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Posted: Jan 07, 2009

Characterizing the electronic properties of carbon nanotubes

(Nanowerk Spotlight) One of the issues we addressed in our Nanotechnology Spotlight yesterday was the challenge of separating metallic from semiconducting single-walled carbon nanotubes (SWCNT) after production. Developing a rapid and parallel technique for the electronic characterization of high-density arrays of SWCNT devices is essential for future large-scale production processes of nanoelectronics components.
Currently used methods for electronic characterization of SWCNT devices and arrays are time consuming to set up, slow to execute, and not suitable for large-scale deployment. These methods include direct electron transport measurements – which require that each device be individually measured in three-terminal configuration (source, drain, gate) – and scanning probe microscopy, which can map the surface potential along a nanotube; again, a very slow technique with limited ability to be integrated into the process flow for microelectronic fabrication and characterization.
More recent demonstrations of sorting nanotubes with DNA have been more promising but are very early stage (see: There is something fishy about these nanotubes).
Researchers in Germany have now presented Voltage-Contrast Scanning Electron Microscopy (VC-SEM) as a new technique for the characterization of molecular electronic devices and device arrays. They show that by VC-SEM it is possible to simultaneously characterize an entire high-density array of SWCNT devices, and distinguish metallic from semiconducting nanotube devices, without having to individually measure the electron transport characteristics of each device. They also show that their technique can be extended to characterize defects and device failure in nanotubes.
"VC-SEM caused by self-charging of the device under electron irradiation (VC-SEM Type 1) was developed in the 1960s by the microelectronic industry for failure location in circuits" Dr. Aravind Vijayaraghavan tells Nanowerk. "In our recent work, we have advanced VC-SEM under the influence of externally applied bias and electric field (VC-SEM Type II and III) for the electronic characterization of nanoelectronic devices. VC-SEM serves to fill a void in the electronic characterization techniques for molecular devices, particularly in highly integrated arrays and circuits. It is a visual technique, works on a variety of insulating substrates that are typical in the microelectronics industry, and can be integrated into the process flow in device fabrication."
Vijayaraghavan, a member of the Molecular Nanostructures group of Ralph Krupke at the Institute of Nanotechnology (INT) at the Research Center Karlsruhe in Germany, is first author of a paper in Nano Research ("Imaging electronic structure of carbon nanotubes by voltage-contrast scanning electron microscopy").
VC-SEM involves tuning the electronic band structure and imaging the potential profile along the length of the nanotube. The resultant secondary electron (SE) contrast allows to distinguish between metallic and semiconducting carbon nanotubes and to follow the switching of semiconducting nanotube devices, as confirmed by in situ electrical transport measurements.
The technique developed by the INT team, including graduate students from the University of Karlsruhe, involves imaging the SE emission from a SWCNT device under three-terminal (source-drain-gate) bias conditions. Vijayaraghavan explains the principle: "Surface potentials modify SE yield – positive potential reduces yield and negative potential enhances yield. Also, transverse electric fields can deflect electrons away from the detector. Semiconducting and metallic SWCNTs show different potential drop along their length, due to their different conductivities, and this results in differences in their SE image."
Imaging Electronic Structure of Carbon Nanotubes by Voltage-Contrast Scanning Electron Microscopy
Top: An array of 10 devices, each comprised of an individual SWCNT connected to source (upper) and drain (lower) electrodes, laying on an insulating silicon dioxide surface with a conducting silicon back-gate underneath. When this array is imaged under such three-terminal bias conditions, devices containing a metallic SWCNT will show similar contrast for source and drain electrodes (2,4,7,8), devices with a semiconducting SWCNT will show different contrast of source compared to both drain and gate (1,3,6,9) while devices containing a break or defect will show similar contrast of the source and gate. Bottom: Closer inspection of semiconducting SWCNTs under such bias conditions reveals that at low gate bias, when the nanotube is in its OFF state, the contrast along the nanotube changes gradually from drain to source, while at high gate bias, when the device is in its ON state, the contrast along the nanotube is uniform from drain to source. (Gate bias from left to right is -15V, -5V, 0V, 5V, 15V) (Image: Dr. Vijayaraghavan, Dr. Krupke, Forschungszentrum Karlsruhe)
The researchers point out that VC-SEM can probe the potential distribution along a carbon nanotube in a similar way to scanning probe techniques like electrostatic force microscopy or sliding contact measurements. However, the acquisition times for VC-SEM are orders of magnitude shorter than for scanning probe techniques, and there is no need to explicitly bias each source electrode.
Vijayaraghavan and his colleagues show that electron irradiation can cause charges to be implanted into the insulating substrate adjacent to a nanotube that is being probed. "These implanted charges cause inhomogeneous local electric fields that perturb the electronic system of the nanotube and result in metal-insulation transitions," he says (see: "Metal-Semiconductor Transition in Single-Walled Carbon Nanotubes Induced by Low-Energy Electron Irradiation"). "Fortunately, our recent work has shown that this is reversible by applying a high current or high bias annealing procedure (see: "Reversible Metal-Insulator Transitions in Metallic Single-Walled Carbon Nanotubes"). In fact, such creation and annealing of defects can be followed in situ by VC-SEM."
The team in Karlsruhe is developing this technique for statistical analysis of high-density nanotube device arrays. With it they now can determine what fractions of devices are metallic, semiconducting, and defective. Structural defects in the nanotube, such as a missing carbon atom or a bond-rotation that causes a chirality change, might form during growth or subsequent processing steps like sonication and acid-treatment.
"We are currently working on extending the technique to defect location and characterization, which will be published soon" says Vijayaraghavan. "VC-SEM can be used to rapidly screen through a large number of devices to identify which ones are interesting for further characterization by complementary techniques like near-field Raman spectroscopy."
By . Michael is author of two books by the Royal Society of Chemistry: Nano-Society: Pushing the Boundaries of Technology (RSC Nanoscience & Nanotechnology) and Nanotechnology: The Future is Tiny. Copyright © Nanowerk

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