Transition metal oxides are in the very focus of current worldwide research interest as highly promising material candidates for various forefront applications. Spectacular phenomena such as high-Tc superconductivity (HTSC), colossal magneto-resistivity (CMR) and high-efficiency thermoelectricity (TE) have already been uncovered for these materials. Many of the key phenomena stem from strong electron-correlation effects that tend to localize the electrons in these oxides. As a consequence, a so-called pseudogap is opened up that is sensitive to, e.g. temperature, external electrical or magnetic field, optical excitation, mechanical pressure and chemical doping. Another common feature is that many of the most attracting functional oxide materials possess a multilayered crystal structure. Along with the aforementioned HTSC, CMR and TE materials, the examples include the ion-conductive materials extensively used in Li-ion batteries, solid-oxide fuel cells (SOFCs), oxygen permeation membranes and sensors.
In conventional multilayered materials different atomic layers are stacked to form a crystallographically coherent crystal. Besides such materials also incoherent oxide materials are known (so-called misfit-layered materials) in which two or more nanoscale blocks are incoherently coupled in forming a kind of self-assembled nanocomposite crystal. What we should do is to explore and creatively utilize the consequences of lost coherency between different layers, as such materials are likely to provide us with possibilities for incorporating multiple and even seemingly contradictory functions into a single material. When the basic understanding is gained of the role of each layer in such oxides exhibiting uncommon properties as a whole, it is possible to synthesize new compounds with tailored properties by finely controlling e.g. carrier concentrations of the individual blocks as well as the inter-block interactions.
Moreover, layered crystals – both commensurate and incommensurate – are potential hosts for additional intercalated guest species to be accommodated between the adjacent layers/blocks. Therefore, synthetic strategies based on topotactic soft-chemical intercalation routes are likely to become increasingly important “layer-engineering” tools for state-of-the-art tailoring of advanced functional materials. Another prominent synthesis approach is to deposit the material layer-by-layer from the gas phase. Atomic layer deposition (ALD) and molecular layer deposition (MLD) techniques provide us with exciting possibilities not fully exploited yet. Just to highlight some examples, these techniques enable us (i) to combine inorganic and organic layers into completely new hybrid materials, (ii) to coat various novel/functional/sensitive substrate materials such as steel, graphene, polymers, nanocellulose and other biomaterials with thin oxide layers, and (ii) to transfer a nano-structure-based function to a functional oxide material grown in a conformal manner on top of a substrate with the desired nanostructure.
- J. Malm, E. Sahramo, M. Karppinen & R.H.A. Ras, Photo-controlled wettability switching by conformal coating of nanoscale topographies with ultrathin oxide films, Chemistry of Materials 22, 3349 (2010).
- T. Hirvikorpi, M. Vähä-Nissi, T. Mustonen, E. Iiskola & M. Karppinen, Atomic layer deposited aluminum oxide barrier coatings for packaging materials, Thin Solid Films 518, 2654 (2010).
- K. Uusi-Esko & M. Karppinen, Extensive series of hexagonal and orthorhombic RMnO3 (R = Y, La, Sm, Tb, Yb, Lu) thin films by atomic layer deposition, Chemistry of Materials 23, 1835 (2011).
- J.T. Korhonen, P. Hiekkataipale, J. Malm, M. Karppinen, O. Ikkala & R.H.A. Ras, Inorganic nanotube aerogels by atomic layer deposition onto native nanocellulose templates, ACS Nano 5, 1967 (2011).
- T. Hirvikorpi, M. Vähä-Nissi, J. Nikkola, A. Harlin & M. Karppinen, Thin Al2O3 barrier coatings onto temperature-sensitive packaging materials by atomic layer deposition, Surface and Coatings Technology 205, 5088 (2011).
Cicada wings are superhydrophibic (water repellent) due to their specific pillar-like surface nanostructure. When they are coated with a thin layer of ZnO by ALD, the coating alters the surface chemistry, but not the characteristic pillar-like surface topography. Therefore, the surface still exhibits high contact angles, i.e. water stays on the nanopillars as a droplet. However, when this material is exposed to the UV light, the change in the intrinsic wetting properties of the ZnO coating transforms the surface to superhydrophilic, and water spreads to a flat layer.
Some strongly-correlated-electron oxides of transition metals are already applied as electro-functional materials. The demand is, however, remarkably increasing such that, e.g. in the next-generation sustainable energy technologies and in the currently strongly emerging new spintronics technologies, oxide materials will be playing the key roles. Our aim is to expand the variety of oxide-material candidates with potentially usable functions. The practical work comprises design, synthesis, modelling and characterization efforts. To extend the frontier of new-material research unique/uncommon synthesis approaches, such as high-pressure (HP) and combined ALD/MLD techniques, are employed. For sample characterization, the emphases are on (i) detailed crystal structure determination techniques, and (ii) techniques that are capable in probing the states of individual oxygen and metal species in terms of occupancy, valence, orbital and spin.
- L. Karvonen, M. Valkeapää, R.S. Liu, J.M. Chen, H. Yamauchi & M. Karppinen, O-K and Co-L XANES study on oxygen intercalation in perovskite SrCoO3-δ, Chemistry of Materials 22, 70 (2010).
- E.-L. Rautama & M. Karppinen, R-site varied series of RBaCo2O5.5 compounds with precisely controlled oxygen content, Journal of Solid State Chemistry 183, 1102 (2010).
- S. Vasala, J.-G. Cheng, H. Yamauchi, J.B. Goodenough & M. Karppinen, Sr2Cu(W1-xMox)O6: a quasi-two-dimensional magnetic system with a possibility of large magnetic frustration,submitted (2012).
From O-K and Co-L edge XANES spectroscopy it is revealed that upon increasing oxygen content of SrCoO3-δ from 2.50 to 2.82 oxygen intercalation proceeds as follows: (i) O2 is first absorbed on the surface as O2-, (ii) then reductively split into Ox-, and (iii) finally in the bulk reoxidized to Oz- (0 < z < x).
Examples of Exciting Material Families