Second-order Peaks Research Articles

Emission Diversity of ZnO Nanocrystals with Different Growth Temperatures

ABSTRACTScanning electronic microscopy (SEM), X ray diffraction (XRD) and photoluminescence (PL) have been applied to the study of structural and optical properties of ZnO nanocrystals prepared by the ultrasonic spray pyrolysis (USP) at different temperatures. The variation of temperatures and times at the growth of ZnO films permits modifying the ZnO phase from the amorphous to crystalline, to change the size of ZnO nanocrystals (NCs), as well as to vary their photoluminescence spectra. The study has revealed three types of PL bands in ZnO NCs: defect related emission, the near-band-edge (NBE) PL, related to the LO phonon replica of free exciton (FE) recombination, and its second-order diffraction peaks. The PL bands, related to the LO phonon replica of FE, and its second-order diffraction in the room temperature Pl spectrum testify on the high quality of ZnO films prepared by the USP technology.

On the expected order of kinetics in a series of thermoluminescence (TL) and thermally stimulated conductivity (TSC) peaks

As reported in the literature, both in experimental results and in simulated glow curves, in a series of TL peaks associated with a series of trapping states and a single recombination center, the peaks tend to be of first order. In the present work we show theoretically and demonstrate by examples of numerical simulations that the last peak in a series obtained by a model of a single recombination center and multiple traps may be of second order whereas the lower-temperature peaks are usually of first order. This is the case even when retrapping is significantly faster than recombination. In some cases, the last peak has a long tail, longer than that of second-order peaks, which has to do with a different mechanism that has been discussed in a recent paper. Similar simulations of a more complex and more realistic situation of a model with multiple trapping states and multiple recombination centers have been performed. The prevalence of first-order appearance of both the curves of free electrons, associated with thermally stimulated conductivity (TSC) and of TL, evaluated for randomly chosen sets of trapping parameters, is shown by histograms. The occurrence of a small number of very high values of the symmetry factor is also discussed.

Study of Microwave Backscattering From Two-Dimensional Nonlinear Surfaces of Finite-Depth Seas

This paper presents a study of the microwave backscattering from 2-D time-evolving nonlinear surfaces of a sea with finite depth by using the second-order small-slope approximation. According to the shallow-water dispersion relation, the revised nonlinear hydrodynamic choppy wave model in connection with an experiment-verified sea spectrum for finite-depth water is employed to construct the wave profiles in the finite-depth sea. The numerical results show that the discrepancy between the choppy surfaces of the infinite-depth sea and their finite-depth counterparts for monostatic normalized radar cross section is much smaller than that between the linear surfaces and the nonlinear choppy surfaces. Furthermore, the comparison of the Doppler spectra of the backscattered echoes from the linear and nonlinear choppy sea surfaces shows that the nonlinear hydrodynamic features significantly impact the Doppler spectrum. In particular, the Doppler spectrum for nonlinear finite-depth sea presents much higher second-order peaks and increased spectral amplitudes in the frequency range around the Doppler peak frequency, which reiterates the importance of the role that the nonlinear hydrodynamic effect of waves played in the interpretation of backscattering from finite-depth nearshore seas from the qualitative point of view.

Emission spectrum of the driven nonlinear oscillator

Motivated by recent "circuit QED" experiments we investigate the noise properties of coherently driven nonlinear resonators. By using Josephson junctions in superconducting circuits, strong nonlinearities can be engineered, which lead to the appearance of pronounced effects already for a low number of photons in the resonator. Based on a master equation approach we determine the emission spectrum and observe for typical circuit QED parameters, in addition to the primary Raman-type peaks, second-order peaks. These peaks describe higher harmonics in the slow noise-induced fluctuations of the oscillation amplitude of the resonator and provide a clear signature of the nonlinear nature of the system.

