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News Article | February 28, 2017
Site: www.eurekalert.org

WASHINGTON, D.C., Feb. 28, 2017 -- Limitations of the piezoelectric array technologies conventionally used for ultrasonics inspired a group of University College London researchers to explore an alternative mechanism for generating ultrasound via light, also known as the photoacoustic effect. Coupling this with 3-D printing, the group was able to generate sounds fields with specific shapes for potential use in biological cell manipulation and drug delivery. Piezoelectric materials generate mechanical stress in response to an applied electric field, resulting in a usable and precisely controllable force that can, for example, be used to create sound waves. But achieving this control with conventional piezoelectric arrays requires both complicated electronics and large numbers of extremely small individual components which are expensive and difficult to manufacture. The photoacoustic effect, in contrast, occurs when a short pulse or modulated source of light is absorbed by a material, producing a sound wave. As the group reports in this week's Applied Physics Letters, from AIP Publishing, their work focuses on using the photoacoustic effect to control ultrasound fields in 3-D. "One useful feature of the photoacoustic effect is that the initial shape of the sound that's generated is determined [by] where the light is absorbed," said Michael Brown, a doctoral student at the Biomedical Ultrasound Group of the Department of Medical Physics and Biomedical Engineering at University College London. "This can be used to create tightly focused intense points of sound just by depositing an optical absorber on a concave surface, which acts like a lens." More generally, it's possible to manufacture samples with nearly any surface shape by using a 3-D printer and a transparent material. "By depositing an optical absorber on this surface, which can be done via spray painting, a sound wave of nearly any shape can be created by illuminating this sample with a laser," Brown said. "If you carefully tailor the design of the surface and therefore the shape of the acoustic wave, it's possible to control where the sound field will focus and even create fields focused over continuous shapes. We're using letters and numbers." This is particularly significant because, in theory, the ability to control the shape of the wavefront -- the surface over which the sound wave has a constant phase, somewhat like the edge of the wave -- enables a large degree of control over the resulting field. "But actually designing a wavefront that generates a desired pattern becomes more challenging as the complexity of the target increases," Brown said. "A clear 'best' design is only available for a few select cases, such as the generation of a single focus." To overcome this limitation, the group "developed an algorithm that allows users to input a desired sound field in 3-D, and it then outputs a 3-D printable surface profile that generates this field," Brown said. "Our algorithm allows for precise control of the intensity of sound at different locations and the time at which the sound arrives, making it quick and easy to design surfaces or 'lenses' for a desired application." Brown and his colleagues demonstrated the effectiveness of their algorithm by creating a lens designed to generate a sound field shaped like the numeral 7. After illuminating the lens by a pulsed laser, they recorded the sound field and the desired "7" was clearly visible with high contrast. "It was the first demonstration of generating a multi-focal distribution of sound using this approach," Brown said. There are many potential uses for the tailored optoacoustic profiles created by the group. "Highly intense sound can cause heating or exert forces on objects, such as in acoustic tweezers," Brown said. "And similar single-focus devices are already being used for cleaving cell clusters and targeted drug delivery, so our work could be useful within that area." The group is also interested in the effects of propagating through tissue, which introduces distortions to the shape of wavefronts caused by variations in the speed of sound. "If the structure of the tissue is known beforehand via imaging, our approach can be used to correct for these aberrations," Brown said. "Manipulating the shape and time during which the focused sound is generated can also be useful for the maneuvering and controlling biological cells and other particles." Going forward, Brown and his group hope to investigate the use of other light sources and what advantages they might offer. "One limitation of our work was the use of a single-pulsed laser," Brown said. "This meant that the temporal shape of the sound generated from the sample was only one short pulse, which limited the complexity of the fields that could be generated. In the future, we're interested in using alternative modulated optical sources to illuminate these devices." The article, "Generating arbitrary ultrasound fields with tailored optoacoustic surface profiles," is authored by Michael Brown, Daniil Nikitichev, Bradley E. Treeby and Ben Cox. The article will appear in the journal Applied Physics Letters Feb. 28, 2017 (DOI: 10.1063/1.4976942). After that date, it can be accessed at http://aip. . Applied Physics Letters features concise, rapid reports on significant new findings in applied physics. The journal covers new experimental and theoretical research on applications of physics phenomena related to all branches of science, engineering, and modern technology. See http://apl. .


