Optical properties of photonic crystals filled with metal quantum dots

The features of optical-range electromagnetic wave passage through the photonic crystal filled with metal quantum dots are studied. The possibility of localizing electromagnetic radiation within the photonic crystal with a nonequilibrium temperature increase in a small localization region to 10³–10⁶ K under femtosecond excitation by a laser pulse with an energy of 10^?3 J was shown.     

Ultrafast spectroscopy of CdS/CdSe quantum dots

Results from the nonresonance spectroscopy of CdS/CdSe quantum dots (composites of CdSe–CdS nanoparticles (core–shell)) are presented. The nonlinear optical properties of CdS/CdSe QDs in PMMA are studied with fs pulses at 1053 nm using the transient lens technique. QDs generate rapidly oscillating signals with amplitude rise times of around 200 fs and decay times of around 500 fs, while pure PMMA polymer only generates an oscillating signal whose envelope coincides with its autocorrelation function.     

Optical properties of photonic crystals filled with iodine vapor

Properties of electromagnetic waves in the photonic crystal with pores filled with iodine vapor are considered. The conditions of the permittivity resonance appearance in the visible region of the spectrum, caused by the electronic transition in iodine molecules, are determined. For the resonant photonic crystal under study, dispersion curves, group velocities of electromagnetic waves, and reflection spectra are calculated for various globule diameters.     

Hole mobility and trapping in PVK films doped with CdSe/CdS and CdSe quantum dots

Experimental study of the hole mobility in polyvinylcarbazole (PVK) films doped with two kinds of nanocrystals, on bare core CdSe and core-shell CdSe/CdS quantum dots, with concentrations ranging from 3 · 10¹⁰ to 3 · 10¹⁵ cm⁻³, is presented. The quantum dots investigated were made using colloidal chemistry. The hole mobility was measured using the time-of-flight technique as a function of the applied electrical field in the range 10⁵–10⁶ V/cm and for temperatures from 20°C to 50°C. The transient curves, being featureless on a linear plot, show on a double logarithmic scale a sharp inflection point indicating a dispersive carrier drift process. The recovered values of the mobility are in the range 3 · 10⁻⁸–10⁻⁶ cm²·V⁻¹·s⁻¹ and their field and temperature dependences can be analyzed formally within the framework of the Gaussian disorder model proposed by B?ssler. The energetic disorder is, within the experimental accuracy, independent of the concentration and type of quantum dots for the CdSe quantum dots at all concentrations and for the CdS/CdSe quantum dots up to 10¹⁴ cm⁻³. The spatial disorder factors are very large (from 5.3 to 8.7) and do not depend in a systematic way upon the type and concentration of quantum dots (QDs). The experiments show that the apparent mobility does not change considerably with concentration, but it was found that the samples with CdSe/CdS quantum dots at concentrations from 10¹⁵ to 3 · 10¹⁵ cm⁻³ show a decreased photocurrent response. The dependence of the time-integrated transients (corresponding to the full charge value) upon the quantum-dot concentration has been determined. Differences in total photogenerated charge for pure and doped polymer films imply that the quantum dots of that type are the hole traps with capture times much more smaller than the transit time and with emission times a few orders longer than the transit time. CdSe quantum dots without a shell do not seem to exhibit the same properties as core shells and do not produce considerable changes in the charge transfer, even at a density of 10¹⁵ cm⁻³.     

Stability and luminescence properties of CsPbBr3/CdSe/Al core-shell quantum dots

To improve the stability and luminescence properties of CsPbBr₃ QDs, we proposed a new core-shell structure for CsPbBr₃/CdSe/Al quantum dots (QDs). By using a simple method of ion layer adsorption and a reaction method, CdSe and Al were respectively packaged on the surface of CsPbBr₃ QDs to form the core-shell CsPbBr₃/CdSe/Al QDs. After one week in a natural environment, the photoluminescence quantum yields of CsPbBr₃/CdSe/Al QDs were greater than 80%, and the PL intensity remained at 71% of the original intensity. Furthermore, the CsPbBr₃/CdSe/Al QDs were used as green emitters for white light-emitting diodes (LEDs), with the LEDs spectrum covering 129% of the national television system committee (NTSC) standard color gamut. The core-shell structure of QDs can effectively improve the stability of CsPbBr₃ QDs, which has promising prospects in optoelectronic devices.     

