arXiv subjects

J. M. J. Lopes

Publications and source records attributed to J. M. J. Lopes.

9 recordsLinked to original sources

arXiv2023

Large-area synthesis of ferromagnetic Fe$_{5-x}$GeTe$_{2}$/graphene van der Waals heterostructures with Curie temperature above room temperature

Van der Waals (vdW) heterostructures combining layered ferromagnets and other two-dimensional (2D) crystals are promising building blocks for the realization of ultra-compact devices with integrated magnetic, electronic and optical functionalities. Their implementation in various technologies depends strongly on the development of a bottom-up scalable synthesis approach allowing to realize highly uniform heterostructures with well-defined interfaces between different 2D layered materials. It also requires that each material component of the heterostructure remains functional, which ideally includes ferromagnetic order above room temperature for 2D ferromagnets. Here, we demonstrate large-area growth of Fe$_{5-x}$GeTe$_{2}$/graphene heterostructures achieved by vdW epitaxy of Fe$_{5-x}$GeTe$_{2}$ on epitaxial graphene. Structural characterization confirmed the realization of a continuous vdW heterostructure film with a sharp interface between Fe$_{5-x}$GeTe$_{2}$ and graphene. Magnetic and transport studies revealed that the ferromagnetic order persists well above 300 K with a perpendicular magnetic anisotropy. In addition, epitaxial graphene on SiC(0001) continues to exhibit a high electronic quality. These results represent an important advance beyond non-scalable flake exfoliation and stacking methods, thus marking a crucial step toward the implementation of ferromagnetic 2D materials in practical applications.

cond-mat.mtrl-sci↗

arXiv2023

Stacking order effects on the energetic stability and electronic properties of $n$-doped graphene/h-BN van der Waals heterostructures on SiC(0001)

Heterostructures made of stacked 2D materials with different electronic properties are studied for their potential in creating multifunctional devices. Graphene (G) and hexagonal boron nitride (h-BN) van der Waals (vdW) systems have been extensively researched, including recent studies on synthesizing h-BN on graphene/SiC(0001) templates. These studies suggest that h-BN encapsulation could occur in addition to vdW epitaxy. This work presents theoretical research on G/h-BN heterostructures on SiC(0001) with a carbon buffer layer. The results show an energetic preference for h-BN encapsulation below a single layer of graphene: G/h-BN/SiC in bilayer systems and G/h-BN/G/SiC in trilayer systems. Electronic structure calculations reveal that the linear energy band dispersion of graphene is maintained in bilayer systems but with Dirac points at different energy positions due to the varied electron doping level of graphene. In trilayer systems, the doping level of graphene also depends on the stacking order. The electronic band structure of G/h-BN/G/SiC features two Dirac points below the Fermi level but with different energies. Less stable systems like G/G/h-BN/ and h-BN/G/G/SiC display parabolic bands near the Fermi level. Additional structural characterizations were performed based on simulations of C-1s core-level-shift (CLS) and carbon K-edge X-ray absorption near-edge spectroscopy (XANES) to aid future experimental spectroscopy in these graphene/h-BN vdW systems.

cond-mat.mtrl-sci↗

arXiv2021

Spin dependent charge transfer in MoSe2/hBN/Ni hybrid structures

We present magneto-photoluminescence measurements in a hybrid 2D semiconductor/ferromagnetic structure consisting of MoSe2/hBN/Ni. When the Nickel layer is magnetized, we observe circularly polarized photoluminescence of the trion peak in MoSe2 monolayer under linearly polarized excitation. This build-up of circular polarization can reach a measured value of about 4% when the magnetization of Ni is saturated perpendicularly to the sample plane, and changes its sign when the magnetization is reversed. The circular polarization decreases when the hBN barrier thickness increases. These results are interpreted in terms of a spin-dependent charge transfer between the MoSe2 monolayer and the Nickel film. The build-up of circular polarization is observed up to 120 K, mainly limited by the trion emission that vanishes with temperature.

