Step‐Edge‐Induced Patterning and Orientation Control of Crystalline Organic Semiconductor Films

Orientation control over micropatterned crystalline organic semiconductor film is crucial to achieve high device performance with small variation in organic electronics. Here, using Au patterned SiO2 substrates, a method is reported to realize high‐resolution patterning and orientation control of crystalline organic thin films simultaneously. The vacuum deposited N,N‐dioctyl‐3,4,9,10‐perylene tetracarboxylic diimide (PTCDI‐C8) molecules can selectively diffuse to and nucleate on prepatterned Au stripes, resulting in patterned crystalline films with a preferential orientation distribution. Scanning tunneling microscope and high‐resolution atomic force microscope images reveal that the PTCDI‐C8 molecules first assemble on Au with perylene diimide cores in a laying down configuration, providing nucleation sites at the step edges for directional growth along the a‐axis of the molecule crystal. Subsequent deposition of molecules leads to orientated crystalline films growth due to the π packing of the perylene diimide cores, resulting in orientation control on micropatterned crystalline films. The finding provides a new strategy to grow oriented crystalline films for high device performance uniformity.


Introduction
Highresolution organic semiconductor (OSC) device arrays with crystalline films are of crucial importance for high per formance and high integrated level organic electronics. [1][2][3] In particular, building block elements such as organic fieldeffect As extensively employed in inorganic semiconductors, single crystalline films can be directly epitaxial grown on single crystal substrates to guarantee a same orientation over the entire sub strate wafer. [14,15] Unfortunately, owing to the absence of single crystal substrates, organic semiconductors are typically grown on amorphous surfaces with randomly oriented domains. [16,17] Much effort has been dedicated to controlling the orientation by directed growth either in vacuum or solution as well as by employing external force/magnetic/electrical fields. Using sol vents as media, various solutionbased techniques were devel oped to control or confine the flow or evaporation direction of the molecule containing solvents, which in return results in manipulating the crystallographic order of OSC molecules. [18][19][20][21][22] For example, Bao et al. demonstrated a shearing force to con trol the contraction direction of the threephase contact line of the solution, so as to guide the assembly of molecule along the moving direction of the scraper. [23,24] Dip coating also pro vides an effective process to control the direction of crystal pre cipitation by withdrawing the substrate from the solution that containing the object materials. When the evaporation rate of a solvent at the meniscus front is larger than that in the bulk solu tion, a preferential nucleation and growth orientated organic films along the pulling direction can be achieved. [25,26] Simi larly, offcenter spincoating or drop casting were also employed to control the growth of crystals by depinning of threephase contact line. [27,28] Another strategy to grow orientated crystals is to confine the solution on templates, e.g., wetting/dewetting Si pillars and photoresist stripes, where the crystalline films are grown along the templates during the evaporation of sol vents. [29,30] In addition, the molecular packing orientation can be further tuned by an external magnetic field or electric field. [31][32][33][34] However, the solutionbased methods require high solubility of OSC materials in organic solvents and the residual solvent in the obtained crystals might degrade the quality of crystals with poor electronic and photoelectric performance. [35] With advantages of fine multilayer thickness control and interface engineering, physical vapor deposition is also devel oped as one of the mainstream methods for high quality organic crystalline film preparation. [36] Mimic to inorganic semiconductors, weak epitaxy growth was developed to epitaxi ally grow organic crystalline films on molecule templates. [37,38] However, typically the molecule templates are also formed on amorphous surfaces, e.g. psexiphenyl on SiO 2 , with ran domly oriented domains in size of micrometers. [39] Anisotropic The white double head arrow in (b) represents the orientation of the stripes and the white dash parallelogram is marked in (c) to label the unit cell and the two arrows are drawn to indicate the direction of a-axis and b-axis. α represents the angle between the a-axis and the b-axis. d) Top view and side view of the molecular packing of (001) plane cutting from PTCDI-C 8 crystal.
