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Infrared Spectroscopy of Three-Dimensional Self-Assembled Monolayers: N-Alkanethiolate Monolayers on Gold Cluster Compounds Michael J. Hostetler, Jennifer J. Stokes, and Royce W. Murray* Kenan Laboratories of Chemistry, University of North Carolina, Chapel Hill, North Carolina 27599-3290 Received March 15, 1996X Transmission infrared spectroscopy has been used to probe the structure of alkanethiolate monolayers adsorbed onto nanometer-sized gold clusters. The alkyl chain lengths vary between propanethiolate and tetracosanethiolate; specifically the C3, C4, C5, C6, C7, C8, C10, C12, C16, C20, and C24 alkanethiolates have been examined as solid suspensions in KBr pellets. It has been found that the smaller chain lengths (C3, C4, and C5) are relatively disordered, with large amounts of gauche defects present, and thus most resemble the free alkanes in the liquid state. The longer length alkanethiolates are predominantly in the all trans zigzag conformation. There are detectable amounts of near surface gauche defects, the amount of which decreases with increasing chain length, and a reasonably high percentage of end-gauche defects, the relative amount of which increases with increasing chain length. Internal gauche defects cannot be detected. A model is proposed to explain these observations, and the data are compared with that collected for alkanethiolates self-assembled onto the more traditional two-dimensional systems.
Self-assembled monolayers on flat surfaces (twodimensional or 2D-SAMs) have attracted a great deal of attention in the past 15 years1san interest arising, in part, from the wide range of technologies in which 2DSAMs can play a significant role.2 As a result, many characterization and structure-function correlation studies have been reported. A major limitation has been the scope of the analytical tools useable to further probe these structures. Along with a series of important reports by Schiffrin et al.3a-c and others,3d-f we recently described an investigation4a of alkanethiol monolayers self-assembled on small, well-defined gold clusters. These results, further on-going work,4b and recent small angle X-ray scattering (SAXS) data4c on dodecanethiolate-stabilized cluster samples prepared as described herein show the clusters to have an 11.8-14.1 Å range of gold core radii, which is compatible with cuboctahedra of gold containing 309 or 561 atoms or a mixture of the two. Considering other analytical results and assuming monodisperse clusters,4a * To whom correspondence should be addressed. E-mail:
[email protected]. X Abstract published in Advance ACS Abstracts, June 15, 1996. (1) (a) Dubois, L. H.; Nuzzo, R. G. Annu. Rev. Phys. Chem. 1992, 43, 437-463. (b) Ulman, A. An Introduction to Ultrathin Organic Films; Academic: New York, 1991. (c) Bain, C. D.; Whitesides, G. M. Angew. Chem., Int. Ed. Engl. 1989, 28, 506-512. (2) (a) Frisbie, C. D.; Rozsnyai, L. F.; Noy, A.; Wrighton, M. S.; Lieber, C. M. Science 1994, 265, 2071-2074. (b) Huang, J.; Dahlgran, D. A.; Hemminger, J. C. Langmuir 1994, 10, 626-628. (c) Willner, I.; Riklin, A.; Shoham, B.; Revenizon, D.; Katz, E. Adv. Mater. 1993, 13, 912-915. (d) Chaudhury, M. K.; Whitesides, G. M. Science 1992, 255, 12301232. (3) (a) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801-802. (b) Brust, M.; Fink, J.; Bethell, D.; Schiffrin, D. J.; Kiely, C. J. J. Chem. Soc., Chem. Commun. 1995, 1655-1656. (c) Brust, M.; Bethell, D.; Schiffrin, D. J.; Kiely, C. J. Adv. Mater. 1995, 7, 795-797. (d) Leff, D. V.; Ohara, P. C.; Heath, J. R.; Gelbart, W. M. J. Phys. Chem. 1995, 99, 7036-7041. (e) Badia, A.; Gao, W.; Singh, S.; Demers, L.; Cuccia, L.; Reven, L. Langmuir 1996, 12, 1262-1269. (f) Badia, A.; Singh, S.; Demers, L.; Cuccia, L.; Brown, G. R.; Lennox, R. B. Chem. Eur. J. 1996, 2, 359-363. (4) (a) Terrill, R. H.; Postlethwaite, T. A.; Chen, C.-H.; Poon, C.-D.; Terzis, A.; Chen, A.; Hutchison, J. E.; Clark, M. R.; Wignall, G.; Londono, J. D.; Superfine, R.; Falvo, M.; Johnson, C. S., Jr.; Samulski, E. T.; Murray, R. W. J. Am. Chem. Soc. 1995, 117, 12537-12548. (b) Hostetler, M. J.; Green, S. J.; Stokes, J. J.; Murray, R. W. J. Am. Chem. Soc. 1996, 118, 4212-4213. (c) Unpublished results: Hostetler, M. J., and Murray, R. W., University of North Carolina, Chapel Hill, and Londono, J. D., and Wignall, G., Oak Ridge National Laboratory, November 1995.
