A new method for generating high-energy proton beams could one day improve the precision of proton therapy for treating cancer. Developed by an international research collaboration headed up at the National University of Singapore, the technique involves accelerating H₂⁺ ions and then using a novel two-dimensional carbon membrane to split the high-energy ion beam into beams of protons.

One obstacle when accelerating large numbers of protons together is that they all carry the same positive charge and thus naturally repel each other. This so-called space–charge effect makes it difficult to keep the beam tight and focused.

“By accelerating H₂⁺ ions instead of single protons, the particles don’t repel each other as strongly,” says project leader Jiong Lu. “This enables delivery of proton beam currents up to an order of magnitude higher than those from existing cyclotrons.”

Lu explains that a high-current proton beam can deliver more protons in a shorter time, making proton treatments quicker, more precise and targeting tumours more effectively. Such a proton beam could also be employed in FLASH therapy, an emerging treatment that delivers therapeutic radiation at ultrahigh dose rates to reduce normal tissue toxicity while preserving anti-tumour activity.

Industry-compatible fabrication

The key to this technique lies in the choice of an optimal membrane with which to split the H₂⁺ ions. For this task, Lu and colleagues developed a new material – ultraclean monolayer amorphous carbon (UC-MAC). MAC is similar in structure to graphene, but instead of an ordered honeycomb structure of hexagonal rings, it contains a disordered mix of five-, six-, seven and eight-membered carbon rings. This disorder creates angstrom-scale pores in the films, which can be used to split the H₂⁺ ions into protons as they pass through.

Scaling the manufacture of ultrathin MAC films, however, has previously proved challenging, with no industrial synthesis method available. To address this problem, the researchers proposed a new fabrication approach in which the emergence of long-range order in the material is suppressed, not by the conventional approach of low-temperature growth, but by a novel disorder-to-disorder (DTD) strategy.

DTD synthesis uses plasma-enhanced chemical vapor deposition (CVD) to create a MAC film on a copper substrate containing numerous nanoscale crystalline grains. This disordered substrate induces high levels of randomized nucleation in the carbon layer and disrupts long-range order. The approach enabled wafer-scale (8-inch) production of UC-MAC films within just 3 s – an order of magnitude faster than conventional CVD methods.

Disorder creates precision

To assess the ability of UC-MAC to split H₂⁺ ions into protons, the researchers generated a high-energy H₂⁺ nanobeam and focused it onto a freestanding two-dimensional UC-MAC crystal. This resulted in the ion beam splitting to create high-precision proton beams. For comparison they repeated the experiment (with beam current stabilities controlled within 10%) using single-crystal graphene, non-clean MAC with metal impurities and commercial carbon thin films (8 nm).

Measuring double-proton events – in which two proton signals are detected from a single H₂⁺ ion splitting – as an indicator for proton scattering revealed that the UC-MAC membrane produced far fewer unwanted scattered protons than the other films. Ion splitting using UC-MAC resulted in about 47 double-proton events over a 20 s collection time, while the graphene film exhibited roughly twice this number and the non-clean MAC slightly more. The carbon thin film generated around 46 times more scattering events.

The researchers point out that the reduced double-proton events in UC-MAC “demonstrate its superior ability to minimize proton scattering compared with commercial materials”. They note that as well as UC-MAC creating a superior quality proton beam, the technique provides control over the splitting rate, with yields ranging from 88.8 to 296.0 proton events per second per detector.

“Using UC-MAC to split H₂⁺ produces a highly sharpened, high-energy proton beam with minimal scattering and high spatial precision,” says Lu. “This allows more precise targeting in proton therapy – particularly for tumours in delicate or critical organs.”

“Building on our achievement of producing proton beams with greatly reduced scattering, our team is now developing single molecule ion reaction platforms based on two-dimensional amorphous materials using high-energy ion nanobeam systems,” he tells Physics World. “Our goal is to make proton beams for cancer therapy even more precise, more affordable and easier to use in clinical settings.”

