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.     

Micro‐Raman investigation of transition‐metal‐doped ZnO nanoparticles

We measured the Raman spectra of ZnO nanoparticles (ZnO‐NPs), as well as transition‐metal‐doped (5% Mn(II), Fe(II) or Co(II)) ZnO nanoparticles, with an average size of 9 nm. A typical Raman peak at 436 cm⁻¹ is observed in the ZnO‐NPs, whereas Zn_1−xMn_xO, Zn_1−xFe_xO and Zn_1−xCo_xO presented characteristic peaks at 661, 665 and 675 cm⁻¹, respectively. These peaks can be related to the formation of Mn₃O₄, Fe₃O₄ and Co₃O₄ species in the doped ZnO‐NPs. Moreover, these samples were analyzed at various laser powers. Here, we observed new vibrational modes (512, 571 and 528 cm⁻¹), which are specific to Mn, Fe and Co dopants, respectively, and ZnO‐NPs did not reveal any additional modes. The new peaks were interpreted either as disorder activated phonon modes or as local vibrations of Mn‐, Fe‐ and Co‐related complexes in ZnO. Copyright © 2007 John Wiley & Sons, Ltd.     

Vibrational Raman spectra of CdSx Se1‐x magic‐size nanocrystals

Resonant Raman scattering spectra of ultrasmall (<2 nm) magic‐size nanocrystals (NCs) are reported. The spectra of CdS and CdS_x Se_1‐x NCs, resonantly excited with 325 nm and 442 nm laser lines, correspondingly, reveal broad features in the range of bulk optical phonons. The relatively large width, ～50 cm^‐1, and downward shift, ～20 cm^‐1, of the Raman bands with respect to the longitudinal optical phonon in bulk crystals and large NCs are discussed based on the breaking of the translational symmetry and bond distortion in these ultrasmall NCs. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

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.     

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 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.     

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.     

Size effects on phonon localization and Raman enhancement in silicon nanotips

Silicon nanotip arrays exhibit a wide variety of interesting optical and electronic properties associated with their dimensionality. We here investigate the effect of size‐induced changes on phonon localization and explain the enhanced Raman response. The occurrence of normally forbidden transitions in the photoluminescence spectra provides evidence for the predicted localization effect. Spatially resolved Raman spectroscopy reveals a continuous change of the silicon Raman peak position and peak width along the nanotip that is attributed to a smooth change between bulk properties at the base to size‐induced phonon confinement in the apex of the nanotip. This approach allows to exclude heating effects that normally overwhelm the phonon confinement signature. The Raman spectra are in excellent agreement with the spatial correlation model and the extracted correlation length is comparable to the tip dimensions. The observed phonon confinement coincides with an enhancement of the Raman scattering efficiency at the tip apex and results in a 40‐fold increase of the sample's Raman intensity compared with bulk silicon. These results provide a step toward the integration of Si based optoelectronic devices. Copyright © 2012 John Wiley & Sons, Ltd.     

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.     

Annealing temperature dependence of Raman scattering in Si/SiO2 superlattice prepared by magnetron sputtering

Si/SiO₂ superlattices were prepared by magnetron sputtering, and the deposition temperature and annealing temperature had a great influence on the superlattice structure. In terms of SEM images, the mean size of Si nanocrystals annealed at 1100 °C is larger than that of nanocrystals annealed at 850 °C. It was found that the films deposited at room temperature are amorphous. With increasing deposition temperature, the amorphous and crystalline phases coexist. With increasing annealing temperature, the Raman intensity of the peak near 470 cm⁻¹ decreases, and the intensity of that at 520 cm⁻¹ increases. Also, on increasing the annealing temperature, the Raman peak near 520 cm⁻¹ shifts and narrows, and asymmetry emerges. A spherical cluster is used to model the nanocrystals in Si/SiO₂ superlattices, and the observed Raman spectra are analyzed by combining the effects of confinement on the phonon frequencies. Raman spectra from a variety of nanocrystalline silicon structures were successfully explained in terms of the phonon confinement effect. The fitted results agreed well with the experimental observations from SEM images.     