www.advmatinterfaces.de liquid crystal films were also used to align OSC molecules upon deposition, with the orientation parallel to the dominant direction of charge transfer. [40] Microscopically viewed by scan ning tunneling microscope (STM), atomic step was reported to induce the azimuthal alignment of pentacene molecules in submonolayer coverage. [41] Previously, we proposed a template induced area selective growth method for the surface patterning of organic semiconductors. [42][43][44] With the method, single crystal microorganic fieldeffect transistor arrays and microorganic light emitting diode arrays over large area were successfully fabricated. [45][46][47] In this work, we further report the orientation control of N,Ndioctyl3,4,9,10perylene tetracarboxylic diimide (PTCDIC 8 ) crystalline film grown on Au lines patterned SiO 2 . Highresolution atomic force microscope (AFM) images reveal that the orientation is a reflection of the molecular packing direction. The step edges of the Au lines function as predeter mined nucleation sites with a preferential stacking direction for perylene diimide cores of PTCDIC 8 molecules, achieving high resolution patterning and orientation control of the crystalline films simultaneously. It has been reported that the molecular packing along the ππ orientation is more favorable for charge transport. [48,49] Therefore, the method we report here should have potential application to improve electrical performance of electronic devices.

Results and Discussion
The molecular structure of PTCDIC 8 is shown in the inset of Figure 1a, which is a typical ntype organic semiconductor with electron mobility over 1 cm 2 V −1 s −1 . [50] Figure 1a shows a 40 µm × 40 µm AFM image of 30 nm PTCDIC 8 film grown on thermal oxidized SiO 2 surface, with optimized deposition parameters of substrate temperature of 170 °C and deposi tion rate of 1 nm min −1 , respectively. The long alkyl chain substituents on both sides of perylene diimide core make the molecules more flexible, which may promote orderly arrange ment of molecules on a hightemperature substrate, thereby forming layered films. [51] Figure 1a further reveals randomly oriented domains surrounded by cracks as domain boundaries. The cracks originate from the thermal mismatch between the organic film and the Si/SiO 2 substrate when the sample was cooled down to room temperature. [43,52] A close look at the surface, shown in the 4 µm × 4 µm AFM image in Figure 1b, demonstrates that each domain consists of layered stripes with a single orientation which is marked by the white two way arrows. The layer step is measured to be 2.11 ± 0.07 nm in height, which is consistent with that of 2.07 nm calculated by Bragg equation of (001) dspacing from the Xray diffraction (XRD) pattern ( Figure S1, Supporting Information), suggesting upright standing of the molecule on SiO 2 substrate. Figure 1c shows the molecular resolution AFM image of the film, which allows to propose a reasonable packing of PTCDIC 8 molecules. The unit cell parameters are measured to be a = 0.48 ± 0.02 nm, b = 0.87 ± 0.04 nm, and α = 82° ± 2°, respectively, from the fast Fourier transform of images ( Figure S2, Supporting Informa tion). The unit cell of the crystalline films is consistent with that of (001) plane cutting from the PTCDIC 8 single crystal (the top view and side view shown in Figure 1d), where the unit cell parameters are a′ = 0.47 nm, b′ = 0.85 nm and α′ = 82.8°, respectively. [53] To further investigate the molecular packing in the crystal line film, topography and phase images of the 30 nm PTCDIC 8 film grown on SiO 2 were measured by AFM. As shown in Figure 2a, the image containing three domains with different orientations is exhibited, where three double head arrows are used to guide the eyes to distinguish the orientation of the stripes. The zoom in topography image of the area marked by white dashed square is shown in Figure 2b. The dashed white curve in Figure 2b indicates a grain boundary between two crystallographic domains of PTCDIC 8 , while an obviously lay ered structure can be observed on both sides. Figure 2c is the corresponding phase image of Figure 2b, where the molecular packing can be roughly identified, and the aaxis direction of the two domains are marked by white double head arrows. The marked direction is parallel to the corresponding orientation of www.advmatinterfaces.de the stripes indicated by the arrows in Figure 2a. Furthermore, we also studied the molecular packing in different layers. The highresolution AFM images in Figure 2d,e show that the molecular packing is consistent on both sides of the step. In this case, after imaging different samples for many regions, we confirm that the orientation of the stripe reflects the molecular packing direction in the domain, which is parallel to the aaxis of the crystal.
