Introducing Design Strategies to Preserve N-Heterocycles Throughout the On-Surface Synthesis of Graphene Nanostructures.

Despite the impressive advances in the synthesis of atomically precise graphene nanostructures witnessed during the last decade, advancing in compositional complexity faces major challenges. The concept of introducing the desired functional groups or dopants in the molecular precursor often fails due to their lack of stability during the reaction path. Here, a study on the stability of different pyridine and pyrimidine moieties during the on-surface synthesis of graphene nanoribbons on Au(111) is presented. Combining bond-resolved scanning tunneling microscopy with X-ray photoelectron spectroscopy, the thermal evolution of the nitrogen dopants throughout the whole reaction sequence is tracked. A comparative experimental and ab initio electronic characterization confirms the presence of dopants in the final structures, revealing also that the pyridinic nitrogen leads to a significant band downshift. The results demonstrate that, by using synthetic strategies to lower the reaction temperatures, one can preserve specific N-heterocycles throughout all the reaction steps of the synthesis of graphene nanoribbons and beyond the interibbon coupling reaction that leads to nanoporous graphene.

Atomically Sharp Lateral Superlattice Heterojunctions Built-In Nitrogen-Doped Nanoporous Graphene.

Nanometer scale lateral heterostructures with atomically sharp band discontinuities can be conceived as the 2D analogues of vertical Van der Waals heterostructures, where pristine properties of each component coexist with interfacial phenomena that result in a variety of exotic quantum phenomena. However, despite considerable advances in the fabrication of lateral heterostructures, controlling their covalent interfaces and band discontinuities with atomic precision, scaling down components and producing periodic, lattice-coherent superlattices still represent major challenges. Here, a synthetic strategy to fabricate nanometer scale, coherent lateral superlattice heterojunctions with atomically sharp band discontinuity is reported. By merging interdigitated arrays of different types of graphene nanoribbons by means of a novel on-surface reaction, superlattices of 1D, and chemically heterogeneous nanoporous junctions are obtained. The latter host subnanometer quantum dipoles and tunneling in-gap states, altogether expected to promote interfacial phenomena such as interribbon excitons or selective photocatalysis.

Basic aspects of the charge density wave instability of transition metal trichalcogenides NbSe3and monoclinic-TaS3.

NbSe3and monoclinic-TaS3(m-TaS3) are quasi-1D metals containing three different types of chains and undergoing two different charge density wave Peierls transitions atTP1andTP2associated with type III and type I chains, respectively. The nature of these transitions is discussed on the basis of first-principles DFT calculation of their Fermi surface (FS) and electron-hole response function. Because of the stronger inter-chain interactions, the FS and electron-hole response function are considerably more complex for NbSe3thanm-TaS3; however a common scenario can be put forward to rationalize the results. The intra-chain inter-band nesting processes dominate the strongest response for both type I and type III chains of the two compounds. Two well-defined maxima of the electron-hole response for NbSe3are found with the (0a*, 0c*) and (1/2a*, 1/2c*) transverse components atTP1andTP2, respectively, whereas the second maximum is not observed form-TaS3atTP2. Analysis of the different inter-chain coupling mechanisms leads to the conclusion that FS nesting effects are only relevant to set the transversea*components in NbSe3. The strongest inter-chain Coulomb coupling mechanism must be taken into account for the transverse coupling alongc*in NbSe3and along botha*andc*form-TaS3. Phonon spectrum calculations reveal the formation of a giant 2kFKohn anomaly form-TaS3. All these results support a weak coupling scenario for the Peierls transition of transition metal trichalcogenides.

Localized electronic vacancy level and its effect on the properties of doped manganites.

Oxygen vacancies are common to most metal oxides and usually play a crucial role in determining the properties of the host material. In this work, we perform ab initio calculations to study the influence of vacancies in doped manganites [Formula: see text], varying both the vacancy concentration and the chemical composition within the ferromagnetic-metallic range ([Formula: see text]). We find that oxygen vacancies give rise to a localized electronic level and analyse the effects that the possible occupation of this defect state can have on the physical properties of the host. In particular, we observe a substantial reduction of the exchange energy that favors spin-flipped configurations (local antiferromagnetism), which correlate with the weakening of the double-exchange interaction, the deterioration of the metallicity, and the degradation of ferromagnetism in reduced samples. In agreement with previous studies, vacancies give rise to a lattice expansion when the defect level is unoccupied. However, our calculations suggest that under low Sr concentrations the defect level can be populated, which conversely results in a local reduction of the lattice parameter. Although the exact energy position of this defect level is sensitive to the details of the electronic interactions, we argue that it is not far from the Fermi energy for optimally doped manganites ([Formula: see text]), and thus its occupation could be tuned by controlling the number of available electrons, either with chemical doping or gating. Our results could have important implications for engineering the electronic properties of thin films in oxide compounds.

