- Email: [email protected]
Mobile devices powered just by tapping your finger
NEWS
They found that the Fe-NCB displayed its highest activity at 900(C, and durability tests showed its performance to degrade by only 3-4%. Then, working with Nissan’s North America Technical Centre, the catalyst was added to a series of membrane
Materials Today Volume 18, Number 9 November 2015
electrode assemblies, and characterized under automotive test conditions. By optimizing the stack structure, they found that high current densities could be obtained with this catalyst, as well as improved durability, a reduction in resistive
losses and better water management. The team see this as a promising candidate for future polymer electrolyte membrane fuel cells, and are continuing to work with Nissan to develop it further. Laurie Winkless
A nano-solution to bone tissue engineering?
NEWS
A team of chemical engineers have developed highly-porous, bioactive titania scaffolds that encourage bone tissue to regenerate itself, speeding up the healing process. Repairing bones has long been a lengthy process – the damaged bone sections must first be realigned and then stabilized (typically using a cast), but then the patient must just wait for the healing process to happen. But today’s biomedical literature is full of materials that could speed this process up – bio-inspired scaffolds that can encourage bone tissue regeneration. One material that has attracted particular attention is the ceramic, titanium dioxide (TiO2). It is not only biocompatible, but porous structures made from it display excellent mechanical properties. However, questions have remained about its bioactivity, that is, its ability to bond to bone without the need for a secondary coating.
A team of Latvian chemical engineers recently published work in Materials Letters [ Loca, et al., Mater. Lett. (2015), doi:10. 1016/j.matlet.2015.07.017] to demonstrate that nano-TiO2 might be the answer. They took inspiration from the fact that all bioactive materials form a layer called ‘‘bonelike apatite’’ on their surfaces – it’s this that allows them to bond directly to native bone. Knowing that its formation is influenced by the surface (nanostructured surfaces stimulate more growth than microstructured) the team from Riga Technical University coated highly-porous TiO2 ceramic scaffolds with TiO2 nanopowder. The scaffolds were produced using polyurethane foams with fully interconnected pore structure as sacrificial templates. After removal of the template, some of the resulting TiO2 scaffolds were additionally coated with a TiO2 nanoparticles (average size 15 nm).
The bioactivity of these scaffolds (coated and non-coated) was evaluated by immersing them in simulated body fluid (SBF) for 21 days – a well-established in vitro test method. The results showed that the surface microstructure of the uncoated TiO2 scaffolds remained unchanged throughout the test. In contrast, after just seven days, agglomerates of white, spherical particles were observed on the coated scaffolds, and by day 21, the particles formed a uniform layer across the entire structure. Further analysis showed this layer to comprise of calcium and phosphorus, indicating that it was, in fact apatite. This work has confirmed that nanostructured TiO2 scaffolds display in vitro bioactivity. It is hoped that such structures could find use in bone tissue regeneration applications. Laurie Winkless
Mobile devices powered just by tapping your finger We produce small amounts of energy in everything we do, movements like walking and tapping on a keyboard release energy that is then mostly dissipated. However, new research by a team from India and Germany has shown a way to develop flexible and biodegradable devices that generate power from such common movement that could lead to a new generation of electronic devices that never need to be charged. Although there has been many studies into nanogenerators that are able to capture such energy and convert it into electricity to power mobile devices, this investigation – as reported in ACS Applied Materials & Interfaces [Tamang, et al., ACS Appl. Mater. Interfaces (2015), doi:10.1021/acsami. 5b04161] – looked to improve nanogenerators in terms of their recharging and biodegradability. The device they developed uses a flexible, biocompatible polymer film made from polyvinylidene fluoride (PVDF),
474
before DNA is added to improve the material’s ability to harvest energy from everyday motion and then turn it into electrical power. This breakthrough could resolve those perennial problems around portable electronics of their short battery life and need for power sources dependent on fossil fuels while offering biocompatibility, flexibility and low cost. The device, which was shown to light up 22–
55 green or blue light-emitting diodes powered only by gentle tapping, is capable of harvesting energy from mechanical stresses including human touch, walking, machine vibration and football juggling. The nanogenerator exhibited high piezoelectric energy conversion efficiency that facilitated the immediate switching on of the diodes. Using a flexible piezoelectric film meant the nanogenerator could avoid the usual stretching, poling and inclusions of inorganic nanoparticles to induce the electroactive phase, especially as stretching has a negative effect on the performance and lifetime of such devices, while electrical poling consumes power and reduces production yield. The DNA–PVDF piezoelectric composite polymer is eco-friendly and has great flexibility, and can be moulded into different configurations with less volume and weight. As lead researcher Dipankar Mandal points out, they ‘‘exploited the electrical
Materials Today Volume 18, Number 9 November 2015
