From The DOE’s Lawrence Berkeley National Laboratory: “Microscopy Images Could Lead to New Ways to Control Excitons for Quantum Computing”

From The DOE’s Lawrence Berkeley National Laboratory

2.7.23
Alison Hatt

The unit-cell averaged electron microscopy-derived composite image shows excitons in green. The moiré unit cell outlined in the lower right of the exciton map is about 8 nanometers in size. (Credit: Sandhya Susarla and Peter Ercius/Berkeley Lab)

Excitons are drawing attention as possible quantum bits (qubits) in tomorrow’s quantum computers and are central to optoelectronics and energy-harvesting processes. However, these charge-neutral quasiparticles, which exist in semiconductors and other materials, are notoriously difficult to confine and manipulate. Now, for the first time, researchers have created and directly observed highly localized excitons confined in simple stacks of atomically thin materials. The work confirms theoretical predictions and opens new avenues for controlling excitons with custom-built materials.

“The idea that you can localize excitons on specific lattice sites by simply stacking these 2D materials is exciting because it has a variety of applications, from designer optoelectronic devices to materials for quantum information science,” said Archana Raja, co-lead of the project and a staff scientist at Lawrence Berkeley National Laboratory’s (Berkeley Lab) Molecular Foundry [below], whose group led the device fabrication and optical spectroscopy characterization.

The team fabricated devices by stacking layers of tungsten disulfide (WS2) and tungsten diselenide (WSe2). A small mismatch in the spacing of atoms in the two materials gave rise to a moiré superlattice, a larger periodic pattern that arises from the overlap of two smaller patterns with similar but not identical spacing of elements. Using state-of-the-art electron microscopy tools, the researchers collected structural and spectroscopic data on the devices, combining information from hundreds of measurements to determine the probable locations of excitons.

“We used basically all the most advanced capabilities on our most advanced microscope to do this experiment,” said Peter Ercius, who led the imaging work at the Molecular Foundry’s National Center for Electron Microscopy. “We were pushing the boundaries of everything we can do, from making the sample to analyzing the sample to doing the theory.”

Theoretical calculations, led by Steven Louie, a faculty senior scientist at Berkeley Lab and distinguished professor of physics at UC Berkeley, revealed that large atomic reconstructions take place in the stacked materials, which modulate the electronic structure to form a periodic array of “traps” where excitons become localized. Discovery of this direct relationship between the structural changes and the localization of excitons overturns prior understanding of these systems and establishes a new approach to designing optoelectronic materials.

The team’s findings are described in a paper published in the journal Science [below] with postdoctoral fellows Sandhya Susarla (now a professor at Arizona State University) and Mit H. Naik as co-lead authors. Next the team will explore approaches to tuning the moiré lattice on demand and making the phenomenon more robust to material disorder.

Science

See the full article here .

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LBNL campus

Berkeley Lab campus Aerial View

Bringing Science Solutions to the World

In the world of science, The Lawrence Berkeley National Laboratory (Berkeley Lab) is synonymous with “excellence.” Thirteen Nobel prizes are associated with Berkeley Lab. Seventy Lab scientists are members of the The National Academy of Sciences, one of the highest honors for a scientist in the United States. Thirteen of our scientists have won the National Medal of Science, our nation’s highest award for lifetime achievement in fields of scientific research. Eighteen of our engineers have been elected to the The National Academy of Engineering, and three of our scientists have been elected into The Institute of Medicine. In addition, Berkeley Lab has trained thousands of university science and engineering students who are advancing technological innovations across the nation and around the world.

Berkeley Lab is a member of the national laboratory system supported by The DOE through its Office of Science. It is managed by the University of California and is charged with conducting unclassified research across a wide range of scientific disciplines. Located on a 202-acre site in the hills above The University of California-Berkeley campus that offers spectacular views of the San Francisco Bay, Berkeley Lab employs approximately 3,232 scientists, engineers and support staff. The Lab’s total costs for FY 2014 were $785 million. A recent study estimates the Laboratory’s overall economic impact through direct, indirect and induced spending on the nine counties that make up the San Francisco Bay Area to be nearly $700 million annually. The Lab was also responsible for creating 5,600 jobs locally and 12,000 nationally. The overall economic impact on the national economy is estimated at $1.6 billion a year. Technologies developed at Berkeley Lab have generated billions of dollars in revenues, and thousands of jobs. Savings as a result of Berkeley Lab developments in lighting and windows, and other energy-efficient technologies, have also been in the billions of dollars.

Berkeley Lab was founded in 1931 by Ernest Orlando Lawrence, a University of California-Berkeley physicist who won the 1939 Nobel Prize in physics for his invention of the cyclotron, a circular particle accelerator that opened the door to high-energy physics. It was Lawrence’s belief that scientific research is best done through teams of individuals with different fields of expertise, working together. His teamwork concept is a Berkeley Lab legacy that continues today.

History

1931–1941

The laboratory was founded on August 26, 1931, by Ernest Lawrence, as the Radiation Laboratory of the University of California-Berkeley, associated with the Physics Department. It centered physics research around his new instrument, the cyclotron, a type of particle accelerator for which he was awarded the Nobel Prize in Physics in 1939.

E.O. Lawrence’ first cyclotron.

LBNL 88 inch cyclotron.

LBNL 88 inch cyclotron.

Throughout the 1930s, Lawrence pushed to create larger and larger machines for physics research, courting private philanthropists for funding. He was the first to develop a large team to build big projects to make discoveries in basic research. Eventually these machines grew too large to be held on the university grounds, and in 1940 the lab moved to its current site atop the hill above campus. Part of the team put together during this period includes two other young scientists who went on to establish large laboratories; J. Robert Oppenheimer founded The DOE’s Los Alamos Laboratory, and Robert Wilson founded The DOE’s Fermi National Accelerator Laboratory.

1942–1950

Leslie Groves visited Lawrence’s Radiation Laboratory in late 1942 as he was organizing the Manhattan Project, meeting J. Robert Oppenheimer for the first time. Oppenheimer was tasked with organizing the nuclear bomb development effort and founded today’s Los Alamos National Laboratory to help keep the work secret. At the RadLab, Lawrence and his colleagues developed the technique of electromagnetic enrichment of uranium using their experience with cyclotrons. The “calutrons” (named after the University) became the basic unit of the massive Y-12 facility in Oak Ridge, Tennessee. Lawrence’s lab helped contribute to what have been judged to be the three most valuable technology developments of the war (the atomic bomb, proximity fuse, and radar). The cyclotron, whose construction was stalled during the war, was finished in November 1946. The Manhattan Project shut down two months later.

1951–2018

After the war, the Radiation Laboratory became one of the first laboratories to be incorporated into the Atomic Energy Commission (AEC) (now The Department of Energy . The most highly classified work remained at Los Alamos, but the RadLab remained involved. Edward Teller suggested setting up a second lab similar to Los Alamos to compete with their designs. This led to the creation of an offshoot of the RadLab (now The DOE’s Lawrence Livermore National Laboratory) in 1952. Some of the RadLab’s work was transferred to the new lab, but some classified research continued at Berkeley Lab until the 1970s, when it became a laboratory dedicated only to unclassified scientific research.

Shortly after the death of Lawrence in August 1958, the UC Radiation Laboratory (both branches) was renamed the Lawrence Radiation Laboratory. The Berkeley location became the Lawrence Berkeley Laboratory in 1971, although many continued to call it the RadLab. Gradually, another shortened form came into common usage, LBNL. Its formal name was amended to Ernest Orlando Lawrence Berkeley National Laboratory in 1995, when “National” was added to the names of all DOE labs. “Ernest Orlando” was later dropped to shorten the name. Today, the lab is commonly referred to as “Berkeley Lab”.

The Alvarez Physics Memos are a set of informal working papers of the large group of physicists, engineers, computer programmers, and technicians led by Luis W. Alvarez from the early 1950s until his death in 1988. Over 1700 memos are available on-line, hosted by the Laboratory.

The lab remains owned by the Department of Energy , with management from the University of California. Companies such as Intel were funding the lab’s research into computing chips.

