J. Phys. Chem. C 2009, 113, 9251–9255
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Ultrathin Films of Diindenoperylene on Graphite and SiO2 Yu Li Huang,† Wei Chen,*,†,‡ Han Huang,† Dong Chen Qi,† Shi Chen,† Xing Yu Gao,† Jens Pflaum,§,| 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, Institute of Experimental Physics VI, UniVersity of Wuerzburg, Wuerzburg, Germany, and BaVarian Center for Applied Energy Research, Am Hubland, 97047 Wuerzburg, Germany ReceiVed: December 8, 2008; ReVised Manuscript ReceiVed: April 12, 2009
In situ low-temperature scanning tunneling microscopy (LT-STM), synchrotron-based high-resolution photoemission spectroscopy, and near-edge X-ray absorption fine structure measurements were used to study the supramolecular packing, molecular orientation, and electronic structures of ultrathin films of diindenoperylene (DIP) on graphite and SiO2. LT-STM measurements reveal that monolayer DIP on highly ordered pyrolytic graphite (HOPG) adopts a long-range ordered “brick-wall” arrangement with their molecular π-planes orientated parallel to the graphite surface, arising from the DIP-graphite interfacial π-π interaction. In contrast, DIP molecules stand upright on inert SiO2 due to the weak interfacial interaction. The ionization potential (IP) of DIP films largely depends on their molecular orientation, i.e., the IP for the standing-up DIP film on SiO2 is 0.40 ( 0.05 eV lower than that of the lying-down film on graphite. This is attributed to the intrinsic surface dipoles that originate from the intramolecular dipolar C-H bonds exposed at the surfaces of the standing-up thin film. Introduction Organic thin-film-based electronics have attracted much attention for low-cost, large-scale, and flexible electronic device applications, including organic light-emitting diodes (OLEDs), organic field-effect transistors (OFETs), organic solar cells, and organic spintronics.1-13 It is known that the interfaces between the active organic layers and the substrate, i.e., molecule/ electrode or molecule/dielectric interfaces, play a crucial role in determining the device performance.14-19 For example, in sexithienyl thin-film transistors (TFTs), it has been demonstrated that only the first two molecular layers next to the dielectric interface determine the charge transport.20 Therefore, intensive research efforts have been devoted to understand these molecule-substrate interface properties, including the supramolecular packing, molecular orientation of the thin films, interfacial electronic energy level alignment, tuning the hole or electron injection barriers, interfacial charge or energy transfer, reducing the charge carrier trapping at the interface, and the surface transfer doping of organic active layers.14-41 In order to understand the thin-film growth mechanism and hence to facilitate the fabrication of organic thin films with desired properties, it is necessary to carry out systematic investigations of the growth and electronic structures of organic molecules on well-defined substrates. Diindenoperylene (DIP) represents one of the most promising organic semiconductors for application in organic electronic devices (Figure 1). The long exciton diffusion length of at least 100 nm in DIP thin films facilitates potential applications in excitonic organic photovoltaic cells.42 A balanced charge carrier transport along the c′ direction in DIP single crystals enables * Corresponding author. E-mail:
[email protected]. † Department of Physics, National University of Singapore. ‡ Department of Chemistry, National University of Singapore. § University of Wuerzburg. | Bavarian Center for Applied Energy Research.
Figure 1. Schematic drawing of the DIP molecular structure.
its use in ambipolar OFETs.43 During the growth on top of copper hexadecafluorophthalocyanine (F16CuPc) thin films, DIP molecules can self-organize into organic nanodots with high crystallinity.44 The growth of DIP on inert substrates such as SiO2 induces a strong tendency of self-ordering along the long molecular axis to form highly ordered thin films.45 In this paper, we use in situ low-temperature scanning tunneling microscopy (LT-STM), synchrotron-based high-resolution photoemission spectroscopy (PES), and near-edge X-ray absorption fine structure (NEXAFS) measurements to study the supramolecular packing, molecular orientation, and electronic structures of ultrathin films of DIP on graphite and SiO2. Experimental Section The LT-STM experiment was carried out in an Omicrometer LT-STM interfaced to a Nanonis controller.46,47 STM imaging was performed at 77 K. A freshly cleaved highly ordered pyrolytic graphite (HOPG) substrate was thoroughly degassed in ultrahigh vacuum (UHV) at around 800 K overnight before DIP deposition. DIP was deposited from a low-temperature Knudsen cell onto HOPG at room temperature (RT) in a separate UHV growth chamber connected to the LT-STM via a gate valve. Prior to the deposition, DIP was purified by gradient vacuum sublimation. The deposition rate of DIP was monitored by a quartz-crystal microbalance (QCM) during evaporation and
10.1021/jp810804t CCC: $40.75 2009 American Chemical Society Published on Web 05/04/2009
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Figure 2. STM images for the lying-down DIP monolayer on HOPG: (a) 20 × 20 nm2 (Vtip ) 2.4 V and Itunnel ) 70 pA) and (b) its corresponding detailed 5 × 5 nm2 images (Vtip ) 2.4 V and Itunnel ) 70 pA). (c) Schematic drawing for the proposed molecular packing structure of the lying-down DIP monolayer on HOPG.
