Numerical experiments on phonon properties of isotope and vacancy-type disordered graphene

We have studied phonon properties of graphene theoretically with different concentrations of ¹³C isotope and vacancy-type defects. The forced vibrational method, which is based on the mechanical resonance to extract the pure vibrational eigenmodes by numerical simulation, has been employed to compute the phonon density of states (PDOSs) and mode pattern of isotope-disordered graphene as well as a combined isotope and vacancy-type defective graphene structure. We observe a linear reduction of the E_2g mode frequencies with an increase in ¹³C concentration due to the reduced mass variation of the isotope mixture. We find a downshift of the E_2g mode of 65 cm^− 1, which is a very good agreement with the experimental results, and the phonon frequencies described by the simple harmonic oscillator model. The vacancy-type defects break down the phonon degeneracy at the Г point of the LO and TO modes, distort and shift down the phonon density of states significantly. The PDOS peaks for the combined isotope and vacancy-type defects show the remarkable increase in the low-frequency region induced by their defect formations. Due to phonon scattering by ¹³C isotope or vacancies, some graphene phonon wave functions become localized in the real space. Our numerical experiments reveal that the lattice vibrations in the defective graphene show the remarkably different properties such as spatial localization of lattice vibrations due to their random structures from those in the perfect graphene. The calculated typical mode patterns for in-plane K point optical phonon modes indicate that the features of strongly localized state depend on the defect density, and the phonon is localized strongly within a region of several nanometers in the random percolation network structures. In particular, for in-plane K point optical phonon modes, a typical localization length is on the order of ≈ 7 nm for isotope impurities, ≈ 5 nm for vacancy-type defects and ≈ 6 nm for mixed-type defects at high defect concentrations of 30%. Our findings can be useful for the interpretation of experiments on infrared, Raman, and neutron-diffraction spectra of defective graphene, as well as in the study of a wide variety of other physical properties such as thermal conductivity, specific heat capacity, and electron–phonon interaction.     

Raman and surface-enhanced Raman scattering of a series of size-separated polyynes

Solutions of hydrogen-capped polyynes were prepared by laser ablation of graphite powder in n-hexane and subjected to size separation by high-performance liquid chromatography. Solutions of size-selected polyynes C_nH₂ (n = 8–16) were investigated by normal Raman (NR) and surface-enhanced Raman scattering (SERS) spectroscopy. A main band appearing in the 2000–2200 cm⁻¹ region of the NR spectra showed a systematic downward shift as the chain length increased. The observed NR bands were assigned to Raman-active CC stretching vibrational modes by comparison with calculations based on density functional theory. Raman bands observed in SERS spectra were very broad and located at frequencies lower than the NR bands. A systematic band shift with increasing chain length was also observed for one of the bands. This band was thus assigned to a counterpart of the strong band in the NR spectra. These results made it possible to assign the origins of previously reported SERS bands of mixed polyyne solutions.     

Structural analysis of polycrystalline graphene systems by Raman spectroscopy

A theoretical model supported by experimental results explains the dependence of the Raman scattering signal on the evolution of structural parameters along the amorphization trajectory of polycrystalline graphene systems. Four parameters rule the scattering efficiencies, two structural and two related to the scattering dynamics. With the crystallite sizes previously defined from X-ray diffraction and microscopy experiments, the three other parameters (the average grain boundaries width, the phonon coherence length, and the electron coherence length) are extracted from the Raman data with the geometrical model proposed here. The broadly used intensity ratio between the C–C stretching (G band) and the defect-induced (D band) modes should be used to measure samples with crystallite sizes larger than the phonon coherence length, which is found equal to 32 nm. The Raman linewidth of the G band is more appropriate to characterize the crystallite sizes below the phonon coherence length, down to the average grain boundaries width, which is found to be 2.8 nm. “Ready-to-use” equations to determine the crystallite dimensions based on the Raman spectroscopy data are given.     

