Reflectance Spectroscopy Measurements Research Articles

(Invited) Quasi-Two-Dimensional Thermoelectricity in Snse

Stannous selenide is a layered semiconductor with a puckered honeycomb lattice that constitutes a polar analogue of black phosphorus, and of great interest as a thermoelectric material [1-3]. Hole doped SnSe supports a large Seebeck coefficient at high conductivity. We combine experimental techniques in the form of angle resolved photo-emission spectroscopy, optical reflection spectroscopy and magnetotransport measurements to map out a multiple-valley valence band structure and a quasi two-dimensional dispersion. The quasi two-dimensional dispersion realizes the low-dimensional Hicks-Dresselhaus thermoelectric, which contributes to the high Seebeck coefficient reported at high carrier density [1-2]. We further demonstrate that the hole accumulation layer in exfoliated SnSe transistors exhibits a field effect mobility of up to 250 cm2/Vs at T=1.3 K. Unintentional hole doping and persistent photoconductivity has been observed, suggesting material quality can be further improved. SnSe is thus found to be a high quality, quasi two-dimensional semiconductor.

One-Pot electrochemical fabrication of Single-Crystalline SnO nanostructures on Si and ITO substrates for Catalytic, sensor and energy storage applications

In this work, SnO nanostructures were prepared by a facile and reproducible electrochemical technique from an oxygenated aqueous solution of Sn2+ onto Si(100) and indium tin oxide (ITO) coated glass that are widely used as substrates in various applications. The nanoscale structure, morphology and chemical compositions of the as-electrodeposited SnO were characterized by field emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). Our results showed that SnO nanobelts with very smooth and flat surfaces could be electrodeposited on Si(100). While the vertically grown nanobars of SnO, including small number of nanobelt, nanodisks and aggregated nanoparticles, could be synthesized on ITO. Photocatalytic, antireflective, capacitive, and electrochemical sensing behavior of the SnO nanostructures were explored using cyclic voltammetry, UV–vis absorption and reflection spectroscopy and photocurrent measurements. The as-electrodeposited SnO nanobelts on Si(100) exhibit an antireflective effect (reduction in reflectance from 75% to 15% in the 200–400 nm wavelength range) and the high electrochemical double layer capacitance (Csp: 4.63 mF/cm2). In addition, these SnO nanobelts on Si(100) showed an excellent amperometric response toward H2O2 sensing with a low detection limit of 5 μM and a high sensitivity of 2.6 μA/μM cm2. The fast and uniform photocurrent responses were observed for the as-electrodeposited SnO nanobars on the ITO electrodes under visible light.

Preparation of electrospun heterostructured hollow SnO2/CuO nanofibers and their enhanced visible light photocatalytic performance

Heterostructured SnO2/CuO nanofibers with a hollow morphology were successfully fabricated by a one-step electrospinning method. The electrospun nanofibers were transformed into hollow nanostructures in the presence of camphene after a calcination process, and the obtained samples were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), UV-vis diffuse reflection spectroscopy (DRS), photoluminescence spectra (PL), and photodegradation measurements. The scanning electron microscopy (SEM) images displayed a rough and hollow structure for the obtained nanofibers. X-ray photoelectron spectroscopy (XPS) and energy-dispersive X-ray spectroscopy (EDX) identified the molecular composition and chemical interactions of the nanofibers. Photoluminescent (PL) measurements indicated that a recombination of the photoinduced electrons and holes was further inhibited due to the hollow nanostructure. Furthermore, the photodegradation of methylene blue suggested that the heterostructured SnO2/CuO hollow nanofibers possessed higher charge separation and photodegradation abilities than those of the other samples under visible light irradiation. This work can be potentially applied to the fabrication of other inorganic oxide photocatalysts with enhanced photodegradation activity in the field of environmental remediation.

