Reeti Bajpai

Graphene supported platinum nanoparticle counter-electrode for enhanced performance of dye-sensitized solar cells.

Composites of few layered graphene (G) and platinum (Pt) nanoparticles (NP) with different loadings of Pt were used as counter electrode (CE) in dye-sensitized solar cell (DSSC). NPs were deposited directly on to G using pulsed laser ablation method (PLD). DSSCs formed using the composite CEs show improved performance compared to conventional Pt thin film electrode (Std Pt) and unsupported Pt NPs. Composite with 27% loading of Pt shows 45% higher efficiency (η = 2.9%), greater short circuit current (J(sc) = 6.67 mA cm(-2)), and open circuit voltage (V(oc) = 0.74 V) without any loss of the fill factor (FF = 58%) as compared to the cells fabricated using Std Pt electrodes. Values of η, J(sc) and V(oc) for DSSC using Std Pt CE were 2%, 5.05 mA cm(-2) and 0.68 V, respectively. Electrochemical impedance spectroscopy using I(-)(3)/I(-) redox couple confirm lower values of charge transfer resistance for the composite electrodes, e.g., 2.36 Ω cm(2) as opposed to 7.73 Ω cm(2) of Std Pt. The better catalytic activity of these composite materials is also reflected in the stronger I(-)(3) reduction peaks in cyclic voltammetry scans.

Plasma modified flexible bucky paper as an efficient counter electrode in dye sensitized solar cells

Platinum (Pt) -free counter electrodes (CEs) for dye sensitized solar cells (DSSCs) were developed using freestanding flexible single wall carbon nanotube (SWNT) films called bucky papers (BPs). BP was irradiated with microwave plasma, created using a mixture of Ar (1%) and H2 (99%) gases, for 2 h. Raman scattering measurements revealed that no significant defects were created in the SWNTs as a result of the treatment. Plasma-treated BP (P-BP) developed vertically oriented, micron sized, pillar-like structures on its surface, while its base was still a dense random mesh of SWNTs. This unique flexible film had a larger accessible surface area and better catalytic properties. The plasma treatment improved the efficiency of BP-based DSSCs from 2.44% to 4.02%, which is comparable to Pt thin film (4.08%). The P-BP based solar cell operated with an open circuit voltage of 0.73 V and a fill factor of 0.70. It also had much higher efficiencies than films of randomly oriented plasma treated SWNTs. Using electrochemical impedance spectroscopy, the charge transfer resistances of P-BP and Pt were found to be 1.46 and 1.73 Ω cm2, respectively.

Dramatic enhancement of the emission current density from carbon nanotube based nanosize tips with extremely low onset fields.

Nanostructures based on multiwalled carbon nanotubes (MWNTs) are fabricated using plasma of the mixture of hydrogen and nitrogen gases. The plasma-sharpened tips of nanotubes contain only a few tubes at the apex of the structure and lead to the dramatic enhancement in the emission current density by a factor >10(6) with the onset field as low as 0.16 V/microm. We propose that the nature of the tunneling barrier changes significantly for a nanosize tip at very high local electric field and may lead to the saturation in the emission current density.

Enhanced Field Emission and Improved Supercapacitor Obtained from Plasma‐Modified Bucky Paper

The surface morphology of bucky papers (BPs) made from single-walled carbon nanotubes (CNTs) is modified by plasma treatment resulting in the formation of vertical microstructures on the surface. The shapes of these structures are either pillarlike or conelike depending on whether the gas used during plasma treatment is Ar or CH(4) . A complex interplay between different factors, such as the electric field within the plasma sheath, polarization of the CNT, intertubular cohesive forces, and ion bombardment, result in the formation of these structures. The roles played by these factors are quantitatively and qualitatively analyzed. The final material is flexible, substrate-free, composite-free, made only of CNTs, and has discrete vertically aligned structures on its surface. It shows enhanced field emission and electrochemical charge-storage capabilities. The field enhancement factor is increased by 6.8 times, and the turn-on field drops by 3.5 times from an initial value of 0.35 to 0.1 V μm(-1) as a result of the treatment. The increase in Brunauer-Emmett-Teller surface area results in about a fourfold improvement in the specific capacitance of the BP electrodes. Capacitance values before and after the treatments are 75 and 290 F g(-1) , respectively. It is predicted that this controlled surface modification technique could be put to good use in several applications based on macroscopic CNT films.

