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Simple Fabrication of an Organic Laser by Microcontact Molding of a Distributed Feedback Grating Sergej Hermann, R. Clayton Shallcross,* and Klaus Meerholz*
Semiconducting nanocrystals (NCs) have attracted a great deal of attention for applications in optoelectronic devices due to their size-tunable optical and energetic properties as well as the ability to be inexpensively processed from solution.[1–3] Incorporation of NCs into such devices relies on the ability to precisely control their morphology, either as pure materials or hybrid blends with organic polymers, on nanometer length scales to facilitate photoinduced (e.g., solar fuel[4,5] and photovoltaic[6] applications) and dark (e.g., field effect transistors[7,8] and lightemitting diodes)[9] charge transfer. Directed lithographic selfassembly of NCs on the nanoscale has also been employed to fabricate patterned features that can act as photonic structures (e.g., gratings[10,11] or lasers),[12] nanoelectronic circuits[13] and sensors.[14] All the aforementioned applications rely on controlling the interaction of the ligand-capped NCs with a variety of materials and substrates to afford the desired nanoscale morphology (i.e., nanoscale phase separation, crack-free film formation and high-fidelity nanoscale pattern replication for hybrid photovoltaics, nanocrystal solar cells/transistors and photonic structures/sensors, respectively). Previously, Armstrong et al. have published on the formation of high-fidelity CdSe NC diffraction gratings that demonstrated impressive transmission diffraction efficiency.[10] These nanoscale diffraction gratings are produced via a room temperature microcontact molding procedure utilizing poly(dimethylsiloxane) (PDMS) replicas of commercially-available masters. The NC gratings are coherent over large areas (> 0.5 cm2) and show high-fidelity replication of the master pattern. Here, we demonstrate another application of such NC gratings; the ability to provide distributed feedback (DFB) in a planar waveguide. While grating structures are routinely used for DFB in optical gain systems, the formation of such gratings typically requires optical interference lithography,[15] electron beam lithography[16] or elevated-temperature embossing[12,17] techniques, which are time consuming and/or energy intensive. Combining the developed synthesis and low-temperature manufacturing techniques (i.e., the microcontact molding S. Hermann, R. C. Shallcross, K. Meerholz Department of Chemistry University of Cologne Luxemburger Str. 116, 50939, Cologne, (Germany) E-mail: [email protected]; [email protected] R. C. Shallcross Department of Chemistry University of Arizona Tucson, Arizona 85721, USA
DOI: 10.1002/adma.201401616
Adv. Mater. 2014, DOI: 10.1002/adma.201401616
procedure) with a fluorescent organic polymer, which is capable of amplified spontaneous emission (ASE) and therefore optical gain, we report on the first hybrid NC/polymer system that demonstrates a reduced lasing threshold and increased differential lasing efficiency when compared to the all-polymer reference device. Device Introduction: We investigate devices consisting of a polymeric dielectric cladding layer coated with a red-emissive polymer (REP) featuring optical gain (see SI for chemical structure and optical characterization). Due to the fact that conjugated polymers in general have a relatively high index of refraction (n = 1.7 … 1.8) compared with the dielectric cladding used here (n = 1.55), the REP (n = 1.73) will simultaneously act as a planar waveguide for emitted light. Under pulsed laser excitation (λ = 532 nm, τ = 7 ns, frep = 10 Hz), REP shows ASE around 662 nm (Figure S2) with a threshold intensity of Ithr = 17 µJ cm−2 (Figure S3). This energy density corresponds to a power density of 2.4 kW cm−2, which is comparable with record values reported in the literature in the red range beyond 650 nm (2.8 kW cm−2 for Dow Red F and 0.5 kW cm−2 for 4% BEH in m-EHOP).[18,19] Much higher ASE thresholds of 40 to 280 kW cm−2 are known from other organic red materials like Rhodamine 700 or different spiro-copolymers.[20,21] DFB is introduced by either stamping a grating of CdSe NCs atop the REP in device type A or by introducing a grating at the interface of bottom dielectric and REP by holographic interference lithography in device type B (Figure 1; see “Experimental” for details). Type A devices are explored with gratings composed of NCs with three different core diameters of 4.5, 3.2 and 2.5 nm. According to their color appearance, the NCs are referred to as R (red), O (orange) and Y (yellow), respectively (Figure S4). For fabrication of both device types, the grating period Λ is chosen to match the Bragg condition (Equation (1) in the second order (i.e., m = 2). m ⋅ λBragg = 2 ⋅ neff ⋅ Λ
(1)
Here λBragg is the wavelength of the diffracted mode waveguided in the REP film and neff is it's effective index of refraction. Assuming (i) the effective index neff to be somewhere between 1.55 (dielectric cladding) and 1.73 (REP) and (ii) that the emitter provides the highest optical gain around the ASEmaximum at λASE = 662 nm, the most suitable grating period can be estimated to be in the range of 400 to 430 nm. Conveniently, the commercially available master templates that are used for creating the PDMS replica stamps for NC stamping are available with 2400 lines mm−1, providing for a period of 417 nm that fits well within this desired range.
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Figure 1. Schematic illustration of the two device types used in this study. Device type A uses microcontact-printed CdSe NC gratings (light grey) as the DFB component, while device type B uses a photolithographically-prepared grating in transparent dielectric cladding layer. The DFB interaction regions are shown by the dashed lines. In both device types, the red-emitting polymer (REP) is excited from the top by a pulsed 532 nm laser, focused to a stripe. Laser emission is observed from the edge of the samples (see arrow).
