Adding Chirality to Semiconductors Could Improve Light-Based Technology

A robust, scalable method to introduce chirality into the band structure of semiconductors could advance photonic technologies that rely on controlling light polarization, like displays, sensors, and optical communications, by giving scientists simultaneous control over light, spin, and charge.

Chiral materials are often created through exciton-coupling, a process where light excites nanomaterials to form excitons that interact and share energy with each other. Historically, exciton-coupled chiral materials were made from organic molecules. Creating chiral materials from inorganic semiconductors has proven challenging due to the precise control needed over nanomaterial interactions.

Researchers from Cornell University, Rochester Institute of Technology (RIT), and several European institutions collaborated on a method to form chiral films out of three different inorganic semiconductor nanoclusters through an evaporative deposition process. The resulting films all showed circular dichroism, a strong indicator of chirality.

“Circular dichroism means the material absorbs left-handed and right-handed circularly polarized light differently, like how screw threads dictate which way something twists,” Cornell professor Richard D. Robinson said.

The researchers directed highly concentrated solutions of asymmetric, semiconductor magic-sized nanoclusters of achiral cadmium sulfide (CdS), cadmium selenide (CdSe), and cadmium telluride (CdTe) through a controlled drying meniscus front. Magic-sized nanoclusters are atomically and electronically identical, enabling greater wavefunction overlap and coupling between neighboring nanoclusters. Also, the bandgap, composition, and size of semiconducting magic-sized clusters can be synthesized to tailor the chiroptic responses over a range of wavelengths.

A scanning electron micrograph of a chiral magic-sized cluster film with micrometer-scale bands being broken into their nanometer-scale filament units. False color indicates right-handed (red), achiral (white), and left-handed (blue) domains. Courtesy of Robinson Group/Cornell University.

Using meniscus-guided evaporation, the researchers twisted the linear nanocluster assemblies into helical chains, forming homochiral domains several square millimeters in size. By controlling the evaporation geometry, the researchers achieved various domain shapes and sizes that could move smoothly between left- and right-handed chirality.

“We realized that by carefully controlling the film’s drying geometry, we could control its structure and its chirality,” Robinson said. “We saw this as an opportunity to bring a property usually found in organic materials into the inorganic world.”

The chiroptic films achieved via meniscus-guided deposition demonstrated an exceptionally large light-matter response, surpassing previously reported record values for inorganic semiconductor materials by nearly two orders of magnitude. G-factors (the anisotropy factors that measure the degree of chirality in a system) reached magnitudes as high as 1.30 for drop-cast films and 1.06 for patterned films, approaching theoretical limits.

RIT professor Steve Weinstein developed a theoretical framework to interpret the nanocluster fiber deposition patterns observed by the team. This framework helped the team recognize important mechanisms for achieving chiroptic films, including the fluid flows responsible for twisting the fibers.

“Our investigation examines the large-area films that are formed and the mechanisms behind them,” Weinstein said. “The result is the highest chiral signal reported for inorganic materials to date. This evaporation-driven technique not only induces nanometer-scale fiber twists but also allows us to tune their chiroptical properties by controlling the flow parameters.”

The use of three different semiconducting nanocluster systems highlights the scope of the method, indicating that it could apply to other nanocluster species or colloidal nanoplatelets.

“I’m excited about the versatility of the method, which works with different nanocluster compositions, allowing us to tailor the films to interact with light from the ultraviolet to the infrared,” researcher Thomas Ugras said. “The assembly technique imbues not only chirality but also linear alignment onto nanocluster fibers as they deposit, making the films sensitive to both circularly and linearly polarized light, enhancing their functionality as metamaterial-like optical sensors.”

The technique for forming chiral films from achiral, solution-processable semiconductors could be used to design and fabricate complex chiroptical materials in ways that are both scalable and versatile. These materials could be used for holographic displays, quantum computing, ultralow-power devices, and medical diagnostics.

For example, chiral nanomaterials could be integrated into wearable sensors to detect the way glucose rotates polarized light, enabling highly sensitive glucose monitoring through the skin. This technology would eliminate the need for painful finger pricks, paving the way for continuous, more comfortable glucose tracking.

Beyond nanocluster films, the study provides insight into natural chiral structures, such as DNA, and offers a pathway to extend the technique to other chiral molecules and nanomaterials for engineering twisted structures.

Future work by the team will focus on extending the technique to other materials, such as nanoplatelets and quantum dots, and refining the technique for industrial-scale manufacturing processes that coat devices with semiconductor thin films.

“We want to understand how factors like cluster size, composition, orientation, and proximity influence chiroptic behavior,” Robinson said. “It’s a complex science, but demonstrating this across three different material systems tells us there’s a lot to explore and it opens new doors for research and applications.”

The research was published in Science (www.doi.org/10.1126/science.ado7201).