Raman scattering as a probe of phonon confinement and surface optical modes in semiconducting nanowires

Raman scattering is shown to be an effective probe of optical and surface optical phonons in highly crystalline semiconducting nanowires (SNWs). We show that the confinement model of Richter et al. well describes the nanowire diameter dependence of the asymmetric broadening of the one-phonon band in Si nanowires observed at ∼520 cm^-1. We also show that the use of high laser flux (∼0.1 mW/μm²) leads to a second mechanism that can asymmetrically broaden the 520 cm^-1 Raman band. This broadening has nothing to do with confinement, and can qualitatively be understood in terms of inhomogeneous laser heating. A model is presented that supports this explanation. The production of SNWs via the vapor–liquid–solid growth mechanism leads, in many cases, to an instability in the nanowire diameter or cross-sectional area. In the second part of this review, we show that this instability activates the surface optical (SO) phonon Raman scattering. Examples of this phenomenon are shown for GaP and ZnS nanowires. The former and latter have, respectively, cylindrical and rectangular cross sections. We show that the cross-sectional shape of the nanowire is important for a quantitative analysis of these SO modes. PACS 78.67.-n; 78.67.Lt; 78.30.-j; 78.30.Fs; 72.10.Di     

Raman scattering studies of individual polar semiconducting nanowires: phonon splitting and antenna effects

Results from Raman scattering experiments on individual crystalline GaP nanowires are presented which indicate that the shape of the nanowire, i.e., the high aspect ratio, may be responsible for two new phenomena involving optical phonons: (1) a shape-inducedsplitting of both the longitudinal optical (LO) and transverse optical (TO) phonons at the center of the Brillouin zone (q=0), and (2) a Raman scattering “antenna” effect which masks the normal Raman polarization selection rules. We suggest that (1) stems from the asymmetry in the long range dipolar sums that control the electromagnetic LO-TOsplitting, and we identify the Raman antenna effect (2) with the internal electric field created by Mie resonances in the nanowire driven by the incident laser field. Although these effects are reported here for GaP, they are expected to be general effects observable in many semiconducting nanowire systems. PACS 78.67.-n; 78.67.Lt; 78.30.-j; 78.30.Fs; 72.10.Di     

Structural and electrical properties of core–shell structured GaP nanowires with outer Ga2O3 oxide layers

This paper presents a review of our current experimental research on GaP nanowires grown by a vapor deposition method. Their structural, electrical, opto-electric transport, and gas-adsorption properties are reviewed. Our structural studies showed that a GaP nanowire consisted of a core–shell structure with a single-crystalline GaP core and an outer Ga₂O₃ layer. The individual GaP nanowires exhibited n-type field effects. Their electron mobilities were in the range of about 6 to 22 cm²/V s at room temperature. When the nanowires were illuminated with an ultraviolet light source, an abrupt increase of conductance occurred resulting from carrier generation in the nanowire and de-adsorption of adsorbed OH^- or O₂ ^- ions on the Ga₂O₃ surface shell. Using an intrinsic Ga₂O₃ shell layer as a gate dielectric, top-gated GaP nanowire field-effect transistors were fabricated and characterized. Like other metal oxide nanowires, the carrier concentration and mobility of GaP nanowires were significantly affected by the surface molecular adsorption of OH or O₂. The GaP nanowire devices were fabricated as sensors for NO₂, NH₃, and H₂ gases by using a simple metal decoration technique. PACS 73.63.-b; 72.80.Ey; 85.35.-p     

Amorphous-like Raman spectra of GaP microcrystals

Raman scattering from gas-evaporated GaP microcrystals smaller than about 400 Å has been investigated. As the crystalline size decreases from ～400 to ～170 Å, drastic changes in the Raman spectrum are observed; the TO and surface phonon peaks broaden and shift, and they strongly overlap with each other, finally transforming into a broad structure; broad bands located at around 80 and 200 cm^-1 appear and grow rapidly. The microcrystals smaller than about 250 Å show spectra very similar to those of amorphous GaP, even though the electron diffraction patterns prove that they are crystalline. The amorphous-like Raman signals seem to come from the surface layers of the microcrystals.     

