Optical Reflectance Spectra Research Articles

Modified polymer network gel preparation on Ag/ZnO quasi sphere nanostructure with enhanced structural and optical properties

In this present work, we investigated the structural, optical and morphological properties of Ag/ZnO Nanostructure prepared via modified polymer network sol-Gel method. Ag/ZnO nanostructures were synthesized with different initial concentrations of Ag precursor. The prepared samples were characterized by XRD, FT-IR, UV, PL and FESEM analysis. The studies reveals that ZnO nanoparticles are belongs to Hexagonal phase structure and it is confirmed by XRD analysis. In the optical reflection spectra of Ag/ZnO Nanostructure, the surface Plasmon resonance peak appears due to Ag Nanoparticles at 398 nm and that confirms the incorporation of Ag+ ions in the ZnO lattice structure. The Tauc plot revealed that Band gap value of Ag/ZnO is smaller than pure ZnO. Moreover Ag/ZnO Nanostructure exhibited the enhanced optical property.

High throughput fabrication of large-area colloidal crystals via a two-stage electrophoretic deposition method

An effective and high-throughput method is demonstrated to produce a colloidal crystal with impressive crystallinity. The fabrication procedure involves a two-stage electrophoresis process in which a vertical electrophoretic deposition of polystyrene microspheres (495 nm) is conducted on an ITO substrate to form a colloidal crystal in size of 4 × 4 cm2, followed by a horizontal electrophoresis to improve the packing crystallinity. Relevant parameters associated with the horizontal electrophoresis step including voltage polarity, bipolar voltage offset, and operation frequency are investigated. Their effects over the crystallinity of the colloidal crystals are verified quantitatively and qualitatively by structural observation via scanning electron microscope and optical reflectance spectra. We determine that the imposition of an alternately-changing bias of 80/0/-120/0 V at 10 Hz (25 ms in each bias) enables the production of colloidal crystals with the largest grain size (106 μm) and the smallest grain boundary (170 nm), as compared to samples prepared from a single vertical electrophoresis step. We attribute the improved crystallinity of the colloidal crystal to the bipolar bias that engenders localized movements of the microspheres, rendering them to pack more closely with reduced defects.

Microstructure, optical and electrical properties of thin films of gallium-phosphorus-titanium alloys synthesized by asymmetric bipolar pulsed direct current magnetron sputtering

Thin films of gallium-phosphorus-titanium (Ga-P-Ti) alloys were prepared on glass substrates at 573 K by an asymmetric bipolar pulsed direct current sputtering technique using an argon atmosphere and targets made from gallium phosphide (GaP) powder and metallic titanium (Ti), at the surface ratios of 8:1, 5:1, 2:1 and 1:1 GaP to Ti (GaP:Ti) on the sputtered area. Examination by X-ray diffraction, transmission electron microscopy, and field emission scanning electron microscope indicated that the as-deposited films from the sputtering targets having GaP:Ti ratios of 8:1, 5:1, and 2:1 were polycrystalline with the cubic zinc-blende crystal structure having GaP as the host material, i.e., Ti-doped GaP. Elemental compositions of the film obtained from the target at a GaP:Ti ratio of 5:1 closely resembled the theoretically predicted intermediate band compound Ga4P3Ti. It was projected that the Ga4P3Ti compound could be fabricated by co-sputtering of GaP and Ti from a single target having the surface area ratio GaP:Ti of 3.5:1. Optical transmission and reflection spectra, temperature dependence of electrical resistivity, and light response of the electrical resistivity showed semiconductor-like behavior for the films obtained from the targets with the GaP:Ti of 8:1 and 5:1, and were metal-like for those deposited from the other targets. Optical band gaps determined from the transmission spectrum of the semiconducting films by Tauc's expression for indirect transition were 1.2–1.5 eV. The results of the study could provide an alternative route for fabricating the intermediate band material based on the Ga-P-Ti system.

