Tagged: Scanning tunneling microscopy (STM) Toggle Comment Threads | Keyboard Shortcuts

  • richardmitnick 9:44 am on February 20, 2021 Permalink | Reply
    Tags: "Tuning Electrode Surfaces to Optimize Solar Fuel Production", , Bismuth vanadate, , , Combining STM and LEIS allowed the scientists to identify the atomic structure and chemical elements on the topmost surface layer of this photoelectrode material., , Interfacial energetics, LEIS-Low-energy ion scattering spectroscopy, Photoelectrochemical performance, Photoelectrodes, , Scanning tunneling microscopy (STM), The experimental and computational results both indicated that the bismuth-rich surfaces lead to more favorable surface energetics and improved photoelectrochemical properties for water splitting., X-ray photoelectron spectroscopy   

    From DOE’s Brookhaven National Laboratory(US): “Tuning Electrode Surfaces to Optimize Solar Fuel Production” 

    From DOE’s Brookhaven National Laboratory(US)

    February 18, 2021
    Ariana Manglaviti
    (631) 344-2347

    Peter Genzer
    (631) 344-3174

    An electrode material with modified surface atoms generates more electrical current, which drives the sunlight-powered reactions that split water into oxygen and hydrogen—a clean fuel.

    Through a tight coupling of experiment and theory, scientists showed at the atomic level how changes in the surface composition of a photoelectrode play a critical role in photoelectrochemical performance.

    Scientists have demonstrated that modifying the topmost layer of atoms on the surface of electrodes can have a remarkable impact on the activity of solar water splitting. As they reported in Nature Energy on Feb. 18, bismuth vanadate electrodes with more bismuth on the surface (relative to vanadium) generate higher amounts of electrical current when they absorb energy from sunlight. This photocurrent drives the chemical reactions that split water into oxygen and hydrogen. The hydrogen can be stored for later use as a clean fuel. Producing only water when it recombines with oxygen to generate electricity in fuel cells, hydrogen could help us achieve a clean and sustainable energy future.

    “The surface termination modifies the system’s interfacial energetics, or how the top layer interacts with the bulk,” said co-corresponding author Mingzhao Liu, a staff scientist in the Interface Science and Catalysis Group of the Center for Functional Nanomaterials (CFN)[below], a U.S. Department of Energy (DOE) Office of Science User Facility at Brookhaven National Laboratory. “A bismuth-terminated surface exhibits a photocurrent that is 50-percent higher than a vanadium-terminated one.”

    “Studying the effects of surface modification with an atomic-level understanding of their origins is extremely challenging, and it requires tightly integrated experimental and theoretical investigations,” said co-corresponding author Giulia Galli from the University of Chicago(US) and DOE’s Argonne National Laboratory(US).

    “It also requires the preparation of high-quality samples with well-defined surfaces and methods to probe the surfaces independently from the bulk,” added co-corresponding author Kyoung-Shin Choi from the University of Wisconsin–Madison(US).

    Choi and Galli, experimental and theoretical leaders in the field of solar fuels, respectively, have been collaborating for several years to design and optimize photoelectrodes for producing solar fuels. Recently, they set out to design strategies to illuminate the effects of electrode surface composition, and, as CFN users, they teamed up with Liu.

    “The combination of expertise from the Choi Group in photoelectrochemistry, the Galli Group in theory and computation, and the CFN in material synthesis and characterization was vital to the study’s success,” commented Liu.

    Bismuth vanadate is a promising electrode material for solar water splitting because it strongly absorbs sunlight across a range of wavelengths and remains relatively stable in water. Over the past few years, Liu has perfected a method for precisely growing single-crystalline thin films of this material. High-energy laser pulses strike the surface of polycrystalline bismuth vanadate inside a vacuum chamber. The heat from the laser causes the atoms to evaporate and land on the surface of a base material (substrate) to form a thin film.

    “To see how different surface terminations affect photoelectrochemical activity, you need to be able to prepare crystalline electrodes with the same orientation and bulk composition,” explained co-author Chenyu Zhou, a graduate researcher from Stony Brook University working with Liu. “You want to compare apples to apples.”

    As grown, bismuth vanadate has an almost one-to-one ratio of bismuth to vanadium on the surface, with slightly more vanadium. To create a bismuth-rich surface, the scientists placed one sample in a solution of sodium hydroxide, a strong base.

    “Vanadium atoms have a high tendency to be stripped from the surface by this basic solution,” said first author Dongho Lee, a graduate researcher working with Choi. “We optimized the base concentration and sample immersion time to remove only the surface vanadium atoms.”

