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Menke MA, Li BM, Arnold MG, Mueller LE, Dietrich R, Zhou S, Kelley-Loughnane N, Dennis P, Boock JT, Estevez J, Tabor CE, Sparks JL. Silky Liquid Metal Electrodes for On-Skin Health Monitoring. Adv Healthc Mater 2024; 13:e2301811. [PMID: 37779336 DOI: 10.1002/adhm.202301811] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/07/2023] [Revised: 09/15/2023] [Indexed: 10/03/2023]
Abstract
Next generation on-skin electrodes will require soft, flexible, and gentle materials to provide both high-fidelity sensing and wearer comfort. However, many commercially available on-skin electrodes lack these key properties due to their use of rigid hardware, harsh adhesives, uncomfortable support structures, and poor breathability. To address these challenges, this work presents a new device paradigm by joining biocompatible electrospun spider silk with printable liquid metal to yield an incredibly soft and scalable on-skin electrode that is strain-tolerant, conformable, and gentle on-skin. These electrodes, termed silky liquid metal (SLiM) electrodes, are found to be over five times more breathable than commercial wet electrodes, while the silk's intrinsic adhesion mechanism allows SLiM electrodes to avoid the use of harsh artificial adhesives, potentially decreasing skin irritation and inflammation over long-term use. Finally, the SLiM electrodes provide comparable impedances to traditional wet and other liquid metal electrodes, offering a high-fidelity sensing alternative with increased wearer comfort. Human subject testing confirmed the SLiM electrodes ability to sense electrophysiological signals with high fidelity and minimal irritation to the skin. The unique properties of the reported SLiM electrodes offer a comfortable electrophysiological sensing solution especially for patients with pre-existing skin conditions or surface wounds.
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Affiliation(s)
- Maria A Menke
- Department of Chemical, Paper, and Biomedical Engineering, Miami University, Oxford, OH, 45056, USA
- Air Force Research Laboratory, Materials and Manufacturing Directorate, Wright-Patterson AFB, Dayton, OH, 45433, USA
| | - Braden M Li
- Air Force Research Laboratory, Materials and Manufacturing Directorate, Wright-Patterson AFB, Dayton, OH, 45433, USA
- Air Force Life Cycle Management Center, Human Systems Division, Wright-Patterson AFB, Dayton, OH, 45433, USA
| | - Meghan G Arnold
- Department of Chemical, Paper, and Biomedical Engineering, Miami University, Oxford, OH, 45056, USA
| | - Logan E Mueller
- Department of Chemical, Paper, and Biomedical Engineering, Miami University, Oxford, OH, 45056, USA
| | - Robin Dietrich
- Air Force Research Laboratory, Materials and Manufacturing Directorate, Wright-Patterson AFB, Dayton, OH, 45433, USA
| | - Shijie Zhou
- Department of Chemical, Paper, and Biomedical Engineering, Miami University, Oxford, OH, 45056, USA
| | - Nancy Kelley-Loughnane
- Air Force Research Laboratory, Materials and Manufacturing Directorate, Wright-Patterson AFB, Dayton, OH, 45433, USA
| | - Patrick Dennis
- Air Force Research Laboratory, Materials and Manufacturing Directorate, Wright-Patterson AFB, Dayton, OH, 45433, USA
| | - Jason T Boock
- Department of Chemical, Paper, and Biomedical Engineering, Miami University, Oxford, OH, 45056, USA
| | - Joseph Estevez
- Naval Air Warfare Center, Weapons Division, China Lake, CA, 93555, USA
| | - Christopher E Tabor
- Air Force Research Laboratory, Materials and Manufacturing Directorate, Wright-Patterson AFB, Dayton, OH, 45433, USA
| | - Jessica L Sparks
- Department of Chemical, Paper, and Biomedical Engineering, Miami University, Oxford, OH, 45056, USA
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2
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Li BM, Reese BL, Ingram K, Huddleston ME, Jenkins M, Zaets A, Reuter M, Grogg MW, Nelson MT, Zhou Y, Ju B, Sennik B, Farrell ZJ, Jur JS, Tabor CE. Textile-Integrated Liquid Metal Electrodes for Electrophysiological Monitoring. Adv Healthc Mater 2022; 11:e2200745. [PMID: 35734914 DOI: 10.1002/adhm.202200745] [Citation(s) in RCA: 4] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/04/2022] [Revised: 05/12/2022] [Indexed: 01/27/2023]
