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Red-emitting narrow-band phosphors are of utmost importance for next-generation white-light phosphor-converted light-emitting diodes (pc-wLEDs) for improved efficacy and optimized correlated color temperatures. A promising representative crystallizing in an ordered variant of the UCr4C4 structure type is Sr[Li2Al2O2N2]:Eu2+ (SALON:Eu2+) emitting at a desirable wavelength of 614 nm. Despite an expected eightfold coordination of the Eu2+ ions by four N3− and O2− ions, respectively, the exact local coordination symmetry and a 1:1 ratio between the two types of ligands is not straightforwardly proven by X-ray diffraction. Low-temperature luminescence spectroscopy in conjunction with ligand field theory are powerful alternatives to resolve local features of Eu2+ as its excited 4f65d1 configuration reacts sensitively to the polarity of the ligands. The dominant emission at 614 nm shows pronounced vibronic fine structure at 10 K. In addition, weak emission bands can be resolved at 570 and 650 nm even at a low doping concentration of 0.5 mol% and are assigned to Eu2+ ions being eightfold coordinated by N3− and O2− ions in a ratio different from 1:1. Due to the feature of vierer ring-type channels in SALON:Eu2+, those Eu2+ centers are sufficiently close for mutual energy transfer, which is characterized by time-resolved luminescence at 10 K.

Red-emitting narrow-band phosphors are of utmost importance for next-generation white-light phosphor-converted light-emitting diodes (pc-wLEDs) for improved efficacy and optimized correlated color temperatures. A promising representative crystallizing in an ordered variant of the UCr4C4 structure type is Sr[Li2Al2O2N2]:Eu2+ (SALON:Eu2+) emitting at a desirable wavelength of 614 nm. Despite an expected eightfold coordination of the Eu2+ ions by four N3− and O2− ions, respectively, the exact local coordination symmetry and a 1:1 ratio between the two types of ligands is not straightforwardly proven by X-ray diffraction. Low-temperature luminescence spectroscopy in conjunction with ligand field theory are powerful alternatives to resolve local features of Eu2+ as its excited 4f65d1 configuration reacts sensitively to the polarity of the ligands. The dominant emission at 614 nm shows pronounced vibronic fine structure at 10 K. In addition, weak emission bands can be resolved at 570 and 650 nm even at a low doping concentration of 0.5 mol% and are assigned to Eu2+ ions being eightfold coordinated by N3− and O2− ions in a ratio different from 1:1. Due to the feature of vierer ring-type channels in SALON:Eu2+, those Eu2+ centers are sufficiently close for mutual energy transfer, which is characterized by time-resolved luminescence at 10 K.

Recently, graphene triangulene chains have been synthesized, and their magnetic response has been analyzed by scanning tunneling microscopy methods by Mishra et al. [Nature (London) 598, 287 (2021)]. Motivated by this study, we determine the exchange bilinear and biquadratic constants of the triangulene chains by calculating two-spin rotations in the spirit of the magnetic force theorem. We then analyze open-ended, odd-numbered chains, whose edge states pair up forming a triplet ground state. We propose three experimental approaches that enable us to trigger and control a singlet-triplet spin transition. Two of these methods are based on applying a mechanical distortion to the chain. We finally show that the transition can be controlled efficiently by the application of an electric field.

Finite-length armchair graphene nanoribbons can behave as one-dimensional topological materials, that may show edge states in their zigzag-terminated edges, depending on their width and termination. We show here a full solution of tight-binding graphene rectangles of any length and width that can be seen as either finite-length armchair or zigzag ribbons. We find exact analytical expressions for both bulk and edge eigenstates and eigenenergies. We write down exact expressions for the Coulomb interactions among edge states and introduce a Hubbard-dimer model to analyze the emergence and features of different magnetic states at the edges, whose existence depends on the ribbon length. We find ample room for experimental testing of our predictions in N=5 armchair ribbons. We compare the analytical results with ab initio simulations to benchmark the quality of the dimer model and to set its parameters. A further detailed analysis of the ab initio Hamiltonian allows us to identify those variations of the tight-binding parameters that affect the topological properties of the ribbons.

