Phonon Confinement Model Research Articles

Raman scattering studies in dilute magnetic semiconductorZn1−xCoxO

Raman spectra of ZnO and Co substituted ${\mathrm{Zn}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x}\mathrm{O}$ (ZCO) were carried out using the Raman microprobe system with an ${\mathrm{Ar}}^{+}$ ion laser source of $514.5\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ wavelength. The shift towards the lower frequency side of the nonpolar ${E}_{2}^{\text{low}}$ mode and the broadening due to Co substitution in ZnO were analyzed using the phonon confinement model. The magnetic measurements showed ferromagnetic behavior with the maximum saturation magnetization $(1.2{\ensuremath{\mu}}_{B}∕\mathrm{Co})$ for 10% Co substitution, which decreased with at further increase in Co concentrations. The intensities of ${E}_{1}(\mathrm{LO})$ at $584\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ and multiphonon modes at $540\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ were increased with an increase in Co substitution. The additional Raman modes in ceramic targets of ZCO spectra for higher concentration of Co substitution $(x=15%--20%)$ were identified to be due to the spinel $\mathrm{Zn}{\mathrm{Co}}_{2}{\mathrm{O}}_{4}$ secondary phase.

Structural and textural changes of CVD-SiC to indentation, high temperature creep and irradiation

The structure and microtexture of different SiC-based CVD coatings have been studied by RMS, in their as-processed state and after high temperature annealing, creep, indentation and irradiation. Both annealing and creep resulted in the same degree of SiC crystal growth and decrease of stacking faults. A slight influence of stress was however observed on the structure and texture of the co-deposited free-carbon, likely as a consequence of the intergranular creep mechanism. Room temperature indentation induces substantial structural disorders (dislocations, stacking faults, small grains) near the contact and more extended damages due to dislocation slip parallel to the compact Si–C planes. These structural changes were found to depend on the single crystal orientation and therefore, on the texture of polycrystalline SiC. Room temperature proton irradiation produced only small amounts of disorders. The specific alterations of the Raman features were explained using a phonon confinement model. This approach supports the existence of very low scale defects, likely as punctual defects.

Etching temperature dependence of optical properties of the electrochemically etched n-GaAs

The GaAs granular films have been prepared by electrochemical anodic etching of n-GaAs in HCl electrolyte at different etching temperatures. The microstructure and optical properties of the films were investigated by micro-Raman spectrum, atomic force microscopy (AFM) and photoluminescence (PL) spectroscopy. Raman spectra reveal marked redshift and broadening, which could be explained by phonon confinement model. Results show the GaAs nanocrystalline films have formed during the anodic etching process under certain chemical conditions. Two “infrared” PL bands at ∼860 nm and ∼920 nm and a strongly enhanced visible PL band envelope around 550 nm were observed in the film prepared at etching temperature of 50 °C. The “green” PL band envelope is attributed to both quantum confinement in GaAs nanocrystals and PL of Ga2O3 and As2O3. The results reveal that the energy band structure of GaAs granular films is closely related to the etching temperatures.

Temperature dependence of the first-order Raman scattering in silicon nanowires

Temperature dependence of the first-order Raman scattering in silicon nanowires was studied using an amended phonon confinement model. The experimental band position and linewidth both showed a better agreement with the calculated data if the temperature effect was considered. Furthermore, a slight improvement in crystal quality was observed.

Raman properties of silicon nanoparticles

The Raman spectra of silicon nanoparticles in the size range between d = 3.5 – 60 nm have been studied experimentally. Scattering processes up to second order are being observed. The experimental results are analyzed in the framework of the phonon confinement model. While this model describes qualitatively the observations for first-order scattering processes, it is not applicable for scattering processes of higher order. From the analysis of second-order scattering, we determine a redshift of the TO phonon at the X and L points.

