DFT Study of Water Adsorption and Decomposition on a Ga-Rich GaP(001)(2×4) Surface

We investigate the adsorption and decomposition states of a water molecule on a Ga-rich GaP(001)(2×4) surface using the PBE flavor of density functional theory (DFT). We selected the GaP(001)(2×4) mixed dimer surface reconstruction model to represent the Ga-rich GaP(001)(2×4) surface. Because our focus is on reactions between a single water molecule and the surface, the surface water coverage is kept at 0.125 ML, which corresponds to one water molecule in the (2×4) unit cell. We report here the geometries and energies for an exhaustive set of adsorption and decomposition states induced by a water molecule on the (2×4) unit cell. Our results support a mechanism in which (1) the first step is the <i>molecular adsorption</i>, with the water molecule forming a Lewis acid–Lewis base bond to the sp² Ga atom of either the first-layer Ga–P mixed dimer or the second layer Ga–Ga dimers using an addition reaction, (2) which is followed by dissociation of the adsorbed H₂O to form the <i>HO/H decomposition state</i> in which the hydroxyl moiety bonds with surface sp² Ga atoms, while the hydrogen moiety binds with the first-layer P atom, (3) which is followed by the <i>O/2H decomposition state</i>, in which the oxygen moiety forms bridged Ga–O–Ga structures with surface Ga dimers while one H bonds with the first-layer P atom and the other to surface sp² Ga atoms. (4) We find that driving off the hydrogen as H₂ leads to the <i>surface oxide state</i>, bridged Ga–O–Ga structures. This surface oxide formation reaction is exothermic relative to the energy of H₂O plus the reconstructed surface. These results provide guidelines for experiments and theory to validate the key steps and to obtain kinetics data for modeling the growth processes

Thermodynamic Control of Two-Dimensional Molecular Ionic Nanostructures on Metal Surfaces

Bulk molecular ionic solids exhibit fascinating electronic properties, including electron correlations, phase transitions, and superconducting ground states. In contrast, few of these phenomena have been observed in low-dimensional molecular structures, including thin films, nanoparticles, and molecular blends, not in the least because most of such structures have been composed of nearly closed-shell molecules. It is therefore desirable to develop low-dimensional ionic molecular structures that can capture potential applications. Here, we present detailed analysis of monolayer-thick structures of the canonical TTF–TCNQ (tetrathiafulvalene 7,7,8,8-tetracyanoquinodimethane) system grown on low-index gold and silver surfaces. The most distinctive property of the epitaxial growth is the wide abundance of stable TTF/TCNQ ratios, in sharp contrast to the predominance of a 1:1 ratio in the bulk. We propose the existence of the surface phase diagram that controls the structures of TTF–TCNQ on the surfaces and demonstrate phase transitions that occur upon progressively increasing the density of TCNQ while keeping the surface coverage of TTF fixed. Based on direct observations, we propose the binding motif behind the stable phases and infer the dominant interactions that enable the existence of the rich spectrum of surface structures. Finally, we also show that the surface phase diagram will control the epitaxy beyond monolayer coverage. Multiplicity of stable surface structures, the corollary rich phase diagram, and the corresponding phase transitions present an interesting opportunity for low-dimensional molecular systems, particularly if some of the electronic properties of the bulk can be preserved or modified in the surface phases