NANO LETTERS
Nanofiber Frequency Doublers J. Brewer,†,‡ M. Schiek,§ A. Lu1 tzen,§,| K. Al-Shamery,§ and H.-G. Rubahn*,†,⊥
2006 Vol. 6, No. 12 2656-2659
Department of Chemistry and Physics, UniVersity of Southern Denmark, DK-5230 Odense M, Denmark, MEMPHYS, Center for Biomembrane Physics, UniVersity of Southern Denmark, DK-5230 Odense M, Denmark, Institute of Pure and Applied Chemistry, UniVersity of Oldenburg, D-26129 Oldenburg, Germany, Kekule-Institute, UniVersity of Bonn, D-53121 Bonn, Germany, and Mads Clausen Institute, UniVersity of Southern Denmark, DK-6400 Soenderborg, Denmark Received June 8, 2006; Revised Manuscript Received July 25, 2006
ABSTRACT Nanoscaled, needle-shaped frequency doublers have been generated via self-assembled surface growth from functionalized quaterphenylene molecules with a designed large hyperpolarizability. The nanofiber frequency doublers exhibit very weak fluorescence centered around 430 nm but emit a strong, resonance-enhanced second-harmonic signal when excited with infrared 80 fs laser pulses. The frequency doublers are employed to correlate second-harmonic response and morphology via two-dimensional true second-harmonic images of individual nanoaggregates obtained with the help of a femtosecond laser scanning microscope.
Bottom-up nanoengineering is the art of fabricating quantitative amounts of self-similar nanoshaped aggregates from designed molecules with specific properties. In the photonic domain, the primordial step to that, molecular engineering has enabled the design of molecules with specific optical properties such as a large two-photon absorption cross section1,2 or large second-harmonic (SH) response,3,4 Such tailor-made molecules are expected to develop into an exciting new group of functionalized nanomaterials.5 In the present work, we show a working example of the complete route from molecular design to massive parallel production of new nanophotonic elements, namely, nanoscaled frequency doublers. Besides its obvious importance for future integrated optical circuits, optical second-harmonic generation (SHG) is also a powerful technique for understanding the correlation between morphology and optoelectronic response of nanoaggregates. In the past, SHG has been implemented as a means to probe many different samples, for example, the interface of silicon nanocrystals6 or to study the crystalline nature of organic nanocrystals.7,8 The most promising approach for generating second-order nonlinearly optically active nanofibers is to grow them from organic molecules with optimized hyperpolarizabilities. However, it has turned out that the growth of morphologically well-defined nanofibers is possible only from a * To whom correspondence should be addressed. E-mail: rubahn@ fysik.sdu.dk. † Department of Chemistry and Physics, University of Southern Denmark. ‡ MEMPHYS, Center for Biomembrane Physics, University of Southern Denmark. § Institute of Pure and Applied Chemistry, University of Oldenburg. | Kekule-Institute, University of Bonn. ⊥ Mads Clausen Institute, University of Southern Denmark. 10.1021/nl0613196 CCC: $33.50 Published on Web 08/08/2006
© 2006 American Chemical Society
restricted class of molecules.9 In fact, the only successful route is to keep a para-quaterphenylene basis unit and to functionalize it symmetrically or asymmetrically by side groups. Very recently, it has been shown that such newly synthesized molecules, for example, p-methyloxylated quaterphenylene (MOP4)10 and p-chlorinated p-quaterphenylene (ClP4),11 form well-aligned, blue-light-emitting nanofibers on cleaved muscovite mica substrates. Because of their selfassembly growth process12 the nanofibers consist of molecules oriented nearly parallel to the substrate surface and perpendicular to the long axis of the fibers. Para-hexaphenylene (p6P) nanofibers, for example, have typical dimensions of a few hundred nanometers wide, a few tens of nanometers high, and several hundred micrometers long. They show, among other interesting photonic properties, waveguiding,13,14 large quantum yield, and a highly anisotropic and polarized fluorescence emission.15 Nonlinear optical activity of p6P nanofibers has also been reported in the past.16,17 However, in the nonlinear response based on two-photon luminescence, that is, independent of excitation wavelength, the same luminescence spectrum of the nanofibers is observed. The amount of true secondharmonic generation from the p6P nanofibers is reported to be on the order of a few percent of the two-photon luminescence intensity at infrared femtosecond excitation around 800 nm.16 Note that the investigations reported in ref 16 have been performed with nanofibers on their original growth substrate mica. From our present investigations on transferred nanofibers, we assign the reported SHG mainly to surface second-harmonic generation from a continuous
