Long-Lived Negative Photocharging in Colloidal CdSe Quantum Dots

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Surfaces, Interfaces, and Catalysis; Physical Properties of Nanomaterials and Materials

Long-Lived Negative Photocharging in Colloidal CdSe Quantum Dots Revealed by Coherent Electron Spin Precession Rongrong Hu, Zhen Wu, Yuanyuan Zhang, Dmitri R. Yakovlev, Pan Liang, Gang Qiang, Jiaxing Guo, Tianqing Jia, Zhenrong Sun, Manfred Bayer, and Donghai Feng J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.9b02341 • Publication Date (Web): 13 Aug 2019 Downloaded from pubs.acs.org on August 13, 2019

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The Journal of Physical Chemistry Letters

Long-Lived Negative Photocharging in Colloidal CdSe Quantum Dots Revealed by Coherent Electron Spin Precession Rongrong Hu,† Zhen Wu,† Yuanyuan Zhang,† Dmitri R. Yakovlev, ‡,§ Pan Liang,† Gang Qiang,‡ Jiaxing Guo,† Tianqing Jia,† Zhenrong Sun,† Manfred Bayer,‡,§ and Donghai Feng*,†,∥ †

State Key Laboratory of Precision Spectroscopy, East China Normal University,

Shanghai 200062, China ‡

Experimentelle Physik 2, Technische Universität Dortmund, 44221 Dortmund,

Germany § Ioffe

Institute, Russian Academy of Sciences, 194021 St. Petersburg, Russia

Collaborative Innovation Center of Extreme Optics, Shanxi University, Shanxi 030006, China ∥

Corresponding Author *E-mail: [email protected].

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ACS Paragon Plus Environment


ABSTRACT: Photo-induced charging in CdSe colloidal quantum dots (QDs) is investigated by time-resolved pump−probe spectroscopy that is sensitive to electron spin polarization. This technique monitors the coherent spin dynamics of optically oriented electrons precessing around an external magnetic field. By addition of 1-octanethiol to the CdSe QD solution in toluene, an extremely long-lived negative photocharging is detected which lives up to one month in N2 atmosphere and hours in air atmosphere at room temperature. 1-octanethiol not only acts as a hole acceptor, but also results in a reduction of the oxygen-induced photo-oxidation in CdSe QDs, allowing an air-stable negative photocharging. Two types of negative photocharging states with different spin precession frequencies and very different lifetimes are identified. These findings have important implications for understanding the photophysical processes in colloidal nanostructures. KEYWORDS: Photocharging, electron spin, pump−probe, hole acceptor, colloidal quantum dots TOC GRAPHIC: 500 400 300

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Photo-induced charge separation is a common phenomenon in colloidal quantum dots (QDs) with important consequences for photocatalytic and photovoltaic applications.1−5 Charge separation relies on extracting one carrier of a photo-generated electron−hole pair from the QD to the QD surface or surrounding. Electron or hole acceptors are often intentionally introduced in order to efficiently extract electrons or holes from the QD core.6 Based on charge separation, photocharging or photodoping methods lead to either negative or positive charging of the QDs.7−13 Stably charged QDs are promising for the realization of zero-threshold optical gain,14 and provide an important platform for emerging quantum technologies.15 Controlled QD single charging facilitates longer electron spin lifetimes, which otherwise are limited by the strong electron−hole exchange interaction in neutral colloidal QDs.16,17 The resident electron in negatively charged QDs can easily be removed by photo-oxidation in air exposure. Therefore, anaerobic conditions are typically used in order to inhibit the photo-oxidation and to obtain long-lived negative photocharging (NPC) states. In solutions of CdSe colloidal QDs, stable NPC at room-temperature has recently been anaerobically prepared with assistance of a reducing hole quencher

