Letter pubs.acs.org/Langmuir
Evolution of Conformational Order During Self-Assembly of n‑Alkanethiols on Hg Droplets: An Infrared Spectromicroscopy Study Christopher B. Babayco,†,⊥ Pauline J. Chang,‡ Donald P. Land,† Richard A. Kiehl,‡ and Atul N. Parikh*,§,∥ †
Department of Chemistry, ‡Department of Electrical & Computer Engineering, §Department of Biomedical Engineering, and Department of Chemical Engineering & Materials Science, University of California, Davis, California 95616, United States
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S Supporting Information *
ABSTRACT: This Letter describes Fourier-transform infrared spectroscopy evidence for the evolution of conformational order and coverage during the formation of n-alkanethiol monolayers on microdroplets of mercury from the solution phase. At the highest coverages obtained by self-assembly, the monolayer is characterized by predominantly all-trans conformational order. For partial monolayers obtained at arbitrarily quenched incubation periods, we find a continuous evolution of the chain conformational order with monolayer coverage. Analyzing these results in light of previously reported models from X-ray scattering reveals a complex self-assembly process in which the density-dependent evolution of the chain conformational order is coupled with that of molecular orientation and density.
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it is now well documented9 that n-alkanethiols (CH3(CH2)nSH; n > 8) organize onto Au[111] to form a 31/2 × 31/2 R30° epitaxially commensurate lattice (and may exhibit c(4 × 2) superlattice modulations) with the underlying crystallographic texture of Au atoms. The average molecule in this SAM occupies an area of 18.7 Å2 in the plane perpendicular to the molecular axis, which is tilted ∼30° from the surface normal in a direction ∼8−10° away from that of the next-nearest neighbor.8 This well-ordered SAM can then serve as an inert organic phase within which useful conducting thiols can be embedded either through simultaneous coadsorption or sequential insertion,11 and thus MmM junctions can be constructed by simply placing bare Hg droplets in contact with the thiol-covered Au surface. However, imperfections in the Au lattice (e.g., pinholes and step edges and kinks between adjoining Au terraces) and kinetic entrapment of defects and grain boundaries during the self-assembly process often produce defects in the packing of alkanethiols.12 The presence of these defects in turn can potentially produce direct metal− metal contacts (leading to conditions for amalgamation of Hg with the counter metal and producing an electrical short), impeding the characterization of the electrical properties of the MmM junctions. A general strategy proposed to circumvent this issue is to build organic bilayers between the two juxtaposed metals.7 Of particular usefulness is the bilayer motif in which two independent monolayers are assembled one onto each of two (e.g., Au and Hg) contacting metallic surfaces. Indeed, previous work by Slowinski and co-workers
INTRODUCTION The ability to sandwich a defect-free organic layer between two metals is central to the design of experimental test beds for the measurements of the electron transport characteristics of metal−molecule−metal (MmM) junctions1−7 relevant to the area of molecule-based hybrids of (bio)organic/inorganic electronic devices,5 including functional devices for information processing.6 In this regard, the use of liquid mercury as one of the two contacting metals is attractive for a variety of reasons.1 First, liquid mercury can be morphed into discrete droplets, which can be brought into MmM contact with (and withdrawn from) desired regions of an organically derivatized metal, allowing for the assembly and disassembly of an MmM junction in a spatially defined manner. This capability allows for the electrical probing of many junctions of the same structure, which is critical for the characterization of structural integrity and reproducibility. Second, the Hg surface is devoid of structural features (e.g., edges, steps, terraces, and pits), which often produce undesirable defects in the structures of the adsorbed organic layer. Third, being liquid, Hg conforms to the topography of the underlying solid surface and allows for the rapid formation of conformal contacts in real time. Together these properties render Hg droplets a very suitable probe for the characterization of both spatially resolved and dynamically assembled molecular conduction pathways in functionalized MmM junctions.1,7 A particularly useful class of materials for the second contacting metal is that of coinage metals (e.g., Au and Ag). This is because a versatile class of thiol-terminated organic molecules (RSH) and their mixtures spontaneously selfassemble or coadsorb onto coinage metals, forming highly ordered self-assembled monolayers (SAMs).8−10 For instance, © XXXX American Chemical Society
