Some materials make excellent light, but refuse to carry electricity. Cambridge researchers say they have now found a way around that problem, building a new kind of LED from insulating nanoparticles once seen as impossible to power.
The team at the Cavendish Laboratory at the University of Cambridge used tiny organic “molecular antennas” to funnel electrical energy into lanthanide-doped nanoparticles, or LnNPs, and created what it describes as the first LEDs built from these materials. The findings were published in Nature.
LnNPs are known for producing highly stable and highly pure light. They emit light in the second near infrared region, which can travel deep into biological tissue, making them attractive for medical imaging and sensing technologies.
But the nanoparticles are electrical insulators, so they cannot easily carry electric current. That has blocked their use in electronic devices such as LEDs.
“These nanoparticles are fantastic light emitters, but we couldn’t power them with electricity. It was a major barrier preventing their use in everyday technology,” said Professor Akshay Rao, who led the research at the Cavendish Laboratory.
“We’ve essentially found a back door to power them. The organic molecules act like antennas, catching charge carriers and then ‘whispering’ it to the nanoparticle through a special triplet energy transfer process, which is surprisingly efficient.”
To make the devices, the researchers built a hybrid material combining organic molecules with inorganic nanoparticles. They attached an organic dye called 9-anthracenecarboxylic acid, or 9-ACA, to the surface of the LnNPs.
In the LEDs, electrical charges are directed into the 9-ACA molecules instead of the nanoparticles. The molecules absorb the energy and enter an excited triplet state. The team said that energy is then transferred to the lanthanide ions inside the nanoparticles with more than 98 percent efficiency, causing the insulating nanoparticles to emit bright, highly pure light.
The resulting devices, called LnLEDs, operate at about 5 volts. The team said they produce electroluminescence with an extremely narrow spectral width, giving them purer light output than competing technologies such as quantum dots.
“The purity of the light in the second near-infrared window emitted by our LnLEDs is a huge advantage,” said Dr Zhongzheng Yu, a lead author of the study and postdoctoral research associate at the Cavendish Laboratory.
“For applications like biomedical sensing or optical communications, you want a very sharp, specific wavelength. Our devices achieve this effortlessly, something that is very difficult to do with other materials.”
The researchers said the technology could support medical devices that see deep inside the body, as well as optical communications systems with less interference and detectors that identify specific chemicals or biological markers.
The team reported a peak external quantum efficiency greater than 0.6 percent for its NIR-II LEDs, which it described as a strong result for an early generation device. It also said there are clear paths to improve performance further.
“This is just the beginning. We’ve unlocked a whole new class of materials for optoelectronics,” said Dr Yunzhou Deng, postdoctoral research associate at the Cavendish Laboratory.
“The fundamental principle is so versatile that we can now explore countless combinations of organic molecules and insulating nanomaterials. This will allow us to create devices with tailored properties for applications we haven’t even thought of yet.”
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