cond-mat.mtrl-sci↗

arXiv2019

Acoustically modulated optical emission of hexagonal boron nitride layers

We investigate the effect of surface acoustic waves on the atomic-like optical emission from defect centers in hexagonal boron nitride layers deposited on the surface of a LiNbO$_3$ substrate. The dynamic strain field of the surface acoustic waves modulates the emission lines resulting in intensity variations as large as 50% and oscillations of the emission energy with an amplitude of almost 1 meV. From a systematic study of the dependence of the modulation on the acoustic wave power, we determine a hydrostatic deformation potential for defect centers in this two-dimensional material of about 40 meV/%. Furthermore, we show that the dynamic piezoelectric field of the acoustic wave could contribute to the stabilization of the optical properties of these centers. Our results show that surface acoustic waves are a powerful tool to modulate and control the electronic states of two-dimensional materials.

cond-mat.mes-hall↗

arXiv2019

Acoustoelectric transport at gigahertz frequencies in coated epitaxial graphene

Epitaxial graphene (EG) produced from SiC surfaces by silicon sublimation is emerging as a material for electronic applications due to its good electronic properties and availability over large areas on a semiconducting substrate. In this contribution, we report on the transport of charge carriers in EG on SiC using high-frequency ($>$ 1 GHz) surface acoustic waves (SAWs). In our devices, the EG is coated with hydrogen-silsesquioxane, SiO$_2$ and a ZnO layer. This allows the efficient generation of SAWs and is compatible with the deposition of a metal top gate. Measurements of frequency- and time-resolved power scattering parameters confirm the generation and propagation of SAWs with frequencies of up to more than 7 GHz. Furthermore, the ZnO coating enhances the acoustoelectric currents by two orders of magnitude as compared to our previous uncoated samples. These results are an important step towards the dynamic acoustic control of charge carriers in graphene at gigahertz frequencies.

cond-mat.mes-hall↗

arXiv2019

Luminescent defects in a few-layer h-BN film grown by molecular beam epitaxy

We report on luminescent centers contained in a few-layer-thick hexagonal boron nitride (h-BN) film grown on Ni by molecular beam epitaxy. After transfer to a SiO$_2$/Si substrate, sharp lines are observed in photo- and cathodoluminescence spectra in both the ultraviolet and the visible range. Spatially resolved measurements reveal that the luminescent centers responsible for these lines are localized within microscopic multi-layer islands that form at the nucleation centers of the h-BN film. The comparison of their energy, polarization and phonon replica emission with previous theoretical predictions suggest that the N$_\mathrm{B}$V$_\mathrm{N}$ anti-site could be one of the light emitters present in our sample. Moreover, we have also observed evidences of other kinds of centers that could be associated to defects containing carbon or oxygen. The characterized luminescent defects could have potential applications as quantum light sources.

cond-mat.mes-hall↗

arXiv2018

Spectrally narrow exciton luminescence from monolayer MoS2 exfoliated onto epitaxially grown hexagonal BN

The strong light-matter interaction in transition Metal dichalcogenides (TMDs) monolayers (MLs) is governed by robust excitons. Important progress has been made to control the dielectric environment surrounding the MLs, especially through hexagonal boron nitride (hBN) encapsulation, which drastically reduces the inhomogeneous contribution to the exciton linewidth. Most studies use exfoliated hBN from high quality flakes grown under high pressure. In this work, we show that hBN grown by molecular beam epitaxy (MBE) over a large surface area substrate has a similarly positive impact on the optical emission from TMD MLs. We deposit MoS$_2$ and MoSe$_2$ MLs on ultrathin hBN films (few MLs thick) grown on Ni/MgO(111) by MBE. Then we cover them with exfoliated hBN to finally obtain an encapsulated sample : exfoliated hBN/TMD ML/MBE hBN. We observe an improved optical quality of our samples compared to TMD MLs exfoliated directly on SiO$_2$ substrates. Our results suggest that hBN grown by MBE could be used as a flat and charge free substrate for fabricating TMD-based heterostructures on a larger scale.

cond-mat.mtrl-sci↗

arXiv2015

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.

cond-mat.mtrl-sci↗

arXiv2012

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 $(6\sqrt3\times6\sqrt3)R30^{\circ}$ periodicity which forms the intrinsic interface structure between epitaxial graphene and SiC(0001). We show that this interface structure leads to a nonvanishing 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.

cond-mat.mtrl-sci↗