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Directed self-assembled crystalline oligomer domains on graphene and graphite

This content has been downloaded from IOPscience. Please scroll down to see the full text. 2014 Nanotechnology 25 035602 (http://iopscience.iop.org/0957-4484/25/3/035602) View the table of contents for this issue, or go to the journal homepage for more

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IOP PUBLISHING

NANOTECHNOLOGY

Nanotechnology 25 (2014) 035602 (8pp)

doi:10.1088/0957-4484/25/3/035602

Directed self-assembled crystalline oligomer domains on graphene and graphite Frank Balzer1 , Henrik H Henrichsen2 , Mikkel B Klarskov2 , ¨ Timothy J Booth2 , Rong Sun1,4 , Jurgen Parisi3 , Manuela Schiek3 and 2 Peter Bøggild 1

Mads Clausen Institute, University of Southern Denmark, DK-6400 Sønderborg, Denmark CNG Center for Nanostructured Graphene, Department of Micro- and Nanotechnology, Technical University of Denmark, Building 345 East, DK-2800 Kongens Lyngby, Denmark 3 Energy and Semiconductor Research Laboratory, Institute of Physics, University of Oldenburg, D-26111 Oldenburg, Germany 2

E-mail: [email protected]

Received 18 July 2013, in final form 8 October 2013 Published 20 December 2013 Online at stacks.iop.org/Nano/25/035602 Abstract We observe the formation of thin films of fibre-like aggregates from the prototypical organic semiconductor molecule para-hexaphenylene (p-6P) on graphite thin flakes and on monolayer graphene. Using atomic force microscopy, scanning electron microscopy, x-ray diffraction, polarized fluorescence microscopy, and bireflectance microscopy, the molecular orientations on the surface are deduced and correlated to both the morphology as well as to the high-symmetry directions of the graphitic surface: the molecules align with their long axis at ±11◦ with respect to a high-symmetry direction. The results show that the graphene surface can be used as a growth substrate to direct the self-assembly of organic molecular thin films and nanofibres, both with and without lithographical processing. (Some figures may appear in colour only in the online journal)

1. Introduction

also technologically [12, 13]. Unlike many other growth substrates, graphene is crystalline, transparent, flat, chemically stable, and electrically conducting. This gives it a prospect of being used as combined growth substrate and electrode material in a device. A more immediate use is in research of organic molecular crystals, due to the unique electron transparency of graphene. Furthermore, it could become a platform for as-grown transmission electron microscopy investigations of oligomer crystals; see figure 1. It has already been observed that para-hexaphenylene (p-6P), vacuum deposited on graphite and graphene substrates at room temperature, forms textured domains of parallel aligned fibre-like structures [14]. Here, more systematic investigations on the optical and morphological properties are presented, and it is shown that the tendency of directing the self-assembly is strong enough to determine the structure and orientation of domains in films as thick as a few ten nanometres. Atomic

Small π -conjugated organic semiconductor molecules have witnessed a strong increase of interest during recent years, especially in the field of organic electronics [1] and organic photovoltaics [2]. Organic nanowires and nanofibres grown from conjugated molecules such as the para-phenylenes [3], hexathiapentacene [4], and thiophene–phenylene co-oligomers [5]—in solution [6], in pores [7], and on surfaces [8]—are increasingly used in prototypical devices. Organic logic elements [6], field-effect transistors [9], and photonic devices [10, 11] have already been realized. Graphene as a growth substrate for organic crystals has interesting perspectives within research but 4 Present address: Solid State Physics, Lund University, SE-221 00 Lund,