Imaging single quantum dots in three-dimensional photonic crystals

Performing fluorescence wide-field microscopy we have imaged single semiconductor quantum dots deep inside a 3-dimensional photonic crystal prepared from colloidal polymer beads. Exploring the emission diffraction patterns in defocused images of quantum dots we demonstrate that the direction-dependent photonic stop band imprints an anisotropy to the angular emission of a single quantum dot. Hence a single, quasi-point-like emitter is manipulated to radiate its photons only to certain well-defined directions by means of the anisotropic light propagation in photonic crystals. The experiments thus provide new routes to evaluate local, frequency selective optical properties in 3-dimensional photonic crystals employing single emitters.     

Size-dependent electronic and optical properties of an exciton in CdSe/CdS/CdSe/CdS multilayer spherical quantum dot

The electronic and optical properties of a single exciton in a CdSe/CdS/CdSe/CdS quantum dot is studied by using effective mass approximation with parabolic confinement. The Coloumbic interaction between electron and hole is included by Hartree potential. A self-consistent technique is used to calculate the energy eigenvalue and wavefunction of exciton. Based on this approximation we investigate the effect of core size, shell thickness, well width on exciton binding energy, absorption spectra, and oscillator strength. The results provide the tuning possibility of electronic and optical properties of multilayer quantum dot with layer thickness.     

In situ growth of CdSe/CdS quantum dots inside and outside of MWCNTs

The CdSe/CdS quantum dots were grown in situ inside and outside of the multiwalled carbon nanotubes in an innocuous solvent and at a lower temperature. The CdSe/CdS-CNTs nanoheterostructures were characterized by TEM, HRTEM, EDS, XPS and PL. The CdSe/CdS nanocrystals with diameters about 5 nm exhibit an improved PL emission.     

Advances in photonic crystals with MEMS and with semiconductor quantum dots

We discuss photonic crystals (PCs) with a microelectromechanical system (MEMS) and semiconductor quantum dots (QDs) as novel classes of PC devices. Integration of MEMS structures into PC devices enables one to realize several kinds of functional devices, such as modulators, switches, and tunable filters for highly integrated photonic circuits. We describe the basic concept of MEMS-integrated PC devices and show numerical and experimental demonstrations of MEMS-integrated functional PC devices. On the other hand, QDs are promising candidates for active media in PC devices. Spontaneous emission control of QD emission in PC nanocavities is especially important for novel optoelectronic devices and quantum information devices. In PC nanocavities, the interaction between QD excitons and photons is enhanced dramatically. The control of spontaneous emission spectrum and the enhancement of the luminescence intensity of InAs QDs by PC nanocavities are demonstrated at telecommunication wavelengths. The Purcell effect for ensemble and single QDs in PC nanocavities are also discussed.     

核壳结构硒化镉/硫化镉/巯基乙酸量子点载流子输运特性

采用水溶液法合成了巯基乙酸(TGA) 包覆的CdSe 量子点. 通过X 射线粉末衍射和高分辨透射显微镜检测结果证实, 合成得到闪锌矿结构CdSe 量子点. 能谱图和傅里叶变换红外光谱图结果说明, 在核CdSe 纳米粒子表面与配体TGA 之间有CdS 壳层结构形成. 利用样品表面光电压(SPV) 谱, 指认CdSe 量子点精细能带结构以及各自对应的激发态特征: 475 nm (2.61 eV) 和400 nm (3.1 eV) 两个波长处的SPV 响应峰分别与CdSe 核和CdS 壳层带-带隙跃迁相对应; 370 nm (3.35 eV) 附近SPV 响应峰可能与TGA 中羰基与巯基或羧基之间发生的n →π^* 跃迁有关. 场诱导表面光电压谱结果证实, 合成的CdSe 量子点具有明显的N 型表面光伏特性, 而上述n→π^* 跃迁则具有P 型表面光伏特性. 荧光光谱谱线均匀增宽以及SPV 响应峰位蓝移, 说明样品具有明显的量子尺寸效应. 结合不同pH 值下合成的CdSe 量子点的SPV 谱和表面光声谱发现, SPV 响应强度与表面光声光谱信号强度变化趋势恰好相反. 上述样品表面光伏效应表明, CdSe 量子点表面和相界面处的精细电子结构以及光生载流子的输运特性均与量子点的尺寸大小存在某种内在联系. 关键词： 硒化镉量子点 光生载流子 表面光电压谱 表面光声光谱     

