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.     

Size-dependent phonon transmission across dissimilar material interfaces

In this paper, we study the size effects on the phonon transmission across material interfaces using the atomistic Green's function method. Layered Si and Ge or Ge-like structures are modeled with a variety of confined sizes in both transverse and longitudinal directions. The dynamical equation of the lattice vibration (phonon waves) is solved using the Green's function method and the phonon transmission is calculated through the obtained Green's function. Phonon transmission across a single interface of semi-infinite Si and Ge materials is studied first for the validation of the methodology. We show that phonon transmission across an interface can be tuned by changing the mass ratio of the two materials. Multi-layered superlattice-like structures with longitudinal size confinement are then studied. Frequency-dependent phonon transmission as a function of both the number of periods and the period thickness is reported. A converged phonon transmission after ten periods is observed due to the formation of phonon minibands. Frequency-dependent phonon transmission with transverse size confinement is also studied for the interface of Si and Ge nanowire-like structures. The phonon confinement induces new dips and peaks of phonon transmission when compared with the results for a bulk interface. With increasing size in the transverse direction, the phonon transmission approaches that of a bulk Si/Ge interface.     

Characterization of chemically synthesized CdS nanoparticles

II-VI semiconductor nanoparticles are presently of great interest for their practical applications such as zero-dimensional quantum confined materials and for their applications in optoelectronics and photonics. The optical properties get modified dramatically due to the confinement of charge carriers within the nanoparticles. Similar to the effects of charge carriers on optical properties, confinement of optical and acoustic phonons leads to interesting changes in the phonon spectra. In the present work, we have synthesized nanoparticles of CdS using chemical precipitation technique. The crystal structure and grain size of the particles are studied using XRD. The UV-visible absorption, photoluminescence and Raman spectra of the sample are recorded and discussed briefly     

Role of interactions in the far-infrared spectrum of a lateral quantum-dot molecule

We study the effects of electron-electron correlations and confinement potential on the far-infrared spectrum of a lateral two-electron quantum-dot molecule by exact diagonalization. The calculated spectra directly reflect the lowered symmetry of the external confinement potential. Surprisingly, we find interactions to drive the spectrum towards that of a high-symmetry parabolic quantum-dot. We conclude that far-infrared spectroscopy is suitable for probing effective confinement of the electrons in a quantum-dot system, even if interaction effects cannot be resolved in a direct fashion.     

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.     

Longitudinal optical phonon-plasmon coupling in luminescent 3C-SiC nanocrystal films

Glycerol-passivated 3C-SiC nanocrystal (NC) solid films with tunable blue photoluminescence show abnormal longitudinal optical (LO) phonon behavior. As the NC size increases, the LO phonon intensity increases in the Raman spectra of the solid films and is even larger than that of the transverse optical mode. The Raman spectra cannot be fitted by using only the phonon confinement model. When further considering the coupling between the LO phonon and plasmon induced by the surface deformation potential in the glycerol layer, good agreement is achieved between the experiments and theory. This indicates that the coupled LO phonon-plasmon effect arising from the surface bonding structure plays a crucial role in the modified LO phonon behavior.     

Phonon-induced polariton superlattices

We show that the coherent interaction between microcavity polaritons and externally stimulated acoustic phonons forms a tunable polariton superlattice with a folded energy dispersion determined by the phonon population and wavelength. Under high phonon concentration, the strong confinement of the optical and excitonic polariton components in the phonon potential creates weakly coupled polariton wires with a virtually flat energy dispersion.     

Photoluminescence of tetrahedral quantum-dot quantum wells

Taking into account the tetrahedral shape of a quantum-dot quantum well (QDQW) when describing excitonic states, phonon modes, and the exciton-phonon interaction in the structure, we obtain within a nonadiabatic approach a quantitative interpretation of the photoluminescence spectrum of a single CdS/HgS/CdS QDQW. We find that the exciton ground state in a tetrahedral QDQW is bright, in contrast to the dark ground state for a spherical QDQW. The position of the phonon peaks in the photoluminescence spectrum is attributed to interface optical phonons. We also show that the experimental value of the Huang-Rhys parameter can be obtained only within the nonadiabatic theory of phonon-assisted transitions.     

