Density Functional Theory Study of Atomic Layer Deposition of Zinc Oxide on Graphene
© Ali and Hashim. 2015
Received: 20 May 2015
Accepted: 8 July 2015
Published: 22 July 2015
The dissociation of zinc ions (Zn2+) from vapor-phase zinc acetylacetonate, Zn(C5H7O2)2, or Zn(acac)2 and its adsorption onto graphene oxide via atomic layer deposition (ALD) were studied using a quantum mechanics approach. Density functional theory (DFT) was used to obtain an approximate solution to the Schrödinger equation. The graphene oxide cluster model was used to represent the surface of the graphene film after pre-oxidation. In this study, the geometries of reactants, transition states, and products were optimized using the B3LYB/6-31G** level of theory or higher. Furthermore, the relative energies of the various intermediates and products in the gas-phase radical mechanism were calculated at the B3LYP/6-311++G** and MP2/6-311 + G(2df,2p) levels of theory. Additionally, a molecular orbital (MO) analysis was performed for the products of the decomposition of the Zn(acac)2 complex to investigate the dissociation of Zn2+ and the subsequent adsorption of H atoms on the C5H7O2 cluster to form acetylacetonate enol. The reaction energies were calculated, and the reaction mechanism was accordingly proposed. A simulation of infrared (IR) properties was performed using the same approach to support the proposed mechanism via a complete explanation of bond forming and breaking during each reaction step.
Two-dimensional (2D) sheets of sp 2-hybridized carbons known as graphene have attracted considerable attention because of their exceptional optical, electrical, chemical, and mechanical properties that impart graphene with the promising ability to develop next-generation functional nanomaterials for various applications [1–3]. To tailor graphene to targeted applications, considerable efforts have been made to control and modify the properties of graphene through various functionalization routes . Furthermore, many studies have been conducted to develop semiconducting material/graphene hybrid structures using either vapor-phase [5–7] or liquid-phase techniques [8–11]. In the past few decades, zinc oxide (ZnO) nanostructures have been considered in many works for optoelectronic and photovoltaic device applications [9–11]. Recently, ZnO/graphene hybrid nanostructure was reported to have excellent potential for use in transparent flexible electrical and optical devices, including flexible photovoltaics, displays, and light emitters [10–15]. The vapor-phase deposition of ZnO using β-diketonates such as acetylacetonate as the Zn precursor was reported as a promising route to grow ZnO nanostructures [12, 14]. Most studies on ZnO/graphene hybrid structures have focused on their structural morphologies and electronic properties [8, 16], whereas few have paid attention to the reaction mechanisms of the semiconducting species at reaction sites on the graphene surface [17, 18]. To our knowledge, no study has reported the reaction mechanism for the vapor-phase deposition of ZnO on graphene utilizing acetylacetonate as a Zn source. In this article, we report the possible reaction mechanism for the deposition of ZnO on pre-oxidized graphene via the injection of vaporized zinc acetylacetonate in the presence of hydrogen.
Until the early 1990s, quantum chemists used the ab initio Hartree–Fock (HF) approach along with second-order Møller-Plesset perturbation theory as starting points to solve Schrödinger’s equation . Further calculations based on experimental data were carried out for the sake of accuracy through quadratic configuration interaction or coupled cluster theory in the case of small molecules [19, 20]. It is only possible to solve the Schrödinger equation for a one-electron system. Thus, in the late 1980s, density functional theory (DFT) coupled with local density corrected approximation (LDA) was developed as an alternative approximation method to derive reliable solutions to the Schrödinger equation for many-electron systems. In computational physics and chemistry, the HF method is one of the approximation methods that is used to determine the wave function and energy of a quantum many-body system in a stationary state. However, according to the HF approximation, electrons move independently, meaning that both the electron–electron repulsion energy and their total energy are overestimated [20, 21]. The limiting HF energy is therefore higher than the experimental energy. The electron correlation energy is the term used to describe the coupling or correlation of electron motions and is defined as the difference between the HF energy and the experimental energy [20, 22, 23].
The Spartan 14 quantum chemistry package (Wavefunction, USA) was used to perform all calculations in this study . Equilibrium geometries were optimized by the B3LYP density functional method using the 6-311G** basis set; the developer of Spartan chose the Gaussian exponents for polarization functions to give the lowest energies for the modeled molecules. The polarization of the s orbitals on hydrogen atoms is crucial to accurately describe the bonding in acetylacetonate systems, particularly the hydrogen bonding. Furthermore, the 6-31G** basis set provides the p-type polarization functions for hydrogen. This can improve the total energy of the system along with the results for systems with large anions and can impose more flexibility . Zn-containing structures were also optimized with larger basis sets and higher levels of theory .
