Si nanocrystals (Si nc) were formed by the implantation of Si+ into a SiO2 film on (100) Si, followed by high-temperature annealing. The microstructure of the Si nc produced by a high-dose (3 × 1017 cm-2) implantation has been extensively investigated using high-resolution transmission electron microscopy (HRTEM). For most of the Si nc (∼90%) with diameters larger than 6 nm, their configurations are characterized by the existence of nanotwins. The twinning structures include single twins, double twins, and multiple twins. The other planar defects such as stacking faults are also observed in some nanoparticles. However, in the Si nc smaller than 5 nm, no evident microstructural defects are observed. Possible reasons that no evident microstructural defects are found in the smaller Si nc are discussed. The microstructural defects inside the Si nc have a great influence on the optical properties.
Si nanocrystals were formed by the implantation of Si+ into a SiO2 film, deposited on (100) Si, followed by high-temperature annealing. Transmission electron microscopy (TEM) was used to examine the effect of implantation dose on the microstructure of the Si nanocrystals (Si nc) in the SiO2 film. The size and spatial distribution and concentration of the Si nc in four passivated samples with different implantation doses were investigated using the dark-field imaging technique. The thickness of all the samples was determined by electron energy-loss spectroscopy (EELS). The structure of the Si nc in all the samples was determined using selected area electron diffraction. It was found that the average diameter of the Si nc changes from 2.7 to 3 and to 3.3 nm for the samples with implantation doses of 6 × 1016, 8 × 1016 and 1 × 1017 cm−2; however, it ranges from 2 to 22 nm in the sample with an implantation dose of 3 × 1017 cm−2. The size of the Si nc is comparatively homogeneous throughout the whole implanted layer in the samples with implantation doses of 6 × 1016 and 8 × 1016 cm−2, while in the samples with implantation doses of 1 × 1017 and 3 × 1017 cm−2, the Si nc in the middle region of the implanted layer are bigger than those near the surface or the bottom of the layer. From TEM experimental results, the concentration of the Si nc is estimated to be 6 × 1018, 4 × 1018 and 4 × 1018 cm−3 for the samples with implantation doses of 6 × 1016, 8 × 1016 and 1 × 1017 cm−2, respectively. For the sample with an implantation dose of 3 × 1017 cm−2, the concentration for the Si nc of 3 nm is around 5 × 1018 cm−3; the concentration for Si nc of 6 nm is around 2 × 1018 cm−3; and the concentration for the 12 nm group is about 1 × 1018 cm−3. In addition, the concentration determined from TEM experiments is compared with the calculated one. Combining the TEM results with a Monte Carlo simulation, we also discuss the sputtering effect and the depth distribution of the Si ions implanted in SiO2.
Tin oxide (SnO2) ultrathin films were deposited by pulsed laser deposition (PLD) onto SiO2/Si and quartz substrates, at various nominal thicknesses ranging from isolated nanoparticles (NPs) to ∼300 nm-thick films, under an oxygen background pressure of 10 mTorr. The microstructural and surface morphologies of the NP-based SnO2 films were characterized by x-ray diffraction and atomic force microscopy, as a function of their nominal film thickness. The PLD-SnO2 films were found to be composed of NPs (in the 1–6 nm range), whose size increases with the film thickness. The energy band gap, as determined from the absorption edge, was found to shift to higher values with decreasing the film thickness (i.e., decreasing the NPs size). It was found that an annealing at 700 °C under O2 ambient is a prerequisite to get a photoluminescence (PL) emission from the PLD-SnO2 films. The PL of the annealed SnO2 films was found to consist of two broad emission bands, regardless of the SnO2 film thickness. The first band is composed of 3 PL subbands peaking at 3.20, 3.01, and 2.90 eV, while the second one is centered on 2.48 eV. In spite of the observed band-gap widening (as confirmed by theoretical calculation), we show that surface state (e.g., oxygen vacancies) dominate completely the PL emission of SnO2 NPs, which becomes more luminescent as the NPs size decreases while the PL energy remains unchanged. The PL properties of the PLD-SnO2 NPs are discussed in terms of defects and/or oxygen vacancies related transitions.
The microstructure of the Si nanocrystals ͑Si nc͒ has been investigated using conventional and highresolution transmission electron microscopy ͑HRTEM͒. For most of the nanocrystals ͑Ͼ90% ͒ larger than 10 nm, HRTEM observations show that they are formed by the coalescence of smaller ones. Two kinds of coalescence, one being the preferential attachments of small particles to the ͕111͖ facets of a seed nanoparticle, and the other being an ordered combination of two or more small nanocrystals with appropriate orientations, have been observed.Si nanocrystals ͑Si nc͒ embedded in a SiO 2 matrix have attracted much interest as a candidate system to act as an efficient light emitter. Although the physical mechanism for the light emission remains unclear, a lot of progress has been made in both the preparation 1-4 and characterization 3-6 of the Si nc. In order to better understand and control the physical properties of the Si nc, it is fundamental and necessary to study the main factors that affect the crystal growth and microstructure development of these nanocrystals. For the nanocrystals embedded within a matrix, the coarsening is usually attributed to Ostwald ripening, 7 in which the crystal growth takes place by diffusion of atoms between neighboring nanocrystals. Recent studies 8-10 of TiO 2 and ZnS nanocrystals growing under hydrothermal conditions have shown that the oriented attachment or coalescence plays an important role in the coarsening of nanocrystals. In addition, the coalescence of small particles by twinning was also reported in FePt nanocrystals. 11,12 In the process of the oriented attachment or coalescence, the nanoparticles can themselves act as the building blocks for crystal growth. However, for the Si nc embedded in a matrix such as SiO 2 , coalescence or oriented attachment has not been observed using highresolution transmission electron microscopy ͑HRTEM͒.In this Rapid Communication, we report the conventional and HRTEM observations of the Si nc produced in a SiO 2 film by ion implantation and annealing. The Si nc range from 2 to 22 nm diam., and have a peculiar size distribution with the depth of the implanted layer. For most of the nanocrystals larger than 10 nm, HRTEM observations show that they are formed by the coalescence of smaller ones. Two kinds of coalescence, one being the preferential and ordered attachments of small particles to the ͕111͖ facets of a seed nanoparticle, and the other being an ordered combination ͑by ͕111͖ twinning͒ of two or more small nanocrystals with appropriate orientations in SiO 2 , have been observed. The high concentration of Si ions is essential for the coalescence.The Si nc were produced by a high-dose ͑3 ϫ 10 17 cm −2 ͒ implantation of Si + into SiO 2 film and annealing ͑1100°C͒. For a detailed experimental procedure, see Ref. 13. The specimens for TEM examination were prepared in a crosssectional orientation ͓͑011͔ zone axis for the Si substrate͒ using conventional techniques of mechanical polishing and ion thinning. Dark-field examination was carried out...
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