Graphite films can shield electronic devices from electromagnetic (EM) radiation, but current techniques for manufacturing them take several hours and require processing temperatures of around 3000 °C. A team of researchers from the Shenyang National Laboratory for Materials Science at the Chinese Academy of Sciences has now demonstrated an alternative way of making high-quality graphite films in just a few seconds by quenching hot strips of nickel foil in ethanol. The growth rate for these films is more than two orders of magnitude higher than in existing methods, and the films’ electrical conductivity and mechanical strength are on par with those of films made using chemical vapour deposition (CVD).
All electronic devices produce some EM radiation. As devices become ever smaller and operate at higher and higher frequencies, the potential for electromagnetic interference (EMI) grows, and can adversely affect the performance of the device as well as that of nearby electronic systems.
Graphite, an allotrope of carbon built from layers of graphene held together by van der Waals forces, has a number of remarkable electrical, thermal and mechanical properties that make it an effective shield against EMI. However, it needs to be in the form of a very thin film for it to have a high electrical conductivity, which is important for practical EMI applications because it means that the material can reflect and absorb EM waves as they interact with the charge carriers inside it.
At present, the main ways of making graphite film involve either high-temperature pyrolysis of aromatic polymers or stacking up graphene (GO) oxide or graphene nanosheets layer by layer. Both processes require high temperatures of around 3000 °C and processing times of an hour. In CVD, the required temperatures are lower (between 700 to 1300 °C), but it takes a few hours to make nanometre-thick films, even in vacuum.
High-performance films
A team led by Wencai Ren has now produced high-quality graphite film tens of nanometres thick within a few seconds by heating nickel foil to 1200 °C in an argon atmosphere and then rapidly immersing this foil in ethanol at 0 °C. The carbon atoms produced from the decomposition of ethanol diffuse and dissolve into the nickel thanks to the metal’s high carbon solubility (0.4 wt% at 1200 °C). Because this carbon solubility greatly decreases at low temperature, the carbon atoms subsequently segregate and precipitate from the nickel surface during quenching, producing a thick graphite film. The researchers report that the excellent catalytic activity of nickel also aids the formation of highly crystalline graphite.
Using a combination of high-resolution transmission microscopy, X-ray diffraction and Raman spectroscopy, Ren and colleagues found that the graphite they produced was highly crystalline over large areas, well layered and contained no visible defects. The electron conductivity of the film was as high as 2.6 x 105 S/m, similar to films grown by CVD or high-temperature techniques and pressing of GO/graphene films.
To test how well the material could block EM radiation, the team transferred films with a surface area of 600 mm2 onto substrates made of polyethylene terephthalate (PET). They then measured the film’s EMI shielding effectiveness (SE) in the X-band frequency range, between 8.2 and 12.4 GHz. They found an EMI SE of more than 14.92 dB for a film approximately 77 nm thick. This value increases to more than 20 dB (the minimum value required for commercial applications) in the entire X-band when they stacked more films together. Indeed, a film containing five pieces of stacked graphite films (around 385 nm thick in total) has an EMI SE of around 28 dB, which means that the material can block 99.84% of incident radiation. Overall, the team measured an EMI shielding of 481,000 dB/cm2/g across the X-band, outperforming all previously reported synthetic materials.
The researchers say that to the best of their knowledge, their graphite film is the thinnest among reported shielding materials, with an EMI shielding performance that can satisfy the requirement for commercial applications. Its mechanical properties are also favourable. The material’s fracture strength of roughly 110 MPa (extracted from stress–strain curves of the material placed on a polycarbonate support) is higher than that of graphite films grown by the other methods. The film is flexible, too, and can be bent 1000 times with a bending radius of 5 mm without losing its EMI shielding properties. It is also thermally stable up to 550 °C. The team believes that these and other properties mean that it could be used as an ultrathin, lightweight, flexible and effective EMI shielding material for applications in many areas, including aerospace as well as electronics and optoelectronics.
Just for fun, here are 10 physics trivia questions to provide some amusement during the global lockdown and to test your knowledge of physics past and present. Don’t worry, you don’t need to calculate anything or know any physics either.
