J. Ghatak

Synthesis and characterization of ZnO thin film grown by electron beam evaporation

Highly transparent, conducting, highly oriented, and almost single phase ZnO films have been deposited by simple e-beam evaporation method, and the deposition parameters were optimized. The films were prepared by (a) evaporation of ZnO at different substrate temperatures and (b) evaporation of ZnO at room temperature and subsequent annealing of the films in oxygen ambient at different temperatures. The characterizations of the film were performed by optical absorption spectroscopy (UV-visible), Fourier transform infrared spectroscopy, resistivity measurement, transmission electron microscopy (TEM), photoluminescence, and x-ray diffraction measurement. Absorption spectra revealed that the films were highly transparent and the band gap of the pre- and postannealed films was in good agreement with the reported values. The band gap of the films increases on increasing the substrate temperature as well as annealing temperature, whereas the resistivity of the film decreases with substrate temperature and increa...

Synthesis of gold-silicon core-shell nanoparticles with tunable localized surface plasmon resonance

Gold-silicon core-shell nanoparticles embedded in silica matrix, evident by transmission electron microscopy and x-ray photoelectron spectroscopy were synthesized by atom beam cosputtering followed by thermal annealing. Optical absorption studies revealed localized surface plasmon resonance (LSPR), which showed regular redshift from 500to583nm with increase in annealing temperature. The observed redshifts in the LSPR peaks are in close agreement with the theoretical calculations assuming Si nanoshells surrounding Au nanoparticles. The Au–Si core-shell formation is explained by Au–Si liquid nanodroplet formation at temperatures higher than the eutectic temperature, followed by phase separation during subsequent cooling.

High-resolution transmission electron microscopy mapping of nickel and cobalt single-crystalline nanorods inside multiwalled carbon nanotubes and chirality calculations

The nickel and cobalt nanorods of the diameters in the range of 6–20nm with lengths of 0.29–0.9μm are formed using multiwalled carbon nanotubes as templates. The nickel and cobalt nanorods as described in our letter are perfect single crystals inside the nanotube with their Miller planes inclined with respect to the tube axis in a particular fashion. The (111) planes of face-centered-cubic nickel and cobalt are inclined at angles 39.6° and 39.4°, respectively, while the hexagonal-closed-packed cobalt (002) planes incline at an angle 53.4°. The inclination of these planes is studied in detail and results are discussed in terms of elastic energy and surface tension. The chirality of the carbon nanotubes, in intimate contact with the nanorod, is determined using the mapping of Ni and C atoms in a graphene sheet. We believe this could pave a way for synthesizing the tubes with known chirality.

One-step method for the self-assembly of metal nanoparticles onto facetted hollow silica tubes

An optimized synthetic approach has been developed for the exclusive formation of facetted silica tubes using a simple organic molecule i.e. ammonium tartrate as a template. The synergy of two phenomena: (a) aggregation of ammonium tartrate and (b) interaction of a hydrolysed TEOS intermediate with the ammonium tartrate aggregate, was achieved under optimized conditions (i.e. 0 °C and pH ca. 11) which resulted in the complete formation of facetted silica tubes as the sole morphology. The typical length and width of these facetted tubes were of the order of 60–100 µm and 0.4–1.2 µm, respectively. Furthermore, a novel one-step method has been developed to anchor Au and Ag nanoparticles onto these tubes. This involves surfactant coated nanoparticles and the anchoring does not necessitate the pre-functionalization of the silica walls. The method is fast, economic and in addition it can be adapted for the synthesis of other hybrid nanomaterials.

Formation and growth of SnO2 nanoparticles in silica glass by Sn implantation and annealing

Nanocrystalline Sn particles have been formed in silica glass through 50 keV Sn− implantation followed by annealing in N2 at 650 °C for 30 min. Samples prepared this way have been annealed in air for 1 h, separately at four different temperatures, 400, 600, 800, and 1000 °C, each at a given temperature. Annealing at temperatures higher than 400 °C has been found to result in oxidation of the Sn nanoparticles (NPs) and formation of the SnO2 phase as confirmed from optical absorption (OA), transmission electron microscopy, and Raman scattering measurements. For the sample annealed at 600 °C, Raman scattering data showed three bands at about 525, 629, and 771 cm−1, the last two corresponding to the A1g and B2g classical Raman modes of rutile SnO2. Increase in annealing temperature resulted in an increase in the intensities of the A1g and B2g modes showing better crystallinity. Also, the A1g peak shifted toward a higher wave number with a steady decrease in the intensity at 525 cm−1. This is in line with the ...

