Langmuir 2008, 24, 6609-6615
6609
On the Importance of Purity for the Formation of Self-Assembled Monolayers from Thiocyanates Cai Shen,† Manfred Buck,*,† James D. E. T. Wilton-Ely,‡ Tobias Weidner,§ and Michael Zharnikov§ EaStCHEM School of Chemistry, UniVersity of St. Andrews, St. Andrews KY16 9ST, United Kingdom, Chemistry Research Laboratory, UniVersity of Oxford, Mansfield Road, Oxford OX1 3TA, United Kingdom, and Angewandte Physikalische Chemie, UniVersita¨t Heidelberg, INF 253, 69120 Heidelberg, Germany ReceiVed February 8, 2008. ReVised Manuscript ReceiVed March 12, 2008 Assembly of dodecyl thiocyanate (C12SCN) from ethanol solution onto Au(111)/mica substrates was investigated by scanning tunneling microscopy (STM), near edge X-ray absorption fine structure spectroscopy (NEXAFS), X-ray photoelectron spectroscopy (XPS), and infrared reflection-absorption spectroscopy (IRRAS). Contrary to previous reports, thiolate monolayers formed by cleavage of the S-CN bond can be obtained whose quality is at least as good as that of self-assembled monolayers (SAMs) formed directly from the thiol analogue of C12SCN, dodecanethiol (C12SH). However, the achievable quality is strikingly dependent on the purity of the thiocyanate with even low levels of contamination impeding the formation of structurally well-defined monolayers.
Introduction Adsorption of organosulfur compounds is a standard way to form self-assembled monolayers (SAMs) on coinage metals, in particular on gold, and it is the ease of preparation and flexibility in the design of the molecular structure which enables the tailoring of surface properties for a diversity of applications in (bio)sensors, nanotechnology, or electrochemistry.1–5 While thiols have been most popular, they are not an optimal choice from the chemical point of view, as they are prone to oxidation to disulfides which in the case of dithiols can seriously affect the formation of SAMs6 or makes the quality of the resulting SAMs critically dependent on the details of the preparation conditions.7,8 In the search for alternatives, a number of other options based on different precursors have been explored including acetyl protected thiols7,9–11 or Bunte salts, that is, organic thiosulfates.12,13 Common drawbacks of these approaches are that the layers, even though chemical analysis shows that they consist of thiolates, * To whom correspondence should be addressed. E-mail: mb45@ st-andrews.ac.uk. † University of St. Andrews. ‡ University of Oxford. § Universita¨t Heidelberg.
(1) Mrksich, M. Chem. Soc. ReV. 2000, 29, 267–273. (2) Gooding, J. J.; Mearns, F.; Yang, W. R.; Liu, J. Q. Electroanalysis 2003, 15, 81–96. (3) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Chem. ReV. 2005, 105, 1103–1169. (4) Schilardi, P. L.; Dip, P.; Claro, P. C. D.; Benitez, G. A.; Fonticelli, M. H.; Azzaroni, O.; Salvarezza, R. C. Chem.sEur. J. 2005, 12, 38–49. (5) Thom, I.; Ha¨hner, G.; Buck, M. Appl. Phys. Lett. 2005, 87, 024101. (6) Tour, J. M.; Jones, L.; Pearson, D. L.; Lamba, J. J. S.; Burgin, T. P.; Whitesides, G. M.; Allara, D. L.; Parikh, A. N.; Atre, S. V. J. Am. Chem. Soc. 1995, 117, 9529–9534. (7) Azzam, W.; Wehner, B. I.; Fischer, R. A.; Terfort, A.; Wo¨ll, C. Langmuir 2002, 18, 7766–7769. (8) Tai, Y.; Shaporenko, A.; Rong, H. T.; Buck, M.; Eck, W.; Grunze, M.; Zharnikov, M. J. Phys. Chem. B 2004, 108, 16806–16810. (9) Shaporenko, A.; Elbing, M.; Baszczyk, A.; von Hanisch, C.; Mayor, M.; Zharnikov, M. J. Phys. Chem. B 2006, 110, 4307–4317. (10) Niklewski, A.; Azzam, W.; Strunskus, T.; Fischer, R. A.; Wo¨ll, C. Langmuir 2004, 20, 8620–8624. (11) Kang, Y. K.; Won, D. J.; Kim, S. R.; Seo, K. J.; Choi, H. S.; Lee, G. H.; Noh, Z. S.; Lee, T. S.; Lee, C. J. Mater. Sci. Eng. C 2004, 24, 43–46. (12) Lukkari, J.; Meretoja, M.; Kartio, I.; Laajalehto, K.; Rajamaki, M.; Lindstrom, M.; Kankare, J. Langmuir 1999, 15, 3529–3537. (13) Lusk, A. T.; Jennings, G. K. Langmuir 2001, 17, 7830–7836.