Abstract
Next generation textile-based wearable sensing systems will require flexibility and strength to maintain capabilities over a wide range of deformations. However, current material sets used for textile-based skin contacting electrodes lack these key properties, which hinder applications such as electrophysiological sensing. In this work, a facile spray coating approach to integrate liquid metal nanoparticle systems into textile form factors for conformal, flexible, and robust electrodes is presented. The liquid metal system employs functionalized liquid metal nanoparticles that provide a simple "peel-off to activate" means of imparting conductivity. The spray coating approach combined with the functionalized liquid metal system enables the creation of long-term reusable textile-integrated liquid metal electrodes (TILEs). Although the TILEs are dry electrodes by nature, they show equal skin-electrode impedances and sensing capabilities with improved wearability compared to commercial wet electrodes. Biocompatibility of TILEs in an in vivo skin environment is demonstrated, while providing improved sensing performance compared to previously reported textile-based dry electrodes. The "spray on dry-behave like wet" characteristics of TILEs opens opportunities for textile-based wearable health monitoring, haptics, and augmented/virtual reality applications that require the use of flexible and conformable dry electrodes.
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Affiliation(s)
- Braden M Li
- Department of Textile Engineering, Chemistry and Science, North Carolina State University, Raleigh, NC, 27606, USA.,Air Force Research Laboratory, Materials and Manufacturing Directorate, Wright-Patterson AFB, Dayton, OH, 45433, USA.,Air Force Life Cycle Management Center, Human Systems Division, Wright-Patterson AFB, Dayton, OH, 45433, USA
| | - Brandon L Reese
- Department of Physics, Miami University, Oxford, OH, 45056, USA.,UES Inc, Dayton, OH, 45432, USA
| | - Katherine Ingram
- Air Force Research Laboratory, 711th Human Performance Wing, Airman Systems Directorate, Wright-Patterson AFB, Dayton, OH, 45433, USA
| | - Mary E Huddleston
- Air Force Research Laboratory, 711th Human Performance Wing, Airman Systems Directorate, Wright-Patterson AFB, Dayton, OH, 45433, USA
| | - Meghan Jenkins
- Air Force Research Laboratory, 711th Human Performance Wing, Airman Systems Directorate, Wright-Patterson AFB, Dayton, OH, 45433, USA
| | - Allison Zaets
- Air Force Research Laboratory, 711th Human Performance Wing, Airman Systems Directorate, Wright-Patterson AFB, Dayton, OH, 45433, USA
| | - Matthew Reuter
- Air Force Research Laboratory, 711th Human Performance Wing, Airman Systems Directorate, Wright-Patterson AFB, Dayton, OH, 45433, USA
| | - Matthew W Grogg
- Air Force Research Laboratory, 711th Human Performance Wing, Airman Systems Directorate, Wright-Patterson AFB, Dayton, OH, 45433, USA
| | - M Tyler Nelson
- Air Force Research Laboratory, 711th Human Performance Wing, Airman Systems Directorate, Wright-Patterson AFB, Dayton, OH, 45433, USA
| | - Ying Zhou
- Department of Textile Engineering, Chemistry and Science, North Carolina State University, Raleigh, NC, 27606, USA
| | - Beomjun Ju
- Department of Textile Engineering, Chemistry and Science, North Carolina State University, Raleigh, NC, 27606, USA
| | - Busra Sennik
- Department of Textile Engineering, Chemistry and Science, North Carolina State University, Raleigh, NC, 27606, USA
| | - Zachary J Farrell
- Air Force Research Laboratory, Materials and Manufacturing Directorate, Wright-Patterson AFB, Dayton, OH, 45433, USA.,UES Inc, Dayton, OH, 45432, USA
| | - Jesse S Jur
- Department of Textile Engineering, Chemistry and Science, North Carolina State University, Raleigh, NC, 27606, USA
| | - Christopher E Tabor
- Air Force Research Laboratory, Materials and Manufacturing Directorate, Wright-Patterson AFB, Dayton, OH, 45433, USA
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3
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Abstract
