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  • richardmitnick 1:38 pm on November 12, 2020 Permalink | Reply
    Tags: "Charges Cascading Along a Molecular Chain", , , , , , Scanning transmission electron microscopy   

    From DOE’s Lawrence Berkeley National Laboratory: “Charges Cascading Along a Molecular Chain” 

    From DOE’s Lawrence Berkeley National Laboratory

    November 12, 2020
    Rachel Berkowitz

    Removing one charged molecule from a one-dimensional array causes the others to alternately turn ‘on’ or ‘off,’ paving the way for information transfer in tiny circuits.

    STEM (scanning transmission electron microscopy) image of a one-dimensional array of F4TCNQ molecules (yellow-orange) on a gate-tunable graphene device. Credit: Berkeley Lab.

    Small electronic circuits power our everyday lives, from the tiny cameras in our phones to the microprocessors in our computers. To make those devices even smaller, scientists and engineers are designing circuitry components out of single molecules. Not only could miniaturized circuits offer the benefits of increased device density, speed, and energy efficiency – for example in flexible electronics or in data storage – but harnessing the physical properties of specific molecules could lead to devices with unique functionalities. However, developing practical nanoelectronic devices from single molecules requires precise control over the electronic behavior of those molecules, and a reliable method by which to fabricate them.

    Now, as reported in the journal Nature Electronics, researchers have developed a method to fabricate a one-dimensional array of individual molecules and to precisely control its electronic structure. By carefully tuning the voltage applied to a chain of molecules embedded in a one-dimensional carbon (graphene) layer, the team led by researchers at Lawrence Berkeley National Laboratory (Berkeley Lab) found it could control whether all, none, or some of the molecules carry an electric charge. The resulting charge pattern could then be shifted along the chain by manipulating individual molecules at the end of the chain.

    “If you’re going to build electrical devices out of individual molecules, you need molecules that have useful functionality and you need to figure out how to arrange them in a useful pattern. We did both of those things in this work,” said Michael Crommie, a senior faculty scientist in Berkeley Lab’s Materials Sciences Division, who led the project. The research is part of a U.S. Department of Energy (DOE) Office of Science-funded program on Characterization of Functional Nanomachines, whose overarching goal is to understand the electrical and mechanical properties of molecular nanostructures, and to create new molecule-based nanomachines capable of converting energy from one form to another at the nanoscale.

    The key trait of the fluorine-rich molecule selected by the Berkeley Lab team is its strong tendency to accept electrons. To control the electronic properties of a precisely aligned chain of 15 such molecules deposited on a graphene substrate, Crommie, who is also a UC Berkeley professor of physics, and his colleagues placed a metallic electrode underneath the graphene that was also separated from it by a thin insulating layer. Applying a voltage between the molecules and the electrode drives electrons into or out of the molecules. In that way, the graphene-supported molecules behave somewhat like a capacitor, an electrical component used in a circuit to store and release charge. But, unlike a “normal” macroscopic capacitor, by tuning the voltage on the bottom electrode the researchers could control which molecules became charged and which remained neutral.

    A one-dimensional array of molecules switch from electrically charged (blue dot) to neutral (empty dot) when an odd number of molecules is removed from the end of the pattern. This forces an electron into what used to be the second-to-the-last molecule, causing the other molecules to switch their charge state, thus shifting the alternating pattern of charges. Credit: Berkeley Lab.

    Previous studies of molecular assemblies, the molecules’ electronic properties could not be both tuned and imaged at atomic length scales. Without the additional imaging capability the relationship between structure and function cannot be fully understood in the context of electrical devices. By placing the molecules in a specially designed template on the graphene substrate developed at Berkeley Lab’s Molecular Foundry nanoscale science user facility, Crommie and his colleagues ensured that the molecules were completely accessible to both microscope observation and electrical manipulation.

    As expected, applying a strong positive voltage to the metallic electrode underneath the graphene supporting the molecules filled them with electrons, leaving the entire molecular array in a negatively charged state. Removing or reversing that voltage caused all the added electrons to leave the molecules, returning the entire array to a charge neutral state. At an intermediate voltage, however, electrons fill only every other molecule in the array, thus creating a “checkerboard” pattern of charge. Crommie and his team explain this novel behavior by the fact that electrons repel each other. If two charged molecules were to momentarily occupy adjacent sites, then their repulsion would push one of the electrons away and force it to settle one site farther down the molecular row.

