Soumyendu Roy

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.

High-strength Zr-Nb-(Cu,Ni,Al) composites with enhanced plasticity

Zr73.5Nb9Cu7Ni1Al9.5 and Zr66.4Nb6.4Cu10.5Ni8.7Al8.0 composites of bcc β-Zr(Nb) dendrites embedded in a nanocrystalline matrix were prepared by slow cooling from melt. The increase of Nb content from 6.4 to 9 at. % slightly reduces the strength, but considerably improves the plastic elongation under uniaxial compressive loading from ep=0.6% to 14.8%. The interaction of strain with dendrites and the nanocrystalline matrix is suggested as origin of the improvement of the mechanical properties.

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.

SIMS and RBS study of thermally annealed Pd/β-SiC interfaces

The Pd/β-SiC interfaces were studied using secondary ion mass spectrometry (SIMS) and Rutherford backscattering spectrometry (RBS). This was done with the intent of clarifying any reaction or inter-diffusion at the interface upon prolonged annealing at different temperatures in air. SIMS study indicates that the interface is stable up to 400 °C for at least 12 h. However, at 800 °C, the interface was completely degraded with significant inter-diffusion of palladium and silicon. The RBS study confirms the SIMS observations.

Ohmic contacts to 3C-SiC for Schottky diode gas sensors

Abstract The basic objective of this work was to investigate a suitable ohmic metallization to 3C-SiC epitaxial layer with relatively low process temperature, which is important for on-chip circuit integration. In this study, the ohmic contacts are associated with the catalytic-metal/SiC Schottky diode gas sensors operable at 400 °C. Therefore, the contacts must be reliable for efficient operation of the sensors at this temperature in air ambient. Several metallization schemes were studied and the observed electrical characteristics were correlated to the interface properties by secondary ion mass spectrometry. Nickel contacts to moderately doped (∼10 17 cm −3 ) 3C-SiC epilayer did not exhibit linear current–voltage relationship even after annealing at 600 °C. On the other hand, aluminum exhibited considerably low intercontact resistance in the as-deposited condition. But with increase in annealing temperature the current flow between a couple of Al contacts was impeded presumably due to very high affinity of this metal for oxygen. Titanium exhibited good ohmic properties after annealing at 400 °C. However, higher temperature (600 °C) annealing in air ambient (intended to examine its stability limit) caused surface oxidation of the Ti layer. This problem was minimized by deposition of 100 nm Au overlayer (on top of 400 nm Ti) which would increase the surface conductivity of the ohmic contacts and ensure stable wire bonding.

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.

Novel In Situ Nanostructure-Dendrite Composites in Zr-Base Multicomponent Alloy System

Abstract The evolution of a nanostructure-dendrite composite microstructure of two Zr-base alloys solidified through different casting routes is presented. The alloys were designed by adding different amounts of Nb to the Zr-based multicomponent glass-forming alloy system. The refractory metal Nb promotes the formation of a primary phase having dendritic morphology, whereas the residual melt solidifies to a nanostructured/amorphous matrix. The volume fraction and the morphology of the dendritic phase varied with the Nb content and the adopted casting route. A correlation between the alloy composition and adopted casting method with evolved microstructures and mechanical properties is revealed. These composites exhibit a unique combination of high fracture strength up to 1922 Mpa, as well as plastic strain over 15.8% under uniaxial compression testing at room temperature. The high strength of these composites is imparted by the nanostructured matrix, whereas the large plastic strain is a consequence of the retardation of excessive localized shear banding in the matrix by ductile dendrites. The significant work hardening of the composites prior to fracture is attributed to dislocation multiplication in the solid solution-strengthened dendritic phase.