Phase contrast AFM images. (a) Core-shell structures of Si NCs and Si oxide having rodlike structures derived from porous Si. The histograms (i) and (ii) give the particle size distribution of the length and cross section for 20 particles captured in the AFM image. The average aspect ratio of the particles captured in the image is 2.1. (b) Spherical core-shell structures of Si NCs surrounded by Si oxide, produced by forced oxidation of ball milled Si. The histograms (iii) and (iv) show that the sizes of the core vary from 1 to 10 nm. The shell sizes are found to have much greater size dispersion (40 to 145 nm). (c) STM image of porous Si film grown on , (100) orientation, -type Si wafer by anodization in HF-methanol solution with current density and etching time 15 min. The structures shown in (a) were obtained by exfoliating the layers of three such films and sonicating them in ethanol.
HRTEM bright field images of Si NCs (a) produced by mechanical milling. A large number of particles with sizes less than 5 nm are identifiable. The inset is the corresponding SADP, (b) an isolated spherical Si nanodot with diameter , and (c) Si NC derived from porous Si. The aspect ratio of the particle captured is . The insets (i) and (ii) are the respective SAD pattern and the fast Fourier transform images of the exfoliated porous Si NC.
Room temperature PL spectra of porous Si film prepared with the previously stated formation parameters for excitation energies varying from 325 to 475 nm (i.e., 3.81 to 2.61 eV) at intervals of 25 nm recorded after four weeks of storage. The intensity of emission for excitations below 2.61 eV becomes indistinguishable with instrument noise.
PL spectra of (a) colloidal suspension formed by sonication and centrifugation of exfoliated layers of porous Si films (sample S2) in ethanol, and (b) colloids of Si NCs formed by forced external oxidation of 100 h ball-milled Si (sample S3) in acetonitrile. The inset of (a) is the photograph showing intense visible luminescence from the Si NCs under 350 nm UV excitation. The inset of (b) shows the PL spectra of the S3 in ethanol under excitations with the 457, 476, 488, and 514 nm lines of laser. The second peak in this case is not detected as the emission is recorded up to 650 nm.
Variation of PL peak positions (for the higher energy peaks) for differently synthesized samples (S2 and S3) in different backgrounds (ethanol and acetonitrile), with variation in excitation energy.
Normalized PL spectra of S2 in ethanol for excitation wavelengths ranging from 320–260 nm. New features like peak splitting appear in the higher energy peak for excitation below 300 nm. The same phenomenon was observed for S3.
Schematic representation of (a) exfoliation of porous Si films to form core-shell structures of Si NC and silicon oxide and (b) band structure and schematic mechanisms of PL from pS1, S2, and S3.
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