- Email: [email protected]
ZnS nanocrystals with gradient chemical composition by plasmonic nanoantennas
Optics and Laser Technology 121 (2020) 105821
Contents lists available at ScienceDirect
Optics and Laser Technology journal homepage: www.elsevier.com/locate/optlastec
Full length article
Direct enhancement of luminescence of CdxZn1−xSeyS1−y/ZnS nanocrystals with gradient chemical composition by plasmonic nanoantennas
T
⁎
Nikita A. Toropov , Aisylu N. Kamalieva, Roman O. Volkov, Ekaterina P. Kolesova, Dominika-Olga A. Volgina, Sergey A. Cherevkov, Aliaksei Dubavik, Tigran A. Vartanyan ITMO University, 197101 St. Petersburg, Russia
H I GH L IG H T S
of new nanocrystals was studied in the near field of plasmonic antennas. • Fluorescence their direct contact fluorescence enhancement was observed. • Upon • That was explained by the Purcell effect.
A R T I C LE I N FO
A B S T R A C T
Keywords: Plasmon Quantum dot Fluorescence Enhancement Purcell effect
Flat photonic materials such as thin films or metasurfaces form the basis of new optics and laser technology. However, their characteristics, for example, absorption and luminescence, are drastically limited in contrast to bulk materials. This problem can be overcome using plasmonic nanostructures. While the quantum dots absorption is enhanced by the plasmonic nanostructure near fields in almost all circumstances, more sophisticated approaches are needed to obtain the fluorescent enhancement because of the concurrent quenching by the metallic surface. Here we report on the results of an illustrative experiment demonstrating the luminescence enhancement of a new type of semiconductor nanocrystals CdxZn1−xSeyS1−y/ZnS (alloyed quantum dots) at their direct contact with a monolayer of silver nanoparticles. This type of nanocrystals consists of a CdSe core, a ZnS shell, and a transition layer in between where the composition smoothly changes, thus representing an alloy with variable x and y. We found that at the resonant plasmon–exciton coupling of such nanostructures, the fluorescence enhancement is accompanied by the reduction of the luminescence decay time – the main signature of the Purcell effect. Unlike previously studied compositions, these new hybrid structures do not require additional components to avoid fluorescence quenching which is promising for practical applications in flat optical devices.
1. Introduction Semiconductor nanocrystals or quantum dots (QDs) exhibit the quantum confinement effect which is propitious for optoelectronic devices [1,2]. This impressive effect allows us to tune the QDs spectrum through changing their sizes by several angstroms. QDs are characterized by high quantum yields of fluorescence and narrow and symmetric profiles of emission spectra, as well as wide absorption bands admitting the use of a single source for the excitation of different QDs. In addition, in contrast to organic dyes, QDs possess high thermal stability and photostability [3]. In the preceding decades, QD production has followed two main routes: epitaxial QDs were formed via the vacuum deposition technique ⁎
inside bulk semiconductor crystals, while colloidal QDs were chemically-synthesized in solutions. Colloidal QDs are advantageous for investigations into single-dot optical properties as well as for engineering the superstructures and superlattices based on nanocrystals [4,5]. The investigation into the metal-enhanced fluorescence of semiconductor nanocrystals and organic dyes started about 15 years ago [6,7]. The authors [6] described the dependence of the emission signal on the distance between CdSe QDs and Au nanoparticles supporting localized plasmon oscillations. The authors [8] showed the luminescence enhancement from near-surface quantum-well structures using surface plasmons in Ag gratings, which was considered as a conversion of surface plasmon’s modes into radiative modes. Thus, up to now, there is still no robust interpretation of fluorescent enhancement or
Corresponding author. E-mail address: [email protected] (N.A. Toropov).
https://doi.org/10.1016/j.optlastec.2019.105821 Received 4 February 2019; Received in revised form 18 June 2019; Accepted 5 September 2019 0030-3992/ © 2019 Elsevier Ltd. All rights reserved.
Optics and Laser Technology 121 (2020) 105821
N.A. Toropov, et al.
Alloyed CdxZn1−xSeyS1−y/ZnS semiconductor quantum dots were synthesized by the ‘hot injection’ method as described in [22]. Hydrophobic 7 nm QDs (where 2.4 nm is an estimated core diameter) stabilized with trioctylphosphine oxide and oleic acid molecules were produced as a result. The detailed synthesis procedures together with the TEM images of the CdxZn1−xSeyS1−y/ZnS QDs can be found in the Supporting Information Section [4]. For further samples preparation, QDs were dissolved in chloroform. Fig. 2 plots the absorption spectrum of quantum dots in chloroform. Each of the substrates with Ag nanoparticles and a pure quartz substrate were covered with 200 µL of QDs solution via a spin-coating technique and dried at normal temperature and pressure. The thickness of the QDs layer was estimated by concentration measurements of QDs dissolved from the surface and was about a few monolayers in average. The absorption spectra of the samples were measured with an SF-56 spectrophotometer (OKB-Spektr); the fluorescence spectra were recorded on an RF-5301PC spectrofluorophotometer (Shimadzu). The decay kinetics of fluorescence was carried out by a MicroTime 100 microscope (PicoQuant) equipped with a 409-nm wavelength laser and a 20× objective with numeric aperture of 0.95. The shape of silver nanoparticles affects the frequency of localized plasmons. Usually for island films fabricated through vacuum deposition, the plasmonic absorption band varies from 320 nm to 800 nm [25] and even more, depending on the deposition conditions and substrate materials. But subsequent heat treatment contributes to the nanoparticles uniformity, which leads to the plasmonic band narrowing. Upon contact or at small distances between metallic particles and QDs, fluorescence intensity is decreased due to a number of reasons, for example, the trapping of excited electrons or a resonant energy transfer from QDs to the metallic nanoparticle and subsequent radiationless relaxation transitions [4,10,17,27]. That means, on the one hand, that quantum dots should be placed quite far from silver nanoparticles to prevent quenching processes. On the other hand, quantum dots should be affected by the near fields of silver nanoparticles for the Purcell effect to take place. To find the optimal distance Ropt, we took into account that the effectiveness of the resonant energy transfer from QDs to nanoparticle is decreasing with the distance between them like ~ R−6 [28]. At the same time, the near field of silver nanoparticles E(R) is proportional to R−3 [29,30]. Using these assessments and calculations [30], we chose R = 3, 5, 10, and 13 nm for checking in experiments. As a separating layer, a silicon dioxide deposited at the same vacuum chamber was used. Since the frequency of plasmonic oscillations are sensitive to the dielectric permittivity of the surrounding medium, the optical density spectra of the island films are red-shifted (Fig. 1) and their extinction (absorption + scattering) cross-sections are also changed.
