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Tailoring the Electron Affinity and Electron Emission of Diamond (100) 2 × 1 by Surface Functionalization Using an Organic Semiconductor Dongchen Qi,† 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, and Department of Chemistry, National UniVersity of Singapore, 3 Science DriVe 3, Singapore 117543 ReceiVed June 27, 2008. ReVised Manuscript ReceiVed August 29, 2008
We present here a synchrotron-based photoemission spectroscopy (PES) study of the organic functionalization of bare single crystalline diamond C(100) 2 × 1 surface with an organic semiconductorscopper phthalocyanine (CuPc). Our results reveal that CuPc undergoes chemical reactions with the bare diamond surface by covalently bonding to diamond dimers. The functionalizing molecules induce an interface dipole layer that increasingly reduces the work function and electron affinity of diamond with increasing CuPc coverage, transforming bare diamond from a positive electron affinity to a negative electron affinity (NEA) surface. Meanwhile, a significantly enhanced secondary electron emission yield accompanied the reduced electron affinity, as a result of the lowered electron emission barrier and enhanced electron conduction from diamond to vacuum through the grafted molecules. The work function and electron affinity modulation contributed by the induced interface dipole is estimated to be 0.7 eV, on top of the band bending contributions. Our results highlight the utilization of organic semiconductor molecular functionalization as a means to tailor the surface electronic properties of diamond and other conventional inorganic semiconductors. The unique combination of NEA, high electron emission, and organic semiconductor functionality on diamond could lead to the integration of molecular electronic function with diamond devices.
Introduction Recent progress in synthesizing diamond films using chemical vapor deposition (CVD) has led to significant advancements in the development of diamond-based electronic devices. 1-7 Among the numerous outstanding properties of diamond such as high thermal conductivity, extreme hardness, high carrier mobility, and surface conductivity, its true negative electron affinity (NEA) makes diamond an excellent candidate for electron-emitting devices such as field emission displays,1,3-5 electron multipliers,8 and cold cathode emitters.2,9,10 Figure 1. Ball and stick model of bare diamond C(100) 2 × 1. * Corresponding authors. E-mail:
[email protected] (X.G.); phyweets@ nus.edu.sg (A.T.S.W.). † Department of Physics. ‡ Department of Chemistry.
(1) Rouse, A. A.; Bernhard, J. B.; Sosa, E. D.; Golden, D. E. Appl. Phys. Lett. 2006, 75, 3417–3419. (2) Geis, M. W.; Efremow, N. N.; Woodhouse, J. D.; McAleese, M. D.; Marchywka, M.; Socker, D. G.; Hochedez, J. F. IEEE Electron DeVice Lett. 1991, 12, 456–459. (3) Wang, C.; Garcia, A.; Ingram, D. C.; Lake, M.; Kordesch, M. E. Electron. Lett. 1991, 27, 1459–1461. (4) Xu, N. S.; Latham, R. V.; Tzeng, Y. Electron. Lett. 1993, 29, 1596– 1597. (5) Nutzenadel, C.; KuTtel, O. M.; Groning, O.; Schlapbach, L. Appl. Phys. Lett. 1996, 69, 2662. (6) Hokazono, A.; Kawarada, H.; Ishikura, T.; Nakamura, K.; Yamashita, S. Diamond Relat. Mater. 1997, 6, 339–343. (7) Gluche, P.; Aleksov, A.; Vescan, A.; Ebert, W.; Kohn, E. IEEE Electron DeVice Lett. 1997, 18, 547–549. (8) Mearini, G. T.; Krainsky, I. L.; Wang, Y. X.; Dayton, J. A., Jr.; Ramesham, R.; Rose, M. F. Thin Solid Films 1994, 253, 151–156.
Diamond (100) is the most easily grown crystallographic face by CVD with very high quality and also the most technologically important single crystalline diamond surface. The bare C(100) surface naturally exhibits a 2 × 1 reconstruction in which surface atoms with unsaturated valences pair up to form dimers.11 The dangling bonds of each dimer pair form a highly strained double bond with σ and π components (Figure 1). The highly strained double (9) Geis, M. W.; Efremow, N. N.; Krohn, K. E.; Twichell, J. C.; Lyszczarz, T. M.; Kalish, R.; Greer, J. A.; Tabat, M. D. Nature 1998, 393, 431– 435. (10) Ito, T.; Nishimura, M.; Yokoyama, M.; Irie, M.; Wang, C. Diamond Relat. Mater. 2000, 9, 1561–1568. (11) Furthmuller, J.; Hafner, J.; Kresse, G. Phys. ReV. B 1996, 53, 7334– 7351.