When deposited onto the amorphous SiO 2 substrate surface, the PTCDIC 8 molecules involve absorption, diffusion, desorp tion/nucleation process during the layered film growth. [42] The nucleation refers to aggregation of molecules at a random posi tion over a critical size and evolving to relatively stable clus ters. [39] Further deposition of molecules leads to the anisotropic lateral growth owing to the directional π-π stacking of perylene diimide cores. It is widely observed that molecules can diffuse on crystalline substrate surfaces and nucleate at atomic step edges or kinks in a specific configuration. [41,43,54] This gives us a hint that the molecular packing might be artificially manipu lated microscopically by controlling the molecular alignment during the initial nucleation process. Similar to the atomic step edges, an Au line array with width of 0.5 µm, height of 10 nm, and periodicity of 2 µm on SiO 2 substrate was fabricated as template by standard electron beam lithography (EBL) to induce an orientated growth of PTCDIC 8 stripes. As a result, shown in the AFM image of Figure 3, the topological morphology of the sample with 30 nm PTCDIC 8 demonstrates a wellcontrolled orientation of patterned film induced by the Au line array template, where the dark regions are Au, and the bright ones are PTCDIC 8 layered films. All stripes have similar orienta tion indicated by the red arrow with an angle of about 34° to the dashed black lines that perpendicular to Au stripes. Con sidering the relationship of the molecular packing and stripe orientation revealed by molecular resolution AFM in Figure 2, the similar orientated PTCDIC 8 stripes in the channel indi cate that the presence of Au lines could induce a preferential stacking direction of perylene diimide cores.
In order to analyze the orientation distribution of the domains obtained on bare and the Au lines patterned SiO 2 substrates comprehensively, PTCDIC 8 films in largearea were prepared and then characterized by scanning electron micro scope (SEM), where a large number of crystal domains can be counted for statistical results. Figure 4a,b shows the high resolution SEM images of PTCDIC 8 films on the bare and the Au lines patterned SiO 2 surface, respectively. The detailed stripe orientation of those domains can be clearly distinguished. To define the orientation, a white dash arrow that denoting the [010] direction on the silicon substrate is marked as the refer ence line. The Au lines are intentionally set to be perpendic ular to Si [010] during the EBL process, as shown in Figure 4b. With the marked reference lines, an orientation angle ϕ of the stripe to Si [010] can be measured, as those ϕ 1 and ϕ 2 labeled in Figure 4a,b, respectively. Totally more than 1000 domains are calculated on bare SiO 2 surface, which the position is taken from every 100 µm on the same surface by moving of SEM sample holder stage, as shown in Figure S3 in the Sup porting Information. In the same way, the stripe orientation is also measured in the Au lines patterned area with more than 1000 stripes. Figure 4c shows the statistical diagram of the ori entation distribution of the domains on bare SiO 2 , where the probability of these stripe orientation in each 5° is roughly uniformly distributed between 2% and 4%. As expected, the even distribution of the orientation confirms the randomly ori entated domain on bare SiO 2 owing to the isotropicity of the surface. As a comparison shown in Figure 4d, the orientation distribution statistics on patterned area gives two obvious maxi mums, peaked at −41° and 35° in Gauss fitting curve and dem onstrating a preferential orientation control over the patterned films. We attribute the two preferential orientations to the sym metric effect of Au lines and will take the −41° as the example for the further discussion.
