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Blinking Suppression in Highly-Excited CdSe/ZnS Quantum Dots by Electron Transfer under Large Positive Gibbs (Free) Energy Change Elizabeth Mariam Thomas, Sushant Ghimire, Reiko Kohara, Ajith Nair Anil, Ken-ichi Yuyama, Yuta Takano, K George Thomas, and Vasudevan Pillai Biju ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b03010 • Publication Date (Web): 13 Aug 2018 Downloaded from http://pubs.acs.org on August 14, 2018

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Blinking Suppression in Highly-Excited CdSe/ZnS Quantum Dots by Electron Transfer under Large Positive Gibbs (Free) Energy Change Elizabeth Mariam Thomas,1,2,† Sushant Ghimire,1,3,† Reiko Kohara,1,3 Ajith Nair Anil,1,3 Ken-ichi Yuyama,1,3 Yuta Takano,1,3 K. George Thomas,2 Vasudevanpillai Biju1,3,* 1

Research Institute for Electronic Science, Hokkaido University, Sapporo, Hokkaido 001-

0020, Japan; 2School of Chemistry, Indian Institute of Science Education and Research Thiruvananthapuram (IISER-TVM), Thiruvananthapuram 695551, India; 3Graduate School of Environmental Science, Hokkaido University, Sapporo, Hokkaido 060-0810, Japan †

Equal contribution

*Address correspondence to [email protected]

Abstract Semiconductor quantum dots with stable photoluminescence are necessary for next generation optoelectronic and photovoltaic devices. Photoluminescence intensity fluctuations of cadmium and lead chalcogenide quantum dots have been extensively investigated since the first observation of blinking in CdSe nanocrystals in 1996. In a quantum dot, blinking originates from stochastic photo-charging, non-radiative Auger recombination and delayed neutralization. So far, blinking is suppressed by defect passivation, electron transfer, and shell preparation, but without any deep insight into free energy change of electron transfer. We report real-time detection of significant blinking suppression for CdSe/ZnS quantum dots exposed to N, N-dimethylaniline, which is accompanied by a considerable increase in the time-averaged photoluminescence intensity of quantum dots. Although the Gibbs (free) energy change (∆Get = +2.24 eV), which is estimated electrochemically and from DFT calculations, is unfavourable for electron transfer from N, N-dimethylaniline to a quantum dot in the minimally-excited (band-edge) state, electron transfer is obvious when a quantum dot is highly-excited. Nonetheless, ∆Get crosses from the positive to negative scale as the solvent dielectric constant exceeds 5, favouring electron transfer from N, N-dimethylaniline to a quantum dot excited to the band-edge state. Based on single-molecule photoluminescence 1 ACS Paragon Plus Environment

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and ensemble electron transfer studies, we assign blinking suppression to the transfer of an electron from N, N-dimethylaniline to the hot hole state of a quantum dot. Besides blinking suppression by electron transfer, complete removal of blinking is limited by short-living OFF states induced by the negative trion. Keywords: Quantum dots, Electron transfer, Blinking suppression, Photoluminescence, Hot hole, Hot electron, Oxidation

TOC Graphic

Colloidal semiconductor quantum dots (QDs) follow their legacy of most attractive fluorophores, thanks to their size-based tunable and narrow-band photoluminescence (PL), broad absorption band, incomparable photostability, and the ability to activate multiple excitons.1,2 These appealing properties enflamed the concepts of QD-based LEDs,3-5 displays,6-8 lasers,9-12 and high efficiency solar cells.13-15 Moreover, the above properties make QDs the best replacements of conventional fluorescent tags in bioimaging and sensing.16-20 Nevertheless, the Auger-assisted ultrafast non-radiative carrier recombination becomes a dominant relaxation pathway in charged QDs, which is due to momentum conservation, small size and close proximity between charge carriers.21-27 Also, band-edge 2 ACS Paragon Plus Environment

