Visible Reflectance Measurements Research Articles

Doping‐Dependent Phase Fractions in Hydrothermally Synthesized Mn‐Doped CuFeO2

Using a simple hydrothermal method, delafossite materials are produced and investigated by means of X‐ray diffraction, ultraviolet–visible reflectance measurements, and electron microscopy. Up to nominally , the synthesis yields delafossite material, which generally consists of both the 2H and R3 phases without any impurities. Lattice parameters and the indirect optical bandgap of delafossite close to about 1.3 eV decrease upon increasing Mn content, which confirms the successful doping with Mn up to an actual Mn content of . The 2H phase fraction shows a pronounced and peaked dependence on Mn content with a maximum 2H mass fraction of about 27% at nominally , whereas the hexagonal particle morphology and the platelet size of about 500 nm as measured by both scanning and transmission electron microscopy are independent of the latter.

The combined use of SEM-EDX, Raman, ATR-FTIR and visible reflectance techniques for the characterisation of Roman wall painting pigments from Monte d\u2019Oro area (Rome): an insight into red, yellow and pink shades

The aim of this work has been the identification of the painter’s materials employed in the wall decoration of some destroyed buildings dating approximately between the first century B.C. and the first century A.D. This research originates from a previously started joined archaeological and analytical investigation concerning a varied group of findings that resulted from a rescue excavation performed by Soprintendenza Archeologica in the area of Monte d’Oro in Rome. The focus of this study progression has been directed to a numerous selection of monochrome red, pink and yellow-pigmented fragments. The analyses were performed by means of scanning electron microscopy energy dispersive X-ray spectroscopy (SEM-EDX) combined with Raman and Fourier transform infrared (FTIR) spectroscopies; visible reflectance measurements have also been carried out and the relevance of this technique in such a kind of archaeological studies has been highlighted. Most attention has been given to the assessment of the performances of non-destructive techniques achieved by portable Raman, and visible reflectance instrumentation to test their diagnostic capabilities. In addition to the expected and well-known pigments such as cinnabar, red ochre, hematite for the reds and yellow ochre for the yellows, the study highlighted a diffuse use of mixed colours and in some cases the possible presence of overlapped painted layers and confirmed the presence of gildings. Among the mixtures of pigments, the most singular outcome concerns the pink fragments revealing the possible application of bone white, which seems to be rather uncommon as a pigment in Roman wall decorations.

Crystal Structure, Optical Properties, and Electronic Structure of Calcium Strontium Tungsten Oxynitrides CaxSr1–xWO2N

Novel calcium strontium tungsten oxynitrides CaxSr1–xWO2N (x = 0.25 and 0.5) have been synthesized. The crystal and electronic structures, electron-density distribution, and optical properties of CaxSr1–xWO2N and CaxSr1–xWO4 (x = 0, 0.25, and 0.5) have been investigated by neutron, synchrotron, and X-ray powder diffraction; transmission electron microscopy energy-dispersive spectroscopy (TEM-EDS); scanning electron microscopy; UV–visible reflectance measurements; and ab initio density functional theory (DFT)-based calculations. Precursor materials CaxSr1–xWO4 (x = 0, 0.25, 0.5, and 1) with a scheelite-type structure were prepared by solid-state reactions, and heated at 900 °C for 5 h under an ammonia flow. The main phase in the product for the composition x = 1 was metallic tungsten W, whereas cubic Pm3̅m perovskite-type oxynitrides CaxSr1–xWO2N were obtained for the compositions x = 0, 0.25, and 0.5. The unit-cell parameter a of the cubic perovskite-type CaxSr1–xWO2N obtained from the Rietveld analysis of synchrotron X-ray and neutron powder diffraction data decreases with an increase of Ca concentration x (0 ≤ x ≤ 0.5), which indicates the substitution of Ca for Sr. The existence of nitrogen in CaxSr1–xWO2N was confirmed by (1) the refined occupancy factor in the Rietveld analysis of neutron data and (2) EDS. The maximum-entropy-method electron-density analysis combined with the DFT calculations indicates W–N and W–O covalent bonds in CaxSr1–xWO2N, which are formed by the overlapping of W 5d and anion 2p orbitals. The minimum electron density at the W–N bond is higher than that at the W–O one, which indicates that the W–N bond is more covalent due to the smaller difference in the electronegativity between W and N atoms compared to the W and O ones. The oxidation number of W in CaxSr1–xWO2N was estimated to be 5.2 by bond valence sum, which indicates the W5+ ion with the 5d1 electron configuration. Precursor oxides CaxSr1–xWO4 with W6+ having the 5d0 electron configuration are white and insulating, whereas the CaxSr1–xWO2N oxynitrides with the W5+ ion having the 5d1 configuration are black and exhibit metallic character. These results indicate the insulator–metal transition from the d0 oxide CaxSr1–xWO4 to the d1 oxynitride CaxSr1–xWO2N.