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Figure 2. Chemical structure of the CuPc molecule.
bonds of bare diamond surface dimers exhibit reactivity patterns similar to organic molecules with CdC bonds such as alkenes and olefins and are able to undergo cycloaddition reactions with various unsaturated organic molecules such as 1,3-butadiene, acetylene, allyl alcohol, and acrylic acid. 12-15 Consequently, bare diamond can serve as an excellent platform for various surface organic functionalization, offering a wide range of possibilities for tailoring its chemical/physical properties and thus device characteristics. We previously discovered that the bare diamond surface electron affinity and electron emission yield can be deliberately tuned by organic functionalization with various simple organic molecules, 15 and this tuning effect is directly related to a dipole layer formed by the terminal C-H bonds of the covalently bonded organic molecules. 16 Most studies to date have focused on simple organic molecules with a few unsaturated bonds. We therefore investigate the functionalization of bare diamond surface with more complex organic molecules, especially those with conjugated π-electron systems, some of which are known to behave as semiconductors and may therefore function as active components in diamond-based organic electronic devices. Copper phthalocyanine (CuPc), a common organic semiconductor with excellent chemical stability and remarkable electronic properties such as a high hole mobility, represents one of the most promising candidates for organic electronics;17 it therefore serves as an excellent organic semiconductor model system. The CuPc molecule has a planar structure constituting four aromatic rings around a porphyrin-like central ring with a copper atom at its center (Figure 2). The electronic structures and growth modes of CuPc films on various substrates (metals, inorganic, and organic semiconductors) have been extensively investigated.18-31 Due to its (12) Hossain, M. Z.; Aruga, T.; Takagi, N.; Tsuno, T.; Fujimori, N.; o, T.; Nishijima, M Jpn. J. Appl. Phys 1999, 38, L1496–L1498. (13) 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, 732–733. (14) Wang, G. T.; Bent, S. F.; Russell, J. N.; Butler, J. E.; D’Evelyn, M. P. J. Am. Chem. Soc. 2000, 122, 744–745. (15) Ouyang, T.; Gao, X.; Qi, D.-C.; Wee, A. T. S.; Loh, K. P. J. Phys. Chem. B 2006, 110, 5611–5620. (16) Qi, D.-C.; Liu, L.; Gao, X.; Ouyang, T.; Chen, S.; Loh, K. P.; Wee, A. T. S. Langmuir 2007, 23, 9722. (17) Facchetti, A. Mater. Today 2007, 10, 28–37. (18) Dufour, G.; Poncey, C.; Rochet, F.; Roulet, H.; Sacchi, M.; De Santis, M.; De Crescenzi, M. Surf. Sci. 1994, 319, 251–266. (19) Molodtsova, O. V.; Knupfer, M. J. Appl. Phys. 2006, 99, 053704. (20) Peisert, H.; Knupfer, M.; Schwieger, T.; Auerhammer, J. M.; Golden, M. S.; Fink, J. J. Appl. Phys. 2002, 91, 4872.
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many unsaturated bonds, CuPc is expected to react readily with suitable inorganic semiconductor surfaces with dangling bonds. This has been confirmed by photoemission spectroscopy (PES) and scanning tunneling microscopy (STM) studies of the adsorption of CuPc molecules on Si(111) and Si(100). 18,28-31 However, to our knowledge no studies have been carried out on diamond (100) 2 × 1 surface, which bears certain structural similarities to the Si (100) 2 × 1 surface. In this work, we investigate the in situ adsorption of CuPc molecules on bare diamond C(100) 2 × 1, focusing on the interactions at the hybrid organic-inorganic interface, as well as the modulation of the interfacial electronic structures (e.g., electron affinity, work function, secondary electron emission yield, etc.). The interactions and electronic structures at the interface are explored by synchrotron-based PES. Experimental Section 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 Torr. 32 The diamond sample used in this study was a 4 mm × 4 mm boron-doped (∼1016 cm-3) single crystal diamond with (100) 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 (100) 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.33 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 hν ) 60 eV) was recorded to monitor the presence of a 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. (21) Lozzi, L.; Santucci, S.; La Rosa, S.; Delley, B.; Picozzi, S. J. Chem. Phys. 2004, 121, 1883. (22) Peisert, H.; Knupfer, M.; Fink, J. Surf. Sci. 2002, 515, 491–498. (23) Kera, S.; Casu, M. B.; Bauchspiess, K. R.; Batchelor, D.; Schmidt, T.; Umbach, E. Surf. Sci. 2006, 600, 1077–1084. (24) Biswas, I.; Peisert, H.; Nagel, M.; Casu, M. B.; Schuppler, S.; Nagel, P.; Pellegrin, E.; Chasse, T. J. Chem. Phys. 2007, 126, 174704-5. (25) Molodtsova, O. V.; Zhilin, V. M.; Vyalikh, D. V.; Aristov, V. Y.; Knupfer, M. J. Appl. Phys. 2005, 98, 093702-5. (26) Baffou, G.; Mayne, A. J.; Comtet, G.; Dujardin, G.; Sonnet, P.; Stauffer, L. Appl. Phys. Lett. 2007, 91, 073101-3. (27) Gorgoi, M.; Zahn, D. R. T. Org. Electron. 2005, 6, 168–174. (28) Rochet, F.; Dufour, G.; Roulet, H.; Motta, N.; Sgarlata, A.; Piancastelli, M. N.; De Crescenzi, M. Surf. Sci. 1994, 319, 10–20. (29) Kanai, M.; Kawai, T.; Motai, K.; Wang, X. D.; Hashizume, T.; Sakura, T. Surf. Sci. 1995, 329, L619-L623. (30) Wang, L.; Qi, D.-C.; Liu, L.; Chen, S.; Gao, X.; Wee, A. T. S. J. Phys. Chem. C 2007, 111, 3454–3458. (31) Wang, L.; Chen, S.; Qi, D.-C.; Gao, X.; Wee, A. T. S. Surf. Sci. 2007, 601, 4212–4216. (32) 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, 1031–1034. (33) Strobel, P.; Riedel, M.; Ristein, J.; Ley, L.; Boltalina, O. Diamond Relat. Mater. 2005, 14, 451–458.
