PHYSICAL REVIEW 8 VOLUME 31, NUMBER 8 15 APRIL 1985 Extended x-ray-absorption fine-structure and near-edge-structure studies on evaporated small clusters of Au A. Balerna, E. Bernieri, P. Picozzi, A. Reale, and S. Santucci Istituto di Fisica, UniUersita de L'Aquila, L'Aquila, Italy E. Burattini Consiglio Nazionale delle Ricerche and Istituto 1Vazionale di Fisica Nucleare, Laboratori Nazionali di Frascati, 00044 Frascati, Italy S. Mobilio Istituto Nazionale di Fisica Nucleare, Laboratori Nazionali Frascati, 00044 Frascati, Italy (Received 31 May 1984) The L3 Au edge of small evaporated gold clusters has been studied by x-ray-absorption spectros- copy at the Frascati synchrotron radiation facility. Sample discontinuity and particle size were con- trolled by optical transmission measurements and electron microscopy analysis. The extended x- ray-absorption fine-structure spectra showed evidence of nearest-neighbor-distance contraction, whose value reached 2.5%%uo for the smallest clusters. The behavior of the nearest-neighbor-distance contraction versus cluster diameter agreed with a macroscopic liquid-drop model. Increases in the nearest-neighbor-distance fluctuations around the equilibrium positions were found due to the higher mobility of the surface atoms with respect to the bulk ones. The slight increase of statical disorder together with the general features of the x-ray-absorption near-edge structure spectra al- lowed us to exclude structural changes from the fcc bulk metal structure to the icosahedral struc- ture, even for clusters of 50 atoms. I. INTRODUCTION During the last decade, the chemical and physical prop- erties of small metal clusters have been an active area of research. ' ' In fact the enhancement of chemical reac- tivity, ' the lowering of the melting temperature, ' and the magnetic behavior' * of the metal clusters are peculiar properties widely exploited in many technological applica- tions, among them heterogeneous catalysis. ' Many theoretical and experimental studies have been and are be- ing performed on the electronic and structural properties of metal clusters under different conditions such as differ- ing chemical environment, sample preparation, size, and substrates. ' ' ' ' Many problems are still unsolved: for instance, it is unknown if the cluster-size-dependent chemical reactivity is due to a change in their electronic or in their structural properties and how these properties depend on the sample preparation method and on the in- teraction with substrates. " In this paper we report an investigation on the structur- al properties of Au clusters, evaporated on a weakly in- teracting substrate, whose average diameters range from 11 up to 60 A. In the past, experiments using diffraction have been made on Au clusters in order to measure the lattice pa- rameters as a function of the cluster size. ' Lattice- parameter contractions have been observed, but there is great disagreement among the numerical values obtained on clusters of same diameter: for instance, for a 35-A cluster diameter values ranging from 0. 4% to 2.7%%uo are reported. This is due to the diffraction method which be- comes less and less sensitive as the cluster size decreases. In order to determine the effective amount of contractions and to check also changes in the crystallographic cluster structure, ' we performed a complete structural investi- gation using EXAFS ' (extended x-ray-absorption fine structure) and XANES (x-ray-absorption near-edge structure). The layout of this paper is as follows: In Sec. II we re- port the sample preparation; we briefly describe the details regarding their characterization using optical transmit- tance and electron microscopy measurements and describe the x-ray-absorption spectra. Section III contains the x- ray-absorption and EXAFS data analysis. In Sec. IV we discuss the results and their interpretation. Section V is a summary of the main results. II. SAMPLE PREPARATION AND CHARACTERIZATION' ABSORPTION MEASUREMENTS Cluster samples were prepared under vacuum on a 6- pm polymer film by consecutive evaporation of gold (pur- ity=99. 99%) and Mylar to achieve the optimum metal thickness for x-ray measurements. The amount of Au deposited on each layer of the different samples studied is reported in Table I; the Mylar thickness was always around 200 A. Gold and Mylar depositions were both controlled with a quartz-crystal detector. In order to check that the multilayer samples were discontinuous, optical transmission measurements were made at normal incidence in the spectral range 0.25 — 2. 5 pm. It is known that the transmittance spectra of discon- 31 5058 1985 The American Physical Society