arXiv subjects

K. G. M. Nair

Publications and source records attributed to K. G. M. Nair.

14 recordsLinked to original sources

arXiv2008

Flux dependent 1.5 MeV self-ion beam induced sputtering from Gold nanostructured thin films

We discuss four important aspects of 1.5 MeV Au2+ ion-induced flux dependent sputtering from gold nanostrcutures (of an average size 7.6 nm and height 6.9 nm) that are deposited on silicon substrates: (a) Au sputtering yield at the ion flux of 6.3x10^12 ions cm-2 s-1 is found to be 312 atoms/ion which is about five times the sputtering yield reported earlier under identical irradiation conditions at a lower beam flux of 10^9 ions cm-2 s-1, (b) the sputtered yield increases with increasing flux at lower fluence and reduces at higher fluence (1.0x10^15 ions cm-2) for nanostructured thin films while the sputtering yield increases with increasing flux and fluence for thick films (27.5 nm Au deposited on Si) (c) Size distribution of sputtered particles has been found to vary with the incident beam flux showing a bimodal distribution at higher flux and (d) the decay exponent obtained from the size distributions of sputtered particles showed an inverse power law dependence ranging from 1.5 to 2.5 as a function of incident beam flux. The exponent values have been compared with existing theoretical models to understand the underlying mechanism. The role of wafer temperature associated with the beam flux has been invoked for a qualitative understanding of the sputtering results in both the nanostructured thin films and thick films.

cond-mat.mtrl-sci↗

arXiv2008

Ion beam induced enhanced diffusion from gold thin films in silicon

Enhanced diffusion of gold atoms into silicon substrate has been studied in Au thin films of various thicknesses (2.0, 5.3, 10.9 and 27.5 nm) deposited on Si(111) and followed by irradiation with 1.5 MeV Au2+ at a flux of 6.3x10^12 ions cm-2 s-1 and fluence up to 1x10^15 ions cm-2. The high resolution transmission electron microscopy measurements showed the presence of gold silicide formation for the above-mentioned systems at fluence greater than equal to 1x1014 ions cm-2. The maximum depth to which the gold atoms have been diffused at a fluence of 1x10^14 ions cm-2 for the cases of 2.0, 5.3, 10.9 and 27.5 nm thick films has been found to be 60, 95, 160 and 13 nm respectively. Interestingly, at higher fluence of 1x1015 ions cm-2 in case of 27.5 nm thick film, gold atoms from the film transported to a maximum depth of 265 nm in the substrate. The substrate silicon is found to be amorphous at the above fluence values where unusually large mass transport occurred. Enhanced diffusion has been explained on the basis of ion beam induced, flux dependent amorphous nature of the substrate, and transient beam induced temperature effects. This work confirms the absence of confinement effects that arise from spatially confined structures and existence of thermal and chemical reactions during ion irradiation.

cond-mat.mtrl-sci↗

arXiv2008

Flux dependent MeV self-ion- induced effects on Au nanostructures: Dramatic mass transport and nano-silicide formation

We report a direct observation of dramatic mass transport due to 1.5 MeV Au2+ ion impact on isolated Au nanostructures of an average size 7.6 nm and a height 6.9 nm that are deposited on Si (111) substrate under high flux (3.2x10^10 to 6.3x10^12 ions cm-2 s-1) conditions. The mass transport from nanostructures found to extend up to a distance of about 60 nm into the substrate, much beyond their size. This forward mass transport is compared with the recoil implantation profiles using SRIM simulation. The observed anomalies with theory and simulations are discussed. At a given energy, the incident flux plays a major role in mass transport and its re-distribution. The mass transport is explained on the basis of thermal effects and creation of rapid diffusion paths at nano-scale regime during the course of ion irradiation. The unusual mass transport is found to be associated with the formation of gold silicide nanoalloys at sub-surfaces. The complexity of the ion-nanostructure interaction process has been discussed with a direct observation of melting (in the form of spherical fragments on the surface) phenomena. The transmission electron microscopy, scanning transmission electron microscopy and Rutherford backscattering spectroscopy methods have been used.

cond-mat.mtrl-sci↗

arXiv2007

Optical characterization of GaN by N+ implantation into GaAs at elevated temperature

