Symposium X: Frontiers of Materials Research

Symposium X Alessandra Lanzara_800x800Alessandra Lanzara, University of California, Berkeley, and Lawrence Berkeley National Laboratory

Engineering Two-Dimensional Heterostructures with a Twist 

Written by Arthur L. Robinson

The past few years have seen exciting new opportunities emerging from simply stacking and/or twisting together atom-thick layers of the same or different materials. The lattice mismatch or rotational misalignment introduced by such stacking gives rise to long-range Moiré patterns that lead to modification of the electronic band structure, which in turn gives rise to the appearance of unexpected properties, such as Mott-like behavior and superconductivity, even in weakly interacting systems such as graphene.

In her Thursday Symposium X presentation, Alessandra Lanzara of the University of California, Berkeley, and the Lawrence Berkeley National Laboratory described recent investigations by her group on twisted and strained graphene and transition metal dichalcogenide (TMD) heterostructures as a function of twisting angle and gating. Using angle-resolved photoemission spectroscopy, the group studied the effect of such misalignments on the electronic structure of these materials, yielding insight on the key parameters that lead to the onset of strong correlation and novel behavior in these materials.

Lanzara opened her talk by introducing the importance of topology as an essential theoretical tool in understanding the properties of materials. Until recently, thinking about transitions in crystalline solids has been based on order parameters related to symmetry breaking and correlations. Topology has now joined these as an organizing principle of matter. In general, topological properties are those that are preserved under continuous deformation. For example, in a topological insulator there is no sharp phase transition, but the insulator property is preserved as the electronic band structure is continuously deformed.

Symposium X Alessandra Lanzara 2_800x800

With the addition of topology, said Lanzara, it is now possible to describe the various states of materials now on a single diagram with a correlation energy on one axis and the spin–orbit coupling on the other. Close to the origin, conventional metals and insulators are well described by band theory. As the correlations increase, Mott insulators come to the fore, whereas topological insulators and semimetals come to the fore when spin–orbit coupling increases. But the future may lie in the panoply of exotic behaviors like Weyl insulators that arise as both correlations and spin–orbit coupling grow.

“What new cooperative phenomena and particles will occur when you bring together correlation, spin orbit coupling, and topology?” Lanzara asked next. Taking a hint from physicist Richard Feynman’s famed questions about two-dimensional pages, the question became “What would the properties of materials be if we could really arrange the atoms the way we want them?” But how would one go about exploring this immense space? One way to arrange atoms is by means of heterostructures consisting of stacks of materials with different properties with relevant aspects being dimensionality, coupling to the lattice, order (spin, charge, orbitals, and Cooper pairs in superconductors), and electrostatic doping. Outcomes of building these structures include emergent phenomena at interfaces, such as ferromagnetism, superconductivity, and metal-to-insulator transitions.

From here Lanzara rapidly reviewed some considerations, such as electron screening, and methods for controlling the electronic structure in the context of searching for new phenomena. In particular, her group found that twisting the layers in the heterostructure provided a new level of band-structure control. In fact, when Lanzara was being introduced as the speaker for this Symposium X, the MC used the term twist-tronics.

After discussing engineering of topology and strong correlation, including local inversion-symmetry breaking in the heterostructure layers that gives rise to spin–orbit coupling, Lanzara turned toward the possibility of an even larger phase space for materials design, moving from periodic crystals with both long- and short-range order, to quasiperiodic crystals with order but are not periodic, to Floquet crystals that are periodic in time, and ending with amorphous materials with no long-range order but perhaps some short-range order. After asking if amorphous systems can be used for materials engineering, she reported some early results on the amorphous topological insulator Bi2Se3. One task was to find a replacement for the momentum quantum numbers (kx, ky, kz) in crystals. The group was thinking of an (average) bulk Hamiltonian as spherically symmetric in k-space, resulting in a wavefunction parameterized by k2 and the angles q and f in a spherical coordinate system.

Lanzara summed up her presentation by declaring that two-dimensional heterostructures constitute an incredible, highly tunable platform for exploring correlation, symmetry breaking, and topology. The electronic structure of two-dimensional van der Waals materials is extremely easy to modify, including effects such as symmetry breaking to induce gap opening and renormalization effects due to screening, and spin–orbit coupling and other many-body interactions. But questions still ripe for investigation include: Can we design new types of many-body topological properties and new particles? And what new phases can result from the interplay between them?

Symposium X—Frontiers of Materials Research features lectures aimed at a broad audience to provide meeting attendees with an overview of leading-edge topics.