Predicting potentially toxic elements in tropical soils from iron oxides, magnetic susceptibility and diffuse reflectance spectra

Environmental impacts can be more accurately assessed with the aid of spatial characterization of potentially toxic elements (PTEs). In fact, developing cost-effective, environmentally friendly spatial characterization methods for PTEs can facilitate the expeditious, accurate, detailed diagnosis of soil in large areas. In this study, we used three geomorphic surfaces of Oxisols to assess the ability of chemical and X-ray diffraction analyses of iron oxides, diffuse reflectance spectroscopy (DRS) and magnetic susceptibility (MS) measurements to predict the contents in PTEs (Ba, Co, Cr, Cu, Mn, Mo, Ni, Pb and Zn) and their spatial variability. Magnetic susceptibility and diffuse reflectance spectroscopy allowed well-calibrated prediction models for Ba, Co, Cu, Mn and Ni to be developed, whereas DRS-calibrated methods afforded more accurate prediction of Ba and Mn contents, and magnetic susceptibility-calibrated methods of Co and Ni contents. The correlation between PTEs and free iron contents, and their spatial pattern, testifies to the goodness of the proposed methods for predicting the contents of potentially toxic elements in soils.

Effect of electron irradiation on the optical properties of SrTiO3: An experimental and theoretical investigations

Diffuse reflectance spectroscopy (DRS) measurement has been carried out on pure and electron irradiated single crystal samples of SrTiO3. Interestingly, it has been observed that electron-irradiated sample have new absorption peak in optical absorption spectra. In order to understand the origin of a new absorption peak in electron irradiated sample, first principle calculation for optical spectra considering defects at Sr, Ti and O site using GGA + U approximation have been carried out. It has been observed that the features present in the experimental optical absorption spectra match with that of simulated one having oxygen vacancy. Present investigation suggests that high energy electron irradiation produces oxygen defects in SrTiO3 and DRS is found to be a useful technique to probe the signature of such defects experimentally.

Composites of Laminar Nanostructured ZnO and VOx-Nanotubes Hybrid as Visible Light Active Photocatalysts

A series of hybrid heterostructured nanocomposites of ZnO with V2O5 nanotubes (VOx-NTs) in different mixing ratios were synthesized, with the aim of reducing the recombination of photoinduced charge carriers and to optimize the absorption of visible light. The study was focused on the use of heterostructured semiconductors that can extend light absorption to the visible range and enhance the photocatalytic performance of ZnO in the degradation of methylene blue as a model pollutant. The addition of VOx-NTs in the synthesis mixture led to a remarkable performance in the degradation of the model dye, with hybrid ZnO (stearic acid)/VOx-NTs at a ratio of 1:0.06 possessing the highest photocatalytic activity, about seven times faster than pristine zinc oxide. Diffuse reflectance spectroscopic measurements and experiments in the presence of different trapping elements allowed us to draw conclusions regarding the band positions and photocatalytic degradation mechanism. The photocatalytic activity measured in three subsequent cycles showed good reusability as no significant loss in efficiency of dye degradation was observed.

Influence of reaction time on the structural, optical and electrical performance of copper antimony sulfide nanoparticles using solvothermal method

The less toxic and cost effective ternary Cu-Sb-S nanoparticles and thin films were synthesized and deposited using solvothermal and drop casting method. The reactions were carried out at different timings as 12–48 h, in steps of 12 h using ethylene glycol as solvent and polyvinylpyrrolidone (PVP) as surfactant. Systematic analysis revealed that due to the influence of different reaction time, significant and unique changes were occurring on the crystal structure, optical and electrical properties of the material. The synthesized nanopowders and deposited films were characterized by means of X-ray diffraction, Raman analysis, field emission scanning electron microscope with energy dispersive spectrometer, UV–Vis–NIR diffuse reflectance spectroscopy and hall measurement. XRD results showed that as the time increases crystallinity improves and phase transformation from chalcostibite to tetrahedrite occurs. The Optical performance revealed that the bandgap of nanoparticles were in the range of 1.21–1.49 eV. Hall measurements showed that the deposited Cu12Sb4S13 and CuSbS2 films exhibited p-type conductivity with carrier concentration ranging from 1016-1019 cm−3, indicating a promising p-type absorber material for photovoltaic applications.

