Chemical Functionalization of Diamond Surfaces by Reaction with

Nov 13, 2007 - Hao Wang, Jon-Paul Griffiths, Russell G. Egdell, Mark G. Moloney, and ... Diamond possesses a range of physical and chemical properties...
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Langmuir 2008, 24, 862-868

Chemical Functionalization of Diamond Surfaces by Reaction with Diaryl Carbenes Hao Wang, Jon-Paul Griffiths, Russell G. Egdell, Mark G. Moloney, and John S. Foord* Chemistry Research Laboratory, Department of Chemistry, UniVersity of Oxford, Mansfield Road, Oxford OX1 3TA, United Kingdom ReceiVed August 31, 2007. In Final Form: NoVember 13, 2007 A rapid route to the chemical functionalization of hydrogen-terminated diamond surfaces deposited by chemical vapor deposition involving their reaction with substituted diaryl carbenes has been investigated. To avoid difficulties in the handling of highly reactive compounds, the carbene is generated in situ from the thermal decomposition at 400 K of a thin film of the corresponding diaryl diazomethane precursor deposited at the diamond interface. X-ray photoelectron spectroscopy (XPS) has been used to verify that surface functionalization using two starting compounds, bis(4iodophenyl) diazomethane and bis(4-nitrophenyl) diazomethane, can be achieved using this approach in agreement with recent theoretical studies. The surface grafting density is measured to be around 1014 cm-2 in each case. The chemistry observed is found to be insensitive to the detailed properties of the diamond film and to the presence of oxygen contamination at the hydrogen-terminated diamond surface. We further demonstrate the utility of the approach, in the case of the bound nitrophenyl compound, by its reduction to the corresponding primary amine followed by reaction with fluorescein isothiocyanate to achieve fluorescent tagging of the diamond interface.

1. Introduction Diamond possesses a range of physical and chemical properties which make it an attractive candidate for use in the fabrication of chemical and biological sensors.1-4 For example, in electrochemicaldevices,whereboron-dopeddiamondfindsapplication,5-10 the overpotentials for solvent decomposition tend to be large, providing a wider “potential window” within which electroanalytical measurements can be formed than is possible with most other electrode materials. In addition, diamond electrodes normally show very low background currents, a high degree of reproducibility, and a resistance to surface poisoning or “electrode fouling”. Surface-induced conductivity at hydrogen-terminated diamond interfaces,11 in the vapor phase or in solution, arises as a result of the interaction with adsorbed electron acceptors, and this unique property can be exploited in the fabrication of chemically sensitive field effect transistor (CHEMFET) type devices.12-15 The nature of the conduction mechanism, along with the low electrical noise in diamond, enables much greater * To whom correspondence should be addressed. E-mail : john.foord@ chem.ox.ac.uk. Telephone :+44 (0) 1865 275967. (1) Nebel, C. E.; Shin, D.; Rezek, B.; Tokuda, N.; Uetsuka, H.; Watanabe, H. J. R. Soc. Interface 2007, 4, 439-461. (2) Xiao, X.; Wang, J.; Liu, C.; Carlisle, J. A.; Mech, B.; Greenberg, R.; Guven, D.; Freda, R.; Humayun, M. S.; Weiland, J.; Auciello, O. J. Biomed. Mater. Res., Part B 2006, 77B, 273-281. (3) Carlisle, J. A. Nat. Mater. 2004, 3, 668-669. (4) Yang, W.; Auciello, O.; Butler, J. E.; Cai, W.; Carlisle, J. A.; Gerbi, J. E.; Gruen, D. M.; Knickerbocker, T.; Lasseter, T. L.; Russell, J. N.; Smith, L. M.; Hamers, R. J. Nat. Mater. 2002, 1, 253-257. (5) Swain, G. M. Semicond. Semimet. 2004, 77, 121-148. (6) Swain, G. M. J. Electroanal. Chem. 2004, 22, 181-277. (7) Swain, G. M.; Anderson, A. B.; Angus, J. C. MRS Bull. 1998, 23, 56-60. (8) Einaga, Y.; Sato, R.; Olivia, H.; Shin, D.; Ivandini, T. A.; Fujishima, A. Electrochim. Acta 2004, 49, 3989-3995. (9) Rao, T. N.; Ivandini, T. A.; Terashima, C.; Sarada, B. V.; Fujishima, A. New Diamond Front. Carbon Technol. 2003, 13, 79-88. (10) Compton, R. G.; Foord, J. S.; Marken, F. Electroanalysis 2003, 15, 13491363. (11) Strobel, P.; Riedel, M.; Ristein, J.; Ley, L. Nature (London) 2004, 430, 439-441. (12) Bennett, A.; Gaudin, O.; Williams, O. A.; Foord, J. S.; Jackman, R. B. Mater. Res. Soc. Symp. Proc. 2007, 956, 221-227. (13) Rubio-Retama, J.; Hernando, J.; Lopez-Ruiz, B.; Hartl, A.; Steinmuller, D.; Stutzmann, M.; Lopez-Cabarcos, E.; Garrido, J. A. Langmuir 2006, 22, 58375842.

