Plenary Session Featuring The Fred Kavli Distinguished Lectureship in Materials Science

Plenary 2_270x180Jenny Nelson, Imperial College London

Optimizing Solar Energy Conversion in Molecular Electronic Materials

Written by Rahul Rao

Today, the solar energy world is ruled by crystalline silicon. But recent advances in materials have spawned a host of alternative materials seeking to challenge silicon’s dominance. One class of challengers are molecular electronic materials: carbon-based organic semiconductors that can be excited by visible light.

At Monday’s Plenary Session, Jenny Nelson spoke about the titular materials, how they’ve rapidly developed, and how her group’s work has contributed to bring these organic materials closer to viable solar cells.

Just several decades ago, these materials barely registered on solar researchers’ radars. Most of them had been fashioned from fullerenes, limiting their variety. Additionally, such materials tend to be disordered and suffer from a poor dielectric constant, hurting their prowess as semiconductors.

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But by the 1990s, researchers had learned how to circumvent those restrictions by interfacing two different disordered materials—one to donate electrons, another to accept them. Since then, the materials themselves have diversified from fullerenes into a wide range of other materials that researchers could easily process and readily customize. As a bonus, researchers have found that their coupled molecular electronic materials’ properties are easy to computationally predict.

The result: solar cells made from molecular electronic materials have blossomed in efficiency, from less than 2.5% in 2002 to 11% in 2016 to over 19% in 2022.

Still, any organic solar cells have a long way to go before they can match their crystalline silicon counterparts, many of which now clear the 30% mark. Nelson said that researchers need to quantify and understand the loss in materials.

One way of doing that is to take a solar cell and run it in reverse, like an LED—pushing electrons in and watching the light that comes out. Because a physical relationship exists between that emitted light and the light absorbed when the device is operating as designed, researchers can study how close their device’s efficiency is to its ideal limit.

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Through this method, by 2015, Nelson’s group had helped demonstrate that organic solar cells shed much of their energy through non-radiative loss—something that researchers could address by improving their materials. Recent efforts to do just that have helped drive much of the rapid efficiency jumps these solar cells experienced from the late 2010s.

Their efficiency could rise even farther if researchers can understand which molecular properties lead to which changes in a solar cell’s performance. Nelson’s group probed this computationally, outfitting a model of a solar cell device with a molecular model. They found that varying just four molecular parameters controlled a device’s voltage, current density, and fill factor.

And molecular electronic materials may not need separate donor and acceptor components forever. A smattering of recent research has suggested that single-molecule materials—for instance, polymers built with donor and acceptor subunits—have already reached at least 11% efficiency. Rearranging and reshaping polymers might boost that even further.

Finally, Nelson mentioned that optimal solar cell materials already exist in nature: in the photosystems of photosynthesizing plants. If materials scientists studied them, she said, their findings could light possible paths into a bright future of bountiful organic solar cells.

The Kavli Foundation is dedicated to advancing science for the benefit of humanity, promoting public understanding of scientific research and supporting scientists and their work.

Plenary & Featured Talks available online through June 30, 2022

Today is the last day of the 2022 MRS Spring Meeting & Exhibit. A great benefit of the hybrid design is that some of the talks are available on the virtual platform through June 30, 2022. This includes the Plenary and Featured Talks - which means all of the presentations given in Symposium X: Frontiers of Materials Research!

As a preview to the Symposium X speakers, here are two of the interviews done with MRS-TV.

"From Atom to System—Tera-Scale Energy Transition with Better Batteries” – Symposium X speaker Y. Shirley Meng


The Light Stuff: Enabling Sustainable, Product-Selective Photocatalysts with Plasmonics – Symposium X speaker Jennifer Dionne of Stanford University

Blogger: Judy Meiksin

Plenary Session Featuring The Fred Kavli Distinguished Lectureship in Materials Science - MRS-TV Interview

Sossina Haile of Northwestern University discusses her Plenary address to the 2022 MRS Spring Meeting, titled “Vignettes in Solid State Electrochemistry for Sustainable Energy Technologies.” Haile's presentation can be viewed online through June 30, 2022.



