Water-soluble photoluminescent silicon quantum dots.

For silicon quantum dots to be used in biomedical applications it is essential that they have a substantial photoluminescence quantum yield in the visible region, have a fast radiative recombination rate, and are water soluble and hydrophilic to prevent aggregation and precipitation in a biological environment. The chemical process used to terminate the surfaces of the silicon quantum dots changes the internal electronic structure and thus plays an important role in the resultant emission wavelength and radiative lifetime, and ultimately determines the solubility. [18] Silicon quantum dots with an oxide surface passivation typically display a dipole-forbidden yellow-red emission with radiative lifetimes of 10 3 –10 6 s. [18, 26] This slow rate of recombination limits the use of oxide-passivated silicon quantum dots in biological imaging. However, silicon quantum dots with a hydrogen or carbon surface passivation have electric-dipole-allowed direct band gap transitions that lead to blue photoluminescence with fast recombination rates of 10 8 –10 9 s. [18, 20]

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