Strain Induced Resistance for Graphene Force Sensors

1. Strain Induced Resistance for Graphene Force Sensors Daniel S. Wood Phys 570x

2. Outline • Background • Motivation • Devices • Conclusions

3. Background • Strain Induced Resistance • Bend test • 0 to 2.3mm curvature  no change • 2.3 to 0.8mm curvature  > 10 times resistance, linear relationship • Original properties achieved upon unloading • Stretching • 0-10% no change, 10-25% linear relationship • Pre-strained substrates  increase durability of graphene to 30 % strain

4. Motivation Monitoring of large impacts to athlete’s heads could potentially reduce injury

5. Motivation • Large Force Sensor • Monitoring of large impacts to athlete’s heads could potentially reduce injury • Current devices are bulky resulting in limited use in sports • NEED : small, non invasive force sensor. Low sensitivity but withstand high maximum force • Small Force Sensor • Current AFM tips use spring constant as limiting factor in force calculations. As low as 1 pN sensitivity • NEED : device that can have more force sensitivity to increase microscopy resolution • Solution : Strain Induced Resistance for Graphene Force Sensors

6. Devices – Large Force Sensor Assuming a graphene sheet 1 cm x 1 cm Know that resistivity / strain is a linear relationship: Resistivity can be measured easily to 1mΩ (extreme cases to 1µΩ) Simple stress / strain equations shows force sensitivity and max force applied Human tolerance is 1E4 N, Graphene based sensor 10 times larger

7. Devices – Large Force Sensor • Utilize high tolerance of graphene under stretching to create LFS. • Graphene is 1 cm^2 (initially strained ~10%) • As force acts on device, graphene is stretched up to 30% (2E5N) • Strain causes a measureable change in resistivity Force

8. Devices –Small Force Sensor Assuming a graphene sheet 1 um x 1 um, and thickness Know that resistivity / radius of curvature is a linear relationship: Resistivity can be measured easily to 1µΩ Two point bend equations shows force sensitivity and max force applied AFM tolerance is 1E-12N, Graphene sensor is 100 times more sensitive.

9. Devices –Small Force Sensor • Utilize high sensitivity of graphene under bending to create SFS. • Two graphene sheets (1 um^2) are placed on either side of AFM cantilever • As tip moves across surface, change in cantilever height occurs • Vertical movement of cantilever • Rotation of cantilever • Change in resistivity  graphene curvature  force on AFM tip in many directions

10. Conclusions • Strain of Graphene • Can be measured by changes in resistivity • Force on graphene can be used for both LFS and SFS depending on orientation alone • LFS • Many injuries in sports including car racing and football could be avoided or alleviated if small sensors could be made • Sensors of 1cm^2 and minimal thickness have tolerance of 10X human body • SFS • AFM is an important tool in nanotechnology, increases in sensitivity is an important aspect of microscopy • AFM can resolve force of 1E-12 N • Graphene microsheet could increase sensitivity for imaging by 100X

11. Questions?