Megan A. Hahn

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.

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.

Photoluminescence enhancement of colloidal quantum dots embedded in a monolithic microcavity

We demonstrate an enhancement of the spontaneous emission from colloidal CdSe quantum dots embedded in a half-wavelength one-dimensional cavity. When embedded in the cavity, the emission of the quantum dots is enhanced by a factor of 2.7. We also show a strong amplification by one order of magnitude in the absorption of the CdSe quantum dots due to the cavity effect.

Organic photonic bandgap microcavities doped with semiconductor nanocrystals for room-temperature on-demand single-photon sources

We report the first experimental observation of fluorescence from single semiconductor nanocrystals (colloidal quantum dots) in microcavities. In these room-temperature experiments we observed photon antibunching from single CdSe nanocrystals doped into a chiral one-dimensional photonic bandgap liquid-crystal microcavity. The chirality resulted in high-purity, circular polarization of definite handedness of the emitted single photons. We also report the fabrication of chiral microcavities for telecom wavelengths doped with PbSe nanocrystals as well as a solution-processed-polymer microcavity with a defect layer doped with CdSe nanocrystals between two distributed Bragg reflectors. These systems with their low host fluorescence background are attractive for on-demand single-photon sources for quantum information and communication.

Small-angle rotation in individual colloidal CdSe quantum rods.

CdSe quantum rods (QRs) are very promising novel materials with unique electronic and optical properties. In this paper, we utilize a broad spectrum of techniques including high-resolution annular dark field scanning transmission electron microscope imaging, electron nanodiffraction, and computer simulations to study the internal structure of individual QRs. Random small-angle rotations are commonly found between various sections within individual QRs which can be resolved into twists around the c-axis and bends. The possible origins of these small-angle rotations are further studied. We propose that imperfect oriented attachment coupled with electrostatic interactions between smaller nanoparticles during the growth process results in such small-angle rotations. These small-angle rotations may significantly affect the electronic and mechanical properties of CdSe QRs.

Retraction: Non-blinking semiconductor nanocrystals

This corrects the article DOI: 10.1038/nature08072

Study of the internal structure of individual CdSe quantum rods using electron nanodiffraction

Electron beam nanodiffraction was used to study the internal structure of colloidal CdSe quantum rods (QRs). Small-angle (2°–3°) rotations between various sections of individual colloidal CdSe QRs were observed indicating imperfection in the QRs. These imperfections may significantly impact the electronic, optical, and mechanical properties of the QRs.

Chiral photonic bandgap microcavities doped with single colloidal semiconductor quantum dots

Various single colloidal quantum dots were investigated in chiral liquid crystal 1-D photonic bandgap microcavities for visible and telecom wavelengths. The microcavity environment provides circular polarization of definite handedness. Narrow, circularly-polarized microcavity resonances of 3-5 nm width were observed on the band edges of the stopbands.

Room-temperature single photon sources with definite circular and linear polarizations based on single-emitter fluorescence in liquid crystal hosts

Experimental results of two room-temperature, robust and efficient single-photon sources with definite circular and linear polarization using single-emitter fluorescence in cholesteric and nematic liquid crystal hosts are discussed. For single emitters, we used nanocrystal quantum dots, single color centers in nanodiamonds, and single dye molecules. Single-photon sources based on single emitters in liquid crystals are the room temperature alternatives to cryogenic single-photon sources base on semiconductor heterostructured quantum dots in microcavities prepared by molecular beam epitaxy.