Two-Dimensional Insulator–Metal Transition in Electrolyte-Gated Indium Oxide Thin-Film Transistors Fabricated by Aerosol Jet Printing, as Published in Adv. Electron. Mater.
Product Introduction
Equipment Parameters
I. Abstract of the Paper’s Core Research
This study develops an indium chloride precursor ink tailored for aerosol jet printing, which is paired with a printable ionic‑gel electrolyte to fabricate layered aerosol‑jet‑printed bottom-contact, top‑gate In₂O₃ field‑effect transistors. By employing room‑temperature electrical measurements, Hall effect tests, and 4 K ultra‑low‑temperature transport characterization, we comprehensively elucidate the two‑dimensional carrier transport behavior. The key findings are as follows:
For aerosols only Indium oxide precursor ink system
A precursor solution prepared with acetonitrile as the primary solvent and ethylene glycol as a co‑solvent exhibits stable aerosolization without agglomeration. Subsequent air annealing at 400 °C yields a 30 nm‑thick polycrystalline indium oxide film with a surface roughness of only 0.6 nm, free of pinhole defects, and featuring X‑ray diffraction peaks that match the standard cubic phase of In₂O₃.
Multilayer Integrated Aerosol Printing Process Flow
The In₂O₃ semiconductor channel, the ionic‑gel gate dielectric, and the PEDOT:PSS gate electrode are sequentially fabricated without requiring multiple substrate transfers. The ionic gel—comprising an [EMI][TFSI] salt and a block copolymer—exhibits an interfacial capacitance as high as 100 μF·cm⁻², significantly enhancing carrier‑modulation capability.
Excellent room-temperature electrical properties of transistors
The device exhibits a subthreshold swing of 75 mV/decade, an on–off ratio as high as 10⁶, and a gate leakage current at the nA level. Hall measurements confirm a maximum carrier sheet density of 7 × 10¹⁴ cm⁻² and a Hall mobility of 11 cm²·V⁻¹·s⁻¹, with performance comparable to that of vacuum-deposited indium oxide devices.
Solution-Printed Oxides: First Observation of a Two-Dimensional Insulator - Metallic transformation
Low-temperature electrical measurements at 4 K reveal that, at a critical carrier concentration of 2.06 × 10¹⁴ cm⁻², the film resistance crosses the quantum resistance (h/e² ≈ 25.9 kΩ), and the transport mechanism transitions from localized hopping to metal-like conduction—representing a novel physical discovery in aerosol-printed metal oxide systems.
Electrostatic gate-controlled redox without side reactions
The device switching is governed by electrostatic control of the interfacial double layer, with no oxygen‑vacancy‑mediated redox reactions; it exhibits negligible electrical hysteresis and excellent reversibility over multiple scan cycles.
II. Complete Process Flow for All-Printed Indium Oxide Transistors Using Aerosol Jet Printing
1) Ink formulation: A 0.02 M anhydrous indium(III) chloride–acetonitrile solution, with 10 vol% ethylene glycol added to enhance atomization and spreading; the ionic‑gel ink is composed of an ionic liquid, a PS‑PMMA block copolymer, and ethyl acetate.
2) Substrate pretreatment: Pre‑photolithographic Cr/Au source and drain electrodes; SiO₂‑coated silicon wafers undergo air‑plasma hydrophilic activation.
3) Optomec aerosol‑jet equipment uses a 150 μm nozzle to print In₂O₃ channels, with a carrier gas flow of 10 sccm and a sheath gas flow of 40 sccm; the substrate is maintained at a constant temperature of 60°C to accelerate drying.
4) Annealing at 400°C for 1 hour yields a dense polycrystalline indium oxide thin film.
5) Layer-by-layer printing of the ionic gel gate dielectric and the PEDOT:PSS gate electrode to fabricate a top-gate device.
6) Electrical and transport characterization is performed using a room-temperature probe station and a 9-T superconducting magnet–based low-temperature Hall system.
III. Core Process Advantages of Aerosol Jet Printing (Comparison with Spin Coating, Inkjet, and Vacuum Sputtering)
Maskless Digital Microchannel Customization
CAD enables freehand design of 200-μm‑scale precision transistor channels; spin coating cannot achieve patterning, and photolithography is costly, making this approach well suited for rapid iteration across multiple device specifications.
In-situ layer-by-layer printing of multilayer heterogeneous materials
A single device enables the continuous deposition of three functional layers—semiconductor, ionic gel, and conductive gate—eliminating the need for multiple evaporation steps and substrate transfers, thereby simplifying the fabrication process for multilayer devices.
Non-contact, gentle deposition compatible with flexible substrates.
