Colloidal Bi2S3 Nanocrystals: Quantum Size Effects and Midgap States |
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Authors: | Mauro Aresti Michele Saba Roberto Piras Daniela Marongiu Guido Mula Francesco Quochi Andrea Mura Carla Cannas Mauro Mureddu Andrea Ardu Guido Ennas Vasco Calzia Alessandro Mattoni Anna Musinu Giovanni Bongiovanni |
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Affiliation: | 1. Dipartimento di Fisica, Università degli Studi di Cagliari, Cittadella Universitaria, S.P. Monserrato‐Sestu km 0.7, Monserrato, Italy;2. Dipartimento di Scienze Chimiche e Geologiche, Università degli Studi di Cagliari, Cittadella Universitaria, S.P. Monserrato‐Sestu km 0.7, Monserrato, Italy;3. CNR‐IOM Cagliari, Cittadella Universitaria, Monserrato, (Ca), Italy |
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Abstract: | Among solution‐processed nanocrystals containing environmentally benign elements, bismuth sulfide (Bi2S3) is a very promising n‐type semiconductor for solar energy conversion. Despite the prompt success in the fabrication of optoelectronic devices deploying Bi2S3 nanocrystals, the limited understanding of electronic properties represents a hurdle for further materials developments. Here, two key materials science issues for light‐energy conversion are addressed: bandgap tunability via the quantum size effect, and photocarrier trapping. Nanocrystals are synthesized with controlled sizes varying from 3 to 30 nm. In this size range, bandgap tunability is found to be very small, a few tens of meV. First principles calculations show that a useful blueshift, in the range of hundreds of meV, is achieved in ultra‐small nanocrystals, below 1.5 nm in size. Similar conclusions are envisaged for the class of pnictide chalcogenides with a ribbon‐like structure Pn4Ch6]n (Pn = Bi, Sb; Ch = S, Se). Time‐resolved differential transmission spectroscopy demonstrates that only photoexcited holes are quickly captured by intragap states. Photoexcitation dynamics are consistent with the scenario emerging in other metal–chalcogenide nanocrystals: traps are created in metal‐rich nanocrystal surfaces by incomplete passivation by long fatty acid ligands. In large nanocrystals, a lower bound to surface trap density of one trap every sixteen Bi2S3 units is found. |
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Keywords: | Bi2S3 colloidal nanocrystals time‐resolved spectroscopy midgap states quantum size effects |
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