Plenary Session Featuring The Fred Kavli Distinguished Lectureship in Materials Science

Plenary_Sossina and Carolyn_800 wideSossina Haile, Northwestern University

Vignettes in Solid State Electrochemistry for Sustainable Energy Technologies

Written by Don Monroe

Warming and precipitation changes will hit equatorial regions the hardest, said Sossina Haile of Northwestern University, and developed countries have the responsibility and resources to explore technologies to deal with the changes they are largely responsible for. Her group looks for overlaps between energy technologies, fundamental materials properties, and fabrication of precise structures to get meaningful measurements. Haile presented four examples of solid-state phenomena at successively higher temperatures, which draw on understanding of materials chemistry, thermochemistry, and electrochemistry.

Her first vignette concerned the use of superprotonic conductors at “warm” temperatures. Specifically, the electrical conductivity of cesium dihydrogen phosphate jumps up by three or four orders of magnitude when heated above about 250°C, reflecting rapid reorientation of proton-bearing phosphate groups and the transfer of the proton between neighboring phosphates. One challenge with these materials is that they are water soluble, so they need to be stabilized using a high-humidity environment.

Haile’s research team exploited the proton conduction of this solid acid as the central layer of fuel cells that generate electricity directly from the reaction of hydrogen with oxygen. In this device every electron brings a hydrogen atom, she said, meaning a 100% faradaic efficiency. Adding a reforming catalyst at the anode side creates a cell that uses alcohols as the starting source for hydrogen. There may even be ways to use liquid ammonia as a hydrogen source. “Superprotonic conductors are pure proton conductors,” she said, and “have lots of intriguing technological possibilities.”

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Haile’s second, “hot” vignette involved protonic ceramic electrolytes, such as doped barium zirconate, a perovskite. She described a reversible cell, which can act either as a fuel cell or for electrolysis, similar to a storage battery. The conductivity of this material is good at temperatures around 500°C.

The proton conducting material avoids many of the downsides seen in oxygen-conducting systems, she said, including degradation of materials and dilution of the fuel source. However, current materials are hampered by some electronic conduction.

In her third example, Haile described studies of ceria-zirconia materials, which are commonly used in catalysis at “hotter” temperatures. Although the oxidation state of zirconium is largely fixed, that of cerium can be 3+ or 4+. The researchers used glancing-angle x-ray absorption near-edge structure (XANES) to find that the 3+ state of cerium is much more prominent at the surface. Somewhat surprisingly, this result was largely independent of the zirconium concentration and of the surface orientation. “The relationship between the Ce3+ on the surface and catalytic activity remains a mystery,” she said.

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The final example involves not electrochemistry but thermochemistry, thermally cycling non-stoichiometric perovskites to produce hydrogen. At “fiery” temperatures as high as 1500°C, entropy drives thermal reduction and oxygen release from the materials. Upon quenching to a lower temperature, perhaps 800°C, the material reacts with steam and CO2 to form H2 and CO.

The second process is driven primarily by enthalpy. This driver led Haile and her team to explore materials such as “STM55,” SrTi0.5Mn0.5O3. To achieving the right balance between reduction and oxidation to improve the production rate per cycle, she said, “It’s really important to recognize that the thermodynamics govern the fuel production rate. The whole thing is about thermodynamics.”

Still, “[we’re] waiting for the mechanical engineers to design the reactors so we can design the materials, and the mechanical engineers are saying ‘What can your material do for us, so we can design the reactor,’” she said. “It’s all a bit of a Catch-22.”

The Kavli Foundation is dedicated to advancing science for the benefit of humanity, promoting public understanding of scientific research and supporting scientists and their work.

