Low Threshold Multiexciton Optical Gain in Colloidal CdSe/CdTe Core/Crown Type-II Nanoplatelet Heterostructures

Colloidal cadmium chalcogenide core/crown type-II nanoplatelet heterostructures, such as CdSe/CdTe, are promising materials for lasing and light emitting applications. Their rational design and improvement requires the understanding of the nature of single- and multiexciton states. Using pump fluence and wavelength-dependent ultrafast transient absorption spectroscopy, we have identified three spatially and energetically distinct excitons (in the order of increasing energy): interface-localized charge transfer exciton (X-CT, with electron in the CdSe core bound to the hole in the CdTe crown), and CdTe crown-localized X-CdTe and CdSe core-localized X-CdSe excitons. These exciton levels can be filled sequentially, with each accommodating two excitons (due to electron spin degeneracy) to generate one to six exciton states (with lifetimes of >> 1000, 209, 43.5, 11.8, 5.8, and 4.5 ps, respectively). The spatial separation of these excitons prolongs the lifetime of multiexciton states. Optical gain was observed in tri(XXCTXCdTe) and four (XXCTXCdTe) exciton states. Because of the large absorption cross section of nanoplatelets, an optical gain threshold as low as similar to 43 mu J/cm(2) can be achieved at 550 nm excitation for a colloidal solution sample. This low gain threshold and the long triexciton (gain) lifetime suggest potential applications of these 2D type-II heterostructures as low threshold lasing materials.