Recent developments in autonomous engineered matter have introduced the ability for intelligent materials to process environmental stimuli and functionally adapt1-4. To formulate a foundation for such an engineered living material paradigm, researchers have introduced sensing5-11 and actuating12-16 functionalities in soft matter. Yet, information processing is the key functional element of autonomous engineered matter that has been recently explored through unconventional techniques with limited computing scalability17-20. Here we uncover a relation between Boolean mathematics and kinematically reconfigurable electrical circuits to realize all combinational logic operations in soft, conductive mechanical materials. We establish an analytical framework that minimizes the canonical functions of combinational logic by the Quine-McCluskey method, and governs the mechanical design of reconfigurable integrated circuit switching networks in soft matter. The resulting mechanical integrated circuit materials perform higher-level arithmetic, number comparison, and decode binary data to visual representations. We exemplify two methods to automate the design on the basis of canonical Boolean functions and individual gate-switching assemblies. We also increase the computational density of the materials by a monolithic layer-by-layer design approach. As the framework established here leverages mathematics and kinematics for system design, the proposed approach of mechanical integrated circuit materials can be realized on any length scale and in a wide variety of physics.
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Affiliation(s)
- Charles El Helou
- Department of Mechanical Engineering, The Pennsylvania State University, University Park, PA, USA
| | - Benjamin Grossmann
- Materials and Manufacturing Directorate, Air Force Research Laboratory, Wright-Patterson AFB, OH, USA
| | - Christopher E Tabor
- Materials and Manufacturing Directorate, Air Force Research Laboratory, Wright-Patterson AFB, OH, USA
| | - Philip R Buskohl
- Materials and Manufacturing Directorate, Air Force Research Laboratory, Wright-Patterson AFB, OH, USA
| | - Ryan L Harne
- Department of Mechanical Engineering, The Pennsylvania State University, University Park, PA, USA.
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4
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Gomez EF, Wanasinghe SV, Flynn AE, Dodo OJ, Sparks JL, Baldwin LA, Tabor CE, Durstock MF, Konkolewicz D, Thrasher CJ. 3D-Printed Self-Healing Elastomers for Modular Soft Robotics. ACS Appl Mater Interfaces 2021; 13:28870-28877. [PMID: 34124888 DOI: 10.1021/acsami.1c06419] [Citation(s) in RCA: 17] [Impact Index Per Article: 5.7] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/12/2023]
Abstract
Advances in materials, designs, and controls are propelling the field of soft robotics at an incredible rate; however, current methods for prototyping soft robots remain cumbersome and struggle to incorporate desirable geometric complexity. Herein, a vat photopolymerizable self-healing elastomer system capable of extreme elongations up to 1000% is presented. The material is formed from a combination of thiol/acrylate mixed chain/step-growth polymerizations and uses a combination of physical processes and dynamic-bond exchange via thioethers to achieve full self-healing capacity over multiple damage/healing cycles. These elastomers can be three dimensional (3D) printed with modular designs capable of healing together to form highly complex and large functional soft robots. Additionally, these materials show reprogrammable resting shapes and compatibility with self-healing liquid metal electronics. Using these capabilities, subcomponents with multiple internal channel systems were printed, healed together, and combined with functional liquid metals to form a high-wattage pneumatic switch and a humanoid-scale soft robotic gripper. The combination of 3D printing and self-healing elastomeric materials allows for facile production of support-free parts with extreme complexity, resulting in a paradigm shift for the construction of modular soft robotics.
