Ultraviolet luminescence of ZnO infiltrated into an opal matrix

Technology for the infiltration of zinc oxide into a three-dimensional opal lattice using chemical deposition from a solution was developed. Samples of ZnO-opal composites, whose luminescence at room temperature mainly occurs in the ultraviolet spectral range, were obtained. The filling ratio was monitored by two different techniques: (i) checking the increase in the mass of the sample and (ii) checking the shift of the peak in the optical reflection spectrum of samples filled with ZnO in comparison with the initial opal matrices. The results obtained by these two methods are consistent with each other. Optimum conditions for synthesizing ZnO-filled opals in order to attain the highest intensity of ultraviolet luminescence were determined. It was shown that using “raw” opals, whose voids are incompletely filled with the semiconductor material, leads to a severalfold increase in the intensity of the edge excitonic emission band at room temperature. The results obtained can be used in the development of efficient directed laser light sources in the ultraviolet spectral range based on the “photonic crystal” effect.     

Luminescence from ZnO quantum dots deposited with synthetic opal

Photoluminescence from ZnO layers of varied thickness deposited onto the surface of synthetic opal has been studied. Narrow peaks of luminescence in the excitonic spectral range, related to quantum confinement of the electron wave functions, have been observed. The formation of ZnO quantum dots (QD) within the opal voids in the second subsurface layer has been confirmed by atomic force microscopy and by studying the angular dependence of the luminescence spectra.     

Composite system based on CdSe/ZnS quantum dots and GaAs nanowires

The possibility of fabricating a composite system based on colloidal CdSe/ZnS quantum dots and GaAs nanowires is demonstrated and the structural and emission properties of this system are investigated by electron microscopy and photoluminescence spectroscopy techniques. The good wettability and developed surface of the nanowire array lead to an increase in the surface density of quantum dots and, as a consequence, in the luminosity of the system in the 600-nm wavelength region. The photoluminescence spectrum of the quantum dots exhibits good temperature stability in the entire range 10–295 K. The impact of surface states on energy relaxation and the role of exciton states in radiative recombination in the quantum dots are discussed.     

Interaction of CdSe/ZnS quantum dots: Among themselves and with matrices

Photoluminescence (PL) of CdSe/ZnS quantum dots (QDs) deposited on Si, fused silica, Au film, shows red-shift; and blue-shift whenever two peaks are present, particularly on silica nanospheres. The red-shift with increasing density of QDs is attributed to interaction between QDs with PL emerging from the lower bonding state, and the second peak is attributed to molecular complexes on the surface of QD interacting with its surrounding matrices. The second peak is too weak to be detected on Si wafer with native oxide. The couplings between QD/QD and QDs/silica spheres via the molecular complexes are explained with a simple model. We also demonstrate that the band-gap of photonic crystals consisting of silica spheres can be stabilized by dehydration, annealing at high temperatures up to 1000 °C.     

Characteristics of pH sensors fabricated by using protein-mediated CdSe/ZnS quantum dots

The pH sensors using protein-mediated CdSe/ZnS quantum dots in an electrolyte-insulator-semiconductor (EIS) structure have been investigated. The hydrophobic cylindrical cavity of the chaperonin (GroEL) protein template was used to trap CdSe/ZnS quantum dots on hydrophobically treated SiO₂ surface. The CdSe/ZnS quantum dot with a small diameter of 3.98 nm is observed by atomic force microscope. A fair pH response with a sensitivity of 39 mV/pH and a linearity of 99.48% are obtained by using CdSe/ZnS quantum dot based EIS sensor, while those values are found to be 53 mV/pH and 99.95% for bare SiO₂ based EIS pH sensors. The pH response and linearity of CdSe/ZnS based quantum dot sensors are inferior (slightly) as compared to the bare SiO₂ sensors owing to the initial negative charges of CdSe quantum dots membrane, which has been explained by energy band diagrams. It is expected that this kind of quantum dot membrane can be useful in future bio-molecule detections.     

Effect of conjugation with biomolecules on photoluminescence and structural characteristics of CdSe/ZnS quantum dots

The photoluminescence and photoluminescence excitation spectra, the X-ray diffraction patterns, and the effect of conjugation with biomolecules upon these characteristics are studied for silanized CdSe/ZnS quantum dots. Along with the band of annihilating excitons in the quantum dots, the luminescence spectra exhibit emission associated with defects. It is established that the emission spectrum of defects involves at least two components. It is shown that the defects are located mainly at the small-sized quantum dots; the defects responsible for the long-wavelength component are located mainly at the quantum dots larger in size than the quantum dots, at which the defects responsible for the short-wavelength component are located. It is found that conjugation with biomolecules induces not only the blue shift of the excitonic band, but transformation of the emission spectra of defects and an increase in the contribution of defects to the luminescence spectrum as well. The changes observed in the emission spectrum of defects are attributed to the formation of certain emission centers. It is shown that, when conjugated with biomolecules, the quantum dots experience increasing compression strains. This effect is responsible for the blue shift of the luminescence band of the quantum dots.     

