Liquid exfoliation of layered metal oxides and their langmuir-blodgett films
Prof. dr. ir. J.E. ten Elshof
Research on 2D materials has been booming since the experimental discovery of graphene. As one of the key enablers in 2D materials development, a mature synthesis route to form 2D materials is the way to push those new materials forward from the lab to practical applications. Insight into the exfoliation process is undoubtedly helpful to develop and establish a mature fabrication route. As a member of the family of 2D materials, metal oxide nanosheets include more than 30 compounds, and they show advantages in several applications, such as thin film growth and the design of functional devices. This thesis reports results from exfoliation studies on layered metal oxides and the formation of their Langmuir-Blodgett (LB) films.
In situ experiments showed that the exfoliation of layered titanates occurs within seconds, and that the mechanism is different from the prevalent theory of exfoliation. A study of the exfoliation process under surfactant deficient conditions shows that it is possible to selectively remove nanosheets from the suspension that are of small size and defective. The formation of titanium oxide LB films was investigated to understand the formation process of metal oxide nanosheet layers at liquid-air interfaces (LAI). The results show that at short exfoliation time the nanosheet concentration at the LAI is higher than after long exfoliation time, even though the nanosheet concentration in the bulk of the solution increases with time. These findings can help to reduce the fabrication time of metal oxide nanosheets and their LB films significantly.
A study on the influence of LB parameters on the coverage of LB films shows that the coverage is not determined by the nanosheet concentration, but only by the surface pressure during depositions. In combination with the study of the exfoliation process under surfactant deficient conditions, a step exfoliation process has been established to ensure the fabrication of high quality nanosheet films. Moreover, a protocol has been established to fabricate potassium calcium niobate (KCNO) crystals and calcium niobate (CNO) nanosheets with different sizes. The influence of CNO nanosheet size on properties of PZT films was studied by using CNO nanosheets as seed layers.