B. K. Panigrahi

Structural and electronic properties of nitrogen ion implanted ultra nanocrystalline diamond surfaces

Enhanced electron field emission (EFE) properties have been observed for nitrogen implanted ultra-nanocrystalline diamond (UNCD) films grown by microwave plasma enhanced CVD. X-ray photoelectron spectroscopy (XPS) measurements show that sp2 fraction and C-N bonding increase upon N-implantation and annealing. Significant difference in current-voltage (I-V) curves at the grain and grain boundary has been observed from scanning tunneling spectroscopic (STS) measurement. From the variation of normalized conductance (dI/dV)/(I/V) versus V, bandgap is measured to be 4.8 eV at the grain and 3.8 eV at the grain boundary for as prepared UNCD. Upon nitrogen implantation and annealing, the bandgap decreases for both grain and grain boundary and density of states are introduced in the bandgap. Current imaging tunneling spectroscopy (CITS) imaging shows that the grain boundaries have higher conductivity than the grains and are the prominent electron emitters. The enhancement in EFE properties upon nitrogen implantatio...

Surface plasmon enhanced photoluminescence and Raman scattering of ultra thin ZnO-Au hybrid nanoparticles

We report substantially enhanced photoluminescence (PL) and Raman signal from ZnO nanoparticles (NPs) deposited on Si with Au NPs. The resonant excitation of surface plasmon of Au NPs is responsible for the enhanced PL intensity and Raman signal from the ZnO-Au hybrid NPs. Low temperature PL measurements exhibit 14 fold enhancements in the PL intensity of ZnO-Au sample in comparison with ZnO/Si sample. It is found that, at low temperature, the donor bound exciton (DX) has dominant contribution in the PL emission of ZnO-Au hybrid NPs. At high temperature, the free exciton (FX) has dominant contribution in PL emission of these NPs. The thermal activation energies of DX and FX are found to be 5 meV and 50 meV, respectively. The defect related visible PL emission of ZnO is completely suppressed in the ZnO-Au hybrid NPs.

Quantification of oxide particle composition in model oxide dispersion strengthened steel alloys.

Oxide dispersion strengthened ferritic steels (ODS) are being considered for structural components of future designs of fission and fusion reactors because of their impressive high-temperature mechanical properties and resistance to radiation damage, both of which arise from the nanoscale oxide particles they contain. Because of the critical importance of these nanoscale phases, significant research activity has been dedicated to analysing their precise size, shape and composition (Odette et al., Annu. Rev. Mater. Res. 38 (2008) 471-503 [1]; Miller et al., Mater. Sci. Technol. 29(10) (2013) 1174-1178 [2]). As part of a project to develop new fuel cladding alloys in India, model ODS alloys have been produced with the compositions, Fe-0.3Y2O3, Fe-0.2Ti-0.3Y2O3 and Fe-14Cr-0.2Ti-0.3Y2O3. The oxide particles in these three model alloys have been studied by APT in their as-received state and following ion irradiation (as a proxy for neutron irradiation) at various temperatures. In order to adequately quantify the composition of the oxide clusters, several difficulties must be managed, including issues relating to the chemical identification (ranging and variable peak-overlaps); trajectory aberrations and chemical structure; and particle sizing. This paper presents how these issues can be addressed by the application of bespoke data analysis tools and correlative microscopy. A discussion follows concerning the achievable precision in these measurements, with reference to the fundamental limiting factors.

Ultraviolet and blue photoluminescence from sputter deposited Ge nanocrystals embedded in SiO2 matrix

Ge nanocrystals (NCs) embedded in silicon dioxide (SiO2) matrix are grown by radio-frequency magnetron sputtering and studied in order to understand the origin of ultraviolet (UV) and blue photoluminescence (PL) from the NC-SiO2 system. Ge NCs of diameter 7–8 nm are formed after postdeposition annealing, as confirmed by transmission electron microscopy and Raman scattering studies. Optical Raman studies indicate the presence of strain in the embedded Ge NCs. Polarization dependent low frequency Raman studies reveal surface symmetrical and surface quadrupolar acoustic phonon modes of Ge NCs. PL studies with 488 nm excitation shows a broad emission band peaked at ∼545 nm, which is attributed to oxygen deficient defects in the SiO2 matrix. PL studies with 325 nm excitation show additional strong peaks in the 377–400 nm region. Time resolved PL studies in the UV-blue range show double exponential decay dynamics in the nanosecond time scale, irrespective of the NC size. Comparative studies of PL emission from ...

Raman scattering studies of cobalt nanoclusters formed during high energy implantation of cobalt ions in a silica matrix

Nanoscale cobalt clusters are synthesized in a silica glass matrix through implantation of high energy cobalt ions. Surface acoustic symmetrical vibrational modes of cobalt nanoclusters are detected using Raman spectroscopy. Mode intensity is found to depend strongly on excitation wavelengths. X-ray diffraction studies reveal face-centered-cubic phase of cobalt nanoclusters in postannealed samples. Postannealing in vacuum has led to significant growth of the cobalt nanoclusters in the matrix.

