One- and Two-Photon Excited Fluorescence of CdSe and CdSe/ZnS Quantum Dots in n-Hexane

One- and two-photon absorption and excited fluorescence of the CdSe and the core-shell structure CdSe/ZnS quantum dots （QDs） in n-hexane is investigated. The linear and nonlinear absorption coefficients are measured and the two-photon-absorption cross sections of the QDs are also obtained. For both one-photon fluorescence and two-photon fluorescence, the emission efficiency of CdSe/ZnS is much higher than that of CdSe, originating from the effective surface passivation of the core-shell structure.     

Improved Luminescence Properties and Thermal Stability of ZnS Quantum Dots by Organic and Inorganic Passivation

ZnS quantum dots(QDs) synthesized in water and ethanol solutions were coated with polystyrene (PS) and SiO2 shells,respectively.The band edge emission was enhanced by nearly five times after PS coating and by about thirteen times after SiO2 coating,because the surface trap states were removed.From the photoluminescence properties of ZnS QDs coated with PS and SiO2 shells we have detected the improvement of thermal stability.This is due to the fact that the surface passivation can prevent the further growth of the ZnS QDs and the diffusion of oxygen on the surface of ZnS QDs during thermal oxidation.     

Photoluminescence of ZnSe—ZnS Single Quantum Wells Grown by Vapour Phase Epitaxy

We study the photoluminescence (PL) of ultra thin layer ZnSe quantum Wells in ZnS barriers.Samples with different well widths are grown by vapour phase epitaxy and the PL spectra of these samples are measured by the excitation of a 500W Hg lamp.The peak positions of the bands coming from the excitonic luminescence show a larger blueschift with respect to the energy of free excitons in the ZnSe bulk material.The observed variation of the full width at half maximum and peak position of the bands in the spectra with the well width are interpreted to the formation of the ZnSxSe1-x alloy layer due to the interdiffusion in the interfaces between ZnSe and ZnS.According to the behaviour of the excitons in the smaller conduction band offset,the exciton binding energy is estimated from the dependence of the PL intensity on the temperature.from this result,excitons seem to show nearly three-dimensional characteristics.     

Time-Resolved Photoluminescence Spectroscopy： A Novel Technique for Determination of Luminescence of Quantum Dots

The time-resolved photoluminescence （PL） spectroscopy measured by the gradually increasing start delay time is utilized as a tool for the determination of the luminescence of quantmn dots （QDs）. The luminescence evolution of self-assembled CdSe QDs during the luminescence decay is fully revealed in terms of the experiment technique. The characteristic narrow luminescence lines of self-assembled CdSe QDs are obtained with increasing start delay time.     

Enhanced performance of a solar cell based on a layer-by-layer self-assembled luminescence down-shifting layer of core–shell quantum dots

In this paper, core–shell quantum dots(QDs) with two polar surface functional groups(ZnSe/ZnS–COOH QDs and ZnSe/ZnS–NH_2 QDs) are synthesized in an aqueous phase. Photoluminescence(PL) and absorption spectra clearly indicate luminescence down-shifting(LDS) properties. On the basis of QDs, surface functional group multilayer LDS films(MLDSs) are fabricated through an electrostatic layer-by-layer(LBL) self-assembly method. The PL intensity increases linearly with the number of bilayers, showing a regular and uniform film growth. When the M-LDS is placed on the surface of a Si-based solar cell as an optical conversion layer for the first time, the external quantum efficiency(EQE) and shortcircuit current density(Jsc) notably increases for the LDS process. The EQE response improves in a wavelength region extending from the UV region to the blue region, and its maximum increase reaches more than 15% between 350 nm and 460 nm.     

The effect of annealing on structural,optical and electrical properties of ZnS/porous silicon composites

ZnS films were prepared by pulsed laser deposition (PLD) on porous silicon (PS) substrates. This paper investigates the effect of annealing temperature on the structural, morphological, optical and electrical properties of ZnS/PS composites by x-ray diffraction (XRD), scanning electron microscope (SEM), photoluminescence (PL) and I–V characteristics. It is found that the ZnS films deposited on PS substrates were grown in preferred orientation along β-ZnS (111) direction, and the intensity of diflraction peak increases with increasing annealing temperature, which is attributed to the grain growth and the enhancement of crystallinity of ZnS films. The smooth and uniform surface of the as-prepared ZnS/PS composite becomes rougher through annealing treatment, which is related to grain growth at the higher annealing temperature. With the increase of annealing temperature,the intensity of self-activated luminescence of ZnS increases, while the luminescence intensity of PS decreases, and a new green emission located around 550 nm appeared in the PL spectra of ZnS/PS composites which is ascribed to the defect-center luminescence of ZnS. The I-V characteristics of ZnS/PS heterojunctions exhibited rectifying behavior, and the forward current increases with increasing annealing temperature.     

