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1. Printing flexible two-dimensional indium tin oxide for multifunctional transparent bioelectrodes

   https://doi.org/10.1557/s43580-025-01320-w  

   Rahman, Md Saifur; Ong, Samuel A; Tiwari, Anand P; Scheideler, William J (June 2025, MRS Advances)

   Transparent conductive oxides (TCOs) are a high-performance material system that could enable new wearable sensors and electronics, but traditional fabrication methods face scalability and performance challenges. In this work, we utilize liquid metal printing to produce ultrathin two-dimensional (2D) indium tin oxide (ITO) films with superior microstructural, optical, and electrical properties compared to conventional techniques. We investigate the dynamics of grain growth and its influence on conductivity and the optical properties of 2D ITO, demonstrating the tunability through annealing and multilayer deposition. Additionally, we develop Au-decorated transparent electrodes, showcasing their adhesion and flexibility, low contact impedance, and biocompatibility. Leveraging the transparency of these electrodes, we enable enhanced simultaneous multimodal biosignal acquisition by integrating biopotential-based methods, such as electrocardiogram (ECG) or bioimpedance sensing (e.g., impedance plethysmography, IPG), with optical modalities like photoplethysmography (PPG). This study establishes CLMP-fabricated flexible 2D TCOs as a versatile platform for advanced bioelectronic systems and multimodal diagnostics. 

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2. Scalable Fabrication of Flexible Two-Dimensional Indium Tin Oxide via Continuous Liquid Metal Printing

   Rahman, Md Saifur; Tiwari, Anand P; Scheideler, William J (December 2024, 2024 IEEE International Flexible Electronics Technology Conference (IFETC))

   Abstract— In this study, we present continuous liquid metal printing (CLMP) to produce flexible and transparent indium tin oxide (ITO) layers compatible with plastic substrates with low glass transition temperatures. By leveraging the low melting point of an indium-tin (In-Sn) alloy, we achieve spontaneous two-dimensional (2D) oxide growth at low temperatures, following Cabrera-Mott (CM) oxidation kinetics. A robotically controlled roller deforms the molten alloys, depositing a thin native oxide (ITO) via van der Waals adhesion across large areas (approximately 1200 cm²) in mere seconds. The printed 2D ITO is highly crystalline with large plate-like grains with an average size of 55 nm. They demonstrate low resistivity (approximately 714 μΩ⋅cm) and transparency (>92% in visible light) with an optical bandgap of 3.71 eV. Mechanical testing reveals superior adhesion, 2X greater bendability, and 3X better scratch resistance of flexible 2D ITO. Finally, we demonstrate an application towards flexible transparent electrocardiogram electrodes based on flexible 2D ITO. 

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3. Perspective on integrated photonic devices using transparent conductive oxides: Challenges and opportunities

   https://doi.org/10.1063/5.0179441  

   Wang, Alan X; Hsu, Wei-Che (February 2024, Applied Physics Letters)

   Transparent conductive oxides (TCOs) are gaining increasingly high research interest for integrated photonic devices due to the strong plasma dispersion effect and process compatibility with versatile optoelectronic platforms. In this perspective article, the authors gave a brief review of research efforts both on theoretical modeling and experimental demonstration of integrated photonic devices, especially on high-efficiency electro-optic modulators through the integration with plasmonics and silicon photonics. In addition, the authors discussed the challenge and opportunity associated with TCO photonic devices and the application in photonic integrated circuits (PICs) with emphasis on high mobility materials, high-speed E-O modulators, and large-scale integration. Finally, we conclude that collaboration with existing silicon photonics foundry is a necessary route to incorporate TCOs into existing PIC ecosystems. 

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4. Photonic Lift-off Process to Fabricate Ultrathin Flexible Solar Cells

   https://doi.org/https://doi.org/10.1021/acsami.1c12382  

   Liu, W.; Turkani, V. S.; Akhavan, V.; Korgel, B. A. (September 2021, ACS applied materials interfaces)

   null (Ed.)

   A microsecond time-scale photonic lift-off (PLO) process was used to fabricate mechanically flexible photovoltaic devices (PVs) with a total thickness of less than 20 μm. PLO is a rapid, scalable photothermal technique for processing extremely thin, mechanically flexible electronic and optoelectronic devices. PLO is also compatible with large-area devices, roll-to-roll processing, and substrates with low temperature compatibility. As a proof of concept, PVs were fabricated using CuInSe2 nanocrystal ink deposited at room temperature under ambient conditions on thin, plastic substrates heated to 100 °C. It was necessary to prevent cracking of the brittle top contact layer of indium tin oxide (ITO) during lift-off, either by using a layer of silver nanowires (AgNW) as the top contact or by infusing the ITO layer with AgNW. This approach could generally be used to improve the mechanical versatility of current collectors in a variety of ultrathin electronic and optoelectronic devices requiring a transparent conductive contact layer. 

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5. 2D Amorphous Homojunction Transistors Exfoliated from Liquid Metals

   https://doi.org/10.1002/adfm.202512590  

   Le, Minh Nhut; Rahman, Md Saifur; Agnew, Simon A; Ong, Samuel W; Scheideler, William J (July 2025, Advanced Functional Materials)

   Abstract Ultrathin 2D metal oxides are a high‐performance class of transparent conducting materials capable of overcoming the traditional limitations of inorganic flexible electronics. The low temperature, thermodynamically favorable synthesis of 2D oxides at liquid metal interfaces offers the potential for printing these materials over large areas at unprecedented speeds with sub‐nanometer scale precision. However, these native oxides are sub‐stoichiometric and highly conductive, so new strategies are needed that can precisely engineer the electrostatics and enhance stability. In this work, the crystalline vs. amorphous phase of 2D oxides is engineered via alloying of ternary In_1‐ySn_yO_xand ultralow deposition temperatures (120–160 °C) are afforded by In‐Sn eutectics. This approach is extended to rapid assembly of nanoscale (3–5 nm per layer) vertical 2D homojunctions with electrostatically favorable grading from high density of states front channels to lower density of states back‐channels. Detailed materials characterization reveals how this platform enhances electron mobility while improving resilience under bias‐stress in metal oxide transistors. Devices based on amorphous 2D oxide homojunctions with high‐k sol‐gel ZrO_xdielectrics achieve excellent electron mobility (30 cm²/V·s), steep switching (SS of 100 mV dec⁻¹), I_on/offof 10⁷and 10X reduced bias‐stress shifts, presenting an ideal strategy for high‐performance flexible oxide electronics. 

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