Torsionally Stabilized Nano Impedance Microscopy
Figure 4: The effect of laser optical illumination on the resistance (a) and capacitance (b) of monolayer ZnPP maquettes illustrated as 2D and 3D histograms of the properties. The top 3D histograms in the insets are the properties with illumination, the bottom without illumination. Red curves in the 2D histograms are with illumination; black curves are without exposure. l = 425 nm. Kathan-Galipeau et al., ACS Nano 4835, copyright 2011, with permission of ACS.
significance. In this case the reduction of resistance and increase in capacitance in the presence of the laser illumination is clear. For these proteins the electronic transport mechanisms are complex, but the capacitance differences can be understood in terms of the polarization in the porphyrin complexes within the protein. Te dielectric constant determined from capacitance can be related to the polarization volume in the molecule. When the light is absorbed, the excited electron resides in a more delocalized orbital. Te difference in the delocalization and the degree of coupling between the porphyrins can be compared from these measurements.
Summary Te examples presented illustrate how TR-NIM can be
used to determine properties of hard and soſt materials. It is an advance over earlier probe-based impedance imaging in that it can characterize soſt materials such as organic monolayers and engineered proteins. Te approach opens a pathway to bringing frequency and time-dependent probes to bear on questions of the behavior of complex molecules, such as proteins. Accessing the real and imaginary components of properties adds a new dimension of information at the local scale that will be valuable in the development of hybrid devices.
Acknowledgments Tis research was supported by the Nano/Bio Interface
Center through the National Science Foundation NSEC DMR08-32802
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