Designing Metallic and Insulating Nanocrystal Heterostructures to Fabricate Highly Sensitive and Solution Processed Strain Gauges for Wearable Sensors

Abstract

All-solution processed, high-performance wearable strain sensors are demonstrated using heterostructure nanocrystal (NC) solids. By incorporating insulating artificial atoms of CdSe quantum dot NCs into metallic artificial atoms of Au NC thin film matrix, metal-insulator heterostructures are designed. This hybrid structure results in a shift close to the percolation threshold, modifying the charge transport mechanism and enhancing sensitivity in accordance with the site percolation theory. The number of electrical pathways is also manipulated by creating nanocracks to further increase its sensitivity, inspired from the bond percolation theory. The combination of the two strategies achieves gauge factor up to 5045, the highest sensitivity recorded among NC-based strain gauges. These strain sensors show high reliability, durability, frequency stability, and negligible hysteresis. The fundamental charge transport behavior of these NC solids is investigated and the combined site and bond percolation theory is developed to illuminate the origin of their enhanced sensitivity. Finally, all NC-based and solution-processed strain gauge sensor arrays are fabricated, which effectively measure the motion of each finger joint, the pulse of heart rate, and the movement of vocal cords of human. This work provides a pathway for designing low-cost and high-performance electronic skin or wearable devices.

Thin Film Protected Flexible Nanoparticle Strain Sensors: Experiments and Modeling

“…Strain sensors based on NP films, in particular, have been of growing interest due to their increased sensitivity [19][20][21] when compared to existing metal strain sensors that incorporate thin film technology [23]. In addition, the low processing temperatures required in the case of NP-based strain sensing devices, render them fully compatible with flexible substrate technology [24].…”

“…Particularly in the field of flexible electronics, strain sensors are of great importance in emerging technologies such as the internet of things. Over the last decade, many novel nanomaterials have been used in strain sensing applications such as carbon nanotubes [ 11 , 12 , 13 ], nanowires [ 5 , 14 , 15 ], MoS 2 [ 16 ], graphene [ 17 ], and nanoparticles (NPs) [ 6 , 18 , 19 , 20 , 21 , 22 ].…”

“…Such conductivity mechanisms are governed by the quantum tunneling effect which relates exponentially to inter-NP distance [ 28 ]. Many research groups focus their interest on increasing the sensitivity of NP-based sensors, and usually, this can be achieved by incorporating NP films with varying conductivities, so as to manipulate the charge transport mechanisms of the device [ 19 , 20 , 22 , 29 ]. Lee et al [ 19 ] studied the combination of metallic and insulated NPs as sensitive materials, and thereby combining Au NPs, CdSe NPs, and nanocracks, a gauge factor of up to 5045 was achieved.…”

“…Many research groups focus their interest on increasing the sensitivity of NP-based sensors, and usually, this can be achieved by incorporating NP films with varying conductivities, so as to manipulate the charge transport mechanisms of the device [ 19 , 20 , 22 , 29 ]. Lee et al [ 19 ] studied the combination of metallic and insulated NPs as sensitive materials, and thereby combining Au NPs, CdSe NPs, and nanocracks, a gauge factor of up to 5045 was achieved. In the field of bio-inspired sensing devices, cracks have often been employed, so as to radically increase the sensor’s sensitivity [ 19 , 30 , 31 , 32 ]; Han et al [ 31 ], created a crack-based strain-sensor by depositing Au NPs on top of a cracked Polydimethylsiloxane (PDMS) substrate, obtaining a sensitivity of 5888.…”

“…Lee et al [ 19 ] studied the combination of metallic and insulated NPs as sensitive materials, and thereby combining Au NPs, CdSe NPs, and nanocracks, a gauge factor of up to 5045 was achieved. In the field of bio-inspired sensing devices, cracks have often been employed, so as to radically increase the sensor’s sensitivity [ 19 , 30 , 31 , 32 ]; Han et al [ 31 ], created a crack-based strain-sensor by depositing Au NPs on top of a cracked Polydimethylsiloxane (PDMS) substrate, obtaining a sensitivity of 5888. However, high sensitivities are also possible without cracks on the substrate.…”