News Article | February 15, 2017
Site: www.spie.org

Photonic circuits aligned to single NV centers are fabricated in diamond using femtosecond laser writing, to enable an integrated platform for the excitation and collection from these optically active spin defects. Diamond is considered by many to be the perfect material. Apart from its remarkable beauty when suitably cut, it is the hardest naturally occurring bulk material, has a record high thermal conductivity and offers excellent transparency from the ultraviolet to far infrared. However it is another characteristic which has quantum optics scientists excited about diamond. Analogous to semiconductors and conventional electronics, the key to making diamond functional is an impurity: a point defect called the nitrogen-vacancy (NV) center – ‘nature’s single photon source’. The NV center, which is present in both naturally occurring and synthetically fabricated diamond, consists of a nitrogen with a neighboring empty site replacing carbon atoms in the diamond lattice. The optically active defect boasts long room temperature spin coherence time, making them attractive as quantum bits[1]. Unlike classical computers which rely on digital 0s and 1s, quantum bits can be in 0 and 1 states simultaneously, enabling an exponential speed increase for certain calculations. Quantum computers are particularly useful for solving challenging multivariable problems such as nanoscale simulations in modern science or macroscale problems like predicting the world climate or fluctuations in the stock market. In addition, due to the magnetically sensitive ground state of NV centers, they can be used to measure weak magnetic fields with nanoscale resolution, which has triggered significant research into diamond-based optical magnetometers[2]. An integrated optics platform in diamond would be beneficial for magnetometry due to the enhanced interaction provided by waveguides, and quantum computing, in which NV centers could be optically linked together for long-range quantum entanglement, due to stability and integration provided by monolithic waveguides. However, it remains a challenge to fabricate optical waveguides in diamond, particularly in 3D architectures, due to its hardness and chemical inertness. In an international collaboration between University of Calgary, Politecnico di Milano and the Institute for Photonics and Nanotechnologies (IFN) – CNR, we recently demonstrated the fabrication of 3D optical waveguides in bulk diamond using focused ultrashort laser pulses[3] in a laboratory at CNST-IIT Milano (Figure 1). As confirmed by optically detected magnetic resonance, mRaman spectroscopy and photoluminescence measurements, we showed that the high repetition rate laser writing produced a waveguide with preserved crystallinity. Crucially, we found that the remarkable properties of the NV centers (Figure 2) were maintained, allowing photons to be efficiently carried between the defects, a crucial step in building a scalable quantum photonic platform. Figure 2. Inset shows cross sectional microscope image of buried diamond waveguide, with the optical mode guided between two laser written modification tracks separated by 13 mm. The photoluminescence spectrum inside the waveguide is the same as the pristine diamond, demonstrating preserved nitrogen vacancy properties, crucial for applications in quantum computing and magnetometry. The concentration of NV centers depends on the purity of the diamond, however the defects are randomly distributed throughout the volume. It is highly desirable to deterministically produce NVs on demand with submicron resolution, prealigned with existing photonic circuits. Recently, Chen et al. demonstrated that femtosecond laser static exposures produced vacancies in the bulk of diamond. After annealing at 1000°C, the laser formed vacancies diffused toward nitrogen impurities to produce on-demand and high quality single NVs [4]. We have taken these pioneering works of laser fabrication of optical waveguides[3, 5] and NVs[4] a step further, by incorporating these important building blocks on the same integrated diamond chip, to enable the robust excitation and collection of light at NVs[6]. Because a single laser microfabrication system is used, the alignment between NVs and waveguides is achieved with submicron resolution. Using confocal photoluminescence microscopy and wide-field EMCCD imaging, we demonstrated the coupling of single NVs using optical waveguides (Figure 3). Optically addressed NV centers could open the door for more sophisticated quantum photonic networks in diamond. For example, in quantum grade diamond, the optically linked single NVs could be exploited for single photon sources or solid state qubits. In lower purity diamond, the laser writing of high density NV ensembles within waveguides could enable robust excitation and collection of the fluorescence signal for magnetometry. Figure 3. Below: 532-nm wavelength excitation of single NV center using optical waveguide. Above: NV signature (650 nm – 800 nm) is captured from above using EMCCD imaging (shown) or confocal photoluminescence collection raster scan (not shown). This work was funded by the FP7 DiamondFab CONCERT Japan project, DIAMANTE MIUR-SIR grant, and FemtoDiamante Cariplo ERC reinforcement grant. Shane Eaton received his PhD at University Toronto in 2008. He is now a research associate with IFN. He is the winner of the prestigious SIR Italian project, to study laser microfabrication of quantum photonics in diamond. His h-index is 22 and he has over 50 papers. Belén Sotillo received her PhD at the University of Madrid in 2014 with the highest distinction. She has been author or co-author of several papers published in international journals (h-index of 7). Currently she is a postdoctoral researcher with IFN characterizing the laser-material interaction in diamond. Roberta Ramponi is the director of IFN-CNR and professor of physics at the Politecnico di Milano. She has been the president of the EOS and is now a member of the Board of the Stakeholders and the Executive Board of Photonics21. She has more than 130 journal papers. Andrea Chiappini received his PhD in Physics from the University of Trento in 2006. Since August 2007, he has been the Principal Investigator on the research area “Sol-gel Photonics” at the Institute of Photonic and Nanotechnologies UOS Trento. He is coauthor of 50 papers and his h-index is 17. Maurizio Ferrari received his PhD in Physics from the University of Trento in 1980. He is currently a Director of Research heading the IFN-CNR Trento unit. He is an SPIE Fellow, co-author of more than 400 publications, several book chapters, and is involved in numerous research projects concerning glass photonics. JP Hadden, Paul E. Barclay Institute for Quantum Science and Technology University of Calgary Calgary, Canada JP Hadden completed his PhD at the University of Bristol in 2013. His thesis focused on the use solid immersion lenses for enhanced photon collection efficiency from color centers in diamond. He joined Paul Barclay’s group in 2015 to investigate coupling between mechanical motion and colour centres in diamond. Paul Barclay completed his PhD in Applied Physics at Caltech in 2007. In 2008 he joined HP Labs where he developed diamond nanophotonic devices. Since 2011, he has been a group leader at the University of Calgary and the National Institute for Nanotechnology, where he develops quantum and optomechanical nanophotonic devices. 1. Hensen, B., et al., Loophole-free Bell inequality violation using electron spins separated by 1.3 kilometres. Nature, 2015. 526(7575): p. 682-686. 4. Chen, Y.-C., et al., Laser writing of coherent colour centres in diamond. Nature Photonics, 2016. 5. Courvoisier, A., M.J. Booth, and P.S. Salter, Inscription of 3D waveguides in diamond using an ultrafast laser. Applied Physics Letters, 2016. 109(3): p. 031109. 6.  J. P. Hadden, V. Bharadwaj, B. Sotillo, S. Rampini, R. Osellame, T. T. Fernandez, A. Chiappini, C. Armellini, M. Ferrari, R. Ramponi, P. E. Barclay and S. M. Eaton, Waveguide-coupled single NV in diamond enabled by femtosecond laser writing (arXiv:1701.05885).


