Dependence Of Spectral Features Research Articles

Efficient Treatment of Large Active Spaces through Multi-GPU Parallel Implementation of Direct Configuration Interaction.

We have extended our graphical processing unit (GPU)-accelerated direct configuration interaction program to multiple devices, reducing iteration times for configuration spaces of 165 million determinants to only 3 s using NVIDIA P100 GPUs. Similar improvements in the one- and two-particle reduced density matrix formation allow for fast analytical energy gradients and electronic properties. Our parallel algorithm enables the calculation of arbitrarily large configuration spaces (limited only by available system memory), with iteration times of 13 min for an active space of 18 electrons in 18 orbitals (2.4 billion determinants) using six consumer grade NVIDIA 1080Ti GPUs. These advances enable routine molecular dynamics simulations, geometry optimizations, and absorption spectrum calculations for molecules with large configuration spaces, a task that has heretofore required massive computational effort. In this work, we demonstrate the utility of our program by generating the absorption spectrum for diphenyl acetylene at the floating occupation molecular orbital complete active space configuration interaction level of theory. Several active spaces were investigated to assess the dependence of spectral features on orbital space dimension.

Spectral characterization of an industrial EUV light source for nanolithography

The emission spectra from an industrial, droplet-based, laser-produced plasma, extreme ultraviolet light source for nanolithography are here presented and analyzed. The dependence of spectral features on the CO2-drive-laser intensity is studied by changing the beam spot size at constant pulse energy and duration. We characterize the spectrum by fitting the results of atomic structure calculations to the short-wavelength region (7–11 nm), where the contributions from various charge states can be resolved, and obtain the relative contributions of charge states Sn9+ –Sn15+ . These relative contributions are compared to charge state populations as calculated with the non-equilibrium plasma kinetics code flychk. The calculations are shown to be in good qualitative agreement with the results, showing that the effective plasma temperature, and with it, the shape of the unresolved, main emission feature at 13.5 nm, is a remarkably weak function of laser intensity under this source normal operating conditions.

Understanding the excitation wavelength dependent spectral shift and large exciton binding energy of tungsten disulfide quantum dots and its interaction with single-walled carbon nanotubes

Herein, we investigate the origin of excitation wavelength dependent spectral features and high fluorescence quantum yield in fluorescent 2D tungsten disulfide (WS2) quantum dots (QDs) of average size 2.4 nm. The as-prepared WS2 QDs possess high optical bandgap and reasonably high fluorescence quantum yield ~15.4% in the green region without any functionalization. The broad photoluminescence (PL) spectrum consists of multiple peaks owing to emissions from excitonic transitions and surface defect-related transitions. The excitation wavelength-dependent spectral redshift and narrowing of line shape in the PL peak are analyzed carefully, and it is attributed to the selective excitation/recombination of carriers from different energy levels. The temperature-dependent PL analysis yields an exciton binding energy of ~301 meV in the QDs. Furthermore, we study the interaction between fluorescent WS2 QDs and single-walled carbon nanotubes (SWCNTs) and explore the mechanism of systematic quenching of PL of QDs by SWCNTs. The nature of the Stern–Volmer plot is found to be linear, and the time-resolved fluorescence measurements reveal that the quenching follows primarily the static behavior. Our study further reveals that defect sites in SWCNTs primarily act as the binding sites for WS2 QDs and form non-fluorescent complexes for effective quenching of the PL. The strong interaction between the WS2 QDs and the SWCNTs is evidenced from the spectral shift in the X-ray photoelectron spectroscopy and Raman peaks. Our study reveals the origin of excitation wavelength dependent PL emission from WS2 QDs and the nature of the interaction between WS2 QDs and SWCNTs, which are important for their applications in biomedical imaging and sensing, such as surface-enhanced Raman scattering, etc.