1 What was Brian Cox’s role in the band D:REAM, which had a hit in the 1990s with Things Can Only Get Better? A Vocals B Drums C Keyboards D Guitar
2 Which activity did madcap German theoretical physicist Theodor Kaluza successfully do for the first time after only ever having studied it in a book? A Chess B Swimming C Knitting D Yoga
3 Where did pioneering physicist James Joule carry out his early experiments on thermodynamics? A In a brewery B On a farm C In a river D In his bedroom
4 What has animal-loving Queen guitarist and former astrophysicist Brian May got in his garden? A Satellite dish B Physics lab C Cat sanctuary D Telescope
5 What pet belonging to top Scottish physicist James Clerk Maxwell appears on his statue in Edinburgh? A Cat B Hamster C Dog D Budgie
6 When Isaac Newton died, which of these was not in his possession? A Cheese toaster B Brown teapot C Mohair bed D Shower curtain
7 In TV’s The Big Bang Theory, Leonard, Raj, Howard and Sheldon are researchers at which US institution? A Caltech B Berkeley C Stanford D UCLA
8 Physicists Sabine Hossenfelder and Tim Palmer recently recorded a spoof coronavirus version of which song? A Stayin’ Alive – The Bee Gees B Sick and Tired – The Cardigans C It’s the End of the World as we Know It – REM D Heaven Knows I’m Miserable Now – The Smiths
9 Who has never appeared in the Physics World “Once a Physicist” column (which features people who once studied physics)?A Olympic canoeist B Opera singer C Prize-winning poker player D Professional footballer
10 Which US institution once offered a professorship to the Italian scientist Galileo Galilei? A College of William and Mary B Harvard University C Penn State University D Yale University
Stuck on any questions? We’ll reveal the answers on the next episode of the Physics World Weekly podcast on Thursday 16 April and on this blog on Friday 17 April. In the meantime, you can see what the rest of the world thinks by checking our online polls at #physicsCoronavirusQuiz on Twitter.
Update: Answers: 1 C 2 B 3 B 4 D 5 C 6 D 7 A 8 C 9 D 10 B (although we’re not entirely sure about that last one any more!)
Recent developments in 3D printing of metallic biomaterials enable fabrication of orthopaedic implants tailored for bone reconstruction following trauma or bone tumours, or for use in joint replacements, spinal implants and reconstructive surgery. It’s also possible to fabricate these implants with surface properties that can promote bone-tissue regeneration and minimize the risk of implant-associated infections.
Insertion of any implant in the human body, however, triggers an immune response that can influence the above-mentioned biofunctionalities.
Aiming to minimize the risk of implant-associated infections, a multidisciplinary research team from the Netherlands involving engineers from TU Delft and biologists from Erasmus MC has examined the in vitro immune response triggered by porous titanium implants printed using selective laser melting (SLM). The team examined three types of SLM implants: untreated; surface biofunctionalized using plasma electrolytic oxidation (PEO); and PEO-biofunctionalized in the presence of silver nanoparticles (PEO+Ag) to provide antibacterial functionality (Biomed. Mater. 10.1088/1748-605X/ab7763).
“Most of the research on implants is focused on how they can stimulate formation of the new tissue, with very scarce data on the inflammatory response they elicit,” explains Lidy Fratila-Apachitei from Delft University of Technology. “Our implants are designed to stimulate bone regeneration and prevent implant associated infections. The inflammatory responses triggered by these implants may affect both these biofunctionalities. Therefore, we were interested to know how inflammatory cells (macrophages) respond to our implants.”
In vitro behaviour
Implanted biomaterials trigger responses that activate both pro-inflammatory and anti-inflammatory, pro-healing macrophages. The fine balance between these two types is influenced by the biomaterial’s properties. To investigate this, the researchers examined the response of human peripheral blood monocyte-derived macrophages cultured on the three different titanium implants.
The untreated implants triggered a strong pro-inflammatory response in macrophages, including relatively high levels of genes that may have detrimental effects on healing, combined with early anti-inflammatory effects. PEO-treated implants showed a higher potential to induce pro-healing macrophages, while incorporation of silver nanoparticles led to cytotoxic effects.
Cell type matters
The team also examined the culture of human mesenchymal stromal cells on the same implants and observed that they survived on all surfaces. This finding indicates that the PEO+Ag implants were not cytotoxic for these cells, in agreement with previous research, and suggests different levels of silver toxicity for different cell types.
The researchers emphasize that macrophage sensitivity to the presence of silver nanoparticles may lead to a compromised immune response in vivo. Therefore, the team is working on finding the optimum concentration of silver nanoparticles that would ensure both macrophage viability and antibacterial function of the implant. Thereafter, the researchers will continue to explore the immunomodulatory effects observed with the aim of achieving immune responses that may favour bone regeneration.
In 2018 the nanotechnology community was wowed by two breakthroughs in graphene research made by a team led by Pablo Jarillo-Herrero of the Massachusetts Institute of Technology (MIT) in the US. Their discoveries led to the rapid emergence of a field called twistronics, which offers a new and very promising technique for adjusting the electronic properties of graphene by rotating adjacent layers of the material.
Calling graphene a wonder material may sound trite, but it is an eminently suitable moniker for a material that continues to amaze condensed-matter physicists like Jarillo-Herrero. I chatted with him about what has happened since his team revealed in 2018 that magic-angle graphene is both a high-temperature superconductor and a Mott insulator.