Si–Si optical phonon behavior in localized Si clusters of Si<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Ge<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript> alloy nanocrystals

Raman spectra acquired from Si _x Ge_1−x -nanocrystal-embedded SiO₂ films show dependence of the Si–Si optical phonon frequency on Si content. The frequency upshifts, and peak intensity increases as the silicon concentration increases. For a given Si content, the frequency remains unchanged with annealing temperature. Spectral analysis and density functional theory calculation reveal that the optical Si–Si phonon is related to the formation of localized Si clusters surrounded by Si/Ge atomic layers in the Si _x Ge_1−x nanocrystals and the intensity enhancement arises from the larger cluster size. The synergetic effect of surface tensile stress and phonon confinement determines the Si–Si optical phonon behavior.     

Modified phonon confinement model for size dependent Raman shift and linewidth of silicon nanocrystals

A modified phonon confinement model considering the size distribution, an improved phonon dispersion curve and a confinement function is developed for the calculation of size dependent Raman spectra of the silicon (Si) nanocrystals. The model is capable of simultaneous calculations of the Raman shift, intensity and linewidth. The calculated size dependent redshift and linewidth of Raman spectra are in good agreement with the available experimental data in literature and better than previously reported theoretical results. The rapid rise in the redshift and linewidth for relatively smaller Si nanocrystals are well reproduced. The asymmetric behavior of Raman spectra is also obtained from the present model.     

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.     

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.     

Temperature‐dependent,micro‐Raman spectroscopic study of barium titanate nanoparticles

A comparative, temperature‐dependent (80–500 K at 5 K intervals), micro‐Raman spectroscopic study of 300 and 50 nm diameter ceramic BaTiO₃ nanoparticles was carried out with the purpose of elucidating the nanoparticle size effect on the temperature dependence of the polar and non‐polar phonons. A method for calibrating Raman intensities, along with an iterative spectral fitting algorithm, is proposed for concurrent Raman band position and intensity analysis, increasing the analytical abilities of single temperature point Raman spectroscopy. The 300 nm particles exhibit all three phase transitions, whereas the 50 nm particles do not show evidence of these phase transitions in the same temperature range. The Curie temperature appears to be a phonon converging point, irrespective of the phonon symmetry. An attempt was made to qualitatively relate the temperature‐dependent Raman spectra to complimentary non‐spectroscopic methods, such as heat capacity and X‐ray diffraction studies. The study proves that the temperature‐dependent behavior of the polar phonon, 265 cm⁻¹, can be utilized as a sensitive phase transition probe. Copyright © 2014 John Wiley & Sons, Ltd.     

Origin of the forbidden phonons in Raman scattering spectra of uranium‐doped Ca2CuO3, a spin 1/2 chain system

The Raman scattering spectra of uranium‐doped Ca₂CuO₃ were investigated. The small doping of uranium (≤5%) in this one‐dimensional spin 1/2 chain system induced three new first‐order scattering bands and two new multiphonon bands in the structure of forbidden phonons. The first‐order bands were found to agree well with the existing theoretical results from the ab initio and tight‐binding calculation. Among them, the 470 and 665 cm⁻¹ bands appeared as the basic wavenumbers of which the multiphonon overtones were composed. The grain size effect in this strongly anisotropic system was proposed not to originate from the classical phonon confinement but rather as a result of the segmentation of one‐dimensional spin chains due to doping, which in turn allowed the new vibrational modes and implied the appearance of higher overtones in the scattering spectra. Copyright © 2008 John Wiley & Sons, Ltd.     

First and second‐order Raman scattering of B6O

First and second‐order Raman spectra of B₆O and their dependence on the wavelength of the excitation line from IR (infrared) to deep UV (ultraviolet) has been studied. The first‐order Raman spectra contain 11 well‐resolved lines of the 12 expected modes 5 A_1g + 7 E_g (space group R‐3m, point group D_3d). The second‐order Raman spectra contains eight lines that are resolved only in the case of the 244‐nm excitation line. Copyright © 2009 John Wiley & Sons, Ltd.     

