Dark-matter detectors usually conjure up images of large underground facilities, but relatively small quantum sensors such as atomic clocks and magnetometers have also joined the search for the elusive stuff. In this episode of the Physics World Weekly podcast, Andrei Derevianko at the University of Nevada, Reno explains how it is done.
We are also joined this week by Physics World columnist Caitlin Duffy, who is doing a PhD on superconductivity at the High Field Magnetic Laboratory in the Netherlands. Duffy talks about the benefits of moving country to pursue career opportunities and the professional and personal challenges of working abroad during the pandemic.
Although silicon photodiodes are widely employed in a host of light-detection technologies, scaling them up is difficult and expensive. Researchers at the Georgia Institute of Technology (Georgia Tech) in the US have now compared the performance of these diodes with that of organic polymer-based diodes, which are easy to fabricate over large areas. Somewhat to their surprise, the researchers found that the organic devices match their inorganic counterparts in all areas apart from one: response time. “The result goes against conventional wisdom that switching to organic materials that can lead to scalable devices would mean giving up on performance,” says team member Bernard Kippelen.
Silicon photodiodes (SiPDs) are very efficient detectors of ultraviolet, visible and near-infrared light. One of the metrics that quantifies their performance is noise equivalent power (NEP), which is defined as the optical power that produces a signal-to-noise ratio (SNR) of one. Since a photodiode’s performance varies with its area and the bandwidth over which measurements take place, researchers also use another parameter, the specific detectivity, to compare the performance of different devices. Higher values of specific detectivity mean that the photodiode can detect fainter levels of light.
Small-area, low-noise SiPDS fare well against these metrics. They boast specific detectivities of around 1012 cm Hz1/2W-1 in the visible and infrared parts of the electromagnetic spectrum when evaluated at a low bandwidth. However, maintaining this performance when the devices are fabricated over larger areas requires stringent control of crystal defects in the photodiode material. This can be difficult to achieve, and team leader Canek Fuentes-Hernandez notes that knowledge of how well SiPDs actually perform can be patchy. “Unfortunately, these metrics are seldom measured, and unverified approximations can lead to large errors when estimating their values,” he tells Physics World.
By directly measuring these key metrics, Fuentes-Hernandez, Kippelen and colleagues found that low-noise, large-area solution-processed flexible organic photodiodes are just as efficient as small-area SiPDs at detecting faint light in the visible range. The organic devices also show electronic noise current values in the range of tens of femtoamperes and noise equivalent power values of a few hundred femtowatts. Both values compare well with silicon when measured at a low bandwidth.
Organic electronic devices
In their work, members of the Georgia Tech team studied P3HT:ICBA organic photodiodes on indium tin oxide/polyethylenimine ethoxylated and MoOx/Ag electrodes. The polyethylenimine electrodes are stable in air and also allowed the researchers to produce photovoltaic devices that exhibit low levels of dark current (the electrical current that flows through a device even when no light shines on it). These low dark currents mean that the material can be used in photodetectors designed to capture faint signals of visible light.
Like other organic polymer-based electronic devices, these photodiodes can be made using simple solution-processing and inkjet printing techniques. That makes it possible to coat them onto a variety of surfaces, including flexible ones like those employed in displays and solar cells. Organic thin films also absorb more efficiently than silicon, so the overall thickness of the active light-absorbing layer in organic photodiodes is very small. Indeed, the active layer of the Georgia Tech team’s photodiodes is just 500 nm thick. “A gram of the material could coat the surface of an office desk,” Fuentes-Hernandez says. “Even if you scale their area up, the overall volume of your detector remains small with organics. If you increase the area of a silicon detector, you have a larger volume of materials that at room temperature will generate a lot of electronic noise.”
Direct measurements revealed that a device based on these materials can detect as little as a few hundred thousand photons of visible light every second – equivalent to the magnitude of light that reaches our eye from a single star in a dark sky, explains Fuentes-Hernandez. This sensitivity, combined with their ability to be coated onto large, arbitrarily-shaped substrates, means that organic photodiodes “now offer some clear advantages over state-of-the-art SiPDs in applications requiring response times in the range of tens of microseconds,” he adds.
Expanding the range of applications
According to the team, organic photodiodes could be used in medical applications like pulse oximeters, which use light to measure heart rate and blood oxygen levels. The flexibility of these photodiodes might also allow multiple such devices to be placed on different areas of the body, and the researchers say they could detect a tenth of the light that conventional devices require. This would make it possible to build wearable health monitors that yield better physiological information.
There is just one snag: at 35 microseconds, the organic devices’ response times are significantly longer than those of SiPDS, which typically have response times of picoseconds or nanoseconds. The researchers, who report their work in Science, say they are working to improve response times to expand the range of possible applications for the devices. “The slower response time of our current devices comes from the fact that we use materials that are processed from inks using printing or coating techniques that are not as ordered as crystalline materials,” Kippelen explains. “As a result, the carrier mobility and the velocity of the carriers that can move through these materials are lower, so you can’t get the same fast signals you get with silicon. But for many applications you don’t need picosecond or nanosecond response time.”
Focused ultrasound is an emerging therapeutic technology that uses ultrasonic energy to target tissue deep in the body, precisely and non-invasively. The potential applications are wide-ranging: from ablation of tumours and other lesions to blood–brain barrier opening, immunomodulation and neuromodulation, to name just a few.
“We’ve made a lot of strides in paediatric cancer, now more than 80% of our patients are long-term survivors,” Kim explained. “However, this has come at a significant cost – the acute and late effects of current multimodal therapy in children are substantial.” What’s more, treatment success is not distributed equally. “The prognosis for metastatic, recurrent solid tumours is dismal and has not significantly improved over the past three decades,” she added.
As such, there’s still a vital need for improved treatments for paediatric cancers. Addressing this goal, Kim and colleagues at Children’s National set up the multidisciplinary IGNITE (image guided non-invasive therapeutic energy) team. The group aims to develop and clinically translate focused ultrasound applications that will minimize treatment side effects and increase efficacy, thereby improving the care of paediatric cancer patients.
Kim explained that focused ultrasound offers a range of advantages over other therapies – it is non-invasive, involves no ionizing radiation, is image guided for accuracy and produces multiple biological effects. It also offers the flexibility for combination with other treatments. “These characteristics make focused ultrasound ideal for development in paediatric cancer,” she said.
