6 CHEMICAL SENSORS: FUNDAMENTALS. VOLUME 2: NANOSTRUCTURED MATERIALS Figure 1.5. Photoluminescence spectra of ZnS nanoparticles and ZnS clusters in zeolite Y (ZnS/Y). For photoluminescence spectral measurement of ZnS nanoparticles, the excitation at exc = 280 nm was used. (Reprinted with permission from Chen et al. 1997. Copyright 1997 American Institute of Physics.) size, the position of both surface plasmon adsorption and the fluorescence peak are shifted to shorter wavelengths as well (see Figure 1.5). For the smallest of metallic nanoclusters, with dimensions ca. <2 nm, the surface plasmon absorption disappears. Since so few atoms comprise discrete nanoclusters of this size, the spacing between adjacent energy levels becomes comparable to the thermal energy kT-- especially at lower temperatures and smaller nanocluster diameters (Fahlam 2007). Mechanical properties are size-dependent as well. Many of the mechanical properties of nano- materials are different from those of the bulk materials, including hardness, elastic modulus, fracture toughness, scratch resistance, and fatigue strength, among others. One of the most familiar mechanical phenomena involving size dependency is the Hall-Petch effect, which is characteristic of polycrystalline solids (Law et al. 2004). The yield strength and hardness of a microstructured polycrystalline mate- rial typically increase with decreasing grain size, owing to the progressively more effective disruption of dislocation motion by grain boundaries. However, studies on solids composed of nanoscale grains suggest that the Hall-Petch relation breaks down at a critical grain size, below which a material softens. Atomistic modeling carried out by Schiotz and Jacobsen (2003) points to a transition from dislocation- mediated yielding to grain boundary sliding at very small crystallite sizes as the primary explanation for the anomalous maximum in the strength of metallic polycrystalline solids. Phonon transport is expected to be greatly impeded in nanomaterials (i.e., d < , where d is the diameter and is the phonon mean free path), due to increased boundary scattering and reduced phonon group velocities stemming from phonon confinement (Law et al. 2004). Detailed models of phonon heat conduction in cylindrical (Zou and Balandin 2001) and rectangular (Lu et al. 2003)