Effects of surface passivation on the exciton dynamics of CdSe nanocrystals as observed by ultrafast fluorescence upconversion spectroscopy

The exciton dynamics of CdSe nanocrystals are intimately linked to the surface morphology. Photo-oxidation of the selenium surfaces of the nanocrystal leads to an increase in radiative decay efficiency from both the band edge and deep trap emission states. The addition of the primary amine hexadecylamine curtails nonradiative excitonic decay attributed to the dangling surface selenium orbitals by passivation of those trap sites by the methylene protons on the amine, leading to enhanced band edge emission and the absence of deep trap emission. Furthermore, CdSeZnSe core/shell nanocrystals are not immune from contributions from surface states because of the alignment of the band structures of the core and shell materials.     

Magnetic quantum dots: synthesis, spectroscopy, and magnetism of Co2+ - and Ni2+-doped ZnO nanocrystals

We report a method for the preparation of colloidal ZnO-diluted magnetic semiconductor quantum dots (DMS-QDs) by alkaline-activated hydrolysis and condensation of zinc acetate solutions in dimethyl sulfoxide (DMSO). Mechanistic studies reveal that Co(2+) and Ni(2+) dopants inhibit nucleation and growth of ZnO nanocrystals. In particular, dopants are quantitatively excluded from the critical nuclei but are incorporated nearly isotropically during subsequent growth of the nanocrystals. The smaller nanocrystal diameters that result upon doping are explained by the Gibbs-Thompson relationship between lattice strain and crystal solubility. We describe methods for cleaning the nanocrystal surfaces of exposed dopants and for redispersion of the final DMS-QDs. Homogeneous substitutional doping is verified by high-resolution low-temperature electronic absorption and magnetic circular dichroism (MCD) spectroscopies. A "giant Zeeman effect" is observed in the band gap transition of Co(2+):ZnO DMS-QDs. MCD and Zeeman spectroscopies are used to quantify the magnitude of the p-d exchange interaction (N(0)beta) that gives rise to this effect. N(0)beta values of -2.3 +/- 0.3 eV (-18 500 cm(-1)) for Co(2+):ZnO and -4.5 +/- 0.6 eV (-36 300 cm(-1)) for Ni(2+):ZnO have been determined. Ligand-to-metal charge-transfer transitions are observed in the MCD spectra of both Co(2+):ZnO and Ni(2+):ZnO DMS-QDs and are analyzed in the context of an optical electronegativity model. The importance of these charge-transfer states in determining N(0)beta is discussed. Ferromagnetism with T(C) > 350 K is observed in aggregated nanocrystals of Co(2+):ZnO that unambiguously demonstrates the existence of intrinsic high-T(C) ferromagnetism in this class of DMSs.     

Exciton-plasmon interaction in a composite metal-insulator-semiconductor nanowire system

We report about the synthesis and optical properties of a composite metal-insulator-semiconductor nanowire system which consists of a wet-chemically grown silver wire core surrounded by a SiO2 shell of controlled thickness, followed by an outer shell of highly luminescent CdSe nanocrystals. With microphotoluminescence (micro-PL) experiments, we studied the exciton-plasmon interaction in individual nanowires and analyzed the spatially resolved nanocrystal emission for different nanowire length, SiO2-shell thickness, nanocrystal shape, pump power, and emission polarization. For an SiO2 spacer thickness of approximately 15 nm, we observed an efficient excitation of surface plasmons by excitonic emission of CdSe nanocrystals. For nanowire lengths up to approximately 10 microm, the composite metal-insulator-semiconductor nanowires ((Ag)SiO2)CdSe act as a waveguide for 1D-surface plasmons at optical frequencies with efficient photon outcoupling at the nanowire tips, which is promising for efficient exciton-plasmon-photon conversion and surface plasmon guiding on a submicron scale in the visible spectral range.     

