Ultrafast Photoluminescence Quenching of Initially Excited States in

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C: Physical Processes in Nanomaterials and Nanostructures

Ultrafast Photoluminescence Quenching of Initially Excited States in CdSe Quantum Dots Functionalized with a Charge Acceptor Dye Rafael López-Arteaga, Cesar A. Guarin, Oscar Alejandro Herrera Cortes, and Jorge Peon J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b00949 • Publication Date (Web): 14 Aug 2019 Downloaded from pubs.acs.org on August 20, 2019

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The Journal of Physical Chemistry

Ultrafast Photoluminescence Quenching of Initially Excited States in CdSe Quantum Dots Functionalized with a Charge Acceptor Dye Rafael López-Arteaga†, Cesar A. Guarin§, Oscar Alejando Herrera Cortes†, Jorge Peon†*

† Universidad Nacional Autónoma de México, Instituto de Química, Ciudad Universitaria, Ciudad de México, 04510, México

§ Cátedras CONACyT - Universidad Autónoma Metropolitana, San Rafael Atlixco, ColVicentina, Ciudad de México, 09310, México

AUTHOR INFORMATION

Corresponding Author *e-mail [email protected]

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ABSTRACT

CdSe quantum dots have interesting carrier transfer characteristics and can be used as photon collectors in certain kinds of hybrid photovoltaic devices. Some of these systems work through a charge transfer process from an excitonic state to a surface-adsorbed organic dye. In this letter we explore the carrier transfer timescales through the characterization of the ultrafast photoluminescence behavior of the nanocrystal excitonic states in the presence of adsorbed molecular charge acceptors. We show that upon physisorption of the cyanine dye Indocyanine-Green, significant emission quenching due to carrier transfer can take place in a direct way from the initially pumped states in 5.7 nm diameter CdSe dots. We show that such transfer takes place independently of the excess energy above the band gap. Importantly, this near-instantaneous quenching is responsible for the loss of an important fraction of the excitonic population on a time scale much faster than intra band (hole and electron) excitonic relaxation. The time scales for the

excitonic

quenching

and

relaxation

were

addressed

by

femtosecond

photoluminescence up-conversion experiments. These experiments showed that the time


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constants associated with the accumulation of the band-edge excitons remain unchanged upon dye physisorption, however, the signal amplitude is significantly reduced as function of the addition of Indocyanine Green. The transient photoluminescence from the spectral region associated with states that act as intermediaries during excitonic relaxation (like the 1P3/21P and the 2S1/21S states) show a significant reduction in the amplitude of the exponential components but there was no difference in the transient’s time constants. These features indicate that the yield of accumulation into these transiently populated states, is diminished by the presence of the cyanine dye due to near instantaneous exciton quenching of the initially formed states.

TOC GRAPHICS


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1. INTRODUCTION Cadmium Selenide (CdSe) semiconductor nanocrystals or quantum dots (QD) are interesting materials that have a wide set of applications that include solar cells,1-4 lightemitting diodes,5 and biomedical imaging.6 These systems are interesting prospects for photon collectors in solar cells due to their broad and tunable absorption spectra which arises from a manifold of overlapped excitonic transitions.7 Additionally, chalcogenide QDs have large absorption coefficients and long lived band-edge excited states (BEE).8 Charge separation in these systems may be initiated by direct excitation of the nanocrystal into one of their excitonic states, which can be followed by charge transfer (CT) to a molecular acceptor physiosorbed at their surface.

The confinement effects in these systems imply that exciton diffusion is not a relevant variable, given that the carrier’s wavefunctions spread throughout the particle. On the other hand, the confinement also implies that the electron-hole interaction is enhanced.9 Importantly, the aforementioned effects could be differently weighed for the different excitonic states (initially formed vs intermediary excitons vs fully relaxed and at


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the band-edge). Therefore, knowledge of the earliest dynamics that take place immediately after excitation into the several different excitonic states is crucial for the understanding of the factors that determine the efficiency of the carrier transfer process.10 In particular, it is important to have direct measurements that detect which states are involved in the CT and excitonic relaxation events. These experiments can give insights into the balance of the kinetic competition between intraband relaxation and the electron or hole transfer channels. In fact, although CT from photoexcited QDs has been the subject of several studies and reviews,9, 11-17 the role of the initially excited (IE) or initially pumped states (states above the 1S states formed directly upon excitation), has been addressed to a much smaller extent.18

Our group has recently carried out the first detailed description of the transient spontaneous emissions after excitation into states which are several quanta above the band edge exciton (BEE) through photoluminescence (PL) measurements with femtosecond resolution by the fluorescence up-conversion technique.19 Those experiments showed how different transient excitonic states can be formed in a cascade-


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like process. They also showed that more than one channel is involved for the population build up in the lower excitonic states. More specifically, a clear result from those studies is that the lowest excitons (among them, the 1P3/21P, 2S3/21S and 1S3/21S states) are formed in two different time scales in CdSe QD: One channel corresponds to a rapid sub200 fs population rise, related to an ultrafast hole relaxation event. A second pathway for the population accumulation of the lower excitons, which is responsible for between 70% and 90% of their accumulation, takes place in more than a picosecond, and takes place through a ladder type relaxation process in which at least one transient exciton works as an intermediary, implying a slower population build up.

