Nanocrystalline surfaces

Nanocrystalline Ti02 surfaces are prepared by coating conducting glass with a paste containing colloidal semiconductor particles, followed by a sintering process. For the solar cell type applications of nanocrystalline surfaces under discussion here,... 

The latter two factors in this equation depend on kinetic factors, while LHE(A) depends on the nature of the light-absorbing sensitizer and the active (surface) area of the nanocrystalline surface. Measurements are normally carried out by using monochromatic light and the IPCE values are then calculated using the following equation ... 

The structure and composition of a nanocrystalline surface may have a particular importance in terms of chemical and physical properties because of their small size. For instance, nanocrystal growth and manipulation relies heavily on surface chemistry [261]. The thermodynamic phase diagrams of nanocrystals are strongly modified from those of the bulk materials by the surface energies [262]. Moreover, the electronic structure of semiconductor nanocrystals is influenced by the surface states that He within the bandgap but are thought to be affected by the surface reconstruction process [263]. Thus, a picture of the physical properties of nanocrystals is complete only when the structure of the surface is determined. 

The onset of theoretical studies also suggests that the inhibition properties ate due to soft epitaxy adsorption and organic layer formation on the dense metal surfaces, whereas on stepped or nanocrystalline surfaces, such a homogeneous layer cannot be formed and molecules able to form a covalent bond with the surface will have better inhibition power. 

Mchale J M ef a/1996 Surface energies and thermodynamic stability in nanocrystalline aluminas Science 277 788... 

ReflEXAES can be used for near-surface structural analysis of a wide variety of samples for which no other technique is appropriate. As with EXAES, ReflEXAES is particularly suited for studying the local atomic structure around particular atomic species in non-crystalline environments. It is, however, also widely used for the analysis of nanocrystalline materials and for studying the initial stages of crystallization at surfaces or interfaces. ReflEXAES was first proposed by Barchewitz [4.135], and after several papers in the early nineteen-eighties [4.136, 4.168-4.170] it became an established (although rather exotic) characterization technique. Most synchrotron radiation sources now have beam-lines dedicated to ReflEXAES experiments. 

Thin film coatings of nanocrystalline semiconductors, as collections of quantum dots (QD or Q-dot) attached to a solid surface, resemble in many ways semiconductor colloids dispersed in a liquid or solid phase and can be considered as a subsection of the latter category. The first 3D quantum size effect, on small Agl and CdS colloids, was observed and correctly explained, back in 1967 [109]. However, systematic studies in this field only began in the 1980s. 

From a theoretical point of view, the stability of nanocrystalline diamond was discussed by several authors. Badziag et al. [25] pointed out that, according to semi-empirical quantum chemistry calculations, sufficiently small nanocrystalline diamond (3-5 nm in diameter) may be more stable than graphite by forming C-H bonds at the growing surface. Barnard et al. [26] performed the ab initio calculations on nanocrystalline diamond up to approximately 1 nm in diameter. The results revealed that the surfaces of cubic crystals exhibit reconstruction and relaxations comparable to those of bulk diamond, and the surfaces of the octahedral and cubooctahedral crystals show the transition from sp to sp bonding. 

According to the characterizations by TEM and XRD, the sample prepared from a CH4/H2 plasma was composed of nanocrystalline diamond and disordered microcrystalline graphite. Then nondiamond carbon was effectively removed with an increase in [CO]. It is therefore concluded that the VDOS of the nanocrystalline diamond and DEC films extracted from the HREELS data is in qualitative agreement with the characterizations of TEM and XRD. Although the HREELS probes only the region near the surface, the agreement suggests that the surface dynamics do not differ dramatically from those of the bulk. 

N. Kossovsky, A. Gelman, E. Sponsler, H. Hnatyszyn 1994, (Surface modified nanocrystalline ceramics for drug delivery application), Biomaterials 15, 1201. 

Elbaum R, Vega S, Hodes G (2001) Preparation and surface structure of nanocrystalline cadmium sulfide (sulfoselenide) precipitated from dimethyl sulfoxide solutions. Chem Mater 13(7) 2272-2280... 

Mogyorosi, K., Balazs, N., Sranko, D.F., Tombacz, E., Dekany, 1., Oszko, A., Sipos,P., and Dombi, A. (2010) The effect of particle shape on the activity of nanocrystalline Ti02 photocatalysts in phenol decomposition. Part 3 the importance of surface quality. Applied Catalysis B Environmental, 96 (3-4), 577-585. 

More serious errors may result when the grain-size of a specimen is small compared with the size of an indentation. Then, since all crystals are elastically anisotropic a rigid indenter will produce differing amounts of elastic strain in the grains depending on their orientations. This will create an effective roughening of the surface and increase the friction coefficient. This may result in overestimates of hardnesses. For example, this may underlie reports of nanocrystalline materials being harder than diamond. 

The same skepticism applies to nanocrystalline diamond that is reported to be harder than diamond single crystals (Sumiya and Irifune, 2007). Other cases in which rough surfaces may have skewed the measurements are TiN/SiN coatings (Kauffmann et al., 2005) and (AlMgB14 + TiB2) mixtures (Cook et al., 2000). 

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