Monolayers in Three Dimensions: NMR, SAXS, Thermal, and Electron Hopping Studies of Alkanethiol Stabilized Gold Clusters

Gold clusters stabilized by chemisorbed monolayers of octane-, dodecaneor hexadecanethiolate have been investigated in solution and in the solid phase. These materials can be pumped free of solvent to form a dark brown solid that can be re-dissolved in nonpolar solvents. Their exceptional stability suggests they be viewed as cluster compounds. The self-assembled alkanethiolate monolayers stabilizing the metal clusters can be investigated using techniques that are insufficiently sensitive for study of a monolayer on a flat surface, e.g., 'H and I3C NMR, elemental analysis, differential scanning calorimetry (DSC), thermogravimetry (TGA), and diffusion-ordered NMR spectroscopy (DOSY). Results from such measurements, combined with small-angle X-ray scattering (SAXS) data on solutions of the clusters and images from scanning tunneling (STM), and atomic force microscopy (AFM), are consistent with a small, monodisperse (12 8, radius) gold core, which modeled as a sphere contains -400 Au atoms and -126 alkanethiolate chains, or if modeled as a cuboctahedral structure contains 309 Au atoms and -95 alkanethiolate chains. High-resolution NMR spectra of cluster solutions display well-defined resonances except for methylenes nearest the gold interface; the absence of the latter resonances is attributed to a combination of broadening mechanisms based on the discontinuous change in magnetic susceptibility at the metal-hydrocarbon interface and residual dipolar interactions. Films of the dry, solid cluster compound on interdigitated array electrodes exhibit current-potential responses characteristic of electron hopping conductivity in which electrons tunnel from Au core to Au core. The electron hopping rate decreases and the activation barrier increases systematically at longer alkane chain length. The results are consistent with electron transport rate control being a combination of thermally activated electron transfer to create oppositely charged Au cores (cermet theory) and distance-dependent tunneling (8 = 1.2 A-1) through the oriented alkanethiolate layers separating them. The self-organization of alkanethiols chemisorbed on planar Au surfaces has generated intense interest in understanding its chemical basis.' The two-dimensional character of self-assembled monolayers places limits, however, on the experiments by which they can be probed when they are chemisorbed on planar Au surfaces, e.g.,I wetting, helium diffraction, scanning probe, interfacial electron transfers, FTIR spectroscopy, etc. An *Address correspondence to this author at the University of North +Department of Chemistry, University of Illinois, Urbana, IL. * W. R. Grace and Co, Washington Research Center, 7379 Route 32, 3 Department of Chemistry, National Sun Yat-Sen University, Kaohsiung, Department of Chemistry, University of Oregon, Eugene, OR97403II Oak Ridge National Laboratories. # Department of Physics and Astronomy. University of North Carolina. @ Abstract published in Advance ACS Abstracts, December 1, 1995. (1) (a) DuBois L. H.; Nuzzo, R. G. Annu. Rev. Chem. Phys. 1992, 43, 437. (b) Laibinis, P. E.; Whitesides, G. M.; Allara D. L.; Tao, Y.-T.; Parikh, A. N.; Nuzzo, R. G. J. Am. Chem. Soc. 1991,133, 7152. (c) Chidsey, C. E. D.; Liu, G.; Scoles, G.; Wang, J. Langmuir, 1990, 6, 1804. (d) Fenter, P.; Eisenberger, P.; Liang, K. S. Phys. Rev. Let?. 1993, 70, 2447. (e) Chidsey, C. E. D. Science 1991,251,919. (0 Chailapakul, 0.; Sun, L.; Xu C.; Crooks, R. M. J. Am. Chem. SOC. 1993, 115, 12439. (g) Nemetz, A; Fischer T.; Ulman, A,; Knoll, W. J. Chem. Phys. 1993, 987. (h) Schneider, T. W.; Buttry, D. A. J. Am. Chem. SOC. 1993,115, 12391. (i) Widrig, C. A,; Alves, C. A.; Porter, M. D. J. Am. Chem. Soc. 1991, 123, 2805. (j) Kim, Y.-T.; McCarley, R. L.; Bard A. J. J. Phys. Chem. 1992, 96, 7416. Carolina. Columbia, MD 2 1044-4098.