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Self-assembled coronene nanofibers: optical waveguide effect and magnetic alignment Ken Takazawa,* Jun-ichi Inoue and Kazutaka Mitsuishi To fabricate organic nanofibers that function as active optical waveguides with semiconductor properties, a facile procedure was developed to grow single crystalline nanofibers via p–p stacking of the polycyclic aromatic molecule, coronene, through solution evaporation on a substrate. The fabricated nanofibers with millimeter-scale lengths have well-defined shapes, smooth surfaces, and low-defect structures. The nanofibers are demonstrated to function as efficient active waveguides that propagate their fluorescence (FL) along the fiber axis over their entire length. We further demonstrate that the nanofibers can be highly aligned on the substrate when solution evaporation is conducted in a magnetic field of 12 T. The mechanism of the magnetic alignment can be elucidated by considering the anisotropy of the diamagnetic susceptibility of a single coronene molecule and the crystal structure of a nanofiber. Owing to the high degree of alignment, the nanofibers rarely cross each other, allowing for measurement of the
Received 20th December 2013 Accepted 1st February 2014
waveguiding properties of single isolated nanofibers. The nanofibers propagate their FL of l > 500 nm with a low propagation loss of 0–3 dB per 100 mm, indicating that the nanofibers function as sub-
DOI: 10.1039/c3nr06760b
wavelength scale, low-loss waveguides. Thus, they are promising building blocks for miniaturized
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optoelectronic circuits.
Introduction Single crystalline, one-dimensional nanostructures, e.g., nanobers and nanowires, constructed from organic molecules have attracted considerable attention because of their potential applications in novel nanoscale devices.1–6 In the past decade, fabrication of organic semiconductor nanobers with high carrier mobility has been extensively studied because these nanobers can be used to build organic electronic devices, which have many advantages over their conventional inorganic counterparts, such as a vast choice of building block molecules, high exibility, and low cost. Progress in molecular synthesis and fabrication processes of one-dimensional structures has resulted in great improvements in carrier mobility. In addition to electronic device applications, organic nanobers have recently attracted increased attention for their photonic device applications. It has been demonstrated that nanobers selfassembled from certain classes of dyes and luminescent molecules function as efficient active waveguides that propagate their uorescence (FL) along the ber axis.2,5–13 Their subwavelength dimensions and ability to guide and manipulate light in a micrometer-sized area make these nanobers promising for application in miniaturized photonic circuits. Our aim in this study is to fabricate organic nanobers that can potentially function as both active waveguides with high
National Institute for Materials Science, 3-13 Sakura, Tsukuba, 305-0003, Japan. E-mail: [email protected]
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propagation efficiency and semiconductors with high carrier mobility. Such nanostructures could allow the development of organic optoelectronic devices, which possess the same advantages as other organic devices over their inorganic counterparts. A widely used approach for the fabrication of organic semiconductor nanobers has been the solution-phase self-assembly of polycyclic aromatic molecules (PAMs). When PAMs stack face-to-face via strong p–p interactions and form one-dimensional structures, they exhibit high carrier mobility along the stacking axis.14–26 Therefore, our strategy is to fabricate p-stacked nanobers through solution-phase selfassembly of PAMs and seek those that function as active waveguides. Coronene, which is a simple PAM composed of six aromatic rings, was chosen as the building block molecule for the nanobers (inset, Fig. 1a). Because of the planar disc-like molecular geometry, molecules in a coronene single crystal form a one-dimensional columnar stack via p–p interactions with an exceptionally short plane-to-plane distance of ˚ 27–30 Therefore, a single crystalline nanober of coronene 3.43 A. is expected to exhibit high carrier mobility along the stacking axis (the ber axis). However, in crystallization by either sublimation or precipitation, coronene tends to not only p-stack, but also stack in a manner lateral to the aromatic core. Therefore, needle-shaped bulk crystals are formed rather than nanobers or nanowires. In this study, we have developed a facile process to fabricate single-crystalline ultra-long nanobers of coronene by
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Results and discussion
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Fabrication and characterization of coronene nanobers
(a) Absorption (red curve) and FL (blue curve) spectra of the coronene solution in chloroform. FL spectrum of coronene crystals with l ¼ 405 nm laser excitation (black curve). Inset: chemical formula of coronene. (b) Optical micrograph of needle-shaped coronene crystals. Inset: FL microscopy image of the crystals (excitation: l ¼ 400–440 nm, detection: l > 460 nm).
Fig. 1
utilizing self-assembly at the solution–substrate interface upon solution evaporation. Although nanober/nanowire fabrication from coronene has been reported by several authors,31,32 those fabricated by this method have an aspect ratio (length/width ratio) greater than 1000, which is far larger than those of previously reported structures. Optical microscopy investigations revealed that our nanobers function as efficient active waveguides that propagate their FL over their entire length, which is a millimeter-scale distance. This remarkable waveguiding capability is largely attributed to the structural characteristics of the nanobers such as well-dened shapes, smooth surfaces, and low-defect structures, all of which suppress optical losses by scattering. Moreover, we show that the coronene nanobers can be aligned using a strong magnetic eld. The alignment signicantly reduces nanober crossing, which cannot be avoided in randomly oriented nanobers. This allows for the investigation of the optical properties of single isolated nanobers. We quantitatively investigate the waveguiding properties of single coronene nanobers and demonstrate their superior waveguiding properties.