S0743-7463(96)00249-1 CCC: $12.00
such cuboctahedra would bear stabilizing monolayers of ∼95 and ∼174 alkanethiolate ligands, respectively. (If a mixture of cuboctahedra is actually present, these numbers of ligands will differ by approximately 10% from the values of the individual cluster sizes.) These cluster molecules offer exceptional properties in comparison to other metal cluster compounds,5 including being readily soluble, air stable, isolable, black solids that exhibit significant electron-hopping conductivity.4a Furthermore, these substances are easily characterized by solutionbased techniques (such as NMR spectroscopy),4a,b are amenable to further functionalization and exchange reactions,3b,4b and participate in both solution- and surfacebased electrochemical reactions.4b They hold promise for utility in a wide range of science and technologies.6 We refer to the gold cluster monolayers as “threedimensional” self-assembled monolayers (3D-SAMs) because they can be investigated in bulk samples (solid or solution) and because each monolayer is confined upon a molecule-sized spatial element. There are two structural motifs which may provoke structural and reactivity differences between monolayers of the same alkanethiol on flat (2D) surfaces versus 3D gold cluster surfaces. The first is the higher concentration of surface defect-like sites on the cluster; i.e., of the 252 Au atoms on the surface of a 561 atom cuboctahedron, there are 12 on corner sites, 96 on edge or ridge sites, 96 on Au(100) terraces, and 48 on Au(111) terraces. Thus, classically defined defect sites account for ∼43% of all available sites on the gold cluster core’s surface. Recent thermogravimetric analyses indicate that as much as 68% of the proposed surface sites can be occupied.7a Considering this result as well as STM studies on the migration of alkanethiolates on flat Au(111) surfaces,8 we assume that the majority (if not all) of defect sites on the cluster molecules will be occupied, which leads to a higher density of alkanethiolates (up to (5) (a) Schmid, G. Chem. Rev. 1992, 92, 1709-1727. (b) Hayat, M. A., Ed. Colloidal Gold; Academic Press: New York, 1989; Vol. 1. (c) Schon, G.; Simon, U. Colloid Polym. Sci. 1995, 273, 101-117. (6) Schon, G.; Simon, U. Colloid Polym. Sci. 1995, 273, 202-218. (7) (a) Unpublished results: Hostetler, M. J.; Clark, M. R.; Murray, R. W. (b) This calculation assumes that all of the defect sites and onethird of the terrace sites can be occupied. Higher occupancy should be observed for the smaller cluster compound.
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62% coverage, or 0.62 thiolate/surface gold for the 561 atom cluster)7b as compared to 2D Au(111) surfaces (33% coverage).1a The second major difference is the cluster surface’s high radius of curvature. Nominally, the methylene units of the alkane chains become progressively less densely packed as one moves further away from the core. This obviously creates a potential for the outermost functional groups to exhibit greater liberty of movement than those near the core. On the basis of the above discussion, the question which arises is how similar are these 3D-SAMs to the classical 2D-SAMs? In particular and with relevance to this study: does the structure of a 2D-SAM bear resemblance to a 3D-SAM, and can this latter system shed light onto question not easily answered by analytical studies on 2DSAMs. We have initiated investigation of this question by using infrared spectroscopy to probe the structure of 3D-SAMs. This choice is especially relevant, since reflectionabsorption infrared spectroscopy (RAIRS) has been one of the principal tools in understanding the structure of the alkanethiolate chains on 2D-SAMs,9 e.g., in deducing the presence of the all-trans zigzag methylene chain and its orientation relative to the surface normal. Unfortunately, signal-to-noise limitations prevented an assessment of the relative importance of gauche defects, although it was clear that their concentration was small. In this regard, the work of Pemberton and co-workers proved valuable.10 They compared the surface-enhanced Raman spectra (SERS) of alkanethiols adsorbed on smooth and on roughened gold surfaces and noted the presence of small amounts of both gauche and trans C-S and C-C vibrations. These results were in line with one of the models predicted by a molecular dynamics study11 which indicated that near surface gauche defect concentrations will be either very large (99%) or relatively small (∼1%). We plan to relate this œuvre, as well as that from very detailed IR studies on the conformational behavior of straight chain alkane hydrocarbons,12 to our interpretation of the conformation of n-alkanethiolates chemisorbed on gold clusters. The analysis was aided by varying the alkanethiol employed to prepare the cluster samples from propanethiol to tetracosanethiol, specifically C3, C4, C5, C6, C7, C8, C10, C12, C16, C20, and C24. Our analysis of the IR spectra indicates that, in spite of the differences described above, the 2D and 3D SAMs share more similarities than differences.
