Copper Phthalocyanine on Hydrogenated and Bare Diamond (001)-2

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Copper Phthalocyanine on Hydrogenated and Bare Diamond (001)-2  1: Influence of Interfacial Interactions on Molecular Orientations Dongchen Qi ,† Jiatao Sun,‡ Xingyu Gao,*,† Li Wang,†,§ Shi Chen,†,‡ Kian Ping Loh,‡ and Andrew T. S. Wee*,† ‡

† Department of Physics, National University of Singapore, 2 Science Drive 3 Singapore 117542, Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543, and § Department of Physics, Nanchang University, Nanchang 330031, P. R. China

Received April 6, 2009. Revised Manuscript Received August 21, 2009 The molecular orientations of copper phthalocyanine (CuPc) organic semiconductor molecules on hydrogenated and bare diamond (001)-2  1 surfaces are studied using synchrotron-based photoemission spectroscopy (PES) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. Angular-dependent NEXAFS reveals that the CuPc molecular assemblies are orientationally ordered and lying down on hydrogenated diamond, whereas they undergo a molecular reorientation on bare diamond from lying down at submonolayer coverage to standing up in multilayers. The molecular film on bare diamond also exhibits an order-disorder-order transition in the molecular orientations. The distinct molecular orientation within the CuPc films on both diamond (001) surfaces is explained in terms of the interplay between intermolecular interactions and molecule-substrate interactions.

Introduction With the rapid development of microelectronics and the continuing shrinkage of device dimensions, hybrid organicinorganic systems are expected to revolutionize future technologies and devices.1-3 Through combining the best properties and features of both inorganic and organic materials, we have an unprecedented opportunity to construct functional systems with tunable chemical, electric, mechanical, and biological properties. On the basis of this concept, a wide range of applications (biosensing, optoelectronics, drug delivery, etc.) are being developed, and many routes toward such functional systems are being pursued. In addition to the commonly adopted inorganic semiconductor such as silicon, there have been ongoing interests in the organic functionalization of diamond surfaces,4-12 in view of its exceptional mechanical, electrical, and chemical properties.13 In particular, *Corresponding authors: Tel (65)-6516-6362/2774/2603; e-mail phygaoxy@ nus.edu.sg (X.G.); [email protected] (A.T.S.W.). (1) Bent, S. F. Surf. Sci. 2002, 500, 879. (2) Buriak, J. Chem. Rev. 2002, 102, 1271. (3) Filler, M. A.; Bent, S. F. Prog. Surf. Sci. 2003, 73, 1. (4) Hamers, R. J.; Butler, J. E.; Lasseter, T.; Nichols, B. M.; Russell, J. N.; Tse, K. Y.; Yang, W. Diamond Relat. Mater. 2005, 14, 661. (5) Wang, J.; Firestone, M. A.; Auciello, O.; Carlisle, J. A. Langmuir 2004, 20, 11450. (6) Knickerbocker, T.; Strother, T.; Schwartz, M. P.; Russell, J. N.; Butler, J.; Smith, L. M.; Hamers, R. J. Langmuir 2003, 19, 1938. (7) Hossain, M. Z.; Aruga, T.; Takagi, N.; Tsuno, T.; Fujimori, N.; Ando, T.; Nishijima, M. Jpn. J. Appl. Phys 1999, 38, L1496. (8) Hovis, J. S.; Coulter, S. K.; Hamers, R. J.; D’Evelyn, M. P.; Russell, J. N.; Butler, J. E. J. Am. Chem. Soc. 2000, 122, 732. (9) Wang, G. T.; Bent, S. F.; Russell, J. N.; Butler, J. E.; D’Evelyn, M. P. J. Am. Chem. Soc. 2000, 122, 744. (10) Ouyang, T.; Gao, X.; Qi, D. C.; Wee, A. T. S.; Loh, K. P. J. Phys. Chem. B 2006, 110, 5611. (11) Qi, D.-C.; Liu, L.; Gao, X.; Ouyang, T.; Chen, S.; Loh, K. P.; Wee, A. T. S. Langmuir 2007, 23, 9722. (12) Qi, D.-C.; Gao, X.; Wang, L.; Chen, S.; Loh, K. P.; Wee, A. T. S. Chem. Mater. 2008, 20, 6871. (13) Wort, C. J. H.; Balmer, R. S. Mater. Today 2008, 11, 22.

