Showing papers by "Anvar A. Zakhidov published in 2013"

Nonlinear optical properties of boron doped single-walled carbon nanotubes

Abstract: Single-walled carbon nanotubes (SWCNTs) exhibit excellent nonlinear optical (NLO) properties due to the delocalized π electron states present along their tube axis. Using the open aperture Z-scan method in tandem with X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy, we demonstrate the simultaneous tailoring of both electronic and NLO properties of SWCNTs, from ultrafast (femtosecond) to relatively slow (nanosecond) timescales, by doping with a single substituent, viz., boron. SWCNTs were doped via a wet chemical method using B2O3, and the boron content and bonding configurations were identified using XPS. While in the ns excitation regime, the nonlinear absorption was found to increase with increasing boron concentration in the SWCNTs (due to the increasing disorder and enhanced metallicity of the SWCNTs), the saturation intensity in the fs excitation regime decreased. We attribute this counter-intuitive behavior to excited state absorption on ns timescales, and saturable absorption combined with weak two-photon transitions on fs timescales between van Hove singularities.

Tunable interplay between 3d and 4f electrons in Co-doped iron pnictides

Abstract: We study the interplay of $3d$ and $4f$ electrons in the iron pnictides CeFe${}_{1\ensuremath{-}x}$Co${}_{x}$AsO and GdFe${}_{1\ensuremath{-}y}$Co${}_{y}$AsO, which correspond to two very different cases of $4f$-magnetic moment. Both CeFeAsO and GdFeAsO undergo a spin-density-wave (SDW) transition associated with Fe $3d$ electrons at high temperatures, which is rapidly suppressed by Fe/Co substitution. Superconductivity appears in a narrow doping range: $0.05lxl0.2$ for CeFe${}_{1\ensuremath{-}x}$Co${}_{x}$AsO and $0.05lyl0.25$ for GdFe${}_{1\ensuremath{-}y}$Co${}_{y}$AsO, showing a maximum transition temperature ${T}_{\mathrm{sc}}$ of about 13.5 K for Ce and 19 K for Gd. In both compounds, the $4f$ electrons form an antiferromagnetic (AFM) order at low temperatures over the entire doping range and Co $3d$ electrons are ferromagnetically ordered on the Co-rich side; the Curie temperature reaches ${T}_{\mathrm{C}}^{\mathrm{Co}}\ensuremath{\approx}$ 75 K at $x=1$ and $y=1$. In the Ce compounds, the N\'eel temperature ${T}_{\mathrm{N}}^{\mathrm{Ce}}$ increases upon suppressing the SDW transition of Fe and then remains nearly unchanged with further increasing Co concentration up to $x\ensuremath{\simeq}0.8$ (${T}_{\mathrm{N}}^{\mathrm{Ce}}\ensuremath{\approx}$ 4 K). Furthermore, evidence of Co-induced polarization on Ce moments is observed on the Co-rich side. In the Gd compounds, the two magnetic species of Gd and Co are coupled antiferromagnetically to give rise to ferrimagnetic behavior in the magnetic susceptibility on the Co-rich side. For $0.7\ensuremath{\le}yl1.0$, the system undergoes a possible magnetic reorientation below the N\'eel temperature of Gd (${T}_{\mathrm{N}}^{\mathrm{Gd}}$). Our results suggest that the effects of both electron hybridizations and magnetic exchange coupling between the $3d$-$4f$ electrons give rise to a rich phase diagram in the rare-earth iron pnictides.