Science mission

From the 1950s through the present, Berkeley Lab has maintained its status as a major international center for physics research, and has also diversified its research program into almost every realm of scientific investigation. Its mission is to solve the most pressing and profound scientific problems facing humanity, conduct basic research for a secure energy future, understand living systems to improve the environment, health, and energy supply, understand matter and energy in the universe, build and safely operate leading scientific facilities for the nation, and train the next generation of scientists and engineers.

The Laboratory’s 20 scientific divisions are organized within six areas of research: Computing Sciences; Physical Sciences; Earth and Environmental Sciences; Biosciences; Energy Sciences; and Energy Technologies. Berkeley Lab has six main science thrusts: advancing integrated fundamental energy science; integrative biological and environmental system science; advanced computing for science impact; discovering the fundamental properties of matter and energy; accelerators for the future; and developing energy technology innovations for a sustainable future. It was Lawrence’s belief that scientific research is best done through teams of individuals with different fields of expertise, working together. His teamwork concept is a Berkeley Lab tradition that continues today.

Berkeley Lab operates five major National User Facilities for the DOE Office of Science:

The Advanced Light Source (ALS) is a synchrotron light source with 41 beam lines providing ultraviolet, soft x-ray, and hard x-ray light to scientific experiments.

The DOE’s Lawrence Berkeley National Laboratory Advanced Light Source.
The ALS is one of the world’s brightest sources of soft x-rays, which are used to characterize the electronic structure of matter and to reveal microscopic structures with elemental and chemical specificity. About 2,500 scientist-users carry out research at ALS every year. Berkeley Lab is proposing an upgrade of ALS which would increase the coherent flux of soft x-rays by two-three orders of magnitude.

Berkeley Lab Laser Accelerator (BELLA) Center

A view of BELLA, the Berkeley Lab Laser Accelerator. Credit: Roy Kaltschmidt-Berkeley Lab.

The DOE Joint Genome Institute supports genomic research in support of the DOE missions in alternative energy, global carbon cycling, and environmental management. The JGI’s partner laboratories are Berkeley Lab, DOE’s Lawrence Livermore National Laboratory, DOE’s Oak Ridge National Laboratory (ORNL), DOE’s Pacific Northwest National Laboratory (PNNL), and the HudsonAlpha Institute for Biotechnology . The JGI’s central role is the development of a diversity of large-scale experimental and computational capabilities to link sequence to biological insights relevant to energy and environmental research. Approximately 1,200 scientist-users take advantage of JGI’s capabilities for their research every year.

LBNL Molecular Foundry

The LBNL Molecular Foundry is a multidisciplinary nanoscience research facility. Its seven research facilities focus on Imaging and Manipulation of Nanostructures; Nanofabrication; Theory of Nanostructured Materials; Inorganic Nanostructures; Biological Nanostructures; Organic and Macromolecular Synthesis; and Electron Microscopy. Approximately 700 scientist-users make use of these facilities in their research every year.

The DOE’s NERSC National Energy Research Scientific Computing Center is the scientific computing facility that provides large-scale computing for the DOE’s unclassified research programs. Its current systems provide over 3 billion computational hours annually. NERSC supports 6,000 scientific users from universities, national laboratories, and industry.

DOE’s NERSC National Energy Research Scientific Computing Center at Lawrence Berkeley National Laboratory.

Cray Cori II supercomputer at National Energy Research Scientific Computing Center at DOE’s Lawrence Berkeley National Laboratory, named after Gerty Cori, the first American woman to win a Nobel Prize in science.

NERSC Hopper Cray XE6 supercomputer.

NERSC Cray XC30 Edison supercomputer.

NERSC GPFS for Life Sciences.

The Genepool system is a cluster dedicated to the DOE Joint Genome Institute’s computing needs. Denovo is a smaller test system for Genepool that is primarily used by NERSC staff to test new system configurations and software.

NERSC PDSF computer cluster in 2003.

PDSF is a networked distributed computing cluster designed primarily to meet the detector simulation and data analysis requirements of physics, astrophysics and nuclear science collaborations.

Cray Shasta Perlmutter SC18 AMD Epyc Nvidia pre-exascale supercomputer.

NERSC is a DOE Office of Science User Facility.

The DOE’s Energy Science Network is a high-speed network infrastructure optimized for very large scientific data flows. ESNet provides connectivity for all major DOE sites and facilities, and the network transports roughly 35 petabytes of traffic each month.

Berkeley Lab is the lead partner in the DOE’s Joint Bioenergy Institute (JBEI), located in Emeryville, California. Other partners are the DOE’s Sandia National Laboratory, the University of California (UC) campuses of Berkeley and Davis, the Carnegie Institution for Science , and DOE’s Lawrence Livermore National Laboratory (LLNL). JBEI’s primary scientific mission is to advance the development of the next generation of biofuels – liquid fuels derived from the solar energy stored in plant biomass. JBEI is one of three new U.S. Department of Energy (DOE) Bioenergy Research Centers (BRCs).

Berkeley Lab has a major role in two DOE Energy Innovation Hubs. The mission of the Joint Center for Artificial Photosynthesis (JCAP) is to find a cost-effective method to produce fuels using only sunlight, water, and carbon dioxide. The lead institution for JCAP is the California Institute of Technology and Berkeley Lab is the second institutional center. The mission of the Joint Center for Energy Storage Research (JCESR) is to create next-generation battery technologies that will transform transportation and the electricity grid. DOE’s Argonne National Laboratory leads JCESR and Berkeley Lab is a major partner.

From The Pritzker School of Molecular Engineering At The University of Chicago: “Researchers use light to control magnetic fields at nanoscale”

From The Pritzker School of Molecular Engineering

At

The University of Chicago

9.30.22
Sarah C.P. Williams

Researchers from the University of Chicago’s Pritzker School of Molecular Engineering (PME) discovered how to use a laser beam (red) to control the spin of electrons (purple) within a 2-D semiconductor, letting them precisely manipulate magnetic order within the material. (Photo courtesy of High Lab)

In thin, two-dimensional semiconductors, electrons move, spin and synchronize in unusual ways. For researchers, understanding the way these electrons carry out their intricate dances— and learning to manipulate their choreography—not only lets them answer fundamental physical questions, but can yield new types of circuits and devices.

One correlated phase that such electrons can take on is magnetic order, in which they align their spin in the same direction. Traditionally, the ability to manipulate magnetic order within a 2-D semiconductor has been limited; scientists have used unwieldy, external magnetic fields, which limit technological integration and potentially conceal interesting phenomena.

Now, researchers from the University of Chicago’s Pritzker School of Molecular Engineering (PME) have discovered how to use nanoscale, low-power laser beams to precisely control magnetism within a 2-D semiconductor. Their approach, described online in the journal Science Advances [below], has implications for both studying the emergence of the correlated phase as well as designing new optoelectronic and spintronic devices.

“The fact that we can now use light to manipulate electrons in this way means we have unprecedented control over this magnetic order,” said Asst. Prof. Alex High, the senior author of the new work.

Controllable magnets

High’s lab focused on transition metal dichalcogenides (TMDs), a family of semiconductors that can be exfoliated into single, two-dimensional flakes, measuring just three atoms thick. Scientists had previously hypothesized that electrons within TMDs could assume a correlated phase, with their spin aligned in the same direction to lower the system energy—this ferromagnetic phase is what we colloquially call magnetism. Generating or modeling this transition to the correlated state, however, has been difficult.

High has long been interested in how light can be controlled and, in turn, can alter states of matter. His team wondered whether, instead of external magnetic fields, miniscule beams of light could be used to create a correlated magnetic phase. They aimed a tightly-focused laser beam, less than a micron (one-thousandth of a millimeter) in diameter at a monolayer TMD. They flashed the laser for nanoseconds at a time, while also monitoring the TMD with an optical probe that let them track the activity of its electrons.

The probe revealed that the pulsing laser was impacting the spin-polarization of electrons within a 5 micron by 8 micron area of the TMD, spreading a correlated phase outward from the laser. In other words, the electrons were aligning their spin; the researchers could control the magnetic order of electrons within the tiny area.

