Multiplicity of atomic reconfigurations in an electrochemical Pb single-atom transistor

F.-Q. Xie, X.-H. Lin, A. Gross, F. Evers, F. Pauly, and Th. Schimmel

Phys. Rev. B 95, 195415 – Published 15 May 2017

Abstract

One focus of nanoelectronics research is to exploit the physical limits in size and energy efficiency. Here, we demonstrate a device in the form of a fully metallic atomic-scale transistor based on a lead (Pb) single-atom quantum point contact. The atomic configuration of the point contact determines the conductance of the Pb atomic-scale transistor. The conductance multiplicity of the Pb single-atom transistor has been confirmed by performing switching between an electrically nonconducting “off-state” and conducting “on-states” at $1 G_{0} (G_{0} = 2 e^{2} / h$ , where $e$ is the electron charge, and $h$ Planck's constant), $2.0 G_{0}$ , $3.0 G_{0}$ , $1.5 G_{0}$ , $2.4 G_{0}$ , $2.7 G_{0}$ , $2.8 G_{0}$ , and $5.4 G_{0}$ , respectively. Our density-functional calculations for various ideal Pb single-atom contacts explain the atomic-configuration-related conductance multiplicity of the Pb single-atom transistor. The performance of the Pb single-atom transistors indicates that both the signatures of atomic valence and conductance quantization play roles in electron transport and bistable reconfiguration. The bistable reconfiguration of the electrode tips is an underlying mechanism in the switching of the Pb atomic-scale transistors. The absolute value of the electrochemical potential applied to the gate electrode is less than 30 mV. This merit suggests Pb [besides silver (Ag)] atomic-scale transistors as potential candidates for the development of electronic circuits with low power consumption. The dimension of the switching unit in the Pb single-atom transistor is in the range of 1 nm, which is much smaller than the projected scaling limit of the gate lengths in silicon transistors (5 nm). Therefore, the metallic single-atom transistors may provide perspectives for electronic applications beyond silicon.

Received 23 December 2016
Revised 12 April 2017

DOI:https://doi.org/10.1103/PhysRevB.95.195415

Physics Subject Headings (PhySH)

Ballistic transport Electrochemistry Metals

Condensed Matter, Materials & Applied PhysicsInterdisciplinary Physics

Authors & Affiliations

F.-Q. Xie^1,*, X.-H. Lin², A. Gross², F. Evers³, F. Pauly⁴, and Th. Schimmel^1,5,6,†

¹Institute of Applied Physics, Karlsruhe Institute of Technology, Campus South, D-76128 Karlsruhe, Germany
²Institute of Theoretical Chemistry, Ulm University, D-89069 Ulm, Germany
³Institute I-Theoretical Physics, University of Regensburg, D-93040 Regensburg, Germany
⁴Department of Physics, University of Konstanz, D-78464 Konstanz, Germany
⁵Institute of Nanotechnology, Karlsruhe Institute of Technology, Campus North, D-76021 Karlsruhe, Germany
⁶Herbert Gleiter Institute of Nanoscience, Nanjing University of Science and Technology, 210094 Nanjing, China

