Langmuir 2007, 23, 2887-2891
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Characterization of Self-assembled Monolayers from Lithium Dialkyldithiocarbamate Salts Randy D. Weinstein*,† and Joshua Richards Department of Chemical Engineering, VillanoVa UniVersity, VillanoVa, PennsylVania 19085
Susan D. Thai, Donna M. Omiatek, and Carol A. Bessel Department of Chemistry, VillanoVa UniVersity, VillanoVa, PennsylVania 19085
Christopher J. Faulkner, Siti Othman, and G. Kane Jennings*,‡ Department of Chemical Engineering, Vanderbilt UniVersity, NashVille, Tennessee 37235 ReceiVed October 3, 2006. In Final Form: December 5, 2006 We report the formation of self-assembled monolayers (SAMs) onto gold substrates by exposure to lithium dialkyldithiocarbamate salts [(Li+(R2DTC-), where R ) n-propyl, n-butyl, n-octyl, n-decyl, n-dodecyl, or n-octadecyl] in ethanol or methylene chloride. The crystallinity and composition of the monolayers were assessed by polarized modulation infrared reflection absorption spectroscopy (PM-IRRAS), wettability was characterized by contact angles of water and hexadecane, thickness was measured by spectroscopic ellipsometry, and barrier properties determined by electrochemical impedance spectroscopy. While the shorter R2DTC-s formed monolayers with liquid-like packing, monolayers prepared from the longest R2DTC- (where R ) n-octadecyl) exhibit similar thickness, crystallinity, wettability, and capacitance as monolayers prepared from n-octadecanethiol. The hydrocarbon chains within the monolayers prepared from (C18)2DTC- are less canted on average than those prepared from n-octadecanethiol. Nonetheless, the (C18)2DTC- SAM exhibits an order of magnitude lower resistance against the penetration of redox probes, which is attributed to a higher density of pinhole defect sites.
Introduction Monolayer films on gold surfaces have been used to control protein adsorption,1 mediate electron transfer,2 initiate polymer film growth,3,4 and modulate friction coefficients.5 These monolayers are most often prepared from alkanethiols6 due to the commercial availability of many n-alkanethiols and ω-terminated alkyl thiols, the synthetic ease of preparing many others,7 and the well-characterized assemblies and structures that the thiols afford. Monolayers on gold can also be prepared from other sulfur-containing molecules including dialkyl disulfides,8 dialkyl sulfides,9 dithiocarboxylic acids,10,11 Bunte salts,12 and di- and tridentate thiols.13 The advantages of employing different head groups in the formation of SAMs include the abilities to * Authors to whom correspondence should be addressed. † E-mail:
[email protected]. Fax: (610) 519-7354. ‡ E-mail:
[email protected]. Fax: (615) 343-7951. (1) Prime, K. L.; Whitesides, G. M. Science 1991, 252, 1164. (2) Cui, X. D.; Primak, A.; Zarate, X.; Tomfohr, J.; Sankey, O. F.; Moore, A. L.; Moore, T. A.; Gust, D.; Harris, G.; Lindsay, S. M. Science 2001, 294, 571. (3) Shah, R. R.; Merreceyes, D.; Husemann, M.; Rees, I.; Abbott, N. L.; Hawker, C. J.; Hedrick, J. L. Macromolecules 2000, 33, 597. (4) Bantz, M. R.; Brantley, E. L.; Weinstein, R. D.; Moriarty, J.; Jennings, G. K. J. Phys. Chem. B 2004, 108, 9787. (5) Kim, H. I.; Graupe, M. B.; Oloba, O.; Koini, T.; Imaduddin, S.; Lee, T. R.; Perry, S. S. Langmuir 1999, 15, 3179. (6) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Chem. ReV. 2005, 105, 1103. (7) Laibinis, P. E.; Palmer, B. J.; Lee, S.-W.; Jennings, G. K. The Synthesis of Organothiols and Their Assembly into Monolayers on Gold. In Thin Films; Ulman, A., Ed.; Academic Press: Boston, 1998; Vol. 24, pp 1-41. (8) Nuzzo, R. G.; Fusco, F. A.; Allara, D. L. J. Am. Chem. Soc. 1987, 109, 2358. (9) Takiguchi, H.; Sato, K.; Ishida, T.; Abe, K.; Yase, K.; Tamada, K. Langmuir 2000, 16, 1703. (10) Colorado, R.; Villazana, R. J.; Lee, T. R. Langmuir 1998, 14, 6337. (11) Lee, T.-C.; Hounihan, D. J.; Colorado, R.; Park, J.-S.; Lee, T. R. J. Phys. Chem. B 2004, 108, 2648.
