E1 Symmetry Research Articles

A remarkable enhancement in photocatalytic activity of facilely synthesized Terbium@Zinc oxide nanoparticles by flash combustion route for optoelectronic applications

The facile synthesis of pristine and Terbium@Zinc oxide (Tb@ZnO) samples was produced by fast combustion route. The roughly spherical shaped morphology of Tb@ZnO was investigated by field emission scanning electron microscopy (FESEM) with agglomerated images of small size of particles. The dimension of particle shrinkages with increased the doping contents of Tb into ZnO lattice. The optical band gap of Tb@ZnO sample varies with the concentration of Tb and found to be decreased as 3.276, 3.267, 3.261 and 3.260 eV on increasing concentration of Tb from 1 to 5 wt%. The non-polar phonon mode with E2 symmetry was observed at 99 and 437 cm−1 while polar mode with A1 and E1 symmetry was reported at 330 and 580 cm−1, respectively, for pristine and Tb@ZnO samples. The photocatalytic activity of Tb@ZnO NPs has been investigated by the decolorization of methylene green (MG) dye under black light irradiation (367 nm). A remarkable photocatalytic performance of Tb@ZnO was noticed compared to the pure one and the highest percentage of activity was obtained for 1.0 wt% Tb@ZnO. Current outputs proposed the use of synthesized Tb@ZnO NPs in optoelectronic and photocatalyst applications.

The NH2 scissors band of methylamine

For the first time the ν4 NH2 scissors band has been assigned in a high-resolution infrared spectrum. A rotationally resolved spectrum of methylamine was recorded using two infrared spectroscopic techniques. A White-type multi-pass cell device coupled to the Bruker IFS 125 HR Fourier transform spectrometer was implemented on the AILES beamline at the SOLEIL synchrotron facility and a room-temperature spectrum of the whole band in the region of 1540–1710 cm−1 was recorded with a resolution of 0.001 cm−1. Then, a low-temperature high-resolution spectrum in the 1622.5–1655.6 cm−1 range of Q and R branches of the ν4 band was recorded using a quantum cascade laser spectrometer. Preliminary assignment was carried out in the NH2 scissors band region, and about 2200 transitions for K from 0 to 6 have been assigned for A, B, and E1 symmetry species. The simultaneous fit of assigned lines using a group-theoretical effective Hamiltonian was not successful; instead simple polynomial series expansions were used for each symmetry and K value.

Surface optical phonon modes in hexagonal shaped Al0.97Ga0.03N nanostructures

Raman modes of plasma assisted molecular beam epitaxy grown, c-plane oriented and hexagonal shaped cylindrical Al0.97Ga0.03N nanostructures are reported. Apart from the group theoretically allowed optical phonons at zone center (wave vector, q = 0), two additional phonon modes around 763 and 846 cm−1 are observed in between the reported values of longitudinal optical (LO q=0) and transverse optical (TO q=0) phonon modes. The observed phonon mode frequencies are matched with the simulated dispersion curve for the surface optical (SO) phonon modes and subsequently assigned as SO(A 1) and SO(E 1) with A 1 and E 1 symmetries, respectively. A change in the dielectric constant of the surrounding medium using CCl4 is demonstrated to show a significant red shift of the additional modes with respect to those in air, and they show good agreement with the calculated SO phonon frequencies. The origin of SO phonon modes are attributed to the relaxation of Raman selection rules away from the Brillouin zone centre because of the presence of periodic surface modulations of sharp edges at the corners of well-faceted Al0.97Ga0.03N hexagonal crystallites in the sample. An appropriate quantification is also made in this regard to find out the Fourier component of the surface potential responsible for activating the SO mode.