Effect of annealing on structural, optical and electrical properties of ZnS/porous silicon composites

ZnS films were prepared by pulsed laser deposition (PLD) on porous silicon (PS) substrates. This paper investigates the effect of annealing temperature on the structural, morphological, optical and electrical properties of ZnS/PS composites by x-ray diffraction (XRD), scanning electron microscope (SEM), photoluminescence (PL) and I--V characteristics. It is found that the ZnS films deposited on PS substrates were grown in preferred orientation along β-ZnS (111) direction, and the intensity of diffraction peak increases with increasing annealing temperature, which is attributed to the grain growth and the enhancement of crystallinity of ZnS films. The smooth and uniform surface of the as-prepared ZnS/PS composite becomes rougher through annealing treatment, which is related to grain growth at the higher annealing temperature. With the increase of annealing temperature, the intensity of self-activated luminescence of ZnS increases, while the luminescence intensity of PS decreases, and a new green emission located around 550~nm appeared in the PL spectra of ZnS/PS composites which is ascribed to the defect-center luminescence of ZnS. The I--V characteristics of ZnS/PS heterojunctions exhibited rectifying behavior, and the forward current increases with increasing annealing temperature.     

Structure and Magnetic Properties of （In,Mn）As Based Core-Shell Nanowires Grown on Si（111） by Molecular-Beam Epitaxy

We report the structure and magnetic properties of （In,Mn）As based core-shell nanowires grown on Si （111） by molecular-beam epitaxy. Compared to the core InAs nanowire with a flat side facet and consistent diameter, the core-shell nanowire shows a rough sidewall and an inverse tapered geometry. X-ray diffraction, transmission electron microscopy and energy-dispersive x-ray spectroscopy show that （In,Mn）As is formed on the side facets of In As nanowires with a mixture ofwurtzite and zinc-blende structures. Two ferromagnetic transition temperatures of （In,Mn）As from magnetic measurement data are observed： one is less than 25 K, which could be attributed to the magnetic phase with diluted Mn atoms in the InAs matrix, and the other is at ～300 K, which may originate from the undetectable secondary phases such as MnAs nanoclusters. The synthesis of （In,Mn）As based core-shell nanowires provides valuable information to exploit a new type of spintronic nano-materials.     

Improved carrier transport in Mn:ZnSe quantum dots sensitized La-doped nano-TiO2 thin film

Mn:ZnSe/ZnS/L-Cys core-shell quantum dots(QDs)sensitized La-doped nano-TiO2 thin film(QDSTF)was prepared.X-ray photoelectron spectroscopy(XPS),nanosecond transient photovoltaic(TPV),and steady state surface photovoltaic(SPV)technologies were used for probing the photoelectron behaviors in the Mn-doped QDSTF.The results revealed that the Mn-doped QDSTF had a p-type TPV characteristic.The bottom of the conduction band of the QDs as a sensitizer was just 0.86 eV above that of the La-doped nano-TiO2 thin film,while the acceptor level of the doped Mn2+ions was located at about 0.39 eV below and near the bottom of the conduction band of the QDs.The intensity of the SPV response of the Mn-doped QDSTF at a specific wavelength was ~2.1 times higher than that of the undoped QDSTF.The region of the SPV response of the Mn-doped QDSTF was extended by 191 nm to almost the whole visible region as compared with the undoped QDSTF one.And the region of the TPV response of the Mn-doped QDSTF was also obviously wider than that of the undoped QDSTF.These PV characteristics of the Mn-doped QDSTF may be due to the prolonged lifetime and extended diffusion length of photogenerated free charge carriers injected into the sensitized La-doped nano-TiO2 thin film.     

Uptake of transferrin-conjugated quantum dots in single living cells

We study the uptake and distribution of transferrin （Tf）-conjugated CdSe/CdS/ZnS quantum dots （QDs） in single living HeLa cells with both fluorescence confocal microscopy and three-dimensional （3D） reconstruction technique. By increasing the co-incubation time or the dosage of QDs-Tf, we find that the uptake of QDs-Tf bioconjugates in the cells increases correspondingly, but with different uptake rates. Additionally, the distribution of QDs-Tf, in single live HeLa cells is time dependent. To our knowledge, this is the first study on quantitatively analyzing the uptake and distribution of bioconjugated QDs in single living cells. Such QDs nanoplatform can be further modified for developing biomedical evaluation tool in cancer diagnosis and targeted drug delivery.     

