New Pentacene Crystalline Phase Induced by Nanoimprinted

Mar 27, 2012 - Department of Photonics, Advanced Optoelectronic Technology Center, National Cheng Kung University, Tainan 701, Taiwan. ‡ Department ...
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New Pentacene Crystalline Phase Induced by Nanoimprinted Polyimide Gratings Wei-Yang Chou,*,† Ming-Hua Chang,† Horng-Long Cheng,† Yung-Chun Lee,‡ Chung-Chih Chang,§ and Hwo-Shuenn Sheu∥ †

Department of Photonics, Advanced Optoelectronic Technology Center, National Cheng Kung University, Tainan 701, Taiwan Department of Mechanical Engineering, National Cheng Kung University, Tainan 701, Taiwan § Department of Physics, ROC Military Academy, Kaohsiung 830, Taiwan ∥ National Synchrotron Radiation Research Center, Hsinchu 301, Taiwan ‡

ABSTRACT: Control of the polymorphism of organic semiconductor films is very important in optoelectronic devices, such as organic thin-film transistors, solar cells, and organic light-emitting diodes. Here, we show the use of polyimide nanogratings (PINGs) patterned by nanoimprint lithography to control the polymorphism of deposited pentacene films. Interestingly, a new crystalline phase is first found in the pentacene thin-film formed on the PI-NGs. This new phase, with a d-spacing of 1.35 nm, is more stable than the bulk and thin-film phases in the pentacene film, discussed in terms of X-ray diffraction (XRD) measurements and quantum theory calculations. The response mechanism of the formation of this new phase is characterized by XRD and atomic force microscopy; the polymorphism and surface morphology show a strong dependence on the width of PINGs. The size-dependent PI-NG responses of the pentacene film can also control the phase transition from the thin-film to the bulk phase.

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depends on polymorphism,15 grain size,16−18 and molecular ordering of the pentacene films.19 However, to expand their applications further, there is a need to employ the findings in academic organic semiconductor research to the industries where the stability issue needs to be addressed. In this study, a new stable phase of pentacene film induced by nanoimprinted dielectric is found. This new phase is more stable than the thinfilm and bulk phases in the pentacene films, indicating that pentacene-based devices with this new phase can be applied to industrial production. Pentacene crystallizes through various polymorphisms, which are characterized by interplanar spacing (d001) of 1.41, 1.45, 1.50, or 1.54 nm.20 Two polymorphisms in the pentacene film with d001 spacing of 1.54 and 1.45 nm were grown on SiO2 substrate, respectively, referred to as thin-film and bulk phases.21,22 To improve the performance and stability of

INTRODUCTION Organic semiconductors (OSCs) have been increasingly used since the past decade. Organic devices have shown great potential in high-performance devices, such as organic thin-film transistors (OTFTs),1,2 solar cells,3,4 flexible devices,5,6 and memory devices.7,8 The great progress in this field has been driven by new possible applications of organic/polymeric nanostructures in micro- and optoelectronics. Scaling down organic devices to the nanometer range is possible because OSC films are formed by nanostructures; however, scaling down the size of electronic devices requires advanced methods to pattern them. A large number of studies have demonstrated various patterning methods for achieving high-performance organic devices, including rubbing,9 alternative patterning,10 and photoalignment,11 to improve the regular molecular ordering of OSCs12 or to control their molecular alignment.13 Crystalline pentacene, an ambipolar organic material,14 is the most widely studied OSC due to its high carrier mobility, good semiconducting properties, and environmental stability. The field-effect mobility (μ) of pentacene-based devices highly © 2012 American Chemical Society

Received: January 3, 2012 Revised: March 9, 2012 Published: March 27, 2012 8619

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Figure 1. Schemes of nanoimprinting and pentacene film growth processes. (a−c) The etching mask of Cr nanogratings transferring from the silicon mold to PMMA layer. (d−f) The PI-nanogratings achieved after anisotropic reactive ion etching and acetone lift-off processes. (g) Pentacene film, 60 nm thick, deposited on the PI-nanogratings. (h) The different incident direction of X-ray, where ∥ and ⊥ referred as measurements made with nanograting direction parallel and perpendicular to the direction of incident X-rays, respectively.

