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Two Mechanisms of Fluorescence Intermittency in Single Core/Shell Quantum Dot Igor Sergeevich Osad'ko, Ivan Yurievich Eremchev, and Andrei Vitalievich Naumov J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b04885 • Publication Date (Web): 02 Sep 2015 Downloaded from http://pubs.acs.org on September 5, 2015

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The Journal of Physical Chemistry

Two Mechanisms of Fluorescence Intermittency in Single Core/Shell Quantum Dot Igor S. Osad’ko1, Ivan Yu. Eremchev1, Andrei V. Naumov1,2,* 1

Institute for Spectroscopy of the Russian Academy of Sciences, Molecular Spectroscopy

Department, Moscow Troitsk, 142190 Russia 2

Moscow State Pedagogical University, Chair of the Theoretical Physics, Institute of the

Physics, Technology and Information Systems, Moscow, 119991 Russia ABSTRACT The temporal evolution of fluorescence intensity of a single blinking CdSe/ZnS nanocrystal (quantum dot, QD) has been measured with continuous wave (CW) laser excitation at room temperature. Tracks exhibit ON-/OFF-fluctuations with large amplitude and intensity fluctuations with small amplitude in on interval. The QD photon distribution function was obtained by statistical analysis of this fluorescence track. The experimental QD photon distribution function was compared with analogous function calculated with Monte-Carlo technique on the base of two mechanisms for intermittency which take into account both ionization/neutralization processes in QD core, as well as diffusive fluctuations of atoms on the core-shell interface. We show that combined model based on these two mechanisms sufficiently describes the shape of measured photon distribution function.

* Corresponding author. E-mail: [email protected]; Web-page: www.single-molecule.ru; Tel: +7 (910) 470-67-03

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1. INTRODUCTION In a year after fluorescence intermittency in single core/shell semiconductor quantum dots was discovered 1 a physical mechanism resulting in such effect was proposed by Efros and Rosen. 2 In accordance with the Efros-Rosen theory core/shell QD irradiated by continuous wave (CW) laser light emits bright fluorescence only from neutral core. Fluorescence disappears after Auger ionization of the core. In accordance with Efros-Rosen theory Auger ionization is realized with the help of two excitions: one exciton annihilates and its energy promotes to ejection of the electron from other exciton. Such a model predicted a single time tA of Auger ionization and exponential distribution of ON-states. However, soon power-law distributions of both ON- and OFF-intervals were found in experiments.

3-7

The power-law distribution of ON-intervals could be explained only by

introducing of multiple ways (i.e. characteristic times) for Auger ionization. However, it was not clear where the different Auger times arise from? The question had no answer at that time. Soon solution of this problem was offered by Osad’ko in Refs. 8-9 In these works such core/shell NCs were considered, whose core has electronic states j localized on the core surface (core-shell interface). In such modified charging model, Auger ionization is realized with ejection of the electron from state j localized on the core surface. If Auger ionization of the core is realized with involving electrons occupying state j then we will be able to explain where the different Auger times arise from. Hence, we have many times tAj of Auger ionization. Such modified charging model is able to explain power-law distribution of both OFF- and ON-intervals. 8-10 Unfortunately, this modified version of the charging model was missed by QD community (see, for instance, review

11

and references therein). Therefore,

question concerning many ionization times emerges some time even in recent works. By now a lot of investigations of single QD fluorescence were reported. Main efforts were undertaken to find conditions which could suppress blinking in fluorescence of QDs.

12-15

Influence of many external factors on single QD luminescence was studied with the aim to clarify details of physical model for single QDs. 16-20 The following relation between fluorescence quantum yield Q and fluorescence life time t of NC was found in the works of Bawendi group 21:

QON ,OFF =

γ em ON , OFF nr

γ em + Γ

= γ emt ON , OFF .

(1)

Here γ em is the probability of light emission, t ON , OFF is time of fluorescence decay, and ΓnrON , OFF is rate of non-radiative decay. The upper index shows that rate of non-radiative transitions in

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ON- and OFF-intervals differs with each other. Eq.(1) could be explained with the help of the modified charging model. Authors of Ref. 20 refer NCs with such fluctuations to NC of A-type. Later new facts which were in disagreement with modified charging model have been found.

22-26

First, one was discovered fluctuations of weak fluorescence in OFF-intervals.

Second, fluorescence intensity in ON-intervals exhibits large fluctuation as well. analysis of new data was performed by Frantsuzov et al.

