Negligible Electronic Interaction between Photoexcited Electron-Hole Pairs and Free Electrons in Phosphorus-Boron Co-Doped Silicon Nanocrystals

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(1)Kobe University Repository : Kernel タイトル Title. Negligible Electronic Interaction between Photoexcited Electron-Hole Pairs and Free Electrons in Phosphorus-Boron Co-Doped Silicon Nanocrystals. 著者 Author(s). Limpens, Rens / Fujii, Minoru / Neale, Nathan R. / Gregorkiewicz, Tom. 掲載誌・巻号・ページ Journal of Physical Chemistry C,122(11):6397-6404 Citation 刊行日 Issue date. 2018-03-22. 資源タイプ Resource Type. Journal Article / 学術雑誌論文. 版区分 Resource Version. publisher. 権利 Rights. © 2018 American Chemical Society. This is an open access article published under an ACS AuthorChoice License, which permits copying and redistribution of the article or any adaptations for non-commercial purposes.. DOI. 10.1021/acs.jpcc.7b12313. JaLCDOI URL. http://www.lib.kobe-u.ac.jp/handle_kernel/90004899. Create Date: 2018-06-18.

(2) This is an open access article published under an ACS AuthorChoice License, which permits copying and redistribution of the article or any adaptations for non-commercial purposes.. Article pubs.acs.org/JPCC. Cite This: J. Phys. Chem. C 2018, 122, 6397−6404. Negligible Electronic Interaction between Photoexcited Electron− Hole Pairs and Free Electrons in Phosphorus−Boron Co-Doped Silicon Nanocrystals Rens Limpens,*,† Minoru Fujii,‡ Nathan R. Neale,† and Tom Gregorkiewicz§ †. National Renewable Energy Laboratory, 15013 Denver West Parkway, Golden, Colorado 80401, United States Department of Electrical and Electronic Engineering, Kobe University, Rokkodai, Nada, Kobe 657-8501, Japan § Van der Waals-Zeeman, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands ‡. S Supporting Information *. ABSTRACT: Phosphorus (P) and boron (B) co-doped Si nanocrystals (NCs) have raised interest in the optoelectronic industry due to their electronic tunability, optimal carrier multiplication properties, and straightforward dispersibility in polar solvents. Yet a basic understanding of the interaction of photoexcited electron−hole (e−h) pairs with new physical features that are introduced by the co-doping process (free carriers, defect states, and surface chemistry) is missing. Here, we present the first study of the ultrafast carrier dynamics in SiO2-embedded P−B co-doped Si NC ensembles using induced absorption spectroscopy through a two-step approach. First, the induced absorption data show that the large fraction of the dopants residing on the NC surface slows down carrier relaxation dynamics within the first 20 ps relative to intrinsic (undoped) Si NCs, which we interpret as enhanced surface passivation. On longer time-scales (picosecond to nanosecond regime), we observe a speeding up of the carrier relaxation dynamics and ascribe it to doping-induced trap states. This argument is deduced from the second part of the study, where we investigate multiexciton interactions. From a stochastic modeling approach we show that localized carriers, which are introduced by the P or B dopants, have minor electronic interactions with the photoexcited e−h pairs. This is understood in light of the strong localization of the introduced carriers on their original P- or B-dopant atoms, due to the strong quantum confinement regime in these relatively small NCs (<6 nm).. ■. INTRODUCTION Simultaneous doping of Si nanoparticles with P and B atoms is opening a new field of research where charge compensation (introducing an equal number of donors and acceptors per nanoparticle) enables a pathway to control the optical, electronic, and surface properties,1−3 raising interest in fields ranging from the photovoltaic industry to the bioimaging sector.4,5 The exotic optoelectronic effects arise from charge compensation in which the free charges from each doping species compensate and recombine. As a result, in the optimal situation one ends up with highly doped NCs (with doping concentrations close to the solubility limit of P and B atoms in bulk Si)6 without any free carriers. Upon excitation, these charge-compensated Si NCs exhibit microsecond emission involving donor and