J. Phys. Chem. C 2007, 111, 16489-16497
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Epitaxial Growth of Large Pentacene Crystals on Si(001) Surfaces Functionalized with Molecular Monolayers Kevin P. Weidkamp,† Rudolf M. Tromp,‡ and Robert J. Hamers*,† UniVersity of WisconsinsMadison, Department of Chemistry, 1101 UniVersity AVenue, Madison, Wisconsin 53706, and T.J. Watson Research Center, IBM Research DiVision, Yorktown Heights, New York 10598 ReceiVed: June 13, 2007; In Final Form: August 11, 2007
The nucleation and growth of pentacene thin films are controlled largely by the energies associated with the interfaces. We have used low-energy electron microscopy (LEEM) and photoemission electron microscopy (PEEM) to investigate the nucleation and growth of pentacene thin films on Si(001) surfaces modified with two different molecular monolayers. Clean Si(001)-(2 × 1) surfaces were modified with either 1,5cyclooctadiene or 1-dodecene prior to pentacene growth to study the effects of exposed π bonds at the interface, orientation of those π bonds relative to each other, and rigidity of the molecular layer on pentacene nucleation, growth, and crystalline orientation. Both molecular monolayers weaken the substrate-pentacene interaction sufficiently to allow for low pentacene nucleation density and good pentacene diffusion, leading to the growth of pentacene grains as large as 100 µm. Pentacene grows epitaxially on both functionalized surfaces, adopting an orthorhombic unit cell that follows the orientation of the underlying Si surface reconstruction. Our results show that in addition to improving the ultimate size of pentacene crystals, molecular monolayers are able to impose the substrate orientation on pentacene nuclei and thereby control the crystalline orientation of the thin film.
1. Introduction Organic semiconducting thin films have generated much interest recently with the emergence of organic and molecular electronics.1-3 Organic-inorganic hybrid and completely organic devices that use organic thin film transistors (OTFTs) are poised to enable new advances in electronic applications where mechanical flexibility, low-temperature fabrication, and lowcost manufacturing are desired. These applications include flexible electronic displays, radio frequency identification tags, and other electronics with lower requirements for switching speed. Pentacene has shown a great deal of promise for these applications because of its high mobility, low-temperature deposition, and crystalline growth.4-6 However, the limited crystal quality in films produced by vacuum deposition has resulted in device performance that trails results obtained with single-crystal material. Recent studies have determined several characteristics of pentacene growth on different substrates in an effort to understand what factors lead to optimal film growth and optimal conductivity in the active channel region.7-21 Pentacene deposited on clean Si(001)-(2 × 1) surfaces, for example, shows a chemisorbed first layer that negatively affects the growth process and has undesirable electronic and structural properties.7,13,22 Growth and properties of pentacene films have also been studied on SiO2,12,16,18,23 metals,9,14,15,19-21,24 polymers,5,8,10,25 and chemically modified Si(001) substrates.7,13,26 Pentacene crystal size and molecular orientation on these surfaces varies greatly depending on the electronic and structural * Corresponding author. Tel.: (608) 262-6371; fax: (608) 262-0453; e-mail:
[email protected]. † University of WisconsinsMadison. ‡ T.J. Watson Research Center.
properties of the pentacene-substrate interface.27 Recent studies have shown that chemically modified Si(001) surfaces have greatly improved pentacene growth.7,17,26 In addition to studies of pentacene growth, chemical attachment schemes for many organic molecules28,29 have been studied for the Si(001)-(2 × 1) surface, making it a versatile and well-understood surface both structurally30-32 and chemically. Cycloaddition reactions provide a pathway for complete and well-defined molecular monolayers (ML)28,33-35 on Si(001)-(2 × 1), and they also improve the quality of pentacene films, leading to the growth of large grains.7,17,26 These molecular layers are stable during pentacene deposition and are not altered by the growth.13 Because of the versatility of this type of attachment, it is possible to attach molecules selectively36 and leave different functional groups or parts of molecules exposed at the surface, thereby making it possible to study the growth of pentacene crystals in a variety of different chemical and structural environments. While it is now recognized that functionalization of the Si(001) surface improves the subsequent growth of pentacene, many important questions remain unresolved. In particular, little is known how the chemical and physical properties of the molecular layers, such as exposed π bonds or molecular rigidity, affect subsequent pentacene growth. Here, we report experiments investigating how the molecular structure of the organic overlayer affects the subsequent nucleation and growth of a pentacene film. By combining the information from multiple techniques, including photoemission electron microscopy (PEEM), low-energy electron diffraction (LEED), low-energy electron microscopy (LEEM), ultraviolet photoelectron spectroscopy (UPS), and X-ray photoelectron spectroscopy (XPS), these experiments provide a detailed picture of the chemistry, growth properties, and crystallographic orientation of the pentacene-substrate interfaces. We investigate