MRS Frontiers Reception—Building Communities

Thursday evening activities opened with a lot of fun and creativity. Beginning with PowerPoint Karaoke, risk-taking participants explained the science behind a one-slide PPT which they saw for the first time and had only 30-seconds to prepare! Typically, the topic was not in their field of study. This exercise demonstrates the importance of clearly communicating research in pictures via PPT as well as poster presentations. It also points toward the places where materials advancements in one specialized area informs another area of study. Taking an example from the scientific talks given during the week, when Yuan Yang of Columbia University reported on biological imaging of chemical bonds by using the stimulated Raman scattering (SRS) microscope, Yang also pointed out the advantages of SRS microscopy for imaging lithium ion transport during dendrite formation in lithium ion batteries.    

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After PPT Karaoke, the MRS volunteers involved in introducing new topical areas of study to the MRS membership engaged the Karaoke crowd as well as many other Meeting attendees to brainstorm new directions at the frontiers of materials research. The topical areas included materials for sustainability and artificial intelligence along with biomaterials, quantum materials, and responsive & adaptive materials. During this reception (accompanied with food and beverage), materials researchers came up with questions such as whether self-healing materials can be accomplished at the quantum level. They considered combined specialties such as external stimuli responsive systems for drug delivery. In the biomaterials area, scientists reached for interaction with nature such as naturopathic drug development; interaction between cells, tissues, and sound waves; and a biomimetic system that can fool the real biological systems.

As the news editor for MRS Bulletin, I’ll be looking for these studies coming to fruition!

-Judy Meiksin  


Symposium FF03: Building Advanced Materials via Particle-Based Crystallization and Self-Assembly of Molecules with Aggregation-Induced Emission

Qian Chen, University of Illinois at Urbana-Champaign

Direct Imaging of Layer-by-Layer Growth in a Colloidal Nanoparticle Superlattice

Written by Alana F. Ogata

Who doesn’t love a good movie? Qian Chen’s talk was filled with movies at the nanoscale as she presented recent in situ transmission electron microscopy (TEM) studies of nanoparticle superlattices. In situ TEM circumvents the limitations of vacuum-conditions in traditional TEM and enables direct imaging in a liquid cell. Chen utilizes low dose in situ TEM to study the crystallization of nanoparticles during the formation of superlattices. The audience next watches a video of cubic nanocrystals accumulating layer-by-layer at the crystal surface revealing the growing morphology of a superlattice. Beyond watching movies, molecular dynamic simulations reveal a 2-step nucleation mechanism comprised of an amorphous precursor nucleation domain and crystal nucleation within the amorphous precursor. In situ TEM movies are captivating but typically suffer from criticisms that come from electron beam radiation damage and challenges controlling the local environment. Careful optimization of in situ TEM studies presented in this talk, Chen points out, was extensive and detailed in a 75-page supplementary document.


Symposium EN03: Green Electrochemical Energy Storage Solutions—Materials, Processes and Devices

Yunya Zhang, University of Virginia

Converting Eggs to Flexible, All-Solid Supercapacitors

Written by Tianyu Liu

How do you typically enjoy an egg? Hard boil? Blend it with rice? For Yunya Zhang from the University of Virginia, he chose not to eat the egg but transformed it into a functional device—a supercapacitor that can store electrical energy!

Zhang’s supercapacitor was derived from eggs exclusively. The research group synthesized graphene-like porous carbon sheets by pyrolysis of blends of yolk, white, and shell pieces, blended the yolk and white with potassium hydroxide to solidify the liquid into a gel as an electrolyte, and used a piece of eggshell membrane as a separator. With the electrodes, electrolyte, and separator at hand, the researchers assembled an all-egg-based supercapacitor that could store electricity! Zhang concluded his presentation by acknowledging his PhD advisor, who allowed him to conduct the “crazy” work.


Symposium MS02: Mechanics of Nanocomposites and Hybrid Materials

Qing Hua Wang, Arizona State University

Defect-Mediated Chemical Modification of Semiconducting 2D Metal Chalcogenides

Written by Jahlani Odujole

Qing Hua Wang and her research team were aiming to create nanoscrolls by using a novel functionalization method. Nanoscrolls are less popular than carbon nanotubes, which have gained recent popularity. She presented two videos of movement (at 55 K and 355 K) to demonstrate how functionalization can be used to attach proteins. The researchers determined that a combination of air plasma and water vapor was essential for rolling nanoscrolls onto a SiO2 substrate. Different bonding energies were also observed for different orientations of the crystallographic lattice structure. This research allows for the chemistry of two-dimensional (2D) materials to be used to modify properties. The analysis of defects was presented as a means to characterize and gain a better understanding of 2D materials. The ability to grow chains of molecules form the basis of their future work.