Given those results, we set out to study how the relaxation dynamics may be are altered by the presence of an organic dye at the surface of 5.7 nm CdSe nanocrystals. In these experiments, the changes in the transient emissions from the intermediary states will indicate the time scales for carrier transfer to/from the acceptor, and the potential involvement of the initial (directly formed upon excitation) and intermediate excitonic sates.


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It is important to notice that in order to make a clear measurement of the QD excitonic evolution, it is crucial that in the initial excitation, only the nanocrystal is brought to a higher excitonic state while the organic dye remains in its ground state. The latter is also important since any changes in the photon absorption upon the addition of different equivalents of the dye could trivially alter the photoluminescence experiments due to competing absorption; both, of the initial excitation light, and of the emitted light itself. In the present contribution a careful design of the case-study system was made to avoid these complications and produce a system where the overall intensities and time evolutions of the emissions are solely due to the nanocrystal excited states. These requirements were met through the use of an organic dye with negligible absorbance in the spectral region of both the excitation and the emission of several higher excitonic states of the particular QD under study.19-21 Specifically, we studied the ultrafast deactivation dynamics of higher and intermediary excitonic states of 5.7 nm core CdSe QDs in the presence of the charge transfer quenching agent Indocyanine Green (ICG).22


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Cyaninic and hemicyaninic dyes have been used as sensitizers in organic solar cells as electron acceptors,23 and as both electron and hole donors, even when forming J-aggregates.24 It is well known that core QD’s can induce the disaggregation of sulfonate-functionalized cyanine dimmers due to the strong affinity of sulfonate groups for the Cd2+ ions at the surface of the QD.25 ICG is an anionic functionalized sodium salt with two sulfonate ends that can act as anchor groups. This molecule has been extensively used as a fluorescent probe for tumor imaging,26-29 and some studies have focused in its interaction with nanoparticles.30-31 In this contribution we study the QD photoluminescence quenching of different excitonic states by ICG monomers physiosorbed to its surface. It should be also mentioned that it has been previously verified that the sulphonate groups present in the ICG dye are themselves not responsible for quenching effects like the ones observed in the present study (see below).32 With regards to the electron accepting properties of this cyanine, according to Jasieniak,33 the valence band of the BEE state of CdSe QD of 5.7 nm of diameter lies approximately at 5.36 eV, while the conduction band of lies at -3.15 eV with respect to the vacuum level. Barros et al.22 determined the HOMO and LUMO energy levels of the ICG and found that


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they were at -5.16 eV and -3.63 eV when referenced to the vacuum level. This indicates that the driving force for electron transfer is much larger than for hole transfer. Furthermore, considering the charge-separated state coulombic interaction, if hole transfer were to take place in this system, this would imply a doubly charged species in the ICG (a quaternized nitrogenic cation) side which would contribute unfavorably to hole transfer events. Such considerations point to a dominant electron transfer channel, rather than hole transfer.

The present contribution starts with static spectroscopies and nanosecond time resolved studies through the Time-Correlated Single-Photon Counting (TCSPC) technique. Such measurements were made in order to detect the fraction of excitonic states that are quenched by charge transfer events which occur before or during intraband relaxation (at times near zero, below the resolution of TCSPC). These measurements also revealed the fraction of quenching events that take place after the system has relaxed to the BEE state upon excitation with 3.1 eV photons (excess energy: 1 eV). The TCSPC results were expanded through femtosecond fluorescence up-


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conversion measurements to characterize the changes in the sub-picosecond emission signals from transient states which act as intermediaries in the relaxation processes. The latter results provide a direct evidence for the participation of the initially formed or initially excited (IE),34 states in the overall CT quenching of the QD excitation.

2. EXPERIMENTAL SECTION 2.1. MATERIALS All reagents were purchased from Sigma-Aldrich. Cadmium oxide (CdO 99.99%), trioctylphosphine oxide (TOPO 99%), hexadecylamine (HDA 98%), dioctylamine (DOA 97%), tributylphosphine (TBP 97%), calcium chloride (CaCl2 93%), coumarin 153 (99%), allura red (80%), methanol (HPLC-grade), ethanol (HPLC-grade), acetone (HPLC-grade) were used as received. Selenium pellets (99.999%) were pulverized under Argon (Ar) to facilitate the formation of the precursor solution. Oleic acid (90%) was distilled at 180 °C and high vacuum. CH2Cl2 (HPLC-grade) was distilled in the presence of CaCl2 to


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eliminate water. Indocyanine Green (ICG 90%) was washed for 48 h via Soxhlet under acetone reflux. Nile Blue A (95%) were recrystallized twice from methanol.