Adjustable nozzle-to-substrate distance of 1–5 mm, low printing temperature, and compatibility with PI high‑temperature‑resistant flexible films after 400°C annealing, enabling expanded applications in flexible transparent electronics manufacturing.
The ink has a broad compatibility range, and the precursor solution exhibits stable atomization.
It can handle metal‑salt precursor systems, overcoming inkjet viscosity limitations; the resulting thin films exhibit uniformity and low surface roughness, with no large‑area pinholes.
Consumable utilization has improved significantly.
Ink is deposited only in the device’s functional areas, while spin coating and vacuum deposition are applied to the entire wafer, resulting in substantial material waste; this also reduces the cost of using precious and rare metals such as indium.
IV. Five Core Technological Innovations of This Study
Innovation 1: A complete process for aerosol‑direct‑write all‑electrolyte‑gate indium oxide transistors
An In₂O₃ semiconductor–ionic gel multilayer EGT device was fabricated via additive manufacturing, eliminating reliance on vacuum thin-film deposition equipment. Lai.
Innovation 2: Ultra-High-Capacitance Printable Ionic Gel Gate Dielectric Formulation
Block copolymer–crosslinked ionic liquid inks can stabilize aerosol atomization, with interfacial capacitances exceeding 100 μF, enabling ultra-low gate‑voltage control below 1 V.
Innovation: Two-dimensional insulator–metal transition observed for the first time in a solution-processed oxide system.
Leveraging the ultra‑high carrier injection capability of the electrolyte gate, we have realized a two‑dimensional transport phase transition in aerosol‑grown polycrystalline In₂O₃ thin films, thereby expanding the platform for fundamental research on printed semiconductors.
Innovation 4: Electrostatic Redox-Gate Control Mechanism Without Oxidation
By relying solely on the electrostatic interactions of the interface double layer to modulate charge carriers, the formation of oxygen vacancies in the oxide is suppressed, resulting in reduced hysteresis and a significant improvement in cycling stability.
Innovative 5-Hall Effect Quantitative Characterization of Two-Dimensional Carrier Transport in Printed Structures
A gate-voltage‑dependent Hall measurement scheme has been established to directly quantify carrier concentration and mobility, thereby precisely elucidating the semimetallic transport mechanisms under high carrier densities.
V. Current Industry Challenges and Corresponding Solutions
Challenge 1: Vacuum-based indium oxide deposition equipment is expensive, spin coating cannot achieve patterning, and inkjet printing produces rough lines.
✅ Maskless micrometer-scale direct writing of aerosols, with high pattern resolution and equipment costs far lower than those of sputtering or MBE systems.
Challenge 2: Conventional gate‑dielectric capacitors have low capacitance, requiring transistor drive voltages as high as several tens of volts.
✅ Printable ionic gel forms a nano‑double layer, with an interfacial capacitance of 100 μF·cm⁻², and the device can be fully switched on and off at just 0.8 V.
Challenge 3: In solution‑processed oxide transistors, the carrier concentration is low, resulting in weak on‑state current.
✅ The electrolyte gate can inject an ultra-high surface carrier density of 7×10¹⁴ cm⁻², significantly enhancing the on-state current.
Challenge 4: Most oxide‑based gate control is accompanied by oxygen‑vacancy‑induced redox processes, leading to significant device hysteresis and poor reproducibility.
✅ Pure electrostatic control mechanism, no lattice‑level redox reactions, and highly reproducible cyclic voltammograms across multiple scans.
Challenge 5: Conventional printed oxides struggle to enable two-dimensional electronic transport physics studies.
✅ A uniform, ultrathin In₂O₃ thin film combined with a high‑capacitance ionic gate enables stable triggering of the two‑dimensional insulator–metal phase transition, thereby establishing a low‑cost, low‑temperature transport experimental platform.
VI. Future Diversified Industrial Application Scenarios
1 Low-Power Flexible Transparent Display Backplane
Aerosol‑based batch printing of In₂O₃ transistor arrays, featuring ultra‑low drive voltage and compatibility with thin, flexible displays.
2 Miniature Wearable Physiological Sensing Chip
A low-power switch array on a flexible substrate, designed for the front-end circuitry of electrocardiogram and sweat-signal acquisition.
3 Room-Temperature Two-Dimensional Electron Physics Research Platform
Low-cost aerosol-based fabrication of oxide two-dimensional transport devices, serving as an alternative to expensive vacuum epitaxy equipment.
4 Micro‑MEMS Pressure/Optoelectronic Sensing and Actuation Unit
Small-batch customized microtransistors, tailored for lightweight manufacturing in miniature sensing systems.
5 Large-area printed electronics mass-production R&D line
Roll-to-roll–compatible aerosol-based process enables rapid prototyping of large‑size transparent circuits, shortening the product development cycle.