Graphene is a free-standing sheet of carbon just one atom thick that was first isolated in 2004 by Andre Geim and Konstantin Novoselov – earning the University of Manchester duo the 2010 Nobel Prize for Physics. Since then, researchers have shown that the material has a range of notable and potentially useful properties ranging from high-electron mobility to great physical strength.
Layers of graphene stack upon each other to make the familiar material graphite. Twisted graphene can be made from two sheets of graphene by rotating the sheets away from the usual stacking angle.
Jarillo-Herrero has been working on twisted graphene since 2009. But the big breakthrough came in 2018, when his team reported how it had stacked two sheets of graphene on top of each other and then twisted the sheets so that the angle between them was 1.1°. At this theoretically predicted “magic angle”, the researchers had expected to observe a range of interesting physics. This is because carbon atoms in the two overlapping graphene crystals create a moiré superlattice.
They were not disappointed and made two very important discoveries. First, they found that magic-angle graphene is a Mott insulator. This is a material that should be a metal but is instead an insulator because of strong interactions (correlations) between electrons.
Then, they added a few extra charge carriers to this Mott-insulator state by applying a small electric field, which turned magic-angle graphene into a superconductor at temperatures below 1.7 K. Despite the low temperature, the proximity to the Mott insulator state and low electron density of the material mean that the material resembles a high-temperature superconductor. So, with a simple twist, Jarillo-Herrero’s team had created a system where small adjustments in terms of angle and electric field creates two iconic states of condensed matter physics – a Mott insulator and a high-temperature superconductor.
It has only been two years since Jarillo-Herrero’s team described its results in two papers, and the work has been cited more than 1200 times. He says that most of the citations are from theorists and that there are now hundreds of theory groups working on the system. It takes longer for experimentalists to learn how to make and characterize magic-angle graphene, but Jarillo-Herrero reckons that there are about 20–25 experimental groups that already have results on the material.
Our discovery is the tip of the iceberg, there is so much more underneath
Pablo Jarillo-Herrero
Despite this huge effort, the physics of magic-angle graphene is still in its infancy and Jarillo-Herrero says that there are many things to study. Beyond superconductivity and the strongly correlated states – both of which occur at low temperatures – he says that there is much to learn about the material at higher temperatures.
He is also keen to study other 2D materials that can be twisted. One possibility is bilayer graphene on top of bilayer graphene with a twist between the two bilayers. Twisted bilayers are expected to have interesting correlated-electron physics with magnetic properties that are different to those of the original twisted graphene.
Jarillo-Herrero’s team is also looking at twisting materials unrelated to graphene such as 2D superconductors and 2D magnets. There are theoretical predictions, for example, that a special kind of magnetism called moiré magnetism occurs when 2D magnets are twisted on top of each other.
Moiré patterns Magic angle graphene superlattice. (Courtesy: P Jarillo-Herrero)
“Our discovery is the tip of the iceberg, there is so much more underneath,” he says, “There is a lot of work for many years to come”.
Jarillo-Herrero is frank and honest about the technological applications of magic-angle materials. “Don’t expect any applications in for 30–40 years – which is the normal timescale for a new material.”
He believes that one technology that could emerge is a superconducting transistor that can be switched between superconducting and normal states. Such devices could be used in cryogenic classical computers, which would run at very low temperatures to avoid the power dissipation problems associated with high-speed silicon processors. Other options include using twisted materials to make superconducting single-photon detectors or superconducting quantum bits for quantum computing.
However, he points out that the technology is still in its infancy in terms of making large and consistent samples of twisted materials.
In the nearer future, Jarillo-Herrero says that magic-angle materials have a wide range of fascinating physics that will be explored. One avenue is quantum simulation, whereby twisted graphene is used as a proxy for a more complicated material such as a high-temperature superconductor.
And, of course, he points out that there should be lots of interesting physics – including topological properties – lurking in twisted graphene itself, which will keep researchers busy for years to come.
Cities and countries worldwide are working towards being net carbon neutral. In this episode of the Physics World Weekly podcast the science journalist Kate Ravilious talks to about some of the ways that people and governments are trying to achieve this goal.
Linear accelerators (linacs) for treating cancer are among the most complicated technologies used in hospitals. As well as being difficult to operate, mistakes cause damage to a linac or leave it in a state in which it could do harm to patients. Training new medical physicists in how to use linacs is difficult, however, because the machines are normally in use treating patients. In this episode, Physics World’s Tami Freeman talks to Marco Carlone in Canada, who has founded a company called Linax Technologies to develop a linac simulator that could be used for training purposes.
Finally Margaret Harris and Hamish Johnston talk about space-weather super-storms and how they can damage electrical grids. They also explain how a new map from the US Geological Survey identifies power lines in the US that are most vulnerable to super-storms.
