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An inventory of cosmological mysteries

The Eridanus supervoid — (Courtesy: NASA/JPL-Caltech and Planck US Data Center at IPAC/ESO/Piquito veloz)

Over thousands of years, humans have never tired of pondering the universe and our place in it. But as our knowledge has advanced, the specific questions we ask have changed. What’s Eating the Universe and Other Cosmic Questions by physicist and writer Paul Davies is a whistle-stop tour of the biggest mysteries that cosmologists are investigating today.

Each of the 30 chapters is devoted to a different question. The chapters are only about two to five pages long each, but they are not there to provide answers, nor even to deeply explore any theories. The author briefly explains where each question arose from, and some of the suggested solutions, occasionally opining on whether he thinks any are convincing.

This structure makes for a nice overview of the state of cosmology. After all, science is driven by questions, so summarising the questions that scientists are currently asking is a good way of describing the state of the field. Some chapters look at problems I had already heard of, such as the mysterious fine-tuning of fundamental constants that allows life to exist. Others were new to me – the eponymous chapter details an unexpected void-like cold spot that astronomers have found in the constellation of Eridanus. Speculations follow that our universe might be spontaneously engulfed by a collision with another, or by the quantum vacuum decaying to a lower energy level. If you can bear to contemplate such scary prospects, this book is a fun way of making sure you’re all caught up on where cosmology is at today.

2021 Allen Lane £16.99hb 192pp

Polycrystalline thermoelectric breaks record for heat conversion efficiency

Researchers in Korea and the US have created the most efficient thermoelectric material to date. The material, polycrystalline tin selenide, boasts a heat-to-electricity conversion efficiency of nearly 20%, and could be used in devices that capture waste heat from industrial power plants as well as heat generated by combustion engines in cars, ships and tankers.

Over 65% of the energy produced worldwide is lost as waste heat. Thermoelectric power generators, which are semiconductor-based electronic devices, can turn this heat into electricity via the Seebeck effect. However, to do this well, they need to be made from materials that have an extremely low thermal conductivity while also being good electrical conductors. This is a tricky combination to achieve, especially since the devices may also be exposed to heat sources as hot as 400-500 °C, and thus need to be robust to high temperatures.

Poor mechanical properties

In 2014, a team led by chemist and materials designer Mercouri Kanatzidis at Northwestern University discovered that single-crystal tin selenide (SnSe) was better at converting waste heat to useful electricity than any other known material, with a record-high thermoelectric figure of merit, ZT, of 2.6 at 924 K. Subsequently, another research group found that the ZT of bromine-doped n-type single SnSe crystals was even higher, reaching 2.8 at 773 K. However, the single-crystal version of SnSe is tricky to synthesize and has poor mechanical properties, so it cannot be mass produced.

The polycrystalline form of SnSe, in contrast, is a simple, binary, inexpensive and Earth-abundant material with good mechanical properties. Its conduction properties should also be in its favour: polycrystalline samples are generally understood to have lower lattice thermal conductivities than single crystals thanks to the additional scattering of phonons (vibrations of the crystal lattice) at crystal grain boundaries. Surprisingly, though, other researchers found that the lattice thermal conductivity for polycrystalline SnSe was higher than for the corresponding single-crystal version.

Removing the “skin”

The Northwestern team’s initial measurements of polycrystalline SnSe proved similarly disappointing. However, when the researchers took a closer look at their sample, they found that a thin layer of oxidized tin had formed on its surface. This heat-conducting “skin” is 150 times more thermally conductive than tin selenide itself, so Kanatzidis and In Chung of Seoul National University developed a new synthesis technique to minimize its presence. Their approach involved reducing the tin starting material as well as the tin selenide compound using hydrogen and argon and then annealing the ensemble to high temperatures.

Inexpensive thermoelectric material works at room temperature

The resulting material has a heat-to-electricity conversion efficiency of nearly 20%, with a ZT of 3.1 at a temperature of 783 K. This is far higher than any other bulk thermoelectric system studied to date, which is good news since the commercialization of thermoelectric technology has been seriously limited by low ZTs and the presence of toxic elements like lead (Pb) or rare ones like telluride (Te). Indeed, the ZT of most thermoelectric materials is less than 2, while that of PbTe is around 2.5.

The material in this work, which the team describes in Nature Materials, is a p-type material. Since thermoelectric devices require a pair of p- and n-type materials with similar thermoelectric properties, members of the team say they now hope to develop an n-type counterpart.

Flexible X-ray detector constructed without harmful heavy metals

A wearable, flexible X-ray detector that is constructed without using harmful heavy metals has been developed by researchers from China and the US. The prototype, which is made from metal–organic frameworks layered with gold electrodes and plastic, could provide a safer and more environmentally friendly route to the next generation of radiology devices.