Localized Surface Plasmon Resonance in Gold Nanocluster Arrays on Opaque Substrates

We report on the investigation of the localized surface plasmon resonance (LSPR) in periodical Au nanostructures. The arrays of Au nanoclusters and dimers were fabricated on Si and Si/SiO2 surfaces by electron beam lithography. Diameters and periods of nanoclusters with disk shape vary in the range of 30–150 and 130–200 nm, respectively. Because of the opaque nature of the substrates, optical reflection spectroscopy was chosen to probe the plasmonic properties of the metal nanostructures. From a comparison of experimental reflection spectra with those numerically simulated by the Finite Difference Time Domain (FDTD) method, we determined the model structural parameters of the plasmonic nanostructures. These parameters were further used for the calculation of absorptance spectra of the plasmonic structures for which absorptance in the substrate was subtracted. LSPR positions were determined from the maxima of the absorptance spectra. This study reveals a strong dependence of the LSPR position on nanocluster size, distance between nanoclusters, as well as on the SiO2 layer thickness in the nanometer range. In the case of dimer arrays, the plasmon anisotropy in the dimers leads to a splitting of the LSPR plasmon into two modes with orthogonal polarizations. The absorptance spectra reveal a transverse LSPR mode corresponding to the excitation of plasmons in nanoclusters induced by scattered fields from the neighboring ones. This research provides a pathway for a fast and cost-effective determination of the LSPR position from optical reflection spectra. A broad field of potential applications of metal structures with well-controlled plasmonic properties includes surface-enhanced infrared absorption, photoluminescence, and Raman scattering as well as signal transmission in silicon photonics.

A first-principles study of the properties of P-43m-Si3×2 (X = N, P and As)

The new P-43m-Si3P2 and P-43m-Si3As2 structures are predicted using the first-principles approach based density functional theory (DFT). The elastic constants, structural stability, phonon dispersion spectra, band structures, density of states, and optical properties of P-43m-Si3×2 (X=N, P and As) have been analyzed. The values of the elastic constants indicate that their structures are mechanically stable. Each elastic constant of the Si3N2 is greater than the corresponding elastic constants of Si3P2 and Si3As2. The Young's moduli, shear moduli, bulk moduli, Pugh ratios and Poisson's ratios of P-43m-Si3×2 are calculated at 0 GPa. Si3N2 has a larger Young's modulus, so it has higher hardness and good resistance to deformation. The bulk moduli of P-43m-Si3×2 are isotropic. The shear modulus of Si3As2 is anisotropic. The Pugh ratios of P-43m-Si3×2 are 0.50, 0.49 and 0.39, respectively. Their Poisson's ratios are 0.28, 0.29 and 0.33, respectively. The results show that they are brittle materials at zero pressure. The calculated phonon spectra confirm that they are dynamically stable. The calculated enthalpy of formation indicates their thermodynamic stability. The energy band gaps of P-43m-Si3×2 calculated by HSE06 hybrid function are 0.786, 0.955 and 0.343 eV, respectively. Si3N2 has a direct bandgap, Si3P2 and Si3As2 have indirect bandgaps. The dielectric functions, refractive indices, optical reflectance spectra, absorption coefficients, conductivities and loss functions of P-43m-Si3×2 are calculated. The calculated static dielectric constants of P-43m-Si3×2 are 5.207, 9.237 and 10.072, respectively. The maximum values of the loss functions of P-43m-Si3×2 are 6.408, 5.672 and 5.276 eV, respectively.

Aluminum film over nanosphere surface for Deep Ultraviolet plasmonic nanosensors

Deep ultraviolet (DUV) plasmonic nanosensors are emerging as potential candidates to detect biomolecules with enhanced sensitivity, as these molecules strongly absorb light in the DUV region. In the present work, we fabricated aluminium film over nanospheres surface (AlFON) and successfully demonstrated, for the first time, an AlFON based DUV plasmonic nanosensor for detecting volatile organic compounds (VOCs) such as ethanol, isopropyl alcohol (IPA) and toluene, and bovine serum albumin (BSA) protein. The AlFON was fabricated by thermally evaporating aluminium thin film onto a two-dimensional photonic crystal (2DPC). The 2DPC was prepared by drop casting colloidal polystyrene spheres (PSS) on quartz substrates that self-assembled to form a monolayer of hexagonally close packed PSS with periodic interstices. The optical reflection spectrum of 70 nm AlFON exhibited a sharp minimum at 289 nm in the DUV wavelength range which corresponds to strong absorption in V-shaped nanocavities due to excitation of localized cavity plasmons. The reflection minimum—or LSPR wavelength—is sensitive to the changes in the refractive index of the surrounding medium. The LSPR mode of the V-shaped nanocavities in 70 nm AlFON was exploited to sense VOCs and BSA. The 70 nm AlFON exhibited 244 nm/RIU sensitivity in detecting VOCs and was capable of sensing 1 mM concentration of BSA in water. The AlFON is physically stable and has a self-limiting Al2O3 oxide layer of less than 5 nm thickness as elucidated from x-ray photoelectron spectroscopy measurements. The effect of higher aluminum film thickness, i.e. 100 nm and 120 nm on the sensitivity of AlFON also was studied. The performance of AlFON based DUV plasmonic nanosensors is better than the previously reported Al nanostructure based nanosensors and can still be enhanced by optimizing the structure of 2DPC and AlFON.