Distinguished Keynote

NovoselovSir Kostya Novoselov, The University of Manchester

Materials for the Future

Written by Sophia Chen

Two-dimensional structures like graphene and hexagonal boron nitride make a versatile palette for designing next-generation materials, physicist Kostya Novoselov said in Wednesday’s keynote, titled, “Materials for the Future.”

Novoselov, now at the National University of Singapore, won the 2010 Nobel Prize in Physics for his seminal experiments characterizing the two-dimensional (2D) material graphene. Since then, graphene’s profile has risen among commercial products, both because of its desirable properties and because of its ease of production. Graphene can be found under the hood of all Ford cars, said Novoselov, where they serve noise absorption purposes. Montreal-based company ORA has made headphones with graphene for its stiffness and lightness. Huawei uses graphene films to cool their phones. Graphene is also used as an electrical resistance standard, he said.

Researchers are now pursuing new materials from stacking layers of 2D structures—not just graphene, but also boron nitride, molybdenum diselenide, and tungsten disulfide. This class of materials are broadly known as van der Waals heterostructures, as each atomically thin layer is bonded to the next via van der Waals forces. Researchers have already created LEDs with this type of structure.

A heterostructure’s properties derive from the strong interaction of many electrons between different layers. Thus, researchers can tune a heterostructure’s electronic properties by changing the order and number of layers. Researchers have found surprising materials properties from rotating one layer of atoms with respect to the other slightly. In one 2018 study, researchers created a superconductor by stacking two layers of graphene and rotating one layer by about 1.1 degrees.

Novoselov also discussed several promising future applications for two-dimensional materials. They could be used to design adaptive intelligent materials that respond to external triggers. One example might be a capsule that can open and close to deliver medicine inside the body depending on environmental conditions. Two-dimensional materials could also be used as components in a new type of computer known as neuromorphic computing, he said.

The use of two-dimensional structures could simplify the materials for our electronics. Novoselov pointed out that modern silicon-based devices use a dizzying number of elements ranging from semiconductors to metals to rare earth elements—more than those found in the human body. The increase in material complexity began around 1990, when manufacturers strived to cram more transistors on chips. Two-dimensional materials could potentially reduce the need for rare elements, as they can be oriented and stacked in many configurations to achieve a range of properties.

Plenary Session Featuring The Fred Kavli Distinguished Lectureship in Materials Science

Plenary_HeadshotSir J. Fraser Stoddart, Northwestern University

Artificial Molecular Machines Going from Solution to Surfaces

Written by Don Monroe

Fraser Stoddart has been a pioneer in molecular machines, as recognized by sharing the 2016 Nobel Prize for Chemistry. A useful feature for these structures is the “mechanical bond,” such as that which holds together interlocking molecules, such as a ring-shaped molecule surrounding a dumbbell-shaped one. Among chemistry advances, “a new chemical bond is extremely rare,” he noted.

In his Kavli lecture, Stoddart focused on artificial molecular pumps that exploit this feature and add extra elements to achieve unidirectional motion. But he stressed that these pumps “don’t operate like the mechanical ones” that humans have used for millennia. “It’s a world of difference.”

In the nanomolecular pumps, the free-energy terrain is changed, allowing the molecules to jump around between different accessible states. “It’s all about kinetics,” rather than thermodynamics, he said. The kinetics of association and dissociation can be modulated by changing the charge state of radicals, for example by changing oxidizing or reducing conditions chemically, or electrochemically with an applied voltage.

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Many of the structures Stoddart described use a “pumping cassette” that loads a charged ring-shaped radical onto a “collecting chain” where it is mechanically bound. This process can be repeated to load additional rings, with little increase in the free-energy cost. His research team has loaded as many as 80 rings onto a star polyethylene glycol, incorporating 344 positive charges.

Attaching pumping cassettes to both ends of a chain can double the loading. Stoddart noted that this technique can create a symmetrical loading of molecules, which could in principle be used to make palindromic polymers of the rotaxane ring molecules.

