Transmission electron microscopy and Monte Carlo simulations of ordering in Au-Cu clusters produced in a laser vaporization source

Au-Cu bimetallic alloy clusters are produced in a laser vaporization source starting from Au-Cu alloy targets with different stoichiometric compositions. The clusters are deposited on two different substrates—amorphous carbon and crystalline MgO—and are characterized by electron diffraction and high-resolution electron microscopy. The experiments show that the overall chemical composition in the clusters is the same as the chemical composition of the target material; but the crystal structure of the Au-Cu alloy clusters differs from their known bulk crystal structure. Electron microscopy experiments provide evidence that no chemical ordering exists between Au and Cu atoms and that the clusters are solid solutions. Monte Carlo simulations using the second moment tight-binding approximation, however, predict ${\mathrm{Cu}}_3\mathrm{Au}$ clusters ordered in the core but with a disordered mantle. The possible origins of the differences between experiment and Monte Carlo simulations are discussed.