- Email: [email protected]
Metal sandwich solution
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is not the case and the team shows that when a direct current is applied to their hybrid semiconductor, a plasmon instability is created at a particular wavenumber, which induces terahertz emission, which can then be exploited using a surface grating to split it. Changing the density of conduction electrons in the material or the DC voltage allows the team to tune the cutoff wavenumber and so the frequency of the emission. ‘‘Our work demonstrates a new approach for efficient energy conversation from a dc
Materials Today Volume 18, Number 9 November 2015
electric field to coherent, high-power and electrically tunable terahertz emission by using hybrid semiconductors,’’ explains Iurov. ‘‘Additionally, our proposed approach based on hybrid semiconductors can be generalized to include other novel two-dimensional materials, such as hexagonal boron nitride, molybdenum disulfide and tungsten diselenide.’’ The team will next investigate how the critical wave vector and the required drift current might be optimized for different
system designs perhaps by increasing the number of two-dimensional layers used. The researchers will also investigate the effect of temperature on plasmon instability in order to see how the device will perform in a realistic temperature regime. ‘‘We will work with experimentalists to actually produce this source of radiation under laboratory conditions,’’ Iurov told Materials Today. David Bradley
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Bend me, shape me Flexibility has always been one of the promises of hybrid electronics approaches that add an organic structural element to the conventional inorganic semiconductors. In recent years, circuitry that is not only bendable but often stretchable is beginning to emerge from research laboratories to open up entirely new form factors for the military, for consumer gadgets and for medical applications. Benjamin Leever of the Air Force Research Laboratory (AFRL) at Wright-Patterson Air Force Base, near Dayton, Ohio, USA, and colleagues are at the forefront of such efforts, which they refer to as flexible hybrid electronics. Leever recently offered insights into the developments taking place in the AFRL to delegates at the 250th National Meeting & Exposition of the American Chemical Society (ACS), which took place in Boston, Massachusetts, during August, 2015. ‘‘Basically, we are using a hybrid technology that mixes traditional electronics with flexible, high-performance electronics and new 3D printing technologies,’’ explains Leever. ‘‘In some cases, we incorporate ‘inks,’ which are based on metals, polymers and organic materials, to tie the system together electronically. With our technology, we can take a razor-thin silicon integrated circuit, a few hundred nanometers thick, and place it on a flexible, bendable or even foldable, plastic-like substrate material,’’ he adds.
The Wright-Patterson team turned to liquid gallium alloys as an interconnect material for their hybrid circuitry. ‘‘While these liquid alloys typically oxidize within minutes and become essentially useless,’’ Leever says, ‘‘the team has been able to dramatically reduce the effects of the oxidation through the use of ionic species confined to the walls of microvascular channels within the flexible substrates.’’ The resulting materials are thin and foldable so could be used to pack circuitry into tight spaces or squeeze it through gaps into voids wherein it unfurls, such as within a complex curved volume like an aircraft wing, for instance. Such circuits might be used to measure stresses and strains on wings. Similarly, flexible devices could be incorporated into a flightsuit to monitor aircrew health without risk of body movements cracking a
circuit board. Similar technology could be incorporated into civil engineering structures such as bridges and skyscrapers and in the tracksuits and footwear of athletes and for in-home, wearable tele-healthcare devices for the sick and infirm. ‘‘Overall, the military has the advantage of being able to move ahead with potentially higher risk research,’’ Leever points out. ‘‘Commercial investors want a clear demonstration before making an investment. The military can pursue possibly transformational applications at earlier stages if we see a promising approach to realize and advance a technology’s revolutionary potential. When we are successful, the commercial sector directly benefits.’’ ‘‘In the near-term we are investigating approaches to transition from single-layer circuits to multi-layered circuits with printed vias. electrical interconnects between the layers of a multi-layered circuit board, which will maximize the functionality we can add to a small space,’’ Leever told Materials Today. ‘‘The key challenge here is adapting printing processes to carefully print different materials in quick succession. For the community overall, the most important next step is developing and maturing the manufacturing technology that will enable us to advance from demonstrations in the lab to actual products for military, medical, and consumer use.’’ David Bradley
