Synthesis, and crystal and electronic structure of sodium metal phosphate for use as a hybrid capacitor in non-aqueous electrolyte

Energy storage devices based on sodium have been considered as an alternative to traditional lithium based systems because of the natural abundance, cost effectiveness and low environmental impact of sodium. Their synthesis, and crystal and electronic properties have been discussed, because of the importance of electronic conductivity in supercapacitors for high rate applications. The density of states of a mixed sodium transition metal phosphate (maricite, NaMn1/3Co1/3Ni1/3PO4) has been determined with the ab initio generalized gradient approximation (GGA)+Hubbard term (U) method. The computed results for the mixed maricite are compared with the band gap of the parent NaFePO4 and the electrochemical experimental results are in good agreement. A mixed sodium transition metal phosphate served as an active electrode material for a hybrid supercapacitor. The hybrid device (maricite versus carbon) in a nonaqueous electrolyte shows redox peaks in the cyclic voltammograms and asymmetric profiles in the charge-discharge curves while exhibiting a specific capacitance of 40 F g(-1) and these processes are found to be quasi-reversible. After long term cycling, the device exhibits excellent capacity retention (95%) and coulombic efficiency (92%). The presence of carbon and the nanocomposite morphology, identified through X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) studies, ensures the high rate capability while offering possibilities to develop new cathode materials for sodium hybrid devices.

Simultaneous tunability of the electronic and phononic gaps in SnS2 under normal compressive strain

Controlled variation of the electronic properties of. two-dimensional (2D) materials by applying strain has emerged as a promising way to design materials for customized applications. Using density functional theory (DFT) calculations, we show that while the electronic structure and indirect band gap of SnS2 do not change significantly with the number of layers, they can be reversibly tuned by applying biaxial tensile (BT), biaxial compressive (BC), and normal compressive (NC) strains. Mono to multilayered SnS2 exhibit a reversible semiconductor to metal (S-M) transition with applied strain. For bilayer (2L) SnS2, the S-Mtransition occurs at the strain values of 17%,-26%, and -24% under BT, BC, and NC strains, respectively. Due to weaker interlayer coupling, the critical strain value required to achieve the S-Mtransition in SnS2 under NC strain is much higher than for MoS2. From a stability viewpoint, SnS2 becomes unstable at very low strain values on applying BC (-6.5%) and BT strains (4.9%), while it is stable even up to the transition point (-24%) in the case of NC strain. In addition to the reversible tuning of the electronic properties of SnS2, we also show tunability in the phononic band gap of SnS2, which increases with applied NC strain. This gap increases three times faster than for MoS2. This simultaneous tunability of SnS2 at the electronic and phononic levels with strain, makes it a potential candidate in field effect transistors (FETs) and sensors as well as frequency filter applications.

Friction properties of bio-mimetic nano-fibrillar arrays

Nano-fibrillar arrays are fabricated using polystyrene materials. The average diameter of each fiber is about 300 nm. Experiments show that such a fibrillar surface possesses a relatively hydrophobic feature with a water contact angle of 142 degrees. Nanoscale friction properties are mainly focused on. It is found that the friction force of polystyrene nano-fibrillar surfaces is obviously enhanced in contrast to polystyrene smooth surfaces. The apparent coefficient of friction increases with the applied load, but is independent of the scanning speed. An interesting observation is that the friction force increases almost linearly with the real contact area, which abides by the fundamental Bowden-Tabor law of nano-scale friction.

Electrochemical fabrication and electronic behavior of polypyrrole nano-fiber array devices

Electrochemically active Polypyrrole (PPy) nano-fiber array device was fabricated via electrochemical deposition method using aluminum anodic oxide (AAO) membrane as template. After alkaline treatment electrochemically active PPy nano-fiber lost electrochemical activity, and became electrochemically inactive PPy. The electronic properties of PPy nano-fiber array devices were measured by means of a simple method. It was found that for an indium-tin oxide/electrochemically inactive PPy nano-fiber device, the conductivity of nano-fiber increased with the increase of voltage applied on the two terminals of nano-fiber. The electrochemical inactive PPy nano-fiber might be used as a nano-fiber switching diode. Both Au/electrochemically active PPy and Au/electrochemically inactive PPy nano-fiber devices demonstrate rectifying behavior, and might have been used for further application as nano-rectifiers. (c) 2005 Elsevier B.V. All tights reserved.

Calculation of the electronic structure of carbon films using electron energy loss spectroscopy.

