Rizwan Ur Rehman Sagar

Flexible, Low Cost, and Platinum-Free Counter Electrode for Efficient Dye-Sensitized Solar Cells

A platinum-free counter electrode composed of surface modified aligned multiwalled carbon nanotubes (MWCNTs) fibers was fabricated for efficient flexible dye-sensitized solar cells (DSSCs). Surface modification of MWCNTs fibers with simple one step hydrothermal deposition of cobalt selenide nanoparticles, confirmed by scanning electron microscopy and X-ray diffraction, provided a significant improvement (∼2-times) in their electrocatalytic activity. Cyclic voltammetry and electrochemical impedance spectroscopy suggest a photoelectric conversion efficiency of 6.42% for our modified fibers, higher than 3.4% and 5.6% efficeincy of pristine MWCNTs fiber and commonly used Pt wire, respectively. Good mechanical and performance stability after repeated bending and high output voltage for in-series connection suggest that our surface modified MWCNTs fiber based DSSCs may find applications as flexible power source in next-generation flexible/wearable electronics.

Synthesis of scalable and tunable slightly oxidized graphene via chemical vapor deposition

Semiconducting, large sheets of carbon as an active material in optoelectronic research are missing and reduced graphene oxide (rGO) can be a good candidate. However, chemical synthesis cannot produce large sheets of rGO (i.e. maximum: 20-30μm) as well as high quality rGO due to the restraints of fabrication method. Thus, a novel strategy for the synthesis of large sheets of semiconducting rGO is urgently required. Large area slightly oxidized graphene (SOG) is fabricated at the interface of silicon dioxide (SiO2) and silicon via Chemical Vapor Deposition (CVD) method, herein for the first time. Carbon atoms bond with oxygen functionalities (i.e. CO, COH) at the time of diffusion in SiO2 allowing for C/O ratios from 7 to 10 adjustable by the variation of SiO2 thickness, indicating the tunable oxidation. Moreover, electronic structure and morphology of SOG are similar to the chemically grown rGO. The fabrication mechanism of SOG is also investigated.

Large, Linear, and Tunable Positive Magnetoresistance of Mechanically Stable Graphene Foam–Toward High-Performance Magnetic Field Sensors

Here, we present the first observation of magneto-transport properties of graphene foam (GF) composed of a few layers in a wide temperature range of 2-300 K. Large room-temperature linear positive magnetoresistance (PMR ≈ 171% at B ≈ 9 T) has been detected. The largest PMR (∼213%) has been achieved at 2 K under a magnetic field of 9 T, which can be tuned by the addition of poly(methyl methacrylate) to the porous structure of the foam. This remarkable magnetoresistance may be the result of quadratic magnetoresistance. The excellent magneto-transport properties of GF open a way toward three-dimensional graphene-based magnetoelectronic devices.

Angle dependent magnetotransport in transfer-free amorphous carbon thin films

Amorphous carbon thin films were fabricated on a SiO2 substrate using a chemical vapor deposition (CVD) technique. The structural and surface morphology of the films were analyzed by Raman spectrometry and high resolution transmission electron microscopy (HRTEM), respectively. The atomic ratio % of C(sp 2) and C(sp 3) bonds was estimated using x-ray photoelectron spectroscopy (XPS). The film shows angular magnetoresistance (MR) of 18% and 1.6% at 2 K and room temperature respectively. The mechanism of this angular MR was discussed by use of a grain boundary scattering (GBS) model.

Effect of Annealing Atmosphere Induced Crystallite Size Changes on the Electrochemical Properties of TiO2 Nanotubes Arrays

TNAs (Titanium dioxide nanotube arrays) were synthesized by electrochemical anodization and these TNAs were annealed in different gas atmosphere such as argon, air, hydrogen and nitrogen. This annealing in different atmosphere brought variation in crystallite size (27 ~ 33 nm), which influences on electrochemical properties. The specific capacity of Ar, Air, N2 and H2-annealed TNAs was around ~165, 185, 177 and 190 mAh g, respectively. The crystallite size of anatase TNAs seemed to be responsible for the change in lithium storage capacity, indicating that structural changes of TNAs were playing major role in electrochemical properties.

Facile metal-free reduction-based synthesis of pristine and cation-doped conductive mayenite

In the present study we synthesized conductive nanoscale [Ca24Al28O64]4+(4e−) (hereafter denoted as C12A7:e−) material, and reduced graphene oxide (rGO) was produced, which was unexpected; graphene oxide was removed after melting the sample. The conductivity of C12A7:e− composites synthesized at 1550 °C was 1.25 S cm−1, and the electron concentration was 5.5 × 1019 cm−3. The estimated BET specific surface area of the highly conductive sample was 20 m2 g−1. Pristine C12A7:e− electride was obtained by melting the composite powder, but the nano size of C12A7:e− particles could not be preserved; the value of conductivity was ∼28 S cm−1, electron concentration was ∼1.9 × 1021 cm−3, and mass density was 93%. For C12A7−xVx:e−, where x = 0.25 to 1, the conductivity improved to a maximum value of 40 S cm−1, and the electron density improved to ∼2.2 × 1021 cm−3; this enhancement in conductivity was also proposed by a theoretical study but lacked any associated experimental support.

Large unsaturated room temperature negative magnetoresistance in graphene foam composite for wearable and flexible magnetoelectronics

Room temperature positive magnetoresistance (PMR) in graphene is a conventional phenomenon but we observed large negative magnetoresistance (NMR) in GF/polydimethylsiloxane (GF/PDMS) at room temperature for the first time. The largest NMR ~ 35% was detected at 250 K, while PMR is observed below 200 K. Furthermore, PMR at all temperatures is observed in regular GF specimens, hence, NMR is the result of the infiltration with the electrically insulating polymer. Forward interference and wavefunction shrinkage model has been employed to understand the transport mechanism in GF/PDMS. A critical temperature ~ 224 K for switching between NMR and PMR is observed at the crystallization temperature of PDMS, suggesting a change in polymer chain conformation may be a major reason leading to NMR in GF/PDMS specimens thus role of mechanical properties of PDMS in NMR cannot be ignored and observed locally via specially resolved atomic force microscopy. In addition, storage modulus and heat flow study shows similar transition temperature (~ 200 K) of NMR to PMR and provide an evidence of mechanical stable specimens. As is known, large, tunable, and unsaturated NMR at room temperature is very useful for future facile practical shapeable magnetoelectronic devices.