Pseudo Band Research Articles

Mixed Sr and Ba Tri-Stannides/Plumbides AII(Sn1−xPbx)3

The continuous substitution of tin by lead (M IV ) allows for the exploration geometric criteria for the stability of the different stacking variants of alkaline-earth tri-tetrelides A II M 3 IV . A series of ternary Sr and Ba mixed tri-stannides/plumbides A II (Sn 1 − x Pb x ) 3 (A II = Sr, Ba) was synthesized from stoichiometric mixtures of the elements. Their structures were determined by means of single crystal X-ray data. All structures exhibit close packed ordered A M 3 layers containing M kagomé nets. Depending on the stacking sequence, the resulting M polyanion resembles the oxygen substructure of the hexagonal (face-sharing octahedra, h stacking, Ni 3 Sn-type, border compound BaSn 3 ) or the cubic (corner-sharing octahedra, c stacking, Cu 3 Au-type, border compound SrPb 3 ) perovskite. In the binary compound BaSn 3 (Ni 3 Sn-type) up to 28% of Sn can be substituted against Pb (hP8, P 6 3 / mmc, x = 0.28(4): a = 726.12(6), c = 556.51(6) pm, R1 = 0.0264). A further increased lead content of 47 to 66% causes the formation of the BaSn 2.57 Bi 0.43 -type structure with a ( hhhc ) 2 stacking [hP32, P 6 3 / mmc, x = 0.47(3): a = 726.80(3), c = 2235.78(14) pm, R1 = 0.0437]. The stability range of the BaPb 3 -type sequence ( hhc ) 3 starts at a lead proportion of 78% (hR36, R 3 ¯ m, a = 728.77(3), c = 2540.59(15) pm, R1= 0.0660) and reaches up to the pure plumbide BaPb 3 . A second new polymorph of BaPb 3 forms the Mg 3 In-type structure with a further increased amount of cubic sequences [ ( hhcc ) 3 ; hR48, a = 728.7(2), c = 3420.3(10) pm, R1 = 0.0669] and is thus isotypic with the border phase SrSn 3 of the respective strontium series. For the latter, a Pb content of 32% causes a small existence region of the PuAl 3 -type structure [hP24, P 6 3 / mmc, a = 696.97(6), c = 1675.5(2) pm, R1 = 0.1182] with a ( hcc ) 2 stacking. The series is terminated by the pure c stacking of SrPb 3 , the stability range of this structure type starts at 75% Pb (cP4, Pm 3 ¯ m; a = 495.46(9) pm, R1 = 0.0498). The stacking of the close packed layers is evidently determined by the ratio of the atomic radii of the contributing elements. The Sn/Pb distribution inside the polyanion (’coloring’) is likewise determined by size criteria. The electronic stability ranges, which are discussed on the basis of the results of FP-LAPW band structure calculations are compared with the Zintl concept and Wade’s/mno electron counting rules. Still, due to the presence of only partially occupied steep M-p bands the compounds are metals exhibiting pseudo band gaps close to the Fermi level. Thus, this structure family represents an instructive case for the transition from polar ionic/covalent towards (inter)metallic chemistry.

Voltammetric Determination of Serine Using L‐Cysteine Modified Platinum Electrode and Brief Exploration from the Theoretical Perspective

AbstractA new method for the detection of serine was described using L‐cysteine chemically modified electrode, which was prepared by cyclic voltammetry. The factors, such as pH, scan rate, and soaking time for the experimental process were discussed. Under the optimal conditions, the linear range for serine concentration was 3×10−7∼8×10−5 mol L−1, the detection limit was 2.34×10−7 mol L−1 (S/N=3), and the relative standard deviation was 2.14 % (n=10) for 3×10−7 mol L−1 serine solution. The method was applied to the detection of serine in apple cider vinegar sample with a recovery of 97.5 %–101.9 %. To further discuss the change of the property of platinum surface on atomic level, theoretical calculations were carried out. The change of adsorption energy indicated that the possible interaction of Pt and cysteine was O−C−O bridge adsorption. The analysis about the density of states (DOS) of the system and pseudo band gap showed an increase in the Pt surface activity.

