Key Scientific Results of Research and Development Consortium of NanoFluorescent Materials Ltd. Scientific Board

1. Fedorov A.V., Baranov A.V., Masumoto Y. (2002) Coherent control of the fundamental transition in a single quantum dot. - Solid State Commun., 124, 311-315.

A method of coherent control of the resonant spontaneous optical phonon assisted secondary emission has been proposed for the study of exciton dynamics in a single semiconductor quantum dot (QD). A theoretical analysis of the time-integrated emission excited by a couple of phase-locked pulses of light resonant to fundamental transition of QD has shown that a dependence of the signal envelope function on the mutual delay between the pulses is directly determined by the dephasing rate of the transition.

2. Baranov A.V., Davydov V.G., Fedorov A.V., Ren H.W., Sugou S., Masumoto Y. (2002) Interferometric coherency measurement of stress-induced InGaAs/GaAs quantum dots at the resonant luminescence phonon sideband. - Phys. Rev. B, 66, 075326-(1-7).

Dephasing of the lowest-energy electronic transition of In0.1Ga0.9As/GaAs stress-induced quantum dots in inhomogeneously broadened system has been investigated by the interferometric double pulse excitation and time-integrated detection of optical-phonon sideband in their resonant-photoluminescence spectra. It was found that combination of the narrow phonon resonance and spectral filtering of the signals allow us to determine the dephasing time in inhomogeneously broadened ensemble of quantum dots. The dephasing time of about 100 ps at 2 K shows that a pure dephasing is still essential in the studied system, most likely, as a result of coupling of electronic excitations in quantum dots with optical phonons of surroundings via deformation potential.

3. Baranov A.V., Fedorov A.V., Rukhlenko I.D., Masumoto Y. (2003) Intraband carrier relaxation in quantum dots embedded in doped heterostructures. - Phys. Rev. B, 68, 205318-(1-7).

The effect of bulk plasmon-LO-phonon excitations inherent to doped areas of semiconductor heterostructures upon the electronic dynamics of quantum dots spaced apart from the areas is studied. An effective mechanism of intraband carrier relaxation in quantum dots is proposed for such heterostructures. The mechanism involves interaction between the quantum dot carriers and the electric potential induced by bulk plasmon-LO-phonon modes of the doped areas. It is shown that the interaction opens two relaxation windows with the spectral positions and widths controlled, correspondingly, by the free-carrier concentrations and the bulk plasmon-LO-phonon mode dispersion of the doped areas. The relaxation rates related to the mechanism are calculated for quantum dots spaced apart by different distances z₀ from doped substrate ~the area! with the different free-carrier concentrations n0. The estimations carried out for InAs quantum dots and the GaAs substrate yield the relaxation rates of about 10⁸ s^-1 for n₀=10¹⁸ cm^-3 and z₀=100 nm. The rates increase by two to three orders of magnitude with decreasing the distance down to 20 nm. A manifestation of the mechanism is shown for a system of self-assembled InAs/GaAs quantum dots separated by a distance of 100 nm from an n-doped GaAs substrate by means of the photoluminescence spectroscopy. From the experiment it has been found that L^- and L⁺ plasmon-LO-phonon modes of the GaAs substrate are involved in the intraband carrier relaxation in the quantum dots. The data are in good agreement with the theoretical predictions.

4. Artemyev M.V., Möller B., Woggon U. (2003) Unidirectional alignment of CdSe nanorods. - Nano Letters, 3, 509-512.

Cadmium selenide nanorods, unique nanostructures which possess bright and up to 100% polarized photoluminescence, now can be arranged unidirectionally in a single line. The method is based on the chemical attachment of nanorods to the surface of a single cleaved semiconductor nanolayer and exploits a combination of wet-chemical and epitaxial growth techniques to produce templated semiconductor nanostructures. The nanotemplate here is a ZnS layer with a thickness less than the diameter of the nanorods. Binding the nanorods to the template, the limited space provokes an enhanced alignment of the rods along the ZnS layer which can be proved by the high polarization degree of the nanorods emission.

5. Fedorov A.V., Baranov A.V., Rukhlenko I.D., Masumoto Y. (2003) New many-body mechanism of intraband carrier relaxation in quantum dots embedded to doped heterostructures. - Solid State Commun., 128, 219-223.

