From:
http://www.ba.ipcf.cnr.it/pdf/Comparellitesidoc.pdf
1.3 Size-Dependent Properties of Nanocrystals
The properties of crystalline solids are ordinarily catalogued without reference to their size. It is only in the regime below 10 nm where this variable comes into play. In the past decade, tailoring of materials characteristics by size control has been demonstrated in many inorganic solids belonging to one of the most technologically important classes of materials: semiconductors.
There are two major effects which are responsible for these size variations in nanocrystal properties. First, in nanocrystals the number of surface atoms is a large fraction of the total. Second, the intrinsic properties of the interior of nanocrystals are transformed by quantum size effects. In any material, surface atoms make a distinct contribution to the free energy, and the large changes in thermodynamic properties of nanocrystals (melting temperature depression, solid-solid phase transition elevation) can ultimately be traced to this. The surfaces of nanocrystals have until recently been thought of as largely disordered, yielding spherical or ellipsoidal shapes [16]. More recent work shows that nanocrystals assume regular shapes, with the same well-defined facets as are present in extended crystals [17, 18]. This opens up the possibility of manipulating the surface energy of nanocrystals in a controlled manner. The ability to manipulate the energy of nanocrystal surfaces will have practical consequences.
Independent of the large number of surface atoms, semiconductor nanocrystals with the same interior bonding geometry as a known bulk phase often exhibit strong variations in their optical and electrical properties with size [19, 20]. These changes arise through systematic transformations in the density of electronic energy levels as a function of the size of the interior, known as quantum size effects.
Herewith we report a briefly description of the most striking size-dependent properties of semiconductors and metal nanocrystals.
Melting temperature depression.
In a wide variety of materials ranging from metals to semiconductors to insulators, a decrease in solid to liquid transition temperature has been observed with decreasing nanocrystal size [21-25]. An understanding of this depression can be obtained by considering the factors that contribute to the total energy of a nanocrystal: in a system containing only a few hundred atoms, a large fraction of these atoms will be located on the surface. As surface atoms tend to be coordinatively unsaturated, there is a large energy associated with this surface. The key to understanding this melting point depression is the fact that the surface energy is always lower in the liquid phase compared to the solid phase. In the dynamic fluid phase, surface atoms move to minimize surface area and unfavourable surface interactions. In the solid phase, rigid bonding geometries cause stepped surfaces with high-energy edge and corner atoms. By melting, the total surface energy is thus reduced. The smaller the nanocrystal, the larger the contribution made by the surface energy to the overall energy of the system and thus the more dramatic the melting temperature depression. As melting is believed to start on the surface of a nanocrystal, this surface stabilization is an intrinsic and immediate part of the melting process [26, 27]
[21] Coombes, C. J. J. Phys. 2 (1972) 441.
[22] Buffat, P.; Borel, J.-P. Phys. Rev. A 13 (1976) 2287.
[23] Castro, T.; Reifenberger, R.; Choi, E.; Andres, R. P. Phys. ReV. B:Condens. Matter 42 (1990) 8548.
[24] Beck, R. D.; St. John, P.; Homer, M. L.; Whetten, R. L. Science 253 (1991) 879.
[25] Martin, T. P.; Naher, U.; Schaber, H.; Zimmermann, U. J. Chem.Phys. 100 (1994) 2322.
