Simultaneous Creation and Recovery of Trap States on Quantum Dots

Article pubs.acs.org/JPCC

Simultaneous Creation and Recovery of Trap States on Quantum Dots in a Photoirradiated CdSe−ZnO System Karel Ž ídek,† Kaibo Zheng,† Mohamed Abdellah,†,§ Pavel Chábera,† Tõnu Pullerits,*,† and Masanori Tachyia*,‡ †

Department of Chemical Physics, Lund University, Box 124, 21000 Lund, Sweden National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8565, Japan § Department of Chemistry, Faculty of Science, South Valley University, Qena 83523, Egypt ‡

S Supporting Information *

ABSTRACT: We study changes in the steady-state absorption and ultrafast transient absorption kinetics of the photoirradiated CdSe quantum dot−ZnO system. The changes enable us to reconstruct kinetics of trap creation, which are analyzed with respect to three possible models: trap creation without recovery, trap creation and recovery, and trap creation with an upper limit for trap number accommodated on a quantum dot. We demonstrate that only the model of parallel trap creation and recovery can explain our experimental data. The evidence points toward oxygen generating trapping sites on QD surface and simultaneously passivating the trapping sites by their oxidation.

1. INTRODUCTION

pulsed excitation. The changes in the TA kinetics were interpreted as a creation of trapping states on the QDs. Here we focus on the origin of the changes in the system dynamics. We demonstrate that the results can provide us with valuable information about mechanism of trap creation. We analyze TA kinetics of the photoirradiated QD−ZnO system by assuming three possible models: (i) trap creation without recovery, (ii) parallel trap creation and recovery, and (iii) trap creation with an upper limit for trap number accommodated in a QD. We prove that only the model of light-induced trap creation and recovery, which creates and “repairs” the trap states at the same time, can reproduce our data. The TA analysis together with an observation of a continuous decrease in the QD size with irradiation indicates that oxygen photooxidation and photocorrosion are responsible for the observed effects.

Quantum dots (QDs) have found their application in almost every field of natural sciencephotovoltaics is not an exception.1−3 One of the most often-used concepts in the QD-based photovoltaics is QD-sensitized solar cell, which employs a composite material of QDs (serving as an electron donor) attached to a metal oxide (MO), which acts as an electron acceptor.4 The rapid electron injection from QD to MO maintains the charge separation and enables the light energy harvesting. Importantly, often a decisive question for the photovoltaic applications is the lifetime of a solar cell.1 Here the charge separation in the QD−MO system leads to difficulties. Charged QDs are prone to photodegradation,5−8 and free electrons in MO activate a number of photoreactions.9,10 Therefore, attention paid to the complicated problem has been increasing over the past years.1,6,7,11−14 The photodegradation is typically connected to oxygen interaction with QD surface or ligands on the surface.12,15 One of the important aspects of photodegradation is its manifestations in different experiments, which can, for instance, complicate the data interpretation. It is well-known that the photodegradation affects carrier dynamics in QDs.13 It has been, however, very little studied what effect the photodegradation has on the dynamics in the QD−MO system.7 We have previously reported on the transient absorption (TA) dynamics in the CdSe QD−ZnO system during photodegradation.7 The photodegradation was highly nonlinear with respect to the irradiation light intensity, which was accompanied by hugely pronounced QD degradation under the © XXXX American Chemical Society

2. EXPERIMENTAL SECTION Experimental details of the sample preparation and photodegradation experiment are identical to our previous report in ref 7. Therefore, we only briefly summarize the most important parameters. QDs with diameter 2.6 and 3.1 nm were prepared by a reaction of TOP-Se and CdO under a constant temperature (200 and 240 °C, respectively). By the preparation we obtain QDs with a narrow size distribution, which therefore feature the absorption spectrum (recorded on an Agilent 845x spectrometer) consisting of narrow excitonic bands typical for Received: September 12, 2014 Revised: November 4, 2014

A

dx.doi.org/10.1021/jp509242r | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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deposition of QDs on ZnO, the absorption spectrum of QDs (overlapping excitonic bands) is summed up with scattering contribution of ZnO (background signal), which is only minor in the case of ZnO NPs (Figure 1A, black line); however, it is more pronounced for ZnO NWs due to their size comparable to the visible light wavelength (Figure 2A, green line). The TA kinetics were recorded on a femtosecond laser system (∼80 fs pulse duration, 1 kHz repetition rate). Samples of QD−ZnO were excited by a strong pump beam (wavelength of 470 nm, fluence of 760 mW/cm2), which also served as the source of photodegradation. All TA experiments were performed in a low-oxygen atmosphere, which allows us to measure photodegradation with a suitable rate, such that although the degradation takes place, it is not distorting the TA kinetics during the measurement of one trace. The measurements were carried out by using a very weak probe beam (wavelength of 542 nm).

quantum-confined system (see Figure 1A, dashed line, and Figure 2A, blue line).

