Fluorine Adatoms Research Articles

Diffusion of fluorine adatoms on doped graphene

We calculate the diffusion barrier of fluorine adatoms on doped graphene in the diluted limit using Density Functional Theory. We found that the barrier Δ strongly depends on the magnitude and character of the graphene's doping (δn): it increases for hole doping (δn < 0) and decreases for electron doping (δn > 0). Near the neutrality point the functional dependence can be approximately by Δ = Δ0 – αδn, where α ≃ 6 × 10−12 meV cm2. This effect leads to significant changes of the diffusion constant with doping even at room temperature and could also affect the low temperature diffusion dynamics due to the presence of substrate induced charge puddles. In addition, this might open up the possibility to engineer the F dynamics on graphene by using local gates.

Tuning the transport gap of functionalized graphene via electron beam irradiation

We demonstrate a novel method to tune the energy gap ϵ1 between the localized states and the mobility edge of the valence band in chemically functionalized graphene by changing the coverage of fluorine adatoms via electron-beam irradiation. From the temperature dependence of the electrical transport properties we show that ϵ1 in partially fluorinated graphene CF0.28 decreases upon electron irradiation up to a dose of 0.08 C cm−2. For low irradiation doses (<0.1 C cm−2) partially fluorinated graphene behaves as a lightly doped semiconductor with impurity bands close to the conduction and valence band edges, whereas for high irradiation doses (>0.2 C cm−2) the electrical conduction takes place via Mott variable range hopping.

Chlorine Adsorption on Graphene: Chlorographene

We perform first-principles structure optimization, phonon frequency, and finite temperature molecular dynamics calculations based on density functional theory to study the interaction of chlorine atoms with graphene predicting the existence of possible chlorinated graphene derivatives. The bonding of a single chlorine atom is ionic through the transfer of charge from graphene to chlorine adatom and induces negligible local distortion in the underlying planar graphene. Different from hydrogen and fluorine adatoms, the migration of a single chlorine adatom on the surface of perfect graphene takes place almost without barrier. However, the decoration of one surface of graphene with Cl adatoms leading to various conformations cannot be sustained due to strong Cl–Cl interaction resulting in the desorption through the formation of Cl2 molecules. On the contrary, the fully chlorinated graphene, chlorographene CCl, where single chlorine atoms are bonded alternatingly to each carbon atom from different sides of graphene with sp3-type covalent bonds, is buckled. We found that this structure is stable and is a direct band gap semiconductor, whose band gap can be tuned by applied uniform strain. Calculated phonon dispersion relation and four Raman-active modes of chlorographene are discussed.

Spin-half paramagnetism in graphene induced by point defects

Using magnetization measurements, we show that point defects in graphene - fluorine adatoms and irradiation defects (vacancies) - carry magnetic moments with spin 1/2. Both types of defects lead to notable paramagnetism but no magnetic ordering could be detected down to liquid helium temperatures. The induced paramagnetism dominates graphene's low-temperature magnetic properties despite the fact that maximum response we could achieve was limited to one moment per approximately 1000 carbon atoms. This limitation is explained by clustering of adatoms and, for the case of vacancies, by losing graphene's structural stability.

Modeling of epitaxial graphene functionalization

A new model for graphene epitaxially grown on silicon carbide is proposed. Densityfunctional theory modeling of epitaxial graphene functionalization by hydrogen, fluorine,methyl and phenyl groups has been performed, with hydrogen and fluorine showing a highprobability of cluster formation in high adatom concentration. It has also been shown thatthe clusterization of fluorine adatoms provides midgap states in formation, due tosignificant flat distortion of graphene. The functionalization of epitaxial graphene usinglarger species (methyl and phenyl groups) renders cluster formation impossible, due tothe steric effect, and results in uniform coverage with the energy gap opening.

Stable antiferromagnetic graphone

Density functional modeling of atomic structure and calculation of electronic structure of one-side one-sublattice functionalized graphene (graphone) are performed for hydrogen and fluorine adatoms. It is shown here that using fluorine for functionalization not only enhances stability of compound but also provides switch of magnetic ground state from ferro- to antiferromagnetic. Half-metallic ferromagnetic state in fluorine-based graphone is also discussed.

