Article pubs.acs.org/Langmuir
Electron-Beam Nanopatterning and Spectral Modulation of Organic Molecular Light-Emitting Single Crystals Luana Persano,*,† Andrea Camposeo,† Dario Pisignano,†,‡ Andrea Burini,§ Peter Spearman,§,∥ and Silvia Tavazzi§ †
National Nanotechnology Laboratory (NNL), Istituto Nanoscienze-CNR, via Arnesano, I-73100 Lecce, Italy Dipartimento di Matematica e Fisica “Ennio De Giorgi”, Università del Salento, via Arnesano, I-73100 Lecce, Italy § Dipartimento di Scienza dei Materiali, Università degli Studi di Milano Bicocca, via Cozzi 53, I-20125 Milano, Italy ∥ Centre for Materials Research, SEC Faculty, Kingston University, Penrhyn Road Kingston upon Thames, Surrey KT1 2EE, United Kingdom ‡
S Supporting Information *
ABSTRACT: The nanopatterning of light-emitting molecular crystals with semiconducting properties can be crucial for the development of future optoelectronic and nanoelectronic devices based on organic materials. In this respect, electron-beam writing is a powerful tool to realize patterns at the nanoscale, but it is still rarely applied to active organic materials. Here, sub-100-nm-scale nanopatterning is performed on the surface of quaterthiophene monocrystals by direct maskless electron-beam writing. Gratings are produced on organic crystals with periods ranging from 80 nm to 1 μm and single-line lateral dimensions ranging from 20 to 500 nm, with electron-beam exposure doses between 100 and 1500 μC/cm2. The morphological and texturing properties of the pattern are discussed, together with the interaction mechanisms between the electron beam and the crystal. The resulting modulation of the light emission is consistent with Bragg scattering from the patterned periodic features.
1. INTRODUCTION Organic semiconductors are widely used in many applications, especially in the fields of field-effect and light-emitting devices.1−3 The development of these materials has several advantages, such as relatively easy fabrication, compatibility with low-temperature processing, relatively simple thin-film device fabrication, and tunability of the emission wavelength based on the richness of synthetic organic chemistry. Among other organic materials, monocrystals can be useful for many reasons, offering advantages especially where the control of a few intrinsic physical mechanisms is fundamental, such as in laser devices and field-effect transistors.4−11 Compared to amorphous films commonly used in polymer optoelectronics, crystalline materials typically exhibit electronic states with welldefined polarizations and peculiar propagation properties of the emitted light because of their high anisotropy.12 Use of crystals might be a strategy of breakthrough impact with respect to amorphous or polycrystalline films also because of the lower amount of defects that act as carrier traps, enhancing the charge-transport mobilities.2,3 Moreover, gain narrowing has been demonstrated in a number of organic single-crystalline materials by investigating the amplified spontaneous emission (ASE), which is characterized by spectral narrowing.4−7,13−15 Strong exciton−photon coupling, leading to novel laser devices such as the polariton laser, has also been demonstrated in © 2014 American Chemical Society
microcavities embedding single crystals and thin crystalline films of anthracene.16 All of these applications require the tailoring and control of optical and electronic properties and/or the inclusion of the active crystalline material in a functional structure. Surface nanopatterning is a possible step toward the realization of various architectures in which the emission is spectrally or spatially controlled. Although nanostructuring of semiconductor surfaces is a key point in the development of future optoelectronic and nanoelectronic devices and technologies, only a few methods for gently patterning organic crystals have been reported, and they mainly work at the micrometer scale. For instance, patterned self-assembled monolayers have been used to grow isolated crystals in the form of arrays with interdistances from about 5 μm to tens11 or hundreds17 of micrometers, and patterning assisted by soft stamps has been demonstrated to provide micrometer-scale crystalline isolated wires with enhanced mobility performances.18 Producing periodic nanostructures at the 100-nm and sub100-nm scales would have potential applications in the fields of new optical devices including, in addition to laser applications, Received: September 3, 2013 Revised: November 10, 2013 Published: January 9, 2014 1643
