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WIREs Comput Mol Sci
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Mechanical properties of silicon nanowires

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Abstract Silicon nanowires (SiNWs) are at the top of the list of materials used in conventional electromechanical devices as well as in strained nanotechnology. Both experimental and theoretical studies showed the size‐dependent character of mechanical properties of SiNWs. However, the surface contaminations, local surface strains, ‘boundary conditions’, native oxide, equipment‐induced errors, and the errors caused by postprocessing of results lead to softening of Young's modulus and extension of the region where the size dependency is seen by experimentalists. Application of improved potentials or advanced theoretical modeling such as inclusion of explicit treatment of temperature and quantum‐mechanical effects allows to show specificity of Young's modulus to the size and shape in case of small (width <4 nm) nanowires. The ductile‐brittle transitions of SiNWs at different temperatures are revealed. Some suggestions on postprocessing techniques are discussed. © 2012 John Wiley & Sons, Ltd. This article is categorized under: Structure and Mechanism > Molecular Structures

Relative shift in the resonance frequency as a function of position of the laser spot. The numbers indicate the measured resonance frequency. The inset shows one of the measurements made on the 80 μm long cantilever. (Reprinted with permission from Ref 23. Copyright 2010 IOP Publishing Ltd.)

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Normalized resonance frequency shift as a function of laser power. Laser spot at the cantilever tip. (Reprinted with permission from Ref 23. Copyright 2010 IOP Publishing Ltd.)

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Young's modulus and Poisson's ratio as a function of width of the wire. Values were estimated using different methods. (Reprinted with permission from Refs 19 and 54. Copyright 2010 The American Physical Society, and 2011 American Chemical Society.)

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Schematic depiction of different manipulations in three‐point bending tests. Silicon nanowire of length L is loaded by a lateral load P or PI in order to have the same deflection wc. Resulted force F and displacement ur are used to describe loaded nanowire. (Reprinted with permission from Ref 51. Copyright 2010 American Institute of Physics.)

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Ductile–brittle domain map with the nanowire (NW) diameter in the x‐axis and temperature in the y‐axis. Each symbol corresponds to 10 independently repeated simulations with random initial velocities. A symbol ○ represents ‘ductile’ fracture. A symbol × represents ‘brittle’ fracture. For ▵ symbols, out of 10 simulations, a SiNW can fail in a brittle or ductile manner or sometimes in a mixture of both. (Reprinted with permission from Ref 48. Copyright 2010 Elsevier Ltd.)

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Apparent strain to failure (black data points) and critical resolved stress (red data points) as a function of the diameter of SiNWs. Stress was not measured directly in each test but was inferred from the strain measurements assuming ideal nonlinear elastic behavior, and then the critical resolved stress was determined. (Reprinted with permission from Ref 25. Copyright 2011 American Institute of Physics.)

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Elastic modulus as a function of SiNW radius extracted from (left) AFM bending and (right) nanoindentation. Represented here are the values of elastic modulus extracted from the experimental bending by using tension‐only (black squares) and bending‐tension models (red triangles), and from nanoindentation by using cube‐corner (black circles) and Berkovich (red triangles) indentors. (Reprinted with permission from Ref 27. Copyright 2011 Wiley‐VCH Verlag GMbH & Co. KGaA, Weinheim.)

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AFM images of the SiNW before and after measuring its mechanical properties during bending tests. (Reprinted with permission from Ref 27. Copyright 2011 Wiley‐VCH Verlag GMbH & Co. KGaA, Weinheim.)

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Young's modulus of low‐pressure chemical vapor laterally deposited SiNWs as a function of their diameter. (a) Young's modulus of pristine SiNWs (red dots), their averaged value (151 GPa, red dashed line) compared with Young's modulus of bulk Si (111) (170 GPa, black solid line); (b) Young's modulus of rapid thermally oxidized SiNWs (red dots) compared with averaged values of Young's modulus of pristine SiNWs (red dashed line) and Young's modulus of bulk Si (111) (black solid line). (Reprinted with permission from Ref 26. Copyright 2010 Wiley‐VCH Verlag GMbH & Co. KGaA, Weinheim.)

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(a) Optical microscope image of a HF defect in a 14 nm thick device layer of a silicon‐on‐insulator wafer. (b) Scanning electron microscope image of HF defects in the 14 nm thick silicon‐on‐insulator wafer. The inset shows a close‐up image. The buried oxide layer below the HF defect is etched by the HF solution. (c) White light interferometric picture of a 40 nm thick cantilever type SiNW with defects. (Reprinted with permission from Ref 24. Copyright 2010 IOP Publishing Ltd.)

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