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2006, Applied Surface Science
Photoluminescence excitation measurements have been performed on single, unstrained oxide-embedded Si nanocrystals. Having overcome the challenge of detecting weak emission, we observe four broad peaks in the absorption curve above the optically emitting state. Atomistic calculations of the Si nanocrystal energy levels agree well with the experimental results and allow identification of some of the observed transitions. An analysis of their physical nature reveals that they largely retain the indirect band-gap structure of the bulk material with some intermixing of direct band-gap character at higher energies. Finite-sized nanostructures and bulk random alloys lack the translational symmetry of the underlying bulk-periodic solids they are drawn from. Therefore their wave functions represent a mix of bulk bands over different wave vectors and band indices [1,2]. The additional shift in energies present in nanostructures due to quantum confinement and enhanced many-electron interactions in confined space lead to clear spectroscopic manifestations in nanostructures relative to the reference bulk material [3]. This includes changing of a bulk indirect transition to a nanostructure quasidirect transition [4], as well as more exotic effects such as Coulomb and spin blockade, appearance of many-electron multiplets, violations of Hund's rule and the Aufbau principle, etc. [5]. The modern theory of nanostructures treats such single nanostructures atomistically as a giant molecule rather than via continuum-based effective mass methods [3,6]. However, such high-resolution theoretical calculations cannot be compared with experimental data from ensemble measurements, where size (and shape) dispersion even at a very small scale smears out discrete features both in emission and absorption. Single-dot spectroscopic techniques have been previously applied to self-assembled and colloidal direct band-gap material quantum dots (QDs) of III-V [5,7,8] and II-VI group elements [9]. They have indeed revealed, in conjunction with theory, significant novel nanostructure effects forming the basis for the current understanding of QD physics. Experimentally, the spectrum of nanocrystals can be probed by emission and absorption spectroscopy. While the emission peak position corresponds to the effective optical band gap, the absorption measurements can provide information over a wide energy range, allowing for a more detailed comparison to calculations. So far only ensemble studies were performed on the absorption spectrum of Si nanocrystals by photolumi-nescence excitation (PLE) or transmission methods [10,11], preventing us from observing single Si nanodot features. PLE of individual quantum dots was demonstrated for direct band-gap materials [12-14], but it is much more difficult to perform on single Si nanocrystals due to their low emission rate, stemming from ∼μs exciton lifetimes [15]. At the same time, understanding the electronic structure of Si nanocrystals * Corresponding author: ilyas@kth.se relevant for light absorption is central to their application as phosphors [16], biolabels [17], sensitizers [18], downshifters [19], or photon multipliers [20]. In this Rapid Communication we report successful single-dot spectroscopy studies of silicon quantum dots, revealing the absorption states above the emission level. The experimental difficulty of detecting weak PLE signals from single Si nanocrystals under varying excitations was solved by introducing a stable, focusable, and tunable light source to the sensitive detection system, as described in the Supplemental Material [21]. Previously we could access only the emission state of individual Si nanocrystals in photoluminescence [4,22] and decay measurements [15,23]. The Si quantum dot origin of the emission was evidenced by the observed variation in emission peak position and lifetime, the sharp narrowing of the linewidth at lowered temperature, a signature of biexciton recombination at high excitation, and a Si transverse optical (TO)-phonon sideband in the spectra. Here we present spectroscopic results over a broad energy range (1.5-2.0 eV above the emission state) for Si nanocrystals. A typical spectrum is shown in Fig. 1 (circles, right), where several distinct absorption features can be identified, which are not seen in ensemble absorption measurements (dashed line). We have calculated the energy states and absorption spectra of Si nanocrystals using a set of well-tested theoretical tools based on the empirical pseudopotential method [25]. By employing this atomistic method one no longer needs to use the effective-mass based (continuum) approximations, with their significant flaws [26-28]. Unlike the (atomistic) local density approximation (LDA) methods, the theory discussed here is free from the well-known LDA errors on band gap and effective masses [29], both rather detrimental to obtaining a physically correct description of quantum confinement. In this "modern theory of QDs" one includes a fairly complete description of single-particle effects (multiband interactions; multivalley coupling; spin-orbit interactions; surface or interface effects) [3,28,30]. We solved the atomistic Schrödinger equation explicitly for QD architecture consisting of a thousand to multiple millions of atoms, with the atoms located at specific positions, each carrying its own (screened) pseudopotential [25]. These semiempirical pseudopotentials were obtained 2469-9950/2016/93(16)/161413(5) 161413-1
