Research

While "super-sizing" seems to be the driving force of our food industry, the direction of materials research has been quite the opposite: the dimensions of most technological devices are getting ever smaller. These advances in nanotechnology have a tremendous impact on parts of the economy as diverse as information, energy, health, agriculture, security, and transportation. Some of the examples include data storage at densities greater than one terabit per square inch, high-efficiency solid-state engines, single-cell diagnostics of complex diseases (e.g. cancer), and the development of ultra-light yet super-strong materials for vehicles, with the component sizes comprising these technological devices reduced to the sub-micron scale.

Hear about our research from Professor Greer herself!

The functionality of these devices directly depends on their structural integrity and mechanical stability, driving the necessity to understand and to predict mechanical properties of materials at reduced dimensions. Yield and fracture strengths, for example, have been found to deviate from classical mechanics laws and therefore can no longer be inferred from the bulk response or from the literature. Unfortunately, the few existing experimental techniques for assessing mechanical properties at that scale are insufficient, not easily accessible, and are generally limited to thin films. In order to design reliable devices, a fundamental understanding of mechanical properties as a function of feature size is desperately needed; with the key remaining question whether materials really are stronger when the instrumental artifacts are removed, and if so then why and how.

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MATERIALS WITH CONTROLLED MICROSTRUCTURAL ARCHITECTURE: ENERGY SCIENCES AND STRUCTURAL APPLICATIONS

Researchers: Lucas Meza, Viki Chernow, Lauren Montemayor, Jan Rys (visiting student)
Dongchan Jang (postdoc)

Collaborators: Mike Baskes (LANL), Bill Carter, Alan Jacobsen (HRL), Lorenzo Valdevit (UC Irvine), John Hutchinson (Harvard), Frank Greer (JPL), Jen Dionne (Stanford), Eugen Rabkin (Technion), David Srolovitz (UPenn), and Yong Wei Zhang (IHCP, Singapore)


Hierarchical Design of Architected Materials

Micro-truss structure.

One of the major limitations in the development of current energy technology is the lack of advanced materials suitable for energy applications. We have already reached the limits in the capabilities of bulk materials for applications in fields like thermoelectrics, solar cells, and structural materials. With the advent of micro and nanoscale fabrication techniques, we have been able to make great advances many of these critical areas of energy technology.

A hollow nickel-phosphorus micro-truss is the lowest density structure ever made. Read the Science report here. Photo by Dan Little, HRL Laboratories LLC.

As society moves towards employing architecture to create new materials with unprecedented properties, the critical length scales of individual structural members and of the material microstructure become comparable with one another, and leaps not strides need to be made to understand the governing deformation mechanisms in such material systems.

We explore the potential of using micro and nanoscale truss materials for the devlopment of materials for advanced energy applications. Using a technique known as Two Photon Lithography (TPL) Direct Laser Writing (DLW), we create arbitrarily complex 3D structures with features on nanometer length scales in a process analogous to rapid prototyping. TPL DLW technology has now matured to a point where there are reliable commercial products that are capable of creating nanostructures with ~100 nm laterial feature sizes. We use these structures as a scaffold to deposit materials onto using techniques like ALD, CVD, and electroless deposition. The polymer can then be etched out, leaving behind a hollow nanoscale truss structure. The following research thrusts are currently being pursued.

  1. STRUCTURAL MECHANICS in HIERARCHICAL MATERIALS. (L. Montemayor, L. Meza, D. Chen, J. Rys)
    Creation of extremely strong yet ultra-light materials can be achieved by capitalizing on the hierarchical design of metallic nano-trusses which promise superb thermomechanical properties at extremely low mass densities (lighter than aerogels), making these solid foams dieal for many scientific and technological applications. Yet, the dominant deformation mechanisms in such "meta-materials", where individual constituent size is comparable to the characteristic length scale of the material, are essentially unknown. This gap in current knowledge stems from severe limitations in fabrication methodologies, experimental data, and simulations that model each relevant scale while capturing the overall structural complexity.
      In this project we explore the combination of stretching-dominated truss geometries and nanoscale material strengthening effects to create ultralight materails with very high stregths, fracture toughness, and stiffnesses. It is our goal with this research to create materials that will outperform current lightweight strucutural materials like honeycombs and foams. Theoretical aspects of this work are being conducted in collaboation with Prof. D. Kochmann's group. Various aspects of this project are funded by the NSF, NASA, and DARPA.

