Optical spectra and exciton radiative lifetimes in bulk transition metal dichalcogenides.

The optical response of layered transition metal dichalcogenides (TMDCs) exhibits remarkable excitonic properties which are important from both fundamental and device application viewpoints. One of these phenomena is the observation of intralayer/interlayer excitons. While much effort has been done to characterize excitons in monolayer TMDCs and their heterostructures, a quite limited number of works have addressed the exciton spectra of their bulk counterparts. In this work, we employ ab initio many-body perturbation calculations to investigate the exciton dynamics and spectra of bulk 2H-MX2 (M = Mo, W, and X = S, Se). For molybdenum-based systems, we find the presence of interlayer excitons at energies higher than the first bright exciton (XA), with non-negligible strength intensity. Our results also show that interlayer excitons in tungsten-based systems are almost degenerate in energy with XA and possess very small oscillator strengths when compared with molybdenum-based systems. At room temperature, and considering the thermal exciton fine-structure population for the XA-exciton, we estimate effective radiative lifetimes in the range of ∼4-14 ns. For higher energy excitons we predict longer effective lifetimes of tens of nanoseconds.

Nanogap-based all-electronic DNA sequencing devices using MoS2 monolayers.

The realization of nanopores in atom-thick materials may pave the way towards electrical detection of single biomolecules in a stable and scalable manner. In this work, we theoretically study the potential of different phases of MoS2 nanogaps to act as all-electronic DNA sequencing devices. We carry out simulations based on density functional theory and the non-equilibrium Green's function formalism to investigate the electronic transport across the device. Our results suggest that the 1T'-MoS2 nanogap structure is energetically more favorable than its 2H counterpart. At zero bias, the changes in the conductance of the 1T'-MoS2 device can be well distinguished, making possible the selectivity of the DNA nucleobases. Although the conductance fluctuates around the resonances, the overall results suggest that it is possible to distinguish the four DNA bases for energies close to the Fermi level.

Ab initio modelling of spin relaxation lengths in disordered graphene nanoribbons.

The spin-dependent transport properties of armchair graphene nanoribbons in the presence of extrinsic spin-orbit coupling induced by a random distribution of nickel adatoms is studied. By combining a recursive Green's function formalism with density functional theory, we explore the influence of ribbon length and metal adatom concentration on the conductance. At a given length, we observed a significant enhancement of the spin-flip channel around resonances and at energies right above the Fermi level. We also estimate the spin-relaxation length, finding values on the order of tens of micrometers at low Ni adatom concentrations. This study is conducted at singular ribbon lengths entirely from fully ab initio methods, providing indirectly evidence that the Dyakonov-Perel spin relaxation mechanism might be the dominant at low concentrations as well as the observation of oscillations in the spin-polarization.

Phase transition and electronic structure investigation of MoS2-reduced graphene oxide nanocomposite decorated with Au nanoparticles.

In this work a simple approach to transform MoS2 from its metallic (1T' to semiconductor 2H) character via gold nanoparticle surface decoration of a MoS2 reduced graphene oxide (rGO) nanocomposite is proposed. The possible mechanism to this phase transformation was investigated using different spectroscopy techniques, and supported by density functional theory theoretical calculations. A mixture of the 1T'- and 2H-MoS2 phases was observed from the Raman and Mo 3d high resolution x-ray photoelectron spectra analysis in the MoS2-rGO nanocomposite. After surface decoration with gold nanoparticles the concentration of the 1T' phase decreases making evident a phase transformation. According to Raman and valence band spectra analyzes, the Au nanoparticles (NPs) induce a p-type doping in MoS2-rGO nanocomposite. We proposed as a main mechanism to the MoS2 phase transformation the electron transfer from Mo 4d xy,xz,yz in 1T' phase to AuNPs conduction band. At the same time, the unoccupied electronic structure was investigated from S K-edge near edge x-ray absorption fine structure spectroscopy. Finally, the electronic coupling between unoccupied electronic states was investigated by the core hole clock approach using resonant Auger spectroscopy, showing that AuNPs affect mainly the MoS2 electronic states close to Fermi level.