The role of longitudinal surface collective excitations of doped elements of heterostructure in electronic dynamics of quantum dots was studied theoretically. A new mechanism of intraband quantum dot carrier relaxation is proposed where an interaction between the quantum dot carriers and electric potential induced by charge density fluctuations in the element with frequencies of the surface plasmon-LO-phonon modes is responsible for the process. It has been found that two branches of the surface plasmon-LO-phonon modes of the doped element are effectively involved in the relaxation even in the case when a distance between the element and the quantum dots is relatively long (50-150 nm). The calculations show that relaxation rates about 10¹² s^-1 are expected at the distance of 100 nm rising by an order of magnitude at a distance of 30 nm.

6. Baranov A., Rakovich Yu., Donegan J., Perova T., Moore R., Talapin D., Rogach A., Masumoto Y., Nabiev I. (2003) Effect of ZnS shell thickness on the phonon spectra in CdSe quantum dots. -Phys. Rev. B, 68, 165306-(1-7).

The evolution of the optical phonon spectra of colloidal core/shell CdSe/ZnS quantum dots with an increase of the shell thickness from 0.5 to 3.4 monolayers has been studied by resonant Raman spectroscopy. The results obtained suggest that the ZnS shell changes its structure from amorphous to partly crystalline as the thickness increases. Simultaneously, an increase in Raman scattering by surface (core/shell interface) phonons and the redshift of the corresponding phonon band have been observed and assigned to variations in the shell structure. The disorder present in the shell provides a major contribution to the line shape of the Raman spectra at higher ZnS coverage. A method to control the quality of quantum dots based on Raman spectroscopy is proposed.

7. Wargnier R., Baranov A., Maslov V., Stsiapura V., Sukhanova A., Pluot M., Nabiev I. (2004) Energy transfer in aqueous solutions of oppositely charged nanoassemblies of CdSe/ZnS core/shell quantum dots and in quantum dot-nanogold systems. - Nano Lett., 4, 451-457.

We demonstrate that the nanoscale assemblies of water-solubilized, oppositely charged CdSe/ZnS core/shell quantum dots and of quantum dots and nanogold particles make it possible to develop FRET-based sensors with a donor quenching efficiency close to 100%. The cw and time-resolved photoluminescence analysis of nanoassemblies with different donor-to-acceptor ratios demonstrates a prevalence of 1:1 complexes. Experimental data perfectly fit the mathematical simulation based on long-range FRET and a static model of the assembling of the oppositely charged nanoparticles. These assemblies are stable in biological fluids, suggesting new applications.

8. Sukhanova A., Devy J., Venteo L., Kaplan H., Artemyev A., Oleinikov V., Klinov D., Pluot M., Cohen J.H.M., Nabiev I. (2004) Biocompatible fluorescent nanocrystals for immunolabeling of membrane proteins and cells. - Anal Biochem., 324, 60-67.

A methodology for simple convenient preparation of bright, negatively or positively charged, water-soluble CdSe/ZnS core/shell nanocrystals (NCs) and their stabilization in aqueous solution is described. Single NCs can be detected using a standard epifluorescent microscope, ensuring a detection limit of one molecule coupled with an NC. NCs solubilized in water by DL-Cys were stabilized, to avoid aggregation, by poly(allylamine) and conjugated with polyclonal anti-mouse antibodies (Abs). NC-Abs conjugates were tested in dot-blots and exhibited retention of binding capacity within several nanograms of antigen detected. We further demonstrated the advantages of NC-Abs conjugates in the immunofluorescent detection and three-dimensional (3D) confocal analysis of p-glycoprotein (p-gp), one of the main mediators of the MDR phenotype, overexpressed in the membrane of MCF7r breast adenocarcinoma cells. Immunolabeling of p-gp with NC-Abs conjugates was 4200-, 2600-, and 420-fold more resistant to photobleaching than its labeling with fluorescein isothiocyanate-Abs, R-phycoerythrin-Abs, and AlexaFluor488-Abs, respectively. The labeling of p-gp with NC-Abs conjugates was highly specific, and the data were used for confocal reconstruction of 3D images of the p-gp distribution in the MCF7r cell membrane. Finally, we demonstrated the applicability of NC-Abs conjugates obtained by the method described to specific detection of antigens in paraffin-embedded formaldehyde-fixed cancer tissue specimens, using immunostaining of cytokeratin in skin basal carcinoma as an example. We conclude that the NC-Abs conjugates may serve as easy-to-do, highly sensitive, photostable labels for immunofluorescent analysis, immunohistochemical detection, and 3D confocal studies of membrane proteins and cells.