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trapping states and swapping centres of non-radiative recombination introduce low intensity events in the PL intensity trajectories of single QDs.28 Nonetheless, following ultrafast Auger recombination, a single exciton left in a QD may recombine radiatively. These processes not only lower the PL quantum efficiency (QE) of single QD but also induce PL blinking, imposing fundamental flaws in the evolution of single- and multi- photon QD technology.28-32 PL blinking means inconsistent photon emission, which is featured by disordered and stochastic ON, non-luminescent OFF and hazy GREY events.28-32 Because of the far-fetched promises of QDs as optical gain media,9-12,32 bioimaging probes,16-20 and active components of photovoltaic13-15 and electrooptical devices,3-8 studies of blinking and the development of methods for blinking suppression are ongoing challenges. In the blinking process, the dark OFF state is the result of photo-induced charging due to either the resonant tunnelling of an electron to the surface trap state or the Auger autoionization.21-31, 33-35 Recent investigations of blinking and blinking suppression in size-,36-38 shape-39,40 and composition-41-44 controlled graded and giant QDs focus on multiple excitons, charge neutralization, and elimination of surface defects. Multiple excitons are formed in charged QDs on continuous, high intensity/energy excitation, and any excess charge such as in a positive or negative trion results in non-radiative Auger recombination.24-26,45 In a highlyexcited QD, which is at the cost of either multiple photons or a high energy single photon, the excess energy excites a hole deep in the valance band and an electron high in the conduction band, thereby increasing the probability of ionization.46 In trions, the rate of non-radiative recombination is much higher than that of radiative recombination, which is due to strong carrier-carrier interactions, leaving the QD in the OFF state for a long time. After unstipulated waiting, the PL is restored by the reverse quantum tunnelling or thermal activation of the trapped electrons from the surface to the core.

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Although Auger dynamics of ionized or charged QDs have been investigated by different groups,21,22,24-26,28,30,33-48 charging and blinking are unescapable under high energy or high intensity photoactivation. Details of PL blinking are enlightened in the literatures by Marcus and co-workers,49 which is in terms of diffusion-controlled electron transfer, Klimov and coworkers, which is by controlling carrier loss,36 Fisher and Osborne,37 which is in terms of charge tunnelling and self-trapping, Mulvany and co-workers,45 which is in terms of carrier trapping at the band-edge and ionization, and so forth. In QDs, the blinking follows the truncated or distributed power law behaviour in which the average ON and OFF times ramble.31,50-53 In other words, there isn’t any characteristic scale length for an ON or OFF event. Therefore, to see an ionized QD switches from the OFF to ON state, one should wait open-endedly, stalling the applications of QDs to on-demand light sources or multi-photon devices. In addition to the above size-, shell-, shape- and composition- based blinking suppression, several approaches override the undesired blinking of QDs, such as by surface passivation using ligands and polymers,54-59 and interfacing with noble metals,60,61 fullerene (C60),62 TiO2 nanoparticles,63 and ITO glass.64 These approaches comprise elimination of defects and blockage of electron transfer to the surface/shell and or neutralization of ionized QDs. Nevertheless, the blinking disorder disorders from dot to dot within a given sample, from sample to sample for given type of QD, and from one type of a QD to another, attracting new methods of blinking suppression. After the first report on blinking suppression in CdSe/ZnS QDs through surface passivation using β-mercaptoethanol,54 several related reports appeared, involving capping agents such as mercaptopropionic acid,55 oligo(phenylenevinylene),56 trehalose,57 dithiothreitol,58 and amines.65-68 Among these molecules, the roles of amines on PL blinking and QE of QDs are puzzling.65-70 For example, Bullen and Mulvaney showed that the photoactivation of a CdSe QD solution supplemented with primary amines enhances the PL 4 ACS Paragon Plus Environment

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QE.66 El-Sayed and co-workers observed PL quenching for a CdSe QD sample supplemented with n-butylamine.69 Conversely, Kamat and co-workers found that n-butylamine passivates the surface of QD, suppresses non-radiative recombination at the surface vacancies, and enhances the PL QE of CdSe QDs.70 Successively, blinking suppression was observed for CdTe single QDs supplemented with ethylenediamine.67 The proposed roles of these amines are to cancel the surface electron traps formed by Cd, and render long-living ON events and high PL QE to QDs. However, the neutralization of a hot hole in a positive trion by amines but without lowering the PL QE is unexplored. In this study, we find substantial elimination of the long-living OFF states in the PL intensity trajectories of CdSe/ZnS single QDs exposed to high intensity laser and treated with N, N-dimethylaniline (DMA). By analysing single QDs in real-time, we observe suppression of blinking immediately after the addition of DMA to QDs. This blinking suppression enhances the PL intensities of single QDs, whereas the PL QE remains unchanged at the ensemble solution phase in presence of DMA, even after prolonged or high intensity photoactivation. These results enable us to precisely correlate blinking suppression with electron transfer from DMA to the hot hole state of a QD. While the proposed electron transfer mechanism of blinking suppression is valid only in a hot hole-bearing QD, the Gibbs (free) energy change (∆Get = +2.24 eV) of electron transfer, which is estimated from electrochemical experiments and DFT calculations, does not support electron transfer from DMA to a QD excited to the band edge state. Furthermore, the blinking suppression via electron transfer to the hot hole state of QD is well correlated with the HOMO (-5.46 eV) of DMA and the hole states (< -5.39 eV against the vacuum level) of a CdSe/ZnS QD. In other words, electron transfer from DMA becomes significant only to highly-excited QDs, as in single particle measurements. Nonetheless, the free energy change of electron transfer crosses to the negative scale as the solvent dielectric constant exceeds 5.


