Flexible Electronic Devices Based on Two-Dimensional Carbon Nanomaterials for Wearable Applications


Student thesis: Doctoral Thesis

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Awarding Institution
Award date27 Aug 2019


Carbon, as a fundamental and the most indispensable element in the planet to construct organic life, has been attracting great attention for lots of applications. Past decades of years have witnessed the prosperous developments of carbon nanomaterials with different dimensions. Two-dimensional (2D) carbon materials, especially graphene, has been regarded as the basic unit to construct other types of carbon materials such as fullerene, carbon nanotubes and carbon foams. Due to its excellent electrical and mechanical properties, graphene has a huge potential to be researched in the current time.

With the recent prosperous development of medical diagnosis, artificial intelligence and electronic skins, wearable electronic devices have drawn much considerable attention in our daily life. Among these different kinds of wearable devices, flexible pressure sensors based on the change of electrical resistance have received extensive attention due to some special properties such as fast response and high sensitivity, light weight, easy processing, and low cost. In actual pressure sensor applications, the contact area will change by motion induced pressure change, which can come from touching, pressing, heartbeat, or respiration induced pulse, further resulting in the change of electrical resistance. In order to detect the pressure from the external forces accurately, flexible pressure sensors that can be processed easily, with high sensitivity (especially with a wide pressure window ranged from 0-20 kPa) are highly desirable. In this work, graphene membranes (GMs) with layer-by-layer structures have been successfully fabricated via a facile self-assembly and air-drying method. Due to the excellent electrical properties of graphene films, the GM-based pressure sensor exhibited excellent sensitivity of 52.36 kPa-1 and repeatability in the pressure range of 0-50 kPa. The prepared sensor can be stable and sensitive under both low and high pressures considering its actual application. In addition, the pressure sensor shows excellent performance in wearable applications to detect human motions such as pulse, breath as well as several other types of intense motion. As far as we can concern, compared with most graphene-related pressure sensors, our device shows better sensitivity and stability, which will make a difference in the fields of wearable applications. It’s hoped the facilely prepared layer-by-layer GM-based pressure sensor will have huge potential in smart soft devices for future applications.

Organic memory devices with ultrahigh density data storage are highly required for future electronics with the booming development of the information technology. Among different kinds of memory devices, resistance random access memory has drawn extensive attention as the nonvolatile memory possesses simple structures, easy processing, low cost and mechanical flexibility. In this work, memory devices based on biodegradable carboxymethyl cellulose and graphene oxide (CMC-GO) nanocomposites are demonstrated for the first time. The device with the Al/CMC-GO/Al/SiO2 structure shows excellent write-once-read-many-times (WORM) switching properties such as low operation voltage of 2.22 V, high ON/OFF ratio of ~105, long retention time of >10,000 s, good stability and durability, etc. The flexible memory on polyethylene terephthalate substrate (Al/CMC-GO/Al/PET structure) has a huge potential to be applied in wearable high-performance devices for data storage. The resistive switching effect can be observed in the CMC-GO nanocomposites because the hybrid sites of CMC-GO nanocomposite serve as the trapping centers to form the conduction filament in the conduction process. This newly designed cellulose-based polymer nanocomposites are quite cheap and easy processed for large scale manufacturing of memory devices and can further contribute to future data storage applications such as portable stretchable displays, wearable electronics and electronic skins in the coming age of artificial intelligence.

In conclusions, two-dimensional carbon nanomaterial especially graphene and its derivatives have been fabricated into graphene-based pressure sensor and graphene-based biodegradable memory device. They can exhibit brilliant performance as flexible electronic devices for wearable applications. What’s more, the synthesis of graphene-cellulose nanocomposites provides a new way in wearable soft electronics. We have the reason to believe the 2D carbon nanomaterials will continue to have great potential to be applied in the future applications such as energy storages, information changes and wearable devices, etc.