UV and Blue Photoluminescence from Silicon Nanocolloids
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silicon nanocolloids at room temperature. EXPERIMENTAL Silicon nanocolloid was prepared by means of gas evaporation and matrix isolation techniques [16,171. We used He gas (99.9999 %) as an inert gas atmosphere and methanol or 2-propanol (99.9 %) as an organic liquid. A Pyrex-glass chamber was evacuated to about 2.0 X 10'6 Torr with a turbomolecular pump and then filled with methanol or 2-propanol vapor which had previously been degassed several times by a freeze and thaw technique. The methanol or 2-propanol vapor was frozen onto the inner wall of the glass chamber, the outer wall of which was refrigerated with liquid nitrogen, and then the glass chamber was filled with He gas after re-evacuation. Under refrigeration, silicon nanocrystallites were prepared by the resistant heating technique where small pieces of silicon with a purity of 99.9999 % were heated in a tungsten crucible covered with alumina followed by sublimation into He gas atmosphere and the formed nanocrystallites adhered to the surface of the frozen methanol or 2-propanol. As it returned to room temperature, silicon nanocrystallites were recovered as flocculants in an organic liquid, which could be dispersed with ultrasonic irradiation. The prepared silicon nanocolloid was yellow in color. We prepared five samples (I to V) of silicon nanocolloid with different particle sizes by varying the pressure of the He gas from 0.5 to 5 Torr. The concentration of the colloidal solution determined by measuring the weight of the evaporated silicon nanocrystallites in some of the solutions varied from 1.1 to 2.5 mg/ml. In order to normalize the concentration, the initial solutions were diluted with methanol or 2-propanol to 1.0 mg/ml for photoluminescence and 0.13 mg/ml for UV-visible absorption measurements. The size of silicon nanocrystallites was determined from direct observation by transmission electron microscopy (TEM) using a Hitachi H-8100 system operated at 200 kV. Samples for TEM observation were prepared by dropping silicon nanocolloids onto amorphous carbon film deposited on a copper sheet mesh and leaving it to dry in air. The chemical analysis of nanocrystallites was conducted utilizing an energy dispersive X-ray (EDX) unit attached to the TEM. The IR spectra were measured using a Horiba FT-210 system immediately after the silicon nanocolloid was dropped onto an NaCl substrate and dried in Ar gas atmosphere. The UV-visible absorption spectra of the colloidal samples were measured with a Hitachi U-3210 spectrophotometer. The photoluminescence spectra were measured with a Hitachi F3010 fluorescence spectrophotometer excited by a 150 W xenon lamp, where the excitation energy was set at the range of ultraviolet. The external quantum efficiency was measured with fluorescence spectrophotometer by a relative measurement using Fluorescein as a standard material with the quantum efficiency of 97 %. Several alkaline solids were tested as sodium hydroxide, purity of 96 %, potassium hydroxide, 85 % and calcium hydroxide, 99.9 %. We have used pH values of the
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