Both hexagonal wurtzite and cubic zinc blend GaN phases were synthesized in GaAs by 50 keV N+ implantation at 400 deg C and subsequent annealing at 900 deg C for 15 min in N2 ambient. Crystallographic structural and Raman scattering studies revealed that GaN phases were grown for fluence above 2x1017 cm-2. Temperature-dependent photoluminescence study showed sharp direct band-to-band transition peak ~3.32 eV at temperature <= 200K. The intermediate bandgap value, with respect to ~3.4 eV for hexagonal and ~3.27 eV for cubic phases of GaN is an indicative for the formation of mixed hexagonal and cubic phases.

cond-mat.mtrl-sci↗

arXiv2007

Nitrogen ion beam synthesis of InN in InP (100) at elevated temperature

InN phase is grown in crystalline InP(100) substrates by 50 keV N+ implantation at an elevated temperature of 400 deg C followed by annealing at 525 deg C in N2 ambient. Crystallographic structural and Raman scattering studies are performed for the characterization of grown phases. Temperature- and power-dependent photoluminescence studies show direct band-to-band transition peak ~1.06 eV at temperatures <=150K. Implantations at an elevated temperature with a low ion beam current and subsequent low temperature annealing step are found responsible for the growth of high-quality InN phase.

cond-mat.mtrl-sci↗

arXiv2007

Ferromagnetism in cobalt doped n-GaN

Ferromagnetic ordering is reported in the post-annealed samples of Co doped n-GaN formed by Co+ implantation. A maximum Curie temperature ~ 250K is recorded for the sample with 8 atomic percent Co. Particle induced x-ray emission-channeling study confirmed the substitutional Co in Ga lattice site. Local atomic arrangement around magnetic impurities is also analyzed using Raman study. A disordered model with carrier mediated coupling of localized magnetic moments is made responsible for the observed ferromagnetic ordering.

cond-mat.mtrl-sci↗

arXiv2007

Multi-phonon Raman scattering in GaN nanowires

UV Raman scattering studies show longitudinal optical (LO) mode up to 4th order in wurtzite GaN nanowire system. Frohlich interaction of electron with the long range electrostatic field of ionic bonded GaN gives rise to enhancement in LO phonon modes. Good crystalline quality, as indicated by the crystallographic as well as luminescence studies, is thought to be responsible for this significant observation. Calculated size dependence, incorporating size corrected dielectric constants, of electron-phonon interaction energy agrees well with measured values and also predict stronger interaction energy than that of the bulk for diameter below ~3 nm.

cond-mat.mtrl-sci↗

arXiv2006

Study of ion beam induced mixing in nano-layered Si/C multilayer structures

The effects of ion beam induced atomic mixing and subsequent thermal treatment in Si/C multilayer structures are investigated by use of the technique of grazing incidence X-ray diffraction (GIXRD) and Raman spectroscopy. The [Si (3.0 nm) / C (2.5 nm)]x10 /Si multilayer films were prepared by electron beam evaporation under ultra high vacuum (UHV) environment. The layer thicknesses were measured using in-situ quartz crystal oscillator. These multilayer films were subjected to 40 keV Ar+ ion irradiation with fluences 5E-16 (low fluence) and 1E-17 ions / cm2 (high fluence).The as-prepared and irradiated multilayer samples were annealed at 773 K for one hour. The GIXRD and Raman spectroscopy results reveal the formation of different phases of SiC in these multilayer structures. Deposition induced reactions at the nano-structured interface and subsequent room temperature Ar ion irradiation at low fluence result in formation of the hexagonal SiC phase. High fluence Ar+ ion irradiation and subsequent annealing at 773 K for one hour leads to precipitation of the cubic-SiC phase.

cond-mat.mtrl-sci↗

arXiv2005

Optical absorption and photoluminescence spectroscopy of the growth of silver nanoparticles

Results obtained from the optical absorption and photoluminescence (PL) spectroscopy experiments have shown the formation of excitons in the silver-exchanged glass samples. These findings are reported here for the first time. Further, we investigate the dramatic changes in the photoemission properties of the silver-exchanged glass samples as a function of postannealing temperature. Observed changes are thought to be due to the structural rearrangements of silver and oxygen bonding during the heat treatments of the glass matrix. In fact, photoelectron spectroscopy does reveal these chemical transformations of silver-exchanged soda glass samples caused by the thermal effects of annealing in a high vacuum atmosphere. An important correlation between temperature-induced changes of the PL intensity and thermal growth of the silver nanoparticles has been established in this Letter through precise spectroscopic studies.