We propose a method to determine the magnetic exchange interaction and onsite anisotropy tensors of extended Heisenberg spin models from density functional theory including relativistic effects. The method is based on the Liechtenstein-Katsnelson-Antropov-Gubanov torque formalism, whereby energy variations upon infinitesimal rotations are performed. We assume that the Kohn-Sham Hamiltonian is expanded in a nonorthogonal basis set of pseudoatomic orbitals. We define local operators that are both Hermitian and satisfy relevant sum rules. We demonstrate that in the presence of spin-orbit coupling a correct mapping from the density functional total energy to a spin model that relies on the rotation of the exchange field part of the Hamiltonian can not be accounted for by transforming the full Hamiltonian. We derive a set of sum rules that pose stringent validity tests on any specific calculation. We showcase the flexibility and accuracy of the method by computing the exchange and anisotropy tensors of both well-studied magnetic nanostructures and of recently synthesized two-dimensional magnets. Specifically, we benchmark our approach against the established Korringa-Kohn-Rostoker Green's function method and show that they agree well. Finally, we demonstrate how the application of biaxial strain on the two-dimensional magnet T−CrTe2 can trigger a magnetic phase transition.

Spin-crossover (SCO) active iron(II) complexes are an integral class of switchable and bistable molecular materials. Spin-state switching properties of the SCO complexes have been studied in the bulk and single-molecule levels to progress toward fabricating molecule-based switching and memory elements. Supramolecular SCO complexes featuring anchoring groups for metallic electrodes, for example, gold (Au), are ideal candidates to study spin-state switching at the single-molecule level. In this study, we report on the spin-state switching characteristics of supramolecular iron(II) complexes 1 and 2 composed of functional 4-([2,2′-bithiophen]-5-ylethynyl)-2,6-di(1H-pyrazol-1-yl)pyridine (L1) and 4-(2-(5-(5-hexylthiophen-2-yl)thiophen-2-yl)ethynyl)-2,6-di(1H-pyrazol-1-yl)pyridine (L2) ligands, respectively. Density functional theory (DFT) studies revealed stretching-induced spin-state switching in a molecular junction composed of complex 1, taken as a representative example, and gold electrodes. Single-molecule conductance traces revealed the unfavorable orientation of the complexes in the junctions to demonstrate the spin-state dependence of the conductance., QN/van der Zant Lab

Spin-crossover (SCO) active iron(II) complexes are an integral class of switchable and bistable molecular materials. Spin-state switching properties of the SCO complexes have been studied in the bulk and single-molecule levels to progress toward fabricating molecule-based switching and memory elements. Supramolecular SCO complexes featuring anchoring groups for metallic electrodes, for example, gold (Au), are ideal candidates to study spin-state switching at the single-molecule level. In this study, we report on the spin-state switching characteristics of supramolecular iron(II) complexes 1 and 2 composed of functional 4-([2,2′-bithiophen]-5-ylethynyl)-2,6-di(1H-pyrazol-1-yl)pyridine (L1) and 4-(2-(5-(5-hexylthiophen-2-yl)thiophen-2-yl)ethynyl)-2,6-di(1H-pyrazol-1-yl)pyridine (L2) ligands, respectively. Density functional theory (DFT) studies revealed stretching-induced spin-state switching in a molecular junction composed of complex 1, taken as a representative example, and gold electrodes. Single-molecule conductance traces revealed the unfavorable orientation of the complexes in the junctions to demonstrate the spin-state dependence of the conductance.

This work concerns a novel blue emitting LED phosphor activated by Eu2+. It was found that this luminescent material can be efficiently pumped by near UV or UV-A radiation sources while it exhibits a high quantum yield at a doping level between 0.3 and 3.0%. More impressively, this material has a very high quenching temperature T1/2 and will thus be applicable on-chip in high power phosphor converted LEDs or even in laser diodes. The emission and thermal stability are almost identical to that of BAM:Eu2+ while the absorption band is broader and reaches into the near UV range. Therefore, Na3RbMg7(PO4)6:Eu2+ is a promising candidate as a blue emitter in high CRI full conversion LEDs. To investigate the site preference of Eu2+, DFT and Ligand Field Theory based calculations were performed to successfully predict the emission spectrum. An unusual decay behavior was observed at low temperatures and the underlying mechanism involving spin-forbidden transitions of Eu2+ is discussed.

