Spherical Semiconductor Research Articles

Charge Polarization and Current Distribution in a Conductive Particle in the Rayleigh Region

An investigation of the interaction of a conductive sphere with an electromagnetic wave with attention given to space-charge effects would appear timely, as there is much current interest in the electromagnetic properties of mesoscopic structures. In this study, the polarization of a small semiconductor sphere immersed in a dynamic electric field is explored analytically and numerically. In one approach, suitable for a sphere with low to moderate charge carrier concentration, the Poisson's equation is coupled with the transport equations of the carriers, leading to a quasi-static formulation under the weak-field approximation. Screening effects of the charges on the interior field are revealed, along with a current distribution that is essentially uniform over much of the volume of the sphere. Frequency dependence of the total induced dipole moment of the sphere displays strong dispersion and absorption near the bulk plasma frequency. Validity of the quasi-static assumption is assessed by comparison to results of calculations based on a full-wave formulation. As the nominal carrier concentration exceeds 1020 cm-3, the quasi-static solutions for interior field and current distribution begin to deviate from the full-wave solution and the latter must be employed to provide a realistic account of the charge-wave interaction within the sphere.

Symbiosis Between Photoactive Nanoparticles and their Organic Ligands

Photoactive spherical metal and semiconductor nanoparticles (NPs) are smart systems that exhibit unique properties, such as a high surface-to-volume ratio, a broad absorption spectrum and size-dependent properties. They are capped with a considerable number of ligands required to give rise to stable organic and aqueous NP colloidal solutions. In addition, the ligands can also be used to introduce functionality at the NP periphery. In this case, the NP could act as a 3D-scaffold, which would make a high local concentration of a functional moiety at the NP periphery possible, moreover, the photophysical properties of the NP could be tuned. The combined action of the organic capping and the inorganic core can exert an encapsulating effect, i.e. the organic capping could establish specific interactions with nearby molecules and this would enable the molecules to approach or interact with the NP surface. Therefore, the NP core and the ligand can work together providing the overall hybrid system with new properties or capacities. The relevance of the cooperative action between the spherical photoactive core and the capping are shown in this report with several recent examples developed by my research group, some of them in collaboration with other groups.

THz波段亚波长半导体球形阵列光学特性的数值模拟

THz波段亚波长半导体球形阵列光学特性的数值模拟

Lamb shift of non-degenerate energy level systems placed between two infinite parallel conducting plates

In this paper, the Lamb shift in systems with non-degenerate energy levels, put in the electromagnetic environment provided by two infinite parallel conducting plates, is analyzed. An explicit formula giving the relative Lamb shift (as compared to the standard one in a vacuum) is derived for spherical semiconductor quantum dots (QDs), via a careful mathematical treatment of divergences in the calculations using the theory of distributions. This result settles a controversy between two different formulas existing in the current literature. Its sensitive dependence on the plates separation may be viewed as an indirect manifestation of the Lamb shift and may be used for the fine tuning of the QD non-degenerate energy spectrum in some experimental contexts.

Ultralow-Threshold Two-Photon Pumped Amplified Spontaneous Emission and Lasing from Seeded CdSe/CdS Nanorod Heterostructures

Ultralow-threshold two-photon pumped amplified spontaneous emission (2ASE) and lasing in seeded CdSe/CdS nanodot/nanorod heterostructures is demonstrated for the first time. Such heterostructures allow the independent tunability of the two-photon absorption (2PA) cross-section (σ(2)) through varying the CdS rod size, and that of the emission wavelength through varying the CdSe dot size. With an enhanced σ(2), 2ASE in these heterostructures is achieved with an ultralow threshold fluence of ~1.5 mJ/cm(2), which is as much as one order less than that required for spherical semiconductor NCs. Importantly, by exploiting this unique property of the seeded nanorods exhibiting strong quantum confinement even at relatively large rod sizes, a near reciprocal relation between the 2ASE threshold and the 2PA action cross-section (σ(2)η) (where η is the quantum yield) was found and validated over a wide volume range for II-VI semiconductor nanostructures. Ultrafast optical spectroscopy verified that while the Auger processes in these heterostructures are indeed suppressed, ASE in these samples could also be strongly affected by a fast hole-trapping process to the NR surface states. Lastly, to exemplify the potential of these seeded CdSe/CdS nanodot/nanorod heterostructures as a viable gain media for achieving two-photon lasing, a highly photostable microsphere laser with an ultralow pump threshold is showcased.