synthesized.14 All other reagents were acquired from standard sources and were used as received. Synthesis. The gold cluster compounds were synthesized using a slightly modified literature procedure.4a Briefly, hydrogen tetrachloroaurate (aq) was extracted into toluene using the phase transfer reagent tetraoctylammonium bromide. The resulting organic phase was then reacted with an equimolar amount of alkanethiol followed by reduction with a 10-fold molar excess of sodium borohydride (aq). After stirring at room temperature for 3 h, the organic phase was collected and the solvent removed on a rotary evaporator. The black precipitate was suspended in 30 mL of ethanol, filtered, and washed with 100 mL of ethanol and 100 mL of acetone. The black material (which ranged in texture from a fine powder to a wax, depending upon the alkane chain length) was dried in vacuo for 1 h to give a material free of significant residual alkanethiol. The purity of the sample was determined by the absence of resonances due to free alkanethiol in the 1H NMR spectrum (CDCl3), in particular the R-CH2 resonance at ∼2.5 ppm. Infrared Spectroscopy. Approximately 3 mg of the gold cluster compound was mixed with approximately 1 g of potassium bromide, and the resulting suspension was ground via mortar and pestle into a fine powder. A portion of this material was pressed into a transparent disk at 20 000 psi. The infrared spectra were collected in the transmission mode on a Mattson Galaxy 6000 spectrometer. The spectra were acquired over 256 scans (∼5.5 min) from which a background spectrum (empty cell) was automatically subtracted. The data which are presented have not been manipulated except to straighten out the baseline. The intensities of the bands in each spectrum were not normalized for differing concentrations or sample thicknesses; thus, it is not valid to directly compare the intensities of similar bands in different spectra. However, relative band intensities can be determined by comparing, in the same spectrum, the intensity of a band which changes with alkane chain length (such as the symmetric CH2 stretching mode) with the intensity of a band which is not expected to vary with alkane chain length (such as the symmetric CH3 stretching mode).12
Experimental Section Chemicals. HAuCl4‚xH2O was either purchased (Aldrich 99.99%) or synthesized.13 The alkanethiols were purchased (Aldrich) except for eicosane- and tetracosanethiol, which were (8) (a) Stranick, S. J.; Parikh, A. N.; Tao, Y.-T.; Allara, D. L.; Weiss, P. S. J. Phys. Chem. 1994, 98, 7636-7646. (b) Stranick, S. J.; Parikh, A. N.; Allara, D. L.; Weiss, P. S. J. Phys. Chem. 1994, 98, 11136-11142. (9) (a) Nuzzo, R. G.; Fusco, F. A.; Allara, D. L. J. Am. Chem. Soc. 1987, 109, 2358-2368. (b) Nuzzo, R. G.; Dubois, L. H.; Allara, D. L. J. Am. Chem. Soc. 1990, 112, 558-569. (c) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 35593568. (10) Bryant, M. A.; Pemberton, J. E. J. Am. Chem. Soc. 1991, 113, 8284-8293. (11) Hautman, J.; Klein, M. L. J. Chem. Phys. 1989, 91, 4494-5001. (12) (a) Maroncelli, M.; Qi, S. P.; Strauss, H. L.; Snyder, R. G. J. Am. Chem. Soc. 1982, 104, 6273-6247. (b) Snyder, R. G.; Schachtschneider, J. H. Spectrochim. Acta 1963, 19, 85-116. (c) Schachtschneider, J. H.; Snyder, R. G. Spectrochim. Acta 1963, 19, 117-168. (d) Snyder, R. G. J. Chem. Phys. 1967, 47, 1316-1360. (e) Snyder, R. G.; Strauss, H. L.; Elliger, C. A. J. Phys. Chem. 1982, 86, 5145-5150. (f) Snyder, R. G.; Maroncelli, M.; Strauss, H. L.; Hallmark, V. M. J. Phys. Chem. 1986, 90, 5623-5630. (13) (a) Handbook of Preparative Inorganic Chemistry; Brauer, G., Ed.; Academic Press: New York, 1965; pp 1054-1059. (b) Block, B. P. Inorg. Synth. 1953, 4, 14-17.
Results and Discussion The infrared spectra of alkanethiolate-stabilized gold clusters (KBr pellets) were collected over the range 4003200 cm-1. The n-alkanethiolate chain lengths were varied from C3 to C24. The excellent band definition found in these spectra, and the detailed background of assignments in previous12 work, allowed a fairly complete set of mode assignment to be made (Table 1). These represent a broad gamut of vibrational modes; we will discuss each set of modes separately and, hopefully, integrate in each section trends as a function of chain length, as well as the relationship of band structure to the dominant and defect structure of the alkyl chain. A separate paper15 will describe the effect of ligand substitution and temperature on the microenvironment of the gold cluster compound. The essential conclusions that will be made are as follows: (a) the S-H bond of the alkanethiol is broken upon adsorption; (b) a majority of the alkanethiolate ligands are in an all-trans zigzag conformation; (c) there is a noticeable lack of internal kink defects; (d) the near surface defect concentration appears to be higher for the shorter chains, whereas, the chain end-gauche defect concentration is higher for the longer chains; and (e) the overall structure of alkanethiolates bound to small gold clusters is remarkably similar to that seen for these ligands bound to flat gold surfaces. C-H Stretching Region. The C-H stretching region (2800-3000 cm-1) has been used in previous infrared spectroscopic studies on 2D-SAMs to determine the (14) Analogous syntheses are described in the Supplementary Material of: Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321-335. (15) Hostetler, M. J.; Stokes, J. J.; Murray, R. W. Manuscript in preparation.
