Raman scattering studies of crystalline GaP nanowires reveal a strong additional peak in the first order spectrum that can be clearly assigned to surface optical (SO) phonons. The frequency of this SO peak is found to be sensitive to the dielectric constant of the surrounding medium in which the nanowire is embedded. A theory for the SO phonons in cylindrical wires is presented, the SO mode dispersion ω(q) and the experimental peak frequency are then used to predict the wavelength of the dominant Fourier component of the surface potential responsible for activating the SO mode. Interestingly, this SO phonon wavelength is found to agree with the wavelength of diameter modulation observed for some nanowires.Semiconducting nanowires represent interesting solid state systems that have shown promise recently for new electronic devices, electrooptic devices, and sensors. 1-4 Measurements of their fundamental physical properties are starting to appear in the literature. [5][6][7] In particular, there have been reports of Raman scattering studies in polar semiconducting nanowires in which peaks in the Raman spectra have been identified with the TO and LO phonon modes at zero wavevector (q ) 0). The frequencies of these peaks are similar to that reported for the bulk. However, a "shape effect" for polar semiconducting nanowires has also been reported that stems from the long-range nature of the electromagnetic fields associated with these phonons. 7 The shape effect has been reported to lead to new optic modes whose frequency should differ by 2-10 cm -1 from that of the q ) 0 optic mode in bulk. 7 In this paper, we present results on surface optical (SO) phonons in GaP nanowires. In the perfect nanowire, these modes should not be observable by Raman scattering. However, we observe strong scattering from these modes, and it appears likely that they are activated by periodic oscillations in the wire cross section (or diameter) along its length. An additional Raman peak identified with SO modes in core-shell GaP@GaN nanowires has been reported previously. 8 However, the present work, we believe, is the first to definitively make the SO assignment by virtue of the effect of the overlaying dielectric medium. A theory for the SO modes in a cylindrical polar semiconducting nanowire is also presented here which can explain qualitatively the experimental observations, such as the shift in the SO band frequency with (1) the value of the dielectric constant of the surrounding optical medium, and (2) the period of the diameter oscillation along the wire. Thus, SO modes may be quite useful in detecting small diameter oscillations in the nanowires.The modes at the surface of a nonmagnetic object are determined by the dielectric properties of the materials on both sides of the surface and also by the shape of the object. For a recent review of these SO modes, the reader is referred to ref 9. Fuchs and Kliewer first predicted surface optical phonons in 1965. 10 Ibach, using inelastic electron scattering, observed them for the first time....

We have studied the impact of excitation laser power density on the Raman spectrum of small-diameter (5−15 nm) silicon nanowires. At low power densities, a Lorentzian line is observed at 520 cm -1 , the same value as that of the zone center LO (TO) phonon in bulk silicon. With increasing laser illumination, the Raman band downshifts and asymmetrically broadens on the low-frequency side. Our results contradict the traditionally accepted notion that a downshifted and asymmetrically broadened line in Si nanowires is due to quantum confinement effects. Rather, we suggest that the downshifting can be due to a laser heating effect of the nanowire and that the asymmetric line shape is due to a Fano interference between scattering from the k ) 0 optic phonon and electronic continuum scattering from laser-induced electrons in the conduction band.

