Thomas Mühl

Synthesis and properties of filled carbon nanotubes

Abstract Single- and multi-walled carbon nanotubes are very interesting nanoscaled materials with many possible applications in nanoelectronics. Especially, nanotubes filled with ferromagnetic materials (Fe, Co, Ni) may have significant potential in data storage. Such structures may help to exceed the best available storage densities (>65 Gb/inch2) and show in the case of Fe-filled nanotubes higher coercivities compared to bulk Fe. In addition, metal-filled carbon nanotubes are promising nanowires with excellent oxidation protection. In this paper we describe the synthesis of Fe-, Ni- and Co-filled carbon nanotubes by using the chemical vapor deposition method. Varying the deposition conditions we have obtained filled nanotubes with relatively uniform core diameters and different thicknesses of the carbon walls. The core diameters vary between 15 and 30 nm and the thickness of the carbon shells between 2 and 60 nm. The length of the tubes amounts up to 30 μm. The filled carbon nanotubes are characterised by scanning and transmission electron microscopy and energy dispersive X-ray analysis. The magnetic behaviour of the aligned Fe-filled tubes is investigated using alternating gradient magnetometry measurements and electron holography. The hysteresis loops indicate a magnetic anisotropy. The coercivity depends on the direction of the applied magnetic field. The observed enhanced coercivities are significantly higher than in bulk Fe.

Magnetic force microscopy sensors using iron-filled carbon nanotubes

Probes for magnetic force microscopy (MFM) were prepared by pinning iron-filled multiwall carbon nanotubes to conventional scanning force microscopy probes. These nanotube MFM probes reveal a great potential for high spatial resolution of both topography and magnetic stray field. The ends of the high aspect ratio iron nanowires within the nanotubes can be considered as stationary effective magnetic monopole moments which opens the possibility of quantitative stray field measurements in a straightforward manner. The carbon shells around the iron nanowires provide wear resistance and oxidation protection.

Magnetic properties of aligned Fe-filled carbon nanotubes

We report on the magnetic properties of Fe-filled multiwalled carbon nanotubes(MWNTs) grown by chemical vapor deposition(CVD) on Si substrates with ferrocene as precursor. The MWNTs are aligned perpendicularly to the substrate plane. X-ray diffraction analyses indicate the presence of both bcc and fcc iron with a relatively strong texture. Magnetometry measurements show a pronounced magnetic anisotropy with the easy axis perpendicular to the substrate plane and parallel to the axis of the aligned MWNTs, respectively. The low-temperature behavior suggests a negligible coupling between the two iron phases. We accessed the magnetic properties of individual Fe-filled MWNTs by electron holography using a transmission electron microscope(TEM).

Spin-coherent transport in ferromagnetically contacted carbon nanotubes

The spin-coherent quantum transport through multiwall carbon nanotubes contacted by ferromagnetic Co pads is investigated experimentally. At 4.2 K, the devices show a remarkable increase of the magnetoresistance (MR) ratio with decreasing junction bias, reaching a maximum MR ratio of 30% at a junction bias current of 1 nA. The experimental results suggest the transport to be dominated by spin-dependent tunneling processes at the Co/nanotube interfaces and governed by the local magnetization. We also observe an asymmetry of the magnetoresistance peak position and width which is attributed to a local exchange biasing in the electrode material.

Iron filled carbon nanotubes as novel monopole-like sensors for quantitative magnetic force microscopy

We present a novel ultrahigh stability sensor for quantitative magnetic force microscopy (MFM) based on an iron filled carbon nanotube. In contrast to the complex magnetic structure of conventional MFM probes, this sensor constitutes a nanomagnet with defined properties. The long iron nanowire can be regarded as an extended dipole of which only the monopole close to the sample surface is involved in the imaging process. We demonstrate its potential for high resolution imaging. Moreover, we present an easy routine to determine its monopole moment and prove that this calibration, unlike other approaches, is universally applicable. For the first time this enables straightforward quantitative MFM measurements.

