QUANTUM-SIZE EFFECTS IN THE THERMODYNAMIC PROPERTIES OF METALLIC NANOPARTICLES

THE properties of nanometre-scale metallic particles differ from those of the same material(1,2) in bulk Conduction electrons, because of th eir wave-like nature, can have only certain discrete values of kinetic energy or wavelength. Such 'quantum-size' effects have been observed in two-dimensional electron gases in semiconductors(3,4), and in atomi c-scale metallic point contacts(5). Also present are 'Coulomb-charging ' effects: these are purely classical in origin, and occur when the en ergy required to add one electron to a conducting sphere exceeds the m ean thermal energy k(B)T. Thermal fluctuations in the total charge on the particle are then suppressed(6). In theory, the combination of qua ntum-size and Coulomb-charging effects should cause the properties of small metallic particles to depend sensitively on whether they have an odd or even number of electrons(7). Odd-even effects have been observ ed in experiments on tunnelling between discrete electronic levels of single metal particles(8), but their influence on thermodynamic proper ties remains to be demonstrated. Here we report measurements of the he at capacity and electronic magnetic susceptibility of small metallic c lusters. Our results show definitive evidence for odd-even effects, th us confirming that quantum and classical size effects strongly influen ce the thermodynamic properties of small particles.