organic-inorganic composites could be used as materials for light-absorbing coatings in different industrial applications.

Lead release and species transformation of commercial minium pigments in aqueous phase under UV-irradiation

This study reports the lead ions release and species transformation of minium pigment under UV-irradiation in aqueous phase. The effects of fulvic acid (FA) and pH on lead ions release were investigated. Lower pH and higher FA concentration facilitate the release of lead ions. During photoreaction, electron donors (FA) are provided to scavenge photogenerated holes and enhance the charge separation, leading to the increased lead ions release. When significant amounts of FA exist in the water, they will complex with the released lead ions preferentially. Then, lead ions react with CO2 and proton resulting in the formation of insoluble Pb3(CO3)2(OH)2 after FA consumed. Correspondingly, the dissolved lead ion concentration in water showed a trend of increasing first and then decreasing. Based upon characterization and experimental results, lead ions release and lead species transformation mechanisms of minium were eventually speculated. This research is helpful for better understanding the environmental behavior and risk of semiconductor pigments in natural water.

Photoluminescence imaging of modern paintings: there is plenty of information at the microsecond timescale

Luminescent emission from artworks is usually employed by conservators and restorers as a visual and quick method for assessing the presence of protective varnishes and restoration materials, as well to evaluate the painting conservation status. Besides these mere qualitative observations, the analysis of both spectral and temporal features of the optical emission from a painting can provide useful information about the pictorial materials employed by artists. In this work we propose Multi-spectral Time-gated Photoluminescence Imaging to differentiate between luminescent pigments, on the basis of their different emission spectra and lifetimes. We present the application of the approach on two modern paintings from Fondazione Maimeri Private Collection, on which we differently probe the fast-living emission from surface varnish and lake pigment and the long-living trap state emissions from different semiconductor pigments. With the aid of the time gated detection we record the trap state emission of semiconductor pigments, otherwise covered by the intense fluorescence of the organic compound. The method is complemented with Macro-XRF analysis, to confirm the nature of the identified inorganic pigments, as zinc white and cadmium yellow. The protocol represents a rapid, non-invasive and reliable way for assessing the identification of luminescent pigments and their spatial distribution.

Sunlight-Mediated Lead and Chromium Release from Commercial Lead Chromate Pigments in Aqueous Phase.

Lead chromate pigments are included in a group of the most widely used pigments, which account for 3% of worldwide lead consumption. This study reports the photoactivity of commercial lead chromate pigment (i.e., chrome yellow) under simulated sunlight. It underwent photodissolution in the presence of organic acid and dissolved organic matter in the aqueous phase, releasing Pb(II) and Cr(III). Pb(II) was released more readily than Cr(III) which mainly formed hydroxides and oxides. The photodissolution can be activated by light with a wavelength <514 nm. The reaction is mediated by the reduction of Cr(VI) in the pigment by self-generated electrons. The kinetics were mainly affected by the electron-hole separation efficiency which can be enhanced by electron donors. The reaction rate decreases with increasing solution pH as the photodissolution process consumes protons. The photodissolution of the chrome yellow pigment was further confirmed in a river water sample under natural sunlight, with 11.28% of lead and 2.56% of chromium released in 7 h. This study highlights the importance of considering photochemical processes in risk assessments and regulations of commercial semiconductor pigments, which are currently based on their solubility.

Combined photoluminescence and Raman microscopy for the identification of modern pigments: explanatory examples on cross-sections from Russian avant-garde paintings

In conservation science, the identification of painting materials is fundamental for the study of artists’ palettes, for dating and for understanding on-going degradation phenomena. For these purposes, the study of stratigraphic micro-samples provides unique information on the complex heterogeneity of the pictorial artworks. In this context, we propose a combined-microscopy approach based on the application of time-resolved photoluminescence (TRPL) micro-imaging and micro-Raman spectroscopy. The TRPL device is based on pulsed laser excitation (excitation wavelength = 355 nm, 1 ns pulse width) and time-gated detection, and it is suitable for the detection of photoluminescent emissions with lifetime from few nanoseconds to hundreds of microseconds. In this work, the technique is beneficially applied for identifying different luminescent semiconductor and mineral pigments, on the basis of their spectral and decay kinetic emission properties. The spatial heterogeneities, detected in the micro-sample, are investigated with Raman spectroscopy (785-nm in CW mode) for a further identification of the paint composition on basis of the molecular vibrations associated with the crystal structure. The effectiveness and limits of the proposed combined method is discussed through analysis of a corpus of stratigraphic micro-samples from Russian Avant-garde modern paintings. In the selected samples, the method allows the identification of modern inorganic pigments such as cadmium-based pigments, zinc white, titanium white, chrome yellow, ultramarine and cinnabar.