Received: April 18, 2013 Revised: June 9, 2013
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monolayer phases comprising molecules oriented parallel to the surface normal through a coexistence region consisting of both lying-down (surface-parallel) and standing-up (surfacenormal) molecules. However, the characterization of the evolution of the conformational properties of the alkanethiol monolayers on Hg during this process is not yet available, which is the main subject of the work reported herein. Specifically, using Hg droplets, we report Fourier transform infrared (FTIR) vibrational spectroscopy-based evidence of the chain molecular conformation at variable monolayer coverages. We find that the densest structures of alkanethiol monolayers formed on liquid Hg consists of the average molecular chain in the predominantly all-trans conformation. The characterization of the evolution of chain conformational order during the selfassembly process reveals a gradual and continuous rise in the chain conformational order (and a reduction in the concentration of gauche conformers) with the surface coverage.
clearly indicates the benefits of employing both liquid Hg as a substrate and two opposing monolayers in the construction of MmM junctions.4,13 From the vantage point of molecular self-assembly, the organization of alkanethiols on Hg14−17 presents an interesting case. The binding of S to Hg is strong (∼200 kJ mol−1) and comparable to the S−Au binding energy (∼418 kJ mol−1). However, the liquid surface of Hg, unlike Au, lacks static structural order and defects. Thus, the corrugation potential for the adsorption of alkanethiols on Hg can be best approximated by a smooth isotropic potential field (featureless surface) devoid of the epitaxial constraints responsible for the crystalline-like SAM structures on Au. A cartoon representation of these two limiting cases is provided in Figure 1. Indeed, the
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RESULTS AND DISCUSSION Representative grazing incidence FTIR reflection absorption spectra in the 2050−2250 cm−1 frequency region as a function of the incubation period for arbitrarily selected incubation times are shown in Figure 2. Each spectrum is characterized by the Figure 1. Schematic depicting differences in the substrate−adsorbate potential energy surface for the adsorption of n-alkanethiols on Au (top) and Hg (bottom). On Au, differences in Au−S binding energies between crystallographically differentiated sites and the energy barrier for lateral hopping render the potential energy surface corrugated. The Hg surface, by contrast, offers little or no barrier to lateral diffusion, and the potential energy surface is featureless.
most recent grazing incidence X-ray diffraction (GIXRD) results14,15 suggest that on a Hg surface Langmuir monolayers of alkanethiols in the densest molecular areas (∼19 Å2/ molecule, obtained by cooling the as-prepared samples to ∼10 °C) adopt a crystalline-like hexagonal packing consisting of untilted molecules,16 which remains stable at room temperature. At slightly lower molecular areas of about 20.35−20.66 Å2, typically obtained by solution self-assembly (rather than Langmuir compression), the monolayer exhibits a rectangular packing consisting of molecules tilted at ∼27° in the nearestneighbor direction.16 This expanded lattice (or reduced packing density) of alkanethiol molecules within the solution-phase assembled monolayers on Hg has been proposed to produce the “rotator” phase in which pseudohexagonally packed alkanechain-bearing molecules exhibit free rotation about their long axis.14,16 In either case, the densest Langmuir phase consisting of hexagonally packed untilted molecules or solution-phase assembled monolayers in rectangularly packed 27° tilted molecules, the apposition of the two alkanethiol monolayers, one on Hg and the other on the coinage metal, in an MmM junction should provide an efficient organic phase within which molecular conduction pathways can be engineered.4,18 Beyond structural considerations of densely packed monolayers, the presence of intramolecular defects also raises the question of how conformational order evolves during the selfassembly process. Previous X-ray scattering studies16 suggest that the self-assembly of alkanethiols on the free surface of Hg involves a gradual orientational transition; with increasing surface coverage, alkanethiol molecules reorient from a laterally disordered phase of surface-parallel molecules to ordered
Figure 2. Time-dependent Fourier transform infrared spectra in the C−D stretching-mode region (2050−2250 cm−1) of deuterated ndodecanethiol (CD3(CD2)11SH) monolayers on liquid mercury (Hg) droplets. The schematic depicts the experimental configuration in which an FTIR microscope fitted with a grazing incidence angle objective (GIO) is positioned above the Hg droplets. The XY translational stage allows for sequential measurements of multiple droplets.