inadvertent quenching [9,10]. In the last few years, the enhancement process has been attributed to the Purcell effect. The pioneering work of Purcell [11] generalized that the fluorescence intensity is determined both by the intrinsic characteristics of the quantum emitter and can not only be decreased by the material environment in which the quantum dot is located, but can also be enhanced due to altering the local electromagnetic density of states as well [12–15]. In particular, when QDs are placed near a metallic nanoparticle supporting localized plasmon oscillations, the fluorescence intensity can be enhanced. This allows a metallic nanoparticle to act as a nanoantenna, strongly modifying the electric fields around it and therefore changing the fluorescence by enhancing the light harvesting [16] and altering the radiative and non-radiative decay rates of quantum dots [10,12,17]. At the present time, to achieve higher quantum yields of fluorescence and chemical stability, colloidal quantum dots are synthesized in the form of heterostructures comprising luminescent cores and shells that serve as potential barriers for the electrons confined in the cores. There has been significant interest recently in improving the performance of fluorophores like CdSe and CdSe/ZnS nanocrystals through their combination with gold and silver nanostructures, motivated by both fundamental interest and potential applications. On the one hand, excitons in QDs and plasmons in nanoparticles are relevant objects for impressive research work in the course of the plasmon–exciton (plexciton [18]) strong coupling regime, when the rate of the coherent energy exchange between plasmons and excitons exceeds the rate of the energy losses in the system. In contrast to the metal-enhanced luminescence occurring in the weak coupling regime, in the strong coupling regime plasmon–exciton systems demonstrate such effects as Rabi splitting of fluorescence spectra, optical nonlinearities, and magnetooptical response [19]. On the other hand, QDs with plasmonic particles may be potentially harnessed in many applications, for instance, for sensors and tools for biomedical assays, for quantum information processing, solar cells, light-emitting devices and many other applications where strong coupling is not needed [3,10,20,21]. Recently, a single-step approach for the synthesis of quantum dots CdxZn1−xSeyS1−y/ZnS with chemical composition gradients [22] or alloyed quantum dots was proposed. The main advantage of such QDs is the possibility to tune their properties in a wider range, with the particular aim of increasing their luminescence. However, even such QDs need further improvements. One of the ways for fluorescence enhancement is QDs resonance coupling to the near field of a plasmonic nanoantenna, or the Purcell effect. In this case, enhancement of fluorescence intensity is associated with a lifetime reduction in fluorescence. In the present work we show a clear correlation between fluorescence enhancement and a lifetime decrease for alloyed QDs CdxZn1−xSeyS1−y/ZnS resonantly coupled to silver nanoparticles at different distances. To the best of our knowledge, the Purcell effect for such kind of quantum dots CdxZn1−xSeyS1−y/ZnS caused by nanostructures supporting localized plasmon oscillations were investigated for the first time.
3. Results and discussion The absorptive properties of QDs on top of Ag nanoparticles covered with 10-nm silicon dioxide are presented in Fig. 2. An example of this sample elucidates that adding the quantum dots on the covered island film slightly changes the reflectivity (due to high refractive index of quantum dots material themselves [31,32]) rather than make a contribution to the plasmon frequency shifting. The fluorescence spectrum of quantum dots separated from the silver island film with the silicon dioxide layer is presented in Fig. 2. This spectrum was measured using a 10-nm excitation band centered at a wavelength of 405 nm and a 3 nm spectral slit of a registering monochromator. It is crucial for fluorescence enhancement that a spectral overlap between localized plasmonic oscillations in Ag particles and the QDs excitonic band was reached. For quantitative analysis of QD fluorescence, a sequence of decay kinetics measurements was performed using the laser microscope (Fig. 3). The ordinate axis in the picture corresponds to the intensity of luminescence of quantum dots placed at the quartz substrate, the quartz
2. Materials and methods Metal nanoparticles demonstrating a plasmonic absorption band (Fig. 1) were formed as an island film on a surface of the quartz substrates by physical vapour deposition in a vacuum chamber from a tungsten boat containing a silver of special-purity grade (99.99%). Silver was chosen because silver nanostructures have low-losses at optical frequencies [23,24]. Further thermal annealing of the samples at 200 °C led to the dewetting process, resulting in the hemispherical nanoparticles formation at the substrate surface [25]. The area-filling fraction was less than 50% [26]. The equivalent thickness of silver film and growth rate were determined using a quartz microbalance; they turned out to be 5 nm and 0.01 nm/s, correspondingly. During the growth process of silver film and thermal annealing, the residual gas pressure was kept to the order of ~10−7 Torr. 2
Optics and Laser Technology 121 (2020) 105821
N.A. Toropov, et al.
Fig. 1. Sketch of samples investigated; absorption spectra of the silver nanoparticles film (Ag NPs), and the same films covered with silicon dioxide layers. Layer thicknesses of SiO2 3, 5, 10, and 13 nm mark the corresponding curves.
substrate with Ag nanoparticles, and the same particles covered with silicone dioxide with different thicknesses. The character of decay kinetics and the initial fluorescent signals of QDs are different and strongly depend on the presence or absence of Ag nanoparticles and silicon dioxide layers. Fig. 3 plots the raw data of kinetics measured. Generally, there are many radiative and non-radiative channels of the exciton relaxation characterized by different decay times, for example, in a core of quantum dots, their shell, blinking contribution, and energy transfers [3,4,27,33]. Because of that, the decay kinetics is complicated and can be fitted by a multi-exponentials function with high accuracy: t
t
I (t ) = A1 e− τ1 + A2 e− τ 2 + ⋯,
(1)
where t is the time elapsed after the excitation pulse. However, an example approximation given in Fig. 4 demonstrates a good agreement of the biexponential function with the experimentally-obtained results. For this reason, further analysis was restricted to the pre-exponential factors and the decay times obtained for two exponential fits presented in Fig. 4. It should be noted that for correct comparison all measurements and approximations were performed in identical conditions. Taking into account the values of the fluorescence intensities (Fig. 3), τ1 and τ2 (Table 1), we plotted the dependence of fluorescence intensity of QDs and its averaged lifetime 〈τ 〉 on the SiO2 spacer layer thickness, R (Fig. 5), where 〈τ 〉 is determined by the following formula:
Fig. 2. (Left axis) Absorbance (optical density) of the alloyed quantum dots solution in chloroform (orange curve) used for the layer preparation and that of the Ag nanoparticles (Ag NPs) film covered by 10-nm thick silicon dioxide layer and, on top of it, by a layer of QDs. (Right axis) Fluorescence spectrum for the same film (blue curve). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 3. Fluorescence kinetics of alloyed quantum dots on the quartz surface (QDs), QDs on the surface with Ag nanoparticles (Ag + QDs), and QDs on the surface with Ag nanoparticles covered with 3, 5, 10, and 13 nm silicon dioxide layers (Ag + SiO2(3/5/10/13) + QDs).
