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Mr. SK Firoz Islam
Y8109070
Theoretical Study of Electronic, Transport and Thermoelectric Properties of Spin-Orbit Coupled Two-Dimensional Electron Systems in Presence of Magnetic Field.
3rd June, 2014 (Tuesday)
6 pm
FB-382
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M. Hemanadhan
Y8209866
Study of excited-state energy density functionals constructed by splitting k-space for homogeneous electron gas.
2nd June, 2014 (Monday)
4 pm
FB-382
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Abhishek Chowdhury
Y7209061
Multi-element focused ion beamlets for localized low energy ion matter interaction.
30 May, 2014 (Friday)
4 pm
FB-382
In this thesis a low energy multiple ion beamlet system (MIBS) is developed and utilized for non-destructive investigation of ion matter interactions at micrometer length scales. Since low-energy ion beams (1-5 keV) cannot penetrate deep inside the substrate, therefore the excitations are mainly limited to surface and sub-surface layers. Until now researchers had mainly considered single, broad ion beams (diameter ~ 0.1-2 cm) which made local perturbations challenging. The focused multiple ion beamlets can be employed for tailoring and creation of localized resistive, conducting or optically active regions in matter in a patterned manner.
The beamlets are extracted from a compact wave driven plasma source developed in the laboratory. Two types of beamlet "shower" electrodes are employed in the experiments: one with a honeycomb structure with notched apertures and another a 5×5 array of through apertures. Detailed experiments are performed to optimize the plasma source parameters for obtaining uniform beamlet current, as confirmed by angular measurements of the ion saturation current near the extraction electrode. A computer controlled switching technique is developed that can control motion of individual beamlets for patterned irradiation.
The MIBS is employed to systematically investigate low-energy ion irradiation induced changes in sheet resistivity and Debye temperatures in metallic nano-films of Ag, Cu and Al of thickness d/lo ~ 2-5, where d is the film thickness and lo is the bulk mean free path. The number of defects and impurities in the nano-film is varied in a controlled manner by varying the ionic mass number and beam fluence. Both atomic (Ne, Ar, Kr) and molecular (H2, N2) gases are employed in the investigation. Low temperature measurements are carried out for pristine and irradiated films to determine the residual sheet resistance (RRS). An empirical formula relating RRS with ionic mass number and beam fluence is proposed for the first time. Large diffusion of impurities into the film driven by thermal gradients at the interface is found to give rise to the unexpectedly large observed values of sheet resistance, as confirmed from energy dispersive x-ray spectroscopy. The Debye temperature as determined from Bloch-Grüneisen fitting of sheet resistance versus temperature data, is found to decrease with both ionic mass number and fluence, primarily due to the change in bulk modulus of the nano-film. The surface morphology of the films are investigated using atomic force microscopy (AFM) measurements. The experimental results are verified using well known ion matter interaction simulation tools such as SRIM, TRIM and TRIDYN_HZDR.
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Mihir Sarkar
Y7209062
Charge particle beam interaction with matter from a perspective of lithography.
27th May, 2014 (Tuesday)
2 pm
FB-382
The interaction of energetic charged particles with matter results in a variety of events which are interesting from the point of view of science, and offers exciting possibilities for exploring novel processing applications. In that context, we study two loosely connected aspects of charged particle matter interaction, one of which relates to the charge exchange process that occurs to high speed ions passing through the stripper medium of a tandem accelerator. The other one is electron and ion beam induced lithography which occurs by ionization of a target resist medium. The investigations are motivated by the unique possibilities of charge particle beam lithography for prototyping organic devices.
The charge exchange process is studied by using negative carbon ion (12C-) beam and analyzing the effect of the stripper gas pressure on the average charge (q_avg), charge state fractions, and transmission of the incident beam. It is shown that the equilibrium charge state istribution in the outgoing beam is slowly altered by increment in the stripper gas pressure. For particular incident ion energy the transmission shows a maximum with variation in stripper gas pressure due to the interplay between populations of charge changing target atoms and elastic scattering centers.
The charge particle based lithography process was taken up with MeV roton beams extracted from the tandem accelerator and keV electron beams of a dual-beam SEM/FIB. The effects of proton beam exposure dose and post exposure processing conditions on the quality of the developed micro-structures are analyzed. The exposure characteristics of 30 keV electron beam lithography in a thick chemically amplified resist SU-8 are discussed. The widening of the developed structures in excess of a particular equienergy deposition density contour follows non-monotonic variation. The sidewall profiles indicate that the crosslink profile of the resist proceeds in a reaction-diffusion environment in which the photoacid diffusion itself is controlled by crosslinking density.
Next, wetting behavior of an electron beam written 3D micro-structure pattern on ITO/Glass substrate by two functional inks used for inkjet printing material deposition is examined. The patterned substrate facilitates all additive deposition of an organic semiconductor over the micro-structure pattern. The possibility of convenient processing f array of thin film transistors and similar potential applications are discussed.
