Tuning the Electron Affinity and Secondary Electron Emission of

The tuning of electron affinity and secondary electron emission on diamond (100) surfaces due to cycloaddition with 1,3-butadiene is investigated by ...
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Langmuir 2007, 23, 9722-9727

Tuning the Electron Affinity and Secondary Electron Emission of Diamond (100) Surfaces by Diels-Alder Reaction Dongchen Qi,† Lei Liu,† Xingyu Gao,*,† Ti Ouyang,‡ 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 May 3, 2007. In Final Form: July 11, 2007 The tuning of electron affinity and secondary electron emission on diamond (100) surfaces due to cycloaddition with 1,3-butadiene is investigated by photoemission experiments and density functional theory (DFT) calculations. A significant reduction in electron affinity up to 0.7 eV and enhancement of secondary electron emission were observed after 1,3-butadiene adsorption. The lowering of vacuum level via 1,3-butadiene adsorption is supported by DFT calculations. The C-H bonds in the covalently bonded organics on diamond contribute to the enhanced secondary electron emission and reduced electron affinity in a mechanism similar to that of C-H bonds on hydrogenated diamond surfaces. This combination of strong secondary emission and low electron affinity by the organic functionalization of diamond has potential applications in diamond-based molecular electronic 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 The biocompatibility and wide electrochemical potential window of diamond have also led to investigations on the organic functionalization of diamond, with potential applications in bioelectronics,11 electrochemistry,12 and pH sensors.13 There are several routes whereby surface functionalization can be achieved. * Corresponding author. Tel: (65)-6516-6362/2774/2603. E-mail address: [email protected] (X.G.); [email protected] (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. (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) Hamers, R. J.; Butler, J. E.; Lasseter, T.; Nichols, B. M.; Russell, J. N.; Tse, K. Y.; Yang, W. Diamond Relat. Mater. 2005, 14, 661-668. (12) 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. (13) Garrido, J. A.; Hartl, A.; Kuch, S.; Stutzmann, M.; Williams, O. A.; Jackmann, R. B. Appl. Phys. Lett. 2005, 86, 73504-73504.

Wang and co-workers14 performed the electrochemical reduction of aryldiazonium salts on nanocrystalline diamond and demonstrated the covalent coupling of the functionalized surface to DNA and proteins. Knickerbocker and co-workers15 used ultraviolet light to initiate a coupling reaction between a hydrogenterminated diamond surface and functionalized alkenes. This reaction mechanism is motivated by the principle that photoattachment reactions can be initiated via photoexcitation of electrons and holes in the surface space-charge region, followed by nucleophilic attack by an alkene at the surface.16 The cycloaddition reaction is a powerful method for the formation of C-C bonds and could provide another route to the controlled functionalization of dimer-reconstructed C(100)-2×1 diamond, since dimers on the clean surface are linked by a σ bond and a highly strained π bond17-19 and might be expected to exhibit reactivity patterns similar to those of organic molecules with CdC bonds such as alkenes and olefins. The Diels-Alder ([4+2] cycloaddition) reaction on clean diamond C(100)-2×1 has been previously investigated by electron energy loss spectroscopy (EELS)20 and Fourier transform infrared spectroscopy (FTIR);21 these studies showed that 1,3-butadiene is readily chemisorbed on C(100)-2×1 mainly by [4+2]-type cycloaddition, while [2+2] cycloaddition is not favored because of its much higher activation barrier.22 These findings were further supported by the FTIR study of the reaction of cyclopentene with diamond, which shows a very low sticking coefficient of (14) Wang, J.; Firestone, M. A.; Auciello, O.; Carlisle, J. A. Langmuir 2004, 20, 11450-11456. (15) Knickerbocker, T.; Strother, T.; Schwartz, M. P.; Russell, J. N.; Butler, J.; Smith, L. M.; Hamers, R. J. Langmuir 2003, 19, 1938-1942. (16) Strother, T.; Knickerbocker, T.; Russell, J. N.; Butler, J. E.; Smith, L. M.; Hamers, R. J. Langmuir 2002, 18, 968-971. (17) Thoms, B. D.; Butler, J. E. Surf. Sci. 1995, 328, 291-301. (18) Mercer, T. W.; Pehrsson, P. E. Surf. Sci. 1998, 399, 327-331. (19) Prelas, M. P. A.; Bigelow, L. K.; Popovici, G. Handbook of Industrial Diamonds and Diamond Films; Marcel Dekker: New York, 1998. (20) Hossain, M. Z.; Aruga, T.; Takagi, N.; Tsuno, T.; Fujimori, N.; Ando, T.; Nishijima, M. Jpn. J. Appl. Phys. 1999, 38, L1496-L1498. (21) Wang, G. T.; Bent, S. F.; Russell, J. N.; Butler, J. E.; D’Evelyn, M. P. J. Am. Chem. Soc. 2000, 122, 744-745. (22) Fitzgerald, D. R.; Doren, D. J. J. Am. Chem. Soc. 2000, 122, 1233412339.

