Scanning Conductance Microscopy and High Frequency Scanning Gate Microscopy of Carbon Nanotubes and Polyethylene based N
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O9.8.1
Scanning Conductance Microscopy and High Frequency Scanning Gate Microscopy of Carbon Nanotubes and Polyethylene based Nanofibers Cristian Staii1, Rui Shao2, Nicholas J.Pinto3, Dawn A.Bonnell2, Alan T. Johnson, Jr.1 Department of Physics and Astronomy and Laboratory for Research on the Structure of Matter, University of Pennsylvania, 209 South 33rd Street, Philadelphia, Pennsylvania 19104 2 Department of Material Science and Engineering, University of Pennsylvania, 3231 Walnut Street, Philadelphia, Pennsylvania 19104 3 Department of Physics and Electronics, University of Puerto Rico, Humacao, Puerto Rico 00791 1
ABSTRACT
We present two new approaches that significantly enhance the analytic power of Scanning Conductance Microscopy (SCM) and Scanning Gate Microscopy (SGM). First, we present a quantitative model that explains the phase shifts observed in SCM, by considering the change in the total capacitance of the tip-sample-substrate system. We show excellent agreement with data on samples of (conducting) single wall nanotubes and insulating polyethylene oxide (PEO) nanofibers. This model is also used to determine the dielectric constant of PEO nanofibers, a general approach that can be extended to other dielectric nanowires. Second, we extend the SGM to frequencies up to 15MHz, and use it to image changes in the impedance of carbon nanotube field effect transistor (CNFET) circuits induced by the SGM-tip gate. We show that these measurements are consistent with a simple RC parallel circuit model of the CNFET, with a time constant of 0.3 µs. INTRODUCTION
Many types of hybrid Scanning Probe Microscopies (SPM) have been developed to study the local electrical properties of nanoscale objects. Scanning Conductance Microscopy (SCM), and Scanning Gate Microscopy (SGM) are particularly powerful approaches. SCM can probe the conductivity of nanoscale structures without electrical contacts, and it has been used to image single wall carbon nanotubes (SWNTs), to prove the insulating character of λ-DNA [1], and to distinguish between conducting and insulating polyaniline/polyethylene oxide (Pan/PEO) nanofibers [2]. Although a model for SCM has previously been proposed [1], it does not account for all these observations. We introduce an improved model for the tip –sample –substrate geometry that makes SCM a more powerful quantitative technique. We show that SCM data can be predicted by considering the change in the total capacitance of the tip-samplesubstrate system. We also present experimental data for carbon nanotubes and PEO nanofibers that are in excellent agreement with the predictions of the above model. As a very important application we are using this technique to measure the dielectric constant of PEO nanofibers.
O9.8.2
SGM is a very powerful technique for measuring the local electronic properties of nanoscale circuits. SGM can be used to image Schottky barriers at the metal contacts of a carbon nanotube field effect transistor (CNFET) [3], and gate-susceptible defects along its length [4]. Here, we combin
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