Researchers the University of Pennsylvania Think ‘Small’ to make Progress Towards Better Fuel Cells

Graduate student Jennifer Lee uses a large transmission electron microscope, housed in the Singh Center, to take a closer look at the nanomaterials and nanocrystals that are synthesized in the lab. Credit: University of Pennsylvania

As renewable sources such as wind and solar are quickly changing the energy landscape, scientists are looking for ways to better store energy for when it’s needed. Fuel cells, which convert chemical energy into electrical power, are one possible solution for long-term energy storage, and could someday be used to power trucks and cars without burning fuel. But before fuel cells can be widely used, chemists and engineers need to find ways to make this technology more cost-effective and stable.

A new study from the lab of Penn Integrates Knowledge Professor Christopher Murray, led by graduate student Jennifer Lee, shows how custom-designed nanomaterials can be used to address these challenges. In ACS Applied Materials & Interfaces, researchers show how a fuel cell can be built from cheaper, more widely available metals using an atomic-level design that also gives the material long-term stability. Former post-doc Davit Jishkariani and former students Yingrui Zhao and Stan Najmr, current student Daniel Rosen, and professors James Kikkawa and Eric Stach, also contributed to this work.

The chemical reaction that powers a fuel cell relies on two electrodes, a negative anode and a positive cathode, separated by an electrolyte, a substance that allows the ions to move. When fuel enters the anode, a catalyst separates molecules into protons and electrons, with the latter traveling toward the cathode and creating an electric current.

Catalysts are typically made of precious metals, like platinum, but because the chemical reactions only occur on the surface of the material, any atoms that are not presented on the surface of the material are wasted. It’s also important for catalysts to be stable for months and years because fuel cells are very difficult to replace.

When not busy at the microscope or analyzing data, researchers in the Murray group work on synthesizing new nanomaterials. Credit: University of Pennsylvania

Chemists can address these two problems by designing custom nanomaterials that have platinum at the surface while using more common metals, such as cobalt, in the bulk to provide stability. The Murray group excels at creating well-controlled nanomaterials, known as nanocrystals, in which they can control the size, shape, and composition of any composite nanomaterial.

In this study, Lee focused on the catalyst in the cathode of a specific type of fuel cell known as a proton exchange membrane fuel cell. “The cathode is more of a problem, because the materials are either platinum or platinum-based, which are expensive and have slower reaction rates,” she says. “Designing the catalyst for the cathode is the main focus of designing a good fuel cell.”

The challenge, explains Jishkariani, was in creating a cathode in which platinum and cobalt atoms would form into a stable structure. “We know cobalt and platinum mixes well; however, if you make alloys of these two, you have added atoms of platinum and cobalt in a random order,” he says. Adding more cobalt in a random order causes it to leach out into the electrode, meaning that the fuel cell will only function for a short time.

To solve this problem, researchers designed a catalyst made of layered platinum and cobalt known as an intermetallic phase. By controlling exactly where each atom sat in the catalyst and locking the structure in place, the cathode catalyst was able to work for longer periods than when the atoms were arranged randomly. As an additional unexpected finding, the researchers found that adding more cobalt to the system led to greater efficiency, with a 1-to-1 ratio of platinum to cobalt, better than many other structures with a wide range of platinum-to-cobalt ratios.

The Xeuss 2.0 X-ray scattering instrument, which came to the LRSM in 2018, helps researchers characterize the structures of a wide range of hard and soft materials.  Credit: University of Pennsylvania

The next step will be to test and evaluate the intermetallic material in fuel cell assemblies to make direct comparisons to commercially-available systems. The Murray group will also be working on new ways to create the intermetallic structure without high temperatures and seeing if adding additional atoms improve the catalyst’s performance.

This work required high-resolution microscopic imaging, work that Lee previously did at Brookhaven National Lab but, thanks to recent acquisitions, can now be done at Penn in the Singh Center for Nanotechnology. “Many of the high-end experiments that we would have had to travel to around the country, sometimes around the world, we can now do much closer to home,” says Murray. “The advances that we’ve brought in electron microscopy and X-ray scattering are a fantastic addition for people that work on energy conversion and catalytic studies.”

