Nano News: 04/23/15

Ultra-sensitive sensor detects individual electrons

A Spanish-led team of European researchers at the University of Cambridge has created an electronic device so accurate that it can detect the charge of a single electron in less than one microsecond. It has been dubbed the 'gate sensor' and could be applied in quantum computers of the future to read information stored in the charge or spin of a single electron ("Probing the limits of gate-based charge sensing").

Silicon chip used for the design of the gate sensor. (Image: TOLOP) In the same Cambridge laboratory in the United Kingdom where the British physicist J.J. Thomson discovered the electron in 1897, European scientists have just developed a new ultra-sensitive electrical-charge sensor capable of detecting the movement of individual electrons. "The device is much more compact and accurate than previous versions and can detect the electrical charge of a single electron in less than one microsecond," says M. Fernando González Zalba, leader of this research from the Hitachi Cambridge Laboratory and the Cavendish Laboratory. Details of the breakthrough have been published in the journal 'Nature Communications' and its authors predict that these types of sensors, dubbed 'gate sensors', will be used in quantum computers of the future to read information stored in the charge or spin of a single electron. "We have called it a gate sensor because, as well as detecting the movement of individual electrons, the device is able to control its flow as if it were an electronic gate which opens and closes," explains González Zalba. The researchers have demonstrated the possibility of detecting the charge of an electron with their device in approximately one nanosecond, the best value obtained to date for this type of system. This has been achieved by coupling a gate sensor to a silicon nanotransistor where the electrons flow individually. In general, the electrical current which powers our telephones, fridges and other electrical equipment is made up of electrons: minuscule particles carrying an electrical charge travelling in their trillions and whose collective movement makes these appliances work. However, this is not the case of the latest cutting-edge devices such as ultra-precise biosensors, single electron transistors, molecular circuits and quantum computers. These represent a new technological sector which bases its electronic functionality on the charge of a single electron, a field in which the new gate sensor can offer its advantages.

Scientists use nanoscale building blocks and DNA 'glue' to shape 3D superlattices

Taking child's play with building blocks to a whole new level—the nanometer scale—scientists at the U.S. Department of Energy's (DOE) Brookhaven National Laboratory have constructed 3D "superlattice" multicomponent nanoparticle arrays where the arrangement of particles is driven by the shape of the tiny building blocks. The method uses linker molecules made of complementary strands of DNA to overcome the blocks' tendency to pack together in a way that would separate differently shaped components. The results, published in ("Superlattices Assembled through Shape-Induced Directional Binding"), are an important step on the path toward designing predictable composite materials for applications in catalysis, other energy technologies, and medicine.

Controlling the self-assembly of nanoparticles into superlattices is an important approach to build functional materials. The Brookhaven team used nanosized building blocks—cubes or octahedrons—decorated with DNA tethers to coordinate the assembly of spherical nanoparticles coated with complementary DNA strands. (click on image to enlarge) "If we want to take advantage of the promising properties of nanoparticles, we need to be able to reliably incorporate them into larger-scale composite materials for real-world applications," explained Brookhaven physicist Oleg Gang, who led the research at Brookhaven's Center for Functional Nanomaterials (CFN), a DOE Office of Science User Facility. "Our work describes a new way to fabricate structured composite materials using directional bindings of shaped particles for predictable assembly," said Fang Lu, the lead author of the publication. The research builds on the team's experience linking nanoparticles together using strands of synthetic DNA. Like the molecule that carries the genetic code of living things, these synthetic strands have complementary bases known by the genetic code letters G, C, T, and A, which bind to one another in only one way (G to C; T to A). Gang has previously used complementary DNA tethers attached to nanoparticles to guide the assembly of a range of arrays and structures. The new work explores particle shape as a means of controlling the directionality of these interactions to achieve long-range order in large-scale assemblies and clusters. Spherical particles, Gang explained, normally pack together to minimize free volume. DNA linkers—using complementary strands to attract particles, or non-complementary strands to keep particles apart—can alter that packing to some degree to achieve different arrangements. For example, scientists have experimented with placing complementary linker strands in strategic locations on the spheres to get the particles to line up and bind in a particular way. But it's not so easy to make nanospheres with precisely placed linker strands. "We explored an alternate idea: the introduction of shaped nanoscale 'blocks' decorated with DNA tethers on each facet to control the directional binding of spheres with complementary DNA tethers," Gang said. When the scientists mixed nanocubes coated with DNA tethers on all six sides with nanospheres of approximately the same size, which had been coated with complementary tethers, these two differently shaped particles did not segregate as would have been expected based on their normal packing behavior. Instead, the DNA "glue" prevented the separation by providing an attractive force between the flat facets of the blocks and the tethers on the spheres, as well as a repulsive force between the non-pairing tethers on same-shaped objects.

