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Effect of Fluorination on the Molecular Packing of Perfluoropentacene and Pentacene Ultrathin Films on Ag (111) Swee Liang Wong,† Han Huang,† Yu Li Huang,† Yu Zhan Wang,† Xing Yu Gao,† Toshiyasu Suzuki,§ Wei Chen,*,†,‡ and Andrew Thye Shen Wee*,† Department of Physics, National UniVersity of Singapore, 2 Science DriVe 3, 117542, Singapore; Department of Chemistry, National UniVersity of Singapore, 3 Science DriVe 3, 117543, Singapore; and Institute of Molecular Science, Okazaki, Aichi 4448787, Japan ReceiVed: NoVember 6, 2009; ReVised Manuscript ReceiVed: March 18, 2010
The growth of perfluoropentacene (PFP) and pentacene (PEN) ultrathin films on Ag(111) has been investigated using low-temperature scanning tunneling microscopy. To understand the influence that perfluorination of the parent molecule has on its resultant packing structure, the results are compared against each other in the framework of morphological differences. Perfluorination leads to a different packing structure in the first monolayer. We observed only one closely packed arrangement with periodic dislocation lines for PFP molecules, while for PEN molecules, there are two coexisting arrangements in the first monolayer. Monolayers of each molecule are commensurate with the underlying substrate with long axes of both molecules aligned in the [11j0] direction along the silver surface. The disparity in arrangements is attributed to the difference in peripheral atoms of the two molecules. Additional photoemission spectroscopy studies reveal that PFP physisorbs on Ag (111). Introduction Molecular thin films of organic compounds have garnered considerable interest in recent years due to their applicability in low-cost and flexible devices such as organic field-effect transistors, organic photovoltaic cells, and organic light-emitting diodes. Advantages of such devices include their light weight and flexibility as compared to their inorganic counterparts, making them prime candidates for mobile applications or “smart clothing”, and their compatibility with low cost substrates such as plastics.1 However, the anisotropy of their molecular structures and hence charge mobility would mean that the molecular orientation they adopt on the substrate surface will affect the charge transport across these molecules. Furthermore, grain boundaries between different domains can influence the device performance negatively.2-4 Such surface shapes and molecular packing arrangements are highly dependent on a complex balance of intermolecular and molecule-substrate interfacial interactions5-14 that vary between different molecules. Among these organic molecules, pentacene (C22H14) has been widely studied as a p-type semiconductor for OFETs on various substrates such as SiO2/Si15 and on polymer gate dielectrics16 as the pentacene (PEN) thin film transistor has the highest fieldeffect hole mobility recorded among organic materials (in excess of 5 cm2 V-1 s-1 on SiO2/Si substrates).15 However, PEN OFETs only have an electron mobility of up to 0.04 cm2 V-1 s-1 17 due to its redox potentials.18 Thus, when one considers a complementary circuit to this molecule,19 its corresponding candidate should be one with suitable electron mobility and of similar dimensions to minimize mismatch in lattice parameters for better growth. Its functionalized n-type counterpart, perfluoropentacene (C22F14, PFP), which has similar physical dimensions and * Corresponding authors. E-mail
[email protected] (W.C.), phyweets@ nus.edu.sg (A.T.S.W.). † Department of Physics, National University of Singapore. ‡ Department of Chemistry, National University of Singapore. § Institute of Molecular Science.
