OPTICAL-PROPERTIES OF A FAMILY OF AU-NANOPARTICLE-CONTAINING ALUMINA MEMBRANES IN WHICH THE NANOPARTICLE SHAPE IS VARIED FROM NEEDLE-LIKE (PROLATE) TO SPHEROID, TO PANCAKE-LIKE (OBLATE)

The optical properties of a family of alumina membranes that contain A u nanoparticles with diameters of 52 nm and aspect ratios (length divi ded by diameter) varying from 0.38 (oblate or pancake-like) to 24 (pro late or needle-like) have been studied. The dynamic Maxwell-Garnett (D MG) theory was used to model the optical properties of these Au nanopa rticle-containing membranes, The trend of a redshift in lambda(max) fo r the plasmon resonance band with decreasing aspect ratio (observed pr eviously for pn,late particles) is observed over this entire aspect ra tio range. Indeed, the redshift with decreasing aspect ratio becomes m uch more dramatic for the oblate particles. Because of this enhanced s ensitivity of lambda(max) to the aspect ratio, we suggest that the DMG theory can be used to obtain the aspect ratio for oblate Au nanoparti cles of unknown length by matching the experimental lambda(max) with a DMG calculated value using aspect ratio as the adjustable parameter. (C) 1997 Elsevier science S.A.