A new stable polymorph with a d-spacing of 1.35 nm was found in a pentacene film formed on PI-NGs. This polymorph is assumed to be induced primarily by the geometry of the PINGs during pentacene film growth; it has neither been observed nor expected in past studies.19,20,23,30−34 The stability of the 1.35 nm phase was studied by exposing the pentacene film to ambient conditions for a long time and calculating by use of second-order Møller−Plesset perturbation (MP2) theory. The results indicated that the percentage of 1.35 nm phase in the pentacene film increased, and the electronic coupling energy of the 1.35 nm phase was higher than that of the 1.41 nm phase, resulting in phase transition from the thinfilm and bulk phases to the 1.35 nm phase. Therefore, the 1.35 nm phase is the most stable phase in the polymorphism of thin pentacene films, indicating that improvement in stability of pentacene-based devices using this new phase can be expected. A growth model for pentacene deposition on a PI-NG structure is proposed.

OTFTs, considerable attention was placed on the control of pentacene polymorphic transformation from the thin-film to the bulk phase using improved device fabrication techniques.15,23,24 The bulk phase structure is believed to have high electronic coupling between adjacent pentacene molecules, resulting in a high μ value for pentacene-based OTFTs. The growth model of a pentacene film on a SiO2 substrate was previously demonstrated.25 The polymorphism of the pentacene film strongly depends on the thickness and roughness of the deposited surface. A thick, rough pentacene film can induce the bulk phase, whereas a thin, flat film induces the thin-film phase.21 In the present study, a dielectric with nanograting (NG) structures is used to control the polymorphism of pentacene films. The width reduction of polyimide-NGs (PI-NGs) induces a favorable bulk crystal polymorph in the pentacene films, desirable for both OTFT and organic solar cell applications. NG structures are inserted in the pentacene-based device to control effectively the polymorphism of pentacene film compared with other methods, such as solvent annealing23,26 or postannealing.15,18 Solvent annealing completely transforms the thin-film to the bulk phase, but it damages the pentacene active layer and degrades the electronic performance of the organic devices. The postannealing method is not efficient in controlling the phase transformation. In contrast, inserting an NG structure can efficiently control pentacene polymorphism, and the electronic performance of OTFTs can be simultaneously improved. We previously applied an NG dielectric to fabricate OTFTs27 and achieved outstanding μ and extra high anisotropy of charge transport compared with those reported in the literature.9,12,28,29 The nanostructure-embedded device has a high potential application in nanoscale lithography-free patterning of OSC films in integrated circuits.

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EXPERIMENTAL METHODS Silicon molds with nanogroove structures were fabricated using the standard photolithographic method. 1H,1H,2H,2H-Perfluorodecyltrichlorosilane was first deposited on the silicon mold surface using vapor deposition for antisticking and to ensure easy mold release after imprinting. A Cr layer was deposited on silicon molds with nanogrooves of various periods. The thickness of the Cr layer was 50 nm and the ratio of the ridge to the trench on the mold was approximately 1, as shown in the top part of Figure 1a. An 80 nm thick PI film, which acted as the dielectric and modification layer, was spin-coated on a 300 nm thick SiO2 layer. The PI film was soft-baked at 100 °C for 5 min and hardbaked at 220 °C for 60 min. A PMMA film was spun on the PI layer as the sacrificial layer, as shown at the bottom of Figure 8620

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Figure 2. Scanning electron microscope cross-section image of pentacene film deposited on PI-nanogratings. Green circle line marks the pillar-like pentacene grains grown on the sidewall of the PI-NG.