was further calibrated by counting the adsorbed molecule coverage in large-scale LT-STM images at coverages below 1 monolayer (1 monolayer ) one full monolayer of close-packed DIP molecules with their conjugated π-planes oriented parallel to the HOPG surface). A deposition rate of 0.02 ML/min was chosen in our LT-STM experiment. The NEXAFS and PES measurements were carried out at the Surface, Interface and Nanostructure Science (SINS) beamline of the Singapore Synchrotron Light Source.28-31 The NEXAFS measurements were performed in total-electron-yield (TEY) mode with a photon energy resolution of 0.1 eV. The sample vacuum level shift was determined from PES spectra at the low-kinetic-energy onset (secondary electron cutoff) using a photon energy of 60 eV with negative 5 V sample bias. The sample work function φ was obtained through the equation φ ) hυ - W, where W is the spectrum width (the energy difference between the substrate Fermi level and low-kineticenergy onset).14-19 DIP molecules were in situ deposited on the thoroughly degassed HOPG and Si(111) with native oxide (referred to as SiO2 henceforth) at RT. The deposition rate for DIP was precalibrated by a QCM under similar growth conditions. The actual deposition rate was further calibrated by monitoring the attenuation in intensity of the Au 4f7/2 peak before and after deposition on a sputter-cleaned poly(Au) sample.48
DIP monolayer phase, which adopts a distinct “brick-wall” supramolecular arrangement. Figure 2b displays the corresponding high-resolution STM image, clearly revealing the submo-
Results and Discussion We first use LT-STM to understand the molecular packing structure of DIP on HOPG at submonolayer coverages. All the STM images were collected at 77 K. The planar DIP molecule spontaneously aggregates to form single-layer islands on HOPG upon the deposition at RT. Figure 2a shows a typical molecularly resolved 20 × 20 nm2 STM image for the long-range ordered
Figure 3. C K-edge NEXAFS spectra of 10 nm DIP on (a) HOPG and (b) SiO2. (c) Thickness-dependent C K-edge NEXAFS spectra of DIP on HOPG and on SiO2. (d) C 1s core level spectrum for 10 nm DIP on HOPG. The binding energy is relative to the Fermi level position of the electron analyzer.
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Figure 4. Synchrotron PES spectra for lying-down DIP on HOPG and standing-up DIP on SiO2: (a) valence band spectra at the low-bindingenergy part, (b) corresponding near the EF region spectra (all binding energies are relative to the Fermi level position of the electron analyzer), and (c) PES spectra at the low-kinetic-energy part (secondary electron cutoff). All spectra are measured with photon energy of 60 eV. (d) Replotted valence band spectra relative to the vacuum level.
lecular features of individual DIP molecules originating from their highest occupied molecular orbitals (HOMOs). The unit cell is highlighted in Figure 2b with a ) 1.23 ( 0.05 nm, b ) 1.73 ( 0.05 nm, and R ) 40° ( 2°. The ellipse in Figure 2b represents a single DIP molecule, whose the size coincides well with the previously observed flat-lying DIP on Au(111) with their extended π-plane oriented parallel to the substrate surface.49,50 Similar to other planar molecules such as 3,4,9,10-perylenetetracarboxylic-dianhydride (PTCDA) and copper(II) phthalocyanine (CuPc) on HOPG,21,28,29 the flat-lying configuration of the planar DIP molecules on HOPG is mainly stabilized through the molecular-substrate interfacial π-π interactions, in particular the dispersion force. The “brick-wall” supramolecular arrangement also facilitates the close-packing of the monolayer DIP on HOPG. Such preferential lying-down configuration of DIP on HOPG was further confirmed by the angle-dependent NEXAFS measurements for a thicker DIP film. Figure 3a shows the angledependent C K-edge NEXAFS spectra of 10 nm DIP on HOPG. The first three sharp absorption peaks at the absorption edge from 284 to 289 eV with grazing incidence light (θ ) 20°) are due to resonant transitions from the C 1s core levels of the various carbon atoms into unoccupied molecular orbitals. In principle, these π* resonance peaks of planar DIP molecules with a lying-down configuration are greatly enhanced at grazing incidence since the electronic field vector E of the incident linear polarized synchrotron light has a large projection along the direction of the π* orbitals.51 The maximum intensity of the π* resonances is observed at grazing incidence (θ ) 20°), whereas the maximum intensity of the σ* resonances is observed at normal incidence (θ ) 90°). This indicates that the DIP
molecules adopt a lying-down configuration on HOPG with the molecular π-plane slightly tilting away from the HOPG surface plane. In contrast, DIP molecules stand upright on the inert SiO2 substrate, as revealed by the NEXAFS spectra in Figure 3b with a reversed angular dependence. Such standing-up molecular orientation is commonly observed for other planar molecules such as pentacene and CuPc, resulting from the weak interfacial interactions with the inert SiO2 substrate.28-31,39 Figure 3c shows the thickness-dependent C K-edge NEXAFS spectra of DIP on HOPG near the π* absorption region, using clean HOPG and DIP on SiO2 as references. In order to enhance the contrast of these π* absorption peaks, we choose the grazing incidence absorption spectra for DIP on HOPG, while using the normal incidence absorption spectrum for DIP on SiO2. Obviously, the intensities of the first (at 284.4 ( 0.1 eV) and third (at 287.3 ( 0.1 eV) peaks gradually increase upon the sequential deposition of DIP. Figure 3d shows the C 1s core level spectrum for 10 nm DIP on HOPG, which possesses a single core level peak at the binding energy of 284.3 ( 0.05 eV. This suggests that the observed three π* absorption peaks originate from the resonant excitation from this C 1s core level to different unoccupied peaks (π* peaks). In order to assign these three π* absorption peaks, we compare the NEXAFS spectra of DIP on HOPG with that for PTCDA thin film on HOPG, which has a similar molecular structure with a perylene core terminated by two anhydride groups.28,29,52,53 As such, we attribute the first two absorption peaks to the resonance excitation from the C 1s core level to the π* unoccupied orbitals mainly localized on the perylene core and the third absorption peak to the resonant excitation to the π* unoccupied orbitals mainly localized on the two indeno groups.
