Influence of phonon confinement,surface stress,and zirconium doping on the Raman vibrational properties of anatase TiO2 nanoparticles

We present a comprehensive analysis of the Raman spectra of pure and zirconium‐doped anatase TiO₂ nanoparticles. To account for the wavenumber shifts of the E_g(ν₆) mode as a function of particle size (L) and dopant concentration (x), a modification of the standard phonon confinement model (PCM) is introduced, which takes into account the contribution of surface stress by means of the Laplace–Young equation. Together with X‐ray diffraction (XRD) and transmission electron microscopy data, our analysis shows that the surface stress contribution to the observed blue shift of the Raman wavenumber is of the same magnitude as the spatial phonon confinement effect. Annealing experiments show that Zr‐doped nanoparticles exhibit retarded grain growth and delayed anatase‐to‐rutile phase transition by up to 200 K compared to pure anatase TiO₂. XRD shows that Zr doping leads to a unit cell expansion of the anatase structure. Applying the modified PCM to the x‐dependent variations of the E_g(ν₆) Raman mode, the mode‐Grüneisen parameter is found to increase abruptly at x > 0.07 with a concomitant mode softening. This coincides with the x range over which the Zr cations are reported to be displaced from their position in the tetrahedral lattice, and where Zr precipitation occurs upon annealing. The results have implications for the interpretation of Raman spectra of ionic metal oxide nanoparticles and how these are modified upon cation doping. Copyright © 2011 John Wiley & Sons, Ltd.     

Prediction of superconductivity of Ta2AlC: in situ Raman spectrometry and density functional investigations

In this paper, in situ Raman spectra of Ta₂AlC are measured in the temperature range of 80–500 K at ambient pressure. The frequencies of the Raman modes decrease with increasing temperature, which have been explained by the anharmonic and thermal expansion effects. The line‐width of E_2g (ω₃) mode increases at elevated temperatures, which is found to be due to the anharmonic phonon–phonon scatterings. On the other hand, the line‐widths of E_2g (ω₁) and A_1g (ω₄) modes decrease continuously with increasing temperature, which is explained by the electron–phonon couplings of these two phonon modes with the Ta 5d electrons. The electron–phonon coupling strengths are obtained both in experiments and density functional calculations. Finally, Ta₂AlC is predicted to be a new superconductive MAX phase. Copyright © 2014 John Wiley & Sons, Ltd.     

Physical insight of superconductivity of Nb2AlC: in situ Raman spectrometry investigation

Superconductivity of Nb₂AlC has been previously reported, but the origin is not clear. In this paper, in situ Raman spectra of Nb₂AlC are measured in the temperature range from 80 to 380 K at ambient pressure. The line‐width of E_2g (ω₁) mode increases with temperature which originates from the anharmonic phonon–phonon scattering. On the contrary the line‐widths of E_2g (ω₂) and A_1g (ω₄) modes decrease continuously at elevated temperature. The phenomenon is explained by the electron–phonon coupling. The origin of superconductivity is therefore interpreted by the coupling of Nb 4d electrons with E_2g (ω₂) and A_1g (ω₄) phonon modes. Copyright © 2013 John Wiley & Sons, Ltd.     

Raman spectroscopy to probe residual stress in ZnO nanowire

Raman spectroscopic studies are performed to probe the stress along the length of a bent ZnO nanowire. The zone‐centre E₂^high optical phonon shows a systematic red shift as the junction point of the two arms of the nanowire is approached. The mechanism of the red shift is discussed on the basis of the tensile strain. From the red shift of the phonon peak position, the strain at different regions on the nanowire is estimated. Stress in the bent nanowire is also investigated using photoluminescence (PL) spectroscopy. Results of both Raman and PL study confirm that the bent nanowire is under tensile strain. Copyright © 2011 John Wiley & Sons, Ltd.     

Raman tensor elements for multiferroic BiFeO3 with rhombohedral R3c symmetry

A quantitative assessment of the Raman spectrum emitted from a coarse‐grained polycrystal of multiferroic BiFeO₃ has been carried out by means of a polarized Raman microprobe. The dependence of the intensity of Raman phonon modes has been first theoretically modeled as a function of crystal rotation. Then, the Raman tensor elements have been experimentally determined from the analysis of the A_g and E_g vibrational modes. Copyright © 2009 John Wiley & Sons, Ltd.     

纳米晶锐钛矿相TiO2的非简谐效应和声子局域

对晶粒尺寸为194,86和56nm的纳米晶锐钛矿相TiO₂,进行了从83到723K的变温拉曼散射测量,并对E_g(1)模式进行了详细研究.根据非简谐效应和声子局域模型,对E_g(1)拉曼峰进行了拟合与计算.结果表明,以上三种纳米晶粒的晶格振动机理,在本质上是相同的.三声子过程对频率蓝移起主要作用.为了得到很好的拟合,需要同时考虑三声子和四声子过程.随着温度的升高,四声子过程增强,并对三声子过程起抵消作用.与非简谐衰减相关的声子寿命随着晶粒 关键词： 2')" href="#">纳米晶TiO₂ 拉曼散射 非简谐耦合 声子局域     

An analytical form for the Raman shift dependence on size of nanocrystals

An analytical form of the Raman shift dependence on the size of nanocrystals is presented. On the basis of the hard confinement model, this form describes the deviations from Raman shifts in infinite crystals as Δω = π²A[1 − exp(−η)]/12x²η², where η = L/12ax and x = (A/Γ₀)^1/2, L standing for the crystal size, Γ₀ for the intrinsic band linewidth, a for the lattice parameter and A for a suitable phonon curve parameter. It works in those cases where the average phonon curve shows a quadratic dependence on the phonon quasi‐momentum in the range of interest. Copyright © 2008 John Wiley & Sons, Ltd.     