    “We can make all the molecules empty of charge, or all full, or alternating. We call that a collective charge pattern because it’s determined by electron-electron repulsion throughout the structure,” said Crommie.

    Calculations suggested that in an array of molecules with alternating charges the terminal molecule in the array should always contain one extra electron since that molecule does not have a second neighbor to cause repulsion. In order to experimentally investigate this type of behavior, the Berkeley Lab team removed the final molecule in an array of molecules that had alternating charges. They found that the original charge pattern had shifted over by one molecule: sites that had been charged became neutral and vice versa. The researchers concluded that before the charged terminal molecule was removed, the molecule adjacent to it must have been neutral. In its new position at the end of the array, the formerly second molecule then became charged. To maintain the alternating pattern between charged and uncharged molecules, the entire charge pattern had to shift by one molecule.

    If the charge of each molecule is thought of as a bit of information, then removing the final molecule causes the entire pattern of information to shift by one position. That behavior mimics an electronic shift register in a digital circuit and provides new possibilities for transmitting information from one region of a molecular device to another. Moving a molecule at one end of the array could serve as flipping a switch on or off somewhere else in the device, providing useful functionality for a future logic circuit.

    “One thing that we found really interesting about this result is that we were able to alter the electronic charge and therefore the properties of molecules from very far away. That level of control is something new,” said Crommie.

    With their molecular array the researchers achieved the goal of creating a structure that has very specific functionality; that is, a structure whose molecular charges may be finely tuned between different possible states by applying a voltage. Changing the charge of the molecules causes a change in their electronic behavior and, as a result, in the functionality of the entire device. This work came out of a DOE effort to construct precise molecular nanostructures that have well-defined electromechanical functionality.

    The Berkeley Lab team’s technique for controlling molecular charge patterns could lead to new designs for nanoscale electronic components including transistors and logic gates. The technique could also be generalized to other materials and incorporated into more complex molecular networks. One possibility is to tune the molecules to create more complex charge patterns. For example, replacing one atom with another in a molecule can change the molecule’s properties. Placing such altered molecules in the array could create new functionality. Based on these results the researchers plan to explore the functionality that arises from new variations within molecular arrays, as well as how they can potentially be used as tiny circuit components. Ultimately, they plan to incorporate these structures into more practical nanoscale devices.

    The Molecular Foundry is a DOE Office of Science user facility located at Berkeley Lab.

    See the full article here .


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    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.

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  • richardmitnick 11:13 am on June 8, 2020 Permalink | Reply
    Tags: "Crystalline ‘nanobrush’ clears way to advanced energy and information tech", Advanced Materials, , Atom probe tomography-APT at the Center for Nanophase Materials Sciences a DOE Office of Science User Facility at ORNL., , , Scanning transmission electron microscopy   

    From Oak Ridge National Laboratory: “Crystalline ‘nanobrush’ clears way to advanced energy and information tech” 


    From Oak Ridge National Laboratory

    June 8, 2020
    Dawn M Levy

    A nanobrush made by pulsed laser deposition of CeO2 and Y2O3 with dim and bright bands, respectively, is seen in cross-section with scanning transmission electron microscopy. Credit: Oak Ridge National Laboratory, U.S. Dept. of Energy.

    A team led by the Department of Energy’s Oak Ridge National Laboratory synthesized a tiny structure with high surface area and discovered how its unique architecture drives ions across interfaces to transport energy or information. Their “nanobrush” contains bristles made of alternating crystal sheets with vertically aligned interfaces and plentiful pores.

    “These are major technical accomplishments and may prove useful in advancing energy and information technologies,” said ORNL’s Ho Nyung Lee, who led the study published in Nature Communications. “This is an excellent example of work that is only feasible with the unique expertise and capabilities available at national labs.”

    The team’s researchers hail from DOE national labs Oak Ridge and Argonne and Massachusetts Institute of Technology, or MIT, University of South Carolina, Columbia, and University of Tennessee, Knoxville.