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Table 1. Mode Assignments for n-Alkane Thiolate Monolayers on Au Clustersa position intensity
assignment
position intensity assignment position intensity assignment position intensity assignment
638 670 714 733 779 837
w w s m s w
ν(C-S)G ν(C-S)D ν(C-S)T P1T P1G P1D
891 1034 1045 1086 1198 1220
s sh w s sh s
CH3(CH2)2S P2T,G 1282 RD 1325 RT,G 1333 βT; RG 1373 TD 1419 TG,T 1440
s m sh s m sh
TT; WG WT,G WD U δS δG,T
1452 2868 2927 2958
s s s s
R r+ dr-
474 640 715 725 741 779 796 870
w w m m s m w m
ω ν(C-S)G ν(C-S)T P1T PT,G Px Px Px
895 910 966 1049 1066 1095 1187 1211
w m w w w s sh s
CH3(CH2)3S Px 1261 β 1290 (β, R)E 1337 RT 1348 RG 1355 RT 1377 Tx; Wx 1419 Tx; Wx 1429
s s sh m sh sh m sh
Tx; Wx Tx; Wx Wx WE Wx U δS δG
1456 1463 2834 2859 2870 2924 2954
s sh s sh s s s
R δT d+S d+ r+ dr-
457 640 716 727 742 764 783 904
m w sh s m w w w
ω ν(C-S)G ν(C-S)T P1T PT,G Px Px β
924 964 1062 1078 1105 1179 1203 1250
m m w w s w s s
CH3(CH2)4S Px 1267 (β, R)E 1294 RG 1329 (W, R)E 1340 RT 1362 Tx; Wx 1377 Tx; Wx 1419 Tx; Wx 1439
s s m m m s s sh
Tx; Wx Tx; Wx Wx WE Wx U δS δD
1458 1464 2850 2856 2872 2920 2954
s sh sh s s s s
R δ d+cryst d+liq r+ dr-
457 482 642 723 758 786 814 857
w w w s w w w w
ω ω ν(C-S)G P; ν(C-S)T Px Px Px Px
892 917 1111 1176 1198 1238 1252
w w m w s s s
CH3(CH2)5S β 1279 Px 1302 RT 1338 (P, β)E 1363 Tx; Wx 1377 Tx; Wx 1419 Tx; Wx 1438
s s m m s s sh
Tx; Wx Tx; Wx WE Wx U δS δD
1456 1465 2848 2870 2898 2918 2954
s sh s s sh s s
R δ d+ r+ d+FR dr-
w, br w s w w w w w, br
ω ν(C-S)G P; ν(C-S)T Px Px Px β RT
1113 1170 1194 1223 1240 1265 1284 1294
m w m m m m sh m
CH3(CH2)6S RT 1316 (P, β)E 1340 Tx; Wx 1363 Tx; Wx 1377 Tx; Wx 1419 Tx; Wx 1438 Tx; Wx 1456 Tx; Wx 1464
w m m s s sh s sh
Wx WE Wx U δS δD R δ
2848 2870 2898 2918 2954
s s sh s s
d+ r+ d+FR dr-
465 621 642 719 743 766 810 827 880
w w w s w w sh m w
ω ν(C-S)D ν(C-S)G P; ν(C-S)T Px Px Px Px (β, R)E
892 914 945 1005 1031 1117 1131 1165 1192
w w m m m m w w m
CH3(CH2)7S β 1217 Px 1232 (β, R)E 1255 RT 1268 RT 1287 RT 1296 RT 1342 (P, β)E 1365 Tx; Wx 1377
m s s sh sh m m s s
Tx; Wx Tx; Wx Tx; Wx Tx; Wx Tx; Wx Tx; Wx WE Wx U
1415 1440 1456 1464 2850 2872 2898 2920 2954
s sh s sh s s sh s s
δS δD R δ d+ r+ d+FR dr-
467 640 719 750 770 804 891
w w s sh w w w
ω ν(C-S)G P; ν(C-S)T Px Px Px β
1118 1190 1207 1221 1245 1252 1267
w w w m sh m w
CH3(CH2)9S RT 1277 Tx; Wx 1296 Tx; Wx 1342 Tx; Wx 1365 Tx; Wx 1377 Tx; Wx 1414 Tx; Wx 1438
w m m s s s sh
Tx; Wx Tx; Wx WE Wx U δS δD
1456 1464 2850 2872 2897 2920 2954
s s s s sh s s
R δ d+ r+ d+FR dr-
463 638 719 760 774 810 825 891
w w s w w w w w
ω ν(C-S)G P; ν(C-S)T Px Px Px Px β
943 1072 1122 1190 1201 1211 1232 1240
m w, br m w sh m sh m
CH3(CH2)11S (β, R)E 1255 (W, R)E 1265 RT 1277 Tx; Wx 1294 Tx; Wx 1344 Tx; Wx 1365 Tx; Wx 1377 Tx; Wx 1423
m sh m s s s s sh
Tx; Wx Tx; Wx Tx; Wx Tx; Wx WE Wx U δS
1441 1459 1463 2850 2872 2897 2920 2954
sh sh s s s sh s s
δD R δ d+ r+ d+FR dr-
474 643 721 761 788 831 897 1023
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Table 1 (Continued) position intensity
assignment
position intensity assignment position intensity assignment position intensity assignment
461 645 719 765 777 800 822 889
m w s w w w w w
ω ν(C-S)G P; ν(C-S)T Px Px Px Px β
947 1016 1093 1126 1184 1201 1223 1244
w w w m m m m m
CH3(CH2)15S (β, R)E 1263 RT 1285 RT 1294 RT 1342 Tx; Wx 1367 Tx; Wx 1377 Tx; Wx 1422 Tx; Wx 1440
m sh m m s s sh sh
Tx; Wx Tx; Wx Tx; Wx WE Wx U δS δD
1467 2848 2873 2897 2918 2954
s s s sh s s
δ d+ r+ d+FR dr-
459 640 719 781 802 875 890 1076
m w s w w w w w
ω ν(C-S)G P; ν(C-S)T Px Px (β, R)E β (W, R)E
1105 1126 1182 1196 1213 1230 1246 1259
w w w m m m m m
CH3(CH2)19S RT 1275 RT 1294 Tx; Wx 1342 Tx; Wx 1365 Tx; Wx 1375 Tx; Wx 1422 Tx; Wx 1440 Tx; Wx 1460
m s m s s sh sh sh
Tx; Wx Tx; Wx WE Wx U δS δD R
1468 2848 2873 2897 2918 2954
s s s sh s s
δ d+ r+ d+FR dr-
459 642 719 757 785 802 889 1065
m w s w w w w w
ω ν(C-S)G P; ν(C-S)T Px Px Px β RT
1086 1109 1128 1193 1207 1220 1234 1246
w w w w w w w w
CH3(CH2)23S RG,T 1258 RT 1272 RT 1294 Tx; Wx 1342 Tx; Wx 1367 Tx; Wx 1375 Tx; Wx 1422 Tx; Wx 1439
w w m m s m sh sh
Tx; Wx Tx; Wx Tx; Wx WE Wx U δS δD
1459 1468 2848 2871 2897 2918 2954
sh s s sh sh s s
R δ d+ r+ d+FR dr-
a In this table the following abbreviations12 were used: D, unknown defect site; E, end-gauche defect; T, trans; G, gauche; x, unknown portion of various progression bands; S, CH2 groups next to S-Au bond; w, weak; s, strong; v, very; br, broad; sh, shoulder; m, medium; FR, Fermi resonance; δ, methylene scissoring; W, methylene wagging; T, methylene twisting-rocking; P, methylene rocking-twisting; R, C-C stretching; R, methyl asymmetric bending; U, methyl symmetric bending; β, methyl rocking; r+ or r-, symmetric or antisymmetric methyl C-H stretching, respectively; d+ or d-, symmetric or antisymmetric methylene C-H stretching, respectively.