The IGNITE team opened its first trial in 2015 – a study of MR-guided high-intensity focused ultrasound (MR-HIFU) for treating painful osteoid osteomas (a benign bone tumour) in children. This was followed by a trial of MR-HIFU for paediatric solid tumours.
“We learned that MR-HIFU ablation of osteoid osteomas and solid tumours appears to be safe and feasible,” said Kim, noting that most of the osteoid osteomas exhibited complete responses to the therapy. Osteoid osteomas, however, have an ideal location and size for HIFU ablation, while many solid lesions are larger, harder to reach and could only be partially ablated.
The researchers thus turned their attention to combinations of focused ultrasound with other therapies, such as chemotherapy. Their next clinical trial was a phase I study of MR-HIFU with LTLD, a heat-activated form of the cancer drug doxorubicin, to treat paediatric solid tumours. After systemic administration of the drug, ultrasonic heating to above 42 °C rapidly releases the encapsulated doxorubicin in the targeted tumour vasculature.
“The results are too early to state, but MR-HIFU ablation with LTLD may overcome some of the limitations of ablation in terms of incomplete treatment,” noted Kim. “However, it still doesn’t address that some tumours still not targetable and some are located in metastatic sites that are not reachable.”
To tackle this so-far untreatable subset of paediatric cancers, the team next considered combining HIFU with immunotherapy. “There’s growing evidence of modulation of immunity through HIFU,” Kim explained. “We know that paediatric cancers are typically considered non-immunogenic; so how can we make them immunogenic?”
The researchers performed a pre-clinical study of mouse neuroblastoma treated using HIFU combined with immune checkpoint inhibitors (αCTLA-4 and αPD-L1). They found that the combination caused significant intra-tumour infiltration of macrophages and helper T cells, leading to prolonged survival of the mice. HIFU or checkpoint inhibitors alone did not have the same effect. The study established that HIFU can effectively induce immune sensitization in a previously unresponsive tumour, promising a novel modality to overcome therapeutic resistance.
“We think that focused ultrasound has potential to replace current local control mechanisms,” Kim concluded. “However, there are limitations. The future for most paediatric cancer applications will be combination approaches using the various bioeffects of focused ultrasound. We have ongoing pre-clinical and clinical applications that really have the potential to change treatment paradigms in paediatric cancer medicine.”
A new type of high-power quantum cascade laser that works without bulky cooling equipment could usher in a host of novel imaging applications by making it easier to generate terahertz radiation outside the laboratory. The laser, which was developed by researchers at the Massachusetts Institute of Technology in the US and the University of Waterloo, Canada, can operate at temperatures of up to 250 K – some 40 K higher than the previous record, and attainable with a compact cooler rather than a specialist cryogenic system.
Terahertz (THz) radiation falls between the infrared and microwave regions of the electromagnetic spectrum, with wavelengths in the range of 3 mm – 30 µm. While many molecules absorb light at these wavelengths (making it possible to use THz radiation to identify their molecular “fingerprint”), THz radiation passes straight through everyday materials such as paper, cloth and plastics. This means that THz radiation, like X-rays, can be used to “see” inside objects that are opaque to visible light. Unlike X-rays, however, photons in the THz region have relatively low energies, making THz radiation non-ionizing and therefore safe for biological and medical use. A further benefit is that THz radiation has a shorter wavelength than microwave radiation, which means it can create higher-resolution images.
Underused
While all of this sounds good on paper, radiation between 0.1 and 10 THz is underused in practice due to the lack of practical sources and detectors in this range. Generating THz beams that are intense enough to be useful is also a challenge. Quantum cascade lasers (QCLs) are one option, as their microscopic structures can be tuned to produce coherent THz radiation. However, these lasers must be kept at very low temperatures to function.
Researchers led by Qing Hu and Zbig Wasilewski have now partially overcome this barrier by developing a QCL that produces light in the THz region with only a modicum of cooling. At 250 K, the new laser’s maximum operating temperature is significantly higher than the previous maximum of 210 K that Jérome Faist’s group at ETH Zurich in Switzerland achieved in 2019 – a record that was itself much higher than the 2012 record of 200 K.
Quantum cascade lasers
The temperature-sensitive nature of QCLs stems from the way they are constructed. Unlike standard semiconductor lasers, which generate photons when electrons and holes combine inside a material with a given electronic energy band gap, QCLs consist of tailor-made quantum wells and barriers made of thousands of thin layers of semiconductors. Each electron that travels through the device “cascades” through a series or “staircase” of these quantum wells (QWs) as it passes through the semiconductor layers. In the process, the electron emits multiple photons at frequencies that are set by the structure of the layers.
At higher temperatures, electrons tend to “leak” over the barriers of the QWs, disrupting the laser’s output. Hu and colleagues’ breakthrough came after they managed to reduce this leakage by developing new semiconductor band structures – an improvement which, in turn, enabled them to double the height of the barriers. They also devised a novel configuration in which lower lasing levels of each step of the quantum-well staircase are quickly depopulated of electrons by scattering phonons (vibrations of the quantum lattice) into a ground state. This state then serves as the “injector” of electrons into the upper level of the next step, Hu explains, and the process repeats so that lasing can occur.
Complex structures
The structures the team created are very complex and contain close to 15 000 interfaces between QWs and barriers, half of which are less than seven atomic layers thick, Wasilewski explains. The quality of these interfaces is, he adds, paramount to the THz laser’s performance.
One near-term application for the new THz source would involve the real-time imaging of skin during skin-cancer screenings, Hu says. Cancer cells show up “very dramatically” in THz light, he explains, because they contain more water and blood than normal cells, and water strongly absorbs THz signals. The technology could also be used to detect drugs like methamphetamine and heroin and explosives like TNT since these molecules have a spectral fingerprint within the THz frequency range too.
The researchers, who report their work in Nature Photonics, say that in the future it should be possible to generate THz radiation with a QCL without the need for a cooler at all. They now plan to further increase their device’s maximum operating temperature while also lowering its lasing threshold to reduce heat dissipation. “This will enable a compact and portable laser system operating in a continuous way, which is more useful in applications,” Hu tells Physics World. “For example, a single-frequency continuous laser based on a distributed feedback structure is essential for high-resolution spectroscopy and sensing.”