    The bristles of their multilayer crystal, or “supercrystal,” are grown freestanding on a substrate. Former ORNL postdoctoral fellow Dongkyu Lee synthesized the supercrystals using pulsed laser epitaxy to deposit and build up alternating layers of fluorite-structure cerium oxide (CeO2) and bixbyite-structure yttrium oxide (Y2O3). Realization of the nanoscale bristles was made possible by the development of a novel precision synthesis approach that controls atom diffusion and aggregation during the growth of thin-film materials. Using scanning transmission electron microscopy, or STEM, former ORNL postdoctoral fellow Xiang Gao was surprised to discover atomically precise crystalline interfaces within the bristles.

    Scanning transmission electron microscope (200 kV Jeol prototype) equipped with a 3rd-order spherical aberration corrector. Materialscientist.

    To see the distribution of CeO2 and Y2O3 within the nanobrush, ORNL’s Jonathan Poplawsky measured samples from the bristles using atom probe tomography, or APT, at the Center for Nanophase Materials Sciences, a DOE Office of Science User Facility at ORNL. “APT is the only technique available that is capable of probing the three-dimensional positions of atoms in a material with sub-nanometer resolution and 10 parts per million chemical sensitivity,” Poplawsky said. “APT clarifies the local distributions of atoms within a nanosized object and was an excellent platform for providing information about the 3D structure of the interface between the cerium oxide and yttrium oxide layers.”

    For a 2017 paper [Advanced Science], the ORNL-led researchers used epitaxy by pulsed laser deposition to precisely synthesize nanobrushes with bristles containing only one compound. For the 2020 paper, they used the same method to layer two compounds, CeO2 and Y2O3, fabricating the first hybrid bristles with interfaces between the two materials. Traditionally, interfaces are aligned laterally by layering different crystals in thin films, whereas in the novel nanobrushes when grown on a particular surface, interfaces are aligned vertically through surface energy minimization in bristles that are only 10 nanometers wide — about 10,000 times thinner than a human hair.

    “This is a truly innovative way to build crystalline nanoarchitectures, providing unprecedented vertical interfaces that were never thought viable,” Ho Nyung Lee said. “You cannot achieve these perfect crystalline architectures from any other synthesis method.”

    He added, “There are many ways to utilize interfaces, which is why 2000 Nobel Prize winner Herbert Kroemer said, ‘the interface is the device.’” Conventionally, depositing layers of thin film materials on substrates creates interfaces that are horizontally aligned, allowing ions or electrons to move along the substrate’s 2D plane. The ORNL-led achievement is proof of concept that it is possible to create vertically aligned interfaces through which electrons or ions can be transported out of the substrate’s plane. Moreover, architectures like the nanobrush could be combined with other nanoscale architectures to create devices for quantum technologies and sensing as well as energy storage.

    The low-energy configuration of the fluorite structure caused the formation of unique chevron patterns, or inverted “V” shapes. A slight mismatch between different structures of fluorite and bixbyite crystal subunits causes mismatch of the electronic charges at their interfaces, causing oxygen atoms to vacate the fluorite side, which leads to the formation of functional defects. The spaces that are left behind can form interfacial oxygen ions and create an atomic-scale channel through which the ions can flow. “We are using the interfaces not only to artificially create oxygen ions, but also to guide ion movement in a more deliberate way,” Lee said.

    With the help of ORNL’s Matthew Chisholm, Gao used STEM to uncover the atomic structure of the crystal and electron energy-loss spectroscopy to reveal chemical and electronic insights about the interface. “We observed that a quarter of oxygen atoms are lost at the interfaces,” said Chisholm. “We were also surprised by the chevron growth pattern. It was critical at the beginning to really understand how the interfaces form within the bristles.”

    The nanobrush has a high porosity, and its architecture is advantageous for applications needing large surface area to maximize electronic and chemical interactions, such as sensors, membranes and electrodes. But how could the scientists determine the porosity of their material? Neutrons — neutral particles that pass through materials without destroying them — provided an excellent tool for characterizing porosity of the bulk material. The scientists used resources of the Spallation Neutron Source, a DOE Office of Science User Facility at ORNL, for extended Q-range small-angle neutron scattering that determined the upper limit of porosity to be 49%. “Quickly grown bristles can provide about 200 times as much surface area as a 2D thin film,” said ORNL co-author Michael Fitzsimmons.