    To confirm that this chemical treatment changed the composition of the top surface layer, the scientists turned to low-energy ion scattering spectroscopy (LEIS) and scanning tunneling microscopy (STM) at the CFN.

    In LEIS, electrically charged atoms with low energy—in this case, helium—are directed at the sample. When the helium ions hit the sample surface, they become scattered in a characteristic pattern depending on which atoms are present at the very top. According to the team’s LEIS analysis, the treated surface contained almost entirely bismuth, with an 80-to-20 ratio of bismuth to vanadium.

    “Other techniques such as x-ray photoelectron spectroscopy can also tell you what atoms are on the surface, but the signals come from several layers of the surface,” explained Liu. “That’s why LEIS was so critical in this study—it allowed us to probe only the first layer of surface atoms.”

    In STM, an electrically conductive tip is scanned very close to the sample surface while the tunneling current flowing between the tip and sample is measured. By combining these measurements, scientists can map the electron density—how electrons are arranged in space—of surface atoms. Comparing the STM images before and after treatment, the team found a clear difference in the patterns of atomic arrangements corresponding to vanadium- and bismuth-rich surfaces, respectively.

    The multiprobe surface analysis system in the CFN Proximal Probes Facility.

    “Combining STM and LEIS allowed us to identify the atomic structure and chemical elements on the topmost surface layer of this photoelectrode material,” said co-author Xiao Tong, a staff scientist in the CFN Interface Science and Catalysis Group and manager of the multiprobe surface analysis system used in the experiments. “These experiments demonstrate the power of this system for exploring surface-dominated structure-property relationships in fundamental research applications.”

    Simulated STM images based on surface structural models derived from first-principle calculations (those based on the fundamental laws of physics) closely matched the experimental results.

    “Our first-principle calculations provided a wealth of information, including the electronic properties of the surface and the exact positions of the atoms,” said co-author and Galli Group postdoctoral fellow Wennie Wang. “This information was critical to interpreting the experimental results.”

    After proving that the chemical treatment successfully altered the first layer of atoms, the team compared the light-induced electrochemical behavior of the treated and nontreated samples.

    “Our experimental and computational results both indicated that the bismuth-rich surfaces lead to more favorable surface energetics and improved photoelectrochemical properties for water splitting,” said Choi. “Moreover, these surfaces pushed the photovoltage to a higher value.”

    Many times, particles of light (photons) do not provide enough energy for water splitting, so an external voltage is needed to help perform the chemistry. From an energy-efficiency perspective, you want to apply as little additional electricity as possible.

    “When bismuth vanadate absorbs light, it generates electrons and electron vacancies called holes,” said Liu. “Both of these charge carriers need to have enough energy to do the necessary chemistry for the water-splitting reaction: holes to oxidize water into oxygen gas, and electrons to reduce water into hydrogen gas. While the holes have more than enough energy, the electrons don’t. What we found is that the bismuth-terminated surface lifts the electrons to higher energy, making the reaction easier.”

    Because holes can easily recombine with electrons instead of being transferred to water, the team did additional experiments to understand the direct effect of surface terminations on photoelectrochemical properties. They measured the photocurrent of both samples for sulfite oxidation. Sulfite, a compound of sulfur and oxygen, is a “hole scavenger,” meaning it quickly accepts holes before they have a chance to recombine with electrons. In these experiments, the bismuth-terminated surfaces also increased the amount of generated photocurrent.

    “It’s important that electrode surfaces perform this chemistry as quickly as possible,” said Liu. “Next, we’ll be exploring how co-catalysts applied on top of the bismuth-rich surfaces can help expedite the delivery of holes to water.”

    The work by Choi and Galli was supported by the National Science Foundation and used computational resources of the University of Chicago’s Research Computing Center. The work at the CFN was supported by the DOE Office of Science and carried out in the Materials Synthesis and Characterization and Proximal Probes Facilities.

    See the full article here .


    Please help promote STEM in your local schools.

    Stem Education Coalition

    Brookhaven Campus.

    BNL Center for Functional Nanomaterials.



    BNL RHIC Campus.

    BNL/RHIC Star Detector.

    BNL/RHIC Phenix.

