Vacancy Defects on Surfaces of Au Nano-particles Embedded in MgO
Physical Review Letters, 83 (1999) 4856.
Nanometer-scale science and technology is currently of importance because of the unique properties promised for the next generation of optical and electronic devices. Surface phenomena dominate nano-material properties because of their high surface-to-bulk ratio. We have demonstrated the capability to characterize vacancy clusters on the surface of Au nanoparticles embedded in MgO and found that the presence of these clusters affects the optical properties of the system.
Gold nanoparticles in Magnesia (Figure 1) were fabricated by ion implantation followed by subsequent annealing at the ORNL - Surface Modification and Characterization Research Center.
Figure 1 - TEM of Au Nano-particles in MgO
It has been reported that the surface plasma resonance frequency (SPRF) of gold nanoparticles embedded in magnesia is shifted to the "red" if the nanoparticles are generated in an O2 annealing atmosphere instead of an H2 annealing atmosphere. Our group, working with Allen Mills of Lucent Technology and R. Suzuki and S. Ishibashi of the Electrotechnical Laboratory, used positron lifetime spectroscopy and Doppler broadening of positron annihilation radiation to reveal the origin of the red shift of SPRF.
As shown in Figure 2, the positron decay from Au nanoparticles in MgO displays a different lifetime depending on how the nano-particles are prepared. Nanoparticles prepared by annealing in O2 show a longer lived state labeled v4. This state is associated with clusters of four atomic vacancies located on the surface of the Au particles. The surface vacancy clusters are not seen on the nanoparticles prepared in H2.
Figure 2 - Positron lifetime spectra for MgO and embedded Au nanoparticle layers generated by annealing in O2 or H2, respectively.
The v4 clusters are related to the "red" shift in the following way: for Au nanoparticles generated in O2, electrons are transferred from the Au nanoparticles to the interface vacancy clusters, resulting in a reduced electron density on the nanoparticles, therefore the frequency decreases.