2.2. CdSe QUANTUM DOTS SYNTHESIS.

CdSe QDs were synthetized following a reported procedure.35-36 Briefly, in a three-mouth flask, Cadmium Oleate was produced under Ar atmosphere at 140 °C from 0.3 mmol (38.5 mg) of CdO and 3 mmol (0.962 mL) of distilled oleic acid. The reaction was cooled down to 50 °C and 15 mmol (5.858 g) of TOPO and 24 mmol (5.913 g) of HDA were added under Ar flow. The reaction was heated at 110 °C and under high vacuum for 30 minutes to eliminate water and other volatile solvents. The Selenium precursor solution containing 3 mmol (236.9 mg) of Se, 6 mmol (1.54 mL) of TBP and 21 mmol (6.47 mL) of DOA was quickly injected to the reaction flask (at 300 °C). To obtain the desirable size of CdSe QDs, the reaction was stopped at a specific reaction time blowing lab air and adding 15 mL of dried CH2Cl2. The reaction crude was separated into two centrifuge tubes and a mixture 1:1 methanol:ethanol was added in equal volume to precipitate the CdSe QDs pellets. The tubes were centrifuged for 5 minutes at 5000 rpm. The pellets were decanted,


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redispersed in dried CH2Cl2 and stored at 4 °C. Size and concentration of CdSe QDs were determined from stablished calibration curves.37 All spectroscopic studies were carried out within the next week after the synthesis. It is well known that this synthetic procedure leads to surface atoms that have a lower coordination number. This leads to unsaturated dangling bonds and reconstruction of atomic positions.38

2.3. STEADY STATE AND TRANSMISSION ELECTRON MICROSCOPY CHARACTERIZATION Absorption and emission spectra were recorded in a Cary-50 Bio (Varian) spectrophotometer and a Cary Eclipse (Varian) fluorimeter, respectively, in a 10 mm path length quartz cell. All composite spectra were recorded in dried CH2Cl2. Small volume additions (10 µL) of a highly concentrated solution of ICG (4 µM) were made to a 3 mL 14 nM QD dispersion to avoid dilution effects. After each addition, the composite solution was allowed to reach the equilibrium for 25 minutes. The same quartz cell was used for the nanosecond PL lifetimes measurements. Transmission Electron Microscope (TEM) images were acquired using a JEM-2010F FASTEM (Jeol). Samples were prepared by


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placing a drop of diluted toluene dispersion of NC on the surface of Lacey-Carbon grids (300 mesh, purchased from Electron Microscopy Sciences). Then, solvent was slowly evaporated under low-vacuum for 30 minutes.

2.4. NANOSECOND TIME-CORRELATED SINGLE-PHOTON COUNTING SYSTEM PL lifetimes were acquired in a custom-built confocal microscope upgraded with a TCSPC system.19,

39-40

A 405 nm (LDH-D-C-405, PicoQuant) and a 485 nm (LDH-D-C-485,

PicoQuant) picosecond laser were focused into a 1 cm quartz cell with a 0.25 NA objective (f = 6.1 mm, Melles Griot). The collected PL was passed through a 510 nm longpass dichroic mirror (Chroma T510lpxrxt), a 405 nm Notch Filter (Chroma ZET405nf), a 425 nm long-pass emission filter (Chroma ET425lp) and a 700 nm short-pass filter (Thorlabs FESH0700). The PL was focused to a 50 μm avalanche photodiode (PD-050CTE, Micro Photon Devices). The laser controller (PDL-800-D, PicoQuant) and the photodiode were synchronized through a TCSPC card (PicoHarp 300, PicoQuant). The intensity of the laser was controlled to obtain less than 1% of detection events at 1 MHz repetition rate. The IRF was determined with Allura Red. PL lifetimes were obtained in


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SymphoTime 64 software (PicoQuant) using the Levenberg-Marquardt iteration algorithm.

2.5. FEMTOSECOND FLUORESCENCE UP-CONVERSION The femtosecond fluorescence up-conversion setup has been described in other contributions.41-42 A regeneratively amplified (1 kHz, 800 nm) pulse train of 70 fs pulses with an average power of 1 W was split in two. One part was used to obtain the second harmonic by frequency doubling in a 0.5 mm β-BBO crystal. The intensity of the second harmonic beam was controlled using a variable neutral density filter placed before the crystal. The polarization of the second harmonic was set with a wave plate for magic angle conditions with respect to the up-conversion acceptance direction, for the excitation of the samples. The excitation beam was modulated at 1/3 of the laser repetition rate with a phase-locked chopper to detect the up-conversion signal with a lock-in amplifier (Stanford Research Systems). The samples were studied in a 1 mm flow cell and the PL was collected and refocused with a pair of parabolic mirrors to the up-conversion β-BBO crystal. About 5% of the fundamental beam was crossed with the PL signal to generate


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the frequency sum signal. The signal was collected with a CaF2 lens, focused into a double 10 cm monochromator (Oriel) and detected with a photomultiplier tube. The instrument response function (IRF) for the experiment was determined to be Gaussian with a full width half maximum (fwhm) of 350 fs. Considering the S/N ratio of the experiments and from the inspection of exponential functions, the shortest lifetimes that can be measured with this setup have time constants of ~45% of the IRF. Solvent-only traces were taken back-to-back with the QD experiments to ensure the absence of signals near t = 0. Small volume aliquots (16 µL) of a highly concentrated solution of ICG (4 mM) were added to the quartz-cell flow system of 10 mL of QD 6 µM to avoid dilution effects and were allowed to reach equilibrium for 25 minutes before each measurement. The resulting photon flux controlled with a variable neutral density filter resulted in ~0.075 nJ/pulse and an irradiance of ~10μJ/cm2 to ensure an average number of generated excitons per QD of 0.05 for all measurements.