Lee also experienced first-hand how chemistry research directly connects to real world challenges. She recently presented this work at the International Precious Metals Institute conference and says that meeting members of the precious-metals community was enlightening. “There are companies looking at fuel cell technology and talking about the newest design of the fuel cell cars,” she says. “You get to interact with people that think of your project from different perspectives.”

Murray sees this fundamental research as a starting point towards commercial implementation and real world application, emphasizing that future progress relies on the forward-looking research that’s happening now. “Thinking about a world where we’ve displaced a lot of the traditional fossil fuel-based inputs, if we can figure out this interconversion of electrical and chemical energy, that will address a couple of very important problems simultaneously.”

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New, durable catalyst for key fuel cell reaction may prove useful in eco-friendly vehicles

U of Pennsylvania: Large Scale Production of Graphene + Graphene Updates and Videos

Draw a line with a pencil and it’s likely that somewhere along that black smudge is a material that earned two scientists the 2010 Nobel Prize in Physics. The graphite of that pencil tip is simply multiple layers of carbon atoms; where those layers are only one atom thick, it is known as graphene.

The properties of a material change at the nanoscopic scale, making graphene the strongest and most conductive substance known. Instead of marking mini-golf scores on paper, this form of carbon is suited for making faster and smaller electronic circuitry, flexible touchscreens, chemical sensors, diagnostic devices, and applications yet to be imagined.

Graphene is not yet as ubiquitous as plastic or silicon, however, and producing the material in bulk remains a challenge. Because graphene’s properties rely on it being only one atom thick, until recently, it was only possible to make it in small patches or flakes.

Physicists at Penn have discovered a way around these limitations, and have spun out their research into a company called Graphene Frontiers. Graphene Frontiers

 

More About Graphene

Turning saltwater into clean drinking water is an expensive, energy-intensive process, but could the wonder material graphene make it more accessible?

New Discovery Could Unlock Graphene’s Full Potential – 

Read More:

Follow this direct link to Seeker.com for more information and Videos about the ‘Wonder Material’ of Graphene.

Seeker.com



Graphene sieve turns seawater into drinking water

“Graphene-oxide membranes have attracted considerable attention as promising candidates for new filtration technologies. Now the much sought-after development of making membranes capable of sieving common salts has been achieved. New research demonstrates the real-world potential of providing clean drinking water for millions of people who struggle to access adequate clean water sources.
The new findings from a group of scientists at The University of Manchester were published today in the journal Nature Nanotechnology. Previously graphene-oxide membranes have shown exciting potential for gas separation and water filtration.”

U Penn: Computer modeling for designing drug-delivery nanocarriers

Previous work by some of the researchers uncovered a counter-intuitive relationship that suggested that adding more targeting molecules on the nanocarrier’s surface is not always better, as increases in stability may come with decreases in targeting specificity. Understanding the role the fluttering of the target cell’s surface plays in this equation is necessary for better design of nanocarriers.

Credit: University of Pennsylvania

A team of University of Pennsylvania researchers has developed a computer model that will aid in the design of nanocarriers, microscopic structures used to guide drugs to their targets in the body. The model better accounts for how the surfaces of different types of cells undulate due to thermal fluctuations, informing features of the nanocarriers that will help them stick to cells long enough to deliver their payloads.

The study was led by Ravi Radhakrishnan, a professor in the departments of bioengineering and chemical and biomolecular engineering in Penn’s School of Engineering and Applied Science, and Ramakrishnan Natesan, a member of his lab.

Also contributing to the study were Richard Tourdot, a Radhakrishnan lab member; David Eckmann, the Horatio C. Wood Professor of Anesthesiology and Critical Care in Penn’s Perelman School of Medicine; Portonovo Ayyaswamy, the Asa Whitney Professor of Mechanical Engineering and Applied Mechanics in Penn Engineering; and Vladimir Muzykantov, a professor of pharmacology in Penn Medicine.

