Quasi-Direct Optical Transitions in Silicon Nanocrystals with Intensity Exceeding the Bulk (original) (raw)
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Strong Absorption Enhancement in Si Nanorods
We report two orders of magnitude stronger absorption in silicon nanorods relative to bulk in a wide energy range. The local field enhancement and dipole matrix element contributions were disentangled experimentally by single-dot absorption measurements on differently shaped particles as a function of excitation polarization and photon energy. Both factors substantially contribute to the observed effect as supported by simulations of the light-matter interaction and atomistic calculations of the transition matrix elements. The results indicate strong shape dependence of the quasidirect transitions in silicon nanocrystals, suggesting nanostructure shape engineering as an efficient tool for overcoming limitations of indirect band gap materials in optoelectronic applications, such as solar cells. B ulk and thin film silicon are widely used in modern photovoltaics, where the fundamental limit of efficiency in a single p−n junction cell (∼29.5%) is largely defined by the bandgap energy. 1 Nanostructuring of silicon, for example, in the form of nanowires, was shown to be promising in shortening carrier collection length and suppressing reflection of the incident light. 2 At the limit of the smallest nanostructures, where the size approaches the exciton Bohr radius (∼5 nm), quantum confinement sets in, changing the basic material properties, such as bandgap and k-space structure. 3 This effect can push the efficiency limit higher by allowing a multijunction concept to be realized in the same material. 4,5 It can also provide new functionality for this ubiquitous material in photovoltaics and beyond, 6 where ordered 3D arrays of such nanoparticles 7 can form new energy bands suitable for the direct readout. 8 Nanocrystals of silicon can also be used complementarily in solar cells for photon energy downshifting, 9 or photon multiplication by generating more low-energy photons than incoming high-energy quanta. 10 On the other hand, their enhanced photoluminescence 11 (PL) makes them attractive in several new applications, such as phosphors in white light-emitting diodes (LEDs), 12 or as biomarkers. 13,14 Because of strong absorption at high energies 15 these nanoparticles can also be considered as sensitizers for energy transfer to atomic or molecular species. 16 In general, the electric field experienced by a nanoparticle under external illumination depends on its shape and dielectric contrast with the environment. 17 The incoming field for ablated particles may be locally enhanced or reduced through particle polarizability, depending on the electric field orientation relative to the particle major axis. As a result the absorption cross-section becomes slightly reduced for the perpendicular field orientation and substantially enhanced for the parallel one. 18 This effect was noticed previously for Si nanostructures as strong polarization dependence in nanowire and nanorod absorption. 19−22 Another important effect of the nanoparticle geometry on its interaction with light is the degeneracy lifting and relaxation of transition selection rules in an asymmetric particle. 23 The exact interplay between these effects depends on material properties as well as on the incoming photon energy and particle geometry. For silicon nanocrystals in particular, the energy-dependent quasidirect transitions 15,24 may also be nanoparticle shape-dependent. So for both fundamentals and applications an important question is what maximum absorption enhancement could be achieved by the nanoparticle shape control and under which conditions. Here we investigate experimentally and theoretically both the local field enhancement and the exciton energy level structure effects on silicon nanoparticle absorption. We carried out photoluminescence and photoluminescence excitation measurements for single Si nanorods and close-to-spherical nanoparticles as a function of excitation polarization and energy. Indeed, the nanorods exhibit stronger absorption than spherical counterparts, which can reach as large as a factor of 50. The quantitative contributions of the local-field effect and intrinsic interband transitions in the observed enhancement were extracted from the experimental data with the help of numerical simulations of the wave-particle interaction. Atom
Single-dot absorption spectroscopy and theory of silicon nanocrystals
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
Journal of Nanoscience and Nanotechnology, 2008
Total energy calculations within the Density Functional Theory have been carried out in order to investigate the structural, electronic, and optical properties of un-doped and doped silicon nanostructures of different size and different surface terminations. In particular the effects induced by the creation of an electron-hole pair on the properties of hydrogenated silicon nanoclusters as a function of dimension are discussed in detail showing the strong interplay between the structural and optical properties of the system. The distortion induced on the structure by an electronic excitation of the cluster is analyzed and considered in the evaluation of the Stokes shift between absorption and emission energies. Besides we show how many-body effects crucially modify the absorption and emission spectra of the silicon nanocrystals. Starting from the hydrogenated clusters, different Si/O bonding at the cluster surface have been considered. We found that the presence of a Si O Si bridge bond originates significative excitonic luminescence features in the near-visible range. Concerning the doping, we consider B and P single-and co-doped Si nanoclusters. The neutral impurities formation energies are calculated and their dependence on the impurity position within the nanocrystal is discussed. In the case of co-doping the formation energy is strongly reduced, favoring this process with respect to the single doping. Moreover the band gap and the optical threshold are clearly red-shifted with respect to that of the pure crystals showing the possibility of an impurity based engineering of the absorption and luminescence properties of Si nanocrystals.
