Determination method of energy band gap of carbon nanotube by high temperature dependence of current
Maeda, Masatoshi; kamimura, Takafumi; Iwasaki, Shin; Matsumoto, Kazuhiko
Japan

We have established the new approach to measure the energy band gap (Eg) of the carbon nanotube (CNT). CNT is the useful element for the future nano devices such as a CNT FET, biological sensors with high sensitivity, etc. For these applications, the determination of the Eg of CNT is quite important as Eg influences the device performances. An Eg of semi-conducting nanotube is so far, mainly measured by a photo-luminescence (PL). In this measuring method, however it is necessary to set CNT isolated from the substrate. Therefore, PL technology cannot apply to the CNT devices for the measurement of the Eg in which CNT directly contacts to the substrate. Another method to determine the Eg is to measure a diameter of CNT by atomic force microscope (AFM). The diameter of CNT is directly related to the Eg by the simple formation. In the present research, we have established new approach for the determination of the Eg of individual CNT by measuring the temperature dependence of the CNT current from room temperature to 900°C. The CNT was grown between two electrodes by the thermal CVD. The sample was then, set in an electric furnace and electrodes were connected to the bias source. During the heat-up process, Ar gas was supplied and DC bias was applied between electrodes and the CNT current was monitored. During the heat-up process, the CNT current starts to increase drastically around 700°C. The CNT current in logarithm scale is almost linearly proportional to the inverse of the temperature. This is owing to electrons directly excited by the thermal energy from the valence band to the conduction band. The Eg was obtained from the slope of the CNT current, and it was found to be Eg = 0.57 eV. After the measurement of CNT current at high temperature, the diameter of CNT was confirmed by AFM. The diameter of the CNT is d = 1.41 nm. Therefore, the Eg of CNT can be calculated from the CNT diameter using the simple relation, Eg = 2 γ0 ac-c / d [1], and found to be Eg = 0.563 eV. The Eg obtained from the temperature dependence of CNT current and from the diameter are well coincided. Therefore, it was confirmed that the Eg of CNT can be determined from the temperature dependence of CNT current.
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