^*fangqing.xie@kit.edu
^†thomas.schimmel@kit.edu

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Issue

Vol. 95, Iss. 19 — 15 May 2017

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Images

Figure 1
Scheme of the experimental setup to fabricate Pb atomic-scale transistors, surface analysis, and the model of a single-atom transistor. (a) Pb is deposited and dissolved electrochemically in the narrow gap between two gold working electrodes on a glass substrate, and the conductance of the contact is measured simultaneously. The potentials of the working electrodes with respect to the lead reference electrode and the lead counter electrode are set by a potentiostat. (b) Confocal optical microscope image of as-prepared Au electrodes. The Au electrodes were insulated with a polymer film except for a shining triangle range for the electrochemical deposition of Pb. A schematic diagram of two Pb microcrystallines in gray is placed across the gap between the two gold electrodes to illustrate a point-contact formation. (c) Scanning electron microscopy image of a shadow-sputtered gap. (d) Scanning electron microscopy image of deposited Pb microcrystals on the Au electrodes. (e) Schematic model of a single-atom transistor.
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Figure 2
Quantum conductance switching sequences of lead single-atom transistors. The transistors were controlled by an electrochemical potential applied to the gate electrode. By variation of the gate potential, a digital switching of the single-atom transistor was implemented between a nonconducting “off-state” and a quantized conducting “on-state” at (a) $1 G_{0}$ , (b) $2 G_{0}$ , and (c) $3 G_{0} (G_{0} = 2 e^{2} / h)$ , respectively. The upper graph (blue) in each panel gives the electrochemical potential (“gate voltage”) as a function of time, while the lower graph (red) is the simultaneously recorded source-drain conductance of the single-atom transistor on the same time scale.
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Figure 3
Conductance multiplicity of the lead single-atom transistor. These graphs (left) illustrate that the Pb single-atom transistor can implement quantum conductance switching between an “off-state” and “on-state” at (a) $1.5 G_{0}$ , (b) $2.4 G_{0}$ , (c) $2.7 G_{0}$ , (d) $2.8 G_{0}$ , and (e) $5.4 G_{0}$ , respectively. The conductance is calculated using density-functional theory for various single-atom and dimer contacts oriented along the main crystallographic directions $〈 100 〉, 〈 110 〉$ , and $〈 111 〉$ (right). The calculated conductance values are listed beside the atomic geometries. Because the dimer contact in the direction of $〈 111 〉$ has the same conductance as its single-atom contact partner, its atomic configuration is not displayed here. Each graph shows nine switching cycles. The conductance is plotted on the same scale in the five graphs, while the axes of conductance are labeled in different length units in each panel for the convenience of plotting adapted grid lines. Time axes are scaled to meet the length of the individual switching sequences.
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Figure 4
Demonstration of switching and state-hold in two lead atomic-scale transistors. The transistors switched between the “off-state” and quantized “on-states.” These on-states are (a) $7 G_{0}$ and (b) $10 G_{0}$ . The quantum conductance switching (red) of the atomic-scale transistors, shown in panels (a) and (b), is directly controlled by the gate electrode potential (blue). These graphs also demonstrate that a conductance level can be kept stable if the gate potential is kept on a “hold” level (see arrows).
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Figure 5
Detailed comparison of two switching sequences, (a) 0– $2.7 G_{0}$ and (b) 0– $10 G_{0}$ . The data of the two sequences was recorded at a sampling rate of 50 000 1/s. Graphs (a) and (b) show only five cycles for each switching, the small empty circles indicate measured data, and the lines are guidance for the eye. Graphs (c), (d), (e), and (f) display closing and opening events of the third cycle in each sequence, respectively. The conductance histogram for each switching sequence of 68 cycles in total is presented in (g) and (h), respectively. The full width at half maximum is 0. $06 G_{0}$ around $2.67 G_{0}$ , and 0. $30 G_{0}$ around 10. $13 G_{0}$ . For the histograms, the fast-recorded conductance data was collected in bin sizes of 0. $02 G_{0}$ .
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Figure 6
Demonstration of rectangular-wave-driven switching of a Pb atomic-scale transistor between 0 and $14 G_{0}$ . The rectangular gate voltage wave was generated by an NI LabVIEW signal generation virtual instrument. The experiment was performed on a silicon wafer with a pair of Au electrodes separated by a gap of 2 µm and covered with a layer of photoresist SU-8 that is 2 µm thick. The optical microscope image is shown in panel (a). A rectangular window is opened in the SU-8 film over the Au electrodes with dimensions of $200 \times 32 μ m^{2}$ . The window is used for electrochemical deposition-dissolution. A point contact is formed across the gap in the window. In panel (b), the upper graph (blue) displays a rectangular switching sequence of the electrochemical potential applied to the gate electrode. The parameters characterizing the sequence are an amplitude of 50 mV with a shift of −5 mV (−55 mV to 45 mV), a frequency of 2 Hz, and a 75% duty cycle. The lower graph (red) shows the conductance switching between 0 and $14 G_{0}$ following the rectangular wave of the electrochemical potential with a delay of around 0.17 s.
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