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We have developed methods for creating microscale inorganic light-emitting diodes (LEDs) and for assembling and interconnecting them into unusual display and lighting systems. The LEDs use specialized epitaxial semiconductor layers that allow delineation and release of large collections of ultrathin devices. Diverse shapes are possible, with dimensions from micrometers to millimeters, in either flat or "wavy" configurations. Printing-based assembly methods can deposit these devices on substrates of glass, plastic, or rubber, in arbitrary spatial layouts and over areas that can be much larger than those of the growth wafer. The thin geometries of these LEDs enable them to be interconnected by conventional planar processing techniques. Displays, lighting elements, and related systems formed in this manner can offer interesting mechanical and optical properties.

Compound semiconductors like gallium arsenide (GaAs) provide advantages over silicon for many applications, owing to their direct bandgaps and high electron mobilities. Examples range from efficient photovoltaic devices to radio-frequency electronics and most forms of optoelectronics. However, growing large, high quality wafers of these materials, and intimately integrating them on silicon or amorphous substrates (such as glass or plastic) is expensive, which restricts their use. Here we describe materials and fabrication concepts that address many of these challenges, through the use of films of GaAs or AlGaAs grown in thick, multilayer epitaxial assemblies, then separated from each other and distributed on foreign substrates by printing. This method yields large quantities of high quality semiconductor material capable of device integration in large area formats, in a manner that also allows the wafer to be reused for additional growths. We demonstrate some capabilities of this approach with three different applications: GaAs-based metal semiconductor field effect transistors and logic gates on plates of glass, near-infrared imaging devices on wafers of silicon, and photovoltaic modules on sheets of plastic. These results illustrate the implementation of compound semiconductors such as GaAs in applications whose cost structures, formats, area coverages or modes of use are incompatible with conventional growth or integration strategies.

A convenient process for generating large-scale, horizontally aligned arrays of pristine, single-walled carbon nanotubes (SWNTs) is described. The approach uses guided growth, by chemical vapor deposition (CVD), of SWNTs on miscut single-crystal quartz substrates. Studies of the growth reveal important relationships between the density and alignment of the tubes, the CVD conditions, and the morphology of the quartz. Electrodes and dielectrics patterned on top of these arrays yield thin-film transistors that use the SWNTs as effective thin-film semiconductors. The ability to build high-performance devices of this type suggests significant promise for large-scale aligned arrays of SWNTs in electronics, sensors, and other applications.

Daniel Shir graduated with distinction and obtained a B.S. degree in materials science and engineering from the Pennsylvania State University in 2005. He is currently pursuing a Ph.D. degree in materials science and engineering at the University of Illinois at Urbana−Champaign under Professor John A. Rogers's guidance. Yun-Suk Nam obtained a Ph.D. degree in chemical engineering from Sogang University, Seoul, South Korea, in 2004. He is currently a postdoctoral researcher in the Department of Materials Science and Engineering at the University of Illinois at Urbana−Champaign. Seokwoo Jeon was born in Seoul, Korea, in 1975. He received his B.S. degree in 2000 and his master's degree with Professor Shinhoo Kang from Seoul National University in 2003 after one year as an exchange graduate student with Professor Paul V. Braun at the University of Illinois at Urbana−Champaign (UIUC). He is currently pursuing his Ph.D. degree in materials science and engineering at UIUC under the direction of Professor John A. Rogers. His research interests include soft lithography, 3D nanopatterning, microfluidic systems, and optically functional materials and devices. John A. Rogers obtained B.A. and B.S. degrees in chemistry and in physics from the University of Texas, Austin, in 1989. From MIT, he received S.M. degrees in physics and in chemistry in 1992 and the Ph.D. degree in physical chemistry in 1995.

