Understanding Electrical Conduction and Nanopore Formation During Controlled Breakdown

Abstract

ionic current therefore provides a simple single-molecule biosensing technique. [1][2][3][4] Indeed, over the past several decades, nanopores have proven to be versatile single-molecule sensing devices with applications ranging from DNA [5,6] and protein sequencing, [7,8] to ultra-dilute analyte detection, [9][10][11][12] polymer data storage, [13,14] and enzymology. [15] Nanopore sensors can be classified as either biological [16] or solid-state. [17] Biological nanopores generally consist of barrel shaped proteins that self-insert into lipid or synthetic membranes. Solid-state nanopores, however, are typically formed in thin (<50 nm) dielectrics such as SiN x , [18] TiO 2 , [19] and HfO 2 [20] or 2D materials such as graphene, [21][22][23] MoS 2 , [6] and hBN. [24] The ability to fabricate solid-state nanopores of different diameters and operate them in a wide range of environmental conditions makes them particularly attractive for many of the applications discussed above. [1,17] In the past, solid-state nanopores were typically fabricated using focused charged particle beams to locally sputter material from the membrane. [25][26][27][28] However, this requires specialized equipment, trained operators, and is a labor intensive process thus limiting the availability of this technique to the wider research community.To overcome these issues, a technique called controlled breakdown (CBD) has been developed to fabricate nanopores in solid-state membranes. [29][30][31] In this method, an electric Controlled breakdown has recently emerged as a highly appealing technique to fabricate solid-state nanopores for a wide range of biosensing applications. This technique relies on applying an electric field of approximately 0.4-1 V nm −1 across the membrane to induce a current, and eventually, breakdown of the dielectric. Although previous studies have performed controlled breakdown under a range of different conditions, the mechanism of conduction and breakdown has not been fully explored. Here, electrical conduction and nanopore formation in SiN x membranes during controlled breakdown is studied. It is demonstrated that for Si-rich SiN x , oxidation reactions that occur at the membrane-electrolyte interface limit conduction across the dielectric. However, for stoichiometric Si 3 N 4 the effect of oxidation reactions becomes relatively small and conduction is predominately limited by charge transport across the dielectric. Several important implications resulting from understanding this process are provided which will aid in further developing controlled breakdown in the coming years, particularly for extending this technique to integrate nanopores with on-chip nanostructures.

Fast Fabrication of Solid-State Nanopores for DNA Molecule Analysis

“…When the trans-membrane voltage reached 8 V or above, instantaneous breakdown events were observed, and the resulting pore size was larger than 15 nm. Previous studies have represented the mechanism of CDB as follows: (i) oxidation reactions of chloride ions (or reduction reactions of hydrogen ions) occur at the solution-membrane interface to supply (or remove) electrons; (ii) the electrons travel through charge traps in the SiN x membrane, which forms a highly conductive path and increases damage due to Joule heating [43,46,47]. Here, we measured the leakage current traces for the SiN x membrane before and after plasma treatment at a 600 mV applied voltage.…”

Localised solid-state nanopore fabrication via controlled breakdown using on-chip electrodes

“…2. Following breakdown, reservoirs on both sides of the membrane were filled with 3.6 M LiCl, 10 mM Tris, and 0.1 mM EDTA at pH 8 solution and the device was left overnight for the ionic current through the nanopore to stabilise [50,65]. Following this, a linear ionic current as a function of the applied voltage was measured (S1.10 in the ESM).…”

“…Another technique that has been demonstrated relies on illuminating a plasmonic nanostructure with a laser during CBD. This results in nanopore formation at the center of the plasmonic hotspot [50]. To date, this is the only CBD method that has been used to integrate nanopores with on-chip nanostructures.…”

Localised Solid-State Nanopore Fabrication via Controlled Breakdown using On-Chip Electrodes

“…Note that, due to the asymmetry of this device geometry, the conduction and breakdown characteristics will change significantly depending on the direction of the applied electric field (as explained in Ref. 49 ). The results shown here were obtained by applying a negative voltage to the on-chip electrodes and grounding the electrolyte solution.…”

“…understood and may be the result of increased electron transfer processes at the membrane-electrolyte interface or increased electron transport across the dielectric 49 . Our results nonetheless indicated that it may be possible to confine nanopore formation to a specific region by locally heating the membrane.…”