Filippo S. Boi

Synthesis and structure of free-standing germanium quantum dots and their application in live cell imaging

Free-standing Ge quantum dots around 3 nm in size were synthesized using a bench-top colloidal method and suspended in water and ethanol. In the ethanol solution, the photoluminescence of the Ge quantum dots was observed between 650 and 800 nm. Structural and optical properties of these colloidal Ge quantum dots were investigated by utilizing X-ray diffraction, X-ray absorption spectroscopy, Raman spectroscopy, and photoluminescence spectroscopy and transmission electron microscopy. The structure of the as-prepared Ge quantum dots that were found is best described by a core–shell model with a small crystalline core and an amorphous outer shell with a surface that was terminated by hydrogen-related species. As-prepared Ge quantum dots were suspended in cell growth medium, and then loaded into cervical carcinoma (HeLa) cells. The fluorescent microscopy images were then collected using 405 nm, 488 nm, 561 nm and 647 nm wavelengths. We observed that, based on fluorescence measurements, as-prepared Ge quantum dots can remain stable for up to 4 weeks in water. Investigation of toxicity, based on a viability test, of as-prepared uncoated Ge quantum dots in HeLa cells was carried out and compared with the commercial carboxyl coated CdSe/ZnSe quantum dots. The viability tests show that Ge quantum dots are less toxic when compared to commercial carboxyl coated CdSe/ZnS quantum dots.

Controlling the quantity of α-Fe inside multiwall carbon nanotubes filled with Fe-based crystals: The key role of vapor flow-rate

The growth control of α-Fe inside multiwall carbon nanotubes has challenged researchers for more than a decade owing to the coexistence of this phase with Fe3C and γ-Fe. Previously, long heating treatments of 20 h have been used to decompose the encapsulated Fe-phases in C and Fe; however, these methods were limited by an unusual oxidation process leading to nanotube decomposition. In this letter, we report an alternative chemical vapour deposition approach that through an accurate control of the ferrocene-vapour flow-rate allows to achieve the direct encapsulation of 95% of α-Fe without additional heating treatments.

Cm-size free-standing self-organized buckypaper of bucky-onions filled with ferromagnetic Fe3C

Novel cm-size free-standing buckypapers of bucky-onions filled with a single-phase of ferromagnetic Fe3C single crystals were serendipitously discovered. These buckypapers are obtained directly in situ as the dominant product of the pyrolysis of ferrocene. Vibrating sample magnetometry also revealed an extremely large coercivity of 0.120 tesla and a saturation magnetization of 41 emu g−1.

The Origin of Long-Period Lattice Spacings Observed in Iron-Carbide Nanowires Encapsulated by Multiwall Carbon Nanotubes

Structures comprising single-crystal, iron-carbon-based nanowires encapsulated by multiwall carbon nanotubes self-organize on inert substrates exposed to the products of ferrocene pyrolysis at high temperature. The most commonly observed encapsulated phases are Fe₃C, α-Fe, and γ-Fe. The observation of anomalously long-period lattice spacings in these nanowires has caused confusion since reflections from lattice spacings of ≥ 0.4 nm are kinematically forbidden for Fe₃C, most of the rarely observed, less stable carbides, α-Fe, and g-Fe. Through high-resolution electron microscopy, selective area electron diffraction, and electron energy loss spectroscopy we demonstrate that the observed long-period lattice spacings of 0.49, 0.66, and 0.44 nm correspond to reflections from the (100), (010), and (001) planes of orthorhombic Fe₃C (space group Pnma). Observation of these forbidden reflections results from dynamic scattering of the incident beam as first observed in bulk Fe₃C crystals.With small amounts of beam tilt these reflections can have significant intensities for crystals containing glide planes such as Fe₃C with space groups Pnma or Pbmn.

Controlling high coercivities in cm-scale buckypapers with unusual stacking of vertically aligned and randomly entangled Fe-filled carbon nanotubes

The synthesis of cm-scale buckypapers consisting of an unusual stacking of vertically aligned and randomly entangled Fe-filled carbon nanotubes is revealed through an advanced high flow-rate Cl-assisted chemical vapour deposition approach. The produced films show very attractive room-temperature coercivities as high as 1040 Oe (82 763 A m−1), much higher than those reported in previous Fe-filled buckypapers reports. We attribute the high coercivity to the presence of pinning effects due to the unusual stacking of the two CNT morphologies. The high coercivity, high surface areas, well-ordered porosity and high magnetization make the produced films ideal candidates also for numerous other applications as data storage systems, molecular-capture material, filter membranes, scaffolds for retinal cell transplantation, immune shielding and flexible magnetic sensors.

Observation of large coercivities in radial carbon nanotube structures filled with Fe3C and FeCo single-crystals by viscous boundary layer pyrolysis of ferrocene and cobaltocene

Viscous boundary layer chemical vapor synthesis is a novel technique that uses the viscous boundary layer between a laminar pyrolysed metallocene/Ar vapor flow and a rough surface to induce the nucleation and growth of radial carbon nanotube (CNT) structures highly filled with ferromagnetic materials. Here we report the synthesis and characterization of radial structures comprising multiwall CNTs filled with large quantities of Fe3C and FeCo alloys and low quantities of γ-Fe in the forms of small single crystals. Surprisingly high saturation magnetizations up to 80 emu g−1 and a very high coercivity of 1400 Oe at room temperature are found. Such values of magnetization suggest that no room-temperature magnetic interaction is present between γ-Fe and the ferromagnetic crystals in our samples. The presence of such large coercivity values may be associated with the small size of the encapsulated particles which is strongly dependent on the high evaporation temperature of the precursors for fixed pyrolysis temperatures and vapour flow rate. The addition of Cl-species is also considered in the attempt to slow down the growth-rate of the radial CNT-structure and further investigate their growth mechanism.