Iron-filled carbon nanotubes as probes for magnetic force microscopy

Iron-filled carbon nanotubes (Fe-CNTs) were used to prepare probes for magnetic force microscopy (MFM) by attaching them to the tips of conventional atomic force microscopy cantilevers. An optimized chemical vapor deposition process, employing a two stage furnace and ferrocene as a precursor, supplied the homogeneously filled Fe-CNTs required for the MFM probes. These can be regarded as cylindrically shaped single-domain nanomagnets that are protected from oxidation by a carbon shell. Carbon nanotubes are known to possess both great mechanical stability and elasticity, which lead to a much longer lifetime of these probes compared to conventional magnetically coated probes. It is shown that the prepared probes are suitable for magnetic imaging and so far show no sign of deterioration. Even very long nanotubes can be used as probes, which implies that they are extraordinarily stiff. It is also shown that attached Fe-CNTs can subsequently be tailored by electron-beam induced oxidation (e.g., to remove distur...

Mössbauer transmission and back scattered conversion electron study of Fe nanowires encapsulated in multiwalled carbon nanotubes

Fe57 transmission Mossbauer spectroscopy (TMS) and back scattered conversion electron Mossbauer spectroscopy (CEMS) measurements were carried out on Fe-filled multiwalled carbon nanotubes (Fe-MWCNTs) grown by chemical vapor deposition with ferrocene as precursor. Samples of Fe-MWCNTs material deposited on the inner wall of the quartz tube reactor and samples of aligned Fe-MWCNTs grown perpendicularly to the oxidized Si substrate were characterised by the TMS method. The data show that Fe phases encapsulated within the carbon nanotubes comprise α-Fe, γ-Fe, and Fe3C in different percentage ratio depending on the sample preparation. These results are in a good accordance with the previously measured magnetic characteristics and with the structural data found by x-ray diffraction as well by selected area electron diffraction methods and allow a new complementary characterization of the Fe(Fe-alloy)-MWCNT systems. The CEMS method applied for the characterization of metal containing MWCNTs reveals that close to...

Perpendicular magnetization of long iron carbide nanowires inside carbon nanotubes due to magnetocrystalline anisotropy

Single crystal iron carbide nanowires contained in multiwalled carbon nanotubes have been prepared by aerosol-based thermal chemical vapor deposition. Investigations by transmission electron microscopy reveal the crystallographic [010] axis of the orthorhombic Fe3C nanowires to be predominantly aligned along the nanotube axis. Despite the high aspect ratio of the Fe3C nanowires, magnetic force microscopy measurements imply single domain behavior with the easy magnetic axis of the nanowires perpendicular to the wire axis. In agreement with the structural results, these findings show that the magnetic behavior is dominated by the magnetocrystalline anisotropy contribution, causing the easy axis to be along the [001] direction.

Parallel nanolithography in carbon layers with conductive imprint stamps

Nanometer-scale lithography in amorphous carbon layers was carried out by locally oxidizing the carbon under the tip of a scanning probe microscope. Although this patterning technique is able to yield very small structures, its speed is severely limited due to the serial character of the writing process. We exploit the potential of local carbon oxidation to give a parallel lithography approach which uses prepatterned stamps for electron-induced parallel structuring of the carbon film. This technique allows the transfer of complex, three-dimensional patterns into a carbon resist layer within a single process step.

NANOMETER-SCALE LITHOGRAPHY IN THIN CARBON LAYERS USING ELECTRIC FIELD ASSISTED SCANNING FORCE MICROSCOPY

Resist patterning by scanning probe microscopy is a promising method to create structures in the nanometer range beyond the resolution of conventional electron beam or photo lithography. In conventional resist processing one has to remove either the exposed or unexposed resist in a solvent by an additional step. In this article we demonstrate the possibility of directly writing nano-scaled patterns in a thin amorphous carbon layer, which can be used as an etching mask, by a scanning force microscope. Above a threshold voltage between tip and sample small trenches can be created, whereby the carbon is completely removed from the exposed areas. Evidence is given that the mechanism responsible for the trench formation is a local field-induced oxidation of the carbon layer underneath the tip.