Catálogo de publicaciones - revistas

<jats:title>Abstract</jats:title> <jats:p>Recently reported large values of exciton–polariton nonlinearity of transition metal dichalcogenide (TMD) monolayers coupled to optically resonant structures approach the values characteristic for GaAs-based systems in the regime of strong light-matter coupling. Contrary to the latter, TMD-based polaritonic devices remain operational at ambient conditions and therefore have greater potential for practical nanophotonic applications. Here, we present the study of the nonlinear properties of Ta<jats:sub>2</jats:sub>O<jats:sub>5</jats:sub> slab waveguide coupled to a WSe<jats:sub>2</jats:sub> monolayer. We confirm that the hybridization between the waveguide mode and the exciton resonance in WSe<jats:sub>2</jats:sub> gives rise to the formation of guided exciton–polaritons with Rabi splitting of 36 meV. By measuring transmission of ultrashort optical pulses through this TMD-based polaritonic waveguide, we demonstrate the strong nonlinear dependence of the output spectrum on the input pulse energy. We develop a theoretical model that shows agreement with the experimental results and gives insights into the dominating microscopic processes which determine the nonlinear pulse self-action: Coulomb exciton–exciton interaction and scattering to an incoherent excitonic reservoir. Based on the numerical simulation of nonlinear phenomena in our polariton system, we conclude that it may support a quasi-stationary solitonic regime of pulse propagation at intermediate pump energies. Our results provide an important step for the development of nonlinear on-chip polaritonic devices based on 2D semiconductors.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Identifying environmentally inert, ferromagnetic two-dimensional (2D) materials with high Curie temperatures (<jats:italic>T</jats:italic> <jats:sub>c</jats:sub>) down to the single layer limit has been an obstacle to fundamental studies of 2D magnetism and application of 2D heterostructures to spin-polarized devices. To address this challenge, the growth, structure and magnetic properties of a 2D Cr-silicate single layer on Pt(111) was investigated experimentally and theoretically. The layer was grown by sequentially depositing SiO and Cr followed by annealing in O<jats:sub>2</jats:sub>. Scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), and low energy electron microscopy all indicated a well-ordered layer that uniformly covered the surface, with STM and LEED indicating that the silicate relaxed to its favored lattice constant. Further experimental characterizations demonstrated that the Cr was nominally 3+ but with a lower electron density than typical trivalent Cr compounds. Comparison with theory identified a Cr<jats:sub>2</jats:sub>Si<jats:sub>2</jats:sub>O<jats:sub>9</jats:sub> structure that resembles a single layer of a dehydrogenated dioctahedral silicate. Magnetic circular dichroism in x-ray absorption spectroscopy revealed a ferromagnetically ordered state up to at least 80 K. Theoretical analysis revealed that the Cr in a dehydrogenated Cr-silicate/Pt(111) is more oxidized than Cr in freestanding Cr<jats:sub>2</jats:sub>Si<jats:sub>2</jats:sub>O<jats:sub>9</jats:sub>H<jats:sub>4</jats:sub> layers. This greater oxidation was found to enhance ferromagnetic coupling and suggests that the magnetism may be tuned by doping. The 2D Cr-silicate is the first member of a broad series of possible layered first-row transition metal silicates with magnetic order; thus, this paper introduces a new platform for investigating 2D ferromagnetism and the development of magnetoelectronic and spintronic devices by stacking 2D atomic layers.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Two-dimensional semiconducting transition metal dichalcogenides (TMDs) have attracted significant interest due to their unique optoelectronic properties. More often, these materials are enclosed inside a dielectric layer that can work as an insulator for field-effect transistors. The insulating layer is typically grown with atomic layer deposition (ALD). Here, we study the effects on bare and hBN-covered monolayer MoS<jats:sub>2</jats:sub> and WSe<jats:sub>2</jats:sub> flakes with ALD TiO<jats:sub>2</jats:sub> films. Our results reveal a significant shift and decrease in intensity in photoluminescence and Raman signals of the monolayer TMDs. Further analysis suggests that these changes are caused by chemical doping, strain, and dielectric screening after the ALD. Our study not only sheds light on the impact of ALD on the properties of TMDs, but also indicates ALD can be an alternative method to engineer the doping, strain and dielectric environment for potential optoelectronics and photonics applications.