João Marcelo Jordão Lopes

Cluster coarsening and luminescence emission intensity of Ge nanoclusters in SiO2 layers

SiO2 layers 180 nm thick are implanted with 120 keV Ge+ ions at a fluence of 1.2×1016 cm−2. The distribution and coarsening evolution of Ge nanoclusters are characterized by Rutherford backscattering spectrometry and transmission electron microscopy and the results are correlated with photoluminescence measurements as a function of the annealing temperatures in the 400 °C⩽T⩽900 °C range. At 400 °C we observe a monomodal array of clusters characterized by a mean diameter 〈φ〉=2.2 nm which increases to 〈φ〉=5.6 nm at 900 °C. This coarsening evolution occurs concomitantly with a small change of the total cluster–matrix interface area and an increase of the Ge content trapped in observable nanoclusters. However, at 900 °C a significant fraction of up to about 20% of the Ge content still remains distributed in the matrix around the nanoparticles. The results are discussed in terms of possible atomic mechanisms involved in the coarsening behavior that lead to the formation of the oxygen deficiency luminescence ce...

Contribution of the buffer layer to the Raman spectrum of epitaxial graphene on SiC(0001)

We report a Raman study of the so-called buffer layer with (6 p 3◊ 6 p 3)R30 periodicity which forms the intrinsic interface structure between epitaxial graphene and SiC(0001). We show that this interface structure leads to a non-vanishing signal in the Raman spectrum at frequencies in the range of the D- and G-band of graphene and discuss its shape and intensity. Ab initio phonon calculations reveal that these features can be attributed to the vibrational density of states of the buffer layer.

Influence of the silicon carbide surface morphology on the epitaxial graphene formation

Graphene grown on SiC(0001) by Si depletion has a stepped surface with terraces and step heights up to 10 times larger than those observed in the original SiC surface. This is due to an additional step bunching that usually occurs during graphene formation. In this work, we show that such process can be suppressed by controlling the initial step structure of the SiC surface. In this case, the graphene monolayer is formed on the SiC without modification of the original surface morphology. We observe that the absence of step bunching during growth has no influence on the graphene structural quality.

Synthesis of atomically thin hexagonal boron nitride films on nickel foils by molecular beam epitaxy

Hexagonal boron nitride (h-BN) is a layered two-dimensional material with properties that make it promising as a dielectric in various applications. We report the growth of h-BN films on Ni foils from elemental B and N using molecular beam epitaxy. The presence of crystalline h-BN over the entire substrate is confirmed by Raman spectroscopy. Atomic force microscopy is used to examine the morphology and continuity of the synthesized films. A scanning electron microscopy study of films obtained using shorter depositions offers insight into the nucleation and growth behavior of h-BN on the Ni substrate. The morphology of h-BN was found to evolve from dendritic, star-shaped islands to larger, smooth triangular ones with increasing growth temperature.

Effect of annealing atmosphere on the structure and luminescence of Sn-implanted SiO2 layers

Sn nanoclusters are synthesized in 180 nm SiO2 layers after ion implantation and heat treatment. Annealings in N2 ambient at high temperatures (T⩾700°C) lead to the formation of Sn nanoclusters of different sizes in metallic and in oxidized phases. High-resolution transmission electron microscopy (TEM) analyses revealed that the formed larger nanoparticles are composed by a Sn metallic core and a SnOx shell. The corresponding blue-violet photoluminescence (PL) presents low intensity. However, for heat treatments in vacuum, the PL intensity is increased by a factor of 5 and the TEM data show a homogeneous size distribution of Sn nanoclusters. The low intensity of PL for the N2 annealed samples is associated with Sn oxidation.

Acousto-electric transport in epitaxial monolayer graphene on SiC

We report on the piezoelectric excitation and acoustic charge transport by gigahertz surface acoustic waves (SAWs) in epitaxial monolayer graphene (EG) on SiC. The GHz SAWs frequencies were generated by interdigital transducers fabricated on a piezoelectric island on the SiC substrate. Acoustic transport studies in a Hall bar geometry show that the propagating SAW field transports carriers in EG, the transport direction being determined by the direction of the acoustic beam. Carrier transport is driven by drift in the piezoelectric field induced by the SAW in EG.

Synthesis of quasi-free-standing bilayer graphene nanoribbons on SiC surfaces

Scaling graphene down to nanoribbons is a promising route for the implementation of this material into devices. Quantum confinement of charge carriers in such nanostructures, combined with the electric field-induced break of symmetry in AB-stacked bilayer graphene, leads to a band gap wider than that obtained solely by this symmetry breaking. Consequently, the possibility of fabricating AB-stacked bilayer graphene nanoribbons with high precision is very attractive for the purposes of applied and basic science. Here we show a method, which includes a straightforward air annealing, for the preparation of quasi-free-standing AB-bilayer nanoribbons with different widths on SiC(0001). Furthermore, the experiments reveal that the degree of disorder at the edges increases with the width, indicating that the narrower nanoribbons are more ordered in their edge termination. In general, the reported approach is a viable route towards the large-scale fabrication of bilayer graphene nanostructures with tailored dimensions and properties for specific applications.

Low temperature aging effects on the formation of Sn nanoclusters in SiO2∕Si films and interfaces

The formation of Sn nanocrystals (NCs) in ion implanted SiO2∕Si films is investigated using Rutherford backscattering spectrometry and transmission electron microscopy. Low temperature and long time aging treatments followed by high temperature thermal annealings lead to the formation of a dense bidimensional NC array located at the SiO2∕Si interface. This behavior is discussed considering the formation of small Sn clusters with a significantly improved thermal stability. The present experimental results are in good agreement with recent theoretical predictions that small Sn clusters can have their melting temperature enhanced in more than 1000°C.

Coincident-site lattice matching during van der Waals epitaxy

Van der Waals (vdW) epitaxy is an attractive method for the fabrication of vdW heterostructures. Here Sb2Te3 films grown on three different kind of graphene substrates (monolayer epitaxial graphene, quasi freestanding bilayer graphene and the SiC (6√3 × 6√3)R30° buffer layer) are used to study the vdW epitaxy between two 2-dimensionally (2D) bonded materials. It is shown that the Sb2Te3 /graphene interface is stable and that coincidence lattices are formed between the epilayers and substrate that depend on the size of the surface unit cell. This demonstrates that there is a significant, although relatively weak, interfacial interaction between the two materials. Lattice matching is thus relevant for vdW epitaxy with two 2D bonded materials and a fundamental design parameter for vdW heterostructures.

Molecular beam epitaxy of graphene on ultra-smooth nickel: growth mode and substrate interactions

Graphene is grown by molecular beam epitaxy using epitaxial Ni films on MgO(111) as substrates. Raman spectroscopy and scanning tunneling microscopy reveal the graphene films to have few crystalline defects. While the layers are ultra-smooth over large areas, we find that Ni surface features lead to local non-uniformly thick graphene inclusions. The influence of the Ni surface structure on the position and morphology of these inclusions strongly suggests that multilayer graphene on Ni forms at the interface of the first complete layer and metal substrate in a growth-from-below mechanism. The interplay between Ni surface features and graphene growth behavior may facilitate the production of films with spatially resolved multilayer inclusions through engineered substrate surface morphology.