Li  Et 3 BH  8 with a lifetime up to hours, limited by spontaneous electron trapping.18 Stable NPC states can be identified by the absorption bleaching at the bandedge. Electron spin coherence measurements represent a powerful tool to distinguish charged QDs from their neutral counterparts. The technique exploits the fact that the electron spin 3


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signals are distinctly different between neutral and singly charged QDs.19 In neutral QDs, the photoexcited excitons have strong electron−hole exchange interactions, making the spin of all carriers (exciton, electron and hole) relax very fast (e.g., below 1 ps at room temperature).16,17 Once the QD is singly charged with an electron or a hole, the electron−hole exchange interaction vanishes, leading to a much longer electron spin lifetime which is as long as hundreds of ps or even a few ns.19,20 Different types of charging can be distinguished by measuring the electron Larmor precession frequency in a transverse magnetic field.20,21 In this paper, we reveal the photocharging evolution in a toluene solution of CdSe colloidal QDs through electron spin coherence detection. By addition of hole acceptor 1-octanethiol (OT) organic molecules, an extremely long-lived NPC is observed, which persists over one month in N2 atmosphere. Even in air atmosphere, it lives up to several hours, indicating an efficient reduction of the oxygen-induced photo-oxidation. Analysis of the spin precession frequency indicates the existence of two types of conduction electrons in the NPC states, i.e., two types of NPC states. Colloidal CdSe QDs in toluene with octadecylamine stabilizing ligands were commercially obtained from Hangzhou Najing Technology Co., Ltd. Toluene solutions of CdSe QDs and OT were mixed in an airtight cuvette with varying molar ratios of OT to QD (ROT). The CdSe QD diameter is ~2.4 nm, as estimated from the wavelength of the first exciton absorption peak at 508 nm for the as-grown QDs22 (Figure S1a). After 4


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adding OT, the absorption peak is redshifted up to 5 nm for high OT concentrations, and the photoluminescence is quenched (Figure S1). Magnet

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Figure 1. Prepump−pump−probe measurements of CdSe QDs (diameter 2.4 nm) with OT. (a) Experimental configuration. (b) Sketch giving sample information, including the cuvette dimensions and the QD solution as well as the laser spot sizes. The prepump spot diameter is ~10 mm, much larger than the pump and probe spots. The charging detection position is ~15 mm away from the solution/air interface. (c) Time-resolved ellipticity signals for different prepump conditions. The sample is prepared under air atmosphere. ROT = 700. The black arrows denote the start point of time-counting the prepump on or off. The prepump is a broadband femtosecond laser with the central wavelength of 515 nm and the fluence of 10 µJ/cm2. B = 0.43 T. (d) FFT spectra of signals in panel c. Some of the plots are offset vertically for better distinction of the two observed frequencies.

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Coherent Electron Spin Dynamics. The electron spin coherence measurements are carried out using a three- or two-beam pump−probe setup. Figure 1a shows the three-beam prepump−pump−probe experimental configuration. The circularly-polarized pump pulses generate an electron spin polarization, which subsequent dynamics is monitored by the changes in the ellipticity of the linearly-polarized probe pulses. The linearly polarized prepump pulses are optionally used for generating photocharging in the QDs. More experimental details can be found in the Supporting Information.

The sample information is sketched in Figure 1b. Spin sensitive measurements are used to identify the charging states in the spatial overlap areas of the pump and probe beams (below we call this area the charging detection position). Time-resolved ellipticity signals for different prepump illumination times are shown in Figure 1c. The sample has a molar ratio of OT to QD of ROT = 700 and is prepared under air atmosphere. The charging detection position is ~15 mm away from the solution/air interface. The oscillating ellipticity signals in Figure 1c arise from the Larmor precession of electron spins in a transverse magnetic field of B = 0.43 T. Before any prepump illumination, the spin signal amplitude is very weak (see the black curve at the bottom). After switching on the prepump, the spin signal increases remarkably. Figure 1d gives the fast Fourier transform (FFT) spectra of the spin dynamics. Upon prepump illumination, the spin signal is strong, showing a prominent spin precession frequency with a peak at v1 = 9.48 GHz. In comparison, the spin signal is rather weak before prepump illumination with a 6