electron mobility in OFETs of more than 0.2 cm2 V-1 s-1,20 is hence a desirable candidate. However, the change in atomic makeup of the molecule is expected to alter the intermolecular and molecule-substrate interactions and thus the resultant packing structure, which is crucial to its implementation and compatibility in devices. Experimental studies have been carried out for PFP on Au (111),21 Cu (111),22 and SiOx23-25 substrates but not for PFP on Ag (111). PEN deposition on Ag (111) has been studied extensively and is found to have interesting growth modes, such as an ordered second layer over a disordered first and also the coexistence of two ordered molecular arrangements in the first layer.26-30 Therefore, in this paper, low-temperature scanning tunneling microscopy (LT-STM) studies of PFP and PEN growth on Ag (111) are conducted with specific attention paid to the molecular packing and orientation of the first and second monolayer. Comparative analyses are then made in the framework of morphological differences to investigate the influence that the change in peripheral atoms has on the packing structure. In addition, photoemission spectroscopy measurements are conducted to determine the electronic structures of PFP adsorption on Ag (111) and compared against previous reports for PEN on Ag (111). Experimental Section LT-STM experiments were carried out in a custom-built multichamber ultrahigh-vacuum system with base pressure lower than 6.0 × 10-11 mbar, housing an Omicron LT-STM. STM imaging was carried out in constant current mode with a chemically etched tungsten tip at 77 K maintained by liquid nitrogen cooling.29 Clean Ag (111) surfaces with wide atomically flat terraces were obtained after a few cycles of Ar+ ion bombardment followed by subsequent annealing at 700 K in the growth chamber, and the cleanliness was inspected via largescale STM images. PFP (99.9%, sublimed)18 and PEN (Sigma-
10.1021/jp910581b 2010 American Chemical Society Published on Web 05/04/2010
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Figure 1. (a) STM image of the first monolayer of PFP deposited on Ag (111) (30 × 30 nm2, Vtip ) -1.80 V, I ) 100 pA). (b) STM image showing both molecules and the underlying silver substrate (3 × 6 nm2, Vtip for underlying silver substrate ) 1.2 V, Vtip for PFP overlayer ) -2.00 V, I ) 100 pA for both). (c) Schematic drawing for the proposed molecular packing structure of lying-down PFP on Ag (111) with the unit cell indicated. Lattice vectors of substrate are also indicated by vectors a1 and a2.
Aldrich, 99.9%, sublimed) are deposited from different Knudsen cells (MBE Komponenten, Germany) at 400 and 420 K, respectively, onto the Ag (111) substrate at room temperature. The deposition rates of PFP and PEN were measured by a quartz crystal microbalance and further calibrated from molecular coverages in the LT-STM images. In our experiments, the deposition rates were kept constant at ∼0.15 monolayer (ML) min-1 for PFP and 0.20 ML min-1 for PEN on Ag (111) (1 ML corresponds to a closely packed molecular layer). Throughout the deposition, the chamber pressure was maintained below 5.0 × 10-10 mbar. Synchrotron photoemission measurements were carried out at the Surface, Interface and Nanostructure Science (SINS) beamline in the Singapore Synchrotron Light Source (SSLS)31-33 with resolution of 0.05 eV. The spectra at photoemission secondary electron cutoff were recorded with a bias voltage of -5.0 V to allow the observation of the lowenergy secondary electron cutoffs. Results and Discussion Figure 1a shows a typical LT-STM image of the first PFP monolayer on Ag (111). The substrate terraces are completely covered with PFP in a closely packed arrangement, each molecule represented by a single rodlike feature. On closer inspection, there are periodic dislocation lines in the PFP molecular network. This phenomena is also reported in the network of halogenated molecules, ZnPcCl8, on Ag (111)34,35 and will be elaborated on later. Occasionally, by employing two different tip voltage biases, both the atomic structure of the underlying substrate and the molecular structure of PFP monolayer can be obtained in the same image,36 as shown in Figure 1b. The PFP molecular plane is parallel to Ag (111) with its long axis in the [11j0] direction. The flat-lying configuration allows the molecules to have maximum exposure of their extended conjugated π-plane of electrons to the substrate surface for electronic coupling between the π-orbital of the molecules and the Ag d-bands.29,37 These findings are consistent with previous reports of flat-lying PFP adsorbed on other metals.21,22
The closely packed arrangement of PFP on Ag (111) has a rhombic unit cell with measured dimensions of a ) 1.86 ( 0.05 nm, b ) 0.90 ( 0.05 nm, and an included angle of 60 ( 2°, commensurate with the Ag (111) substrate with an overlayer matrix of
[ ] 6 0 0 3
