Size-dependent Luminescence Research Articles

Vapor-Phase Synthesis and Surface Passivation of ZnSe Nanocrystals

The vapor-phase synthesis of II–VI nanocrystals (quantum dots) has several advantages over traditional liquid-phase techniques, including better compatibility with existing operations in the microelectronics industry. On the other hand, it makes size control and surface passivation a more challenging task. In this paper we report the synthesis and surface passivation of luminescent ZnSe nanocrystals by reacting vapors of dimethylzinc:triethylamine (DMZ–TEA) adduct with hydrogen selenide gas (both diluted in hydrogen) in a counterflow jet reactor operating at 300 K and low pressures (120--60 Torr). The two reactants flow into a cylindrical chamber from separate inlets of a vertical opposed-flow configuration. Homogeneous nucleation of ZnSe occurs through an irreversible reaction between the precursors. The nuclei subsequently grow by surface reactions and cluster--luster coalescence to form nanocrystals that exhibit size-dependent luminescence. Surface passivation was achieved by introducing vapors of 1-pentanethiol into the reactor. The thiol vapors do not interfere with the nucleation and growth of the primary nanocrystals, but significantly change the morphology and size of the final coagulated particles collected on transmission electron microscopy (TEM) grids. The chemisorption of thiol vapors on the surface of the nanocrystals also leads to smaller degradation of the photoluminescence (PL) intensity after 1 month. Structural and morphological characterization of the particles was performed using HR-TEM, stoichiometric analysis using electron diffraction and Auger electron spectroscopy (AES), while PL and Raman spectroscopy were used to study their size-dependent optical properties.

Synthesis and Size Control of Luminescent ZnSe Nanocrystals by a Microemulsion−Gas Contacting Technique

A scalable method for controlled synthesis of luminescent compound semiconductor nanocrystals (quantum dots) using microemulsion-gas contacting at room temperature is reported. The technique exploits the dispersed phase of a microemulsion to form numerous identical nanoreactors. ZnSe quantum dots were synthesized by reacting hydrogen selenide gas with diethylzinc dissolved in the heptane nanodroplets of a microemulsion formed by self-assembly of a poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) amphiphilic block copolymer in formamide. A single nanocrystal is grown in each nanodroplet, thus allowing good control of particle size by manipulation of the initial diethylzinc concentration in the heptane. The ZnSe nanocrystals exhibit size-dependent luminescence and excellent photostability.

Size-correlated spectroscopy and imaging of rare-earth-doped nanocrystals.

Isolated europium-doped metal-oxide nanoparticles were probed by size-correlated high-numerical-aperture (far-field) imaging techniques. A modified Digital Instruments Bioscope atomic force microscope mounted upon a Nikon TE300 inverted microscope was used to interrogate (dry) particles ranging in size from 2 to 150 nm on the surface of a glass or quartz coverslip. These experiments revealed several interesting features of doped-nanoparticle luminescence such as Poissonian occupation statistics, size-dependent luminescence efficiency enhancement for particle sizes of <10 nm, and correlation of interesting transient behavior at particle sizes of <5 nm.

Quantum structures in SiC

Two types of nanostructures in SiC matrices are studied. Heterocrystalline structures are based on different polytypes, e.g. 3C-SiC and 4H-SiC. Multi-quantum well structures with 3C wells embedded in 4H barriers show photoluminescence (PL) that can be interpreted in terms of a type-II heterostructure character and a built-in electric field due to the pyroelectricity of 4H. Germanium is incorporated in 4H-SiC matrices in form of nanometer-sized crystals. Their size, crystal structure and orientation depend on the preparation conditions. A strong size-dependent luminescence from quantum states is predicted.

Synthesis and Size Control of Luminescent II-VI Semiconductor Nanocrystals by a Novel Microemulsion-Gas Contacting Technique

ABSTRACTA scalable, room-temperature technique for controlled synthesis of luminescent II-VI nanocrystals has been developed by using the dispersed phase of stable, well-characterized microemulsions as templates for nanoparticle synthesis. The microemulsions were formed by self-assembly of poly (ethylene oxide)-poly (propylene oxide)-poly (ethylene oxide) (PEO-PPO-PEO) amphiphilic block copolymer and heptane in formamide. By adjusting the surfactant to dispersed phase ratio, stable microemulsions were obtained with droplet diameter of ∼40nm. These microemulsions avoid problems or rapid droplet-droplet coalescence that hamper reverse micelles and lead to polydisperse particle populations. Luminescent ZnSe quantum dots were synthesized by reacting diethylzinc (dissolved in the heptane dispersed phase) with hydrogen selenide gas (diluted in hydrogen). The gas was bubbled through the microemulsion, dissolved in the formamide, and diffused to the nanodroplet interfaces to react with diethylzinc. The experiments indicate that a single nanocrystal is formed in each nanodroplet by coalescence of clusters (nuclei) and smaller crystals. The energy released during coalescence is sufficient to anneal the clusters into high-quality crystals. The process allows precise control of nanocrystal size by adjusting the initial concentration of diethylzinc in heptane. The “as grown” nanocrystals exhibit size-dependent luminescence, narrow and symmetric emission, good monodispersity (confirmed by TEM analysis), and excellent photochemical stability. The technique is currently being extended to the synthesis of CdSe nanocrystals with promising preliminary results.