Fig. 4. Fluorescence decay kinetics (raw data) of alloyed QDs separated from Ag nanoparticles by a 10-nm thick film of silicon dioxide (red circles) and its biexponential approximation (black solid line). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) 3
Optics and Laser Technology 121 (2020) 105821
N.A. Toropov, et al.
4. Conclusions
Table 1 Pre-exponential factors (arb. units) and corresponding decay times (ns) of QDs with/without Ag nanoparticles.
Ag + QDs Ag + SiO2(3 nm) + QDs Ag + SiO2(5 nm) + QDs Ag + SiO2(10 nm) + QDs Ag + SiO2(13 nm) + QDs QDs
A1
τ1
A2
τ2
90,000 76,989 60,262 62,424 59,279 55,622
2.54 2.01 2.24 2.44 2.49 3.53
107,400 90,537 70,297 64,855 73,255 68,494
16.79 13.84 14.54 13.30 15.19 19.83
In this article, we performed the simple and very illustrative experiment to observe the luminescence enhancement of the thin layer of a new type of quantum dots through the Purcell effect by direct contact with plasmonic nanoparticles. This type of quantum dots consists of a core and a shell with smooth radial changes of their chemical composition. It was shown that at the resonant interaction of such semiconductor nanocrystals with localized plasmons, the decay time of their luminescence is reduced. It can be argued that this type of ‘plasmon–quantum dots’ hybrid structure is promising for practical applications and the development of highly efficient flat optical devices. Acknowledgments We thank Kristina Rizvanova for help with measurements. This work was partially supported by the RFBR (16-02-00932 A) and the Council on grants from the President of the Russian Federation, the Government of Russia (08-08). Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.optlastec.2019.105821. References [1] A.P. Alivisatos, Semiconductor clusters, nanocrystals, and quantum dots, Science 271 (1996) 933. [2] S. Jindal, S.M. Giripunje, Potential effect of CuInS2/ZnS core-shell quantum dots on P3HT/PEDOT:PSS heterostructure based solar cell, Opt. Laser Technol. 103 (2018) 212. [3] U. Resch-Genger, M. Grabolle, S. Cavaliere-Jaricot, R. Nitschke, T. Nann, Quantum dots versus organic dyes as fluorescent labels, Nat. Methods 5 (2008) 763. [4] T.K. Kormilina, E.A. Stepanidenko, S.A. Cherevkov, A. Dubavik, M.A. Baranov, A.V. Fedorov, A.V. Baranov, Y.K. Gun'ko, E.V. Ushakova, A highly luminescent porous metamaterial based on a mixture of gold and alloyed semiconductor nanoparticles, J. Mater. Chem. C 6 (2018) 5278. [5] Y. Yu, D. Yu, C.A. Orme, Reversible, tunable, electric-field driven assembly of silver nanocrystal superlattices, Nano Lett. 17 (2017) 3862. [6] O. Kulakovich, N. Strekal, A. Yaroshevich, S. Maskevich, S. Gaponenko, I. Nabiev, U. Woggon, M. Artemyev, Enhanced luminescence of CdSe quantum dots on gold colloids, Nano Lett. 2 (2002) 1449. [7] A.E. Ragab, A. Gadallah, M.B. Mohamed, I.M. Azzouz, Effect of silver NPs plasmon on optical properties of fluorescein dye, Opt. Laser Technol. 52 (2013) 109. [8] T. Sadi, J. Oksanen, J. Tulkki, Effect of plasmonic losses on light emission enhancement in quantum-wells coupled to metallic gratings, J. Appl. Phys. 114 (2013) 223104. [9] A.E. Ragab, A. Gadallah, M.B. Mohamed, I.M. Azzouz, Photoluminescence and upconversion on Ag/CdTe quantum dots, Opt. Laser Technol. 63 (2014) 8. [10] S.Y. Lee, K. Nakaya, T. Hayashi, M. Hara, Quantitative study of the gold-enhanced fluorescence of CdSe/ZnS nanocrystals as a function of distance using an AFM probe, Phys. Chem. Chem. Phys. 11 (2009) 4403. [11] E.M. Purcell, Spontaneous emission probabilities at radio frequencies, Phys. Rev. 69 (1946) 681. [12] J. Li, A.V. Krasavin, L. Webster, P. Segovia, A.V. Zayats, D. Richards, Spectral variation of fluorescence lifetime near single metal nanoparticles, Sci. Rep. 6 (2016) 21349. [13] A.E. Krasnok, A.P. Slobozhanyuk, C.R. Simovski, S.A. Tretyakov, A.N. Poddubny, A.E. Miroshnichenko, Y.S. Kivshar, P.A. Belov, An antenna model for the Purcell effect, Sci. Rep. 5 (2015) 12956. [14] M. Agio, D.M. Cano, The Purcell factor of nanoresonators, Nat. Photonics 7 (2013) 674. [15] Y. Lu, R. Sokhoyan, W. Cheng, S.G. Kafaie, A.R. Davoyan, R.A. Pala, K. Thyagarajan, H.A. Atwater, Dynamically controlled Purcell enhancement of visible spontaneous emission in a gated plasmonic heterostructure, Nat. Commun. 8 (2017) 1631. [16] E. Wientjes, J. Renger, A.G. Curto, R. Cogdell, N.F. van Hulst, Nanoantenna enhanced emission of lightharvesting complex 2: the role of resonance, polarization, and radiative and non-radiative rates, Phys. Chem. Chem. Phys. 16 (2014) 24739. [17] A.G. Bakanov, N.A. Toropov, T.A. Vartanyan, Optical properties of planar nanostructures based on semiconductor quantum dots and plasmonic metal nanoparticles, Opt. Spectrosc. 120 (2016) 477. [18] N.T. Fofang, N.K. Grady, Z. Fan, A.O. Govorov, N.J. Halas, Plexciton dynamics: exciton−plasmon coupling in a J-aggregate−Au nanoshell complex provides a mechanism for nonlinearity, Nano Lett. 11 (2011) 1556. [19] D. Melnikau, A.A. Govyadinov, A. Sánchez-Iglesias, M. Grzelczak, L.M. Liz-Marzán, Y.P. Rakovich, Strong magneto-optical response of nonmagnetic organic materials
Fig. 5. Dependence of fluorescence intensities and corresponding decay times of alloyed QDs CdxZn1−xSeyS1−y/ZnS in the distance between QD layers and Ag nanoparticle island films.
