Theoretical study of structural and electronic properties of 2H-phase transition metal dichalcogenides

Computational physics and chemistry are called to play a very important role in the development of new technologies based on two-dimensional (2D) materials, reducing drastically the number of trial and error experiments needed to obtain meaningful advances in the field. Here, we present a thorough theoretical study of the structural and electronic properties of the single-layer, double-layer, and bulk transition metal dichalcogenides MoS2, MoSe2, MoTe2, WS2, WSe2, and WTe2 in the 2H phase, for which only partial experimental information is available. We show that the properties of these systems depend strongly on the density functional theory approach used in the calculations and that inclusion of weak dispersion forces is mandatory for a correct reproduction of the existing experimental data. By using the most accurate functionals, we predict interlayer separations, direct and indirect band gaps, and spin-orbit splittings in those systems for which there is no experimental information available. We also discuss the variation of these properties with the specific chalcogen and transition metal ato

Tunable plasmons in regular planar arrays of graphene nanoribbons with armchair and zigzag-shaped edges

Recent experimental evidence for and the theoretical confirmation of tunable edge plasmons and surface plasmons in graphene nanoribbons have opened up new opportunities to scrutinize the main geometric and conformation factors, which can be used to modulate these collective modes in the infrared-to-terahertz frequency band. Here, we show how the extrinsic plasmon structure of regular planar arrays of graphene nanoribbons, with perfectly symmetric edges, is influenced by the width, chirality and unit-cell length of each ribbon, as well as the in-plane vacuum distance between two contiguous ribbons. Our predictions, based on time-dependent density functional theory, in the random phase approximation, are expected to be of immediate help for measurements of plasmonic features in nanoscale architectures of nanoribbon devicesC.V.G. acknowledges the financial support of the “Secretaria Nacional de Educación Superior, Ciencia, Tecnología e Innovación” (SENESCYT-ECUADOR

Dielectric screening and plasmon resonances in bilayer graphene

The plasmon structure of intrinsic and extrinsic bilayer graphene is investigated in the framework of ab initio time-dependent density-functional theory (TDDFT) at the level of the random-phase approximation (RPA). A two-step scheme is adopted, where the electronic ground state of a periodically repeated slab of bilayer graphene is first determined with full inclusion of the anisotropic band structure and the interlayer interaction; a Dyson-like equation is then solved self-consistently in order to calculate the so-called density-response function of the many-electron system. A two-dimensional correction is subsequently applied in order to eliminate the artificial interaction between the replicas. The energy range below ∼30 eV is explored, focusing on the spectrum of single-particle excitations and plasmon resonances induced by external electrons or photons. The high-energy loss features of the π and σ+π plasmons, particularly their anisotropic dispersions, are predicted and discussed in relation with previous calculations and experiments performed on monolayer and bilayer graphene. At the low-energy end, the energy-loss function is found to be (i) very sensitive to the injected charge carrier density in doped bilayer graphene and (ii) highly anisotropic. Furthermore, various plasmon modes are predicted to exist and are analyzed with reference to the design of novel nanodevicesM.P. and M.G. acknowledge financial support by the European Commission, the European Social Fund, and the Regione Calabria, (POR) Calabria - FSE 2007/2013, and the hospitality of CIC nanoGUNE and the Donostia International Physics Center (DIPC). V.M.S. acknowledges the partial support from the Basque Departamento de Educación, UPV/EHU (Grant No. IT-756-13) and the Spanish Ministry of Economy and Competitiveness MINECO (Grant No. FIS2013-48286-C2-1-P

Imaging and Controlling Coherent Phonon Wave Packets in Single Graphene Nanoribbons

The motion of atoms is at the heart of any chemical or structural transformation in molecules and materials. Upon activation of this motion by an external source, several (usually many) vibrational modes can be coherently coupled, thus facilitating the chemical or structural phase transformation. These coherent dynamics occur on the ultrafast time scale, as revealed, e.g., by nonlocal ultrafast vibrational spectroscopic measurements in bulk molecular ensembles and solids. Tracking and controlling vibrational coherences locally at the atomic and molecular scales is, however, much more challenging and in fact has remained elusive so far. Here, we demonstrate that the vibrational coherences induced by broadband laser pulses on a single graphene nanoribbon (GNR) can be probed by femtosecond coherent anti-Stokes Raman spectroscopy (CARS) when performed in a scanning tunnelling microscope (STM). In addition to determining dephasing (~ 440 fs) and population decay times (~1.8 ps) of the generated phonon wave packets, we are able to track and control the corresponding quantum coherences, which we show to evolve on time scales as short as ~ 70 fs. We demonstrate that a two-dimensional frequency correlation spectrum unequivocally reveals the quantum couplings between different phonon modes in the GNR.Comment: 18 page

Tunable Graphene Electronics with Local Ultrahigh Pressure

We achieve fine tuning of graphene effective doping by applying ultrahigh pressures (> 10 GPa) using Atomic Force Microscopy (AFM) diamond tips. Specific areas in graphene flakes are irreversibly flattened against a SiO2 substrate. Our work represents the first demonstration of local creation of very stable effective p-doped graphene regions with nanometer precision, as unambiguously verified by a battery of techniques. Importantly, the doping strength depends monotonically on the applied pressure, allowing a controlled tuning of graphene electronics. Through this doping effect, ultrahigh pressure modifications include the possibility of selectively modifying graphene areas to improve their electrical contact with metal electrodes, as shown by Conductive AFM. Density Functional Theory calculations and experimental data suggest that this pressure level induces the onset of covalent bonding between graphene and the underlying SiO2 substrate. Our work opens a convenient avenue to tuning the electronics of 2D materials and van der Waals heterostructures through pressure with nanometer resolution