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Coronene is soluble in only a few organic solvents such as tetrahydrofuran and chloroform. In this study, chloroform was used as the solvent for nanober fabrication via solution evaporation on a substrate. Fig. 1a shows the absorption and FL spectra of a 0.1 mM coronene solution in chloroform. The absorption spectrum (red curve) shows an absorption band with vibronic structures due to the S1–S0 transition in the wavelength range 360–450 nm.33,34 The FL spectrum (blue curve) shows a mirror image of the absorption band with a mirror position at 430 nm, which corresponds to the origin of the S1–S0 transition.33,34 As a preliminary experiment for nanober fabrication, a 3.0 mM coronene solution in chloroform was drop-cast on a glass substrate, and the solution was allowed to evaporate under ambient conditions. Anisotropic needle-shaped crystals similar to the single crystals grown by either sublimation or precipitation were obtained aer solution evaporation (Fig. 1b). The lengths and widths of the crystals were 10–100 mm and 5–10 mm, respectively. These crystals emit bright green FL when excited by light at the energy of the S1–S0 transition (inset, Fig. 1b). The FL spectrum of the crystal was recorded by laser excitation at l ¼ 405 nm (black curve in Fig. 1a). The spectrum shows an FL band with some vibronic structures for l > 450 nm, which agrees with a previously reported FL spectrum of coronene single crystals.27,28 The FL band of the crystals red shis about 50 nm with respect to that of the solution (blue curve). The red shi was attributed to the strong p–p interaction between coronene molecules in the crystal, which leads to the J-aggregate-like spectral properties. Crystal growth via solution evaporation is largely inuenced by conditions such as solution concentration, evaporation speed, and surface properties of the substrate.15,35,36 To fabricate nanobers, we conducted solution evaporation by widely varying these conditions. When approximately 10 ml of a 3.0 mM chloroform solution was drop-cast on a glass substrate and evaporated in a container lled with chloroform vapor, which signicantly reduces the evaporation speed, nanobers with lengths of up to 800 mm were obtained aer solution evaporation (Fig. 2a). Atomic force microscopy (AFM) observations showed that the nanobers have a well-dened rectangular cross-section with widths of 100–1000 nm and heights of 50–200 nm. A scanning electron microscopy (SEM) image of the nanobers revealed that the nanobers have very smooth surfaces (Fig. 2c). These results indicate that a simple reduction in the evaporation speed has a large inuence on the selfassembly kinetics of coronene in solution and leads to a drastic change in the geometry of the resultant structures, i.e., the aspect ratio increases by approximately two orders of magnitude from the needle-shaped bulk crystals (10) to nanobers (1000). Fig. 3a shows the FL microscopy image of the sample area shown in Fig. 2a. The image shows that the tips of each nanober exhibit brighter FL compared to the ber body. This is a characteristic feature of nanobers that function as active
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(a) FL microscopy image of coronene nanofibers (excitation: l ¼ 400–440 nm; detection: l > 460 nm). (b) FL microscopy image of the nanofiber, whose tips are indicated by white arrows in (a), recorded by exciting the position indicated as Ex. with a focused laser beam (l ¼ 405 nm). FL spots at both tips (white arrows) confirm that the nanofiber functions as an active waveguide. Positions of the white circles agree with those in (a), indicating that the propagating FL leaks from the crossing positions of the nanofibers.
Fig. 3
investigate the optical or electronic properties of single isolated nanobers. Magnetic alignment of coronene nanobers (a) Optical micrograph of coronene nanofibers. (b) AFM image of coronene nanofibers (left panel) and cross-section along the white line in the left panel (right panel). (c) SEM image of coronene nanofibers.
Fig. 2
waveguides.2,5–13 To conrm that they function as active waveguides, a FL microscopy image of the nanober, whose tip positions are indicated by arrows in Fig. 3a, was recorded by exciting it with a focused laser beam at l ¼ 405 nm (Fig. 3b). Bright FL was observed at both ber tips (white arrows) as well as the position of the laser excitation (indicated as Ex. by a blue arrow). This clearly shows that the FL generated by laser excitation propagates along the nanober and is output from both tips, thus conrming that the nanober functions as an active waveguide. In addition, the image shows FL spots at several other positions (white circles). The positions of these spots agree with the positions where the nanobers cross other nanobers (white circles in Fig. 3a), implying that a portion of the propagating FL leaks from the crossing positions. These leakages prevent a precise measurement of the waveguiding efficiency (propagation loss) of the nanober. Because nanobers grow in random directions during solution evaporation, they form a network-like structure, where bent nanobers cross each other. For such samples, it is generally not possible to
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The alignment of organic nanobers on a substrate is an important factor governing their application in devices. For example, in fabrication of electronic devices such as nanoberbased eld effect transistors (FETs), the nanobers must be aligned to connect the source and drain electrodes. Besides such device fabrication purposes, highly aligned nanobers, which rarely cross each other, allow for investigation of optical and electronic properties of single isolated nanobers. To this end, we attempted to fabricate highly aligned coronene nanobers on a substrate using a strong magnetic eld. The magnetic alignment has been demonstrated for various supramolecular structures, including biological macromolecules.37–46 Furthermore, fabrication of organic FETs using magnetic alignment has been reported.47 When molecular materials have a highly ordered internal molecular arrangement, they exhibit anisotropy in the diamagnetic susceptibility (c). Because of the anisotropy, the materials in a magnetic eld (B) tend to orient in a specic direction with respect to the applied eld direction (B) so that the extra energy from the eld, which is given by Emag ¼ cB2, is minimized (note c < 0). To obtain highly aligned coronene nanobers, solution evaporation was conducted in a strong magnetic eld of B ¼ 12 T that is applied parallel to the substrate. Fig. 4a and b show an optical micrograph and FL microscopy image, respectively, of the sample obtained aer
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Fig. 5 (a) Diamagnetic susceptibility of a coronene molecule. The axis of the largest diamagnetic susceptibility (Z axis) is perpendicular to the aromatic disc plane. The Z t B orientation gives the lowest magnetic energy. (b) The crystal structure of a coronene nanofiber. Black bars represent side views of coronene molecules. When the long fiber axis is perpendicular to B, the magnetic energy is minimized.
Optical micrograph (a) and FL microscopy image (b) of magnetically aligned nanofibers. The black arrow indicates the direction of the applied magnetic field.