Probing high‐energy ion‐implanted silicon by micro‐Raman spectroscopy

The effect of ion implantation (4 MeV¹²C²⁺, 5 MeV¹⁶O²⁺, and 8 MeV²⁸Si²⁺) on [110] silicon wafers in channeling and random orientation is investigated by micro‐Raman spectroscopy. The profiles were measured using Scanning Electron Microscope (SEM) showing that the ions were penetrating deeper inside the wafer in the channeling case creating a 1–2 µm wide strongly modified region and agreeing with the d‐nuclear reaction analysis measurements. Micro‐Raman spectroscopy was employed for the assessment of the lattice damage, probing the side surface of the cleaved wafers at submicron step. The phonon modifications show strong lattice distortions in zones parallel to the front surface of the wafers and at depths, which agree with the results of the characterization techniques. In these strongly damaged zones, there is a substantial reduction in the phonon intensity, a small shift in wavenumber position, and a large increase in the phonon width. On the basis of a modification of the phonon confinement model that takes under consideration the laser beam profile, the reduction in intensity of scattered light, and the nanocrystallite size distribution from the simulation of the lattice displacements, the main characteristics of the Raman spectra could be reproduced for the random C and O implantations. The results indicate that at a critical doping level, the induced defects and lattice distortions relax by breaking the silicon single crystal into nanocrystallites, thus creating the observed zones of strongly distorted lattice. Copyright © 2014 John Wiley & Sons, Ltd.     

Single‐Cell Investigations of Silver Nanoparticle–Bacteria Interactions

Interaction of bacteria with citrate‐reduced silver nanoparticles (AgNPs) of size 25 nm ± 8.5 nm is studied using Raman spectroscopy in conjunction with plasmon resonance imaging of single bacterial cells. Distribution of isolated nanoparticles (NPs) inside Escherichia coli (ATCC 25922; E. coli) is observed by hyperspectral imaging (HSI) as a function of incubation time. Time‐dependent degradation of bacterial DNA upon incubation of AgNPs with E. coli is proven by Raman spectroscopic studies. While attachment of NPs is evident in HSI, molecular changes are evident from the surface‐enhanced Raman spectra of adsorbed DNA and its fragments. Distinct enhancement of DNA features is observed upon interaction of AgNPs and the number of such distinct features increases with incubation time, reaches a maximum, and decreases afterwards. This systematic interaction of DNA with the NPs system and its gradual chemical evolution is proven by investigating isolated plasmid DNA. A comparative Raman study with silver ions has shown that DNA features are observable only when bacteria are incubated with AgNPs. Energetics of interaction examined with microcalorimetry suggests the exothermicity of ?1.547 × 10¹⁰ cal mol^?1 for the NP–bacteria system. Specific interaction of AgNPs with exocyclic nitrogen present in the bases, adenine, guanine, and cytosine, leads to the changes in DNA.     

Resonant Raman scattering in CdSxSe1−x nanocrystals: effects of phonon confinement,composition, and elastic strain

Optical phonon modes, confined in CdS_xSe_1−x nanocrystal (NC) quantum dots (≈2 nm in radius) grown in a glass matrix by the melting‐nucleation method, were studied by resonant Raman scattering (RRS) spectroscopy and theoretical modeling. The formation of nanocrystalline quantum dots (QDs) is evidenced by the observation of absorption peaks and theoretically expected resonance bands in the RRS excitation spectra. This system, a ternary alloy, offers the possibility to investigate the interplay between the effects of phonon localization by disorder and phonon confinement by the NC/matrix interface. Based on the concept of propagating optical phonons, which is accepted for two‐mode pseudo‐binary alloys in their bulk form, we extended the continuous lattice dynamics model, which has successfully been used for nearly spherical NCs of binary materials, to the present case. After determining the alloy composition for NCs (that was evaluated with only 2–3% uncertainty using the bulk longitudinal optical phonon wavenumbers) and the NC size (using atomic force microscopy and optical absorption data), the experimental RRS spectra were described rather well by this theory, including the line shape and polarization dependence of the scattering intensity. Even though the presence of a compressive strain in the NCs (introduced by the matrix) masks the expected downward shift owing to the phonons' spatial quantization, the asymmetric broadening of both Raman peaks is similar to that characteristic of NCs of pure binary materials. Although with some caution, we suggest that both CdSe‐like and CdS‐like optical phonon modes indeed are propagating within the NC size unless the alloy is considerably heterogeneous. Copyright © 2011 John Wiley & Sons, Ltd.