Light: Science & Applications
Bright trions in direct-bandgap silicon nanocrystals revealed by low-temperature single-nanocrystal spectroscopy2015 •
2009 •
Physical Review B
High luminescence in small Si/SiO2 nanocrystals: A theoretical study2010 •
In recent years many experiments have demonstrated the possibility to achieve efficient photoluminescence from Si/SiO2 nanocrystals. While it is widely known that only a minor portions of the nanocrystals in the samples contribute to the observed photoluminescence, the high complexity of the Si/SiO2 interface and the dramatic sensitivity to the fabrication conditions make the identification of the most active structures at the experimental level not a trivial task. Focusing on this aspect we have addressed the problem theoretically, by calculating the radiative recombination rates for different classes of Si-nanocrystals in the diameter range of 0.2-1.5 nm, in order to identify the best conditions for optical emission. We show that the recombination rates of hydrogenated nanocrystals follow the quantum confinement feature in which the nanocrystal diameter is the principal quantity in determining the system response. Interestingly, a completely different behavior emerges from the OH-terminated or SiO2-embedded nanocrystals, where the number of oxygens at the interface seems intimately connected to the recombination rates, resulting the most important quantity for the characterization of the optical yield in such systems. Besides, additional conditions for the achievement of high rates are constituted by a high crystallinity of the nanocrystals and by high confinement energies (small diameters).
Building-integrated photovoltaics is gaining consensus as a renewable energy technology for producing electricity at the point of use. Luminescent solar concentrators (LSCs) could extend architectural integration to the urban environment by realizing electrode-less photovoltaic windows. Crucial for large-area LSCs is the suppression of reabsorption losses, which requires emitters with negligible overlap between their absorption and emission spectra. Here, we demonstrate the use of indirect-bandgap semiconductor nanostructures such as highly emissive silicon quantum dots. Silicon is non-toxic, low-cost and ultra-earth-abundant, which avoids the limitations to the industrial scaling of quantum dots composed of low-abundance elements. Suppressed reabsorption and scattering losses lead to nearly ideal LSCs with an optical efficiency of η = 2.85%, matching state-of-the-art semi-transparent LSCs. Monte Carlo simulations indicate that optimized silicon quantum dot LSCs have a clear path to η > 5% for 1 m 2 devices. We are finally able to realize flexible LSCs with performances comparable to those of flat concentrators, which opens the way to a new design freedom for building-integrated photovoltaics elements. T he continuous increase in performance of silicon-based photovoltaic (Si-PV) systems and the economic incentive programmes that have characterized the fiscal policies of many western countries have rendered Si-PVs the dominant technology for producing electrical energy from renewable sources in sustainable civil and industrial buildings. Furthermore, in the last few years, the retail cost of Si-PV modules has dropped by ∼70%, which now makes it possible to recover the economic investment for rendering a small building energetically independent in less than a decade 1. However, the situation is radically different in highly urbanized environments, where architecture develops predominantly in terms of height, and the rooftop surfaces commonly used for the installation of Si-PV modules become increasingly insufficient to collect all the energy required for building operations. At present, an area of ∼7 m 2 is needed for 1 kW of peak power. This makes it more difficult to achieve the desired goal of the so-called 'nearly zero energy building' decreed by the EU, according to which all newly constructed buildings will be required to be energetically neutral by 2020 (ref. 2). Hence, it is of growing relevance to realize innovative PV systems that integrate into existing and newly constructed buildings without altering their aesthetic appearance or affecting the quality of life of their occupants. Luminescent solar concentrators (LSCs) 3,4 could help achieve this goal by transforming conventional energy-passive glazing systems into semi-transparent PV windows 5 , effectively converting the facades of urban buildings into distributed energy generation units 6–9. Importantly, integrating