Professor Greer describes the use architected microstructure to develop light-weight structural materials.

Professor Greer demonstrates the low-density Ni microtruss.


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.

Note: Click on the project titles to toggle contents.

Materials by Design: 3-Dimensional Architected Structural Meta-Materials

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. Mechanics of 3-Dimensional Hollow Nanolattices
    Researchers: L. Montemayor, L. Meza, A. Mateos, N. Clarke
    Sponsorship: NSF, DARPA (MCMA)

    The creation of extremely strong and ultra-light materials can be achieved by capitalizing on the hierarchical design of metallic and ceramic nanolattices. 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 reliable commercial products exist that are capable of creating nanostructures with ~100 nm lateral feature sizes. We use these structures as a scaffold to deposit materials onto using techniques like ALD, CVD, and sputtering deposition. The polymer can then be etched out, leaving behind a hollow nanoscale truss structure.

    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. Nanostructured 3-D Architectures for Biomedical Applications:
    From Patterning to Control Cell Adhesion to Designing Cell Scaffolding Organ Transplants
    Researchers: O. Tertuliano, A. Maggi
    Sponsorship: ICB and EAS Discovery Fund

    Biofilm formation is ubiquitous and poses significant problems ranging from pipeline corrosion to human diseases. Biofilms are also detrimental for medical implants because they significantly reduce the implant’s lifespan. Polydimethylsiloxane (PDMS) is the material of choice for a vast number of implants, which tend to lose their functionality over time as a result of cell-adhesion and subsequent biofilm formation. Attempts at impairing the cells’ affinity for the PDMS surface have been made, for example by means of adding poly(ethylene glycol) chains or by mimicking shark skin hence by radically increasing surface roughness, yet these methods have yielded little success.

    In the spirit of likening the physical origins of cell adhesion to hydrophilicity we developed a nanofabrication process aimed at creating a nano-textured PDMS surface that allows control of hydrophobicity. We demonstrate that the water contact angle increases from 107º (non-textured surface) to 174º (pattered surface). In this process, 300 nm-diameter alumina nanoparticles were first dispersed via spin coating on the surface of PDMS. Subsequent SF6/O2 plasma treatment led to etching away the regions not protected by the nano-particles to reveal a nano-texture with superhydrophobic surfaces (CA = 174º). The resulting topology was similar to that of the lotus leaf, which is known for its super-hydrophobic properties. Samples varying in pitch and etch depth were prepared and tested for cell adhesion, which was quantified using fluorescence microscopy.

    Engineering mechanically and biologically compatible regenerative bone implants has proven an arduous and multifaceted pursuit. Most current hip implants employ a titanium alloy as a scaffold for bone remodeling, which involves regenerating a trabecular bone network and profits from bearing load applied through exercise. Titanium is stiffer and stronger than the native hierarchical network of trabecular bone, which causes the latter to carry less of a load and hinders the native tissue remodeling. These types of implants are not biodegradable or porous, which further inhibits osteointegration.

    To tackle this we are designing, fabricating and characterizing trabecular bone inspired nano-scaffolds for bone regeneration; we to the innate architecture to emulate characteristics that would optimize integration. The goals are thus to create complex nanostructures via tow photon lithography (TLP) digital laser writing (DWL) and deposit hydroxyapatite (HA), a biodegradable via sputtering. We mechanically characterize the HA structures via in-situ compression testing in our SEMentor. We have also been able to grow cells on the nanostructures, confirmed via fluorescent microscopy. We further aim to study the effect of structure and stiffness on osteoblast adhesion and thus osteointegration.

  3. Energy Materials: Nano-porous Electrodes for Li-O2 and Li-ion Batteries
    Researchers: C. Xu and D. Tozier (Li-O2), X. Wendy Gu and X. Xia
    Sponsorship: Bosch and CI2

    Li-O2 batteries have a theoretical energy density of 5200Wh/kg, compared to 250Wh/kg of current Li-ion batteries. The successful implementation of Li-O2 batteries in electric vehicles would mean an EV could travel more than 300 miles before a charge is needed. However many challenges remain with current Li-O2 technology, most prominent of which is chemical reversibility during battery cycling. Much research has focused on improving the chemical stability of cathode material. Au is a noble metal indicated to be stable against the strongly oxidizing main discharge product (Li2O2) and other intermediate species (Peng et al, Science, 2012).

