Showing papers by "Anvar A. Zakhidov published in 2015"

Magnetic field effects in hybrid perovskite devices

Abstract: Perovskite photovoltaics are the fastest-advancing solar technology but the mechanisms responsible for their performance are not clear. The observation of magnetic field effects in hybrid perovskites may help to explain their high efficiencies.

Exciton versus free carrier photogeneration in organometal trihalide perovskites probed by broadband ultrafast polarization memory dynamics.

Abstract: We studied the ultrafast transient response of photoexcitations in two hybrid organic-inorganic perovskite films used for high efficiency photovoltaic cells, namely, CH(3)NH(3)PbI(3) and CH(3)NH(3)PbI(1.1)Br(1.9) using polarized broadband pump-probe spectroscopy in the spectral range of 0.3-2.7 eV with 300 fs time resolution. For CH(3)NH(3)PbI(3) with above-gap excitation we found both photogenerated carriers and excitons, but only carriers are photogenerated with below-gap excitation. In contrast, mainly excitons are photogenerated in CH(3)NH(3)PbI(1.1)Br(1.9). Surprisingly, we also discovered in CH(3)NH(3)PbI(3), but not in CH(3)NH(3)PbI(1.1)Br(1.9), transient photoinduced polarization memory for both excitons and photocarriers, which is also reflected in the steady state photoluminescence. From the polarization memory dynamics we obtained the excitons diffusion constant in CH(3)NH(3)PbI(3), D≈0.01 cm(2) s(-1).

Optical, Electrical, and Electromechanical Properties of Hybrid Graphene/Carbon Nanotube Films

Abstract: By combining a graphene layer and aligned multiwalled carbon nanotube (MWNT) sheets in two different configurations, i) graphene on the top of MWNTs and ii) MWNTs on the top of the graphene, it is demonstrated that optical, electrical, and electromechanical properties of the resulting hybrid films depend on configurations.

Three-dimensionally ordered macro-/mesoporous Ni as a highly efficient electrocatalyst for the hydrogen evolution reaction

Abstract: Three-dimensionally (3D) ordered macro-/mesoporous (3DOM/m) Ni is fabricated by the chemical reduction deposition method using lyotropic liquid crystals (LLC) to template the mesostructure within the regular voids of a colloidal crystal (opal). The thereby achieved structural advantages of combining well-ordered bicontinuous mesopores with 3D interconnected periodic macropores, such as abundant exposed catalytically active sites, efficient mass transport, and high electrical conductivity, make this non-noble metal structure an excellent hydrogen evolution reaction (HER) electrocatalyst. The 3DOM/m Ni exhibits a low onset overpotential of 63 mV (vs. RHE) and a small Tafel slope of 52 mV per decade, as well as a long-term durability in alkaline medium. These distinct features of the 3DOM/m Ni render it a promising alternative to Pt-based HER electrocatalysts.

Fabrication of biscrolled fiber using carbon nanotube sheet

Abstract: Fabrication of yarns or other shaped articles from materials in powder form (or nanoparticles or nanofibers) using carbon nanotube/nanofiber sheet as a platform (template). This includes methods for fabricating biscrolled yarns using carbon nanotube/nanofiber sheets and biscrolled fibers fabricated thereby.

Fabrication of nanofiber ribbons and sheets

Abstract: Fabricating a nanofiber ribbon or sheet with a process that includes providing a primary assembly by arranging carbon nanotube nanofibers in aligned arrays, the arrays having a degree of inter-fiber connectivity, drawing the carbon nanotube nanofibers from the primary assembly into a sheet or ribbon, and depositing the sheet or ribbon on a substrate.

Tunable color parallel tandem organic light emitting devices with carbon nanotube and metallic sheet interlayers

Abstract: Parallel tandem organic light emitting devices (OLEDs) were fabricated with transparent multiwall carbon nanotube sheets (MWCNT) and thin metal films (Al,Ag) as interlayers. In parallel monolithic tandem architecture, the MWCNT (or metallic films) interlayers are an active electrode which injects similar charges into subunits. In the case of parallel tandems with common anode (C.A.) of this study, holes are injected into top and bottom subunits from the common interlayer electrode; whereas in the configuration of common cathode (C.C.), electrons are injected into the top and bottom subunits. Both subunits of the tandem can thus be monolithically connected functionally in an active structure in which each subunit can be electrically addressed separately. Our tandem OLEDs have a polymer as emitter in the bottom subunit and a small molecule emitter in the top subunit. We also compared the performance of the parallel tandem with that of in series and the additional advantages of the parallel architecture over the in-series were: tunable chromaticity, lower voltage operation, and higher brightness. Finally, we demonstrate that processing of the MWCNT sheets as a common anode in parallel tandems is an easy and low cost process, since their integration as electrodes in OLEDs is achieved by simple dry lamination process.

