Nanocrystalline arrays

Nanoparticles bearing a so-called three-dimensional self-assembled monolayer (3D-SAM) [136] coating or shell are ideal construction units for programmed, hierarchical self-assembly of larger objects [137]. Several research groups have successfully assembled functionalized nanoparticles into large nanocrystalline arrays... 

Fig. 16 TEM image of a nanocrystalline array of thiol-derivatized Au particles [from Sarathy et ai (reproduced with permission from ref. 96(6))].

Fig. 3.2. XRD patterns from the nanocrystalline arrays of Au particles of different mean diameters (4.2, 2.1, and 1.0 nm). Inset of the figure displays the UV-vis spectra of these particles. (Reprinted with permission from [66], 1997, American Chemical Society).

We have been interested in investigating the size-dependent electronic structure and reactivity of metal clusters deposited on solid substrates. Thus, we have shown that when the cluster size is small (SI nm), an energy gap opens up. Bimetallic clusters show additive effects due to alloying and cluster size in their electronic properties. Small metal clusters of Cu, Ni and Pd show enhanced chemical reactivity with respect to CO and other molecules. Metal clusters and colloids, especially those with protective ligands, have been reviewed in relation to nanomaterials. We have recently developed methods of preparing nanoparticles of various metals as well as nanocrystalline arrays of thiolized nanoparticles of Au, Ag and Pt. In Fig. 16, we show the TEM image of thiol-derivatized Au... 

Nanocarbon emitters behave like variants of carbon nanotube emitters. The nanocarbons can be made by a range of techniques. Often this is a form of plasma deposition which is forming nanocrystalline diamond with very small grain sizes. Or it can be deposition on pyrolytic carbon or DLC run on the borderline of forming diamond grains. A third way is to run a vacuum arc system with ballast gas so that it deposits a porous sp2 rich material. In each case, the material has a moderate to high fraction of sp2 carbon, but is structurally very inhomogeneous [29]. The material is moderately conductive. The result is that the field emission is determined by the field enhancement distribution, and not by the sp2/sp3 ratio. The enhancement distribution is broad due to the disorder, so that it follows the Nilsson model [26] of emission site distributions. The disorder on nanocarbons makes the distribution broader. Effectively, this means that emission site density tends to be lower than for a CNT array, and is less controllable. Thus, while it is lower cost to produce nanocarbon films, they tend to have lower performance. 

Molecular electron transfer is the basis for many important natural and commercial processes. During the past decade photochemists have relied upon supramolecular arrays of molecules to facilitate their understanding of the chemical and physical basis for this fundamentally important process. It therefore seems appropriate that several chapters in this volume examine thermally and photo-chemically induced electron transfer in supramolecular assemblies consisting of inorganic molecular building blocks such as covalently linked donor-acceptor dyads, transition metal clusters, and nanocrystalline semiconductor particles. 

The goal of materials research is really the reverse process, the bottom-up method. In this approach, it is hoped that perfect well-controlled nanoparticles, nanostrucmres, and nanocrystals can be synthesized, which may be compacted into macroscopic nanocrystalline samples, or assembled into superlattice arrays, which may, in mrn, be used in a variety of applications such as nanoelectronic or magnetic devices. Some scientists have even envisioned a time when so-called molecular assemblers will be able to mechanically position individual atoms or molecules, one at a time, in some predefined way (Drexler, 1986). The feasibility of such machines has been hotly debated but, regardless, such systems engineering goals are not really within the scope of this chapter. At present, methods for synthesizing metal and ceramic clusters and nanoparticles fall in one of two broad categories liquid phase techniques or vapor/aerosol methods. 

Recently reported meso- and macroscale self-assembly approaches conducted, respectively, in the presence of surfactant mesophases [134-136] and colloidal sphere arrays [137] are highly promising for the molecular engineering of novel catalytic mixed metal oxides. These novel methods offer the possibility to control surface and bulk chemistry (e.g. the V oxidation state and P/V ratios), wall nature (i.e. amorphous or nanocrystalline), morphology, pore structures and surface areas of mixed metal oxides. Furthermore, these novel catalysts represent well-defined model systems that are expected to lead to new insights into the nature of the active and selective surface sites and the mechanism of n-butane oxidation. In this section, we describe several promising synthesis approaches to VPO catalysts, such as the self-assembly of mesostructured VPO phases, the synthesis of macroporous VPO phases, intercalation and pillaring of layered VPO phases and other methods. 

Self-assembly-based networks Ordered superlattices composed of nanosized semiconducting sulfides have been synthesized within lyotropic phases. Hexagonal-packed arrays of nanocrystalline CdS (or similar structures such as ZnS, Cdi cZn tS, and CdSe) have been produced, a mineral copy of an (ethylene oxide)lo-oleyl/water mesophase presenting periodicities ranging between 7 and 10 nm. 

In attempts to develop field-deployable sensors, efforts focus on, e.g., enhancing sensor performance, reducing sensor cost and size, and simplifying fabrication. We are therefore developing a compact PL-based O2 sensor to evaluate a fully integrated platform, where the PL excitation source is an OLED array and the PD is a p-i-n structure based on thin films of hydrogenated amorphous Si (a-Si H) and related materials, or nanocrystalline Si [18]. 

To summarize, the results show that the use of the compact OLED-based sensor array is a viable approach for simultaneous monitoring of these multiple anal5des. As described later, work is in progress to develop more compact sensors, where additionally pm-thick thin-film PD arrays, such as those based on amorphous or nanocrystalline Si (nc-Si), are structurally integrated with the OLED/sensing component module. ... 

Electrical transport measurements on layer-by-layer assemblies of nanocrystals on conducting substrates have been carried out with a sandwich configuration [691-693]. Nanocrystalline films with bulk metallic conductivity have been realized with Au nanocrystals of 5 and llnm diameter spaced with ionic and covalent spacers [692, 693]. The conductivity of monolayered two-dimensional arrays of metal nanocrystals has been examined with patterned electrodes [694-699], Structural disorder and interparticle separation distance are found to be key factors that determine the conductivity of such layers [694-697]. The conductivity of the layers can be enhanced by replacing the alkane thiol with an aromatic thiol in situ [698,699]. The interaction energy of nanocrystals in such organizations can be continually varied by changing the interparticle distance. 

Kabashin AV, Meunier M (2002) Fabrication of photoluminescent Si-based layers by air optical breakdown near the silicon surface. Appl Surf Sci 186 578-582 Kalkan AK, Bae S, Li H, Hayes DJ, Fosash SJ (2000) Nanocrystalline Si thin films with arrayed void-column network deposited by high density plasma. J Appl Phys 88(1) 555-561 Krishnamurthy A, Rasmussen DH, Suni II (2011) Galvanic deposition of nanoporous Si onto 6061 Al alloy Ifom Aqueous HF. J Electrochem Soc 158(2) D68-D71... 

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