It has been a great 5 days, but the MRS spring meeting is soon coming to a close. Here are some of my final takeaways from my experience at this year's conference:
Perovskite solar cells are still hot and improving in efficiency. Despite lingering stability concerns, these materials continue to show great promise to be used as high-efficiency and ultrathin solar cells.
Solar energy collection itself is still huge. Several different symposiums, including the student-run "Harnessing the power of solar", indicate the scale of the research effort going into solar energy. Funding efforts from the department of energy through their "Sunshot" and "FOCUS" initiatives are also accelerating the development of next-generation solar harvesting technology. There is no doubt that solar power is playing a prominent role in our steady national and global transition away from fossil fuels as energy sources.
The field of 2-dimensional materials is rapidly expanding. MoS2 and WS2 no longer stand alone as 2-D semiconductors; they are joined by other transition metal chalcogenides, black phosphorus (now made more stable through hBN protection), and emerging ternary 2-D materials. As more 2-D materials are discovered, our engineering capabilities also increase, bringing us one step closer to developing these materials into useful technologies.
Science is still fun. Huge credit goes to the Materials research society for making this meeting fun and comfortable, with a great mix of talks, posters, and downtime for one on one communication and collaboration between researchers.
Also, I was happy this year to have been selected as a blogger for the conference. It has been a great opportunity to look at the conference with a new depth and freshness in order to communicate it with others. Thank you all for a great conference, and have a safe journey home!
As promised, the final sessions of the NT4 (non-graphene 2D materials) is all about raman spectroscopy. Almost everyone that works with 2D materials, including myself, has some experience with raman spectroscopy, a powerful tool that probes the phonon modes of a material and acts like a fingerprinting tool to identify the material. The talks this afternoon, starting with NT4.10.01, show that a new parameter must be considered when doing raman spectroscopy: temperature. In particular, high-temperature raman studies, up to 600C, can reveal new physics that may be of crucial importance. For example, the most studied MoS2 raman modes are the E2g and A1g modes; from the location of these modes, it can be predicted whether the crystal is a monolayer, bilayer, trilayer, or bulk. In the first NT4 talk of the afternoon, "The Effects of Substrate on the Properties of Transition Metal Dichalcogenides" (NT4.10.01), Dr. Yong Zhang shows that these raman modes can vary linearly as the temperature increases. The coefficients of this shift with temperature are shown to change for different substrates. In addition to the peak location, the full width-half max of the spectra also vary with temperature; in this way, temperature-modulated raman spectroscopy is demonstrated as a tool for analyzing 2-D material samples.
Clathrin inspired self-assembly of rationally designed mesoscale systems
Written by Devesh Mistry
Clathrin proteins have tri-legged shapes which allow them to self-assemble into a hexagonal lattice which then forms spherical structures called vesicles. This process is used inside cells to transport small molecules. Yifan Kong of Stanford University has been able to mimic this process synthetically with tri-legged Janus particles.
Each side of a Janus particle has different properties. This can be utilized in several ways such as producing particles which react anisotropically with the surrounding medium. Using a photolithographic method, Kong produced his synthetic structures from bimetallic strips. On release from the substrate on which they were formed, the structures adopt curved geometries due to material stresses at the interface between the two metals.
The structures float and collect at the interface between two liquids. Their curved geometry means it becomes energetically favorable to self-assemble into a two-dimensional hexagonal lattice. As one of the metals used in the structures was nickel, a magnet can be used to slowly attract the particles through the top liquid. As this happens the structures slowly self-assemble into a three-dimensional vesicle within the top liquid. By doping the lower liquid with a fluorescent dye it can be seen that the lower liquid is contained within the vesicle.
Mechanically compliant electrode and dielectric elastomers from PDMS-PEG copolymers.
Written by Devesh Mistry
Dielectric elastomers use the attraction between two electrodes separated by an elastomer to produce an electrically switchable actuation. As a high voltage (typically kilovolts) is applied, the distance between the electrodes is reduced due to their mutual attraction. Since volume must be conserved, the elastomer must expand in the plane of the electrodes thus providing actuation options in multiple dimensions.
Typically there is a stress at the interface between the electrodes and the elastomer due to noncompliance of the electrodes. This often leads to delamination and limits the number of cycles a dielectric elastomer can undergo before it is destroyed. Anne Skov of Technical University of Denmark has been investigating the production of conductive elastomer electrodes using PDMS-PEG copolymers and conductive carbon nanotubes. Since the electrodes are compliant, the stress at the interface is greatly reduced and the system can survive many more actuation cycles.
The inclusion of carbon nanotubes also introduces ionic bonding to the elastomer which provides a method of self-healing as such bonds can reform when the material is heated. This is especially important for dielectric elastomer actuators as the high voltages involved can lead to electrical breakdown and damage which weakens the system.
Design of enzymatic reaction networks: Toward life-like materials
Written by Devesh Mistry
Inside every living cell there are complex networks of reactions. Researchers know what each reaction does individually, but how they link together to ultimately provide life is somewhat of a mystery. By designing relatively simple reaction networks involving enzymes, Sjoerd Postma of Radboud University is looking to couple oscillatory reaction networks with materials such as gels to give properties such as additivity.