Morphological Characterization of Stamped Gratings: Figure 2 summarizes the morphological characterization of the stamped CdSe NC gratings used in type A devices, where the nanocrystal regions are bright and the polymer substrate is dark. We find some irregularities and interruptions of the transferred pattern in the case where the PDMS stamps are inked with relatively low NC concentrations (2 mg mL−1; Figure 2a). For the highest concentrations explored here (10 mg mL−1; Figure 2b), the gratings are free of visible defects, where the high contrast between the bright nanocrystal regions and the dark substrate regions suggests that the substrate regions are void of any nanocrystals. For median concentrations of 5 mg mL−1, the uniformity is somewhere between the two former cases. The AFM topography (Figure 2d), in particular the line scan (Figure 2e), indicates that we obtain truncated half-sine-like gratings (which is confirmed by SEM cross sections, see Figure 2c). Even though
the identical stamp is used for all gratings, we find that the reproducibility of the stamping is not perfect, leading to slightly different grating periods (Λ = 416.4 … 433.2 nm) and modulation depths defined as peak-to-valley distance (M = 95.3 … 127.5 nm) between stampings. The observed concentrationdependent morphology of the 1D truncated half-sine-like nanocrystal structures is consistent with those seen in the original report by Armstrong et al. when sinusoidal PDMS replicas (produced from holographic master gratings) are used.[10] The gratings in type B devices exhibit a sinusoidal profile with peakto-valley modulation depth of M ≈ 60 nm (see Figure S7). Lasing Characterization: To investigate the lasing performance of the prepared DFB devices, they are pumped optically with pulsed light (λ = 532 nm, τ = 7 ns, frep = 10 Hz) from the top (i.e., the REP/air interface) at normal incidence (Figure 1). We observe no lasing with R-samples, and only one
Figure 2. Top: SEM top-view images of the samples R.2 (a) and R.10 (b) and a cross-section image (c). Bottom: AFM topography (d) measurement on the sample Y.10 and the corresponding cross-section profile (e) in equal lateral and height scaling. The results shown here are representative for all other samples.
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of the O-samples showed a narrow line at a very high pump intensity (damaging the REP in a few seconds), preventing meaningful measurements. By contrast, all type A devices with Y-gratings showed lasing (see Figure 3). We attribute the negative results with the R- and O-NCs to their absorption overlapping with the PL-range of the emitter polymer causing a loss channel, which is not the case for the Y-NCs. Also, the reference device (type B) exhibits lasing. From a Lorentz fit on the data (inset in Figure 3) a line width of Δλ ≈ 0.2 nm for both device types is obtained, which corresponds to the resolution limit of the spectrograph and CCD camera. Table 1 summarizes the lasing properties for two type A devices with partly interrupted (Y.2) and defect-free (Y.10) grating lines and a type B device. With increasing concentration of the NCs in the ink used for stamping and therefore increasing uniformity of the gratings (compare Figure 2), we observe a reduction to 57% of the threshold pump intensity Ithr and nearly tripling of the differential laser efficiency η (Equation (2)), defined as the slope of the laser output as a function of pump power:
Table 1. Device and lasing properties of type A with Y-NC gratings and type B. The device properties include the grating period Λ and modulation depth M, the lasing wavelength λlasing, the effective refractive index neff of the lasing mode calculated using Equation (1), the threshold pump intensity for lasing Ithr from Figure 4 and the differential laser efficiency normalized to the type B device. Type A
B
DFB
Λ [nm]
M [nm]
λlasing [nm]
neff (exp.)
Ithr [µJ cm−2]
ηnorm
Y.2
416.4
95.3
665.5
1.60
79 ± 4
1.4
Y.10
423.5
95.9
665.5
1.57
45 ± 3
4.0
bottom cladding
426.8
58.3
659.9
1.55
65 ± 4
1.0
Adv. Mater. 2014, DOI: 10.1002/adma.201401616
η=
dPout dPpump
(2)
The observed lasing thresholds of type A and type B devices of 45 and 65 µJ cm−2 correspond to the power density of 6.4 and 9.3 kW cm−2 respectively, which are well below the thresholds reported for Alq3:DCM (22…105 kW cm−2),[22–25] F8BT:MEHPPV (33.6 kW cm−2)[26] or Rhodamine 700 (10…28 kW cm−2).[21] Finally, we compare the lasing performance of the optimized type A device with the reference device (type B) under identical conditions (Figure 4). Obviously, the new type A device features a 31% lower threshold, but more importantly, a differential laser efficiency, which is increased by factor 4. We attribute this finding to a much stronger distributed feedback in type A devices. Table 2 summarizes the effective index of refraction (one calculated from experimental data according to Equation (1) and one obtained from numerical simulation),[27] the intensity distribution of the guided mode, the refractive index contrast on the grating, and the Fourier coefficients of the different grating shapes of the two device types. As demonstrated in the simulation for the 0-order TE-Mode (Figure S5), the additional 50 nm layer of CdSe QDs (with high refractive index, n ca. 2.1) on top of the emitter significantly influences the intensity distribution in the waveguide, leading to a higher mode confinement and a higher effective index of refraction neff as observed experimentally. The higher spatial mode confinement to the waveguide leads to an increase of fill-factor, defined as the fraction of the mode intensity guided in the waveguide core, from 43% (type B) to 61% (type A). In addition to that, a 60% higher grating modulation depth could be achieved in type A devices compared to type B. The waveguiding in DFB structures can be analyzed by the coupled mode theory[28] describing the interaction between the forward and backward propagating waves. The coupling coefficient κ, determining the strength of the DFB can be written as