Phenothiazine Semiconducting Polymer for Light-Emitting Diodes

Abstract: Phenothiazine alternating copolymer poly{4,8-bis(5-dodecylthiophene-2-yl)benzo[1,2-b:4,5-b′]dithiophene-alt-(10-decylphenothiazine)} (P1) is synthesized by Stille coupling polymerization. The synthesized polymer has an electrochemical bandgap of 1.61 eV and displays good photoluminescence with a quantum yield of 63.3%. The polymer is tested as a light-emitting material in polymeric light-emitting diodes (PLEDs) with a device structure of indium tin oxide (ITO)/poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS)/polymer/Cs2CO3/Al. The devices display an orange-yellow electroluminescence (Commission Internationale de l'Eclairage (CIE) 1931 color-space chromaticity diagram: x = 0.5355 and y = 0.4611) with a maximum brightness of 3130 cd m−2 at an applied voltage of 10 V.

Synthesis and characterization of a novel symmetrical sulfone-substituted polyphenylene vinylene (SO2EH-PPV) for applications in light emitting devices.

Abstract: A novel symmetrical alkylsulfonyl-substituted poly(phenylenevinylene) derivative, poly [2,5-bis-(2'-ethylhexylsulfonyl)-1,4-phenylene)vinylene] (SO2EH-PPV), was synthesized via palladium-catalyzed Stille coupling, and its electronic and optical properties were investigated. The novel PPV derivative was characterized by NMR, UV-visible absorption, photoluminescence, gel permeation chromatography, infrared spectroscopy, and cyclic voltammetry (CV). The polymer with Mw of 27,800 and a polydispersity index of 2.6 is readily soluble in common organic solvents, such as THF, chloroform, and toluene. The fluorescence quantum yield of the polymer, determined against rhodamine 6G in dilute aqueous solutions, was 0.95. The HOMO and LUMO levels of SO2EH-PPV were calculated to be -6.0 and -3.61 eV, respectively. The results obtained by CV suggest that SO2EH-PPV is a strong electron acceptor polymer. Single layer stable polymer light-emitting diode devices with the configuration of (ITO/PEDOT:PSS/SO2EH-PPV polymer/Al) were fabricated exhibiting a green light emission.

Ionically reconfigurable organic photovoltaic and photonic devices with tunable common electrode

Abstract: The present invention is directed to a novel type of monolithic hybrid technology. The invention is directed to photonic devices with a minimum of three (3) electrodes and by an inventive process for incorporating mobile ions into organic components of high performance organic photovoltaic (OPV) devices, organic photodetectors and other hybrid photonic devices (such as tandems of OPV), through a novel unique device architecture of a hybrid “Ionic-NT-OPV” structure, in which the ionic components are separated from the OPV by a common nanoporous charge collecting electrode (symbolically depicted as a nanotube: NT), permeable to ions of ionic component inside an inter-connected microchamber.

Weavable dye sensitized solar cells exploiting carbon nanotube yarns

Abstract: Weavable Dye Sensitized Solar Cells (DSSC) made with flexible yarns of conductive multiwalled carbon nanotubes (MWNTs) were produced having a power conversion efficiency above 3%. This was achieved with a specific design and careful consideration of the yarn function in the DSSC. Fermat yarns of MWNTs individually coated with mesoporous TiO2 layer were twisted together and coated with more mesoporous TiO2 to create a 3 dimensional photo electrode to overcome electron diffusion length issues. Archimedian yarns of MWNTs coated with a thin layer of platinum worked as a counter electrode to complete the architecture used in this DSSC.

Superconducting properties of FeSexTe1−x thin film with a composition close to antiferromagnetic ordering

Abstract: In this study Te-rich iron chalcogenide (FeSexTe1−x) thin films with a composition close to antiferromagnetic ordering have been deposited on SrTiO3 (STO) substrates The superconducting critical transition temperature (Tc) of the FeSe01Te09 thin film on STO substrate ranges from ∼125 to ∼133 K The upper critical field is as high as 114 T, which is much higher than that of the FeSe05Te05 thin film on STO substrate (∼49 T) The self-field critical current density () at 2 K of 18 × 105 A cm−2 is much higher than that of the FeSe05Te05 thin film, and the FeSe01Te09 thin film also demonstrates superior pinning properties under applied magnetic field Compared to FeSe05Te05, which was considered as the optimum composition, FeSe01Te09 presents even more promise for high field applications because of its high upper critical field and high critical current density