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the wide electrochemical potential window and unique biocompatibility of diamond allow potential applications in bioelectronics,4-6 electrochemistry,14 and pH sensors.15 The cycloaddition reaction, a widely used reaction scheme in organic synthesis to form new carbon-carbon bonds and rings,16 is an effective route to the controlled organic functionalization of semiconductor surfaces.17 Because of the strained alkene-like double bonds of diamond surface dimers, cycloaddition reactions with various simple organic molecules have been demonstrated to occur readily on bare diamond (001)-2  1 surfaces.7-11 Moreover, it was recently reported that organic semiconductor molecules with extended conjugated π-electron systems can be covalently coupled onto diamond surface, tailoring its surface electronic properties.12 Being technologically important, the incorporation of electrically functional organic semiconductors with diamond opens up many unprecedented opportunities for the development of diamond-based molecular electronic devices. Organic semiconductors have attracted much attention for lowcost, large-scale, and flexible electronic device applications,18-21 including organic light-emitting diodes (OLEDs), organic solar cells, organic field effect transistors (OFETs), and organic spintronics. Intensive research has been devoted to studying the growth (14) Hartl, A.; Schmich, E.; Garrido, J. A.; Hernando, J.; Catharino, S. C. R.; Walter, S.; Feulner, P.; Kromka, A.; Steinmuller, D.; Stutzmann, M. Nat. Mater. 2004, 3, 736. (15) Garrido, J. A.; Hartl, A.; Kuch, S.; Stutzmann, M.; Williams, O. A.; Jackmann, R. B. Appl. Phys. Lett. 2005, 86, 73504. (16) Carruthers, W. Cycloaddition Reactions in Organic Synthesis; Pergamon Press: New York, 1990. (17) Hamers, R. J.; Coulter, S. K.; Ellison, M. D.; Hovis, J. S.; Padowitz, D. F.; Schwartz, M. P.; Greenlief, C. M.; Russell, J. N. Acc. Chem. Res. 2000, 33, 617. (18) Forrest, S. R. Nature 2004, 428, 911. (19) Hagfeldt, A.; Gratzel, M. Acc. Chem. Res. 2000, 33, 269. (20) Crone, B.; Dodabalapur, A.; Lin, Y. Y.; Filas, R. W.; Bao, Z.; LaDuca, A.; Sarpeshkar, R.; Katz, H. E.; Li, W. Nature 2000, 403, 521. (21) Dimitrakopoulos, C. D.; Malenfant, P. R. L. Adv. Mater. 2002, 14, 99.

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of organic thin films with well-controlled properties, such as molecular orientation,22-24 supramolecular organization,25-27 and well-defined surface or interface morphologies or nanostructures.28-31 It is widely accepted that the interface between the active organic layers and the substrate, i.e., molecule-electrode or molecule-dielectric interface, plays a crucial role in improving device performance, since the anisotropic charge carrier transport properties are highly dependent on the ordering and orientation of the molecular units at the interfaces. The molecular orientation at the interface, which can in turn influence the crystalline structure and orderliness of subsequently grown molecular films, is found to be governed by a delicate interplay between the intermolecular interactions and the molecule-substrate interactions.32-34 Understanding their relationships is crucial to optimizing the morphology, structures, and electronic properties of organic thin films for device applications. Until now, most research is devoted to studying the interfaces between organic semiconductors and several technologically important substrates including metals and conducting polymer electrodes, organic heterojunctions, and inorganic semiconductors such as silicon. Much less attention has been paid to understanding interfacial molecular structures between organic semiconductors and diamond. In this regard, diamond (001) provides an ideal platform to examine how the interfacial interaction strength influences molecular organization at their interfaces. The surface reactivity of diamond can be easily tuned by varying the surface termination. Bare diamond surface exhibits reactivity toward unsaturated molecules via cycloaddition, and hence this interface is dominated by strong covalent bonding interactions.11,12 In contrast, hydrogen termination of diamond passivates all the dangling bonds of reactive dimers, rendering the hydrogenated diamond surface chemically inert. As a result, the interfacial interactions between hydrogenated diamond surface and organic semiconductors are expected to be dominated by weak van der Waals-type interactions. Diamond thus provides a unique opportunity to realize these two distinct interfacial interactions on the same substrate, which can be exploited in different device architectures. Clarifying the interfacial properties between diamond and organic semiconductors, therefore, has important implications for understanding and engineering organic/inorganic interfaces. Copper(II) phthalocyanine (CuPc) is an extensively studied organic semiconductor with high chemical stability, excellent electronic properties, and high quality film structures and is a potential candidate for organic electronics and novel functional devices;35-38 it therefore serves as an excellent archetypical (22) Chen, W.; Huang, H.; Chen, S.; Chen, L.; Zhang, H. L.; Gao, X. Y.; Wee, A. T. S. Appl. Phys. Lett. 2007, 91, 114102. (23) Chen, W.; Huang, H.; Chen, S.; Gao, X. Y.; Wee, A. T. S. J. Phys. Chem. C 2008, 112, 5036. (24) Chen, W.; Chen, S.; Huang, H.; Qi, D. C.; Gao, X. Y.; Wee, A. T. S. Appl. Phys. Lett. 2008, 92, 063308. (25) Heutz, S.; Salvan, G.; Jones, T. S.; Zahn, D. R. T. Adv. Mater. 2003, 15, 1109. (26) Yim, S.; Heutz, S.; Jones, T. S. Phys. Rev. B 2003, 67, 165308. (27) Heutz, S.; Cloots, R.; Jones, T. S. Appl. Phys. Lett. 2000, 77, 3938. (28) Xue, J.; Rand, B. P.; Uchida, S.; Forrest, S. R. Adv. Mater. 2005, 17, 66. (29) Yang, F.; Shtein, M.; Forrest, S. R. Nat. Mater. 2005, 4, 37. (30) Yang, F.; Sun, K.; Forrest, S. R. Adv. Mater. 2007, 19, 4166. (31) Lunt, R. R.; Benziger, J. B.; Forrest, S. R. Adv. Mater. 2007, 19, 4229. (32) Witte, G.; W€oll, C. J. Mater. Res. 2004, 19, 1889. (33) Schreiber, F. Phys. Status Solidi 2004, 201, 1037. (34) Forrest, S. R. Chem. Rev. 1997, 97, 1793. (35) Facchetti, A. Mater. Today 2007, 10, 28. (36) McKeown, N. B. Phthalocyanine Materials: Synthesis, Structure, and Function; Cambridge University Press: New York, 1998. (37) Zhu, F.; Wang, H. B.; Song, D.; Lou, K.; Yan, D. H. Appl. Phys. Lett. 2008, 93, 103308. (38) Zhu, F.; Yang, J.; Song, D.; Li, C.; Yan, D. Appl. Phys. Lett. 2009, 94, 143305.