The induction of nanographitic phase on Fe coated diamond films for the enhancement in electron field emission properties

A thin layer of iron coating and subsequent post-annealing (Fe-coating/post-annealing) is seen to significantly enhance the electron field emission (EFE) properties of ultrananocrystalline diamond (UNCD) films. The best EFE properties, with a turn on field (E0) of 1.98 V/μm and current density (Je) of 705 μA/cm2 at 7.5 V/μm, are obtained for the films, which were Fe-coated/post-annealed at 900 °C in H2 atmosphere. The mechanism behind the enhanced EFE properties of Fe coated/post-annealed UNCD films are explained by the microstructural analysis which shows formation of nanographitic phase surrounding the Fe (or Fe3C) nanoparticles. The role of the nanographitic phase in improving the emission sites of Fe coated/post-annealed UNCD films is clearly revealed by the current imaging tunneling spectroscopy (CITS) images. The CITS images clearly show significant increase in emission sites in Fe-coated/post-annealed UNCD films than the as-deposited one. Enhanced emission sites are mostly seen around the boundarie...

Creation of self-organized gold nanostructures by keV ion beam irradiation

We synthesized Au nanostructures on glass by low-energy ion irradiation of a thin Au film. The Au films, deposited on a glass substrate, were irradiated by 50 keV Si− ions with different fluences. The UV–vis absorption spectroscopy of irradiated samples indicates the surface plasmon resonance peak, which is a signature of the formation of Au nanostructures. The surface morphology of the films has been characterized by atomic force microscopy, which supports the UV–vis spectroscopy findings. The metal content or the film thickness of the samples, as measured by Rutherford backscattering spectroscopy, decreased with increasing fluence, showing sputtering by ion irradiation and tailing in the lower energy edge with irradiation, revealing recoil implantation. The formation of Au nanostructures by this method can be explained on the basis of an interplay between surface instability due to ion beam sputtering and surface diffusion.

Direct observation of enhanced emission sites in nitrogen implanted hybrid structured ultrananocrystalline diamond films

A hybrid-structured ultrananocrystalline diamond (h-UNCD) film, synthesized on Si-substrates by a two-step microwave plasma enhanced chemical vapour deposition (MPECVD) process, contains duplex structure with large diamond aggregates evenly dispersed in a matrix of ultra-small grains (∼5 nm). The two-step plasma synthesized h-UNCD films exhibit superior electron field emission (EFE) properties than the one-step MPECVD deposited UNCD films. Nitrogen-ion implantation/post-annealing processes further improve the EFE properties of these films. Current imaging tunnelling spectroscopy in scanning tunnelling spectroscopy mode directly shows increased density of emission sites in N implanted/post-annealed h-UNCD films than as-prepared one. X-ray photoelectron spectroscopy measurements show increased sp2 phase content and C–N bonding fraction in N ion implanted/post-annealed films. Transmission electron microscopic analysis reveals that the N implantation/post-annealing processes induce the formation of defects in...

Direct observation and mechanism of increased emission sites in Fe-coated microcrystalline diamond films

The electron field emission (EFE) properties of microcrystalline diamond (MCD) films are significantly enhanced due to the Fe coating and post-annealing processes. The 900 °C post-annealed Fe coated diamond films exhibit the best EFE properties, with a turn on field (E0) of 3.42 V/μm and attain EFE current density (Je) of 170 μA/cm2 at 7.5 V/μm. Scanning tunnelling spectroscopy (STS) in current imaging tunnelling spectroscopy mode clearly shows the increased number density of emission sites in Fe-coated and post-annealed MCD films than the as-prepared ones. Emission is seen from the boundaries of the Fe (or Fe3C) nanoparticles formed during the annealing process. In STS measurement, the normalized conductance dI/dVI/V versus V curves indicate nearly metallic band gap, at the boundaries of Fe (or Fe3C) nanoparticles. Microstructural analysis indicates that the mechanism for improved EFE properties is due to the formation of nanographite that surrounds the Fe (or Fe3C) nanoparticles.

Realization of magnetic anisotropy and L10 CoPt ordered phase by Pt+ ion irradiation on a Co/Pt bilayer film

Ion-beam synthesis of a CoPt layer was achieved by Pt+ ion irradiation on Pt/Co bilayer films, and further growth and ordering of the CoPt layer was seen upon 4 MeV Si+ ion irradiation at 300 °C. Rutherford backscattering (RBS) results show that the ion-beam mixing increases as a function of Pt+ ion fluence, and at a fluence of 1015 ions cm−2 the formation of an ordered face-centered tetragonal (FCT) CoPt phase in the mixed region was clearly observed from the grazing-incidence x-ray diffraction (GIXRD) measurements. The magneto-optic Kerr effect results show that the coercivity increases with increase in Pt+ ion fluence.