This paper presents methods for solution casting and transfer printing collections of individual single-walled carbon nanotubes (SWNTs) onto a wide range of substrates, including plastic sheets. The deposition involves introduction of a solvent that removes surfactant from a suspension of SWNTs as it is applied to a substrate. The subsequent controlled flocculation (cF) produces films of SWNTs with densities that can be varied between a few tubes per square micron to thick multilayers in a single deposition step and with orientation determined by the direction of solution flow. High-resolution rubber stamps inked in this manner can be used to print patterns of tubes with geometries defined by the relief structure on the surface of the stamp. Thin film transistors fabricated with these techniques demonstrate their potential use in flexible "macroelectronic" systems.

This paper demonstrates the use of arrays of networks of single wall carbon nanotubes (SWNTs) and electrical breakdown procedures for building thin film transistors (TFTs) that have good, reproducible performance and high current output. Channel length scaling analysis of these TFTs indicates that the resistance at the source/drain contacts is a small fraction of the device resistance, in the linear regime. When measured with the channel exposed to air or coated by poly(methyl methacrylate) (PMMA), these transistors operate in the unipolar p mode. By spin-coating the polymer polyethylenimine (PEI) on the channel region, these transistors can be switched to operate in the unipolar n mode. Patterning the exposure of a single channel to PMMA and PEI yields p−n diodes. These results indicate that SWNT-TFTs can provide the building blocks of complex complementary circuits for a range of applications in macroelectronics, sensors, and other systems.

We report the use of networks of single-walled carbon nanotubes (SWNTs) with high and moderate coverages (measured as number of tubes per unit area) for all of the conducting (i.e., source, drain, and gate electrodes) and semiconducting layers, respectively, of a type of transparent, mechanically flexible, thin-film transistor (TFT). The devices are fabricated on plastic substrates using layer-by-layer transfer printing of SWNT networks grown using optimized chemical vapor deposition (CVD) procedures. The unique properties of the SWNT networks lead to electrical (e.g., good performance on plastic), optical (e.g., transparent at visible wavelengths), and mechanical (e.g., extremely bendable) characteristics in this "all-tube" TFT that would be difficult, or impossible, to achieve with conventional materials.Invisible circuits based on transparent transistors have broad potential applications in consumer, military, and industrial electronic systems. [1,2] In backlit display devices, for example, transparent active-matrix circuits can increase the aperture ratio and battery life. Transparent electronic materials that can be printed on low-cost, flexible, plastic substrates are potentially important for new applications, such as bendable heads-up display devices, see-through structural health monitors, sensors, and steerable antennas. [3][4][5] More advanced systems, such as electronic artificial skins [6] and canopy window displays, will require materials that can also tolerate the high degrees of mechanical flexing (i.e., high strains) needed for integration with complex curvilinear surfaces. Most examples of transparent TFTs (TTFTs) use thin films of inorganic oxides as the semiconducting and conducting layers. [7][8][9] Although the electrical properties of these oxides can be good (mobilities and conductivities as high as 20 cm 2 V -1 s -1[10] and 4.8 × 10 3 X -1 cm -1 , [11] respectively), their mechanical characteristics are not optimally suited for use in flexible and mechanically robust devices. For example, the tensile fracture strains for ZnO and indium tin oxide (ITO) thin films are less than 0.03 % [12] and 1 %, [13] respectively.Aligned arrays [14] or random networks [15,16] of individual SWNTs represent alternative classes of transparent semiconducting and conducting materials. In networks with high coverages of SWNTs, especially when in the form of small bundles, the metallic tubes (normally present with semiconducting tubes in a 1:2 ratio) form a percolating network that behaves like a conducting "film". [17,18] At moderate coverages, only the semiconducting tubes form such a percolating network and the film shows semiconducting properties. [19] Unlike the oxides, the SWNT films have excellent mechanical properties due to their high elastic moduli (1.36-1.76 TP nm/tube diameter nm) [20] and fracture stresses (100-150 GPa) [21] of the tubes. SWNT-based semiconductors have been used in flexible TFTs. [15,[22][23][24] In one case, solution-deposited SWNT networks also formed the gate electrodes. [25] A...