Langmuir 1998, 14, 845-854
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Structural Evolution of Hexadecanethiol Monolayers on Gold during Assembly: Substrate and Concentration Dependence of Monolayer Structure and Crystallinity Roger H. Terrill, Troy A. Tanzer, and Paul W. Bohn* Department of Chemistry, Beckman Institute and Materials Research Laboratory, University of Illinois at Urbana-Champaign, 600 South Mathews, Urbana, Illinois 61801 Received May 19, 1997. In Final Form: December 8, 1997 The structures of hexadecanethiol (HDT) self-assembled monolayers (SAM) on gold assembled for up to 11 days from ethanolic HDT solutions are reported. Gold|HDT films were periodically removed from 0.001, 0.1, 1, or 10 mM HDT solution and analyzed by Fourier transform infrared external reflection spectroscopy (FTIR-ERS). FTIR-ERS data provide evidence for an ongoing increase in SAM crystallinity and, in some cases, a decrease in average chain tilt angle with respect to the surface normal during this long assembly time. SAM crystallinity, judged by line shape and position, increased more rapidly for more concentrated HDT solutions but appears to converge on the same final value independent of HDT concentration in the range of 10-6 to 10-2 M. Computer-optimized fits to the ensemble of C-H stretching bands between 2800 and 3000 cm-1 made on ∼50 different SAMs which were prepared under a wide range of conditions return surprisingly consistent average tilt and twist angles. For Au on Cr-primed glass the absolute values of the average cant and twist angles were 20 ( 2° and 49 ( 3°, respectively, and for Au on HF-etched Si〈111〉 they were 22 ( 2° and 51 ( 1°, respectively. A clear trend in SAM cant angles, decreasing with increasing time, is observed for the glass|Cr|Au|HDT system. Two structural models were compared: one in which all chains are identical and one in which alternating alkanethiol chains have orthogonal backbone planes.
Introduction Self-assembled monolayers (SAMs) of alkanethiols on gold, silver and copper have been studied extensively1 within the past decade. By virtue of their general integrity and by choice of chemical functionality at the ω-terminus, alkanethiol SAMs can be prepared which control surface properties such as wetting, adhesion, corrosion, and binding affinity. Densely packed alkanethiol SAMs function as nearly impermeable dielectrics at the metal solution interface2 and as patternable monolayer resists.3 The need for extended immersion (or assembly) times to make reproducible and low defect density monolayers has been a topic of interest in the literature for some time. Scanning tunneling microscopy (STM) studies illustrate an annealing processes driven by heating4 or soaking5 on a 10-100 hour time scale for long-chain thiols, indicating that structurally sensitive applications may benefit in terms of reproducibility by extended immersion times even though wetting, ellipsometry,6 and nanogravimetric re* To whom correspondence should be addressed. (1) (a) Ulman, A. An Introduction to Ultrathin Films: From Langmuir-Blodgett to Self-Assembly; Academic Press, Inc.: New York, 1991. (b) Whitesides, G. M.; Laibinis, P. E. Langmuir 1990, 6, 87. (2) (a) Becka, A. M.; Miller, C. J. J. Phys. Chem. 1992, 96, 26572668. (b) Miller, C. J.; Gratzel, M. J. Phys. Chem. 1991, 95, 5225-5233. (c) Miller, C. J.; Cuendet, P.; Gratzel, M. J. Phys. Chem. 1991, 95, 877886. (3) (a) Corbitt, T. S.; Crooks, R. M.; Ross, C. B.; Mampden-Smith, M. J.; Schoer, J. K. Adv. Mat. 1993, 5 (12), 935-938. (b) Xia, Y.; Mrksich, M.; Kim, E.; Whitesides, G. M. J. Am. Chem. Soc. 1995, 117, 95769577. (c) Kumar, A.; Whitesides, G. M. Appl. Phys. Lett. 1993, 63 (14), 2002. (4) Delamarche, E.; Michel, B.; Kang, H.; Gerber, Ch. Langmuir 1994, 10, 4103-4108. (5) Delamarche, E.; Michel, B.; Gerber, Ch.; Anselmetti, D.; Guntherodt, H.-J.; Wolf, H.; Ringsdorf, H. Langmuir 1994, 10, 28692871. (6) Biebuyck, H. A.; Bain, C. D.; Whitesides, G. M. Langmuir 1994, 10 (6), 1825-1831.