C(100) 2 × 1 Electron Affinity and Electron Emission
Figure 3. C 1s PES spectra (photon energy, 350 eV) of CuPc on bare 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.
After careful degassing for several days, CuPc (purity 99.99%, Sigma-Aldrich) was evaporated in situ on diamond at room temperature using a standard Knudsen cell (MBE-Komponenten, Germany). The nominal thickness of CuPc film was estimated from the attenuation of the bulk diamond C 1s peak intensity before and after deposition. PES were 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. 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. 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).
Results and Discussion Covalent Functionalization of Diamond (100) 2 × 1 by CuPc. As a result of its highly strained double bonds (π plus σ) formed between surface dimer atoms, bare diamond C(100) 2 × 1 is expected to be reactive with CuPc molecules which also possess numerous unsaturated bonds. Indeed, the chemical reaction occurring at the CuPc and bare diamond interface is confirmed by C 1s and N 1s PES spectra. Figure 3 shows the C 1s PES spectra of bare diamond with increasing CuPc thickness. Bare diamond exhibits a pronounced surface state at about 0.9 eV lower binding energy (BE) than the main peak at 284.40 eV, which was previously attributed to the π-bonded dimers on the (100) surface.34 Upon the deposition of CuPc molecules, there is an apparent intensity decrease of the dimer related surface state, which (34) Graupner, R.; Maier, F.; Ristein, J.; Ley, L.; Jung, C. Phys. ReV. B 1998, 57, 12397–12409.
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disappears with the emergence of new components from CuPc above the thickness of 2.7 Å. The radical suppression of the bare diamond surface state has been previously observed during the adsorption of 1,3-butadiene and other simple unsaturated organic molecules, and it is attributed to the cycloaddition reaction that breaks the dimer π-bonds.15 Similarly, the gradual decrease of the surface state peak in Figure 3 also suggests a chemical reaction between CuPc and diamond dimer that eliminates the dimer π-bonds. However, neither the CuPc component nor additional components related to newly formed bonds between diamond and CuPc can be resolved below the thickness of 2.7 Å, possibly due to overlaps with the dominant diamond components. Subsequent depositions result in further attenuations of the bulk diamond peak, while CuPc components become dominant with three distinctive components (labeled C1, C2, SC2), which are characteristic of CuPc films.18-22 These three components have been attributed to the aromatic carbon of the benzene rings (C1), pyrrole carbon linked to nitrogen (C2), and a π-π* shakeup satellite of C2 carbon (SC2), 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 peak. The well-formed CuPc components (C1, C2, and SC2) at large thicknesses, as well as the existence of the π-π* shakeup satellite of C2 carbon (SC2), clearly indicate the integrity of CuPc molecules deposited after all diamond dimers are passivated by the first CuPc monolayer. During the least-squares fitting procedure, it was found that fixing both the relative energy positions and intensities of individual peaks of CuPc cannot yield satisfactory fitting results. This is attributed to the reacted first monolayer of molecules; therefore the intensity ratios of CuPc components were treated as free fitting parameters. It was found that the intensity ratio of C1 to C2 on bare diamond increases from 1.3 at low CuPc coverage (2.7 Å) to 3.0 at high coverage (36 Å). The increase in C1/C2 ratio is attributed to the change of CuPc molecular orientation due to the different attenuation of the pyrrolic (C2) and aromatic carbon (C1) signals, respectively. When CuPc molecules lie flat on the surface, the C 1s PES signals from pyrrolic and aromatic carbon atoms were similarly attenuated, whereas for molecules standing up on the surface the electrons from the pyrrolic C atoms (C2) are attenuated by aromatic rings (C1) closer to the vacuum interface.23,24 Therefore, the apparent increase in C1/C2 ratio as a function of film thickness suggests a change of CuPc molecular orientation from lying-down at submonolayer coverages to standing up in multilayers. This orientation transition may be associated with the transition from the dominant molecule-substrate covalent interactions in the first monolayer to weak intermolecular interactions in multilayers. The corresponding N 1s spectra are shown in Figure 4. The spectral shape of submonolayer CuPc deposited on bare diamond contrasts strongly with those of the multilayer films (right panel of Figure 4). The initial deposition of 0.15 Å CuPc leads to the formation of two distinct peaks located at around 397.8 eV (peak N1) and 398.8 eV (peak N2), respectively. Subsequent depositions lead to the gradual
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Figure 4. N 1s PES spectra (photon energy, 500 eV) of CuPc on bare diamond with increasing thickness. N 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.
