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(a) Schematic and (b) scanning electron microscopy image of the AuNP device under study. The AuNPs were assembled on a SiO2/Si substrate with pre-made Au/Cr electrodes, and the SiO2 surface was modified with (3-aminopropyl)-trimethoxysilane (APTMS) to increase the adhesion to AuNPs. The 12 nm-diameter AuNPs were surface-modified with molecules with a carbon number of 3 or 8; these are marked with triangle symbols. (c) The zero-bias resistances R as a function of temperature T show opposite trends for MPA-metallic (MPA-II-1) and MPA-insulating (MPA-IV-16) devices. The inset shows R versus T −1/2 curves for temperatures down to T = 40 mK; the metallic device (MPA-II-1) is represented by red curve, and the insulating device (MPA-IV-3 “QD”) represented by blue squares. Here the devices were measured in a mK cryostat, and the wiring resistance gave a constant background to the device resistance.
(a) The current-voltage (IVb ) curves for the insulating device (MPA-IV-3 “QD”) at T = 300 K, 120 mK, and 40 mK. Below 1 K, the IVb curve became non-linear. (b) IVb curves for device MPA-IV-7 “SET” at Vg = 0 V, and Vg = −3.04 V, at which the Coulomb gap shrank to a minimal value. (c) Family of IVb curves for device “QD” under various Vg biases. From bottom to top, Vg = 0 V, −2.58 V, −5.09 V, −7.58 V, and −9.5 V. The single Coulomb gap evolved into three plateaus at Vg > 2 V. Here, the curves are vertically shifted for clarity. Curves in (b) and (c) were measured at 40 mK.
(a) Schematic illustration of the formation of charge puddles and single-electron tunneling through the barriers. (b) Intensity plot of the differential conductance dI/dVb as a function of gate voltage Vg and (source-drain) bias voltage Vb for the “SET” device at 40 mK. As shown by the lines, the Coulomb diamond structure signifies single electron tunneling in this device. (c) For the “QD” device, the Vg -dependence of the plateaus is clearly shown in the intensity plot of dI/dVb as a function of Vg and Vb . (d) Family of dI/dVb vs. Vb curves for the “QD” device at various Vg from 1.9 V to 9.5 V. Here the curves are shifted vertically for clarity. The vertical scale bar is 1 μS. In the vicinity of zero-bias Vb ∼ 0, the peak position revealed a similar Coulomb diamond structure, a feature of quantum dots. A clear signature of quantum-level repulsion was observed at Vg = 7.6 V.
(a) The source-drain voltage drop (VDS ) against Vg curves for the “SET” device at a fixed bias current Ib . These curves show Coulomb oscillations and a change in the Coulomb gap at various magnetic fields. In a zero magnetic field, the oscillation period was 8.86 V, and the gap change was approximately 33 μV. In fields larger than μ 0 H = 0.4 T, a different oscillation with a period of 2.04 V (marked by the red arrows) was found. The large-period oscillation only survived up to μ 0 H = 0.4 T, but the smaller oscillation persisted up to μ 0 H = 5.75 T. (b) dI/dVb against Vb curves for the “QD” device in various magnetic fields of 0 T, 0.3 T, 0.5 T, and 0.6 T. The conduction peaks signifying the current steps were smeared out when μ 0 H was larger than 0.6 T. (c) 4-probe measurements on the metallic devices (MPA-II-9) shows a positive magnetoresistance (MR) of 0.2% in a magnetic field μ 0 H = 3 T at 100 mK. The resistance at μ 0 H = 0 was 107 Ω.
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