acceptor states that is red-shifted relative to their intrinsic Si NC cousins, even to values that are below the bandgap of bulk Si.7 The ability to tune the optical properties of Si NCs could enable their use in quantum dot (QD) solar cells, for which quantum-confined intrinsic Si NCs exhibit a bandgap shifted away from the optimal value of thermodynamical power conversion efficiency albeit with highly efficient carrier multiplication.8,9 Therefore, these co-doped NCs offer the unique potential for enabling efficient carrier © 2018 American Chemical Society. multiplication with an energy threshold value of around 2Eg and an optical bandgap (Eg) sufficiently red-shifted to be relevant for QD solar cells, as we have recently shown in ref 10. Although important advances have been made to increase the stability, optical activity, and functionality of these co-doped Si NCs,1,2,7 a pathway toward practical applications will only be possible with a complete understanding of the carrier dynamics in these systems, a research direction that has remained unexplored. To this end we note that it is well-known that high doping concentrations of solely P or B quench the optical activity,11−14 being often assigned to strong nonradiative Auger interactions between the free carriers and the photoexited e−h pairs, i.e., trion recombination. Interestingly, ensembles of chargecompensated, P−B co-doped Si NCs also show reduced photoluminescence quantum yield values,7 assigned to the same trion formation in unintentionally created uncompensated NCs (NCs that are doped with an unequal number of P- and Bdopants and which therefore cannot fully charge compensate Received: December 14, 2017 Revised: February 8, 2018 Published: March 5, 2018 6397. DOI: 10.1021/acs.jpcc.7b12313 J. Phys. Chem. C 2018, 122, 6397−6404.

(3) Article. The Journal of Physical Chemistry C. Figure 1. Optical characterization of the co-doped (in red) and intrinsic Si NCs (in black). (a) PL spectra taken at an excitation wavelength of λexc = 405 nm, displaying a significantly red-shifted PL spectrum for the co-doped ensemble as a result of the donor−acceptor-pair recombination process, as discussed in the main text. The dashed lines function as guides to the eye and represent Gaussian (red) and log-normal (black) fitting curves. (b) Linear absorption spectra indicating the charge carrier absorption feature for the co-doped Si NC ensemble. The dashed lines function as a guide to the eye. (c) XRD pattern of the co-doped Si NCs, showing two main components, the SiO2 band and the (111) Si peak. The dashed red line represents the total fitting function of the XRD pattern as elaborated in the main text. Similar PL and identical absorption data have been presented for these materials in refs 10 and 15.. ■. their free carriers). Here we present the first detailed study of the ultrafast carrier dynamics of solid-state ensembles of P−B co-doped Si NCs and show that this theory cannot explain the observed reduction in the optical activity for NCs in a highly confined regime (<6 nm in diameter). In order to arrive at this conclusion, we investigated the electronic interactions between free carriers and the photoexcited e−h pairs by applying a universal modeling procedure to the nonlinear excitation regime of the carrier dynamics, an approach that might be valuable for other doped NC systems as well.. ■. RESULTS AND DISCUSSION Preliminary Considerations. Before addressing the details of this work, we need to clarify the terminology used. For easyreading purposes we used the term “free carrier” in the Introduction to describe carriers introduced by the doping sites. Defining a carrier as “free” in a quantum-confined system in which electrons and holes are strongly localized is conflicting by definition. Furthermore, due to increased ionization energies in quantum-confined systems, carriers that are introduced by the dopants could feel an additional localization-potential to its original doping atom. Whether a complete localization occurs and whether this would result in a significant reduction of Coulomb interactions between photoexcited e−h pairs and the introduced carriers are so far unknown