Fig. 4
solution evaporation in the eld. Nearly straight nanobers aligned perpendicular to B were obtained. Since the anisotropy in c is the origin of the magnetic alignment, the direction of the alignment with respect to B is closely related to the internal molecular arrangement (crystal structure) of the nanobers. To elucidate the mechanism of the magnetic alignment of coronene nanobers, we rst consider the anisotropy in c of a single coronene molecule. For a molecule consisting of a p-conjugated ring, c dominantly originates from a diamagnetic ring current, which is induced by an external magnetic eld. Thus, c of a coronene molecule, which consists of six aromatic rings, is strongly anisotropic, and the anisotropy is expressed as |cZ| [ |cX| z |cY|, where cX, cY, and cZ are the diamagnetic susceptibilities along the molecular axes dened in Fig. 5a. Consequently, a coronene molecule in a magnetic eld tends to orient such that the ring plane is parallel to the eld (Z t B) (Fig. 5a). Note that this discussion does not imply that single coronene molecules in solution or on a substrate orient in such a direction in the 12 T eld. The energy difference between the two orthogonal orientations that provide the highest and lowest magnetic energies (ZkB and Z t B, respectively) is given by DE ¼ (cZ cX(Y))B2 ¼ DcB2. |DE| for a single coronene molecule (Dc z 4900 1012 m3 mol1)48 is approximately 3 1024 J, which is three orders of magnitude smaller than the thermal energy at room
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temperature (|DE| kTroom z 4 1021 J, k: Boltzmann constant) even in a strong magnetic eld of 12 T. Thus, the orientation of molecules in a solution thermally uctuates. Much less, the molecules on a substrate cannot be magnetically oriented because the interaction between the molecules and the surface strongly governs their orientation. However, as coronene molecules in the drop-cast solution aggregate because of an increase in their concentration via solution evaporation, |DE| of the aggregate (nanober) linearly increases with the number of aggregated molecules (N), which is expressed as DE ¼ NDcB2. When |DE| of a nanober is larger than kTroom, it aligns in the evaporating solution. Since |DE| of a single coronene molecule is three orders of magnitude small than kTroom, N z 1000 is a critical aggregation number for magnetic alignment. Furthermore, when |DE| becomes large enough to maintain alignment until the solvent is completely evaporated, highly aligned nanobers are obtained on the substrate. Here, we conrm the above-mentioned mechanism of magnetic alignment by considering the crystal structure of coronene nanobers. Bulk single crystals of coronene, which are grown by sublimation or precipitation, have an anisotropic needle-like shape, and the molecules stack via p–p interactions along the long axis of the needle (space group P21/n, a ¼ 16.10 ˚ b ¼ 4.695 0.005 A, ˚ c ¼ 10.15 0.05 A, ˚ and b ¼ 110.8 0.05 A, 0.2 ).28,29 Therefore, molecules in the nanober are naturally assumed to p-stack along their long axis and have a crystal structure similar to the bulk single crystals, apart from some differences in the lattice constants (for a more precise determination of the crystal structure of the nanobers, X-ray or electron diffraction studies, which are not performed in this work, are necessary). Fig. 5b schematically illustrates the molecular arrangement in a coronene nanober based on the assumption that it has the same crystal structure as the bulk single crystals. As shown in Fig. 5b, when the nanober aligns perpendicular to the eld direction, the condition Z t B, which provides the minimum Emag, is fullled for all the coronene molecules in the nanober. This is consistent with the experimental result that the coronene nanobers align perpendicular to the eld direction, supporting the mechanism of the magnetic alignment and assumed crystal structure. The fact
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that the nanobers have a similar crystal structure to that of the bulk single crystals suggests that the nanobers exhibit high carrier mobility along the ber axis because molecules in the crystals stack with a very short plane-to-plane distance.27–30
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Waveguiding properties of single coronene nanobers Because of the high degree of alignment, the crossing between nanobers is considerably reduced in the magnetically aligned samples. Owing to this advantage, we can investigate the waveguiding properties of single isolated nanobers. Fig. 6a shows an FL microscopy image of a nanober with a length of 536 mm and a width of 500 nm recorded by exciting its various
Fig. 6 (a) FL microscopy images of a 536 mm long nanofiber recorded by exciting at various positions, indicated by Ex., with a focused laser beam (l ¼ 405 nm). X represents the distance between Ex. and one of the fiber tips indicated by Det. (b) Upper panel: spatially resolved FL spectrum measured at Ex., corresponding to the spectrum of light that is coupled to the nanofiber. Lower panels: spatially resolved FL spectra measured at Det. for X ¼ 60–484 mm.
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positions with a focused laser spot (l ¼ 405 nm). X represents the distance between the excitation position (indicated as Ex.) and the le tip (indicated as Det.), i.e., the propagation distance. Because this nanober does not cross any other nanobers, no FL spot was observed other than at both tips and the excited positions. Such ultra-long, isolated nanobers cannot be observed in samples prepared outside a magnetic eld. The absence of redundant FL spots also implies that the nanober does not have distinct structural defects. When there is a defect in a nanober waveguide, the propagating FL is scattered by the defect, and the FL spot is observed at the defect position.36 Fig. 6a shows that the FL at Det. gradually weakens with an increase in X, indicating a propagation loss. To investigate the wavelength dependence of this loss, we measured the FL spectra at Det. for various X and those at Ex. by a spatially resolved FL microscopy technique.36,49,50 The top panel in Fig. 6b shows the spectrum measured at Ex., which corresponds to the FL spectrum of the nanober and the spectrum of light coupled to the nanober. The spectrum agrees with that of the bulk single crystals (Fig. 1a), further supporting that the nanobers have a crystal structure similar to the bulk single crystals. The lower panels in Fig. 6b show the FL spectra measured at Det. for X ¼ 60–484 mm. These spectra clearly show that the FL on the short wavelength side (l < 500 nm) decays with X, indicating a propagation loss. However, the decay in the long wavelength region (l > 550 nm) is small, showing that the nanober functions as a low-loss waveguide in this wavelength region. To quantitatively analyze the propagation loss, the FL intensities at Det. for several different wavelengths (l ¼ 450– 650 nm) were plotted as a function of X (symbols in Fig. 7a). These data points were well tted to an exponential decay function expressed by I(X) ¼ I0 exp(aX), where I0 is the normalized intensity of the coupled light and a is a tting parameter (solid lines). If the guided light is assumed to not be reected by the end facets of the ber, the propagation loss for a guided distance of X mm is given by L(X) ¼ 10 log[I(X)/I0] dB/X mm ¼ 10 log[exp(aX)] dB per X mm. The propagation losses for X ¼ 100 mm (L(100)) were evaluated using a and plotted as a function of the wavelength (Fig. 7b). The propagation loss for several nanobers was investigated, and they showed a similar wavelength dependence to that shown in Fig. 7b. Namely, for the short wavelength region of l < 500 nm, the nanobers exhibited a relatively large propagation loss of L(100) # 10 dB, but for the long wavelength region of l > 500 nm, L(100) was only 0–3 dB. These results indicate that for the operation wavelength range of l > 500 nm the coronene nanobers can function as sub-wavelength scale, low-loss waveguides. The low-loss waveguiding behaviour of the coronene nanobers for l > 500 nm is largely attributed to their structural characteristics. Namely, they have a well-dened shape, smooth surface, and low-defect structure, all of which suppress the loss of propagating FL through scattering. In addition to the structural characteristics, spectral properties strongly inuence the propagation loss. The relatively large propagation loss for l < 500 nm is related to spectral properties of the nanobers. For a precise understanding of the relationship between spectral properties and propagation loss, it is necessary to
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component of the lower branch polaritons increases as the energy approaches the exciton absorption band. Because the exciton component interacts with the phonons, the polaritons in the short wavelength region, which have a large exciton component, rapidly decay, resulting in a large propagation loss. In contrast, the polaritons in the long wavelength region, which are dominated by a photon component, weakly interact with the phonons and therefore can propagate with low loss. We are currently investigating the photon–exciton interaction in coronene nanobers to reveal the physics behind their waveguiding behavior and to gain knowledge in the choice of molecules for fabricating active waveguides with a wide low-loss spectral range.
