Optical Reflectance Studies Research Articles

Structural, band profiles and optical reflectivity studies of ALi2B (A = Cu, Ag; B = Ge, Sn, Pb) compounds

Abstract The subject of given project is to highlight the basic elastic, electronic and optical reflectivity of ALi2B (A = Cu, Ag; B = Ge, Sn, Pb). For the calculation of these properties, we used the full potential linearized augmented plane wave (FP-LAPW) procedures carry through Wien2k package. Specifically, the Perdew, Burke and Ernzerhof’s generalized gradient approximation (PBE-GGA) and Wu and Cohen generalized gradient approximation (WC-GGA) have been used. The obtained results fit well with existing experimental data. Different elastic parameters, such as constant of elasticity, elastic moduli, Poisson’s ratio, anisotropy factor and Cauchy pressure, are calculated for the first time for the compounds. The elastic properties clearly summarized the compound’s elastically stability and brittleness in both zinc blend phase. The band structure results for ALi2B shows that the compounds are metallic having overlapping bands across the electron chemical potential. The valance band highest energy state is composed from combination of Cu-d and Sn-p state, disclosed through these compound’s total (DOS) plots, whereas the conduction band primarily constitution is from Li-p, Sn-s Cu-s and Sn-p states. The intraband transitions play vital rule in the description of the optical reflectivity of the ALi2B compounds.

Synergistic effect on silicon surface under low energy he ion irradiation at elevated temperatures

One of the major challenges in nuclear fusion, nanolithography, and various industries is the effect of low energy ion irradiation on the surface morphology of materials. For example, the effect of low energy irradiation on plasma-facing materials' lifetime in fusion reactors, the effect on the optical systems in nanolithography laser produced plasma devices, and the effect on mass production of effective and flexible anti-reflecting coatings in photovoltaic (PV) industry. Silicon and carbides in particular, showed interest in nuclear fusion as potential wall materials, as in mirror designs for nanolithography devices, and as coatings in PV system components. Recently innovative novel Si nanostructures such as nanocones (NCs) demonstrated great feasibility in several areas including the next generation PV cells, batteries, supercapacitors, nano-optoelectronics, photonics, and energy storage devices. Their unique electronic properties and significantly higher absorption over a wide broadband range, offer paramount potential in various fields including microelectronics, optoelectronics, photonics, photovoltaics, biology, sensors, templet for selective growth (on nanostructured substrates), fast electronic devices, and even in high- density data storage. We tailor Si NCs using simultaneous low energy helium (He) ion irradiation on planar Si (111) and (100) and Ta metal masks. In a series of successive experiments, three key parameters, temperature, flux, and presence of Ta metal impurity, were tuned to study the progression of nanostructure development, as well as the effect of these nanostructures on key material properties. Before performing He ion sputtering experiments, 100 eV Ar+ ion irradiations, for 10 min duration, were also performed in-situ in each case for removing any preexisting SiO2 form planer Si substrates. Clear and distinctive progressions of NC development were found during the tuning of each parameter. During the temperature dependent studies, we clearly observed a sequential progression from thicker and less dense cones (773 K) to finer and more densely packed NCs (1073 K). On the other hand, during the case of flux dependent studies, it was found that increasing flux increases the size of the NCs, with low flux resulting in sparsely placed nanodots, and high fluxes resulting in large conglomerates. Finally, the effect of Ta impurity shows an increased propensity for NCs development. In addition, using optical-reflectivity studies we clearly observed a significant reduction (from ∼65 to almost zero) in optical reflectivity.