Investigation of micropipes in 6H–SiC by Raman scattering

The spectra of 6H–SiC crystals including micropipes have been examined for the Si face using Raman scattering. The first-order Raman features reveal that the intensity of the transverse optical phonon band centered at ∼796 cm −1 is sensitive to the micropipes. And the second-order Raman features of the micropipes in bulk 6H–SiC are well-defined using the selection rules for second-order scattering in wurtzite structure. It is found that there are some second-order peaks missing for the micropipe-including sample, which may be induced by the reduction of the incident laser intensity at around the micropipe, especially the uneven surface in the inner wall of the micropipe. These features might also be employed to characterize other structural defects such as screw-dislocations and threading edge dislocations.

Higher-order interference of low-coherence optical fiber sensors

The higher-order interference noise that is caused by multireflection at the fiber sensor's end surface in low-coherence fiber sensor array is proposed. The generation of the higher-order interference noise and its quantity and amplitude are theoretically analyzed. The second-order interference noises are experimentally demonstrated. The results show that the second-order noises arise in any sensor array composed of more than two sensors and the number of the second-order peaks is proportional to the third power of the sensor's number. The ratio of the amplitude of the second-order noise to that of the signal peak is proportional to the reflectivity of the sensor's end surface. In a sensor array, when the reflectivity is more than 10(-5), the amplitude of the second-order noise is higher than other noises and it becomes a main factor that determines the signal-to-noise ratio of the sensor arrays. Therefore, reducing the higher-order interference noise can improve the multiplexing capacity of the sensor array.

The structures and properties of FeSin/FeSi\hbox{$_{\mathsf{n}}^{+}$}+n/FeSi\hbox{$_{\mathsf{n}}^{-}$}−n (n = 1 ~ 8) clusters

The geometry, stability, and electronic properties of iron-doped silicon clusters FeSi n /FeSi\hbox{$_{n}^{+}$}+n/FeSi\hbox{$_{n}^{-}$}−n (n = 1 ~ 8) have been systematically investigated using the density functional theory (DFT) approach at the B3LYP/6-311+G* level. Our results show that the ground state structures of FeSi n /FeSi\hbox{$_{n}^{+}$}+n/FeSi\hbox{$_{n}^{-}$}−n change from planar to three-dimensional for n > 3. Bipyramidal structures, or their face-capped isomers, are favored for the larger clusters. For neutral FeSi n clusters, their ground state structures are the trigonal, tetragonal, capped tetragonal, capped pentagonal, and combined tetragonal bipyramids for n = 4 ~ 8, respectively. The lowest-energy structures of the anionic FeSi\hbox{$_{n}^{-}$}−n clusters essentially retain similar frameworks to their neutral counterparts, while those of the cationic FeSi\hbox{$_{n}^{+}$}+n clusters are significantly deformed; this is confirmed by their calculated ionization potential and electronic affinity values. For most of the stable structures, the spin electronic configurations are s = 1 or 2 for neutral FeSi n , s = 3/2 or 5/2 for ionic FeSi\hbox{$_{n}^{+}$}+n/FeSi\hbox{$_{n}^{-}$}−n. The average binding energy values generally increase with increasing cluster size, indicating the clusters can continue to gain energy during the growth process. Fragmentation and second-order energy peaks (maxima) are found at n = 2, 5, and 7 for FeSi n /FeSi\hbox{$_{n}^{-}$}−n, n = 4 and 6 for FeSi\hbox{$_{n}^{+}$}+n, suggesting that these clusters possess higher relative stability. Furthermore, the HOMO-LUMO gap values show that anionic FeSi\hbox{$_{n}^{-}$}−n have greater chemical reactivity than cationic FeSi\hbox{$_{n}^{+}$}+n and neutral FeSi n , except when n = 7.

