Sometimes when high technology lets you down, the path to a brighter future can be opened by the humble contents of a stationery drawer. That appears to be the lesson from a breakthrough that could lead to better solar cells and light emitting diodes (LEDs), made possible by a roll of sticky tape.
Phosphorus sheets a single atom thick are known as phosphorene, and have interesting properties that scientists are keen to explore. However, making such sheets has proven a challenge, and even when very thin sheets have been produced, it has not always been apparent if they are one, two or several atoms thick.
“Consequently, many important fundamental properties, such as exciton dynamics, remain underexplored,” a team led by Dr. Yuerui Lu of the Australian National University report inLight: Science and Applications.
Lu and his colleagues solved the problem with Scotch tape from the local office supplies store, which they applied to a crystal of black phosphorus. When peeled off, the tape took with it a thin strip of phosphorus.
Thicknesses varied, sometimes just a single monolayer, forming true phosphorene, and other times sheets two or more atoms thick stuck to the tape. However, the team realized color changed with thickness, allowing them to measure it using optical interferometry.
Multiple sticky tapings gave the team a selection of sheets with a variety of depths, and they set about exploring the behavior of the new material, although Lu told IFLScience that they do peel it off the tape first.
The idea of using sticky tape was borrowed from similar work on graphene, but phosphorene has the added advantage of being a semiconductor, with the potential to replace silicon in many parts of the electronics industry.
“Because phosphorene is so thin and light, it creates possibilities for making lots of interesting devices, such as LEDs or solar cells,” said Lu, “It shows very promising light emission properties.”
Most significantly, the thickness of the phosphorene changes its properties. “By changing the number of layers we can tightly control the band gap, which determines the material’s properties, such as the color of LED it would make,” said Lu.
The band gap for an isolated layer of phosphorene was measured at 1.75 electron volts, which is equivalent to 700 nanometers, the wavelength of the reddest light we can see. Thicker sheets have progressively smaller band gaps and thereforeinfrared wavelengths.
Visible light LEDs have much wider applications than those with wavelengths too long to see, but Lu told IFLScience that “While in principal it is hard to get shorter wavelengths, some coatings can give higher band gaps.”
Likewise, Lu believes phosphorene adjusted to have a variety of suitable band gaps could be the future of multijunction solar cells, which offer enormously increased efficiency by stacking cells on top of each other. This is because the top junction captures high-energy photons while allowing those with lower energies through, which are then collected by junctions with lower band gaps placed beneath.