10.1021/la701285h CCC: $37.00 © 2007 American Chemical Society Published on Web 08/17/2007

Tuning Electron Affinity of Diamond (100) Surfaces

cyclopentene.23 Very recently, it has also been demonstrated how the dimer structure influences reaction pathways for organic molecules by studying the different products when acrylonitrile24 and 1,2-cyclohexanedione25 are reacted with Si (100) and diamond (100) surface dimers. We have previously reported the adsorption of a series of unsaturated organic molecules including allyl alcohol, acrylic acid, allyl chloride, as well as acetylene and 1,3-butadiene on the clean diamond C(100)-2×1.26 Chemisorption via cycloaddition of these molecules was identified by high-resolution EELS (HREELS) and synchrotron radiation-based spectroscopy. Meanwhile, the organic adsorbed surfaces all showed substantially lowered electron affinity and enhanced secondary electron emission compared to the clean surface. The terminal C-H bonds of the bonded organics were proposed to explain this lowering of diamond electron affinity, similar to the NEA scenario of hydrogenated diamond.27 However, it should be noted that our previous studies only qualitatively demonstrated the lowering of diamond electron affinity by organic adsorption. Therefore, quantitative determination of the electron affinity value of functionalized diamond as a function of molecular uptake is needed to precisely tune diamond electron affinity by cycloaddition reactions. Second, the interpretation of the lowering of electron affinity as being the result of a dipole layer formed by the terminal C-H bonds of bonded organics would require a certain orientation of the C-H bonds so that they could contribute to a net perpendicular component of the dipole moment relative to the surface.27 Since the C-H bond orientation has hardly been derived from previous spectroscopy studies, density functional theory (DFT) calculations are needed to optimize the geometries of the reaction products. In this work, we investigate the adsorption of 1,3-butadiene on clean diamond (100)-2×1 using synchrotron-based photoemission spectroscopy (PES) and DFT calculations. The effect of chemisorbed 1,3-butadiene molecules on the electron affinity as well as the secondary electron emission of diamond is investigated both experimentally and theoretically. Choosing 1,3butadiene as the model organic molecule to investigate the tuning of diamond electron affinity is based on two considerations. First, [4+2] cycloaddition of 1,3-butadiene on C(100)-2×1 surfaces is already well established, and shows the highest sticking coefficient among all the organics studied on diamond surfaces.20-22,26 Second, the relatively simple and symmetric chemical structure of 1,3-butadiene reduces the complexity in modeling the reaction product by DFT calculations. Experimental Section Experiments were carried out at the SINS beamline of the Singapore Synchrotron Light Source (SSLS) in an ultrahigh vacuum (UHV) chamber with a base pressure of 1 × 10-10 Torr.28 The diamond sample used in this study was a 4 mm × 4 mm borondoped single-crystal diamond with (100) orientation, grown epitaxially on a type IIb diamond single-crystal substrate. The diamond sample was mounted on a direct heating sample holder with a silicon substrate underneath; diamond was heated by direct-heating of the (23) 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. (24) Schwartz, M. P.; Barlow, D. E.; Russell, J. N.; Butler, J. E.; D’Evelyn, M. P.; Hamers, R. J., Jr. J. Am. Chem. Soc. 2005, 127, 8348-8354. (25) Schwartz, M. P.; Barlow, D. E.; Russell, J. N.; Weidkamp, K. P.; Butler, J. E.; D’Evelyn, M. P.; Hamers, R. J. J. Am. Chem. Soc. 2006, 128, 11054-11061. (26) Ouyang, T.; Gao, X.; Qi, D. C.; Wee, A. T. S.; Loh, K. P. J. Phys. Chem. B 2006, 110, 5611-5620. (27) Maier, F.; Ristein, J.; Ley, L. Phys. ReV. B 2001, 64, 165411. (28) Yu, X.; Wilhelmi, O.; Moser, H. O.; Vidyaraj, S. V.; Gao, X.; Wee, A. T. S.; Nyunt, T.; Qian, H.; Zheng, H. J. Electron. Spectrosc. Relat. Phenom. 2005, 144, 1031-1034.

