Nanostructures and 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 seperation 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.
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.
Magnetic Properties of Nanowire Arrays
The ATLAS technique for facbrication 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 ditribution 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 paricles 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).
Faceting of Vicinal Surfaces
- R. Verre, R.G.S. Sofin, V. Usov, K. Fleischer, D. Fox, G. Behan, H. Zhang and I.V. Shvets
- Surface Science, 606(23-24):1815-1820 (2012)
Shallow Angle Deposition of Nanowire Arrays and their Magnetic Properties
- S.K. Arora, B.J. O'Dowd, P. Thakur, N.B. Brookes, B. Ballesteros, P. Gambardella, and I.V. Shvets
- Current Nanoscience Vol 9, Issue 5 (2013) 609-614
- S. K. Arora, B.J. O'Dowd, D.M. Polishchuk, A.I. Tovstolytkin, P. Thakur, N.B. Brookes, I.V. Shvets.
- Journal of Applied Physics, 114(13), 133903-133903-7.
Reflectance Anisotropy Spectroscopy of Nanoparticle Arrays
- R. Verre, K. Fleischer, O. Ualibek, and I. V. Shvets.
- Applied Physics Letters, 100, 031102 (2012)
- R. Verre, K. Fleischer, J. F. McGilp, D. Fox, G. Behan, H. Zhang and I. V. Shvets.
- Nanotechnology 23, 035606 (2012)
- O. Ualibek, R. Verre, B. Bulfin, V. Usov, K. Fleischer, J.F. McGilp and I.V. Shvets.
- Nanoscale, 2013, 5, 4923