Sweden. 0957-4484/14/035602+08$33.00

1

c 2014 IOP Publishing Ltd Printed in the UK

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PL-B873-CU). Bireflectance of the samples was observed by the same microscope, in which the sample was illuminated with polarized white light in an episcopic configuration, and with a Leitz DMRME polarization microscope equipped with a Tucsen IS130 CMOS camera. Surface morphologies were imaged with atomic force microscopes (JPK NanoWizard and Veeco CPII) in intermittent contact mode (BudgetSensors Tap 300: force constant 40 N m−1 , tip radius smaller than 10 nm), and with scanning electron microscopes (Hitachi S-4800 and FEI Helios Nanolab 600). The thicknesses of the graphite/graphene flakes (1-, 2-, 3-layer graphene or multilayer graphite) were estimated from optical microscope images of the flakes before p-6P deposition [20, 21]. X-ray diffraction (XRD) was performed with a PANalytical X‘PertPro MPD diffractometer in Bragg–Brentano geometry ˚ Cu Kα). The sample was mounted in a sample (λ = 1.542 A, spinner to eliminate preferred orientation effects.

Figure 1. A 1 keV SEM image of a 10 nm p-6P film deposited onto a suspended single-layer graphene flake supported by a Quantifoil holey carbon grid [15]. The p-6P has clearly formed nanoscopic fibre-like aggregates, visible on the left-hand side of the sample (sample temperature during deposition 353 K), and there is no clear change in nanofibre morphology from the supported to the unsupported part of the flake. Reproduced from [14].

3. Results and discussion force microscopy (AFM) and scanning electron microscopy (SEM) together with polarized-light microscopy (PLM) are used to determine the relative orientation of molecules and molecular structures with respect to individual flakes of graphene and graphite.

The grown organic thin films showed blue fluorescence after excitation with UV light, regardless of whether they condensed on SiO2 or on graphite/graphene. From some exfoliated flakes the fluorescence was polarized, and these flakes were also bireflectant and showed pleochroism [22]. In figure 2, overview optical microscope images of two p-6P/graphite/SiO2 /Si samples are presented, each stitched together from 40 single microscope images [23]. In (a), the sample has not been treated after exfoliation, whereas (b) has had ZEP resist applied, baked, and removed to simulate a lithographic process. The images were taken under polarized white-light episcopic illumination, and were observed through a second, crossed linear polarizer. On a blue background, various graphite/graphene flakes are visible with sizes between a few micrometres and up to a few hundred micrometres; two characteristic flakes are marked by arrows. Distinct differences between the two samples are revealed by their bireflectance. On SiO2 /Si, p-6P forms a fluorescing film, which only gives rise to little signal for the bireflectance measurements. This is expected, since the amorphous structure of SiO2 does not support the formation of domains of mutually parallel oriented lying molecules. On about 70–80% of the graphene/graphite flakes from the simulated lithographically processed sample the bireflectance is even smaller—the flakes appear black. For the non-processed samples in figure 2(a), 80–90% of the flakes show strong bireflectance, resulting in bright colours in the microscope image. Obviously the film formation is quite sensitive to the initial sample preparation, where in this case the chemical ZEP resist treatment effectively prevents ordered p-6P domains from forming. This is confirmed by high-resolution imaging such as AFM and SEM; see below. Similar results have been obtained by others for the growth of pentacene on graphene with and without PMMA residues [12]. However, previous experiments do show that ordered p-6P domains can form on lithographically processed flakes [14], too, which obviously is an advantage for technological uses.