Infrared dielectric functions, phonon modes, and free-charge carrier properties of high-Al-content AlxGa1−xN alloys determined by mid infrared spectroscopic ellipsometry and optical Hall effect

We report on the analysis of a combined mid-infrared spectroscopic ellipsometry and mid-infrared optical Hall effect investigation of wurtzite structure c-plane oriented, crack-free, single crystalline, and high-Al-content AlxGa1−xN layers on 4H-SiC. For high-Al-content AlxGa1−xN, a two mode behavior is observed for both transverse and longitudinal branches of the infrared-active modes with E1 symmetry, while a single mode behavior is found for the longitudinal modes with A1(LO) symmetry. We report their mode dependencies on the Al content. We determine and discuss static and high frequency dielectric constants depending on x. From the analysis of the optical Hall effect data, we determine the effective mass parameter in high-Al-content AlxGa1−xN alloys and its composition dependence. Within the experimental uncertainty limits, the effective mass parameters are found isotropic, which depend linearly on the Al content. The combination of all data permits the quantification of the free electron density N and mobility parameters μ.

DNA-wrapped carbon nanotubes aligned in stretched gelatin films: Polarized resonance Raman and absorption spectroscopy study

We present the study of DNA-wrapped single-walled carbon nanotubes (SWNTs) embedded in the stretched gelatin film by the polarized resonance Raman spectroscopy and visible-NIR optical absorption. The polarized dependent absorption spectra taken along and normal to the stretching direction demonstrate a comparatively high degree of the alignment of isolated SWNTs in the gelatin matrix. The analysis of Raman spectra of isolated SWNTs in the gelatin stretched films showed that the degree of the alignment of carbon nanotubes along the stretching direction is about 62%. The dependence of the peak position of G+-band in Raman spectra on the polarization angle θ between the polarization of the incident light and the direction of the stretching of films was revealed. This shift is explained by the different polarization dependence of the most intensive A and E1 symmetry modes within the G+-band. The performed studies of embedded DNA-wrapped nanotubes in the gelatin film show the simple method for obtaining the controlled ordered biocompatible nanotubes inside a polymer matrix. It can be used for manufacturing sizable flexible self-transparent films with integrated nanoelectrodes.

Disposable Plasmonics: Plastic Templated Plasmonic Metamaterials with Tunable Chirality.

Development of low-cost disposable plasmonic substrates is vital for the applicability of plasmonic sensing. Such devices can be made using injection-molded templates to create plasmonic films. The elements of these plasmonic films are hybrid nanostructures composed of inverse and solid structures. Tuning the modal coupling between the two allows optimization of the optical properties for nanophotonic applications.

Room temperature structure vibrational and dielectric properties of Ho modified YMnO3

The structural, vibrational, and dielectric properties of bulk Ho-doped Y1−xHoxMnO3 (x = 0, 0.03, 0.05) solids prepared by standard solid-state reaction route were investigated. X-ray diffraction (XRD) patterns confirmed the hexagonal P63cm structure of Y1−xHoxMnO3 (x = 0.0, 0.03, 0.05) ceramics. Rietveld refinements of XRD data revealed that the doping ions led to unit cell contraction in three directions due to nearly equal ionic radii of Ho3+ ion (0.901 Å) substituted at the Y-site ion. The grain size of Ho-doped solids varied from 5 to 10 μm. For pristine h-YMnO3, the experimentally observed Raman scattering lines at around 151, 305, 460, and 682 cm−1 are of A1 symmetry, those at 410 cm−1 are of E1 symmetry, and the lines at 139 and 219 cm−1 are of E2 symmetry. Another interesting observation is the existence of an A1 line at 682 cm−1 and an E1 line at about 410 cm−1 which are much stronger than the remaining lines of A1 and E1 symmetries, respectively. The high value of dielectric constant and dielectric loss tangent at low frequency is explained by space charge polarization and the saturation in the high-frequency region is due to the electric dipoles not being in pace with the frequency of the applied electric field.

Coupled Exciton and Charge-Transfer Resonances in the Raman Enhancement of Phonon Modes of CdSe Quantum Dots (QDs)

We report the observation of the enhancement of a transverse optical (TO) phonon mode of e1 symmetry and a normally forbidden surface optical (SO) phonon mode of b1 symmetry in CdSe quantum dots (QDs) due to adsorption of 4-mercaptopyridine molecules. The former is observed in 3, 4, and 5 nm diameter particles, while the latter is observed only in the 2 nm particles. Maximum enhancement of the phonon modes is obtained through a coupling of the charge-transfer transition and the exciton transition which are in resonance with the laser energy. Selection rules using Herzberg–Teller (vibronic) coupling are invoked to explain the observed enhancements.

