Role of Crystal Structure and Chalcogenide Redox Properties on the Oxidative Assembly of Cadmium Chalcogenide Nanocrystals

Abstract

Oxidative assembly of metal chalcogenide nanocrystals (NCs) enables the formation of 2-D (dense) and 3-D porous structures without the presence of intervening ligands between particles that can moderate transport properties. This route has been demonstrated to be successful for a range of single-component structures including CdQ, PbQ, and ZnQ (Q = S, Se, Te). En route to the controllable assembly of multicomponent nanostructures, the roles of Q redox properties (2Q2− → Q22− + 2e) responsible for particle cross-linking and the native structure (cubic zinc blende vs hexagonal wurtzite) in the kinetics of assembly in single-component CdQ NCs are evaluated using time-resolved dynamic light scattering (TR-DLS). For wurtzite CdQ, the rates follow the ease of oxidation, with telluride as the fastest, followed by selenide and sulfide. However, when comparing CdS wurtzite (w) and zinc blende (zb), the cubic NCs exhibit surprisingly slow kinetics. NMR studies reveal the zb structure to have lower ligand coverage (by a factor of 4) relative to that of w, and the formation of free disulfide (the product of ligand oxidation) is slow. This is attributed to differences in the surface energies of w and zb facets, with w having polar (0001) facets of high energy compared to the neutral facets of the zb structure. The zb-CdS NCs prepared by low-temperature synthesis methods are likely to suffer from surface defects that may moderate reactivity. EPR studies suggest that zb-CdS has paramagnetic sulfur vacancies not present in w-CdS. These data suggest that structure plays an unexpectedly large role in the kinetics of CdQ NC oxidative assembly, providing a useful lever to moderate activities in multicomponent assemblies.

Atomically dispersed Pb ionic sites in PbCdSe quantum dot gels enhance room-temperature NO2 sensing

“…Based on prior work demonstrating (1) slow gelation kinetics for cubic polymorphs (i.e., PbSe) relative to hexagonal polymorphs, and (2) facilitated cation exchange on ligand-stripped surfaces 52 , 53 , our strategy for targeting Pb x Cd 1− x Se QD gels involves initial synthesis of hexagonal (wurtzite) CdSe, subsequent gelation (induced by ligand stripping), and ultimately ion-exchange of Cd 2+ for Pb 2+ as illustrated in Fig. 1b .…”

Visualizing Heterogeneity of Monodisperse CdSe Nanocrystals by Their Assembly into Three-Dimensional Supercrystals

“…52 Conversely, NCs of the same size and composition but with different phases, or with different phases present on their surfaces, can have different ligand coverages of their surfaces. 63 For example, NMR studies on CdS NCs of similar sizes revealed that CdS NCs with ZB structure had lower ligand coverage by a factor of 4 as compared to NCs with W structure. 63 This different ligand coverage, in turn, can influence the solvation of the NC ligand shells 40 and thus the interactions among NCs.…”

“…Also playing important roles are the choice of surface ligands; ,,,− in fact, transitions between the ZB and W phases can be induced by introducing different ligands , or by changing the NC surface, and the phase of CdSe NCs can depend even on the length of the ligands . Conversely, NCs of the same size and composition but with different phases, or with different phases present on their surfaces, can have different ligand coverages of their surfaces . For example, NMR studies on CdS NCs of similar sizes revealed that CdS NCs with ZB structure had lower ligand coverage by a factor of 4 as compared to NCs with W structure .…”

“…Conversely, NCs of the same size and composition but with different phases, or with different phases present on their surfaces, can have different ligand coverages of their surfaces . For example, NMR studies on CdS NCs of similar sizes revealed that CdS NCs with ZB structure had lower ligand coverage by a factor of 4 as compared to NCs with W structure . This different ligand coverage, in turn, can influence the solvation of the NC ligand shells and thus the interactions among NCs. ,, As ligand–ligand interactions are one of the key factors determining the NC self-assembly process, ,− polytypism has the potential to significantly affect CdSe NC self-assembly.…”

CuGaS2 and CuGaS2–ZnS Porous Layers from Solution-Processed Nanocrystals

“…Besides toxicity, if not properly controlled, such solution-based ligand-stripping strategies can result in large concentrations of surface traps that may strongly limit the material performance [4]. An alternative strategy to remove organic ligands from NC-based layers and cross-link the NCs to facilitate charge transport involves using a non-oxygen-transferring oxidant [5,6,7]. This oxidizing agent produces chalcogen-chalcogen bonds between the NCs, potentially reducing surface recombination sites and facilitating charge transfer between NCs [8,9].…”