In single molecule force spectroscopy experiments, force probes chemically modified with synthetic, single-stranded DNA oligomers produced characteristic steady-state forces connected by abrupt steps between plateaus, as the probes moved away from a graphite substrate. The force plateaus represent peeling of a small number of polymer molecules from the flat surface. The final force jump in the retraction region of the force–distance curves can be attributed to a single DNA molecule detaching from the graphite surface. Previously, Manohar et al. (Nano Lett. 2008, 8, 4365) reported the peeling forces of the pyrimidine oligomers as 85.3 ± 4.7 and 60.8 ± 5.5 pN for polythymine and polycytosine, respectively. We measured the force–distance curves for purine oligomers on a graphite surface and found the peeling forces to be 76.6 ± 3.0 and 66.4 ± 1.4 pN for polyadenine and polyguanine, respectively. Using a refined model for peeling a single freely jointed polymer chain from a frictionless substrate, we determined a ranking of the effective average binding energy per nucleotide for all four bases as T ≥ A > G ≥ C (11.3 ± 0.8, 9.9 ± 0.5, 8.3 ± 0.2, and 7.5 ± 0.8 k B T, respectively). The binding energy determined from the peeling force data did not scale with the size of the base. The distribution of peeling forces of polyguanine from the graphite surface was unusually broad in comparison to the other homopolymers, and often with inconsistent chain extensions, possibly indicating the presence of secondary structures (intra- or intermolecular) for this sequence.

We report the experimental observation of a trapped rainbow in adiabatically graded metallic gratings, designed to validate theoretical predictions for this unique plasmonic structure. Onedimensional graded nanogratings were fabricated and their surface dispersion properties tailored by varying the grating groove depth, whose dimensions were confirmed by atomic force microscopy. Tunable plasmonic bandgaps were observed experimentally, and direct optical measurements on graded grating structures show that light of different wavelengths in the 500-700-nm region is "trapped" at different positions along the grating, consistent with computer simulations, thus verifying the "rainbow" trapping effect.slow light | surface dispersion engineering | surface plasmons S ince Ebbesen et al.'s report on extraordinary optical transmission through plasmonic hole arrays was published in 1998 (1), the study of plasmonics and metamaterials (2) has progressed at a rapid pace and led to the discovery of phenomena with unique optical properties. For example, recent theoretical investigations reported the "trapped rainbow" storage of terahertz waves in metamaterials (3) and plasmonic graded structures (4), and generated considerable interest for slow-light applications. It was predicted that tapered waveguides with a negative refractive index core (3) and graded metallic grating structures (4, 5) were capable of slowing a broadband rainbow to a standstill. By varying the nanotopology of metal surfaces, the optical properties of surface plasmon polaritons (SPPs) can be tailored via so-called surface dispersion engineering (6-8). Moreover, by scaling the feature size of the graded grating structures down to the nanometer scale, it was theoretically predicted that telecommunication waves and even visible waves can also be trapped (9, 10).The intrinsic slow-light properties of SPP modes in 1D metallic grating structures can be seen from their dispersion relations, where their group velocity v g is found to decrease significantly as the photonic band edge is approached. Our recent investigations of simple 1D metallic gratings demonstrated that the surface dispersion properties can be tuned by systematically varying the groove depth and grating period. The dispersion relations for adiabatically graded gratings vary monotonically with position, so that incoming waves at different wavelengths can be trapped or localized at different positions along the propagation direction of the grating.Advances in nanofabrication and characterization techniques now permit the experimental demonstration of this interesting class of structures. Rainbow trapping has not yet been unambiguously demonstrated in the visible regime for either metamaterials (3) or plasmonic structures (4, 5, 9), although in related studies photonic crystal nanocavities with graded hole size were recently shown to exhibit adiabatically reduced group velocities for photonic modes at telecommunication frequencies (11). Two preliminary efforts were recently reported to realize the trap...

