Enhanced Thermal Transport at Covalently Functionalized Carbon Nanotube Array Interfaces

Nanotechnology Today - “Cool it!” That’s a prime directive for microprocessor chips and a promising new solution to meeting this imperative is in the offing. Researchers with the U.S. Department of Energy (DOE)’s Lawrence Berkeley National Laboratory (Berkeley Lab) have developed a “process friendly” technique that would enable the cooling of microprocessor chips through carbon nanotubes.

Frank Ogletree, a physicist with Berkeley Lab’s Materials Sciences Division, led a study in which organic molecules were used to form strong covalent bonds between carbon nanotubes and metal surfaces. This improved by six-fold the flow of heat from the metal to the carbon nanotubes, paving the way for faster, more efficient cooling of computer chips. The technique is done through gas vapor or liquid chemistry at low temperatures, making it suitable for the manufacturing of computer chips.

“We’ve developed covalent bond pathways that work for oxide-forming metals, such as aluminum and silicon, and for more noble metals, such as gold and copper,” says Ogletree, who serves as a staff engineer for the Imaging Facility at the Molecular Foundry, a DOE nanoscience center hosted by Berkeley Lab. “In both cases the mechanical adhesion improved so that surface bonds were strong enough to pull a carbon nanotube array off of its growth substrate and significantly improve the transport of heat across the interface.”

Cooling microprocessor chips through the combination of carbon nanotubes and organic molecules as bonding agents is a promising technique for maintaining the performance levels of densely packed, high-speed transistors in the future.

Ogletree is the corresponding author of a paper describing this research in Nature Communications. The paper is titled “Enhanced Thermal Transport at Covalently Functionalized Carbon Nanotube Array Interfaces.” Co-authors are Sumanjeet Kaur, Nachiket Raravikar, Brett Helms and Ravi Prasher.

Overheating is the bane of microprocessors. As transistors heat up, their performance can deteriorate to the point where they no longer function as transistors. With microprocessor chips becoming more densely packed and processing speeds continuing to increase, the overheating problem looms ever larger. The first challenge is to conduct heat out of the chip and onto the circuit board where fans and other techniques can be used for cooling. Carbon nanotubes have demonstrated exceptionally high thermal conductivity but their use for cooling microprocessor chips and other devices has been hampered by high thermal interface resistances in nanostructured systems.

“The thermal conductivity of carbon nanotubes exceeds that of diamond or any other natural material but because carbon nanotubes are so chemically stable, their chemical interactions with most other materials are relatively weak, which makes for high thermal interface resistance,” Ogletree says. “Intel came to the Molecular Foundry wanting to improve the performance of carbon nanotubes in devices. Working with Nachiket Raravikar and Ravi Prasher, who were both Intel engineers when the project was initiated, we were able to increase and strengthen the contact between carbon nanotubes and the surfaces of other materials. This reduces thermal resistance and substantially improves heat transport efficiency.”

Sumanjeet Kaur, lead author of the Nature Communications paper and an expert on carbon nanotubes, with assistance from co-author and Molecular Foundry chemist Brett Helms, used reactive molecules to bridge the carbon nanotube/metal interface – aminopropyl-trialkoxy-silane (APS) for oxide-forming metals, and cysteamine for noble metals. First vertically aligned carbon nanotube arrays were grown on silicon wafers, and thin films of aluminum or gold were evaporated on glass microscope cover slips. The metal films were then “functionalized” and allowed to bond with the carbon nanotube arrays. Enhanced heat flow was confirmed using a characterization technique developed by Ogletree that allows for interface-specific measurements of heat transport.

“You can think of interface resistance in steady-state heat flow as being an extra amount of distance the heat has to flow through the material,” Kaur says. “With carbon nanotubes, thermal interface resistance adds something like 40 microns of distance on each side of the actual carbon nanotube layer. With our technique, we’re able to decrease the interface resistance so that the extra distance is around seven microns at each interface.”

Although the approach used by Ogletree, Kaur and their colleagues substantially strengthened the contact between a metal and individual carbon nanotubes within an array, a majority of the nanotubes within the array may still fail to connect with the metal. The Berkeley team is now developing a way to improve the density of carbon nanotube/metal contacts. Their technique should also be applicable to single and multi-layer graphene devices, which face the same cooling issues.

