🔬 condensed matter

Electronic access to glass transition in supercooled ionic liquids using ambipolar transistor

This paper demonstrates that an ambipolar PdSe $_2$ field-effect transistor can serve as an electrical probe to resolve ion-specific relaxation dynamics and infer rheological parameters of supercooled ionic liquids, enabling the characterization of glass transition phenomena and polymer confinement effects within solid-state device architectures where conventional rheometry is inapplicable.

Original authors: Tanima Kundu, Rahul Paramanik, Aishee Saha, Shibnath Mandal, Antarik Sarkar, Bipul Karmakar, Subhadeep Datta

Published 2026-06-17

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CC BY 4.0

Original authors: Tanima Kundu, Rahul Paramanik, Aishee Saha, Shibnath Mandal, Antarik Sarkar, Bipul Karmakar, Subhadeep Datta

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine you are trying to understand how a thick, sticky liquid (like honey or cold syrup) behaves as it gets colder and colder, eventually turning into a solid-like glass. Usually, to study this, scientists use big machines that physically stir or squeeze the liquid. But what if you only have a tiny drop of liquid, trapped inside a microscopic electronic chip? You can't stick a spoon in there. You can't squeeze it.

This paper introduces a clever new way to "feel" the liquid's stickiness and movement using electricity instead of mechanical force. Here is how they did it, explained simply:

The Setup: A Tiny Electronic Gate

The researchers built a special electronic switch (a transistor) made from a material called PdSe2. Think of this material as a two-way street for electricity; it can carry both positive and negative charges.

Instead of a normal battery, they used a drop of ionic liquid (a salt that is liquid at room temperature) as the "gate" to control the switch. When they applied a voltage, the ions in the liquid moved to the surface of the switch, creating a super-thin layer of charge that turned the switch on or off.

The Problem: The Liquid Gets "Lazy"

As they cooled this liquid down, the ions started to move slower and slower, like people in a crowded room trying to dance as the music slows to a crawl. Eventually, they get stuck, and the liquid turns into a "glass" (a solid that looks like a liquid).

In a normal lab, you'd need a lot of liquid to measure this "stiffening." But here, they had a tiny drop. So, they used the electronic switch as a sensor.

The Experiment: The "Hysteresis" Loop

They ran a test where they turned the voltage up and down quickly.

At warm temperatures: The ions were fast and energetic. They could keep up with the voltage changes instantly. The electronic signal went up and down in a perfect, tight loop.
As it got colder: The ions got sluggish. They couldn't keep up with the voltage changes. The electronic signal started to lag behind, creating a wide, stretched-out loop (called hysteresis).

The width of this loop told them exactly how "stuck" the ions were. A wider loop meant the liquid was getting much more viscous (thicker).

The Special Trick: Sorting the Ions

This ionic liquid has two types of ions: small, fast ones (like EMIM+) and big, bulky ones (like TFSI−).
Because their electronic switch is "ambipolar" (it can handle both positive and negative traffic), they could test the ions separately:

When they pushed the voltage one way, the small ions rushed to the surface.
When they pushed it the other way, the big, bulky ions had to move.

They found that the big ions were much slower to relax (get back to normal) than the small ones. This allowed them to measure the specific "personality" of each ion type as the liquid cooled.

The "Glass" Moment

They discovered that as the liquid cooled, it didn't just get slowly thicker. Instead, the liquid started to break apart into tiny, isolated islands of movement surrounded by a rigid, frozen matrix.

The Analogy: Imagine a busy highway where cars are moving freely. As it gets colder, traffic jams form. Eventually, the highway breaks into isolated pockets of moving cars surrounded by a gridlock of frozen cars. The researchers could measure exactly when the "moving pockets" became too small to connect to each other.

They defined a specific temperature where the liquid effectively stopped behaving like a liquid and started acting like a glass. They called this the point where the "mobile fraction" of the liquid collapsed.

The Polymer Test

To prove their method was sensitive, they mixed the liquid with a polymer (a plastic-like material) to make a "ionogel." This is like putting the liquid inside a sponge.

The Result: The polymer acted like a cage, making it even harder for the ions to move.
The Finding: Their electronic sensor immediately detected that the liquid turned into a glass at a higher temperature than the pure liquid. It showed that the "cage" made the ions freeze sooner.

The Bottom Line

The researchers successfully turned a tiny electronic chip into a super-sensitive "electronic finger" that can feel the viscosity and glassy behavior of a liquid in a space too small for any traditional machine. They didn't just measure that it got cold; they measured how the ions slowed down, how they broke into fractal clusters, and how different ions behaved differently, all without ever touching the liquid with a mechanical tool.