Improved Quantitative SERS Enabled by Surface Plasmon Enhanced Elastic Light Scattering.

The application of surface-enhanced Raman spectroscopy (SERS) for everyday quantitative analysis is hindered by the point-to-point variability of SERS substrates that arises due to the heterogeneous distribution of localized electromagnetic fields across a suite of plasmonic nanostructures. Herein, we adopt surface-enhanced elastic scattering as a SERS internal standard. Both elastic and inelastic (i.e., Raman) scattering are simultaneously enhanced by a given “hot spot”, and thus, the surface-enhanced elastic scattering signal provides a localized intrinsic internal standard that scales across all of the plasmon-enhanced electromagnetic fields within a substrate. Elastically scattered light originates from the amplified spontaneous emission (ASE) of the commercial laser, leading to the formation of a low-wavenumber pseudo band that arises from the interaction of the ASE and the edge filter. A theoretical model was developed to illustrate the underlying mechanism supporting this normalization approach. The normalized Raman signals are independent of the incident laser intensity and the density of “hot spots” for numerous SERS substrates. Following “hot-spot” (HS) normalization, the coefficient of variation for the tested SERS substrates decreases from 10 to 60% to 2%–7%. This approach significantly improves SERS quantitation of four chloroanilines and enables collection of highly reproducible analyte adsorption results under both static and dynamic imaging conditions. Overall, this approach provides a simple means to improve SERS reproducibility without the need to use additional chemicals as internal standards.

Analytical model of subthreshold swing of a gate and channel engineered double gate MOSFET

AbstractWe have developed a 2‐dimensional analytical model to study the subthreshold swing and channel conductance of a gate and channel engineered double gate MOSFET. Basic diffusion equation has been considered to model the drain to source current of the device in subthreshold region and expressions of the device parameters like subthreshold swing and channel conductance have been derived. Subthreshold swing variations of the device with varied materials, channel, and effective oxide layer thickness and channel length have also been investigated. Changes in subthreshold swing owing to different doping profile concentrations have been explored. Effect of similar device parameters on the channel conductance of the device has been examined as well. Another important observation of the work is the pseudo flat band condition of the device structure based on the intersecting point of body potentials of the device at different thickness of the channel. The analytical results of the proposed model have been validated with the numerical results from commercially available 2‐dimensional device simulator.

Slow photon amplification of gas-phase ethanol photo-oxidation in titania inverse opal photonic crystals

Here we describe the successful fabrication of six titania inverse opal (TiO2 IO) photocatalysts with fcc[111] pseudo photonic band gaps (PBGs) tuned to span the UV–vis region. Photocatalysts were fabricated by a colloidal crystal templating and sol-gel approach – a robust and highly applicable bottom-up scheme which allowed for precise control over the geometric and optical properties of the TiO2 IO photocatalysts. Optical properties of the TiO2 IO thin films were investigated in detail by UV–vis transmittance and reflectance measurements. The PBG along the fcc[111] direction in the TiO2 IOs was dependent on the inter-planar spacing in the [111] direction, the incident angle of light and the refractive index of the medium filling the macropores in the IOs, in agreement with a modified Bragg’s law expression. Calculated photonic band structures for the photocatalysts revealed a PBG along the Γ→L direction at a/λ∼0.74, in agreement with the experimental optical data. By coupling the low frequency edge of the PBG along the [111] direction with the electronic absorption edge of anatase TiO2, a two-fold enhancement in the rate of gas phase ethanol photo-oxidation in air was achieved. This enhancement appears to be associated with a ‘slow photon’ effect that acts to both enhance TiO2 absorption and inhibit spontaneous emission (i.e. suppress electron-hole pair recombination).