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light-extraction enhancement for electroluminescent devices, high-density data storage, sensors, and biomedical materials. In all of these fields, electron-beam writing (EBW) is largely employed in research because of its relative ease of use and combination of submicrometer patterning capability and excellent overlay accuracy.19,20 However, whereas EBW is almost ubiquitously used for high-resolution lithographic experiments on polymeric resists, the nanopatterning of active, conjugated materials by EBW has been only poorly explored and is largely limited to amorphous polymer systems. For instance, direct maskless lithography by EBW was recently reported on the prototype conjugated polymer poly[2methoxy-5-(2′-ethylhexyloxy)-1,4-phenylene vinylene] (MEHPPV).21 Other reports have focused on EBW patterning of poly(3-octylthiophene) and polyfluorene derivatives,22,23 whereas submicrometer-scale and direct EBW on organic semiconductor monocrystals is basically unexplored. Previous nanolithography studies on amorphous polymeric films did not allow for the prediction of the performances of EBW approaches on crystalline molecular semiconductors, where a peculiar behavior of the beam−organic interaction can be highlighted by the inherent material anisotropy. In fact, to date, electron-beam irradiation on nonlinear organic crystals has been reported only on samples previously coated with a thin, highly conductive layer. Optical channel waveguides (with line widths of a few micrometers) and Mach−Zehnder modulators24 have been realized with such a method through the reduction of the refractive index25 in the exposed regions of the crystals. Several interplaying physicochemical processes, such as electron scattering, charge conduction, and thermal diffusion at the molecular scale, can potentially affect the beam−semiconductor interaction and shape sample volumes irradiated by electrons differently than in microscopically disordered polymeric films. Here, we report for the first time the sub100-nm patterning of monocrystals of quaterthiophene (4T, Figure 1), which is an example of an organic semiconductor crystalline material showing self-waveguiding of the emitted light, ASE emission from two vibronic replicas at 510 and 553 nm for fluences larger than 0.1 mJ cm−2, and a stimulated emission cross section on the order of 10−15 cm2.8,9 Through direct EBW, we demonstrate arrays of dots with diameters of 15−40 nm and overall densities of up to about 280 dots/μm2. The possible mechanisms at the origin of the observed surface texturing and an extensive characterization of the morphological and optical properties of the related materials were investigated and are also reported.
Figure 1. SEM micrograph of the edge of a 4T crystal (acceleration voltage of the electron beam = 5 kV), sketch of the 4T molecule, and 4T unit cell viewed along the c* axis (i.e., along the normal to the exposed ab face). being b and the angle β being 91.81°), and the cell parameters are a = 6.085 Å, b = 7.858 Å, and c = 30.483 Å.31 The unit cell viewed along the c* axis (i.e., along the normal to the exposed ab face) is shown in Figure 1, together with a scanning electron microscopy (SEM) image of the edge of a 4T crystal. For EBW, a Raith 150 system with an acceleration energy of 20 keV, an aperture of 60 μm, a current of 0.9 nA, and a step size ranging from 20 to 50 nm was used. EBW doses were in the range 100−1500 μC/ cm2. 