(544ec) Theoretical Studies on the Gas-Phase Synthesis and Properties of Semiconducting Nanomaterials

Homogenous gas-phase nanomaterials formation is a complex phenomenon in which hundreds, or possibly thousands of species, undergo simultaneous reaction. Understanding semiconducting nanomaterials formation from the pyrolysis of mixtures of silane (SiH₄) and germane (GeH₄) at even the mildest conditions is still incomplete. Intentional synthesis of semiconducting nanomaterials can benefit from an improved mechanistic understanding of formation. Semiconducting silicon-germanium (SiGe) clusters have attracted great interest for their use as optoelectronic, sensor, and photovoltaic materials. Conversely, defects arise in semiconductor processing because these SiGe particles deposit on the growing substrate. Since these clusters are important for the fine processing of semiconductors and the synthesis of novel materials, computational modeling can play a very important role in narrowing the gap between controlled experimental studies and practical operating conditions.

Recently, automated network generation techniques have allowed the kinetics of inorganic nanoparticle formation, such as Si nanoparticles¹, to be described at the mechanistic level. Rate coefficients must be estimated for every elementary step comprising the mechanistic model, and kinetic correlations are used to make this tractable. One common method for predicting activation barriers (E_a) is the Evans-Polanyi correlation; however, these structure-activity correlations require detailed thermochemical information for each reacting species. There are limited studies available that predict the thermochemical properties of SiGe clusters. For this purpose, we conducted a computational study of hydrogenated silicon-germanium alloy clusters (Si_xGe_yH_z,1<X+Y≤6) to predict the thermodynamic properties. The optimized geometries of the Si_xGe_yH_z clusters were investigated systematically using quantum chemical calculations and statistical thermodynamics². All electronic energies for the clusters were calculated using Gaussian-n methods, which use B3LYP geometries and higher-level corrections based on single point energies. To validate our approach, we compared our methodology to other popular composite methods such as the CBS-QB3 method and to available experimental data. Our studies have established trends in thermodynamic properties (standard enthalpy of formation (ΔH^o_f), standard entropy (S^o), and heat capacity (C_p)), as a function of cluster composition and structure. These learnings will be discussed in the context of tailored nanomaterials design.

1. Adamczyk, A. J.; Reyniers, M. F.; Marin, G. B.; Broadbelt, L. J., Kinetics of Substituted Silylene Addition and Elimination in Silicon Nanocluster Growth Captured by Group Additivity. ChemPhysChem 2010, 11 (9), 1978-1994.

2. Adamczyk, A. J.; Broadbelt, L. J., Thermochemical Property Estimation of Hydrogenated Silicon Clusters. J. Phys. Chem. A 2011, 115 (32), 8969-8982.