“This new technique provides us a handy way to manipulate electron correlation, making the study of the correlated phases much more practical than it has been in the past,” said postdoctoral fellow Kai Hao, co-first author of the paper.

“One of the things that makes this really attractive is the rather straightforward nature of it,” said graduate student Andrew Kindseth, who also contributed to the new work. “In many ways, it’s as simple as just shining a circularly polarized laser on this material.”

A New Research Platform

The new technique for controlling magnetism in atomically thin semiconductors offers a jumping off point for a plethora of new studies, the researchers said.

Besides magnetic phases, TMD systems have also been hypothesized to form more exotic correlated electronic phases such as Wigner crystals, charge density waves, Mott states and superconductivity. The capability to locally manipulate the electron spins in TMDs within an ultrashort timescale and with nanoscale precision may provide previously inaccessible information, which will further aid the theoretical study of these exotic phases.

On the application side, there is an urgent need for novel optoelectronic and spintronic devices to meet the explosive growth in the information industry. The demonstration of efficient optical control of spin order has great potential for device applications. Immediate impacts include building on-chip spin sources, tunable optical isolators, and efficient fan-out in spintronic circuits.

“The capability to optically manipulate magnetic memory and generate spin amplification in TMDs – materials widely studied for next-generation technologies – will push optoelectronics and spintronics in new directions,” said graduate student Robert Shreiner, a co-first author of the paper.

Science paper:
Science Advances

See the full article here .

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The Pritzker School of Molecular Engineering is the first school of engineering at the University of Chicago. It was founded as the Institute for Molecular Engineering in 2011 by the university in partnership with Argonne National Laboratory. When the program was raised to the status of a school in 2019, it became the first school dedicated to molecular engineering in the United States. It is named for a major benefactor, the Pritzker Foundation.

The scientists, engineers, and students at PME use scientific research to pursue engineering solutions. The school does not have departments. Instead, it organizes its research around interdisciplinary “themes”: immuno-engineering, quantum engineering, autonomous materials, and water and energy. PME works toward technological advancements in areas of global importance, including sustainable energy and natural resources, immunotherapy-based approaches to cancer, “unhackable” communications networks, and a clean global water supply. The school plans to expand its research areas to address more issues of global importance.

IME was established in 2011, after three years of discussion and review. It was the largest academic program founded by the University of Chicago since 1988, when the Harris School of Public Policy Studies was established.

Matthew Tirrell was appointed founding Pritzker Director of IME in July 2011. The Pritzker Directorship honors the Pritzker Foundation, which donated a large gift in support of the institute. Tirrell is a researcher in biomolecular engineering and nanotechnology. His honors include election to The National Academy of Engineering, The American Academy of Arts and Sciences, and The National Academy of Sciences. He became dean of PME in 2019.

The William Eckhardt Research Center (WERC), which houses the school and part of the Physical Sciences Division, was constructed between 2011 and 2015. The WERC was named for alumnus William Eckhardt, in recognition of his donation to support scientific research at the university.

In 2019, the school received more than $23.1 million in research funding. From 2011 to 2019, faculty at the school have filed 69 invention disclosures and have created six companies.

On May 28, 2019, the University of Chicago announced a $100 million commitment from the Pritzker Foundation to support the institute’s transition to a school—the first school of molecular engineering in the U.S. The Pritzker Foundation helped establish the school with a new donation of $75 million, adding to an earlier $25 million donation that supported the institute and the construction of the Pritzker Nanofabrication Facility. In 2019, PME became the university’s first new school in three decades.

PME offers a graduate program in molecular engineering for both Master and Ph.D. students, as well as an undergraduate major and minor in molecular engineering offered with the College of the University of Chicago.

The institute began accepting applications to its doctoral program in fall 2013. The first class of graduate students was matriculated the following fall. In 2019, the school had 28 faculty members, 91 undergraduate students, 134 graduate students, and 75 postdoctoral fellows.

The graduate program curriculum includes various science and engineering disciplines, product design, entrepreneurship, and communication. The program is interdisciplinary, featuring a connected art program called STAGE Lab. STAGE Lab creates plays and films in the context of scientific research at PME.

The undergraduate major was added in spring 2015. It was the first engineering major offered at the University of Chicago. In 2018, the first undergraduate class received degrees in molecular engineering. When the school was established in 2019, it announced plans to expand its undergraduate offerings.

David Awschalom, a professor at PME, said the school has contributed to Chicago becoming a hub for quantum education and research. PME offers an advanced degree in quantum science and engineering. It also partnered with Harvard University to launch the Quantum Information Science and Engineering Network, a graduate student training program in quantum science and engineering. Participating students are paired with two mentors—one from academia and one from industry. The program was funded by a $1.6 million award from the National Science Foundation.

The school’s partnership with The DOE’s Argonne National Laboratory provides additional opportunities for research and innovation. Argonne’s facilities include the Advanced Photon Source, the Argonne Leadership Computing Facility, and the Center for Nanoscale Materials. The lab also has experience licensing new technology for industrial and commercial applications.
PME’s educational outreach initiatives include K-12 programs with events and internships throughout the year. In 2019, with the establishment of PME, the school also launched a partnership with City Colleges of Chicago. The multi-year program connects City College students interested in STEM fields with PME faculty and labs, with the goal of enabling these students to transfer into four-year STEM degree programs.

The University of Chicago is an urban research university that has driven new ways of thinking since 1890. Our commitment to free and open inquiry draws inspired scholars to our global campuses, where ideas are born that challenge and change the world.

We empower individuals to challenge conventional thinking in pursuit of original ideas. Students in the College develop critical, analytic, and writing skills in our rigorous, interdisciplinary core curriculum. Through graduate programs, students test their ideas with University of Chicago scholars, and become the next generation of leaders in academia, industry, nonprofits, and government.

University of Chicago research has led to such breakthroughs as discovering the link between cancer and genetics, establishing revolutionary theories of economics, and developing tools to produce reliably excellent urban schooling. We generate new insights for the benefit of present and future generations with our national and affiliated laboratories: DOE’s Argonne National Laboratory, DOE’s Fermi National Accelerator Laboratory , and the Marine Biological Laboratory in Woods Hole, Massachusetts.
The University of Chicago is enriched by the city we call home. In partnership with our neighbors, we invest in Chicago’s mid-South Side across such areas as health, education, economic growth, and the arts. Together with our medical center, we are the largest private employer on the South Side.

In all we do, we are driven to dig deeper, push further, and ask bigger questions—and to leverage our knowledge to enrich all human life. Our diverse and creative students and alumni drive innovation, lead international conversations, and make masterpieces. Alumni and faculty, lecturers and postdocs go on to become Nobel laureates, CEOs, university presidents, attorneys general, literary giants, and astronauts. The University of Chicago is a private research university in Chicago, Illinois. Founded in 1890, its main campus is located in Chicago’s Hyde Park neighborhood. It enrolled 16,445 students in Fall 2019, including 6,286 undergraduates and 10,159 graduate students. The University of Chicago is ranked among the top universities in the world by major education publications, and it is among the most selective in the United States.

The university is composed of one undergraduate college and five graduate research divisions, which contain all of the university’s graduate programs and interdisciplinary committees. Chicago has eight professional schools: the Law School, the Booth School of Business, the Pritzker School of Medicine, the School of Social Service Administration, the Harris School of Public Policy, the Divinity School, the Graham School of Continuing Liberal and Professional Studies, and the Pritzker School of Molecular Engineering. The university has additional campuses and centers in London, Paris, Beijing, Delhi, and Hong Kong, as well as in downtown Chicago.

University of Chicago scholars have played a major role in the development of many academic disciplines, including economics, law, literary criticism, mathematics, religion, sociology, and the behavioralism school of political science, establishing the Chicago schools in various fields. Chicago’s Metallurgical Laboratory produced the world’s first man-made, self-sustaining nuclear reaction in Chicago Pile-1 beneath the viewing stands of the university’s Stagg Field. Advances in chemistry led to the “radiocarbon revolution” in the carbon-14 dating of ancient life and objects. The university research efforts include administration of DOE’s Fermi National Accelerator Laboratory and DOE’s Argonne National Laboratory, as well as the U Chicago Marine Biological Laboratory in Woods Hole, Massachusetts (MBL). The university is also home to the University of Chicago Press, the largest university press in the United States. The Barack Obama Presidential Center is expected to be housed at the university and will include both the Obama presidential library and offices of the Obama Foundation.

