S. Dash

Surface optical modes in GaN nanowires

We investigate the optical phonons in crystalline GaN nanowires using Raman spectroscopy. Reduced phonon lifetime in the nanostructures is attributed to the increased anisotropy in lattice vibrations. Apart from the group theoretically allowed optical phonons, new phonon modes around 652 cm−1 and 691 cm−1 have been observed. In view of its good agreement with values in GaN, the observed phonon mode is assigned as surface optical (SO) phonon. This could be attributed to the surface modulation along the GaN nanowire diameter and it is quantitatively evaluated with observed surface morphology and the calculated dispersion relation corresponding to SO phonon modes. The modulation in the surface morphology, observed in the present study, is typical of the vapour-liquid-solid growth process. The instability in the surface phonon potential activates the SO phonon modes, which is well explained in this present study.

Humidity-dependent friction mechanism in an ultrananocrystalline diamond film

Friction behaviour of an ultrananocrystalline diamond film deposited by the microwave plasma-enhanced chemical vapour deposition technique is studied in a controlled humid atmosphere. The value of friction coefficient consistently decreases while increasing the humidity level during the tribology test. This value is 0.13 under 10% relative humidity conditions, which is significantly decreased to 0.004 under 80% relative humidity conditions. Such a reduction in friction coefficient is ascribed to passivation of dangling covalent bonds of carbon atoms, which occurs due to the formation of chemical species of hydroxyl and carboxylic groups such as C?COO, CH3COH and CH2?O bonding states.

Energy efficient reduced graphene oxide additives: Mechanism of effective lubrication and antiwear properties

Optimized concentration of reduced graphene oxide (rGO) in the lube is one of the important factors for effective lubrication of solid body contacts. At sufficiently lower concentration, the lubrication is ineffective and friction/wear is dominated by base oil. In contrast, at sufficiently higher concentration, the rGO sheets aggregates in the oil and weak interlayer sliding characteristic of graphene sheets is no more active for providing lubrication. However, at optimized concentration, friction coefficient and wear is remarkably reduced to 70% and 50%, respectively, as compared to neat oil. Traditionally, such lubrication is described by graphene/graphite particle deposited in contact surfaces that provides lower shear strength of boundary tribofilm. In the present investigation, graphene/graphite tribofilm was absent and existing traditional lubrication mechanism for the reduction of friction and wear is ruled out. It is demonstrated that effective lubrication is possible, if rGO is chemically linked with PEG molecules through hydrogen bonding and PEG intercalated graphene sheets provide sufficiently lower shear strength of freely suspended composite tribofilm under the contact pressure. The work revealed that physical deposition and adsorption of the graphene sheets in the metallic contacts is not necessary for the lubrication.

Response of magnetron sputtered AlN films to controlled atmosphere annealing

The present investigation deals with the examination of the response of amorphous AlN films to post-deposition annealing environments such as high vacuum (HV) and nitrogen atmosphere (NA). The c/a ratio values from GIXRD for both cases are around 1.602. The XPS profile of NA-AlN shows a deficiency of nitrogen on the surface, whereas the oxygen impurity level is negligible in the case of NA compared with HV. The PL spectra substantiate the nitrogen vacancies in NA-AlN. The amorphous AlN exhibits a nanoindentation hardness of 18 GPa.

Mass spectrometry based evolved gas analysis system for thermal decomposition studies

An experimental facility for evolved gas analysis by mass spectrometry (EGA-MS) has been built in-house and extensively used to study the temperature programmed decomposition (TPD) of a number of inorganic solids. Fractional extent of reactionα acquired from real time multiple ion detection trend analysis mass spectra of gases released from thermally impressed specimen has been used to obtain functional transformf(α) of non-isothermal solid state kinetic rate expressions. The corresponding model integral functionsg(α) based on mechanisms like random nucleation, diffusion and interface motion have been used to establish kinetics control regimes for specific decomposition sequences. From ln[g(α)/T2] vs 1/T plots Arrhenius parameters like activation energy and pre-exponential factor could be determined. Signature of the rate controlling mechanism governing the gas release behaviour was found in the crystallographic transformation brought about by the temperature programme. This paper describes the scope and capabilities of our EGA-MS facility with typical results on temperature programmed decomposition of CuSO4·5H2O and AlNH4(SO4)2·12H2O.

