Graphene Interface on Pentacene

Article pubs.acs.org/Langmuir

Effect of Water Layer at the SiO2/Graphene Interface on Pentacene Morphology Manisha Chhikara,† Egon Pavlica,† Aleksandar Matković,‡ Radoš Gajić,‡ and Gvido Bratina*,† †

Laboratory of Organic Matter Physics, University of Nova Gorica, Vipavska 13, SI-5000 Nova Gorica, Slovenia Center for Solid State Physics and New Materials, Institute of Physics, University of Belgrade, Pregrevica 118, 11080 Belgrade, Serbia

‡

ABSTRACT: Atomic force microscopy has been used to examine early stages of pentacene growth on exfoliated single-layer graphene transferred to SiO2 substrates. We have observed 2D growth with mean height of 1.5 ± 0.2 nm on as-transferred graphene. Threedimensional islands of pentacene with an average height of 11 ± 2 nm were observed on graphene that was annealed at 350 °C prior to pentacene growth. Compellingly similar 3D morphology has been observed on graphene transferred onto SiO2 that was treated with hexamethyldisilazane prior to the transfer of graphene. On multilayer graphene we have observed 2D growth, regardless of the treatment of SiO2. We interpret this behavior of pentacene molecules in terms of the influence of the dipolar field that emerges from the water monolayer at the graphene/SiO2 interface on the surface energy of graphene.

I. INTRODUCTION Graphene’s 2D crystal lattice may serve as a template for the growth of organic molecules.1−3 Studying the growth of small organic molecules such as pentacene on graphene is not only interesting for fundamental research but also important for the optimization of graphene-based optoelectronic devices.4 The performance of these devices is significantly affected by the morphology of thin organic semiconductor (OS) layers, which act as charge-transporting layers. One of the most thoroughly studied organic semiconductors in the area of organic electronics is certainly pentacene (C22H14). This is because it was demonstrated relatively early that hole mobilites on the order of several cm2/(V s) can be achieved in pentacene-based organic thin film transistors (OTFTs). 5,6 Also, several experimental reports demonstrate that the principal charge transport in OTFTs occurs through the first monolayer of OS deposited on the gate dielectric.7−9 Because the charge transport is strongly dependent on the state of crystalline order within the OS layer, it is extremely important to know and to be able to control the mechanisms that govern the ordering of OS molecules on the substrate in the early stages of crystal growth. Early stages of pentacene crystal growth have been studied thoroughly on dielectric substrates such as SiO2, metals, and poly(methyl methacrylate) (PMMA).10−13 However, relatively few reports are available that focus on the initial stages of growth of pentacene on graphene and/or graphenerelated materials.14,15 Pentacene growth on relatively thick substrates such as SiO2 is governed by the conditions at a single interface, namely, the one between the substrate and the pentacene islands. The morphology of the layer in an early stage of growth, i.e., in the © 2014 American Chemical Society

phase beyond the nucleation but before the completion of the first molecular layer, is therefore governed solely by the energy balance among the substrate surface energy, the pentacenelayer surface energy, and the energy of interface formation.16 Conditions become less straightforward as graphene is introduced. Once graphene is transferred onto a substrate, a new interface is formed. Because of the single-atom dimensions of graphene, this interface cannot be neglected when considering the energetics of the pentacene/graphene system. Indeed, Zhou et al.17 using scanning tunneling microcopy and density functional theory observed that pentacene morphology on epitaxial graphene is strongly influenced by the underlying Ru(0001) substrate. Graphene grown on a metal such as Ru(0001) exhibits a Moiŕe pattern and represents selective adsorption sites for pentacene molecules. An even more ubiquitous substrate for graphene deposition, SiO2, may represent a challenge in predicting the morphology of pentacene on graphene. Namely, water molecules easily attach to the silanol groups on SiO2 and form a thin water layer.18−20 Shim et al.21 have observed an ∼0.4-nm-thick water layer trapped between graphene and an atomically flat mica substrate using atomic force microscopy (AFM). They suggested that this water layer suppresses the charge transfer between graphene and mica. Lafkioti et al.18 observed that adsorbed molecules from the ambient air such as water can cause graphene doping, resulting in a hysteresis effect and the relatively poor performance of graphene-based devices. Received: July 25, 2014 Revised: September 8, 2014 Published: September 12, 2014 11681