2. Experimental details Highly doped Si(100) wafers with 90 nm dry thermally grown SiO2 served as carrier substrates. The wafers were marked with a coordinate system defined by standard optical lithography, in order to ease the relocation of found flakes. After a N2 /O2 plasma cleaning process, pieces of natural graphite were exfoliated onto the wafers using tape (Nitto Denko Corporation) to produce randomly dispersed flakes of graphite and graphene [16, 17]. Some of the few-layer graphene flakes on the wafers were then patterned with electron-beam lithography, as described previously [18]. This involved applying ZEP520A electron-beam resist on the wafer, which was removed in Anisol for 45 min, followed by 135 min baking on a 300 ◦ C hotplate. After processing the graphene and graphite flakes on the SiO2 /Si, they were transferred into a vacuum chamber with a base pressure of 1 × 10−8 mbar. Without further cleaning, an 80 nm thick p-6P film (TCI America) was ˚ s−1 , deposited from an effusion cell at a rate of 0.1–0.2 A monitored by a water-cooled quartz microbalance (Inficon XTC/2). During deposition, the substrate was kept at room temperature, unless otherwise noted. Details of the deposition set-up have been described elsewhere [19]. The optical and morphological characterizations of the grown organic films were carried out ex situ. Emitted polarized fluorescence after excitation with a high-pressure Hg lamp (excitation wavelength λexc ≈ 365 nm) was observed with a Nikon Eclipse ME-600 epifluorescence microscope. A computer-controlled rotational stage (Thorlabs PRM1Z8) rotated the sample, and the fluorescence was recorded through a fixed linear polarizer with a digital camera (PixeLINK 2

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Figure 2. Polarization microscope images (white-light reflectance, crossed polarizers) of two SiO2 /Si samples covered with graphite and graphene flakes, on which 80 nm p-6P has been deposited. The sample in (a) has not been treated after exfoliation while the sample in (b) has had resist deposited, baked, and removed. Two flakes are marked by yellow arrows. Note that the slight raster structure comes from stitching of 40 single microscope images. In the XRD spectrum (c), the vertical red solid lines denote reflections from upright standing p-6P molecules (β-phase and γ -phase), the dot–dashed blue lines reflections from lying p-6P molecules, and the dashed green lines reflections from the substrates [24, 25].

Figure 3. (a) Optical microscope image of part of a graphite flake (‘g’) on SiO2 /Si (‘s’) before p-6P deposition. After p-6P deposition a bireflectance (polarizer directions EW and NS) (b) and a fluorescence (c) image of the same area show different growth modes. The dotted yellow lines trace the edge of the flake. SEM images of p-6P deposited on graphite flakes and on SiO2 /Si in (d) and (e) reveal the morphological differences between bireflectant (e) and non-bireflectant p-6P (d).

X-ray diffraction shows the growth of upright molecules in the γ -structure and in the β-structure [26], as well as ¯ (2 1 1), ¯ the condensation of lying molecules with the (1 1 1), ¯ and (3 1 2) ¯ faces and—to lesser extent—with the (3 0 2) parallel to the substrate; see figure 2(c). The lying orientations have been found for, for example, fibre growth of p-6P on muscovite mica and KCl [27, 28]. The occurrence of the γ -phase and the β-phase is typical for growth on SiO2 at room temperature [29]. In figure 3, the difference in growth is made more obvious by showing an optical microscope image of a graphite flake before organic material has been deposited, (a), and the corresponding bireflectance image after p-6P deposition (b). On some parts of the flake bireflectant domains are visible, but other parts of the same flake appear just dark, independent

of the rotation of the sample in between the polarizers. The corresponding unpolarized fluorescence microscope image (c) shows that, although minor differences in fluorescence intensity are visible, the difference in bireflectance is not due to a lack of adsorbed organic material but due to a different morphology. A closer inspection by scanning electron microscopy, figures 3(d) and (e), reveals that on bare SiO2 /Si p-6P grows in the form of short (a few tens of nanometres to a few hundred nanometres long) needle-like entities on top of flat islands. These aggregates are growing randomly, forming a rather rough and open layer, which leads to a lower amount of bireflectance. On the part of the graphite flake which does not show bireflectance, the p-6P morphology is very similar to that on the substrate. On the bireflectant 3

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Figure 4. Optical microscope images of a graphite flake before (a) and after ((b), (c)) p-6P deposition. Some single, double, and triple graphene layers are denoted by yellow numbers. To visualize the different organic domains, in (b) the sample is observed in between crossed polarizers (EW and NS directions). In (c), the flake is illuminated with UV light, and the blue fluorescence is observed through a linear polarizer (EW direction). The red square in (a) marks the part of the flake which is shown in more detail in figure 5.