One-step synthesis of highly water-soluble magnetite colloidal nanocrystals

A high-temperature solution-phase hydrolysis approach has been developed for the synthesis of colloidal magnetite nanocrystals with well-controlled size and size distribution, high crystallinity, and high water solubility. The synthesis was accomplished by the hydrolysis and reduction of iron(III) cations in diethylene glycol with a rapidly injected solution of sodium hydroxide at an elevated temperature. The high reaction temperature allows for control over size and size distribution and yields highly crystalline products. The superior water solubility is achieved by using a polyelectrolyte, that is, poly(acrylic acid) as the capping agent, the carboxylate groups of which partially bind to the nanocrystal surface and partially extend into the surrounding water. The direct synthesis of water-soluble nanocrystals eliminates the need for additional surface modification steps which are usually required for treating hydrophobic nanocrystals produced in nonpolar solvents through the widely recognized pyrolysis route. The abundant carboxylate groups on the nanocrystal surface allow further modifications, such as bioconjugation, as demonstrated by linking cysteamine to the particle surface. The monodisperse, highly water-soluble, superparamagnetic, and biocompatible magnetite nanocrystals should find immediate important biomedical applications.     

CdS纳米晶与多肽分子相互作用研究

研究了半导体CdS纳米晶的表面功能化及荧光光谱特性, 并利用静电/配位自组装方法实现了多肽和CdS纳米晶的生物无机偶联, 研究了纳米晶多肽偶联体系的荧光光谱以及多肽与CdS纳米晶之间的相互作用. 结果表明: 含巯基多肽对CdS纳米晶表面形成完善包覆, 消除CdS纳米晶表面缺陷, 使CdS纳米晶荧光增强; 含端氨基多肽使CdS纳米晶荧光出现先升后降趋势; 其余不含巯基和氨基的多肽均猝灭CdS纳米晶荧光, 猝灭机制属于形成化合物所引起的静态猝灭, 它们的结合常数约为2×10⁴, 结合位点数约为0.87～1.00.     

Direct growth of shape-controlled nanocrystals on nanotubes via biological recognition

The new biological approach was examined to fabricate shape-controlled Ag nanocrystals grown directly on surfaces, inspired by nature that various shapes of nanocrystals are produced accurately and reproducibly in biological systems. Here we demonstrate the direct growth of hexagon-shaped Ag nanocrystals on sequenced peptide-coated nanotubes via biological recognition. When the peptide, Asn-Pro-Ser-Ser-Leu-Phe-Arg-Tyr-Leu-Pro-Ser-Asp, recognizing and effecting the Ag nanocrystal growth on the (111) face, was sequenced and incorporated onto template nanotube surfaces, the biomineralization of Ag ions on the nanotubes led the isotropic hexagon-shaped Ag nanocrystal coating under pH control of the growth solution. Multiple Ag nanocrystal shapes were observed when the peptide mineralized Ag ions without the template nanotubes, and therefore the template nanotube has a significant influence on regulating the majority of Ag nanocrystals into the hexagonal shape. This biological approach, using specific peptide sequences on surfaces to control nanocrystal shapes, may be developed as a simple and economical method to produce building blocks with desired physical properties for new generation of electronics, sensors, and optical devices.     

Colloidal transition-metal-doped ZnO quantum dots

Methods for introducing new magnetic, optical, electronic, photophysical, or photochemical properties to semiconductor nanocrystals are attracting intense applications-oriented interest. In this communication, we report the preparation and electronic absorption spectroscopy of colloidal ZnO DMS-QDs. Our synthetic procedure involves modification of literature methods known to yield highly crystalline and relatively monodisperse nanocrystals of pure ZnO to allow introduction of transition-metal dopants. We use ligand-field electronic absorption spectroscopy as a dopant-specific optical probe to monitor dopant incorporation during nanocrystal growth and to verify internal substitutional doping in Co2+:ZnO and Ni2+:ZnO DMS-QDs. To the best of our knowledge, these are the first free-standing oxide DMS-QDs reported. The synthesis of colloidal oxide DMS-QDs introduces a new category of magnetic semiconductor materials available for detailed physical study and application in nanotechnology.     

Energy transfer dynamics of nanocrystal-polymer composites

Steady-state and time-resolved photoluminescence spectroscopy are used to examine the photoluminescent properties of nanocrystal-polymer composites consisting of colloidal PbS nanocrystals blended with poly(2-methoxy-5(2-ethylhexyloxy)-p-phenylene vinylene). Quenching of the emission from the conjugated polymer due to the PbS nanocrystals is observed along with band edge emission from the ligand capped PbS nanocrystals. A decrease in the photoluminescence lifetime of MEH-PPV is also observed in the thin film nanocrystal-polymer composite materials. Photoluminescence excitation spectroscopy of the PbS nanocrystal emission from the composite shows features attributed to MEH-PPV providing evidence of a F?rster transfer process.     

