Materials challenges of two-dimensional materials for flexible sensing applications
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Sensors are playing an increasingly important role in our lives because they enable the detection of environmental changes and, therefore, initiate a response accordingly. Sensors convert detected physical or chemical changes, for example, motion, radiation, heat, acidity, chemicals, etc., to useful and readable signals. Field-effect transistors (FETs), a class of semiconductor device in which the electrical current is controlled through an applied gate voltage, are promising for many sensing applications. Even though FETs-based sensors have been well-developed, flexible version of such sensors remains a big challenge and requires new materials and new sensing designs. Two-dimensional (2D) materials such as graphene and transition metal dichalcogenides (TMDs) are promising candidates for FET-based sensors due to their flexibility, transparency and potential for high electrical performance. Because of the atomically thin nature of 2D materials, their electrical properties are extremely sensitive to their atomic-scale structure as well as to their surfaces and interfaces with other materials. Specifically, defects, dopants, attached molecules or change in the band structure due to strain can shift the Fermi level resulting in a measured change in current. The goal of this work is to meet the challenges faced by the 2D TMD-based electronics in sensor applications by developing a fundamental understanding of the impact of materials processing, structure, interfaces and surfaces on resultant electronic properties. Furthermore, a simplified strategy for chemical and biological electrical sensors is developed to bridge the current sensing technology to the use of next generation flexible 2D transducers. To improve 2D TMD-based electronics, this work demonstrates a solution-processed molecular doping technique to control the electronic band structure and the electrical performances of TMD-based devices. Charge transfer doping due to the electron transfer between TMD semiconductors and the redox-active molecular dopants is demonstrated to be a promising tool to control the carrier concentration as well as the Fermi level of MoS2 and WSe2. Understanding the impact of external strain on the flexible devices is crucial toward their practical application. This work investigates the electronic properties and stability of flexible TMD FETs under mechanical strain. The interesting mechano-electric properties of TMDs provide a new opportunity for transparent and flexible mechanical strain sensors. Furthermore, the fundamental physics and the controllability of this strain sensitivity are studied. FET-based potentiometric sensors provide a promising technique for the detection of chemical and biological species without the use of secondary bio-labels. This work first focuses on the comparison between two commonly used potentiometric sensing platforms – ion-sensitive field-effect transistor (ISFET) based on nano-materials, and a similar, but simplified, extended-gate FET (EGFET) in which the sensor surface is separated from the transducer. It is then demonstrated that the sensor sensitivity depends on the sensing surface instead of sensor platform. As a result, the following demonstration of biochemical sensing is based on EGFETs. In addition, EGFETs provide a more reliable operation and ready compatibility with any commercialized transistors currently as well as 2D TMD-based transistors in the future. Finally, EGFET is proved a promising candidate for practical sensing application by a direct comparison between EGFET potentiometric biosensors with impedimetric biosensors. The work presented in this thesis demonstrates initial first steps toward the sensing applications using 2D TMD semiconductors. Despite the current challenges faced by 2D TMD-based FETs in biochemical sensing applications, the proposed EGFET configuration provides a readily available biosensing technique for current technologies and the future compatibility to 2D TMD-based transducers.