New research introduces a contactless way to see how mirror-image materials respond differently to circularly polarized light, without first building them into a complete electronic device. The researchers developed a novel method based on light-induced charge separation that allows to directly probe how the material’s structure acts like a microscopic filter, influencing how electrons separate and move. The advance is important because it gives scientists a faster, cleaner way to test promising materials without metal contacts and other parts of a finished device affecting the results. The approach could ultimately help researchers develop new mirror-image materials for technologies that use circularly polarized light, electrical charge, and electron spin.

Imagine two nearly identical materials sitting side by side. They are mirror images of each other, much like your left and right hands. Shine ordinary light on them, and they seem much the same. But shine light that twists in one direction, and one material responds more strongly. Twist the light the other way, and its mirror-image partner takes the lead.

That is the story behind a new study from researchers at the Institute of Chemistry and the Center for Nanoscience and Nanotechnology at the Hebrew University of Jerusalem, published in Small. Led by Dr. Joanna Dehnel and Dr. Igal Levine investigated chiral 2D perovskites, materials that come in two mirror-image forms—much like a left and right hand.

The researchers wanted to know what would happen to the flow of electrons when these mirror-image materials interacted with circularly polarized light, light whose electric field rotates as it travels.

But there was a problem.

How do you measure the intrinsic electronic properties of a material without changing it?

Scientists normally study a material’s electrical behavior by turning it into a device and attaching electrical contacts. But those contacts can introduce defects and other effects that can obscure the intrinsic electrical response of the material, or make it difficult to know whether the signal comes from the material itself or from the device built around it.

The team took a different approach.

They introduced a technique called circularly polarized time-resolved surface photovoltage, or CP-TRSPV. Instead of requiring a complete device, the method allows researchers to observe how electrical charges separate inside the material without adding a top electrical contact. It can also follow what happens over an unusually wide range of time, from nanoseconds to milliseconds.

Think of it as listening to what the material has to say before wiring it into a device.

And the material had quite a story to tell.

Mirror-image materials preferred opposite kinds of light

When the researchers switched between right- and left-circularly polarized light, the two mirror-image materials responded in opposite ways.
The R-form responded more strongly to right-circularly polarized light, while the S-form responded more strongly to left-circularly polarized light.

As an important comparison, a racemic sample, a mixture without an overall mirror-image preference, showed no significant difference between the two kinds of light.

The result suggested that the crystal’s chiral structure itself was responsible for the effect.

Even more surprising was the size of the electrical response.

The researchers measured a photovoltage anisotropy factor, called gSPV, as large as −0.7 in the R material and 0.17 in the S material. The electrical difference was roughly 1,000 times larger than the difference seen simply in how the materials absorbed the two types of circularly polarized light.

That meant something more interesting was happening than one material simply absorbing more light.

The crystal acts like a filter for electrons

The results are consistent with an effect known as chiral-induced spin selectivity, or CISS.

Electrons have a quantum property called spin. In these chiral materials, the crystal structure can favor electrons with one spin orientation over another. In simple terms, the material can behave somewhat like a microscopic filter, allowing certain electrons to move or separate more efficiently than others.
The researchers were also able to watch the electrical response unfold over time.

The initial spin-related process happens extremely quickly. But the difference it creates in the number of separated electrical charges can remain much longer, from nanoseconds into the millisecond range. The researchers stress that this does not mean the electron spins themselves survive for milliseconds. Instead, the initial filtering creates different populations of separated charges, and that difference remains visible as the charges later become trapped, released or recombine.

A faster path toward future technologies
The novel methodology could help scientists better understand materials being explored for spintronics and optoelectronics, technologies that use not only electrical charge and light, but also electron spin.

Chiral perovskites are particularly interesting because their structure may help control charge and spin without requiring an external magnetic field. Potential applications include circularly-polarized light detectors, spin-based electronics and other devices that respond differently to different forms of polarized light.

The new measurement method could also speed up the search for useful materials.

Instead of building a complete electronic device every time scientists create a new chiral material, researchers could first use the contactless technique to test how strongly the material responds, in a rapid manner that can also be extended to high-throughout studies. The authors say the approach could potentially be extended to many different chiral materials and chemical compositions.

The study turns a difficult-to-observe quantum effect into an electrical signal researchers can follow through time.