Blue-shifted Emission in CdTe/ZnSe Heterostructured Nanocrystals

Properties of colloidal heterostructured nanocrystals are largely determined by the spatial distribution of photogenerated carriers across the junction of semiconductor materials that form the heterostructure. The two known types of carrier distributions are identified based on whether both carriers reside within the same (type I) or opposite (type II) sides of the heterojunction. Here we demonstrate the existence of another type of spatial carrier distribution in heteronanocrystals, which corresponds to the localization of both charges along the material junction. Such localization. pattern was realized in novel CdTe/ZnSe heteronanocrystals, where the expected type I infrared emission was dominated by more intense photoluminescence in the 570-600 nm range, corresponding to the recombination of carriers within an interfacial alloy layer, formed by the cation and anion exchange between CdTe and ZnSe phases. Fabricated heteronanocrystals exhibit excellent optical characteristics including near-single-exponential lifetimes, enhanced emission stability, and fluorescence emission quantum yields of up to 24%

Composition-Tunable Properties Of CdSxTe1-x Alloy Nanocrystals

Ternary CdSxTe1-x semiconductor quantum dots with both homogeneous and gradient composition have been fabricated via pyrolysis of organometallic precursors. The nanocrystal structure, size, and composition were characterized by UV-visible absorption and fluorescence spectroscopy, transmission electron microscopy, energy-dispersive X-ray elemental analysis, and X-ray diffractrometry. It was found that the band gap of homogeneously alloyed CdSxTe1-x is highly nonlinear with the crystalline composition, which was evidenced by a significant red-shift in the fluorescence of these nanocrystals with respect to the emission wavelength of their CdS and CdTe binary compounds. This effect, also known as optical bowing, seems to be enhanced in CdSxTe1-x nanocrystals because of large differences in atomic radii and electronegativities of S and Te chalcogens. Properties of gradient ternary alloys were found to be markedly different from those of homogeneous CdSxTe1-x. Their absorption and emission profiles, for instance, had a relatively low spectral overlap leading to large Stokes shifts of up to 150 nm. Other properties of fabricated CdSxTe1-x, nanocrystals and their significance to applications in areas of biomedical imaging, solar cells, and quantum dot-based LEDs are discussed

Synthesis And Characterization Of Type II ZnSe/CdS Core/Shell Nanocrystals

High-quality ZnSe/CdS core/shell nanocrystals, exhibiting a type II carrier localization regime, were fabricated via a traditional pyrolysis of organometallic precursors. The two-step synthesis involved fabrication of 4.5-6 nm ZnSe seeds followed by a subsequent deposition of the CdS shell. An efficient spatial separation of electrons and holes between the core and the shell was observed for heterostructures containing more than three monolayers of CdS, which was primarily evidenced by the spatially indirect emission tunable from 480 to 610 nm for a fixed core diameter. Because of a large (type II) offset of band edges at the core/shell interface, fabricated nanocrystals exhibited a relatively low spectral overlap between emission and absorption profiles, with associated Stokes shifts of up to 110 nm. The quantum yield of as-prepared samples was 12-18% and was further improved to 20% after purification of nanocrystals through multiple hexane/methanol extractions. Novel properties of synthesized ZnSe/CdS nanocrystals as well as their applicability to practical realizations in areas of biomedical imaging, solar sells, and quantum dot-based lasers are discussed

Tuning the Morphology of Au/CdS Nanocomposites through Temperature-Controlled Reduction of Gold-Oleate Complexes

A general synthetic strategy for controlling the shape of gold domains grown onto CdS semiconductor nanocrystals is presented. The colloidal growth of Au nanoparticles is based on the temperature-controlled reduction of Au-oleate complexes on the surface of CdS and allows for precise tuning of nanoparticle diameters from 2.5 to 16 nm simply by adjusting the temperature of the growth solution, whereas the shape of Au/CdS nanocomposites can be controllably switched between matchsticks and barbells via the reaction rate. Depending on the exact morphology of Au and CdS domains, fabricated nanocomposites can undergo evaporation-induced self-assembly on a substrate either through end-to-end coupling of Au domains, resulting in the formation of one-dimensional chains or via side-by-side packing of CdS nanorods, leading to the onset of two-dimensional superlattices