Optical properties of GaN/AlN quantum dots

We present here a review of the peculiar optical properties of GaN/AlN quantum dots. These systems show unusually large exciton binding energies and band-offsets. Moreover, when grown along the (0001) axis in the wurtzite phase, the optical properties are dominated by huge on-axis internal electric fields, leading to a very low oscillator strength and complex dynamical behavior. It is also possible to grow GaN quantum dots in the cubic phase or along nonpolar axis of the wurtzite cell. We discuss properties of ensembles of quantum dots, as well as of single quantum dots studied by micro-photoluminescence. To cite this article: P. Lefebvre, B. Gayral, C. R. Physique 9 (2008).     

Fractional decay of quantum dots in real photonic crystals

We show that fractional decay may be observable in experiments using quantum dots and photonic crystals with parameters that are currently achievable. We focus on the case of inverse opal photonic crystals and locate the position in the crystal where the effect is most pronounced. Furthermore, we quantify the influence of absorptive loss and show that it is a limiting but not prohibitive effect.     

Interactions between CdSe/CdS quantum dots and DNA through spectroscopic and electrochemical methods

The interaction of CdSe/CdS quantum dots (QDs) with Herring sperm-DNA (hs-DNA) has been studied by UV-vis spectroscopy and electrochemical method. Cu(phen)₂^2+/1+ (phen = 1, 10-phenanthroline) was used as an indicator for electroactive dsDNA or ssDNA. The apparent association constant has been deduced (4.94 × 10³ M⁻¹ and 2.39 × 10² M⁻¹) from the absorption spectral changes of the dsDNA-QDs and ssDNA-QDs. The results of dissociation method suggest that Cu(phen)₂^2+/1+ is more easily dissociated from dsDNA or ssDNA modified gold electrode (dsDNA/Au or dsDNA/Au) in presence of QDs. The dissociation rate constant (k) of Cu(phen)₂^2+/1+ on dsDNA/Au is 4.48 times higher than that in absence of QDs, while k is 2.34 times higher than that in absence of QDs on ssDNA/Au in Tris buffer with low ionic strength (pH 7.0, 0.5 mM NaCl). The results illuminate that hs-DNA has high affinity for QDs due to electrostatic force, hydrogen bonds, and van der Waals interactions, and the binding force of QDs with dsDNA is stronger than ssDNA.     

Preparation of fluorescent nanocomposites with CdSe/CdS quantum dots by dispersion polymerization in supercritical carbon dioxide

A new method for preparing fluorescent polymer nanocomposites based on dispersion copolymerization of methyl methacrylate and styrene in the presence of CdSe/CdS quantum dots in supercritical carbon dioxide medium is proposed. A one-step scheme of the formation of fine poly(methyl methacrylate)/polystyrene/CdSe/CdS nanocomposites is realized, and the factors influencing their photoluminescent characteristics and photochemical stability under prolonged exposure to natural conditions (sunlight) and intense UV irradiation in air are examined.     

Study of CdSe/CdS quantum dots in solutions and gels by small-angle X-ray scattering

The sizes of semiconductor nanocrystals of CdSe/CdS quantum dots (QDs) synthesized by the colloidal method were estimated using small-angle X-ray scattering. The distribution of QD nanocrystals in organic solvents of different polarities and in polymer gels and matrices is studied. Structural invariants of scattering QD particles (heterogeneities of the electron density)—namely, inertia radii and sizes, forms, and dispersive composition of particles—are determined. The contribution of scattering by QDs in solutions and gels is calculated. The effective sizes of particles and their aggregates are determined, and the parameters of the distribution over the QD sizes in organic solvents and polymer matrices are estimated. The typical distance between particles in samples is determined. The position of the maximum at the beginning of the small-angle scattering curves corresponds to the distance d _m = 2π/h ₀ between the planes (here, h ₀ is the position of the maximum on the scale h). It is 74–76.9 ? for solutions, 60 ? for gels, and 99 ? for polymer matrices with concentrations of up to 0.15% and 77 ? for those with the concentrations exceeding 0.15%, which is close to the estimation of the sizes of separate CdSe QDs that was obtained from the distribution histograms (60–80 ?). This result shows that CdSe/CdS QDs introduced in the polymer matrices disperse to form either separate particles or small aggregates and located at a distance on the order of 80 ? from each other.     