properties of the DNA molecules to generate useful piezoelectric power that can be implemented to run portable devices.’’ The team now hope to modify the device to
function as a self-powering system in implantable biomedical devices, where the nanogenerator could generate electricity from the blood flow of the patient, and
NEWS
could also find uses in structural monitoring, and even in determining the quality of fruit and in tea grading. Laurie Donaldson
Scientists from Korea’s Pohang University of Science and Technology have managed to tune the band gap in black phosphorus into a unique state of matter as an improved conductor, a finding that could allow greater flexibility in the design and optimization of electronic and optoelectronic devices such as telecommunication lasers and solar panels. In the area of 2D materials, graphene has of course been receiving much attention due to its properties as an excellent conductor of heat and electricity. However, the muchtouted material has the major drawback of having no band gap, which is crucial to determining its electrical conductivity – the smaller the band gap, the more efficiently current can move across the material and the stronger the current. As graphene has a band gap of zero in its natural state, its semiconductor potential cannot be realized since the conductivity cannot be closed down. Attempts to open a band gap in graphene have proved difficult without reducing its quality, so the Korean team used black phosphorus, the stable form of white
phosphorus, as a 2D semiconductor before inducing the important property of graphene in other 2D semiconductors to get round this problem. As Keun Su Kim points out, ‘‘we tuned BP’s band gap to resemble the natural state of graphene, a unique state of matter that is different from conventional semiconductors’’. The study, published in Science [Kim, et al., Science (2015), doi:10.1126/science. aaa6486], demonstrated how the electronic state of black phosphorus could be tuned from a semiconductor to an efficient conductor depending on the strength of electric field applied. At a zero band gap, its
electronic state becomes a ‘Dirac semimetal state’, which is similar to the intrinsic state of graphene. Electrons were transferred from a potassium dopant to the surface of the black phosphorus, which confined the electrons and allowed the team to manipulate this state. Potassium produces the strong electrical field required to tune the size of the band gap. The doping process induced a large Stark effect that tuned the band gap so that the valence and conductive bands moved closer together, reducing the band gap. The vertical electric field therefore modulates the band gap and tunes the material from a moderate-gap semiconductor to a bandinverted semimetal. The potential of this unique electronic state of black phosphorus needs to be investigated further as it could find also applications in engineering where the band gap could be adjusted for devices dependent on knowledge of their exact behavior, as well as in the realization of high performance and very small transistors for the semiconductor industry. Laurie Donaldson
even as a carrier frequency for wireless telecommunications. Godfrey Gumbs and Andrii Iurov at Hunter College of the City University of
New York and the University of New Mexico, Albuquerque, working with Danhong Huang of the Air Force Research Laboratory at Kirtland Air Force Base, New Mexico, and Wei Pan of Sandia National Laboratory, also in Albuquerque, have investigated hybrid semiconductors that combine two-dimensional (2D) crystalline layers and a thick conducting material. They point out such structures could consist of a single or pair of sheets of graphene, silicene, or a 2D electron gas. The team explains that their approach exploits instabilities in surface plasmon resonance and essentially disproves a conclusion by others that when a layer lies above a thick conductor, one can simply replace one of the frequencies in a two-layer plasma dispersion equation by the surface plasmon frequency of the underlying substrate. This
Tunable terahertz generation A DC electric field could be used to generate tunable terahertz radiation, according to researchers in the USA. The discovery opens up what has remained an essentially unnavigable part of the electromagnetic spectrum that lies between the microwave and infrared regions. Coherent artificial terahertz sources might soon be possible and lead to new analytical spectroscopic and imaging techniques [G. Gumbs, et al. J. Appl. Phys. 118 (2015) 054303]. Terahertz radiation is known to be distinguish between living tissues of different water content or density and has already been used in airport security as the infamous ‘‘naked body’’ scanners. However, it has serious potential in identifying tumors and other lesions in the body as well as carrying out single-molecule imaging, detection of explosives or hidden weapons, or
475
NEWS
Tuning black phosphorus for improved conductivity
They found that the Fe-NCB displayed its highest activity at 900(C, and durability tests showed its performance to degrade by only 3-4%. Then, working with Nissan’s North America Technical Centre, the catalyst was added to a series of membrane
Materials Today Volume 18, Number 9 November 2015
electrode assemblies, and characterized under automotive test conditions. By optimizing the stack structure, they found that high current densities could be obtained with this catalyst, as well as improved durability, a reduction in resistive
losses and better water management. The team see this as a promising candidate for future polymer electrolyte membrane fuel cells, and are continuing to work with Nissan to develop it further. Laurie Winkless
A nano-solution to bone tissue engineering?