organic film (“wetting layer”) on the growth substrate and from the nanofiber surfaces. Here we show SHG from nanofibers that have been grown from asymmetrically functionalized para-phenylenes with electron push and pull groups, namely, nanofibers made from 4-amino, 4′′′-methoxy-1,1′:4′,1′′:1′′:4′′,1′′′-quaterphenylene (MOP4NH2). Because of the intrinsic high hyperpolarizabilities of the molecular building blocks with high crystalline order, the resulting nanofibers are also expected to possess large nonlinear susceptibilities. The synthesis of MOP4NH2 and related molecules is complex and will be described in detail elsewhere.18 It starts with a building block consisting of a single phenyl ring substituted by a protective group and a reactive group in para positions. Additional phenyl rings are added stepwise at reactive groups using Suzuki cross-coupling reactions to finally give the p-quaterphenylene core bearing functional groups at the 4,4′′′ positions. Fibers were transferred from the mica growth substrate to glass substrates using a standard procedure.15 The nonlinear optical properties of the fibers were investigated with two different approaches. For measurements of polarized optical spectra, a Spectra Physics Tsunami femtosecond resonator was used to excite the fibers at wavelengths between 750 and 850 nm. The resulting light emission from the fibers was sent through a series of lenses and band-pass filters to a cooled CCD spectrometer (spectral resolution 1 nm). The same setup was also used for cw UV excitation of the sample with a HeCd laser at an emission wavelength of 325 nm. A laser scanning confocal microscope (Zeiss LSM 510 META) coupled with a Spectra Physics Mai-Tai (wideband, 710-990 nm) Ti:sapphire laser for two photon excitation experiments served to obtain three-dimensional SHG images of individual needles. The excitation power of the laser at different wavelengths was measured directly at the sample position. The SHG signal from the sample could be recorded simultaneously in transmission with a photomultiplier on top of the sample and in reflection with a photomultiplier below the sample. The morphology of the fibers was imaged by an atomic force microscope (AFM), mounted on an inverted epi-fluorescence microscope. Because the fiber’s fluorescence is very weak compared to that from fibers made of para-hexa-phenylene, it was not possible to obtain epi-fluorescence images of the MOP4NH2 fibers. Figure 1 shows instead a dark-field image of the needles, which reveals their good mutual alignment. Next, we have transferred MOP4NH2 nanofibers from mica to glass in order to avoid second-harmonic generation from the underlying substrate. Upon irradiation with femtosecond pulses, a strong second-harmonic peak was observed (Figure 2a), shifting as expected with excitation wavelength. The accompanying fluorescence signal, which appeared at excitation wavelengths shorter than 800 nm was about 80 times smaller compared to the signal due to SHG. Upon cw excitation at 325 nm, we have recorded spatially integrated fluorescence spectra with a total fluorescence intensity about 100 times weaker than that obtained for p6p nanofibers (Figure 2b). The SHG intensity was measured as Nano Lett., Vol. 6, No. 12, 2006
Figure 1. Darkfield image (113 × 85 µm2) of MOP4NH2 fibers.
a function of the excitation light’s polarization angle with respect to the fixed transition dipole moment vector of the MOP4NH2 molecules, revealing a polarization ratio P ) (Imax - Imin)/(Imax + Imin) ) 0.96. The large polarization ratio of the fibers indicates good alignment of the MOP4NH2 molecules and thus a high degree of crystallinity of the fibers. This crystallinity is conserved during the transfer process to glass. With the laser scanning microscope (LSM), spatially resolved nonlinear optical experiments (SHG-LSM) have been performed on indiVidual fibers. Figure 2c shows the integrated SHG intensity as a function of excitation wavelength for fixed power and for a single fiber. There is an obvious resonance enhancement of the SHG at the twophoton transition frequency between 780 and 800 nm. To perform subsequent optical (SHG-LSM) and morphological (AFM) investigations on the same nanofibers, we have marked the positions of the optically imaged fibers on the sample by ablating patterns around the fibers. These patterns could then be identified in an inverted optical microscope, which was situated below the AFM and thus it was possible to obtain AFM images of the very same fibers that had been imaged in the LSM. The resulting threedimensional plots for a chain of three nanofibers are shown in Figure 3. It is noted that the SH intensity shows pronounced bright and dark spots along the long axes of the fibers. To understand this behavior, we note that the SH signal from the fibers will originate mainly from three different sources: the nanofiber bulk, IV, the interface between fiber and surface, IS1, and the upper surface of the fibers, IS2. Let us first discuss IV. If the SH emission would be a bulk effect, then it should be proportional to the number of emitters and thus proportional to the integrated height profile along the fiber. A comparison of the integrated height and the integrated intensity profiles (Figure 4) shows, however, that the SH signal intensity does not simply correlate to the amount of deposited MOP4NH2. This is different from twophoton fluorescence, where the measured two-photon luminescence intensity profile for a p6P fiber transferred to glass shows very good agreement with the height profile of the same fiber, measured via atomic force microscopy (Figure 5). A closer look at Figure 3 reveals that the SH intensity increases at inhomogeneous areas along the fiber such as 2657
Figure 2. (a) Solid line: Spatially integrated SHG from MOP4NH2 nanofibers excited with fs pulses at 780 nm (800 W/cm2). Dotted line: same as the black line, but for excitation with 800 nm. (b) Spatially integrated fluorescence spectrum from MOP4NH2 excited at 325 nm with a cw laser (2 mW/cm2). (c) Femtosecond induced SHG from individual MOP4NH2 fibers as a function of excitation wavelength. The excitation intensity was 8.9 × 105 W/cm2. A clear resonance enhancement is observed.