For some science students, the path from undergraduate degree to career of choice is a linear and seemingly friction-free transition. For others, less so. Stephanie Mottershead, a project manager at Surrey Satellite Technology Ltd (SSTL), a UK manufacturer of small satellite systems, definitely falls into the second category. After completing a BSc in geology and environmental science at the University of Bristol, Mottershead walked straight into a tanking graduate jobs market and a wider economy reeling from the aftershocks of the 2008 financial crash.
Needs must, though, and Mottershead spent the next decade or so on a very different career trajectory, working as a professional singer in London and on various luxury cruise ships. Her passion for science undimmed, she also managed to study part-time for an Open University (OU) degree in physics and astronomy. “I loved the planetary-science modules in my OU course and it gradually dawned on me that I could do something with this professionally,” she explains. “Rather than continue with music as a career and science as a hobby, I made the decision to switch things around.”
Mottershead’s reinvention took off in 2017 when she secured a European Space Agency (ESA) scholarship to attend the International Space University’s Space Studies Program in Cork, Ireland. This nine-week residential “summer space camp” brings together around 250 delegates each year, among them early-career scientists and engineers intent on forging a path in the space industry alongside established professionals seeking to broaden their knowledge of the space sector beyond their current discipline. “The Space Studies Program was an incredible experience,” says Mottershead. “To meet and learn from a network of experts and peers like that really opened my eyes to the range of career paths available within the space industry.”
Own the space
Spurred on by her experience in Cork, Mottershead wrapped up her OU degree in double-quick time and set her sights on UCL’s MSc in space science and engineering. “The 12-month programme at UCL aligned with my desire to combine planetary science and space engineering,” she says. “I wanted to pursue both and the mix of taught-course modules ties everything together really well.”
Stephanie Mottershead: “The MSc is the most intense thing I’ve ever done, but massively rewarding at the same time.” (Courtesy: Stephanie Mottershead)
Another factor in Mottershead’s choice of UCL is the reputation of the teaching and research staff. “You get to learn from world-leading experts in all fields of space science,” she explains. “For our individual research projects, we also got to work with scientists and engineers at UCL’s Mullard Space Science Laboratory (MSSL) – all of whom are actively involved on high-profile space missions like the recently launched Solar Orbiter (see box, below).”
The taught element of the MSc course, which takes up the first six months of the programme, is followed by an individual research project and dissertation, a group research project (involving the full MSc cohort), plus exams for good measure. “There’s no let up at all,” adds Mottershead. “It’s the most intense thing I’ve ever done, but massively rewarding at the same time.”
In terms of specifics, the lecture programme comprises a series of compulsory modules: space data systems and processing; space instrumentation and applications; space science, environment and satellite missions; and space systems engineering. That core learning is reinforced with a series of optional modules, including planetary atmospheres; solar physics; high-energy astrophysics; space plasma and magnetospheric physics; remote sensing; and global monitoring and security.
Although she was awarded an MSc with distinction, Mottershead admits there were times during the course when she and other students struggled with the workload. “Alongside the MSc, I was working 20 hours a week to pay the bills,” she explains. “With hindsight, I realize that wasn’t such a great idea. It’s important for prospective students to commit 100% to this course and minimize any distractions.”
My space
With so much going on, Mottershead ranks the group research exercise as one of the high-points of her experience, with all 24 MSc students working collectively to scope out a full-lifecycle mission concept and spacecraft design – in this case, a mission to Uranus.
Working to a three-week deadline, the students break into smaller functional teams to cover all aspects of the mission architecture, including the fundamental scientific questions the mission will seek to address; spacecraft design and scientific instrumentation; trajectory and orbital planning; as well as all of the telecommunications.
“It’s meant to be tough,” explains Mottershead. “The idea is to give students a taste of what a big science project looks like in terms of the intensity, the teamworking and multidisciplinary collaboration, as well as the granular scope of the work programme. It was stressful for sure, but also incredibly satisfying.”
The individual research dissertation provided another highlight, allowing Mottershead to collaborate with senior MSSL scientists (Andrew Coates and Geraint Jones) on a new mission concept. “I subsequently got to present this work to an audience of space-industry professionals, including representatives from NASA and ESA, when I was invited to take a slot at the International Planetary Probe Workshop [IPPW] in Oxford,” she adds.
After graduating last autumn, Mottershead took her first steps into the space industry with a project management role at SSTL. The company, a wholly owned subsidiary of aerospace giant Airbus, bills itself as “the world’s leading small-satellite manufacturer”, serving an international customer base that includes the likes of NASA, ESA and SpaceX.
“Right now,” notes Mottershead, “every day is different and I’m learning a lot – and fast – about people management, collaboration and coordination across different project teams. Longer term, I’m keen to get closer again to the hands-on space science. I have a strong personal interest in remote sensing – using satellites to study climate change, deforestation and disaster monitoring.”