prepare films with different structures, packing densities, electrontransfer properties, and thermal stabilities. Dialkyldithiocarbamates are another class of ligands that spontaneously adsorb to gold surfaces to form monolayers (Figure 1). The recent work of Wei and co-workers14 showed that several n-alkane, branched-alkane, aromatic, and ring substituted dithiocarbamates could adsorb onto gold surfaces and particles upon exposure to CS2 and the appropriate secondary amine. These ligands offer some advantages over their traditional thiol counterparts. First of all, R and R′ groups can be independently selected, which enables surfaces to be coated with films that contain a variety of structures and packing densities. Dialkyldithiocarbamates also have interatomic distances between the two sulfur atoms at almost the perfect distance for placing each of the two sulfur atoms on adjacent gold atoms and providing for two-point ligation with resonance generated between the two sulfur atoms (see Figure 1).10,15,16 The resonant bidentate structure of the anchored dithiocarbamate has been shown to produce a characteristically different molecule-metal coupling compared to n-alkanethiols, which is believed to make these ligands applicable for molecular electronics.17 The resonance structure can incorporate both of the sulfur-carbon bonds as well as the carbon-nitrogen bond due to the free electron pairs on the sulfur (12) Lukkari, J.; Meretoja, M.; Kartio, I.; Laajalehto, K.; Rajamaki, M.; Lindstrom, M.; Kankare, J. Langmuir 1999, 15, 3529. (13) Park, J.-S.; Vo, A. N.; Barriet, D.; Shon, Y.-S.; Lee, T. R. Langmuir 2005, 21, 2902. (14) Zhao, Y.; Perez-Segarra, W.; Shi, Q.; Wei, A. J. Am. Chem. Soc. 2005, 127, 7328. (15) Ulman, A. Chem. ReV. 1996, 96, 1533. (16) Querner, C.; Reiss, P.; Bleuse, J.; Pron, A. J. Am. Chem. Soc. 2004, 126, 11574. (17) Morf, P.; Raimondi, F.; Nothofer, H. G.; Schnyder, B.; Yasuda, A.; Wessels, J. M.; Jung, T. A. Langmuir 2006, 22, 658.
10.1021/la062905h CCC: $37.00 © 2007 American Chemical Society Published on Web 01/30/2007
2888 Langmuir, Vol. 23, No. 5, 2007
Figure 1. Assembly of dialkyldithiocarbamate salts into monolayers on gold.
and nitrogen atoms. Recently, Li and Kosov18 computationally showed that a dithiocarbamate used to anchor molecular wires to gold electrodes exhibited improved electron transport properties with strong molecule-electrode coupling, which allowed for enhanced electrical conductance, most likely due to the resonance and availability of the free electrons. Monolayers formed with these two-point attachments with resonance are, in some cases, more robust to temperature, solvents, and exchanges with other ligands than single attached ligands such as n-alkanethiols. Dimethyl- and diethyl- dithiocarbamate monolayers on gold were shown to be stable in water for longer than a week over the pH range of 1-12 while the same monolayers also showed negligible exchange with solutions of dodecanethiol in ethanol at room temperature for a 1 week exposure.14 The monolayer formed from dibutyldithiocarbamate showed stability up to 85 °C in water for 12 h.14 Computational studies have also predicted that monolayers generated from several dialkyldithiocarbamates would be excellent wear inhibitors for engines, creating and sustaining monolayers on the iron oxide surfaces even when exposed to high temperatures.19 Dithiocarbamates have also been used to stabilize nanoparticles.14,20 In this article, we describe SAMs prepared from lithium dialkyldithiocarbamate salts through characterization of crystallinity and composition by polarized modulation infrared reflection absorption spectroscopy (PM-IRRAS), wettability by contact angles, thickness by spectroscopic ellipsometry, and barrier properties by electrochemical impedance spectroscopy. As the R2DTC- adsorbates must be synthesized, we have limited our analysis to those chain lengths in which starting materials are commercially available. We compare the properties of the most crystalline SAM formed from a LiR2DTC salt (Li(C18)2DTC) with those of a SAM prepared from octadecanethiol. Our work demonstrates that R2DTC- anions of short to moderate chain length form SAMs that are a less crystalline alternative to thiolate SAMs but that a SAM prepared from (C18)2DTC- is comparable to a SAM prepared from C18SH in thickness, wettability, and crystallinity. Experimental Section Materials. Gold shot (99.99%) and silicon(100) wafers were obtained from J&J Materials and Montco Silicon, respectively. Chromium-coated tungsten rods were obtained from R.D. Mathis. Octadecanethiol (Sigma-Aldrich), isooctane (Fisher), and 100% ethanol (AAPER) were all used as received while deionized water (16.7 MΩ) was purified with a Modu-Pure system. For the synthesis of the dithiocarbamates, dipropylamine, dibutylamine, dioctylamine, n-butyllithium (2.5 mol/L in hexanes), diethyl ether, and carbon disulfide were supplied by Sigma-Aldrich. Didodecylamine and didecylamine were purchased from TCI-EP (Tokyo, Japan) and dioctadecylamine was obtained from Fluka Chemika. Note: n-butyl lithium is extremely flammable and water-sensitiVe and should be handled under inert conditions. Details of the synthesis and (18) Li, Z. Y.; Kosov, D. S. J. Phys. Chem. B 2006, 110, 9893. (19) Zhou, Y. H.; Jiang, S. Y. T. C.; Yamaguchi, E. S.; Frazier, R.; Ho, A.; Tang, Y. C.; Goddard, W. A. J. Phys. Chem. A 2000, 104, 2508. (20) Vickers, M. S.; Cookson, J.; Beer, P. D.; Bishop, P. T.; Thiebaut, B. J. Mater. Chem. 2006, 16, 209.