Surface‐enhanced Raman scattering of 4,4'‐diaminostilbene: Dependence of spectral features and resonant enhancement on surface coverage

AbstractAt any applications of surface‐enhanced Raman scattering (SERS) spectroscopy, from detection of small ions to recognition of large biological molecules, an understanding the spectral profiles and its transformation occurring under changes of local environment is an important stage essential for a reliable chemical analysis. In this work, we showed that the adsorption of 4,4'‐diaminostilbene on Ag nanoparticles is accompanied by a dramatic change in the SERS profile observed at transition from submonolayer to multilayer coverage. A profile deconvolution analysis allowed revealing the spectral signatures inherent for the surface species existing at submonolayer and multilayer adsorption corresponding to the hot spots and ordinary conditions, respectively. A resonance character of Raman enhancement was detected in the case of 4,4'‐diaminostilbene adsorbed in hot spots regions. A feasibility of ligand to metal charge transfer was proved on the basis of ultraviolet–visible data. The overtones and combination bands were observed in the SERS spectra at resonance conditions. The spectral signatures of the molecules inside and outside hot spots regions were assigned. A considerable activation of rings vibrations together with a suppression of ethylenic bridge vibrations was revealed in both cases. The most prominent features of 4,4'‐diaminostilbene linked with two Ag nanoparticles in the range 200–1,800 cm‐1 are the medium bands at 581 and 1249 cm‐1 corresponding to CCC bending and C–N stretching modes, respectively. The obtained results can serve as a basis for development of new powerful Raman techniques for evaluation of surface coverage and “hot spots mapping,” which are of great interest for surface science and plasmonics.

X-ray Emission Spectrum of Liquid Ethanol: Origin of Split Peaks.

The X-ray emission spectrum of liquid ethanol was calculated using density functional theory and a semiclassical approximation to the Kramers-Heisenberg formula including core-hole-induced dynamics. Our spectrum agrees well with the experimental spectrum. We found that the intensity ratio between the two peaks at 526 and 527 eV assigned as 10a' and 3a″ depends not only on the hydrogen bonding network around the target molecule but also on the intramolecular conformation. This effect is absent in liquid methanol and demonstrates the high sensitivity of X-ray emission to molecular structure. The dependence of spectral features on hydrogen-bonding as well as on dynamical effects following core excitation are also discussed.

Mechanical exfoliation and layer number identification of MoS2 revisited

In this paper we accurately revisit the mechanical exfoliation and layer number determination of MoS2. By modelling the exfoliation itself as a random vertical (lateral) exfoliation (fragmentation) phenomenon, a rationale is given to optimise the number of iterations in the scotch-tape peeling technique and we rigorously demonstrate that since the second cleavage the occurrence of monolayers is the most likely. For the unambiguous layer number identification, experiments have been carried out with a unified complementary approach based on optical microscopy, atomic force microscopy, resonant and non resonant Raman spectroscopy, and photo-luminescence spectroscopy. The experimental analysis has been carried out on a statistically significant set of few-layer MoS2 flakes (from one to five layers). The work stresses the strong need of such complementary multi-technique approach to really unambiguously determine the layer number of flakes (that neither optical microscopy, nor AFM alone can give). Optical microscopy contrast analysis experiments (carried out on flakes deposited onto 270 nm SiO2/Si(1 0 0) substrates) demonstrate that for few-layer MoS2 (from the mono to the epta-layer) the optical contrast is weakly depending on the radiation wavelength and is a non-monotonic function of the layer number. Thus, flakes from ten to twelve layers exhibit the same contrast of monolayers, as demonstrated by parallel AFM analysis. AFM clearly shows that the stacking between the silicon oxide substrate and the first MoS2 layer is strongly unpredictable and likely depending on the degree of surface contamination of the substrate. A parallel systematic resonant Raman study is performed ranging from one to five layers clearly exhibiting systematic layer dependent spectral features in the one-five layers range. For the first time a layer estimation number based on a quantitative analysis of resonant Raman and PL spectra is proposed.

