Author Correction: Carbon nanotubes as emerging quantum-light sources

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Valley Subband Splitting in Bilayer Graphene Quantum Point Contacts

We report a study of one-dimensional subband splitting in a bilayer graphene quantum point contact in which quantized conductance in steps of 4e^{2}/h is clearly defined down to the lowest subband. While our source-drain bias spectroscopy measurements reveal an unconventional confinement, we observe a full lifting of the valley degeneracy at high magnetic fields perpendicular to the bilayer graphene plane for the first two lowest subbands where confinement and Coulomb interactions are the strongest and a peculiar merging or mixing of K and K^{'} valleys from two nonadjacent subbands with indices (N,N+2), which are well described by our semiphenomenological model.

Publisher Correction: Carbon nanotubes as emerging quantum-light sources

In the version of this Perspective originally published, the x-axis label of Fig. 1d was missing; it should have read 'Wavelength (nm)'. The units of the y axis of Fig. 3b were incorrect; they should have been meV. And the citation of Fig. 3c in the main text was incorrect; it should have been to Fig. 3b. These issues have now been corrected.

Carbon nanotubes as emerging quantum-light sources

Progress in quantum computing and quantum cryptography requires efficient, electrically triggered, single-photon sources at room temperature in the telecom wavelengths. It has been long known that semiconducting single-wall carbon nanotubes (SWCNTs) display strong excitonic binding and emit light over a broad range of wavelengths, but their use has been hampered by a low quantum yield and a high sensitivity to spectral diffusion and blinking. In this Perspective, we discuss recent advances in the mastering of SWCNT optical properties by chemistry, electrical contacting and resonator coupling towards advancing their use as quantum light sources. We describe the latest results in terms of single-photon purity, generation efficiency and indistinguishability. Finally, we consider the main fundamental challenges stemming from the unique properties of SWCNTs and the most promising roads for SWCNT-based chip integrated quantum photonic sources.

Controlled modification of mono- and bilayer graphene in O₂, H₂ and CF₄ plasmas

In this work, covalent modification of mono- and bilayer graphene is achieved using tetrafluoromethane (CF₄), oxygen and hydrogen RF plasma. Controlled modification of graphene is usually difficult to achieve, in particular with oxygen plasma, which is rather aggressive and usually leads to etching of graphene. Here we use x-ray photoelectron spectroscopy and Raman spectroscopy to show that mild plasma conditions and fine tuning of the number of functional groups can be obtained in all plasmas by varying parameters such as exposure time and sample position inside the chamber. We found that even for the usual harsh oxygen treatment the defect density could be lowered, down to one defect for 3.5 × 10⁴ carbon atoms. Furthermore, we show that CF₄ plasma leads to functionalization without etching and that graphene becomes an insulator at saturation coverage. In addition, the reactivity of mono- and bilayer graphene was studied revealing faster modification of monolayer in oxygen and CF₄ plasma, in agreement with previous works. In contrast, similar modification rates were observed for both mono- and bilayer during hydrogenation. We attribute this discrepancy to the presence of more energetic species in the hydrogen plasma such as positive ions that could play a role in the functionalization process.

Electroluminescence in single layer MoS2

We detect electroluminescence in single layer molybdenum disulfide (MoS2) field-effect transistors built on transparent glass substrates. By comparing the absorption, photoluminescence, and electroluminescence of the same MoS2 layer, we find that they all involve the same excited state at 1.8 eV. The electroluminescence has pronounced threshold behavior and is localized at the contacts. The results show that single layer MoS2, a direct band gap semiconductor, could be promising for novel optoelectronic devices, such as two-dimensional light detectors and emitters.

Phonon-assisted electroluminescence from metallic carbon nanotubes and graphene

We report on light emission from biased metallic single-wall carbon nanotube (SWNT), multiwall carbon nanotube (MWNT) and few-layer graphene (FLG) devices. SWNT devices were assembled from tubes with different diameters in the range 0.7-1.5 nm. They emit light in the visible spectrum with peaks at 1.4 and 1.8 eV. Similar peaks are observed for MWNT and FLG devices. We propose that this light emission is due to phonon-assisted radiative decay from populated pi* band states at the M point to the Fermi level at the K point. Since for most carbon nanotubes as well as for graphene the energy of unoccupied states at the M point is close to 1.6 eV, the observation of two emission peaks at approximately 1.6 +/- approximately 0.2 eV could indicate radiative decay under emission or absorption of optical phonons, respectively.

Raman spectroscopic evidence for hot-phonon generation in electrically biased carbon nanotubes

We provide spectroscopic evidence for hot-phonon generation in biased single-walled carbon nanotubes by measuring simultaneously the Stokes and anti-Stokes Raman lines of the G mode and of the radial-breathing mode as a function of current bias. Using Bose-Einstein statistics we can directly calculate the phonon temperature from the intensity ratio of the anti-Stokes to Stokes lines. Upon nanotube biasing we observe (i) an increase of the G mode phonon temperature in contrast to the radial-breathing mode phonons that remain thermalized at room temperature, and (ii) no softening of the G mode. Based on these observations, we exclude current-induced thermal heating of the nanotube.