Spectra Of Bulk Research Articles

Excitonic effects in the K and L2,3 edges spectra of bulk and monolayer black phosphorus from first-principles

In this study, the K and L2,3 X-ray absorption near edge structure (XANES) of bulk and monolayer black phosphorus (BP) were calculated and compared. Calculations were performed in two different levels of theory, with and without considering the excitonic effects. The present calculations indicated that both bulk and monolayer BP showed strong anisotropy in their core absorption spectra. The results demonstrated that the electron-hole coupling and formation of the bound exciton had a significant effect on the edge spectra of both bulk and monolayer BP in a way that the excitonic effect modified the oscillator strengths and the energy positions of the main spectral structures, intensely. The edge spectra of bulk were in a good agreement with the existing experimental core spectra. As the focus shifted from bulk to monolayer, all the K and L2,3 absorption spectra were red-shifted. When excitonic effects were included in the calculations, the spectral features were red-shifted, as well. Decomposition of L2,3 edge to L2 and L3 edges indicated that the contribution of the L3 edge was dominant. Local densities of states (DOS) calculations showed that p-like and d-like states of phosphorus atoms play the main role in the electron transitions of K and L2,3 edges spectra, respectively.

Sensitivity of quantum walks to a boundary of two-dimensional lattices: approaches based on the CGMV method and topological phases

In this paper, we treat quantum walks in a two-dimensional lattice with cutting edges along a straight boundary introduced by Asboth and Edge (2015 Phys. Rev. A 91 022324) in order to study one-dimensional edge states originating from topological phases of matter and to obtain collateral evidence of how a quantum walker reacts to the boundary. Firstly, we connect this model to the CMV matrix, which provides a 5-term recursion relation of the Laurent polynomial associated with spectral measure on the unit circle. Secondly, we explicitly derive the spectra of bulk and edge states of the quantum walk with the boundary using spectral analysis of the CMV matrix. Thirdly, while topological numbers of the model studied so far are well-defined only when gaps in the bulk spectrum exist, we find a new topological number defined only when there are no gaps in the bulk spectrum. We confirm that the existence of the spectrum for edge states derived from the CMV matrix is consistent with the prediction from a bulk-edge correspondence using topological numbers calculated in the cases where gaps in the bulk spectrum do or do not exist. Finally, we show how the edge states contribute to the asymptotic behavior of the quantum walk through limit theorems of the finding probability. Conversely, we also propose a differential equation using this limit distribution whose solution is the underlying edge state.

Size and shape-tailored hydrothermal synthesis and characterization of nanocrystalline LaPO4:Eu3+ phosphor

A facile hydrothermal route has been demonstrated for the synthesis of Eu3+ doped LaPO4 with distinct and well-controlled morphologies (spherical and rod-like). Morphological control has been established by fine-tuning the pH of the precursor solution, and formation of different morphologies at different pH values has been explained by taking into consideration the chemical potentials of the various species present in the precursor solution. At constant pH of the mother liquor, an increase in the autoclaving temperature from 120°C to 140°C is seen to change the aspect ratio of the nanorods. A red shift in the charge transfer band is observed in the excitation spectra of the nanocrystalline samples (both nanorod and nanosphere) with respect to the bulk. A change in the (red/orange) peak intensity ratio in the photoluminescence emission spectra of bulk, spherical and rod-like nanocrystalline samples reveals that the host morphology plays a vital role in determining the local symmetry of Eu3+ in LaPO4 matrix. Decay-time measurement of the as-prepared morphologies at 620nm (for 5D0–7F2 transition) leads to a maximum value for nanorods (3.7ms), while the corresponding values for nanospheres and bulk samples are somewhat smaller (2.58ms and 2.99ms respectively). This clearly indicates the influence of shape anisotropy on the photoluminescence properties of the as-synthesized nanoarchitectures.