orientation of the methylene chains.9 This was achieved by judicious use of the dipole selection rule inherent in RAIRS.16 Our experiments, based on transmission IR spectroscopy and of monolayers that are randomly oriented with respect to the beam, are on both accounts not sensitive to this type of information. Nevertheless, the C-H stretching region still held valuable information. For example, the position of the peaks and the increase in intensity of the methylene stretching vibrations (relative to the methyl stetching vibration) with chain length indicated that the structural integrity of the alkanethiolate was maintained during formation of the clusters and that solvent entrapment or multilayer formation does not play a significant role in monolayer formation.9c More interesting, though, are the actual peak values for the symmetric (d+) and antisymmetric (d-) CH2 stretching vibrations, which can be used as a sensitive indicator of the ordering of the alkyl chains.1,9 For example, in crystalline polyethylene12e,f d- lies at 2920 cm-1 and d+ is found at 2850 cm-1. In solution, these values become 2928 and 2856 cm-1, respectively; the higher energies for the methylene stretching vibrations of polyethylene in solution are thought to arise from a greater incidence of gauche defects. Thus, studies of 2DSAMs have correlated observed d- and d+ peak values with the degree of chain ordering. For example, Nuzzo et al.9a,b reported that the d- and d+ values for hexadecanethiolate monolayers on a gold surface appear at 2920 and 2850 cm-1, respectively, and concluded that the number of gauche defects in the methylene chains was small. Similarly, Porter et al.9c report the RAIR spectra of a wide range of alkanethiolate monolayers on a 2D gold surface and compare their d- and d+ values to those of (16) Chabal, Y. J. Surf. Sci. Rep. 1988, 8, 211-357.
crystalline hexadecanethiol (2918, 2851 cm-1) and liquid heptanethiol (2924, 2855 cm-1). They found that monolayers with chains longer than hexanethiol were highly ordered, whereas the smaller molecules most resembled the liquid state, presumably with a higher density of gauche defects. In the transmission FTIR spectra of our 3D-SAM system, both the antisymmetric d- and symmetric d+ CH2 stretches (Figure 1, see dotted vertical lines) undergo changes with alkanethiolate chain length that are consistent with this previously established pattern. A crystalline microenvironment is seen for alkyl chains C6 or greater, as indicated by the antisymmetric d- CH2 stretches lying between 2918 and 2920 cm-1 and symmetric d+ stretch values between 2848 and 2850 cm-1. Pentanethiolate (Table 1) behaves as if it were on the border between the two states with a d- at 2920 cm-1 but a d+ at 2856 cm-1 (although there is a shoulder at ca. 2850 cm-1). Propanethiolate and butanethiolate most resemble the liquid state. Thus, our results while qualitatively similar to those of Porter et al.9c differ in that crystalline behavior is preserved for even shorter alkanethiolate chains. The latter result is somewhat surprising in light of the high radius of curvature of the gold core of the cluster molecules, which might have been presumed to encourage disordering. However, the higher surface density of ligands on the 3D-SAMs (vide supra) should promote ordering. In related work, Badia et al.3f examined the C-H stretching region of the C14, C16, and C18 gold cluster compounds. A detailed investigation of the d+ and dmodes of these materials as a function of temperature revealed order-disorder transitions that were commensurate with relatively sharp melting transitions measured by differential scanning calorimetry (DSC).
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Figure 1. Transmission infrared spectra of the alkanethiolatestabilized gold cluster compound: (a) propanethiolate, (b) hexanethiolate, (c) octanethiolate, (d) dodecanethiolate, (e) eicosanethiolate. The intensities of the bands were not corrected for either sample concentration or the thickness of the disk, and thus it is not valid to compare the absolute intensities of similar bands in the different spectra. The relative intensities of peaks can be compared by using the ∼2954 cm-1 (r-) peak as an internal yardstick.
Figure 2. Transmission infrared spectra of the alkanethiolatestabilized gold cluster compound: (a) propanethiolate, (b) hexanethiolate, (c) octanethiolate, (d) dodecanethiolate, (e) eicosanethiolate. The intensities of the bands were not corrected for either sample concentration or the thickness of the disk, and thus it is not valid to compare the absolute intensities of similar bands in the different spectra. The relative intensities of peaks can be compared by using the ∼1377 cm-1 (U) peak as an internal yardstick.
This alkanethiolate chain ordering behavior becomes especially intriguing in that DSC studies performed in this lab (data not shown) indicate that the C8, C10, and C12 thiolate cluster compounds exhibit relatively broad melt or glass transitions at temperatures less than 273 K. Thus, unlike the longer chain materials, these compounds, while appearing to be in a macroscopically melted state (the above materials are highly viscous gums at room temperature) are microscopically crystalline. We will report more fully on the temperature dependent behavior of these compounds in the future.15 1400-1500 cm-1. The literature reports four bands in this region associated with straight chain alkanes:12 δ, the scissoring motion of an all-trans methylene chain (∼1467 cm-1); δG, the scissoring motion of a methylene group next to a gauche bond (∼1460 cm-1); R, the methyl antisymmetric bending vibration (∼1458 cm-1); and a band found near 1440 cm-1, the identity of which is somewhat controversial. Two of the more likely alternatives that have been considered for this latter band are that it arises either from the mixing of methylene wagging and scissoring modes for an end-gauche defect17 or from the scissoring of a methylene group next to two gauche bonds. The latter has been favored on the basis of a number of considerations including the absence of this band from the spectra of shorter alkanes (butane and pentane), in which a double gauche bond is not as likely to appear.
