Rapid trapping as the origin of non-radiative recombination in

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Letter

Rapid trapping as the origin of non-radiative recombination in semiconductor nanocrystals Federico Pevere, Fatemeh Sangghaleh, Benjamin Bruhn, Ilya Sychugov, and Jan Linnros ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.8b00581 • Publication Date (Web): 13 Jul 2018 Downloaded from http://pubs.acs.org on July 14, 2018

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ACS Photonics

Rapid trapping as the origin of non-radiative recombination in semiconductor nanocrystals Federico Pevere, Fatemeh Sangghaleh, Benjamin Bruhn, Ilya Sychugov*, Jan Linnros Department of Applied Physics, KTH – Royal Institute of Technology, Isafjordsgatan 22, 16440 Kista, Sweden KEYWORDS quantum dots, photoluminescence, blinking, efficiency, Auger recombination

ABSTRACT

We demonstrate that non-radiative recombination in semiconductor nanocrystals can be described by a rapid luminescence intermittency, based on carrier tunneling to resonant traps. Such process, we call it “rapid trapping (blinking)”, leads to delayed luminescence and promotes Auger recombination, thus lowering the quantum efficiency. To prove our model, we probed oxide- (containing static traps) and ligand- (trap-free) passivated silicon nanocrystals emitting at similar energies and featuring mono-exponential blinking statistics. This allowed us to find analytical formulas and to extract characteristic trapping/de-trapping rates, and quantum efficiency as a function of temperature and excitation power. Experimental single-dot temperature-dependent decays, supporting the presence of one or few resonant static traps, and

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ensemble saturation curves were found to be very well described by this effect. The model can be generalized to other semiconductor nanocrystals, although the exact interplay of trapping/detrapping, radiative and Auger processes may be different, considering the typical times of the processes involved.


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MAIN TEXT The mechanism of non-radiative recombination in semiconductors was first explained by Shockley, Read and Hall (SRH) in 1952 as the trapping and subsequent recombination of electrons and holes by a defect or an impurity in the crystal.

1-2

While the process involves a

diffusive step (long for high-quality bulk crystals), the actual recombination should occur on a very short time scale (~fs), suggestively through multi-phonon emission. As a consequence, for nanoscale systems like semiconductor nanocrystals (NCs) or quantum dots (QDs), where diffusion distances are negligible, such non-radiative process would be very fast and therefore dominant. Thus, in a photoluminescence (PL) experiment NCs with such defects would appear “dark” (cf. silicon NCs with dangling bonds 3) while defect-free particles would appear “bright”, their statistical proportion setting the quantum efficiency (QE) for the ensemble. The corresponding QD exciton-recombination models are depicted in Figure 1(a) -(b). While impurities and defects have been vastly studied in bulk semiconductors, the detailed mechanism of non-radiative recombination in nanocrystals remains unclear. For instance, using single-dot spectroscopy on oxide-passivated silicon nanocrystals (Si-NCs) we found that the PL decay time varies largely for different NCs even emitting at the same wavelength (same bandgap energy).

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Thus, there are not only “bright/dark” but also “grey” nanocrystals and some non-

radiative process must be invoked competing with radiative recombination on a similar time scale, i.e. ~µs, usually assigned to unknown defects at the Si/SiO2 interface.

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Indeed, less

intensive, “grey” nanocrystals have also been observed for direct bandgap QDs phenomenologically associated with ~ns non-radiative processes. These systems can also exhibit


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PL decay time fluctuations

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in which case the ~ns non-radiative processes cannot be assigned

to very-fast charge trapping at surface states 9-11, usually relevant in hot carrier relaxation. A complex scenario is typically used, as shown in Fig. 1(c), to explain “grey” NCs: an excited electron (or hole) can be trapped in the shell and then recombines with the other particle via an unknown non-radiative path.

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Obviously, since electron-hole recombination is a resonant

process, trap sites for both electrons and holes should exist in the shell, as well as midgap states to facilitate this scenario. So a natural question arises if a simpler physical explanation can be more relevant instead of such a complex artificial construction.

Figure 1. Exciton-recombination models in a semiconductor QD. (a) Bright QDs: only radiative recombination, rate Γr. (b) Dark QDs: ultra-fast non-radiative recombination through a midgap state located at the core or at the nanocrystal interface, rate Γnr. (c) Grey NCs: (left) complex scenario where an electron (hole) is trapped at interface states and then recombines with the core hole (electron) via unknown non-radiative mechanism; (right) rapid trapping (blinking) model based on trapping/de-trapping of an electron (hole) at interface states and ultra-fast non-radiative Auger recombination occurring upon next exciton generation. Studies of single QDs (both Si

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and from direct bandgap QDs

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) have also revealed an

ON/OFF intermittency (or blinking) under continuous-wave excitation at long time-scales (~mss), from recording of the PL intensity time trace. Indications of the blinking extension to shorter


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timescales were also obtained via time-correlated measurements.