It was published in the journal Royal Society Open Science.

Nanocarriers can be designed with molecules on their exteriors that only bind to biomarkers found on a certain type of cell. This type of targeting could reduce side effects, such as when chemotherapy drugs destroy healthy cells instead of cancerous ones, but the biomechanics of this binding process are complex.

Previous work by some of the researchers uncovered a counter-intuitive relationship that suggested that adding more targeting molecules on the nanocarrier’s surface is not always better.

A nanocarrier with more of those targeting molecules might find and bind to many of the corresponding biomarkers at once. While such a configuration is stable, it can decrease the nanocarrier’s ability to distinguish between healthy and diseased tissues. Having fewer targeting molecules makes the nanocarrier more selective, as it will have a harder time binding to healthy tissue where the corresponding biomarkers are not over-expressed.

The team’s new study adds new dimensions to the model of the interplay between the cellular surface and the nanocarrier.

“The cell surface itself is like a caravan tent on a windy day on a desert,” Radhakrishnan said. “The more excess in the cloth, the more the flutter of the tent. Similarly, the more excess cell membrane area on the ‘tent poles,’ the cytoskeleton of the cell, the more the flutter of the membrane due to thermal motion.”

The Penn team found that different cell types have differing amounts of this excess membrane area and that this mechanical parameter governs how well nanocarriers can bind to the cell. Accounting for the fluttering of the membrane in their computer models, in addition to the quantity of targeting molecules on the nanocarrier and biomarkers on the cell surface, has highlighted the importance of these mechanical aspects in how efficiently nanocarriers can deliver their payloads.

“These design criteria,” Radhakrishnan said, “can be utilized in custom designing nanocarriers for a given patient or patient-cohort, hence showing an important way forward for custom nanocarrier design in the era of personalized medicine.”

The research was supported by the National Science Foundation through grants DMR-1120901, CBET-1236514 and MCB060006, and the National Institutes of Health through grants U01EB016027, 1R01EB006818-05, HL125462 and HL087936.

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The above post is reprinted from materials provided byUniversity of Pennsylvania. Note: Materials may be edited for content and length.

University of Pennsylvania: In the Future Seeing Your Dentist May ‘Go Nano’ ~ Using nanoparticles to break up plaque and prevent cavities

The bacteria that live in dental plaque and contribute to tooth decay often resist traditional antimicrobial treatment, as they can “hide” within a sticky biofilm matrix, a glue-like polymer scaffold.

A new strategy conceived by University of Pennsylvania researchers took a more sophisticated approach. Instead of simply applying an antibiotic to the teeth, they took advantage of the pH-sensitive and enzyme-like properties of iron-containing nanoparticles to catalyze the activity of hydrogen peroxide, a commonly used natural antiseptic. The activated hydrogen peroxide produced free radicals that were able to simultaneously degrade the biofilm matrix and kill the bacteria within, significantly reducing plaque and preventing the tooth decay, or cavities, in an animal model.

“Even using a very low concentration of hydrogen peroxide, the process was incredibly effective at disrupting the biofilm,” said Hyun (Michel) Koo, a professor in the Penn School of Dental Medicine’s Department of Orthodontics and divisions of Pediatric Dentistry and Community Oral Health and the senior author of the study, which was published in the journal Biomaterials. “Adding nanoparticles increased the efficiency of bacterial killing more than 5,000-fold.”

The paper’s lead author was Lizeng Gao, a postdoctoral researcher in Koo’s lab. Coauthors were Yuan Liu, Dongyeop Kim, Yong Li and Geelsu Hwang, all of Koo’s lab, as well as David Cormode, an assistant professor of radiology and bioengineering with appointments in Penn’s Perelman School of Medicine and School of Engineering and Applied Science, and Pratap C. Naha, a postdoctoral fellow in Cormode’s lab.

