Advanced Functional Materials Research in Ramki Kalyanaraman's group
Advanced functional materials show new, improved or unusual behaviors either as a result of their structure, composition or size. A seen numerous times in history, new materials often lead to revolutionary changes and so the design the discovery of new materials is one the most important activities of our society.
CHALLENGES: As the sophistication of new functional materials increases, the technology to manufacture them also grows in complexity and cost. Therefore one of the key challenges in this area is to couple simple manufacturing approaches with materials discoveries. Our pathway to discovering new materials is by utilizing nanomanufacturing routes based on natural processes, such as self-organization. From an application perspective our research is on materials and devices for new and improved solar cells, magnetic applications, bio and chemical sensors, and electronics.
Oxide Resistant Nanoparticles for Plasmon Sensing
Oxide Nanowire Gas Sensor
In this discovery-driven activity with collaborators at UTK and SIUE we discovered enhanced oxidation resistance of Silver (Ag) nanoparticles in contact with Cobalt (Co). Over the past few years we have been investigating dewetting self-organization of bilayer liquid films, such as of Ag and Co. That work is described in detail under our Nanomanufacturing page. During the course of this study we have found new types of nanostructures (such as the horizontally stacked one depicted in Fig. 1) as well serendipitously discovered the higher stability of AgCo as compared to Co, which has propelled this direction of research.
Fig. 2 shows TEM and EELS data comparing nanoparticles of pure Ag (left side)and AgCo (right side) after storage in air for 50 days. The pure Ag particle has become more diffusedue to the formation of an oxide. However, the Ag in the Ag-Co NP system did not show any sign of degradation to its surface and no oxide was found in it. We have attributed this to a cathodic protection effect due to galvanic coupling with the Co playing the role of a sacrificial anode to stabilize Ag.
Fig. 3 shows broadband optical absorbance from Ag nanoparticles after various times. It is clear that with increasing time, the Ag plasmon at ~475 nm is decreasing in intensity and is almost non-existent after ~180 days. This degradation is primarily a result of oxidation, as evidenced from the TEM results in Fig. 2.
Fig. 4 shows the broadband optical absorbance of AgCo nanoparticles after various times. It is clear that with increasing time, the AgCo plasmon at ~485 nm is virtually unchanged in position or intensity. This is consistent with the oxide-free behavior evidenced from the TEM resuls in Fig. 2.
Fig. 5 compares the sensing properties of AgCo vas Ag for samples that were heated in air for variosu temperatures. SPR or surface plasmon resonance sensing is based on detecting the refractive index change when molecules bind or change chemistry. Sensitivity is defined as the rate of change in Localized SPR wavelength with refractive index, usually in units of nm/RIU. From Fig. 5, it is evident that the sensitivity of the Ag-Co bimetallic arrays is much more stable even after heating as compared to Ag which shows highly erratic behaviors.
In this collaborative discovery-driven research with Prof. Khenner at WKU we are utilizing computer models to synthesize new types of nanostructures via self-organization. Over the last decade, core-shell nanostructures have been widely utilized to exploit the novel behavior of composite nanoparticles. One reason for the dominance of core-shell structures is the well developed solution chemistry route to fabricating such structures. However, new and improved functions could be achieved by alternate structures, such has horizontally and vertically stacked, embedded, etc. As shown in Fig. 1, recent modeling studies of bilayer self-organization predicted a number of new nanostructures may be accessible and we are pursuing experimental studies to verify if the models predictions are correct.
In Fig. 2 the time evolution of the bilayer system consisting of Ag on Co on SiO2 substrate is shown. As evident, this evolution is tending to a nanostructure (longer time t2) in which the Ag will be vertically stacked on the Co. This evolution is a complex function of various processing and materials parameters and is well captured by the nonlinear model that includes transient thermal temperature under laser melting.
In Fig. 3 the model prediction of the final state of a bilayer system starting from a Co film on Ag is shown (left figure). It predicts a vertically stacked structure. The TEM study of nanoparticlesmade with similar film thickness as used the model also shows a vertically stacked nanostructure (left image) suggesting good correlation between model and experiment.
