Black phosphorus is a unique optoelectronic material with a strong, tunable emission of mid infrared to visible wavelengths. A better understanding of the physics of this system is of great interest because of its technical application fields.
Intriguing results were achieved by the group around Prof. Javey in Berkely, who analyzed the dependency of the photoluminescence quantum yield in black phosphorus [1]. When the thickness decreased to approx. 4 nm, they observed a decrease in photoluminescence due to enhanced surface carrier recombination. This had actually been expected. Unexpectedly, the intensity increased significantly as the thickness further decreased.  The average quantum yield value for mono layers lay at about 30%. This behavior arises from the free-carrier to excitonic transition in black phosphorus thin films, and differs from the behavior of conventional semiconductors, where the photoluminescence quantum yield declines with decreasing thickness.
There was another surprising result: The carrier recombination speed on the surface of black phosphorus is two orders of magnitude lower than the lowest value reported in the literature for any semiconductor with or without passivation. This is interesting for photonic applications in general and photonic embedded circuits in particular.  A material system of black phosphorus thus results in significantly higher quantum yields than conventional semiconductors.  
The photoluminescence measurements were carried out with a Cryostation s200 from Montana Instruments. The closed-cycle cryostats from MI allow a very high numerical aperture at minimal vibration levels inside the sample chamber.
The full article is available here:
[1] Higashitarumizu, N., Uddin, S.Z., Weinberg, D. et al. Anomalous thickness dependence of photoluminescence quantum yield in black phosphorous. Nat. Nanotechnol. 18, 507–513 (2023). https://doi.org/10.1038/s41565-023-01335-0

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