Image and Text Guide
Schematic (top) and optical micrograph (bottom) of a bottom-contact top-gate van der Pauw device: On a Si/SiO₂ substrate, Cr/Au source–drain electrodes (5/30 nm) are first defined by photolithography, followed by printing an In₂O₃ semiconductor channel (200 × 200 μm) using Optomec AJP. Above this, an [EMI][TFSI]+PS‑PMMA‑PS ionic gel gate dielectric and a PEDOT:PSS gate electrode are sequentially deposited. The four probe contacts are labeled 1–4.
b is the wide-angle XRD (Cu Kα) pattern of a 30-nm-thick printed In₂O₃ thin film: the four main peaks corresponding to the cubic phase of In₂O₃ (Ia-3, a = 10.117 Å)—211, 222, 400, and 440—are clearly resolved. The full width at half maximum of the 222 peak yields an estimated average vertical crystallite size of approximately 10 nm.
c is the 2×2 μm AFM height map of the same sample: the thin film is dense with no pinholes, exhibiting a nanoparticle morphology, and its surface roughness is only 0.6 nm.
a shows the ID–VG transfer curves (filled symbols) and the absolute gate current |IG| (open triangles) measured across the source–drain electrode pairs 12/13/14 at VD = 0.1 V and a scan rate of 50 mV·s⁻¹: the device turns on at VG ≈ −0.4 V, with a subthreshold swing S = 75 mV·dec⁻¹ and an ON/OFF ratio of 10⁶; at VG = 1 V, ID exceeds 10 μA, while the gate–drain leakage current is only about 3 nA. The curves for the different electrode pairs overlap closely, demonstrating that the film is uniform and isotropic.
b shows the ID–VD output characteristics of the same device at VG = 0.1–0.8 V (in 0.1-V steps): in the low‑VD region, the linear I–V behavior indicates good source–drain ohmic contacts; in the high‑VD region, the current saturates. The device operates at a gate voltage below 1 V, exhibiting the typical characteristics of a low‑voltage, high‑performance EGT.
Curves of the raw Hall voltage VH and the field‑zero‑corrected Hall resistance RHall as a function of ±90 kOe magnetic field, for VG = 0.4–1.5 V: When VG > 0 V (device ON), the Hall signal is clear, linear with H, and of electron type; the absolute value of the slope decreases with increasing VG, corresponding to a rise in carrier density.
c shows the dependence of the Hall electron density nHall (solid red dots) and the displacement‑current‑based density nFET (open black dots) on VG: nHall increases monotonically, reaching 5.6 × 10¹⁴ cm⁻² at VG = 1.5 V, which is among the highest electron densities directly observed in solution‑processed metal‑oxide EGTs; nFET is slightly higher than nHall, as the displacement current accounts for both free and trapped charges.
d shows the dependence of the Hall mobility, μHall = 1/(e R nHall), on VG: it reaches a peak of 11 cm²·V⁻¹·s⁻¹ at VG = 0.6 V (nHall = 1.25 × 10¹⁴ cm⁻²), comparable to that of state-of-the-art solution‑processed In₂O₃ TFTs; however, as VG and the electron density are further increased, μHall instead decreases by more than 30%. This mobility peak coincides precisely with the critical density for the subsequent low‑temperature transport IMT.
Figure a shows the logarithmic–linear plots of sheet resistance R(T) as a function of VG for two devices (A/B) at eight values of nHall: at low carrier densities (<2.06×10¹⁴ cm⁻², shown in light blue and below), R increases sharply upon cooling (indicating strong localization and insulating behavior); when nHall ≥ 2.06×10¹⁴ cm⁻², R(T) levels off, and the low‑temperature resistance drops by nearly an order of magnitude to below h/e² (25.9 kΩ). The transition point lies precisely near the quantum resistance, a hallmark of a two‑dimensional metal–insulator transition.
b The relationship between the conductance G = 1/R and nHall at 4 K: At the lowest n, G approaches zero; the first non-zero conductance occurs at nHall ≈ 2.06 × 10¹⁴ cm⁻², with G ≈ e²/h, after which it increases approximately linearly, reaching G ≈ 0.45 mS at nHall = 7.28 × 10¹⁴ cm⁻².
c is the reduced activation energy. The ln W–ln T plot of W = −d ln R/d ln T shows that, when nHall ≤ 1.01 × 10¹⁴ cm⁻², the data fall on the Efros–Shklovskii variable-range hopping line with a slope of −1/2 (characteristic of a Coulomb gap and strong localization); whereas when nHall ≥ 2.06 × 10¹⁴ cm⁻², ln W crosses zero and continues to decrease upon cooling at higher carrier densities, signaling the emergence of non‑activated metallic transport.
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