3. RESULTS AND DISCUSSION 3.1. Steady-State Absorption. We will first focus on effects of photodegradation on the steady-state absorption. It has been shown previously by Tvrdy et al. that a white lamp irradiation (850 mW/cm2) in the atmospheric environment over a long period (tens of minutes) changes significantly the absorption spectrum of the QD−MO system.18 In particular, the irradiation leads to a blue-shift of the QD excitonic bands and smearing out of the bands. We have reproduced the same result (again in ambient atmosphere) by using the irradiation with a femtosecond laser (30 mW/cm2, 1 kHz repetition rate, 80 fs pulses). The femtosecond pulses lead to the similar changes already on the scale of a few minutes, while the irradiation power was more than one order of magnitude lower. This is a consequence of the nonlinear nature of the photodegradation, which we have demonstrated in our previous article.7 As expected, the photodegradation leads to a blue-shift of the QD excitonic band, which indicates changes in the mean QD size.17 We have determined for each absorption spectrum of irradiated samples the lowest absorption band position by using the second derivative of the spectra (see Figure 2B). The band position can be consequently recalculated into the mean QD size according to the work of Yu et al.17 We can observe that the QD size changes in two different stages (see Figure 1C). During the first 3 min, the mean QD size rapidly decreases by approximately 0.12 nm, and it is followed by a slower decrease for the remaining period. As we will discuss later, in conjunction with the transient absorption measurements, this is likely a consequence of the oxidative process on the QD surface. The oxidation creates first a layer of SeO2 on the QD surface (the rapid QD size decrease), which later undergoes the photocorrosion (the slower QD size decrease).15 The change in the QD size should manifest itself also in the magnitude of absorption because the absorption cross section of QDs decreases with decreasing QD size (approximately D3 dependence for CdSe QDs, where D stands for the QD diameter).17 As we observe a decrease of the mean QD size by 5.7% (see Figure 1C), we should also see the accompanying decrease in absorption (sample optical density) by 18%. Experimentally, we can compare absorption traces in a featureless spectral region where we do not observe any bands shift with changing the QD size. At 450 nm we, indeed,

Figure 1. (A) Absorption spectra of CdSe QD system−ZnO NPs under different irradiation times (solid lines) compared to absorption of the original QDs (dashed black line). The samples were irradiated by 470 nm femtosecond pulses (30 mW/cm2, 1 kHz repetition rate, 80 fs pulses) (B) Second derivative of the absorption spectrum after 1 min laser irradiation. The minimum position was obtained by fitting with the Gaussian function (red line). (C) Mean QD size deduced from the lowest absorption band position17 for pure QDs (star) and QD−ZnO system under the laser irradiation (black squares).

Figure 2. (A) Absorption spectra of the QDs studied by TA spectroscopy. (B) Typical TA kinetics of the pure QDs under low excitation intensity (open circles), QDs attached to ZnO before irradiation (filled circles), and QD−ZnO system under a long-term (42 min) irradiation (filled diamonds). All curves are fitted by a multiexponential fit (solid lines, see text for details). (C) Scheme of the measured samples and dominating processes.

Surface capping of the as-prepared QDs was exchanged by 2mercaptopropionic acid (MPA), which serves as a linker maintaining attachment of QDs to ZnO nanowires (500−600 nm long NWs prepared by hydrothermal method) or a layer of ZnO nanoparticles (commercial colloidal NPs, Sigma-Aldrich, 1.5 μm thick layer prepared by doctor blade method). After B