  2. LOW K THERMOELECTRIC MATERIALS. (L. Meza)
    By creating materials with features on the same order as the phonon mean free path in a meterial, it is possible to artificially lower the thermal conductivity. This phenomenon has been exploited in materials known as superlattices, where, by layering thin film materials, it is possible to achieve ultralow thermal conductivites. In this work, we will be fabricating bulk superlattice materials in the form of nanoscale truss structures. Because these materials will have the effect of impeding phonon transport while leaving electrical conductivity relatively the same, they have a great potential for applications as thermoelectric materials. Thermal experiments are being conducted in collaboration with Prof. A. Minnich's group

  3. PHOTOVOLTAICS. (V. Chernow)
    Nanophotonic light trapping provides the potential to improve the current generation of solar cells as well as to create high-efficiency cells using novel materials and desgins. In addition to their intrinsic potential for improving light trapping, periodic geometries facilitate a more direct comparison between theory (and computations) and experiment. The main light trapping mechanism in periodic arrays is the interference between different modes propagating through the structure. In 3D photonic crystals, optical modes interfere either constructuively or desctructively, leading to either high transmission through the structure or high reflectance.
      The nano-trusses fabricated in this project allow for tuning the periodicity of 3-dimensional lattice such that the resulting photonic crystal has (as close to) zero reflection losses. As solar cells today broadly utilize anti-reflective coatings (ARCs) to collect as much light as possible - our nano-trusses will serve as extremely efficient and lightweight, damage-tolerant ARCs. Optical measurements and theory are being conducted in collaboraiton with Prof. J. Dionne's group. Funding for this project is provided by the Dow-Resnick

  4. LITHIUM ION BATTERIES. (X. Wendy Gu)
    Most commercial Li-ion batteries have positive and negative electrodes, which are continuously intercalated by Li atoms. Lithiation is a fundamental process that takes place in Si anodes in Li-ion based batteries when Li+ cations enter the Si lattice during electric charge. Silicon is particularly attractive as anode material due to its exceedingly high theoretical lithium storage capacity of 4,200 mAh/g. However, upon lithiation, Si loses its crystalline structure and the alloy becomes amorphous, with an attendant volume increase in the range of 300-400%. This volume increase creates large internal stresses that may induce plastic deformation, damage and fracture of the anode, and commonly lead to device failure after only a few charge cycles. The nano-trusses will integrate the capability of long-term battery cycling with minimal irreversible structural changes that commonly degrade storage capacity in solid electrolytes, in conjunction with close-to-zero global expansion coefficient, extremely light weight, and macro-scale flexibility.
      The focus of this project is to design, develop, and evalute nano and micro-truss silicon-based metamaterials for Li ion batteries applications. We are designing hollow periodic Si nano-trusses with all characteristic dimensions below 1 µm such that no overall volumetric expansion during lithiation-delithiation cycling occurs, which has been the major cause of degradation and performance loss in Si-based Li ion batteries. This work is being conducted in collaboration with Prof. M. Ortiz's group who are using computational models to identify promising nano-truss designs and to predict their performance and mechnical reliability over multiple cycles.

MECHANICAL PROPERTIES AND DEFORMATION MECHANISMS IN NANO-SCALE SOLIDS

National Science Foundation Office of Naval Research

Researchers: Zachary Aitken, Xun (Wendy) Gu
Dongchan Jang, Seok-Woo Lee (postdocs)

Collaborators: Chris Weinberger (Sandia NL), Huajian Gao (Brown University), Karin Dahmen (UIUC), and Dennis Kochmann (Caltech)


A long-standing problem in materials science is that of predicting mechanical properties from knowing the initial microstructure and the collective behavior of individual defects. The objective of this project is to develop a physical understanding of plasticity mechanisms operating in single-crystalline, amorphous, and nano-crystalline solids, when material dimensions are reduced to the nanometer scale (or to the size of their instrinsic microstructure) subjected to homogeneous deformation.