The work built off a seminal finding by Gao and colleagues, published in 2007 in Nature Nanotechnology, showing that nanoparticles, long believed to be biologically and chemically inert, could in fact possess enzyme-like properties. In that study, Gao showed that an iron oxide nanoparticle behaved similarly to a peroxidase, an enzyme found naturally that catalyzes oxidative reactions, often using hydrogen peroxide.

When Gao joined Koo’s lab in 2013, he proposed using these nanoparticles in an oral setting, as the oxidation of hydrogen peroxide produces free radicals that can kill bacteria.

“When he first presented it to me, I was very skeptical,” Koo said, “because these free radicals can also damage healthy tissue. But then he refuted that and told me this is different because the nanoparticles’ activity is dependent on pH.”

Gao had found that the nanoparticles had no catalytic activity at neutral or near-neutral pH of 6.5 or 7, physiological values typically found in blood or in a healthy mouth. But when pH was acidic, closer to 5, they become highly active and can rapidly produce free radicals.

The scenario was ideal for targeting plaque, which can produce an acidic microenvironment when exposed to sugars.

Gao and Koo reached out to Cormode, who had experience working with iron oxide nanoparticles in a radiological imaging context, to help them synthesize, characterize and test the effectiveness of the nanoparticles, several forms of which are already FDA-approved for imaging in humans.

Beginning with in vitro studies, which involved growing a biofilm containing the cavity-causing bacteria Streptococcus mutans on a tooth-enamel-like surface and then exposing it to sugar, the researchers confirmed that the nanoparticles adhered to the biofilm, were retained even after treatment stopped and could effectively catalyze hydrogen peroxide in acidic conditions.

They also showed that the nanoparticles’ reaction with a 1 percent or less hydrogen peroxide solution was remarkably effective at killing bacteria, wiping out more than 99.9 percent of the S. mutans in the biofilm within five minutes, an efficacy more than 5,000 times greater than using hydrogen peroxide alone. Even more promising, they demonstrated that the treatment regimen, involving a 30-second topical treatment of the nanoparticles followed by a 30-second treatment with hydrogen peroxide, could break down the biofilm matrix components, essentially removing the protective sticky scaffold.

Moving to an animal model, they applied the nanoparticles and hydrogen peroxide topically to the teeth of rats, which can develop tooth decay when infected with S. mutans just as humans do. Twice-a-day, one-minute treatments for three weeks significantly reduced the onset and severity of carious lesions, the clinical term for tooth decay, compared to the control or treatment with hydrogen peroxide alone. The researchers observed no adverse effects on the gum or oral soft tissues from the treatment.

“It’s very promising,” said Koo. “The efficacy and toxicity need to be validated in clinical studies, but I think the potential is there.”

Among the attractive features of the platform is the fact that the components are relatively inexpensive.

“If you look at the amount you would need for a dose, you’re looking at something like 5 milligrams,” Cormode said. “It’s a tiny amount of material, and the nanoparticles are fairly easily synthesize, so we’re talking about a cost of cents per dose.”

In addition, the platform uses a concentration of hydrogen peroxide, 1 percent, which is lower than many currently available tooth-whitening systems that use 3 to 10 percent concentrations, minimizing the chance of negative side effects.

Looking ahead, Gao, Koo, Cormode and colleagues hope to continue refining and improving upon the effectiveness of the nanoparticle platform to fight biofilms.

“We’re studying the role of nanoparticle coatings, composition, size and so forth so we can engineer the particles for even better performance,” Cormode said.

Explore further: Nanoparticles release drugs to reduce tooth decay

More information: Lizeng Gao et al. Intrinsic peroxidase-like activity of ferromagnetic nanoparticles, Nature Nanotechnology (2007). DOI: 10.1038/nnano.2007.260

Lizeng Gao et al. Nanocatalysts promote Streptococcus mutans biofilm matrix degradation and enhance bacterial killing to suppress dental caries in vivo, Biomaterials (2016). DOI: 10.1016/j.biomaterials.2016.05.051

 

 

The “Nano-Tooth Fairy” – Nanoparticles Release Drugs to Reduce Tooth Decay

Therapeutic agents intended to reduce dental plaque and prevent tooth decay are often removed by saliva and the act of swallowing before they can take effect. But a team of researchers has developed a way to keep the drugs from being washed away.