Hu and his colleagues are also developing compact THz imaging systems with a high dynamic range, which is required to penetrate thicker layers of material and allows for faster imaging. “We are also developing broadband THz radiation amplifiers, akin to the amplifiers used at the front end in cell phones and radio receivers,” he says. “Since most THz signals are quite weak, be they from the universe (90% of photons in the universe are in the THz range) or from earthly sources, amplification will greatly ease the demanding requirements for detection and subsequent signal processing.”
You may have noticed that not everyone agrees with the outcome of the 2020 US Presidential election. But looking beyond the ALL CAPS TWEETS of Donald Trump, one claim circulating on social media is that some of Joe Biden’s votes look suspicious because they don’t adhere to “Benford’s law “.
So do the claims stack up? In short, no – but the reasons are interesting.
Named after the US physicist Frank Benford, the law relates to frequencies of first digits in large sets of numbers. Benford described the law in a 1938 paper, though it had been observed in 1881 by Canadian astronomer Simon Newcomb.
According to the law, in many big, natural datasets far more numbers begin with a 1 than any other digit. Numbers start with 1 for roughly 30% of the data, followed by the digit 2 for 17.6% of the data, whereas 9 is the leading digit just 5% of the time. Remarkably, this rule applies to everything from distributions of river lengths and volcano sizes to molecular weights.
Small digits galore: a Benford distribution of first digits in a large natural dataset
Benford’s curve is also observed in human systems. Take a huge random sample of streets of varying sizes, for example, and you’d expect more addresses starting with 1 than those starting with 9. It can even shed light on financial fraud: with a legitimate tax return you might expect profit and expense totals to approximate a Benford curve, but if the books have been cooked, you might see more figures rounded off to 0 or 5.
But back to elections. Data scientists have previously analysed vote tallies from elections in Iran, Ukraine and elsewhere – examining the first, second and final digits of vote tallies. However, the efficacy of using Benford’s law to identify electoral fraud is contentious, with one 2011 study concluding that finding meaningful patterns is like “seeing cats, dogs, and crows in clouds”.
One proponent of “Benfordizing” election results is Walter Mebane, a political scientist from the University of Michigan, but he sees no signs of foul play in the recent US election.
In the latest episode of the Radiolab podcast, Mebane explains why the US electoral vote counts don’t follow the law. Essentially it is because the US has a two-party political system and voter precincts are drawn up to be roughly the same size within a given district. If precincts register 1000 votes, for example, and are split roughly evenly between Trump and Biden, you’d expect most tallies to start with 4s, 5s and 6s, not 1s and 2s.
In a working paper published on 10 November, Mebane looks deeper at the US election data using a 2BL test, based on the second digits and Benford’s law digit probabilities, along with other statistical tools.
The bottom line: there are no signs of irregularity in the officially declared precinct vote counts data from Fulton County, GA, Allegheny County, PA, Milwaukee, WI, and Chicago, IL, as some have claimed.
You can find more information in this blog post by data scientist Jenifer Golbeck and this video by mathematician and author Matt Parker.
Researchers have shown that certain metal-organic materials can act as permanent magnets at temperatures of up to 242 °C, while remaining magnetized in external magnetic fields as strong as 7500 oersteds – 25 times higher than other “molecular magnets” reported previously. Both values are comparable to various purely inorganic magnets available on the market today, suggesting a range of possible applications for magnets made from these lightweight and abundant materials.
Room-temperature magnets are usually made from pure metals, metal oxides or intermetallic compounds. Despite their ubiquity – they are crucial components of data processing and storage devices, electrical motors, renewable energy technologies and more – they suffer from several drawbacks. They are heavy, require a lot of energy to fabricate, and are made from raw materials that are sometimes difficult to source – especially for widely used rare-earth-based magnets like NdFeB and SmCo.
Promising alternatives
Magnets made from molecular building blocks such as organic ligands and paramagnetic metal ions are promising alternatives to purely inorganic magnets. As well as having similar magnetic behaviour to that of traditional magnets, their properties can be precisely tailored and optimized post-synthesis thanks to the flexibility of molecular and coordination chemistry, says team leader Rodolphe Clérac of the University of Bordeaux and CNRS in France. Indeed, researchers have already made magnetic structures that have no inorganic equivalent, including single-molecule magnets, single-chain magnets and 2D/3D networks with magnetically ordered phases.
Another advantage of molecule-based magnets is that they have very low densities (around 1 g/cm3) compared to their purely inorganic counterparts, which typically have densities over 5 g/cm3. This makes them attractive for emerging technologies such as magnetoelectronics, magnetic sensing and data storage, even though their maximum energy product – a measure of a magnet’s strength – is much lower. The snag is that most molecule-based magnets made to date can only operate at relatively low temperatures, which prevents them from being used more widely.
Increasing operating temperatures
To increase the operating temperatures of these magnets, researchers have tried linking radicals (that is, species containing at least one unpaired electron), to metal ions in 2D or 3D co-ordination networks. The strong magnetic coupling between the free electron spins of the radicals and the metal ion spins produce magnetically ordered phases that boast critical temperatures (that is, the temperature above which a material’s intrinsic magnetic moments cease to be regularly ordered) as high as 400 K in some cases. There is a price to pay, however, in that the room-temperature coercivity (a measure of the magnetic field needed to reduce the magnetization of a ferromagnetic material to zero) of these materials is low – on the order of hundreds of oersteds at best.
Clérac and colleagues have now used co-ordination chemistry – the combination of metals and ligands at the molecular level – to make a lightweight magnet with an ordering temperature of up to 242 °C and a 7500 oersted coercivity at room temperature. The magnet is made by the chemical reduction of pre-assembled co-ordination networks consisting of metal ions of chromium (an abundant metal) and inexpensive organic molecules known as pyrazines (pyz).
Enhanced magnetic interactions
Clérac and colleagues used lithium 1,2–dihydroacenaphthylenide in tetrahydrofuran to reduce two 2D co-ordination networks, CrCl2(pyz)2 and Cr(OSO2CH3)2(pyz)2. This strategy enhances magnetic interactions between the Cr ions and pyz molecules. Although structurally similar, these two materials have very different physical properties: while Cr(OSO2CH3)2(pyz)2 is antiferromagnetic below 10 K and an insulator, CrCl2(pyz)2 is ferrimagnetic below 55 K and an electrical conductor even at room temperature.
