Semiconductor Microspheres Research Articles

Ultraviolet photodetector improvement based on microspheres

Owing to the ideal optical cavity structure, the microsphere has always played a significant role in micro/nano optoelectronic devices for its high light-confined ability and large surface-volume ratio, which have been applied in lots of areas including intracellular lasers, tunable lasers, and so on. Furthermore, the microsphere cavity will be a useful and efficient means to improve the photodetector (PD) performance by enhancing the light absorption efficiency and light energy in the arrays. Herein, the wide-bandgap semiconductor microspheres are introduced in the PD, which are synthesized by a simple and universal hydrothermal method. The sensitive ultraviolet PD is constructed and obtained with a metal-semiconductor-metal type device structure. The oxide defect concentration in the ZnO microspheres is modulated through thermal treatment to improve the performance, including higher photo-to-dark current ratio near 600, responsivity of 7.6✕10−5 A/W, detectivity of 4.6✕107 Jones, and response speed. Moreover, the microspheres fabricated flexible device presents great detecting performance with different bending degrees. The results display the excellent prospect of semiconductor microspheres to flexible ultraviolet high-performance PDs, which possess the broad and potential development prospect in flexible even on-chip optoelectronic devices.

Applying the resonant-state expansion to realistic materials with frequency dispersion

The dispersive resonant-state expansion, developed for an accurate calculation of the resonant states in open optical systems with frequency dispersion, is applied here to realistic materials, such as metallic nanoparticles and semiconductor microspheres. The material permittivity is determined by fitting the measured indices of refraction and absorption with a generalized Drude-Lorentz model containing a number of poles in the complex frequency plane. Each Drude or Lorentz pole generates an infinite series of resonant states. Furthermore, for small nanoparticles, each of these poles produces a distinct surface plasmon polariton mode. The evolution of these multiple surface modes with increasing radius traces the transition from the electrostatic limit to significant retardation and radiation. Treating the optical phonon range in a semiconductor microsphere, a reststrahlen band separating the resonant states is found. Considering a small energy range around the semiconductor band gap, the transition from absorption to gain is described by inverting the Lorentz pole weight, which results in the formation of lasing resonant states. Interestingly, the series of resonant states converging towards the absorption pole from the lower frequency side reshapes for a gain pole into a clockwise loop approaching the pole from the higher frequency side, being separated from a series spanning from low to high frequencies and containing the lasing modes.

Efficient mesoscopic perovskite solar cells from emulsion-based bottom-up self-assembled TiO2 microspheres

Mesoscopic perovskite solar cells (PSCs) usually show superior photovoltaic performance to their planar counterparts because of the mesoporous electron transport layers (ETLs) benefiting for perovskite crystal growth and electron transport. However, the preparation of high-quality mesoporous ETLs needs selecting sizable semiconductor nanocrystals commonly within 20–50 nm, smaller or larger nanocrystals will lead to either too narrow pores to effectively infiltrate perovskite materials or too small surface area to efficiently extract charges from perovskite materials. Using sizable semiconductor microspheres can solve the issue, whereas it is also not easy to synthesis sizable semiconductor microspheres. Here, a novel emulsion-based bottom-up self-assembly strategy is used to prepare small size (about 150 nm) TiO2 microspheres from ultra-small (about 3.6 nm) TiO2 nanocrystals. The as-prepared mesoscopic PSCs can yield a champion PCE as high as 19.27% with a much smaller current–voltage hysteresis compared with those of the planar one. Specially, the emulsion-based bottom-up self-assembly strategy is a general way for preparing microspheres from wide kinds of semiconductor nanocrystals, so it will greatly expand the material selection range for preparing efficient mesoscopic PSCs and even inverted mesoscopic devices.

