A thin crystal of hexagonal boron nitride is a relative of graphite, but transparent and insulating. Occasionally, single defects appear in its lattice: a boron or nitrogen atom is missing, leaving a trap for electrons. Such a trap can absorb light and re-emit it, like a tiny flashlight.
Until now, researchers mostly studied defects in the crystal's interior, away from the surface. Now, a new type of defect has been discovered that sits right on the surface. These defects glow in the green-yellow part of the spectrum — exactly where the human eye is most sensitive. Under a powerful laser, their glow fades: the light forces the defect to change shape, and it stops acting like a flashlight. But if the crystal is heated, the defect returns to its original state and starts glowing again. The energy required for this return turned out to be quite small—24.5 millielectronvolts. That's like the energy of thermal motion at room temperature.
There is also another way to permanently quench the defects: treat the surface chemically, attaching lithium and functional groups to it. This destroys the defect irreversibly. By combining laser and chemistry, you can draw arbitrary glowing patterns on the crystal surface: dark lines on a glowing background. It's like engraving, but instead of a burin, you use a light beam.
Such surface defects are especially interesting because they are in direct contact with the environment. This makes them sensitive to the slightest changes around them: temperature, pressure, the presence of molecules. That means they could become the basis for ultra-sensitive sensors. Moreover, they are easy to integrate into optical circuits and electronic devices, since the hBN crystal is only a few atomic layers thick.
The activation energy of 24.5 meV is roughly equal to the thermal energy at room temperature, so defects can spontaneously 'turn on' and 'turn off' even without external influence.