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organic semiconductor for the present study. The CuPc molecule has a planar structure with 4-fold symmetry and is comprised of four aromatic rings around a porphyrin-like central ring with a copper ion (Cu2þ) at its center (Figure 1). Depending on the growth parameters, especially the substrate temperature, bulk CuPc molecular solids exist in several different crystalline polymorphs, and the two most important monoclinic crystalline forms are denoted as by R and β forms (cf. Figure 1 and Table 1).39 In general, films in R-form can be more easily grown by vacuum deposition on substrates at room temperature, whereas the β-form is observed if the substrate is kept at elevated temperatures (>210 °C) during deposition. In both polymorphs, the molecules form quasi-onedimensional molecular columns along the b-axis with the molecular planes parallel to each other (π;π stacking), while the adjacent columns are arranged into a herringbone-like structure. The major difference between the two polymorphs is the angle between the normal direction of molecular plane and the stacking direction (b-axis), which is about 26° in the R-form and 45° in the β-form. The adsorption behaviors and growth mode of CuPc films on various substrates (metals, inorganic and organic semiconductors) have been extensively investigated.34,40-54 However, to our knowledge, no studies have been carried out on diamond surfaces. In this study, we report and compare the in situ adsorption of CuPc molecules on hydrogenated diamond C(001) 2  1:H as well as on bare diamond C(001) 2  1, focusing on the differences in interfacial molecular orientation and its orderliness. The inertness of the hydrogenated diamond surface is verified by synchrotronbased photoemission spectroscopy (PES). The evolution of molecular orientation and order in the first few layers is investigated by angular-dependent near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. Finally, we will show that the molecular orientation of CuPc molecules on diamond during initial growth stages is closely correlated to the competition between moleculesubstrate interactions and intermolecular interactions.

Experimental Section Sample Preparations. Experiments were carried out at the SINS beamline of Singapore Synchrotron Light Source (SSLS) in an ultrahigh-vacuum (UHV) chamber with a base pressure of 1  10-10 mbar.55 The diamond sample used in this study was a (39) Leznoff, C. C.; Lever, A. B. P. Phthalocyanines: Properties and Applications; VCH: New York, 1989. (40) Dufour, G.; Poncey, C.; Rochet, F.; Roulet, H.; Sacchi, M.; De Santis, M.; De Crescenzi, M. Surf. Sci. 1994, 319, 251. (41) Molodtsova, O. V.; Knupfer, M. J. Appl. Phys. 2006, 99, 053704. (42) Peisert, H.; Knupfer, M.; Schwieger, T.; Auerhammer, J. M.; Golden, M. S.; Fink, J. J. Appl. Phys. 2002, 91, 4872. (43) Lozzi, L.; Santucci, S.; La Rosa, S.; Delley, B.; Picozzi, S. J. Chem. Phys. 2004, 121, 1883. (44) Peisert, H.; Knupfer, M.; Fink, J. Surf. Sci. 2002, 515, 491. (45) Rocco, M. L. M.; Frank, K. H.; Yannoulis, P.; Koch, E. E. J. Chem. Phys. 1990, 93, 6859. (46) Kera, S.; Casu, M. B.; Bauchspiess, K. R.; Batchelor, D.; Schmidt, T.; Umbach, E. Surf. Sci. 2006, 600, 1077. (47) Wang, L.; Qi, D.-C.; Liu, L.; Chen, S.; Gao, X.; Wee, A. T. S. J. Phys. Chem. C 2007, 111, 3454. (48) Evangelista, F.; Ruocco, A.; Pasca, D.; Baldacchini, C.; Betti, M. G.; Corradini, V.; Mariani, C. Surf. Sci. 2004, 566, 79. (49) Koma, A. Prog. Cryst. Growth Charact. Mater. 1995, 30, 129. (50) Yamashita, A.; Hayashi, K. Adv. Mater. 1996, 8, 791. (51) Peisert, H.; Biswas, I.; Zhang, L.; Knupfer, M.; Hanack, M.; Dini, D.; Batchelor, D.; Chasse, T. Surf. Sci. 2006, 600, 4024. (52) Biswas, I.; Peisert, H.; Nagel, M.; Casu, M. B.; Schuppler, S.; Nagel, P.; Pellegrin, E.; Chasse, T. J. Chem. Phys. 2007, 126, 174704. (53) Resel, R.; Ottmar, M.; Hanack, M.; Keckes, J.; Leising, G. J. Mater. Res. 2000, 15, 935. (54) 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. (55) Yu, X. J.; Wilhelmi, O.; Moser, H. O.; Vidyaraj, S. V.; Gao, X. Y.; Wee, A. T. S.; Nyunt, T.; Qian, H. J.; Zheng, H. W. J. Electron Spectrosc. Relat. Phenom. 2005, 144, 1031.

