Open Access
Add to BibSonomy
Abstract
In this work, we show controlled spatial and spectral modulation of local dielectric polarization in amorphous organic Alq3: DCM host: guest medium. We use self-strained silicon-dioxide microbeams to apply tensile strain on thin films at different DCM doping. From the measured spectral shift of the emission peak, we estimate the orientational polarizability (and in turn dielectric polarizability) in the strained host: guest medium at different guest doping. Orientational polarizability in the thin film follows a linear relationship with applied tensile strain at different guest material doping while the strain sensitivity shows an inverse relationship with guest doping.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The emission properties of organic fluorophores are profoundly affected by the polarizability of the surrounding medium. Under increased solvent polarity, the emission spectrum of the fluorophore shows bathochromatic shift that depends strongly on the polar nature of the fluorophore [1]. This phenomenon known as solvatochromism, has been widely studied and reported for fluorescence molecules in solvents with different dielectric properties [2]. A similar effect has also been observed in doped host: guest solid-state thin films [3], where the highly dipolar nature of the guest molecules in relatively non-polar host results in a bathochromic shift in fluorescence emission of the guest molecules as the guest doping concentration is changed. The increase of guest doping results in the reduced intermolecular distance between neighboring guest molecules affecting the dipolar interactions. Previous works studying such phenomenon (also referred as solid-state solvation) include the spectral tuning of the luminescent laser dye 4-(dicyanomethylene)-2-methyl-6-(julolidin-4-ylvinyl)- 4H-pyran) (DCM2) (ground state dipole moment = 11 D [4]) and 4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran DCM (ground state dipole moment 5.6 D [5,6] doped in N,N′-Bis(3-methylphenyl)-N,N′-diphenylbenzidine (TPD) [4] (ground state dipole moment 1.5 D [7]) and tris-(8-hydroxyquinoline)aluminum (Alq3) [8] (ground state dipole moment 3.9 D [9]). Alongside solvatochromic spectral shift, increased guest molecular concentration in host medium may lead to reduced excited state lifetime and photoluminescence quantum yield of the medium through the formation of local molecular aggregates [6,10,11]. More recently, external mechanical deformation has been successfully used to trigger solid-state solvation in doped host: guest systems [12,13] and modulate the dynamics of charge-transfer states [14]. However, the dependence of the sensitivity of dielectric polarizability of an organic host: guest medium to external mechanical stimuli at different guest molecule doping concentration remains unknown. While axial strain has been widely proposed as a tool for modulating physical properties of organic semiconducting media for flexible device applications [15–18], understanding the energy shift for different doping under strain is critical to leverage the true potential of solvatochromism.
In our study, we use Alq3: DCM as an archetype host: guest system (Fig. 1(a) and (b)) [19–21]. DCM molecules suspended in relatively non-polar Alq3 background matrix undergo solid-state solvation (under no mechanical stimuli) which triggers a pronounced redshift of the emission spectrum. Following the electronic transition by absorption of a photon, the solute molecules reach an initial excited state commonly referred to as the Franck- Condon state [22]. For molecules having a large transition dipole moment, an equilibrium excited state is established by the interaction between the excited solute molecules and the surrounding electrostatic medium. In a doped host: guest medium, this local polarizable medium is created by polar guest molecules and the local electric field becomes stronger with guest doping. This phenomenon is called self- polarization. Figure 1(c) shows a simple schematic of the mechanism of solvatochromism in an amorphous organic thin- film under external mechanical tension. As the thin film is axially stretched, the local molecular density decreases triggering a decrease in self polarization and a reduction in the energy required for relaxation from the short-lived Franck-Condon state. As a result, a blueshift in the emission spectrum of the molecule is observed. From the measured spectral variation of DCM emission and axial strain on the thin film, we can extract the spatial modulation of the dielectric polarizability under axial strain and estimate the strain sensitivity of dielectric polarization in the host: guest medium.
