Exciton storage by Mn(2+) in colloidal Mn(2+)-doped CdSe quantum dots

Colloidal Mn (2+)-doped CdSe quantum dots showing long excitonic photoluminescence decay times of up to tau exc = 15 mus at temperatures over 100 K are described. These decay times exceed those of undoped CdSe quantum dots by approximately 10 (3) and are shown to arise from the creation of excitons by back energy transfer from excited Mn (2+) dopant ions. A kinetic model describing thermal equilibrium between Mn (2+ 4)T 1 and CdSe excitonic excited states reproduces the experimental observations and reveals that, for some quantum dots, excitons can emit with near unity probability despite being approximately 100 meV above the Mn (2+ 4)T 1 state. The effect of Mn (2+) doping on CdSe quantum dot luminescence at high temperatures is thus completely opposite from that at low temperatures described previously.     

Co2+掺杂水溶性ZnS量子点的制备及其光致发光性能

采用共沉淀法,以3-巯基丙酸为表面修饰剂,成功制备出Co2+掺杂水溶性ZnS量子点。采用X射线衍射仪、透射电子显微镜、原子发射光谱仪、紫外-可见吸收光谱仪和荧光分光光度计等,研究了Co2+掺杂剂及掺杂量对ZnS量子点的晶体结构、形貌和发光性能等的影响。结果表明:所得产物均为ZnS立方型闪锌矿结构,量子点呈不规则球形,粒径主要集中在5.2 nm左右;掺杂样品发红色荧光,发光性能明显增强,属于Co2+形成的杂质能级(4A1—4T1)与缺陷的复合发光。同时,利用红外吸收光谱对Co2+掺杂水溶性ZnS量子点的形成机理进行了初步探讨。     

Spin-polarizable excitonic luminescence in colloidal Mn2+-doped CdSe quantum dots

The photoluminescence of colloidal Mn2+-doped CdSe nanocrystals has been studied as a function of nanocrystal diameter. These nanocrystals are shown to be unique among colloidal doped semiconductor nanocrystals reported to date in that quantum confinement allows tuning of the CdSe bandgap energy across the Mn2+ excited-state energies. At small diameters, the nanocrystal photoluminescence is dominated by Mn 2+ emission. At large diameters, CdSe excitonic photoluminescence dominates. The latter scenario has allowed spin-polarized excitonic photoluminescence to be observed in colloidal doped semiconductor nanocrystals for the first time.     

Sub-picosecond spin relaxation of bright excitons and imbalance suppression in asymmetric Cdse/Zns nanocrystal quantum dots under an applied magnetic field

The ultrafast spin dynamics of the bright exciton in CdSe/ZnS nanocrystal quantum dots has been investigated using a circularly polarized pump-probe experiment. A remarkably fast spin flip (-500 fs) of the bright exciton was observed at 4 K, which is attributed to the anisotropic electron-hole exchange interaction and the random positioning of nanocrystal quantum dots. In the presence of an applied magnetic field (5 T), the exciton spin parallel to the external magnetic field was favored due to Zeeman splitting. We found that this imbalance can possibly be suppressed by the state-blocking and the mixing of the 1(L) and 1(U) states in asymmetric quantum dots.     

Energetic pinning of magnetic impurity levels in quantum-confined semiconductors

Donor- and acceptor-type (D/A) impurities play central roles in controlling the physical properties of semiconductors. With continued miniaturization of information processing devices, the relationship between quantum confinement and D/A ionization energies becomes increasingly important. Here, we provide direct spectroscopic evidence that impurity D/A levels in doped semiconductor nanostructures are energetically pinned, resulting in variations in D/A binding energies with increasing quantum confinement. Using magnetic circular dichroism spectroscopy, the donor binding energies of Co2+ ions in colloidal ZnSe quantum dots have been measured as a function of quantum confinement and analyzed in conjunction with ab initio density functional theory calculations. The resulting experimental demonstration of pinned impurity levels in quantum dots has far-reaching implications for physical phenomena involving impurity-carrier interactions in doped semiconductor nanostructures, including in the emerging field of semiconductor spintronics where magnetic-dopant-carrier exchange interactions define the functionally relevant properties of diluted magnetic semiconductors.     

Growth and Spectroscopy of CdSe: Mn Quantum Dots

In this paper we report the growth and optical properties of ZnSe/CdSe:Mn magnetic quantum dots by Atomic Layer Epitaxy. For the uncapped samples, dot densities of the order of 10⁹ cm^–2 were measured by Atomic Force Microscopy. The ensemble dot photoluminescence (PL) was observed over a range of energies between 2.1 and 2.5 eV, and a spectrally broad emission at 2.15 eV from the internal Mn²⁺ transition was observed at high Mn concentrations. Single dot spectroscopy was carried out by confocal microscopy and the PL line width was measured as a function of Mn concentration. For large Mn contents the temporal change in magnetization causes a broadening of the single dot PL line of up to 4 meV FWHM. However, for low concentrations the single dot PL line widths were resolution limited at <0.2 meV.     