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Figure 1. Crystalline structures of R- and β-forms of CuPc crystals. Table 1. Lattice Parameters of r- and β-Form of CuPc Crystals

R-form β-form

a (A˚)

b (A˚)

c (A˚)

β (deg)

mol/unit cell

25.92 19.6

3.79 4.79

23.92 14.6

90.4 120.6

4 2

4 mm  4 mm boron-doped (∼1016 cm-3) single crystal diamond with (001) orientation grown epitaxially to a thickness of about 1 μm on a type IIb diamond single crystal substrate. Prior to insertion into the UHV chamber, the diamond sample was cleaned by microwave hydrogen plasma at 800 °C to obtain a smooth, hydrogen-terminated 2  1 reconstructed (001) surface. After the plasma treatment, the diamond was transferred into the UHV chamber and heated to 200-400 °C to remove all residual surface contaminations while leaving the hydrogen termination intact. The surface cleanliness was verified by PES, and a sharp 2  1 pattern was observed by low-energy electron diffraction (LEED). To obtain a bare diamond C(100) 2  1 surface, the sample was annealed to about 1000 °C, and ultraviolet photoelectron spectroscopy (UPS) spectra (at hv = 60 eV) were recorded to monitor the presence of a negative electron affinity (NEA) peak in the low kinetic energy part of the spectra. If the NEA peak was observed, the annealing was repeated 20-30 °C higher until the NEA peak disappeared completely. This careful annealing procedure, with hydrogen coverage monitoring, fully dehydrogenates the diamond surface while at the same time avoids graphitization of diamond surface due to possible overheating. Organic Molecule Deposition. After careful degassing for several days, CuPc (purity 99.99%, Sigma-Aldrich) was sublimed in situ on diamond substrate kept at room temperature using a standard Knudsen cell (MBE-Komponenten, Germany), without further purification. The sublimation temperature of CuPc source was kept at 380 °C. Accurate temperature control is realized through a feedback loop, consisting of a thermocouple inside the Langmuir 2010, 26(1), 165–172

effusion cell and a power supply that controls the output power of the heating elements. The chamber pressure was maintained at less than 2  10-9 mbar during deposition. The nominal thickness of CuPc film was estimated from the attenuation of the bulk diamond C 1s peak intensity before and after deposition. PES Measurements. PES was recorded using a hemispherical analyzer (Omicron EAC2000-125) at normal emission angle with constant pass energy of 5 eV. The energy resolution is about 50 meV for all PES measurements. No charging effect was observed at the applied photon fluxes. The binding energies of all PES spectra were calibrated and referenced to the Fermi level of a sputtered gold foil in electrical contact with the diamond sample. The intensities of all PES spectra were normalized to the total incoming photon flux as measured by the photocurrent I0 of a refocusing mirror in front of the chamber. The secondary electron emission cutoff (for determination of vacuum level) was measured with the sample at -5 V bias to overcome the work function of electron analyzer (4.38 ( 0.05 eV). Thus, the sample work function (φ) was obtained through the equation φ = hν - W, where W is the spectrum width (energy difference between substrate Fermi level and secondary electron emission cutoff). NEXAFS Measurements. NEXAFS spectra of N K-edge were recorded in Auger electron yield (AEY) mode by detecting N KVV Auger electrons using the electron analyzer. Linearly polarized synchrotron light was used in the measurement. All spectra were normalized to the incident photon flux using I0 as a monitor of the photon beam intensity and later normalized to the same absorption edge step height well above threshold. The energy resolution was set at 0.1 eV for all NEXAFS measurements. Computational Details. In this work, we have performed density functional theory (DFT) calculations to determine the adsorption binding energy of CuPc molecules on hydrogenated diamond surface. The Kohn-Sham equations are solved in a DOI: 10.1021/la901204x

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plane-wave basis set, using projector augmented waves (PAW)56 to describe the electron-ion interaction, as implemented in the Vienna Ab-initio Simulation Package (VASP).57-59 Exchange and correlation effect are described by the generalized gradient approximation (GGA) in the form of Perdew, Burke, and Ernzerhof (PBE).60 The kinetic energy cutoff for the plane-wave basis set is 400 eV. In modeling the adsorption of CuPc molecule on the fully hydrogenated C(100)-2  1 surface, a slab model with a 3  6 supercell is employed. The slab model contains five carbon atomic layers with 36 C atoms per layer and 36 H atoms for the hydrogenated C(100)-2  1 surface, plus one hydrogen atomic layer with 72 H atoms to saturate the bottom dangling bonds. The thickness of the vacuum between the adsorbate and neighboring diamond surface is set at about 20 A˚, which is enough to separate the interaction between the periodic images. In the ionic structure relaxation, the bottom three carbon layers and the underlying hydrogen layer are fixed. The positions of the CuPc adsorbate and the remaining atoms are allowed to relax until the forces acting on each unconstrained atom were below 0.05 eV/A˚. The induced dipole moments in the vertical direction are corrected.61 Because of the large computational resources required, we considered seven possible adsorption models, in which CuPc molecule adopts lying-down or standing-up configurations. The (adsorption) binding energy for the adsorbate systems, Ead, was calculated by the equation Ead = Ediamond þ ECuPc - ETotal, where Ediamond and ECuPc are the energies of the fully hydrogenated diamond slab and the isolated CuPc molecule, and ETotal is the total energy of the system for CuPc adsorbed on the fully hydrogenated diamond surface.