Our in-situ experiments are performed in a unique instrument "SEMentor" (SEM + Nanoindenter + Mentor = SEMentor), which is capable of conducting nano mechanical experiments at room temperature down to the cryogenic temperatures while simultaneously capturing video.

The stress-strain data obtained by the SEMentor, combined with the pre- and post- mortem TEM microstructural characterization and modeling efforts present a powerful method for gaining a more thorough understanding of plasticity at the nano-scale. This combination of techniques provides us with critical information for the development of constitutive laws that govern flow stress as a function of size and initial microstructure through an experimentally-based understanding of the underlying phenomena.


The SEMentor


Tension on 100nm-diameter nano crystalline Pt nano-pillar with intentionally fabricated notch (courtesy of Wendy Gu).

Some specific on-going efforts of this project are:

  1. Investigation of how surfaces affect material strength and defect nucleation: The need for understanding the effect of free surfaces on defect behavior is furthered by their weakening, as the ideal surface strength has been reported to be lower than the ideal bulk strength. more info
  2. Interrogation of the properties of individual boundaries: Internal boundaries can act as barriers to and/or sources of plasticity. Nanoscale mechanical testing allows us to probe the specific properties of grain boundaries by characterizing and isolating specific boundaries under homogeneous deformation.
  3. Enhancing the mechanical properties of the nano-materials through properly designing and constructing the internal interfaces: Depending on their interfacial structure (e.g. coherent vs. incoherent), relative orientation (e.g. regular grain boundary vs. mirror-symmetric twin boundary), or global configuration (e.g. random network vs. highly parallel and perpendicular or inclined to the loading axis), the materials' mechanical properties vary drastically. We will investigate the effect of the interface characteristics on the mechanical properties of the nano-materials, and will try to create the superior nano-materials for various structural applications.
  4. Assessment of mechanical response of nano-pillars through uniaxial compression and tension experiments: When metals have one dimension reduced to nanoscale sizes, the strength is found to increase dramatically. The extent and type of strengthening is being investigated in many different classes of metals in order to fully understand the unique deformation mechanisms. The SEMentor allows for in-situ tensile testing that continues to enhance the current understanding of mechanical behavior of materials at the nano-scale by providing information on their ductility, fracture and ultimate tensile strengths. It has also allowed for experimental insight into the complex tension-compression asymmetry found in some BCC materials and will allow for the experimental investigation of a similar tension-compression asymmetry in FCC materials, as predicted computationally.
  5. Post-deformation cross-sectional TEM analysis of nano-scale specimens, which will shed light on dislocation activity (crystals) and on possible nano- crystallization (bulk metallic glasses) during and after the deformation. While the SEMentor is not capable of capturing the motion of individual mobile dislocations, a specific dislocation network (or lack thereof) evolved as a result of applied deformation can be visualized and analyzed via TEM.
  6. Application of current understanding of nanoscale systems to develop macroscopic materials while maintaining the unique properties found only at these small length scales. These building blocks will be used to engineer new materials and study their resulting deformation mechanisms. Emphasis will be put on developing structures that harness both material and higher-level structural properties to develop composites with unique physical response.
  7. Fracture mechanics at the nanoscale: Uniaxial tension tests are performed on micron-sized nanocrystalline pillars that contain nanoscale stress concentrations such as sharp cracks and notches. This provides experimental tests of intriguing theoretical work predicting flaw tolerance at the nanoscale (Gao 2003, PNAS) and provides insight on constitutive relations in nanoscale fracture.
  8. Individual Ni3Al Nano-Cubes under Pressure: Defect-free crystals of all sizes generally require the application of close-to-theoretical stresses to deform. Testing this classical tenet is challenging even in bulk or micro-sized samples, because defects naturally emerge during either crystal growth or sample fabrication. At the nano-scale, however, it is possible to create dislocation-free mechanical specimens, enabling either confirmation or refutal of this classical hypothesis. more info