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In conclusion, we report on the first demonstration of laser emission from a hybrid NC/polymer system utilizing a DFB grating made of high-refractive CdSe nanocrystals that are stamped directly on top of the emitter layer. We observe a resolution-limited 0.21 nm narrow emission that exhibits very pronounced threshold behavior with reduced lasing threshold and fourfold improvement in differential lasing efficiency when compared to the all-polymer reference device. Furthermore, we elucidate the effect of the NC size and concentration used to ink the PDMS mold in respect to the lasing efficiency of the red-emitting polymer/NC DFB grating device, providing design rules for the purposeful implementation of such nanocrystal gratings with a variety of optical gain materials that emit throughout the visFigure 4. Output characteristics of the optimized type A device (squares, leftY-axis) and the ible and infrared spectral range. Although the type B device (triangles, right Y-axis) as a function of pump intensity. Please note the different scales and the offset for the two axes, reflecting the different differential laser efficiencies of material used here (CdSe) limits the spectral the two devices. The inset shows the simulated intensity distribution of zero-order TE-mode range to the red part of the spectrum (i.e., the for both device types. red absorption of larger CdSe nanocrystals is detrimental to the lasing process), nanomaω terials with little to no absorption in the visible spectral region κ = ∫ E 0* ( x , y )ε l ( x , y )E 0 ( x , y ) dxdy (3) 4 (e.g., ZnS or TiO2) can be used to extend the described principle to shorter wavelengths. where εl (x, y) is the lth Fourier component of the periodic dieThe microcontact molding procedure is very simple and less time consuming when compared to typical DFB structures prolectric perturbation.[29] We perform a Fourier analysis of both duced by thermally assisted imprint molding, as well as optical grating shapes (Figure S6) and find, assuming an equal modand electron beam lithography. It provides for large-area gratulation depth for the coefficient of the first harmonic (correings with nanoscale periods, defined by the master template, of sponding to the fundamental period of grating), an 11% higher high quality and reproducibility that can be prepared at room value for type A devices. Therefore, according to Equation (3), a temperature. Furthermore, since the NCs are already pre-dried higher DFB strength for type A devices can be expected. in the PDMS mold, the printing method alleviates any issues In summary, this partly qualitative analysis of spatial typically encountered when solution-processing multi-layered mode confinement, refractive index contrast and shape of the structures/devices utilizing materials with similar solubility in grating, including the modulation depth, makes the observed common organic solvents. overall advance of type A devices plausible. The major contribution to the stronger DFB in type A devices most probably comes from the 3- to 5-fold refractive index modulation in the DFB structure due to application of CdSe with a significantly higher refractive index n ≈ 2.1 and the empty grating grooves Experimental Section in between (n = 1.0), but also from higher fill factor and larger NC Synthesis and Characterization: We synthesized monodisperse Fourier coefficient. CdSe NCs in three different sizes, following a previously published Table 2. Comparison of the properties determining the strength of DFB in the two device types. Property exp.
simul.
4
Device type A (Y.10) Threshold intensity Ithr
45 µJ
cm−2
Device type B 79 µJ cm−2
Differential laser efficiency η
4.0
1.0
Index contrast at DFB grating
+0.5 / −0.6
+0.16 / −0.02
neff (experiment, s. Table 1)
1.57 … 1.60
1.55
Grating modulation depth
96 nm
58 nm
neff (simulation, s. Figure S5)
1.63
1.57
Fill factor
61%
43%
Fourier coeff. for 1st harmonic
1.11
1.00
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procedure by Peng et al.[30] The NCs were isolated from excess ligand and unreacted precursors via two precipitation-solvation cycles, which has been previously described in detail.[10] See Figure S4 in SI for absorption and photoluminescence spectra of the CdSe NCs. According to their color appearance, we will denote the NCs as R (red), O (orange) and Y (yellow) respectively. The CdSe NC core size can be estimated via an equation published by Peng et al.[31] using the first excitonic absorption feature, corresponding to diameters of 4.5, 3.2, and 2.5 nm for R, O, and Y, respectively. Stamping: The stamping procedure is identical to that previously published by Armstrong et al.[10] Briefly, PDMS replicas, which could be used several times, were prepared from an aluminum master holographic optical diffraction grating with a sine-shaped topography (Λ = 417 nm, M = 124 nm). The PDMS replicas are thus sinusoidal in shape. Then, the entire area of the PDMS stamp (ca. 1.3 cm2) is flooded via drop-casting with a toluene solution (75 µL) of the NCs with a defined concentration (see text). The toluene solvent is allowed
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I=
AE p gl e − 1) g (
(4)
where AEP describes spontaneous emission rate, which is proportional to the pump energy EP. By fitting this expression to experimental data, the net gain g could be obtained for different pump intensities (Figure S3). Simulations: The calculations of intensity distribution and effective index of refraction for devices under investigation were performed using a Maxwell solver CAMFR.[27] For this we assumed simplified device geometries, where all films are considered to be planar. For type A devices, a 50 nm thin CdSe Layer is assumed instead of a 100 nm peakto-valley modulated grating.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Adv. Mater. 2014, DOI: 10.1002/adma.201401616
Acknowledgements
