In the atomic layer deposition (ALD) of cobalt (Co) and ruthenium (Ru) metals using nitrogen plasma, the structure and composition of the post N-plasma NH x -terminated (x = 1 or 2) metal surfaces are not well-known but are important in the subsequent metal-containing pulse. In this paper, we use the low-index (001) and (100) surfaces of Co and Ru as models of the metal polycrystalline thin films. The (001) surface with a hexagonal surface structure is the most stable surface, and the (100) surface with a zigzag structure is the least stable surface but has high reactivity. We investigate the stability of NH and NH2 terminations on these surfaces to determine the saturation coverage of NH x on Co and Ru. NH is most stable in the hollow hexagonal close-packed site on the (001) surface and the bridge site on the (100) surface, while NH2 prefers the bridge site on both (001) and (100) surfaces. The differential energy is calculated to find the saturation coverage of NH and NH2. We also present results on mixed NH/NH2-terminations. The results are analyzed by thermodynamics using Gibbs free energies (ΔG) to reveal temperature effects on the stability of NH and NH2 terminations. Ultra-high vacuum and standard ALD operating conditions are considered. Under typical ALD operating conditions, we find that the most stable NH x -terminated metal surfaces are 1 ML NH on the Ru(001) surface (350–550 K), 5/9 ML (0.56 ML) NH on the Co(001) surface (500–650 K) and a mixture of NH and NH2 on both Ru(100) and Co(100) surfaces.

Pyrolysis of lignocellulose biomass to produce various fuels and chemicals has gained increasing interest in recent decades. An in-depth understanding of the biomass pyrolysis reaction mechanisms is essential for the advancement of pyrolysis techniques. Quantum chemistry (QC) modeling is a powerful approach for the pyrolysis mechanism investigation at the atomic/molecular level. Despite a short history of only about 2 decades, its application to the biomass pyrolysis mechanism exploration has been well-developed, along with the fast advances of supercomputer and computational codes in the new century. This review addresses the recent progress on the pyrolysis mechanism of the three basic biomass components (cellulose, hemicellulose, and lignin) by QC modeling. On the basis of the QC modeling results reported in the literature, the current review critically summarizes the key developments about the pyrolysis chemistry of biomass by focusing on their microscopic elementary reactions, the formation routes of typical products, bimolecular interactions within or between biomass components, and catalytic effects of various catalysts. Notably, there are great gaps between the theoretical models employed in QC modeling and the natural biomass substance in the pyrolysis process. Therefore, a brief analysis of the challenges and future research perspectives is provided for the biomass pyrolysis mechanism research.

Cobalt is a potential candidate in replacing copper for interconnects and has been applied in the trenches and vias in semiconductor industry. A non-oxidizing reactant is required in plasmaenhanced atomic layer deposition (PE-ALD) of thin films of metals to avoid O-contamination. PE-ALD of Co has been demonstrated experimentally, but the growth mechanism and key reactions are not clear. In this paper, the reaction mechanism of metal cyclopentadienyl (Cp, C5H5) precursors (CoCp2) and NHx-terminated Co surface is studied by density functional theory (DFT) calculations. The Cp ligands are eliminated by CpH formation via a hydrogen transfer step and desorb from metal surface. The surface facet plays an important role in the reaction energies and activation barriers. The results show that on the NHx-terminated surfaces corresponding to ALD operating condition (temperature range 550K to 650K), the two Cp ligands are eliminated completely on Co(100) surface during the metal precursor pulse, resulting in Co atom deposited on the Co(100) surface. But the second Cp ligand reaction of hydrogen transfer is thermodynamically unfavourable on the Co(001) surface, resulting in CoCp fragment termination on Co(001) surface. The final terminations after metal precursor pulse are 3.03 CoCp/nm 2 on NHxterminated Co(001) surface and 3.33 Co/nm 2 on NHx-terminated Co(100) surface. These final structures after metal precursor pulse are essential to model the reaction during the following Nplasma step.

The double-layer model of oxygen adsorption is proposed for the initial oxidation of the Ti(0001) surface based on diffusion barrier analysis.