Structure-Property-Performance Relationship of Ultrathin Pd-Au Alloy Catalyst Layers for Low-Temperature Ethanol Oxidation in Alkaline Media.

Pd-containing alloys are promising materials for catalysis. Yet, the relationship of the structure-property performance strongly depends on their chemical composition, which is currently not fully resolved. Herein, we present a physical vapor deposition methodology for developing PdxAu1-x alloys with fine control over the chemical composition. We establish direct correlations between the composition and these materials' structural and electronic properties with its catalytic activity in an ethanol (EtOH) oxidation reaction. By combining X-ray diffraction (XRD) and X-ray photelectron spectroscopy (XPS) measurements, we validate that the Pd content within both bulk and surface compositions can be finely controlled in an ultrathin-film regime. Catalytic oxidation of EtOH on the PdxAu1-x electrodes presents the largest forward-sweeping current density for x = 0.73 at ∼135 mA cm-2, with the lowest onset potential and largest peak activity of 639 A gPd-1 observed for x = 0.58. Density functional theory (DFT) calculations and XPS measurements demonstrate that the valence band of the alloys is completely dominated by Pd particularly near the Fermi level, regardless of its chemical composition. Moreover, DFT provides key insights into the PdxAu1-x ligand effect, with relevant chemisorption activity descriptors probed for a large number of surface arrangements. These results demonstrate that alloys can outperform pure metals in catalytic processes, with fine control of the chemical composition being a powerful tuning knob for the electronic properties and, therefore, the catalytic activity of ultrathin PdxAu1-x catalysts. Our high-throughput experimental methodology, in connection with DFT calculations, provides a unique foundation for further materials' discovery, including machine-learning predictions for novel alloys, the development of Pd-alloyed membranes for the purification of reformate gases, binder-free ultrathin electrocatalysts for fuel cells, and room temperature lithography-based development of nanostructures for optically driven processes.

Spin-orbit coupling prevents spin channel suppression of transition metal atoms on armchair graphene nanoribbons.

We investigate the spin-dependent electronic and transport properties of armchair graphene nanoribbons including spin-orbit coupling due to the presence of nickel and iridium adatoms by using ab initio calculations within the spin-polarized density functional theory and non-equilibrium Green's function formalism. Our results indicate that the intensity of the spin-flip precession is a direct consequence of the relaxed adsorption sites of the adatoms. We point out that d orbitals of Ni and Ir result in strong dependence on the spin-conserved and spin-flip transmission probabilities. In particular, we show that the presence of spin-orbit coupling can lead to an enhancement of the transmission probabilities especially around resonances arising due to weak coupling with specific orbitals.

Ultrafast charge transfer dynamics pathways in two-dimensional MoS2-graphene heterostructures: a core-hole clock approach.

Two-dimensional van der Waals heterostructures are attractive candidates for optoelectronic nanodevice applications. The charge transport process in these systems has been extensively investigated, however the effect of coupling between specific electronic states on the charge transfer process is not completely established yet. Here, interfacial charge transfer (CT) in the MoS2/graphene/SiO2 heterostructure is investigated from static and dynamic points of view. Static CT in the MoS2-graphene interface was elucidated by an intensity quenching, broadening and a blueshift of the photoluminescence peaks. Atomic and electronic state-specific CT dynamics on a femtosecond timescale are characterized using a core-hole clock approach and using the S1s core-hole lifetime as an internal clock. We demonstrate that the femtosecond electron transfer pathway in the MoS2/SiO2 heterostructure is mainly due to the electronic coupling between S3p-Mo4d states forming the Mo-S covalent bond in the MoS2 layer. For the MoS2/graphene/SiO2 heterostructure, we identify, with the support of density functional calculations, new pathways that arise due to the high density of empty electronic states of the graphene conduction band. The latter makes the transfer process time in the MoS2/graphene/SiO2/Si twice as fast as in the MoS2/SiO2/Si sample. Our results show that ultrafast electron delocalization pathways in van der Waals heterostructures are dependent on the electronic properties of each involved 2D material, creating opportunities to modulate their transport properties.

Two-dimensional exciton properties in monolayer semiconducting phosphorus allotropes.

