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Zapping silicon onto paper key to good wearable electronics
Materials Today Volume 18, Number 7 September 2015
NEWS
News Nano sandwich improves rechargeable batteries Researchers from Kansas State University have shown how miniature ‘sandwiches’ comprised of nanosheets could help improve common rechargeable lithiumion batteries used in cellphones and other rechargeable electronics. The team was exploring the lithium cycling of molybdenum disulfide (MoS2) sheets, where one molybdenum atom is sandwiched between two sulfur atoms. In their study, published in Scientific Reports [David, et al., Sci. Rep. (2015), doi:10. 1038/srep09792], silicon carbonitridewrapped molybdenum disulfide sheets demonstrated improved stability as a battery electrode with little capacity fading, and able to store over double as much lithium (or charge) than bulk molybdenum disulfide shown in other studies. Sulfur is well known for forming intermediate polysulfides that dissolve in the organic electrolyte of the battery, leading to capacity fading. This study demonstrated that the capacity drop in the molybdenum disulfide sheets could also be due to loss of sulfur into the electrolyte. To reduce this dissolution, wrapping the sheets in a few
Representation of a molybdenum disulfide (MoS2) sheet covered with SiCN ceramic.
layers of the high-temperature ceramic silicon carbonitride, which is produced by heating liquid silicon-based polymers, offers much higher chemical resistance toward the liquid electrolyte. Once the reactions had taken place, the cells showed that the silicon carbonitride protected against mechanical and chemical degradation with liquid organic electrolyte. The team had previously demonstrated that exfoliated sheets of tungsten disulfide (which has a similar structure to molybdenum disulfide) can store more than twice the amount of charge (or lithium) capacity than their bulk crystals. However, such large lithium capacity is short-lived as it starts to react irreversibly with the organic
electrolyte identified as the main reason for capacity fading As team leader Gurpreet Singh said, ‘‘The silicon carbonitride-wrapped molybdenum disulfide sheets show stable cycling of lithium ions irrespective of whether the battery electrode is on copper foil-traditional method or as a self-supporting flexible paper as in bendable batteries.’’ The self-standing paper electrode is therefore useful for lightweight batteries, especially as the glass-like coating of the silicon carbonitride is known for its high chemical and thermal stability. It allows diffusion of lithium ions through it to reversibly react with molybdenum disulfide, but protects the reaction of molybdenum disulfide with the organic electrolyte during successive cycling. There are interesting potential applications for the work in a range of innovative consumer electronics, and the team now hope to test the stability of the electrode material to improve our understanding of how the molybdenum disulfide cells would act in electronic devices that are recharged hundreds of times. Laurie Donaldson
Zapping silicon onto paper key to good wearable electronics A route to low-power, fast, wearable electronics at a remarkably low cost has been designed by researchers in the Netherlands and Japan. They claim that using lasers to print silicon onto paper could enable the large-scale manufacture of effective flexible, wearable electronics containing wireless sensors. Stretchable and even edible electronics may also be possible.
The quest to develop wearable electronics has mostly focused on organic and metal-oxide based inks, due to their ability to be printed onto a wide range of flexible materials easily. However, the electronic performance of these inks is not as good as that of silicon electronics. ‘‘The printed organic semiconductor’s mobility for holes is too low and that for electrons is even
lower. The printed metal oxide semiconductor has got higher electron mobility but its hole mobility is almost zero,’’ explains Ryoichi Ishihara, who led the research team based at Delft University of Technology and the Japan Advanced Institute of Science and Technology in Ishikawa. High quality polycrystalline silicon provides high mobilities for both electrons and 1369-7021/http://dx.doi.org/10.1016/j.mattod.2015.06.018
358
holes, he says. This is essential for the configuration of highly reliability, fast and low-power circuits. The ability to print the electronics is the key to keeping costs down. Silicon integrated circuit chips made using the traditional route could be mounted onto low-cost substrates, but the robotics used to make these – while constantly reducing in price – will always impede the costs dropping as low as that for printing. Routes to printing silicon ink have been designed previously, but the high temperatures (3508C) required in the annealing step rule out their use on many flexible materials including paper, polyethylene terephthalate and polyethylene naphthalate. ‘‘We were able to reduce the formation temperature of polysilicon significantly (to 1508C) in the solution process of Si, so inexpensive substrates with a low thermal budget such as paper can now be used,’’ Ishihara says. This work is published in Applied Physics Letters [Trifunovic, et al., Appl. Phys. Lett. (2015), doi:10.1063/1.4916998].
