Line Shape Analysis Research Articles

Understanding the origin of mobility enhancement in wedge-shaped c-GaN nanowall networks utilizing spectroscopic techniques

Recently, the electron mobility in wedge-shaped c-GaN nanowall networks has been estimated to cross the theoretical mobility limit for bulk GaN. Significant blue-shift of the bandgap has also been observed. Both the findings are explained in terms of two-dimensional electron gas (2DEG) formed at the central vertical plane of the walls due to the polarization charges at the two inclined faces. Carrier concentration and mobility have earlier been determined from thermoelectric power and conductivity measurements with the help of a statistical model. Due to the network nature of the system, direct measurements of these quantities from Hall experiments are not possible. Search for a better way to estimate mobility in this system thus becomes important. Since, strain can also lead to the blue-shift of the bandgap, it is also imperative to evaluate carefully the role of strain. Here, using Raman spectroscopy, we have estimated carrier concentration and mobility in these nanowall networks with varied average tip-widths. Depth distribution of strain and luminescence characteristics are also studied. The study reveals that strain has no role in the bandgap enhancement. Moreover, the electron mobility, which is determined from the lineshape analysis of the A1(LO)-plasmon coupled mode in Raman spectra, has been found to be significantly higher than the theoretical limit of mobility for bulk GaN for the same electron concentration. These results thus corroborate the picture of polarization induced vertical 2DEG formation in these walls as predicted theoretically.

Topological examination of the bacteriophage lambda S holin by EPR spectroscopy

The S protein from bacteriophage lambda is a three-helix transmembrane protein produced by the prophage which accumulates in the host membrane during late gene expression. It is responsible for the first step in lysing the host cell at the end of the viral life cycle by multimerizing together to form large pores which permeabilize the host membrane to allow the escape of virions. Several previous studies have established a model for the assembly of holin into functional holes and the manner in which they pack together, but it is still not fully understood how the very rapid transition from monomer or dimer to multimeric pore occurs with such precise timing once the requisite threshold is reached. Here, site-directed spin labeling with a nitroxide label at introduced cysteine residues is used to corroborate existing topological data from a crosslinking study of the multimerized holin by EPR spectroscopy. CW-EPR spectral lineshape analysis and power saturation data are consistent with a three-helix topology with an unstructured C-terminal domain, as well as at least one interface on transmembrane domain 1 which is exposed to the lumen of the hole, and a highly constrained steric environment suggestive of a tight helical packing interface at transmembrane domain 2.

Synthesis, crystal structure and NMR-study new mononuclear paramagnetic Er (III) complex based on imine derivatives of thiacalix[4]arene

The synthesis, crystal structure and NMR-study of new paramagnetic Er (III) complex based on imine derivatives of thiacalix[4]arene was carried out. Temperature dependences of the 1H NMR spectra of complex Er(III) and calix[4]arene with N-contained substituents (I) have been analyzed using the line shape analysis, taking into account the temperature variation of paramagnetic chemical shifts, within the frame of the dynamic NMR method. Two kinds of molecular dynamics in I are reported for the CDCl3 solution. The first one conformational dynamics is conditioned by the pinched cone A ↔ pinched cone B interconversion with activation free energy ΔG≠(298 K) = 40 ± 3 kJ/mol (observed at low temperature). The second one dynamics is connected with rearrangements in coordination environment (with ΔG≠(298 K) = 58 ± 3 kJ/mol). Due to substantial temperature dependence of paramagnetic shifts, the complex I can be considered as NMR thermosensor reagent for local temperature monitoring. Owing to the large magnetic moments typical for many Ln-ions and especially high value of magnetic anisotropy for Er cation (III) adopting capped trigonal prismatic geometry of coordination sphere one may expect that 1-Er can potentially exhibit SMM behavior.

High-resolution Compton spectroscopy using x-ray microcalorimeters.