Raman-scattering spectrum of GaN straight nanowires

The Raman spectrum of GaN straight nanowires deposited on a LaAlO₃ crystal substrate was studied. The E₂ (high) phonon frequency at 560 cm^-1 shows a 9 cm^-1 shift compared with the calculated value. The low-energy shift and band broadening of the Raman modes result from the nanosize effect. The unique property of the low intensity ratio of I_E2/I_A1(LO) on the Raman spectrum from the GaN straight nanowires was observed. Received: 5 June 2000 / Accepted: 7 June 2000 / Published online: 2 August 2000     

纳米磷化镓粉体还原氦过程的Raman光谱分析

利用Raman光谱对还原氮后的纳米磷化镓(GaP)粉体进行了表征。结果表明：纳米GaP粉体表面含有Ga-O，P-O和H-O化学键。此外，进行氮还原过程后，在Raman位移约为1700～3300cm^-1范围内(相当于709～800nm或1．55～1．75eV)，纳米GaP的Raman光谱出现了一个宽、强荧光发射峰；而在未进行通氮处理的纳米GaP Raman光谱中，没有观察到该荧光峰的存在。本文对该荧光发射峰的起因作了初步分析。     

Raman spectroscopy of ZnS nanostructures

We report Raman scattering results of wurtzite ZnS nanowires, nanocombs, and nanobelts. The Raman spectrum obtained from ZnS nanowires exhibits first‐order phonon modes at 272, 284, and 350 cm⁻¹, corresponding to A₁/E₁ transverse optical, E₂ transverse optical, and A₁/E₁ longitudinal optical phonons, respectively. Several multiphonon modes are also observed. The longitudinal optical phonon mode varies in wavenumber for nanocombs and nanobelts, indicating that the residual strain varies during the morphological change from ZnS nanowires to nanocombs and ultimately to nanobelts. Interestingly, a surface optical (SO) phonon mode varies in wavenumber depending on the shape and surface roughness of the ZnS nanostructures. The surface modulation wavelengths of the ZnS nanowires, nanocombs, and nanobelts are estimated using the SO phonon dispersion relations and the observed SO phonon wavenumbers. Copyright © 2012 John Wiley & Sons, Ltd.     

纳米磷化镓粉体还原氮过程的Raman光谱分析

利用Raman光谱对还原氮后的纳米磷化镓(GaP)粉体进行了表征。结果表明:纳米GaP粉体表面含有Ga-O,P-O和H-O化学键。此外,进行氮还原过程后,在Raman位移约为1700~3300cm-1范围内(相当于709~800nm或1.55~1.75eV),纳米GaP的Raman光谱出现了一个宽、强荧光发射峰;而在未进行通氮处理的纳米GaPRaman光谱中,没有观察到该荧光峰的存在。本文对该荧光发射峰的起因作了初步分析。     

硅纳米线拉曼光谱的研究

声子限制效应会引起本征硅纳米线拉曼光谱红移及不对称宽化,但研究发现其并非引起硅纳米线拉曼光谱改变的主要因素。研究表明,由于在拉曼光谱测量中,通常使用的入射激光功率都在5 mW以上,激光加热会导致很高的局部温度,从而引起拉曼光谱大幅度红移并对称宽化,这是硅纳米线拉曼光谱红移的主要影响因素。另外,激光功率很高时,由激光激发的载流子会与声子发生Fano型干涉,从而使硅纳米线拉曼光谱发生Fano型红移和不对称宽化。除此之外,对小直径本征硅纳米线,声子限制效应导致波矢选择定弛则弛豫,使不在布里渊区中心的声子也可以参与拉曼散射,因而其拉曼光谱中除常见的几个拉曼峰外还会出现新拉曼峰。     

Synthesis and characterization of GaP nanochains with a twin-modulated quasi-periodic structure

GaP nanochains have been synthesized by hydrogen-assisted thermal evaporation, and characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), photoluminescence (PL), and Raman spectroscopy. GaP nanochains possess a (111) twin crystal plane-modulated quasi-periodic structure, that gives a strong green photoluminescence at 618 nm. While the Raman spectrum of the nanochains is similar to that of the GaP crystal, the intensity of the longitudinal optical (LO) peak is stronger than that of the transverse optical (TO) peak, which is supposedly related to the nanochain microstructures.     

Pressure dependent optical phonon anharmonicity in GaP

The asymmetric Raman line shape of the TO(Γ) phonon in GaP has been carefully measured under hydrostatic pressure up to 87 kbar (room temperature). At high pressure the peak becomes symmetric and a weak sideband emerges. These results can be explained as due to the third order anharmonic interaction of TO(Γ) with the combination modes TA + LA(≈ X). A calculation of this effect, based on the measured phonon dispersion of GaP, is successful in fitting the observed line shape at all pressures. The anharmonic coupling strength for the decay TO(Γ) → TA + LA(≈ X) is found to decrease linearly with pressure by an appreciable amount.     