Synthesis of colloidal Mn2+:ZnO quantum dots and high-TC ferromagnetic nanocrystalline thin films

We report the synthesis of colloidal Mn(2+)-doped ZnO (Mn(2+):ZnO) quantum dots and the preparation of room-temperature ferromagnetic nanocrystalline thin films. Mn(2+):ZnO nanocrystals were prepared by a hydrolysis and condensation reaction in DMSO under atmospheric conditions. Synthesis was monitored by electronic absorption and electron paramagnetic resonance (EPR) spectroscopies. Zn(OAc)(2) was found to strongly inhibit oxidation of Mn(2+) by O(2), allowing the synthesis of Mn(2+):ZnO to be performed aerobically. Mn(2+) ions were removed from the surfaces of as-prepared nanocrystals using dodecylamine to yield high-quality internally doped Mn(2+):ZnO colloids of nearly spherical shape and uniform diameter (6.1 +/- 0.7 nm). Simulations of the highly resolved X- and Q-band nanocrystal EPR spectra, combined with quantitative analysis of magnetic susceptibilities, confirmed that the manganese is substitutionally incorporated into the ZnO nanocrystals as Mn(2+) with very homogeneous speciation, differing from bulk Mn(2+):ZnO only in the magnitude of D-strain. Robust ferromagnetism was observed in spin-coated thin films of the nanocrystals, with 300 K saturation moments as large as 1.35 micro(B)/Mn(2+) and T(C) > 350 K. A distinct ferromagnetic resonance signal was observed in the EPR spectra of the ferromagnetic films. The occurrence of ferromagnetism in Mn(2+):ZnO and its dependence on synthetic variables are discussed in the context of these and previous theoretical and experimental results.     

Colloidal gallium indium oxide nanocrystals: a multifunctional light-emitting phosphor broadly tunable by alloy composition

We demonstrate compositionally tunable photoluminescence in complex transparent conducting oxide nanocrystals. Alloyed gallium indium oxide (GIO) nanocrystals with variable crystal structures are prepared by a colloidal method throughout the full composition range and studied by different structural and spectroscopic methods, including photoluminescence and X-ray absorption. The structures and sizes of the GIO nanocrystals can be simultaneously controlled, owing to the difference in the growth kinetics of In(2)O(3) and Ga(2)O(3) nanocrystals and the polymorphic nature of both materials. Using the synthesized nanocrystal series, we demonstrate the structural and compositional dependences of the photoluminescence of GIO nanocrystals. These dependences, induced by the interactions between specific defect sites acting as electron donors and acceptors, are used to achieve broad emission tunability in the visible spectral range at room temperature. The nature of the photoluminescence is identified as donor-acceptor pair recombination and changes with increasing indium content owing to the changes in the energy states of, and interactions between, donors and acceptors. Structural analysis of GIO nanocrystals by extended X-ray absorption fine structure spectroscopy reveals that In(3+) occupies only octahedral, rather than tetrahedral, sites in the spinel-type γ-Ga(2)O(3) nanocrystal host lattice, until reaching the substitutional incorporation limit of ca. 25%. The emission decay dynamics is also strongly influenced by the nanocrystal structure and composition. The oxygen vacancy defects, responsible for the observed photoluminescence properties, are also implicated in other functional properties, particularly conductivity, enabling the application of colloidal GIO nanocrystals as integrated optoelectronic materials.     