Figure 1). The PI-NGs were formed on the SiO2 substrate. A 60 nm thick pentacene film was deposited on the PI-NGs at a deposition rate of approximately 0.02 nm/s (step g of Figure 1). Figure 2 shows the cross-sectional image of the pentacene film on the PI-NGs obtained from scanning electron microscopy (SEM), showing that the pentacene film is continuous on the PI-NG. Pentacene polymorphisms and molecular orientation were investigated using X-ray diffraction (XRD) measurements. To examine the orientation of pentacene molecules, XRD measurements were taken with the X-ray incident direction parallel and perpendicular to the pentacene NGs (step h of Figure 1). Figure 3a shows an atomic force microscope (AFM) image of 80 nm wide PI-NGs on the surface of a SiO2 substrate where there are no PI residues at the bottom of the trenches. Pentacene was then deposited on the PI-NGs at various widths, as shown in Figure 3b−e. The AFM images indicate that the grains of the pentacene films formed on the PI-NGs are larger than those deposited on the trenches. The pentacene grains grown on the trenches were similar to those directly grown on the SiO2 surface. The pentacene film on the sidewall of the PINGs had pillar-like grains, as shown in the inset of Figure 3d. Interestingly, there were two similarly sized pentacene grains on the PI-NGs with widths of 400 and 600 nm, resulting in a continuous boundary line at the center of the PI-NGs, as shown in Figure 3b,c. Figure 3d shows that, when the width of the PINGs was extended to 800 nm, an additional grain appeared at the center of the PI-NG due to three nucleation sites (Figure 3a shows two at the edges and another at the center of the PI-NG during the early stage of pentacene growth). The grain size of the pentacene film located at the grating center was almost twice as large as that located at the grating edge, attributable to the limitation of the grating edge during pentacene grain growth. The confinement effect of the pentacene film growth on the PI-NGs rapidly decreased when the PI-NG width was

1a. A loading force was applied since the unbaked PMMA layer retained some adhesion and mobility, for a few minutes to embed Cr gratings into the PMMA layer. To bind the Cr gratings, the PMMA film was baked at 90 °C for a few minutes, while the loading force was maintained. The Cr nanogratings were partially embedded in the PMMA layer as an etching mask for subsequent anisotropic RIE to remove the polymers (PMMA and PI) that were not protected by the transferred Cr pattern. The residual PMMA layer was dissolved using chemical etching in acetone; this allowed the Cr gratings to be lifted off to obtain a PI-NGs substrate, as shown in Figure 1b−f. In the experiment, a 60 nm thick pentacene film was deposited on the PI-NGs at a deposition rate of approximately 0.02 nm/s.

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RESULTS AND DISCUSSION The nanogroove structure designed for aligning pentacene films is shown in Figure 1. Chromium (Cr)-coated silicon molds with 800, 1200, 1600, and 2400 nm periodic gratings, which consisted of 300 nm deep and 400, 600, 800, and 1200 nm wide trenches, respectively, were prepared for the nanoimprinting process. The molds were treated with a 10 nm thick antiadhesion layer, 1H,1H,2H,2H-perfluorodecyltrichlorosilane. A sacrificial polymethylmethacrylate (PMMA) layer was spincoated onto an 80 nm thick PI (RN-1349, Nissan Chemical) film, previously formed on a SiO2 substrate. The PMMA layer maintained certain mobility before hot baking. A loading force was applied for a few minutes to imprint the mold (placed exactly on top of the substrate) into the PMMA layer, as shown in steps a and b in Figure 1. After baking, the PMMA film was hardened during the imprinting process. As the mold was separated from the substrate, Cr-gratings were partially embedded in the PMMA film to act as an etching mask for subsequent anisotropic reactive ion etching on the PMMA and PI films. After chemical etching in acetone, the PMMA film was dissolved, and the Cr gratings were lifted off (steps c−f in 8621

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Figure 4. XRD spectra of pentacene films deposited upon different wide PI-nanogratings in which the direction of incident X-rays (a) parallel to the direction of nanogratings (referred to as NG∥) and (b) perpendicular to the direction of nanogratings (referred to as NG⊥). The insets of panels a and b reveal that the XRD spectra were measured under exposing the pentacene film to ambient conditions over two months.