The detailed understanding of the electronic properties of carbon-based materials requires the determination of their electronic structure and more precisely the calculation of their joint density of states (JDOS) and dielectric constant. Low electron energy loss spectroscopy (EELS) provides a continuous spectrum which represents all the excitations of the electrons within the material with energies ranging between zero and about 100 eV. Therefore, EELS is potentially more powerful than conventional optical spectroscopy which has an intrinsic upper information limit of about 6 eV due to absorption of light from the optical components of the system or the ambient. However, when analysing EELS data, the extraction of the single scattered data needed for Kramers Kronig calculations is subject to the deconvolution of the zero loss peak from the raw data. This procedure is particularly critical when attempting to study the near-bandgap region of materials with a bandgap below 1.5 eV. In this paper, we have calculated the electronic properties of three widely studied carbon materials; namely amorphous carbon (a-C), tetrahedral amorphous carbon (ta-C) and C60 fullerite crystal. The JDOS curve starts from zero for energy values below the bandgap and then starts to rise with a rate depending on whether the material has a direct or an indirect bandgap. Extrapolating a fit to the data immediately above the bandgap in the stronger energy loss region was used to get an accurate value for the bandgap energy and to determine whether the bandgap is direct or indirect in character. Particular problems relating to the extraction of the single scattered data for these materials are also addressed. The ta-C and C60 fullerite materials are found to be direct bandgap-like semiconductors having a bandgaps of 2.63 and 1.59eV, respectively. On the other hand, the electronic structure of a-C was unobtainable because it had such a small bandgap that most of the information is contained in the first 1.2 eV of the spectrum, which is a region removed during the zero loss deconvolution.

Electronic comparison of InAs wurtzite and zincblende phases using nanowire transistors

We compare the electronic characteristics of nanowire field-effect transistors made using single pure wurtzite and pure zincblende InAs nanowires grown from identical catalyst particles. We compare the transfer characteristics and field-effect mobility versus temperature for these devices to better understand how differences in InAs phase govern the electronic properties of nanowire transistors. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Controlled amine functionality in self-assembled monolayers via the hidden amine route: chemical and electronic tunability.

A synthetic strategy for fabricating a dense amine functionalized self-assembled monolayer (SAM) on hydroxylated surfaces is presented. The assembly steps are monitored by X-ray photoelectron spectroscopy, Fourier transform infrared- attenuated total reflection, atomic force microscopy, variable angle spectroscopic ellipsometry, UV-vis surface spectroscopy, contact angle wettability, and contact potential difference measurements. The method applies alkylbromide-trichlorosilane for the fabrication of the SAM followed by surface transformation of the bromine moiety to amine by a two-step procedure: S(N)2 reaction that introduces the hidden amine, phthalimide, followed by the removal of the protecting group and exposing the free amine. The use of phthalimide moiety in the process enabled monitoring the substitution reaction rate on the surface (by absorption spectroscopy) and showed first-order kinetics. The simplicity of the process, nonharsh reagents, and short reaction time allow the use of such SAMs in molecular nanoelectronics applications, where complete control of the used SAM is needed. The different molecular dipole of each step of the process, which is verified by DFT calculations, supports the use of these SAMs as means to tune the electronic properties of semiconductors and for better synergism between SAMs and standard microelectronics processes and devices.

Measuring the electrical properties of semiconductor nanowires using terahertz conductivity spectroscopy

Accurately measuring the electronic properties of nanowires is a crucial step in the development of novel semiconductor nanowire-based devices. With this in mind, optical pump-terahertz probe (OPTP) spectroscopy is ideally suited to studies of nanowires: it provides non-contact measurement of carrier transport and dynamics at room temperature. OPTP spectroscopy has been used to assess key electrical properties, including carrier lifetime and carrier mobility, of GaAs, InAs and InP nanowires. The measurements revealed that InAs nanowires exhibited the highest mobilities and InP nanowires exhibited the lowest surface recombination velocity. © 2013 Copyright SPIE.

Controlling electronic structures by irradiation in single-walled SiC nanotubes: a first-principles molecular dynamics study

Using first-principles molecular dynamics simulations, the displacement threshold energy and defect configurations are determined in SiC nanotubes. The simulation results reveal that a rich variety of defect structures (vacancies, Stone-Wales defects and antisite defects) are formed with threshold energies from 11 to 64 eV. The threshold energy shows an anisotropic behavior and exhibits a dramatic decrease with decreasing tube diameter. The electronic structure can be altered by the defects formed by irradiation, which suggests that the electron irradiation may be a way to use defect engineering to tailor electronic properties of SiC nanotubes.