CdZnO coated film: A material for photovoltaic applications

Abstract The present study reports structural and optical parameters of wide band gap oxide thick film prepared by screen-printing followed by sintering route. Characterization of the samples was carried out with UV-spectroscopy, XRD, SEM, and Photoluminous study. The XRD and SEM studies reveal that the film deposited is polycrystalline, double phase, and porous with unsymmetrical grain distributions. Optical diffused reflection spectroscopy and Pl measurements give optical band gap of 2.87 eV and near band edge emission at 430 nm.

Petal-like CdS nanostructures coated with exfoliated sulfur-doped carbon nitride via chemically activated chain termination for enhanced visible-light–driven photocatalytic water purification and H2 generation

Novel composite photocatalysts consisting of petal-like cadmium sulphide (CdS) nanoparticles and varying amounts of exfoliated sulfur-doped carbon nitride (SCN) were successfully prepared. The as-obtained materials were characterized by field emission scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, ultraviolet–visible diffuse reflection spectroscopy, photoluminescence spectroscopy and photocurrent-time measurement. Results indicate a strong electric coupling interaction between SCN and CdS due to the heterojunction formed at the amine functionalities sites and oxidized chain terminations of SCN. Two typical pollutants like rhodamine B (RhB) and colorless bisphenol A (BPA) were used for the evaluation of photocatalytic activity. The best-performing CdS/SCN composite (i.e., CS5) synthesized exhibited enhanced visible-light-driven photocatalytic RhB efficiency of about 8.71 and 4.06 times higher than those of pure exfoliated SCN and CdS, respectively. As for BPA degradation, the CS5 composite was 9.00 and 3.61 times more efficient than that of exfoliated SCN and CdS, respectively. These excellent performances were found to be attributable to the remarkable charge carrier separation between CdS and exfoliated SCN with the aid of heterojunction interfacial structures. More importantly, the exfoliated SCN substantially reinforced the photostability of the CdS nanoparticles. Evaluation of the photocatalytic H2 evolution showed that the visible-light H2 production rate of the best-performing CS5 composite was also much greater than the constituents at 247.72 μmol h−1 g−1. Cyclic tests demonstrated the stability of the CS5 composite over repeated use. A possible mechanism was proposed to explain the photocatalytic reaction process. This study provides new insights into the preparation of highly efficient and stable sulfide-based composite photocatalysts, which are promising for implementation in wide-ranging environmental applications.

Dynamical anisotropic response of black phosphorus under magnetic field

Black phosphorus (BP) has emerged as a promising material candidate for next generation electronic and optoelectronic devices due to its high mobility, tunable band gap and highly anisotropic properties. In this work, polarization resolved ultrafast mid-infrared transient reflection spectroscopy measurements are performed to study the dynamical anisotropic optical properties of BP under magnetic fields up to 9 T. The relaxation dynamics of photoexcited carrier is found to be insensitive to the applied magnetic field due to the broadening of the Landau levels and large effective mass of carriers. While the anisotropic optical response of BP decreases with increasing magnetic field, its enhancement due to the excitation of hot carriers is similar to that without magnetic field. These experimental results can be well interpreted by the magneto-optical conductivity of the Landau levels of BP thin film, based on an effective k · p Hamiltonian and linear response theory. These findings suggest attractive possibilities of multi-dimensional control of anisotropic response (AR) of BP with light, electric and magnetic field, which further introduces BP to the fantastic magnetic field sensitive applications.

Principal frequency component analysis based on modulate chopper technique used in diffuse reflectance spectroscopy measurement.