The researchers say that their molecule-based metal-organic magnets compare well with traditional inorganic magnets, while also boasting better magnetic properties and a higher critical temperature than previous molecule-based magnets. “The post-synthetic chemical reduction of co-ordination networks we have demonstrated is a general, simple and effective approach that could allow for the preparation of a new generation of high-temperature lightweight magnets with yet unrealized applications in emergent technologies,” Clérac says.
In the future, members of the team (which also includes researchers from the European Synchrotron Radiation Facility, ESRF), say they plan to develop a completely new family of molecule-based magnets with adjustable properties. “We also hope to find novel materials that could combine magnetic properties with high electrical conductivity at room temperature,” Clérac tells Physics World.
“Focused ultrasound is a rapidly expanding field – and if you look at research publications covering focused ultrasound in the brain, there’s an exponential rise in interest,” said Nir Lipsman, a neurosurgeon at Sunnybrook Health Sciences Centre. “A lot of this is due to the myriad ways in which focused ultrasound can interact with the brain.”
Lipsman was speaking at last week’s 7th International Symposium on Focused Ultrasound, where he described some recent advances in treating brain tumours with focused ultrasound (FUS). While ultrasound may be familiar to most for its use in medical imaging, FUS – which works by focusing multiple ultrasound beams onto targets inside the body – also serves as a therapeutic modality, exploiting a wide range of mechanisms to achieve different physiological effects.
In the brain, high-frequency FUS can be used to generate discrete targeted lesions, while low-frequency FUS, in combination with injected microbubbles, can facilitate drug delivery across the blood–brain barrier (BBB). Other potential applications include transient modulation of brain circuitry, targeted uncaging of nanodroplets for drug delivery and hyperthermia to enhance radiotherapy.
Tackling the BBB
Many of these applications could play a significant role in treating brain tumours, such as glioblastoma multiforme (GBM), the most common malignant brain cancer, which has limited treatment options. One big obstacle is it’s currently not possible to deliver targeted therapies across the BBB and into the brain. Various technologies have been investigated to breach the BBB, but all have cost limitations or associated risks.
FUS, on the other hand, provides a safe and precise way to bypass the BBB. Following injection of microbubbles, FUS is targeted through the skull onto a discrete part of the brain. Microbubbles exposed to ultrasound energy oscillate and physically pull apart cells in the BBB, allowing any circulating drugs to pass through, in concentrations that could ordinarily not be achieved.
“This is a mechanical process that opens up the BBB and does so reversibly,” said Lipsman. “This could be a window that allows us to gain access and deliver therapies.”
Lipsman was part of the team that performed the first human BBB opening in 2015, publishing the results of this first-in-human study last year. The team treated five patients with malignant brain tumours using MR-guided FUS alongside chemotherapy, demonstrating that the approach was safe and feasible.
Since this phase I trial, the technology has evolved to allow treatment of much larger volumes, while upgraded software allows real-time visualization of BBB opening. The team is now running a phase II trial in patients with GBM, to define the safety profile of BBB opening in larger volumes. Lipsman noted that the procedure, which takes 2–3 hr, is performed with the patient awake and usually as a day procedure. “This is the first time that non-invasive day surgery of BBB opening to enhance the delivery of chemotherapy has been performed in large volumes,” he said.
Nir Lipsman with a participant in the trial of BBB opening in patients with GBM. (Courtesy: Kevin Van Paassen, Sunnybrook Health Sciences Centre)
The team is also investigating the use of FUS in patients with metastatic tumours to the brain. In breast cancer, for example, Herceptin can effectively treat breast metastases everywhere in the body, except the brain. The problem is that Herceptin is several orders of magnitude too large to get through the BBB and into the brain.
To address this, the team is now running an ongoing trial of FUS-enhanced Herceptin delivery in breast metastases to the brain. Results from the first two patients show that FUS can safely and reversibly open the BBB, and in highly sensitive regions of the brain. The next steps include radiolabelling the drugs to visualize their passage into the brain.
Liquid biopsy
FUS can also contribute to neuro-oncology in other ways, such as providing an improved way to determine whether a suspicious brain lesion is a tumour, and whether it requires surgical resection or could be treated using chemotherapy or radiotherapy. Currently, this diagnosis requires a brain biopsy, in itself a risky procedure.
Instead, the idea is to use FUS to open the BBB and enable a “liquid biopsy” based on a blood test. Opening the BBB results in increased detection of cell-free DNA – which can be tested to ascertain whether a lesion is cancerous. And the greater volume of BBB opened, the more cell-free DNA is detected in the blood.
“A means of opening up the BBB and potentially allowing us to peripherally detect what a lesion is by taking a blood test could lead to a more promising way of making a clinical diagnosis,” said Lipsman. “This will mitigate the need for open neurosurgery exclusively for the purpose of biopsy.”
Lipsman concluded that the many applications of FUS to oncology are arguably its most exciting. “We’re interested in developing this treatment, as are many centres around the world, for primary brain tumours and metastatic brain tumours. And we’re very interested in working with paediatric colleagues and developing applications for children,” he said. “The goal over the next five to 10 years is to run as many trials as we can. This is arguably one of the most exciting and most promising areas of clinical neurosciences.”
Researchers have found a way to grow layers of two-dimensional (2D) materials with predictable interlayer twists, dispensing with the need to stack and twist separately-grown layers by hand. The new technique uses curved growth surfaces and could provide a significant boost to the field of “twistronics” – a new approach to tuning the electronic properties of materials and engineering future devices.
In recent years, physicists and materials scientists have explored ways of using the weak (van der Waals) coupling between stacked, atomically-thick layers of material to manipulate the material’s properties. The most famous example is graphene, a 2D sheet of carbon atoms. Graphene does not normally have an electronic band gap, but it can be made to develop one when placed on top of hexagonal boron nitride (hBN), a 2D material with a similar lattice constant. If these stacked layers of graphene and hBN are twisted, however, the angle between the graphene and hBN lattices increases, reducing the van der Waals coupling and causing the band gap to disappear. In this fashion, graphene can be made to act like a metal or a semiconductor simply by varying the angle between layers. What is more, placing two layers of graphene together and rotating them at a “magic” angle of 1.1° relative to each other transforms metallic graphene into a superconductor.
This emerging science of “twistronics” thus offers a way of controlling a material’s electronic properties that does not require changing its chemical make-up, as conventional techniques (such as doping) do. But there is a problem: creating these stacks of materials usually requires researchers to exfoliate or synthesize layers of 2D materials separately before stacking and twisting them – a painstaking manual process.