All thermal correction energies were calculated using the 6-311G**, 6-311++G**, and 6-311++G(2df,2pd) (for Zn-containing reactions) basis sets. Calculations involving anions and absolute acidity (e.g., dipole moment calculations) require extra care when selecting the basis sets because extra electrons are weakly coupled to specific atoms or groups of atoms. Thus, the basis sets should provide diffuse s- and p-type functions on non-hydrogen atoms. This is usually designated by the “+” sign, as in 6-311++G**. The second “+” sign indicates that a diffuse function is added to hydrogen [32, 33]. To obtain more accurate energy calculations, single-point calculations were performed at the B3LYP/ 6-311G** optimized geometry using the B3LYP/6-311 + G**, MP2/6-311 + G**, B3LYP/6-311 + G(2df,2p), and MP2/6-311 + G(2df,2p) levels of theory.
Results and Discussion
Dissociation of Zn2+ from the Zn(acac)2 Complex
Mechanism of the Dissociation Reaction
The reaction coordinate diagram shows that the initial transition state was obtained at a lower energy barrier (24.70 kcal/mol) than the final transition state (61.78 kcal/mol). Thus, TS1 is considered to be the rate-limiting step on which the overall reaction kinetics depend. The reaction follows a typical interchange substitution mechanism profile as the secondary bonding with the square planar complex of Zn(acac)2 were detected at the reaction intermediates. Because the association of H atoms to the square planar complex is the longest step in the pathway, it can be considered to be the actual rate-determining step of the overall reaction. Hence, the reaction mechanism can be classified as an interchange substitution mechanism that is associatively activated.
Computed atomic charges calculated for the keto and enol tautomers of acetylacetonate molecule
Computed bond orders and bond lengths for the keto and enol tautomers of the acetylacetonate molecule
Simulation of IR Spectroscopy
The results of the IR simulations compared to published experimental results
C–C vibration out of bending
Bands around 1000
C–C vibration out of bending
C–C stretching and C–O bonds
C–O vibrations of the epoxy groups
Presence of νC–O bond
Attributed to the C=C stretching among the graphene C network
C–OH stretching, the C=C stretching
Conversion of the carbonyl group from C=C–C=O into transient structure C+–C=C–O+
Peaks around 1478 due to the increase of O−C=O vibrations during the conversion of carbonyl group.
Attributed to aromatic carbon double bonds
Complete transformation of the carbonyl group into C+–C=C–O+
Corresponding to the C=O stretching vibrations from carbonyl and carboxylic groups
Vibrations at 1700 indicating C=O bonds
As long as the reaction proceeds, the intensity of the previously stated peaks continues to change according to the continuous movement of the graphene oxide layer to accept the Zn–H group. The permanent bonds are constructed via the strong attachment of the Zn–H group to the oxygen sites. A remarkable peak was observed at 1730 cm−1 (Fig. 4(d)), indicating the complete transformation of the carbonyl group into C+–C=C–O+. In the next step of the simulation, a peak corresponding to Zn–O bond formation was detected (Fig. 4(e)) at 550 cm−1. In the corresponding geometry for the same simulation step, the Zn–H bonds have been constructed between the three surrounding O atoms. In fact, the Zn–O peak was observed in the early stages of the simulation with low transmittance intensity. These peaks could be captured as a result of the tendency of Zn2+ (as a Lewis acid) to form complexes dominated by highly directional covalent interactions with the oxygen networks before the Zn–O covalent bonds are finally formed.
In this study, we have investigated the gas-phase reactions involved in the deposition of zinc and the adsorption of Zn2+ to the oxygen network to produce ZnO/graphene composites. The energies of reactants, transition states, and products were calculated, and a reaction mechanism for the dissociation of Zn2+ from its complex was proposed. The energy barrier for the dissociation of Zn2+ from the acetylacetonate complex was found to be 61.78 kcal/mol. Furthermore, the results of a molecular orbital study indicated the complete abstraction of Zn2+ from the acetylacetonate complex. The calculated IR results were in good agreement with experimental IR results reported in literature, validating the findings of the current study. The proposed route of growth involves a self-terminating reaction due to H capping at the end of the H–Zn–3O group. This supports the possibility of achieving atomic layer deposition (ALD) rather than chemical vapor deposition (CVD) while deposition occurs from the gas phase.
AAA thanks Malaysia-Japan International Institute of Technology (MJIIT) for the scholarship. This work was supported by Nippon Sheet Glass Corp, Hitachi Foundation, MJIIT, Universiti Teknologi Malaysia, Malaysia Ministry of Education and Malaysia Ministry of Science, Technology and Innovation through various research grants.
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