Marcus Freitag

Fabrication and electrical characterization of polyaniline-based nanofibers with diameter below 30 nm

We fabricate and electrically characterize electrospun nanofibers of doped polyaniline/polyethylene oxide (PAn/PEO) blend with sub-30 nm diameter. Fiber diameters near 5 nm are obtained for optimized process parameters. Scanning conductance microscopy (SCM) shows that fibers with diameter below 15 nm are electrically insulating; the small diameter may allow complete dedoping in air or be smaller than phase-separated grains of PAn and PEO. Electrical contacts to nanofibers are made by shadow mask evaporation with no chemical or thermal damage to the fibers. Single fiber I–V characteristics show that thin fibers conduct more poorly than thick ones, in agreement with SCM data. I–Vs of asymmetric fibers are rectifying, consistent with formation of Schottky barriers at the nanofiber-metal contacts.

Controlled creation of a carbon nanotube diode by a scanned gate

We use scanning gate microscopy to precisely locate the gating response in field-effect transistors (FETs) made from semiconducting single-wall carbon nanotubes. A dramatic increase in transport current occurs when the device is electrostatically doped with holes near the positively biased electrode. We ascribe this behavior to the turn-on of a reverse biased Schottky barrier at the interface between the p-doped nanotube and the electrode. By positioning the gate near one of the contacts, we convert the nanotube FET into a rectifying nanotube diode. These experiments both clarify a longstanding debate over the gating mechanism for nanotube FETs and indicate a strategy for diode fabrication based on controlled placement of acceptor impurities near a contact.

Role of single defects in electronic transport through carbon nanotube field-effect transistors.

The influence of defects on electron transport in single-wall carbon nanotube field-effect transistors (CNFETs) is probed by combined scanning gate microscopy (SGM) and scanning impedance microscopy (SIM). SGM images are used to quantify the depletion surface potential, and from this the Fermi level, at individual defects along the CNFET length. SIM is used to measure the voltage distribution along the CNFET. When the CNFET is in the conducting state, SIM reveals a uniform potential drop along its length, consistent with diffusive transport. In contrast, when the CNFET is off, potential steps develop at the position of depleted defects. High-resolution imaging of a second set of weak defects is achieved in a new tip-gated SIM mode.

Local electronic properties of single-wall nanotube circuits measured by conducting-tip AFM

We use conducting-tip atomic force microscopy (AFM) to measure local electronic properties of single-wall carbon nanotube (SWNT) circuits on insulating substrates. When a voltage is applied to the tip and AFM feedback is used to position the tip, images formed from the tip-sample tunnel current have single tube resolution (near 1 nm diameter), more than an order of magnitude better than simultaneously acquired topographic AFM images. By finding points where the tip-sample current is zero, it is possible to measure the electrochemical potential within the circuit, again with nanometer resolution. Such measurements provide compelling evidence that nanotubes within a bundle have only weak electronic coupling. Finally, the AFM tip is used as a local electrostatic gate, and the gating action can be correlated with the structure of the SWNT bundle sample. This technique should be useful for a broad range of circuits containing SWNTs and other molecules.

Tip-gating effect in scanning impedance microscopy of nanoelectronic devices

Electronic transport in semiconducting single-walled carbon nanotubes is studied by combined scanning gate microscopy and scanning impedance microscopy (SIM). Depending on the probe potential, SIM can be performed in both invasive and noninvasive modes. High-resolution imaging of the defects is achieved when the probe acts as a local gate and simultaneously as an electrostatic probe of local potential. A class of weak defects becomes observable even if they are located in the vicinity of strong defects. The imaging mechanism of tip-gating scanning impedance microscopy is discussed.

Carbon nanotubes as a tip calibration standard for electrostatic scanning probe microscopies

Scanning surface potential microscopy (SSPM) is one of the most widely used techniques for the characterization of electrical properties at small dimensions. Applicability of SSPM and related electrostatic scanning probe microscopies for imaging of potential distributions in active micro- and nanoelectronic devices requires quantitative knowledge of tip–surface contrast transfer. Here we demonstrate the utility of carbon-nanotube-based circuits to characterize geometric properties of the tip in the electrostatic scanning probe microscopies. Based on experimental observations, an analytical form for the differential tip–surface capacitance is obtained.

Local variation of metal-semiconducting carbon nanotube contact barrier height

Carbon nanotubes provide great promise for future use as electronic devices. Previously we have used a conducting-tip atomic force microscope to measure the local field effect in a metal-semiconducting C nanotube-metal device. Here we propose a consistent electrostatic model that incorporates the image force, electric field and tip potential and describes how the latter reduces the potential barrier seen by thermionically emitted carriers in the metal-nanotube junction. The model describes a position-dependent barrier change, consistent with experimental data.

Material contrast by combined scanning tunneling and force microscopy imaging of single-walled carbon nanotubes

A newly developed combination of tunneling and force microscopy enables near-atomic point resolution, and tubes can be identified without the need of a conducting substrate. This is a crucial step for the characterization of electronic devices based on individual single-wall tubes. Images of the spatial current distribution taken in constant force mode show strongly enhanced contrast between tubes and gold or silicon dioxide substrates, respectively, which is attributed to the unusual elastic properties of the nanotubes.

Locally measured electronic properties of single wall carbon nanotubes

We use a conducting-tip atomic force microscope (CT-AFM) to measure the local electronic properties of single wall carbon nanotube (SWNT) circuits. Tunnel current images of SWNTs in circuits can have 1 nm resolution, much better than that of standard AFM. By using the tip as a mobile electrode, we are able to measure variations of the local electrochemical potential within a SWNT bundle with millivolt resolution. We find that most of a bundle’s resistance comes from hopping between the nanotubes in the bundle while the individual nanotubes are highly conductive. The conducting AFM tip can also be used as a local electrostatic gate of the SWNT circuit, and we make the observation that the gating of a SWNT bundle occurs at an isolated “hot spot.”

Nanoscale Characterization of Carbon Nanotube Field‐Effect Transistors

We use scanning gate microscopy and scanning impedance microscopy to map the gating behavior as well as the electrostatic potential distribution in field‐effect transistors made of single‐wall carbon nanotubes. By applying an ac bias, both Schottky barriers at the source and drain electrodes are imaged simultaneously. The potential profile in the device shows a strong dependence on the backgate‐induced conductivity state. In an “ON” state, the potential profile is essentially linear, suggesting diffusive conduction. In contrast, the nanotube transistor shows pronounced potential steps in an “OFF” state. The steps are correlated with defect sites along the nanotube length. In a self‐gating regime, scanning impedance microscopy can be used to image defects with increased resolution. Weak defects in the vicinity of strong ones become visible.