Figure 5. SEM (a) and AFM (b) images of p-6P on the same graphene flake (single and triple layer), a detail from the flake presented in figure 4 (red square). Orientational domains as well as the growth mode on bare SiO2 are clearly visible. In (c) and (d), colour-coded spatially resolved distributions of the polarization angle φpol (c) and of the extinction angle φext (d) are shown. The dotted squares depict the area shown in (a) and (b).

part (e), the p-6P morphology is distinctly different: more compact aggregates form. In figure 4, optical microscope images of a single graphene/graphite flake are presented; the flake shows a large degree of bireflectance. The flake consists of several layers, starting with one, two, and three layers, denoted by yellow numbers in (a). Both the bireflectance image in (b) as well as the polarized fluorescence image in (c), taken after p-6P deposition, reveal the formation of domains. Note that different colours in bireflection compared to, for example, figure 3(b) are solely due to a different thickness of the graphite flake and thus stem from varied optical path lengths for light reflected from the graphite/SiO2 and SiO2 /Si interfaces. Here no obvious difference in the overall appearance is observed between single, double, and triple layers, suggesting the same growth mode independent of the number of graphene layers. In general, a smaller tendency for oriented growth on single graphene layers than on multilayers is observed. To investigate the orientations of the textured growth phase, SEM as well as intermittent contact AFM was used to image the samples. Figure 5 shows the texture of different domains and the disordered organic structure on the bare SiO2 substrate. Since the p-6P growth on the bare substrate is more open, here the average height is about 20 nm larger than that for the film on the graphene/graphite flake. Emission characteristics on a micrometre scale were obtained by rotating the sample, either during unpolarized UV illumination or during illumination with linearly polarized

white light. The emitted fluorescence and reflected white light, respectively, were observed by a digital camera through a linear polarizer. The sample was rotated in steps of 1φ = 5◦ over 360◦ with a fixed polarizer position, leading to a stack of N = 73 images. From a series of polarized fluorescence microscope images, the angle for the maximum fluorescence intensity was determined. Beforehand, the rotation and image drifts were corrected by image processing [30, 31]. The discrete Fourier coefficients I˜γ (x, y) were calculated from the intensity x,y variation In of each pixel at position (x, y) via [32, 7] I˜γ (x, y) =

X 1 N−1 I x,y e−i2πγ n/N . N n=0 n

For the polarized fluorescence, the dominant Fourier coefficient is the one with γ = 2, corresponding to a cos2 φ dependence as expected from Malus’ law for the emission of linear polarized light. The polarization angle, and thus the mean molecule orientation within a pixel, is obtained by  φpol (x, y) = 12 arg I˜γ =2 (x, y) . For the bireflectance images, the fourth discrete Fourier coefficient (γ = 4) was obtained, and from this the extinction angle:   π 1 φext (x, y) = arg I˜γ∗ =4 (x, y) + i I˜γ =4 (x, y) + . 2 8 In figure 5, the spatially resolved distributions of the polarization angle φpol (x, y), (c), and of the extinction angle 4

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Figure 6. (a) Distribution of φpol (x, y) (0◦ –180◦ , red solid line) and of φext (x, y) (180◦ –360◦ , blue dashed line) for a multilayer flake. A correlation plot (b) demonstrates the direct relation between φpol and φext . High-symmetry directions deduced from the morphology of the graphite flake are denoted by white horizontal and vertical lines. Absolute values for the angles relate to the orientation of the investigated flake.