Curved surfaces
A team led by Song Jin of the University of Wisconsin–Madison in the US has now overcome this challenge by exploiting non-Euclidean (curved) surfaces and a type of crystal imperfection called a screw dislocation to grow twisted 2D crystals. In 2D materials, these screw dislocations cause a spiral structure to form in which all the layers throughout the stack are connected and the orientation of every layer is aligned – sort of like a ramp in a multi-storey car park, Jin explains.
In their experiments, the Wisconsin researchers placed nanoparticles of silicon oxide under the centres of their spirals of 2D materials. This nanoparticle disrupts the previously flat surface, creating a curved foundation for their 2D crystal (made, in this case, from the transition metal dichalcogenides tungsten disulphide or tungsten diselenide) to grow on.
In this situation, instead of an aligned spiral in which the edge of each layer lies parallel to the previous layer, the researchers found that the 2D crystal forms a multilayer spiral that continually twists in a predictable way from one layer to the next. The angle of the interlayer twist stems from a mismatch between the flat 2D crystals and the curved surfaces they grow on, they explain, and two different types of spiral are possible. When a spiral structure grows directly over the nanoparticle, it creates a pattern that lead author Yuzhou Zhao dubs a “fastened spiral”. A structure grown over an off-centre nanoparticle, in contrast, is termed an “unfastened spiral”.
Models and measurements
To explain this behaviour, Zhao developed a mathematical model that predicts the twist angles of spirals based on the geometry of the curved surfaces. These modelled shapes generally agree with the structures he grew in the laboratory. In addition, electron microscopy measurements by Chengyu Zhang and Paul Voyles, also at Wisconsin, show that lattices of atoms on neighbouring twisted layers form an overlapping interference (moiré) pattern – as expected.
While the researchers used tungsten disulphide and tungsten diselenide to show how twisted material could be grown, they note that the concept of twisting spirals could be extended to other 2D materials. “We can now follow a rational model rooted in mathematics to create a stack of 2D layers with a controllable twist angle between every layer,” Zhao says. “Being able to directly synthesize twisting 2D materials in this way will allows us to study novel quantum physics in these materials.”
Full details of the research are reported in Science.
Vacuum technology is a ubiquitous presence in all manner of fundamental and applied physics research endeavours – from synchrotron light sources to semiconductor fabrication, from electron microscopy to quantum computing, and plenty more besides. Yet as any vacuum end-user knows all too well, cutting-edge research projects rarely stick to the script. As such, the process of designing, developing and manufacturing a turnkey vacuum chamber to support a diverse set of scientific applications is tricky at best and, when extrapolated over an extended timeframe, vanishingly hard to get right – not least when it comes to matching the chamber’s technical specifications versus research priorities that are, by definition, always fluid and always evolving.
Now, however, the Czech vacuum equipment manufacturer STREICHER Pilsen appears to have come up with an elegant and potentially game-changing concept that has the potential to both simplify and future-proof the design of scientific vacuum systems. In short: STREICHER Pilsen’s S-Cube modular vacuum chamber system offers researchers a controlled vacuum space where modification is the norm rather than the exception – an “evolve-as-you-grow” technology model that offers built-in flexibility, scalability and, as a result, many more design degrees of freedom versus conventional vacuum equipment. There’s an environmental upside as well, with the modular S-Cube approach enabling vacuum users to extend the lifetime of their vacuum chambers (rather than making cyclical investments in new hardware as research priorities change).
“Our S-Cube modular vacuum concept enables modification of the existing vacuum chamber by replacing only some of the walls or by adding another extension module,” explains Jiří Lopata, CEO of STREICHER Pilsen. “There is no need to manufacture a new chamber as the scientists’ research requirements change – a feature that ultimately translates into significantly lower capital outlay over time. Put simply: if you can adapt your existing vacuum system, you don’t need to invest in a new one.”
The new rules of vacuum
Near term, the priority for Lopata and colleagues is to raise awareness of the S-Cube product concept within STREICHER Pilsen’s established customer base – far from straightforward in the grips of a global pandemic. For context, the company has been manufacturing vacuum equipment and components for more than 25 years, serving OEMs and technology start-ups across a range of industries – chemical and pharmaceutical, semiconductor, food processing, heat treatment and steel manufacturing. Big science is another core market segment, with activities to date spanning high-profile projects for the Extreme Light Infrastructure (ELI) beamlines in central Europe; the Joint Institute for Nuclear Research (Dubna, Russia); and the Fusion for Energy project (which coordinates the European Union’s contribution to the ITER experimental fusion reactor in France).
According to Lopata, this diverse customer base – spanning industry and R&D – provides the foundation for STREICHER Pilsen’s core competency. “Our design and engineering teams have first-hand experience and deep domain knowledge about the production of custom vacuum equipment,” he explains. “S-Cube is a logical progression of that collective know-how. For us, it’s all about supporting the complex applications and ever-changing vacuum requirements of our customers, whether they’re in a research or an industry setting.”
Make it easy S-Cube users are able to design their own complex vacuum system by connecting a mix of basic building blocks comprising cubic, hexagonal and cuboid chamber modules. (Courtesy: STREICHER Pilsen)
For scientific end-users of vacuum systems, what’s particularly attractive about the S-Cube concept is its combination of simplicity, modularity and ease of customization – features that would be otherwise unthinkable with a conventional welded vacuum chamber. In effect, S-Cube users are able to design their own complex vacuum system by connecting – via standardized interfaces – a mix of basic building blocks comprising cubic, hexagonal and cuboid vacuum chamber modules (all available in a range of custom sizes, with wall lengths from 450 mm up to 2 m).
Yet while flexibility and customer choice are a given, there’s also a broader emphasis on system-level S-Cube solutions (see “Two heads are better than one”, below). “We always support our customers –not only with the design and production of the vacuum chamber, but with the entire vacuum system,” says Emil Černy, head of engineering at STREICHER Pilsen. “That means expert guidance regarding the optimum choice of vacuum pumps, valves, electrical feedthroughs and vacuum gauges, as well as easy-to-use systems for vacuum process control and data visualization.”
Customers also have a range of more granular options in terms of S-Cube functionality – for example, direct integration of an optical table on independent supports; specification of aluminium chamber walls within a stainless-steel system frame; as well as polished or sand-blasted internal surfaces. For now, all S-Cube chambers are compatible with low, medium and high-vacuum applications, though a planned option for metal sealing will ensure future support for ultrahigh-vacuum (UHV) systems.