and difficult to assess without addressing the ultrafast carrier dynamics. This nuance forms the base of our observations, and in this light we will further use the term “localized carrier” for a carrier that is introduced by substitutional P and/or B atoms. We direct the interested reader to a comprehensive review on doped Si NCs that, among a detailed overview of some advances in the field of doped Si NCs, discusses this aspect from both a theoretical and experimental perspective.16 Optical Characterization of Co-Doped and Intrinsic Si NCs. In Figure 1a we present the PL spectra of both Si NC ensembles and report on a significantly red-shifted PL spectrum for the P−B co-doped Si NCs. Similar PL spectra have been reported before in recent publications,10,15 in which we assigned the red shift (toward energy values below the bandgap of bulk Si, i.e., 1.12 eV, and therefore unexplainable purely by quantum confinement) to the recombination between donor and acceptor states of the P and B impurity atoms.16−18 By fitting to a Gaussian distribution, we find a fwhm of 228 ± meV and 220 ± 2 meV for PL spectra of the doped and intrinsic ensembles of Si NCs, respectively. A larger broadening is expected for the co-doped ensemble due to the variation in the binding energy in these systems (i.e., the energy difference between the donor/acceptor states and the conduction band and valence band minimum and maximum, respectively), which depends not only on the NC size (i.e., the confinement energy. METHODS. The samples used in this study are similar to the ones in previous work.10,15 SiO2-embedded solid-state dispersions of P−B co-doped Si NCs and intrinsic (undoped) Si NCs have been produced by a cosputtering technique and a subsequent annealing step in which the thermodynamic driving force for forming SiO2 results in Si NCs precipitating within the SiO2 matrix. Nominal P and B concentrations of 1.25 atom % have been used during the production process. The PL spectrum of the intrinsic NCs and the IR-VIS PL spectrum of the co-doped NCs are investigated by combining two Ocean Optics spectrometers, the OceanFX and the NIRQuest for the visible and near-infrared part of the spectrum, respectively. A continuous wave excitation at a wavelength of λexc = 405 nm is used for both measurements. Note that previous investigations10,15 presented similar PL spectra for the same samples, upon λexc = 340 nm excitation. Linear absorption spectra have been duplicated from ref 10 and are measured with a dual beam mode PerkinElmer Lambda 950 spectrometer. X-ray diffraction (XRD) patterns are measured on a Rigaku D/MAX2500. Induced absorption (IA) experiments were conducted on a pump−probe setup, using a femtosecond transient absorption spectrometer (Helios, Ultrafast Systems) with a 4 W Ti:sapphire amplifier (Libra, Coherent) laser source, operating at 1 kHz and 100 fs pulse width. 6398. DOI: 10.1021/acs.jpcc.7b12313 J. Phys. Chem. C 2018, 122, 6397−6404.

(4) Article. The Journal of Physical Chemistry C. Figure 2. Normalized IA traces (at Δt = 40 ps) of the intrinsic (black) and co-doped (red) Si NCs at a pump wavelength of λPump = 500 nm and a probe wavelength of λProbe = 1200 nm. The IA data of the co-doped NCs is a trace taken from the linear excitation regime in ref 10. The co-doped NC ensemble shows a suppression of the initial fast quenching mechanism that is typical for Si NCs, within the first 20 ps (left part of the x-axis). The dashed lines function as guides to the eye. At longer timescales (right part of the x-axis) we report on a reduced IA signal for the co-doped NCs. Potential responsible mechanisms are discussed in the main text. Markers “A” and “B” define the IA magnitude at 1 and 500 ps, respectively.. Econf)19 but also on Coulomb interactions between the ionized impurity atoms, determined by their spatial separations.20 As a result, significant spectral broadening is typically observed21,22 that surpasses that of the intrinsic NC ensemble that is solely defined by the size-dispersion. In Figure 1b we display the linear absorption of both samples, featuring nonzero absorption at energies of <1.0 eV, which is below the optical bandgap of