Furthermore, the recent

puzzling observation of delayed luminescence, reported for several direct-bandgap nanocrystals such as CdSe, Cu+-doped CdSe and CuInS2 NCs, has also suggested the presence of a very fast blinking, with a detrimental effect on the QE.

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Note that, during a long accumulation, the

blinking obviously would result in a “grey” PL intensity, although the exact effect on observed efficiencies has not been deciphered. In addition, many blinking QDs (e.g. CdSe-QDs) exhibit power-law ON- and OFF- time distributions with heavy tails, making impossible to evaluate average times without introducing artificial truncations.

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These kinetically broad

distributions seem to hold even when blinking is very fast (~ns-µs), as shown by the delayed luminescence,

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thus hindering a clear understanding of the underlying physical process as

well as its analytical formulation at different time scales. In contrast to non-normalized laws lacking an average ON- and OFF- time (and therefore also average PL intensity), mono-exponential blinking statistics have been reported for oxidepassivated Si-NCs exhibiting clear ON-OFF intensity levels.

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On the other hand,

functionalization of the surface with organic ligands has recently allowed to obtain Si-NCs featuring high quantum yields (~70%) both in colloidal solutions Those reportedly show no blinking 27

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and in polymer solids.

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and feature near-unity internal quantum efficiency (IQE)

, an ideal reference sample without “grey” particles. In this letter, we demonstrate that non-radiative recombination in semiconductor nanocrystals

with strong Auger recombination can be consistently explained as a very fast carrier-trapping phenomenon leading to rapid luminescence intermittency. We call it “rapid trapping” or equivalently “rapid blinking” and it is based on carrier tunneling to and from resonant trap states present in the shell of the NC. In our model, there is no need to assume any slow (~µs-ns)


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unknown non-radiative process involving complex scenarios,

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5-6, 12

while the true loss

mechanism can be just ultra-fast Auger recombination set up by rapid carrier trapping, as depicted in Figure 1(c) (right schematic). For the case considered here, Si/SiO2-NCs, the static nature of the traps is confirmed by this work, previous blinking data microelectronics studies of the “random telegraphic noise” effect.

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as well as numerous Characteristic non-

radiative times seen in the experiment can then vary broadly, defined only by the core-trap separation, naturally bridging over to normal blinking, observed at longer time scales and eventually to dark nanocrystals for very long trapping times (typically observed at low temperatures). Furthermore, our model may explain the increase of non-radiative recombination towards room temperature simply as an effect of phonon-induced broadening of the emission peak giving access to more trap states by resonant tunneling. In order to validate such a rapid blinking scenario, we used Si-NCs featuring monoexponential blinking statistics and similar emission energies (i.e. experiencing similar quantum confinement effects) which allowed: (i) to study the physical effect in its purity, unmasked by broad kinetics, and to adequately model it by means of a static three-state system; (ii) to find analytical formulas and extract switching rates and resulting QE as a function of experimental parameters, important for applications. Two different experiments validating the model were designed: (i) in pulsed excitation the PL decay would reveal a fast, temperature-dependent component indicative of the rate of resonant transfer to trap states, but also a slow tail due to delayed luminescence as carriers return from traps; (ii) under continuous excitation in such nanocrystals, a decreased PL efficiency would be observed well before the onset of normal Auger recombination, i.e. before the excitation level of one exciton per NC. Here we prove the presence of this non-radiative process, and it is shown to dominate carrier dynamics in oxide-


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passivated Si NCs. Its exact manifestation in other semiconductor NCs will depend on the characteristic times of trapping/de-trapping, radiative and Auger processes involved. Experimental details are mainly outlined in the Supporting Information. In brief, for single-dot measurements, Si-NCs embedded in oxide were fabricated from a low-doped Si wafer via electron-beam lithography, reactive ion etching and self-limiting oxidation

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resulting in well-

separated (~2 µm, i.e. resolvable in far-field PL) nanocrystals embedded in oxide walls (Fig. S1). For ensemble studies, we compared two samples of Si-NCs with different passivation. In the first, oxide-passivated NCs were formed in a Si-implanted SiO2 layer after thermal annealing. 33 In the second, dodecene-passivated NCs were encapsulated into a poly(methyl methacrylate) (PMMA) matrix via heat-promoted polymerization.