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Affiliation(s)
- Eliot F Gomez
- Air Force Research Laboratory, Materials and Manufacturing Directorate, Wright-Patterson AFB, Ohio 45433, United States
- UES Inc., Dayton, Ohio 45432, United States
| | - Shiwanka V Wanasinghe
- Department of Chemistry and Biochemistry, Miami University, 651 E High Street, Oxford, Ohio 45056, United States
| | - Alex E Flynn
- Air Force Research Laboratory, Materials and Manufacturing Directorate, Wright-Patterson AFB, Ohio 45433, United States
| | - Obed J Dodo
- Department of Chemistry and Biochemistry, Miami University, 651 E High Street, Oxford, Ohio 45056, United States
| | - Jessica L Sparks
- Department of Chemical, Paper, and Biomedical Engineering, Miami University, 650 E High Street, Oxford, Ohio 45056, United States
| | - Luke A Baldwin
- Air Force Research Laboratory, Materials and Manufacturing Directorate, Wright-Patterson AFB, Ohio 45433, United States
| | - Christopher E Tabor
- Air Force Research Laboratory, Materials and Manufacturing Directorate, Wright-Patterson AFB, Ohio 45433, United States
| | - Michael F Durstock
- Air Force Research Laboratory, Materials and Manufacturing Directorate, Wright-Patterson AFB, Ohio 45433, United States
| | - Dominik Konkolewicz
- Department of Chemistry and Biochemistry, Miami University, 651 E High Street, Oxford, Ohio 45056, United States
| | - Carl J Thrasher
- Air Force Research Laboratory, Materials and Manufacturing Directorate, Wright-Patterson AFB, Ohio 45433, United States
- UES Inc., Dayton, Ohio 45432, United States
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5
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Abstract
Liquid metals are a promising functional material due to their unique combination of metallic properties and fluidity at room temperature. They are of interest in wide-ranging fields including stretchable and flexible electronics, reconfigurable devices, microfluidics, biomedicine, material synthesis, and catalysis. Transformation of bulk liquid metal into particles has enabled further advances by allowing access to a broader palette of fabrication techniques for device manufacture or by increasing area available for surface-based applications. For gallium-based liquid metal alloys, particle stabilization is typically achieved by the oxide that forms spontaneously on the surface, even when only trace amounts of oxygen are present. The utility of the particles formed is governed by the chemical, electrical, and mechanical properties of this oxide. To overcome some of the intrinsic limitations of the native oxide, it is demonstrated here for the first time that 2D graphene-based materials can encapsulate liquid metal particles during fabrication and imbue them with previously unattainable properties. This outer encapsulation layer is used to physically stabilize particles in a broad range of pH environments, modify the particles' mechanical behavior, and control the electrical behavior of resulting films. This demonstration of graphene-based encapsulation of liquid metal particles represents a first foray into the creation of a suite of hybridized 2D material coated liquid metal particles.
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Affiliation(s)
- Megan A Creighton
- Air Force Research Laboratory, Materials and Manufacturing Directorate, Wright-Patterson Air Force Base, Dayton, OH, USA.
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6
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Abstract
Gallium alloys with other low melting point metals, such as indium or tin, to form room-temperature liquid eutectic systems. The gallium in the alloys rapidly forms a thin surface oxide when exposed to ambient oxygen. This surface oxide has been previously exploited for self-stabilization of liquid metal nanoparticles, retention of metastable shapes, and imparting stimuli-responsive behavior to the alloy surface. In this work, we study the effect of water as an oxidant and its role in defining the alloy surface chemistry. We identify several pathways that can lead to the formation of gallium oxide hydroxide (GaOOH) crystallites, which may be undesirable in many applications. Furthermore, we find that some crystallite formation pathways can be reinforced by typical top-down particle synthesis techniques like sonication. This improved understanding of interfacial interactions provides critical insight for process design and implementation of advanced devices that utilize the unique coupling of flexibility and conductivity offered by these gallium-based liquid metal alloys.