Raman spectroscopy characterization of titania nanoparticles produced by flame pyrolysis: The influence of size and stoichiometry

A systematic study of the shift and linewidth of the Eg Raman peak at 144cm−1 of anatase TiO2 nanopowders, produced by a flame aerosol technique, is here presented. The analysis was performed as a function of the crystal domain size and of the degree of oxidation. In the nanopowders, a clear contribution of the stoichiometry defects to the peak shift was evidenced, while the peak width seems to be less affected by the oxygen content. The Raman peak behavior due to size reduction has been interpreted in the framework of a phonon quantum confinement model. A critical review of the different approaches to this model, adopted in the literature to explain the behavior of the anatase Raman spectra as a function of the domain size, is presented. In particular, the hypothesis of a three-dimensional isotropic model for the dispersion relations is discussed. This analysis evidences general limits in the application of the phonon confinement model to the study and characterization of nanoparticles and nanostructured materials, showing how an uncritical use of the confinement theory can yield distorted results.

Characterization of Si nano-polycrystalline films at the nanometer level using resonant Raman scattering

We have measured a resonant Raman scattering from polycrystalline silicon (poly-Si) films with thicknesses of 24–381nm at the depth resolution of approximately 5nm and found that poly-Si films are under compressive stress. The main Raman peak in poly-Si films tends to shift to a higher frequency as the thickness of the underlayer of the silicon dioxide (SiO2) film becomes greater. From this result, it has been considered that the compressive stress becomes larger with an increase in the thickness of the underlayer of the SiO2 film. We have tried to separate the observed Raman shifts into those caused by the crystallite size effect and those caused by stress in the poly-Si films. We did this using the resonant Raman scattering technique and calculation by the phonon confinement model. It has been found that the crystallite size obtained from the Raman measurement roughly agrees with the size obtained from the x-ray measurement. This result suggests that the phonon-dispersion curve does not change significantly under the compressive stress of 102MPa order.

Optical and microstructural investigations of porous silicon

Raman scattering and photoluminescence (PL) measurements on (100) oriented n-type crystalline silicon (c-Si) and porous silicon (PS) samples were carried out. PS samples were prepared by anodic etching of c-Si under the illumination of light for different etching times of 30, 60 and 90 min. Raman scattering from the optical phonon in PS showed the redshift of the phonon frequency, broadening and increased asymmetry of the Raman mode on increasing the etching time. Using the phonon confinement model, the average diameter of Si nanocrystallites has been estimated as 2.9, 2.6 and 2.3 nm for 30, 60 and 90 min samples, respectively. Similar size of Si crystallites has been confirmed from the high resolution transmission electron microscopy (HRTEM). Using 2TO phonon mode intensity, we conjectured that the disordered Si region around the pores present in 30 min PS dissolved on etching for 90 min. The photoluminescence (PL) from PS increased in intensity and blue shifted with etching time from 2.1–2.3 eV. Blue shifting of PL is consistent with quantum confinement of electron in Si nanocrystallites and their sizes are estimated as 2.4, 2.3 and 2.1 nm for 30, 60 and 90 min PS, respectively which are smaller than the Raman estimated sizes due to temperature effect. Unambiguous dominance of quantum confinement effect is reported in these PS samples.

Surface disorder in c-Si induced by swift heavy ions

The disorders induced in crystalline silicon (c-Si) through the process of electronic energy loss in the swift heavy ion irradiation were investigated. A number of silicon <1 0 0> samples were irradiated with 65 MeV oxygen ions at different fluences, 1×1013 to 1.5×1014 ions/cm2, and characterized by the Raman spectroscopy, the optical reflectivity, the X-ray reflectivity, the atomic force microscopy (AFM) and the X-ray diffraction (XRD) techniques. The intensity, redshift, phonon coherence length and asymmetric broadening associated with the Raman peaks reveal that stressed and disordered lattice zones are produced in the surface region of the irradiated silicon. The average crystallite size, obtained by analyzing Raman spectrum with the phonon confinement model, was very large in the virgin silicon but decreased to<100 nm dimension in the ion irradiated silicon. The results of the X-ray reflectivity, AFM and optical reflectivity of 200–700 nm radiation indicate that the roughness of the silicon surface has enhanced substantially after ion irradiation. The diffusion of oxygen in silicon surface during ion irradiation is evident from the oscillation in the X-ray reflectivity spectrum and the sharp decrease in the reflectivity of 200–400 nm radiation. The rise in temperature, estimated from the heat spike model, was high enough to melt the local silicon surface. The results of XRD indicate that lattice defects have been induced and a new plane <2 1 1> has been formed in the silicon <1 0 0>after ion irradiation. The results of the present study show that the energy deposited in crystalline silicon through the process of electronic energy loss ∼0.944 keV/nm per ion is sufficient to induce disorders of appreciable magnitude in the silicon surface even at a fluence of ∼1013 ions/cm2.