News Article | March 1, 2017
Site: www.eurekalert.org

Harvard researchers have developed a lightweight, portable nanofiber fabrication device that could one day be used to dress wounds on a battlefield or dress shoppers in customizable fabrics. The research was published recently in Macromolecular Materials and Engineering. There are many ways to make nanofibers. These versatile materials -- whose target applications include everything from tissue engineering to bullet proof vests -- have been made using centrifugal force, capillary force, electric field, stretching, blowing, melting, and evaporation. Each of these fabrication methods has pros and cons. For example, Rotary Jet-Spinning (RJS) and Immersion Rotary Jet-Spinning (iRJS) are novel manufacturing techniques developed in the Disease Biophysics Group at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) and the Wyss Institute for Biologically Inspired Engineering. Both RJS and iRJS dissolve polymers and proteins in a liquid solution and use centrifugal force or precipitation to elongate and solidify polymer jets into nanoscale fibers. These methods are great for producing large amounts of a range of materials- - including DNA, nylon, and even Kevlar -- but until now they haven't been particularly portable. The Disease Biophysics Group recently announced the development of a hand-held device that can quickly produce nanofibers with precise control over fiber orientation. Regulating fiber alignment and deposition is crucial when building nanofiber scaffolds that mimic highly aligned tissue in the body or designing point-of-use garments that fit a specific shape. "Our main goal for this research was to make a portable machine that you could use to achieve controllable deposition of nanofibers," said Nina Sinatra, a graduate student in the Disease Biophysics Group and co-first author of the paper. "In order to develop this kind of point-and-shoot device, we needed a technique that could produce highly aligned fibers with a reasonably high throughput." The new fabrication method, called pull spinning, uses a high-speed rotating bristle that dips into a polymer or protein reservoir and pulls a droplet from solution into a jet. The fiber travels in a spiral trajectory and solidifies before detaching from the bristle and moving toward a collector. Unlike other processes, which involve multiple manufacturing variables, pull spinning requires only one processing parameter -- solution viscosity -- to regulate nanofiber diameter. Minimal process parameters translate to ease of use and flexibility at the bench and, one day, in the field. Pull spinning works with a range of different polymers and proteins. The researchers demonstrated proof-of-concept applications using polycaprolactone and gelatin fibers to direct muscle tissue growth and function on bioscaffolds, and nylon and polyurethane fibers for point-of-wear apparel. "This simple, proof-of-concept study demonstrates the utility of this system for point-of-use manufacturing," said Kit Parker, the Tarr Family Professor of Bioengineering and Applied Physics and director of the Disease Biophysics Group. "Future applications for directed production of customizable nanotextiles could extend to spray-on sportswear that gradually heats or cools an athlete's body, sterile bandages deposited directly onto a wound, and fabrics with locally varying mechanical properties."