Conventional radiation detectors for X-ray imaging – whether integrated into large, immobile instruments like CT scanners or the small bitewing detectors used by dentists – are typically fashioned into rigid panels. For some applications, however, flexible sensors would be more appropriate, for example in allowing detectors to conform to rounded body parts or to be moulded into the inside of confined spaces. To this end, researchers have turned to so-called metal–organic frameworks – flexible, semiconducting materials that respond to electromagnetic radiation by producing a measurable electric current.

Unfortunately, however, many of these metal–organic frameworks have a drawback that they share with conventional, rigid X-ray detectors: they are partly made from lead, the mining and use of which brings both environmental and health risks.

“Although the encapsulated detectors present a low risk of exposure to patients, there is a significant health risk to the manufacturing and maintenance personnel involved,” explains mechanical engineer Shenqiang Ren of the State University of New York at Buffalo.

In their study, Ren and colleagues produced a lead-free metal–organic framework by mixing a nickel chloride salt with 2,5-diaminobenzene-1,4-dithiol (DABDT) for several hours. The nickel linked the DABDT molecules, resulting in a compound that was more sensitive to 20 keV X-rays (suitable for use in diagnostic imaging applications) than recently reported flexible detectors made from amorphous selenium, Cs₂TeI₆ or gallium(III) trioxide. To make a basic sensor array, the team sandwiched individual pixels of their metal–organic framework between gold electrodes before attaching each to a plastic surface and linking them up with copper wiring. Finally, they coated the whole device in a flexible, silicone-based polymer.

Proof-of-concept

Tests imaging an aluminium letter “H” placed on the detector array confirmed that a significantly lower current output was recorded by the pixels covered by the metal shape than those that were unimpeded. The detector, Ren says, “manifests a high detection sensitivity and low detection limit, demonstrating a proof-of-concept wearable X-ray detector for radiation monitoring and imaging”.

While the researchers’ prototype featured only a 12 x 12 grid – and measured 6 x 6 cm – the design is inherently scalable to both higher resolutions and pixel densities. “Traditional X-ray imagers are rigid and uncomfortable to wear on the body or to place near the examined organ,” Ren explains. “The flexible detector shown in this work could be mounted onto objects with curved geometries and complex surfaces, such as wearable electronics.” Alongside applications for medical imaging, the flexible sensor could also be used for dosimetry measurements and for non-destructive object scanning.

“The need to implement large area, low-power operated and conformable radiation detectors has recently triggered a quest for novel material platforms able to reliably detect ionizing radiation,” comments Beatrice Fraboni, a physicist from the University of Bologna, Italy, who was not involved in the present study. The metal–organic framework developed by Ren and colleagues, she adds, is a “novel and promising candidate that well compares with the other material platforms so far proposed for this task – organic semiconductors and perovskites – and grants a lead-free, and thus sustainable, option”.

Imalka Jayawardena – an engineer from the University of Surrey who was also not involved in the study – agrees, noting that metal–organic frameworks have also shown promise in other areas, including photodetection and energy storage. “This work certainly shows the potential of such material systems for future X-ray detector technologies that should not only have high sensitivity but can also curved to conform to complex geometries,” he says.

With their initial study complete, the researchers are now moving to explore the structural design and controlled sensing performance of a variety of different metal–organic frameworks, alongside improving the resolution of their detector design and exploring the potential for large-scale manufacturing.

The study is described in Nano Letters.

Planning for space travel

For much of the last year and a half, planning international travel, sometimes even national travel, has been for most of us wishful thinking. But Bergita and Urs Ganse, the authors of The Spacefarer’s Handbook, have something even more ambitious on their minds. They are scientist siblings, Urs a theoretical space physicist, and Bergita a space physician, and their book is a thorough guide to space travel for biological beings.

From building resilient spacecraft, to the physics of orbital navigation and atmospheric re-entry, it leaves no stone unturned. The chapters about life in space demonstrate just how much more complex it is to send a person into space than to send a machine, detailing how everything from sleep cycles to our sense of taste are affected by a microgravity environment.

The chapter on space medicine might be off-putting for some would-be space farers, with sections on “puffy face-bird legs syndrome”, “shrinking hearts” and “G-measles”. However, the exploration of the psychological impacts of space travel may be surprisingly relevant for readers right now – astronauts are not the only ones who have found themselves enclosed in a small space with very few other people during the last year.

