Coaxial atomic force microscope probes for imaging with dielectrophoresis

Abstract

We demonstrate atomic force microscope ͑AFM͒ imaging using dielectrophoresis ͑DEP͒ with coaxial probes. DEP provides force contrast allowing coaxial probes to image with enhanced spatial resolution. We model a coaxial probe as an electric dipole to provide analytic formulas for DEP between a dipole, dielectric spheres, and a dielectric substrate. AFM images taken of dielectric spheres with and without an applied electric field show the disappearance of artifacts when imaging with DEP. Quantitative agreement between our model and experiment shows that we are imaging with DEP. © 2011 American Institute of Physics. ͓doi:10.1063/1.3585670͔The atomic force microscope ͑AFM͒ has proved to be a useful tool to study dielectric materials at the nanoscale. 1,2 Many AFM imaging modes have emerged to study dielectric materials. Scanning polarization force microscopy has been used to study the dielectric properties of adsorbed liquid films. 3 Electrostatic force microscopy has been used to quantitatively measure the dielectric constant 4,5 and dielectric spectrum 6 of thin films. Techniques such as scanning near field microwave microscopy, 7 scanning nonlinear dielectric microscopy, 8,9 and scanning capacitance microscopy 10 directly observe electrical signals that vary with local dielectric properties. Coaxial AFM probes have previously been used for microassembly 11 and near field optical and microwave microscopy. 12 In this paper, we demonstrate dielectrophoretic imaging with a coaxial AFM probe. Imaging with dielectrophoresis ͑DEP͒ can provide topographical information as well as information about the local dielectric constant and spectrum. We begin by modeling a coaxial probe as an electric dipole to find an analytic formula for the dielectrophoretic force due to a dielectric sphere and dielectric substrate. The simulated electric field of a coaxial AFM probe is found to be well approximated by a dipolar electric field. We topographically image microspheres with a coaxial probe and find many artifacts due to the similar size of the tip and the microspheres. Imaging the same spheres under the same imaging conditions with an applied electric field improves the imaging quality and removes artifacts from tip shape. A dipole model of dielectrophoretic imaging with the coaxial tip provides an accurate fit to topographical scans of microspheres showing that imaging with DEP allows for greatly improved noncontact imaging.We model a coaxial probe as an electric dipole to find the dielectrophoretic force between a coaxial probe and dielectric spheres. Consider a dipole p ជ in vacuum oriented in the z-direction, as shown in the inset of Fig. 1. A nearby dielectric sphere of permittivity P and radius a will become polarized and experience a dielectrophoretic force, 13where 0 is the permittivity of free space and E is the electric field created by the dipole. In a vacuum, the sphere is pulled toward electric field maxima; this is positive DEP. The z-component of the force on the dipole due to the sphere is found by substituting the e...

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