On 2D-SAM alkanethiolates on gold surfaces, typically only the all-trans scissoring mode (δ) has been seen in this region.9 However, at low temperatures this mode becomes a doublet, a finding explained by factor group splitting.1a We initiate our discussion of this spectral region for 3D-SAMs on cluster molecules with propanethiol, since it behaves somewhat differently from the longer cluster ligands. In the free state, propanethiol exhibits only two bands between 1400 and 1500 cm-1 (R and the δ).18 For the propanethiolate-stabilized Au cluster, a new mode appears at 1419 cm-1 (Figure 2, top), which we assign to the scissoring of a methylene group adjacent to the Au-S bond (δS). It is consistent that long chain alkanecarboxylate 2D-SAMs on silver surfaces have a similar band assigned as the scissoring of a methylene group adjacent to the electron-withdrawing COO- head group.19 The 1419 cm-1 band is seen in the spectra of all other alkanethiolates (Figure 2, Table 1) and, as expected, exhibits, for longer alkyl chains, a progressively lower intensity relative to that of the methylene scissoring mode δ. For alkanethiolates larger than C3 (Figure 2), four bands are typically seen between 1400 and 1500 cm-1 (except for C16, Table 1, a result likely linked to the resolution of the spectrophotometer). On the basis of the above discussion, three of these modes can be straightforwardly assigned: methylene scissoring at ∼1463 cm-1, δ; methyl antisymmetric bending at ∼1456 cm-1, R; and CH2-S
(17) The defects described in this paper have the following meaning: end-gauche, a single gauche C-C bond on the end of an all-trans methylene chain; internal kink, a single gauche C-C bond in an otherwise all-trans chain; double-gauche, two sequential gauche C-C bonds either at the end of or in an all-trans chain.
(18) Hayashi, M.; Shiro, Y.; Murata, H. Bull. Chem. Soc. Jpn. 1966, 39, 112-117. (19) Smith, E. L.; Porter, M. D. J. Phys. Chem. 1993, 97, 8032-8038.
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methylene scissoring at ∼1419 cm-1, δS. As expected, the intensity of the methylene scissoring mode δ increases relative to those of the other two vibrations and the peak position decreases in energy as the length of the alkyl chain increases. The remaining weak band at 1440 cm-1 could, in principle, be assigned to a methylene scissoring vibration next to a double gauche bond. However, there are several difficulties with this interpretation. First, unlike the case of the free alkanes, this band is present in the spectra of all alkanethiolates larger than C3. Second, other bands associated with such a defect site are absent (see below). Third, the intensity of the 1440 cm-1 band scales with the intensity of the other bands associated with a chain endgauche defect (see below). That is, from C4 to C10, the relative intensity of the ∼1440 cm-1 band increases with chain length, after which the intensity appears to level off and may in fact decrease somewhat as the chain length is further increased. We thus assign this band as arising from the mixing of methylene wagging and scissoring motions for a methylene group next to a chain end-gauche defect. 1200-1400 cm-1. The literature describes several important sets of bands in this spectral region. The CH3 symmetric bending vibration, also known as the umbrella mode (U), appears at ∼1375 cm-1. This mode is essentially isolated to the motion of the methyl group and is thus relatively insensitive to the conformation of the rest of the chain. In addition, the twisting-rocking (Tx) and wagging progression bands (Wx) can be found between 1180 and 1380 cm-1. The presence of these progression bands as a series of well-resolved peaks is a strong indicator of crystallinity. For example, in the liquid-phase IR spectra of linear alkanes, these bands are weak, broad features, whereas sharp features are seen for the crystalline phase.12a,d This region of the spectrum is also rich in defect-related bands. Snyder has pointed out that there are four wagging bands between 1300 and 1400 cm-1 which can be assigned to defect structures12d and whose peak values are approximately independent of alkyl chain length to within 5 cm-1.20 These bands are found at 1345 cm-1 for a chain end-gauche defect, 1366 and 1306 cm-1 for an internal kink defect, and 1353 cm-1 for a double gauche defect. As an example, heptadecane, in the solid state near its melting transition, has a concentration of end-gauche and internal kinks between 5 and 10% and of double gauche defects less than 1%. The concentration of these defects generally increases with increasing chain length. In the RAIR spectra of 2D-SAMs, the umbrella mode has been observed as a weak band.1a In addition, Porter identified a set of wagging progression bands for alkanecarboxylates adsorbed on a 2D silver surface and cited this as evidence for a high degree of chain order.19 Interestingly, defect-related modes have not yet been observed in this region even though the presence of endgauche defects has been determined indirectly.1a Spectra of 3D-SAM alkanethiolates on Au clusters display (Figure 2) a large number of bands between 1200 and 1380 cm-1, and except for the U mode at 1373-1377 cm-1 and one defect-related mode (vide infra), we have assigned these bands to either a twisting-rocking or wagging progression mode. In general, we do not distinguish between these two possibilities in our assignments; however, we note that the number of bands
increases with chain length and that nearly a full set of both the Wx and Tx progression bands can be detected up to C12 (band overlapping occurs with the longer chains). This important observation is strong evidence that the microenvironment of the alkanethiol monolayers on gold clusters mimics well the crystallinity found in crystalline alkanes and in well-ordered 2D-SAM systems. In the 3D-SAM system, we cannot with confidence assign any bands to either internal kink defects or double gauche defects (the latter result is relevant to our assignment of the defect scissoring mode described above). While there is consistently a band near 1366 cm-1, which is consistent with an internal kink defect, its accompanying band at 1306 cm-1 (which usually has equal or stronger intensity) is absent, and thus the presence of this defect cannot be established. However, there is good evidence for the chain endgauche defect wagging mode by the presence of a band between 1338 and 1348 cm-1 for all chain lengths between C4 and C24. Furthermore, the energy for this band stabilizes at 1342 cm-1 for chain lengths greater than C7, a behavior not expected for a progression band. The intensity of this band increases (relative to the U mode which is used as an internal yardstick) with chain length up to C10, whereupon it levels off or decreases slightly. On the basis of relative intensity correlations from the literature,12a a conservative estimate of the chain endgauche defect concentration is between 10 and 25% (of the total chains) for all chain lengths, a value much higher than that seen for similar lengths of alkanes in the solid state. This difference is plausibly suggested as being due to the fact that the alkanethiolate monolayers are bonded to a highly curved surface so that the chain packing density near the surface will always be greatest and, consequently, the outermost groups will have more freedom as chain length increases. 