Experimental assessment of the phonon confinement in TiO2 anatase nanocrystallites by Raman spectroscopy

TiO₂ aerogels prepared by sol‐gel method and followed by supercritical drying have been annealed at temperatures ranging between 400 and 550 °C. The obtained TiO₂ anatase crystallites with the mean size between 6.4 and 13.9 nm, as obtained from transmission electron microscopy measurements, have been further investigated by Raman spectroscopy. It was found that the peak position and full width at half maximum (FWHM) of the TiO₂ anatase Raman bands located around 144, 398, and 638 cm⁻¹ are influenced by crystallite dimension. These spectral changes can be assigned to the combined action of several nanosize effects such as phonon confinement, phonon coupling, strain, and stoichiometry defects. Surprisingly, the best discrimination of the FWHM change with the nanocrystallite mean size was achieved for the 638 cm⁻¹ band, whereas the best discrimination for the peak position was found for the 398 cm⁻¹ band. The critical size values obtained from the peak position and FWHM evaluation were between 12.7 and 13.1 nm. Taking into consideration that only the phonon confinement and inhomogeneous strain can induce an asymmetric broadening of the Raman signal, the bands asymmetry was evaluated, and the critical size values of the nanocrystallites were determined to be between 10 and 11 nm. For a symmetric size distribution of TiO₂ anatase crystallites with dimensions between 6.4 and 13.9 nm, the obtained result indicates that the phonon confinement contribution to the overall size effects is more than 75%. No evidence about the influence of the phonon coupling and vacancies on the Raman features was observed. The comparison of the data derived from the experimental analysis with those obtained by applying the theoretical phonon confinement model indicates the necessity of developing an improved phonon confinement model. The asymmetry approach can be applied for a great variety of nanostructures, as a measure of the confinement effect. Copyright © 2011 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.     

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.     

纳米二氧化钛材料的相变和声子限制效应(英文)

拉曼光谱用于研究二氧化钛材料的相变和声子限制效应。化学溶液方法制备了TiO2纳米晶材料。其平均粒度为 6 8- 2 7 9nm。最低频率 1 5 2cm- 1 Eg 模随粒度减小出现蓝移和加宽。在声子限制模型下 ,理论上对不同粒度的TiO2 纳米晶 (6 8,1 0 3和 2 7 9nm)的频移和线宽进行计算 ,结果与实验吻合得很好。研究了TiO2 纳米晶锐钛矿 -金红石相变 ,其相变温度为 6 5 0 - 6 90℃ ,比体块TiO2 的相变温度 1 0 0 0℃低 ,表明了相变的尺寸效应     

Raman spectroscopy for study of interplay between phonon confinement and Fano effect in silicon nanowires

A combined effect of doping (type and species) and size on Raman scattering from silicon (Si) nanowires (NWs) has been presented here to study interplay between quantum confinement and Fano effects. The SiNWs prepared from low doping Si wafers show only confinement effect, as evident from the asymmetry in the Raman line‐shape, irrespective of the doping type. On the other hand SiNWs prepared from wafer with high doping shows the presence of electron–phonon interaction in addition to the phonon confinement effect as revealed from the presence of asymmetry and antiresonence in the corresponding Raman spectra. This combined effect induces an extra asymmetry in the lower energy side of Raman peak for n‐type SiNWs whereas the asymmetry flips from lower energy side to the higher energy side of the Raman peak in p‐type SiNWs. Such an interplay can be represented by considering a general Fano‐Raman line‐shape equation to take care of the combined effect in SiNWs. Copyright © 2015 John Wiley & Sons, Ltd.     