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observe 15% decrease in absorption. Furthermore, the absorption steadily decreases together with irradiation time. The above-described experiment was performed for the QDs attached to ZnO. To check that the sample photostability is caused by the charge separation, we repeated the same experiment for QDs deposited on SiO2. Here the changes in the absorption spectra over the same time range were negligible (within error bars of fitting). The QD−SiO2 and QD−ZnO samples differ in electron dynamics. In the latter case, the QDs are left positively charged over the microsecond time range.7 On the contrary, the pure QDs stay excited only on the nanosecond time scale, and only a small part becomes charged by Auger ionization. Both longer lifetimes of excitation and QD charging contribute to faster photodegradation of QDs in the QD−MO system.5−8 3.2. Transient Absorption Measurements. The photoinduced changes do not only appear in the steady-state absorption. Also, the transient absorption changes in both its amplitude and kinetics, as we have shown previously.7 In the following text we will demonstrate that the changes in steady state and transient absorption can be explained by the same processes. The measurements were carried out by using a strong excitation pulse (λpump = 470 nm), which also served as a source of photodegradation. In order to measure the TA kinetics scan by scan, the photodegradation had to be slowed down. This was achieved by placing the sample in a low-oxygen atmosphere. The kinetics were detected by a very weak probe beam (λprobe = 542 nm). The wavelength of the probe beam was selected to match the spectral maximum of the bleach of the lowest excitonic statesthe dominating component of the TA signal. The bleach is induced by a limited number of electron states in the quantized conduction band of CdSe QDs. Therefore, it is sensitive only to the electron dynamics and can be used to track down the electron recombination, trapping, or injection, i.e., any change in electron population of QD lowest excited states. We will first turn to the electron dynamics in the neat QD (without attachment to MO). Figure 2B (open circles) illustrates a typical TA kinetics for the QDs measured under a low excitation fluence. It features a fast subpicosecond onset of bleach due to electron relaxation to its lowest excited state. It is followed by a long-term recombination on the nanosecond time scale. The kinetics is similar to many previously reported measurements of single-exciton recombination in QD,19−21 and therefore we will not discuss it in more detail. Instead, we will focus on the TA dynamics in the QD−MO system, which is the main subject of the article. The data (filled circles in Figure 2B) represent the TA kinetics of the system before the sample photodegradation and can be fitted by a three-exponential function. This is a result of multiple processes involved. The electron injection from QDs directly attached to MO takes place on the subnanosecond time scale, and it is connected to the first two components.6,19,22 The injection can manifest itself in the TA kinetics as a two-exponential decay due to injection via charge transfer state (CTS) or heterogeneous electron injection, as we showed in our previous article.6 Moreover, a small fraction of the QDs can lack a direct connection to MO, and the electron injection is inhibited or very slow. Those QDs are responsible for the long-lived nanosecond component of the TA decay.23

The TA decay is also affected by the high excitation intensity which can lead to multiexciton effects.16 However, as it is demonstrated in the Supporting Information, conclusions of the article are not sensitive to assignment of the subnanosecond decay components to exact processes. It only assumes that there exist an additional fast process of electron trapping created through photodegradation, which competes with the other processes in the TA decay. The filled diamonds in Figure 2B represent the kinetics of the system after photodegradation. The fast component of the TA decay has become much shorter due to the photodegradation. The long component, however, has stayed almost the same. We consider that the speeding up of the fast component is due to quenching of the excited state by traps generated on QDs by the photodegradation. This conclusion forms the foundation of the following modeling of the data. 3.3. Trap Creation Models. In the following paragraphs we will present three different models of traps’ generation on QDs: (i) trap creation without recovery (TC model), (ii) parallel trap creation and recovery (TCR model), and (iii) trap creation with an upper limit for trap number accommodated on a QD (LT model). QDs require a specific description of the trap states, since a QD represents a tiny nanocrystal, which can have only integer number of traps on its surfaces. It is therefore appropriate to describe QDs as an ensemble of those with no trap p(0), one trap p(1), ..., and n traps p(n). The distribution p(n) will, of course, change with irradiation time τ because the photodegradation will take place. Therefore, in our theory all p(n) will be time-dependent p(n;τ). Note that we use τ for the irradiation time to avoid confusion with the time t used for the time dependence of the TA signal. Trap Formation without Recovery. We start with the simplest model of traps generation, where photoirradiation constantly induces traps with a rate K. The probability p(n;τ) that a QD contains n traps after irradiation for time τ then satisfies the following equations: dp(0; τ ) = −Kp(0; τ ) dτ

(1)

d p(n ; τ ) = Kp(n − 1; τ ) − Kp(n; τ ) dτ (n = 1, 2, 3, ...)