Metallic Diluted Dimerization in VO2 Tweeds.

The observation of electronic phase separation textures in vanadium dioxide, a prototypical electron-correlated oxide, has recently added new perspectives on the long standing debate about its metal-insulator transition and its applications. Yet, the lack of atomically resolved information on phases accompanying such complex patterns still hinders a comprehensive understanding of the transition and its implementation in practical devices. In this work, atomic resolution imaging and spectroscopy unveils the existence of ferroelastic tweed structures on ≈5 nm length scales, well below the resolution limit of currently used spectroscopic imaging techniques. Moreover, density functional theory calculations show that this pretransitional fine-scale tweed, which on average looks and behaves like the standard metallic rutile phase, is in fact weaved by semi-dimerized chains of vanadium in a new monoclinic phase that represents a structural bridge to the monoclinic insulating ground state. These observations provide a multiscale perspective for the interpretation of existing data, whereby phase coexistence and structural intermixing can occur all the way down to the atomic scale.

Anion ordering transition and Fermi surface electron-hole instabilities in the (TMTSF)2ClO4 and (TMTSF)2NO3 Bechgaard salts analyzed through the first-principles Lindhard response function.

The first-principles electron-hole Lindhard response function has been calculated and analyzed in detail for two (TMTSF)2 X (X = ClO4 and NO3) Bechgaard salts undergoing different anion-ordering (AO) transitions. The calculation was carried out using the real triclinic low-temperature structures. The evolution of the electron-hole response with temperature for both relaxed and quenched salts is discussed. It is shown that the 2k F response of the quenched samples of both salts display a low temperature curved and tilted triangular continuum of maxima. This is not the case for the relaxed samples. (TMTSF)2ClO4 in the AO state exhibits a more quasi-1D response than in the non AO state and relaxed (TMTSF)2NO3 shows a sharp maximum. The curved triangular plateau of the quenched samples results from multiple nesting of the warped quasi-1D Fermi surface which implies the existence of a large q range of electron-hole fluctuations. This broad maxima region is around 1% of the Brillouin zone area for the X = ClO4 salt (and X = PF6) but only 0.1% for the X = NO3 salt. It is suggested that the strong reduction of associated SDW fluctuations could explain the non detection of the SDW-mediated superconductivity in (TMTSF)2NO3. The calculated maxima of the Lindhard response nicely account for the modulation wave vector experimentally determined by NMR in the SDW ground state of the two salts. The critical AO wave vector for both salts is located in regions where the Lindhard response is a minimum so that they are unrelated to any electron-hole instability. The present first-principles calculation reveals 3D effects in the Lindhard response of the two salts at low temperature which are considerably more difficult to model in analytical approaches.

Siesta: Recent developments and applications.

A review of the present status, recent enhancements, and applicability of the Siesta program is presented. Since its debut in the mid-1990s, Siesta's flexibility, efficiency, and free distribution have given advanced materials simulation capabilities to many groups worldwide. The core methodological scheme of Siesta combines finite-support pseudo-atomic orbitals as basis sets, norm-conserving pseudopotentials, and a real-space grid for the representation of charge density and potentials and the computation of their associated matrix elements. Here, we describe the more recent implementations on top of that core scheme, which include full spin-orbit interaction, non-repeated and multiple-contact ballistic electron transport, density functional theory (DFT)+U and hybrid functionals, time-dependent DFT, novel reduced-scaling solvers, density-functional perturbation theory, efficient van der Waals non-local density functionals, and enhanced molecular-dynamics options. In addition, a substantial effort has been made in enhancing interoperability and interfacing with other codes and utilities, such as wannier90 and the second-principles modeling it can be used for, an AiiDA plugin for workflow automatization, interface to Lua for steering Siesta runs, and various post-processing utilities. Siesta has also been engaged in the Electronic Structure Library effort from its inception, which has allowed the sharing of various low-level libraries, as well as data standards and support for them, particularly the PSeudopotential Markup Language definition and library for transferable pseudopotentials, and the interface to the ELectronic Structure Infrastructure library of solvers. Code sharing is made easier by the new open-source licensing model of the program. This review also presents examples of application of the capabilities of the code, as well as a view of on-going and future developments.

Fermi surface electron-hole instability of the (TMTSF)2PF6 Bechgaard salt revealed by the first-principles Lindhard response function.