dx.doi.org/10.1021/la502970q | Langmuir 2014, 30, 11681−11688

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Hz. A silicon-etched probe (HQ:NSC15/AlBs) with a rectangular cantilever (325 kHz, 40 N/m) and backside coated with Al were used for imaging. The relative humidity level was 25−30% during the imaging. Statistical analysis of the pentacene islands was carried out using Gwyddion software.27

Important effects on graphene morphology due to the interfacial water layer between mica and graphene were reported by Severin et al.22 The graphene/substrate interface, therefore, expectedly plays a significant role in pentacene layer morphology. Our approach, described in this article, includes an alteration of the graphene/ SiO2 interface in order to remove the water layer from it. To this end we have used two methods: heating the graphene/SiO2 samples prior to pentacene deposition and treating SiO2 substrates with hexaethyldisilazane (HMDS) prior to graphene transfer. The effectiveness of removing the interfacial water layer in graphene/SiO2 systems has been documented in ref 23. However, the self-assembly of HMDS molecules is a known method of rendering the SiO2 surface hydrophobic.18 Our results indicate that the removal of the interfacial water layer results in a 2D−3D transition in the morphology of the pentacene layer deposited on graphene on SiO2. We associate this transition with the absence of an interfacial electric dipole caused by the water layer, which affects the surface energy of graphene and consequently changes the free-energy change upon pentacene/graphene interface formation, resulting in 3D pentacene islands on the graphene surface. The resulting morphology of pentacene is a consequence of competition between intermolecular and substrate−molecule interactions, and morphology in turn is sensitive to the surface, chemical, and structural properties of the underlying substrate.24 Yoshikawa et al.25 observed that pentacene on SiO2, which was treated with HMDS, forms higher islands due to reduced surface energy (43.6 mJ/m2) than on bare SiO2 (61.4 mJ/m2). As a result, pentacene molecules aggregate spontaneously on SiO2 treated with HMDS, and this effect is more pronounced on SiO2 treated with octadecyltrichlorosilane (OTS) where the surface energy reduces to 28.1 mJ/m2. In this work, we have studied the initial stages of growth of pentacene on exfoliated single-layer graphene flakes. Our results show that the pentacene morphology is significantly different on as-transferred graphene and annealed graphene surfaces. Also, a different pentacene morphology than on astransferred graphene is observed when the underlying substrate is treated with HMDS. Hence, we focused on studying the effect of the treatment of the underlying substrate and the annealing impact on the final morphology of pentacene.

III. RESULTS AND DISCUSSION In our experiments we have elected to use the statistical analysis of pentacene islands on graphene as a probe for molecular ordering on the surface. This approach is well documented28−31 and yields valuable information on principal mechanisms that result in a given OS overlayer morphology. In this approach the island size distributions (ISDs) are determined from the AFM images and compared as one or more parameters governing the deposition process of the OS layer in question. With island size we refer to the area of the island, as determined by the software through pixel counting. Typically substrate temperature, coverage, and molecular flux are the growth parameters of choice. Figure 1 shows ISDs and the AFM images of the pentacene islands on as-transferred graphene. Pentacene was evaporated at

Figure 1. AFM images of pentacene islands on single-layer graphene for different coverages of (a) θ = 0.07, (b) θ = 0.17, and (c) θ = 0.37. (d) Island size distribution of pentacene islands for the abovementioned coverages (θ = 0.07 (black line), θ = 0.17 (red line), θ = 0.37 (blue line), and θ = 0.62 (green line)). The mean height of the first-layer pentacene islands is 1.5 ± 0.2 nm at all coverages. The color code in the bar represents the heights of the islands in the images. The false color bar is from 0 (dark) to 10 nm (bright).