Optical properties of InGaN quantum dots

We present time-resolved and spatially-resolved photoluminescence (PL) measurements of InGaN inclusions in a GaN matrix. The structures were grown by metal-organic chemical vapor deposition on sapphire and Si(111) substrates. Nonresonant pulsed excitation yields a broad PL peak, while resonant excitation into the nonresonant PL intensity maximum results in an evolution of a sharp resonant PL peak, having a spectral shape defined by the excitation laser pulse and a radiative decay time close to that revealed for PL under nonresonant excitation. Observation of a resonantly excited narrow PL line gives clear proof of the quantum dot (QD) nature of luminescence in InGaN–GaN samples. Cathodoluminescence (CL) and micro-PL measurements demonstrate sharp emission lines from single QD states. The recombination dynamics of single QD’s and the whole QD ensemble were investigated. Monoexponential decay was observed for the PL of single QD’s. For similar transition energies different time constants were obtained. Therefore the nonexponential decay observed for the whole ensemble is attributed to the coexistence of QD’s having similar ground-state transition energies, but significantly different electron–hole overlap.     

Optical properties of nanostructured 2D metal-dielectric photonic crystals with a lattice defect

Optical properties of 2D nanocomposite-based photonic crystals with a lattice defect are studied. The nanocomposite comprises metallic nanospheres dispersed in a transparent matrix and is characterized by an effective resonant permittivity. Transmission spectrum for s-polarized waves at oblique incidence is calculated. Spectral manifestation of the splitting of the defect mode when its frequency coincides with the resonant frequency of the nanocomposite is studied. The essential dependence of the splitting on the angle of incidence and concentration of metallic nanospheres in the nanocomposite matrix is established. Specific features of spatial distribution of the electric field intensity in defect modes of crystals are analyzed.     

Optical properties of Si-doped InAs/InP quantum dots

InAs quantum dots (QDs) were grown on InP substrates by low pressure-metalorganic chemical vapor deposition. Disilane (Si₂H₆) was used as an n-type dopant. The positions of Si doping were varied: buffer layer, capping layer, modulation doping, and QD itself. Surface treatment of InP by Si₂H₆ was also performed to see the effect of Si on InAs QD. Photoluminescence (PL) and atomic force microscopy (AFM) were used to characterize optical and structural properties of QDs, respectively. It was found that the PL peak positions varied from 0.73 to 0.88 eV with the position of Si doping. PL peak blue shift in modulation doped sample was explained in terms of state filling effect. It was found that Si doping at QD itself was the most effective way to obtain the strongest integrated PL intensity without degrading the QD size distribution.     

Size-dependent optical properties and carriers dynamics in CdSe/ZnS quantum dots

Size-dependence of optical properties and energy relaxation in CdSe/ZnS quantum dots （QDs） were investigated by two-colour femtosecond （fs） pump-probe （400/800 nm） and picosecond time-resolved photoluminescence （ps TRPL） experiments. Pump-probe measurement results show that there are two components for the excited carriers relaxation, the fast one with a time constant of several ps arises from the Auger-type recombination, which shows almost particle sizeindependence. The slow relaxation component with a time constant of several decades of ns can be clearly determined with ps TRPL spectroscopy in which the slow relaxation process shows strong particle size-dependence. The decay time constants increase from 21 to 34 ns with the decrease of particle size from 3.2 to 2.1 nm. The room-temperature decay lifetime is due to the thermal mixing of bright and dark excitons, and the size-dependence of slow relaxation process can be explained very well in terms of simple three-level model.     

Optical properties of one-dimensional soft photonic crystals with ferrofluids

We review the recent theoretical study on the optical properties of one-dimensional soft photonic crystals (1D SPCs) with ferrofluids. The proposed structure is composed of alternating ferrofluid layers and dielectric layers. For the ferrofluid, single domain ferromagnetic nanoparticles can align to a chain under the stimuli of an external magnetic field, thus changing the microstructure of the system. Meanwhile, nonlinear optical responses in ferrofluids are also briefly reviewed.