Interplay of self-assembly and viscoelasticity in chiral liquid crystal gels

Electro-optic and photonic devices that are robust against external stresses and deformation offer several technological advantages. A variety of device applications based on chiral liquid crystals (LCs) exhibiting blue phases (BPs), chiral nematic (N*), and ferroelectric Smectic (SmC*) phases have already been demonstrated. Chiral LC-gels are, therefore, among the best soft materials that can further extend device performance and functionality by combining electro-optic and photonic properties with mechanical stability. In view of this, a composite material composed of two highly chiral calamitic LCs and a low-molecular-weight organogelator was developed and investigated using a variety of experimental techniques. Optical microscopy, selective reflection, and x-ray diffraction studies show that the LC characteristics are very well retained in the chiral LC-gels, namely, BPIII-gel, BPII-gel, BPI-gel, N*-gel, twist grain boundary A phase (TGBA)-gel, and SmC*-gel, which form sequentially as the sample is gradually cooled from the isotropic phase. Rheological studies establish that the chiral LC-gels exhibit several features of a soft viscoelastic solid. This has been understood in terms of the mode of self-assembly of the gelator fibers and the ubiquitous defects pertinent to different LC phases. The major highlights of the study are the formation of a defect-mediated strong N*-gel with enhanced viscoelastic moduli and the induction of a TGBA-gel. The stretchable nature of a free-standing film of the BPI-gel that retains its shape over time is another interesting aspect of the present work.

Optical constants of crystalline Bi2Sr2CaCu2O by Brillouin light scattering spectroscopy

Room-temperature optical constants of crystalline Bi2Sr2CaCu2O were determined using data extracted from Brillouin light scattering spectra. Optical extinction coefficient-to-refractive index ratios at a wavelength of 532 nm were obtained from bulk phonon peak linewidth and frequency shift measurements and range from for directions close to the crystallographic c-axis. These ratios, and optical extinction coefficients, absorption coefficients, and imaginary parts of the dielectric function determined from these ratios and known refractive index, are in general agreement with values found in optical reflectance studies, but are 5–7 times larger than those extracted from optical interference measurements.

Dynamic all-optical control in ultrashort double-pulse laser ablation

Double-pulse femtosecond laser ablation of thin aluminum films and bulk aluminum counterintuitively demonstrated a strong (60–70%) raise of the thickness-dependent thresholds for inter-pulse delays of 20–200 ps, preventing material removal at above-threshold fluencies. Time-resolved optical pump-probe reflection and double-pump transmission studies were performed and confirmed the variation of the ablation threshold depending on the interpulse delay. To support the experimental measurements, the process of double-pulse laser ablation was modelled with the combined atomistic-continuum model. The applied model can describe the laser-induced non-equilibrium phase transition processes at atomic precision, whereas the effect of free carriers, playing a determinant role for the case of ultrashort laser pulses, is accounted for in the continuum. The simulations revealed the underlying pre-ablative laser-induced stress dynamics in the hot, acoustically relaxed Al melt, crucially sensitive to the second pump-pulse compressive pressurization. The results of theoretical and experimental study enable efficient dynamic all-optical control of ultrafast laser ablation.

Direct comparison of the results of arsenic ion implantation in n– and p–type Hg0.8Cd0.2Te

Optical reflectance in the visible wavelength range, transmission electron microscopy, and the Hall-effect measurements with mobility spectrum analysis have been used for the direct comparison of the results of arsenic ion implantation in samples with n– and p–type conductivity fabricated on the basis of a Hg1-xCdxTe film with x = 0.22 grown by molecular beam epitaxy on a Si substrate. Optical reflectance studies showed that the effect of ion implantation on the properties of the surface was very similar in n– and p–type material. Transmission electron microscopy showed that defect patterns were also similar in n– and p–type samples in terms of the thicknesses of implantation-damaged layers and types of defects formed. Electrical studies demonstrated that low- and mid-mobility electrons induced by the implantation in n– and p–type material had similar average concentration and mobility. It was concluded that the nature of donor defects that produced these electrons was the same in n– and p–type HgCdTe, and it was suggested that the defects in question were interstitial mercury atoms captured by dislocation loops and by quasi-point defects formed as a result of implantation.