A simple solvothermal route towards the morphological control of ZnO and tuning of its optical and photocatalytic properties

ZnO nanoparticles with different morphologies were solvothermally synthesized by controlling the alkali (sodium hydroxide) concentration in an isopropanol solution. The products were characterized by means of powder X-ray diffraction, UV-visible absorption spectra, scanning electron microscopy, transmission electron microscopy, and selected area electron diffraction. The morphologies of the formed ZnO nanocrystals were dependent on the concentration of the alkali, and with increases of sodium hydroxide concentration, the ZnO nanocrystals evolved from rod to hexagonal bipyramid, and then to a flower-like nanostructure. The flower-like nanostructure resulted from the etching of the hexagonal bipyramid by the excess alkali. The photoluminescence and photocatalytic properties of the prepared ZnO were investigated. The difference of green emission among the ZnO nanocrystals indicated that a higher sodium hydroxide concentration led to a higher level of defects. The size, the surface structure and defects in the ZnO nanocrystals affected its photo-degradation characteristics.     

Organic Excitonic State Management by Surface Metallic Coupling of Inorganic Lanthanide Nanocrystals

The ability to harness charges and spins for control of organic excitonic states is critical in developing high-performance organic luminophores and optoelectronic devices. Here we report a facile strategy to efficiently manipulate the electronic energy states of various organic phosphors by coupling them with inorganic lanthanide nanocrystals. We show that the metallic atoms exposed on the nanocrystal surface can introduce strong coupling effects to 9-(4-ethoxy-6-phenyl-1,3,5-triazin-2-yl)-9H-carbazole (OCzT) and some organic chromophores with carbazole functional groups when the organics are approaching the nanocrystals. This unconventional organic–inorganic hybridization enables a nearly 100 % conversion of the singlet excitation to fast charge transfer luminescence that does not exist in pristine organics, which broadens the utility of organic phosphors in hybrid systems.     

Localized surface plasmon resonances of anisotropic semiconductor nanocrystals

We demonstrate that anisotropic semiconductor nanocrystals display localized surface plasmon resonances that are dependent on the nanocrystal shape and cover a broad spectral region in the near-IR wavelengths. In-plane and out-of-plane dipolar resonances were observed for colloidal dispersions of Cu(2-x)S nanodisks, and the wavelengths of these resonances are in good agreement with calculations carried out in the electrostatic limit. The wavelength, line shape, and relative intensities of these plasmon bands can be tuned during the synthetic process by controlling the geometric aspect ratio of the disk or using a postsynthetic thermal-processing step to increase the free carrier densities.     

Synthesis of hybrid CdS-Au colloidal nanostructures

We explore the growth mechanism of gold nanocrystals onto preformed cadmium sulfide nanorods to form hybrid metal nanocrystal/semiconductor nanorod colloids. By manipulating the growth conditions, it is possible to obtain nanostructures exhibiting Au nanocrystal growth at only one nanorod tip, at both tips, or at multiple locations along the nanorod surface. Under anaerobic conditions, Au growth occurs only at one tip of the nanorods, producing asymmetric structures. In contrast, the presence of oxygen and trace amounts of water during the reaction promotes etching of the nanorod surface, providing additional sites for metal deposition. Three growth stages are observed when Au growth is performed under air: (1) Au nanocrystal formation at both nanorod tips, (2) growth onto defect sites on the nanorod surface, and finally (3) a ripening process in which one nanocrystal tip grows at the expense of the other particles present on the nanorod. Analysis of the hybrid nanostructures by high-resolution TEM shows that there is no preferred orientation between the Au nanocrystal and the CdS nanorod, indicating that growth is nonepitaxial. The optical signatures of the nanocrystals and the nanorods (i.e., the surface plasmon and first exciton transition peaks, respectively) are spectrally distinct, allowing the different stages of the growth process to be easily monitored. The initial CdS nanorods exhibit band gap and trap state emission, both of which are quenched during Au growth.     

Colloidal HgTe nanocrystals with widely tunable narrow band gap energies: from telecommunications to molecular vibrations

A convenient, aqueous-based synthesis of stable HgTe nanocrystals with widely size-tunable room temperature emission between wavelengths of 1.2 to 3.7 mum is demonstrated. By the choice of the thiols, applied as stabilizers, we optimized the growth dynamics, the luminescence quantum yields (up to 40%), and a ligand-exchange procedure, required to transfer the nanocrystals from water to nonpolar organic solvents. The latter is greatly improved and facilitated by the use of mercaptoethylamine as initial stabilizer. The possibility to tune the HgTe nanocrystal sizes from 3 to 12 nm and to control their surface functionalities (hydrophobic and hydrophilic) makes them very promising for the development of infrared optical devices, emitting in the wavelength region between the telecommunications and the molecular vibrations.     