the co-doped NC ensemble (Figure 1a). This is an indication of the presence of at least one or more localized carriers per nanoparticle, i.e., uncompensated Si NCs resulting from a nonstoichiometric P/B ratio.23 The effect of these localized electrons or holes on the dynamics of the photoexcited e−h pairs is under debate. Whereas it seems natural to assume that nonradiative trion decay would dominate the dynamics, as proposed in refs 13 and 23, it is also argued that the large binding energies of the donor/acceptor states, which are significantly higher than the bulk ionization energy (∼45 meV) due to quantum confinement, suppress Auger interactions between the e−h pairs and the localized carriers.24 In the next experiments we will address this issue in more detail. However, before doing so, we first finish the basic characterization and focus on quantifying the average NC sizes of the ensembles. While both the intrinsic and the P−B co-doped Si NCs were prepared by sputtering thin films of silicon substoichiometric dioxide (SiOx where x < 2) followed by high temperature annealing, the NC size distributions of these two samples are not necessarily identical. Incorporation of the dopant impurities could have impact on the Si diffusion rates25 and even the self-organization nature of the nanoparticles,26 a research area that has remained unexplored. Since the red-shifted PL spectrum of the codoped NCs relates to donor−acceptor transitions and not the band-to-band excitonic recombination, it is nontrivial to derive the average NC size from it. For this purpose we performed XRD measurements on the co-doped NCs (Figure 1c). Apart from the crystalline-Si (111) peak, a significant contribution of the surrounding SiO2 matrix appears. We excluded this effect in the analysis by making use of a Lorentzian fit of the SiO2 peak,. with the peak position and fwhm retrieved from the pure SiO2 XRD signal, as shown in the Supporting Information (Figure S1). Broadening of the (111) Si diffraction peak (fitted with a Voigt function) is ascribed to the small size of the NCs and can be modeled by the Scherrer equation.27 From the XRD pattern we conclude on an average size of the co-doped Si NCs of 5.8 nm ±0.5 nm, larger than that of the intrinsic NCs with a diameter of 3.8 nm ±0.5 nm, as derived from their PL spectrum following ref 28. We estimated the error in the NC size of the co-doped NCs and the intrinsic NCs from the Voigt fitting procedure and the experimental error in ref 28, respectively. Note that the size difference is rather significant. Further research on the growth mechanism might be able to explore the origin of this alteration. We do mention that only minor size differences have been observed between singly doped P and B NCs and P−B co-doped NCs for the same annealing condition as used for our samples.29 Apparently, the main NC growth variations can thus be found between intrinsic and doped NCs, regardless of the singly or co-doping nature. Ultrafast Carrier Dynamics in the Linear Excitation Regime. To study the effect of co-doping on the ultrafast carrier dynamics, we present the IA decay data for both NC ensembles (intrinsic and co-doped) in Figure 2. The experiments have been conducted at a pump wavelength of λPump = 500 nm and a near-infrared (NIR) probe of λProbe = 1200 nm. The traces have been normalized at a time delay of Δt = 40 ps. Note that the measurements have been performed in the linear excitation regime, where the average number of absorbed photons per NC is significantly less than 1, Nabs ≪ 1. Experimental validation of the linear excitation regime is presented in Figure S2 in the Supporting Information. We report on two important features appearing upon co-doping of the Si NCs: (1) an increased level of surface passivation and (2) the appearance of an additional nonradiative recombination channel in the subnanosecond regime. First, it is well-known that the ultrafast dynamics of single-excitons in intrinsic Si NCs typically feature nonradiative recombination within the first 100 ps, illustrating relaxation pathways, such as the self-trapped 6399. DOI: 10.1021/acs.jpcc.7b12313 J. Phys. Chem. C 2018, 122, 6397−6404.

