Highly Fluorescent and Stable Black Phosphorus Quantum Dots in Water

Although 2D black phosphorus (BP) shows excellent optical and electronic properties, there are few reports on the photoluminescence (PL) properties of BP nanostructures because of the low yield of mechanical exfoliation, instability in water, and relatively weak emission. Herein, liquid exfoliation is combined with surface passivation to produce fluorescent BP quantum dots (BPQDs) with a high yield. The BPQDs exhibit strong PL in both ethanol and water and the absolute fluorescent quantum yield in water reaches 70%. Moreover, the BPQD solution exhibits stable PL for 150 d under ambient conditions and better photostability than conventional organic dyes and heavy-metal semiconducting nanostructures with intense fluorescence. The experiments and theoretical calculation reveal that the intense and stable PL originates from the intrinsic band-to-band excitation states and two surface states related to the P-OH and P-O-CH2CH3 bonding structures introduced by passivation. The polar water molecules remove many nonradiative centers and simultaneously increase the P-related fluorescent groups on the surface of BPQDs. Therefore, PL from the BPQDs in water is enhanced largely. The excellent fluorescent properties of BPQDs in an aqueous solution bode well for bioimaging and the negligible biotoxicity and distinct cell images suggest large potential in the biomedical and display fields.