Two-dimensional (2D) functional materials for sensing and piezoelectric applications
Creators: name: KHAN, Hareem
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 401605
subject: Functional materials
classificationCode: 401804
subject: Nanoelectronics

description: Due to the distinctive physicochemical properties of two dimensional (2D) materials, therehas been an increase in research being conducted in this area. Owing to property changes that occur with reduction in dimension such as large surface area to volume ratios and a lack of centrosymmetry, the functionality of 2D materials spans a wide range of applications. In 2D structures, any external stimuli affects the whole structure and an enhanced effect is thus seen as the bulk is synonymous to the whole structure.

In this thesis, the effect of the external stimuli (including chemical, electrical and mechanicalstimuli, etc.) such as gas molecules as well as the resultant electrical polarisation effects due to an external mechanical stress on various 2D materials is studied. The core of the work involves producing and studying 2D metal oxides and sulfides for sensing applications as these two groups of metal compounds offer great functionalities. This work further explores the application of such materials within the scope of chemical (gas-sensing) and physical (piezoelectric) sensing applications.

The 2D materials that are studied for the gas sensor applications are tin disulphide (SnS2) and tungsten oxide (WO3); this is due to their suitable electronic properties and favourable surface energies with paramagnetic NO2. The piezoelectric studies focus on zinc oxide (ZnO) and tin monosulfide (SnS) due to their acknowledged piezoelectricity tendencies. The investigation done by the author was implemented by carrying out tailored synthesis - giving access to tuneable properties, characterisation of the material, and finally studying the gassensing and piezoelectric characterisations.

With the current air pollution levels, resulting in hazardous living and working environments - in many highly industrialised nations, suitable and reliable gas sensing technologies are necessary to inform decision making and mitigate the effects of pollution. However, current technologies of gas sensors still fail to reach the set targets in sensitivity, selectivity and stability for many pollutant gases and particularly the highly toxic NO2. Hence, further research is necessary to find better sensing alternatives. The choice of highly sensitive 2D materials as the sensing material in conjunction with physisorption gas response mechanism can fulfil this criteria. Physisorption, which emerges at lower operating temperatures, can potentially offer selectivity, if the synergy between the target gas and the polarisation property of the sensitive material’s surface are explored.

As such, tungsten oxide (WO3) is a material with a favorable surface energy value for NO2 adsorption facilitating the charge transfer and thus resulting in selective sensing at a low operating temperature. Hence, in the first part of thesis, the author used this material in the implementation of a low cost, selective and sensitive sensor. A novel, facile, and scalable wet-chemical synthesis method was used to fabricate the WO3. The resultant sensor demonstrated an extraordinary response factor of ~30 selectively to ultralow 40 parts per billion (ppb) NO2 at a relatively low operating temperature of 150°C.

Further it is noted that the current technological demands of sensors are heading towards the use of wireless, passive sensors in areas such as biomedical and food monitoring and particularly for real-life applications that exist in harsh environments. This class of sensors provide numerous advantages such as their remote sensing capabilities, non-intrusive nature, and the elimination of the use of wire or solder that may deteriorate. Passivity of sensors provides benefits such as longer lifetimes, low maintenance and low-cost. In the next part of the thesis, the author employed a novel inductive-capacitive antenna platform which is both wireless and passive for the NO2 gas-sensing. This type of wireless sensor is simple to not only design but also to fabricate. 2D tin disulphide (SnS2) was integrated as the sensing material for the NO2 detection. The planar SnS2 in limited layers was nominated as it provides a large active surface area for the enhanced adsorption of NO2 and has a small interlayer binding energy. Low electronegativity and suitable electronic band positions potentially enhances gas adsorption facilitating physisorption and parts per billion (ppb) level sensitivity. The obtained LC gas sensor was successfully implemented as a wireless sensor of selectively NO2 at limit of detection (LOD) of ~ 0.6 ppm.

In the area of physical sensing, the use of piezoelectric materials is an important consideration. Piezoelectricity refers to the associated electrical polarisation as a result of applied mechanical stress in a material. Materials with this property can be utilized in future low-power or self-powered sensing applications such as nanogenerators. The piezoelectric properties in 2D materials, which are not seen in their bulk counterparts, arise due to the breakdown in inversion symmetry as they are reduced to few layer thicknesses. Recently, a great interest has developed in these types of materials due their high crystallinity and ability to withstand enormous strain. The author of the thesis started this investigation with a conventional non-layered piezoelectric material zinc oxide (ZnO). ZnO is a well-known piezoelectric material when in its bulk form. 1D morphologies have been studied extensively but there is limited work on 2D structures and the effect of thickness reduction to the piezoelectric properties. Using the recently developed technique of delamination of the interfacial oxide layer on a liquid metal droplet, the author synthesised nanosheets of different thicknesses and then utilising Piezoresponse Force Microscopy (PFM), the author studied the electromechanical co-efficient variation with material thickness. 

In the next and final part, the author then worked towards using the acquired techniques to experimentally study the piezoelectric properties of tin sulfide prompted by recent theoretical depictions of colossal piezoelectric tendencies in the material. The puckered lattice structure of SnS provides this increased flexibility but the major challenge to this study till date had been the synthesis of crystalline, homogeneous and large monolayer SnS nanosheets. The author achieved this by executing the optimised liquid metal delamination in a sulphur-rich environment to synthesise sulphides instead of oxides. Material characterisations established the formation of ultrathin, highly crystalline monolayers of SnS featuring a notable carrier mobility of ~35 cm<sup>2</sup> V<sup>-1</sup>s<sup>-1</sup>and a piezoelectric coefficient of ~26 pm V<sup>-1</sup>. This material was then used to form unprecedented energy-harvesting devices. The piezoelectric nanogenerator (PENG) fabricated demonstrated an average peak output voltage of ~150 mV at 0.7% strain.

In summary, the author of the PhD has made significant contributions to the areas of physisorption-based gas sensing of NO2, liquid metal synthesis of sulphides and the piezoelectric properties of ultrathin non-layered oxide and layered metal oxides and sulphides. The Ph.D. study has introduced novel pathways to fabricate low-cost 2D
materials for enhanced sensing.