Size and phonon-confinement effects on low-frequency Raman mode of anatase TiO 2 nanocrystal

Raman spectra of anatase TiO 2 nanocrystals with 2.2–25.5 nm grain sizes were measured at temperature range 83–293 K. The size dependences of the frequency, the bandwidth and the lineshape of the low-frequency mode agree well with those calculated on the basis of the phonon confinement model with the theoretical phonon dispersion relationships. The correlations of both Raman frequency and bandwidth to the grain size L satisfy a L −1.3 law. The contributions of three-phonon anharmonic processes to the frequency and bandwidth at various grain sizes were obtained from the temperature dependence of the Raman spectra, and the results show that the phonon coupling is increased in anatase TiO 2 nanocrystal.

Transformation of nanodiamond into carbon onions: A comparative study by high-resolution transmission electron microscopy, electron energy-loss spectroscopy, x-ray diffraction, small-angle x-ray scattering, and ultraviolet Raman spectroscopy

The structural properties of both nanodiamond particles synthesized by detonation and the products of their transformation into carbon onions via vacuum annealing at 1000 and 1500°C have been studied using high-resolution transmission electron microscopy (HRTEM), electron energy-loss spectroscopy, x-ray diffraction (XRD), small-angle x-ray scattering (SAXS), and Raman spectroscopy. The advantages of UV Raman spectroscopy over visible Raman spectroscopy for the analysis of these carbon nanomaterials are demonstrated. It was found that the synthesized nanodiamond particles have a composite core-shell structure comprising an ordered diamond core covered by a disordered (amorphous) outer shell formed by the mixed sp2∕sp3 bonding of carbon atoms. The observed structure of the nanodiamond particles are comparable with the structure of the bucky diamond clusters comprising a diamond core and a reconstructed surface which stabilizes the cluster at the average diameter of ∼30Å, as predicted recently from theoretical studies. Assuming a spherical shape for the particles and employing a two-step boundary model of electron density distribution developed in this work to describe the SAXS patterns produced by the core-shell structure of the nanodiamond particles, it was evaluated that the average diameter of the core is ∼30Å and the average thickness of the shell is ∼8Å; values which are in agreement with results obtained from HRTEM and XRD measurements. A discrepancy between these results and average diamond crystallite size obtained from Raman spectra by applying the phonon confinement model (35–45Å) is discussed. It is hypothesized from analysis of broadening of the XRD diamond peaks that at the nanoscale under influence of the particle shape, which is not strictly of a cubic (or spherical) symmetry, a slight hexagonal distortion of the cubic diamond structure appears in the nanodiamond particles. The transformation of the nanodiamond into carbon onions proceeds from the amorphous outer shell of the particles inwards towards the particles’ diamond core. UV Raman spectroscopy effectively senses the initial stage of the transformation revealing a reconstruction of the mixed sp2∕sp3 bonding of carbon atoms located in the outer shell, into sp2-bonded carbon atoms similar to those in nanocrystalline graphite. It is shown that intershell distance in carbon onions formed from nanodiamonds depends on the temperature of the transformation and relates to the linear thermal expansion coefficient of the graphite structure along the stacking direction of the graphene layers (the c axis). In accordance with SAXS results, there is evidence for an increase of the average particle size of the synthesized nanodiamond [48(3)Å] after transformation into carbon onions [58(10)Å].