Unveiling the Key Factors in the High Performance of SnO2 Anode Material for the Advanced Lithium-Ion Batteries

Lithium-ion batteries (LIBs) have been recognized as major part in the development of electrical devices such as portable device, energy storage system (ESS) and electric vehicle (EV) due to high energy storage-demand. Graphite material is the most widely used as anode part in commercial LIBs due to its low cost and good cycle life. However, due to limited capacity (~372mAh/g) of graphite, alternative anode materials based on conversion or alloying reaction have been extensively studied, which shows exceptional high capacity. Among these anode materials, SnO2 has been attracted as one of the promising anode materials, which stores high electric energy by conversion and alloying reaction during first discharge process: (1) SnO2 + 4Li++ 4e- → Sn + 2Li2O (Conversion reaction; 711mAh/g), (2) Sn + xLi++ xe- → LixSn (0<<x<<4.4) (Alloying reaction; 993mAh/g of Sn or 783mAh/g of SnO2). In this lithium storage mechanism, only alloying reaction has been contributed to reversible capacity after the first irreversible discharge process.[1] Currently, there are several reports that some nano-structured and composite SnO2 materials deliver exceptional high reversible capacity beyond their theoretical capacity based on alloying reaction without clear explanation of additional capacity.[2,3] In our previous work, for elucidating the additional capacity and the detailed lithium storage mechanism of nano-structured SnO2, we performed systematic analysis about the mesoporous SnO2 anode material using synchrotron radiation based analyses such as XRD and XAS, and reported novel electrochemical reaction mechanism. Ordered mesoporous SnO2 prepared by hard templating showed a reversible capacity of about 1100 mAh/g, which is higher than the predicted value based on the alloying reaction mechanism of Sn, formed from conversion reaction of SnO2 with lithium. Synchrotron XAS analysis combined with XRD revealed that some portion of the Li2O phase decomposes to form the SnOx phase with Sn upon delithiation (additional conversion reaction), parallel with dealloying of LixSn (alloying reaction), which leads to unexpected high capacity of an ordered mesoporous SnO2 material.[4] Namely, high performance of SnO2 anode material is highly related to the additional conversion reaction during electrochemical cycling and stable structure for accommodating volume expansion. In this study, to understand the main reason for the difference in electrochemical performance between bulk and mesoporous SnO2, we performed XAS experiment about bulk SnO2 anode material in same procedure with our previous report, and carried out quantitative analysis using EXAFS spectra of bulk and mesoporous sample as well. Besides, for maximizing the performance of SnO2 anode material for LIBs, we prepared SnO2/rGO composite, synthesized by simple refluxing method using graphene oxide, SnCl4 and ascorbic acid. Composition of metal oxide nanoparticles and rGO systems is the efficient methods to improve electrochemical performance in terms of rate performance and cyclability. In metal oxide/rGO composite anode materials, rGO provide faster lithium ion diffusion pathway through rGO sheets and metal-O-C linkage at the interface, and ensure stable structure during volume expansion aroused from electrochemical cycling by strong interaction between rGO sheets and metal oxide nanoparticles.[5] The discharge/charge capacity of bulk, mesoporous, and SnO2/rGO composite were 1644, 1073mAh/g; 2059, 1148mAh/g; and 1752, 1185mAh/g, respectively. Except the irreversible capacity during the initial discharge, reversible capacity is similar to each other. However, compared to bulk and mesoporous SnO2, SnO2/rGO composite material shows prominent cyclability, and its capacity retention after 100th cycles is over 100%. This phenomenon may be related to the stable rGO source and exfoliation process during electrochemical cycling, respectively.[6] Using synchrotron radiation based techniques, we found that high reversible capacity of SnO2/rGO composite comes from alloying reaction and partial conversion reaction as same as bulk and mesoporous SnO2. Additionally, through further XAS experiment using cycled samples, we confirmed key factors in the high performance of SnO2/rGO composite anode material. More detailed discussion will be presented at the time of meeting. Reference [1] I. A. Courtney et al., Journal of The Electrochemical Society 144 (1997) 2045-2052 [2] Z. Ying et al., Appl. Phys. Lett. 87 (2005) 113108 [3] J. Lin et al., ACS Nano 7, (2013) 6001–6006 [4] H. Kim et al., Chemistry of materials 26 (2014) 6361-6370 [5] X.-Y. Shan et al., J. Mater. Chem. A 2 (2014) 17808–17814 [6] J. Wang et al., J. Am. Chem. Soc. 133 (2011) 8888–8891