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spin precession frequency of v2 = 10.73 GHz. From these frequencies, the two g factor values of g1  1.57 and g 2  1.78 are evaluated from the equation g  hv / (  B B) , where h , v and  B are the Planck constant, the Larmor precession frequency, and the Bohr magneton, respectively. The exciton and hole spins relax too fast at room temperature to be resolved in our measurements.21 This allows us to exclude the neutral exciton and the hole as origin of the two spin signals. Therefore, we attribute both spin components to the electron spin precession in photocharged QDs. A detailed study on the origin of the two Larmor frequencies in the coherent spin dynamics of colloidal CdSe QDs can be found in ref 21. The electron spin signal can originate from either negatively or positively photocharged QDs, but analysis of the spin dephasing time helps us to determine the type of photocharging. In negatively charged QDs, the electron spin is polarized via the optical excitation of a negative trion (X-). The coherent spin signal is contributed by the resident electron, i.e. its lifetime is not limited by the X- recombination (Figure S2). In comparison, in the positively charged QDs, the electron spin signal comes from the photogenerated electron of the positively charged trion (X+) (Figure S3). In this case, the lifetime of the X+ complex sets an upper limit to the spin lifetime. In small CdSe QDs with a diameter of 2.4 nm, X+ decays fast with a lifetime below 50 ps due to nonradiative Auger recombination.23 The electron spin signal shown in Figure 1c has a spin dephasing time significantly longer than 50 ps. The spin dephasing time T2* of the v1 and v2 7



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components are 730 and 250 ps, respectively (Figure S4). Therefore, both spin components can be assigned to negatively photocharged QDs, which is consistent with the hole accepting nature of the OT molecules.24 The two spin components with different precession frequencies indicate that there are two types of NPC in the CdSe QDs with OT molecules, which originate from two types of conduction electrons in the QD core.21 Note that without OT, the spin signal in as-grown CdSe QDs is very weak and cannot be detected both with and without the prepump illumination (Figure S5).

As shown in Figure 1c, before the prepump illumination, we only observe the second type of NPC (NPC2), which is corresponding to the Larmor precession frequency v2 = 10.73 GHz, even with a continuous irradiation of the pump and probe light up to 1 hour (Figure S6). However, switching on the prepump for 10 min leads to the appearance of the first type of NPC (NPC1, v1 = 9.48 GHz), while the spin signal related to NPC2 is not observable anymore. After 15 min illumination and switching off the prepump, the NPC1 spin signal is still strong, e.g., for switch-off times up to 10 min as shown by the topmost curve in Figure 1c. This shows that the NPC1 state lives more than 10 min in the solution of CdSe QDs prepared under air atmosphere.

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Figure 2. Influence of the charging detection position on the negative photocharging related to v1 = 9.48 GHz. Blue, magenta and black data sets correspond to the charging detecton position being ~5, 15 and 25 mm away from the solution/air interface, respectively. The sample is prepared under air atmosphere with a volume of 300 μL. ROT = 700. The prepump illumination time is 10 min. The prepump is a broadband femtosecond laser with the central wavelength of 515nm and the fluence of 10 µJ/cm2.