Size‐dependent evolution of phonon confinement in colloidal Si nanoparticles

A size‐dependent evolution of phonon confinement is revealed in Si nanoparticles (NPs) via Raman spectroscopy. By introducing a variable confinement factor, α, into a well‐known phenomenological phonon confinement model (PCM) developed by Richter et al., acceptable fits are achieved to downshifted and asymmetrically broadened Raman spectra of Si NPs with different diameters, d, from 2.4 nm to 6.3 nm. A comparative study using Raman spectra of colloidal Si NPs, for the first time, shows an apparent positive linear correlation between α and the Si NP size. Based on the PCM, the amplitude of the atomic vibration (phonon) at the real physical boundary of NPs is proportional to e^−α/2, which indicates that the amplitude of the first order optical phonon is relatively larger at the edges for smaller Si nanostructures despite of their stronger phonon confinement weighed by α/d². Copyright © 2015 John Wiley & Sons, Ltd.     

Optical properties of a hydrogenic impurity in a confined Zn1−xCdxSe/ZnSe spherical quantum dot

The effect of longitudinal optical phonon field on the ground state and low lying-excited state energies of a hydrogenic impurity in a Zn_1−xCd_xSe/ZnSe strained quantum dot is investigated for various Cd content using the Aldrich-Bajaj effective potential. We consider the strain effect considering the internal electric field induced by the spontaneous and piezoelectric polarizations. Calculations have been performed using Bessel function as an orthonormal basis for different confinement potentials of barrier height. Polaron induced photoionization cross section of the hydrogenic impurity in the quantum dot is investigated. We study the oscillator strengths, the linear and third-order nonlinear optical absorption coefficients as a function of incident photon energy for 1s-1p and 1p-1d transitions with and without the polaronic effect. It is observed that the potential taking into account the effects of phonon makes the binding energies more than the obtained results using a Coulomb potential screened by a static dielectric constant and the optical properties of hydrogenic impurity in a quantum dot are strongly affected by the confining potential and the radii. It is also observed that the magnitude of the absorption coefficients increases for the transitions between higher levels with the inclusion of phonon effect.     

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.     

Supersonic cluster beam deposition of nanostructured titania

Nanostructured titanium dioxide films have been deposited by supersonic cluster beam deposition (CBD). Nanoparticles are produced by a pulsed microplasma cluster source (PMCS) and selected by aerodynamic separation effects. The as-deposited film is a complex mixture where amorphous material coexists, at the nanoscale, with anatase and rutile crystal phases. The nanocrystalline fraction of the film is characterized by crystal size ranging from 100 nm to less than 5 nm. We have characterized the film structure by transmission electron microscopy, Raman spectromicroscopy, X-ray diffraction, and UV-visible spectroscopy showing that correlations exist between cluster size and film properties. In particular if very small clusters are deposited, the film shows a predominant rutile phase whereas larger clusters form films with mainly anatase structure. Our observations suggest that phonon confinement effects are responsible for a significant shift and broadening observed for the Raman peaks. In addition, optical gap tuning is provided by mass selection: large clusters assembling generates a film with 3.22 eV optical gap, while smallest clusters 3.52 eV.     

The role of photodarkening and Auger recombination in the dynamics of the optical response for Cd(S,Se) nanoparticles

We first recall the main linear optical properties of semiconductor-doped glasses in the strong confinement regime. We then discuss the origin of the photodarkening effect and its consequences on the luminescence and absorption spectra. Photodarkening and, at high excitation intensity, Auger processes strongly modify carrier recombination as studied by time-resolved luminescence measurements. This is further supported by using nonlinear optical techniques such as optical phase conjugation. We finally recall the origin of the nonlinear response of such media in the resonant regime and the size dependence of the corresponding figure of merit.     

Bi_2O_3-Li_2O玻璃的热致变色研究

发现氧化和氧化锂所形成的玻璃有着明显的热致变色现象 .温度系数随氧化含量的增加而上升 ,反映了热致变色现象主要来源于氧化 .这种热致变色现象与半导体同样 ,来源于玻璃中的电子 声子相互作用而产生的随温度变化的光学能隙 .氧化重金属氧化物玻璃中高电子密度和低声子能量的化学键是产生强电子 声子相互作用的主要原因     

Radiative properties of lanthanide and transition metal ions in nanocrystals

We examine the physical factors that influence the radiative transitions of small-radius optical centers in ellipsoidal nanoparticles. The main objective of this work is to reveal to the extent to which changes in the phonon subsystem and electron-phonon interaction caused by spatial confinement effects affect the radiative characteristics of optical centers in nanocrystals.     