II. EXPERIMENTAL SECTION Single-layer graphene (SLG) flakes of about 20 × 15 μm2 in size were exfoliated from KISH graphite and transferred onto Si wafers with a 300-nm-thick SiO2 layer.26 Selected samples were annealed in an Ar atmosphere at 350 °C for 3 h. The height of as-transferred (pristine) graphene flakes on SiO2 was found to be 1 ± 0.1 nm, and the height of graphene after annealing was found to be 0.5 ± 0.05 nm. In addition, graphene flakes were also transferred onto SiO2 treated with HMDS. Prior to transfer, HMDS was spin-coated at 1500 rpm on SiO2 and subsequently held at 120 °C for 1 h. Submonolayer coverages of pentacene layers (Sigma-Aldrich, purity >99.8%) were evaporated on graphene samples in a high-vacuum chamber at a base pressure of 10−8 Torr. Deposition was performed systematically in the range of coverages of 0.07 < θ < 0.65, and pentacene growth was examined at substrate temperatures (Ts) of 29 and 60 °C. Coverage was monitored by an in situ quartz-crystal thickness monitor that was calibrated by ex-situ AFM measurements. The nominal pentacene deposition rate was 1 nm/min. An AFM (Veeco CP-II) operating in noncontact mode (NC mode) was used to characterize the morphology of pentacene layers. The NC mode has allowed imaging without influencing the shape of pentacene islands on the graphene surface due to tip−sample forces. The scan rate was 1

Ts = 29 °C. Figure 1a−c shows the AFM micrographs of pentacene islands on graphene for coverages of θ = 0.07, 0.17, 0.37, and 0.62. In all cases, we observe 2D pentacene islands with an average height of 1.5 ± 0.2 nm, which corresponds to the thin-film phase of pentacene exhibiting an interplanar spacing of 1.54 nm5. The average island area of pentacene islands at θ = 0.07 was found to be 0.09 ± 0.02 μm2. With increasing coverage, the existing islands on the surface start to attract incoming molecules that are attached to the perimeter of the island. The island size grows so that the mean island area becomes 0.3 ± 0.05 μm2 at a coverage of θ = 0.17. At θ = 0.37 (Figure 1c), the mean island area increases to 0.8 ± 0.1 μm2. With coverage increasing to 0.62 (Figure 2a), the island area increases because of the coalescence of islands. In addition to 11682


