arXiv subjects

S. Amirthapandian

Publications and source records attributed to S. Amirthapandian.

15 recordsLinked to original sources

arXiv2025

Charge compensation by phase segregated Sb2Se3 phase in Bi1.95Sb0.05Se3 topological insulator by laser fluence

We report the charge compensation in topological insulator thin films due to the laser fluence-induced segregation of the Sb2Se3 phase. Sb doped Bi2Se3 films were deposited on commercial Si substrates coated with 300 nm of amorphous SiO2 using the Pulsed Laser Deposition technique at different laser fluence of 1.25 J-cm-2, 1.87 J-cm-2, 2.75 J-cm-2, and 3.25 J-cm-2. The grazing x-ray diffraction measurements revealed the growth of rhombohedral Bi2Se3 films on SiO2 with additional peaks corresponding to the Sb2Se3 peaks. The phase fraction of the segregated Sb2Se3 systematically reduces as the laser fluence increases. The magnetoresistance measured at temperatures 3K-20K was compared across different samples deposited at various laser fluences. The sample deposited at 1.25 J-cm-2 exhibits a bulk-insulating nature with an order decrease in the carrier density. The weak antilocalization quantum correction to the magneto conductivity was analyzed using the Hikami Larkin Nagaoka Theory. As the laser fluence increases the deduced value of coefficient alpha, number of conduction channels was found to be 0.5, 1.5, 2.5, and 5.0, and the quantum phase coherence length lphi ranges from 10 nm to 150 nm. The power factor gamma of the temperature dependence of lphi also varies as 0.37, 0.45, 0.69, and 0.92, respectively. The charge compensation due to the non-TI, ptype Sb2Se3 phase is believed to cause the reduction in the bulk carrier density n, and enhanced surface state contribution.

cond-mat.mtrl-sci↗

arXiv2018

Nano-molar to milli-molar level Ag (I) determination using absorption of light by ZnS QDs without organic ligand

Despite of useful applications of Ag, it is hazardous to health and environment and hence early detection is required. Crystal defects influencing optical property is less likely utilized for Ag detection using prevalent UV-Vis technique. Quantum dots (QDs) ZnS prepared by soft chemical route are exploited for the detection of Ag ions in aqueous solution for nM to mM concentrations. UV-Vis and photoluminescence (PL) measurements reveal quantum confinement effect and presence of defects in ZnS nanoparticles (NPs). Controlled synthesis of ZnS NPs allows appearance of defects related peak at 400-550 nm in UV-Vis spectra. A plausible mechanism is presented and elucidated by XRD and PL studies for chemical interaction between ZnS with Ag ion. The interaction affects prominently and defects related absorption of ZnS NPs and allows single step Ag ion detection in aqueous medium. This absorption method is extended to other ions (Na, K, Mg, Ca, Ba ions) including S prone heavy and toxic ions like Pb, Hg and Cd. Among them, Ag ion shows the best response with ZnS NPs. This demonstration using a portable and cost effective technique with no organic component opens up possibility for other inorganic materials.

cond-mat.mtrl-sci↗

arXiv2017

In-situ formation of SiGe alloy by electron beam evaporation and the effect of post deposition annealing on the energy band gap

We report the synthesis of polycrystalline (poly)-SiGe alloy thin films through solid state reaction of Si/Ge multilayer thin films on Si and glass substrates at low temperature of 500 °C. The pristine thin film was deposited using electron beam evaporation with optimized in-situ substrate heating. Our results show the co-existence of amorphous Si (a-Si) phase along with the poly-SiGe phase in the pristine thin film. The a-Si phase was found to subsume into the SiGe phase upon post deposition annealing in the temperature range from 600 to 800 °C. Additionally, dual energy band gaps could be observed in the optical properties of the annealed poly-SiGe thin films. The stoichiometric evolution of the pristine thin film and its subsequent effect on the band gap upon annealing are discussed on the basis of diffusion characteristics of Si in poly-SiGe.