    One of ten national laboratories overseen and primarily funded by the Office of Science of the U.S. Department of Energy (DOE)(US), Brookhaven National Laboratory conducts research in the physical, biomedical, and environmental sciences, as well as in energy technologies and national security. Brookhaven Lab also builds and operates major scientific facilities available to university, industry and government researchers. The Laboratory’s almost 3,000 scientists, engineers, and support staff are joined each year by more than 5,000 visiting researchers from around the world. Brookhaven is operated and managed for DOE’s Office of Science by Brookhaven Science Associates, a limited-liability company founded by Stony Brook University, the largest academic user of Laboratory facilities, and Battelle, a nonprofit, applied science and technology organization.

  • richardmitnick 10:19 am on December 14, 2020 Permalink | Reply
    Tags: "Researchers Pinpoint More Precise Method for Atomic-Level Manufacturing", , Hydrogen depassivation lithography (HDL), , Scanning tunneling microscopy (STM),   

    From The University of Texas at Dallas: “Researchers Pinpoint More Precise Method for Atomic-Level Manufacturing” 

    From The University of Texas at Dallas

    Dec. 11, 2020
    Kim Horner

    Building a silicon-based qubit, or quantum bit, the basic unit of information in a quantum computer, starts with an atomically flat silicon surface (left) coated with a layer of hydrogen. On the right, areas where UT Dallas researchers removed hydrogen atoms are highlighted.

    Quantum computers have the potential to transform fields such as medicine, cybersecurity and artificial intelligence by solving hard optimization problems that are beyond the reach of conventional computing hardware.

    But the technology to manufacture the devices on a large scale does not yet exist.

    Researchers at The University of Texas at Dallas have developed a technique that could remove one of the challenges to scaling the production of silicon quantum devices. The researchers outlined their method, which provides greater control and precision during the fabrication process, in a study published in the July print edition of the Journal of Vacuum Science & Technology B. Silicon is the preferred material for the base of quantum devices because of its compatibility with conventional semiconductor technology.

    The study’s corresponding author, Dr. Reza Moheimani, the James Von Ehr Distinguished Chair in Science and Technology and a professor of systems engineering in the Erik Jonsson School of Engineering and Computer Science, received a $2.4 million U.S. Department of Energy grant in 2019 to develop technology for atomically precise manufacturing, the process of building new materials and devices atom by atom.

    Moheimani’s team is addressing a range of challenges to quantum device fabrication.

    “Our latest work increases the precision of the fabrication process,” Moheimani said. “We’re also working to increase throughput, speed and reliability.”

    The researchers’ method for building a silicon-based qubit, or quantum bit, the basic unit of information in a quantum computer, starts with an atomically flat silicon surface coated with a layer of hydrogen, which prevents other atoms or molecules from getting absorbed into the surface. Next, researchers use a scanning tunneling microscope (STM), which features a probe with an atomically sharp tip, functioning as a micro-robotic arm, to remove atoms of hydrogen selectively from the surface. The STM was designed for imaging atomic features on a surface, however, researchers also use the device to manipulate atoms in a mode called hydrogen depassivation lithography (HDL).

    The painstaking process involves positioning the tip over an atom of hydrogen, adding a high-frequency signal to the tip-sample bias voltage and ramping up the amplitude of the high-frequency signal until the atom of hydrogen detaches from the surface, revealing silicon underneath. After a predetermined number of hydrogen atoms are selectively removed from the surface, phosphine gas is introduced in the environment and after a specific process, atoms of phosphorus are adsorbed to the surface, where each functions as a qubit.

    The problem with conventional HDL is that it can be easy for the operator to pluck the wrong atom of hydrogen resulting in creation of qubits at unwanted locations. Using the STM for HDL requires a higher voltage than for imaging, which too often causes the tip to crash into the surface sample, forcing the operator to start over.

    The researchers were working on their solution to the STM tip-crash problem when they discovered a more precise method for manipulating the surface atoms.

    “Conventional lithography cannot achieve the requisite atomic precision,” Moheimani said. “The issue is that we are using a microscope to do lithography; we’re using a device to do something it’s not designed for.”

    The researchers found that they could achieve higher precision by performing HDL in imaging mode, rather than the conventional lithography mode, with some adjustments to the voltage and a change to the STM’s feedback control system.

    “We realized that we could actually use this method to remove hydrogen atoms in a controlled fashion,” Moheimani said. “This came as a surprise. It’s one of those things that happens during experiments, and you try to explain it and take advantage of it.”

    Quantum computers are expected to be able to store more information than current computers. Current transistors, which relay information, cannot be made any smaller, said Hamed Alemansour, a mechanical engineering doctoral student and lead author of the study.