Figure 2. C 1s PES spectra (photon energy, 350 eV) of CuPc on hydrogenated diamond with increasing thickness. C 1s spectra are all normalized to the same height for better viewing. Solid lines through the experimental data points demonstrate the results of the least-squares fitting.

Results and Discussion PES Studies of CuPc on Hydrogenated Diamond Surface. The strong chemical reaction involving covalent bond formation between CuPc and the surface dimers of bare diamond (001)-2  1 studied by synchrotron-based PES has been reported.12 The reactive nature of bare diamond surface originates from the highly strained double bonds with σ and π components of the surface dimers. Consequently, passivation by hydrogen atoms is expected to make hydrogenated diamond surface nonreactive toward alkene containing organic molecules such as CuPc. The evolution of C 1s PES spectra of hydrogenated diamond (001) surface with increasing CuPc thickness is shown in Figure 2. The cleanliness of pristine hydrogenated diamond surface after mild annealing (400-600 °C) to desorb loosely adsorbed hydrocarbon molecules is confirmed by the single C 1s bulk component located at 284.3 eV. Subsequent step-by-step deposition of CuPc leads to continuous changes to the C 1s line shape from that of the diamond substrate to that of the deposited CuPc film. At large thickness (50 A˚), the signal from the diamond substrate is completely attenuated, and the C 1s spectrum is characteristic of CuPc films with three distinctive peaks (labeled C1, C2, and SC2 in Figure 2). Both the relative peak positions and area ratios of individual CuPc components agree well with previous studies of CuPc films.40-44 They are attributed to the aromatic carbon of the benzene rings (C1), pyrrole carbon linked to nitrogen (C2), and a shakeup satellite of C2 carbon (SC2) related to intramolecular π-π* transitions, respectively. Although there should be an additional satellite feature associated with the aromatic carbon (C1) hidden within the C2 feature, the C2 peak can be fitted very well using a single Voigt-shape peak. (56) (57) (58) (59) (60) (61)

Bl€ochl, P. E. Phys. Rev. B 1994, 50, 17953. Kresse, G.; FurthmAuller, J. Phys. Rev. B 1996, 54, 11169. Kresse, G.; Joubert, D. Phys. Rev. B 1999, 59, 1758. Perdew, J. P.; Zunger, A. Phys. Rev. B 1981, 23, 5048. Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. Rev. Lett. 1996, 77, 3865. Makov, G.; Payne, M. C. Phys. Rev. B 1995, 51, 4014.

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Figure 3. N 1s PES spectra (photon energy, 500 eV) of CuPc on hydrogenated diamond with increasing thickness.

During the fitting process, the energy separations and the intensity ratio of individual peaks of CuPc were fixed, whereas their absolute energy positions and widths were free fitting parameters. As shown in Figure 2, the C 1s spectra measured after each deposition step of CuPc on hydrogenated diamond surface can be satisfactorily fitted by a superposition of spectra from diamond and CuPc with different spectral weights. Additional components are not necessary in the fitting process. After each deposition step, there were no observable BE shifts for both diamond and CuPc components within an experimental error of 0.05 eV. The corresponding N 1s spectra of CuPc on hydrogenated diamond appear as one dominant peak with its corresponding satellite located at 1.6-1.7 eV higher in BE (N-S), typical of CuPc bulk films (Figure 3) as well.44 The FWHM of the main peak is around 1.0 eV, and the binding energy is unchanged at all Langmuir 2010, 26(1), 165–172

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Figure 4. (a) UPS spectra (photon energy, 60 eV) of hydrogenated diamond after CuPc deposition of increasing thickness; (b) the corresponding secondary electron emission at the low kinetic energy region. The kinetic energy scale in (b) is corrected for an applied bias voltage of -5 V, and the cutoffs (vertical line) indicate the positions of the vacuum level relative to the Fermi level (EF). The secondary electron emission peaks are all normalized to the same height.

CuPc coverages, indicating the integrity of Cu-N bonds and hence the molecular structure. It should be noted that the chemical states of the aza-bridging nitrogens (the outer four nitrogens bonded with two carbon atoms) and the pyrrolic nitrogens (the inner four nitrogens bonded with carbon atoms and the central Cu metal atom) are very similar, and thus they cannot be separated in the main peak.44 In both the C 1s (Figure 2) and N 1s (Figure 3) spectra, the absence of additional components other than those from bulk diamond and CuPc films, especially at the earliest stages of CuPc deposition, clearly indicates that no chemical reaction occurs at the interface due to the hydrogen passivation. More importantly, the diamond C 1s main line exhibits no shifts upon CuPc deposition. This suggests that the electronic levels of the hydrogenated diamond substrate remain essentially unperturbed by CuPc deposition. In particular, interfacial charge transfer, which is generally observed between semiconductor/metal or semiconductor heterojunctions due to their Fermi energy difference,62 is negligible at this interface. Otherwise, the charge redistribution across the interface would modify the space charge layer in hydrogenated diamond, causing electrostatic band bending which would be manifested as a BE shift of diamond core-level states.63 Valence band spectra of hydrogenated diamond with deposited CuPc films at different thicknesses are shown in Figure 4a. The spectrum of hydrogenated diamond is typical for a diamond C(100) 2  1:H surface.64 Subsequent depositions of CuPc lead to gradual attenuation of diamond-associated valence band features and the emergence of several new components originating from CuPc. At a nominal thickness of 50 A˚, various molecular orbital derived states can be clearly resolved, while diamond features completely disappear. The spectrum of 50 A˚ CuPc film is typical of that for bulk CuPc with the HOMO peak at 1.70 ( 0.05 eV and its edge at 1.25 ( 0.05 eV (indicated by the vertical dashed lines in Figure 4a).42,44 No additional spectral structures close to the (62) L€uth, H. Solid Surfaces, Interfaces and Thin Films; Springer-Verlag: New York, 2001. (63) Qi, D.-C.; Chen, W.; Gao, X.; Wang, L.; Chen, S.; Loh, K. P.; Wee, A. T. S. J. Am. Chem. Soc. 2007, 129, 8084. (64) Maier, F.; Ristein, J.; Ley, L. Phys. Rev. B 2001, 64, 165411.