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to evaporate with the stamp tilted at an angle of ca. 15–20°, where the grating grooves are parallel to the tilt angle, under a cap to ensure slow evaporation of the solvent, which took approximately 10 min. Using this procedure, only NCs remain in the recessed regions of the PDMS stamp due to the dewetting process that is described in detail in the manuscript by Armstrong et al.[10] The dried NC-coated stamps are manually brought into contact with the substrate and removed again, transferring the 1D nanocrystal structures from the recessed stamp regions to the bare substrate. At this point it is important to ensure the translation-only (in the z-direction) movement of the stamp, because any rotation during removal will compromise the grating quality. Devices Fabrication: Since the devices are pumped optically from the top and the emission is collected from the edge, a defect-free edge of the emitter layer is desirable. To achieve this, the substrates were scratched with a glasscutter to give pre-determined breaking points. Then the scratched substrates were coated with a ca. 20 µm thick dielectric cladding layer OrmoClear (microresist technology GmbH), burying the scratch and yielding a smooth surface for further processing. To be crosslinked, the OrmoClear was pre-baked according to its processing guidelines on a hot plate at 80 °C and exposed to UV-light (λ = 365 nm) under inert gas atmosphere. For the type B devices an additional ≈1.5 µm OrmoClear layer was spin-coated, pre-baked and exposed with an interference pattern of UV laser light (HeCd, 325 nm), yielding after developing a sine-shaped corrugated surface with modulation depth of M ≈ 60 nm (Figure S7). The OrmoClear layers were covered by a 200 nm thick red-emitting spiro co-polymer (REP) layers. For type A devices a grating of CdSe-NCs is deposited by microcontact molding as described above onto the emitter layer. By systematically varying the size of NCs (R, O and Y) and their concentration in the toluene solution (2, 5, and 10 mg mL−1) we obtained nine devices with varying uniformity and lasing ability. In the following, the samples are denoted by “NC-type.Concentration”, e.g. “R.2”, meaning that R-NCs were deposited using a concentration of 2 mg mL−1. Lasing Investigation: To investigate and characterize the lasing performance of the prepared devices, they were pumped optically with a pulsed SHG Nd-YAG laser (λ = 532 nm, τ = 7 ns, frep = 10 Hz) from the top. We used a rectangular-shaped excitation area of 200 µm × 1 mm. The beam was focused with a cylindrical lens to a stripe of 200 µm width and cut on one side by a razor blade 1 mm away from the sample edge. To provide a nearly flat-top intensity profile over the excitation area the beam was initially expanded to a diameter of ca. 20 mm. The same setup was used to characterize the emitter polymer in absence of DFB gratings. By varying the length of the excitation stripelthe outputIas a function of this length for different pump intensities was collected. Assuming the polymer to have an optical amplification g, which includes all losses and therefore can be negative, straightforward mathematics yields an expression for output[32]
We would like to thank Merck KGaA (Darmstadt) for providing the red emitter polymer. Further, we acknowledge funding by the State of Northrhine-Westfalia and the Europäischer Fonds für Regionale Entwicklung (EFRE) through the PROTECT project, which is part of the Centre of Organic Production Technologies COPT.NRW.
Received: April 10, 2014 Revised: June 14, 2014 Published online: [1] D. V. Talapin, J.-S. Lee, M. V. Kovalenko, E. V. Shevchenko, Chem. Rev. 2010, 110, 389. [2] T. Nann, W. M. Skinner, ACS Nano 2011, 5, 5291. [3] I. J. Kramer, E. H. Sargent, Chem. Rev. 2013. [4] P. Rodenas, T. Song, P. Sudhagar, G. Marzari, H. Han, L. Badia-Bou, S. Gimenez, F. Fabregat-Santiago, I. Mora-Sero, J. Bisquert, U. Paik, Y. S. Kang, Adv. Energy Mater. 2013, 3, 176. [5] R. C. Shallcross, G. D. D’Ambruoso, J. Pyun, N. R. Armstrong, J. Am. Chem. Soc. 2010, 132, 2622. [6] I. J. Kramer, E. H. Sargent, ACS Nano 2011, 5, 8506. [7] M. S. Kang, A. Sahu, D. J. Norris, C. D. Frisbie, Nano Lett. 2010, 10, 3727. [8] P. Guyot-Sionnest, J. Phys. Chem. Lett. 2012, 3, 1169. [9] J. Kwak, W. K. Bae, D. Lee, I. Park, J. Lim, M. Park, H. Cho, H. Woo, D. Y. Yoon, K. Char, S. Lee, C. Lee, Nano Lett. 2012, 12, 2362. [10] R. C. Shallcross, G. S. Chawla, F. S. Marikkar, S. Tolbert, J. Pyun, N. R. Armstrong, ACS Nano 2009, 3, 3629. [11] M. J. Hampton, J. L. Templeton, J. M. DeSimone, Langmuir 2010, 26, 3012. [12] Y. Chan, P. T. Snee, J.-M. Caruge, B. K. Yen, G. P. Nair, D. G. Nocera, M. G. Bawendi, J. Am. Chem. Soc. 2006, 128, 3146. [13] T. S. Mentzel, D. D. Wanger, N. Ray, B. J. Walker, D. Strasfeld, M. G. Bawendi, M. A. Kastner, Nano Lett. 2012, 12, 4404. [14] V. P. Pattani, C. Li, T. A. Desai, T. Q. Vu, Biomed Microdevices 2008, 10, 367. [15] I. D. W. Samuel, G. A. Turnbull, Chem. Rev. 2007, 107, 1272. [16] A. J. C. Kuehne, M. Kaiser, A. R. Mackintosh, B. H. Wallikewitz, D. Hertel, R. A. Pethrick, K. Meerholz, Adv. Funct. Mater. 2011, 21, 2564. [17] V. C. Sundar, H.-J. Eisler, T. Deng, Y. Chan, E. L. Thomas, M. G. Bawendi, Adv. Mater. 2004, 16, 2137. [18] R. Xia, G. Heliotis, Y. Hou, D. D. Bradley, Organic Electronics 2003, 4, 165. [19] R. Gupta, M. Stevenson, A. Dogariu, M. D. McGehee, J. Y. Park, V. Srdanov, A. J. Heeger, H. Wang, Appl. Phys. Lett. 1998, 73, 3492. [20] B. H. Wallikewitz, D. Hertel, K. Meerholz, Chem. Mater. 2009, 21, 2912. [21] B. H. Wallikewitz, G. O. Nikiforov, H. Sirringhaus, R. H. Friend, Appl. Phys. Lett. 2012, 100, 173301. [22] S. Klinkhammer, T. Woggon, U. Geyer, C. Vannahme, S. Dehm, T. Mappes, U. Lemmer, Appl. Phys. B 2009, 97, 787. [23] D. Schneider, T. Rabe, T. Riedl, T. Dobbertin, M. Kröger, E. Becker, H.-H. Johannes, W. Kowalsky, T. Weimann, J. Wang, P. Hinze, Appl. Phys. Lett. 2004, 85, 1886. [24] M. Koschorreck, R. Gehlhaar, V. G. Lyssenko, M. Swoboda, M. Hoffmann, K. Leo, Appl. Phys. Lett. 2005, 87, 181108. [25] B. Schütte, H. Gothe, S. I. Hintschich, M. Sudzius, H. Fröb, V. G. Lyssenko, K. Leo, Appl. Phys. Lett. 2008, 92, 163309.