Growth of carbon nanotubes at temperatures compatible with integrated circuit technologies

The potential applications of carbon nanotubes grown for semiconductor and sensor devices are immense. But, this growth must be CMOS compatible and over large-areas, in excess of 4 inches in diameter, for any industrial interest. Reports of low-temperature growth of carbon nanotubes have mostly resulted in false dawns in the context of CMOS production, with direct integration for mass manufacturing remaining a challenge, as lower synthesis temperatures matched to manufacturing result in nanotubes with high defect levels. We report a unique ‘top-down’ synthesis method that allows energy delivery directly to the catalyst, resulting in higher quality nanotube growth at compatible low substrate temperatures. This growth is demonstrated over a large-area, whilst maintaining the silicon substrate below 350 °C. Long-range ordering of carbon nanotubes is supported by well developed second-order Raman peaks and HREM. The methodology developed is suitable to produce many nano-material systems, including graphene and silicon nanowires.

Order and disorder in the wetting layer on Ru(0001)

The growth of an intact water monolayer on Ru(0001) has been investigated by comparing the ordering of O and Ru, determined by low-energy electron diffraction (LEED), with that of the top layer of O and H, as probed by He atom scattering (HAS). Although LEED shows that water forms an extended commensurate (square root 3 x square root 3) R30 degrees structure as the coverage approaches 0.67 monolayer, the HAS distributions are insensitive to the exact water coverage and show a very low specular reflectivity, indicating a disordered water layer. The angular profile from a D2O monolayer shows a broad diffuse peak in the angular scattering distribution at a momentum exchange similar to the position of the second-order (1/3, 1/3) peaks, but the maxima show little variation with scattering azimuth. H2O shows a slightly higher He reflectivity and more clearly resolved angular structure, with broad, faint peaks appearing close to the first-order diffraction positions. The origin of this disorder is discussed based on density functional calculations for the monolayer which find that water forms chains of flat and H-down molecules within a hexagonal hydrogen-bonding network, rather than the ice bilayer usually assumed. This arrangement leads to long-range order in the O location, but disorder in the O height and the proton orientation. We discuss how this combination of lateral order in the adsorption site, but disorder in the water orientation, is reflected in the sharp square root 3 LEED pattern but diffuse, broad peaks in He scattering.

Growth and characterization of ZnGeP 2 single crystals by the modified Bridgman method

A good quality ZnGeP 2 (ZGP) single crystal 15 mm in diameter and 40 mm in length was grown in a vertical three-zone tubular furnace by the modified Bridgman method, i.e. real-time temperature compensation technique (RTTCT) with descending ampoule. The starting material is high-pure and single-phase polycrystal of ZnGeP 2 synthesized by the single-temperature zone and mechanical oscillation method (STZMOM). The as-grown single crystal was characterized by various methods, including X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive analyzer of X-ray (EDAX), and IR and UV spectrophotometers. It is found that there is a cleavage face of (1 1 2) and second-order XRD peaks of the {1 1 2} faces are observed, the IR transmittance of a sample of 2 mm thickness is above 55% in the range from 6000 to 800 cm −1, the absorption edge is near 612.5 nm and the band gap is about 2.02 eV. The absorption coefficient ( α) is within 0.015–0.022 cm −1 at the spectral region 2–8 μm. The crystal has a stoichiometry ratio Zn:Ge:P=1:1.12:1.91 which is close to the ideal stoichiometry ratio of 1:1:2. All results demonstrate that the modified growth method is a new and promising method for the ZGP single crystal and the quality of as-grown crystal is good.