Raman scattering and field-emission properties of RuO2 nanorods

We report Raman scattering and field emission properties of rutile RuO2 nanorods obtained by metalorganic chemical vapor deposition. The RuO2 nanorods have lengths up to several micrometers and diameters in the range of 10–50 nm. The nanosize dependencies of the peak shift and the broadening of the three first-order Raman modes agree well with those calculated on the basis of the phonon confinement model. The unique geometrical features of RuO2 nanorods exhibit a strong effect on field enhancement (β∼1153), which results in a low threshold field (Eth∼4.9V∕μm) defined at the beginning of emission. The low turn-on field for driving a current of 10μA∕cm2 is about 10.3V∕μm, which is comparable with amorphous carbon film. Our results indicate that RuO2 nanorods provide an excellent alternative for field emitter due to several advantages, including nanometer structure, natural conductor, enhanced resistance to oxidation, and long-term stability.

Compressive and tensile stress in colloidal CdSe semiconductor quantum dots

Compressive and tensile stress in colloidal CdSe quantum dots (QDs) is examined using resonance Raman spectroscopy. We find that the dispersion of the longitudinal optical phonon mode with size does not follow theoretical calculations based on phonon confinement models. To account for these deviations, the presence of compressive or tensile stress in the QDs is proposed. The influence of surface reconstruction on QD behavior is evidenced by differences in hexadecylamine (HDA) and trioctylphospine oxide (TOP/TOPO) passivation. We find that CdSe QDs passivated by HDA exhibit compressive stress, while CdSe QDs passivated by TOP/TOPO exhibit tensile stress. Evidence is provided that the CdSe-HDA QD stress is directly to a passivant driven surface reconstruction effect.

Morphology and stress study of nanostructured porous silicon as a substrate for PbTe thin films growth by electrochemical process

Porous silicon layers (PSL) were produced by stain etching from a HF:HNO3 500:1 mixture with etching time varying in the range of 1 up to 10 min. The samples have presented nanometric porosity as a function of etching time, characteristic of heavily doped p type silicon. The residual stress and the correlation length of the layers were obtained through the analysis of the micro-Raman spectra using a phonon confinement model including a term to account for the amorphous phase. The residual compressive stress tends to increase as expected due to the contribution of smaller crystallites to be more representative as the etching time increases. PbTe thin films were electrodeposited on PSL from aqueous alkaline solutions of Pb(CH3COO)2, disodium salt of ethylendiaminetetraacetic acid (EDTA) and TeO2 by galvanostatic and potentiostatic method. It was also obtained nanostructured PbTe thin films with polycrystalline morphology evidenced by X-ray Diffraction (XRD) spectra. Scanning Electron Microscopy (SEM) analysis has demonstrated good films reproducibility with an average grain size of 100 nm.

Raman scattering and photoluminescence studies on O + implanted porous silicon

The effects of 125 keV O + implantation on porous silicon (PS) in the fluence range from 10 14 to 10 16 cm −2 have been investigated using Raman scattering and photoluminescence techniques. PS has been prepared by anodically etching (100) cut n-type crystalline silicon (c-Si). Raman spectra of PS have been generated using a model of phonon confinement in Si nanocrystallites and compared with experiment. As the fluence increases, the Si nanocrystallite size estimated from the Raman spectra decreases. Unimplanted PS shows a visible photoluminescence (PL) peak at 2.02 eV, with 0.3 eV FWHM coming from electron confinement in Si nanocrystallites. Its intensity decreases due to the creation of non-radiative recombination centers and the peak position increases due to nanocrystallites size reduction with increase of fluence. On O + implantation, PS shows an additional visible PL at 2.35 eV with 0.3 eV FWHM due to amorphous silicon (a-Si) nanozones created by O + single ion impact. Evidence for this PL peak is provided from O + implanted c-Si. 2.35 eV PL intensity in O + implanted PS is 10 times higher than that in O + implanted c-Si.