    The SEMentor


RADIATION DAMAGE TOLERANCE IN INTERFACE-CONTAINING METALLIC NANO STRUCTURES

Researchers: Rachel Liontas (graduate student) and Dongchan Jang (postdoc)

Collaborators: Peter Hosemann (UC Berkeley), Mike Demkowicz (MIT), Khalid Hattar (Sandia NL), and Amit Misra (LANL)

Due to imminent demand for clean and sustainable energy sources, there is an emerging focus to utilize nuclear energy as a viable source of energy. New reactors should be significantly safer and more reliable, should have an increased lifetime, proliferation resistance and efficiency trough higher operating temperatures. The demanding service conditions like higher neutron doses, exposure to higher temperatures, and corrosive environments will pose a significant challenge for structural materials in these new reactors. Therefore, the development of new, revolutionary materials systems immune to radiation damage is crucial.

During irradiation, energetic neutrons displace atoms from their lattice sites, creating collision cascades, and affecting the material in an unfavorable way. These displacement cascades involve thousands of atoms, and subsequent defect reactions create stable damage aggregates like vacancy clusters, dislocation loops, and - sometimes - the bubbles, notorious for their role in swelling and embrittlement of irradiated materials. In order to develop a fundamental understanding on the damaging effects of radiation on material health, it is necessary to study these effects in isolated (as opposed to an ensemble) microstructural features. This requires testing "small" samples, whose dimensions are comparable with the intrinsic microstructure size. In order to create innovative materials capable of enhanced radiation resistance, it is imperative to develop a fundamental understanding of deformation processes occurring in the irradiated microstructure. This will serve as foundation for the development of predictive models, which in turn will guide the design codes and defect-assessment criteria that ensure structural integrity and safety of nuclear reactor operation.

Figure 1. SEM images of tension experiment setup of interface-containing nano-pillars: (a) As-plated 100nm-diameter Cu-Fe pillars- before tension. (b) After tension- fracture by shear. (c) After tension- fracture at the interface.

This project seeks to obtain quantitative influence of helium sink strength and proximity on helium bubble nucleation and growth in He-irradiated nano-scale metallic structures, and the resulting deformation mechanisms and mechanical properties by applying experimental and computational methods. Nano-scale tension and compression experiments, both ex-situ and in-situ, on low energy He-irradiated samples combined with site-specific microstructural characterization and modeling efforts present a powerful method for gaining this understanding. This systematic approach provides critical information for identifying key factors that govern He bubble nucleation and growth upon irradiation as a function of both sink strength and sink proximity through an experimentally confirmed physical understanding.

We study different interfaces in order of increasing affinity for vacancy and helium absorption: grain boundaries, interfaces and periodically arranged metallic glass/metal multi-layers spanning the entire nano-pillar volume. A FIB-less nano-pillar fabrication approach with precise initial microstructure control (both dislocation density and grain boundaries) and experimentally determined stress-strain relationships are enhanced by in-situ SEM observations coupled with TEM microstructural characterization of the same samples before and after deformation and atomistic modeling.

DEFORMATION RESPONSE OF VERTICALLY ALIGNED CARBON NANOTUBE (VACNT) FORESTS

ARO through Institute for Collaborative Biotechnologies

Researchers: Nisha Mohan, Ee Jane Lim (undergrad)
Sid Pathak (formerly)

Collaborators: A. Needleman (Univ of North Texas), B. Cola (GA Tech), J. Hart (Univ of Michigan)

Keck Institute for Space Studies

Fig. 1 SEM images reveal the hierarchical morphology of the (top) 141.5 μm thick VACNT films (magnification 260×), which consist of (middle) nominally vertical aligned CNTs visible at a lower magnification of 30k×, and (bottom) a complex intertwined network seen at higher magnifications of 240k×. SEM pictures are taken at a 60 deg tilt angle. (bottom inset) Individual multiwalled CNTs of outer diameter 8.8±2.1 nm are visible in the TEM image.