Kohn anomalies and nonadiabaticity in doped carbon nanotubes

The high-frequency Raman-active phonon modes of metallic single-walled carbon nanotubes (SWCNT) are thought to be characterized by Kohn anomalies (KAs) resulting from the combination of SWCNT intrinsic one-dimensional nature and a significant electron-phonon coupling (EPC). KAs are expected to be modified by the doping-induced tuning of the Fermi energy level ${ϵ}_{F}$, obtained through the intercalation of SWCNTs with alkali atoms or by the application of a gate potential. We present a density-functional theory (DFT) study of the phonon properties of a (9,9) metallic SWCNT as a function of electronic doping. For such study, we use, as in standard DFT calculations of vibrational properties, the Born-Oppenheimer (BO) approximation. We also develop an analytical model capable of reproducing and interpreting our DFT results. Both DFT calculations and this model predict, for increasing doping levels, a series of EPC-induced KAs in the vibrational mode parallel to the tube axis at the $\mathbf{\ensuremath{\Gamma}}$ point of the Brillouin zone, usually indicated in Raman spectroscopy as the ${G}^{\ensuremath{-}}$ peak. Such KAs would arise each time a new conduction band is populated. However, we show that they are an artifact of the BO approximation. The inclusion of nonadiabatic effects dramatically affects the results, predicting KAs at $\mathbf{\ensuremath{\Gamma}}$ only when ${ϵ}_{F}$ is close to a band crossing ${E}_{X}$. For each band crossing, a double KA occurs for ${ϵ}_{F}={E}_{X}\ifmmode\pm\else\textpm\fi{}\ensuremath{\hbar}\ensuremath{\omega}∕2$, where $\ensuremath{\hbar}\ensuremath{\omega}$ is the phonon energy. In particular, for a $1.2\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ metallic nanotube, we predict a KA to occur in the so-called ${G}^{\ensuremath{-}}$ peak at a doping level of about ${N}_{\mathrm{el}}∕C=\ifmmode\pm\else\textpm\fi{}0.0015$ atom $({ϵ}_{F}\ensuremath{\approx}\ifmmode\pm\else\textpm\fi{}0.1\phantom{\rule{0.3em}{0ex}}\mathrm{eV})$ and, possibly, close to the saturation doping level $({N}_{\mathrm{el}}∕C\ensuremath{\sim}0.125)$, where an interlayer band crosses the ${\ensuremath{\pi}}^{*}$ nanotube bands. Furthermore, we predict that the Raman linewidth of the ${G}^{\ensuremath{-}}$ peak significantly decreases for $\ensuremath{\mid}{ϵ}_{F}\ensuremath{\mid}\ensuremath{\geqslant}\ensuremath{\hbar}\ensuremath{\omega}∕2$. Thus, our results provide a tool to determine experimentally the doping level from the value of the KA-induced frequency shift and from the linewidth of the ${G}^{\ensuremath{-}}$ peak. Finally, we predict KAs to occur in phonons with finite momentum $\mathbf{q}$ not only in proximity of a band crossing but also each time a new band is populated. Such KAs should be observable in the double-resonant Raman peaks, such as the defect-activated $D$ peak, and the second-order peaks $2D$ and $2G$.

Scattering of light by a periodic structure in the presence of randomness V Detection of successive peaks in a periodic structure

We address the problem of detecting periodic structures hidden behind roughness. We have shown that if r(0) is the coherence length of the scattered radiation, due to the random part of the surface, and Lambda is the wavelength of the periodic part of the surface, then with a matched filtering method that we introduce, and by using simple computations with the intensity data, it is possible to detect the hidden first-order peak even when (r(0)/Lambda) approximately 0.11. Here we advance the method to bring out very weak second-order peaks, which we demonstrate for what we believe is the first time. The unmistakable presence of both the first- and second-order peaks, which have identical shapes as the zeroth-order peak, is strong evidence of the hidden periodicity and serves as a novel method for the detection of weak periodicities hidden behind strong randomness.

On the Singular Nature of the Second-Order Peaks in HF Radar Sea Echo

Electromagnetic (EM) scattering from the sea surface concerned with second-order Doppler spectra for finite-length pulses is theoretically analyzed in the scattering regime typical of, but not confined to, high-frequency (HF) radars. The Doppler spectra of the second-order cross section consist of three different parts: continuum and two pairs of peaks-the second harmonic and corner reflection peaks. This paper is the first investigation of fundamental properties of these peaks from the perspective of their use in measurements of surface currents. It is shown that these peaks are of singular nature in the following sense: The main (singular) contribution is due to particular pairs of waves, despite the fact that waves of many different wavelengths and directions contribute to these peaks. This fact opens a possibility to employ these peaks in remote sensing of vertical profiles of surface currents. Using the number of waves in the pulse (the pulse length) LGt1 as a large parameter, an asymptotic description of these peaks is developed. Height, width, and position of the peaks are explicitly found in terms of L. The peak positions, to the leading order, do not depend on the pulse length, although a small explicitly found O(L-1) shift has to be taken into account for the corner reflection peaks. The heights are ~lnL and ~L1/2 for the second harmonic and corner reflection peaks, respectively. The results open the way for wider use of the second-order peaks for probing surface currents