    In this project we use Au microlattices as the cathode of Li-O2 batteries, the fabrication of the sacrificial polymer scaffold is done at HRL. Unlike conventional cathodes, they are carbon and binder free, which decrease cathode decomposition; the large pore sizes also prevent any pore clogging of Li2O2 during discharge. The highly ordered and controllable structure of the microlattice allow us to decouple geometry and material effects on battery performance, something that is unattainable in conventional stochastic cathodes. Spectroscopy techniques such as Raman and FTIR are used to identify discharge and charge products. This work is done in collaboration with Robert Bosch Research and Technology Center (RTC), and funded by the Bosch Energy research Network (BERN).

    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. In-situ SEM lithiation is performed on Si nano-lattices to develop an understanding of how electrode structural morphology affects electrochemical performance. 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 of Nano-sized Solids

National Science Foundation Office of Naval Research
  1. Fracture and Deformation of Small-Scale Materials

    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).

  2. Fracture and Deformation in Small-scale Metallic Glasses: Nano-pillars and Nano-lattices
    Researchers: D. Chen, R. Liontas, N. Peter

    Stable amorphous phases in binary alloys have been fabricated since 1960, following their introduction by Klement, et al. Obtainable through extremely fast cooling, such amorphous metal alloys, or metallic glasses exhibit unique properties and represent materials with some of the highest known elastic moduli and elastic limits. However, due to their random structure and known plasticity carriers such as dislocations in crystals, failure in metallic glasses is catastrophic at the elastic limit, resulting in significant research being focused on introducing ductility and toughness into these strong, yet brittle glasses.

    Our research is directed at the nanoscale, where interesting phenomena arise in metallic glasses. For example, the aforementioned catastrophic failure mode transitions from that of shear banding to a ductile necking-to-shear banding failure under tension in pillar-shaped samples less than ~100 nm in dimensions. This so-called brittle-to-ductile transition also seems to be enhanced in ion-irradiated samples, suggesting an interplay between deformation mode and the extent of local damage or relaxation and lack thereof. These nanoscale properties not only help to reveal the underlying fundamental structure-to-deformation relationships in metallic glasses, but also provide a means for introducing ductility in them via hierarchy. In nanotrusses, for example, truss members with tube thickness less than ~100 nm possess ductility that can be proliferated to micron or even millimeter dimensions. In other hierarchical materials such as foams and nanoporous materials, these properties may one day allow us to make ductile metallic glasses that both deform and harden like steels and retain superior strength and stiffness.

  3. Processing-Microstructure-Device Performance in CIGS Thin Film Solar Cells
    Researchers: S. Luo
    Sponsorship: CTEE

    We examine Cu(In,Ga)Se2 (CIGS) thin films fabricated by (1) selenization of pre-sputtered Cu-In-Ga mixture and (2) co-evaporation of each constituent, and link their performance disparity to differences in film morphology and microstructure. We also investigate mechanical properties of the CIGS material such as elastic modulus and yield strength via nanoindentation and nanopillar compression. Current effort includes using density functional theory and molecular dynamics simulations to reveal the diffusion and incorporation mechanisms of Na in CIGS thin film.

  4. Crackling Noise and Microplasticity
    Researchers: X. Ni
    Collaboration: LIGO

    Crackling noise is the noise arising from a nonlinear conversion of energy from slow varying external condition to high-frequency random events that are correlated with defect activities in materials. We are motivated by measuring crackling noise for advanced instrumental requirement (e.g. the LIGO suspension systems), and an in-depth investigation of crackling noise should lead to novel approach to studying critical phenomena such as scaling behavior and universality for slip statistics. Crackling noise is also a promising feature for developing non-destructive method of characterizing deformation properties of materials.