The handbook was originally written in German but has been translated and published in English this year. The occasional phrasing betrays this, but it does not detract from the book as a fun and informative read. There are many pictures and diagrams, which help to explain both the challenges of these far-reaching voyages, and the ingenuity that might help us to achieve them.

2020 Springer £19.99pb 307pp

The great upheaval in physics – through the eyes of a scientist who was there

If you have studied physics beyond school, you will likely have heard of the Rayleigh–Jeans law – an equation that accurately approximates the spectral radiance of blackbody radiation at long wavelengths. The equation is named after the English physicists John Rayleigh and James Jeans, the latter being the subject of a new biography: Sir James Jeans: Scientist, Philosopher and Musician, written by his son Christopher Jeans (co-authored with Alexandr Kozenko).

This book details Jeans’ extensive research in physics and astronomy, but, as the title suggests, it also explores his family life and his love of music. With its large dimensions and its numerous images, it feels like something of a family photo album – one in which the photos often feature household names like Albert Einstein and Marie Curie. Some pages even feel like a virtual museum, with photographs of intriguing artefacts, such as the chair in which Jeans did all his writing, and a letter from his correspondence with Ernest Rutherford.

In this comprehensive biography, Jeans’ personality comes through, as does the author’s fascination with his eminent father’s life. For those of us without such an immediate connection to the book’s subject, it offers a tangible record of the great upheaval in physics that happened in the last century, through the life of a scientist who was there at the heart of it.

2021 Cosmogonic Press £25pb 294pp

Wooden satellite to launch in 2023, glowing thread is made from wood

Wood is an amazing material. It can be strong, flexible or lightweight – and sometimes all of the above depending upon the tree. Wood can be used on its own, or it can be processed to create materials with an even wider range of desirable properties.

So, it should come as no surprise that scientists in Japan are looking at how wood could be used in space. Previous studies have shown that wood can withstand the extremes of temperature experienced in space – it holds up well from in the -150–150 °C range – and it is not affected significantly by near-vacuum conditions.

Now, Koji Murata at Kyoto University’s Biomaterials Design Lab and colleagues are sending several samples of wood to the International Space Station (ISS). The samples are destined for Japan’s Kibō research module on the ISS, where they will be placed outside in space.

The team is particularly interested in how wood is eroded by collisions with atomic oxygen, which is present in low Earth orbit. The ultimate goal of Murata’s team is to build a wooden satellite called LignoSat, which they hope to launch in 2023.

Enhanced glow

One amazing property of wood that the Japanese team will not be studying is the material’s ability to glow in the dark – at least faintly. Researchers in China and the UK have devised a way of enhancing this phosphorescence by crosslinking lignin molecules from wood with an acrylic polymer. The result is a thread that will glow for about 1 s when exposed by UV light – which is nearly 1000 times longer than the natural glow of wood.

The team is interested in glowing wood because most phosphorescent materials are either toxic or difficult to work with. They used their thread to create luminescent textiles that they say could be used to prevent the counterfeiting of luxury goods.

The team hope that biodegradable phosphorescent materials can be made from lignin, which is an abundant waste product of the paper industry. The team includes Tony James at the UK’s University of Bath and Zhijun Chen at the Engineering Research Center of Advanced Wooden Materials at China’s Northeast Forestry University. They describe their research in Cell Reports: Physical Science.

The physics of rollercoasters

Rollercoasters are so thrilling because our bodies frequently experience changing forces. To a physicist, they also offer a perfect example of classical physics in action, which can be used as a teaching tool. This video offers a brief introduction to rollercoaster mechanics and how technology developments are bringing new opportunities for thrill-seekers. Find out more by reading the article “Twists, turns, thrills and spills: the physics of rollercoasters”, originally published in the August issue of Physics World.

Solar polarization paradox resolved at last

A paradox that puzzled a generation of solar and atomic physicists – and occasionally pitted theories from one field against the other – has been resolved. The paradox concerns the polarization of light at a specific point in the solar spectrum, and previous attempts to explain it required either an extreme reduction in the Sun’s magnetic field (which solar physicists deemed unlikely) or changes to the physics of atom-photon interactions (which atomic physicists did not observe). The latest research, however, indicates that no such adjustments are needed. Instead, careful numerical modelling showed that the discrepancy between solar observations and atomic theory vanishes after complex, previously neglected interactions are taken into account.

Most sunlight is at least partly linearly polarized, and the reasons are generally well-understood. Because atoms in the solar atmosphere absorb more photons from “behind” them (that is, from the direction of the Sun), conservation of angular momentum means that some of their spin sublevels become more populated than others. When the atoms then decay back to their ground state, they emit more photons from these populated sublevels, leading to polarized emissions.

