Abstract: In article number 1703909, Gang Chen, Guihua Yu, and co-workers present a novel concept of 2D nanofluidic lithium-ion transport channels based on stacked Co3O4nanosheets for high-performance lithium batteries. This unique nanoarchitecture exhibits exceptional capacity and outstanding long-term cycling stability for lithium-ion storage at high-rates in both half- and full-cells.
Despite being a promising electrode material, bulk cobalt oxide (Co3O4) exhibits poor lithium ion storage properties. Nanostructuring, e.g. making Co3O4 into ultrathin nanosheets, shows improved performance, however, Co3O4-based nanomaterials still lack long-term stability and high rate capability due to sluggish ion transport and structure degradation.
|Nanofluidic channels possess desired properties to address above issues. However, while these unique structures have been studied in hollow nanotubes and recently in restacked layered materials such as graphene, it remains challenging to construct nanofluidic channels in intrinsically non-layered materials.|
|Motived by the large number of non-layered materials, e.g. transition metal oxides, which hold great promise in battery applications, scientists aim to extend the concept of nanofluidic channels into these materials and improve their electrochemical properties.|
|Nanofluidic channels feature a unique unipolar ionic transport when properly designed and constructed. By controlling surface charge and channel spacing, enhanced and selective ion transport can be achieved in these channels by constructing them with dimensions comparable to the double Debye length and opposite surface charge with respect to the transporting ion.|
|In a new study published in Advanced Materials (“Engineering 2D Nanofluidic Li-Ion Transport Channels for Superior Electrochemical Energy Storage”), researchers have developed a Co3O4-based two-dimensional (2D) nano-architecture possessing nanofluidic channels with specially designed interlayer characteristics for fast lithium ion transport, leading to exceptional performance in lithium ion batteries ever reported for this material.|
|“Such constructed 2D nanofluidic channels in non-layered materials manifest a general structural engineering strategy for improving electrochemical properties in a vast number of different electrode materials,” Guihua Yu, a professor in Materials Science and Engineering, Mechanical Engineering, at the Texas Materials Institute, University of Texas at Austin. “The enhanced and selective ion transport demonstrated in our study is of broad interest to many applications where fast kinetics of ion transport is essential.”|
|Illustration of lithium ion transport in 2D nanofluidic channels. (Reprinted with permission by Wiley-VCH)|
|On the one hand, an intercalated molecule acts as interlayer pillar in the stacked oxide, constituting transport channels with proper spacing. On the other hand, negatively charged functional groups anchored on the nanosheets surface facilitate transport of positively charged lithium ions inside the channels.|
|“Satisfying aforementioned conditions for unipolar ionic transport, combined with other advantageous features – extra storage capacity contributed by the surface functional groups, buffered structural stress from the interlayer spacing, and shortened lithium ion diffusion distance due to the ultrathin nanosheet morphology – the resulting nanoarchitecture exhibit exceptional electrochemical performance as tested in lithium-ion batteries,” notes Yu.|
|In a next step, the researchers are going to extend the concept of 2D nanofluidic channels to other electrode materials with or without layering structures. With ability to further tune interlayer spacing, they expect some promising energy storage applications in beyond-lithium-ion batteries.|
|It might also be interesting to examine this structural engineering strategy in other applications, for example, catalysis.|
|Design and LIBs application of Co3O4 nanosheets with 2D nanofluidic channels. (a) The synthetic route from Co-based layered hydroxide precursor to Co3O4 nanosheets with 2D nanofluidic channels. (b) Cycling performance of a full cell (anode: Co3O4 nanosheets /cathode: commercial LiCoO2). (Reprinted with permission by Wiley-VCH)|
|Constructing 2D nanofluidic channels for energy storage application is still in its infancy and the success of using non-layered materials demonstrated in this study promises a bright future in this direction with a broader material coverage.|
|“We are also taking this research direction even further by looking into the transport and storage properties for energy storage systems based on larger charge-carrying ions, such as Na+ and Mg2+, ” concludes Yu. “In order to realize that, an important challenge is to tune the channel spacing in a controlled manner. It is also imperative to investigate structural stability and scalability of this specially designed nanoarchitecture for its utilization in practical applications.”|
|@Michael Berger © Nanowerk|
In an advance that helps pave the way for next-generation electronics and computing technologies—and possibly paper-thin gadgets —scientists with the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) developed a way to chemically assemble transistors and circuits that are only a few atoms thick.