核/壳结构ZnS:Mn/ZnS量子点光发射增强研究

利用水溶性前驱体材料在水性介质中制备了ZnS:Mn和ZnS:Mn/ZnS核/壳结构量子点(QDs,quantum dots),并用X射线衍射(XRD)、光致发光(PL)对ZnS:Mn和ZnS:Mn/ZnS核/壳结构QDs的结构和发光性能进行研究.ZnS:Mn和ZnS:Mn/ZnS QDs XRD谱与标准谱吻合,根据De...     

CdSe/ZnSe量子点的合成与荧光特性

采用低温成核生长与一步法相结合的方式合成了CdSe/ZnSe核壳结构量子点,并通过吸收光谱、荧光光谱、X射线衍射等分析手段证明了ZnSe壳层包覆成功.对加入空穴传输材料后CdSe/ZnSe量子点的荧光变化情况进行了深入的研究.稳态光谱结果表明.空穴传输材料对量子点发光有较强的淬灭作用;时间分辨光谱结果显示,随着空穴传输材料分子浓度的增加,量子点的荧光寿命明显缩短,其荧光淬灭过程可以解释为静态淬灭和动态淬灭过程.静态淬灭来源于量子点表面与空穴传输材料间的相互作用;而动态淬灭则来源于量子点到空穴传输材料的空穴转移过程.因此,量子点的壳层结构及空穴传输材料的种类对量子点的荧光淬灭起关键作用.     

ZnS:Eu/ZnS核/壳结构量子点自发辐射性能研究

利用水溶性前驱体材料在水性介质中制备了ZnS:Eu和ZnS:Eu/ZnS核/壳结构量子点,并利用XRD、TEM和PL对ZnS:Eu和ZnS:Eu/ZnS核/壳结构量子点的结构和发光性能进行了研究.ZnS:Eu和ZnS:Eu/ZnS量子点XRD谱显示:ZnS:Eu和ZnS:Eu/ZnS量子点具有β-ZnS结构,且随着Zn...     

Femtosecond optical nonlinearities of CdSe quantum dots

Femtosecond differential absorption measurements of the quantum-confined transitions in CdSe microcrystallites are reported. Spectral hole burning is observed, which is accompanied by an induced absorption feature on the high-energy side. The spectral position of the burned hole depends on the excitation wavelength. For excitation on the low-energy side of the lowest quantum-confined transition, a slight shift of the hole towards the line center is observed. The hole width increases with pump intensity and the magnitude of the induced transparency saturates at the highest excitation level. The results are consistently explained by bleaching of one-pair states and induced absorption caused by the photoexcited two electron-hole pair states. It is concluded that the presence of one electron in the excited state prevents further absorption of photons at the pair-transition energy and accounts for the major portion of the bleaching of the transition     

Effect of CdSe/ZnS quantum dots on color purity and electrical properties of polyfluorene based hybrid light emitting diode

We report enhanced color purity of hybrid organic-inorganic light emitting diode based on polyfluorene-CdSe/ZnS quanum dot (QD) blend as emissive layer. Effect on structural, optical and electrical properties of different doping concentration (0–100 wt.%) of QD in polyfluorene (PFO) was studied. Photoluminescence and electroluminescence spectra confirm the β-formation of PFO by incorporation of CdSe/ZnS QD. Photoluminescence (PL) of blend film was also compared with another method based on one dimensional photonic band gap (1D-PBG) structure that has been used for color purity. In both the cases, that is, QD doped device and 1D-PBG based structures the narrowing of PL spectra was observed. But the fabrication of QD-doped device for color purity is easier than fabricating 1D-PBG structure using multilayer dielectric coating. The present study might find application for QD based color displays, where color purity is an important requirement.     

高荧光效率深红色硒化镉核壳量子点的制备

采用二次注入镉前驱体的方法,以十八烯(ODE)为 溶剂,通过优化前驱体注入速度、摩尔比等实验条件, 抑制量子点(QDs)生长的奥斯瓦尔德熟化过程,制备出粒径7.5n m以上,发光效率在15%左右的硒化镉(CdSe)QDs 核;通过对制备的QDs核进行包覆,形成了CdSe/ZnCdS的核壳结构;获得了单分散性的荧 光波长大于 660nm且发光效率高达43%的深红光QDs。所制备的QDs有望在生物影 像和植物工厂等领域得到应用。     

Electrically pumped lasing from CdSe quantum dots

Fabrication of a ZnSe-based laser diode which employs a fivefold CdSe quantum dot stack separated by ZnSSe spacer layers of high S content is reported. For the first time, electrically pumped room-temperature lasing from such quantum dots was obtained at a wavelength around 560.5 nm. The threshold current density is 7.5 kA/cm ²     

Polaron effect-dependent third-order optical susceptibility in a ZnS/CdSe quantum dot quantum well

The effect of the electron-phonon interaction on the third-harmonic is investigated theoretically for electrons confined in a core-shell quantum dot. The interactions of electrons with different phonon modes in the core-shell system, including the confined longitudinal optical (LO) and the interface optical (IO) phonon modes, are investigated. We carried a detailed calculation of third-harmonic generation (THG) process on a ZnS/CdSe core-shell quantum dot as a function of pump photon energy with different incident photon energy and under different sizes. The results reveal that the polaron effects are quite important especially around the peak value of the third-order susceptibility. By increasing the size of the quantum dots, the peaks of χ_THG⁽³⁾ will shift to lower energy, and the intensities of the peaks will increase.     