Synthesis and superconductivity in spark plasma sintered pristine and graphene-doped FeSe$_{0.5}$Te$_{0.5}$

Abstract: Abstract Here, we present a new ball-milling and spark plasma sintering (SPS)-based technique for the facile synthesis of FeSe0.5Te0.5 superconductors without the need for pre-alloying. This method is advantageous since it is quick and flexible for incorporating other dopants such as graphene for vortex pinning. We observed that FeSe0.5Te0.5 exhibits a coexistence of ferromagnetic (FM) and superconductivity signature plausibly arising from a FM core-superconducting shell structure. More importantly, the Hc2 values observed from resistivity data are higher than 7 T, indicating that SPS process synthesized FeSe0.5Te0.5 samples could lead to next-generation superconducting wires and cables.

Thermal property tuning in aligned carbon nanotube films and random entangled carbon nanotube films by ion irradiation

Abstract: Ion irradiation effects on thermal property changes are compared between aligned carbon nanotube (A-CNT) films and randomly entangled carbon nanotube (R-CNT) films. After H, C, and Fe ion irradiation, a focusing ion beam with sub-mm diameter is used as a heating source, and an infrared signal is recorded to extract thermal conductivity. Ion irradiation decreases thermal conductivity of A-CNT films, but increases that of R-CNT films. We explain the opposite trends by the fact that neighboring CNT bundles are loosely bonded in A-CNT films, which makes it difficult to create inter-tube linkage/bonding upon ion irradiation. In a comparison, in R-CNT films, which have dense tube networking, carbon displacements are easily trapped between touching tubes and act as inter-tube linkage to promote off-axial phonon transport. The enhancement overcomes the phonon transport loss due to phonon-defect scattering along the axial direction. A model is established to explain the dependence of thermal conductivity changes on ion irradiation parameters including ion species, energies, and current.

Microwave conductance properties of aligned multiwall carbon nanotube textile sheets

Abstract: Understanding the conductance properties of multi-walled carbon nanotube (MWNT) textile sheets in the microwave regime is essential for their potential use in high-speed and high-frequency applications. To expand current knowledge, complex high-frequency conductance measurements from 0.01 to 50 GHz and across temperatures from 4.2 K to 300 K and magnetic fields up to 2 T were made on textile sheets of highly aligned MWNTs with strand alignment oriented both parallel and perpendicular to the microwave electric field polarization. Sheets were drawn from 329 and 520 μm high MWNT forests that resulted in different DC resistance anisotropy. For all samples, the microwave conductance can be modeled approximately by a shunt capacitance in parallel with a frequency-independent conductance, but with no inductive contribution. This is consistent with diffusive Drude conduction as the primary transport mechanism up to 50 GHz. Further, it is found that the microwave conductance is essentially independent of both temp...

Effect of high-pressure fluorination on electrical properties of multi-walled carbon nanotubes sheet

Abstract: High-pressure fluorination of multi-walled carbon nanotubes (MWNTs) sheet was performed at room temperature. This fluorine treatment increases the electrical conductivity of MWNTs sheet. In addition, low-temperature conductivity measurement data gives a good fit to variable-range hopping (VRH) model. Thus, VRH model was adopted to study and understand the increase in electrical conductivity. Estimation of the degree of disorder from the experimental data and VRH model indicate that degree of disorder of fluorinated multi-walled carbon nanotubes (FMWNTs) is higher than that of MWNTs. Higher degree of disorder of FMWNTs sheet substantiate the increase in electrical conductivity when compared to MWNTs. Scanning Electron Microscopy (SEM) and Raman analysis shows that change in structural disorder of FMWNTs network. Effect of fluorination on activation energy was also studied. The activation energy reduces with lower temperatures. The estimated values of temperature coefficient of resistance also show the increase in degree of disorder of FMWNTs. The change in electrical properties of FMWNTs is a much-needed requirement in the application of nano-electronics.

Production and application of ribbon and sheet of nanofiber and twisted yarn and non-twisted yarn of nanofiber

Abstract: PROBLEM TO BE SOLVED: To provide a yarn, ribbon and sheet of nanofibers; to provide a method for producing the yarn, ribbon and sheet; and to provide applications of the yarn, ribbon and sheet.SOLUTION: In some embodiments, a yarn, ribbon and sheet of nanotubes include carbon nanotubes. Specifically, such carbon nanotubes of the present invention provide the following unique properties and property combinations, for example: extreme toughness; resistance to failure at knots; high levels of electrical and thermal conductivities; high absorption of energy that occurs reversibly; up to 13% of strain-to-failure as compared with a few percent in other fibers with similar toughness; very high resistance to creep; retention of strength even when heated in air at 450°C for one hour; and very high radiation resistance and UV resistance even when irradiated in air.

Nanoimprinted solar cells with large molecular weight P3HT nanogratings with enhanced molecular alignment

Abstract: We demonstrate efficient polymer solar cells made from large molecular weight (MW) poly(3-hexylthiophene-2,5-diyl) (P3HT) nanostructures made by nanoimprint lithography (NIL). We found NIL allows the polymer with higher MW (> 40 kDa) to organize more strongly than lower MW, which is desired for efficient charge transport but difficult to achieve previously. Solar cells with large MW P3HT nanogratings show a high power conversion efficiency of 4.4%, which is among the best reported for the same materials.