Electrochemical fabrication of nanoporous gold-supported manganese oxide nanowires based on electrodeposition from eutectic urea/choline chloride ionic liquid

In this study an electrochemical approach is proposed for the fabrication of nanoporous gold (NPG)-supported manganese oxide nanowires. This method consists in: (i) electrodepositing an Au–Mn alloy, (ii) selectively corroding Mn under electrochemical control, forming simultaneously the NPG support and the functional Mn-oxide decorating the nanopores. The electrodeposition process is carried out from a bath based on eutectic urea/choline chloride ionic liquid. The electrochemical dealloying/oxidation step is performed in an aqueous solution. The oxidation process, performed by cyclic voltammetry (CV) in aqueous solution, has been monitored in situ by visible reflectivity measurements. The crystal structure and the morphology of the electrodeposited Au–Mn alloy have been examined by XRD and SEM, respectively. The 3D morphology of the nanostructure has been characterised by FIB/FEG-SEM. The specific capacity of NPG-supported nanostructured Mn-oxide has been measured by CV and found to be notably better than that of unsupported electrodeposited Mn-oxide nanowires.

Electrodeposition of manganese oxide from eutectic urea/choline chloride ionic liquid: An in situ study based on soft X-ray spectromicroscopy and visible reflectivity

In this work, we describe a novel approach to the fabrication of MnO2, consisting in Mn electrodeposition from a choline chloride/urea eutectic ionic liquid bath followed by controlled electrochemical oxidation in an aqueous solution. Both electrodeposition and electro-oxidation processes have been studied by electrochemical and in situ spectroelectrochemical approaches. Synchrotron-based soft X-ray scanning microscopy and absorption spectroscopy were used to measure in situ the Mn valence distribution on the electrodes and within the cell. The oxidation process was followed in situ by visible reflectivity measurements: the different stages of the oxidation process could be discriminated dynamically on the basis of relative reflectivity changes. The capacitance of the electrochemically fabricated films has been measured with cyclic voltammetry and electrochemical impedance spectrometry.

The darkening of zinc yellow: XANES speciation of chromium in artist's paints after light and chemical exposures

The color darkening of selected brushstrokes of the masterpiece A Sunday on La Grande Jatte – 1884 (by Georges Seurat) has been attributed to the alteration of the chromate pigment zinc yellow. The pigment originally displays a bright greenish-yellow color but may undergo, after aging, darkening to a dull, ocher tone. We used XANES to probe the oxidation state of Cr on paint reconstructions, and show that color changes are associated with the reduction of Cr(VI) to Cr(III). Paint mixtures containing the pigment and linseed oil to mimic mixtures used in La Grande Jatte were subjected to artificial aging in the presence of light, SO2, and variable air humidity—50 and 90% relative humidity. High relative humidity led to the largest degree of Cr(VI) reduction whereas low relative humidity promoted light-induced alterations. These results are corroborated by visible reflectance measurements on the same laboratory samples and contribute to a better understanding of the chemical reactivity of chromate pigments, which are present in many historical works of art.

Insights into Masolino's wall paintings in Castiglione Olona: Advanced reflectance and fluorescence imaging analysis

Abstract Investigations of wall paintings by Masolino da Panicale in the Baptistery of Castiglione Olona were conducted using a multi-analytical approach, based on in situ fluorescence imaging, multispectral reflectance imaging and non-destructive analysis of a historical cross-section. Fluorescence imaging of wall paintings, mainly focused on the discrimination and identification of organic compounds, has been conducted with a novel approach, which incorporates the use of spectrally-resolved and lifetime-resolved imaging devices, applied together for the first time for the analysis of paintings. Analysis of the UV-induced emissions from the surface of the paintings allowed the documentation of the selective use of a carmine-based red lake, identified using micro-Raman spectroscopy and Scanning Electron Microscopy. Visible reflectance measurements highlighted the distribution and use of different inorganic pigments. Finally, the in situ study, with the support of early 20 th century photographs, provided additional information on Masolino's style and revealed details and decorative elements of the painting, which are indiscernible under standard illumination.

