Ionic Current Rectification, Breakdown, and Switching in Heterogeneous Oxide Nanofluidic Devices

Abstract

We investigate several ion transport behaviors in sub-20 nm nanofluidic channels consisting of heterogeneous oxide materials. By utilizing distinct isoelectric points of SiO2 and Al2O3 surfaces and photolithography to define the charge distribution, nanofluidic channels containing positively and negatively charged surfaces are created to form an abrupt junction. This method provides much more robust surface charges than previous approaches by surface chemical treatment. The fabricated nanofluidic diodes exhibit high rectification of ion current and achieve record-high rectification factors (ratio of forward current to reverse current) of over 300. The current-voltage property of the device follows the theoretical model quantitatively, except that at low ion concentrations the forward current degrades and the reverse current is greater than theoretical prediction, which can be attributed to access resistance and breakdown of water molecules. The breakdown effect characterized by a negative conductance followed by a rapid increase of current is observed in a double junction diode. The occurrence of the breakdown is found to be enhanced by the abruptness of the junction between the heterogeneous nanochannels. Finally, we demonstrate ionic switching in a three-terminal nanofluidic triode in which the ionic flow can be electrically regulated between different channel branches. The study provides insight into the ion transport behavior in nanofluidic devices containing heterogeneous surfaces.

Robust Sandwich‐Structured Nanofluidic Diodes Modulating Ionic Transport for an Enhanced Electrochromic Performance

“…The sandwich‐structured nanofluidic diodes exhibited a pH‐tunable ion rectification property because the surface charges of three components were sensitive to the pH value of the electrolyte as discussed above 13. At low pH values, the exterior surface covered with WO 3 layer carried a little negative charges due to a weak deprotonation of the surface hydroxyls, while the inner walls of AAO nanopores and the exterior surface covered with NiO were both positively charged owing to the protonation of hydroxyls.…”

“…The three inorganic oxides that made up the membrane have distinct pI values 12. In a neutral aqueous solution, the surface of AAO nanoporous membrane (pI ≈ 8) is close to an electrical neutrality, while NiO (pI = 10–11) and WO 3 (pI = 0.2–0.5) carries positive and negative charges respectively, resulting from the protonation and deprotonation of surface hydroxyls 13. The two oppositely charged exterior surfaces are essential to realize ion rectification characteristic in this sandwich‐structured nanoporous membrane.…”

“…To evaluate the effect of ionic transport on the electrochromic performance, several systems were established on the basis of different electrolyte properties (aqueous or organic solution) and membrane structures (nanoporous or nonporous membrane). As shown in Figure 6 a, a linear I–V property indicated that the ion rectification behavior of the sandwich‐structured nanoporous membrane could not be obtained in the LiClO 4 PC electrolyte, owing to no charge generation on the surface of the membrane in an organic solution 13. In this case, the membrane demonstrated a reversible change in the absorbance (ΔA = 1.68) at the wavelength of 750 nm in response to the alternately applied redox potentials of +1 and −1.5 V (Figure 6b).…”

Toward Complementary Ionic Circuits: The npn Ion Bipolar Junction Transistor

“…Ideally, these ionic transistors should have a high ratio between the on and off current to minimize leakage, a fast switching speed, a high current gain, be possible to integrate into a larger system and they should also be functional at physiological conditions. Recently, nanofluidic transistors and diodes have received a lot of attention [4][5][6] . In these devices the surface charge along the walls of the thin channels modulates the conductivity through the device.…”

Sub-10 nm transparent all-around-gated ambipolar ionic field effect transistor

“…With a view point of the PNPS formulation without constraints, a positive gate voltage changed the polarity of the gated region only, and then, the nanochannel behaved as the npn nanofluidic transistor for negatively charged nanochannels. For the npn nanofluidic transistor, the application of a drain voltage could cause a reverse bias in one of the two pn junctions and hence the ionic current should be saturated, 39 leading to a unipolar behavior. Moreover, numerical results with negative gate voltage had non-negligible error compared to the experimental results.…”