10.1021/jp074560x CCC: $37.00 © 2007 American Chemical Society Published on Web 10/16/2007
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whether it is possible to direct the azimuthal growth of pentacene crystals on a macroscopic level or the orientation of molecules within those crystals on a smaller scale by arranging an anisotropic array of exposed π bonds at the interface. By comparing the influence of a rigidly attached cyclic molecule having exposed unsaturated bonds with the influence of a molecule having longer alkyl chains, we also gain insight into how the mechanical flexibility of the molecule affects pentacene growth. The results of these studies provide a broader understanding of the types of molecular interactions that control the nucleation and growth of organic semiconductors at surfaces. 2. Experimental Procedures All LEEM, PEEM, and LEED experiments were performed on an IBM LEEM II microscope that has been described previously.37 Base pressure for this ultrahigh vacuum (UHV) system was 1 × 10-10 Torr. PEEM images were taken by exposing the sample surface to UV light from a Hg discharge lamp while biasing the sample at -15 kV. The contrast observed in the PEEM images results from differences in the density of photoemitted electrons reflecting differences in surface electronic properties. Images were taken either every 10 or 20 s during pentacene growth, using a computer-controlled shutter to limit exposure of the sample to UV light and preventing desorption of pentacene.7 An adjoining chamber was equipped with a He(I) ultraviolet photon source, a hemispherical electron energy analyzer, and a detector (SPECS) for conducting UPS experiments. The analyzer pass energy was set to 1.5 eV for all UPS measurements. XPS experiments were performed on a different UHV instrument to measure C/Si area ratios. This second chamber included a monochromatized Al KR source, a hemispherical analyzer, and a 16-channel detector (Physical Electronics). For these experiments, the source was operated at 350 W, and the pass energy was set to 11.75 eV, corresponding to an analyzer resolution of 0.18 eV. Clean Si(001) surfaces (n-type, As-doped, 0.004 Ω cm) were prepared by first outgassing at 950 K for >1 h and then flash annealing to 1600 K. To study single-domain surfaces with all surface dimers aligned, some intentionally miscut samples (ntype, As-doped, 0.05 Ω cm), miscut by 4 ( 0.25° toward the 〈110〉 direction, were used. This small miscut leads to a surface consisting of small terraces separated by two-atom-high steps where dimer rows of all terraces are parallel.38,39 1,5-Cyclooctadiene (COD, 98+%, Aldrich) and 1-dodecene (95%, Aldrich) were introduced to the system through separate variable leak valves. Both liquids were purified by multiple freeze-pumpthaw cycles, and the purity was verified by in situ mass spectrometry. Exposure was measured in Langmuirs (L ) 1 × 10-6 Torr s) based on the pressure as measured by an ion gauge. Pentacene was evaporated from a quartz crucible within a Knudsen cell after being outgassed at 130 °C for several hours prior to thin film growth. Growth was monitored visually by PEEM with rates and coverages measured from these images using a computer algorithm that determines the percentage of the area that is covered by the pentacene film. Coverages are expressed in ML, where we define 1 ML as the amount of pentacene required to form a complete crystalline layer in which the molecules are standing with the long axis nearly vertical on the surface, slightly canted from normal, as is seen with pentacene growth on weakly interacting substrates.12,24,40 This operational definition is used because coverages defined in this manner can be easily determined from the PEEM images. Because the first layer is not usually complete before subsequent
Figure 1. Schematic images of the attachement of (a) COD and (b) dodecene to surface silicon dimers.
TABLE 1: Area Ratios Taken from C(1s) and Si(2p) Peaks in XPS Spectra for Three Different Organic Terminating Layers: CP, COD, and Dodecenea molecule
C/Si ratio
ratio to CP
CP COD Dodecene
0.11 0.18 0.15
1 1.6 1.4
a Also given is the C/Si ratio normalized to that of CP-terminated Si for comparison of coverage based on number of C atoms in the organic molecule.