Your vacuum, your way
When it comes to market-facing activity, it’s clear that S-Cube innovation also extends to the online presentation of the product offering. A case in point is the Configurator, a dedicated website that allows customers to design and optimize their own S-Cube vacuum chamber via an intuitive and easy-to-use interface. “Think collaborative product design,” says Černy. “The Configurator is our shop window – a great way for new customers to get to know the basic features of the S-Cube system.”
With the help of the online tools, for example, the user is able to equip removable chamber walls with a range of built-in accessories (hinges, flanges, locks and viewports), integrate an optical table, as well as define key pressure and temperature cycles. What’s more, all system variants are displayed immediately in a preview that can be exported to PDF for proofing and discussion with the STREICHER Pilsen design team.
“Ultimately, our goal is to make the interface with S-Cube technology as easy to use and as accessible as possible,” concludes Černy. “The Configurator puts the design capability in the hands of the customer, while our in-house technical team provides the specialist support to move them rapidly from a v1.0 specification to a finalized S-Cube chamber configuration.”
To fast-track the commercial roll-out of the S-Cube product portfolio, STREICHER Pilsen has teamed up with fellow vacuum manufacturer Edwards in what, the vendors hope, will be a win-win for vacuum customers in research and industry.
The formal collaboration kicked off earlier this year and sees STREICHER Pilsen offering a range of Edwards’ technologies – pumps, sensors, leak detectors and control systems – to end-users interested in purchasing not just a vacuum chamber, but a turnkey S-Cube vacuum system. This approach to the market will open up wider commercial opportunities for STREICHER Pilsen, while Edwards stands to gain new sales channels for its diverse product portfolio as well as its after-sales support and field-service teams.
“Our two companies have complementary rather than competing technology portfolios,” explains Radim Hlavaty, Edwards’ regional account manager (Czech Republic and Slovakia). “It’s also a logical collaboration in terms of proximity, given that Edwards has a large manufacturing facility in the Czech Republic.”
Notwithstanding the commercial drivers, there are other significant upsides to consider, notes Emil Černy, head of engineering at STREICHER Pilsen. “We can see real synergies for our respective design teams and application engineers,” he explains. “On our side, we’re benefiting directly from Edwards’ specialist capabilities in areas like vacuum simulation and system-level optimization, as well as gaining insights from their product development roadmaps for next-generation vacuum pumps.”
Once upon a time, air travel was risky, costly and highly polluting. But these days modern airliners are the safest way to travel long distances, as well as being cheaper, quieter and more fuel-efficient than their predecessors. If you need to cross the US, a seat on an Airbus will get you there on less than half the fuel of an average American car – and save you three days.
Unfortunately, this efficiency is a double-edged sword, encouraging more and more people to fly or send cargo by plane. Aviation’s contribution to the world’s CO2 emissions has risen from zero to 2% within the last century despite the fact that the litres of fuel burned per passenger-kilometre has plummeted – from 10.3 l/100 passenger-km for the De Havilland Comet 4 in 1958, when commercial flight with jet engine aircraft was new, to 2.42 l/100 passenger-km with today’s Airbus A330neo-900. Furthermore, with around 107,000 commercial flights taking place every day prior to the COVID-19 pandemic, noise pollution from airports is a huge concern.
Aviation’s contribution to the world’s CO2 emissions has risen from zero to 2% within the last century despite the fact that the fuel burned per passenger-kilometre has plummeted
Green solutions
So, what can be done to make aviation greener?Well let’s consider Boeing’s 787 Dreamliner – a modern airliner that looks much like its noisier, less-efficient ancestor, the 707. Each has a cylindrical fuselage with a square-root-of-x nose profile, wings swept back about 35°, and jet engines sticking forward from under the wings. This shape is set by the laws of mechanics and by compressible fluid flow.
Look closer, however, and differences emerge. The Dreamliner’s engines are fatter in front, so they have a high “bypass ratio” – they put less kinetic energy into the airstream for the same amount of thrust. The engine exhaust nozzles are scalloped with a chevron shape, which creates vortices in the exhaust fumes. When high-speed exhaust mixes with the slower-moving air, the vortices make less noise than the chaotic mixing of earlier engines.
More subtly, the leading edge of the 707’s aluminium wing is straight but the Dreamliner’s is curved, which reduces fuel-wasting drag. Although the shape is too complex to be affordably built with aluminium, it is made possible thanks to a composite material – carbon-fibre-reinforced plastic (CFRP). Besides allowing aircraft components such as the wings, tail and fuselage to have a wide variety of shapes, CFRP is stiffer and has a higher strength-to-weight ratio than aluminium or steel. These factors make the aeroplane lighter, so less fuel is needed to get it off the ground and keep it airborne.
One challenge of CFRP is in the factory, where it starts as a flexible fabric or tape infused with uncured epoxy resin. This is laid on moulds by hand or by automated tape-laying machines. To make it rigid, it then has to be “cured” or baked in an autoclave at temperatures of up to 180 °C, depending on the blend of CFRP. The problem is that ensuring that every region of the part stays at exactly the right temperature is tricky, calling for careful process control and sometimes for instruments embedded within the fabric. To verify that every ply of material (there can be as many as 100 layers) lies smoothly on the one below it with no wrinkles, voids or delaminations, aircraft manufacturers use inspection methods, such as machine-vision, for automatic checks while the tape is being applied, and X-ray and ultrasound to verify each part before it’s built into a larger structure.
When lightning strikes
A big issue for CFRP is that it has a very high resistance, which is a problem when a typical commercial airliner is struck by lightning at least once a year. A bolt can carry as much as 200 kA of electrical current, which enters at one location, such as a wingtip, and exits at another, like the tail. Aeroplanes that have skin and sub-structure of mostly aluminium easily handle the current and the heat it produces at the point of attachment: engineers only need to include good conductive paths between components, skins thick enough to prevent melt-through, and internal sealant over gaps or joints to prevent arcs and sparks.
Part of the problem is that CFRP conducts electricity anisotropically. Current flows fairly easily along the fibres, which are 2000 times less conductive than copper but 30 times more conductive than silicon. However, current flows poorly between the fibres or between ply layers. Electrical potential from a lightning strike may therefore arc through the air to get around non-conductive zones, or the current may heat fibres and create a spark of vaporized material – either of which could cause a fire in a fuel tank.