(5) Article. The Journal of Physical Chemistry C. Figure 3. Modeling the Auger interactions of the co-doped NC ensemble by making use of two different methods. Both figures are produced from the same data set, which is derived at λPump = 500 nm and λProbe = 1200−1350 nm. The original traces from which these interactions are constructed are presented in the Supporting Information and can be found in the data set of ref 10 as well. (a) Average number of excitons per NC (see the main text for the derivation) as a function of time for several excitation intensities, with Nabs = 0.8, 1.75 and 2.8, according to the method of Schaller et al.40 The red dotted lines represent three-charge interaction fits. (b) Scaled IA intensities (scaling is provided by the method of Klimov et al.)41 followed by double exponential fits to derive the biexciton lifetimes of all three Nabs traces. The log-lin scale is used for clarity.. exciton in the vicinity of an oxidized surface,30,31 phononassisted cooling,32 and/or the presence of “dark” NCs.33 Especially for the oxide-embedded Si NCs in our work, where dangling bond-initiated defect states are easily formed, a significant ultrafast quench is likely to occur. This was previously observed and quantified in sputter-produced oxideembedded Si NCs, through ultrafast IA measurements by Trinh et al.34 We do note that hydrogen passivation of dangling bonds (e.g., by low-temperature annealing under H-ambient)35,36 can significantly increase the optical activity of intrinsic Si NC ensembles, enabling photoluminescence quantum yields of around 35% for SiO2-embedded Si NCs.37 We argue that the absence, or at least a significant reduction, of this quenching behavior for the co-doped NCs originates from an increased level of surface passivation by the termination of dangling bonds, as previously observed by electron spin resonance (ESR) upon the incorporation of P.38 Moreover, the surface layer of the P and B atoms might also minimize the effect of the self-trapped exciton by reducing the interplay with detrimental oxygen-related defect states at the surface. Both scenarios are not unlikely given the fact that a significant amount of impurity atoms are located at or in the vicinity of the Si/SiO2 interface.29 We recall that previous investigations do indeed show that the optical activity of singly doped Si NCs increases for small concentrations of both Pdopants12,13,38 and B-dopants.14 For higher doping concentrations, however, the optical activity is significantly quenched. This is often observed through standard PL measurements on the μs−ms timescales12,14 or more sophisticated PLQY measurements7 showing values far below the efficiency limit of intrinsic SiO2-embedded Si NCs.37 This relates to our second observation. On the right part of the x-axis in Figure 2 we report on an additional recombination channel appearing in the co-doped NC ensemble on longer time scales (>50 ps). Such an effect has previously been proposed to arise from (1) trion formation between photoexcited e−h pairs and localized carriers in uncompensatively doped NCs, giving rise to Auger quenching13,23 and/or (2) sub-bandgap trap states induced by interstitial impurities, an idea that was based on electrical transport measurements, density functional theory calculations, and ESR results.12,24 As mentioned, there is no clear consensus on this issue, and depending on the NC size, both of these scenarios might be valid at the same time. Though the above experiment cannot distinguish between the two theories, in the. next experiment we shed some light on this issue by investigating the Coulomb interactions in the co-doped NCs in the multiexciton regime. Auger Interactions in the Nonlinear Excitation Regime. We now move to probing the charge dynamics under high light intensity conditions that allows us to quantify multiexciton interactions. We assume that electronic interactions between localized carriers and photoexcited e−h pairs would increase the Auger recombination rate, thereby shortening the biexciton lifetime. The presence of additional carries would increase the number of initial states and should therefore strengthen the transition rate of Auger recombination. Note that this scenario would only be true if the biexciton recombination can be treated as a superposition of independent positive and negative trion decay Auger pathways.39 So to be conclusive, we will define both the biexciton lifetime and its interaction