Abstract This thesis is dedicated to realizations and physical understanding of electronic and photocatalytic properties after decorating transition metals to the semiconducting molybdenum disulfide. Synthesized via facile wet chemical methods, the MoS₂-Au, MoS₂-Au-Ni and MoS₂-Ag-Ni composites were formed as binary or ternary compounds. The Au nanoparticles are stably joined to the MoS₂ matrix without deteriorating layered structures of the host. After introducing the Au nanoglue as a common buffer, a metallic contact is reached between Ni and MoS₂, and attributed to new electron migration channel via MoS₂ edge contact. Adapting the Ag as the buffer element can attach the Ni to the basal plane of the MoS₂ beside edge contact. The Ni-Ag-MoS₂ composite effectively splits water under visible light irradiation and produce hydrogen. The excellent photocatalytic activity is attributed to effective charge migration through dangling bonds at the MoS2-Ag-Ni alloy interface and the activation of MoS₂ basal planes. / Original papers The original publications are not included in the electronic version of the dissertation. W. Cao, V. Pankratov, M. Huttula, X. Shi, S. Saukko, Z. Huang, M. Zhang. Gold nanoparticles on MoS2 layered crystal flakes. Materials Chemistry and Physics, 158, 89−95 (2015). DOI: 10.1016/j.matchemphys.2015.03.041 X. Shi, S. Posysaev, M. Huttula, V. Pankratov, J. Hoszowska, J.-Cl. Dousse, F. Zeeshan, Y. Niu, A. Zakharov, T. Li, O. Miroshnichenko, M. Zhang, X. Wang, Z. Huang, S. Saukko, D. L. González, S. van Dijken, M. Alatalo, W. Cao. Metallic contact between MoS₂ and Ni via Au nanoglue. Small, 14, 1704526 (2018). DOI: 10.1002/smll.201704526 http://jultika.oulu.fi/Record/nbnfi-fe2018060525279 X. Shi, M. Huttula, V. Pankratov, J. Hoszowska, J.-Cl. Dousse, F. Zeeshan, Y. Niu, A. Zakharov, Z. Huang, G. Wang, S. Posysaev, O. Miroshnichenko, M. Alatalo, W. Cao. Quantification of bonded Ni atoms for Ni-MoS₂ metallic contact through X-ray photoemission electron microscopy. Microscopy and Microanalysis, 24, 458−459 (2018). DOI: 10.1017/S1431927618014526 http://jultika.oulu.fi/Record/nbnfi-fe2018082834233 X. Shi, M. Zhang, W. Cao, X. Wang, M. Huttula. Efficient photocatalytic hydrogen evolution via activated multilayer MoS₂. Manuscript. X. Shi, Z. Huang, M. Huttula, T. Li, S. Li, X. Wang, Y. Luo, M. Zhang, W. Cao. Introducing magnetism into 2D nonmagnetic inorganic layered crystals: a brief review from first-principles aspects. Crystals, 8, 24 (2018). DOI: 10.3390/cryst8010024 http://jultika.oulu.fi/Record/nbnfi-fe201802153441

For over half a century, the technological promise of spins manipulable by a small voltage has captivated the interest of experimental and theoretical researchers alike. However, if thin-film multiferroics are to be incorporated into future data storage devices, a much greater understanding of their behaviour and how they differ from their bulk counterparts is required. In this thesis, we probe the fundamental multiferroic properties of BiFeO₃ films through a combination of state-of-the-art diffraction and microscopy techniques. We investigate the coupling between magnetic, ferroelectric, and structural order, with a focus on domains, and how the domain structure may be manipulated in order to tailor the multiferroic properties of the material. Using non-resonant magnetic x-ray scattering (NXMS) and neutron diffraction, we study the magnetic and structural properties of (111)_pc-oriented BiFeO₃ films. Contrary to the general belief that to they grow as a rhombohedral monodomain, we find that they comprise a sub-micron texture of monoclinic domains. The magnetic structure is found to be intimately coupled to the structure, resulting in the propagation vector being locked to the monoclinic b-axis. This magnetoelastic coupling opens up a route to strain-engineer the magnetic domains via epitaxial strain. By growing BiFeO₃ films on a lower-symmetry, TbScO₃ substrate, we are able to engineer a magnetic, structural and ferroelectric monodomain, coherent over the entire film, constituting an increase in the domain size by over five orders of magnitude. We directly demonstrate the coupling between ferroelectric and magnetic order parameters of the cycloidal magnetic structure. Using NXMS polarimetry to measure directly the magnetic polarity, we show that upon switching the ferroelectric polarisation, the magnetic polarity switches accordingly---a major rearrangement of the magnetic structure, with each spin rotating by 90 degrees on average. This goes counter to idea that magnetic and ferroelectric order parameters are only weakly coupled in type-I multiferroics. Finally, using photoemission electron microscopy we are able to directly image the sub-micron magnetostructural domain structure. We further show that there is a strong interfacial coupling between the magnetostructural domains of BiFeO₃ with a ferromagnetic overlayer. The BiFeO₃ domains are found to impose a uniaxial anisotropy in the overlayer, opening up a route to control ferromagnetic domains.