Conclusions
Fig. 7 (a) FL intensities at several wavelengths of l ¼ 450–650 nm extracted from the spectra shown in Fig. 6b and plotted as a function of X (symbols). Lines are best fitted curves. (b) Plot of the optical losses for a guided distance of 100 mm (dB per 100 mm) as a function of the wavelength.
consider the interaction between the propagating FL and molecular excitons, i.e., the photon–exciton interaction. Two mechanisms for the loss in the short wavelength region are distinguished by the strength of the photon–exciton interaction. When the interaction is weak, the nanobers are simply considered as optical waveguides, where the FL is conned by the difference in the refractive index between the nanober and surrounding media. According to this picture, the FL in the long wavelength region, where it is energetically away from the exciton absorption band, propagates for a long distance without being reabsorbed by molecules. However, in the short wavelength region, where it is close to the absorption band, the FL exhibits large loss due to reabsorption. On the other hand, when the photon–exciton interaction is strong, the FL propagation should be viewed as propagation of exciton polaritons (the lower branch exciton polaritons), which are mixed states of photons and excitons.51,52 In such a case, the loss must be understood as the decay of the exciton polaritons through processes such as scattering with acoustic phonons. An exciton
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A facile procedure to grow single-crystalline nanobers of coronene through solution evaporation on a glass substrate was developed with the aim of fabricating organic nanobers that function as both active waveguides and semiconductors. The fabricated nanobers with millimeter-scale length have welldened shapes, smooth surfaces, and low-defect structures. The nanobers functioned as efficient active waveguides that propagated their FL along the ber axis over their entire length. To align these nanobers on the substrate, solution evaporation was conducted in a strong magnetic eld of 12 T. Nanobers that highly aligned perpendicular to the applied eld direction were obtained. The mechanism of the magnetic alignment was elucidated by considering the anisotropy in the diamagnetic susceptibility of a single coronene molecule and the crystal structure of a nanober. Because of the high degree of alignment, the nanobers rarely crossed each other, which allowed for measurement of the waveguiding properties of single isolated nanobers. The nanobers were observed to propagate their FL of l > 500 nm with a low propagation loss of 0–3 dB per 100 mm, indicating that the nanobers can function as subwavelength scale, low-loss waveguides in that wavelength range. To explore the possibility of their application to optoelectronic devices, the next steps are fabrication of FETs consisting of magnetically aligned nanobers and investigation of their semiconductor properties.
Experimental Absorption and FL spectra of coronene solution The absorption and FL spectra of the coronene solution in chloroform were recorded using a spectrometer (Ocean Optics, USB2000). For absorption spectroscopy, a combination of a halogen and deuterium lamp coupled to an optical ber was used for excitation. In FL spectroscopy, an output of a continuous wave diode laser (Coherent, Radius405, l ¼ 405 nm) was used for excitation. Nanober fabrication Coronene and chloroform were purchased from Tokyo Kasei and Wako Pure Chemical, respectively, and were used without
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further purication. The sample solution (3.0 mM) was prepared by dissolving coronene in chloroform by sonication for 12 h. A microscope cover glass (18 mm 18 mm) was blown with compressed dry air to remove dust. Approximately 10 ml of the sample solution was drop-cast onto the substrate using a micropipette and immediately placed in a glass container (a Petri dish with a lid), which was lled with chloroform vapor by depositing a few drops of chloroform in the bottom. Aer the solvent was evaporated in a few hours, the sample was extracted from the container and dried under ambient conditions. Magnetic alignment of nanobers A superconducting magnet that generates a maximum magnetic eld of 12 T in a 10 cm diameter room temperature bore was used. The glass container containing the substrate, which was prepared in the same way described above, was placed at the eld center of the magnet bore. AFM and SEM The topography of the sample was obtained using AFM (Veeco, Caliber) in the tapping mode. The electron microscopy image was recorded using a SEM (JEOL, JSM-7000F). The sample was carbon coated to avoid charging. FL microscopy imaging and spatially resolved FL microscopy The output of the diode laser at l ¼ 405 nm was coupled to an epi-illumination FL microscope (Olympus, BX-51). The laser beam was focused onto the sample with a 5 objective lens (NA ¼ 0.13). FL from the sample was collected using the same objective lens and recorded using a color charge-coupled device (CCD) camera (Jenoptic, ProgRes C10) through a long-pass lter that blocks the excitation laser. In spatially resolved FL microscopy, FL was imaged onto the entrance slits of an imaging monochromator (Acton Research, SpectraPro 2150). FL that passed through the slits was recorded using a liquidnitrogen-cooled, back-illuminated CCD camera (Princeton Instruments, Spec10, 1340 400 pixels). The captured image was spectrally and spatially resolved along the horizontal and vertical axes of the CCD camera, respectively. The FL spectrum was obtained by extracting a horizontal cross-section of the image.
Acknowledgements We thank Dr T. Takamasu and K. Sakoda for helpful discussions. This work was supported by the Grant-in-Aid for Scientic Research (no. 24540332), Japan Society for the Promotion of Science.