To microscopically explore the molecular packing of PTCDI C 8 deposited to Au patterned SiO 2 surfaces, we viewed the self assembly of molecules on Au single crystalline surface by STM. First, a submonolayer of PTCDIC 8 molecules was deposited on Au (111) surface at 170 °C in ultrahigh vacuum system and the obtained highresolution STM image is shown in Figure 5a. The molecules arrange in a stripeshaped selfassembly struc ture on the Au (111) surface, where two adjacent bright spots in the molecular stripe correspond to a PTCDIC 8 molecule. The unit cell of the structure is measured to be a 1 = 1.73 ± 0.04 nm, b 1 = 2.03 ± 0.03, and α 1 = 93° ± 1° respectively, indicating a lay down configuration of molecules with the perylene diimide cores parallel to Au surface and the alkyl chains positioned upward. [43,55] Subsequent deposition of PTCDIC 8 molecules leads to the formation of new layer with a different structure, as the STM image shown in Figure S4 in the Supporting Infor mation. The layer step height is 1.99 ± 0.10 nm (see Figure S4, Supporting Information), and the unit cell of the structure is measured to be a 2 = 0.49 ± 0.01 nm, b 2 = 0.84 ± 0.01 and www.advmatinterfaces.de α 2 = 82° ± 2° respectively, providing an edgeon configuration which is consistent with the results of films grown on SiO 2 . [43] The layer dependent assembly structure of the PTCDIC 8 molecules on Au (111) surface reveals the existence of a wetting layer owing to the strong metal bonds to molecules. With the wetting layer that the alkyl chains of the molecules positioned upwards (the possible top and front views schematically shown in Figure 5b), the Au surface is passivated to screen the strong metal bonds of Au. The following deposition of PTCDIC 8 con tinues to grow with the edgeon configuration, same to that on the SiO 2 surface. Starting from the second layer, the molecules stand obliquely on the first layer in the same arrangement as in bulk single crystal. [56,57] As a result, we speculate the area selective growth and orienta tion control of PTCDIC 8 crystalline films on Au lines patterned SiO 2 as following: firstly, when a low quantity of PTCDIC 8 is deposited on the Au patterned surface, the molecules diffuse on the surface and are selectively adsorbed to Au. Owing to the disordered state of thermal deposited Au on SiO 2 , [58] the per ylene diimide cores of the molecules will lay on the Au surface randomly. An increase of the deposited molecules on surface leads to dense packed wetting layer on Au surface, resulting in a passivated surface, similar to molecule assembled on Au (111) surface. Meanwhile, the molecules in the passivation layer at the side wall of Au lines (step edges), with perylene diimide cores randomly oriented, provide new nucleation centers for the subsequently deposited molecules (see Figure 5c). Among them, the specific molecules with orientation matching the crystallography of PTCDIC 8 grown on SiO 2 are expected to provide highest nucleation probability due to the overlap of the molecular π-π orbitals. This π-π stacking further leads to the directional growth of molecules upon deposited, resulting in the orientation control of the crystalline stripes. Based on the preferential orientation of −41° from experimental statistics, the aaxis and baxis of the crystalline stripe are illustrated by the red parallelogram in Figure 5d. Obviously, the direction of perylene diimide core of PTCDIC 8 molecule in the crystalline stripes is not fully parallel to the one in the wetting layer as indicated by the right white arrow in Figure 5d. The tilt is estimated to be about 10° and attributed to weakening molecule interaction with upward positioning of alkyl chains, similar to other molecules with π-π interaction on metal surfaces. [59,60] The orientation maximum deviation in symmetry and broad angle distribution in experiments could be attributed to relative roughness of the EBL patterned Au lines (estimated in nanometers by EBL tech nique). [61] In comparison to the atomic and molecular scale in Å, this nanometer scale roughness leads to a direction fluctuation of perylene diimide core plane which absorbed to Au step edges, thus resulting in a broad distribution of stripe orientation. We believe the orientation controlling of PTCDIC 8 packing can be further improved through optimizing the flatness of the Au steps or employing microcrystal facet. [62] In summary, using Au patterned SiO 2 substrate, we dem onstrate that the highresolution patterning and orientation control of crystalline films can be simultaneously realized by modulating the diffusion and absorption of PTCDIC 8 mole cules on the Au lines patterned surface. The step edges of the Au pattern play a crucial role in the nucleation and stacking of molecules, which further determine the orientation of the crys talline films through π-π stacking mechanism. In principle, it

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is also feasible for other πconjugated organic molecules, which prefer to passivate the Au surface by forming a selfassembled monolayer on the surface in a layflat configuration. [62][63][64] This finding provides a new strategy to prepare single crystallike films to achieve organic electronic devices with small device to device and batch to batch performance variation.