Optoelectronic characterization of ZnS/PS systems

ZnS thin films are deposited on porous silicon （PS） substrates with different porosities by pulsed laser deposition （PLD）. The photoluminescence （PL） spectra of the samples are measured at room temperature. The results show that the PL intensity of PS after deposition of ZnS increases and is associated with a blue shift. With the increase of PS porosity, a green emission at about 550 nm is observed in the PL spectra of ZnS/PS systems, which may be ascribed to the defect-center luminescence of ZnS films. Junction current- voltage （I-V） characteristics were studied. The rectifying behavior of I-V characteristics indicates the formation of ZnS/PS heterojunctions, and the forward current is seen to increase when the PS porosity is increased.     

Effects of annealing temperature on shape transformation and optical properties of germanium quantum dots

The influences of thermal annealing on the structural and optical features of radio frequency(rf) magnetron sputtered self-assembled Ge quantum dots(QDs) on Si(100) are investigated.Preferentially oriented structures of Ge along the(220) and(111) directions together with peak shift and reduced strain(4.9%to 2.7%) due to post-annealing at 650 ℃ are discerned from x-ray differaction(XRD) measurement.Atomic force microscopy(AFM) images for both pre-annealed and post-annealed(650 ℃) samples reveal pyramidal-shaped QDs(density ~ 0.26×10~(11) cm~(-2)) and dome-shape morphologies with relatively high density ~ 0.92×10~(11) cm~(-2),respectively.This shape transformation is attributed to the mechanism of inter-diffusion of Si in Ge interfacial intermixing and strain non-uniformity.The annealing temperature assisted QDs structural evolution is explained using the theory of nucleation and growth kinetics where free energy minimization plays a pivotal role.The observed red-shift ~ 0.05 eV in addition to the narrowing of the photoluminescence peaks results from thermal annealing,and is related to the effect of quantum confinement.Furthermore,the appearance of a blue-violet emission peak is ascribed to the recombination of the localized electrons in the Ge-QDs/SiO_2 or GeO_x and holes in the ground state of Ge dots.Raman spectra of both samples exhibit an intense Ge-Ge optical phonon mode which shifts towards higher frequency compared with those of the bulk counterpart.An experimental Raman profile is fitted to the models of phonon confinement and size distribution combined with phonon confinement to estimate the mean dot sizes.A correlation between thermal annealing and modifications of the structural and optical behavior of Ge QDs is established.Tunable growth of Ge QDs with superior properties suitable for optoelectronic applications is demonstrated.     

Effect of In_xGa_(1-x)As Interlayer on Surface Morphology and Optical Properties of GaSb/InGaAs Type-Ⅱ Quantum Dots Grown on InP (100) Substrates

The effects of indium composition in InGaAs interlayer on morphology of GaSb/InGaAs quantum dots(QDs)and on optical properties of GaSb/fnGaAs QD material system are studied.AFM images show that the change of the indium composition in InGaAs interlayer can alter the GaSb QD morphology.It is found that low indium composition in InGaAs interlayer can promote the formation of QDs,while high indium composition can inhibit the formation of QDs.The photoluminescence(PL) spectra of GaSb/InGaAs QDs at 8K under low excitation power indicate that the third root of the excitation power is linear with the peak position,which provides a direct evidence for their luminescence belonging to type-Ⅱ material optical transition.The PL spectra at 8K under an excitation power of 90 mW show that the optical properties of GaSb/InGaAs QD material system can be affected by the indium composition in the InGaAs interlayer,and the PL peak position is linear with the indium composition.The optical properties of GaSb/InGaAs QDs can be improved by adjusting the indium composition in the InGaAs interlayer.     

Observation of enhanced two-photon absorption in ZnS quantum dots at an ultraviolet wavelength

Two-photon absorption(2PA) in zinc sulphide(ZnS) and Mn2+-doped ZnS quantum dots is reported by the z-scan technique,with nanosecond pulsed laser radiation at 355 nm.The observed values of the 2PA cross section of all the samples are 105 times larger than that of bulk ZnS.     

Photoluminescence from site-selected coupling between quantum dots and microtoroid cavities

By using CdSe/ZnS quantum dots （QDs）, we study the effect of cavity quantum electrodynamics on the coupling of the microtoroid cavity. When with whispering gallery （WG） modes, the microtoroid cavity demonstrates high quality factor and small mode volume at visible wavelengths. Accordingly, fiber tapers allow QDs to adhere into the cavity and further permit the control of site-selected coupling. From the luminescence spectra, QDs are modulated effectively by cavity modes, Variable modulations are observed by changing QD coupling conditions. Therefore, based on experimental and theoretical research, strong and tunable Purcell enhancement can be realized by this system.     