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Self-assembled coronene nanofibers: optical waveguide effect and magnetic alignment Ken Takazawa,* Jun-ichi Inoue and Kazutaka Mitsuishi To fabricate organic nanofibers that function as active optical waveguides with semiconductor properties, a facile procedure was developed to grow single crystalline nanofibers via p–p stacking of the polycyclic aromatic molecule, coronene, through solution evaporation on a substrate. The fabricated nanofibers with millimeter-scale lengths have well-defined shapes, smooth surfaces, and low-defect structures. The nanofibers are demonstrated to function as efficient active waveguides that propagate their fluorescence (FL) along the fiber axis over their entire length. We further demonstrate that the nanofibers can be highly aligned on the substrate when solution evaporation is conducted in a magnetic field of 12 T. The mechanism of the magnetic alignment can be elucidated by considering the anisotropy of the diamagnetic susceptibility of a single coronene molecule and the crystal structure of a nanofiber. Owing to the high degree of alignment, the nanofibers rarely cross each other, allowing for measurement of the
Received 20th December 2013 Accepted 1st February 2014
waveguiding properties of single isolated nanofibers. The nanofibers propagate their FL of l > 500 nm with a low propagation loss of 0–3 dB per 100 mm, indicating that the nanofibers function as sub-
DOI: 10.1039/c3nr06760b
wavelength scale, low-loss waveguides. Thus, they are promising building blocks for miniaturized
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optoelectronic circuits.
Introduction Single crystalline, one-dimensional nanostructures, e.g., nanobers and nanowires, constructed from organic molecules have attracted considerable attention because of their potential applications in novel nanoscale devices.1–6 In the past decade, fabrication of organic semiconductor nanobers with high carrier mobility has been extensively studied because these nanobers can be used to build organic electronic devices, which have many advantages over their conventional inorganic counterparts, such as a vast choice of building block molecules, high exibility, and low cost. Progress in molecular synthesis and fabrication processes of one-dimensional structures has resulted in great improvements in carrier mobility. In addition to electronic device applications, organic nanobers have recently attracted increased attention for their photonic device applications. It has been demonstrated that nanobers selfassembled from certain classes of dyes and luminescent molecules function as efficient active waveguides that propagate their uorescence (FL) along the ber axis.2,5–13 Their subwavelength dimensions and ability to guide and manipulate light in a micrometer-sized area make these nanobers promising for application in miniaturized photonic circuits. Our aim in this study is to fabricate organic nanobers that can potentially function as both active waveguides with high
National Institute for Materials Science, 3-13 Sakura, Tsukuba, 305-0003, Japan. E-mail: [email protected]
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propagation efficiency and semiconductors with high carrier mobility. Such nanostructures could allow the development of organic optoelectronic devices, which possess the same advantages as other organic devices over their inorganic counterparts. A widely used approach for the fabrication of organic semiconductor nanobers has been the solution-phase self-assembly of polycyclic aromatic molecules (PAMs). When PAMs stack face-to-face via strong p–p interactions and form one-dimensional structures, they exhibit high carrier mobility along the stacking axis.14–26 Therefore, our strategy is to fabricate p-stacked nanobers through solution-phase selfassembly of PAMs and seek those that function as active waveguides. Coronene, which is a simple PAM composed of six aromatic rings, was chosen as the building block molecule for the nanobers (inset, Fig. 1a). Because of the planar disc-like molecular geometry, molecules in a coronene single crystal form a one-dimensional columnar stack via p–p interactions with an exceptionally short plane-to-plane distance of ˚ 27–30 Therefore, a single crystalline nanober of coronene 3.43 A. is expected to exhibit high carrier mobility along the stacking axis (the ber axis). However, in crystallization by either sublimation or precipitation, coronene tends to not only p-stack, but also stack in a manner lateral to the aromatic core. Therefore, needle-shaped bulk crystals are formed rather than nanobers or nanowires. In this study, we have developed a facile process to fabricate single-crystalline ultra-long nanobers of coronene by
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Results and discussion
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Fabrication and characterization of coronene nanobers
(a) Absorption (red curve) and FL (blue curve) spectra of the coronene solution in chloroform. FL spectrum of coronene crystals with l ¼ 405 nm laser excitation (black curve). Inset: chemical formula of coronene. (b) Optical micrograph of needle-shaped coronene crystals. Inset: FL microscopy image of the crystals (excitation: l ¼ 400–440 nm, detection: l > 460 nm).
Fig. 1
utilizing self-assembly at the solution–substrate interface upon solution evaporation. Although nanober/nanowire fabrication from coronene has been reported by several authors,31,32 those fabricated by this method have an aspect ratio (length/width ratio) greater than 1000, which is far larger than those of previously reported structures. Optical microscopy investigations revealed that our nanobers function as efficient active waveguides that propagate their FL over their entire length, which is a millimeter-scale distance. This remarkable waveguiding capability is largely attributed to the structural characteristics of the nanobers such as well-dened shapes, smooth surfaces, and low-defect structures, all of which suppress optical losses by scattering. Moreover, we show that the coronene nanobers can be aligned using a strong magnetic eld. The alignment signicantly reduces nanober crossing, which cannot be avoided in randomly oriented nanobers. This allows for the investigation of the optical properties of single isolated nanobers. We quantitatively investigate the waveguiding properties of single coronene nanobers and demonstrate their superior waveguiding properties.
