Nanoscale characterization of ZnO, PbTiO3, CH3NH3PbI3 and thin film growth of Cu2OSeO3

Abstract

Scanning probe microscopy (SPM) is a method to image and measure on the nanometre and atomic scale which has enabled significant progress in science and engineering. In this thesis this technique was used to study the electrical and piezoelectric properties of different materials. Materials were chosen based on their potential applications in science and technology. Each chapter in this dissertation is allocated to a specific material. ZnO with different structures has seen widespread research interest because of piezoelectricity properties that are used in a wide range of applications. In this dissertation the response of ZnO nano and microstructure platelets in dark and under laser illumination was studied using c-AFM. The results show that the grain boundaries in ZnO platelets are sensitive to light with energy below the band gap. This is due to the defect levels at grain boundaries and photoexcited carriers. The piezoelectric response of ZnO nano and microstructure platelets was studied by the PFM technique. Application of electrical field alters the piezoresponse in individual grains. These observations suggest new pathways for using conducting ZnO in optoelectronics devices which rely on grain and grain boundary engineering. Electrical conduction at domain walls of PbTiO3 (PTO) single crystals was also studied in this dissertation. PTO is a well-known ferroelectric material that has considerable ferroelectric and piezoelectric properties. It has applications in electrical devices like piezoelectric transducers and up to now studies are only focussed on piezoelectric domains for this material. Here, the study is focus on domain wall conductivity conducted by conductive atomic force microscopy (c-AFM). Naturally occurring 180° domains exhibit current flow along the delineating domain wall. The domain wall current flow shows Schottky-like rectifying behaviour. The nanoscale effects of photochemically-active additives on grain boundaries in CH3NH3PbI3 solar cells was studied. Scanning probe microscopy under light illumination, in particular Kelvin probe force microscopy, was applied to study the surface potential changes under laser light illumination. The recently found improvement in efficiency of BQ added solar cells can be clearly seen in vanishing contact potential differences at grain boundaries under illumination, rendering the material more uniform in solar cell operating conditions. Our findings shed light onto halide perovskite materials and functional additive design for improved solar cell performance. IIn addition to SPM measurements on ZnO and PTO, the growth of Cu2OSeO3 thin films is also discussed in this dissertation. The focus was to study the magnetic skyrmion systems in this material. The insulator Cu2OSeO3 has gained interest because of the possibility of electrical control of magnetism in insulator materials, which is energetically more efficient in comparison to metallic systems. Thin films were grown by the pulsed laser deposition (PLD) technique on MgO and NdGaO3 substrates. The grown thin films were characterized by XRD and AFM. The formation of skyrmions was confirmed by small angle neutron scattering