Catálogo de publicaciones - revistas

<jats:title>Abstract</jats:title> <jats:p>The optical field generated by a nanoplasmonic probe is revealed in tip-enhanced Raman spectroscopy – TERS – experiments. The TERS intensity profile of nano-objects smaller than the probe’s apex has a donut-like shape which resembles the magnitude of the field generated by a point-dipole source, being well described by the Dyadic Green’s function. Having prior knowledge on the excitation field generated by the TERS probe, we measured the width of shear solitons caused by lattice reconstruction in low-angle twisted bilayer graphene, a prominent platform for twistronics, and the extend of defect-induced light emission from graphene edges.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The recent observation of room temperature spin-dependent photoluminescence (PL) emission from hexagonal boron nitride’s (h-BN’s) defect centers motivates for performing a complementary low-temperature photophysical study of quantum emitters under relatively high magnetic fields. Here, we investigate the PL emission dynamics of h-BN’s visible single-photon emitters under an applied out-of-plane magnetic field at cryogenic temperatures. The PL intensity of the emitters in our work strikingly exhibits strong magnetic field dependence and decreases with the increased magnetic field. A substantial decrease in the integrated PL intensity of the emitters by up to one order of magnitude was observed when the applied field is increased from 0T to 7T. The observed reversible photodarkening of PL emission due to the applied magnetic field is in very well agreement with the predictions of a recent joint experimental and theoretical study and can happen only if the spin-selective, non-radiative, and asymmetric intersystem crossing transitions proceed from the triplet excited state to the lowest-lying spin-singlet metastable state and from the metastable state to the triplet ground state. Our results not only shed more light on the light emission paths of defect centers in h-BN but also show the use of the magnetic field as an efficient control knob in the development of magneto-optical devices.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We report on observation of the infrared photoresistance of twisted bilayer graphene (tBLG) under continuous quantum cascade laser illumination at a frequency of 57.1 THz. The photoresistance shows an intricate sign-alternating behavior under variations of temperature and back gate voltage, and exhibits giant resonance-like enhancements at certain gate voltages. The structure of the photoresponse correlates with weaker features in the dark dc resistance reflecting the complex band structure of tBLG. It is shown that the observed photoresistance is well captured by a bolometric model describing the electron and hole gas heating, which implies an ultrafast thermalization of the photoexcited electron-hole pairs in the whole range of studied temperatures and back gate voltages. We establish that photoresistance can serve a highly sensitive probe of the temperature variations of electronic transport in tBLG. </jats:p>

<jats:title>Abstract</jats:title> <jats:p>The quest for electric-field control of nanoscale magnetic states such as skyrmions, which would impact the field of spintronics, has led to a challenging search for multiferroic materials or structures with strong magnetoelectric coupling and efficient electric-field control. Here we report a theoretical prediction that such phenomena can be realized in two-dimensional (2D) bilayer FE/PMM and trilayer FE/PMM/FE heterostructures (two-terminal and three-terminal devices), where FE is a 2D ferroelectric and PMM is a polar magnetic metal with strong spin-orbit coupling. Such a PMM has strong Dzyaloshinskii-Moriya interactions (DMI) that can generate skyrmions, while the FE can generate strong magnetoelectric coupling through polarization-polarization interactions. In trilayer heterostructures, contact to the metallic PMM layer enables multiple polarization configurations for electric-field control of skyrmions. We report density-functional-theory calculations for particular material choices that demonstrate the effectiveness of these arrangements, with the key driver being the polarization-polarization interactions between the PMM and FE layers. The present findings provide a method to achieve strong magnetoelectric coupling in the 2D limit and a new perspective for the design of related spintronics. </jats:p>

<jats:title>Abstract</jats:title> <jats:p>Chemical exfoliation is an attractive approach for the synthesis of graphene due to low cost and simplicity. However, challenges still remain in the characterization of solution-processed graphene, in particular with atomic resolution. Through this work we demonstrate the X-ray pair distribution function as a novel approach to study the solution-processed graphene or other 2D materials with atomic resolution, directly in solution, produced by liquid-phase and electrochemical exfoliations. The results show the disappearance of long-range atomic correlations, in both cases, confirming the production of single and few-layer graphene. In addition, a considerable ring distortion has been observed as compared to graphite, irrespective of the solvent used: the normal surface angle to the sheet of the powder sample should be less than 6o, compatible with ripples formation observed in suspended graphene. We attribute this effect to the interaction of solvent molecules with the graphene nanosheets.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>With high theoretical capacity and suitable operating potential, SiOx is regarded as one of the most promising anode materials for high-energy density lithium-ion batteries, but it suffers from large volume change during charge/discharge and low electronic conductivity, leading to poor cycling stability and rate capability. To overcome these problems, a SiOx@rGO film with porous structure are prepared through vacuum filtration and self-propagation reduction method, which can be directly used as a free-standing anode for lithium-ion batteries. The self-propagation process of graphene oxide to graphene can be completed rapidly within 1 second, and endows the film with developed pores due to the instantaneous release of substantial gases. The porous structure is beneficial for exposing massive active sites, facilitating fast ion transport and buffering the volume change of the SiOxduring charge/discharge. Moreover, the reduced graphene oxide (rGO) sheets construct a conductive framework for rapid electron transfer in the film. As a result, the SiOx@rGO film exhibits high lithium storage capacity (1191 mAh g-1 at 0.1 A g-1), excellent cycling stability (82% capacity retention after 100 cycles) and good rate capability (349 mAh g-1 at 3.2 A g-1). This study not only provides a high-performance film anode material for lithium-ion batteries, but also develops a simple and efficient method for constructing porous film electrodes for various energy storage devices. </jats:p>