3. RESULTS AND DISCUSSION 3.1. STEADY STATE


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The absorption and emission spectra of the CdSe nanocrystals are shown in Figure 1. As can be seen, the BEE transition occurs at 624 nm indicating a QD size of 5.7 nm.37 Also, the features in the absorption spectrum at shorter wavelengths comprise clear signatures of the different excitonic states.36, 43 Furthermore, the photoluminescence spectrum of the QD sample presents a FWHM of less than 80 meV, indicating a size monodispersity of less than 5%.44 The PL quantum yield was 0.12 compared against Nile Blue A, similar to the PL yield reported in other contributions with this synthetic procedure .19,45-47 TEM images were obtained to characterize the size monodispersity of the synthetic method (Figure S1). The absorption spectrum of the ICG dye shows the transition to the first singlet state around 818 nm and weak absorption transitions to the higher singlet states in a broad range of the visible spectrum. It is clear that from the overlap of both spectra, a selective excitation of the QD is possible at all wavelengths below the BEE. In particular, excitation at 400 nm will imply a relative photon absorption of more than 75 times that of the QD in relation to the small absorption of ICG at this wavelength for the highest ICG concentration of our experiments (8 equivalents, see below).


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Figure 1. Absorption spectrum (green) and structure of Indocyanine Green; absorption (red) and photoluminescence (orange) spectra of CdSe quantum dots.

Importantly, for the objectives of the present study (in regards to the initially excited (IE) states), the absorption spectra of the QD can be deconvoluted into its separate excitonic transitions following previously published procedures.19,

48-50

Based on this

deconvolution, it was possible to assign the excitation and the emission to regions of excitonic transitions. This analysis also allows the identification of a continuous cubic feature which, in previous studies, has been associated to band-like transitions in CdSe and that can be accessed while exciting the QDs above the BEE (see Supporting Information Figure S2).48, 51-52


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Figure 2A shows the absorption spectra of the QD solution upon consecutive additions of ICG and enough equilibration time to form a stable composite system. It is clear that the absorption of the solution remains constant in the region of absorption of the QD and only increases in the region of the near-IR transition of the ICG molecules (Figure 2A, Inset). The addition of up to 8 equivalents of ICG to the suspension corresponds to an absorbance of the QD system of 0.048 at 400 nm and that of the ICG of 0.007 at 818 nm for a 1 cm optical pathlength ([QD] = 14 nM). This implies that the changes in the QD emission signal (see below) are not due to a trivial screening effect by the absorption of the ICG chromophore.

It is relevant to establish the molecular situation of the ICG adsorbate in the QD composite. It has been determined that physiosorbed organic molecules with sulphonate groups at the end of hydrocarbon chains like those of ICG, coexist with the native surface groups from the synthesis process (carboxilates, phosphates and amines). It is also well established that these cyaninic dyes exist at the QD surface in the form of a monomer.25 For our case, the cyaninic absorption and emission spectrum reveals that the spectra in


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the 700-900 nm region correspond to the monomeric form of ICG. Following previous work,25 this can be concluded from comparisons of spectra of ICG in different solvents where the molecule exists in different aggregation states. A detailed description of this evaluation is included in the Supporting Information (see Figure S3).

Figure 2. (A) Absorption spectra of CdSe quantum dots with different additions of equivalents of Indocyanine Green. Inset: Change in the absorption value at selected wavelengths. (B) Photoluminescence spectra of CdSe quantum dots (λexc = 405 nm) as a function of consecutive additions of equivalents of Indocyanine Green. Inset: Change in steady state photoluminescence emission using different excitation wavelengths.

Upon addition of up to 8 ICG equivalents, the nanocrystal steady state PL (from the BEE) is quenched down to 30% of its value without ICG (see Figure 2B, Inset). This variation is independent of the excitation wavelength (Figures S4 and S5). Such reduction in the net emission from the nanocrystals speaks about the charge transfer quenching


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channels which compete with the radiative deactivation after electronic excitation. As mentioned, the reduction in the QD emission is not due to a trivial screening effect by the ICG, neither of the excitation nor of the emission light. Given that at the optical excitation (400 nm), the ICG absorbance accounts for less than 1% of the optical density, this absorbance is not responsible for the QD emission reduction (600-630 nm), even at the highest dye concentration (see the negligible absorption changes at 624 nm and 634 nm, inset of Figure 2A).

As can be seen in Figure 2, the reduction of the emission from the QD is not accompanied by significant emission from the ICG dye which’s fluorescence yield remains less than 10-4 (associated to a near negligible QD to dye energy transfer channel). Consistently, the Förster overlap integral equals 8 x10-14 M-1cm3 which is at least one order of magnitude smaller than that in ideal energy transfer pairs (See also Figure S4 and S5).53 The fact that the major channel after excitation corresponds to CT instead of energy transfer is consistent with previous studies where both channels are in principle possible in similar systems.46 As we show below, the detailed time-resolved


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measurements of the different spectral regions will give more insight about the excitonic states involved in the carrier transfer events.