Dental plaque is made up of bacteria enmeshed in a sticky matrix of polymers—a polymeric matrix—that is firmly attached to teeth. The researchers, led by Danielle Benoit at the University of Rochester and Hyun Koo at the University of Pennsylvania’s School of Dental Medicine, found a new way to deliver an antibacterial agent within the plaque, despite the presence of saliva.

Their findings have been published in the journal ACS Nano.

“We had two specific challenges,” said Benoit, an assistant professor of biomedical engineering. “We had to figure out how to deliver the anti-bacterial agent to the teeth and keep it there, and also how to release the agent into the targeted sites.”

To deliver the agent—known as farnesol—to the targeted sites, the researchers created a spherical mass of particles, referred to as a nanoparticle carrier. They constructed the outer layer out of cationic—or positively charged—segments of the polymers. For inside the carrier, they secured the drug with hydrophobic and pH-responsive polymers.

The positively-charged outer layer of the carrier is able to stay in place at the surface of the teeth because the enamel is made up, in part, of HA (hydroxyapatite), which is negatively charged. Just as oppositely charged magnets are attracted to each other, the same is true of the nanoparticles and HA. Because teeth are coated with saliva, the researchers weren’t certain the nanoparticles would adhere. But not only did the particles stay in place, they were also able to bind with the polymeric matrix and stick to dental plaque.

Since the nanoparticles could bind both to saliva-coated teeth and within plaque, Benoit and colleagues used them to carry an anti-bacterial agent to the targeted sites. The researchers then needed to figure out how to effectively release the agent into the plaque.

Farnesol is released from the nanoparticle carriers into the cavity-causing dental plaque. Credit: Michael Osadciw/University of Rochester 

A key trait of the inner carrier material is that it destabilizes at acidic—or low pH—levels, such as 4.5, allowing the drug to escape more rapidly. And that’s exactly what happens to the pH level in plaque when it’s exposed to glucose, sucrose, starch, and other food products that cause tooth decay. In other words, the nanoparticles release the drug when exposed to cavity-causing eating habits—precisely when it is most needed to quickly stop acid-producing bacteria.

The researchers tested the product in rats that were infected with Streptococcus mutans—a microbe that causes tooth decay. “We applied the test solutions to rats’ mouths twice daily for 30 seconds, simulating what a person might do using a mouth rinse morning and night,” said Hyun Koo, a professor in the Department of Orthodontics and co-senior author of the work. “When the drug was administered without the nanoparticle carriers, there was no effect on the number of cavities and only a very small reduction in their severity. But when it was delivered by the nanoparticle carriers, both the number and severity of the cavities were reduced.”

Plaque formation and tooth decay are chronic conditions that need to be monitored through regular visits to the dental office. The researchers hope their results will someday lead to better—and perhaps permanent—treatments for dental plaque and tooth decay, as well as other biofilm-related diseases.

Penn Engineers Develop Graphene-based Biosensor That Works in Three Ways at Once

One of nanotechnology’s greatest promises is interacting with the biological world the way our own cells do, but current biosensors must be tailor-made to detect the presence of one type of protein, the identity of which must be known in advance.

University of Pennsylvania engineers have now devised a new kind of graphene-based biosensor that works in three ways at once. Because proteins trigger three different types of signals, the sensor can triangulate this information to produce more sensitive and accurate results. By taking advantage of the unique integration of multiple physical sensing modes on the same chip, this sensor device can extend the protein-concentration sensing range by a thousand-fold.

This extended range could be particularly useful in early diagnosis of certain cancers, where the blood biomarker concentration varies by orders of magnitude from patient to patient. The ability to make multiple detections of the same biomarker on the same chip also has the potential to reduce false positives and negatives in medical diagnostic tests.