Elastic-wave transmission through self-similar anisotropic Cantor-like multilayers

Through the study of elastic wave propagation in Cantor-like anisotropic multilayers, this work analyzes the influence of medium geometry on the transmission of elastic waves to yield a better understanding of the connection between topological ordering and physical properties. Cantor-like multilayers, whose homothetic dimension is modified by changing the length of the central segment, are made of one anisotropic material with two orientations. The influence of the combination of self-similarity and anisotropy on the global transmission is investigated by means of iteration order, homothetic dimension and layer orientations. The propagation is described by the stiffness matrix algorithm. The results reveal that the homothetic dimension scales the resonance frequencies and the frequency ranges of the pseudo band gaps, and that layer orientation influences the speed of quasi-transverse waves to enhance the effect of self-similarity. Finally, an extensive study on various frequency ranges is conducted. It is demonstrated that self-similarity may be used to tune the position and the width of the pseudo band gaps to minimize the global acoustic transmission.

First principle investigations of the structural, electronic and magnetic properties of predicted new zirconium based full-Heusler compounds, Zr2MnZ (Z=Al, Ga and In)

The crystal structure, electronic and magnetic properties of predicted new full-Heusler compounds Zr2MnZ (Z=Al, Ga, In) were studied within the density functional theory (DFT) framework. These materials exhibit unique properties that connect the spin gapless semiconducting character with the completely compensated ferrimagnetism. Magnetically ordered Zr2MnZ (Z=Al, Ga, In) compounds crystallize in inverse Heusler structure are stable against decomposition and have zero magnetic moment per formula unit, in agreement with Slater–Pauling rule. The Zr2MnAl compound presents semiconducting properties with an energy band gap of 0.41eV in the majority spin channel and a zero band gap in the minority spin channel. By substituting completely the Al in Zr2MnAl via Ga and In elements, semiconducting pseudo band gaps are formed in the majority spin channels due to different neighborhoods around the manganese atoms, which decreases the energy of Mn triple degenerated anti-bonding states.

Synthesis of Silica Particles with Precisely Tailored Diameter

A modified seeded growth process of silica particles with a continuous addition of tetraethyl orthosilicate (TEOS) was presented to control the diameter of silica particles. The diameter of particles was monitored by dynamic light scattering to control the addition of TEOS. The increase in the diameter of the silica particles with time and the addition of TEOS was investigated. The diameter of silica seeds increased from 193 nm to 446 nm in 4 h. The final diameter of silica particles was tailored within the range of ±5 nm to the target diameter. Silica particles with diameter of 446 nm were synthesized and assembled into photonic crystals with a pseudo band gap centered at just 1000 nm. The feasibility and practicability of this modified seeded growth process was verified.

Optical simulation of colloidal photonic crystals with controllable size spheres of silica

In this work, optical properties of colloidal silica crystals are investigated theoretically. Numerical calculations are discussed and compared to experimental measurements. The existence of photonic band gap (or stop band) is inferred by analyzing the transmission spectra and dips of low transmission are typically correlated with photonic band gaps. The position of the main dip in simulated spectra matches the pseudo photonic band gap expected by the calculated photonic band gap diagram. Positions of stop band show a strong dependence on the diameter of silica spheres. We note that positions of the main dips deduced from simulated and measured spectra are very near. Good agreement between measured and theoretical results is then reported.

Detailed photoluminescence studies of thin film Cu2S for determination of quasi-Fermi level splitting and defect levels

We have studied chalcocite (Cu2S) layers prepared by physical vapor deposition with varying deposition parameters by calibrated spectral photoluminescence (PL) and by confocal PL with lateral resolution of Δ x≈0.9 μm. Calibrated PL experiments as a function of temperature T and excitation fluxes were performed to obtain the absolute PL-yield and to calculate the splitting of the quasi-Fermi levels (QFLs) μ=Ef,n−Ef,p at an excitation flux equivalent to the AM 1.5 spectrum and the absorption coefficient α(ℏω), both in the temperature range of 20 K≤T≤400 K. The PL-spectra reveal two peaks at E#1=1.17 eV and E#2=1.3 eV. The samples show a QFL-splitting of μ>700 meV associated with a pseudo band gap of Eg=1.25 eV. The high-energy peak shows an unexpected temperature behavior, namely, an increase of PL-yield with rising temperature at variance with the behavior of QFL-splitting that decreases with rising T. Our observations indicate that, contrary to common believe, it is not the PL-yield, but rather the QFL-splitting that is the comprehensive indicator of the quality of the excited state in an illuminated semiconductor. A further examination of the lateral variation of opto-electronic properties by confocal PL and the surface contour shows no detectable correlation between Cu2S grains/grain boundaries and the PL-yield or QFL-splitting.