Synthesis Of ZnSe/CdS/ZnSe Nanobarbells Showing Photoinduced Charge Separation

We report on it colloidal synthesis of barbell-shaped nanocrystals comprising a type II hetero-junction of ZnSe and US domains and showing compelling evidence of photoinduced charge separation at the interface of ZnSe and US materials. The nanobarbells were fabricated in a two-step procedure by growing ZnSe caps onto polar facets of US nanorods. Under present synthetic conditions, minimal growth of ZnSe shell in the lateral direction and focusing of the barbell length distribution were observed. Formation of epitaxial interfaces between nearly lattice-matched ZnSe and US crystal phases was primarily evidenced by the observation of spatially indirect fluorescence and long radiative lifetimes corresponding to the decay of charge transfer states

Ultrafast Carrier Dynamics In Type II ZNSE/CDS/ZNSE Nanobarbells

We employ femtosecond transient absorption spectroscopy to get an insight into ultrafast processes occurring at the interface of type II ZnSe/CdS heterostructured nimocrystals fabricated via colloidal routes and comprising a barbell-like arrangement of ZnSe tips and CdS nanorods. Our study shows that resonant excitation of ZnSe tips results in an unprecedently fast transfer of excited Electrons into CdS domains of nanobarbells (\u3c0.35 ps), whereas selective pumping of CdS components loads to a relatively slow injection of photoinduced holes into ZnSe tips (tau(h) = 95 ps). A qualitative thermodynamic description of observed electron processes within the classical limit of Marcus theory was used to identify a specific charge transfer regime associated with the ultrafast electron injection into CdS. Potential photocatalytic applications of the observed fast separation of carriers along the main axis of ZnSe/CdS barbells are discussed

Suppression Of The Plasmon Resonance In Au/cds Colloidal Nanocomposites

The nature of exciton-plasmon interactions in Au-tipped CdS nanorods has been investigated using femtosecond transient absorption spectroscopy. The study demonstrates that the key optoelectronic properties of composite heterostructures comprising electrically coupled metal and semiconductor domains are substantially different from those observed in systems with weak interdomain coupling. In particular, strongly coupled nanocomposites promote mixing of electronic states at semiconductor-metal domain interfaces, which causes a significant suppression of both plasmon and exciton excitations of carriers

Radiative Recombination Of Spatially Extended Excitons In (znse/cds)/cds Heterostructured Nanorods

We report on organometallic synthesis of luminescent (ZnSe/CdS)/CdS semiconductor heterostructured nanorods (hetero-NRs) that produce an efficient spatial separation of carriers along the main axis of the structure (type II carrier localization). Nanorods were fabricated using a seeded-type approach by nucleating the growth of 20-100 nm CdS extensions at [000 +/- 1] facets of wurtzite ZnSe/CdS core/shell nanocrystals. The difference in growth rates of CdS in each of the two directions ensures that the position of ZnSe/CdS seeds in the final structure is offset from the center of hetero-NRs, resulting in a spatially asymmetric distribution of carrier wave functions along the heterostructure. Present work demonstrates a number of unique properties of (ZnSe/CdS)/CdS hetero-NRs, including enhanced magnitude of quantum confined Stark effect and subnanosecond switching of absorption energies that can find practical applications in electroabsorption switches and ultrasensitive charge detectors

The Effect of Dielectric Friction on the Rate of Charge Separation in Type Ii Znse/Cds Semiconductor Nanorods

The effect of dielectric friction on the rate of charge separation in type II ZnSe/CdS semiconductor nanorods has been investigated using picosecond transient absorption spectroscopy. The spatial separation of an excited electron-hole pair was estimated from the redshift in band edge absorption corresponding to the decrease in the exciton binding energy. The present study identifies a considerable effect of the solvent polarity on the rate of charge separation in semiconductor heterostructures, which should be taken into account when selecting nanorod caging media, such as solvents or polymer matrices. © 2009 American Institute of Physics

Linker-Free Modification Of TiO2 Nanorods With PbSe Nanocrystals

We report on a colloidal synthesis of Pbse/TiO2 heterostructures, comprising small-diameter PbSe nanocrystals epitaxially grown onto the surface of TiO2 nanorods. The deposition of lead selenide onto prefabricated TiO2 nanocrystals proceeds via formation of a thin PbSe shell that subsequently breaks into sub-2-nm islands. Additional precursor injections are then used to increase the size of PbSe nanocrystals up to 5 nm. In the case of small-size PbSe, a 2.1-ns transfer of photoinduced carriers into TiO2 domain was evidenced through quenching of the PbSe band gap emission. Overall, the present synthesis demonstrates a colloidal approach to all-inorganic modification of TiO2 surfaces with semiconductor nanocrystals, which provides a viable alternative to a more common supramolecular assembly of nanocrystal-oxide composites