Raman spectra of structures with CdTe-, ZnTe-, and CdSe-based quantum dots and their relation to the fabrication technology

Quantum-dot structures based on the CdTe, ZnTe, and CdSe semiconductors are prepared by molecular-beam epitaxy, colloid chemistry methods, and ball milling, and their Raman spectra are studied. Localized longitudinal phonons are observed in all spectra. The dependence of the localized phonon frequency on the thickness of the ZnTe barrier in CdTe/ZnTe quantum-dot superlattices is used to derive the dispersion relation for longitudinal phonons in ZnTe. The Raman spectra of ensembles of colloidal quantum dots differ from the spectra of the other objects by the absence of tellurium bands and a strong intensity of the longitudinal phonon band of CdTe. It is revealed that the spectra depend on the technology employed to prepare quantum-dot structures.     

Influence of spin–orbit interactions on the polaron properties in wurtzite semiconductor quantum well

We have study the simultaneous effect of Rashba and Dresselhaus spin–orbit interactions on the polaron properties in wurtzite semiconductor quantum wells. The linear and cubic contributions of the bulk Dresselhaus spin–orbit coupling and the effects of phonon confinement on electron–optical-phonon interaction Hamiltonians are taken into account. We have found analytical solutions for the polaron energies as well as polaron effective mass within the range of validity of perturbation theory. It is shown that the polaron energy and effective mass correction are both significantly enhanced by the spin–orbit coupling. Wave number dependent phonon contribution on the electron energy has minima and varies differently of the spin-up and spin-down states. Polaron self-energy due to interface optical phonon modes has larger values than of the confined optical phonon modes ones. The polaron effective mass exhibits anisotropy and the contribution of the Dresselhaus spin–orbit coupling term on the polaron effective mass is dominated by Rashba one.     

Phonon confinement effects in Raman spectra of porous silicon at non‐resonant excitation condition

Light emitting porous silicon samples with different porosities, i.e. crystalline sizes, were produced from the low level doped p‐type silicon wafers by the anodization process. The effects of strong phonon confinement, redshift and broadening, were found on the O(Γ) phonon mode of the Raman spectra recorded at non‐resonant excitation condition using a near infrared 1064 nm laser excitation wavelength. Similarly, the blueshift of the photoluminescence peak was observed by reducing the crystalline sizes. Vibrational and optical findings were analysed within the existing models of confinement on the vibrational and electronic states of silicon nanocrystals. Since the energy of the photoluminescence peak of small nanocrystals also depends on the oxygen content on the surface of nanocrystals, the surface oxidation states were examined using infrared and energy dispersive spectroscopy. The partial coverage of the surface of nanocrystals was found due to the sample exposure to air. As a consequence, the photoluminescence energy did not increase as would be expected from the quantum confinement model. These results further indicate that the oxygen passivation along with the quantum confinement determines the electronic states of the silicon nanocrystals in porous silicon. Copyright © 2014 John Wiley & Sons, Ltd.     

抛物量子线中弱耦合极化子的有效质量和光学声子平均数

讨论电子与体纵光学(LO)声子弱耦合时对抛物量子线中极化子性质的影响.采用Tokuda改进的线性组合算符法、Lagrange乘子和变分法,导出了抛物量子线中弱耦合极化子的有效质量和光学声子平均数随拉格朗日乘子变化的规律及极化子振动频率随量子线约束强度的变化规律.并以ZnS量子线为例进行了数值计算,结果表明:抛物量子线中弱耦合极化子的有效质量m^*和光学声子平均数N随着拉格朗日乘子u的增加而增大;该结论与体材料中结论基本一致,但量子线中的效应比体材料更明显,表明量子线对电子约束的增强,使极化子效应更明显.同时,极化子振动频率λ随约束强度ω₀的增强而增大.