sults7 detect the presence of an adsorbate monolayer in just a few minutes of immersion time. However, studies8 that have addressed the assembly time scale question directly have not arrived at a consensus. Thus, the present work seeks to clarify the dependence of SAM structure on assembly time, and we demonstrate that hexadecanethiol (HDT) SAMs continue to change in average structure throughout an 11-day immersion (assembly time) in ethanolic solutions of HDT. Studies based on electron, X-ray and He diffractions,9 STM,10 and atomic force microscopy (AFM)11 support a picture in which the sulfur headgroups of n-alkanethiol chains form a x3×x3 (R30°) overlayer (Figure 1) on the predominantly 〈111〉 surface of polycrystalline gold films. Infrared12 and Raman13 spectroscopies inform about the alkyl chain configurations and indicate a slightly canted, (7) (a) Schessler, H. M.; Karpovich, D. S.; Blanchard, G. J. J. Am. Chem. Soc. 1996, 118, 9645-9651. (b) Pan, W.; Durning, C. J.; Turro, N. J. Langmuir 1996, 12, 4469-4473. (8) (a) Schneider, T. W.; Buttry, D. A. J. Am. Chem. Soc. 1993, 115, 12391-7. (b) Buck, M.; Grunze, M.; Eisert, F.; Fischer, J.; Traeger, F. J. Vac. Sci. Technol., A 1992, 10, 926-929. (c) Frubose, D.; Doblhofer, K. J. Chem. Soc., Faraday Trans. 1995, 91, 1949-1953. (d) Karpovich, D. S.; Blanchard, G. J. Langmuir 1994, 10, 3315-3322. (e) Peterlinz, K. A.; Georgiadis, R. Langmuir 1996, 12, 4731-4740. (9) (a) Strong, L.; Whitesides, G. M., Langmuir 1988, 4, 546-558. (b) Dubois, L. H.; Zegarski, B. R.; Nuzzo, R. G. J. Chem. Phys. 1993, 98, 678-688. (c) Fenter, P.; Eisenberg, P.; Li, j.; Camillone, N., III.; Bernasek, S.; Scoles, G.; Ramanarayanan, T. A.; Liang, K. S. Langmuir 1991, 7, 437-439. (e) Chidsey, C. E. D.; Liu, G.; Rowntree, P.; Scoles, G. J. Chem. Phys. 1989, 91, 4421-4423. (10) Widrig, C. A.; Alves, C. A.; Porter, M. D. J. Am. Chem. Soc. 1991, 113, 2805-2810. (11) Alves, C. A.; Smith, E. L.; Porter, M. D. J. Am. Chem. Soc. 1992, 114, 1222-1227. (12) (a) Dubois, L. H.; Nuzzo, R. G. Annu. Rev. Phys. Chem. 1992, 43, 437. (b) Nuzzo, R. G.; Dubois, L. H.; Allara, D. L. J. Am. Chem. Soc. 1990, 112, 558. (c) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, D. E. D. J. Am. Chem. Soc. 1987, 109, 3559-3568. (d) Laibinis, P.; Whitesides, G. M.; Allara, D. L.; Tao, Y.-T.; Parikh, A. N.; Nuzzo, R. G. J. Am. Chem. Soc. 1991, 113, 7152. (e) Chang, S.-C.; Chao, J.; Tao, Y.-T. J. Am. Chem. Soc. 1994, 116, 6792.
S0743-7463(97)00508-8 CCC: $15.00 © 1998 American Chemical Society Published on Web 01/28/1998
846 Langmuir, Vol. 14, No. 4, 1998
Figure 1. Representation of the x3×x3(R30°) overlayer structure for thiol S-headgroups (filled circles) binding to a gold surface (open circles).
Figure 2. Alkanethiol chain configuration showing chain cant angle (R), chain twist angle (β), and azimuthal angle (γ) with respect to gold surface crystal axis.
nearly all-trans phase for well-formed alkanethiols, CH3(CH2)nSH, for n > 7. In addition to the periodicity of the headgroup binding, alkanethiol SAM structure is defined by the configuration of the alkane chain, i.e., the all-trans chain tilt (R), twist (β), and azimuthal angle (γ) with respect to the underlying gold lattice, illustrated in Figure 2. Evidence that the alkanethiol molecules pack on the surface with alternating chain twist orientations is consistent with the structure of crystalline alkanes as well as IR,14 STM,15 and diffraction results16 and theoretical studies.17 Within this general framework, however, there is experimental evidence for a range of headgroup binding patterns and, thus, a range of possible chain conforma(13) Bryant, M. A.; Pemberton, J. E. J. Am. Chem. Soc. 1991, 113, 3629-3637, 8284-8293. (14) Nuzzo, R. G.; Korenic, E. M.; Dubois, L. H. J. Chem. Phys. 1990, 93, 767-773. (15) (a) Stranick, S. J.; Parikh, A. N.; Tao, Y.-T.; Allara, D. L.; Weiss, P. S. J. Phys. Chem. 1994, 7636-7646. (b) Schonenberger, C.; Jorritsma, J.; Sondag-Huethorst, J. A. M.; Fokkink, L. G. J. J. Phys. Chem. 1995, 99, 3259-3271. (c) Poirier, G. E.; Tarlov, M. J. Langmuir 1994, 10 (9), 2853-2856. (16) (a) Camilone, N., III.; Chidsey, C. E. D.; Liu, G.-Y.; Scoles, G. J. Chem. Phys. 1993, 98, 3503. (b) Camilone, N., III.; Chidsey, C. E. D.; Eisenberger, P.; Fenter, P.; Li, J.; Liang, K. S.; Liu, G.-Y., Scoles, G. J. Chem. Phys. 1993, 99, 744. (c) Fenter, P.; Eisenberger, P.; Liang, K. S. Phys. Rev. Lett. 1993, 70, 2447. (17) (a) Pertsin, A. J.; Grunze, M. Langmuir 1994, 10, 3668-3674. (b) Ullman, A.; Eilers, E. J.; Tillman, N. Langmuir 1989, 5, 1147. (c) Sellers, H.; Ullman, A.; Shnidman, Y.; Eilers, J. E. J. Am. Chem. Soc. 1993, 115, 9389.