9. Stsiapura V., Sukhanova A., Artemyev A., Pluot M., Cohen J.H.M., Nabiev I. (2004) Functionalized nanocrystal-tagged fluorescent polymer beads : synthesis, physicochemical characterization and immunolabeling application. - Analyt. Biochem., 334, 257-265.

A methodology for incorporating solubilized CdSe/ZnS core/shell nanocrystals (NCs) into functionalized carboxylated polystyrene latexes 0.3-1 µm in diameter via a swelling procedure was developed and used for the production of homogeneous, highly fluorescent polymeric beads (HFPBs), which were found to be comparable in brightness to standard polymeric microspheres doped with organic fluorophores and more photostable than the latter by more than 50 times (Fluoresbrite yellow-orange microspheres were used as an example). The three-dimensional (3D) confocal analysis of individual 1-µm HFPB demonstrated that the beads were doped with the NCs almost homogeneously. HFPBs 0.3 µm in diameter were conjugated with anti-mouse polyvalent immunoglobulins and used for immunofluorescent detection of p-glycoprotein, a mediator of the multidrug resistance phenotype, overexpressed in the membrane of MCF7r breast adenocarcinoma cells. The photostability of NCs-tagged HFPBs offers obvious advantages for the reconstruction of 3D confocal fluorescence images of antigen distribution, and their exceptionally high brightness combined with photostability permits the detection of a single antigen molecule using a standard epifluorescence microscope.

10. Artemyev M.V., Kisiel D., Abmiotko S., Antipina M.N., Khomutov G.B., Kislov V.V., Rakhnyanskaya A.A. (2004) Self-organized, highly luminescent CdSe nanorod-DNA complexes. - J. Am. Chem. Soc., 126, 10594-10597.

DNA molecules are useful building blocks and nanotemplates for controllable fabrication of various bioinorganic nanostructures due to their unique physical-chemical properties and recognition capabilities and the synthetic availability of desired nucleotide sequences and length. We have synthesized novel DNA complexes with positively charged, highly luminescent CdSe nanorods that can be self-organized into filamentary, netlike, or spheroidal nanostructures. DNA-CdSe-nanorod filaments possess strongly linearly polarized photoluminescence due to the unidirectional orientation of nanorods along the filaments.

11. Möller B.M., Woggon U., Artemyev M.V., Schöps O. (2004) Photonic molecules doped with semiconductor nanocrystals. - Phys. Rev. B, 70, 115323-1 - 115323-5.

We report on coherent cavity field coupling in linear chains and arrays of exactly size-matched spherical microcavities doped with CdSe quantum dots. The spatial distribution and the dominant polarization type of both the weakly and strongly coupled cavity resonances are studied spectrally and spatially resolved in various coupled resonator geometries. Both experiment and theory show strong photon mode coupling with pronounced mode splitting as well as weak coupling with no significant loss in Q-factor depending on the emitter position and orientation.

12. Le Thomas N., Herz E., Schöps O., Woggon U., Artemyev M. (2005) Exciton fine structure in single CdSe nanorods. - Phys. Rev. Lett., 94, 016803-1 - 016803-4.

We study the optical properties of excitons in one-dimensional (1D) nanostructures at low temperatures. In single CdSe=ZnS core-shell nanorods we observe a fine structure splitting and explain it by exchange interaction. Two peaks are observed with different degrees of linear polarization of DLP < 0.85 and DLP > 0.95. For small nanorod radii R<_aB/2, an increase in the photoluminescence decay time is found when the temperature increases from 10 to 80 K. The observations are explained by a radius-dependent change in the symmetry of the 1D-exciton ground state which transforms from a dark state into bright states below a critical radius of Rcrit ~ 3.7 nm.

13. Woggon U., Herz E., Schöps O., Artemyev M.V., Arens Ch., Rousseau N., Schikora D., Lischka K., Linvinov D., Gerthsen D. (2005) Hybrid epitaxial-colloidal semiconductor nanostructures. - Nano Letters, 5, 483-490.

