D. C. Agarwal

Crystal growth behaviour in Au-ZnO nanocomposite under different annealing environments and photoswitchability

The growth of gold nanoparticles and ZnO nanorods in atom beam co-sputtered Au-ZnO nanocomposite (NC) system by annealing at two different ambient conditions is demonstrated in this work. Annealing in a furnace at 600 °C (air environment) confirmed the formation of ZnO nanorods surrounded with Au nanoparticles. In-situ annealing inside a transmission electron microscope (TEM) led to the formation of gold nanocrystals with different polygonal shapes. TEM micrographs were obtained in real time at intermediate temperatures of 300 °C, 420 °C, and 600 °C under vacuum. The growth mechanisms of Au nanocrystals and ZnO nanorods are discussed in the framework of Au-Zn eutectic and Zn-melting temperatures in vacuum and air, respectively. Current-voltage responses of Au-ZnO NC nanorods in dark as well as under light illumination have been investigated and photoswitching in Au-ZnO NC system is reported. The photoswitching has been discussed in terms of Au-ZnO band-diagram.

Swift heavy ion induced structural and optical modifications in LiF thin film

Thermally deposited 200 nm polycrystalline films of lithium fluoride (LiF) grown on glass substrates were irradiated with 150 MeV Ag ions at various fluences between 1 × 1011 and 2 × 1013 ions cm−2. The irradiation induced structural and optical modifications were studied using glancing angle x-ray diffraction (GAXRD), optical absorption and photoluminescence (PL) spectroscopy. The GAXRD results show that the films are polycrystalline and the average grain size (estimated from the widths of the GAXRD peak using the Scherrer formula) decreases systematically from 46.3 nm for the pristine sample to 18.3 nm for the sample irradiated at a fluence of 3 × 1012 ions cm−2. Thereafter, it remains constant. This reduction is attributed to strain induced fragmentation of grains. The optical absorption studies show dominant absorption bands of F3 (385 nm) and F2 (445 nm) colour centres. It is observed that the concentration of the colour centres increases with ion fluence and gets saturated at higher fluences. This can be correlated with GAXRD results in the sense that as the density of grain boundaries increases the concentration of colour centres also increases. The variation with fluence in PL intensities of the F2 and colour centres is studied. The intensity of both bands (F2 and ) increases up to a fluence of 1 × 1012 ions cm−2, followed by an exponential decrease, which is due to the increase in the non-radiative transition rate in the presence of defect-rich material.

Electronic excitation induced tuning of surface plasmon resonance of Ag nanoparticles in fullerene C70 matrix

We report the electronic excitation induced controlled tuning of the surface plasmon resonance (SPR) wavelength of Ag nanoparticles (NPs) in fullerene C70 matrix. The transformation of fullerene C70 into amorphous carbon (a:C) under ion irradiation is used to tune the SPR wavelength of C70–Ag nanocomposite thin films. A 100 nm blue shift was recorded for irradiation at a fluence of 3 × 1013 ions cm−2 by 120 MeV Ag ions. A growth of Ag NPs from 7.0 ± 0.8 to 11.0 ± 0.4 nm with increasing fluence was observed by transmission electron microscopy and it is explained in the framework of thermal spike model. The transformation of fullerene C70 into amorphous carbon with ion irradiation was confirmed by Raman spectroscopy. This work demonstrates the possibility to locally excite the SPR at a desired wavelength and therefore, acquiring multiple SPR bands at a single substrate which could be useful in developing more efficient optical sensors.