The steady state PL excitation emission spectra (detection wavelength : 670 nm) for the QD and QD-dye composites are shown in Figure S6A. Importantly, the normalized excitation spectra show the same profile after scaling independently of the number of ICG equivalents added to the suspension (up to 8 ICG equivalents), and shows a good match with the QD-only sample upon appropriate scaling. This result implies that the reduction in the BEE emission is reduced to approximately the same proportion independently of the excitation wavelength in the 400 to 650 nm region. Such independence on the excitation wavelength indicates that the decrease of the PL is independent of the excitonic state which is initially excited.

3.2. NANOSECOND DYNAMICS Figure 3 summarizes the TCSPC results for the CdSe/ICG systems with up to 8 molar equivalents of the dye. Importantly, these measurements were made in back to back fashion using the same QD solution and taking equal photon accumulation times for all


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the decay traces (60 s). This was made so that the actual signal intensities as a function of time can be compared as for different ICG concentrations (0 to 8 equivalents). These histograms show that the earliest resolved signals systematically decrease with increasing concentration of the ICG (from 0 equivalents of a QD only solution (14 nM) to 8 equivalents or 112 nM of ICG; such low concentrations were made in order to avoid inner filter or reabsorption effects at the 400 nm spectral region). Comparing the TCSPC results, it is also clear that the traces decay with faster rates in the nanoseconds time scale (see Figure 3A inset). Both these trends were observed for 405 nm and 485 nm excitation as can be seen in Figure S7. As mentioned before, the decrease in the total amplitude of the signal (as measured by the level at earliest resolved times) is not due to a screening effect of neither de excitation light, or the ICG fluorescence itself, as in both regions, the ICG makes a negligible contribution to the absorbance, while the signal amplitude is decreased by a factor of 0.43 for 8 ICG equivalents in comparison to the QDonly suspensions. This feature suggests that the amount of the BEE formed at the earliest times that TCSPC can resolve was significantly reduced due to the participation of early deactivation channels or directly from the IE states, this was further explored with the


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appropriate time resolution in the PL Up-Conversion measurements (see below). It is common that the results of TCSPC experiments are shown as a normalized plot in which the time bin (time channel) with the largest signal (near t=0) accumulates the same amount of counts in the different experiments (in our case, upon variations of the added amount of ICG). However, such traditional way to acquire and present the data would impede the visualization of the reduction of the near time equal zero signals while still showing the changes in the decay times in the nanosecond regime, this is shown in the insets of Figures 3A and S7.

Figure 3. (A) Decay histograms of CdSe quantum dots as a function of consecutive additions of equivalents of Indocyanine Green for constant TCSPC photon accumulation time (60 s) and λexc=405 nm. Inset: Decay histograms of CdSe quantum dots as function of consecutive additions of equivalents of Indocyanine Green at constant maximum count of photons for the most intense time bin, and λexc=405 nm. (B) Change in the photoluminescence maximum of CdSe quantum dots signal at t = 0 as function of added


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equivalents of Indocyanine Green. Inset: Change in the intensity averaged lifetimes of photoluminescence CdSe quantum dots as function of added equivalents of Indocyanine Green.

To further illustrate the points of the previous paragraph, in Figure 3B we show the total signal amplitude of the TCSPC experiments of Figure 3A and Figure S7 as a function of the added equivalents of ICG. As can be seen, the luminescence signal near t = 0 (which scales with the total signal amplitude) decreases monotonically upon additions of ICG. Such effect points to a drop of the population of the BEE states which are formed after the formation of IE states, due to the presence of the ICG molecules.

As mentioned, besides the important decrease of the signal amplitude, there is a decrease in the QD emission yield associated to slower quenching processes that are reflected in a reduction in the average decay time in the nanoseconds time scale (inset of Figure 3A).The corresponding average time constants are reduced from 30.6 ns to 23.4 ns upon the addition of 8 eq. of ICG (Tables S2 and S3). From these results, the drop in the steady state BEE emission yield for 8 ICG equivalents corresponds to a 0.5


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factor from the change in the near t = 0 amplitude, and an additional 0.76 factor from the ns time-scale quenching (steady state relative intensities are: Iss(8 eq.)/Iss(0 eq.)) = 0.38).

3.3. FEMTOSECOND FLUORESCENCE UP-CONVERSION RESULTS First, we describe the fs and ps evolution of the emission signals of the QD-only samples (no ICG added). The dynamics follow the same general trend previously reported by our group for the BEE and higher excitons of 4.4 nm and 5.4 nm QDs.19 Figure 4A shows the early time evolution of the emission signals form the BEE at the detection wavelength of 634 nm. As can be seen, the fluorescence up-conversion technique nicely resolves the signal accumulation which is proportional to the BEE population upon excitation into higher excitonic states (λexc: 400 nm). This transient shows an accumulation of the signal in two-time scales: sub-200 fs and 1.3 ps. The sub 200 fs component appears as a steplike, near instantaneous accumulation of the signal and is highlighted by a blue circle in Figure 4A. As mentioned in our previous contribution, this ultrafast feature is fully reproducible and has been observed in other studies.19, 54 The 1.3 ps component is well resolved and corresponds to approximately 60% of the total signal amplitude at the


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maximum which is reached at 3 ps. These time constants measure the rate of formation of the BEE state which’s population gathers from the relaxation processes of the higher and intermediate excitons.8, 19, 51, 55-62 In the 12 ps scale of the graph, the beginnings of an early decay component of the signal starts to show up. The inset of Figure 4A shows a wider view of the emission signal and allows visualization of a near 20 ps decay. In congruence with previous studies,8 the 20 ps component is followed by the slower components of diverse time constants that span from hundreds of ps to tens of ns (notice from the TCSPC experiments, that the average lifetime of this state was 30 ns). To demonstrate that at this photon fluence no trion or biexciton Auger processes were present, different PL transients were obtained varying the irradiance energy. The same behavior was observed for all photon fluences (Figure S8).