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Coronene is soluble in only a few organic solvents such as tetrahydrofuran and chloroform. In this study, chloroform was used as the solvent for nanober fabrication via solution evaporation on a substrate. Fig. 1a shows the absorption and FL spectra of a 0.1 mM coronene solution in chloroform. The absorption spectrum (red curve) shows an absorption band with vibronic structures due to the S1–S0 transition in the wavelength range 360–450 nm.33,34 The FL spectrum (blue curve) shows a mirror image of the absorption band with a mirror position at 430 nm, which corresponds to the origin of the S1–S0 transition.33,34 As a preliminary experiment for nanober fabrication, a 3.0 mM coronene solution in chloroform was drop-cast on a glass substrate, and the solution was allowed to evaporate under ambient conditions. Anisotropic needle-shaped crystals similar to the single crystals grown by either sublimation or precipitation were obtained aer solution evaporation (Fig. 1b). The lengths and widths of the crystals were 10–100 mm and 5–10 mm, respectively. These crystals emit bright green FL when excited by light at the energy of the S1–S0 transition (inset, Fig. 1b). The FL spectrum of the crystal was recorded by laser excitation at l ¼ 405 nm (black curve in Fig. 1a). The spectrum shows an FL band with some vibronic structures for l > 450 nm, which agrees with a previously reported FL spectrum of coronene single crystals.27,28 The FL band of the crystals red shis about 50 nm with respect to that of the solution (blue curve). The red shi was attributed to the strong p–p interaction between coronene molecules in the crystal, which leads to the J-aggregate-like spectral properties. Crystal growth via solution evaporation is largely inuenced by conditions such as solution concentration, evaporation speed, and surface properties of the substrate.15,35,36 To fabricate nanobers, we conducted solution evaporation by widely varying these conditions. When approximately 10 ml of a 3.0 mM chloroform solution was drop-cast on a glass substrate and evaporated in a container lled with chloroform vapor, which signicantly reduces the evaporation speed, nanobers with lengths of up to 800 mm were obtained aer solution evaporation (Fig. 2a). Atomic force microscopy (AFM) observations showed that the nanobers have a well-dened rectangular cross-section with widths of 100–1000 nm and heights of 50–200 nm. A scanning electron microscopy (SEM) image of the nanobers revealed that the nanobers have very smooth surfaces (Fig. 2c). These results indicate that a simple reduction in the evaporation speed has a large inuence on the selfassembly kinetics of coronene in solution and leads to a drastic change in the geometry of the resultant structures, i.e., the aspect ratio increases by approximately two orders of magnitude from the needle-shaped bulk crystals (10) to nanobers (1000). Fig. 3a shows the FL microscopy image of the sample area shown in Fig. 2a. The image shows that the tips of each nanober exhibit brighter FL compared to the ber body. This is a characteristic feature of nanobers that function as active
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(a) FL microscopy image of coronene nanofibers (excitation: l ¼ 400–440 nm; detection: l > 460 nm). (b) FL microscopy image of the nanofiber, whose tips are indicated by white arrows in (a), recorded by exciting the position indicated as Ex. with a focused laser beam (l ¼ 405 nm). FL spots at both tips (white arrows) confirm that the nanofiber functions as an active waveguide. Positions of the white circles agree with those in (a), indicating that the propagating FL leaks from the crossing positions of the nanofibers.
Fig. 3
investigate the optical or electronic properties of single isolated nanobers. Magnetic alignment of coronene nanobers (a) Optical micrograph of coronene nanofibers. (b) AFM image of coronene nanofibers (left panel) and cross-section along the white line in the left panel (right panel). (c) SEM image of coronene nanofibers.
Fig. 2
waveguides.2,5–13 To conrm that they function as active waveguides, a FL microscopy image of the nanober, whose tip positions are indicated by arrows in Fig. 3a, was recorded by exciting it with a focused laser beam at l ¼ 405 nm (Fig. 3b). Bright FL was observed at both ber tips (white arrows) as well as the position of the laser excitation (indicated as Ex. by a blue arrow). This clearly shows that the FL generated by laser excitation propagates along the nanober and is output from both tips, thus conrming that the nanober functions as an active waveguide. In addition, the image shows FL spots at several other positions (white circles). The positions of these spots agree with the positions where the nanobers cross other nanobers (white circles in Fig. 3a), implying that a portion of the propagating FL leaks from the crossing positions. These leakages prevent a precise measurement of the waveguiding efficiency (propagation loss) of the nanober. Because nanobers grow in random directions during solution evaporation, they form a network-like structure, where bent nanobers cross each other. For such samples, it is generally not possible to
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The alignment of organic nanobers on a substrate is an important factor governing their application in devices. For example, in fabrication of electronic devices such as nanoberbased eld effect transistors (FETs), the nanobers must be aligned to connect the source and drain electrodes. Besides such device fabrication purposes, highly aligned nanobers, which rarely cross each other, allow for investigation of optical and electronic properties of single isolated nanobers. To this end, we attempted to fabricate highly aligned coronene nanobers on a substrate using a strong magnetic eld. The magnetic alignment has been demonstrated for various supramolecular structures, including biological macromolecules.37–46 Furthermore, fabrication of organic FETs using magnetic alignment has been reported.47 When molecular materials have a highly ordered internal molecular arrangement, they exhibit anisotropy in the diamagnetic susceptibility (c). Because of the anisotropy, the materials in a magnetic eld (B) tend to orient in a specic direction with respect to the applied eld direction (B) so that the extra energy from the eld, which is given by Emag ¼ cB2, is minimized (note c < 0). To obtain highly aligned coronene nanobers, solution evaporation was conducted in a strong magnetic eld of B ¼ 12 T that is applied parallel to the substrate. Fig. 4a and b show an optical micrograph and FL microscopy image, respectively, of the sample obtained aer
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Fig. 5 (a) Diamagnetic susceptibility of a coronene molecule. The axis of the largest diamagnetic susceptibility (Z axis) is perpendicular to the aromatic disc plane. The Z t B orientation gives the lowest magnetic energy. (b) The crystal structure of a coronene nanofiber. Black bars represent side views of coronene molecules. When the long fiber axis is perpendicular to B, the magnetic energy is minimized.
Optical micrograph (a) and FL microscopy image (b) of magnetically aligned nanofibers. The black arrow indicates the direction of the applied magnetic field.