<jats:title>Abstract</jats:title> <jats:p>The interaction of high-quality transition metal trichalcogenides (TMTs) single crystals FePX3 (X: S, Se) with water molecules is studied using near-edge X-ray absorption fine structure (NEXAFS) and X-ray photoelectron spectroscopy (XPS) in a wide range of temperature and partial pressure of H2O. The physisorption nature of interaction between H2O and FePX3 is found at low temperatures and relatively small concentrations of water molecules, that is supported by the DFT results. When temperature of the FePX3 samples and partial pressure of H2O are increased, the interaction at the interface is defined by two competing processes -- adsorption of molecules at high partial pressure of H2O and desorption of molecules due to the increased surface mobility and physisorption nature of interaction. Our intensive XPS/NEXAFS experiments accompanied by DFT calculations bring new understanding on the interaction of H2O with surface of a new class of 2D materials, TMTs, pointing to their stability and reactivity, that is important for further applications in different areas, like sensing and catalysis. </jats:p>

<jats:title>Abstract</jats:title> <jats:p>Silicon (Si) is a new candidate anode material for lithium-ion batteries. The porous treatment of Si anode has been proved to be effective. In order to improve the interface performance and energy density of batteries, we start from the current collector and make further improvements. Combined with the advantages of new two-dimensional material MXene in electrochemical aspects, we make MXene replace the traditional Cu foil as current collector of Si anode. The prepared MXene paper is both flexible and lightweight. After coating the Si slurry on it, the assembled half cells and 5V-class full cells can achieve normal lithium-ion intercalation and deintercalation. Moreover, compared with the battery using Cu current collector, the volume expansion of porous silicon in the battery with MXene is further alleviated, and the cycle stability performance is also improved. </jats:p>

<jats:title>Abstract</jats:title> <jats:p>Though promising in renewable energy, osmotic energy is limited by the unsatisfactory conversion performance caused by defective ion transport and selectivity of semipermeable membranes. As an emerging family of two-dimensional (2D) materials, MXenes have been attracting extensive interests for constructing osmotic membrane due to its natural 2D nanoconfined space, hydrophilicity and abundant surface terminations. The regulation of the surface charge density of MXenes plays an important role for the improvement of osmotic energy conversion. Herein, we systematically investigate Mo2TiC2Tx MXene membranes for osmotic energy harvesting. Benefitting from the improved surface negative-charged density treated by alkali solution, and the 2D nanoconfined space, the Mo2TiC2Tx MXene membrane shows improved cation selectivity and permeability performance. The osmotic voltage (Vos) increases to 83 mV with an improved cation transference number (t+) of 0.95 at 0.5M/0.01M alkali KCl solution (pH=9), while osmotic voltage (Vos) is 74 mV with a cation transference number (t+) of 0.9 at 0.5M/0.01M alkali KCl solution (pH=7). The output power density (Pmax) reaches up to 13.1 W m-2 with an energy conversion efficiency (ηmax) of 40.5% at 0.5M/0.01M alkali KCl solution (pH=9), which is superior to many of other 2D osmotic membranes. The modification method of surface charge density for Mo2TiC2Tx MXene osmotic membrane has shown great perspective in renewable osmotic energy harvesting. </jats:p>

<jats:title>Abstract</jats:title> <jats:p>Femtosecond laser-pulse excitation provides an energy efficient and fast way to control magnetization at the nanoscale, providing great potential for ultrafast next-generation data manipulation and nonvolatile storage devices. Ferromagnetic van der Waals materials have garnered much attention over the past few years due to their low dimensionality, excellent magnetic properties, and large response to external stimuli. Nonetheless, their behaviour upon fs laser-pulse excitation remains largely unexplored. Here, we investigate the ultrafast magnetization dynamics of a thin flake of Fe<jats:sub>3</jats:sub>GeTe<jats:sub>2</jats:sub> (FGT) and extract its intrinsic magnetic properties using a microscopic framework. We find that our data is well described by our modelling, with FGT undergoing a slow two-step demagnetization, and we experimentally extract the spin-relaxation timescale as a function of temperature, magnetic field and excitation fluence. Our observations indicate a large spin-flip probability in agreement with a theoretically expected large spin-orbit coupling, as well as a weak interlayer exchange coupling. The spin-flip probability is found to increase when the magnetization is pulled away from its quantization axis, opening doors to an external control over the spins in this material. Our results provide a deeper understanding of the dynamics van der Waals materials upon fs laser-pulse excitation, paving the way towards two-dimensional materials-based ultrafast spintronics.</jats:p>