Estimation of tissue oxygen saturation from RGB images and sparse hyperspectral signals based on conditional generative adversarial network

PurposeIntra-operative measurement of tissue oxygen saturation ({hbox {StO}}_2) is important in detection of ischaemia, monitoring perfusion and identifying disease. Hyperspectral imaging (HSI) measures the optical reflectance spectrum of the tissue and uses this information to quantify its composition, including {hbox {StO}}_2. However, real-time monitoring is difficult due to capture rate and data processing time.MethodsAn endoscopic system based on a multi-fibre probe was previously developed to sparsely capture HSI data (sHSI). These were combined with RGB images, via a deep neural network, to generate high-resolution hypercubes and calculate {hbox {StO}}_2. To improve accuracy and processing speed, we propose a dual-input conditional generative adversarial network, Dual2StO2, to directly estimate {hbox {StO}}_2 by fusing features from both RGB and sHSI.ResultsValidation experiments were carried out on in vivo porcine bowel data, where the ground truth {hbox {StO}}_2 was generated from the HSI camera. Performance was also compared to our previous super-spectral-resolution network, SSRNet in terms of mean {hbox {StO}}_2 prediction accuracy and structural similarity metrics. Dual2StO2 was also tested using simulated probe data with varying fibre number.Conclusions{hbox {StO}}_2 estimation by Dual2StO2 is visually closer to ground truth in general structure and achieves higher prediction accuracy and faster processing speed than SSRNet. Simulations showed that results improved when a greater number of fibres are used in the probe. Future work will include refinement of the network architecture, hardware optimization based on simulation results, and evaluation of the technique in clinical applications beyond {hbox {StO}}_2 estimation.

The band structure of CuInTe2 studied by optical reflectivity

CuInTe2 is a semiconductor with high potential for use as a thermoelectric material and as the absorber in thin film solar cells. Studying the optical reflectivity spectra of CuInTe2 single crystals resolves resonances at 1.054 eV and 1.072 eV, which are assigned to the A and B free excitons. Photoluminescence spectra exhibited a peak due to the A free exciton at 1.046 eV. Varshni coefficients were found for both excitons. Zero temperature bandgaps EgA = 1.060 eV and EgB = 1.078 eV were determined for the A and B valence sub-bands, respectively. The splitting due to crystal-field ΔCF and spin-orbit effects ΔSO were calculated as −26.3 meV and 610 meV, respectively, using the determined EgA and EgB and a literature value of EgC.

P-Si(100)/InGaN thin film structure and investigation of its physical properties: N2 gas flow effect

Abstract In this study, effect of N2 gas flow rates on structural, optical and morphological properties of InGaN thin films grown by Radio Frequency Magnetron Sputtering (RFMS) method on p-Si (100) substrate have been investigated. X-ray diffraction (XRD) measurements confirmed that InGaN thin films have been successfully deposited and exhibited diffraction peak belong to (002) plane for each film. The surface roughness of InGaN thin films has been determined by using Atomic Force Microscopy (AFM) and the roughness value is about 13 nm with a maximum height of 36 nm and a maximum depth of 37 nm. The surface roughness value Rq\RMS value is 15 nm, which is almost consistent with the linear roughness value for 0 sccm. The energy band gap of the film was determined by using the optical reflectance spectra and the value of the band gap energy is found to be 2.01, 1.77 eV and 2.26 eV for 0 sccm, 1 sccm and 2 sccm N2 gas flow rates, respectively

Локализация экситона Ванье-Мотта на органополупроводниковом интерфейсе ленгмюровская пленка / CdS"

Low temperature (T=2 K) optical reflection spectra have been studied for the organic-semiconductor structures prepared by deposition of Langmuir-Blodgett films on CdS surface. The spectra are measured in the resonant spectral range of the exciton state in CdS. Theoretical analysis of the spectra is carried out with an account of the spatial dispersion effects and the exciton-free surface “dead” layer. The conclusion has been drawn of the interface localization of the Wannier-Mott exciton due to organic film deposition on the crystal surface.