Fig. 4
solution evaporation in the eld. Nearly straight nanobers aligned perpendicular to B were obtained. Since the anisotropy in c is the origin of the magnetic alignment, the direction of the alignment with respect to B is closely related to the internal molecular arrangement (crystal structure) of the nanobers. To elucidate the mechanism of the magnetic alignment of coronene nanobers, we rst consider the anisotropy in c of a single coronene molecule. For a molecule consisting of a p-conjugated ring, c dominantly originates from a diamagnetic ring current, which is induced by an external magnetic eld. Thus, c of a coronene molecule, which consists of six aromatic rings, is strongly anisotropic, and the anisotropy is expressed as |cZ| [ |cX| z |cY|, where cX, cY, and cZ are the diamagnetic susceptibilities along the molecular axes dened in Fig. 5a. Consequently, a coronene molecule in a magnetic eld tends to orient such that the ring plane is parallel to the eld (Z t B) (Fig. 5a). Note that this discussion does not imply that single coronene molecules in solution or on a substrate orient in such a direction in the 12 T eld. The energy difference between the two orthogonal orientations that provide the highest and lowest magnetic energies (ZkB and Z t B, respectively) is given by DE ¼ (cZ cX(Y))B2 ¼ DcB2. |DE| for a single coronene molecule (Dc z 4900 1012 m3 mol1)48 is approximately 3 1024 J, which is three orders of magnitude smaller than the thermal energy at room
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temperature (|DE| kTroom z 4 1021 J, k: Boltzmann constant) even in a strong magnetic eld of 12 T. Thus, the orientation of molecules in a solution thermally uctuates. Much less, the molecules on a substrate cannot be magnetically oriented because the interaction between the molecules and the surface strongly governs their orientation. However, as coronene molecules in the drop-cast solution aggregate because of an increase in their concentration via solution evaporation, |DE| of the aggregate (nanober) linearly increases with the number of aggregated molecules (N), which is expressed as DE ¼ NDcB2. When |DE| of a nanober is larger than kTroom, it aligns in the evaporating solution. Since |DE| of a single coronene molecule is three orders of magnitude small than kTroom, N z 1000 is a critical aggregation number for magnetic alignment. Furthermore, when |DE| becomes large enough to maintain alignment until the solvent is completely evaporated, highly aligned nanobers are obtained on the substrate. Here, we conrm the above-mentioned mechanism of magnetic alignment by considering the crystal structure of coronene nanobers. Bulk single crystals of coronene, which are grown by sublimation or precipitation, have an anisotropic needle-like shape, and the molecules stack via p–p interactions along the long axis of the needle (space group P21/n, a ¼ 16.10 ˚ b ¼ 4.695 0.005 A, ˚ c ¼ 10.15 0.05 A, ˚ and b ¼ 110.8 0.05 A, 0.2 ).28,29 Therefore, molecules in the nanober are naturally assumed to p-stack along their long axis and have a crystal structure similar to the bulk single crystals, apart from some differences in the lattice constants (for a more precise determination of the crystal structure of the nanobers, X-ray or electron diffraction studies, which are not performed in this work, are necessary). Fig. 5b schematically illustrates the molecular arrangement in a coronene nanober based on the assumption that it has the same crystal structure as the bulk single crystals. As shown in Fig. 5b, when the nanober aligns perpendicular to the eld direction, the condition Z t B, which provides the minimum Emag, is fullled for all the coronene molecules in the nanober. This is consistent with the experimental result that the coronene nanobers align perpendicular to the eld direction, supporting the mechanism of the magnetic alignment and assumed crystal structure. The fact
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that the nanobers have a similar crystal structure to that of the bulk single crystals suggests that the nanobers exhibit high carrier mobility along the ber axis because molecules in the crystals stack with a very short plane-to-plane distance.27–30
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Waveguiding properties of single coronene nanobers Because of the high degree of alignment, the crossing between nanobers is considerably reduced in the magnetically aligned samples. Owing to this advantage, we can investigate the waveguiding properties of single isolated nanobers. Fig. 6a shows an FL microscopy image of a nanober with a length of 536 mm and a width of 500 nm recorded by exciting its various
Fig. 6 (a) FL microscopy images of a 536 mm long nanofiber recorded by exciting at various positions, indicated by Ex., with a focused laser beam (l ¼ 405 nm). X represents the distance between Ex. and one of the fiber tips indicated by Det. (b) Upper panel: spatially resolved FL spectrum measured at Ex., corresponding to the spectrum of light that is coupled to the nanofiber. Lower panels: spatially resolved FL spectra measured at Det. for X ¼ 60–484 mm.
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positions with a focused laser spot (l ¼ 405 nm). X represents the distance between the excitation position (indicated as Ex.) and the le tip (indicated as Det.), i.e., the propagation distance. Because this nanober does not cross any other nanobers, no FL spot was observed other than at both tips and the excited positions. Such ultra-long, isolated nanobers cannot be observed in samples prepared outside a magnetic eld. The absence of redundant FL spots also implies that the nanober does not have distinct structural defects. When there is a defect in a nanober waveguide, the propagating FL is scattered by the defect, and the FL spot is observed at the defect position.36 Fig. 6a shows that the FL at Det. gradually weakens with an increase in X, indicating a propagation loss. To investigate the wavelength dependence of this loss, we measured the FL spectra at Det. for various X and those at Ex. by a spatially resolved FL microscopy technique.36,49,50 The top panel in Fig. 6b shows the spectrum measured at Ex., which corresponds to the FL spectrum of the nanober and the spectrum of light coupled to the nanober. The spectrum agrees with that of the bulk single crystals (Fig. 1a), further supporting that the nanobers have a crystal structure similar to the bulk single crystals. The lower panels in Fig. 6b show the FL spectra measured at Det. for X ¼ 60–484 mm. These spectra clearly show that the FL on the short wavelength side (l < 500 nm) decays with X, indicating a propagation loss. However, the decay in the long wavelength region (l > 550 nm) is small, showing that the nanober functions as a low-loss waveguide in this wavelength region. To quantitatively analyze the propagation loss, the FL intensities at Det. for several different wavelengths (l ¼ 450– 650 nm) were plotted as a function of X (symbols in Fig. 7a). These data points were well tted to an exponential decay function expressed by I(X) ¼ I0 exp(aX), where I0 is the normalized intensity of the coupled light and a is a tting parameter (solid lines). If the guided light is assumed to not be reected by the end facets of the ber, the propagation loss for a guided distance of X mm is given by L(X) ¼ 10 log[I(X)/I0] dB/X mm ¼ 10 log[exp(aX)] dB per X mm. The propagation losses for X ¼ 100 mm (L(100)) were evaluated using a and plotted as a function of the wavelength (Fig. 7b). The propagation loss for several nanobers was investigated, and they showed a similar wavelength dependence to that shown in Fig. 7b. Namely, for the short wavelength region of l < 500 nm, the nanobers exhibited a relatively large propagation loss of L(100) # 10 dB, but for the long wavelength region of l > 500 nm, L(100) was only 0–3 dB. These results indicate that for the operation wavelength range of l > 500 nm the coronene nanobers can function as sub-wavelength scale, low-loss waveguides. The low-loss waveguiding behaviour of the coronene nanobers for l > 500 nm is largely attributed to their structural characteristics. Namely, they have a well-dened shape, smooth surface, and low-defect structure, all of which suppress the loss of propagating FL through scattering. In addition to the structural characteristics, spectral properties strongly inuence the propagation loss. The relatively large propagation loss for l < 500 nm is related to spectral properties of the nanobers. For a precise understanding of the relationship between spectral properties and propagation loss, it is necessary to
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component of the lower branch polaritons increases as the energy approaches the exciton absorption band. Because the exciton component interacts with the phonons, the polaritons in the short wavelength region, which have a large exciton component, rapidly decay, resulting in a large propagation loss. In contrast, the polaritons in the long wavelength region, which are dominated by a photon component, weakly interact with the phonons and therefore can propagate with low loss. We are currently investigating the photon–exciton interaction in coronene nanobers to reveal the physics behind their waveguiding behavior and to gain knowledge in the choice of molecules for fabricating active waveguides with a wide low-loss spectral range.