High fliers: Chris Owen (sixth from left) and other principal investigators responsible for the 10 scientific instruments on the Solar Orbiter mission. “This is probably a once-in-a-career kind of picture,” explains Owen, “and was taken a few hours before the mission launch, when we got up close to the launcher.” (Courtesy: Yannis Zouganelis/ESA)
It’s unlikely that Chris Owen, professor of physics at UCL’s Mullard Space Science Laboratory (MSSL), will ever forget where he was on the evening of 9 February 2020. Hardly surprising given that he was high-fiving colleagues in Cape Canaveral, Florida, after witnessing the successful launch of ESA’s €1.4 billion Solar Orbiter mission – a spacecraft that, once it reaches its operational orbit in two years’ time, will use an array of 10 scientific instruments to deliver the most detailed images yet of the Sun and its outer atmosphere as well as unprecedented in situ measurements of the solar wind, the continuous stream of high-speed charged particles emanating from the Sun’s surface.
Owen, for his part, is principal investigator for one of those scientific instruments – the mission’s Solar Wind Analyser (SWA) – and heads up an international science and engineering consortium that will, over the next decade, use the SWA’s sensors to better understand the physics of space weather. Here he talks to Physics World about big-science collaboration and the benefits for UCL’s MSc students of proximity to the staff and projects at MSSL.
What role will the SWA play on Solar Orbiter?
The Sun emits a constant stream of high-speed charged particles into the heliosphere (a vast, bubble-like region that extends from the Sun to the edge of the Solar System). This solar wind influences Earth’s near-space system – sometimes disrupting satellite communications and, on occasion, even damaging or destroying orbiting spacecraft. The SWA will carry out a range of in situ measurements on this particle stream – something akin to “sniffing” the solar wind.
Who built the SWA?
The SWA is a case study in international collaboration. There are three sensors on the instrument: the one we built here at MSSL will study electrons in the solar wind; the sensor for protons and alpha particles was built mostly in France and the Czech Republic; while a US team supported by NASA led the development of the sensor for heavy ions such as iron, carbon and oxygen. All three sensors are controlled by a central electronics box built in Italy, while Airbus UK developed the spacecraft as well as the heatshield technology that will protect it while orbiting close to the Sun.
How do your MSc students benefit from MSSL’s involvement in high-profile missions like Solar Orbiter?
At MSSL, we are the largest university-based space science lab in the UK, covering all core space science and engineering disciplines in one department. At any given time, we’re participating in a range of big science projects that, by definition, have to be multidisciplinary and international in scope. That means our MSc students get to learn from leading space scientists and engineers involved in defining and delivering new space missions. Our MSc is attractive in terms of its subject coverage and scope, but also because of MSSL’s strategic position within the space science ecosystem. We have a lot of connections within the space industry and that can open doors for our researchers and students.
Are there opportunities for MSc students to get involved directly with missions like Solar Orbiter?
Absolutely – that’s the norm rather than exception at MSSL. Last year, for example, one of my MSc students undertook a project to investigate spacecraft charging in the solar wind – and specifically the impact that this behaviour might have on the SWA’s measurements of charged particles. The student in question used simulation tools to build and “test” a virtual Solar Orbiter spacecraft in an effort to understand the underlying physics, with the behaviours predicted by her simulations feeding into the wider ESA research on this issue.
What does the future hold for MSSL and your MSc programme?
These are exciting times for MSSL. We’re working on a number of high-profile future missions, including Plato (to drive new discoveries in exoplanetary science), Euclid (mapping of the dark Universe) and the PanCam scientific cameras for the ExoMars 2020 Rover (scheduled to arrive on Mars in April 2023). All of these missions will translate into incredible learning and research opportunities for our MSc students.
An international team led by researchers at Stanford University has developed a new device for connecting the brain directly to silicon-based technologies. The interface has the potential to improve human prosthetics and enable technology that could one day restore vision or speech in patients (Science Advances 10.1126/sciadv.aay2789).
The device, which contains hundreds of microwires, can be gently inserted into the brain and connected to an external silicon chip that records the electrical brain signals transmitted by each wire. The team – also from the Francis Crick Institute, University College London, ETH Zurich and Austin, TX-based technology company Paradromics – has already successfully tested the device on the retinal cells of rats and in the brains of living mice.
‘Electronic movies’
As first author Abdulmalik Obaid, a PhD Candidate at Stanford University, explains, the team’s main goal was to close the gap between the power of modern electronics and what is currently available in existing brain–machine interfaces.
“The difficulty with using modern electronics is there is a mismatch between the three-dimensional architecture of the brain and largely two-dimensional electronics. We sought to overcome these limitations by employing commercial silicon chips, such as the ones used in high-speed cameras and microdisplays, and combining them with arrays of microscopic wires,” Obaid says.
