Abstract
We have studied the dependence of concentration quenching of luminescence (donor–acceptor energy transfer) on the thickness d of dye-doped polymeric films (HITC:PMMA) and found its strong inhibition at small values of d. This phenomenon is tentatively explained by a limited number of acceptors, which donors’ excitation can reach in thin samples, if the film’s thickness is comparable to the diffusion length of the energy transfer. The latter mechanism, along with effective reduction of the dye concentration, is responsible for an inhibition of the concentration quenching of dye molecules impregnating porous alumina membranes. The elongation of emission kinetics in thick (≥3 μm) HITC:PMMA films is cautiously attributed to the samples’ crystallinity.
1 Introduction
Energy transfer between donors and acceptors [1] (which can be atoms, molecules, quantum dots, etc.) is an important physical phenomenon, whose applications range from biophysics [2] and phosphors [3] to laser technology [4, 5] and photovoltaics [6]. The common utilizations of the energy transfer in technology include sensitization of emission [4, 5, 7] and photon cutting [8, 9] (also known as cross relaxation [4, 5]). Depending on emission and absorption spectra of donors and acceptors, their oscillator strengths, the rates of dephasing and relaxation, molecular concentrations, etc., energy transfer can be coherent or incoherent and reversible or irreversible [1]. The most common and thoroughly studied is the energy transfer occurring in the weak coupling Forster regime, which is incoherent and irreversible, and whose rate is determined by the overlap of the emission spectrum of donors and the absorption spectrum of acceptors [1, 10]. When the rate of the energy transfer, commonly resulting in quenching of donors’ luminescence, increases with an increase of the molecular concentration, this phenomenon (primarily studied in this work) is referred to as the concentration quenching.
Quantum emitters [11, 12], including dye molecules [13, 14], are widely used in a variety of nanophotonics [15, 16] fundamental studies [17, 18] and applications [19, 20]. At high dye concentrations, which are of importance at stimulated emission [21, 22] and strong coupling [23], [24], [25], [26], the concentration quenching of luminescence and its inhibition in vicinity to metallic surfaces and lamellar metal/dielectric metamaterials have been studied [27], [28], [29]. While the particular focus of these and other studies [30] was on effects of non-local metallic environments, the effects of the nanoscopic samples’ geometry on luminescence kinetics in purely dielectric systems received much less attention. In this study, we have demonstrated and explained the dependence of luminescence concentration quenching and its inhibition on the (i) thickness of dye-doped polymeric films deposited on glass substrates and (ii) concentration of dye molecules impregnating porous anodic alumina membranes. Both effects are discussed in terms of the reduced number of acceptors available to donors in thin films and fibers. Furthermore, the (iii) experimentally observed elongation of emission kinetics in thick (≥3 μm) HITC:PMMA films is cautiously attributed to the samples’ crystallinity.
2 Experimental samples
The experimental samples in our studies were (1) PMMA polymeric films doped with HITC dye, deposited on glass, and (2) anodic alumina membranes impregnated with dye-doped polymer (HITC:PMMA), Figure 1. The latter 50 μm thick membranes (acquired from Redox Inc.) had 30 ± 5 nm wide open pores (filling factor f = 15%), propagating wall-to-wall, perpendicular to the membrane’s faces. The dye concentration ranged between
The samples were prepared by dissolving 2-[7-(1,3-dihydro-1,3,3-trimethyl-2H-indol-2-ylidene)-1,3,5-heptatrienyl]-1,3,3-trimethyl-3H-indoliumiodide (HITC) dye and poly (methyl methacrylate) (PMMA) polymer in dichloromethane (DCM). The solution was then sonicated for t = 90 min at room temperature. Finally, the dye doped polymer solutions were spin coated onto micro glass slides (obtained from VWR International) or anodic alumina membranes, discussed above, Figure 1, using a spin-coater from Specialty Coating Systems (Model P6700). The thickness measurements of the polymeric films (on glass) were done using a stylus profilometer (Bruker Dektak XT).
