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John O | March 2018

Study looks into the physics-defying quantum properties of HOIP materials


by josh perry, editor
jperry@coolingzone.com

 

researchers at the georgia institute of technology in atlanta led an international study into the semiconducting properties of hybrid organic-inorganic perovskites (hoip) that defy physics by producing stable quantum properties despite intense molecular motion.

 


georgia tech graduate research assistant felix thouin in carlos silva's lab stands at a table where laser light in the visible range undergoes processing to ready it for use in measuring materials qualities. (georgia tech / allison carter)

 

 

according to a report from georgia tech, researchers from five research institutions in four countries were finally able to measure a typical hoip and found that its quantum properties were as solid as molecularly-rigid semiconductors, including those based on graphene.

 

hoip semiconductors are of particular interest because of their ability to absorb and emit light, which makes them useful for applications ranging from optoelectronics to lasers to led. also, hoip semiconductors, unlike those produced from graphene, are capable of being made in low temperatures from solutions.

 

the 3-d stack of layered 2-d sheets that compose hoip materials self-assembles and still holds onto its 2-d quantum properties. the article added, “those sheets are held together by interspersed layers of another molecular structure that is a bit like a sheet of rubber bands. that makes the scaffolding wiggle like a funhouse floor.”

 

it is unusual to have quantum properties that are stable with that much movement and at room temperature, rather than the super-cooled environments in which quantum properties are typically explored.

 

“hoips are great semiconductors because their electrons do an acrobatic square dance,” the article said. “energy levels in a system can free the electrons to run around and participate in things like the flow of electricity and heat. the orbits, which are then empty, are called electron holes, and they want the electrons back.”

 

it continued, “the electrons and holes race around each other like dance partners pairing up to what physicists call an ‘exciton.’ excitons act and look a lot like particles themselves, though they’re not really particles.”

 

excitons give semiconductors desirable properties when electricity is applied and some form biexcitons, which enhance the energy even higher. researchers saw biexciton formation as high as 80-90 percent in hoip, which makes them ideal for lasers and led.

 

the research was recently published in physical review materials. the abstract stated:

 

“with strongly bound and stable excitons at room temperature, single-layer, two-dimensional organic-inorganic hybrid perovskites are viable semiconductors for light-emitting quantum optoelectronics applications. in such a technological context, it is imperative to comprehensively explore all the factors—chemical, electronic, and structural—that govern strong multiexciton correlations.

 

“here, by means of two-dimensional coherent spectroscopy, we examine excitonic many-body effects in pure, single-layer (pea)2pbi4 (pea = phenylethylammonium). we determine the binding energy of biexcitons—correlated two-electron, two-hole quasiparticles—to be 44±5 mev at room temperature.

 

“the extraordinarily high values are similar to those reported in other strongly excitonic two-dimensional materials such as transition-metal dichalcogenides. importantly, we show that this binding energy increases by ∼25% upon cooling to 5 k.

 

“our work highlights the importance of multiexciton correlations in this class of technologically promising, solution-processable materials, in spite of the strong effects of lattice fluctuations and dynamic disorder.”

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