Carola Meyer

Excited State Spectroscopy in Carbon Nanotube Double Quantum Dots

We report on low-temperature measurements in a fully tunable carbon nanotube double quantum dot. A new fabrication technique has been used for the top-gates in order to avoid covering the whole nanotube with an oxide layer as in previous experiments. The top-gates allow us to form single dots and control the coupling between them, and we observe 4-fold shell filling. We perform inelastic transport spectroscopy via the excited states in the double quantum dot, a necessary step toward the implementation of new microwave-based experiments.

Purification and optical spectroscopy of N@C60

Using multi-step high performance liquid chromatography (HPLC) it was possible to prepare a chromatographically pure sample of N@C60. Invoking EPR spectroscopy of solid samples prepared after each HPLC step, the increase of relative N@C60 concentration could unambiguously be determined. The UV/Vis spectrum of N@C60 is indistinguishable within experimental error from that of C60, confirming negligible coupling between nitrogen in its atomic ground state and C60 cage molecular wave functions. The observed large dipolar width of the EPR spectrum of the pure paramagnetic compound indicates weak spin exchange between neighboring paramagnetic centers which is further proof for a very small spin transfer from the encapsulated atom to the fullerene cage.

Manipulating InAs nanowires with submicrometer precision.

InAs nanowires are grown epitaxially by catalyst-free metal organic vapor phase epitaxy and are subsequently positioned with a lateral accuracy of less than 1 μm using simple adhesion forces between the nanowires and an indium tip. The technique, requiring only an optical microscope, is used to place individual nanowires onto the corner of a cleaved-edge wafer as well as across predefined holes in Si(3)N(4) membranes. The precision of the method is limited by the stability of the micromanipulators and the precision of the optical microscope.

Observation of breathing-like modes in an individual multiwalled carbon nanotube.

We study collective vibrational breathing modes in the Raman spectrum of a multiwalled carbon nanotube. In correlation with high-resolution transmission electron microscopy, we find that these modes have energies differing by more than 23% from the radial breathing modes of the corresponding single-walled nanotubes. This shift in energy is explained with intershell interactions using a model of coupled harmonic oscillators. The strength of this interaction is related to the coupling strength expected for few-layer graphene.

Fabrication of coupled graphene–nanotube quantum devices

We report on the fabrication and characterization of all-carbon hybrid quantum devices based on graphene and single-walled carbon nanotubes. We discuss both carbon nanotube quantum dot devices with graphene charge detectors and nanotube quantum dots with graphene leads. The devices are fabricated by chemical vapor deposition growth of carbon nanotubes and subsequent structuring of mechanically exfoliated graphene. We study the detection of individual charging events in the carbon nanotube quantum dot by a nearby graphene nanoribbon and show that they lead to changes of up to 20% of the conductance maxima in the graphene nanoribbon, acting as a well performing charge detector. Moreover, we discuss an electrically coupled graphene-nanotube junction, which exhibits a tunneling barrier with tunneling rates in the low GHz regime. This allows us to observe Coulomb blockade on a carbon nanotube quantum dot with graphene source and drain leads.

Experimental Steps Towards the Realisation of a Fullerene Quantum Computer

In connection with the realization of a fullerene quantum computer, the construction of linear chains of endohedral fullerenes and their orientation with respect to the external magnetic field is required. We show here that the orientation of these chains can be achieved by embedding them in a liquid crystal matrix. The orientation is demonstrated for the elongated C 7 0 endohedral fullerene.

Index assignment of a carbon nanotube rope using tip-enhanced Raman spectroscopy

We used tip-enhanced Raman spectroscopy to study the diameter-dependent Raman modes in a contacted carbon nanotube (CNT) rope. We show that with the near-field tip enhancement a large number of nanotubes within a rope can be identified, even if the nanotube modes cannot be distinguished in the far-field signal. Several metallic and semiconducting nanotubes can be identified and assigned to nanotube families. Additionally, we provide a tentative chiral index assignment.

Defects induced on chemical vapour deposition carbon nanotubes during peapod synthesis on substrates

Individual carbon nanotubes are filled with fullerene molecules directly on the substrate. Two different oxidation techniques for opening the tubes prior to the filling, annealing in air, and acid treatment, are compared. High-resolution transmission electron microscopy images indicate that both methods induce defects on the sidewalls of the nanotubes. In the case of acid treatment, the inner walls can be damaged without affecting the outer walls, while the inner walls are opened along with the outer ones by heating in air. The effect of acid treatment on the tubes is much stronger than known from bulk samples. In contrast to previous studies, we find amorphous carbon inside the nanotubes after oxidation, and an additional high-temperature annealing step is needed to remove these plugs in order to open the tubes for filling.

Synthesis and Functionalization of Fullerenes Encapsulating Atomic Phosphorus

Synthesis, purification, and electron spin properties of the endohedral fullerene P@C 60 are reported. Highly enriched material could be obtained for the first time, making this molecule accessible for further experiments. We report the first successful chemical modification of P@C 60 via cyclo-propanation to the C 2v symmetric monoadducts P@C 61 (COOCH 3 ) and P@C 61 (COOC 2 H 5 ). X-band electron spin resonance data are presented and discussed. The phosphorus atom shows anisotropic hyperfine coupling in the modified fullerenes, in contrast to the unmodified case, and in contrast to similarly modified N@C 60 .

Spin-dependent electronic hybridization in a rope of carbon nanotubes

Institut fu¨r Theoretische Physik A, RWTH Aachen, 52056 Aachen, Germany(Dated: November 18, 2010)We demonstrate single electron addition to different strands of a carbon nanotube rope. Anti-crossings of anomalous conductance peaks occur in quantum transport measurements through theparallel quantum dots forming on the individual strands. We determine the magnitude and thesign of the hybridization as well as the Coulomb interaction between the carbon nanotube quantumdots, finding that the bonding states dominate the transport. In a magnetic field the hybridizationis shown to be selectively suppressed due to spin effects.