Experimental Section
Materials: PTCDI-C 8 was purchased from Sigma-Aldrich and used without further purification. Si (100) substrates with 300 nm thermally grown oxide layer were purchased from silicon materials. The metal materials of chromium and gold were purchased from the ChemPur company with a purity of 99.9%. Poly(methyl methacrylate) (PMMA) was purchased from Allresist GmbH. Commercially available acetone, ethanol, and deionized water were obtained from Sigma-Aldrich and used as received unless otherwise stated.
Preparation of Line-Shaped Au Patterns: For e-beam lithography (EBL), a layer of PMMA with a thickness of about 300 nm was spin coated on the surface of clean SiO 2 . The line-shaped Au patterns with a width of 0.5 µm and periodicity of 2 µm was engraved on the PMMA layer, controlled by the pattern generator by selecting the appropriate electron beam exposure. Next, the substrates with the exposure pattern were developed in a developer. After that, 2 nm Cr and 10 nm Au were deposited on top by the thermal evaporation system. The samples were soaked in acetone for 20 min to remove the PMMA resist. Acetone, ethanol, and water were used to clean the resist for 30 min, respectively.
Deposition of Organic Films: The substrates with Au patterns were brought into a homemade vacuum chamber under a pressure of 5 × 10 −4 Pa for the deposition of organic molecules. The PTCDI-C 8 molecules were placed in a quartz crucible, and heated to 280 °C for thermal evaporation. The temperature of the substrate was measured using thermocouples and a temperature of 170 °C was used in this study and the deposition rate is controlled at 1 nm min −1 . During the deposition process, the film thickness was monitored by a quartz microbalance mounted adjacent to the sample. After deposition of PTCDI-C 8 molecules, the samples were cooled down to room temperature and taken out of the vacuum chamber for characterizations.
Characterization of Organic Films: Atomic force microscopy (AFM) measurements for morphology were performed in air by a Multimode Nanoscope IIIa instrument (Digital Instrument) in tapping mode with n + -silicon tip. (Type: PPP-NCH-W, resistivity: 0.01-0.02 Ωcm, thickness: 4 ± 1 µm/length: 125 ± 10 µm/width: 30 ± 7.5 µm, resonance frequency: 204-497 kHz, force constant: 10-130 N m −1 , tip height: 10-15 µm). The AFM images were analyzed by WSxM software. The overall topography of the samples was characterized by Zeiss Crossbeam XB1540 SEM. XRD c) 3D schematic diagram of PTCDI-C 8 adsorption on Au step-edge and subsequent growth of crystalline films on SiO 2 surface. d) Top view of the schematic illustration of step-induced oriented growth of PTCDI-C 8 molecules on SiO 2 surface with a −41° angle between the a-axis and the horizontal line. The unit cell is marked by red parallelograms; the white arrows represent the planar orientation of perylene diimide core of the first-and secondlayer molecules and the deflection angle between them is highlighted with a red circle. Hydrogen atoms of the molecules are omitted in (c) and (d).

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.