Day: July 9, 2024

1. What Iran’s moderate new President Masoud Pezeshkian might try to change — and what he definitely won’t  CBS News
2. Iran has voted in a reformist president — but change remains distant  CNBC
3. Reformist Pezeshkian beats hard-liner to win Iran presidential election, promising outreach to West  The Associated Press

Silicon computer chips have served us well for more than half a century. The tiniest features on chips currently sold are approximately 3 nanometers — a startlingly small size given that a human hair is roughly 80,000 nanometers wide. Reducing the size of features on chips will help us meet our endless need for more memory and processing power in the palm of our hand. But the limit of what can be achieved with standard materials and processes is near.

Researchers at the U.S. Department of Energy’s (DOE) Princeton Plasma Physics Laboratory (PPPL) are applying their expertise in physics, chemistry and computer modeling to create the next generation of computer chips, aiming for processes and materials that will produce chips with smaller features. 

“All of our existing electronic devices use chips made up of silicon, which is a three-dimensional material. Now, many companies are investing a lot in chips made up of two-dimensional materials,” said Shoaib Khalid, an associate research physicist at PPPL. The materials actually exist in three dimensions, but they are so thin –– often made up of only a few layers of atoms — that scientists have taken to calling them 2D.

Khalid, together with PPPL’s Bharat Medasani and Anderson Janotti from the University of Delaware, investigated one potential silicon replacement: a 2D material known as a transition-metal dichalcogenide (TMD). Their new paper, published in the journal 2D Materials, details the variations that can occur in the atomic structure of TMDs, why they happen and how they affect the material. Information about these variations lays the groundwork for refining the processes needed to create next-generation computer chips. Ultimately, the goal is to design plasma-based manufacturing systems that can create TMD-based semiconductors made to the precise specifications required for the application.

TMD: A tiny metal sandwich

A TMD can be as thin as three atoms high. Think of it like a tiny metal sandwich. The bread is made of a chalcogen element: oxygen, sulfur, selenium or tellurium. The filling is a layer of transition metal — any metal from groups 3 to 12 in the periodic table of elements.

A bulk TMD has five or more layers of atoms. The atoms are arranged in a crystal structure or lattice. Ideally, the atoms are organized in a precise and consistent pattern throughout the lattice. In reality, small alterations can be found in the pattern. One spot in the pattern might be missing an atom, or an atom might be found in an odd location. Scientists call these alterations defects, but they can have a beneficial impact on the material.

Some TMD defects, for example, can make the semiconductor more electrically conductive. Good or bad, it is critical that scientists understand why defects happen and how they will affect the material so they can incorporate or eliminate these defects as necessary. Understanding common defects also allows the researchers to explain the results from past experiments with TMDs.

“When bulk TMDs are made, they have excess electrons,” Khalid said, adding that researchers were not sure why these excess negatively charged particles were present. “In this work, we explain that the excess electrons can be caused by hydrogen.”

The researchers came to this conclusion after calculating the amount of energy that would be required to form different kinds of TMD defects. They looked at defects involving chalcogen vacancies, which were previously known to be present in TMDs, and defects involving hydrogen because this element is often present during the chip manufacturing process. Researchers are particularly interested in finding out which defects require minimal formation energy because these are the ones that are likely to occur — it doesn’t take much energy for them to happen!

The team then investigated the role of each of the low-formation-energy defects. Specifically, they wanted to know how each defect configuration might impact the electrical charge of the material. The researchers found that one of the defect configurations involving hydrogen provides excess electrons, which creates negatively charged semiconductor material, known as an n-type. Computer chips are made using combinations of n-type semiconductor material and positively charged, or p-type, material.

Shedding light on missing chalcogens

The other type of defect explored in the paper is known as a chalcogen vacancy: a missing atom of oxygen, sulfur, selenium or tellurium, depending on the type of TMD. The researchers focused on explaining the results of past experiments on flakes of the bulk TMD material molybdenum disulfide. The experiments, which involved shining light on the TMD, showed unexpected frequencies of light coming from the TMD. These unexpected frequencies, the researchers found, could be explained by the movement of electrons related to the chalcogen vacancy.

“This is a common defect. They can often see it from the images of scanning tunneling microscopes when they grow the TMD film,” Khalid said. “Our work provides a strategy to investigate the presence of these vacancies in the bulk TMDs. We explained past experimental results shown in molybdenum disulfide, and then we predicted a similar thing for other TMDs.”

The process suggested by the researchers involves analyzing the TMD for defects using measurement techniques called photoluminescence to see which frequencies of light are emitted by the material. The peak frequency of light can be used to determine the electron configurations of the atoms in the TMD and the presence of chalcogen defects. The journal article includes information about the frequencies that would be emitted by five types of TMDs with chalcogen vacancies, including molybdenum disulfide. The results, therefore, provide a guideline for investigating chalcogen vacancies in future experiments.

Plants are known to respond to seasonal changes by budding, leafing, and flowering. As climate change stands to shift these so-called phenological stages in the life cycle of plants, access to data about phenological changes – from many different locations and in different plants – can be used to draw conclusions about the actual effects of climate change. However, conducting such analyses require a large amount of data and data collection of this scale would be unthinkable without the help of citizen scientists.

“The problem is that the quality of the data suffers when fewer people engage as citizen scientists and stop collecting data,” says first author Karin Mora, research fellow at Leipzig University and iDiv.

Mobile apps like Flora Incognita could help solve this issue. The app allows users to identify unknown wild plants within a matter of seconds. “When I take a picture of a plant with the app, the observation is recorded with the (exact) location as well as a time stamp,” explains co-author Jana Wäldchen from the Max Planck Institute for Biogeochemistry (MPI-BGC), who developed the app with colleagues from TU Ilmenau. “Millions of time-stamped plant observations from different regions have been collected by now.” Although satellite data also records the phenology of entire ecosystems from above, they do not provide information about the processes taking place on the ground.

Plants show synchronised response

The researchers developed an algorithm that draws on almost 10 million observations of nearly 3,000 plants species identified between 2018 and 2021 in Germany by users of Flora Incognita. The data show that each individual plant has its own cycle as to when it begins a flowering or growth phase. Furthermore, the scientists were able to show that group behaviour arises from the behaviour of individuals. From this, they were able to derive ecological patterns and investigate how these change with the seasons. For example, ecosystems by rivers differ from those in the mountains, where phenological events start later.

The algorithm also accounts for the observational tendencies of Flora Incognita users, whose data collection is far from systematic. For example, users record more observations on the weekend and in densely populated areas.

“Our method can automatically isolate these effects from the ecological patterns,” Karin Mora explains. “Fewer observations don’t necessarily mean that we can’t record the synchronisation. Of course, there are very few observations in the middle of winter, but there are also very few plants that can be observed during that time.”

It is known that climate change is causing seasonal shifts – for example, spring is arriving earlier and earlier. How this affects the relationship between plants and pollinating insects and therefore potentially also food security is still being subject to further research. The new algorithm can now be used to better analyse the effects of these changes on the plant world.