〈τ 〉 = (A1 τ 12 + A2 τ 22)/(A1 τ 1 +A2 τ 2).
(2)
Analysis of the plotted dependences of fluorescence intensities and decay times in the distance between QDs and Ag nanoparticles, R, (Fig. 5) leads to the conclusion that the experimental observations are due to the Purcell effect. Indeed, starting with the QDs that do not interact with metal nanoparticles and going through the spacer layer thicknesses of 13 nm and 10 nm, the fluorescence intensities of QDs in the near fields of Ag nanoparticles are enhanced while the average lifetimes are simultaneously reduced in accordance with expectations for the Purcell effect. Despite being at smaller distances, the lifetime behavior becomes more complicated, the fluorescence intensity continues to rise. For this reason, QDs with a gradient component composition are more promising candidates for applications in high-efficient light-emitting devices or sensors because they do not require a buffer layer separating them from plasmonic nanostructures. Indeed, the largest fluorescence enhancement was observed in the case of direct contact between the QDs and silver nanoparticles. In other words, the optimal thickness of the dielectric layer Ropt is 0 nm for the alloyed CdxZn1-xSeyS1-y/ZnS quantum dots. Contrary to the ordinary core–shell CdSe/ZnS quantum dots, the shell thickness of which is commensurate with the size of their nucleus, in alloyed QDs the shell is much larger than the core. From the curve obtained (Fig. 5), it can be seen that the maximum fluorescence is observed when we do not have a dielectric layer; quantum dots just are put on the surface of the nanoparticles. Based on this data, it can be concluded that the shells of alloyed QDs themselves perform the function of an insulating layer. A slight increase of the fluorescence lifetime that accompanies the growth of the fluorescence intensity on the smallest thicknesses of the spacer layer shows that, in addition to the Purcell effect, the non-radiative losses reduction is responsible for the observed phenomena in this case. Nevertheless, the fluorescence lifetime is still smaller than in the case of the isolated QDs. 4
Optics and Laser Technology 121 (2020) 105821
N.A. Toropov, et al.
coupled to plasmonic nanostructures, Nano Lett. 17 (2017) 1808. [20] Y. Yang, Y. Zheng, W. Cao, A. Titov, J. Hyvonen, J.R. Manders, J. Xue, P.H. Holloway, L. Qian, High-efficiency light-emitting devices based on quantum dots with tailored nanostructures, Nat. Photonics 9 (2015) 259. [21] R.M. Elnoby, M.H. Mourad, S.L. Hassab Elnaby, M. Abou Kana, Monocrystalline solar cells performance coated by silver nanoparticles: Effect of NPs sizes from point of view Mie theory, Opt. Laser Technol. 101 (2018) 208. [22] W.K. Bae, K. Char, H. Hur, S. Lee, Single-step synthesis of quantum dots with chemical composition gradients, Chem. Mater. 20 (2008) 531. [23] C. Wang, H. Chen, L. Sun, W. Chen, Y. Chang, H. Ahn, X. Li, S. Gwo, Giant colloidal silver crystals for low-loss linear and nonlinear plasmonics, Nat. Commun. 6 (2015) 7734. [24] A.A. High, R.C. Devlin, A. Dibos, M. Polking, D.S. Wild, J. Perczel, N.P. de Leon, M.D. Lukin, H. Park, Visible-frequency hyperbolic metasurface, Nature 522 (2015) 192–196. [25] N.A. Toropov, N.B. Leonov, T.A. Vartanyan, Influence of silver nanoparticles crystallinity on localized surface plasmons dephasing times, Phys. Status Solidi B 255 (2018) 1700174. [26] A. Berthelot, G.C. des Francs, H. Varguet, J. Margueritat, R. Mascart, J. Benoit, J. Laverdant, From localized to delocalized plasmonic modes, first observation of
[27]
[28]
[29]
[30] [31] [32]
[33]
5
superradiant scattering in disordered semi-continuous metal films, Nanotechnology 30 (2019) 015706. A. Ishikawa, K. Osono, A. Nobuhiro, Y. Mizumoto, T. Torimoto, H. Ishihara, Theory for self-consistent interplay between light and nanomaterials strongly modified by metallic nanostructures, Phys. Chem. Chem. Phys. 15 (2013) 4214. A.O. Govorov, J. Lee, N.A. Kotov, Theory of plasmon-enhanced Förster energy transfer in optically excited semiconductor and metal nanoparticles, Phys. Rev. B 76 (2007) 125308. K.L. Kelly, E. Coronado, L. Zhao, G.C. Schatz, The optical properties of metal nanoparticles: the influence of size, shape, and dielectric environment, J. Phys. Chem. B 107 (2003) 668. D.V. Guzatov, V.V. Klimov, Radiative decay engineering by triaxial nanoellipsoids, Chem. Phys. Lett. 412 (2005) 341. M. Debenham, Refractive indices of zinc sulfide in the 0.405-13-µm wavelength range, Appl. Opt. 23 (1984) 2238. M.P. Lisitsa, L.F. Gudymenko, V.N. Malinko, S.F. Terekhova, Dispersion of the refractive indices and birefringence of CdSxSe1-x single crystals, Phys. Status Solidi 31 (1969) 389. W. Qin, R.A. Shah, P. Guyot-Sionnest, CdSeS/ZnS alloyed nanocrystal lifetime and blinking studies under electrochemical control, ACS Nano 6 (2012) 912.
