Quantum dots are extremely tiny fluorescent semiconductor dots. Although they are small in size, they play a very important role in many fields such as health and electronic technology. At the same time, their shortcomings are also obvious-they are toxic and very expensive. Made of metal.
In recent years, scientists have been working to find Yimu to come together and conduct joint research. An economical and harmless alternative to quantum dots. With current technology, it has become very easy to manufacture carbon-based dots (also known as carbon nanodots, or carbon dots), and it is non-toxic and economical. It can be said to have the potential of quantum dot substitutes, but it also has a Defects-less light is emitted.
Recently, a research team from the University of Illinois has made new discoveries. With the help of femtosecond nano-imaging technology, they have observed the shape and distribution of good and bad emitters in the carbon nanodot cluster. The next step is to screen high-quality emitters from them. Replace the existing metal quantum dots.
At present, their research results have been published on PNAS.
Femtosecond nano-imaging technology allows carbon nanodots to be discovered
Researchers from the University of Illinois at Urbana-Champaign and the University of Delaware in Baltimore County came together through a collaborative project of the Beckman Institute for Advanced Science and Technology in Illinois to conduct joint research.
”Before conducting this research, we don’t know if all carbon nanodots are just general emitters, or if they contain both good emitters and bad emitters.” University of Illinois Chemistry, who led the research Professor Martin Gruebele said. “But what we can be sure of is that once we find a way to prove that the projectiles are good or bad, then we can definitely screen out good ones,” he added.
This means that if you want to determine whether the emitters in the carbon nanodot cluster are good or bad, the first thing you need to do is to be able to see them.
But this faces two huge challenges: first, these carbon nanodots are too small, with a diameter of less than 10nm; second, when excited, they emit light within a few picoseconds. In other words, the volume of carbon nanodots is too small and the excitation is too fast, so that it is difficult to capture and observe in the two dimensions of space and time.
As a result, they used optical methods to excite carbon nanodots to image the electron-phonon dynamics in a single dot and the nanoscale heat transfer between two carbon dots. The tip of the scanning tunneling microscope is used as the detector of the optically excited state, and the electron tunnel is optically blocked to record the image of the carrier dynamics in the time range of 0.1-500 picoseconds (ps).
They also imaged the coupling of photons in a single carbon dot with conduction electrons in gold as an ultrafast energy transfer mechanism between two adjacent carbon dots. The electron density of the excited state migrates from the whole to molecular level (1nm) surface defects, and then the non-uniformity of a single point is relaxed to a long-lived fluorescent state or back to the ground state.
”If you use the previous single-molecule absorption scanning tunneling microscope, you can only image the excited state without time resolution. Obviously this will not work.” Greebele said, “However, the most important point is that In this research, we used femtosecond-level nano-imaging technology, which combines femtosecond time resolution and nano-spatial resolution to record quantum dots in an excited state.”
Relying on this femtosecond-level nano-imaging technology, the Greuebelle team passed Research and observation have found that there are two ways of energy excitation: one is to directly emit fluorescence; the other is to release in the form of thermal energy first, and then it may produce fluorescence.
As shown in the figure below (the length of the white scale in the figure is 5nm), the left side represents that under the imaging of an ordinary scanning tunneling microscope, the carbon nanodot is just a point without any features; the right side represents the time-resolved single molecule Under the imaging of the absorption scanning tunneling microscope, the laser excitation is initially distributed on the entire carbon nanodots, but within a few picoseconds, the excitation migrates to the highly localized area of the surface, and these processes are all recorded.
Screening out perfect carbon nanodots is expected to replace metal quantum dots
The researchers then conducted a more in-depth observation. Gruebele said, “We found that among a large number of carbon nanodots, about 20% of the carbon nanodots emit strong light, which is a perfect emitter, and about 80% remain. The carbon nanodots have a short luminous state before they release heat.” In other words, there are both good emitters and bad emitters in the carbon nanodot cluster, and the ratio of good to bad is about 1:4.
He also pointed out, “This is very important because we have seen that there are different populations, which means that if a certain method is used to screen and only the perfect emitter is selected, then the population of carbon nanodots can be purified. And purification.” So
far, the first step is completed. They successfully observed the carbon nanodots and analyzed the pros and cons of the emitters in the carbon nanodot cluster.
With the help of femtosecond nano-imaging technology, researchers have other important discoveries in observation, such as why some carbon nanodots never emit light. This indicates that researchers may hope to create a perfect emitter through artificial synthesis.
”Metal quantum dots are often used to monitor the health of living cells, but this is far from ideal. Choosing non-toxic and economical carbon nanodots will be a major advancement. We are now using femtosecond nano imaging technology. A lot of new discoveries have been gained.” Greebele said, “Next, whether to screen out the perfect emitter in the carbon nanodot cluster through some methods, or to create a perfect emitter through synthesis, this is just a multiple choice question.”
Gruebele The research of the professor’s team has made carbon nanodots a successful step in replacing metal quantum dots. Next, it is just around the corner to replace the current expensive and toxic metal quantum dots by screening, separating, or directly synthesizing carbon nanodots.