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RESULTS AND DISCUSSION We demonstrate real-time blinking suppression of single QDs by electron transfer from DMA to the hot hole state of QD, wherein the free energy change of electron transfer is positive for the band-edge state. A uniform tethering of QDs on a glass substrate is a prerequisite for the real-time detection of blinking suppression of the same single QDs with and without DMA. Figure 1 shows the steps involved in the tethering of QDs on a glass substrate. Here, a clean glass coverslip (ca 150 µm thick) functionalized with 3-mercaptopropyl trimethoxysilane provides free thiol functional groups to which ZnS-shelled CdSe QDs were tethered (Figure 1a). Single QDs were uniformly distributed at the microscopic level, by applying a 10 pM

Fig. 1| Single QD sample preparation. (a) Steps involved in the preparation of single QD sample, (b) PL image of single QDs tethered on a glass coverslip and observed in an inverted fluorescence microscope, by exciting with 532 nm laser (80 Wcm-2). The scale bar in (b) is 20 µm. Here, single QDs are immersed in hexadecane.


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QD solution. Details of sample preparation are provided in the methods section. Figure 1b shows PL image of a single QD sample, shined with 532 nm laser, and recorded using an EMCCD camera. To understand blinking suppression by DMA, first, we recorded and examined the PL intensity trajectories of more than 100 single QDs immersed in hexadecane, but without any DMA. Typical PL intensity trajectories of three pristine single QDs and the corresponding PL intensity histograms are shown in Figure 2a. The high intensity of excitation light (80 Wcm-2),

Fig. 2| Dot-by-dot variation and the origin of PL blinking. (a) PL intensity trajectories (left panel) and histograms (right panel) of three representative single CdSe/ZnS QDs tethered on a glass coverslip and immersed in hexadecane. The sample was excited with 532 nm light (80 Wcm-2). (b) Schematic presentation of blinking mechanism involving (i) biexciton in a high intensity photoirradiated QD, (ii) a QD with a hot electron, (iii) a QD with a hot hole, (iv) a negative trion, and (v) a positive trion.


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an indispensable parameter in single particle imaging, induces ON and OFF events in the trajectories, durations of both varied from dot-to-dot. These variations are contributed by difference in the distribution of charge carrier trap states in the core, shell or core-shell interface. By referring to reports on carrier dynamics in highly-excited QDs,21-31, 33-35 we illustrate various excited states of a QD in Figure 2b. Once an electron or a hole is trapped in a defect or if a QD is ionized, the non-radiative relaxation dominates in the overall deactivation pathway of a successively photoactivated QD, providing OFF states in the PL intensity trajectories. Mechanisms of blinking are precisely discussed in a recent report by Mulvany and co-workers.45 By considering electron transfer from DMA to QD in the current work, we discuss blinking mechanism using positive and negative trion states. As shown in Figure 2b, the OFF states can be correlated with the positive [Figure 2b(v)] and negative [Figure 2b(iv)] trions formed respectively from the positively [Figure 2b(iii)] and negatively [Figure 2b(ii)] charged states. The random distribution of ON and OFF durations in PL trajectories is the result of inhomogeneity in the neutralization of an ionized QD. Furthermore, the total number of photons emitted by a QD is determined by the relative distributions and durations of the ON, OFF and GREY events. Because of the dot-by-dot variations of ON and OFF times, the current study focuses on real-time blinking suppression of the same single QDs, by observing the PL intensity trajectories before and after the addition of DMA to QDs. Blinking is sophisticated by the interplay among factors such as trap-state density,45,54 photo-brightening,58 temperature,71 pH,72 and hot carriers.25 Besides these factors, the above dot-by-dot variations of blinking attracted us to real-time blinking suppression by applying DMA, to the same single QD. To understand the role of DMA on blinking suppression, we studied the PL intensity trajectories of the same single CdSe/ZnS QD under the continuous photoirradiation before and after the addition of DMA (Figure 3a). Here, DMA was