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These two samples have identical

emission spectra (Fig. S2) centered at ~1.7 eV with a fwhm of ~300 meV. The microphotoluminescence (µPL) system consisted of an inverted microscope, a spectrometer with a cooled EMCCD camera, an avalanche photodiode for decay acquisitions and a liquid-He cryostat for low-temperature studies. The excitation source was a 405-nm diode laser which could be modulated from cw up to the MHz range for time-resolved measurements. In total, the PL decay and spectrum of 17 QDs were measured at 2, 10, 20, 40, 70, 140, 220 and 300 K. Since their emission range was 1.6-1.9 eV with fwhm of ~150 meV, we can state that all the nanocrystals probed in this work experience a similar quantum confinement effect and can differ only in the passivation type (oxide or ligands). Figure 2(a) shows the PL spectra of a single oxide-passivated Si-QD measured at different temperatures, from 2 to 300 K. By decreasing the temperature, the no-phonon (NP) PL linewidth becomes narrower, revealing characteristic silicon TA (~ 15 meV) and TO (~ 60 meV) phonon replicas, and the emission energy is blue-shifted. This blue-shift naturally follows from the


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widening of the Si bandgap (see also Fig. S3). 5, 35-36 For the same dot, the low-temperature PL decays are presented in Fig. 2(b). Although at 300 K the decay can be well-fitted by a monoexponential function, at low temperatures a bi-exponential decay becomes apparent. The temperature dependence of the NP-linewidth presence of material-specific phonon replicas

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and its peak position, as well as the clear strongly indicate emission from quantum-

confined excitons in silicon: only the measured particles satisfying these stringent requirements were considered as single Si-NCs in our study. Indeed, if we exclude the unlikely event of two distinct and adjacent Si-NCs emitting at the same energy, the temperature evolution of the PL spectrum should reveal two narrow NP emission lines at low temperatures when two different adjacent emitters are measured. The lack of clear singlet-triplet emission line splitting 36, 39-41 can be attributed to the mixing of dark and bright excitons in the presence of trap states. 42

Figure 2. (a) Normalized PL spectra and (b) decays of an oxide-passivated Si-QD (dot306) taken at different temperatures under cw and modulated excitation, respectively. Each spectrum has an arbitrary intensity offset and decays show binned data (circles) and mono-/bi- exponential fittings (dashed lines) in a semi-log scale. The decay at 300 K is re-scaled in the inset for clarity.


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As in Fig. 2(b) for dot306, our decay measurements on many individual objects (see, for example, Fig. S5 for better signal-to-noise ratio data) reveal that the PL intensity decay can be generally described as = + , (1)

where Γ1 and Γ2 are respectively the fast and slow rates (Γ1 > Γ2), both in the kHz range. Thus, usually the decay is not governed by a single rate, as we would expect from a fully radiative recombination process,

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but other processes rather than the radiative must be involved.

Obviously, Γ1 cannot be directly associated to Auger recombination (ΓA > 1 GHz) 43 and both Γ1 and Γ2 vary with temperature, as shown later. The existence of a second recombination path in a single Si nanocrystal (Γ2), with a lifetime exceeding the radiative lifetime, suggests the presence of delayed luminescence, which was already observed in other types of NCs featuring power-law blinking statistics. 19-22 In those works, given the claimed link between blinking and delayed PL, the latter was extending on timescales orders of magnitude longer than the radiative lifetime. Conversely, here the first-reported delayed contribution from single Si-NCs does not show broadly distributed kinetics, but decays mono-exponentially with the temperature-dependent rate Γ2. It is important to note that we cannot discard the possibility to have delayed emitted photons at much longer times also in our material system, given the limited time scale (~µs-ms) that can be probed experimentally for single Si-QDs. However, our results are indeed consistent with a link between delayed PL and “rapid blinking”, since our oxide-passivated Si-NCs feature monoexponential blinking statistics instead of power-laws.

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In addition, the delayed PL here

reported at the single-dot level could also contribute to the long (> 100 µs) tail observed in spectrally-resolved PL decay of Si-NCs ensembles, 44 which cannot be explained by variations in non-radiative rates (SRH recombination is a much faster process).


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The rapid blinking model we propose here is based on the energy diagram of Fig. 3(a) drawn for a Si-NC emitting at 1.75 eV (as dot306) and on the corresponding state-transition diagram of Fig. 3(b). Once an exciton is generated (state ), the electron in the Si core can tunnel to one of the N static resonant traps present in the neighboring oxide, leaving the NC charged (state ). In that case, fast non-radiative Auger recombination becomes dominant upon further photon absorption (state ).

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The same non-radiative process happens when a biexciton is

present in the nanocrystal (state ). The finite number of available (resonant) trap states depends on the energy-level broadening which grows linearly with temperature (see Fig. S4).

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The nature of these trap states is not clear but their static character is confirmed by this work, previous blinking data

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and microelectronics studies of this material system.

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In those

works, mono-exponential charge switching statistics corresponding to single oxide traps have been reported for small area MOSFETs. The location of a trap was determined by analyzing the dependence of trapping and de-trapping rates with carrier concentration: while a weak dependence was found for a Si trap, an increase of trapping rate with carrier concentration characterizes a trap in the gate oxide.

