Catálogo de publicaciones - revistas

<jats:title>Abstract</jats:title> <jats:p>Dirac nodal line semimetals are a novel class of topological materials in which the valence and conduction bands touch along lines in the reciprocal space, with linear dispersion. These materials attract a growing attention, but the experimental realizations for two-dimensional systems are sparse. We report here the first experimental realization of a two-dimensional hexagonal monolayer&amp;#xD;Cu<jats:sub>2</jats:sub>Ge, grown by evaporation of Ge on a Cu(111) substrate. Through a combination of LEED, XPS and ARPES measurements, we show that the surface presents all characteristics expected from calculations for a free-standing Cu<jats:sub>2</jats:sub>Ge monolayer. More specifically, the preservation of the two concentric nodal lines around the Γ point indicates weak interactions between the Cu<jats:sub>2</jats:sub>Ge surface and its Cu(111) substrate, making it an ideal system for the study of Dirac nodal line materials.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Nanomeshes, often referred to as phononic crystals, have been extensively explored for their unique properties, including phonon coherence and ultralow thermal conductivity (κ). However, experimental demonstrations of phonon coherence are rare and indirect, often relying on comparison with numerical modeling. Notably, a significant aspect of phonon coherence, namely the disorder-induced reduction in κ observed in superlattices, has yet to be experimentally demonstrated. In this study, through atomistic modeling and spectral analysis, we systematically investigate and compare phonon transport behaviors in graphene nanomeshes, characterized by 1D line-like hole boundaries, and silicon nanomeshes, featuring 2D surface-like hole boundaries, while considering various forms of hole boundary roughness. Our findings highlight that to demonstrate disorder-induced reduction in κ of nanomeshes, optimal conditions include low temperature, smooth and planar hole boundaries, and the utilization of thick films composed of 3D materials.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Like in any other semiconductor, point defects in transition-metal dichalcogenides (TMDs) are expected to strongly impact their electronic and optical properties. However, identifying defects in these layered two-dimensional materials has been quite challenging with controversial conclusions despite the extensive literature in the past decade. Using first-principles calculations, we revisit the role of chalcogen vacancies and hydrogen impurity in bulk TMDs, reporting formation energies and thermodynamic and optical transition levels. We show that the S vacancy can explain recently observed cathodoluminescence spectra of MoS$_2$ flakes and predict similar optical levels in the other TMDs. In the case of the H impurity, we find it more stable sitting on an interstitial site in the Mo plane, acting as a shallow donor, and possibly explaining the often observed n-type conductivity in some TMDs. We also predict the frequencies of the local vibration modes for the H impurity, aiding its identification through Raman or infrared spectroscopy.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We explore the structural evolution of two-dimensional (2D) MoO3 films beyond the monolayer, which have been prepared by physical vapor deposition and post-oxidation onto a Pd(100) surface, and characterized by the tools of surface science and density functional theory (DFT) calculations. According to DFT, the most stable oxide layers are stoichiometric, and derive their energetic stability from the low cost of creating 2D freestanding layers from the orthorhombic bulk phase, good matching to Pd, and the particularly strong adhesion to the substrate. The observed 2D MoO3 layers are distinguished by well-ordered linear defects, such as domain boundaries in the monolayer, and misfit dislocations in the bilayer. Applying reactive oxidation preparation conditions results in the formation of ordered arrays of nanostructures, nanowires and nanoclusters, in the MoO3 bilayer. The formation of such linear structures is accounted for in the DFT by models of missing row defects of various orientations and stoichiometries. Their relative stability is rationalized in terms of the number of broken Mo-O bonds, the polar character of the nanostructure edges and the interaction strength with the Pd substrate. Comparison with similar WO3 layers on Pd(100) is provided.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Monolayer graphene is a promising material for a wide range of applications, including sensors, optoelectronics, antennas, EMR shielding, flexible electronics, and conducting electrodes. Chemical vapor deposition (CVD) of carbon atoms on a metal catalyst is the most scalable and cost-efficient method for synthesizing high-quality, large-area monolayer graphene. The usual method of transferring the CVD graphene from the catalyst to a target substrate involves a polymer carrier which is dissolved after the transfer process is completed. Due to often unavoidable damage to graphene, as well as contamination and residues, carrier mobilities are typically 1 000 − 3 000 cm^2/(V s), unless complex and elaborate measures are taken. Here, we report on a simple scalable fabrication method for flexible graphene field-effect transistors that eliminates the polymer interim carrier, by laminating the graphene directly onto office lamination foils, removing the catalyst, and depositing Parylene N as a gate dielectric and encapsulation layer. The fabricated transistors show field and Hall-effect mobilities of 7 000 − 10 000 cm^2/(V s) with a residual charge-carrier density of 2 × 10^11 1/cm^2 at room temperature. We further validate the material quality by terahertz time domain spectroscopy (THz-TDS) and observation of the quantum Hall effect at low temperatures in a moderate magnetic field of ∼ 5 T. The Parylene encapsulation provides long-term stability and protection against additional lithography steps, enabling vertical device integration in multilayer electronics on a flexible platform.