Moving away from solution chemistry, Stoddart illustrated the tethering of molecular pumps to a metalorganic-framework membrane. The result is what he termed “mechanisorption” to the membrane. Unlike the well-known physisorption and chemisorption, driven by van der Waals or chemical bonding, respectively, this process is intrinsically far from equilibrium, and is made possible by mechanical bonding.

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Stoddart also mentioned the potential for molecular nanotopology (formerly called chemical topology) to form various interlocking ring-like structures, including knots, belts, and Möbius strips. (The linear molecules employed for his molecular pumps do not satisfy this description.) “There are eight million knots, so we can keep chemists and materials scientists occupied for centuries,” he said, since only about a dozen have been made so far.

Although Stoddart admitted that he is “not an applications scientist,” he expressed the hope that the tools and techniques his group has developed could be helpful for battery technology and hydrogen storage as well as capture of CO2 and methane. He also expects that there will be huge opportunities in medical science, in view of the profound importance of biological molecular pumps.

The Kavli Foundation is dedicated to advancing science for the benefit of humanity, promoting public understanding of scientific research and supporting scientists and their work.

Plenary Session Featuring The Fred Kavli Distinguished Lectureship in Materials Science

Plenary_Paul AlivisatosPaul Alivisatos, University of California, Berkeley
Recent Advances in the Study of Colloidal Nanocrystals Enabled by In Situ Liquid Cell Transmission Electron Microscopy

Written by Sophia Chen

Paul Alivisatos received this year’s Fred Kavli Distinguished Lectureship in Materials Science. A professor at the University of California, Berkeley, Alivasatos is slated to become the next president of the University of Chicago this fall.

Alivisatos researches the use of electron microscopy to study nanocrystals inside liquid cells—pockets of liquid sandwiched between layers of graphene containing the material of interest. The liquid cell environment allows researchers to observe dynamics in systems ranging from biological cells to battery interfaces.

In his presentation, Alivisatos discussed his optimism about the future of nanoscience, particularly the integration of new data science tools for analyzing electron microscopy images. These new data science tools allow researchers to find patterns in large datasets quickly. “I’m enamored of them,” said Alivisatos of the available data science tools. “I think they’re really going to accelerate what we’re going to be able to do.”

His research team is working to understand the interaction between electron beam and liquid cell. They have used convolutional neural networks, a common machine learning technique, to study the motion of gold nanoparticles in a liquid cell. The neural network could classify electron microscope images of the nanoparticles into different types of motion, such as Brownian motion or a random walk.

To reap the power of these new data science tools, researchers must integrate them with more “classical,” physically motivated approaches, said Alivisatos. His group has studied the chemical environment inside the liquid cell after being probed by the electron microscope. Using old-school concepts like redox couples, they were able to control some activity inside the liquid cell.

Alivisatos also presented recent work from his group on why some crystals become chiral. Curiously, some crystals grow to become left- or right-handed on the macroscopic scale even when its constituent microscopic parts are the opposite chirality. Studying tellurium crystals, the research group found that while the presence of ligands of certain handedness will bias a crystal to match its chirality, these ligands are neither necessary nor sufficient to determine the crystal’s final chirality.

One virtual audience member asked Alivisatos for advice for younger researchers. Alivisatos described the benefits of sharing your work openly, and its contribution to others’ research. “If there’s one thing I would say to early career colleagues, it’s to recognize that you’re part of a community,” said Alivisatos.

The Kavli Foundation is dedicated to advancing science for the benefit of humanity, promoting public understanding of scientific research and supporting scientists and their work.

Plenary Session Featuring The Fred Kavli Distinguished Lectureship in Materials Science

Dario-gilDarío Gil, IBM T. J. Watson Research Center
Scaling the Scientific Method to Enter the Era of Accelerated Materials Discovery

“Right now is the most exciting time in computing in probably the last 60 years,” said Darío Gil, director of research at IBM T. J. Watson Research Center, “where we are witnessing the convergence of different ways to represent and process information.”

