Hybridization in a rope of carbon nanotubes
We have shown that quantum dots (QDs) are formed on the individual strands of a carbon nanotube (CNT) rope at low temperatures. Quantum transport measurements revealed that the parallel QDs interact not only capacitively but also show hybridization of electronic states, which appears as anticrossings in the stability diagram (see figure above). We can control the strand, to which an electron is added, as well as the degree of hybridization by exploiting the differential gating effect present in these devices. This hybridization can be suppressed due to spin effects, if a magnetic field is applied. The device was characterized using tip-enhanced Raman spectroscopy. We found signatures of in total seven CNTs, of which three appeared to be semiconducting and four were metallic. Diameters and chiralities were assigned to the different CNTs within the rope, nicely completing the picture gained from the transport experiments.
(a) Quantum transport measurement of the CNT device shows clear additional resonances (dashed lines) within the coulomb blockade indicating a parallel quantum dot formed on a different strand on the CNT rope. (b) TERS measurements at two laser energies on the carbon nanotube rope, which was used for quantum transport spectroscopy (a). Different radial breathings modes ωi indicate CNTs with different diameters. Signatures of five different tubes within the rope are indicated in these measurements. Only a very weak silicon signal is visible in the far field when the tip is not approached.
Breathing-like modes in a multiwalled carbon nanotube
We observed collective vibrational modes, so-called breathing-like modes (BLMs), within a multiwalled carbon nanotube (MWCNT). To achieve this, we developed a procedure to investigate the same CNT by high-resolution transmission electron microscopy (HRTEM) and Raman spectroscopy. The Raman shift of a radial breathing mode (RBM) of an individual single-walled CNT depends on the inverse diameter. We found a deviation of >23% from this law for the individual tubes forming a MWCNT determining the diameter of the individual CNTs within the MWCNT using HRTEM. A model of coupled harmonic oscillators, however, reproduced the measured Raman shifts very well. Thus, we proved for the first time that RBMs couple to BLMs in a MWCNT. This can be used to investigate the coupling strength between CNTs in a MWCNT, which is of interest for, e.g., nanoelectromechanical devices.
Low-frequency Raman spectrum of the MWCNT measured at two different wavelengths (blue and red line) and its simulation using the harmonic oscillator model for the coupled modes as described in the text. Inset: HRTEM micrograph of the same MWCNT. The nanotube shells appear bright.