X-ray Compton spectroscopy is one of the few direct probes of the electron momentum distribution of bulk materials in ambient and operando environments. We report high-resolution inelastic x-ray scattering experiments with high momentum and energy transfer performed at a storage-ring-based high-energy x-ray light source facility using an x-ray transition-edge sensor (TES) microcalorimeter detector. The performance was compared with a silicon drift detector (SDD), an energy-resolving semiconductor detector, and Compton profiles were measured for lithium and cobalt oxide powders relevant to lithium-ion battery research. Spectroscopic analysis of the measured Compton profiles demonstrates the high-sensitivity to the low-Z elements and oxidation states. The line shape analysis of the measured Compton profiles in comparison with computed Hartree-Fock profiles is usually limited by the resolution of the semiconductor detector. We have characterized an x-ray TES microcalorimeter detector for high-resolution Compton scattering experiments using a bending magnet source at the Advanced Photon Source with a double crystal monochromator, providing monochromatic photon energies near 27.5keV. The momentum resolution below 0.16 atomic units (a.u.) was measured, yielding an improvement of more than a factor of 7 over a state-of-the-art SDD for the same scattering geometry. Furthermore, the lineshapes of narrow valence and broad core electron profiles of sealed lithium metal were clearly resolved using an x-ray TES compared to smeared and broadened lineshapes observed when using the SDD. High-resolution Compton scattering using the energy-resolving area detector shown here presents new opportunities for spatial imaging of electron momentum distributions for a wide class of materials with applications ranging from electrochemistry to condensed matter physics.

NMR lineshape analysis using analytical solutions of multi-state chemical exchange with applications to kinetics of host–guest systems

Nuclear magnetic resonance (NMR) lineshape analysis is a powerful tool for the study of chemical kinetics. Here we provide techniques for analysis of the relationship between experimentally observed spin kinetics (transitions between different environments A,B,dots) and corresponding chemical kinetics (transitions between distinct chemical species; e.g., free host and complexed host molecule). The advantages of using analytical solutions for two-, three- or generally N-state exchange lineshapes (without J-coupling) over the widely used numerical calculation for NMR spectral fitting are presented. Several aspects of exchange kinetics including the generalization of coalescence conditions in two-state exchange, the possibility of multiple processes between two states, and differences between equilibrium and steady-state modes are discussed. ‘Reduced equivalent schemes’ are introduced for spin kinetics containing fast-exchanging states, effectively reducing the number of exchanging states. The theoretical results have been used to analyze a host–guest system containing an oxoporphyrinogen complexed with camphorsulfonic acid and several other literature examples, including isomerization, protein kinetics, or enzymatic reactions. The theoretical treatment and experimental examples present an expansion of the systematic approach to rigorous analyses of systems with rich chemical kinetics through NMR lineshape analysis.

Polarized Raman spectroscopy on topological semimetal Co3Sn2S2

AbstractWe present polarized Raman spectroscopy of the topological semimetal Co3Sn2S2, which was recently shown to host a Weyl semimetal phase. Stokes Raman spectra were obtained with the incident light parallel to the c‐axis of Co3Sn2S2. Two major phonon Raman peaks were observed at 289 and 386 cm−1 over continuous background emission signals. The intensity of the low‐wavenumber (289 cm−1) peak showed no polarization dependence. The high‐wavenumber (386 cm−1) peak and the continuous background signal were strongly polarized in the incident light polarization direction. These responses were almost independent of the in‐plane crystal orientation to the incident polarization, as is the manifestation of the point group symmetry of the unit cell of Co3Sn2S2. According to the group theory and Raman tensor analyses, the low‐ and high‐wavenumber Raman signals are attributed to Γ point phonon modes with and symmetries, respectively. Furthermore, line shape analyses revealed that the high‐wavenumber mode exhibited asymmetric peak feature well described by the Breit–Wigner–Fano function. These results suggest the Fano resonance between the phonon scattering with the continuous electronic background associated with low energy excitations near the Fermi energy. The clarified phonon energies and symmetries, as well as the electronic contribution to the Raman scattering, will not only be useful as a fingerprint to readily verify the experimentally grown or theoretically calculated crystal structure but also suggest importance of Raman spectroscopy as an effective tool to study low energy excitations and their interactions in Co3Sn2S2.

ATP line splitting in association with reduced intracellular magnesium and pH: a brain 31 P MR spectroscopic imaging (MRSI) study of pediatric patients with myelin oligodendrocyte glycoprotein antibody-associated disorders (MOGADs).

