Atomistic lattice-gas models for thermodynamically and kinetically directed assembly are applied to Ru nanocluster formation on a monolayer of graphene supported on Ru(0001) at 309 K. Nanocluster density, mean size, height distribution, and spatial ordering are analyzed by kinetic Monte Carlo simulations. Both models can reproduce the experimental data, but additional density functional theory analysis favors the former.

Extended abstract of a paper presented at Microscopy and Microanalysis 2010 in Portland, Oregon, USA, August 1 – August 5, 2010.

KTiOPO4 (KTP) is a nonlinear optical crystal presently used for second harmonic generation and electro-optic applications. The properties (ionic conductivity and damage susceptibility) of KTP crystals can vary depending on the specific technique and conditions used for growth. Consistent defect mechanisms have been determined to explain the observed AC conductivity and damage results of KTP grown by the flux and high and low temperature hydrothermal techniques. The presence of nonstoichiometry on the K and O sublattices in KTP, increasing in magnitude with temperature, is proposed. Using these defect mechanisms, the predominant defects compensating for the formation of vacant potassium sites (VK's) in flux and hydrothermal materials are vacant oxygen sites (VO's) and OH−'s, respectively. The presence of a more varied distribution of OH− sites at room temperature in high temperature hydrothermal material with higher AC conductivity indicates the importance of specific OH− sites in the lattice that may enhance the mobility of ionic carriers. The correlation of higher AC conductivity to increased average current and damage in electric field treated KTP is explained on the basis of the proposed compensating defects (VO 's and OH− 's) which set the [VK] and [Ti3+]. The similarity of the linear optical properties of KTP grown by the various techniques is confirmed by the insensitivity of the absorption edge to the nonstoichiometry or defects present.

Evidence supporting the temperature dependent defect mechanism of nonstoichiometry on the potassium and oxygen sublattices in KTP is presented. The primary compensating defects for the formation of vacant potassium sites in typical flux grown KTP are vacant oxygen sites. Protons (OH-) are the principal defect compensating for the formation of vacant potassium sites in high temperature hydrothermal KTP. A model of the ionic conductivity in high temperature hydrothermal KTP is proposed in which specific protons participate in cooperative motion over a limited distance with the potassium vacancies migrating along the “channels” in the structure in the Z-direction. The higher activation energy measured for ionic conductivity in flux grown KTP (0.5 eV) relative to high temperature hydrothermal (0.3 eV) is suggested to be due to the energy required to dissociate from a defect complex, such as a (VO - VK). The correlation of ionic conductivity to damage susceptibility appears to be due to the levels of compensating defects for vacant potassium sites in KTP, which are related to the concentrations of Ti3+ formed in the crystals. Further study is ongoing to understand the specific mechanisms involved in the ionic conductivity and damage in KTP grown by the flux and hydrothermal techniques.

We report infrared (IR) measurements of protons (H+), present as OH−, in KTiOPO4 grown by various techniques. The IR spectra are sensitive to the growth technique used. In high temperature hydrothermally grown crystals we observe IR bands which have strongly temperature dependent linewidths. The Arrhenius-type activation energies for these linewidths are small (6–15 meV). The IR bands are polarized and this information can aid in making OH− site assignments.