Climate change presents formidable, complex challenges for urban planners and policymakers around the world, from protecting the public from health risks to managing the threats posed by extreme weather events. To address these challenges and offer advice and solutions, Sami G. Al-Ghamdi, founder of the Urban Lab at KAUST, has compiled and edited a book entitled ‘Sustainable Cities in a Changing Climate: Enhancing Urban Resilience’[1].
The book offers a comprehensive guide for urban planners, policymakers and stakeholders, addressing the pressing challenges posed by climate change. Al-Ghamdi aims to provide practical tools and strategies to build resilience against climate threats, such as extreme weather and public health risks.
“Cities are facing unprecedented pressures, including rising temperatures, flash flooding, increasing incidence of vector-borne diseases and sea level rise. These challenges threaten the long-term sustainability of many cities and the well-being of their inhabitants,” says Al-Ghamdi. “By bringing together authors with a diverse range of expertise, this book aims to provide a comprehensive and timely exploration of the critical intersection between urban development, climate change and resilience.”
Al-Ghamdi’s own interdisciplinary research forms the basis for many of the themes touched on in the book.
Supporting the Middle East and beyond
The Middle East is one of the most water scarce regions in the world, and desertification and extreme heat present a significant threat to livelihoods and public health, with elderly and vulnerable people particularly at risk. Efficient and sustainable water management is crucial. So is the need for resilient energy systems — that power air-conditioning, for instance — highlighting the need to build and maintain a disruption-free energy network.
“The Middle East’s reliance on energy-intensive desalinated water is unsustainable,” says Al-Ghamdi. We need to adopt energy-efficient technologies, increase wastewater recycling and raise awareness about the environmental impacts of desalinated water consumption. Our energy networks also need to work seamlessly with all forms of energy generation.”
Effective management of the rapid growth and spread of urban areas is also important globally. Urban planners must “expect the unexpected” and, for example, ensure that built environments can cope with the intensifying storms and heavy rainfall in the MENA region.
“During rainstorms in Saudi Arabia, our coastal cities can act as dams that block water runoff, resulting in severe flash flooding and danger to life,” says Al-Ghamdi. “We need natural solutions for absorbing and diverting water to prevent flooding, hence the need for green-blue-grey infrastructure (GBGI).”
Natural flood management solutions
GBGI is a comprehensive solution for sustainable urban flood risk management. It involves combining green infrastructure such as parks and green roofs, blue infrastructure including rivers and wetlands, and grey infrastructure (stormwater drains, retention basins). By integrating these elements, cities can better manage flood risks while enhancing urban resilience.
“Such systems help regulate urban temperatures, manage stormwater and reduce flood risks. For example, Copenhagen’s Cloudburst Management Plan incorporates GBGI to handle extreme rainfall events,” says Mohammad M. Al-Humaiqani, a postdoc at Urban Lab. “GBGI also improves urban biodiversity, enhances recreational spaces and adapts to a rapidly evolving climate challenge.”
Helping urban communities adapt to climate change is an important strand throughout the book. Al-Ghamdi emphasises the importance of involving everyone in local planning and green initiatives, for example through workshops, education programs and participatory planning sessions in local communities.
Developing sustainable energy systems
Diversifying from fossil fuels to renewables and improving energy efficiency are pertinent goals for many countries. In regions like the Middle East, energy demand is high and climate conditions are harsh, adding extra pressure on the energy system.
“Policymakers should promote the development of decentralized energy systems, such as microgrids, that operate independently during disruptions,” says Al-Ghamdi. “Local microgrids specific to individual cities or even critical facilities like hospitals can maintain power when the wider national grid is under pressure.”
Boosting healthcare resilience
Al-Ghamdi’s book also covers climate-related public health challenges. It highlights the importance of developing local heat action plans, like the one implemented in Ahmedabad in India that includes an early warning system and a public education scheme, and modifying healthcare facilities to cope with heatwaves.
“Integrating climate considerations builds a resilient healthcare sector capable of withstanding multifaceted threats, thus safeguarding communities in the long term,” says Furqan Tahir, a postdoc at Urban Lab. “Healthcare systems need to be proactive and to foster interdisciplinary collaboration and data-driven decision-making processes.”
Another health risk posed by climate change is the rise of vector-borne diseases carried by blood-feeding insects. For example, mosquitoes are expanding their geographical range, bringing several associated diseases, such as malaria and Dengue Fever.
