The mobile ions in an electrolyte, in the form of room-temperature ionic liquid (IL) or ionic gel composed of IL suspended in a matrix of PMMA-based copolymer, are used to gate (accumulate or deplete) low-dimensional electron systems via the electric field effect. Sheet carrier density swings up to 10^15 cm-2 can be induced, and hence electrolyte gating can be used to study systems where the relevant electron/hole density scales are significant fractions of the Brillouin zone volume (especially transition metal oxide interfaces and thin films, and even normal metal films). By comparison, typical gate dielectrics can sustain electric fields that affect carrier density changes up to 10^13 cm-2.
Ionic gels formed by a matrix of PS-PMMA-PS copolymer in ionic liquids (e.g., EMI-TFSA) are used to gate the bare surface of undoped SrTiO3 (STO), a semiconducting perovskite oxide. Accumulating a high concentration of cations over the crystal surface causes electronic reconstruction, forming a two-dimensional and highly-mobile system of electrons direclty beneath the surface, residing on the Ti sites and partially filling the Ti 3d conduction band. Carrier density swings on the order of 10^14 cm-2 electrons can be induced by tuning the gate voltage within the electrochemical window of the electrolyte. The strong electric fields of the mobile ions in the electrolyte bear close resemblance to the "polar catastrophe" caused by the polarity of lanthanum aluminate (LAO) overlayer in LAO/STO heterostructures. Low-temperature, high-field magnetotransport measurements show that the Kondo effect due to the magnetism of +3-coordinated Ti ions is the dominant scattering mechanism at high electron density [Ref 1].
Figure taken from [Ref 2], showing the analogy between the LAO/STO interface and electrolyte-gated STO.
Ionic liquid gating of strontium titanate nanostructures
We aim to study the superconducting properties of strontium titanate interfaces at the nanoscale using ionic liquid gating in conjunction with nanostructured metal gates. We are interested in the following questions: What is the phase diagram in small devices, and how does it compare to bulk data? What information about the nature of the transport can we extract by measuring mesoscopic effects?
SEM micrograph of a 1 μm long by 300 nm wide hall bar of ionic liquid gated STO; Differential resistance of hall bar as a function of DC bias current, showing 7 nA critical current; Universal conductance fluctuations of resistance.
Using nanopatterned gates, we can define a 1 μm long by 300 nm wide hall bar. This allows us to both study superconductivity at the nanoscale in STO and access the mesoscopic transport regime. Furthermore, these gates can be used to modulate the Fermi level and effective width of the Hall bar by changing the gate voltages. This device shows a clear superconducting feature at 12 mK, with a critical current of around 7 nA. The critical current feature is strongly suppressed as a function of temperature, disappearing around 200 mK. The density of electrons can be tuned between 1-2x10^13 cm-2, dramatically changing superconducting features. The conductance as a function of magnetic field shows reproducible fluctuations, called universal conductance fluctuations, which occur when the phase coherence length is comparable to the size of the nanostructure. The magnitude of the fluctuations is ~0.05 e^2/h, and decreases with temperature as expected. We still see fluctuations at temperatures up to 500mK.
In-situ x-ray studies
Understanding how the ions in the electrolyte respond to applied potential is key to maximizing the interfacial capacitance and achieving the highest carrier densities. Since the ions also scatter the carriers in the SrTiO3, understanding how the ions order on the surface is important to maximize carrier mobility.
By using a bright source of synchrotron x-rays at the Stanford Synchrotron Radiation Lightsource (just up the hill from Stanford), we are able to measure the distribution of ions above the SrTiO3 surface in the ionic liquid BMPY-FAP. Surprisingly, the ions form alternating positively and negatively charged layers on the surface, and they rotate when a bias is applied.
Formation of layered structure in BMPY-FAP. Reflectivity. Symbols are data and lines are best fit models. The extracted charge density shows cation enrichment in the first layer at positive bias. The increase in layer spacing at positive bias suggests that the long cation axis has rotated perpendicular to the surface.
We have shown that, in addition to electrostatic effects, electrochemistry can play a role in changing the electrical properties of materials. For example, we used x-ray absorption spectroscopy to show that in thin gold films the relatively large change in carrier density observed by other groups using electrolyte gating is caused by reversible oxidation of the gold surface.
Formation of Au2O3 surface layer at negative gate voltage. (a) Sheet resistance and (b) carrier density as a function of applied bias to the electrolyte (DEME-BF4). (c) Normalized absorption coefficient versus incident x-ray energy (XANES) at -2 V, 0V, and 2V and (d) difference between 0 V spectrum and other spectra. The large changes in resistance and carrier density at -2 V coincide with the appearance of Au2O3 features in the spectrum. (e), (f) The same measurement at higher grazing angle (with calculated penetration depth of 150 nm) shows no change as a function of gate voltage, indicating that the oxidation is a surface effect.
- Menyoung Lee, J. R. Williams, Sipei Zhang, C. Daniel Frisbie, and D. Goldhaber-Gordon, "Electrolyte Gate-Controlled Kondo Effect in SrTiO3," Physical Review Letters 107, 256601 (2011) [See Ref. 2, the accompanying Physics Viewpoint]. Supplementary info.
- Johann Kroha, "Tuning Correlations in a 2D Electron Liquid," Physics 4, 106 (2011).
- Trevor Petach, M. Lee, R. C. Davis, A. Mehta, and D. Goldhaber-Gordon. "Mechanism for the Large Conductance Modulation in Electrolyte-gated Thin Gold Films," Physical Review B, 90(8), 081108 (2014).
Trevor Petach, A. Mehta, R. Marks, B. Johnson, M. F. Toney, and D. Goldhaber-Gordon. "Voltage-Controlled Interfacial Layering in an Ionic Liquid on SrTiO3," ACS Nano, 10(4), 4565-4569 (2016).