Phonon Confinement Model Research Articles

InP nanocrystals via surfactant-aided hydrothermal synthesis

In the presence of surfactant, potassium stearate, quantum-confined InP nanocrystals (NCs) were hydrothermally synthesized in aqueous ammonia. Powder x-ray diffraction (XRD) patterns give the zinc blende phase of InP with lattice constant a=5.8377±8×10−4 Å. Transmission electron microscopy (TEM) micrographs show that the as-prepared InP NCs are spherical secondary particles (120 nm) consisting of spherical nanocrystals and rod-like nanocrystals grown in the direction perpendicular to the [111] direction, which is different from those grown by solution–liquid–solid process. X-ray photoelectron spectra indicate that the nanocrystals have a stoichiometric ratio of In:P=1.2:1 and their surfaces are capped with stearate ions. Powder XRD, TEM images, Raman spectra, optical absorption and photoluminescence spectra of InP NCs grown with surfactant were compared with those of the InP NCs grown without surfactant, and it indicated that the as-prepared InP NCs via surfactant-aided synthesis are quantum confined: their transverse optical and longitudinal optical vibration modes exhibit frequency redshifting and asymmetric broadening which is consistent with the phonon confinement model of nanoparticles. Compared with bulk values, InP NCs exhibit a band-edge emission band centered at 1.81 eV (685 nm) with a blueshift of 0.44 eV at 300 K. A wide distribution in size and the two-morphology nature of the NCs resulted in featureless absorption spectra and broadening of the emission band. The formation mechanism of the InP NCs is discussed and attributed to an in situ decomposition process of an indium polyphosphide amorphous precursor.

Phonon confinement in oxide-coated silicon nanowires

Raman spectroscopy of micron-long crystalline Si nanowires covered with a thick SiO2 layer showed a downshift and asymmetric broadening of the Raman first order TO phonon peak when compared with the bulk (q=0) mode. The Raman shift and broadening were attributed to phonon confinement in the nanowires. A phenomenological phonon confinement model, incorporating the effects of nanowire diameter distribution, is presented. This model is shown to accurately describe the broadening of the Raman peak and is consistent with the microstructure of Si nanowires. In addition to the work a distribution of the phonon wave vector was directly taken into consideration replacing the diameter distribution to fit the Raman TO peak. The effects of the nano-Si:SiO2 boundary on the Raman spectra are discussed in terms of softening of the phonon confinement.

Raman and photoluminescence properties of Ge nanocrystals in silicon oxide matrix

Ge nanocrystals (nc-Ge) embedded in silicon oxide thin films have been fabricated using rf magnetron co-sputtering deposition and post-growth thermal annealing method. Raman scattering and photoluminscence (PL) measurements have been used to characterize their crystallization and light emission properties. The Ge crystallinity and nanocrystal size obtained using a three-peak fitting method based on a phonon confinement model have been found to increase with the increase of annealing temperature. The observed blue photoluminescence band located at ∼3.1 eV and a weaker band at ∼2.4 eV under the excitation of 325 nm laser did not show significant size dependence while their relative intensities were related to the addition of H 2 or O 2 into the sputtering ambient. The photoluminescence mechanism is discussed.

Ion-channeling and Raman scattering study of damage accumulation in silicon

Damage was introduced into Si(100) using 245 keV Si+ ions implanted to a wide range of doses with implant temperatures of −195, 25 or 100 °C. The accumulation of this damage was monitored with Rutherford backscattering and ion channeling (RBS-C) and by following the intensity and lineshape variation of the first-order (1-O) Raman peak of silicon. For all implant temperatures the RBS-C data showed the expected trend with dose. For −195 °C and room temperature implants, the decrease in intensity of the 1-O Raman peak shows a similar trend to the RBS-C data, but in each case the threshold dose is about a decade lower than its RBS-C counterpart. On implantation at 100 °C the sensitivity of the Raman spectra to low damage concentrations is more dramatic and decreases continuously over the full dose range, from 5×1012 to 2×1016 Si/cm2, examined in this study. This suggests that the intensity of the 1-O Raman peak is particularly sensitive to the types of defect structures that are stable in silicon during irradiation at elevated temperatures. The phonon confinement model is discussed in light of these results.