Quadrupole effects in photoabsorption in ZnO quantum dots

Electronic transitions in the quantum confined states in the conduction band of spherical ZnO semiconductor quantum dot are studied. Photoabsorption spectra and oscillator strengths have been obtained for various sizes of quantum dot using effective mass approximation. Electric quadrupole effect has been included in addition to the electric dipole effect. The nonlinear contribution to photoabsorption spectra has also been studied. Results for transition energies of the quantum confined states are found to be in good agreement with the available experimental data.

Third harmonic generation in intraband transitions of spherical silicon quantum dots

A theoretical study of the third harmonic generation (THG) is reported involving intraband transitions in the conduction band of spherical Si semiconductor quantum dot surrounded by SiO2, Si3N4, and SiC matrix. The wave function and energies of a singly charged Si dot are calculated using the effective mass approximation. A finite barrier height is considered at the interface of the dot and the surrounding matrix. The results show that the THG coefficient of the silicon quantum dot strongly depends on the radius of the dot and the surrounding matrix.

Effect of rare earth substitution on the composite prepared by nickel ferrite with graphite nanosheet

In this work, the composite NiNdxFe2−xO4/NanoG was prepared by coprecipitation reaction of Nd3+ substituted nickel ferrite with graphite nanosheet (NanoG). The structures of NiNdxFe2−xO4 (x=0.03, 0.06, 0.09) and NiNdxFe2−xO4/NanoG were characterized by scanning electron microscopy, energy dispersive spectroscopy, X-ray diffraction and laser particle size appliance. Results show that when x=0.06, the particle size of NiNd0.06Fe1.94O4 is in the range of 60–70nm and can be chemically precipitated on the surface of NanoG. The saturation magnetization of NiNd0.06Fe1.94O4 is 41.79emu/g while it is 26.24emu/g for NiNd0.06Fe1.94O4/NanoG. The conductivity of NiNd0.06Fe1.94O4/NanoG is in the semiconductor sphere. Measurement of electromagnetic parameters shows that the minimum reflection loss of NiNd0.06Fe1.94O4 is −13.15dB at 11.88GHz. When the mass ratio of NiNd0.06Fe1.94O4 to NanoG reaches 4:1, the minimum reflection loss of NiNd0.06Fe1.94O4/NanoG is −17.03dB at 9.29GHz.

Second-harmonic generation in integrated nanosphere chains

The second-harmonic generation in a three-dimensional subwavelength photonic circuit consisting of spherical semiconductors with large second-order nonlinear susceptibility has been investigated by using a multiple-scattering method. The second-harmonic fields along a single chain, around a corner, and split at forked structures have been demonstrated using the three-dimensional confinement effects of subwavelength light. Our results show that the second-harmonic generations can be realized in the three-dimensional subwavelength photonic circuit and provide an alternative route for the design of integrated three-dimensional nonlinear devices.

General Bottom‐Up Construction of Spherical Particles by Pulsed Laser Irradiation of Colloidal Nanoparticles: A Case Study on CuO

The development of a general method to fabricate spherical semiconductor and metal particles advances their promising electrical, optical, magnetic, plasmonic, thermoelectric, and optoelectric applications. Herein, by using CuO as an example, we systematically demonstrate a general bottom-up laser processing technique for the synthesis of submicrometer semiconductor and metal colloidal spheres, in which the unique selective pulsed heating assures the formation of spherical particles. Importantly, we can easily control the size and phase of resultant colloidal spheres by simply tuning the input laser fluence. The heating-melting-fusion mechanism is proposed to be responsible for the size evolution of the spherical particles. We have systematically investigated the influence of experimental parameters, including laser fluence, laser wavelength, laser irradiation time, dispersing liquid, and starting material concentration on the formation of colloidal spheres. We believe that this facile laser irradiation approach represents a major step not only for the fabrication of colloidal spheres but also in the practical application of laser processing for micro- and nanomaterial synthesis.