Formation of silicon nanocrystals in SiO2 by oxireduction reaction induced by impurity implantation and annealing

Silicon nanocrystals are candidates for complete integration of optical and electronic functions in Si-based devices. In this work, we investigate the feasibility of producing them by a method based on impurity-induced oxireduction of SiO2. Thermodynamic calculations suggested Mg as a suitable impurity due to its chemical strength in oxireducing the SiO2 matrix and simultaneously avoiding the formation of Si-based compounds. The samples were obtained by 1×1017 Mg+/cm2 implantations into fused silica followed by thermal anneal in vacuum. Transmission electron microscopy (TEM), and Raman and photoluminescence measurements confirmed the presence of Si nanocrystals. The average nanocrystal size was evaluated according to the phonon confinement and quantum confinement models relative to the Raman and photoluminescence results, respectively, to be about 10 nm, in agreement with the TEM results.

Raman spectrum of array-ordered crystalline silicon nanowires

Array-ordered single-crystal silicon nanowires were fabricated by the nanochannel-aluminal and CVD method. The average length and diameter of the nanowires is about 10 μm and 60 nm, respectively. A study of the Raman spectrum of the nanowires shows that the Raman shift to low frequency is due to the quantum confinement effect, which is discussed by using the phonon confinement model. Also we determine the peaks of the Raman spectrum to be corresponding to that of crystal silicon (c-Si).

Raman spectrum of array-ordered crystalline silicon nanowires

Array-ordered single-crystal silicon nanowires were fabricated by the nanochannel-aluminal and CVD method. The average length and diameter of the nanowires is about 10 μm and 60 nm, respectively. A study of the Raman spectrum of the nanowires shows that the Raman shift to low frequency is due to the quantum confinement effect, which is discussed by using the phonon confinement model. Also we determine the peaks of the Raman spectrum to be corresponding to that of crystal silicon (c-Si).

Investigation of nanostructured porous silicon by Raman spectroscopy and atomic force microscopy

A set of porous silicon layers was produced by stain etching using a solution of HF:HNO 3 in a composition ratio of 500:1 with etching time varying from 1 to 10 min. The residual stress and the correlation length of the layers were obtained through the analysis of the micro-Raman spectra using a phonon confinement model including a term to account for the amorphous phase. The correlation length, which corresponds to the crystallite size, is found to decrease as the etching time increases, and agrees with the values obtained by atomic force microscopy analysis. The residual compressive stress tends to increase with etching time, as expected by the smaller crystallite size. However, an oscillatory behavior is detected for etching times higher than 4 min. This result correlates well with the photoluminescence measurements and can be understood by the porous silicon layer formation observed with the atomic force microscopy.

Simultaneous micro-Raman and photoluminescence study of spark-processed germanium: Report on the origin of the orange photoluminescence emission band

Microstructure and origin of the orange photoluminescence (PL) band of visible luminescing spark-processed germanium (sp-Ge) has been studied using simultaneous micro-Raman and -PL spectroscopy and scanning electron microscope. Instability of the sp-Ge film to energetic electron beams (⩾5 keV) during SEM measurements suggested that sp-Ge is not composed exclusively of purely Ge–Ge bonded material (elemental Ge) but also of other form(s) of material(s) of different chemistry. Indeed, micro-Raman spectra of the films showed that sp-Ge is composed of mainly substoichiometric germanium oxides (GeOxs) and elemental Ge (in the form of nano- or micro-crystals). Further, it is proposed that the Ge particles were embedded in thick GeOx (0&amp;lt;x&amp;lt;2) layers. Sizes of the Ge nanocrystals were estimated using a phonon confinement model and it was found that the spark processing method can generate Ge crystals with sizes as small as 6–8 nm. However, the micro-PL of sp-Ge indicated unambiguously that the orange PL band (near 1.9 eV) does not originate from or involve these Ge nanocrystals but the Ge–O bonded material, i.e., GeOxs.