Electronic materials: buckling down for flexible electronics.

T he success of electronic paper, roll-up displays and many other potential applications of flexible electronics will depend on the availability of electronic materials that can be stretched, compressed and bent. Previous efforts to develop electronic materials that can be mechanically deformed without breaking have mainly focused on small organic molecules and polymers. Unfortunately, these materials often have low charge-carrier mobilities and short operating lifetimes, so electronic devices made from them cannot compete with devices made from inorganic materials such as silicon and gallium arsenide. Inorganic materials, however, are notorious for their brittleness. Th ey tend to fracture under a tensile strain of about 1%, so it is impractical to incorporate them directly into fl exible electronic devices. On page 201 of this issue, John Rogers and co-workers at the University of Illinois at UrbanaChampaign and the Argonne National Laboratory, both in the US, overcome this problem by demonstrating that spectacular bendability, compressibility and stretchability can be accomplished for both silicon and gallium arsenide1. Th eir approach relies on controlling how single-crystal nanoribbons made from these materials buckle when they are attached to a fl exible substrate. Th is work opens new avenues for controlling the three-dimensional shape of inorganic nanostructures and off ers immediate opportunities for fabricating fl exible electronic devices with superior performance from the inorganic materials currently used in microelectronics. Buckling has been applied to a variety of fabrication tasks since it was first demonstrated in 1998 (ref. 2). In a typical approach, a thin layer of the inorganic material, which is naturally stiff, is deposited on top of a relatively thick substrate made of an elastic polymer (elastomer) that has been stretched by mechanical force or thermal expansion (Fig. 1a). When the stress in the elastic substrate is relieved, a sinusoidal pattern will develop in the stiff layer in an effort to minimize the total elastic strain energy of the system. Both the wavelength and amplitude of the wave-like pattern depend on the thickness of each layer, as well as their mechanical properties. Because the wavelength increases linearly with the thickness of the stiff layer, there is a simple correlation between the buckling wavelength and the elastic moduli of the materials, which can be used to measure the elastic properties of polymeric thin films3. The phenomenon of buckling has also been used to turn intrinsically rigid inorganic materials into flexible ones, and several groups have shown that buckled structures with built-in stress can be compressed, stretched or bent without being damaged4–6. However, the maximum strain that such structures can accommodate is limited. Buckled silicon ribbons, for example, can only sustain tensile strains of less than 15% (ref. 4). Now Rogers and co-workers have overcome this limitation by patterning the surface of a pre-strained elastic substrate with reactive groups, allowing them to control how the ribbons buckle when the strain is released. They have demonstrated that the stretchability of nanoribbons made of single-crystal Inorganic nanoribbons can be attached to an elastic surface at selected positions to make wave-like structures that maintain their semiconducting properties when stretched or compressed. These nanostructures will prove to be immediately useful in fl exible electronics. ELECTRONIC MATERIALS