Growth mechanisms of Pd clusters and CVD grapheme flakes: kinetic explanations

Growth mechanisms of Pd clusters and CVD grapheme flakes: kinetic explanations
김승철Michael Rutenberg SchoenbergZhengtang LuoNicole KawamotoA. T. Charlie JohnsonAndrew M. Rappe
Pd deposition; Kinetic simulation; kinetic Monte Carlo; Ferroelectric substrate; CVD graphene
Issue Date
Asian Consortium on Computational Material Science Working Group Meeting
Although every system evolves toward low free energy states, it is often trapped at certain state or prefers certain evolution path to others due to kinetic origins. We present two phenomena that happen due to selective activations of certain kinetic processes: polarization-dependent catalytic activity of Pd clusters [1] and formation of zigzag edged CVD graphene [3]. Since Inoue and his coworkers observed polarization-dependent catalytic activity (i.e. catalytic activity of surfaces depends on polarization direction of materials) of Pd deposited on polar LiNbO3 [2], it has been thought that surface charges on polar substrates alter the activity of catalytic metal clusters by charging them. However, this seems to be inconsistent with other well-known fact that surface charges of any polar material are compensated to keep it from diverging electrostatic energy, thus little charges will be remained. Using density functional theory (DFT) calculations along with kinetic Monte Carlo (KMC) simulations, we provide other feasible explanation that polarization-dependent catalysis is due to a size effect rather than a charging effect [1]. The negatively polarized surface of LiNbO3 on which positive surface charges are induced becomes more oxygen-rich for charge compensation. When they are deposited, Pd atoms need to break many Pd-O bonds to move on the substrate, thus their thermal motions will be suppressed during the deposition. Pd atoms will form widely spread small clusters or planar geometries at moderate temperature due to this kinetic suppression. On the contrary, the positive surface becomes lithium-rich, has small number of Pd-O bonds, thus large clusters will be formed due to fast thermal motions. The geometric difference is large enough to explain the observed phenomenon of polarization-dependent catalysis.
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