Alexandre Felten

Radio-frequency plasma functionalization of carbon nanotubes surface O2, NH3, and CF4 treatments

Inductive coupled rf-plasma at 13.56 MHz was used to modify multiwalled carbon nanotubes (MWCNTs). This technique can be easily used to tailor the chemical composition of carbon nanotubes by attaching a wide variety of functional groups at their surface: oxygen-, nitrogen-, and fluorine-containing groups have been grafted. The influence of various plasma conditions (power, type of gas, treatment time, pressure, position of the CNT sample inside the chamber) on the functionalization of the MWCNT surface was analyzed by x-ray photoelectron spectroscopy. The results show that for too high oxygen plasma power, chemical etching occurs at the surface of the CNT, thus destroying its structure. On the other hand, for optimal values of the plasma parameters, functional groups (hydroxide, carbonyl, carboxyl, amine, fluorine, etc.) were found to bond to the CNT surface, suggesting that both the concentration and type of the functional groups are in close connection with the plasma conditions. These results were compared to interaction energies predicted by ab initio calculations for different functional groups under consideration, showing that functionalization by oxygen plasma produces mainly functional groups with lower interaction energy.

Gas Sensing with Au-Decorated Carbon Nanotubes

The sensing properties of carbon nanotubes (CNTs) decorated with gold nanoparticles have been investigated by means of combined theoretical and experimental approaches. On one hand, first-principles and nonequilibrium Greens functions techniques give access to the microscopic features of the sensing mechanisms in individual nanotubes, such as electronic charge transfers and quantum conductances. On the other hand, drop coating deposition of carbon nanotubes decorated with gold nanoparticles onto sensor substrates and their characterization in the detection of pollutants such as NO(2), CO, and C(6)H(6) provide insight into the sensing ability of nanotube mats. Using the present combined approaches, the improvement in the detection of some specific gases (NO(2) and CO) using Au-functionalized nanotubes is explained. However, for other gases such as C(6)H(6), the Au nanoparticles do not seem to play a crucial role in the sensing process when compared with pristine CNTs functionalized with oxygen plasma. Indeed, these different situations can be explained by identifying the relationship between the change of resistance (macroscopic feature) and the shift of the Fermi level (microscopic feature) after gas adsorption. The understanding of the sensing ability at the atomic level opens the way to design new gas sensors and to tune their selectivity by predicting the nature of the metal that is the most appropriate to detect specific molecular species.

Gold clusters on oxygen plasma functionalized carbon nanotubes: XPS and TEM studies

Oxygen plasma treated multi-walled carbon nanotubes (MWCNTs) have been decorated with gold nanoclusters by thermal evaporation. Transmission electron microscopy (TEM) shows that the nature and extent of gold coverage can be varied by simultaneously changing the parameters used for the plasma treatment and the gold evaporation time. The evaporated gold clusters on oxygen plasma treated MWCNTs have a more dense distribution than the clusters evaporated on as-synthesized MWCNTs. Analyses of the valence band and the core levels by x-ray photoelectron spectroscopy (XPS) suggest poor charge transfer between the gold clusters and the MWCNTs.

Heterostructures Produced from Nanosheet-Based Inks

The new paradigm of heterostructures based on two-dimensional (2D) atomic crystals has already led to the observation of exciting physical phenomena and creation of novel devices. The possibility of combining layers of different 2D materials in one stack allows unprecedented control over the electronic and optical properties of the resulting material. Still, the current method of mechanical transfer of individual 2D crystals, though allowing exceptional control over the quality of such structures and interfaces, is not scalable. Here we show that such heterostructures can be assembled from chemically exfoliated 2D crystals, allowing for low-cost and scalable methods to be used in device fabrication.

Controlled modification of mono- and bilayer graphene in O, H and CF plasmas

In this work, covalent modification of mono- and bilayer graphene is achieved using tetrafluoromethane (CF₄), oxygen and hydrogen RF plasma. Controlled modification of graphene is usually difficult to achieve, in particular with oxygen plasma, which is rather aggressive and usually leads to etching of graphene. Here we use x-ray photoelectron spectroscopy and Raman spectroscopy to show that mild plasma conditions and fine tuning of the number of functional groups can be obtained in all plasmas by varying parameters such as exposure time and sample position inside the chamber. We found that even for the usual harsh oxygen treatment the defect density could be lowered, down to one defect for 3.5 × 10⁴ carbon atoms. Furthermore, we show that CF₄ plasma leads to functionalization without etching and that graphene becomes an insulator at saturation coverage. In addition, the reactivity of mono- and bilayer graphene was studied revealing faster modification of monolayer in oxygen and CF₄ plasma, in agreement with previous works. In contrast, similar modification rates were observed for both mono- and bilayer during hydrogenation. We attribute this discrepancy to the presence of more energetic species in the hydrogen plasma such as positive ions that could play a role in the functionalization process.