Figure 1. Raman spectra excited at 514.5 nm of (a) the “black” diamond sample and (b) the “white” diamond sample.

sensitivity than is possible with Si-based analogues. Diamond is thought to be “biocompatible”, suggesting “in vivo” applications.1,2 Overall, the outstanding optical and electronic properties of diamond, coupled with great chemical and mechanical stability, make it an excellent “platform” for sensor fabrication in general, if longevity and stable performance is needed. A real possibility of realizing the potential applications of diamond has emerged with the development of low cost chemical vapor deposition (CVD) techniques for the growth of thin films or free-standing wafers, which are now available through several commercial sources.16-18 In contrast to the high-pressure, hightemperature (HPHT) techniques, diamond CVD produces material in the “wafer” form with electronic and optical properties which can exceed those of gem-quality diamond and which can be controllably doped as n-type and p-type. Although early coatings (14) Haertl, A.; Garrido, J. A.; Nowy, S.; Zimmermann, R.; Werner, C.; Horinek, D.; Netz, R.; Stutzmann, M. J. Am. Chem. Soc. 2007, 129, 1287-1292. (15) Rezek, B.; Shin, D.; Watanabe, H.; Nebel, C. E. Sens. Actuators, B 2007, B122, 596-599.

10.1021/la702701p CCC: $40.75 © 2008 American Chemical Society Published on Web 01/05/2008

Chemical Functionalization of Diamond Surfaces

Langmuir, Vol. 24, No. 3, 2008 863 Scheme 1

were polycrystalline in nature and relatively rough, other synthetic forms such as single crystal and ultra-nanocrystalline materials are now beginning to emerge,17,18 enabling optimization for particular applications. To obtain a selective chemical response, the diamond sensor interface often needs to be chemically functionalized so that it demonstrates a specific interaction with a target analyte. For example, Yang et al. have functionalized diamond with DNA, for hybridization studies with solution DNA, and demonstrated the superior stability of functionalized diamond surfaces when compared to several other interfaces.4 The problem of diamond surface functionalization is quite general, since a number of possible applications of this material demand careful control of interfacial molecular properties. As-grown CVD diamond surfaces normally have hydrogen chemically bonded to them and are chemically inert. A number of different approaches have been examined to functionalize such a surface based on surface oxidation or halogenation,19the use of organic peroxides,20 electrochemical reduction of diazonium salts,21-23 or photochemical attachment of molecules containing vinyl end groups.4,24-27 The latter two approaches seem to be emerging as the most favored. However, differing “attachment” schemes possess particular advantages and limitations depending on the specific circumstances, and there is general continuing interest in the development of new routes for surface functionalization, particularly of the inert hydrogen-terminated diamond interfaces. We investigate a new surface chemical route to diamond surface modification in the present work. Our approach derives from a recently developed process for the surface modification of organic and inorganic polymers, and relies upon the high reactivity of carbenes derived from diazo compounds under actinic irradiation to insert or add to diverse chemical functions.28,29 This permits (16) May, P. W. Philos. Trans. R. Soc. London, Ser. A 2000, 358, 473-495. (17) Williams, O. A.; Nesladek, M. Phys. Status Solidi A 2006, 203, 33753386. (18) Isberg, J.; Hammersberg, J.; Johansson, E.; Wikstrom, T.; Twitchen, D. J.; Whitehead, A. J.; Coe, S. E.; Scarsbrook, G. A. Science (Washington, DC, U.S.) 2002, 297, 1670-1672. (19) Ohtani, B.; Kim, Y.; Hashimoto, K.; Fujishima, A.; Uosaki, K. Chem. Lett. 1998, 953. (20) Tsubota, T.; Hirabayashi, O.; Ida, S.; Nagaoka, S.; Nagata, M.; Matsumoto, Y. J. Ceram. Soc. Jpn. 2002, 110, 669-675. (21) Kuo, T.-C.; McCreery, R. L.; Swain, G. M. Electrochem. Solid-State Lett. 1999, 2, 288-290. (22) Wang, J.; Firestone, M. A.; Auciello, O.; Carlisle, J. A. Langmuir 2004, 20, 11450-11456. (23) Uetsuka, H.; Shin, D.; Tokuda, N.; Saeki, K.; Nebel, C. E. Langmuir 2007, 23, 3466-3472. (24) Nebel, C. E.; Shin, D.; Takeuchi, D.; Yamamoto, T.; Watanabe, H.; Nakamura, T. Diamond Relat. Mater. 2006, 15, 1107-1112. (25) Knickerbocker, T.; Strother, T.; Schwartz, M. P.; Russell, J. N.; Butler, J.; Smith, L. M.; Hamers, R. J. Langmuir 2003, 19, 1938-1942. (26) Strother, T.; Knickerbocker, T.; Russell, J. N.; Butler, J. E.; Smith, L. M.; Hamers, R. J. Langmuir 2002, 18, 968-971. (27) Haertl, A.; Schmich, E.; Garrido, J. A.; Hernando, J.; Catharino, S. C. R.; Walter, S.; Feulner, P.; Kromka, A.; Steinmueller, D.; Stutzmann, M. Nat. Mater. 2004, 3, 736-742. (28) Griffiths, J.-P.; Moloney, M. G. Fudan Xuebao, Ziran Kexueban 2005, 44(5), 772-773. (29) Moloney, M. G.; Ebenezer, W.; Awenat, K. U.S. Patent, USP6,699,527, 2004.

