LMA

Interfaces in heterostructures

  Interfaces in heterostructures

 

Joining together two materials produces an interface between them. The breaking of the translational symmetry of each material when approaching the interface is often the source of novel properties that in many circumstances differ drastically from those of the original materials.
The surface of a nanomaterial is the simplest example of an interface, and it is well known that its properties can be remarkably different to the bulk due to the different charge distribution, strain relaxation, perturbed magnetic interactions or chemical interaction with the environment. A paradigmatic example of the importance of the surface in Nanoscience is the case of nanoparticles. The surface properties of nanoparticles are the origin for their particular electric, magnetic and chemical properties. The crystal lattice at the surface of nanoparticles is relaxed; presents an atomic ordering different from that of the bulk, strain relaxation, surface reconstruction and faceting. The electronic structure of the surface is different due to the different coordination and bonding of the surface atoms, and in many cases they acquire a strong chemical activity that confers them catalytic functionality. Magnetic nanoparticles present remarkably different magnetic properties due to the different exchange interactions and magnetic anisotropy of the spins located at the surface.
Heterostructures are the most attractive nanostructure where the physical properties of the whole system is inherently linked to that of the interface. In Nanoscience and Nanotechnology, there are multitude of examples of heterostructures in which interfaces play a key role. Strained-enhanced electron mobility in transistor channels has contributed to further scale reduction and increase in the number of transistors per area unit in semiconductor industry. The interface between thin LaAlO3-SrTiO3 interfaces behaves as a 2D electron gas, in contradiction with the insulating nature of both materials. The Rashba effect is the interaction between spin and angular momentum of electrons that appears at the interface of materials with high spin-orbit coupling materials. This exotic interface phenomenon could be the basic principle of the conversion between charge and spin currents in the spintronic devices of the future decades.

Transmission Electron Microscopy of heterostructure interfaces

The physics of interfaces is intrinsically a nanoscale phenomenon that requires nanocharacterization techniques with high spatial (atomic) resolution. Transmission Electron Microscopy and, in particular, aberration corrected TEM is among the most suitable due to the formidable combination of quantitative imaging and analytical capabilities with atomic resolution.
Magnetic Nanoparticles. A first example of the potential of aberration corrected TEM is the study of ultra small core-shell nanoparticles. Only TEM can provide direct information on the morphology and structure of materials such as magnetite nanoparticles coated with ultrathin MgO shellS. Magnetite nanoparticles are biocompatible, their size can be tailored and their magnetic properties can be tuned from ferromagnetic to superparamagnetic behavior, so they are an ideal system for biomedical applications such as magnetic separation of biomolecules, magnetic fluid hyperthermia, magnetic resonance imaging, and target delivery. However, they are subject to degradation (further oxidation) that damages their properties, and they may suffer from aggregation due to dipolar interaction that reduces their performance in biomedical applications. Coating the particles with other material that protects their original properties and reduces the risk of agglomeration is beneficial for real-life applications.

Figure 1. Aberration corrected HRTEM image of a magnetite nanoparticle epitaxially coated by a 1-nm-thick MgO layer. The insets show the FFT calculated from the areas marked with white squares.

Among the possible coatings, inorganic materials offer a better protection from the core oxidation. Magnesium oxide (MgO) is particularly suitable due to its thermal stability (high melting point) and negligible lattice mismatch with magnetite. Thus, a group of INA researchers produced core-shell magnetite@MgO nanoparticles by a combination of co-precipitation and sol-gel methods. The result of their synthetic method can only be properly characterized by aberration corrected HRTEM in the FEI Titan Cube of the LMA. The results of the aforementioned synthetic method are ultra-small (<10 nm) magnetite nanoparticles perfectly coated by an epitaxial MgO layer of approximately 1 nm of thickness. Only aberration corrected HRTEM can certify the presence of this protective shell, determine its size and its perfectly ordered cube-on-cube epitaxial growth over the magnetite core.
Epitaxial magnetic oxide thin films. There is an aberration corrected TEM technique decisive in the characterization of interfaces, particularly in the case of oxides, which is atomic resolution chemical mapping. The combination of an aberration corrected probe in STEM with Electron Energy Loss Spectroscopy (EELS) of core-loss edges allow the chemical mapping of the chemical species present in a material. This type of experiment provides the ultimate characterization of the chemical structure and electronic states of interfaces, as it allows the column-by column determination of the chemical composition and, in many cases, the electronic state (oxidation state) of the oxide material. This is extremely useful to analyze possible chemical diffusion and charge redistribution phenomena at the interface, together with structural defects like roughness or surface steps with chemical sensitivity.
This technique is based on the concept of the STEM-EELS 3D data cube resulting from spectrum imaging experiments. Most of the EELS experiments performed nowadays rely on the possibility of collecting spectra in selected positions of a sample by controlling the position of the incident beam with the scan coils of the microscope in STEM mode. Consequently, the incident beam can be scanned along the sample with a certain dwell time and EELS spectra collected in each point enabling the spectrum imaging, i.e. 1D or 2D arrays of EELS spectra collected pixel by pixel in a region of the sample. The concept is illustrated in Figure 2. STEM-EELS spectrum imaging in aberration corrected STEM microscopes enables atomic resolution chemical mapping such as the examples shown below.

