Enhancement of Biexciton Emission Due to Long-Range Interaction of

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

Enhancement of Biexciton Emission Due to LongRange Interaction of Single Quantum Dots and Gold Nanorods in a Thin-Film Hybrid Nanostructure Victor Krivenkov, Simon Goncharov, Pavel Samokhvalov, Ana SánchezIglesias, Marek Grzelczak, Igor Nabiev, and Yury P. Rakovich J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b03549 • Publication Date (Web): 07 Jan 2019 Downloaded from http://pubs.acs.org on January 11, 2019

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The Journal of Physical Chemistry Letters

Enhancement of biexciton emission due to long-range interaction of single quantum dots and gold nanorods in a thin-film hybrid nanostructure Victor Krivenkov*,1, Simon Goncharov1, Pavel Samokhvalov1, Ana Sánchez-Iglesias2, Marek Grzelczak3, Igor Nabiev1,4, and Yury Rakovich*,1,3,5,6 1

National Research Nuclear University MEPhI (Moscow Engineering Physics

Institute), 115409 Moscow, Russian Federation 2 CIC

biomaGUNE, Paseo de Miramón 182, 20014 Donostia-San Sebastián, Spain

3 Donostia

International Physics Center, Paseo Manuel Lardizabal 4, 20018 Donostia-

San Sebastián, Spain 4

Laboratoire de Recherche en Nanosciences, LRN-EA4682, Université de Reims

Champagne-Ardenne, 51100 Reims, France 5

Centro de Física de Materiales (MPC, CSIC-UPV/EHU) and Donostia International

Physics Center, Paseo Manuel de Lardizabal 5, 20018 Donostia - San Sebastian, Spain 6

IKERBASQUE, Basque Foundation for Science, Maria Diaz de Haro 3, 48013 Bilbao,

Spain

1 ACS Paragon Plus Environment

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AUTHOR INFORMATION Corresponding

Authors:

Victor

Krivenkov:

[email protected];

[email protected]

ABSTRACT Semiconductor quantum dots (QDs) are known for their ability to multiphoton emission caused by recombination of biexcitons (BX). However, the quantum yield (QY) of BX emission is low due to the fast Auger process. Plasmonic nanoparticles (PNPs) provide an attractive opportunity to accelerate the BX radiative recombination.

Here,

we

demonstrate

the

PNPs-induced

distance-controlled

enhancement of the BX emission of single QDs. Studying the same single QD before and after its integration with the PNPs, we observed the plasmon-mediated increase in the QY of BX emission. Remarkably, the enhancement of the BX emission remains pronounced even at the distances of 170 nm. We explain this effect by the efficient coupling, which results in the trade-off between resonance energy transfer from QD to GNRs and the Purcell effect at small QD-PNPs separations, and the predominant influence of the Purcell effect at longer distances. Our findings constitute a reliable approach to managing the efficiency of multiexciton emission over a wide span of distances, thus paving the way to new applications.

TOC GRAPHIC


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KEYWORDS Multiphoton emission, plasmonic coupling, quantum dots, gold nanorods, energy transfer.

Since the discovery of quantization of the excitonic states in semiconductor nanocrystal QDs,1 their unique optical and photophysical properties have been extensively studied,2 which paved the way to various applications, such as photovoltaic and optoelectronic devices, light-emitting diodes and displays, photocatalytic and photoelectrochemical systems for solar fuel production, biolabeling and bioimaging.3-5 In addition, during the last two decades, disruptive development has occurred in QD-based quantum optics and quantum technologies. This includes research in cavity electrodynamics, superradiance, non-classical light emission and single-photon emission.6-7 One of the distinguishing features of QDs is their ability to generate simultaneously two excitons.8 This effect can be observed in the photoluminescence (PL) spectra and using photon correlation spectroscopy (PCS) as a practically simultaneous emission of two photons from a single QD as a result of the biexciton/exciton cascade.9 This phenomenon has an important implication for quantum technologies because the photons emitted by the BX and exciton can be entangled in polarization steaming from the spin of the BX.10 However, the practical implementation of these effects is hampered by Auger ionization, a process in which two excited electrons interact with 3 ACS Paragon Plus Environment


