Evolution of the Ultrafast Photoluminescence of Colloidal Silicon Nanocrystals with Changing Surface Chemistry Zhenyu Yang, †,‡ Glenda B. De los Reyes, ‡,§ Lyubov V. Titova, §,∥ Ilya Sychugov, ⊥ Mita Dasog, † Jan Linnros, ⊥ Frank A. Hegmann,* ,§ and Jonathan G. C. Veinot* ,† † Department of Chemistry, University of Alberta, Edmonton, Alberta T6G 2G2, Canada § Department of Physics, University of Alberta, Edmonton, Alberta T6G 2E1, Canada ⊥ Materials and Nano Physics Department, ICT School, KTH-Royal Institute of Technology, 16440 Kista, Sweden * S Supporting Information ABSTRACT: The role of surface species in the optical properties of silicon nanocrystals (SiNCs) is the subject of intense debate. Changes in photo- luminescence (PL) energy following hydrosilylation of SiNCs with alkyl- terminated surfaces are most often ascribed to enhanced quantum confinement in the smaller cores of oxidized NCs or to oxygen-induced defect emission. We have investigated the PL properties of alkyl-functionalized SiNCs prepared using two related methods: thermal and photochemical hydrosilylation. Photochemi- cally functionalized SiNCs exhibit higher emission energies than the thermally functionalized equivalent. While microsecond lifetime emission attributed to carrier recombination within the NC core was observed from all samples, much faster, size-independent nanosecond lifetime components were only observed in samples prepared using photochemical hydrosilylation that possessed substantial surface oxidation. In addition, photochemically modified SiNCs exhibit higher absolute photoluminescent quantum yields (AQY), consistent with radiative recombination processes occurring at the oxygen-based defects. Correlating spectrally- and time-resolved PL measurements and XPS-derived relative surface oxidation for NCs prepared using different photoassisted hydrosilylation reaction times provides evidence the PL blue- shift as well as the short-lived PL emission observed for photochemically functionalized SiNCs are related to the relative concentration of oxygen surface defects. KEYWORDS: silicon nanocrystals, photoluminescence, nanosecond lifetime, surface functionalization F ollowing the discovery of light-emitting porous silicon by Canham, 1 many materials based on nanostructured silicon (e.g., silicon nanocrystals (SiNCs)) have been investigated as candidate active systems for optoelectronics devices, ultrafast data communication, and data storage, as well as fluorescent labels and biological sensors. 2−8 Despite these important practical advances, no clear consensus regarding the origin of optical emission from SiNCs exists. 9−17 Bulk Si does not show efficient luminescence because of its indirect bandgap. 18 However, efficient photoluminescence (PL) from nanostruc- tured silicon has been widely observed; it has been explained in the context of quantum confinement and defect and surface states. 17,19−29 As expected, PL resulting from the influence of quantum confinement in SiNC cores is size dependent and shifts in the maximum emission energy are inversely proportional to nanocrystal size. In addition, the PL dynamics associated with SiNC core emission range throughout the nanosecond (ns) to microsecond (μs) time scales. Long-lived PL (i.e., μs time scale) observed in the yellow to infrared spectral region is often attributed to quasi-direct 30 and phonon-assisted radiative recombination in the core states of larger SiNCs; 14,31 short- lived blue-green emission (i.e., ns time scale) is often attributed to quasi-direct recombination in smaller (diameter <2 nm) SiNCs. 11,29,32−35 Surface chemistry has also been implicated in the PL and emission dynamics of SiNCs. 36 Nanosecond PL in the blue- green and yellow-red spectral regions has been attributed to fast recombination of carriers in surface states (or defects) introduced as a result of ligand passivation and oxidation. 37−43 For blue-emitting alkyl-functionalized SiNCs, charge transfer to surface states has been suggested as one possible mechanism for nanosecond PL decay times. 44−46 For yellow/orange-emitting SiNCs, the origin of similarly short- lifetime PL and a concomitant blue-shifted PL maximum has been attributed to various silicon−oxygen species on the NC surfaces. 15 Contrasting this, suggestions that surface oxidation leads to a decrease in SiNC size that induces a blue-shift of the PL emission maximum have appeared. 47 Complicating the community’s understanding, other reports claim surface oxidation induces a red-shift of the PL maximum. 17,33 The authors explained this latter observation by invoking the presence of surface silanones (SiO) that purportedly introduce new electronic states that lead to “trapped-electron- Received: July 28, 2014 Article pubs.acs.org/journal/apchd5 © XXXX American Chemical Society A DOI: 10.1021/acsphotonics.5b00143 ACS Photonics XXXX, XXX, XXX−XXX