Alternating,all-trans polyenynes: Model compounds for poly(diacetylene)s with defined conjugation length

The syntheses of polyenynes as model compounds for poly(diacetylene)s (PDAs) are described. Variation of properties (UV–VIS, Raman, NMR and bond geometries) as a function of the chain length was investigated. After extrapolation to infinite chain length these data were compared to those for PDAs. From UV–VIS spectra a value of λ = 551 nm (2.25 eV) was calculated corresponding to the electronic transition of a single polyenyne chain. This energy is located at the low energy end of a yellow PDA solution spectrum. From Raman scattering v_(C?C) = 2108–2128 cm^?1 and v_(C?C) = 1505–1532 cm^?1 were calculated after extrapolation. Similarly sp-C¹³C NMR data yielded a shift of δ = 100 ppm. These data are almost identical to data known for yellow PDA solutions. Bond geometries are almost identical to those of poly(diacetylene)s and theoretical data.     

Resonant Raman spectroscopy on enriched C carbon nanotubes

Isotopically enriched single-wall carbon nanotubes with different ¹³C concentrations were investigated by resonant Raman spectroscopy. Linear reductions of the Raman frequencies with an increase of ¹³C concentration are observed for the different nanotube Raman modes, and the effect of the reduced mass variation of the isotope mixture on the phonon frequencies is described through a simple harmonic oscillator model. In addition to the frequency dependence, the Raman linewidths as a function of the ¹³C concentration were also investigated and an expression describing this is presented. We observe an increase in the G band linewidth, as the ¹³C:¹²C ratio approaches unity. Measurements with different excitation energies were performed and the frequency dispersions of the D and G′ bands with laser energy were observed to be the same for ¹²C and ¹³C nanotubes, suggesting no changes in the electronic structure after isotope enrichment. Through analysis of the radial breathing modes in the Raman spectra obtained with different excitation energies, a relation between these modes frequency and the ¹³C nanotubes diameter was also established.     

Pressure‐dependent Raman modes near the cubic‐tetragonal transition in strontium titanate

The pressure dependence of the Raman frequency shifts of various Raman modes is calculated at room temperature using the volume data from the literature for the cubic‐tetragonal transition in SrTiO₃. The isothermal mode Grüneisen parameters of those Raman modes are obtained, which decrease with increasing pressure for this molecular crystal. Calculated Raman frequencies are then used to predict the damping constant and the inverse relaxation time of those Raman modes as a function of pressure by means of the pseudospin‐phonon (PS) coupled model and the energy fluctuation (EF) model to describe the cubic‐tetragonal transition in SrTiO₃. Also, the values of the activation energy are extracted for the Raman modes studied using both models (PS and EF). Our predicted damping constant and the inverse relaxation time for the Raman modes, can be compared with the experimental measurements close to the cubic‐tetragonal transition in SrTiO₃.     

Characterizing intrinsic charges in top gated bilayer graphene device by Raman spectroscopy

In this work we study the behavior of the optical phonon modes in bilayer graphene devices by applying top gate voltage, using Raman scattering. We observe the splitting of the Raman G band as we tune the Fermi level of the sample, which is explained in terms of mixing of the Raman (E_g) and infrared (E_u) phonon modes, due to different doping in the two layers. We theoretically analyze our data in terms of the bilayer graphene phonon self-energy which includes non-homogeneous charge carrier doping between the graphene layers. We show that the comparison between the experiment and theoretical model not only gives information about the total charge concentration in the bilayer graphene device, but also allows to separately quantify the amount of unintentional charge coming from the top and the bottom of the system, and therefore to characterize the intrinsic charges of bilayer graphene with its surrounding environment.     

Determination of residual stress in PZT films produced by laser ablation with X-ray diffraction and Raman spectroscopy

The stresses in lead zirconate titanate (PZT) films, produced by pulsed laser deposition with different ratios Zr/Ti, 92/8, 65/35 and 55/45, was studied using Raman spectra and X-ray diffraction techniques. Based on lattice parameters and the elastic constants of PZT the films stresses were estimated from XRD measurements using the calculated d-spacing in the stressed and unstressed states. The results revealed the presence of compressive stress in PZT with composition 55/45 and tensile stress in the others. On the other hand, analysing the Raman phonon frequency in the A₁(TO₃) and E(LO₃) vibration modes and taking into account the phonon frequency under zero stress and the stress under which the phonon frequency becomes zero the stress in these films was estimated. The residual stresses extracted from the A₁(TO₃) mode are consistent with those extracted from the E(LO₃) mode and with those measured by X-ray diffraction technique.     

