Origin of the surface-induced first hyperpolarizability in the C ₆₀/SiO₂ system: SCC-DFTB insight

Using the self-consistent charge density functional tight binding (SCC-DFTB) method, C₆₀ molecules physisorbed on an α-quartz slab are shown to display a first hyperpolarizability, whereas, owing to their symmetry, both the α-quartz slab and C₆₀ molecule have no first hyperpolarizabilities. A larger first hyperpolarizability is achieved when the lowest-lying (five- or six-membered) ring is situated in between two hydroxyl rows, rather than on top, because this situation favors orbital overlaps and charge transfer. Further analysis has demonstrated that (i) the first hyperpolarizability originates from the MO overlap and field-induced charge transfers from the neighboring substrate/adsorbate moieties but not to geometric relaxation of the C₆₀ molecules at the interface and that (ii) larger first hyperpolarizabilities are associated with low surface coverage and with small distances between C₆₀ and the surface. This contribution is a clear illustration of the emergence of second-order nonlinear optical responses (first hyperpolarizability) as a result of breaking the centrosymmetry.