Figure 5. Binding energy shifts of C1, N1, and diamond peak (Figures 3 and 4) as a function of CuPc thickness on bare diamond.
decrease of N2 intensity until it disappears at a thickness of 2.7 Å. Beyond the thickness of 2.7 Å, the spectra are dominated by the N1 peak with a small satellite peak located at 1.7 eV higher BE (N-S), resembling that of bulk CuPc films on other substrates.22 Therefore the N1 component is attributed to nitrogen atoms of pristine CuPc molecules, while N2 is related to the interfacial states of nitrogen atoms covalently bonded to diamond surface dimers. It should be noted that the N1 and N2 components at submonolayer coverages may originate from the two inequivalent types of nitrogen atoms (aza-bridging and pyrrolic) within the same CuPc molecule. Indeed nitrogen atoms at the aza-bridging sites (the outer four nitrogens bonded to two carbon atoms) are known to exhibit a higher reactivity than the pyrrolic ones (the inner four nitrogens bonded to carbon atoms and the central Cu metal atom, see Figure 2). This was demonstrated previously in potassium-doped CuPc, where it was found the K+ ions are prone to bond close to the azabridging sites. 25 More recently, using scanning tunneling microscopy (STM) and density functional theory (DFT) calculations, Baffou et al. showed the anchoring of phthalocyanine (Pc) molecules on the 6H-SiC(0001)3 × 3 surface through the formation of Si-N bonds between two adjacent Si dangling bonds of SiC and two opposite aza-bridging N of the Pc molecule, while the inner pyrrolic N remain intact. 26 Therefore, in our case it is very likely that a few azabridging nitrogen atoms within a CuPc molecule are directly forming covalent bonds with the diamond dimers and constitute the N2 component in Figure 4, whereas the remaining unreacted nitrogen species including both the pyrrolic and the aza-bridging ones form the N1 component. However, the involvement of pyrrolic nitrogen in the interfacial chemical reaction cannot be completely ruled out at the present stage. Previous studies on the adsorption of CuPc on Si(111) 7 × 7 and Si(100) 2 × 1 have shown strong chemical interactions between molecules and silicon dangling bonds through the formation of Si-C and Si-N bonds which lead
to a planar adsorption geometry of CuPc molecules.18,28-31 Similarly, our PES results also indicate that a similar chemical reaction occurs between diamond and CuPc molecules. The chemical reaction breaks the highly strained π bonds of surface dimers and directly couples the dimer atoms to C and N atoms of CuPc molecules through cycloaddition reactions, thus functionalizing the diamond surface. Although a detailed reaction scheme and configuration of CuPc on diamond cannot be formulated at the present stage, the observation of covalent bonding between nitrogen atoms and diamond dimers points to a lying-down geometry of CuPc molecules upon adsorption, which is in agreement with the evolution of C1/C2 ratio in C 1s PES spectra discussed previously. Figure 5 summarizes the binding energy shifts of the CuPc and diamond core-level components as a function of CuPc thickness. Below a thickness of 3 Å, the N1 peak shifts abruptly by 0.5 eV (Figure 4). The N1 peak continues to shift to higher BE at a much slower rate with increasing coverage and stabilizes above the thickness of 18 Å. As the CuPc components can only be resolved beyond the thickness of 2.7 Å in the C 1s spectra (Figure 3), data points for the C1 peak are not available at submonolayer coverage. Nevertheless, the trend of the C1 core-level shift is consistent with that of the N1 peak, indicating a “band bending” like energy shift in the CuPc film. Similar energy shifts of the CuPc film have been previously observed on gold20 and hydrogen-terminated silicon surfaces, 27 and it was found that the conventional electrostatic band bending model can hardly explain these energy shifts due to the very limited charge carrier density of intrinsic CuPc (∼107 cm-3) films. Instead they are attributed to the change in polarization (i.e., the distribution of charge) in the molecular layer from the vicinity of the interface to thick CuPc films. However, molecular polarization alone cannot account for the rather large energy shift (over 0.8 eV). Indeed, a partial charge transfer across the interface accompanying covalent bond formation should also contribute to the observed energy
C(100) 2 × 1 Electron Affinity and Electron Emission
Figure 6. Valence band spectra (photon energy, 60 eV) of diamond with CuPc of increasing thicknesses.
