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 MoS/graphene/SiO heterostructure is investigated from static and dynamic points of view. Static CT in the MoS-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 MoS/SiO heterostructure is mainly due to the electronic coupling between S3p-Mo4d states forming the Mo-S covalent bond in the MoS layer. For the MoS/graphene/SiO 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 MoS/graphene/SiO/Si twice as fast as in the MoS/SiO/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.

We have synthesized boron-doped single wall carbon nanotubes in a high vacuum chemical vapor deposition (CVD) system using a new boron precursor. Transmission electron microscopy was used in order to confirm the presence of single wall carbon nanotubes and field emission scanning electron microscopy to allow a qualitative characterization of the produced tubes. To estimate the doping level, we compared the Raman spectra with pure single wall carbon nanotubes and we found an upshifted G band as an evidence of doping. X-ray photoelectron spectroscopy analysis and ab initio electronic structure calculations reveals the presence of substitutional boron atoms incorporated on the tubes. We have also developed a simple method to determine quantitatively in which temperature range the carbon nanotubes are produced more efficiently by high vacuum CVD.

New techniques to manipulate the electronic properties of few layer 2D materials, unveiling new physical phenomena as well as possibilities for new device applications have brought renewed interest to these systems. Therefore, the quest for reproducible methods for the large scale synthesis, as well as the manipulation, characterization and deeper understanding of these structures is a very active field of research. We here report the production of nitrogen doped bilayer graphene in a fast single step (2.5 minutes), at reduced temperatures (760 °C) using microwave plasma-enhanced chemical vapor deposition (MW-PECVD). Raman spectroscopy confirmed that nitrogen-doped bilayer structures were produced by this method. XPS analysis showed that we achieved control of the concentration of nitrogen dopants incorporated into the final samples. We have performed state of the art parameter-free simulations to investigate the cause of an unexpected splitting of the XPS signal as the concentration of nitrogen defects increased. We show that this splitting is due to the formation of interlayer bonds mediated by nitrogen defects on the layers of the material. The occurrence of these bonds may result in very specific electronic and mechanical properties of the bilayer structures.

Multiwalled carbon nanotubes (MWCNTs) grown by spray pyrolysis have been decorated with silver nanoparticles prepared via the silver mirror reaction. Good dispersion of silver nanostructures was obtained on the surface of MWCNTs, resulting in an efficient and simple wet chemistry method for increasing the reactivity of the carbon nanotubes surfaces. High-resolution transmission electron microscopy showed the orientations of the crystallography planes of the anchored silver nanoparticles and revealed their size distribution. Raman spectroscopy results confirm that the composite material preserves the integrity of the MWCNTs. Scanning electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy were also employed for sample characterization.

Phosphorus-doped multiwalled carbon nanotubes (P-MWNTs) have been successfully synthesized by spray pyrolysis methods using a solution of ferrocene and triphenylphosphine in toluene. Electron microscopy images reveal corrugated tubes with a special morphology, similar to a carbon necklace. P-MWNTs are shorter compared to undoped tubes grown in the same conditions using ferrocene and toluene as precursors. Raman spectroscopy characterization suggests the formation of more defective tubes as the phosphorus in the precursor solution was increased. X-ray photoelectron spectroscopy (XPS) revealing the chemical environment of the phosphorus atoms clearly indicates the presence of substitutional phosphorus in the nanotubes.

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Multiwalled carbon nanotubes (MWCNTs) synthesized by spray pyrolysis were decorated with cobalt oxide nanoparticles using a simple synthesis route. This wet chemistry method yielded nanoparticles randomly anchored to the surface of the nanotubes by decomposition of cobalt nitrate hexahydrate diluted in acetone. Electron microscopy analysis indicated that dispersed particles were formed on the MWCNTs walls. The average size increased with the increasing concentration of cobalt nitrate in acetone in the precursor mixture. TEM images indicated that nanoparticles were strongly attached to the tube walls. The Raman spectroscopy results suggested that the MWCNT structure was slightly damaged after the nanoparticle growth.

Colloidal suspensions of oxocarbon-encapsulated gold nanoparticles have been synthesized in a one-step procedure by pulsed-laser ablation (PLA) at 532 nm of a solid gold target placed in aqueous solution containing CO2 absorbers, but without any stabilizing agent. Multi-wavelength surface enhanced Raman spectroscopy allows the identification of adsorbed amorphous carbon and graphite, Au-carbonyl, Au coordinated CO2-derived bicarbonates/carbonates and hydroxyl groups around the AuNPs core. Scanning electron microscopy, energy dispersive x-ray analysis and high resolution transmission electron microscopy highlight the organic shell structure around the crystalline metal core. The stability of the colloidal solution of nanocomposites (NCs) seems to be driven by solvation forces and is achieved only in neutral or basic pH using monovalent hydroxide counter-ions (NaOH, KOH). The NCs are characterized by a blue shift of the localized surface plasmon resonance (LSPR) band typical of metal-ligand stabilization by terminal π-back bonding, attributed to a core charging effect caused by Au-carbonyls. Total organic carbon measurements detect the final content of organic carbon in the colloidal solution of NCs that is about six times higher than the value of the water solution used to perform PLA. The colloidal dispersions of NCs are stable for months and are applied as analytical probes in amino glycoside antibiotic LSPR based sensing.