cond-mat.mtrl-sci↗

arXiv2017

Flipping growth orientation of nanographitic structures by plasma enhanced chemical vapor deposition

Nanographitic structures (NGSs) with multitude of morphological features are grown on SiO2/Si substrates by electron cyclotron resonance - plasma enhanced chemical vapor deposition (ECR-PECVD). CH4 is used as source gas with Ar and H2 as dilutants. Field emission scanning electron microscopy, high resolution transmission electron microscopy (HRTEM) and Raman spectroscopy are used to study the structural and morphological features of the grown films. Herein, we demonstrate, how the morphology can be tuned from planar to vertical structure using single control parameter namely, dilution of CH4 with Ar and/or H2. Our results show that the competitive growth and etching processes dictate the morphology of the NGSs. While Ar-rich composition favors vertically oriented graphene nanosheets, H2-rich composition aids growth of planar films. Raman analysis reveals dilution of CH4 with either Ar or H2 or in combination helps to improve the structural quality of the films. Line shape analysis of Raman 2D band shows nearly symmetric Lorentzian profile which confirms the turbostratic nature of the grown NGSs. Further, this aspect is elucidated by HRTEM studies by observing elliptical diffraction pattern. Based on these experiments, a comprehensive understanding is obtained on the growth and structural properties of NGSs grown over a wide range of feedstock compositions.

cond-mat.mtrl-sci↗

arXiv2017

Enhanced superconductivity and superconductor to insulator transition in nano-crystalline molybdenum thin films

Disorder driven superconductor to insulator transition via intermediate metallic regime is reported in nano-crystalline thin films of molybdenum. The nano-structured thin films have been deposited at room temperature using DC magnetron sputtering at different argon pressures. The grain size has been tuned using deposition pressure as the sole control parameter. A variation of particle sizes, room temperature resistivity and superconducting transition has been studied as a function of deposition pressure. The nano-crystalline molybdenum thin films are found to have large carrier concentration but very low mobility and electronic mean free path. Hall and conductivity measurements have been used to understand the effect of disorder on the carrier density and mobilities. Ioffe-Regel parameter is shown to correlate with the continuous metal-insulator transition in our samples.

cond-mat.supr-con↗

arXiv2016

Localized Charge Transfer Process and Surface Band Bending in Methane Sensing by GaN Nanowires

The physicochemical processes at the surfaces of semiconductor nanostructures involved in electrochemical and sensing devices are strongly influenced by the presence of intrinsic or extrinsic defects. To reveal the surface controlled sensing mechanism, intentional lattice oxygen defects are created on the surfaces of GaN nanowires for the elucidation of charge transfer process in methane (CH4) sensing. Experimental and simulation results of electron energy loss spectroscopy (EELS) studies on oxygen rich GaN nanowires confirmed the possible presence of 2(ON) and VGa-3ON defect complexes. A global resistive response for sensor devices of ensemble nanowires and a localized charge transfer process in single GaN nanowires are studied in situ scanning by Kelvin probe microscopy (SKPM). A localized charge transfer process, involving the VGa-3ON defect complex on nanowire surface is attributed in controlling the global gas sensing behavior of the oxygen rich ensemble GaN nanowires.

cond-mat.mtrl-sci↗

arXiv2016

Optical band gap and associated band-tails in nanocrystalline AlN thin films grown by reactive IBSD at different substrate temperatures

AlN thin films have been grown on Si (100) substrates by reactive ion beam sputter deposition (IBSD) at different substrate temperatures varying from room temperature (RT) to 500oC. Substrate temperature induced microstructural transition from amorphous at RT, nanocrystalline at 300oC to microcrystalline at 400oC has been observed by Transmission Electron Microscopy (TEM). Average surface roughness (Ra) and morphology has been explored by using Atomic Force Microscopy (AFM). UV-VIS spectrophotometry has been employed to probe the substrate temperature induced changes in optical band-gap (Eg) of grown thin films in reflectance mode. It was found that Eg was increased from 5.08 to 5.21 eV as substrate temperature was increased from RT to 500oC. Urbach energy tail (Eu) along with weak absorption tail (WAT) energy (Et) have been estimated to account for the optical disorder which was found to decrease with associated increase in Eg.