What’s more, their method yields functional structures at a scale large enough to begin thinking about real-world applications and commercial scalability.
They report their research online July 11 in the journal Nature Nanotechnology.
The scientists controlled the synthesis of a transistor in which narrow channels were etched onto conducting graphene, and a semiconducting material called a transition-metal dichalcogenide, or TMDC, was seeded in the blank channels. Both of these materials are single-layered crystals and atomically thin, so the two-part assembly yielded electronic structures that are essentially two-dimensional. In addition, the synthesis is able to cover an area a few centimeters long and a few millimeters wide.
“This is a big step toward a scalable and repeatable way to build atomically thin electronics or pack more computing power in a smaller area,” says Xiang Zhang, a senior scientist in Berkeley Lab’s Materials Sciences Division who led the study.
Zhang also holds the Ernest S. Kuh Endowed Chair at the University of California (UC) Berkeley and is a member of the Kavli Energy NanoSciences Institute at Berkeley. Other scientists who contributed to the research include Mervin Zhao, Yu Ye, Yang Xia, Hanyu Zhu, Siqi Wang, and Yuan Wang from UC Berkeley as well as Yimo Han and David Muller from Cornell University.
Their work is part of a new wave of research aimed at keeping pace with Moore’s Law, which holds that the number of transistors in an integrated circuit doubles approximately every two years. In order to keep this pace, scientists predict that integrated electronics will soon require transistors that measure less than ten nanometers in length.
Transistors are electronic switches, so they need to be able to turn on and off, which is a characteristic of semiconductors. However, at the nanometer scale, silicon transistors likely won’t be a good option. That’s because silicon is a bulk material, and as electronics made from silicon become smaller and smaller, their performance as switches dramatically decreases, which is a major roadblock for future electronics.
Researchers have looked to two-dimensional crystals that are only one molecule thick as alternative materials to keep up with Moore’s Law. These crystals aren’t subject to the constraints of silicon.
In this vein, the Berkeley Lab scientists developed a way to seed a single-layered semiconductor, in this case the TMDC molybdenum disulfide (MoS2), into channels lithographically etched within a sheet of conducting graphene. The two atomic sheets meet to form nanometer-scale junctions that enable graphene to efficiently inject current into the MoS2. These junctions make atomically thin transistors.
“This approach allows for the chemical assembly of electronic circuits, using two-dimensional materials, which show improved performance compared to using traditional metals to inject current into TMDCs,” says Mervin Zhao, a lead author and Ph.D. student in Zhang’s group at Berkeley Lab and UC Berkeley.
Optical and electron microscopy images, and spectroscopic mapping, confirmed various aspects related to the successful formation and functionality of the two-dimensional transistors.
In addition, the scientists demonstrated the applicability of the structure by assembling it into the logic circuitry of an inverter. This further underscores the technology’s ability to lay the foundation for a chemically assembled atomic computer, the scientists say.
“Both of these two-dimensional crystals have been synthesized in the wafer scale in a way that is compatible with current semiconductor manufacturing. By integrating our technique with other growth systems, it’s possible that future computing can be done completely with atomically thin crystals,” says Zhao.
Explore further: Excitonic dark states shed light on TMDC atomic layers
More information: Large-scale chemical assembly of atomically thin transistors and circuits, Nature Nanotechnology, DOI: 10.1038/nnano.2016.115
University of Akron researchers have developed new materials that function on a nanoscale, which could lead to the creation of lighter laptops, slimmer televisions and crisper smartphone visual displays.
Known as “giant surfactants” – or surface films and liquid solutions – the researchers, led by Stephen Z.D. Cheng, dean of UA’s College of Polymer Science and Polymer Engineering, used a technique known as nanopatterning to combine functioning molecular nanoparticles with polymers to build these novel materials.
The giant surfactants developed at UA are large, similar to macromolecules, yet they function like molecular surfactants on a nanoscale, Cheng says. The outcome? Nanostructures that guide the size of electronic products.
