- Email: [email protected]
Photochemical attachment of amine linker molecules on hydrogen terminated diamond
Diamond & Related Materials 15 (2006) 1107 – 1112 www.elsevier.com/locate/diamond
Photochemical attachment of amine linker molecules on hydrogen terminated diamond C.E. Nebel a,⁎, D. Shin a , D. Takeuchi a , T. Yamamoto a,b , H. Watanabe a , T. Nakamura c a
b
Diamond Research Center, AIST, Central 2, Tsukuba 305-8568, Japan Graduate School of Pure and Applied Science, University of Tsukuba, Tsukuba 305-8577, Japan c Center for Advanced Carbon Materials, AIST, Central 5, Tsukuba 305-8568, Japan Available online 27 January 2006
Abstract The photochemical attachment of 10-amino-dec-1-ene molecules protected with trifluoro acetamide acid group (“TFAAD”) on hydrogen terminated single crystalline CVD diamond, using a high-pressure mercury lamp with a peak emission around 250 nm, is described and characterized by XPS measurements. Angular resolved XPS experiments reveal oriented molecular bonding on diamond with protecting amide groups at the top. The time dependence of photoattachment follows an exponential law, with a time constant of only 1.7 h due to spin-coating of TFAAD. Based on total photoyield spectroscopy (TPYS) and spectrally resolved photoconductivity (SPC) data, measured on high quality single crystalline diamond, we introduce a model where valence-band electrons are optically excited with energies of 4.7 to 5.2 eV into empty states of TFAAD molecules. This transition is stimulated by the negative electron affinity of H-terminated diamond, which shifts the photochemical reaction window of diamond below its optical gap (5.47 eV) into a regime where commercially available UV light sources emit and organic molecules like TFAAD are still transparent. © 2005 Elsevier B.V. All rights reserved. Keywords: Surface photochemical functionalization; Single crystalline CVD diamond; Total photoyield spectroscopy; Spectrally resolved photoconductivity; XPS; DNA attachment
1. Introduction Many applications in biological, environmental, and medical science involve chemically or biologically modified surfaces that must be stable in contact with electrolytes for long periods of time to be used on a continuous basis. While a variety of biologically sensitive electrical devices have been fabricated as early as the 1970s, the limited stability in comparatively harsh environments, characteristic of physiological conditions, has greatly hampered their practical utility [1–4]. Diamond shows unusual chemical and physical properties which makes it a perfect interface to biological molecules and biological systems [5–7]. Recently, it has been shown that hydrogen terminated ultra-nano- and nanocrystalline diamond can be photochemically modified [7,8] to achieve biomolecular recognition properties and to control nonspecific binding of proteins to the surface [9,10]. Taking advantage of the limited semicon⁎ Corresponding author. Tel.: +81 29 861 4836; fax: +81 29 861 2773. E-mail address: [email protected] (C.E. Nebel). 0925-9635/$ - see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2005.11.041
ducting properties of ultra-nano- or nanocrystalline diamond, these biomolecular properties have been used to detect binding events by changes in electrical properties. The covalent modification of ultra-nano- and nanocrystalline diamond surfaces can be achieved starting with a clean hydrogen terminated surface. Then a photochemical reaction with organic molecules containing a vinyl (C_C) group typically located at one end of the molecule is stimulated [7,11,12]. The initial chemical functionalization is accomplished by placing a small volume (a few microliters) of longchain organic compounds onto the H-terminated nano-crystalline diamond surface, forming a thin liquid film. The sample is sealed into a chamber with quartz window and then illuminated for a given period of time with ultraviolet light from a mercury lamp with a dominant emission at 254 nm in nitrogen atmosphere which is schematically shown in Fig. 1. XPS data show that ultra-nanocrystalline CVD diamond can be chemically modified through a UV mediated reaction with functionalized alkenes. The mechanism is however unclear up-to-now. On one side, the UV excitation at 254 nm (4.86 eV) is
1108
C.E. Nebel et al. / Diamond & Related Materials 15 (2006) 1107–1112
Fig. 1. The diamond surface is modified by photochemical reaction with 10amino-dec-1-ene molecules protected with trifluoro acetamide group (“TFAAD”). Ultraviolet light (250 nm) from a mercury lamp is used for excitation, which gives rise to covalent bonding of the molecules.
boundaries, that show absorption in the sub-band gap regime which may trigger the chemical reaction [13]. In this paper we report for the first time about the mechanism of photochemical attachment of amine linker molecules on high quality intrinsic single crystalline CVD diamond. We characterize the chemical bonding mechanism by total photoyield spectroscopy (TPYS) and spectrally resolved photoconductivity (SPC) experiments, using hydrogen terminated or oxidized single crystalline CVD diamond films and 10-amino-dec-1-ene molecules protected with trifluoroacetic acid group (“TFAAD”) as linkers. In parallel, we apply photochemical attachment experiments, using a high pressure ultraviolet mercury grid lamp with emission around 250 nm and 10 mW/cm2 intensity. The bonding is characterized by angle resolved XPS. Finally, a model of chemical reactions is introduced and discussed in this paper. 2. Experimental
significantly below the critical threshold of absorption of diamond, as the indirect optical gap of single crystalline diamond is 5.47 eV. But, on the other side, ultra-nanocrystalline CVD diamond consists of only 95% sp3-bonded carbon, 5% is sp2-bonded amorphous graphitic carbon located in lots of grain
For our experiments we selected high quality undoped, single crystalline diamond with H-termination. These films have been grown homoepitaxially, using microwave plasma chemical vapor deposition (CVD), on 3 × 3 mm synthetic (100) Ib diamond substrates. Growth parameters are: substrate
Fig. 2. a) XPS survey spectrum of a hydrogen-terminated single crystalline diamond surface that was exposed to TFAAD and 10 mW/cm2 UV illumination (250 nm) for 2 h. b) The C(1s) spectrum reveals two additional small peaks at 292.9 and 288.5 eV which are attributed to carbon atoms in the CF3 cap group and in the C_O group, respectively. c) The ratio of the F(1s) signal (peak area) to that of the total C(1s) signal as a function illumination time is time dependent, follows an exponential increase (dashed line), with a characteristic time constant τ of 1.7 h. d) Angle resolved (with respect to the surface normal) XPS experiments show an increase of the F(1s) / C(1s) peak intensities, rising from 48° to 78°.