(2)

The first term on right-hand side of eq 2 shows the formation of a trap on a QD with n − 1 traps, which converts it into a QD with n traps, while the second term shows the formation of a trap on a QD with n traps, which converts it into a QD with n + 1 traps. The solution of eqs 1 and 2 obeys a Poisson distribution: p(n; τ ) = ⟨m⟩n

e−⟨m⟩ n!

(3)

with the mean number ⟨m⟩ of traps in a QD proportional to the irradiation time τ, i.e., ⟨m⟩ = Kτ. The fraction of QDs with no trap can be calculated from eq 3 and ⟨m⟩ = Kτ as p(0; τ ) = exp( −Kτ ) = exp(−κs)

(4)

If we denote the time required for one scan by Δτ and use the scan number s as the time variable (τ = sΔτ), the last part of eq 4 is obtained with κ = KΔτ. C



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Trap Creation and Recovery. We can extend the previous model by an additional process of trap recovery featuring rate K′. The distribution p(n;τ) can be then described by the following equations: dp(0; τ ) = −Kp(0; τ ) + K ′p(1; τ ) dτ

(5)

d p(n ; T ) = Kp(n − 1; τ ) − Kp(n; τ ) + (n + 1)K ′ dτ × p(n + 1; τ ) − nK ′p(n; τ )

(n = 1, 2, 3, ...)

(6)

Equations 5 and 6 are a straightforward extension of eqs 1 and 2. The third term on the right-hand side of eq 6 shows the disappearance of a trap on a QD with n + 1 traps, which converts it into a QD with n traps, while the fourth term shows the disappearance of a trap on a QD with n traps, which converts it into a QD with n −1 traps. The solution of eqs 5 and 6 again obeys a Poisson distribution (see eq 3); however, the mean number of traps ⟨m⟩ per QD is now given by ⟨m⟩ = K /K ′[1 − exp( −K ′τ )]

Figure 3. (A) Normalized TA decay for four representative scans (0, 3, 6, 9). (B) Ratio between normalized kinetics of the TA scans and initial (s = 0) scan. The long-lived signal ratio (see eq 18) was obtained as the mean value in time interval between 55 and 150 ps. For longer delays the noise in ratio increases due to the rapid TA signal decay. (C) Number of QDs with no trap (black squares) calculated from eq 18.

Δα(t , s) = f (t , s)Δα(t , 0)

(7)

The original decay Δα(t,0) is different, depending on whether it occurs by electron injection via charge transfer state, heterogeneous injection, or even other processes. However, eq 14 holds in either case, as we discuss in detail in the Supporting Information. From eq 14 one obtains

It is possible as well to derive p(0;τ) from eqs 3 and 7: p(0; τ ) = exp[−κ /κ′(1 − exp( −K ′τ ))] = exp[−κ /κ′(1 − exp(−κ′s))]

(8)

Δα(t , s)/Δα(t , 0) = f (t , s)

Trap Creation with an Upper Limit for the Traps Number. In the first model we assumed that there is no upper limit to the number of traps a QD can accommodate. If there is such upper limiting number z, trap creation has to slow down when the increasing number of traps is approaching the limit. We obtain following set of equations:

The decay f(t,s) due to photoirradiation-generated traps is theoretically calculated based on the distribution p(n,s) of numbers of traps on a QD after s scans f (t ; s ) =

(n = 1, 2, 3, ...)

(10)

We assume that the rate constant for generation of a new trap in a QD with n traps by photoirradiation is given by K[1 − n/ z], where K is the rate for creation of a trap on QD with no trap. The solution of eqs 9 and 10 obeys a binomial distribution ⎛

⎞⎛

⎞

(nz )⎜⎝ ⟨mz⟩ ⎟⎠ ⎜⎝1 − ⟨mz⟩ ⎟⎠

f (t ; s) = p(0, s)

z−n

(11)

for t ≫ 1/kq

Δα(t , s)/Δα(t , 0) = p(0, s)

(17)

for t ≫ 1/kq

(18)

Equation 18 indicates that the observed ratio Δα(t,s)/Δα(t,0) should become constant at long times, and this constant value gives the value of p(0,s). In Figure 3B, we plot the observed ratio Δα(t,s)/Δα(t,0) as a function of delay time t for different values of scan number s. The ratio Δα(t,s)/Δα(t,0) indeed becomes constant at long times. We obtained the value of p(0,s) from this constant value and plotted it as a function of scan number s in Figure 3C. 3.5. Data-Model Comparison and Discussion. We have determined the p(0;s) as a function of the scan number and compared it to the three studied models for trap creation (see Figure 4A). Two models, which do not include any traps recovery, i.e., TC and LT models, imply that the number of