“Part of our mission at the Molecular Foundry is to help develop solutions for technology problems posed to us by industrial users that also raise fundamental science questions,” Ogletree says. “In developing this technique to address a real-world technology problem, we also created tools that yield new information on fundamental chemistry.”

This work was supported by the DOE Office of Science and the Intel Corporation.

# # #

The Molecular Foundry is one of five DOE Nanoscale Science Research Centers (NSRCs), national user facilities for interdisciplinary research at the nanoscale, supported by the DOE Office of Science. Together the NSRCs comprise a suite of complementary facilities that provide researchers with state-of-the-art capabilities to fabricate, process, characterize, and model nanoscale materials, and constitute the largest infrastructure investment of the National Nanotechnology Initiative. The NSRCs are located at DOE’s Argonne, Brookhaven, Lawrence Berkeley, Oak Ridge and Sandia and Los Alamos national laboratories. For more information about the DOE NSRCs, please visit science.energy.gov/bes/suf/user-facilities/nanoscale-science-research-centers/.

Lawrence Berkeley National Laboratory addresses the world’s most urgent scientific challenges by advancing sustainable energy, protecting human health, creating new materials, and revealing the origin and fate of the universe. Founded in 1931, Berkeley Lab’s scientific expertise has been recognized with 13 Nobel prizes. The University of California manages Berkeley Lab for the U.S. Department of Energy’s Office of Science. For more, visit www.lbl.gov.

The DOE Office of Science is the single largest supporter of basic research in the physical sciences in the United States and is working to address some of the most pressing challenges of our time. For more information, please visit science.energy.gov.

Contact: Lynn Yarris lcyarris@lbl.gov 510-486-5375 DOE/Lawrence Berkeley National Laboratory

Layer-Dependent Electrocatalysis of MoS2 for Hydrogen Evolution

Researchers at North Carolina State University have shown that a one-atom thick film of molybdenum sulfide (MoS2) may work as an effective catalyst for creating hydrogen. The work opens a new door for the production of cheap hydrogen.

Hydrogen holds great promise as an energy source, but the production of hydrogen from water electrolysis – freeing hydrogen from water with electricity – currently relies in large part on the use of expensive platinum catalysts. The new research shows that MoS2 atomically thin films are also effective catalysts for hydrogen production and – while not as efficient as platinum – are relatively inexpensive. (A Q&A with Cao on how this research differs from earlier studies of other catalysts for hydrogen production can be found on NC State’s research blog.)

“We found that the thickness of the thin film is very important,” says Dr. Linyou Cao, an assistant professor of materials science and engineering at NC State and senior author of a paper describing the work. “A thin film consisting of a single layer of atoms was the most efficient, with every additional layer of atoms making the catalytic performance approximately five times worse.”

Researchers found MoS2 thin films are effective catalysts for hydrogen production. (Click to enlarge.)

The effect of the thin films’ thickness came as a surprise to researchers, because it has long been thought that catalysis normally takes place along the edges of the material. Because thin films have very little ‘edge,’ conventional wisdom held that thin films were essentially catalytically inactive.

But the researchers discovered that a material’s thickness is important because the thinner the MoS2 thin film is, the more conductive it becomes – and the more conductive it becomes, the more effective it is as a catalyst.

“The focus has been on creating catalysts with a large ‘edge’ side,” Cao says. “Our work indicates that researchers may want to pay more attention to a catalyst’s conductivity.”

Cao developed the technique for creating high-quality MoS2 thin films at the atomic scale in 2013. The current production of hydrogen from the atomically thin film is powered by electricity. His team is working to develop a solar-powered water-splitting device that uses the MoS2 thin films to create hydrogen.

The paper, “Layer-dependent Electrocatalysis of MoS2 for Hydrogen Evolution,” is published online in Nano Letters. Lead author of the paper is Yifei Yu, a Ph.D. student at NC State. Co-authors include Yanpeng Li, a Ph.D. student at NC State; Dr. Shengyang Huang, a former visiting scholar at NC State; and Drs. Stephan Steinmann and Weitao Yang of Duke University. The research was supported by U.S. Army Research Office grant W911NF-13-1-0201.

-shipman-

Note to Editors: The study abstract follows.

