arXiv subjects

Marvin Metzelaars

Publications and source records attributed to Marvin Metzelaars.

7 recordsLinked to original sources

arXiv2025

Probing enhanced electron-phonon coupling in graphene by infrared resonance Raman spectroscopy

We report on resonance Raman spectroscopy measurements with excitation photon energy down to 1.16 eV on graphene, to study how low-energy carriers interact with lattice vibrations. Thanks to the excitation energy close to the Dirac point at $\mathbf{K}$, we unveil a giant increase of the intensity ratio between the double-resonant 2D and 2D$^\prime$ peaks with respect to that measured in graphite. Comparing with fully \textit{ab initio} theoretical calculations, we conclude that the observation is explained by an enhanced, momentum-dependent coupling between electrons and Brillouin zone-boundary optical phonons. This finding applies to two dimensional Dirac systems and has important consequences for the modeling of transport in graphene devices operating at room temperature.

cond-mat.mes-hall↗

arXiv2024

Benchmarking the integration of hexagonal boron nitride crystals and thin films into graphene-based van der Waals heterostructures

We present a benchmarking protocol that combines the characterization of boron nitride (BN) crystals and films with the evaluation of the electronic properties of graphene on these substrates. Our study includes hBN crystals grown under different conditions and scalable BN films deposited by either chemical or physical vapor deposition (CVD or PVD). We explore the complete process from boron nitride growth, over its optical characterization by time-resolved cathodoluminescence (TRCL), to the optical and electronic characterization of graphene by Raman spectroscopy after encapsulation and Hall bar processing. Within our benchmarking protocol we achieve a homogeneous electronic performance within each Hall bar device through a fast and reproducible processing routine. We find that a free exciton lifetime of 1 ns measured on as-grown hBN crystals by TRCL is sufficient to achieve high graphene room temperature charge carrier mobilities of 80,000 cm$^2$/(Vs) at a carrier density of |n| = 10$^{12}$ cm$^{-2}$, while respective exciton lifetimes around 100 ps yield mobilities up to 30,000 cm$^2$/(Vs). For scalable PVD-grown BN films, we measure carrier mobilities exceeding 10,000 cm$^2$/(Vs) which correlates with a graphene Raman 2D peak linewidth of 22 cm$^{-1}$. Our work highlights the importance of the Raman 2D linewidth of graphene as a critical metric that effectively assesses the interface quality (i.e. surface roughness) to the BN substrate, which directly affects the charge carrier mobility of graphene. Graphene 2D linewidth analysis is suitable for all BN substrates and is particularly advantageous when TRCL or BN Raman spectroscopy cannot be applied to specific BN materials such as amorphous or thin films. This underlines the superior role of spatially-resolved spectroscopy in the evaluation of BN crystals and films for the use of high-mobility graphene devices.

cond-mat.mtrl-sci↗

arXiv2024

Electrically Controlled Interfacial Charge Transfer Induced Excitons in MoSe2-WSe2 Lateral Heterostructure

Controlling excitons and their transport in two-dimensional (2D) transition metal dichalcogenides (TMDs) heterostructures is central to advancing photonics and electronics on-chip integration. We investigate the controlled generation and manipulation of excitons and their complexes in monolayer (1L) MoSe2-WSe2 lateral heterostructure (LHS), directly grown via water-assisted chemical vapor deposition. Using a field-effect transistor design by incorporating a few-layer graphene back gate, single-layer graphene edge contact and encapsulation with few-layer hexagonal boron nitride, we achieve precise electrical tuning of exciton complexes and their transfer across 1D interfaces. At cryogenic temperatures (4 K), photoluminescence and photocurrent maps reveal the synergistic effect of local electric field and interface phenomena in the modulation of excitons, trions, and free carriers. We observe spatial variations in exciton and trion densities driven by exciton-trion conversion under electrical manipulation. The first-principle density functional theory calculation reveals significant band modification at the lateral interfaces and graphene-TMDs contact region. Furthermore, we demonstrate the versatility of 2D TMDS LHS in hosting and manipulating quantum emitters, achieving precise control over narrow-band emissions through modulating carrier injection and electrical biasing. This work extends the boundary of the present understanding of excitonic behaviour within lateral heterojunctions, highlighting the potential for controlled exciton manipulation across 1D interfaces and paving the way for next-generation electro-optical quantum devices.

