Alka Ingale

Swift heavy ion induced modification of C60 thin films

Modification of thin film samples of C60 on Si and quartz substrates, induced by irradiation of 110 MeV Ni ions at various fluences, was studied. The pristine and irradiated samples were investigated using Raman spectroscopy, electrical conductivity, and optical absorption spectroscopy. The Raman data and band gap measurements indicate that swift ions at low fluences result in aggregate formations involving multiple molecular units such as dimers or polymers. High fluence irradiation results in submolecular formations and amorphous semiconducting carbon, indicating overall damage (fragmentation) of the fullerene molecules. These submolecular units in the amorphous carbon network have been identified as nanocrystalline graphite.

Role of electron energy loss in modification of C60 thin films by swift heavy ions

This paper presents a comparative study of the effects of irradiation by swift heavy ions (SHIs) with Se values ranging from 80 to 1270 eV/A and fluence ranges varying between 1010 and 1014 ions/cm2 incident on thin films of C60. The control over Se is exercised through the choice of ion species for irradiation (O, Ni, and Au). Structural changes in C60 were investigated quantitatively using Raman spectroscopy. The results indicate that at low fluences polymer formation takes place whereas at high fluences there is complete fragmentation of C60, resulting in amorphous carbon formation. Measured values of band gap and in situ resistivity decrease with fluence. This result is consistent with the structural modifications observed by Raman spectroscopy. The composition of the polymer fraction formed (e.g., the content of two dimensional polymerized network of C60 molecules) as well as that of a-C (e.g., the content of nanographite) also vary with Se of the ion used. A phenomenological model, taking into accou...

Structural and particulate to bulk phase transformation of CdS film on annealing: A Raman spectroscopy study

We have studied the effect of annealing (at temperatures 300, 400, and 500 °C) on nanoparticulate CdS film on silicon substrate deposited by hybrid Langmuir–Blodgett chemical bath deposition technique. Width, intensity, and the line shape of the longitudinal optical phonon in Raman spectra and photoluminescence (PL) spectra suggest that the annealing brings about coalescence of nanoparticles at 300 °C, increasing the size of the nanoparticles and also increases the red region PL related to surface states. Annealing at 400 °C leads to two different coexistent states, showing larger nanoparticles and bulklike film. On further annealing at 500 °C, PL shows a drastic redshift to PL of bulklike CdS. This is well brought out by resonance Raman measurements and later confirmed by scanning electron microscope data. Most interesting observation is that PL and Raman results together are indicative of phase transformation of nanoparticulate hexagonal CdS film to bulklike cubic CdS film on annealing at 500 °C, unlike...

A comparative study on nanotextured high density Mg-doped and undoped GaN

Nanotextured high density Mg-doped and undoped GaN were obtained using photoelectrochemical etching. Interesting features are observed in the temperature dependent photoluminescence (PL) studies of these nanotextured materials. First, the PL intensity of the excitonic emissions shows more than three orders of enhancement. At low temperature, the peak energy shows a blueshift with temperature. This phenomenon is attributed to the formation of excitonic band-tail states. Second, the excitonic emissions in the nanotextured samples are redshifted compared to the as-grown GaN suggesting strain relaxation. Third, the blue luminescence band (2.7–2.9eV in Mg-doped GaN) shows a large redshift, which is not consistent with strain relaxation calculated from excitonic band. Furthermore, temperature dependence of the blue luminescence band energy shows an asymmetric S-shaped behavior in nanotextured GaN. All these observations are explained by invoking an increase in carrier localization due to an increase in potentia...

Crystalline quality of ZnSe thin films grown on GaAs by pulsed laser deposition in He and Ar ambients

Abstract Epitaxial films of ZnSe with a thickness of approximately 1 μm, were deposited on (001) semi-insulating GaAs by pulsed laser deposition using a third harmonic Nd-YAG laser at 10 −6 torr vacuum and at various pressures of He and Ar gases. The best crystalline quality of the ZnSe epilayer was obtained when the deposition was carried out at 10 −4 torr of He. The full width at half maximum of the X-ray diffraction rocking curve of the ZnSe film deposited at 400°C in 10 −4 torr of He was approximately 230 arcsecs. A comparison of the reciprocal lattice space mappings showed that ZnSe epilayers deposited in Ar ambient have a much larger variation of lattice spacing and strain than those grown in vacuum and He ambient. The scanning electron micrographs showed that the epilayers of ZnSe deposited at 10 −4 torr of He have the least particulate density. Possible reasons for the observed improvement in the crystalline quality are discussed.

