Elastically strained nanowires and atomic sheets
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Introduction Strain is a universal phenomenon pertinent to the synthesis, fabrication, and applications of all types of materials. Strain can have two distinct regimes: In the plastic regime, materials generally undergo irreversible changes, such as failure or property degradation. Elastic deformation, on the other hand, can induce reversible changes to materials properties, from electronic and chemical to optical; this is called elastic strain engineering (ESE). A particularly successful application of strain engineering is pushing the limit of miniaturization of silicon-based field-effect transistors.1 By alternating deposition of Si and Ge layers, the interlayer lattice mismatch creates controllable strain in the silicon crystal. The precisely controlled strain breaks the crystal field symmetry and reduces the scattering of carriers by phonons, resulting in substantially increased carrier mobility, which allows for further reduction in the channel size.
“Magnified” strain effects in nanowires The strength of a material depends very strongly on its dimensions. Typically, smaller structures can tolerate larger deformations before yielding. For a given rod structure under a bending deformation, the smaller the radius of the rod (distance to the rod axis), the larger the strain it can sustain at the
maximum elastic loading stress.2 Therefore, it is anticipated that ESE can be profitably explored in materials with reduced dimensions, such as nanowires and atomic sheets. Rapid progress in device miniaturization has led to the quick rise of flexible, nanoscale devices for which one-dimensional nanowires and atomic sheets such as graphene are particularly promising candidate materials. These materials possess unusual electronic properties, frequently arising from giant surface effects and strong quantum confinement. Thanks to their reduced dimensions, these materials also exhibit superb mechanical properties, some of which (e.g., graphene) are among the strongest human-made materials. Moreover, unlike in strain-engineering in the conventional microelectronics industry, where uniform strain is often employed to enhance materials properties, in these low-dimensional wires and sheets, inhomogeneous strain fields can also be generated, providing unprecedented opportunities to explore paradigms of ESE. Therefore, nanowires and atomic sheets are ideal platforms to explore novel elastic strain effects and device concepts at the nanoscale. Wong et al. were among the first to address the size effects of mechanical properties of nanotubes and SiC nanorods using atomic force microscopy (AFM).3 They found that the strengths of SiC nanorods were substantially greater
Dapeng Yu, School of Physics, Peking University; [email protected] Ji Feng, School of Physics, Peking University; [email protected] James Hone, Department of Electronic Engineering, Columbia University; [email protected] DOI: 10.1557/mrs.2014.6
© 2014 Materials Research Society
MRS BULLETIN • VOLUME 39 • FEBRUARY 2014 • www.mrs.org/bulletin
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ELASTICALLY STRAINED NANO
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