model. The previous experiments have been conducted in the linear excitation regime (Nabs ≪ 1). Now, by increasing the pump intensity, we enter the nonlinear excitation regime (Nabs > 1) in which Auger recombination between two (or more) photoexcited e−h pairs dominates the carrier dynamics in the first hundreds of picoseconds. Upon increasing the excitation density, we clearly see a fast Auger-related component arising at short time scales (Supporting Information, Figure S2). Verification of the nonlinear regime is performed by monitoring the A/B ratio for several excitation intensities, with A being the IA magnitude at 1 ps and B at 500 ps, as indicated in Figure 2. This data set allows us to determine the average biexciton lifetime, similar to the procedure used by Schaller et al.40 (and Trinh et al.34 for Si NCs). By (1) normalizing all traces at 500 ps (point B), where all nonlinear processes are expected to be finished and only single-exciton behavior remains, and (2) taking the ratio of the resulting nonlinear transients to the linear transient, we end up with the time-development of the average number of e−h pairs (excitons). We display these dynamics in Figure 3a for the co-doped NC ensemble at different excitation conditions. This normalization method enables us to exclude any effect arising from e−h pair dynamics in the single-exciton regime so that we can solely focus on the nonradiative recombination due to the multiexciton Auger interactions, as visualized in Figure 3a. The excitation distribution within an ensemble (Nabs) at t = 0 is given by Poisson statistics: 6400. DOI: 10.1021/acs.jpcc.7b12313 J. Phys. Chem. C 2018, 122, 6397−6404.

(6) Article. The Journal of Physical Chemistry C. Figure 4. Dynamics of the average number of excitons per NC for the intrinsic (black) and co-doped Si NCs (red). (a) Direct comparison of the average number of excitons per NC as a function of time for both NC ensembles. For clarity we use a log−log scale. The red dotted lines represent the three-charge interaction model, perfectly agreeing for both samples, reflecting a biexciton lifetime of 28 and 140 ps for the intrinsic and co-doped Si NCs. (b) r3 dependence of the derived biexciton lifetimes as a function of the NC volume, perfectly in line with the values observed before by Beard et al., for intrinsic Si NCs.43 The black dashed line is a linear fit to the values of Beard et al., and the red dashed line is the same fit overlaid on our data set. (c) Scheme of the symmetric three-charge interaction model in which the electrons and holes are considered as independent charges and three of them have to interact in order to give rise to Auger recombination.. pn (0) =. Jσ n (−Jσ ) e n!. biexponential decay to produce the data in Figure 3b. Although no information is provided on the type of interactions, this method allows for a simple separation of the biexciton recombination from higher order exciton recombinations (in this case the three-exciton recombination is significantly present, as expected for the highest excitation intensity with Nabs = 2.8). We therefore fit the scaled traces by a double exponential decay. The longer lifetime represents the biexciton lifetime and is found here to be around 131−136 ps (with a fitting error of around ±20 ps, the precise errors are displayed in Figure 3b) for all three excitation intensities, confirming the fits from the stochastic model in Figure 3a. By using the stochastic interaction model of eqs 1−3, we define the average biexciton lifetime for the intrinsic Si NC to be 1/Γ2 = 28 ps, as presented by the black dynamics and the red dashed fitting curve in Figure 4. For completeness we add the interaction dynamics of the co-doped Si NC ensemble (the green line of Figure 3a) in red and use a log−log scale. Taking into account the difference in the average NC size (3.8 ± 0.5 nm for the intrinsic and 5.8 ± 0.5 nm for the co-doped Si NC ensemble), the lifetimes follow the expected universal r3 dependence for both direct and indirect semiconductor nanoparticles.42 This is illustrated by Figure 4b, in which we plot our derived biexciton lifetimes in comparison with the ones observed by Beard et al.,43 for intrinsic Si NCs, showing striking similarity. Hence, within the experimental error of both the NC size estimation and the rigorous modeling procedure of the ultrafast carrier dynamics, we do not observe a shortening of the biexciton lifetime as a result of localized carriers