IR and UV–visible spectroscopic studies of the adsorption of 3-fluorobenzoic acid on Au(1 1 1), Au(1 0 0) and Au(1 1 0) electrodes in neutral solution

3-Fluorobenzoic acid adsorbed on Au(1 1 1), Au(1 0 0) and Au(1 1 0) electrodes has been investigated using in situ IR reflection spectroscopy, UV–visible reflectivity measurements, and differential capacity measurements. As determined by IR reflection spectroscopy, the vertically adsorbed 3-fluorobenzoate is the predominant species at positive potentials, and the flat-lying type is adsorbed at negative potentials. In the UV–visible reflectivity measurements, the reflectivity changes for the vertically adsorbed and flat-lying 3-fluorobenzoates on the Au(1 0 0) electrode were observed at different frequency regions. No UV–visible reflectivity signal was observed for the Au(1 1 0) electrode, and it was proved that the UV–visible reflectivity change has a strong single crystal surface dependence. For the vertical configuration, a large contribution between the 5d orbital of Au and the 2p orbital of oxygen in 3-fluorobenzoate was found for the cluster model by DFT calculation.

Low-temperature growth of polycrystalline Ge thin film on glass by in situ deposition and ex situ solid-phase crystallization for photovoltaic applications

Poly-crystalline germanium (poly-Ge) thin films have potential for lowering the manufacturing cost of photovoltaic devices especially in tandem solar cells, but high crystalline quality would be required. This work investigates the crystallinity of sputtered Ge thin films on glass prepared by in situ growth and ex situ solid-phase crystallization (SPC). Structural properties of the films were characterized by Raman, X-ray diffraction and ultraviolet–visible reflectance measurements. The results show the transition temperature from amorphous to polycrystalline is between 255 °C and 280 °C for in situ grown poly-Ge films, whereas the transition temperature is between 400 °C and 500 °C for films produced by SPC for a 20 h annealing time. The in situ growth in situ crystallized poly-Ge films at 450 °C exhibit significantly better crystalline quality than those formed by solid-phase crystallization at 600 °C. High crystalline quality at low substrate temperature obtained in this work suggests the poly-Ge films could be promising for use in thin film solar cells on glass.

Synthesis and characterization of color variants of nitrogen- and fluorine-substituted TiO2

Reaction pathways to nitrogen- and fluorine-doped TiO2 have been investigated and the compositions analyzed by neutron powder diffraction, nitrogen analyses and titrations of Ti3+. The reported formation of TiNF under pyrolysis of (NH4)2TiF6 in NH3 could not be reproduced when H2O was carefully excluded. If special precautions are not taken to exclude H2O, a product of the previously reported olive-green appearance is obtained; TiN0.05O1.89F0.06, in which 1% of Ti is trivalent. The prolonged hydrolysis and ammonolysis leading to this product proceeds via the intermediates (NH4)1−xTiOF3−x, which adopts the hexagonal tungsten–bronze structure, and/or TiOF2. The latter is therefore suggested as a convenient starting material for doped anatase pigments. Reflectance measurements couple the green color with valence to conduction band excitations (Eg = 2.3 eV) and intraband transitions involving Ti3+. The green phase can be converted to a brilliant yellow in the presence of water vapor at 400–600 °C, which further hydrolyzes the fluoride and oxidizes the titanium, yielding TiN0.04O1.92F0.04 in a particular case. The yellow color and band gap (Eg = 2.4 eV) may be promising for applications as a pigment or photocatalyst. Neutron powder diffraction characterizations and UV–Visible reflectance measurements indicate homogenous doping throughout the bulk. The combined results suggest that the green and yellow phases are part of a homogeneity range adjacent to TiO2, in which the band gap narrowing results largely from nitrogen for oxygen subsitution and the green color is linked to formation of Ti3+ defects.

