Reversibly bistable materials could revolutionize flexible electronics

(Nanowerk Spotlight) Rollable displays and other flexible, stretchable electronic systems are often enabled by the successful integration of nanostructured materials. Most commercially available flexible electronic circuits and devices are fabricated on flexible plastic substrates, such as polymeric amides, PEEK polymers, or transparent conductive polyester films. Although these substrates can be easily bent and rolled up, they cannot be used to fabricate rollable display-integrated gadgets that are fixed at a rigid perpendicular position on their own.

Expect future generations of smartphones to be flexible. Already, Apple has been granted a patent for 'Flexible electronic devices' (U.S. Patent 8,929,085) that comprises a flexible housing and a flexible display screen and Samsung has a similar patent application pending (US 14/272,715).

This is where the need for a mechanically flexible and reversibly bistable material – a material that can assume two stable states: a flexibly rolled state and an independently unbent state – arises.

For the first time, a research team at King Abdullah University of Science and Technology, (KAUST) has now used a reversibly bistable material to demonstrate flexible electronics. In addition, the scientists introduced a performance metric – the cumulative impact budget — which takes into account the impact force imparting an impulse on the silicon fabric during the mechanical deformation of the substrate.

The results have just been published in today's online edition of Applied Physics Letters ("Functional integrity of flexible n-channel metaloxidesemiconductor field-effect transistors on a reversibly bistable platform").

Digital photograph of transferred MOSFET silicon die onto metallic bistable structure. (Image: Prof. Muhammad Mustafa Hussain, KAUST)

"We investigated the mechanical and electronic aspects of flexible, inorganic field-effect transistors physically supported by a mechanically bistable metallic substrate," Muhammad Mustafa Hussain, an Associate Professor of Electrical Engineering at KAUST, summarizes the effort. "Our work combines basic and applied studies with findings that are supported by experimental data – semiconductor device analysis, scanning electron microscopy, energy-dispersive X-ray, and high-speed imaging – and theoretical discussion: impulse-momentum theory and approximation, and prediction of the kinetic energy losses and magnitude of impulsive forces with respect to impact speeds."

"This is the first demonstration and discussion of the effectiveness of a reversibly bistable material for free-form electronics," Nasir Alfaraj, a PhD student in Hussain's group and the paper's first author, tells Nanowerk. "The material we used to form the bistable substrate is a porous iron-carbon metal alloy, which is inexpensive and commonly used in the fabrication of commercially available cycling safety wristbands and a variety of ankle bracelets for orthopedic health care. This motivated us to work with low-cost, commonly available material and extend its functionality through the integration of logic and control components in order to create practical flexible display devices."

To fabricate their device, the team attached a flexible silicon-based metal-oxide-semiconductor field-effect transistor (MOSFET) on a mechanically flexible and optically-semitransparent porous silicon onto this metallic bracelet.

This material platform has two stable and reversible mechanical states: stretched and rolled. Surface, cross-sectional, and elemental composition nanoscale examinations of the thin metallic structure, along with electrical measurements of the transistors, show that although the distribution of nanopores throughout the structure allows the metal alloy to internally absorb strain energy and hence achieve flexibility. Nevertheless, the transistor devices on the thin silicon fabric maintained their integrity after accumulating an impulsive force budget about 300 times higher than the force that average adults experience as a result of their weight.

This work could have a significant impact on the electronics industry and open the door to commercializing flexible, large electronic devices. Reversibly bistable flexible transistors can be used in a variety of applications, including optoelectronic devices in which LEDs are controlled by reversibly bistable flexible transistors. As such transistors can handle high drive currents, they can be used to realize foldable display devices.

The researchers note that reversibly bistable electronics can also aid in the development of practical orthopedic tools and technologies that employ electronic devices required to handle high physical force loads.

As a next step, the team plans to design and demonstrate a naturally flexible and reversibly bistable polymer to be integrated with state-of-the-art logic and radio-frequency electronic devices.