High-speed observation of semiconductor microsphere generation by laser ablation in the air

We have succeeded in fabricating zinc oxide (ZnO) microsphere by laser ablation in the air and demonstrated ultraviolet whispering gallery mode lasing from an optically pumped ZnO microsphere. However, fabricated microspheres could not be controlled in size, and ejection direction of microsphere was random, because the generation mechanism was attributed to the droplet ejection by laser ablation. In this report, the effect of pulse duration on microsphere generation was investigated, and we introduced optical vortex to control the size and the ejection direction of droplets. The behavior of ablated target was visualized using a high-speed camera. As a result of time-resolved observation, it was found that many ZnO microspheres can be efficiency generated by the long pulse laser irradiation. In addition, it was observed that droplets were scattered randomly by Gaussian beam irradiation. In the case of optical vortex irradiation, straight flight of the droplet was achieved. Furthermore, the size of droplets changed with pulse energy and ablation spot size with optical vortex. From these results, controlled fabrication of ZnO microspheres is expected with optical vortex laser ablation.

Interference-Induced Broadband Absorption Enhancement for Plasmonic-Metal@Semiconductor Microsphere as Visible Light Photocatalyst

A fundamental study is performed on the surface plasmon resonance (SPR) of metal@semiconductor microsphere photocatalyst to uncover its broadband absorption mechanism over the visible wavelength region. Finite element method studies show that an interference pattern is uniquely generated inside the semiconductor microsphere due to the optical reflection and refraction at the interface between the microsphere and the catalytic medium. By embedding plasmonic nanoparticles (NPs) into the microsphere, an interference-induced broadband absorption enhancement over the entire visible region can be achieved as compared to other plasmonic structures. Based on the properties of the interference, the broadband absorption enhancement can be obtained everywhere inside the microsphere and is particularly large at the microsphere hot-zone. Studies also show that microsphere consisting of higher refractive index semiconductor can maximize the interference-induced broadband absorption enhancement. Besides, NPs with differ...

Fabrication of single-crystalline microspheres with high sphericity from anisotropic materials.

Microspheres with high sphericity exhibit unique functionalities. In particular, their high symmetry makes them excellent omnidirectional optical resonators. As such perfect micrometre-sized spheres are known to be formed by surface tension, melt cooling is a popular method for fabricating microspheres. However, it is extremely difficult to produce crystalline microspheres using this method because their surfaces are normally faceted. Only microspheres of polymers, glass, or ceramics have been available, while single-crystalline microspheres, which should be useful in optical applications, have been awaiting successful production. Here we report the fabrication of single-crystalline semiconductor microspheres that have surfaces with atomic-level smoothness. These microspheres were formed by performing laser ablation in superfluid helium to create and moderately cool a melt of the anisotropic semiconductor material. This novel method provides cooling conditions that are exceptionally suited for the fabrication of single-crystalline microspheres. This finding opens a pathway for studying the hidden mechanism of anisotropy-free crystal growth and its applications.

Fabrication and characterization of spherical micro semiconductor crystals by laser ablation method

We have been establishing the technique of fabricating spherical semiconductor microcrystals with suitable diameters for whispering gallery mode (WGM) lasing. Concretely, semiconductor microspheres were synthesized by ablating various semiconductor-sintered targets with focused pulsed laser at high fluences. In this report, dependences of fabricated zinc oxide (ZnO) microstructures on laser wavelengths were investigated. Lasing characteristics and photoluminescence of ZnO microspheres were determined, and photoluminescence of Sb-doped ZnO microspheres were determined. Additionally, it was also found that Sb-doped ZnO and aluminum nitride microspheres can be similarly synthesized. By developing this method, which does not require complex processing, it is expected that efforts in the application of WGM lasing are accelerated in many kinds of semiconductors.