(12)

The fraction of QDs without a trap p(0; τ) can be calculated from eqs 11 and 12 as p(0; τ ) = exp( −Kτ ) = exp(−κs)

(16)

By combining eq 15 with eq 17, we have

with the mean number ⟨m⟩ of traps per QD given by ⟨m⟩ = z[1 − exp( −Kτ /z)]

∑ p(n , s) exp(−nkqt )

This equation expresses that the quenching rate of the excited state by n traps is proportional to the number n of traps and to the quenching rate kq by a single trap. At the same time the contributions from all fractions p(n,s) of QDs with n traps after s scans have to be summed up. Although we do not know a priori the electron trapping rate, it is expected to take place on the picosecond time scale.6,28 In eq 16 the contributions of terms with n ≥ 1 will be close to zero at time scales where time t ≫ 1/kq. Therefore, we have

(9)

⎡ ⎡ dp(n; τ ) n − 1⎤ n⎤ = K ⎢1 − ⎥p(n − 1; τ ) − K ⎢⎣1 − ⎥⎦p(n; τ ) ⎣ dτ z ⎦ z

p( n ; τ ) =

(15)

n

dp(0; τ ) = −Kp(0; τ ) dτ

n

(14)

(13)

where we use again the scan number s as time variable. 3.4. Transient Absorption Decay Analysis. The observed TA signal after s scans (see Figure 3A) can be decomposed into two componentsthe original TA decay before photodegradation Δα(t,0) and the decay due to photoirradiation-generated traps, f(t,s):24−27 D



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Scheme 1. Simplified Scheme of Possible Photoreactions of CdSe QDs Triggered by Oxygen and Water Vapors (Adopted from Ref 15)

Figure 4. (A) Dependence of number of QDs with no trap (p0) on scan number. Data points are fitted by expected behavior of the TC and LT models (red line) and the TCR model (blue line); see text for details. (B) Mean number of traps per QD (symbols) fitted by the expected dependence on scan number (line). (C) Selected TA scans (open circles) compared to the fit by using the TCR model assuming heterogeneous injection (thin solid lines) perfectly overlapping with the CTS-mediated injection (thick solid lines).

the water molecules can only originate from the ambient humidity. Even though the QD surface can be fully oxidized, the SeO2 layer does not inhibit the photodegradation. The oxidized layer can undergo the so-called photocorrosion assisted again by water molecules. In this case the oxidized layer is dissociated into Cd 2+ and SeO 3 2−. The photocorrosion therefore continuously decreases the size of the QD and simultaneously broadens the QD size distribution, as we have observed in the steady-state absorption. As a consequence, the TA signal is highly affectedthe number of QDs with TA signal on the probe wavelength (original absorption maximum) significantly decreases. At the same time the mean number of traps per QD stays low because the photooxidation of the surface tends to recover (oxidize again) the trapping site on the QD surfaces. It is worth stressing here that the information about traps refers only to the electron traps on QDs because the TA measurements on the chosen probe wavelength are insensitive to the hole dynamics. According to our measurements, even the nanosecond component amplitude is reduced due to the photodegradation. This might seem surprising, as the component is connected to the QDs without direct attachment to ZnO (for instance due to agglomeration).23 At the same time the charge separation, which is responsible for the QD charging and fast photodegradation, takes place only on the QD−MO interface. The influence on the nanosecond component can be explained by hole migration within the agglomerated QDs.29−31 The holes left behind after electron injection into ZnO recombine only on the microsecond time scale.7,32 This is a very long period during which the hole can be transferred between different QDs and cause photodegradation in any QDnot only those with the direct attachment. Finally, we will demonstrate that the TCR model can also reproduce all measured TA kinetics. By using the mean number of trap states for each scan, we can calculate the p(n,s), which is the Poisson distribution. Knowledge of p(n,s) and TA kinetics before photodegradation Δα(t,0) (black circles in Figure 4C) allows us to calculate also the expected TA kinetics (eqs 14 and 15), where we need to insert also the parameter of trapping rate. By using the previously published results, where the fast component lifetime before and after photodegradation changes from 8 to about 5 ps, we can estimate the trapping lifetime.7