Over the past four decades, ATP, the obligatory energy molecule for keeping all cells alive and functioning, has been thought to contribute only one set of signals in brain 31 P MR spectra. Here we report for the first time the observation of two separate β-ATP peaks in brain spectra acquired from patients with myelin oligodendrocyte glycoprotein antibody-associated disorders (MOGADs) using 3D MRSI at 7 T. In voxel spectra with β-ATP line splitting, these two peaks are separated by 0.46 ± 0.18 ppm (n = 6). Spectral lineshape analysis indicates that the upper field β-ATP peak is smaller in relative intensity (24 ± 11% versus 76 ± 11%), and narrower in linewidth (56.8 ± 10.3 versus 41.2 ± 10.3 Hz) than the downfield one. Data analysis also reveals a similar line splitting for the intracellular inorganic phosphate (Pi ) signal, which is characterized by two components with a smaller separation (0.16 ± 0.09 ppm) and an intensity ratio (26 ± 7%:74 ± 7%) comparable to that of β-ATP. While the major components of Pi and β-ATP correspond to a neutral intracellular pH (6.99 ± 0.01) and a free Mg2+ level (0.18 ± 0.02 mM, by Iotti's conversion formula) as found in healthy subjects, their minor counterparts relate to a slightly acidic pH (6.86 ± 0.07) and a 50% lower [Mg2+ ] (0.09 ± 0.02 mM), respectively. Data correlation between β-ATP and Pi signals appears to suggest an association between an increased [H+ ] and a reduced [Mg2+ ] in MOGAD patients.

(Invited) Unravelling Water-Ion Dynamics in Reverse Osmosis Membranes with Nuclear Magnetic Resonance Spectroscopy

Fundamental understanding of the charge (ions and water) transport in polymeric membranes used at the forefront of the water-energy nexus challenge is yet uncertain and depends on the complex interplay between polymer structure and dynamics that facilitate transport. Herein, we investigated the dynamics-transport of proton and ion transport in a rather neutrally charged organic membrane (i.e., polyamide) using solid-state NMR spectroscopy. Specifically, the role of competing ions' effect in complex ionic mixtures at the membrane interface has been explored on a molecular level. Proton and sodium ion diffusion measurements are made using pulsed-field gradient NMR diffusometry. The energy barriers for proton and ion transport within the active layer of the reverse osmosis (RO) membrane are assessed using magic angle spinning (MAS) and static NMR spectral line shape analysis. We have investigated the dynamics-transport of water and sodium ions in complex ionic mixtures of polyamide polymeric media using solid-state NMR spectroscopy. Through monitoring 13C, 23Na, and 1H NMR nuclei, quantitative insights into the diffusion of water (1H), sodium mobile ion (Na+), and polymer membrane dynamics (13C) are extrapolated. Furthermore, the spin-lattice (T1) relaxation time of sodium and water ions is investigated to obtain quantitative insight into how sodium and water ions’ rotational movements occur in the absence and presence of other competing seawater cations such as potassium. Room temperature spectra and spin-lattice relaxation times indicated the presence of both mobile and rigidly held ionic species in the membrane at different levels of relative humidity. The concentration and temperature dependence of relaxation times and chemical shifts have been investigated. NMR relaxation measurements detect how the water rotation is modified by sodium and potassium cations in the membrane. Results show that at low relative humidity and in the presence of competing ions, there are sodium ion resonances for crystalline sodium ions and rigidly held sodium ions on the surface. With increasing relative humidity and decreasing competing ions, the resonance suggests solution-like dynamic rotational movements of sodium ions in polyamide. The results confirm the change in activation energy barrier for sodium and hydrogen ion permeation in polyamide upon the presence of potassium ions.

Photoexcited carrier dynamics in semi-insulating 4H-SiC by Raman spectroscopy

The photoexcited carrier dynamics of high-purity (HPSI) and vanadium-doped semi-insulating (VDSI) 4H-SiC irradiated by lasers with different wavelengths and powers were investigated. Raman spectra were measured at room temperature and the photoexcited carrier concentrations were extracted from the Raman line shape analysis of longitudinal optical phonon–plasmon coupled mode. It was found that the longitudinal optical (LO) peaks of HPSI and VDSI did not shift with laser power variations, due to a low concentration of photoexcited carriers, when a 532- nm laser was used. However, when a 355- nm laser was adapted, the relationship between the photoexcited carrier concentrations and the laser power was found to be nonlinear because of the dominance of trap-assisted Auger (TAA) recombination. The coefficient of TAA recombination was laser power–dependent. The proposed carrier dynamic model deepens the understanding of the physical mechanism of semi-insulating SiC irradiated by nanosecond laser and provides an insight into the interpretation of experimental phenomena related to laser energy in optoelectronic devices.