From the cover: nanoscale imaging of chemical interactions: fluorine on graphite.

Using C60-functionalized scanning tunneling microscope tips, we have investigated the adsorption of fluorine on graphite. Based on characteristics of the accompanying electron standing waves, we are able to distinguish the fluorine adatoms that have bonded ionically to the graphite surface from those that have formed covalent bonds with the surface. This result permits determination of the ratio of ionic to covalent C-F bonds on graphite obtained by gas phase fluorination, which seems to be temperature-independent between 200 and 300 degrees C under the reaction conditions used.

The kinetics of the low-pressure chemical vapour deposition of molybdenum on silicon from molybdenum hexafluoride and hydrogen

The kinetics of the low-pressure chemical vapour deposition of molybdenum on silicon from molybdenum hexafluoride and hydrogen are studied at 600 K and 8–315 Pa using a continuous flow perfectly mixed reactor, equipped with a microbalance for in situ rate measurements. A significant contribution of reaction of the hexafluoride with silicon, which diffuses through the growing metal layer, to the overall rate obscures the kinetics of the reduction of molybdenum hexafluoride with hydrogen. A model is developed to obtain the intrinsic rates of the latter process. A ten-step elementary surface network is able to describe these rate data. According to this mechanism molybdenum hexafluoride adsorbs on the growing metal surface, leading to adsorbed fluorine atoms and solid molybdenum. The fluorine adatoms are removed from the surface by reaction with adsorbed hydrogen atoms. Trench fill experiments and their modelling furnish additional arguments for the proposed kinetic scheme.

Surface diffusion model accounting for the temperature dependence of tungsten etching characteristics in a SF6 magnetoplasma

To explain the influence of the substrate temperature Ts on the etching characteristics of tungsten in a SF6 magnetoplasma, we have extended the surface diffusion model originally developed for the etching of the W–F system at constant (ambient) temperature. It allows us to understand our experimental observations which include the influence of Ts on the anisotropy and the fact that the lateral (spontaneous) etch rate of W as a function of 1/Ts does not follow an Arrhenius law. The model is valid as long as the pressure is sufficiently low (⩽0.5 mTorr) to neglect the influence of coadsorption and passivation effects, likely related to oxygen contamination of the gas phase coming from the fused silica discharge tube interacting with fluorine atoms. Consistency of the model is well demonstrated by observing that the lateral to vertical etch rate ratio as a function of 1/Ts, under different plasma conditions, leads to a unique value of the activation energy (R=0.65 eV) for the associative desorption of WF6, the volatile reaction product of tungsten with fluorine adatoms.

Thermal decomposition of a silicon-fluoride adlayer: Evidence for spatially inhomogeneous removal of a single monolayer of the silicon substrate.

The thermal decomposition of silicon-fluoride adlayers, produced by exposing a clean Si(100) surface to a molecular beam of fluorine, have been investigated under ultrahigh-vacuum conditions with x-ray photoelectron spectroscopy and mass spectrometry. The only gas phase reaction products detected from temperature-programmed desorption are ${\mathrm{SiF}}_{2}$ and ${\mathrm{SiF}}_{4}$, the relative yield of each product depending strongly on the coverage of fluorine adatoms. For fluorine coverages below 3--4 monolayers, the major reaction product is ${\mathrm{SiF}}_{2}$, whereas above 3--4 monolayers the relative yield of ${\mathrm{SiF}}_{4}$ increases continuously, while that of ${\mathrm{SiF}}_{2}$ remains constant. Independent of the initial coverage of fluorine, the thermal decomposition is terminated near 800 K by the removal of one monolayer of the Si substrate in the form of ${\mathrm{SiF}}_{2}$. Results are presented that suggest the removal of this ``final'' monolayer proceeds inhomogeneously, leaving separate domains in which the local coverage of fluorine is either zero or near saturation.