1000-1200 cm-1. The literature describes two relevant sets of bands in this region of the spectrum. The first set is the C-C stretching modes (R), commonly found between 950 and 1150 cm-1. These bands have been examined by Pemberton et al.10 using Raman spectroscopy to analyze the free alkanethiols (solid and liquid) and surfaceenhanced Raman spectroscopy (SERS) to study alkanethiolate monolayers. For the free molecules in the liquid state, both the RT (trans C-C bonds) and RG (gauche C-C bonds) modes were observed (the latter at 1065-1080 cm-1) in high abundance. Chemisorption on either roughened silver or gold surfaces caused the intensity of RG to drop dramatically; a further decrease in intensity occurred upon adsorption on a smooth gold or silver surface or by reducing the temperature. The presence of a gauche C-C stretching vibration in the spectra of the adsorbed alkanethiol is indicative of either a near surface defect or an internal kink in the monolayer. The other important spectral features in this region are two weak chain endgauche defect bands,12a which involve the combination of either a methylene and methyl rock (∼1165 cm-1) or a methylene wag and a C-C stretch (∼1078 cm-1). As with the wagging end-gauche defect bands, the peak values for these modes are independent of chain length to within 5 cm-1.20 In the spectra of alkanethiolate-stabilized gold clusters (Figure 2), an RG band can be identified for only the shortest (C3-C5) and longest (C24) alkanethiolates. In general, this region of the spectrum is well-resolved for the shortest (C3-C5) and the longest (C16-C24) chains, whereas the middle length (C6-C12) chains usually have only one well-resolved peak at ∼1120 cm-1 (RT) as well as a poorly defined hump which may contain information on RG. On the basis of information in the C-S stretching
(20) This statement is a slight simplification; these peak values reach a steady value ((5 cm-1) only once the following chain lengths are exceeded (see ref 12a): 1366 cm-1, >C5; 1353 cm-1, >C4; 1341 cm-1, >C3; 1306 cm-1, >C5; 1164 cm-1, >C4; 1078 cm-1, >C4; 955 cm-1, >C11; 873 cm-1, >C8.
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region (vide infra), it is likely that an RG band is present in all of the spectra. In regard to end-gauche defects, there is some indication for the aforementioned bands in this region (Table 1); however, their intensity is often weak. Nevertheless, good evidence exists for at least one of these bands in six of the cluster compounds (C5, C6, C7, C8, C12, C20), with no obvious correlation with chain length. 700-1000 cm-1. This region of the spectrum has two sets of bands12b,c of interest to this discussion (the C-S stretching vibration, which is also found in this region, is discussed separately below). The first is the methylene rocking mode, which has a principal band (P1), located between 719 and 740 cm-1, and progression bands (Px), located between 720 and 1065 cm-1. These progression rocking modes can be used as an indicator of internal defects; i.e., four or more trans CH2 groups in a row would show up as a band between 718 and 723 cm-1, while fewer (as might arise from an internal gauche defect) would give a band between 723 and 738 cm-1.12d The other band of interest in this region is the in-plane CH3 rocking mode (β), located at ∼890 cm-1, with higher values for shorter (eC5) chain lengths. This mode is also sensitive to the presence of an end-gauche defect, giving rise (in combination with a C-C stretching vibration) to bands at ∼878 and ∼960 cm-1. Once again, the peak positions of these defect bands are independent of chain length within 5 cm-1.20 The methylene rocking modes of propanethiol have been identified in the literature,17 and as with the C-S mode (vide infra), there exists a distinct set of progression bands for two different conformers, which have been labeled TT and GT, where the first letter describes the conformation (trans or gauche) about the C-C bond adjacent to the C-S bond and the second letter describes the conformation about the C-S bond. For example, at -120 °C, the TT conformer of propanethiol has a P1 band at 736 cm-1, whereas, for the GT conformer, this band appears at 780 cm-1. In general, Px bands for the gauche conformer are found at higher energy. From FTIR spectra (Figure 3) of the alkanethiolate monolayers on the gold clusters, we have analogously assigned (Table 1) the P1 and Px modes.21 The shorter chain alkanethiolates (C3-C5) have two P1 modes, which can be assigned to the TT and GT conformers. For the larger molecules, only the TT P1 mode (labeled P for molecules larger than C5, since this band is likely an envelope of the P1 mode and higher order Px modes) can be observed. Furthermore, neither bands nor shoulders can be detected between 723 and 738 cm-1, indicating the amount of internal kinks is small. These data again emphasize the transition in the ligand microenvironment from liquid-like (C3-C5) to crystalline (g6) that occurs as the chain length increases. There are also several bands in this region which can be assigned (Table 1) with reasonable confidence either to a methyl rocking mode (β) or to one of the end-gauche defect modes described above (βE). We note that, unlike what has been observed for free alkanes, the peak position of the higher energy end-gauche defect mode in this region settles near a value of 945 cm-1 (instead of 960 cm-1) as the alkyl chain length increases. C-S Stretching Vibration. The peak position of the C-S stretching mode can be used as a diagnostic for the orientation of the adjacent C-C bond. For example, IR (21) The mode assignments for the progression bands between 720 and 900 cm-1 can only be considered tentative due to the lack of a viable model system. It is likely that some of the bands in this region arise from defect structures, such as the end-gauche conformer (see below). However, we feel it is better to err on the conservative side and only assign obvious bands to defect modes.