NEWS
The team first skimmed the liquid polysilane directly on a paper surface using a doctor-blade in an oxygen- and water-free environment. Next, they polymerised the film using UV light and then annealed the layer by exposing it to an XeCl excimer laser. The paper is not damaged because the duration of the laser pulse is so short: in the range of tens of nanoseconds. ‘‘The process is unexpectedly easy!’’ explains Ishihara. ‘‘Several measurements
confirmed that the layer consists of silicon crystals and nothing else and a thin-film transistor made using the layer exhibited mobilities as high as those of the conventional poly-Si.’’ Ishihara expects the process to scale-up easily for large-scale manufacturing. Doctor-blade coating is already used in the rollto-roll printing process used for printing newspapers and the lasers are used for annealing materials in the manufacture of smartphone displays, he explains. More optimization is however required before electronics made this way are ready to hit the shelves. Once this is done, ‘‘the most immediate application will be wearable electronics having ultra-high frequency radiofrequency identification (RF-ID) tag with sensors,’’ says Ishihara. These could be used for medical, fitness, identification or security purposes. ‘‘The process can also be expanded to solar cells, and will also realize stretchable and even edible electronics.’’ Nina Notman
act as artificial muscle because these cells can both contract or expand when they are bent, a phenomenon not observed in polymer gels, nanotubes and other materials tested for devices based on artificial muscle. Before they could construct an artificial muscle from onion epidermal cells, the team had to treat the biostructure with dilute sulfuric acid to remove the hemicellulose, a protein that gives the cells some rigidity. Then were then able to coat these newly flexible cells with a layer of gold to give them an electrode layer. A current applied to the gold electrode caused the onion cells to bend or stretch like a muscle cell, the team reports. ‘‘We intentionally made the top and bottom electrodes a different thickness so
that the cell stiffness becomes asymmetric from top to bottom,’’ explains Shih. This anisotropy gives the team control over how the artificial muscle structure responds electrically. A lower voltage (less than 50 V) makes them expand and flex downwards just 30 mm, toward the thicker bottom electrode layer. Conversely, a high voltage (50–1000 V) causes the cells to contract and so flex upwards, up to 1 mm, toward the thinner top layer. ‘‘We found that the single-layer lattice structure can generate unique actuation modes that engineered artificial muscle has never achieved before,’’ adds Shih. By combining two onion muscles the team was able to fashion a pair of ‘‘tweezers’’ which they could then use to grab hold of a tiny cotton ball.
A polysilicon layer was coated onto paper with the aid of a pulsed laser light (credit: R. Ishihara, M. Trifunovic/TUDelft).
