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High internal emission efficiency of silicon nanoparticles emitting in the visible range Bart van Dam, Clara I Osorio, Mark Hink, Remmert Muller, A. Femius Koenderink, and Katerina (Dohnalova) Newell ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.7b01624 • Publication Date (Web): 10 Apr 2018 Downloaded from http://pubs.acs.org on April 10, 2018
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ACS Photonics
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High
internal
emission
efficiency
of
silicon
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nanoparticles emitting in the visible range
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Bart van Dama, Clara I. Osoriob, Mark A. Hinkc, Remmert Mullerb, A. Femius Koenderinkb,a,
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Katerina Dohnalovaa*
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a
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Netherlands.
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b
Center for Nanophotonics, AMOLF, Science Park 104, 1098 XG Amsterdam, The Netherlands.
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c
Section of Molecular Cytology and van Leeuwenhoek Centre for Advanced Microscopy,
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Swammerdam Institute for Life Sciences, University of Amsterdam, Science Park 904, 1098XH,
Institute of Physics, University of Amsterdam, Science Park 904, 1098 XH, Amsterdam, The
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Amsterdam, the Netherlands.
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*
Presently Katerina Newell
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Correspondence can be addressed to:
[email protected] 14
KEYWORDS
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silicon nanoparticles, local density of optical states, photoluminescence, decay rate, quantum
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efficiency, transition dipole moment
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Abstract
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Light emitting silicon nanoparticles (Si-NPs) are interesting for lighting applications due to their
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non-toxicity, chemical robustness and photo-stability, however, are not practically considered
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due to their low emission efficiencies. While large Si-NPs emitting in the red to infrared spectral
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region show ensemble emission quantum efficiencies up to 60%, the emission efficiencies of
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smaller Si-NP, emitting in the visible spectral range are far lower, typically below 10-20%. In
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this work, we test this efficiency limit by measuring for the first time the internal quantum
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efficiency (IQE), i.e. the higher bound of the emission quantum efficiency, considering only the
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emissive NPs within the ensemble, of Si-NPs emitting in the visible spectral range between 350
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and 650 nm. On the basis of photoluminescence decay measurements in a Drexhage geometry,
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we show that Si-NPs with organic passivation (C:Si-NPs) can have high direct bandgap-like
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radiative rates, which enable a high IQE over ~50%. In this way, we demonstrate that Si-NPs can
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in principle be considered a competitive candidate as a phosphor in lighting applications and
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medical imaging also in the visible spectral range. Moreover, our findings show that the reason
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for the much lower ensemble emission efficiency is due to the fact that the ensemble consists of
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a low fraction of emissive NPs, most likely due to a low PL ‘blinking’ duty cycle.
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Silicon has become the cornerstone of CMOS technologies, implemented in microelectronics,
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photovoltaics and photodetector technologies. Unfortunately, as an active emitter in lighting or
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photonic applications silicon is hindered by its indirect bandgap. The indirect bandgap is
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characterized by inefficient band-edge absorption and low radiative rates, which result in
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impractically low photon fluxes and emission efficiencies in the presence of non-radiative
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channels. The limitations of the indirect bandgap can be overcome with silicon nano-particles
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(Si-NPs).1–3 Si-NPs show room temperature size-tunable luminescence and have many
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advantageous properties: Si-NPs are photo-chemically robust and stable due to covalent bonding
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of ligands,4,5 they offer spectrally broad photoluminescence (PL), tunable from the near-infrared
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(IR) to the ultraviolet (UV),2,3,6 are non-toxic (bio-compatible and bio-degradable7,8) and can be
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bio-functionalized by a large diversity of covalently bonded ligands.9,10 Since their discovery,1
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the emission efficiency of Si-NPs emitting in the near-IR spectral region has been significantly
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improved, with reports of external quantum efficiencies (EQEs) exceeding 60%11,12 (EQE is for
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an ensemble of emitters given by the ratio of the total number of emitted and absorbed photons).
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The EQE could see even further improvement as the internal quantum efficiency (IQE), i.e. the
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emission quantum efficiency of the subset of the ensemble of emitters that is emissive, has been
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reported to reach unity.13–15 The high IQE, which is given by the internal competition between
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radiative and non-radiative processes, show that bright subsets within the ensemble of NPs exist
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for which non-radiative channels are already completely suppressed.
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For emission in the visible spectral region below ~600 nm, the situation is very different.
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EQEs of intrinsic PL are only sparsely reported on and have not exceeded 20%,3 which is
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typically argued to be the consequence of the increased number of surface defects with smaller
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Si-NPs.3,16,17 Furthermore, tunability of the PL by the size of the Si-NP through the visible
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spectral range seems inaccessible for Si-NPs in the presence of oxygen defect states,18 rendering
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the most commonly studied oxide passivated Si-NPs (O:Si-NPs) unsuitable for e.g. lighting
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applications.
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These limitations can be overcome with a class of Si-NPs capped with organic molecules
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(C:Si-NPs) which, extend emission into the visible spectral region.2,3,19–22 Moreover, the
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emission rates in C:Si-NPs have been shown to approach those of direct bandgap materials,19–24
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suggesting significant enhancement of the radiative recombination rates by 2-3 orders of
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magnitude compared to hydrogen or oxide passivated Si-NPs.3,19,22–25 These enhanced radiative
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rates are typically interpreted in terms of the formation of direct bandgap-like optical transitions
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due to the electronegative environment,24 tensile strain induced by the organic ligands23, or are
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related to extrinsic emission sites.26–28 Nevertheless, despite the greatly enhanced radiative rates
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in C:Si-NPs, the EQE remains comparatively low in the visible range (