Three-Photon Absorption in Seeded CdSe/CdS Nanorod Heterostructures

Size-dependent three-photon absorption (3PA) and three-photon-excited photoluminescence (PL) of CdSe/CdS nanorod heterostructures have been investigated using Z-scan and upconversion PL techniques with 150 fs laser pulses at 1300 nm. The PL from these rods shows clear cubic power dependence, and the 3PA cross section (σ3) was found to be as high as 1.5 × 10–75 cm6 s2 photon–2 for 39 nm long rods, which is 2–4 orders of magnitude larger than those previously reported for spherical semiconductor nanocrystals under ∼100 fs laser pulse excitation. Importantly, by exploiting the unique property of the seeded nanorods to exhibit strong quantum confinement even at relatively large rod sizes, a superlinear dependence of the 3PA cross section on nanoparticle volume was found and validated over a wide volume range. A simple derived relation that quantitatively describes σ3 and takes into account the laser pulse duration is proposed, thereby providing a clear basis of comparison between the σ3 values of various II–V...

The electric field effects on the binding energies and the nonlinear optical properties of a donor impurity in a spherical quantum dot

In this study, we have investigated the influence of an external electric field on the electronic properties of the ground and excited states and studied the linear and the third-order nonlinear optical properties (i.e., absorption coefficients and refractive indices) in a spherical semiconductor quantum dot of a parabolic confinement with an on-center shallow hydrogenic impurity. In the calculations, a variational procedure was employed within the effective-mass approximation. We found that the binding energies of the ground and excited states, the absorption coefficients, and the refractive index changes of 0s-1p and 1p-2d transitions depend on the applied electric field. The results show that the existence of the electric field has great influence on the optical absorption coefficients and the refractive index changes. Also, we have found that the magnitudes of the absorption coefficient and the refractive index change of the spherical quantum dot increase for transitions between higher levels.

Photoabsorption and photoelectric process in Si nanocrystallites

Using the effective mass approximation, we investigate the photoabsorption and photoelectric process in the conduction band of a single electron charged spherical Si semiconductor quantum dot nanostructure embedded in the amorphous SiO2 matrix. We consider the potential barrier at the interface as of (i) infinite and (ii) finite heights. The effect of self-energy associated with surface polarization due to the charging of the quantum dot is also considered. Our results of photoabsorption coefficient for (1s-1p) transition are found to be in good agreement with the available experimental data.

Selective Pulsed Heating for the Synthesis of Semiconductor and Metal Submicrometer Spheres

Heiße Sache: Zur Synthese von Halbleiter- und Metallkügelchen muss das anisotrope Kristallwachstum eingedämmt werden, dies ist bei Anwendung herkömmlicher Heizmethoden aber schwer zu erreichen. Selektives Heizen von Nanopartikel-Kolloiden durch Laserpulse führt dagegen zu Halbleiter- und Metallkügelchen mit Submikrometergröße. Detailed facts of importance to specialist readers are published as ”Supporting Information”. Such documents are peer-reviewed, but not copy-edited or typeset. They are made available as submitted by the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

Selective Pulsed Heating for the Synthesis of Semiconductor and Metal Submicrometer Spheres

Canned heat: Inhibition of anisotropic crystal growth is critical for synthesizing semiconductor/metal spheres, but it is kinetically difficult to achieve in methods relying on conventional heating. Selective heating of colloidal nanoparticles by pulsed laser irradiation allows size-tailored semiconductor and metal submicrometer spheres to be synthesized.

