Nanomanufacturing Research in Ramki Kalyanaraman's group

Nanomanufacturing is the field that deals with the synthesis, fabrication and processing of materials whose nanoscale components provide new and useful behavior that can be harnessed in applications over multiple length scales, such as a single nanowire electrical conductor to structural materials made of nanoparticles. The ability to efficiently, smartly and cheaply manufacture the required nanomaterial is critical towards realizing future breakthrough technologies that will arise due to nanoscale behaviors. We are especially interested in building multi-functional 3D structures, such as that shown in the figure on the right, that combine several different useful properties like sensing by surface plasmons, energy harvesting of solar energy and data storage by magnetism.

CHALLENGES: How does one put multiple useful functions together? We approach this by a layered solution, in which one or more layers provide the necessary function by a suitable combination of material and structure. In order to create the useful layers, we have been inspired by routes found in nature, i.e. self-organization. Over the past half a decade our focus has been on understanding the fundamental behavior of self-organization in ultrathin metal, semiconductor and oxide ceramic films, under nanosecond pulsed laser melting. We have become one of the leaders in the field of pulsed laser dewetting, which is now growing as a potential means to fabricate a variety of useful nanostructures. Please choose one of the topics to read about this and other ways we make nano- and macro- materials.

Bilayer Liquid Self-Organization Transient Marangoni Dewetting Spinodal Dewetting Laser Interference Self-Organization Dynamic Self-Organization

Bilayer Liquid Self-Organization

In this research we are coming up with ways to fabricate multimetal alloy and composite nanoparticles with desired physical properties by self-organization. Future multi-functional materials that combine various physical effects, such as ferromagnetism, strong localized surface plasmon resonance, magneto-optics, and catalysis, could be made from mixtures and/or alloys of multiple metals. Surfaces comprised of such nanomaterials will enable unique platforms to perform biocatalysis and biosensing as well as to control the polarization and current of photons, and electrons. Since the resulting properties of such nanomaterials will be determined by size, spacing, shape, and composition, appropriate cost-effective synthesis techniques must be investigated. Towards this goal, nanoscale pattern forming by a self-organizing route, in which intrinsic material parameters control nanostructure formation, is a strong candidate because of the inherent predictability and robustness of such a process. In this research we are focused on the laser melting of ultrathin multilayers and their subsequent evolution into fascinating patterns that also have useful functions. As depicted in Fig. 1, understanding how to control the final nanoscale structure, size, spacing, shape and composition are central to our research. Our initial studies have been primarily on immiscible metallic liquids, such as of Ag and Co, and this has yielded us a rich array of scientific and technological breakthroughs, including discovery of new nanostructures and physical properties.

H. Krishna et al ACS Nano 2010; M. Khenner et al Phys. Fluids 2011; S. Yadavali et al, Phys. Rev. B.; R. Sachan et al. Nanotechnology 2012.

The relationship between the size of the bimetallic nanoparticles and the processing parameters, such as thickness of the bilayer films is critical to establishing the relation between size, spacing and composition and eventually to determine if the technique is viable as a manufacturing process. By performing experiments in conjunction with self-organization theory we establish a parameter space that provides the range of independent control possible for particle size and composition. We found that the nanoparticle size could be changed independently by one order of magnitude while composition could be changed independently by ~85% for the Ag-Co system. As a result, large and predictable tunability in the physical properties is feasible, as demonstrated for the localized surface plasmon resonance in this Ag-Co system.

R. Sachan et al Nanotechnology 2012

Nanosecond Transient Marangoni Dewetting

In this discovery-driven topic we are exploring the role of thermal transients on self-organization. When an nanoscopically thin film is heated by a nanosecond laser pulse, the film temperature is a non-monotonic function of local film height variations, as shown in Fig. 1. This is a result of the thickness variation in optical reflectivity, as well as the thickness and time dependence of energy absorption and heat flow. As a result transient temperature gradients can result along the plane of the deforming liquid surface that produce transient thermocapillary or marangoni effects. Consequently self-organization phenomenon, such as by dewetting, can be modified and result in new length scales of pattern formation. We have seen excellent correlation between experiments and theoretical works on this subject.

J. Trice et al Phys. Rev. Lett 2008; H. Krishna et al PCCP 2009; N. Shirato et al, J. Appl. Phys. 2009.

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Spinodal Dewetting

In this research we are investigating pulsed laser dewetting of single layer films. Ordered metal nanoparticle arrays have become an active area of research due to their potential applications in nonlinear optics and nanophotonics below the diffraction limit. However, practical realization of such applications require that cost-effective and reliable processes be developed for the manufacturing of ordered metal nanoarrays. Natural phenomena that lead to the formation of patterns with characteristic length scales and predictable time scales could be exploited in designing such nanomanufacturing processes. One route is to exploit instabilities of a thin fluid film leading to spatial patterns with length and time scales that depend on the thermophysical material properties such as interfacial tension, contact angle with the substrate, fluid viscosity and, for ultrathin films, long range dispersion forces such as the van der Waal's interaction. An example especially pertinent to thin film pattern formation is the hydrodynamic dewetting instability such as spinodal dewetting which occurs when attractive intermolecular forces exceed the stabilizing effect of interfacial tension. Under such conditions, spontaneous film thickness fluctuations could be amplified and this will result in the breakup of the film leading eventually to the formation of drops/particles with well defined spatial order. In this work we have investigated this possibility by utilizing pulsed laser melting of nanoscopically thin metal films, as depicted in Fig. 1.

Some representative publications on this topic can be found here.C. Favazza et al Nanotechnology 2006; H. Krishna et al Nanotechnology 2010; J. Trice et al, Phys. Rev. B. 2007.

In Fig. 3 the morphology from spontaneous dewetting of ultrathin Ag films on SiO_{2} under nanosecond laser melting is found to be film thickness dependent. For films below a certain thickness the intermediate stages of the morphology consisted of bicontinuous structures, while above it it consisted of regularly-sized holes. Ag dewetting is consistent with the spinodal dewetting instability and this morphology transition behavior is very similar to previous observations in polymer films. Based on the behavior of free energy curvature that incorporates intermolecular forces, we estimated that the experimentally observed morphological transition thickness corresponds to a minima in the free energy curvature, similar to previous results in polymer films.

H. Krishna et al Nanotechnology 2010

Laser Interference Self-Organization

Dynamic Self-Organization

Contact us for more information via email to ramki @ utk dot edu

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