40 × 40 and 50 × 50 μm2 gratings with periods (Λ) in the range of 80−1000 nm and single-line lateral dimensions (L) ranging from 20 to 500 nm were produced on flat regions of 4T single crystals. Each grating was written both parallel and perpendicular to the optical a and b axes. The gratings produced on 4T were stable over many months under ambient storage conditions. The morphological characterization of the nanopatterned crystal surfaces was carried out by tapping-mode atomic force microscopy (AFM) in air (22 °C and 40% humidity) using a Nanoscope IIIa controller with a MultiMode head (Veeco). Phosphorus-doped Si tips were employed with a 6−10-nm nominal curvature radius and a resonant frequency of 280 kHz. The same AFM system was used for mechanical characterization by force−distance spectroscopy (phosphorus-doped Si tips with a nominal elastic constant of 20−80 N/m). Confocal microscopy was performed using an A1R MP confocal system (Nikon) coupled to an inverted microscope (Eclipse Ti, Nikon). The samples were excited by a laser (λexc = 408 nm) through an oil-immersion objective with numerical aperture (NA) of 1.4. Microphotoluminescence (μ-PL) spectra were obtained by focusing a laser beam (diode laser with λ = 410 nm) onto the samples (spot size
2. EXPERIMENTAL SECTION Our source material was synthesized according to established procedures.26,27 Millimeter and larger-sized 4T single crystals were grown in a horizontal furnace with three heating zones by physical vapor transport28−30 of the sublimated material under a stream of nitrogen. The PVT system was formed of a reactor tube, four glass growth tubes, and a flat-bottom tube as crucible for the starting 4T powder. The growth tube was 13 cm long and had internal and external diameters of 1.5 and 1.8 cm, respectively. The reactor tube was 80 cm long, and it had internal and external diameters of 1.8 and 2.2 cm, respectively. We grew crystals of the low-temperature polymorph31 with a thin plate habit and thicknesses ranging from hundreds of nanometers to a few micrometers. Single crystals were selected by observation under a polarizing microscope with crossed polarizer and analyzer. The largest flat surface corresponds to the ab crystal plane. This polymorph has four molecules per unit cell arranged in a herringbone packing, a monoclinic structure (the monoclinic axis 1644
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of about 30 μm) through a microscope objective with NA = 0.3 and analyzing the emission (collected by the same objective) with a monochromator (iHR320, JobinYvon) equipped with a CCD detector (Simphony, JobinYvon).
(indentation depth below 1 nm) was largely unchanged, thus ruling out significant mechanical/structural changes at very small deformations (Figure 2b). On the contrary, the EBW patterns were clearly appreciated by AFM phase imaging, where dots appeared brighter than the pristine crystal (Figure 3).
3. RESULTS AND DISCUSSION By means of EBW, surface texturing on 4T single crystals was obtained at doses larger than 500 μC/cm2, as measured by AFM in tapping mode. Interestingly, when observed at the nanoscale, the textured crystal surface did not show a continuous array of homogeneously written parallel lines, as in previous experiments on amorphous organics or on commonly used EBW resists.21 Instead, each single stripe was composed of aligned arrays of single dots, spaced by the imposed EBW step size. With increasing exposure dose, the dot diameter increased monotonically to 35 ± 8 nm at 1500 μC/ cm2 (Figure 2a). For example, 250-nm-period parallel lines,
Figure 2. (a) Diameter of the patterned features vs EBW exposure dose. With increasing exposure dose, the dot diameter increases monotonically to 35 ± 8 nm at 1500 μC/cm2. The continuous line is a guide for the eye. Insets: Topographic (left) and phase (right) AFM micrographs of patterned stripes with a period of 250 nm. Exposure dose = 1000 μC/cm2, step size = 50 nm. Scale bar = 40 nm. Vertical scales: 4 nm (left) and 19° (right). (b) Force−displacement (Z) curve on as-grown (circles) and nanopatterned (solid lines) 4T. The arrows indicate the displacement directions for the different curves. Inset: Corresponding force−indentation (δ) curves for as-grown (open symbols) and patterned (solid symbols) 4T. The measured stiffness is almost the same before and after EBW, ruling out significant surface mechanical modifications upon patterning with the used tips (elastic constant = 20−80 N/m).
Figure 3. AFM phase micrographs of patterned features with different lateral dimensions (L) and periods (Λ). Exposure dose = 1000 μC/ cm2. The bottom part of the top-right micrograph (with stripes parallel to the b axis) shows the edge of the exposed region. Vertical scale: 0− 90°.