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An analogous behavior of the ON-OFF blinking rate

with excitation power (i.e. carrier concentration) was found for Si/SiO2-NCs as well. 14


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Figure 3. (a) Electron energy diagram and schematic PL spectra for a rapid blinking Si/SiO2-NC. Energy shift of conduction (∆ECB) and valence (not shown) band and bandgap energy (Eg) as found by Seguini et al. 45 The light blue bell is a qualitative sketch of the electron wavefunction and dashed lines indicate energy-level thermal broadening. N trap states are present in the SiO2 shell. (b) Corresponding state-transition diagram. States: unexcited, exciton, electron trapped at ith trap, biexciton and positively-charged exciton. Rates: ΓG generation, Γr radiative, ΓA Auger, ΓON-OFFi (ΓOFFi-ON) trapping (de-trapping) rate to (from) ith trap. For the system considered here (Si-NCs), Auger recombination is strong (ΓA >> ΓON-OFFi and ΓA >> ΓOFFi-ON) and the NCs relax very fast from states and . By neglecting high excitations (those affect only amplitudes A1,2, but not eigenvalues Γ1,2), the decay constants of ON-state population can be found either by solving Kolmogorov equations or by using a probability density (PD) approach (more details in Supporting Information). Both methods confirm that the PL decay for a single static trap has a bi-exponential form, as in Eq. (1) with Γ2 < Γr < Γ1. In general, with N distinct resonant traps the PL decay can be shown to be the sum of (N+1) decaying exponential functions, arising from the electron trapping/de-trapping processes. Furthermore, the PD approach can be used to find also the PL decay of a NC with monoexponential ON-OFF and power-law OFF-ON distributions, which could correspond to traps fluctuating in energy or real space over time. Hence,

() = erfc , if ≪ (2#)

1 '() * erfc + () = $ − ,- , if ≫ (2) (%)


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where = /0/11 2 , erfc is the complementary error function and θ is the truncation time

(more details in Supporting Information). The result is similar to what was found experimentally for example in CuInS2-NCs. 22 Due to the need of artificial truncation of the power-law OFF-ON distribution, it is not possible to extract switching rates from such a PL decay curve, neither we can have information regarding the quantum efficiency. On the other hand, unmasked by broad kinetics, Si-NCs allow to extract such useful parameters from detailed analytical analysis based on eigenvalues of the observed decay constants Γ1,2 here. Correct amplitudes A1,2 of Eq. (1) can be found by simulating the decays using a full Monte-Carlo approach, taking into account both high excitations and the experimental square-wave pulse shape used here (Fig. S5 and related text). In order to estimate the radiative rate Γr, we first consider that it is passivation-independent for Si-NCs (i.e. core-related emission) 25, 27, 46 and our single-dot sample have a similar photonic embedding as the one used by Miura et al. 46 which was also oxidized. Thus, given the emission energies of our Si-NCs (1.6-1.9 eV), the radiative rate can be estimated to be in the range of 1020 kHz. In addition, Γr can be considered to decrease by a factor of ~3-5 only from 300 to 2 K, as indicated by recent temperature-dependent decay measurements on ensembles of ligandpassivated Si-NCs 47-48 and from the absence of clear triplet-state emission in our sample. Indeed, the transition from singlet to triplet state would be accompanied by a ~1-2 orders of magnitude decrease of Γr. 36, 39-41 Measured bi-exponential decays allow the extraction of trapping/de-trapping rates (or rapid blinking switching rates). Given the three decay rates (Γ1, Γ2, Γr), the rapid blinking switching rates and the ON-state duty cycle δON (luminescence efficiency) can be found as (from Eq. (S14))

Γ/0/11 = (Γ − Γ )(Γ − Γ )/Γ , (3#)


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Γ/11/0 = Γ Γ /Γ , (3)

6/0 =

Γ Γ . (38) Γ (Γ + Γ − Γ )

