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Light-responsive chiral nanomaterials are attractive for their applications as metamaterials, in hyperbolic geometry, as high-sensitivity bioanalysis agents, and as catalysts for asymmetric reactions. Nanocarbons belong to the family of light-responsive materials that includes fullerenes (spheres), carbon nanotubes (pillars), and graphene (sheets). Several studies have sought to introduce chirality to these materials, or to produce separate chiral configurations. Compared with some semiconductor inorganic nanoparticles (e.g., cadmium telluride), these materials are expected to be less toxic and thus may be considered as strong candidates for bioapplications. The synthetic methods for imparting chirality, however, are not transferable from one structure to another, and for the past decade, the goal of achieving chiral nanocarbons based on graphene has proved elusive. For example, methods that are suitable for the synthesis of chiral fullerenes cannot be extended to the preparation of enantiopure carbon nanotubes (those that have only one chiral geometry) because the growth of the graphene network requires a temperature in the range of several hundred degrees celsius, at which traditional asymmetric catalysts cannot survive. Another problem lies in the inherent geometry of the flat graphene sheet. In contrast with some fullerenes and nanotubes, with potential asymmetric arrangement of carbon atoms, flat graphene does not have defect-free, surface-configurational chiral symmetry (see Figure 1). In our work, we have thus focused on exploiting conformational flexibility as the key element to introduce chiral geometry into a nanocarbon structure. In our technique, we cut a nano-sized graphene quantum dot (GQD) from a flat sheet of graphene. This allows us to tune the wavelength of light that interacts with the material by quantum confinement of electrons or holes. With this size-control of the graphene sheet, we make the material similar to both noble metal and semiconductor nanostructures. We are able to attach chiral ligands to the free edge of the GQD, and these ligands control the conformation of the graphene sheet. We achieved helical buckling (twisting) of the GQDs through covalent attachment of moieties of the amino acid L/D-cysteine to their edges (L/D-GQDs). This gave rise to chiroptical activity, as a result of chiral interactions at the ‘crowded’ edges.1 We detected the chiroptical response in circular dichroism (CD) spectra as bands at about 210–220 and 250–265nm, with inverted signs for opposite handedness of the cysteine edge ligands. The chiroptical peaks at 210–220nm correspond to the hybridized molecular orbitals of the chiral amino acids and atoms of the graphene edges. We obtained various experimental and modeling information, including density functional theory calculations of CD spectra with probabilistic distributions of GQD isomers. These results indicate that the band at 250–265nm originates from the 3D twisting of the graphene sheet (see Figure 2). This can be attributed to the chiral excitonic transitions. We carried out a biocompatibility test of the L/D-GQDs by exposing them to Hep G2 (human liver cancer) cells. The results revealed the L/D-GQDs' general biocompatibility, as well as a noticeable difference in the toxicity of the stereoisomers (those with the same molecular formula, but with different atom orientations), where the D-GQD was slightly more toxic. Our molecular dynamics simulations demonstrated that D-GQDs have a stronger tendency to accumulate within the cellular membranes than L-GQDs. The emergence of such nanoscale chirality in GQDs that are decorated with biomolecules is expected to be a general stereochemical phenomenon for a variety of flexible sheets of nanomaterials. In summary, chiral nanocarbons are attractive materials for use as bioanalysis agents, drug delivery systems, and chiral catalysts. Compared with other nanocarbons, flat graphene sheets do not have the inherent chiral configuration required for chiroptical activity, and manipulation of their conformation is necessary. We have fabricated chiral graphenes by cutting out nano-sized GQDs and attaching chiral ligands to their edges, which induces a twist. The biocompatibility of chiral GQDs paves the way for their use in the development of photo-responsible drug delivery vehicles and more selective phototherapies. In addition, the twisted electronic states can lead to polarization-based optoelectronic devices and (photo)catalysts. In our future work we will focus on exploring these possibilities. This work was supported by the Center for Photonic and Multiscale Nanomaterials, which is funded by the National Science Foundation (NSF) Materials Research Science and Engineering Center program DMR 1120923. Other support came from the NSF projects 1403777, 1411014, 463474, the Japan Society for the Promotion of Science (KAKENHI 2510001), and the US Department of Energy Basic Energy Sciences (DE-SC0002619). The authors also thank the University of Michigan's Electron Microbeam Analysis Laboratory for its assistance with electron microscopy, and acknowledge the NSF for funding the JEOL 2010F analytical electron microscope used in this work.