The University of Chicago’s students, faculty, and staff have included 100 Nobel laureates as of 2020, giving it the fourth-most affiliated Nobel laureates of any university in the world. The university’s faculty members and alumni also include 10 Fields Medalists, 4 Turing Award winners, 52 MacArthur Fellows, 26 Marshall Scholars, 27 Pulitzer Prize winners, 20 National Humanities Medalists, 29 living billionaire graduates, and have won eight Olympic medals.

The University of Chicago is enriched by the city we call home. In partnership with our neighbors, we invest in Chicago’s mid-South Side across such areas as health, education, economic growth, and the arts. Together with our medical center, we are the largest private employer on the South Side.

Research

According to the National Science Foundation, University of Chicago spent $423.9 million on research and development in 2018, ranking it 60th in the nation. It is classified among “R1: Doctoral Universities – Very high research activity” and is a founding member of the Association of American Universities and was a member of the Committee on Institutional Cooperation from 1946 through June 29, 2016, when the group’s name was changed to the Big Ten Academic Alliance. The University of Chicago is not a member of the rebranded consortium, but will continue to be a collaborator.

The university operates more than 140 research centers and institutes on campus. Among these are the Oriental Institute—a museum and research center for Near Eastern studies owned and operated by the university—and a number of National Resource Centers, including the Center for Middle Eastern Studies. Chicago also operates or is affiliated with several research institutions apart from the university proper. The university manages DOE’s Argonne National Laboratory, part of the United States Department of Energy’s national laboratory system, and co-manages DOE’s Fermi National Accelerator Laboratory, a nearby particle physics laboratory, as well as a stake in the Apache Point Observatory in Sunspot, New Mexico.
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SDSS Telescope at Apache Point Observatory, near Sunspot NM, USA, Altitude 2,788 meters (9,147 ft).

Apache Point Observatory, near Sunspot, New Mexico Altitude 2,788 meters (9,147 ft).
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Faculty and students at the adjacent Toyota Technological Institute at Chicago collaborate with the university. In 2013, the university formed an affiliation with the formerly independent Marine Biological Laboratory in Woods Hole, Mass. Although formally unrelated, the National Opinion Research Center is located on Chicago’s campus.

From The University of Texas-Dallas : “Physicists Shine Light on Solid Way To Extend Excitons’ Life”

From The University of Texas-Dallas

June 17, 2022
Stephen Fontenot,
UT Dallas,
972-883-4405
stephen.fontenot@utdallas.edu,

Dr. Anton Malko’s Optics and Ultrafast Spectroscopy Laboratory focuses on the science and engineering of excitonic processes in various novel nanomaterials and hybrid structures. Malko and fellow researchers tested ultrathin semiconductors made with a method called laser-assisted synthesis technique in a recent study.

Optics researchers at The University of Texas at Dallas have shown for the first time that a new method for manufacturing ultrathin semiconductors yields material in which excitons survive up to 100 times longer than in materials created with previous methods.

The findings show that excitons, quasiparticles that transport energy, last long enough for a broad range of potential applications, including as bits in quantum computing devices.

Dr. Anton Malko, professor of physics in the School of Natural Sciences and Mathematics, is corresponding author of a paper published online March 30 in Advanced Materials that describes tests on ultrathin semiconductors made with a recently developed method called laser-assisted synthesis technique (LAST). The findings show novel quantum physics at work.

Semiconductors are a class of crystalline solids whose electrical conductivity is between that of a conductor and an insulator. This conductivity can be externally controlled, either by doping or electrical gating, making them key elements for the diodes and transistors that underpin all modern electronic technology.

Two-dimensional transition metal dichalcogenides (TMDs) are a novel type of ultrathin semiconductor consisting of a transition metal and a chalcogen element arranged in one atomic layer. While TMDs have been explored for a decade or so, the 2D form that Malko examined has advantages in scalability and optoelectronic properties.

“LAST is a very pure method. You take pure molybdenum or tungsten, and pure selenium or sulfur, and evaporate them under intense laser light,” Malko said. “Those atoms are distributed onto a substrate and make the two-dimensional TMD layer less than 1 nanometer thick.”

A material’s optical properties are partially determined by the behavior of excitons, which are quasiparticles that can transport energy while remaining electrically neutral.

“When a semiconductor absorbs a photon, it creates in the semiconductor a negatively charged electron paired with a positive hole, to maintain neutral charge. This pair is the exciton. The two parts are not completely free from each other — they still have a Coulomb interaction between them,” Malko said.

Malko and his team were surprised to discover that excitons in LAST-produced TMDs lasted up to 100 times longer than those in other TMD materials.

“We quickly found that, optically speaking, these 2D samples behave totally differently from any we’ve seen in 10 years working with TMDs,” he said. “When we started to look deeper at it, we realized it’s not a fluke; it’s repeatable and dependent on growth conditions.”

These longer lifetimes, Malko believes, are caused by indirect excitons, which are optically inactive.

“These excitons are used as a kind of reservoir to slowly feed the optically active excitons,” he said.

Lead study author Dr. Navendu Mondal, a former UT Dallas postdoctoral researcher who is now a Marie Skłodowska-Curie Individual Fellow at Imperial College London, said he believes the indirect excitons exist due to the abnormal amount of strain between the monolayer TMD material and the substrate on which it grows.

“Strain-controlling in atomically thin monolayer of TMDs is an important tool to tailor their optoelectronic properties,” Mondal said. “Their electronic band-structure is highly sensitive to structural deformations. Under enough strain, band-gap modifications cause formation of various indirect ‘dark’ excitons that are optically inactive. Through this finding, we reveal how the presence of these hidden dark excitons influences those excitons created directly by photons.”

Malko said the built-in strain in 2D TMDs is comparable to what would be induced by pressing on the material with externally placed micro- or nanosize pillars, although it is not a viable technological option for such thin layers.

“That strain is crucial for creating these optically inactive, indirect excitons,” he said. “If you remove the substrate, the strain is released, and this wonderful optical response is gone.”

Malko said the indirect excitons can be both electronically controlled and converted into photons opening a path to the development of new optoelectronic devices.

“This increased lifespan has very interesting potential applications,” he said. “When an exciton has a lifespan of only about 100 picoseconds or less, there is no time to use it. But in this material, we can create a reservoir of inactive excitons that live much longer — a few nanoseconds instead of hundreds of picoseconds. You can do a lot with this.”

Malko said the results of the research are an important proof-of-concept for future quantum-scale devices.

“It’s the first time we know of that anyone has made this fundamental observation of such long-living excitations in TMD materials — long enough to be usable as a quantum bit — just like an electron in a transistor or even just for light harvesting in a solar cell,” he said. “Nothing in the literature can explain these superlong exciton lifetimes, but we now understand why they have these characteristics.”

The researchers next will try to manipulate excitons with an electric field, which is a key step toward creating quantum-level logic elements.

“Classical semiconductors have already been miniaturized down to the doorstep before quantum effects change the game entirely,” Malko said. “If you can apply gate voltage and show that 2D TMD materials will work for future electronic devices, it’s a huge step. The atomic monolayer in 2D TMD material is 10 times smaller than the size limit with silicon. But can you create logic elements at that size? That’s what we need to find out.”

Other key contributors to this research are Dr. Yuri Gartstein, associate professor of physics at UT Dallas who did computational modeling that explained the reservoir behavior and coupling between different exciton species; and Dr. Masoud Mahjouri-Samani and graduate student Nurul Azam from Auburn University, who developed and used the LAST method to create the semiconductor material.

Funding for the research came from the U.S. Department of Energy, Basic Energy Sciences program (BES award #DE-SC0010697).

See the full article here .