Contents lists available at ScienceDirect
Optics and Laser Technology journal homepage: www.elsevier.com/locate/optlastec
Full length article
Direct enhancement of luminescence of CdxZn1−xSeyS1−y/ZnS nanocrystals with gradient chemical composition by plasmonic nanoantennas
T
⁎
Nikita A. Toropov , Aisylu N. Kamalieva, Roman O. Volkov, Ekaterina P. Kolesova, Dominika-Olga A. Volgina, Sergey A. Cherevkov, Aliaksei Dubavik, Tigran A. Vartanyan ITMO University, 197101 St. Petersburg, Russia
H I GH L IG H T S
of new nanocrystals was studied in the near field of plasmonic antennas. • Fluorescence their direct contact fluorescence enhancement was observed. • Upon • That was explained by the Purcell effect.
A R T I C LE I N FO
A B S T R A C T
Keywords: Plasmon Quantum dot Fluorescence Enhancement Purcell effect
Flat photonic materials such as thin films or metasurfaces form the basis of new optics and laser technology. However, their characteristics, for example, absorption and luminescence, are drastically limited in contrast to bulk materials. This problem can be overcome using plasmonic nanostructures. While the quantum dots absorption is enhanced by the plasmonic nanostructure near fields in almost all circumstances, more sophisticated approaches are needed to obtain the fluorescent enhancement because of the concurrent quenching by the metallic surface. Here we report on the results of an illustrative experiment demonstrating the luminescence enhancement of a new type of semiconductor nanocrystals CdxZn1−xSeyS1−y/ZnS (alloyed quantum dots) at their direct contact with a monolayer of silver nanoparticles. This type of nanocrystals consists of a CdSe core, a ZnS shell, and a transition layer in between where the composition smoothly changes, thus representing an alloy with variable x and y. We found that at the resonant plasmon–exciton coupling of such nanostructures, the fluorescence enhancement is accompanied by the reduction of the luminescence decay time – the main signature of the Purcell effect. Unlike previously studied compositions, these new hybrid structures do not require additional components to avoid fluorescence quenching which is promising for practical applications in flat optical devices.
1. Introduction Semiconductor nanocrystals or quantum dots (QDs) exhibit the quantum confinement effect which is propitious for optoelectronic devices [1,2]. This impressive effect allows us to tune the QDs spectrum through changing their sizes by several angstroms. QDs are characterized by high quantum yields of fluorescence and narrow and symmetric profiles of emission spectra, as well as wide absorption bands admitting the use of a single source for the excitation of different QDs. In addition, in contrast to organic dyes, QDs possess high thermal stability and photostability [3]. In the preceding decades, QD production has followed two main routes: epitaxial QDs were formed via the vacuum deposition technique ⁎
inside bulk semiconductor crystals, while colloidal QDs were chemically-synthesized in solutions. Colloidal QDs are advantageous for investigations into single-dot optical properties as well as for engineering the superstructures and superlattices based on nanocrystals [4,5]. The investigation into the metal-enhanced fluorescence of semiconductor nanocrystals and organic dyes started about 15 years ago [6,7]. The authors [6] described the dependence of the emission signal on the distance between CdSe QDs and Au nanoparticles supporting localized plasmon oscillations. The authors [8] showed the luminescence enhancement from near-surface quantum-well structures using surface plasmons in Ag gratings, which was considered as a conversion of surface plasmon’s modes into radiative modes. Thus, up to now, there is still no robust interpretation of fluorescent enhancement or
Corresponding author. E-mail address: [email protected] (N.A. Toropov).
https://doi.org/10.1016/j.optlastec.2019.105821 Received 4 February 2019; Received in revised form 18 June 2019; Accepted 5 September 2019 0030-3992/ © 2019 Elsevier Ltd. All rights reserved.
Optics and Laser Technology 121 (2020) 105821
N.A. Toropov, et al.
Alloyed CdxZn1−xSeyS1−y/ZnS semiconductor quantum dots were synthesized by the ‘hot injection’ method as described in [22]. Hydrophobic 7 nm QDs (where 2.4 nm is an estimated core diameter) stabilized with trioctylphosphine oxide and oleic acid molecules were produced as a result. The detailed synthesis procedures together with the TEM images of the CdxZn1−xSeyS1−y/ZnS QDs can be found in the Supporting Information Section [4]. For further samples preparation, QDs were dissolved in chloroform. Fig. 2 plots the absorption spectrum of quantum dots in chloroform. Each of the substrates with Ag nanoparticles and a pure quartz substrate were covered with 200 µL of QDs solution via a spin-coating technique and dried at normal temperature and pressure. The thickness of the QDs layer was estimated by concentration measurements of QDs dissolved from the surface and was about a few monolayers in average. The absorption spectra of the samples were measured with an SF-56 spectrophotometer (OKB-Spektr); the fluorescence spectra were recorded on an RF-5301PC spectrofluorophotometer (Shimadzu). The decay kinetics of fluorescence was carried out by a MicroTime 100 microscope (PicoQuant) equipped with a 409-nm wavelength laser and a 20× objective with numeric aperture of 0.95. The shape of silver nanoparticles affects the frequency of localized plasmons. Usually for island films fabricated through vacuum deposition, the plasmonic absorption band varies from 320 nm to 800 nm [25] and even more, depending on the deposition conditions and substrate materials. But subsequent heat treatment contributes to the nanoparticles uniformity, which leads to the plasmonic band narrowing. Upon contact or at small distances between metallic particles and QDs, fluorescence intensity is decreased due to a number of reasons, for example, the trapping of excited electrons or a resonant energy transfer from QDs to the metallic nanoparticle and subsequent radiationless relaxation transitions [4,10,17,27]. That means, on the one hand, that quantum dots should be placed quite far from silver nanoparticles to prevent quenching processes. On the other hand, quantum dots should be affected by the near fields of silver nanoparticles for the Purcell effect to take place. To find the optimal distance Ropt, we took into account that the effectiveness of the resonant energy transfer from QDs to nanoparticle is decreasing with the distance between them like ~ R−6 [28]. At the same time, the near field of silver nanoparticles E(R) is proportional to R−3 [29,30]. Using these assessments and calculations [30], we chose R = 3, 5, 10, and 13 nm for checking in experiments. As a separating layer, a silicon dioxide deposited at the same vacuum chamber was used. Since the frequency of plasmonic oscillations are sensitive to the dielectric permittivity of the surrounding medium, the optical density spectra of the island films are red-shifted (Fig. 1) and their extinction (absorption + scattering) cross-sections are also changed.