Sol-Gel Spin Coating Synthesis of TiO<sub>2</sub> Nanostructure and Its Optical Characterization

This work focuses on the sol-gel spin coating technique of TiO2 nanostructure synthesis and its characterization. Though various methods have been used to fabricate TiO2 nanostructure, much effort has not been exerted to achieve better photoresponsive and narrowly dispersed TiO2 nanostructure using the sol-gel spin coating method. Therefore, it is imperative to realize the synthesis of TiO2 nanostructures, and investigate their properties. In this work, TiO2 is synthesized by sol-gel spin coating technique using titanium tetraisopropoxide, isopropanol, acetic acid and deionized water as starting materials and deposited on borosilicate glass substrates. The effects of annealing temperatures (300˚C, 400˚C and 500˚C) on the structural and optical properties of the films were investigated by different techniques: Scanning Electron Microscopy (SEM), optical microscopy and UV-visible spectrophotometry. The optical characterization showed the direct band gap at 3.7 eV, 3.6 eV and 3.4 eV for 300˚C, 400˚C and 500˚C, respectively, and the optical transmittance and reflectance spectra showed a greater performance at 500˚C. The grain sizes obtained from SEM annealed at 300˚C, 400˚C and 500˚C are found to be about 6.0 nm, 5.0 nm and 4.0 nm respectively. The grain size of TiO2 nanostructure films decreased with increasing annealing temperatures. The results clearly indicated that the sol-gel spin coating synthesis of TiO2 nanostructure and post-thermal treatment at 500˚C cooled naturally at room temperature result in better photoresponsive and narrowly dispersed TiO2 nanostructure films with higher photoresponsive and good optical properties.

Changes of optical transition models caused by crystal structural changes in CaSe2O5

The present work concerns calcium pyroselenites CaSe2O5 with two independent crystal structures, their difference physical properties and phase transformations. α-CaSe2O5 in orthorhombic form shows 2D calcium oxide anionic layers whereas new compound β-CaSe2O5 crystallizes in monoclinic space group C2/c. It's a new structure type with 1D calcium oxide thick chains which are further connected by Se2O5 dimers. Rietveld refinement indicates that the weight fraction for α/β-CaSe2O5 mixture phase was found to be 84.5% and 15.5% for α- and β-CaSe2O5, respectively. Optical diffuse reflectance spectrum coupled with Kubelka–Munk fit gives a result that α-CaSe2O5 adopts direct optical transition model while β-CaSe2O5 is an indirect semiconductor. The experimental result matches well with the prediction of theory calculation. The optical bandgap values were fitted to be 4.74 and 4.12 eV for α- and β-CaSe2O5, respectively. Thermal analysis coupled with variable temperature powder XRD indicates the phase transformation between these two phases.

Unique 3D framework formed by adding MIIO4 groups into high Sb/P ratio phosphatoantimonates

Abstract Explorations on a mixed metal phosphatoantimonate system led to the discovery of two new anhydrous phosphatoantimonates, namely, Cs4MSb6P4O28 (M=Mg, Zn), which represent the first examples of quinary AI – MII –SbV – PV– O systems. Single-crystal structure analysis reveals that the two title compounds crystallize in the tetragonal space group I41/a (No. 88). Their structures feature complicated 3D frameworks with interesting tunnel structures comprised of corner sharing MO4 tetrahedra, SbO6 octahedra and PO4 tetrahedra, with the Cs+ cations sitting in the tunnels to balance the valence. Optical reflectance spectrum measurements show that these two compounds are insulators with band gaps of about 4.5 eV.