Continuum Schrödinger Operators Associated With Aperiodic Subshifts

We study Schrödinger operators on the real line whose potentials are generated by an underlying ergodic subshift over a finite alphabet and a rule that replaces symbols by compactly supported potential pieces. We first develop the standard theory that shows that the spectrum and the spectral type are almost surely constant and that identifies the almost sure absolutely continuous spectrum with the essential closure of the set of energies with vanishing Lyapunov exponent. Using results of Damanik–Lenz and Klassert–Lenz–Stollmann, we also show that the spectrum is a Cantor set of zero Lebesgue measure if the subshift satisfies the Boshernitzan condition and the potentials are aperiodic and irreducible. We then study the case of the Fibonacci subshift in detail and prove results for the local Hausdorff dimension of the spectrum at a given energy in terms of the value of the associated Fricke–Vogt invariant. These results are elucidated for some simple choices of the local potential pieces, such as piecewise constant ones and local point interactions. In the latter special case, our results explain the occurrence of so-called pseudo bands, which have been pointed out in the physics literature.

Microwave transmission through one-dimensional hybrid quasi-regular (fibonacci and Thue-Morse)/periodic structures

The transmission properties of hybrid quasi-periodic photonic systems (HQPS) made by the combination of one-dimensional periodic photonic crystals (PPCs) and quasi-periodic photonic crystals (QPCs) were theoretically studied. The hybrid quasi-periodic photonic lattice based on the hetero-structures was built from the Fibonacci and Thue-Morse sequences. We addressed the microwave properties of waves through the one-dimensional symmetric Fibonacci, and Thue-Morse system i.e., a quasi-periodic structure was made up of two different dielectric materials (Rogers and air), in the quarter wavelength condition. It shows that controlling the Fibonacci parameters permits to obtain selective optical filters with the narrow passband and polychromatic stop band filters with varied properties which can be controlled as desired. From the results, we presented the self-similar features of the spectra, and we also presented the fractal process through a return map of the transmission coefficients. We extracted powerfully the band gaps of hybrid quasi-periodic multilayered structures, called pseudo band gaps, often containing resonant states, which could be considered as a manifestation of numerous defects distributed along the structure. The results of transmittance spectra showed that the cutoff frequency could be manipulated through the thicknesses of the defects and the type of dielectric layers of the system. Taken together, the above two properties provide favorable conditions for the design of an all-microwave intermediate reflector.

High-resolution rovibrational analysis of CH281BrF in the range 920–980 cm−1

The high-resolution infrared spectrum of CH281BrF has been studied in part of the atmospheric window between 920 and 980cm−1, the region characterized by the v9 and v5+v6 absorptions. The v9 emerges as a pseudo a-type band, with selection rule ΔKa=ΔKc=0, ±2, and v5+v6 exhibits a predominant a-type structure. The interactions connecting these levels and the dark 3v6 state have been explored. The anomalous v9 transitions have been ascribed to Coriolis interaction with v4. The analysis yields a set of accurate upper-state parameters for v9 and v5+v6. Spectroscopic information on 3v6 has been also obtained.

Spectral Calibrated and Confocal Photoluminescence of Cu2S Thin-Film Absorber

ABSTRACTWe have studied Cu2S absorber layers prepared by physical vapor deposition (PVD) by calibrated spectral photoluminescence (PL) and by confocal PL as function of temperature T and excitation fluxes to obtain the absolute PL-yield at an excitation flux equivalent to the AM1.5 spectrum and to calculate the splitting of the quasi-Fermi levels (QFL) µ = Ef,n-Ef,p and the absorption coefficient α(E), both in the temperature range 20 K ≤ T ≤ 400 K. The PL-spectra reveal two peaks at E1 = 1.17 eV and E2 = 1.3 eV, of which the low energy peak is only detectable at temperatures T < 200 K. The samples show an impressive QFL-splitting of µ > 700 meV at 300 K associated with a pseudo band gap of Eg = 1.25 eV. The high energy peak shows an unexpected temperature behavior, namely an increase of the PL-yield with rising temperature at variance with the behavior of QFL-splitting that decreases with rising T from extrapolated T = 0K value of µ = 1.3 eV. The PL-yield versus temperature will be discussed in terms of different defect states in the band gap. Our observations indicate that, contrary to common believe, it is not the PL-yield, but rather the QFL-splitting that is the comprehensive indicator of the quality of the excited state in an illuminated semiconductor. A further examination of the lateral variation of the opto-electronic properties by confocal PL shows a strong correlation between the QFL-splitting, the Urbach energy EU and the optical band gap Eopt, respectively.

