Demetrios Anglos

Surface Functionalized Carbogenic Quantum Dots

Quantum dots are semiconductor nanocrystals that inherently fluoresce at specific wavelengths in the visible, enabling a number of potential applications to be realized. However, conventional quantum dots are based on metallic elements, which has raised concerns over toxicity, stability and high cost. As a result, the search for more benign substitutes is a worthwhile yet challenging undertaking. Recently a new type of visible emitters has been reported exclusively based on functionalized carbon nanoparticles. The carbon dots were 5 nm in diameter and were produced via laser ablation of graphite. Surface oxidation with nitric acid and subsequent covalent grafting of organic moieties afforded light-emitting derivatives. Notably, the light emitted by these dots depends on the wavelength of light used for excitation. It was suggested that the tethered modifier stabilizes the surface of the carbon nanoparticles helping to generate energy traps that emit light when stimulated, an effect described as emission from passivated surfaces. Because of its origin the emission is size-dependent, i.e., the smaller the size of the dots the better their photoluminescence efficiency. In another intriguing approach, photoluminescent carbon dots 3 nm in size were directly fabricated by electrochemical shocking of multi-wall carbon nanotubes. The demonstrated photoluminescence adds another dimension to the versatility of carbon-based

Laser Diagnostics of Painted Artworks: Laser-Induced Breakdown Spectroscopy in Pigment Identification

Laser-induced breakdown spectroscopy (LIBS) was employed for the in situ analysis of pigments used in painting. LIBS spectra were collected from a wide variety of pigments in powder form and in oil color test samples. Appropriate emission lines for the identification of the metallic elements in the pigments examined are proposed. Under optimal experimental parameters, the technique is minimally destructive; two pulses from a laser beam focused on the sample surface result in the formation of a small crater with typical diameter around 40 μm and depth of no more than 10 μm. Furthermore, recording LIBS spectra from successive laser pulses on the same spot of a model oil painting resulted in information regarding the pigment composition of several paint layers, showing the capability of the technique in performing depth profile analysis. Finally, a test case is presented in which an 18th century oil painting, subjected to partial restoration, was examined by LIBS, and the different pigments used in the original and in the restored part of the work were clearly identified. The results of our studies demonstrate the applicability of LIBS in the rapid, in situ, and practically nondestructive determination of pigments in painted artworks.

The application of LIBS for the analysis of archaeological ceramic and metal artifacts

Abstract A bench-top laser-induced breakdown spectroscopy (LIBS) system has been used in the examination of pottery, jewelry and metal artifacts found in archaeological excavations in central and eastern Crete, Greece. The objects date from the Middle and Late Minoan periods (ca. 20th–13th century b . c .) through Byzantine and Venetian to Ottoman times (ca. 5th–19th century a.d .). The spectral data indicates the qualitative and often the semi-quantitative elemental composition of the examined materials. In the case of colored glazed ceramics, the identity of pigments was established while in the case of metal and jewelry analysis, the type of metal or metal alloy used was determined. The analyses demonstrate the potential of the LIBS technique for performing routine, rapid, on-site analysis of archaeological objects, which leads to the quick characterization or screening of different types of objects.

Random laser action in organic–inorganic nanocomposites

Random laser action is demonstrated in organic–inorganic, disordered hybrid materials consisting of ZnO semiconductor nanoparticles dispersed in an optically inert polymer matrix. The ZnO particles provide both the gain and the strong scattering power that leads to light trapping due to multiple elastic scattering, whereas the polymer matrix offers ease of material fabrication and processability in view of potential applications. Excitation of the nanohybrids by a laser pulse with duration shorter than the ZnO photoluminescence lifetime leads to a dramatic increase in the emitted light intensity accompanied by a significant spectral and temporal narrowing above a certain threshold of the excitation energy density. Critical laser and material parameters that influence the observed laser-like emission behavior are investigated in a series of nanocomposites.

Pigment identification in painted artworks: a dual analytical approach employing laser-induced breakdown spectroscopy and Raman microscopy

The combined application of two laser-based analytical techniques—laser-induced breakdown spectroscopy (LIBS) and Raman microscopy—for pigment identification on painted artworks is demonstrated. Detailed spectral data are presented from analyses performed on a 19th century Byzantine icon, which was examined in order to identify the pigments used in the original painted structure, as well as in interventions carried out subsequently for restorative purposes. LIBS measurements yielded elemental analytical data which suggest the presence of certain pigments and, in addition, provide information on the stratigraphy of the paint layers. Identification of most pigments and of the materials used in the preparation layer was performed by Raman microscopy.