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We use in situ high-resolution PES to study the orientationdependent energy level alignment of the standing-up (on SiO2) and lying-down (on HOPG) DIP thin films. Figure 4 shows the PES valence band (VB) spectra at the low-binding-energy region (Figure 4, parts a and b) and spectra at the low-kinetic-energy region (Figure 4c). The hole injection barriers (∆h) can be measured from the energy difference between the substrate Fermi level and the HOMO leading edge.14-19 The ionization potential (IP) equals to the sum of ∆h and sample work function of φ. As shown in Figure 4a, the PES spectra show the apparent angle dependence of the valence band features for DIP films. Both the φ and ∆h of the standing-up DIP are 0.20 ( 0.05 eV lower than that of the lying-down film. This leads to deduction that the IP of the standing-up DIP film is 0.40 ( 0.05 eV lower than that of the lying-down film, as shown by the replotted valence band PES spectra relative to the vacuum level for both lying-down and standing-up thin films of DIP in Figure 4d. The linear extrapolation of the low-binding-energy onset corresponds to the IP of the films, which are highlighted in the Figure 4d. The IP is 5.0 ( 0.05 eV for the standing-up DIP film and 5.4 ( 0.05 eV for the lying-down DIP film. Such orientationdependent IP can be attributed to the intrinsic surface dipoles that originate from the intramolecular dipolar C-H bonds exposed at the surfaces of the standing-up thin film, i.e., an upward pointing surface dipole that reduces the IP and which has been reported in other ordered molecular thin films.33,34,54-58 Conclusion In summary, in situ LT-STM and synchrotron-based highresolution PES and NEXAFS measurements were used to study the supramolecular arrangement, molecular orientation, and electronic structures of ultrathin DIP films on HOPG. It is found that the DIP-graphite interfacial π-π interaction facilitates the lying-down configuration of DIP films on HOPG. Monolayer DIP on HOPG adopts a long-range ordered “brick-wall” arrangement. In contrast, DIP molecules stand upright on inert SiO2 due to the weak interfacial interaction. Orientationdependent IPs have been observed for the standing-up and lyingdown DIP films. The IP for the standing-up DIP film is 0.40 ( 0.05 eV lower than that of the lying-down film, attributed to the intrinsic surface dipoles that originate from the intramolecular dipolar C-H bonds exposed at the surfaces of the standing-up thin film. Such detailed investigations of the growth mechanism, molecular orientation, and electronic structure of DIP ultrathin films have important implications for DIP-based organic electronic devices, such as OFETs or organic solar cells. Acknowledgment. The authors acknowledge the support from the Singapore A*STAR Grant R-398-000-036-305 and ARF Grants R-144-000-196-112 and R-143-000-392-133. References and Notes (1) Dimitrakopoulos, C. D.; Malenfant, P. R. L. AdV. Mater. 2002, 14, 99. (2) Ho, P. K. H.; Kim, J. S.; Burroughes, J. H.; Becker, H.; Li, S. F. Y.; Brown, T. M.; Cacialli, F.; Friend, R. H. Nature 2000, 404, 481. (3) Yang, F.; Shtein, M.; Forrest, S. R. Nat. Mater. 2005, 4, 37. (4) Xue, J.; Rand, B. P.; Uchida, S.; Forrest, S. R. AdV. Mater. 2005, 17, 66. (5) Peumans, P.; Yakimov, A.; Forrest, S. R. J. Appl. Phys. 2003, 93, 3693. (6) Forrest, S. R. MRS Bull. 2005, 30, 28. (7) Brumbach, M.; Placencia, D.; Armstrong, N. R. J. Phys. Chem. C 2008, 112, 3142. (8) Blom, P. W. M.; Mihailetchi, V. D.; Koster, L. J. A.; Markov, D. E. AdV. Mater. 2007, 19, 1551. (9) Muccini, M. Nat. Mater. 2006, 5, 605.
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