Effect of vacancy defects on phonon properties of hydrogen passivated graphene nanoribbons

The phonon properties of hydrogen-passivated armchair graphene nanoribbons (AGNRs) with different vacancy concentrations are investigated theoretically. We calculate the change in the phonon density of states (PDOSs) due to a broad range of vacancies and hydrogen passivation effects using forced vibrational method. A large downshift of prominent Raman active Г point LO mode phonons with an increase of vacancy concentration or decrease of ribbon widths are observed. We find an increasing peak intensities for the C–H stretching mode with the decrease of ribbon width or the increase of defect density. An inserted vacancy concentration of 10% and higher induce the broadening and distorting of the PDOS peaks significantly. The localization properties of phonon due to defects were also studied. The typical mode pattern of K point iTO mode phonons show the spatial localized vibrations persuaded by armchair edges or vacancies, which are in conceptually good agreement with the large D band of the Raman spectra comes from the armchair-edges or the imperfections of crystal. The typical displacement pattern for C–H stretching mode shows a random displacement of H atoms in contrast to C atoms. Our simulation results show the significant impact of vacancy defects on the vibrational properties of GNRs.     

Exchange rate constants of invisible protons in proteins determined by NMR spectroscopy

Although labile protons that are exchanging rapidly with those of the solvent cannot be observed directly, their exchange rate constants can be determined by indirect detection of scalar-coupled neighboring nuclei. We have used heteronuclear NMR spectroscopy to measure the exchange rate constants of labile protons in the side chains of lysine and arginine residues in ubiquitin enriched in carbon-13 and nitrogen-15 at neutral pH. Exchange rate constants as fast as 40x10(3) s(-1) were thus measured. These results demonstrate that NMR spectroscopy is a powerful tool for the characterization of lysine NH3(+) and arginine NH groups in proteins at physiologically relevant pH values.     

Phonon dynamics in AlN lattice contaminated by oxygen

The phonon dynamics of wurtzite aluminum nitride contaminated by oxygen were investigated by employing the Raman back scattering, the Fourier Transform Infrared (FT-IR) reflectivity and absorption, and the Brillouin scattering techniques on unseeded polycrystalline samples of AlN built of single crystallites 1–5 mm in size. The six Raman active zone center optic modes were observed and identified. Throughout the oxygen contamination range (∼1, ∼2, and ∼6 at.%) of three samples investigated, the widths of the principal Raman modes were found to decrease with increasing the oxygen content in the single crystal. This behavior is interpreted as a change in the nature of the oxygen defect when the oxygen concentration exceeds 1 at.%. The FT-IR reflectivity spectrum exhibits two-mode behavior at low oxygen concentration, one-mode behavior tends to be dominant when the oxygen concentration increases, and only one-mode behavior can be observed at high oxygen concentration. These changes in the reststrahlen band with oxygen concentration support the hypothesis of a transition in the oxygen accommodation defect as the concentration of oxygen increases. The oxygen effects on the AlN optical parameters were investigated by calculating these optical parameters from the reflectivity data of single crystallites differing in their oxygen concentration. The FT-IR absorption measurements showed several absorption bands in the multiple-phonon region. A tentative interpretation is proposed in which these bands are considered to be due to oxygen impurity absorption and to a combination of several phonon branches at the Brillouin-zone boundaries. The absorption spectrum in the one-phonon region allowed us to obtain a reliable data on the phonon density of states function in bulk AlN. Lastly, three different configurations were used in Brillouin scattering measurements to achieve a complete determination of the elastic stiffness constants of AlN.     