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Affiliation(s)
- Megan A Creighton
- National Research Council, Washington, DC 20001, United States
- Materials and Manufacturing Directorate, Air Force Research Laboratory, Wright-Patterson Air Force Base, Dayton, Ohio 45433, United States
| | - Michelle C Yuen
- National Research Council, Washington, DC 20001, United States
- Materials and Manufacturing Directorate, Air Force Research Laboratory, Wright-Patterson Air Force Base, Dayton, Ohio 45433, United States
| | - Michael A Susner
- UES, Inc., Dayton, Ohio 45431, United States
- Materials and Manufacturing Directorate, Air Force Research Laboratory, Wright-Patterson Air Force Base, Dayton, Ohio 45433, United States
| | - Zachary Farrell
- UES, Inc., Dayton, Ohio 45431, United States
- Materials and Manufacturing Directorate, Air Force Research Laboratory, Wright-Patterson Air Force Base, Dayton, Ohio 45433, United States
| | - Benji Maruyama
- Materials and Manufacturing Directorate, Air Force Research Laboratory, Wright-Patterson Air Force Base, Dayton, Ohio 45433, United States
| | - Christopher E Tabor
- Materials and Manufacturing Directorate, Air Force Research Laboratory, Wright-Patterson Air Force Base, Dayton, Ohio 45433, United States
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7
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Morris NJ, Farrell ZJ, Tabor CE. Chemically modifying the mechanical properties of core-shell liquid metal nanoparticles. Nanoscale 2019; 11:17308-17318. [PMID: 31513218 DOI: 10.1039/c9nr06369b] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/10/2023]
Abstract
Eutectic gallium-indium (EGaIn) is a room temperature liquid metal that can be readily fabricated into nanoparticles which naturally form a thin, passivating gallium oxide shell. These core-shell nanoparticles are of interest for a variety of stimuli-responsive applications, which often utilize physical deformation of the particles to release the molten, conductive payload from within the gallium oxide shell. In the present work, we introduce a variety of chemical strategies to produce EGaIn nanoparticles which exhibit a wide range of gallium oxide shell thicknesses. These chemically modified oxide thicknesses are then correlated to the core-shell liquid nanoparticles' mechanical properties by subjecting the particles to orthogonal characterization techniques; XPS for measurement of the gallium oxide shell thickness and nanoindentation for measurement of particle stiffness and elastic modulus. Additionally, nanoindentation is used to determine the onset of particle rupture and resultant conductivity. Ultimately, quantification of the relationships between chemical treatment and derivative mechanical properties in liquid metal nanoparticles will enable advanced applications of these colloids, such as in tailored self-healing and responsive electronic devices.
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Affiliation(s)
- Nicholas J Morris
- UES, Inc., 4401 Dayton Xenia Rd, Dayton, OH 45432, USA and Air Force Research Laboratory, Dayton, OH, USA.
| | - Zachary J Farrell
- UES, Inc., 4401 Dayton Xenia Rd, Dayton, OH 45432, USA and Air Force Research Laboratory, Dayton, OH, USA.
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8
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Thrasher CJ, Farrell ZJ, Morris NJ, Willey CL, Tabor CE. Mechanoresponsive Polymerized Liquid Metal Networks. Adv Mater 2019; 31:e1903864. [PMID: 31403234 DOI: 10.1002/adma.201903864] [Citation(s) in RCA: 77] [Impact Index Per Article: 15.4] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/18/2019] [Revised: 07/20/2019] [Indexed: 05/15/2023]
Abstract
Room-temperature liquid metals, such as nontoxic gallium alloys, show enormous promise to revolutionize stretchable electronics for next-generation soft robotic, e-skin, and wearable technologies. Core-shell particles of liquid metal with surface-bound acrylate ligands are synthesized and polymerized together to create cross-linked particle networks comprising >99.9% liquid metal by weight. When stretched, particles within these polymerized liquid metal networks (Poly-LMNs) rupture and release their liquid metal payload, resulting in a rapid 108 -fold increase in the network's conductivity. These networks autonomously form hierarchical structures that mitigate the deleterious effects of strain on electronic performance and give rise to emergent properties. Notable characteristics include nearly constant resistances over large strains, electronic strain memory, and increasing volumetric conductivity with strain to over 20 000 S cm-1 at >700% elongation. Furthermore, these Poly-LMNs exhibit exceptional performance as stretchable heaters, retaining 96% of their areal power across relevant physiological strains. Remarkable electromechanical properties, responsive behaviors, and facile processing make Poly-LMNs ideal for stretchable power delivery, sensing, and circuitry.