Investigation of europium(Ⅲ)-doped ZnS for immunoassay

Biofunctional europium(Ⅲ)-doped ZnS(ZnS:Eu) nanocrystals are prepared by a sol–gel method. The characteristic luminescence of ZnS:Eu is used as a probe signal to realize sensitive immunoassay. The luminescence intensity of the Eu~(3+) in the ZnS matrix shows strong concentration dependence, and the optimal doping concentration is 4%. However,the emission wavelengths of the ZnS:Eu nanocrystals are not dependent on doping concentration nor the temperature(from 100 K to 300 K). Our results show that these features allow for reliable immunoassay. Human immunoglobulin, used as a target analyte, is captured by antibody modified ZnS:Eu probe and is finally enriched on gold substrate for detection.High specificity of the assay is demonstrated by control experiments. The linear detection range is 10 nM –800 nM, and the detection limit is about 9.6 nM.     

Structural, Optical and Electrical Properties of ZnS/Porous Silicon Heterostructures

ZnS films are deposited by pulsed laser deposition on porous silicon （PS） substrates formed by electrochemical anodization of p-type （100） silicon wafer. Scanning electron microscope images reveal that the surface of ZnS films is unsmoothed, and there are some cracks in the ZnS films due to the roughness of the PS surface. The x-ray diffraction patterns show that the ZnS films on PS surface are grown in preferring orientation along cubic phase β-ZnS （111） direction. White light emission is obtained by combining the blue-green emission from ZnS films with the orange-red emission from PS layers. Based on the I-V characteristic, the ZnS/PS heterojunction exhibits the rectifying junction behaviour, and an ideality factor n is calculated to be 77 from the I-V plot.     

Upconversion properties of Er^3＋/Yb^3＋ co-doped TeO2-Nb2O5-Li2O glasses

The Er^3＋/Yb^3＋ co-doped TeO2-Nb2O5-Li2O glass is prepared by conventional melting method, and its upconversion spectra are measured. The intense green upconversion luminescence upon excitation with a 976 nm laser diode is observed with the naked eyes. The dependence of luminescence intensity on the ratio of Yb^3＋/Er^3＋ is discussed in detail, and the relationship between the ratio of green luminescence intensity to red luminescence intensity and the ratio of Yb^3＋/Er^3＋ is also studied, The luminescence intensity increases with the ratio of Yb^3＋/Er^3＋ increasing. The ratio of Yb^3＋/Er^3＋ plays a more important role than the concentration of Er^3＋ in determining the upconversion luminescence intensity. The ratio of green luminescence intensity to red luminescence intensity reaches a maximum when ratio of Yb^3＋/Er^3＋ is 3. Thus the glass could be one of the potential candidates for LD pumping solid-state lasers.     

Correct Evaluation of the Effect of Transverse Effective Charges on Phonons in AlAs Quantum Dots

An improved valence force field model (VFFM) is suggested to calculate the phonon modes in both bulk specimens and quantum dots (QDs) of AlAs taking account of the effect of transverse effective charges (TOs) correctly.The resultant dispersions of AlAs bulk phonons are in accord better with the results carefully fitted to the experimental data by using 11-parameters rigid-ion model, than those got by ordinary VFFM, especially in the region of near Г point. For AlAs QDs, TCs are evaluated bond by bond for each phonon mode of QD and its effect on the change of the force on atoms is taken into account to modify further the phonon spectrum. The frequency spectra and densities ofphonon states of d/fferent irreducible representations calculated by using improved VFFM are compared with the results of ordinary VFFM. The correct evaluation of the TOs is not only important in calculating the phonon spectrum of both bulk and QD specimens accurately, but is also in the further discussion of the electron-phonon (e-ph) interaction, which can be directly related to TCs of ions in QD.     

The influence of interfacial barrier engineering on the resistance switching of In2O3：SnO2/TiO2/In2O3：SnO2 device

The I-V characteristics of In2O3：SnO2/TiO2/In2O3：SnO2 junctions with different interracial barriers are inves- tigated by comparing experiments. A two-step resistance switching process is found for samples with two interfacial barriers produced by specific thermal treatment on the interfaces. The nonsynchronous occurrence of conducting filament formation through the oxide bulk and the reduction in the interracial barrier due to the migration of oxygen vacancies under the electric field is supposed to explain the two-step resistive switching process. The unique switching properties of the device, based on interracial barrier engineering, could be exploited for novel applications in nonvolatile memory devices.