Nanostructures and Interfaces

Our work in the area of Nanostructures and Interfaces is wide ranging. Key areas of interest include step-bunching of vicinal surfaces, and self-assembled nanostructure arrays with tunable magnetic/optical properties. Other areas of research include resistive switching and electronic properties at interfaces.

Step-bunching and Faceting of Vicinal Crystals

The APRG is an established leader in the area of step-bunching of vicinal crystals such as Si (111) and Al2O3 (c-plane sapphire). The templates have a small miscut angle (typically <6°) in certain directions which gives rise to an array of atomic steps perpendicular to the miscut direction. In the case of Si, the step-bunching effect is brought about by carrying out a DC-anneal sequence in UHV, while for Al2O3 the procedure involves a high temperature anneal in air. These processes result in a highly regular step and terrace morphology. We have spent several years exploring the parameter-space of these procedures, and are now in a position where we can control the separation and height of the steps through choice of annealing conditions. These studies have also shed new light on the surface properties of these materials, in terms of surface energies, adatom mobilities and crystal terminations.

AFM image of highly ordered step array on vicinal sapphire.

AFM image of highly ordered step array on vicinal sapphire.


Shallow Angle Deposition of Nanostructure Arrays

We use our step-bunched templates (Si, Al2O3, MgO, etc.) as a template for the fabrication of ordered arrays of nanostructures via a glancing angle deposition technique, which we have called ATLAS (Atomic Terrace Low Angle Shadowing). (For details on our ATLAS deposition chambers, click here.)

In this technique, the step-bunched template is held at a very shallow angle with respect to the deposition flux (typically <6°) which allows the material to be deposited only on the step-bunches or the flat terraces between the steps, depending on whether the flux is directed in the 'uphill' or 'downhill' directions.

This technique is not specific to any particular material for deposition, and we have been able to create nanostructures of Ni, Fe, Co, Ag, Au and Cu By carrying out uphill depositions.

AFM image of Ni nanowire

AFM image of Ni nanowire arays on step-bunched Si substrate. The is directed in the 'uphill' direction as indicated by the arrow. The dashed grey line indicates the macroscopic surface plane.


Magnetic Properties of Nanowire Arrays

The ATLAS technique for fabrication of arrays of nanowires composed of magnetic material allows the exploration of magnetic properties on the nanoscale across a well-controlled parameter-space. Some of the behaviours we have analysed include:

  • Coercivity as a function of temperature for wires of varying thickness
  • Reversal mechanisms as a function of wire thickness
  • Influence of various capping layers, and the effect on magnetisation as they age
  • Ferromagnetic resonance of wire arrays
  • Influence of dipolar coupling on coercivity as a function of inter-wire separation
  • Study of dipolar coupling using remanence measurements
  • Use of FORC analysis to determine influence of superparamagnetic behaviour

IMAGE: FORC distribution plots for an Fe nanowire array (top) and Co nanoparticle array (bottom). The features in these plots reveal the distribution of coercivities and influence of superparamagnetic particles within the array.


Optical Properties of Nanoparticle Arrays

By depositing material using the ATLAS technique onto substrates ata elevated temperature, we are able to grow elongated nanoparticles rather than continuous nanowires. The ellipsoidal shape of these particles as well as their very small inter-particle gap (<5nm) give rise to an anisotropy in their optical properties, which we analyse using RAS (Reflectance Anisotropy Spectroscopy). (See details on our equipment here.) Since our deposition chamber is equipped with a strain-free window, we are able to carry out "in situ" RAS measurements as sample growth continues. We typically use faceted sapphire step-templates to grow these nanoparticle arrays due to their transparency in the visible spectrum.

Using RAS, we have shown that the resonance energy of the nanoparticle array can be tuned through appropriate choice of deposited thickness and substrate temperature. Furthermore, we have shown that the deposition technique is applicable to a range of materials for deposition (Cu, Au, Ag, etc.), which allows us to specify a resonance energy anywhere in the visible spectrum, or equal to the frequency of any laser with an output in the visible spectrum.

IMAGE: SEM image of Ag nanoparticle array on faceted Al2O3 (top), and in-situ RAS response as deposition thickness is increased (bottom).


Selected Publications

Faceting of Vicinal Surfaces

  • Title: Equilibrium faceting formation in vicinal Al2O3 (0001) surface caused by annealing
    Authors: R. Verre, R.G.S. Sofin, V. Usov, K. Fleischer, D. Fox, G. Behan, H. Zhang and I.V. Shvets
    Source: Surface Science, 606(23-24):1815-1820 (2012)

    Title: Revealing electromigration on dielectrics and metals through the step-bunching instability
    Authors: V. Usov, C. Ó Coileáin, A. N. Chaika, S. I. Bozhko, V. N. Semenov, S. Krasnikov, O. Toktarbaiuly, S. Stoyanov, and I. V. Shvets
    Source: Phys. Rev. B 102, 035407 (2020)

    Title: Step bunching with both directions of the current: Vicinal W(110) surfaces versus atomistic-scale model
    Authors: O. Toktarbaiuly, V. Usov, C. Ó Coileáin, K. Siewierska, S. Krasnikov, E. Norton, S. I. Bozhko, V. N. Semenov, A. N. Chaika, B. E. Murphy, O. Lübben, F. Krzyżewski, M. A. Załuska-Kotur, A. Krasteva, H. Popova, V. Tonchev, and I. V. Shvets
    Source: Phys. Rev. B 97, 035436 (2018)

Shallow Angle Deposition of Nanowire Arrays and their Magnetic Properties

Reflectance Anisotropy Spectroscopy of Nanoparticle Arrays