Er3 +:Nd3 + concentration dependent spectral features of lithium-niobate-tellurite amorphous media

We report the Er3+/Nd3+co-dopants concentration dependent on thermal, structural, absorption and emission properties of tellurite glasses of composition (70-x-y)TeO2–15Li2CO3–15Nb2O5–(x)Er2O3–(y)Nd2O3 with x=0 and 1.0mol%, and 0≤y≤1.0mol%. Glasses are synthesized via melt-quenching method and characterized. The XRD pattern confirmed the amorphous nature of thermally stable and transparent glasses. Physical properties such as refractive index, molar volume, density, polarizability and molar refractions are determined. The thermal parameters particularly the thermal stability is from 281°C to 302°C and Hruby criterion of around 0.82 to 0.93 revealed a wide glass formation range. The structural properties of the glass represented by the FTIR spectrum indicate that as Nd2O3 content increases, the sharp infrared absorption peaks shifted from 474.7cm−1 to 499.4cm−1. These peaks are due to NbO, TeOTe and OTeO bond linkage bending vibration. For TeO4 trigonal bipyramid, the peak occurred at 672.5cm−1 to 680.4cm−1 whereas for TeO3 trigonal pyramid, two infrared band peaks were observed at 784.7–790.2cm−1 and 883.5–891.7cm−1. The absorption peaks around 1382.7cm−1 is due to the TeONb stretching vibration while peaks at 1633.7–1637.2cm−1 and 3390.1–3433.2cm−1 are due to the stretching vibrations of the hydroxyl group participating in the strong metal bonding as well as in the hydrogen bonding, respectively. Raman spectra displayed a peak shift of NbO bending vibration at 205.3 to 213.3cm−1, symmetric stretching and bending vibrations of Te-eqO-axTe at 354.6 to 356.9cm−1 and 375.3 to 380.9cm−1. For TeO4 trigonal bipyramid, the peak occurred at 622.8cm−1 and the TeO3 trigonal pyramid occurred at 845.4 to 849.6cm−1. Both direct and indirect optical band gap energies are increased with 2.84 to 3.12eV and 2.53 to 2.67eV, respectively. Urbach energy is decreased with the increase of Nd2O3 contents from 0.31 to 0.23eV. The room temperature up-conversion photoluminescence under 725nm excitations displayed two prominent bands centred at 616nm (orange) and 633nm (red). The manifested strong green band (497nm) in the down-converted luminescence spectra is attributed to the energy transfer from erbium to neodymium ion. The highest luminescence intensity is achieved for x=y=1.0mol%. The influence of co-dopants on the emission properties is analysed using rare earth ions' partial energy level diagram. The excellent luminescence properties of these glasses suggest their suitability for solid state lasers and other photonic applications.

Intensity dependent spectral features in high harmonic generation

We study the influence of the driving laser intensity on the spectral features of high harmonic generation in a semi-infinite gas cell. The effects of the harmonic dipole phase and the dispersion phase mismatches induced by the ionized medium during the harmonic generation process are revealed and the interplay between the macroscopic response and the single-atom response is discussed. We consider the conditions for generation of a narrow bandwidth, bright, and highly coherent high harmonic source.

Diameter dependence of interwall separation and strain in multiwalled carbon nanotubes probed by X-ray diffraction and Raman scattering studies

