SAM Structure and Dynamics
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| Figure 1 |
Self-assembled monolayers (SAMs) have garnered significant interest recently owing to their broad spectrum of potential scientific and technological applications. From a scientific standpoint, such monolayers can serve as tunable models for understanding soft materials, such as biological interfaces. Many more applied functions have also been suggested including corrosion resistance, tribology, and nanotechnology, i.e. chemical sensors and molecular electronics. The most heavily studied class of SAMs is alkanethiols and, among these, 1-decanethiol (C10) adsorbed on gold is the archetype.
The majority of these studies have focused on structural characteristics utilizing a large ensemble of real- and reciprocal-space surface probe techniques. Taken together, these experiments have mapped out the distinctive phase diagram for ultrathin alkanethiol films. For the purposes of this study, the salient result from these experiments is that there are two principal thermodynamically stable ordered structures for C10/Au(111): a lying-down, low-density "striped" phase with (11.5´Ö3)R30º symmetry (Figure 2 inset) and a high-density "standing" phase with c(4Ö3´2Ö3)R30º symmetry. Understanding the internal and external vibrational characteristics of SAMs is a direct route to dissecting the interplay of forces involved in the process of self assembly. Development of more sophisticated and functional self-organizing structures will only be possible once these subtleties are understood.
| Figure 2 |
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Vollmer et al. have searched extensively for collective vibrations in several shorter chain-length alkanethiols adsorbed on two copper surfaces [1]. Using helium atom scattering (HAS), they observed an FTz mode (similar to Figure 4 movie) in the disordered, physisorbed monolayers and multilayers but saw no quantized inelastic features for the dense ordered phase of either thiol chemisorbed on either surface. The absence of such soft modes was attributed to two characteristics: the short alkane chain length (C2H5SH and C7H15SH, hereafter C2 and C7, respectively) and the relatively strong sulfur-copper interaction. By studying C10/Au(111), both of these concerns can be addressed.
| Figure 3 |
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To prepare a highly-ordered SAM, one must first prepare a highly-ordered substrate. The Au(111) crystal used in these studies was cleaned by repeated cycles of sputtering and annealing until contaminant levels were below our Auger detection limit and helium reflectivity was maximized. Surface crystallinity was confirmed by high quality helium diffraction from the (23´Ö3) "herringbone" reconstruction showing an unusually robust full five Bragg orders of diffraction with a concomitant low level of diffuse background (Figure 1). The average domain size, extracted from the FWHM of the specular diffraction peak with the instrument function deconvoluted, is at least 400 Å. A characteristic diffraction scan of the well-known, chemisorbed, low-density (11.5´Ö3) striped phase of C10/Au(111) is shown in Figure 2. An average domain size of at least 400 Å is inferred from the width of the diffraction peaks as there is no broadening added to the substrate specular peak width.
After verifying the quality of the SAM, TOF spectra were obtained at incident beam energies of 21.5 meV, 32.3 meV, and 43.1 meV to ascertain the optimal condition for phonon spectroscopy of this system. Representative spectra are displayed in Figure 3. At the lowest incident energy studied, approximately the same as that which resulted in an FTz mode for C2 and C7 on Cu, inelastic features were barely perceptible above the background (Figure 3a). Energy-transfer peaks emerged when the incident energy was increased to 43.1 meV but the multiphonon background intensity was significant (Figure 3c). With the goal of measuring discrete low-energy vibrations in mind, Ei = 32.3 meV was selected as a compromise because a clear inelastic peak was observed and the multiphonon intensity was moderate (Figure 3b).
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| Figure 4 (click for movie [~300 kb]) |
Dispersion measurements performed at this beam energy reveal an Einstein mode at DE = ± 8 meV (Figure 3, lower panel). We attribute this feature to the external vibration of the entire molecule with polarization perpendicular to the surface (Figure 4). Moreover, internal modes of the hydrocarbon chain are expected to be somewhat lower in energy.
In summary, the dynamics of the well-ordered (11.5´Ö3) striped phase of
decanethiol chemisorbed on Au(111) were studied using high-resolution helium atom scattering. In
contrast to ethanethiol and heptanethiol chemisorbed on copper, a dispersionless low-energy vibrational
mode was observed at 8 meV and was attributed to the frustrated external translation perpendicular to
the surface. Further investigations involving the standing phase for this system will assist in
identifying the source of this discrepancy. Together, these studies will aid in the understanding of
the forces that control the organization of self-assembled structures and contribute to the development
of novel nanoscale materials.
[1] S. Vollmer, P. Fouquet, G. Witte, C. Boas, M. Kunat, U. Burghaus, Ch. Wöll Surf. Sci. 462 135 (2000).
88. "Surface vibrations of a highly-ordered low-density alkanethiol monolayer measured using helium atom scattering"
