Two-phonon Modes Research Articles

Effect of fuel ratio on combustion synthesis and properties of magnetic nanostructures

We report a simple one-step solution combustion method for the preparation of ultrafine Ni, NiO and Ni/NiO nanostructures. The fuel-to-oxidizer ratio can be tuned to control the formation of either nanoparticles (Ni or NiO) or nanocomposites (Ni/NiO). The resultant nanostructures are characterized by x-ray diffraction, scanning electron microscopy, Raman spectroscopy and vibrating sample magnetometer. Processing under fuel rich conditions leads to the formation of Ni/NiO nanocomposite while fuel lean environment results in primarily NiO. The processing conditions influence the stoichiometry and the size (6.8 to 39.5 nm) of NiO nanoparticles. The intensity of one- phonon Raman longitudinal optical mode is found to be higher than two-phonon optical mode for non-stoichiometric NiO. The saturation magnetization and coercivity of the as-synthesized powders are influenced by the ratio of Ni to NiO, stoichiometry of NiO and pore size.

Structural analysis of hydrogenated nanocrystalline silicon thin films as a function of substrate temperature during deposition

In this contribution, the micro- and macro-structure of plasma grown hydrogenated nanocrystalline silicon (nc-Si:H) thin films were followed with respect to substrate temperature ( T s ) ranging from 80 °C to 200 °C. nc-Si:H films were deposited by plasma enhanced chemical vapor deposition technique using silane gas highly diluted by hydrogen and high RF power density. Micro-structure analysis was performed with grazing angle X-ray diffraction (GAXRD) and dispersive Raman spectroscopies. Parallelly, morphological properties of the films were investigated via field emission scanning electron microscopy (FE-SEM) by taking both surface and cross-sectional micrographs. GAXRD results suggest the presence of 4–5 nm nanocrystallites with the size almost independent of T s . The detailed analysis of the Raman spectra reveals one- and two-phonon modes due to amorphous and crystalline silicon (c-Si), where all c-Si related peaks shifted to lower frequencies. Raman nanocrystalline volume fraction is found to be greater than 50% for all T s ; it increases together with the short-range order at elevated T s . This micro-structural improvement with T s is considered to be slight when compared to the FE-SEM-observed drastic changes of morphology, particularly the size of large conglomerates (35–250 nm). The behavior of micro- and macro-structure with T s is correlated well with the previously determined bonding and lateral conductivity measurements. It seems to be the macro-structure that accounts for bonding and electrical properties of nc-Si:H thin films.

Extended x-ray absorption fine structure and micro-Raman spectra of Bridgman grown Cd1−xZnxTe ternary alloys

We have performed low-temperature micro-Raman scattering and extended x-ray absorption fine-structure (EXAFS) measurements on the Bridgman-grown bulk zinc-blende Cd1−xZnxTe (1.0 ≧̸ x ≧̸ 0.03) ternary alloys to comprehend their structural and lattice dynamical properties. The micro-Raman results are carefully appraised to authenticate the classical two-phonon mode behavior insinuated by far-infrared (FIR) reflectivity study. The composition-dependent EXAFS experiments have revealed a bimodal distribution of the nearest-neighbor bond lengths—its analysis by first-principles bond-orbital model enabled us to estimate the lattice relaxations around Zn/Cd atoms in CdTe/ZnTe to help evaluate the necessary force constant variations for constructing the impurity-perturbation matrices. The simulated results of impurity vibrational modes by average–t–matrix Green’s function (ATM-GF) theory has put our experimental findings of the gap mode ∼153 cm−1 near x ≈ 1 on a much firmer ground.

Multimode phonon cooling via three-wave parametric interactions with optical fields

We discuss the possible cooling of different phonon modes via three-wave mixing interactions of vibrational and optical modes. Since phonon modes exhibit a variety of dispersion relations or frequency spectra with diverse spatial structures, depending on the shape and size of the sample, we formulate our theory in terms of relevant spatial mode functions for the interacting fields in any given geometry. Our general formulation is applicable to cooling of any low-frequency excitation mode in matter which can give appreciable inelastic light scattering via the modulation of the optical dielectric function. We discuss the possibility of Dicke-like collective effects in phonon cooling and present explicit results for simultaneous cooling of two-phonon modes via the anti-Stokes up-conversions. We show that the bimodal cooling should be observable experimentally.

