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Thermal decomposition of Zn[(C6H5)2PSSe]2 single-source precursor for the chemical vapour deposition of binary and ternary zinc chalcogenides: a theoretical study
© Opoku et al. 2015
Received: 9 March 2015
Accepted: 9 May 2015
Published: 17 June 2015
The mechanistic pathways for the formation of zinc selenide, zinc sulphide and ternary zinc selenide sulphide in the gas phase decomposition of Zn[(C6H5)2PSSe]2 single-source precursor has been explored on the singlet and doublet potential energy surfaces using density functional theory method in order to understand the thermodynamic and kinetic properties. The optimized geometries and the predicted energies were obtained using the M06 functional and a combination of the basis sets LACVP* for Zn and 6-31(d) for light atoms. The rate constants of each elementary reaction have been calculated using the transition state theory. The results indicate that the steps that lead to ternary ZnSexS1−x formation on both the singlet and doublet potential energy surfaces is favoured kinetically and thermodynamically over those that lead to ZnSe and ZnS formation. Density functional theory calculations of the gas phase decomposition of the complex indicate that the deposition of ternary ZnSexS1−x in chemical vapour deposition may involve more than one step but the steps that lead to its formation are consistent with a dominant role for both thermodynamic and kinetic factors. The theoretical study showed important insights as a general tool to anticipate the gas phase decomposition mechanism of a novel precursor when direct experimental measurements are not available.
Transition metal chalcogenides have been of considerable technological applications such as solar energy conversion, solar control coatings, microelectronic devices, catalysts, sensors, optical filters and laser sources (Yamaguchi et al. 1996; Teteris 2003; Savadogo 1998; Sang et al. 2002). Structural information such as geometrical and electronic configurations, molecular dynamics, and thermodynamic and magnetic properties of the dichalcogenophosphinato complexes is important to understand the different factors influencing their practically useful properties (Artem’ev et al. 2014).
The mixed crystals of II–VI compound semiconductors have attracted much attention for applications in optical devices (El Haj Hassan et al. 2007). Indeed, the easiest way to change artificially the electronic and optical properties of semiconductors is by forming their alloys (Gunshor and Nurmikko 1997). It seems therefore very interesting to study ZnS and ZnSe mixed in the ZnSxSe1−x ternary alloys.
An experimental observation of labile and reactive intermediates on the surface is ultimately challenging and difficult, since the experiments have great difficulties in measuring the intermediates and the transition states in the high-temperature system with complicated reactions. Optimising the CVD conditions such that higher-purity materials are obtained at a higher growth rate requires knowledge of the deposition chemistry obtained by performing specially designed experiments and/or modelling and simulation (Opoku et al. 2014). Due to the difficulty of assessing such a reaction mechanism by experiment, theoretical calculations can be an excellent means of exploring these processes on a molecular scale. Knowledge of thermodynamic and kinetic parameters obtained from density function theory calculation is important to understand and optimise deposition conditions require for selective growth process in chemical vapour deposition. Therefore, understanding the kinetics of gas phase decomposition reactions of single source precursor is becoming more and more important. The good correspondence between DFT results and experimental data has led to DFT playing a pivotal role in the prediction of the reaction rates of complex species important for gas-phase reactions of single-source precursors (Hohenberg and Kohn 1964).
All the calculations were done using the M06 hybrid density functional. The M06 is a novel hybrid meta functional with good accuracy and has been parameterized for modelling organometallic and inorganometallic thermochemistry, non-covalent interactions and kinetics for systems containing transition metal elements (Zhao and Truhlar 2008; Zhao et al. 2014). Open shell systems were treated using unrestricted density functional theory. Geometry optimizations were performed using a standard valence LACVP* basis set as implemented in the Spartan Molecular Modelling program (Wave function 2010). For the first- and second-row elements, LACVP implies a 6-31G double-ξ basis set. For the zinc atoms, LACVP uses a nonrelativistic effective core potential (LACVP* uses the 6-31G* basis set for all light elements and the Hay-Wadt ECP and basis set for Zn; see: Hay and Wadt 1985a, b; Wadt and Hay 1985), where the valence part is essentially of double-ξ quality. The starting geometries of the molecular systems were constructed using Spartan’s graphical model builder and minimized interactively using the sybyl force field (Clark et al. 1989). Local minima were optimized using the Spartan ‘10 v1.1.0 Molecular Modelling program (Wave function 2010). A normal mode of analysis was performed to verify the nature of the stationary point and equilibrium geometries were characterized by the absence of imaginary frequencies. The transition state structures were located by series of constrained geometry optimization in which the breaking bonds were fixed at various lengths and optimized the remaining internal coordinates. The approximate stationary points located from such a procedure were then fully optimized using the standard transition state optimization procedure in Spartan (Aniagyei et al. 2013). All first-order saddle points were shown to have an imaginary vibrational frequency along the reaction coordinate.