Finally, 30 keV electron beam lithography is investigated in a novel multilayer of resist which includes an intermediate metallic layer in the stack. The feasibility of the process and the possible line width resolution has been analyzed by obtaining the spatial distribution of electron energy deposition density profile through the multilayers of resist. As a test case application, the lithography process is shown to facilitate fabrication of a vertical organic transistor based on poly (3-hexylthiophene) (P3HT) as the active material.
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P. K. Rout
Y7209866
Interface superconductivity and giant proximity effect in La-based cuprate heterostructures
23rd May, 2014 (Friday)
3 pm
FB-382
A range of novel physical phenomena like high mobility two-dimensional electron gas, quantum Hall effect, magnetism, and interface superconductivity are observed at interfaces between complex oxides, which completely differ from the ones of the constituent compounds. Recent observation of superconductivity at the interface of various cuprate heterostructures is one such exciting phenomenon. Here, we will present the observation of interface superconductivity in the bilayer systems composed of various combinations of cuprates like La2-xSrxCuO4 and La2-x-yNdySrxCuO4. The nature of the interfacial layer in such composite systems will be discussed. The second part of the seminar will deal with the superconductor (S)-normal metal (N)-superconductor (S) Josephson junctions based on high temperature cuprate superconductors. Unlike conventional SNS junctions, the supercurrent in these junctions can flow through barriers as thick as 50 nm, which is order of magnitude larger than the coherence length. We will discuss our results in the context of "giant proximity effect".
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D. N. Patel
Y5209062
Spectroscopic investigations of brass plasma and synthesis of nanoparticles.
19th May, 2014 (Monday)
12 noon
FB-382
Laser ablation of materials has applications in thin films, laser etching, micromachining and nanotechnology. A comprehensive study of laser ablation of brass is carried out in different ambient conditions viz. vacuum, air and liquid to understand composite/alloy target plasma and formation of nano particles in plasma plumes. The brass plasma is created using Nd:YAG
laser operated at 10 Hz with 1064 nm and 266 nm. Optical emission spectroscopy (OES) measurement at threshold of brass shows the
preferential vaporization of Zn. The influence of magnetic field on plasma plume dynamics shows the splitting, oscillations and rotations of the
plume. The behavior of the expanding plume in various ambient; vacuum, air and liquid, shows distinct confinement of plume in water ambient. The high temperature, density and pressure of plume in water ambient facilitate the formation of metastable phases. Nanoparticles formed in the plumes show a compositional change with the size of nanoparticles in the vapor phase. The nanoparticles are characterized by scanning electron microscopy (SEM), energy dispersive X-ray (EDX) measurement, photoluminescence (PL) and Raman spectroscopy. The smaller size particles (nanoparticles size ~55 nm and size distribution FWHM ~ 7.5 nm) are reported in water ambient, however, the larger size particles (micron size particles ~ 2.5 µm and size distribution FWHM ~ 1.7 µm) are observed in air ambient. Smaller size particles are mostly Zn enriched. The photoluminescence (PL) measurement of the particles deposited in water ambient shows the peak at 380 nm and the PL peaks at 380 nm, 415 nm and 440 nm are observed when the particles are deposited in air ambient. The measurement of particles size shows the decrease in size away from the crater periphery in air however, in the case of ablation in water ambient nanosized particles are observed away from the crater periphery and nano rod shaped structures are reported at the crater periphery. The Raman measurement on the deposited structures confirms the ZnO rod at the crater periphery.
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Pabitra Mandal
Y6109067
Anomalous magnetic response of CaFe1.94Co0.06As2 superconductor and nonlinear response of the driven vortex state in NbS2 superconductor.
17th May, 2013
11:15 am
FB-382
In recent times studies on the new class of Iron-Pnictide superconductors have revealed a complex doping phase diagram. It is found that in a certain doping range, superconductivity in these system emerges via the suppression of magnetic order. As a part of the thesis, we have developed a high sensitivity magneto-optical imaging technique for sensitively imaging small changes local magnetic field distribution in underdoped 122- CaFe0.94Co0.06As2 single crystals. Our investigations reveal evidence of an unusual coexistence of magnetic and superconducting property in this material. Details of this will be discussed. From our work we suggest that there exists an the inherent coexistence of magnetism along with superconductivity may also be responsible for the anomalously strong pinning found in this class of superconductors.
I will also present our investigation into novel phase transformations in the driven vortex matter in NbS2 superconductor. We investigate the scaling of the current-voltage characteristic of the superconductor using universal scaling ansatz proposed by Meng-Bo Luo et al., PRL 98, 267002 (2007). This study attempts to understand the depinning transition and what is the nature of the driven vortex phase.