Weinstein et al. purification of the lithium dialkyldithiocarbamates (MdLi in Figure 1) are presented elsewhere,21,22 and products were verified by melting points and 1H and 13C NMR spectroscopy. The chain lengths of R2DTC-s chosen for this study were those whose starting materials were commercially available. The acidic form (MdH in Figure 1) of the dialkyldithiocarbamates is not stable,14 and hence we chose to explore the lithium salts since lithium was the smallest available metal ion and produced a stable dialkyldithiocarbamate. An alternative approach to forming the monolayers is to use the secondary amine and carbon disulfide in solution and expose the gold surface directly to that solution14 if the counterion would pose problems. Sample Preparation. Gold substrates were prepared by sequentially evaporating 100 Å of chromium at a rate of 1 Å/s and ∼1500 Å of gold at a rate of 3 Å/s onto silicon [Si(100)] wafers inside a diffusion-pumped chamber with a base pressure of 4 × 10-6 Torr. Gold substrates prepared by this approach are known to be polycrystalline with a predominate (111) structure.8 Prior atomic force microscopy images of substrates prepared in this manner show Au(111) crystallites that are ∼50 nm in diameter.7 SAM Preparation. SAMs were prepared by immersing evaporated gold films into solutions containing 1 mM lithium dialkyldithiocarbamate salts in ethanol or dichloromethane at room temperature for 4-24 h. As controls, SAMs were prepared by immersing evaporated gold substrates into a solution of octadecanethiol (1 mM) in ethanol at room temperature for the same adsorption time. Upon removal, the samples were rinsed with fresh solvent and water and then dried in a stream of nitrogen. Infrared Spectroscopy. Polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) data were collected using a Bruker PMA-50 attachment to a Bruker Tensor 26 infrared spectrometer equipped with a liquid-nitrogen-cooled mercurycadmium-telluride (MCT) detector and a Hinds Instruments PEM90 photoelastic modulator. The source beam was modulated at a frequency of 50 kHz with half-wavelength (λ/2) retardation and set at 80° incident to the sample surface. Spectra for SAMs on gold substrates were collected over 10 min (760 scans) at a resolution of 4 cm-1. The differential reflection spectra (∆R/R) were calculated from the s- and p-polarized signals simultaneously collected by a lock-in-amplifier. All reported IR spectra were repeated at least twice using independent sample preparations. The estimated error in reported spectra based on the standard deviations of band absorbances measured after these independent preparations was less than 10%. Electrochemical Impedance Spectroscopy (EIS). Electrochemical impedance measurements of SAM-coated gold samples were performed with a CMS300 Electrochemical Impedance System (Gamry Instruments) interfaced to a personal computer. A Teflon cell equipped with a gold-coated silicon counter electrode and a Ag/AgCl/saturated KCl reference electrode contained an aqueous solution of 0.1 M Na2SO4, 1 mM K3Fe(CN)6, and 1 mM K4Fe(CN)6‚3H2O. The working electrode in contact with the aqueous solution was a confined circle with an area of 1 cm2. The measurements were made at the open circuit potential with a 5 mV ac perturbation that was controlled between 50 mHz and 20 kHz. Film resistance and capacitance values were determined by fitting the EIS data with a Randles model equivalent circuit23 using software provided by Gamry. Spectroscopic Ellipsometry. Ellipsometric thicknesses were determined from a J.A. Woollam M-2000DI variable angle spectroscopic ellipsometer. Thicknesses were fit to data taken at 75° from the surface normal over wavelengths from 400 to 700 nm using a refractive index of 1.45 at all wavelengths. Optical constants of the underlying gold used in the preparation of each sample were taken prior to monolayer deposition and used as a baseline for thickness measurements. Reported thickness values and errors (21) Dunbar, A.; Omiatek, D. M.; Thai, S. D.; Kendrex, C. E.; Grotzinger, L. L.; Boyko, W. J.; Weinstein, R. D.; Skaf, D. W.; Bessel, C. A.; Denison, G. M.; DeSimone, J. M. Ind. Eng. Chem. Res. 2006, 45, 8779. (22) Sakai, S.; Aizawa, T.; Fujinami, T. J. Org. Chem. 1974, 39, 1970. (23) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications, Second ed.; Wiley: New York, 2001.
SAMs from Lithium Dialkyldithiocarbamate Salts
Langmuir, Vol. 23, No. 5, 2007 2889 Table 1. Effect of R2DTC- Chain Length on the Advancing Water and Hexadecane Contact Angles, the Charge-transfer Resistances, and Capacitances of the SAMs chain length
θa(H2O) (deg)
θa(HD) (deg)
Rct (kΩ cm2)
Cf (µF/cm2)
18 12 10 8
114 108 104 98
48 34 25