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Figure 1. Reaction scheme of [4+2] cycloaddition for 1,3-butadiene on the diamond C(100)-2×1 surface. silicon 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 surface. The diamond sample was further cleaned in situ by annealing to about 200400 °C to desorb loosely bound oxygen and other gaseous contamination. The surface cleanliness was verified by X-ray photoelectron spectroscopy (XPS), and a sharp 2×1 pattern was observed by low-energy electron diffraction for the hydrogenated surface. To obtain a clean diamond C(100)-2×1 surface, the sample was annealed to about 1000 °C, and ultraviolet photoelectron spectroscopy (UPS) spectra (at hν ) 60 eV) were recorded to monitor the presence of an 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, along with hydrogen coverage monitoring, fully dehydrogenates the diamond surface while at the same time avoiding graphitization of the diamond surface due to possible overheating. 1,3-Butadiene (Fluka >99%) was introduced into the UHV chamber through a precision leak valve without further purification. All doses were done at room temperature with the chamber pressure being monitored by an ion gauge. The dosing pressures used in this work were in the range of 10-8-10-6 Torr and were not corrected to account for the relative positions of the leak valve with respect to the sample and ionization gauge. All dosages were expressed in Langmuirs (L), where 1 L ) 1 × 10-6 Torr‚s. Secondary electron emission spectra were recorded using UPS with the photon energy set to 60 eV. Emitted photoelectrons were analyzed using a hemispherical electrostatic analyzer (Omicron EAC2000-125) at a constant pass energy of 5 eV. The energy resolution was 50 meV for all the UPS measurements. In order to overcome the work function of the analyzer (4.30 ( 0.05 eV), a -5.0V bias was applied to the sample. No charging effect was observed at the applied photon fluxes. All spectra were normalized to the photocurrent (I0) of the refocusing mirror in front of the chamber. Computational Methodology. In order to understand the geometry and the electron affinity of a diamond surface upon adsorption of 1,3-butadiene molecules, we performed first-principles DFT calculations using the plane wave basis VASP code29,30 within the generalized gradient approximation (GGA).31 Ultrasoft pseudopotentials32 were employed as the ionic potential for all the elements. Sampling k-points with 0.05 Å-1 separation in the Brillouin zone were used. All structures were optimized and relaxed such that the change in energy upon ion displacement was less than 1 meV.

Results and Discussion Enhanced Secondary Electron Emission and Reduced Electron Affinity by Diels-Alder Reaction. Figure 1 shows the [4+2] cycloaddition reaction scheme for 1,3-butadiene molecules on a clean diamond C(100)-2×1 surface.20,21 Upon reaction, the 1,3-butadiene molecules break the diamond dimer π bonds and form cyclohexene-like structures on the surface.26 The secondary electron emission of the diamond surface under different surface conditions and 1,3-butadiene dosages is investigated by recording the low-kinetic-energy regions of UPS spectra at a photon energy of 60 eV, as shown in Figure 2a. The (29) Kresse, G.; FurthmAuller, J. Phys. ReV. B 1996, 54, 11169-11186. (30) Kresse, G.; FurthmAuller, J. Comput. Mater. Sci. 1996, 6, 15-50. (31) Perdew, J. P.; Wang, Y. Phys. ReV. B 1992, 45, 13244-13249. (32) Vanderbilt, D. Phys. ReV. B 1990, 41, 7892-7895.