An electrochemical cell for 2-dimensional surface optical reflectance during anodization and cyclic voltammetry

We have developed an electrochemical cell for in situ 2-Dimensional Surface Optical Reflectance (2D-SOR) studies during anodization and cyclic voltammetry. The 2D-SOR signal was recorded from electrodes made of polycrystalline Al, Au(111), and Pt(100) single crystals. The changes can be followed at a video rate acquisition frequency of 200 Hz and demonstrate a strong contrast between oxidizing and reducing conditions. Good correlation between the 2D-SOR signal and the anodization conditions or the cyclic voltammetry current is also observed. The power of this approach is discussed, with a focus on applications in various fields of electrochemistry. The combination of 2D-SOR with other techniques, as well as its spatial resolution and sensitivity, has also been discussed.

Optical spectroscopy study on pressure-induced phase transitions in the three-dimensional Dirac semimetal Cd3As2

We report a room-temperature optical reflectivity study performed on [112]-oriented Cd$_3$As$_2$ single crystals over a broad energy range under external pressure up to 10 GPa. The abrupt drop of the band dispersion parameter ($z$-parameter) and the interruption of the gradual redshift of the bandgap at $\sim$4~GPa confirms the structural phase transition from a tetragonal to a monoclinic phase in this material. The pressure-induced increase of the overall optical conductivity at low energies and the continuous redshift of the high-energy bands indicate that the system evolves towards a topologically trivial metallic state, although a complete closing of the band gap could not be observed in the studied pressure range. Furthermore, a detailed investigation of the low-pressure regime suggests the possible existence of an intermediate state between 2 and 4~GPa , that might be a precursor of the structural phase transition or due to the lifted degeneracy of the Dirac nodes. Several optical parameters show yet another anomaly at 8~GPa, where low-temperature superconductivity was found in an earlier study.

Preparation and characterization of screen-printed SnS0.5Se0.5 alloy films

In present investigation, tin sulphoselenide (SnS0.5Se0.5) thick films were deposited on glass substrate by simple and economical screen-printing method followed by sintering process. The structural, morphological, elemental, optical and electrical properties of the sintered films were studied through X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), energy dispersive X-ray analysis (EDAX), UV–Vis spectroscopy and current–voltage (I–V) analysis respectively. XRD analysis confirms the polycrystalline nature of films, exhibiting orthorhombic structure with preferred orientation of grains along (111) direction. The FE-SEM image indicates that the film bears granular grains and porosity. The presence of Sn, S and Se elements in films were confirmed by EDAX analysis. The optical studies for the films were analyzed through reflectance spectra. Optical reflectance studies indicated a direct band gap of 1.21 eV for the films. The electrical conductivity of films was found to be ~ 10−4 Ω−1 cm−1 at room temperature.

Optical transmittance and reflectance studies and evidence of weak electron–phonon interaction in Type-I Ge clathrate Ba8Ga16Ge30

The optical properties of ternary type-I Ge clathrate Ba8Ga16Ge30 are investigated by optical reflectance and transmittance measurements. The refractive index and extinction coefficient are calculated from the reflectance spectrum via the modified Kramers–Kronig analysis method between 0.5 and 3.2 eV. The photon energy dependence of the optical absorption coefficient reveals that Ba8Ga16Ge30 is an indirect band gap semiconductor with a gap energy Eg of 0.69 eV at 5.7 K and 0.66 eV at 285 K. The temperature dependence of Eg can be satisfactorily described by an equation based on the Bose–Einstein statistics model. When compared with those of common elemental, III–V, and II–VI semiconductors, the small temperature coefficient dEg/dT of the Ba8Ga16Ge30 can be considered to represent the weak electron–phonon interaction in the Ba8Ga16Ge30 clathrate.

