By using an atomic force microscope based on a quartz tuning fork sensor, a 3-dimensional description of the interface between liquid hexadecane and a highly oriented pyrolytic graphite surface can be achieved at room temperature. The C16H34 monolayer in contact with the substrate surface exhibits a lamellar structure whereas no observation at the liquid/graphite interface by scanning tunnelling microscopy was reported for this alkane. The second layer shows very weak corrugations corresponding to lamella boundaries. Force/distance curves show at least four oscillations separated by 0.4 nm except for the first period with a 0.38 nm distance that corresponds to the layer closer the substrate. Such a description agrees well with molecular dynamics results obtained on alkane/solid interfaces.
We describe a method to perform noncontact atomic force microscopy (NC-AFM) in liquid and in air using a quartz crystal tuning fork. The latter, thanks to its increased stability originating from its higher stiffness, compared to cantilevers, enables very low oscillation amplitudes and thus, should yield a very high sensitivity to the force interactions. However, applying conventional NC-AFM technique fails, due to the phase lock loop demodulator becoming unstable as the input signal to noise ratio decreases when lowering the oscillation amplitude. Therefore we have developed so-called zero phase mode atomic force microscopy based on phase modulation. Comparisons with conventional frequency modulation mode using the same mechanical setup demonstrate gains in sensitivity and in scanning rates. High resolution topographic images, as well as simultaneous dissipation images can be recorded using this very convenient and effective technique.
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