We report the first-principles DFT calculation of the electron-hole Lindhard response function of the (TMTSF)2PF6 Bechgaard salt using the real triclinic low-temperature structure. The Lindhard response is found to change considerably with temperature. Near the 2k F spin density wave (SDW) instability it has the shape of a broad triangular plateau as a result of the multiple nesting associated with the warped quasi-one-dimensional Fermi surface. The evolution of the 2k F broad maximum as well as the effect of pressure and deuteration is calculated and analyzed. The thermal dependence of the electron-hole coherence length deduced from these calculations compares very well with the experimental thermal evolution of the 2k F bond order wave correlation length. The existence of a triangular plateau of maxima in the low-temperature electron-hole Lindhard response of (TMTSF)2PF6 should favor a substantial mixing of q-dependent fluctuations which can have important consequences in understanding the phase diagram of the 2k F SDW ground state, the mechanism of superconductivity and the magneto-transport of this paradigmatic quasi-one-dimensional material. The first-principles DFT Lindhard response provides a very accurate and unbiased approach to the low-temperature instabilities of (TMTSF)2PF6 which can take into account in a simple way 3D effects and subtle structural variations, thus providing a very valuable tool in understanding the remarkable physics of molecular conductors.

Unfolding method for periodic twisted systems with commensurate Moiré patterns.

We present a general unfolding method for the electronic bands of systems with double-periodicity. Within density functional theory with atomic orbitals as basis-set, our method takes into account two symmetry operations of the primitive cell: a standard expansion and a single rotation, letting to elucidate the physical effects associated to the mutual interactions between systems with more than one periodicity. As a result, our unfolding method allows studying the electronic properties of vertically stacked two-dimensional homo- or heterostructures. We apply our method to study [Formula: see text] single-layer graphene, [Formula: see text] twisted single-layer graphene, and [Formula: see text] graphene- [Formula: see text] tungsten disulfide heterostructure with an interlayer angle of [Formula: see text]. Our unfolding method allows observing typical mini gaps reported in heterostructures, as well as other electronic deviations from pristine structures, impossible to distinguish without an unfolding method. We anticipate that this unfolding method can be useful to compare with experiments to elucidate the electronic properties of two-dimensional homo- or heterostructures, where the interlayer angle can be considered as an additional parameter.

Guidelines for Selecting Interlayer Spacers in Synthetic 2D-Based Antiferromagnets from First-Principles Simulations.

Following the recent synthesis of graphene-based antiferromagnetic ultrathin heterostructures made of Co and Fe, we analyse the effect of the spacer between both ferromagnetic materials. Using density functional calculations, we carried out an exhaustive study of the geometric, electronic and magnetic properties for intercalated single Co MLs on top of Ir(111) coupled to monolayered Fe through n graphene layers (n = 1, 2, 3) or monolayered h-BN. Different local atomic arrangements have been considered to model the Moiré patterns expected in these heterostructures. The magnetic exchange interactions between both ferromagnets ( J C o - F e ) are computed from explicit calculations of parallel and anti-parallel Fe/Co inter-layer alignments, and discussed in the context of recent experiments. Our analysis confirms that the robust antiferromagnetic superexchange-coupling between Fe and Co layers is mediated by the graphene spacer through the hybridization of C's p z orbitals with Fe and Co's 3d states. The hybridization is substantially suppressed for multilayered graphene spacers, for which the magnetic coupling between ferromagnets is critically reduced, suggesting the need for ultrathin (monolayer) spacers in the design of synthetic graphene-based antiferromagnets. In the case of h-BN, p z orbitals also mediate d(Fe/Co) coupling. However, there is a larger contribution of local ferromagnetic interactions. Magnetic anisotropy energies were also calculated using a fully relativistic description, and show out-of-plane easy axis for all the configurations, with remarkable net values in the range from 1 to 4 meV.

Coexistence of Elastic Modulations in the Charge Density Wave State of 2 H-NbSe2.

Bulk and single-layer 2 H-NbSe2 exhibit identical charge density wave order (CDW) with a quasi-commensurate 3 × 3 superlattice periodicity. Here we combine scanning tunnelling microscopy (STM) imaging at T = 1 K of 2 H-NbSe2 with first-principles density functional theory (DFT) calculations to investigate the structural atomic rearrangement of this CDW phase. Our calculations for single-layers reveal that six different atomic structures are compatible with the 3 × 3 CDW distortion, although all of them lie on a very narrow energy range of at most 3 meV per formula unit, suggesting the coexistence of such structures. Our atomically resolved STM images of bulk 2 H-NbSe2 unambiguously confirm this by identifying two of these structures. Remarkably, these structures differ from the X-ray crystal structure reported for the bulk 3 × 3 CDW which in fact is also one of the six DFT structures located for the single-layer. Our calculations also show that due to the minute energy difference between the different phases, the ground state of the 3 × 3 CDW could be extremely sensitive to doping, external strain or internal pressure within the crystal. The presence of multiphase CDW order in 2 H-NbSe2 may provide further understanding of its low temperature state and the competition between different instabilities.