cond-mat.mtrl-sci↗

arXiv2004

'Spillout' effect in gold nanoclusters embedded in c-Al2O3(0001) matrix

Gold nanoclusters are grown by 1.8 MeV Au^\sup{2+} implantation on c-Al\sub{2}O\sub{3}(0001)substrate and subsequent air annealing at temperatures 1273K. Post-annealed samples show plasmon resonance in the optical (561-579 nm) region for average cluster sizes ~1.72-2.4 nm. A redshift of the plasmon peak with decreasing cluster size in the post-annealed samples is assigned to the 'spillout' effect (reduction of electron density) for clusters with ~157-427 number of Au atoms fully embedded in crystalline dielectric matrix with increased polarizability in the embedded system.

cond-mat.mtrl-sci↗

arXiv2004

Blue luminescence of Au nanoclusters embedded in silica matrix

Photoluminescence study using the 325 nm He-Cd excitation is reported for the Au nanoclusters embedded in SiO2 matrix. Au clusters are grown by ion beam mixing with 100 KeV Ar+ irradiation on Au [40 nm]/SiO2 at various fluences and subsequent annealing at high temperature. The blue bands above ~3 eV match closely with reported values for colloidal Au nanoclusters and supported Au nanoislands. Radiative recombination of sp electrons above Fermi level to occupied d-band holes are assigned for observed luminescence peaks. Peaks at 3.1 eV and 3.4 eV are correlated to energy gaps at the X- and L-symmetry points, respectively, with possible involvement of relaxation mechanism. The blue shift of peak positions at 3.4 eV with decreasing cluster size is reported to be due to the compressive strain in small clusters. A first principle calculation based on density functional theory using the full potential linear augmented plane wave plus local orbitals (FP-LAPW+LO) formalism with generalized gradient approximation (GGA) for the exchange correlation energy is used to estimate the band gaps at the X- and L-symmetry points by calculating the band structures and joint density of states (JDOS) for different strain values in order to explain the blueshift of ~0.1 eV with decreasing cluster size around L-symmetry point.

cond-mat.mtrl-sci↗

arXiv2004

Acoustic vibrations of a silica-embedded gold nanoparticle: elastic anisotropy

Spherical gold nanoclusters are grown by 1.8 MeV Au++ ion implantation into an amorphous silica matrix and subsequent air annealing at 873 K. Ultraviolet and visible light absorption confirms the presence of a dipolar surface plasmon peak at the expected location for a gold sphere in silica. Grazing incidence X-ray diffraction shows peaks corresponding to fcc gold with lattice spacing close to that of bulk gold, as well as allowing estimation of cluster size using Scherrer's formula. Low frequency Raman scattering reveals a relatively narrow peak, suggesting a narrow distribution of nanocrystal diameters. Acoustic phonon frequencies corresponding to the spheroidal quadrupolar vibrations of a continuum sphere with the anisotropic elasticity of gold are calculated using a novel method of molecular dynamics and extrapolation to the continuum limit using relatively small numbers of point masses. The study confirms high energy ion implantation as a method capable of producing gold nanocrystals with spherical shape and well controlled size.

cond-mat.other↗

arXiv2004

Growth of silver nanoclusters embedded in soda glass matrix

Temperature-controlled-growth of silver nanoclusters in soda glass matrix is investigated by low-frequency Raman scattering spectroscopy. Growth of the nanoclusters is ascribed to the diffusion-controlled precipitation of silver atoms due to annealing the silver-exchanged soda glass samples. For the first time, Rutherford backscattering measurements performed in this system to find out activation energy for the diffusion of silver ions in the glass matrix. Activation energy for the diffusion of silver ions in the glass matrix estimated from different experimental results is found to be consistent.

cond-mat.mtrl-sci↗

arXiv2004

Blueshift of plasmon resonance with decreasing cluster size in Au nanoclusters embedded in silica matrix

Gold nanoclusters are grown by 1.8 MeV Au++ implantation in silica matrix and subsequent air annealing in the temperature range of 873K-1273K. Post-annealed samples show plasmon resonance in the optical region (~ 2.38-2.51 eV) for average cluster radii ~1.04-1.74 nm. A blueshift of the plasmon peak is observed with decreasing cluster size in the annealed samples. Similar trend of blueshift with decreasing cluster size in case of Au nanoclusters embedded in the porous alumina matrix [B. Palpant, et al., Phys. Rev. B 57, 1963 (1998)] has convinced us to assume a possible role of 'rind' like porosity at the Au nanocluster-matrix interface with available open volume defects in the amorphous silica matrix.

cond-mat.mtrl-sci↗