Gil is referring to the convergence of bits, neurons, and qubits enabled by the “cloud” programming environment with the help of artificial intelligence (AI) that changes the way problems can be solved. It is ushering us into the era of accelerated discovery. This is particularly critical for addressing global problems such as the current pandemic and climate change. 

To put this in perspective, Gil said the discovery of a new material—from the design concept to commercialization—takes about a decade, using capital of USD$10 million to USD$100 million. IBM wants to cut this effort by 90 percent.

Gil said, “We envision in the future inserting this technology [of quantum computing] to work in tandem with the AI-enriched simulation step of materials discovery loop.”

As our society is getting more and more digitalized, materials discovery in the field of semiconductors needs to accelerate significantly. IBM is aware that all of the materials going into computer chips must be as sustainable as possible. Gil focused his talk on the R&D of photoresists. Photoresists are a light-sensitive material used for forming semiconductor patterning.

Currently, photoresists carry potential toxic risks, so the research community needs to search for new photoacid generators (PAG). Gil showed step-by-step the advantages of using, first, the deep search method, which can complete complex queries on a photoacid generator—38 million edges (connections between entities or nodes that hold information that can also hold information)—in 0.1 s. This process led researchers to a PAG that was used for other applications but never tested for extreme ultra-violet (EUV) lithography.

AI-enriched simulation was then used to augment the material dataset with predicted properties. Generative modeling—a new capability in AI—accepted the information on materials properties and design constraints and filled in the gaps by generating 1000 PAG cation candidates with targeted properties. The next step is autonomous chemical synthesis in order to reduce trial and error and increase reliability and achieve scalability. This is where Cloud-based AI-driven autonomous laboratories become useful and IBM was able to show, on November 19, 2020, the first PAG material formed through this process.

Quantum computing works in tandem with AI, offering another revolution in discovery acceleration. A classical computer can be used for solving easy problems. However, for hard problems such as simulating materials, classical computers can provide only an approximation. “But there’s another technology that alters the equation between what’s possible to solve and what will be possible to solve,” said Gil, “and that is the world of quantum computers.”

IBM uses superconducting qubits, and has made its quantum computing accessible worldwide from the Cloud. In a nutshell, researchers write their programs which they send to the quantum computer that converts the 0s and 1s into microwave pulses that travel to the quantum processor. “We perform superposition, entanglement, and interference operations to perform the computation,” Gil said, then send the information back. Gil said that over 360 billion quantum circuits have been executed by quantum computers to date over the past four years with over 260k users. 

The Kavli Foundation is dedicated to advancing science for the benefit of humanity, promoting public understanding of scientific research and supporting scientists and their work.

Plenary Session Featuring The Fred Kavli Distinguished Lectureship in Materials Science

John-rogersJohn A. Rogers, Northwestern University
Functional Materials for Bioelectronic Neural Interfaces

John A. Rogers’ research has expanded from bendable, stretchable electronic systems that mount on the surface of the skin like temporary tattoos to include cellular-scale electronic/optoelectronic devices that inject into the deep brain and thin microfluidic cuffs that softly encircle the surfaces of peripheral nerves.

Much of the motivation for this work is that some medical disorders are difficult to treat with traditional pharmaceutical approaches, Rogers said, which has led to the rise of the concept of bioelectronic medicines. Addressing these disorders with various engineering platforms—such as deep brain stimulators to treat depression and Parkinson’s—is becoming possible because of the collaborations of researchers in different fields, in many cases with materials science in the lead. Rogers himself has appointments in the department of materials science and engineering, electrical and computer engineering, chemistry, biomedical engineering, mechanical engineering—and neurological surgery.