Fast quantification of water content in glycols by compact 1H NMR spectroscopy

Glycols are key chemicals for many applications in different fields of activities. Being highly hydroscopic, glycols contain usually water. The presence of water, even in tiny amounts, can affect their chemical and physical properties. Therefore, the accurate determination of water content is essential for any intended applications. In this context, a novel method using low-field Nuclear Magnetic Resonance (NMR) spectroscopy is introduced. The proposed approach offers a straightforward, fast, low-cost, and versatile solution for water quantification in glycols without the need for reagents or calibration data. It is demonstrated by quantifying the water concentration up to 11 wt% in aqueous ethylene glycol (EG) and triethylene glycol (TEG) mixtures with the help of lineshape analysis of the corresponding proton spectra. The limit of detection, achieved within 1 min of measuring time, was 0.05 wt% for water in EG and 0.15 wt% in TEG. The excellent agreement between the NMR results and those from the Karl-Fischer titration indicates that the proposed NMR-based approach has a great potential to be used as an alternative to the Karl-Fischer method. In addition, it is expected that the same methodology can be applied for water quantification in more complex glycolic solutions and other mixtures.

Atomic-Scale Rectification and Inelastic Electron Tunneling Spectromicroscopy.

The phenomenon of rectification describes the emergence of a DC current from the application of an oscillating voltage. Although the origin of this effect has been associated with the nonlinearity in the current-voltage I(V) relation, a rigorous understanding of the microscopic mechanisms for this phenomenon remains challenging. Here, we show the close connection between rectification and inelastic electron tunneling spectroscopy and microscopy for single molecules with a scanning tunneling microscope. While both techniques are based on nonlinear features in the I(V) curve, comprehensive line shape analyses reveal notable differences that highlight the two complementary techniques of nonlinear conductivity spectromicroscopy for probing nanoscale systems.

Phospholipid headgroups govern area per lipid and emergent elastic properties of bilayers

Phospholipid bilayers are liquid-crystalline materials whose intermolecular interactions at mesoscopic length scales have key roles in the emergence of membrane physical properties. Here we investigated the combined effects of phospholipid polar headgroups and acyl chains on biophysical functions of membranes with solid-state 2H NMR spectroscopy. We compared the structural and dynamic properties of phosphatidylethanolamine and phosphatidylcholine with perdeuterated acyl chains in the solid-ordered (so) and liquid-disordered (ld) phases. Our analysis of spectral lineshapes of 1,2-diperdeuteriopalmitoyl-sn-glycero-3-phosphoethanolamine (DPPE-d62) and 1,2-diperdeuteriopalmitoyl-sn-glycero-3-phosphocholine (DPPC-d62) in the so (gel) phase indicated an all-trans rotating chain structure for both lipids. Greater segmental order parameters (SCD) were observed in the ld (liquid-crystalline) phase for DPPE-d62 than for DPPC-d62 membranes, while their mixtures had intermediate values irrespective of the deuterated lipid type. Our results suggest the SCD profiles of the acyl chains are governed by methylation of the headgroups and are averaged over the entire system. Variations in the acyl chain molecular dynamics were further investigated by spin-lattice (R1Z) and quadrupolar-order relaxation (R1Q) measurements. The two acyl-perdeuterated lipids showed distinct differences in relaxation behavior as a function of the order parameter. The R1Z rates had a square-law dependence on SCD, implying collective mesoscopic dynamics, with a higher bending rigidity for DPPE-d62 than for DPPC-d62 lipids. Remodeling of lipid average and dynamic properties by methylation of the headgroups thus provides a mechanism to control the actions of peptides and proteins in biomembranes.

The Influence of Deuterium Isotope Effects on Structural Rearrangements, Ensemble Equilibria, and Hydrogen Bonding in Protic Ionic Liquids.