Figure 1c depicts the proposed model. Figure 2a shows a LT-STM image of 1.4 ML PEN on Ag (111), with the lower half covered by bilayer PEN. The PEN molecules, like PFP, are also adsorbed in a flat lying configuration. However, there exist two different arrangements, closepacking (CP) and loose packing (LP), with comparable coverage for PEN on Ag (111), as labeled in the upper half of Figure 2a. The two molecular rows belonging to separate packing arrangements differ by an angle of ∼20° as shown in Figure 2a. The detailed STM images of the two separate molecular packing structures in the monolayer are shown in Figure 2b,c. The unit cell parameters of the CP arrangement from the STM images are a ) 0.91 ( 0.05 nm, b ) 1.52 ( 0.05 nm, and an included angle of 88 ( 2° while for the LP arrangement, the lattice vectors are a ) 1.05 ( 0.05 nm and b ) 1.52 ( 0.05 nm with an included angle of 67 ( 2°. These results are similar to those experiments conducted on PEN on Ag (111) in ref 26. However, in contrast to the findings in which the molecular long axes of PEN in the two coexisting arrangements have a 10° difference, we observe that in both the CP and LP arrangements the molecular long axes are in the [11j0] direction of the Ag (111) surface. The angle (∼20°) as indicated in Figure 2a between the two arrangements is caused by a difference in packing arrangement rather than a change in orientation of the molecules with an abrupt transition from one arrangement to the other at the boundaries. On the basis of the STM image in Figure 2a, by comparing the relative longitudinal translation of the
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Figure 2. (a) STM image of 1.4 ML PEN on Ag (111) (35 × 35 nm2, Vtip ) -1.44 V, I ) 100 pA). CP and LP indicate the areas of closely and loosely packed arrangement, respectively. (b) STM image of corresponding detailed 5 × 10 nm2 image of the CP structure (Vtip ) 1.81 V, I ) 100 pA). (c) 5 × 10 nm2 image of the LP structure (Vtip ) -1.50 V, I ) 100 pA). (d) Schematic drawing for the proposed molecular packing structure of lying-down PEN on Ag (111) with the unit cell indicated (blue for compact arrangement and red for loose packed arrangement). Lattice vectors of substrate are also indicated by vectors a1 and a2.
molecules38 and taking into consideration the ∼20° change in direction along the molecular rows, the model comprising the coexisting arrangements is proposed in Figure 2d. The commensurate matrix for the loosely packed arrangement is
[ ] 4 2 2 6
and that for the closely packed arrangement is
[ ] 3 1/2 2 6
For an angle of 20° to occur between the two rows of molecules, adjacent molecules within the closely packed arrangement have to alternate between two nonequivalent adsorption sites. The loosely packed arrangement of molecules is likely to be the result of the molecule-substrate interactions dominating over the intermolecular interactions, leading to the PEN molecules adopting equivalent adsorption sites on Ag (111). In contrast, the closely packed molecular arrangement is due to a balance between the intermolecular and molecule-substrate interactions,
the intermolecular interactions being stronger in this case because of the shorter distances between molecules. This results in adjacent PEN molecules having different adsorption sites on the silver surface. The variation in packing structure between the first monolayer of the two molecules is attributed to the difference in peripheral atoms. For PFP, intermediate or coexisting configurations of PFP molecules are not observed due to the higher electronegativity of fluorine atoms. Unlike for PFP on Cu(111),22 which has an intramolecular dipole moment, it has been shown recently that there is no such molecular distortion for PFP on Ag(111).39 Hence, the repulsion is likely due to periphery electrostatic repulsions. The effects of such intermolecular repulsion on the molecular arrangement is also observed in ZnPcF8 molecules deposited on Ag(111) where there is only one molecular configuration as compared to multiple arrangements of ZnPcCl8 on Ag (111)34 and also other fully halogenated molecules such as CoPcF16 on Au(111) where self-assembly is not possible.40 In contrast, for PEN molecules that possess hydrogen atoms which have weaker electrostatic repulsion between them, a coexisting loosely packed arrangement other than the closely packed one is allowed. The dislocation lines in PFP molecular film is a result of a competition between substrate-molecule
PFP and PEN Ultrathin Films on Ag (111)
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Figure 3. STM image of (a) bilayer PFP deposited on Ag (111) at room temperature (150 × 150 nm2, Vtip ) -2.24 V, I ) 100 pA) and (b) the corresponding detailed image of the bilayer surface (30 × 30 nm2, Vtip ) -2.18 V, I ) 100 pA). STM image of (c) bilayer PEN deposited on Ag (111) at room temperature (100 × 100 nm2, Vtip ) 1.39 V, I ) 100 pA) and (d) the corresponding detailed image of the bilayer surface (12 × 12 nm2, Vtip ) -1.44 V, I ) 100 pA).
