Photoluminescence Quantum Yields from Crystalline and Amorphous Silicon Nanoparticles
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Photoluminescence Quantum Yields from Crystalline and Amorphous Silicon Nanoparticles Rebecca Anthony, and Uwe Kortshagen Mechanical Engineering, University of Minnesota, 111 Church St. SE, Minneapolis, MN, 55455 ABSTRACT The optical properties of amorphous and crystalline silicon nanoparticles are studied. By tuning the power of the non-thermal plasma reactor, the structure of the particles is adjusted from amorphous to crystalline nanoparticles. The microstructure is studied using transmission electron microscopy, X-ray diffraction, and Raman vibrational spectroscopy. The photoluminescence quantum yields of crystalline nanoparticle samples are consistently higher than those of amorphous nanoparticles. INTRODUCTION Photoluminescence from nanoscale silicon has been well studied. In general, it is thought that the silicon must be in nanocrystalline form to emit light; however, several groups [1,2] have reported photoluminescence from amorphous silicon nanoparticles embedded in various solid-state matrices. Further investigation into the optical properties of amorphous silicon nanoparticles compared with silicon nanocrystals is needed. Using a nonthermal plasma reactor, the crystallinity of silicon nanoparticles may be adjusted according to process parameters. In this work, we examine the photoluminescence from amorphous and crystalline silicon nanoparticles produced in the same reactor, as a function of input power and of nanoparticle size. The nanocrystals consistently show bright photoluminescence, while the optical signal from amorphous nanoparticles is low or non-existent. EXPERIMENTAL DETAILS The silicon nanoparticles were synthesized in a non-thermal low-pressure plasma reactor, as described previously [3]. The size of the nanoparticles is controlled by adjusting the flowrate of inert gas through the reactor tube between 25 and 100 sccm. To change the crystallinity of the nanoparticles, the 13.56 MHz radiofrequency input power to the reactor is adjusted between 20 and 100 watts. Crystalline and amorphous particles of four sizes were synthesized, from three to five nanometers in diameter. After air-free synthesis, the particles were functionalized in a liquid-phase hydrosilylation process to remove surface hydrogen and replace it with 1-dodecene, an organic ligand that helps to limit oxidation and to improve particle solubility in non-polar solvents [4]. The nanoparticle samples were characterized using an FEI Tecnai T-12 Transmission Electron Microscope for TEM images, a Bruker-AXS Microdiffractometer for X-ray Diffraction (XRD) spectra, and a confocal Raman microscope (Witec alpha300 R confocal Raman microscope with UHTS300 spectrometer and DV401 CCD) for Raman vibrational spectroscopy. The photoluminescence quantum yield of the
nanoparticles was measured using an LED excitation source at 390 nm, an integrating sphere, and a USB2000 spectrometer (Ocean Optics, Inc.). DISCUSSION Nanoparticle Crystallinity To verify the changing crystallinity of the nanoparticles, several samples were synthesized under the sam
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