Recrystallization of ion-irradiated germanium due to intense electronic excitation

Germanium single crystals were irradiated at room temperature by 1.5MeV energy germanium ions and high energy silver ions of 100MeV. Based on the transmission and high-resolution electron microscopic investigations, we present the experimental evidence of complete recrystallization of the amorphized germanium layer, formed by the self-ion-implantation, due to intense electronic excitations generated by the swift Ag ions. This phenomenon is observed at room temperature—far below the solid phase epitaxial growth temperature and that at which low energy ion beam induced epitaxial crystallization takes place. The results are explained in the light of local transient melting due to a high rate of energy deposition by the silver ions and its subsequent cooling. Based on the calculations on thermal spike concept in combination with the nonequilibrium thermodynamics, we obtain a reasonably good estimate for the experimental observation.

MeV heavy ion induced recrystallization of buried silicon nitride layer: Role of energy loss processes

We report on MeV heavy ion beam induced epitaxial crystallization of a buried silicon nitride layer. Transmission electron micrographs and selected area diffraction patterns are used to study the recrystallization of an ion beam synthesized layer. We observe complete recrystallization of the silicon nitride layer having good quality interfaces with the top and substrate Si. Recrystallization is achieved at significantly lower temperatures of 100, 150, and 200°C for oxygen, silicon, and silver ions, respectively. The fact that recrystallization is achieved at the lowest temperature for the oxygen ions is discussed on the basis of the energy loss processes.

Observation of a universal aggregation mechanism and a possible phase transition in Au sputtered by swift heavy ions.

Two exponents delta for the size distribution of n-atom clusters, Y(n) approximately n{-delta}, have been found in Au clusters sputtered from embedded Au nanoparticles under swift heavy ion irradiation. For small clusters, below 12.5 nm in size, delta has been found to be 3/2, which can be rationalized as occurring from a steady state aggregation process with size independent aggregation. For larger clusters, a delta value of 7/2 is suggested, which might come from a dynamical transition to another steady state where aggregation and evaporation rates are size dependent. In the present case, the observed decay exponents do not support any possibility of a thermodynamic liquid-gas-type phase transition taking place, resulting in cluster formation.

Anomalous diffusion of Au in mega-electron-volt Au implanted SiO2∕Si(100)

Thermal annealing induced redistribution behavior of Au (3 MeV, 6.0×1015 ions cm−2), implanted into SiO2∕Si(100) substrates, has been investigated using Rutherford backscattering spectrometry and cross-sectional transmission electron microscopy (XTEM). Sequential annealing in the temperature range of 550−750 °C has been found to result in rejection of Au atoms from Si toward the SiO2∕Si interface. Above 750 °C a significant fraction of the implanted Au atoms has been found to back diffuse into deeper regions, well below the projected range Rp of Au. Direct annealing of a sample at 850 °C also shows a similar anomalous diffusion of Au into deeper regions, well below Rp. However, direct annealing at 1050 °C has been found to result in an enhanced accumulation of Au at a deeper layer. XTEM micrographs for the same sample show the presence of Au-rich nanoparticles and dislocations, decorated with Au-rich nanoparticles in this region. Trails of Au-Si liquid nanodroplets along with dislocations extending into t...

Size distribution of sputtered particles from Au nanoislands due to MeV self-ion bombardment

Nanoisland gold films, deposited by vacuum evaporation of gold onto Si(100) substrates, were irradiated with 1.5MeV Au2+ ions up to a fluence of 5×1014ionscm−2 and at incidence angles up to 60° with respect to the surface normal. The sputtered particles were collected on carbon-coated grids (catcher grid) during ion irradiation and were analyzed with transmission electron microscopy and Rutherford backscattering spectrometry. The average sputtered particle size and the areal coverage are determined from transmission electron microscopy measurements, whereas the amount of gold on the substrate is found by Rutherford backscattering spectrometry. The size distributions of larger particles (number of atoms/particle, n⩾1000) show an inverse power law with an exponent of ∼−1 in broad agreement with a molecular-dynamics simulation of ion impact on cluster targets.Nanoisland gold films, deposited by vacuum evaporation of gold onto Si(100) substrates, were irradiated with 1.5MeV Au2+ ions up to a fluence of 5×1014ionscm−2 and at incidence angles up to 60° with respect to the surface normal. The sputtered particles were collected on carbon-coated grids (catcher grid) during ion irradiation and were analyzed with transmission electron microscopy and Rutherford backscattering spectrometry. The average sputtered particle size and the areal coverage are determined from transmission electron microscopy measurements, whereas the amount of gold on the substrate is found by Rutherford backscattering spectrometry. The size distributions of larger particles (number of atoms/particle, n⩾1000) show an inverse power law with an exponent of ∼−1 in broad agreement with a molecular-dynamics simulation of ion impact on cluster targets.