CdTe和CdS量子点的非线性特性对比

应用Top-hat Z-scan技术在波长为532 nm,脉宽为190 fs激光脉冲下研究了CdTe和CdS量子点的光学非线性吸收和非线性折射特性。实验结果表明:在飞秒激光脉冲作用下,CdTe量子点的非线性吸收表现为饱和吸收,CdS量子点表现为反饱和吸收。CdTe量子点的非线性折射表现为自散焦,CdS量子点表现为自聚焦。尺寸分别为2.6、2.4 nm的CdTe量子点和CdS量子点的非线性吸收系数分别为-9.2610-14、0.7810-14 m/W,非线性折射率系数分别为-0.8610-20、1.4610-20 m2/W,三阶非线性极化率分别为2.7210-15、1.3610-15 esu。表明相近尺寸下不同材料的镉类半导体量子点的光学非线性吸收和非线性折射特性不同,并对其机理进行分析。     

Photoluminescence properties of cadmium-selenide quantum dots embedded in a liquid-crystal polymer matrix

The photoluminescence properties of cadmium-selenide (CdSe) quantum dots with an average size of ～3 nm, embedded in a liquid-crystal polymer matrix are studied. It was found that an increase in the quantum-dot concentration results in modification of the intrinsic (exciton) photoluminescence spectrum in the range 500–600 nm and a nonmonotonic change in its intensity. Time-resolved measurements show the biexponential decay of the photoluminescence intensity with various ratios of fast and slow components depending on the quantum-dot concentration. In this case, the characteristic lifetimes of exciton photoluminescence are 5–10 and 35–50 ns for the fast and slow components, respectively, which is much shorter than the times for colloidal CdSe quantum dots of the same size. The observed features of the photoluminescence spectra and kinetics are explained by the effects of light reabsorption, energy transfer from quantum dots to the liquid-crystal polymer matrix, and the effect of the electronic states at the CdSe/(liquid crystal) interface.     

Photoluminescence of germanium quantum dots formed by pulsed laser ablation

Time-resolved studies of photoluminescence in the energy range 1.4–3.2 eV are carried out for GeO_x films (x ≤ 2) containing Ge nanoclusters. The relaxation times vary in the range from 50 ns to 20 μs. The films are produced by single-stage pulsed laser ablation and deposited from the direct and reverse fluxes of particles of the erosion spray in oxygen and argon atmosphere. The dependences of the photoluminescence properties of the films on the conditions of deposition and on doping with gold are studied. The photoluminescence bands, with peaks at the energies 1.42–2.1 eV, are narrow, slow, and close to each other in energy, whereas the other bands, with peaks near 2.5 eV and in the range 2.7–2.9 eV, are broad, faster, and only slightly overlapping in energy. The results are interpreted in the context of the model of photoluminescence in Ge quantum dots, whose dimensions are controlled by the conditions of their formation.     

Luminescence properties of InAs/GaAs quantum dots prepared by submonolayer migration-stimulated epitaxy

The results of an investigation of the luminescence properties of an ensemble of InAs quantum dots, obtained by submonolayer migration-stimulated epitaxy on singular and vicinal GaAs(100) surfaces, are reported. The largest width at half-height of the photoluminescence line is observed in samples with a 3° disorientation, indicating that the size-variance of the quantum dots is largest in this case. Quasiequilibrium quantum dots are formed either with a long sample holding time in an arsenic flow or with a larger quantity of deposited indium. Fiz. Tekh. Poluprovodn. 31, 912–915 (August 1997)     

Photoluminescence enhancement of quantum dots with photonic structures

Photoluminescence (PL) of CdSe/ZnS quantum dots (QDs) deposited on Si and photonic crystals with packed silica spheres, are investigated with the purpose of establishing enhancement of PL. Quantum dots with size ～2.4 nm are introduced into the solution containing silica spheres of ～345 nm size. Upon evaporation, QDs are incorporated into the photonic crystals made of close-pack spheres. We found that PL is red- or blue-shifted (from interaction with molecular complexes) with increase of QD-density. The broad PL emission is partly from particle size distribution, but mainly due to the interaction with local molecular complexes with surface quantum size effects. We have also observed enhanced interaction when the emission is close to the photons at the Brillouin zone boundary.     

Photoluminescence of localized excitons in InGan quantum dots

Photoluminescence spectra of samples with ultrathin InGaN layers embedded in AlGaN and GaN matrices are studied experimentally in the temperature range of 80 to 300 K. It is shown that the temperature dependences can be understood in the context of Eliseev’s model and that, in the active region of the structures under study, the dispersion σ of the exciton-localization energy depends on the average In content in InGaN-alloy layers. Furthermore, the Urbach energy E _U, which characterizes the localization energy of excitons in the tails of the density of states, was determined from an analysis of the shape of the low-energy slope of the spectrum. It is shown that σ and E _U, quantities representing the scale of the exciton-localization effects, vary linearly with the photoluminescence-peak wavelength in the range from the ultraviolet to the green region of the spectrum.     