The paradox of the sodium D₁line

The current polarization puzzle originated in 1996, when astrophysicists Jan Stenflo of ETH Zurich in Switzerland and Christoph Keller (then at the US National Solar Observatory in Arizona) used an instrument called ZIMPOL (Zurich Imaging Polarimeter) to study solar radiation at a wavelength of around 589nm. This wavelength corresponds to a perturbation in the Sun’s spectrum caused by sodium atoms in the solar chromosphere absorbing photons and then emitting them from multiple sublevels of an excited state.

While theory predicted that emissions from this so-called sodium D₁ line should not be polarized, Stenflo and Keller’s observations suggested otherwise. “If the line in question were not polarizable, you would see a decrease in polarization with respect to the baseline, because the transition would simply be absorbing [partially polarized] radiation and re-emitting non-polarized radiation,” explains solar physicist Ernest Alsina Ballester of Switzerland’s Università della Svizzera Italiana (USI), now working at the Instituto de Astrofísica de Canarias (IAC) in Tenerife, Spain. “What you get is polarization added.”

Two years later, Egidio Landi Degl’Innocenti of the University of Florence, Italy tried to explain this anomaly by noting that emissions from the D₁ line could be polarized if the ground state involved in this transition was itself polarized. However, Landi Degl’Innocenti readily admitted to a problem with this proposal: it could only be true if the magnetic field in the solar chromosphere was less than 0.01 G. Any higher, and the ground state would be depolarized by the so-called Hanle effect. Unfortunately, such a tiny magnetic field conflicted with observations of other spectral lines (largely by Stenflo and Keller) as well as arguments from plasma physics suggesting that the field must be orders of magnitude stronger. “If the chromosphere is magnetized there was no available explanation for this spectral line to be produced,” summarizes Alsina Ballester. “This became known as the paradox of the sodium D₁ line.”

A search for explanations

Over the next quarter-century, various researchers proposed creative solutions to this paradox. Stenflo and others suspected that the quantum theory of scattering needed modification. “This seemed to be more compelling than the idea that the chromosphere was truly unmagnetized, which would have led to many problems in our current understanding of the solar atmosphere,” Alsina Ballester says. Atomic physicists duly designed laboratory experiments to test this idea using broadband radiation, lasers and other light sources, but nothing conclusive was found.

The first real hint appeared in 2013, when Luca Belluzzi and Javier Trujillo Bueno of the IAC showed that if the radiation anisotropy the sodium atoms perceive varies with wavelength across the tiny absorption bandwidth of the D₁transition, this could produce polarized emission without a polarized ground state. Previously, Alsina Ballester explains, “it had typically been assumed that it was a good approximation to neglect these variations”. The only problem was that at the time, there was no theoretical framework that would have allowed Belluzzi and Trujillo Bueno to incorporate a magnetic field into this model and thereby extract predictions to compare with astronomical observations.

Linear polarization of sunlight — **Close agreement** Observations and theoretical predictions of the linear polarization of sunlight across the solar sodium D1 and D2 spectral lines. (Courtesy: IRSOL – IAC)

In 2017, Véronique Bommier of the Paris Observatory developed such a framework. According to Alsina Ballester, a key aspect of Bommier’s work is that it accounts for frequency correlations between incoming and scattered radiation. Armed with this information, Belluzzi (now at the USI), Trujillo Bueno and Alsina Ballester constructed a detailed prediction of the polarization of the light from the sodium D₁ line. They then made their own observations using ZIMPOL-3 – a third-generation version of Stenflo and Keller’s instrument – and found almost perfect agreement with their predictions.

Scattering theory enters the mainstream

Bommier is impressed with the new research, which appears in Physical Review Letters. “They obtain excellent agreement between theory and observation, and I think that this is, in itself, a kind of proof that this is a polarization profile of the sodium D₁line,” she says. In her view, however, the study’s most important result is that the researchers observed no net polarization when they integrated their results across the entire absorption spectrum of the D₁line (opposite polarization is seen at different wavelengths, producing a sum consistent with zero). This differs from Stenflo and Keller’s original observations, and Bommier argues that the net polarization they saw may have been an observational artefact. “This was the paradox,” she says. “There was a disagreement between quantum mechanics and Stenflo’s original observations.”

The study’s authors declined to comment on this suggestion, saying only that “a careful analysis is still needed”. Stenflo’s response is to point out that the new observations show that the polarized D₁ profile shape varies with spatial location on the Sun, and at some places there is more net polarization than at other places. “The authors selected one particular observed D₁ profile and showed that it could be explained in terms of their theory when accounting for the spectral structure of the pumping radiation in the scattering process,” Stenflo says. “While this is a significant result, they have not yet demonstrated that the theory can provide a consistent explanation of all types of observed D₁ profiles.”