shifts. It is clear from Figure 5 that the core-level BE shifts occur most significantly within the submonolayer regime (below 4 Å), in agreement with the scenario of charge transfer induced by an interfacial chemical reaction. 35 Moreover, the “band bending” direction within the CuPc film indicates that electrons are transferred from the CuPc to diamond most probably via the newly formed covalent bonds, leaving the CuPc layers at the interface positively charged (electron depletion). Similar interfacial charge transfer and p-doping of CuPc has been observed on certain organic substrates, such as on conducting polymers (CPs)36 and on self-assembled monolayers (SAMs).37 The proposed charge transfer direction is further corroborated by the downward band bending in the diamond surface region (electron accumulation), as indicated by the subtle increase in diamond C 1s BE after CuPc deposition (Figures 3 and 5). Additional information on the electronic structures of the CuPc functionalized diamond surface can be extracted from the valence band structures shown in Figure 6. As with the corresponding C 1s spectrum, the valence band spectrum of bare diamond is characterized by a significant surface state near the valence band edge (1.2 eV from EF) which is related to the π-bonded surface dimer states. Therefore, the rapid intensity decrease of the surface state after the initial CuPc deposition is consistent with an interfacial chemical reaction. Further depositions lead to an overall attenuation of the diamond features and the emergence of CuPc orbitals with its highest occupied molecular orbital (HOMO) state resolved (35) Ishii, H.; Sugiyama, K.; Ito, E.; Seki, K. AdV. Mater. 1999, 11, 605– 625. (36) Peisert, H.; Knupfer, M.; Zhang, F.; Petr, A.; Dunsch, L.; Fink, J. Appl. Phys. Lett. 2003, 83, 3930–3932. (37) Chen, W.; Gao, X. Y.; Qi, D. C.; Chen, S.; Chen, Z. K.; Wee, A. T. S. AdV. Funct. Mater. 2007, 17, 1339–1344.
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(above a thickness of 5.7 Å). A closer inspection of the HOMO state reveals a shift toward higher binding energy with further CuPc deposition, consistent with the shifts of CuPc core-level components and therefore attributed to the same origins. Tailoring Surface Electronic Properties of Diamond by Organic Functionalization. The merit of organic functionalization of semiconductors is the promise of combining properties of both organic and inorganic materials, thereby presenting new functionalities to conventional inorganic semiconductor surfaces such as lubrication, chemical sensing, and biocompatibility.38 Moreover, the incorporation of organic layers at the interface can modify the surface electrostatic potential by giving rise to an interface dipole layer. Such a dipole layer, which either results from the dipole moments of the grafted molecules themselves or from the charge transfer between substrate and molecules or both, can in turn change the surface electron affinity and work function as well as secondary electron emission yield.39,40 Secondary electron emission recorded in the low kinetic energy part of the PES spectra (with photon energy of 60 eV) was used to monitor the variations of diamond surface work function and electron emission yield induced by CuPc functionalization (Figure 7a). The cutoff of the emission peak indicates the position of vacuum level (Evac) relative to the Fermi energy (EF) and hence the work function value (φ). The secondary electron emission from bare diamond surface is characterized by a broad and low intensity distribution with its emission cutoff at 5.3 eV (φ ) 5.3 eV), which is typical for a bare diamond surface with positive electron affinity (PEA). 16,41 The initial deposition of CuPc (0.15 Å) on bare diamond significantly shifts the vacuum level (VL) to lower kinetic energy by 0.6 eV, indicating a drastic reduction of the diamond work function by the same amount. Meanwhile, the secondary electron emission shows a sharp peak centered at 5.3 eV with its peak height enhanced over 3 times as compared to that of bare diamond. Subsequent depositions lead to further reduction of the work function which eventually drops to 3.6 eV at the thickness of 36 Å. The emission peak has the narrowest fwhm (full width at half-maximum) of about 0.5 eV at the thickness of 0.45 Å, and becomes moderately broadened with subsequent depositions. A quantitative determination of the electron affinity value χ requires knowledge of the position of conduction band minimum (EC) relative to the vacuum level Evac, which is not directly accessible in Figure 7a. However, it can still be determined indirectly by combining the position of diamond valence band maximum (EV) and its band gap (Eg) according to EC - EF ) Eg - (EF - EV). Although the position of EV is not visible in the valence band spectra of bare diamond shown in Figure 6 due to the presence of surface states, its separation to experimentally more accessible spectra features (38) Bent, S. F. Surf. Sci. 2002, 500, 879–903. (39) Borriello, I.; Cantele, G.; Ninno, D.; Iadonisi, G.; Cossi, M.; Barone, V. Phys. ReV. B 2007, 76, 035430-11. (40) He, T.; Ding, H.; Peor, N.; Lu, M.; Corley, D. A.; Chen, B.; Ofir, Y.; Gao, Y.; Yitzchaik, S.; Tour, J. M. J. Am. Chem. Soc. 2008, 130, 1699–1710. (41) Diederich, L.; Aebi, P.; Kuttel, O. M.; Schlapbach, L. Surf. Sci. 1999, 424, 314–320.