Phonon anomalies and structural transition in spin ice Dy2Ti2O7: a simultaneous pressure‐dependent and temperature‐dependent Raman study

We revisit the assignment of Raman phonons of rare‐earth titanates by performing Raman measurements on single crystals of O¹⁸ isotope‐rich spin ice and nonmagnetic pyrochlores and compare the results with their O¹⁶ counterparts. We show that the low‐wavenumber Raman modes below 250 cm⁻¹ are not due to oxygen vibrations. A mode near 200 cm⁻¹, commonly assigned as F_2g phonon, which shows highly anomalous temperature dependence, is now assigned to a disorder‐induced Raman active mode involving Ti⁴⁺ vibrations. Moreover, we address here the origin of the ‘new’ Raman mode, observed below T_C ~ 110 K in Dy₂Ti₂O₇, through a simultaneous pressure‐dependent and temperature‐dependent Raman study. Our study confirms the ‘new’ mode to be a phonon mode. We find that dT_C/dP = + 5.9 K/GPa. Temperature dependence of other phonons has also been studied at various pressures up to ~8 GPa. We find that pressure suppresses the anomalous temperature dependence. The role of the inherent vacant sites present in the pyrochlore structure in the anomalous temperature dependence is also discussed. Copyright © 2012 John Wiley & Sons, Ltd.     

Multiphonon resonant Raman scattering in N‐doped ZnO

Multiphonon 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.     

Raman study of structural disorder in ZnO nanopowders

Raman scattering spectroscopy has been used for the characterization of zinc oxide nanoparticles obtained by mechanical activation in a high‐energy vibro‐mill and planetary ball mill. Raman modes observed in spectra of nonactivated sample are assigned to Raman spectra of the ZnO monocrystal, while the spectra of mechanically activated samples point out to the structural and stoichiometric changes, depending on the milling time and the choice of equipment. Observed redshift and peak broadening of the E₂^high and E₁ (LO) first‐order Raman modes are attributed to increased disorder induced by mechanical milling, followed by the effects of phonon confinement due to correlation length decrease. The additional modes identified in Raman spectra of activated ZnO samples are related to the surface optical phonon modes, due to the intrinsic surface defects and presence of ZrO₂as extrinsic defects introduced by milling in zirconia vials. Copyright © 2009 John Wiley & Sons, Ltd.     

MgB2 and AlB2 at high pressures

We have studied the effect of pressure on vibrational and electronic properties of MgB₂ and AlB₂ by ab initio calculations and Raman spectroscopy. The comparison between the calculations and the Raman data puts the common assignment of a broad spectral feature near 600 cm^?1 in MgB₂ to the E _2g phonon into question. At variance with MgB₂ the Raman spectra of AlB₂ exhibit a well-defined E _2g mode indicating that the anomalous Raman spectrum encountered in MgB₂ is not related to the metallicity of the samples nor is it intrinsic to crystals of the AlB₂ structure type. A theoretical estimate of the pressure dependence of T _c in MgB₂ shows that the experimentally observed decrease of T _c under pressure is predominantly due to phonon frequency shifts.     

The vibrational spectra of the cyanide ligand revisited: the ν(CN) infrared and Raman spectroscopy of Prussian blue and its analogues

A careful analysis of the Raman spectra of the M′_x[M(CN)₆]_y Prussian blue species has enabled a general model for the interpretation of the ν(CN) vibrational spectra. The spectral patterns are derived from those of the metal ions with local O_h symmetry. Two limiting models are discussed. A ‘localized mode’ model, involving matrix‐isolated species, is in much better accord with the observations than a ‘factor group’ model. The use of the infrared feature as fingerprint of specific M CN M′ units arises from the axis‐specific nature of individual T_1u modes. The interpretation of the A_1g and E_g Raman features is done in terms of localized vibrations, with involvement of additional energy terms from the lattice motions. Copyright © 2011 John Wiley & Sons, Ltd.     