the introduction of color,30 biocidal activity, or biocompatibility31 and facilitates adhesion onto an otherwise inactive polymeric material. It is based on the attachment chemistry summarized in Scheme 1, in which a ring-substituted diaryldiazo compound 1 is deposited as a thin film on the hydrogenated diamond surface. The thin film is then heated to decompose 1 and form the corresponding diaryl carbene 2 in situ, which may then insert into the C-H or C-C bonds at the diamond surface to produce a new C-C bond. The substituents X and X′ on the aryl rings, which may be the same or different, may serve both to attenuate and control the reactivity of the carbene and to act as linking groups to enable the attachment of diverse functions to the diamond surface. The method is experimentally very simple and rapid, can be carried out on conductors and insulators alike, and avoids the obvious difficulties associated with handling highly reactive, volatile carbenes. Recently, a theoretical prediction appeared indicating that addition of carbenes to a diamond surface ought to be feasible.32 We confirm that this is indeed the case in the present study and demonstrate its application through the attachment of fluorescent functional groups to the diamond interfaces studied. 2. Experimental Section 2.1. Diamond Sample Preparation and Physical Measurements. Diamond samples produced by CVD techniques can have significantly different properties, depending on the substrates, nucleation, and growth conditions which are employed. To investigate if the chemistry occurring was particularly sensitive to the type of diamond sample, three samples of different character were studied in this work, all grown by microwave-assisted CVD (MWCVD). The first two interfaces investigated were the growth side of thick free-standing wafers displaying a sharply faceted morphology and a polycrystalline structure with a grain size of approximately 10 µm. One sample was grown using low methane/hydrogen ratios and displayed a typical “white” appearance representative of high phase purity; the second was grown at higher methane/hydrogen concentrations and displayed a black appearance, because of the incorporation of some sp2 material. Both samples exhibited high electrical resistivity. Raman spectra excited using Ar ion laser radiation at 514.5 nm are shown in Figure 1. Each sample shows a “diamond’ peak at ∼1330 cm -1, while the black diamond shows the expected broad peak at ∼1550 cm-1 due to sp2 impurities. The sensitivity to sp2 material is 50-100 times that for diamond at this wavelength.33 A third sample was purchased commercially and supplied as an electrochemical electrode from Windsor Scientific (U.K.). The material is doped with B to a concentration of around 1020 cm-3 and is therefore electrically conducting. It is polished to a root mean square (rms) roughness below 10 nm; further details of the material are published elsewhere.33 Before each sample was used in the experiments described below, it was subjected to a surface hydrogenation treatment, which we have used routinely in the past for studies of hydrogen-induced surface conductivity. The sample was immersed in acid baths and then rinsed in water and exposed to pure hydrogen plasma in a (30) Awenat, K.; Davis, P. J.; Moloney, M. G. Chem. Commun. 2005, 990992. (31) Griffiths, J.-P.; Moloney, M. G. GB Patent, PCT/GB2006/000139, 2006. (32) Xu, Y.-J.; Zhang, Y.-F.; Li, J.-Q. J. Org. Chem. 2005, 70, 6089-6092. (33) Bachmann, P. K.; Wiechert, D. U. Diamond Relat. Mater. 1992, 1, 42233.

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Wang et al. Scheme 2

a

a Reagents: (i) sodium nitrite, sulfuric acid, water, 0 °C; (ii) potassium iodide, water 50 °C; (iii) chromium oxide, acetic acid, water, 100 °C; (iv) hydrazine hydrate; (v) mercury(II) oxide, NaOH, EtOH, H2O.

Scheme 3

a

a Reagents: (i) nitric acid, sulfuric acid -40 °C; (ii) chromium trioxide, acetic acid, water 100 °C; (iii) tosyl hydrazine, THF, ethanol, 78 °C; (iv) 1 M sodium hydroxide(aq).