Electrical and Thermal Transport in Coplanar Polycrystalline Graphene-hBN Heterostructures.

We present a theoretical study of electronic and thermal transport in polycrystalline heterostructures combining graphene (G) and hexagonal boron nitride (hBN) grains of varying size and distribution. By increasing the hBN grain density from a few percent to 100%, the system evolves from a good conductor to an insulator, with the mobility dropping by orders of magnitude and the sheet resistance reaching the MΩ regime. The Seebeck coefficient is suppressed above 40% mixing, while the thermal conductivity of polycrystalline hBN is found to be on the order of 30-120 Wm-1 K-1. These results, agreeing with available experimental data, provide guidelines for tuning G-hBN properties in the context of two-dimensional materials engineering. In particular, while we proved that both electrical and thermal properties are largely affected by morphological features (e.g., by the grain size and composition), we find in all cases that nanometer-sized polycrystalline G-hBN heterostructures are not good thermoelectric materials.

Piezoelectric monolayers as nonlinear energy harvesters.

We study the dynamics of h-BN monolayers by first performing ab-initio calculations of the deformation potential energy and then solving numerically a Langevine-type equation to explore their use in nonlinear vibration energy harvesting devices. An applied compressive strain is used to drive the system into a nonlinear bistable regime, where quasi-harmonic vibrations are combined with low-frequency swings between the minima of a double-well potential. Due to its intrinsic piezoelectric response, the nonlinear mechanical harvester naturally provides an electrical power that is readily available or can be stored by simply contacting the monolayer at its ends. Engineering the induced nonlinearity, a 20 nm2 device is predicted to harvest an electrical power of up to 0.18 pW for a noisy vibration of 5 pN.

Band selection and disentanglement using maximally localized Wannier functions: the cases of Co impurities in bulk copper and the Cu(111) surface.

We have adapted the maximally localized Wannier function approach of Souza et al (2002 Phys. Rev. B 65 035109) to the density functional theory based SIESTA code (Soler et al 2002 J. Phys.: Condens. Mater. 14 2745) and applied it to the study of Co substitutional impurities in bulk copper as well as to the Cu(111) surface. In the Co impurity case, we have reduced the problem to the Co d-electrons and the Cu sp-band, permitting us to obtain an Anderson-like Hamiltonian from well defined density functional parameters in a fully orthonormal basis set. In order to test the quality of the Wannier approach to surfaces, we have studied the electronic structure of the Cu(111) surface by again transforming the density functional problem into the Wannier representation. An excellent description of the Shockley surface state is attained, permitting us to be confident in the application of this method to future studies of magnetic adsorbates in the presence of an extended surface state.

Real-Time TD-DFT Simulations in Dye Sensitized Solar Cells: The Electronic Absorption Spectrum of Alizarin Supported on TiO2 Nanoclusters.

The structural and electronic properties of the alizarin dye supported on TiO2 nanoclusters have been examined by means of time-dependent density-functional (TD-DFT) calculations performed in the time-domain framework. The calculated electronic absorption spectrum of free alizarin shows a first band centered at 2.67 eV that upon adsorption features a red shift by 0.31 eV, in agreement with both experimental and previous theoretical work. This red shift arises from a relative stabilization of the dye LUMO when adsorbed. To analyze the dependence of the electronic properties of the dye-support couple on the size of metal-oxide nanoparticles, different models of (TiO2)n nanoclusters have been used (with n = 1, 2, 3, 6, 9, 15, and 38). As a conclusion, the minimal model is good enough to theoretically reproduce the main feature in the spectrum (i.e., the energy shift of the main band upon binding to TiO2). However, it fails in creating intermediate states which could play a significant role under real experimental conditions (dynamics of the electronic transfer). Indeed, as the size of the nanocluster grows, the dye LUMO moves from the edge to well inside the conduction band (Ti 3d band). On the other hand, to assess the consistency of the time-domain approach in the case of such systems, conventional (frequency-domain) TD-DFT calculations have been carried out. It is found that, as far as the functional and basis set are equivalent, both approaches lead to similar results. While for small systems the standard TD-DFT is better suited, for medium to large sized systems, the real-time TD-DFT becomes competitive and more efficient.