Raman spectrum of Si nanowires: temperature and phonon confinement effects

The Raman spectrum of Si nanowires (NWs) is a matter of controversy. Usually, the one-phonon band appears broadened and shifted. This behaviour is interpreted in terms of phonon confinement; however, similar effects are observed for NWs with dimensions for which phonon confinement does not play any relevant role. In this context, the temperature increase induced by the laser beam is recognized to play a capital role in the shape of the spectrum. The analysis of the Raman spectrum, under the influence of the heating induced by the laser beam, is strongly dependent on the excitation conditions and the properties of the NWs. We present herein an analysis of the Raman spectrum of Si NWs based on a study of the interaction between the laser beam and the NWs, for both ensembles of NWs and individual NWs, taking account of the temperature increase in the NWs under the focused laser beam and the dimensions of the NWs.     

Raman study of activated quasi-modes due to misorientation of ZnO nanowires

We present Raman scattering study of wurtzite ZnO nanowires deposited by metal–organic chemical vapor deposition (MOCVD) on (0001) sapphire. It is shown that the misorientation of the nanowires, i.e. the inclination with respect to the vertical direction, mixes A1 and E1 Raman modes, giving rise to new modes for which the propagation directions make various angles with the C-axis of ZnO nanowires. Two particular bands depending on the tilt of the nanowires are observed at 400 and 585 cm^?1. They are attributed to TO and LO quasi-phonons, and explained using Loudon's model. Morphological information of the nanowires was extracted and an average orientation of nanowires is calculated.     

The role of phase-matching and nanocrystal-size effects in three-wave mixing and CARS processes in porous gallium phosphide

Nonlinear-optical interactions, such as second-harmonic and sum-frequency generation and coherent anti-Stokes Raman spectroscopy (CARS), are investigated in porous GaP for the first time by means of a novel laser source based on mode-locked picosecond Nd³⁺:YVO₄ laser and subsequent continuum generation in an optical fiber. The efficiency of the former two nonlinear optical processes is shown to be strongly dependent on the wavelengths of the interacting waves and tends to increase with the decrease of the excitation wavelength. The power of the generated second-harmonic and sum-frequency increases by a factor of 2 and 30, respectively, compared to the crystalline GaP. In contrast, the CARS signal in porous GaP is found to be less efficient than one in crystalline GaP. The observed results are explained in terms of competition of the phase-matching effects in GaP nanocrystals and the enhanced photon lifetime in scattering porous GaP layers. PACS 42.25.Dd; 42.65.Ky; 42.70.Nq; 81.07.Bc     

Raman characterization of high-pressure phase transition in AlN nanowires

High-pressure phase transition of AlN nanowires was investigated in the range of 0-33.1 GPa by in situ Raman spectrum method in diamond anvil cell (DAC). The A1 (LO) vibration mode exhibits considerably asymmetry and broadening, indicating the occurrence of wurtzite-to-rocksalt phase transition. The Raman signal of high-pressure phase can be assigned to the disorder activated Raman scattering of rocksalt AlN. After fully releasing pressure, the Raman characterization of high-pressure phase was quenched. According to the pressure dependence of phonon frequency of AlN nanowires, the difference of transiton path between AlN nanowires and bulk materials was discussed and the mode Grüneisen parameters were determined.     

Coherent anti-Stokes Raman scattering and normal anti-Stokes Raman scattering due to LO phonons in GaP using cw diode lasers

Coherent anti-Stokes Raman scattering (CARS) and normal anti-Stokes Raman scattering (NARS) have been measured in (001) GaP at room temperature due to the 403 cm⁻¹ LO phonons using a continuous wave (CW) 785.0 nm fixed-wavelength pump laser and a CW Stokes laser tunable in the 800-830 nm wavelength range. CARS measurements are normally made using pulsed lasers. The use of CW diode lasers allows a more accurate comparison between the measured and calculated values of the CARS signal. The pump and Stokes laser beams were linearly polarized perpendicular to each other, same as the pump and normal Stokes/anti-Stokes scattered light for the GaP sample used in this work. The pump and Stokes laser powers incident upon the GaP sample, located in the focal plane of a 20 mm effective focal length lens, were <20 and 50 mW, respectively. The diameter of the laser beams in the focal plane of the focusing lens was determined to 40±5 μm. The pump and Stokes laser beam intensities incident upon the 0.3 mm thick GaP sample were <2 and 5 kW cm², respectively. The powers of the CARS and NARS signals were measured using a Raman spectrometer. The signal output of the Raman spectrometer was calibrated using the pump laser and several neutral density filters. The Raman linewidth (full-width at half-maximum) of the LO phonons was determined to be 0.95±0.05 cm⁻¹, using the variation of the CARS signal with the wavelength of the Stokes laser. The measured powers of the CARS and NARS signals are about a factor of 5 and 1.5, respectively, smaller than those calculated from the corresponding theoretical expressions.     