“In Jeddah, Dengue is now a seasonal illness,” says Al-Ghamdi. “To proactively identify future hotspots for vector-borne diseases, we need detailed climate modeling to quantify temperature, rainfall and humidity distributions on daily, monthly and yearly scales. This comprehensive data will underpin developing control and mitigation measures to be incorporated into health policies, ensuring better preparedness and response.”
Positive future planning
Al-Ghamdi is keen that urban planners and other stakeholders take a positive stance on tackling uncertainty, and place robust and rigorous research and practical tools at the heart of urban development.
“Our ambition is to inspire transformative action that ensure our urban areas not only survive but flourish amidst the evolving climate landscape,” concludes Al-Ghamdi. “This work reflects our dedication to fostering resilient, inclusive and adaptive urban communities for a better future.”
Hydrogen is a clean-burning fuel that could help to reduce fossil fuel consumption, but the flammable gas can be challenging to store. KAUST researchers have now calculated that vast amounts of hydrogen could be inexpensively stockpiled in pipes at the bottom of lakes and reservoirs, potentially boosting hydrogen’s role in tackling climate change[1].
Renewable energy sources such as solar and wind are intermittent, so any excess electricity output must be saved to fill gaps in supply. This can be achieved by powering electrolyzers that split water into hydrogen and oxygen. The hydrogen can be kept until it is needed and then fed into fuel cells to regenerate electricity; the only waste product produced is water.
Hydrogen can also be used in a range of industrial processes, offering a key benefit over storing the electricity in batteries. “Hydrogen can decarbonize sectors that electricity and batteries cannot decarbonize, including shipping, aviation, steel making and ammonia production,” says team member Julian Hunt, a research scientist at KAUST.
Relatively small quantities of hydrogen can be kept in pressurized containers, while larger amounts are stored in depleted natural gas reservoirs or underground salt caverns. Yet these sites are not widely available, and this approach can require transporting hydrogen over long distances.
The international team led by KAUST proposes that hydrogen could be stored in lakes and reservoirs close to where the gas is produced and consumed. The gas would be contained in polyethylene pipes filled with gravel to weigh them down. Crucially, the hydrogen would be at the same pressure as the surrounding water, so that as the water gets deeper, the pressure increases and the pipes’ energy storage capacity rises. Pipes at a depth of 200 meters could offer a lifetime storage cost of US$0.17 per kilogram of hydrogen, which Hunt says would be much more economical than using pressurized containers.
As a case study, the team calculated that California’s Oroville Lake, a 210-meter-deep reservoir, could offer a total energy storage capacity of 86 gigawatt-hours — enough to power 8,000 houses for a year.
The team estimates that worldwide there are 1,760 lakes and 3,403 reservoirs deep enough for this kind of system. These bodies of water could collectively store 12 petawatt-hours, amounting to 40% of global annual electricity consumption. More than 80% of that capacity would be found in the five largest lakes, including the Caspian Sea.
To ensure the pipes remained undisturbed, it would be important to monitor the movement of ships on these lakes. “If a large object were dropped from the surface, or a boat sank and hit the pipeline, it could damage the pipes and release hydrogen,” says Yoshihide Wada, who led the team. Still, any leaking hydrogen would simply bubble to the surface and dissipate harmlessly in the atmosphere, the researchers add.
Although there are no immediate plans to build a hydrogen storage system like this, the team is collaborating with KAUST colleague Thomas Finkbeiner to test a similar system in the Red Sea that will use compressed air to store energy.
A way to select a suite of mangrove bacteria that can transform plastic has been developed that potentially offers a new strategy in the global toolkit of plastic waste cleanup. Researchers have assessed the impact of polyethylene terephthalate (PET) particles and seawater intrusion on the microbiome of mangrove soil and then experimented with an enrichment culture to select a suite of PET-transforming microbes[1].
Plastic ocean pollution is growing globally at an alarming rate, with plastic fragments found even in deep oceans far from from human habitation. Mangroves are important biodiversity hotspots that offer a range of ecosystem services but are increasingly at risk from many stressors including plastic pollution.