New MRS Energy & Sustainability Editor Y. Shirley Meng

Editor-in-Chief Shirley Meng says that her vision for the MRS Energy & Sustainability journal is to be the number 1 journal for energy materials research "that showcases the convergent research among science, technology, social science, economics, and policy." She says the bright picture of energy and sustainability cannot be realized with only scientists and engineers but requires the need to bring together the society at large to work with them.

 


 


Symposium EN03: Green Electrochemical Energy Storage Solutions—Materials, Processes and Devices

Chuan Xia, Rice University

Direct Electrosynthesis of Pure Aqueous H2O2 Solutions Up to 20% by Weight Using a Solid Electrolyte

Written by Tianyu Liu

Have you used hydrogen peroxide for disinfection and cleaning at home? Where did you get the chemical? From a pharmacy? A technology developed by Chuan Xia and co-workers from Rice University might allow you to make high-purity hydrogen peroxide on-site and on-demand. No more complaining about the traffic jam on your way to pharmacy shops! Xia and co-workers designed an electrochemical cell allowing H2 and O2 gases to react peacefully and produce hydrogen peroxide. O2 stream flowed into the catalyst-loaded cathode of the cell and was reduced to HO2-. H2 gas entered the anode of the cell and was oxidized to H+. These electrochemically generated HO2- and H+ ions met each other in a porous, solid-state electrolyte and yielded H2O2. Xia said that their cell exhibited over 90% selectivity of H2O2, and could convert ~25 vol.% of H2 gas.


Symposium SB09.03/SB06.11.03: Interfacing Bio/Nano Materials with Cancer and the Immune System

David Mooney, Harvard University

Viscoelasticity and Cancer

Written by Alana F. Ogata

Understanding the time-dependent mechanical properties of matrices in the body can reveal important mechanisms in cancer biology. When a cell is placed in a collagen medium, the cell will reach out to grab, pull, and push itself through the matrix, leading David Mooney to hypothesize that mechanical forces can provide feedback to the cell about its environment. Mooney uses alginate hydrogels to effectively decouple key variables, such as stiffness and ligand density, to study viscoelasticity and its effects on cancer cell proliferation. Upon comparison of hydrogels with identical stiffness, cancer cells rapidly proliferate through more viscoelastic gels. When exploring the effects of viscoelasticity on tumor volume in mice, more tumor growth was observed in viscoelastic conditions compared to tumor growth in elastic conditions. Outside of cancer invasion, viscoelasticity also plays a key role in providing mechanical cues to regulate the immune system. Fascinating results from David Mooney’s talk provide insight into how immune cells respond to elastic-to-viscoelastic transitions and how mechanical checkpoints may largely impact the fate of immune cells in tumors.


Best Poster Award Winners – Thursday

Poster Session_800x800

 

Alexander Hernandez Oendra, ETH Zurich

EN09.13.08 Template Stripping of Perovskite Thin Films for Dry Interfacing and Surface Structuring

 

Rachel Smith, Massachusetts Institute of Technology

SB01.12.19 Manufacturing Biohybrid Textiles through a Robust Fiber Based Cell-Free Expression System

 

Lazaro Padilha, Universidade Estadual de Campinas

EN10.16.07 Multi-Exciton Interactions and Size—An Interplay for Efficient Two-Photon Induced Gain in Perovskite Quantum Dots

 

 

 


Symposium EN06: Development in Catalytic Materials for Sustainable Energy – Bridging the Homogeneous/Heterogeneous Divide

Qi Dong, Boston College

Understanding the Role of Electrolyte in Electrochemical CO2 Reduction Reactions

Written by Tianyu Liu

Researchers have unveiled the role of water with different concentrations in electrochemical CO2 reduction reactions (CO2RR). Wait, did you ask what the water concentration mean? I had the same question when Qi Dong from Boston College began to present in the Thursday morning session of Symposium EN06. It turned out that Dong's work was associated with a unique solution system—water-in-salt.

Water-in-salt systems are solutions with highly concentrated salts. The salt concentration is so high that water molecules could be treated as solutes dispersed among salt ions. Utilizing Li-salt-based water-in-salt solutions, Dong and co-workers investigated the reaction kinetics of CO2RR over the Au surface and at different water concentrations. They revealed that the rate-determining step of CO2RR on Au should be the direct transfer of electrons to CO2, rather than proton-coupled electron transfer.