The growth kinetics of colloidal ZnO nanoparticles in alcohols

We have studied the synthesis of ZnO nanostructures over a wide range of parameters to determine the kinetics of the nanocrystals growth. The initial rapid nucleation and growth is kinetically controlled, the subsequent ZnO nanocrystals growth is thermodynamically controlled through the diffusion limited Ostwald coarsening. The ZnO coarsening rates increased with number of alcohol’s alkyl group carbons and temperature increase, pointing to importance of the solvent viscosity, dielectric constants, surface energy and the bulk solubility. The results are consistent with the Lifshitz–Slyozov–Wagner model. For all alcohols, in the NaOH induced reaction, a lower activation energy was observed compared to the aqueous reaction. A lower ZnO solubility, obtained by the water synthesis could be responsible for these observations. Our results point to the importance of the reactant selection in controlling the kinetics of the nanostructure formation, their size and the nature of the surface defects responsible for their luminescence.     

Study of the Growth of Capped ZnO Nanocrystals: A Route to Rational Synthesis

We report the study of complex and unexpected dependencies of nanocrystal size as well as nanocrystal‐size distribution on various reaction parameters in the synthesis of ZnO nanocrystals using poly(vinyl pyrollidone) (PVP) as a capping agent. This method establishes a qualitatively different growth mechanism to the anticipated Ostwald ripening behavior. The study of size‐distribution kinetics and an understanding of the observed non‐monotonic behaviors provides a route to rational synthesis. We used a simple, but accurate, approach to estimate the size‐distribution function of nanocrystals from the UV‐absorption spectrum. Our results demonstrate the accuracy and generality of this approach, and we also illustrate its application to various semiconducting nanocrystals, such as ZnO, ZnS, and CdSe, over a wide size range (1.8–5.3 nm).     

Temperature programmed desorption analysis of sol-gel-derived titania films

Sol-gel-derived titania films were analyzed by temperature programmed desorption (TPD) and X-ray diffraction (XRD) techniques. The relationship between the TPD curves measured for two types of titania gel films and their crystal structures was investigated. On the basis of the analyses, a preparation process for a titania sol solution containing anatase nanocrystals was designed and developed. Using this process, a colloidal anatase titania sol solution was prepared by heating aqueous titanium hydroxide containing HCl at 60°C for 2 h. The nanocrystal structure of the titania films obtained by coating the sol on glass substrates was confirmed by TPD and XRD measurements.     

Photoluminescence brightening via electrochemical trap passivation in ZnSe and Mn(2+)-doped ZnSe quantum dots

Spectroelectrochemical experiments on wide-gap semiconductor nanocrystals (ZnSe and Mn(2+)-doped ZnSe) have allowed the influence of trap electrochemistry on nanocrystal photoluminescence to be examined in the absence of semiconductor band filling. Large photoluminescence electrobrightening is observed in both materials upon application of a reducing potential and is reversed upon return to the equilibrium potential. Electrobrightening is correlated with the transfer of electrons into nanocrystal films, implicating reductive passivation of midgap surface electron traps. Analysis indicates that the electrobrightening magnitude is determined by competition between electron trapping and photoluminescence (ZnSe) or energy transfer (Mn(2+)-doped ZnSe) dynamics within the excitonic excited state, and that electron trapping is extremely fast (k(trap) ≈ 10(11) s(-1)). These results shed new light on the complex surface chemistries of semiconductor nanocrystals.     

Luminescence in colloidal Mn-doped semiconductor nanocrystals

Recent advances in nanocrystal doping chemistries have substantially broadened the variety of photophysical properties that can be observed in colloidal Mn²⁺-doped semiconductor nanocrystals. A brief overview is provided, focusing on Mn²⁺-doped II–VI semiconductor nanocrystals prepared by direct chemical synthesis and capped with coordinating surface ligands. These Mn²⁺-doped semiconductor nanocrystals are organized into three major groups according to the location of various Mn²⁺-related excited states relative to the energy gap of the host semiconductor nanocrystals. The positioning of these excited states gives rise to three distinct relaxation scenarios following photoexcitation. A brief outlook on future research directions is provided.