Atomistics of Asymmetric Lateral Growth of Colloidal Zincblende CdSe Nanoplatelets

Abstract

Shape anisotropy in colloidal zincblende CdSe nanoplatelets (NPLs) has a direct bearing on their optical and electronic properties. The lateral aspect ratio of NPLs provides an additional knob with which one can control their optical characteristics even at a fixed thickness. For example, one can regulate the polarized emission and assembly behaviors of NPLs by varying the lateral aspect ratio while keeping the optical transition at the energy fixed by the NPL thickness. Given the isotropic nature of the zincblende crystal structure, such control over shape anisotropy is intriguing yet not fully understood. In this study, based on combined experimental and simulation works, we systematically investigate the asymmetric lateral growth behavior of colloidal 4.5 monolayer thick (5 Cd and 4 Se layers) CdSe NPLs. Experimentally, it is found that NPLs with rectangular and square shapes have different growth directions, <100> and <110>, respectively. The cadmium acetate (Cd(ac) 2 )-to-Se precursor ratio turns out to have an important role in altering the lateral growth kinetics of NPLs, and the deficiency of Se under high Cd-to-Se precursor ratio conditions slows down the growth and changes the growth direction from <100> to <110>. First-principles calculations reveal that the observed growth behaviors can be explained in light of the irregular wedge-shaped conformations at the NPL side surfaces that expose {110} and {111} faces. Specifically, it is identified that the adsorption of Cd(ac) 2 on {110} ({111}) surfaces becomes an energetically unfavorable process in the low (high) Se coverage regime, explaining the fast <100>-direction (slow <110>-direction) growth into rectangular (square) NPLs. The presented mechanism for the lateral growth of zincblende NPLs provides an atomic-level understanding of the transition processes of NPL morphology and growth direction, with the control of unique anisotropic properties of NPLs on the line.

Triethyl-Borates as Surfactants to Stabilize Semiconductor Nanoplatelets in Polar Solvents and to Tune Their Optical Properties

“…The lattice spacings of 0.301 and 0.223 nm belonged to the (200) and (220) crystal planes of NPLs, respectively. This value was lower than that reported in the literature ( Yoon et al, 2021 ) because the surface strain of the nanocrystal changed after ligand exchange. The surface strain is one of the reasons for the changes in the optical properties of NPLs ( Zhou et al, 2015 ) ( Dufour et al, 2019 ).…”

Impacts of Dopant and Post-Synthetic Heat-Treatment on Carrier Relaxation of Cu2+-Doped CdSe Nanoplatelets

“…Since colloidal CdSe NPLs are composed of alternating layers of Cd and Se along ⟨001⟩ direction and lateral growth happen perpendicular to the thickness of the NPLs, therefore, lateral growth direction for such NPLs is ⟨100⟩ direction. Lateral growth along ⟨100⟩ in colloidal ZB CdSe rectangular NPLs was also observed earlier Figure f shows Cd and Se atoms’ CP atomic arrangement in 2D pristine ZB CdSe NPLs.…”

“…Lateral growth along ⟨100⟩ in colloidal ZB CdSe rectangular NPLs was also observed earlier. 33 Figure 1f shows Cd and Se atoms' CP atomic arrangement in 2D pristine ZB CdSe NPLs. Thickness (T) is oriented along the crystallographic [001] direction.…”

Efficient, Selective CO2 Photoreduction Enabled by Facet-Resolved Redox-Active Sites on Colloidal CdS Nanosheets

“…25−28 Accordingly, their crystal lattice termination can be regarded as two groups: Cdterminated large basal planes versus Cd and S interlaced small edge facets, both of which can potentially serve as two kinds of distinct active panels for reduction and oxidation reactions. 26,29 On the other hand, recent studies on the electronic structure of the colloidal Cd chalcogenide (CdE; E = S, Se, or Te) nanoplatelets (NPLs, similar to NSs but with relatively small lateral dimensions) suggest that photogenerated electrons and holes can theoretically migrate toward designated facets by subtle surface engineering. 30,31 Specifically, photoexcited electrons tend to be localized at the deep trap states on the edge facets, 29,32 whereas band-edge holes can be fully delocalized to the large basal plane upon substituting the Sterminated layer for the pristine Cd-terminated layer.…”

“…Colloidal CdS NSs are chosen for the distinct surface configuration and the unique electronic structure. Colloidal CdS NSs possess atomically precise thickness, consisting of n + 1 atomic layers of Cd and n atomic layers of S with an interlaced arrangement along the out-of-plane [001] direction. − Accordingly, their crystal lattice termination can be regarded as two groups: Cd-terminated large basal planes versus Cd and S interlaced small edge facets, both of which can potentially serve as two kinds of distinct active panels for reduction and oxidation reactions. , On the other hand, recent studies on the electronic structure of the colloidal Cd chalcogenide (CdE; E = S, Se, or Te) nanoplatelets (NPLs, similar to NSs but with relatively small lateral dimensions) suggest that photogenerated electrons and holes can theoretically migrate toward designated facets by subtle surface engineering. , Specifically, photoexcited electrons tend to be localized at the deep trap states on the edge facets, , whereas band-edge holes can be fully delocalized to the large basal plane upon substituting the S-terminated layer for the pristine Cd-terminated layer. ,, Altogether, with a proper degree of control, the two-dimensional (2D) anisotropy in shape can match the balanced migration of photoinduced charge carriers with the spatial distribution of surface redox sites by surface engineering on CdS NSs.…”