presence of a number of overlapping peaks with distinguishable maxima at 2080, 2190, and 2215 cm−1. Previous studies of the vibrational spectra of compounds containing specifically deuterated aliphatic chains provide mode assignments.19−23 The lower-frequency strong band near 2100 cm−1 (2090−2120 cm−1) and the higher-frequency band near 2200 cm−1 (2180− 2205 cm−1) can be assigned to the methylene C−D symmetric (νsym) and methylene antisymmetric (νantisym) stretches, B
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respectively,19 whereas the weaker peak at 2215 cm−1 is due to methyl C−D asymmetric stretching mode absorptions.20 Additionally, overlapping weaker and broader peaks at 2160 and 2060 cm−1 are also discernible, which can be attributed to the symmetric stretching fundamental (νsym) in Fermi resonance with the first overtone of the CD2 or CD3 bending mode.19,22 There are several noteworthy features in these data, as will be discussed below. The conformational properties of the aliphatic tails of the monolayers obtained at the saturation coverage (>60 min) are characterized by the positions of νsym and νantisym peaks. It is well appreciated in the literature20,21 that the precise positions of CD2 symmetric (νsym) and CD2 antisymmetric (νantisym) stretching bands are strong indicators of the intramolecular chain conformation: the peak frequencies of these vibrational modes are typically reported to lie in the narrow ranges of 2084−2087 and 2189−2194 cm−1 for all-trans extended chains and in the distinctly different ranges of 2095−2097 and 2196− 2198 cm−1 for conformationally disordered chains, latter characterized by a significant presence of gauche conformers. The positions of these peaks are also influenced, albeit to a much smaller degree, by intermolecular interactions controlled by the packing of alkane tails (∼1−2 cm−1 for CH2 and 60 min incubation period), of 2084.7 and 2189.5 cm−1 can be interpreted to indicate that the alkyl chains in Hg-supported SAMs adopt a predominantly all-trans conformational state with a small (if any) concentration of gauche conformers. It is interesting to recall that perfectly aligned chains adopting a perpendicular orientation would in principle produce no signal at high angles of incidence because of the surface selection rule.24,25 The fact that nonvanishing vibrational signals are observed in our experiments may reflect the effects of the curved geometry of Hg droplets, which renders locally vertically oriented chains relative to the substrate at a nonperpendicular angle with respect to the impinging light. They may also suggest that the coverages we obtain by self-assembly correspond to the previously reported rectangular packing corresponding to molecular areas of 20.35−20.66 Å2 with an off-normal orientation (27° molecular tilt) of the monolayer chains.15,16 The latter study also predicts the chain packing to adopt the “rotator” phase characterized by the rapid rotation of the carbon−carbon chain around the chain axis26 resulting in each chain occupying a cone around the Hg− S bond. Our results indicating the preponderance of dominantly all-trans chains with a small concentration of gauche conformers are qualitatively consistent with such a packing motif. Note however that we cannot differentiate the rotator phase from the crystalline phase using the stretching vibrational data alone.26 Next, we analyze the spectral changes that report on the kinetics of self-assembly of n-alkanethiols by considering spectral properties for intermediate incubation periods (