Fig. 1. (a) Molecular structures of the materials used in this work: (a) Alq3 & (b) DCM. (c) Simple schematic representation of solid- state solvation under tensile strain. Here the center molecular dipole is shown to be surrounded by solvent molecules. Under tensile strain, the molecular density decreases and the resulting reduction in self-polarization triggers a blueshift in the emission of DCM molecules at a specific doping.
The solvatochromic spectral shift due to solute-solvent interaction in a dielectric medium is generally represented by the Lippert- Mataga equation [23,24]:
Here, ${\upsilon _a}$ and ${\upsilon _f}$ refer to the peak absorption and emission wavenumbers of solute molecule in cm−1, $\varepsilon$ and n refer to the dielectric constant and refractive index of the surrounding solid-state solvent medium respectively, h and c refer to the Planck’s constant and the speed of light respectively, μE and μG refer to the dipole moment of the solute molecules in the excited and ground-state respectively, a is the radius of the cavity from Onsager reaction field theory [25], and C refers to a constant that represents the unperturbed spectral shift of the solute emission. The term $\frac{{\varepsilon - 1}}{{2\varepsilon + 1}} - \frac{{{n^2} - 1}}{{2{n^2} + 1}}$ generally referred to as the orientational polarizability [26], represents the local dielectric polarization of the medium. The first term $\frac{{\varepsilon - 1}}{{2\varepsilon + 1}}$ represents the effect on Stokes shift due to both electronic and molecular reorientation of the solvent dipoles around the excited solute molecules. On the other hand, the second term $\frac{{{n^2} - 1}}{{2{n^2} + 1}}$ represents the high-frequency response due to electronic reorientation. Therefore, the difference between the two terms accounts for the net dielectric polarization due to molecular reorientation in a solute-solvent medium. In solid-state doped thin films, orientational polarizability increases as the concentration of polar guest molecules in a relatively non- polar background host matrix is increased.
We utilize suspended silicon dioxide (SiO2) microbeam structures and exploit the residual stress in the thermally grown SiO2 layer to generate axial strain on the thin film. The microbeam structures are fabricated on 300 nm SiO2/Si substrates using standard photolithography and reactive ion etching (RIE) process. A 30 nm Alq3: DCM thin film followed by 50 nm 2,2′,2"-(1,3,5-Benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi) is deposited using vapor thermal evaporation. Finally, the microbeams were released using XeF2 gas-phase etching [27]. The 50 nm TPBi acts as a protective layer against possible fluorination [28,29] and photophysical degradation of the host: guest medium during the XeF2 gas-phase etching. The residual compressive stress in the thin film buckles the microbeams during the release process [30] resulting in axial strain (as high as 1%) on the overlying organic thin-film [13]. Figure 2(a) shows false-color SEM micrographs of representative SiO2 released microbeams with overlying organic thin film.
Fig. 2. Effect of axial strain on solvatochrmic shift for a 1.5% DCM doped Alq3: DCM thin film. (a) False color SEM micrograph of SiO2 microbeams (length 15 µm) released using XeF2 gas phase etching following thermal evaporation. (b) Normalized PL intensity profile at three different positions on the beam. The normalized PL intensity at position P (maximum strain) shows a clear blueshift with respect to normalized intensity at positions M and N(zero strain). (c) Axial tensile strain along the released SiO2 microbeams with the zero strain positions (M & N) and maximum strain position highlighted(P).(d) Peak emission wavelength of DCM and fractional change in orientational polarizability along the microbeam extracted from photoluminescence measurements.