A Size‐Dependent Analysis of the Structural, Surface, Colloidal, and Thermal Properties of Ti1–xB2 (x = 0.03–0.08) Nanoparticles

We report the size‐dependent structural, surface, colloidal, and thermal properties of the well‐known refractory material titanium diboride prepared at the nanoscale by a size‐controlled inorganic molten‐salt technique. A combined analysis of the powder X‐ray diffraction data through a modified Williamson–Hall plot, a size–strain plot, and Rietveld fitting methods indicates that TiB2 nanocrystals with average crystallite sizes smaller than D ≈ 22 nm prefer to form defects at the titanium site rather than experience strain. The resulting composition is Ti1–xB2 [x = 0.03(1)–0.08(1)]. The size‐induced defect formation is accompanied by anisotropic lattice contraction that decreases primarily the cell parameter c. Transmission electron microscopy revealed that the nanocrystals are embedded in an amorphous matrix. A comparison between the electron spin resonance spectra of bulk and nanosized samples indicated that only the nanosized samples yield observable signals. This signal is associated with unpaired electrons trapped in the boron–oxygen species that surround the Ti1–xB2 nanocrystals. These species provide stability to the Ti1–xB2 nanocrystals in aqueous dispersions, as evidenced by electrokinetic measurements. Upon heating under an Ar atmosphere, the boron–oxygen species evaporate from ca. 1000 °C, and the Ti vacancies in the interior nanocrystalline core vanish. TiB2 nanocrystals show excellent chemical stability against decomposition up to 1500 °C.

Highly sensitive SERS analysis of the cyclic Arg-Gly-Asp peptide ligands of cells using nanogap antennas.

The cyclic RGD (cRGD) peptide ligands of cells have become widely used for treating several cancers. We report a highly sensitive analysis of c(RGDfC) using surface enhanced Raman spectroscopy (SERS) using single dimer nanogap antennas in aqueous environment. Good agreement between characteristic peaks of the SERS and the Raman spectra of bulk c(RGDfC) with its peptide's constituents were observed. The exhibited blinking of the SERS spectra and synchronization of intensity fluctuations, suggest that the SERS spectra acquired from single dimer nanogap antennas was dominated by the spectrum of single to a few molecules. SERS spectra of c(RGDfC) could be used to detect at the nanoscale, the cells' transmembrane proteins binding to its ligand. SERS of cyclic RGD on nanogap antenna.

Contactless Electroreflectance Spectroscopy of ZnO with Different Polarization-Direction Probe Beams

Contactless electroreflectance (CER) spectra of ZnO bulk has been measured at 300K. It is observed the difference between the CER spectra using by the polarization E of probe light perpendicular ( Ec) and parallel ( Ec) to the c-axis of the m-plane ZnO. In addition, a mercury lamp was focused on the sample to reduce its strength of electric field. It was observed that the CER spectrum was blue shifted with Hg lamp on. Hence, the observed features were attributed to excitonic transitions. The experimental spectra were fitted by Lorentzian lineshapes. The energies of the A ( B), B(A), and C transitions can be determined.

Resources of polarimetric sensitivity in spin noise spectroscopy

We attract attention to the fact that the ultimate (shot-noise-limited) polarimetric sensitivity can be enhanced by orders of magnitude leaving the photon flux incident onto the photodetector on the same low level. This opportunity is of crucial importance for present-day spin noise spectroscopy, where a direct increase of sensitivity by increasing the probe beam power is strongly restricted by the admissible input power of the broadband photodetectors. The gain in sensitivity is achieved by replacing the 45-deg polarization geometry commonly used in conventional schemes with balanced detectors by geometries with stronger polarization extinction. The efficiency of these high-extinction polarization geometries with enhancement of the detected signal by more than an order of magnitude is demonstrated by measurements of the spin noise spectra of bulk n:GaAs in the spectral range 835-918 nm. It is shown that the inevitable growth of the probe beam power with the sensitivity gain makes spin noise spectroscopy much more perturbative, but, at the same time, opens up fresh opportunities for studying nonlinear interactions of strong light fields with spin ensembles.