Many reaction networks involve feedback loops. By designing a network involving positive and negative feedback loops, Postma has been able to produce a system capable of producing an oscillatory concentration of the enzyme trypsin. The amplitude and frequency of oscillations can be tuned by altering parameters such as the flow rate of the network’s “fuel.”
Going forward, Postma hopes to couple the oscillatory concentration of trypsin to control the dynamic process of additivity of a gel material. This would lead to transitions between liquid and gel states and hence to life-like materials.
Programming Liquid Crystal Elastomers—Elasticity, Actuations, and Beyond
Written by Arthur L. Robinson
Displays for products like smart phones and televisions based on liquid crystals pervade modern society and accordingly are a $1 billion industry that includes the currently fashionable wearable electronics, but these applications by no means exhaust what is possible with liquid crystals. In his Outstanding Young Investigator Award talk, Tim White of the Air Force Research Laboratory (AFRL) noted that it has been long known that liquid-crystalline materials in polymeric forms also exhibit exceptional characteristics in applications like transparent armor or bulletproof vests, as well as in optics and photonics. For example, researchers at the Air Force Research Laboratory and elsewhere have been working on exploiting the anisotropic properties of liquid crystals for controlling mechanical properties with the eventual goal of functional devices, such as mechanical actuators, based on shapes or surfaces that adapt to external stimuli. Among the many types of liquid crystals, he concentrated on programmable thermotrophic liquid crystals in the calamitic class. He described three main elements of such materials: Monomers, liquid crystal alignment, and shape and topology. The monomers are rod-like with aliphatic chains at the ends and polar head groups. Traditionally alignment relative to the film’s confining surfaces was by mechanical means but the interest now is in photo-alignment. Shape determines functionality and shape changes introduce multi-functionality. Shape derives from patterns and is the result of introducing anisotropy into the mechanical properties of liquid-crystal films.
White distinguished between liquid-crystal polymers with no cross-linking between chains (e.g., Kevlar), glassy liquid-crystal polymer networks with extensive cross-linking and a limited shape response, and liquid-crystal elastomers with an intermediate level of cross-linking, owing to a low glass transition temperature, but exceptional shape change. For example, heat causes a phase change and increases the volume at the microscale; the local volume increase amplifies the mechanical response; and the mechanical properties change as a result, as in the change in the length of a wire, similar to the change in a muscle fiber. Orientation is a key factor. Stretching orthogonal to the orientation of the chains results in a linear behavior like rubber, but stretching in the direction of the chains introduces what White called “soft elasticity” in which there is a region with large strain at low stress, a feature useful for flexible devices. He then reviewed methods of preparing and aligning liquid-crystal elastomers and aligning them (mechanically by hanging a weight on them, magnetically, and optically by adding a photoactive group before cross-linking and illuminating). To locally program the elastomer, a technique White called blueprinting was adopted. A blue-light laser sequentially patterns the direction of orientation to create active or inactive spots100-µm in diameter as the sample is moved through the beam on a two-dimensional (2D) stage, a process that takes about 80 minutes. The next step was to impose a designed response to heating (or more generally, a stimulus) so that the patterning generated a useful result. One way to accomplish this is by means of topological defects. For example, the group demonstrated a 3 × 3 array in which, when heated through the nematic–isotropic transition, cones arose out of each element of the array capable of lifting a weight 700 times heavier than themselves about 5 mm, an achievement on the way to real actuation as it compares to human muscle. The same effect can be stimulated by light, a solvent, or electromechanically.
Exploiting “soft elasticity” in flexible hybrid electronics in the near term emphasizes combining flexible high-performance electronics like solar cells with printed electronics like interconnects, but the future looks toward innovative multi-material and multi-functional printing approaches to enable advanced capabilities such as reconfigurable antennas, embedded electronics, and “ruggedized” survivable electronics. Inspiration comes from Nature from wood grains to muscles and tendons and ligaments. The ACL, whose injury plagues athletes, is designed to be flexible vertically but more robust laterally. Much of the effort is toward integrating stretchability with ruggedness. To monitor the effect on local strain of global stretching, White’s group used a technique called 2D digital-image correlation to map strain in a variety of samples including the AFRL logo printed in a liquid-crystal elastomer film, where the letters were the soft regions. The relevance to flexible electronics was demonstrated in an experiment with a stretched metal-coated elastomer film comprising an elastic and a soft elastic region. The electrical resistance on stretching was unchanged in the elastic region but dramatically increased in the soft region. To close, White looked at a way to speed up the time to produce samples from 1-2 days to a minute or so. The more homogenous network properties associated with the new methods allowed more strain and a sharper transition, which permitted films with cones lifting up to 9 mm. To progress to actual functional devices will require collaboration with the mechanical design community, but the pattern programmability suggests a huge world of possibilities.
