Low‐temperature synthesis and Raman scattering of Mn‐doped ZnO nanopowders

Nanocrystalline Mn‐doped zinc oxides Zn_1−xMn_xO (x = 0–0.10) were synthesized by the sol–gel technique at low temperature. The calcination temperature of the as‐prepared powder was found at 350 °C using differential thermal analysis. A thermogravimetric analysis showed that there is a mass loss in the as‐prepared powder till 350 °C and an almost constant mass till 800 °C. The X‐ray diffraction patterns of investigated nanopowders calcined at 350 °C correspond to the hexagonal ZnO structure without any foreign impurities. The average grain size of the nanocrystal that was observed around ∼25–40 nm from transmission electron microscopy matched well with the crystallite size calculated from the line shape of X‐ray diffraction. The chemical bonding structure in Zn_1−xMn_xO nanopowders was examined using X‐ray photoelectron spectroscopy techniques, which indicate substitution of Mn²⁺ ions into Zn²⁺ sites in ZnO lattice. Micro Raman spectroscopy confirmed the insertion of Mn ions in the ZnO host matrix, and similar wurtzite structure of Zn_1−xMn_xO (x < 10%) nanocrystals. Temperature‐dependent Raman spectra of the nanocrystals displayed suppression of luminescence and enhancement in full width at half maximum in pure ZnO nanocrystals with increase in temperature, which suggests an enhancement in particle size at elevated temperature. Copyright © 2009 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.     

Surface optical phonon—assisted electron Raman scattering in a semiconductor quantum disc

We have carried out a theoretical calculation of the differential cross section for the electron Raman scattering process associated with the surface optical phonon modes in a semiconductor quantum disc.electron states are considered to be confined within a quantum disc with infinite potential barriers.The optical phonon modes we have adopted are the slab phonon modes by taking into consideration the Frohlich interaction between an electron and a phonon.The selection rules for the Raman process are given.Numerical results and a discussion are also presented for various radii and thicknesses of the disc,and different incident radiation energies.     

Direct observation of surface‐enhanced Raman scattering in ZnO nanocrystals

We have been able to observe the surface‐enhanced Raman scattering (SERS) from 4‐mercaptopyridine (4‐Mpy) molecules adsorbed on ZnO nanocrystals, which display 10³ enhancement factors (EFs). An excitation wavelength‐dependent behavior is clearly observed. Another molecule BVPP is also observed to have surface‐enhanced Raman signals. The chemical enhancement is most likely responsible for the observed enhancement, since plasmon resonances are ruled out. The research is important not only for a better understanding of the SERS mechanism, but also for extension of the application of Raman spectroscopy to a variety of adsorption problems on a semiconductor surface. Copyright © 2009 John Wiley & Sons, Ltd.     

高性能ZnO纳米块体材料的制备及其拉曼光谱学特征

利用六面顶高压设备制备了高密度、低脆性、纳米级的ZnO块体材料，用MDI/JADE5 X射线衍射仪(Cu靶)和XL30S-FEG场发射扫描电子显微镜对高压样品的相组成、晶粒尺寸及微观形貌进行了表征.利用E55+FRA106/5傅里叶变换激光拉曼光谱仪通过ZnO块体样品位于50—500cm^-1之内的拉曼光谱, 研究了极性半导体纳米材料的拉曼光谱学特征.发现在极性半导体ZnO纳米块体材料中，没有出现明显的尺寸限制效应. 关键词： ZnO纳米块体 拉曼光谱 尺寸限制效应     

Titanium dioxide synthesized using titanium chloride: size effect study using Raman spectroscopy and photoluminescence

Titanium dioxide nanocrystals were prepared by the wet chemical method and characterized by X‐ray diffraction (XRD), transmission electron microscopy (TEM), Raman scattering (RS) and photoluminescence techniques. The XRD pattern shows the formation of single phase anatase structure of average sizes ∼7 nm (sample A) and ∼15 nm (sample B) for two samples. Additionally, TEM and RS were used to confirm the anatase crystal structure for both samples. The PL spectra show that the intensity of the sample A is more than that of sample B, which has been attributed to defect(s) and particle size variation. A modified phonon confinement model incorporating particle size distribution function and averaged dispersion curves for two most dispersive phonon branch (Γ‐X direction) have been used to interpret the size effect in Raman spectra. The obtained Raman peak shift and full width at half‐maximum agree well with the experimental data. Our observations suggest that the phonon confinement effects are responsible for a significant shift and broadening for the Raman peaks. Copyright © 2009 John Wiley & Sons, Ltd.     

