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Thesis Format

Integrated Article

Degree

Doctor of Philosophy

Program

Chemical and Biochemical Engineering

Supervisor

Dr. Zhang, Jin

Abstract

Magnetoresistance (MR) refers to the change of a material’s electrical resistance under the presence of an external magnetic field. The discovery of new MR phenomena (e.g., giant magnetoresistance (GMR) and tunneling magnetoresistance (TMR)) since the 1980s initiates the revolution of novel electric devices in the fields of data storage, position sensing, current sensing, non-destructive monitoring, biomedical sensing systems, etc. However, current devices display inadequate MR at the low magnetic field/room temperature, limited working range, and bulky size, which hinders the further application of MR sensors/devices. Therefore, the main goal of this thesis is to develop two-dimensional MR materials with high performance and their potential applications.

First, a new hybrid nanosheet was designed and developed by integrating reduced graphene oxide (rGO) and FeCo nanoparticles (NPs). A facile solvothermal process was developed to produce FeCo/rGO hybrid nanosheets with significant MR (21 ± 6%) at the low magnetic field (10 kOe) and room temperature. In addition, we demonstrated that the wireless magnetic field sensing system with FeCo/rGO hybrid nanosheets and ZigBee radio modules was able to achieve real-time detection and data collection of a working mobile phone. By adjusting the mass ratio of rGO adding to the system, we obtained the tunable MR of FeCo/rGO hybrid nanosheets. On the other hand, it was found that the formation of Co-Mn oxides NPs on rGO hybrid nanosheets was increased as the mass ratio of rGO added in the reaction exceeded 50 wt.%. The effects of imported ions on the MR of hybrid nanosheets were investigated. The formation of Co-Mn oxides NPs on hybrid nanosheets in this solvothermal process could lead to relatively high MR (3.5% ~ 4.5%) compared with other structures containing Co and Mn under the same conditions.

In addition to the chemical synthesis, we constructed FeCo/rGO hybrid nanosheets by using a laser-assisted physical deposition process, i.e., the matrix-assisted pulsed laser evaporation (MAPLE). The FeCo/rGO hybrid nanosheets prepared by MAPLE displayed MR with a level of 0.7% at ambient temperature and low magnetic fields (10 kOe), which is larger than or close to the reported MR of physically prepared FeCo-based granular materials/structures with higher FeCo ratios.

Finally, a mechanically flexible nanocomposite hydrogel with MR properties was developed. FeCo/rGO hybrid nanosheets were incorporated with the hydrogel matrix by using a photo-initiated polymerization process. The significant enhancements in mechanical properties compared with hydrogel matrix (toughness (0.11 MPa, 2.0x higher), Young's modulus (0.48 MPa, 1.5x higher), and maximal tensile stress (0.22 MPa, 1.7x higher)) and negative MR (-1.4 ± 0.3%) at room temperature and low magnetic fields were observed.

This work provides the solution to the challenges of developing ideal MR materials/structures and offers a better understanding of designing graphene-based nanocomposites with large MR. We believe that the flexibility and integrability of FeCo/rGO hybrid nanosheets not only benefit the design of MR sensors but also pave the way for extending the application of MR devices in the foreseeable future.

Summary for Lay Audience

Magnetoresistance (MR) can be defined as the variation of a material’s electrical resistance under the presence of an external magnetic field. Nowadays, MR sensors and devices are employed in magnetic storage (recording), position sensing, current sensing, non-destructive monitoring, biomedical sensing systems, etc. The performance of MR sensors at room temperature and low magnetic fields is important in these applications. However, current devices display inadequate MR at the low magnetic field/room temperature, limited working range, and bulky size. Therefore, the main goal of this thesis is to design and construct ideal MR materials/structures with large MR under these conditions.

We designed and developed a hybrid nanosheet based on reduced graphene oxide (rGO) and FeCo nanoparticles (NPs). The chemically synthesized FeCo/rGO hybrid nanosheets displayed significant MR at the low magnetic field and room temperature. A hybrid nanosheets-based wireless magnetic field sensing system was constructed, which achieved real-time detection and data collection of a working mobile phone. Meanwhile, we discovered the formation of Co-Mn oxides NPs on rGO hybrid nanosheets as the mass ratio of rGO added in the reaction exceeded 50 wt.%. The hybrid nanosheets with Co-Mn oxides NPs displayed relatively high MR compared with other structures containing Co and Mn under similar conditions.

In addition to the chemical path, we constructed FeCo/rGO hybrid nanosheets with a laser-assisted physical deposition process, i.e., the matrix-assisted pulsed laser evaporation (MAPLE). The MAPLE-prepared FeCo/rGO hybrid nanosheets exhibited MR close to or larger than the MR of other physically prepared FeCo-based granular materials/structures in previous works. Finally, by combining FeCo/rGO hybrid nanosheets with the hydrogel matrix, we developed a nanocomposite hydrogel with significant enhancements in mechanical properties and negative MR. The studies in this thesis provide a better understanding of designing and developing graphene-based nanocomposites with large MR. We believe that the findings in this thesis could be applied to overcome the obstacles of MR sensors and devices regarding sensitivity and cost-efficiency and benefit the future applications of MR devices.

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Available for download on Friday, September 01, 2023

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