Optical Transmission Spectroscopy Research Articles

Nitrogen doped single layer graphene for CZTS-based thin film solar cells

CdS thin films are commonly utilized as a buffer layer in chalcopyrite thin-film solar cells. However, due to the toxic nature of Cadmium (Cd), ongoing efforts are being directed towards exploring alternative options. In this contribution, doped graphene film with remarkable optical and electrical properties has been introduced for the first time as an alternative buffer layer, replacing CdS in CZTS thin-film solar cell application. In this study, nitrogen-doped graphene (N-doped graphene) film was utilized as a substitute buffer layer in the CZTS thin-film solar cell structure, replacing the conventional CdS thin film. For comparative analysis, CZTS/N-doped graphene and CZTS/CdS traditional solar cell structures were fabricated and separately characterized. The CZTS thin films produced were examined through EDX, XRD, SEM, Raman, optical transmission and PL spectroscopy measurements. According to performed analyses, the Cu-poor and Zn-rich kesterite CZTS thin films exhibited a uniform and dense polycrystalline microstructure as observed in surface and cross-sectional SEM images. XRD spectra of the kesterite CZTS thin film displayed predominant peaks corresponding to the (112), (220/204), and (312/116) diffraction planes of the kesterite CZTS phase. Raman spectra showed a dominant peak at ∼336 cm−1 associated with the kesterite CZTS phase. PL emission spectra indicated transitions from the conduction band to defect levels. The CVD-grown doped graphene film exhibited a 3.43 I2D/IG ratio and a 25 cm−1 FWHM of the 2D peak, indicating a single-layer graphene according to Raman analysis. Permanent nitrogen doping with a 2% atomic concentration was confirmed by XPS measurement. The optical transmission measurement of the single layer doped graphene film showed a 95% transmittance value. Nitrogen doping was contributed to decrease the sheet resistance of the graphene film. The Glass/Mo/CZTS/N-doped graphene/i-ZnO/ITO/Al solar cell displayed a VOC of 0.267 V, JSC of 24.6 mA/cm2, FF of 36.46, and η of 2.37%, showing higher FF and Jsc values but lower conversion efficiency compared to the Glass/Mo/CZTS/CdS/i-ZnO/ITO/Al traditional solar cell structure. Hence, the superior working function and transparency properties position the N-doped graphene film as a competitive buffer layer for use in CZTS-based thin-film solar cells.

Transient Photocurrents and Defect States in Hierarchically Structured ZnO Nanowires

ZnO‐nanostructured thin films can be used as electron transport layers in ordered heterojunction quantum‐dot solar cells. An increased interface between electron and hole‐transporting layers allows for a fast separation of the electron–hole pairs preventing their recombination. Consequently, an improvement of the overall solar cell efficiency is expected. ZnO hierarchical nanorod thin films can provide the desired increase in interface area. In this work, their properties are studied and compared with simple ZnO nanorod films. The ZnO nanorod films are synthesized hydrothermally by placing the quartz/indium‐tin‐oxide substrates in a solution of zinc nitrate hexahydrate and sodium hydroxide at 90 °C. Hierarchical structures are obtained through a second deposition step. Scanning electron microscopy is used to characterize the size and shape of the nanorods. Simple ZnO films display hexagonal nanorods with a diameter of around 150 nm while hierarchical films show additional 40 nm diameter nanorods growing on the side surfaces of the primary 150 nm nanorods. Optical transmittance spectroscopy reveals a bandgap around 3.35 eV. Current–voltage measurements in the high‐field regime of 104 V cm−1 are used to determine the density of trap states Nt through the application of trap‐filled limit current theory.