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each other, with subsequent recombination of one of these electrons with a hole and the excitation of the other electron to higher energy states. Being extremely inefficient in bulk semiconductors, due to constraints imposed by energy and translational-momentum conservation, the Auger effect becomes highly efficient in QDs because of the relaxation of momentum conservation in these nanostructures.11 Moreover, being essentially a phenomenon of three-body interaction, the Auger effect plays the most prominent role in the nonradiative recombination of multi-exciton states, which leads to a strong decrease in the quantum yield (QY) of the associated optical transitions. As a result, even though the binding energy of BX in QDs may be higher than the thermal energy, it is difficult to observe a well-resolved BX line in the PL spectra at room temperature. In measurement of the second-order photon correlation g(2) using PCS, when the split emission signal is detected using two detectors, the Auger effect manifests itself in a change in the photon statistics and a strongly suppressed signal at zero delay.12 Consequently, in order to make BX transitions observable, it is necessary to mitigate the Auger effect. Two main strategies have been proposed and implemented for this purpose. The first is aimed at suppressing the efficiency of Auger recombination by means of an engineering reduced overlap between the electron and hole wave functions in QDs. One of approaches along this way is growing of so-called “giant” core/shell heterostructured QDs.13 Using this approach, BXs were efficiently generated and detected.14 Another possibility is to fabricate QDs with a graded composition of the interfacial layer using alloyed ternary or quaternary compounds, which ensures “smoothing” of the confining potential and suppresses the intraband transition involved in the dissipation of the electron–hole recombination energy potential.15 The BX signature in the PL spectra of QDs with a modified confinement potential is direct evidence of Auger recombination


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suppression.16 It was also demonstrated that the use of the type II (or quasi-type II) localization regime can be very successful in the separation of electrons and holes in the core and shell regions of the QDs heterostructures, which leads to repulsive BX interaction energies and reduced Auger recombination efficiencies.17-19 Another strategy implemented to overcome the losses due to the Auger effect (but not necessarily suppressing it) is to accelerate the radiative recombination of BXs and make it more efficient by means of QD coupling with PNPs or nanostructures. This strategy makes use of the fact that the luminescence efficiency and the radiation decay rate of QDs can be significantly altered by the formation of coupled systems.20-21 The observed accelerated radiative recombination rate (i.e., the increased PL intensity) is attributed to the enhancement of the local field due to the excitation of plasmon resonances in metal nanostructures, which can increase the light absorption and alter the radiative decay rates of nearby QDs (the Purcell effect22). Related phenomenon that leads to an increase in the emission intensity is the redistribution of the local density of states at the optical frequencies of the emitter. In the case of a strong overlap between the GNRs absorption and QD emission bands, the PNPs can stimulate secondary radiation with an increased intensity at the same frequency as the PL emitted by the QDs.23 The effects described above should manifest themselves both for exciton and BX states, albeit to different extent. It was shown that the PL intensity of BXs will be enhanced stronger than that of single excitons, because the BX excitation rate (i.e. emission intensity) is proportional to the fourth power of the local electric field, whereas the excitation rate of excitons is proportional to the square of this field. Moreover, the BX QY can be increased due to the increase of radiative rate of BX close to PNPs.24