Phase imaging allows a higher spatial resolution and accuracy to be retained in analyzing the patterned surface than amplitude micrographs. Indeed, the sine of the phase shift is proportional to the amount of energy locally dissipated because of inelastic processes at the surface, including surface energy hysteresis and viscoelasticity, thus allowing a local estimation of the eventual electron-beam-induced compositional contrast.33−36 For instance, previous experiments on blends of polymer materials or semicrystalline materials attributed different phase signals to components of different crystallization stages and stiffnesses.33−38 Here, the whole native surface was made of crystalline material, and impinging electrons locally modified the surface. This local modification is attributable to top-downcreated carbon contaminations, induced by the electron beam and related to hydrocarbons and other molecules present in the exposure chamber; their adsorption or polymerization on the crystal can affect the surface at the nanoscale as well. These effects have also been exploited to pattern silicon39 and might be at the origin of the observed surface texturing, contributing
exposed at 1000 μC/cm2 with a step size of 50 nm, showed almost nontopological dots with diameters of about 23 ± 3 nm and subnanometer heights (insets of Figure 2a). The dots were more clearly visualized by phase imaging (right inset of Figure 2a), as better described below. The distance between neighboring dots in the lines matched the EBW step size. To obtain information on the possible mechanisms of electron-beam texturing, we investigated the nanoscale mechanical properties of the pattern. Force−distance spectroscopy32 within and outside the exposed crystal surface allowed a slight reduction (∼18%) of adhesion forces between the cantilever tip and the patterned dots to be detected; however, the material response to forces in the range of 0.1−0.5 μN 1645
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to the increased phase shift signal and reduction of local adhesion forces.40 However, the electron beam interacts with 4T not only at surface level, but also deeper in the crystal layers. As reported in Figure 3, the eventual anisotropic behavior of the electron diffusion within the crystal was probed by realizing each pattern with the stripe direction parallel and perpendicular to the unitcell axes (a and b). In fact, phase imaging indicated that the interaction of the electron beam with the organic material was largely independent of the crystal axis, because the textured areas (in terms of both lateral resolution and phase contrast) were not affected by the orientation with respect to a particular axis. The effect of the material properties, such as density, atomic numbers, atomic weights, and sample thickness, on the trajectories of electrons and distribution of delivered energy were investigated by Monte Carlo simulations using the CASINO platform.41 A typical distribution of electronic paths originating from an incident 20 keV electron beam in a multilayer composed of a 4T crystal (1-μm thickness) on top of a SiO2/Si substrate is shown in Figure 4a, which indicates that the beam penetrated the sample almost without deflection for a depth of about 10 nm. Scattering events then spread the electronic paths laterally up to a few micrometers. This is shown in detail in Figure 4b, displaying two-dimensional views of energy absorbed by samples having different 4T thicknesses in the range we grew by physical vapor deposition (from top to bottom: 500 nm, 1 μm, and 5 μm, respectively). We found that a fraction on the order of 0.1% of the total energy of impinging electrons was delivered near the crystal surface, that is, at depths of less than 30 nm in the organic material. Overall, more than 80% of the energy was delivered to the sample within a maximum depth of 1.6 μm below the exposed surface (right panels of Figure 4b). It should be pointed out that the broadening of the irradiated volume due to scattering was quite similar to that found in amorphous layers such as MEH-PPV and polymethylmethacrylate resist. Discrete, elastic scattering events undergone by electrons indeed lead to analogous envelopes of the electronic paths. The distinctive dotted nanopatterning observed here at the surface of the crystal are likely to originate from mechanisms such as top-down-created carbon dots, unaffected by scattering-induced broadening of the electron-irradiated volume and occurring in the first few nanometers near the surface, as evidenced by Figure 4a. Instead, local melting phenomena can be basically ruled out for our exposure conditions (20 keV beam delivering a current of about 0.9 nA). In fact, even in the most favorable adiabatic approximation (heat exchange of less than 10%), taking into account the molar heat capacity of 4T (∼330 J/K mol)42 allows a local temperature increase of a few degrees to be estimated for our simulated exposed unit volume (∼6 × 104 nm3). In general, exposing organic samples to electron beams mainly causes structural changes and variations in crystallinity.43 In this case, the material resistance to electron damage is defined by a critical dose (namely, the damage threshold in units of electron charge delivered through a unit cross section of the sample), which is correlated with the material thermal stability.44 In the case of oligothiophene crystals, which have a melting point about 200 °C, the critical electron dose at 10 keV should be higher than 10−2 C/cm2,43,44 which is 1 order of magnitude larger than the values here used for EBW. Although well-retained, such pristine features of 4T are affected as a consequence of patterning in their light emission. In Figure 5a,b, we show confocal fluorescence micrographs of a
Figure 4. (a) Two-dimensional visualization of the electron scattering trajectories (incident electron energy = 20 keV, blue lines) in a multilayer system (4T/SiO2/Si), highlighting the electron-beam path in the 4T crystal within the first tens of nanometers from the surface (z = 0 in the plot). (b) Two-dimensional plot (x, z) of absorbed energy density in 4T/SiO2/Si samples. From top to bottom, the 4T crystal thickness is 500 nm, 1 μm, and 5 μm, respectively. Shown energies are integrated along the transverse direction (y axis). Darker regions indicate higher amounts of absorbed energy. Right panels: Corresponding energy absorbed by a volume of sample (∼6 × 104 nm3) aligned with the electron-beam impinging point and at a depth z below the crystal surface. Number of simulated electrons = 200.