Formation of self-affine nanostructures on ZnO surfaces by swift heavy ions

The topography evolution using the high energy heavy ion irradiation revealed the fact that ion bombardment produces self-affine nanostructures, creating peculiar surface morphologies and regular structures on the surface of the ZnO thin film at certain fluences. The self-affine nanopatterns produced on the surface of ZnO thin film under swift heavy ion irradiation are different types of nanostructures such as nanodimensional grains aligned like a linear array. Scanning probe microscopy is used for investigating the ZnO surfaces and UV-visible spectroscopy for studying the effect of surface modification on optical properties. The two-dimensional power spectral density of the irradiated ZnO thin films have been evaluated for each image to extract the value of growth factor (β) and roughness exponent (α). The exponent n increases from 2.2 to 4.0 up to a critical value of fluence and beyond which it decreases. These values suggest that ion assisted∕induced diffusion process plays a crucial role in the evolution of self-affine nanostructures on ZnO surface.The topography evolution using the high energy heavy ion irradiation revealed the fact that ion bombardment produces self-affine nanostructures, creating peculiar surface morphologies and regular structures on the surface of the ZnO thin film at certain fluences. The self-affine nanopatterns produced on the surface of ZnO thin film under swift heavy ion irradiation are different types of nanostructures such as nanodimensional grains aligned like a linear array. Scanning probe microscopy is used for investigating the ZnO surfaces and UV-visible spectroscopy for studying the effect of surface modification on optical properties. The two-dimensional power spectral density of the irradiated ZnO thin films have been evaluated for each image to extract the value of growth factor (β) and roughness exponent (α). The exponent n increases from 2.2 to 4.0 up to a critical value of fluence and beyond which it decreases. These values suggest that ion assisted∕induced diffusion process plays a crucial role in the evolut...

Thermal and ion induced annealing of nanocrystalline ZnO thin film deposited by atom beam sputtering

ZnO thin films were deposited using atom beam sputtering and their modifications have been shown by two processes: (a) thermal annealing of ZnO thin films in oxygen and (b) athermal annealing by irradiation of these films by swift heavy ions (SHIs) in a high vacuum chamber. The as-deposited films showed the nanocrystalline nature with a preferred orientation along the c-axis of the hexagonal structure as revealed by x-ray diffraction (XRD) and Raman spectra. The influence of the thermal annealing and athermal annealing on the structural and surface modifications of these thin films were investigated. XRD and Raman spectroscopy confirmed the improvement in the crystallinity of ZnO thin film by both thermal annealing and SHI irradiation. The Zn–O bonding was confirmed by Fourier transform infrared spectroscopy and the interpretation of IR spectra corroborated the XRD and Raman results. Surface morphology was investigated by atomic force microscopy. The AFM study of the films implied no significant change in the roughness of the films in both types of annealing conditions. It was concluded that the modification of nanocrystalline ZnO thin film could be possible by both thermal and athermal annealing. Results indicate that transient annealing by SHI irradiation induces the highly textured c-axis oriented ZnO thin film for device applications, comparable to those of high temperature annealing.

Synthesis, characterizations, and thermal induced structural transformation of silver-fullerene C60 nanocomposite thin films for applications in optical devices

Nanocomposite thin films of fullerene C60 containing Ag nanoparticles (NPs) were synthesized by thermal codeposition. The surface plasmon resonance (SPR) band of the nanocomposite film was observed in the region 450–550 nm, showing a large redshift with increasing metal concentration. This is explained by the Maxwell–Garnett effective medium theory considering the absorbing nature of fullerene C60. The C60–Ag nanocomposite thin film with lowest Ag concentration was annealed at increasing temperatures in neutral atmosphere. The SPR band showed first a small redshift after annealing at small temperature then progressive blueshift at higher temperature. This behavior of SPR is explained by the increased particle–particle interaction due to the compaction of the fullerene C60 film upon annealing at low temperature and the transformation of fullerene C60 matrix into amorphous carbon at higher temperature. Rutherford backscattering spectrometry and transmission electron microscopy were used to quantify Ag metal...

A study on the formation of Ag nanoparticles on the surface and catcher by ion beam irradiation of Ag thin films

Irradiation of Ag thin films with 100 MeV Ag ions leads to the formation of Ag nanoparticles on the surface as well as on the catcher, due to electronic energy loss mediated sputtering of Ag. The experimentally determined sputter yield of Ag is found to be three orders of magnitude higher than the values expected for bulk Ag, which is explained on the basis of the inelastic thermal spike model. The confinement of energy in the nanoparticles having size smaller than the electron mean free path (λ) and higher surface coverage area results in a higher sputtering yield. Transmission electron microscopy was performed to study the size distribution of nanoparticles on the catcher. The variation of sputtered particle yield with the number of constituent atoms follows an inverse power law with the value of exponent (δ) ~ 0.33, at a fluence of 1 × 1013 ions cm−2. With increase in fluence up to 1 × 1014 ions cm−2, an additional value of exponent of δ ≈ 1 arises. The size of Ag nanoparticles is decreased with increased fluence due to ion-induced sputtering. The irradiated sample is found to have partially embedded nanoparticles showing localized surface plasmon resonance.

