We used time-lapsed scanning tunneling microscopy between 43 and 50 K and density functional theory (DFT) to explore the basic surface diffusion steps of cobalt phthalocyanine (CoPc) molecules on the Ag(100) surface. We show that the CoPc molecules translate and rotate on the surface in the same temperature range. Both processes are associated with similar activation energies; however, the translation is more frequently observed. Our DFT calculations provide the activation energies for the translation of the CoPc molecule between the nearest hollow sites and the rotation at both the hollow and the bridge sites. The activation energies are only consistent with the experimental findings, if the surface diffusion mechanism involves a combined translational and rotational molecular motion. Additionally, two channels of motion are identified: the first provides only a channel for translation, while the second provides a channel for both the translation and the rotation. The existence of the two channels explains a higher rate for the translation determined in experiment.

Diffusion and decay of alloyed Cu=Ag islands are investigated in the size range from 1 to 40 nm 2 on Ag(100) at room temperature with fast-scanning tunneling microscopy and density functional theory. While islands at sizes above 7 nm 2 show the diffusion and decay behavior expected for dynamics based on single atom hopping, islands smaller than 4 nm 2 diffuse faster and decay slower than predicted by standard theory. This anomalous behavior at unexpected large island sizes is related to a size dependent dealloying of the Cu=Ag islands. DOI: 10.1103/PhysRevLett.107.046101 PACS numbers: 68.43.Jk, 68.35.bd, 68.37.Ef, 68.43.Bc The physical properties of nanoscale systems can change abruptly with decreasing system size. These characteristics are often used to tune the electronic or optical performance of quantum dots [1] or 3D nanoparticles [2]. On surfaces, the occurrence of such scaling effects was observed in the emergence of preferred heights in thin metal films [3,4]. Later it was detected for lateral scaling of quantum dots and adatom islands [5,6]. All effects investigated so far are so-called quantum size effects, which are related to electron confinement. Such confinement occurs when the size of a nanostructure becomes comparable to the electron's de Broglie wavelength. For metals, this wavelength is typically in the range of a few atomic distances, which is still small compared to the size of nanoparticles used for technological applications.In this Letter, we present a novel type of scaling effect not related to electron confinement and found in nanoscale structures consisting of up to 100 atoms. We investigate diffusion and decay of Cu and alloyed Cu=Ag islands on Ag(100) by fast-scanning tunneling microscopy (STM) and density functional theory (DFT) and reveal a transition region from 4 to 7 nm 2 for this heteroepitaxial metal system. Above 7 nm 2 the islands are composed of a Cu=Ag alloy, and application of standard theory to the diffusion and decay behavior of these islands indicates that the island dynamics is based on single atom movements. In the transition region, the islands dealloy. Below a size of 4 nm 2 the islands consist only of Cu atoms adsorbed not exclusively in hollow sites of the Ag(100) surface. The diffusion of the pure Cu islands is faster and their decay is slower than predicted by standard theory. DFT relates the displacement of the atoms from hollow sites to the strain induced by the large lattice mismatch between copper and silver. These islands show a stronger internal bond and a weaker bond to the surface explaining the anomalous diffusion and decay behavior.Measurements are performed with a commercial fast-scanning STM (SPECS ''STM 150 Aarhus'') at and close to room temperature (RT) and with a custom-built low-temperature STM [7] at 5 K. Both setups are housed in an UHV environment (base pressure 2 Â 10 À10 mbar). The fast-scanning STM is equipped with a stabilization algorithm for recording of image sequences (movies).The Ag(100) surface is cleaned by standard sputtering a...

Arrhenius plots are often used to determine energy barriers and prefactors of thermally activated processes. The precision of thus determined values depends crucially on the precision of the temperature measurement at the sample surface. We line out a procedure to determine the absolute temperature of a metal sample in a cryogenic scanning tunneling microscope between 5 K and 50 K with sub-Kelvin precision. We demonstrate the dependence of prefactor and diffusion energy on this calibration for diffusion of CO on Cu(111) and on Ag(100) measured in the temperature range from 30 K to 38 K and 19 K to 23 K, respectively.

The diffusion of carbon monoxide molecules on Cu(111) is investigated in time-lapsed scanning tunneling microscopy in a temperature range from 30 to 38 K. An asymptotic theory of adsorbate diffusion predicted a trio interaction that changes the diffusion barrier of three particles diffusing in close proximity beyond the change induced by the long-range interaction between three pairs of molecules. Distance-dependent variations in the diffusion energy confirm this theoretical prediction. In future, the theory can better assist experiments for a broader exploration, not only for diffusion, but also for nucleation and reaction.

Water diffusion across the surfaces of materials is of importance to disparate processes such as water purification, ice formation, and more. Despite reports of rapid water diffusion on surfaces the molecular level, details of such processes remain unclear. Here, with scanning tunneling microscopy, we observe structural rearrangements and diffusion of water trimers at unexpectedly low temperatures (<10 K) on a copper surface, temperatures at which water monomers or other clusters do not diffuse. Density functional theory calculations reveal a facile trimer diffusion process involving transformations between elongated and almost cyclic conformers in an inchworm-like manner. These subtle intermolecular reorientations maintain an optimal balance of hydrogen-bonding and water–surface interactions throughout the process. This work shows that the diffusion of hydrogen-bonded clusters can occur at exceedingly low temperatures without the need for hydrogen bond breakage or exchange; findings that will influence Ostwald ripening of ice nanoclusters and hydrogen bonded clusters in general.