Its late friday morning, and I was looking to learn something new by joining a symposium outside of my field of study. So many choices! However, based on my long passion for spaceflight, I decided to join in on EE12:"Radiation Damage in Materials - A grand Multiscale Challenge". A grand way to describe a grand problem, radiation damage affects many industries, but in particular I am aware of the threat solar radiation poses to astronauts that leave the protective magnetic field surrounding the earth. I joined the symposium to get a quick snapshot of the research that goes into making materials resistant to radiation damage. My snapshot brought me to Rigen Mo's talk, "Enhanced Radiation Damage Resistance of Ni-Based Concentrated Solid Solution Alloys as Investigated by In Situ TEM" (EE12.8.09). In this talk, many intentionally designed radiation-tolerant alloys were presented that demonstrated a range of tolerances to controlled radiation exposure (generally of Kr++ ions). One takeaway was that these particular materials show great radiation hardness due to their compositional complexity; the alloys have no long-distance order and a complex physical form, which ultimately suppresses defect generation, retards migration, and promotes recombination, which result in greater general radiation hardness. This method for achieving radiation hardness could provide a great deal of tunability in the engineering of similar materials. In addition, I was impressed by the elegant means of in-situ analysis via transmission electron microscopy. This talk was my first snapshot into the details of radiation hardness engineering, and I am excited at the idea of learning more about this relevant field.
Gold Award Recipients: Chia-Hao Chuang, Massachusetts Institute of Technology Peter Dieme, Wake Forest University Mathieu Grisolia, Centre National de la Recherche Scientifique/Thales Qianqian Lin, The University of Queensland Yuchuan Shao, University of Nebraska-Lincoln Hanze Ying, University of Illinois at Urbana-Champaign Lei Zhang, The Pennsylvania State University
Silver Award Recipients: Hamed Arami, University of Washington Amay Bandodkar, University of California, San Diego Ryan Hufschmid, University of Washington Won Jun Jo, Massachusetts Institute of Technology Yoonseob Kim, University of Michigan, Ann Arbor Kwon-Hyeon Kim, Seoul National University Jinxing Li, University of California, San Diego Elena Liang, University of California, Irvine Thomas Lonjaret, Ecole des Mines de Saint-Etienne Steven Naleway, University of California, San Diego Simiao Niu, Georgia Institute of Technology Nuri Oh, University of Illinois at Urbana-Champaign Abdon Pena-Francesch, Pennsylvania State University Kasra Sardashti, University of California, San Diego Yifei Yu, North Carolina State University
Dino Di Carlo, University of California, Los Angeles
Microstructured Materials for Cell Analysis and Regeneration
Written by Aditi Risbud
On Thursday evening, Dino Di Carlo of the University of California, Los Angeles presented his work on using microfluidics to manufacture three-dimensional microstructured materials. Di Carlo’s research leverages microstructures to enable function in biology at the cellular scale.
The first strategy Di Carlo discussed was structuring magnetic materials to manipulate magnetically-labeled cells by amplifying magnetic field gradients. In particular, applying large forces on cells to initiate signaling is challenging because the “force envelope” diminishes as the particle size decreases.
Using a technique called magnetic ratching cytometry, researchers can leverage structured ferromagnetic materials at the microscale to generate effects at the nanoscale. In particular, changing the pitch between arrays of micromagnets allows the researchers to sort cells in their equilibrium configuration by magnetic content. This separation based on relative expression level or relative volume is useful in applications such as detecting cancer cells.
The next strategy is using microfluidically-created particles that flow into a wound and anneal together to form a microstructured scaffold with porosity that allows tissue integration. This technique addresses the “foreign body response” caused by traditional implants that limits transport of nutrients and interferes with drug delivery.
To heal or augment function, Di Carlo says, the tissue must integrate seamlessly in situ. Using microporous annealed particle (MAP) gels, a slurry of particles is introduced into a wound or blood to vessel. These gels are then annealed to form a scaffold network connected to the surrounding tissue, with the negative “void space” creating porosity. This microstructured technique leads to accelerated healing of wounds.
Lastly, Di Carlo’s group can structure particles themselves in three dimensions to enable emergent physical properties. They used a technique called optical transient liquid molding, which employs two-dimensional light patterns to create three-dimensional structures, a so-called “extrusion of an extrusion.”
Di Carlo and his colleagues have also developed freely available software, called uFlow, to enable automated three-dimensional structural design of materials such as shaped fibers, “Janus” fibers, or chemically heterogeneous fibers.
Di Carlo said receiving the Outstanding Young Investigator Award is “particularly an honor because I’m relatively new to the materials field.”
The Materials Research Society and the Sociedad Mexicana de Materiales participate in an annual Poster Award Exchange Program. The three most outstanding poster winners from the XXIV International Materials Research Congress 2015 had their posters on display at the 2016 MRS Spring Meeting.
The authors of the award-winning posters are:
Fernando Buendía, Universidad Nacional Autónoma de México
Francisco Manuel Lino Zapata, Instituto Potosino de Investigación Científica y Tecnológica A.C.
Yasab Ruíz Hernández, Universidad Nacional Autónoma de México