Resonant Raman scattering and photoluminescent properties of nonpolar a-plane ZnO thin film on LiGaO2 substrate

Nonpolar a-plane ZnO thin films are grown on LiGaO2 substrates and their optical and vibronic characteristics are investigated. Raman spectroscopy under nonresonant and resonant excitations shows the phase purity and resonant multiphonon process. From multiphonon Raman peaks up to the 6th order in the resonant spectra, the LO phonon frequencies for wave vectors away from the Γ point are estimated using the cascade model. Polar LO phonons with E1 symmetry play an important role in the indirect transitions of hot excitons. The optical anisotropy has been studied on the basis of polarization-dependent photoluminescence. The origin of photoluminescence is studied and a donor binding energy of 74.6 meV is found.

Morphology of InN nanorods using spectroscopic Raman imaging

We report the vibrational properties of vertical and oblique InN nanorods (NRs) grown by molecular beam epitaxy (MBE). Surface optical (SO) Raman mode at 561 cm−1, belonging to E1 symmetry [SO(E1)], is identified along with symmetry allowed Raman modes of E2(low), E2(high), and E1(LO) at 87, 489, and 589 cm−1, respectively, corresponding to wurtzite InN phase. Usually, SO phonon modes arise due to breakdown of translational symmetry of surface potential at surface defects, which are attributed by the surface roughness. Intensity distribution of E1(LO) and SO(E1) phonon modes over a specified area have been analysed using Raman area mapping with an optical resolution of 400 nm. Imaging with E1(LO) phonon mode, originating from the bulk of the sample, distinguishes the vertical NRs alone. We are able to resolve NR morphologies in both vertical and oblique cases with additional Raman mapping analysis of SO(E1) phonon mode, emerging from the surface irregularities, which are confined to the tip of MBE grown NRs. Copyright © 2013 John Wiley & Sons, Ltd.

Optical phonons in the bulk and on the surface of ZnO and ZnTe/ZnO nanowires in Raman spectra

Raman spectra of ZnO and core-shell ZnTe/ZnO nanowires have been measured under conditions of nonresonant and resonant excitations by Ar+ and He-Cd lasers. The optical vibration frequencies that are characteristic of the wurtzite structure of ZnO crystals have been determined. The Raman-active longitudinal optical (LO) phonons in all the studied structures have a mixed A1 and E1 symmetry. Surface optical modes with frequencies of 460–470 cm−1 have been found. Surface optical modes of single-layer nanowires have been analyzed using the calculation performed in the previous work by P.M. Chassaing and his colleagues, and those of double-layer nanowires have been analyzed in terms of the expressions obtained in the present work. The size of heterogeneities of the nanowires along their axis has been determined.

Analytical study of two-dimensional degenerate metamaterial antennas

Dispersion curves of metamaterial steerable antennas composed of two-dimensional arrays of metallic unit structures with the C4v and C6v symmetries are calculated both qualitatively by the tight-binding approximation and quantitatively by the finite-difference time-domain method. Special attention is given to the case of eigenmodes of the E symmetry of the C4v point group and those of the E1 and E2 symmetries of the C6v point group, since they are doubly degenerate on the Γ point of the Brillouin zone so that they naturally satisfy the steerability condition. We show that their dispersion curves have quadratic dependence on the wave vector in the vicinity of the Γ point. To get a linear dispersion, which is advantageous for steerable antennas, we propose a method of controlled symmetry reduction. The present theory is an extension of our previous one [Opt. Express 18, 27371 (2010)] to two-dimensional systems, for which we can achieve the deterministic degeneracy due to symmetry and the controlled symmetry reduction becomes available. This design of metamaterial steerable antennas is advantageous in the optical frequency.

