Scientists grow organic semiconductor crystals vertically for first time

UCLA-led breakthrough could literally reshape solar cells and electronic devices

 

Our smartphones, tablets, computers and biosensors all have improved because of the rapidly increasing efficiency of semiconductors.

Since the turn of the 21st century, organic, or carbon-based, semiconductors have emerged as a major area of interest for scientists because they are inexpensive, plentiful and lightweight, and they can conduct current in ways comparable to inorganic semiconductors, which are made from metal-oxides or silicon.

Now, materials scientists from the California NanoSystems Institute at UCLA have discovered a way to make organic semiconductors more powerful and more efficient.

Their breakthrough was in creating an improved structure for one type of organic semiconductor, a building block of a conductive polymer called tetraaniline. The scientists showed for the first time that tetraaniline crystals could be grown vertically.

The advance could eventually lead to vastly improved technology for capturing solar energy. In fact, it could literally reshape solar cells. Scientists could potentially create “light antennas” — thin, pole-like devices that could absorb light from all directions, which would be an improvement over today’s wide, flat panels that can only absorb light from one surface.

The study, led by Richard Kaner, distinguished professor of chemistry and biochemistry and materials science and engineering, was recently published online by the journal ACS Nano.

The UCLA team grew the tetraaniline crystals vertically from a substrate, so the crystals stood up like spikes instead of lying flat as they do when produced using current techniques. They produced the crystals in a solution using a substrate made of graphene, a nanomaterial consisting of graphite that is extremely thin — measuring the thickness of a single atom. Scientists had previously grown crystals vertically in inorganic semiconducting materials, including silicon, but doing it in organic materials has been more difficult.

Tetraaniline is a desirable material for semiconductors because of its particular electrical and chemical properties, which are determined by the orientation of very small crystals it contains. Devices such as solar cells and photosensors work better if the crystals grow vertically because vertical crystals can be packed more densely in the semiconductor, making it more powerful and more efficient at controlling electrical current.

“These crystals are analogous to organizing a table covered with scattered pencils into a pencil cup,” said Yue “Jessica” Wang, a former UCLA doctoral student who now is a postdoctoral scholar at Stanford University and was the study’s first author. “The vertical orientation can save a great deal of space, and that can mean smaller, more efficient personal electronics in the near future.”

Once Kaner and his colleagues found they could guide the tetraaniline solution to grow vertical crystals, they developed a one-step method for growing highly ordered, vertically aligned crystals for a variety of organic semiconductors using the same graphene substrate.

“The key was deciphering the interactions between organic semiconductors and graphene in various solvent environments,” Wang said. “Once we understood this complex mechanism, growing vertical organic crystals became simple.”

Kaner said the researchers also discovered another advantage of the graphene substrate.

“This technique enables us to pattern crystals wherever we want,” he said. “You could make electronic devices from these semiconductor crystals and grow them precisely in intricate patterns required for the device you want, such as thin-film transistors or light-emitting diodes.”

The paper’s other authors were UCLA graduate students James Torres, Shan Jiang and Michael Yeung; Adam Stieg, associate director of shared resources at CNSI and the scientific director of the Nano and Pico Characterization Lab; Yves Rubin, UCLA professor of chemistry and biochemistry; and Xiangfeng Duan, UCLA professor of chemistry and biochemistry. Co-author Santanu Chaudhuri is a principal research scientist at the Illinois Applied Research Institute at University of Illinois at Urbana–Champaign.

UCLA

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Antimony nanocrystals for batteries

TEM image (false coloured) of monodisperse antimony nanocrystals. 
(Photo: Maksym Kovalenko Group / ETH Zurich)

Researchers from ETH Zurich and Empa have succeeded for the first time to produce uniform antimony nanocrystals. Tested as components of laboratory batteries, these are able to store a large number of both lithium and sodium ions. These nanomaterials operate with high rate and may eventually be used as alternative anode materials in future high-energy-density batteries.




The hunt is on – for new materials to be used in the next generation of batteries that may one day replace current lithium ion batteries. Today, the latter are commonplace and provide a reliable power source for smartphones, laptops and many other portable electrical devices.

On the one hand, however, electric mobility and stationary electricity storage demand a greater number of more powerful batteries; and the high demand for lithium may eventually lead to a shortage of the raw material. This is why conceptually identical technology based on sodium-ions will receive increasing attention in coming years. Contrary to lithium batteries, researched for more than 20 years, much less is known about materials that can efficiently store sodium ions.

Antimony electrodes?

A team of researchers from ETH Zurich and Empa headed by Maksym Kovalenko may have come a step closer to identifying alternative battery materials: they have become the first to synthesise uniform antimony nanocrystals, the special properties of which make them prime candidates for an anode material for both lithium-ion and sodium-ion batteries. The results of the scientists’ study have just been published in Nano Letters.

For a long time, antimony has been regarded as a promising anode material for high-performance lithium-ion batteries as this metalloid exhibits a high charging capacity, by a factor of two higher than that of commonly used graphite. Initial studies revealed that antimony could be suitable for rechargeable lithium and sodium ion batteries because it is able to store both kinds of ions. Sodium is regarded as a possible low-cost alternative to lithium as it is much more naturally abundant and its reserves are more evenly distributed on Earth.

For antimony to achieve its high storage capability, however, it needs to be produced in a special form. The researchers managed to chemically synthesize uniform – so-called “monodisperse” – antimony nanocrystals that were between ten and twenty nanometres in size.