We report strong isotope effects for the protic ionic liquid triethylammonium methanesulfonate [TEA][OMs] by means of deuterium solid-state NMR spectroscopy covering broad temperature ranges from 65 K to 313 K. Both isotopically labelled PILs differ in non-deuterated and fully deuterated ethyl groups of the triethyl ammonium cations. The N-D bond of both cations is used as sensitive probe for hydrogen bonding and structural ordering. The 2 H NMR line shape analysis provides the deuteron quadrupole coupling constants and the characteristics of a broad heterogeneous phase with simultaneously present static and mobile states indicating plastic crystal behavior. The temperatures where both states are equally populated differ by about 80 K for the two PILs, showing that deuteration of the ethyl groups in the trialkylammonium cations tremendously shifts the equilibrium towards the static state. In addition, it leads to a significant less cooperative transition, associated with a significantly reduced standard molar transition entropy.

Heat Treatment-Induced Conductivity Enhancement in Sulfide-Based Solid Electrolytes: What is the Role of the Thio-LISICON II Phase and of Other Nanoscale Phases?

Sulfide-based solid Li+ electrolytes are one of the most promising electrolyte classes for solid-state battery applications. These solid electrolytes are typically prepared by means of high-energy ball milling, often followed by a heat treatment step for enhancing the Li+ ion conductivity. In many cases, heat treatment-induced conductivity enhancements have been attributed to the formation of a superionic thio-LISICON II phase. However, the chemical composition and structure of this phase as well as the origin of the conductivity enhancement are still under debate. Here, we have carried out a comprehensive study on the thiophosphate-based electrolyte system (1 – x) Li3PS4 + x LiI with x = 0–0.5. By combining electrochemical impedance spectroscopy, X-ray diffraction, and Raman spectroscopy as well as 7Li NMR line-shape analysis and high-resolution multidimensional 31P solid-state NMR measurements, we show that the widely used concept of a thio-LISICON II phase governing the ionic conductivity of heat-treated samples cannot explain the experimental observations. Double-quantum constant-time 31P NMR proves that P2S64– units are embedded in the amorphous phase of the ball-milled pristine samples. Upon heat treatment, the amorphous phase with the embedded P2S64– units is transformed into different nanoscale crystalline phases, a thio-LISICON II phase, a β-Li3PS4 phase, and a Li4PS4I-related phase. A structural model of the thio-LISICON II phase needs to explain the coupling pattern from two-dimensional double-quantum NMR presented here, showing two phosphorus environments with an approximate ratio of 2:1. Furthermore, our results indicate that in heat-treated samples, a highly disordered nanoscale Li4PS4I-related phase exists with an ionic conductivity even exceeding that of the thio-LISICON II phase.

Zinc gallate (ZnGa2O4) epitaxial thin films: determination of optical properties and bandgap estimation using spectroscopic ellipsometry

Electronic grade ZnGa2O4 epitaxial thin films were grown on c-plane sapphire substrates by metal-organic chemical vapor deposition and investigated using spectroscopic ellipsometry. Their thickness, roughness and optical properties were determined using a Multiple Sample Analysis based approach by the regression analysis of optical model and measured data. These samples were then compared to samples which had undergone ion etching, and it was observed that etching time up to four minutes had no discernible impact on its optical properties. Line shape analysis of resulting absorption coefficient dispersion revealed that ZnGa2O4exhibited both direct and indirect interband transitions. The modified Cody formalism was employed to determine their optical bandgaps. These values were found to be in good agreement with values obtained using other popular bandgap extrapolation procedures.

Line Shape Analysis of Dynamic NMR Spectra for Characterizing Coordination Sphere Rearrangements at a Chiral Rhenium Polyhydride Complex.