Diffuse reflectance spectroscopy (DRS) is significantly affected from the interference of the ambient light and dark current of the instrument. Optical choppers, together with lock-in/synchronous amplification, can overcome these interferences. However, in spectral measurement, the sampling rate of the spectrometer is different from the Δ-pulse sampling, which is not high enough because of the integration time. In addition, the energy distribution is not perfectly concentrated as expected in modulate chopper technology. Therefore, in this study, based on the modulate chopper technique, we proposed a principal frequency component analysis (PFCA) method for DRS. This technique not only effectively eliminated the interference and dark current of the instrument but also improved the measurement precision using the energy of different frequencies. First, experiments were designed to successfully verify the function of optical choppers, eliminating the interference of the ambient light. Second, a set of 64 mixture solutions was designed and measured by DRS using the PFCA method to prove the feasibility of the proposed method. The solution was mixed with intralipid-20% suspension, India ink, and rhodamine B. These samples were analyzed by DRS under different conditions: no-chopper with overlapping and averaging, chopper demodulated by Fourier transform, and chopper demodulated by PFCA. The partial least square regression analysis was implemented to predict the concentration. Compared to the result of three methods, DRS equipped with chopper using the PFCA method showed the best results. The results of this study showed that the PFCA method not only satisfactorily eliminated the interference signals but also extracted useful information as much as possible, improving the analysis accuracy.

Quasi-two-dimensional thermoelectricity in SnSe

Stannous selenide is a layered semiconductor that is a polar analogue of black phosphorus, and of great interest as a thermoelectric material. Unusually, hole doped SnSe supports a large Seebeck coefficient at high conductivity, which has not been explained to date. Angle resolved photo-emission spectroscopy, optical reflection spectroscopy and magnetotransport measurements reveal a multiple-valley valence band structure and a quasi two-dimensional dispersion, realizing a Hicks-Dresselhaus thermoelectric contributing to the high Seebeck coefficient at high carrier density. We further demonstrate that the hole accumulation layer in exfoliated SnSe transistors exhibits a field effect mobility of up to $250~\mathrm{cm^2/Vs}$ at $T=1.3~\mathrm{K}$. SnSe is thus found to be a high quality, quasi two-dimensional semiconductor ideal for thermoelectric applications.

Photoelectrochemical studies of InGaN/GaN MQW photoanodes

The research interest in photoelectrochemical (PEC) water splitting is ever growing due to its potential to contribute towards clean and portable energy. However, the lack of low energy band gap materials with high photocorrosion resistance is the primary setback inhibiting this technology from commercialisation. The ternary alloy InGaN shows promise to meet the photoelectrode material requirements due to its high chemical stability and band gap tunability. The band gap of InGaN can be modulated from the UV to IR regions by adjusting the In concentration so as to absorb the maximum portion of the solar spectrum. This paper reports on the influence of In concentration on the PEC properties of planar and nanopillar (NP) InGaN/GaN multi-quantum well (MQW) photoanodes, where NPs were fabricated using a top-down approach. Results show that changing the In concentration, while having a minor effect on the PEC performance of planar MQWs, has an enormous impact on the PEC performance of NP MQWs, with large variations in the photocurrent density observed. Planar photoanodes containing MQWs generate marginally lower photocurrents compared to photoanodes without MQWs when illuminated with sunlight. NP MQWs with 30% In generated the highest photocurrent density of 1.6 mA cm−2, 4 times greater than that of its planar counterpart and 1.8 times greater than that of the NP photoanode with no MQWs. The InGaN/GaN MQWs also slightly influenced the onset potential of both the planar and NP photoanodes. Micro-photoluminescence, diffuse reflectance spectroscopy and IPCE measurements are used to explain these results.

Broadband spectroscopy for characterization of tissue-like phantom optical properties

Abstract Optical phantoms are widely used for evaluating the performance of biomedical optical modalities, and hence, absorbing and scattering materials are required for the construction of optical phantoms. Towards that aim, new readily available and inexpensive black Ink (Parker) as a simulating absorber as well as Intralipid 20% as a simulating scatterer are thoroughly investigated. Broadband Transmittance and Diffuse reflectance spectroscopic measurements were performed in the visible range 400 – 700 nm. Optical properties of the phantom materials are determined. Analytical expressions for absorption and scattering coefficient related to the concentrations and wavelength of the Parker ink and Intralipid are also presented and discussed. The results show nonlinear trend in the absorption coefficient of Parker ink over the examined visible spectral range. Furthermore, Intralipid scattering coefficient variation across the mentioned spectral range shows a tissue-like scattering trend. The findings demonstrate the capability of the broadband transmission and diffuse reflectance for characterizing tissue-like phantom materials in the examined spectral range.