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Tungsten disulfide (WS<jats:sub>2</jats:sub>), as a typical member of transition metal chalcogenides (TMDs), has attracted extensive research interest in optoelectronics, especially photodetectors. However, the performance of photodetectors based on monolayer WS<jats:sub>2</jats:sub> is restricted to weak light absorption. Here, AgInGaS quantum dots (AIGS-QDs) with a large absorption coefficient and high quantum efficiency are integrated onto WS<jats:sub>2</jats:sub> atomic layers to achieve excellent photoelectric performance. Notably, the observed photoluminescence (PL) quenching and the reduction of the decay time of PL in the WS<jats:sub>2</jats:sub>/AIGS-QDs heterojunction confirm the interfacial charge transfer from AIGS-QDs to WS<jats:sub>2</jats:sub> layer. The results show that type II energy band arrangement leads to the efficient separation of photoexcited carriers at the interface between WS<jats:sub>2</jats:sub> and AIGS-QDs. This WS<jats:sub>2</jats:sub>/AIGS-QDs photodetector achieves an ultrahigh responsivity (<jats:italic>R</jats:italic>) of 3.3 × 10<jats:sup>3</jats:sup> A W<jats:sup>−1</jats:sup>, an external quantum efficiency (EQE) of 7.8 × 10<jats:sup>6</jats:sup>% and a detectivity (<jats:italic>D</jats:italic>*) of 1.3 × 10<jats:sup>13</jats:sup> Jones. Our work provides promising potential for future high-performance monolayer TMD-based photodetectors.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The growing demand for safe, cost-efficient, high-energy and high-power electrochemical energy storage devices has stimulated the development of aqueous-based supercapacitors with high capacitance, high rate capability, and high voltage. 2D titanium carbide MXene-based electrodes have shown excellent rate capability in various dilute aqueous electrolytes, yet their potential window is usually narrower than 1.2 V. In this study, we show that the potential window of Ti3C2Tx MXene can be efficiently widened to 1.5 V in a cost-effective and environmentally benign polyethylene glycol (PEG) containing molecular crowding electrolyte. Additionally, a pair of redox peaks at -0.25 V/-0.05 V vs. Ag (cathodic/anodic) emerged in cyclic voltammetry after the addition of PEG, yielding an additional 25% capacitance. Interestingly, we observed the co-insertion of the molecular crowding agent PEG-400 during the Li+ intercalation process based on in-situ X-ray diffraction analysis. As a result, Ti3C2Tx electrodes presented an interlayer space change of 4.7 Å during a complete charge/discharge process, which is the largest reversible interlayer space change reported so far for MXene-based electrodes. This work demonstrates the potential of adding molecular crowding agents to improve the performance of MXene electrodes in aqueous electrolytes and to enlarge the change of the interlayer spacing.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Synaptic devices with tunable weight hold great promise in enabling non-von Neumann architecture for energy efficient computing. However, conventional metal-insulator-metal (MIM) based two-terminal memristors share the same physical channel for both programming and reading, therefore the programming power consumption is dependent on the synaptic resistance states and can be particularly high when the memristor is in the low resistance states. Three terminal synaptic transistors, on the other hand, allow synchronous programming and reading and have been shown to possess excellent reliability. Here we present a binary oxide based three-terminal MoS2 synaptic device, in which the channel conductance can be modulated by interfacial charges generated at the oxide interface driven by Maxwell-Wagner instability. The binary oxide stack serves both as an interfacial charge host and gate dielectrics. Both excitatory and inhibitory behaviors are experimentally realized, and the presynaptic potential polarity can be effectively controlled by engineering the oxide stacking sequence, which is a unique feature compared with existing charge-trap based synaptic devices and provides a new tuning knob for controlling synaptic device characteristics. By adopting a three-terminal transistor structure, the programming channel and reading channel are physically separated and the programming power consumption can be kept constantly low (~ 50 pW) across a wide dynamic range of 105. This work demonstrates a complementary metal oxide semiconductor (CMOS) compatible approach to build power efficient synaptic devices for artificial intelligence (AI) applications.