“Mangrove ecosystems are exposed to high levels of plastic and their soils have been reported to contain diverse microbial communities including plastic-active microorganisms,” explains Diego Javier Jiménez Avella, a research scientist in the Microbial EcoGenomics and Biotechnology Laboratory (MEGBLab) at KAUST, who led this research project. “So we thought these soils could be a good source of microbes with potential for breaking down plastics. Yet microbial diversity and metabolic activities in mangrove soils are still largely unknown.”
Analyzing the collective genomic information of two bacterial consortia showed that some bacterial species have novel enzymes capable of breaking down and transforming PET. The novel bacterial genus Mangrovimarina plasticivorans is a particularly important member of these consortia as it carries two genes that code synthesis of monohydroxyethyl terephthalates hydrolases — enzymes that are capable of degrading a PET byproduct.
These results are important as they increase our ecological understanding of PET transformation in nature and describe a novel bacterial genus and enzymes potentially capable of degrading PET. This is also the first time researchers have demonstrated that a bacterial consortia derived from mangrove soils can transform a fossil-fuel-based hydrolysable plastic.
“Engineering microbiomes to effectively transform plastics is an exciting research theme in microbiology and biotechnology,” explains Jiménez. “It is also a daunting task: bioremediation of microplastics in natural marine ecosystems is challenging due to low effectiveness, problems with scalability, testing, implementation, evaluation and legislation.”
The team’s approach to designing microbial inoculants and/or enzyme cocktails capable of accelerating PET degradation could be broadly applied using microbial inocula from a range of terrestrial and aquatic ecosystems. This in turn could identify more novel plastic-degrading microbes or enzymes.
“These laboratory-scale findings are a step to addressing plastic pollution and require further research and development — such as optimization and scalability — before they can be practically applied,” notes Alexandre S. Rosado, principal investigator at KAUST and leader of the MEGBLab.
Led by KAUST scientists, the research team — a collaboration that began in 2021 with eight institutions in Colombia, Brazil, USA, Germany, Australia, U.K. and Saudi Arabia — anticipates that broad use of this approach could help the design of efficient microbial consortia targeting plastic transformation both in the laboratory and in large-scale industrial settings.
The team are continuing to investigate the selection of plastic-transforming microbial communities from Red Sea mangroves and enzymatic activity of putative novel PET-degrading enzymes found in this study.
An underwater metasurface that performs far better than conventional resonators has been developed by Mohamed Farhat and Ying Wu from KAUST, working with colleagues from University Bourgogne Franche-Comté in France. A new pattern is helping to overcome some of the challenges of communication and sensing in water[1].
Electromagnetic waves are heavily attenuated as they pass through water, which limits their range. Instead, acoustic or sound waves provide a more viable option. But the resonators or cavities that confine and enhance acoustic waves, which are crucial for these communication systems, do not operate as well as their electromagnetic counterparts, particularly underwater, due to increased inherent losses.
“Previously, state of the art ultrasound resonators relied on conventional resonant systems, which resulted in low quality-factors or short-lived resonances that decayed only after a few cycles of oscillations. This situation is even worse underwater, where viscous damping or increased leaking create additional losses,” says Farhat.
“Our work presents a significant improvement compared to previous designs because it achieves an exceptionally high Q-factor within a simple underwater acoustic device,” explains Farhat. Waves can be trapped within a cavity and the level of confinement is quantitatively measured by its quality factor, or Q: the higher the Q, the longer the wave energy stays trapped inside.
Farhat and his co-workers developed an underwater resonator that included a metasurface: a thin silicon film imprinted with a periodic pattern that repeats over a distance shorter than the wavelength of the wave. This pattern can be engineered to control the way the metasurface interacts with the wave.
The team used a dicing machine to create an array of 0.1-millimeter slits in a silicon substrate of periodicity of 1 millimeter. The cavity consisted of two of these metasurfaces separated by a gap of 0.8 millimeter. The researchers characterized their structure by immersing it in water and using a transducer to create ultrasound waves with frequencies between 0.5 and 3 megahertz. They could then measure how the ultrasound transmitted through or reflected from the metasurface. In this way they were able to demonstrate a Q-factor of 350 for one-megahertz ultrasonic waves.
Crucial to this success was the unusual way their cavity traps the acoustic wave. In most resonators, the energy of the trapped wave needs to be less than a certain threshold. But the structure made by Farhat and team is not fully closed; the open resonator supports quasi-bound states in the continuum localized within a compact region of space even though their energy lies above the threshold.