MWCVD reactor at a temperature of 750 °C for 30 min. After this treatment, the sample plate heater was switched off and the microwave power was gradually reduced, so that the sample cooled to about 200 °C in the presence of the plasma. At this point, the microwave power supply was switched off, and the sample was allowed to cool to room temperature in flowing hydrogen before being removed from the reactor. All XPS spectra were measured using a Scienta ESCA 300 apparatus, excited using a monochromatic Al KR X-ray source at 1486.6 eV. Fluorescence measurements were conducted using a Leica KY-F75U fluorescence microscope. 2.2. Synthesis of Chemical Precursors for Reaction with Diamond. Two substituted diazomethanes 1 were synthesized for reaction with diamond. The first contained iodine (X ) X′ ) I), since several I core levels have high cross sections in XPS, thereby facilitating the detection of the surface bound molecule by this spectroscopic technique. The second contained nitro functionalities (X ) X′ ) NO2) to investigate the wet chemistry of the functionalized material. Synthetic details are described below. As outlined in Scheme 2, 4,4′-diaminodiphenylmethane (3a) in concentrated sulfuric acid was added to sodium nitrite solution to produce the bisdiazonium salt (3b) which was treated with potassium iodide, extracted with dichloromethane (DCM), and purified to produce diiododiphenylmethane (3c). This was then reacted with chromium trioxide in acetic acid, followed by extraction with toluene to produce 4,4′-diiodobenzophenone (4), which was then converted to 4,4′-diiodobenzophenone hydrazone (5) by reaction with hydrazine hydrate. Finally, the first target molecule for reaction with the diamond substrates, bis(4-iodophenyl) diazomethane (1a, Scheme 2), was produced by reaction of 5 with mercury(II) oxide under alkaline conditions and subsequent extraction and purification. 4,4′-Dinitrodiphenylmethane (6b, Scheme 3) was prepared from diphenylmethane (6a) by reaction with concentrated sulfuric and nitric acid, and this was subsequently oxidized by chromium trioxide to produce 4,4′-dinitrobenzophenone (7a), which was in turn reacted with tosylhydrazine to produce 7b. After reaction with aqueous sodium hydroxide, the second target molecule, bis(4-nitrophenyl) diazomethane (1b) was readily obtained. Further information concerning the chemical synthetic work is given in the Supporting Information.

3. Results and Discussion After the hydrogenation treatment described in the preceding section and before carrying out any chemical processing, the diamond samples were analyzed by XPS to establish the initial chemical state of the surface, after exposure to atmospheric

Figure 2. Survey XPS spectra of (i) the hydrogenated diamond surface of the “black” diamond sample (inset shows narrow scan of the C 1s region) and (ii) the surface of the sample after deposition of a thin film of compound 1a.

conditions. A typical wide scan spectrum which is representative of the initial surfaces used throughout this work is shown in Figure 2(i). The main features in the spectrum are the C 1s photoemission peak at ∼284 eV binding energy and the C KLL Auger peak at an apparent binding energy of ∼1215 eV, confirming the elemental purity of the sample. An energy loss feature to higher binding energy of the C 1s peak is characteristic of the excitation of diamond plasmons, and the high-resolution scan of the C 1s spectral region (see Figure 2(i) inset) shows a single peak profile but with an asymmetric tailing to higher binding energies. As has been established elsewhere, we attribute this feature to the presence of C-H bonds at the surface, and also to the presence of weakly bound hydrocarbons which tend to form on diamond surfaces when exposed to the atmosphere, since the peak tailing is diminished by low temperature annealing in Vacuo.34 A small O 1s feature is visible in the spectrum at ∼532 eV binding energy. (34) Goeting, C. H.; Marken, F.; Gutierrez-Sosa, A.; Compton, R. G.; Foord, J. S. New Diamond Front. Carbon Technol. 1999, 9, 207-228.