News Article | March 1, 2017
Site: phys.org

There are many ways to make nanofibers. These versatile materials—whose target applications include everything from tissue engineering to bullet proof vests—have been made using centrifugal force, capillary force, electric field, stretching, blowing, melting, and evaporation. Each of these fabrication methods has pros and cons. For example, Rotary Jet-Spinning (RJS) and Immersion Rotary Jet-Spinning (iRJS) are novel manufacturing techniques developed in the Disease Biophysics Group at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) and the Wyss Institute for Biologically Inspired Engineering. Both RJS and iRJS dissolve polymers and proteins in a liquid solution and use centrifugal force or precipitation to elongate and solidify polymer jets into nanoscale fibers. These methods are great for producing large amounts of a range of materials- - including DNA, nylon, and even Kevlar—but until now they haven't been particularly portable. The Disease Biophysics Group recently announced the development of a hand-held device that can quickly produce nanofibers with precise control over fiber orientation. Regulating fiber alignment and deposition is crucial when building nanofiber scaffolds that mimic highly aligned tissue in the body or designing point-of-use garments that fit a specific shape. "Our main goal for this research was to make a portable machine that you could use to achieve controllable deposition of nanofibers," said Nina Sinatra, a graduate student in the Disease Biophysics Group and co-first author of the paper. "In order to develop this kind of point-and-shoot device, we needed a technique that could produce highly aligned fibers with a reasonably high throughput." The new fabrication method, called pull spinning, uses a high-speed rotating bristle that dips into a polymer or protein reservoir and pulls a droplet from solution into a jet. The fiber travels in a spiral trajectory and solidifies before detaching from the bristle and moving toward a collector. Unlike other processes, which involve multiple manufacturing variables, pull spinning requires only one processing parameter—solution viscosity—to regulate nanofiber diameter. Minimal process parameters translate to ease of use and flexibility at the bench and, one day, in the field. Pull spinning works with a range of different polymers and proteins. The researchers demonstrated proof-of-concept applications using polycaprolactone and gelatin fibers to direct muscle tissue growth and function on bioscaffolds, and nylon and polyurethane fibers for point-of-wear apparel. "This simple, proof-of-concept study demonstrates the utility of this system for point-of-use manufacturing," said Kit Parker, the Tarr Family Professor of Bioengineering and Applied Physics and director of the Disease Biophysics Group. "Future applications for directed production of customizable nanotextiles could extend to spray-on sportswear that gradually heats or cools an athlete's body, sterile bandages deposited directly onto a wound, and fabrics with locally varying mechanical properties." A schematic of the pull spinning apparatus with a side view illustration of a fiber being pulled from the polymer reservoir. The pull spinning system consists of a rotating bristle that dips and pulls a polymer jet in a spiral trajectory . Credit: Leila Deravi/Harvard University Explore further: Techniques offer better, tunable production of nanofibers for bulletproof vests, cellular scaffolding More information: Leila F. Deravi et al. Design and Fabrication of Fibrous Nanomaterials Using Pull Spinning, Macromolecular Materials and Engineering (2017). DOI: 10.1002/mame.201600404


