Three main directions: in situ spectroscopy, oxide thin film meso-materials, oxide thin film growth.
In situ Spectroscopy
In situ characterization gives the opportunity to investigate intrinsic electronic properties of epitaxial systems by electron spectroscopy as well as take advantage of the two ways of generating particles for deposition, i.e., electron beam deposition and pulsed laser deposition. Because of both the in situ thin film synthesis, eliminating anomalies due to the interaction with ambient air, as well the fact that probing depth are of the order of the thin film thicknesses, a true comparison of electronic properties and transport properties are possible.
- Current investigations, using the COMAT system at Twente, a UHV pulsed laser deposition (PLD) system with various in situ spectroscopies and imaging techniques (XPS, UPS, XPD, STM, AFM, PFM) inspired on the system above, are aimed at controlling the octaeader rotations by epitaxy and study their effect on properties of various magnetic perovskite-type oxides such as SrRuO3 and LaSrMnO3. Both in situ photoelectron spectroscopy as well in situ photoelectron diffraction are being employed and with a soon to be installed in situ nano-probe system based on scanning tunneling microscopy, transport measurements can simultaneously be performed on a local scale.
- In a separate effort, in the MURI program for superconductor coated conductors a real time diagnostic procedure was demonstrated, which was made possible by Fourier Transform Infrared reflectometry (appeared in Applied Physics Letters, 2007). Not only does this technique monitor the exact temperature of the surface on which a film is growing, it was also found that the reflectivity spectra give useful information regarding the state of the growing film. For the case of high rate Electron-Beam deposition of YBa2Cu3O7 on metal tape we discovered that the nucleation and growth of the superconducting phase are wildly dependent on the oxygen pressure and activity during and just after the deposition as revealed by FTIR.
Oxide thin film meso-materials
In this research (funded under TOP-NWO) we want to make a leap forward in inorganic materials development, and design and assemble such objects to form larger, hierarchical structures. The aim is to develop strategies that combine material synthesis and (self-)assembly of nanosized “building blocks” (spheres, cubes, wires, rods, core-shell structures, etc.), into mesoscale architectures with 3D spatial control over the location of elements of the assembly. In such hierarchical structures, the traditional molecular length scale is far extended, and collective effects determine the property of the assembly.
- nano-wires by self-organization: Single crystalline complex oxide nano-wires by self-organization. This is achieved by exploiting two unexpected particularities of the terminating atomical planes of single crystalline perovskite transition metal oxides: for example 1), we found that in the case of (110) DyScO3 the two possible terminating surfaces, DyO and ScO2, order into line-like patterns and 2) the difference in surface diffusivity of the deposited metallic and ferromagnetic SrRuO3 on the two terminations, driving the self organization process. The SrRuO3 wires are metallic, yet electrically isolated from each other over macroscopic length scales, single crystalline and fully oriented, which could be applied in cross-wire architechtures or as a patterned electrode material in combination with ferroelectric oxides such as Pb(Zr,Ti)O3.
- Pulsed laser deposition on 2D nanosheets: 2-dimensional metal oxide nanosheets of Ti0.87O2 and Ca2Nb3O10 were used as 2D templates for guided growth of functional oxide films such as SrRuO3 (Nijland, ACS Appl. Mater. Interfaces 2013) and (La,Sr)MnO3. Nanosheet films were synthesized and placed on silicon substrates by Langmuir-Blodgett deposition. Using pulsed laser deposition, SrRuO3 films were formed on the substrates containing the nanosheets seed layers. The presence of nanosheets had a clear impact on both the morphology and the crystallographic orientation of the films. The nanosheets also had a clear effect on the magnetic properties of the films, which showed anisotropic behavior only when a nanosheets seed layer was used. A monolayer consisting of a mixture of both types of nanosheets was made to illustrate that nanosheets can be used to locally control the structure of films on a single substrate. This promising possibility may pave the way to films with position dependent properties that are determined by the local crystallographic orientation (Nijland, 2015). We obtained atomic scale roughness by introducing a SrTiO3 interlayer. Currently the properties of these films are studied.
Oxide thin film Growth (enabler)
- Co-development of a high-pressure reflection high-energy energy diffraction (RHEED) to study the growth of complex oxides during pulsed laser deposition (PLD). This new technique allowed the in situ study of oxide thin film growth at the most favorable deposition conditions, that is high temperature to enable epitaxial growth and high deposition oxygen pressure to enable stable phase formation. As a direct result of this development, many new technologies have been developed. (APL 1997)
- Hybrid Pulsed Laser Deposition/Molecular Beam Synthesis (MBE) system for oxide growth with in situ XPS1, UPS2 and RHEED3 . Unlike PLD, MBE growth requires rate control for the individual components of the materials (e.g., Sr and Ru and O in SrRuO3). The use of Electron Impact Emission Spectrometry was studied for such purposes as well as the generation of atomic oxygen.
- Epitaxially stabilized oxides with imposed crystal symmetries, such as CuO, which occurs as the monoclinic mineral tenorite in nature, but we have evidence now that CuO can assume higher crystal symmetry when grown on a suitable substrate. (PRB 2008, PRL
- In a quite different approach we have monitored the crystallization of both YBa2Cu3O7 and YBa2Cu4O8 from dense glassy precursors using X-ray scattering and an ambient controlled hot stage (APL, 2007). This general method makes possible the growth of films from constituents which are too volatile or where the phase equilibrium is unfavorable.
- One of the topics that has been studied with prof. T.H. Geballe at Stanford is a simple predictive method to select potentially interesting oxide materials based on an ionic approach (PRB 2006).