Charge Pumping in Carbon Nanotubes

We demonstrate charge pumping in semiconducting carbon nanotubes by a traveling potential wave. From the observation of pumping in the nanotube insulating state we deduce that transport occurs by packets of charge being carried along by the wave. By tuning the potential of a side gate, transport of either electron or hole packets can be realized. Prospects for the realization of nanotube based single-electron pumps are discussed.

Figure 1

(a) Schematic of the device showing the transducer and carbon nanotube. (b) Atomic force microscope image of the contacted nanotube. Scale bar is 500 nm. The gray scale height range is smaller for the central square to highlight the nanotube. The SAW pumps charge from the source (S) to the drain (D), while the carrier type is controlled by the nearby gate (G).

Figure 2

(a) Color scale plot of the dc current versus gate and source-drain bias voltage at $T=5\mathrm{K}$. Red indicates positive and blue negative currents. The superimposed rhomboid, taken at a threshold current of 0.5 nA, gives an indication of the location of the insulating region. Insets: Schematic diagrams showing the difference in Schottky barrier height presented to electrons and holes. (b) Line scan taken at $V_{\mathrm{sd}}=0.5\mathrm{V}$.

Figure 3

(a),(b) Dependence of the SAW-induced current on rf frequency for two values of the gate voltage, showing distinct peaks at the resonant SAW frequency. rf power applied to the transducer is $+20\mathrm{dBm}$. (c) Dependence of the SAW-induced current on gate voltage. Positive current corresponds to electron flow in the direction of SAW propagation. Inset: Dependence of the current on dc bias for four fixed values of the gate voltage, showing a transition from low to high conductance at the current peak. rf power is $+20\mathrm{dBm}$. (d)–(f) Schematic band diagrams depicting the proposed mechanism of charge transport. The conduction ($E_C$) and valence ($E_V$) bands are bent by the SAW allowing injection of electrons or holes depending on the phase ($\varphi$) of the wave [24].

Figure 4

SAW-induced current versus gate voltage for fixed values of rf power 17.3 (blue), 17.9, 18.8, 19.1, 19.4, 19.7, and 20 dBm (red). Inset: Points correspond to $|I_{\mathrm{SAW}}|=0.1\mathrm{nA}$. This small threshold current was arbitrarily chosen to indicate the region in the $P-V_g$ plane in which a SAW-induced current is absent. The dashed lines are a guide to the eye.