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Fig. 3| Blinking suppression of single QDs using DMA. (a) PL intensity trajectory of a single QD tethered on a glass substrate and immersed in hexadecane before and after addition of a DMA (10 mM in hexadecane) solution, and (b,e) the corresponding PL intensity histograms (b) before and (e) after addition of DMA. (c,d,f,g) ON and OFF time distributions for 100 single QDs (c,f) before and (d,g) after addition of DMA. (h) PL intensity trajectory of a single QD immersed in a DMA solution (10 mM in hexadecane). All the samples were excited with 532 nm cw laser (80 Wcm-2).

introduced at 150 s after beginning of PL intensity measurements. The corresponding PL intensity histograms that illustrate the total PL intensity of ON and OFF events, before and


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after addition of DMA, are shown in Figure 3b and e. Interestingly, after supplementing QDs with DMA, the occurrence of low intensity events (OFF) dramatically decreased and the time averaged total PL intensity of single QDs increased by a factor of 2.3. Indeed, the increase in intensity is due to disappearance of long-living OFF events, which does not mean an increase in the ensemble level PL QE (Φ f =0.79) in presence of DMA. In other words, under high intensity photoirradiation, the time averaged PL intensities of pristine QDs decrease due to long OFF events, which is partially recovered by DMA. To verify blinking suppression and enhanced PL intensity, we further plotted the ON and OFF time distributions in a log-log scale for 100 single QDs, both before and after addition of DMA (Figure 3c,d,f,g). With and without DMA, the OFF-time distribution of single QDs closely follows the power law behaviour, whereas the ON-time distribution shows a neartruncated behaviour. These observations are consistent with previous observations by pioneers in the QD subject.50-53 Interestingly, in presence of DMA, the ON time increased by an order of magnitude and the OFF time decreased by an order of magnitude, wiping out OFF durations exceeding a few seconds. For pristine QDs, the non-exponential recovery from OFF state to ON state is attributed to either the de-trapping of electrons from exponentially distributed trap states or the hopping transport from the core, surface, or interface, involving randomly distributed states.50,73 On the other hand, the deviation of ON time distribution from the power law statistics is explained on the basis of diffusion controlled mechanism and biexciton dynamics.30, 51-53, In presence of DMA, the durations of ON events dominate over the OFF events, which is attributed to electron transfer from DMA to a QD with a hot hole [Figure 2b(iii)] or a positive trion with a hot hole, which is experimentally clarified below within the frames of Marcus theory of electron transfer. Before clarification of electron transfer from DMA to the hot hole state, we consider possibilities such as electron transfer to the minimally-excited state (band-edge, single 10 ACS Paragon Plus Environment

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exciton), highly-excited non-ionized state, or the positive trion state. Among these three possibilities, first, we examined electron transfer from DMA to QD in the minimally-excited state, which is by estimating the free energy change of electron transfer in polar and nonpolar solvents and correlating the free energy change with ensemble PL QE and PL lifetime of QDs in presence of DMA. The Gibbs (free) energy (∆ ) change of electron transfer is given by the following equation.74

