Negative Thermal Quenching and Size-Dependent Optical Characteristics of Highly Luminescent Phosphorene Nanocrystals

Highly luminescent and direct band gap black phosphorus (BP) nanocrystals (NCs) synthesized using a cost-effective sonochemical exfoliation route are reported. Microscopic studies have confirmed the exfoliation of few-layer nanoflakes and BP NCs with variable average size between approximate to 2 and approximate to 8 nm. Structural and vibrational characteristics reveal the superior crystalline quality by retaining the phase and crystalline symmetry in NCs. Size-dependent absorption characteristics are observed by controlling the nanoparticle size and show a progressive blue shift near band edge absorption on reducing the size, useful for selective multicolor photodetection. Temperature-dependent photoluminescence measurements from room temperature (300 K) down to 10 K are performed. Most interestingly, BP NCs with the lowest diameter exhibit a novel negative thermal quenching behavior as compared to positive thermal quenching observed usually in bulk semiconductors and BP NCs with larger diameter. Metal-semiconductor-metal type (Au/BP-NCs/Au) photodetectors are fabricated to study their photoresponse properties, which reveal the potential application of BP NCs as optical modulators.