2. Methods
We pattern the microbeam features on thermally deposited 300 nm SiO2/ Si substrates using standard photolithography methods and reactive-ion-etching (RIE) techniques. SPR 220 (3.0) was used as the photoresist in photolithography(spin speed 500 rpm, approximate thickness 2 µm). To promote photoresist adhesion, a vapor phase deposition of hexamethyldisilazane (HMDS) was carried out before the photolithography step. The samples were exposed in a contact aligner tool for 4 seconds and developed using AZ 726 MIF developer. The patterned SiO2 regions were etched using CF4 and CHF3 gases (flow rate 15 sccm) at 20 mTorr and 150W RF power settings. Following the RIE, a short deep reactive ion etching (DRIE) using SF6 as the etchant (flow rate 390 sccm) and C4F8(flow rate 0- 30 sccm) as the inert gas was performed to etch a few microns of exposed silicon to facilitate the access of XeF2 gas. A short, low-power(100W) plasma stripping was carried out to strip the deposited C4F8 from silicon sidewalls.
Following the fabrication of the microbeams, thin films of 30 nm Alq3: DCM homogeneous blend and 50 nm TPBi were deposited using vapor thermal evaporation at low pressure (2e−7 torr). The evaporation rate for Alq3 was varied between 1–2 A°/s. The molecular doping of DCM in the thin film was controlled by adjusting the DCM evaporation rate during the evaporation process. Following the thermal evaporation, the microbeams were released using three cycles of XeF2 etching. The gas pressure was set at 3.0 Torr and time for each cycle was kept at 20 s.
The strained thin films were characterized using a 450 nm continuous wave (CW) laser with a diffraction-limited spot. The photoluminescence signal was collected using a 100X 0.9 NA objective (Nikon TU Plan Apo EPI 100X 0.9) and was analyzed using a high-resolution spectrometer (Princeton Instruments IsoPlane SCT 320) coupled to a highly-sensitive CCD camera (Princeton Instruments Pixis 400). To perform PL measurement along the strained microbeam axis, a piezo-nanopositioner stage was used in 1 µm steps to move the sample under the laser excitation spot.
3. Results and discussion
3.1 Strain tuned solvatochromism in Alq3: DCM thin film
Figure 2 shows the effect of axial strain on the emission properties of Alq3: DCM thin-film for 1.5% DCM doping. Figure 2(b) shows normalized PL intensity profiles at three different positions on the microbeam – positions M, N and P. Figure 2(c) shows the axial strain along the microbeam calculated from the out-of-plane deflection. The surface topology of the microbeam and the extracted out-of-plane beam deflection profile are shown in supplementary materials (section 1). Due to the nature of deflection, the strain generated along the microbeams can range from 0.2 - 0.3% compressive (at the anchor position) to about 0.9% tensile (at the position of maximum deflection). In Fig. 2(c), positions M & N represent the location of zero strain on the released microbeams. Position P represents the maximum strain location on the beam and the position of maximum out-of-plane deflection. From the figure, we see that the axial strain gradually increases as we approach position P from position M, reaches the maximum value at position P(∼0.9%), and gradually decreases as we approach position N. Figure 2(d) shows the peak photoluminescence emission wavelength measured along the microbeam on the left y-axis. We observe a gradual blueshift in the peak emission wavelength from the zero strain positions (M & N) to the maximum strain position (position P). We observe a blueshift of nearly 6 nm for an axial tensile strain of about 0.9% (also seen in Fig. 2(b)) as a result of solvatochromism under tensile strain. We note that no significant change in the PL linewidth was observed, suggesting that the axial strain did not affect the homogeneous distribution of molecules in the host: guest medium. We also include the axial strain profile and the measured peak emission wavelength of DCM for 2.5% and 5% doped thin films in the supplymentary materials (sections 2 and 3 respectively).
