Bridge Enhanced Nanoscale Impedance Microscopy (25060)
A conductive atomic force microscopy (cAFM) adjunct has been developed by Northwestern scientists that is capable of quantitatively measuring the magnitude and phase of alternating current flow through the tip/sample junction with a five order of magnitude improvement in sensitivity. Significant improvement in sensitivity and spatial resolution will enable the study of electronic behavior in nanomaterials and biological samples.
ADVANTAGES: The device provides spatially resolved quantitative current magnitude and phase shift values in materials with 10 nm precision, a 6 orders of magnitude improvement versus previously available techniques. Quantitative measurement of frequency dependent electronic behavior of conductive pathways with capacitances on the order of 10-18 farads, 5 orders greater sensitivity than present day results at this spatial resolution scale, has been demonstrated.
SUMMARY:Macroscopic impedance spectroscopy techniques have been employed to characterize alternating current charge transport for a variety of materials systems and devices. Modeling of this frequency dependent behavior has revealed underlying electrolytic surface reactions, doping levels of semiconductors, the properties of interfaces in organic and inorganic multi-layer devices, and charge transport in percolation network systems. However, these macroscopic methods only reveal an ensemble average of the underlying contributions of individual pathways, defects, film thickness variations, electrochemical reactions, and failure mechanisms. Scanning probe impedance measurement techniques based on the conductive Atomic Force Microscope (cAFM) has enabled probing current flow and resistivity variations on conductive surfaces with nanoscale spatial resolution. However, fringe capacitance (1-100 picoF) between the sample and the probe imposes a serious detection limit.
In an effort to improve the sensitivity of nanoscale impedance microscopy (NIM), a variable resistor/capacitor (RC) bridge circuit to cancel the spurious contribution to the AC current flow caused by fringe capacitance has been devised. This addition improves the detection limit of NIM by at least five orders of magnitude, enabling the detection of impedance values that are typical for many nanostructures, nano-electrochemical cells, and biological systems. This technique is referred to as bridge enhanced nanoscale impedance microscopy (BE-NIM).
The sensitivity and spatial resolution of BE-NIM was implemented in a series of five 20 nm tall by 750 nm wide gold electrodes patterned on a 500 nm thick silicon oxide grown on n-type silicon (1-10 Ω/cm). The electrodes were connected to micron sized gold squares of varying capacitances and imaged. The resulting topography, magnitude, and phase images (Fig a-c) are compared to the same region imaged using the standard NIM approach, (Fig d-f). The contribution of the ~1pF fringe capacitance in NIM, is evident. A comparison of the phase images (Fig. c vs. f) from the two techniques further reveals the strengths of BE-NIM.
This drastic improvement in sensitivity and spatial resolution will facilitate the study of electronic behavior in cutting edge hybrid nanomaterials and biological samples for the purpose of research, reliability testing, failure analysis, and basic science studies.
Liam Pingree and Mark Hersam
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