Addressable Direct-Write Nanoscale Filament Formation and Dissolution by Nanoparticle-Mediated Bipolar Electrochemistry

Abstract

Nanoscale conductive filaments, usually associated with resistive memory or memristor technology, may also be used for chemical sensing and nanophotonic applications; however, realistic implementation of the technology requires precise knowledge of the conditions that control the formation and dissolution of filaments. Here we describe and characterize an addressable direct-write nanoelectrochemical approach to achieve repeatable formation/dissolution of Ag filaments across a ∼100 nm poly(ethylene oxide) (PEO) film containing either Ag⁺ alone or Ag⁺ together with 50 nm Ag-nanoparticles acting as bipolar electrodes. Using a conductive AFM tip, formation occurs when the PEO film is subjected to a forward bias, and dissolution occurs under reverse bias. Formation-dissolution kinetics were studied for three film compositions: Ag|PEO-Ag⁺, Ag|poly(ethylene glycol) monolayer-PEO-Ag⁺, and Ag|poly(ethylene glycol) monolayer-PEO-Ag⁺/Ag-nanoparticle. Statistical analysis shows that the distribution of formation times exhibits Gaussian behavior, and the fastest average initial formation time occurs for the Ag|PEO-Ag⁺ system. In contrast, formation in the presence of Ag nanoparticles likely proceeds by a noncontact bipolar electrochemical mechanism, exhibiting the slowest initial filament formation. Dissolution times are log-normal for all three systems, and repeated reformation of filaments from previously formed structures is characterized by rapid regrowth. The direct-write bipolar electrochemical deposition/dissolution strategy developed here presents an approach to reconfigurable, noncontact in situ wiring of nanoparticle arrays-thereby enabling applications where actively controlled connectivity of nanoparticle arrays is used to manipulate nanoelectronic and nanophotonic behavior. The system further allows for facile manipulation of experimental conditions while simultaneously characterizing surface conditions and filament formation/dissolution kinetics.

Keywords: conductive AFM; conductive filament; formation−dissolution kinetics; resistive switching; solid polymer electrolyte.

Figure 1.

Single filament formation. (A) Schematic representation of the four phases of filament behavior; initial nucleation at C-AFM tip with a positive substrate bias (i), leading to filament growth (ii), which contacts the surface at time Δτ_f (iii). Under reverse, i.e., negative, substrate bias, dissolution occurs (iv). (B) An I–V characteristic of a single filament, starting at a substrate bias of −1 V ((1) OFF state) and sweeping to +1 V ((2) Formation), showing no measurable current until the filament is formed ((3) ON state). The filament remains stable under a small negative bias before resetting to a nonconductive state ((4) Dissolution). Curve acquired at 0.6 V s⁻¹, with a maximum readable current of ±600 nA. (C) Current vs time trace showing the time difference (Δτ_f) between application of a formation voltage and the resulting increase in current corresponding to filament formation. The junction is stable under an applied positive bias to the substrate, which is stepped down in −200 mV increments to −1 V, at which the filament remains conductive until dissolution occurs.

Figure 2.

Formation/dissolution time distributions for simple filaments (system I). (A) Formation (τ_f) and (B) dissolution time (τ_d) distributions for simple filaments in 0.5 wt % (40 nm thickness, green), 1 wt % (80 nm thickness, blue) and 2 wt % (130 nm thickness, red) films of PEO with formation bias V_substrate − V_tip = +0.6 V, and dissolution bias of −1.0 V. The solid lines are skewnormal on standard time (formation) and log time (dissolution) curve fits. (C) Formation (τ_f) and (D) dissolution (τ_d) time distributions for simple filaments in a 1 wt % (80 nm) PEO film with a formation bias of +0.6 V and a dissolution bias of −0.6 V. Insets to (B) and (D): The dissolution time distributions are clearly log-normal when plotted directly vs time.

Figure 3.

PDT-functionalized Ag-coated substrate after spin-coating a 1 wt % PEO layer (system II). (A) Plan-view SEM image. (B) FIB cross-sectional image of sample, showing glass substrate, Au, Ag, and PEO layers. A protective Pt layer was deposited on the PEO layer to preserve film integrity during the cross-sectioning process. (C) Formation (τ_f) and (D) dissolution (τ_d) time distributions.

Figure 4.

AgNP-functionalized sample after spin-coating 1 wt % PEO film (system III). (A) Plan-view SEM image showing in-plane distribution of nanoparticles. (B) Cross-sectional SEM image showing vertical placement of nanoparticles (white arrows overlaid) in the PEO film. (C) Formation (τ_f) and (D) dissolution (τ_d) time distributions together with multipeak fits (black line, overlay; individual peaks shown below). Peaks are numbered corresponding to formation condition: (1) shorted nanoparticle against tip or substrate, (2) formation between tip and substrate in area away from AgNP, and (3) formation between tip and substrate through AgNP acting as a bipolar electrode.

Figure 5.

Schematic representations of the filament formation conditions based on the vertical and horizontal position of the nanoparticle in the PEO film for system III. Arrow color gradients represent qualitative changes in electric potential within the film (quantitative simulation results given in Supporting Information). (A) AgNP shorted against the AFM tip or substrate. (B) Filament formation in the absence of a AgNP. The filament grows directly from the tip to the substrate as in system II. (C) The presence of a proximal nanoparticle deflects filament formation away from the nanoparticle, increasing τ_f. (D) With the tip positioned over a AgNP, the formation of two conductive filaments is required to produce a conductive pathway.

Figure 6.

Repeated formation and dissolution of single filaments (cyclic formation/dissolution). Plot of dissolution times, (τ_d in ln[ms]), of single filaments vs repetition number for (A) system I and (C) systems II and III. (B) and (D) show distributions of the data from panels (A) and (C), respectively, with Gaussian fit lines overlaid.