This paper describes a noncontact calibration procedure for lateral force microscopy in air and liquids. The procedure is based on the observation that the sensitivity of a force microscope may be calibrated using the raw thermal noise spectrum of the cantilever and its known spring constant, which can be found from the same uncalibrated thermal noise spectrum using Sader's method (Rev. Sci. Instrum.1999, 70, 3967-3969). In addition to the power spectrum of the cantilever thermal noise, this noncontact calibration method only requires knowledge of the plan view dimensions of the cantilever that could be measured using an optical microscope. This method is suitable for in situ force calibration even in viscous fluids through a two-step calibration procedure, where the cantilever thermal spectra are captured both in air and in the desired liquid. The lateral calibration performed with the thermal noise technique agrees well with sensitivity values obtained by the wedge calibration procedure. The approach examined in this paper allows for complete calibration of normal and lateral forces without contacting the surface, eliminating the possibility for any tip damage or contamination during calibration.

We report the experimental observation of a trapped rainbow in adiabatically graded metallic gratings, designed to validate theoretical predictions for this unique plasmonic structure. Onedimensional graded nanogratings were fabricated and their surface dispersion properties tailored by varying the grating groove depth, whose dimensions were confirmed by atomic force microscopy. Tunable plasmonic bandgaps were observed experimentally, and direct optical measurements on graded grating structures show that light of different wavelengths in the 500-700-nm region is "trapped" at different positions along the grating, consistent with computer simulations, thus verifying the "rainbow" trapping effect.slow light | surface dispersion engineering | surface plasmons S ince Ebbesen et al.'s report on extraordinary optical transmission through plasmonic hole arrays was published in 1998 (1), the study of plasmonics and metamaterials (2) has progressed at a rapid pace and led to the discovery of phenomena with unique optical properties. For example, recent theoretical investigations reported the "trapped rainbow" storage of terahertz waves in metamaterials (3) and plasmonic graded structures (4), and generated considerable interest for slow-light applications. It was predicted that tapered waveguides with a negative refractive index core (3) and graded metallic grating structures (4, 5) were capable of slowing a broadband rainbow to a standstill. By varying the nanotopology of metal surfaces, the optical properties of surface plasmon polaritons (SPPs) can be tailored via so-called surface dispersion engineering (6-8). Moreover, by scaling the feature size of the graded grating structures down to the nanometer scale, it was theoretically predicted that telecommunication waves and even visible waves can also be trapped (9, 10).The intrinsic slow-light properties of SPP modes in 1D metallic grating structures can be seen from their dispersion relations, where their group velocity v g is found to decrease significantly as the photonic band edge is approached. Our recent investigations of simple 1D metallic gratings demonstrated that the surface dispersion properties can be tuned by systematically varying the groove depth and grating period. The dispersion relations for adiabatically graded gratings vary monotonically with position, so that incoming waves at different wavelengths can be trapped or localized at different positions along the propagation direction of the grating.Advances in nanofabrication and characterization techniques now permit the experimental demonstration of this interesting class of structures. Rainbow trapping has not yet been unambiguously demonstrated in the visible regime for either metamaterials (3) or plasmonic structures (4, 5, 9), although in related studies photonic crystal nanocavities with graded hole size were recently shown to exhibit adiabatically reduced group velocities for photonic modes at telecommunication frequencies (11). Two preliminary efforts were recently reported to realize the trap...

We used friction force microscopy measurements to determine the yield strength of several structurally similar Langmuir-Blodgett (LB) bilayer films deposited on a hydrophobic substrate. Film failure was initiated by increasing the load applied by the probe of the atomic force microscope in the course of continuous scanning at nominally the same location on the sample. This film failure was readily detected in friction versus load curves, as well as by imaging of trenches created due to removal of the film. The depths of the trenches formed in the course of these yield strength experiments were consistent with complete removal of these bilayer films, as evidenced by comparisons to film thicknesses measured by ellipsometry. The structure of the LB bilayer was modified by replacing a tetra-chain amphiphile bearing four quaternary ammonium groups with a polymeric surfactant resulting in little change in the yield strength. On the other hand, the addition of a polyanionic gluing layer at the central interface of the bilayers almost doubled the yield strength of the films. To uncover any possible structural effects created by changes in the terminal functionality, the hydrocarbon top layer of the bilayer was replaced with a perfluorinated capping layer. In spite of the changes in frictional properties, the yield strength of this film also appeared to be dominated by the presence of the glued interface.