Asymmetrical Optical Response of Opal Photonic Crystals with Graded Thickness

The influence of thickness gradient and structural order on the spectral response of opal photonic crystals (PhCs) grown by evaporation-induced self-assembly (EISA) are presented. SEM imaging and angle resolved optical transmission spectroscopy are used to investigate the evolution of the PBG along a thickness gradient for opals grown from five different colloidal sphere concentrations at two different evaporation rates. The degradation of structural order along the thickness gradient is demonstrated, the occurrence of which attenuates the PBG with the thinning of the opal film and results in asymmetrical angle-resolved transmission spectra. The asymmetry in transmission intensity becomes more pronounced for opals grown from lower volume fractions, where secondary Bragg reflections also appear at low incident angles.

Detailed Study of the Correlation between Cross-Linking of Thick SU-8 and UV-NIR Optical Transmission/Photoluminescence Spectroscopy.

SU-8 polymers are promising materials for various applications due to their low cost, excellent thermal stability, and outstanding mechanical properties. Cross-linking of SU-8 is a crucial process that determines the properties of the materials. This study investigates the effect of cross-linking of free-standing SU-8 films on optical transmission and PL emission under various curing conditions. Our findings show that an increase in the cross-linking density reduces optical transmission and causes a red shift of the PL emission band peaks. By directly measuring the optical response of the isolated SU-8, we remove any uncertainty due to the substrate's presence. Moreover, we show that optical transmission and PL spectroscopy are two non-distractive techniques that can be employed to monitor the curing of the SU-8. This finding enhances our understanding of the cross-linking process in SU-8 and paves the way to further enhance the properties of the SU-8 polymer for various electronics and optoelectronics applications.

Near- and far-field study of polarization-dependent surface plasmon resonance in bowtie nano-aperture arrays.

Bowtie nano-apertures can confine light into deep subwavelength volumes with extreme field enhancement, making them a useful tool for various applications such as optical trapping, deep subwavelength imaging, nanolithography, and sensors. However, the correlation between the near- and far-field properties of bowtie nano-aperture arrays has yet to be fully explored. In this study, we experimentally investigated the polarization-dependent surface plasmon resonance in bowtie nano-aperture arrays using both optical transmission spectroscopy and photoemission electron microscopy. The experimental results reveal a nonlinear redshift in the transmission spectra as the gap size of the bowtie nanoaperture decreases for vertically polarized light, while the transmission spectra remain unchanged with different gap sizes for horizontally polarized light. To elucidate the underlying mechanisms, we present simulated charge and current distributions, revealing how the electrons respond to light and generate the plasmonic fields. These near-field distributions were verified by photoemission electron microscopy. This study provides a comprehensive understanding of the plasmonic properties of bowtie nano-aperture, enabling their further applications, one of which is the optical switching of the resonance wavelength in the widely used visible spectral region without changing the geometry of the nanostructure.

Correlation between crystallite size, resistance to oxidation, and phase transformation of germanium-doped Fe3O4 nanocrystalline thin films by using a sputtering target of α-Fe2O3

We investigated the correlation between the crystallite size, resistance to oxidation, and phase transition of Ge-doped Fe3O4 nanocrystalline thin films. We prepared thin films on water-cooled glass substrates by radiofrequency sputtering with Ge-tipped α-Fe2O3 ceramic disks as targets. The addition of Ge caused a reductive phase transition from α-Fe2O3 with a corundum structure to Ge-doped Fe3O4 with an inverse spinel structure, resulting in a crystallite size of <10 nm. We annealed three samples with average sizes of 5, 8, and 10 nm by changing Ge addition concentrations at 673 K in air for up to 323 days until reaching near-thermal equilibrium. All samples maintained their magnetization at almost a constant value over long-term heat treatment. The results of Raman spectroscopy and optical transmittance spectroscopy suggest that there was a phase transition to Ge-doped γ-Fe2O3 at 5 nm, Ge-doped Fe3−δO4 with iron vacancies δ at 8 nm, and Ge-doped Fe3O4 at 10 nm. In other words, we retained Fe3O4 down to a relatively small crystallite size of 10 nm because of the improved oxidation resistance imparted by Ge doping.