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The University of Texas at Dallas is a Carnegie R1 classification (Doctoral Universities – Highest research activity) institution, located in a suburban setting 20 miles north of downtown Dallas. The University enrolls more than 27,600 students — 18,380 undergraduate and 9,250 graduate —and offers a broad array of bachelor’s, master’s, and doctoral degree programs.

Established by Eugene McDermott, J. Erik Jonsson and Cecil Green, the founders of Texas Instruments, UT Dallas is a young institution driven by the entrepreneurial spirit of its founders and their commitment to academic excellence. In 1969, the public research institution joined The University of Texas System and became The University of Texas at Dallas.

A high-energy, nimble, innovative institution, UT Dallas offers top-ranked science, engineering and business programs and has gained prominence for a breadth of educational paths from audiology to arts and technology. UT Dallas’ faculty includes a Nobel laureate, six members of the National Academies and more than 560 tenured and tenure-track professors.

From Lawrence Berkeley National Lab: “2D Electronics Get an Atomic Tuneup”

From Lawrence Berkeley National Lab

August 20, 2020
Theresa Duque
tnduque@lbl.gov
(510) 495-2418

Scientists at Berkeley Lab, UC Berkeley demonstrate tunable, atomically thin semiconductors.

Electron microscopy experiments revealed meandering stripes formed by metal atoms of rhenium and niobium in the lattice structure of a 2D transition metal dichalcogenide alloy. (Image courtesy of Amin Azizi.)

TO TUNE THE BAND GAP, a key parameter in controlling the electrical conductivity and optical properties of semiconductors, researchers typically engineer alloys, a process in which two or more materials are combined to achieve properties that otherwise could not be achieved by a pristine material.

But engineering band gaps of conventional semiconductors via alloying has often been a guessing game, because scientists have not had a technique to directly “see” whether the alloy’s atoms are arranged in a specific pattern, or randomly dispersed.

Now, as reported in Physical Review Letters, a research team led by Alex Zettl and Marvin Cohen – senior faculty scientists in the Materials Sciences Division at the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab), and professors of physics at UC Berkeley – has demonstrated a new technique that could engineer the band gap needed to improve the performance of semiconductors for next-generation electronics such as optoelectronics, thermoelectrics, and sensors.

For the current study, the researchers examined monolayer and multilayer samples of a 2D transition metal dichalcogenide (TMD) material made of the alloy rhenium niobium disulfide.

Electron microscopy experiments revealed meandering stripes formed by metal atoms of rhenium and niobium in the lattice structure of the 2D TMD alloy.

A statistical analysis confirmed what the research team had suspected – that metal atoms in the 2D TMD alloy prefer to be adjacent to the other metal atoms, “which is in stark contrast to the random structure of other TMD alloys of the same class,” said lead author Amin Azizi, a postdoctoral researcher in the Zettl lab at UC Berkeley.

Calculations performed at Berkeley Lab’s National Energy Research Scientific Computing Center (NERSC) by Mehmet Dogan, a postdoctoral researcher in the Cohen lab at UC Berkeley, demonstrated that such atomic ordering can modify the material’s band gap.

NERSC at LBNL

NERSC Cray Cori II supercomputer, named after Gerty Cori, the first American woman to win a Nobel Prize in science

NERSC Hopper Cray XE6 supercomputer, named after Grace Hopper, One of the first programmers of the Harvard Mark I computer

NERSC Cray XC30 Edison supercomputer

NERSC GPFS for Life Sciences

The Genepool system is a cluster dedicated to the DOE Joint Genome Institute’s computing needs. Denovo is a smaller test system for Genepool that is primarily used by NERSC staff to test new system configurations and software.

NERSC PDSF computer cluster in 2003.

PDSF is a networked distributed computing cluster designed primarily to meet the detector simulation and data analysis requirements of physics, astrophysics and nuclear science collaborations.

Future:

Cray Shasta Perlmutter SC18 AMD Epyc Nvidia pre-exascale supeercomputer

NERSC is a DOE Office of Science User Facility.

Optical spectroscopy measurements performed at Berkeley Lab’s Advanced Light Source revealed that the band gap of the 2D TMD alloy can be additionally tuned by adjusting the number of layers in the material.

LBNL ALS

Also, the band gap of the monolayer alloy is similar to that of silicon – which is “just right” for many electronic and optical applications, Azizi said. And the 2D TMD alloy has the added benefits of being flexible and transparent.

The researchers next plan to explore the sensing and optoelectronic properties of new devices based on the 2D TMD alloy.

Co-authors with Azizi, Cohen, and Zettl include Jeffrey D. Cain, Mehmet Dogan, Rahmatollah Eskandari, Emily G. Glazer, and Xuanze Yu.

The Advanced Light Source and NERSC are DOE Office of Science user facilities co-located at Berkeley Lab.

This work was supported by the DOE Office of Science. Additional funding was provided by the National Science Foundation.

See the full article here .

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Please help promote STEM in your local schools.

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LBNL campus

LBNL Molecular Foundry

Bringing Science Solutions to the World
In the world of science, Lawrence Berkeley National Laboratory (Berkeley Lab) is synonymous with “excellence.” Thirteen Nobel prizes are associated with Berkeley Lab. Seventy Lab scientists are members of the National Academy of Sciences (NAS), one of the highest honors for a scientist in the United States. Thirteen of our scientists have won the National Medal of Science, our nation’s highest award for lifetime achievement in fields of scientific research. Eighteen of our engineers have been elected to the National Academy of Engineering, and three of our scientists have been elected into the Institute of Medicine. In addition, Berkeley Lab has trained thousands of university science and engineering students who are advancing technological innovations across the nation and around the world.

Berkeley Lab is a member of the national laboratory system supported by the U.S. Department of Energy through its Office of Science. It is managed by the University of California (UC) and is charged with conducting unclassified research across a wide range of scientific disciplines. Located on a 202-acre site in the hills above the UC Berkeley campus that offers spectacular views of the San Francisco Bay, Berkeley Lab employs approximately 3,232 scientists, engineers and support staff. The Lab’s total costs for FY 2014 were $785 million. A recent study estimates the Laboratory’s overall economic impact through direct, indirect and induced spending on the nine counties that make up the San Francisco Bay Area to be nearly $700 million annually. The Lab was also responsible for creating 5,600 jobs locally and 12,000 nationally. The overall economic impact on the national economy is estimated at $1.6 billion a year. Technologies developed at Berkeley Lab have generated billions of dollars in revenues, and thousands of jobs. Savings as a result of Berkeley Lab developments in lighting and windows, and other energy-efficient technologies, have also been in the billions of dollars.

Berkeley Lab was founded in 1931 by Ernest Orlando Lawrence, a UC Berkeley physicist who won the 1939 Nobel Prize in physics for his invention of the cyclotron, a circular particle accelerator that opened the door to high-energy physics. It was Lawrence’s belief that scientific research is best done through teams of individuals with different fields of expertise, working together. His teamwork concept is a Berkeley Lab legacy that continues today.

A U.S. Department of Energy National Laboratory Operated by the University of California.

From MIT News: “Exploring interactions of light and matter”

From MIT News

June 30, 2020
David L. Chandler

Juejun Hu pushes the frontiers of optoelectronics for biological imaging, communications, and consumer electronics.

MIT professor Juejun Hu specializes in optical and photonic devices, whose applications include improving high-speed communications, observing the behavior of molecules, and developing innovations in consumer electronics. Image: Denis Paste

Growing up in a small town in Fujian province in southern China, Juejun Hu was exposed to engineering from an early age. His father, trained as a mechanical engineer, spent his career working first in that field, then in electrical engineering, and then civil engineering.

“He gave me early exposure to the field. He brought me books and told me stories of interesting scientists and scientific activities,” Hu recalls. So when it came time to go to college — in China students have to choose their major before enrolling — he picked materials science, figuring that field straddled his interests in science and engineering. He pursued that major at Tsinghua University in Beijing.

He never regretted that decision. “Indeed, it’s the way to go,” he says. “It was a serendipitous choice.” He continued on to a doctorate in materials science at MIT, and then spent four and a half years as an assistant professor at the University of Delaware before joining the MIT faculty. Last year, Hu earned tenure as an associate professor in MIT’s Department of Materials Science and Engineering.