LSCs 'invisibly' into urban environments would facilitate the public acceptance of PV power and enable seamless integration in this territory, essentially reducing to zero the so-called 'cost of land' for PV installation. A typical LSC consists of a plastic optical waveguide doped with fluorophores or of a glass slab coated with active layers of emissive materials. Direct, as well as the diffused sunlight that penetrates the matrix, is absorbed by the fluorophores and is re-emitted at a longer wavelength. The luminescence, guided by total internal reflection, propagates to the waveguide edges, where it is converted into electricity by small PV cells installed along the slab perimeter (Fig. 1a). Because the LSC area exposed to sunlight is much larger than the waveguide edges, LSCs can, in principle, greatly increase the flux of radiation incident onto the perimeter PV devices and thus boost the photocur-rent 4,6. Importantly for building-integrated PV (BIPV) applications, LSCs can be produced with unmatched design freedom in terms of shape, transparency, colour and flexibility, which architects can use to enhance a building's aesthetic value. In addition to this, the LSC design uniquely enables the harvest of solar radiation using electro-deless semitransparent waveguides, with essentially no aesthetic impact 7–11 , which is paramount for the realization of PV windows. Finally, because the perimeter PV cells are indirectly illuminated by luminescence originating across the whole waveguide, LSCs are essentially unaffected by the efficiency losses and electrical stresses caused by shadowing of the device that occur in bulk and thin-film PVs. Despite tremendous advancements in device design 6,10–12 , the wide use of LSCs has long been hindered by the lack of suitable emitters. Organo-metallic chromophores suffer from limited spectral coverage, while organic dyes and conventional core-only colloidal quantum dots 13,14 exhibit a large overlap between the absorption and emission spectra, which leads to strong reabsorption of the guided luminescence in large-area LSCs 10,15. However, recent breakthroughs in the design of quantum dots have leap-frogged LSC technology to such a degree that they could be commercialized for integrated PV applications in the near future 7,8,14,16,17. Colloidal quantum dots feature a broad absorption spectrum that can be tuned to harvest solar radiation across the whole visible spectrum and to the near-infrared (NIR). Their luminescence can be tuned
ACS Photonics
Evolution of the Ultrafast Photoluminescence of Colloidal Silicon Nanocrystals with Changing Surface Chemistry2015 •
Advanced Functional Materials
Light-Emission Performance of Silicon Nanocrystals Deduced from Single Quantum Dot Spectroscopy2008 •
New Journal of Physics
Excitation dependence of photoluminescence in silicon quantum dots2007 •
physica status solidi (a)
Effect of X-ray irradiation on the blinking of single silicon nanocrystals2015 •
Physical Review Letters
Probing the Spontaneous Emission Dynamics in Si-Nanocrystals-Based Microdisk Resonators2010 •
2007 •
Journal of the Optical Society of America B
Structure of whispering gallery mode spectrum of microspheres coated with fluorescent silicon quantum dots2013 •
2008 •
2012 •
2006 Conference on Optoelectronic and Microelectronic Materials and Devices
Spectral Properties and dynamics of carriers in Si quantum dots in SiN matrices2006 •
Current Opinion in Solid State and Materials Science
Silicon nanostructures from electroless electrochemical etching2005 •
Physica E: Low-dimensional Systems and Nanostructures
Towards spectroscopy of a few silicon nanocrystals embedded in silica2009 •
Towards the First Silicon Laser
Optical Gain From Silicon Nanocrystals A critical perspectives2003 •
Surface Science Reports
Porous silicon: a quantum sponge structure for silicon based optoelectronics2000 •
2010 •
Physical Review A
Fundamental photophysics and optical loss processes in Si-nanocrystal-doped microdisk resonators2008 •
Journal of Luminescence
Radiative channel competition in silicon nanocrystallites2005 •
Journal of Physics D: Applied Physics
Ultrafast decay of femtosecond laser-induced grating in silicon-quantum-dot-based optical waveguides2008 •
Applied Physics Letters
Exciton-phonon coupling in a CsPbBr3 single nanocrystalThe Journal of Physical Chemistry C
Quantum Confinement Explains Pump-Dependent Luminescence from Butyl-Terminated Si Quantum DotsScientific reports
Origin of the Photoluminescence Quantum Yields Enhanced by Alkane-Termination of Freestanding Silicon Nanocrystals: Temperature-Dependence of Optical Properties2016 •
2014 •
Characterization of Silion Nanocrystals
Characterization of Si Nanocrystals2010 •
1996 •
Applied Physics Letters
Temperature-dependent photoluminescence of nanocrystalline ZnO thin films grown on Si (100) substrates by the sol–gel process2005 •
Superlattices and Microstructures
Long coherence times in self-assembled semiconductor quantum dots2002 •
Physics Reports
Physical properties of elongated inorganic nanoparticles2011 •
2012 •