With Prof's Seal and Cho at UCF, we are researching nanostructured materials to make excellent room temperature hydrogen sensors. In order for hydrogen fuel to contribute significantly to our energy portfolio, highly sensitive and fast responding hydrogen sensors are needed that can operate at low and room temperatures. Current technology based on oxide thin film sensors can only operate well at temperature of 100 to 150 C. Our research is focused on utilizing nanostructured oxides as sensing materials. As shown in Fig. 1, we have made periodic oxide nanostructure arrays for room temperature sensing that have >100 times higher sensitivity then films.
In Fig. 2 the cyclic resistance change in a SnO2 thin film (in green) is compared to that in a nanowire array (in red) in the presence of cycling of hydrogen gas of different concentrations at room temperature. A dramatic improvement is seen in the sensivity with periodic nanoarrays as well as an extremely fast response and recovery time. over more then 170 times is seen with the periodic nanoarrays.
In Fig. 3 the experimentally exhibited electrical response of the SnO2
nanoarray to the varying concentrations of hydrogen shows that the minimum electrical response corresponds to a resistance change of approximately 137 times. This is evidence that nanoarray SnO2 is more than capable of detecting low concentrations of hydrogen at room temperature. The electrical response of the thin film
SnO2 is so minute that it is relatively insensitive.
In this discovery-driven research with Dr. Ganagopadhyay and Prof Nussinov at WU we are utilizing nanosecond laser processing to control the magnetism in nanoparticles and nanowires. When the size of a magnetic material is decreased to a length scale comparable with the magnetic domain wall or exchange force distance, the material starts exhibiting unusual magnetic behaviors. For instance, well-studied ferromagnetic material like cobalt exhibits unique magnetic properties when it is in the form of nanoparticles (Fig. 1). For recording media application, perpendicular recording (using out-of-plane oriented data bit) has several advantages over parallel recording (in-plane), such as high density data storage, high stability, and short bit length. In order to make this technology possible, out-of-plane oriented magnetic data bit with large magnetic anisotropy and greater stability are desired. Similarly, nanowires with high length to width aspect ratio exhibit large coercivity, large anisotropy and high remanence due to the shape and crystalline anisotropy. Ferromagnetic nanowires have potential applications in magnetic MEMS (micro-electric-mechanical-system). Nanowires can also be utilized for sensing, manipulating and separating biological cells and these applications require high coercivities and stability at ambient conditions.
In. Fig. 2 zero field magnetic force microscopy (MFM) image of a one dimensional patterned Co nanoparticles produced by pulsed laser interference irradiation is shown. These single domain nanoparticles all have their magnetization direction oriented out-of-plane, as evidenced by the nature of the MFM contrast. This effect is a result of magnetostrictiion resulting from strain due to rapid cooling rates during laser processing. By choosing materials with different magnetostrictive signs and nanoparticle size one can tailor the orientation of the magnets.
In. Fig. 3 Surface-Magneto Optical Kerr Effect (SMOKE) measurements of magnetization behavior in Co nanoarrays is shown for polar (perpendicular to substrate) and longitudinal (parallel to substrate) geometry measurements. The hysteresis behavior shows that it is easier to switch the orientation along the plan as compare to perpendicular to it. Such behavior is evidence for the magnetic anisotropy in such nanoparticles.
One extremely interesting magnetic state that has gained momentum in recent years is the skyrmionic state. It is characterized by a vortex where the edge magnetic moments point opposite to the core. Although skyrmions have many possible realizations, in practice, creating them in a laboratory is a difficult task to accomplish. In this work, different methods for skyrmion generation and customization are suggested. Skyrmionic behavior was numerically observed in minimally customized simulations of spheres, hemisphere, ellipsoids, and hemiellipsoids, for typical Cobalt parameters, in a range approximately 40–120 nm in diameter simply by applying a field. In. Fig. 4 a vector plot of the skyrmion state in a sphere of radius 59 nm is shown. The slice is along the equator of the sphere. Only a subset of local magnetic moments is shown for clarity.