4T crystal patterned with features of Λ = 1 μm perpendicular to the a crystal axis (micrographs of features parallel to the a crystal axis are displayed in Figure S3 of the Supporting Information). Here, the sample was excited by a tightly focused laser beam (spot size of a few hundred nanometers), thus allowing the variation of the PL intensity upon patterning to be spatially resolved. The unexposed areas of the 4T crystal are clearly visible as bright stripes in both cross-sectional (Figure 5a) and planar (Figure 5b) fluorescence micrographs. The spatially resolved PL intensity profiles reported in Figure 5c clearly indicate a modulation of the emission intensity across 1646
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Figure 5. (a−c) Confocal fluorescence micrographs of a 4T crystal patterned with stripes perpendicular to the a crystal axis. The periodicity, Λ, of the pattern is 1 μm. (a) Cross-sectional fluorescence micrograph, obtained by collecting a sequence of images at different values of the z coordinate. (b) Top view. The black arrow indicates the direction of the a axis. Markers: 5 μm. (c) PL intensity profile of panel b, obtained by integrating the emission signal along the crystal thickness (z coordinate). (d) Emission spectra of the pristine 4T crystal (blue continuous line) and the patterned crystal (with grating period, Λ = 1 μm) and features parallel to either the a (open red circles) or b (black continuous line) crystal axis. (e) Emission spectra of a 4T crystal structured by a grating with Λ = 450 nm and features parallel to the a (open red circles) or b (black continuous line) crystal axis.
with lower refractive indexes (air and SiO2) forms a slab waveguide, which favors the self-waveguiding of the emitted light.45 The presence of the periodic structure realized by EBW allowed part of the waveguided light to be outcoupled. In particular, the grating with features parallel to the b axis resulted in an enhancement of the bands at about 610 and 690 nm. By considering the Bragg relation
the exposed/unexposed regions. This effect can be appreciated by the enhanced fluorescent contrast in confocal micrographs, which clearly shows surface texturing. In fact, electrons traveling across 4T deliver part of their energy to the crystal (Figure 4), which can induce local modifications and/or decomposition of the 4T molecules, affecting the π-conjugated structure as previously observed in nonlinear organic single crystals.25 A further contribution to emission alterations coming from the formation of the dots shown in Figure 3 cannot be ruled out, because the emission of organics is typically very sensitive to the presence of impurities and/or contaminants. Overall, these mechanisms lead to a remarkable periodic modulation of the PL intensity across the pattern. The overall PL emission spectra are compared in Figure 5d,e. These spectra were collected using a μ-PL setup, collecting the optically excited luminescence from an entire emitting subvolume of patterned crystal over an area of excitation of about 700 μm2 (spot size of 30 μm, including many patterning stripes). By this approach, the effect of the grating on the overall emission properties could be evaluated. For both the patterned and pristine crystals, the spectrum showed broad bands centered at about 520, 553, 608, and 688 nm, given by the overlap of the room-temperature 4T excitonic progression45 (510, 551, 599, and 656 nm) and a well-known progression due to an impurity, as reported in the literature (525, 566, 620, and 678 nm).26,27 However, the emission of the structured region displayed an increase in the intensity at the lowest energies, particularly when the grating (with Λ = 450 nm) was produced with features parallel to the b axis (Figure 5e). We attribute these differences to the presence of a structure with a period comparable to the emission wavelength, which can modify the emission spectrum through Bragg scattering effects. The 4T crystal surrounded by materials