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Fermi level, which would otherwise indicate the appearance of interfacial gap states associated with chemical bond formations or strong charge transfer,63 can be identified in Figure 4a. Therefore, the evolution of the valence band spectra is consistent with the nonreactive nature of the heterojunction between CuPc and hydrogenated diamond as concluded from the core-level PES spectra. Figure 4b shows the secondary electron emission recorded in the low kinetic energy part of the UPS spectra. Pristine hydrogenated diamond exhibits a strong and sharp secondary emission peak with the emission cutoff at around 4.0 ( 0.1 eV, indicating the work function of 4.0 eV which is typical of a NEA diamond surface.63,65,66 This cutoff position barely changes with subsequent CuPc depositions, indicating a common vacuum level alignment across the CuPc/diamond interface without the formation of an interface dipole.67,68 The formation of an interface dipole is commonly associated with charge transfer and charge redistribution (e.g., chemical bond formation, Pauli repulsion) at the interface. For example, a significant reduction of the work function of bare diamond surface upon molecular chemisorption is commonly observed due to the intramolecular interface dipole layer induced.10-12 Physisorption of molecular acceptors with sufficiently high electron affinity on hydrogenated diamond often leads to interfacial charge transfer which builds up a substantial interface dipole.63,69 The absence of an interface dipole barrier at the interface between hydrogenated diamond and CuPc indicates that neither chemical reaction nor charge transfer occurs at the interface. Consequently, the molecule-substrate interactions are characterized by weak van der Waals-type interactions. NEXAFS of CuPc on Both Diamond Surfaces. In order to elucidate the influence of molecule-substrate interaction at the CuPc/diamond interface on molecular orientations, angulardependent NEXAFS was used to characterize the molecular (65) 314. (66) 219. (67) (68) (69)

Diederich, L.; Aebi, P.; Kuttel, O. M.; Schlapbach, L. Surf. Sci. 1999, 424, Diederich, L.; Kuttel, O. M.; Aebi, P.; Schlapbach, L. Surf. Sci. 1998, 418, Ishii, H.; Sugiyama, K.; Ito, E.; Seki, K. Adv. Mater. 1999, 11, 605. Cahen, D.; Kahn, A. Adv. Mater. 2003, 15, 271. Strobel, P.; Riedel, M.; Ristein, J.; Ley, L. Nature 2004, 430, 439.

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Figure 5. Angular-dependent N K-edge NEXAFS spectra for CuPc film with increasing thickness on (a) hydrogenated diamond and (b) bare diamond. The incidence angle θ is defined as the angle between the direction of the incident light and the normal direction of the substrate. All spectra are normalized to have the same absorption edge step height. The insets show the schematic film structures during growth.

orientational structures of CuPc on hydrogenated and bare diamond surfaces, respectively. Although an orientation transition from lying-down to standing-up for CuPc on bare diamond was already suggested by inspecting the evolution of C 1s corelevel spectra,12 more accurate information on the molecular orientation within molecular assemblies can be acquired by angular-dependent NEXAFS.70 In molecular systems, NEXAFS monitors the resonant excitations from the core level of a specific atomic species of a molecule (e.g., C 1s or N 1s) to its unoccupied molecular orbitals (i.e., π* or σ*). The intensity of the resonances has a strong polarization dependence with the incident synchrotron light; i.e., the resonance is strongest when the electric field vector E of the incident linearly polarized light is parallel to the π* or σ* molecular orbital and weakest when E is perpendicular to the π* or σ* orbital. For a flat, conjugated molecular system such as CuPc, the π* and σ* orbitals are directed essentially out-of-plane and in-plane, respectively.70 Therefore, polarization-dependent or angular-dependent NEXAFS is able to determine the molecular orientation. It is important to note that the molecular tilting angle as determined from NEXAFS is averaged over the whole probing area, and the molecules in fact have a Gaussian-like tilt angle distribution rather than a fixed tilt angle.71 Moreover, the molecular orientational order can be inferred from the intensity modulation amplitude defined as the (70) St€ohr, J. NEXAFS Spectroscopy; Springer: Berlin, 1992. (71) Petrovykh, D. Y.; Perez-Dieste, V.; Opdahl, A.; Kimura-Suda, H.; Sullivan, J. M.; Tarlov, M. J.; Himpsel, F. J.; Whitman, L. J. J. Am. Chem. Soc. 2005, 128, 2.