Terrill et al.
tions. Various studies suggest the presence of low binding density SAM structures. For example STM studies sometimes show periodic missing row structures,18 in which every third or fifth row of molecules are missing depending on adsorption and annealing histories. These observations are consistent with the idea that during assembly, a SAM may sample many configurational states before finding a final, low-energy structure and with the present observations of slow changes in average SAM structure changes over extended immersion times. Depending on SAM chain length, assembly, and annealing histories, STM and low-temperature He diffraction results identify various (e.g., n×x3, nx3×x3 (n ) 1-6)) superlattices defined both by headgroup binding and possibly by chain twist alternation patterns relative to the Au〈111〉 surface. Accordingly, multiple SAM phases are implied by calculations that show only small energetic distinctions between them. For example molecular dynamics calculations19 suggest that the 2 × 1 and 2 × 4 superstructures (relative to x3×x3 binding) that originate in chain twist alternation differ in interchain stabilization by only 0.9 kcal/mol; such structures are observed by STM to coexist with one another in asadsorbed and ∼100 °C postannealed SAMs, respectively.20 More densely packed phases might occur if some sulfur binds directly atop Au atoms. Such an arrangement is theoretically21 disfavored versus interstital binding by 6 kcal/mol but might still be exothermic depending on intermolecular chain-chain interactions. In fact, thiols bind to Ag with a lower cant angle (5°) and have a lower penalty for on-top binding (3.3 kcal/mol).21 Calculations indicate that a ∼30-38° cant angle on gold is minimum in energy for the surface,17b but the lowest energy conformation of the system of bound and excess diffusing thiols may appear at higher packing densities. Importantly, STM and AFM images often reveal a high density of “defect” structures assigned to void defects,15a,22 liquidlike,23 physisorbed thiols,15b or low-density, e.g., missing-row, phases.15b,e This study explores the evolution of alkanethiol crystallinity and chain ordering over many decades of assembly time, as a function of hexadecanethiol concentration and for two differently textured types of polycrystalline gold films, i.e., either Au anchored by a thin Cr layer onto glass microscope slides, or Au deposited directly onto HFetched Si(111). For both types of Au film, spectral shifts and line shape changes were observed over the range of assembly times. These changes reflect, by analogy to the pure alkanethiol materials, a crystallization process, which is consistent with the healing of defects or filling of low-density phases by diffusing alkanethiols. On the other hand, spectral fits indicate a remarkably constant average chain configuration, R and β, and only for the case of the rougher surface (Au on Cr-primed glass) is there a well-resolved trend in R with assembly time. We also find that models of the experimental spectra are relatively insensitive to details of the unit cell, i.e., whether the alkyl chains have identical β or the packing is identical with alkanethiol crystals and alternating chains are (18) Schonenberger, C.; Jorritsma, J.; Sondag-Huethorst, J. A. M.; Fokkink, L. G. J. J. Phys. Chem. 1995, 99, 3259-3271. (19) Mar, W.; Klein, M. Langmuir 1994, 10, 188. (20) Schonenberger, C.; Jorritsma, J.; Sondag-Huethorst, J. A. M.; Fokkink, L. G. J. J. Phys. Chem. 1995, 99, 3259-3271. (21) Sellers, H.; Ullman, A.; Shnidman, Y.; Eilers, J. E. J. Am. Chem. Soc. 1993, 115, 9389-9401. (22) Stranick, S. J.; Kamna, M. M.; Krom, A. N.; Parikh, A. N.; Allara, D. L.; Weiss, P. S. J. Vac. Sci., Technol. B 1994, 12, 2004-2007. (23) Poirier, G. E.; Tarlov, M. J.; Rushmeier, H. E. Langmuir 1994, 10, 3383-3386.
Structural Evolution of Hexadecanethiol Monolayers
twisted in β by exactly 90°. Two-chain models in which interchain twist angles were optimized resulted in a dihedral of