Eventually, such a technique could be used in an all-purpose biosensor, which could identify a wide range of proteins through their mass, as well as their optical and electrical properties.

A biosensor that did not have to be fine-tuned to detect only specific proteins would have a host of biomedical applications in diagnostic devices.

The study, published in the journal NanoLetters, was conducted by Ertugrul Cubukcu, assistant professor in the departments of Materials Science and Engineering and Electrical and Systems Engineering in Penn’s School of Engineering and Applied Science, and members of his lab, Alexander Y. Zhu, Fei Yi, Jason C. Reed and Hai Zhu.

“In a typical single mode biosensor you have two proteins that interact strongly. You attach protein A to your sensor and, when protein B binds to it, the sensor transduces that binding into some sort of electrical signal,” Cubukcu said,” But it’s kind of a dumb sensor in that it can only tell you if that kind of binding has occurred.

“But let’s say you have proteins A, B, C and D, all with different physical properties, like charge and mass. If you had a sensor that was sensitive to several of those properties, you could tell the difference between those binding events without starting with corresponding proteins for all of them.”

The more sensing modes operating at once, the better a sensor is at distinguishing between similar proteins. Proteins A and B might have the same mass but different charges, while proteins B and C have the same charges but different optical properties.

A multimodal sensor, pulling in data from multiple categories, could narrow the identity of a protein by comparing those values to a large database. Such an ability could potentially enable it to be applied to samples where the protein’s contents are unknown, a major upgrade on current technology which generally involves custom-building sensors to detect the presence of pre-defined sets of proteins.

The team’s sensors consist of a base of silicon nitride, coated with a layer of graphene, a single-atom-thick lattice of carbon atoms. Being carbon based means that graphene is an attractive bonding surface for proteins, which means that the device doesn’t need to be “functionalized” with proteins that are apt to interact with the ones the sensor aims to detect.

Graphene’s extreme thinness and unique electrical properties also allow for the mechanical, electrical and optical modes to operate simultaneously without interfering with one another.

“In the mechanical mode, the graphene is like the skin of a drum,” said Alexander Zhu, the first author of the study, who was then an undergraduate working in Cubukcu’s lab. “As proteins bind, the total mass changes and the resonance of the drum changes as a function of the total mass.

“In the electrical mode, we can look at how electrons travel across the graphene. The conductance is a function of the total available carriers inside, so, if you have something binding to the graphene, that changes the number of carriers and therefore the conductance properties.

“Finally, in the optical mode, we have a source of visible light and shine it on the sensor and measure the reflection. When nothing is bound, it’s seeing just air, but, as soon as proteins bind, we can measure the change in the refractive index.”

In their study, the researchers tested their sensor with known samples of proteins in order to demonstrate that all three modes can work simultaneously.

“We’ve shown that one sample provides all three shifts,” Yi said, “in the mass, electrical and optical readouts.”

Further work from Cubukcu’s group will investigate the feasibility of using this multimodal sensor to identify proteins from unknown samples.

The research was supported by the National Science Foundation under grants IIP-1312202 and ECCS-1408139.

“Programmable Matter” using Nanocrystals

When University of Pennsylvania nano-scientists created beautiful, tiled patterns with flat nano-crystals, they were left with a mystery: why did some sets of crystals arrange themselves in an alternating, herringbone style, even though it wasn’t the simplest pattern? To find out, they turned to experts in computer simulation at the University of Michigan and the Massachusetts Institute of Technology.

These transmission electron microscope images show the two different patterns the nano-crystals could be made to pack in. 

The result gives nanotechnology researchers a new tool for controlling how objects one-millionth the size of a grain of sand arrange themselves into useful materials, it gives a means to discover the rules for “programming” them into desired configurations.

The study was led by Christopher Murray, a professor with appointments in the Department of Chemistry in the School of Arts and Sciences and the Department of Materials Science and Engineering in the School of Engineering and Applied Sciences. Also on the Penn team were Cherie Kagan, a chemistry, MSE and electrical and systems engineering professor, and postdoctoral researchers Xingchen Ye, Jun Chen and Guozhong Xing. 