Rogers foresees a major continuing role for materials scientists, not only in systems that offer electronic interfaces but also optical, microfluidic, thermal coupling schemes, and others. In terms of functional capabilities, advances in materials for these multimodal platforms could lead to approaches in restoring or extending organ function through interfaces that include both diagnostic and therapeutic capabilities, in closed feedback loops. New approaches to patterning and processing known materials and in combining them together in unusual ways serve as additional routes for further miniaturizing implantable devices of these types, and for developing soft, stretchable platforms that more naturally interface with soft biological tissues. Related advances in materials science enable complex, extended network architectures and associated capabilities for “full 3D volumetric integration” with living organisms. Other directions include bioresorbable active and passive materials for implantable devices that function for a time period that matches a biological process such as wound healing, and then naturally disappear to remove unnecessary device load from the body without a secondary surgery.

Creating electronic materials and integrated circuits that can interface with the brain, biology’s most sophisticated electronic system, serves as an example of work in this broader area, where biocompatible semiconductors and other supporting materials must come together to support high performance operation. “Our feeling is that the ultimate solution is a hybrid one, where diverse classes of both organic and inorganic materials heterogeneously integrate into systems where the best properties of the best materials enable the highest levels of function and performance at the biotic/abiotic interface.”

This concept of hybrid electronics requires attention not only to materials but also to materials structures and materials interfaces. In his talk, Rogers showed examples of nanoscale forms of device-grade monocrystalline silicon—as nanomembranes and nanoribbons created directly from bulk wafers. He demonstrated routes for forming bulk quantities of such materials and concepts in manufacturing science that allow their deterministic assembly into layouts and formats that serve as starting points for constructing functional systems. He demonstrated how devices formed in this fashion can interface across large areas of the brain to record extended electrical processes associated with seizures, as well as responses to auditory and visual stimuli, in a range of animal models, from rodents to non-human primates. “The unmatched spatio-temporal resolution of these systems allows their use as tools to facilitate research into fundamental aspects of neuroscience,” Rogers said, “and, potentially in the future, as advanced diagnostics for various kinds of surgical operations like those used to treat chronic forms of epilepsy.”

Rogers also detailed some research that supports the use of optical forms of neuromodulation, as cellular-scale, injectable light sources that operate in a wireless fashion to turn on or off targeted neural circuits in freely behaving animal models—for studies that improve our understanding of basic processes in the brain. He believes that this broader frontier in neuroscience will rely critically on advanced neurotechnologies built upon innovation in materials science.

The Kavli Foundation is dedicated to advancing science for the benefit of humanity, promoting public understanding of scientific research and supporting scientists and their work.

Plenary Session Featuring the Fred Kavli Distinguished Lectureship in Materials Science

IMG_8542_800x800Sharon C. Glotzer, University of Michigan

Engineering Matter Across Scales

Written by Arthur L. Robinson

The ability to design and make the perfect material with just the right properties to do what we want, how we want, and when we want is the holy grail of materials research says Sharon Glotzer of the University of Michigan. Such “materials on demand” require control over thermodynamics, kinetics, nonequilibrium behavior and structure across many length and timescales. In her Fred Kavli Distinguished Lecture in Materials Science during Monday evening’s Plenary Session, Glotzer took it upon herself to demonstrate how atomic and molecular crystal structures—made possible by chemical bonds at Angstrom scales—can be realized with noninteracting nanoparticles and colloids via entropic bonds at nanometer to micron scales. Beyond their importance in understanding and engineering the self-assembly of colloidal crystals and nanoparticle superlattices, Glotzer says, the fact that some of the most complex structures in metallurgy and in molecular crystals can be realized without explicit attraction of any kind, reveals fundamental insights into what is needed to engineer matter across scales.

Physical matter is held together by chemical bonds (ionic, covalent, hydrogen, metallic, and so on). It is a combination of quantum theory, which describes interatomic interactions, and statistical thermodynamics, which governs free-energy minimization, that determine all possible crystal structures in nature. Stable crystals can be predicted if we know all the interatomic forces and can minimize the free energy subject to thermodynamic constraints. In principle, the chemical bonding structure of any set of N atoms can be computed, with varying degrees of accuracy depending on the approximations used.

