History of Nanocrystal Quantum Dots

In “Nanocrystal Quantum Dots: From Discovery to Modern Development” Alexander L. Efros and Louis E. Brus describe the history of nanocrystal research starting with stained glass all the way to liquid dispersed colloidal nanoparticles. The article begins by describing the quantum confinement effect which explains the size dependence of semiconductor nanoparticles. As nanoparticles decrease in size, the energy of the band gap responsible for fluorescence increases. In other words, the smaller the nanoparticle, the more confined the electron becomes. With careful control of the size distribution of nanoparticles, a majority of the visible spectrum can be covered. Starting with research into nanocrystals dispensed in glass, the annealing temperature was connected to particle size by the discovery that lower annealing temperatures caused a blue shift in the absorption spectra of different samples. The article then describes how the control of nanocrystal size was applied to liquid colloids and the realization of the importance of nucleation in the formation of nanocrystals. The authors describe the process of starting nucleation with the addition of precursor chemicals, then lowering the reaction temperature to limit the growth of nanocrystals. The crystals still grow at this slower rate, leading to difficulty controlling the final size of the nanocrystals. To combat this problem, capping reagents were added to bond to the surface of the nanocrystals and provide steric hindrance to the growth of the nanocrystals. Initially organometallics that bonded phenols to the surface were used but it was discovered that trioctylphosphine provided better results. The article then goes on to describe the development of the “particle in a box” equation that is used to describe the size dependence of absorption spectra for nano sized particles. These breakthroughs in research also led to the understanding of Coulomb Interactions and Bohr radius. In cases where the particles are much smaller than their Bohr radius this is known as “strong confinement” and in cases where the particle is much larger than their Bohr radius it is known as “weak confinement”. This also leads to a description of the intermediate regime where the particles have properties between bulk solid state materials and strongly confined nanocrystals.  The article then goes on to describe the future development possibilities for nanocrystals including “room-temperature luminescence dynamics” and using nanocrystals as “fluorescent biological labels”.  Efros and Brus give a highly detailed description of the history of nanocrystals and include detailed scientific explanation of nanocrystal properties. To learn more, read the full article here.

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