Figure 4(a) shows measured decay rates Γ1 and Γ2 of different oxide-passivated single Si-NCs as a function of temperature. The extracted rapid blinking ON-OFF rates and ON-state duty cycles from Eq. (3a) and (3c) are illustrated in Fig. 4(b). As a support of our analytical expressions, Monte-Carlo simulations confirmed that the as-extracted parameters are within the simulation statistical uncertainty range (see Fig. S5 and related text). It is important to note that, due to the intrinsically low emission rate (~10 kHz), it was not possible to measure each dot through all the temperatures. For T>150 K only Γ1 can be measured since Γ2 becomes much smaller than Γ1, reducing the signal below the noise level. In that range Γ2 was evaluated from Eq. (3c), using known values of efficiency δON (10% and 30% respectively at 300 and 220 K). 49 As the temperature decreases below 150 K, ΓON-OFF drops and δON increases due to less resonant traps available (cf. Fig. 3). Consequently, both fast and slow rates can be clearly resolved for several nanocrystals. However, some dots show mono-exponential decays also at low temperatures, which are characterized by one rate lying in between fast and slow rates (green circles). This result can be explained if the dots exhibit suppressed rapid trapping, e.g. for nanocrystals with fewer or totally absent resonant traps. The OFF-ON rates seem to exhibit a more complex temperature behavior (Fig. S7). The difference between ON-OFF and OFF-ON rates could be due to the significant exciton binding energy of Si-NCs, ~200 meV for our emission energies.

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Finally, the contribution of delayed luminescence in the total signal is

substantial (> 75%) over the whole temperature range (Fig. S7), highlighting the importance of this process in nanocrystal luminescence. Regarding other types of QDs the temperature dependence of the trapping/de-trapping dynamics as well as the related absolute value of rates


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can vary due to different radiative recombination rate, Auger process, phonon coupling, singlettriplet splitting and state intermixing w.r.t. to Si-QDs. For example, in CdSe-QDs the trapping can be faster (~ns) and the temperature-dependence of the dynamics is shown to be not strong at least from 20 K to room temperature. 22

Figure 4. Temperature-dependent data of different Si/SiO2-NCs. (a) Fast (blue dots) and slow (red dots) rates from bi-exponential fitting of measured PL decays. Rates from mono-exponential fitting (green dots), calculated slow rates (stars) and assumed radiative rate range (grey area). (b) Rapid blinking ON-OFF rates (purple triangles) and ON-state duty cycles (orange dots) extracted from Eq. (3a) and (3c), respectively. In both plots error bars are uncertainty on data and thick lines are trends.


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Since the properties of the trapping/de-trapping effect are now established, we are in a position to evaluate its manifestation in a continuous-wave excitation experiment. By solving the rate equations in steady-state conditions (see Supporting Information), the PL integrated intensity of a rapid blinking NC can be found as =

9Φ;< 9Φ;< ≈ , (4) 9Φ;< 9Φ;< (9Φ;< ) 1+ 1+ + Γ 6/0 Γ 6/0 Γ 6/0 Γ=

where the approximation is valid if Auger recombination is very strong, ΓA >> σΦex with σ as the absorption cross-section and Φex as the excitation photon flux. The internal quantum efficiency is then @A =

Γ 6/0 (5) Γ 6/0 + 9Φ;
10 kHz comparable with the radiative rate of the nanocrystal and generally increasing with temperature. As predicted by our rapid trapping scenario, there is a significant reduction of quantum efficiency under continuous wave excitation for oxide-covered Si-NCs with respect to trap-free, ligand-passivated ones. Ligand-passivated Si-NCs featuring near-unity IQEs 25, 27 seem to lack this non-radiative mechanism. Thus, the oxide matrix imposes inherent limitations on the luminescence efficiency of silicon nanocrystals due to its intrinsic trap sites. Apparently, the quality of the shell, and not only the interface, needs to be considered for the fabrication of highly efficient luminescent semiconductor nanoparticles. Although the combination of trapping/de-trapping, radiative and Auger processes is specific for a certain material system, our rapid blinking model is intended also for other semiconductor NCs. In direct bandgap QDs such as for instance CdSe QDs, these processes may be observed at significantly shorter time scales while decays due to a complex scenario of inter-related trap states, characterized by power-law blinking statistics, become largely non-linear. Since radiative recombination for these systems is on par with the Auger recombination rate it may be less dramatic than for the Si-NC system. Yet, we believe that similar fundamental trapping mechanisms are at play explaining non-radiative recombination.

ASSOCIATED CONTENT Supporting Information Experimental details, additional data, discussion and theory.


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AUTHOR INFORMATION Corresponding Author *[email protected] Author Contributions F.P. and F.S. carried out measurements and analyzed the data. B.B. fabricated the samples. F.P. and I.S. built the theoretical model. F.P. and F.S. wrote the first manuscript draft and I.S. and J.L. contributed to the final version. I.S. and J.L. supervised the overall work. Funding Sources Swedish Research Council (VR) through an individual contract and through a Linné grant (ADOPT), SPIE through an individual scholarship. ACKNOWLEDGMENT The authors thank A. Marinins and T. Chulapakorn for the help with sample preparation. Financial support from the Swedish Research Council (VR) through an individual contract and through a Linné grant (ADOPT) is gratefully acknowledged. F.P. thanks SPIE for an individual scholarship. ABBREVIATIONS NC nanocrystal; QD quantum dot; PL photoluminescence; CW continuous-wave; QE quantum efficiency; IQE internal quantum efficiency.