Phonon deformation potentials for the corundum structure of sapphire

A theoretical Raman polarization analysis is proposed for the corundum structure of sapphire (α‐Al₂O₃) and validation experiments conducted with the purpose of retrieving the full set of phonon deformation potentials (PDPs). From the theoretical side, the change in force constants under stress/strain has been expressed in matrix form, and close‐form solutions were obtained for the eigenvalues that take into account the local dependence of oblique phonons on crystallographic orientation (i.e. uncoupling the effects of local crystal orientation and stress tensor from the shifts of Raman bands). From the experimental side, controlled (uniaxial) stress fields were applied to sapphire parallelepiped bars (along known crystallographic axes) while Raman spectra were systematically recorded along the bar thickness. An untextured alumina polycrystal with fine grain size was also investigated according to the same procedure. As a result of this set of experiments, PDPs for both A_1g and E_g vibrational bands could be retrieved. Validation of PDP constants was obtained by measuring the steeply graded stress fields developed ahead of a surface crack propagated along an arbitrary crystallographic direction in the R‐plane of the sapphire crystal. Copyright © 2011 John Wiley & Sons, Ltd.     

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

In 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.     

Polarized Raman scattering in helimagnetic β‐MnO2

A rutile β‐MnO₂ film was grown on MgO substrate using plasma‐assisted molecular beam epitaxy (PAMBE) monitored by reflection high‐energy electron diffraction (RHEED). Polarized Raman spectra at various temperatures were obtained to investigate the influence of the helimagnetic structure on the vibrational modes of β‐MnO₂. A red shift of E_g modes indicates a gradual formation of spin angles between neighboring Mn⁴⁺ ions. The intensities of the E_g and A_1g modes with y‐polarized incidence increase remarkably below the Néel temperature. A new view as vibrational mode projection (VMP) indicates the interactions between the magnetic component of incident light and the helimagnetic structure. Copyright © 2008 John Wiley & Sons, Ltd.     

Raman polarization analysis of highly crystalline polyethylene fiber

The complex orientation dependence in space of Raman active vibrations in the orthorhombic structure of polyethylene (PE) is discussed in terms of Raman tensor elements as intrinsic physical parameters of the lattice. Building upon the symmetry assignment of these vibrational modes, we systematically studied, from both theoretical and experimental viewpoints, the changes of polarized intensity for the A_g and the B_2g + B_3g vibrational modes with respect to PE molecular orientation. After explicitly expanding the Raman selection rules associated with the A_g and the B_2g + B_3g modes, introducing them into general expressions of the orientation distribution function, and validating them by means of a least‐square fitting procedure on experimental data, we compare here two mesostructural models for a highly crystallized and self‐aligned PE fiber structure. Stereological arguments are shown concerning the arrangement of orthorhombic fibrils in such a sample that unfold the correct values of five independent Raman tensor elements for orthorhombic PE. Copyright © 2010 John Wiley & Sons, Ltd.     

Electron tunneling spectroscopy of the phonon spectrum of MgB<Subscript>2</Subscript>

The specific features of the phonon spectrum of the MgB₂ compound (T _c = 38 K) are investigated by tunneling spectroscopy. It is demonstrated that both the position and the energy width of the fundamental optical mode E _2g in the phonon spectrum are in good agreement with inelastic X-ray spectroscopy data but differ substantially from Raman spectroscopy results. Among possible factors responsible for this discrepancy, the anharmonic and nonadiabatic effects that are characteristic of the MgB₂ system are discussed.     

Raman spectra of MgB2 at high pressure and topological electronic transition

Raman spectra of MgB₂ ceramic samples were measured as a function of pressure up to 32 GPa at room temperature. The spectrum at normal conditions contains a very broad peak at ∼590 cm⁻¹ related to the E _2g phonon mode. The frequency of this mode exhibits a strong linear dependence in the pressure region from 5 to 18 GPa, whereas, beyond this region, the slope of the pressure-induced frequency shift is reduced by about a factor of two. The pressure dependence of the phonon mode up to ∼5 GPa exhibits a change in the slope, as well as a “hysteresis” effect in the frequency vs. pressure behavior. These singularities in the E _2g mode behavior under pressure support the suggestion that MgB₂ may undergo a pressure-induced topological electronic transition.     

Fracture‐induced nanoscrolls from CVD‐grown monolayer molybdenum disulfide

We report a simple and effective way of fabricating molybdenum disulfide (MoS₂) nanoscrolls by self‐rolling up fractured monolayer CVD‐grown MoS₂ microflakes. Morphological results reveal that MoS₂ nanoscrolls are formed only at newly formed edges, owing to an orientation‐specific fracture behavior. Using Raman spectroscopy, we show that the E¹_2g Raman peak (A_1g peak) for MoS₂ nanoscrolls significantly red‐shifts (blue‐shifts), indicating structural change. The proposed mechanism is that the newly formed edges induced by fracture behavior self‐roll up to nanoscrolls to minimize the surface free energy, meanwhile, the serious lattice contradiction of upper sulfur plane controls the rolling directions. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

‘Soft’ optical phonons and the morphotropic phase transition of the Pb(Ti1−x, Zrx)O3 system

Room temperature Raman scattering results for the Pb(Ti_1?x, Zr_x)O₃ system in its tetragonal ferroelectric phase are analyzed. For x ≤ 0.25, the square of the frequency of the ‘soft’ E(TO) phonon is linear in the Zr concentration. In addition, we find that the morphotropic phase transition at x = 0.535 may be associated with an instability of this ‘soft’ phonon. It is shown that changes in the short-range harmonic forces play an important role in the x-dependence of the ‘soft’ E(TO) phonon frequency.