The IBS develops a graphene-carbon nanotube element using wrinkled oxidized film:

Bendable, transparent, and stretchable properties enable unlimited applications

Director Young-hee Lee, a professor at Sungkyunkwan University's Center for Integrated Nanostructure Physics, and an affiliate of the IBS, has led his team to successfully develop an electronic element using an unbreakable, wrinkled oxidized film that can be stretched by up to 20%.

Research into nanomaterials is being actively conducted for the purpose of developing an electronic element that can be bent, folded, and stretched. Although graphene and carbon nanotubes feature outstanding electronic-movement properties, and can withstand deformation, their applicability is limited because the oxidized material (used as an insulation film) is easily fractured. Despite this limitation, the Korean research team discovered that the wrinkled oxidized film can be used as an insulator, enabling the development of transparent and stretchable electronic elements.

The research was conducted by Sang-hoon Chae, a Ph.D. student working under Professor Lee at the research center, in collaboration with researchers at Pittsburgh University and the University of California.

Graphene was discovered in 2004 by Professors Geim and Novoselov, subsequent winners of the 2010 Nobel Prize for Physics. Graphene is a hexagonal honeycomb structure consisting of a single layer of carbon atoms. Its composition lends it outstanding conductivity, charge motility, and enormous potential for applications, which is why so much research on the material has been undertaken, and the reason it has been referred to as a "new dream material". Much of the ongoing research worldwide is focused on graphene when used with carbon nanotube.

Carbon nanotube is a tubular material with an inner diameter of two nanometers or less (1 nm = one billionth of a meter). This makes it a leading contender to replace silicon as the main material for semiconductors used in electronics.

Since the successful synthesis of graphene and carbon nanotube, researchers across the globe have been conducting research into various applications: touch screens, transparent conductive electrodes, and high-speed electronic elements. In particular, applied research has been devoted to bendable electronic elements, using the unique and combined properties of graphene and carbon nanotube. If conductive graphene is used as the electrode, and semiconductive carbon nanotube as the electronic path, elements can be created that have outstanding electrical and mechanical properties.

In order to produce a completely stretchable electronic element, the insulation film (which controls the movement of the electronics) must be just as stretchable as the electrode and electronic path. Yet conventional insulation layers made of oxidized film fracture easily.

Flat, nano-level oxidized film can endure forces encountered during bending, but fracture easily when stretched; polymer insulation film can endure forces during stretching, but leaks too much current. Therefore, the key to developing a stretchable electronic material is to select a stretchable insulation film.

The team deposited an aluminum oxide film (Al2O3, featuring a high-dielectric constant) on a copper plate, before coating it with a methacrylic-resin polymer (PMMA). The copper plate was then removed using a solution that dissolves copper. In the process, the aluminum layer becomes wrinkled after it has undergone the stressing process.

The next step is attach the wrinkled oxidized film to the graphene electrode to be used as the transistor's insulation layer. Next, the semiconducting carbon nanotube is attached, creating an electronic element whose oxidized film does not fracture, which can stretch by up to 20% when unwrinkled.

The research-center team also proved that, because the wrinkling in oxidized materials was created naturally, the oxidized film can be stretched in many directions. This could therefore be considered the world's first prototype of a truly stretchable element, because the basic electronic-element unit and all the transistor parts (electrode, electronic path, and insulation layer) were made of materials capable of withstanding deformation. The element also revealed 80% transmittance; all of the materials involved were transparent.

The research outcome should be viewed as a new class of technology that is distinct from the conventional silicon semiconductor. Silicon materials and oxidized silicon film are opaque and easily fractured, which limits their application potential in transparent, stretchable elements and displays. In addition, using wrinkled oxidized film and graphene-nanotube overcomes these limitations, creating an element that is transparent and stretchable by up to 20%. Its unlimited applicability makes it extraordinarily significant.

"This research has greatly expanded the applicability of electronic elements like graphene and carbon nanotube as the new dream material," said Professor Young-hee Lee on the significance of this research. He added: "...beyond being bendable, the stretchable transparent element has...unlimited application potential for displays, foldable and wearable computers, and sensors to be attached to skin."

The research results were published as "Transferred Wrinkled Al2O3 for Highly Stretchable and Transparent Graphene-Carbon Nanotube Transistors" in the March issue of "Nature Materials" (March 4, online edition).