First-Principles Study of Nonradiative Recombination in Silicon Nanocrystals: The Role of Surface Silanol

The photophysical properties of emissive silicon nanomaterials depend strongly on the chemical composition and structure of their surfaces, and the development of a causal, microscopic understanding of this relationship is highly desirable. One surface-dependent property of interest is the propensity for nonradiative recombination (NRR). In this work, we apply ab initio theoretical methods to investigate the mechanism of NRR in a silicon nanocrystal with a single surface silanol group. Ab initio multiple spawning simulations of an electronically excited cluster model (Si₇H₁₁OH) indicate ultrafast nonradiative decay to the electronic ground state. A multireference electronic structure study demonstrates that this nonradiative decay occurs near conical intersections between the ground and first excited electronic states of the cluster. These intersections are accessed after stretching of the bond between the silanol silicon atom and an adjacent silicon atom. The presence of this intersection in a true nanomaterial is confirmed by optimization of a similar conical intersection in a silicon nanocrystal (oblate, major diameter 1.4 nm, minor diameter 1.0 nm) with a silanol group on the surface (Si₄₄H₄₅OH). This intersection was identified using a graphics processing unit accelerated implementation of the configuration interaction singles natural orbital complete active space configuration interaction method. All intersections identified in this work are predicted to be at least 4.3 eV above the ground state minimum energy. This confirms the widely held view that silanol groups do not introduce efficient pathways for nonradiative recombination of excitons created upon absorption of visible light. That such an assignment is made entirely from first-principles underscores the value of conical intersection optimization as a tool for elucidating semiconductor photophysics