Figure 2. The spectrum imaging 3D cube.

An example of a model atomic resolution STEM-EELS chemical mapping is shown in Figure 3, where a thin film of (110)-oriented LCMO observed along a (100) zone axis is shown. It shows how the La and Mn lattice can be nicely resolved independently with a perfect correspondence with the atomic column positions observed in the HAADF image. A certain intermixing can be appreciated at the interface, extending up to 3-4 unit cells, which could be due to either actual chemical diffusion or interface steps. This type of defects may affect the macroscopic properties of the heterostructures.

Figure 3. Atomic resolution STEM-EELS of a La0.67Ca0.33MnO3 / SrTiO3 (110) interface.

This information is extremely valuable to fine-tune the growth conditions to produce atomic sharp interfaces. This is the case of the second example, a bilayer of LaCoO3-La0.67Sr0.33MnO3 grown on SrTiO3 (100), illustrated in Figure 4. In this case, the growth conditions have been optimized to present perfect atomic flat interfaces with no significant intermixing down to a single unit cell.

Figure 4. Atomic resolution STEM-EELS of a LaCoO3-La0.67Sr0.33MnO3 bilayer grown on SrTiO3 (100). Sample courtesy of Francisco Rivadulla.

Strain-engineered ferroelectric thin films. Strain engineering is a strategy that is being exploited in the last years to induce additional functionalities to thin films. An interesting example is the case of ferroelectrics. Ferroelectricity is based on the breaking of the inversion symmetry caused by a polar displacement of atoms in the lattice that produces a relative displacement of the positive and negative charges to induce a net electric dipole. Epitaxial strain of a ferroelectric thin film grown on a substrate is bound to distort the lattice and affect their ferroelectric properties. For instance, thin films of a common ferroelectric such as PbTiO3 present a complex domain structure of 90º ferroelastic and ferroelectric domains when it is grown epitaxial on DyScO3/SrRuO3 (substrate/buffer), presenting a configuration of a- and c-domains, as shown in Figure 5. This strain in this domain configuration can be analyzed by Geometrical Phase Analysis (GPA) to probe the deformation and rotation of the PbTiO3 lattice with respect to the SrRuO3 buffer (taken as a reference). In addition to the expected rotation of the lattice expected in the a-domains with respect to the c-domains, the combination of epitaxial growth and the coexistence of both types of domains induces a gradient of the y-component of strain (εyy) in the c-domains near the interface between two consecutive a-domains. Apparently, the presence of a-domains induces higher tetragonality in the acute angle of the c-domain than in the obtuse angle.

Figure 5. Upper panel: atomic structure of PbTiO3 and domain structure present in epitaxial PbTiO3 grown on DyScO3/SrRuO3 substrates. Lower panel, from left to right: HAADF image of the structure, rotation map determined by GPA, and y-component of strain determined by GPA.

The consequence of this phenomenon in the ferroelectric behavoir of PbTiO3 has been studied by a quantitative analysis of the HAADF-STEM images. An advanced algorithm can be developed to determine the position of the different atomic columns in the image, and the relative position of the Ti atoms with respect to the Pb atoms is measured as a characteristic feature of the presence and orientation of the electric polarization.
As an example, the electric polarization determined in distant areas with respect to the interfaces (squares I and II in Figure 6 corresponds to the expected polarization orientation in their respective domains. However, the quantitative analysis of the polarization orientation near the interfaces shows a clear rotation of the polarization of the c-domains towards the substrate plane, which clearly correlates with the strain gradient previously detected. This phenomenon, the coupling between strain gradients and ferroelectric polarization, is called flexoelectricity, and this works demonstrates its potential to tune the orientation of ferroelectric polarization by physical methods, such as interface epitaxy.

Figure 6. Upper left: HAADF-STEM image analysed. Upper right: examples of determined polarization orientation in normal a- and c-domains of PbTiO3. Bottom left: strain map (y-component, color coded) superimposed with the polarization orientation determined; the coupling between them demonstrates the presence of flexoelectric rotation of the polarization. Bottom left: line profiles of the polarization angle with respect to the substrate plane.

Highlights

  • Flexoelectric tuning of polar rotations in ferroelectric thin films.
    G. Catalan, A. Lubk, A. H. G. Vlooswijk, E. Snoeck, C. Magén, A. Janssens, G. Rispens, G. Rijnders, D.H.A. Blank and B. Noheda
    Nature Materials 10, 963-967 (2011).
  • Ultra-thin MgO coating of superparamagnetic magnetite nanoparticles by combined co-precipitation and sol-gel synthesis.
    L. De Matteis, L. Custardoy, R. Fernández-Pacheco, C. Magén, J. M. de la Fuente, C. Marquina, and M. R. Ibarra
    Chem. Mater. 24, 451-456 (2012).