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www.MaterialsViews.com [26] S. Klinkhammer, X. Liu, K. Huska, Y. Shen, S. Vanderheiden, S. Valouch, C. Vannahme, S. Bräse, T. Mappes, U. Lemmer, Opt. Express 2012, 20, 6357. [27] P. Bienstman, CAMFR: (CAvity Modelling FRamework) Universität Ghent, Ghent 2001. [28] H. Kogelnik, C. V. Shank, J. Appl. Phys 1972, 43, 2327.
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[29] A. Yariv, P. Yeh, Photonics: Optical electronics in modern communications Oxford University Press, New York 2007. [30] X. Peng, J. Thessing, Struct. Bonding 2005, 118, 79. [31] W. W. Yu, L. Qu, W. Guo, X. Peng, Chem.Mater. 2003, 15, 2854. [32] M. D. McGehee, R. Gupta, S. Veenstra, E. K. Miller, M. A. Díaz-García, A. J. Heeger, Phys. Rev. B 1998, 58, 7035.
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Adv. Mater. 2014, DOI: 10.1002/adma.201401616
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Simple Fabrication of an Organic Laser by Microcontact Molding of a Distributed Feedback Grating Sergej Hermann, R. Clayton Shallcross,* and Klaus Meerholz*
Semiconducting nanocrystals (NCs) have attracted a great deal of attention for applications in optoelectronic devices due to their size-tunable optical and energetic properties as well as the ability to be inexpensively processed from solution.[1–3] Incorporation of NCs into such devices relies on the ability to precisely control their morphology, either as pure materials or hybrid blends with organic polymers, on nanometer length scales to facilitate photoinduced (e.g., solar fuel[4,5] and photovoltaic[6] applications) and dark (e.g., field effect transistors[7,8] and lightemitting diodes)[9] charge transfer. Directed lithographic selfassembly of NCs on the nanoscale has also been employed to fabricate patterned features that can act as photonic structures (e.g., gratings[10,11] or lasers),[12] nanoelectronic circuits[13] and sensors.[14] All the aforementioned applications rely on controlling the interaction of the ligand-capped NCs with a variety of materials and substrates to afford the desired nanoscale morphology (i.e., nanoscale phase separation, crack-free film formation and high-fidelity nanoscale pattern replication for hybrid photovoltaics, nanocrystal solar cells/transistors and photonic structures/sensors, respectively). Previously, Armstrong et al. have published on the formation of high-fidelity CdSe NC diffraction gratings that demonstrated impressive transmission diffraction efficiency.[10] These nanoscale diffraction gratings are produced via a room temperature microcontact molding procedure utilizing poly(dimethylsiloxane) (PDMS) replicas of commercially-available masters. The NC gratings are coherent over large areas (> 0.5 cm2) and show high-fidelity replication of the master pattern. Here, we demonstrate another application of such NC gratings; the ability to provide distributed feedback (DFB) in a planar waveguide. While grating structures are routinely used for DFB in optical gain systems, the formation of such gratings typically requires optical interference lithography,[15] electron beam lithography[16] or elevated-temperature embossing[12,17] techniques, which are time consuming and/or energy intensive. Combining the developed synthesis and low-temperature manufacturing techniques (i.e., the microcontact molding S. Hermann, R. C. Shallcross, K. Meerholz Department of Chemistry University of Cologne Luxemburger Str. 116, 50939, Cologne, (Germany) E-mail: [email protected]; [email protected] R. C. Shallcross Department of Chemistry University of Arizona Tucson, Arizona 85721, USA
DOI: 10.1002/adma.201401616
Adv. Mater. 2014, DOI: 10.1002/adma.201401616
procedure) with a fluorescent organic polymer, which is capable of amplified spontaneous emission (ASE) and therefore optical gain, we report on the first hybrid NC/polymer system that demonstrates a reduced lasing threshold and increased differential lasing efficiency when compared to the all-polymer reference device. Device Introduction: We investigate devices consisting of a polymeric dielectric cladding layer coated with a red-emissive polymer (REP) featuring optical gain (see SI for chemical structure and optical characterization). Due to the fact that conjugated polymers in general have a relatively high index of refraction (n = 1.7 … 1.8) compared with the dielectric cladding used here (n = 1.55), the REP (n = 1.73) will simultaneously act as a planar waveguide for emitted light. Under pulsed laser excitation (λ = 532 nm, τ = 7 ns, frep = 10 Hz), REP shows ASE around 662 nm (Figure S2) with a threshold intensity of Ithr = 17 µJ cm−2 (Figure S3). This energy density corresponds to a power density of 2.4 kW cm−2, which is comparable with record values reported in the literature in the red range beyond 650 nm (2.8 kW cm−2 for Dow Red F and 0.5 kW cm−2 for 4% BEH in m-EHOP).[18,19] Much higher ASE thresholds of 40 to 280 kW cm−2 are known from other organic red materials like Rhodamine 700 or different spiro-copolymers.[20,21] DFB is introduced by either stamping a grating of CdSe NCs atop the REP in device type A or by introducing a grating at the interface of bottom dielectric and REP by holographic interference lithography in device type B (Figure 1; see “Experimental” for details). Type A devices are explored with gratings composed of NCs with three different core diameters of 4.5, 3.2 and 2.5 nm. According to their color appearance, the NCs are referred to as R (red), O (orange) and Y (yellow), respectively (Figure S4). For fabrication of both device types, the grating period Λ is chosen to match the Bragg condition (Equation (1) in the second order (i.e., m = 2). m ⋅ λBragg = 2 ⋅ neff ⋅ Λ
