Until recently, however, our technology was behind our imagination, and this field remained almost exclusively theoretical. The electrical properties of carbon nanotubes, a class of macromolecules discovered almost a decade ago, may play a central role in future molecular electronics because carbon nanotubes are the perfect system on which general idea of molecular electronics can be the perfect system since the first reported individual nanotube transport experiment. Their unique structure makes them potentially useful as basic elements for generations of highly integrated circuits. So far, electronic devices based on individual nanotube molecules have mimicked the performance of conventional integrated circuits, such as the field effect transistor. For that reason, nanotubes provide the missing link which brings together the fields of conventional microelectronics and the designer molecular electronics of the future. The excitement regarding basic physics in nanotubes has been equally profound. The electrons in nanotubes are confined to one dimension, which radically changes particle interactions.
This theory has processed four different experiments. The first series of experiments addresses the surface analyses of SWNT and Peapod. We measured those NT materials by TEM and AFM. The height of SWNT was 1.55nm and the length was several mm. In the case of Peapod, the diameter of Peapod is about 1.4~1.5nm, which is consistent. Distance between the lines is ten collinear circles approximately 0.7 nm in diameter and the lines are spaced 0.3nm from the tubule wall.
The second series of experiments does the transport properties of individual SWNT and Peapod. The fabrication of the tubeFET, the single-molecule field effect transistor, is relatively straightforward, and integration of multiple devices into a circuit may eventually be possible by using molecular self-assembly techniques. The $I-V_{bias}$ curve shows that the resistance saturates around 1MΩ. The nonlinearity at a room temperature and the asymmetric dependence of the conductance on the gate voltage polarity indicate that this sample is semiconducting. In case of Peapod, $I-V_{bias}$ curve shows similar properties, which the resistance saturates 0.9 MΩ.
In the NEXT, we have discovered two extraordinary phenomena of SWNT and Peapod about the memory effects and UV effects in the third category. Each tubeFET was measured $I-V_{gate}$ curves to check the memory effects and $I-V_{bias}$ curves to check the conductance changes with UV and the visible light. The $I-V_{gate}$ of SWNT tubeFET was acquired at ambient conditions and a bias voltage of V=10mV. The threshold voltages are shifted with respect to each other: 2.7V for the curve starting at - 10V('down'sweep) and 6.8V for the one starting at +10V('up'sweep). The readout data also demonstrated the memory effects of SWNT tubeFET. In case of Peapod tubeFET, they didn't show any memory effects because of their Schottky barrier and the bandgap energy. The basic cause of this memory effects is the trapped negative charges in the bulk oxide. When a SWNT was exposed first to very UV light at 365nm and the visible at 532nm. There is no photoresponse at all to light, while there is on to the exposure to the UV, and visible light. But in case of Peapod, when we were irradiating UV, the currents increased 4~5 times. The mechanism of the increasing current hasn't been unclear yet, but it has been unambiguously established that oxygen chemisorption plays a profound role in enhancing the photosensitivity of Peapod.
The fourth series of experiments addresses the new fabricating method for Peapod, which reduced annealing time from 65 hours to 30min using by a microwave oven. $C_{60}$ is contained almost exclusively by 1.5nm diameter SWNTs and each $C_{60}$ is spaced approximately 1.0nm center-to-center. The microwave can produce more defects to act as the channel of $C_{60}$ molecules. So $C_{60}$ molecules can move easily inside of the tube, and does not need too long time like the traditional method.
Those data and suggested mechanism should be useful for nanotube applications. Our experiments and new attempts suggest that we are already at the heart of many proposed approaches to nano-electronics and it will be quite striking that the nanoscale memory will come here soon.