efficient energy storage devices, supercapacitors and wear-resistant and tough armored materials, according to a team at Drexel University in Philadelphia, Pennsyl-
vania, USA. Babak Anasori and his collea¨ ping University, gues at Drexel and Linko Sweden, have demonstrated how to sandwich together two-dimensional sheets of
Metal sandwich solution A technique for fusing different elements in layers to make a uniform and stable composite with predictable properties could open up routes to faster, smaller and more
476
molybdenum, titanium and carbon that would otherwise not stick together. ‘‘By sandwiching one or two atomic layers of a transition metal like titanium, between monoatomic layers of another metal, such as molybdenum, with carbon atoms holding them together, we discovered that a stable material can be produced,’’ Anasori explains. ‘‘It was impossible to produce a 2D material having just three or four molybdenum layers in such structures, but because we added the extra layer of titanium as a connector, we were able to synthesize them.’’ The team reports details in the journal ACS Nano and explains how each new combination of atom-thick layers offers new opportunities and properties [Anasori, et al., ACS Nano (2015), doi:10.1021/ acsnano.5b03591]. The team begins with what they refer to as an ordered MAX phase material, M3AX2, for example Mo2TiAlC2, which can be separated into thin 2D sheets of metal and metal carbide by acid etching away the aluminum, to generate MXenes, named by analogy with graphene, etc. Ordered MXenes have the formula M0 2M00 C2 or M0 2M00 3C3, where M0 and M00 are two different early transition metals and M0 layers sandwich M00 carbide layers. The team has now worked their way through the early transition metals making new MXenes of
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different compositions and testing their properties. ‘‘We had reached a bit of an impasse, when trying to produce a molybdenum containing MXenes,’’ Anasori explains. ‘‘By adding titanium to the mix we managed to make an
ordered molybdenum MAX phase, where the titanium atoms are in center and the molybdenum on the outside.’’ Theoretical calculations (density functional theory, DFT) carried out by colleagues at Oak Ridge National Laboratory, suggest to the Drexel team that they could, in principle, make as many as 25 new materials with different combinations of transition metals, that they had assumed were not worth trying before. The new layering method gives researchers a large number of possibilities for tuning the properties for a wide range of applications. Now, Anasori plans to make more materials by replacing titanium with vanadium, niobium, and tantalum and other metals, which could unearth a vein of new physical properties that support energy storage and other applications in thermoelectrics, batteries, catalysis, photovoltaics, electronic devices, structural composites and many other fields. ‘‘We want to explore the limits of this approach to making new MAX phases and their MXenes experimentally,’’ team leader Michel Barsoum told Materials Today. ‘‘Just because DFT calculations predict something, does not always mean you will get it in the lab. Second, we need to now carefully characterize the 2D materials we made and find out in what applications they would be most suitable,’’ he told us. David Bradley
fuse sufficient distance before they break down and they release their energy as an electric current. Furis and colleagues have carried out scanning laser microscopy with linearly polarized light and exploited photoluminescence to optically probe the molecular
structure of phthalocyanine crystals in a thin film. The new imaging technique reveals the structural defects and grain boundaries in the crystal that act as bumps and potholes to exciton traffic. Recently, the US Department of Energy identified ‘‘determining the mechanisms by which the absorbed energy (exciton) migrates through the system prior to splitting into charges that are converted to electricity’’ as an important part of the research needed to boost photovoltaic research and development. ‘‘One of today’s big challenges is how to make better photovoltaics and solar technologies,’’ echoes Furis, who directs UVM’s program in materials science, ‘‘and to do that we need a deeper understanding of exciton diffusion.’’ Such insights should allow the team to remove the roadblocks to open up an electron superhighway in the thin film by very
Exciton times for photovoltaics Organic semiconductors have promised flexibility for electronic devices for many years, but there remain obstacles in their development into inexpensive TV screens that roll up, roofing tiles that double as solar panels and solar-powered fabrics. Now, Madalina Furis of the University of Vermont and her colleagues have turned to a colorful old friend, the blue dye molecule, which they say will lift the barriers on an ‘‘electron superhighway’’ [M. Furis, et al. Nat. Commun. (2015) 6]. Excitons are neutral species and are not pushed along by a voltage, nevertheless they bounce from one tightly stacked phthalocyanine molecule to the next carrying their payload of solar-driven energy with them until it is released as an electric current. In order to work efficiently and effectively the excitons in organic thin film photovoltaic materials must be able to dif-