More efficient designs possible Nanopatterning, or self-assembling molecular materials, is the genius behind the small, light and fast world of modern-day gadgetry, and now it has advanced one giant step thanks to the UA researchers who say these new materials, when integrated into electronics, will enable the development of ultra-lightweight, compact and efficient devices because of their unique structures.
During their self-assembly, molecules form an organized lithographic pattern on semiconductor crystals, for use as integrated circuits. Cheng explains that these self-assembling materials differ from common block copolymers (a portion of a macromolecule, comprising manyunits, that has at least one feature which is not present in the adjacent portions) because they organize themselves in a controllable manner at the molecular level.
“The IT industry wants microchips that are as small as possible so that they can manufacture smaller and faster devices,” says Cheng, who also serves as the R.C. Musson and Trustees Professor of Polymer Science at UA.
He points out that the current technique can produce the spacing of 22 nanometers only, and cannot go down to the 10 nanometers or less necessary to create tiny, yet mighty, devices. The giant surfactants, however, can dictate smaller-scale electronic components.
“This is exactly what we are pursuing – self-assembling materials that organize at smaller sizes, say, less than 20 or even 10 nanometers,” says Cheng, equating 20 nanometers to 1 /4,000th the diameter of a human hair.
Team work has commercial applications
An international team of experts, including George Newkome, UA vice president for research, dean of the Graduate School, and professor of polymer science at UA; Er-Qiang Chen of Peking University in China; Rong-Ming Ho of National Tsinghua University in Taiwan; An-Chang Shi of McMaster University in Canada; and several doctoral and postdoctoral researchers from Cheng’s group, have shown how well-ordered nanostructures in various states, such as in thin films and in solution, offer extensive applications in nanotechnology.
The team’s study is highlighted in a pending patent application through the University of Akron Research Foundation and in a recent journal article, “Giant surfactants provide a versatile platform for sub-10-nm nanostructure engineering” published in Proceedings of the National Academy of Sciences of the United States of America (110, 10078-10083, 2013).
“These results are not only of pure scientific interest to the narrow group of scientists, but also important to a broad range of industry people,” says Cheng, noting that his team is testing real-world applications in nanopatterning technologies and hope to see commercialization in the future.
Fluorescent tetrapod nanocrystals could light the way to the future design of stronger polymer nanocomposites. A team of researchers with the U.S. Dept. of Energy (DOE)’s Lawrence Berkeley National Laboratory (Berkeley Lab) has developed an advanced optomechanical sensing technique based on tetrapod quantum dots that allows precise measurement of the tensile strength of polymer fibers with minimal impact on the fiber’s mechanical properties.
In a study led by Paul Alivisatos, Berkeley Lab Dir. and the Larry and Diane Bock Prof. of Nanotechnology at the Univ. of California (UC) Berkeley, the research team incorporated into polymer fibers a population of tetrapod quantum dots (tQDs) consisting of a cadmium-selenide (CdSe) core and four cadmium-sulfide (CdS) arms. The tQDs were incorporated into the polymer fibers via electrospinning, among today’s leading techniques for processing polymers, in which a large electric field is applied to droplets of polymer solution to create micro- and nano-sized fibers. This is the first known application of electrospinning to tQDs.
Fluorescent tetrapod quantum dots or tQDs (brown) serve as stress probes that allow precise measurement of polymer fiber tensile strength with minimal impact on mechanical properties. Inserts show relaxed tQDs (upper) and stressed (lower).
Image: LBN Laboratory.
“The electrospinning process allowed us to put an enormous amount of tQDs, up to 20% by weight, into the fibers with minimal effects on the polymer’s bulk mechanical properties,” Alivisatos says. “The tQDs are capable of fluorescently monitoring not only simple uniaxial stress, but stress relaxation and behavior under cyclic varying loads. Furthermore, the tQDs are elastic and recoverable, and undergo no permanent change in sensing ability even upon many cycles of loading to failure.”
Alivisatos is the corresponding author of a paper describing this research in NANO Letters. Co-authors were Shilpa Raja, Andrew Olson, Kari Thorkelsson, Andrew Luong, Lillian Hsueh, Guoqing Chang, Bernd Gludovatz, Liwei Lin, Ting Xu and Robert Ritchie.