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was determined by plotting a histogram of island areas. When we plotted the IDSs from Figure 1d scaled according to eq 1 from ref 36, we found that the histograms do not scale. We observe from Figure 1d that the position of the peak shifts toward higher values of the island area with increasing coverage. At the same time, the height of the ISD decreases with increasing coverage. Islands start to merge, and the island density decreases significantly. With increasing coverage to θ = 0.62, the island area increases and the centroid of the ISD curve shifts toward a higher island area. The distribution broadens with coverage. From the absence of dynamic scaling, we can conclude that pentacene on graphene grows far from the diffusion-limited growth regime. To demonstrate the role of substrate temperature on pentacene morphology, we compare in Figure 2 the surface morphology of pentacene evaporated on graphene at Ts = 29 °C (Figure 2a) to that at Ts = 60 °C (Figure 2b). At Ts = 29 °C, the coverage is 0.62, and at Ts = 60 °C, the coverage is 0.68. Figure 2a shows that before the completion of the first monolayer there is significant second-layer growth. Threedimensional islands observed on the top of the first monolayer are 20 nm high. The island density of the first-monolayer islands is 0.38 ± 0.05 μm−2. The mean island area is 1.6 ± 0.02 μm2. With increasing substrate temperature (Ts) to 60 °C, the island density decreases to 3.0 ± 0.01 μm−2 (Figure 2b) because of the coalescence of islands and the mean island area increases to 37 ± 2 μm2. We did not observe any significant growth of the second layer at Ts = 60 °C. In our previous results we have already observed that the island density decreases rapidly with increasing substrate temperature from 10 to 60 °C.37 This behavior can be explained by the model of nucleation and growth of thin layers that takes into account different processes occurring on the substrate surface such as surface diffusion and re-evaporation.38 As the substrate temperature is increased, surface diffusion along the edges of islands is enhanced, resulting in enlarged compact islands and reduced island density.39 The interaction between organic molecules and graphene can be weak or strong. It has been observed previously that pentacene molecules have a lyingdown orientation on epitaxial grown Ru(0001), which is a result of the weak molecule−substrate interaction.17,40 On the contrary, Hong et al.40 using STM investigated that organic molecules such as 7,7,8,8,-tetracyanoquinodimethane and 2,2,6,6,-tetramethyl-1-piperridinyloxy on graphene grown on Ir(111) exhibit strong chemical bonding with their long molecular axis parallel to the graphene surface. In addition to the pentacene−graphene interaction, the pentacene morphology can be influenced by the interaction between graphene and SiO2, which is characterized by weak dispersion-type forces. Sabio et al.19 suggest that the energy of water molecules with the first graphene layer varies with the distance from the interface z as z−3 and is on the order of ∼100 meV/nm2. Graphene flakes exfoliated under ambient conditions can have one or more layers of water molecules between SiO2 and graphene and/or on the graphene layer. The dipolar electric field that originates from water molecules adsorbed at the interface can therefore play a crucial role in the determination of the morphology and electronic properties of graphene. Because of the presence of water, the measured height difference between SiO2 and graphene is 1 nm and is in close agreement with the earlier reports.41,42 At such a distance between graphene and SiO2, dispersive forces are dominant. Therefore, water at the interface can change the interaction

Figure 2. 10 × 10 μm2 AFM images of pentacene islands on singlelayer graphene flakes at substrate temperatures of (a) 29 °C and (b) 60 °C. The mean height of pentacene islands is 1.5 ± 0.2 nm in both cases. The mean island area in panel a is 1.6 ± 0.02 μm2, and the coverage is 0.62. The mean island area in panel b is 37 ± 2 μm2, and the coverage is 0.68. The false color scale is adjusted from 0 (dark) to 10 nm (bright).

the islands on graphene, we can see substantially smaller 2D islands in the SiO2 region of the image. We see that pentacene islands on graphene are also more compact than on SiO2. The compactness of the islands is a result of edge diffusion, a process in which molecules detach from individual islands and diffuse along the edges. This can be a result of the reduced molecule−substrate interaction that can be attributed to the lower surface energy of graphene32 as compared to that of SiO2.13 Bright elongated features in this image are 3D pentacene islands whose average height amounts to 20 nm. The nucleation of the 3D islands on the first monolayer prior to its completion is a signature of a pseudo-Stranski−Krastanov growth mode, which is not observed on SiO2. It indicates that beyond a certain island size, the formation of the graphene/ pentacene interface becomes energetically less favorable, leading to a formation of 3D structures on top of the existing islands. This also implies that the growth of pentacene on graphene cannot be described in the framework of diffusionlimited aggregation (DLA).33,34 Here, incoming molecules undergo random walks before attaching irreversibly to the growing islands. In this regime, highly ramified pentacene islands have been observed on SiO2 and are a result of strong molecule−substrate interaction.31,35 One of the important signatures of DLA is also the dynamic scaling of a characteristic dimension of the system, which scales with the variation of one of the deposition parameters. For example, Ruiz et al.31 report on the dynamic scaling of ISD of pentacene islands with coverage, within the low-coverage regime (θ < 0.5) on SiO2. Pentacene islands on graphene, in our experiments instead, do not follow dynamic scaling. ISDs for coverages of 0.07 (squares), 0.17 (filled circles), 0.37 (open circles), and 0.62 (triangles) are shown in Figure 1d. The pentacene islands were localized by using a threshold algorithm, and the area of the islands was extracted via pixel counting. Subsequently, the ISD 11683