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>In strongly correlated quantum materials, electrons behave in ways that often extend beyond the confines of conventional Fermi-liquid theory. Interesting results include the observation of low-temperature metallic behavior in systems that are highly resistive. Here we provide an overview of experiments in which insulators exhibit characteristics of a metal such as the Shubnikov–de Haas-like quantum oscillations, focusing on recent findings in the correlated insulating states of two-dimensional WTe2. We discuss the status of current research, clarify the debates and challenges in interpreting the experiments, rule out extrinsic explanations and discuss promising future directions.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Gaining growing attention in spintronics is a class of magnets displaying zero net magnetization and spin-split electronic bands called alter magnets. Here, by combining density functional theory and symmetry analysis, we show that RuF<jats:sub>4</jats:sub> monolayer is a two-dimensional d-wave altermagnet. Spin-orbit coupling leads to pronounced spin splitting of the electronic bands at the Γ point by ∼100 meV and turns the RuF<jats:sub>4</jats:sub> into a weak ferromagnet due to nontrivial spin-momentum locking that cants the Ru magnetic moments. The net magnetic moment scales linearly with the spin-orbit coupling strength. Using group theory we derive an effective spin Hamiltonian capturing the spin-splitting and spin-momentum locking of the electronic bands. Disentanglement of the altermagnetic and spin-orbit coupling induced spin splitting uncovers to which extent the altermagnetic properties are affected by the spin-orbit coupling. Our results move the spotlight to the nontrivial spin-momentum locking and weak ferromagnetism in the two-dimensional altermagnets relevant for novel venues in this emerging field of material science research.&amp;#xD;</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Research interests in two-dimensional (2D) materials have seen exponential growth owing to their unique and fascinating properties. The highly exposed lattice planes coupled with tunable electronic states of 2D materials have created manifold opportunities in the design of new platforms for energy conversion and sensing applications. Still, challenges in understanding the electrochemical characteristics of these materials arise from the complexity of both intrinsic and extrinsic heterogeneities that can obscure structure–activity correlations. Scanning electrochemical probe microscopic investigations offer unique benefits in disclosing local electrochemical reactivities at the nanoscale level that are otherwise inaccessible with macroscale methods. This review summarizes recent progress in applying techniques of scanning electrochemical microscopy (SECM) and scanning electrochemical cell microscopy (SECCM) to obtain distinctive insights into the fundamentals of 2D electrodes. We showcase the capabilities of electrochemical microscopies in addressing the roles of defects, thickness, environments, strain, phase, stacking, and many other aspects in the heterogeneous electron transfer, ion transport, electrocatalysis, and photoelectrochemistry of representative 2D materials and their derivatives. Perspectives for the advantages, challenges, and future opportunities of scanning electrochemical probe microscopy investigation of 2D structures are discussed.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>2D transition metal dichalcogenides (TMDs) are leading materials for next-generation optoelectronics, but fundamental problems stand enroute to commercialization. These problems include firstly, the widely debated defect and strain-induced origins of intense low-energy broad luminescence peaks (L-peak) observed at low temperatures. Secondly, role of oxygen in tuning properties via chemisorption and physisorption is intriguing but challenging to understand. Thirdly, physical understanding of benefits of hBN encapsulation is inadequate. Using a series of samples, we decouple contributions of oxygen, defects, adsorbates, and strain on optical properties of monolayer MoS2. Defect-origin of L-peak is confirmed by temperature and power-dependent photoluminescence (PL) measurements, with a dramatic redshift ~ 130 meV for oxygen-assisted chemical vapour deposition (O-CVD) samples (c.f. exfoliated). Anomalously, O-CVD samples show high A-exciton PL at room temperature (c.f. exfoliated), but reduced PL at low temperatures, attributed to strain-induced direct-to-indirect bandgap-crossover in low-defect O-CVD MoS2. These observations are consistent with our density functional theory calculations, and supported by Raman spectroscopy. In exfoliated samples, charged O-adatoms are identified as thermodynamically favourable defects, and create in-gap states. Beneficial effect of encapsulation originates from reduction of charged O-adatoms and adsorbates. This experimental-theoretical study uncovers the type of defects in each sample, enables an understanding of the combined effect of defects, strain and oxygen on band structure, and enriches understanding of effects of encapsulation. This work proposes O-CVD for creating high-quality materials for optoelectronics.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The formation of morphotropic phase boundaries (MPBs) is a pivotal strategy in piezoelectric ceramics and crystals, primarily used to enhance the electromechanical coupling. However, the application of this strategy in van der Waals (vdW) piezoelectrics and ferroelectrics has been limited, largely due to challenges in achieving phase coexistence and enabling possible polarization rotation. In this study, we address this gap by synthesizing a Selenium doped vdW ferroelectric, CuInP2(S1-xSex)6, with a doping range of 0 ≤ x ≤ 0.15, to create an MPB. Our findings indicate the presence of an MPB near x=0.05, situated between monoclinic and trigonal phases. This boundary was confirmed using X-ray diffraction and transmission electron microscope techniques. Remarkably, the composition at x=0.05 shows a high dielectric constant (εr=13.8) and an impressive local effective piezoelectric coefficient (d33eff=51 pm/V) at 80 K. Additionally, an unusual softening of the Young’s modulus was observed near MPB. These results are crucial for enhancing electromechanical coupling in vdW layered materials and herald new avenues for the design and optimization of piezoelectric and electromechanical properties in these materials.</jats:p>