Novel method enhances size-controlled production of luminescent quantum dots

Luminescence is a process where an object absorbs light at a specific wavelength and then re-emits it at a different wavelength. This occurs when electrons in the material are excited to a higher energy state, then subsequently decay to lower energy states, including the ground state, emitting light in the process. Luminescence is useful in various technological applications due to its highly efficient and reproducible nature.

One of the materials with the highest luminescence efficiency are Quantum Dots (QDs), currently used in high-resolution displays, LEDs, solar panels, and sensors. QDs are semiconductor nanoparticles whose emissive characteristics are directly linked to their size due to quantum confinement. By monitoring and controlling the crystal growth during synthesis, scientists can plan the desired luminescence.

In a study published in Scientific Reports, a research team led by Andrea de Camargo at the University of São Paulo’s São Carlos Physics Institute (IFSC-USP) presented a novel approach to monitoring QD formation. This method involves using in situ luminescence analysis, which enables scientists to observe QD synthesis in real-time, without affecting the reaction, and to monitor crystal growth by observing the emitted light color.

QDs are synthesized by mixing cadmium (Cd2+) and tellurium (Te2-) precursor solutions in the presence of a size control reagent. The reaction starts via telluride and cadmium ion clustering, and as it proceeds, additional units of CdTe join the cluster spherically, in a process known as self-assembly. The size of the QDs can be estimated by monitoring the emission frequencies.

QDs with a diameter of 1-2 nanometers emit in the blue and green regions of the visible spectrum, while larger QDs, measuring 4-5 nm, emit at lower frequencies, as yellow and red respectively. This novel method has several advantages over the conventional synthesis strategy, as it allows for real-time monitoring, providing more spectra per unit of time and avoiding unnecessary waste.

Quantum confinement gives QDs the capacity to confine electrons in three dimensions, making quantum phenomena more evident. The study’s main contribution relates to the development and application of a highly versatile in situ luminescence measurement system, which enabled the inference of the size of the crystalline nanoparticles and the characterization of the formation of intermediate compounds in the chemical reactions.

The existence of QDs was first predicted theoretically in 1937 by Herbert Fröhlich, and in the 1980s, Alexey Ekimov and Louis Brus independently observed quantum confinement in semiconductor nanoparticles. In the 1990s, French-American physicist Moungi Bawendi developed significantly enhanced methods of QD synthesis. In 2023, Ekimov, Brus, and Bawendi were awarded the Nobel Prize for Chemistry for their work in the field.

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