“Layer-dependent Electrocatalysis of MoS2 for Hydrogen Evolution”

Authors: Yifei Yu, Shengyang Huang, Yanpeng Li, and Linyou Cao, North Carolina State University; Stephan Steinmann and Weitao Yang, Duke University

Published: Jan. 16, 2014, Nano Letters

DOI: 10.1021/nl403620g

Abstract: The quantitative correlation of the catalytic activity with microscopic structure of heterogeneous catalysts is a major challenge for the field of catalysis science. It requests synergistic capabilities to tailor the structure with atomic scale precision and to control the catalytic reaction to proceed through well-defined pathways. Here we leverage on the controlled growth of MoS2 atomically thin films to demonstrate that the catalytic activity of MoS2 for the hydrogen evolution reaction decreases by a factor of ~4.47 for the addition of every one more layer.

Similar layer dependence is also found in edge-riched MoS2 pyramid platelets. This layer-dependent electrocatalysis can be correlated to the hopping of electrons in the vertical direction of MoS2 layers over an interlayer potential barrier. Our experimental results suggest the potential barrier to be 0.119V, consistent with theoretical calculations. Different from the conventional wisdom, which thinks that the number of edge sites is important, our results suggest that increasing the hopping efficiency of electrons in the vertical direction is a key for the development of high-efficiency two-dimensional material catalysts.

Contact: Matt Shipman matt_shipman@ncsu.edu 919-515-6386 North Carolina State University

Phase-transition-driven growth of compound semiconductor crystals from ordered metastable nanorods

Nanotechnology Today - Research teams at the HZB and at the University of Limerick, Ireland, have discovered a novel solid state reaction which lets kesterite grains grow within a few seconds and at relatively low temperatures. For this reaction they exploit a transition from a metastable wurtzite compound in the form of nanorods to the more stable kesterite compound.

At the EDDI Beamline at BESSY II, the scientists could observe this process in real-time when heating the sample: in a few seconds Kesterite grains formed. The size of the grains was found to depend on the heating rate. With fast heating they succeeded in producing a Kesterite thin film with near micrometer-sized crystal grains, which could be used in thin film solar cells. These findings have now been published in the journal “Nature Communications”.

The transformation from a layer of closely packed nanorods (top left) to a polycrystalline semiconductor thin film (top right) can be observed in by in-situ X-ray diffraction in real time. The intensities of the diffraction signals are color coded in the image at the bottom. A detailed analysis of the signals reveals that the transformation of the nanorods into kesterite crystals takes only 9 to 18 seconds. Picture: R. Mainz/A. Singh

Grain formation during growth of kesterite solar cells observed in real-time.

As starting material for the formation of the kesterite film serves a “carpet of nanorods”: With the help of solution-based chemical processing, the chemists around Ajay Singh and Kevin Ryan at the University of Limerick have fabricated films of highly ordered wurtzite nanorods, which have exactly the same composition as kesterite Cu2ZnSnS4.

With the help of real-time X-ray diffraction at the EDDI beamline of BESSY II, HZB physicists around Roland Mainz and Thomas Unold could now observe how a phase transition from the metastable wurtzite phase to the stable kesterite phase leads to a rapid formation of a thin film with large kesterite grains. “It is interesting to see that the complete formation of the kesterite film is so fast”, says Mainz. And the faster the samples are heated up, the larger the grains grow. Mainz explains that at low heating rate, the transition from wurtzite to kesterite starts at lower temperature at which many small grains form – instead of a few larger grains. Additionally, more defects are formed at lower temperatures. During fast heating, the transition takes place at higher temperature at which grains with less defects form.

Moreover, the comparison of the time-resolved evolution of the phase transition during slow and during fast heating shows that not only the grain growth is triggered by the phase transition, but also the phase transition is additionally accelerated by the grain growth. The HZB physicists have developed a model which can explain these findings. By means of numerical model calculations, they demonstrated the accordance of the model with the measured data.

Novel synthesis pathway for thin film semiconductors with controlled morphology.

The work points towards a new pathway for the fabrication of thin microcrystalline semiconductor films without the need of expensive vacuum technology. Cu2ZnSnS4-based kesterite semiconductors have gained increasing attention in the past, since they are a promising alternative for the Cu(In,Ga)Se2 chalcopyrite solar cells which already achieved efficiencies above 20%.