Structural studies and Raman spectroscopy of forbidden zone boundary phonons in Ni‐doped ZnO ceramics

We present X‐ray diffraction and Raman spectroscopy studies of Ni‐doped ZnO (Zn_1−xNi_xO, x = 0.0, 0.03, 0.06, and 0.10) ceramics prepared by solid‐state reaction technique. The presence of the secondary phase along with the wurtzite phase is observed in Ni‐doped ZnO samples. The E₂(low) optical phonon mode is seen to be shifted to a lower wavenumber with Ni incorporation in ZnO and is explained on the basis of force‐constant variation of ZnO bond with Ni incorporation. A zone boundary phonon is observed in Ni‐doped samples at ∼130 cm⁻¹ which is normally forbidden in the first‐order Raman scattering of ZnO. Antiferromagnetic ordering between Ni atoms via spin‐orbit mechanism at low temperatures (100 K) is held responsible for the observed zone boundary phonon. Copyright © 2008 John Wiley & Sons, Ltd.     

Raman study of V/III flux ratio effect in InP/InAlAs/InP heterostructures grown by MOCVD

Micro‐Raman measurements have been carried out in order to study the V/III flux ratio effect in InP/InAlAs/InP heterostructures grown by metal‐organic chemical vapor deposition (MOCVD). Photoluminescence (PL) studies in InP/InAlAs/InP heterostructures [1] , [2] show a strong dependence of the PL band linewidth on V/III molar ratio. In addition to the observation of the two‐mode behavior and the disorder activated modes in InAlAs alloy, an analysis of Raman spectra shows a line shape broadening and wavenumber shift of Raman peaks for various V/III molar ratios, with minimum linewidth and lattice mismatch occurring at V/III = 50. Also, a strong dependence on the composition modulation of the AlAs‐like longitudinal optic (LO_AlAs−like) phonon was observed due to clustering. Calculation of the in‐plane strain shows that the lattice mismatch between the epilayer and the substrate is relatively insensitive to flux ratio variation within the range investigated. Therefore, the high arsenic overpressures used have an insignificant adverse effect on the quality of the hetero‐interfaces. Copyright © 2009 John Wiley & Sons, Ltd.     

Photoluminescence and resonant Raman scattering in N-doped ZnO thin films

A combination of studies on photoluminescence and resonant Raman scattering in N-doped ZnO thin films were carried out at room temperature. In the photoluminescence spectra, a transformation of radiative recombination mechanism from free-exciton to donor-acceptor-pair transition was observed. An enhancement of resonant Raman scattering processes as well as longitudinal optical (LO) phonon overtones up to the sixth order were observed in the Raman spectra. Also, the nature of the 1LO phonon underwent a transformation from a pure A₁(LO) mode to a quasimode with mixed A₁ and E₁ symmetry. The underlying mechanisms accounting for the influences of N doping on the optical properties of ZnO were related to the incorporation of extrinsic defects in the crystal lattice.     

CdSe和ZnO量子点的拉曼光谱研究

本文介绍了用拉曼光谱研究CdSe和ZnO两种Ⅱ Ⅳ族量子点材料的结果,对拉曼峰进行了指认。观察到的光学声子峰位的移动被认为是由量子限制效应引起。     

ZnO晶体的偏振拉曼散射的深入研究

利用拉曼选择定则,设计了ZnO单晶的直角偏振几何配置。在室温下测量了ZnO单晶的各种振动模式的偏振拉曼散射光谱。与原先的文献相比较,初步讨论了各种振动模式的线宽和强度的变化原因。除ZnO晶体中包括非极性和极性拉曼基频振动,准横光学和准纵光学模式和振动属性被指认外,它们的高阶拉曼散射模式首先被确定。本研究结果为深入了解ZnO晶体和薄膜的宏观性质和微观结构提供了依据。     

Raman investigation of defective SiC nanocrystals

Phonon confinement effect and surface optical mode in SiC nanocrystal have been investigated through Raman spectroscopy. Considering high density of stacking faults in SiC grains, the correlation length of RWL (proposed by Richter, Wang, Li to explain phonon confinement in nano silicon) model is determined as a distance between nearby stacking faults. Thus, homogeneous region becomes thin slices in cylindrical SiC grains, which redefines weighting function. Effect of anisotropy of phonon dispersion curve is also analyzed during calculation. The additional 875‐cm⁻¹ band is attributed to defects and amorphous SiC, which is confirmed by transmission electron microscopy. SiC grains are approximated as column array with grain boundary substances regarded as surrounding medium, which explains surface optical phonon mode at 915 cm⁻¹. Copyright © 2015 John Wiley & Sons, Ltd.     

Ultraviolet Raman scattering in ZnO nanowires: quasimode mixing and temperature effects

We use near‐resonance Raman scattering to investigate zinc oxide (ZnO) nanowires grown by chemical vapor deposition on Si substrates. We discuss the role of quasimode mixing on the wavenumber of the longitudinal optical (LO) bands, and we perform Raman measurements with different excitation powers to investigate possible laser heating effects. We find that in the Raman spectra of as‐deposited nanowires grown along the c‐axis of wurtzite, the LO bands are located slightly below the E₁(LO) mode of bulk ZnO. We perform a calculation of the expected LO wavenumber in an ensemble of randomly oriented nanowires. Our analysis shows that light refraction, together with the orientation‐dependent cross‐section of the nanowires for the incoming light, counterbalances quasimode mixing effects in the as‐grown product, giving rise to LO bands that are barely redshifted relative to the E₁(LO) mode. In the case of ZnO nanowires that have been mechanically removed (scratched) and subsequently deposited onto separate Si substrates, we observe clear laser‐induced heating. Temperature effects account well for the Raman wavenumber shifts displayed by the LO bands in the Raman spectra of the scratched nanowires. Copyright © 2010 John Wiley & Sons, Ltd.     

