Todd D. Krauss

Non-blinking semiconductor nanocrystals

The photoluminescence from a variety of individual molecules and nanometre-sized crystallites is defined by large intensity fluctuations, known as ‘blinking’, whereby their photoluminescence turns ‘on’ and ‘off’ intermittently, even under continuous photoexcitation. For semiconductor nanocrystals, it was originally proposed that these ‘off’ periods corresponded to a nanocrystal with an extra charge. A charged nanocrystal could have its photoluminescence temporarily quenched owing to the high efficiency of non-radiative (for example, Auger) recombination processes between the extra charge and a subsequently excited electron–hole pair; photoluminescence would resume only after the nanocrystal becomes neutralized again. Despite over a decade of research, completely non-blinking nanocrystals have not been synthesized and an understanding of the blinking phenomenon remains elusive. Here we report ternary core/shell CdZnSe/ZnSe semiconductor nanocrystals that individually exhibit continuous, non-blinking photoluminescence. Unexpectedly, these nanocrystals strongly photoluminesce despite being charged, as indicated by a multi-peaked photoluminescence spectral shape and short lifetime. To model the unusual photoluminescence properties of the CdZnSe/ZnSe nanocrystals, we softened the abrupt confinement potential of a typical core/shell nanocrystal, suggesting that the structure is a radially graded alloy of CdZnSe into ZnSe. As photoluminescence blinking severely limits the usefulness of nanocrystals in applications requiring a continuous output of single photons, these non-blinking nanocrystals may enable substantial advances in fields ranging from single-molecule biological labelling to low-threshold lasers.

Robust Photogeneration of H2 in Water Using Semiconductor Nanocrystals and a Nickel Catalyst

Robust Reduction A major challenge in the design of artificial photosynthesis catalysts has been their instability under the reaction conditions—a problem that plants and other autotrophs address by perpetually reproducing their biochemical machinery. Han et al. (p. 1321, published online 8 November) now demonstrate a system for photoreductive hydrogen generation in water that manifests undiminished activity for weeks at a time. Semiconductor nanoparticles for light absorption were combined with a soluble nickel complex for the catalytic chemistry. The system currently requires a sacrificial electron donor, but its robustness shows promise for future pairing with an integrated oxidation catalyst. A photoreduction system combining nanoparticulate light absorbers with a soluble molecular catalyst proves stable for weeks. Homogeneous systems for light-driven reduction of protons to H2 typically suffer from short lifetimes because of decomposition of the light-absorbing molecule. We report a robust and highly active system for solar hydrogen generation in water that uses CdSe nanocrystals capped with dihydrolipoic acid (DHLA) as the light absorber and a soluble Ni2+-DHLA catalyst for proton reduction with ascorbic acid as an electron donor at pH = 4.5, which gives >600,000 turnovers. Under appropriate conditions, the precious-metal–free system has undiminished activity for at least 360 hours under illumination at 520 nanometers and achieves quantum yields in water of over 36%.

Synthesis and characterization of PbSe quantum dots in phosphate glass

The controlled synthesis of PbSe nanocrystal quantum dots with narrow size distributions was achieved through phase decomposition of the PbSe solid solution in a phosphate glass host. Structural characterization by electron microscopy and x-ray diffraction shows that the dots have mean diameters between 2 and 15 nm. The exciton Bohr radius aB=46 nm in PbSe, so these quantum dots provide unusual and perhaps unique access to the regime of strong quantum confinement. The optical absorption spectra are compared to the predictions of a theoretical treatment of the electronic structure. The theory agrees well with experiment for dots larger than ∼7 nm, but for smaller dots there is some deviation from the theoretical predictions.

The structural basis for giant enhancement enabling single-molecule Raman scattering

We find that giant surface-enhanced Raman scattering for adsorbates on silver surfaces is present only on surfaces that exhibit self-similar fractal topology as inferred from atomic force microscopy. The fractal character results in localizing the energy of incident photons to volumes of a few nanometers on a side, millions of times smaller than the diffraction limit. Consistent with this finding, we have found an enhancement in spontaneous Raman cross section of >13 orders of magnitude for adsorbates on silver surfaces demonstrated to be fractal. The location of “hot spots” on the fractal surfaces is found to be hypersensitive to incident wavelength and polarization even though the observed Raman scattering is strictly linear in incident intensity. These observations are consistent with localization of the photon energy facilitated by the disordered nature of fractal organization through interference between the incident wave and scattered radiation from silver nanoparticle surface plasmons. We also present a surface preparation method that consistently produces fractal topologies that support single-molecule Raman scattering.