Silver islands formed after air annealing of amorphous Ag–Cu–Mn–O sputtered films

Ag–Cu–Mn–O films were deposited on glass substrates by co-sputtering of manganese and Ag 60Cu 40 targets in an Ar–O 2 reactive mixture. The currents applied to the targets were fixed to obtain a (Ag+Cu)/Mn atomic ratio close to 3. As-deposited films were X-ray amorphous. Air annealing at 350 °C did not modify the film aspect. On the other hand, higher annealing temperatures (450 and 550 °C) led to the formation of crystals that were eye-visible. X-ray diffraction analysis of such films revealed the formation of crystalline silver phase. This result was confirmed by X-ray energy dispersive spectroscopy (EDS) analyses and UV–visible reflectance measurements. Observations by scanning electron microscopy showed that annealed films are composed of silver islands of approx. 100 μm in diameter dispersed on oxide surface. Furthermore, silver islands exhibited a highly facetted morphology. The effect of the film thickness on the shape of silver islands was studied. Chemical etching in hydrochloric acid was performed to dissolve the silver islands and to analyse by EDS and observe by scanning electron microscopy the surface of the remaining oxide. Finally, the effect of copper on the stability of Ag–Cu–Mn–O films was discussed.

Effect of the oxygen flow rate on the structure and the properties of Ag–Cu–O sputtered films deposited using a Ag/Cu target with eutectic composition

Ag–Cu–O films were deposited on glass substrates by reactive sputtering of a composite Ag 60Cu 40 target in various Ar–O 2 mixtures. The films were characterised by energy dispersive X-ray analysis, X-ray diffraction, UV–visible spectroscopy and using the four point probe method. The structure of the films is strongly dependent on the oxygen flow rate introduced in the deposition chamber. The variation of the oxygen flow rate allows the deposition of the following structures: Ag–Cu–(O) solid solution, nc-Ag + nc-Cu 2O, nc-Ag + nc-(Ag,Cu) 2O and finally X-ray amorphous. UV–visible reflectance measurements confirm the occurrence of metallic silver into the deposited films. The increase of the oxygen flow rate induces a continuous increase of the film oxygen concentration that can be correlated to the evolution of the film reflectance and the film electrical resistivity. Finally, the structural changes vs. the oxygen content are discussed in terms of reactivity of sputtered atoms with oxygen.

Addition of silver in copper nitride films deposited by reactive magnetron sputtering

Silver–copper nitride thin films were deposited on glass substrates by reactive co-sputtering of silver and copper targets. The films were characterized by energy dispersive X-ray spectroscopy to determine the silver to copper atomic ratio and by X-ray diffraction to determine the film structure. From the experimental values of lattice constant and UV–visible reflectance measurements, the position of silver atoms in Cu3N films was discussed. Finally, the effect of silver on the film electrical resistivity was presented.

Toward aerosol optical depth retrievals over land from GOES visible radiances: determining surface reflectance

Frequent observations of aerosol over land are desirable for aviation, air pollution and health applications. Thus, a method is proposed here to correct surface effects and retrieve aerosol optical depth using visible reflectance measurements from the Geostationary Operational Environmental Satellite (GOES). The surface contribution is determined from temporal compositing of visible imagery, where darker pixels correspond to less atmospheric attenuation and surface reflectance is deduced from the composite using radiative transfer. The method is applied to GOES‐8 imagery over the eastern US. Retrieved surface reflectance is compared with separate retrievals using a priori ground‐based observations of aerosol optical depth. The results suggest that surface reflectances can be determined to within ±0.04. The composite‐derived surface reflectance is further analysed by retrieving aerosol optical depth and validating retrievals with Aerosol Robotic Network (AERONET) observations. This analysis indicates that the retrieved optical depth is least biased, hence the surface reflectance is most accurate, when the composite time period varies seasonally. Aerosol optical depth retrievals from this validation are within ±0.13 of AERONET observations and have a correlation coefficient of 0.72. While aerosol optical depth retrieval noise at low optical depths may be limiting, the retrieval accuracy is adequate for monitoring large outbreaks of aerosol events.

Mapping net primary production and related biophysical variables with remote sensing: Application to the BOREAS region

Maps of net and gross primary production, autotrophic respiration, biomass, and other biophysical variables were generated for 106 km2 of boreal forest in central Canada (the Boreal Ecosystem‐Atmosphere (BOREAS) region) using a production efficiency model (PEM) driven with remotely sensed observations at 1 km2 spatial resolution. The PEM was based on carbon yields of absorbed photosynthetically active radiation for both gross and net primary production (GPP and NPP), accounting for environmental stress and autotrophic respiration (Ra). Physiological control was modeled using remotely sensed maps of air temperature, vapor pressure deficit, and soil moisture. The accuracy of the inferred variables was generally within 10–30% of point measurements at the surface and independent model results (both at the stand level). Biomass maps were derived from visible reflectance measurements and were also compared to independently derived maps. Area‐averaged GPP was 604 g C m−2 yr−1 compared with average canopy respiration of 428 g C m−2 yr−1 and NPP of 235 g C m−2 yr−1. Net annual carbon uptake in net primary production for the region totaled 175 teragrams. Canopy carbon exchange (GPP and Ra) differed widely between land cover types even though the model does not use land cover information. Extensive areas of the least productive cover types (e.g., lowland needleleaf species) accounted for the greatest amount of NPP.