Microstructure, magnetic, optical and catalytic activity of Li–Co–Cd nanoferrites

Nanoparticles of Lithium mixed ferrite of the chemical formula Li0.1 (Co1 − x Cdx)0.8 Fe2.1O4; x = 0.0, 0.2, 0.4, 0.6 and 0.8, had been successfully prepared by sol–gel technique. Samples were structurally characterized by X-ray diffraction, Energy dispersive spectroscopy, Transmission Electron Microscope and Fourier Transform infra-red spectroscopy. From vibrating sample magnetometer measurements, saturation magnetization and coercivity were determined. The saturation magnetization and coercive field, generally, decreased with x. The Curie temperature Tc is determined through the change of relative permeability with temperature. The direct energy gap values for the synthesized samples were determined from the diffused reflectance spectroscopy measurements using double beam spectrometer. In order to study the oxidation of methylene blue, H2O2 and nanopowder of ferrites were added to the aqueous solution of MB with stirring and the efficiency of the catalyst was evaluated.

Study of Semimagnetic Mn-Doped WO3 Nanoparticles Synthesised by Precipitation Method: Hydrogenation Creates a Promising DMS

Abstract Tungsten oxide (WO3) nanoparticles doped with different amounts of manganese ions (W1−x Mn x O3, where x = 0.011, 0.022 and 0.044) were synthesised by hydraulic acid-assisted precipitation, followed by thermal calcinations. The powders were characterised by X-ray fluorescence (XRF), X-ray diffraction (XRD), diffuse reflectance spectroscopy (DRS) and magnetic measurements. The monoclinic structure at room temperature (∼293 K) found for un-doped WO3 was preserved even with Mn doping. However, doping with Mn ions caused decease in unit-cell volume and slight increase in crystallite size (CS) of host WO3. The hydrogenation was observed to corrode the crystallites without changing in crystalline structure. Controllable room-temperature ferromagnetic (RT-FM) properties were obviously observed with hydrogenated WO3 doped with Mn. In addition, there existed an optimum doping concentration of Mn in WO3 to obtain superior FM properties. Therefore, Mn-doped WO3 nanopowders, owning to these amazingly tunable magnetic properties, could be considered a potential candidate for many applications partially required FM properties such as optical phosphors and catalysts.

Iodobismuthate(III) and Iodobismuthate(III)/Iodocuprate(I) Complexes with Organic Ligands

Pyridine ligand complexes of [Bu4N][BiI4] were prepared using chelating ligands 2,2′‐bipyridyl (2,2′‐Bpy), 1,10‐phenanthroline (Phen), and 4‐nitro‐1,10‐phenanthroline (NO2Phen), producing monomeric complexes [Bu4N][(2,2′‐Bpy)BiI4], [Bu4N][(Phen)BiI4], and [Bu4N][(NO2Phen)BiI4], and bridging ligands 4,4′‐bipyridyl (4,4′‐Bpy), pyrazine (Pyz), and aminopyrazine (NH2Pyz) resulting in formation of polymers [Bu4N]n[(4,4′‐Bpy)BiI4]n and [Bu4N]2n[(RPyz)Bi2I8]n (R = H, NH2). The latter contain edge‐sharing Bi2I8 dimers. Organic ligand BiIII/CuI clusters [Bu4N]2[L2Bi2Cu2I10] {L = PPh3, P(OPh)3} and [Bu4N]2[PyBi2Cu2I10] (Py = pyridine) have been prepared. All bismuthate(III) centers are distorted octahedra and all cuprate(I) centers are tetrahedral, with organic ligands bonded to CuI. The first neutral BiI3/CuI organic ligand complex [BiCu3I6(PPh3)6] is reported. Diffuse reflectance spectroscopy measurements reveal strong absorption bands for both iodobismuthate(III) and iodocuprate(I)/bismuthate(III) complexes in the UV and visible range. Despite the similarity in absorption bands, DFT calculations support a distinct shift in transition from a mixed halide/metal‐to‐ligand charge transfer (X/MLCT) to a metal–halide cluster‐centered transition upon incorporation of copper(I) into the cluster.