Reduction of the transverse effective charge of optical phonons in ZnO under pressure

From Raman scattering on a-plane wurtzite ZnO crystals we obtained a decreasing splitting between longitudinal and transversal optical phonons with A1 and E1 symmetry as a function of hydrostatic pressure up to 5.5 GPa. Consequently, the transverse effective charge (eT∗) exhibits a strong reduction with increasing pressure, yielding 2.17–14.6×10−3 P/GPa and 2.04–13.7×10−3 P/GPa (in units of the elementary charge) for the A1 and E1 phonons, respectively. We find a clear systematic in the linear pressure coefficient of eT∗ with bond polarity for the series of wide-band gap semiconductors SiC, AlN, GaN, and ZnO.

Disorder‐induced Raman scattering effects in one‐dimensional ZnO nanostructures by incorporation and anisotropic distribution of Dy and Li codopants

AbstractIn Dy3+ and Li+ codoped ZnO nanowires, the additives accumulate preferentially in {0001} planes, resulting in serious breakdown of the translational symmetry in ab plane and modification of the phonon oscillation field. Not only acoustic overtones, silent optical modes, surface optical (SO) phonon modes, and multi‐phonon processes can be effectively observed in the nonresonant Raman scattering (RS) and the Fourier‐transform infrared (FTIR) spectra, but the quasi‐LO and TO modes of mixed A1 and E1 symmetry also show a noticeable red shift from E1 symmetry (in ab plane) to A1 symmetry (along c axis). The presence of dislocations and internal strain at the surface layer rich in additives, coming from the segregation of additives, forms a quasi‐bilayer system, resulting in the appearance and enhancement of SO phonon modes in RS and FTIR spectra. The Fano interference, originating from the interaction between the discrete scattering from phonons and the continuum scattering from laser‐induced electrons in the doped nanostructures, leads to typical asymmetric lineshapes on the lower wavenumber sides. Copyright © 2010 John Wiley & Sons, Ltd.

Multiphonon resonant Raman scattering in N‐doped ZnO

AbstractMultiphonon resonant Raman scattering in N‐doped ZnO films was studied, and an enhancement of the resonant Raman scattering process as well as longitudinal optical (LO) phonon overtones up to the sixth order were observed at room temperature. The resonant Raman scattering intensity of the 1LO phonon in N‐doped ZnO appears three times as strong as that of undoped ZnO, which mainly arises from the defect‐induced Raman scattering caused by N‐doping. The nature of the 1LO phonon at 578 cm−1 is interpreted as a quasimode with mixed A1 and E1 symmetry because of the defects formed in the ZnO lattice. In addition, the previously neglected impurity‐induced two‐LO‐phonon scattering process was clearly observed in N‐doped ZnO. Copyright © 2009 John Wiley & Sons, Ltd.

Decay of zone-center phonons in AlN with A1, E1, and E2 symmetries

Raman studies are reported for the A1(TO), E1(TO), E22, and A1(LO) symmetry phonons of AlN from 20to375K. By applying anharmonic decay theory to the observed temperature dependences of the phonon energies and linewidths, we determine the phonon decay mechanisms of these zone-center vibrations. Thermal expansion is taken into account using published temperature-dependent coefficients. The A1(TO), E1(TO), and E22 vibrations are described by symmetric two-phonon decay. The A1(LO) band is interpreted by an asymmetric two-phonon decay. Phonon lifetimes are obtained based on the observed linewidths and the dependence allows us to estimate the impurity-related phonon lifetime for each vibration. The latter ranges from 2.9to9.1ps.

Decay of zone-center phonons in GaN with A1, E1, and E2 symmetries

We report Raman studies of the A1(TO), E1(TO), E22, A1(LO), and E1(LO) symmetry phonons of GaN from 20to325K. By applying anharmonic decay theory to the observed temperature dependences of the phonon energies and linewidths, we determine the phonon decay mechanisms of these zone-center vibrations. Thermal expansion is taken into account using published temperature-dependent coefficients. The A1(TO) and E1(TO) vibrations are described by symmetric two-phonon decay. The E22 decays via the creation of three phonons. Both A1(LO) and E1(LO) bands are interpreted by an asymmetric two-phonon decay, with a minor contribution to the decay of the former from the three-phonon creation. Phonon lifetimes are obtained based on the observed linewidths.