石墨烯量子点/CdS/CdSe共敏化太阳能电池

以三维锐钛矿TiO₂微球为上层光散射层材料, 以商业纳米TiO₂为下层连接材料, 采用刮刀法制备了一种新颖的双层TiO₂薄膜, 并应用于量子点敏化太阳能电池(QDSSC)。其中, 石墨烯量子点(GQDs)采用滴液法引入, CdS/CdSe量子点采用连续离子层吸附法(SILAR)制备。采用场发射扫描电镜、透射电镜、X射线衍射、紫外-可见漫反射光谱及荧光光谱对样品进行表征。实验还制备了CdS/CdSe量子点敏化及石墨烯量子点/CdS/CdSe共敏化太阳能电池, 并研究了石墨烯量子点及CdS不同敏化周期及对电池性能影响。研究结果表明, 石墨烯量子点及CdS不同敏化周期对薄膜的光学性质、电子传输及载流子复合均有较大影响。优选条件下, TiO₂/QGDs/CdS(4)/CdSe电池的光电转换效率为1.24%, 光电流密度为9.47 mA/cm², 显著高于TiO₂/CdS(4)/CdSe电池的这些参数(0.59%与6.22 mA/cm²)。这主要是由于TiO₂表层吸附石墨烯量子点后增强了电子的传输, 减少了载流子的复合。     

Shake and break--shaking-induced changes of size quantization in aggregated quantum dots

We show how simple mechanical agitation of precipitated CdSe quantum dot aggregates causes partially reversible color changes (clearly visible to the eye) in the absorption spectrum of the CdSe (about 4 nm size). The color changes, which are due to changes in size quantization, are not accompanied by change in quantum dot size. This phenomenon is explained by partial deaggregation of the precipitates, leading to reduced charge overlap between neighboring dots. Shaking was shown to result in a looser aggregate structure. It is suggested that CdSO3 particles (an initial product of the CdSe formation reaction) act as weak bridges between CdSe quantum dots, mediating the interparticle interactions and allowing the deaggregation to occur on shaking.     

Multicolor quantum dot encoding for polymeric particle-based optical ion sensors

Multicolor quantum dot-encoded polymeric microspheres are prepared with controllable and uniform doping levels that function as chemical sensors on the basis of bulk optode theory. TOP/TOPO-capped CdSe quantum dots and CdTe quantum dots capped with CdS (lambdaem = 610 and 700 nm, lambdaex = 510 nm) are blended with a THF solution of poly(methyl methacrylate-co-decyl methacrylate), poly(n-butylacrylate), or poly(vinyl chloride) plasticized with bis(2-ethylhexyl) sebacate without a need for ligand exchange. Polymeric microspheres are generated under mild, nonreactive conditions with a particle caster that breaks down a polymer stream containing the quantum dots into fine droplets by the vibration of a piezocrystal. The resulting microspheres exhibit uniform size and fluorescence emission intensities. Fluorescent bar codes are obtained by subsequent doping of two quantum dots with different colors and mass ratios into the microspheres. A linear relationship is found between the readout fluorescence ratio of the two types of nanocrystals and the mixing ratio. Quantum dot-encoded ion sensing optode microspheres are prepared by simultaneous doping of sodium ionophore X, chromoionophore II, a lipophilic tetraphenylborate cation exchanger, and TOPO-capped CdSe/CdS quantum dot as the fluorescent label. A net positive charge of the quantum dots is found to induce an anion-exchange effect on the sensor function, and therefore, an increased concentration of the lipophilic cation exchanger is required to achieve proper ion sensing properties. The modified quantum dot-labeled sodium sensing microspheres show satisfactory sodium response between 10(-4) and 0.1 M at pH 4.8, with excellent selectivity toward common interferences. The amount of the carried positive charges of the CdSe quantum dots is estimated as 2.8 mumol/g of quantum dots used in this study.     

