Sara Maccagnano-Zacher

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.

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

Reduction of Contrast in ADF-STEM Images Due To Amorphous Layer

A study of high-resolution ADF imaging in aberration-corrected STEMs was carried out by multislice simulation. The presence of amorphous layers at the surface of a crystalline specimen is shown to significantly alter the visibility of the atomic columns in ADF images. After propagating through an amorphous layer a portion of the beam passes without any alteration while scattered electrons introduce a Gaussian background. An amorphous layer at the beam entry surface appears to have slightly more of an effect on the ADF image contrast than that of an amorphous layer at the exit surface, and this difference increases with increasing atomic number. With a constant crystal layer thickness, the reduction of contrast as a function of increasing amorphous layer is found to have the same behavior and trend, regardless of the initial crystal layer thickness.