27-28

22

23

Overall

and the model of “multiple

recombination centers” (MRC) was proposed. They supposed that ON-/OFF-fluctuations don’t result from ionization/neutralization processes and the reason to these fluctuations is of another type. Fluorescence quantum yield was presented in the following form

Q(i) =

γ em γ em + Γ(i )

= γ emt (i)

(2)

Here index  describes fluctuations of non-radiative rate due to stochastic processes going in socalled “multiple recombination centers”. In accordance with Eq.(2) all intensity fluctuations are considered on equal footing. However, measurements of a number of fluorescence time traces reveal that plenty of time traces exhibit ON-/OFF-fluctuations with large amplitude and intensity fluctuations with small amplitude, i.e. expression for fluorescence quantum yield looks as follows:

QON , OFF (i) =

γ em ON , OFF nr

γ em + Γ

(i )

= γ emt ON ,OFF (i)

(3)

Here intensity fluctuations with large amplitude are described by upper index “ON” or “OFF” whereas fluctuations with small amplitude are described by index . Additionally it was recently found surprising fact that QDs can demonstrate various types of fluctuations and therefore, NCs can be related to NCs of A- or B-type.20 MRC model could not describe fluctuations in NCs of B-type. A physical model for QDs fluorescence with the quantum yield Q ON , OFF (i ) described by Eq. (3) has been recently proposed by Osad’ko.

29

The model was named the combined model

because one takes into account important features of both the modified charging model and the MRC model. In accordance with the combined model intensity ON-/OFF-fluctuations with large amplitude emerge due to processes of ionization/neutralization of the core in core/shell QDs (the charging model), whereas intensity fluctuations with small amplitude marked by index  result from fluctuations of non-radiative transitions. Therefore, additionally so-called two-level systems (TLS) have been invoked as part of the combined model. TLS can, for instance, describe two possible positions of several atoms situated on core/shell interface. TLS model was successfully used in spectroscopy of single

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molecules embedded in polymer or glass matrix for describing jumps of fluorescence lines in frequency scale.

30-32

Such jumps have been observed also in fluorescence of single QDs.

33

Although MRC model doesn’t discuss jumps of fluorescence lines in frequency scale we think it will be able to describe such jumps as well. In the present work we demonstrate that the photon distribution in blinking fluorescence from single CdSe/ZnS core/shell QDs measured in our experiment can be described invoking two mechanisms for blinking: the improved charging model and the TLS model. 2. EXPERIMENTAL Luminescence time traces were recorded using a scanning confocal microscope. Single CdSe/ZnS quantum dots (Sigma-Aldrich, Lumidot 610, product number 694614; average sizes of QD with ZnS shell are 5.2 nm; QDs are capped with hexyldecylamine/trioctylphosphine oxide) were located on the surface of a glass substrate. Luminescence of the quantum dots was excited by continuous laser radiation with a wavelength of 532 nm (Coherent Verdi V6). Luminescence of single quantum dots was acquired with a single-channel detector operating in the photon counting mode. Preliminary selection of the proper concentration of QDs, as well as laser focusing procedure and targeting on the selected QD was performed in wide-field epifluorescence mode with highly sensitive EM CCD camera (PCO Sensicam EM). The additional confirmation of separate locations of considered QDs was obtained by scanning of surface with combined atomic-force microscope (Nano Scan Technology). The description of the experimental setup in more details is presented in Ref. 34. The spin coating method was used for sample preparation: a drop of a strongly diluted solution of quantum dots in toluene was placed onto a thin glass plate, after which the plate was spinned up to 3000 rpm. The solution concentration was adjusted so that the characteristic distance between single quantum dots on the substrate surface was more than 280 nm (diffraction limited spot size at a given excitation wavelength and for a given microlens). The experimental setup consisted of a confocal microscope with a scanning piezostage (Nano Scan Technology, Russia). Focusing of the exciting light on a sample and collecting of NC luminescence were performed using high aperture microscopic objective (Nikon CFI 100x, NA 0.95). Interference filters (Thorlabs) were used to eliminate the scattered laser light. Detection of NC luminescence was performed in the photon counting mode using an avalanche photodiode (EG&G SPCM-200-PQ) and a gated photon counter (Stanford Research SR400). The following parameters were set for the avalanche photodiode: dark counts − 20 photocounts per second, the quantum efficiency ~ 60%, and the dead time ~ 100 ns. Excitation was performed using a Coherent Verdi V6 laser operating at a wavelength of 532 nm. The ACS Paragon Plus Environment

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experimental parameters were as follows: the signal acquisition time − 1 ms and the laser intensity ~ 10 W/cm2. To focus the lens on a single QD, a preliminary scanning of a sample by piezo-movement stage relative to the diffraction spot and measuring of luminescence in each spatial point were performed. After that, a QD of interest was chosen in the luminescent image and a luminescence time trace was measured. For analysis we choose one single QD with pronounced blinking-flickering behavior, which is obviously stand-alone. Fig. 1a presents a blinking fluorescence trace of a single CdSe/ZnS NC measured in the 30 s time interval, and Fig. 1b provides the distribution function of photons in this trace.