News Article | February 15, 2017
Site: phys.org

The announcement was hailed as a breakthrough in optics and was named among Science Magazine's top discoveries of 2016. But the lens had a limitation – it could only focus one color at a time. Now, the same team has developed the first flat lens that works within a continual bandwidth of colors, from blue to green. This bandwidth, close to that of an LED, paves the way for new applications in imaging, spectroscopy and sensing. The research is published in Nano Letters. One of the major challenges in developing a flat, broadband lens has been correcting for chromatic dispersion, the phenomenon where different wavelengths of light are focused at different distances from the lens. "Traditional lenses for microscopes and cameras—including those in cell phones and laptops—require multiple curved lenses to correct chromatic aberrations, which adds weight, thickness and complexity," said Federico Capasso, Robert L. Wallace Professor of Applied Physics and Vinton Hayes Senior Research Fellow in Electrical Engineering. "Our new breakthrough flat metalens has built-in chromatic aberrations corrections so that a single lens is required." Correcting for chromatic dispersion—known as dispersion engineering—is a crucial topic in optics, and an important design requirement in any optical systems that deals with light of different colors. The ability to control the chromatic dispersion of flat lenses broadens their applications and introduces new applications that have not yet been possible. "By harnessing chromatic aspects, we can have even more control over the light," said Reza Khorasaninejad, a Research Associate in the Capasso Lab and first author of the paper. "Here, we demonstrate achromatic flat lenses and also invent a new type of flat lens with reverse chromatic dispersion. We showed that one can break away from the constraints of conventional optics, offering new opportunities only bound by the designer's imagination." To design an achromatic lens—a lens without chromatic dispersion—the team optimized the shape, width, distance, and height of the nanopillars that make up the heart of the metalens. As in previous research, the researchers used abundant titanium dioxide to create the nanoscale array. This structure allows the metalens to focus wavelengths from 490 nm to 550 nm, basically from blue to green, without any chromatic dispersion. "This method for dispersion engineering can be used to design various ultrathin components with a desired performance," said Zhujun Shi, a PhD student in the Capasso Lab and co-first author of the paper. "This platform is based on single step lithography and is compatible with high throughput manufacturing technique such as nano-imprinting." Harvard's Office of Technology Development has filed patent applications on a portfolio of flat lens technologies and is working closely with Capasso and members of his research group to catalyze commercialization of this technology through a startup company. Explore further: Meta-lens works in the visible spectrum, sees smaller than a wavelength of light More information: M. Khorasaninejad et al. Achromatic Metalens over 60 nm Bandwidth in the Visible and Metalens with Reverse Chromatic Dispersion, Nano Letters (2017). DOI: 10.1021/acs.nanolett.6b05137


News Article | February 16, 2017
Site: www.sciencedaily.com

Although scientists have been able to levitate specific types of material, a pair of UChicago undergraduate physics students helped take the science to a new level. Third-year Frankie Fung and fourth-year Mykhaylo Usatyuk led a team of UChicago researchers who demonstrated how to levitate a variety of objects -- ceramic and polyethylene spheres, glass bubbles, ice particles, lint strands and thistle seeds -- between a warm plate and a cold plate in a vacuum chamber. "They made lots of intriguing observations that blew my mind," said Cheng Chin, professor of physics, whose ultracold lab in the Gordon Center for Integrative Science was home to the experiments. In their work, researchers achieved a number of levitation breakthroughs, in terms of duration, orientation and method: The levitation lasted for more than an hour, as opposed to a few minutes; stability was achieved radially and vertically, as opposed to just vertically; and it used a temperature gradient rather than light or a magnetic field. Their findings appeared Jan. 20 in Applied Physics Letters. "Magnetic levitation only works on magnetic particles, and optical levitation only works on objects that can be polarized by light, but with our first-of-its-kind method, we demonstrate a method to levitate generic objects," said Chin. In the experiment, the bottom copper plate was kept at room temperature while a stainless steel cylinder filled with liquid nitrogen kept at negative 300 degrees Fahrenheit served as the top plate. The upward flow of heat from the warm to the cold plate kept the particles suspended indefinitely. "The large temperature gradient leads to a force that balances gravity and results in stable levitation," said Fung, the study's lead author. "We managed to quantify the thermophoretic force and found reasonable agreement with what is predicted by theory. This will allow us to explore the possibilities of levitating different types of objects." (Thermophoresis refers to the movement of particles by means of a temperature gradient.) "Our increased understanding of the thermophoretic force will help us investigate the interactions and binding affinities between the particles we observed," said Usatyuk, a study co-author. "We are excited about the future research directions we can follow with our system." The key to obtaining high levitation stability is the geometrical design of the two plates. A proper ratio of their sizes and vertical spacing allows the warm air to flow around and efficiently capture the levitated objects when they drift away from the center. Another sensitivity factor is that the thermal gradient needs to be pointing upward -- even a misalignment of one degree will greatly reduce the levitation stability. "Only within a narrow range of pressure, temperature gradient and plate geometric factors can we reach stable and long levitation," Chin said. "Different particles also require fine adjustment of the parameters." The apparatus offers a new ground-based platform to investigate the dynamics of astrophysical, chemical and biological systems in a microgravity environment, according to the researchers. Levitation of macroscopic particles in a vacuum is of particular interest due to its wide applications in space, atmospheric and astro-chemical research. And thermophoresis has been utilized in aerosol thermal precipitators, nuclear reactor safety and the manufacturing of optical fibers through vacuum deposition processes, which apply progressive layers of atoms or molecules during fabrication. The new method is significant because it offers a new approach to manipulating small objects without contacting or contaminating them, said Thomas Witten, the Homer J. Livingston Professor Emeritus of Physics. "It offers new avenues for mass assembly of tiny parts for micro-electro-mechanical systems, for example, and to measure small forces within such systems. "Also, it forces us to re-examine how 'driven gases,' such as gases driven by heat flow, can differ from ordinary gases," he added. "Driven gases hold promise to create new forms of interaction between suspended particles." Levitation of materials in ground-based experiments provides an ideal platform for the study of particle dynamics and interactions in a pristine isolated environment, the paper concluded. Chin's lab is now looking at how to levitate macroscopic substances greater than a centimeter in size, as well as how these objects interact or aggregate in a weightless environment. "There are ample research opportunities to which our talented undergraduate students can contribute," Chin said.