cond-mat.mtrl-sci↗

arXiv2015

Novel single phase vanadium dioxide nanostructured films for methane sensing near room temperature

Methane (CH_4) gas sensing properties of novel vanadium dioxide (VO_2) nanostructured films is reported for the first time. The single phase nanostructures are synthesized by pulsed dc-magnetron sputtering of V target followed by oxidation in O_2 atmosphere at 550 ^oC. The partial pressure of O_2 is controlled to obtain stoichiometric VO_2 with the samples showing rutile monoclinic crystalline symmetry and regions of rod shaped nano-architectures. These nanostructured films exhibit a reversible semiconductor to metal transition in the temperature range of 60-70 ^oC. Gas sensing experiments are carried out in the temperature span from 25 ^oC to 200 ^oC in presence of CH_4. These experiments reveal that the films respond very well at temperatures as low as 50 ^oC, in the semiconducting state.

cond-mat.mtrl-sci↗

arXiv2015

Optical Properties of Mono-Dispersed AlGaN Nanowires in the Single-Prong Growth Mechanism

Growth of mono-dispersed AlGaN nanowires of ternary wurtzite phase is reported using chemical vapour deposition technique in the vapour-liquid-solid process. The role of distribution of Au catalyst nanoparticles on the size and the shape of AlGaN nanowires are discussed. These variations in the morphology of the nanowires are understood invoking Ostwald ripening of Au catalyst nanoparticles at high temperature followed by the effect of single and multi-prong growth mechanism. Energy-filtered transmission electron microscopy is used as an evidence for the presence of Al in the as-prepared samples. A significant blue shift of the band gap, in the absence of quantum confinement effect in the nanowires with diameter about 100 nm, is used as a supportive evidence for the AlGaN alloy formation. Polarized resonance Raman spectroscopy with strong electron-phonon coupling along with optical confinement due to the dielectric contrast of nanowire with respect to that of surrounding media are adopted to understand the crystalline orientation of a single nanowire in the sub-diffraction limit of about 100 nm using 325 nm wavelength, for the first time. The results are compared with the structural analysis using high resolution transmission microscopic study.

cond-mat.mtrl-sci↗

arXiv2015

Raoult's Formalism in Understanding Low Temperature Growth of GaN Nanowires using Binary Precursor

Growth of GaN nanowires are carried out via metal initiated vapor-liquid-solid mechanism, with Au as the catalyst. In chemical vapour deposition technique, GaN nanowires are usually grown at high temperatures in the range of 900-1100 ^oC because of low vapor pressure of Ga below 900 ^oC. In the present study, we have grown the GaN nanowires at a temperature, as low as 700 ^oC. Role of indium in the reduction of growth temperature is discussed in the ambit of Raoult's law. Indium is used to increase the vapor pressure of the Ga sufficiently to evaporate even at low temperature initiating the growth of GaN nanowires. In addition to the studies related to structural and vibrational properties, optical properties of the grown nanowires are also reported for detailed structural analysis.

cond-mat.mtrl-sci↗

arXiv2015

Direct Evidence of Mg Incorporation Pathway in Vapor-Liquid-Solid Grown p-type Nonpolar GaN Nanowires