The study is reported in Nature Nanotechnology.

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In this work, Al and W are compared as individual dopants as well as co-dopants to arrive to an optimal cathode active material design. The objective is to improve the energy density of the materials without compromising cycle life; a feat which was previously thought unattainable for Ni-rich, Co-free layered oxide materials.

The findings emphasize the importance of understanding the effect of chemical composition and synthesis conditions on the morphology of the material particles. In turn, this morphology plays a determinant role in the cycling performance of the electrode.

In addition to conventional material characterization methods (such as x-ray diffraction, scanning electron microscopy, incremental capacity analysis, etc.), measurements of the particles’ strength were also analyzed to provide better insight on how the material will perform in an expanding-contracting electrode. Mechanical resilience if often overlook when studying and designing cathode materials, however, particularly in materials that are prone to microcracking, this information provides an important piece of the puzzle to understand the degradation mechanisms of the electrode.

This led to the development of a Co-free cathode material which can provide a capacity of 260 mAh/g on the first cycle while retaining 95% capacity after 50 cycles in half cells cycled to 4.3 V. At a lower upper-cutoff voltage of 4.06 V, this material delivers 220 mAh/g with no observable capacity loss after 100 cycles.

Ines Hamam has obtained her PhD in materials engineering (in 2024) and her MSc in physics (in 2020) from the University of Dalhousie under the supervision of world-renowned battery expert Dr Jeff Dahn. She is now a technologist at BMW furthering the world effort of transport electrification.

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Researchers in Japan have directly visualized the formation and evolution of quasiparticles known as excitons in carbon nanotubes for the first time. The work could aid the development of nanotube-based nanoelectronic and nanophotonic devices.

Carbon nanotubes (CNTs) are rolled-up hexagonal lattices of carbon just one atom thick. When exposed to light, they generate excitons, which are bound pairs of negatively-charged electrons and positively-charged “holes”. The behaviour of these excitons governs processes such as light absorption, emission and charge carrier transport that are crucial for CNT-based devices. However, because excitons are confined to extremely small regions in space and exist for only tens of femtoseconds (fs) before annihilating, they are very difficult to observe directly with conventional imaging techniques.

Ultrafast and highly sensitive

In the new work, a team led by Jun Nishida and Takashi Kumagai at the Institute for Molecular Science (IMS)/SOKENDAI, together with colleagues at the University of Tokyo and RIKEN, developed a technique for imaging excitons in CNTs. Known as ultrafast infrared scattering-type scanning near-field optical microscopy (IR s-SNOM), it first illuminates the CNTs with a short visible laser pulse to create excitons and then uses a time-delayed mid-infrared pulse to probe how these excitons behave.

“By scanning a sharp gold-coated atomic force microscope (AFM) tip across the surface and detecting the scattered infrared signal with high sensitivity, we can measure local changes in the optical response of the CNTs with 130-nm spatial resolution and around 150-fs precision,” explains Kumagai. “These changes correspond to where and how excitons are formed and annihilated.”

According to the researchers, the main challenge was to develop a measurement that was ultrafast and highly sensitive while also having a spatial resolution high enough to detect a signal from as few as around 10 excitons. “This required not only technical innovations in the pump-probe scheme in IR s-SNOM, but also a theoretical framework to interpret the near-field response from such small systems,” Kumagai says.

The measurements reveal that local strain and interactions between CNTs (especially in complex, bundled nanotube structures) govern how excitons are created and annihilated. Being able to visualize this behaviour in real time and real space makes the new technique a “powerful platform” for investigating ultrafast quantum dynamics at the nanoscale, Kumagai says. It also has applications in device engineering: “The ability to map where excitons are created and how they move and decay in real devices could lead to better design of CNT-based photonic and electronic systems, such as quantum light sources, photodetectors, or energy-harvesting materials,” Kumagai tells Physics World.