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Mr. Ajeet Kumar Sharma
Y8109061
Distributions of first passage times in stochastic processes in a living cell
8th May, 2014 (Thursday)
4 pm
FB-382
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Mr Suman Banerjee
Y5209865
Opto-electronic analysis of planar, bulk and hybrid planar-mixed heterojunction organic solar cells
8 May, 2014 (Thursday)
11 am
Samtel Centre Seminar Room
he electrical response to incident light on a thin film stack system was analysed with an opto-electronic model by considering interference effect in a thin film stack system and solving the basic semiconductor equations to describe various characteristics of organic solar cells. The model was then used to describe incident light wavelength dependent photocurrent response (spectral response) of Schoottky diode, planar and bulk-heterojunction solar cells. Important parameters of organic materials. Such as, exciton diffusion length, exciton dissociation
efficiency and exciton blocking efficiency are estimated. Quantitative analysis of the role of different thin film layers and optimised film
thicknesses of efficient solar cell devices are estimated using the model.
The model is used further to explain the current-voltage (I-V) characteristics of organic solar cell. A general model for planar mixed-heterojunction solar cell is developed and used for planar and bulk heterojunction solar cell by considering some special cases. The role of mobility and contact property (extraction rate) on the I-V characteristics of planar and bulk-heterojunction solar cells are explained using the model. Many devices were made and characterised using the experimental setup designed in the lab. Device characteristics were also explained using the model developed.
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Mr. Samar Layek
Y6209862
Structural and Magnetic Properties of Transition Metal based Oxides/hydroxides Synthesized Using Specific Methods
31st March, 2014 (Monday)
6 pm
FB-382
Transition metal oxides/hydroxides are important in many applications. Three oxide systems namely iron oxide/hydroxides, Nickel oxide and Barium ferrite are investigated. Interesting results are obtained as the properties can be controlled by the synthesis method and parameters.
Hematite nanoparticles synthesized by using different combustion methods show variety of magnetic properties depending of the particle size and specific choice of fuel used for preparation. Maghemite nanorods of length 20 nm show preferential magnetic ordering.
Mixed iron oxide/hydroxides are one of the potential candidates for the fluoride and heavy metal ions purification. The effect of dilute amount of Ca, Mg substitution on the phase formation, structural and magnetic properties is studied in details. Increasing Ca doping leads to phase formation. Simultaneous incorporation of Ca, Mg leads to completely amorphous phase.
The effect of transition metal and rare earth metal substitution on the structural and magnetic properties on NiO nanoparticles is studied. Mn
doping changes the antiferromagnetic ordering in NiO to completely ferromagnetic ordering whereas weak ferromagnetic nature is found for other dopants. The value of magnetization depends on the concentration of doping and the particular dopant.
Enhancement of the magnetic properties in single phase doped BiFeO3 ceramics is found by mechanical activation. The magnetization is seen to increase either by doping in Bi/Fe site or by decreasing crystallite size below a certain limit.
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Durgesh Chand Tripathi
Y5109066
Carrier Transport in Organic Semiconductor Diodes with Doped/Undoped Interfaces
29th March (Saturday), 2014
5 pm
SCDT Seminar Room
Organic semiconductors are currently being used for many large area electronic applications. With rising complexity of multilayer structures such as in organic light emitting diodes, the doped/undoped (high-low) organic interface has become an important determinant of device performance. However, underlying mechanisms of charge transport across the doped/undoped interface is not understood.
In this thesis, we study charge transport across the doped-undoped interface using capacitance-voltage (C-V) and temperature dependent current density-voltage characteristics. The specially designed device structures for this purpose have been fabricated using a state-of-the-art Cluster tool. The prototype materials m-MTDATA and F4-TCNQ are used as matrix and dopant, respectively and the doping is achieved by co-evaporation.
The carrier transport mechanism at doped/undoped interface is sensitive to the barrier height and carrier accumulation at the interface. It is found that the charge transport across homo-interfaces is controlled by tunneling through alignment of local density of states on the two sides; whereas at hetero-interfaces it is controlled by the offset.
The characteristic peak observed in C-V in such structures is found to depend on device configuration, and the peak height is sensitive to the layer thickness. A competition between the diffusion dominated and drift dominated transport is responsible for occurrence of the peak. A functional relation between small signal capacitance and voltage has been established in the diffusion regime prior to the C-V peak. It has also been shown that the measured mobility by impedance technique in space charge limited regime is dependent on device structures needing careful interpretation of its field dependence.
Electroluminescent transient technique has been used to measure diffusivity and mobility simultaneously in organic diodes. In contrast to mobility, field dependence of diffusivity showed a specific minimum at a critical field, which suggests a convenient normalization procedure, called “shift normalization”, to annihilate temperature dependence. This method has been used for a unified description of mobility in prototypical small molecule Alq3 from which all the parameters of Gaussian disorder transport models can be inferred independently.