Polymer nanocomposites are polymers that contain fillers of nanoparticles dispersed throughout the polymer matrix. Exhibiting a wide range of enhanced mechanical properties, these materials have great potential for a broad range of biomedical and material applications. However, rational design has been hampered by a lack of detailed understanding of how they respond to stress at the micro- and nanoscale.
“Understanding the interface between the polymer and the nanofiller and how stresses are transferred across that barrier are critical in reproducibly synthesizing composites,” Alivisatos says. “All of the established techniques for providing this information have drawbacks, including altering the molecular-level composition and structure of the polymer and potentially weakening mechanical properties such as toughness. It has therefore been of considerable interest to develop optical luminescent stress-sensing nanoparticles and find a way to embed them inside polymer fibers with minimal impact on the mechanical properties that are being sensed.”
The Berkeley Lab researchers met this challenge by combining semiconductor tQDs of CdSe/CdS, which were developed in an earlier study by Alivisatos and his research group, with electrospinning. The CdSe/CdS tQDs are exceptionally well-suited as nanoscale stress sensors because an applied stress will bend the arms of the tetrapods, causing a shift in the color of their fluorescence. The large electric field used in electrospinning results in a uniform dispersal of tQD aggregates throughout the polymer matrix, thereby minimizing the formation of stress concentrations that would act to degrade the mechanical properties of the polymer. Electrospinning also provided a much stronger bond between the polymer fibers and the tQDs than a previous diffusion-based technique for using tQDs as stress probes that was reported two years ago by Alivisatos and his group. Much higher concentrations of tQDs could also be achieved with electrospinning rather than diffusion.
When stress was applied to the polymer nanocomposites, elastic and plastic regions of deformation were easily observed as a shift in the fluorescence of the tQDs even at low particle concentrations. As particle concentrations were increased, a greater fluorescence shift per unit strain was observed. The tQDs acted as non-perturbing probes that tests proved were not adversely affecting the mechanical properties of the polymer fibers in any significant way.
“We performed mechanical tests using a traditional tensile testing machine with all of our types of polymer fibers,” says Shilpa Raja, a lead author of the Nano Letters paper along with Andrew Olson, both members of Alivisatos’ research group. “While the tQDs undoubtedly change the composition of the fiber—it is no longer pure polylactic acid but instead a composite—we found that the mechanical properties of the composite and crystallinity of the polymer phase show minimal change.”
The research team believes their tQD probes should prove valuable for a variety of biological, imaging and materials engineering applications.
“A big advantage in the development of new polymer nanocomposites would be to use tQDs to monitor stress build-ups prior to material failure to see how the material was failing before it actually broke apart,” says co-lead author Olson. “The tQDs could also help in the development of new smart materials by providing insight into why a composite either never exhibited a desired nanoparticle property or stopped exhibiting it during deformation from normal usage.”
For biological applications, the tQD is responsive to forces on the nanoNewton scale, which is the amount of force exerted by living cells as they move around within the body. A prime example of this is metastasizing cancer cells that move through the surrounding extracellular matrix. Other cells that exert force include the fibroblasts that help repair wounds, and cardiomyocytes, the muscle cells in the heart that beat.
“All of these types of cells are known to exert nanoNewton forces, but it is very difficult to measure them,” Raja says.
“We’ve done preliminary studies in which we have shown that cardiomyocytes on top of a layer of tQDs can be induced to beat and the tQD layer will show fluorescent shifts in places where the cells are beating. This could be extended to a more biologically-relevant environment in order to study the effects of chemicals and drugs on the metastasis of cancer cells.”
Another exciting potential application is the use of tQDs to make smart polymer nanocomposites that can sense when they have cracks or are about to fracture and can strengthen themselves in response.
“With our technique we are combining two fields that are usually separate and have never been combined on the nanoscale, optical sensing and polymer nanocomposite mechanical tunability,” Raja says. “As the tetrapods are incredibly strong, orders of magnitude stronger than typical polymers, ultimately they can make for stronger interfaces that can self-report impending fracture.”