Kesterite has similar physical properties as the chalcopyrite semiconductors, but consist only of elements which are abundantly present in the earth crust. The new procedure could also be interesting for the fabrication of micro- and nanostructured photoelectric devices as well as for semiconductor layers consisting of other materials, says Mainz. “But we continue to focus on kesterites, because this is a really exciting topic at the moment.”

Contact: Dr. Roland Mainz roland.mainz@helmholtz-berlin.de 49-030-806-242-737 Helmholtz-Zentrum Berlin für Materialien und Energie

A self-propelled biohybrid swimmer at low Reynolds number

CHAMPAIGN, Ill. — The alien world of aquatic micro-organisms just got new residents: synthetic self-propelled swimming bio-bots.

A team of engineers has developed a class of tiny bio-hybrid machines that swim like sperm, the first synthetic structures that can traverse the viscous fluids of biological environments on their own. Led by Taher Saif, the University of Illinois Gutgsell Professor of mechanical science and engineering, the team published its work in the journal Nature Communications.

“Micro-organisms have a whole world that we only glimpse through the microscope,” Saif said. “This is the first time that an engineered system has reached this underworld.”

The bio-bots are modeled after single-celled creatures with long tails called flagella – for example, sperm. The researchers begin by creating the body of the bio-bot from a flexible polymer. Then they culture heart cells near the junction of the head and the tail. The cells self-align and synchronize to beat together, sending a wave down the tail that propels the bio-bot forward.

Engineers developed the first tiny, synthetic machines that can swim by themselves, powered by beating heart cells. | Photo by Alex Jerez Roman, Beckman Institute for Advanced Science and Technology

This self-organization is a remarkable emergent phenomenon, Saif said, and how the cells communicate with each other on the flexible polymer tail is yet to be fully understood. But the cells must beat together, in the right direction, for the tail to move.

“It’s the minimal amount of engineering – just a head and a wire,” Saif said. “Then the cells come in, interact with the structure, and make it functional.”

The team also built two-tailed bots, which they found can swim even faster. Multiple tails also opens up the possibility of navigation. The researchers envision future bots that could sense chemicals or light and navigate toward a target for medical or environmental applications.

“The long-term vision is simple,” said Saif, who is also part of the Beckman Institute for Advanced Science and Technology at the U. of I. “Could we make elementary structures and seed them with stem cells that would differentiate into smart structures to deliver drugs, perform minimally invasive surgery or target cancer?”

The swimming bio-bot project is part of a larger National Science Foundation-supported Science and Technology Center on Emergent Behaviors in Integrated Cellular Systems, which also produced the walking bio-bots developed at Illinois in 2012.

“The most intriguing aspect of this work is that it demonstrates the capability to use computational modeling in conjunction with biological design to optimize performance, or design entirely different types of swimming bio-bots,” said center director Roger Kamm, a professor of biological and mechanical engineering at the Massachusetts Institute of Technology. “This opens the field up to a tremendous diversity of possibilities. Truly an exciting advance.”

1/17/2014 | Liz Ahlberg, Physical Sciences Editor | 217-244-1073; eahlberg@illinois.edu Photo by Alex Jerez Roman, Beckman Institute for Advanced Science and Technology

Micro-windmills that generate wind energy and may become an innovative solution to cell phone batteries

Nanotechnology Today - A UT Arlington research associate and electrical engineering professor have designed a micro-windmill that generates wind energy and may become an innovative solution to cell phone batteries constantly in need of recharging and home energy generation where large windmills are not preferred.

Smitha Rao and J.-C. Chiao designed and built the device that is about 1.8 mm at its widest point. A single grain of rice could hold about 10 of these tiny windmills. Hundreds of the windmills could be embedded in a sleeve for a cell phone. Wind, created by waving the cell phone in air or holding it up to an open window on a windy day, would generate the electricity that could be collected by the cell phone’s battery.

Rao’s works in micro-robotic devices initially heightened a Taiwanese company’s interest in having Rao and Chiao brainstorm over novel device designs and applications for the company’s unique fabrication techniques, which are known in the semiconductor industry for their reliability.

“The company was quite surprised with the micro-windmill idea when we showed the demo video of working devices,” Rao said. “It was something completely out of the blue for them and their investors.”

One of Rao's micro-windmills is placed here on a penny.