Artificial muscles know their onions Researchers at the National Taiwan University have turned to the epidermal cells of the onion, Allium cepa, to help them make an artificial muscle. The new muscle responds to an applied voltage like a pair of tweezers pincing or opening depending on the direction of the voltage and so expanding or contracting in different directions, a first for artificial muscles. The team reports details in the journal Applied Physics Letters [Chien-Chun Chen, et al. Appl. Phys. Lett. 106 (2015) 183702] ‘‘The initial goal was to develop an engineered microstructure in artificial muscles for increasing the actuation deformation [the amount the muscle can bend or stretch when triggered],’’ explains project leader Wen-Pin Shih. ‘‘One day, we found that the onion’s cell structure and its dimensions were similar to what we had been making.’’ The onion epidermis is the soft and tissue-like layer beneath the onion’s surface. This fragile and translucent layer is composed of block-shaped cells that form a tightly-packed lattice. Shih and his colleagues recognized that these epidermal cells might be useful for accomplishing a difficult task in making devices that could
359
NEWS
Materials Today Volume 18, Number 7 September 2015
NEWS
News Nano sandwich improves rechargeable batteries Researchers from Kansas State University have shown how miniature ‘sandwiches’ comprised of nanosheets could help improve common rechargeable lithiumion batteries used in cellphones and other rechargeable electronics. The team was exploring the lithium cycling of molybdenum disulfide (MoS2) sheets, where one molybdenum atom is sandwiched between two sulfur atoms. In their study, published in Scientific Reports [David, et al., Sci. Rep. (2015), doi:10. 1038/srep09792], silicon carbonitridewrapped molybdenum disulfide sheets demonstrated improved stability as a battery electrode with little capacity fading, and able to store over double as much lithium (or charge) than bulk molybdenum disulfide shown in other studies. Sulfur is well known for forming intermediate polysulfides that dissolve in the organic electrolyte of the battery, leading to capacity fading. This study demonstrated that the capacity drop in the molybdenum disulfide sheets could also be due to loss of sulfur into the electrolyte. To reduce this dissolution, wrapping the sheets in a few
Representation of a molybdenum disulfide (MoS2) sheet covered with SiCN ceramic.
layers of the high-temperature ceramic silicon carbonitride, which is produced by heating liquid silicon-based polymers, offers much higher chemical resistance toward the liquid electrolyte. Once the reactions had taken place, the cells showed that the silicon carbonitride protected against mechanical and chemical degradation with liquid organic electrolyte. The team had previously demonstrated that exfoliated sheets of tungsten disulfide (which has a similar structure to molybdenum disulfide) can store more than twice the amount of charge (or lithium) capacity than their bulk crystals. However, such large lithium capacity is short-lived as it starts to react irreversibly with the organic
electrolyte identified as the main reason for capacity fading As team leader Gurpreet Singh said, ‘‘The silicon carbonitride-wrapped molybdenum disulfide sheets show stable cycling of lithium ions irrespective of whether the battery electrode is on copper foil-traditional method or as a self-supporting flexible paper as in bendable batteries.’’ The self-standing paper electrode is therefore useful for lightweight batteries, especially as the glass-like coating of the silicon carbonitride is known for its high chemical and thermal stability. It allows diffusion of lithium ions through it to reversibly react with molybdenum disulfide, but protects the reaction of molybdenum disulfide with the organic electrolyte during successive cycling. There are interesting potential applications for the work in a range of innovative consumer electronics, and the team now hope to test the stability of the electrode material to improve our understanding of how the molybdenum disulfide cells would act in electronic devices that are recharged hundreds of times. Laurie Donaldson
Zapping silicon onto paper key to good wearable electronics A route to low-power, fast, wearable electronics at a remarkably low cost has been designed by researchers in the Netherlands and Japan. They claim that using lasers to print silicon onto paper could enable the large-scale manufacture of effective flexible, wearable electronics containing wireless sensors. Stretchable and even edible electronics may also be possible.
The quest to develop wearable electronics has mostly focused on organic and metal-oxide based inks, due to their ability to be printed onto a wide range of flexible materials easily. However, the electronic performance of these inks is not as good as that of silicon electronics. ‘‘The printed organic semiconductor’s mobility for holes is too low and that for electrons is even
lower. The printed metal oxide semiconductor has got higher electron mobility but its hole mobility is almost zero,’’ explains Ryoichi Ishihara, who led the research team based at Delft University of Technology and the Japan Advanced Institute of Science and Technology in Ishikawa. High quality polycrystalline silicon provides high mobilities for both electrons and 1369-7021/http://dx.doi.org/10.1016/j.mattod.2015.06.018
358
holes, he says. This is essential for the configuration of highly reliability, fast and low-power circuits. The ability to print the electronics is the key to keeping costs down. Silicon integrated circuit chips made using the traditional route could be mounted onto low-cost substrates, but the robotics used to make these – while constantly reducing in price – will always impede the costs dropping as low as that for printing. Routes to printing silicon ink have been designed previously, but the high temperatures (3508C) required in the annealing step rule out their use on many flexible materials including paper, polyethylene terephthalate and polyethylene naphthalate. ‘‘We were able to reduce the formation temperature of polysilicon significantly (to 1508C) in the solution process of Si, so inexpensive substrates with a low thermal budget such as paper can now be used,’’ Ishihara says. This work is published in Applied Physics Letters [Trifunovic, et al., Appl. Phys. Lett. (2015), doi:10.1063/1.4916998].
