H-bond Enhanced Adsorption And Dissociation Of Polar Molecules On Silicene And Si/Ge(100)-2×1 Surfaces

The modification of Si/Ge（100）-2×1 surface with molecules（inorganic and organic）, metal atoms, or their combinations has attracted enormous interests in the past three decades, since it holds the potential application in the fields of chemical sensors, molecular electronics, and nanolithography. Previous researches mainly focused on surface reactions. For instance, cycloaddition between alkene and Si=Si/Ge=Ge dimer, nucleophilic and electrophilic reactions, aromatic reactions, and self-assembly. In comparision, there are relatively less attention aimed at the influence of dipole-dipole interctions on the thermodynamics and kinetics of adsorption of molecules on Si/Ge（100）. It is well known, however, that vast majority of molecules possess dipoles, such as HF（1.91 D）, H₂O（1.85 D）, and NH₃（1.47 D）. When they interact with Si/Ge（100）-2×1 surface, the dipole interactions between molecules themselves such as H-bonding, and between molecule and the surface, including electrostatic interaction and dative bonding will become complicated. In this respect, figuring out their effects on the adsorption and dissocation of molecules on Si/Ge（100） surface will deepen our understanding on the characteristics of Si/Ge（100） surface. Herein, we studied the adsorption and dissociation of three typical polar molecules, i.e., NH₃, H₂ O, and HF, on Si/Ge（100） using first-principles calculations.Overall, H-bonding and dative bonding always show a synergistic effect, together with the electrostatic attraction, dominating the adsorption and dissociation of polar molecules on Si/Ge（100） surface. Both H₂ O and HF are found to be energetically favored to cluster with the pre-adsorbed H₂ O molecule on Si（100）/Ge（100） surface. As the second H₂ O molecule acting as a catalyst, the dissociation barriers of H₂ O on Si（100） are nearly lowered by an order of magnitude, and reduced from ⁰.7 to ⁰.4 eV on Ge（100）. This explains the experimental observation that H₂ O molecules could be dissociated with a unity sticking coefficient on Si（100）-2×1 surface in low temperatures（especially as low as 80K）, and could be partially dissociated on Ge（100） at ¹00K, respectively. Furthermore, H₂ O molecule dissociates spontaneously on Si（100） and the barriers are lowered to ⁰.2 eV on Ge（100） when catalyzed by HF, providing an efficient approach for dissociation of H₂ O on Si（100）/ Ge（100） surface.The synergistic effect is dependent on the electronegativity of X atom of the additional molecule（X=N, O, and F）, dative bonding, and polarity of X-H bond of pre-adsorbed molecule. With the formation of more H-bonds, the synergistic effect is enhanced, leading to the spontaneous dissocation of the pre-adsorbed H₂ O or NH₃ molecule on Si（100）, indicating that the coverage is a key factor for the dissocation of polar molecules on Si/Ge（100）.Furthermore, we have investigated the adsorption mechanism of H₂ O and NH₃ on silicene by using ab initio methods. We have demonstrated that Pauli exclusion and H-bonding played a critical role in the adsorption of H₂ O on silicene via comparing the adsorption of an isolated H₂O/NH₃ molecule, and H₂ O dimer on silicene and Si（100） surfaces. This explains that the isolated H₂ O is inert on silicene while NH₃ tends to chemisorption. However, the Pauli exclusion can be overcome by the synergetic effect of Si…O dative bonding and H-bonding with the additional H₂ O molecule. As a consequence, H₂ O molecules are readily to chemisorb on silicene in the form of cluster via H-bonding, with significantly stabilized as compared to the pre-adsorbed H₂ O. The NH₃ molecules are also energetically favored to cluster via H-bond. As catalyzed by the additional molecule, the dissociation barrier of H₂ O is lowered from 0.70 to 0.33 eV, while it is from 1.18 to 0.88 eV for NH₃.Additionally, we have studied the adsorption of TM（Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn） on benzoquinone（BQ）, by using the first-principles methods. It is found that Sc/Ti atom is energetically favored to bind with 1, 2-BQ（OBQ） to form OBQ-Sc/Ti complex when compared to their bulk structures. Notably, the coordination number of Sc/Ti is only two, thereby providing enough empty d orbitals for H₂ storage without geometrical blocking. Four H₂ molecules can be accommodated by each OBQ-Sc complex via Kubas-like interaction, with the adsorption energy of 0.20 eV/ H₂ and the H₂ storage capacity of ⁵.0 wt%. For practical application, two types of structures have been proposed for H₂ storage using BQ molecule as building block, which would guarantee the dispersing of Sc atom and avoid clustering of OBQ-Sc complexes. In particular, for Sc-decorated oxygen-terminated zigzag graphene nanoribbons, the capacity of H₂ storage reaches up to 6.0 wt%.