DFT calculation for the electronic properties and quantum capacitance of pure and doped Zr2CO2 as electrode of supercapacitors

Defect and doping are effective methods to modulate the physical and chemical properties of materials. In this report, we investigated the structural stability, electronic properties and quantum capacitance (Cdiff) of Zr2CO2 by changing the dopants of Si, Ge, Sn, N, B, S and F in the substitutional site. The doping of F, N, and S atoms makes the system undergo the semiconductor-to-conductor transition, while the doping of Si, Ge, and Sn maintains the semiconductor characteristics. The Cdiff of the doped systems are further explored. The B-doped system can be used as cathode materials, while the systems doped by S, F, N, Sn atoms are promising anode materials of asymmetric supercapacitors, especially for the S-doped system. The improved Cdiff mainly originates from Fermi-level shifts and Fermi-Dirac distribution by the introduction of the dopant. The effect of temperature on Cdiff is further explored. The result indicates that the maximum Cdiff of the studied systems gradually decreases with the increasing temperature. Our investigation can provide useful theoretical basis for designing and developing the ideal electrode materials for supercapacitors.

Local atomic structure analysis around Mg atom doped in GaN by X-ray absorption spectroscopy and spectrum simulations

The identification of the incorporated site of magnesium (Mg) and hydrogen (H) required for p-type formation in gallium nitride (GaN) power devices has been demonstrated by X-ray absorption spectroscopy (XAS). In this study, the fluorescence line of Mg with 3 × 1019 atoms cm−3 was successfully separated from that of Ga using a superconducting tunnel junction array detector with high sensitivity and high energy resolution, and consequently the Mg K-edge XAS spectra of such dilute samples were obtained. The site of Mg atoms incorporated into the GaN lattice was identified as the Ga substitutional site by comparing the experimental XAS spectrum with the simulated spectra calculated by density functional theory. In addition, the presence or absence of H around Mg can be determined through distinctive characteristics expected from the spectrum simulations.

Impurity Doping in Mg(OH)2 for n-Type and p-Type Conductivity Control

Magnesium hydroxide (Mg(OH)2) has a wide bandgap of about 5.7 eV and is usually considered an insulator. In this study, the energy levels of impurities introduced into Mg(OH)2 are predicted by first-principles calculations. A supercell of brucite Mg(OH)2 consisting of 135 atoms is used for the calculations, and an impurity atom is introduced either at the substitutional site replacing Mg or the interlayer site. The characteristics of impurity levels are predicted from density-of-states analysis for the charge-neutral cell. According to the results, possible shallow donors are trivalent cations at the substitutional site (e.g., Al and Fe) and cation atoms at the interlayer site (Cu, Ag, Na, and K). On the other hand, an interlayer F atom can be a shallow acceptor. Thus, valence control by impurity doping can turn Mg(OH)2 into a wide-gap semiconductor useful for electronics applications.

Local electronic descriptors for solute-defect interactions in bcc refractory metals

The interactions between solute atoms and crystalline defects such as vacancies, dislocations, and grain boundaries are essential in determining alloy properties. Here we present a general linear correlation between two descriptors of local electronic structures and the solute-defect interaction energies in binary alloys of body-centered-cubic (bcc) refractory metals (such as W and Ta) with transition-metal substitutional solutes. One electronic descriptor is the bimodality of the d-orbital local density of states for a matrix atom at the substitutional site, and the other is related to the hybridization strength between the valance sp- and d-bands for the same matrix atom. For a particular pair of solute-matrix elements, this linear correlation is valid independent of types of defects and the locations of substitutional sites. These results provide the possibility to apply local electronic descriptors for quantitative and efficient predictions on the solute-defect interactions and defect properties in alloys.

Modifications in the Structural and Optical Properties of ZnO Nanophosphor on Doping with Tb

Background: The characteristic visible emission from ZnO being attributed to the defect energy states can be tailored by doping as well as by synthesis techniques. Rare-earth elements, among various dopants, are interesting because of their unique emission properties in the visible region. Terbium (Tb), in particular, is reported to contribute significantly to the creation of the defect energy states when doped in ZnO. This study investigated the Tb concentration dependent modifications in the structural and optical properties of ZnO nanophosphor. Methods: Tb (0.1, 0.5, 01.0 mol%) doped nanophosphor powder samples prepared by low temperature precipitation method, were sintered in air at 700oC using a home-built temperature controlled (±1oC) muffle furnace. Powder XRD and EDX spectra at room temperature were recorded using Philips X perts x-ray spectrometer while Jeol JSM-7600F was used to record SEM images. Photoluminescence spectra excited by the 280, 300, 380 and 460nm radiation from a Xe lamp were recorded using Carry 8000 spectrophotometer. Raman spectra excited by 514.5nm radiation from an Ar-ion laser, was investigated using Morrison microscope Olympus Bx 41 while UV-VIS absorption spectra were recorded on UV- 1800 UV-VIS Spectrophotometer. Results: FTIR and XRD spectra showed that the basic ZnO wurtzite crystal structure remained unchanged on doping. However, XRD data analysis indicated that the 0.1 mol% Tb might be incorporated in ZnO unit cell at an interstitial and / or substitutional site(s) while at 0.5 and 1.0 mol% doping levels migration of Tb to the surface could be the dominant process. This was further confirmed by Raman and photoluminescence studies. Broad emission (122nm FWHM) peaking around 510nm was observed when the doped samples were excited with 280 and 300nm radiation while characteristic ZnO emission was observed with 380 and 460nm radiation. The calculated chromaticity color coordinates (x,y) of the emission excited by 280nm in 0.5 mol% doped ZnO were: x=0.29 and y=0.31, which are very close to those of the daylight at noon. Conclusion: Concentration dependent lattice distortions were observed; it was concluded that at 0.1mol% concentration level Tb was incorporated in ZnO lattice resulting in interstitial or substitutional defects. On the other hand, at 0.5 and 1.0 mol% doping levels diffusion of Tb to the surface producing strain due to "hydrostatic like pressure" seemed to be the dominating process; maximum strain was observed at 0.5mol% doping. The calculated chromaticity color coordinates of the 280nm excited emission from ZnO:Tb (0.5mol%) were found to be very close to those of the "day light at noon” indicating the suitability of the material for the realization of white light sources.

Activation Energy for the Post Implantation Annealing of 1019 cm-3 and 1020 cm-3 Ion Implanted Al in 4H SiC

The activation energy for the electrical activation of 1x1019 cm-3 and of 1x1020 cm-3 ion implanted Al in 4H-SiC has been estimated. Ion implantation temperature and dose rate were in the range 430-500°C and around 1011 cm2s-1, respectively. Post implantation annealing temperatures varied between 1500 °C and 1950 °C. The annealing time per each annealing temperature was sufficiently long that the sheet resistance of the implanted layer could be equal to the stationary value at the applied annealing temperature. The Arrhenius plots of the room temperature sheet resistances with respect to the post implantation annealing temperatures featured an exponential trend for both the implanted Al concentrations. The activation energies of these plots are the activation energy for placing an implanted Al atom in a substitutional site, i.e. the electrical activation energy. Activation energies around 1 eV, equal within errors for the two implanted Al concentrations, were found.