There are some molecules that can show luminescence at two different excited states, a process known as dual emission which has led to the use of these molecules in thermometric applications. In most cases organic molecules have been used but researchers are also keen to try out inorganic colloidal semiconductor crystals as they show a good resistance to photodegradation. However, the inorganic nanocrystals investigated so far have suffered from low thermometric sensitivity and poor ratiometric detection.
Daniel Gamelin’s team has been studying dual emission in Mn2+ doped nanocrystals for some time and has finally overcome these problems. “Prior results from our group had identified the possibility of accessing this basic phenomenon in Mn-doped CdSe nanocrystals, but encountered big difficulties because of the need to work with very small nanocrystals in order to have the correct energy alignment of the two emissive states. Unfortunately, in those very small nanocrystals, we could never get all of the nanocrystals to be doped, so our signals were always contaminated with undoped nanocrystals and the results were very unreliable. To get around this problem, we separated the tasks of doping and energy gap tuning. We started with doping of wide-gap ZnSe nanocrystals so that we could grow them large enough that every nanocrystal contained Mn. We could then tune the semiconductor energy gap down to the correct range for this dual emission effect by growing a narrower-gap shell of ZnCdSe around the Mn-doped ZnSe cores. This turned out to provide precisely the right control over both doping and energy gap tuning to open up these materials for very clean, high-temperature dual emission,” he explains to Materials Today.
The nanocrystals that the team produced have two distinct emissive excited states with a 10 000-fold difference in luminescence rates. This means that very small changes in temperature result in a very large change in relative peak intensities with dual emission being the dominant component of the overall luminescence.
Temperature ranges from cryogenic to well above room temperature can be achieved, where the dual-emission window is tuned by changing the energy gap during nanocrystal fabrication.