Light emission from black phosphorus research suggests new possibilities.

Numerous types of lighting, both indoors and outdoors, are beginning to be replaced by energy-efficient LEDs (light-emitting diodes). Materials research, which significantly improved the quality and intensity of light that can be achieved with these devices, has been a major contributor to the rise of LEDs. However, these materials need to be handled with extreme precision, including surfaces that are as free of imperfections as possible.

Ali Javey, a senior faculty scientist at the Lawrence Berkeley National Laboratory (Berkeley Lab), said of the materials used in today’s LED devices, “They’re more efficient and last longer.” However, “to get good efficiencies, you really have to treat the [material] surfaces to perfection—you have to watch and control the surface chemistry, and even then, you still have some light losses,” she said of the materials. Additionally, Javey teaches computer science and electrical engineering at the University of California, Berkeley.

According to Javey and his group, their most recent research on black phosphorus (BP), a material of interest due to its electronic properties, uncovered a tantalizing capability for light emission at the surface of BP.

“Previously, our group and others demonstrated very bright midwave infrared LEDs. Because of fundamental material features, traditional semiconductor-based mid-IR LEDs are inefficient. In that wavelength band, BP has a natural advantage.”

Senior Faculty Scientist Ali Javey, a professor of electrical engineering and computer sciences at the University of California, Berkeley.

It turns out that it is possible to cause extremely thin layers of BP to emit useful quantities of light at particular wavelengths. Furthermore, it will do so regardless of the surface. In fact, they can even allow BP to oxidize, analogous to rust, and it will continue to emit light in the mid-infrared (IR) range without losing any efficiency.

“The surface is not modified in any special way by us. We do no unique science.” We do not apply any special layers of protection,” he stated.

The findings that the Javey team discovered were published in the Nature Nanotechnology journal.

The work offers exciting applications as well as a fundamental discovery regarding the properties of BP.

Javey stated, “Black phosphorus is extremely useful for midwave IR light emission and detection.” Midwave IR LEDs with extremely high brightness have previously been demonstrated by our group and others. Due to fundamental material properties, conventional semiconductor-based mid-IR LEDs are inefficient. BP enjoys an innate benefit in that frequency range.”

Javey went on to say that applications in spectroscopy, night vision, sensing, and other areas make use of the mid-IR range. The novel optoelectronic applications of layered materials, such as photodetectors and light-emitting devices, are highlighted by our findings because of their distinctive material properties.”

The Javey lab has been studying the “magical properties” of BP for some time. Getting light from thin black phosphorus They reported in 2022 that BP can be induced to emit or detect infrared (IR) light dynamically and reversibly at room temperature under mechanical strain in a range of desirable wavelengths—2.3 to 5.5 micrometers, which span the short- to mid-wave infrared.

Co-author Shiekh Uddin stated, “In this new paper, we look at how the light emission mechanism changes when we change the thickness of BP.” When electrons and holes, or negatively and positively charged particles, collide in thick BP units, they can produce light.”

Diminished under a couple of nanometers, in any case, the electrons and openings at the BP surface are restricted to the point that they consolidate like magnets drawn together in a pocket. This energized state, called exciton, radiates light more proficiently than disconnected electrons and openings.

Co-author Naoki Higashitarumizu stated, “Importantly, we find the surface is less detrimental to the luminescence efficiency owing to the inherent crystal structure of BP.” It turns out that BP has a surface recombination velocity that is unusually low. That indicates the rate at which carriers—electrons or holes—are lost on the surface of the material without producing light.

According to Uddin, BP actually has a surface recombination velocity that is two orders of magnitude lower than that of other materials. This is true even when the surface has been damaged or oxidized by exposure to the environment. Accordingly, we can accomplish splendid light discharge in any event when BP is made extremely slender.

Pushing ahead, Javey said, “We accept the low surface recombination isn’t restricted to BP yet ought to likewise be pertinent to other layered materials with comparable precious stone designs.”

More information: Naoki Higashitarumizu et al, Anomalous thickness dependence of photoluminescence quantum yield in black phosphorous, Nature Nanotechnology (2023). DOI: 10.1038/s41565-023-01335-0

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