Coupled-wave analysis of apodized volume gratings

This work presents the use of longitudinal refractive index modulation (apodization) in photosensitive glass for improved sidelobe suppression in volume holographic optical elements. We develop a numerical model for both uniform and apodized volume holograms based on rigorous coupled-wave analysis. We validate the model by comparison with a transmissive 1.55- mum uniform volume grating in photothermorefractive glass. We then apply our numerical model to calculate the spectral response of apodized gratings. The numerical results demonstrate that apodization of the refractive index modulation envelope improves spectral selectivity and reduces first and second-order side-lobe peaks by up to 33 and 65 dB, respectively. We suggest a method for creating apodization in volume holograms with approximately Gaussian spatial refractive index profile.

InGaN multiple‐quantum‐well light‐emitting diodes on an AlN/sapphire template by metalorganic chemical vapor deposition

An InGaN multiple-quantum-well (MQW) LED grown on AlN/sapphire template is reported. Comparing with the conventional LED on sapphire using a low temperature (LT) buffer layer, the LED on AlN/sapphire template shows better electrical and optical characteristics. The crystalline quality of the LED structure was investigated by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The zeroth, -first- and -second-order satellite peaks and the Pendellosung fringes can be seen clearly in (0004) XRD ω/2θ rocking curves for the LED on AlN/sapphire template. While, for the LED on sapphire, only the zeroth and -first-order satellite peaks can be seen. The full width at half maximum (FWHM) of GaN in (0004) ω scan is about 82.8 and 231.6 arcsec for the LED grown on AlN/sapphire template and sapphire, respectively. The dislocation density is 5 × 107–3 × 108 cm−2 for the LED on AlN/sapphire template and 2–5 × 109 cm−2 for the LED on sapphire. The reverse leakage current of the LED on AlN/sapphire template is over one order of magnitude lower than that on sapphire due to the low threading dislocation density in the epilayer. The optical power of the LED on AlN/sapphire template increased sublinearly up to 300 mA injection current. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Study of far-infrared reflection and Raman scattering spectra in reactive ion etched ZnTe

Far-infrared reflection and Raman scattering measurements have been carried out on reactive ion etched p-ZnTe samples. The averaged thickness of the surface damaged layer is found to be in the range of 1.0-1.5µm and the etch-induced defect density is in the order of 1018cm-3. The Raman intensity ratio between the second-order Raman peaks and the first-order longitudinal optical phonons reveals an increase trend with the radio frequency (rf) power. With the aid of related theories, we discuss the effects of the rf plasma power and the concentration of CH4/H2 on the damage, disorder and the second-order Raman structures in p-ZnTe samples.

Concentration-Dependent Formation Mechanisms in Mesophase Silica−Surfactant Films

The development of surfactant-templated mesoporous films grown at the air/water interface was investigated using specular and off-specular X-ray reflectivity techniques. The samples were prepared in acidic conditions using cetyltrimethylammonium bromide (CTABr, 0.075 M) and different concentrations (0.27−0.88 M) of tetramethyloxysilane (TMOS). At CTABr/TMOS molar ratio between 0.277 and 0.093, the final films exhibited at least three orders of diffraction, corresponding to a real space repeat distance of 46 A. This long-range order was not observed when the TMOS content was increased to 0.88 M at constant CTABr concentration. In each case, the induction period prior to film formation was monitored using diffuse X-ray scattering. The intensity of the specular reflectivity peak, first and second-order diffraction peaks, and Yoneda wing were recorded as a function of time. The reflected intensity of the specular peak oscillated during the lengthy induction period, indicating that at some concentrations the f...