Surface-state origin for the blueshifted emission in anodically etched porous silicon carbide

Anodic etching of SiC yields a highly monodisperse distribution of nanometer dimension porous structures which extend to a significant depth. Cathodoluminescence (CL) studies of the porous layers yield luminescence peaks in the UV region, above the band gap energy of bulk SiC. Higher etching current densities produce porous silicon carbide (PSiC) with peak CL emission wavelengths deeper in the ultraviolet. Photoluminescence (PL) is also blueshifted in anodically etched PSiC, although not to the extent of the CL emission, suggesting that different emissive states are accessed in CL and PL. Raman investigations of the polar A1 LO mode, which couples strongly to the macroscopic electric field accompanying the LO phonon, were conducted in an attempt to discern whether quantum confinement effects could effectively explain the blueshifted emission. The principal feature of the Raman spectra was a significant low-frequency shoulder on the A1 LO mode, the magnitude of which correlates with the magnitude of the blueshift in PL and the intensity of the blueshifted CL emission. The shoulder was fit quantitatively with a model incorporating the effects of extraordinary LO modes and longitudinal and transverse Fröhlich modes. The Fröhlich mode widths derived from the fit are too wide to be due solely to Fröhlich modes and likely indicate the combined effects of diffuse scattering, broadening of spectral lines, and violation of the symmetry selection rules. The preponderance of the evidence, especially the inability to fit the low-frequency shoulder in the Raman spectra with a phonon confinement model, support an interpretation in which defect structures or surface states are responsible for the UV emission.

Characterization of Ge nanocrystals embedded in SiO2 by Raman spectroscopy

Ge nanocrystals formed in a SiO2 matrix by ion implantation were studied by Raman spectroscopy. It is shown that Raman analysis based on the phonon confinement model yields a successful explanation of the peculiar characteristics resulting from the nanocrystals. A broadening and a shift in the Raman peak are expected to result from the reduced size of the crystals. Asymmetry in the peak is attributed to the variations in the size of the nanocrystals. These effects were observed experimentally for the Ge nanocrystals prepared by ion implantation and explained theoretically by incorporating the effect of size and size distribution into the theoretical description of the Raman shift. A comparison with the transmission electron microscopy images indicated that this analysis could be used to estimate the structural properties of nanocrystals embedded in a host matrix. The evolution of nanocrystal formation with annealing temperature, i.e. the size growth, was monitored by Raman spectrometry for several samples and the corresponding nanocrystal sizes were estimated using the phonon confinement model.

Synthesis of nanocrystalline Si particles from a solid-state reaction during a ball-milling process

Silicon particles with nanometric size were synthesized by mechanical milling using a high-energy ball miller. The Si nanoparticles, observed using Raman spectroscopy, were formed according to a solid-phase reaction between amorphous SiO 2 and Al powders. The crystalline size was determined by fitting the Raman spectrum with a phonon confinement model.

Raman line-shape analysis of nano-structural evolution in cation-ordered ZrTiO 4-based dielectrics

It is known that the microwave dielectric characteristics of ZrTiO 4 are greatly influenced by processing conditions such as the degree of Sn-modification and the cooling rate after sintering. The effects of these processing variables on the nano-structural characteristics of ZrTiO 4-based dielectrics were studied by analyzing the Raman line-shape parameters using the phonon-confinement model. It was shown that the nano-structural shape of the cation-ordered domains underwent a sequential change from thin-slab form to platelet shape, and finally to spherical shape with increasing Sn-content or cooling rate.

Polarization leakage and asymmetric Raman line broadening in microwave dielectric ZrTiO 4

Nano-scale structural characteristics and broken symmetry associated with the formation of incommensurately ordered phase from the high-temperature disordered phase of ZrTiO 4 were studied using Raman scattering method. For this purpose, the Raman peak at 815 cm −1 caused by the A g -symmetry normal mode was employed as a nano-structural probe. A polarization leakage was observed in the polarized Raman spectra of single-crystal ZrTiO 4. The observed leakage was attributed to the random orientation of the short-range ordered domains in the beginning stage of the normal-to-incommensurate transformation. The computational result based on the phonon-confinement model indicated that the asymmetric Raman line broadening in the incommensurately ordered ZrTiO 4 single-crystal was directly related to the existence of the faulted boundary having the average periodicity of 3.9 nm along the a-axis.

