Tiny antennas to bring electrical power to the un-powerable nanoparticles

A new technique uses ‘molecular antennas’ to funnel electrical energy into insulating nanoparticles, creating a new class of ultra-pure near-infrared LEDs for medical diagnostics, optical communications, and sensing.

Researchers at the Cavendish Laboratory, University of Cambridge have developed a new method to electrically power insulating nanoparticles, a feat previously thought impossible under normal conditions. By attaching organic molecules that act as tiny antennas, they have created the first-ever light-emitting diodes (LEDs) from these materials. The breakthrough, published in Nature, opens the door to a new generation of devices with applications ranging from deep-tissue biomedical imaging to high-speed data communication.

The focus of the study is a class of materials known as lanthanide-doped nanoparticles (LnNPs). These particles are renowned for their exceptional ability to emit light that is incredibly pure and stable, particularly in the second near-infrared range, which can penetrate deep biological tissues. However, a major hurdle has always been their electrically insulating nature, meaning they couldn’t be incorporated into modern electronic devices like 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.

The team’s innovative solution was to create an organic-inorganic hybrid material. They attached an organic dye with a functional group anchor, called 9-anthracenecarboxylic acid (9-ACA), to the surface of the LnNPs. In their newly designed LEDs, charges are injected into these 9-ACA molecules, acting as molecular antenna, rather than the nanoparticles themselves. The molecules capture this energy and enter an excited state known as triplet state. Normally, this triplet state is considered “dark”, or wasted, in many other optical systems. But in this system, the energy from the triplet start is transferred with over 98% efficiency to the lanthanide ions inside the insulating nanoparticles, causing it to light up brilliantly.

This new method allows the team’s “LnLEDs” to be turned on with a low operating voltage of around 5 volts and to produce electroluminescence with an exceptionally narrow spectral width, making it significantly purer than that of competing technologies like quantum dots (QDs).

“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.”

This discovery unlocks a wide range of potential applications. With their ability to emit exceptionally pure light when powered electrically, these nanoparticles could enable the development of next-generation medical devices. Tiny, injectable, or wearable LnLEDs could be used for deep-tissue imaging to detect diseases like cancer, monitor organ function in real-time, or activate light-sensitive drugs with pinpoint precision. The purity and narrow spectral width of the emitted light also hold promise for faster, clearer optical communications systems, potentially allowing more data to be transmitted with less interference. The technology could also lead to highly sensitive devices for detecting specific chemicals or biological markers.

The team has already demonstrated a peak external quantum efficiency of over 0.6% for their NIR-II LEDs, an extremely promising result for a first-generation device, and has identified clear strategies for further improvement.

“This is just the beginning. We’ve unlocked a whole new class of materials for optoelectronics,” added 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.”

This work was supported in part by a UK Research and Innovation (UKRI) Frontier Research Grant (EP/Y015584/1) and Postdoctoral Individual Fellowships (Marie Skłodowska-Curie Fellowship grant scheme).