We studied the shape and energy position of near-band-edge photoluminescence spectra of InN epitaxial layers with different doping levels. We found that the experimental spectra of InN layers with moderate doping level can be nicely interpreted in the frames of the “free-to-bound” recombination model in degenerate semiconductors. For carrier concentrations above $n>\mathrm{}5\mathrm{}\times {10}^{18}{\mathrm{cm}}^{-3}$ the emission spectra can also be modeled satisfactorily, but a contribution due to a pushing up of nonequilibrium holes over the thermal delocalization level in the valence band tails should be considered in the model. The emission spectra of samples with low doping level were instead explained as a recombination from the bottom of the conduction band to a shallow acceptor assuming the same value of the acceptor binding energy estimated from the spectra of highly doped samples. Analyzing the shape and energy position of the free-electron recombination spectra we determined the carrier concentrations responsible for the emissions and found that the fundamental band gap energy of InN is $E_g=692\mathrm{}\pm 2\mathrm{meV}$ for an effective mass at the conduction-band minimum $m_{n0\mathrm{}}{=0.042m}_0.$

Direct assembling of N-graphene, i.e. nitrogen doped graphene, in a controllable manner was achieved using microwave plasmas at atmospheric pressure conditions. The synthesis is accomplished via a single step using ethanol and ammonia as carbon and nitrogen precursors. Tailoring of the high-energy density plasma environment results in a selective synthesis of N-graphene (~0.4% doping level) in a narrow range of externally controlled operational conditions, i.e. precursor and background gas fluxes, plasma reactor design and microwave power. Applying infrared (IR) and ultraviolet (UV) irradiation to the flow of free-standing sheets in the post-plasma zone carries out changes in the percentage of sp², the N doping type and the oxygen functionalities. X-ray photoelectron spectroscopy (XPS) revealed the relative extension of the graphene sheets π-system and the type of nitrogen chemical functions present in the lattice structure. Scanning Electron microscopy (SEM), Transmission Electron microscopy (TEM) and Raman spectroscopy were applied to determine morphological and structural characteristics of the sheets. Optical emission and FT-IR spectroscopy were applied for characterization of the high-energy density plasma environment and outlet gas stream. Electrochemical measurements were also performed to elucidate the electrochemical behavior of NG for supercapacitor applications.

One of the greatest challenges in the commercialization of graphene and derivatives is production of high quality material in bulk quantities at low price and in a reproducible manner. The very limited control, or even lack of, over the synthesis process is one of the main problems of conventional approaches. Herein, we present a microwave plasma-enabled scalable route for continuous, large-scale fabrication of free-standing graphene and nitrogen doped graphene sheets. The method’s crucial advantage relies on harnessing unique plasma mechanisms to control the material and energy fluxes of the main building units at the atomic scale. By tailoring the high energy density plasma environment and complementarily applying in situ IR and soft UV radiation, a controllable selective synthesis of high quality graphene sheets at 2 mg/min yield with prescribed structural qualities was achieved. Raman spectroscopy, scanning electron microscopy, high resolution transmission electron microscopy, X-ray photoelectron spectroscopy and Near Edge X-ray-absorption fine-structure spectroscopy were used to probe the morphological, chemical and microstructural features of the produced material. The method described here is scalable and show a potential for controllable, large-scale fabrication of other graphene derivatives and promotes microwave plasmas as a competitive, green, and cost-effective alternative to presently used chemical methods.

The deformation potentials of the $E_1(\mathrm{TO})$ and $E_2$ modes of InN are determined by combining infrared spectroscopic ellipsometry, Raman scattering, and x-ray diffraction measurements, and using a reported value of the mode Grüneisen parameter. The deformation potentials are obtained for two sets of stiffness constants. Strain-free values of the InN $E_1(\mathrm{TO})$ mode of 477.9 ${\mathrm{cm}}^{\text{−1}}$ and of the $E_2$ mode of 491.1 ${\mathrm{cm}}^{\text{−1}}$ have been determined.

We present a comprehensive study of the $\mathrm{InN}$ dielectric function from the midinfrared range up to $6.5\mathrm{eV}$ employing spectroscopic ellipsometry at room temperature. The single-crystalline $\mathrm{InN}$ films with Hall concentrations varying between $7.7\times {10}^{17}$ and $1.4\times {10}^{19}{\mathrm{cm}}^{-3}$ were grown by molecular-beam epitaxy on sapphire substrates and show high structural quality. Free-carrier and phonon properties are determined precisely. The onset of the absorption edge reveals a distinct blueshift with increasing free-electron concentration, from $0.65\mathrm{eV}$ for the highest resistive material up to $0.80\mathrm{eV}$. In the ultraviolet region, electronic interband transitions are detected at 4.84, 5.41, 5.59, and $6.10\mathrm{eV}$, the second being the dominating one, and tentatively assigned in the Brillouin zone.