Femtosecond measurement of nonlinear absorption and refraction in CdS, ZnSe, and ZnS

The nonlinear index of refraction and the two‐photon absorption coefficient of CdS, ZnSe, and ZnS were measured using the Z‐scan technique with 100‐femtosecond pulses at 610 nm, 780 nm, and 1.27 μm. The results are in reasonable agreement with measurements made at similar wavelengths with longer pulses. A simple theoretical model accounts for the dispersion of the nonlinearities and confirms their electronic nature.

Mysteries of TOPSe Revealed: Insights into Quantum Dot Nucleation

We have investigated the reaction mechanism responsible for QD nucleation using optical absorption and nuclear magnetic resonance spectroscopies. For typical II-VI and IV-VI quantum dot (QD) syntheses, pure tertiary phosphine selenide sources (e.g., trioctylphosphine selenide (TOPSe)) were surprisingly found to be unreactive with metal carboxylates and incapable of yielding QDs. Rather, small quantities of secondary phosphines, which are impurities in tertiary phosphines, are entirely responsible for the nucleation of QDs; their low concentrations account for poor synthetic conversion yields. QD yields increase to nearly quantitative levels when replacing TOPSe with a stoiciometric amount of a secondary phosphine chalcogenide such as diphenylphosphine selenide. Based on our observations, we have proposed potential monomer identities, reaction pathways, and transition states and believe this mechanism to be universal to all II-VI and IV-VI QDs synthesized using phosphine based methods.

Colloidal Semiconductor Quantum Dots with Tunable Surface Composition

Colloidal CdS quantum dots (QDs) were synthesized with tunable surface composition. Surface stoichiometry was controlled by applying reactive secondary phosphine sulfide precursors in a layer-by-layer approach. The surface composition was observed to greatly affect photoluminescence properties. Band edge emission was quenched in sulfur terminated CdS QDs and fully recovered when QDs were cadmium terminated. Calculations suggest that electronic states inside the band gap arising from surface sulfur atoms could trap charges, thus inhibiting radiative recombination and facilitating nonradiative relaxation.

Multiple Exciton Generation in Single-Walled Carbon Nanotubes

Upon absorption of single photons, multiple excitons were generated and detected in semiconducting single-walled carbon nanotubes (SWNTs) using transient absorption spectroscopy. For (6,5) SWNTs, absorption of single photons with energies corresponding to three times the SWNT energy gap results in an exciton generation efficiency of 130% per photon. Our results suggest that the multiple exciton generation threshold in SWNTs can be close to the limit defined by energy conservation.

Bright Fluorescence from Individual Single-Walled Carbon Nanotubes

Single-walled carbon nanotubes (SWNTs) have unique photophysical properties but low fluorescence efficiency. We have found significant increases in the fluorescence efficiency of individual DNA-wrapped SWNTs upon addition of reducing agents, including dithiothreitol, Trolox, and β-mercaptoethanol. Brightening was reversible upon removal of the reducing molecules, suggesting that a transient reduction of defect sites on the SWNT sidewall causes the effect. These results imply that SWNTs are intrinsically bright emitters and that their poor emission arises from defective nanotubes.

Flow Cytometric Analysis To Detect Pathogens in Bacterial Cell Mixtures Using Semiconductor Quantum Dots

Compared to a common green organic dye, semiconductor quantum dots (QDs) composed of CdSe/ZnS core/shell bioconjugates display brighter fluorescence intensities, lower detection thresholds, and better accuracy in analyzing bacterial cell mixtures composed of pathogenic E. coli O157:H7 and harmless E. coli DH5alpha using flow cytometry. For the same given bacterial mixture, QDs display fluorescence intensity levels that are approximately 1 order of magnitude brighter compared to the analogous experiments that utilize the standard dye fluorescein isothiocyanate. Detection limits are lowest when QDs are used as the fluorophore label for the pathogenic E. coli O157:H7 serotype: limits of 1% O157:H7 in 99% DH5alpha result, corresponding to 106 cells/mL, which is comparable to other developing fluorescence-based techniques for pathogen detection. Finally, utilizing QDs to label E. coli O157:H7 in cell mixtures results in greater accuracy and more closely approaches the ideal fluorophore for pathogen detection using flow cytometry. With their broader absorption spectra and narrower emission spectra than organic dyes, QDs can make vast improvements in the field of flow cytometry, where single-source excitation and simultaneous detection of multicolor species without complicating experimental setups or data analysis is quite advantageous for analyzing heterogeneous cell mixtures, both for prokaryotic pathogen detection and for studies on eukaryotic cell characteristics.