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We study the ultrafast structural dynamics, in response to electronicexcitations, in heterostructures composed of Au$_{923}$ nanoclusters onthin-film substrates with the use of femtosecond electron diffraction. Variousforms of atomic motion, such as thermal vibrations, thermal expansion andlattice disordering, manifest as distinct and quantifiable reciprocal-spaceobservables. In photo-excited, supported nanoclusters thermal equilibrationproceeds through intrinsic heat flow, between their electrons and theirlattice, and extrinsic heat flow between the nanoclusters and their substrate.For an in-depth understanding of this process, we have extended thetwo-temperature model to the case of 0D/2D heterostructures and used it todescribe energy flow among the various subsystems, to quantify interfacialcoupling constants, and to elucidate the role of the optical and thermalsubstrate properties. When lattice heating of Au nanoclusters is dominated byintrinsic heat flow, a reversible disordering of atomic positions occurs, whichis absent when heat is injected as hot substrate-phonons. The present analysisindicates that hot electrons can distort the lattice of nanoclusters, even ifthe lattice temperature is below the equilibrium threshold for surfacepre-melting. Based on simple considerations, the effect is interpreted asactivation of surface diffusion due to modifications of the potential energysurface at high electronic temperatures. We discuss the implications of such aprocess in structural changes during surface chemical reactions.

Ultrafast heat flow in heterostructures of Au nanoclusters on thin-films: atomic-disorder induced by hot electrons