3.2 Modeling of solvatochromic spectral shift under strain
From the measured peak PL emission, one can extract the local orientational polarization (Δf) along the strained thin films for different DCM doping concentrations. We rewrite equation Eq. (1) as:
where,We use the reported values of m and C of 7.94X103 cm−1 and 2.74X103 cm−1 respectively [5,31] and the DCM absorption peak wavelength of 480 nm [32] to extract the orientational polarizability based on the measured PL emission peaks. We assume a negligible change in the absorption spectrum of DCM due to axial tension on the thin film. The solvatochromic shift due to molecular reorientation takes place over a long time scale (∼10−9 s) compared to light absorption(∼10−15 s), justifying the assumption [22]. The spatial variation of the change in orientational polarizability along the 1.5% doped DCM doped thin film is plotted in Fig. 2(d) (right y-axis). The orientational polarizability decreases gradually from the zero-strain position and reaches its minimum value at the maximum tensile strain position on the microbeam. Based on the results, ∼9% change in the orientational polarization was estimated under axial tension of 0.9%. The spatial variation of the orientational polarizability for a 2.5% and 5% DCM doped strained thin film are available in the supplementary materials (sections 2 and 3 respectively).
From our measurement, we observe a linear relationship between the peak DCM emission energy and the axial tensile strain for different DCM doping concentrations as shown in Fig. 3. However, the rate of a solvatochromic shift under strain was different at different DCM doping concentrations. We observed the highest slope of 0.016 eV/% in the 1.5% doped film, reducing to 0.006 eV/% for 5% doped film.
Fig. 3. Linear fit of DCM peak emission energy with axial strain at different DCM doping concentrations: (a) 1.5%; (b) 2.5%; (c) 5%. Even though DCM peak emission energy shift consistently follows a linear relationship with axial strain, the range of peak emission energy modulation under strain decreases with DCM doping. The error bar in the figures represents the range of DCM peak emission energies from different samples.
To understand the relationship of DCM peak emission energy to external axial strain, we use Claussius- Mossoti relation that relates the dielectric response of a homogeneous medium, $\frac{{\varepsilon - 1}}{{\varepsilon + 2}}$ with molecular polarizability, α as [33]:
Here, n refers to the molecular density of the medium (N/V where we assume the number of molecules dispersed in volume V is N), and ɛ0 refers to the free space dielectric constant. Since the high-frequency refractive index is not affected by axial tension on the thin film, any effect on the dielectric polarizability under external strain must be due to changes in the dielectric function, $\Delta f(\varepsilon ) = \frac{{\varepsilon - 1}}{{2\varepsilon + 1}}$.
Under strain, the thin film undergoes local volumetric change (Δv) that modulates the molecular density at a specific doping level. We estimate the volumetric change under axial tension by considering a small volumetric segment of the thin film as shown in Fig. 4(a). The length, width, and height of the segment are presented as X, Y, and Z. The out-of-plane deflection of the microbeams elongates the thin-film uniaxially (along the x-direction in Fig. 4(a)) and therefore, due to the Poisson effect, the thin film is compressed along y and z directions. We represent the elongation of the thin film along the microbeam axis as Δx and the orthogonal compression as Δy and Δz. Defining the Poisson ratio of the thin film as ν, the orthogonal compressions can be written as $\Delta y ={-} \nu Y{\varepsilon _x}$ and $\Delta z ={-} \nu Z{\varepsilon _x}$.
Fig. 4. (a) Simple schematic representation of an organic thin film under axial tensile strain. While the dimensions of the unstrained thin film are mentioned in the figure, the figure is not drawn to scale. (b) Strain sensitivity of the orientational polarization in Alq3: DCM host: guest medium at different DCM doping. The error bars represent the range of strain sensitivity extracted from PL measurements on microbeams at different DCM doping.
Therefore, under uniaxial strain, the volumetric change in the thin film (Δv) due to small axial elongation under strain, can be related to the initial volume, V as [34]:
To the first order, the dielectric polarizability under tensile strain, $\Delta f(\varepsilon )$ is related to the volumetric change by the following linear relationship:
3.3 Strain tuned dielectric polarization at different DCM doping
We define the strain sensitivity of orientational polarization as the fractional change in orientational polarization per unit axial strain i.e. $\frac{{\Delta f(\varepsilon ) - \Delta f(0)}}{{\Delta f(0)}}$ where $\Delta f(0)$ is the orientational polarization of an unstrained thin film. Figure 4(b) shows the estimated strain sensitivity of orientational polarization of Alq3: DCM thin films at different DCM doping concentrations. As the DCM doping increases, orientational polarizability and the resulting solvatochromic shift in the thin film becomes less sensitive to the external uniaxial strain.