Governments and manufacturers have therefore developed ways to avoid arcs and sparks. Metal foils, for example, are used on the outside of a plane to guide lightning current and protect composite skins. Other techniques include using dielectric material to isolate current and direct it away from sensitive components, and minimizing resistance in fastener installations. There are also laboratory experiments and software that can simulate lightning current, allowing us to get a better theoretical and experimental understanding of lightning attachment, arc conditions and ignition processes. Engineers can then use this knowledge to design aircraft with large margins of safety above the strongest lightning strike. These efforts have paid off. Aircraft built with CFRP wings and fuselages are routinely struck by lightning, just like their aluminium siblings, but none has had an ignition. The challenge now is to reduce the weight of these solutions so fuel economy gets even better.
While heating and arcing are direct effects of a lightning strike, it can also cause indirect effects by inducing voltage, current or force in components where the lightning current doesn’t reach. For example, a fast-changing lightning current can induce a voltage that flips a bit in nearby electronics, or it can induce eddy-current forces so strong they bend or break some components. Designers use modelling and testing to verify that indirect effects cause no problems, or that counter-measures such as shunt diodes or shielded cables keep a current spike from reaching a sensitive component.
Sweeping wings
On an aeroplane, the wings’ job is to lift the craft against gravity. It does this by deflecting air downward, which imparts downward momentum to the air and an equal upward momentum to the wings. The structure inside the wing transfers the force to the fuselage (the body of the plane) thereby allowing the aircraft to fly.
(a) Early planes featured straight, thick wings – indeed, some modern small aircraft still do. While these are structurally efficient, requiring little mass to safely transfer lifting force to the body, a tapered wing is even better. The structure is lighter than for straight wings because most lift is produced by the thick region near the fuselage, meaning it only has to transfer that lift a short distance.
A plane can also be made more stable if there is slight sweep back on the leading edge. For a plane flying straight, the wings have equal cross-section to the airstream. When the craft twists to the right, perhaps from a gust of turbulence, a swept left-wing presents a larger cross-section to the wind than the right wing. This helps the aeroplane quickly straighten out after small disturbances.
For planes with jet engines, the wings are swept back even further than for their slower, non-jet-engine counterparts – in some cases they can even be angled at more than 40°.
Jet engines allow planes to fly near the speed of sound, at transonic speeds. Air flowing over curved or thick parts of the plane must move even faster than the speed the plane is travelling, often meaning it reaches supersonic speeds (faster than the speed of sound). So when that supersonic airflow moving over those thicker parts of the wing collides with slower moving air, it creates shockwaves. A shock irreversibly transforms the air’s kinetic energy into heat and that loss of energy creates lots of drag.
The engineers who built early jets solved the problem by sweeping the wings. (b) With a straight wing’s profile, in the thick region, airflow is supersonic. (c) With a swept wing, the air follows a gentler path. Its arrival at the thick part of the wing is delayed by a factor of 1/cos(θ), where θ is the sweep angle. Air farther from the wing has more time to shift out of the way, so air near the wing has a wider “channel”. It needn’t go supersonic to get around the wing.
The next generation
Though the shape of airliners has changed little over the last 60 years, the cantilevered swept-wing shape may, over the next decade or two, finally give way to a new design thanks to NASA and companies like Boeing, which are developing the “transonic truss-braced wing” that will exhibit little or no sweep.
Conventional truss-braced wings with no sweeping V-shape are an old idea and the design is still popular for small aeroplanes. In fact, the world’s best-selling aircraft, the four-seat Cessna 172, has a straight truss-braced wing. Until now, however, no jet airliner has used an unswept wing, truss-braced or not, because it produces too much drag at transonic speeds, those that are near or at the speed of sound (see box above). Airliners usually travel at Mach 0.8–0.85, where the Mach number is the fraction of the speed of sound in the local atmospheric condition (343 m/s in dry air at 20 °C) – typically the actual speed is 246–271 m/s (885–974 km/h) depending on air temperature.
New, thin airfoils made possible by CFRP, plus structural bracing by thin CFRP trusses, finally allow for wings with little or no sweep that can travel at these speeds without producing too much drag. The advantages of an unswept wing include the fact that it is structurally more efficient than a swept-back wing. To have a given wingspan, a swept wing must be longer than an unswept one. The wing’s lift then acts with a longer moment arm, which increases the bending moment on the wing’s structure. To resist this, the structure of the swept wing must be stronger, and therefore heavier, than an unswept one.
The external truss also reduces stress on the wing’s internal structure so the wing can be thinner top-to-bottom, giving it a small frontal cross-section and thereby reducing drag. To cut drag more, the wing is narrower front-to-back than a traditional wing, which lets laminar flow cover a larger fraction of the wing surface. The wing is also longer relative to its front-to-back width, or “chord”, and this higher aspect ratio reduces the energy lost when high-pressure air from below the wing flows around the wingtip to the low-pressure region above the wing. An upward-folding tip allows this long-winged aircraft to fit into the small gates of regional airports.
Electric aeroplanes
Another way to make planes greener is to develop hybrid propulsion, rather like we now have hybrid cars. The idea is that jet turbines would boost power for take-off while batteries or fuel cells would provide steady power for cruise. Of course, it would be even better if aeroplanes could fly purely on renewable energy (such as solar or hydro) or nuclear power, which emit no carbon.
Alas, an all-electric airliner to replace today’s planes is not on the cards. Batteries have a specific energy (measured in joules per kilogram) that is orders of magnitude lower than the specific energy of hydrocarbon fuel. This figure will improve, but not enough. That’s because when weight matters, as it does in aeroplanes, batteries will always suffer from the fact that they have to carry their own oxidizer, whereas fuel-burning engines use oxygen from air. Therefore, a battery-powered aeroplane or helicopter can only work over tens or perhaps hundreds of miles, not over the thousands of miles where an aeroplane’s speed makes it most useful.
A battery-powered aeroplane or helicopter can only work over tens or perhaps hundreds of miles, not over the thousands of miles where an aeroplane’s speed makes it most useful
There are a few prospects on the horizon for getting around the fundamental limits of batteries. One option is to use lasers or microwaves to transmit energy from the ground to aircraft in flight. The beam of photons creates voltage and current in dipole antenna elements or photovoltaic arrays on the aircraft, with diodes siphoning off the power to run motors or electronics. This has been done at a small scale. One of the best demonstrations was by the SHARP project in Canada back in 1987 using microwaves, but more recent efforts include Lockheed Martin and LaserMotive (now PowerLight) flying an unmanned aerial vehicle (UAV) using laser beams. However, the method faces regulatory and cost challenges before it can scale up to commercial flight.
