Research ArticlePHYSICAL SCIENCES

Metal nanoparticle film–based room temperature Coulomb transistor

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Science Advances  14 Jul 2017:
Vol. 3, no. 7, e1603191
DOI: 10.1126/sciadv.1603191
  • Fig. 1 Device design.

    (A) Schematic drawing. (B) Transmission electron micrograph of the CoPt nanoparticle monolayer as assembled by the Langmuir-Blodgett method. (C and D) Light micrographs of the final devices with three pairs of source and drain contacts as well as a local back-gate electrode. The gap between the leads and thus the length of the nanoparticle channel is 1, 2, or 3 μm (top to bottom).

  • Fig. 2 Output characteristics.

    (A) Current-voltage curves for different temperatures show the transition from a system governed by Coulomb blockade to almost ohmic conduction. Lowest and highest temperatures are included as insets to provide a good comparability and are fitted with the respective transport mechanism. (B) Same data plotted logarithmically as differential conductance over bias voltage. The decrease of the blockade-regime width, changes in threshold voltage, and increase in zero-bias conductance (at 90 K and above) can be seen. (C) The Arrhenius plot allows for a linear fit of the logarithmic zero-bias conductance outside of the Coulomb blockade regime. The slope indicating the activation energy is similar for all channel lengths. (D) The activation energy decreases exponentially with the applied electric field and is not dependent on channel length or gating geometry.

  • Fig. 3 Temperature dependency.

    (A) The local back gate induces clear oscillations with reproducing features during a bidirectional gate-voltage sweep for different temperatures. (B) The highest found on/off ratios for three different channel lengths measured with a locally gated sample decay exponentially with increasing temperature. (C) A sample gated with the global substrate shows very periodic oscillations with similar percentages. (D) Smaller nanoparticles (2.3 nm diameter) lead to higher on/off ratios and more pronounced oscillations at higher temperatures. Minor changes in bias voltage adjust the current and render the curve perfectly smooth. Even above room temperature, clear Coulomb oscillations are visible and efficiencies of 5% could be reached. A larger distance to the gate electrode and smaller particles elongate the oscillation period from an average of 24.3 V (A) to 54.7 V (C) and 96.9 V (D). Note that the origin of the higher frequency oscillations seen in (C) and (D) is not entirely clear; they are assumed to result from some external influence and only appeared from time to time.

  • Fig. 4 Voltage dependency.

    (A) Oscillations induced by a local gate for different bias voltages at a temperature of 10 K. For the lowest shown voltage, an averaged curve has been added as a guide to the eye. The shape and distinctive features of each curve are reproducing at different bias voltages. A shift in minimum position can very well be seen. (B) Corresponding output characteristics and differential conductance plot. The colored lines mark the bias voltages displayed in (A). (C) The bias voltage dependency of the on/off ratio illustrates the different stages of electrical transport in the system. The maximum performance is reached around the threshold voltage.

  • Fig. 5 Oscillation shift.

    (A) Optical micrograph of a device with a 3-μm-long channel and three parallel 500-nm-wide gate electrodes. (B) Each gate can be used to induce oscillations. At the same bias voltage, the main features are shifted. (C) Plotting the position of the minimum with respect to the applied bias voltage yields a linear correlation that is independent of temperature. Each gate position corresponds to a certain slope. The measured data roughly match the simulated slopes but reveal a shift in the lithographic alignment.

  • Fig. 6 Particle size comparison.

    (A) Size distributions including Gaussian fits. a.u., arbitrary units. (B) Period of oscillation for different channel lengths, temperatures, and bias voltages. The horizontal lines mark the average value for each set of data points. The period seems to be only dependent on particle size and separation between channel and gate electrode. With these two parameters, it can be tuned over a wide voltage range. (C) Bias voltage dependent on/off ratios at low temperature for the three sizes. The best performance can be obtained by using small particles. (D) The activation energy is inversely proportional to the particle diameter. The results of this work supplement the data from another publication (31).

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/3/7/e1603191/DC1

    text S1. Capacitance.

    fig. S1. Particle characterization.

    fig. S2. Monolayer preparation.

    fig. S3. Hysteresis.

    fig. S4. Capping.

    fig. S5. Transport mechanism.

    fig. S6. 2D plot of the device characteristics.

    fig. S7. Voltage-dependent oscillations.

    fig. S8. Position of minimum.

    fig. S9. Influence of gate position.

    References (41, 42)

  • Supplementary Materials

    This PDF file includes:

    • text S1. Capacitance.
    • fig. S1. Particle characterization.
    • fig. S2. Monolayer preparation.
    • fig. S3. Hysteresis.
    • fig. S4. Capping.
    • fig. S5. Transport mechanism.
    • fig. S6. 2D plot of the device characteristics.
    • fig. S7. Voltage-dependent oscillations.
    • fig. S8. Position of minimum.
    • fig. S9. Influence of gate position.
    • References (41, 42)

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