“As these chips are inherently flat, we combine them with easily tailored arrays of microscopic wires, which are precisely spaced to be minimally invasive,” he adds. “This approach lifts the scalable, but two-dimensional, silicon technology to the third dimension of the brain, allowing us to record ‘electronic movies’ of neural activity over large brain regions.
To scale-up and be able to record larger numbers of neurons, Obaid explains the research team had to create a brain–machine interface that was not only capable of recording from thousands of signals simultaneously, but also capable of integrating well with the brain and causing minimal damage. In doing so, he and his colleagues developed a way to create large-scale arrays of very thin microwires and figured out how to connect them to silicon-based devices, such as high-speed cameras and microdisplays, in order to take advantage of advances in those technologies.
With a silicon chip attached to the top and the wires at the bottom gently inserted into the brain, the microwire array can take a movie of neural activity. (Courtesy: Andrew Brodhead/Stanford News Service)
“The combination of these two allows us to record more data from the brain, as well as being less invasive than previous approaches,” he says. “Another key benefit of this design is it allows us to simultaneously record different brain regions at different depths. This is important to study different neuroscience questions, or for brain–machine interfaces that need to reach different areas of the brain.”
Robotic limbs
Moving forward, Obaid says that the device has a broad range of potential applications in neuroscience research and brain–machine interfaces for clinical applications – particularly in view of its longevity and stability, which allows the team to study processes like learning in the brain.
“This is the neuroscience question we’re most interested in studying,” says Obaid. “In terms of clinical applications, while we are a way out, we’re particularly interested in applications for prosthetics, particularly speech assistance. The goal is that, through this device, the recording of more signals from the brain can improve the quality of prosthetics and enhance our understanding of the brain, both in healthy and diseased states.”
The team is currently testing the stability and longevity of the devices in the brain through long-term animal studies. Based on these studies, the researchers are also exploring what the neural activity recorded through the device can tell them about both short-term and long-term changes in the brain during learning.
“With the density and high resolution made possible by this technology, we hope that in the future it can be used to help improve human prosthetics, such as devices that can translate electric signals from the brain into robotic limbs, as well as devices that could restore vision or speech in a patient,” adds Obaid.
When I watch films, TV shows or sports I often find myself thinking about the physics of the situation. Here in North America, we’re approaching the end of ice hockey season. The main aim of this sport is simply to get the puck into the net. But how far could a player actually hit a puck, if the net and edge of the rink weren’t there? Could you even make the puck loop all around the rink? These kinds of questions are ideal tools for teaching physics, as you can start with the most basic scenario and build upon it to reach the complex reality.
How far could a player actually hit a puck, if the net and edge of the rink weren’t there? Could you even make the puck loop all around the rink? Such questions are ideal tools for teaching physics
To answer how far you can hit a puck, there are three basic layers. You start with the ice, which is a very slippery surface – so it’s safe to assume that the friction between the puck and the ice is negligible. You also ignore air resistance, which only leaves the downward gravitational force and the normal force (the upward pushing force from the ice), which balance each other out. No net force means no movement, so you apply a pushing force, such as a hit from a hockey stick, which results in the puck travelling at a constant speed forever.
All of this is simple mechanics, but it’s not quite realistic. Although ice is very slippery, there will be a frictional force between it and the puck, which acts against the forward motion – meaning you must account for it. At a fundamental level, friction is a complicated interaction, but these complexities can be captured with a simple model of friction, which is found experimentally for different materials. Assuming the coefficient of friction is about 0.1 for our puck on ice, using some basic kinematics and Newton’s handy laws, that gives a stopping distance of just over 1000 m when the puck is hit with a starting speed of 160 km/hr.
While more realistic than never stopping, this scenario is still not believable, as air resistance also needs to be taken into account. Although the collisions between air molecules and objects can be complex, like with friction there is a model to describe the scenario. Unfortunately, when putting this into the equation for acceleration there’s a snag – the acceleration can be used to determine the change in velocity, but the magnitude of the acceleration now also depends on the velocity.
It’s possible to write the acceleration as the derivative of velocity with respect to time, turning this equation into a differential equation. But there is another route in the form of numerical calculations, which allow you to take a problem and break it down into many smaller and simpler problems. In this case, the motion of a sliding hockey puck can be modelled in small time steps, let’s say 0.1 seconds. During that tenth of a second, the hockey puck will indeed decrease in speed. However, the change in speed will be small – small enough that the acceleration can be calculated and assumed constant, allowing the motion during this short interval to be determined. With the help of a computer (because intervals of 0.1 s means a lot of data points), you get a plot of time versus puck position, which shows for a puck of mass 170 g, the stopping distance is 227 m. Turns out, air resistance plays a significant role.