Technical Summary: Electronic Access to Glass Transition in Supercooled Ionic Liquids using Ambipolar Transistor

Problem Statement
Characterizing the relaxation dynamics and viscosity of supercooled liquids as they approach the glass transition remains a central challenge in integrated electronic architectures. Conventional rheometric techniques (e.g., rotational shear, oscillatory methods) require milliliter-scale volumes and mechanical stability, rendering them incompatible with strong confinement, thin films, nanofluidic geometries, or solid-state device environments. This experimental gap prevents the direct probing of microscopic dynamics in the tiny volumes relevant to modern nanoelectronics and soft condensed matter physics.

Methodology
The authors demonstrate a method to transduce ion dynamics directly into electronic transport using an ambipolar Palladium Diselenide (PdSe2) field-effect transistor (FET) gated by an ionic liquid.

Device Architecture: Multilayered PdSe2 flakes were exfoliated onto SiO2/Si substrates. The devices were gated using either a droplet of the prototypical ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMIM-TFSI) or a polymer-confined ionogel (EMIM-TFSI mixed with PVDF-HFP).
Measurement Protocol: The study employs a combined static and dynamic measurement approach:
1. Static Hysteresis: Transfer curves ( $I_{ds}$ vs. $V_{ig}$ ) are swept at various temperatures. The hysteresis width ( $\Delta V_H$ ) reflects the delay in ionic relaxation relative to the gate sweep rate.
2. Dynamic Transients: Gate-voltage pulses (steps from 0 to $\pm$ 2.5/3 V) are applied, and the drain current transients ( $I_{ds}(t)$ ) are recorded.
Data Analysis: The current transients are fitted to the Kohlrausch-Williams-Watts (KWW) stretched-exponential function to extract the relaxation time ( $\tau$ ) and the stretch exponent ( $\beta$ ). The ambipolar nature of PdSe2 allows for polarity-selective gating, enabling the independent resolution of cation (EMIM $^+$ ) and anion (TFSI $^-$ ) relaxation kinetics.
Viscosity Estimation: Viscosity ( $\eta$ ) is semiquantitatively inferred by combining the extracted relaxation times with a diffusion estimate derived from Fick's law and the classical Stokes-Einstein relation.

Key Results

Ion-Specific Dynamics: The transistor successfully resolves distinct relaxation kinetics for EMIM $^+$ and TFSI $^-$ . The smaller cation (EMIM $^+$ ) exhibits faster relaxation ( $\tau_{EMIM^+} < \tau_{TFSI^-}$ ) due to lower steric hindrance and a smaller hydrodynamic radius.
Non-Arrhenius Behavior: The temperature dependence of the relaxation time for both ions deviates from Arrhenius behavior and follows the Vogel-Fulcher-Tammann (VFT) scaling, a hallmark of fragile glass-forming systems.
- For pure EMIM-TFSI, the fragility index ( $m$ ) is calculated as 98 (EMIM $^+$ ) and 142 (TFSI $^-$ ), firmly placing the liquid in the "fragile" regime.
- The relaxation time grows from $\sim$ 1 s near room temperature to $>100$ s as the temperature approaches 200 K.
Ergodicity Loss and Percolation: The authors introduce an electrically accessible quantity, $p_{eq}(T)$ $p_{e q} (T)$ , representing the fraction of the ionic medium that equilibrates within the experimental timescale (20 s).
- As temperature decreases, $p_{eq}$ collapses sharply, indicating the fragmentation of mobile liquid regions into percolating fractal clusters.
- The characteristic temperature ( $T_{eq}$ ) where $p_{eq} = 0.5$ is identified as $\sim$ 213 K for the pure liquid and $\sim$ 231 K for the ionogel.
Effect of Polymer Confinement: In the PVDF-HFP confined ionogel, the characteristic temperatures shift upward by 15–30 K, and the extracted viscosity increases by nearly two orders of magnitude at 280 K compared to the neat liquid. The ionogel exhibits a lower fragility index ( $m \approx 73$ ), suggesting a transition toward "stronger" glass-forming behavior due to reduced free volume and restricted cooperative mobility.

Significance and Claims
The paper claims to establish an ambipolar PdSe2 transistor as a device-integrated platform capable of probing the rheology and glass physics of supercooled ionic media where conventional rheometry is inapplicable.

Scalability: The method enables rheological characterization at the picoliter scale within solid-state architectures.
Sensitivity: The approach is sensitive enough to detect structural constraints imposed by polymer matrices and to resolve ion-specific relaxation dynamics.
Physical Insight: By extracting VFT parameters and tracking the collapse of equilibrated regions ( $p_{eq}$ ), the method provides a direct route to quantify the ergodic-to-nonergodic crossover and the depletion of configurational entropy in nanoscale environments.

The study concludes that this electrical probing technique offers a viable alternative for characterizing supercooled liquids in functional electronic devices, bridging the gap between macroscopic rheology and microscopic device physics.