CityU’s discovery of silicon’s ultra-large elasticity brings about revolutionary changes to medical equipment

​​A recent research led by the Department of Mechanical and Biomedical Engineering (MBE) at City University of Hong Kong (CityU) has shown that silicon nanowires can be stretched above 10% elastic strain, several times higher than that of previous findings. 

A few samples even reached 16% tensile strain, which is very close to the theoretical elastic limit of silicon (17 to 20%). This groundbreaking discovery will likely bring about revolutionary changes to flexible electronics and medical equipment for the well-being of society. 

The paper titled “Approaching the Ideal Elastic Strain Limit in Silicon Nanowires,” presented by Zhang Hongti, a doctoral student in MBE, as the first author, was published in Science Advances, a periodical of the American Association for the Advancement of Science (AAAS), in August. 

Dr Lu Yang, supervisor of Zhang Hongti and Assistant Professor in MBE, is the leader of this research. He explained that this ultra-large elasticity could be mainly attributed to the pristine defect-scare, nanosized single-crystalline structure and atomically smooth surfaces of the material itself. Also, the uniaxial tensile testing platform developed at CityU helped avoid silicon nanowires from being damaged during the sample fixing and testing. 

Dr Lu pointed out that silicon is an essential material in the semiconductor industry for the production of various types of electronic devices and microchips which are closely related to human life. However, bulk silicon crystals are hard but brittle and easily broken, which has imposed constraints on their use for producing electronic devices that require to be stretched and deformed, such as those installed inside or outside the human body for monitoring health.
 
Nowadays, there are more and more electronic and mechatronic applications that require large deformation. “The new findings of super stretchy silicon nanowires can significantly expand their use in such areas and improve health technologies.” Dr Lu said. “For example, it allows manufacturers to produce micro medical sensors flexibly attached to a human body which will enable patients to move more freely.”

In the past decades, researchers found that materials can generally endure a higher degree of deformation when their sizes are reduced to small scales. For instance, the elasticity of silicon can increase by about 2 to 3% when its size is reduced to micro- or nanometer scales. However, this value is still far from the theoretical elastic limit of silicon ~17%.
 
As Dr Lu’s team has discovered that defects in the surface of silicon crystals will significantly affect its elasticity, they chose to use high-quality silicon nanowires with single-crystalline structure for the testing. By using micro-mechanical devices assisted with an in-situ SEM (Scanning Electron Microscope) nanoindenter to perform uniaxial tensile tests, they found that the wires were able to quickly recover to their original form after they were elastically strained above 10%, which is five times higher than that of previous findings. A few samples even reached 16%, very close to their theoretical elastic limit.
 
The partnering organisations for this research include the Massachusetts Institute of Technology; University of California, Los Angeles; Xiamen University; and IBM Research Centre. Dr Lu and his team have been recently awarded about HK$700,000 from the Research Grants Council of Hong Kong for conducting further research on “Elastic Strain Engineering” of the nanosized silicon.


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