φext (x, y), (d), are shown for the same flake. In both cases, patches with well-defined angles are observed. Integral distributions of both angles (φpol : red solid line, φext : blue dashed line) are shown in the polar plot in figure 6(a); maxima are marked by red and blue ticks, respectively, at the outer border. Six maxima for φpol and 12 maxima for φext were found within 180◦ . Within the experimental error of ±3◦ , each maximum in the polarization angle distribution corresponds to a maximum in the extinction angle distribution. This supports the correlation of φpol with the long molecule axis for p-6P [33]. The additional maxima for the extinction angle distribution, which do not agree with a maximum of the polarization angle distribution, correspond to one shifted by 90◦ . This is expected, since the reflected light intensity becomes zero four times within 360◦ , i.e. whenever the fast or slow axis of the crystalline organic film domain is parallel to one of the polarizer directions. This assumption is further supported by the direct correlation between the polarization angle and the extinction angle; see figure 6(b). For each pixel of the image, an angle φpol (x, y) corresponds either directly to the same value in φext (x, y), as demonstrated by the points lying on the diagonal (yellow dashed line), or to an angle shifted by 90◦ . The orientation of the long molecular axis with respect to the substrate is clearly confirmed with both methods. Although it is not possible for the bireflectance analysis to differentiate between the fast and slow axes without an additional retarder plate, bireflectance is of major advantage compared to fluorescence, since the use of white light instead of UV light does not harm the organic thin film [34]. Concluding, from these measurements, the existence of six different molecular domains on the graphite/graphene flakes is deduced. They are grouped in three pairs of two with (23 ± 3)◦ in between, and (60 ± 3)◦ in between the three pairs. To correlate φpol (x, y) and φext (x, y) with the morphology, the local fibre orientation at (x, y), θdomain (x, y), is determined from SEM and AFM images such as shown in figure 7. The

Figure 7. White-light microscope image with a single polarization filter in front of the camera. Coloured domains have their origin in the film’s pleochroism. This graphite flake has been subjected to the described lithographic processing although it was not targeted by the lithography. The shape of the graphite flake is entirely a result of the mechanical cleaving process favouring symmetry directions of the highly oriented graphite. The inserted AFM images illustrate the morphology of the structured domains and on the SiO2 next to the flake.

local molecule orientational angles are defined as βmol,pol (x, y) = φpol (x, y) − θdomain (x, y) and βmol,ext (x, y) = φext (x, y) − θdomain (x, y). These angles, mapped to the range from 0◦ to 180◦ , describe how the long molecular axes are placed within the fibres with respect to their local long axis [35, 36, 7, 33]. Due to the direct correspondence of polarized fluorescence and bireflectance, a distribution of βmol,pol and βmol,ext will give the same result. Such a distribution of βmol,pol (x, y) obtained from a single graphite flake is shown in figure 8(a). Clear maxima at 90◦ ± 14◦ (dashed vertical lines) are observed. The 5

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Figure 8. (a) Distribution of orientation angles βmol,pol of p-6P molecules within the bireflectant domains. Dashed vertical lines depict (90 ± 14)◦ . Directions of the p-6P long axes (thin blue arrows) on the graphite lattice are sketched in (b). In (c), a model for the observed orientations is presented. As in (b), high-symmetry directions of the graphite lattice are depicted by bold orange arrows. General nanofibre orientations are symbolized by blue boxes, and the molecular orientations from (b) by inclined lines within the boxes; the insets illustrate ¯ and (1¯ 1¯ 1) orientations, having mirrored molecule arrangements. Each ‘nanofibre’ represents a single domain orientation. the p-6P (1 1 1)

