First-Principles Study on the Surface Chemistry of 1.4 nm Silicon Nanocrystals: Case of Hydrosilylation Rong Wang, Xiaodong Pi,* and Deren Yang* State Key Laboratory of Silicon Materials and Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China * S Supporting Information ABSTRACT: As a leading surface-modification approach, hydrosilylation is critical to the practical use of silicon nanocrystals (Si NCs). However, the effect of hydrosilylation- induced surface chemistry on the electronic and optical properties of Si NCs is rather limitedly understood. By means of first-principles calculation at 0 K we show thermodynamically favored surface bonding for hydrosilylation of 1.4 nm Si NCs and the relative reactivity of alkenes and alkynes. The optical properties of hydrosilylated Si NCs are elucidated on the basis of their energy-level schemes and radiative recombination rates. The chain length (up to C12) of ligands hardly affects the absorption and emission of Si NCs. The increase of the surface coverage (up to 29%) of ligands causes the absorption onset to slightly redshift, hardly rendering changes to the light emission from Si NCs. As an added advantage, hydrosilylation may lead to enhanced light emission from Si NCs. Radiative recombination is very sensitive to surface chemistry for Si NCs. Only the coexistence of CC and functional groups at the NC surface significantly modifies the electronic structures and optical behavior of Si NCs. 1. INTRODUCTION As one of the most important semiconductor nanostructures, silicon nanocrystals (Si NCs) have been not only advancing the development of Si-based microelectronics, 1,2 optoelectronics 3,4 and photovoltaics 5,6 but also extending Si technology into bioimaging 7,8 and photosensitizing. 9,10 While Si NCs may be present in porous Si, 11 dielectric matrix (e. g., SiO x and SiN x ) ,12,13 and silicon pillars, 14 they can also be freestand- ing. 15,16 In fact, freestanding Si NCs have recently gained great popularity due to their easily accessible surface and flexible incorporation in device structures. 16−19 In most applications of freestanding Si NCs, surface modification should be first carried out to render the dispersibility of freestanding Si NCs in desired media. 16,20−25 Given the vulnerability of freestanding Si NCs to oxidation, surface modification also serves to retard oxidation of freestanding Si NCs in air. 17,26,27 As a leading surface-modification approach, hydrosilylation that is either thermally or photochemially initiated has been carried out for freestanding Si NCs in both liquid phase and gas phase. 20−30 The key advantages of hydrosilylation include mild reaction conditions, minimal reaction byproducts, and freedom in the incorporation of a variety of functional groups at the surface of Si NCs. 26 It is well known that the technological importance of Si NCs largely originates from their remarkable optical properties. 3,15,16 Surface chemistry may significantly impact the optical behavior of Si NCs together with quantum confinement. 31−34 Recently intensified motivation to realize the practical use of freestanding Si NCs has led to enthusiastic investigation on the hydro- silylation-induced surface chemistry for Si NCs. First, both radical and nonradical mechanisms have been proposed for the reaction of hydrosilylation. 27,35−37 Hydrosilylation-induced bonding at the surface of Si NCs was experimentally studied in the case of hydrosilylation with alkenes. 23,38 Light emission from Si NCs hydrosilylated with alkenes and alkynes was measured. 17,26 It was shown that both the energy and the intensity of the photoluminescence (PL) from Si NCs were changed by hydrosilylation. Significantly, hydrosilylation actually quenched the PL from Si NCs when conjugated alkynes were used. 26 It has been realized that factors intrinsic to the experimental design of the hydrosilylation of Si NCs often complicate the explanation of experimental phenomena. 26 In the mean time, direct surface characterization of Si NCs remains experimen- tally challenging. 35 Therefore, theoretical simulation for hydrosilylation of Si NCs is highly desired. However, to date there only exists limited theoretical simulation for hydro- silylation of Si NCs. Reboredo et al. first showed that an exciton-mediated mechanism was responsible for the optically activated hydrosilylation of Si NCs by means of ab initio calculation. 39 They then pointed out that the steric effect prevented full surface coverage of Si NCs with unreconstructed surface. For full surface coverage of a Si NC with reconstructed surface, alkyl ligands raised both the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the Si NC and led to small changes in the Received: August 6, 2012 Revised: August 12, 2012 Published: August 13, 2012 Article pubs.acs.org/JPCC © 2012 American Chemical Society 19434 dx.doi.org/10.1021/jp307785v | J. Phys. Chem. C 2012, 116, 19434−19443