Enhancement of ferromagnetism and multiferroicity in Ho doped Fe rich BiFeO3

Crystalline solid solution of BiHoXFe1+XO3 (X=0, 0.05, 0.1, and 0.15) ceramics have been successfully synthesized by slow step sintering schedule. At particular value of X (X=0.1) the sample showed good crystallinity with almost impurity free phases. It was observed that at particular concentration of X (X=0.1), the sample showed enhanced M–H loop by Ho doping in presence of excess Fe. Furthermore, the M–H loop gets reduced with the increase in value of X (X=0.15). Compared to the pure BFO, the dielectric properties for X=0.1 composition are improved due to the decreased oxygen vacancies by the stabilized oxygen octahedron. Raman spectra over the frequency range of 100–1500cm−1 have been systematically investigated with different concentrations of X. Besides the changes of the peak position and the line width of the all modes, the prominent frequency shift, the line broadening and variation of the intensity for the two-phonon mode between 1270 and 1280cm−1 were observed with increase in value of X. All these results indicate the existence of strong spin–phonon coupling in Ho doped Fe rich BFO.

Effect of magnetic annealing on phonon behaviors of nanocrystalline BiFeO 3

Influence of magnetic annealing at 823 K up to 10 T (T) on the phonon behaviors of nanocrystalline BiFeO 3 was investigated by Raman spectroscopy. The frequencies of fundamental Raman modes increase obviously with increasing annealing magnetic field, and the intensity of the 1260 cm −1 two-phonon mode decreases. The pronounced anomalies of Raman phonon modes under magnetic annealing are attributed to the change of the spin-phonon coupling due to the modulation of spiral spin order. Furthermore, the temperature dependence of Raman peak positions, for the two prominent modes (147 and 176 cm −1), show no notable anomaly around T N except the sample annealed under 10 T magnetic field; meanwhile, in this sample, another obvious phonon anomaly occurs at ∼150 K (another magnetic phase transition point), which indicate that stronger magnetic annealing with 10 T intensely enhances the spin-phonon coupling, and possibly increases magnetoelectric coupling of nanocrystalline BiFeO 3 due to severely modulation of spiral spin order.

Scanning Raman spectroscopy of graphene antidot lattices: Evidence for systematic p-type doping

We have investigated antidot lattices, which were prepared on exfoliated graphene single layers via electron-beam lithography and ion etching, by means of scanning Raman spectroscopy. The peak positions, peak widths, and intensities of the characteristic phonon modes of the carbon lattice have been studied systematically in a series of samples. In the patterned samples, we found a systematic stiffening of the G band phonon mode, accompanied by a line narrowing, while the 2D two-phonon mode energies are found to be linearly correlated with the G mode energies. We interpret this as evidence for p-type doping of the nanostructured graphene.

Probing the Intrinsic Properties of Exfoliated Graphene: Raman Spectroscopy of Free-Standing Monolayers

The properties of pristine, free-standing graphene monolayers prepared by mechanical exfoliation of graphite are investigated. The graphene monolayers, suspended over open trenches, are examined by means of spatially resolved Raman spectroscopy of the G-, D-, and 2D-phonon modes. The G-mode phonons exhibit reduced energies (1580 cm(-1)) and increased widths (14 cm(-1)) compared to the response of graphene monolayers supported on the SiO(2)-covered substrate. From analysis of the G-mode Raman spectra, we deduce that the free-standing graphene monolayers are essentially undoped, with an upper bound of 2 x 10(11) cm(-2) for the residual carrier concentration. On the supported regions, significantly higher and spatially inhomogeneous doping is observed. The free-standing graphene monolayers show little local disorder, based on the very weak Raman D-mode response. The two-phonon 2D mode of the free-standing graphene monolayers is downshifted in frequency compared to that of the supported region of the samples and exhibits a narrowed, positively skewed line shape.

Raman study of BiFeO3 with different excitation wavelengths

Raman spectra of bismuth ferrite (BiFeO3) over the frequency range of 100-1500cm(-1) have been systematically investigated with different excitation wavelengths. The intensities of the two-phonon modes are enhanced obviously under the excitation of 532 nm wavelength. This is attributed to the resonant behavior when incident laser energy closes to the intrinsic bandgap of BiFeO3. The Raman spectra of BiFeO3 excited at 532nm were measured over the temperature range from 77 to 678K Besides the abnormal changes of the peak position and the linewidth of the A, mode at 139 cm(-1), the prominent frequency shift, the line broadening and the decrease of the intensity for the two-phonon mode at 1250cm(-1) were observed as the temperature increased to Neel temperature (T-N). All these results indicate the existence of strong spin-phonon coupling in BiFeO3. (c) 2008 Elsevier B.V. All rights reserved.

