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Physics Of Complex Inorganic Nano-Materials

Gertjan Koster

Designing oxide materials for advanced materials experiments and novel applications.

Thin film oxides are studied with two main goals: 1) New experiments with the aim to reveal (novel) electronic or magnetic degrees of freedom in a condensed matter system often require ad hoc designed materials either to meet the conditions they’re being interrogated at or match the used probes. 2) These very same materials systems (oxides) are expected to host a plethora of potentially technologically relevant properties, which further motivates their exploration in order to identify and demonstrate functionality.

Both goals can be achieved by having expertise in three areas: in situ spectroscopy of oxide thin films, the development of oxide thin film templates and the study of oxide thin film growth. Current focus application areas are: chemical sensors, brain inspired electronics and catalysis and energy conversion.

In situ Spectroscopy

In situ thin film (spectroscopic) characterization gives the opportunity to investigate intrinsic electronic properties of epitaxial systems by X-ray or electron spectroscopy as well as take advantage of different techniques for deposition, i.e., electron beam deposition and pulsed laser deposition. Because of the in situ-ness of the 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. Collaborations exist with UU, UBC/CLS, Soleil/Orsay, UvA/LCLS, PSI, Napels, DTU and EMAT, Antwerp.

  • 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 octahedra 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 an in situ nano-probe system based on scanning tunneling microscopy, transport measurements can simultaneously be performed on a local scale.
  • In collaboration with the university of Amsterdam (Golden) a vacuum suitcase was developed, enabling us transport thin film samples under a ultra-high vacuum conditions to synchrotron facilities such as Bessy in Berlin and Diamond in the UK, where spectroscopic X-ray characterization has been performed (XAS, HAXPES, XPS etc.)
  • Development and designing experiments dedicated to obtain high temporal and spatial resolution information of the electronic and magnetic structure of thin films. Among others by using X-ray or E-beam transparent thin film samples (collaborations with UU, UvA and EMAT, Antwerp).
  • In a separate effort, in the MURI program for superconductor coated conductors a real time diagnostic tool was demonstrated, based on Fourier Transform Infrared reflectometry (appeared in Applied Physics Letters, 2007). 

Oxide thin film templates

  • Atomic termination control of single crystal surfaces
  • The precise chemical nature of the terminating crystal plane of a substrate crystal has profound impact on the nucleation and growth of oxide thin films. The group has vast experience in developing recipes to achieve 100% pure unique termination
  • 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. This promising possibility may pave the way to films with position dependent properties that are determined by the local crystallographic orientation (Nijland et al., Adv. Func. Mat. 2015). We obtained atomic scale roughness by introducing a SrTiO3 interlayer. Currently the properties of these films are studied.
  • Development of buffer layers for technical substrates such as Si and GaN, collaboration with Josef Stefan Institute (Slovenia).

Oxide thin film Growth

  • 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.
  • 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.
  • In situ monitoring of the transient PLD plasma plume composition and chemical conversions in relation to the film growth (APL materials 2016/2017).