Semiclassical analysis of two-level collective population inversion using photonic crystals in three-dimensional systems

I theoretically demonstrate the population inversion of collective two-level atoms using photonic crystals in three-dimensional (3D) systems by self-consistent solution of the semiclassical Maxwell-Bloch equations. In the semiclassical theory, while electrons are quantized to ground and excited states, electromagnetic fields are treated classically. For control of spontaneous emission and steady-state population inversion of two-level atoms driven by an external laser which is generally considered impossible, large contrasts of electromagnetic local densities of states (EM LDOS’s) are necessary. When a large number of two-level atoms are coherently excited (Dicke model), the above properties can be recaptured by the Maxwell-Bloch equations based on the first-principle calculation. In this paper, I focus on the realistic 1D PC’s with finite structures perpendicular to periodic directions in 3D systems. In such structures, there appear pseudo photonic band gaps (PBG’s) in which light leaks into air regions, unlike complete PBG’s. Nevertheless, these pseudo PBG’s provide large contrasts of EM LDOS’s in the vicinity of the upper photonic band edges. I show that the realistic 1D PC’s in 3D systems enable the control of spontaneous emission and population inversion of collective two-level atoms driven by an external laser. This finding facilitates experimental fabrication and realization.

Preparation of porous titanium oxide films onto indium tin oxide for application in organic photovoltaic devices

In this work, porous ordered TiO2 films were prepared through sol gel route by using a monolayer of polystyrene spheres as template on indium-tin oxide/glass substrate. These films were characterized by SEM, AFM, Raman spectroscopy, UV–vis absorbance and XRD. The UV–vis absorbance spectrum show a pseudo band gap (PBG) with maxima at 460nm arising from the light scattering and partial or total suppression of the photon density of states, this PBG can be controlled by the size of the pore. We also propose the use of this porous film as electron acceptor electrode in organic photovoltaic cells; we show that devices prepared with porous titania displayed higher efficiencies than devices using compact titania films as electrode. Such behaviour was observed in both bilayer and bulk heterojunction devices.

How to characterize terrains on 4 Vesta using Dawn Framing Camera color bands?

We present methods for terrain classification on 4 Vesta using Dawn Framing Camera (FC) color information derived from laboratory spectra of HED meteorites and other Vesta-related assemblages. Color and spectral parameters have been derived using publicly available spectra of these analog materials to identify the best criteria for distinguishing various terrains. We list the relevant parameters for identifying eucrites, diogenites, mesosiderites, pallasites, clinopyroxenes and olivine+orthopyroxene mixtures using Dawn FC color cubes. Pseudo Band I minima derived by fitting a low order polynomial to the color data are found to be useful for extracting the pyroxene chemistry. Our investigation suggests a good correlation (R2=0.88) between laboratory measured ferrosilite (Fs) pyroxene chemistry vs. those from pseudo Band I minima using equations from Burbine et al. (Burbine, T.H., Buchanan, P.C., Dolkar, T., Binzel, R.P. [2009]. Planetary Science 44, 1331–1341). The pyroxene chemistry information is a complementary terrain classification capability beside the color ratios. We also investigated the effects of exogenous material (i.e., CM2 carbonaceous chondrites) on the spectra of HEDs using laboratory mixtures of these materials. Our results are the basis for an automated software pipeline that will allow us to classify terrains on 4 Vesta efficiently.

