Introduction. Colloidal 2D nanoplatelets (NPLs) demonstrate remarkable optoelectronic properties. In particular, they have a narrow and –at room temperature- homogeneously broadened emission linewidth, fast exciton... [ view full abstract ]
Introduction. Colloidal 2D nanoplatelets (NPLs) demonstrate remarkable optoelectronic properties. In particular, they have a narrow and –at room temperature- homogeneously broadened emission linewidth, fast exciton recombination and high fluorescence quantum efficiency. These advantageous properties can be further tuned in heterostructures, where in the case of 2D materials a second semiconductor can be grown either on top or bottom facets (core/shell nanoplatelets), or on the lateral facets of the nanoplatelet (core/crown nanoplatelets). Here, we present results on a ternary architecture, CdSe/CdS/CdTe core/crown nanoplatelets. Due to the peculiar band alignment of the three semiconductors, the CdS layer acts as a hole tunneling barrier, while electron relaxation from the CdTe crown into the CdSe remains possible.
Methods. CdSe/CdS/CdTe core/crown nanoplatelets were synthesized following literature protocols (Tessier et al., Nano Lett. 2014, 14, 207−213; Pedetti et al. J. Am. Chem. Soc. 2014, 136, 16430−16438) and drop cast on a sapphire substrate to form a close-packed thin film. We loaded the samples in a close-cycle cryostat and cooled them to 4K to investigate the temperature dependent emission properties. We excited the samples with a 405 nm pulsed laser and collected the fluorescence spectra and time-resolved fluorescence decay.
Results and discussion. Due to the staggered band offset between CdSe and CdTe, we observed emission from an indirect transition around 650 nm. As CdS forms a barrier for hole relaxation between crown and core regions, the CdSe/CdS/CdTe yielded an additional emission peak from the CdSe core, in contrast with CdSe/CdTe core/crown nanoplatelets without a barrier. The resulting dual emission was investigated as a function of temperature. The different nature of both emission peaks (direct in CdSe vs. indirect across the CdSe/CdTe interface) yielded a spectrally and temporally stable indirect transition as a function of temperature, while the emission rate of the CdSe emission increased at lower temperatures, and the spectral position shifted to shorter wavelengths. We finally explored the possibility of using the dual emission as a ratiometric sensor.