Efficiently producing single-walled carbon nanotube rings and investigation of their field emission properties

Single-walled carbon nanotube (SWNT) rings with a diameter of about 100 nm have been prepared by thermally decomposing hydrocarbon in a floating catalyst system. These rings appeared to consist mostly of SWNT toroids. High resolution transmission electron microscopy showed that these rings were composed of tens of SWNTs with a tightly packed arrangement. The production of SWNT rings was improved through optimizing various growth parameters, such as growth temperature, sublimation temperature of the catalyst, different gas flows and different catalyst components. The growth mechanism of the SWNT rings is discussed. In the field emission measurements we found that field emission from a halved ring is better than that from a whole SWNT ring, which contributed to the better emission from two opened ends of the nanotubes of the halved SWNT ring.

Electronic investigation of self-organized InAs quantum dots

Deep Level Transient Spectroscopy (DLTS) has been applied to investigate the electronic properties of self-organized InAs quantum dots. The energies of electronic ground states of 2.5ML and 1.7ML InAs quantum dots (QDs) with respect to the conduction band of bulk GaAs are about 0.21 eV and 0.09 eV, respectively. We have found that QDs capture electrons by lattice relaxation through a multi-phonon emission process. The samples are QDs embedded in superlattices with or without a 500 Angstrom GaAs spacing layer between every ten periods of a couple of GaAs and InAs layers. The result shows that the density of dislocations in the samples with spacer layers is much lower than in the samples without the spacer layers.

Electronic investigation of self-organized InAs quantum dots

Deep Level Transient Spectroscopy (DLTS) has been applied to investigate the electronic properties of self-organized InAs quantum dots. The energies of electronic ground states of 2.5ML and 1.7ML InAs quantum dots (QDs) with respect to the conduction band of bulk GaAs are about 0.21 eV and 0.09 eV, respectively. We have found that QDs capture electrons by lattice relaxation through a multi-phonon emission process. The samples are QDs embedded in superlattices with or without a 500 Angstrom GaAs spacing layer between every ten periods of a couple of GaAs and InAs layers. The result shows that the density of dislocations in the samples with spacer layers is much lower than in the samples without the spacer layers.

Ab initio study on the physical properties of CoN3 and RhN3 with skutterudite structure

The elastic and electronic properties of hypothetical CoN3 and RhN3 with cubic skutterudite structure were studied by first principles calculations based on density functional theory. By choosing different initial geometries, two local minima or modifications were located on the potential energy surface, termed as modifications I and II. Both compounds are mechanically stable. For each compound, modification I is lower in energy than II. Thermodynamically stable phases can be achieved by applying pressures. Modification II is lower in energy than I at above 50 GPa for both compounds.

Crystal structures and elastic properties of superhard IrN2 and IrN3 from first principles

First principles calculations were performed to investigate the structural, elastic, and electronic properties of IrN2 for various space groups: cubic Fm-3m and Pa-3, hexagonal P3(2)21, tetragonal P4(2)/mnm, orthorhombic Pmmn, Pnnm, and Pnn2, and monoclinic P2(1)/c. Our calculation indicates that the P2(1)/c phase with arsenopyrite-type structure is energetically more stable than the other phases. It is semiconducting (the remaining phases are metallic) and contains diatomic N-N with the bond distance of 1.414 A. These characters are consistent with the experimental facts that IrN2 is in lower symmetry and nonmetallic. Our conclusion is also in agreement with the recent theoretical studies that the most stable phase of IrN2 is monoclinic P2(1)/c. The calculated bulk modulus of 373 GPa is also the highest among the considered space groups. It matches the recent theoretical values of 357 GPa within 4.3% and of 402 GPa within 7.8%, but smaller than the experimental value of 428 GPa by 14.7%. Chemical bonding and potential displacive phase transitions are discussed for IrN2. For IrN3, cubic skutterudite structure (Im-3) was assumed.

Structures and elastic properties of OsN2 investigated via first-principles density functional calculations

The structure, elastic, and electronic properties of OsN2 at various space groups: cubic Fm-3m, Pa-3, and orthorhombic Pnnm were studied by first-principles calculations based on density functional theory. Our calculation indicates that the structure in orthorhombic Pnnm phase is energetically more stable compared with cubic systems. It is metallic, mechanically stable and contains diatomic N-N units with the bond distance 1.418 A. These characters are consistent with experimental facts that OsN2 is orthorhombic and metallic. The calculated bulk modulus 394 GPa is also the highest among the considered space groups, slightly larger than previous value 358 GPa. The calculated elastic anisotropic factors and directional bulk modulus showed that OsN2 possess high elastic anisotropy.