Discovery of Novel Materials in Actuator Applications
Creators: name: Tran, Kevin
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 340704
subject: Theoretical quantum chemistry
classificationCode: 340303
subject: Nanochemistry

description: Many of the current devices that society uses daily as a necessity, rely on some form of motion. This can range from the motion in large devices such as motors in cars, to the motion in speakers present in smaller devices such as smartphones. The motion that is produced comes from actuators, which are devices that can undergo dimensional changes in response to stimuli from the environment external to the device or within the device itself. Such stimuli can include temperature, pressure, mechanical strains and electrical charges.
The materials often found in actuators are often referred to as stimuli responsive materials, making them ideal for various environmental conditions. However, as technology advances, so too will the efficiency and performance needs of these devices. To limit waste in the production process, or to reduce the dimensions of the devices, the materials required will also become smaller. Therefore, it is essential to find materials that can make up these newer devices while maintaining their functionality and providing sufficient outputs.
Materials that respond to electricity or injected charge are ideal for actuators as they are known to generate mechanical deformations that are highly accurate within the micro- and nanometre range. This includes piezoelectric materials that are able to convert electrical energy into mechanical deformations and vice versa.
In this thesis, the effect of external stimuli, specifically application of electric field and charge, on the structure and properties of a variety of nanomaterials will be investigated. This work focuses on using a computational approach and specifically density functional theory (DFT) calculations to screen the piezoelectric and electromechanical properties of one-dimensional (1D) and two-dimensional (2D) nanomaterials for their potential application in actuator devices. Other applications, namely as gas sensing materials is also explored. 
The nanomaterials studied in this work include ultra-thin 2D zinc oxide (ZnO), 1D black phosphorus (BP) nanotubes, and the 2D monolayer and bilayer group-IV monochalcogenides (germanium sulfide (GeS), germanium selenide (GeSe), tin sulfide (SnS) and tin selenide (SnSe)) as they all have either outstanding piezoelectric or structural properties suitable for actuator devices. 
ZnO is known to be a highly piezoelectric material, producing the largest piezoelectric responses among tetrahedrally bonded semiconductors. With its non-centrosymmetric buckled structure and distinct polar surfaces, ZnO is the ideal material for piezoelectric based applications. However, when reduced to fewer than 8 atomic layers thick, 2D ZnO sheets become planar similar to a sheet of graphene, and as a result lose their piezoelectric properties. In this thesis it is shown that by introducing defects to the system, the centrosymmetric structure of graphitic ZnO nanosheets can be broken, restoring their polarity. In particular, it was determined that Zn vacancies can not only restore the piezoelectric properties of ZnO nanosheets but can produce piezoelectric responses that exceed the bulk value by 120%. 
The group-IV monochalcogenides are both highly piezoelectric and flexible due to their similar structure with BP and non-centrosymmetric structure. Having a puckered orthorhombic structure allows them to undergo high strains making them ideal for actuator applications. It is not known, however, how their 2D monolayers respond to charge injection. All 4 monolayers can achieve enhanced strain responses after hole or charge injection, with monolayer SnS achieving an electromechanical strain response up to -12.7% greater by contracting. The volumetric work density and actuation stress produced by these materials upon charge injection means they could be ideal for nano-based actuators. 
As the group-IV monochalcogenides exist as layered materials, they can be synthesized as bilayers. By stacking two layers of the same material or two different materials, bilayers could yield properties that are not seen in either the bulk or monolayer counterparts. In this work, the piezoelectric response for different stacking arrangements were investigated. Overall, bilayer SnSe was calculated to have a 130% increase in the piezoelectric response compared to the monolayer. 
BP monolayers are known to have outstanding strain responses due to their puckered orthorhombic structure. The flexibility offered by BP allows it to easily deform in the presence of external stimuli, with charge injection causing the monolayer to expand significantly. However, it is not known how their 1D counterpart, nanotubes, will respond to the injected charge. In this work, the electromechanical response of BP nanotubes comprised of 4 different diameters are examined. It was shown that a NT with 15 BP units can expand in diameter up to 30.2%, a value which is larger than other 1D nanomaterials, such as carbon and molybdenum disulfide nanotubes. Hence the expansion in diameter enables further applications for the material in drug delivery and nanochannel applications.
In addition to the potential applications in piezoelectric and electromechanical actuators, 2D nanomaterials are ideal candidate materials for gas sensing devices due to their high surface area. The sensitivity of monolayer group-IV monochalcogenides to toxic gases, including NH3, NO2 and SO2 was determined using DFT calculations. For a 0.25 ML coverage, the reducing gas, NH3, was physisorbed on the monolayer and acted as an electron donor. The sensitivity to the oxidising gases, NO2 and SO2 were significantly stronger, with both gases withdrawing electrons from the monolayers. However, the binding of the gases to the monolayers was shown to be controllable with an applied electric field. An electric field of -0.8 eV/Å changes the adsorption type of NH3 from being physisorbed to weakly chemisorbed with the direction of charge transfer reversing so the gas withdraws charge from the monolayers. The sensitivity of NO2 was greatly enhanced with a +0.8 eV/Å electric field making the gas strongly chemisorbed on the monolayers. Similarly, the sensitivity to SO2 was also increased but to a lesser extent. 
To summarise, nanomaterials that are highly piezoelectric and flexible are promising candidates for actuator-based devices as they can generate sufficient strain responses to produce accurate displacements in a large range. Furthermore, the strain responses of monolayer group-IV monochalcogenides can be used to enhance the storage and sensing ability of these materials to pollutant gases. The results presented in this thesis have provided an insight into the possible candidate materials for piezoelectric and electromechanical actuator devices.