Rao’s designs blend origami concepts into conventional wafer-scale semiconductor device layouts so complex 3-D moveable mechanical structures can be self-assembled from two-dimensional metal pieces utilizing planar multilayer electroplating techniques that have been optimized by WinMEMS Technologies Co., the Taiwanese fabrication foundry that took an initial interest in Rao’s work.

“The micro-windmills work well because the metal alloy is flexible and Smitha’s design follows minimalism for functionality.” Chiao said.

WinMEMS became interested in the micro-electro mechanical system research and started a relationship with UT Arlington. Company representatives visited with the UT Arlington team several times in 2013 to discuss collaboration.

An agreement has been established for UT Arlington to hold the intellectual properties while WinMEMS explores the commercialization opportunities. UT Arlington has applied for a provisional patent.

Currently, WinMEMS has been showcasing UT Arlington’s works on its website and in public presentations, which include the micro-windmills, gears, inductors, pop-up switches and grippers. All of those parts are as tiny as a fraction of the diameter of a human hair.

These inventions are essential to build micro-robots that can be used as surgical tools, sensing machines to explore disaster zones or manufacturing tools to assemble micro-machines.

“It’s very gratifying to first be noticed by an international company and second to work on something like this where you can see immediately how it might be used,” said Rao, who earned her Ph.D in 2009 at UT Arlington. “However, I think we’ve only scratched the surface on how these micro-windmills might be used.”

The micro windmills were tested successfully in September 2013 in Chiao’s lab. The windmills operate under strong artificial winds without any fracture in the material because of the durable nickel alloy and smart aerodynamic design.

“The problem most MEMS designers have is that materials are too brittle,” Rao said. “With the nickel alloy, we don’t have that same issue. They’re very, very durable.”

The micro-windmills can be made in an array using the batch processes. The fabrication cost of making one device is the same as making hundreds or thousands on a single wafer, which enables for mass production of very inexpensive systems.

“Imagine that they can be cheaply made on the surfaces of portable electronics,” Chiao said, “so you can place them on a sleeve for your smart phone. When the phone is out of battery power, all you need to do is to put on the sleeve, wave the phone in the air for a few minutes and you can use the phone again.”

Chiao said because of the small sizes, flat panels with thousand of windmills could be made and mounted on the walls of houses or building to harvest energy for lighting, security or environmental sensing and wireless communication.

He added that it has been fulfilling to see his former student succeed and help move innovation toward the marketplace.

“To see a company recognize that and seek you out for your expertise speaks volumes about what UT Arlington means to the world,” he said proudly.

The University of Texas at Arlington is a comprehensive research institution of more than 33,300 students and 2,300 faculty members in the epicenter of North Texas. It is the second largest institution in the University of Texas System. Research expenditures reached almost $78 million last year. Visit www.uta.edu for more information.

###

The University of Texas at Arlington is an Equal Opportunity and Affirmative Action employer.

Media Contact: Herb Booth, Office:817-272-7075, Cell:214-546-1082, hbooth@uta.edu News Topics: energy, engineering, innovation, research, sustainability

Gel–Liposome-Mediated Co-Delivery of Anticancer Membrane-Associated Proteins and Small-Molecule Drugs for Enhanced Therapeutic Efficacy

Nanotechnology Today: Gel–Liposome-Mediated Co-Delivery of Anticancer Membrane-Associated Proteins and Small-Molecule Drugs for Enhanced Therapeutic Efficacy.

Researchers have developed a technique for creating nanoparticles that carry two different cancer-killing drugs into the body and deliver those drugs to separate parts of the cancer cell where they will be most effective. The technique was developed by researchers at North Carolina State University and the University of North Carolina at Chapel Hill.

“In testing on laboratory mice, our technique resulted in significant improvement in breast cancer tumor reduction as compared to conventional treatment techniques,” says Dr. Zhen Gu, senior author of a paper on the research and an assistant professor in the joint biomedical engineering program at NC State and UNC-Chapel Hill.

“Cancer cells can develop resistance to chemotherapy drugs, but are less likely to develop resistance when multiple drugs are delivered simultaneously,” Gu says. “However, different drugs target different parts of the cancer cell. For example, the protein drug TRAIL is most effective against the cell membrane, while doxorubicin (Dox) is most effective when delivered to the nucleus. We’ve come up with a sequential and site-specific delivery technique that first delivers TRAIL to cancer cell membranes and then penetrates the membrane to deliver Dox to the nucleus.”