∆ = − + − +

1 1 2 1 1 + − − − 1, 2

where E00 is the zero-zero transition energy for CdSe/ZnS QD, EOX is the oxidation potential of the donor (DMA), ERED is the reduction potential of the acceptor (QD), rA and rD are respectively the effective radii of reduced acceptor (QD) and oxidized donor (DMA), RCC is the centre to centre distance between the ions, is the dielectric constant of the solvent in which electron transfer is investigated, and is the dielectric constant of the solvent in which EOX and ERED are electrochemically estimated. We estimated the E00 value for CdSe/ZnS QD at 1.89 eV, which is from the emission spectrum (Figure 4c), EOX and ERED values respectively for DMA and QD at 0.19 eV and -1.15 eV, which is from cyclic voltammetry and differential pulse voltammetry (DPV) experiments (Figure 4a,b) in dichloromethane ( = 8.93, rA at 2.38 nm, which is from Image j analysis of TEM images of over 200 QDs, and rD at 0.4 nm, which is from DFT calculations. Based on equation 1 and the above parameters, the ∆G0 value is estimated at +2.24 eV for electron transfer from DMA to QD in hexadecane ( = 2.049). Interestingly, from calculations, the above free energy change crosses to the negative scale upon increasing the solvent dielectric constant beyond 5. In other words, based on ∆G0 values, we rule out electron transfer from DMA to the minimally-excited state of QD in less polar solvents such as hexane ( = 1.89), cyclohexane ( = 2.02, hexadecane ( = 2.049), carbon 11 ACS Paragon Plus Environment

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Fig. 4| Redox potentials of electron acceptor and donor and the PL properties of QDs. (a,b) DPVs of (a) a solution of QD (100 nM) in dichloromethane, and (b) a solution of DMA (10 mM) in dichloromethane. The electrolyte is a solution (0.1 M) of tetrabutylammonium hexafluorophosphate (TBAPF6) in dichloromethane. (c) PL spectra of a QD solution (10 nM in hexadecane), with and without DMA. The concentration of DMA was increased from 0 to 15 mM at 1.5 mM steps. (d) PL decay profiles of a QD solution (10 nM in hexadecane), with (15 mM) and without DMA. (e) A minimally-excited QD-DMA system in which electron transfer from DMA to QD is unfavourable.

tetrachloride ( = 2.24), 1,4-dioxane ( = 2.25), benzene ( = 2.27), xylenes ( = 2.27~2.57), toluene ( = 2.38), diethyl ether ( = 4.27 and chloroform ( = 4.81). To examine any electron transfer from DMA to minimally-excited QDs, we recorded the ensemble PL spectra of a QD solution in presence of different concentrations of DMA. As seen in Figure 4c, the PL intensity of a QD solution remained essentially unchanged after addition of up to 15 mM DMA. During ensemble spectral measurements, the excitation light 12 ACS Paragon Plus Environment

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(532 nm) intensity was kept at 6 µWcm-2, which is far below the biexciton threshold of CdSe/ZnS QD. In other words, hot carriers or trion state was absent during PL spectral measurements. The negligibly small (10 min) recorded the PL intensity trajectories of single QDs by systematically increasing and decreasing the excitation power. The PL intensity trajectories recorded for the same single QD irradiated with 30 to 150 Wcm-2 532 nm laser in the ascending and descending power orders are shown in Figure 6a,b. Here, after every 60 s, the laser power was increased


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Fig. 6| Excitation power-dependent PL of QDs. (a,b) PL intensity trajectories of a single QD in presence of 10 mM DMA solution (in hexadecane), under (a) increasing and (b) decreasing laser power. The QD was excited with 532 nm cw laser, and after every 60s, the power was increased or decreased at 30 Wcm-2 steps in the 30 to 150 Wcm-2 range. (c) PL decay profiles of a QD solution (10 nM) in hexadecane at low (350 mW cm-2, black trace) and high (150 Wcm-2, red trace) intensity excitation conditions.

during the first 300 s and then decreased during the next 300 s. As expected, the rate of ionization of a QD increased with increase in the excitation power. In other words, the ON time decreased, and the number and durations of OFF events increased with increase in power, which is consistent with the generalized blinking mechanism shown in Figure 2b. As seen in Figure 6b (during 540 to 600 s), the ON time decreased, and the number of OFF events increased even after the excitation intensity was returned to the original level. In other words, persistent blinking enhancement and decrease of average PL intensity are observed for QDs with (Figure 6b) or without DMA after prolonged exposure to high intensity light. The persistence of enhanced blinking even after the excitation intensity was decreased is attributed to photoinduced formation of charge carrier traps, an effect that is opposite to the photoinduced PL intensity enhancement. However, suppressed PL blinking continued for 10 min or more for single QDs exposed to DMA (10 mM solution in hexadecane) and moderate intensity (80 Wcm-2 or less) excitation light (532 nm). 16 ACS Paragon Plus Environment