Influence of Charging Detection Position. For the sample prepared under air atmosphere, the detection position relative to the solution/air interface in the spin measurements has a strong influence on the NPC1 lifetime. As shown in Figure 2, for a sample with a volume of 300 μL with ROT = 700, the NPC1 lives more than 45 min when the detection position is near the bottom of the cuvette (~25 mm away from the solution/air interface). When the detection position is shifted upward to be closer to the solution/air interface, the NPC1 lifetime becomes shorter. It is less than 20 min for the ~15 mm detection position. For the ~5 mm position, the NPC1 state is not observed. 9


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Figure 3. Comparison of prepump−pump−probe measurements in air and N2 atmosphere. The volume of two samples is 100 μL. ROT = 700. The charging detection position is ~5 mm away from the solution surface. The black arrows denote the start point for time-counting the prepump on or off. The prepump is a broadband femtosecond laser with the central wavelength of 515 nm and the fluence of 10 µJ/cm2. B = 0.43 T. (a) Spin signals for the sample prepared under air atmosphere. (b) FFT of signals in panel a. (c) Spin signals for the sample prepared under N2 atmosphere. (d) FFT of signals in panel c. Influence of Oxygen. Comparative measurements are performed for two samples prepared under either air or N2 atmospheres. The volume of both samples is 100 μL and ROT = 700. The detection position is ~5 mm away from the solution surface. The spin signals in air atmosphere in the prepump−pump−probe measurements are shown in 10


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Figure 3a and the corresponding FFT spectra are given in Figure 3b. The spin signal amplitude increases twice when the prepump is turned on, and it keeps the same Larmor precession frequency of v2 = 10.73 GHz corresponding to the NPC2 state. Once switching off the prepump after 10 min prepump illumination, the spin signal recovers immediately and becomes the same as before any prepump illumination. This reveals that the lifetime of NPC2 is short and cannot be evaluated in our measurements with a time resolution of about one minute. For the sample prepared under N2 atmosphere, the prepump−pump−probe measurements show very different appearances. Firstly, the frequency changes from 10.73 GHz to 9.48 GHz, evidencing that the photocharging state is changed from NPC2 to NPC1. Secondly, after 10 min illumination and switching off the prepump, the NPC1 lives longer than 100 min. Molar Ratio Dependence. The lifetime of NPC1 strongly depends on the OT concentration. To that end, a set of CdSe QD solutions with different OT concentrations ROT = 7, 70, 700, 7000 and 30000 were prepared under air atmosphere. Their NPC1 lifetime as a function of OT concentration is shown in Figure 4. For ROT = 7, the NPC1 lifetime is equal to zero, which means that no NPC1 spin signal is observable. For ROT = 70 to 30000, the NPC1 spin signal appears, and its lifetime increases with OT concentration. For ROT = 30000, the NPC1 lifetime reaches a few hours despite the sample preparation under air atmosphere.

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Intrinsic Lifetime of NPC1. To address this problem, a 400 μL CdSe QD solution was prepared under N2 atmosphere with ROT = 30000 to suppress the oxygen effects. Also, the diffusion of QDs in solution should be taken into account for evaluating the long lifetime of photocharging. We measured that the diffusion of QDs across a distance of ~4 mm takes several tens of minutes (Figure S7), which is significantly shorter than the intrinsic lifetime of NPC1. In order to reduce the QD diffusion effect, a 473 nm CW laser (with a stronger power than the pulsed laser) is used for illumination and the laser spot size is beam-expanded by a combination of cylindrical and concave lenses for illuminating the 12


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whole volume of the QD solution. Furthermore, the sample is kept in the dark and only irradiated by the pump and probe light used for the spin measurements. Under this condition, the lifetime of the NPC1 state is extremely long. As shown in Figure 5, after an initial decay during 2 days, it turns over to a much slower relaxation and 25% of the NPC1 signal survives over 30 days. Absorbance (a.u.)