Influence of bismuth on optical and electrical properties of thin Ge2Sb2Te5 films prepared by DC ion plasma sputtering

The structure of Ge2Sb2Te5 thin films doped with Bi has been studied using TEM, Raman spectroscopy, and Optical Transmission Spectroscopy. Thin films were obtained by direct current (DC) ion–plasma sputtering with thicknesses from 60 to 200 nm. TEM images revealed an amorphous film structure with bismuth nanocrystallites with a size of ∼1 to ∼20 nm. A shift in the Raman peaks of Bi–doped Ge2Sb2Te5 films compared to undoped samples was observed. A shift of the fundamental absorption edge to the long wavelength region of the spectrum along with an increase in the concentration of bismuth were detected in the optical transmission spectra of the films. A decrease in the optical band gap of bismuth-doped films was found. It has been shown that with increasing bismuth content in the films, their conductivity also increases. It has been found from studies of the current–voltage film characteristics reversible switching effect takes place.

Enhanced structural and optical properties of semipolar (112¯2) AlGaN film with insertion of AlN/AlGaN superlattice

The high-quality semipolar (112¯2) AlGaN films with high Al contents were successfully deposited on (101¯0) m-plane sapphire substrates with the insertion of AlN/AlGaN superlattice (SL) by metal-organic chemical vapor deposition technology. The dependence of structural and optical properties of the (112¯2) AlGaN film on the deposition parameters for the inserted AlN/AlGaN SL was investigated extensively premised on the characterization results of the optical microscope, atomic force microscopy, relative optical transmittance spectroscopy, high-resolution x-ray diffraction, and photoluminescence spectroscopy. It was discovered that the insertion of the AlN/AlGaN SL grown under an optimized stabilization time of 10 s between the deposition of AlN and AlGaN sublayers could be used to make significant enhancements in surface morphological characteristics, crystal quality, and optical properties of the (112¯2) AlGaN film. The mechanism for the defects reduction in the (112¯2) AlGaN film was revealed to be owing to the increase in bending and annihilating probability of the threading dislocations, basal-plane stacking faults, and other structural defects promoted by introducing sufficiently high desorbed Ga atom-induced vacancy concentration in the optimized thermal treatment process.

Effect of direct current bias on MZO window layer in CdTe solar cells

The band alignment between magnesium-doped zinc oxide (MZO) and cadmium telluride (CdTe) is critical for enhancing the efficiency of CdTe solar cells. In order to study the effect of direct current (DC) bias on the MZO film, various DC biases were applied during the radio frequency (RF) sputtering of the MZO films, with the target bias ranging from 79 V to 121 V. The compositional, structural, chemical, and electronic properties of the MZO films were investigated. The optical transmission and x-ray photoelectron spectroscopy results show that the DC bias could modify the position of the Fermi level while having minimal impact on the band gap of the MZO film. The MZO films with different DC biases were employed as the window layer in CdTe solar cells. The device without DC bias exhibited a low efficiency of 7.7%, corresponding to a cliff-like conduction band offset near −0.1 eV. The optimal DC bias for the MZO film was found to be 99 V, leading to an improvement in efficiency to 12.9%, with a spike-like conduction band offset near 0.2 eV. Further increases in the conduction band offset, however, decrease the photo-generated current, negatively impacting device performance. The results emphasize the significance of DC bias in the RF sputtering process of MZO films with regard to the functionality of CdTe solar cells.

Complex dielectric function of H-free a-Si films: Photovoltaic light absorber

a-Si layers could serve as a low-cost precursor for fabricating a poly-Si absorber for photovoltaic cells. We prepared a-Si films onto glass substrates by RF-magnetron sputtering. The Ar-gas pressure was adjusted from 0.7 to 4.5 Pa to reach average rates (≳1.0nm/s), and increase manufacturing efficiency. We analyzed the structural and optical properties of the a-Si films using Raman and optical transmission spectroscopies. The Cody-Lorentz model was adopted to calculate their complex refractive index.