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Figure 7. (a) Secondary electron emission of bare diamond with increasing CuPc thickness (photon energy, 60 eV). The kinetic energy scale is corrected for an applied bias voltage of -5 V. The cutoff of emission (vertical line in graph) indicates the position of vacuum level (VL) relative to Fermi level (EF). The secondary electron emission peaks are all normalized to have the same height for better viewing. The actual peak height varies significantly as reported in (c). (b) Secondary electron emission of hydrogenated diamond with increasing CuPc thickness (photon energy, 60 eV). (c) Electron affinity and secondary electron emission intensity of bare diamond as a function of CuPc thickness.
such the C 1s core-level line of bulk diamond (Figure 3) is constant and can be used to infer the position of EV. The energy distances of EV to the C 1s core-level of bulk diamond was previously determined by an independent calibration experiment to be 283.9 ( 0.1 eV.42 The C 1s binding energy (bulk component) of bare diamond was previously determined to be 284.40 eV; therefore, EV is determined to be 0.5 ( 0.1 eV below the Fermi level (EF) at the surface region. With the diamond band gap of 5.5 eV, the conduction band minimum EC is then calculated to be 5.0 eV above EF. The electron affinity of the bare diamond surface is therefore calculated to be χ ) (Evac - EF) - (EC - EF) ) φ - (EC EF) ) 5.3 - 5.0 ) 0.3 eV ( 0.1 eV, consistent with the PEA nature of bare diamond surface. The electron affinity value for the functionalized diamond surface with different CuPc coverage is determined in the same way and is listed in Table 1 along with other determined energy levels. (42) Maier, F.; Ristein, J.; Ley, L. Phys. ReV. B 2001, 64, 165411.
Table 1. Energy Levels of Bare Diamond and Organic Functionalized Diamond with Increasing CuPc Thicknessesa diamond surface with CuPc [Å]
CB(1s)b [eV]
EF - EVc [eV]
EC - EFc [eV]
φc [eV]
χc [eV]
bare 0.15 0.45 1.2 2.7 5.7 9.0 18 36
284.40 284.55 284.60 284.62 284.56 284.55 284.55 284.55 N.A.
0.5 0.6 0.7 0.7 0.7 0.7 0.7 0.7 0.7
5.0 4.9 4.8 4.8 4.8 4.8 4.8 4.8 4.8
5.3 4.7 4.5 4.2 4.1 4.0 3.8 3.8 3.6
+0.3 -0.2 -0.3 -0.6 -0.7 -0.8 -1.0 -1.0 -1.2
a CB(1s): C 1s binding energy of bulk diamond component. EF - EV: energy distance between Fermi level and valence band maximum. EC EF: energy distance between conduction band minimum and Fermi level. φ: work function. χ: electron affinity. b With an uncertainty of 0.05 eV. c With an uncertainty of 0.1 eV.
The dependence of electron affinity against CuPc thickness is plotted in Figure 7c. It is obvious that the electron affinity is continuously reduced with increasing CuPc coverage, and the initial deposition of 0.15 Å already transformed bare
C(100) 2 × 1 Electron Affinity and Electron Emission
diamond from a PEA surface to a NEA one. The lowest reached electron affinity is -1.2 eV, comparable to NEA of the hydrogenated diamond surface.42,43 Figure 7c also displays the secondary electron emission peak height as a function of CuPc thickness. In the submonolayer range (below 5 Å), the emission intensity rises quickly with CuPc thickness and reaches about 30 times that of bare diamond at the thickness of 1.2 Å. Further depositions cause the emission intensity to decline exponentially with the film thickness, which becomes comparable again to that of bare diamond after the formation of CuPc multilayers but with the sharp emission peak still present, indicating of a narrow energy distribution of secondary electrons emitted. It is apparent from Figure 7c that both the electron affinity and secondary electron emission intensity change most within the first CuPc monolayer coverage (below 5 Å) where these molecules are in direct contact with the bare diamond surface and undergo chemical reactions as discussed previously. Our previous work demonstrated that cycloaddition reaction of diamond with organic molecules could induce a significant lowering of diamond electron affinity (work function) with enhanced secondary electron emission.15,16 Density functional theory (DFT) calculations reveal that the terminal C-H bonds of covalently bonded molecules form a dipole layer lowering the diamond vacuum level in a way analogous to that on the hydrogenated diamond surface. Recently, firstprinciple calculations of organic molecule adsorption on the silicon (100) surface demonstrated the tuning of silicon electron affinity through a dipole layer formed by adsorbed molecules39 and was experimentally confirmed by adsorbing a series of organic molecules with varying dipole moments or electron-donating abilities to the silicon substrate.40 Therefore, a similar mechanism is proposed to explain the observed work function and electron affinity variation of bare diamond surface after CuPc deposition. The numerous C-H bonds of the outer benzene rings inside CuPc molecules could potentially contribute to such a dipole layer which modifies the electrostatic potentials outside the surface and in turn influences the surface potentials. Although the chemical reaction between diamond and CuPc molecules constrains the initially deposited molecules to lie flat, the formation of new covalent bonds leads inevitably to the loss of conjugation of the molecular system and eventually distorts the molecular geometry.31,39 As a result, the terminal C-H bonds of the outer benzene rings are possibly bent out of the molecular plane and point toward the vacuum side to form a dipole layer. Additional interface dipoles may originate from the charge separation as a result of the interfacial charge transfer via newly formed covalent bonds. As discussed in the previous section, the dipole moment is therefore pointing from the diamond side (electron-accepting) toward the molecular side (electron-donating). With increasing coverage of adsorbed molecules, the planar average of the electrostatic potential field of the dipole layer provides a potential step that lowers the vacuum level. Although the internal energy distribution of secondary electrons inside diamond is largely unperturbed by the organic functionalization, the ability to probe those electrons by the analyzer (43) Cui, J. B.; Ristein, J.; Ley, L. Phys. ReV. Lett. 1998, 81, 429–432.