吸附于纳米磷化镓粉体表面碱性品红的拉曼光谱(英文)

本文中,对吸附于纳米磷化镓(GaP)粉体表面的碱性品红拉曼光谱进行了研究。通过将吸附碱性品红与纯碱性品红晶体样品的拉曼光谱进行对比、分析可知,碱性品红在纳米GaP粉体表面发生了化学吸附。在吸附碱性品红样品的拉曼光谱中,位于1200~1320cm-1范围内的光谱特征表明可能有新的化学键(P-O-C+或Ga-O-C+)形成。碱性品红分子的中央碳正离子(C+)与GaP表面具有孤对电子的氧原子形成配位键。红外光谱结果表明,氧原子与纳米GaP粉体表面的磷原子或镓原子键合,以P-O,Ga-O或P-O-Ga形式存在于GaP表面。碱性品红分子的呼吸振动,N-苯环伸缩振动,以及苯环C-C伸缩振动散射强度与纯碱性品红晶体样品相比皆有所增强。N-苯环伸缩振动散射强度增加意味着N原子是除C+离子以外的另一个可以与GaP表面发生化学作用的活性中心,这种化学作用是由N原子与GaP表面存在共轭效应造成的。     

Synthesis of GaP nanowires with Ga2O3 coating

Zinc-blende-type face centred cubic structure GaP nanowires with Ga₂O₃ coating were synthesized by heating Ga₂O₃ and red phosphorus powder with the assistance of NH₃ and Ar at 1323 K. The GaP/Ga₂O₃ nanowires have a uniform size distribution with diameters ranging from tens of nm to hundreds of nm and lengths up to several micrometres. The inner GaP nanowires have almost a single-crystal structure with twin defects and have the 111 direction as preferential growth direction. Outer Ga₂O₃ layers were polycrystalline and acted as a protection layer for the inner GaP nanowires to permit their use at high temperature. The GaP/Ga₂O₃ structure may have potential applications in future nanodevice design. PACS 81.07.Bc; 81.16.Pr     

Magnetic resonance in Ge0.99Mn0.01 nanowires

The contributions of several different subsystems to the magnetic properties of Ge_0.99Mn_0.01 nanowires are distinguished. The ferromagnetic resonance spectrum is found to have four components, two of which have the same temperature dependence and a Lorentzian shape. Presumably, these components correspond to the excitation of spin waves in the Mn³⁺ ion subsystem under the simultaneous influence of exchange and dipole-dipole interactions. There is also another Lorentzian-shaped component corresponding to resonance in the subsystem of localized Mn²⁺ centers. The fourth spectrum component has an asymmetric Dyson shape and is related to the resonance of mobile paramagnetic centers. A correlation is found between the temperature dependences of the spectral parameters of the magnetic resonances of the localized centers (Mn³⁺ and Mn²⁺ ions) and the charge carrier subsystem. This correlation indicates that the ferromagnetic exchange between the localized centers is due to carrier spin transport.     

Raman spectroscopic and photoluminescence study of single-crystalline SnO2 nanowires

Single-crystalline SnO₂ nanowires with sizes of 4-14 nm in diameter and 100-500 nm in length were produced in a molten salt approach by using hydrothermal synthesized precursor. Structural characters of the nanowires were examined by X-ray diffraction and high-resolution electron transmission microscopy. Raman, photoluminescence and X-ray photoelectron spectra of the samples were examined under heat treatments. Three new Raman modes at 691, 514 and 358 cm⁻¹ were recorded and assigned. The former two are attributed to activation of original Raman-forbidden A_2uLO mode and the third is attributed to defects in small-sized nanowires. A strong photoluminescence is observed at about 600 nm, the temperature effects is examined and the origin of the PL process is discussed via X-ray photoelectron spectra.