Extending to other low-dimensional systems

Kumagai thinks the team’s approach could be extended to other low-dimensional systems, enabling insights into local dynamics that have previously been inaccessible. Indeed, the researchers now plan to apply their technique to other 1D and 2D materials (such as semiconducting nanowires or transition metal dichalcogenides) and to explore how external stimuli like strain, doping, or electric fields affect local exciton dynamics.

“We are also working on enhancing the spatial resolution and sensitivity further, possibly toward single-exciton detection,” Kumagai says. “Ultimately, we aim to combine this capability with in operando device measurements to directly observe nanoscale exciton behaviour under realistic operating conditions.”

The technique is detailed in Science Advances.

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Researchers in Denmark have produced the acoustic equivalent of a rainbow, creating a structure that spatially decomposes sound into its component frequencies in free space. Developing such a structure had proven difficult due to the complexity required, but the team at Danmarks Tekniske Universitet (DTU) managed it thanks to an advanced structural design technique. The new architecture could be used to make devices tailored to emit or receive certain frequencies of sound.

Optical rainbows occur when white light is split into its different spectral components, for example by passing through dispersive media such as prisms or droplets. Although acoustic rainbows are less well-known, they follow the same principle, being the spatial decomposition of sound in free space where waves oscillating at different frequencies propagate in different directions. They have previously been created in confined media using arrays of resonant structures that “trap” sound at different positions in space depending on their frequency. Examples include waveguides, solid and/or fluid mixtures and devices known as acoustic circulators.

Acoustic spectral decomposition also occurs in several natural structures, including the outer ear structures, or pinnae, of mammals such as bats, cetaceans and primates. Indeed, the pinnae of primates (including humans) have an intricate geometry that generates complex interference phenomena via scattering of sound waves, thereby enabling the animals to localize external sources of sound.

An acoustic scattering structure

While researchers have previously attempted to imitate such biological designs, these efforts were largely unsuccessful. The new work, which was co-led by Rasmus Ellebæk Christiansen and Efren Fernandez-Grande at the DTU, succeeded in part thanks to a new technique known as computational morphogenesis, or topology optimization.

This technique, which the researchers describe in Science Advances, builds on an earlier morphogenetic design framework for tailoring passive acoustic scattering structures with dimensions on the order of a few wavelengths. Using an iterative process, the team spatially redistributed sound-reflecting material in an air background inside a specified region of space. This enabled them to tailor the sound field emitted from the created structure to match a predefined target emission pattern across a specified frequency band, mimicking naturally-occurring “sound shaping” structures.

“Such a technique is possible today thanks to the rapid growth in computational power in recent years that has allowed us to model and synthesize sound on the large scale,” Ellebæk Christiansen explains. When combined with advanced production techniques like additive manufacturing (also known as 3D printing), he adds that the team benefitted from “nearly unlimited design freedom”, with the new technique enabling the design of metamaterials and nonintuitive structures hitherto deemed unrealizable.

“Our approach to designing the structures is to re-formulate the device design problem carefully and meticulously as a mathematical optimization problem and to use topology optimization to solve this problem,” he explains. “In this way, we do not rely on simplified design rules derived from underlying physics models, on design intuition or on prior design experience to come up with our device geometry. Instead, we use rigorous mathematical modelling and simulation coupled with advanced numerical algorithms.”

Towards new and very different structures/geometries

The geometry and topology of the metamaterial the team created has several features reminiscent of structures present in the pinnae that spatio-spectrally decompose sound, Ellebæk Christiansen says. However, he tells Physics World that the technique may also enable them to develop structures/geometries that offer new possibilities never realized in nature.

One option, Fernandez-Grande suggests, would be to design acoustic materials that reflect different frequencies of sound in different ways – for example, by scattering high frequencies diffusely and redirecting low frequencies towards an absorbing surface. “It might also help in the development of acoustic lenses – that is, sound sources (such as loudspeakers) that control how different frequencies are radiated in space,” Fernandez-Grande adds.