broadening of the distribution and the error of ±4◦ mainly stems from the finite resolution of the optical microscope set-up. Symmetric values with respect to 90◦ depict fibres with a mirrored molecule arrangement (mirror plane perpendicular to the substrate surface and along the local fibre orientation). Therefore, the long molecule axis for all domains is almost perpendicular (76◦ ± 4◦ ) to the fibre direction. This angle is in good agreement with the 76◦ found for p-6P nanofibres ¯ and (2 1 1) ¯ being the contact on muscovite mica, with (1 1 1) planes [19, 37, 38, 27]; these orientations have also been found by XRD; see figure 2(c). Due to the particular shapes of some graphite flakes— graphene tends to rip along armchair directions [39]— high-symmetry directions are extracted from the optical and AFM images. For the graphite flake in figure 7, the 30◦ separation between the two straight sides indicates that these are oriented in the zigzag and armchair directions. Knowing the characteristic directions of the graphite lattice, the nanofibre-like morphological structures, and the molecular orientation within the structures, their relation can be compared in the plots shown in figures 6 and 8. All the nanofibre orientations are offset by an average of 5◦ to a characteristic graphite lattice direction—at this point it is unknown if this is the armchair or zigzag direction. All the molecular orientations are on average 76◦ relative to their respective nanofibre orientations. This is also obvious in figure 6 from the vertical and horizontal white lines which depict these high-symmetry directions. Thus symmetry reduces the six observed domain orientations to a single domain type. The close line-up with the graphitic lattice orientations indicates epitaxial growth and suggests a growth according to that of p-6P on graphene on Ir(1 1 1) [40]. Using low-energy electron diffraction (LEED), Hlawacek et al have found that p-6P molecules align with their long axes at ±11◦ with respect to the armchair direction of graphene [41]. The value of (11 ± 4)◦ found here is in good agreement with that. Note that for a single fibre direction only a single molecule orientation is realized: the fibre long axis does not serve as a mirror axis. This is due to the combination of preferred

molecule orientations on the surface and the bulk packing of the molecules into fibres [3]. The three high-symmetry directions, however, do serve as mirror axes for the fibres, resulting in six different domains from two different fibre types; see figure 8(c). These results differ from those of an STM study [42], where it has been found that p-6P molecules align their long molecular axis approximately 2◦ off the armchair lattice direction and columns of parallel molecules to be approximately 4◦ off the zigzag orientation (and ±88◦ between the two). This might be explainable by the substantially lower growth temperature of T = 105 K, resulting in a different adsorption structure. It has been reported for the growth of pentacene on graphene that polymer residues from PMMA [12] lead to the formation of upright molecules, whereas on the clean graphene surface the molecules prefer a lying-down state due to an increased interaction of the pentacene π -electrons with the graphene surface. A similar reason might be responsible in this case, where few-monolayer contaminations by, for example, water or hydrocarbons [43] might prevent the condensation of lying molecules.

4. Conclusions We find that thick p-6P layers form nanostructured crystalline domains on graphite and graphene similar to those found on mica substrates and in accordance with prior LEED studies on graphene. Polarized optical microscopy reveals the condensation of two types of fibre on the surface into six domains. White-light bireflectance microscopy can give the same information of molecular orientation as fluorescence microscopy. This is a huge advantage, since UV-bleaching can be avoided. Hence not only can the molecule orientations on the graphitic surface be determined, but it can be easily used to visualize graphene domains on the carrier substrate [44, 45]. Ordered p-6P domains are formed even on lithographically processed flakes, having a great potential for technological uses. 6

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Acknowledgments

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We thank B Laursen, Nano-Science Center, Department of Chemistry, University of Copenhagen, Denmark, and K Thilsing-Hansen and J Kjelstrup-Hansen, SDU, for technical assistance in initial experiments. HH, PB, and FB thank the Danish Research Council for Technology and Production Sciences (FTP) for financial support. PB acknowledges support from The Center for Nanostructured Graphene (CNG) sponsored by the Danish National Research Foundation, DNRF58. HH also thanks the Danish National advanced Technology Foundation (HTF) for financial support. MS thanks the Hanse Wissenschaftskolleg (HWK), Germany, for financial support.

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Directed self-assembled crystalline oligomer domains on graphene and graphite.

We observe the formation of thin films of fibre-like aggregates from the prototypical organic semiconductor molecule para-hexaphenylene (p-6P) on grap...
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