Vertically aligned carbon nanotube (VACNT) forests have been earmarked for a range of potential applications in areas such as energy dissipation devices, electrical interconnects, thermal interface materials, micro-electro-mechanical-systems (MEMS) and microelectronics. However while individual carbon nanotubes are known for a variety of exceptional properties, the mechanical behavior of VACNT forests is more complicated due to their hierarchical structure which spans multiple length scales (Fig 1). The collective behavior of these materials is expected to rely heavily on the properties of the individual CNTs, as well as on the variations in the collective inter-tube interactions and inherent property gradients of the microstructure, which in turn are controlled by their synthesis techniques.

We have been studying the mechanical response of VACNT forests using a combination of different techniques, such as uniaxial compression and indentation. In combination with traditional ex-situ (in air) methods, we also conduct in-situ experiments in a custom-built nano-mechanical deformation instrument, SEMentor, comprised of a Scanning Electron Microscope (SEM) and a nanoindenter, which allow real-time observations of the evolved morphology of the VACNT during the test.

Our results indicate that the compressive response VACNT cylindrical bundles exhibit unique deformation properties, namely resilience, deformability and energy dissipation, all of which increase at faster strain rates. These materials appear to combine the best aspects of different foam characteristics, such as their ability to switch between various buckled morphologies which makes them capable of undergoing large strains of ~60-80% without generating damaging peak stresses (similar to metallic foams), as well as their high resilience and close to 100% recovery upon load release leading to a higher energy dissipation (as in polymeric foams) - see video. Furthermore, their multifunctional nature, high electrical (comparable to copper) and thermal (similar to metals) conductivities, and relative ease of manufacture identifies VACNT bundles as a completely new class of material systems entering the "white space" in the material property chart, as shown in Fig 2.

In contrast, during indentation similarly grown VACNT films first deform by a sudden shear offset, manifested as a ~20 μm displacement burst, and exhibit marginal recoverability during cycling (see video). Subsequent localized buckle formation and onset of densification are found to be a strong function of substrate proximity. We are currently analyzing the constraint-driven differences in boundary conditions that govern both recoverability and morphological deformation signature under indentation as compared with compression.

One possible reason that VACNTs can have such high variance in recoverability is differences in tortuosity, or the “curviness” and “tangled-ness” of the individual tubes. In order to quantitatively determine the tortuosity we analyzed SEM images of a sample in two ways - a human-assisted method to extract an average radius of curvature (ARC), and using image analysis methods present in FIJI (FIJI’s Is Just ImageJ) to detect the total tube length (TTL). ARC allows us to define a “figure of merit” to evaluate trends among images against which we can compare TTL.

To produce ARC results, we trace small regions of characteristic curves in the front most layers of nanotubes and fit the traces to a circle, from which we extract a radius of curvature. This average is adjusted by a weighting factor generated by evaluating a randomly-centered grid of 35 points as 0 (“straight”) or 1 (“curved”).

TTL results are achieved by reducing the image to a “skeleton”, i.e. a network of single-pixel width (1-D) lines which can easily be analyzed for qualities such as Euclidian distance and start/end points. The number of tubes in an image are analyzed based on the number of start/end points at the top and bottom edges of an image, assuming that tubes run from top to bottom. The average tortuosity for the image is calculated as the ratio of the total Euclidian length to the total length of all tubes, assuming all tubes are the height of the image.

Fig. 3 Image of a nanotube forest analyzed via ARC.

Fig. 3 Skeletonized image of a nanotube forest.

INNOVATIVE MATERIALS FOR LOW-TEMPERATURE ELECTRONICS FOR SPACE APPLICATIONS

Researchers: Lucas Meza (graduate student) and Seok-Woo Lee (postdoc)

Collaborators: A. Shapiro, P. Beauchamp (JPL), J. Beauchamp (Caltech), J. Lunine (Cornell)


Images courtesy of "Kiss Study Final Report: Titan: Scientific and Engineering Challenges"
by J. Lunine, J. Jackson, M. Mojarradi, and J.R. Greer"

NASA's planned future missions to planetary bodies of extreme environments, such as Titan, Europa and Earth's Moon all of which have very low surface temperatures, represent some of the most severe challenges for the functionality of electronics. In this project we investigate the deformation mechanisms of two material systems, namely carbon nanotube (CNT) bundles and Sn based soldering materials, for potential applications in cryogenic microelectronics.