News Article | February 21, 2017
Site: www.chromatographytechniques.com

Although scientists have been able to levitate specific types of material, a pair of UChicago undergraduate physics students helped take the science to a new level. Third-year Frankie Fung and fourth-year Mykhaylo Usatyuk led a team of UChicago researchers who demonstrated how to levitate a variety of objects—ceramic and polyethylene spheres, glass bubbles, ice particles, lint strands and thistle seeds—between a warm plate and a cold plate in a vacuum chamber. “They made lots of intriguing observations that blew my mind,” said Cheng Chin, professor of physics, whose ultracold lab in the Gordon Center for Integrative Science was home to the experiments. In their work, researchers achieved a number of levitation breakthroughs, in terms of duration, orientation and method: The levitation lasted for more than an hour, as opposed to a few minutes; stability was achieved radially and vertically, as opposed to just vertically; and it used a temperature gradient rather than light or a magnetic field. Their findings appeared Jan. 20 in Applied Physics Letters. “Magnetic levitation only works on magnetic particles, and optical levitation only works on objects that can be polarized by light, but with our first-of-its-kind method, we demonstrate a method to levitate generic objects,” said Chin. In the experiment, the bottom copper plate was kept at room temperature while a stainless steel cylinder filled with liquid nitrogen kept at negative 300 degrees Fahrenheit served as the top plate. The upward flow of heat from the warm to the cold plate kept the particles suspended indefinitely. “The large temperature gradient leads to a force that balances gravity and results in stable levitation,” said Fung, the study’s lead author. “We managed to quantify the thermophoretic force and found reasonable agreement with what is predicted by theory. This will allow us to explore the possibilities of levitating different types of objects.” (Thermophoresis refers to the movement of particles by means of a temperature gradient.) “Our increased understanding of the thermophoretic force will help us investigate the interactions and binding affinities between the particles we observed,” said Usatyuk, a study co-author. “We are excited about the future research directions we can follow with our system.” The key to obtaining high levitation stability is the geometrical design of the two plates. A proper ratio of their sizes and vertical spacing allows the warm air to flow around and efficiently capture the levitated objects when they drift away from the center. Another sensitivity factor is that the thermal gradient needs to be pointing upward—even a misalignment of one degree will greatly reduce the levitation stability. “Only within a narrow range of pressure, temperature gradient and plate geometric factors can we reach stable and long levitation,” Chin said. “Different particles also require fine adjustment of the parameters.” The apparatus offers a new ground-based platform to investigate the dynamics of astrophysical, chemical and biological systems in a microgravity environment, according to the researchers. Levitation of macroscopic particles in a vacuum is of particular interest due to its wide applications in space, atmospheric and astro-chemical research. And thermophoresis has been utilized in aerosol thermal precipitators, nuclear reactor safety and the manufacturing of optical fibers through vacuum deposition processes, which apply progressive layers of atoms or molecules during fabrication. The new method is significant because it offers a new approach to manipulating small objects without contacting or contaminating them, said Thomas Witten, the Homer J. Livingston Professor Emeritus of Physics. “It offers new avenues for mass assembly of tiny parts for micro-electro-mechanical systems, for example, and to measure small forces within such systems. “Also, it forces us to re-examine how ‘driven gases,’ such as gases driven by heat flow, can differ from ordinary gases,” he added. “Driven gases hold promise to create new forms of interaction between suspended particles.” Levitation of materials in ground-based experiments provides an ideal platform for the study of particle dynamics and interactions in a pristine isolated environment, the paper concluded. Chin’s lab is now looking at how to levitate macroscopic substances greater than a centimeter in size, as well as how these objects interact or aggregate in a weightless environment. “There are ample research opportunities to which our talented undergraduate students can contribute,” Chin said.