分子束外延PbTe单晶薄膜的反常拉曼光谱研究

采用分子束外延(MBE)方法在BaF₂(111)衬底上生长了高质量的PbTe单晶薄膜， 拉曼光谱测量观察到了表面氧化物的振动模、布里渊区中心(q≈0)纵光学(LO)声子振动模以 及声子-等离子激元耦合模振动.随着显微拉曼光谱仪激光光斑聚焦深度的改变，各拉曼散射 峰的峰位、积分强度、半高宽等都表现出不同的变化趋势. 随着激光光斑聚焦位置从样品表 面上方3μm处变化到表面下方3μm处，PbTe外延薄膜的LO声子频率从119cm^-1移 动到124cm^{-1关键词： PbTe外延薄膜 拉曼散射 纵光学声子}     

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.     

压电晶体拉曼散射的统一量子论

发展了拉曼散射的一个广义量子理论，它能同时说明非极性模和极性模的作用.在场论中， 光被纵光学和横光学模的拉曼散射能在一个统一的理论框架内描述. 关键词： 拉曼散射 声子 量子场论     

ZnO纳米棒的拉曼和发光光谱研究(英文)

本文对采用湿化学方法合成的ZnO纳米棒样品的拉曼光谱和发光光谱进行了研究。由扫描电镜结果可知,合成的ZnO纳米棒具有很好的尺寸发布均匀性,直径在30 nm左右,长度大于1微米。采用显微拉曼光谱技术,得到了632.8 nm波长激发的拉曼光谱,并和体相样品的拉曼光谱进行了对比分析;由325 nm激光波长激发得到的荧光光谱可知样品具有很好的紫外发光性质。     

Raman scattering by folded acoustic phonons in InGaN/GaN superlattices

We use Raman scattering to investigate the folded longitudinal acoustic (LA) phonons in a series of In_xGa_1−xN/GaN superlattices (SLs) grown by molecular beam epitaxy with different compositions (15% < x < 38%) and SL periods (from 8 to 20 nm). A novel, ultralow wavenumber filtering module, which provides access to ultralow wavenumber Raman modes on single‐grating spectrometers, has been used to perform the Raman measurements. Zone‐folding effects are observed, and the wavenumber behavior of the folded LA modes is well reproduced with a linear dispersion for the folded LA modes as predicted by elastic continuum theory. We employ the wavenumber of the doublets to evaluate the period of the SLs. Copyright © 2011 John Wiley & Sons, Ltd.     

Softening behavior of acoustic phonon mode in ZnO nanoparticles: the effect of impurities and particle size variation with temperature

Decay dynamics of the acoustic phonon mode in ZnO nanoparticles, synthesized using the wet chemical technique, is investigated. It is well established that optic phonon modes in a semiconductor favor an anharmonic decay dynamics; in contrast, acoustic modes evidence a rather complex decay behavior, manifesting their dependence on other parameters such as particle size, impurity species, etc. At lower temperatures (T < 500 K), the anharmonic decay process, caused by the weakening of the bond strength, is responsible for the observed decrease in the acoustic mode wavenumbers. However, particle growth due to the coalescence sintering process is prominent at higher temperatures (>600 K) and governs the softening behavior of the acoustic phonon mode towards the Rayleigh line. On the other hand, the precursor species and reaction byproducts on the surface of ZnO nanoparticles induce an anomalous softening behavior in the decay dynamics at specific temperatures by damping the acoustic phonon mode. Copyright © 2011 John Wiley & Sons, Ltd.     

Micro‐Raman scattering spectroscopy study of Li‐doped and undoped ZnO needle crystals

ZnO nanostructures have attracted great attention for possible applications in optoelectronic and spintronic devices. The electrical resistivity because of carriers can be improved by the introduction of Li ions, as Li is a possible dopant for achieving p‐type ZnO. We have carried out a comprehensive micro‐Raman scattering study of the phonons in 1% Li‐ and undoped ZnO needle crystals grown and annealed at 1073 K for 1 and 2 h under oxygen environment. Phonon mode of doped and undoped ZnO does not show any measurable shift for the doping concentration of 1%. As line width is related to point defect density, we find for both Li‐ and undoped ZnO crystals the crystallinity is improving towards the tip of the needle crystals. Copyright © 2008 John Wiley & Sons, Ltd.