We describe a highly sensitive new type of calorimeter based on the deflection of a ‘‘bimetallic’’ micromechanical sensor as a function of temperature. The temperature changes can be due to ambient changes, giving a temperature sensor or, more importantly, due to the heat absorbed by a coating on the sensor, giving a heat sensor. As an example we show the results of using the sensor as a photothermal spectrometer. The small dimensions and low thermal mass of the sensor make it highly sensitive and we demonstrate a sensitivity of roughly 100 pW. By applying a simple model of the system the ultimate sensitivity is expected to be of the order of 10 pW. The thermal response time of the cantilever can also be determined, giving an estimate of the minimum detectable energy of the sensor. This we find to be 150 fJ and again from our model, expect a minimum value of the order of 20 fJ.
Using frequency-modulation atomic force microscopy ͑FM-AFM͒ at sub-nanometer vibration amplitudes, we find in the system n-dodecanol/graphite that solvation layers may extend for several nanometers into the bulk liquid. These layers maintain crystalline order which can be imaged using FM-AFM. The energy dissipation of the vibrating tip can peak sharply upon penetration of molecular layers. The tip shape appears critical for this effect.A notable property of liquid near the liquid-solid interface is the presence of solvation layers, i.e., ordering of liquid molecules due to boundary conditions imposed by the solid. Such layering has been observed via oscillatory solvation forces when confining the liquid between the substrate and a solid probe using the surface force apparatus 1 and atomic force microscopy ͑AFM͒. 2,3 While solvation forces are routinely detected via the modulation of the force, the lateral structure of the solvation shells-beyond the first adsorbed monolayer-has rarely been directly observed. One exception are long-chain alkanes adsorbed from solution, for which a second layer has been imaged using scanning tunneling microscopy ͑STM͒. 4 Unfortunately, STM does not lend itself to imaging of "higher" layers as the tunneling current decreases exponentially with the thickness of the confined ͑nonconductive͒ liquid. The lamellar structure of a second solvation layer of hexadecane was imaged recently using a tuning fork AFM. 5 There remain open questions about the structure of higher solvation layers, namely, whether they exhibit lateral order. These questions will become increasingly important for high-resolution AFM imaging in liquid, in particular, for hydration layers in biological systems. 6,7 The development of instrumentation capable of smallamplitude, frequency-modulated AFM ͑FM-AFM͒ ͑Ref. 8͒ in liquid environments 9 has dramatically increased the resolution and sensitivity achievable in AFM studies in liquid. We apply FM-AFM to a linear alcohol ͑dodecanol͒ on an atomically flat graphite substrate slightly above the bulk freezing temperature. We find not only spectroscopic evidence of multiple solvation layers, i.e., force oscillations but also obtain real-space topography images of the alcohol molecules in higher layers, demonstrating that the solvation layers in this system have a crystalline structure. Further we sometimes observe, depending on the condition of the tip, sharp peaks in the mechanical dissipation just as a solvation layer is squeezed out of the tip-sample gap.To achieve the sensitivity necessary for molecular resolution AFM in liquid, a commercial AFM ͑Molecular Imaging Picoscope͒ was modified. Changes to the optical beam deflection sensor include replacement of the standard laser source with a home-built, rf-modulated diode laser 9 and modification of the focusing optics to achieve a smaller numerical aperture. Using standard silicon cantilevers ͑type NCLR, Nanosensors, Neuchatel, Switzerland and type ACLA, AppNano, Santa Clara, CA͒ with a spring constant of k c Ϸ 40 N / m, a ...
We have performed simultaneous force and conductivity measurement of hexadecane liquid confined between a conducting atomic force microscope tip and a graphite surface. Both the current and the force data reveal discrete solvation layering of the hexadecane near the surface. We typically observe that the current does not vary with load in a simple way as the layer closest to the surface is compressed, but increases markedly prior to the expulsion of material from the tip-sample gap. We infer that even for a nanoscale asperity there is conformation change of the confined hexadecane under the tip apex prior to squeeze out of the molecules.
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