inadvertent quenching [9,10]. In the last few years, the enhancement process has been attributed to the Purcell effect. The pioneering work of Purcell [11] generalized that the fluorescence intensity is determined both by the intrinsic characteristics of the quantum emitter and can not only be decreased by the material environment in which the quantum dot is located, but can also be enhanced due to altering the local electromagnetic density of states as well [12–15]. In particular, when QDs are placed near a metallic nanoparticle supporting localized plasmon oscillations, the fluorescence intensity can be enhanced. This allows a metallic nanoparticle to act as a nanoantenna, strongly modifying the electric fields around it and therefore changing the fluorescence by enhancing the light harvesting [16] and altering the radiative and non-radiative decay rates of quantum dots [10,12,17]. At the present time, to achieve higher quantum yields of fluorescence and chemical stability, colloidal quantum dots are synthesized in the form of heterostructures comprising luminescent cores and shells that serve as potential barriers for the electrons confined in the cores. There has been significant interest recently in improving the performance of fluorophores like CdSe and CdSe/ZnS nanocrystals through their combination with gold and silver nanostructures, motivated by both fundamental interest and potential applications. On the one hand, excitons in QDs and plasmons in nanoparticles are relevant objects for impressive research work in the course of the plasmon–exciton (plexciton [18]) strong coupling regime, when the rate of the coherent energy exchange between plasmons and excitons exceeds the rate of the energy losses in the system. In contrast to the metal-enhanced luminescence occurring in the weak coupling regime, in the strong coupling regime plasmon–exciton systems demonstrate such effects as Rabi splitting of fluorescence spectra, optical nonlinearities, and magnetooptical response [19]. On the other hand, QDs with plasmonic particles may be potentially harnessed in many applications, for instance, for sensors and tools for biomedical assays, for quantum information processing, solar cells, light-emitting devices and many other applications where strong coupling is not needed [3,10,20,21]. Recently, a single-step approach for the synthesis of quantum dots CdxZn1−xSeyS1−y/ZnS with chemical composition gradients [22] or alloyed quantum dots was proposed. The main advantage of such QDs is the possibility to tune their properties in a wider range, with the particular aim of increasing their luminescence. However, even such QDs need further improvements. One of the ways for fluorescence enhancement is QDs resonance coupling to the near field of a plasmonic nanoantenna, or the Purcell effect. In this case, enhancement of fluorescence intensity is associated with a lifetime reduction in fluorescence. In the present work we show a clear correlation between fluorescence enhancement and a lifetime decrease for alloyed QDs CdxZn1−xSeyS1−y/ZnS resonantly coupled to silver nanoparticles at different distances. To the best of our knowledge, the Purcell effect for such kind of quantum dots CdxZn1−xSeyS1−y/ZnS caused by nanostructures supporting localized plasmon oscillations were investigated for the first time.
3. Results and discussion The absorptive properties of QDs on top of Ag nanoparticles covered with 10-nm silicon dioxide are presented in Fig. 2. An example of this sample elucidates that adding the quantum dots on the covered island film slightly changes the reflectivity (due to high refractive index of quantum dots material themselves [31,32]) rather than make a contribution to the plasmon frequency shifting. The fluorescence spectrum of quantum dots separated from the silver island film with the silicon dioxide layer is presented in Fig. 2. This spectrum was measured using a 10-nm excitation band centered at a wavelength of 405 nm and a 3 nm spectral slit of a registering monochromator. It is crucial for fluorescence enhancement that a spectral overlap between localized plasmonic oscillations in Ag particles and the QDs excitonic band was reached. For quantitative analysis of QD fluorescence, a sequence of decay kinetics measurements was performed using the laser microscope (Fig. 3). The ordinate axis in the picture corresponds to the intensity of luminescence of quantum dots placed at the quartz substrate, the quartz
2. Materials and methods Metal nanoparticles demonstrating a plasmonic absorption band (Fig. 1) were formed as an island film on a surface of the quartz substrates by physical vapour deposition in a vacuum chamber from a tungsten boat containing a silver of special-purity grade (99.99%). Silver was chosen because silver nanostructures have low-losses at optical frequencies [23,24]. Further thermal annealing of the samples at 200 °C led to the dewetting process, resulting in the hemispherical nanoparticles formation at the substrate surface [25]. The area-filling fraction was less than 50% [26]. The equivalent thickness of silver film and growth rate were determined using a quartz microbalance; they turned out to be 5 nm and 0.01 nm/s, correspondingly. During the growth process of silver film and thermal annealing, the residual gas pressure was kept to the order of ~10−7 Torr. 2
Optics and Laser Technology 121 (2020) 105821
N.A. Toropov, et al.
Fig. 1. Sketch of samples investigated; absorption spectra of the silver nanoparticles film (Ag NPs), and the same films covered with silicon dioxide layers. Layer thicknesses of SiO2 3, 5, 10, and 13 nm mark the corresponding curves.
substrate with Ag nanoparticles, and the same particles covered with silicone dioxide with different thicknesses. The character of decay kinetics and the initial fluorescent signals of QDs are different and strongly depend on the presence or absence of Ag nanoparticles and silicon dioxide layers. Fig. 3 plots the raw data of kinetics measured. Generally, there are many radiative and non-radiative channels of the exciton relaxation characterized by different decay times, for example, in a core of quantum dots, their shell, blinking contribution, and energy transfers [3,4,27,33]. Because of that, the decay kinetics is complicated and can be fitted by a multi-exponentials function with high accuracy: t
t
I (t ) = A1 e− τ1 + A2 e− τ 2 + ⋯,
(1)
where t is the time elapsed after the excitation pulse. However, an example approximation given in Fig. 4 demonstrates a good agreement of the biexponential function with the experimentally-obtained results. For this reason, further analysis was restricted to the pre-exponential factors and the decay times obtained for two exponential fits presented in Fig. 4. It should be noted that for correct comparison all measurements and approximations were performed in identical conditions. Taking into account the values of the fluorescence intensities (Fig. 3), τ1 and τ2 (Table 1), we plotted the dependence of fluorescence intensity of QDs and its averaged lifetime 〈τ 〉 on the SiO2 spacer layer thickness, R (Fig. 5), where 〈τ 〉 is determined by the following formula:
Fig. 2. (Left axis) Absorbance (optical density) of the alloyed quantum dots solution in chloroform (orange curve) used for the layer preparation and that of the Ag nanoparticles (Ag NPs) film covered by 10-nm thick silicon dioxide layer and, on top of it, by a layer of QDs. (Right axis) Fluorescence spectrum for the same film (blue curve). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 3. Fluorescence kinetics of alloyed quantum dots on the quartz surface (QDs), QDs on the surface with Ag nanoparticles (Ag + QDs), and QDs on the surface with Ag nanoparticles covered with 3, 5, 10, and 13 nm silicon dioxide layers (Ag + SiO2(3/5/10/13) + QDs).