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To understand the role of DMA on blinking suppression, we examined the PL intensity trajectories of single QDs after removing DMA. At first, PL intensity trajectories of single QDs immersed in a DMA solution (10 mM in hexadecane) were recorded for 300 s, during which blinking was suppressed. Successively, DMA was removed by washing the sample with hexadecane, which was followed by recording PL intensity trajectories of single QDs in hexadecane. Interestingly, after removal of DMA, long-living OFF events reappeared in the trajectories. This result suggests that DMA does not cause any permanent change to QDs. Further, as the excitation intensity was increased to 150 Wcm-2, the PL lifetime of a pristine QD sample decreased from 15.2 ns to 9.5 ns, which is associated with a fast component in the decay profile (Figure 6c). The decreased PL lifetime of QD and the fast decay component under high intensity excitation are attributed to an increase in the rate of non-radiative Auger recombination. Interestingly, under high intensity excitation, the PL lifetime of a QD sample supplemented with DMA was longer (12.5 ns) than that of a QD only sample. These results support electron transfer from DMA to highly-excited QDs, which is correlated with blinking suppression.

Conclusion While the large positive free energy change of electron transfer helps us to rule out electron transfer from DMA to a minimally-excited QD, DMA suppresses blinking of single QDs by acting as an electron donor to a QD with a hot hole. Our hypothesis about excited state dynamics and electron transfer processes in a QD excited with photons of energy close to the band-gap energy and supplemented with DMA are summarized in Figure 7. First, a state with a hot hole (ii), a hot electron (iv), or both a hot hole and a hot electron (v) can be formed from the biexciton state (i) generated under high intensity excitation. Following further absorption


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CB

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CB

(iv)

CB (ii)

(i) VB

VB

VB

-5.46 eV HOMO DMA

QD Negative trion (Short OFF states)

Neutralization (Blinking suppression)

CB

CB (vi)

-3.06 eV

(v)

(iii) VB

VB HOMO DMA

-5.39 eV HOMO DMA

CB ∆G0 = +2.24 eV VB -5.46 eV HOMO DMA

Fig. 7| Mechanism of blinking suppression in a highly excited QD. (i) a biexciton state, (ii) a hot hole state that accepts an electron from DMA, (iii) a positive trion that accepts an electron from DMA, (iv) a hot electron state, (v) a QD with a hot electron and a hot hole, accepting electron from DMA, and (vi) a negative trion.

of a photon, a negative trion can be formed from the state (iv) and a positive trion can be formed from the state (ii). A negative trion (vi) can also be formed as a result of electron transfer from DMA to the hot hole state (v) and the successive absorption of a photon. By considering the general mechanism shown in Figure 2b, blinking is caused by a positive and a negative trion. Therefore, by correlating Figure 2 with Figure 7, blinking suppression becomes effective by neutralization of the ionized states (ii), (iii), (iv) and (vi). In the current work, blinking is suppressed by electron transfer from DMA to the hot hole states (ii) and (iii). The absence of long-living OFF states (Figure 3a, h) in presence of DMA suggests that such states were contributed by positive trions. This blinking suppression is valid when the Gibbs (free) energy change of electron transfer is positive in less polar solvents such as toluene, hexadecane, cyclohexane, chloroform and diethyl ether, for which the dielectric constant is below 5. In solvents with dielectric constant exceeding 5, the free energy change


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becomes negative, enabling electron transfer from DMA to the band-edge state of a QD and lowering the PL QE. Nonetheless, complete removal of blinking needs neutralization of the negatively charged states (iv) and (vi) as well, which is not accomplished with the electron donor DMA. The short-living OFF states remained in the PL intensity trajectories (Figure 3a, h) even after the addition of DMA should be due to negative trions. Therefore, for complete blinking suppression, a QD needs an electron donor (D) and an acceptor (A), which neutralize both the negatively and positively charged QDs, but without quenching the PL of QDs or forming an electron donor-acceptor (D-A) pair between them.