The rate constants were computed using the transition state theory for the selected reaction pathways (Benson 1960; Glasstone et al. 1941) and assuming that the transmission coefficient, κ is equal to 1.
Results and discussion
Optimized Geometry of Zn[(C6H5)2PSSe]2 precursor
Comparison of the calculated geometries of Zn[(C6H5)2PSSe]2 and Zn[( i Pr)2PSSe]2 precursor at the M06/LACVP* level of theory (bond lengths in angstroms and bond angles in degrees)
Overall decomposition of Zn[(C6H5)2PSSe]2 precursor
The calculated Gibbs free energy of activation and reaction energy necessary for the formation of the (C6H5)2PSSe–Zn–SeSP(C6H5) intermediate on the double potential energy surface through a singlet transition state TS1/s are +44.12 and +32.10 kcal/mol, respectively. The consequent decomposition of the (C6H5)2PSSe–Zn–SeSP(C6H5) intermediate through singlet transition state TS2/d requires a barrier of +27.77 kcal/mol (Figure 2) and a reaction energy of −8.80 kcal/mol to form the (C6H5)2PSSe–Zn–Se intermediate. It is possible for the (C6H5)2PSSe–Zn–Se intermediate to decompose in two ways. On the singlet surface, the decomposition of the (C6H5)2PSSe–Zn–SeSP(C6H5) intermediate to form the (C6H5)2PSSe–Zn–S intermediate has an activation barrier of +28.49 kcal/mol and reaction energy of −2.59 kcal/mol. The formation of the singlet (C6H5)2PSSe–Zn–Se intermediate by the dissociation of a phenyl radical from the (C6H5)PSSe–Zn–Se intermediate through the doublet transition state requires an activation barrier of +56.24 kcal/mol and reaction energy of −4.01 kcal/mol. A singlet (C6H5)PSSe–Zn–S intermediate has been found to be 12.89 kcal/mol more stable than the C6H5)PSSe–Zn–Se intermediate.
Calculated activation barriers and reaction energy of the last step of the various reactions of the Zn[(C6H5)PSSe]2 and Zn[( i Pr)2PSSe] 2 b complexes
INT4/s → P1/s
INT4/s → P2/s
INT5/s → P3/s
INT5/s → P4/s
INT6/d → P5/s
INT6/d → P6/s
INT6/d → P7/s
INT7/s → P8/s
INT7/s → P9/s
INT7/s → P10/s
Deposition rates from the phenyl phosphinato complex, Zn[(C6H5)2PSSe]2 have been observed to be lower than those from the isopropyl phosphinato complex. These differences are attributed to the higher dissociation energy of the P–C bond in phenyl phosphinato complex and also greater electron withdrawing nature of the phenyl substituent, as compared to isopropyl. The higher activation energy for phenyl phosphinato is consistent with cleavage of the stronger phosphinato P–C bond before or during the rate-determining step of the deposition process (Table 2).
Thus, the overall barrier for the decomposition of the (C6H5)PSSe–Zn–Se intermediate to give the ternary ZnSexS1−x is higher than the activation barrier for the ZnSe formation pathway. The reaction which would lead back to the reactant is energetically less favourable due to the larger barrier. The ternary ZnSexS1−x dissociation pathway is only 7.79 kcal/mol more stable than the ZnSe dissociation pathway. Moreover, it is possible for the (C6H5)PSSe–Zn–S intermediate to decompose in two ways. The formation of the singlet ZnS + [(C6H5)PSeS]− through a singlet transition state has an activation barrier of +80.12 kcal/mol and a reaction energy of −2.74 kcal/mol. The formation of singlet ternary ZnSexS1−x + [(C6H5)PS]− through the singlet transition state by the dissociation of the Zn–S2 and Zn–Se1 bonds from the (C6H5)PSSe–Zn–S intermediate has an activation barrier of +66.96 kcal/mol and reaction energy of −15.22 kcal/mol.