NEWS
The team first skimmed the liquid polysilane directly on a paper surface using a doctor-blade in an oxygen- and water-free environment. Next, they polymerised the film using UV light and then annealed the layer by exposing it to an XeCl excimer laser. The paper is not damaged because the duration of the laser pulse is so short: in the range of tens of nanoseconds. ‘‘The process is unexpectedly easy!’’ explains Ishihara. ‘‘Several measurements
confirmed that the layer consists of silicon crystals and nothing else and a thin-film transistor made using the layer exhibited mobilities as high as those of the conventional poly-Si.’’ Ishihara expects the process to scale-up easily for large-scale manufacturing. Doctor-blade coating is already used in the rollto-roll printing process used for printing newspapers and the lasers are used for annealing materials in the manufacture of smartphone displays, he explains. More optimization is however required before electronics made this way are ready to hit the shelves. Once this is done, ‘‘the most immediate application will be wearable electronics having ultra-high frequency radiofrequency identification (RF-ID) tag with sensors,’’ says Ishihara. These could be used for medical, fitness, identification or security purposes. ‘‘The process can also be expanded to solar cells, and will also realize stretchable and even edible electronics.’’ Nina Notman
act as artificial muscle because these cells can both contract or expand when they are bent, a phenomenon not observed in polymer gels, nanotubes and other materials tested for devices based on artificial muscle. Before they could construct an artificial muscle from onion epidermal cells, the team had to treat the biostructure with dilute sulfuric acid to remove the hemicellulose, a protein that gives the cells some rigidity. Then were then able to coat these newly flexible cells with a layer of gold to give them an electrode layer. A current applied to the gold electrode caused the onion cells to bend or stretch like a muscle cell, the team reports. ‘‘We intentionally made the top and bottom electrodes a different thickness so
that the cell stiffness becomes asymmetric from top to bottom,’’ explains Shih. This anisotropy gives the team control over how the artificial muscle structure responds electrically. A lower voltage (less than 50 V) makes them expand and flex downwards just 30 mm, toward the thicker bottom electrode layer. Conversely, a high voltage (50–1000 V) causes the cells to contract and so flex upwards, up to 1 mm, toward the thinner top layer. ‘‘We found that the single-layer lattice structure can generate unique actuation modes that engineered artificial muscle has never achieved before,’’ adds Shih. By combining two onion muscles the team was able to fashion a pair of ‘‘tweezers’’ which they could then use to grab hold of a tiny cotton ball.
A polysilicon layer was coated onto paper with the aid of a pulsed laser light (credit: R. Ishihara, M. Trifunovic/TUDelft).
Artificial muscles know their onions Researchers at the National Taiwan University have turned to the epidermal cells of the onion, Allium cepa, to help them make an artificial muscle. The new muscle responds to an applied voltage like a pair of tweezers pincing or opening depending on the direction of the voltage and so expanding or contracting in different directions, a first for artificial muscles. The team reports details in the journal Applied Physics Letters [Chien-Chun Chen, et al. Appl. Phys. Lett. 106 (2015) 183702] ‘‘The initial goal was to develop an engineered microstructure in artificial muscles for increasing the actuation deformation [the amount the muscle can bend or stretch when triggered],’’ explains project leader Wen-Pin Shih. ‘‘One day, we found that the onion’s cell structure and its dimensions were similar to what we had been making.’’ The onion epidermis is the soft and tissue-like layer beneath the onion’s surface. This fragile and translucent layer is composed of block-shaped cells that form a tightly-packed lattice. Shih and his colleagues recognized that these epidermal cells might be useful for accomplishing a difficult task in making devices that could
359
NEWS
Materials Today Volume 18, Number 7 September 2015