Carbon-13 NMR of hydroxyethylcellulose: An accurate method for structural determination

The high-resolution carbon-13 NMR spectrum of hydroxyethylcellulose (HEC) with about 2.5 moles of ethylene oxide (MS 2.5) average substitution per anhydroglucose ring (AHG) is presented. From models, the CMR chemical shifts for all of the different carbon atoms are assigned. Direct measurement of the relative intensities of the CMR signals for certain carbon atoms in HEC permits rapid and accurate computation of (1) the average chain length of poly(ethylene oxide); (2) the degree of substitution of ethylene oxide, and (3) the average relative degree of substitution of the alcohol groups on the AHG ring.     

Qualitative Evolution of Asymmetric Raman Line-Shape for NanoStructures

A qualitative evolution of an asymmetric Raman line-shape function from a Lorentzian line-shape is discussed here for application in low dimensional semiconductors. The step-by-step evolution reported here is based on the phonon confinement model which is successfully used in literature to explain the asymmetric Raman line-shape from semiconductor nanostructures. Physical significance of different terms in the theoretical asymmetric Raman line-shape has been explained here. Better understanding of theoretical reasoning behind each term allows one to use the theoretical Raman line-shape without going into the details of theory from first principle. This will enable one to empirically derive a theoretical Raman line-shape function for any material if information about its phonon dispersion relation, size dependence, etc., is known.     

Rotor Synchronized MAS Two-Dimensional Exchange NMR in Solids. Principles and Applications

The principles of the rotor synchronized magic angle spinning (MAS) two-dimensional exchange NMR, first proposed by Veeman and coworkers are reviewed, with particular emphasis on situations where chemical exchange in solids proceeds in concert with molecular reorientation. Calculations of cross peak intensities as function of the ratio between the chemical shift anisotropy and the spinning rate are presented for several cases. These calculations emphasize the advantage of using slow spinning rates (ω_R < ω_LΔσ) in such experiments when detailed information about mechanistic pathways in solids is sought. Three applications of the method to solid systems using carbon-13 NMR are described. These include: (a) Trimethylsulfoxonium iodide, in which the molecules undergo 120°-jumps about the molecular C₃ symmetry axis; (b) Tropolone, where the tautomeric hydrogen shift is found to be a consequence of the self diffusion within the crystal lattice, and in general accompanied by molecular reorientation. Here the two-dimensional pattern is used to obtain information about the various mechanisms of the diffusion process; (c) Bullvalene, where a quantitative analysis of the cross peak intensities as function of the mixing time provides kinetic information on two independent processes, viz. symmetric threefold jumps and a concerted Cope rearrangement-molecular reorientation reaction.     

High temperature crystallographic and low temperature magnetic phase transitions in Fe doped Sr2RuMnO6

Fe doped Sr₂RuMnO₆ (SRMO) double perovskites (Sr₂RuMn_1-xFe_xO₆, x = 0, 0.1, 0.2 and 0.3) were prepared by solid-state route. Both x-ray diffraction and Raman spectroscopy were performed to investigate the crystal structure of the synthesized double perovskites. Rietveld refinement of the x-ray diffraction patterns confirmed a phase transition from tetragonal to cubic space group as a function of doping concentration of iron. Raman spectroscopy at room temperature and group theory analysis revealed the phonon modes associated with the space group of the samples. The temperature dependent Raman spectroscopy showed an anharmonic behaviour of the phonon modes of the Fe doped SRMO samples. The temperature evolution of the phononic modes in the range of 300 K–620 K is predominantly influenced by the lattice degrees of freedom. The presence of several oxidation states Mn (2⁺, 3⁺ and 4⁺) and Fe (3⁺ and 4⁺) was confirmed by an X-ray photoemission spectroscopy analysis of the highest doped sample (x = 0.3). The magnetic properties measurements showed that the samples were completely paramagnetic at room temperature. The samples exhibit antiferromagnetism at very low temperatures and we conclude that they exhibit ferrimagnetic ground state in the mid temperature region.     