Time-Resolved Photoluminescence Microscopy Combined with X-ray Analyses and Raman Spectroscopy Sheds Light on the Imperfect Synthesis of Historical Cadmium Pigments.

Recent studies have shown that modern pigments produced after the Second Industrial Revolution are complex systems characterized by a high level of heterogeneities. Therefore, it is fundamental to adopt a multianalytical approach and highly sensitive methods to characterize the impurities present within pigments. In this work we propose time-resolved and spectrally resolved photoluminescence (PL) microscopy for the mapping of luminescent crystal defects and impurities in historical cadmium-based pigments. PL analysis is complemented by X-ray diffraction, X-ray fluorescenceand Raman spectroscopies, andbyscanning electron microscopy to determine the chemical composition and crystal structure of samples. The study highlights the heterogeneous and complex nature of historical samples that can be associated with the imperfect manufacturing processes tested during the period between the 1850s and 1950s. The results also allow us to speculate on a range of synthesis processes. Since it is recognized that the stability of paints can be related to pigments synthesis, this research paves the way to a wider study on the relationship between synthesis methods and deterioration of cadmium pigments and paints. This rapid and immediate approach using PL can be applied to other semiconductor pigments and real case studies.

Determining The Presence of Photocatalytic Titanium White Pigments via Embedded Paint Sample Staining: A Proof of Principle

ABSTRACTTwentieth century paints often contain titanium dioxide and zinc oxide based white pigments that can range from photostable to highly photocatalytic. Photocatalytic pigments can cause the degradation of paint upon UV exposure, whereas photostable pigments may be benign or can protect paintings from degradation. Hence, knowing whether or not a pigment is photocatalytic is of high importance for risk assessment and the subsequent decision making process concerning storage and exposure conditions of objects. Here we present a proof of principle, focused on titanium white paints, for an easy-to-use and low-tech application of a commercial photocatalytic activity indicator ink (PAII) on embedded paint samples or cross sections. This test determines, qualitatively, if a photocatalytic pigment is present in a white paint sample. The PAII paint sample staining application shows an obvious color change, within five minutes of UV irradiation, for paint samples containing photocatalytic pigments. A microscope with a camera and a UV source are the only necessary equipment for the application of this method. A quantitative image processing protocol is also proposed as an extension of the staining method by applying open source software analysis to measure the color change using photographs. The test was evaluated on reference paints with well-characterized pigments and applied on samples from modern paintings by Piet Mondriaan, Robert Ryman, and Lucebert, indicating the presence of harmful photocatalytic pigments in these cases. The novel application of a commercial ink on paint samples offers a simple test, not just for assessment of photocatalytic activity of titanium white pigments, but which may in future be applied for the detection of photoactive forms of zinc white and other potentially harmful semiconductor pigments in art objects.

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.

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.

Photoluminescence properties of zinc white: an insight into its emission mechanisms through the study of historical artist materials

While the photophysical properties of ZnO nanostructures have been widely explored, less research has focused on the bulk material present in artist pigments. This study is based on the analysis of historical pastels, representative of artist materials available at the turn of the twentieth century, and of the pure powder pigment as the control sample. The study of the intensity of the photoluminescence emission as a function of the fluence and of the nanosecond and microsecond emission decay kinetic properties allows the elucidation of the emission mechanisms in control ZnO and historical samples containing ZnO. Data suggest that in historical samples the near-band-edge free-exciton photoluminescence emission, typically occurring in the pure semiconductor, is influenced by the interaction of the pigment with surrounding organic binding material. Conversely, crystal defects, typically expected in historical samples following the imperfect synthesis process available at the beginning of the twentieth century, introduce minor modifications to the photoluminescence emission. The study further suggests that zinc carboxylates, detected in all historical samples and known to introduce characteristic groups on the surface of ZnO, could be responsible for changes in emission mechanisms. Research demonstrates how photoluminescence decay kinetics and the study of the dependence of the emission intensity on the fluence are powerful methods for elucidating the nature of the mechanism processes in luminescent semiconductor pigments.

Multianalytical Study of Historical Luminescent Lithopone for the Detection of Impurities and Trace Metal Ions.