Image shows the structure of the nanoparticle (left), and how the nanoparticles home in on a tumor and shrink it (right)

Gu’s research team developed nanoparticles with an outer shell made of hyaluronic acid (HA) woven together with TRAIL. The HA interacts with receptors on cancer cell membranes, which “grab” the nanoparticle. Enzymes in the cancer cell environment break down the HA, releasing TRAIL onto the cell membrane and ultimately triggering cell death.

When the HA shell breaks down, it also reveals the core of the nanoparticle, which is made of Dox that is embedded with peptides that allow the core to penetrate into the cancer cell. The cancer cell encases the core in a protective bubble called an endosome, but the peptides on the core cause the endosome to begin breaking apart. This spills the Dox into the cell where it can penetrate the nucleus and trigger cell death.

“We designed this drug delivery vehicle using a ‘programmed’ strategy,” says Tianyue Jiang, a lead author in Dr. Gu’s lab. “Different drugs can be released at the right time in their right places,” adds Dr. Ran Mo, a postdoctoral researcher in Gu’s lab and the other lead author.

“This research is our first proof of concept, and we will continue to optimize the technique to make it even more efficient,” Gu says. “The early results are very promising, and we think this could be scaled up for large-scale manufacturing.”

The paper, “Gel–Liposome-Mediated Co-Delivery of Anticancer Membrane-Associated Proteins and Small-Molecule Drugs for Enhanced Therapeutic Efficacy,” is published online in Advanced Functional Materials. Co-authors of the paper are Adriano Bellotti, an undergraduate at NC State, and Dr. Jianping Zhou, a professor at China Pharmaceutical University.

-shipman-

Contact: Matt Shipman matt_shipman@ncsu.edu 919-515-6386 North Carolina State University For Immediate Release Matt Shipman | News Services | 919.515.6386 Dr. Zhen Gu | 919.515.7944 Release Date: 01.06.14

Operando transmission electron microscopy electrochemical liquid cell Li-ion battery; Si anode Si lithiation delithiation

Demonstration of an Electrochemical Liquid Cell for Operando Transmission Electron Microscopy Observation of the Lithiation/Delithiation Behavior of Si Nanowire Battery Anodes

In the search for long-lasting, inexpensive rechargeable batteries, researchers develop more realistic methods to study the materials in action.

RICHLAND, Wash. – Researchers have developed a way to microscopically view battery electrodes while they are bathed in wet electrolytes, mimicking realistic conditions inside actual batteries. While life sciences researchers regularly use transmission electron microscopy to study wet environments, this time scientists have applied it successfully to rechargeable battery research.

The results, reported in December 11's issue of Nano Letters, are good news for scientists studying battery materials under dry conditions. The work showed that many aspects can be studied under dry conditions, which are much easier to use. However, wet conditions are needed to study the hard-to-find solid electrolyte interphase layer, a coating that accumulates on the electrode's surface and dramatically influences battery performance.

Liquid battery electrolytes makes this view of an uncharged electrode (top) and a charged electrode (bottom) a bit fuzzy. Image courtesy of Gu et al, Nano Letters 2013

"The liquid cell gave us global information about how the electrodes behave in a battery environment," said materials scientist Chongmin Wang of the Department of Energy's Pacific Northwest National Laboratory. "And it will help us find the solid electrolyte layer. It has been hard to directly visualize in sufficient detail."

Ebb, Flow, Swell

Even though electricity seems invisible, storing and using it in batteries has some very physical effects. Charging a battery jams electrons into the negative electrode, where positively charged lithium ions (or another metal ion such as sodium) rush in to meet and hold onto the electrons. Those ions have to fit within pores within the electrode.

Powering a device with a battery causes the electrons to stream out of the electrode. The positive ions, left behind, surge through the body of the battery and return to the positive electrode, where they await another charging.

Wang and colleagues have used high-powered microscopes to watch how the ebbing and flowing of positively charged ions deform electrodes. Squeezing into the electrode's pores makes the electrodes swell, and repeated use can wear them down. For example, recent work funded through the Joint Center for Energy Storage Research — a DOE Energy Innovation Hub established to speed battery development — showed that sodium ions leave bubbles behind, potentially interfering with battery function.