The observed trend in strain sensitivity of orientational polarization for different doping concentrations can be explained by the physical origin of the solvatochromic shift in the host: guest Alq3: DCM system. At very low concentrations, the emission properties of organic fluorophores are strongly affected by the electric field from surrounding fluorophores in a host: guest medium [35]. The increase in DCM doping reduces the intermolecular separation between DCM dipoles, enhances the dielectric polarization and the net local field on a molecular dipole. This leads to an increase in the energy required for molecular orientation after photoexcitation and a stronger stabilization of the excited state. In our experiment, the mechanical properties of the system are dominated by the host Alq3 matrix. Therefore, under uniaxial tensile strain, the intermolecular distance between neighboring DCM molecules dispersed in Alq3 matrix increases. Besides, as the thin film is uniaxially elongated, compression due to the Poisson effect also reduces intermolecular distance orthogonal to the elongation axis. These two mechanisms result in a net molecular density and net dielectric polarizability in the medium that governs the energy shift due to solvatochromism. At low DCM doping level, due to low molecular density and large intermolecular distance, uniaxial elongation of the thin film leads to a reduction in net molecular density and reduces the net local electric field on a molecular dipole. Therefore, we observe a larger modulation of the orientational polarizability under strain at lower DCM doping concentration. . However, as the DCM doping increases, due to reduced intermolecular separation, orthogonal compression of the thin film due to the Poisson effect counteracts the change in orientational polarization due to elongation under axial tensile strain. This results in lower net change in orientation polarization, thereby reducing the strain sensitivity. We note that aggregation of molecules at high concentrations could also contribute to the reduction in the sensitivity. To confirm this, we performed PL measurements on 5% DCM doped compressively strained organic thin films and observed a similar trend in the orientational polarizability and peak DCM emission (supplementary materials, section 5). Thus, we conclude that the observed change in the orientational polarizability (and in turn dielectric polarizability) is indeed due to external strain and has negligible contribution from molecular aggregation. Our observation of solvatochromic shift under strain, therefore, reveals the critical role of molecular doping in the modulation of dielectric polarizability of host: guest medium under external mechanics.
4. Conclusion
Using self-strained SiO2 microbeams to apply controlled axial tensile strain on an organic host: guest medium, we show precise modulation of the solvatochromic spectral shift in a molecular doped host: guest medium at different guest doping concentrations. Our measurements show that the spectral shift due to solvation at different doping concentrations of the guest molecule follows a linear relationship with applied strain. From the solvatochromic spectral shift, using Lippert- Mataga equation we show that the dielectric polarizability of the host: guest medium shows greater sensitivity at low guest molecule doping. As the dielectric nature of the medium is externally increased by guest molecular doping, short-range intermolecular interactions strongly dominate the solvation properties in the medium and the dielectric properties of the medium become less sensitive to external mechanical stimuli. The reported results can greatly aid in the design of wearable and large-scale flexible optoelectronic devices based on the excitonic modulation under external mechanical stimuli.
Funding
Air Force Office of Scientific Research (FA9550-17-1-0208).
Acknowledgments
The authors sincerely acknowledge the Lurie Nanofabrication Facility at the University of Michigan where the fabrication of the devices was performed.
Disclosures
The authors declare that there are no conflicts of interest related to this article.
See Supplement 1 for supporting content.