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This finding opened the way for numerous experiments aimed at enhancement of the BXs QY to the level of excitons, which for a single QD, can be monitored using PCS, where a strong plasmon-induced increase in the cross-correlated signal is expected at a zero delay between the detectors. To date, efficient BX emission as a result of interaction with the plasmon system has been achieved in various configurations, including QDs integrated with PNPs,24-27 sharp metal tips or nanocones,28-29 thin films of noble metals,30-31 and other plasmonic nanostructures.32-33 Also, a combination of the strategies described above was also used to enhance the BX emission.34-35 Along with matching the resonance of the emitter with the plasmon modes, the enhancement can be maximized when the distance between the metal and QDs systems is optimal (between 10 and 20 nm20, 36). For this reason, in most of the studies cited above, the experiments were carried out at distances of up to 15 nm. It should be noted that most of these experiments were not carried out on the same single QD before and after the coupling with the plasmon structure, which sometimes makes it difficult the correct interpretation of the results obtained. In this work, a combination of QDs (with a CdSe core and ZnS/CdS/ZnS multishell) and gold nanorods (GNRs) was used to study the effect of coupling on the BX emission. The procedures and details of the synthesis, as well as the optical spectra of QDs and GNRs and their corresponding images can be found in the Supporting information. The size of the GNRs was 39.1 ± 1.3 by 20.5 ± 0.5 nm, their extinction spectrum is presented in Figure S1a and TEM image is shown in Figure S1c. The diameter of the QDs was ~5 nm, their extinction and luminescence spectra are presented in Figure S1a and TEM image is shown in Figure S1b. First, taking into account the reported optimal distances between QDs and GNR for PL enhancement, a thin-film hybrid structure with a total structure thickness of 20 nm


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was fabricated (Figure 1a) as described in SI (Scheme 1 in Figure S3). Careful adjustment of the concentration of QDs combined with scanning confocal fluorescence imaging was used to ensure that the average distance between the individual QDs incorporated in the PMMA film was larger than the laser excitation spot (SI, Figure S6). Subsequently, the emission properties of the same single QD were studied in situ before and after GNRs deposition. Before GNRs deposition, measurements of the time trace of the PL intensity (Figure 1b) revealed typical blinking behavior with clear on/off periods, which led to a bimodal distribution in the histogram of the count rate (Figure 1c). This phenomena is well known for QDs and is caused either by the quenching of an exciton by the charged state of the QD, which was preliminarily ionized with involvement of nonradiative Auger recombination, or/and by trap induced recombination.37-38 In the absence of GNRs, the PL decay of QDs shows a biexponential behavior (Figure 1d) with fast and slow lifetimes of 2.4 ns and 23 ns (weighting factors 0.25 and 0.75, respectively), which yields an average lifetime of 18 ns, calculated using equation S2 (SI).



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Figure 1. (a) Schematics of the hybrid QD-GNRs system. (b) The time trace of the PL intensity of a single QD before (black) and after (red) GNRs deposition, and the background noise (blue). (c) Photon correlation histograms indicating the distribution of count rate observed in the time trace from single QD before (black) and after (red) GNRs deposition. (d) PL decay curves recorded from a single QD before (black line) and after (red line) GNR deposition. The observed PL properties were drastically altered after GNRs deposition. First, a strong PL quenching, accompanied by a reduction of off-states, was observed in the time trace of the PL intensity (Figure 1b). Secondly, after GNR deposition, the PL decay was still biexponential with an average lifetime of 1.5 ns, which is 12 times shorter than that for the same QD not interacting with GNRs (Figure 1d). The first decay component was found to be 350 ps with a weight of 0.75 and the second one 4.8


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ns with a weighting factor of 0.25. The observed reduction in the PL lifetimes indicates an acceleration of the exciton recombination rate and redistribution of the efficiency of the radiative recombination channels. At the same time, the counts rate distribution shows continuous signal of the same intensity (Figure 1c) that demonstrates that QD does not blink and is always in the same state, presumably accelerated and quenched neutral “on”-state. This result allows us to associate the first ultrafast decay component with BX emission.29 It is well known that shortening of the PL lifetime can be caused not only by the effects of the plasmon–exciton interaction (i.e., the Purcell effect), but also by the appearance of an additional non-radiative channel associated with the Förster resonance energy transfer (FRET) from QD to GNRs. To determine which of these effects is dominant, the integrated PL intensity as the area under the photon count time trace was estimated with the exclusion of the background noise (Figure 1b). It turned out that, after GNRs deposition, the integrated PL intensity of QD decreased by about 3.1-fold, which indicates a corresponding drop in QY. This clearly points out to a change in the radiative recombination rate. Indeed, if the radiative recombination rate were not modified, then the observed PL lifetime should vary in direct proportion to QY: 𝑘𝑟𝑎𝑑