REFERENCES


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(1) Hall, R. N., Electron-Hole Recombination in Germanium. Phys. Rev. 1952, 87 (2), 387-387. (2) Shockley, W.; Read, W. T., Statistics of the Recombinations of Holes and Electrons. Phys. Rev. 1952, 87 (5), 835-842. (3) Meyer, B. K.; Petrova‐Koch, V.; Muschik, T.; Linke, H.; Omling, P.; Lehmann, V., Electron spin resonance investigations of oxidized porous silicon. Appl. Phys. Lett. 1993, 63 (14), 19301932. (4) Sangghaleh, F.; Bruhn, B.; Schmidt, T.; Linnros, J., Exciton lifetime measurements on single silicon quantum dots. Nanotechnology 2013, 24 (22), 225204. (5) Hartel, A. M.; Gutsch, S.; Hiller, D.; Zacharias, M., Fundamental temperature-dependent properties of the Si nanocrystal band gap. Phys. Rev. B 2012, 85 (16), 165306. (6) Hartel, A. M.; Gutsch, S.; Hiller, D.; Zacharias, M., Intrinsic nonradiative recombination in ensembles of silicon nanocrystals. Phys. Rev. B 2013, 87 (3), 035428. (7) Ebenstein, Y.; Mokari, T.; Banin, U., Fluorescence quantum yield of CdSe/ZnS nanocrystals investigated by correlated atomic-force and single-particle fluorescence microscopy. Appl. Phys. Lett. 2002, 80 (21), 4033-4035. (8) Fisher, B. R.; Eisler, H.-J.; Stott, N. E.; Bawendi, M. G., Emission Intensity Dependence and Single-Exponential Behavior In Single Colloidal Quantum Dot Fluorescence Lifetimes. J. Phys. Chem. B 2004, 108 (1), 143-148. (9) Kambhampati, P., Hot Exciton Relaxation Dynamics in Semiconductor Quantum Dots: Radiationless Transitions on the Nanoscale. J. Phys. Chem. C 2011, 115 (45), 22089-22109. (10) Kambhampati, P., On the kinetics and thermodynamics of excitons at the surface of semiconductor nanocrystals: Are there surface excitons? Chem. Phys. 2015, 446, 92-107.


20

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ACS Photonics

(11) Krause, M. M.; Kambhampati, P., Linking surface chemistry to optical properties of semiconductor nanocrystals. PCCP 2015, 17 (29), 18882-18894. (12) Frantsuzov, P. A.; Volkán-Kacsó, S.; Jankó, B., Universality of the Fluorescence Intermittency in Nanoscale Systems: Experiment and Theory. Nano Lett. 2013, 13 (2), 402-408. (13) Bruhn, B.; Valenta, J.; Sangghaleh, F.; Linnros, J., Blinking Statistics of Silicon Quantum Dots. Nano Lett. 2011, 11 (12), 5574-5580. (14) Bruhn, B.; Qejvanaj, F.; Sychugov, I.; Linnros, J., Blinking Statistics and ExcitationDependent Luminescence Yield in Si and CdSe Nanocrystals. J. Phys. Chem. C 2014, 118 (4), 2202-2208. (15) Efros, A. L.; Rosen, M., Random telegraph signal in the photoluminescence intensity of a single quantum dot. Phys. Rev. Lett. 1997, 78 (6), 1110-1113. (16) Efros, A. L.; Nesbitt, D. J., Origin and control of blinking in quantum dots. Nat. Nanotechnol. 2016, 11 (8), 661-71. (17) Sher, P. H.; Smith, J. M.; Dalgarno, P. A.; Warburton, R. J.; Chen, X.; Dobson, P. J.; Daniels, S. M.; Pickett, N. L.; O'Brien, P., Power law carrier dynamics in semiconductor nanocrystals at nanosecond timescales. Appl. Phys. Lett. 2008, 92 (10), 101111. (18) Davanço, M.; Hellberg, C. S.; Ates, S.; Badolato, A.; Srinivasan, K., Multiple time scale blinking in InAs quantum dot single-photon sources. Phys. Rev. B 2014, 89 (16), 161303. (19) Rabouw, F. T.; Kamp, M.; van Dijk-Moes, R. J. A.; Gamelin, D. R.; Koenderink, A. F.; Meijerink, A.; Vanmaekelbergh, D., Delayed Exciton Emission and Its Relation to Blinking in CdSe Quantum Dots. Nano Lett. 2015, 15 (11), 7718-7725.