We have explored the performance of an integrated multianalytical approach to the analysis of a series of microsamples of historical lithopone (a coprecipitate of ZnS + BaSO4) produced at the beginning of the 20th century, based on the combination of spectrally- and lifetime-resolved photoluminescence (PL) microscopy imaging and electron paramagnetic resonance (EPR) spectroscopy. Multispectral imaging of the PL emission from microsamples revealed the presence of different luminescence centers emitting in the visible spectrum, which we have hypothesized as trace Cu and Mn impurities unintentionally introduced into the ZnS crystal lattice during synthesis, which act as deep traps for electrons. Time-resolved PL imaging analyses highlighted the microsecond decay-kinetic behavior of the emission, confirming the trap state nature of the luminescence centers. EPR confirmed the presence of Cu and Mn, further providing information on the microenvironment of defects in the ZnS crystalline lattice related to specific paramagnetic ions. The multianalytical approach provides important insights into the historical synthesis of lithophone and will be useful for the rapid screening and mapping of impurities in complex semiconductor pigments and other artists' materials.

Observation of microwave superfluid phenomena of multiple phase magnetic fluid

We observe superfluid phenomena by microwaves irradiation to multiple phase magnetic fluid in room temperature or room pressure. Ferromagnetism transformation of diamagnetic or paramagnetic particles in multiple phase magnetic fluid containing constant rate of ferromagnetic particles, diamagnetic or paramagnetic particles mixing organic polyphenol and irradiation of microwaves is, observed by superexchange interaction. Superfluid phenomena are observed by irradiation of microwaves to aforementioned multiple phase of magnetic fluid containing ferromagnetism transformed diamagnetic or paramagnetic particles with ferromagnetic particles. Mixing semiconductor pigments amplifying superfluid energy by photosensitivity is observed. Visible light LED selecting wavelength is irradiated to superfluid condition of aforementioned multiple phase magnetic fluid thus magnetic field and energy of superfluid is enhanced by light quantum amplification effect.

Understanding the (in)stability of semiconductor pigments by a thermodynamic approach

Several artists' pigments are semiconductors. Some of these materials appear stable over time, whereas others already show remarkable signs of degradation after limited time periods. The (in)stability of these pigments can be understood using a thermodynamic approach. For several pigment-related materials, the thermodynamic oxidation and reduction potential (ϕox and ϕred) were determined and evaluated considering the absolute energy positions of the valence and conduction band edges and the water redox potentials. The positions of ϕox and ϕred can be used in a fast screening of the stability of semiconductor pigments towards photoinduced corrosion in an aqueous/humid environment. This theoretical approach corresponds well with experimental data on pigment permanence and degradation phenomena found in literature.

Electrochemical photodegradation study of semiconductor pigments: influence of environmental parameters.

Chemical transformations in paintings often induce discolorations, disturbing the appearance of the image. For an appropriate conservation of such valuable and irreplaceable heritage objects, it is important to have a good know-how on the degradation processes of the (historical) materials: which pigments have been discolored, what are the responsible processes, and which (environmental) conditions have the highest impact on the pigment degradation and should be mitigated. Pigment degradation is already widely studied, either by analyzing historical samples or by accelerated weathering experiments on dummies. However, in historic samples several processes may have taken place, increasing the complexity of the current state, while aging experiments are time-consuming due to the often extended aging period. An alternative method is proposed for a fast monitoring of degradation processes of semiconductor pigments, using an electrochemical setup mimicking the real environment and allowing the identification of harmful environmental parameters for each pigment. Examples are given for the pigments cadmium yellow (CdS) and vermilion (α-HgS).

Optical properties of Ta (3-x)Zr (x)N (5-x)O (x) semiconductor pigments

Tantalum zirconium oxynitride phases Ta (3-x)Zr (x)N (5-x)O (x) (0 ≤ x ≤ 0.66) having the Ta 3N 5 structure and Ta (1-x)Zr (x)N (1− x)O (1+x) (0 ≤ x ≤ 0.28) with the TaON structure have been prepared by solid state-gas syntheses. Oxide precursors have been prepared by a novel sol-gel process variant, using a mixture of acetic acid and acetic anhydride as solvent. The optical properties of the brilliantly coloured pigment quality Ta (3-x)Zr (x)N (5-x)O (x) (0 ≤ x ≤ 0.66) semiconducting oxynitrides have been examined by UV-VIS remission spectroscopy. Their applicability for an industrial scale production and a widespread use as brilliant, lightfast and inert inorganic colouring agents has been investigated. These pigments may be useful for colouring paints, plastics and medium temperature ceramic applications, replacing comtemporary toxic metal containing pigments.