477
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Materials Today Volume 18, Number 9 November 2015
is not the case and the team shows that when a direct current is applied to their hybrid semiconductor, a plasmon instability is created at a particular wavenumber, which induces terahertz emission, which can then be exploited using a surface grating to split it. Changing the density of conduction electrons in the material or the DC voltage allows the team to tune the cutoff wavenumber and so the frequency of the emission. ‘‘Our work demonstrates a new approach for efficient energy conversation from a dc
Materials Today Volume 18, Number 9 November 2015
electric field to coherent, high-power and electrically tunable terahertz emission by using hybrid semiconductors,’’ explains Iurov. ‘‘Additionally, our proposed approach based on hybrid semiconductors can be generalized to include other novel two-dimensional materials, such as hexagonal boron nitride, molybdenum disulfide and tungsten diselenide.’’ The team will next investigate how the critical wave vector and the required drift current might be optimized for different
system designs perhaps by increasing the number of two-dimensional layers used. The researchers will also investigate the effect of temperature on plasmon instability in order to see how the device will perform in a realistic temperature regime. ‘‘We will work with experimentalists to actually produce this source of radiation under laboratory conditions,’’ Iurov told Materials Today. David Bradley
NEWS
Bend me, shape me Flexibility has always been one of the promises of hybrid electronics approaches that add an organic structural element to the conventional inorganic semiconductors. In recent years, circuitry that is not only bendable but often stretchable is beginning to emerge from research laboratories to open up entirely new form factors for the military, for consumer gadgets and for medical applications. Benjamin Leever of the Air Force Research Laboratory (AFRL) at Wright-Patterson Air Force Base, near Dayton, Ohio, USA, and colleagues are at the forefront of such efforts, which they refer to as flexible hybrid electronics. Leever recently offered insights into the developments taking place in the AFRL to delegates at the 250th National Meeting & Exposition of the American Chemical Society (ACS), which took place in Boston, Massachusetts, during August, 2015. ‘‘Basically, we are using a hybrid technology that mixes traditional electronics with flexible, high-performance electronics and new 3D printing technologies,’’ explains Leever. ‘‘In some cases, we incorporate ‘inks,’ which are based on metals, polymers and organic materials, to tie the system together electronically. With our technology, we can take a razor-thin silicon integrated circuit, a few hundred nanometers thick, and place it on a flexible, bendable or even foldable, plastic-like substrate material,’’ he adds.
The Wright-Patterson team turned to liquid gallium alloys as an interconnect material for their hybrid circuitry. ‘‘While these liquid alloys typically oxidize within minutes and become essentially useless,’’ Leever says, ‘‘the team has been able to dramatically reduce the effects of the oxidation through the use of ionic species confined to the walls of microvascular channels within the flexible substrates.’’ The resulting materials are thin and foldable so could be used to pack circuitry into tight spaces or squeeze it through gaps into voids wherein it unfurls, such as within a complex curved volume like an aircraft wing, for instance. Such circuits might be used to measure stresses and strains on wings. Similarly, flexible devices could be incorporated into a flightsuit to monitor aircrew health without risk of body movements cracking a
circuit board. Similar technology could be incorporated into civil engineering structures such as bridges and skyscrapers and in the tracksuits and footwear of athletes and for in-home, wearable tele-healthcare devices for the sick and infirm. ‘‘Overall, the military has the advantage of being able to move ahead with potentially higher risk research,’’ Leever points out. ‘‘Commercial investors want a clear demonstration before making an investment. The military can pursue possibly transformational applications at earlier stages if we see a promising approach to realize and advance a technology’s revolutionary potential. When we are successful, the commercial sector directly benefits.’’ ‘‘In the near-term we are investigating approaches to transition from single-layer circuits to multi-layered circuits with printed vias. electrical interconnects between the layers of a multi-layered circuit board, which will maximize the functionality we can add to a small space,’’ Leever told Materials Today. ‘‘The key challenge here is adapting printing processes to carefully print different materials in quick succession. For the community overall, the most important next step is developing and maturing the manufacturing technology that will enable us to advance from demonstrations in the lab to actual products for military, medical, and consumer use.’’ David Bradley
efficient energy storage devices, supercapacitors and wear-resistant and tough armored materials, according to a team at Drexel University in Philadelphia, Pennsyl-
