Nanoscale materials and devices for energy storage, spintronics and biosensors
Our research group focuses on the synthesis, characterization and device fabrication of nanoscale materials for energy storage, nano-electronics and bio-sensing applications. We seek materials solutions to critical problems hindering advances in these technologically vital areas through the manipulation of materials functionality via microstructural engineering. The research conducted in our group is multidisciplinary in nature and includes collaborations with faculty members in the Colleges of Engineering and Science as well as with academic and national laboratories.
Advanced Energy Storage Materials and Devices
The need for higher capacity, portable, wearable and implantable energy storage devices has enormously incremented in recent years. Whereas incremental improvements can be anticipated in Li-ion battery technology, its chemistry, fundamentally limits the attainable capacity in such devices. The Li-O2 pair, on the other hand provides a theoretical energy density rivaling that of liquid fuels. The implementation of these so-called Li-Air batteries, is hindered by significant challenges including: the reactivity of high capacity Li metal anodes with current liquid and polymer electrolytes and the lack of a multi-functional air cathode required for efficient and rechargeable devices. In addition, during charge/discharge cycling, Li can buildup in the electrolyte, forming dendrites and resulting in thermal runoff, rapid discharge and ultimately failure of the cell with potential catastrophic results.
Our research aims at extending the safety, capacity and performance of Li-based batteries. To this effect we are developing novel solid-state electrolytes (SSE) and hierarchical cathode structures. The goal is the synthesis of SSE having ionic conductivities on par liquid electrolytes (1.0 x 10-3 S/cm) that when combined with cathode multi-functional materials will enable batteries based on Li-Air chemistries to deliver up 10-fold capacity advantage over Li-ion. We synthesize cubic-phase fast ionic conducting garnet electrolytes based on Li7La3Zr2O12 (LLZO). We have readily achieved ionic conductivities up to 2.0 x 10-4 S/cm in ceramic structures not fully densified. To further improve their ionic conductivity we are investigating modifications of the garnet lattice structure through aliovalent substitution with single and dual dopants such as Bi, Al, W and Nb. As an example, when site-substituted onto the 16a Zr4+ site in the garnet lattice, Bi5+ displaces additional Li+, thereby optimizing the Li+ occupancy and allowing for increased ionic conductivity in Bi-doped LLZO. To incorporate these SSE into commercial battery devices, we are developing flexible and high-temperature stable composite polymer-ceramic electrolytes. These SSE are also readily incorporated into thin film batteries employing vapor phase growth techniques such as sputter-deposition.
The realization of Li-Air batteries has been equally hindered by the lack of an Air-cathode capable of providing the multi-functionality required for device operation. This includes a cathode microstructure that accommodates solid state oxide and peroxide reaction products while providing adequate transport of oxygen and Li toward the active, electron conducting surfaces; the development of oxygen resistant cathode materials that are impervious to high oxidation potentials and the implementation of an efficient redox process at the cathode surface so that lithium oxidation is fully reversible. Our research in this area will focus initially on thin film cathodes, wherein utilizing multilayered and laminated materials we seek to develop nano-porous carbon thin films incorporating judiciously chosen catalytic nanoparticles in contact with thin films providing additional functionality for electron transport and oxidation reversibility.
Battery devices employing our materials will be fabricated and tested in collaboration with the Battery Innovation Center and bench-marked in performance against state-of-the art electrolytes, anode and cathode components utilizing industry standard tests and methodologies. In parallel we will conduct extensive materials characterization studies of solid-state electrolyte and air-cathode materials for additional functionality improvements based on fundamental understanding in collaboration with Argonne National Laboratories.
Spintronics Materials and Devices
Spintronics or spin electronics, involves the study and manipulation of the intrinsic spin of the electron and its associated magnetic moment, in addition to its fundamental electronic charge. Spintronics emerged from discoveries in the 1980s related to spin-dependent electron transport phenomena in solid-state devices. This includes the observation of spin-polarized electron injection from a ferromagnetic material to a metal and the discovery of giant magnetoresistance independently by Albert Fert and Peter Grünberg for which they were awarded the Physics Novel Prize in 2007. The use of semiconductors for spintronics began with the theoretical proposal of a spin field-effect-transistor by Purdue Professor Supriyo Datta in 1990. These discoveries have culminated in the development of the magnetic tunnel junction (MTJ), a device whose magnetoresistance is controlled by spin-transfer-torque (STT) switching of the magnetization orientation of a nano-magnet. MTJs are the building blocks of current spintronics memory and logic devices.
Our spintronics research merges the Computational and Fundamentals of Magnetism expertise of the Datta group (ECE) with the Magnetic Materials and Device Fabrication expertise of the Marinero group to develop high-speed electronic and novel memory devices. The group is a key component of the “Spintronics: Atoms to Systems” pre-eminent team, competitively selected in 2015 by the College of Engineering as one of the strategic areas for growth at Purdue University. The pre-eminent team aims to develop cross-disciplinary education and research programs at Purdue spanning fundamental science to circuits and systems, including physical theory and modeling, materials science, device and circuit implementation and novel architecture designs.