Fig. 4. Fluorescence decay kinetics (raw data) of alloyed QDs separated from Ag nanoparticles by a 10-nm thick film of silicon dioxide (red circles) and its biexponential approximation (black solid line). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) 3
Optics and Laser Technology 121 (2020) 105821
N.A. Toropov, et al.
4. Conclusions
Table 1 Pre-exponential factors (arb. units) and corresponding decay times (ns) of QDs with/without Ag nanoparticles.
Ag + QDs Ag + SiO2(3 nm) + QDs Ag + SiO2(5 nm) + QDs Ag + SiO2(10 nm) + QDs Ag + SiO2(13 nm) + QDs QDs
A1
τ1
A2
τ2
90,000 76,989 60,262 62,424 59,279 55,622
2.54 2.01 2.24 2.44 2.49 3.53
107,400 90,537 70,297 64,855 73,255 68,494
16.79 13.84 14.54 13.30 15.19 19.83
In this article, we performed the simple and very illustrative experiment to observe the luminescence enhancement of the thin layer of a new type of quantum dots through the Purcell effect by direct contact with plasmonic nanoparticles. This type of quantum dots consists of a core and a shell with smooth radial changes of their chemical composition. It was shown that at the resonant interaction of such semiconductor nanocrystals with localized plasmons, the decay time of their luminescence is reduced. It can be argued that this type of ‘plasmon–quantum dots’ hybrid structure is promising for practical applications and the development of highly efficient flat optical devices. Acknowledgments We thank Kristina Rizvanova for help with measurements. This work was partially supported by the RFBR (16-02-00932 A) and the Council on grants from the President of the Russian Federation, the Government of Russia (08-08). Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.optlastec.2019.105821. References [1] A.P. Alivisatos, Semiconductor clusters, nanocrystals, and quantum dots, Science 271 (1996) 933. [2] S. Jindal, S.M. Giripunje, Potential effect of CuInS2/ZnS core-shell quantum dots on P3HT/PEDOT:PSS heterostructure based solar cell, Opt. Laser Technol. 103 (2018) 212. [3] U. Resch-Genger, M. Grabolle, S. Cavaliere-Jaricot, R. Nitschke, T. Nann, Quantum dots versus organic dyes as fluorescent labels, Nat. Methods 5 (2008) 763. [4] T.K. Kormilina, E.A. Stepanidenko, S.A. Cherevkov, A. Dubavik, M.A. Baranov, A.V. Fedorov, A.V. Baranov, Y.K. Gun'ko, E.V. Ushakova, A highly luminescent porous metamaterial based on a mixture of gold and alloyed semiconductor nanoparticles, J. Mater. Chem. C 6 (2018) 5278. [5] Y. Yu, D. Yu, C.A. Orme, Reversible, tunable, electric-field driven assembly of silver nanocrystal superlattices, Nano Lett. 17 (2017) 3862. [6] O. Kulakovich, N. Strekal, A. Yaroshevich, S. Maskevich, S. Gaponenko, I. Nabiev, U. Woggon, M. Artemyev, Enhanced luminescence of CdSe quantum dots on gold colloids, Nano Lett. 2 (2002) 1449. [7] A.E. Ragab, A. Gadallah, M.B. Mohamed, I.M. Azzouz, Effect of silver NPs plasmon on optical properties of fluorescein dye, Opt. Laser Technol. 52 (2013) 109. [8] T. Sadi, J. Oksanen, J. Tulkki, Effect of plasmonic losses on light emission enhancement in quantum-wells coupled to metallic gratings, J. Appl. Phys. 114 (2013) 223104. [9] A.E. Ragab, A. Gadallah, M.B. Mohamed, I.M. Azzouz, Photoluminescence and upconversion on Ag/CdTe quantum dots, Opt. Laser Technol. 63 (2014) 8. [10] S.Y. Lee, K. Nakaya, T. Hayashi, M. Hara, Quantitative study of the gold-enhanced fluorescence of CdSe/ZnS nanocrystals as a function of distance using an AFM probe, Phys. Chem. Chem. Phys. 11 (2009) 4403. [11] E.M. Purcell, Spontaneous emission probabilities at radio frequencies, Phys. Rev. 69 (1946) 681. [12] J. Li, A.V. Krasavin, L. Webster, P. Segovia, A.V. Zayats, D. Richards, Spectral variation of fluorescence lifetime near single metal nanoparticles, Sci. Rep. 6 (2016) 21349. [13] A.E. Krasnok, A.P. Slobozhanyuk, C.R. Simovski, S.A. Tretyakov, A.N. Poddubny, A.E. Miroshnichenko, Y.S. Kivshar, P.A. Belov, An antenna model for the Purcell effect, Sci. Rep. 5 (2015) 12956. [14] M. Agio, D.M. Cano, The Purcell factor of nanoresonators, Nat. Photonics 7 (2013) 674. [15] Y. Lu, R. Sokhoyan, W. Cheng, S.G. Kafaie, A.R. Davoyan, R.A. Pala, K. Thyagarajan, H.A. Atwater, Dynamically controlled Purcell enhancement of visible spontaneous emission in a gated plasmonic heterostructure, Nat. Commun. 8 (2017) 1631. [16] E. Wientjes, J. Renger, A.G. Curto, R. Cogdell, N.F. van Hulst, Nanoantenna enhanced emission of lightharvesting complex 2: the role of resonance, polarization, and radiative and non-radiative rates, Phys. Chem. Chem. Phys. 16 (2014) 24739. [17] A.G. Bakanov, N.A. Toropov, T.A. Vartanyan, Optical properties of planar nanostructures based on semiconductor quantum dots and plasmonic metal nanoparticles, Opt. Spectrosc. 120 (2016) 477. [18] N.T. Fofang, N.K. Grady, Z. Fan, A.O. Govorov, N.J. Halas, Plexciton dynamics: exciton−plasmon coupling in a J-aggregate−Au nanoshell complex provides a mechanism for nonlinearity, Nano Lett. 11 (2011) 1556. [19] D. Melnikau, A.A. Govyadinov, A. Sánchez-Iglesias, M. Grzelczak, L.M. Liz-Marzán, Y.P. Rakovich, Strong magneto-optical response of nonmagnetic organic materials
Fig. 5. Dependence of fluorescence intensities and corresponding decay times of alloyed QDs CdxZn1−xSeyS1−y/ZnS in the distance between QD layers and Ag nanoparticle island films.