The full lithiation or sodiation of antimony leads to large volumetric changes. By using nanocrystals, these modulations of the volume can be reversible and fast, and do not lead to the immediate fracture of the material. An additional important advantage of nanocrystals (or nanoparticles) is that they can be intermixed with a conductive carbon filler in order to prevent the aggregation of the nanoparticles.
Ideal candidate for anode material

Electrochemical tests showed Kovalenko and his team that electrodes made of these antimony nanocrystals perform equally well in sodium and in lithium ion batteries. This makes antimony particularly promising for sodium batteries because the best lithium-storing anode materials (Graphite and Silicon) do not operate with sodium.

Highly monodisperse nanocrystals, with the size deviation of ten percent or less, allow identifying the optimal size-performance relationship. Nanocrystals of ten nanometers or smaller suffer from oxidation because of the excessive surface area. On the other hand, antimony crystals with a diameter of more than 100 nanometres aren’t sufficiently stable due to aforementioned massive volume expansion and contraction during the operation of a battery. The researchers achieved the best results with 20 nanometer large particles.
Performance not so size dependent

Another important outcome of this study, enabled by these ultra-uniform particles, is that the researchers identified a size-range of around 20 to 100 nanometres within which this material shows excellent, size-independent performance, both in terms of energy density and rate-capability.

These features even allow using polydisperse antimony particles to obtain the same performance as with very monodisperse particles, as long as their sizes remain within this size-range of 20 to 100 nanometres.

“This greatly simplifies the task of finding an economically viable synthesis method”, Kovalenko says. “Development of such cost-effective synthesis is the next step for us, together with our industrial partner.” Experiments of his group on monodisperse nanoparticles of other materials show much steeper size-performance relationships such as quick performance decay with increasing the particle size, placing antimony into a unique position among the materials which alloy with lithium and sodium.
More expensive alternative

Does this mean that an alternative to today’s lithium-ion batteries is within our grasp? Kovalenko shakes his head. Although the method is relatively straightforward, the production of a sufficient number of high-quality uniform antimony nanocrystals is still too expensive.

“All in all, batteries with sodium-ions and antimony nanocrystals as anodes will only constitute a highly promising alternative to today’s lithium-ion batteries if the costs of producing the batteries will be comparable,” says Kovalenko.


It will be another decade or so before a sodium-ion battery with antimony electrodes could hit the market, the ETH-Zurich professor estimates. The research on the topic is still only in its infancy. “However, other research groups will soon join the efforts,” the chemist is convinced.

Lithium-ion batteries

A current lithium-ion battery comprises two electrodes – a cathode and an anode. The anode is often made of graphite, the cathode of metal oxides such as cobalt oxide. The lithium ions lodge themselves in these materials during the charging or discharging processes. The two electrodes are separated by a e wall permeable only for lithium ions traveling between the two electrodes, but not for electrons. During the discharge of a battery, the lithium ions shift from the anode to the cathode. As the electrons do not fit through, they take a detour via an electronic device, which is powered by the resulting electron flux. Electrons and ions meet again in the cathode. When the battery is charging, the ions and electrons are enforced to flow in the opposite direction. For the battery to work effectively and for a long time, the ions need to be able to move in and out of the electrode materials easily. The shape and size of the electrode materials should not change much through the recurrent absorption and release of the ions.

Nanocrystals bond silicone to PTFE

Tetrapod shape aids bonding (CAU, Xin Jin)

The potential for silicone in medical applications keeps growing.

In the newest development, researchers in Germany have discovered a way to join silicone and polytetrafluoroethylene (PTFE) using nano-scaled crystal linkers as internal staples. A major side benefit is that it's a purely mechanical process, ensuring no change in the chemical structure of the polymers.

Potential applications include breathing masks, implants or sensors.

"If the nano staples make even extreme polymers like Teflon (PTFE) and silicone stick to each other, they can join all kinds of other plastic materials", says Professor Rainer Adelung, who runs the functional nano materials group at the Institute of Materials Science in Kiel that participated in the announcement of the discovery.

The key to the approach is the use of very tiny crystals made of zinc oxide that are shaped like tetrapods with four legs protruding from the point of origin. They interlock and form strong bonds, and have been used in larger forms in coastal protection.

Here's how it works:  zinc oxide crystals are distributed carefully on a heated layer of PTFE, kind of like sprinkling sugar on partially baked cookies. After silicone is poured on top, the polymer sandwich is heated to 100°C for less than sixty minutes.

"It's like stapling two non-sticky materials from the inside with the crystals: When they are heated up, the nano tetrapods in between the polymer layers pierce the materials, sink into them, and get anchored", says Xin Jin, the first author of the publication, who is currently working on her PhD thesis. Her supervisor, Yogendra Kumar Mishra, adds: "If you try to pull out a tetrapod on one arm from a polymer layer, the shape of the tetrapod will simply cause three arms to dig in deeper and to hold on even firmer."

The peel strength of the composite structure is 200 Newtons per meter, which is described as similar to peeling sticky tape off glass. "The stickiness we have achieved with the nano tetrapods is remarkable, because as far as we could verify, no one has ever made silicone and Teflon stick to each other at all," says co-author Lars Heepe, PhD student from the Zoological Institute of Kiel University. 

Adelung said fundamental research on the project will continue as practical applications are pursued. One of the business partners, nanoproofed GmbH, is currently developing a product for painting on top of silicone.

The work was conducted within the German Research Foundation (DFG)-funded Collaborative Research Center 677"Function by Switching".

Source: http://www.plasticstoday.com/articles/nanocrystals-bond-silicone-ptfe0829201201