3 Results and discussions
3.1 Spectroscopic and kinetics studies
The spectroscopic properties of HITC:PMMA films were studied in recent Ref. [27]. The emission and excitation spectra were collected using the spectrofluorimeter Fluorolog 3 (from Horiba) and the absorption spectra were taken using the spectrophotometer Lambda 900 (form PerkinElmer). Similar measurements were carried out in the present study. The absorption and excitation bands have the maxima at 762 nm, while the emission band has its peak position at 772 nm, Figure 2. Knowing the spectrum of absorption coefficients and the dye concentration, we have determined the spectrum of absorption cross sections
In the emission kinetics measurements, the samples were excited with ∼150 fs pulses of a mode locked Ti:Sapphire laser (Mira 900 from Coherent). The laser spot was ∼ 2.5 mm in diameter, the average power was ∼ 60 mW, and the repetition rate was 76 MHz. The emission kinetics were recorded using a visible and near-IR streak camera (Model C5680 from Hamamatsu). The time resolution, determined by the jitter of the laser and the wide-open entrance slit of the streak camera, was ∼100 ps. The emission decay rates were determined by fitting the experimental kinetics with exponential functions, as explained below.
3.2 Donors and acceptors
In our experiments, not aggregated HITC dye molecules serve as donors. Quenching centers (acceptors) are, generally, of different nature and their concentration c A can be either dependent on or independent of the molecular concentration ν, c A ∝ ν x , where x = 0, 1, 2, … Quadratic dependence of the energy transfer rate on the HITC dye concentration, reported in Ref. [27], is consistent with c A ∝ ν 2 and direct energy transfer between donors and acceptors [1], Figure 3a. On the other hand, at high concentration of donors c D and efficient energy transfer between them, the migration of electronic excitation over donors can accelerate the energy transfer to acceptors and shorten donors’ emission kinetics. In this case of migration-assisted energy transfer, Figure 3b, the quadratic dependence of the transfer rate on the dye concentration is predicted at c A ∝ ν [1]. We infer that the latter regime of donor–acceptor energy transfer takes place in our experiments, as described below.
3.3 Determination of the Förster radius
Knowing the experimental spectra of emission F(ω) and absorption cross section μ(ω), we determined the Förster radius for energy migration over HITC molecules (the distance between two molecules at which the transfer rate is equal to the emission decay rate [1]) to be equal to
Here χ
2 is the factor describing relative orientation of dipoles (χ
2 = 2/3 for random orientations)
3.4 Emission kinetics of thin HITC:PMMA films on glass
In the emission kinetics analysis, we first treated all kinetics as single exponential and determined their effective decay rates γ by equating the area under the normalized experimental emission kinetics
The emission kinetics of low concentrated HITC dye (
At the same time, at large dye concentrations, the emission kinetics had significantly higher decay rates (larger at
At the dye concentration equal to v = 40 g/l, the emission kinetics were measured at various thicknesses of HITC:PMMA films, ranging from 11 nm to 3.6 µm. In the data set represented by solid black circles in Figure 5, the film thickness was controlled by viscosity of the solution at unchanged spinning parameters. The corresponding emission decay rates were maximal at the film thicknesses d ∼ 1 µm and they decreased significantly at smaller values of d. The characteristic length scale of the decay rate reduction, d
0 = 33 nm, was determined by fitting the data points with the function C + B(1 − exp(−d/d
0)), solid line in Figure 5. A qualitatively similar behavior (and the fitting) have been demonstrated in the series of HITC:PMMA films with dye concentrations equal to
3.5 Diffusion length of the excitation migration
We infer that shortening of the emission kinetics in thin films (at high dye concentrations) is due to a relatively small number of acceptors to which donors can transfer their excitation, leading to inhibition of the Förster energy transfer and inhibition of the concentration quenching. Naively, one would think that this effect can be significant only at film thickness d ≤ R F. However, as we show below, the characteristic length of the excitation migration can be larger than this. We assume that the energy migration over donors (delivering excitation energy to acceptors) is diffusion, with the diffusion coefficient (in 1D approximation) given by [34]
where ∆t is the time of the excitation “hop” between two molecules, related to the energy transfer rate W as
Thus, at
Therefore,
At
3.6 Inhibition of concentration quenching in anodic alumina membranes
In the next particular experiment, we impregnated anodic alumina membranes with HITC:PMMA at different dye concentrations ν and studied the emission kinetics and the decay rates as the function of ν. The ∼80 nm dye-doped polymeric films with the same dye concentrations, deposited on glass, served as control samples.