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The OT molecules are known to act as hole acceptors for CdSe QDs,24 which leads to the QD negatively photocharged. For the NPC1 states, electrons are lastingly resident in the conduction band of the QD core, which can be further confirmed by steady-state absorption spectroscopy. After photocharging, the absorption shows a photobleaching of the bandedge absorption, as shown in the inset of Figure 5, which is the typical 13



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characteristic for stable NPC in CdSe colloidal QDs.8,25 In comparison, a hole occupation in the valence band does not result in absorption bleaching in CdSe or CdS colloidal QDs.26−30 The fractional bleach at the first excitonic absorption maximum is ~0.086 in our case. Therefore, the average number of resident electrons in each QD

n  2( A0  A) / A0 =0.17 , where A0 and A are the excitonic absorption before and after photocharging. This means that ~17% of the QDs are singly charged for the conditions used in Figure 5. The origin of the two Larmor frequencies (v1 and v2) in the coherent spin dynamics of colloidal CdSe QDs was investigated in ref 21. In short, the v1 frequency is associated with an electron confined in the QD center, and the v2 frequency arises from an electron additionally localized in the vicinity of QD surface. The present study shows the existence of two types of NPC states, further validating the existence of two types of conduction electrons. Note that the commonly used method of absorption bleaching is not possible to distinguish such two types of electrons, which detects only presence of electron in the QD and cannot distinguish details of its localization. It is known that in the presence of oxygen, the conduction electron in negatively charged QDs is efficiently removed by the oxygen adsorbed in the solution.8,9,31−34 As shown in Figure 3, oxygen can effectively remove the first type of conduction electrons (v1 = 9.48 GHz), but not the second type (v2 = 10.73 GHz). The reason is that the formation time is long for the NPC1 state, while it is short for the NPC2 state. The NPC1 14


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formation time is in the one-minute order of magnitude (see, e.g., Figure 1c). If the NPC1 formation time is longer than its decay time due to the oxygen-induced photo-oxidation and/or the QD diffusion, NPC1 spin signal will not appear. This is also the reason why only the focused pump and probe beams do not show signatures of the NPC1 state as the NPC1 formation is slower than the QD-diffusion-induced decay. In comparison, the formation of the NPC2 state relies on fast hole trapping during a time, which is shorter than the pump pulse duration or the exciton spin relaxation time given by the electron−hole exchange interaction.21 Thus, the NPC2 formation is not affected by oxygen. The anaerobic conditions prevent photo-oxidation and are effective to maintain a long-lived NPC1 state. In the present study, even under air atmosphere, enough OT molecules make the NPC1 states stable up to several hours, because the thiol hole-accepting ligands in the solution help to reduce the photo-oxidation of CdSe QDs.35,36 As demonstrated in Figure 2, the NPC1 charging lifetime is sensitive to the detection position relative to the solution/air interface, implying that the oxygen influence varies at different positions. To the best of our knowledge, this position dependent phenomenon has not been reported previously, and its origin is unclear and needs further investigation.

In summary, we have demonstrated the existence of two types of NPC states in CdSe colloidal QDs with hole acceptor ligands using electron spin coherence measurements. We have measured the photocharging evolution dynamics and the influence of detection 15



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position, oxygen atmosphere and hole acceptor concentration. The observed extremely long-lived charge separated states, are potentially useful for applications in low-threshold lasing,14 photocatalysis and spintronics. The existence of different types of NPC and analysis of their evolution provide a better understanding of the photophysical processes in colloidal nanostructures.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: Sample information, details about the optical and spectroscopic measurements, spin excitation scheme for charged QDs, electron spin dephasing time for the two types of negative photocharging states, time-resolved ellipticity signals in as-grown colloidal CdSe QDs, pump−probe measurements for CdSe QDs with OT hole acceptor, and QD diffusion measurements.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Notes 16


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The authors declare no competing financial interest. ACKNOWLEDGEMENTS This work was partially supported by the National Key Research and Development Program of China (Grant 2018YFA0306303), the Deutsche Forschungsgemeinschaft in the frame of the ICRC TRR 160 (Project B1), the National Natural Science Foundation of China (Grants 11374099, 11474097, 11727810 and 61720106009), the Science and Technology Commission of Shanghai Municipality (Grants 19ZR1414500 and 16520721200), and the 111 project of China (Grant B12024).

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Long-Lived Negative Photocharging in Colloidal CdSe Quantum Dots

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