Gold Nanoclusters at the Interface Au/GaAs(001): Preparation, Characterization, and Plasmonic Spectroscopy

Local plasmons of gold nanoclusters formed at Au/GaAs interface are observed and investigated. The gold nanoclusters are prepared by thermal annealing of thin Au films deposited on GaAs(001) surface. In order to prevent chemical interaction between Au and GaAs at high temperatures, a thin layer of GaN is formed on GaAs substrate surface by chemical nitridation in hydrazine sulfide solution prior to deposition of Au. Typical shapes and sizes of Au nanoclusters are obtained from STM characterization of samples in ultrahigh vacuum followed by measuring and calculating their optical reflectivity spectra. In general, the reflectivity spectra of Au/GaAs samples reveal two resonant peaks at the energies about 1.55 and 2.15 eV. The former peak dominates for oxidized GaAs(001) substrates, while the latter peak is clearly visible for nitridized GaAs substrates. Theoretical analysis of the optical spectra of Au/GaAs interface using a model of Au nanospheroids allows us to assign the reflectance peak at 2.15 eV to local plasmons of Au nanoclusters which look like oblate islands on a Au/GaAs(001) sample. In turn, the peak at 1.55 eV is assigned to plasmons of Au nanoclusters prolate perpendicularly to the surface of GaAs crystal in whose near-surface region these Au clusters are buried.

Influence of gold nanoholes and nanoslits arrays on Raman spectra and optical reflectance of graphene oxide.

We report the effect of gold nanostructured substrates, fabricated by interference lithography technique (IL), on the Raman spectra and optical reflectance of graphene oxide (GO) layers. For purposes of comparison two gold nanostructured substrates, nanoslits (AuNSs) and circular nanoholes (AuNHs) were compared with a non-nanostructured gold substrate. Effects induced by the gold nanostructured substrates are discussed in terms of the ID/IG ratio and the FWHM of the G band (FWHM(G)) as a function of the G band intensity (IG), showing that both ID/IG and FWHM(G) parameters are highly sensitive to the number of GO layers (nGO), which would allow to identify the number of GO layers in a reliable way. Optical reflectance spectra (R(λ)) reveal that plasmons are generated on the surface of nanostructured substrates by the incident radiation. Dips in R(λ) are ascribed as coupling by surface plasmon polaritons described by Bloch waves (BW-SPP). A peak in R(λ) is also observed and it is ascribed to visible radiation produced by Förster resonance energy transfer and Purcell effect. The relevance of these results lies in the possibility of designing colorimetric plasmonic sensors, based on few layers of GO with an excellent control of nGO and with potential in detection of molecules by fluorescent absorption.

A Magneto‐Reflectivity Study of CuGaSe2 Single Crystals

CuGaSe2 single crystals are studied using magneto‐reflectivity at 4.2 K in magnetic fields B up to 20 T. The A and B free excitons, observed in the optical reflectivity spectra, blue shift with increasing B. A low‐field perturbation approach within the anisotropic hydrogenic model is used to fit the dependence of the spectral position of these excitons on B. The A and B exciton reduced masses of 0.115 m0 and 0.108 m0 (m0 is the free electron mass), Rydbergs of 12.9 and 12.2 meV, Bohr radii 5.08 and 5.4 nm, and effective hole masses of 0.64m0 and 0.48 m0, respectively, are determined.

A Tunable Nanoplasmonic Mirror at an Electrochemical Interface

Designing tunable optical metamaterials is one of the great challenges in photonics. Strategies for reversible tuning of nanoengineered devices are currently being sought through electromagnetic or piezo effects. For example, bottom-up self-assembly of nanoparticles at solid | liquid or liquid | liquid interfaces can be used to tune optical responses by varying their structure either chemically or through applied voltage. Here, we report on a fully reversible tunable-color mirror based on a TiN-coated Ag substrate immersed in an aqueous solution of negatively charged Au-nanoparticles (NPs). Switching electrode potential can be used to fully control the assembly/disassembly of NPs at the electrode | electrolyte interface within a 0.6 V wide electrochemical window. The plasmon coupling between the electrode and the adsorbed NP array at high positive potentials produces a dip in the optical reflectance spectrum, creating the “absorber” state. Desorption of NPs at low potentials eliminates the dip, returning ...

