Counting single electrons with a carbon-nanotube transistor

Detection and manipulation of individual electrons are hot topics in nanoscale electronics due to the promise of ultra-low-energy-dissipation devices and information processing in molecular circuits. Single-electron detectors made from metal or semiconducting material have been reported, but these have the drawback of operating at millikelvin temperatures.¹ Carbon nanotubes, on the other hand, have been used at temperatures up to 200K to detect single electrons as they hop onto random defects in a silicon oxide layer.² However, the electron traps in this case are poorly controlled.

Here we show, for the first time, real-time detection of transfer of single electrons between a gold (Au) nanoparticle and a carbon nanotube at a temperature close to 150K. This temperature is three orders of magnitude higher than that for one of the previous detectors. We also demonstrate that nanotube transistors can probe electrons in other molecular systems, which the other microfabricated detectors cannot do. Moreover, the single-electron-detection measurements allow for full characterization of the system circuit, shown schematically in Figure 1(b).

Figure 1. (a) Atomic force microscope image of the device geometry, showin a single-wall carbon nanotube (SWNT) spanning the gap between source (S) and drain (D) electrodes, with a gold (Au) nanoparticle on top of it. (b) Schematic of the measurement setup. The conductance G_tube is the ratio of the current measured by the ammeter (A) to the source-drain voltage along the nanotube, V_SD. (c–f) Energy levels in the nanotube, particle, and gate as the gate voltage is raised. Each time an empty energy level of the particle matches the electrochemical potential of the tube, an electron is transferred onto the particle, which is detected by the nanotube transistor. (Reprinted with permission.³ Copyright 2007 American Chemical Society.)

The carbon-nanotube transistors were fabricated using standard techniques, with nanotubes grown by chemical-vapor deposition on a silicon dioxide-covered silicon substrate. Au nanoparticles were deposited onto the wafer from aqueous suspension and positioned on top of the nanotube by atomic force microscope manipulation, as shown in Figure 1(a).

The transfer of single electrons into the Au nanoparticle can be detected by measuring the conductance G_tube of the nanotube, which is extremely sensitive to the presence of electric charges: see Figure 2(a). As the gate voltage, V_G, is swept, the conductance is gradually turned off as for typical p-doped semiconducting single-wall nanotubes (SWNTs). However, we additionally observe abrupt conductance jumps (vertical red bars) that indicate discrete electron transfer events from the nanotube into the particle. Each transferred electron changes the electrostatic potential in the particle and, in turn, the charge density in the nanotube, which effectively shifts the conductance curve horizontally in V_G.

Figure 2. Detection of single electrons by changes in the tube conductance G_tube(V_G). (a) Vertical red bars indicate conductance jumps as the gate voltage, V_G, is swept from –4 to –1V. The inset shows the relation between G_tube(V_G) and the number of electrons on the Au particle. (b) Tube conductance over a smaller range of V_G. Each color corresponds to a different sweep. The inset shows two sweeps of G_tube(V_G) in black and red. Jumps appear at different V_G values as a result of the stochastic nature of the electron transfer. (Reprinted with permission.³ Copyright 2007 American Chemical Society.)

Figure 3. Fluctuations in time of the electron number due to thermal excitation. Tube conductance at 50K, for V_G=−1.35V, experiences two levels. We attribute the extra level to the electrochemical potential of the tube, E_F, that matches the center of the Coulomb gap. The insets show the energy levels in the tube and in the Au particle for different numbers of electrons. The thermal energy kT is shown in red. (Reprinted with permission.³ Copyright 2007 American Chemical Society.)

To unequivocally confirm that these discrete jumps correspond to single-electron events, we performed repetitive scans in V_G, as shown in Figure 2(b). A collection of curves is obtained that are periodically spaced in gate voltage, with a period ΔV_G^shift of about 60 mV. This periodicity suggests that adjacent curves differ by a single electron in the Au particle and that the observed jumps between curves represent transfers of individual electrons.

The detection of single electrons allows the characterization of Au-nanoparticle electrical properties, by observing the time dependence of the electron transfer versus temperature. For instance, Figure 3 shows the tube conductance at 50K when the gate voltage is set at a fixed value. The tube conductance fluctuates between two values on a time scale of several hundred seconds, corresponding to an electron going back and forth into the Au particle due to thermal excitation and changing the number of electrons. These fluctuations of electron number due to thermal excitation can provide information on the energy separation between electron states of the Au particle.³

In conclusion, we have demonstrated well-controlled single-electron detection in a simple, well-defined, highly resistive molecular circuit consisting of a carbon-nanotube transistor and a Au nanoparticle. Nanotube transistors are shown to be excellent detectors of single electrons at high temperatures. We have exploited such single-electron counting and its low transfer rate to electrically characterize the Au particle.

Single-electron counting with nanotubes offers great promise for probing the electronic properties of nanoscale systems. Beyond the metallic nanoparticles studied here, the technique should work with organic molecules, biomolecules, and semiconducting particles, whose high electrical resistivity results in currents too small to measure with conventional electronics. Our next steps are to investigate photoelectric effects in cadmium selenide particles and charge transfer in the biomolecules that are involved in photosynthesis and respiration.