In accord with Ref. [27], the HITC:PMMA films on glass demonstrate growth of the emission decay rates with increase of ν (concentration quenching) that could be fitted with the formula
In membrane samples impregnated with HITC:PMMA at low dye concentrations, the decay rates were almost similar to those in the control samples (at the same dye concentrations). However, at high dye concentrations, the decay rates in membranes were significantly smaller than those in the HITC:PMMA films on glass, Figure 6.
Based on the model of the excitation migration discussed above, we argue that ∼30 nm in diameter HITC:PMMA polymer fibers formed in pores of alumina membranes were so thin that they inhibited the long-range diffusion energy transfer and the corresponding concentration quenching. One can further infer that the statistically uniform distribution of the nanometer-scale HITC:PMMA dispersed phase is equivalent to a reduced dye concentration fν, where f is the filling factor for a dye-doped polymer.
The reduction of the effective dye concentration
The same mechanism is also expected to contribute to inhibition of concentration quenching in thin films in vicinity of metallic nanostructures [27], [28], [29, 35]. However, the inhibition of the concentration quenching in metallic nanostructures is stronger than that on top of glass [28].
3.7 Inhibition of concentration quenching in thick HITC:PMMA films
Another intriguing phenomenon was observed in thick highly doped HITC:PMMA films (d ≥ 3.0 μm,
4 Summary
To summarize, (i) we have studied the dependence of concentration quenching of luminescence (donor–acceptor energy transfer) on the thickness d of dye-doped polymeric films (HITC:PMMA) and found its strong inhibition at small values of d. (ii) This phenomenon is tentatively explained by the limited number of acceptors, which can be reached by donors’ excitation in thin samples, if the film’s thickness is comparable to the diffusion length of the energy transfer (determined by the Förster radius and the dye concentration). (iii) The same phenomenon is consistent with inhibition of the concentration quenching of dye embedded in anodic alumina membranes. (iv) The elongation of the emission kinetics in thick HITC:PMMA films is cautiously attributed to the samples’ crystallinity.
According to Ref. [37], the photonic density of states (PDOS) and, correspondingly, the radiative decay rates of emitters implanted into a dielectric matrix are getting reduced in vicinity of a dielectric-air interface. The same mechanism should cause a reduction of spontaneous emission rates in thin HITC:PMMA films in our study. However, the comparison of the model [37] and our experiment suggests that the PDOS reasoning is insufficient to explain the strong reduction of the decay rates observed at high dye concentration ν = 40 g/l (∼2.5-fold, Figure 5). This and seeming absence of the PDOS effect in low doped HITC:PMMA thin films, ν = 2 g/l and ν = 3.6 g/l, is the subject of the further study to be published elsewhere.
Funding source: National Science Foundation
Award Identifier / Grant number: (AFOSR)/ FA9550-18-1-0417
Award Identifier / Grant number: NSF /1830886 and 1856515
Award Identifier / Grant number: Department of Defense (DoD) / W911NF1810472
-
Author contribution: All the authors have accepted responsibility for the entire content of this submitted manuscript and approved submission.