New Complexes of Methyldiphenylphosphonium Cyclopentadienylide, Representative of a Class of Ligands Heretofore Much Neglected

Methyldiphenylphosphonium cyclopentadienylide, C5H4PMePh2 (II), has been synthesized and characterized spectroscopically and crystallographically and has been found to exhibit properties consistent with the zwitterionic structure IIb. The new group 6 coordination complexes (η5-C5H4PMePh2)M(CO)3 have been synthesized via substitution reactions of II with M(CO)6 (M = Cr, Mo) or W(CO)3(MeCN)3. Comparisons of ν(CO) values of these complexes with those of the isoelectronic (η6-C6H6)M(CO)3 and [(η5-C5H5)M(CO)3]- suggest that the electron-donating ability of the ylide is less than that of the cyclopentadienyl anion but much greater than that of benzene. Thermal and photochemical substitutions of the CO ligands of (η5-C5H4PMePh2)M(CO)3 (M = Cr, Mo) by equimolar amounts of PMe3 and PPh3 were not observed, but the ylide is displaced photochemically from (η5-C5H4PMePh2)Mo(CO)3 by excess PMe3 to form fac-Mo(CO)3(PMe3)3, while (η5-C5H4PMePh2)Mo(CO)3 reacts with I2 to form [(η5-C5H4PMePh2)Mo(CO)3I]I. The crystal structures of the free ylide and all four of its coordination complexes show that in each case one of the ylide phenyl rings not only eclipses the C5H4 ring but also is oriented toward it in an edge-on fashion. The intramolecular edge−face orientations involve interactions of an aromatic hydrogen atom with the C5H4 aromatic π systems and are rationalized in terms of electrostatic attractive forces between the slightly positive phenyl hydrogen atom and the negatively charged C5H4 ring. The electronic structures of II and of (η5-C5H4PMePh2)Cr(CO)3 have been investigated using ab initio methodologies; it is found that the near-degenerate HOMO and HOMO-1 orbitals of the free ylide exhibit symmetries very similar to those of the corresponding, doubly degenerate HOMO of the cyclopentadienyl anion (E1 symmetry) and that a lower energy, almost fully symmetric orbital corresponds to the fully symmetric bonding A1 MO of the cyclopentadienyl anion. In none of these three orbitals does there appear to be significant π involvement of an orbital on the phosphorus atom, consistent with the low P−C bond order implicit in the zwitterionic structure IIb. The primary ylide−metal interactions in (η5-C5H4PMePh2)Cr(CO)3 involve donation of the HOMO and HOMO-1 orbitals into the dxz and dyz orbitals of the chromium, while the calculated ylide−Cr(CO)3 bond dissociation energy is about 30% higher than the analogous ring−metal bond dissociation energy of (η6-C6H6)Cr(CO)3.

Surface vibrational spectroscopy on shear-aligned poly(tetrafluoroethylene) films.

Sum-frequency vibrational spectroscopy was used to obtain the first surface vibrational spectra of shear-deposited highly oriented poly(tetrafluoroethylene) (PTFE, Teflon) thin films. The surface PTFE chains appeared to lie along the shearing direction. Vibrational modes observed at 1142 and 1204 cm-1 were found to have the E1 symmetry, in support of some earlier analysis in the long-lasting controversy over the assignment of these modes.

Optical Phonons in Hexagonal AlxInyGa1-x-yN (y ? 0.12)

We report on first studies of the phonon mode properties of hexagonal AlxInyGa1—x—yN films. The 200–330 nm thick quaternary films (0 ≤ x ≤ 0.40, y ≈ 0.12), grown by MOCVD on 6H-SiC substrates, are pseudomorphically strained to the GaN buffer layers. We observe and differentiate the influence of strain and alloying on the frequency of the GaN-like E1(TO) phonon mode of AlxInyGa1—x—yN using infrared spectroscopic ellipsometry. Assuming the effective electron mass value of GaN, the free-electron concentrations are estimated from the A1-LO-phonon–plasmon coupled mode frequencies and increase by about two orders of magnitude in going from x = 0 to x = 0.40. We further obtain the composition dependence of the polarity for the AlN-like phonon mode with E1 symmetry. A possibly disorder-induced absorption band occurs above the GaN-like E1(TO) mode.