Research on the light emission of black phosphors leads to new applications

(Nanowerk NewsEnergy saving LEDs (light emitting diodes) have started to replace many types of lighting both indoor and outdoor. The main reason for the advent of LEDs was material research which greatly increased the quality and intensity of light that could be achieved with these devices. But the material must be handled with a very high degree of precision, including a surface that is as free from blemishes as possible.

“They are more efficient and last longer,” said Lawrence Berkeley National Laboratory Senior Scientist (Berkeley Lab) Ali Javey, of the materials that today’s LED devices consist of, “but to get good efficiency, you really have to treat (the material) ) surface perfectly – you have to pay attention and control the surface chemistry and even then, you still have a small loss.” Javey is also a professor of electrical engineering and computer science at the University of California, Berkeley.

Javey and his team say their recent research on black phosphorus (BP) – a material of interest for its electronic properties – reveals a tantalizing ability for light emission on BP’s surface.

It turns out that very thin BP layers can be stimulated to emit useful amounts of light at certain wavelengths. What’s more, it will do so regardless of the surface. In fact, they can even allow BP to oxidize (think rust) and will still emit light in the mid-infrared (IR) without losing efficiency.

“We didn’t do anything special on the surface. We don’t do special chemistry. We don’t put a special layer of protection in place,” he said.

Javey’s team published their findings in the journal Natural Nanotechnology (“Anomalous thickness dependence of photoluminescence quantum results in black phosphorus”).

The work represents a fundamental discovery about the properties of BP, and it represents exciting prospects for application.

“Black phosphorus is great for the emission and detection of medium-wave IR light,” says Javey. “Our group and others have demonstrated very bright midwave IR LEDs before. Mid-IR LEDs using conventional semiconductors are not very efficient due to the underlying material properties. BP has an inherent advantage in that wavelength range.”

The mid-IR range is attractive for applications in night vision, sensing, spectroscopy and more, continues Javey. “Our findings emphasize the unique material characteristics of layered materials for new optoelectronic applications such as light-emitting devices and photo detectors.”

Getting Light from Thin Black Phosphorus

Javey’s lab has been investigating BP’s “magic properties” for some time. By 2022, they report that, under mechanical stress, BP can be induced to dynamically emit or detect infrared (IR) light in the desired wavelength range – 2.3 to 5.5 micrometers, which includes short to midwave infrared – and to do reversibly at room temperature.

“In this new paper, we look at how the mechanism of light emission changes as we change the thickness of the BP,” said co-author Shiekh Uddin. Within a thick BP unit, electrons and holes – that is, negatively and positively charged particles – can produce light when they collide with each other.”

Thinning below a few nanometers, however, the electrons and holes on the BP surface are so constrained that they combine like a magnet being pulled together in a pocket. This excited state, called an exciton, emits light more efficiently than isolated electrons and holes.

“Importantly, we found that the surface is less detrimental to luminescence efficiency due to the inherent crystal structure of the BP,” said co-author Naoki Higashitarumizu. It turns out that BP has an unusually low surface recombination speed. It is a measure of how quickly the carriers—electrons or holes—disappear on the surface of a material without producing light.

In fact, BP’s surface recombination speed is two orders of magnitude lower than that of other materials, said Uddin. This applies even when the surface has oxidized or been damaged by environmental exposure. As a result, we were able to achieve a bright glow even when the BP was made very thin.

Looking ahead, Javey said, “We believe low surface recombination is not limited to BP but should also apply to other layered materials with similar crystal structures.”