layers begin to nucleate, coverages of >1 ML often correspond to situations in which the first layer has yet to fill in completely, but the total amount of pentacene on the surface would be more than enough to fill a single complete layer. 3. Results 3.1. Terminating Layers. Previous work has shown that organic molecules, specifically those with a single degree of unsaturation, form complete and self-terminating MLs on the Si(001) surface.28,34,41 COD is known to react with the two Si surface dimer atoms through a [2 + 2] cycloaddition reaction that leaves one CdC double bond unreacted, as shown in Figure 1a.34,42 Since dodecene has only one functional group, it is expected to react in a manner analogous to other simple alkenes,28,33,43 leading to the product depicted in Figure 1b. The result is a long carbon chain protruding from the surface with a strong Si-C bond at the interface.44 Because the chemical attachment of both of these molecules requires the presence of unreacted dimers at the surface, the reactions are self-terminating, proceeding only until the accessible surface attachment sites are filled. XPS experiments as a function of exposure verify the self-terminating nature and show that the sticking coefficient of alkenes on Si(001) is nearly unity, so that exposures of only a few Langmuirs are needed to form a complete ML. A clean Si(001) surface can therefore act as a versatile template for different types of organic MLs. Table 1 shows the ratios of carbon/silicon peak areas extracted from XPS measurements after 20 L exposure to cyclopentene (CP), COD, and dodecene. In the case of unsaturated hydrocarbons, 20 L exposure was sufficient to saturate the Si(001)(2 × 1) surface.33,36 Cyclopentene, which has been shown to form a self-terminating ML on the Si(001) surface,33 was used as a control. Carbon/silicon (C/Si) area ratios were calculated by normalizing the C(1s) peak area to the area of the Si(2p) doublet, which acts as an internal standard and corrects for slight differences in signal intensity between experiments. In calculating the C/Si ratios, atomic sensitivity factors used for carbon and silicon were 0.296 and 0.283, respectively, as specified for our analyzer setup. These C/Si ratios provide a basic quantitative
Growth of Large Pentacene Crystals on Si(001) Surfaces
Figure 2. He(I) UPS spectra of (a) the clean Si(001)-(2 × 1) 4° miscut surface, (b) the surface after exposure to 20 L COD, and (c) the CODterminated Si surface after deposition of 1 ML of pentacene.
comparison of the surface reactions by describing the relative surface densities of carbon atoms. The data in Table 1 show that the C/Si ratio for COD (0.18) is larger than that of CP (0.11), indicating that the CODterminated Si surface has 1.6 times as many C atoms per unit area as the CP-terminated Si surface. Taking into account the fact that COD has eight C atoms and CP has only five, this result indicates that both CP and COD have nearly the same number of molecules per unit area. Previous STM studies (not shown) have shown that the saturation coverage is between 0.5 and 1.0 molecule per dimer.33,34 The C/Si ratio of dodecene (0.15) is slightly smaller than that of cyclooctadiene (0.18), even though dodecene has 10 C atoms per molecule and cyclooctadiene has only eight. The lower C/Si ratio demonstrates that MLs of dodecene may not be as densely packed as those of either COD or CP. The poorer packing density is likely associated with steric effects from the long alkyl chain. Nevertheless, our data indicate that dodecene forms a selfterminating ML with a density of one molecule per -three to four dimers on the Si(001) surface, consistent with the estimated diameter of the alkyl chain. 3.2. Pentacene Growth on COD/Si(001). Figure 2 shows He I UPS spectra from a Si(001) 4° miscut sample that was functionalized with a ML of COD and a layer of pentacene. Although the results shown here are for COD on a 4° miscut surface, similar results were obtained for COD-modified onaxis Si(001) substrates. Figure 2a is the spectrum from a clean, annealed Si(001) surface, showing peaks at 0.6, 1.9, and 2.9 eV binding energy. The peak at 0.6 eV has been attributed to the dimer reconstruction at the surface.45 Figure 2b, a spectrum taken after exposing the same sample to 20 L COD, shows the disappearance of the Si dimer peak and the appearance of peaks at 1.8 (shoulder), 3.8, and 6.2 eV. The peak at 6.2 eV is in the region (5-10 eV) that is characteristic of C-C single bonds and thus is due to the COD molecule. Peaks at 1.9 and 3.8 eV indicate the presence of intact π bonds. The presence of CdC π bonds after adsorption of COD is consistent with previous infrared measurements34 and photoemission measurements42 of COD adsorption with Si(001). These investigations concluded that only one of the two CdC double bonds is involved in bonding to Si, thereby leaving a second CdC group exposed at the vacuum interface. Consequently, functionalization of Si(001) with COD produces a surface consisting of an array of CdC bonds lying parallel to the surface plane.34 Figure 2c shows photoemission data on a sample that was terminated with COD, followed by deposition of 1 ML of pentacene. Deposition of pentacene leads to new peaks at 1.0, 2.1 (shoulder), 2.5, 3.3 (shoulder), and 3.7 eV. These peaks correspond closely to those we previously reported for multilayer
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Figure 3. PEEM images showing the growth of pentacene on the CODterminated Si(001) surface at four different coverages: (a) 0.20 ML, (b) 0.57 ML, (c) 0.88 ML, and (d) 1.08 ML. The sample was held at room temperature, and the growth rate was ∼1 ML/25 min or 0.04 ML/min. The arrows in panel d demonstrate the predominate axes adopted by the second layer.
