Probing the electronic structure of small copper particles: 63Cu NMR at 1.5 K

Abstract

We have investigated the ⁶³Cu NMR (at 1.5 and 4.2 K) of a series of samples comprising silica-supported colloidal copper particles with average diameter (d) ranging from 1000 to 34 Å. Substantial reductions have been observed in the metal Knight shift of the NMR of sub-80 Å particles in our measurement field of 12.7 T. The magnitude of this reduction in the Knight shift is both temperature- and size-dependent; the effect being most pronounced for the smallest copper particles at our base temperature of 1.5 K.

In the lower-size regime of colloidal copper particles (d⩽ 80 Å), there exists a fundamental and qualitative difference in the electronic structure of the particles as compared with that of bulk copper metal. Specifically, the magnetic properties of the colloidal metal particles are dependent upon whether each individual particle contains either an even or an odd number of copper atoms. We find that the ⁶³Cu NMR Knight shift distinguishes the behaviour of even (and odd) numbered copper particles. In the case of even-number particles, the reduction in the Knight shift arises from the loss of the paramagnetic susceptibility, this being most pronounced at low temperatures.

The observed reduction in the NMR Knight shift of copper from its bulk metallic value is strong evidence for the existence of quantum size effects in colloidal copper particles at low temperatures.