Facile one-step transfer process of graphene

Chemical vapour deposition (CVD) is emerging as a popular method for growing large-area graphene on metal substrates. For transferring graphene to other substrates the technique generally used involves deposition of a polymer support with subsequent etching of the metal substrate. Here we report a simpler one-step transfer process. Few-layer graphene (FLG) grown on a Cu substrate were transferred to a silanized wafer by just pressing them together. Hydrogen bonding between the hydroxyl group on FLG and the amine group on silane molecules facilitate the transfer.

Effect of top metal contact on electrical transport through individual multiwalled carbon nanotubes

The effect of position of top metal contact on the electrical transport through individual multiwalled carbon nanotubes (MWNTs) has been investigated using gas injection system in situ in scanning electron microscope to deposit the top platinum metal contacts at different desired sites on the side contacted MWNTs in bridging structure. Current-voltage measurements reveal a significant improvement in electrical properties of the tubes after the top contact is made. This improvement has been found to be independent of position of top contact, i.e., whether the top contact is made on the ends or at any other site of the tube.

Investigation of Metal-Induced Enhancement in Electrical Conductance of Multiwalled Carbon Nanotubes

We here segregate the contributions of contact improvement and change in multiwalled carbon nanotubes (MWNT) inherent properties in electrical conductance enhancement caused by metal deposition. The conductance of individual MWNTs enhances greatly due to the platinum and tungsten deposition even at the locations of the tube where no beneath metal contact is present. The change in conductance is explained in terms of the change in the density of states at Fermi level, due to charge transfer between metal atoms and nanotube as well as by radial stress created on the tube. This type of improved conduction is different from the high bias-assisted tunneling type carrier transport and in our study, even at zero bias an increment of 140% in the typical conductance value has been experimentally observed. These results are important for electronic device perspective of nanowire research, mainly the interconnect applications in real electronic devices.

Healing of Broken Multiwalled Carbon Nanotubes Using Very Low Energy Electrons in SEM: A Route Toward Complete Recovery

We report the healing of electrically broken multiwalled carbon nanotubes (MWNTs) using very low energy electrons (3-10 keV) in scanning electron microscopy (SEM). Current-induced breakdown caused by Joule heating has been achieved by applying suitably high voltages. The broken tubes were examined and exposed to electrons of 3-10 keV in situ in SEM with careful maneuvering of the electron beam at the broken site, which results in the mechanical joining of the tube. Electrical recovery of the same tube has been confirmed by performing the current-voltage measurements after joining. This easy approach is directly applicable for the repairing of carbon nanotubes incorporated in ready devices, such as in on-chip horizontal interconnects or on-tip probing applications, such as in scanning tunneling microscopy.

Ceasing of voltage switching amongst graphitic shells in multiwalled carbon nanotubes: A route toward stability

Weakly interacting graphitic shells of different resistivities within multiwalled carbon nanotubes (MWNTs) cause instability in current and thus limit their reliability for electronic device applications. We here demonstrate voltage switching amongst graphitic shells of MWNTs by applying current sweeps with observed switching time in the range of 100–400 ms. We further demonstrate ceasing of this switching behaviour by local metal deposition on the MWNTs. After metal deposition, the graphitic shells behave like resistive wires connected altogether. This concept of metal deposition benefits in the higher conductivity and stable currents for MWNTs and proves their strong candidature as interconnecting wires.

Structural Changes In Nickel Nanaorods Encapsulated In Carbon Nanotubes By 200 KV Electron Irradiation In TEM

Nickel nanorods encapsulated inside multiwalled carbon nanotubes were synthesized using microwave plasma chemical vapor deposition (MPCVD) technique. HRTEM study of these tubes exhibits long, good crystalline nickel filled CNTs with well‐graphitized walls. Different orientations of filled metal with respect to graphitic walls have been observed and the orientations were found to get affected by the electron accelerating voltages of 200 KV. We report that by deliberate e‐irradiation at specific sites the crystalline planes of nickel can change their orientation. With continuous irradiation for 10–15 minutes the nickel starts evolving out of the tube. We propose that metal at the tip of the tube may be exposed by employing e‐beam irradiation and it may be useful to study the true electrical properties of both the multiwalled carbon nanotubes and Ni nanorod inside the tube.