Microwave-assisted synthesis of mesoporous CdS quantum dots modified by oleic acid

To obtain stable mesoporous CdS quantum dots, an in situ method that uses oleic acid as a soft template was introduced, and the oleic acid modified CdS quantum dots were then treated with microwaves. The structure and properties of the samples were characterized using X-ray diffraction (XRD), Fourier transform infrared (FT-IR), high-resolution transmission electron microscopy (HRTEM), thermogravimetry (TG) and Brunauer–Emmett–Teller (BET) gas sorptometry. It was found that microwave treatment had a positive effect on the increase in surface area, improving the mesostructure and narrowing the pore size distribution of mesoporous CdS.     

Photocatalytic degradation of reactive black 5 azo dye by zinc sulfide quantum dots prepared by a sonochemical method

Zinc sulfide (ZnS) nanoparticles (NPs) with an average particle size of 2 nm were successfully synthesized under ultrasonic irradiation without any surfactant and high temperature treatment. Prepared NPs were characterized by powder X-ray diffraction, transmission electron microscopy, high resolution transmission electron microscopy, Fourier transform infrared spectroscopy, dynamic light scattering analysis, and UV–vis absorption spectroscopy. The energy band gap of ZnS NPs was measured by UV–vis absorption spectroscopy. The photocatalytic activity of semiconducting sulfide quantum dots for degradation of an azo dye called reactive black 5 (RB5) was investigated. Results showed that the dye can be photocatalytically degraded with high rate by ZnS NPs under UV light irradiation. The kinetics of removal of RB5 in aqueous solutions was studied in a series of experiments which were varied in the amount of ZnS NPs, contact time, pH, dye concentration, and temperature. The experimental data were fitted very well in the pseudo-second order kinetic model. 95% of dye was successfully removed in 10 min using 0.2 g ZnS NPs in a neutral pH. A possible molecular mechanism for photocatalytic degradation of dye by ZnS NPs was also given.     

Photoluminescence of InAs quantum dots coupled to a two-dimensional electron gas

Self-assembled InAs quantum dots have been extensively studied by a variety of experimental techniques. Works have been done on the transport properties of the InAs dots located near a two-dimensional electron gas (2DEG). However, there have been few reports on the optical properties of the InAs dots located closely to 2DEG. In this work, InAs dots samples with 2DEG and without 2DEG growth by solid source molecular beam epitaxy were studied using photoluminescence measurements. Different photoluminescence behaviors between the InAs dots and the InAs dots near the 2DEG were observed. It was found that the emission efficiency of the InAs dots was significantly enhanced by the existence of the nearby 2DEG and the thermal activation energy of the InAs dots was decreased by the 2DEG. It was speculated that the 2DEG at the AlGaAs/GaAs interface worked as an electron reservoir to the InAs dots. As a result, the conduction band between the dots and 2DEG is lowered, and thus the thermal activation energy of PL is lowered. It was concluded that in this way the optical properties of the InAs quantum dots could be tailored for optical applications.     

Photoluminescence of InSb quantum dots in GaAs and GaSb matrices

The photoluminescent properties of quantum dots formed in the deposition of an InSb thin film (1–3 monolayers) on GaAs(100) and GaSb(100) surface are investigated. The results indicate the importance of As-Sb substitution reactions in the formation of quantum dots on a GaAs surface. Fiz. Tekh. Poluprovodn. 31, 68–71 (January 1997)     

Photoluminescence from germanium quantum wells and quantum dots in silicon grown by MBE at low temperature

Semiconductors - The low-temperature (T=2 K) photoluminescence (PL) has been studied in Si/Ge structures grown by MBE at a low (250–350°C) temperature of Ge deposition. The luminescence...     

Photoluminescence properties and chemical bond variations of SiN x :H films with silicon quantum dots

Hydrogenated silicon nitride(SiNx :H) thin films are deposited on p-type silicon substrates by plasma enhanced chemical vapor deposition(PECVD) using a gas mixture of ammonia and silane at 230 °C.The chemical compositions and optical properties of these films,which are dealt at different annealing temperatures,are investigated by Fourier transform infrared(FTIR) absorption spectroscopy and photoluminescence(PL) spectroscopy,respectively.It is shown that the FTIR presents an asymmetric Si-N stretching mode,whose magnitude is enhanced and position is shifted towards higher frequencies gradually with the increase of the annealing temperature.Meanwhile,it is found that the PL peak shows red shift with its magnitude decreasing,and disappears at 1100 °C.The FTIR and PL spectra characteristics suggest that the light emission is attributed to the quantum confinement effect of the carriers inside silicon quantum dots embedded in SiNx : H thin films.     