Methods We obtained CdSe/ZnS QDs (PL maximum~655nm) from Invitrogen Corporation Inc., DMA, hexadecane and dichloromethane (DCM) from Tokyo Chemical Industry. Single-molecule samples were prepared by tethering QDs to coverslips, as shown in Figure 1. Coverslips were cleaned by washing successively with water and acetone, which was followed by soaking of coverslips in a solution of 3-mercaptopropyl trimethoxysilane (0.2 % v/v) in acetone for 30 minutes at room temperature. In this step, the silanol groups on the surface of coverslips reacted with the silane, providing us with thiol functional groups. Subsequently, 10 pM solution of CdSe/ZnS QDs in hexadecane was placed uniformly on one side of the silanized coverslip for 30 min at room temperature, which resulted in the tethering of QDs to glass surface via disulphide bond formation. After tethering QDs to coverslips, the samples were thoroughly washed successively with toluene and hexane, which was under ultrasonication for removing any QD non-specifically bound to the sample surface. Electrochemical measurements were performed under Argon atmosphere, with BAS CV50W electrochemical analyser using graphite working electrode, platinum wire counter electrode, and Ag/AgCl reference electrode. The working electrode, reference electrode, and 19 ACS Paragon Plus Environment

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counter electrode were connected to the device and immersed in a sample solution mixed with the supporting electrolyte tetrabutylammonium hexafluorophosphate (TBAPF6). Dry DCM was used to dissolve TBAPF6, DMA, and QD. All potentials are referenced to the ferrocene/ferrocenium couple (Fc/Fc+) as the internal standard. DPVs were obtained at the pulse amplitude of 50 mV, pulse width of 50 ms, a pulse period of 200 ms and a scan rate of 20 mV/s. All solutions were deaerated for 10 min by nitrogen gas purging prior to the electrochemical measurements. The geometries of DMA were optimized using the Gaussian 09 (Revision D.01) program76 with the B3LYP77 function. The 6-311++G(d,f)78 basis set was used with diffuse functions on all atoms. Energy levels were calculated at the same levels of the theory. Single-molecule images and trajectories were recorded in an inverted optical microscope (Olympus IX 70) that was equipped with a 40 X objective lens (Olympus, NA 40) and an iXon3 EMCCD camera (Andor Technology). The excitation light source used for single QD imaging was a 532 nm cw laser (Millennia IIs, Spectra-Physics). All single-molecule measurements were carried out continuously in hexadecane, with or without DMA. The bin time is 30 ms for all trajectories. The thicknesses of hexadecane and DMA solution (in hexadecane) layers were kept constant for avoiding any changes in excitation intensity during acquisition of images and trajectories. In single-molecule experiments, the concentration of DMA solution was varied from 1 nM to 100 mM. Blinking suppression was observed in presence of 1 to 15 mM DMA solution. Among these concentrations, 10 mM is found optimum for stable PL for 10 min or more under continuous illumination of QDs. The excitation source used for lifetime measurements was 400 nm (150 fs) pulses generated from the SHG crystal of an optical parametric amplifier (Coherent OPA 9400). The OPA was pumped at 200 kHz by a regenerative amplifier (Coherent RegA 9000) seeded from


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a mode-locked Ti:Sapphire laser (Coherent Mira 900F). The fluorescence lifetime system in this research is an assembly of a polychromator (Chromex, model 250IS) and a photoncounting streak camera (Hamamatsu, model C4334). The fluorescence signals from samples were filtered through a 580 nm long-pass filter, focused at the entrance slit of the polychromator, and detected using the streak camera. Fluorescence decays were analyzed by fitting with the third order equation, and the average fluorescence lifetime was calculated. The absorption and emission spectra were recorded in a UV/Visible spectrophotometer (Evolution 220, ThermoFisher Scientific) and a Fluorescence spectrophotometer (Hitachi FL4500), respectively. The ensemble experiments were carried out using 10 nM QD solutions in hexadecane by the successive addition of DMA at 1.5 mM intervals. Acknowledgements This work was carried out in Hokkaido University under the support (to V. B.) of JSPS Grant-in-Aid for Scientific Research on Innovative Areas (Photosynergetics program Grant 17H01099). E. M. T. is grateful to CSIR for a Research Fellowship. S. G. acknowledges a MEXT fellowship for Graduate Research. Also, this work was supported by MEXT Dynamic Alliance for Open Innovation Bridging Human, Environment and Materials. Author contributions V. B. proposed and guided the research. E. M. T. and S. G. conducted single-molecule experiments. R. K., K. Y. and A. N. A. conducted ensemble PL measurements. E. M. T., R. K. and Y. T. conducted electrochemical measurements and DFT calculations. All authors helped to analyze the data and draft the manuscript.

Competing Interests The authors declare no competing interests. 21 ACS Paragon Plus Environment

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