Ф=𝑘

𝑟𝑎𝑑 + ∑𝑘𝑛𝑜𝑛𝑟𝑎𝑑

=

𝑘𝑟𝑎𝑑 𝑘𝛴

𝜏𝛴

= 𝜏𝑟𝑎𝑑,

(1)

where krad is the radiative recombination rate, Σknonrad is the sum of the rates of all nonradiative processes, kΣ is the sum of krad and Σknonrad, τΣ is the measured PL lifetime, and τrad is the radiative lifetime. In the absence of any other mechanisms of quenching, the efficiency of FRET can be expressed either in terms of the relative decrease of the integrated QD’s PL intensity (E = 1 ˗ PLQD+GNRs/PLQD), or in terms of modification of the PL lifetime as E = 1 ˗ 9 ACS Paragon Plus Environment


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QD+GNRs/QD. Significantly, no correlation was observed between the value of E estimated from the data on PL quenching (E=0.67) and changes in the PL lifetime (E=0.92). This discrepancy indicates that FRET alone cannot be fully responsible for the observed effects and for given sample the interplay between the FRET and the Purcell effect takes place with the dominant contribution of FRET into PL quenching and the pronounced influence of the Purcell effect on the enhancement of recombination rate. To gain a deeper insight into the mechanisms of the plasmon-mediated enhancement and assess the contribution of the BXs to the QDs PL we have utilized PCS. When monitoring the PL of a single QD in the mono-exciton regime, the drop in the coincidence (i.e. in the cross-correlation function 𝑔(2)(τ)) between the two arms of the PCS setup at zero delay (τ = 0) is expected. This implies a single-photon emission mode and means that, after its recombination, an exciton in a QD produces one photon, and then it takes some time before emitting another. Although even an ideal single-photon emitter might produce a nonzero g(2) value due to jitter, the rise of the central peak at τ = 0 above this noise level in pulsed excitation regime reflects an increased probability of detecting two photons during a single laser cycle. In the case of single QD, this indicates BX emission.39 Figure 2a shows the measured cross-correlation function g(2)(τ) of the selected QD before the GNRs deposition. It displays the reduced coincidence rate around τ=0, that is, photon anti-bunching, which is typical of the statistics of photons of a single quantum emitter. The ratio of the area of each peak of g(2)(τ) to the average area of the side peaks (with the exception of the central peak) is shown as red dots on Figure 2c. It can be seen that the area of the central peak of the g(2)(τ = 0) does not exceed 25% of the average


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area of the side peaks (Figures 2a,c), which also implies the single-photon QD emission regime and rules out the possibility of clustering effect in g(2) measurements.40 It is known that for a single QD, the BX QY can be calculated using the following equation:39 Ф𝐵𝑋 Ф𝑋

𝛥𝑡

≈

∫ ―𝛥𝑡𝑔(2)(τ)𝑑τ 𝑇 + 𝛥𝑡 ∫𝑇 ― 𝛥𝑡𝑔(2)(τ)𝑑τ

𝑐𝑒𝑛𝑡𝑟𝑎𝑙 𝑝𝑒𝑎𝑘 𝑎𝑟𝑒𝑎

= 𝑎𝑣𝑒𝑟𝑎𝑔𝑒 𝑠𝑖𝑑𝑒 𝑝𝑒𝑎𝑘 𝑎𝑟𝑒𝑎,

(2)

where ФBX and ФX are the QYs of BX and single exciton, respectively. According to this equation for the data presented in Figure 2, the BX QY is around 0.25 of the exciton QY.