Violet phosphorus transmission and photoconductance spectroscopy

Violet phosphorus is a semiconducting allotrope of phosphorus with a layered crystalline structure consisting of orthogonally oriented layers of phosphorus chains composed of P2[P8]P2[P9] repeating units. Here, we report optical transmission spectroscopy and photoconductivity measurements of exfoliated flakes of violet phosphorus in the thin-film bulk limit. The violet phosphorus was synthesized by chemical vapour transport, and subsequently protected from oxidation with an inert gas environment. A peak photoconductive responsivity of R = 7 mA W−1 at photon energy 2.8 eV was observed. The spectral dependence of optical transmission and photoconductivity of violet phosphorus leads us to identify optical transitions at van Hove singularities corresponding to energies E 1 = 1.80 ± 0.05 eV and E 2 = 1.95 ± 0.05 eV. Density functional theory was applied to the calculation of violet phosphorus (vP) bandstructure, and a dipole transition analysis shows that optical transitions at the Z and A 0 points of the Brillouin zone are in agreement with experimental observations. Exposure to ambient environmental conditions for several minutes is sufficient to significantly reduce vP photoconductivity, while longer exposure leads to blistering due to oxidation. Thus, a locally inert chemical environment is essential to accessing vP intrinsic optoelectronic properties.

The Synthesis Regimes Effect on Powders and Ceramics of MgAl<sub>2</sub>O<sub>4</sub> Doped with Cr<sup>3+</sup>

This work presents the investigation of nanopowders from magnesium aluminum spinel doped with Cr3+. Different synthesis conditions were used to determine the influence on lattice parameter. The following synthesis parameters varied: time of calcination, cold pressing and alternative precursor of Cr3+. The nanopowders properties were measured by XRD, optical transmission and EDX spectroscopy. The determined average size of synthesized nanoparticles is around 70 nm. Ceramic samples are given by hot pressing and studied by transmission spectroscopy and luminescent spectroscopy. The excitation and emission spectra demonstrates 2E - 4A2 (687 nm), 4A2 - 4T1 (400 nm) and 4A2 - 4T2 (532 nm) transitions.

Ground-based Optical Transmission Spectroscopy of the Nearby Terrestrial Exoplanet LTT 1445Ab

Nearby M-dwarf systems currently offer the most favorable opportunities for spectroscopic investigations of terrestrial exoplanet atmospheres. The LTT 1445 system is a hierarchical triple of M dwarfs with two known planets orbiting the primary star, LTT 1445A. We observe four transits of the terrestrial world LTT 1445Ab (R = 1.3 R ⊕, M = 2.9 M ⊕) at low resolution with Magellan II/LDSS3C. We use the combined flux of the LTT 1445BC pair as a comparison star, marking the first time that an M dwarf is used to remove telluric variability from time-series observations of another M dwarf. We find Hα in emission from both LTT 1445B and C, as well as a flare in one of the data sets from LTT 1445C. These contaminated data are removed from the analysis. We construct a broadband transit light curve of LTT 1445Ab from 620 to 1020 nm. Binned to 3 minute time bins, we achieve an rms of 49 ppm for the combined broadband light curve. We construct a transmission spectrum with 20 spectrophotometric bins each spanning 20 nm and compare it to models of clear, 1× solar composition atmospheres. We rule out this atmospheric case with a surface pressure of 10 bars to 3.2σ confidence, and with a surface pressure of 1 bar to 3.1σ confidence. Upcoming secondary eclipse observations of LTT 1445Ab with the James Webb Space Telescope will further probe the cases of a high-mean-molecular-weight atmosphere, a hazy or cloudy atmosphere, or no atmosphere at all on this terrestrial world.

Composition of the Cytochrome c Complex with Cardiolipin by Thermal Lens Spectrometry.