Quantitative analysis of the phonon confinement effect in arbitrarily shaped Si nanocrystals decorated on Si nanowires and its correlation with the photoluminescence spectrum

Arrays of single‐crystalline Si nanowires (NWs) decorated with arbitrarily shaped Si nanocrystals (NCs) are grown by a metal‐assisted chemical etching process using silver (Ag) as the noble metal catalyst. The metal‐assisted chemical etching‐grown Si NWs exhibit strong photoluminescence (PL) emission in the visible and near infrared region at room temperature. Quantum confinement of carriers in the Si NCs is believed to be primarily responsible for the observed PL emission. Raman spectra of the Si NCs decorated on Si NWs exhibit a red shift and an asymmetric broadening of first‐order Raman peak as well as the other multi‐phonon modes when compared with that of the bulk Si. Quantitative analysis of confinement of phonons in the Si NCs is shown to account for the measured Raman peak shift and asymmetric broadening. To eliminate the laser heating effect on the phonon modes of the Si NWs/NCs, the Raman measurement was performed at extremely low laser power. Both the PL and Raman spectral analysis show a log‐normal distribution for the Si NCs, and our transmission electron microscopy results are fully consistent with the results of PL and Raman analyses. We calculate the size distribution of these Si NCs in terms of mean diameter (D₀) and skewness (σ) by correlating the PL spectra and Raman spectra of the as‐grown Si NCs decorated on Si NWs. Copyright © 2015 John Wiley & Sons, Ltd.     

Raman spectroscopy of sol–gel derived titanium oxide thin films

Raman spectra of TiO₂ films prepared via the sol–gel process were studied by UV and visible Raman spectroscopy. The evolution of the phases of TiO₂ films during annealing was investigated, and the relative intensities of the Raman bands excited with 325 nm were found to be distinct from those of the bands excited with 514 nm. The transmittance and FTIR spectra of the films annealed at different temperatures were characterized. The crystallization process of the powders and thin films treated by different annealing methods were also studied with Raman spectroscopy. The results show that the change in the relative intensities is caused by the resonance Raman effect. The anatase to rutile transition of the powder occurs at 700 °C, while that of the thin film occurs at 800 °C. The analysis of Raman band shape (peak position and full width at half‐maximum) after conventional furnace annealing and rapid thermal annealing indicates the influence of the non‐stoichiometry and phonon confinement effect. Copyright © 2011 John Wiley & Sons, Ltd.     

Synthesis and photoluminescence studies of silicon nanoparticles embedded in silicon compound films

High-density silicon nanoparticles with well-controlled sizes were grown onto cold substrates in amorphous SiN _x and SiC matrices by plasma-enhanced chemical vapor deposition. Strong, tunable photoluminescence across the whole visible light range has been measured at room temperature from such samples without invoking any post-treatment, and the spectral features can find a qualitative explanation in the framework of quantum confinement effect. Moreover, the decay time was for the first time brought down to within one nanosecond. These excellent features make the silicon nanostructures discussed here very promising candidates for light-emitting units in photonic and optoelectronic applications.      

Raman investigation of silicon nanocrystals: quantum confinement and laser‐induced thermal effects

We present a detailed experimental and theoretical Raman investigation of quantum confinement and laser‐induced local thermal effects on hydrogenated nanocrystalline silicon with different nanocrystal sizes (3.6–6.2 nm). The local temperature was monitored by measuring the Stokes/anti‐Stokes peak ratio with the laser power density range from ~120 to 960 kW/cm². In combination with the three‐dimensional phonon confinement model and the anharmonic effect, which incorporates the three‐phonon and four‐phonon decay processes, we revealed an asymmetrical decay process with wavenumbers ~170 and 350 cm^–1, an increasing anharmonic effect with nanocrystal sizes, and a shortening lifetime with enhanced temperature and decreasing nanocrystal dimension. Furthermore, we demonstrated experimentally that for Si nanocrystals smaller than 6 nm, the quantum confinement effect is dominant for the peak shift and line broadening. Copyright © 2011 John Wiley & Sons, Ltd.     

Micro‐Raman investigation of nanosized zinc ferrite: effect of crystallite size and fluence of irradiation

Zinc ferrite nanoparticles of different sizes ranging from 12 to 62 nm were synthesized by using the nitrate route. These nanoparticles were irradiated by a 100 MeV oxygen beam at two fluences: 1 × 10¹³ and 5 × 10¹³ ions/cm². It was observed that modes corresponding to cubic spinel phase were retained after the irradiation in all the systems. The variation in the parameters of various modes follows phonon confinement, while this effect seems to violate in irradiated specimen. It was found that the irradiation‐induced changes in the modes F_2g(2) and F_2g(3) depend on whether the crystallite size of the pristine sample is less than, equal to or greater than the phonon confinement length, while this length is not dominant for the irradiation‐induced changes in the mode A_1g. The changes in various parameters of the modes are attributed to the combined effect of the restructuring of the chemical species and ion‐induced defects. Copyright © 2011 John Wiley & Sons, Ltd.     