    He added, “What we learn may advance applications of neutron science in the process. Whereas thin films do not provide sufficient surface area for neutron spectroscopy studies, ORNL’s novel nanobrush architecture does, and could be a platform for learning more about interfacial materials when an even brighter neutron beam becomes available at SNS’s Second Target Station, which is a funded construction project.”

    Theoretical calculations of the material system from the electronic and atomic level supported findings about oxygen-vacancy creation at the interfaces. MIT contributor Lixin Sun performed density functional theory calculations and molecular dynamics simulations under the direction of Bilge Yildiz.

    “Our theoretical calculations revealed how this interface can accommodate a largely different chemistry at this type of unique interface compared to bulk materials,” said Yildiz. The MIT calculations predicted the energy needed to remove a neutral oxygen atom to form a vacancy close to the interface or in the middle of a cerium oxide layer. “In particular, we found that a large fraction of oxygen ions is removed at the interface without deteriorating the lattice structure.”

    Lee said, “Indeed, these critical interfaces could form inside of nanobrush architectures, making them more promising than conventional thin films in many technological applications. Their much greater surface area and larger number of interfaces — potentially, thousands inside each bristle — may prove a game changer in future technologies in which the interface is the device.”

    The DOE Office of Science supported the research. The work used resources of the Center for Nanophase Materials Sciences and the Spallation Neutron Source, which are DOE Office of Science User Facilities at ORNL, as well as resources of the National Energy Research Scientific Computing Center and the Advanced Photon Source, DOE Office of Science User Facilities at Lawrence Berkeley National Laboratory and Argonne National Laboratory, respectively.

    See the full article here .

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    ORNL is managed by UT-Battelle for the Department of Energy’s Office of Science. DOE’s Office of Science is the single largest supporter of basic research in the physical sciences in the United States, and is working to address some of the most pressing challenges of our time.


  • richardmitnick 11:52 am on March 10, 2020 Permalink | Reply
    Tags: "UCLA-led research team produces most accurate 3D images of ‘2D materials’", , , , , Scanning transmission electron microscopy, The researchers examined a single layer of molybdenum disulfide a frequently studied 2D material.,   

    From UCLA Newsroom: “UCLA-led research team produces most accurate 3D images of ‘2D materials’” 

    From UCLA Newsroom

    March 9, 2020
    Wayne Lewis

    Image showing the 3D atomic coordinates of molybdenum (blue), sulfur (yellow) and added rhenium (orange). A 2D image is shown beneath the 3D model.

    Scientists develop innovative technique to pinpoint coordinates of single atoms.

    A UCLA-led research team has produced in unprecedented detail experimental three-dimensional maps of the atoms in a so-called 2D material — matter that isn’t truly two-dimensional but is nearly flat because it’s arranged in extremely thin layers, no more than a few atoms thick.

    Although 2D-materials–based technologies have not yet been widely used in commercial applications, the materials have been the subject of considerable research interest. In the future, they could be the basis for semiconductors in ever smaller electronics, quantum computer components, more-efficient batteries, or filters capable of extracting freshwater from saltwater.

    The promise of 2D materials comes from certain properties that differ from how the same elements or compounds behave when they appear in greater quantities. Those unique characteristics are influenced by quantum effects — phenomena occurring at extremely small scales that are fundamentally different from the classical physics seen at larger scales. For instance, when carbon is arranged in an atomically thin layer to form 2D graphene, it is stronger than steel, conducts heat better than any other known material, and has almost zero electrical resistance.

    But using 2D materials in real-world applications would require a greater understanding of their properties, and the ability to control those properties. The new study, which was published in Nature Materials, could be a step forward in that effort.

    The researchers showed that their 3D maps of the material’s atomic structure are precise to the picometer scale — measured in one-trillionths of a meter. They used their measurements to quantify defects in the 2D material, which can affect their electronic properties, as well as to accurately assess those electronic properties.

    “What’s unique about this research is that we determine the coordinates of individual atoms in three dimensions without using any pre-existing models,” said corresponding author Jianwei “John” Miao, a UCLA professor of physics and astronomy. “And our method can be used for all kinds of 2D materials.”