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overgrown layers.44 The number and shape of overgrown 3D islands depends particularly on the film/substrate and the deposition parameters. The driving force for 3D nucleation due to relaxation from the elastic regime competes with the surface energy for the formation of 3D growth. However, at Ts = 60 °C, only 3D growth is observed (Figure 3b) and the 3D islands exhibit an average height of 25 ± 4 nm. Therefore, this change in morphology on annealed graphene compared to that on asprepared graphene can be attributed to the water layer that exists between SLG and SiO2. This may suggest that the removal of water at the interface can change the surface energy of graphene, which can lead to the 3D growth of pentacene islands on annealed graphene. Paek et al.45 have computed the vdW interaction energy and strain energy at the graphene/SiO2 interface using Lennard-Jones potential parameters. They estimated the vdW interaction energy in the range of 0.93 ± 0.07−1.56 ± 0.08 eV/nm2. Also, the strain energy in graphene ranges from 0.25 to 0.36 eV/nm2. The mean distance between graphene and SiO2 at the interface is 0.43 ± 0.02 nm for an interaction energy of 0.93 ± 0.07 eV/nm2. However, the interaction energy also depends on the morphology, surface roughness of the substrate, and bending modulus of graphene.46 In our second approach, we have investigated the morphology of pentacene on graphene that was transferred onto HMDS-treated SiO2, and the results for Ts = 29 °C are illustrated in Figure 4. We observe that pentacene molecules on

between SiO2 and graphene and can influence the pentacene morphology on graphene. To investigate the effect of the interfacial water layer on the morphology of pentacene, we have altered the graphene/SiO2 interface in two ways: first, we annealed graphene/SiO2 at 350 °C for 3 h in Ar, and second, prior to the transfer of graphene, we have treated SiO2 with HMDS. Annealing the graphene sample can help in depleting the water layer. Coupling between graphene and SiO2 is stronger during annealing, and graphene is adhered to the substrate.43 On annealed samples we have systematically observed a reduced height difference between SiO2 and graphene relative to as-transferred graphene. Our AFM measurements indicated a value of 0.5 ± 0.05 nm after annealing, which argues that the majority of water molecules were removed from the interface. Figure 3a shows the

Figure 3. 10 × 10 μm2 AFM images of pentacene islands on graphene annealed at 350 °C for 3 h in an Ar atmosphere. Pentacene was deposited at (a) Ts = 29 °C and (b) Ts = 60 °C. The mean height of the 2D islands is 1.5 ± 0.2 nm, and the mean height of the 3D islands is 20 nm. The mean height of pentacene islands on SiO2 is 1.5 ± 0.2 nm. The height of the islands observed at Ts = 60 °C is 25 nm in panel b. The false color scale is from 0 (dark) to 20 nm (bright).

pentacene morphology on such annealed graphene. At Ts = 29 °C, pentacene arranges in part in 2D islands (darker patches) with a mean area of 1.5 ± 0.03 μm2 that are similar to the islands observed on as-transferred samples (Figure 2a). Significant numbers of pentacene molecules aggregate in 3D islands (that appear as bright elongated features). The average height of the 3D islands on the graphene surface is 11 ± 2 nm, and their average length is 0.8 ± 0.2 μm. Three-dimensional islands are also observed on the first pentacene layer with the same mean height as on graphene. We are therefore witnessing incomplete Stranski−Krastanov growth in the sense that the second 3D layer nucleates before the first is completed. The growth of first monolayer is restricted by the accumulation of strain energy as a result of a lattice misfit. The underlying substrate has a strong influence on the strained first monolayer. The formation of 3D islands reduces the strain energy, which overcompensates for the surface energy, i.e., the contribution from the adhesive forces between the substrate and the