Home > Press > First-ever videos show how heat moves through materials at the nanoscale and speed of sound: Groundbreaking observations could help develop better, more efficient materials for electronics and alternative energy Abstract: Using a state-of-the-art ultrafast electron microscope, University of Minnesota researchers have recorded the first-ever videos showing how heat moves through materials at the nanoscale traveling at the speed of sound. This video made with the University of Minnesota ultrafast electron microscope (UEM) shows the initial moments of thermal-energy motion in an imperfect semiconducting material. The video shows nanoscale waves of energy, called phonons, moving at about 6 nanometers (0.000000006 meters) per picosecond (0.000000000001 second). Credit: College of Science and Engineering The research, published today in Nature Communications, provides unprecedented insight into roles played by individual atomic and nanoscale features that could aid in the design of better, more efficient materials with a wide array of uses, from personal electronics to alternative-energy technologies. Energy in the form of heat impacts all technologies and is a major factor in how electronic devices and public infrastructure are designed and engineered. It is also the largest form of waste energy in critical applications, including power transmission and especially transportation, where, for example, roughly 70 percent of the energy in gasoline is wasted as heat in automobile engines. Materials scientists and engineers have spent decades researching how to control thermal energy at the atomic level in order to recycle and use it to dramatically increase efficiencies and ultimately drive down the use of fossil fuels. Such work would be greatly aided by actually watching heat move through materials, but capturing images of the basic physical processes at the heart of thermal-energy motion has presented enormous challenges. This is because the fundamental length scales are nanometers (a billionth of a meter) and the speeds can be many miles per second. Such extreme conditions have made imaging this ubiquitous process extraordinarily challenging. To overcome these challenges and image the movement of heat energy, the researchers used a cutting-edge FEI Tecnai™ Femto ultrafast electron microscope (UEM) capable of examining the dynamics of materials at the atomic and molecular scale over time spans measured in femtoseconds (one millionth of a billionth of a second). In this work, the researchers used a brief laser pulse to excite electrons and very rapidly heat crystalline semiconducting materials of tungsten diselenide and germanium. They then captured slow-motion videos (slowed by over a billion times the normal speed) of the resulting waves of energy moving through the crystals. "As soon as we saw the waves, we knew it was an extremely exciting observation," said lead researcher David Flannigan, an assistant professor of chemical engineering and materials science at the University of Minnesota. "Actually watching this process happen at the nanoscale is a dream come true." Flannigan said the movement of heat through the material looks like ripples on a pond after a pebble is dropped in the water. The videos show waves of energy moving at about 6 nanometers (0.000000006 meters) per picosecond (0.000000000001 second). Mapping the oscillations of energy, called phonons, at the nanoscale is critical to developing a detailed understanding of the fundamentals of thermal-energy motion. "In many applications, scientists and engineers want to understand thermal-energy motion, control it, collect it, and precisely guide it to do useful work or very quickly move it away from sensitive components," Flannigan said. "Because the lengths and times are so small and so fast, it has been very difficult to understand in detail how this occurs in materials that have imperfections, as essentially all materials do. Literally watching this process happen would go a very long way in building our understanding, and now we can do just that." ### In addition to Flannigan, researchers involved in the study are University of Minnesota materials science graduate student Daniel R. Cremons and chemical engineering graduate student Dayne A. Plemmons. The research was funded primarily by the National Science Foundation through the University of Minnesota Materials Research Science and Engineering Center. For more information, please click If you have a comment, please us. Issuers of news releases, not 7th Wave, Inc. or Nanotechnology Now, are solely responsible for the accuracy of the content.


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Abstract: A method to rotate single particles, cells or organisms using acoustic waves in a microfluidic device will allow researchers to take three dimensional images with only a cell phone. Acoustic waves can move and position biological specimens along the x, y and z axes, but for the first time researchers at Penn State have used them to gently and safely rotate samples, a crucial capability in single-cell analysis, drug discovery and organism studies. The research, published today in Nature Communications, was led by Tony Jun Huang, professor of engineering science and mechanics and Huck Distinguished Chair in Bioengineering Science. Huang and his group created an acoustofluidic rotational manipulation (ARM) method that traps bubbles in a series of small cavities inside a microfluidic device. Acoustic transducers similar to ultrasound imaging transducers create an acoustic wave in the fluid, making the bubbles vibrate, which creates microvortexes in the flowing liquid that are tunable so the sample rotates in any direction and at any desired speed. "Currently confocal microscopes are required in many biological, biochemical and biomedical studies, but many labs do not have access to a confocal microscope, which costs more than $200,000," said Huang. "Our ARM method is a very inexpensive platform and it is compatible with all the optical characterization tools. You can literally use a cell phone to do three-dimensional imaging." To demonstrate the device's capabilities, the researchers rotated C. elegans, a model organism about a millimeter in length frequently used in biological studies. They also acoustically rotated and imaged a HeLa cancer cell. Existing methods of manipulating small objects depend on the optical, magnetic or electrical properties of the specimen, and/or damage the specimen due to laser heating. The ARM method, on the other hand, uses a gentle acoustic wave generated by a power similar to ultrasound imaging, and at a lower frequency. The device is also compact and simple to use. "Our method is a valuable platform for imaging and studying the effect of rotation at the single cell level," said co-lead author Adem Ozceki, graduate student in engineering science and mechanics. "More important, with the capacity to rotate large numbers of cells in parallel, researchers will be able to perform high-throughput single-cell studies. " In addition to its applicability to a large range of biological and physical science investigations, ARM technology shows excellent biocompatibility in a HeLa cell viability test in which 99.2 percent of cells survived manipulation. ### Also contributing to "Rotational manipulation of single cells and organisms using acoustic waves" were former group member Daniel Ahmed, Ph.D.; graduate students Nagagireesh Bojanala, Nitesh Nama, Awani Upadhyay, Yuchao Chen; and Wendy Hanna-Rose, associate professor of biochemistry and molecular biology; all from Penn State. The National Institutes of Health; National Science Foundation; and the Center for Nanoscale Science, an NSF Materials Research Science and Engineering Center at Penn State supported this work. Components of the work were conducted at the Penn State Materials Research Institute's Nanofabrication Laboratory. For more information, please click If you have a comment, please us. Issuers of news releases, not 7th Wave, Inc. or Nanotechnology Now, are solely responsible for the accuracy of the content.