Shifting to soft matter, Glotzer explains that the entities playing the role of “atoms,” being big and complex, are different. Examples are dendrimers molecular, DNA, proteins, micelles, nanoparticles, including those functionalized with ligands, and viruses. Scores of three-dimensional crystal structures can self-assemble in solution from soft-matter building blocks. Most are isostructural to atomic crystals, though with larger lattice spacings. In every case, interparticle interactions combined with thermodynamics dictate crystal morphologies.


Particle shape plays a big role in dictating colloidal crystal structure because the anisotropy can create an effective “valence” that dictates the number and bond orientation of neighboring particles. Clathrates, structures consisting of polyhedral cages with large pores that can be used for host-guest chemistry, represent a challenging target for colloidal assembly. Here, particles of sizes ranging from a few nanometers to a couple of microns self-assemble in solution to form crystals where the “atoms” are replaced by particles made of atoms. Typically these particles are metals like Au or Ag, semiconductors like CdTe or CdS, or polymer like PS or PMMA, and functionalized with molecular organic ligands or DNA. The particles can be charged or neutral, be magnetic or not, spherical or rod-like or polyhedral. Particles can interact via electrostatic interactions mediated by the solvent, van der Waals interactions, magnetic dipole interactions, h-bonds between ligands, and excluded-volume interactions.  Regardless of what interactions are present, they conspire to produce net interparticle repulsion and attraction that—combined with thermodynamics—dictate the preferred arrangement of particle positions and orientations. Today these colloidal crystals can be predicted, designed, and synthesized.

Very counterintuitive, says Glotzer, is that even in the absence of any explicit interparticle interactions, colloidal particle can form crystals due solely to particle shape and excluded-volume interactions. In these cases, free energy minimization is the same thing as entropy maximization. Entropy alone can drive self-assembly of an incredible diversity of colloidal crystal structures and with extraordinary complexity, both with and without atomic or molecular analogues. Upon crowding, hard particles organize to maximize the system entropy by maximizing the number of allowed microstates. Lots of questions remain: What else is possible with entropy alone? What crystals structures are not possible with entropy alone; if not, why not? What about multi-atomic systems of shapes that are not necessarily hard? To what extent is entropy helping to order nanoparticles into colloidal crystals? Can we engineer entropy to engineer target crystals?

Finishing up, Glotzer described entropic bonding as a process that selects for the set of interparticle orientations and positions that maximizes system entropy in analogy with chemical bonding as a process that selects for the set of interatomic orientations and positions that minimizes the total energy among atoms. In this analogy, entropy creates an emergent “valence.” With an entropic analog to the Schrödinger equation based on “shape orbitals” that can be placed on a lattice and optimized for maximum overlap to find the bonding orbital and free energy of a unit cell, Glotzer and colleague Thi Vo developed a predictive microscopic theory of entropic bonding based on a pseudopotential between a quasiparticle and a hard shape. They replaced the electron probability density (wave function) in the Schrödinger equation with the quasiparticle probability density, where quasiparticles serve as a proxy for an effective entropic attraction between shapes in a Schrödinger-like equation. Applying their theory to a hard-shape system of two hard cubes in a box at a density of 0.55, they found their model predicted a simple cubic “lattice” that would have lower energy than fcc or bcc, whereas for a system of truncated tetrahedra, the preferred lattice would progress from that of a quasicrystal to diamond to -Sn to bcc as the degree of truncation increases. These predictions match those from molecular-assembly simulations made in 2012.

For takeaways, Glotzer noted that entropic forces can be directional, effect valence, and act as bonds; there is now a predictive microscopic theory of entropic bonds; and this allows the use of approaches used for atomic crystals to be used for colloidal crystals.

The Kavli Foundation is dedicated to advancing science for the benefit of humanity, promoting public understanding of scientific research and supporting scientists and their work.