No abstract

Two-dimensional (2D) crystals have renewed opportunities in design and assembly of arti cial lattices without the constraints of epitaxy. However, the lack of thickness control in exfoliated van der Waals (vdW) layers prevents realization of repeat units with high delity. Recent availability of uniform, waferscale samples permits engineering of both electronic and optical dispersions in stacks of disparate 2D layers with multiple repeating units. We present optical dispersion engineering in a superlattice structure comprised of alternating layers of 2D excitonic chalcogenides and dielectric insulators. By carefully designing the unit cell parameters, we demonstrate > 90 % narrowband absorption in < 4 nm active layer excitonic absorber medium at room temperature, concurrently with enhanced photoluminescence in cm 2 samples. These superlattices show evidence of strong light-matter coupling and exciton-polariton formation with geometry-tunable coupling constants. Our results demonstrate proof of concept structures with engineered optical properties and pave the way for a broad class of scalable, designer optical metamaterials from atomically-thin layers.

Crystalline formation of high magnetic-moment thin films through low-temperature annealing processes compatible with current semiconductor technologies is crucial for the development of next generation devices, which can utilise the spin degree of freedom. Utilising in-situ aberration corrected electron microscopy, we report a 235 °C crystallisation process for a Co-based ternary Heusler-alloy film whose initial nucleation is initiated by as few as 27 unit cells. The crystallisation occurs preferentially in the ⟨111⟩ crystalline directions via a two-dimensional (2D) layer-by-layer growth mode; resulting in grains with [110] surface normal and [111] plane facets. This growth process was found to reduce the crystallisation energy by more than 50% when compared to bulk samples whilst still leading to the growth of highly ordered grains expected to give a high degree of spin-polarisation. Our findings suggest that the 2D layer-by-layer growth minimises the crystallisation energy allowing for the possible implementation of highly spin-polarised alloy films into current chip and memory technologies.

We investigated growth, annealing conditions and magnetic properties of the Heusler alloy Fe2+xVyAl by means of x-ray diffraction, magnetic hysteresis and exchange-bias measurements. Ferromagnetic Heusler alloy films were obtained by sputtering Fe2VAl and Fe3VAl targets and performing post-growth annealing. The characteristic (2 2 0) Heusler alloy peaks were seen in the x-ray diffraction measurements and corresponding ferromagnetic behaviours were observed. In addition, antiferromagnetic Heusler alloy films were deposited by employing Al pegs on Fe3VAl sputtering targets. The deposited films had elemental ratios close to the predicted Fe2.5V0.5Al phase, and a 16 Oe exchange-bias was measured in a Fe2.3V0.7Al/Co60Fe40 system at 100 K.

Spintronics is a new and emerging field in nanotechnology, which has been evolving rapidly. It aims to exploit the spin degree of freedom in order to realise advanced electronic devices. With the recent improvement in the storage media devices following to the discovery of giant magnetoresistance effect, it is envisioned that the electronic devices have several advantages over the conventional electronics in respect to the storage capacity, speed and power consumption. One avenue towards next generation spintronic devices is to develop half-metallic ferromagnets (HMFs) with 100% spin polarisation (P) and Curie temperature (T C ) above room temperature (RT). HMFs have unique properties, in which the majority spins have a metallic band structure, whereas the minority spins have a semiconducting band with the Fermi level (E F ) lying within an energy gap. P of HMFs has been predominantly estimated using Julliere's formula in a magnetic tunnel junction (MTJ) or measured by the Andreev reflection (AR) at low temperature. Both methods are very sensitive to the surface/interface spin polarisation.Alternative optical methods such as photoemission have also been employed. However, these methods require a complicated and expensive set-up. Therefore, it is of paramount importance to directly and easily measure the band-gap of HMFs.The main aim of this study is to develop a new technique to directly measure the band-gap (E g ) of HMFs at RT. For that, a simple experimental set-up has been designed This technique allows measuring E g of HMFs at RT for the first time. It can therefore be used to provide simple optimisation of growth conditions.iii