in uncompensated NCs. Therefore, based on the biexciton lifetime and the successful three-charge interaction fits, we show that the electronic interactions between the localized carriers and the photoexcited e−h pairs are relatively weak, at least in comparison to the strong nature of the biexciton interactions. This is most likely resulting from the large binding energies of the localized carriers due to the strong quantum confinement effect in these NCs. Figure 4c displays a scheme of the symmetric threecharge interaction model. On the basis of these results, we postulate that the localized carriers in uncompensated NCs are not responsible for the additional nonradiative recombination channel in the ps−ns range (Figure 2b). As such, defect states due to unintentionally generated interstitial impurities are the most plausible. (1). with pn(0) representing the population fraction of n excitons per NC at time t = 0, J the excitation fluence per unit area, and σ the absorption cross section. The product of the last two determines the average number of absorbed photons per NC, Nabs. This sets the boundary condition for the following set of differential equations: d p (t ) = Γn + 1pn + 1 (t ) − Γnpn (t ) dt n. (2). with Γn being the decay rate from n to (n − 1) excitons per NC. For intrinsic Si NCs, the Coulomb interactions are typically modeled by making use of a three-charge model34 with the biexciton recombination rate (Γ2) governing the multiexciton nonradiative recombination as follows:. Γn =. ⎛ 1 ⎞2 ⎜ n⎟ (n − 1)Γ 2 ⎝2 ⎠. (3). In this model, electrons and holes are considered as independent charges and three of them have to interact to induce Auger recombination. The perfect fits to this model (Figure 3a) indicate that the photogenerated electrons and holes in the co-doped systems indeed behave as independent charges while undergoing Auger recombination, just as is the case for intrinsic Si NCs.34 Since the interaction model in eq 3 is constructed for a symmetric situation in which an equal number of electrons and holes are present, this suggests that only minor electronic interactions exist between the localized carries and photoexcited e−h pairs in uncompensated NCs. Otherwise, an interaction model with an asymmetric distribution of electrons and holes would be required to accurately describe the dynamics. Moreover, by successfully fitting the nonlinear IA transients obtained for different Nabs values using the same Γ2 and interaction model (Figure 3a), we verify the modeling procedure and determine the biexciton lifetime of the co-doped Si NCs to be 1/Γ2 = 140 ps. We also confirmed this biexciton lifetime value by making use of the method of Klimov et al.41 In brief, raw intensities were normalized to the magnitude at long times (>500 ps), single exciton dynamics were removed by subtracting the traces in the nonlinear regime, and the resulting traces were fitted to a 6401. DOI: 10.1021/acs.jpcc.7b12313 J. Phys. Chem. C 2018, 122, 6397−6404.

(7) Article. The Journal of Physical Chemistry C. the carrier concentration (whether free or localized), which agrees with the decreased optical activity in co-doped samples found in this work and prior studies. These results show that co-doped Si NCs in the strong quantum confinement regime (offering sub-1.1 eV energy transitions) could be leveraged for QD-based multiexciton optoelectronics applications similar to those of intrinsic Si NCs. Their optical activities are not limited by the presence of some remaining free carriers in uncompensated NCs, a scenario that would have been unsolvable since it is inherent to the NC formation procedure by precipitation in a glass matrix (being the only synthetic method currently available for the preparation of these interesting nanostructures). Hence, to exploit these materials for charge extraction in any device concept, experimentalists must limit the presence of interstitial doping sites, potentially by developing new synthesis techniques.. explanation for the observed quench. This model could explain the previous observed PL quenching behavior in P or B singly doped Si NCs as well, in contrast to the often considered nonradiative trion recombination.13,23,44 As such, it may well be that charge compensation in P−B co-doped NCs is of lesser importance to the PL activity of these ensembles than previously assumed. We do recall that the interplay between localized carriers and photoexcited e−h pairs should strongly depend on