On the relationship between plasticity and spectral characteristics of swelling soils: The 3–5 μm wavelength region

Soil swelling is a potential geotechnical hazard that causes considerable problems leading to serious economic consequences in construction sectors, principally light-weight infrastructure development. Simple identification of swelling soils and evaluation of their geotechnical properties is a constant focus of research in geotechnical engineering, including the application of spectroscopic techniques. Potential of laboratory spectroscopy in the 3–5μm wavelength region was investigated for compositional analyses and classification of swelling soils. Spectra of bulk and fine fraction (passing the ASTM 0.075mm sieve aperture) soil samples were investigated. The relationships between soil spectra and plasticity were explored using various statistical analyses techniques such as cluster analysis and cross tabulation. Classification of soils based on spectral characteristics showed significant correlations with soil plasticity classes. Strong statistical measures of associations such as high chi-square test statistics, likelihood ratio and contingency coefficients were obtained. Correlations varied from bulk to fine soil fraction. A higher level of agreement with plasticity was achieved for the spectra of fine fraction soils than for the bulk soil samples. The study showed that spectroscopy in the 3–5μm wavelength region is useful for geotechnical investigation of swelling soils.

Computing Raman and infrared wavenumbers of nanostructures: application to silicon nanowires

The characterization of nanostructures with spectroscopic methods is a fundamental tool in nanoscience. For novel nanostructures, the interpretation of spectral features is a challenging task. To address this issue, we present the “Symmetry‐Filtered Molecular Dynamics (SFMD)” method to calculate Raman and infrared wavenumbers from molecular dynamics (MD) simulations, employing only the symmetry of the atomic structure. Explicit and expensive calculations of the electric polarizability or the dipole moment are not required. Therefore, our method can be easily used with any standard MD software. On the basis of the density functional tight‐binding method for the MD simulations, we apply our method to bulk silicon and small‐diameter hydrogen‐passivated silicon nanowires. For bulk silicon, we study the wavenumber shift of the Raman peak with temperature and obtain results that are in good agreement with experiments. We further show that thermal lattice expansion is a minor effect (22%) and that temperature‐driven anharmonic effects (78%) are the main contributions to that wavenumber shift. By analyzing the bond lengths of different silicon nanowires, we found that surface stress manifests as a 0.37% shortening of bonds only in the outermost silicon layer. We further analyzed the diameter‐dependent wavenumber shift of a Raman peak in silicon nanowires. We found that the main contribution to the wavenumber shift comes from the phonon confinement effect and surface stress leads to an additional shift of 9–22%. Our results indicate that our method is able to produce quantitative results that can be compared with experiments. We propose our method to be used for the understanding of Raman and infrared spectra of novel bulk and nanostructures. Copyright © 2012 John Wiley &amp; Sons, Ltd.

Micro-Raman studies on thermally evaporated Ag2Se thin films

Silver selenide (Ag2Se) thin films of thickness between 80 and 160 nm were prepared by thermal evaporation method. From XRD studies, the structure of the prepared film is confirmed to exhibit orthorhombic structure with polycrystalline nature. The average grain size of Ag2Se has been determined as 43 nm. EDAX spectra of these films were taken, and its composition ratio has been analyzed. Micro-Raman spectra of bulk and their films have been recorded with He-Ne laser source at room temperature. Raman peaks are observed at 232, 455, 566 and 815 cm−1. These peaks are assigned to their corresponding modes of vibrations.