News Article | February 23, 2017
Site: www.cemag.us

Ionotronic devices rely on charge effects based on ions, instead of electrons or in addition to electrons. These devices open new opportunities for creating electrically switchable memories. However, there are still many technical challenges to overcome before this new kind of memories can be produced. Researchers at Aalto University in Finland have visualized how oxygen ion migration in a complex oxide material causes the material to alter its crystal structure in a uniform and reversible fashion, prompting large modulations of electrical resistance. They performed simultaneous imaging and resistance measurements in a transmission electron microscope using a sample holder with a nanoscale electrical probe. Resistance-switching random access memories could utilize this effect. “In a transmission electron microscope, a beam of high-energy electrons is transmitted through a very thin specimen. Various detectors collect the electrons after their interaction with the sample, providing detailed information about the atomic structure and composition of the material. The technique is extremely powerful for nanomaterials characterization, but if used conventionally, it does not allow for active material manipulation inside the microscope. In our study, we utilized a special sample holder with a piezo-controlled metallic probe to make an electrical nanocontact. This in situ method allowed us to apply short voltage pulses and thereby control the migration of oxygen ions in our sample,” explains Academy of Finland Research Fellow Lide Yao from the Department of Applied Physics. The researchers found that migration of oxygen ions away from the contact area results in an abrupt change in the oxide lattice structure and an increase of electrical resistance. Reversal of the voltage polarity fully restores the original material properties. Electro-thermal simulations, performed by PhD candidate Sampo Inkinen, showed that a combination of current-induced sample heating and electric-field-directed ion migration causes the switching effect. “The material that we investigated in this study is a complex oxide. Complex oxides can exhibit many interesting physical properties including magnetism, ferroelectricity, and superconductivity, and all these properties vary sensitively with the oxidation state of the material. Voltage-induced migration of oxygen ions does change the amount of oxidation, triggering strong material responses. While we have demonstrated direct correlations between oxygen content, crystal structure, and electrical resistance, the same ionotronic concept could be utilized to control other material properties,” says Professor Sebastiaan van Dijken, who is a coauthor on the paper. “In the current study, we employed a special sample holder for simultaneous measurements of the atomic-scale structure and electrical resistance. We are now developing an entirely new and unique holder that would allow for transmission electron microscopy measurements while the specimen is irradiated by intense light. We plan to investigate atomic scale processes in perovskite solar cells and other optoelectronic materials with this setup in the future,” adds Yao. Nature Communications has published the results. The in situ transmission electron microscopy study was performed at Aalto University’s Nanomicroscopy Center for high-resolution material characterization and part of Finland’s national research infrastructure, OtaNano.