They collaborated with Sharon Glotzer, a professor of chemical engineering at Michigan, and Ju Li, a professor of nuclear science and engineering at MIT.

Their research was featured on the cover of the journal Nature Chemistry.

“The excitement in this is not in the herringbone pattern,” Murray said, “It’s about the coupling of experiment and modeling and how that approach lets us take on a very hard problem.”

Previous work in Murray’s group has been focused on creating nanocrystals and arranging them into larger crystal superstructures. Ultimately, researchers want to modify patches on nanoparticles in different ways to coax them into more complex patterns. The goal is developing “programming matter,” that is, a method for designing novel materials based on the properties needed for a particular job.

“By engineering interactions at the nanoscale,” Glotzer said, “we can begin to assemble target structures of great complexity and functionality on the macroscale.”

Glotzer introduced the concept of nanoparticle “patchiness” in 2004. Her group uses computer simulations to understand and design the patches.

Recently, Murray’s team made patterns with flat nanocrystals made of heavy metals, known to chemists as lanthanides, and fluorine atoms. Lanthanides have valuable properties for solar energy and medical imaging, such as the ability to convert between high- and low-energy light.

They started by breaking down chemicals containing atoms of a lanthanide metal and fluorine in a solution, and the lanthanide and fluorine naturally began to form crystals. Also in the mix were chains of carbon and hydrogen that stuck to the sides of the crystals, stopping their growth at sizes around 100 nanometers, or 100 millionths of a millimeter, at the largest dimensions. By using lanthanides with different atomic radii, they could control the top and bottom faces of the hexagonal crystals to be anywhere from much longer than the other four sides to non-existent, resulting in a diamond shape.

To form tiled patterns, the team purified the nano-crystals and mixed them with a solvent. They spread this mixture in a thin layer over a thick fluid, which supported the crystals while allowing them to move. As the solvent evaporated, the crystals had less space available, and they began to pack together.

The diamond shapes and the very long hexagons lined up as expected, the diamonds forming an argyle-style grid and the hexagons matching up their longest edges like a foreshortened honeycomb. The hexagons whose sides were all nearly the same length should have formed a similar squashed honeycomb pattern, but, instead, they lined up in an alternating herringbone style.

“Whenever we see something that isn’t taking the simplest pattern possible, we have to ask why,” Murray said.

They posed the question to Glotzer’s team.

“They’ve been world leaders in understanding how these shapes could work on nanometer scales, and there aren’t many groups that can make the crystals we make,” Murray said. “It seemed natural to bring these strengths together.”

Glotzer and her group built a computer model that could recreate the self-assembly of the same range of shapes that Murray had produced. The simulations showed that if the equilateral hexagons interacted with one another only through their shapes, most of the crystals formed the foreshortened honeycomb pattern, not the herringbone.

“That’s when we said, ‘Okay, there must be something else going on. It’s not just a packing problem,'” Glotzer said. Her team, which included graduate student Andres Millan and research scientist Michael Engel, then began playing with interactions between the edges of the particles. They found that that if the edges that formed the points were stickier than the other two sides, the hexagons would naturally arrange in the herringbone pattern.

The teams suspected that the source of the stickiness was those carbon and hydrogen chains. Perhaps they attached to the point edges more easily, the team members thought. Since experiment doesn’t yet offer a way to measure the number of hydrocarbon chains on the sides of such tiny particles, Murray asked MIT’s Ju Li to calculate how the chains would attach to the edges at a quantum mechanical level.

Li’s group confirmed that, because of the way that the different facets cut across the lattice of the metal and fluorine atoms, more hydrocarbon chains could stick to the four edges that led to points than the remaining two sides. As a result, the particles become patchy.

“Our study shows a way forward making very subtle changes in building block architecture and getting a very profound change in the larger self-assembled pattern,” Glotzer said. “The goal is to have knobs that you can change just a little and get a big change in structure, and this is one of the first papers that shows a way forward for how to do that.”