Sandipan Pramanik

Thermal graphene metamaterials and epsilon-near-zero high temperature plasmonics

The key feature of a thermophotovoltaic (TPV) emitter is the enhancement of thermal emission corresponding to energies just above the bandgap of the absorbing photovoltaic cell and simultaneous suppression of thermal emission below the bandgap. We show here that a single layer plasmonic coating can perform this task with high efficiency. Our key design principle involves tuning the epsilon-near-zero frequency (plasma frequency) of the metal acting as a thermal emitter to the electronic bandgap of the semiconducting cell. This approach utilizes the change in reflectivity of a metal near its plasma frequency (epsilon-near-zero frequency) to lead to spectrally selective thermal emission and can be adapted to large area coatings using high temperature plasmonic materials. We provide a detailed analysis of the spectral and angular performance of high temperature plasmonic coatings as TPV emitters. We show the potential of such high temperature plasmonic thermal emitter coatings (p-TECs) for narrowband near-field thermal emission. We also show the enhancement of near-surface energy density in graphene-multilayer thermal metamaterials due to a topological transition at an effective epsilon-near-zero frequency. This opens up spectrally selective thermal emission from graphene multilayers in the infrared frequency regime. Our design paves the way for the development of single layer p-TECs and graphene multilayers for spectrally selective radiative heat transfer applications.

High Density Integration of Carbon Nanotube Spin Valves

In this work we report fabrication and characterization of short-channel multi-walled carbon nanotube (MWCNT) based spin valves. These devices are embedded in hexagonally ordered nanopores in an anodic alumina template and show spin relaxation length of 0.28 μm at 8K.

Thickness Dependent Interlayer Magnetoresistance in Multilayer Graphene Stacks

Chemical Vapor Deposition grown multilayer graphene (MLG) exhibits large out-of-plane magnetoresistance due to interlayer magnetoresistance (ILMR) effect. It is essential to identify the factors that influence this effect in order to explore its potential in magnetic sensing and data storage applications. It has been demonstrated before that the ILMR effect is sensitive to the interlayer coupling and the orientation of the magnetic field with respect to the out-of-plane (c-axis) direction. In this work, we investigate the role of MLG thickness on ILMR effect. Our results show that the magnitude of ILMR effect increases with the number of graphene layers in the MLG stack. Surprisingly, thicker devices exhibit field induced resistance switching by a factor of at least ~107. This effect persists even at room temperature and to our knowledge such large magnetoresistance values have not been reported before in the literature at comparable fields and temperatures. In addition, an oscillatory MR effect is observed at higher field values. A physical explanation of this effect is presented, which is consistent with our experimental scenario.

Magnetoresistance effects in multilayer graphene as grown on ferromagnetic substrates and implications for spin filtering

Multilayer graphene (MLG) or thin graphitic films as grown on nickel (Ni) or cobalt (Co) has been recently proposed as a promising platform for realizing highly efficient spin filters. However, graphene forms chemisorption interface with Ni and Co, which significantly affects the electronic properties of the interfacial layers as well as the growth of the subsequent graphene layers. Such systems can give rise to various types of magnetoresistance (MR) effects that are completely unrelated to spin filtering. It is, therefore, important to understand these MR effects in order to identify the spin filtering related signal. In this work we highlight on the various MR effects that are observed in Ni/MLG systems and that are also unrelated to spin filtering. In particular, an “interlayer magnetoresistance” (ILMR) effect manifests in these systems, which can result in large MR values that are comparable to state-of-the-art magnetic tunnel junctions at similar operating conditions. Preliminary measurements on Co/MLG samples also indicate presence of ILMR effect.

Fabrication of Highly Ordered Cylindrical Nanopores with Modulated Diameter Using Anodic Alumina

We propose two novel methods for fabrication of highly ordered cylindrical nanopores with modulated diameter using anodic aluminum oxide (AAO) template. These are (a) multistep anodization in which each step is separated from the next by a controlled wet etching process, and (b) lateral hard anodization of aluminum in malonic acid. This latter method also circumvents the typical sample burning problem which always accompanies surface hard anodization. Further, this method enables high temperature, high voltage anodization studies without requiring a buffer layer of aluminum oxide on the surface.

Dual-band quasi-coherent radiative thermal source

We design, fabricate and characterize the spectral, polarization, angular and temperature dependence of a microstructured SiC thermal infrared source; achieving independent control of the frequency and polarization of thermal radiation in two spectral bands.

High-field magnetoresistance effects and temperature-dependent spin relaxation in rubrene nanowire spin valves

In this work we discuss the high-field magnetoresistance effects and temperature-dependent spin relaxation in rubrene nanowire spin valves. Rubrene thin film spin valves have been studied by several groups in the past since this material can potentially offer long spin relaxation length (Ls). However, the Ls values reported so far have been low, typically ~10 nm at low temperatures (~10K). Recently we have reported a vertical spin valve device using rubrene spacer, in which rubrene is patterned in a nanowire array geometry. Such patterning leads to significant suppression of spin relaxation and we have reported spin relaxation length of ~47 nm at ~10K in rubrene nanowires. In this work we present the high field magnetoresistance measurements performed on rubrene nanowires and discuss the possible origins of these effects. Further, we discuss possible origin of the observed temperature-dependence of spin relaxation length.