Electrical conductivity of traditional materials such as metals is relatively independent of current flow or applied voltage. On the other hand, novel applications arise from voltage- (or current-) dependent conduction, enabling the semiconductor industry to flourish (the interesting material behavior stems from doping) for instance. Recently, a new approach to tailor the electrical response of a material has been gaining popularity that relies on conductivity modification of transition metal oxides through the formation of a substrate-electrolyte interface, referred to as electrolyte gating.

Upon electrolyte gating, not only does conductivity increase by orders of magnitude, but its temperature dependence also changes qualitatively—in other words an insulator-to-metal transition is observed. Such alternations have often been ascribed to interfacial effects, for example, the presence of an electric double layer; however, such an explanation does not necessarily account for all the observations. In a recent study, Ivan Božović of Brookhaven National Laboratory and Yale University and colleagues identified a new mechanism to explain the observed insulator-to-metal transition upon electrolyte gating. The study is published in npj Quantum Materials.

The researchers examined the alteration in electrical conductivity of thin tungsten oxide (WO₃) films subjected to electrolyte gating using two different electrolytes: sodium fluoride salt in polyethylene glycol (PEG-NaF)—a polymer electrolyte, and diethylmethyl (2-methoxyethyl) ammonium bis (trifluoromethylsulfonyl) imide (DEME-TFSI)—an ionic liquid. The sample resistivity—seen in (b) in the Figure—was found to have strong dependence on applied voltage as well as temperature for different sample sizes. The resistivity versus temperature trends also change sign as the gate voltage is increased, exhibiting a clear insulator-to-metal transition with resistivity dropping by 5-6 orders of magnitude when the gate voltage is increased from 0 V to 2 V.

To identify whether the changes in electrical transport behavior were due to surface or bulk effects, different sized WO₃ films were studied (thicknesses of 10 nm, 30 nm, 50 nm, and 70 nm). The conductivity was found to be independent of film thickness, suggesting that the behavioral change results from a bulk effect rather than a surface-dominated phenomenon. All of these experiments were carried out in operando.

Often the formation of a solid-electrolyte interface leads to intercalation of some species that changes electrical properties; for example, Li-ion batteries work based on this intercalation reaction. In this study, at equilibrium, the concentration of intercalated species was uniform throughout, thus affecting bulk changes. The first logical contender for intercalation is the cation present in the salt; negative voltage is applied to the sample–see Figure (a), hence cations are attracted toward the substrate. The researchers carried out experiments with just the electrolyte solvent (no salt) and the results were very close to that with salt, thus demonstrating that salt ions do not intercalate.

Next, since the experiments were performed in air, the researchers examined whether oxygen in air could be intercalating. Equivalent tests in vacuum, ozone, and ambient atmosphere produced similar results, thus suggesting that the intercalating species must be something else. X-ray diffraction, Hall Effect, and Fourier transform infrared spectroscopy operando experiments on gated films surmised that the intercalated ions are positively charged and very small. Also, explicitly H₂-annealed samples were found to exhibit very similar trends (both quantitatively and qualitatively), thus leading the research team to deduce that hydrogen intercalation was the cause of the observed behavior.

Shriram Ramanathan from Purdue University, who was not associated with this study, says that the work provides an interesting new insight into the properties of WO₃ which is a highly tunable system. He is hopeful that this result will further motivate researchers to closely look at chemical mechanisms that govern modification of emerging oxide semiconductors and application of novel approaches to characterize advanced materials in operando.

Electrolyte gating thus appears to be an interesting approach to tune electrical properties of metal oxides and transition metal oxides. Depending on the choice of the substrate-electrolyte pair, the conductivity enhancement results from bulk (e.g., intercalation) or surface (e.g., double layer) effects or a combination of both. An accurate characterization of the relative importance of these two phenomena would facilitate engineering solutions and innovative applications.

Read the article in npj Quantum Materials.