Carbon Nanotubes | EPFL Graph Search

This thesis studies carbon nanotubes using state-of-the-art computational methods. Using large-scale quantum-mechanical calculations, based on density-functional-theory, we investigate several important aspects of the physics and chemistry of single-walled-nanotubes. The focus is on the effect of defects, namely adparticles and vacancies, on the structural, electronic and dynmical properties of the nanotubes. We also present preliminary results of simulations exploring a possible route for the formation of SWNTs from graphene nanoflakes. Adparticles include atomic hydrogen, oxygen, sulfur at different concentrations as well as nitrogen--oxides. Chemisorption of hydrogen as well as oxygen and isoelectronic species results in the formation of clusters on the sidewall, with characteristic structures corresponding to characteristic signatures in the electronic spectra. Especially in the case of oxygen, we find that relatively high energy barriers separate different structures: this shows that not only thermodynamically favored configurations are relevant for the understanding of oxygen chemisorption but the presence of traps cannot be neglected. The fingerpint of these traps is confirmed by scanning-tunneling spectroscopy. Trends with size and chirality of the nanotubes and oxygen coverage are studied in detail and also explained in terms of simple chemical descriptors. The importance of large-scale atomistic models is also emphasized to obtain convergent results and thus reliable predictions, and comparison is made of results we obtain using different gradient-corrected exchange-correlation functionals and in part with hybrid functionals. The study of nitrogen-oxides faces the difficulty to correctly represent physisorption, also with empirically corrected gradient-corrected exchange-correlation and hybrid functionals. Still our calculations of vibrational frequencies of different molecules on the sidewall, once compared with experiment, are able to distinguish the specific species observed in infrared spectra. Part of our work is centered on the comparison of widely used classical potentials (reactive force fields) with DFT results. Specifically we use them to study hydrogen and oxygen chemisorption, and especially examine the validity of several different force-fields for the description of the structure and energetics of single and double vacancies. In all cases, we find rather limited agreement with DFT results, showing the intrinsic difficulty to represent the subtle and intrinsic quantum effects governing the physico-chemical behavior of carbon nanotubes. Still, the wealth of results we have obtained might be useful for an improvement of these classical schemes for a specific application or other semi-classical models.

Alignment of energy levels in amorphous oxides and at semiconductor-water interfaces

Zhendong Guo

We conduct a detailed investigation of defects in two representative amorphous oxides: amorphous Al2O3 (am-Al2O3) and TiO2 (am-TiO2), by combining ab initio molecular dynamics (MD) simulations and hybrid functional calculations. Our results indicate that oxygen vacancies and interstitials occur neither in am-Al2O3 nor in am-TiO2 due to structural rearrangements which assimilate the defect structure, and that the injection of extra holes can lead to the formation of O-O peroxy linkages. In am-Al2O3, hydrogen is found to be amphoteric. Based on localized Wannier functions, we identify the nominal charge state and the composition of the defect core units related to C and N impurities in am-Al2O3, which are found to depend on the total charge set in the simulation cell. Through the adopted electron counting rule, we assess that carbon and nitrogen impurities are only found in neutral and in singly positive charge states, respectively, indicating that none of them gives charge transition levels in am-Al2O3. In addition, those defect core units are shown to incorporate a varying number of oxygen atoms, by which their formation energy is dependent on the oxygen chemical potential. In addition, we propose an exchange mechanism for hole transport in am-TiO2 that relies on the simultaneous breaking and forming of O-O peroxy linkages that share one O atom. Through the use of nudged-elastic-band calculations, a hopping path as long as 1.2 nm with barriers of 0.3-0.5 eV is demonstrated, suggesting that hole diffusion through O-O peroxy linkages is viable in am-TiO2. In this work, we also determine the band alignment between various semiconductors and liquid water by combining MD simulations of atomistic interface models, electronic-structure calculations at the hybrid-functional and GW levels, and a computational standard hydrogen electrode (SHE). Our study comprises GaAs, GaP, GaN, CdS, ZnO, SnO2, rutile and anatase TiO2. For each semiconductor, we generate atomistic interface models with liquid water at the pH corresponding to the point of zero charge. The MD simulations are started from initial configurations, in which the water molecules are either molecularly (m) or dissociatively (d) adsorbed on the semiconductor surface. The calculated band offsets are found to be strongly influenced by the adsorption mode at the semiconductor-water interface, leading to differences larger than 1 eV between m and d models of the same semiconductor. We then assess the accuracy of various ab initio electronic-structure schemes in determining the band alignment. In the last part, we try to evaluate photocatalysts for water-splitting by considering the surface coverage and the energy alignment. We determine surface concentrations of water molecules, protons, and hydroxyl ions adsorbed at the semiconductor-water interfaces mentioned above as a function of pH. This is achieved through the calculation of the acidity constants at the surface sites, which are derived from ab initio MD simulations and a grand-canonical formulation of adsorbates. It is finally shown how the potential of a semiconductor material as photocatalysts for water splitting can be inferred by combining the nature of the surface coverage and the alignment of the band edges to the relevant redox levels.