So what about hitting the puck around an entire hockey rink (about 180 m, in the shape of a rounded rectangle) with one shot? In this scenario, the motion of the puck can be split into two parts. The simple part is the motion along the straight edges of the rink – the wall would create a different interaction with the air, and change the drag coefficient. For a first approximation we can assume the puck follows the same calculation as above. But when the puck travels around the rounded corners of the rink, which have a radius of curvature of 8.5 m, the boundary wall will add two new forces to the calculation. First, the normal force from the wall, which pushes the puck sideways in order to get it to turn. Second, the friction between the wall and the puck.
There are also two ways a puck could travel around this bend. It could “roll” along the wall, in which case there will still need to be some type of wall frictional force that causes the puck to increase its angular velocity. In order to be completely rolling, the angular velocity of the puck would have to be equal to the linear speed of the puck multiplied by the radius of the puck (which is true for any rolling without slipping object).
The other way the puck could travel around the corners is by completely sliding without rolling. In this version, the angular velocity of the puck would stay at zero and there would just be a kinetic frictional force. Of course, the coefficient of friction between the rubber puck and the wall would likely be much higher than for the ice–rubber interaction. But it gets even more complicated.
For this wall–puck friction, the magnitude of the frictional force depends on the normal force for the wall pushing on the puck to make it turn. This normal force depends on the radius of curvature of the wall and the speed of the puck. So, again, the wall–puck frictional force depends on the speed of the puck.
It’s possible that the puck could “bounce” when it transitions from the straight part of the wall to the curved part. This would not only cause it to lose kinetic energy (and slow down), but it would also mean that it loses contact with the wall. This is a pretty tough problem, and to solve it you probably need some more experimental data on the interaction between the puck and the wall. Consider this your homework the next time you’re watching a game!
A light-emitting silicon-based material with a direct band gap has been created in the lab, 50 years after its electronic properties were first predicted. This feat was achieved by an international team led by Erik Bakkers at Eindhoven University of Technology in the Netherlands. They describe the new nanowire material as the “Holy Grail” of microelectronics. With further work, light-emitting silicon-based devices could be used to create low-cost components for optical communications, computing, solar energy and spectroscopy.
Silicon is the wonder material of electronics. It is cheap and plentiful and can be fabricated into ever smaller transistors that can be packed onto chips at increasing densities. But silicon has a fatal flaw when it comes to being used as a light source or solar cell. The semiconductor has an “indirect” electronic band gap, which means that electronic transitions between the material’s valence and conduction bands involve vibrations in the crystal lattice. As a result, it is very unlikely that an excited electron in the conduction band of silicon will decay to the valence band by emitting light. Conversely, the absorption of light by silicon does not tend to excite valence electrons into the conduction band – a requirement of a solar cell.
In contrast, electronic transitions in direct band gap semiconductors do not involve lattice vibrations, so these materials emit copious amounts of light when electrons are excited – and are very good at converting light into electricity.
Incompatible with silicon processing
As a result, direct band gap materials such as gallium arsenide are used to create LEDs, lasers and solar cells. Unfortunately, these materials are hard to integrate into silicon processing, making it difficult and expensive to create devices that combine the electronic properties and scalability of silicon with the optical properties of direct band gap materials. Such devices could, in principle, lead to faster and more efficient telecoms and computing systems in which information is transmitted and processed using light alone.
50 years ago, researchers first calculated that a silicon-germanium alloy with a hexagonal crystal structure should have a direct band gap. The problem is that under ambient conditions both silicon and germanium have diamond-like crystal structures.
In 2015 Bakkers’ team developed a way of creating hexagonal germanium and silicon germanium nanowires. This involves first growing an extremely thin (about 35 nm diameter) gallium arsenide nanowire substrate. Germanium is then deposited on the nanowire, increasing its thickness by a factor of 10. The gallium arsenide nanowires have a hexagonal cross section and as a result, the surrounding germanium shell also has a hexagonal crystal structure. This is because the gallium arsenide provides a structural template for hexagonal growth; and the very high surface to volume ratio of the nanowire allows hexagonal growth to occur.
Once they created the hexagonal germanium shell, the researchers were able to deposit silicon-germanium or silicon to create hexagonal crystals of those materials. “We were able to do this such that the silicon atoms are built on the hexagonal template, and by this forced the silicon atoms to grow in the hexagonal structure,” explains Eindhoven’s Elham Fadaly.
Defects and impurities
In 2015, however, they were unable to make the nanowires emit light – a problem that the team said was related to the presence of defects and impurities in the crystal structure.
Now, the Eindhoven team has joined forces with researchers in Germany and Austria to create higher quality nanowires that emit light. They measured the emission by firing a laser at the nanowires in order to excite the electrons and then detected the infrared light that is emitted. “Our experiments showed that the material has the right structure, and that it is free of defects. It emits light very efficiently,” says Eindhoven’s Alain Dijkstra.
