Non-blinking semiconductor nanocrystals

Abstract

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.

Near-Infrared Photoluminescence Enhancement in Ge/CdS and Ge/ZnS Core/Shell Nanocrystals: Utilizing IV/II–VI Semiconductor Epitaxy

“…33,34,58À61 Because the resulting nanocrystals still show the NIR photoluminescence characteristic of Ge, these nanocrystals must have either a core/shell or graded Ge/ZnS composition. 62 Why this etching or exchange occurs with ZnS and not with CdS is presently unknown, but there are some clues: On the basis of lattice parameters, ZnS leads to compression (À4.4%) while CdS leads to expansion (þ3.1%) of the Ge core (Table 1). In addition, according to Pearson's theory of hard and soft acids and bases (HSAB), 63,64 Zn 2þ is a borderline acid while Cd 2þ is a soft acid.…”

Fluxionality of gold nanoparticles investigated by Born-Oppenheimer molecular dynamics

“…12 A-D. These four structures are all characterized by a 4 atoms core and a 30 atoms shell, and they are all very close in energy (within K B T , Table I It should be noted that interactions between core and shell in core-shell nanoparticles have been shown to be of relevance for the steadiness of photon emission in semiconducting nanoparticles [100]. A simulation at 400 K was performed showing a consistent sampling of the conformational space compared to the MD simulation at 300 K.…”

The many aspects of quantum dots

“…They have been widely used in bio-imaging applications for a number of years, and researchers have recently developed methods to stop the fluorescence from quantum dots switching on and off in a random manner (a problem known as 'blinking') 9 . More recently the optical properties of colloidal quantum dots have been exploited in lighting to improve energy efficiency and make the output of light-emitting diodes seem more natural 10 , and applications in digital cameras, displays and solar power are also being investigated 10,11 .…”