C.E. Nebel et al. / Diamond & Related Materials 15 (2006) 1107–1112
temperature 800 °C, microwave power 750 W, total gas pressure 25 Torr, total gas flow 400 sccm with 0.025% CH4 in H2. To achieve H-termination after growth, the CH4 is switch off and the diamond is exposed to a pure hydrogen plasma for 5 min with otherwise identical parameter. A detailed discussion of sample properties has been given in Refs. [14–17]. To summarize the most important quality features: 1) strong room temperature free-exciton emission intensities at 5.27 and 5.12 eV are detected. 2) The surface is atomically flat with stepedge growth where terraces run parallel to the (110) direction [14–17]. 3) Hall-effect experiments show transfer doping induced surface conductivity with a room temperature hole mobilities of up to 340 cm 2 /V [18]. 4) Wetting angle experiments result in typical angles of N 94° [19]. 5) Total photoyield spectroscopy experiments reveal negative electron affinities of − 1.1 eV [20]. The diamond surfaces are modified by photochemical reactions with 10-amino-dec-1-ene molecules protected with trifluoroacetic acid group (“TFAAD”) [7,11,12]. The chemical functionalization is accomplished by placing 2 μm of TFAAD onto the H-terminated single-crystalline diamond surface, spincoating with 4000 rounds/min for 20 s to form a 5 μm thick liquid TFAAD on the diamond film. Then, the sample is sealed into a chamber with quartz window and nitrogen atmosphere, followed by UV illumination for a given period of time. The ultraviolet light was generated in a high-pressure mercury grid lamp with peak emission at 250 nm and 10 mW/cm2 intensity. The chemical attachment is characterized using a X-ray photoelectron (XPS) system with a monochromatized AlKα source (1486.6 eV). To investigate the photoattachment process, we have performed total photoyield spectroscopy (TPYS), where the quantum efficiency of photoelectron emission is measured as a function of photon energy. Details are described in Ref. [20].
1109
Spectrally resolved photoconductivity experiments in planar contact geometry have been performed on H-terminated diamond with 500 × 100 μm active area. These areas have been produced by photolithography and oxygen plasma etching. On each side of the bar (500 μm separation), Au contacts with ohmic properties to H-terminated diamond have been thermally evaporated. A Xenon lamp is used as light source for a 30 cm monochromator with a grating, blazed at 250 nm, and the light is chopped at 1.7 Hz. The photocurrent is detected by a lock-in amplifier. To normalize the SPC spectra, we have measured the light intensity with a pyroelectric detector. Typical applied electric fields in case of H-termination and TFAAD coverage are in the range of 20 V/cm. Please note that TFAAD is an insulating liquid. All dark- and photocurrents are due to transport in diamond. After oxidation of the diamond surface, electric fields of 2 kV/cm were applied to generate detectable photosignals. 3. Results Fig. 2a shows a XPS survey spectrum of a hydrogenterminated single crystalline diamond surface that was exposed to TFAAD and 10 mW/cm2 UV illumination (250 nm) for 2 h. The sample was rinsed in chloroform and methanol (each 5 min in ultrasonic) before introduced into the XPS system. The overall spectrum shows a strong fluorine peak with a binding energy of 689 eV, a O(1s) peak at 531 eV, a N(1s) peak at 400 eV and a large C(1s) bulk peak at 284.5 eV. The C(1s) spectrum reveals two additional small peaks at 292.9 and 288.5 eV (see Fig. 2b) which are attributed to carbon atoms in the CF3 cap group and in the C_O group, respectively. From these experiments, we conclude that UV light of about 250 nm (5 eV) initiates the attachment to H-terminated single crystalline diamond. The ratio of the F(1s) XPS signal (peak area) to that of
Fig. 3. a) Comparison of spectrally resolved photoconductivity (SPC, full line) and total photoyield spectroscopy (TPYS, open triangles) spectra, measured in the regime 4.5 to 6 eV. b) Photoexcited electrons leaving diamond are detected by TPYS. Missing electrons are holes in diamond, which can be detected by spectrally resolved photoconductivity (SPC).
