Gregory Van Lier

Ab initio study of the elastic properties of single-walled carbon nanotubes and graphene

Abstract The first all-electron ab initio study of Youngs modulus and Poisson ratio for a number of closed single-walled nanotubes is presented. At the Hartree–Fock 6-31G ∗ level, the results obtained compare well with experimental as well as previous theoretical studies, predicting a Youngs modulus higher than 1 TPa. The calculated Youngs modulus for a graphene layer is found to be smaller than for its (5,5)-nanotube counterpart.

Electronic structure and aromaticity of graphene nanoribbons.

We analyse the electronic structure and aromaticity of graphene nanoribbons and carbon nanotubes through a series of delocalisation and geometry analysis methods. In particular, the six-centre index (SCI) is found to be in good agreement with the mean bond length (MBL) and ring bond dispersion (RBD) geometry descriptors. Based on DFT periodic calculations, three distinct classes of aromaticity patterns have been found for armchair graphene nanoribbons, appearing periodically as the width of the ribbon is increased. The periodicity in the band gap is found to be related to these aromaticity patterns. Also, the appearance of such distinct aromaticity distribution is explained within the framework of the Clars sextet theory. Both delocalisation and geometry analysis methods are shown to be very fast and reliable tools for easily analysing the aromaticity in carbon nanosystems.

Super-Robust, Lightweight, Conducting Carbon Nanotube Blocks Cross-Linked by De-fluorination

We produced large binder-free multi-walled carbon nanotube (MWNT) blocks from fluorinated MWNTs using thermal heating and a compressing method in vacuo. This technique resulted in the formation of covalent MWNT networks generated by the introduction of sp(3)-hybridized carbon atoms that cross-link between nanotubes upon de-fluorination. The resulting carbon nanotube blocks are lighter than graphite, can be machined and polished, and possess average bending strengths of 102.2 MPa, a bending modulus of 15.4 GPa, and an electrical conductivity of 2.1 x 10(2) S/cm. Although each nanotube exhibits a random structure in these blocks, the mechanical properties are 3 times higher than those obtained for commercial graphite. On the basis of theoretical molecular dynamics simulations, a model is presented for the nanotube interconnecting mechanism upon de-fluorination.

Spectroscopy and Defect Identification for Fluorinated Carbon Nanotubes

Finely tuned: Carbon nanotubes are exposed to a CF(4) radio-frequency plasma (see picture). High-resolution photoelectron spectroscopy shows that the treatment effectively grafts fluorine atoms onto the MWCNTs, altering the valence electronic states. Fluorine surface concentration can be tuned by varying the exposure time.Multi-wall carbon nanotubes (MWCNTs) were exposed to a CF(4) radio-frequency (rf) plasma. High-resolution photoelectron spectroscopy shows that the treatment effectively grafts fluorine atoms onto the MWCNTs, altering the valence electronic states. Fluorine surface concentration can be tuned by varying the exposure time. Evaporation of gold onto MWCNTs is used to mark active site formation. High-resolution transmission electron microscopy coupled with density functional theory (DFT) modelling is used to characterise the surface defects formed, indicating that the plasma treatment does not etch the tube surface. We suggest that this combination of theory and microscopy of thermally evaporated gold atoms onto the CNT surface may be a powerful approach to characterise both surface defect density as well as defect type.

Cost effective calculation of molecular charge distributions and gas phase deprotonation energies using density functional methods

Abstract Major advances have been made in density functional theory and linear scaling methods. However, routine application of these methods to larger systems possessing no symmetry still remains difficult. Moreover, no clear standard exists for the use of small basis sets in the calculation of atomic or molecular properties with density functional methods. In this work an evaluation is made of the performance of different density functional methods for small and often used basis sets. Deprotonation energies are calculated for a series of small compounds and compared with high-level quantum chemical calulations. Furthermore, atomic populations calculated with these methods are compared with high-level results. The results show that the use of density functional calculations with smaller basis sets is justified for calculations on larger systems, retaining relatively good accuracies.

Theoretical prediction of the solubility of fluorinated C60

Although extensive theoretical and experimental research has been conducted on fluorinated fullerenes, little detailed information exists on their solubility in different solvents. However, this solubility is crucial for their processability and possible application. In this work, we predict the solubility of fluorinated C(60) in various polar and non-polar solvents, based on a correlation between experimentally measured solubilities for C(60) from the literature and theoretically predicted solubilisation energies. These solubilisation energies are predicted using the polarizable continuum model (PCM) at the ab initio Hartree-Fock 6-31+G* level of theory. In particular, the solubilities are predicted for C(60)F(2)(n) (n = 1-10) isomers, part of the addition route to saturnene C(60)F(20). An increasing solubility is found for more polar solvents with higher degree of fluorination. With these results, we can determine the minimal fluorination degree necessary for possible solubilisation in a given solvent, and offer new perspectives to separate and purify species with different degrees of fluorination.