Excitons play a key role in technological applications since they have a strong influence on determining the efficiency of photovoltaic devices. Recently, it has been shown that the allotropes of phosphorus possess an optical band gap that can be tuned over a wide range of values including the near-infrared and visible spectra, which would make them promising candidates for optoelectronic applications. In this work we carry out ab initio many-body perturbation theory calculations to study the excitonic effects on the optical properties of two-dimensional phosphorus allotropes: the case of blue and black monolayers. We elucidate the most relevant optical transitions, exciton binding energy spectrum as well as real-space exciton distribution, particularly focusing on the absorption spectrum dependence on the incident light polarization. In addition, based on our results, we use a set of effective hydrogenic models, in which the electron-hole Coulomb interaction is included to estimate exciton binding energies and radii. Our results show an excellent agreement between the many-body methodology and the effective models.

Anomalous Temperature Dependence of the Band Gap in Black Phosphorus.

Black phosphorus (BP) has gained renewed attention due to its singular anisotropic electronic and optical properties that might be exploited for a wide range of technological applications. In this respect, the thermal properties are particularly important both to predict its room temperature operation and to determine its thermoelectric potential. From this point of view, one of the most spectacular and poorly understood phenomena is indeed the BP temperature-induced band gap opening; when temperature is increased, the fundamental band gap increases instead of decreases. This anomalous thermal dependence has also been observed recently in its monolayer counterpart. In this work, based on ab initio calculations, we present an explanation for this long known and yet not fully explained effect. We show that it arises from a combination of harmonic and lattice thermal expansion contributions, which are in fact highly interwined. We clearly narrow down the mechanisms that cause this gap opening by identifying the peculiar atomic vibrations that drive the anomaly. The final picture we give explains both the BP anomalous band gap opening and the frequency increase with increasing volume (tension effect).

Optical spectrum of bottom-up graphene nanoribbons: towards efficient atom-thick excitonic solar cells.

Recently, atomically well-defined cove-shaped graphene nanoribbons have been obtained using bottom-up synthesis. These nanoribbons have an optical gap in the visible range of the spectrum which make them candidates for donor materials in photovoltaic devices. From the atomistic point of view, their electronic and optical properties are not clearly understood. Therefore, in this work we carry out ab-initio density functional theory calculations combine with many-body perturbation formalism to study their electronic and optical properties. Through the comparison with experimental measurements, we show that an accurate description of the nanoribbon's optical properties requires the inclusion of electron-hole correlation effects. The energy, binding energy and the corresponding excitonic transitions involved are analyzed. We found that in contrast to zigzag graphene nanoribbons, the excitonic peaks in the absorption spectrum are a consequence of a group of transitions involving the first and second conduction and valence bands. Finally, we estimate some relevant optical properties that strengthen the potential of these nanoribbons for acting as a donor materials in photovoltaic.

Voltage-driven ring confinement in a graphene sheet: assessing conditions for bound state solutions.

We have systematically studied the single-particle states in quantum rings produced by a set of concentric circular gates over a graphene sheet placed on a substrate. The resulting potential profiles and the interaction between the graphene layer and the substrate are considered within the Dirac Hamiltonian in the framework of the envelope function approximation. Our simulations allow microscopic mapping of the character of the electron and hole quasi-particle solutions according to the applied voltage. General conditions to control and operate the bound state solutions are described as functions of external and controllable parameters that will determine the optical properties ranging from metallic to semiconductor phases. Contrasting behaviors are obtained when comparing the results for repulsive and attractive voltages as well as for variation of the relative strength of the graphene-substrate coupling parameter.

Anisotropy induced localization of pseudo-relativistic spin states in graphene double quantum wire structures.

We study the single-particle properties of Dirac Fermions confined to a double quantum wire system based on graphene. We map out the spatial regions where electrons in a given subband display the largest occupation probability induced by spatial anisotropic effects associated to the interaction strength between the graphene wires and the substrate. Here, the graphene-substrate interaction is considered as an ad hoc parameter which destroys the zero-gap observed in the relativistic Dirac cone characteristic of graphene electronic energy dispersions. Furthermore, the results indicate that the character of quasi-extended spin states, viewed by multisubband probability density function, is highly sensitive to spatial asymmetries and to the graphene-substrate interaction strength.