Si nanoparticles

In order to overcome these problems, hybridization of both materials (C and Si) in one electrode material by HTC seemed to be a promising option [75]. For this purpose, pre-formed silicon nanoparticles were dispersed into a dilute solution of glucose followed by hydrothermal treatment at 180 °C. The carbon-coated particles were then further treated at 750 °C in order to improve the conductivity and structural order of the carbon layer. It was shown that the hydrothermal treatment, following by high temperature carbonization, resulted in formation of a few nanometer thin layer of SiOx layer on the Si nanoparticles, effectively leading to a Si/SiOx/C nanocomposite. Some TEM micrographs of these materials are shown in Fig. 7.8. 

The lithium-storage properties of these Si SiOx/C nanocomposite electrodes were investigated in different electrolyte systems and compared to pure Si nanoparticles. From all the analyzed systems, the Si SiOx-C nanocomposite in conjunction with the solvent vinylene carbonate (VC) to form the solid-electrolyte interface showed the best lithium storage performance in terms of a highly reversible lithium-storage capacity (1100 mAh g-1), excellent cycling performance, and high rate capability (Fig. 7.9). 

Belomoin G, Therrien J, Smith A, Rao S, Twesten R, Chaieb S, Nayfeh MH, Wagner L, Mitas L. Observation of a magic discrete family of ultrabright Si nanoparticles. Appl. Phys. Lett. 2002 80 841-843. 

Our TEM data suggested that this decomposition results in the precipitation of Si nanoparticles, which are the nuclei of SiNWs, clothed with shells of silicon oxide. 

In view of the foregoing discussion on hydrosilylation of Si nanoparticles, the understanding of the effect of PA termination on the dot properties becomes vital. 

Total energy calculations have been performed to understand the role of oxidation on the structural, electronic and optical properties of Si nanoclusters. Our aim is to explain the peculiar properties of aged porous Si samples, heavily oxidized Si nanoparticles and embedded Si nanocrystals. We have studied two types of structures isolated H-covered clusters, replacing Si-H bonds with various Si-0 bonds and Si nanoclusters embedded in a SiOz matrix. Regarding the isolated clusters we find that the optoelectronic properties depend on the type and the number of Si-O bonds at the cluster surface. For the embedded systems our results show that a close interplay between chemical and structural effects plays a key role in the light emission processes. 

One of the great issues in the field of silicon clusters is to understand their photoluminescence (PL) and finally to tune the PL emission by controlling the synthetic parameters. The last two chapters deal with this problem. In experiments described by F. Huisken et al. in Chapter 22, thin films of size-separated Si nanoparticles were produced by SiLL pyrolysis in a gas-flow reactor and molecular beam apparatus. The PL varies with the size of the crystalline core, in perfect agreement with the quantum confinement model. In order to observe an intense PL, the nanocrystals must be perfectly passivated. In experiments described by S. Veprek and D. Azinovic in Chapter 23, nanocrystalline silicon was prepared by CVD of SiH4 diluted by H2 and post-oxidized for surface passivation. The mechanism of the PL of such samples includes energy transfer to hole centers within the passivated surface. Impurities within the nanocrystalline material are often responsible for erroneous interpretation of PL phenomena. 

Due to the larger size of the Si nanoparticles collected in the filter, most filter samples do not show any visible photoluminescence. However, as we could observe with an IR-sensitive video camera, they usually luminesce in the IR. For the following experiments, we selected a few samples that showed some weak PL below 700 nm that could be observed by eye. After HF treatment and subsequent oxidation, the samples clearly showed enhanced PL in the visible as a result of the reduced particle size. To accelerate the oxidation, the samples were exposed for 2-A h to water vapor at 150 °C in an oven. 

Si nanoparticles. However, to clarify this issue, it will be necessary to measure the complete PL spectra in the IR by employing a sensitive semiconductor detector. 

We have shown that, in order to exhibit intense PL, the Si nanocrystals must be perfectly passivated. A simple way to achieve this is through natural aerial oxidation. We have followed this process by measuring the photoluminescence as a function of time. Stable conditions are achieved after approximately 6 months. This indicates that the oxidation of Si nanoparticles is a self-limiting process. 

The oxide shell of silicon nanoparticles can be etched away by exposing the samples to HF vapor. The subsequent oxidation reduces the size of the crystalline core and shifts the PL of the nanoparticles to shorter wavelengths. This technique can also be applied to reduce the size of Si nanoparticles collected in much larger amounts in the exhaust line of the flow reactor, and to shift their photoluminescence, which is normally in the IR, into the visible. 

This work was supported by PROCOPE, a bilateral cooperation between France and Germany. G. L. thanks the Alexander-von-Humboldt Foundation for a fellowship. Finally, we are grateful to Jion Gong for preparing the size-selected deposit and to Henri Perez for his advice and help in the experiments devoted to the HF etching and passivation of the Si nanoparticles collected on the filter. 

Si is an attractive material because it is inert, non-toxic, abundant, economical, and Si nanoparticles exhibit size-dependent visible luminescence (Fig. 1). Although much research has been conducted on the optical and electric... 

The Si-nanocrystal-containing SiOx (x = 0.6) powders, which were supplied by DENKA Co. (Tokyo, Japan), were dispersed in methanol. Then a mixture of HF (49 wt. %) and HN03 (69 wt. %) (10/1, v/v) was added to the suspension to dissolve the oxide and to decrease the Si nanocrystal sizes. Then, the solution was sonicated. As the etching time increased, the nanocrystal size decreased and photoluminescence (PL) from the nanocrystals continuously shifted from red to green. Because etched Si nanoparticles are hydrophobic, the nanoparticles remained loosely assembled in the suspension after sonication. These assemblies were collected on a polyvinylidene fluoride (PVDF) membrane fdter and washed three times with methanol. 

To make the Si nanoparticles hydrophilic, the surface H termination of the nanocrystals was transformed to propionic acid (PA) termination by the following photo-initiated hydrosilylation (Fig. 3) [9]. First, the Si nanoparticles... 

Fig. 7 shows the FTIR spectra of the Si nanoparticles. Curve (a) is the spectrum of dried Si nanoparticles after hydrosily lation, while curve (b) is an experimental spectrum from pure acrylic acid for comparison. Upon comparing these spectra, peaks attributed to C=C bonds are clearly present in spectrum (b), but are almost completely absent in spectrum (a). Simultaneously, the Si-CH2 peak is observed in spectrum (a). These observations confirm that the hydrosilylation proceeds as expected and Si-C bonds replace the C=C and Si-H bonds. Although the nanoparticles were dried in air, the Si-0 peak at 1080 cm 1 is quite small. 

Fig. 8 shows the optical absorption, PL and PL excitation (PLE) spectra of the Si nanoparticles. The weak... 

Figure 7. FTIR spectra of (a) Si nanoparticles after hydrosilylation and (b) acrylic acid.

Figure 9. Spontaneously ordered Si nanoparticles at an octanol/aqueous suspension interface (a) low magnification image and (b) outlined part in image (a).

FIGURE 22.10 Use of sol-gel processing to produce SI nanoparticles in a glass matrix, (a-c) The time indicates the length of the heat treatment. 

HRTEM) showed that these were crystalline Si nanoparticles (which have potentially interesting luminescent properties). So not only does this type of study produce an interesting material, but it also sheds light on Ostwald ripening and devitrification of a glass. 

Figure 18 Dielectric constant of Si nanoparticles as a function of size.

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