From CdSe Nanoplatelets to Quantum Rings by Thermochemical Edge Reconfiguration

Abstract

The variation in the shape of colloidal semiconductor nanocrystals (NCs) remains intriguing. This interest goes beyond crystallography as the shape of the NC determines its energy levels and optoelectronic properties. While thermodynamic arguments point to a few or just a single shape(s), terminated by the most stable crystal facets, a remarkable variation in NC shape has been reported for many different compounds. For instance, for the well-studied case of CdSe, close-to-spherical quantum dots, rods, two-dimensional nanoplatelets, and quantum rings have been reported. Here, we report how two-dimensional CdSe nanoplatelets reshape into quantum rings. We monitor the reshaping in real time by combining atomically resolved structural characterization with optical absorption and photoluminescence spectroscopy. We observe that CdSe units leave the vertical sides of the edges and recrystallize on the top and bottom edges of the nanoplatelets, resulting in a thickening of the rims. The formation of a central hole, rendering the shape into a ring, only occurs at a more elevated temperature.

Stimulated Emission through an Electron–Hole Plasma in Colloidal CdSe Quantum Rings

“…Next, we proceed with a quantitative decomposition in Figure 1 c using a series of step functions, describing free carrier absorption from a 2D continuum of states (C, dashed lines) and the associated exciton absorption peaks (X, solid lines). 9 Notably, the exciton position energies, HH–LH splitting and the SO–CB transition extracted from the fit, line up with the values reported for weakly confined NPLs thicker than 8.5 ML (2.3 nm) ( Supporting Information ). 18 , 22 Finally, we extract the exciton binding energy Δ HH as 107 ± 1 meV, matching the trend for decreasing exciton binding energy for increasing thickness.…”

Two-Dimensional CdSe-PbSe Heterostructures and PbSe Nanoplatelets: Formation, Atomic Structure, and Optical Properties

“…As can be seen, the NCs have a toroidal shape and are highly crystalline, in agreement with our previous work. 18 Although several particles are fully perforated, others have a membrane in the center with a thickness of the original CdSe NPL. 18 After the cation exchange, the shape of the NCs is mostly preserved ( Figure 6 c).…”

“… 18 Although several particles are fully perforated, others have a membrane in the center with a thickness of the original CdSe NPL. 18 After the cation exchange, the shape of the NCs is mostly preserved ( Figure 6 c). The FFT pattern of a single PbSe quantum ring (inset) shows that the NCs are highly crystalline, as the characteristic reflections from rs-PbSe are observed.…”

“…CdSe NPLs were converted into CdSe quantum rings by a previously reported procedure. 18 Briefly, selenium was dispersed in OLAM to yield a concentration of 7.9 mg Se/mL OLAM. 1.0 mL of CdSe NPLs with an absorbance of 0.2 (in a 1 cm cuvette) at the first exciton transition after being diluted 300 times was precipitated and redispersed in 3 mL of ODE and 1.5 mL of OLAM.…”

“…In this respect, we remark that a thermochemical reconfiguration of CdSe NPLs into quantum rings also occurs via the vertical facets and vertices. 18 …”