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Affiliation(s)
- Carl J Thrasher
- Air Force Research Laboratory, Materials and Manufacturing Directorate, Wright-Patterson AFB, Dayton, OH, 45433, USA
| | - Zachary J Farrell
- Air Force Research Laboratory, Materials and Manufacturing Directorate, Wright-Patterson AFB, Dayton, OH, 45433, USA
- UES Inc., Dayton, OH, 45432, USA
| | - Nicholas J Morris
- Air Force Research Laboratory, Materials and Manufacturing Directorate, Wright-Patterson AFB, Dayton, OH, 45433, USA
- UES Inc., Dayton, OH, 45432, USA
| | - Carson L Willey
- Air Force Research Laboratory, Materials and Manufacturing Directorate, Wright-Patterson AFB, Dayton, OH, 45433, USA
- UES Inc., Dayton, OH, 45432, USA
| | - Christopher E Tabor
- Air Force Research Laboratory, Materials and Manufacturing Directorate, Wright-Patterson AFB, Dayton, OH, 45433, USA
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9
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Joshipura ID, Ayers HR, Castillo GA, Ladd C, Tabor CE, Adams JJ, Dickey MD. Patterning and Reversible Actuation of Liquid Gallium Alloys by Preventing Adhesion on Rough Surfaces. ACS Appl Mater Interfaces 2018; 10:44686-44695. [PMID: 30532957 DOI: 10.1021/acsami.8b13099] [Citation(s) in RCA: 17] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/09/2023]
Abstract
This work reports a simple approach to form, study, and utilize rough coatings that prevent the adhesion of gallium-based liquid metal alloys. Typically, liquids with large interfacial tension do not wet nonreactive surfaces, regardless of surface topography. However, these alloys form a surface oxide "skin" that adheres to many substrates, even those with low surface energy. This work reports a simple approach to render closed channels and surfaces, including soft materials, to be "oxide-phobic" via spray-coating (NeverWet, which is commercially available and inexpensive). Surface spectroscopic techniques and metrology tools elucidate the coatings to comprise silica nanoparticles grafted with silicones that exhibit dual length scales of roughness. Although prior work shows the importance of surface roughness in preventing adhesion, the present work confirms that both hydrophobic and hydrophilic rough surfaces prevent oxide adhesion. Furthermore, the coating enables reversible actuation through submillimeter closed channels to form a reconfigurable antenna in the gigahertz range without the need for corrosive acids or bases that remove the oxide. In addition, the coating enables open surface patterning of conductive traces of liquid metal. This shows it is possible to actuate liquid metals in air without leaving neither metal nor oxide residue on surfaces to enable reconfigurable electronics, microfluidics, and soft electrodes.
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Affiliation(s)
| | | | | | | | - Christopher E Tabor
- Materials and Manufacturing Directorate , Air Force Research Laboratories , Wright Patterson , Ohio , United States
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10
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Mahmoud, Tabor CE, El-Sayed MA, Ding Y, Wang ZL. A New Catalytically Active Colloidal Platinum Nanocatalyst: The Multiarmed Nanostar Single Crystal. J Am Chem Soc 2008; 130:4590-1. [DOI: 10.1021/ja710646t] [Citation(s) in RCA: 256] [Impact Index Per Article: 16.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Affiliation(s)
- Mahmoud
- Laser Dynamics Laboratory, School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332-0400 and School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0245
| | - Christopher E. Tabor
- Laser Dynamics Laboratory, School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332-0400 and School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0245
| | - Mostafa A. El-Sayed
- Laser Dynamics Laboratory, School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332-0400 and School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0245
| | - Yong Ding
- Laser Dynamics Laboratory, School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332-0400 and School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0245
| | - Zhong Lin Wang
- Laser Dynamics Laboratory, School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332-0400 and School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0245
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