Nevertheless, Stenflo hails the present paper as a step forward for the field. In the past, he notes, the D₁ paradox was seen as a fringe problem. Now that new generations of solar telescopes are making the polarization of solar emissions increasingly observable, Stenflo thinks astronomers are beginning to appreciate that the D₁line provides diagnostic access to complex magnetic fields that govern the dynamics and heating of the solar corona. “The resolution of the two-decade old solar paradox that is provided by the present paper is a major step in moving polarized scattering physics into the mainstream of solar physics,” he concludes.

Molecular engineering of photoinduced charge separation

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A key factor in natural photosynthesis, solar fuels and photovoltaic cells is how to control photoinduced charge separation and recombination in donor-acceptor systems. This talk highlights photoinduced electron transfer in donor-acceptor linked molecules, dye- sensitized solar cells, and bulk heterojunction solar cells for energy and biological applications.

The research initiatives, “Dynamic Exciton: Emerging Science and Innovation,” focus on manipulating charge-transfer states for energy conversion.

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Hiroshi Imahori completed his doctorate in organic chemistry at Kyoto University, Japan. From 1990–1992, he was a post-doctoral fellow at the Salk Institute for Biological Studies, US. In 1992, he became an assistant professor, The Institute of Scientific and Industrial Research (ISIR), Osaka University, Japan, and then moved to the Graduate School of Engineering, Osaka University, as an associate professor. Since 2002, he has been a professor of chemistry, Graduate School of Engineering, Kyoto University. He received the JSPS (Japan Society for the Promotion of Science) Prize (2006), CSJ (Chemical Society of Japan) Award for Creative Work (2006), Osaka Science Prize (2007), NISTEP (National Institute of Science and Technology Policy) Researcher Award (2007) and was named fellow of The Electrochemical Society (2016). His current interests involve solar-energy conversion and organic functional materials.

Electrochemistry and coupled chemo-mechanical phenomena in batteries beyond intercalation

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Lithium (Li) metal has been considered an ideal anode for high-energy rechargeable Li batteries, while Li nucleation and growth at the nanoscale remain mysterious as to achieving reversible stripping and deposition. A few decades of research have been dedicated to this topic.

Breakthroughs in novel electrolytes have been seen in the last few years, where the efficiency of lithium deposition exceeds 99.6%. In this illustration, cryogenic-transmission electron microscopy (Cryo-TEM/Cryo-FIB) was used to reveal the evolving nanostructure of Li deposits at various transient states in the nucleation and growth process, in which a disorder-order phase transition was observed as a function of current density and deposition time. More importantly, complementary techniques such as titration gas chromatography (TGC) reveal significant insights about the phase fraction of solid electrolyte interphases (SEI) and electrochemical deposited Li (EDLi).

While cryo-EM has made significant contributions to enabling lithium metal anodes for batteries, its applications in electrochemical interphases such as those in solid state batteries, beyond lithium batteries, are still in their infancy. Therefore, a few new perspectives will be discussed about how future advanced imaging and spectroscopic techniques can help to accelerate the innovation of novel energy storage materials and architectures.

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Dr Y Shirley Meng received her PhD in Advance Materials for Micro & Nano Systems from the Singapore-MIT Alliance in 2005, after which she worked as a Postdoctoral Research Fellow and Research Scientist at the Massachusetts Institute of Technology. She currently holds the Zable Chair Professor in Energy Technologies and Professor in Materials Science & Nanoengineering at the University of California San Diego. She is the Principal Investigator of the Laboratory for Energy Storage and Conversion research group. Starting in 2005, Dr Meng served as the Founding Director of the Sustainable Power and Energy Center, until she was named Inaugural Director of the Institute for Materials Discovery and Design in 2020. She has received prestigious awards including the 2020 Michael Faraday Medal of the Royal Chemical Society; 2019 International Battery Association Battery IBA Research Award; 2018 and 2019 Blavatnik Awards for Young Scientists Finalist; 2018 American Chemical Society ACS Applied Materials & Interfaces Young Investigator Award; 2017 IUMRS-Singapore Young Scientist Research Award; 2016 ECS C W Tobias Young Investigator Award; and 2011 NSF CAREER Award. Dr Meng is a Fellow of The Electrochemical Society; Editor in Chief of MRS Energy & Sustainability; and co-founder of Unigrid LLC. She is also the author and co-author of more than 210 peer-reviewed journal articles and two book chapters, and holds five issued patents.

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An inventory of cosmological mysteries