Raman scattering spectra in the normal phase of 2H–NbSe2

Polarized Raman spectra have been measured on the layer-structure compound 2H–NbSe2 single crystal, in a temperature range from 15 to 300 K. The temperature dependence of phonon energies can be described by the Klemens model which assumes that a zone-centre phonon decays into two acoustical phonons with opposite momenta in the same acoustic branch. The low-energy broad peak in the Raman spectra was found to shift to lower energies with decreasing temperature in the normal phase. The results show that the broad peak can be considered as a two-phonon soft mode described by one soft phonon plus one normal phonon mode, with the vectors of the two phonons .

Dynamical Symmetry Breaking Induced by Ultrashort Laser Pulses in KTaO3

Raman selection rules were observed to be violated dynamically under resonant and coherent pumping of the Raman-active two-phonon mode in the cubic perovskite-type oxide KTaO 3 . In this study, two intense femtosecond laser pulses having near infrared frequencies were used to excite the crystal with a relative time delay within ±200 fs. The frequency difference of the pulses was set to be almost equal to the frequency of one of the Raman-active two-phonon modes, with both phonons being at the Brillouin-zone (BZ) edge point. As a result, not only the conventional multistep coherent anti-Stokes Raman scattering (CARS) signals, but a new series of multistep signals with the frequency spacing of originally Raman-inactive single phonons were observed in the wavelength range from near infrared to the whole visible region. Importantly, the observed frequency spacing of the multistep signals does not correspond to the single-phonon frequency at the Γ-point but that at the BZ-edge point. In addition, the angle spacing of the observed signals is almost constant; it is also quite different from the angle distribution of the conventional CARS signals. A model mechanism of BZ folding from the BZ edge to the Γ-point due to coherent accumulation of two phonons at the BZ edge is presented to explain all aspects of this new phenomenon. Two phonons at the BZ edge and those at the Γ-point are anharmonically coupled within a decay time of coherent phonons, and those originally at the BZ edge become Raman-active. This novel phenomenon should provide a new tool to observe phonon modes at the BZ edge other than inelastic neutron scattering.

Raman and IR Spectroscopy of Chemically Processed Single-Walled Carbon Nanotubes

IR and Raman spectroscopy has been used to study the evolution of the vibrational spectrum of bundled single-walled carbon nanotubes (SWNTs) during the purification process needed to remove metal catalyst and amorphous carbon present in arc-derived SWNT soot. We have carried out a systematic study to define the different outcomes stemming from the purification protocol (e.g., DO, DO/HCl, DO/HNO(3), H(2)O(2), H(2)O(2)/HCl), where dry oxidation (DO) or refluxing in H(2)O(2) was used in a first purification step to remove amorphous carbon. The second step involves acid reflux (HCl or HNO(3)) to remove the residual growth catalyst (Ni-Y). During strong chemical processing, it appears possible to create additional defects where carbon atoms are eliminated, the ring structure is now open, localized C=C bonds are created, and O-containing groups can be added to this defect to stabilize the structure. Evolution of SWNT skeletal disorder obtained via chemical processing was studied by Raman scattering. Higher intensity ratios of R- and G-band (I(R)/I(G)) are more typically found in SWNT materials with low D-band intensity and narrow G-band components. Using IR transmission through thin films of nanotubes, we can resolve the structure due to functional groups that were present in the starting material or added through chemical processing. After high-temperature vacuum annealing of the purified material at 1100 degrees C, IR spectroscopy shows that most of the added functional groups can be removed and that the structure that remains is assigned to the one- and two-phonon modes of SWNTs.

Understanding the Nature of the DNA-Assisted Separation of Single-Walled Carbon Nanotubes Using Fluorescence and Raman Spectroscopy

Certain DNA sequences form a highly stable monolayer at the surface of single-walled carbon nanotubes, enabling their dispersion in aqueous solution. Ion-exchange chromatography can be used to separate these systems on the basis of their diameter in distinct, eluted fractions. Raman and fluorescence spectroscopy are used to characterize the nature of the separation by a comparison of radial breathing mode intensities at 457-, 532-, 633-, and 785-nm excitation wavelengths. Trends reveal a strong diameter dependence of the fractions, with larger diameters eluting later than smaller diameters. Average diameters of each fraction varied from 0.816 and 1.084 nm for the first and last eluted fractions, respectively. This conclusion is supported by a systematic shifting of the Raman tangential mode with increasing elution as well as changes in the relative intensities of radial breathing modes. Fluorescence emission from each fraction reveals the absence of relative shifting or broadening and hence uniformity in the chemical environment and the absence of doping-related complications for each fraction. The separation also reveals the fine structure of the G‘ two-phonon mode, which is described using a composite model with contributions from transition-dependent phonons.