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Figure 4. (A) Up-Conversion photoluminescence decay for the 1S3/21S exciton (λexc = 400 nm) detected at 634 nm. Inset: Same transient plotted to 200 ps. (B) UpConversion photoluminescence decay for the 1P3/21P exciton detected at 550 nm and for the 2S1/21S exciton detected at 500 nm (λexc = 400 nm).

As shown in our previous study,19 the spontaneous emission signals can also be resolved for several transiently populated intermediary excitonic states at shorter wavelengths. These signals correspond to transient luminescence produced by the higher excitons which are undergoing accumulation and relaxation in the fs and ps time scales. The up-conversion results for the emission at 550 nm and 500 nm are shown in Figure 4B. These signals are characterized by the rapid accumulation of the respective excitonic state (from the spectral zones, 500 nm corresponds approximately to the 2S1/21S region, and 550 nm corresponds to the 1P3/21P state). After the early signal appearance, these intermediary states decay in time scales which are related to the accumulation of the BEE and the formation of trapped states.19 Specifically, the 550 nm decay corresponds to 5.4 ps, and the one at 500 nm, to 2.9 ps (see Table S6).


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The rise-decay features for the intermediate excitonic states are defined by the aforementioned ladder type mechanism which occurs in parallel with the rapid (sub 200 fs) channel that populates the lower excitonic states. The sequential nature of the slower accumulations can be nicely visualized by the time-resolution of the higher excitonic states and is described by the scheme presented in our previous contribution.19 The scheme considers the possibility that upon 400 nm excitation, different kinds of excitons may be formed. More specifically, the sub-200 fs component that results in an ultrafast accumulation of the lower states is proposed to occur through a direct hole relaxation step (maintaining the electron in a given state).63-66 On the other hand, the stepwise relaxation is considered to be related to an Auger-type electron to hole energy transfer, forming holes deep in the valence band, which is followed by a slower step in which the electron and/or hole relaxation dynamics produce a series intermediaries which redound in a slower (picosecond) secondary or additional accumulation of the BEE population.

Next, we present the description of the ways these early signal change upon the addition of the ICG molecules at the nanocrystal surface. First, as can be seen in Figure


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5A, the accumulations and decays of the BEE photoluminescence signal show a reduction in the overall amplitude proportional to the added equivalents of ICG. This clearly indicates that the accumulation of the lowest excitonic state is reduced by the presence of the ICG molecules in their surface. This same effect can be seen for the other excitons: at 550 nm around the 1P3/21P region in Figure 5B, and at 500 nm for the 2S1/21S region in Figure 5C. As mentioned before, these effects cannot be explained by a trivial screening effect by the dye, neither for the excitation nor the emission light. Interestingly, the maximum amplitude of the 500 nm signal is reduced to approximately 20% of its value without ICG, while the signal maximum for the BEE emission with 8 equivalents comprises approximately 50% of the signal without any ICG. Additionally, the timeconstants for the decay of the intermediary state at 500 nm do not show significant changes upon addition of up to 8 equivalents of ICG and remain within 10% of the value without ICG (2.9 ps). Similar statements can be made about the time constants associated with the rise and decay of the 550 nm emissions. The time scales for the ultrafast and 1.3 ps rises observed for the BEE at 634 nm also are not significantly changed upon addition of ICG. Given that only the amplitude of the 500 and 550 nm


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transients are reduced, but not the respective time constants, it can be concluded that the population of previous or initially excited (IE) states is reduced by the ICG quenching and

not the states responsible for these emissions themselves. This includes the 1P3/21P and the 2S1/21S states.


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Figure 5. (A) Up-Conversion photoluminescence signals for the 1S3/21S exciton (λexc = 400 nm) detected at 634 nm as function of Indocyanine Green additions. (B) UpConversion photoluminescence traces for the 1P3/21P exciton region (λexc = 400 nm) detected at 550 nm as function of Indocyanine Green additions. (C) Up-Conversion photoluminescence traces for the 2S1/21S exciton region (λexc = 400 nm) detected at 500 nm as function of Indocyanine Green additions. The previous statement is consistent with the negligible changes in the shape of the excitation spectra in the region of 400-500 nm (Figure S6B) upon addition of ICG equivalents. An analysis of the changes of the relative amplitude of the rapid (step-like)

vs the 1.3 ps accumulation of the BEE emission gives more insight into this effect: When the emission signal of the BEE is normalized to its maximum level (around 3 ps), the traces show that the relative contribution of the rapid accumulation (step-like feature) is reduced in comparison with the contribution related to the slower picosecond component. A careful analysis of this effect is included in the Supporting Information in Figures S9 to S12 (the relative contribution of the ultrafast vs picosecond accumulation was assessed as shown in Figure S12). From this quantitative analysis it is concluded that, while the fast component accounts for 60% of the total signal at its maximum without ICG, upon addition of 8 equivalents of the organic dye, its contribution drops to about 40%.