Oxidation and reactivity of nitrogen- and phosphorus-doped heterofullerenes

Density functional theory (DFT) calculations suggest significantly different oxidation behaviour for phosphorus-doped heterofullerenes compared to their pure and nitrogen-doped counterparts, due to formation of a phosphene oxide. This oxide is not thermally labile, suggesting stable phosphofullerenes are likely to be C(59)POH and (C(59)PO)(2). In contrast, azafullerenes form stable epoxides when oxidised. We calculate the effect of oxidation on radical pairing and hydrogen passivation. Notably while the C(59)N radical behaves as a donor, C(59)PO will be an acceptor.

Charge delocalisation in hydrofullerenes and substituted hydrofullerenes: effect of deprotonation

The delocalisation that occurs upon deprotonation of a series of hydrofullerenes and substituted hydrofullerenes is analysed. The charges and bond lengths of the acidic and basic forms of each system are considered. We find approximately one electron delocalising from the deprotonated C atom to the rest of the system on deprotonation. The softness of the cage and the bond alternation permit delocalisation of the resulting negative charge over the main part of the fullerene cage, following the pattern of conjugated bonds, showing a clear ‘delocalisation path’. The same paths were found for hydrofullerenes and substituted hydrofullerenes, so that the properties of different functional groups are seen not to influence the delocalisation pattern in a significant way. This delocalisation greatly stabilises the basic form and explains the high acidities of these hydrocarbon systems.The deprotonation energies for the two most stable C60H2 and the two most stable C70H2 isomers have been calculated at the ab initio HF/6-31G* level of theory. 5,6-C70H2 was found to be more acidic than 1,2-C60H2. This inversion as compared to 1H NMR predictions results from the difference in cage structure of C60 and C70. The delocalising charge reaches the equatorial band of six rings present in the C70 structure, giving rise to a better stabilisation of the basic form than for 1,2-C70H2.

Effect of the fluorination technique on the surface-fluorination patterning of double-walled carbon nanotubes

Double-walled carbon nanotubes (DWCNTs) are fluorinated using (1) fluorine F2 at 200 °C, (2) gaseous BrF3 at room temperature, and (3) CF4 radio-frequency plasma functionalization. These have been comparatively studied using transmission electron microscopy and infrared, Raman, X-ray photoelectron, and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. A formation of covalent C–F bonds and a considerable reduction in the intensity of radial breathing modes from the outer shells of DWCNTs are observed for all samples. Differences in the electronic state of fluorine and the C–F vibrations for three kinds of the fluorinated DWCNTs are attributed to distinct local surroundings of the attached fluorine atoms. Possible fluorine patterns realized through a certain fluorination technique are revealed from comparison of experimental NEXAFS F K-edge spectra with quantum-chemical calculations of various models. It is proposed that fluorination with F2 and BrF3 produces small fully fluorinated areas and short fluorinated chains, respectively, while the treatment with CF4 plasma results in various attached species, including single or paired fluorine atoms and –CF3 groups. The results demonstrate a possibility of different patterning of carbon surfaces through choosing the fluorination method.

Looking at bulk-heterojunction organic photovoltaics from two viewpoints: morphology development and charge transfer

In this paper, a combined experimental and theoretical study was performed on the P3HT:PCBM system used in organic photovoltaics. Fast-scanning differential chip calorimetry, an advanced thermal analysis technique, was used to simulate the thermal annealing used in the production of P3HT:PCBM solar cells to increase the degree of crystallinity, and thus efficiency. The main advantage of this technique for stuying the thermal annealing are the very high rates of heating and cooling that can be used, up to 106 K.s-1, permitting one to avoid crystallization during cooling. In parallel with the experimental study, the charge transfer between donor (P3HT) and acceptor (PCBM) at the interface is studied using density functional theory. The charge separation between donor and acceptor present for the ground state of the combined system, diminished when the first triplet was investigated. This was explained by the formation of a bridge state, formed after population by the LUMO with one electron. Such a molecular orbital can facilitate charge transfer.