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Page 22 of 26

(20) Whitham, P. J.; Knowles, K. E.; Reid, P. J.; Gamelin, D. R., Photoluminescence Blinking and Reversible Electron Trapping in Copper-Doped CdSe Nanocrystals. Nano Lett. 2015, 15 (6), 4045-51. (21) Whitham, P. J.; Marchioro, A.; Knowles, K. E.; Kilburn, T. B.; Reid, P. J.; Gamelin, D. R., Single-Particle Photoluminescence Spectra, Blinking, and Delayed Luminescence of Colloidal CuInS2 Nanocrystals. J. Phys. Chem. C 2016, 120 (30), 17136-17142. (22) Marchioro, A.; Whitham, P. J.; Knowles, K. E.; Kilburn, T. B.; Reid, P. J.; Gamelin, D. R., Tunneling in the Delayed Luminescence of Colloidal CdSe, Cu+-Doped CdSe, and CuInS2 Semiconductor Nanocrystals and Relationship to Blinking. J. Phys. Chem. C 2016, 120 (47), 27040-27049. (23) Verberk, R.; Orrit, M., Photon statistics in the fluorescence of single molecules and nanocrystals: Correlation functions versus distributions of on- and off-times. J. Chem. Phys. 2003, 119 (4), 2214-2222. (24) Mobarok, M. H.; Purkait, T. K.; Islam, M. A.; Miskolzie, M.; Veinot, J. G., Instantaneous Functionalization of Chemically Etched Silicon Nanocrystal Surfaces. Angew. Chem. Int. Ed. 2017, 56 (22), 6073-6077. (25) Marinins, A.; Zandi Shafagh, R.; van der Wijngaart, W.; Haraldsson, T.; Linnros, J.; Veinot, J. G. C.; Popov, S.; Sychugov, I., Light-Converting Polymer/Si Nanocrystal Composites with Stable 60–70% Quantum Efficiency and Their Glass Laminates. ACS Appl. Mater. Interfaces 2017, 9 (36), 30267–30272. (26) Nishimura, H.; Ritchie, K.; Kasai, R. S.; Goto, M.; Morone, N.; Sugimura, H.; Tanaka, K.; Sase, I.; Yoshimura, A.; Nakano, Y.; Fujiwara, T. K.; Kusumi, A., Biocompatible fluorescent


22

Page 23 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Photonics

silicon nanocrystals for single-molecule tracking and fluorescence imaging. J. Cell Biol. 2013, 202 (6), 967-83. (27) Sangghaleh, F.; Sychugov, I.; Yang, Z.; Veinot, J. G. C.; Linnros, J., Near-Unity Internal Quantum Efficiency of Luminescent Silicon Nanocrystals with Ligand Passivation. ACS Nano 2015, 9 (7), 7097-7104. (28) Ralls, K. S.; Skocpol, W. J.; Jackel, L. D.; Howard, R. E.; Fetter, L. A.; Epworth, R. W.; Tennant, D. M., Discrete resistance switching in submicrometer silicon inversion layers individual interface traps and low frequency (1/f?) noise. Phys. Rev. Lett. 1984, 52 (3), 228-231. (29) Kirton, M. J.; Uren, M. J., Noise in solid-state microstructures - a new perspective on individual defects, interface states and low-frequency (1/f) noise. Adv. Phys. 1989, 38 (4), 367468. (30) Fang, W.; Simoen, E.; Aoulaiche, M.; Luo, J.; Zhao, C.; Claeys, C., Distinction between silicon and oxide traps using single-trap spectroscopy. Phys. Status Solidi A 2015, 212 (3), 512517. (31) Simoen, E.; Fang, W.; Aoulaiche, M.; Luo, J.; Zhao, C.; Claeys, C., Random telegraph noise: The key to single defect studies in nano-devices. Thin Solid Films 2016, 613, 2-5. (32) Bruhn, B.; Sangghaleh, F.; Linnros, J., Fabricating single silicon quantum rods for repeatable single dot photoluminescence measurements. Phys. Status Solidi A 2011, 208 (3), 631--634. (33) Chulapakorn, T.; Sychugov, I.; Suvanam, S. S.; Linnros, J.; Wolff, M.; Primetzhofer, D.; Possnert, G.; Hallén, A., Si-nanoparticle synthesis using ion implantation and MeV ion irradiation. Phys. Status Solidi C 2015, 12 (12), 1301-1305.


