Luminescence refers to the result of a process in which an object absorbs light at one wavelength and then re-emits it at another wavelength. The phenomenon is used in a wide array of techno­logical appli­cations involving highly efficient and reproducible emitting devices that can easily be miniaturized. The materials with the highest luminescence efficiency include quantum dots (QDs), currently used in high-reso­lution displays, LEDs, solar panels, and sensors. Func­tionalization of the surface of QDs with various types of molecules permits inter­action with cellular structures or other molecules of interest for the purpose of investi­gating molecular-level biological processes.

Monitoring and control of crystal growth during synthesis of QDs in solution permits intelligent planning of the desired lumines­cence. Researchers led by Andrea de Camargo from the University of São Paulo’s São Carlos Physics Institute (IFSC-USP) in Brazil, and collaborators at Kiel University in Germany present a novel approach to the monitoring of QD formation. “We used cadmium telluride (CdTe) as a model system and controlled nano­particle growth in a heated aqueous solution via in situ luminescence analysis,” says Pedro Felipe Garcia Martins da Costa.

The technique enables scientists to see what is happening in the solution in real time without affecting QD synthesis, so that they can monitor crystal growth by observing the color of the light emitted. “QDs are synthesized by mixing cadmium and tellurium precursor solutions in the presence of a size control reagent. The tempera­ture is raised and the chemical reaction begins via telluride and cadmium ion clustering. As the reaction proceeds, additional units of CdTe join the cluster spheri­cally in a process known as self-assembly. Nano­particle size can be estimated thanks to rapid and precise monitoring of the emission frequencies. QDs of CdTe with a diameter of 1 to 2 nanometers emit in the blue and green regions of the visible spectrum. Larger QDs, measuring 4 to 5 nm, emit at lower frequencies, as yellow and red respectively,” says postdoc Leonnam Gotardo Merizio.

According to Costa, the novel method has several advantages over the conventional synthesis strategy. “In the conven­tional technique, you have to take small samples of the solution to measure QD size, but the in situ technique lets you do so as the process is under way, without having to interfere with the reaction medium to take samples so that more spectra can be obtained per unit of time, reaction volume isn’t affected, and unneces­sary waste is avoided. The emission color of the QDs of interest can therefore be controlled far more precisely. The equipment that delivers the excitation light via optical fiber at the appropriate wavelength also collects the emitted light and determines its charac­teristic frequency in the RGB color system. It’s worth noting that control of the RGB system is relevant to image formation in several luminescent devices, such as monitors and smartphone displays,” he explains.

QDs synthesized in this way, he added, were also characterized by means of X-ray diffraction, transmission electron microscopy, ultra­violet-visible absorption spectro­scopy, and infrared vibration spectro­scopy. “Quantum confinement gives QDs the capacity to confine electrons in three dimensions, making quantum phenomena more evident and charac­terizing them as inter­mediate materials between atoms, molecules and larger crystalline arrays,” Costa says.

“Many papers have been published on the synthesis of QDs made of CdTe. Our study’s main contri­bution relates to the development and application of a highly versatile in situ lumines­cence measurement system. The methodology enabled us to infer the size of the crystalline nanoparticles and to characterize the formation of inter­mediate compounds in the chemical reactions by in situ association with other techniques that permit chemical and/or structural analysis. This evolution of synthesis optimizes chemical yields and saves energy,” Camargo says. (Source: FAPESP)

Reference: P. F. G. M. da Costa et al.: Real-time monitoring of CdTe quantum dots growth in aqueous solution, Sci. Rep. 14, 7884 (2024); DOI: 10.1038/s41598-024-57810-8

Link: São Carlos Institute of Physics, University of São Paulo (IFSC – USP), São Carlos, Brazil