Materials exhibiting circular dichroism absorb light differently depending on whether the light has left or right circular polarisation. Those made of assemblies of inorganic nanoparticles potentially have the highest tunability and the strongest optical properties, but none are so strong as those recorded by an international group led by Cornell University in New York state, USA. In tests with the state-of-the-art Mueller Matrix Polarimeter at Diamond’s B23 beamline, the researchers found that a novel method can produce chiral semiconductors from inorganic, achiral nanoparticles with record-breaking circular dichroism. Published in Science, the work could help advance a range of technologies, from optical communication and drug selection, to improving the brightness and contrast of the latest consumer televisions.
Just as your hands can be left or right – that is, mirror images of each other – so can molecules. The property is called chirality, and it occurs in natural as well as manmade materials. The drug penicillin is derived from a naturally occurring mould that has a chiral structure, for instance – and it is only this structure, not its oppositely handed counterpart, which works as an antibiotic. When light passes through a chiral structure, its two circularly polarised components – themselves referred to as left and right – are absorbed by different amounts, an effect known as circular dichroism (CD).
Materials exhibiting CD are very useful. Shine a light through a drug candidate, and the amount that gets through a CD material can tell you whether the drug has the left- or right-handed structure. CD materials can be used in signal processing in optical communications, too, when alternate light polarisations encode different calculational states. They can even improve the brightness and contrast of displays based on LEDs or OLEDs, which generate circularly polarised light.
One way to make these materials is to start with nanoscale, achiral building blocks, and get them to twist during assembly so that the resultant bulk structures are chiral, and capable of generating CD. Organic molecules have proved best at this in the past, as their bonding is much more flexible for creating big assemblies and their units are identical copies of each other. Now, however, Professor Richard Robinson at Cornell and his colleagues, have shown that inorganic nanoparticles also have a lot of potential for CD materials. His team developed magic-sized cluster assemblies, where each of the nanoclusters are identical and helically assembled. Control of the helical assembly enables them to tune the chiral optical response. One way to think of this would be to compare screws in carpentry. Their methods would be akin to turning handedness and twist ratio of a screw. His team includes researchers from various institutions in Europe, including Dr Giuliano Siligardi and Dr Tamás Jávorfi at Diamond’s B23 beamline. According to Prof. Robinson:
The development of inorganic semiconducting nanoparticles with intense chiroptical properties has lagged. Our work demonstrates that inorganic semiconducting materials can present exceptional properties, promising to address scientific challenges and spawn new areas of research.
Their team’s trick is to use an unusual assembly technique known as meniscus-guided evaporation, which begins with a solution of inorganic nanoparticles sandwiched between two parallel plates. As the solution dries, the shifting evaporation front exerts a torque on the nanoparticles, causing them to twist into chiral chains. According to measurements taken with B23’s Mueller Matrix Polarimeter (MMP), the g-factor – the ratio between the CD and the absorbance that can be a maximum of ±2 – was found to be as high as 1.3 for nanoparticles made of cadmium sulphide (CdS), a record for inorganic assemblies. Dr Siligardi says these measurements could only have been performed at Diamond:
B23 is the only synchrotron beamline in the world with an MMP capable of measuring the linear and circular anisotropies in the UV–visible spectral region of light over areas of several mm2 at 50μm resolution. And with Diamond-II, this will get even better, with resolutions 25 to 100 times smaller.
The work on inorganic chiral assemblies paves the way for more advanced technology that relies on CD. It also opens more avenues for the study of optoelectronic materials with B23’s MMP, says Dr Siligardi:
The MMP allows us to better understand how they work and identify the critical aspects that can be improved and controlled. Theoretical and analytical quality control are crucial for the optimisation of sample preparation to produce reproducible materials that are the sine qua non for any commercial application.
To find out more about the B23 beamline please contact the Principal Beamline Scientist Giuliano Siligardi: [email protected].
Ugras, T. J. et al. Transforming achiral semiconductors into chiral domains with exceptional circular dichroism. Science 387 (6733) 2025. DOI: 10.1126/science.ado7201
Image credits: authors of the publication
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