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Nitul Singh Rajput

Studies of issues involved in Focused ion beam based Nanostructures and developing newer methodologies.
19th June, 2013 (Monday)
11:00 am
FB-382
FIB technology is gradually becoming one of the most popular techniques for micro/nano engineering materials. In FIB, the maskless processes (deposition & milling) are used as bottom-up and top-down fabrication tool to meet the challenges at nanoscale. Despite its successful venture from precise engineering to manufacturing for the study of basic physics, the ion irradiation processes have number of secondary effects. However, in some cases, the secondary effects viz., implantation, damage formation can be exploited fruitfully to modify the sample locally.

In this thesis work, the FIB processes are used for novel structure fabrication and several newer methodologies are developed. A metallic cantilever, fabricated using a very unique technique, tends to change its shape and geometry on ion irradiation.

Investigation shows that ion-matter interaction at nanoscale has a prime role in the bending process. The phenomenon is exploited to create several 3D micro/nano structures as well as used to sense mass of the order of femtogram, lifting a heavy mass... etc.

FIB is also used to fabricate metallic nanowires and nanogap electrodes and their characterizations are made. It is found that insulating base substrate has the prime role in the failure of the nanogap structures. Using milling and deposition, several overhanging 3-D structures are created and their thermal characterization is made.

And lastly we will discuss about the role substrate and the ion beam heating in the FIB-CVD process. It is found that, the ion beam heating is not significant to cause any disrupt change in the growth of the structure.

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Dhirendra Kumar Sinha
Y4109071
Carrier Transport and Electroluminescence in Polyfluorene embedded with CdSe/ZnS Quantum Dots
17th June, 2013 (Monday)
12:00 Noon
Samtel Centre for Display Technologies (Seminar Room)
We simultaneously study the carrier transport and luminescence processes in the model system of a blue-emitting conjugated polymer, Polyfluorene (PFO), embedded with core-shell type CdSe/ZnS Quantum Dots (QDs). The test devices used for these studies have conventional structure of ITO (transparent anode)|PEDOT:PSS (hole-injecting layer)|PFO:QDs
(active layer)| Ca-Al (Cathode), wherein the concentration of the Qds varies between 0-80% by weight. The study of the electroluminescence transients at varying temperatures (10-300 K) is the principal technique which we use in this study.
The thesis focuses primarily on the following three issues:

•The simultaneous determination of the field and temperature dependence of
the diffusivity and the mobility of the charge-carriers in PFO;

•The differences between the mechanisms of PL and EL at QDs and the role
of the charge-carrier mobility in controlling the EL;

•Modifications in the mechanisms controling charge-carrier mobility for a
wide range of QD-loading conditions, and field and temperature variation.

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Gorky Shaw
Y6209863
Static and driven phases of vortex matter in superconductors with intrinsic and nanopatterned pins
06-05-2013 (Monday)
10:00 AM
FB-382
Preamble: The vortex state in superconductors exhibits competition and interplay between effects of interactions, thermal fluctuations and quenched random disorder (natural or artificially generated). Consequently the static and dynamic phases of the vortex state are quite complex. In this seminar, I will present results of my studies on the static and driven phases of vortex matter in high-Tc and low-Tc superconductors with intrinsic as well as artificially patterned pinning centers. 
Static phases of vortex matter in sample with intrinsic pinning centers: Using high sensitivity magneto-optical imaging (MOI) on a Bi2Sr2CaCu2O8 (BSCCO) single crystal we have identified signatures of a novel interaction driven freezing transition from a dilute vortex liquid to solid phase along with a solid-liquid phase coexistence regime. We have constructed a phase boundary for the low field vortex liquid - solid transition line in the H - T vortex phase diagram. We have studied the effect of pinning strength on this phase boundary. We have also evaluated the entropy change associated with the low field phase transformation.
Driven phases of vortex matter: By measuring the velocity time series response of a vortex state in response to a continuous drive applied via a transport current, we have uncovered a new non-equilibrium driven jammed vortex state into which the vortex state organizes. The entry into a jammed vortex state we find is driven either by attempting to steadily accelerate the vortex state or by waiting for long time at constant drive. De-pinning of the jammed vortex state is also highly unusual, and is associated with giant vortex-velocity fluctuations with life-times exhibiting critical divergence on approaching the threshold depinning force value. Our study is compared with recent studies on similar effects found in driven colloidal systems.
Effects of artificial pinning on static phases of vortex matter: We have used Focused Ion Beam (FIB) to generate array of nanopins (blind holes) on surfaces of single crystals of 2H-NbSe2 and BSCCO. Bulk magnetization measurements in these samples provide evidence of a driven weak to strong pinning crossover. Using these artificial pinning centers we show how the local static configuration of vortices can be significantly altered with nanopatterning.

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Neeraj Shukla
Y6109861
Ion beam induced nanostructuring and magnetic ordering
22nd January, 2013
11:00 AM
FB-382
Ion irradiation has a variety of effects on various substrates such as sputtering of materials, adhesion of thin films, intermixing at the interface of multilayer's, scattering effects, ripple formation, defect formation. Low energy ions(few 10s of keV) are utilized for surface modification purposes and the high energy ions (few MeV) are utilized for ion implantation and for inducing defects of various kinds. 
In the present work, we have used low energy (10-30 keV) focused Ga ion beam to fabricate and study micro/nano structures on a substrate and high energy(1-2 MeV) 1H+ and 12C+ ion beams to modify properties of highly oriented pyrolytic Graphite(HOPG). 
By using focused ion beam (FIB) of 30 keV Ga ions, a variety of 1-D and 2-D micro/nano structures are prepared. A 2-D array of metallic patches has been fabricated using improved adhesion of metallic film on glass substrate caused by the ion beam bombardment. Similarly 1-D gratings with nanometer sized grating element have been fabricated by milling selective areas on silver film using Ga FIB. The forward and backward scattering of the incident Ga ions from different material structures is used to modify the nearby FIB fabricated structure. The effect has been investigated in three different geometries/material combinations.

Ferromagnetic ordering has been shown to occur in Highly Oriented Pyrolytic Graphite (HOPG) due to proton irradiation. By varying ion beam parameters, magnetic moment of the order of 10-6 emu has been obtained in optimum conditions. We have obtained 2 orders higher magnetic moment in HOPG by bombarding 12C+ ions. The results are analyzed in terms of defect densities induced in different layers along the thickness traversed by ions. Magneto-resistance studies have been made on selected irradiated HOPG sample at 20 K in the in-plane and out-of-plane orientations of unirradiated and 1H+ and 12C+ ion irradiated HOPG flakes. The results correlate well with the
findings of the VSM studies of irradiated sample.