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Covalent bonding between transition metal atoms is a common phenomenon in honeycomb lattices of layered materials, which strongly affects their electronic and magnetic properties. This work presents a detailed spectroscopic study of α-MoCl3, 2D van der Waals material with covalently bonded Mo2 dimers, with a particular focus on the Mo–Mo bonding. Raman spectra of α-MoCl3 were studied with multiple excitation laser lines chosen in different parts of the absorption spectrum, while polarization measurements aided in the symmetry assignment of the observed modes. Furthermore, far-IR measurements and DFT phonon computations were performed to complete vibrational assignment. Polarized absorption, photoluminescence, and photoelectron spectroscopy supported by DFT calculations were employed to understand the consequences of the Mo–Mo bonding for the electronic structure and the localization/delocalization balance in d3–d3 interactions. A coupling of dimerization-related structural and electronic properties was revealed in the strong resonance Raman enhancement of the Mo–Mo stretching mode at 153 cm−1 when the excitation laser matched the electronic transition between σ-bonding and antibonding orbitals of the Mo2 dimer (σ→σ*). The deeper understanding of the metal-metal bonding and identification of the vibrational and electronic spectroscopic signatures of the dimerization will be of great use for the studies of electron delocalization in magnetic van der Waals materials.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We explore the non-equilibrium transport regime in graphene using a large dc current in combination with a perpendicular magnetic field. The strong in-plane Hall field generated in the graphene channel results in Landau levels that are tilted spatially. The energy of cyclotron orbits in the bulk varies as a function of the spatial position of the guiding center, enabling us to observe a series of compelling features. While Shubnikov-de Haas oscillations are predictably suppressed in the presence of the Hall field, a set of fresh magneto resistance oscillations emerge near the charge neutrality point as a function of dc current. Two branches of oscillations with linear dispersions are evident as we vary carrier density and dc current, the velocities of which closely resemble the TA and LA phonon modes, suggestive of phonon-assisted intra-Landau level transitions between adjacent cyclotron orbits. Our results offer unique possibilities to explore non-equilibrium phenomena in two-dimensional materials and van der Waals heterostructures.&amp;#xD;</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The low crystal symmetry of rhenium disulphide (ReS<jats:sub>2</jats:sub>) leads to the emergence of dichroic optical and optoelectronic response, absent in other layered transition metal dichalcogenides, which could be exploited for device applications requiring polarization resolution. To date, spectroscopy studies on the optical response of ReS<jats:sub>2</jats:sub> have relied almost exclusively in characterization techniques involving optical detection, such as photoluminescence, absorbance,or reflectance spectroscopy. However, to realize the full potential of this material, it is necessary to develop knowledge on its optoelectronic response with spectral resolution. In this work, we study the polarization-dependent photocurrent spectra of few-layer ReS<jats:sub>2</jats:sub> photodetectors, both in room conditions and at cryogenic temperature. Our spectral measurements reveal two main exciton lines at energies matching those reported for optical spectroscopy measurements, as well as their excited states. Moreover, we also observe an additional exciton-like spectral feature with a photoresponse intensity comparable to the two main exciton lines. We attribute this feature, not observed in earlier photoluminescence measurements, to a nonradiative exciton transition. The intensities of the three main exciton features, as well as their excited states, modulate with linear polarization of light, each one acquiring maximal strength at a different polarization angle. We have performed first-principles exciton calculations employing the Bethe-Salpeter formalism, which corroborate our experimental findings. Our results bring new perspectives for the development of ReS<jats:sub>2</jats:sub>-based nanodevices.&amp;#xD;</jats:p>

<jats:title>Abstract</jats:title> <jats:p>As one of two-dimensional (2D) semiconductor materials, transition metal dichalcogenides (TMDs) have sparked enormous potential in next-generation optoelectronics due to their unique and excellent physical, electronic and optical properties. Controllable growth of wafer-scale 2D TMDs is essential to realize various high-end applications, while it remains challenging. Herein, 2-inch 2D WS2 films were successfully synthesized by ambient pressure chemical vapor deposition based on substrate engineering and space-confined strategies. WS2 nucleation density can be effectively modulated depending on the annealing conditions of sapphire substrate. 2D WS2 films with controllable thickness can be fabricated by adjusting the space-confined height. Moreover, our strategies are demonstrated to be universal for the growth of other 2D TMD semiconductors. WS2-based photodetectors with different thicknesses were systematically investigated. Monolayer WS2 photodetector displays large responsivity of 0.355 A/W and high specific detectivity of 1.48 × 1011 Jones. Multilayer WS2 device exhibits negative self-powered photoresponse. Our work provides a new route for the synthesis of wafer-scale 2D TMD materials, paving the way for high performance integrated optoelectronic devices.&amp;#xD;</jats:p>