Electrochemically gated organic photovoltaic with tunable carbon nanotube cathodes

Abstract: We demonstrate an organic photovoltaic (OPV) device with an electrochemically gated carbon nanotube (CNT) charge collector. Bias voltages applied to the gate electrode reconfigure the common CNT electrode from an anode into a cathode which effectively collects photogenerated electrons, dramatically increasing all solar cell parameters to achieve a power conversion efficiency of ∼3%. This device requires very little current to initially charge and the leakage current is negligible compared to the photocurrent. This device can also be viewed as a hybrid tandem OPV-supercapacitor with a common CNT electrode. Other regimes of operation are briefly discussed.

Effects of nanostructure geometry on polymer chain alignment and device performance in nanoimprinted polymer solar cell

Abstract: Among the various organic photovoltaic devices, the conjugated polymer/fullerene approach has drawn the most research interest. The performance of these types of solar cells is greatly determined by the nanoscale morphology of the two components (donor/acceptor) and the molecular orientation/crystallinity in the photoactive layer. This article demonstrates our recent studies on the nanostructure geometry effects on the nanoimprint induced poly(3 hexylthiophene-2,5-diyl) (P3HT) chain alignment and photovoltaic performance. Out-of-plane and in-plane grazing incident X-ray diffractions are employed to characterize the chain orientations in P3HT nanogratings with different widths and heights . It is found that nanoimprint procedure changes the initial edge-on alignment in non-imprinted P3HT thin film to a vertical orientation which favors the hole transport, with an organization height H ≥ 170 nm and width in the range of 60 nm≤ W width w = 60 nm, height h = 170 nm), allowing for the most efficient charge separation, transport and light absorption. We believe this work will contribute to the optimal geometry design of nanoimprinted polymer solar cells.

Superconductivity in an inhomogeneous bundle of metallic and semiconducting nanotubes

Abstract: Using Bogoliubov-de Gennes formalism for inhomogeneous systems, we have studied superconducting properties of a bundle of packed carbon nanotubes, making a triangular lattice in the bundle's transverse cross-section. The bundle consists of a mixture of metallic and doped semiconducting nanotubes, which have different critical transition temperatures. We investigate how a spatially averaged superconducting order parameter and the critical transition temperature depend on the fraction of the doped semiconducting carbon nanotubes in the bundle. Our simulations suggest that the superconductivity in the bundle will be suppressed when the fraction of the doped semiconducting carbon nanotubes will be less than 0.5, which is the percolation threshold for a two-dimensional triangular lattice.

OPV Tandems with CNTS: Why Are Parallel Connections Better Than Series Connections

Abstract: The efficiency of organic photovoltaic cells can be increased in tandem OPV structures with complementary light absorption in top and bottom sub-cells. We demonstrate that strong transparent CNT sheets can be used as an effective charge collector interlayer in OPV and hybrid tandem solar cells. Most importantly we show that CNT sheets can be used in monolithic parallel tandems (P-T) as common a electrode interconnect between top and bottom sub-cells. For achieving good performance one of these subcells in P-T should be of inverted type. We achieved good inversion in OPV, using ZnO nanoparticles, which act as hole barrier layers and invert a typical anode ITO into a cathode. With this inverted bottom cell the efficiency of P-T is significantly improved, as compared to our earlier results. We briefly discuss the modeling analysis of OPV tandems and derive an optimal set of parameters, for highest efficiency P-T. Our simple model shows that for tandems with unbalanced photocurrents but similar open circuit voltages the optimized P-T architecture is always better than conventional series tandem (S-T) geometry. Indeed the experimental comparison of P-T with S-T for hybrids of OPV and dye sensitized solar cells demonstrate the imporved efficiency of the former.