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The lack of change in the time constants, the reduction of the total signal amplitude at 500 nm and 550 nm, and the larger reduction in the rapid component for the BEE, point to the absence of any direct involvement of the  > 500 nm emitting states in the quenching process, and the participation of an “early exciton” or initially excited state in the emission quenching by the ICG molecules. A mechanism in which an early exciton has an additional quenching process and a branched evolution is presented in Scheme 1, where the quenching competes with the relaxation steps that give rise the lower energy states (see Scheme S1 for the complete set of pathways). The proposed scheme is consistent with all the present experimental observations, including the reduction in the signal amplitude of the TCSPC and steady state experiments. The applicability of this kinetic scheme to the present results can be easily verified through simple numerical simulations of the proposed mechanism. The process sequence of Scheme 1 is a simplified version of the mechanism detailed in reference 19 for exciton relaxation from the IE states (see also Scheme S1), with the addition of a carrier transfer step from initially excited states. This process is associated to the kCT rate constant. Potentially, other early or intermediary states can participate in the quenching process (k´CT). The population of


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these states, which can be formed by ultrafast hole relaxation steps, is indicated by NINT2. The simulations are included in Figures S13 and S14.

Scheme 1. A) Kinetic deactivation pathways proposed for the formation of the population of the BEE existing in competition with charge transfer processes that takes place from the initially excited state (at any energy), or, in general, an early exciton. NIES represents the population of initially formed excitonic states and NHES the population of hot excitonic states. The kCT rate constant is related to charge transfer processes from the initially excitonic states forming a population of charge separated products NICG, likewise, k’CT forms these products from an intermediary state NINT2. kUFhR is the rate for a fast relaxation channel into these intermediaries and k’UFhR forms lower energy excitons NBEE through ultrafast hole relaxation. B) Energy level diagram of the proposed CT pathways from the initially pumped states to the ICG molecule.

The mechanism of Scheme 1 includes the important pathways of the associated with the deactivation of the initial exciton population NIES and the hot excitonic states


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population NHES studied in this contribution. As detailed in reference 19, the hot excitonic states may evolve towards the ultrafast formation of the lower energy excitons (NBEE) by a direct ultrafast hole relaxation process kUFhR (associated to the step-like feature in the BEE accumulation), or evolve through a series of intermediary states involving the slower steps: kA, khR1 and khR2 which are associated to the picoseconds rise of the BEE population (see Scheme S1 for these pathways). The present results imply the presence of an ultrafast charge transfer process as a third channel kCT for the evolution of the early excitonic states with initial population NIES, and which is responsible for the reduction of the population, not only of the lower excitonic states, but also of the intermediaries as shown in the results of Figure 5.

The “early excitonic states” or initially excited states which, from the observed signal trends, need to be considered the ones involved in the carrier transfer, most likely possess particular characteristics that enhance the probability of the carrier transfer to the cyaninic dye. These early excitonic states may be formed either directly or rapidly upon excitation. A sub-set of these excitons may possess more favorable properties for


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carrier transfer events.67 It should be noticed that some of these models actually do not show the Marcus Inverted Region.68

One factor that supports an ultrafast transfer rate from the

IE states is the

observation that the coupling matrix elements for charge transfer depend strongly on the density that extends to the outside region of the QD.69 The IE states are likely to be more extended than the more localized states near the band edge responsible for the emissions of Figures 4 and 5. Additionally, the states formed upon (or immediately after) excitation are considered to be mixed with a near continuum of states of the semiconductor.12, 70-71 Additionally, the recently described Auger-assisted electron transfer mechanism might contribute to a larger charge transfer efficiency for the IE states. In this mechanism, the electron transfer event is coupled to a hole excitation process whereby the ET rates have a monotonous increase as a function of driving force.9, 12, 72

Although the mechanism in Scheme 1 is a simple description of the quenching of the emission of the BEE and the different transient states, several more complicated schemes can also be considered. For example, the involvement of other states rapidly


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formed after excitation can also be drawn into the scheme. These short-lived intermediaries can be situated in the path of the kUFhR channel and correspond to additional states undergoing quenching before the formation of the band edge states. The numerical simulation of this scheme is presented in Figure S13 where it is nicely visualized that the tendency of the PL transients can be reproduced. From this result it is clear that both CT pathways can take place from both the initially formed states and trough the presence of rapidly formed intermediary states.