Skin pigmentation characterized by visible reflectance measurements

The epidermal melanin content affects most dermatologic treatments involving light, and can limit the therapeutic success significantly. Therefore, knowledge of the optical properties of skin is required. This study investigates how the concentration of melanin influences visible reflectance spectra of skin and the relationship to threshold radiant energy fluence for melanosomal or melanocyte destruction. Reflectance spectra were measured at 28 pigmented human skin sites in vivo. For Asian and Caucasian subjects, measured reflectance values varied over the same range, while significantly lower values were recorded for African individuals. Epidermal melanin absorption coefficients measured at 694 nm were about 2500 m-1 for African, and 300–1200 m-16 for Caucasian and Asian skin. Twenty-five skin sites were exposed to ruby laser pulses (694 nm), where the pulse duration was long enough to allow heat diffusion between melanosomes. Hypopigmentation occurred, on average, at 12 and 26 J cm-2 for sun-exposed and sun-protected white skin, respectively, while slightly lower threshold values resulted from the measured spectra. As visible reflectance spectra reveal information regarding skin pigmentation and individual threshold doses for melanosomal damage, a use as a diagnostic tool in various dermatological laser treatments is apparent.

A physical retrieval of cloud liquid water over the global oceans using special sensor microwave/imager (SSM/I) observations

A method of remotely sensing integrated cloud liquid water over the oceans using spaceborne passive measurements from the special sensor microwave/imager (SSM/I) is described. The technique is comprised of a simple physical model that uses the 19.35‐ and 37‐GHz channels of the SSM/I. The most comprehensive validation to date of cloud liquid water estimated from satellites is presented. This is accomplished through a comparison to independent ground‐based microwave radiometer measurements of liquid water on San Nicolas Island, over the North Sea, and on Kwajalein and Saipan Islands in the western Pacific. In areas of marine stratocumulus clouds off the coast of California a further comparison is made to liquid water inferred from advanced very high resolution radiometer (AVHRR) visible reflectance measurements. The results are also compared qualitatively with near‐coincident satellite imagery and with other existing microwave methods in selected regions. These comparisons indicate that the liquid water amounts derived from the simple scheme are consistent with the ground‐based measurements for nonprecipitating cloud systems in the subtropics and middle to high latitudes. The comparison in the tropics, however, was less conclusive. Nevertheless, the retrieval method appears to have general applicability over most areas of the global oceans. An observational measure of the minimum uncertainty in the retrievals is determined in a limited number of known cloud‐free areas, where the liquid water amounts are found to have a low variability of 0.016 kg m−2. A simple sensitivity and error analysis suggests that the liquid water estimates have a theoretical relative error typically ranging from about 25% to near 40% depending on the atmospheric/surface conditions and on the amount of liquid water present in the cloud. For the global oceans as a whole the average cloud liquid water is determined to be about 0.08 kg m−2. The major conclusion of this paper is that reasonably accurate amounts of cloud liquid water can be retrieved from SSM/I observations for nonprecipitating cloud systems, particularly in areas of persistent stratocumulus clouds, with less accurate retrievals in tropical regions.

The estimation of ground cover of potato by reflectance measurements

In nine field trials, the proportion of ground covered by potato canopy was estimated using near-infrared and visible reflectance measurements. The data set spanned a range of cultivars, treatments, soil types, soil moisture regimes and growing conditions that ranged from severely stressed to near-potential growth. Near-infrared reflectance of the canopy, corrected for soil reflectance (WDVI), was related to ground cover estimated by a trained expert with or without the use of a grid-frame. Different linear regressions between ground cover and WDVI for the early phase of growth and for the late phase, allowed the calculation of ground cover of potato from WDVI with anabsolute accuracy of 3.5–6.5%. The derived regression lines are expected to have a general applicability for potato grown on soils of temperate regions. It is concluded that reflectance measurement is an objective, fast and reliable method for estimating the proportion of ground covered by potato canopies.