Thermal lens spectrometry along with spectrophotometric titration were used to assess the composition of the complex of oxidized cytochrome c (ferricytochrome c) with 1,1',2,2'-tetraoleyl cardiolipin, which plays a key role in the initiation of apoptosis. Spectrophotometric titration was carried out for micromolar concentrations at which the complex is mainly insoluble, to assess the residual concentration in the solution and to estimate the solubility of the complex. Thermal lens spectrometry was used as a method of molecular absorption spectroscopy, which has two advantages over conventional optical transmission spectroscopy: the higher sensitivity of absorbance measurements and the possibility of studying the light absorption by chromophores and heat transfer in complex systems, such as living cells or tissues. Thermal lens measurements were carried out at nanomolar concentrations, where the complex is mainly in solution, i.e., under the conditions of its direct measurements. From the thermal lens measurements, the ratios of cytochrome c and cardiolipin in the complex were 50 at pH 7.4; 30 at pH 6.8; and 10 at pH 5.5, which fit well to the spectrophotometric data. The molecular solubility of the complex at pH 6.8-7.4 was estimated as 30 µmol/L.

Thin films of solid electrolyte lithium sulfate deposited by e-beam evaporation

Fabrication of lithium sulfate (Li2SO4) thin films has been successfully attempted by the electron beam evaporation technique, and the deposition conditions were systematically optimized to get stable Li2SO4 thin films. The structural, optical, and electrical properties of the fabricated Li2SO4 thin films were investigated using X-ray diffraction, scanning electron microscope, vibrational spectroscopy techniques, optical transmission spectroscopy, and electrical conductivity measurements. The as-deposited Li2SO4 thin films were X-ray amorphous, although they exhibited Bragg peaks corresponding to Li2SO4 when annealed. The fundamental vibrational modes of sulfate anions in thin films were observed through FT-IR and Raman spectroscopy studies. The ionic conductivity value for the thin film was found to be 2.03 × 10-8 S cm−1 at 373 K, which is of three orders of magnitude higher than that of the bulk.

A new setup for optical measurements under controlled environment.

We present a new analytical instrument for studying the optical properties of materials in different gaseous environments at room and controlled elevated temperatures. The system consists of a vacuum chamber, which is equipped with temperature and pressure controllers, a heating band, and a residual gas analyzer and is connected to a gas feeding line via a leak valve. Two transparent view ports located around a sample holder allow for optical transmission and pump-probe spectroscopy using an external optical setup. The capabilities of the setup are demonstrated by conducting two experiments. In the first experiment, we study the photodarkening and bleaching kinetics of photochromic oxygen-containing yttrium hydride thin films illuminated in ultra high-vacuum and correlate it with changes in partial pressures inside the vacuum chamber. In the second study, we investigate changes in the optical properties of a 50nmV film upon hydrogen absorption.

Annealing-induced tunable bandgap and photoluminescence of (InGa)2O3 films deposited by magnetron sputtering

(InGa)2O3 films are grown on quartz substrates by radio frequency magnetron sputtering using In/Ga2O3 target. The crystal structure, morphology, chemical states and optical properties of (InGa)2O3 films after annealing are investigated by X-ray diffraction, scanning electron microscopy, transmission electron microscope, X-ray photoelectron spectroscopy, optical transmission spectroscopy and photoluminescence spectroscopy. The crystallinity of (InGa)2O3 film improves and phase segregation occurs as the annealing temperature is raised to 800 °C. A mass of cavities are formed on the surface of annealed (InGa)2O3 films. The nanocrystal particles are found in the (InGa)2O3 film annealed at 800 °C. The optical bandgap values of (InGa)2O3 films change from 4.68 to 4.89 eV with increasing annealing temperature from room temperature to 800 °C, which is associated with the decrease of oxygen vacancies and loss of In component of (InGa)2O3 films. The variation of emission band in (InGa)2O3 films is influenced by the tunable bandgap, increasing grain boundary and phase segregation. This work investigates the effect of annealing on the optical properties of (InGa)2O3 films and promotes the application of (InGa)2O3 films in the optical field.