We have made a systematic study on the diameter dependent spectral features in X-ray diffraction (XRD) and Raman scattering studies of multiwalled carbon nanotubes (MWCNTs) of various diameters in the range 5−100 nm. High resolution transmission electron microscopy (HRTEM) imaging reveals a systematic decrease in the interwall separation from 3.8 Å down to 3.2 Å as the diameter of nanotubes increases from 5.8 nm to 63.2 nm. Analysis of the XRD patterns shows an exponential decrease in d 002 interlayer spacing with increasing tube diameter, in close agreement with the HRTEM results. Further, XRD profile line width shows inverse diameter dependence and exponential increase in intensity as the diameter of the MWCNTs increases. Raman spectra of different diameter nanotubes show different evolutions of metallic and semiconducting components in the G-band, as found from spectral deconvolution. The frequency and full width at half maximum (FWHM) of the semiconducting component of the G − band gradually decreases as the tube diameter increases. Ratio of intensities of G − band to D-band first shows a sharp fall as the tube diameter increases from 7 nm to 15 nm and then slowly increases with increasing diameter. On the other hand, G′ mode frequency shows large up shift when average diameter is increased from 7 nm to 15 nm and then saturates for higher diameter tubes. Analysis of Raman and XRD data reveals that the lowest diameter (7 nm) MWCNTs have features similar to those of the single walled nanotubes, while the spectral features are distinctly different for higher diameter MWCNTs due to the interaction among tube walls that is very significant for large diameter MWCNTs. Observed diameter dependence of the spectral features is explained in terms of nanotube curvature and atomic vibrations involving interaction among the walls in MWCNTs. The present study demonstrates the power of XRD for nondestructive evaluation of diameter distribution and interwall separation in MWCNTs with wide range of diameters.

Unusual solvent dependent electronic absorption spectral properties of nickel(II) and copper(II) perhaloporphyrins

Electronic absorption spectra of divalent metal ( Ni (II) and Cu (II)) complexes of 2,3,7,8,12,13,17,18-octa(bromo/chloro)-5,10,15,20-tetraphenylporphyrins (MOXTPP; X = Br , Cl ) were examined in various solvents. M (II) perhaloporphyrins exhibited dramatic shifts in their optical absorption spectral features relative to the corresponding metallotetraphenylporphyrins, MTPPs. Copper(II) perhaloporphyrins show significant red-shifts of the absorption bands in coordinating solvents relative to that observed for nickel(II) perhaloporphyrins. The large red-shift of the electronic absorption bands and the gain in intensity of the longest wavelength band (Q(0,0)), of Cu (II) perhaloporphyrins in certain coordinating solvents is comparable to that found in meso-tetraphenylporphinatozinc(II), ZnTPP. The solvent dependent spectral features of M (II) perhaloporphyrins are attributed to a coordinative interaction of the solvent with the core metal ion induced by the electron deficient porphyrin macrocycle.

Application of Two-dimensional Covariance Analysisto Infrared Spectroscopic Studies of Electrolyte Solutions

Two-dimensional correlation analysis is applied, for the first time, to aid in structural studies of electrolyte solutions. Concentration dependent spectral features due to solute–solvent and solute–solute interactions can be separated from each other and their relative correlated changes established. The application of 2D correlation analysis, using a generalized covariance approach, is demonstrated using FTIR–ATR data for sub-molar concentrations of lithium perchlorate in diethyl ether. Although it has been established that, as the electrolyte salt concentration increases, the concentration of diethyl ether in the trans–trans conformation decreases relative to diethyl ether in the trans–gauche conformation, no evidence of these changes could be seen in the methyl and methylene C–H stretching bands. This region of the mid-IR spectra was previously thought to show no solute–solvent dependent bands. The 2D covariance analysis performed in this study shows clearly that methyl and methylene stretching modes can be associated uniquely with either the trans–trans or the trans–gauche conformer.

Infrared Multiple Specular Reflectance of Mono- and Submonomolecular Layers of p-Bromobenzoic Acid on Oxidized Metals

The advantages of Multiple Specular Reflection (MSR) over single reflection grazing incidence infrared spectroscopy are discussed in this paper. A calculation which incorporates the experimental constraints for a multiple reflection setup is performed. The results concerning the angular dependence are confirmed by experiment. The main advantages are a wide angular acceptance and a release of the grazing angle of incidence geometry. Approximately the same sensitivity can be obtained as is theoretically possible with a single reflection at grazing incidence. However, in practice a somewhat higher sensitivity can be obtained with multiple specular reflectance spectroscopy. The sensitivity is illustrated by the measurement of submonolayers of p-bromobenzoic acid chemi-sorbed on an oxidized aluminium flat surface. Comparison with neutron activation analysis data shows that the reflection-absorption intensities are proportional to the surface coverage. A dependence of spectral features on surface coverage can be observed.