NEWS
A team of chemical engineers have developed highly-porous, bioactive titania scaffolds that encourage bone tissue to regenerate itself, speeding up the healing process. Repairing bones has long been a lengthy process – the damaged bone sections must first be realigned and then stabilized (typically using a cast), but then the patient must just wait for the healing process to happen. But today’s biomedical literature is full of materials that could speed this process up – bio-inspired scaffolds that can encourage bone tissue regeneration. One material that has attracted particular attention is the ceramic, titanium dioxide (TiO2). It is not only biocompatible, but porous structures made from it display excellent mechanical properties. However, questions have remained about its bioactivity, that is, its ability to bond to bone without the need for a secondary coating.
A team of Latvian chemical engineers recently published work in Materials Letters [ Loca, et al., Mater. Lett. (2015), doi:10. 1016/j.matlet.2015.07.017] to demonstrate that nano-TiO2 might be the answer. They took inspiration from the fact that all bioactive materials form a layer called ‘‘bonelike apatite’’ on their surfaces – it’s this that allows them to bond directly to native bone. Knowing that its formation is influenced by the surface (nanostructured surfaces stimulate more growth than microstructured) the team from Riga Technical University coated highly-porous TiO2 ceramic scaffolds with TiO2 nanopowder. The scaffolds were produced using polyurethane foams with fully interconnected pore structure as sacrificial templates. After removal of the template, some of the resulting TiO2 scaffolds were additionally coated with a TiO2 nanoparticles (average size 15 nm).
The bioactivity of these scaffolds (coated and non-coated) was evaluated by immersing them in simulated body fluid (SBF) for 21 days – a well-established in vitro test method. The results showed that the surface microstructure of the uncoated TiO2 scaffolds remained unchanged throughout the test. In contrast, after just seven days, agglomerates of white, spherical particles were observed on the coated scaffolds, and by day 21, the particles formed a uniform layer across the entire structure. Further analysis showed this layer to comprise of calcium and phosphorus, indicating that it was, in fact apatite. This work has confirmed that nanostructured TiO2 scaffolds display in vitro bioactivity. It is hoped that such structures could find use in bone tissue regeneration applications. Laurie Winkless
Mobile devices powered just by tapping your finger We produce small amounts of energy in everything we do, movements like walking and tapping on a keyboard release energy that is then mostly dissipated. However, new research by a team from India and Germany has shown a way to develop flexible and biodegradable devices that generate power from such common movement that could lead to a new generation of electronic devices that never need to be charged. Although there has been many studies into nanogenerators that are able to capture such energy and convert it into electricity to power mobile devices, this investigation – as reported in ACS Applied Materials & Interfaces [Tamang, et al., ACS Appl. Mater. Interfaces (2015), doi:10.1021/acsami. 5b04161] – looked to improve nanogenerators in terms of their recharging and biodegradability. The device they developed uses a flexible, biocompatible polymer film made from polyvinylidene fluoride (PVDF),
474
before DNA is added to improve the material’s ability to harvest energy from everyday motion and then turn it into electrical power. This breakthrough could resolve those perennial problems around portable electronics of their short battery life and need for power sources dependent on fossil fuels while offering biocompatibility, flexibility and low cost. The device, which was shown to light up 22–
55 green or blue light-emitting diodes powered only by gentle tapping, is capable of harvesting energy from mechanical stresses including human touch, walking, machine vibration and football juggling. The nanogenerator exhibited high piezoelectric energy conversion efficiency that facilitated the immediate switching on of the diodes. Using a flexible piezoelectric film meant the nanogenerator could avoid the usual stretching, poling and inclusions of inorganic nanoparticles to induce the electroactive phase, especially as stretching has a negative effect on the performance and lifetime of such devices, while electrical poling consumes power and reduces production yield. The DNA–PVDF piezoelectric composite polymer is eco-friendly and has great flexibility, and can be moulded into different configurations with less volume and weight. As lead researcher Dipankar Mandal points out, they ‘‘exploited the electrical
Materials Today Volume 18, Number 9 November 2015
