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.     

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.     

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     

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。     

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).     

Evidence of thermally activated transfer of excited carriers between CdSe/ZnSe quantum dots

Temperature dependent photoluminescence and cathodoluminescence of selfassembled CdSe/ZnSe quantum dots grown by metalorganic vapor phase deposition were investigated. We found an unusual large red shift and a narrowing of the photoluminescence peak with temperature increases. Cathodoluminescence studies of a small number of quantum dots showed that the broad peak observed in the photoluminescence spectra is, in fact, made up of a series of narrower peaks, coming from quantum dots of different sizes. While the intensity of luminescence from small dots drops monotonously with temperature rises, that from the large dots displays a peculiar behavior. It actually increases within the temperature range of 140–170 K, the same range in which the photoluminescence peak shows narrowing. The simultaneous increase of luminescence from some quantum dots and decrease from others are believed to be responsible for the red shift and narrowing of the observed photoluminescence peak. A simple analytically solvable rate equation model was used to understand the spectral data. We suggest that the unusual behaviors observed can be understood as resulting from a transfer of thermally activated carriers from small to large quantum dots.     

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)     

Thermally activated resonance tunneling in asymmetric systems of CdSe/ZnSe double quantum wells with self-assembled quantum dots

In the luminescence study of double quantum wells formed by depositing two CdSe layers with different nominal thicknesses into a ZnSe matrix, a heavy dependence of the photoluminescence spectrum on the thickness of the ZnSe barrier separating the quantum wells, the excitation photon energy, and temperature is observed. The photoluminescence spectra are studied at barrier widths of 34, 50, and 63 monolayers, excitation photon energies of 3.06, 2.71, and 2.54 eV, and temperatures T in the range of 5–200 K. Upon above- (3.06 eV) and below-barrier (2.71 eV) excitation, the photoluminescence spectrum exhibits two bands, I ₁(T) and I ₂(T), corresponding to the annihilation of excitons localized in the quantum dots of the shallow and deep quantum wells. An increase in temperature to ～50 K yields only a slight decrease in the total integrated emission intensity of both bands I _PL(T) and the intensities of each of the two bands, I ₁(T) and I ₂(T). A further increase in temperature results in substantial redistribution of the photoluminescence intensity between the two wells, which is attributed to the tunneling of excitons from the QD (quantum-dot) states of the shallow well to states of the deep well. This process is of the activation character and manifests itself as a sharp decrease in the integrated emission intensity related to the shallow quantum well, I ₁(T), and a simultaneous increase in the integrated emission intensity of quantum dots of the deep quantum well, I ₂(T). The experimentally detected effect is most profound in the range of temperatures T = 110–130 K and in the samples with a barrier thickness of 50 monolayers. It is most likely that the tunneling is of a resonance nature. This inference follows from the fact that the barrier width is much larger than the well widths for both wells, which predetermines only slight penetration of the wave functions into the neighboring well, and the effect of tunneling itself is only slightly supressed, as the barrier thickness is increased. At the same time, the activation energy is at least three time higher that the optical phonon energy, which cannot be explained on the basis of existing theory.     

Injection lasers based on InGaAs quantum dots in an AlGaAs matrix

Arrays of vertically coupled InGaAs quantum dots (QDs) in an AlGaAs matrix have been used in injection lasers. Increase in the band gap of a matrix material by replacement of a GaAs matrix with an AlGaAs one led to dramatic increase in quantum dot localization energy. By using this approach, we reduced the thermal population of the matrix and wetting layer states and thus decreased room temperature threshold current density to 63 A/cm², increased differential efficiency up to 65%, and achieved room temperature continuous wave operation with output power of 1 W. Negative characteristic temperature has been observed in temperature dependence of threshold current density of these lasers in some temperature range. A qualitative explanation assuming a transition from non-equilibrium to Fermi population of QD states is proposed.     

Solution-processed planar white light-emitting diodes based on cadmium-free Cu-In-Zn-S/ZnS quantum dots and polymer

Bright white light-emitting diodes (WLEDs) were fabricated by using a simple solution-processed technique, in which the yellow cadmium-free Cu-In-Zn-S/ZnS core/shell quantum dots (QDs) blending with poly [(9, 9-dioctylfluorenyl-2, 7-diyl)-co-(4, 4′-(N-(p-butylphenyl)) diphenylamine)] (TFB) was used as emissive layers. The color of the electroluminescence (EL) from the device could be tuned from blue-green to white by varying the thermal annealing temperatures, and white EL emission could be obtained under the annealing temperature of 95 °C. A high color rendering index (CRI) of 90 and the Commission Internationale de l'Eclairage (CIE) color coordinates of (0.33, 0.32) were achieved in the WLEDs annealed at 95 °C, respectively. The WLEDs exhibited a low turn-on voltage of 2.5 V and a maximum luminance of 1500 cd/m², which were maintained at 0.1 cd/A over a wide range of luminance from 100 to 1300 cd/m². This work may open up a new way to realize white light in the planar WLEDs based on the cadmium-free QDs.