Jonghoon Choi, Chemical and Biomolecular Engineering, University of Maryland and NIST, 100 bureau drive, stop 8312, Gaithersburg, MD 20899, Vytas Reipa, Biochemical Science, National Institute of Standards and Technology, 100 Bureau Drive, BLDG 227, Gaithersburg, MD 20899, and Nam Sun Wang, Chemical Engineering, University of Maryland, 9000 Rockville Pike, 5766 Bldg 13/3N07, Bethesda, MD 20892.
We show that the photo-assisted dissolution of Si nanocrystals in HF/HNO3 acid mixture facilitates the photoluminescence blue shift. Si nanocrystals were synthesized using the anodic etching of Si wafers followed by the mechanical removal of the porous surface layer from the etched wafers. The subsequent UV-assisted dissolution reaction generated samples that progressed with time from red, to orange, yellow, and green photoluminescence, which indicates a progressive shrinkage in the nanoparticle size. We developed a model that describes this size change during the course of dissolution. The resulting fluorescent Si nanocrystals during the dissolution process were characterized using photoluminescence spectroscopy, TEM, absorption spectroscopy, and time resolved fluorescence spectroscopy. The dissolution process also yielded blue-emitting Si nanocrystals, although presently not as reproducibly as those emitting other colors. Photoluminescence decay time measurements suggest that blue Si nanocrystals undergo a different fluorescence emission mechanism from those emitting at lower energies. Our Si nanocrystals possess high quantum efficiency (10-40%) and cover the entire visible spectrum. These unique properties make them an ideal fluorophore for biological applications.