Our current research focuses at solving two key impediments inherent to current spintronics devices: their inherently slow switching speeds and the excessive energy required to switch the magnetization state of memory and logic devices. We apply our multi-physics computational framework to identify nano-magnet architectures that offer the potential of magnetization switching in the picosecond time scale with record-low values of injected spin currents. One of the critical issues facing nano-magnet devices is the thermal stability of the magnetic orientation: as dimensionality is reduced, the magnetization can randomly flip direction under the influence of the environment temperature. To circumvent, this so-named superparamagnetic limit, it is necessary to utilize ferromagnetic materials with high magnetic anisotropy. However, to switch the magnetization of these high anisotropy materials requires excessive spin currents that result in dielectric breakdown of the MTJ’s oxide barrier component. For the case of magnetic field switching, Marinero and co-workers at the IBM and Hitachi Research Laboratories developed novel magnetic exchange coupled structures to switch high anisotropy materials with the fields available in conventional magnetic recording devices. Ferromagnetic exchange coupling of high with low anisotropy materials resulted in drastic reduction of the field required for switching high anisotropy materials. Inspired by such results, our group at Purdue has conducted an extensive computational assessment of magnetic exchange coupled structures driven by spin currents. We have identified a novel synthetic ferrimagnet (SFM) structure that switches in picosecond time scales with low energy requirements. This novel structure highly simplifies the fabrication of MTJs. We are in the process of fabricating SFM devices that we propose to use as the building blocks for high speed electronics and memory devices.
Kerem Y. Camsari
Bradlee K. Beauchamp
Ahmed Zeeshan Pervaiz
Our biosensor research has two thrusts: a) the development of an ultra-sensitive, room temperature and wearable magnetic sensor array for brain research (cellular and cognitive) based on the detection of magnetic signals associated with brain activity and neuron interactions; b) development of autonomous biosensors (sense, diagnose, remote transmission and self-powered) for the detection and monitoring of diseases transmitted by arthropod vectors such as Dengue, Yellow Fever, Zyka, Chikungkuya and Nile Virus disease.
Techniques to study brain activity rely either on magnetic resonance imaging (MRI) through the change of nuclear relaxation times or on magneto encephalography (MEG) through sensing the extremely weak femto-Tesla (fT) magnetic fields produced by neural activity. Current MEG research requires large, patient-intrusive equipment utilizing SQUID (Superconducting QUantum Interference Device) sensors operating at low cryogenic temperatures. Furthermore, a SQUID detects magnetic flux, reducing their size to increment the system spatial resolution compromises their sensitivity.
Our group is developing fT magnetic sensor arrays for brain research. The sensor is based on the tunable graphene magnetometer developed by Marinero and co-workers at the Hitachi San Jose Research Laboratory (Nano Letters, 10, 341, (2010)) for detecting magnetic fields with nanoscale resolution. It exploits the Lorentz force interaction of charge carriers in high mobility materials with the magnetic field. The sensitivity of the graphene sensor is largely amplified by combining the Hall response with enhanced geometric magnetoresitance (Extraordinary Magnetoresistance, EMR). The demonstrated room temperature sensitivity of 100 V/Tesla of the graphene EMR magnetometer can be dramatically increased for MEG brain research. The charge carrier mobility limits, to first order, the signal response of the EMR sensor. The carrier mobility of the exfoliated samples utilized ranged from 1 - 5 x 103 cm2/V-sec and was limited by structural defects and surface contamination. Recent work on hexagonal boron nitride supported graphene has yielded mobilities ~1.2 x 105 cm2/V-sec, which should enable fT detection. We are developing sensor arrays utilizing these high mobility graphene bilayers that will be fabricated on flexible substrates for wearable headgear to study brain activity in different length and time scales. This project is a collaborative program between the research groups of Professor Marinero and Zhongming Liu of the School of Biomedical Engineering and Dr. Hector Ramon Martinez, Director of the Neurology Institute of the Monterrey Tech. in Mexico.
Biosensors for Arthropod Borne Diseases
Vector monitoring and eradication is essential in combating the world-wide impact of diseases transmitted by mosquitoes, ticks, sandflies and blackflies. According to USAID about half of the world population are presently at risk of Dengue; it is now endemic in over 100 countries and the CDC estimates that as many as 400 million people are infected yearly. There is no vaccine for Dengue and changes in global climate and global transportation are associated with recent outbreaks in non-tropical areas. Similarly, the Zyka virus, which is transmitted by the same mosquito responsible for Dengue (Aedes Aegypti) has recently spread across the Pacific Ocean into Mexico, Central America, the Caribbean and South America. Its effects are most devastating in pregnant women resulting in miscarriage or microcephaly. There is significant fear that the virus will spread to the USA.There are currently no field deployable surveillance tools having in-situ rapid point of care diagnostics for Dengue or Zyka virus to permit its early detection and disease containment. Our research group is developing a platform to integrate into impedance-based bio-sensors diagnostic tools for Dengue and Zyka Virus based on RNA hybridization. Electrochemical biosensors are based on the measurement of the electrical resistance of the electrode surface and on the binding kinetics of molecules between electrolyte and electrode surface. Such diagnostics tools offer simplicity and ease of use, high combined sensitivity and selectivity, and are low cost devices. These sensors can be integrated into systems that transmit the collected information remotely through for example a cellular communication network. Such sensors can also be integrated into systems placed in at-risk areas where mosquitos can be collected and tested on site, thus offering the opportunity of early intervention in at-risk population areas. The same platform will be used to rapidly diagnose the presence of the virus in affected human populations. Currently virus verification, relies on enzyme-linked immune-sorbent assay (ELISA) and complex polymerase chain reaction (PCR) testing. These tests require complex and time-consuming sample preparation, sample transport to specialized laboratories with trained personnel, and are also susceptible to false positive results.
This research project is a collaborative effort between the research groups of Professors Lia Stanciu and Marinero from the School of Materials Engineering and Professor Richard J. Kuhn of the Department of Biological Sciences.
Angel Rafael Monroy