“These so-called bound states in the continuum, firstly discovered in quantum mechanics, do not couple to the surrounding environment and hence possess a diverging quality-factor, which is a measure of the lifetime of the resonance,” explains Farhat.
“Our research advances the field of metamaterials, acoustics, and communications but also holds tremendous promise for practical applications, such as highly efficient acoustic filters, sensors, and transducers, as well as advanced communication and medical imaging systems and non-destructive testing,” concludes Wu.
A promising solar material hampered by stability problems has been fortified by a KAUST team that studied how the material degraded when exposed to air and moisture, and then tweaked its chemical composition to make it more durable[1]. This is a key step towards using the material in solar panels and other devices.
The material is known as a perovskite, a family of inexpensive compounds that are easily processed into thin films that convert light into electricity. These perovskites generally contain three types of ingredients: metals such as tin or lead; halogen ions, including bromide or iodide; and positively-charged ions such as cesium, methylammonium or formamidinium.
Lead-only perovskites typically absorb visible light, but perovskites containing a mix of tin and lead can also absorb near-infrared light. In principle, a solar cell made from a tin-lead perovskite could be teamed with a second solar cell that absorbs different wavelengths of light, forming a partnership that offers a higher power output than a conventional solar panel.
“In these ‘tandem’ solar cells, each of the layers specializes in absorbing a specific part of the solar spectrum,” explains Luis Lanzetta, one of the leading scientists behind the new research. “It means that a larger portion of the photons in sunlight can be converted into electricity.”
Meanwhile, tin-lead perovskites could also be used to detect near-infrared light in biological imaging or medical monitoring devices.
The big problem is that oxygen and moisture in the air rapidly degrade tin-lead perovskites. This process also turns some of the perovskite’s iodide into iodine, causing further damage that significantly reduces the material’s performance in less than an hour.
The KAUST researchers studied exactly how this degradation happens at the atomic level, and used that knowledge to tackle the problem. They made perovskites containing various mixtures of cesium, methylammonium and formamidinium, and found that cesium-rich blends were far more stable than the others. In contrast, methylammonium-rich formulations generated about four times as much iodine as their cesium-rich counterparts.
Computer simulations suggested that cesium can capture iodine in a way that slows perovskite breakdown. In contrast, methylammonium turns iodine into an even more active form called triiodide. “The bad news is that triiodide forms right on top of the perovskite surface, where it is in close contact with the material and able to oxidize it rapidly,” says Lanzetta.
So the researchers added a thin layer containing cesium or rubidium to the material’s surface, and included a chemical agent into the perovskite that could scavenge iodine. These tactics produced perovskite solar cells with a good efficiency of around 18%, and the scavenger ensured the cell’s performance was virtually unchanged after 2 hours exposure to the air.
The team plans to investigate other types of iodine scavenger and test different ways of incorporating it into the perovskite to improve its performance. “We believe that further optimizing of iodine-scavenging additives for perovskite solar cells will lead to highly durable technologies,” says Derya Baran, who led the team.
Two designs of frequency-locked semiconductor laser have been developed that deliver high-purity light with an ultra-narrow linewidth and exceptionally low noise. The lasers, which operate in the near-infrared or visible regions, look set to prove useful as compact, high-quality coherent optical sources that suit chip-scale integration. They suit applications such as LIDAR, atomic clocks, optical gyroscopes, metrology and microwave photonics.
“We aimed to demonstrate the versatility of low-noise semiconductor lasers by developing devices that operate effectively at two different spectral regions – 1310nm [1] and 780nm [2]. This expands their potential for broader applications in fields that require different wavelengths” explained Professor Yating Wan from KAUST, who is the corresponding author for both papers. The research emerged through collaboration between KAUST, Sandia National Labs and the University of California, Santa Barbara in the U.S. The results were recently published in Nature Photonics and Optica.
The first laser, a quantum dot (QD) laser grown directly on silicon, uses an external fiber cavity to stabilize and narrow the emission line and achieves a Lorentzian linewidth of just 16Hz – claimed to be the narrowest ever achieved for an on-chip QD laser. Its emission wavelength of 1310nm and exceptionally narrow linewidth make it well suited for constructing a highly stable microwave synthesizer.