The ability to elastically sustain loads at large deflection angles has earmarked CNTs as uniquely tough materials for energy-absorbing applications. However, while the mechanical properties of individual CNTs in tension are promising, they degrade when the size is increased toward the micron scale, especially under compression. To fulfill the technical requirements for the extreme space conditions, it thus becomes extremely important to project the outstanding properties of CNTs to larger scales, and to understand and control adhesion between the nanotubes and the underlying substrate.

Similarly, Sn is known to undergo both phase transition and a ductile-to-brittle (DTB) transition in these temperature regimes, which can potentially compromise their mechanical integrity in such applications. We use nano-mechanical (both indentation and nano-pillar compression) testing from room down to cryogenic (77K) temperatures to ascertain the effects of temperature on phase and DTB transitions, as well as study the deformation mechanism under these conditions.

NANOMATERIALS FOR PHOTOVOLTAICS

Researchers: Shi Luo (Caltech) and Jiun-Haw Lee (National Taiwan University)

Collaborators: Chee-Wee Liu (National Taiwan University), C.C. Tsai (National Chiao Tung University), and Harry Atwater (Caltech)




Funding for this project is provided by the Caltech-Taiwan Energy Exchange

DEVELOPMENT OF LIGHTWEIGHT, RADIATION- AND DAMAGE-TOLERANT MICRO-TRUSSES

Researchers: David Chen, Jan Rys (graduate students) and Dongchan Jang (postdoc)

Collaborators: Yunfeng Shi (RPI), Bill Carter, Alan Jacobsen, and Toby Schaedler (HRL)


Group News
Mar. 2014: Nisha passes her Ph.D. defense! Congratulations!
Mar. 2014: Congratulations to Ottman for winning the NSF Graduate Research Fellowship and Viki and Arturo for receiving honorable mentions!
Feb. 2014: Greer Group invades TMS 2014. David, Lauren, Lucas, Nico, Chen, Rachel, Seok-Woo, Viki, Wendy, Zach and Julia gives talks and present posters on their work at the TMS.
Feb. 2014: Julia presents her solution at Google Solve for X!
Jan. 2014: Julia and colleagues Paul Sternberg and Axel Scherer lead IdeasLab with Caltech at the World Economic Forum.
Dec. 2013: Lauren, Viki, Jan and David present talks at the MRS conference in Boston.
Nov. 2013: Julia attends the National Academies' Keck Future Initiatives symposium.
Oct. 2013: Zach presents a talk at the ARL-MEDE conference in Baltimore.
Oct. 2013: Wendy, Lucas and Julia attend and present talks at the ECI Conference on Nanomechanical Testing in Faro, Portugal.
Sept 2013: Dongchan, Lucas, and Lauren's work featured on Caltech's frontpage. Read the paper here!
July 2013: Congratulations to Lauren for passing her candidacy!
July 2013: Lucas wins Best Student Paper award at SES 2013! Read all about it here.
July 2013: Julia receives the SES Young Investigator Award at SES 2013 at Brown University!
July 2013: Julia performed Beethoven's 1st piano concerto with the Vienna International Orchestra at Altenberg Stift on July 21, 2013. See the performance here.
July 2013: Julia wins 2013 Nano Letter Young Investigator Lectureship Award! Read here. MCE announcement
April 201: Congratulations to Rachel for passing her qualifying exams!
May 2013 : Congratulations to Nisha on passing her candidacy exam!
May 2013 : Julia delivers a lecture (and apparently an impromptu piano recital) at the National Academy of Engineering's Frontiers of Engineering Symposium in Beijing.
May 2013 : Congratulations to Lucas on passing his candidacy! One step closer to Ph.D.
March 201: Congratulations to Rachel on winning the NSF Graduate Research Fellowship!