Temperature and magnetic field tunability of composite metamaterials containing spherical semiconductor particles in far-infrared and terahertz regimes

The effect of temperature and dc magnetic field variations on effective electromagnetic parameters of metamaterials (MTMs) containing spherical semiconductor particles is studied theoretically. The effect of temperature is taken into account through its influence on semiconductor carrier density and mobility. The effect of dc magnetic field is included using an extension of the Mie theory, describing the interaction of a plane wave with a gyrotropic sphere. The effective parameters such as relative permittivity and permeability are calculated by proper application of the Maxwell Garnett (MG) theory and its extensions to quasi-static condition and multi-phase structures. Based on these theories, the temperature and dc magnetic field tunability of three different MTM structures is investigated. First a single phase medium is considered which contains spherical semiconductor particles of one kind, randomly dispersed in a dielectric host. Then two multi-phase structures containing (a) two kinds of spherical semiconductor particles or (b) spherical particles with core–shell topology are investigated. The two multi-phase MTM structures can exhibit negative index of refraction in far-infrared spectral region. The measure of the temperature and dc magnetic field tunability of effective parameters such as relative permittivity and refractive index of the structures is evaluated and it is shown specifically that the real part of refractive index can be tuned to get negative, zero or positive values in far-IR or THz regimes, but the imaginary part of the index and the Figure of Merit (FOM) are also quite sensitive to the temperature and magnetic field variations. The tunable MTMs can find new applications in THz devices such as switches, tunable mirrors, isolators, converters, polarizers, filters and phase shifters.

Electronic structure and optical properties of quantum-confined lead salt nanowires

In the framework of four-band envelope-function formalism, developed earlier for spherical semiconductor nanocrystals, we study the electronic structure and optical properties of quantum-confined lead-salt (PbSe and PbS) nanowires (NWs) with a strong coupling between the conduction and the valence bands. We derive spatial quantization equations, and calculate numerically energy levels of spatially quantized states of a transverse electron motion in the plane perpendicular to the NW axis, and electronic subbands developed due to a free longitudinal motion along the NW axis. Using explicit expressions for eigenfunctions of the electronic states, we also derive analytical expressions for matrix elements of optical transitions and study selection rules for interband absorption. Next we study a two-particle problem with a conventional long-range Coulomb interaction and an interparticle coupling via medium polarization. The obtained results show that due to a large magnitude of the high-frequency dielectric permittivity of PbSe material, and hence, a high dielectric NW/vacuum contrast, the effective coupling via medium polarization significantly exceeds the effective direct Coulomb coupling at all interparticle separations along the NW axis. Furthermore, the strong coupling via medium polarization results in a bound state of the longitudinal motion of the lowest-energy electron-hole pair (a longitudinal exciton), while fast transverse motions of charge carriers remain independent of each other.

Single-particle-like charge-density excitations in charged spherical semiconductor quantum dots

Single-particle-like charge-density excitations in charged spherical semiconductor quantum dots

Whispering-gallery exciton polaritons in submicron spheres

We show that a semiconductor sphere with a radius comparable with the wavelength of light at the exciton resonance frequency can behave as a high-quality three-dimensional microcavity or ``polariton dot.'' We obtain numerical results for gallium arsenide submicron spheres and demonstrate that, in contrast to a larger sphere or planar microcavity, they can simultaneously possess both large quality factor and high finesse. In the strong-coupling regime the Rabi splitting approaches the bulk polariton splitting.

Nanoparticle size characterization by laser light scattering

Spherical semiconductor nanoparticles (ZnS) were specially fabricated by an inexpensive chemical route. The scattering profile of the nanoparticles was investigated by laser light scattering technique. A beam of polarized light from a diode laser (λ0 ≈ 630 nm) was allowed to fall on the nanospheres embedded in flexible host matrix Polyvinyl Alcohol (PVA). The light scattered from the samples were detected by means of analyzer mounted photodiode array from 10° to 170° in steps of 1°. Signals from the detectors were interfaced with a high resolution data acquisition system and the whole experiment was carried out in differential mode. Size of the nanoparticles was obtained by using Mie theory and verified by T-matrix approach. The results obtained agree with the XRD and TEM results.