(1)
Here λBragg is the wavelength of the diffracted mode waveguided in the REP film and neff is it's effective index of refraction. Assuming (i) the effective index neff to be somewhere between 1.55 (dielectric cladding) and 1.73 (REP) and (ii) that the emitter provides the highest optical gain around the ASEmaximum at λASE = 662 nm, the most suitable grating period can be estimated to be in the range of 400 to 430 nm. Conveniently, the commercially available master templates that are used for creating the PDMS replica stamps for NC stamping are available with 2400 lines mm−1, providing for a period of 417 nm that fits well within this desired range.
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Figure 1. Schematic illustration of the two device types used in this study. Device type A uses microcontact-printed CdSe NC gratings (light grey) as the DFB component, while device type B uses a photolithographically-prepared grating in transparent dielectric cladding layer. The DFB interaction regions are shown by the dashed lines. In both device types, the red-emitting polymer (REP) is excited from the top by a pulsed 532 nm laser, focused to a stripe. Laser emission is observed from the edge of the samples (see arrow).
Morphological Characterization of Stamped Gratings: Figure 2 summarizes the morphological characterization of the stamped CdSe NC gratings used in type A devices, where the nanocrystal regions are bright and the polymer substrate is dark. We find some irregularities and interruptions of the transferred pattern in the case where the PDMS stamps are inked with relatively low NC concentrations (2 mg mL−1; Figure 2a). For the highest concentrations explored here (10 mg mL−1; Figure 2b), the gratings are free of visible defects, where the high contrast between the bright nanocrystal regions and the dark substrate regions suggests that the substrate regions are void of any nanocrystals. For median concentrations of 5 mg mL−1, the uniformity is somewhere between the two former cases. The AFM topography (Figure 2d), in particular the line scan (Figure 2e), indicates that we obtain truncated half-sine-like gratings (which is confirmed by SEM cross sections, see Figure 2c). Even though
the identical stamp is used for all gratings, we find that the reproducibility of the stamping is not perfect, leading to slightly different grating periods (Λ = 416.4 … 433.2 nm) and modulation depths defined as peak-to-valley distance (M = 95.3 … 127.5 nm) between stampings. The observed concentrationdependent morphology of the 1D truncated half-sine-like nanocrystal structures is consistent with those seen in the original report by Armstrong et al. when sinusoidal PDMS replicas (produced from holographic master gratings) are used.[10] The gratings in type B devices exhibit a sinusoidal profile with peakto-valley modulation depth of M ≈ 60 nm (see Figure S7). Lasing Characterization: To investigate the lasing performance of the prepared DFB devices, they are pumped optically with pulsed light (λ = 532 nm, τ = 7 ns, frep = 10 Hz) from the top (i.e., the REP/air interface) at normal incidence (Figure 1). We observe no lasing with R-samples, and only one
Figure 2. Top: SEM top-view images of the samples R.2 (a) and R.10 (b) and a cross-section image (c). Bottom: AFM topography (d) measurement on the sample Y.10 and the corresponding cross-section profile (e) in equal lateral and height scaling. The results shown here are representative for all other samples.
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of the O-samples showed a narrow line at a very high pump intensity (damaging the REP in a few seconds), preventing meaningful measurements. By contrast, all type A devices with Y-gratings showed lasing (see Figure 3). We attribute the negative results with the R- and O-NCs to their absorption overlapping with the PL-range of the emitter polymer causing a loss channel, which is not the case for the Y-NCs. Also, the reference device (type B) exhibits lasing. From a Lorentz fit on the data (inset in Figure 3) a line width of Δλ ≈ 0.2 nm for both device types is obtained, which corresponds to the resolution limit of the spectrograph and CCD camera. Table 1 summarizes the lasing properties for two type A devices with partly interrupted (Y.2) and defect-free (Y.10) grating lines and a type B device. With increasing concentration of the NCs in the ink used for stamping and therefore increasing uniformity of the gratings (compare Figure 2), we observe a reduction to 57% of the threshold pump intensity Ithr and nearly tripling of the differential laser efficiency η (Equation (2)), defined as the slope of the laser output as a function of pump power:
Table 1. Device and lasing properties of type A with Y-NC gratings and type B. The device properties include the grating period Λ and modulation depth M, the lasing wavelength λlasing, the effective refractive index neff of the lasing mode calculated using Equation (1), the threshold pump intensity for lasing Ithr from Figure 4 and the differential laser efficiency normalized to the type B device. Type A
B
DFB
Λ [nm]
M [nm]
λlasing [nm]
neff (exp.)