Figure 3. (a) Three-dimensional representation (9.88 × 1.8 µm2) of the measured second-harmonic intensity (resolution 500 nm) from transferred MOP4NH2 fibers. The “height” scale refers to relative SH intensity. (b) Three-dimensional AFM image of the same MOP4NH2 fibers (height scale 150 nm).
breaks or bends. Bends in the fiber orientation must result in a change of the orientation of the transition dipole moments with respect to the fixed polarization of the excitation light. From measurements, the orientation of the excitation electrical field vector is known in the fibers laboratory coordinate system as is the variation of SH intensity as a function of field vector angle (viz., Malus law). Hence, we can estimate the variation of SH intensity emission along the fibers from the measured morphology and taking into account local changes in volume and dipole moment orientation. A comparison with the measured SH intensity profile has shown that this approach fails in representing the SH emission from the fibers correctly.19 This leaves us with IS1 and IS2 as possible sources of the inhomogeneous SH emission. To discriminate between these sources, we take advantage of the fact that we can measure simultaneously reflected and transmitted SH from the same spots of the very same fibers with a local resolution of the order of 500 nm. Obviously, because the fibers consist of nearly transparent semiconducting material, both the trans2658
Figure 4. Integrated height profiles (solid lines) of transferred MOP4NH2 fibers as well as integrated SHG signals (dotted lines). Profiles for the lower to upper fibers in Figure 3 are shown in parts a-c.
mitted and the reflected SH will contain contributions from both interfaces. However, because of the difference in coefficients for transmitted and reflected radiation, the locally resolved total transmitted and reflected SH intensities are expected to differ from each other if there are local variations in the responsible local dielectric function. This is indeed observed by comparison of SH images of fibers recorded in reflection and transmission. The areas of maximum intensity differ significantly from each other.19 Besides the fact that the nanofibers’ morphology is not perfectly homogeneous along their surfaces, we also note from AFM measurements on transferred nanofibers differNano Lett., Vol. 6, No. 12, 2006
demonstrating the first freely transferrable nanofiber frequency doublers from designed organic molecules we have also presented a three-dimensional SH variant (SH-LSM) of fluorescent laser scanning microscopy.
Figure 5. Integrated height profile (solid line) of a p6p fiber and integrated two-photon luminescence intensity (dotted line) from the same fiber.