The second laser consists of an AlGaAs distributed feedback (DFB) design, which is connected to a SiN micro-ring resonator in order to achieve self-injection locking. This resulted in a spectral linewidth of 105Hz. Its emission wavelength of 780nm aligns perfectly with optical transition of Rubidium-87, which is used in atomic optical traps and optical clocks.
Both devices are engineered for cost-effective scalable mass production. The QD lasers, grown on CMOS-compatible (001) silicon, could potentially be manufactured in foundries on 300mm-sized silicon wafers. Meanwhile, use of commercially available components in DFB lasers brings it closer to market readiness.
Both devices bring significant improvements over most semiconductor lasers that typically have much broader linewidths in the kilohertz or megahertz range. While similar narrow linewidth performance can be achieved with fiber lasers or solid-state lasers, they are much larger devices and are not amenable to chip-scale integration with optoelectronics and electronics.
“These advances bring the performance of semiconductor lasers on par with fibre and solid-state lasers: they provide a competitive alternative that combines the benefits of reduced size and cost with high performance,” commented by Artem Prokoshin, the first author of the Optica paper.
The team plans to reduce the size of devices by bringing the external locking cavities onto the same platform as the laser. “Our primary objective is to develop fully integrated narrow-linewidth lasers, explained Wan. “Currently, both devices we’ve worked on utilize external cavities – a fibre cavity or a micro-ring resonator, both of which are off-chip. Our next step is to integrate these components on-chip.”
While there is clearly more work in refining the devices, Wan says that they are already considering real applications and the commercial potential for the lasers. In particular, KAUST is collaborating with industry partners to explore the opportunities of using the QD lasers in sensors for use in autonomous equipment in the mining sector.
“The goal of this project is to develop a solid-state LIDAR prototype that integrates 3D point cloud processing possibilities,” she explained. “This prototype is specifically designed for mining operations in complex desert environments.”
A computationally efficient statistics-based approach has made it possible to emulate global climate simulations at ultra-high spatial resolution for the first time, shows research by a KAUST-led team[1].
“Climate simulations generated by Earth system models are indispensable for advancing understanding of climate processes, predicting future changes and developing strategies to address the challenges posed by climate change,” says KAUST postdoc Yan Song. “However, generating these simulations requires extensive computation, often taking weeks or months.”
Song, along with colleague Marc Genton and collaborator Zubair Khalid from Lahore University of Management Sciences in Pakistan, took a new look at the intricate Earth system models (ESMs) that describe global climate dynamics with a view to applying statistical methods to improve their efficiency.
“ESMs enable comprehensive and detailed climate simulations, but they are computationally expensive and require massive amounts of data storage, limiting their practicality for ultra-high-resolution applications,” says Song. “Leveraging statistical techniques, we constructed a practical complement for ESMs called a statistical emulator that captures intrinsic spatiotemporal structures of the ESMs and generates fast stochastic approximations.”
Generating simulations with ESMs involves iteratively solving a series of equations for each grid cell globally over time. A statistical emulator instead uses a much less complex statistical approach with trained parameters to generate a stochastic imitation of the simulation output. Then, just the emulator’s stochastic parameters require storage, instead of the full outputs of many climate simulations.
Song, Genton and Khalid’s emulator is based on a mathematical spherical harmonic transformation that converts spatial information on a sphere into the frequency domain to enable more efficient statistical analysis.
“Spherical harmonic transformations are useful for identifying dominant spatial variations and patterns, and enables analysis in the frequency domain, which significantly accelerates computations,” says Song.
Using their emulator approach, the researchers successfully produced emulations of simulated global surface temperatures from the newly published CESM2-LENS2 dataset at a daily timescale and a spatial resolution of 110 km — the first time emulations at such temporal and spatial resolution have been attempted.
Leveraging exascale computing resources, the researchers then extended their method to enable the stochastic emulation of global surface temperatures at an hourly timescale and spatial resolution of just 3 km, for which they have been recognized with a nomination for the Gordon Bell Prize — a prestigious award for an outstanding accomplishment in high-performance computing.
“The problem addressed in this work is of significant value to climate scientists,” Song says.
A reusable medical patch that uses octopus-like suckers to stick to skin can monitor a range of vital signs — and could put an end to the skin injuries caused by traditional adhesive patches, including skin inflammation, tension injuries, blisters, and tears[1].
