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Liquid-metal deposition technique yields 2D ITO with increased flexibility

By Nora M. Hassan May 26, 2020
Schematic of the two-dimensional indium-tin-oxide (ITO) printing process where (a) an indium–tin alloy droplet is placed onto a heated substrate and pressed from the top with another pre-heated substrate, then (b) the liquid metal is pressed into a thin metallic film (the inset shows the crystal structure), and (c) when the top substrate is lifted, the liquid metal forms small spherical droplets leaving a thin oxide film on the substrate. (d) The ITO films do not lose conductivity upon bending; the inset shows the ITO/polymer interface. Credit: Nature Electronics

Indium tin oxide (ITO), a transparent and conductive material, is at the heart of today’s touchscreens and vibrant displays in smartphones, tablets, computers, and TVs. Unfortunately, the brittleness of this material is hindering the trend toward flexible displays and touchscreens that are lighter, thinner, and unbreakable. A team of researchers at RMIT University and University of New South Wales (UNSW), Australia, has succeeded in overcoming this obstacle by demonstrating a new method to deposit atomically thin ITO layers on flexible substrates with much better performance than commercial ITO.

“This was an interesting concept and Dr. Torben Daeneke, my colleague, was suggesting its possibility at the time. However, as it was a first of its kind, we needed to understand some fundamentals to make the printing process with liquid metal happen,” says Kourosh Kalantar-Zadeh, a professor at UNSW and one of the leaders of the project.

The market for flexible displays is expected to be enormous and a range of materials is being developed to achieve transparency, conductivity, and flexibility. Graphene and silver nanowires are at the forefront of these efforts. However, ITO has better optical properties than graphene and better conductivity than nanowires. “Really foldable ITO had not been made before,” says Dorna Esrafilzadeh, a Scientia Fellow at UNSW.

ITO is brittle when deposited in tens of nanometers thick layers, the typical thickness in conventional fabrication techniques, and so would break on bending on a flexible substrate. The researchers realized that atomically thick layers are flexible; they thus used a simple, inexpensive technique to print an atomically thick layer of ITO on an elastic material at lower temperatures and at ambient pressure.

“This is really an exciting work because it provides a simple route to produce indium tin oxide.…It is normally formed by vacuum processing, which is a slow process. Here, the authors form the layer by simply squishing a droplet of a molten alloy of tin and indium between two substrates, which displaces the metal and leaves behind only the oxide. The technique is akin to squishing a waterbed, displacing the water and leaving behind the plastic skin. Here, the skin is an oxide that is ultra-thin, transparent, flexible, and conductive,” says Michael Dickey, a professor at North Carolina State University and who was not involved in the study.

The researchers tried different indium-tin ratios until they achieved the ideal alloy at Sn concentrations of 5% and optimal fabrication parameters. The superior quality of the films was confirmed by measuring key properties including crystallinity, conductivity, and optical properties. The two-dimensional (2D) ITO was found to be highly conductive, flexible, and six times more transparent than monolayer graphene. These ITO films could consequently improve energy efficiency by absorbing up to 10 times less visible light than graphene, an important feat in devices where battery life is crucial. “When the material [is] used in commercial devices you can experience the difference in transparency. It will give an extraordinary quality to colors,” says Torben Daeneke, a research scientist at RMIT.

To demonstrate the effectiveness of the ITO films and the potential of the method to be used in large-scale device fabrication, the researchers created a prototype touchscreen by depositing 2D ITO films on both sides of a glass substrate. More impressive, the 2D ITO films retained conductivity after bending on a flexible polyimide substrate 3000 times. In contrast, a commercial ITO device failed before the fifth bending cycle. “What it means [is] you can fold the display, put it in your pocket, and unfold it and use it as a reader. Like an electronic newspaper that you may have watched in science fiction movies,” says Robi Dotta, a postdoctoral fellow at RMIT.

The researchers fabricated an ITO bilayer by depositing another layer on top of an ITO monolayer using the same method and achieved conductivity similar to that of commercial materials with 10 times less light absorption. By repeating the deposition of more layers, desired properties could be achieved.

“We are going to investigate many other binary combinations of liquid metals and test their conductivity and transparency. We believe that there are many other 2D transparent conductive oxides that are out there to be discovered,” says Nitu Syed, a postdoctoral researcher at RMIT.

  Read the abstract in Nature Electronics.