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Page 24 of 26

(34) Marinins, A.; Yang, Z.; Chen, H.; Linnros, J.; Veinot, J. G. C.; Popov, S.; Sychugov, I., Photostable Polymer/Si Nanocrystal Bulk Hybrids with Tunable Photoluminescence. ACS Photonics 2016, 3 (9), 1575-1580. (35) Lautenschlager, P.; Garriga, M.; Vina, L.; Cardona, M., Temperature dependence of the dielectric function and interband critical points in silicon. Phys. Rev. B 1987, 36 (9), 4821-4830. (36) Brongersma, M. L.; Kik, P. G.; Polman, A.; Min, K. S.; Atwater, H. A., Size-dependent electron-hole exchange interaction in Si nanocrystals. Appl. Phys. Lett. 2000, 76 (3), 351-353. (37) Sychugov, I.; Fucikova, A.; Pevere, F.; Yang, Z.; Veinot, J. G. C.; Linnros, J., Ultranarrow Luminescence Linewidth of Silicon Nanocrystals and Influence of Matrix. ACS Photonics 2014, 1 (10), 998-1005. (38) Bruhn, B.; Valenta, J.; Sychugov, I.; Mitsuishi, K.; Linnros, J., Transition from silicon nanowires to isolated quantum dots: Optical and structural evolution. Phys. Rev. B 2013, 87 (4), 045404. (39) Kovalev, D.; Heckler, H.; Polisski, G.; Koch, F., Optical properties of Si nanocrystals. Phys. Status Solidi B 1999, 215 (2), 871-932. (40) Heckler, H.; Kovalev, D.; Polisski, G.; Zinov'ev, N. N.; Koch, F., Magneto-optical effects in photoluminescence of Si nanocrystals. Phys. Rev. B 1999, 60 (11), 7718-7721. (41) Fernee, M. J.; Littleton, B. N.; Rubinsztein-Dunlop, H., Detection of Bright Trion States Using the Fine Structure Emission of Single CdSe/ZnS Colloidal Quantum Dots. ACS Nano 2009, 3 (11), 3762-3768. (42) Califano, M.; Franceschetti, A.; Zunger, A., Temperature Dependence of Excitonic Radiative Decay in CdSe Quantum Dots: The Role of Surface Hole Traps. Nano Lett. 2005, 5 (12), 2360-2364.


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ACS Photonics

(43) Delerue, C.; Lannoo, M.; Allan, G.; Martin, E.; Mihalcescu, I. I.; Vial, J. C.; Romestain, R.; Muller, F.; Bsiesy, A., Auger and Coulomb charging effects in semiconductor nanocrystallites. Phys. Rev. Lett. 1995, 75 (11), 2228-2231. (44) Linnros, J.; Lalic, N.; Galeckas, A.; Grivickas, V., Analysis of the stretched exponential photoluminescence decay from nanometer-sized silicon crystals in SiO2. J. Appl. Phys. 1999, 86 (11), 6128-6134. (45) Seguini, G.; Castro, C.; Schamm-Chardon, S.; BenAssayag, G.; Pellegrino, P.; Perego, M., Scaling size of the interplay between quantum confinement and surface related effects in nanostructured silicon. Appl. Phys. Lett. 2013, 103 (2), 023103. (46) Miura, S.; Nakamura, T.; Fujii, M.; Inui, M.; Hayashi, S., Size dependence of photoluminescence quantum efficiency of Si nanocrystals. Phys. Rev. B 2006, 73 (24), 245333. (47) Van Sickle, A. R.; Miller, J. B.; Moore, C.; Anthony, R. J.; Kortshagen, U. R.; Hobbie, E. K., Temperature Dependent Photoluminescence of Size-Purified Silicon Nanocrystals. ACS Appl. Mater. Interfaces 2013, 5 (10), 4233-4238. (48) Ghosh, B.; Takeguchi, M.; Nakamura, J.; Nemoto, Y.; Hamaoka, T.; Chandra, S.; Shirahata, N., Origin of the Photoluminescence Quantum Yields Enhanced by Alkane-Termination of Freestanding Silicon Nanocrystals: Temperature-Dependence of Optical Properties. Sci. Rep. 2016, 6, 36951. (49) Valenta, J.; Greben, M.; Gutsch, S.; Hiller, D.; Zacharias, M., Effects of inter-nanocrystal distance on luminescence quantum yield in ensembles of Si nanocrystals. Appl. Phys. Lett. 2014, 105 (24), 243107. (50) Reboredo, F. A.; Franceschetti, A.; Zunger, A., Dark excitons due to direct Coulomb interactions in silicon quantum dots. Phys. Rev. B 2000, 61 (19), 13073-13087.


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(51) Pevere, F.; Sychugov, I.; Sangghaleh, F.; Fucikova, A.; Linnros, J., Biexciton Emission as a Probe of Auger Recombination in Individual Silicon Nanocrystals. J. Phys. Chem. C 2015, 119 (13), 7499-7505. (52) Yang, Z.; De los Reyes, G. B.; Titova, L. V.; Sychugov, I.; Dasog, M.; Linnros, J.; Hegmann, F. A.; Veinot, J. G. C., Evolution of the Ultrafast Photoluminescence of Colloidal Silicon Nanocrystals with Changing Surface Chemistry. ACS Photonics 2015, 2 (5), 595-605.

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