Phenomenological description of phonon confinement in semiconductor nanocrystals

The average function (average of sine and Gaussian functions) is used to investigate the vibrational properties of spherical semiconductor nanocrystals within the framework of phenomenological phonon confinement model [Solid State Commun. 39 (1981) 625]. The results for the phonon confinement and Raman intensity peaks are quite in agreement with the microscopic calculations. It is proposed here that the average function should be used as a phonon weighting function for the spherical nanocrystals, if the phonon confinement model [Solid State Commun. 39 (1981) 625] is being used to investigate their vibrational properties.

Raman spectroscopic studies on bismuth nanoparticles prepared by laser ablation technique

Bi nanoparticles are prepared by means of laser ablation in Ar atmosphere (0.2–10 Torr) with KrF (248 nm) excimer laser of the power 200 mJ. The size of the Bi particles estimated by TEM measurements is in the range 3–10 nm. Raman active E g mode shifts to a higher frequency and becomes broader for a sample prepared in a lower pressure of Ar atmosphere. However, the peak frequency and the bandwidth of A 1g mode show almost no change with the change of the particle size. These experimental results can be well explained by a phonon confinement model of Campbell and Fauchet by taking the phonon dispersion properties that the E g mode of the crystal has a large dependence on the wave numbers near the Γ point, but the A 1g mode is rather independent of the phonon wave numbers.

In-plane control of morphology and tunable photoluminescence in porous silicon produced by metal-assisted electroless chemical etching

Photoluminescent porous silicon (PSi) was produced by Pt-assisted electroless etching of p−-Si (100) in a 1:2:1 solution of HF, H2O2, and methanol. The peak emission wavelength of the PSi could be tuned in the range 500 nm⩽λ⩽600 nm simply by changing the time of etching. The luminescence is sufficiently intense at all wavelengths to be visible by eye. Furthermore, by patterning the metal areas on the surface prior to etching, the luminescence can be controlled spatially. To investigate the relationship among processing variables — principally etch time and spatial proximity to Pt — and morphology, scanning electron microscopy (SEM), true color fluorescence microscopy, and spatially resolved phonon line shape studies were undertaken. SEM images show nanocrystalline features in the region where the luminescence originates, a region which shifts spatially as a function of etch time, as indicated by fluorescence microscopy. Raman scattering measurements of the shift and broadening of the longitudinal optical phonon band interpreted in the context of the phonon confinement model were used to estimate crystallite sizes. As with the luminescence, the crystallite sizes were found to vary as a function of distance from the Pt patterned area and etch time. These results are interpreted in light of an etching mechanism in which H2O2 reduction results in hole injection deep into the valence band, which then drifts spatially and plays a critical role in determining the rate at which Si is removed from the surface.

Lattice-mismatch induced-stress in porous silicon films

We have studied the stress in porous silicon films with different porosities at the interface with the substrate. Micro-Raman spectra were measured along a cleaved cross-section to sample different layer depths. Each spectrum was fit to the phonon confinement model, with the bulk phonon frequency as a free parameter to remove phonon confinement effects. At the interface this frequency increases sharply, indicating a compressive stress on the porous silicon pillars. The stress is due to the lattice mismatch, measured by X-ray diffraction, between the porous film and the bulk silicon substrate. For porosities between 50 and 85% the stress and the lattice mismatch vary, respectively, between 4 and 10 kbar, and between 2.9×10 −3 and 3.5×10 −3. Finally, from the dependence of stress on the lattice mismatch we obtain a microscopic Young's modulus of 156 GPa. This magnitude, mostly dependent on atomic bonding, is close to the bulk silicon value and is much larger than the macroscopic modulus, strongly dependent on the porous structure, reported in the literature.

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.

Infrared and Raman-scattering studies in single-crystalline GaN nanowires

Infrared and Raman-scattering studies of high-purity and -quality GaN nanowires are presented. The nanosize dependences of the peak shift and the broadening of the four first-order Raman modes agree with those calculated on the basis of the phonon confinement model. Additionally, the appearance of one Raman mode at ∼ 254 cm −1 is attributed to zone-boundary phonon activated by surface disorders and finite-size effects. Moreover, the Raman-scattering intensities of certain phonons show a different resonantly enhanced behavior, which can be used to verify the information on the electronic structures and the electron–phonon interaction in GaN nanowires.