Chemical Functionalization of Diamond Surfaces

The use of standard XPS sensitivity factors35 (O, 0.66; C, 0.25) to normalize peak areas to relative atomic concentration shows that the O/C ratio within the XPS sampling depth is around 1.5%. The majority of the oxygen present is likely to be adsorbed water vapor, which again is known to exist at diamond surfaces in ambient atmosphere,11 rather than chemisorbed oxygen, since the signal diminishes significantly if the sample is annealed in Vacuo to 450 K to remove weakly adsorbed material but not chemisorbed oxygen. As verified from conductivity measurements in the case of the B-free samples, the samples exhibited surface conductivity in air, as expected for hydrogenated diamond. The surface preparation and XPS characteristics in the present work are closely similar to those described in other detailed studies of functionalization, with an O contamination level deduced from XPS that would appear to be somewhat lower than typical in the present instance; although the hydrogenated diamond surface is normally described as being inert in the atmosphere, we have reported previously that a slow increase in surface oxygen can be detected over time in the ambient surroundings,36 so it is helpful to work with freshly hydrogenated samples if the oxygen level is to be minimized. Deposition of a thin film of bis(4-iodophenyl) diazomethane (1a) was carried out by preparing a 1% (w/v) solution in DCM and dropping it onto the hydrogenated diamond sample in the laboratory atmosphere. Solvent evaporation left a thin film of organic material, which was just visible under an optical microscope. The resulting layer was then characterized by XPS; a typical wide scan spectrum recorded after deposition on the black diamond sample is shown in Figure 2(ii). The spectrum is dominated by the C 1s peak at ∼284 eV and the I 3d5/2 and 3d3/2 core peaks at ∼622 and 634 eV, respectively, along with C and I Auger structures at 1220 and 970 eV, respectively; other weaker I photoemission peaks at 50 eV (I 4d), 877 eV (I 3p3/2), and 932 eV (I 3p1/2) are also visible. Although 1a contains N, the expected N 1s core line at 399 eV cannot be discerned in the spectrum, but an O 1s peak at 530 eV is detected. Use of XPS sensitivity factors35 suggests an O/C ratio of 0.08 and an I/C ratio of 0.13 within the spectroscopically sampled region. Since the deposited layer should be significantly thicker than the XPS sampling depth, only the deposited layer is analyzed. As expected, the I:C stoichiometry is consistent with the empirical formula for compound 1a. A notable feature of the deposited thin film at this stage is that it is easily removed by washing with DCM to restore an XPS spectrum in which no iodine signal is visible. This indicates that no I-containing species have become covalently attached to the surface. The lack of a detectable N signal in XPS and the presence of some O would appear to indicate that some superficial decomposition and oxidation of the layer occurs, either through reaction with the atmosphere or moisture or through decomposition under the X-ray flux used for XPS analysis. However, this degradation is probably not extensive, since the powdered form of the precursor is fairly stable when handled in the laboratory atmosphere. In further experiments, XPS measurements were made after vacuum annealing of the deposited layer to 400 K for 5 min and after subsequent DCM washing of the layer thereby produced. Wide scan XPS data are shown in Figure 3. It can be seen that heating the layer produces very little apparent change in the XPS spectrum. However, striking differences to unheated layers are found when the heated layer is washed with DCM. Although, (35) Wagner, C. D.; Davis, L. E.; Zeller, M. V.; Taylor, J. A.; Raymond, R. M.; Gale, L. H. Surf. Interface Anal. 1981, 3, 211. (36) Foord, J. S.; Wang, J.; Lau, C. H.; Hiramatsu, M.; Vickers, J.; Jackman, R. B. Phys. Status Solidi A 2001, 186, 227-233.

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Figure 3. Survey XPS spectra of a thin film of compound 1a (a) on the “black” diamond sample; (b) after heating the sample to 400 K for 5 min; (c) after the thermal treatment and subsequent washing with dichloromethane; and (d) with the same preparation treatment but for the “white” diamond sample.

the I signal diminished during the washing procedure, a significant amount of I now remained on the diamond surface and could not be removed by extensive solvent rinsing or sonication. This is in contrast to the situation when the film was washed with DCM without first carrying out the heating process. In this case, all the I is easily removed. The final elemental I/C ratio calculated with the appropriate sensitivity factors35 for the I 3d5/2 and C 1s transitions (C, 0.25; I, 6) is approximately 0.01. Essentially identical results were obtained irrespective of which type of diamond sample was used. In Figure 3, for example, we compare the spectrum for the black and white diamond; similar spectra are observed in both cases. High-resolution XPS spectra of the C 1s region, recorded during the experiments reported above, are shown in Figure 4 along with sensitive scans of the energy loss region to higher binding energies; corresponding high-resolution spectra of the I 3d and O 1s spectral regions are illustrated in Figure 5. As noted above, the spectra for the deposited thin film and the film after heating are relatively similar. The C 1s and O 1s regions also show comparable features. In each case, a low binding energy peak is seen with a broad peak structure extending to higher binding energies. The I 3d spectral region also shows a peak, at 620.5 eV, with a higher energy additional component. A peculiar characteristic of all these spectra is that the extent and intensity of the peak broadening to higher binding energies varied during repeated sample preparations, with the differing spectral regions showing a comparable degree of broadening in any given XPS spectrum. This behavior therefore cannot be associated with normal chemical shift effects that are associated with differing chemical forms. It is instead associated with differential charging of the sample as a result of the photoionization process inherent to XPS, because of the presence of insulating organic molecular phases on the surface of a thickness sufficient to support the buildup of static charge. Molecules in intimate electrical contact with the underlying diamond photoemit in the “parent” photoemission band, whereas others, which may, for example, be in thicker areas of the film, contribute to the broad tail at higher binding energies due to charging. The boron-doped and undoped diamond samples behaved similarly, so it is apparent that the differential charging is associated with the organic thin film rather than the underlying substrate. Apparently, the thick insulating film initially deposited undergoes some chemical