Excimer laser restoration of painted artworks : Procedures, mechanisms and effects

Abstract Excimer laser ablation is demonstrated to afford a novel, highly effective method of restoration of painted artworks. The application relies on the strong UV-absorptivity of these substrates ensuring efficient material removal, with minimal light penetration to the sublayers. On the basis of structural and analytical examinations, 248 nm is shown to be appropriate for achieving sufficient etching rates in combination with good surface morphology. With proper optimization of the irradiation parameters, excimer laser cleaning can be a highly selective process, surpassing by far the degree of selectivity afforded by traditional restoration methods. Furthermore, a number of different laser analytical techniques can be used for on-line monitoring and control, thereby safeguarding against damage. Broadband reflectography, providing structural information about the uncovered layers, and laser-induced breakdown spectroscopy, providing information on the elemental composition of the ablated material, appear to be particularly effective in this respect. Furthermore, the plausible consequences of the laser irradiation on the state of the paintings are investigated in experiments involving model and realistic systems. In particular, the importance of photochemical effects is addressed by chromatographic analysis of irradiated realistic samples for the detection of photoproducts, and by examining the degree of photolysis of photosensitive dopants incorporated in model samples. Potential consequences of laser-induced photomechanical effects are addressed via the use of holographic interferometry. In all, these experiments indicate that optimal fluence ranges can be defined in which damaging effects to the substrate are minimal or insignificant.

Compositional characterization of encrustation on marble with laser induced breakdown spectroscopy

Abstract This study deals with the analysis of encrustation on marble by laser induced breakdown spectroscopy (LIBS), with the aim to obtain quick in-situ information on the in-depth profiling of the encrustation before advancing to conservation treatments. The encrustation examined is formed on exposed marble: (a) as products of the interaction between the stone surface and atmospheric pollutants (dendritic black and thin black encrustation, of approximately 300 and 200 μm thicknesses, respectively); (b) from deposition of soil–dust on marble surfaces (soil–dust crust, 300 μm thick); and (c) from treatments conducted in the past for aesthetic and/or protective purposes (patina samples, 300 μm thick). The crusts examined are multilayer encrustations on un-weathered marble, as revealed by studying cross sections with optical microscopy and scanning electron microscopy coupled with energy dispersive X-ray analysis. The elemental LIBS profiles of black encrustation based on relative spectral line intensity values show that the Fe, Si, Al and Ti content relative to Ca content decrease significantly with depth, expressing, thus, contamination decreasing within the alteration layers, since these elements originate from atmospheric pollution and deposition. In the cases of soil–dust encrustation and patina samples Si I and Al I emissions identified throughout the analyzed crust, indicate deposition of soil–dust and remnants of previous treatments, respectively. Therefore, LIBS, a micro-destructive technique can be used as an autonomous in-situ diagnostic technique to obtain in-depth elemental profiling of encrustation even in cases of highly in-homogeneous layered crusts, such those of un-weathered Pentelic marble.

Laser-Induced Fluorescence in Artwork Diagnostics: An Application in Pigment Analysis

The applicability of laser-induced fluorescence (LIF) spectroscopy as a nondestructive analytical technique for artwork diagnostics is investigated. In this work, LIF is employed in the examination of a set of cadmium sulfide- and cadmium selenide sulfide-based pigments in a series of oil painting test samples. Fluorescence spectra of the oil colors are recorded upon pulsed laser excitation at 532, 355 (Nd:YAG), and 248 nm (KrF excimer). The technique is shown to be suitable for differentiating among the various cadmium pigments used in this study and, furthermore, to be capable of identifying individual components in mixtures of these pigments on the basis of their characteristic fluorescence emission. Future prospects and the potential for the extension of LIF from a research laboratory technique into a conservators tool for artwork diagnostics are discussed.

Analysis of pigments in polychromes by use of laser induced breakdown spectroscopy and Raman microscopy

Abstract Two laser-based analytical techniques, Laser Induced Breakdown Spectroscopy (LIBS) and Raman microscopy, have been used for the identification of pigments on a polychrome from the Rococo period. Detailed spectral data are presented from analyses performed on a fragment of a gilded altarpiece from the church of Escatron, Zaragoza, Spain. LIBS measurements yielded elemental analytical data which suggest the presence of certain pigments and, in addition, provide information on the stratigraphy of the paint layers. Identification of most pigments and of the materials used in the preparation layer was performed by Raman microscopy.

Ultraviolet laser filaments for remote laser-induced breakdown spectroscopy (LIBS) analysis: applications in cultural heritage monitoring

We report experiments with subpicosecond UV laser filaments for the remote analysis of samples related to objects of cultural heritage. The classic laser-induced breakdown spectroscopy (LIBS) technique finds new avenues through femtosecond filamentation, and dynamic remote LIBS becomes possible. Advantages such as self-regulated laser intensity deposition on the target suggest that there is significant potential for using UV femtosecond filaments for the remote analysis of sculpture and large monuments.