Time evolution studies of laser induced chemical changes in InAs nanowire using Raman spectroscopy

We report the study of time evolution of chemical changes on the surface of an InAs nanowire (NW) on laser irradiation in different power density regime, using Raman spectroscopy for a time span of 8–16 min. Mixture of metastable oxides like InAsO4, As2O3 are formed upon oxidation, which are reflected as sharp Raman peaks at ∼240–254 and 180–200 cm−1. Evidence of removal of arsenic layer by layer is also observed at higher power density. Position controlled laser induced chemical modification on a nanometer scale, without changing the core of the NW, can be useful for NW based device fabrication.

Resonance Raman mapping as a tool to monitor and manipulate Si nanocrystals in Si-SiO2 nanocomposite

Specially designed laser heating experiment along with Raman mapping on Si-SiO2 nanocomposites elucidates the contribution of core and surface/interface in the intermediate frequency range (511–514 cm−1) Si phonons. The contribution of core to surface/interface increases with the size of Si nanocrystal, which itself increases on laser irradiation. Further, it is found that resonance Raman is crucial to the observance of surface/interface phonons and wavelength dependent Raman mapping can be corroborated with band edges observed in absorption spectra. This understanding can be gainfully used to manipulate and characterize Si-SiO2 nanocomposite, simultaneously for photovoltaic device applications.

Intersubband plasmon-phonon coupling in GaAsP/AlGaAs near surface quantum well

The investigation of electron-phonon coupling in near surface GaAs1−xPx/AlyGa1−yAs quantum well structures using wavelength and intensity dependent Raman spectroscopy shows that in the near surface quantum well case, coupled modes are situated at the frequency between longitudinal optical (LO) and transverse optical phonons of GaAsP, which is forbidden for the coupled electron-phonon modes in the bulk ternary alloy. The observed “GaAs like” and “GaP like” LO phonon-intersubband plasmon coupled mode frequencies decrease with increase in carrier density. These results corroborate well with the theoretical calculation for variation of two dimensional electron gas-phonon coupling with carrier density in ternary alloys.

Intersubband Plasmon ‐ Phonon Coupling in GaAsP/AlGaAs Single Quantum Well: A Raman Spectroscopy Study

A Raman scattering study of GaAs1−yPy (y = 0.14)/AlxGa1−xAs (x = 0.7) single quantum wells has shown two mode behavior for both barrier and quantum well alloys. In this report we present investigation of origin of an additional mode ∼284 cm−1. From wavelength and intensity dependent Raman measurements, it was concluded that the mode is intersubband plasmom‐phonon coupled mode due to photoexcited carriers. The coupled mode frequency dependence on free carrier density as predicted by theory is qualitatively in agreement with observed coupled mode frequencies in intensity dependent Raman measurements.

Study of InAs nanowire structure using spatially resolved Raman spectroscopy

The structure of tapered InAs nanowire (diameter ∼ 400 to 200 nm from base to tip) along the length (10 µm) is studied using spatially resolved Raman spectroscopy. The observed Raman spectra suggest presence of both wurtzite (WZ) and zincblende (ZB) structure for InAs with increasing content of wurtzite structure towards tip. Polarized Raman measurements shows dominance of E2h (WZ) ∼ 213 cm−1 in x(y, y)x and dominance of TO (ZB) ∼ 215.5 cm−1 x(z, z)x configuration for light polarization perpendicular and parallel to nanowire axis, respectively. The blue and red shift in WZ and ZB phonons (base and center) from their bulk value is attributed to changed near neighbor interaction due to presence of the other structure. However, the blue shift of both E2h (WZ) and TO (ZB) near tip is indicative of additional mechanical compressive stress.