Properties of electrospun CdS and CdSe filled poly(methyl methacrylate) (PMMA) nanofibres

Electrospinning technique was used to fabricate poly(methyl methacrylate) (PMMA) fibres incorporating CdS and CdSe quantum dots (nanoparticles). Different nanoparticle loadings (2, 5 and 10 wt% with respect to PMMA) were used and the effect of the quantum dots on the properties of the fibres was studied. The optical properties of the hybrid composite fibres were investigated by photoluminescence and UV-vis spectrophotometry. Scanning electron microscopy (SEM), X-ray diffraction and FTIR spectrophotometry were also used to investigate the morphology and structure of the fibres. The optical studies showed that the size-tunable optical properties can be achieved in the polymer fibres by addition of quantum dots. SEM images showed that the morphologies of the fibres were dependent on the added amounts of quantum dots. A spiral type of morphology was observed with an increase in the concentration of CdS and CdSe nanoparticles. Less beaded structures and bigger diameter fibres were obtained at higher quantum dot concentrations. X-ray diffractometry detected the amorphous peaks of the polymer and even after the quantum dots were added and the FTIR analysis shows that there was no considerable interaction between the quantum dots and the polymer fibres at low concentration of quantum dots however at higher concentrations some interactions were observed which shows that QDs were present on the surfaces of the fibres.     

Luminescent quantum dots fluorescence resonance energy transfer-based probes for enzymatic activity and enzyme inhibitors

The paper describes the development and characterization of analytical properties of quantum dot-based probes for enzymatic activity and for screening enzyme inhibitors. The luminescent probes are based on fluorescence resonance energy transfer (FRET) between luminescent quantum dots that serve as donors and rhodamine acceptors that are immobilized to the surface of the quantum dots through peptide linkers. Peptide-coated CdSe/ZnS quantum dots were prepared using a one-step ligand exchange process in which RGDC peptide molecules replace trioctylphosphine oxide (TOPO) molecules as the capping ligands of the quantum dots. The peptide molecules were bound to the surface of the CdSe/ZnS quantum dots through the thiol group of the peptide cysteine residue. The peptide-coated quantum dots were labeled with rhodamine to form the FRET probes. The emission quantum yield of the quantum dot FRET probes was 4-fold lower than the emission quantum yield of TOPO-capped quantum dots. However, the quantum dot FRET probes were sufficiently bright to enable quantitative enzyme and enzyme inhibition assays. The probes were used first to test the enzymatic activity of trypsin in solution based on FRET signal changes of the quantum dot-based enzymatic probes in the presence of proteolytic enzymes. For example, exposure of the quantum dot FRET probes to 500 microg/mL trypsin for 15 min resulted in 60% increase in the photoluminescence of the quantum dots and a corresponding decrease in the emission of the rhodamine molecules. These changes resulted from the release of rhodamine molecules from the surface of the quantum dots due to enzymatic cleavage of the peptide molecules. The quantum dot FRET-based probes were used to monitor the enzymatic activity of trypsin and to screen trypsin inhibitors for their inhibition efficiency.     

Pressure dependence of Mn2+ fluorescence in MnS/ZnS core-shell quantum dots

The high pressure photoluminescence spectra of MnS/ZnS core-shells quantum dots were measured using a diamond anvil cell up to 9.4 GPa. Orange emission at 590 nm from the 4T1 --> 6A1 transition of Mn2+ ions was observed. The Mn2+ emission shifted to red with increasing pressure. The experimental pressure coefficient was -48.3 meV/GPa, which is agreement with the calculated value based on the crystal field theory. The redshift is attributed to the increase of crystal field strength and decrease of Racah parameters during compression.     

CdSe量子点的制备及荧光性能改善

主要讨论了CdSe量子点的制备及荧光性能的改善.采用水相合成方法制备了CdSe量子点,并用X射线粉末衍射仪对所合成的量子点进行表征,用荧光分光光度计研究了量子点的荧光性质.结果表明,采用样品处理温度的调节和ZnS壳层的包覆能在一定程度上改善CdSe量子点的荧光性能.     

Colloidal synthesis of ultrathin two-dimensional semiconductor nanocrystals

2D semiconductor quantum wells have been recognized as potential candidates for various quantum devices. In quantum wells, electrons and holes are spatially confined within a finite thickness and freely move in 2D space. Much effort has focused on shape control of colloidal semiconductor nanocrystals(NCs), and synthesis of 2D colloidal NCs has been achieved very recently. Here, recent advances in colloidal synthesis of uniform and ultrathin 2D CdSeNCs are highlighted. Structural and optical property characterization of these quantum-sized 2D CdSe NCs is discussed. Additionally, 2D CdSe NCs doped with Mn 2+ ions for dilute magnetic semiconductors (DMS) are presented.These 2D CdSe-based NCs can be used as model systems for studying quantum-well structures.     