Hostetler et al.
Figure 3. Transmission infrared spectra of the alkanethiolatestabilized gold cluster compound: (a) propanethiolate, (b) hexanethiolate, (c) octanethiolate, (d) dodecanethiolate, (e) eicosanethiolate. The intensities of the bands were not corrected for either sample concentration or the thickness of the disk, and thus it is not valid to compare the absolute intensities of similar bands in the different spectra. The relative intensities of peaks can be compared by using the ∼890 cm-1 (β) peak as an internal yardstick.
studies on propanethiol in both the gas and liquid phases reveal C-S modes at 655 and 706 cm-1 which were assigned to the GT and TT conformers, respectively.18 These two conformations were also observed in relatively high abundance for longer chains in the liquid state; upon condensation to a solid, the number of alkanethiol chains in the GT conformer approaches zero (for alkyl chains gC5).10 The proportion of alkanethiolate chains in the TT conformer is greater in 2D-SAMs on either smooth or roughened gold substrates compared to the same molecule in the liquid state.10 In the 2D films, the amount of gauche conformer is independent of chain length; in addition, the shorter alkanethiolate chains contain less C-S defects on a smooth surface than on a roughened surface, a trend not observed for the longer chain alkanethiolate monolayers.10 In contrast to these results, both recent grazing incidence X-ray diffraction studies of dodecanethiol selfassembled on a Au(111) surface and mechanistic studies on ligand exchange in 2D-SAMs were consistent with dialkyl disulfides (RSSR) as the structural unit on the surface.22 These moieties, in order to avoid unfavorable steric interactions on the surface, require that at least half of the alkanethiolate chains be in the GT conformation. The trends observed for 3D-SAM monolayers on the gold clusters are remarkably similar to those seen by SERS on 2D-SAMs. For example, for all chain lengths the ν(C-S)G band (Figure 3, Table 1) can be detected at ∼640 (22) (a) Fentner, P.; Eberhardt, A.; Eisenberger, P. Science 1994, 266, 1216-1218. (b) Schlenoff, J. B.; Li, M.; Ly, H. J. Am. Chem. Soc. 1995, 117, 12528-12536.
N-Alkanethiolate Monolayers on Gold Cluster Compounds
cm-1. The ν(C-S)T band is well-resolved at 714-716 cm-1 for the C3-C5 alkanethiolates but is partially obscured by the P1 band for the C6-C8 alkanethiolates and completely obscured by the P1 band for the longer chain adsorbates. There are several interesting features associated with these vibrational modes. First, the energy at which they appear is ∼5 cm-1 higher than those seen for the same molecule in a 2D-SAM surface (∼710 and 635 cm-1 for the trans and gauche C-S stretching vibrations, respectively10), suggesting that the gold cluster may not be as electron-withdrawing as bulk gold. Second, the defect density, as determined from the intensity ratios of these two modes, for shorter alkyl chains in 3D-SAMS (C3,C4, ∼30%) is approximately the same as that for similar 2D-SAMs on slightly roughened gold surfaces.10 For the longer alkyl chains, the gauche C-S stretching mode intensity decreases (when normalized to the U mode), giving a defect density of ∼20% for the C5-C12 alkanethiolates and ∼10% for C16-C24. Thus, these longer chains have a defect density similar to those of 2D-SAMs on smooth gold surfaces, a result consistent with the higher crystallinity of the longer chains. However, these intensity ratio data are inconsistent with the binding of dialkyl disulfides to the cluster surface. Third, some of the cluster compounds exhibit additional bands in this region, one at 670 cm-1 for propanethiolate and one at 621 cm-1 for octanethiolate. It is unclear whether these extra bands represent surface defects on the colloid, impurities in the sample, or packing defects. We favor the latter interpretation on the basis of the IR spectra obtained from cluster compounds which have undergone exchange reactions, as will be reported in another paper.15 Finally, we note that molecular dynamics (MD) simulations have predicted that there will be either a ∼1% or ∼99% C-S defect density on 2D-SAM monolayers on gold. While the cluster alkanethiolate monolayers differ somewhat from the MD basis model, the C-S defect density of 10-30% which we observe is most consistent with the low defect density model. CCC Bending Vibration (ω). Our spectroscopic window extends to 400 cm-1, and thus we cannot observe at this time such low-frequency vibrations as the Au-S stretch. It is however possible to detect the CCC skeletal bending vibration between 460 and 475 cm-1, which is seen for all alkanethiolates except propanethiolate.23 In free alkanes, there are two possible bending vibrations involving the CCC skeleton.12c The higher frequency mode (350-535 cm-1, although it is usually observed at the lower energies), termed the chain extension mode, involves C-C stretching vibrations coupled with in-phase CCC bending vibrations and tends to expand or contract the chain. The lower frequency mode (