2πneff 2π 2π sin θ = ± ±m λ λ Λ
where neff is the effective refractive index of the guided mode and m indicates the diffraction order, the observed peaks at the lowest energy correspond to scattering angles of about 10° and 2° (using as neff the refractive index measured for light polarized parallel to the b axis, neff = 1.5),26,27 which are within the collection angle of the used objective. Taking into account our maximum collection angle (∼17°), we do not expect contributions from Bragg scattering in the spectra collected for the grating with features parallel to the a axis, because of the higher refractive index (about 1.75). Overall, these findings demonstrate how the light emission of 4T monocrystals can be tuned by direct EBW nanopatterning. Because PL is collected from the entire emitting volume of patterned crystals, the emission modulation can be enhanced in samples with smaller thicknesses. Indeed, according to the calculated distributions of absorbed energy (Figure 4b), the overall effectiveness of Bragg scattering would be improved with respect to thicker crystals, because the fraction of photons emitted and traveling in the deeper region of samples might also experience higher refractive index contrasts. 1647
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(4) Fichou, D.; Delysse, S. V.; Nunzi, J. M. First evidence of stimulated emission from a monolithic organic single crystal: αOctithiophene. Adv. Mater. 1997, 9, 1178−1181. (5) Hibino, R.; Nagawa, M.; Hotta, S.; Ichikawa, M.; Koyama, T.; Taniguchi, Y. Emission Gain-Narrowing from Melt-Recrystallized Organic Semiconductors. Adv. Mater. 2002, 14, 119−122. (6) Zhu, X. H.; Gindre, D.; Mercier, N.; Frere, P.; Nunzi, J. M. Stimulated Emission from a Needle-like Single Crystal of an EndCapped Fluorene/Phenylene Co-oligomer. Adv. Mater. 2003, 15, 906−909. (7) Ichikawa, M.; Hibino, R.; Inoue, M.; Haritani, T.; Hotta, S.; Araki, K.; Koyama, T.; Taniguchi, Y. Laser Oscillation in Monolithic Molecular Single Crystals. Adv. Mater. 2005, 17, 2073−2077. (8) Fichou, D.; Dumarcher, V.; Nunzi, J. M. One- and two-photon stimulated emission in oligothiophenes single crystals. Opt. Mater. 1999, 12, 255−259. (9) Polo, M.; Camposeo, A.; Tavazzi, S.; Raimondo, L.; Spearman, P.; Papagni, A.; Cingolani, R.; Pisignano, D. Amplified spontaneous emission in quaterthiophene single crystals. Appl. Phys. Lett. 2008, 92, 083311. (10) de Boer, R. W. I.; Gershenson, M. E.; Morpurgo, A. F.; Podzorov, V. Organic single-crystal field-effect transistors. Phys. Status Solidi a 2004, 201, 1302−1331. (11) Briseno, A. L.; Mannsfeld, S. C. B.; Ling, M. M.; Liu, S.; Tseng, R. J.; Reese, C.; Roberts, M. E.; Yang, Y.; Wudl, F.; Bao, Z. Patterning organic single-crystal transistor arrays. Nature 2006, 444, 913−917. (12) Tavazzi, S.; Raimondo, L.; Silvestri, L.; Spearman, P.; Camposeo, A.; Polo, M.; Pisignano, D. J. Dielectric tensor of tetracene single crystals: The effect of anisotropy on polarized absorption and emission spectra. J. Chem. Phys. 2008, 128, 154709. (13) Quochi, F.; Cordella, F.; Mura, A.; Bongiovanni, G.; Balzer, F.; Rubahn, H. G. One-dimensional random lasing in a single organic nanofiber. J. Phys. Chem. B 2005, 109, 21690−21693. (14) Cordella, F.; Quochi, F.; Saba, M.; Andreev, A.; Sitter, H.; Sariciftci, N. S.; Mura, A.; Bongiovanni, G. Optical gain performance of epitaxially grown para-sexiphenyl films. Adv. Mater. 2007, 19, 2252− 2256. (15) Losio, P. A.; Hunziker, C.; Guenter, P. Amplified spontaneous emission in para-sexiphenyl bulk single crystals. Appl. Phys. Lett. 2007, 90, 241103. (16) Kena-Cohen, S.; Forrest, S. R. Room-temperature polariton lasing in an organic single-crystal microcavity. Nature Photon. 