Figure 1.

(a) The experimental fluorescence time trace of single nanocrystal CdSe/ZnS (size of single nanocrystal is 5.2 nm) as measured at room temperature with CW laser excitation at wavelength 532 nm. Exposition time is 10ms. (b) The histogram of photon distribution for the same single QD. The dashed line shows Poisson distribution with the same position of the maximum.

Jumps between ON- and OFF-states are clearly seen in the trace. The presence of OFF-intervals leads to a photon distribution function that is drastically different from the Poisson distribution function typical for luminescence of two-level systems. There are two basic differences from luminescence of two-level systems. First, there is a great probability of detecting a small number of photons in the 10 ms time interval. The peak relating small number of photons in Fig. 1b demonstrates this fact. Second, the distribution with a maximum of 56 photons is substantially broader than the Poisson distribution with the same maximum. This fact indicates complex dynamics of photon emission from a NC. 3. INTENSITY FLUCTUATIONS IN THE CHARGING MODEL Since the combined model is able to describe fluctuations in NCs of A- and B-type, we shall use this model in our work. According to the modified charging model

2, 8-10

NC ionization/neutralization are the main reason for ON-/OFF-fluctuations. electronic states of the neutral core are shown in Fig.2.

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Figure 2.

The lowest electronic levels of the neutral core.

Laser excitation L creates hot electron-hole pairs in the core. They will rapidly lose their energy. Electron and hole create: i) an exciton state (ex) and ii) there is a probability for hot electron to get localized state j on the core surface. In fact we can use instead of the left scheme the right scheme with effective pumping

Lex = L

γ ex γ ex + ∑ j γ j

, Lj = L

γj γ ex + ∑ j γ j

(4)

They describe effective pump of excitons and j-th states by hot electrons. Hole state related to jth electron exists as well but it is not shown in Fig.2. Such effective scheme will be used further. If we neglect the two-exciton states the energy diagram for the modified charging model looks as Fig.3 shows.

Figure 3.

The electronic levels in neutral core (left) and ionized core (right) of NCs of A- and B-type. Bold red and white arrows show processes of ionization and neutralization.  is rate of Auger neutralization and  is rate of tunneling neutralization.   0 in NCs of A-type and   0 in NCs of B-type

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Left part of the diagram shows the lowest electronic levels of the neutral core. Right part of the diagram shows the lowest electronic levels of the charged core because electron j has been already ejected to shell trap k. (ex) means an exciton state in the neutral core, (j) means an electronic state localized on the surface of the neutral core. When j state is populated by an electron created by laser light, hole exists as well in the core and therefore, the core is neutral. (ex+j) means an exciton created in the neutral core by laser light+an electron in j-th state. (ex+j) is two particle state in the neutral core. Right part of the diagram shows the lowest levels of the ionized core+electron in k-th state of the shell. (k) means the ground state of the charged core because an electron was ejected from the core to k-th shell trap. (ex+k) is the exciton (trion) in the charged core because an electron was ejected to k-th trap level in the shell. State (j+k) is not vital for the process of neutralization of the charged core. Therefore, this state is omitted. It should be noted that electronic energy scheme presented in Fig.3 can be used for NCs of A- and B-type. Let us find out what fluorescence trace and what photon distribution function we would have if there were only one way j for the electron to escape with the rate of G1 from the NC core and only one way for the tunnel type neutralization with the rate of g1. The diagram in Fig. 4 depicts this situation.

Figure 4.

The energy diagram for NC of B-type with one channel of slow ionization/neutralization.

The calculation of random photon emission times by the Monte Carlo method according to the diagram in Fig. 4 gives the trace and the photon distribution function presented in Fig. 5. The rate constants K and K1 determine the intensity of luminescence of the neutral and ionized cores. The fluorescence trace and photon distribution in this fluorescence, calculated using the Monte Carlo method and the diagram in Fig. 4, are shown in Fig. 5. ACS Paragon Plus Environment

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Figure 5.

The fluorescence time trace (a) and photon distribution function simulated with the help of the scheme shown in Fif.3 for single QD with single ionization/neutralization channel (solid blue line). The dashed line is nearest Poisson distribution. The histogram on (b) shows experimental single QD photon distribution function. The parameters of the calculation were:

G3 = 104 s −1 , g3 = 1s −1 ,

8 −1 K = 5.6 × 10 3 s −1 , K 1 = 10 2 s −1 , Γ0 = 10 s , g 2A = 0 . Here K1