-
Research funding: The work was supported by National Science Foundation (NSF) grants 1830886 and 1856515, Air Force Office of Scientific Research (AFOSR) grant FA9550-18-1-0417, and Department of Defense (DoD) grant W911NF1810472.
-
Conflict of interest statement: The authors declare no conflicts of interest regarding this article.
References
[1] V. M. Agranovich and M. D. Galanin, “Electronic Excitation Energy Transfer in Condensed Matter,” in Modern Problems in Condensed Matter Sciences, Amsterdam, North-Holland, Elsevier, 1982.Search in Google Scholar
[2] D. L. Andrews and A. A. Demidov, "Resonance Energy Transfer", Wiley, New York, 1999, ISBN 978-0-471-98732-1.Search in Google Scholar
[3] K. Li, M. Shang, H. Lian, and J. Lin, “Recent development in phosphors with different emitting colors via energy transfer,” J. Mater. Chem. C, vol. 4, pp. 5507–5530, 2016. https://doi.org/10.1039/c6tc00436a.Search in Google Scholar
[4] R. C. Powell, Physics of Solid-State Laser Materials, vol. 1, New York, Springer Science & Business Media, 1998.10.1007/978-1-4612-0643-9Search in Google Scholar
[5] A. A. Kaminskii, “Laser Crystals: Their Physics and Properties”, vol. 14, 2nd ed. Berlin, Heidelberg, Springer-Verlag, 1990.10.1007/978-3-540-70749-3Search in Google Scholar
[6] G. Chen, J. Seo, C. Yang, and P. N. Prasad, “Nanochemistry and nanomaterials for photovoltaics,” Chem. Soc. Rev., vol. 42, pp. 8304–8338, 2013. https://doi.org/10.1039/c3cs60054h.Search in Google Scholar PubMed
[7] M. T. Nimmo, L. M. Caillard, W. De Benedetti, et al.., “Visible to near-infrared sensitization of silicon substrates via energy transfer from proximal nanocrystals: further insights for hybrid photovoltaics,” ACS Nano, vol. 7, pp. 3236–3245, 2013. https://doi.org/10.1021/nn400924y.Search in Google Scholar PubMed
[8] B. M. van der Ende, L. Aarts, and A. Meijerink, “Near-Infrared quantum cutting for photovoltaics,” Adv. Mater., vol. 21, pp. 3073–3077, 2009. https://doi.org/10.1002/adma.200802220.Search in Google Scholar
[9] Y. Teng, J. Zhou, X. Liu, S. Ye, and J. Qiu, “Efficient broadband near-infrared quantum cutting for solar cells,” Opt Express, vol. 18, pp. 9671–9676, 2010. https://doi.org/10.1364/oe.18.009671.Search in Google Scholar PubMed
[10] T. Forster, “Intermolecular energy transfer and fluorescence,” Ann. Phys., vol. 2, pp. 55–75, 1948. https://doi.org/10.1002/andp.19484370105.Search in Google Scholar
[11] I. Aharonovich, D. Englund, and M. Toth, “Solid-state single-photon emitter,” Nat. Photonics, vol. 10, pp. 631–641, 2016. https://doi.org/10.1038/nphoton.2016.186.Search in Google Scholar
[12] A. Delga, J. Feist, J. Bravo-Abad, and F. J. Garcia-Vidal, “Quantum emitters near a metal nanoparticle: strong coupling and quenching,” Phys. Rev. Lett., vol. 112, p. 253601, 2014. https://doi.org/10.1103/physrevlett.112.253601.Search in Google Scholar
[13] A. Mokhtari, A. Chebira, and J. Chesnoy, “Subpicosecond fluorescence dynamics of dye molecules,” J. Opt. Soc. Am. B, vol. 7, pp. 1551–1557, 1990. https://doi.org/10.1364/josab.7.001551.Search in Google Scholar