References
1. A. Marini, A. Muñoz-Losa, A. Biancardi, and B. Mennucci, “What is solvatochromism?” J. Phys. Chem. B 114(51), 17128–17135 (2010). [CrossRef]
2. C. Reichardt and T. Welton, Solvents and Solvent Effects in Organic Chemistry4 Edition (Wiley, 2010).
3. C. F. Madigan and V. Bulović, “Solid state solvation in amorphous organic thin films,” Phys. Rev. Lett. 91(24), 247403 (2003). [CrossRef]
4. V. Bulović, R. Deshpande, M. Thompson, and S. Forrest, “Tuning the color emission of thin film molecular organic light emitting devices by the solid state solvation effect,” Chem. Phys. Lett. 308(3–4), 317–322 (1999). [CrossRef]
5. M. Meyer and J. C. Mialocq, “Ground state and singlet excited state of laser dye DCM: Dipole moments and solvent induced spectral shifts,” Opt. Commun. 64(3), 264–268 (1987). [CrossRef]
6. A. Uddin and C. B. Lee, “Exciton behaviours in doped tris (8-hydroxyquinoline) aluminum (Alq3) films,” Phys. Status Solidi (c) 8(1), 80–83 (2011). [CrossRef]
7. P. M. Borsenberger and J. J. Fitzgerald, “Effects of the dipole moment on charge transport in disordered molecular solids,” J. Phys. Chem. 97(18), 4815–4819 (1993). [CrossRef]
8. V. Bulović, A. Shoustikov, M. A. Baldo, E. Bose, V. G. Kozlov, M. E. Thompson, and S. R. Forrest, “Bright, saturated, red-to-yellow organic light-emitting devices based on polarization-induced spectral shifts,” Chem. Phys. Lett. 287(3–4), 455–460 (1998). [CrossRef]
9. A. Curioni, M. Boero, and W. Andreoni, “Alq 3 : Ab initio calculations of its structural and electronic properties in neutral and charged states,” Chem. Phys. Lett. 294(4–5), 263–271 (1998). [CrossRef]
10. A. P. Green and A. R. Buckley, “Solid state concentration quenching of organic fluorophores in PMMA,” Phys. Chem. Chem. Phys. 17(2), 1435–1440 (2015). [CrossRef]
11. X. M. Ding, W. Huang, H. Z. Shi, X. Y. Hou, Z. H. Xiong, S. T. Zhang, G. Y. Zhong, J. He, and Z. Xu, “In situ photoluminescence investigation of doped Alq,” Appl. Phys. Lett. 80(25), 4846–4848 (2002). [CrossRef]
12. W. Chang, G. M. Akselrod, and V. Bulović, “Solid-state solvation and enhanced exciton diffusion in doped organic thin films under mechanical pressure,” ACS Nano 9(4), 4412–4418 (2015). [CrossRef]
13. K. Datta and P. B. Deotare, “Optical determination of Young’s Modulus of nanoscale organic semiconductor thin films for flexible devices,” ACS Appl. Nano Mater. 3(2), 992–1001 (2020). [CrossRef]
14. W. Chang, D. N. Congreve, E. Hontz, M. E. Bahlke, D. P. McMahon, S. Reineke, T. C. Wu, V. Bulović, T. Van Voorhis, and M. A. Baldo, “Spin-dependent charge transfer state design rules in organic photovoltaics,” Nat. Commun. 6(1), 6415 (2015). [CrossRef]
15. M. Matta, M. J. Pereira, S. M. Gali, D. Thuau, Y. Olivier, A. Briseno, I. Dufour, C. Ayela, G. Wantz, and L. Muccioli, “Unusual electromechanical response in rubrene single crystals,” Mater. Horiz. 5(1), 41–50 (2018). [CrossRef]
16. H. H. Choi, H. T. Yi, J. Tsurumi, J. J. Kim, A. L. Briseno, S. Watanabe, J. Takeya, K. Cho, and V. Podzorov, “A large anisotropic enhancement of the charge carrier mobility of flexible organic transistors with strain: a Hall effect and Raman study,” Adv. Sci. 7(1), 1901824 (2020). [CrossRef]
17. M. A. Reyes-Martinez, A. J. Crosby, and A. L. Briseno, “Rubrene crystal field-effect mobility modulation via conducting channel wrinkling,” Nat. Commun. 6(1), 6948 (2015). [CrossRef]