cond-mat.mes-hall↗

arXiv2024

Infrared resonance Raman of bilayer graphene: signatures of massive fermions and band structure on the 2D peak

Few-layer graphene possesses low-energy carriers which behave as massive fermions, exhibiting intriguing properties in both transport and light scattering experiments. Lowering the excitation energy of resonance Raman spectroscopy down to 1.17 eV we target these massive quasiparticles in the split bands close to the K point. The low excitation energy weakens some of the Raman processes which are resonant in the visible, and induces a clearer frequency-separation of the sub-structures of the resonance 2D peak in bi- and trilayer samples. We follow the excitation-energy dependence of the intensity of each sub-structure and, comparing experimental measurements on bilayer graphene with ab initio theoretical calculations, we trace back such modifications on the joint effects of probing the electronic dispersion close to the band splitting and enhancement of electron-phonon matrix elements.

cond-mat.mes-hall↗

arXiv2023

Chemically detaching hBN crystals grown at atmospheric pressure and high temperature for high-performance graphene devices

In this work, we report on the growth of hexagonal boron nitride (hBN) crystals from an iron flux at atmospheric pressure and high temperature and demonstrate that (i) the entire sheet of hBN crystals can be detached from the metal in a single step using hydrochloric acid and that (ii) these hBN crystals allow the fabrication of high carrier mobility graphene devices. By combining spatially-resolved confocal Raman spectroscopy and electrical transport measurements, we confirm the excellent quality of these crystals for high-performance hBN-graphene-based van der Waals heterostructures. The full width at half maximum of the graphene Raman 2D peak is as low as 16 cm$^{-1}$, and the room temperature charge carrier mobilitiy is around 80000 cm$^2$/(Vs) at a carrier density 1$\times$10$^{12}$cm$^{-12}$. This is fully comparable with devices of similar dimensions fabricated using crystalline hBN synthesized by the high pressure and high temperature method. Finally, we show that for exfoliated high-quality hBN flakes with a thickness between 20 nm and 40 nm the line width of the hBN Raman peak, in contrast to the graphene 2D line width, is not useful for benchmarking hBN in high mobility graphene devices.

cond-mat.mes-hall↗

arXiv2022

Ag(II) as spin super-polarizer in metal fluoride clusters

Using quantum mechanical calculations, we examine magnetic (super)exchange interactions in hypothetical, chemically reasonable molecular coordination clusters containing fluoride bridged late transition metals or selected lanthanides, as well as Ag(II). By referencing to analogous species comprising closed shell Cd(II) we provide theoretical evidence that the presence of Ag(II) may modify the magnetic properties of such systems (including metal metal superexchange) to a surprising degree, specifically both coupling sign and strength may markedly change. Remarkably, this happens in spite of the fact that the fluoride ligand is the least susceptible to spin polarization among all monoatomic ligands known in chemistry. In an extreme case of a oxo bridged Ni(II)2 complex, the presence of Ag(II) leads to a nearly 17 fold increase of magnetic superexchange and switching from antiferro- to ferromagnetic coupling. Ag(II), with one hole in its d shell that may be shared with or transferred to ligands, effectively acts as spin superpolarizer, and this feature could be exploited in spintronics and diverse molecular devices.

cond-mat.mtrl-sci↗

arXiv2022

Structural phase transitions and magnetic superexchange in MIAgIIF3 perovskites at high pressure

Pressure induced phase transitions of MIAgIIF3 perovskites where M = K, Rb, Cs, have been predicted theoretically for the first time for pressures up to 100 GPa. The sequence of phase transitions for M = K and Rb consists of an orthorhombic to monoclinic and back to orthorhombic transition, associated with progressive bending of infinite chains of corner sharing [AgF6]4 minus octahedra and their mutual approaching via secondary Ag...F contacts. In stark contrast, only a single phase transition i.e. tetragonal to triclinic, is predicted for CsAgF3, associated with substantial deformation of the Jahn Teller-distorted first coordination sphere of AgII and association of the infinite [AgF6]4 minus chains into a polymeric sublattice. The phase transitions markedly decrease the coupling strength of intra-chain antiferromagnetic superexchange in MAgF3 hosts lattices.

cond-mat.mtrl-sci↗