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Figure 4. Narrow scan C 1s spectra of a thin film of compound 1a on diamond normalized to constant C 1s peak height: (a) after deposition; (b) after heating to 400 K for 5 min; (c) after washing with dichloromethane; (d) expanded scan of the region to higher binding energy of the parent C 1s peak for the sample in (c) presented on an energy loss scale referenced to the C 1s peak maximum; and (e) as for (d) but for a hydrogenated diamond surface.

changes when heated, but the organic matter remains on the sample surface after this treatment and can be washed off to a large extent using DCM solvent. Bearing in mind from the known bulk chemistry that the heat treatment is expected to generate the reactive carbene in situ, the most likely reaction occurring in the bulk of the film is an ensuing dimerization reaction to produce tetra(4-iodophenyl)ethene. These units are soluble in DCM and therefore are easily washed away, and since the heated film has the same C:I stoichiometry as the starting phase, little change occurs in the XPS spectrum as a result of the heating cycle alone. However, after solvent washing, a different picture arises. The C 1s spectrum is restored to a state which is very similar to that seen at the start of the experiments, suggesting that the major part of the deposited phase has dissolved away. That some species become chemically adsorbed during the heating procedure is however clearly demonstrated by the presence of iodine in the XPS spectrum. We now turn our attention to what can be deduced concerning the nature of these chemisorbed species attached to the diamond surface. A useful insight can be obtained by consideration of the I 3d high-resolution spectra in Figure 5. Three peaks for each spin orbit component are clearly discernible in the XPS spectrum of the deposited layer, both before and after thermal annealing. The highest binding energy feature displayed variable broadening as discussed above, and it is simply due to sample charging. In

Wang et al.

Figure 5. Narrow scan I 3d (top) and O 1s (bottom) spectra of a thin film of compound 1a on diamond normalized in intensity to the C 1s peak height: (a) after deposition; (b) after heating to 400 K for 5 min; and (c) after washing with dichloromethane. The I spectrum in (c) is shown at ×5 sensitivity compared to corresponding scans in (a) and (b).

contrast, the peaks at 618 and 620.7 eV remain at constant energy and arise from distinct chemical species. Extensive tabulated XPS chemical shift data are available for I.37 When in the anionic or chemisorbed form, such as in CsI or adsorbed at a silver surface, the binding energy is relatively low (in the range 618619 eV). In contrast, when in a molecular form such as in CH3I or C2H5I, the binding energy rises to around 620.8 eV. XPS studies of CF3I adsorption on diamond indicate an I 3d5/2 binding energy of around 620 eV dropping to 619 eV when dissociation occurs to bond I directly to the diamond, or even lower to 616617 eV if HI is produced on the surface.38 In the light of this information, it seems clear that the main I 3d5/2 peak at 620.7 eV, which is seen both for the initially deposited film and for the final chemisorbed layer, is representative of I bound in the form of compound 1a. The weaker peak at 617 eV is probably associated with some decomposition product such as adsorbed I or HI, in view of the low binding energy. We have previously inferred from the lack of an N 1s signal that some surface decomposition of the deposited layer occurs, and this is a second indicator. The dominant I signal from the adsorbed layer at 620.7 eV thus provides a good indication that the I present is bound (37) Wagner, C. D.; Naumkin, A. V.; Kraut-Vass, A.; Allison, J. W.; Powell, C. J.; Rumble, J. R., Jr. NIST X-ray Photoelectron Spectrscopy Database, NIST Database 20, Version 3.4. (38) Smentkowski, V. S.; Yates, J. T., Jr.; Chen, X.; Goddard, W. A., III. Surf. Sci. 1997, 370, 209-231.

Chemical Functionalization of Diamond Surfaces

to the surface in the molecular form shown schematically in Scheme 1, and thus that the carbene reaction with the hydrogenated diamond surface has indeed taken place. Other indications are consistent with this view. The spectral region above the C 1s peak from the functionalized surface shown in Figure 4b shows weak loss features at around 5 and 7 eV, which are consistent with the shakeup loss structures observed due to π-π* excitations of the aromatic phenyl ring.39 The surface concentration of iodine achieved is also consistent with the geometric properties of the adsorbed bis(4-iodophenyl)methyl residue rather than, for example, a saturated chemisorbed layer of iodine atoms. For a single adsorbed atomic monolayer of a species A adsorbed on a uniform solid B, it is simply shown that the spectral intensity ratio is given by the equation

