DOI: https://doi.org/10.29363/nanoge.hpatom.2022.007
Publication date: 30th October 2021
Colloidal lead halide perovskite quantum dots (QDs) have recently emerged as a unique class of low-cost, versatile semiconductors of high optoelectronic quality. Their photoluminescent properties are highly dependent upon their surfaces. In the example of CsPbBr3 QDs, an experimentally validated strategy for healing the surface trap states and for improving the colloidal stability is to treat CsPbBr3 QDs with didodecyldimethylammonium bromide (DDAB) combined with/without cesium bromide (CsBr) or lead bromide (PbBr2). Although the typical observation of increased photoluminescence quantum yields (PLQYs) and longer radiative lifetimes after surface treated with DDAB have been intensively studied by theoretical and experimental techniques, physical evidence regarding how post-synthesis surface treatments alter the surface structure and recover luminescence is still lacking. Besides, the position of anionic ligands (i.e., oleate) on the surface is not directly addressed. Herein, we demonstrate post-synthetic surface-treated DDAB ligands largely replace native oleate via 1H and 13C NMR. Surface-selective 133Cs and 207Pb solid-state NMR (SSNMR) spectra correlate to the -NH3+, a-CH2, N+-[CH3/CH2-]4 groups of native dodecylammonium, native oleate, and/or surface treated DDAB surface ligands, respectively. The presence of an additional 133Cs NMR signal with unique 133Cs chemical shift in surface-selective 133Cs spectrum demonstrates surface Cs sites are coordinated by oleate ligands, which is confirmed by DNP-enhanced 1H → 13C{133Cs} CP D-HMQC experiments. Comparison of the calculated and experimental 1H{133Cs} S-RESPDOR and 1H{207Pb} S-REDOR dipolar dephasing curves indicate QDs are CsBr terminated after surface treatment with DDAB-only, DDAB + CsBr, and DDAB + PbBr2, with quaternary ammonium ligands partially occupying A sites. These results highlight the utility of high-resolution solid-state NMR spectroscopy for studying how the surface structure of nanomaterials affects optical properties.
Solid-state NMR experiments (Conventional or DNP-enhanced) and data analysis (Y.C. and A.J.R.) were supported by the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences, Materials Science and Engineering Division. The Ames Laboratory is operated for the U.S. DOE by Iowa State University under Contract DE-AC02-07CH11358. A.J.R. acknowledges additional support from the Alfred P. Sloan Foundation through a Sloan research fellowship.
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