Compounded effect of vacancy on interfacial thermal transport in diamond-graphene nanostructures

By employing molecular dynamics simulations, it is observed that the distance of the vacancy(s) from the diamond-graphene interface is a determinant of interfacial resistance. In this study, we explain this relationship in two inter-related approaches. (1) A vacancy situated close to the interface reduces the interfacial resistance, suggesting that phonon dynamics around the vacancy contribute to reduce the interfacial barrier. Vibrational density of state calculations show that phonon scattering processes at an interfacial vacancy(s) influence the interfacial transmission significantly. This is attributed to inelastic mode conversion processes at the vacancy(s) that create modes with frequencies and velocities which match well with those in the interfacial diamond, resulting in the drop of interfacial thermal resistance as the location of vacancy approaches the interface. (2) Radial distribution function analyses indicate the conformity nature of interfacial diamond bonds and the contraction/lengthening of interfacial graphene bonds as the vacancy position is varied. These structural changes improve/weaken the matching of the interfacial bond length and result in mode conversion processes that modify the amount of mismatch in the vibrational density of states and phonon velocities in the two media.     

First-principles, UV Raman, X-ray diffraction and TEM study of the structure and lattice dynamics of the diamond-lonsdaleite system

We report the results of the study of the polycrystalline powder of the diamond-lonsdaleite system by X-ray diffractometry, transmission electron microscopy and UV Raman spectroscopy. The measured data of structural parameters are in good agreement with ab initio calculations. We show that the Raman spectrum is proportional to the phonon density of states of the diamond-lonsdaleite system.     

Characterization of substituted phenol–formaldehyde resins using solid-state carbon-13 NMR

Crosslinked substituted phenol–formaldehyde resins were synthesized from cashew nut shell liquid, 3-n-pentadecylphenol and phenol with formaldehyde. The resulting resins were crosslinked and investigated using carbon-13 NMR in the solid state using cross-polarization, magic angle spinning, and dipolar decoupling. Comparisons were made between the spectra of pure phenol–formaldehyde resins and it was shown possible to distinguish between the resins. It was also shown that the proton-dephased spectrum gave better spectral resolution for the substituted compounds. In addition, the solids carbon-13 technique verified that the degradation of the substituted phenolic resins occurs first with the degradation of the side chain in agreement with suggestions from earlier work.     

Enzymatic synthesis and spectroscopic characterization of 1,3-divernoloyl glycerol fromVernonia galamensis seed oil

cis-12,13-Epoxy-cis-octadecenoic (vernolic) acid occurs in triglycerides of the seed oil ofVernonia galamensis. The seeds also contain a lipase capable of hydrolyzing the triglycerides. Previous investigators incubated the seed ofVernonia anthelmintica and isolated 5.6% yield of 1,3-divernoloyl glycerol. We used crude lipase extract fromV. galamensis seed to synthesize 1,3-divernoloyl glycerol from vernonia oil in pentane at 40% yield. A 94% conversion of the 1,3-divernoloyl glycerol to pure vernolic acid (5.34% oxirane = 98.9% purity) was achieved by a low-energy saponification process. The carbon-13 nuclear magnetic resonance (NMR) spectrum of the 1,3-divernoloyl glyceride indicates a potential for using carbon-13 NMR spectroscopy in the identification of isomeric diglycerides. Thus the paper describes the synthesis, spectroscopic and chemical characterization of 1,3-divernoloyl glycerol, in addition to providing quantitative carbon-13 NMR studies ofV. galamensis oil.     

Temperature-dependent Raman scattering in round pit of 4H-SiC

The Raman spectra of N-doped 4H-SiC single crystal films is investigated between 100 and 600 K. The temperature dependence of the three optical modes is obtained. These measurements reveal that all Raman peaks shift to lower frequencies with increasing temperature, except A₁(LO). The temperature dependence of A₁(LO) phonon modes in the round pit also manifests different features with temperature increasing, but the demarcation temperature point of the blueshift and the redshift in the round pit is higher than that in the outer area. At high temperature, all active phonon modes clearly become broader, but the linewidth of the E₁(TO) phonon mode from round pit increases with temperature more rapidly than that from the outer area, this indicates that the lifetime of the E₁(TO) phonon in round pit is more sensitive than that in the outer area.