    “The kind of technology that’s used now for making transistors has reached its limit. It’s difficult to decrease the size any more through conventional methods,” Alemansour said.

    While a conventional computer uses the precise values of 1s and 0s to make calculations, the fundamental logic units of a quantum computer are more fluid, with values that can exist as a combination of 1s and 0s at the same time or anywhere in between. The fact that a qubit can represent two numbers at the same time allows the quantum computer to process information much faster.

    One of the next challenges, Moheimani said, will be to develop technology to operate multiple STM tips at a time.

    “What if we can use 10 or 100 tips in parallel with each other so we can do the same lithography multiplied by 100 times? What if we can do it 10 times faster? If we can manufacture 100 qubits 10 times faster, we’re 1,000 times better off already,” Moheimani said.

    Other researchers involved with the current study include scientists at Richardson, Texas-based nanotechnology company Zyvex Labs: Dr. John Randall, president and CEO, who co-chairs the Jonsson School’s Industrial Advisory Council and is on the adjunct faculty at UT Dallas; Dr. James Owen, director of anatomically precise manufacturing; and Dr. Ehud Fuchs, research scientist.

    See the full article here .


    Please help promote STEM in your local schools.

    Stem Education Coalition

    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.

  • richardmitnick 8:26 am on September 23, 2020 Permalink | Reply
    Tags: "I focus on how electrons behave within solids.", "Think We Already Know Everything About Electrons? Think Again", , , , Emergent phenomena-in which groups of atoms or electrons act in unexpected ways., Groups of electrons collectively behave differently than we would expect from the way each individual electron acts on its own., , Scanning tunneling microscopy (STM), , Songtian Sonia Zhang, Superconductivity is an example of an emergent phenomenon within physics., , ,   

    From Simons Foundation: Women in STEM-Songtian Sonia Zhang”Think We Already Know Everything About Electrons? Think Again” 

    From Simons Foundation

    Songtian Sonia Zhang. Credit: Rick Soden, Princeton University
    Physicist Songtian Sonia Zhang explores how electrons work within the tiniest objects and finds that they sometimes do unexpected things.

    September 22, 2020
    Marcus Banks

    Songtian Sonia Zhang envisioned a life in finance, until she discovered that learning how electrons work is much more rewarding. A fundamental physicist with a bachelor’s degree from the University of Waterloo in Ontario, Canada, and a doctorate from Princeton University, Zhang has already discovered unexpected behaviors among electrons found in quantum materials like superconductors or magnets. But many mysteries remain about the behavior of these tiny particles. Now beginning a postdoctoral appointment at Columbia University in the physics lab of Dmitri N. Basov, Zhang already has lots of ideas about what she wants to explore next. Our conversation has been edited for clarity.

    You began as a dual major in economics and physics at the University of Waterloo. What prompted the sharper focus on physics instead?

    When I was an undergraduate at Waterloo, I planned to pursue a career in finance, perhaps even on Wall Street. I was interested in physics too, but I never imagined becoming a physicist. That all changed after I completed a physics research project in my third year of college, which happened to overlap with my first job at a financial services firm. This gave me the chance to directly compare finance to physics work — and physics won out handily.

    When I was doing physics, I felt like I was helping to bring new understanding into the world. I know that sounds corny, but it’s true. And it was far more rewarding to me than my work at the financial firm, where I essentially was moving money around. Don’t get me wrong, we need money! But I knew early that physics was for me.

    That sounds clarifying! But the study of physics is broad. How did you narrow your interests?

    During my last semester at Waterloo I researched a special kind of magnet known as ‘spin ice,’ in which the atoms are arranged in a complex lattice pattern. Most magnets have a north and south pole. And if you cut a magnet in two, each new magnet will then also have a north and south pole. But spin ice magnets have such a complex structure that we call them geometrically frustrated. Spin ice magnets can behave like monopoles — that is, a magnet with only one pole instead of the normal two. We still don’t know if monopoles even exist! But the spin ice magnet I studied sure seemed like a monopole, which was fascinating to me. When I first began to study physics, I assumed I would become an astrophysicist. Instead I decided to be more down-to-earth — literally. Today I study the physics of solids, not stars.

    This sounds like quantum physics.

    In many ways, yes. But quantum physics is an extremely broad term that can apply to many things, so in some ways it’s too general. The specific field I work in is condensed matter physics. I focus on how electrons behave within solids. I’m particularly interested in how groups of electrons behave, and especially how their collective behavior cannot be predicted by how each individual in the group acts.

