Please cite this article in press as: D. Timmerman, T. Gregorkiewicz, Mater. Sci. Eng. B (2009), doi:10.1016/j.mseb.2009.02.010 ARTICLE IN PRESS G Model MSB-12009; No. of Pages 3 Materials Science and Engineering B xxx (2009) xxx–xxx Contents lists available at ScienceDirect Materials Science and Engineering B journal homepage: www.elsevier.com/locate/mseb Space-separated quantum cutting in differently prepared solid-state dispersions of Si nanocrystals and Er 3+ ions in SiO 2 D. Timmerman * , T. Gregorkiewicz Van der Waals-Zeeman Institute, University of Amsterdam, Valckenierstraat 65, NL-1018 XE Amsterdam, The Netherlands article info Article history: Received 2 May 2008 Received in revised form 11 February 2009 Accepted 13 February 2009 Keywords: Silicon Erbium Excitation Quantum structures Photoluminescence Quantum cutting Optical properties abstract We investigate the efficiency of Si nanocrystal-mediated photoluminescence of Er 3+ ions in SiO 2 as a function of quantum energy of incoming photons. We demonstrate that for identical nanocrystals the threshold energy of the recently reported space-separated quantum cutting effect does not change with the nanocrystal-to-Er distance. In contrast, the enhancement of excitation efficiency observed above the threshold, in the high-energy photon regime, diminishes at larger separations, thus exhibiting a clear nanocrystal-to-Er distance dependence. © 2009 Elsevier B.V. All rights reserved. 1. Introduction One way of improving the optical properties of indirect semi- conductors is by reducing their dimensionality, e.g. by forming nanocrystals (NCs). When the size of a structure becomes compara- ble to the size of the Bohr radius of an exciton in the used material, quantum confinement effects will set in and alter the electronic structure [1]. Following Heisenberg’s uncertainty principle, spatial localization of carriers will decrease the momentum determina- tion. As a result, k is not a good quantum number anymore. In this way, the k-conservation rule, which severely inhibits rate of radia- tive transitions in indirect bandgap semiconductors like silicon, will relax and the probability for them to take place will increase. In this way, nanostructuring opens the way for silicon-based photonics. For Si-NCs it is generally accepted that the emission of photons is due to band-to-band recombinations of quantum confined e–h pairs [2]. It is possible to create an e–h pair in a NC by absorption of a photon. Since the absorption cross-section of silicon grows for shorter wavelengths, it is frequently inferred that short exci- tation wavelengths are necessary for efficient photoluminescence (PL) from Si-NCs. When the Si-NCs are incorporated in SiO 2 together with Er 3+ ions, they can act as sensitizers of erbium-related luminescence [3–5]. In this case the NCs, which have been excited by a photon, * Corresponding author. E-mail address: d.timmerman@uva.nl (D. Timmerman). can transfer their energy to neighboring Er 3+ ions and bring them in an excited state. Because the excitation of the Er 3+ ions pro- ceeds via the Si-NCs, it is expected that the generation efficiency of erbium-related luminescence will also grow for shorter excitation wavelengths. 2. Space-separated quantum cutting In order to study the energy transfer process between Si-NCs and Er 3+ ions, we performed PL excitation measurements on solid dispersions of small Si-NCs (average diameter 3.1 nm) embedded in a SiO 2 matrix. Some of the dispersions were doped with Er 3+ ions (details of the fabrication procedure can be found in Ref. [6]). In the experiments, we followed the efficiency of the luminescence for different excitation wavelengths of either exciton-related PL, in the samples containing only Si-NCs, or the erbium-related PL in co- doped samples. Subsequently, these measurements were compared with separately measured absorbance to determine the relative quantum efficiency with which absorbed photons are contribut- ing to the luminescence. From these investigations, we concluded on the occurrence of a process of space-separated quantum cutting (SSQC) [7] in which absorption of the single high-energy photon might result in formation of two excitons in two neighboring, NCs, or induce excitation of two Er 3+ ions: 1. NC-PL: When the quantum energy of the incoming photon exceeds two times the nanocrystal bandgap two excitons per one absorbed photon might be generated. Moreover, in contrast 0921-5107/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2009.02.010