But up to this point, the transmission electron microscopes have only been able to accommodate dry battery cells, which researchers refer to as open cells. In a real battery, electrodes are bathed in liquid electrolytes that provide an environment ions can easily move through.

So, working with JCESR colleagues, Wang led development of a wet battery cell in a transmission electron microscope at EMSL, the DOE's Environmental Molecular Sciences Laboratory on the PNNL campus. The team built a battery so small that several could fit on a dime. The battery had one silicon electrode and one lithium metal electrode, both contained in a bath of electrolyte.

Mystery Layer

When the team charged the battery, they saw the silicon electrode swell, as expected. However, under dry conditions, the electrode is attached at one end to the lithium source — and swelling starts at just one end as the ions push their way in, creating a leading edge. In this study's liquid cell, lithium could enter the silicon anywhere along the electrode's length. The team watched as the electrode swelled all along its length at the same time.

"The electrode got fatter and fatter uniformly. This is how it would happen inside a battery," said Wang.

The total amount the electrode swelled was about the same, though, whether the researchers set up a dry or wet battery cell. That suggests researchers can use either condition to study certain aspects of battery materials.

"We have been studying battery materials with the dry, open cell for the last five years," said Wang. "We are glad to discover that the open cell provides accurate information with respect to how electrodes behave chemically. It is much easier to do, so we will continue to use them."

As far as the elusive solid electrolyte interphase layer goes, Wang said they couldn't see it in this initial experiment. In future experiments, they will try to reduce the thickness of the wet layer by at least half to increase the resolution, which might provide enough detail to observe the solid electrolyte interphase layer.

"The layer is perceived to have peculiar properties and to influence the charging and discharging performance of the battery," said Wang. "However, researchers don't have a concise understanding or knowledge of how it forms, its structure, or its chemistry. Also, how it changes with repeated charging and discharging remains unclear. It's very mysterious stuff. We expect the liquid cell will help us to uncover this mystery layer."

This work was supported by the Department of Energy's Offices of Science and of Energy Efficiency and Renewable Energy.

Reference: Meng Gu, Lucas R. Parent, B. Layla Mehdi, Raymond R. Unocic, Matthew T. McDowell, Robert L. Sacci, Wu Xu, Justin Grant Connell, Pinghong Xu, Patricia Abellan, Xilin Chen, Yaohui Zhang, Daniel E. Perea, James E. Evans, Lincoln J. Lauhon, Ji-Guang Zhang, Jun Liu, Nigel D. Browning, Yi Cui, Ilke Arslan, and Chong-Min Wang. Demonstration of an Electrochemical Liquid Cell for Operando Transmission Electron Microscopy Observation of the Lithiation/Delithiation Behavior of Si Nanowire Battery Anodes, Nano Letters Dec. 11, 2013, 6106-6112, DOI:10.1021/nl403402q.

The Department of Energy's Office of Science is the single largest supporter of basic research in the physical sciences in the United States and is working to address some of the most pressing challenges of our time.

The Joint Center for Energy Storage Research (JCESR) is a major partnership that integrates researchers from many disciplines to overcome critical scientific and technical barriers and create new breakthrough energy storage technology. Led by the U.S. Department of Energy's Argonne National Laboratory, partners include national leaders in science and engineering from academia, the private sector, and national laboratories. Their combined expertise spans the full range of the technology-development pipeline from basic research to prototype development to product engineering to market delivery. Funding for JCESR is provided by the U.S. Department of Energy Office of Science.

EMSL, the Environmental Molecular Sciences Laboratory, is a national scientific user facility sponsored by the Department of Energy's Office of Science. Located at Pacific Northwest National Laboratory in Richland, Wash., EMSL offers an open, collaborative environment for scientific discovery to researchers around the world. Its integrated computational and experimental resources enable researchers to realize important scientific insights and create new technologies. Follow EMSL on Facebook, LinkedIn and Twitter.

Interdisciplinary teams at Pacific Northwest National Laboratory address many of America's most pressing issues in energy, the environment and national security through advances in basic and applied science. Founded in 1965, PNNL employs 4,300 staff and has an annual budget of about $950 million. It is managed by Battelle for the U.S. Department of Energy. For more information, visit the PNNL News Center, or follow PNNL on Facebook, Google+, LinkedIn and Twitter.