    Miao is the deputy director of the STROBE National Science Foundation Science and Technology Center and a member of the California NanoSystems Institute at UCLA. His UCLA lab collaborated on the study with researchers from Harvard University, Oak Ridge National Laboratory and Rice University.

    The researchers examined a single layer of molybdenum disulfide, a frequently studied 2D material. In bulk, this compound is used as a lubricant. As a 2D material, it has electronic properties that suggest it could be employed in next-generation semiconductor electronics. The samples being studied were “doped” with traces of rhenium, a metal that adds spare electrons when replacing molybdenum. That kind of doping is often used to produce components for computers and electronics because it helps facilitate the flow of electrons in semiconductor devices.

    To analyze the 2D material, the researchers used a new technology they developed based on scanning transmission electron microscopy, which produces images by measuring scattered electrons beamed through thin samples. Miao’s team devised a technique called scanning atomic electron tomography, which produces 3D images by capturing a sample at multiple angles as it rotates.

    The scientists had to avoid one major challenge to produce the images: 2D materials can be damaged by too much exposure to electrons. So for each sample, the researchers reconstructed images section by section and then stitched them together to form a single 3D image — allowing them to use fewer scans and thus a lower dose of electrons than if they had imaged the entire sample at once.

    The two samples each measured 6 nanometers by 6 nanometers, and each of the smaller sections measured about 1 nanometer by 1 nanometer. (A nanometer is one-billionth of a meter.)

    The resulting images enabled the researchers to inspect the samples’ 3D structure to a precision of 4 picometers in the case of molybdenum atoms — 26 times smaller than the diameter of a hydrogen atom. That level of precision enabled them to measure ripples, strain distorting the shape of the material, and variations in the size of chemical bonds, all changes caused by the added rhenium — marking the most accurate measurement ever of those characteristics in a 2D material.

    “If we just assume that introducing the dopant is a simple substitution, we wouldn’t expect large strains,” said Xuezeng Tian, the paper’s co-first author and a UCLA postdoctoral scholar. “But what we have observed is more complicated than previous experiments have shown.”

    The scientists found that the largest changes occurred in the smallest dimension of the 2D material, its three-atom-tall height. It took as little as a single rhenium atom to introduce such local distortion.

    Armed with information about the material’s 3D coordinates, scientists at Harvard led by Professor Prineha Narang performed quantum mechanical calculations of the material’s electronic properties.

    “These atomic-scale experiments have given us a new lens into how 2D materials behave and how they should be treated in calculations, and they could be a game changer for new quantum technologies,” Narang said.

    Without access to the sort of measurements generated in the study, such quantum mechanical calculations conventionally have been based on a theoretical model system that is expected at a temperature of absolute zero.

    The study indicated that the measured 3D coordinates led to more accurate calculations of the 2D material’s electronic properties.

    “Our work could transform quantum mechanical calculations by using experimental 3D atomic coordinates as direct input,” said UCLA postdoctoral scholar Dennis Kim, a co-first author of the study. “This approach should enable material engineers to better predict and discover new physical, chemical and electronic properties of 2D materials at the single-atom level.”

    Other authors were Yongsoo Yang, Yao Yang and Yakun Yuan of UCLA; Shize Yang and Juan-Carlos Idrobo of Oak Ridge National Laboratory; Christopher Ciccarino and Blake Duschatko of Harvard; and Yongji Gong and Pulickel Ajayan of Rice.

    The research was supported by the U.S. Department of Energy, the U.S. Army Research Office, and STROBE National Science Foundation Science and Technology Center. The scanning transmission electron microscopy experiments were conducted at the Center for Nanophase Materials Sciences, a DOE user facility at Oak Ridge National Laboratory.

    See the full article here .

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    For nearly 100 years, UCLA has been a pioneer, persevering through impossibility, turning the futile into the attainable.

    We doubt the critics, reject the status quo and see opportunity in dissatisfaction. Our campus, faculty and students are driven by optimism. It is not naïve; it is essential. And it has fueled every accomplishment, allowing us to redefine what’s possible, time after time.

    This can-do perspective has brought us 12 Nobel Prizes, 12 Rhodes Scholarships, more NCAA titles than any university and more Olympic medals than most nations. Our faculty and alumni helped create the Internet and pioneered reverse osmosis. And more than 100 companies have been created based on technology developed at UCLA.

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