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is highly dependent on the position of vacuum level at the surface. As a result, the majority of the photoexcited electrons which then thermalize to the conduction band minimum (CBM) of diamond now have a lower barrier for vacuum emission, leading to a giant enhancement and narrower distribution of secondary electron emission upon organic depositions as compared to that from bare diamond. However, the energy distribution profile of bare diamond is still conserved, but it is hiding in the high intensity tail after the sharp emission peak. After the bare diamond surface is completely passivated, the subsequently deposited molecules no longer react with diamond and retain their flat geometry. Consequently, the dipole moments of the terminal C-H bonds average out in every direction owing to the symmetry of molecules, resulting in no net perpendicular component of the dipole moment contributed by the molecules henceforth. Therefore, the work function and electron affinity changes very slowly in the later deposition stage, and this relatively small work function change is attributed to the “band bending”-like energy shift within CuPc multilayers, which will be discussed later. In the meantime, electrons accumulated at the CBM of diamond begin to be attenuated by the unreacted CuPc overlayers through inelastic scattering, leading to an exponential decrease of the secondary electron emission intensity. However, the sharp emission peak feature is still retained for CuPc thickness below 9 Å, which indicates secondary electrons originating from diamond CBM still dominate. The slight shifting of the emission peak to lower kinetic energy direction is consistent with the downward band bending of diamond after organic depositions. At large CuPc thickness (i.e., 36 Å) the secondary electrons originally emitted from diamond CBM are largely inelastically scattered down to molecular unoccupied states, and its energy distribution does not reflects the internal energy distribution of electrons in diamond any more.44,45 As a result, the secondary electron emission of 36 Å CuPc is largely different from that from thinner films. The crucial role of interfacial chemical reaction in inducing the interface dipole is further elaborated by a comparative measurement of the work function of hydrogenated diamond (100) 2 × 1:H with increasing CuPc coverage. The chemical inertness of hydrogenated diamond due to hydrogen passivation effectively prevents diamond from reacting with the deposited CuPc molecules. As shown in Figure 7b, the vacuum level as indicated by the cutoff of the emission peak barely changes with increasing CuPc thickness, indicating a well aligned common vacuum level and therefore the absence of an interface dipole across the interface. The intensity of the secondary emission peak, however, decreases exponentially with CuPc thickness. This suggests that without an interfacial chemical reaction, CuPc molecules cannot establish an interface dipole layer and contribute to the large secondary electron emission. It is also worth mentioning that the relatively small electron affinity of CuPc (∼2.7 eV)20 as compared to the ionization potential of hydrogenated dia(44) Ueno, N.; Sugita, K.; Seki, K.; Inokuchi, H. Phys. ReV. B 1986, 34, 6386. (45) Ueno, N.; Sugita, K.; Koga, O.; Suzuki, S Jpn. J. Appl. Phys 1983, 22, 1613–1617.
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Figure 9. Schematic energy level diagram of CuPc on bare diamond surface. The lowest unoccupied molecular orbital (LUMO) position of CuPc is estimated by adding a CuPc transport band gap of 2.3 eV to the HOMO position.51 All values are estimated to lie within an error of 0.1 eV.
Figure 8. Work function and band bending magnitudes as a function of CuPc thickness on bare diamond surface. The band bending is calculated as the sum of diamond C 1s peak BE shift and CuPc N1 peak BE shift.