In his work at the Institute, he has focused on optical and photonic devices, whose applications include improving high-speed communications, observing the behavior of molecules, designing better medical imaging systems, and developing innovations in consumer electronics such as display screens and sensors.

“I got fascinated with light,” he says, recalling how he began working in this field. “It has such a direct impact on our lives.”

Hu is now developing devices to transmit information at very high rates, for data centers or high-performance computers. This includes work on devices called optical diodes or optical isolators, which allow light to pass through only in one direction, and systems for coupling light signals into and out of photonic chips.

Lately, Hu has been focusing on applying machine-learning methods to improve the performance of optical systems. For example, he has developed an algorithm that improves the sensitivity of a spectrometer, a device for analyzing the chemical composition of materials based on how they emit or absorb different frequencies of light. The new approach made it possible to shrink a device that ordinarily requires bulky and expensive equipment down to the scale of a computer chip, by improving its ability to overcome random noise and provide a clean signal.

The miniaturized spectrometer makes it possible to analyze the chemical composition of individual molecules with something “small and rugged, to replace devices that are large, delicate, and expensive,” he says.

Much of his work currently involves the use of metamaterials, which don’t occur in nature and are synthesized usually as a series of ultrathin layers, so thin that they interact with wavelengths of light in novel ways. These could lead to components for biomedical imaging, security surveillance, and sensors on consumer electronics, Hu says. Another project he’s been working on involved developing a kind of optical zoom lens based on metamaterials, which uses no moving parts.

Hu is also pursuing ways to make photonic and photovoltaic systems that are flexible and stretchable rather than rigid, and to make them lighter and more compact. This could allow for installations in places that would otherwise not be practical. “I’m always looking for new designs to start a new paradigm in optics, [to produce] something that’s smaller, faster, better, and lower cost,” he says.

Hu says the focus of his research these days is mostly on amorphous materials — whose atoms are randomly arranged as opposed to the orderly lattices of crystal structures — because crystalline materials have been so well-studied and understood. When it comes to amorphous materials, though, “our knowledge is amorphous,” he says. “There are lots of new discoveries in the field.”

Hu’s wife, Di Chen, whom he met when they were both in China, works in the financial industry. They have twin daughters, Selena and Eos, who are 1 year old, and a son Helius, age 3. Whatever free time he has, Hu says, he likes to spend doing things with his kids.

Recalling why he was drawn to MIT, he says, “I like this very strong engineering culture.” He especially likes MIT’s strong system of support for bringing new advances out of the lab and into real-world application. “This is what I find really useful.” When new ideas come out of the lab, “I like to see them find real utility,” he adds.

See the full article here .

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Please help promote STEM in your local schools.

Stem Education Coalition

The mission of MIT is to advance knowledge and educate students in science, technology, and other areas of scholarship that will best serve the nation and the world in the twenty-first century. We seek to develop in each member of the MIT community the ability and passion to work wisely, creatively, and effectively for the betterment of humankind.

From MIT News: “Transparent graphene electrodes might lead to new generation of solar cells”

From MIT News

June 5, 2020
David L. Chandler

New roll-to-roll production method could enable lightweight, flexible solar devices and a new generation of display screens.

A new manufacturing process for graphene is based on using an intermediate carrier layer of material after the graphene is laid down through a vapor deposition process. The carrier allows the ultrathin graphene sheet, less than a nanometer (billionth of a meter) thick, to be easily lifted off from a substrate, allowing for rapid roll-to-roll manufacturing. These figures show this process for making graphene sheets, along with a photo of the proof-of-concept device used (b). Courtesy of the researchers.

A new way of making large sheets of high-quality, atomically thin graphene could lead to ultra-lightweight, flexible solar cells, and to new classes of light-emitting devices and other thin-film electronics.

The new manufacturing process, which was developed at MIT and should be relatively easy to scale up for industrial production, involves an intermediate “buffer” layer of material that is key to the technique’s success. The buffer allows the ultrathin graphene sheet, less than a nanometer (billionth of a meter) thick, to be easily lifted off from its substrate, allowing for rapid roll-to-roll manufacturing.

The process is detailed in a paper published yesterday in Advanced Functional Materials, by MIT postdocs Giovanni Azzellino and Mahdi Tavakoli; professors Jing Kong, Tomas Palacios, and Markus Buehler; and five others at MIT.

Finding a way to make thin, large-area, transparent electrodes that are stable in open air has been a major quest in thin-film electronics in recent years, for a variety of applications in optoelectronic devices — things that either emit light, like computer and smartphone screens, or harvest it, like solar cells. Today’s standard for such applications is indium tin oxide (ITO), a material based on rare and expensive chemical elements.

Many research groups have worked on finding a replacement for ITO, focusing on both organic and inorganic candidate materials. Graphene, a form of pure carbon whose atoms are arranged in a flat hexagonal array, has extremely good electrical and mechanical properties, yet it is vanishingly thin, physically flexible, and made from an abundant, inexpensive material. Furthermore, it can be easily grown in the form of large sheets by chemical vapor deposition (CVD), using copper as a seed layer, as Kong’s group has demonstrated. However, for device applications, the trickiest part has been finding ways to release the CVD-grown graphene from its native copper substrate.

This release, known as graphene transfer process, tends to result in a web of tears, wrinkles, and defects in the sheets, which disrupts the film continuity and therefore drastically reduces their electrical conductivity. But with the new technology, Azzellino says, “now we are able to reliably manufacture large-area graphene sheets, transfer them onto whatever substrate we want, and the way we transfer them does not affect the electrical and mechanical properties of the pristine graphene.”

The key is the buffer layer, made of a polymer material called parylene, that conforms at the atomic level to the graphene sheets on which it is deployed. Like graphene, parylene is produced by CVD, which simplifies the manufacturing process and scalability.

As a demonstration of this technology, the team made proof-of-concept solar cells, adopting a thin-film polymeric solar cell material, along with the newly formed graphene layer for one of the cell’s two electrodes, and a parylene layer that also serves as a device substrate. They measured an optical transmittance close to 90 percent for the graphene film under visible light.

The prototyped graphene-based solar cell improves by roughly 36 times the delivered power per weight, compared to ITO-based state-of-the-art devices. It also uses 1/200 the amount of material per unit area for the transparent electrode. And, there is a further fundamental advantage compared to ITO: “Graphene comes for almost free,” Azzellino says.

“Ultra-lightweight graphene-based devices can pave the way to a new generation of applications,” he says. “So if you think about portable devices, the power per weight becomes a very important figure of merit. What if we could deploy a transparent solar cell on your tablet that is able to power up the tablet itself?” Though some further development would be needed, such applications should ultimately be feasible with this new method, he says.

The buffer material, parylene, is widely used in the microelectronics industry, usually to encapsulate and protect electronic devices. So the supply chains and equipment for using the material already are widespread, Azzellino says. Of the three existing types of parylene, the team’s tests showed that one of them, which contains more chlorine atoms, was by far the most effective for this application.

The atomic proximity of chlorine-rich parylene to the underlying graphene as the layers are sandwiched together provides a further advantage, by offering a kind of “doping” for graphene, finally providing a more reliable and nondestructive approach for conductivity improvement of large-area graphene, unlike many others that have been tested and reported so far.

“The graphene and the parylene films are always face-to-face,” Azzellino says. “So basically, the doping action is always there, and therefore the advantage is permanent.”

The research team also included Marek Hempel, Ang-Yu Lu, Francisco Martin-Martinez, Jiayuan Zhao and Jingjie Yeo, all at MIT. The work was supported by Eni SpA through the MIT Energy Initiative, the U.S. Army Research Office through the Institute for Soldier Nanotechnologies, and the Office of Naval Research.

See the full article here .

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Please help promote STEM in your local schools.

Stem Education Coalition

The mission of MIT is to advance knowledge and educate students in science, technology, and other areas of scholarship that will best serve the nation and the world in the twenty-first century. We seek to develop in each member of the MIT community the ability and passion to work wisely, creatively, and effectively for the betterment of humankind.