Smoothing, roughening and sputtering: the complex evolution of immiscible Fe/Bi bilayer system

In this work, swift heavy ion (SHI) induced surface smoothing, roughening and sputtering of thermally immiscible Fe/Bi bilayer system has been investigated. The pristine and irradiated samples were analysed by Rutherford backscattering spectrometry (RBS), grazing angle x-ray diffraction (XRD), atomic force microscopy and scanning electron microscopy including x-ray dispersive energy analyzer. RBS analysis revealed that steepness of the low energy edge of the Bi signal increases at a fluence of 3 ? 1013?ions?cm?2, beyond which the slope of the rear edge decreases. The increased steepness is due to smoothing induced at initial fluence; however, the decrease in the slope of rear edge beyond 3 ? 1013?ions?cm?2 fluence is a result of surface roughening. XRD reveals the increase in the crystalline nature of Bi after irradiation at 3 ? 1013?ions?cm?2. Irradiation at higher fluences from 6 ? 1013 to 1 ? 1014?ions?cm?2 leads to a decrease in the crystalline nature of Bi. Surface roughness of pristine and irradiated samples from AFM analysis revealed that initially roughness decreases with a fluence of 3 ? 1013?ions?cm?2. However, at higher fluences, beyond 3 ? 1013?ions?cm?2, the agglomeration of smaller grains has been observed due to the shear flow mechanism, which results in surface roughening. The observed behaviour of surface smoothing and roughening under SHI irradiation may be explained on the basis of the thermal spike model.

Tailoring of structural and electron emission properties of CNT walls and graphene layers using high-energy irradiation

Structural and electron emission properties of carbon nanotubes (CNTs) and multilayer graphene (MLG) are tailored using high-energy irradiation by controlling the wall thickness and number of layers. Ion irradiation by 100 MeV Ag+ ions at different fluences is used as an effective tool for optimizing defect formation in CNTs and MLGs, as analysed by micro-Raman spectroscopy. It is found that the cross section for defect formation (η) is 3.5 × 10−11 for thin-walled CNTs, 2.8 × 10−11 for thick-walled CNTs and 3.1 × 10−11 for MLGs. High-resolution transmission electron microscopy results also show that thin-walled CNTs and MLGs are more defective in comparison with thick-walled CNTs. Carbon atoms rearrange at a fluence of 1 × 1012 ions cm−2 in thick-walled CNTs to heal up the damage, which aggravates at higher fluences. The observed electron emission parameters of the modified thin-walled CNTs and MLGs are confirmed with the changes in the structures and are optimized at a fluence of 1 × 1011 ions cm−2. However, the electron emission properties of thick-walled CNTs are modified at a fluence of 1 × 1012 ions cm−2. The enhancement in the electron emission properties is due to the rearrangement of bonds and hence modified tips due to irradiation.

Quasi-aligned gold nanodots on a nanorippled silica surface: experimental and atomistic simulation investigations

Quasi-aligned gold nanodots with a periodicity of ∼ 40 nm have been synthesized on a silica substrate by oblique deposition of gold on fast argon atom-beam-created nanoripples of wavelength 40 nm and subsequent annealing. The size distribution of these aligned nanodots resulting from oblique deposition at 85° of 0.5 nm Au film perpendicular to ripples is narrower than the similar deposition on a flat surface. The deposition and annealing process was simulated with a three-dimensional kinetic lattice Monte Carlo technique in order to understand the formation of aligned nanodots. The atomistic simulation and the experimental results suggest that there is an optimal thickness which can result in nanodots aligned along the ripples in the case of depositions perpendicular to the ripples. The nanodots formed after annealing of the films deposited parallel to ripples or on flat surface lack alignment.