Eu2+-activated phosphors of the UCr4C4 structure type have emerged as promising new (ultra-)narrow band phosphors. Especially the UCr4C4-type alkali lithosilicates have caused a sensation in the field of phosphor research in the recent years due to their consistent (ultra)narrow-band luminescence in the cyan and green range that can pave the way to novel backlighting LEDs with high color gamut. The compound “K2Na2[Li3SiO4]4:Eu2+” stands out with a strikingly narrow emission in the cyan range (<0.2 eV or <30 nm full width at half maximum in the energy or wavelength scale, respectively) but the position of the Eu2+ ions within this structure has been so far controversial. High-resolution photoluminescence spectroscopy at 10 K reveals a complex emission spectrum with seven emission bands in the violet to green range. Combination with single-crystal X-ray diffraction (SCXRD) demonstrates that the cation channels in the structure also contain Li+ ions. The presence of emission bands in the violet and blue spectral range reveals that the Eu2+ ions also occupy the available K sites in the structure independently confirmed by advanced ligand field calculations. This study demonstrates that a combination of SCXRD, photoluminescence spectroscopy, and theory can help elucidate advanced structural problems.

Graphene quantum dots (QDs) are intensively studied as platforms for the next generation of quantum electronic devices. Fine tuning of the transport properties in monolayer graphene QDs, in particular with respect to the independent modulation of the tunnel barrier transparencies, remains challenging and is typically addressed using electrostatic gating. We investigate charge transport in back-gated graphene mechanical break junctions and reveal Coulomb blockade physics characteristic of a single, high-quality QD when a nanogap is opened in a graphene constriction. By mechanically controlling the distance across the newly formed graphene nanogap, we achieve reversible tunability of the tunnel coupling to the drain electrode by 5 orders of magnitude, while keeping the source-QD tunnel coupling constant. The break junction device can therefore become a powerful platform to study the physical parameters that are crucial to the development of future graphene-based devices, including energy converters and quantum calorimeters., BN/Cees Dekker Lab, Dynamics of Micro and Nano Systems, QN/van der Zant Lab, QN/Steeneken Lab

Graphene quantum dots (QDs) are intensively studied as platforms for the next generation of quantum electronic devices. Fine tuning of the transport properties in monolayer graphene QDs, in particular with respect to the independent modulation of the tunnel barrier transparencies, remains challenging and is typically addressed using electrostatic gating. We investigate charge transport in back-gated graphene mechanical break junctions and reveal Coulomb blockade physics characteristic of a single, high-quality QD when a nanogap is opened in a graphene constriction. By mechanically controlling the distance across the newly formed graphene nanogap, we achieve reversible tunability of the tunnel coupling to the drain electrode by 5 orders of magnitude, while keeping the source-QD tunnel coupling constant. The break junction device can therefore become a powerful platform to study the physical parameters that are crucial to the development of future graphene-based devices, including energy converters and quantum calorimeters.

In this work we perform molecular dynamics simulations of mixtures of a prototypical protic ionic liquid, 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF4]), with lithium tetrafluoroborate (LiBF4), confined between two borophene walls of three different surface charges, −1, 0 and +1 e/nm2, where e is the elementary charge. The properties of the system are analyzed by means of ionic density profiles, angular orientations of [BMIM]+ cations close to the wall and vibrational densities of states for the salt cations close to the walls. The lateral structure of the first layer close to the surface is also studied on one hand, calculating Minkowski parameters and the Shannon entropy of the patterns of the 2D density maps of the anions placed there and, on the other hand, computing the 2D-Fourier transform of the positions of these anions. Our results are compared with those obtained previously for the same mixtures confined between two graphene walls. Although similarities exist between both cases, interesting differences are observed in the lateral structure that the ionic liquid adopts near borophene interfaces due to their strong anisotropy. In particular, we have observed that borophene induces more markedly ordered 2D patterns in the innermost layer of the ionic liquid electric double layer, specially when they are charged. It is this feature that makes borophene a potential candidate for battery electrode applications with possibilities beyond those of graphene.