High-resolution infrared spectroscopy of the interacting ν9, ν5 + ν6 and 3 ν6 levels of CH 279BrF

The high-resolution (0.0030 cm −1) Fourier transform infrared spectrum of CH 2 79BrF has been studied in part of the atmospheric window between 910 and 980 cm −1, the region of the ν 9 (935.847 cm −1) and ν 5 + ν 6 (961.239 cm −1) bands. The ν 9 fundamental consists of a pseudo a-type band induced by Coriolis coupling with ν 5 + ν 6, in turn exhibiting a predominant a-type structure. Several interactions connecting these levels and the dark state 3 ν 6 have been assessed. The whole data set is treated using Watson’s A-reduced Hamiltonian in the I r representation implemented with first order a- and b- and c-type Coriolis terms. A detailed analysis of the rotational structure yields a set of accurate upper-state parameters up to quartic distortion terms for ν 9 and ν 5 + ν 6. In addition, spectroscopic information about the dark ternary overtone of ν 6 has been obtained.

Spectral narrowing and lasing threshold in self-assembled active photonic crystal

Laser-induced emission from rhodamine-B dye embedded in pseudo band gap opaline photonic crystals is discussed. The photonic crystals are fabricated using rhodamine-B doped polystyrene colloids and show 65% reflectance at the stop band centered at 604nm. The reflectance of the crystal is increased to 74% by coating with a thin layer of gold. Both spontaneous and stimulated emissions of the dye are observed in the photonic stop band environment by exciting the crystal with the second harmonic (532nm) of a Q-switched Nd:YAG laser. The thin layer of gold functioned as a high reflecting end mirror to the dye-doped cavity when the crystal is pumped from the substrate side. Angle-dependent suppression at the stop band wavelength is observed in the spontaneous emission of the dye. Spectrally narrow stimulated emission and lasing is achieved in the gold coated dyed PhC at a threshold pump power of 60mW in a selective direction of 22° from the direction of excitation. By studying emission from several photonic crystals with different number of layers, it is concluded that a sharp threshold for lasing is not observed in uncoated photonic crystals when they contained fewer than 30 ordered layers and lesser than 70% reflectance.

Cadmiumreiche Cadmide des Systems Na/K/Cd

AbstractThe phase formation in the cadmium‐rich section of the ternary system Na/K/Cd was explored by syntheses from the elements. The crystal structures of the compounds in the region of Cd:(Na/K) ratios around 1:5 to 1:6, comprising the known binary compounds Na2Cd11 and K3Cd16 as well as ternary cadmides of the series KNaxCd11–x, were refined on the basis of single‐crystal X‐ray data. The structure of the sodium cadmide Na2Cd11, which was in principle known from early film experiments (Mg2Zn11 structure type, cubic, space group Pm$\bar3$, a = 958.75(9) pm, Z = 3, R1 = 0.042) was confirmed and refined using single crystal data. The border phases of the new series KNaxCd11–x (x = 0.87/0.53; tetragonal, space group I41/amd, a = 1222.98(6)/1215.17(12) pm, c = 766.05(4)/769.05(7) pm, Z = 4, R1 = 0.021/0.024) form the BaCd11 structure type, which was previously unknown for alkaline metal compounds. The barium site of the basic structure type is occupied by potassium, whereas one cadmium site with slightly higher coordination number and interatomic distances is statistically occupied by sodium and cadmium atoms, which leads to the phase width given above. In the structure, the atoms of the two pure Cd positions form channels running along the tetragonal c axis, in which the K and Na/Cd atoms alternate. A similar structural motif is found in the second border compound K3Cd16, which has been described formerly as K0.37Cd2 with a complete disorder of potassium atoms inside the channels. The detailed inspection of single crystal image plate diffractometer data revealed satellite reflections indicating an almost commensurately modulated superstructure. In the new structural model with a quadrupled c axis (tetragonal, space group P4/nnc, a = 915.2(6), c = 1159.0(4) pm, Z = 2) three potassium cations per channel are partially ordered. Both the significant structural factors (strongly changing radius of the countercations) and the electronic criteria, which can be assessed from the pseudo band gap calculated using FP‐LAPW‐DFT methods, are discussed.