1110
C.E. Nebel et al. / Diamond & Related Materials 15 (2006) 1107–1112
ratio observed by angle resolved XPS will increase. Fig. 2d shows the results as a function of angle with respect to the surface normal. As the angle is increased from 48° to 78° the measured F / C intensity ratio increases. It demonstrates that the photochemical modification of H-terminated single crystalline diamond produces oriented molecular layers of TFAAD with the protected amine group at the exposed surface, where it is accessible to further reactions. This is in good agreement with data reported in the literature for nano-crystalline diamond [21]. To attach DNA, we applied the recipe introduced by Hamers and co-workers [7,11,21], where the protected amine is firstly deprotected, leaving behind a primary amine. The primary amine is then reacted with the heterobifunctional crosslinker sulphosuccinimidyl-4-(N-maleimidomethyl)cyclohexane-1carboxylate and finally reacted with thiol-modified DNA to produce the DNA-modified diamond surface. Details will be discussed elsewhere. 4. Discussion and conclusions
Fig. 4. a) Normalized SPC data as detected on H-terminated diamond (full squares) and on oxidized diamond (open diamonds). Also drawn in the DC-dark current level which is in the range of 250 μA. The shoulder in the regime 4.5 to 5.4 eV is only detected on H-terminated diamond. b) Phase variations of the lock-in amplifier as detected on H-terminated diamond (full triangles) and on oxidized diamond (open circles). On H-terminated diamond the phase variation below 4.5 eV indicates noise.
the total C(1s) signal, RFC, as a function illumination time is shown in Fig. 2c. The time dependence of RFC follows an exponential law: RFC(t) = A{1 − exp(− t / τ)}, increase with a characteristic time constant τ of 1.7 h. This is about two times faster than reported for the photoattachment process on nanocrystalline diamond (τnano-d = 3 h), where the TFAAD has not been homogenized by spin coating [7]. To distinguish between random or oriented attachment of TFAAD we have performed angle resolved XPS experiments. As discussed in Ref. [21], electrons ejected by fluorine and carbon in a random film have nearly equal trajectories through the film, so that F(1s) photoelectrons from CF3 cap-groups and C(1s) photoelectrons will undergo similar amounts of inelastic scattering within the film. However, when the film is oriented with CF3 groups at the top, then one expects that the C(1s) intensity will be scattered more effectively and the F(1s) / C(1s)
These experiments show that H-terminated single crystalline diamond surfaces can be chemically modified through UVmediated reactions with alkenes. The chemical mechanism has not been revealed up-to-now. Strother et al. [11] suggested that the chemical reaction is initiated by photoexcitation of electrons in the subsurface region of diamond, followed by a nucleophilic attack of alkenes. As the indirect optical band-gap of single crystalline diamond is 5.47 eV and sub-band gap absorption can be neglected, a valence-band/conduction-band excitation of electrons with 5 eV photons can be ruled out. However, the energy threshold for electron emission of H-terminated diamond surfaces is different with respect to the optical gap. Due to the negative electron affinity, electrons are emitted already at 4.3 eV as measured by total photoyield spectroscopy (TPYS) experiments [20] in UHV. Photoexcited electrons leave behind holes in diamond, which can be detected by spectrally resolved photocurrent (SPC) experiments (see Fig. 3b). Therefore, SPC can be applied instead of TPYS to characterize electron emission under conditions which are typical for the TFAAD attachment and which cannot be applied in TPYS experiments.
Fig. 5. Transmission of TFAAD (full squares), SPC of H-terminated diamond (open circles), SPC with TFAAD on top (open triangles) and the spectral shape of the mercury lamp used for photoattachment (diamonds).
C.E. Nebel et al. / Diamond & Related Materials 15 (2006) 1107–1112
Fig. 6. a) Photoexcitation of valence-band electrons into empty states of TFAAD which generates nucleophilic properties. b) Bond breaking and covalent bonding of TFAAD to diamond, which also involves a electron hole recombination.
Fig. 3a shows a comparison of SPC and TPYS spectra as detected on H-terminated SINGLE crystalline CVD diamond. An overall good agreement between both spectra is obvious as both show clear onsets around 4.5 eV and steep rises. SPC approaches a plateau and shows a second rise for hν N 5.4 eV, while the TPYS spectrum shows the second rise slightly shifted to hν N 5.5 eV. The first rise in TPYS at 4.5 eV is attributed to electron excitations from the valence-band maximum directly into vacuum states and the second rise for hν N 5.5 eV to valence-band maximum/conduction-band minimum photoexcitations of electrons in combination with the negative electron affinity of diamond [20,22]. To elucidate the properties of the SPC spectrum, we compare SPC spectra as detected on H-terminated and oxidized diamond films, which is shown in Fig. 4. Only H-terminated diamond shows the rise in photoconductivity at 4.5 eV. At hν N 5.4 eV, both show increases which we attribute to indirect optical transitions of electrons from the valence band into the conduction band. The shoulder in the regime 4.5 to 5.5 eV on H-terminated diamond is a new feature, which can be detected only on diamond with H-termination. As the shoulders in SPC and in TPYS disappear after oxidation, we attribute the appearance of both to the same phenomenon, namely to electron transitions from the valence-band maximum into vacuum states. Electrons leaving diamond can be detected by TPYS and the missing valence-band electrons (= holes) give rise to a photocurrent, propagating at the surface of diamond which can be detected by SPC. This interpretation is supported by lock-in amplifier phase properties. On H-terminated diamond, a significant phase shift is detected when the absorption shifts from bulk (hν N 5.5 eV) to surface related SPC (4.4 to 5.5 eV). This shift is absent on oxidized diamond, where the SPC signal is due to bulk related photoexcitation and propagation. Based on these results, we use SPC experiments to monitor electron emission during photochemical attachment of TFAAD, covering H-terminated diamond. Fig. 5 summarizes typical results. Firstly, we have measured the optical transmission of TFAAD as a function of photon energy. For these experiments we used thin quartz plates as
1111
substrates for TFAAD. TFAAD is deposited on it with a typical thickness 300 μm, to realize a closed TFAAD layer on quartz. The TFAAD film is transparent up to 4.7 eV. Above this energy the transmission is strongly decreasing. In case of a 5 μm thick TFAAD film (as used in our attachment experiments), we expect a transmission window which extends deeper into the UV. This is shown in the spectrum of SPC detected electron emission of H-terminated diamond covered with a 5 μm layer of TFAAD (open triangles), which shows a maximum at 4.9 eV. The peak is due to a combination of optical transmission of TFAAD (full squares) and rise in SPC (open circles). It indicates that chemical reactions, which are triggered by photoelectrons, can be achieved in a narrow energy regime of 4.7 to 5.2 eV on H-terminated single crystalline diamond. This is indeed the spectral regime of mercury UV light sources. Fig. 5 shows the details of our UV mercury lamp, which fits the excitation window of H-terminated diamond very well. Fig. 6 summarizes schematically the chemical reaction scheme. In the close vicinity of the H-terminated diamond surface, electrons from the diamond valence band are optically excited with photon energies around 5 eV to access empty states of the TFAAD molecules, therefore generating a nucleophilic situation. The C_C bond becomes instable and attacks the positive C\H bond nearby, to trigger the rearrangement with the result that the positive charged hydrogen atom is changing bonds, bonding to one of the broken carbon double bonds. The other carbon bond is covalently bonding to diamond, which attaches the molecule to the surface of diamond. Finally a electron-hole recombination takes place to ensure charge neutrality condition of the reaction. Further experiments are required to confirm this model, especially helpful would be the knowledge of the electronic structure of the TFAAD molecule. To summarize: We demonstrated for the first time, that photochemical attachment of TFAAD molecules on hydrogen terminated single crystalline diamond can be achieved, using the recipe proposed by Hamers et al. [6,7]. Using TPYS and SPC data we introduce a model where valence-band electrons are optically excited into empty states of TFAAD molecules with photon energies in the range 4.7 to 5.2 eV. These transitions are stimulated by the negative electron affinity of Hterminated diamond, which shifts the photochemical reaction window of diamond slightly below the optical gap into a regime where commercially available UV light sources emit and organic molecules like TFAAD are still transparent. References [1] A. Brajter-Toth, J.Q. Chamber (Eds.), Electroanalytical Methods for Biological Materials, Marcel Dekker, New York, 2002. [2] W.-L. Xing, J. Cheng (Eds.), Biochips: Technology and Applications, Springer Verlag, 2003. [3] N.T. Flynn, T.N.T. Tran, M.J. Cima, R. Langer, Langmuir 19 (2003) 10909. [4] A.B. Kharatinov, J. Wasserman, E. Katz, I. Willner, J. Phys. Chem., B 105 (2001) 4205. [5] S. Haymond, G.T. Babcock, G.M. Swain, J. Am. Chem. Soc. 124 (2002) 10634. [6] W. Yang, J.E. Butler, J.N. Russel Jr., R.J. Hamers, Langmuir 20 (2004) 6778.
1112
C.E. Nebel et al. / Diamond & Related Materials 15 (2006) 1107–1112
[7] W. Yang, J.E. Butler, W. Cai, J. Carlisle, D. Gruen, T. Knickerbocker, J.N. Russel Jr., L.M. Smith, R.J. Hamers, Nat. Matters 1 (2002) 253. [8] A. Haertl, E. Schmich, J.A. Garrido, J. Hernando, S.C.R. Catharino, S. Walter, P. Feulner, A. Kromka, D. Steinmueller, M. Stutzmann, Nat. Matters 3 (2004) 736. [9] T.L. Lasseter, B.H. Clare, N.L. Abbott, R.J. Hamers, J. Am. Chem. Soc. 126 (2004) 10220. [10] T. Lasseter Clare, B.H. Clare, B.M. Nichols, N.L. Abbott, R.J. Hamers, Langmuir 21 (2005) 6344. [11] T. Strother, T. Kickerbocker, J.N. Russel Jr., J.E. Butler, L.M. Smith, R.J. Hamers, Langmuir 18 (2002) 968. [12] T. Kickerbocker, T. Strother, M.P. Schwartz, J.N. Russel Jr., J.E. Butler, L.M. Smith, R.J. Hamers, Langmuir 19 (2003) 1938. [13] J.A. Carlisle, O. Auciello, The Electrochemical Society, Interface, Springer, 2003, p. 28.
[14] H. Watanabe, D. Takeuchi, S. Yamanaka, H. Okushi, K. Kajimura, T. Sekiguchi, Diam. Rel. Mat. 8 (1999) 1272. [15] H. Okushi, Diam. Rel. Mat. 10 (2001) 281. [16] H. Okushi, H. Watanabe, S. Ri, S. Yamanaka, D. Takeuchi, J. Cryst. Growth 237–239 (2002) 1269. [17] H. Watanabe, S.-G. Ri, S. Yamanaka, D. Takeuchi, H. Okushi, New Diam. Front. Carbon Technol. 12 (2002) 369. [18] C.E. Nebel, New Diam. Front. Carbon Technol. 15 (2005) 247. [19] B. Rezek, C.E. Nebel, sent to Diam. Rel. Mat. [20] D. Takeuchi, H. Kato, G.S. Ri, T. Yamada, P.R. Vinod, D. Hwang, C.E. Nebel, H. Okushi, S. Yamasaki, APL 86 (2005) 152103. [21] R.J. Hamers, J.E. Butler, T. Lasseter, B.M. Nichols, J.N. Russel Jr., K.-Y. Tse, W. Yang, Diam. Rel. Mat. 14 (2004) 661. [22] D. Takeuchi, S.-G. Ri, H. Kato, C.E. Nebel, S. Yamasaki, Phys. Stat. Sol. 202 (a) (2005) 2098.
Photochemical attachment of amine linker molecules on hydrogen terminated diamond C.E. Nebel a,⁎, D. Shin a , D. Takeuchi a , T. Yamamoto a,b , H. Watanabe a , T. Nakamura c a
b
Diamond Research Center, AIST, Central 2, Tsukuba 305-8568, Japan Graduate School of Pure and Applied Science, University of Tsukuba, Tsukuba 305-8577, Japan c Center for Advanced Carbon Materials, AIST, Central 5, Tsukuba 305-8568, Japan Available online 27 January 2006
Abstract The photochemical attachment of 10-amino-dec-1-ene molecules protected with trifluoro acetamide acid group (“TFAAD”) on hydrogen terminated single crystalline CVD diamond, using a high-pressure mercury lamp with a peak emission around 250 nm, is described and characterized by XPS measurements. Angular resolved XPS experiments reveal oriented molecular bonding on diamond with protecting amide groups at the top. The time dependence of photoattachment follows an exponential law, with a time constant of only 1.7 h due to spin-coating of TFAAD. Based on total photoyield spectroscopy (TPYS) and spectrally resolved photoconductivity (SPC) data, measured on high quality single crystalline diamond, we introduce a model where valence-band electrons are optically excited with energies of 4.7 to 5.2 eV into empty states of TFAAD molecules. This transition is stimulated by the negative electron affinity of H-terminated diamond, which shifts the photochemical reaction window of diamond below its optical gap (5.47 eV) into a regime where commercially available UV light sources emit and organic molecules like TFAAD are still transparent. © 2005 Elsevier B.V. All rights reserved. Keywords: Surface photochemical functionalization; Single crystalline CVD diamond; Total photoyield spectroscopy; Spectrally resolved photoconductivity; XPS; DNA attachment
1. Introduction Many applications in biological, environmental, and medical science involve chemically or biologically modified surfaces that must be stable in contact with electrolytes for long periods of time to be used on a continuous basis. While a variety of biologically sensitive electrical devices have been fabricated as early as the 1970s, the limited stability in comparatively harsh environments, characteristic of physiological conditions, has greatly hampered their practical utility [1–4]. Diamond shows unusual chemical and physical properties which makes it a perfect interface to biological molecules and biological systems [5–7]. Recently, it has been shown that hydrogen terminated ultra-nano- and nanocrystalline diamond can be photochemically modified [7,8] to achieve biomolecular recognition properties and to control nonspecific binding of proteins to the surface [9,10]. Taking advantage of the limited semicon⁎ Corresponding author. Tel.: +81 29 861 4836; fax: +81 29 861 2773. E-mail address: [email protected] (C.E. Nebel). 0925-9635/$ - see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2005.11.041
ducting properties of ultra-nano- or nanocrystalline diamond, these biomolecular properties have been used to detect binding events by changes in electrical properties. The covalent modification of ultra-nano- and nanocrystalline diamond surfaces can be achieved starting with a clean hydrogen terminated surface. Then a photochemical reaction with organic molecules containing a vinyl (C_C) group typically located at one end of the molecule is stimulated [7,11,12]. The initial chemical functionalization is accomplished by placing a small volume (a few microliters) of longchain organic compounds onto the H-terminated nano-crystalline diamond surface, forming a thin liquid film. The sample is sealed into a chamber with quartz window and then illuminated for a given period of time with ultraviolet light from a mercury lamp with a dominant emission at 254 nm in nitrogen atmosphere which is schematically shown in Fig. 1. XPS data show that ultra-nanocrystalline CVD diamond can be chemically modified through a UV mediated reaction with functionalized alkenes. The mechanism is however unclear up-to-now. On one side, the UV excitation at 254 nm (4.86 eV) is
1108
C.E. Nebel et al. / Diamond & Related Materials 15 (2006) 1107–1112
Fig. 1. The diamond surface is modified by photochemical reaction with 10amino-dec-1-ene molecules protected with trifluoro acetamide group (“TFAAD”). Ultraviolet light (250 nm) from a mercury lamp is used for excitation, which gives rise to covalent bonding of the molecules.
boundaries, that show absorption in the sub-band gap regime which may trigger the chemical reaction [13]. In this paper we report for the first time about the mechanism of photochemical attachment of amine linker molecules on high quality intrinsic single crystalline CVD diamond. We characterize the chemical bonding mechanism by total photoyield spectroscopy (TPYS) and spectrally resolved photoconductivity (SPC) experiments, using hydrogen terminated or oxidized single crystalline CVD diamond films and 10-amino-dec-1-ene molecules protected with trifluoroacetic acid group (“TFAAD”) as linkers. In parallel, we apply photochemical attachment experiments, using a high pressure ultraviolet mercury grid lamp with emission around 250 nm and 10 mW/cm2 intensity. The bonding is characterized by angle resolved XPS. Finally, a model of chemical reactions is introduced and discussed in this paper. 2. Experimental
significantly below the critical threshold of absorption of diamond, as the indirect optical gap of single crystalline diamond is 5.47 eV. But, on the other side, ultra-nanocrystalline CVD diamond consists of only 95% sp3-bonded carbon, 5% is sp2-bonded amorphous graphitic carbon located in lots of grain
For our experiments we selected high quality undoped, single crystalline diamond with H-termination. These films have been grown homoepitaxially, using microwave plasma chemical vapor deposition (CVD), on 3 × 3 mm synthetic (100) Ib diamond substrates. Growth parameters are: substrate
Fig. 2. a) XPS survey spectrum of a hydrogen-terminated single crystalline diamond surface that was exposed to TFAAD and 10 mW/cm2 UV illumination (250 nm) for 2 h. b) The C(1s) spectrum reveals two additional small peaks at 292.9 and 288.5 eV which are attributed to carbon atoms in the CF3 cap group and in the C_O group, respectively. c) The ratio of the F(1s) signal (peak area) to that of the total C(1s) signal as a function illumination time is time dependent, follows an exponential increase (dashed line), with a characteristic time constant τ of 1.7 h. d) Angle resolved (with respect to the surface normal) XPS experiments show an increase of the F(1s) / C(1s) peak intensities, rising from 48° to 78°.
C.E. Nebel et al. / Diamond & Related Materials 15 (2006) 1107–1112
temperature 800 °C, microwave power 750 W, total gas pressure 25 Torr, total gas flow 400 sccm with 0.025% CH4 in H2. To achieve H-termination after growth, the CH4 is switch off and the diamond is exposed to a pure hydrogen plasma for 5 min with otherwise identical parameter. A detailed discussion of sample properties has been given in Refs. [14–17]. To summarize the most important quality features: 1) strong room temperature free-exciton emission intensities at 5.27 and 5.12 eV are detected. 2) The surface is atomically flat with stepedge growth where terraces run parallel to the (110) direction [14–17]. 3) Hall-effect experiments show transfer doping induced surface conductivity with a room temperature hole mobilities of up to 340 cm 2 /V [18]. 4) Wetting angle experiments result in typical angles of N 94° [19]. 5) Total photoyield spectroscopy experiments reveal negative electron affinities of − 1.1 eV [20]. The diamond surfaces are modified by photochemical reactions with 10-amino-dec-1-ene molecules protected with trifluoroacetic acid group (“TFAAD”) [7,11,12]. The chemical functionalization is accomplished by placing 2 μm of TFAAD onto the H-terminated single-crystalline diamond surface, spincoating with 4000 rounds/min for 20 s to form a 5 μm thick liquid TFAAD on the diamond film. Then, the sample is sealed into a chamber with quartz window and nitrogen atmosphere, followed by UV illumination for a given period of time. The ultraviolet light was generated in a high-pressure mercury grid lamp with peak emission at 250 nm and 10 mW/cm2 intensity. The chemical attachment is characterized using a X-ray photoelectron (XPS) system with a monochromatized AlKα source (1486.6 eV). To investigate the photoattachment process, we have performed total photoyield spectroscopy (TPYS), where the quantum efficiency of photoelectron emission is measured as a function of photon energy. Details are described in Ref. [20].
1109
Spectrally resolved photoconductivity experiments in planar contact geometry have been performed on H-terminated diamond with 500 × 100 μm active area. These areas have been produced by photolithography and oxygen plasma etching. On each side of the bar (500 μm separation), Au contacts with ohmic properties to H-terminated diamond have been thermally evaporated. A Xenon lamp is used as light source for a 30 cm monochromator with a grating, blazed at 250 nm, and the light is chopped at 1.7 Hz. The photocurrent is detected by a lock-in amplifier. To normalize the SPC spectra, we have measured the light intensity with a pyroelectric detector. Typical applied electric fields in case of H-termination and TFAAD coverage are in the range of 20 V/cm. Please note that TFAAD is an insulating liquid. All dark- and photocurrents are due to transport in diamond. After oxidation of the diamond surface, electric fields of 2 kV/cm were applied to generate detectable photosignals. 3. Results Fig. 2a shows a XPS survey spectrum of a hydrogenterminated single crystalline diamond surface that was exposed to TFAAD and 10 mW/cm2 UV illumination (250 nm) for 2 h. The sample was rinsed in chloroform and methanol (each 5 min in ultrasonic) before introduced into the XPS system. The overall spectrum shows a strong fluorine peak with a binding energy of 689 eV, a O(1s) peak at 531 eV, a N(1s) peak at 400 eV and a large C(1s) bulk peak at 284.5 eV. The C(1s) spectrum reveals two additional small peaks at 292.9 and 288.5 eV (see Fig. 2b) which are attributed to carbon atoms in the CF3 cap group and in the C_O group, respectively. From these experiments, we conclude that UV light of about 250 nm (5 eV) initiates the attachment to H-terminated single crystalline diamond. The ratio of the F(1s) XPS signal (peak area) to that of
Fig. 3. a) Comparison of spectrally resolved photoconductivity (SPC, full line) and total photoyield spectroscopy (TPYS, open triangles) spectra, measured in the regime 4.5 to 6 eV. b) Photoexcited electrons leaving diamond are detected by TPYS. Missing electrons are holes in diamond, which can be detected by spectrally resolved photoconductivity (SPC).
1110
C.E. Nebel et al. / Diamond & Related Materials 15 (2006) 1107–1112
ratio observed by angle resolved XPS will increase. Fig. 2d shows the results as a function of angle with respect to the surface normal. As the angle is increased from 48° to 78° the measured F / C intensity ratio increases. It demonstrates that the photochemical modification of H-terminated single crystalline diamond produces oriented molecular layers of TFAAD with the protected amine group at the exposed surface, where it is accessible to further reactions. This is in good agreement with data reported in the literature for nano-crystalline diamond [21]. To attach DNA, we applied the recipe introduced by Hamers and co-workers [7,11,21], where the protected amine is firstly deprotected, leaving behind a primary amine. The primary amine is then reacted with the heterobifunctional crosslinker sulphosuccinimidyl-4-(N-maleimidomethyl)cyclohexane-1carboxylate and finally reacted with thiol-modified DNA to produce the DNA-modified diamond surface. Details will be discussed elsewhere. 4. Discussion and conclusions
Fig. 4. a) Normalized SPC data as detected on H-terminated diamond (full squares) and on oxidized diamond (open diamonds). Also drawn in the DC-dark current level which is in the range of 250 μA. The shoulder in the regime 4.5 to 5.4 eV is only detected on H-terminated diamond. b) Phase variations of the lock-in amplifier as detected on H-terminated diamond (full triangles) and on oxidized diamond (open circles). On H-terminated diamond the phase variation below 4.5 eV indicates noise.
the total C(1s) signal, RFC, as a function illumination time is shown in Fig. 2c. The time dependence of RFC follows an exponential law: RFC(t) = A{1 − exp(− t / τ)}, increase with a characteristic time constant τ of 1.7 h. This is about two times faster than reported for the photoattachment process on nanocrystalline diamond (τnano-d = 3 h), where the TFAAD has not been homogenized by spin coating [7]. To distinguish between random or oriented attachment of TFAAD we have performed angle resolved XPS experiments. As discussed in Ref. [21], electrons ejected by fluorine and carbon in a random film have nearly equal trajectories through the film, so that F(1s) photoelectrons from CF3 cap-groups and C(1s) photoelectrons will undergo similar amounts of inelastic scattering within the film. However, when the film is oriented with CF3 groups at the top, then one expects that the C(1s) intensity will be scattered more effectively and the F(1s) / C(1s)
These experiments show that H-terminated single crystalline diamond surfaces can be chemically modified through UVmediated reactions with alkenes. The chemical mechanism has not been revealed up-to-now. Strother et al. [11] suggested that the chemical reaction is initiated by photoexcitation of electrons in the subsurface region of diamond, followed by a nucleophilic attack of alkenes. As the indirect optical band-gap of single crystalline diamond is 5.47 eV and sub-band gap absorption can be neglected, a valence-band/conduction-band excitation of electrons with 5 eV photons can be ruled out. However, the energy threshold for electron emission of H-terminated diamond surfaces is different with respect to the optical gap. Due to the negative electron affinity, electrons are emitted already at 4.3 eV as measured by total photoyield spectroscopy (TPYS) experiments [20] in UHV. Photoexcited electrons leave behind holes in diamond, which can be detected by spectrally resolved photocurrent (SPC) experiments (see Fig. 3b). Therefore, SPC can be applied instead of TPYS to characterize electron emission under conditions which are typical for the TFAAD attachment and which cannot be applied in TPYS experiments.
Fig. 5. Transmission of TFAAD (full squares), SPC of H-terminated diamond (open circles), SPC with TFAAD on top (open triangles) and the spectral shape of the mercury lamp used for photoattachment (diamonds).
C.E. Nebel et al. / Diamond & Related Materials 15 (2006) 1107–1112
Fig. 6. a) Photoexcitation of valence-band electrons into empty states of TFAAD which generates nucleophilic properties. b) Bond breaking and covalent bonding of TFAAD to diamond, which also involves a electron hole recombination.
Fig. 3a shows a comparison of SPC and TPYS spectra as detected on H-terminated SINGLE crystalline CVD diamond. An overall good agreement between both spectra is obvious as both show clear onsets around 4.5 eV and steep rises. SPC approaches a plateau and shows a second rise for hν N 5.4 eV, while the TPYS spectrum shows the second rise slightly shifted to hν N 5.5 eV. The first rise in TPYS at 4.5 eV is attributed to electron excitations from the valence-band maximum directly into vacuum states and the second rise for hν N 5.5 eV to valence-band maximum/conduction-band minimum photoexcitations of electrons in combination with the negative electron affinity of diamond [20,22]. To elucidate the properties of the SPC spectrum, we compare SPC spectra as detected on H-terminated and oxidized diamond films, which is shown in Fig. 4. Only H-terminated diamond shows the rise in photoconductivity at 4.5 eV. At hν N 5.4 eV, both show increases which we attribute to indirect optical transitions of electrons from the valence band into the conduction band. The shoulder in the regime 4.5 to 5.5 eV on H-terminated diamond is a new feature, which can be detected only on diamond with H-termination. As the shoulders in SPC and in TPYS disappear after oxidation, we attribute the appearance of both to the same phenomenon, namely to electron transitions from the valence-band maximum into vacuum states. Electrons leaving diamond can be detected by TPYS and the missing valence-band electrons (= holes) give rise to a photocurrent, propagating at the surface of diamond which can be detected by SPC. This interpretation is supported by lock-in amplifier phase properties. On H-terminated diamond, a significant phase shift is detected when the absorption shifts from bulk (hν N 5.5 eV) to surface related SPC (4.4 to 5.5 eV). This shift is absent on oxidized diamond, where the SPC signal is due to bulk related photoexcitation and propagation. Based on these results, we use SPC experiments to monitor electron emission during photochemical attachment of TFAAD, covering H-terminated diamond. Fig. 5 summarizes typical results. Firstly, we have measured the optical transmission of TFAAD as a function of photon energy. For these experiments we used thin quartz plates as
1111
substrates for TFAAD. TFAAD is deposited on it with a typical thickness 300 μm, to realize a closed TFAAD layer on quartz. The TFAAD film is transparent up to 4.7 eV. Above this energy the transmission is strongly decreasing. In case of a 5 μm thick TFAAD film (as used in our attachment experiments), we expect a transmission window which extends deeper into the UV. This is shown in the spectrum of SPC detected electron emission of H-terminated diamond covered with a 5 μm layer of TFAAD (open triangles), which shows a maximum at 4.9 eV. The peak is due to a combination of optical transmission of TFAAD (full squares) and rise in SPC (open circles). It indicates that chemical reactions, which are triggered by photoelectrons, can be achieved in a narrow energy regime of 4.7 to 5.2 eV on H-terminated single crystalline diamond. This is indeed the spectral regime of mercury UV light sources. Fig. 5 shows the details of our UV mercury lamp, which fits the excitation window of H-terminated diamond very well. Fig. 6 summarizes schematically the chemical reaction scheme. In the close vicinity of the H-terminated diamond surface, electrons from the diamond valence band are optically excited with photon energies around 5 eV to access empty states of the TFAAD molecules, therefore generating a nucleophilic situation. The C_C bond becomes instable and attacks the positive C\H bond nearby, to trigger the rearrangement with the result that the positive charged hydrogen atom is changing bonds, bonding to one of the broken carbon double bonds. The other carbon bond is covalently bonding to diamond, which attaches the molecule to the surface of diamond. Finally a electron-hole recombination takes place to ensure charge neutrality condition of the reaction. Further experiments are required to confirm this model, especially helpful would be the knowledge of the electronic structure of the TFAAD molecule. To summarize: We demonstrated for the first time, that photochemical attachment of TFAAD molecules on hydrogen terminated single crystalline diamond can be achieved, using the recipe proposed by Hamers et al. [6,7]. Using TPYS and SPC data we introduce a model where valence-band electrons are optically excited into empty states of TFAAD molecules with photon energies in the range 4.7 to 5.2 eV. These transitions are stimulated by the negative electron affinity of Hterminated diamond, which shifts the photochemical reaction window of diamond slightly below the optical gap into a regime where commercially available UV light sources emit and organic molecules like TFAAD are still transparent. References [1] A. Brajter-Toth, J.Q. Chamber (Eds.), Electroanalytical Methods for Biological Materials, Marcel Dekker, New York, 2002. [2] W.-L. Xing, J. Cheng (Eds.), Biochips: Technology and Applications, Springer Verlag, 2003. [3] N.T. Flynn, T.N.T. Tran, M.J. Cima, R. Langer, Langmuir 19 (2003) 10909. [4] A.B. Kharatinov, J. Wasserman, E. Katz, I. Willner, J. Phys. Chem., B 105 (2001) 4205. [5] S. Haymond, G.T. Babcock, G.M. Swain, J. Am. Chem. Soc. 124 (2002) 10634. [6] W. Yang, J.E. Butler, J.N. Russel Jr., R.J. Hamers, Langmuir 20 (2004) 6778.
1112
C.E. Nebel et al. / Diamond & Related Materials 15 (2006) 1107–1112
[7] W. Yang, J.E. Butler, W. Cai, J. Carlisle, D. Gruen, T. Knickerbocker, J.N. Russel Jr., L.M. Smith, R.J. Hamers, Nat. Matters 1 (2002) 253. [8] A. Haertl, E. Schmich, J.A. Garrido, J. Hernando, S.C.R. Catharino, S. Walter, P. Feulner, A. Kromka, D. Steinmueller, M. Stutzmann, Nat. Matters 3 (2004) 736. [9] T.L. Lasseter, B.H. Clare, N.L. Abbott, R.J. Hamers, J. Am. Chem. Soc. 126 (2004) 10220. [10] T. Lasseter Clare, B.H. Clare, B.M. Nichols, N.L. Abbott, R.J. Hamers, Langmuir 21 (2005) 6344. [11] T. Strother, T. Kickerbocker, J.N. Russel Jr., J.E. Butler, L.M. Smith, R.J. Hamers, Langmuir 18 (2002) 968. [12] T. Kickerbocker, T. Strother, M.P. Schwartz, J.N. Russel Jr., J.E. Butler, L.M. Smith, R.J. Hamers, Langmuir 19 (2003) 1938. [13] J.A. Carlisle, O. Auciello, The Electrochemical Society, Interface, Springer, 2003, p. 28.
[14] H. Watanabe, D. Takeuchi, S. Yamanaka, H. Okushi, K. Kajimura, T. Sekiguchi, Diam. Rel. Mat. 8 (1999) 1272. [15] H. Okushi, Diam. Rel. Mat. 10 (2001) 281. [16] H. Okushi, H. Watanabe, S. Ri, S. Yamanaka, D. Takeuchi, J. Cryst. Growth 237–239 (2002) 1269. [17] H. Watanabe, S.-G. Ri, S. Yamanaka, D. Takeuchi, H. Okushi, New Diam. Front. Carbon Technol. 12 (2002) 369. [18] C.E. Nebel, New Diam. Front. Carbon Technol. 15 (2005) 247. [19] B. Rezek, C.E. Nebel, sent to Diam. Rel. Mat. [20] D. Takeuchi, H. Kato, G.S. Ri, T. Yamada, P.R. Vinod, D. Hwang, C.E. Nebel, H. Okushi, S. Yamasaki, APL 86 (2005) 152103. [21] R.J. Hamers, J.E. Butler, T. Lasseter, B.M. Nichols, J.N. Russel Jr., K.-Y. Tse, W. Yang, Diam. Rel. Mat. 14 (2004) 661. [22] D. Takeuchi, S.-G. Ri, H. Kato, C.E. Nebel, S. Yamasaki, Phys. Stat. Sol. 202 (a) (2005) 2098.