We have explained how DFT underestimates the HOMO-LUMO gap and how we can correct it with the gas phase and also the screening correction. To understand the first one better we work with the Hubbard dimer and how the analytical gap calculation differs from the one obtained with DFT. For the second case we reviewed the simplest image charge calculations and how we can generalize them from a quantum mechanical point of view. With that results we studied how to shift the position of the occupied and unoccupied molecular resonances in ab initio transport calculations of molecules between electrodes to obtain gaps and transport properties. We pointed out the fundamental problem of the previous screening correction implemented in GOLLUM; the arbitrariness. This model depended of an effective distance that has not a clear physical criteria to be chosen. In fact, when we make it match with experimental values the transmission function obtained and thus the conductance were wrong (very difficult to study molecules we do not know). An alternative method that depends only on physical parameters that we know from the beginning is proposed (atoms position and Mulliken population). With it we remove all this arbitrariness improving at least the qualitative and we hope the quantitative agreements with experiments. To test the accuracy of the method we worked with one of the most studied molecules, the BDT molecule coupled to gold leads finding that our results were more complete and precise than the previous ones. Last but no least we show our analytical results for the effect of the application of a voltage to the planes (contacts) and how it can shift the levels up or down changing the transmission function and thus the conductance. In the future we will try to implement this in GOLLUM to make a more complete and precise version of this program.

El carbono es un elemento químico que se encuentra constantemente a nuestro alrededor. Por ejemplo, en combinación con el hidrógeno y el oxígeno forma los carbohidratos: moléculas que proporcionan energía a las células vivas. El carbono tiene diferentes alótropos tridimensionales como el grafito (comúnmente utilizado en las minas de los lápices o en la industria armamentística) o el diamante (un material con una gran dureza y muy demandado en joyería). Además de las variantes tridimensionales, existen alótropos cerodimensionales (como la molécula C60, también conocida como fulereno, descubierta en 1985 y que consiste en una esfera de 60 átomos de carbono) y unidimensionales (los nanotubos de carbono descubiertos en 1991 que consisten en una estructura cilíndrica). Sin embargo, no es hasta 2004, cuando un grupo de científicos dirigidos por Andre Geim y Konstantin Noselov anuncian que podían aislar una monocapa atómica de carbono a partir del grafito, que aparece el alótropo bidimensional del carbono: el grafeno. Este descubrimiento los hizo merecedores del premio Nobel de Física en el año 2010. Hasta el descubrimiento del grafeno se pensaba que los materiales estrictamente bidimensionales no podían existir porque eran termodinámicamente inestables, por lo que este hecho supuso una gran revolución en el ámbito de la física. Pero también fue un descubrimiento muy importante a nivel industrial: las nuevas propiedades que ofrecía el carbono lo convirtieron en un material muy útil, por ejemplo, en el campo de la electrónica (para nuevas baterías, procesadores, cables de alta velocidad, etc.), en la construcción, en la medicina... El grafeno capturó la atención de todos los científicos e investigadores de la época. Sin embargo, en los últimos años la investigación se ha centrado en la aplicación del grafeno a la nanotecnología. Esto requiere el uso de geometrías del grafeno lo suficientemente pequeñas, de manera que adquiere importancia el estudio de láminas de carbono con

Producción Científica, Arsenic has been predicted to present significantly more diverse 2D phases than other elemental compounds like graphene. While practical applications must be based on finite arsenene samples, like nanoribbons, theory has so far focused on the infinite sheet. Our ab initio simulations show the clear contrast between the properties of arsenene nanoribbons and those of the monolayer, ranging from phase stability to electronic structure. We include nanoribbons derived from the buckled, puckered and square/octagon structures of bulk arsenene. The flexibility afforded by different parent structures, widths and edge passivations leads to a rich variety of semiconducting structures with tunable gaps., Xunta de Galicia (projects AGRUP2015/11, ED431E 2018/8 and GRC ED431C 2016/001), Junta de Castilla y León (project VA124G18)