Bi13S18I2: (Re)discovery of a Subvalent Bismuth Compound Featuring [Bi2]4+ Dimers Grown in Sulfur/Iodine Flux Mixtures

Reactions of bismuth in sulfur/iodine flux mixtures were explored at various temperatures and iodine concentrations. The observed products include Bi2S3, BiSI, and Bi13S18I2. The latter compound, formerly reported as “Bi19I3S27” grows as well formed needles from the flux. This enabled extensive crystallographic studies by single crystal XRD and powder synchrotron XRD. These data allow a more accurate assignment of the disordered bismuth sites in the structure, indicating formation of subvalent Bi24+ dimers; this results in a stoichiometry of Bi13S18I2. Thermal decomposition studies and Raman spectroscopy support this structural model, and electronic structure calculations and optical reflectance studies indicate this compound is an indirect band gap semiconductor with band gap of 0.3 eV.

Impact of interfaces on scenario of crystallization of phase change materials

Chalcogenide phase change materials (PCMs), such as Ge-Sb-Te alloys, have outstanding properties, which has led to their successful use for a long time in optical memories (DVDs) and, more recently, in Phase Change Random Access Memories. The latter are the most promising candidate to replace the current FLASH memories. The main feature of PCMs are fast and reversible phase transformations between crystalline and amorphous states with very different transport and optical properties. Controlling their crystallization, however, is a challenge. In the present work, a new picture of the prototypical GeTe and Ge2Sb2Te5 thin film crystallization is provided from optical reflectivity, resistivity and Scanning Transmission Electron Microscopy studies of 100 nm thick films. The crystallization temperature Tx significantly varies as a function of surface oxidation. Whereas Ge2Sb2Te5 has been, and is still, the subject of many publications, the present results reveal that its crystallization mechanism was not fully understood. Heterogeneous crystallization of Ge2Sb2Te5 at the upper film surface, reported so far, results from a surface alteration. Adequately protected Ge2Sb2Te5 and GeTe crystallize through nucleation inside the amorphous phase. Preventing surface alteration due to oxidation as demonstrated in the present study, through suitable capping layer deposition methods, yields a spectacular increase in Tx above the values that have been reported in the literature. The present study demonstrates that interface engineering allows one to select the crystallization mechanism, and hence control the stability of the amorphous phase in PCMs.

Gyroidal titanium nitride as nonmetallic metamaterial

We have synthesized gyroidal TiN metamaterials, studied their optical properties, and compared them with the optical properties of the TiN thin films fabricated using reactive magnetron sputtering. The plasma frequency, ωp, and the corresponding free carrier concentration, N, in the gyroid samples were found to be much lower than those in thin films. Furthermore, the plasma frequency in TiN gyroids was comparable to or smaller than the damping rate, γD. This makes the studied TiN gyroid a poor plasmonic material. At the same time, TiN gyroidal samples have demonstrated bright rainbow pattern in the optical microscopy reflectance study. This phenomenon is tentatively explained by different orientations of the gyroid domains.

Effect of volatile solvent infiltration on optical and electrical characteristics of porous photonic structures

The infiltration of small chain alcohols into the deep nano sized pores of one dimensional porous silicon (PS) based photonic structures have been continuously monitored against time by simultaneous electrical and optical measurements. The in situ optical reflection studies during volatile solvent exposure reveal several dynamic processes; within a limited time duration of solvent exposure the microcavity resonant peak shifts towards higher wavelength, and after prolonged exposure and drying the cavity resonant peak shifts to a new semi-permanent lower wavelength. In situ optical and electrical responses from PS photonic structure-based low-cost multifunctional devices reveal their potential application for a wide range of chemical and biological species detection and monitor their sensor dynamic processes.

Optical reflectance, optical refractive index and optical band gap measurements of nonlinear optics for photonic applications

Single crystals of Pyrrolidinomethylpthalamide (PMP) were grown by the slow evaporation solution growth at room temperature. The grown crystals were subjected to powder X-ray diffraction studies to confirm the crystal structure. The modes of vibration of different molecular groups present in PMP have been identified by FTIR spectral analysis. Optical transference of the grown crystal was investigated by UV–vis-NIR spectrum. The lower optical cut off wavelength for this crystal is observed at 300nm and energy band gap 5.167eV. The optical reflectance and optical refractive index studies have been carried out in this crystal. The optical conductivity and electrical conductivity studies have been carried out on PMP single crystal. Finally Second Harmonic Generation (SHG) for the materials of this work was confirmed using Nd:YAG laser.