Conclusions
Fig. 7 (a) FL intensities at several wavelengths of l ¼ 450–650 nm extracted from the spectra shown in Fig. 6b and plotted as a function of X (symbols). Lines are best fitted curves. (b) Plot of the optical losses for a guided distance of 100 mm (dB per 100 mm) as a function of the wavelength.
consider the interaction between the propagating FL and molecular excitons, i.e., the photon–exciton interaction. Two mechanisms for the loss in the short wavelength region are distinguished by the strength of the photon–exciton interaction. When the interaction is weak, the nanobers are simply considered as optical waveguides, where the FL is conned by the difference in the refractive index between the nanober and surrounding media. According to this picture, the FL in the long wavelength region, where it is energetically away from the exciton absorption band, propagates for a long distance without being reabsorbed by molecules. However, in the short wavelength region, where it is close to the absorption band, the FL exhibits large loss due to reabsorption. On the other hand, when the photon–exciton interaction is strong, the FL propagation should be viewed as propagation of exciton polaritons (the lower branch exciton polaritons), which are mixed states of photons and excitons.51,52 In such a case, the loss must be understood as the decay of the exciton polaritons through processes such as scattering with acoustic phonons. An exciton
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A facile procedure to grow single-crystalline nanobers of coronene through solution evaporation on a glass substrate was developed with the aim of fabricating organic nanobers that function as both active waveguides and semiconductors. The fabricated nanobers with millimeter-scale length have welldened shapes, smooth surfaces, and low-defect structures. The nanobers functioned as efficient active waveguides that propagated their FL along the ber axis over their entire length. To align these nanobers on the substrate, solution evaporation was conducted in a strong magnetic eld of 12 T. Nanobers that highly aligned perpendicular to the applied eld direction were obtained. The mechanism of the magnetic alignment was elucidated by considering the anisotropy in the diamagnetic susceptibility of a single coronene molecule and the crystal structure of a nanober. Because of the high degree of alignment, the nanobers rarely crossed each other, which allowed for measurement of the waveguiding properties of single isolated nanobers. The nanobers were observed to propagate their FL of l > 500 nm with a low propagation loss of 0–3 dB per 100 mm, indicating that the nanobers can function as subwavelength scale, low-loss waveguides in that wavelength range. To explore the possibility of their application to optoelectronic devices, the next steps are fabrication of FETs consisting of magnetically aligned nanobers and investigation of their semiconductor properties.
Experimental Absorption and FL spectra of coronene solution The absorption and FL spectra of the coronene solution in chloroform were recorded using a spectrometer (Ocean Optics, USB2000). For absorption spectroscopy, a combination of a halogen and deuterium lamp coupled to an optical ber was used for excitation. In FL spectroscopy, an output of a continuous wave diode laser (Coherent, Radius405, l ¼ 405 nm) was used for excitation. Nanober fabrication Coronene and chloroform were purchased from Tokyo Kasei and Wako Pure Chemical, respectively, and were used without
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further purication. The sample solution (3.0 mM) was prepared by dissolving coronene in chloroform by sonication for 12 h. A microscope cover glass (18 mm 18 mm) was blown with compressed dry air to remove dust. Approximately 10 ml of the sample solution was drop-cast onto the substrate using a micropipette and immediately placed in a glass container (a Petri dish with a lid), which was lled with chloroform vapor by depositing a few drops of chloroform in the bottom. Aer the solvent was evaporated in a few hours, the sample was extracted from the container and dried under ambient conditions. Magnetic alignment of nanobers A superconducting magnet that generates a maximum magnetic eld of 12 T in a 10 cm diameter room temperature bore was used. The glass container containing the substrate, which was prepared in the same way described above, was placed at the eld center of the magnet bore. AFM and SEM The topography of the sample was obtained using AFM (Veeco, Caliber) in the tapping mode. The electron microscopy image was recorded using a SEM (JEOL, JSM-7000F). The sample was carbon coated to avoid charging. FL microscopy imaging and spatially resolved FL microscopy The output of the diode laser at l ¼ 405 nm was coupled to an epi-illumination FL microscope (Olympus, BX-51). The laser beam was focused onto the sample with a 5 objective lens (NA ¼ 0.13). FL from the sample was collected using the same objective lens and recorded using a color charge-coupled device (CCD) camera (Jenoptic, ProgRes C10) through a long-pass lter that blocks the excitation laser. In spatially resolved FL microscopy, FL was imaged onto the entrance slits of an imaging monochromator (Acton Research, SpectraPro 2150). FL that passed through the slits was recorded using a liquidnitrogen-cooled, back-illuminated CCD camera (Princeton Instruments, Spec10, 1340 400 pixels). The captured image was spectrally and spatially resolved along the horizontal and vertical axes of the CCD camera, respectively. The FL spectrum was obtained by extracting a horizontal cross-section of the image.
Acknowledgements We thank Dr T. Takamasu and K. Sakoda for helpful discussions. This work was supported by the Grant-in-Aid for Scientic Research (no. 24540332), Japan Society for the Promotion of Science.
Notes and references 1 L. Zang, Y. Che and J. S. Moore, Acc. Chem. Res., 2008, 41, 1596–1608. 2 Y. S. Zhao, H. B. Fu, A. D. Peng, Y. Ma, D. B. Xiao and J. N. Yao, Adv. Mater., 2008, 20, 2859–2876.
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