Mapping Dielectric Properties with Torsionally Stabilized Nano Impedance Microscopy: Hard Materials to Biomolecules
Kendra Kathan-Galipeau, Xi Chen, Bohdana Discher, and Dawn A. Bonnell* Nano/Bio Interface Center, Te University of Pennsylvania, Philadelphia, PA 19104
*
bonnell@lrsm.upenn.edu
Introduction During the last twenty-five years of scanning tunneling
microscopy (STM), atomic resolution has become routine in studies of a wide range of materials. While less routine, atomic force microscopy (AFM) also can achieve atomic resolution, even on insulating surfaces. Tese two microscopy approaches have significantly advanced our understanding of the surface physics and chemistry of many important phenomena. A significant contribution of scanning probe microscopy is that it can characterize local properties as well as structure. Surprising levels of spatial resolution have been achieved in scanning probes that can access continuum properties such as resistance, capacitance, etc. [1, 2]. Recently there has been a focus on extending AFM to
examine more complex properties, such as dielectric function and electromechanical coupling, while pushing the limits of spatial resolution of properties ever further. In this paper we describe a new scanning probe microscopy method that accesses local impedance, leading to information about dielectric function and polarization that can be applied to both hard and soſt materials. Combining this with local laser excitation yields quantitative information about excited states and photo conduction.
Scanning Probes and Impedance Most force-based scanning microscopes that probe
electronic properties employ a conducting tip and the application of an electrical signal to the tip-surface junction. In some cases this is done with the probe tip in contact with the surface, yielding, for example, resistance, capacitance, and piezo-force microscopies. In other cases, the tip is above the surface leading to potentiometry and Kelvin probe microscopy. Introducing frequency variation into these local electrical measurements allows the access of increased information about the sample, in the present case dielectric function. Scanning Impedance Microscopy (SIM) can be done in non-contact mode, where the voltage modulation is across the sample of interest [3]. Alternatively, it can be done in contact or near contact mode, which is analogous to macroscopic impedance spectroscopy and is referred to as nano impedance microscopy/ spectroscopy (NIM/NIS) [4, 5]. Impedance spectroscopy has long been recognized as
one of the major techniques for the characterization of AC transport in materials. Impedance is defined as the ratio of the AC voltage excitation to a current response: Z(ω) = V(ω)/I(ω). Impedance, Z, is usually expressed in complex form in terms of its modulus, |Z|, and phase, θ, as Z = |Z| eiθ. Capacitance, resistance, and associated frequency dependence of the impedance depend on the material properties. Typical
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applications of impedance spectroscopy are to differentiate grain boundary, grain interior, and electrode impedances by fitting the data to corresponding equivalent circuit models [6]. However, the general limitation of all macroscopic techniques is that only average transport properties are determined. Te first SPM techniques for accessing AC transport behavior were SIM [3, 4, 6–11] and NIM [4, 7]. Strategies to enhance the sensitivity of these measurements have been suggested [4, 12–19]. Tese approaches are facilitating examinations of a range
of material systems and devices. A limitation, however, is that they cannot be applied to soſt surfaces, such as organic or biomolecular monolayers. Te challenges are to control the tip-surface force so as not to damage the surface, to detect very small signals, and to eliminate the contributions from stray capacitances. Tis is accomplished in torsionally stabilized nano impedance microscopy (TR-NIM), where we introduce sample-tip laser excitation to increase the property information content.
Torsionally Stabilized Nano Impedance Microscopy
Torsional resonance is employed to control the sample-tip
force interaction and topographic imaging, enabling near-field contact with the lowest possible force (10 nN). In torsional mode, the probe is oscillated along the cantilever’s long axis, creating a rotational oscillation. An impedance measurement system is designed to: operate at very low currents typical for SAMs (<1 pA), operate over a significant frequency range (kHz to 100 kHz) to increase signal-to-noise ratios, and compensate for system stray capacitances. Figure 1 illustrates a schematic diagram of the TR-NIM
configuration. Te tip is maintained in proximity of the sample surface by the microscope controller. Te electrical impedance of tip-surface junction is measured using external circuitry consisting of a current amplifier, signal generator, and lock-in amplifier. Tunneling currents across biological molecules are quite small; to measure this current, a high-speed, high-gain current amplifier is required. Te signal generator and current amplifier have a common ground. A reference signal is fed into the lock-in amplifier for synchronization of signal generator and lock-in amplifier. Te signal from the amplifier is summed with the compensating signal to account for the capacitance of the entire system. Tis then enters the lock-in amplifier. Te output of the lock-in amplifier is fed into the microscope controller for analysis. Te spatial resolution can be estimated to be about 20 nm. Te resolution of capacitance is 2 aF. Te total impedance can be modeled as the impedance due to capacitive coupling, which is the impedance due to
doi:10.1017/S1551929511001234
www.microscopy-today.com • 2011 November
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