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Figure 2. (a) Low-kinetic-energy region of normal emission UPS spectra (hV ) 60 eV) of a diamond sample under different surface conditions. The kinetic energy scale is corrected for an applied bias voltage of -5 V. The inset is a magnification of the onset of the emission part, illustrating the shift of the vacuum level. The vertical lines indicate the position of Ec for hydrogenated surface (solid line) and for all the other surface conditions (dotted line). (b) The dependence of the secondary electron emission intensity (area of the sharp emission peak) and the work function on 1,3-butadiene dosage.

emission spectra of the clean diamond surface as well as that exposed to 1,3-butadiene below 10 L dosage are almost identical, featuring a very broad and low-intensity peak above 5.5 eV kinetic energy, which is typical of positive electron affinity (PEA) surfaces.33,34 At a dosage of about 10 L, a very sharp and intense peak centered at 5.10 eV with a full width at half-maximum (FWHM) of 0.23 eV appears, and the intensity is over 20 times larger than that of the clean diamond surface. Meanwhile, the onset of emission shifts about 0.42 eV to lower kinetic energy. Considering this low kinetic energy cutoff as the position of the vacuum level (Evac), the work function of the reacted diamond surface is therefore reduced by 0.42 eV, accordingly. With further dosage, the secondary electron emission increases continuously, but at a much slower rate, and the work function decreases correspondingly as shown in Figure 2b. The FWHM of the secondary electron peak increases from 0.23 eV at 10 L to 0.40 eV at 3300 L. No measurements were done with dosages larger than 3300 L because the diamond surface appears to be almost saturated as shown from the trends of the emission peak intensity and work function in Figure 2b. However, even at the highest dosage, the intensity of the sharp secondary electron emission peak reaches only 60% of the hydrogenated diamond surface. Many electron-emission studies have interpreted the presence of the sharp peak in the secondary electron emission spectra as evidence of an NEA surface without actually determining the (33) Diederich, L.; Kuttel, O. M.; Aebi, P.; Schlapbach, L. Surf. Sci. 1998, 418, 219-239. (34) Diederich, L.; Aebi, P.; Kuttel, O. M.; Schlapbach, L. Surf. Sci. 1999, 424, 314-320.

Qi et al.

Figure 3. Energy band diagram of (a) hydrogenated diamond (100) and (b) clean diamond (100) with subsequent 1,3-butadiene adsorption. The arrow indicates the tuning of electron affinity (vacuum level) by molecular adsorption until reaching the saturation dosage of 3300 L. All values are estimated to lie within an error of 0.15 eV.

electron affinity χ.35-37 It has been pointed out that the sharply peaked energy distribution can also be attributed to a small but positive electron affinity.38 In order to determine the electron affinity of diamond under various surface conditions, the position of the conduction band minimum (CBM) (Ec) must be measured relative to the vacuum level (Evac). There are several ways reported to determine Ec using electronspectroscopy. Diederich et al. used the cutoff energy position of a NEA peak and a low-kinetic energy peak or shoulder below the NEA peak to represent the positions of Ec and Evac, respectively, thus acquiring the electron affinity χ.33,34 However, this method depends on a distinct separation between the vacuum level cutoff and the NEA peak cutoff, which is absent in the present case (Figure 2a), possibly due to variations in crystal quality, surface preparations conditions and applied bias in the work function measurements. Yater et al. calculated the value of χ from the secondary electron emission spectra,38 in which they related a rapid intensity change between 8 and 15 eV in the secondary electron emission spectra to Ec through a consideration of electron scattering and emission mechanisms in the material. However, it has been pointed out that these relationships are affected by the amount of band bending at the surface and thus (35) Krainsky, I. L.; Asnin, V. M.; Mearini, G. T.; Dayton, J. A., Jr. Phys. ReV. B 1996, 53, 7650-7653. (36) Thomas, R. E. In MRS Symposia Proceedings; Dreifus, D., Collins, A., Beetz, C., Humphreys, T., Das, K., Pehrsson, P., Eds.; Materials Research Society: Pittsburg, PA, 1996; Vol. 416, p 263. (37) van der Weide, J.; Zhang, Z.; Baumann, P. K.; Wensell, M. G.; Bernholc, J.; Nemanich, R. J. Phys. ReV. B 1994, 50, 5803. (38) Yater, J. E.; Shih, A.; Abrams, R. Phys. ReV. B 1997, 56, 4410-4416.