Formation and characterization of self-organized CdSe quantum dots

We have investigated the formation and characteristic of self-organized CdSe quantum dots (QDs) on ZnSe(001) surfaces with the use of photoluminescence (PL) and transmission electron microscopy (TEM). Coherent CdSe QDs are naturally formed on ZnSe surfaces, when the thickness of CdSe layers is around 2 ML. The plan-view TEM images exhibit that CdSe QDs have a relatively narrow distribution of QD size, and that the density of CdSe QDs is about 10¹⁰ cm⁻². The base structure of the CdSe dot is rhombic, which has the long axis of about 20 nm in length along direction. The temperature dependence of macro-PL spectra reveals that the behavior of self-organized CdSe QDs is quite different from that of ZnCdSe quantum well (QW), resulting from characteristic features of zero-dimensional structures of QDs. Moreover, the macro-PL results suggest the existence of QW-like continuous state lying over QD states. Micro-PL measurements show several numbers of high-resolved sharp lines from individual CdSe QDs. The linewidth broadening with temperature depends on peak energy position of the QDs. The linewidths of lower energy lines, corresponding to larger size QDs, are more temperature dependent.     

CdSe量子点的制备与荧光特性研究

主要讨论了CdSe量子点的制备及荧光特性。CdSe量子点由化学方法制备,通过选择不同的反应时间得到不同尺度的量子点样品。用荧光方法研究了量子点样品在石英衬底和有机溶剂中的荧光特性。实验表明,这些量子点都有良好的荧光特性。还用无限深球方势阱模型分析了量子点样品的电子态,并根据荧光参数估算了量子点的尺度．各样品荧光峰具有一致的半峰宽,表明CdSe量子点的成核过程在反应开始时同时完成。     

CdSe/ZnS量子点掺杂聚合物光纤放大器增益特性分析

提出了一种半导体量子点CdSe/ZnS掺杂聚合物光纤放大器。测量了CdSe/ZnS量子点吸收和发射光谱,采用二能级结构和速率方程的方法,全面描述了CdSe/ZnS量子点掺杂聚合物光纤放大器的增益性能。计算了放大器增益随量子点掺杂光纤长度、量子点掺杂浓度和信号光强度的变化,给出了不同泵浦光强条件下的增益谱线及半高全宽。结果表明,在mW量级的泵浦条件下,CdSe/ZnS量子点掺杂聚合物光纤放大器可获得35dB以上的增益,获得相同增益所需泵浦光强度只有同类型染料掺杂聚合物光纤放大器的万分之一。泵浦光强与量子点掺杂浓度之间存在最佳对应关系,单位泵浦功率激发的最佳量子点数为6.33×107/mW。在室温下,CdSe/ZnS量子点掺杂聚合物光纤放大器具有550nm～610nm的带宽,含盖了聚合物光纤的低损窗口。     

Exciton enhancement effect on the third harmonic generation in ZnS/CdSe quantum dot quantum well

Exciton enhancement effect on the third-order optical nonlinearities of a ZnS/CdSe quantum dot quantum well (QDQW) has been theoretically studied. The wave functions and eigenenergies of excitons in QDQW have been calculated under the effective-mass approximation. By solving a three-dimensional nonlinear Schrödinger equation and by means of compact density matrix method, the third-order nonlinear susceptibilities for third-harmonic generation (THG) have been calculated in a two energy levels model of QDQW. Firstly, we studied the size effect on THG in QDQW. Then we compared the value of THG with the case that only considering electron states. The results show that the THG is greatly enhanced when compared with the condition just considering electron states.     

Quantum size dependent optical nutation in a core-shell CdSe/ZnS quantum dot

The energy eigenvalues and eigenfunctions have been obtained for a core-shell CdSe/ZnS quantum dot structure under effective-mass approximation. The electric transition dipole moment is calculated for the 1s-2s electronic transition. The optical nutation signal of the transition of electrons has been calculated numerically based on optical Bloch equations. Particularly, we have investigated the quantum size, the core's radius and the shell's thickness, dependent optical nutation. It is shown from calculation results that the optical nutation signal is sensitive to the size and structure change. And the reasons for the variation of the Rabi frequency have been discussed based on the theory of the quantum size confined effect (QSCE).