Nanoparticles on silicon

Figure 10. (A) Schematic illustration of PEI-mediated self-assembly of FePt nanoparticles by alternately adsorbing a layer of PEI and a layer of nanoparticles on a solid surface and TEM images of PEI-mediated assembly of 4 nm Fe58Pt42 nanoparticles on silicon oxide surface (B) one layer of assembly and (C) three layers of assembly [56].

Choi, W. K., Liew,T. H., et al. A Combined Top-Down and Bottom-Up Approach for Precise Placement of Metal Nanoparticles on Silicon. Small,4(3), 330-333 (2008). 

Chattopadhyay, S., Lo, H.C., Hsu, C.H., Chen, LC. and Chen, K.H. (2005) Surface-enhanced Raman spectroscopy using self-assembled silver nanoparticles on silicon nanotips. Chemistry of Materials, 17, 553-9. 

Figure 3. AFM images of the silver nanoparticles on the Xi02(l 00) single crystal at the deposition times of (a) 15s and (b) 180 s. The images were recorded in a tapping mode with driving frequency of 110-150 kHz at a scan rate of 1 Hz by using a silicon cantilever with a normal spring constant of 15Nm (SI-DF20, Seiko instruments).

Wet Preparation of Metal Nanoparticles and their Immobilization on Silicon Substrates... 

Si-C formation technique with hydrogen-terminated silicon substrates can also be used as the covalent attachment of nanomaterials onto silicon surface. The possibility of assembling nanomaterials in order is strongly desired in order to enable efficient utilization of their unique nano-sized properties. Ordered arranging and position controlling of nanomaterials on solid substrates especially on silicon surface have been intensively studied [10]. In this manuscript, the nanoparticle immobilization by thermal Si-C formation will be discussed [11]. 

The multilayer nanocomposite films containing layers of quasi-spherical Fe nanoparticles (d — 5.8 nm) separated by dielectric layers from boron nitride (BN) are synthesized by the repeated alternating deposition of BN and Fe onto a silicon substrate [54]. In this work the authors managed to realize the correlation in the arrangement of Fe nanoparticles between the layers the thin BN layer deposited on the Fe layer has a wave-like relief, on which the disposition of Fe nanoparticles is imprinted as a result, the next Fe layer deposited onto BN reproduces the structure of the previous Fe layer. Thus, a three-dimensional ordered system of the nanoparticles has been formed on the basis of the initial ordered Fe nanoparticle layer deposited on silicon substrate [54]. The analogous three-dimensional structure composed of the Co nanoparticles layers, which alternate the layers of amorphous A1203, has been obtained by the PVD method [55]. 

Fig. 15.10. Band diagrams illustrating the effect of charging of Ni nanoparticles on the surface of silicon. Ni particles are charged positive on p-type silicon and negative on n-type Si.

In fabrication of the catalysts by laser electrodispersion, thermally oxidized silicon wafers with a thickness of Si02 oxide layer of 1 pm were used as a substrate. A substrate with so thick an oxide layer can be regarded as an insulator. In some cases, wafers of crystalline (1 0 0) Si were used, which had on their surface only a thin (l-2nm) layer of a natural oxide. This layer is tunnel-transparent for electrons, and, therefore, charge exchange between supported nanoparticles and silicon is possible. 

Fig. 15.12. Specific catalytic activity vs. surface density of copper nanoparticles on thermally oxidized silicon in reactions involving chlorohydrocarbons (1) CC14 + C8H16 at 150°C, (2) the same at 130°C and e = 10, (3) isomerization of dichlorobutenes at 110°C, (4) isomerization of dichlorobutenes at 130°C, and (5) CC14 + C10H22 at 130°C.

The dissimilarities between the charge states of nickel nanostructures deposited onto substrates of well-conducting p- and n-type silicon (unoxidized) were manifested in different catalytic activities in the reaction of carbon tetrachloride addition to olefins. It was shown that negatively charged nanoparticles on an n-type Si substrate have a two times higher activity, compared with positively charged particles on a p-type Si substrate (see Figure 15.10). 

Janus micelles are non-centrosymmetric, surface-compartmentalized nanoparticles, in which a cross-linked core is surrounded by two different corona hemispheres. Their intrinsic amphiphilicity leads to the collapse of one hemisphere in a selective solvent, followed by self-assembly into higher ordered superstructures. Recently, the synthesis of such structures was achieved by crosslinking of the center block of ABC triblock copolymers in the bulk state, using a morphology where the B block forms spheres between lamellae of the A and C blocks [95, 96]. In solution, Janus micelles with polystyrene (PS) and poly(methyl methacrylate) (PMMA) half-coronas around a crosslinked polybutadiene (PB) core aggregate to larger entities with a sharp size distribution, which can be considered as supermicelles (Fig. 20). They coexist with single Janus micelles (unimers) both in THF solution and on silicon and water surfaces [95, 97]. 

One assembly example is polyethylenamine (PEI)-mediated self-assembly of FePt nanoparticles [56]. PEI is an all -NH-based polymer that can replace oleate/oleylamine molecules around FePt nanoparticles and attach to hydrophilic glass or silicon oxide surface through ionic interactions [52], A PEI/FePt assembly is readily fabricated by dipping the substrate alternately into PEI solution and FePt nanoparticle dispersion. Figure 10 shows the assembly process and TEM images of the 4 nm Fes8Pt42 nanoparticle self-assemblies on silicon oxide surfaces. Characterizations of the layered structures with X-ray reflectivity and AFM indicate that PEI-mediated FePt assemblies have controlled thickness and the surfaces of the assemblies are smooth with root mean square roughness less than 2 nm. 

Xia and coworkers have demonstrated that the polarization of light plays an important role for nanoparticles with anisotropic shapes, especially with tmncated comers [77]. Optical dark-field mode imaging was employed to identify individual silver nanocubes deposited on silicon substrate as shown in Fig. 15.10a. Raman spectra were collected from the nanocubes, which were oriented in different directions with respect to the laser polarization. Subsequent SEM imaging of the same cubes enabled the authors to directly correlate the orientation of the nanocubes with respect to the light polarization and the SERS enhancement factor. They observed dramatic variation in SERS intensity when the nanocubes were oriented at different angles relative to the polarization of excitation laser as shown in Fig. 15.10b. SERS spectra of 1,4-benzenedithiol adsorbed on Ag nanocubes oriented in different directions showed different intensities with respect to the light polarization direction. The individual nanocubes with sharp comers were the most... 

In this paper the dependence of the rate of chemical reactions in a nanoparticle on its size is considered using reaction of oxidation of the silicon nanoparticle as an example. 

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