4. CONCLUSION A system comprised of 5.7 nm QDs and a physiosorbed cyaninic dye with negligible absorption at wavelengths between 390 nm and 635 nm was designed to study the participation of initially excited (IE) states in the overall carrier transfer quenching of the electronic excitation induced in the QD. The detailed measurements of the luminescence dynamics of 5.7 nm QDs reveal the time scale involved in the rapid quenching of the electronically excited states. The TCSPC measurements reveal a reduction in the time constant of the nanoseconds-decay of the emission from the 3S1/21S QD state upon


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physisorption of ICG. However, from femtosecond experiments, it is clear that an important fraction of the signal quenching occurs in the sub 200 fs time scale. This quenching involves excitonic states which are populated either directly by the photon absorption event or rapidly thereafter. In particular, our experiments reveal that the quenching takes place before the steps involved in the rise and decay of the intermediary excitons that emit from 500 to 640 nm. This was demonstrated by time-resolving the luminescence in a spectral region that corresponds to the transient population of states that include excitons in the region of the 1P3/21P, and 2S1/21S absorption features. While the time constant for the decay of these states are not influenced by the dye, their formation yield from “previous” excitons is significantly altered by ICG. From an analysis of the energy alignment of the excitonic states and the orbitals of ICG, and the signals, it appears that the electron is the most likely carrier involved in the transfer related to emission quenching. Luminescence resolved by the femtosecond up-conversion method is particularly suitable for studying the specific states which are implicated in transfer channels that compete with the accumulation of the population of the states where the carriers have fully relaxed after direct excitation with excess energy.


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ASSOCIATED CONTENT Supporting Information. QD TEM images, deconvoluted absorption spectra and deconvolution parameters, absorption spectra of ICG in different environments, QD PL spectra at different excitation wavelengths, QD PL excitation spectra as function of ICG additions, TCSPC histograms for a 485 nm excitation wavelength, best fit parameters for the TCSPC histograms and up-conversion transients, normalized up-conversion transients for the BEE state at different photon fluences, first derivative and amplitude relationship of the up-conversion transient signals, simulated transients for QD and QDICG for the BEE PL up-conversion transients, complete kinetic scheme of the exciton deactivation, kinetic results of the numerical simulations are included in the supporting information file and is available free of charge.

AKNOWLEDGMENTS The authors would like to express their gratitude to Fís. Roberto Hernández Reyes and Dr. Verónica Henao from UNAM for the preparation samples and TEM measurements. Authors acknowledge grants from CONACYT CB 220392, Fronteras de la Ciencia 179,


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and PhD scholarship 257847 and, grant from PAPIIT/DGAPA/UNAM IN212814 for financial support.

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Figure 1. Absorption spectrum (green) and structure of Indocyanine Green; absorption (red) and photoluminescence (orange) spectra of CdSe quantum dots. 85x53mm (300 x 300 DPI)



Figure 2. (A) Absorption spectra of CdSe quantum dots with different additions of equivalents of Indocyanine Green. Inset: Change in the absorption value at selected wavelengths. (B) Photoluminescence spectra of CdSe quantum dots (λexc = 405 nm) as a function of consecutive additions of equivalents of Indocyanine Green. Inset: Change in steady state photoluminescence emission using different excitation wavelengths. 175x55mm (300 x 300 DPI)


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Figure 3. (A) Decay histograms of CdSe quantum dots as a function of consecutive additions of equivalents of Indocyanine Green for constant TCSPC photon accumulation time (60 s) and λexc=405 nm. Inset: Decay histograms of CdSe quantum dots as function of consecutive additions of equivalents of Indocyanine Green at constant maximum count of photons for the most intense time bin, and λexc=405 nm. (B) Change in the photoluminescence maximum of CdSe quantum dots signal at t = 0 as function of added equivalents of Indocyanine Green. Inset: Change in the intensity averaged lifetimes of photoluminescence CdSe quantum dots as function of added equivalents of Indocyanine Green. 170x55mm (300 x 300 DPI)



Figure 4. (A) Up-Conversion photoluminescence decay for the 1S3/21S exciton (λexc = 400 nm) detected at 634 nm. Inset: Same transient plotted to 200 ps. (B) Up-Conversion photoluminescence decay for the 1P3/21P exciton detected at 550 nm and for the 2S1/21S exciton detected at 500 nm (λexc = 400 nm). 170x55mm (300 x 300 DPI)


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Figure 5. (A) Up-Conversion photoluminescence signals for the 1S3/21S exciton (λexc = 400 nm) detected at 634 nm as function of Indocyanine Green additions. (B) Up-Conversion photoluminescence traces for the 1P3/21P exciton region (λexc = 400 nm) detected at 550 nm as function of Indocyanine Green additions. (C) Up-Conversion photoluminescence traces for the 2S1/21S exciton region (λexc = 400 nm) detected at 500 nm as function of Indocyanine Green additions. 85x133mm (300 x 300 DPI)



Scheme 1. A) Kinetic deactivation pathways proposed for the formation of the population of the BEE existing in competition with charge transfer processes that takes place from the initially excited state (at any energy), or, in general, an early exciton. NIES represents the population of initially formed excitonic states and NHES the population of hot excitonic states. The kCT rate constant is related to charge transfer processes from the initially excitonic states forming a population of charge separated products NICG, likewise, k’CT forms these products from an intermediary state NINT2. kUFhR is the rate for a fast relaxation channel into these intermediaries and k’UFhR forms lower energy excitons NBEE through ultrafast hole relaxation. B) Energy level diagram of the proposed CT pathways from the initially pumped states to the ICG molecule. 160x60mm (300 x 300 DPI)


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Ultrafast Photoluminescence Quenching of Initially Excited States in

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