Micrometre-length continuous single-crystalline nm-thin Fe3C-nanowires with unusual 010 preferred orientation inside radial few-wall carbon nanotube structures: the key role of sulfur in viscous boundary layer CVS of ferrocene

A key challenge in the fabrication of carbon nanotubes filled with ferromagnetic nanowires is the control of the number of nanotube-walls together with the nanowire continuity, composition and crystallinity. We report the serendipitous observation of novel radial carbon nanotube structures with few walls (2–5 walls) filled with nm-thin and many-micrometres long continuous single-crystalline Fe3C nanowires. These are the dominant reaction products in chemical vapour synthesis experiments involving the pyrolysis of ferrocene/sulfur mixtures in the viscous boundary layer between a rough surface and a laminar Ar flow. These nanowires are found with an unusual preferred 010 orientation along the nanotube capillary. The properties of these structures are investigated through the use of multiple techniques: SEM, TEM, HRTEM, EDX, STEM, XRD, Raman spectroscopy, FT-IR spectroscopy and VSM.

Boundary layer chemical vapour synthesis of self-organised ferromagnetically filled radial-carbon-nanotube structures

Boundary layer chemical vapour synthesis is a new technique that exploits random fluctuations in the viscous boundary layer between a laminar flow of pyrolysed metallocene vapour and a rough substrate to yield ferromagnetically filled radial-carbon-nanotube structures departing from a core agglomeration of spherical nanocrystals individually encapsulated by graphitic shells. The fluctuations create the thermodynamic conditions for the formation of the central agglomeration in the vapour which subsequently defines the spherically symmetric diffusion gradient that initiates the radial growth. The radial growth is driven by the supply of vapour feedstock by local diffusion gradients created by endothermic graphitic-carbon formation at the vapour-facing tips of the individual nanotubes and is halted by contact with the isothermal substrate. The radial structures are the dominant product and the reaction conditions are self-sustaining. Ferrocene pyrolysis yields three common components in the nanowire encapsulated by multiwall carbon nanotubes, Fe3C, α-Fe, and γ-Fe. Magnetic tuning in this system can be achieved through the magnetocrystalline and shape anisotropies of the encapsulated nanowire. Here we demonstrate proof that alloying of the encapsulated nanowire is an additional approach to tuning of the magnetic properties of these structures by synthesis of radial-carbon-nanotube structures with γ-FeNi encapsulated nanowires.

Observation of local changes of “carbon-to-metal ratio” in the growth mechanism of carbon nanostructures grown from FePd-based and Fe3C catalysts by pyrolysis of ferrocene and dichlorocyclooctadiene-palladium mixtures: the crucial role of Cl

One of the major challenges in the field of carbon nanomaterials consists of understanding their growth mechanism dynamics. Recent reports have shown preliminary steady state chemical vapour deposition (CVD) experiments for the encapsulation of FePd-based alloys inside carbon-based nanostructures. However, very little is known about their growth mechanism dynamics. Here we investigate the possible presence of local changes of “carbon-to-metal ratio” in the growth mechanism of carbon nanostructures grown from FePd-based and Fe3C catalyst-particles by steady state CVD and viscous-boundary-layer chemical vapour synthesis (CVS) experiments involving the pyrolysis of ferrocene/dichlorocyclooctadiene-palladium mixtures and of ferrocene/dichlorobenzene mixtures. In the first case, we demonstrate the observation of an unusual growth mechanism in which a direct transition from a spherically elongated carbon-onion-like (CNOs-like) morphology (low carbon-to-metal ratio condition) to a carbon fiber-like morphology (high carbon-to-metal ratio condition) is present within the same carbon structure. We attribute such transitions to the variation of the Cl-radicals concentration during the pyrolysis process, which implies a variation in the local “carbon to metal ratio” parameter. The reported mechanism is then compared in detail with that of Fe3C filled CNTs obtained by CVD of ferrocene/dichlorobenzene mixtures and with those reported in the literature for other transition metal catalyst systems. In contrast, when a viscous boundary layer is created between the pyrolysed ferrocene/dichlorocyclooctadiene-palladium precursors and a rough surface, radial CNT structures filled with large quantities of both FePd3 and Fe3C crystals are found as major reaction products. The presence of filled-CNOs within the radial structures-core (resulting from the homogeneous nucleation of particles in the viscous boundary layer) suggests that fluctuations in the local “carbon-to-metal ratio” are also present in this type of mechanism. The morphological, cross-sectional and structural properties of the obtained carbon structures are analyzed in detail by SEM, TEM, STEM, HRTEM, ED and X-ray diffraction.

New insights on the dynamics of the γ-Fe/α-Fe phase-transition inside iron-filled carbon nanotubes

One of the challenges in the field of carbon nanotubes (CNTs) is the encapsulation of a single crystalline phase of ferromagnetic α-Fe. The formation of additional γ-Fe and Fe3C phases during CNT-growth generally limits the direct encapsulation of these crystals in the form of a single phase. A solution, the use of post-synthesis annealing, has been considered; however oxidation of the encapsulated metal-phases is commonly found due to the elevated temperatures (T) necessary for the phase-conversion. Here we investigate the dynamics of γ-Fe to α-Fe transition by T-dependent X-ray diffraction in vacuum. We show that a direct γ-Fe to α-Fe transition is present already below 200 °C and becomes significantly fast in the T-range of 300–399 °C. In such a T-range no metal oxidation is found. Rietveld refinement analyses also show that a T-dependent increase in the unit-cell c-axis value of the graphitic CNT-walls is present.