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Table 1. The Electron Affinity Value χE and the Work Function OE for Different Diamond Surface Conditions Determined by the Low-Kinetic-Energy Part of the UPS Normal Emission Spectra at a Photon Energy of 60 eVa diamond

φE (eV)

χE (eV)

χc (eV)

C(100)-2×1:H C(100)-2×1 dose 1 L dose 10 L dose 60 L dose 360 L dose 1360 L dose 3300 L 25% coverage

4.0 ( 0.1 5.3 ( 0.1 5.3 ( 0.1 4.9 ( 0.1 4.8 ( 0.1 4.7 ( 0.1 4.7 ( 0.1 4.6 ( 0.1

-0.7 ( 0.15 0.9 ( 0.15 0.9 ( 0.15 0.5 ( 0.15 0.4 ( 0.15 0.3 ( 0.15 0.3 ( 0.15 0.2 ( 0.15

-2.32 0.28

a

-0.72

χC is the electron affinity value determined by DFT calculation.

are not suitable to determine Ec explicitly.39 Here, we used EcEF ) Egap- (EF - Ev) to determine the position of Ec relative to the Fermi level (EF). Although the diamond valence band maximum (VBM) (Ev) is not directly visible in the UPS valence band spectra, its energy separation from the C 1s of the diamond bulk component in XPS is constant and can be used to determine the position of VBM. This energy separation was derived from an independent calibration experiment40,41 to be 283.9 eV with an uncertainty of 0.1 eV. The C 1s binding energy of the bulk diamond was determined previously to be 284.7 eV for hydrogenated surfaces, and 285.0 eV for clean surfaces and subsequent organic adsorbed surfaces.26 With knowledge of the diamond band gap (5.5 eV), Ec is thus deduced to be 4.7 ( 0.1 and 4.4 ( 0.1 eV above EF, respectively. With the position of vacuum level Evac obtained from the low kinetic energy cutoff in Figure 2a, the electron affinity value χ can be calculated according to χ ) Evac - Ec for different surface conditions, as listed in Table 1. It is now clear how the electron affinity quantitatively decreases with increasing dosage of 1,3-butadiene. However, the electron affinity of the diamond sample at the saturation dosage is still about 0.9 eV higher than that of hydrogenated diamond. From the above analysis, we are able to sketch the energy diagrams of hydrogenated diamond surfaces and clean diamond surfaces with 1,3-butadiene adsorption, as shown in Figure 3. It is known that boron forms the acceptor level in diamond with an ionization energy of EA ) 0.36 eV. By requiring charge neutrality, the bulk Fermi level EF was calculated by Bandis and Diederich at 0.30 eV above the VBM (Ev) for a B doping level of 1016 cm-3.34,42 By comparing with the value of EF - Ev measured at the surface, the band bending (BB) values can be estimated. As shown in Figure 3, all surfaces exhibit a downward band bending with varying magnitudes, in agreement with earlier reports.34,43,44 This downward band bending implies hole depletion, which is commonly related to donor-type surface states.45-48 However, the band bending that usually extends over 200 nm into diamond bulk33 does not influence the precision of the electron affinity value reported (39) Kono, S.; Shiraishi, M.; Goto, T.; Abukawa, T.; Tachiki, M.; Kawarada, H. Diamond Relat. Mater. 2005, 14, 459-465. (40) Maier, F.; Graupner, R.; Hollering, M.; Hammer, L.; Ristein, J.; Ley, L. Surf. Sci. 1999, 443, 177-185. (41) Maier, F.; Ristein, J.; Ley, L. Phys. ReV. B 2001, 64, art. no. 165411. (42) Bandis, C.; Pate, B. B. Phys. ReV. B 1995, 52, 12056-12071. (43) Cui, J. B.; Ristein, J.; Ley, L. Phys. ReV. Lett. 1998, 81, 429-432. (44) Graupner, R.; Maier, F.; Ristein, J.; Ley, L.; Jung, C. Phys. ReV. B 1998, 57, 12397. (45) Bobrov, K.; Comtet, G.; Dujardin, G.; Hellner, L.; Bergonzo, P.; Mer, C. Phys. ReV. B 2001, 63, 165421. (46) Bobrov, K.; Comtet, G.; Dujardin, G.; Hellner, L. Phys. ReV. Lett. 2001, 86, 2633. (47) Bobrov, K.; Mayne, A. J.; Dujardin, G. Nature 2001, 413, 616-619. (48) Luth, H. H. In Solid Surfaces, Interfaces and Thin Films, 4th ed.; Springer: New York, 2001; Chapter 7.

above, since the probing depth of PES (