Investigation of temperature dependent dielectric constant of a sputtered TiN thin film by spectroscopic ellipsometry

The temperature dependence of optical constants of titanium nitride thin film is investigated using Spectroscopic Ellipsometry (SE) between 1.4 and 5 eV in the temperature range of 300 K to 650 K in steps of 50 K. The real and imaginary parts of the dielectric functions e1(E) and e2(E) marginally increase with increase in temperature. A Drude Lorentz dielectric analysis based on free electron and oscillator model are carried out to describe the temperature behavior. With increase in temperature, the unscreened plasma frequency and broadening marginally decreased and increased, respectively. The parameters of the Lorentz oscillator model also showed that the relaxation time decreased with temperature while the oscillator energies increased. This study shows that owing to the marginal change in the refractive index with temperature, titanium nitride can be employed for surface plasmon sensor applications even in environments where rise in temperature is imminent.

Enhancement of electron field emission properties of TiO2−x nanoplatelets by N-doping

Thin films of TiO2−x and TiO2−x−yN2y nano-platelets having better field emission properties compared to one dimensional nanostructures reported in literature were synthesized by spray pyrolysis. Low threshold field of 8 V μm−1 and 4.1 V μm−1, for 1 mA cm2, was observed for TiO2−x and TiO2−x−yN2y respectively, implying enhanced field emission upon N-doping.

Tribological properties of nanocrystalline diamond films deposited by hot filament chemical vapor deposition

The dependence of the structural and morphological properties of nanocrystalline diamond films grown by hot filament chemical vapor deposition on the substrate temperature was studied. Friction coefficients of these films were measured and found to vary from high to ultra low, depending on the chemical nature of the films i.e., sp2 and sp3 phase fractions. For all films, the friction coefficient was found to decrease with increase in sp2/sp3 phase fraction. The wear rate follows the trend of the friction coefficient and was likewise found to depend on the structural and morphological properties of the films. For all the films, the friction coefficient is found to decrease with normal load which is ascribed to sliding induced surface amorphization/graphitization.

Reactive magnetron sputtered wear resistant multilayer transition metal carbide coatings: microstructure and tribo-mechanical properties

To enhance the performance and durability of mechanical components, surface properties need to be modified. In this work, a unique combination of transition metal carbide (TMC) multilayer coatings (TiC/CrC, TiC/ZrC and TiC/WC) were deposited by reactive DC magnetron sputtering on 316LN steel substrates. GIXRD results showed the presence of an amorphous CrC phase in the TiC/CrC multilayer, whereas, crystalline TiC, ZrC and W2C phases were observed in their respective coatings. FESEM analysis indicated the non-columnar structures of TMC layers in all the samples, except the CrC layer in the TiC/CrC multilayer. Solid solutions of the TiC and WC layers at the interface were identified due to the migration of W atoms into the TiC crystal lattices. The poor crystalline nature of the TiC/CrC multilayer led to lower hardness and weak wear resistance. However, enhanced tribo-mechanical properties were observed in TiC/ZrC and TiC/WC multilayer coatings. This is explained by the improved crystallinity and enhanced resistance to plastic deformation. For the selected tribological parameters, abrasive, adhesive and combined abrasive/adhesive wear modes are the major governing factors in determining the wear behaviors of TiC/CrC, TiC/ZrC and TiC/WC coatings, respectively. The chemical stability of deformed wear tracks was observed by Raman spectroscopy.

Facile gamma radiolytic synthesis of synergistic Co3O4-rGO nanocomposite: direct use in photocatalytic water splitting

Nanostructured Co3O4 was synthesized through a gamma (γ)-radiolysis technique using alcoholic (iso-propanol) salt solutions of cobalt ions with and without suspended graphene oxide (GO) nanoparticles, respectively. Formation of Co3O4 accompanied with GO reduction takes place simultaneously (in situ) upon γ-ray exposure carried out at a rate of 5.1 KGy h−1. Reduction of GO and formation of Co3O4 were confirmed by XRD, Raman and UV–vis spectroscopy. XRD and HRTEM results supports the embedding of nano-crystalline Co3O4 in an amorphous matrix. Relatively larger crystallites of cobalt oxide obtained in the presence of rGO proved to be a decisive supporting material for the directional growth of Co3O4. Electrochemical characterization established the fact that rGO is indeed considered as a proficient medium for electrochemical electron transfer process. Photo-assisted H2 generation studies using Co3O4 and Co3O4-rGO nano-composite yielded 3 and 30 μmol h−1 g−1 of hydrogen (H2) generation, respectively, supports the action of rGO as an electron trap.