    You’re saying that groups of electrons collectively behave differently than we would expect from the way each individual electron acts on its own?

    Exactly. We call this overall concept ‘emergent phenomena,’ in which groups of atoms or electrons act in unexpected ways. There are many examples of emergent phenomena in nature that go well beyond quantum physics. Think of individual birds migrating together as a cohesive flock, or a school of fish swimming upriver to spawn. Even though each individual bird or fish moves independently, they become entangled in the group and impossible to distinguish from one another.

    Superconductivity is an example of an emergent phenomenon within physics. Regular electrical conductors carry a current known as electricity; this is how we light lightbulbs, for example, by connecting an electricity source to an object that emits light. These kinds of everyday conductors come with inherent inefficiencies — energy is always lost because the electricity faces resistance as it travels. This is why lightbulbs eventually burn out.

    In contrast, a superconductor operating in extremely low temperatures (−450 F) can keep conducting electricity forever, because the electricity meets no resistance at all. Nobody could have predicted that superconductors could do this. It had to be discovered through observation, and it’s an example of how we are constantly learning about new types of emergent phenomena. Sometimes this is purely about developing knowledge for its own sake, but oftentimes this work has practical applications.

    We’ll loop back to the practical applications in a moment. First, though, what was your most exciting discovery at Princeton?

    At Princeton I studied kagome magnets. The atoms that comprise these magnets are arranged in a lattice which evokes the famous Japanese basket lattice pattern of the same name.

    Scanning tunneling microscopy (STM) image of magnetic adatoms deposited on topological superconductor candidate PbTaSe2. Inset: a 2D enlarged view. Each magnetic adatom can host a Majorana zero mode acting as a topological qubit which has the potential to be used for robust quantum computation. Credit: Songtian Zhang.

    Like the spin ice magnets I previously mentioned, kagome magnets are geometrically frustrated. In our research, we did various things to these magnets, such as observing them within magnetic fields of various strengths or alternating their temperatures. This was all to see how the electrons behaved in different conditions. In high magnetic fields the kagome magnets started acting like negative magnets — meaning they exerted more energy when moving in the same direction as a magnetic field and not when going ‘against the wind,’ so to speak.

    We published these unexpected results in Nature Physics last year.

    The fact that we discovered something totally unexpected shows the importance of keeping an open mind, of not being locked into any one idea. Instead, we are always finding new questions to ask.

    As you begin your postdoctoral work at Columbia, what do you plan to focus on, at least initially, until you discover new questions?

    I’m interested in learning more about topological insulators: objects that, on the surface, conduct electricity but in their interior act as an insulator. When the material is cut, the new surface, which was previously the insulating bulk, becomes conductive and can now support surface currents. Besides topological insulators there are topological superconductors, which can superconduct currents of electricity. The physics community has made some headway in understanding these superconductors, but there’s a lot more work that needs to be done.

    And how do you hope this knowledge will inform our understanding of the natural and physical world?

    Topological superconductors come from the math concept of topology. A good way to think about topology is the relationship between a doughnut, with a hole in the middle, and a ball, which has no holes. In this comparison, the doughnut and the ball are topologically distinct.

    By comparison, the doughnut would be topologically identical to a ring, which also has a hole in the middle. In this example, the number of holes is a topological property that can’t be destroyed without changing the underlying nature of the object.

    In physics, we’re interested in electronic behaviors that are similarly robust, such as in topological superconductors. There’s great potential for topological superconductors to be used for powerful, reliable and robust quantum computation, which will be a giant leap past the computers we use today. I can’t say exactly how my work will contribute to this, but I do know I’m excited to be on the journey. And I know I will enjoy it more than working on Wall Street.

    See the full article here.


    Please help promote STEM in your local schools.

    Stem Education Coalition

    Mission and Model

    The Simons Foundation’s mission is to advance the frontiers of research in mathematics and the basic sciences.

    Co-founded in New York City by Jim and Marilyn Simons, the foundation exists to support basic — or discovery-driven — scientific research undertaken in the pursuit of understanding the phenomena of our world.

    The Simons Foundation’s support of science takes two forms: We support research by making grants to individual investigators and their projects through academic institutions, and, with the launch of the Flatiron Institute in 2016, we now conduct scientific research in-house, supporting teams of top computational scientists.

Compose new post
Next post/Next comment
Previous post/Previous comment
Show/Hide comments
Go to top
Go to login
Show/Hide help
shift + esc
%d bloggers like this: