Determination of the Structure and Orientation of Organic Molecules

Anal. Chem. 2006, 78, 3104-3112

Determination of the Structure and Orientation of Organic Molecules Tethered to Flat Graphitic Carbon by ATR-FT-IR and Raman Spectroscopy Franklin Anariba,† Umamaheswari Viswanathan,‡ David F. Bocian,*,† and Richard L. McCreery*,‡

Department of Chemistry, University of California, Riverside, California 92521, and Department of Chemistry, The Ohio State University, 100 West 18th Avenue, Columbus, Ohio 43210

Mono- and multilayers of nitroazobenzene (NAB), azobenzene (AB), nitrobiphenyl (NBP), biphenyl (BP), and fluorene (FL) were covalently bonded to flat pyrolyzed photoresist films (PPF) by electrochemical reduction of their diazonium derivatives. The structure and orientation of the molecular layers were probed with ATR-FT-IR and Raman spectroscopy. A hemispherical germanium ATR element used with p-polarized light at 65° incidence angle yielded high signal/noise IR spectra for monolayer coverage of molecules on PPF. The IR spectra are dominated by in-plane vibrational modes in the 1000-2000-cm-1 spectral range but also exhibit weaker out-of-plane deformations in the 650-1000-cm-1 region. The average tilt angle with respect to the surface normal for the various molecules varied from 31.0 ( 4.5° for NAB to 44.2 ( 5.4° for FL with AB, NBP, and BP exhibiting intermediate adsorption geometries. Raman intensity ratios of NAB and AB for p- and s-polarized incident light support the conclusion that the chemisorbed molecules are in a predominantly upright orientation. The results unequivocally indicate that molecules electroreduced from their diazonium precursors are not chemisorbed flat on the PPF surface, but rather at an angle, similar to the behavior of Au/thiol self-assembled monolayers, Langmuir-Blodgett films, and porphyrin molecules chemisorbed thermally on silicon and PPF from alkyne and alkene precursors. Recent interest in molecular electronic concepts and devices has stimulated investigations of bonding between conducting “contacts” and a variety of molecules, including aliphatic and aromatic hydrocarbons, heterocyclic molecules, and conducting polymers. To understand electron transport across molecular junctions consisting of single molecules or groups of molecules oriented between conducting contacts, it is necessary to characterize the nature and geometry of the molecule/conductor bond. The current report discusses aromatic molecules bonded to a graphitic carbon surface, which are the basis of one of the reported paradigms for determining electron transport mechanisms through molecular junctions. * To whom correspondence should [email protected]; [email protected]. † University of California. ‡ The Ohio State University.

be

addressed.

3104 Analytical Chemistry, Vol. 78, No. 9, May 1, 2006

E-mail:

Covalent attachment of a variety of molecules to graphitic surfaces via aryldiazonium ion reduction has been reported by various investigators.1-13 The highly reactive phenyl radicals produced electrochemically from diazonium reagents lead to densely packed monolayers on carbon, with the formation of a strong and stable C-C bond between a phenyl ring and the substrate. Carbon surfaces modified via diazonium electroreduction have been used in a number of different types of studies including adhesion promotion in carbon fiber composites,3,4 electroanalysis and electrocatalysis,12,14-16 electrode kinetics,8,15-17 and nanoscale patterning18 and investigations of current-voltage behavior of molecular junctions.19-29 (1) Delamar, M.; Hitmi, R.; Pinson, J.; Saveant, J. M. J. Am. Chem. Soc. 1992, 114, 5883-5884. (2) Allongue, P.; Delamar, M.; Desbat, B.; Fagebaume, O.; Hitmi, R.; Pinson, J.; Saveant, J. M. J. Am. Chem. Soc. 1997, 119, 201-207. (3) Delamar, M.; Desarmot, G.; Fagebaume, O.; Hitmi, R.; Pinson, J.; Saveant, J. Carbon 1997, 35, 801-807. (4) Coulon, E.; Pinson, J.; Bourzat, J.-D.; Commercon, A.; Pulicani, J.-P. Langmuir 2002, 17, 7102-7106. (5) Kariuki, J. K.; McDermott, M. T. Langmuir 1999, 15, 6534-6540. (6) Kariuki, J. K.; McDermott, M. T. Langmuir 2001, 17, 5947-5951. (7) Chen, P.; Fryling, M.; McCreery, R. L. Anal. Chem. 1995, 67, 3115. (8) Chen, P.; McCreery, R. L. Anal. Chem. 1996, 68, 3958. (9) Itoh, T.; McCreery, R. L. J. Am. Chem. Soc. 2002, 124, 10894-10902. (10) Saby, C.; Ortiz, B.; Champagne, G. Y.; Belanger, D. Langmuir 1997, 13, 6805-6813. (11) Ortiz, B.; Saby, C.; Champagne, G. Y.; Belanger, D. J. Electroanal. Chem. 1998, 455, 75-81. (12) Downard, A. J.; Roddick, A. D. Electroanalysis 1995, 7, 376-378. (13) Downard, A. J. Electroanalysis 2000, 12, 1085-1096. (14) Downard, A. J. Langmuir 2000, 16, 9680-9682. (15) DuVall, S.; McCreery, R. L. Anal. Chem. 1999, 71, 4594-4602. (16) DuVall, S.; McCreery, R. L. J. Am. Chem. Soc. 2000, 122, 6759-6764. (17) DuVall, S.; Yang, H.-H.; McCreery, R. L. In Proceedings of the Electrochemical Society; Leddy, J., Vanysek, P., Porter, M. D., Eds.; Electrochemical Society: Pennington, NJ, 1999; Vol. 99, pp 33-36. (18) Brooksby, P.; Downard, A. J. Langmuir 2005, 21, 1672. (19) Ranganathan, S.; Steidel, I.; Anariba, F.; McCreery, R. L. Nano Lett. 2001, 1, 491-494. (20) Anariba, F.; McCreery, R. L. J. Phys. Chem. B 2002, 106, 10355-10362. (21) Solak, A. O.; Ranganathan, S.; Itoh, T.; McCreery, R. L. Electrochem. Solid State Lett. 2002, 5, E43-E46. (22) McCreery, R. L.; Dieringer, J.; Solak, A. O.; Snyder, B.; Nowak, A.; McGovern, W. R.; DuVall, S. J. Am. Chem. Soc. 2003, 125, 10748-10758. (23) McCreery, R. L.; Dieringer, J.; Solak, A. O.; Snyder, B.; Nowak, A. M.; McGovern, W. R.; DuVall, S. J Am Chem Soc 2004, 126, 6200. (24) Nowak, A. M.; McCreery, R. L. Anal. Chem. 2004, 76, 1089-1097. (25) McGovern, W. R.; Anariba, F.; McCreery, R. L. J. Electroanal. Chem. Soc. 2004, 152, E176. (26) McCreery, R. L. Chem. Mater. 2004, 16, 4477-4496. (27) Nowak, A. M.; McCreery, R. L. J. Am. Chem. Soc. 2004, 126, 16621-16631. 10.1021/ac052042h CCC: $33.50

© 2006 American Chemical Society Published on Web 03/30/2006

Figure 1. Relationship between incident light and chemisorbed NAB molecule. The incident angle for IR light in the GATR configuration was 65° relative to the surface normal. Also shown are the dipole derivative vectors for NAB. φ is zero when the transition dipole along the short axis of the molecule is parallel to the surface plane.

The formation of monolayers and multilayers of various diazonium salts has been demonstrated by systematically varying the charge (Q), solution concentration (M), and deposition time (t). The ensuing diazonium films have been characterized by voltammetry,2,3,11,15-17,30 X-ray photoelectron spectroscopy,2,11 timeof-flight secondary ion mass spectroscopy,31 atomic force microscopy,18,30,32,33 Raman spectroscopy,9,34 and Fourier transform infrared reflection-absorbance spectroscopy.5,6,35 Surface spectroscopic techniques such as FT-IR and Raman can provide valuable information about the structure and orientation of films chemisorbed onto surfaces.36 The molecular orientation of self-assembled monolayers deposited onto metal surfaces has been previously determined by infrared spectroscopy.37-41 In addition, FT-IR has been employed to obtain the orientation of LangmuirBlodgett films42 physisorbed to various metals and glass and the adsorption geometry of porphyrin monolayers covalently bonded to silicon surfaces.43-45 The optical properties of carbon, particu(28) McCreery, R. L. Interface 2004, 13, 46. (29) Anariba, F.; Steach, J. K.; McCreery, R. L. J. Phys. Chem. B 2005, 109, 11163. (30) Brooksby, P.; Downard, A. J. Langmuir 2004, 20, 5038. (31) Combellas, C.; Knaoufi, F.; Pinson, P.; Podvorica, F. Langmuir 2005, 21, 280. (32) Anariba, F.; DuVall, S. H.; McCreery, R. L. Anal. Chem. 2003, 75, 38373844. (33) Brooksby, P.; Downard, A. J. J. Phys. Chem. B 2005, 109, 8791. (34) Liu, Y.-C.; McCreery, R. L. J. Am. Chem. Soc. 1995, 117, 11254. (35) Adenier, A.; Cabet-Deliry, E.; Chausse, A.; Griveau, S.; Mercier, F.; Pinson, J.; Ul-Vautrin, C. Chem. Mater. 2005, 17, 491. (36) Greenler, R. G. J. Chem. Phys. 1966, 44, 310. (37) Allara, D. L.; Nuzzo, R. G. Langmuir 1985, 1, 52. (38) Ulman, A.; Eilers, J.; Tillman, N. Langmuir 1989, 5, 1147. (39) Shaporenko, A.; Cyganik, P.; Buck, M.; Ulman, A.; Zharnikov, M. Langmuir 2005, 21, 8204. (40) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559-3568. (41) Alves, C. A.; Porter, M. D. Langmuir 1993, 9, 3507. (42) Hulshof, J.; Feringa, B.; Schoondorp, M.; Schouton, A. J. Langmuir 1992, 8, 1825.

larly its strong absorption coefficient in the UV-visible-IR regions and its inability to support electromagnetic field enhancement in surface-enhanced Raman spectroscopy, make it much less amenable than metals to surface spectroscopy. As a result, vibrational studies of adsorbed films on graphitic surfaces have been limited. The first FT-IR studies of molecules on glassy carbon (GC) were reported for physisorbed Cu3(Fe(CN)6)2 films.46-48 In subsequent work, the quantitative aspects of infrared external reflection spectroscopy for the characterization of thin poly(methyl methacrylate) films on GC were reported,49 with the carbonyl stretch observed for a film thickness down to 7.8 nm. The infrared absorption characteristics of molecular adlayers on carbon substrates differ from those of adlayers on metals in that image dipoles and the associated surface selection rules do not significantly affect the intensities of the vibrational modes. In particular, the IR absorption observed using p-polarized incident light at near-glancing incidence will be largest for vibrations with dipole derivative vectors more parallel to the surface normal, even if an image dipole is weak or absent (Figure 1). IR absorption will also be observed using s-polarized light; however, the largest absorption will occur for vibrations whose dipole derivatives are more parallel to the plane of the surface. The relative IR intensities of vibrations whose dipole derivatives are oriented along different molecular axes can be used to determine the orientation of the (43) Yasseri, A.; Syomin, D.; Malinovskii, V.; Loewe, R.; Lindsey, J.; Zaera, F.; Bocian, D. J. Am. Chem. Soc. 2004, 126, 11944. (44) Wei, L.; Syomin, D.; Loewe, R.; Lindsey, J. S.; Zaera, F.; Bocian, D. F. J. Phys. Chem. B 2005, 109, 6323. (45) Yasseri, A.; Syomin, D.; Loewe, R.; Lindsey, J.; Zaera, F.; Bocian, D. J. Am. Chem. Soc. 2004, 126, 15603. (46) Porter, M. D.; Karweik, D. H.; Kuwana, T.; Theis, W. B.; Norris, G. B.; Tiernan, T. O. Appl. Spectrosc. 1984, 38, 11-16. (47) Datta, M.; Freeman, J.; Jansson, R. Spectrosc. Lett. 1985, 18, 273. (48) Datta, M.; Robert, E. W.; Freeman, J. Spectrosc. Lett. 1986, 19, 129. (49) Porter, M. D.; Bright, T. B.; Allara, D. L. Anal. Chem. 1986, 58, 24612465.

Analytical Chemistry, Vol. 78, No. 9, May 1, 2006

3105

molecules on the surface. This is illustrated in Figure 1, which uses nitroazobenzene (NAB, one of the molecules of interest in the present study) as an example. For this molecule, the relative intensities of the vibrations polarized along the long axis versus those polarized out of the plane of the molecule can be used to obtain the angle between the surface and the long molecular axis (θ), as expressed in eqs 1-3. Similarly, the relative intensities of modes polarized along the short axis versus the out-of-plane vibrations can be used to obtain the rotation angle about the long axis of the molecule (φ), as expressed in eqs 4 and 5:36,37,50-52

(Ilain-plane/Iout-of-plane)monolayer ) (Ilain-plane/Iout-of-plane)iso (tan2 θ/cos2 φ) (1) θ ) tan

-1

{[(I

la

in-plane/Iout-of-plane)monolayer

(I

2

(cos φ)/

la

in-plane/Iout-of-plane)iso]

R ) 90 - θ

} (2)

1/2

(3)

Table 1. Modification Conditions and Film Thicknesses molecule

deposition potential, Va

thickness, nmc

no. of samplesd

NAB NAB (1.9) NAB (4.5) AB AB NBP (1.7) NBP (4.2) BP FL NAB NBP BP FL PANI PANI

0.0 (1)b -0.2 (1) -0.6 (4) -0.2 (1) -0.6 (4) -0.4 (1) -0.6 (10) -0.8 (1) -0.8 (1) physisorbed physisorbed physisorbed physisorbed see text see text

1.0 1.9 4.5 1.2 2.1 1.7 4.2 1.6 1.7

2 5 4 2 1 5 4 4 5 1 2 1 3

6.0e 50e

a Negative potential limit for derivatization scan, stated vs Ag/Ag+ (0.1 M). All scans started at +0.4 V vs Ag/Ag+, at a scan rate of 0.2 V/s. b Number in parentheses indicates number of derivatization cycles. c AFM thickness established by “scratching” technique.32 d Refers to independently prepared mono- or multilayers. e Thickness determined with ellipsometry, with n ) 1.8997 and k ) 0.64

(Isain-plane/Iout-of-plane)monolayer ) (Isain-plane/Iout-of-plane)iso (tan φ)2 (4) φ ) tan-1 {[(Isain-plane/Iout-of-plane)monolayer/ (Isain-plane/Iout-of-plane)iso]1/2} (5) where I is the relative infrared intensity for a given spectrum, Ila refers to the relative infrared intensities for modes with dipole derivatives along the long axis of the molecule, Isa is the relative infrared intensities along the short axis of the molecule, Iiso refers to isotropically oriented material, θ is the angle between the surface and the long molecular axis, R is the average tilt angle of the molecule relative to the surface normal, and φ is the rotation angle about the long axis of the molecule. Raman spectroscopy with polarized incident laser light has also been used to determine the orientation of molecules adsorbed onto highly ordered pyrolytic graphite (HOPG)53 and polished GC54 and to probe the orientation of single-wall carbon nanotubes.55,56 The main factor affecting the Raman scattering from carbon surfaces is the relationship between the incident electric field and the orientation of the molecules. In the case of metal phthalocyanines adsorbed on the basal plane of HOPG, the resonance Raman scattering intensity was maximized when the electric field vector of the incident light was parallel to the plane of the macrocycle.53 For the case of NAB, shown in Figure 1, we expect maximum Raman intensity when the incident electric field is parallel to the long axis of the molecule, which has the largest polarizability. (50) Painter, P. C.; Coleman, M. M.; Koenig, J. L. The Theory of Vibrational Spectroscopy and Its Application to Polymeric Materials; Wiley: New York, 1982. (51) Harrick, N. J.; Mirabella, F. M. Internal Reflection Spectroscopy: Review and Supplement; Harrick Scientific Group: New York, 1985. (52) Zaera, F. Int. Rev. Phys. Chem. 2002, 21, 433. (53) Zhao, J.; McCreery, R. L. Langmuir 1995, 11, 4036. (54) Liu, Y.-C.; McCreery, R. L. Anal. Chem. 1997, 69, 2091. (55) Azoulay, J.; Debarre, A.; Richard, A.; Tchenio, P.; Bandow, S.; Iijima, S. Chem. Phys. Lett. 2000, 331, 347. (56) Yu, Z.; Brus, L. J. Phys. Chem. B 2001, 105, 1123.

3106

Analytical Chemistry, Vol. 78, No. 9, May 1, 2006

The current investigation was undertaken to determine the molecular orientations of monolayers of NAB, azobenzene (AB), nitrobiphenyl (NBP), biphenyl (BP), fluorene (FL), and multilayers of NAB, AB, and NBP on flat graphitic carbon. The orientations were obtained by using a combination of polarized IR and Raman spectroscopy. The studies confirm the molecular structure following chemisorption as well as the average tilt angle with respect to the surface. These results were correlated with the conductivity of molecular electronic junctions made from similar monolayers, which have been previously reported.29 EXPERIMENTAL SECTION Preparation of Pyrolyzed Photoresist Films. The procedure for preparing pyrolyzed photoresist films (PPF) has been described previously.57,58 Briefly, positive photoresist AZ P4330-RS (AZ Electronic Materials, Somerville, NJ) was spin-coated onto clean silicon at 6000 rpm for 30 s before a programmed heat treatment to 1000 °C for 60 min under flowing forming gas (95% N2 + 5% H2 at ∼100 mL/min). As noted previously, PPF is similar in properties and conductivity to GC, with primarily sp2-hybridized bonding.57,58 The PPF surface is flat (