are of poor structural quality and/or the experimental conditions prove critical. Another route has more recently been reported by Ciszek et al.14,15 who demonstrated that thiocyanates form thiolate SAMs through cleavage of the S-CN bond. Compared to other self-assembling organosulfur compounds, this approach promises to depend less critically on the preparation conditions, as thiocyanates are more stable than thiols and no deprotection steps are required. Thiocyanate assembly on gold has been shown to yield thiolate SAMs by a surface mediated cleavage of the S-CN bond, and subsequent desorption of [Au(CN)2]–23 has been suggested.14,15 However, comparison with SAMs formed from thiols showed an inferior quality of the monolayer for the thiocyanate analogues as revealed by cyclic voltammetry,15 sum frequency generation,16 and scanning tunneling microscopy (STM)16 where the latter did not reveal any ordered structure. In contrast, a very recent study of octyl thiocyanates demonstrated that ordered SAMs can be formed, but this is strongly dependent on the preparation conditions.17 While solution-based preparation at room temperature did not yield ordered SAMs, extended areas of crystalline order were observed for preparation at elevated temperature and adsorption from the vapor phase. However, the overall structural quality of the SAMs is still far from that of SAMs formed directly from thiols. Furthermore, high resolution STM revealed a structure very different from the 3 × 3 based structures of alkane thiols, and a missing row structure was suggested. It appears that, so far, it has proven difficult to establish alternatives to thiols or disulfides which yield thiolate SAMs of high quality. The present paper demonstrates that the difficulties in obtaining high quality SAMs from thiocyanate precursors is not necessarily an intrinsic limitation of the concept but might be limited by extrinsic factors such as contamination. (14) Ciszek, J. W.; Tour, J. M. Chem. Mater. 2005, 17, 5684–5690. (15) Ciszek, J. W.; Stewart, M. P.; Tour, J. M. J. Am. Chem. Soc. 2004, 126, 13172–13173. (16) Dreesen, L.; Volcke, C.; Sartenaer, Y.; Peremans, A.; Thiry, P. A.; Humbert, C.; Grugier, J.; Marchand-Brynaert, J. Surf. Sci. 2006, 600, 4052–4057. (17) Choi, Y.; Jeong, Y.; Chung, H.; Ito, E.; Hara, M.; Noh, J. Langmuir 2008, 24, 91–96.
10.1021/la8004272 CCC: $40.75 2008 American Chemical Society Published on Web 05/27/2008
6610 Langmuir, Vol. 24, No. 13, 2008
Experimental Section Dodecanethiol (98%) (C12SH) and dodecyl thiocyanate (97%) (C12SCN) were purchased from Aldrich and Alfa Aesar, respectively. Absolute ethanol (BDH) served as the solvent. Whereas dodecane thiol was used as received, dodecyl thiocyanate was applied both as purchased and after purification. In the purification procedure, one major peak was identified by UV (retention time 21.9 min) with two minor but significant UV-active impurities (11.0 and 27.4 min) present. A Dionex high-performance liquid chromatography (HPLC) machine was used with a P60 pump with a UVD 340V UV detector (UV at 240 nm) and a Phenomenex Jupiter 10 µm Proteo 90 Å column, dimensions 250 × 21.2 mm, at a flow rate of 8 mL/min. Solvents: A ) H2O, B ) MeCN; t ) 0, 40% A; t ) 5, 40% A; t ) 10, 0% A; t ) 30, 100% A. Mica substrates with an epitaxial Au(111) layer 300 nm thick were purchased from Georg Albert PVD, Heidelberg, Germany. The substrates were flame annealed before immersion into solutions of either 15 mM dodecanethiol or 15 mM dodecyl thiocyanate in ethanol. After removal from the solution, samples were rinsed with pure ethanol and dried in a nitrogen stream. STM measurements were carried out in air using a PicoPlus microscope (Molecular Imaging). Tips were mechanically cut from a 0.25 mm Pt/Ir wire (8:2, Goodfellow). STM images which are presented as acquired throughout the paper were recorded in constant current mode using tunneling currents between 8 and 20 pA and a sample bias between 0.75 and 1.2 V (tip positive). X-ray photoelectron spectroscopy (XPS) and near edge X-ray absorption fine structure spectroscopy (NEXAFS) measurements were performed under ultrahigh vacuum (UHV) conditions (a base pressure of
