Parabolic Potential Surfaces Localize Charge Carriers in Nonblinking Long-Lifetime “Giant” Colloidal Quantum Dots

Abstract

Materials for studying biological interactions and for alternative energy applications are continuously under development. Semiconductor quantum dots are a major part of this landscape due to their tunable optoelectronic properties. Size-dependent quantum confinement effects have been utilized to create materials with tunable bandgaps and Auger recombination rates. Other mechanisms of electronic structural control are under investigation as not all of a material’s characteristics are affected by quantum confinement. Demonstrated here is a new structure–property concept that imparts the ability to spatially localize electrons or holes within a core/shell heterostructure by tuning the charge carrier’s kinetic energy on a parabolic potential energy surface. This charge carrier separation results in extended radiative lifetimes and in continuous emission at the single-nanoparticle level. These properties enable new applications for optics, facilitate novel approaches such as time-gated single-particle imaging, and create inroads for the development of other new advanced materials.

Long-Lived and Bright Biexcitons in Quantum Dots with Parabolic Band Potentials

“…Another measure of lifetime which is useful for comparison with time-resolved radioluminescence below is the time to decay to 1/e of the initial intensity, τ 1/e = 139 and 81 ns, respectively. These long lifetimes are strong photophysical evidence of well-separated electron and hole wave functions and closely match modeling of the electron and hole wave functions presented in earlier work . The longer lifetime of the 6.6 nm core compared to the 6.9 nm core is primarily a function of the larger shell into which electrons may delocalize …”

“…22 In these particles, a radial gradient of the core and an abrupt core/shell interface, confirmed with elemental mapping, results in well-separated electron−hole wave functions (i.e., type II structure), 23 manifest by an elongated photoluminescence reaching lifetimes above 500 ns, compared to a CdSe/CdS gQD of comparable size (<50 ns lifetime). 22 Furthermore, despite the conjecture of enhanced fluorescence intermittency arising from Auger recombination facilitated by abrupt potential interfaces, 24−26 the graded core CdZnSe/CdS gQDs are in fact nonblinking despite the discontinuous core/ shell potential interface. 22,26 Here, multiexciton photophysics in the same CdZnSe/CdS gQDs reported previously are studied in the context of technologies and applications in which radiative multiexcitonic emission is valuable or essential.…”

“…These long lifetimes are strong photophysical evidence of well-separated electron and hole wave functions and closely match modeling of the electron and hole wave functions presented in earlier work. 22 The longer lifetime of the 6.6 nm core compared to the 6.9 nm core is primarily a function of the larger shell into which electrons may delocalize. 22 Multiexciton states were examined by using power-dependent photoluminescence measurements.…”

“…22 The longer lifetime of the 6.6 nm core compared to the 6.9 nm core is primarily a function of the larger shell into which electrons may delocalize. 22 Multiexciton states were examined by using power-dependent photoluminescence measurements. Multiexcitons are apparent in a few ways in these experiments.…”

“…For example, Auger recombination limits lasing as this process originates, in most uncharged quantum dots (QDs) from a biexcitonic state. − Auger recombination rates in quasi-spherical nanocrystals of a single composition follow a nearly universal scaling relationship with volume . Several strategies have been developed to suppress Auger recombination including control over nanoparticle size, shape, − core/shell structures, − doping, − and composition variation to adjust band offsets and potential gradients. ,,, Recently, a distinct strategy for Auger suppression was demonstrated through the synthesis of radially alloyed-core/shell CdZnSe/CdS “giant” quantum dots (gQDs) . In these particles, a radial gradient of the core and an abrupt core/shell interface, confirmed with elemental mapping, results in well-separated electron–hole wave functions (i.e., type II structure), manifest by an elongated photoluminescence reaching lifetimes above 500 ns, compared to a CdSe/CdS gQD of comparable size (<50 ns lifetime) .…”