Charge transport mechanism within 2D material films and its role on field effect transistor and capacitor technology

Abstract

Graphene and other layered materials such as transition metal dichalcogenides (TMDs) and MXenes have opened up a plethora of untapped potential for better device architectures and performance. One of the areas where 2D materials are regarded as disruptive technology is field-effect transistors (FETs), where 2D materials can offer high field-effect mobility (μFE ) and faster switching time. Moreover, the solution processability of 2D materials has enabled their usage as inks in large-scale printing processes. Thus, due to the economy of scale, 2D material device manufacturing has become competitive and attractive for commercialisation. However, significant hurdles, such as reproducibility in device performance, need to be addressed. In this thesis, ink formulations optimised for making textile capacitors and inkjet printed FETs are outlined. Initially, the devices’ inks were produced via liquid exfoliation (ultrasonic and electrochemical exfoliation). The 2D material inks were then coated on a desired substrate via either dip coating (textile capacitor) or inkjet printing. Subsequently, a variety of characterisation techniques were employed to understand the 2D material inks properties. For instance, XPS and Raman spectroscopy was used to understand the structural integrity of 2D material flakes. Raman data on the graphene flakes used in this thesis showed a characteristic peak at 1580 cm−1 (E2g) which is indicative of pristine few layer graphene flakes. In addition, XPS data on the MoS2 flakes provided conclusive evidence for the presence of 2H MoS2 which is a semiconductor, thus, has the potential to be used as a channel material in transistor. The solid state textile capacitor outlined in this thesis had a capacitance of ∼ 26 pF cm−2 which is comparable to other textile capacitor present in literature (∼ 50 pF cm−2). The textile capacitor also retained its energy storing capability when bent (bending radius ∼ 2.5 cm) with the C C0 only changing by 0.5%. Moreover, washability tests were also preformed on the textile capacitor. The results of the washability test exhibited a 5% decrease in capacitance after 10 wash cycles, which increased to 15% after 25 wash cycles. This thesis also discusses the fabrication of a complementary metal-oxide-semiconductor (CMOS) using MoS2(n-type metal-oxide-semiconductor, NMOS) and IDT-BT (p-type metal-oxide- semiconductor, PMOS). The MoS2 and IDT-BT transistors demonstrated an Ion/Io f f ratio of ≈ 50 and ≈ 103, while the mobility (μFE ) was measured as ≈ 0.06 ± 0.02 cm2 and ≈ 2.7 x 10−4 ± 5x 10−5 cm2 V−1 respectively. When the NMOS and PMOS were placed in series iv to make a CMOS the voltage gain (| AV,depletion−load |) was measured as 4 which indicates that the CMOS has the ability to function as a NOT gate. Lastly, this thesis also provides an insight into the charge transport characteristics for different films of 2D materials(graphene, MoS2 and Ti3C2) and governing factors that influence the charge transport mechanism. The modelling used to understand the charge transport involved conducting temperature dependant experiments, which provided an insight into the interaction between localized and extended states. The temperature dependent measurements revealed that 3D variable range hopping (3D VRH) is the main electron transport mechanism in graphene. While MoS2 films showed nearest neighbour hopping(NNH) at T > 200 K and 3D variable range hopping at T < 200 K. Such a change in charge transport mechanism (from NNH to 3D VRH) in MoS2 films has not been reported previously in literature to the best of our knowledge. This thesis also reports a average hopping distance (Ravg) and localization length (ξloc) of 0.21 nm and 0.4 nm for MoS2 films, while the graphene films showed a Ravg and ξloc of 14 nm and 13 nm. Both Ravg and ξloc for graphene and MoS2 films is smaller compared to the lateral size of the graphene and MoS2 flakes. This means unhindered charge transport taking place through the basal plane of graphene and MoS2 flakes, but the charge transport face obstruction at the boundaries of flakes. The MXene films in this thesis exhibited similar metallic behavior as epitaxial MXene films reported in literature. The resistivity of the MXene films were measured as ρ ∼ 46-7 μ Ω m (for T = 4.3-300K) which is one order of magnitude higher then epitaxial MXene films (ρ ∼ 6-5 μΩ m for T = 4.3-300K). On further examination using magnetoresistance measurements the presence of weak anti-localization was also confirmed in the MXene films which explained the up-turn in conductivity in MXene films at T < 100 K. These finding will aid future researcher to engineer 2D flakes and inks that can met the specific requirements for given devices applications.