“The patch is designed for easy removal without causing discomfort or pain, unlike conventional glue-based patches,” says Nazek El-Atab, who led the team that developed it. “Our goal is to develop a comprehensive, versatile, skin-attachable device that can revolutionize wearable health monitoring and diagnostic technologies.”
Clinicians routinely use adhesive patches to attach medical devices to patients. These devices might record pulse rate or muscle response, for example, or deliver vital medicines through the skin. Many patches rely on chemical adhesives, but these glues can cause a range of side effects for the skin.
Inspired by the circular suckers found on octopuses’ arms, KAUST researchers have developed a way to rapidly and cheaply create medical patches that carry ‘adhesive miniaturized octopus-like suckers’ (AMOS). The patches are flexible, biocompatible, and breathable, and carry an electrode that can monitor several types of biosignal.
“Previous bioinspired suction-based adhesives have suffered primarily from limited manufacturing flexibility and versatility due to traditional nano-/microfabrication techniques,” explains Aljawharah A. Alsharif, a Ph.D. student under the supervision of El-Atab.
“Other bio-inspired patches that adhere using suction mechanisms tend to face challenges when it comes to manufacturing: traditional nano-/microfabrication techniques limit the required manufacturing flexibility and versatility to produce them. Typically, these adhesives feature tiny hollows or ridges measured in millionths or even billionths of a meter and so fabricating materials with these finely detailed structures can be expensive. Also, they may only be effective on certain types of skin surface.
The AMOS patch overcomes these limitations by using a rapid hybrid 3D printing approach, explains Alsharif. The researchers found that a 3D printing technique called stereolithography could offer the precision they needed to make the AMOS patches. The method uses an ultraviolet laser to accurately build up a resin mold that contains tiny domes and wiggly lines. Then they used that mold to create an AMOS patch from a biocompatible polymer called polydimethylsiloxane (PDMS), which has some inherent stickiness.
After testing patches with different-sized suckers and various patterns, they found that 200 micrometer-wide suckers offered the greatest adhesion. Meanwhile, the patch’s wiggly grooves help moisture escape from the skin, ensuring that the material is highly breathable. “When the patch is lightly pressed on to the skin, the suckers create a vacuum, providing secure adhesion even under various skin conditions such as dry, wet, or hairy surfaces,” says Alsharif. This mode of adhesion also enables the same patch to be reapplied again and again, making it useful for long -erm health monitoring
The researchers fitted the patch with electrodes, attached it to the hairy chest of a male volunteer while he cycled on an exercise bike, and used the device to monitor the subject’s electrocardiogram (ECG) signals. The same patch could also be placed on different parts of the body to record electromyograms (EMG) — which measure muscle response — and electrooculograms (EOG) to monitor eye movements.
The team were also able to ‘road test’ the patch on KAUST computational scientist and associate professor Matteo Parsani. Parsani wore the patch to monitor various biosignals on his 30-day Athar: East to West hand-cycling journey across the kingdom, a distance of more than 3000km.
“The versatility of the AMOS patch allows it to function effectively across different types of biosignal measurements simultaneously, demonstrating its broad applicability and efficiency in biomedical applications,” says Alsharif. “It can also be reused multiple times without significant loss of adhesion.”
The researchers now aim to apply AMOS patches to other measurements, including temperature, glucose and stress levels. “We plan to conduct extensive clinical trials to validate its efficacy in real-world medical applications,” says El-Atab.
The team is also collaborating with other research groups to expand the range of applications for the AMOS patch in other wearable health technologies, says El-Atab.
Novel switches that can effectively turn on and off electromagnetic signals of frequencies up to 260 GHz using a device known as a memristor have been developed by an international team[1].
Switches are the most important part of an electrical circuit. Transistors — which are effectively miniaturized electrically operated switches — are crucial for radiofrequency communication applications such as smartphones, Wi-Fi, Bluetooth and satellite communications. While communication devices typically work at a few to tens of gigahertz (GHz, or billion cycles per second), future sixth generation (6G) systems will require hundreds of gigahertz. Transistors can struggle to provide effective switching between on and off conditions with such high-frequency signals.
The switches have been developed by KAUST scientists Sebastian Pazos, Mario Lanza and their colleagues, along with international collaborators from Ireland (Tyndall National Institute), Spain (Universidad Autónoma de Barcelona), and the United States (University of Texas at Austin).