and intermolecular interactions.34,35 Substrate-molecule interactions results in a site-specific adsorption for both PEN and PFP molecules, as evidenced by the commensurability of the molecular superlattice with the substrate. However, due to the increased intermolecular repulsion between PFP molecules together with its inherent molecular anisotropy, i.e., PFP having more fluorine molecules at the long molecular edges, there is a significant buildup of stress which is released by the formation of dislocation lines. Thus, we only observe dislocation lines along the short axis of the molecules while molecular arrangement along the long axis is quasi-infinite. The closely packed network of PEN on Ag (111) does not display similar dislocation lines probably due to the reduced electrostatic repulsion between hydrogen atoms. Figure 3a shows a LT-STM image of Ag (111) covered by 1.7 ML PFP. Figure 3b shows a detailed LT-STM image of the PFP bilayer on Ag (111). The bilayer PFP surface has a rhombic unit cell with measured dimensions of a ) 1.70 ( 0.05 nm, b ) 0.94 ( 0.05 nm, and an included angle of 67 ( 2°. The second layer shown in Figure 3b no longer has columns of PFP molecules separated by periodic dislocation lines. This is due to the distance of the second layer being further away from the substrate, and hence the substrate influence on the molecules is much weaker. Similarly, Figure 3c shows the LT-STM image of Ag (111) covered by 1.6 ML PEN, while Figure 3d shows a detailed LT-STM image of the bilayer PEN on Ag (111). The bilayer PEN surface has a rhombic unit cell with measured dimensions of a ) 1.88 ( 0.05 nm, b ) 0.69 ( 0.05 nm, and an included angle of 76 ( 2°. For PEN on Ag (111), the second layer exhibits long-range ordering with a closely packed
arrangement in which the long molecular axis is parallel to the substrate surface and possesses an out-of-plane tilt, as reported previously.27 This is in contrast to the dual phase first layer. The growth mode for the first two layers of each organic molecule on Ag (111) is layer by layer, as a full coverage of the first monolayer is observed prior to any formation of second layer islands. Both their bilayer surfaces exhibit long-range ordering with similar close-packed molecular arrangements whereby the long molecular axes are adsorbed parallel to the surface. The underlying layer for both molecules acts as a wetting layer passivating the substrate, diminishing its influence on the second layer. Figure 4 shows the changes that occur to the molecular network of the first monolayer after annealing the bilayer at 380 K for 30 min. The top layer for both molecules have completely desorbed together with additional molecules from the first layer, leaving behind a first monolayer with lower coverage. The two coexisting arrangements of PEN which existed in the first monolayer before annealing are reduced to only a single loosely packed arrangement as shown in Figure 4b, suggesting that the loosely packed arrangement of PEN on Ag (111) is the lower energy configuration. For PFP on Ag (111) as shown in Figure 4a, the only observable arrangement is still that of the closely packed one, confirming that this is the most thermodynamically stable phase at lower coverage and the dislocation lines stabilizes the arrangement. Figure 5a shows the thickness-dependent C 1s core level spectra for PFP molecules adsorbed on Ag (111). The nominal thickness as indicated on the right of the figure is obtained from the attenuation of the Ag 3d photoemission peaks. In Figure
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Figure 4. STM image of the (a) first monolayer of PFP on Ag (111) after annealing of the bilayer at 380 K for 30 min (50 × 50 nm2, Vtip ) -2.00 V, I ) 100 pA) and (b) first monolayer of PEN on Ag (111) after annealing of the bilayer at 380 K for 30 min (50 × 50 nm2, Vtip ) -2.24 V, I ) 100 pA).
