Delocalized Surface Electronic States on Polar Facets of Semiconductor Nanocrystals

Abstract

Wurtzite CdSe@CdS dot@platelet nanocrystals with (001) and (00–1) polar facets as the basal planes and (100) family of nonpolar facets as the side planes are applied for studying surface defects on semiconductor nanocrystals. When they are terminated with cadmium ions coordinated with carboxylate ligands, a single set of absorption features and band-edge photoluminescence (PL) with near unity PL quantum yield and monoexponential PL decay dynamics (lifetime ∼28 ns) are observed. In addition to these spectral signatures, when the surface is converted to sulfur-terminated, a second set of sharp absorption features with decent extinction coefficients and a secondary band-edge PL with low PL quantum yield and long-lifetime (>78 ns) PL decay dynamics are reproducibly recorded. Photochemical analysis confirms that the secondary UV–vis and PL spectral features are quantitatively correlated with each other. Chemical analysis and X-ray photoelectron spectroscopy measurements confirm that such secondary spectral features are well correlated with the sulfide (such as −SH) and disulfide (such as −S–S−) surface sites of a basal plane, which likely form surface hole electronic states delocalized on the entire basal plane. Results suggest that, for studying surface defects on semiconductor nanocrystals, it is essential to prepare a nearly monodisperse surface structure in terms of facets and surface chemical bonding.

Synthesis of Colloidal Quantum Dots with an Ultranarrow Photoluminescence Peak

“…It is interesting to notice that the significant PL peak narrowing occurs mostly on the low-energy side for both types of emitters, which is consistent with elimination of surface-related states at the low energy side of the band-edge. 93 Importantly, the single-dot PL measured using standard single-molecule spectroscopy 94 is as narrow as 5.2 nm (or 28.8 meV) for the blue-emitting alloy/shell QDs (Figure 7c) and 9.7 nm (or 43.9 meV) for the green-emitting QDs (Figure 7d), which are close to the ideal peak position and fwhm for blue and green emitters for display applications. 17 Nevertheless, the PL fwhm values in Figures 7c,d are the narrowest for dotshaped QDs in the specific color windows.…”

Current Status and Challenges of Solar Cells Based on Semiconductor Nanocrystals

“…However, for any emissive defect states, their radiative decay lifetime is typically much longer than the intrinsic excitons. 94,99 This suggests that it is much more competitive for charges at intermediate traps to fulfill a designated charge separation (process "4" in Figure 5B, bottom panel).…”

“…It should be mentioned that delocalized defect states do exist. , Such defect states might also be emissive, meaning a finite k r′ . However, for any emissive defect states, their radiative decay lifetime is typically much longer than the intrinsic excitons. , This suggests that it is much more competitive for charges at intermediate traps to fulfill a designated charge separation (process “4” in Figure B, bottom panel).…”

“…Defects would not only be important for charge separation (see above subsection) but also play a visible role in charge transport. Among different kinds of the trap states, those delocalized surface defect states , might less impact charge transport.…”

“…It should be mentioned that delocalized defect states do exist. 91,99 Such defect states might also be emissive, 99 meaning a finite k r′ . However, for any emissive defect states, their radiative decay lifetime is typically much longer than the intrinsic excitons.…”