Temperature Dependence of Contactless Electroreflance Spectroscopy of C-Plane ZnO Bulk

Contactless electroreflectance（CER）spectra of c-plane ZnO bulk have been measured from 20 to 300 K. Photoreflectance（PR）spectrum of ZnO bulk has also been measured at 300 K. The CER spectrum is red-shifted with respect to the PR spectrum at 300 K. It is known that the average field experienced in the CER is larger than that of the PR. In addition, the features of PR or CER is blue-shifted when the field becomes larger if they belong to band-to-band transitions. Hence, the observed features are excluded from band-to-band transitions and are attributed to excitonic transitions. The CER spectra were then fitted by Lorentzian profiles. The energies of the A（B）, B（A）, and C excitonic transitions at 0K, which were obtained by the best fit of Varshni’s equation, are 3.419, 3.436, and 3.482 eV, respectively.

Deposition from Dopamine Solutions at Ge Substrates: An in Situ ATR-FTIR Study

Deposition from dopamine (DA) solutions at germanium (Ge) model substrates was monitored under stationary conditions using surface sensitive in situ attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy. ATR-FTIR spectra of the interfacial organic layer formed upon contact of TRIS buffered aqueous DA solutions to a Ge internal reflection element (IRE) showed conveniently strong diagnostic IR absorption bands, which were increasing with deposition time up to at least 6 h. Comparison of IR spectra of unreacted pristine DA, surface reacted, and bulk reacted material confirmed chemical reactions of DA to a polymerizate according to the literature. The found IR bands could be assigned to aromatic as well as C-O single bond moieties. The kinetic courses of the diagnostic band integrals showed an initial increase and saturation of the deposition after around 300 min, which could be empirically represented by an exponential damping function revealing a rather small kinetic constant. Highest deposition levels were found at pH = 8.5 (TRIS buffer or NaOH) in contrast to pH = 6.2, where no deposition occurred. Minor deposition was found in the presence of salt or at ZnSe instead of Ge due to the absence of reactive hydroxyl groups. The concentration dependence of DA deposition showed an initial increase and a saturation beginning at around 0.4 mg/mL (0.0022 M), where around 50 nm thick films featuring granular surface morphologies are formed. The adsorbed species are suggested to be smaller bulk reacted DA polymerizate particles with reactive end groups. Rinsing the formed films by pure TRIS buffer resulted in a time dependent release of deposited organic material by ≈23%, which could be represented by an exponential decay function. A saturation of the release after around 100 min and a larger kinetic constant compared to deposition could be determined. The released material is suggested to be larger aggregated bulk reacted DA polymerizate particles loosely bound to the surface by weak interaction forces.

Spectra of optical parameters in bulk and film amorphous alloys of the Se95As5 system containing samarium (Sm) impurities

Reflectance spectra of bulk and film amorphous alloys of the Se95As5 system containing samarium (Sm) impurities are studied in the energy range of 1–6 eV. Spectral dependences of optical constants and derivatives of optical dielectric functions are calculated by the Kramers-Kronig method. Changes in spectra of optical parameters depending on the content of impurities introduced into Se95As5 and conditions of their preparation are explained based on the cluster model. According to the latter, changes in the electron density of states depends on changes in atomic configurations in clusters, i.e., short-range order changes.

Localised Phonon Modes at LiNbO3 (0001) Surfaces

The vibrational properties of the LiNbO3 (0001) surfaces have been investigated both from first principles and with Raman spectroscopy measurements. Firstly, the phonon modes of bulk and of the (0001) surface are calculated by means of the density functional theory. Our calculations reveal the existence of localised vibrational modes both at the positive and at the negative surface. The surface vibrations are found at energies above and within the bulk bands. Phonon modes localised at the positive and at the negative surface differ substantially. In a second step, the Raman spectra of LiNbO3 bulk and of the two surfaces have been measured. Raman spectroscopy is shown to be sensitive to differences between bulk and surface and between positive and negative surface. The calculated and measured frequencies are in agreement within the error of the method.