no chemical interaction of PFP with Ag (111). Similarly, the adsorption of PEN on Ag is reported to be that of physisorption.42 Figure 5b shows the photoemission secondary electron cutoff with increasing amounts of PFP deposited on Ag (111). From the cutoff shift in the spectra we note that there is a maximal downward shift in work function or vacuum level of -0.3 eV after deposition of about 2 ML (∼1.0 nm nominal thickness) of PFP molecules. As there are no strong chemical interactions between molecules and substrate as indicated by core level measurements, this decrease in work function is attributed to an electron push back effect caused by the repulsion between the electrons of the molecule and the free surface electrons of the metal which creates an interface dipole.43 However, we note that for PFP on Cu (111)22 the total work function decrease is a net result of an electron push back effect and an intramolecular dipole which is not verified in our experiment. For PEN molecules deposited on Ag, the work function shift is reported to be -0.6 eV.42 The smaller negative shift in work function for PFP/Ag indicates that the adsorption of PFP on Ag is probably weaker than PEN on Ag. Figure 5. (a) Spectra of C 1s electron binding energy levels of increasing thickness of PFP deposited at room temperature on Ag (111). Peaks 1 and 2 are explained in the text. (b) Spectra at photoemission secondary electron cutoff with a -5.0 V bias applied of increasing thickness of PFP deposited on Ag (111) at room temperature. There is a maximal shift of -0.3 eV in work function as indicated by the dotted lines. Nominal thicknesses for each spectrum are labeled at the side.
5a, peak 1 at binding energy of 285.95 ( 0.05 eV is assigned to the carbon atoms not directly bonded to fluorine in the molecule and peak 2 at a higher binding energy of 287.50 ( 0.05 eV is assigned to the carbon atoms directly bonded to the electronegative fluorine atoms. The assignment of these two peaks separated by ∼1.5 eV is consistent with similar core level measurements reported for PFP on Cu (111).22 There is a small shift in binding energy levels of +0.1 eV for peak 1 from 285.95 ( 0.05 to 286.05 ( 0.05 eV and peak 2 from 287.50 ( 0.05 to 287.60 ( 0.05 eV at the maximum thickness deposited (∼4 ML). This is due to the photohole screening effect where the topmost layer of PFP molecules is no longer in the effective screening range of the electrons in the metal substrate.41 As there are neither new peaks, change in line widths of the peaks, nor chemical shifts of the C 1s peaks, we can conclude that there is
Conclusion In summary, we have compared the growth behaviors of pentacene and perfluoropentacene films deposited on Ag (111) at room temperature. On the basis of LT-STM studies, we have demonstrated that a change in peripheral atoms of the parent pentacene molecule from hydrogen to fluorine results in a significant alteration of the molecular packing structure. The first monolayer of PFP only has a single closely packed arrangement, while that of PEN has two coexisting monolayer phases. In addition, there are periodic dislocation lines present in the PFP monolayer network due to release of built-up intermolecular repulsion and none observed in the PEN monolayer. The differences are proposed