mond (∼4.2 eV)42 also eliminates the possibility of interface electron transfer from diamond to the adsorbed molecules, which would otherwise occur when hydrogenated diamond is exposed to air46 or adsorbs molecules with high electron affinity.47,48 To quantitatively evaluate the interface dipole induced by the adsorbed molecules, the “band bending” contribution to the measured work function variation should be isolated.49 It is noted that both band bendings in diamond and in the organic film, manifested by the binding energy shifts of the corresponding core-level components in the PES spectra, contribute to the decrease in work function upon CuPc deposition. In Figure 8, the decrease in work function is plotted together with the sum of band bending in diamond and CuPc film (i.e., the binding energy shift of diamond C 1s peak and CuPc N1 peak) as a function of CuPc nominal thickness. As evident from Figure 8, the change in work function is substantially larger than the band bending, with the difference gradually enlarging with increasing thickness and stabilizing at about 0.7 eV at a thickness of 5.7 Å. This is indicative of the magnitude of the interface dipole induced by reacted CuPc molecules. This evolution is consistent with the scenario whereby the interface dipole is fully developed within the first monolayer of CuPc and subsequent variations in work function as well as electron affinity are mainly attributed to “band bending” inside the CuPc films. Energy Level Alignment Diagram. Figure 9 depicts the schematic energy level diagram across the heterojunction of CuPc on bare diamond. As discussed in the previous section, pristine bare diamond has a positive electron affinity of +0.3 eV. The first monolayer CuPc molecules undergo a strong (46) Maier, F.; Riedel, M.; Mantel, B.; Ristein, J.; Ley, L. Phys. ReV. Lett. 2000, 85, 3472–3475. (47) Strobel, P.; Riedel, M.; Ristein, J.; Ley, L. Nature 2004, 430, 439– 441. (48) Qi, D.-C.; Chen, W.; Gao, X.; Wang, L.; Chen, S.; Loh, K. P.; Wee, A. T. S. J. Am. Chem. Soc. 2007, 129, 8084–8085. (49) Cahen, D; Kahn, A AdV. Mater. 2003, 15, 271–277.
chemical reaction with the underlying diamond dimers by forming new covalent bonds. The reacted molecules also create an interface dipole which significantly lowers the surface vacuum level below the CBM of diamond, transforming bare diamond to a NEA surface. As a result of the lowered surface barrier, a large number of hot electrons accumulated in the CBM of diamond may transfer to the unoccupied states of molecules and be eventually emitted into the vacuum in the form of a sharp and intense secondary electron emission peak. This is in contrast to the generally observed low conductivity and low emission intensity in organic overlayers. This electron conduction from an electron “reservoir” (diamond in our case) to the emission surface through a molecular layer has been recently demonstrated in the system of self-assembled diamondoid monolayer on metal substrates, where the emitted photoelectrons are monochromatic and form a sharp emission peak in the low kinetic energy part of the photoelectron spectra.50 By analogy to the metal-thiolate bonds formed between the SAMs and metal substrate, the covalent bonds between diamond and CuPc molecules also play an important role in the electron emission process by forming bridges facilitating electron conduction from diamond to molecules. After completion of the first monolayer, the subsequently deposited CuPc molecules are shielded from chemical reaction and retain their integrity. A downward energy shift in all electron energetics levels of CuPc which extends over 5 nm from the interface is also observed. Conclusions In this work, we have studied the organic functionalization of bare diamond C(100) 2 × 1 surface by CuPc, a model organic semiconductor molecule, and how the organic functionalization modulates the surface electronic properties (50) Yang, W. L.; Fabbri, J. D.; Willey, T. M.; Lee, J. R. I.; Dahl, J. E.; Carlson, R. M. K.; Schreiner, P. R.; Fokin, A. A.; Tkachenko, B. A.; Fokina, N. A.; Meevasana, W.; Mannella, N.; Tanaka, K.; Zhou, X. J.; van Buuren, T.; Kelly, M. A.; Hussain, Z.; Melosh, N. A.; Shen, Z. X. Science 2007, 316, 1460–1462. (51) Hill, I. G.; Kahn, A.; Soos, Z. G.; Pascal, J. R. A. Chem. Phys. Lett. 2000, 327, 181–188.
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of diamond. We have shown that CuPc molecules undergo strong chemical reactions with the underlying bare diamond dimers by forming new covalent bonds at the interface, adopting a lying-down geometry. Analysis of the work function and secondary electron emission reveals that the reacted molecules induce an interface dipole of 0.7 eV through distortion of the molecular plane and significantly lowers the surface vacuum level even below the CBM of diamond, transforming bare diamond to a NEA surface. As a result, the secondary electron emission from diamond is greatly enhanced by several orders due to lowered vacuum emission barrier and enhanced electron conduction from the diamond conduction band to the emission surface through attached molecules. After the completion of the first monolayer, the subsequently deposited molecules are shielded from interfacial reactions and experience an orientation transition from lying down to standing-up due to the much reduced molecule-substrate interactions. CuPc in multilayers also exhibit a downward shift in molecular energy levels away from the interface. Through organic functionalization of diamond by a organic semiconductor, a unique combination of NEA, high electron
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emission, and electronic functionality of organic semiconductor is achieved that may open a new route to the integration of molecular electronic function with diamond technology, a promise unparalleled by conventional surface functionalization with small and simple organic molecules. In particular, the C-C bonded interface by organic functionalization is more resistant than C-H interface of hydrogenated diamond to hydrolysis or moisture attack, thereby leading to an alternative route to fabricating stable diamond-based NEA cathodes. More importantly, our present approach may provide a foundation for the utilization of organic semiconductor molecules to tailor the surface electronic properties of conventional inorganic semiconductors via surface engineering and eventually achieve a controllable tuning of device characteristics and functionalities. 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 performed at SSLS under NUS Core Support C380-003-003001, A*STAR/MOE RP 3979908 M and A*STAR 12 105 0038 grants. CM801752J