用拉曼散射光谱估算纳米Ge晶粒平均尺寸

用射频共溅射技术和真空退火方法制备了埋入ＳｉＯ２基底中的纳米Ｇｅ复合膜（ｎｃ－Ｇｅ／ＳｉＯ２）测量了不同温度退火后该复合膜的拉曼散射光谱，其结果与晶体Ｇｅ的拉曼谱相比，纳米Ｇｅ的拉曼峰红移峰形变宽，用拉曼谱的参数计算了纳米Ｇｅ晶粒的平均尺寸，所得结果与声子限域理论模型符合。     

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.     

Finite size effect on Raman frequency of phonons in nano-semiconductors

A comprehensive study on Raman spectroscopy with different excitation wavelengths, sample sizes, and sample shapes for optic phonons (OPs) and acoustic phonons (APs) in polar and non-polar nano-semiconductors has been performed. The study affirms that the finite size effect does not appear in the OPs of polar nano-semiconductors, while it exists in all other types of phonons. The absence of the FSE is confirmed to be originated from the long-range Fröhlich interaction and the breaking of translation symmetry. The result indicates that the Raman spectra of OPs cannot be used as a method to characterize the scale and crystalline property of polar nano-semiconductors.     

离子注入诱导量子阱界面混合效应的光致荧光谱研究

采用多元芯片方法获得了一系列不同离子注入剂量的GaAsAlGaAs非对称耦合量子阱单元,通过光致荧光谱测量,研究了单纯的离子注入导致的界面混合效应.荧光光谱行为与有效质量理论计算研究表明,Al原子在异质结界面的扩散在离子注入过程中已基本完成,而热退火作用主要是去除无辐射复合中心. 关键词： 量子阱 离子注入 光致荧光谱 界面混合     

Micro‐Raman investigation of tin dioxide nanostructured material based on annealing effect

Rutile‐structured nanocrystalline tin dioxide (SnO₂) powder was synthesized by the chemical precipitation method using the precursor SnCl₂• 5H₂O. The SnO₂ powder was annealed at different temperatures, namely, 600, 800 and 1000 °C. Micro‐Raman spectra were recorded for both the as‐grown and annealed SnO₂ nanocrystalline samples. Micro‐Raman spectral measurements on the SnO₂ nanoparticle show the first‐order Raman modes A_1g (633 cm⁻¹), E_1g (475 cm⁻¹) and B_2g (775 cm⁻¹), indicating that the grown SnO₂ belongs to the rutile structure. The first‐order A_1g mode is observed as an intense band, whereas the other two modes show low intensity. The full width at half‐maximum and band area of the Raman lines of SnO₂ nanoparticle annealed at various temperatures were calculated. The effect of high‐temperature annealing on the vibrational modes of SnO₂ was studied. The optical image of SnO₂ nanocrystalline material was used to understand the surface morphology effect. Copyright © 2011 John Wiley & Sons, Ltd.     

Polar propagating optical phonon states and their dispersive spectra in wurtzite nitride superlattices: Quantum size effect

The polar optical phonon states of propagating (PR) modes in wurtzite GaN/Al_xGa_1−xN superlattices (SLs) are investigated within the dielectric continuum model framework. It is proved that the PR phonon modes only appear in wurtzite GaN/Al_xGa_1−xN SL with a small x$x$ Al mole ratio concentration (such as x<0.34$x<0.34$). The analytical phonon states of PR modes and their dispersive equation in the wurtzite GaN/Al_xGa_1−xN SL structures are obtained. Numerical calculations on the dispersive spectra of PR modes and their electrostatic potential properties as well as the quantum size effect are performed for a wurtzite GaN/Al_0.15Ga_0.85N SL. Results reveal that dispersive curves of PR modes in SLs form frequency bands. As the well width of GaN well-layer increases, the frequency bands of PR modes become steeper and narrower. The discussion of electrostatic potentials shows that the wavelength of PR phonon modes with a phase qL=π$qL=\pi$ (0) is 2L$2L$ (L$L$). With the increase of the SL well width, the wave-node number of the PR phonon modes in the barrier regions increases. The present theoretical scheme and numerical results are quite useful for analyzing the dispersive spectra of PR phonon modes and their polaronic effect in wurtzite GaN/AlGaN SL structures.     

