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Fig. (a) Sketch of the DFM secondary scan mode. (b) Semilogarithmic correlation between DFM gate ratio and FET device switching ratio. (c) Numerical simulation results of DFM signal and carrier concentration and mobility dependence. The DFM nanoscale spatial resolution shows a topography (d) and dielectric response (eg) of a single-walled carbon tube with a metal-semiconductor junction inside.
At present, nanomaterials have been increasingly used in various devices in the fields of electronics, optoelectronics, bioelectronics, sensing, and energy. Therefore, understanding and characterizing the electrical properties of nanomaterials is not only an interest in basic scientific research, but also an urgent need to achieve its wide application. However, the traditional field-effect transistor (FET) method encounters problems in the characterization of the electrical properties of nanomaterials, such as complicated device preparation process, difficult to achieve material-electrode ohmic contact, and low detection flux.
A group of researchers from the Suzhou Institute of Nanotechnology and Nanobionics of the Chinese Academy of Sciences Chen Liji and his collaborators jointly developed a new type of functional imaging technology called dielectric force microscopy (DFM) to solve the above problems. A related review was published in a recent Journal of of Chemical Research 48:1788 (2015).
The major contribution of semiconductors and metallic materials to the dielectric response of the external electric field comes from macroscopic polarization caused by carrier migration. Therefore, the carrier concentration in the material and its mobility both determine the dielectric response of the material and also determine its conductivity. With ultra-sensitive detection (~pN) of tiny force by the scanning probe technique, DFM characterizes the dielectric response of nanomaterials by measuring the interaction forces between the induced dipoles of the material and the charge on the tip. This imaging mode “sees†the carriers in nanomaterials without the need for electrode contacts (Figure a). Using single-walled carbon nanotubes (diameter ~1 nm) and zinc oxide nanowires (diameter ~30-50 nm) as research models, DFM successfully measured the dielectric constant of nanomaterials (Nano Letters 7:2729 (2007)). Resolving of semiconductor, metal and metal conductivity (Nano Letters 9:1668 (2009)) and determination of carrier type in semiconductor materials (Journal of Physical Chemistry C 116:7158 (2012)) (Fig. eg). More interestingly, DFM exhibits ~20nm spatial resolution that cannot be achieved with traditional FET methods.
In addition, Chen Lizhen and his collaborators verified the parallel measurement of DFM and FET by comparing the DFM and FET measurements of the same single-walled carbon tube (Nano Research 7:1623 (2014)). Related research results revealed that the gated modulation ratio of the DFM signal (the ratio of the DFM signal at different gate voltages) is proportional to the logarithm of the FET device switching ratio (Figure b). This semi-logarithmic relationship is interpreted and confirmed by the Drude model at the micro level (Fig. c). This model will provide a theoretical framework for the future application of DFM technology in different materials and device systems.
Scanning near-field microwave microscopy developed by Stanford University Professor Zhi-Xun Shen has similar characteristics and functions to DFM in the field of nano-materials electrical property measurement (Review of Scientific Instruments 79:063703 (2008)). Both scanning near-field microwave microscopy and DFM have non-contact measurements and nanoscale spatial resolution. The difference is that scanning near-field microwave microscopy and DFM separately measure the high-frequency and low-frequency dielectric properties of the material. DFM eliminates the need for expensive high-frequency network analyzers and specially-designed scanning probes, making it easy to use in a variety of complex imaging environments. This imaging mode of DFM may be widely used in the future basic research and industrial online monitoring.
The related series of work was funded by the National Natural Science Foundation of China, the Pilot Project of the Chinese Academy of Sciences, the Natural Science Foundation of Jiangsu Province, the Petroleum Research Foundation of the American Chemical Society, and the Suzhou Nanometer Science and Technology Collaborative Innovation Center.
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