18. T. Wang, Photophysics of Novel Optoelectronic Materials under Hydrostatic Pressure (University of Amsterdam, 2019).
19. C. W. Chang, Y. T. Kao, and E. W. G. Diau, “Fluorescence lifetime and nonradiative relaxation dynamics of DCM in nonpolar solvent,” Chem. Phys. Lett. 374(1–2), 110–118 (2003). [CrossRef]
20. B. Beyer and K. Leo, “Efficiency increase of organic solar cells with emissive light-in-coupling layers,” J. Mater. Chem. C 3(41), 10830–10836 (2015). [CrossRef]
21. C. W. Tang, S. A. Vanslyke, and C. H. Chen, “Electroluminescence of doped organic thin films,” J. Appl. Phys. 65(9), 3610–3616 (1989). [CrossRef]
22. J. R. Lakowicz, Principles of Fluorescence Spectroscopy (Springer, 2006).
23. E. Lippert, “Dipolmoment und Elektronenstruktur von angeregten Molekülen,” Zeitschrift fur Naturforsch. - Sect. A J. Phys. Sci. 10(7), 541–545 (1955). [CrossRef]
24. N. Mataga, Y. Kaifu, and M. Koizumi, “Solvent effects upon fluorescence spectra and the dipolemoments of excited molecules,” Bull. Chem. Soc. Jpn. 29(4), 465–470 (1956). [CrossRef]
25. L. Onsager, “Electric Moments of Molecules in Liquids,” J. Am. Chem. Soc. 58(8), 1486–1493 (1936). [CrossRef]
26. A. P. Green, K. T. Butler, and A. R. Buckley, “Tuning of the emission energy of fluorophores using solid state solvation for efficient luminescent solar concentrators,” Appl. Phys. Lett. 102(13), 133501 (2013). [CrossRef]
27. H. F. Winters and J. W. Coburn, “The etching of silicon with XeF2 vapor,” Appl. Phys. Lett. 34(1), 70–73 (1979). [CrossRef]
28. P. R. Hammond, “Laser dye DCM, its spectral properties, synthesis and comparison with other dyes in the red,” Opt. Commun. 29(3), 331–333 (1979). [CrossRef]
29. A. A. Turban, S. L. Bondarev, V. N. Knyukshto, and A. P. Stupak, “Quenching of fluorescence for fluoro derivatives of the laser dye DCM in polar solutions,” J. Appl. Spectrosc. 73(5), 678–685 (2006). [CrossRef]
30. E. Iwase, P.-C. Hui, D. Woolf, A. W. Rodriguez, S. G. Johnson, F. Capasso, and M. Lončar, “Control of buckling in large micromembranes using engineered support structures,” J. Micromech. Microeng. 22(6), 065028 (2012). [CrossRef]
31. E. Lippert, “Dipolmoment und Elektronenstruktur von angeregten Molekülen,” Z. Naturforsch., A: Phys. Sci. 10(7), 541–545 (1955). [CrossRef]
32. A. Luridiana, G. Pretta, D. Chiriu, C. M. Carbonaro, R. Corpino, F. Secci, A. Frongia, L. Stagi, and P. C. Ricci, “A facile strategy for new organic white LED hybrid devices: Design, features and engineering,” RSC Adv. 6(26), 22111–22120 (2016). [CrossRef]
33. P. Van Rysselberghe, “Remarks concerning the Clausius-Mossotti law,” J. Phys. Chem. 36(4), 1152–1155 (1932). [CrossRef]
34. S. D. Senturia, “Introduction,” in Microsystem Design (Kluwer Academic Publishers, 2005), pp. 3–14.
35. R. Farchioni and G. Grosso, Organic Electronic Materials: Conjugted Polymers and Low Molecular Weight Electronic Solids (Springer, 2001).
