Tunable Luminescent Carbon Quantum Dots via Non-Thermal Plasma Synthesis

Luminescent semiconductor quantum dots (QDs) have attracted wide attention due to their multimodal abilities to absorb and emit light, perform photo- and electro-catalysis, and more. These materials can exhibit unique properties belonging to their nanoscale building blocks, as well as ensemble behavior at the micro- or macro-scales. However, typical semiconductor QDs contain toxic heavy metals which are not always environmentally friendly – hence, non-toxic and benign QDs like carbon nanomaterials are preferred for practical applications. In the present study luminescent carbon QDs are synthesized in a non-thermal plasma reactor using methane and argon as the reactant species. Radiofrequency power at 13.56 MHz was supplied through ring electrodes to a flow-through reactor tube held at 4.7 Torr. The QDs formed in the plasma were deposited onto arbitrary substrates via inertial impaction. The synthesized material was characterized using Transmission Electron Microscopy (TEM), Scanning Electron Microscopy (SEM), Raman spectroscopy, and UV-Vis optical absorption. X-ray Diffraction revealed the presence of crystalline graphite peak around 2θ = 24° and the same was corroborated from the lattice fringes visible from the TEM micrographs. From these lattices, the lattice spacing in the micrographs was around 0.33 nm, corresponding to the (002) plane of graphite, and the average QD diameter was 4 nm. Raman spectra exhibited sharp D and G peaks as expected from carbon nanostructures. The UV-Vis absorption spectrum showed a peak around 278 nm, which has been correlated with the ∏ → ∏* transition of the carbon aromatic rings. On ultraviolet excitation of the carbon quantum dots suspended in toluene, the photoluminescence emission peak comes around 500 nm (green region of the visible spectrum) which can then be red shifted with increasing residence time of QDs in the plasma. For other semiconductors produced in plasmas, an increased residence time has resulted in a larger QD size – and PL from carbon QDs is known to shift to lower energies with increasing QD size. We hypothesize that the redshift in PL emission peak is thus an indication of an increasing QD diameter, which we investigate using further TEM and XRD analysis. On adding hydrogen to the gas mixture, we observed that the PL emission intensity increased remarkably, possibly due to passivation of the defect states on the surface with a simultaneous change in emission wavelength. Finally, we found the carbon QD samples to be hydrophobic with a water contact angle of 135° due to the surface C-H bonds, further illustrating their multifunctionality. These exciting properties signify the future prospects for plasma-produced carbon QDs in LEDs, circuits, coatings, and more.