Enhanced infrared absorption spectra of self-assembled alkanethiol monolayers using the extraordinary infrared transmission of metallic arrays of subwavelength apertures

Kenneth R. Rodriguez, Summit Shah, Shaun M. Williams, Shannon Teeters-Kennedy, James V. Coe
2004 Journal of Chemical Physics  
The surface-plasmon-mediated, extraordinary transmission of metallic arrays of subwavelength apertures has been used as the light source for absorption studies of self-assembled monolayers on metal. Enhanced infrared absorption spectra of a sequence of alkanethiol self-assembled monolayers on copper were recorded for carbon chain lengths varying from 8 to 18 atoms. Transition positions and intensities are presented over a large range of the infrared region. The connection between the
more » ... ween the vibrational modes of the CH 2 wagging progression and the infinite methylene chain is explored using a traditional coupled oscillator approach and a new cluster perspective. The push toward nanoscale assembly fosters a need for simple methods to characterize surfaces. The extraordinary transmission of biperiodic metal meshes 1 has been explored in the infrared ͑IR͒ region 2,3 in order to overlap the surface plasmon ͑SP͒ mediated phenomena with the fundamental range of molecular vibrations. Enhancements of more than 100-fold in IR absorption by monolayer coatings on the metallic mesh have been observed by analyzing changes in the IR transmission. 2-4 The phrase "extraordinary transmission" refers to the observation that more light is transmitted than is incident upon the apertures of these optically thick meshes, i.e., light that initially strikes optically thick metal is also being transmitted. This behavior is attributed to the excitation of SPs ͑excitations of the metal's conducting electrons at the surface͒ due to the coupling of photons with the subwavelength pattern of perforations. As SPs propagate along the surface, they can tunnel through the apertures and be emitted on the other side of the mesh as photons without being scattered from the incident beam. Since SPs are polaritons ͑mixed photon-charge oscillation states͒, it is interesting to consider their interactions with molecules on the surface of the supporting metal as probed by IR absorption. These experiments start with the finest available commercial mesh from Precision Eforming ͑Courtland, NY; formerly with Buckbee-Mears Inc.͒ which is currently only available in Ni. The mesh has a square lattice of square holes ͑12.7 m hole-to-hole spacing, 8.0 m hole width tapering to ϳ6.5 m on the opposite side, and 5.0 m thickness͒. The Ni mesh is electrochemically coated with copper 3,5 in order to narrow the transmission resonances ͑enhancing SP lifetimes͒ and to enable a stable coating of alkanethiol ͑these coatings are not stable on nickel oxide 6 ͒. This work was accomplished with apertures having widths in the range of 3-6 m after Cu-coating. The zero-order IR transmission spectrum of a copper coated mesh with ϳ5.5 m hole width is shown with the dotted trace in Fig. 1 . Inset is an SEM image of a similarly coated mesh showing the regular square pattern of the mesh. The percentage of open area is 19% and yet the resonance labeled 1,0 shows transmission of 48%, i.e., Ebbesen's extraodinary transmission effect is exhibited. There are other resonances at higher energies than the 1,0 resonance ͑such as the one labeled 1,1͒ that are broader and extensively overlapped leaving a fairly constant transmission of ϳ5% in the C-H stretching region. The Cu-coated mesh was dipped 7-10 overnight in 20 mM alkanethiol ͑hydrocarbon chains terminated on one end with an SH group͒ in toluene solution and allowed to dry and crystallize for a day. The transmission spectra with an octanethiol coating is given with a solid trace in Fig. 1 ͑1000 scans, 1 cm Ϫ1 resolution, DTGS detector, 6 hr scan time in a Perkin Elmer Spectrum GX at perpendicular incidence and lattice aligned with spectrometer's polarization͒. The alkanethiolate self-assembled monolayer causes a shift in the 1,0 transmission resonance in Fig. 1 which is the wavelength-scanned equivalent of ATR experiments. 11 More importantly, molecular absorptions of the monolayer coating are evident and circled in Fig. 1 . In order to convert these signals to an absorption spectrum, points were chosen along the solid trace, excluding the absorption features, and a spline fit was performed to provide a smooth, effective background. Absorption spectra obtained from -log͑sample/spline background͒ are presented in Fig. 2 for a sequence of alkanethiols (C n H 2nϩ1 SH) with carbon chain lengths of n c ϭ8, 12, 14, 15, 16, and 18 carbons. There is a great deal of IR work on alkanethiol selfassembled monolayers ͑SAMs͒ to guide the interpretation of these spectra 8,12-14 with most work in the C-H stretch region. In our spectra, the absorptions are intense and the peaks are narrow exhibiting full-widths-at-half-maximum of ϳ4 cm Ϫ1 . Octadecanethiol SAMs on Cu have been assayed
doi:10.1063/1.1814052 pmid:15527329 fatcat:if3pwvnxwjeqflbompgmu7to5i