D. S. Misra

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.

Hydrogen in chemical vapour deposited diamond films

Abstract Thin films of diamond were deposited at 20, 40 and 70 Torr by MPCVD and at 40, 100 and 140 Torr by HFCVD while keeping the substrate temperature constant. The films were characterised using micro-Raman spectroscopy, X-Ray diffraction (XRD) and scanning electron microscopy (SEM). Micro-Raman spectra show a sharp peak centered near the natural diamond line (≈ 1332 cm −1 ) accompanied by a non-diamond band at ≈ 1450 − 1550 cm −1 . Intense peaks corresponding to (111), (400) and (311) reflections are observed in XRD. Diamond crystallites with sharp facets are seen in SEM. Plasma diagnostics in the MPCVD system were also performed at various conditions in order to estimate the H atom concentration in the gas phase. Elastic recoil detection analysis was used to measure the H concentration. It is observed that the H concentration shows contrasting trends in the films grown by MPCVD and HFCVD at various growth pressures. H concentration is found to be 1.0, 0.8 and 0.7at % in the films grown at 20, 40 and 70 Torr, respectively, in MPCVD apparatus. Conversely, the H concentration in the films grown by HFCVD increases systematically with growth pressure (0.5, 1.1 and 1.4 at % at 40, 100 and 140 Torr, respectively). Non-diamond carbon impurities correlate well with H content. Results of micro-Raman spectroscopy, SEM and plasma diagnostic are discussed in the framework of a growth model. Results of H content in the diamond films obtained by various techniques are compared.

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.

High density of multiwalled carbon nanotubes observed on nickel electroplated copper substrates by microwave plasma chemical vapor deposition

Abstract High density and long multiwalled carbon nanotubes are deposited using nickel clusters formed from an electrodeposited nickel coating treated by ammonia. Tubes of length 40–50 μm having bamboo shaped structure are aligned nearly parallel to the copper substrate. The inner diameter of the tubes is in the range 20–50 nm. The selected area and nano area diffraction spots clearly show the crystalline nature of nickel clusters. At some locations of the samples in the vicinity of graphite layers, Ni 3 C phase is also detected. The formation of the Ni 3 C phase that is unstable at high temperature supports the idea of a vapor–liquid–solid (VLS) mechanism for the growth of the tubes.

Nature of confinement of phonons in nanocrystalline CVD diamond

Abstract Self-standing nanocrystalline CVD diamond sheets are investigated using Raman spectroscopy and X-ray diffraction. The sheets are composed of diamond phase and an amorphous-carbon-like non-diamond phase whose content increases with increasing gas pressure in the growth chamber. In the samples synthesised at high pressures the 1332 cm−1 diamond Raman peak exhibits asymmetric broadening. Although the scattering from defects can account for the symmetric part of the line-width, the asymmetry is much more than expected for a confined phonon line-shape calculated according to the Gaussian confinement model for the particle sizes obtained from X-ray diffraction. An alternative model for the confinement of phonons is discussed which involves the standing waves arising from the reflection of the phonons from the elastic-dielectric boundary. The calculated line-shapes arising from the contributions from the allowed q-points in the Brillouin-zone are found to agree with the measured spectra.

Chemical vapour deposition of diamond on stainless steel: the effect of Ni-diamond composite coated buffer layer

Abstract Diamond films were grown on nickel and nickel-diamond composite coated stainless steel substrates using microwave plasma chemical vapour deposition (MPCVD). X-ray diffraction (XRD), scanning electron microscopy and Raman spectroscopy studies give clear evidence for the growth of diamond on buffer layered substrates. XRD results further suggest that after CVD treatment the interface nickel ( a = 3.522 A) shows an increase in the lattice constant ( a = 3.563 A). The unit cell expansion can be attributed to the formation of the Ni-C solid solution phase. The new unit cell parameter of Ni match closely with diamond ( a = 3.566 A). In addition, the embedded diamond particles assist in faster nucleation and growth of the diamond crystallites on composite coated substrates.

Stoichiometry of the diamond/silicon interface and its influence on the silicon content of diamond films

Thin films of diamond were grown by microwave plasma chemical vapor deposition at growth pressures of 10, 20, 40, and 60 Torr keeping the substrate temperature constant at 975 °C. Increase in the growth pressure reduced the size of the plasma ball resulting in an increase in the microwave power density (MPD). The films were characterized by scanning electron microscopy, micro-Raman, and photoluminescence (PL) spectroscopy. A systematic variation was observed in surface morphology and quality of the films. The intensity of the peak at 1.68 eV in the PL spectra of the films, which is assigned to Si impurities was also observed to increase consistently with the MPD. The stoichiometry of the diamond/silicon interface was studied by x-ray photoelectron spectroscopy (XPS) and found to be a sensitive function of the MPD. XPS results showed the formation of nonstoichiometric SiC along with other carbon phases in the initial stages of the growth. A correlation was observed between the composition of the interface ...

Studies of defects and impurities in diamond thin films

Abstract Diamond thin films were grown on silicon substrates using microwave plasma chemical vapour deposition at various microwave power densities (MPD). Three sets of the films were grown for various thicknesses. The films were characterised using micro-Raman spectroscopy, photoluminescence (PL), cathodoluminescence (CL) and X-ray photoelectron spectroscopy (XPS). Elastic recoil detection analysis (ERDA) was used to determine concentrations of light impurities (N and O). Micro-Raman spectroscopy shows a systematic variation in the non-diamond to diamond carbon content with MPD. Various defect centers related mainly with nitrogen were observed in PL and CL spectra of the films. A sharp feature was observed at 1.68 eV in all the spectra. This peak is attributed to Si impurity in diamond films. Interestingly, the intensity of the peak increases with increase in MPD. The broad band A in the CL spectra has contributions from both green and blue regions, and the intensity ratio of the green to blue region varies with MPD. The spectra of the films of various thickness were also compared, and it was found that the Si content of the films decreases with increasing thickness. ERDA results indicate that the films contain 0.1–0.6% N and O as impurities. The interface composition of the films as a function of MPD was investigated using XPS, and the increase in the Si content of the films was correlated with the change in composition of the diamond/silicon interface.

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.

Enhancement of (100) texture in diamond films grown using a temperature gradient

Abstract Diamond films with dominant (100) texture were grown with a temperature gradient across the Si (100) substrates using hot filament chemical vapor deposition technique. Deposition was carried out with 0.8% CH 4 in balance hydrogen at an average substrate temperature of 880 °C. The deposition pressure was varied between 20–120 torr. Films were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM) and Fourier Transform Infrared Spectroscopy (FTIR). XRD shows very strong (400) reflection in all the samples. SEM results show a smooth diamond surface comprised of (100) platelets. As the (100) diamond plates were grown on top of the (100) oriented silicon substrate the faces are more or less aligned parallel with the substrate surface, resulting in a relatively smoother diamond surfaces. FTIR studies show novel features in the films. Quantitative analysis was carried out to measure the H content in the films.