We report optical second harmonic generation studies of the organic dye molecule rhodamine 6G spin cast on fused silica surfaces. The concentration dependence of the second harmonic response demonstrates oscillatory behavior with a period corresponding to the concentration required for monolayer surface coverage. This behavior reflects the formation of ordered molecular adlayers which persist for approximately five periods. Polarized SHG studies confirm orientational anisotropy of the dye molecules and allow the orientation within adjacent layers to be determined. Optical absorbance measurements of the films indicate the onset of rhodamine 6G aggregate formation at surface coverages of approximately one monolayer. However, the onset of dimer or aggregate fluorescence is observed to occur only at much higher surface coverages, consistent with the loss of orientational order within the adlayers. Our results indicate strong adsorbate-substrate interaction which gives rise to orientational anisotropy within the first molecular layer. Well-defined order within subsequent layers is determined by interlayer adsorbate-adsorbate aggregation and decays on a length scale of several molecular diameters. These results provide a direct measure of the extent of interfacial ordering at the solid/air interface.
High quality metal thin films and nanostructures are critical building blocks for next generation nanotechnologies 1-3 . They comprise low-loss circuit elements in nanodevices 4,5 , provide new catalytic pathways for water splitting and CO2 reduction technologies 6,7 , and enable the confinement of spatially extended electromagnetic waves to be harnessed for application in information processing 8 , energy harvesting 9,10 , engineered metamaterials, 11 and new technologies that will operate in the quantum plasmonics limit 12 . However, the controlled fabrication of high-definition single-crystal subwavelength metal nanostructures remains a significant hurdle, due to the tendency for polycrystalline metal growth using conventional physical vapor deposition methods, and the challenges associated with placing solution-grown nanocrystals in desired orientations and locations on a surface to fabricate functional devices. Here, we introduce a new scalable, green, wet chemical approach to monocrystalline noble metals that enables the fabrication of ultrasmooth, epitaxial, single-crystal films of controllable thickness. They are ideal for the subtractive manufacture of nanostructure through ion beam milling, and additive crystalline nanostructure via lithographic patterning to enable large area, single-crystal metamaterials and high aspect ratio nanowires. Our single-crystal nanostructures demonstrate improved feature quality and pattern transfer yield, reduced optical and resistive losses, tailored local fields, and greatly improved stability compared to polycrystalline structures, supporting greater local field enhancements and enabling new practical advances at the nanoscale.X. Yuan is thanked for technical assistance with ellipsometry data (supplementary information).
Vibrational wave packet dynamics from a monolayer-covered surface are reported. These dynamics reflect surface vibrational coherence in a monolayer of amphiphilic molecules deposited at the CaF2/air interface. The induced macroscopic polarization following coherent excitation of adsorbate CH3 vibrational modes displays quantum interference effects (quantum beats) and decays on a time scale dependent on the nature of the interfacial environment. These observations provide a link between the degree of interfacial order and the vibrational coherence lifetime and demonstrate that monitoring interfacial wave packet dynamics represents a new method for characterizing these important chemical regions.
Surface-specific five wave mixing spectroscopy is used to examine adsorbate dynamics at the fused silica/air interface. Signals whose temporal response is significantly broader than the instantaneous fourth-order electronic polarizability are attributed to low frequency, adsorbate nuclear motion. Based on the dependence of these dynamics on the moment of inertia and their comparison with dynamics observed in liquids, the five wave mixing temporal response has been assigned to adsorbate intermolecular librational motion. Demonstration of this new fourth-order technique allows one, in principle, to extend all third-order nonlinear bulk-phase spectroscopies to surfaces and interfaces with specificity.
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