Article pubs.acs.org/JPCC
First-Principles Study of Nonradiative Recombination in Silicon Nanocrystals: The Role of Surface Silanol Yinan Shu and Benjamin G. Levine* Department of Chemistry, Michigan State University, East Lansing, Michigan 48824 S Supporting Information *
ABSTRACT: 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.
1. INTRODUCTION Semiconductor nanomaterials exhibit unique and tunable properties that make them appealing building blocks for nextgeneration optoelectronic materials. Many applications of such materials such as light emission and solar energy conversion require the creation of an electronic excitation that lives long enough to perform useful work. A competing physical process is nonradiative recombination (NRR), which converts the energy of this electronic excitation into heat. It has long been known that NRR is facilitated by both bulk and surface defects, but the assignment of a particular defect as a nonradiative center remains very challenging. Such assignments may remain ambiguous after many years of combined experimental and theoretical effort. Drawing on ideas developed in the field of molecular photochemistry,1−5 we recently demonstrated that nanomaterials can undergo NRR via conical intersections between the ground and first excited potential energy surfaces (PESs).6 The coupling between the intersecting electronic states is very large in the vicinity of such intersections, promoting fast and efficient nonradiative transitions. We demonstrated that such intersections can arise due to local distortions of the electronic and nuclear structure at defects. Characterizing the resulting intersections with multireference electronic structure methods © XXXX American Chemical Society
allows the determination of a causal connection between microscopic structure and the propensity for NRR. It would be highly desirable to develop such a causal connection for silicon nanocrystals (SiNCs), which show promise for a variety of light emission applications7−10 and exhibit photophysical properties that depend strongly on surface chemistry.11−15 For example, the oxidation of the silicon surface, which is spontaneous upon contact with air at ambient conditions, dramatically changes the properties of SiNC photoluminescence (PL). Oxidation of a sample of wellpassivated SiNCs both reduces the quantum yield of PL16 and limits the PL energy to the lower portion of the visible range (PL maximum