[14] E. Dulkeith, A. C. Morteani, T. Niedereichholz, et al.., “Fluorescence quenching of dye molecules near gold nanoparticles: radiative and nonradiative effects,” Phys. Rev. Lett., vol. 89, p. 203002, 2002. https://doi.org/10.1103/physrevlett.89.203002.Search in Google Scholar PubMed
[15] R. J. Coles, D. M. Price, J. E. Dixon, et al.., “Chirality of nanophotonic waveguide with embedded quantum emitter for unidirectional spin transfer,” Nat. Commun., vol. 7, pp. 1–7, 2016. https://doi.org/10.1038/ncomms11183.Search in Google Scholar PubMed PubMed Central
[16] P. N. Prasad, “Nanophotonics”, Hoboken, New Jersey, John Wiley & Sons Inc., 2004.10.1002/0471670251Search in Google Scholar
[17] V. A. G. Rivera, O. B. Silva, Y. Ledemi, Y. Messaddeq, and E. MaregaJr., Collective Plasmon-Modes in Gain Media: Quantum Emitters and Plasmonic Nanostructures, Cham; Heidelberg; New York; Dordrecht; London, Springer, 2014. Available at: https://doi.org/10.1007/978-3-319-09525-7.Search in Google Scholar
[18] A. González-Tudela, P. A. Huidobro, L. Martín-Moreno, C. Tejedor, and F. J. García-Vidal, “Theory of strong coupling between quantum emitters and propagating surface plasmons,” Phys. Rev. Lett., vol. 110, p. 126801, 2013. https://doi.org/10.1103/physrevlett.110.126801.Search in Google Scholar PubMed
[19] Y. M. He, G. Clark, J. R. Schaibley, et al.., “Single quantum emitters in monolayer semiconductors,” Nat. Nanotechnol., vol. 10, pp. 497–502, 2015. https://doi.org/10.1038/nnano.2015.75.Search in Google Scholar PubMed
[20] C. Polisseni, K. D. Major, S. Boissier, S. Grandi, A. S. Clark, and E. A. Hinds, “Stable, single-photon emitter in a thin organic crystal for application to quantum-photonic devices,” Opt Express, vol. 24, pp. 5615–5627, 2016. https://doi.org/10.1364/oe.24.005615.Search in Google Scholar
[21] M. A. Noginov, G. Zhu, A. M. Belgrave, et al.., “Demonstration of a spaser-based nanolaser,” Nature, vol. 460, pp. 1110–1112, 2009. https://doi.org/10.1038/nature08318.Search in Google Scholar PubMed
[22] M. A. Noginov, G. Zhu, M. Mayy, B. A. Ritzo, N. Noginova, and V. A. Podolskiy, “Stimulated emission of surface plasmon polaritons,” Phys. Rev. Lett., vol. 101, p. 226806, 2008. https://doi.org/10.1103/physrevlett.101.226806.Search in Google Scholar
[23] P. Törmä and W. L. Barnes, “Strong coupling between surface plasmon polaritons and emitters: a review,” Rep. Prog. Phys., vol. 78, p. 013901, 2014. https://doi.org/10.1088/0034-4885/78/1/013901.Search in Google Scholar
[24] J. Dintinger, S. Klein, F. Bustos, W. L. Barnes, and T. W. Ebbesen, “Strong coupling between surface plasmon-polaritons and organic molecules in subwavelength hole arrays,” Phys. Rev. B, vol. 71, p. 035424, 2005. https://doi.org/10.1103/physrevb.71.035424.Search in Google Scholar
[25] E. K. Tanyi, H. Thuman, N. Brown, S. Koutsares, V. A. Podolskiy, and M. A. Noginov, “Control of the Stokes shift with strong coupling,” Adv. Opt. Mater., vol. 5, p. 1600941, 2017. https://doi.org/10.1002/adom.201600941.Search in Google Scholar