to be due to the increased electrostatic repulsion between halogenated PFP as compared to PEN. For both molecules, the bilayer surface experiences a diminished substrate influence, and the features present in the first monolayer are no longer observed. Photoemission studies show that the adsorption mechanisms for both molecules on Ag do not involve covalent bond formation. In addition, the work function shift for PFP/Ag is measured to be lesser than that of PEN/Ag, indicating the possibility of a weaker PFP adsorption on Ag (111) as compared to PFP. Thus, when hybrid systems comprising differently functionalized molecules derived
PFP and PEN Ultrathin Films on Ag (111) from a single parent molecule are considered, the inherent morphological and electronic changes accompanying the change in atomic makeup should be taken into account. Such changes induced by a change in intermolecular and molecule-substrate interactions can affect the compatibility of different molecules and hence charge transport through these structures. Acknowledgment. The authors acknowledge the support from the Singapore ARF Grants R-144-000-196-112, R-143000-392-133, and R-143-000-406-112. The authors thank Prof. X. S. Wang for discussions. References and Notes (1) Koch, N. ChemPhysChem 2007, 8, 1438–1455. (2) Koch, N.; Vollmer, A.; Salzmann, I.; Nickel, B.; Weiss, H.; Rabe, J. P. Phys. ReV. Lett. 2006, 96, 156803. (3) Salih, A. J.; Lau, S. P.; Marshall, J. M.; Maud, J. M.; Bowen, W. R.; Hilal, N.; Lovitt, R. W.; Williams, P. M. Appl. Phys. Lett. 1996, 69, 2231. (4) Karl, N.; Marktanner, J. Mol. Cryst. Liq. Cryst. Sci. Technol. 2001, 355, 149. (5) Fenter, P.; Schreiber, F.; Zhou, L.; Eisenberger, P.; Forrest, S. R. Phys. ReV. B 1997, 56, 3046. (6) Peisert, H.; Schwieger, T.; Auerhammer, J. M.; Knupfer, M.; Golden, M. S.; Fink, J.; Bressler, P. R.; Mast, M. J. Appl. Phys. 2001, 90, 466–469. (7) Ellis, T. S.; Park, K. T.; Hulbert, S. L.; Ulrich, M. D.; Rowe, J. E. J. Appl. Phys. 2004, 95, 982. (8) Schmitz-Hu¨bsch, T.; Fritz, T.; Staub, R.; Back, A.; Armstrong, N. R.; Leo, K. Surf. Sci. 1999, 437, 163. (9) Schuerlein, T. J.; Schmidt, A.; Lee, P. A.; Nebesny, K. W.; Armstrong, N. R. Jpn. J. Appl. Phys. 1995, 34, 3837–3845. (10) France, C. B.; Schroeder, P. G.; Forsythe, J. C.; Parkinson, B. A. Langmuir 2003, 19, 1274–1281. (11) Kang, J. H.; Zhu, X. Y. Appl. Phys. Lett. 2003, 82, 3248. (12) Yang, J. L.; Yan, D. H. Chem. Soc. ReV. 2009, 38, 2634–2645. (13) Wang, H. B.; Zhu, F.; Yang, J.; Geng, Y.; Yan, D. AdV. Mater. 2007, 19, 2168–2171. (14) Huang, Y. L.; Chen, W.; Chen, S.; Wee, A. T. S. Appl. Phys. A: Mater. Sci. Process. 2009, 95, 107–111. (15) Gundlach, D. J.; Lin, Y. Y.; Jackson, T. N.; Nelson, S. F.; Schlom, D. G. IEEE Electron DeVice Lett. 1997, 18, 87–89. (16) Klauk, H.; Halik, M.; Zschieschang, U.; Schmid, G.; Radlik, W.; Weber, W. J. Appl. Phys. 2002, 92, 5259. (17) Singh, T. B.; Senkarabacak, P.; Sariciftci, N. S.; Tanda, A.; Lackner, C.; Hagelauer, R.; Horowitz, G. Appl. Phys. Lett. 2006, 89, 033512. (18) Sakamoto, Y.; Suzuki, T.; Kobayashi, M.; Gao, Y.; Fukai, Y.; Inoue, Y.; Sato, F.; Tokio, S. J. Am. Chem. Soc. 2004, 126, 8138–8140. (19) Klauk, H.; Zschieschang, U.; Pflaum, J.; Halik, M. Nature 2007, 445, 745–748.
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