IA ) IB P

nASA

∫ DBSBe ∞

A 0

-x/λB

) dx

nASA PADBSB λB

where nA is the number density of the adsorbed species A, DB is the atomic density of atoms in the bulk solid, IB is the inelastic mean free path of the photoemitted electrons from the bulk solid, PA is their transmission probability through the adsorbed layer A, and x is the distance below the solid surface40. Si is the XPS surface sensitivity factors which can be obtained from the conventional bulk factors35 by dividing by the appropriate value of λ; the ratio of bulk factors for the I 3d 5/2 transition in comparison to C 1s is 24, so the ratio of surface sensitivity factors will be around 29 assuming reasonable values for the mean free path. IB has been recently measured in diamond films for Al KR C 1s electrons to be 1.7 nm,41 and the density of diamond is 1.8 × 1023 atoms cm-3. Use of these factors in the equation above, assuming PA ) 1 yields an I atom density in the range 1-2.4 × 1014 cm-2 for the different repeat experiments carried out and, given that each bound complex contains 2 I atoms assuming the structure in Scheme 1, gives a concentration for the bound diphenyl complex of half these values. (The adsorbed layer attenuated the C 1s electron flux by less than 20%, so it is sufficient to ignore this factor for the sake of these ballpark calculations.) It should also be noted that we have assumed that the overlayer makes a negligible contribution to the total measured C 1s intensity. This is justified, since it is clear from the XPS sensitivity factors for C and I (and indeed from the measured data in Figure 3) that the actual C1s contribution from the overlayer is several times less than the I signal intensity, and so it is indeed negligible compared, for example, to the total C 1s intensity in Figure 3c and d. The conclusion is that the surface concentration of the bound complex achieved in the experiments is as a maximum about 1/10 of what is observed in a simple close-packed atomic layer. We do not have detailed knowledge of the bonding geometry of the organic complex to the surface, but estimates based on the Connolly solvent-excluded volume of sensible analogues, or close-packing calculations using reasonable bond distances and packing distances between phenyl rings appropriated to solid benzene would yield an upper limit of around 3 × 1014 molecules cm-2 if the rings were “vertical”; tilted mercaptobiphenyl derivatives on Au have a packing density of ∼1.7 × 1014 cm-2.42 It would not be expected that an ordered close-packing would (39) Nordfors, D.; Nilsson, A.; Maartensson, N.; Svensson, S.; Gelius, U.; Lunell, S. J. Chem. Phys. 1988, 88, 2630-2636. (40) Briggs, D.; Seah, M. P. Practical Surface Analysis - Auger and X-ray Photoelectron Spectroscopy; Wiley: New York, 1990. (41) Zemek, J.; Potmesil, J.; Vanecek, M.; Lesiak, B.; Jablonski, A. Appl. Phys. Lett. 2005, 87, 262114/1-262114/2. (42) Ulman, A. Acc. Chem. Res. 2001, 34, 855-63.

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in fact be achieved in these experiments, given that the functionalization is essentially achieved at the interface between two solids (the diamond and the thin deposited film) where limited diffusion can take place. The number density achieved corresponds reasonably to the maximum which would be anticipated for a disordered layer of the diphenyl complex, assuming the whole diamond surface was involved. By way of comparison, the grafting density achieved corresponding to the cross section of a single vertical phenyl ring on diamond using electrochemical attachment chemistry is 3 × 1014 cm-2, while photochemical attachment of the trifluoroethyl ester of ω-undecenoic acid yields a density of around 1014 cm-2, comparable to what is observed here.26,43 As we have noted above, some small variations from sample to sample were observed in this work in the final concentration of the bound complex, due to unavoidable differences in the sample loading onto the diamond and no doubt other experimental variables. However, no systematic differences could be detected in the behavior of the three different diamond samples employed, showing that the chemistry is insensitive to both the phase purity of the sample as well as the presence of boron as a dopant. Similarly, no systematic variation was observed with changes in the level of oxygen contamination on the initial hydrogenterminated surface; either these centers were inert or they reacted with a similar probability to the dominant hydrogen-terminated regions. In further experiments to investigate this approach to surface functionalization, experiments moved on to studies involving the bis(4-nitrophenyl) diazomethane compound 1b (Scheme 3), which again was deposited by dropping a 1% (w/v) solution in DCM onto the diamond followed by evaporation of the solvent. The sample was then heated in vacuum to 400 K for 5 min, cooled, and washed with DCM as in the experiments described above. As expected, the wide scan spectra now indicated the additional presence of nitrogen instead of iodine at the surface. High-resolution spectra of the N 1s region are shown in Figure 6. Two components are observed in the spectrum of the heated surface, which diminish in intensity after washing in DCM. The high binding energy component is known to be associated with the presence of nitro groups as expected, with the low binding energy component arising from amine functionalities.43 It is wellestablished that the reduction of nitro groups to amine species occurs during XPS analysis43 as a result of chemical processes driven by the emitted secondary electron flux, so although the spectra indicate the presence of the two types of nitrogen species, it is most likely the presence of the reduced form is simply an artifact of the experiment. The ability to functionalize the diamond surface with nitro groups in this way provides further evidence that the molecular integrity and reactivity does indeed follow the paths implied by Scheme 1 above. Calculation of the coverage of the bound complex from the N 1s signal, assuming two nitrogen species per molecule and using the method described in detail above and an appropriate sensitivity factor for nitrogen, yielded a grafting density of around 1014 cm-2, comparable to that of the iodine compound, again as expected if it is the carbene functionality that is responsible for the reaction with the surface. After the heating and DCM washing process noted above, other nitrophenyl-modified diamond samples were reduced by heating in aqueous solutions of 1 M sodium sulfide for 24 h to bring about the wet chemical conversion of the nitro substituent to an amino one prior to XPS analysis. The corresponding XPS spectrum indicates that this reduction process both diminishes (43) Lud, S. Q.; Steenackers, M.; Jordan, R.; Bruno, P.; Gruen, D. M.; Feulner, P.; Garrido, J. A.; Stutzmann, M. J. Am. Chem. Soc. 2006, 128, 16884-16891.