From École Polytechnique Fédérale de Lausanne: “Controlling the optical properties of solids with acoustic waves”

From École Polytechnique Fédérale de Lausanne

29.11.19
Majed Chergui
Nik Papageorgiou

Physicists from Switzerland, Germany, and France have found that large-amplitude acoustic waves, launched by ultrashort laser pulses, can dynamically manipulate the optical response of semiconductors.

One of the main challenges in materials science research is to achieve high tunability of the optical properties of semiconductors at room temperature. These properties are governed by “excitons”, which are bound pairs of negative electrons and positive holes in a semiconductor.

Excitons have become increasingly important in optoelectronics and the last years have witnessed a surge in the search for control parameters – temperature, pressure, electric and magnetic fields – that can tune excitonic properties. However, moderately large changes have only been achieved under equilibrium conditions and at low temperatures. Significant changes at ambient temperatures, which are important for applications, have so far been lacking.

This has now just been achieved in the lab of Majed Chergui at EPFL within the Lausanne Centre for Ultrafast Science, in collaboration with the theory groups of Angel Rubio (Max-Planck Institute, Hamburg) and Pascal Ruello (Université de Le Mans). Publishing in Science Advances, the international team shows, for the first time, control of excitonic properties using acoustic waves. To do this, the researchers launched a high-frequency (hundreds of gigahertz), large-amplitude acoustic wave in a material using ultrashort laser pulses. This strategy further allows for the dynamical manipulation of the exciton properties at high speed.

This remarkable result was reached on titanium dioxide at room temperature, a cheap and abundant semiconductor that is used in a wide variety of light-energy conversion technologies such as photovoltaics, photocatalysis, and transparent conductive substrates.

“Our findings and the complete description we offer open very exciting perspectives for applications such as cheap acousto-optic devices or in sensor technology for external mechanical strain,” says Majed Chergui. “The use of high-frequency acoustic waves, as those generated by ultrashort laser pulses, as control schemes of excitons pave a new era for acousto-excitonics and active-excitonics, analogous to active plasmonics, which exploits the plasmon excitations of metals.”

“These results are just the beginning of what can be explored by launching high-frequency acoustic waves in materials,” adds Edoardo Baldini, the lead author of the article who is currently at MIT. “We expect to use them in the future to control the fundamental interactions governing magnetism or trigger novel phase transitions in complex solids”.

Other contributors

University of the Basque Country
Max Planck Institute for the Structure and Dynamics of Matter
Simons Foundation (Flatiron Institute)
CNRS Joint Research Units

Funding

Swiss National Science Foundation (NCCR:MUST and R’EQUIP), European Research Council (Advanced Grant DYNAMOX), Horizon 2020

See the full article here .

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Please help promote STEM in your local schools.

Stem Education Coalition

EPFL campus

EPFL is Europe’s most cosmopolitan technical university. It receives students, professors and staff from over 120 nationalities. With both a Swiss and international calling, it is therefore guided by a constant wish to open up; its missions of teaching, research and partnership impact various circles: universities and engineering schools, developing and emerging countries, secondary schools and gymnasiums, industry and economy, political circles and the general public.

From Lawrence Berkeley National Lab: “The Beauty of Imperfections: Linking Atomic Defects to 2D Materials’ Electronic Properties”

From Lawrence Berkeley National Lab

November 20, 2019
Theresa Duque
tnduque@lbl.gov
(510) 495-2418

Scientists at Berkeley Lab reveal oxygen’s hidden talent for filling in atomic gaps in TMDs; and the surprising role of electron spin in conductivity.

Scanning tunneling microscopy image of an oxygen substituting sulfur (left), and a sulfur vacancy (right) in tungsten disulfide. In comparison, a strand of human DNA is 2.5 nanometers (nm) in diameter, and a strand of human hair is about 100,000 nm wide. (Credit: Berkeley Lab)

Like any material, atomically thin, 2D semiconductors known as TMDs or transition metal dichalcogenides are not perfect, but their imperfections can actually be a good thing.

Understanding how defects are structured at the atomic scale, how they are created, and how they interact with electrons are the first steps to designing new advanced materials. However, no one has been able to link useful properties like optical absorption and emission, conductivity, or catalytic function to specific defects in TMDs.

Now, two studies led by scientists at the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) have revealed surprising details on how some atomic defects emerge in TMDs, and how those defects shape the 2D material’s electronic properties. Their findings could provide a more versatile yet targeted platform for designing 2D materials for quantum information science and smaller, more powerful next-generation light-based electronics (optoelectronics).

A quantum tip for 2D materials

In the world of materials science, many researchers assumed that the most abundant defects in TMDs were the result of missing atoms or “vacancies” of sulfur in tungsten disulfide (WS2), or selenium vacancies in molybdenum diselenide (MoSe2).

But as reported in Nature Communications, the researchers found that the defects previously observed with other methods were actually created by oxygen atoms replacing sulfur or selenium atoms, said D. Frank Ogletree, a staff scientist at Berkeley Lab’s Molecular Foundry and a co-author of the two studies.

Oxygen, like sulfur and selenium, is part of the oxygen or “chalcogen” family of elements. And since chalcogens share similar properties, there isn’t much change in conductivity when an oxygen atom takes the place of a sulfur or selenium atom in a TMD crystal structure, he said.

Atomic force microscopy image of sulfur vacancy in tungsten disulfide. (Credit: Berkeley Lab)

“In other words, it’s like exchanging one kind of apple for another,” explained co-lead author Bruno Schuler, a postdoctoral researcher at Berkeley Lab’s Molecular Foundry. “So when an oxygen atom fills in for a missing sulfur or selenium atom, it effectively restores the TMD’s electronic properties.”

Co-lead author with Schuler is Sara Barja, who was a postdoctoral researcher in Berkeley Lab’s Materials Sciences Division at the time of the Nature Communications study.

Key to their finding was the use of the Molecular Foundry’s atomic force microscope (AFM), with a single carbon monoxide (CO) molecule acting as an ultrasharp “tip” or probe, and scanning tunneling microscope (STM). They also benefited from state-of-the-art calculations carried out by scientists from Berkeley Lab’s Center for Computational Study of Excited State Phenomena in Energy Materials (C2SEPEM).

When used with AFM, the CO-tip images the surface atoms at a very high resolution that’s not possible with conventional techniques, and precisely pinpoints the defect’s atomic site; STM provides the defect’s unique electronic fingerprint.

The combined insights from both of these methods, combined with detailed calculations performed at Berkeley Lab’s National Energy Research Scientific Computing Center (NERSC), ultimately helped us understand what these defects are and why they behave the way they do,” said author Alexander Weber-Bargioni, who led the studies. Weber-Bargioni is the facility director for Imaging and Manipulation of Nanostructures at Berkeley Lab’s Molecular Foundry.

NERSC at LBNL

NERSC Cray Cori II supercomputer, named after Gerty Cori, the first American woman to win a Nobel Prize in science

NERSC Hopper Cray XE6 supercomputer, named after Grace Hopper, One of the first programmers of the Harvard Mark I computer

NERSC Cray XC30 Edison supercomputer

NERSC GPFS for Life Sciences

The Genepool system is a cluster dedicated to the DOE Joint Genome Institute’s computing needs. Denovo is a smaller test system for Genepool that is primarily used by NERSC staff to test new system configurations and software.

NERSC PDSF computer cluster in 2003.

PDSF is a networked distributed computing cluster designed primarily to meet the detector simulation and data analysis requirements of physics, astrophysics and nuclear science collaborations.

Future:

Cray Shasta Perlmutter SC18 AMD Epyc Nvidia pre-exascale supeercomputer

NERSC is a DOE Office of Science User Facility.

The unexpected power of an orbiting electron’s spin.

In the researchers’ second study, published in Physical Review Letters, they demonstrated how to deliberately create chalcogen vacancies by heating a sample of WS2 in vacuum up to 600 degrees Celsius (1,112 degrees Fahrenheit), resulting in a thermal energy that causes the atoms to vibrate. “The vibrations kick out one of the sulfur atoms, creating an atomic hole in the material’s crystalline structure,” explained lead author Schuler.

