Physics Of Complex Inorganic Nano-Materials

Gertjan Koster

Brief summary of recent research topics

  • Artificial ferromagnetic structures (Boschker et al., PRL 2012); high mobility oxide 2DEGs (Huijben, Koster et al., Adv. Func. Mat. 2013).
  • Self-assembled nano-structures of epitaxial oxides (Kuiper et al., MRC 2011)
  • New methods for the preparation of complex oxide surfaces and interfaces (Nijland et al., ACS Applied Materials & Interfaces, 2014)
  • Photoemission on the ruthanate thin films (Rev. Mod. Phys. 2012)
  • Development of vacuum suitcase for transport of samples to external characterization facilities (HAXPES, in collaboration with Golden, UvA, Slooten et al., Phys. Rev. B 2013).
  • In situ characterization Techniques, XPS and XPD
  • Oxygen sublattice control studied by (S)TEM, XAS and RIXS (Samal et al., PRL 2013).
  • Manipulation of magnetic exchange in SrRuO3 in ultra thin heterostructures as studied by scanning SAGNAC MOKE.

In situ Spectroscopy

Hybrid Pulsed Laser Deposition/Molecular Beam Synthesis system for oxide growth with in situ XPS, UPS and RHEED (developed at the Geballe Laboratory for Advanced Materials at Stanford University) 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.

Examples of oxide systems that are being investigated using photoelectron spectroscopy are the itinerant ferromagnet SrRuO3 and the LaAlO3/SrTiO3 system.

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). 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.

As a recent example, we presented a study of epitaxially stabilized cubic CuO using in situ photoelectron spectroscopy and photoelectron diffraction.
(Appeared in PRB as editors choice)

Oxide thin film meso-materials

Our ability to control the composition, size, and shape of metal oxide-based nanostructures has progressed over the past decade to a level that we can now control the functional properties of these with a high degree of accuracy.

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.

It is our goal to combine and develop techniques that allow the synthesis and organization of novel materials at multiple length scales in three spatial dimensions. The formation of such hierarchical assemblies from functional metal oxide building blocks with magnetic, optical, mechanical, electrical, or otherwise functional properties is a new approach.

For example, we explore new methods to grow 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. (submitted to Nanoletters)

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.

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 (by the PI and co-workers), amongst others:

  1. Ability to control the atomic terminating plane of single crystal complex oxide substrates, such as SrTiO3, and DyScO3. (Applied Physics Letters 1998; Advanced Functional Materials 2010), Twente.
  2. Growth manipulation, such as pulsed laser interval deposition. (Applied Physics Letters 1999, 70 citations), Twente.
  3. Engineering of interfaces in epitaxial oxide heterostructures (Physical Review Letters 2007, 136 citations), Stanford.

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 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 (invited talk, SSRL user meeting, Menlo Park, 2004). 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).

The achieved results mentioned above, i.e., the ability to control growth on an atomic level at the most favorable PLD conditions, have been applied in later years, in the fabrication of complex epitaxial multi-layered structures as well as the study of their properties. The above-mentioned technologies and know-how are nowadays used in many laboratories worldwide for the fabrication of all-oxide devices and artificial materials based on, for instance, oxide superlattices and were key technologies required for the recent discovery of two-dimensional electron gases (2DEGs) at the interface in the LaAlO3-SrTiO3 system. The latter system has been recently recognized as important discovery within oxide materials science (Science, breakthrough of the year 2007).

Further developments oxide growth methods (understanding growth) as well as the usage of in situ photoemission and currently these results are used to fabricate and study, among others:

  1. Artificial materials with enhance or new functionalities, such as artificial ferromagnetic structures (Boschker et al., 2012); high mobility oxide 2DEGs (Huijben, Koster et al., 2013).
  2. Self-assembled nano-structures of epitaxial oxides (Kuiper et al., MRC 2011)
  3. New methods for the preparation of complex oxide surfaces and interfaces (Blok et al., Appl. Phys. Lett. 2011)
  4. Photoemission on the ruthanate thin films (Rev. Mod. Phys. 2011)