Photoluminescence of InAs quantum dots grown on disoriented GaAs substrates

The photoluminescence spectra in an external magnetic field of an ensemble of InAs quantum dots grown by molecular beam epitaxy on a (001) GaAs substrate with a disorientation in the [010] direction are studied. A redistribution of the photoexcited carriers among different groups of dots under the influence of the magnetic field is observed. The concentration of quantum dots is determined by analyzing the data. Fiz. Tekh. Poluprovodn. 33, 1084–1087 (September 1999)     

Optical properties of CdS micro/nanocrystalline structures prepared via a thermal evaporation method

Cadmium sulfide (CdS) micro/nanocrystalline structures were fabricated by thermal evaporation in the absence of a catalyst on ITO-coated glass and Si(100) substrates. The samples prepared at 520 °C showed two structures of CdS: nanowires (NWs) and necklace-like microstructures grown on Si(111) and ITO substrates, respectively. The crystalline structures of the grown CdS necklace-like microstructures and NWs were studied using XRD analysis. The CdS necklaces and NWs structures were formed with hexagonal (wurtzite) structure. Two emission bands (green and red) were observed in the photoluminescence spectra of the prepared samples. The green PL emission in both structures showed a strong blue-shift compared with the optical band gap of CdS. Raman spectra of the grown CdS micro/nanocrystalline structures show redshift in the fundamental (1LO) and overtone (2LO) peaks location compared with Raman bands of bulk CdS.     

CdTe和CdTe/CdS量子点的量子尺寸效应

利用水热法合成了三种不同尺寸的单核CdTe量子点和核壳CdTe/CdS量子点。应用Top-hat Z-scan技术在纳秒、皮秒、飞秒激光脉冲作用下研究了三种不同尺寸单核CdTe量子点的非线性吸收特性。实验结果表明:在不同激光脉冲作用下三种不同尺寸的CdTe量子点的非线性吸收特性均表现为饱和吸收,并且均呈现出随着量子点尺寸的减小,其非线性吸收特性增大的趋势。为了进一步研究量子点尺寸的变化对非线性吸收特性的影响,又在飞秒激光脉冲作用下研究了核壳CdTe/CdS量子点的非线性吸收特性;随着包壳时间的增加,壳层厚度增加,量子点尺寸增加,其非线性吸收特性呈减小趋势,并且核壳CdTe/CdS量子点的非线性吸收特性明显优于单核CdTe量子点;分析讨论了单核CdTe量子点与核壳CdTe/CdS量子点的非线性吸收特性和量子尺寸效应机制,实验结果表明合成的量子点样品均具有良好的量子尺寸效应。     

CdTe/CdS core shell quantum dots for photonic applications

In this work, we show a simple experimental procedure to obtain CdTe/CdS (diameter=3-7 nm) core shell nanocrystals in colloidal suspension. The nanocrystals were characterized by transmission electronic microscopy and powder X-ray diffraction analysis. The optical properties were determined by electronic absorption, excitation and emission spectroscopies and are discussed in terms of chemical changes that occur at the surface of the particles.     

Baric properties of InAs quantum dots

In the context of the deformation potential model, baric dependences of the energy structure of InAs quantum dots in a GaAs matrix are calculated. Under the assumption of the absence of interaction between the spherical quantum dots of identical sizes, the energy dependence of the baric coefficient of energy of the radiative transition in the quantum dot is determined. A similar dependence is also found experimentally in the photoluminescence spectra under uniform compression of the InAs/GaAs structures. Qualitative agreement between the theory and experiment as well as possible causes for their quantitative difference are discussed. It is concluded that such factors as the size dispersion, Coulomb interaction of charge carriers, and tunnel interaction of quantum dots contribute to this difference.     

Electronic properties of interacting quantum dots

The electronic properties of two interacting dots connected to leads are studied in the Kondo regime. The configuration is such that one dot is inserted into the lead while the other is side-connected to it. The situation, where both dots are in the Kondo regime is investigated. We find that the development of a Kondo state related to the connected dot is mediated by the inserted dot. In this case, an anti-resonance appears in the density of states of the inserted dot, at the Fermi level. The equation of motion method is used to calculate the Green's functions of the system.     

Synthesis of CdS quantum dots in an imidazolium based ionic liquid

A solution protocol to synthesize CdS quantum dots (QDs) using 1-ethyl-3-methylimidazolium methanesulfonate ionic liquid is described in this work. Various reaction parameters such as temperature and injection protocols were varied to study the optical and structural properties of the obtained particles. X-ray diffraction (XRD) studies confirmed the formation of cubic and hexagonal phases depending on the reaction conditions. Peak broadening in the diffraction patterns revealed the formation of small sized quantum dots and the morphological studies further showed formation of nano-sized particles in the range of 2–15 nm, with close to spherical shaped morphologies. Blue-shifted band gaps further confirmed formation of very small CdS nanoparticles, which dependent on the reaction temperatures.     

Electrical properties of semiconductor quantum dots

A method, which makes it possible to obtain semiconductor particles V ≈ 10^?20 cm³ in volume (quantum dots) with a concentration of up to 10¹¹ cm^?2 and electrical contacts to each of them, is suggested. High variability in the electrical properties of such particles from a metal oxide (CuO or NiO) after the chemisorption of gas molecules is found.