The scientists also discovered that “spin-orbit interaction” – which relates to the properties of electrons orbiting around an atom’s nucleus and in their own inherent directional spin – plays a significant role in the electronic structure of chalcogen vacancies.

“In many cases the electron orbital and spin are autonomous and do not care about each other,” he said. “But in some cases, as we discovered in our study, they interact and form hybrid states of electronic structure.”

Schuler noted that the impact of spin-orbit interaction on the electronic structure of defect sites in TMDs wasn’t clearly understood before this study.

“It wasn’t even on anyone’s radar. We’re the first to prove it not only by quantitatively determining the magnitude of spin-orbit coupling but also by directly imaging the defect’s electronic orbitals,” he said.

Now that the researchers have successfully demonstrated how to create chalcogen vacancies in TMDs, Schuler said that they plan to explore the engineering of atomic defects in other types of 2D materials, such as the creation of distinct spin-polarized states, which would be useful for realizing atomic quantum light emitters and other such devices.

Co-corresponding author Jeff Neaton, a senior faculty scientist in Berkeley Lab’s Materials Sciences Division and professor of physics at UC Berkeley, said that Berkeley Lab offers a unique venue for carrying out multidisciplinary studies.

“By combining novel experiments at the Molecular Foundry with leading-edge theory, and computing defects’ properties at NERSC with computational methods developed at C2SEPEM, we are steps closer to understanding how common defects can be used to tune optoelectronic properties in 2D materials,” he said.

Participants in the Nature Communications study involved researchers from Berkeley Lab; UC Berkeley; the University of the Basque Country UPV/EHU-CSIC, Basque Foundation for Science, and Donostia International Physics Center, Spain; Ecole Polytechnique Fédérale de Lausanne, Switzerland; the Korea Institute of Science and Technology; Pusan National University, Korea; and the Weizmann Institute of Science, Israel.

Participants in the Physical Review Letters study involved researchers from Berkeley Lab; UC Berkeley; Weizmann Institute of Science, Israel; Technical University of Munich; University of the Basque Country UPV/EHU-CSIC, Basque Foundation for Science, and Donostia International Physics Center, Spain.

Postdoctoral researchers Christoph Kastl and Christopher Chen of the Molecular Foundry grew the tungsten disulfide samples for the Nature Communications and Physical Review Letters studies; and Hyejin Ryu, a doctoral researcher at the Advanced Light Source (ALS), grew samples of molybdenum diselenide for the Nature Communications study.

LBNL ALS

The work for both studies was supported by the U.S. Department of Energy’s Office of Science, including the Computational Materials Sciences Center for Computational Study of Excited State Phenomena in Energy Materials (C2SEPEM); research at the Molecular Foundry, a DOE Office of Science user facility that specializes in nanoscale science; and resources at Berkeley Lab’s National Energy Research Scientific Computing Center (NERSC). Resources at the Advanced Light Source (ALS) were used for the Nature Communications study.

The U.S. National Science Foundation provided additional funding for the Nature Communications study, and the DOE Early Career Research Program provided additional funding for the Physical Review Letters study.

NERSC and the ALS are also DOE Office of Science user facilities.

See the full article here .

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Please help promote STEM in your local schools.

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LBNL campus

LBNL Molecular Foundry

Bringing Science Solutions to the World
In the world of science, Lawrence Berkeley National Laboratory (Berkeley Lab) is synonymous with “excellence.” Thirteen Nobel prizes are associated with Berkeley Lab. Seventy Lab scientists are members of the National Academy of Sciences (NAS), one of the highest honors for a scientist in the United States. Thirteen of our scientists have won the National Medal of Science, our nation’s highest award for lifetime achievement in fields of scientific research. Eighteen of our engineers have been elected to the National Academy of Engineering, and three of our scientists have been elected into the Institute of Medicine. In addition, Berkeley Lab has trained thousands of university science and engineering students who are advancing technological innovations across the nation and around the world.

Berkeley Lab is a member of the national laboratory system supported by the U.S. Department of Energy through its Office of Science. It is managed by the University of California (UC) and is charged with conducting unclassified research across a wide range of scientific disciplines. Located on a 202-acre site in the hills above the UC Berkeley campus that offers spectacular views of the San Francisco Bay, Berkeley Lab employs approximately 3,232 scientists, engineers and support staff. The Lab’s total costs for FY 2014 were $785 million. A recent study estimates the Laboratory’s overall economic impact through direct, indirect and induced spending on the nine counties that make up the San Francisco Bay Area to be nearly $700 million annually. The Lab was also responsible for creating 5,600 jobs locally and 12,000 nationally. The overall economic impact on the national economy is estimated at $1.6 billion a year. Technologies developed at Berkeley Lab have generated billions of dollars in revenues, and thousands of jobs. Savings as a result of Berkeley Lab developments in lighting and windows, and other energy-efficient technologies, have also been in the billions of dollars.

Berkeley Lab was founded in 1931 by Ernest Orlando Lawrence, a UC Berkeley physicist who won the 1939 Nobel Prize in physics for his invention of the cyclotron, a circular particle accelerator that opened the door to high-energy physics. It was Lawrence’s belief that scientific research is best done through teams of individuals with different fields of expertise, working together. His teamwork concept is a Berkeley Lab legacy that continues today.

A U.S. Department of Energy National Laboratory Operated by the University of California.

From NC State: “Researchers show thermally activated delayed photoluminescence from semiconductor nanocrystals”

North Carolina State University

This post is dedicated to J.M. in Maryland, because she loves pretty pictures.

Pyrenecarboxylic acid-functionalized CdSe quantum dots undergo thermally activated delayed photoluminescence. Image: Cedric Mongin

December 18, 2017
Tracey Peake
tracey_peake@ncsu.edu
919.515.6142

Researchers from North Carolina State University have found that the transfer of triplet excitons from nanomaterials to molecules also creates a feedback mechanism that returns some energy to the nanocrystal, causing it to photoluminesce on long time scales. The mechanism can be adjusted to control the amount of energy transfer, which could be useful in optoelectronic applications.

Felix N. Castellano, Goodnight Innovation Distinguished Chair of Chemistry at NC State, had previously shown that semiconductor nanocrystals could transfer energy to molecules, thereby extending their excited state lifetimes long enough for them to be useful in photochemical reactions.

In a new contribution, Castellano and Cédric Mongin, a former postdoctoral researcher currently an assistant professor at École normale supérieure Paris-Saclay in France, have shown that not only does the transfer of triplet excitons extend excited state lifetimes, but also that some of the energy gets returned to the original nanomaterial in the process.

“When we looked at triplet exciton transfers from nanomaterials to molecules, we noticed that after the initial transfer the nanomaterial would still luminesce in a delayed fashion, which was unexpected,” says Castellano. “So we decided to find out what exactly was happening at the molecular level.”

Castellano and Mongin utilized cadmium selenide (CdSe) quantum dots as the nanomaterial and pyrenecarboxylic acid (PCA) as the acceptor molecule. At room temperature, they found that the close proximity of the relevant energy levels created a feedback mechanism that thermally repopulated the CdSe excited state, causing it to photoluminesce.

Taking the experiment one step further, the researchers then systematically varied the CdSe-PCA energy gap by changing the size of the nanocrystals. This resulted in predictable changes to the resultant excited state lifetimes. They also examined this process at different temperatures, yielding results consistent with a thermally activated energy transfer mechanism.

“Depending on relative energy separation, the system can be tuned to behave more like PCA or more like the CdSe nanoparticle,” says Castellano. “It’s a control dial for the system. We can make materials with unique photoluminescent properties simply by controlling the size of the nanoparticle and the temperature of the system.”

The work appears in Nature Chemistry, and was supported by the Air Force Office of Scientific Research (FA9550-13-1-0106) and the U.S. Department of Energy (DE-SC0011979). Mongin is first author and Castellano is corresponding author. Pavel Moroz and Mikhail Zamkov of Bowling Green State University also contributed to the work.

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