The researchers found that by reducing the silicon content of the hexagonal alloy from 35% to 0 they could change the wavelength of the light from 1.5 to 3.5 µm. This partially overlaps the wavelengths of infrared light that are currently used in optical telecoms.
Bakkers believes that the team will soon be able to create a laser using the nanowires: “By now we have realized optical properties that are almost comparable to indium phosphide and gallium arsenide, and the materials quality is steeply improving. If things run smoothly, we can create a silicon-based laser in 2020.”
As well as optical telecoms and optical computing, the new silicon-based material could be used to create low-cost chemical sensors that use infrared spectroscopy.
Keeping busy: Tao Wang at his home during the lockdown period in Wuhan, China. (Courtesy: Tao Wang)
I run a research group made up of more than 20 graduate students, and in a “normal” workday my job is to supervise and direct them on research activities related to optoelectronic devices such as solar cells and light-emitting diodes. I also teach an undergraduate course in polymer physics during our teaching season, with lectures two times a week. I would normally also go to conferences, although not every week.
The city of Wuhan and the residential compounds within it responded differently at different stages of the pandemic. At the beginning of the outbreak, normal life was not affected much as the number of infected people was low. On 23 January, Wuhan was locked down, with nobody able to leave the city; however, in the early days of the lockdown people could still walk freely outside their homes. This was soon changed so that nobody could leave their residential compound except those involved in essential work, as evidence showed that less strict measures were not preventing the spread of the coronavirus.
Life under lockdown
During the lockdown, a lot of medical and other resources were sent to Wuhan, and many volunteers helped deliver groceries to residential compounds, assist the vulnerable, and bring food to doctors and nurses on the front line. At first, patients with mild symptoms were asked to return home and self-isolate – partly due to the shortage of hospital beds and other resources, and partly due to a lack of experience in how to treat a virus that humans had not encountered before. Again, this was soon changed, as the virus continued to spread, clusters of infections appeared, and people with mild conditions developed more serious symptoms.
To deal exclusively with coronavirus patients, Wuhan constructed two new hospitals from scratch in 10 days. Another 16 makeshift hospitals were also built, some of them in one day. Other provinces in China also sent many thousands of doctors and nurses to hospitals in Hubei. This enabled health workers to collect and treat all patients in hospital and closely watch those who have been in close contacts with patients. The number of new cases reduced immediately with these actions, and this – along with a reduced number of patients in hospitals after their cure and discharge – helped to ease the crisis.
I have kept myself fairly busy while self-isolating at home during the lockdown time in Wuhan. Whilst we report our body temperatures every day to local health volunteers and try to keep our life free of chaos and panic, we also try to do some of the work we would expect to do in a normal time. My students and I have online meetings every two weeks, during which we discuss some of the latest literature related to their projects. We finished writing and revising a few manuscripts, and I also wrote two grant proposals (it is proposal writing time between January and March in China). At the beginning of the new semester in March, university students in Wuhan were asked not to return on campus due to the outbreak of COVID-19, and all face-to-face lectures have been turned to online virtual ones. This minimizes the disruption to their studies, while also ensuring their health and safety.
Emerging from the epidemic
For the past 20 days, very few new cases of coronavirus have been reported in Wuhan, and as of today the total number of coronavirus patients is less than 500. So, after 11 weeks of lockdown, people in Wuhan were allowed to leave the city from midnight on 8 April. Thanks to the great achievement of putting down a pandemic in about two months, people in “epidemic-free” residential compounds are now allowed to leave their homes, for example to do grocery shopping in supermarkets. A lot of commercial units have resumed functioning. The authorities are evaluating how to ensure public health and safety in these new circumstances, and when that is settled our students will be allowed to return to campus. I actually tidied up my office today, and I am waiting for our students to be back, which I am sure won’t take long.
With great efforts from people in every country, this extraordinary crisis will surely be overcome, and we will be back to “normal” life. But this new normality won’t be the same as the one that existed before. It is going to change our society in ways we haven’t fully anticipated. I hope the changes are positive rather than negative. We should live in more healthy ways so that we can share this planet with other beings, and that will require everyone to think things over after the disruption is finished. I do see positive things in all nations across the globe: responsibility, selflessness, self-discipline, unity and resolve. As for positive things in my professional life, the lockdown gave me time to look back and think over what I have done in my research activities over the past few years, and particularly to evaluate whether they are as methodologically robust as they could be. I have some thoughts on that and will start from those once I am able to return to my laboratory.
I hope the rest of the world can see hope from my experience in Wuhan. If we stick to social distancing, wash hands and wear masks, this pandemic is certainly controllable.
