In situgrazing-incidence x-ray-diffraction and electron-microscopic studies of small gold clusters

Small gold clusters having diameters of 1--3 nm have been studied by using x-ray-diffraction and electron microscopy. Large amounts of small gold clusters generated by the inert-gas vapor-condensation method were deposited on a silicon wafer cooled down to 92 K to avoid coalescence growth and grazing-incidence x-ray-diffraction intensity from the cold cluster deposit was measured in situ. The diffraction pattern was analyzed and compared quantitatively with calculated patterns from several structural models considering the observed size distribution and found to be explainable approximately by a decahedral structure with geometrically closed shells. Electron-microscopic observations for gold clusters deposited sparsely on an amorphous carbon film at room temperature were also performed and decahedral multiply-twinned clusters 2--3 nm in diameter were frequently observed. This result agrees well with the x-ray statistical result, suggesting that the most dominant structure of small gold clusters prepared under our experimental conditions is the decahedral structure. The present experiments have demonstrated that an extremely weak x-ray-diffraction signal from such a small cluster can be measured quantitatively by preparing a cold cluster deposit.