Journal of Low Temperature Physics

, Volume 45, Issue 1–2, pp 189–232 | Cite as

The microelectronic structure of platinum particles investigated by NMR

  • I. Yu
  • W. P. Halperin
Article

Abstract

The properties of195Pt nuclear magnetic resonance of small particles have been studied over a range of particle sizes from 33 to 200 Å using pulsed NMR techniques. This work was initiated to study size-dependent phenomena and to elucidate their physical origin. An anomalous linewidth, more than an order of magnitude larger than that of bulk platinum for the smallest size sample, was discovered. This was found to be inhomogeneous broadening and to have a size dependence (d)−1, whered is the mean particle diameter of the sample. Within the temperature range of 1.7–77 K, no temperature dependence was observed. As a consequence, the broadening was attributed to an intraparticle Knight shift distribution resulting from electron spin density oscillations associated with the metal surface. Spin-echo envelopes from the small platinum particles were found to decay nonexponentially, indicating the presence of a nuclear spin diffusion process in the magnetic field gradients associated with the Knight shift distribution. The diffusion process was measured by the Hahn (90°–180°) pulse technique and the Carr-Purcell-Meibocm-Gill techniques and analyzed using a spin diffusion constant of platinum computed with the moment method of Redfield and Yu. The computed spin diffusion constant wasDz=4×10−12 cm2/sec and from this an rms magnetic field gradient was determined. Upon analyzing the size-dependent spin-echo results, the field gradient was found to be characteristic of a surface region of thickness 1.5 ± 0.5 lattice constants independent of the size of the particles.

In contrast to these anomalous properties of small platinum particles, the peak position of the resonance lines and the spin-spin and spin-lattice relaxation times were found to be identical to the values for bulk material. A simple model ascribing free electron behavior tos conduction electrons was applied to study quantitatively the effects of electron spin density oscillation on NMR properties. Applying an infinite-barrier boundary condition to the electrons, we computed numerically the electron charge and spin density distributions for platinum particles of diameter up to 300 Å. The computations revealed Friedel oscillations of these densities near the particle surface. From the computed electron spin density of thes electrons, the spatial distribution of the contact Knight shift was obtained, which could account for all the observed NMR properties of small platinum particles—the resonance position, line shape, linewidth, relaxation times, rms field gradient, and the thickness of the surface region.

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Copyright information

© Plenum Publishing Corporation 1981

Authors and Affiliations

  • I. Yu
    • 1
  • W. P. Halperin
    • 1
  1. 1.Department of Physics and Astronomy and Materials Research CenterNorthwestern UniversityEvanston
  2. 2.Department of PhysicsDuke UniversityDurham

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