170 DOI: 10.1021/la901204x

intensity difference between a resonance at grazing incidence angle and normal incidence. In general, a larger intensity difference implies a narrower tilt angle distribution and higher orientational order, and vice versa.71 Figure 5a shows the angular-dependent NEXAFS spectra of the N K-edge at various coverages of CuPc on hydrogenated diamond. The first four sharp absorption peaks labeled A through D are assigned to the transitions from N 1s core level to individual π* orbitals, while the broad absorption features at higher photon energies correspond to transitions to σ* states.45-47 At all CuPc thicknesses, the π* resonances are always much higher at grazing incidence angle (θ = 70°) than those at normal incidence (θ = 0°), whereas the σ* resonances show the opposite behavior. The angular dependence of these features clearly indicates that CuPc molecules are orientationally ordered and nearly parallel to the hydrogenated diamond surface at all thicknesses. Using the intensity ratio of the most prominent π* resonance A at different incidence angles R(πA*) = IA(0°)/ IA(70°) and adopting 100% linearly polarized light, the average tilting angle (β) of molecular plane relative to the substrate can be calculated according to the equation70 1 IðθÞµ1 þ ð3 sin2 θ -1Þð3 cos2 β -1Þ 2

ð1Þ

The resulting average tilting angle β of the molecular plane is plotted against film thickness as shown in Figure 6. For comparison, the calculated values for the R and β polymorphs of CuPc Langmuir 2010, 26(1), 165–172

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Figure 6. Average molecular tilting angle as derived from eq 1 using the intensity variation of resonance A in the angular-dependent NEXAFS spectra for various CuPc film thicknesses on diamond substrates. The dotted lines through the data points serve as guides for the eye only. The squares denote the estimated values as found for the known R-form and β-form single crystal structures of CuPc, assuming the crystals lie on different cleavage planes as depicted in the inset illustrations.

single crystal are also shown in Figure 6, assuming that the deposited crystallites lie with their cleavage plane on the surface as sketched in the inset (also see Figure 1). As evident from Figure 6, CuPc molecules lie nearly flat at all thicknesses on the hydrogenated diamond surface with an average tilting angle around 27 ( 5°, close to the estimated value (26°) for the R-form crystalline structure of CuPc with its a-c plane parallel to the substrate. This indicates that the deposited CuPc molecules possibly self-organize into R-phase crystalline domains or crystallites on top of the hydrogenated diamond surface. The NEXAFS spectra from CuPc on bare diamond in Figure 5b are similar in shape but display an entirely different angular dependence evolution as compared to those of CuPc on hydrogenated diamond. At submonolayer coverage (1.2 A˚), the π* resonances are strongest at grazing incidence (θ = 70°), whereas they become the strongest at normal incidence (θ = 0°) at multilayer coverage. Moreover, at 5.7 A˚ thickness which is around 1 monolayer coverage, the angular dependence of both π* resonances and σ* resonances almost vanishes. Considering that NEXAFS probes a large sample area, the absence of angular dependence essentially indicates a disordered molecular orientation.47 This change of angular dependence against film thickness clearly suggests a reorientation of CuPc molecules from lying down at submonolayer coverage to standing up in multilayers. The transition must start within the first monolayer, resulting in the observed disordered molecular orientation. After the bare diamond surface is completely passivated by a monolayer of the molecules, the molecules subsequently grow standing up with improved orentational order. The calculated average molecular tilting angle β as a function of film thickness shown in Figure 6 reveals a gradual increase of the tilting angle with increasing film thickness, reaching a maximum of 75 ( 5°, which is slightly larger than that of R-CuPc with a-b cleavage plane parallel to the substrate. Therefore, multilayer CuPc on bare diamond are also likely to be in the R-phase. The organization and orientation of organic molecules, particularly conjugated organic semiconductors, on solid substrates has been the subject of extensive research due to its importance to device performance.32,33 In general, it is governed Langmuir 2010, 26(1), 165–172

by a complex balance between the intermolecular interactions and the molecule-substrate interactions. There have been numerous investigations on the molecular orientation of various phthalocyanine (Pc) systems on different substrates.34,42,46-54 Summarizing briefly, Pc molecules tend to adopt a standing-up geometry on most inert surfaces such as oxidized inorganic substrates (ITO and SiOx) as well as polycrystalline metal substrates, where the stronger van der Waals-type intermolecular interactions (e.g., π;π interactions due to intermolecular overlapping of π-orbitals, electrostatic quadrupoles interactions, London dispersion forces due to transient multipole interactions) dominate over the weaker molecule-substrate interactions.54 In contrast, a lyingdown adsorption geometry is favored on most single crystal metals,34,42,46 alkali halides,49,50 and reactive semiconductor surfaces,40,47,72 where the molecule-substrate interactions are stronger than the intermolecular interactions. As shown in this work, no covalent bonds are formed between CuPc molecules and the hydrogenated diamond substrate. In spite of this absence of strong covalent interactions, the CuPc molecule contains N atoms and an extended π-conjugated electron system that allow it to form N 3 3 3 H-C73 and π 3 3 3 H-C74 hydrogen bonding with the terminal H atoms of the hydrogenated diamond. This directional hydrogen bonding energetically favors a planar or near-planar adsorption geometry because the N atoms and especially the π-electrons of molecules can interact with more H atoms on the diamond surface due to the cooperative effect of the π 3 3 3 H-C hydrogen bonding.74 To further support this argument, we performed DFT calculations for the (adsorption) binding energy of both lying-down and standing-up molecules using seven possible adsorption configurations (Figure 7). The DFT calculations indeed confirm the lying-down orientation is energetically favorer over standing-up orientation because the (adsorption) binding energies for all three considered lying-down molecular (72) Baffou, G.; Mayne, A. J.; Comtet, G.; Dujardin, G.; Sonnet, P.; Stauffer, L. Appl. Phys. Lett. 2007, 91, 073101. (73) Jeffrey, G. A. An Introduction to Hydrogen Bonding; Oxford University Press: New York, 1997. (74) Nishio, M.; Umezawa, Y.; Honda, K.; Tsuboyama, S.; Suezawa, H. CrystEngComm 2009, 11, 1757.