不同温度下硒化镉(CdSe)量子点的生长及荧光性研究

研究了以氧化镉（CdO）和硒（Se）粉为前驱体,在三辛基膦（TOP）和油酸中合成无机半导体量子点（quantum dots, QDs）CdSe.研究了在不同的反应温度下粒子的生长,通过紫外吸收光谱（UV-Vis）、荧光发射光谱（PL）、透射电子显微镜（TEM）等手段跟踪反应过程并对样品性能进行了表征.实验结果表明,反应温度和反应时间对量子点的生长和荧光性能有很大的影响.     

Cd^2＋参量对于CdSe／CdS／ZnS量子点荧光特性的影响

测量了CdSe、CdSe／CdS／ZnS量子点的吸收光谱和发射光谱,讨论了两种不同量子点的光谱特性。改变核层内Cd^2＋浓度以及壳层内Cd^2＋／Zn^2＋比例,分别测量多组CdSe／CdS／ZnS量子点发射光谱并计算量子产率,结果表明,Cd^2＋浓度45mmol／L或者Cd^2＋／Zn^2＋比例为1／2时,CdSe／CdS／ZnS量子产率最高,从而确定最佳的Cd^2＋定量参数。     

Exchange Interactions and Ferromagnetism of Pr_(n+1)Co_(3n+5)B_(2n)-type Compounds

In present investigation exchange interactions of Pr_(n+1)Co_(3n+5)B_(2n)-type compounds have been evalu-ated in the light of molecular-field theory. The exchange interactions and ferromagnetism in thesecompounds are discussed in terms of lattice parameters and interatomic distance between Co atom.     

Studies of interaction of amines with TOPO/TOP capped CdSe quantum dots: Role of crystallite size and oxidation potential

This work reports the interaction of aliphatic (triethyl amine, butyl amine) and aromatic amines (PPD, aniline) with CdSe quantum dots of varied sizes. The emission properties and lifetime values of CdSe quantum dots were found to be dependent on the oxidation potential of amines and crystallite sizes. Smaller CdSe quantum dots (size 5 nm) ensure better surface coverage of amines and hence higher quenching efficiency of amines could be realized as compared to larger CdSe quantum dots (size 14 nm). Heterogeneous quenching of amines due to the presence of accessible and inaccessible set of CdSe fluorophores is indicated. PPD owing to its lowest oxidation potential (0.26 V) has been found to have higher quenching efficiency as compared to other amines TEA and aniline having oxidation potentials 0.66 and >1.0 V, respectively. Butyl amine on the other hand, plays a dual role: its post-addition acts as a quencher for smaller and enhances emission for larger CdSe quantum dots, respectively. The beneficial effect of butyl amine in enhancing emission intensity could be attributed to enhance capping effect and better passivation of surface-traps.     

Quantum oscillations in magnetically doped colloidal nanocrystals

Progress in the synthesis of colloidal quantum dots has recently provided access to entirely new forms of diluted magnetic semiconductors, some of which may find use in quantum computation. The usefulness of a spin qubit is defined by its Rabi frequency, which determines the operation time, and its coherence time, which sets the error correction window. However, the spin dynamics of magnetic impurity ions in colloidal doped quantum dots remain entirely unexplored. Here, we use pulsed electron paramagnetic resonance spectroscopy to demonstrate long spin coherence times of ～0.9 μs in colloidal ZnO quantum dots containing the paramagnetic dopant Mn(2+), as well as Rabi oscillations with frequencies ranging between 2 and 20 MHz depending on microwave power. We also observe electron spin echo envelope modulations of the Mn(2+) signal due to hyperfine coupling with protons outside the quantum dots, a situation unique to the colloidal form of quantum dots, and not observed to date.     

One-step and rapid synthesis of high quality alloyed quantum dots (CdSe-CdS) in aqueous phase by microwave irradiation with controllable temperature

In this paper, we presented a seed-mediated approach for rapid synthesis of high quality alloyed quantum dots (CdSe-CdS) in aqueous phase by microwave irradiation with controllable temperature in 1 h. In the synthesis, CdSe seeds were first formed by the reaction of NaHSe and Cd²⁺, and then alloyed quantum dots (CdSe-CdS) were rapidly produced by releasing of sulfide ions from 3-mercaptopropionic acid as sulfide source with microwave irradiation. The alloyed quantum dots synthesized had good optical properties, the quantum yield was up to 25%, and the full width at half maximum of the emission spectrum peak was about 28 nm. The as-prepared alloyed CdSe-CdS QDs were characterized by XRD, XPS and ICP-AES in order to explore the structure and component of the alloyed nanocrystals and the reaction mechanism. We speculate that the alloyed CdSe-CdS quantum dots may exist a gradient internal structure according to our preliminary results.