Doping of III-nitride based compound semiconductor nanowires is still a challenging issue to have a control over the dopant distribution in precise locations of the nanowire optoelectronic devices. Knowledge of the dopant incorporation and its pathways in nanowires for such devices is limited by the growth methods. We report the direct evidence of incorporation pathway for Mg dopants in p-type nonpolar GaN nanowires grown via vapour-liquid-solid (VLS) method in a chemical vapour deposition technique for the first time. Mg incorporation is confirmed using X-ray photoelectron (XPS) and electron energy loss spectroscopic (EELS) measurements. Energy filtered transmission electron microscopic (EFTEM) studies are used for finding the Mg incorporation pathway in the GaN nanowire. Photoluminescence studies on Mg doped GaN nanowires along with the electrical characterization on heterojunction formed between nanowires and n-Si confirm the activation of Mg atoms as p-type dopants in nonpolar GaN nanowires.

cond-mat.mtrl-sci↗

arXiv2015

Growth of InN quantum dots to nanorods: A competition between nucleation and growth rates

Growth evolution of InN nanostructures via a chemical vapor deposition technique is reported using In_2O_3 as precursor material and NH_3 as reactive gas in the temperature range of 550-700 ^oC. Morphology of the nanostructures solely depends on the growth temperature, evolving from quantum dot sized nanoparticles to nanorods. It is found that 630 ^oC is the threshold temperature for nanorod growth. At 630 ^oC, nucleation starts with multifaceted particle having {10-12} surface planes. Subsequently, hexagonal polyhedral NRs are grown along the [0001] direction with non-polar surfaces of m-planes {10-10}. A comprehensive study is carried out to understand the evolution of nanorods as a function of growth parameters like temperature, time and gas flow rate. Change in the morphology of nanostructures is explained based on the nucleation rate and the growth rates during the phase formation. Raman studies of these nanostructures show that a biaxial strain is developed because of unintentional impurity doping with the increase in growth temperature.

cond-mat.mtrl-sci↗

arXiv2015

Photoluminescence of oxygen vacancies and hydroxyl group surface functionalized SnO_2 nanoparticles

We report, for the first time, the luminescence property of the hydroxyl group surface functionalized quantum dots (QDs) and nanoparticles (NPs) of SnO_2 using low energy excitations of 2.54 eV (488 nm) and 2.42 eV (514.5 nm). This luminescence is in addition to generally observed luminescence from 'O' defects. The as-prepared SnO_2 quantum dots (QDs) are annealed at different temperatures in ambient conditions to create varied NPs in size. Subsequently, average size of the NPs is calculated from the acoustic vibrations observed at low frequencies in the Raman spectra and by the transmission electron microscopic measurements. Detailed photoluminescence studies with 3.815 eV (325 nm) excitation reveal the nature of in-plane and bridging 'O' vacancies as well as adsorption and desorption occurred at different annealing temperatures. X-ray photoelectron spectroscopy studies also support this observation. Defect level related to the surface -OH functional groups shows a broad luminescence peak around 1.96 eV in SnO_2 NPs which is elaborated with the help of temperature dependent studies.

cond-mat.mtrl-sci↗

arXiv2015

Influence of oxygen in architecting large scale nonpolar GaN nanowires

Manipulation of surface architecture of semiconducting nanowires with a control in surface polarity is one of the important objectives for nanowire based electronic and optoelectronic devices for commercialization. We report the growth of exceptionally high structural and optical quality nonpolar GaN nanowires with controlled and uniform surface morphology and size distribution, for large scale production. The role of O contamination (~1-10^5 ppm) in the surface architecture of these nanowires is investigated with the possible mechanism involved. Nonpolar GaN nanowires grown in O rich condition show the inhomogeneous surface morphologies and sizes (50 - 150 nm) while nanowires are having precise sizes of 40(5) nm and uniform surface morphology, for the samples grown in O reduced condition. Relative O contents are estimated using electron energy loss spectroscopy studies. Size-selective growth of uniform nanowires is also demonstrated, in the O reduced condition, using different catalyst sizes. Photoluminescence studies along with the observation of single-mode waveguide formation, as far field bright violet multiple emission spots, reveal the high optical quality of the nonpolar GaN nanowires grown in the O reduced condition.

physics.chem-ph↗

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↗