Figure 2. (a) The cross-correlation functions of a single QD before (a) and after (b) GNRs deposition. (c) The ratio of the area of each peak of the cross-correlation function



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to the average area of the side peaks (with exception of the central peak) before (black dots) and after (red dots) GNRs deposition. Solid lines are a guide to the eye. After the GNR deposition, the central peak of the g(2), measured from the same QD, rises significantly, and its amplitude becomes comparable to the amplitude of the side peaks (Figure 2b). An important point is that all the g(2)(τ) peaks became strongly narrowed, which is most likely due to the 12-fold decrease in the average QD PL lifetime (Figure 1d). Along with this effect, the area of the central peak of the g(2)(τ) reached 100% of the average area of the side peaks (Figures 2c), which unequivocally indicates that the BX QY becomes equal to the exciton QY (Equation 2). These results clearly demonstrated that the interaction of QDs with the plasmon system in the developed thin-film structure makes the BX emission more efficient. However, the most striking result of this study is the effect of the separation between the QDs and GNRs on the enhancement of the BX PL. Using the thickness calibration curve (SI, Figure S4), film structures with a total thickness up to 180 nm were fabricated by deposition of PMMA layers with increasing thickness on the top of the initial 10-nm PMMA layer containing QDs, followed by GNRs deposition (SI, Scheme 2 in Figure S3). Figure 3a shows the estimated areas of g(2)(τ) peaks after GNR deposition for different spacer thicknesses. The data show that with an increase in the spacer thickness, the depth of the dip at τ = 0 increases and, therefore, the effect of GNRs on the BX generation in QD is weakening.


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Figure 3. Antibunching properties of single QDs at different distances from the GNRs layer. (a) The ratio of the area of each peak of the cross-correlation function to the average area of the side peaks (with the exception of the central peak) for three different spacer thicknesses. Solid lines are a guide to the eye. (b) Dependence of the ratio of the area of the central peak to the average area of the side peaks of the cross-correlation function on the thickness of the spacer between the QD and GNRs. Inset shows Schematics of the hybrid QD-GNRs system. Figure 3b shows the dependence of the depth of this dip on the spacer thickness, which clearly demonstrates the trend of monotonous decrease. It is remarkable, however, that the ratio of the area of the central peak to the average area of the side 13 ACS Paragon Plus Environment


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peaks and, therefore, the ratio of the BX QY to the exciton QY does not fall below 70% for a spacer thickness up to 170 nm. This shows that the plasmon-mediated enhancement of the BX emission persists even at such exceptionally large distances between QD and GNRs. A more detailed description of the experimental results presented in Figure 3 can be found in Figure S9 and Table S1 in Supplementary Information. The previous studies of the optical properties of hybrid QDs/PNPs structures reported much smaller distances (less than 40 nm) at which the effect of plasmoninduced enhancement of BX PL diminishes or even disappears entirely.28-29, 41 In the general case, the transfer of energy between the plasmon structure and quantum emitter in a coupled system is governed by the dipole–dipole interaction, the precise mechanism of which is determined by the thickness of the spacer. At short distances (R ~10 nm), the non-radiative local field of one dipole can excite the second dipole, the effect is known as FRET. At these distances, the efficiency of FRET decreases as R-6, because each dipole decays as R-3 in the near field. In this regime, if the radiative damping in the decay of the plasmons prevails, the energy transferred from the quantum emitter to the plasmon system can be re-emitted into the far field at the wavelength of the emitter and, due to the amplified local field of the PNPs, the PL of the emitter can be enhanced. In this scenario, to ensure effective improvement of biexciton PL, this secondary emission should dominate the inevitable quenching. On the other hand, the QY of the emitter in the presence of PNPs can also be enhanced through the Purcell effect.42-43 In this case, the PNPs can be considered as a nanoantennas or nanocavities, which modifies the local density of optical states of the plasmonic field, enhancing the radiative rate of the coupled emitter.44 It should be noted that, for a plasmon dipole, the local density of optical states obeys approximately the R-3


