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The electronic properties of carbon nanotubes are such that they may be metallic or semiconducting depending on their diameter and the arrangement of graphitic rings in the walls. They also exhibit exceptional mechanical, thermal and chemical properties. Utilization of these properties has led to the application of nanotubes as scanning probes, electron field emission sources, actuators and nano-electronic devices.

Their nanometric dimensions, high aspect ratio, large surface area and unique thermal, optical and electronic properties have promoted carbon nanotubes as one of the perfect candidates for sensing applications. Nanotube-based physical sensors can measure pressure, flow, temperature and the mass of an attached particle.

As discussed in Chapter 1, researchers at the Georgia Institute of Technology have demonstrated a carbon nanotube–based nanobalance which can weigh sub-micron scale particles. By applying an alternating voltage, they were able to create resonance in the nanotube with a specific frequency which depends on the nanotube length, diameter, density and elastic properties. The mass of the particle was calculated from changes in the resonance frequency that occur on placing the particle over the carbon nanotube. Using this technique, the mass of a carbon sphere was determined to be 22 femtogram, which is by far the smallest mass ever measured. This approach may lead to a technique for the weight measurement of individual biomolecules. This femto balance can also find application in weighing bio-organisms such as viruses.

Cleland and Roukes at the California Institute of Technology have reported the fabrication and characterization of a working nanometre-scale mechanical electrometer. The device has demonstrated a charge sensitivity below a single electron charge per unit bandwidth (~0.1 electrons/Hz at 2.61 MHz) better than that of state-of-the-art semiconductor devices and comparable with the charge detection capabilities of cryogenic single electron transistors.

Prof. Ramgopal Rao and his group at IIT Bombay have developed a MEMS-based accelerometer using a polymer composite (Fig. 4.6), which can sense acceleration levels down to the 100 mg range. A low-cost technique at temperatures below 100°C has been used for the fabrication, which allows easy integration with CMOS.

Single-walled carbon nanotubes have been shown to exhibit piezoresistive effect, i.e. when they are bent or stretched, their electrical resistance changes. Based on this principle, carbon nanotube–based pressure and strain sensors have been developed. The pressure sensor consists of an ultrathin aluminium oxide membrane to which carbon nanotubes are attached. To calibrate the device, the deformation of the membrane in response to applied pressure was measured using white-light interferometry. They then monitored changes in nanotube resistance as a function of strain. They could detect a change even for strains as small as a hundredth of a percent, which in this case were induced by pressures of a few tens of kilopascals. The sensing nanotube was in this case metallic so that the gauge factor was positive. It had a value close to that of the best silicon devices.

Flow sensors have also been realised using SWNT. The SWNT bundles were packed between two metal electrodes and it was observed that they produced electrical signals in response to fluid flow. This is due to the direct scattering of the free carriers from the fluctuating coulombic fields of the ions or polar molecules in the flowing liquid. It was found through experiments that the ionic strength of the flowing liquid significantly affected the induced voltage.

The electronic properties, such as the local density of states, of single-walled carbon nanotubes are shown to be extremely sensitive to the chemical environment, because in this case all the tube atoms are surface atoms. The electrical conductivity and thermoelectric power also vary during exposure to several gas species and most of the nanotube-based chemical sensors are developed based on this principle.

It has been observed that the electrical resistance of individual semiconducting SWNTs changes by up to three orders of magnitude on exposure to NO₂ or NH₃

Fig. 4.6 MEMS accelerometer fabricated using polymer composite. (Source: V Ramgopal Rao, IIT Bombay)

molecules at room temperature. Using this effect, a chemical sensor in which a single semiconducting SWNT was placed in contact with titanium or gold metal electrodes at the end has been fabricated. SWNT-based hydrogen sensors have also been demonstrated by sputter-coated individual and bundled SWNTs with Pd nanoparticles. The conductivity was found to decrease by 50% and 33% for the individual and nanotube bundles respectively, on exposure to an air mixture containing 400 ppm of hydrogen. Oxygen sensors based on change in resistance and thermoelectric power on exposure to oxygen have been reported. For biosensors, various ligands can be connected to a nanotube transducer in order to recognise target molecules specifically. The large surface areas available for molecule adsorption make carbon nanotubes a suitable material for biosensors. Carbon nanotube–based sensors are a thousand times smaller that microelectromechanical systems (MEMS) sensors and consume less power. These two important advantages will foster their use as implantable devices. Such biosensors based on functionalized carbon nanotubes will provide high sensitivity, large linear range, fast response, long life and low detection thresholds for different analytes.