Figure 4. (a) 10 × 10 μm2 AFM image of pentacene islands (white elongated islands) on graphene that was transferred onto an HMDStreated SiO2 substrate temperature was 29 °C. The typical height of 3D islands is 25 nm. The false color scale is adjusted from 0 (dark) to 20 nm (bright). (b) Power spectral density function (PSDF) of pentacene islands of type I lying parallel on a graphene surface in panel a. The length of pentacene islands estimated by the fwhm of PSDF in reciprocal space is 7.6 μm−1. The width of pentacene islands estimated in reciprocal space is 15.6 μm−1. (c) PSDF of pentacene islands of type II on graphene. The length of pentacene islands estimated in reciprocal space on graphene is 10.0 μm−1. The width of pentacene islands estimated in reciprocal space is 13.8 μm−1. 11684


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being 7.57 ± 0.14 μm−1 (indicated by arrow), corresponding to 324 ± 29 nm in real space. The calculation of the width of pentacene islands is less accurate because the PSDF exhibits a relatively constant value over the whole distance. Extrapolating the PSDF with a normal distribution gives a rough estimate of the fwhm as 15.59 ± 0.71 μm−1 in reciprocal space. Using this estimate, we obtain approximately 157 ± 7 nm as the width of pentacene islands. The angle between the abscissa and ordinate is 90° in reciprocal space as indicated in Figure 4b. This implies that pentacene islands are oriented parallel to the x axis of the AFM image. Some of the islands were oriented in a different direction than the majority of the islands as shown in Figure 4a and are indicated by type II. The calculation of PSDF on type II islands was performed in the same way as in the case of type I islands and is illustrated in Figure 4b. The PSDF of these elongated islands is represented in Figure 4c. Contour lines (black dotted lines) represent a 2D Gaussian function. The resulting values of σ1 = 10.03 ± 0.26 μm−1 and σ2 = 13.78 ± 0.50 μm−1 correspond to the average quad dimensions of 244 ± 29 nm in the horizontal direction and 178 ± 29 nm in the vertical direction, respectively. The pentacene islands are oriented 59.5° with respect to the graphene substrate. This value is strikingly similar to the value of the angle between the two directions of the graphene unit cell. We submit that the orientation of the islands on graphene transferred onto an HMDS-treated substrate reflects morphology that is governed by graphene/pentacene interactions, which in turn are a consequence of the underlying molecular ordering of HMDS molecules. To investigate the range of the effect of the graphene/SiO2 interface, we have deposited pentacene onto many-layer graphene (MLG) flakes that were transferred onto SiO2 pretreated with HMDS. As shown in Figure 5a, pentacene forms 2D islands. The island height of 1.5 ± 0.2 nm corresponds to an upright orientation of the pentacene molecules on MLG flakes. The island density is 0.4 ± 0.06 μm−2 at θ = 0.30 in Figure 5a. The mean projected area of the islands is 0.75 ± 0.08 μm2. We note here that the observed height of the islands differs with the height of the pentacene islands on graphite reported in ref 14, which reports a height of 0.22 nm. This value corresponds to a flat-lying orientation of the pentacene molecules, which is observed on weakly interacting substrates. Because the planar orientation of pentacene molecules results in 3D morphology and is frequently a consequence of disorder on the substrate surface, we can speculate that the observed differences in molecular orientation between MLG and graphite reflect an increased density of grain boundaries on graphite, as compared to singledomain MLG. The PSDF of pentacene islands on MLG is shown in Figure 5b and was obtained in the same way as for SLG (Figure 4b). At first glance, these islands are disc-shaped, and their position is random. However, a smoothed PSDF map reveals two characteristic features: (i) The PSDF exhibits straight edges along the directions, which are indicated with dashed lines and originate from the center, and (ii) the highest PSDF contour has a hexagonal shape with a diameter of 7.9 μm−1. The characteristic directions in real space, which correspond to the straight edges of the PSDF map, are also rotated and presented with dashed lines in Figure 5b. The angle between these directions of 54.7° closely resembles the angle of the hexagonal lattice (60°). Indeed, the FFT of a hexagon results in a PSDF of the shape of six symmetric edges with an angle of 60° between