properties of the DNA molecules to generate useful piezoelectric power that can be implemented to run portable devices.’’ The team now hope to modify the device to
function as a self-powering system in implantable biomedical devices, where the nanogenerator could generate electricity from the blood flow of the patient, and
NEWS
could also find uses in structural monitoring, and even in determining the quality of fruit and in tea grading. Laurie Donaldson
Scientists from Korea’s Pohang University of Science and Technology have managed to tune the band gap in black phosphorus into a unique state of matter as an improved conductor, a finding that could allow greater flexibility in the design and optimization of electronic and optoelectronic devices such as telecommunication lasers and solar panels. In the area of 2D materials, graphene has of course been receiving much attention due to its properties as an excellent conductor of heat and electricity. However, the muchtouted material has the major drawback of having no band gap, which is crucial to determining its electrical conductivity – the smaller the band gap, the more efficiently current can move across the material and the stronger the current. As graphene has a band gap of zero in its natural state, its semiconductor potential cannot be realized since the conductivity cannot be closed down. Attempts to open a band gap in graphene have proved difficult without reducing its quality, so the Korean team used black phosphorus, the stable form of white
phosphorus, as a 2D semiconductor before inducing the important property of graphene in other 2D semiconductors to get round this problem. As Keun Su Kim points out, ‘‘we tuned BP’s band gap to resemble the natural state of graphene, a unique state of matter that is different from conventional semiconductors’’. The study, published in Science [Kim, et al., Science (2015), doi:10.1126/science. aaa6486], demonstrated how the electronic state of black phosphorus could be tuned from a semiconductor to an efficient conductor depending on the strength of electric field applied. At a zero band gap, its
electronic state becomes a ‘Dirac semimetal state’, which is similar to the intrinsic state of graphene. Electrons were transferred from a potassium dopant to the surface of the black phosphorus, which confined the electrons and allowed the team to manipulate this state. Potassium produces the strong electrical field required to tune the size of the band gap. The doping process induced a large Stark effect that tuned the band gap so that the valence and conductive bands moved closer together, reducing the band gap. The vertical electric field therefore modulates the band gap and tunes the material from a moderate-gap semiconductor to a bandinverted semimetal. The potential of this unique electronic state of black phosphorus needs to be investigated further as it could find also applications in engineering where the band gap could be adjusted for devices dependent on knowledge of their exact behavior, as well as in the realization of high performance and very small transistors for the semiconductor industry. Laurie Donaldson
even as a carrier frequency for wireless telecommunications. Godfrey Gumbs and Andrii Iurov at Hunter College of the City University of
New York and the University of New Mexico, Albuquerque, working with Danhong Huang of the Air Force Research Laboratory at Kirtland Air Force Base, New Mexico, and Wei Pan of Sandia National Laboratory, also in Albuquerque, have investigated hybrid semiconductors that combine two-dimensional (2D) crystalline layers and a thick conducting material. They point out such structures could consist of a single or pair of sheets of graphene, silicene, or a 2D electron gas. The team explains that their approach exploits instabilities in surface plasmon resonance and essentially disproves a conclusion by others that when a layer lies above a thick conductor, one can simply replace one of the frequencies in a two-layer plasma dispersion equation by the surface plasmon frequency of the underlying substrate. This
Tunable terahertz generation A DC electric field could be used to generate tunable terahertz radiation, according to researchers in the USA. The discovery opens up what has remained an essentially unnavigable part of the electromagnetic spectrum that lies between the microwave and infrared regions. Coherent artificial terahertz sources might soon be possible and lead to new analytical spectroscopic and imaging techniques [G. Gumbs, et al. J. Appl. Phys. 118 (2015) 054303]. Terahertz radiation is known to be distinguish between living tissues of different water content or density and has already been used in airport security as the infamous ‘‘naked body’’ scanners. However, it has serious potential in identifying tumors and other lesions in the body as well as carrying out single-molecule imaging, detection of explosives or hidden weapons, or
475
NEWS
Tuning black phosphorus for improved conductivity