Brilliant Glow of Paint-On Semiconductors Comes from Ornate Quantum Physics

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There is a potentially more attractive way to produce the light, and it is a core strength of the new hybrid semiconductors.

An electron has a negative charge, and an orbit it vacates after having been excited by energy is a positive charge called an electron hole. The electron and the hole can gyrate around each other forming a kind of imaginary particle, or quasiparticle, called an exciton.

“The positive-negative attraction in an exciton is called binding energy, and it’s a very high-energy phenomenon, which makes it great for light emitting,” Silva said.

When the electron and the hole reunite, that releases the binding energy to make light. But usually, excitons are very hard to maintain in a semiconductor.

“The excitonic properties in conventional semiconductors are only stable at extremely cold temperatures,” Silva said. “But in HOIPs the excitonic properties are very stable at room temperature.”

Ornate Quasiparticle Twirling

Excitons get freed up from their atoms and move around the material. In addition, excitons in an HOIP can whirl around other excitons, forming quasiparticles called biexcitons. And there’s more.

Excitons also spin around atoms in the material lattice. Much the way an electron and an electron hole create an exciton, this twirl of the exciton around an atomic nucleus gives rise to yet another quasiparticle called a polaron. All that action can result in excitons transitioning to polarons back. One can even speak of some excitons taking on a “polaronic” nuance.

Compounding all those dynamics is the fact that HOIPs are full of positively and negatively charged ions. The ornateness of these quantum dances has an overarching effect on the material itself.

Wave Patterns Resonate

The uncommon participation of atoms of the material in these dances with electrons, excitons, biexcitons and polarons creates repetitive nanoscale indentations in the material that are observable as wave patterns and that shift and flux with the amount of energy added to the material.

“In a ground state, these wave patterns would look a certain way, but with added energy, the excitons do things differently. That changes the wave patterns, and that’s what we measure,” Silva said. “The key observation in the study is that the wave pattern varies with different types of excitons (exciton, biexciton, polaronic/less polaronic).”

The indentations also grip the excitons, slowing their mobility through the material, and all these ornate dynamics may affect the quality of light emission.

Rubber Band Sandwich

The material, a halide organic-inorganic perovskite, is a sandwich of two inorganic crystal lattice layers with some organic material in between them – making HOIPs an organic-inorganic hybrid material. The quantum action happens in the crystal lattices.

The organic layer in between is like a sheet of rubber bands that makes the crystal lattices into a wobbly but stable dancefloor. Also, HOIPs are put together with many non-covalent bonds, making the material soft.

Individual units of the crystal take a form called perovskite, which is a very even diamond shape, with a metal in the center and halogens such as chlorine or iodine at the points, thus “halide.” For this study, the researchers used a 2D prototype with the formula (PEA)2PbI4.

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