Structural, magnetic, dielectric, vibrational and optical properties of Zr substituted Bi0.90Gd0.10FeO3 multiferroics

We report the evolution of structural, physical and magnetic properties of Zr doped Bi0.90Gd0.10Fe1-xZrxO3, (x = 0.0, 0.03, 0.07 and 0.10) nanoparticles prepared by tartaric acid based sol gel route. Rietveld refinement of X-ray diffraction patterns shows the coexistence of rhombohedral (R3c) and orthorhombic (Pbnm) phase in all the samples. Morphological studies confirm the compositional driven reduction in particle size with increasing Zr concentration due to filling of oxygen vacancies and hence lower grain growth. The magnetic measurements at room temperature indicate that Zr substitution induces weak ferromagnetism and discerns large and non-zero remanant magnetization. Both dielectric constant (ε′) and loss factor (tan δ) of these samples decrease with increasing Zr concentration, indicating improved dielectric properties due to the suppression of oxygen vacancies in co-doped compositions. An anomaly in dielectric constant and dielectric loss is observed in the vicinity of antiferromagnetic Neel temperature. Fourier transform infrared spectra show that the Fe–O bond length decreases with increasing Zr concentration which is consistent with the Rietveld analysis. The UV–visible diffuse reflectance spectroscopy measurements show a significant reduction in the optical band gap value from 2.35 eV to 2.05 eV for Bi0.90Gd0.10Fe1-xZrxO3 samples.

TEM and AFM studies of aluminium nitride films synthesized by pulsed laser deposition

Aluminium nitride (AlN) films were synthesized by pulsed laser ablation of poly-AlN target on Si(100) substrates using a KrF* excimer laser source (λ = 248 nm, τFWHM ≤ 25 ns), with incidence laser fluence of ~ 3 J/cm2 and laser pulse repetition frequencies (LPF) of 3, 10 and 40 Hz, respectively. The depositions were performed in nitrogen pressure of 0.1 Pa and at substrate temperatures of 450 and 800 °C. The AlN structures were studied by transmission electron microscopy, atomic force microscopy (AFM), Fourier transform infrared reflectance (FTIR) spectroscopy and spectroscopic ellipsometry (SE) measurements. The results show that at 450 °C and LPF of 3 Hz the AlN film is entirely amorphous, while at LPF of 10 and 40 Hz nanocrystallites with h-AlN phase appear in the grown films. At 800 °C, well-textured h-AlN nanocrystallites with columnar grains are formed. Growth of nanocrystallites in the 450 °C AlN films, similar to films grown at 800 °C, is possible when the films are deposited onto a high-temperature AlN “seed” layer, as they follow the columnar structure but with small-sized crystallites and a weaker texturing. AFM imaging reveals increasing surface roughness with the degree of crystallinity in the synthesized films. The structural changes are well correlated with the variation in the optical parameters registered by FTIR and SE.

Microstructural and light emission properties of ZnSnP2 thin film absorber: Study of native defects

Thin films of ZnSnP2 were successfully grown on p-type silicon (001), sapphire and glass substrate by e-beam evaporation method. The as-deposited films were characterized using scanning and high resolution transmission electron microscopes, reflectance and transmittance spectroscopy and photoluminescence measurement at low temperature. Polycrystalline nature of the films was verified by high resolution transmission electron microscopy. An optical band gap of 1.71 eV was estimated at room temperature. Two broad luminescence bands at 1.529 eV and at 1.634 eV were observed at 15 K. The light emission characteristics of the ZnSnP2 were explained in terms of donor-acceptor pair recombination mechanism. Tin-on-zinc sites and zinc vacancies/zinc-on-tin sites were considered as donor and acceptor due to presence of native defects in the films. A schematic band model based on the experimental finding was suggested to account for the DA pair recombination.