Upconversion is a phenomenon involving the emission of one photon (after the absorption of multiple photons of reduced energy), which boosts the light from lower to higher frequencies.
In a new study, scientists gained considerable insight on photon upconversion in nanoparticles. Prospective applications of this new research include high-resolution spectroscopy, high-density information storage, anti-counterfeiting measures and scientific imaging.
There are numerous kinds of materials that can exhibit optical behaviour such as upconversaion. Luminescent materials like semiconductor or lanthanide nanoparticles are more attractive than fluorescent organic dyes because they are stable when subjected to light. Lanthanide ions, used in the new study, are capable of transferring the near infrared (NIR) light of an economic continuous wave laser, towards higher-energy visible and even ultraviolet frequencies.
In the study, published by the journal Angewandte Chemie, researchers used advanced spectroscopy and theoretical modelling to determine that the movement of excitation energy has a massive effect on upconversion dynamics. The scientists said that the 'dopant ions spatially separated' (DISS) nanostructures used to reach their conclusion, could be tailored to the manipulate upconversion dynamics.
Upconversion dynamics have been thought to be established exclusively by the emitting ions and their interactions with adjoining sensitizing ions. The new study revealed this does not hold true for nanostructures. The study team showed the luminescence behaviour in nanocrystals is very much dependent on the migration sequence of the excitation energy.
The scientists discovered a strong connection between the arbitrary nature of the energy migration and the upconversion luminescence behaviour by using a combination of advanced spectroscopy and time-resolved Monte Carlo simulation; a mathematical model used to evaluate the risk and uncertainty. The researchers used so-called 'dopant ions spatially separated' (DISS) nanostructures, where activators and sensitizers are situated into various spatial regions of a single nanoparticle. The effect on energy migration was quantitatively portrayed by adjusting the thickness of the migration level of the nanostructure or by changing the migrator ion dopant concentration in the migration stratum.
Hence, the team was able to establish that, due to of its arbitrary nature, the movement of excitation energy between any two points in the crystal takes longer than what would be anticipated from a direct point-to-point energy transfer.
By using this new important insight, the scientists could effectively manage the upconversion luminescence-time behaviour, both the rise and the decay sequence, by manipulating the energy migration paths in explicitly-tailored DISS nanostructures. The study's outcome is noteworthy for the usage of these kinds of materials in a range of scientific applications, including super high-resolution spectroscopy.
This new insight comes after a study published in October revealed that scientists from the National University of Singapore had developed a new kind of crystalline material that was also capable of controlling the upconversion process.
In that study, researchers used hybrid materials known as metal-organic frameworks (MOFs) to change near-infrared radiation into visible light. They found the packing of the molecules accountable for the light upconversion is a major factor in establishing the power of the visible light that can be emitted from these materials. MOFs with expanded structures that had larger voids between molecules were capable of greater light conversion efficiency, the researchers found.
The Singapore researchers also said using this knowledge of MOFs’ intermolecular interactions, the creation of structured molecular solids with desired upconversion qualities is now achievable. Expanding on these findings, the study team is looking to create better upconversion materials with greater light conversion performance. These materials can possibly collect the infrared, ultraviolet and visible light spectrum for solar power applications, the researchers said.
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