In the future, the researchers would like to transition from their current two-dimensional design to one that is fully three-dimensional. “This would offer significantly more design freedom, and acoustic field complexity, which might allow for even better/more elaborate spatio-spectral sound field control,” Ellebæk Christiansen says.

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A team of researchers in Sweden has demonstrated how smart optical metasurfaces can respond far more strongly to incoming light when switched to their conducting states. By fine-tuning the spacing between arrays of nanoantennae on a polymer metasurface, Magnus Jonsson and colleagues at Linköping University were able to generate nonlocal electromagnetic coupling between the antennae – vastly strengthening the metasurface’s optical responses.

Metasurfaces are rapidly emerging as a key component of smart optical devices, which can dynamically manipulate the wavefronts and spectral signals of incoming light. “They work in a way that nanostructures are placed in patterns on a flat surface and become receivers for light,” Jonsson explains. “Each receiver, or antenna, captures the light in a certain way and together these nanostructures allow the light to be controlled as you desire.”

One promising route towards such intelligent metasurfaces is to fabricate their antennae from conducting polymers, such as PEDOT. In such materials, the intrinsic permittivity – which determines how the material responds to electric fields, such as those from incoming light – can be manually switched by altering the oxidation state through a redox reaction. This, in turn, modifies the polymer’s carrier density and mobility, altering the number and behaviour of mobile charge carriers that contribute to its optical properties.

A key measure of how well these materials resonate with light is the “quality factor”, which describes how sharp and long-lived a resonance is. A higher quality factor signifies a stronger, more precise interaction with light, while a lower value indicates weaker and broader responses.

When PEDOT is in its metallic oxidation state, incident light will drive the resonance of surface plasmons: collective oscillations of mobile charges that are confined near the surface of the material. At specific wavelengths, these plasmons can strongly enhance electromagnetic fields – altering properties including the phase, amplitude and spectral composition of the light reflected and transmitted by the metasurface.

Alternatively, when PEDOT is switched to its insulating state, the resulting lack of available charge carriers will significantly suppress surface plasmon formation, leading to diminished optical response.

In principle, this effect offers a useful way to modulate the nanoantennae of smart metasurfaces via redox reactions. So far, however, the surface plasmons generated through this approach have only resonated weakly in response to incident light, and have quickly lost their energy after excitation – even when the polymer is switched to its metallic state. This has made the approach impractical for use in smart, switchable metasurfaces that require strong and coherent plasmonic behaviour.

Jonsson’s team addressed this problem by considering the spacing of PEDOT nanoantennae within periodic arrays. When separated at precisely the right distance, the array generated nonlocal coupling through coherent diffractive interactions – involving the constructive interference of light scattered by each antenna.

As a result, this arrangement supported collective lattice resonances (CLRs) – in which entire arrays of nanoantennae respond collectively and coherently to incident light. This drastically boosted the strength and sharpness of the material’s plasmonic response, boosting its quality factor by up to ten times that of previous conducting polymer nanoantennae. Such high-quality resonances indicate more coherent, longer-lived plasmonic modes.

As before, the researchers could manually switch the nanoantenna array between metallic and insulating states via redox reactions, which reversibly weakened its plasmonic responses as required. This dynamic tuning offers a pathway towards electrically or chemically programmable optical behaviour.

Based on this performance, Jonsson’s team is now confident that this approach could have promising implications for the future of smart optical metasurfaces. “We show that metasurfaces made of conducting polymers seem to be able to provide sufficiently high performance to be relevant for practical applications,” says co-author Dongqing Lin.

For now, the researchers have demonstrated their approach across mid-infrared wavelengths. But with some further tweaks to their fabrication process, allowing for closer spacings between the nanoantennae and smaller antenna sizes, they aim to generate CLRs in the visible spectrum. If achieved, this could open up new opportunities for smart optical metasurfaces in cutting-edge optical applications as wide-ranging as holography, invisibility cloaking and biomedical imaging.

The study is described in Nature Communications.

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