vania, USA. Babak Anasori and his collea¨ ping University, gues at Drexel and Linko Sweden, have demonstrated how to sandwich together two-dimensional sheets of
Metal sandwich solution A technique for fusing different elements in layers to make a uniform and stable composite with predictable properties could open up routes to faster, smaller and more
476
molybdenum, titanium and carbon that would otherwise not stick together. ‘‘By sandwiching one or two atomic layers of a transition metal like titanium, between monoatomic layers of another metal, such as molybdenum, with carbon atoms holding them together, we discovered that a stable material can be produced,’’ Anasori explains. ‘‘It was impossible to produce a 2D material having just three or four molybdenum layers in such structures, but because we added the extra layer of titanium as a connector, we were able to synthesize them.’’ The team reports details in the journal ACS Nano and explains how each new combination of atom-thick layers offers new opportunities and properties [Anasori, et al., ACS Nano (2015), doi:10.1021/ acsnano.5b03591]. The team begins with what they refer to as an ordered MAX phase material, M3AX2, for example Mo2TiAlC2, which can be separated into thin 2D sheets of metal and metal carbide by acid etching away the aluminum, to generate MXenes, named by analogy with graphene, etc. Ordered MXenes have the formula M0 2M00 C2 or M0 2M00 3C3, where M0 and M00 are two different early transition metals and M0 layers sandwich M00 carbide layers. The team has now worked their way through the early transition metals making new MXenes of
NEWS
different compositions and testing their properties. ‘‘We had reached a bit of an impasse, when trying to produce a molybdenum containing MXenes,’’ Anasori explains. ‘‘By adding titanium to the mix we managed to make an
ordered molybdenum MAX phase, where the titanium atoms are in center and the molybdenum on the outside.’’ Theoretical calculations (density functional theory, DFT) carried out by colleagues at Oak Ridge National Laboratory, suggest to the Drexel team that they could, in principle, make as many as 25 new materials with different combinations of transition metals, that they had assumed were not worth trying before. The new layering method gives researchers a large number of possibilities for tuning the properties for a wide range of applications. Now, Anasori plans to make more materials by replacing titanium with vanadium, niobium, and tantalum and other metals, which could unearth a vein of new physical properties that support energy storage and other applications in thermoelectrics, batteries, catalysis, photovoltaics, electronic devices, structural composites and many other fields. ‘‘We want to explore the limits of this approach to making new MAX phases and their MXenes experimentally,’’ team leader Michel Barsoum told Materials Today. ‘‘Just because DFT calculations predict something, does not always mean you will get it in the lab. Second, we need to now carefully characterize the 2D materials we made and find out in what applications they would be most suitable,’’ he told us. David Bradley
fuse sufficient distance before they break down and they release their energy as an electric current. Furis and colleagues have carried out scanning laser microscopy with linearly polarized light and exploited photoluminescence to optically probe the molecular
structure of phthalocyanine crystals in a thin film. The new imaging technique reveals the structural defects and grain boundaries in the crystal that act as bumps and potholes to exciton traffic. Recently, the US Department of Energy identified ‘‘determining the mechanisms by which the absorbed energy (exciton) migrates through the system prior to splitting into charges that are converted to electricity’’ as an important part of the research needed to boost photovoltaic research and development. ‘‘One of today’s big challenges is how to make better photovoltaics and solar technologies,’’ echoes Furis, who directs UVM’s program in materials science, ‘‘and to do that we need a deeper understanding of exciton diffusion.’’ Such insights should allow the team to remove the roadblocks to open up an electron superhighway in the thin film by very
Exciton times for photovoltaics Organic semiconductors have promised flexibility for electronic devices for many years, but there remain obstacles in their development into inexpensive TV screens that roll up, roofing tiles that double as solar panels and solar-powered fabrics. Now, Madalina Furis of the University of Vermont and her colleagues have turned to a colorful old friend, the blue dye molecule, which they say will lift the barriers on an ‘‘electron superhighway’’ [M. Furis, et al. Nat. Commun. (2015) 6]. Excitons are neutral species and are not pushed along by a voltage, nevertheless they bounce from one tightly stacked phthalocyanine molecule to the next carrying their payload of solar-driven energy with them until it is released as an electric current. In order to work efficiently and effectively the excitons in organic thin film photovoltaic materials must be able to dif-
477
NEWS
Materials Today Volume 18, Number 9 November 2015