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Figure 7. Optimized structures of several possible adsorption models (M1-M7) of CuPc on the hydrogenated C(001)-2  1 surface. Values in parentheses in each model are the (adsorption) binding energy (Ead) calculated by DFT calculations.

geometries are ubiquitously larger than those of the standing-up geometries. The smallest difference between the (adsorption) binding energies for lying-down and standing-up models is already greater than 0.22 eV (M1-M5). In particular, the negative (adsorption) binding energy for a certain standing-up model (M6) implies a strongly unfavored adsorption geometry. As a result, the molecule-substrate (hydrogen bonding) interactions are at least comparable in strength to the intermolecular interactions for CuPc on hydrogenated diamond. This competition between the intermolecular and molecule-substrate interactions results in a configuration that maximizes the intermolecular interactions by π;π stacking in bulklike herringbone structures, where at the same time the molecules tend to lie down on the substrate. The R-form CuPc crystal with its a-c plane parallel to the substrate could satisfy these two requirements simultaneously and therefore represents the most likely CuPc film structure on hydrogenated diamond (see Figure 5a, inset). A similar molecular arrangement was previously observed for CuPc grown on other substrates as well.53 In contrast, the strong chemical bonding at the bare diamond/CuPc interface12 causes the first layer molecules to adopt a lying-down geometry (Figure 5b), maximizing the overlaps between the molecular π-orbitals and the electronic wave functions of the substrate. While approaching completion of the first monolayer, the subsequently deposited molecules experience increasing difficulty adopting planar orientations due to steric hindrance of neighboring adsorbed molecules and incline away from the substrate.47 These different molecular orientations result in the observed disordered molecular organization within the first deposited monolayer. After the bare diamond surface is completely wetted by the first monolayer, the substrate is well screened and behaves like a weakly interacting van der Waals substrate such as oxidized substrates. Moreover, the orientationally disordered interfacial CuPc monolayer also induces a degree of roughness on the length scale of the molecules, further weakening the molecule-substrate interactions by reducing the effective adsorption site and preventing the lock-in of the initial lying adsorption geometry.51 As a result, at higher thicknesses the molecule-substrate interaction becomes almost negligible, and the intermolecular interactions and surface energy become the dominant parameters influencing the molecular orientation.

172 DOI: 10.1021/la901204x

In thicker films, the CuPc molecules preferentially grow with face-to-face stacking in a standing-up arrangement with respect to the substrate surface, forming a bulklike herringbone structure between adjacent layers (see Figure 5b, inset).52 This orientation maximizes the intermolecular π;π interactions and minimizes surface energy.47,51 Hence, the molecular orientational order in thick films is significantly improved. This self-ordering behavior is mainly driven by the dominant intermolecular π-π interactions between molecular aromatic rings.

Conclusions In summary, we have studied and compared the molecular orientations of a model organic semiconductor (CuPc) on hydrogenated and bare diamond surfaces by synchrotron-based PES and angular-dependent NEXAFS. On hydrogenated diamond, CuPc molecules are found to weakly interact with diamond substrate via van der Waals-type interactions. Neither chemical reactions nor charge transfer is observed at the interface as indicated by PES. Molecules adopt a nearly lying-down geometry throughout growth with a high degree of orientational order as a result of the competing molecule-substrate interactions and intermolecular interactions. This unexpected lying-down molecular geometry on hydrogenated diamond is supported by DFT calculations. On bare diamond, CuPc molecules undergo chemical reactions with the underlying diamond dimers by forming new covalent bonds at the interface. The molecular orientation experiences a transition from lying-down at submonolayer coverage to standing-up in multilayer films, accompanied by an orderdisorder-order transition in the molecular orientations during growth. These transitions are related to the switch from the dominant molecule-substrate covalent interactions in the first monolayer to weak intermolecular interactions in multilayers. These findings highlight the importance in understanding the complex interactions at the organic/inorganic semiconductor interface and have important implications for the development of diamond-based organic electronic devices. Acknowledgment. The authors gratefully acknowledge the support from National University of Singapore under Grants R-144-000-107-112 and R-144-000-106-305. This work was partly performed at SSLS under NUS Core Support C380-003-003-001, A*STAR/MOE RP 3979908M, and A*STAR 12 105 0038 grants.

Langmuir 2010, 26(1), 165–172