Type-II Colloidal Quantum Wells: CdSe/CdTe Core/Crown Heteronanoplatelets

Abstract

Solution-processed quantum wells, also known as colloidal nanoplatelets (NPLs), are emerging as promising materials for colloidal optoelectronics. In this work, we report the synthesis and characterization of CdSe/CdTe core/crown NPLs exhibiting a Type-II electronic structure and Type-II specific optical properties. Here, based on a core-seeded approach, the CdSe/CdTe core/crown NPLs were synthesized with well-controlled CdTe crown coatings. Uniform and epitaxial growth of CdTe crown region was verified by using structural characterization techniques including transmission electron microscopy (TEM) with quantitative EDX analysis and X-ray diffraction (XRD). Also the optical properties were systematically studied in these Type-II NPLs that reveal strongly red-shifted photoluminescence (up to ∼150 nm) along with 2 orders of magnitude longer fluorescence lifetimes (up to 190 ns) compared to the Type-I NPLs owing to spatially indirect excitons at the Type-II interface between the CdSe core and the CdTe crown regions. Photoluminescence excitation spectroscopy confirms that this strongly red-shifted emission actually arises from the CdSe/CdTe NPLs. In addition, temperature-dependent time-resolved fluorescence spectroscopy was performed to reveal the temperature-dependent fluorescence decay kinetics of the Type-II NPLs exhibiting interesting behavior. Also, water-soluble Type-II NPLs were achieved via ligand exchange of the CdSe/CdTe core/crown NPLs by using 3-mercaptopropionic acid (MPA), which allows for enhanced charge extraction efficiency owing to their shorter chain length and enables high quality film formation by layer-by-layer (LBL) assembly. With all of these appealing properties, the CdSe/CdTe core/crown heterostructures having Type-II electronic structure presented here are highly promising for light-harvesting applications.

Colloidal synthesis and optical properties of type-II CdSe–CdTe and inverted CdTe–CdSe core–wing heteronanoplatelets

“…Similar behaviour was observed earlier for 4 and 5 ML CdSe-CdTe hetero-NPLs prepared by different protocols. [41][42][43] The PL quantum yield of our 5 ML CdSe-CdTe HNPLs, measured at room temperature in chloroform relative to Rhodamine 6G in ethanol, was 23%. This is a rather high value for the HNCs with an indirect optical transition schematically shown in Fig.…”

“…Similar extended PL decay times for type II CdSe-CdTe hetero-NPLs to core CdSe NPLs were observed also in ref. [41][42][43], while the exact 〈τ〉 values may vary within the different batches due to the complex nature of the structure and defect states in hetero-NPLs. However, no data for inverted CdTe-CdSe hetero-NPLs were presented before.…”

“…During the submission of this manuscript we became aware of related work reporting the realization of straight type-II CdSe-CdTe heteronanoplatelets obtained by different protocols. [41][42][43] In these papers the authors demonstrate that the general approach for epitaxial overgrowth of a flat CdSe core with CdTe wings results in characteristic type II optical transitions. Here, we demonstrate that not only straight CdSe-CdTe, but also an inverted type II CdTe-CdSe core-wings system can be successfully obtained by the same method.…”

Temperature dependent radiative and non-radiative recombination dynamics in CdSe–CdTe and CdTe–CdSe type II hetero nanoplatelets

“…[16][17][18][19][20][21][22] Recent studies on the synthesis of type II CdSe-CdTe and CdTe-CdSe core-lateral shell nanoplatelets have investigated their morphology, the type II band offsets as well as the prolongation of the PL decay time with respect to core only nanoplatelets at room temperature. [23][24][25] In this report we concentrate on the temperature dependent photoluminescence (PL) decay dynamics and show that the observed luminescence decay is related to spatially indirect excitons, having a significantly prolonged lifetime with respect to core only nanoplates. We further concentrate on the yet open question whether carriers at the type II junction are present as spatially indirect excitons or separated electron hole pairs.…”

“…The PLE spectrum recorded at a type II transition wavelength of 680 nm clearly proves the formation of a CdSe-CdTe hetero junction, as the PLE spectrum resembles the cumulative absorption spectrum of the CdSe-CdTe nanoplatelets, in line with previous investigations. [23][24][25] The samples were embedded in a poly(lauryl acrylate) host matrix on quartz substrates for low temperature measurements. Fig.…”