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Abstract: Researchers from the University of Illinois at Urbana-Champaign have demonstrated a new approach to modifying the light absorption and stretchability of atomically thin two-dimensional (2D) materials by surface topographic engineering using only mechanical strain. The highly flexible system has future potential for wearable technology and integrated biomedical optical sensing technology when combined with flexible light-emitting diodes. "Increasing graphene's low light absorption in visible range is an important prerequisite for its broad potential applications in photonics and sensing," explained SungWoo Nam, an assistant professor of mechanical science and engineering at Illinois. "This is the very first stretchable photodetector based exclusively on graphene with strain-tunable photoresponsivity and wavelength selectivity." Graphene--an atomically thin layer of hexagonally bonded carbon atoms--has been extensively investigated in advanced photodetectors for its broadband absorption, high carrier mobility, and mechanical flexibility. Due to graphene's low optical absorptivity, graphene photodetector research so far has focused on hybrid systems to increase photoabsorption. However, such hybrid systems require a complicated integration process, and lead to reduced carrier mobility due to the heterogeneous interfaces. According to Nam, the key element enabling increased absorption and stretchability requires engineering the two-dimensional material into three-dimensional (3D) "crumpled structures," increasing the graphene's areal density. The continuously undulating 3D surface induces an areal density increase to yield higher optical absorption per unit area, thereby improving photoresponsivity. Crumple density, height, and pitch are modulated by applied strain and the crumpling is fully reversible during cyclical stretching and release, introducing a new capability of strain-tunable photoabsorption enhancement and allowing for a highly responsive photodetector based on a single graphene layer. "We achieved more than an order-of-magnitude enhancement of the optical extinction via the buckled 3D structure, which led to an approximately 400% enhancement in photoresponsivity," stated Pilgyu Kang, and first author of the paper, "Crumpled Graphene Photodetector with Enhanced, Strain-tunable and Wavelength-selective Photoresponsivity," appearing in the journal, Advanced Materials. "The new strain-tunable photoresponsivity resulted in a 100% modulation in photoresponsivity with a 200% applied strain. By integrating colloidal photonic crystal--a strain-tunable optomechanical filter--with the stretchable graphene photodetector, we also demonstrated a unique strain-tunable wavelength selectivity." "This work demonstrates a robust approach for stretchable and flexible graphene photodetector devices," Nam added. "We are the first to report a stretchable photodetector with stretching capability to 200% of its original length and no limit on detection wavelength. Furthermore, our approach to enhancing photoabsorption by crumpled structures can be applied not only to graphene, but also to other emerging 2D materials." ### In addition to Nam and Kang, study co-authors include Michael Cai Wang and Peter M. Knapp in the Department of Mechanical Science and Engineering at Illinois. The optical characterizations and partial device fabrication were carried out in the Frederick Seitz Materials Research Laboratory and the Micro and Nano Technology Laboratory at Illinois. For more information, please click If you have a comment, please us. Issuers of news releases, not 7th Wave, Inc. or Nanotechnology Now, are solely responsible for the accuracy of the content.


Abstract: Researchers from Iran and Malaysia designed a nanostructure based on carbon nanotubes with antibacterial properties to be used in public places, specially hospitals and clinics. This material can also be used in blood purification filters. The carbon materials solve the problem of blood poisoning caused by hemodialysis. Hemodialysis is a process in which blood is purified for kidney patients. The device stops working gradually due to the accumulation of bacteria and microbes in the filters and membranes. Therefore, numerous studies have been carried out to use antibacterial materials including polymeric compounds and carbon nanostructures in this process. The primary objective of the research was to introduce an antibacterial compound to produce appropriate coatings for isolated areas such as hospitals. However, a potential compound was also suggested to produce blood purification nanofilters taking into consideration blood toxicity due to the use of filters containing metal particles. Polymers produced in this research were firstly introduced as antibacterial agents. However, since the polymers have effective antibacterial effects against various types of resistant hospital bacteria, it seems that they can be used in the production of antibacterial coatings and medical equipment. Results of the research have been published in Journal of Biomedical Materials Research Part A, vol. 103, 2015, pp. 2959-2965. The result of the research has also been patented under Patent No. 74061 entitled "Antibacterial Polyvinyl Chloride Composite" in Iran Patent Office. For more information, please click If you have a comment, please us. Issuers of news releases, not 7th Wave, Inc. or Nanotechnology Now, are solely responsible for the accuracy of the content.

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