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Md. Shamim
Y6209861
Construction of exchange and correlation energy functionals for
excited-states in time-independent density functional theory.
27th December, 2013 (Thursday)
10:00 am
FB-382
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Debaprasad Sahu
Y7109067
Physics of negative ion containing plasmas: volume generation, measurement and wave induced phenomena.
16th December, 2013 (Monday)
10:30 am
FB-382
In this thesis, a novel microwave plasma based volume negative ion source is developed and detailed experiments are performed to optimize negative ion generation for hydrogen (H-)and measurement of negative ion density. Electromagnetic waves of 2.45 GHz (angular frequency 1.53×1010 rad/s) are launched directly into a multicusp (MC) plasma device in the k perp B mode, where k is the wave vector and B is the magnetostatic field. Localized wave induced resonances are created at the periphery of MC (inner radius = 60 mm), depending upon the magnetic field and plasma density. These resonance zones are identified experimentally. The electron cyclotron resonance (ECR) occurs close to the boundary at r = 42 mm, whereas the upper hybrid resonance (UHR) lies r = ~ 22 mm away from MC boundary.
The H- source is divided into two sections namely production (PR) and attachment (AR), separated by a transverse magnetic filter, which deflects the hot electrons (7 - 15 eV) trying to enter from PR and allows only the cold electrons (~ 1 - 2 eV) into AR. The hot electrons in PR are favorable for the generation of vibrationally excited molecules of hydrogen, which pass into AR, where negative ions are produced by their dissociative attachment with the cold electrons. The plasma frequency at the center is ~ (1.2 - 1.4)×1010 rad/s and is comparable to the angular wave frequency indicating sustenance of close to cutoff density plasmas (~ 7.44×1010 cm-3) at the wave frequency. The electron mean free path lies in the range ~ 25 - 30 cm and is comparable to the system size, therefore the plasma is almost collisionless. Measurements of electron energy distribution function (EEDF) in both AR and PR confirm the nature of the distribution and suitability for the aforementioned processes. The H^- density measured using two methods, viz. the extracted current and the second derivative beat method is found to be ~ 5×109 cm-3. The source has also been operated in the pulsed mode. Here, temporal filtering generates negative ion rich plasmas in the afterglow phase where cold electrons (~ 1 eV) are generated. The H- density in the afterglow is measured to be ~ 2×1010 cm-3, using the saturation current ratio method and is higher than that of the continuous mode case.
Finally, a steady state and a time dependent model, using particle and charge balance equations are developed to estimate the negative ion density in both the downstream region (continuous mode) and in the temporal afterglow (pulsed mode). The results of the two models agree reasonably well with the experimentally measured H- density. The compact negative ion source would be useful in a variety of physics studies and applications such as dusty plasma with negative ions, neutral beam injection in fusion, modification of material surfaces, semiconductor fabrication, and ion beam lithography.
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Amit Banerjee
Y7209864
Resonance behavior of FIB grown nanomechanical systems and the role of microstructure.
28th October, 2013 (Monday)
8:00 AM
FB-382
In recent time discovery of novel material properties at nanodimension has motivated studies pertaining to investigating the mechanical properties of materials at nanometer dimension. Due to limitations in detection capability, most of the studies on anomechanical systems have been on systems fabricated out of semiconducting materials. In this thesis we discuss ways to fabricate metallic nanomechanical systems and also develop ways to detect resonance behaviour in these systems. We have fabricated using the focused ion beam induced milling process metallic nanocantilevers by milling self-supporting polycrystalline thin films of Au and Ag. The resonance frequencies and the amplitude of vibration of the metallic nanocantilevers have been experimentally measured. We study the resonance behaviour of our nanocantilever systems within the Euler-Bernoulli formulation and investigate the deviations from the expected behavior. We show the existence of a characteristic dimension in the metallic nanocantilever system at which the dissipation in the system is minimum and the resonance frequency approaches the ideal Euler-Bernoulli limit. We shall discuss issues related to this characteristic dimension and its relationship with the novel microstructure observed in the metallic thin films from which the nanocantilevers are fabricated. We have also fabricated a set of elastically coupled Au nanocantilever system and studied their resonance behaviour. In the second part of this presentation, we shall demonstrate the fabrication and resonance behaviour of a focused electron and ion beam grown C-Pt core-shell type bilayer nanowire resonator system. The resonance frequency of a bilayer system depends on densities and Young’s moduli of the constituting materials. We have experimentally measured the densities and Young’s moduli of the materials that we have used for the present study. The resonance frequency behaviour of the bilayer system is compared with the theoretical predictions. Finally towards the end of the presentation we discuss a few novel method we have developed for sensing nanomechanical vibration.
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