    The experiment setup is based on a Michelson interferometer, because it achieves high displacement resolution by rejecting symmetric noises. The theoretical interferometry output has no dependence on any common motion represented by the common optical path L, and thus the interferometer will finally work as a transducer converting only the differential mode dL to the optical signal. Since the slip events are stochastic process they will serve as the main differential mode signal gained in our Michelson configuration. Collaborating with LIGO, We are now in the process of building a setup aiming for a noise floor of 10-15 m/√Hz above 10Hz, limited only by quantum shot noise. Theoretical aspects of this work are being conducted in collaboration with Prof. Karin Dahmen’s group.

  5. Deformation of small-scale High-Entropy Alloys
    Researchers: A. Giwa

Mechanics of Hard Bio-Materials

  1. Fracture and Deformation in Trabecular Bone: Anisotropy, Hierarchy, and Scaffolds
    Researchers: O. Tertuliano
    Sponsorship: ICB/ARO

    Engineering mechanically and biologically compatible regenerative bone implants has proven an arduous and multifaceted pursuit. Most current hip implants employ a titanium alloy as a scaffold for bone remodeling, which involves regenerating a trabecular bone network and profits from bearing load applied through exercise. Titanium is stiffer and stronger than the native hierarchical network of trabecular bone, which causes the latter to carry less of a load and hinders the native tissue remodeling. These types of implants are not biodegradable or porous, which further inhibits osteointegration. To successfully mimic the trabecular bone in creating better prosthetics (or bone replacement), it is essential to understand its properties at the fundamental level.

    We investigate the mechanical properties of the native human trabecular bone at each level of hierarchy: from several hundred nanometers to several tens of microns. We fabricate micro- and nano-sized cylinders and dogbone shaped samples from decelluarized trabeculae which contains collagen and hydroxyapatite via Focused Ion Beam milling. Stand-alone cylinders are then tested in compression and tension under prescribed nominal displacement rate. Both monotonic and cyclical experiments were performed, and evolved microstructure was analyzed. We present nano-mechanical behavior of site-specific samples within trabeculae and discuss them in the framework of small-scale mechanical deformation.

  2. Triggering Neurological Response in Insects via Electron Beam Perturbation
    Researchers: Z. Aitken
    Collaboration: A. Scherer Group

Nano-Photonics: Polymer Nanolattices as Mechanically Tunable 3D Photonic Crystals

Researchers: V. Chernow

3-dimensional photonic crystals (PhCs) have unique light interaction and propagation properties which make them applicable in numerous areas including low-loss mirrors, sensors, and structures for light management in solar cells. The utility of photonic crystals for particular applications may, however, be limited by their optical response. The optical response, otherwise defined as the position of the photonic bandgap of the PhC, is usually preset at the time of fabrication and constrains the photonic bandgap to a narrow wavelength range. Applied mechanical deformation can be used to alter the dimensions and periodicity of a PhC, which will modify its optical bandwidth and increase the wavelength range of the photonic bandgap.

In this project we explore the relationship between uniaxial compressive strain, ε, and photonic bandgap, λ_stopband, in 3-dimensional polymeric nanolattices subjected to uniaxial compression. Polymer octahedron nanolattices are fabricated using Two Photon Lithography-Direct Laser Writing (TPL-DLW), with unit cell sizes on the order of 4 microns. Uniaxial compression experiments are performed in-situ, inside of a scanning electron microscope (SEM), which enables us to simultaneously collect mechanical data and observe deformation of the photonic crystal. Optical characterization is performed using Fourier Transform Infrared (FTIR) Spectroscopy where reflection spectra are collected for nanolattices under varying amounts of strain. Reflection spectra are used to identify the wavelength position of the photonic bandgap as a function of compressive strain, and reveal that λ_stopband blueshifts as the periodicity of the lattice decreases with increasing strain. Plotting the photonic bandgap position versus strain reveals a linear relationship between λ_stopband and ε. We also demonstrate that bandgap shifts on the order of 3 microns can be achieved when the octahedron nanolattice photonic crystals are compressed by ̴40%.