ences between the nanofiber morphology at the supporting surface and at the interface to vacuum or air. This local variation in morphology adds up differently into transmission and reflection coefficients and thus results in a difference in the local variation of the generated SH intensity in reflection and in transmission. In conclusion, we have synthesized nonlinearly optically active MOP4NH2 molecules and have grown from those nanofibers, which generate true second-harmonic intensity following excitation with femtosecond light pulses. We expect these nanoscaled frequency doublers to be of potential great importance for upconversion of the light from submicrometer-scaled infrared light sources. The second-harmonic signal could be used to image fibers with a spatial resolution of about 500 nm. On individual fibers, a local variation of the SH intensity in transmission and in reflection has been observed simultaneously and compared directly to morphological variations of the very same fibers. In contrast to two-photon luminescence, the second-harmonic intensity does not correlate simply to the amount of light-emitting material but depends sensitively on local surface morphological variations. Hence, although the generation of second-harmonic intensity by itself is due to the hyperpolarizability of the molecular building blocks of the nanofibers and would be close to zero for symmetrically functionalized quaterphenylenes, its local variation along individual nanofibers is dominated by local field enhancement of the responsible effective dielectric function of the nanofibers. The local field enhancement, in turn, is due to local changes in the fiber’s morphology. Further work is in progress to quantitatively measure the nonlinear susceptibility of the fibers. We note that besides
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Acknowledgment. We are grateful to the Danish research agencies SNF (21-03-0469) and STVF (26-04-0253) for supporting this work. J.B. thanks the graduate school of Molecular Biophysics for a stipend. M.S., A.L., and KS thank the German research foundation, DFG, for financial support. References (1) Albota, M.; Beljonne, D.; Bredas, J. L.; Ehrlich, J. E.; Fu, J. Y.; Heikal, A. A.; Hess, S. E.; Kogej, T.; Levin, M. D.; Marder, S. R.; McCord-Maughon, D.; Perry, J. W.; Rockel, H.; Rumi, M.; Subramaniam, G.; Webb, W. W.; Wu, X. L.; Xu, C. Science 1998, 281, 1653. (2) Ventelon, L.; Charier, S.; Moreaux, L.; Mertz, J.; Blanchard-Desce, M. Angew. Chem., Int. Ed. 2001 40-11, 2098-2101. (3) Thalladi, V. R.; Brasselet, S.; Weiss, H.-C.; Blaser, D.; Katz, A. K.; Carrell, H. L.; Boese, R.; Zyss, J.; Nangia, A.; Desiraju, G. R. J. Am. Chem. Soc. 1998, 120, 2563-2577. (4) Ashwell, G. J. J. Mater. Chem. 1999, 9, 1991-2003. (5) Drexler, K. E. Proc. Natl. Acad. Sci. U.S.A. 1981, 78, 5275-5278. (6) Figliozzi, P.; Sun, L.; Jiang, Y.; Matlis, N.; Mattern, B.; Downer, M. C.; Withrow, S. P.; White, C. W.; Mochan, W. L.; Mendoza, B. S. Phys. ReV. Lett. 2005, 94, 047401. (7) Le Floc’h, V.; Brasselet, S.; Roch, J.-F.; Zyss, J. J. Phys. Chem. B 2003, 107, 12403-12410. (8) Brasselet, S.; Le Floc’h, V.; Treussart, F.; Roch, J.-F.; Zyss, J.; Botzung-Appert, E.; Ibanez, A. Phys. ReV. Lett. 2004, 92, 207401. (9) Balzer, F.; Rubahn, H.-G. AdV. Funct. Mater. 2005, 15, 17-24. (10) Schiek, M.; Luetzen, A.; Koch, R.; Al-Shamery, K.; Balzer, F.; Frese, R.; Rubahn, H.-G. App. Phys. Lett. 2005, 86, 153107. (11) Schiek, M.; Luetzen, A.; Koch, R.; Al-Shamery, K.; Balzer, F.; Rubahn, H.-G. Cryst. Growth Des., submitted for publication, 2006. (12) Balzer, F.; Rubahn, H.-G. Appl. Phys. Lett. 2001, 79, 3860-3862. (13) Yanagi, H.; Ohara, T.; Morikawa, T. AdV. Mater. 2001, 13, 14521455. (14) Balzer, F.; Bordo, V. G.; Simonsen, A. C.; Rubahn, H.-G. Appl. Phys. Lett. 2003, 82, 10-12. (15) Brewer, J.; Maibohm, C.; Jozefowski, L.; Bagatolli, L.; Rubahn, H.G.; Nanotechnology 2005, 16, 2396-2401. (16) Balzer, F.; Al-Shamery, K.; Neuendorf, R.; Rubahn, H.-G. Chem. Phys. Lett. 2003, 368, 307-312. (17) Beermann, J.; Bozhevolnyi, S. I.; Bordo, V. G.; Rubahn, H.-G. Opt. Commun. 2004, 237, 423-429. (18) Schiek, M.; Bruhn, T.; Al-Shamery, K.; Koch, R.; Lu¨tzen, A.; Balzer, F.; Brewer, J.; Rubahn, H.-G. Chem.-Eur. J., to be submitted for publication, 2006. (19) Brewer, J.; Rubahn, H.-G. Optical frequency doubling and imaging of functionalized organic nanofibers. Opt. Lett., to be submitted for publication.
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