We present a growth technique which combines wet-chemical growth and molecular beam epitaxy (MBE) to create complex semiconductor nanostructures with nanocrystals as active optical material. The obtained results show that wet-chemically prepared semiconductor nanocrystals can be incorporated in an epitaxally grown crystalline cap layer. As an exemplary system we chose CdSe nanorods and CdSe(ZnS) core-shell nanocrystals in ZnSe and discuss the two limits of thin ( d ~ 2 R) and thick ( d>2 R) ZnSe cap layers of thickness d for CdSe nanorods and nanodots of radii R between 2 and 4 nm. In contrast to the strain-induced CdSe/ZnSe Stranski-Krastanow growth of a quantum dot layer in a semiconductor heterostructure, the technique proposed here does not rely on strain and thus results in additional degrees of freedom for choosing composition, concentration, shape, and size of the nanocrystals. Transmission electron microscopy and X-ray diffractometry show that the ZnSe cap layer is of high crystalline quality and provides all parameters for a consecutive growth of Bragg structures, waveguides, or diode structures for electrical injection.

14. Fedorov A.V., Baranov A.V., Rukhlenko I.D., Gaponenko S.V. (2005) Enhanced intraband carrier relaxation in quantum dots due to the effect of plasmon-LO-phonon density of states in doped heterostructures. - Phys. Rev. B, 71, 205318-(1-8).

The effect of surface plasmon-LO-phonon excitations of the doped elements of heterostructures in the electronic dynamics of quantum dots has been theoretically studied in a double heterostructure. It has been found that, in contrast to a single heterostructure, critical points can arise in the surface plasmon-LO-phonon density of states for layered structures. This results in enhanced quantum-dot intraband carrier relaxation as compared with the single heterostructure. It has been shown that the relaxation rates and spectral positions of relaxation windows strongly depend on the thickness of the layer containing the quantum dots. These effects of the critical points of density of states have been demonstrated using a model n-GaAs/GaAs/ air heterostructure with an InAs quantum dot embedded in the GaAs layer.

15. Möller B.M., Woggon U., Artemyev M.V. Coupled-resonator optical waveguides doped with nanocrystals. Opt Lett. 2005 Aug 15;30(16):2116-8.

Microsphere resonators doped with semiconductor nanocrystals are explored as building blocks for coupled-resonator optical waveguides (CROWs). The evolution of individual cavity modes into coherently coupled waveguide modes is studied using polarization-sensitive microphotoluminescence spectroscopy. To demonstrate the formation of multisphere photon states, we use a bent linear array of microresonators and probe the properties of the cavity photon field by the spatially and spectrally resolved measurement of the nanocrystal emission. Photon mode coupling is evidenced by the observed mode splitting and emission intensity distributions along the CROW structure.

16. Stsiapura V., Sukhanova A., Baranov A., Artemyev M.V., Kulakovich O., Oleinikov V., Cohen J.H.M., Pluot M., Nabiev I. (2006) DNA-assisted formation of quasi-nanowires from fluorescent CdSe/ZnS nanocrystals. - Nanotechnology, 17, 581-587.

Highly ordered quasi-nanowires from fluorescent semiconductor CdSe/ZnS spherical (quantum dots) or rod-like (quantum rods) nanoparticles were produced using DNA as a template. Positively charged nanoparticles were fixed along the negatively charged DNA backbone by electrostatic interaction. After incubation of the solution of DNA and nanoparticles at different stoichiometric ratios the complexes were applied to the hydrophobic surface and stretched using the molecular combing technique. Here, we demonstrate that fluorescent patterns with desirable morphology and properties can be formed by varying the nanoparticle charge and shape and their stoichiometry in the complex with DNA.

17. Schöps O., Le Thomas N., Woggon U., Artemyev M.V. (2006) Recombination dynamics of CdTe/CdS core-shell nanocrystals. - J. Phys Chem B Condens Matter Mater Surf Interfaces Biophys., 110(5), 2074-2079.

The recombination dynamics of zinc-blende-type, deep-red emitting CdTe/CdS core-shell nanocrystals is studied over a wide temperature range. Two characteristic decay regimes are found: a temperature-dependent decay component of a few nanoseconds and a long-living temperature-independent component of approximately 315 ns. The average decay time of the exciton states changes from 20 to 5ns when the temperature is increased from 15 to 295 K. At low temperatures, the observed decay behavior is assigned to thermally induced population and decay of the allowed exchange-split exciton states. At temperatures above T>100 K, nonradiative decay channels involving phonons start to contribute to the exciton recombination. The observed broad distribution in decay times, monitored by stretched exponential fitting functions, we explain by variations in the electron-hole overlap caused by a partly incomplete CdTe/CdS core-shell structure and the nearly energy-degenerated bright and dark state superposition.