graphene aggregate in 3D islands, with an average height of 25 ± 4 nm (Figure 4a). The mean projected area of these islands is 0.026 μm2. The island density of pentacene islands on the graphene surface was found to be 4.0 ± 0.4 μm−2, and the effective coverage is θ = 0.13. SiO2 is hydrophilic in nature because of hydrogen bonding between silanol group and water molecules. Treatment with HMDS changes the surface energy in comparison to bare SiO2 to 43.6 mJ/m2 as compared to 66 mJ/m2 for bare SiO2 as reported in ref 25. Lafkioti et al.18 suggest that HMDS can displace water molecules, which are attached to the hydrogen of silanol group. Therefore, graphene can be screened from the influence of water molecules. We see that the islands in Figure 4a appear to be aligned along two principal directions (labeled type I and type II). To confirm the preferential alignment and to determine the island dimensions, we have performed a fast Fourier transform (FFT) analysis of the images. Each of the islands was included in a 2D FFT. Prior to calculation, a binary version of the AFM images was obtained where the region corresponding to a selected pentacene island was set to value of 1 and the remaining region was set to zero to calculate the mean background height of the image. Such mapping was then transformed into reciprocal space. FFT decomposes the morphology data (information on position and height of individual points of the image) into its harmonic components, representing spectral frequencies present in the morphology data. A windowing function is used to suppress the data at the edges of the image. Figure 4b,c represents the results of FFT of Figure 4a. The images present the absolute value of the complex Fourier coefficient, which is proportional to the square root of the power spectrum density function (PSDF). In an ideal case, when the islands are positioned on a regular grid and are oriented identically, the PSDF function can be devised as a product of PSDF of a single island and of PSDF of the grid. The fact is a consequence of the theorem that Fourier transform of a convolution of functions is the product of Fourier transforms of the individual functions. The resulting PSDF of ideal islands on the regular grid consists of an regular array of peaks, which correspond to PSDF of the grid. The peak width is infinitesimally small only in the case in which an infinitely large area is used to calculate the PSDF. The amplitude of each peak is determined by the value of the PSDF of a single island at the position of the peak. Therefore, PSDF of a single island acts as an “envelope” for the final PSDF. To estimate the envelope of PSDF of Figure 4a, we used a Nadaraya−Watson kernel regression estimate to smooth out the PSDF.47 Using this method, we were able to remove nonuniform individual peaks, which correspond to the PSDF of a grid of irregular island positions. The resulting PSDF function (Figure 4b-c) is highly anisotropic, reflecting the shape of pentacene islands. Figure 4b represents the PSDF for the majority of islands, which are elongated parallel on the graphene flake as shown in Figure 4a. They are indicated as type I in Figure 4a. PSDF shown in Figure 4b can therefore be considered to be a Fourier transform of an “average” single pentacene island. PSDF is elongated in the vertical direction and rapidly decreases in the horizontal direction. The vertical (horizontal) axis in reciprocal space corresponds to the horizontal (vertical) axis in real space. Contour lines (black dotted) correspond to a Gaussian function fitted with four parameters: amplitude, angle of rotation of axes, width in the main direction (σ1), and width in the perpendicular direction (σ2). From Figure 4b, we estimate by σ2 the length of type I pentacene islands in reciprocal space 11685


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Figure 6. 10 × 10 μm2 AFM micrographs of pentacene on (a) singlelayer graphene and (b) bilayer graphene and (c) many layer graphene. Pentacene was evaporated at a substrate temperature of 29 °C. The height of the first layer of pentacene islands is 1.5 ± 0.2 nm in panels a−c. The color scale is from 0 (dark) to 10 nm (bright). (d) Island size distribution of pentacene islands on single-layer (black squares and line), bilayer (red circles and line), and many-layer (blue triangles and line) graphene.