parameters such as the doping concentration and the NC size. Further research is therefore needed to elucidate the effect of localized carriers (and the resulting trion formation) on the optical activity for differently sized NCs with varying doping concentrations. Further Discussion Regarding the Sample Inhomogeneity. On the basis of the below-bandgap NIR linear absorption that is indicative of the presence of localized carriers (Figure 1b), we know that our co-doped Si NC ensemble sample is an inhomogeneous set consisting of compensated and uncompensated NCs. The quantification of the level of inhomogeneity is however extremely challenging since the carriers are (1) localized and (2) present in a small number. Standard methods to determine the concentration of carriers rely on the presence of “free” carriers and a large amount of them (e.g., >10 free carriers per NC have to be present to support a localized surface plasmon resonance).45 This withholds us from performing a more sophisticated modeling procedure that treats an ensemble as a distribution, instead of the average ensemble-driven approach applied in this work. In this light we refer to a recent study based on similar samples that showed that there is (1) no realistic chance of finding NCs that are undoped or solely B-doped, (2) a small percentage of NCs (∼2−3%) that are solely P-doped, and (3) a >97% chance of finding a P−B co-doped Si NC.29 Despite the high probability of finding P−B co-doped NCs and the reduced formation energy for compensated doping,46 individual NCs are not necessarily compensated (the resolution of the atom probe tomography experiment used to derive the doping concentration was insufficient to resolve the level of charge compensation). Thus, within these co-doped NC samples we find a distribution of compensated and uncompensated NCs which together accounts for the ultrafast dynamics observed here.. ■. ASSOCIATED CONTENT. S Supporting Information *. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b12313. Derivation of the Lorentzian shape of the SiO2 signal in the XRD pattern and verification of the nonlinear excitation regime (PDF). ■. AUTHOR INFORMATION. Corresponding Author. *E-mail: rens.limpens@nrel.gov. Telephone: +1 (303) 2754687. ORCID. Rens Limpens: 0000-0002-2417-9389 Minoru Fujii: 0000-0003-4869-7399 Nathan R. Neale: 0000-0001-5654-1664 Tom Gregorkiewicz: 0000-0003-2092-8378 Notes. The authors declare no competing financial interest.. ■. ACKNOWLEDGMENTS R.L. acknowledges the National Renewable Energy Laboratory (NREL) LDRD program for the award of the Nozik Postdoctoral Fellowship to perform this research. N.R.N. was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences, Solar Photochemistry Program under Contract DE-AC36-08GO28308 to NREL. The authors thank Chung Xuan Nguyen for his help with the optical characterization, Arnon Lesage for his efforts regarding the sample production, and Noah Bronstein for building the Ocean Optics PL system.. ■. CONCLUSIONS Making use of induced absorption spectroscopy, we present the first detailed investigation on the ultrafast carrier dynamics of highly confined P−B co-doped Si NCs. We report on a decreased carrier recombination rate within the first 20 ps after photoexcitation and ascribe it to a surface passivation effect by the introduction P and B surface atoms. In contrast, for timescales of >20 ps we observe a new nonradiative recombination channel not present for the intrinsic Si NCs. To determine its origin, we investigated Auger interactions between multiple photoexcited e−h pairs and show that they follow a three-charge interaction model with rates similar to those found for intrinsic NC ensembles, apparently being unaffected by the presence of localized carriers in uncompensated NCs. This indicates that the localized carriers only weakly interact with the photoexcited e−h pairs and are most probably not responsible for the additional nonradiative recombination channel. We thus assign this new nonradiative recombination channel to defect states resulting from the unintentional introduction of interstitial impurities that do not contribute to. ■. REFERENCES. (1) Sugimoto, H.; Fujii, M.; Imakita, K. Synthesis of boron and phosphorus co-doped all-inorganic colloidal silicon nanocrystals from hydrogen silsesquioxane. Nanoscale 2014, 6, 12354−12359. (2) Sugimoto, H.; Fujii, M.; Fukuda, Y.; Imakita, K.; Akamatsu, K. 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