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Figure 6. Narrow scan N 1s of a thin film of compound 1c on diamond normalized to the C 1s peak height: (a) after deposition and heating to 400 K for 5 min; (b) after heat treatment of the thin film and washing in dichloromethane; (c) after reduction by heating in 1 M sodium sulfide solution for 24 h; and (d) after reaction with fluorescein isothiocyanate.

Figure 7. Fluorescence image of (a) a fluorescein modified sample (see text) and (b) a “blank” sample.

the total concentration of nitrogen at the surface and as expected completely converts all nitrogen species present to the amine form. Finally, these reduced samples were reacted with fluorescein isothiocyanate dissolved in tetrahydrofuran to achieve fluorescent tagging of the free amine groups, followed by washing. In Figure 7, we compare a fluorescent image of the treated surface to that of a blank sample, which confirms that a fluorescent image which is chemically stable can be achieved. The fluorescence intensity was fairly uniform over the sample surface.

4. Concluding Remarks The compounds bis(4-iodophenyl) diazomethane and bis(4nitrophenyl) diazomethane have been synthesized and deposited

Wang et al.

in thin film form on hydrogen-terminated surfaces of CVD diamond wafers possessing different characteristics. Upon brief heating of the deposited layer to drive formation of the corresponding carbene and solvent washing, it has been demonstrated using XPS that a stable chemisorbed layer is formed on the diamond surface comprising the substituted diphenyl molecule derived from the original diaryldiazo precursor. The grafting concentration achieved is around 1014 cm-2. In the case of the bis(4-nitrophenyl) precursor, we have also demonstrated how the wet chemical reduction of the chemisorbed nitro groups to amine functionalities can be achieved to bring about the fluorescent tagging of the diamond surface. Similar chemical characteristics are seen for undoped and conductive (boron-doped) diamond and for diamond thin films of varying phase purity. Differing levels of oxygen contamination of the hydrogenated surface also seem to have little effect. The overall chemical routes followed demonstrate how the reactions of carbenes with diamond surfaces can be exploited to bring about surface functionalization without resorting to the difficulties which would be anticipated in a vapor phase processing route involving these reactive compounds. A wide range of functionalities can be incorporated as ring substituents in the diaryl carbene, providing flexibility in the functional groups that can be attached to the diamond surface. Probably the most frequently adopted current routes to diamond functionalization at present are via the electrochemical reduction of diazonium compounds or the photochemically driven reaction with alkenes.4,21-27 The route examined here possesses certain obvious differences. In comparison to the electrochemical route, the reaction of in situ generated carbenes is not limited to conductive diamond. In contrast to the photochemical route, which usually involves extended irradiation times, the reaction with carbenes is very rapid. However, other factors need also to be considered in determining the convenience of a particular functionalization scheme. The photochemical route depends sensitively on the (negative) electron affinity of diamond,44 so it is restricted to the hydrogenated surface, whereas this is unlikely to be the case for the reaction with carbenes, which are known to readily insert into a variety of chemical bonds including C-C bonds. All three routes seem to produce similar grafting densities of around 1014 cm-2, and they clearly differ in the exact nature of the “linker” group which binds the functionality to the diamond. Other differences will no doubt emerge as this carbene route to diamond surface chemistry is further characterized in the future. Acknowledgment. The authors would like to acknowledge Dr Mark Wallace and Ms. Lihui Wang for assistance with fluorescence measurements. Supporting Information Available: Chemical synthesis of the compounds and modified diamond used in this work. This material is available free of charge via the Internet at http://pubs.acs.org. LA702701P (44) Nichols, B. M.; Butler, J. E.; Russell, J. N., Jr.; Hamers, R. J. J. Phys. Chem. B 2005, 109, 20938-20947.