As for solar-powered aircraft, they do already exist, but because the sunlight is so weak, these fragile aircraft fly slowly and with little payload. A third option is to recharge aircraft in flight, just as aerial tankers transfer fuel to military aircraft today. It’s plausible for long-range all-electric flight, but technical challenges remain.
From super to hypersonic airliners
Another development that we could see over the next two or three decades is hypersonic travel, which means craft flying at Mach 5 (6126 km/h) or faster. This kind of speed could get you from Los Angeles to Beijing in no more than two hours. Supersonic airliners – the Concorde and the Tupolev 144 – only flew at half that speed and gave off huge amounts of CO2. They were also horribly noisy at take-off and landing, and emitted sonic booms during cruise. Both are now out of operation.
Hypersonic airliners will do better. These new aircraft will likely use hydrogen – produced by electrolysis using non-fossil energy sources – or low-carbon fuel like methane. With such light molecules, the fuel and air can rapidly mix, which is vital because air moves through the engine so fast it spends only a few milliseconds in the combustion zone.
The future of flight Concept images of a future commercial airliner with straight truss-braced wings (left, courtesy: NASA/The Boeing Company) and a low-sonic-boom hypersonic aircraft (right, courtesy: Lockheed Martin).
Sonic boom is less of a problem now, too. NASA is already developing low-boom aircraft designs, which have a long, sharp nose that reduces the area over which air is compressed to form the leading shockwave. They also keep protuberances, like the engine intake, on top of the fuselage so shockwaves produced there go up, not downward to bother people.
What’s more, thanks to a quirk of the atmosphere, most acoustic energy from hypersonic planes will never reach Earth. If you imagine a sonic boom heading out diagonally down from an aircraft, the sound will meet steadily warmer air as it moves. Given that sound and shockwaves travel faster in warm air, the sonic boom will refract – its path will curve and become more horizontal as it moves lower. So apart from shockwaves that are heading almost straight down, the path eventually curves upward, avoiding the ground altogether. And because hypersonic aircraft fly at 27,000 to 30,000 m – more than twice as high as today’s airliners, because higher altitudes mean thinner air and therefore less drag – nearly 100% of the sonic boom refracts away from the Earth.
Despite these benefits, several challenges of hypersonic flight must be overcome. One is the fact that air at the nose and leading edge of a hypersonic plane can get hotter than 1300 K because the air’s kinetic energy becomes heat energy when it runs into the perpendicular surface and stops. At those temperatures, CFRP, which has revolutionized today’s airliners, simply doesn’t hold its strength. Fortunately, titanium alloys and other advanced, lightweight materials – some with honeycomb structures to reduce heat transport – are being developed to solve this problem.
Another issue is ionizing radiation. When a galactic cosmic-ray particle strikes a nucleus in the upper atmosphere, the nucleus shatters into a spray of secondary particles. The trails of these secondaries create conductive paths in semiconductor electronics. Today’s planes fly at altitudes of 9000–13,000 m, where the secondaries are spread out, so any one electronic device sees only a small effect. But for a hypersonic plane flying at 30,000 m, the secondaries will be tightly clumped still, creating a much stronger radiation environment. It wouldn’t be a problem for passengers; a flight will be so short that the total dose would be lower than in today’s flights. But electronics will have trouble. A single chip may be hit by many secondaries at the same time, and such a sudden rise in conductivity can cause errors in a semiconductor chip, or even damage it permanently. Costly solutions do exist for spacecraft, such as physical shielding or electronics made of expensive materials with large band gaps. The challenge is to devise solutions for aircraft that are affordable and lightweight.
Urban aviation
Sky taxi Aircraft like this helicopter developed by Volocopter could be roaming our city skies taxiing people and cargo. (Courtesy: CC BY-SA 4.0 Matti Blume)
With towns and cities suffering from rising congestion on the roads, there is a growing demand for an aircraft that can land vertically in a small space, load one or two people or a van-sized cargo, and fly above the clogged highways to another spot in the same city. That’s where battery-powered helicopters could be the answer. With four or more rotors, these electric “rotorcraft” will soon be carrying cargo, and perhaps passengers, in cities. Their short range – a few dozen miles – is a perfect match for urban flight.
This new family of aircraft has many environmental benefits. Because they fly at about 320 km/h and use uncongested routes, each aircraft can make as many trips per day as, say, 10 cars or trucks. If that happens, it gets 10 vehicles off the roads, making our commute a little easier. Each aircraft would also mean that 10 cars or trucks wouldn’t need to be built, saving energy and materials. Of course, being electric-powered, the aircraft emit no CO2 in cities where electricity comes from non-fossil sources. It also helps keep engine noise tolerable.
Challenges remain, however. For a two-person taxi service, the aircraft would have to be recharged quickly. The 15-minute charging time of today’s batteries would need to be shortened, or ways would have to be found to quickly swap depleted batteries for charged ones. Batteries would go through up to 20 deep discharge cycles per day, so they would reach end-of-life in a few months as the electrochemical process involved in a rechargeable battery isn’t totally reversible. That’s because with each charge–discharge cycle, some atoms from the electrodes or the electrolyte go to the wrong place, eventually weakening the battery and making it potentially develop too much leakage current to hold charge for long. Discarding or refurbishing the batteries at the required rate will therefore be a challenge.
Helicopters are also noisy, as is any vehicle with many small rotors. There are ways to make them quieter, like ducts around the rotors, but these have costs and trade-offs.
Finally, the density of air traffic would be unprecedented; a thousand aircraft simultaneously flying in an area the size of Los Angeles or London will require a new, and probably automated, approach to air traffic control. Computers, not humans, would have to pilot these vehicles and co-ordinate with traffic control.
In essence, the commercial aviation sector is continuing to refine and develop traditional aircraft as it has throughout its 100-year-old history, constantly seeking to reduce costs and lower their impact on the environment. But there are now new directions for the industry too, in the form of super-efficient new airliners that will look quite different, electric-powered rotorcraft that revolutionize urban transport (see box above), and hypersonic aeroplanes that protect the world’s environment while shrinking the distance between its people. Within decades, air travel may look very different to now.