We show that the long-range dipolar interactions in a crystalline cubic polar semiconducting nanowire give rise to an important splitting of the Raman-active transverse optic ͑TO͒ and longitudinal optic ͑LO͒ phonons at the center of the Brillouin zone. The dipole sums that determine the two LO and two TO phonon frequencies in the nanowire are sensitive to the aspect ratio ͑L/D͒, where L and D are the length and diameter, respectively. In the limit L/D →ϱ, we predict the phonon frequencies for several important polar semiconducting nanowires. Our calculated results are also compared with Raman scattering data obtained on crystalline GaP and GaAs semiconducting wires. Semiconducting nanowires have recently been shown to produce exciting devices, such as chemical sensors, 1 lasers, 2 and field effect transistors. 3 With sufficiently small diameters ͑Dр10 nm͒ one expects quantum confinement to alter the device properties via the electronic and phonon states. 4,5 Raman scattering has been used to study the effects of phonon confinement. 6,7 In agreement with the model first proposed by Richter et al. 8 and extended by Campbell and Fauchet, 9 the nanowire Raman-active optical phonon line is found to downshift and broaden asymmetrically with decreasing wire diameter. Recent work has extended the Raman line shape analysis to include the diameter distribution of the nanowire sample. 10 The essential ideas which evolved from Richter et al.'s model calculations for nanowires are the following: ͑1͒ a proper description of the long wavelength (qϭ0) phonon in the nanowire with a wire axis along the z direction requires a collection of bulk phonon states (q Ќ ) with wave vectors q Ќ in the range (0Ͻq Ќ р1/D), where q Ќ is the wave vector along a direction perpendicular to z, and D is the diameter of the nanowire, and ͑2͒ one should expect firstorder Raman scattering activity in the nanowire to extend over the range of phonon frequencies (0) to (0)ϩ⌬, where ⌬Ϸ (d/dq Ќ )(1/D). This phonon confinement or ''size'' effect on the Raman spectrum is expected to occur in both polar ͑e.g., GaAs, GaP͒ or nonpolar ͑e.g., Si, Ge͒ semiconductoring nanowires.In this Brief Report, we show that there is a second important effect on the Raman spectrum of a semiconductor nanowire that is of an altogether different origin. This ''shape'' effect stems from the long range dipolar interactions within the nanowire. 11 The parameters of the nanowire that determine the strength of this shape effect are the aspect ratio of the wire ͑L/D͒, the high frequency dielectric constant and the ion plasma frequency. Raman spectra on GaAs and GaP nanowires are presented and compared with theory. We report Raman spectra on long nanowires ͑lengths exceeding a few microns͒ with a most probable diameter Dϳ20 nm, where the quantum confinement or ''size'' effect is not important, i.e., the model of Richter et al. produces insignificant changes in the linewidth and peak position relative to that observed for the bulk. For these samples, we anticipate only a ''shape'' eff...

Temperature dependent X-ray diffraction and Raman spectroscopic studies were carried out on the flux grown single crystals of gallium ferrite with Ga:Fe ratio of 0.9:1

We report here our Raman studies on Fe 3Ϫx Zn x O 4 (xϭ0,0.015,0.03) across the Verwey transition in the temperature range 20-300 K. The changes in Raman spectra as a function of doping show that the changes are gradual for samples with higher Zn doping. Allen's formula has been used to estimate the strength of electronphonon interaction from the observed lineshape parameters. These estimates show that there is strong electronphonon coupling in this system and is highest for the T 2g 3 mode in comparison to A 1g and T 2g 2 modes.

We demonstrate room temperature ferroelectricity in the epitaxial thin films of magnetoelectric

We show the effect of composition on structural and magnetic characteristics of pure phase polycrystalline Ga 2-x Fe x O 3 (GFO) for compositions between 0.8 ≤ x ≤ 1.3. X-ray analysis reveals that lattice parameters of GFO exhibit a linear dependence on Fe content in single phase region indicating manifestation of Vegard's law. Increasing Fe content of the samples also leads to stretching of bonds as indicated by the Raman peak shifts. Further, low temperature magnetic measurements show that the coercivity of the samples is maximum for Ga:Fe ratio of 1:1 driven by a competition between decreasing crystallite size and increasing magnetic anisotropy.

We present a theoretical study of the structure-property correlation in gallium ferrite, based on first-principles calculations followed by a subsequent comparison with experiments. The local spin density approximation (LSDA + U ) of the density functional theory has been used to calculate the ground state structure, electronic band structure, density of states and Born effective charges. The calculations reveal that the ground state structure is orthorhombic Pc2 1 n having A-type antiferromagnetic spin configuration, with lattice parameters matching well with those obtained experimentally. Plots of the partial density of states of constituent ions exhibit noticeable hybridization of Fe 3d, Ga 4s, Ga 4p and O 2p states. However, the calculated charge density and electron localization function show a largely ionic character of the Ga/Fe-O bonds which is also supported by a lack of any significant anomaly in the calculated Born effective charges with respect to the corresponding nominal ionic charges. The calculations show a spontaneous polarization of ∼59 μC cm −2 along the b-axis which is largely due to asymmetrically placed Ga1, Fe1, O1, O2 and O6 ions.