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     

Multipeak-structured photoluminescence mechanisms of as-prepared and oxidized Si nanoporous pillar arrays

Silicon dominates the electronic industry, but its poor optical properties mean that it is not preferred for photonic applications. Visible photoluminescence (PL) was observed from porous Si at room temperature in 1990, but the origin of these light emissions is still not fully understood. This paper reports that an Si nanocrystal, silicon nanoporous pillar array (Si-NPA) with strong visible PL has been prepared on a Si wafer substrate by the hydrothermal etching method. After annealing in O₂ atmosphere, the hydride coverage of the Si pillar internal surface is replaced by an oxide layer, which comprises of a great quantity of Si nanocrystal (nc-Si) particles and each of them are encapsulated by an Si oxide layer. Meanwhile a transition from efficient triple-peak PL bands from blue to red before annealing to strong double-peak blue PL bands after annealing is observed. Comparison of the structural, absorption and luminescence characteristics of the as-prepared and oxidized samples provides evidence for two competitive transition processes, the band-to-band recombination of the quantum confinement effect of nc-Si and the radiative recombination of excitons from the luminescent centres located at the surface of nc-Si units or in the Si oxide layers that cover the nc-Si units because of the different oxidation degrees. The sizes of nc-Si and the quality of the Si oxide surface are two major factors affecting two competitive processes. The smaller the size of nc-Si is and the stronger the oxidation degree of Si oxide layer is, the more beneficial for the luminescent centre recombination process to surpass the quantum confinement process is. The clarification on the origin of the photons may be important for the Si nanoporous pillar array to control both the PL band positions and the relative intensities according to future device requirements and further fabrication of optoelectronic nanodevices.     

Raman spectroscopy of synthetic,geological and biological vaterite: a Raman spectroscopic study

Raman spectroscopy was used to study vaterite samples of biological, geological and synthetic origin. The Raman band positions and the full width at half‐maximum (FWHM) of the lattice modes and the internal modes of the carbonate ion of all specimens show no significant differences between vaterites of different origin. With increasing Mg concentrations, synthetic vaterite samples show increasing FWHM in the region of the lattice modes and the three ν₁ bands, whereas no change in luminescence was detected. In contrast, in situ measurements of vaterite areas in freshwater cultured pearls (FWCPs) by laser ablation inductively coupled plasma mass spectrometry (LA‐ICP‐MS) together with the Raman spectra obtained at the same points show that the luminescence intensity in biological samples is affected by the magnesium content. The Raman spectroscopic features of vaterite and parisite‐(Ce) are compared, and their similarities suggest that the structure of vaterite contains at least three crystallographically independent carbonate groups and similar carbonate group layers. A band at 263 cm⁻¹ is observed for the first time in this study, whereas it could be demonstrated that bands previously reported at 130 and 190 cm⁻¹ do not belong to vaterite. Copyright © 2009 John Wiley & Sons, Ltd.     

Raman spectra of CdS nanocrystals in Nafion: longitudinal optical and confined acoustic phonon modes

Quantum confinement effects on the longitudinal optical and acoustic phonons in CdS nanocrystals in the strongly confined regime in the polymer matrix Nafion are studied using Raman spectroscopy. The LO-phonon modes show size-dependent asymmetric broadening though the broadening and asymmetry are less than those predicted by the phonon confinement models. Two types of confined acoustic modes corresponding to n=1, l=0 and n=1, l=2 spheroidal vibrations are observed. Softening of the spheroidal modes is observed in the strongly confined regime.