Dynamic solution nuclear magnetic resonance (NMR) spectroscopy is the typical method of characterizing the dynamic rearrangements of atoms within the coordination sphere for transition metal polyhydride complexes. Line shape fitting of the dynamic NMR spectra can lead to estimates for the activation parameters of the dynamic rearrangement processes. A combination of dynamic 31P-{1H} NMR spectroscopy of metal-bound phosphorus atoms with dynamic 1H-{31P} NMR spectroscopy of hydride ligands may identify hydride ligand rearrangements that occur in conjunction with a phosphorus atom rearrangement. For molecules that exhibit such a coupled pair of rearrangements, dynamic NMR spectroscopy can be used to test theoretical models for the ligand rearrangements. Dynamic 1H-{31P} NMR spectroscopy and line shape fitting can also identify the presence of an exchange process that moves a specific hydride ligand beyond the metal's inner coordination sphere through a proton exchange with a solvent molecule such as adventitious water. The preparation of a new compound, ReH5(PPh3)2(sec-butyl amine), that exemplifies multiple dynamic rearrangement processes is presented along with line shape fitting of dynamic NMR spectra of the complex. Line shape fitting results can be analyzed by the Eyring equation to estimate the activation parameters for the identified dynamic processes.

Line Shape Analysis of Dynamic NMR Spectra for Characterizing Coordination Sphere Rearrangements at a Chiral Rhenium Polyhydride Complex

Line Shape Analysis of Dynamic NMR Spectra for Characterizing Coordination Sphere Rearrangements at a Chiral Rhenium Polyhydride Complex

Paramagnetic Properties and Moderately RapidConformational Dynamics in the Cobalt(II) Calix[4]arene Complex by NMR.

1H NMR measurements are reported for the CD2Cl2/CDCl3 solutions of the Co(II) calix[4]arenetetraphosphineoxide complex (I). Temperature dependences of the 1H NMR spectra of I have been analyzed using the line shape analysis, taking into account the temperature variation of paramagnetic chemical shifts, within the frame of the dynamic NMR method. Conformational dynamics of the 2:1 Co(II) calix[4]arene complexes was conditioned by the pinched cone ↔pinched cone interconversion of I (with activation Gibbs energy ΔG≠(298K) = 40 ± 3 kJ/mol. Due to substantial temperature dependence of paramagnetic shifts, complex I can be used as model compound for designing an NMR thermosensor reagent for local temperature monitoring.

Disentangling Many-Body Effects in the Coherent Optical Response of 2D Semiconductors.

In single-layer (1L) transition metal dichalcogenides, the reduced Coulomb screening results in strongly bound excitons which dominate the linear and the nonlinear optical response. Despite the large number of studies, a clear understanding on how many-body and Coulomb correlation effects affect the excitonic resonances on a femtosecond time scale is still lacking. Here, we use ultrashort laser pulses to measure the transient optical response of 1L-WS2. In order to disentangle many-body effects, we perform exciton line-shape analysis, and we study its temporal dynamics as a function of the excitation photon energy and fluence. We find that resonant photoexcitation produces a blue shift of the A exciton, while for above-resonance photoexcitation the transient response at the optical bandgap is largely determined by a reduction of the exciton oscillator strength. Microscopic calculations based on excitonic Heisenberg equations of motion quantitatively reproduce the nonlinear absorption of the material and its dependence on excitation conditions.

Exploring unusual temperature-dependent optical properties of graphite single crystal by spectroscopic ellipsometry

This study investigated the optical properties of graphite single crystal as a function of temperature between 4.5 and 500 K and within the spectral range of 0.73–6.42 eV through spectroscopic ellipsometry. At room temperature, the complex dielectric function of graphite exhibited broad spectra in the ultraviolet energy region. The Breit–Wigner–Fano (BWF) line shape analysis of the optical absorption spectrum demonstrated that two BWF line shapes were fitted with central energies at 4.85 ± 0.01 eV and 6.21 ± 0.03 eV. These two features were attributed to interference between an excitonic transition at the saddle point (M) in the band structures and collective excitations of the surface plasmons. Analysis of the temperature-dependent asymmetric BWF line shape indicated that the peak position shifted to lower energies under increasing temperatures, with the linewidth becoming narrow and the intensity increasing; this can be explained by the asymmetric factor 1/qBWF, which decreased under increasing temperatures. The temperature dependence of optical absorption for graphite is opposite to that of the interband absorptions observed in silicon. First-principles calculations verified the presence of two resonant conditions in the ultraviolet energy region. The lifetime of surface plasmons was expected to decrease with increased temperatures, leading to a decrease of the 1/qBWF. These results offer insight into quasiparticle band structures and collective excitations of graphite and provide valuable information for the technological development of graphite-based ultraviolet optoelectronic and photonic devices at different temperatures.