HIGH-RESOLUTION ELECTRON-MICROSCOPY AND ELECTRON-ENERGY-LOSS SPECTROSCOPY OF GIANT PALLADIUM CLUSTERS

Combined structural and chemical characterization of cationic polynucl ear palladium coordination compounds Pd(561)L(60)(OAc)(180), where L = 1,10-phenantroline or 2,2'-bipyridine has been carried out by high-re solution electron microscopy (HREM) and analytical electron microscopy methods including electron energy-loss spectroscopy (EELS), zero-loss electron spectroscopic imaging, and energy-dispersive X-ray spectrosc opy (EDX). The cell structure of the cluster matter with almost comple tely uniform metal core size distributions centered around 2.3 +/- 0.5 am was observed. Zero-loss energy filtering allowed to improve the im age contrast and resolution. HREM images showed that most of the palla dium clusters had a cube-octahedral shape. Some of them had a distorte d icosahedron structure exhibiting multiple twinning. The selected-are a electron diffraction patterns confirmed the face centered cubic stru cture with lattice parameter close to that of metallic palladium. The energy-loss spectra of the populations of clusters contained several b ands, which could be assigned to the delayed PdM(4,5)-edge at 362 eV, the Pd M(3)-edge at 533 eV and the Pd M(2)-edge at 561 eV, the NK-edge at about 400 eV, the O K-edge at 532 eV overlapping with the Pd M(3)- edge and the carbon C K-edge at 284 eV. Background subtraction was app lied to reveal the exact positions and fine structure of low intensity elemental peaks, EELS evaluations have been confirmed by EDX. The rec orded series of the Pd M-edges and the N K-edge in the spectra of the giant palladium clusters obviously were related to Pd-Pd- and Pd-ligan d bonding.