Figure 3. Atomic force microscopic images of (a) PI-nanogratings and cross-section profile of the PI-nanogratings along the direction of the green line and (b−e) pentacene film deposited on the different wide PI-nanogrooves, where the widths of these nanogratings are (b) 400, (c) 600, (d) 800, and (e) 1200 nm. The inset in panel d is the pentacene morphology on the sidewall of the nanograting. All scanning images are 4 μm × 4 μm in size.

directions when the widths of the PI-NGs were decreased, indicating that polymorphic transformation from the thin-film to the bulk phase occurred in the pentacene film deposited on narrow PI-NGs. This result was attributed to the migration of pentacene molecules confined by the width of the PI-NGs during the deposition process of the pentacene film. Generally, pentacene growth initially proceeds layer by layer. However, a three-dimensional growth mode appeared early in the pentacene thin film on PI-NGs due to the confinement of pentacene molecule migration. The pentacene molecules then grew on the inclined plane of the pentacene grains, leading to an increase in the tilt angle (θtilt) of the pentacene molecules from the c axis toward the a axis. The polymorph of the pentacene gradually transformed from the thin-film to the bulk phase, as shown in Figure 6a. Therefore, the bulk phase dominated the polymorphs of pentacene at narrow PI-NGs. From the comparison of the XRD results of the pentacene films grown on native PI and PI-NGs (Figure 5b), we found that the surface geometry of the dielectric can be used to control the polymorphs of the pentacene films.38 The bulk phase preferentially formed in the pentacene thin film on narrow PI-NGs. For pentacene samples with PI-NGs, the XRD measurements taken in the NG⊥ measurement mode show that a new polymorph was formed. The d001 spacing and θtilt of this new phase were estimated to be 1.35 nm and 33.53°, respectively. The peak of the 1.35 nm phase was clearly observed in the NG⊥ measurement mode, whereas it was not seen in the NG∥ measurement mode. The NG⊥ measurement mode detected a

increased to 1200 nm because this grating width is too large to confine the pentacene molecule migration during deposition. The polymorphs of the 60 nm thick pentacene films formed on PI-NGs were characterized by XRD measurements in which a Mac Science 18 kW rotating anode X-ray generator (M18XHF) with Cu Kα radiation (50 kV, 200 mA, and λ = 0.15418 nm) was used. The incident beam was monochromatized by a Gr(0002) crystal to eliminate Cu Kβ contamination. Two-directional XRD measurements, taken with grating directions parallel (hereinafter referred to as NG∥) and perpendicular (hereinafter referred to as NG⊥) to the direction of the incident X-rays, were performed on the pentacene samples with PI-NGs. The obtained diffraction profiles are shown in Figure 4. For the pentacene samples with PI-NGs, three crystalline polymorphs were found according to their (00l) spacing. Large-intensity (001′) diffraction peaks at 5.73° and (001) peaks at 6.13° corresponding to the thin-film and bulk phases, respectively, were observed for all samples in both NG∥ and NG⊥ measurement modes, whereas the peak at 6.53° was only observed in the NG⊥ measurement mode. The (001) lattice constants along the c axis of the thin-film and bulk phases were calculated as 1.54 and 1.45 nm, respectively, agreeing with those reported in the literature.20,21,35−37 Figure 5a shows that the intensity ratio of the (001′) to the (001) peak was significantly reduced in both measurement 8622

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Figure 5. (a) XRD intensity ratio of the thin-film phase (001′) to the bulk phase (001), where the directions of incident X-ray are parallel to the direction of nanogratings (marked as NG∥) and perpendicular to the direction of nanogratings (marked as NG⊥). (b) XRD spectra of pentacene films deposited on different wide PI-nanogratings and native PI.

Figure 6. (a) Schematic evolution of pentacene film grown on the ridge and sidewall of a nanograting. The inset SEM picture shows the profile of pentacene film on the PI-NG. (b) Binding energies of 1.35 and 1.41 nm structures as a function of θEF between two neighboring pentacene molecules.