Optical Fabrication of Semiconductor Single-Crystalline Microspheres in Superfluid Helium

ABSTRACTWe successfully fabricated semiconductor microspheres of ZnO, ZnSe, etc., by laser ablation in superfluid helium and investigated their morphology and optical properties. Time-resolved photoluminescence spectroscopy in ultraviolet region of single ZnO microspheres shows luminescence spectra with mode structures and remarkable reduction of the luminescence decay time compared to that of polycrystals or non-spherical microparticles. This indicates strong light-matter interaction due to efficient light-confinement in the ZnO microspheres. In addition, the fabricated ZnSe microspheres also show the photoluminescence spectra with typical mode structures indicating their high sphericity.

Facile synthesis of hierarchical SnO 2 semiconductor microspheres for gas sensor application

Hierarchical SnO 2 microspheres were synthesized by a hydrothermal method at 140 °C using stannic chloride hydrate and sodium hydroxide as starting materials. The individual hierarchical SnO 2 microsphere ranged from 700 to 900 nm in diameter. After these microspheres were heated at 600 °C for 2 h, the spheres were cross-linked into clusters by short SnO 2 nanorods as revealed by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Most importantly, SnO 2 hierarchical microsphere sensor exhibits excellent selectivity and fast response to ethanol. Response and recovery times were 0.6 s and 11 s when the sensor was exposed to 50 ppm ethanol at an operating temperature of 300 °C. Thus, hierarchical structures play a significant role in the field of gas sensing.

Light intensity fluctuations on a semiconductor microsphere calculated by boundary element method

Abstract For a semiconductor microsphere irradiated by monochromatic unpolarized plane light wave, the governing Maxwell equations, which are transformed into Schrodinger equations through Debye potentials, are solved by the boundary element method, one of the integral formulations. The resultant light intensities on the particle surface show noise-like fluctuations depending on various parameters such as the light wavelength, the particle size, the numerical surface element size, etc. The more the numerical surface elements used, the greater the noise extent of the intensities. We think that this noise is related to the fluctuations happening in the real world and that they are somehow made into shape numerically by Green’s function and surface integration. One can consider a numerical surface element as a crystalline or amorphous unit cell. Actually a few experiments with photon energy conversion devices give the consistent results with ours that the energy conversion efficiency is on the increase with the decreasing size of unit cells. It is thus proposed here that this noise from numerical computation may be modeled to be the real thermal fluctuations of photon density on particle surface for photovoltaic cells, photocatalysts, photoluminescence devices, etc.

ZnSe semiconductor hollow microspheres.

Magic bubbles: ZnSe semiconductor microspheres aggregated from nanocrystals of tunable size have been successfully synthesized for the first time by use of gas bubbles. Based on the interior nanocrystal size and structure, the as-obtained samples exhibit different colors.

Spherical cavity-mode laser with self-organized CuCl microspheres

We report lasing of exciton-polaritons in semiconductor microspheres with spherical cavity modes under subpicosecond ultraviolet laser excitation at low temperature. Small bulk crystals of CuCl are melted and formed into fused microspheres with diameters ranging from a few to several tens of micrometers. The threshold for lasing is 1nJ/pulse, which corresponds to 10(9) photons/pulse. We found that the biexciton-to-longitudinal-exciton transition is responsible for the lasing.

Nonlocal response of size-quantized excitons in a semiconductor microsphere

By using nonlocal response theory which determines the selfconsistent motions of induced polarization and radiation field, the linear response of size-quantized excitons in a semiconductor microsphere has been calculated as a function of the radius of the sphere. The induced polarization for resonant light shows a remarkable size-resonant enhancement.

Influence of the electron-phonon interaction on a donor like exciton in a semiconductor microsphere

We study the influence of the electron-optical phonon coupling on a donor like exciton for which the hole is localized at the centre of a spherical microcrystal. This work is performed within the envelope function approximation considering a non degenerated conduction band. The quantum confinement is described by an infinitively deep potential well. The electron-optical phonon coupling is treated within the adiabatic approximation. We determine the Huang-Rhys factor using a one parameter trial wave-function which allows to include the electron-hole correlation. The theoretical model is compared to experiment.