QDs without any trap p(0;s) exponentially decreases with the scan number (see eqs 4 and 17). On the other hand, the TCR model expects a more complicated behavior because QDs with traps can become again QDs without any trap (see eq 8). We can clearly see in Figure 4A that the trap formation on QD in the photoirradiated CdSe−ZnO system is well described only by the model of parallel trap creation and recovery. Form the knowledge of p(0;s) it is possible to calculate the mean number of traps per QD for the TCR model (see Figure 4B). We observe here that even for a prolonged photodegradation, the mean number of electron traps per QDs stays below 1. This might sound surprising in light of the fact that during the photodegradation the TA signal amplitude decreases by 54% and QDs are, apparently, highly affected. The whole observation can be, however, fully explained by the previously described photochemical reactions observed for CdSe QDs capped by a mercaptocarbonic acid (summarized in ref 15, Figure 2). Especially the very strong dependence of photodegradation rate on oxygen concentration is a strong indirect evidence that the processes triggered by oxygen and water vapor stand behind the photodegradation. Nevertheless, the contribution of other parallel processes to photodegradation cannot be excluded. The photoreactions in our QD−MO system are highly pronounced by charge separation and charging of QDs, as we discussed previously. It is worth noting that the QDs on silica surface under analogous laser irradiation do not undergo any significant change in TA shape and amplitude. In the following paragraph we will summarize the proposed photoreactions, which fully rationalize our results (see Scheme 1). Several processes in this case interplay in the system. Photooxidation of the thiol groups produces disulfide molecules from the linker and can induce new traps on the QD surface. The traps can be passivated again by a photooxidation of the exposed surface, which forms a layer of SeO2 on the QD surface. This process leads at the same time to decrease in the effective size of the affected QD, which is in line with the corresponding effects observed in the steady-state absorption. The trap regeneration can be also partly mediated by photoadsorption of water molecules on the CdSe surface.15 However, as our samples were not kept in water at any stage, E



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We use the fact that rates of two competing processes sum up in the TA decay (1/τTA) = (1/τ1) + (1/τTr), and we obtain that the kq in eq 15 is approximately equal to 1/10 ps−1. The calculated kinetics using this value (thin lines in Figure 4C) are in very good agreement with the measured data. The analysis carried out in the article does not depend on the mechanism of electron injection, as we prove in detail in the Supporting Information. To verify this statement even for this step, we have calculated the expected kinetics for electron injection via charge transfer state. By using a similar trapping rate (1/8 ps−1), we obtain practically identical results (thick solid lines). The final information, which can be extracted from the model, lies in the rate of trap creation and recovery during irradiation. The best fit of our data leads to trap creation and recovery rates of κ = (0.13 ± 0.02) scan−1 and κ′ = (0.22 ± 0.04) scan−1, respectively. The values can be converted into the real time as 0.04 min−1 and 0.07 min−1, respectively (one scan took 3 min). The obtained rates of the trap recovery and creation are very similar. This can be again explained by using the previous knowledge about photoactivated processes in CdSe QDs.15 In this case, the act of charge trap creation (or recovery) is triggered by presence of an oxygen or water molecule close to the excited QD. This criterion is common for both processes, and therefore it also results in the rates of the two processes, which are almost equal.

ASSOCIATED CONTENT

S Supporting Information *

Detailed discussion of the applied theory for different models of electron injection (heterogeneous injection and injection via charge transfer state). This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

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4. CONCLUSIONS We have analyzed changes in the steady-state absorption and transient absorption kinetics in the photoirradiated CdSe QDs on ZnO. Three possible scenarios of trap creation during the photodegradation have been studied and compared to experimentally obtained kinetics of trap creation. We have proved that our data can be described only by model, which takes into account both trap creation and recovery. Our work provides a demonstration of how the photoactivated chemical reactions on the surface of CdSe QD display itself in the transient absorption kinetics. It also provides a very universal and simple tool to analyze the trap creation mechanism in the QD−MO by extrapolating amplitudes of the long-lived transient absorption signal.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (T.P.). *E-mail [email protected] (M.T.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge financial support from the Swedish Energy Agency, the Knut and Alice Wallenberg Foundation, and the Swedish Research Council. Collaboration within nmC@LU is acknowledged. F



Article

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Simultaneous Creation and Recovery of Trap States on Quantum Dots

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