Ithr [µJ cm−2]
ηnorm
Y.2
416.4
95.3
665.5
1.60
79 ± 4
1.4
Y.10
423.5
95.9
665.5
1.57
45 ± 3
4.0
bottom cladding
426.8
58.3
659.9
1.55
65 ± 4
1.0
Adv. Mater. 2014, DOI: 10.1002/adma.201401616
η=
dPout dPpump
(2)
The observed lasing thresholds of type A and type B devices of 45 and 65 µJ cm−2 correspond to the power density of 6.4 and 9.3 kW cm−2 respectively, which are well below the thresholds reported for Alq3:DCM (22…105 kW cm−2),[22–25] F8BT:MEHPPV (33.6 kW cm−2)[26] or Rhodamine 700 (10…28 kW cm−2).[21] Finally, we compare the lasing performance of the optimized type A device with the reference device (type B) under identical conditions (Figure 4). Obviously, the new type A device features a 31% lower threshold, but more importantly, a differential laser efficiency, which is increased by factor 4. We attribute this finding to a much stronger distributed feedback in type A devices. Table 2 summarizes the effective index of refraction (one calculated from experimental data according to Equation (1) and one obtained from numerical simulation),[27] the intensity distribution of the guided mode, the refractive index contrast on the grating, and the Fourier coefficients of the different grating shapes of the two device types. As demonstrated in the simulation for the 0-order TE-Mode (Figure S5), the additional 50 nm layer of CdSe QDs (with high refractive index, n ca. 2.1) on top of the emitter significantly influences the intensity distribution in the waveguide, leading to a higher mode confinement and a higher effective index of refraction neff as observed experimentally. The higher spatial mode confinement to the waveguide leads to an increase of fill-factor, defined as the fraction of the mode intensity guided in the waveguide core, from 43% (type B) to 61% (type A). In addition to that, a 60% higher grating modulation depth could be achieved in type A devices compared to type B. The waveguiding in DFB structures can be analyzed by the coupled mode theory[28] describing the interaction between the forward and backward propagating waves. The coupling coefficient κ, determining the strength of the DFB can be written as
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In conclusion, we report on the first demonstration of laser emission from a hybrid NC/polymer system utilizing a DFB grating made of high-refractive CdSe nanocrystals that are stamped directly on top of the emitter layer. We observe a resolution-limited 0.21 nm narrow emission that exhibits very pronounced threshold behavior with reduced lasing threshold and fourfold improvement in differential lasing efficiency when compared to the all-polymer reference device. Furthermore, we elucidate the effect of the NC size and concentration used to ink the PDMS mold in respect to the lasing efficiency of the red-emitting polymer/NC DFB grating device, providing design rules for the purposeful implementation of such nanocrystal gratings with a variety of optical gain materials that emit throughout the visFigure 4. Output characteristics of the optimized type A device (squares, leftY-axis) and the ible and infrared spectral range. Although the type B device (triangles, right Y-axis) as a function of pump intensity. Please note the different scales and the offset for the two axes, reflecting the different differential laser efficiencies of material used here (CdSe) limits the spectral the two devices. The inset shows the simulated intensity distribution of zero-order TE-mode range to the red part of the spectrum (i.e., the for both device types. red absorption of larger CdSe nanocrystals is detrimental to the lasing process), nanomaω terials with little to no absorption in the visible spectral region κ = ∫ E 0* ( x , y )ε l ( x , y )E 0 ( x , y ) dxdy (3) 4 (e.g., ZnS or TiO2) can be used to extend the described principle to shorter wavelengths. where εl (x, y) is the lth Fourier component of the periodic dieThe microcontact molding procedure is very simple and less time consuming when compared to typical DFB structures prolectric perturbation.[29] We perform a Fourier analysis of both duced by thermally assisted imprint molding, as well as optical grating shapes (Figure S6) and find, assuming an equal modand electron beam lithography. It provides for large-area gratulation depth for the coefficient of the first harmonic (correings with nanoscale periods, defined by the master template, of sponding to the fundamental period of grating), an 11% higher high quality and reproducibility that can be prepared at room value for type A devices. Therefore, according to Equation (3), a temperature. Furthermore, since the NCs are already pre-dried higher DFB strength for type A devices can be expected. in the PDMS mold, the printing method alleviates any issues In summary, this partly qualitative analysis of spatial typically encountered when solution-processing multi-layered mode confinement, refractive index contrast and shape of the structures/devices utilizing materials with similar solubility in grating, including the modulation depth, makes the observed common organic solvents. overall advance of type A devices plausible. The major contribution to the stronger DFB in type A devices most probably comes from the 3- to 5-fold refractive index modulation in the DFB structure due to application of CdSe with a significantly higher refractive index n ≈ 2.1 and the empty grating grooves Experimental Section in between (n = 1.0), but also from higher fill factor and larger NC Synthesis and Characterization: We synthesized monodisperse Fourier coefficient. CdSe NCs in three different sizes, following a previously published Table 2. Comparison of the properties determining the strength of DFB in the two device types. Property exp.
simul.
