Until recently it was believed that certain classes of nanoparticles were electrically insulating and could therefore not be made to emit light electrically. In potentially a major breakthrough for LED technology, optical communication and improved medical diagnoses, new research from Cambridge shows a proof-of-concept of how lanthanide-doped nanoparticles (LnNPs) can in fact be used to emit precisely in the near-infrared window with high photostability.
Current technologies involving LnNPs require optical excitation of LEDs as a result of which they were unsuitable for use within normal electrical systems – they always needed an external light source to excite them.
The new systems, while successful, rely on inefficient charging methods thanks to a fluoride host in sodium lanthanide tetrafluoride (NaLnF₄) with ≈8 eV band gap. In other words, the system is not quite ready for mass production. However, the current research, published in Nature last month, is promising because this is the first time such results have been achieved.
The method discovered at the Cavendish laboratory involves injecting electrons and holes through select materials and recombining them on 9-anthracenecarboxylic (9-ACA) ligands to generate singlet and triplet excitons – excited states of these particles – within the organic molecules in the system. This much was known already. But in most existing systems the energy released in these states are considered wasted energy which bring down the efficiency of the system as a while.
In the Nature paper the physicists describe that they were able to convey the charges from the organic ‘antenna’ molecules to the nanoparticles with over 98% efficiency, causing the lanthanide ions to light up.
For students of condensed matter physics the energy transfer would be of particular interest. The Cavendish group were essentially able to conduct a Triplet Energy Transfer from the T1 state of the 9-ACA through a Dexter-type process. They also considered the Förster Resonance Energy Transfer (FRET) process but concluded this would be inefficient due to low energy band overlap between the lanthanide ions and the 9-ACA blue emission.
The set-up works with just 5V of operating potential difference and produces a narrower spectral electroluminescence than even quantum dots. Consequently, for applications where sharp, very precise wavelengths of light are needed – such as optical communications and biomedical imaging – these new LnLEDs hold incredible promise as a ‘new class of materials for optoelectronics.’
Next, the team plans to test the same, or a similar, procedure with other combinations of organic molecules and insulating nanomaterials to expand our understanding of light emission through compact electronic set-ups.