[26] T. U. Tumkur, G. Zhu, and M. A. Noginov, “Strong coupling of surface plasmon polaritons and ensembles of dye molecules,” Opt Express, vol. 24, pp. 3921–3928, 2016. https://doi.org/10.1364/oe.24.003921.Search in Google Scholar
[27] S. Prayakarao, S. R. Koutsares, C. E. Bonner, and M. A. Noginov, “Effect of nonlocal metal–dielectric environments on concentration quenching of HITC dye,” J. Opt. Soc. Am. B, vol. 36, pp. 3579–3587, 2019. https://doi.org/10.1364/josab.36.003579.Search in Google Scholar
[28] S. Koutsares, L. S. Petrosyan, S. Prayakarao, et al.., “Effect of metallic substrates and cavities on emission kinetics of dye-doped polymeric films,” J. Opt. Soc. Am. B, vol. 38, pp. 88–94, 2021. https://doi.org/10.1364/josab.409998.Search in Google Scholar
[29] V. N. Peters, S. Prayakarao, S. R. Koutsares, C. E. Bonner, and M. A. Noginov, “Control of physical and chemical processes with nonlocal metal–dielectric environments,” ACS Photonics, vol. 6, pp. 3039–3056, 2019. https://doi.org/10.1021/acsphotonics.9b00734.Search in Google Scholar
[30] F. Reil, U. Hohenester, J. R. Krenn, and A. Leitner, “Förster-type resonant energy transfer influenced by metal nanoparticles,” Nano Lett., vol. 8, pp. 4128–4133, 2008. https://doi.org/10.1021/nl801480m.Search in Google Scholar
[31] M. Müller, R. Zentel, T. Maka, S. G. Romanov, and C. M. Sotomayor Torres, “Photonic crystal films with high refractive index contrast,” Adv. Mater., vol. 12, pp. 1499–1503, 2000. https://doi.org/10.1002/1521-4095(200010)12:20<1499::aid-adma1499>3.0.co;2-m.10.1002/1521-4095(200010)12:20<1499::AID-ADMA1499>3.0.CO;2-MSearch in Google Scholar
[32] S. Prayakarao, D. Miller, D. Courtwright, C. E. Bonner, and M. A. Noginov, “Non-resonant enhancement of spontaneous emission of HITC dye in metal-insulator-metal waveguides,” J. Opt. Soc. Am. B, vol. 36, pp. 2312–2316, 2019. https://doi.org/10.1364/josab.36.002312.Search in Google Scholar
[33] F. Auzel, “Upconversion and anti-Stokes processes with f and d ions in solids,” Chem. Rev., vol. 104, pp. 139–174, 2004. https://doi.org/10.1021/cr020357g.Search in Google Scholar PubMed
[34] H. Berg, “Chapter 1. Diffusion: Microscopic Theory.” In Random Walks in Biology, Princeton, Princeton University Press, 2018, pp. 5–16.10.1515/9781400820023-004Search in Google Scholar
[35] S. Rout, Z. Qi, L. S. Petrosyan, et al.., “Effect of random nanostructured metallic environments on spontaneous emission of HITC dye,” Nanomaterials, vol. 10, p. 2135, 2020. https://doi.org/10.3390/nano10112135.Search in Google Scholar PubMed PubMed Central
[36] J. D. Stroupe and R. E. Hughes, “The structure of crystalline poly- (methyl methacrylate),” J. Am. Chem. Soc., vol. 80, pp. 2341–2342, 1958. https://doi.org/10.1021/ja01542a086.Search in Google Scholar
[37] E. Snoeks, A. Lagendijk, and A. Polman, “Measuring and modifying the spontaneous emission rate of erbium near an interface,” Phys. Rev. Lett., vol. 74, pp. 2459–2462, 1995. https://doi.org/10.1103/physrevlett.74.2459.Search in Google Scholar
© 2021 Sangeeta Rout et al., published by De Gruyter, Berlin/Boston
This work is licensed under the Creative Commons Attribution 4.0 International License.