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(Color online) (a) Current sensing AFM image of two crossed nanotubes of 1.7 and 0.9 nm diameter. Current-voltage measurements were done at points A–G, on nanotubes NT1 which is connected to a titanium contact, and NT2. The presence of a large defect is estimated at P1, causing a large drop in the current. The image was acquired at a tip-nanotube bias of 20 mV, and a current preamp gain of . Resistance between the nanotubes is estimated at 160 MΩ at 20 mV. (b) Schematic of the nanotube-nanotube junction J1, which are in physical contact. Conductive AFM tip is used to make electrical contact to the nanotubes at specific positions.
Current-voltage measurements (a) before and (b) after the junction J1, show linear electrical behavior close to zero bias. Oscillations are seen at position D, which are due to the influence of junction J1.
Current spectroscopy for tip positions (a) E, (b) F, and (c) G on nanotube NT2, at lengths 270, 390, and 590 nm from J1. The small effective mass of electrons inside the quantum well formed between two junctions produces differential negative resistance. For electrons injected into the nanotube NT2, the energy levels are marked and compared numerically to a model where the effective electron mass is matched to the energy levels. The calculated electron effective masses are 0.003, 0.0035, and respectively. The light effective mass offsets the effect of the long well dimensions, allowing the measurement of the quantized energy levels.
Infinite quantum well energy ratios between a level and level , are compared to the energy ratios at points D, E, F, and G on NT1 and NT2. The ratios show that an asymmetric finite quantum well structure compares closely with the ideal infinite quantum well. First energy levels (underlined) in each case are simulated values, and could not be identified from the measurements possibly due to thermal effects swamping out the small differences.
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