2010, 4, 371−375. (17) Briseno, A. L.; Aizenberg, J.; Han, Y.-J.; Penkala, R. A.; Moon, H.; Lovinger, A. J.; Kloc, C.; Bao, Z. Patterned Growth of Large Oriented Organic Semiconductor Single Crystals on Self-Assembled Monolayer Templates. J. Am. Chem. Soc. 2005, 127, 12164−12165. (18) Lee, H. M.; Kim, J. J.; Choi, J. H.; Cho, S. O. In Situ Patterning of High-Quality Crystalline Rubrene Thin Films for High-Resolution Patterned Organic Field-Effect Transistors. ACS Nano 2011, 5, 8352− 8356. (19) Altissimo, M. E-beam lithography for micro-/nanofabrication. Biomicrofluidics 2010, 4, 026503. (20) Tseng, A. A.; Chen, K.; Chen, C. D.; Ma, K. J. Electron Beam Lithography in Nanoscale Fabrication: Recent Development. IEEE Trans. Electron. Packag. Manuf. 2003, 26, 141−149. (21) Stabile, R.; Camposeo, A.; Persano, L.; Tavazzi, S.; Cingolani, R.; Pisignano, D. Organic-based distributed feedback lasers by direct electron-beam lithography on conjugated polymers. Appl. Phys. Lett. 2007, 91, 101110. (22) Persson, S. H. M.; Dyreklev, P.; Inganas, O. Patterning of poly(3-octylthiophene) conducting polymer films by electron beam exposure. Adv. Mater. 1996, 8, 405−408. (23) Doi, Y.; Saeki, A.; Koizumi, Y.; Seki, S.; Okamoto, K.; Kozawa, T.; Tagawa, S. Nanopatterning of polyfluorene derivative using electron-beam lithography. J. Vac. Sci. Technol. B 2005, 23, 2051− 2055.
4. CONCLUSIONS In summary, we have reported the nanopatterning of 4T monocrystals by direct maskless EBW. Pattern periods ranged from 1 μm to 80 nm, with lateral sizes of the single lines down to 20 nm and depths on the order of nanometers. In contrast to previous experiments on amorphous conjugated polymers, the textured crystal did not show continuously exposed features, but instead exhibited aligned arrays of high-resolution single dots spaced by the EBW step size, as by locally induced conformational changes at the surface. We found no evidence that the interaction of the electron beam with the organics was dependent on the crystal axis because the textured areas were not affected by their orientation with respect to either of the unit-cell axes, at least for the accessible a and b axes of 4T. On the contrary, the modulation of the light emission resulted from the combination of nanopatterning and optical anisotropy along the crystal directions. Finally, the exposure dose was found to play an important role, producing permanent texturing with resolution on the scale of tens of nanometers for values between 500 and 1500 μC/cm2. Future studies focused on the modifications in transport properties induced by EBW nanopatterning might provide a better understanding, together with design guidelines for textured, electrically injected optoelectronic devices based on molecular crystals.
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ASSOCIATED CONTENT
* Supporting Information S
Crystal growth details, three-dimensional AFM view of features patterned on the surface of the organic crystal, and confocal fluorescence micrographs of a patterned 4T single crystal with stripes parallel to the a axis. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors acknowledge A. Papagni for the 4T chemical synthesis, R. Stabile for sample exposures, V. Fasano for assistance during confocal measurements, and A. Borghesi for critically reading the article. D. Drouin is also gratefully acknowledged for helpful discussions about the Casino software. Fondazione Cariplo; the Apulia Regional Project “Network of Public Research Laboratories” M.I.T.T. (#13); and the Italian Ministry of Education, Universities, and Research through FIRB Project RBFR08DJZI ‘Futuro in Ricerca’ are acknowledged for funding.
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REFERENCES
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dx.doi.org/10.1021/la4033833 | Langmuir 2014, 30, 1643−1649