long-range crystallization of pentacene films; thus, the signal of the XRD spectrum at 6.53° was more obvious than that in the NG∥ measurement mode. The polymorph of the 1.35 nm phase is believed to exist in the pentacene film on the sidewall of the NGs. To verify the stability of the 1.35 nm phase, all samples were preserved in ambient for two months. After ambient exposure was done for all pentacene films grown on various PING periods, the 1.35 nm phase of the pentacene film was observed in both XRD measurement modes, as shown by the Figure 4 inset. Interestingly, the peak intensities of the 1.35 nm phase for the ambient degraded pentacene films are relatively enhanced compared with those of fresh pentacene films, indicating that the 1.35 nm phase is a more stable phase under ambient conditions when compared with the thin-film and bulk phases. Accordingly, phase transition occurred from the bulk and thin-film phases to the stable 1.35 nm phase during exposing the pentacene films to ambient conditions. We have previously established a growth model of pentacene film on a SiO2 substrate that was later confirmed by Murakami et al., who observed a pentacene polymorphic transformation from the thin-film to the bulk phase occurring on a concave surface using high-resolution cross-sectional transmission electron microscopy.25,39 When the thickness of the pentacene film increased, steep hillsides gradually formed on the grain and grain boundary. The deposited pentacene molecules on the inclined plane then resulted in an increase in θtilt. Thereafter, the increase in θtilt leads to the transformation of the polymorph of the pentacene film from the thin-film to the bulk phase. A steeper hillside can induce the pentacene molecule to a further tilt. PI-NGs had a step-like morphology to support steep sidewalls for the growth of the pentacene film.

The 1.35 nm phase pentacene was located on the sidewall of the PI-NGs. Ordered pillar-like grains appeared in the pentacene film on the sidewall of the PI-NGs, as shown in Figure 3d, inset. This morphology has never been observed in the literature. The pentacene film on the sidewall of the PI-NG lacks long-range ordering when the incident X-ray beam is parallel to NG. Accordingly, the XRD signal of the 1.35 nm phase cannot be easily observed when the incident X-ray beam is parallel to the NG. The peak intensity of the 1.35 nm structure also increases with a decrease in the width of the PINGs due to the increase in PI-NG density within the XRD beam spot, as shown in Figure 4b. On the basis of the above results, the proposed growth model of the pentacene film deposited on PI-NGs is shown in Figure 6a. The bulk phase of the pentacene film generally forms as a thick film.21,25 The effect of nanoconfinement induced by the PI-NGs led to the early formation of three-dimensional grains during pentacene film growth, resulting in polymorphic transformation of the pentacene film from the thin-film to the bulk phase at lower film thickness. Therefore, the bulk phase grew on the hillside of the thin-film phase. At the early stage of pentacene growth on the gratings and at the trenches, the pentacene molecules, deposited on flat surfaces, had the tendency to be arranged vertically, resulting in the formation of the thin-film phase. Although the growth of the pentacene film on the sidewall of the PI-NGs was layer by layer parallel to the substrate, its formation was similar to sand vertical step. The pentacene film on the sidewall of NG is thick at the corner between the sidewall and the substrate, as shown by the red envelope line in the SEM picture within the NG in the Figure 8623