First- and second-order Raman scattering in nanocrystalline silicon

Confinement effects on first-order and second-order Raman scattering in nanocrystalline silicon are presented. The average dimension of the nanocrystals has been determined by an analysis of the first-order Raman spectra using the phenomenological phonon confinement model. The second-order acoustic bands 2LA and 2TA do not seem to be influenced by confinement effects and are similar to those in the bulk. The second-order optic bands are broadened and shifted in comparison to those in the bulk. The ratio of the integrated intensities $I{(2\mathrm{T}\mathrm{O})}_{\mathrm{int}}/I{(\mathrm{TO})}_{\mathrm{int}}$ depends upon the nanocrystal size and varies between the crystalline and amorphous limits.

Preparation and optical properties of composite thin films with embedded InP nanoparticles

InP nanoparticles embedded in SiO2 thin films were prepared by radio-frequency magnetron co-sputtering. We analyzed the structure and growth behavior of the composite films under different preparation conditions. X-ray diffraction and Raman spectroscopy analyses indicate that InP nanoparticles have a polycrystalline structure. The average size of InP nanoparticles is in the range of 3–10 nm. The broadening and red shift of the Raman peaks were observed, which can be interpreted by the phonon confinement model. Optical transmission spectra indicate that the optical absorption edges of the films can be modulated in the visible light range. The marked blue shift of the absorption edge with respect to that of bulk InP is explained by the quantum confinement effect. The theoretical values of the blue shift predicted by the effective mass approximation model are different from the experimental results for the InP-SiO2 system. Analyses indicate that the exciton effective mass of the InP nanoparticles is not constant and is inverse relative to the particles radius, which may be the main reason that results in the discrepancy between the theoretical and the experimental result. We discussed the possible transition of the direct band gap to the indirect band gap for InP nanoparticles embedded in SiO2 thin films.

Temperature-dependent Raman scattering studies in nanocrystalline silicon and finite-size effects

A comparative study of the temperature-dependent Raman scattering of nanocrystalline and bulk silicon is presented. The nanocrystalline silicon samples were made by a cw laser annealing process, and the characteristic dimensions were determined with a phenomenological phonon confinement model. Experimental results indicate a higher degree of anharmonicity in nanocrystals compared to that in the bulk. The anharmonic constants are found to be highly size dependent and increase greatly with decreasing dimensions. The phonon lifetimes have two contributions, one temperature dependent and the other temperature independent, both decreasing rapidly with decreasing nanocrystal size. The temperature-dependent term ${\ensuremath{\tau}}_{0}$ is important for larger nanocrystals, while the temperature-independent term ${\ensuremath{\tau}}_{1}$ becomes dominant for nanocrystals of sizes less than 4 nm.

UV Raman characteristics of nanocrystalline diamond films with different grain size

Abstract Nanocrystalline diamond films with different size were characterized by ultraviolet (UV) (244 nm) Raman spectroscopy. It was found that a diamond peak at 1333 cm −1 was enhanced, while the D and G peak of graphite as well as photoluminescence was suppressed, compared with that measured by visible (514.5 nm) Raman. With decreasing the particle size from 120 to 28 nm, the diamond peak shifts from 1332.8 to 1329.6 cm −1 , the line width of the peak becomes broader, the intensity ratio of diamond and G peak decreases. The down shift and broadening of the diamond peak depending on the particle size by UV Raman measurements are consistent with the phonon confinement model.

Formation and characterization of germanium nanoparticles

Elemental germanium was mechanically milled with magnesium oxide with the intention of forming disperse nanoparticulate germanium in a soluble matrix. The crystallite size was determined by x-ray diffraction (XRD) and Raman spectroscopy using a phonon confinement model. The crystallite size was found to decrease exponentially with milling time; however, the size determined by XRD was typically five to ten times greater than that by Raman. This was attributed to the presence of two separate crystallite sizes, which were averaged when using the Scherrer equation for the XRD data. Sonication of the powder resulted in the breakup of >20 μm aggregates into individual particles of approximately 40 nm. These particles are thought to compose a single crystal core with a crystallite size of approximately 28 nm surrounded by a layer of smaller crystallites (approximately 5 nm), which showed quantization during Raman spectroscopy. Separation of the germanium from the magnesium oxide was readily achieved using a simple acid leach, although some oxidation of germanium was evident when using an aqueous leach.