films of pentacene deposited directly on Si(001).13 When a static shift is applied to account for the difference in ionization potential between molecules bound at a surface and those in the gas phase, these energies also correlate to those observed in gas phase pentacene.46,47 These data indicate that the first layer of pentacene is physisorbed with its molecules remaining intact. Figure 3 is a set of PEEM images showing various stages of first- and second-layer pentacene growth on COD-modified Si(001) (COD/Si). In PEEM images, the brightness is determined primarily by the total electron yield. The first layer of pentacene appears as light islands on the dark COD/Si background. Subsequent pentacene layers appear darker, with the contrast becoming progressively weaker as the pentacene thickness increases. UPS data (not shown) demonstrate that in the second layer, the pentacene valence band spectrum shifts to larger binding energies, thereby decreasing the photoemission electron yield. The frames shown were taken at coverages of (a) 0.20 ML, (b) 0.57 ML, (c) 0.88 ML, and (d) 1.08 ML, with a growth rate of ∼0.04 ML/min. The estimated exposures include the fact that the second layer that nucleates before the first layer has completely covered the surface. Nucleation of the first layer begins almost immediately (after 20 s layer nuclei that appear in Figure 3d. Finally, the PEEM images also
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Figure 4. (a) LEED pattern from the clean Si(001)-(2 × 1) surface taken at normal electron incidence. The right graphic is a schematic of the pattern with the individual spots labeled. (b-e) LEED patterns taken from pentacene grains after submonolayer growth of pentacene on the COD-terminated Si(001) surface, showing the four different orientations within the first layer. In panels b, c, and e, the underlying Si(001) spots are still apparent. (f-i) Schematic images of the patterns shown in panels b-e with the pentacene spots connected by lines. In the case of panels f and g, these outer rectangles are aligned with the underlying Si(001) spots. Connecting the Si(001) spots and extending the lines divides the rectangle into four quadrants that represent the pentacene unit cells, even though some spots appear to be missing. Pentacene spots in panels h and i appear as arcs, demonstrating a small amount of rotational freedom, and maintain a significant amount of correlation with the underlying Si(001) spots.
reveal that growth of the second layer of pentacene occurs preferentially along axes rotated 45° relative to each other, as denoted in Figure 3d. These second-layer nuclei resemble crosses, each with a long axis and a short axis, with four distinct crystal directions.12 Therefore, we can identify the two primary differences between the first and the second pentacene layers as increased nucleation density in the second layer and orientation of second-layer grains along preferred axes. Similar results have been reported previously on chemically modified silicon surfaces.7,12 While PEEM images such as those in Figure 3 give information about the macroscopic growth characteristics, LEED measurements provide information about the arrangement of molecules within a pentacene grain. Figure 4 shows a series of LEED patterns obtained using the LEEM instrument. Figure 4a shows the pattern from a clean Si(001)-(2 × 1) surface. This image shows four characteristic half-order spots. In this image, the first-order spots were just outside the Ewald sphere and thus are not visible. The four half-order spots result from the doubling of the unit cell upon dimerization of the Si surface to form the Si(001)-(2 × 1) reconstruction, leading to a rectangular unit
Weidkamp et al. cell 3.84 Å × 7.68 Å in size.48 Because the dimer rows of the reconstructed surface rotate by 90° across each atomic step and the spacing between steps is smaller than the electron beam diameter, the LEED measurement probes both (2 × 1) and (1 × 2) dimer orientations, referred to here as the x- and y-directions. In the reciprocal lattice shown in LEED, the halforder spots appear halfway between the (0, 0) spot and the firstorder spots. Figure 4b-e shows four different LEED patterns obtained on different regions of a single Si(001) sample that was prepared by saturating the surface with COD and then depositing 100 µm across along their long dimensions, and they demonstrate fractal character at low coverage (Figure 9a). Growth for the sample shown took place at ∼0.1 ML/min, a slightly faster rate than that of the COD/Si(001) sample shown in Figure 3. Figure 9c shows the onset of second-layer growth before the first layer has completed. Figure 9c,d shows significant orientation of the second-layer pentacene grains, evidenced by fact that all grains have axes corresponding to the directions of the two perpendicular arrows in Figure 9c. This complete orientation carries over into the third pentacene layer as well, as seen in Figure 9e. Orientation of the secondlayer grains along two directions therefore means that first-layer grains necessarily adopt orientations along these directions as well. Figure 10 shows two LEED patterns taken after growth of