4
Device type A (Y.10) Threshold intensity Ithr
45 µJ
cm−2
Device type B 79 µJ cm−2
Differential laser efficiency η
4.0
1.0
Index contrast at DFB grating
+0.5 / −0.6
+0.16 / −0.02
neff (experiment, s. Table 1)
1.57 … 1.60
1.55
Grating modulation depth
96 nm
58 nm
neff (simulation, s. Figure S5)
1.63
1.57
Fill factor
61%
43%
Fourier coeff. for 1st harmonic
1.11
1.00
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procedure by Peng et al.[30] The NCs were isolated from excess ligand and unreacted precursors via two precipitation-solvation cycles, which has been previously described in detail.[10] See Figure S4 in SI for absorption and photoluminescence spectra of the CdSe NCs. According to their color appearance, we will denote the NCs as R (red), O (orange) and Y (yellow) respectively. The CdSe NC core size can be estimated via an equation published by Peng et al.[31] using the first excitonic absorption feature, corresponding to diameters of 4.5, 3.2, and 2.5 nm for R, O, and Y, respectively. Stamping: The stamping procedure is identical to that previously published by Armstrong et al.[10] Briefly, PDMS replicas, which could be used several times, were prepared from an aluminum master holographic optical diffraction grating with a sine-shaped topography (Λ = 417 nm, M = 124 nm). The PDMS replicas are thus sinusoidal in shape. Then, the entire area of the PDMS stamp (ca. 1.3 cm2) is flooded via drop-casting with a toluene solution (75 µL) of the NCs with a defined concentration (see text). The toluene solvent is allowed
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I=
AE p gl e − 1) g (
(4)
where AEP describes spontaneous emission rate, which is proportional to the pump energy EP. By fitting this expression to experimental data, the net gain g could be obtained for different pump intensities (Figure S3). Simulations: The calculations of intensity distribution and effective index of refraction for devices under investigation were performed using a Maxwell solver CAMFR.[27] For this we assumed simplified device geometries, where all films are considered to be planar. For type A devices, a 50 nm thin CdSe Layer is assumed instead of a 100 nm peakto-valley modulated grating.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Adv. Mater. 2014, DOI: 10.1002/adma.201401616
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to evaporate with the stamp tilted at an angle of ca. 15–20°, where the grating grooves are parallel to the tilt angle, under a cap to ensure slow evaporation of the solvent, which took approximately 10 min. Using this procedure, only NCs remain in the recessed regions of the PDMS stamp due to the dewetting process that is described in detail in the manuscript by Armstrong et al.[10] The dried NC-coated stamps are manually brought into contact with the substrate and removed again, transferring the 1D nanocrystal structures from the recessed stamp regions to the bare substrate. At this point it is important to ensure the translation-only (in the z-direction) movement of the stamp, because any rotation during removal will compromise the grating quality. Devices Fabrication: Since the devices are pumped optically from the top and the emission is collected from the edge, a defect-free edge of the emitter layer is desirable. To achieve this, the substrates were scratched with a glasscutter to give pre-determined breaking points. Then the scratched substrates were coated with a ca. 20 µm thick dielectric cladding layer OrmoClear (microresist technology GmbH), burying the scratch and yielding a smooth surface for further processing. To be crosslinked, the OrmoClear was pre-baked according to its processing guidelines on a hot plate at 80 °C and exposed to UV-light (λ = 365 nm) under inert gas atmosphere. For the type B devices an additional ≈1.5 µm OrmoClear layer was spin-coated, pre-baked and exposed with an interference pattern of UV laser light (HeCd, 325 nm), yielding after developing a sine-shaped corrugated surface with modulation depth of M ≈ 60 nm (Figure S7). The OrmoClear layers were covered by a 200 nm thick red-emitting spiro co-polymer (REP) layers. For type A devices a grating of CdSe-NCs is deposited by microcontact molding as described above onto the emitter layer. By systematically varying the size of NCs (R, O and Y) and their concentration in the toluene solution (2, 5, and 10 mg mL−1) we obtained nine devices with varying uniformity and lasing ability. In the following, the samples are denoted by “NC-type.Concentration”, e.g. “R.2”, meaning that R-NCs were deposited using a concentration of 2 mg mL−1. Lasing Investigation: To investigate and characterize the lasing performance of the prepared devices, they were pumped optically with a pulsed SHG Nd-YAG laser (λ = 532 nm, τ = 7 ns, frep = 10 Hz) from the top. We used a rectangular-shaped excitation area of 200 µm × 1 mm. The beam was focused with a cylindrical lens to a stripe of 200 µm width and cut on one side by a razor blade 1 mm away from the sample edge. To provide a nearly flat-top intensity profile over the excitation area the beam was initially expanded to a diameter of ca. 20 mm. The same setup was used to characterize the emitter polymer in absence of DFB gratings. By varying the length of the excitation stripelthe outputIas a function of this length for different pump intensities was collected. Assuming the polymer to have an optical amplification g, which includes all losses and therefore can be negative, straightforward mathematics yields an expression for output[32]
We would like to thank Merck KGaA (Darmstadt) for providing the red emitter polymer. Further, we acknowledge funding by the State of Northrhine-Westfalia and the Europäischer Fonds für Regionale Entwicklung (EFRE) through the PROTECT project, which is part of the Centre of Organic Production Technologies COPT.NRW.
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Adv. Mater. 2014, DOI: 10.1002/adma.201401616