Increased carrier generation rate in Si nanocrystals in SiO2 investigated by induced absorption
Applied Physics Letters, 2011
We report on investigations of optical generation of carriers in Si nanocrystals embedded in SiO 2 matrix by time-resolved induced absorption technique. Results obtained for excitation below and above twice the bandgap energy hm < 2E g and hm > 2E g show very similar decay characteristics (within s resolution % 100 fs). When intensity of the signal is correlated to number of generated excitons, it is found that for the high photon energy excitation, carrier generation rate is considerably enhanced. These results are discussed in terms of carrier multiplication reported previously for semiconductor nanocrystals and photoluminescence quantum yield measurements for similar materials.
Electronic Spectroscopy and Photophysics of Si Nanocrystals: Relationship to Bulk c-Si and Porous Si
Journal of the American Chemical Society, 1995
The structural characterization, electronic spectroscopy, and excited-state dynamics of surface-oxidized Si nanocrystals, prepared in a high-temperature aerosol apparatus, are studied to gain insight into the emission mechanism of visible light from these systems. The results are compared with direct-gap CdSe nanocrystals, indirectgap AgBr nanocrystals, bulk crystalline silicon, and porous silicon thin films. As the size of the Si crystallites decreases to 1-2 nm in diameter, the band gap and luminescence energy correspondingly increase to near 2.0 eV, or 0.9 eV above the bulk 1.1-eV band gap. The absorption and luminescence spectra remain indirect-gap-like with strong transverse optical vibronic origins. The quantum yield increases to about 5% at room temperature, but the unimolecular radiative rate remains quite long, s-l. The luminescence properties of Si nanocrystals and porous Si are consistent, in most respects, with simple emission from size-dependent, volume-quantum-confined nanocrystal states. Room-temperature quantum yields increase not because coupling to the radiation field is stronger in confined systems, but because radiationless processes, which dominate bulk Si emission, are significantly weaker in nanocrystalline Si. An analogous series of changes occurs in nanocrystalline AgBr. While previous work on CdSe and CuCl nanocrystals has revealed size regimes for their spectroscopic properties, the Si and AgBr nanocrystal studies are shown here to reveal additional size regimes for their kinetic properties.
Doping in silicon nanostructures
Physica Status Solidi (A) Applications and Materials Science, 2007
We report on an ab initio study of the structural, electronic and optical properties of boron and phosphorous doped silicon nanocrystals. The scaling with the Si-nanocrystal size is investigated for both the neutral formation energies (FE) and the impurity activation energies. Both these energies scale with the nanocrystal inverse radius. The optical properties reveal the existence of new absorption peaks in the low energy region related to the presence of the impurity. The effects of B and P co-doping show that the formation energies are always smaller than those of the corresponding single-doped cases due to both carriers compensation and minor structural distortion. Moreover in the case of co-doping the electronic and optical properties show a strong reduction of the band gap with respect to the pure silicon nanocrystals that makes possible to engineer the photoluminescence properties of silicon nanocrystals.
Optical absorption spectra of doped and codoped Si nanocrystallites
2008
Silicon nanocrystallites (NCs) and similar nanostructures have been intensively investigated in the last years due to their interesting quantum confinement properties. 1–3 The strong spatial localization of electrons and holes in Si NCs can enhance radiative recombination rates and give rise to luminescence. Among other known applications, research on Si NCs could lead to optoelectronic devices compatible with the consolidated Si technology. Optical gain in Si NCs has been reported, 4, 5 and new devices have recently been suggested. 6, 7
RETRACTED: Absorption spectra of nanocrystalline silicon embedded in SiO2 matrix
Materials Letters, 2000
. Nanocrystalline silicon nc-Si embedded SiO matrix has been formed by annealing the SiO films fabricated by 2 x Ž . plasma-enhanced chemical vapor deposition PECVD technique. Absorption coefficient and photoluminescence of the films have been measured at room temperature. The experimental results show that there is an ''aUrbach-like''b exponential Ž . 1r2 Ž . absorption in the spectral range of 2.0-3.0 eV. The relationship of a hn A hn y E demonstrates that the luminescent g nc-Si have an indirect band structure. The existence of Stokes shift between photoluminescence and absorption edge indicates that radiative combination can take place not only between electron states and hole states but also between shallow trap states of electrons and holes. q