EPFL2019

The role of disorder in the electronic and transport properties of monolayer and bilayer graphene

Fernando Gargiulo

This thesis is devoted to the computational study of the electronic and transport properties of monolayer and bilayer graphene in the presence of disorder arising from both topological and point defects. Among the former, we study grain boundaries in monolayer graphene and stacking domain boundaries in bilayer graphene, whereas among the latter we study hydrogen atoms covalently bound on the graphene crystal lattice. The electronic spectrum of disordered graphene has been studied within a tight-binding framework, which has been coupled to the Landauer-Büttiker theory and Green¿s function techniques in order to have access to the properties of coherent transport of graphene charge carriers. We assess the low-energy equilibrium structures of defective graphene by a combination of ab initio density functional theory, classical potentials, and Monte Carlo methods. We study periodic grain boundaries in monolayer graphene and individuate two classes of defects with opposite effects in terms of scattering of low-energy charge carriers. One class, unexpectedly, is highly reflecting in the limit of low defect density, whereas another is highly transparent. Subsequently, we study disordered grain boundaries in order to predict the intrinsic conductance of realistic polycrystalline graphene samples. In two related works, conducted in collaboration with experimentalists, we identify the atomic structure of periodic grain boundaries imaged by scanning tunneling microscopy, and discuss the valley-filtering capabilities of a line defect of graphene that can be grown in a controllable manner. Next, we investigate the electronic transport of graphene with realistic hydrogen adsorbates, whose equilibrium configurations are obtained by means of Monte Carlo simulations. We find that the conductance of graphene dramatically increases upon formation of cluster adatoms, which we predict to happen spontaneously at room temperature. This is due to the non- resonant nature of a large fraction of hydrogen clusters in the room-temperature distribution, which we further elucidate by means of an analytically solvable model. Finally, we study the behavior, in terms of structural and electronic properties, of twisted bilayer graphene in the limit of zero twist angle. We find a critical angle below which the system arranges in a triangular superlattice of Bernal-stacking domains, separated by a hexagonal network of stacking domain boundaries. The presence of stacking domain boundaries is at the base of our interpretation of an experiment reporting oscillations in the electrical conductance of bilayer graphene subjected to mechanical indentation.

EPFL2015

Atomic Oxygen Chemisorption on Carbon Nanotubes Revisited with Theory and Experiment

Wanda Andreoni, Alessandro Curioni, Fabio Pietrucci

Density-functional-theory based calculations of two single-walled carbon nanotubes of different chirality settle open issues on the sidewall chemisorption of atomic oxygen at low concentrations. Ether groups are the thermodynamically favored configurations. If kinetically trapped in epoxide groups, oxygen introduces characteristic new levels in the gap of the nanotube that are detected with scanning tunneling spectroscopy experiments. Discrepancies with previous predictions are shown to originate from the inadequacy of previous models to describe low-concentration oxygen adsorbated on nanotubes.