Developing Advanced Materials Immune to Radiation Damage

Researchers: R. Liontas and N. Peter
Sponsorship: DOE and National Academies Keck Future Initiatives (NAKFI)

Radiation-producing equipment and facilities are prevalent today in areas including energy production, science, medicine, and space. New concepts in materials design are essential to develop materials for these and future applications that are capable of tolerating irradiation extremes. Radiation damage tolerant systems can be designed by incorporating radiation-defect sinks, such as interfaces, grain boundaries, and free surfaces, or by utilizing inherently radiation-damage tolerant materials, such as metallic glasses. We seek a fundamental understanding of how such individual nano-scale features and materials respond to radiation in order to determine the specific characteristics that make a system radiation damage tolerant. We are interested in irradiation by helium, as helium is an inevitable byproduct of many irradiation processes, particularly in nuclear reactors, where its production can lead to severe embrittlement, degradation of mechanical properties, and in extreme cases catastrophic failure.

This work is conducted through ion-beam implantation of helium, in-situ mechanical testing of individual nanostructures, and microstructural analysis through techniques such as transmission electron microscopy (TEM). We have found a 2-fold increase in the ductility of ~100 nm Ni-P metallic glass nanopillars upon helium implantation thus demonstrating the potential for utilizing such materials in radiation-intensive applications. Currently, we are studying the effects of interfaces in bi-material nanopillars made of Fe and Fe-W metallic glass. Future work will build on these studies to determine the radiation-damage tolerance of systems with increasing complexity, such as metallic glass or nanocrystalline 3-D architected nanolattices containing a high density of free surfaces.

Group News
Oct. 2017: Congratulations to the now Dr. Xiaoyue Ni, Dr. Eric Luo, Dr. Alessandro Maggi, and Dr. Victoria Chernow for successfully defending their theses.
Jun. 2017: Daryl was selected into the Biotechnology Leadership Predoctoral Program in Micro/Nanomedicine at Caltech that will provide him the opportunity to work at biotech companies via internships! Congratulations, Daryl!
Jun. 2017: Congratulations to Bryce for being the first of his class to pass his candidacy exam!
Jun. 2017: Ryan was recently awarded with a CCE Teaching Assistantship Award granted to the division's most exceptional teaching assistants. Congratulations!
Apr. 2017: The Greer troops present at the MRS Spring 2017 meeting! Adenike, Xiaoxing, and Xuan present talks, and Julia presents in Symposium X. Click here to see a video/write-up of her talk.
Dec. 2016: Chen's new paper in PNAS on properties of small-scale Li is featured on the front page of Caltech news today! See it here!
Dec. 2016: Congratulations to Diandra Almasco (and her mentor Carlos Portela) for winning first place for her SURF poster!
Sep. 2016: Congratulations to Amanda for passing her qualifying exams!
Sep. 2016: Congratulations to Dr. Rachel Liontas on completing her PhD!
Aug. 2016: Congratulations to our newest PhD - Dr. Chen Xu on a successful defense!
May 2016 : Congratulations to Ryan, Max, Mike, Arturo, Carlos, Daryl, Alessandro, and Adenike for passing their candidacy exams!
May 2016 : Congratulations to the now Dr. Lucas Meza and Dr. David Chen on successfully defending both of their dissertations!
Mar. 2016: Check out Julia's new video on CNN!
Mar. 2016: Julia was named as one of fifteen 2016 National Security Science and Engineering Faculty Fellows by the DoD! See it here.
Feb. 2016: Zach and Eric's new PNAS paper is featured in a Caltech news article!
Jan. 2016: See the new NBC-NSF video featuring the Greer group that went live today!
Dec. 2015: Greer troops take MRS by storm, with presentations by David Chen, Viki Chernow, Lauren Montemayor (alum), Julia, and posters by Dylan and Adenike. Adenike also gave a talk at the African MRS after.
Dec. 2015: Special issue of MRS Bulletin with a focus on Innovation comes out, with a featured review article by Viki and Lauren!
Nov. 2015: Julia gives a talk at TEDxManhattanBeach. See the talk here.
Oct. 2015: Congratulations to Dylan for his wonderful talk at the 2015 ECS fall conference and to Xiaoxing for winning the best poster award!