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6a inset. As the film thickens, the pentacene molecules near the sidewall of the PI-NGs interact with PI and are then forced to tilt further. Additional lateral interaction at the sidewall of the pentacene film can lead to the formation of the 1.35 nm phase. The perturbation comes from the sidewall of the PI-NG; therefore, the grain formation of the 1.35 nm phase attached to the sidewall of PI-NG has a pillar-like shape, as shown in the SEM picture in Figure 2. The pillar-like grain (marked by a solid green circle in Figure 2) is directly observed by SEM and was consistent with the result obtained by AFM, as shown by the Figure 3d inset. To confirm further that the 1.35 nm phase is a stable crystal polymorph, the stabilization (binding) energy (ΔEb) of the corresponding herringbone edge-to-face dimer structures was estimated using quantum chemical calculations because the dimer structure is the basic repeating unit of the pentacene crystal. Constructing the corresponding dimer geometry as accurately as possible, similar to that in the crystal polymorphs, was done by adopting the observed structural parameters of separation distance, θtilt, and θEF (defined in the Figure 6b inset). The MP2 method was used in the 6-31G(d) basis set to calculate ΔEb, the dipole moment, and the electronic coupling energy (i.e., charge transfer integrals). The correct basis set superposition error was used to calibrate ΔEb. The MP2 method has been widely used for studying binding or stabilization energies (ΔEb) between aromatic molecules,40,41 and the simulated equilibrium distance is in good agreement with the crystal data from XRD measurements.42,43 Dimer calculations were performed for the 1.41 and 1.35 nm phases because the 1.41 nm structure is assumed to be the most stable structure in the pentacene single crystal. The strength of the electrostatic potential at the 1.41 nm structure is the lowest of the four well-known polymorphs of pentacene crystal.20 Figure 6b shows the value of ΔEb as a function of θEF for the two polymorphs. The ΔEb value of the 1.35 nm dimer structure is lower than that of the 1.41 nm dimer structure, suggesting that the former is a more stable structure. The formation of the 1.35 nm dimer structure also induces an extra dipole moment of 1.92 D to enhance intermolecular bonding, thus making the 1.35 nm dimer structure more stable than the 1.41 nm one (dipole moment of 1.66 D). However, the crystal is not the dimer, and the stabilization of the crystal polymorph also depends on the interactions from other surrounding molecules. The embedded-cluster calculations for the 13.5 nm structure cannot be performed as before because of the lack of lattice parameters.43 More recently, using MP2 calculation, the equilibrium dimer structure of the two well-known pentacene polymorphs, the so-called thin-film phase (d001 spacing of 1.54 nm) and the bulk phase (d001 spacing of 1.44 nm), have been successfully simulated.44 The observed results of optimal θEF and the stabilization trends of these dimer states are comparable with the findings from XRD measurements and crystal cluster calculations, respectively. Consequently, the calculated results based on the dimer structures are assumed to be acceptable in discussing the stabilization of crystal polymorphs. However, at the microscopic level, electronic coupling is the key parameter for charge transport in conjugated organic materials. The calculated splitting of the highest occupied molecular orbital (HOMO) is an estimate of the electronic coupling energy for holes. A higher hole mobility is expected from a larger HOMO bandwidth.45 The absolute value of electronic coupling energy (t) for hole transport is approximated as

t=

ε HOMO − ε HOMO − 1 2

(1)

where εHOMO and εHOMO−1 are the energies of the HOMO and HOMO−1 levels, respectively. The electronic coupling energy calculated from eq 1 reveals that the 1.35 nm phase has a larger electronic coupling energy (241 meV) than the 1.41 nm structure (187 meV).

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CONCLUSIONS A new polymorphism of pentacene thin films and the control of polymorphism from thin-film to bulk phase induced by nanoimprinting PI-NGs were investigated using XRD, AFM, and SEM. The polymorphic control mechanism for pentacene films on nanoimprinted PI-NGs is significantly affected by the width of the PI-NGs. The polymorph of pentacene gradually changed from the thin-film to the bulk phase as the width of the NGs was reduced. This process was thought to be driven by the early formation of the three-dimensional growth mode during pentacene film growth, mainly caused by the confinement effect of the nanostructure. This in turn limits the migration capability of the pentacene molecules. The morphology and polymorph of the pentacene film can be controlled well by adjusting the dimensions of the PI-NGs. The NG-induced 1.35 nm phase was thought to be driven by the interaction between the sidewall of the PI-NG and the pentacene molecule, resulting in a further increase in the tilt angle of the pentacene molecule. This polymorph was confirmed to be a more stable dimer structure than the 1.41 nm phase in single crystal pentacene using quantum chemical simulation. Direct evidence observed from XRD spectra for samples held in ambient conditions over two months proved that the 1.35 nm phase is a more stable phase than the others in the pentacene film. The obtained pentacene nanostructure has great potential for achieving anisotropic electrical properties in field-effect transistor applications, as demonstrated in our previous work.27 Control of the pentacene polymorph and the proposed mechanism can also be applied to other organic semiconductors.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +886 6 2757575, ext. 63912. Fax: +886 6 2095040. Email: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Science Council, Taiwan, under grants NSC 99-2112-M-006-014-MY3, NSC 992738-M-006-002, and NSC 97-3114-M-006-001. We are grateful to the National Center for Highperformance computing of Taiwan for computer time and facilities.

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