A correlative approach, combining chlorophyll a fluorescence, reflectance, and Raman spectroscopy, for monitoring hydration induced changes in Antarctic lichen Dermatocarpon polyphyllizum

Lichens are successful colonizers in extreme environments worldwide, and they are considered to have played an important role during the evolution of life. Here, we have used a correlative approach, combining three optical signals (chlorophyll a fluorescence (ChlF), reflectance, and Raman spectra), to monitor hydration induced changes in photosynthetic properties of an Antarctic chlorolichen Dermatocarpon polyphyllizum. We measured these three signals from this lichen at different stages (after 4 h, 24 h, and 48 h) of hydration, and compared the data obtained from this lichen in "dry state" as well as in different "hydrated state". We found that dry state of this lichen has: (1) no variable ChlF, (2) high reflectance, with no red-edge and almost zero photochemical reflectance index (PRI), and (3) low-intensity Raman bands of their carotenoids. Furthermore, 4 h of hydration, increased its relative water content (RWC) by 93%, showed red-edge in reflectance spectra, and changed the maximum quantum yield of PSII photochemistry (Fv/Fm) from 0 to 0.57 ± 0.01. We found that reflectance indices, normalized difference index (NDVI) and PRI, significantly differed between brown and black/green surface areas, at all hydration stages; whereas, a shift in the Raman ν1(CC) band, between brown and black/green surface areas, occurred in 24 h or 48 h hydrated samples. These data indicate that hydration shortly (within 4 h) activated functions of photosynthetic apparatus, and the de novo synthesis of carotenoids occured in 24 h or 48 h. Furthermore, exposure to high irradiance (2000 μmol photons m-2 s-1), in 48 h hydrated lichen, significantly reduced Fv/Fm (signifies photoinhibition) and increased PRI (represents changes in xanthophyll pigments). We conclude that the implication of such a correlative approach is highly useful for understanding survival and protective mechanisms on extremophile photosynthetic organisms.

Hyperspectral imaging system based on a single-pixel camera design for detecting differences in tissue properties.

Optical spectroscopy can be used to distinguish between healthy and diseased tissue. In this study, the design and testing of a single-pixel hyperspectral imaging (HSI) system that uses autofluorescence emission from collagen (400nm) and nicotinamide adenine dinucleotide phosphate (475nm) along with differences in the optical reflectance spectra to differentiate between healthy and thermally damaged tissue is discussed. The changes in protein autofluorescence and reflectance due to thermal damage are studied in ex vivo porcine tissue models. Thermal lesions were created in porcine skin (n=12) and liver (n=15) samples using an IR laser. The damaged regions were clearly visible in the hyperspectral images. Sizes of the thermally damaged regions as measured via HSI are compared to sizes of these regions as measured in white-light images and via physical measurement. Good agreement between the sizes measured in the hyperspectral images, white-light imaging, and physical measurements were found. The HSI system can differentiate between healthy and damaged tissue. Possible applications of this imaging system include determination of tumor margins during surgery/biopsy and cancer diagnosis and staging.

Sol-gel preparation and characterization of Er3+ doped TiO2 luminescent nanoparticles

The present paper reports on down-and up-conversion luminescence behaviour of sol-gel derived erbium doped titanium dioxide with anatase structure. Through combined structural, optical and electron microscope analysis, effective and influence of Er3+ doping into TiO2 lattice has been demonstrated using X-Ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), and optical reflectance spectra. XRD results showed only anatase diffraction peaks of TiO2, indicating the formation of a pure anatase phase even after Er3+ incorporation into TiO2 lattice. Selected Area Electron Diffraction (SAED) confirmed that the synthesized TiO2 nanoparticles are polycrystalline in nature which correlated well with XRD findings. Upon excitation at 320 nm, two down-conversion contributions at 378 nm and 435 nm attributed to indirect band gap and defect-related emissions, respectively were observed from both pure and Er3+ doped TiO2 nanoparticles. On the other hand, strong green up-conversion emission centred at 544 nm ascribed to 4S3/2 → 4I15/2 transition of Er3+ was observed under 980 nm laser excitation for all Er3+ doped TiO2 samples. This result analysis brings insight on understanding of structural, optical and luminescence properties of Er3+ doped TiO2 nanoparticles for use in solar cells and bio imaging devices.