News Article | February 22, 2017
Site: www.eurekalert.org

Almost one year ago, borophene didn't even exist. Now, just months after a Northwestern University and Argonne National Laboratory team discovered the material, another team led by Mark Hersam is already making strides toward understanding its complicated chemistry and realizing its electronic potential. Created in December 2015, borophene is a two-dimensional, metallic sheet of boron, the element commonly used in fiberglass. Although borophene holds promise for possible applications ranging from electronics to photovoltaics, these applications cannot be achieved until borophene is integrated with other materials. Now Hersam's team -- and a bit of serendipity -- have successfully accomplished this integration. "Integrated circuits are at the heart of all of our computers, tablets, and smartphones,'" said Hersam, Walter P. Murphy Professor of Materials Science and Engineering in Northwestern University's McCormick School of Engineering. "Integration is the key element that has driven advances in electronic technology." Supported by the Office for Naval Research and National Science Foundation, the research appeared online on February 22 in the journal Science Advances. Erik Luijten, professor of materials science and engineering at Northwestern University, co-authored the paper. Xiaolong Liu, a student in Northwestern's Applied Physics Graduate Program, is the paper's first author. Because borophene does not appear in nature, scientists must grow it in the laboratory by synthesizing it on a sheet of silver. Hersam's team deposited an organic material (perylene-3,4,9,10-tetracarboxylic dianhydride) on top of the borophene, in an attempt to integrate the two materials. What happened next was a surprise. The organic material, which is known to self-assemble on essentially any material, instead diffused off the borophene and onto the silver sheet. The result was a self-assembled monolayer of the organic material directly next to the borophene, forming a nearly perfect interface. Well-controlled interfaces between distinct materials enable integrated devices, including diodes and photovoltaics. Hersam's surprising technique bypassed the typical challenge to creating a sharp interface -- getting materials to touch but not mix. "This is a nice bit of serendipity because we solved a problem without any additional intervention required," Hersam said. "Borophene did not exist a year ago. Twelve months later, we're already forming essentially perfect interfaces." Not only does Hersam's finding set the stage to explore electronic applications for borophene, it also illuminates the new material's fundamental properties. The next challenge is to move borophene off silver and onto an inert substrate that does not interfere with its electronic properties. "Borophene is unique in its ability to form abrupt interfaces via self-assembly," Hersam said. "We're beginning to understand its chemistry, which will guide our efforts to transfer the material onto appropriate substrates for further integration."


News Article | February 23, 2017
Site: www.cemag.us

Ionotronic devices rely on charge effects based on ions, instead of electrons or in addition to electrons. These devices open new opportunities for creating electrically switchable memories. However, there are still many technical challenges to overcome before this new kind of memories can be produced. Researchers at Aalto University in Finland have visualized how oxygen ion migration in a complex oxide material causes the material to alter its crystal structure in a uniform and reversible fashion, prompting large modulations of electrical resistance. They performed simultaneous imaging and resistance measurements in a transmission electron microscope using a sample holder with a nanoscale electrical probe. Resistance-switching random access memories could utilize this effect. “In a transmission electron microscope, a beam of high-energy electrons is transmitted through a very thin specimen. Various detectors collect the electrons after their interaction with the sample, providing detailed information about the atomic structure and composition of the material. The technique is extremely powerful for nanomaterials characterization, but if used conventionally, it does not allow for active material manipulation inside the microscope. In our study, we utilized a special sample holder with a piezo-controlled metallic probe to make an electrical nanocontact. This in situ method allowed us to apply short voltage pulses and thereby control the migration of oxygen ions in our sample,” explains Academy of Finland Research Fellow Lide Yao from the Department of Applied Physics. The researchers found that migration of oxygen ions away from the contact area results in an abrupt change in the oxide lattice structure and an increase of electrical resistance. Reversal of the voltage polarity fully restores the original material properties. Electro-thermal simulations, performed by PhD candidate Sampo Inkinen, showed that a combination of current-induced sample heating and electric-field-directed ion migration causes the switching effect. “The material that we investigated in this study is a complex oxide. Complex oxides can exhibit many interesting physical properties including magnetism, ferroelectricity, and superconductivity, and all these properties vary sensitively with the oxidation state of the material. Voltage-induced migration of oxygen ions does change the amount of oxidation, triggering strong material responses. While we have demonstrated direct correlations between oxygen content, crystal structure, and electrical resistance, the same ionotronic concept could be utilized to control other material properties,” says Professor Sebastiaan van Dijken, who is a coauthor on the paper. “In the current study, we employed a special sample holder for simultaneous measurements of the atomic-scale structure and electrical resistance. We are now developing an entirely new and unique holder that would allow for transmission electron microscopy measurements while the specimen is irradiated by intense light. We plan to investigate atomic scale processes in perovskite solar cells and other optoelectronic materials with this setup in the future,” adds Yao. Nature Communications has published the results. The in situ transmission electron microscopy study was performed at Aalto University’s Nanomicroscopy Center for high-resolution material characterization and part of Finland’s national research infrastructure, OtaNano.

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