Effect of Nb substitution on the electronic property of lead-free piezoelectric (Na0.41K0.09Bi0.50)TiO3 single crystal: Optical absorption and photoelectron study

Single crystals of undoped and Nb doped sodium and potassium-bismuth titanate at the morphotropic phase boundary (MPB) composition [(Na0.41K0.09Bi0.50)TiO3: NKBT] were grown from high temperature solutions. The effect of Nb doping on the electronic property of NKBT was investigated by optical transmission and photoelectron spectroscopy (PES). The optical absorption shows that the bandgap having the lowest energy is indirect in nature and increases with the increase in the Nb concentration in the lattice. The estimated Urbach energy increases with the increase of Nb concentration signifying an increase in the disorder in the lattice on Nb doping. The PES revealed that the valence band maxima (VBM) exhibit red shift on Nb doping. It also revealed the presence of in-gap states probably due to Ti3+ defect and oxygen vacancy. Moreover, inline with the optical measurement, the PES showed that there is a decrease in the energy gap between the VBM and defect state with the Nb doping. Furthermore, the angle resolved PES measurement confirmed the presence of both the direct and indirect bandgaps in the crystal.

Facile and cost-effective technique to control europium oxidation states in glassy fluorophosphate matrices with tunable photoluminescence

Inorganic fluorophosphate glasses doped with Eu^{2+}/Eu^{3+} are potential candidates for phosphors for commercial white LEDs. This report presents a fast, inexpensive and effective method of controlling the relative concentrations of Eu^{2+}/Eu^{3+} photoluminescent centers in these glasses. The technique consists of a fast quenching of the melt of initial reagents under appropriate conditions. Eu^{2+}/Eu^{3+} ratio was controlled by carrying out the melting under a reducing atmosphere at a temperature between 1000 and 1200 ^circC for periods of 5 to 15 minutes. The reducing atmosphere was provided by a ’double crucible’ technique and did not require special gas lines during the synthesis. The samples were studied by several complementary experimental methods (X-ray diffractometry—XRD, X-ray photoelectron spectroscopy—XPS, photoluminescence—PL—and photoluminescence excitation—PLE—spectroscopies as well as optical transmission spectroscopy). It was shown that the syntheses resulted in amorphous materials with different relative Eu^{2+}/Eu^{3+} concentration ratios, strongly dependent on the preparation conditions: the temperature and the time of melting in a reducing atmosphere. Moreover, changes in these ratios strongly affected the materials’ PL and PLE spectra. Demonstration of reproducible smooth transition from amaranth to blue luminescence color, with white in between, was the most spectacular result of this work.

Evaluation of an Integrated Spectroscopy and Classification Platform for Point-of-Care Core Needle Biopsy Assessment: Performance Characteristics from Ex Vivo Renal Mass Biopsies

PurposeTo evaluate a transmission optical spectroscopy instrument for rapid ex vivo assessment of core needle cancer biopsies (CNBs) at the point of care. Materials and MethodsCNBs from surgically resected renal tumors and nontumor regions were scanned on their sampling trays with a custom spectroscopy instrument. After extracting principal spectral components, machine learning was used to train logistic regression, support vector machines, and random decision forest (RF) classifiers on 80% of randomized and stratified data. The algorithms were evaluated on the remaining 20% of the data set held out during training. Binary classification (tumor/nontumor) was performed based on a decision threshold. Multinomial classification was also performed to differentiate between the subtypes of renal cell carcinoma (RCC) and account for potential confounding effects from fat, blood, and necrotic tissue. Classifiers were compared based on sensitivity, specificity, and positive predictive value (PPV) relative to a histopathologic standard. ResultsA total of 545 CNBs from 102 patients were analyzed, yielding 5,583 spectra after outlier exclusion. At the individual spectra level, the best performing algorithm was RF with sensitivities of 96% and 92% and specificities of 90% and 89%, for the binary and multiclass analyses, respectively. At the full CNB level, RF algorithm also showed the highest sensitivity and specificity (93% and 91%, respectively). For RCC subtypes, the highest sensitivity and PPV were attained for clear cell (93.5%) and chromophobe (98.2%) subtypes, respectively. ConclusionsEx vivo spectroscopy imaging paired with machine learning can accurately characterize renal mass CNB at the time of tissue acquisition.