Polarization Dependence of the C 1s X-ray Absorption Spectra of Individual Multi-Walled Carbon Nanotubes

The polarization dependence (linear dichroism) of the C 1s X-ray absorption spectrum of individual multi-walled carbon nanotubes (MWCNTs) is measured using scanning transmission X-ray microscopy. A very strong dichroic effect is found in the C 1s --> pi* transition, with almost complete disappearance of this transition when the electric-field (E)-vector is aligned parallel to high-quality (low-defect) MWCNTs and maximum intensity when the E-vector is orthogonal to the tube axis. In contrast, there is very little dichroism in the C 1s --> sigma* transitions. The origin of this polarization effect is explained. The magnitude of the polarization dependence is found to differ in MWCNTs synthesized by different methods (arc discharge versus chemical vapor deposition). This is ascribed to differences in densities of sp(2)-type defects. The potential for use of this signal to characterize defects in single-carbon-nanotube devices is discussed.

The Role of Oxygen at the Interface between Titanium and Carbon Nanotubes

We study the interface between carbon nanotubes (CNTs) and surface-deposited titanium using electron microscopy and photoemission spectroscopy, supported by density functional calculations. Charge transfer from the Ti atoms to the nanotube and carbide formation is observed at the interface which indicates strong interaction. Nevertheless, the presence of oxygen between the Ti and the CNTs significantly weakens the Ti-CNT interaction. Ti atoms at the surface will preferentially bond to oxygenated sites. Potential sources of oxygen impurities are examined, namely oxygen from any residual atmosphere and pre-existing oxygen impurities on the nanotube surface, which we enhance through oxygen plasma surface pre-treatment. Variation in literature data concerning Ohmic contacts between Ti and carbon nanotubes is explained via sample pre-treatment and differing vacuum levels, and we suggest improved treatment routes for reliable Schottky barrier-free Ti-nanotube contact formation.

Scanning transmission x-ray microscopy of isolated multiwall carbon nanotubes

Scanning transmission x-ray microscopy (STXM) has been used to study isolated carbon nanotubes for the first time. STXM and transmission electron microscopy were applied to the same type of nanotubes, providing unique information about their composition, and electronic and structural properties. The carbon 1s near-edge x-ray absorption fine structure spectra show significant differences between multiwall carbon nanotube and carbon nanoparticle contaminants. Pristine and acid treated multiwall carbon nanotubes were also compared, highlighting the potential of the technique to differentiate surface functional groups at the nanoscale.

Single- and double- sided chemical functionalization of bilayer graphene

An experimental study on the interaction between the top and bottom layer of a chemically functionalized graphene bilayer by mild oxygen plasma is reported. Structural, chemical, and electrical properties are monitored using Raman spectroscopy, transport measurements, conductive atomic force microscopy and X-ray photoelectron spectroscopy. Single- and double-sided chemical functionalization are found to give very different results: single-sided modified bilayers show relatively high mobility (200-600 cm(2) V(-1) s(-1) at room temperature) and a stable structure with a limited amount of defects, even after long plasma treatment (>60 s). This is attributed to preferential modification and limited coverage of the top layer during plasma exposure, while the bottom layer remains almost unperturbed. This could eventually lead to decoupling between top and bottom layers. Double-sided chemical functionalization leads to a structure containing a high concentration of defects, very similar to graphene oxide. This opens the possibility to use plasma treatment not only for etching and patterning of graphene, but also to make heterostructures (through single-sided modification of bilayers) for sensors and transistors and new graphene-derivatives materials (through double-sided modification).

Measuring point defect density in individual carbon nanotubes using polarization-dependent X-ray microscopy

The presence of defects in carbon nanotubes strongly modifies their electrical, mechanical, and chemical properties. It was long thought undesirable, but recent experiments have shown that introduction of structural defects using ion or electron irradiation can lead to novel nanodevices. We demonstrate a method for detecting and quantifying point defect density in individual carbon nanotubes (CNTs) based on measuring the polarization dependence (linear dichroism) of the C 1s --> pi* transition at specific locations along individual CNTs with a scanning transmission X-ray microscope (STXM). We show that STXM can be used to probe defect density in individual CNTs with high spatial resolution. The quantitative relationship between ion dose, nanotube diameter, and defect density was explored by purposely irradiating selected sections of nanotubes with kiloelectronvolt (keV) Ga(+) ions. Our results establish polarization-dependent X-ray microscopy as a new and very powerful characterization technique for carbon nanotubes and other anisotropic nanostructures.