A Treasure Trove for Nanotechnology Experts

EPFL and NCCR-MARVEL scientists identified more than 1,000 2-D materials, focusing on the feasibility of exfoliation. Credit: EPFL/G.Pizzi

A team from EPFL and NCCR Marvel has identified more than 1,000 materials with a particularly interesting 2-D structure. Their research, published in Nature Nanotechnlogy, paves the way for groundbreaking technological applications.

2-D materials, which consist of a few layers of atoms, are considered the future of nanotechnology. They offer potential new applications and could be used in small, higher-performance and more energy-efficient devices. Two-dimensional materials were first discovered almost 15 years ago, but only a few dozen of them have been synthesized so far.

Now, thanks to an approach developed by researchers from EPFL’s Theory and Simulation of Materials Laboratory (THEOS) and from NCCR-MARVEL for Computational Design and Discovey of Novel Materials, many more promising 2-D materials may be identified. Their work was recently published in the journal Nature Nanotechnology.

The first 2-D material isolated was graphene, in 2004, earning its discoverers a Nobel Prize in 2010. This marked the start of a whole new era in electronics, as graphene is light, transparent and resilient and, above all, a good conductor of electricity. It paved the way to new applications in such fields as photovoltaics and optoelectronics. “To find other materials with similar properties, we focused on the feasibility of exfoliation,” explains Nicolas Mounet, a researcher in the THEOS lab and lead author of the study.

“But instead of placing adhesive strips on graphite to see if the layers peeled off, like the Nobel Prize winners did, we used a digital method.”

The researchers developed an algorithm to review and carefully analyze the structure of more than 100,000 3-D materials recorded in external databases. From this, they created a database of around 5,600 potential 2-D materials, including more than 1,000 with particularly promising properties. In other words, they’ve created a treasure trove for nanotechnology experts.

To build their database, the researchers used a step-by-step process of elimination. First, they identified all of the materials that are made up of separate layers. “We then studied the chemistry of these materials in greater detail and calculated the energy that would be needed to separate the layers, focusing primarily on materials where interactions between atoms of different layers are weak, something known as Van der Waals bonding,” says Marco Gibertini, a researcher at THEOS and the second author of the study.

Of the 5,600 materials initially identified, the researchers singled out 1,800 structures that could potentially be exfoliated, including 1,036 that looked especially easy to exfoliate. This represents a considerable increase in the number of possible 2-D materials known today. They then selected the 258 most promising materials, categorizing them according to their magnetic, electronic, mechanical, thermal and topological properties.

“Our study demonstrates that digital techniques can really boost discoveries of new materials,” says Nicola Marzari, the director of NCCR-MARVEL and a professor at THEOS. “In the past, chemists had to start from scratch and just keep trying different things, which required hours of lab work and a certain amount of luck. With our approach, we can avoid this long, frustrating process because we have a tool that can single out the materials that are worth studying further, allowing us to conduct more focused research.”

It is also possible to reproduce the researchers’ calculations thanks to their software AiiDA, which describes the calculation process for each material discovered in the form of workflows and stores the full provenance of each stage of the calculation.

“Without AiiDA, it would have been very difficult to combine and process different types of data,” explains Giovanni Pizzi, a senior researcher at THEOS and co-author of the study. “Our workflows are available to the public, so anyone in the world can reproduce our calculations and apply them to any material to find out if it can be exfoliated.

More information: Nicolas Mounet et al, Two-dimensional materials from high-throughput computational exfoliation of experimentally known compounds, Nature Nanotechnology (2018). DOI: 10.1038/s41565-017-0035-5

Provided by Ecole Polytechnique Federale de Lausanne

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Making Cheaper (Perovskite) Solar Cells with 20.2 Percent Efficiency

Perovskite New Materials 20 plus id42356EPFL scientists have developed a solar-panel material that can cut down on photovoltaic costs while achieving competitive power-conversion efficiency of 20.2%.
Some of the most promising solar cells today use light-harvesting films made from perovskites – a group of materials that share a characteristic molecular structure. However, perovskite-based solar cells use expensive “hole-transporting” materials, whose function is to move the positive charges that are generated when light hits the perovskite film. Publishing in Nature Energy (“A molecularly engineered hole-transporting material for e cient perovskite solar cells”), EPFL scientists have now engineered a considerably cheaper hole-transporting material that costs only a fifth of existing ones while keeping the efficiency of the solar cell above 20%.
FDT on a Perovskite Surface
This is a 3-D illustration of FDT molecules on a surface of perovskite crystals. (Image: Sven M. Hein / EPFL)

As the quality of perovskite films increases, researchers are seeking other ways of improving the overall performance of solar cells. Inadvertently, this search targets the other key element of a solar panel, the hole-transporting layer, and specifically, the materials that make them up. There are currently only two hole-transporting materials available for perovskite-based solar cells. Both types are quite costly to synthesize, adding to the overall expense of the solar cell.

To address this problem, a team of researchers led by Mohammad Nazeeruddin at EPFL developed a molecularly engineered hole-transporting material, called FDT, that can bring costs down while keeping efficiency up to competitive levels. Tests showed that the efficiency of FDT rose to 20.2% – higher than the other two, more expensive alternatives. And because FDT can be easily modified, it acts as a blueprint for an entire generation of new low-cost hole-transporting materials.
“The best performing perovskite solar cells use hole transporting materials, which are difficult to make and purify, and are prohibitively expensive, costing over €300 per gram preventing market penetration,” says Nazeeruddin. “By comparison, FDT is easy to synthesize and purify, and its cost is estimated to be a fifth of that for existing materials – while matching, and even surpassing their performance.”
Source: Ecole Polytechnique Fédérale de Lausanne

Graphene-perovskite hybrids make new super-detectors: Turning Light into Energy

Graphene Perovskite 081115 324x182EPFL scientists have created the first perovskite nanowire-graphene hybrid phototransistors. Even at room temperature, the devices are highly sensitive to light, making them outstanding photodetectors.

The lead-containing perovskite materials can turn light into electricity with high efficiency, which is why they have revolutionized solar cell technologies. On the other hand, graphene is known for its super-strength as well as its excellent electrical conductivity. Combining the two materials, EPFL scientists have created the first ever class of hybrid transistors that turn light into electricity with high sensitivity and at room temperature. The work is published in Small.

The lab of László Forró at EPFL, where the chemical activity is led by Endre Horváth, used its expertise in microengineering to create nanowires of the perovskite methylammonium lead iodide. This highly non-trivial route for the synthesis of nanowires was developed by him in 2014 and called slip-coating method. The advantage of nanowires is their consistency, while their manufacturing can be controlled to modify their architecture and explore different designs.

Making a device by depositing the perovskite nanowires onto graphene has increased the efficiency in converting light to electrical current at room temperature. “Such a device shows almost 750,000 times higher photoresponse compared to detectors made only with perovskite nanowires,” added Massimo Spina who fabricated the miniature photodetectors. Because of this exceptionally high sensitivity, the graphene/perovskite nanowire hybrid device is considered to be a superb candidate for even a single-photon detection.

This work was founded by the Swiss National Science Foundation. The hybrid devices were fabricated in part at EPFL’s Center for Micro/Nanotechnology.