Calculated rate constants for the gas phase decomposition of Zn[(C6H5)PSSe]2 at 800K
INT4/s → P1/s
1.76 × 10−13
2.83 × 10−4
4.97 × 10−17
INT4/s → P2/s
7.75 × 10−13
5.82 × 10−8
4.51 × 10−20
INT5/s → P3/s
1.44 × 10−11
1.45 × 102
2.10 × 10−9
INT5/s → P4/s
3.05 × 10−9
8.19 × 101
2.50 × 10−7
INT6/d → P5/s
9.34 × 10−3
1.45 × 107
1.36 × 105
INT6/d → P6/s
5.61 × 10−4
8.33 × 101
4.67 × 10−2
INT6/d → P7/s
3.20 × 10−1
1.08 × 1015
3.47 × 1014
INT7/s → P8/s
5.87 × 10−8
1.41 × 10−6
8.30 × 10−14
INT7/s → P9/s
1.98 × 10−7
4.58 × 10−10
9.05 × 10−17
INT7/s → P10/s
1.52 × 10−3
1.21 × 10−1
1.84 × 10−4
The decomposition of the Zn[(C6H5)2PSSe]2 to give the doublet (C6H5)2PSSe–Zn intermediate through the singlet transition state has an activation barrier and reaction energy of +47.29 and +38.12 kcal/mol, respectively. The decomposition of the doublet four-membered (C6H5)2PSSe–Zn intermediate to form ZnSe + [(C6H5)2PS]− has an activation barrier of +17.71 kcal/mol and a reaction energy of −47.89 kcal/mol while the decomposition of the doublet four-membered (C6H5)2PSSe–Zn intermediate to form ZnS + [(C6H5)2PSe]− has an activation barrier of +22.18 kcal/mol and a reaction energy of −40.74 kcal/mol. The formation of ZnSe through the dissociation of Zn–S and P–Se bonds from ( i Pr)2PSSe–Zn intermediate (TS11/s in Scheme 3) which was reported in the work of Opoku et al. (2015b) has been found to have a barrier and reaction energy of +0.82 and −28.57 kcal/mol, respectively on the singlet surface.
Also, the formation of the ternary ZnSexS1−x + [(C6H5)2P]− through direct dissociation of the P–Se1 and P1–S1 from the (C6H5)2PSSe–Zn intermediate has an activation barrier of +12.09 kcal/mol and a reaction energy of −58.63 kcal/mol. The barrier along this route is only 5.62 kcal/mol lower than the barrier for the ZnSe formation pathway. The activation barriers for the formation of the singlet ZnSe and ZnS (17.71 and 22.18 kcal/mol) are higher than the activation barrier for the formation of ternary ZnSexS1−x. Moreover, this decomposition pathway has the lowest activation free-energy barrier and will proceed very rapidly; the corresponding unimolecular and recombination rate constant are 3.20 × 10−1/s and 3.47 × 1014 cm3/mol/s, respectively, see Table 3. Thus overall, the formation of ZnSe and ZnS by the decomposition of the (C6H5)2PSSe–Zn intermediate cannot compete favourably, both kinetically and thermodynamically with the ternary ZnSexS1−x formation pathway.
Pathways initiated via the formation of a ZnSe abstraction from the (C6H5)P(Se)S–Pb intermediate is competitive with the pathways that lead to the formation of ZnS.
Kinetically and thermodynamically the most favourable pathway involves the formation of ternary ZnSexS1–x on both the singlet and doublet potential energy surfaces.
The initial dissociation of phenyl radical to form the (C6H5)2PSSe–Zn–SeSP(C6H5) intermediate is kinetically only 3.47 kcal/mol lower than the [(C6H5)2PS]− dissociation to form the (C6H5)2P(Se)S–Pb intermediate.
The isopropyl precursor appears to be more preferable to the phenyl precursor due to its high growth rate at their mutual deposition temperature.
The spin density map and single occupied molecular orbital analysis shows that it is mainly the coordinating Se- and S-donor atoms that provides the spin and electron contribution.
All the authors contributed equally to the preparation of this manuscript. All authors read and approved the final manuscript.
The authors are very grateful to the National Council for tertiary Education (NTCE), Ghana for a research Grant under the Teaching and Learning Innovation Fund (TALIF-KNUSTR/3/005/2005). We are also grateful to the Computational Quantum Chemistry Laboratory at the Department of Chemistry, Kwame Nkrumah University of Science and Technology (KNUST), Kumasi, Ghana for the use of their facilities for this work.
Compliance with ethical guidelines
Competing interests The authors declare that they have no conflict of interest.
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