Figure 5. (a) A 10 × 10 μm2 AFM image of pentacene islands on many-layer graphene. The angle represents the direction of the edges of pentacene islands on graphene. The color scale is from 0 (dark) to 20 nm (bright). (b) Map of the power spectral density function (PSDF) of pentacene islands on many-layer graphene.

of differences in surface energy among the three types of graphene. We will consider the values of surface energy for graphene (46.7 mJ/m2) and graphite (54.8 mJ/m2)32 as guidelines for the values of surface energy of SLG and MLG, respectively. The interfacial energy of the pentacene/SLG interface is therefore lower than the interfacial energy of the pentacene/MLG interface. It is therefore energetically less consuming to form large interfacial areas between pentacene and SLG than between pentacene and MLG, which leads to large 2D pentacene islands on SLG. We already reported that reduced surface energy on SLG relative to that on BLG results in smaller initial stable clusters than on BLG.37 Lower interfacial energy leads to smaller stable clusters that lead to a broader ISD.47

them. In addition, PSDF of randomly positioned hexagons in an ideal case exhibits a broadened six-star shape that disappears when hexagons are randomly rotated. Therefore, the shape of the PSDF of pentacene on MLG indicates that the edges of pentacene islands prefer common hexagonal directions. From these results we can conclude that the “natural” morphology of pentacene on MLG is 2D island growth, at least in the submonolayer coverage range explored. This type of morphology seems to be immune to the presence or absence of the water-induced interface dipole. This argues for the relatively short range of the effect of the dipole on the kinetics of the pentacene molecules on the surface as suggested in ref 19. Interestingly, as the distance from the graphene surface to the interface is reduced to the nanometer range, the effect of the interface dipole is in an increase in the surface mobility of molecules leading to 2D growth. The absence of this additional stimulus results in 3D growth as shown in Figures 3 and 4. These findings are summarized in Figure 6, where we compare the morphologies (Figure 6a−c) and ISDs (Figure 6d) for pentacene deposited on SLG, BLG, and MLG. All three types of graphene were transferred onto pristine SiO2, and pentacene was evaporated at Ts = 29 °C on all three substrates. Island densities are 0.4 ± 0.07, 0.6 ± 0.07, and 1.3 ±0.12 μm−2 on SLG, BLG, and MLG, respectively. Mean island areas were found to be 1 ± 0.1, 0.6 ± 0.07, and 0.2 ± 0.08 μm2 on SLG, BLG, and MLG, respectively. ISDs for the three types of substrates (SLG, squares; BLG, circles; and MLG, triangles) demonstrate an interesting trend. As we move away from the interface (in the direction SLG → BLG → MLG), the ISDs become narrower and the mean island size moves toward smaller values. The pentacene surface mobility therefore decreases as the graphene-surface/interface distance is increased. We interpret the observed trend in terms

IV. CONCLUSIONS We have studied the evolution of the 2D morphology of pentacene islands on graphene in the coverage range of 0.07 < θ < 0.65 at a substrate temperature of 29 °C. The island size distribution broadens with the coverage, and its centroid moves toward the larger island size. Subsequently, we have investigated the morphology of pentacene islands on pristine SLG samples and graphene annealed at 350 °C in Ar by AFM. Two-dimensional growth is observed on as-prepared graphene, whereas annealed graphene favors mostly 3D growth at a substrate temperature of 29 °C and complete 3D growth at a substrate temperature of 60 °C. The difference in pentacene morphology on an annealed graphene sample is likely to be determined by the depletion of the water layer at the SiO2/ graphene interface. In addition, pentacene on graphene transferred onto HMDS-treated SiO2 also exhibits 3D morphology. However, pentacene on many graphene layers transferred onto HMDS-treated SiO2 exhibits 2D morphology as a result of the change in surface energy of interfacial layers with respect to single-layer graphene. The power spectral density of pentacene islands shows that pentacene islands favor hexagonal directions on many-layer graphene. Moreover, the 11686


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ISD of pentacene islands is broader on SLG than on BLG and MLG and is a result of reduced surface energy.

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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 This work was funded in part by the Slovenian Research Agency, program P1-0055 and EMONA. A.M. and R.G. acknowledge support from the Serbian Ministry of Science through projects OI 171005 and III 45018.

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REFERENCES

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