Visible photoluminescence and quantum confinement effects in amorphous Si/SiO 2 multilayer structures

We have prepared amorphous silicon (a-Si)-based multilayer structures by an electron beam deposition technique and studied their luminescence properties. The structure of a-Si/SiO 2 multilayers was studied by transmission electron microscopy and X-ray diffraction analysis. With a decrease of the a-Si well thickness, the PL peak energy shifts to higher energy. The temperature dependence of the PL spectrum suggests that the blueshift of the mobility edge occurs and the size-dependent visible luminescence is ascribed to radiative recombination of carriers localized in the shallow tail state. The quantum confinement effects of localized carriers will be discussed.

Bioconjugation of Highly Luminescent Colloidal CdSe-ZnS Quantum Dots with an Engineered Two-Domain Recombinant Protein

We present a novel approach, based on molecular self-assembly driven by electrostatic attractions, for conjugating inorganic colloidal semiconductor nanocrystals (quantum dots: QDs) having negatively charged surfaces with a two-domain recombinant protein bearing a positively charged C-terminal leucine zipper domain. Aggregation-free QD/protein conjugate dispersions were prepared. Conjugates retain both properties of the starting materials, i.e., biological activity of the protein and spectroscopic characteristics of the QDs. Such hybrid bio-inorganic conjugates represent a powerful fluorescent tracking tool, because they combine advantages of CdSe–ZnS quantum dots, such as chemical stability and a wide range of size-dependent luminescence emission properties, with a straightforward electrostatic conjugation approach. We describe the design and preparation of a model QD/protein conjugate and present functional characterization of the conjugate using luminescence and bioassays.

Size effects on the luminescence spectrum in amorphous Si/SiO2 multilayer structures

We have studied size effects on the photoluminescence (PL) spectrum in amorphous silicon (a-Si)-based multilayer structures. At low temperatures, the PL spectrum and the PL lifetime are sensitive to the a-Si well thickness in a-Si/SiO2 multilayers. With a decrease of the a-Si well thickness, the PL peak energy and the mobility edge shift to higher energy. The temperature dependence of the PL spectrum shows that the size-dependent visible luminescence is ascribed to radiative recombination of carriers localized in the band-tail state, rather than the band-edge emission in quantum-confined states. The quantum confinement and localization of carriers in the band-tail states will be discussed.

Photoluminescence of granular CdSe films

We have produced granular films with a high density of CdSe clusters by a deposition method using a cluster beam. The photoluminescence spectra of those granular films were measured over the temperature range of 80–300 K in order to explore quantum size effects and interactions between clusters. The results were compared with photoluminescence of a thin film which was made by a conventional vacuum evaporation method. It has been found that the observed blue-shifted broad photoluminescence band of the granular film cannot be explained simply by the calculation using the effective mass approximation derived by Kayanuma with taking into account the size distribution of CdSe clusters in the film. The discrepancy suggests that the luminescence yield may depend strongly on the cluster size. The surface states and connectivity of clusters which may influence the interaction between clusters in such a granular film should be also considered as well as the size-dependent luminescence yield to explain the temperature dependence of peak energy of the photoluminescence.

Size-dependent luminescence of GaAs quantum wires on vicinal GaAs (110) surfaces with giant steps formed by MBE

Two size-dependent photoluminescence peaks are observed for GaAs quantum wires naturally formed on vicinal GaAs(1 1 0) surfaces with giant steps by molecular beam epitaxy. These two peaks are found to have different activation energies from the temperature dependence of these peaks. Monochromatic CL images shows the formation of two group of quantum wires with different size. Excitation intensity dependence of relative intensities of these peaks indicates no thermalization between these two group of wires.

X-Ray Absorption Spectroscopy from H-Passivated Porous Si and Oxidized Si Nanocrystals

AbstractQuantum confinement in nanoscale Si structures is widely believed to be responsible for the visible luminescence observed from anodically etched porous silicon (por-Si), but little is known about the actual size or shape of these structures. Extended x-ray absorption fine structure data from a wide variety of por-Si samples show significantly reduced average Si coordination numbers due to the sizable contribution of surface-coordinated H. (The H/Si ratios, as large as 1.2, were independently confirmed by ir-absorption and α-recoil measurements.) The Si coordinations imply very large surface/volume ratios, enabling the average Si structures to be identified as crystalline particles (not wires) whose dimensions are typically &lt;15 Å. Comparison of the size-dependent peak luminescence energies with those of oxidized Si nanocrystals, whose shapes are known, shows remarkable agreement. Furthermore, near-edge x-ray absorption fine structure measurements of the nanocrystals shows the outer oxide and interfacial suboxide layers to be constant over a wide range of nanocrystal sizes. The combination of these results effectively rules out surface species as being responsible for the observed visible luminescence in por-Si, and strongly supports quantum confinement as the dominant mechanism occurring in Si particles which are substantially smaller than previously reported or proposed.