Abstract
Colloidal CdSe nanocrystals (NCs) have shown promise in applications ranging from LED displays to medical imaging. Their unique photophysics depend sensitively on the presence or absence of surface defects. Using simulations, we show that CdSe NCs are inherently defective; even for stoichiometric NCs with perfect ligand passivation and no vacancies or defects, we still observe that the low energy spectrum is dominated by dark, surface-associated excitations, which are more numerous in larger NCs. Surface structure analysis shows that the majority of these states involve holes that are localized on two-coordinate Se atoms. As chalcogenide atoms are not passivated by any Lewis base ligand, varying the ligand should not dramatically change the number of dark states, which we confirm by simulating three passivation schemes. Our results have significant implications for understanding CdSe NC photophysics, and suggest that photochemistry and short-range photoinduced charge transfer should be much more facile than previously anticipated.
Highlights
Colloidal CdSe nanocrystals (NCs) have shown promise in applications ranging from LED displays to medical imaging
NCs are sometimes referred to as artificial atoms[2], despite the fact that, aside from their absorption spectra, NC photophysics is very different from atomic photophysics: the photoluminescence (PL) spectra of NCs often display a significant Stokes shift[21]; the PL quantum yield (QY) is significantly less than unity[22]; and under constant illumination, single NC PL displays an on-off intermittency known as blinking[23,24,25,26]
In each of these situations, the unusual photophysics of NCs is intimately tied to the existence of surface defect states that compete with the bulk-like PiS states
Summary
Colloidal CdSe nanocrystals (NCs) have shown promise in applications ranging from LED displays to medical imaging. The low-energy absorption spectra of NCs are typically dominated by a bright peak or peaks[3,19] and the way in which these peaks shift with NC size, as well as the spacing between them, can be qualitatively explained by simple particle-in-asphere (PiS) models[20] For this reason, NCs are sometimes referred to as artificial atoms[2], despite the fact that, aside from their absorption spectra, NC photophysics is very different from atomic photophysics: the photoluminescence (PL) spectra of NCs often display a significant Stokes shift[21]; the PL quantum yield (QY) is significantly less than unity[22]; and under constant illumination, single NC PL displays an on-off intermittency known as blinking[23,24,25,26]. This picture is supported by the fact that coreshell NCs have dramatically improved PL QY and significantly reduced blinking[22,34]
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