The DNA tethers lead cubic blocks and spheres to self assemble so that one sphere binds to each face of a cube, resulting in a regular, repeating arrangement. "The DNA permits us to enforce rules: spheres attract cubes (mutually); spheres do not attract spheres; and cubes do not attract cubes," Gang said. "This breaks the conventional packing tendency and allows for the system to self-assemble into an alternating array of cubes and spheres, where each cube is surrounded by six spheres (one to a face) and each sphere is surrounded by six cubes." Using octahedral blocks instead of cubes achieved a different arrangement, with one sphere binding to each of the blocks' eight triangular facets. The method required some thermal processing to achieve the most uniform long-range order. And experiments with different types of DNA tethers showed that having flexible DNA strands was essential to accommodate the pairing of differently shaped particles. "The flexible DNA shells 'soften' the particles, which allows them to fit into arrangements where the shapes do not match geometrically," Lu said. But excessive softness results in unnecessary particle freedom, which can ruin a perfect lattice, she added. Finding the ideal flexibility for the tethers was an essential part of the work. 3D arrangement of cubes and octahedrons

Changing the shape of the blocks from cubes to octahedrons results in a different 3-D arrangement. The scientists used transmission and scanning electron microscopy at the CFN and also conducted x-ray scattering experiments at the National Synchrotron Light Source, another DOE Office of Science User Facility at Brookhaven Lab, to reveal the structure and take images of assembled clusters and lattices at various length scales. They also explained the experimental results with models based on the estimation of nanoscale interactions between the tiny building blocks. "Ultimately, this work shows that large-scale binary lattices can be formed in a predictable manner using this approach," Gang said. "Given that our approach does not depend on the particular particle's material and the large variety of particle shapes available—many more than in a child's building block play set—we have the potential to create many diverse types of new nanomaterials."

Mechanical cloaks of invisibility - without complicated mathematics

A honeycomb is a very stable structure. If it has a larger hole, however, stability is largely lost. What might a honeycomb look like, which survives external forces in spite of a hole? Such stable types of known constructions might be useful in architecture or when developing new construction materials. So far, the mathematical expenditure required has been very high and did not lead to the success desired in mechanics. Researchers of Karlsruhe Institute of Technology (KIT) have now found a new principle that considerably facilitates the mathematical approach and produces promising results with simple means.

Mechanical cloak of invisibility: In a regular honeycomb structure (left), a hole is compensated by a distortion (right). External forces act as if the hole would not exist. (Image: T. Bückmann/KIT) The conception of “coordinate transformation” may sound complex, but such mathematical transformations are rather helpful: A mesh of connected points is drawn onto a rubber skin. Coordinate transformation is simulated by extending and distorting this rubber surface. When the assumed mesh can be mapped onto material distribution, a rather universal design approach results. It can be used to direct e.g. mechanical forces acting on the material along the tracks desired. For light, such transformations are based on the mathematics of transformation optics. So far, however, it has been impossible to transfer this principle to real materials and components in mechanics. The mathematics made impossible requirements on the material. To overcome these difficulties, researchers of the KIT Institute of Applied Physics around first author Tiemo Bückmann found a new, simple method. “We imagined a network of electric resistors,” Bückmann explains. “The wire connections between the resistors may be chosen to be of variable length, but their value does not change. Electric conductivity of the network even remains unchanged, when it is deformed.” The researchers transferred this thought experiment to practice. “In mechanics, this principle is found again when imagining small springs instead of resistors,” Tiemo Bückmann says. “We can make single springs longer or shorter when adapting their shapes, such that the forces between them remain the same. This simple principle saves computation expenditure and allows for the direct transformation of real materials.”

Analysis of a hole in a hexagonal structure: External forces strongly deform the structure, the structure is unstable. By means of the new construction method, this error can be reduced strongly. (Image: T. Bückmann) (click on image to enlarge) The researchers tested their method in a model experiment with a material made of printed polymer. A stable hexagonal honeycomb structure was provided with a hole. Due to its reduced stability, the distorting forces first caused an error of more than 700 percent. After application of the newly developed transformation, the error amounted to 26 percent only. The results have just been published in the ("Mechanical cloak design by direct lattice transformation"). Applications are manifold, as the new method can be used to calculate known composite materials or mechanical support constructions. Even special designs will react as stably as possible to external forces – as if the support construction would not have been deformed.