“A memristor is a two-terminal electronic device that can change its electrical resistance when subjected to a tailored electrical stress,” says Pazos. “A memristor acts like a switch as it toggles between a high-resistance ‘off’ state that blocks the current flow and a low-resistance ‘on’ state that allows current to flow.”
Memristors are useful as radiofrequency switches due to their ultralow power consumption. Conventional RF switches employ semiconductor devices (transistors or diodes) that may require several tens of milliamperes to maintain their state, while memristors are nonvolatile: they “remember” their resistance when the control signal is switched off.
Lanza’s team built their memristor from a 2D layered material called hexagonal boron nitride between two gold electrodes. “Memristors made of single-layer 2D materials have previously been explored for use as RF switches, but they had limited endurance and the device-to-device and cycle-to-cycle variability was severe,” explains Pazos, the leading researcher of this study. “We thought of using multilayered hexagonal boron nitride, instead of monolayers, in an attempt to solve these issues.”
Key to the operation of their radiofrequency switches was the ability to create and control one or more conductive filaments through the multilayer hexagonal boron nitride by application of voltage/current pulses. In this way, their switches exhibited an “off” state with a resistance as low as 7 Ohms and blocked about 99% of the signal.
The losses when transmitting the radiofrequency signal in the “on” state can be as low as 5% at frequencies of 100 GHz. And importantly, while previous monolayer hexagonal boron nitride devices lasted for only 30 cycles between high- and low-resistivity states, the devices from Pazos et al. were sufficiently robust to last 2,000 cycles.
In addition, these switches were combined for the first time into circuits featuring multiple memristors, which improved their signal blocking capabilities well above 200 GHz. “In our group, we have already shown that hexagonal boron nitride memristors can be integrated with CMOS circuits, and we now want to take this further into the high frequency realm with devices such as configurable high frequency communication circuits and on-chip antenna arrays,” says Pazos.
A hybrid approach to advance solar technology aims to generate more powerful solar cells by making better use of the solar energy spectrum while increasing efficiency and stability[1].
Researchers are taking this hybrid approach by combining traditional solar cells in tandem with more innovative materials called perovskites. Perovskites are a wide variety of crystalline materials that share specific general structural characteristics based on a combination of three types of chemical ions. They are being recruited into an ever-expanding range of applications, including solar cells, catalysts, sensors and much more.
Leading this research are KAUST’s solar researchers Esma Ugur, Erkan Aydin and Stefaan De Wolf.
“Our strategy uses a specific additive during the deposition of the halide perovskite layers to keep the perovskite top cell of the structure stable over time in the required operational conditions”, Ugur explains. She says that the impressive progress at KAUST has achieved record performance enhancements, expected to bring cheaper and more effective perovskites into widespread use.
The latest improvements created a more consistent distribution of voltage across the perovskite film. This allowed the team to set two new records for the highest power conversion efficiency (PCE) of any two-junction solar cell under nonconcentrated light.
“We reached one version at 33.2% PCE in April 2024 and another at 33.7% in June,” says Aydin. “These achievements prove the high efficiency potential of perovskite/silicon tandems and they effectively address two major issues where improvements are needed.”
One of the greatest technical achievements came by modifying the way in which positive charge is spread across the key molecule methylenediammonium dichloride (MDACl2). This facilitates the formation of tetrahydrotriazinium (THTZ-H+) in-situ. The cyclic nature of the THTZ-H+ positive ion (cation) allows it to form strong hydrogen bonds with halides in multiple directions within the perovskite lattice. This arrangement enhances the overall PCE and also the stability of the perovskites under prolonged light and heat exposure.
Reflecting on the challenges of the complex teamwork required throughout this research, De Wolf explains that tandem solar cells are significantly more complex structures than conventional solar cells
“Developing our cells from scratch to become top performing champions took several years of efforts by a dedicated team with expertise in various disciplines, including fundamental sciences such as physics and chemistry, materials science, engineering and device engineering.”
The team is optimistic that further advances are on the way. They believe there is still room to achieve higher power outputs, although that may require more fundamental and time-consuming research and development activities.
“We will continue pushing the boundaries of tandem cells,” De Wolf concludes. “While our advances should be applicable worldwide, they will also be especially useful for developing long-term efficient tandem solar cells suitable for hot and humid climates like Saudi Arabia.”
The technology is already on the path to commercialization in collaboration with several major photovoltaic manufacturers worldwide.