On the possibility of using commercial software packages for thermoluminescence glow curve deconvolution analysis.

This paper explores the possibility of using commercial software for thermoluminescence glow curve deconvolution (GCD) analysis. The program PEAKFIT has been used to perform GCD analysis of complex glow curves of quartz and dosimetric materials. First-order TL peaks were represented successfully using the Weibull distribution function. Second-order and general-order TL peaks were represented accurately by using the Logistic asymmetric functions with varying symmetry parameters. Analytical expressions were derived for determining the energy E from the parameters of the Logistic asymmetric functions. The accuracy of these analytical expressions for E was tested for a wide variety of kinetic parameters and was found to be comparable to the commonly used expressions in the TL literature. The effectiveness of fit of the analytical functions used here was tested using the figure of merit (FOM) and was found to be comparable to the accuracy of recently published GCD expressions for first- and general-order kinetics.

Evidence of a tilted and herringbone structure in cadmium behenate Langmuir–Blodgett ultrathin films: Comparison with Langmuir monolayers

The molecular arrangement of monolayers and multilayers of cadmium behenate deposited at the air–water interface or on air-oxidized silicon wafers was investigated by means of grazing incidence x-ray diffraction. Even though it is frequently mentioned that the Langmuir–Blodgett (LB) monolayers of fatty acid salts have usually an in-plane hexagonal packing, we obtained two first-order peaks in all the investigated films, indicating a distorted hexagonal lattice. Whereas these peaks are in-plane in the Langmuir or LB monolayers implying no molecular tilt, they are slightly out of the plane in the 3 and 5 layers samples indicating tilted molecules. In these few layers LB films, second-order peaks are detected and the molecules are tilted towards a next-nearest neighbor (NNN structure), as in the L2′ phase of fatty acids. Using two angles of incidence of the x-ray beam allowing to probe either all the layers or only the external layers, a distribution of the tilt angle in the range 1°–6° is evidenced within each layer of the LB films. Moreover, the herringbone peak is measured in this NNN structure corresponding to the L2′ phase, implying that the herringbone order is long-range in the LB films.

Hydrogen desorption kinetics and Ge 2H 6 reactive sticking probabilities on Ge-adsorbed Si( [formula omitted

Modeling of Si 1− x Ge x (0 1 1) gas-source epitaxy from hydride precursors requires knowledge of reactive sticking probabilities and hydrogen desorption kinetics. We use D 2 temperature programmed desorption (TPD), reflection high energy electron diffraction (RHEED), Auger electron spectroscopy (AES), and kinetic modeling to determine hydrogen desorption kinetics from, and Ge 2H 6 reactive sticking probabilities at, Si and Ge sites on Ge-adsorbed Si(0 1 1) samples with Ge surface coverages θ Ge between 0 and 1 ML. Following Ge adsorption, the samples were exposed to atomic D until saturation coverage at 250 °C. TPD spectra consist of five second-order peaks due to D 2 desorption from, in order of decreasing temperature, Si rest-atom and adatom monodeuterides, Si dideuteride, and Ge rest-atom and adatom monodeuteride phases. The maximum temperatures of all five peaks shift to lower values with increasing θ Ge. We attribute this, as in the case of Ge-adsorbed Si(0 0 1), to a decrease in Si–D and Ge–D binding energies due to long-range electronic interactions. Ge/Si(0 1 1) surface reconstructions, determined by RHEED, are “16×2” with θ Ge<0.4 ML and “2×8” at higher coverages. Quantitative θ Ge values were obtained by fitting the D 2 TPD spectra and validated using in situ AES. Based upon θ Ge vs Ge 2H 6 exposure data and modeling of the adsorption kinetics, we obtain zero-coverage Ge 2H 6 reactive sticking probabilities of 1.9×10 −2 and 4.8×10 −3 at Si and Ge sites, respectively.