New silver(I) coordination polymer containing a rare inorganic graphite-like [AgS]n layer

Abstract A new metal-organic coordination polymer, namely [Ag2(H2trzS)2Cl2]n (1, H2trzS = 1H-1, 2, 4,-triazole-3-thiol), has been successfully synthesized under solvothermal conditions and characterized by elemental analysis, FT-IR spectra and powder X-ray diffraction (PXRD). Single crystal X-ray diffraction analysis reveals that 1 features a two-dimensional (2D) layer structure with a rare inorganic graphite-like [AgS]n layer. Optical diffuse reflectance study reveals an optical band gap of 3.83 ev, making it a wide band gap semiconductor. The densities of states (DOS) calculations reveal that the luminescent emission of 1 originates from the intraligand-charge-transfer.

Optical reflectance, optical refractive index and optical conductivity measurements of nonlinear optics for l-aspartic acid nickel chloride single crystal

Single crystals of l-aspartic acid nickel chloride (LANC) were grown by the slow evaporation technique at room temperature. The grown crystals were subjected to Powder X-ray diffraction studies to confirm the crystal structure. The modes of vibration of different molecular groups present in LANC have been identified by FTIR spectral analysis. Optical transferency of the grown crystal was investigated by UV-Vis-NIR spectrum. The lower optical cut off wavelength for this crystal is observed at 240nm and energy band gap 5.179eV. The optical reflectance and optical refractive index studies have been carried out in this crystal. Finally, the optical conductivity and electrical conductivity studies have been carried out on LANC single crystal.

ZnO nanowire growth by electric current heating method: A study on the effect of substrate temperature

In this study, we report the growth of ZnO nanowire on quartz glass substrates with Au-catalyst assistance by electric current heating of ZnO ceramic bar. The effect of substrate temperature on the properties of ZnO nanostructures has been investigated systematically. Structural analysis indicates that the grown ZnO crystals belong to hexagonal phase with preferential growth along (002) orientation. Scanning electron microscopic studies reveal the aligned ZnO nanowires were grown at 800°C. The typical length and diameter of nanowires are in the uniform ranges of 4–20μm and 20–100nm, respectively, showing their high aspect ratio of about 1000. We have made an attempt to discuss about the change in ZnO nanostructures with different substrate temperatures and the possible mechanism for the growth of nanowires. Optical reflectance studies show the infrared reflectivity was controlled through the substrate temperature.

A new luminescent Cu(I) complex stabilized by Cu⋅⋅⋅Cu interactions

Abstract A novel luminescent Cu(I) compound, [Cu(HtrzS)]n (1 HtrzSH=1H-1,2,4-triazole-3-thiol), was hydrothermally synthesized, which features a two-dimensional (2D) inorganic–organic hybrid framework containing a inorganic [CuS]n layer. Further, this 2D framework is connected by the intermolecular hydrogen bonds into a three-dimensional (3D) supramolecular framework. Optical diffuse reflectance study reveals an optical band gap of 3.44 eV, making it a wide band gap semiconductor. Moreover, this compound exhibits strong photoluminescence at room temperature. The densities of states (DOS) calculations reveal that luminescence emission bands of the compound originate from the ligand-to-metal charge transfer.

Multifunctional Ti1−xTaxO2: Ta doping or alloying?

Useful electronic, magnetic, and optical properties have been proposed and observed in thin films of Ti1−xMxO2 (M=Ta,Nb,V). In this work, we have studied phase formation for films of Ti1−xTaxO2 prepared by pulsed laser deposition. We show that substitutional Ta in TiO2 results in a different material system in terms of its electronic properties. Moss–Burstein shift is ruled out by comparing the electrical transport data of anatase and rutile TiO2. Vegard’s law fit to the blueshift data and the high energy optical reflectivity studies confirm the formation of an alloy with a distinct band structure.