〈τ 〉 = (A1 τ 12 + A2 τ 22)/(A1 τ 1 +A2 τ 2).
(2)
Analysis of the plotted dependences of fluorescence intensities and decay times in the distance between QDs and Ag nanoparticles, R, (Fig. 5) leads to the conclusion that the experimental observations are due to the Purcell effect. Indeed, starting with the QDs that do not interact with metal nanoparticles and going through the spacer layer thicknesses of 13 nm and 10 nm, the fluorescence intensities of QDs in the near fields of Ag nanoparticles are enhanced while the average lifetimes are simultaneously reduced in accordance with expectations for the Purcell effect. Despite being at smaller distances, the lifetime behavior becomes more complicated, the fluorescence intensity continues to rise. For this reason, QDs with a gradient component composition are more promising candidates for applications in high-efficient light-emitting devices or sensors because they do not require a buffer layer separating them from plasmonic nanostructures. Indeed, the largest fluorescence enhancement was observed in the case of direct contact between the QDs and silver nanoparticles. In other words, the optimal thickness of the dielectric layer Ropt is 0 nm for the alloyed CdxZn1-xSeyS1-y/ZnS quantum dots. Contrary to the ordinary core–shell CdSe/ZnS quantum dots, the shell thickness of which is commensurate with the size of their nucleus, in alloyed QDs the shell is much larger than the core. From the curve obtained (Fig. 5), it can be seen that the maximum fluorescence is observed when we do not have a dielectric layer; quantum dots just are put on the surface of the nanoparticles. Based on this data, it can be concluded that the shells of alloyed QDs themselves perform the function of an insulating layer. A slight increase of the fluorescence lifetime that accompanies the growth of the fluorescence intensity on the smallest thicknesses of the spacer layer shows that, in addition to the Purcell effect, the non-radiative losses reduction is responsible for the observed phenomena in this case. Nevertheless, the fluorescence lifetime is still smaller than in the case of the isolated QDs. 4
Optics and Laser Technology 121 (2020) 105821
N.A. Toropov, et al.
coupled to plasmonic nanostructures, Nano Lett. 17 (2017) 1808. [20] Y. Yang, Y. Zheng, W. Cao, A. Titov, J. Hyvonen, J.R. Manders, J. Xue, P.H. Holloway, L. Qian, High-efficiency light-emitting devices based on quantum dots with tailored nanostructures, Nat. Photonics 9 (2015) 259. [21] R.M. Elnoby, M.H. Mourad, S.L. Hassab Elnaby, M. Abou Kana, Monocrystalline solar cells performance coated by silver nanoparticles: Effect of NPs sizes from point of view Mie theory, Opt. Laser Technol. 101 (2018) 208. [22] W.K. Bae, K. Char, H. Hur, S. Lee, Single-step synthesis of quantum dots with chemical composition gradients, Chem. Mater. 20 (2008) 531. [23] C. Wang, H. Chen, L. Sun, W. Chen, Y. Chang, H. Ahn, X. Li, S. Gwo, Giant colloidal silver crystals for low-loss linear and nonlinear plasmonics, Nat. Commun. 6 (2015) 7734. [24] A.A. High, R.C. Devlin, A. Dibos, M. Polking, D.S. Wild, J. Perczel, N.P. de Leon, M.D. Lukin, H. Park, Visible-frequency hyperbolic metasurface, Nature 522 (2015) 192–196. [25] N.A. Toropov, N.B. Leonov, T.A. Vartanyan, Influence of silver nanoparticles crystallinity on localized surface plasmons dephasing times, Phys. Status Solidi B 255 (2018) 1700174. [26] A. Berthelot, G.C. des Francs, H. Varguet, J. Margueritat, R. Mascart, J. Benoit, J. Laverdant, From localized to delocalized plasmonic modes, first observation of
[27]
[28]
[29]
[30] [31] [32]
[33]
5
superradiant scattering in disordered semi-continuous metal films, Nanotechnology 30 (2019) 015706. A. Ishikawa, K. Osono, A. Nobuhiro, Y. Mizumoto, T. Torimoto, H. Ishihara, Theory for self-consistent interplay between light and nanomaterials strongly modified by metallic nanostructures, Phys. Chem. Chem. Phys. 15 (2013) 4214. A.O. Govorov, J. Lee, N.A. Kotov, Theory of plasmon-enhanced Förster energy transfer in optically excited semiconductor and metal nanoparticles, Phys. Rev. B 76 (2007) 125308. K.L. Kelly, E. Coronado, L. Zhao, G.C. Schatz, The optical properties of metal nanoparticles: the influence of size, shape, and dielectric environment, J. Phys. Chem. B 107 (2003) 668. D.V. Guzatov, V.V. Klimov, Radiative decay engineering by triaxial nanoellipsoids, Chem. Phys. Lett. 412 (2005) 341. M. Debenham, Refractive indices of zinc sulfide in the 0.405-13-µm wavelength range, Appl. Opt. 23 (1984) 2238. M.P. Lisitsa, L.F. Gudymenko, V.N. Malinko, S.F. Terekhova, Dispersion of the refractive indices and birefringence of CdSxSe1-x single crystals, Phys. Status Solidi 31 (1969) 389. W. Qin, R.A. Shah, P. Guyot-Sionnest, CdSeS/ZnS alloyed nanocrystal lifetime and blinking studies under electrochemical control, ACS Nano 6 (2012) 912.