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law,45-47 and, hence, the PL of the emitter can be enhanced through the Purcell effect at significantly longer distances between the two components of the coupled system than in the FRET regime. In both cases, the necessary condition for the enhancement is the spectral overlap of the PNPs absorption band with the PL band of the emitter. Another important factor is which of the two processes, absorption or scattering, dominates the extinction spectrum. The dominance of absorption causes PL quenching, while an increase in the scattering component leads to enhanced luminescence.48 For relatively large PNPs (such as the GNRs used in this study), in which scattering predominates over absorption, modification of the radiative rate and PL intensity are results of interplay between the FRET and the Purcell mechanism at short distances (up to 10 nm) and through the Purcell effect alone at much larger distances,45-46, 48-50 which also is related to the regime of long-range surface enhanced fluorescence.51-53 Thus, a possible explanation for the enhancement of BX emission observed in this study is the trade-off between FRET and the Purcell effect at closer distances with the predominant influence of the latter for greater distances. It is noteworthy, that beyond the dipole-dipole approximation, when a quantum emitter interacts with two-dimensional or three-dimensional PNPs arrays,48, 54 or a fragmented metal film,48, 51, 53 that acts as an effective medium, even the 1/R dependency of PL modification can be observed.55 In our case, the coupling between the emitting state of the QDs and the plasmon mode of the layer of aggregated GNRs (SI, Figure S6) is assumed to be very efficient due to the strong overlap of the QD emission and GNRs absorption bands, which promotes the collective plasmon–exciton interaction at long distances up to 170 nm. In summary, our work shows that the biexciton emission can be considerably enhanced due to the long-range interaction of a single QD with a PNPs. Specifically, we



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fabricated a thin-film composite structure with a controllable distance between QDs and PNPs. To directly monitor the changes in the PL properties of BXs in QDs, we carried out PCS and time-resolved PL experiments on the same single QD before and after deposition of PNPs. We have shown that plasmon-mediated BX emission can be observed at distances of up to 170 nm between the constituents of the system. We explain the enhancement of the biexciton PL by modification of the recombination rate and growth of BX emission QY due to trade-off between FRET and the Purcell effect at small separations between a single QD and a PNPs, and the predominant influence of the Purcell effect at longer distances caused by the modification the local density of optical states. Thus, our work contributes in deeper understanding of the mechanisms of BX recombination in the presence of PNPs and provides a new insight into the requirements and strategies for the development of nanostructures with efficient BX emission, which can make a significant impact on materials science, quantum optics, quantum technologies, and sensing.

ASSOCIATED CONTENT Supporting Information The following files are available free of charge. General information on experimental methods; details of synthesis; the description of the procedures for fabrication of the hybrid QD-GNR during PL experiments on a single QD; details of PL decay studies and investigation of QDs using PCS; TEM images of QDs and GNRs, schematics of PMMA layer deposition, a calibration curve for PMMA thickness, AFM image of the GNR layer; fluorescent images of QDs in a PMMA film. (PDF) 16 ACS Paragon Plus Environment

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AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected]; *Email:[email protected] ORCID Viktor Krivenkov: 0000-0003-0280-2296 Yury Rakovich: 0000-0003-0111-2920 Notes The authors declare no competing financial interests. ACKNOWLEDGMENT Authors thank M. Molinari for TEM characterization of QDs and I.S. Vaskan for assistance with AFM profiling measurements. We acknowledge the financial support from the Ministry of Education and Science of the Russian Federation, grant no. 14.Y26.31.0011. Y.R. acknowledges the support from MINECO (Ministerio de Economiá y Competitividad, Spain), Project Fis2016.80174-P (PLASMOQUANTA). REFERENCES (1) Ekimov, A. I.; Onushchenko, A. A. Quantum size effect in three dimensional semiconductor microcrystals. JETP Lett. 1981, 34, 345–349. (2) Rogach, A. L. Semiconductor Nanocrystal Quantum Dots. Springer: Wien, New York, 2008; p 372. (3) Pietryga, J. M.; Park, Y.-S.; Lim, J.; Fidler, A. F.; Bae, W. K.; Brovelli, S.; Klimov, V. I. Spectroscopic and Device Aspects of Nanocrystal Quantum Dots. Chem. Rev. 2016, 116, 10513-10622. (4) Razgoniaeva, N.; Moroz, P.; Lambright, S.; Zamkov, M. Photocatalytic Applications of Colloidal Heterostructured Nanocrystals: What’s Next? The Journal of Physical Chemistry Letters 2015, 6, 4352–4359. (5) Alivisatos, P. The use of nanocrystals in biological detection. Nat. Biotechnol. 2004, 22, 47–52. 17 ACS Paragon Plus Environment


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