Computational investigation of the L 2,3-edge spectra of bulk and (110) surface of rutile TiO 2

The Ti-L 2,3 edge spectra of rutile TiO 2 have been calculated with the time-dependent density functional theory (TDDFT), employing the zero-order regular approximation (ZORA) at both scalar relativistic and spin–orbit coupling levels. The bulk rutile as well as its (110) surface has been considered with the aim of analyzing the sensitivity of the calculated Ti L 2,3 spectra to the different symmetry environments of the Ti atom in the two conditions. A good agreement with the experiment has been obtained in both cases. The inclusion in the theoretical scheme of both the configuration interaction and the spin–orbit coupling is essential to correctly describe the intensity distribution among the L 3- and L 2-edge transitions. The fine structures of the spectra reflect the symmetry environment of the absorbing Ti atom as well as the long range contribution from neighbor Ti atoms.

Molecular orientation and spectral investigations of Langmuir–Blodgett films of selected copper phthalocyanines

Various spectral methods (IR absorption and reflectance-absorption, Raman scattering and UV–Vis absorption) were used for investigations of a family of copper phthalocyanines in non-polarized and polarized light. The spectra of bulk and thin film samples were compared and discussed; an orientation of selected copper phthalocyanines in Langmuir–Blogdett layers and interaction between the phthalocyanine dyes with solid substrates were studied and discussed. Molecular arrangement of the molecular skeleton on the solid substrates was also evaluated. The presence of dye aggregates in the Langmuir–Blodgett layers was shown confirming the data obtained in concentrated solutions and in Langmuir layers.

Density functional theory simulations of B K and N K NEXAFS spectra of h-BN/transitionmetal(111) interfaces

The electronic structure and the corresponding B K and N K near-edge x-ray finestructure (NEXAFS) spectra of epitaxially grown h-BN on Ni(111), Pt(111),and Rh(111) surfaces are investigated by density functional theory. Thecalculations are carried out using the WIEN2k program package applying theaugmented-plane-wave+local orbitals (APW+lo) method. The NEXAFS spectra are simulated using a3 × 3 × 1 super cell and considering the final state rule by means of a (partial) core hole for thecorresponding atom. The influence of a full or partial core hole is shown for theh-BN/Ni(111) system, for which the best agreement with the experimental spectra is foundwhen half a core hole is assumed. All characteristic features of the experimental spectra arewell reproduced by theory, including the angular dependences. The bonding effects areinvestigated by comparing the spectra of bulk h-BN with those of the h-BN/Ni(111)system. An analysis of both the density of states and charge densities reveals strongN-pz–Ni-dz2 bonding/antibonding interactions. In the case of Pt(111) and Rh(111)surfaces, we discuss the effects of the nanomesh structures in terms of simple1 × 1 commensurate models.

Luminescence of uranium ions in sodium fluoride crystals

The luminescence spectra of bulk and fiber sodium fluoride crystals activated with uranium ions have been analyzed. Luminescence of uranium centers at temperatures of 8.5 and 300 K is considered and the features of the luminescence spectra of these centers in crystals of different dimension are analyzed. The luminescence spectra are simulated with allowance for the spectral-line profiles. Possible models of uranium centers are considered.

Voltage-tunable dual-band infrared photodetectors with Si/SiGe metal–semiconductor–metal heterostructure

A simple and low-cost structure of voltage-tunable dual-band near-infrared photodetector (PD) has been proposed, in which the PDs were developed by using Si 0.8Ge 0.2/Si metal–semiconductor–metal (MSM) heterostructure. The Si 0.8Ge 0.2/Si layers were deposited by ultrahigh-vacuum chemical vapor deposition system and a transparent layer of indium–tin oxide (ITO) was used as a metal layer to enhance the entrance of photons. In this study, we found that only one band was detected with a peak wavelength located at 950 nm at zero applied bias. When bias was increased to 1 V, in contrast a dual-band was achieved, where two peak wavelengths were centered at 950- and 1150-nm. It is suggested that the two bands are the absorption of top-Si and bottom-Si 0.8Ge 0.2 layers, respectively. The spectra of Si bulk and Si 0.8Ge 0.2 layer were also measured to verify our results and relating mechanisms are explained here.