Pressure dependence of donor excitation spectra in AlSb

We have investigated the behavior of ground- to bound excited-state electronic transitions of Se and Te donors in AlSb as a function of hydrostatic pressure. Using broadband far-infrared Fourier transform spectroscopy, we observe qualitatively different behaviors of the electronic transition energies of the two donors. While the pressure derivative of the Te transition energy is small and constant, as might be expected for a shallow donor, the pressure derivatives of the Se transition energies are quadratic and large at low pressures, indicating that Se is actually a deep donor. In addition, at pressures between 30 and 50 kbar, we observe evidence of an anticrossing between one of the selenium electronic transitions and a two-phonon mode.

Phonon modes of GaNyP1−y (0.006⩽y⩽0.0285) measured by midinfrared spectroscopic ellipsometry

Midinfrared spectroscopic ellipsometry reveals the two-phonon mode behavior of GaNyP1−y for nitrogen compositions 0.006⩽y⩽0.0285. The single layers (∼350 nm) studied were grown by metalorganic vapor-phase epitaxy on GaP substrates with orientations (001), and (001) with 5° off toward [110]. Line-shape analysis of the midinfrared response allows determination of the transverse- and longitudinal-optical phonon frequencies of the GaP- and GaN-like phonon modes. The polar strength of the GaN lattice resonance increases linearly with y, which can be used to monitor the nitrogen content of GaNyP1−y.

Absorption of infrared radiation by a giant enhancement of two-phonon lattice modes at a dielectric-conductor interface

Absorption of infrared radiation by a giant enhancement of two-phonon lattice modes at a dielectric-conductor interface

Absorption of infrared radiation by a giant enhancement of two-phonon lattice modes at a dielectric-conductor interface

Abstract Recent experimental work has shown that, at certain frequencies of infrared radiation, there is a giant absorption at the interface between a nanosize graphite substrate doped with copper and diamond nanocrystals grown on the substrate. We propose a model for this absorption based on the Born approach which uses a set of dynamie surface dipoles induced in a conducting medium at its interface with a dielectric and where the frequency of radiation corresponds to a combination of two-lattice vibrations. The wavenumber at which the enhancement oceurs can be determined from the fundamental freauency of the vibrations and the functional dependence of the extinction coefficient as a function of wavenumber can be determined from the relaxation times and the plasma frequencies. An analytical expression for the enhancement absorption factor is derived and it is shown how the cut-off of the absorption band can be used to ascertain the size and shape of conductive nanocrystals.

Disorder-induced Raman scattering in Mg xZn 1− xTe mixed crystals

The phonon spectra of the mixed crystal Mg x Zn 1− x Te (0 ≤ X ≤ 0.48) in the zinc-blende phase are investigated by Raman scattering. The phonon frequencies exhibit a dependence on the Mg concentration. The disorder induced one-phonon and two-phonon modes are present in the Raman spectra for X ≥ 0.16. The Raman intensity ratio between first-order and second-order phonon modes was a good indicator to characterize the degree of disorder in the mixed crystals.

Strain Effects in Superlattices of Diluted Magnetic Semiconductor ZnTe/Zn1-xMnxTe

ABSTRACTZnTe/Zn1-xMnxTe superlattices were grown on GaAs (001) substrates by molecular beam epitaxy. The multi-phonon processes including overtones and combinations of optical phonons have been studied by near resonant Raman scattering in the temperature range 13 K to 300 K. The strain arising from lattice mismatch gives rise to a shift in the optical-phonon frequencies. A two-phonon interface mode of superlattice has been observed and identified for the first time. Strain-induced red shifts of exciton energies related to transitions from the conduction subband to the light-hole and heavy-hole subband have been found by photoreflectance measurements. Experimental results agree well with the calculated strain-induced shift in superlattices.

Direct observation of vibrational modes in Hg1−xCdxTe

The vibrational modes in Hg1−xCdxTe epitaxial films and HgTe/CdTe superlattices have been investigated by means of far-infrared (FIR) transmittance spectra at temperatures from 4.2 to 300 K. The observations were put forward in the spectral range from 20 to 350 cm−1, emphasizing the wave-number region at the low-frequency side of the reststrahlen absorption band. A series of single-phonon and two-phonon modes as well as impurities- and defects-induced vibrational modes are observed. FIR transmission seems to be a good probe for characterization of the perfection of Hg1−xCdxTe films.