Probing the phase transition to a coherent 2D Kondo Lattice

Kondo lattices are systems with unusual electronic properties that stem from strong electron correlation, typically studied in intermetallic 3D compounds containing lanthanides or actinides. Lowering the dimensionality of the system enhances the role of electron correlations providing a new tuning knob for the search of novel properties in strongly correlated quantum matter. The realization of a 2D Kondo lattice by stacking a single-layer Mott insulator on a metallic surface is reported. The temperature of the system is steadily lowered and by using high-resolution scanning tunneling spectroscopy, the phase transition leading to the Kondo lattice is followed. Above 27 K the interaction between the Mott insulator and the metal is negligible and both keep their original electronic properties intact. Below 27 K the Kondo screening of the localized electrons in the Mott insulator begins and below 11 K the formation of a coherent quantum electronic state extended to the entire sample, i.e., the Kondo lattice, takes place. By means of density functional theory, the electronic properties of the system and its evolution with temperature are explained. The findings contribute to the exploration of unconventional states in 2D correlated materialsThis work was supported by Ministerio de Ciencia, Innovación y Universidades through grants, PID2021-128011NB-I00 and PID2019-105458RBI00. Ministerio de Ciencia e Innovación and Comunidad de Madrid through grants “Materiales Disruptivos Bidimensionales (2D)” (MAD2DCM)-UAM and “Materiales Disruptivos Bidimensionales (2D)” (MAD2DCM)-IMDEA-NC funded by the Recovery, Transformation and Resilience Plan, and by NextGenerationEU from the European Union. Comunidad de Madrid through grants NMAT2D-CM P20128/NMT-4511 and NanoMagCost. IMDEA Nanoscience acknowledges support from the “‘Severo Ochoa”’ Programme for Centres of Excellence in R&D CEX2020-001039-S. IFIMAC acknowledges support from the “‘María de Maeztu”’ Programme for Units of Excellence in R&D CEX2018-000805-M. M.G. thanks Ministerio de Ciencia, Innovación y Universidades “Ramón y Cajal” Fellowship RYC2020-029317-I. Allocation of computing time at the Centro de Computación Científica at the Universidad Autónoma de Madrid, the CINECA Consortium INF16_npqcd Project, and Newton HPCC Computing Facility at the University of Calabria (MP

Nearly-freestanding supramolecular assembly with tunable structural properties

The synthesis and design of two-dimensional supramolecular assemblies with specific functionalities is one of the principal goals of the emerging field of molecule-based electronics, which is relevant for many technological applications. Although a large number of molecular assemblies have been already investigated, engineering uniform and highly ordered two-dimensional molecular assemblies is still a challenge. Here we report on a novel approach to prepare wide highly crystalline molecular assemblies with tunable structural properties. We make use of the high-reactivity of the carboxylic acid functional moiety and of the predictable structural features of non-polar alkane chains to synthesize 2D supramolecular assemblies of 4-(decyloxy)benzoic acid (4DBA;C[Formula: see text] H[Formula: see text] O[Formula: see text] ) on a Au(111) surface. By means of scanning tunneling microscopy, density functional theory calculations and photoemission spectroscopy, we demonstrate that these molecules form a self-limited highly ordered and defect-free two-dimensional single-layer film of micrometer-size, which exhibits a nearly-freestanding character. We prove that by changing the length of the alkoxy chain it is possible to modify in a controlled way the molecular density of the “floating” overlayer without affecting the molecular assembly. This system is especially suitable for engineering molecular assemblies because it represents one of the few 2D molecular arrays with specific functionality where the structural properties can be tuned in a controlled way, while preserving the molecular pattern

Electronic Excitations ofGraphene, Graphene Nickel interfaces, and Carbon Nanotubes

Scuola di Dottorato "Archimede" Dottorato di Ricerca in Fisica e Tecnologie Quantistiche Ciclo XXV, a.a. 2012Università della Calabri

Plasmon modes in extrinsic graphene: Ab initio simulations vs semi-classical models

Excitation and propagation of surface plasmons in intrinsic and extrinsic graphene are analyzed from the fundamental point of view, using time-dependent density functional theory in linear response regime. Density functional calculations, being set up from first principles, do include anisotropic effects in the unique electronic structure of graphene that cause remarkable consequences even on the THz band. The main signature of this anisotropy is the occurrence of two distinct plasmon modes over a frequency range of 1 to 300THz, where most photonic devices currently operate with large bandwidths and low losses. Further anisotropic features are inherent to the different electromagnetic response of graphene to positive and negative doping concentrations. The Dirac-cone approximation provides a simplified insight, assuming an isotropic graphene band structure near the Fermi level, which is found to be reliable at probing frequencies below ~20 THz and doping levels associated to Fermi energy shifts below/above ±0.3 eV. In these limits, a continuous integral expression derived from the Kubo formula represents an easy-to-use tool capable of catching the main essence of the process