Atomic-scale study of TiO2-GR nanohybrid formation by ALD: the effect of the gas phase precursor.

In the present work, we report on a theoretical-computational study of the growth mechanism of the TiO2-Graphene nanohybrid by atomic layer deposition. Hydroxyl groups (OH) are anchoring sites for interacting with the main ALD titanium precursors (Tetrakis (dimethylamino) Titanium, Titanium Tetrachloride, and Titanium Isopropoxide). Results demonstrate that the chemical nature of the precursor directly affects the reaction mechanism in each ALD growth step. Tetrakis(dimethylamino)titanium is the precursor that presents a higher affinity (lower energy barriers for the reaction) to hydroxylated graphene in the growth process. A complete reaction mechanism for each precursor was proposed. The differences between precursors were discussed through the non-covalent interactions index. Finally, the water molecules help reduce the energy barriers and consequently favor the formation of the TiO2-graphene nanohybrid.

A first-principles study of the atomic layer deposition of ZnO on carboxyl functionalized carbon nanotubes: the role of water molecules.

The formation of heterostructures that combine a large surface area with high surface activity has attracted the attention of the scientific community due to the unique properties and applications of these heterostructures. In this work, we describe - at the atomic level - the full reaction mechanisms involved in the atomic layer deposition of a hybrid ZnO/CNT inorganic structure. First, the pristine CNTs are chemically activated with a carboxylic acid, a process unique to carbon materials. Diethylzinc (DEZ) and water are used as gas-phase precursors to form ZnO. Our findings show that DEZ is physically adsorbed on the CNTs during the exposure of the first precursor. The ligand-exchange to generate chemisorbed ethyl zinc on the O side of the COOH group needs to overcome an energy barrier of 0.06 eV. This is a very small energy if compared to the values (0.5-0.6 eV) obtained in previous studies for OH functionalized surfaces. The height of the barrier is associated with the C[double bond, length as m-dash]O side, which mediates the H proton's exchange from the OH group to the C2H5 ligand. Furthermore, upon exposure to the oxidizing agent (H2O), ethyl zinc exchanges its last ligand as ethane, and it accepts a hydroxyl group through a self-limiting reaction with an energy barrier of 0.88 eV. Notice that the energy barrier of the second ligand-exchange is larger than of the first. We have also analyzed the effect in the saturation of the second precursor: as the quantity of water molecules increases, the long-range interactions tend to repel them. However, the energy barrier of the second ligand-exchange decreases from 1.53 eV to 0.88 eV for one and two water molecules, showing a clear dependence on the oxidizing agent. Non-covalent interactions are used as a tool to visualize the driving forces that take place during each partial reaction in real space. Our study points out the importance of using the right functionalization agent to achieve a controlled and conformal ALD growth at the initial steps of the formation of hybrid ZnO/CNT structures, as well as the role played by the oxidizing agent to lower the energy barrier on the second ALD step.

Understanding the first half-ALD cycle of the ZnO growth on hydroxyl functionalized carbon nanotubes.

We report the adsorption of diethylzinc on hydroxyl functionalized carbon nanotubes. This study intends to understand, at the atomic level, the initial stages of ZnO formation by atomic layer deposition. Our study begins with the molecule physisorbed on the nanotube (initial state of the reaction). The final state of this reaction is when the H atom of the hydroxyl group is abstracted and migrates to one ethyl group of diethylzinc. The oxygen atom relaxes towards the nanotube and forms a strong bond with a carbon atom, while the remaining part of the molecule forms a bond with the H atom and physisorbs on top of the ZnO unit. The probability for this process to happen is very high since the energy to desorb the diethylzinc molecule is higher than the energy needed to break down the O-H bond. Non-covalent interactions and charge density distributions are plotted to confirm the break-down, formation of bonds, and repulsion during the reaction pathway.

Third-order nonlinear optical properties of a multi-layer Al2O3/ZnO for nonlinear optical waveguides.

This is a report of a study of the nonlinear optical properties of samples based on multiple Al2O3/ZnO bilayers fabricated by atomic layer deposition (ALD) in silica. The multi-layer configuration for samples consists of alternated layers of constant thickness of Al2O3 (Δx) and ZnO (Δy) nanolaminates with a total thickness of ∼ 500 nm. The physical properties of the samples were characterized by means of TEM, spectrophotometry and variable angle spectroscopic ellipsometry. The absorptive and refractive contributions to the nonlinearity of the samples were studied by means of z-scan technique using a 100 fs at 800 nm. The nonlinear parameters, ß and n2, are studied using different values of the layers thickness, Δx and Δy, in the nanolaminated stack. The possible applications in optical signal processing system are discussed by means of the figures of merit W and T.