A new approach to self-assemble and tailor complex structures at the nanoscale, developed by an international collaboration led by the University of Cambridge and IBM, opens opportunities to tailor properties and functionalities of materials for a wide range of semiconductor device applications. The researchers have developed a method for growing combinations of different materials in a needle-shaped crystal called a nanowire. Nanowires are small structures, only a few billionths of a metre in diameter. Semiconductors can be grown into nanowires, and the result is a useful building block for electrical, optical, and energy harvesting devices. The researchers have found out how to grow smaller crystals within the nanowire, forming a structure like a crystal rod with an embedded array of gems. Details of the new method are published in the journal ("Synthesis of nanostructures in nanowires using sequential catalyst reactions"). 
Electron microscope images showing the formation of a nickel silicide nanoparticle (colored yellow) in a silicon nanowire. (Image: Stephan Hofmann) "The key to building functional nanoscale devices is to control materials and their interfaces at the atomic level," said Dr. Stephan Hofmann of the Department of Engineering, one of the paper's senior authors. "We've developed a method of engineering inclusions of different materials so that we can make complex structures in a very precise way." Nanowires are often grown through a process called Vapour-Liquid-Solid (VLS) synthesis, where a tiny catalytic droplet is used to seed and feed the nanowire, so that it self-assembles one atomic layer at a time. VLS allows a high degree of control over the resulting nanowire: composition, diameter, growth direction, branching, kinking and crystal structure can be controlled by tuning the self-assembly conditions. As nanowires become better controlled, new applications become possible. The technique that Hofmann and his colleagues from Cambridge and IBM developed can be thought of as an expansion of the concept that underlies conventional VLS growth. The researchers use the catalytic droplet not only to grow the nanowire, but also to form new materials within it. These tiny crystals form in the liquid, but later attach to the nanowire and then become embedded as the nanowire is grown further. This catalyst mediated docking process can 'self-optimise' to create highly perfect interfaces for the embedded crystals. To unravel the complexities of this process, the research team used two customised electron microscopes, one at IBM's TJ Watson Research Center and a second at Brookhaven National Laboratory. This allowed them to record high-speed movies of the nanowire growth as it happens atom-by-atom. The researchers found that using the catalyst as a 'mixing bowl', with the order and amount of each ingredient programmed into a desired recipe, resulted in complex structures consisting of nanowires with embedded nanoscale crystals, or quantum dots, of controlled size and position. "The technique allows two different materials to be incorporated into the same nanowire, even if the lattice structures of the two crystals don't perfectly match," said Hofmann. "It's a flexible platform that can be used for different technologies." Possible applications for this technique range from atomically perfect buried interconnects to single-electron transistors, high-density memories, light emission, semiconductor lasers, and tunnel diodes, along with the capability to engineer three-dimensional device structures. "This process has enabled us to understand the behaviour of nanoscale materials in unprecedented detail, and that knowledge can now be applied to other processes," said Hofmann.
read more "New technique to synthesize nanostructured nanowires"
Electron microscope images showing the formation of a nickel silicide nanoparticle (colored yellow) in a silicon nanowire. (Image: Stephan Hofmann) "The key to building functional nanoscale devices is to control materials and their interfaces at the atomic level," said Dr. Stephan Hofmann of the Department of Engineering, one of the paper's senior authors. "We've developed a method of engineering inclusions of different materials so that we can make complex structures in a very precise way." Nanowires are often grown through a process called Vapour-Liquid-Solid (VLS) synthesis, where a tiny catalytic droplet is used to seed and feed the nanowire, so that it self-assembles one atomic layer at a time. VLS allows a high degree of control over the resulting nanowire: composition, diameter, growth direction, branching, kinking and crystal structure can be controlled by tuning the self-assembly conditions. As nanowires become better controlled, new applications become possible. The technique that Hofmann and his colleagues from Cambridge and IBM developed can be thought of as an expansion of the concept that underlies conventional VLS growth. The researchers use the catalytic droplet not only to grow the nanowire, but also to form new materials within it. These tiny crystals form in the liquid, but later attach to the nanowire and then become embedded as the nanowire is grown further. This catalyst mediated docking process can 'self-optimise' to create highly perfect interfaces for the embedded crystals. To unravel the complexities of this process, the research team used two customised electron microscopes, one at IBM's TJ Watson Research Center and a second at Brookhaven National Laboratory. This allowed them to record high-speed movies of the nanowire growth as it happens atom-by-atom. The researchers found that using the catalyst as a 'mixing bowl', with the order and amount of each ingredient programmed into a desired recipe, resulted in complex structures consisting of nanowires with embedded nanoscale crystals, or quantum dots, of controlled size and position. "The technique allows two different materials to be incorporated into the same nanowire, even if the lattice structures of the two crystals don't perfectly match," said Hofmann. "It's a flexible platform that can be used for different technologies." Possible applications for this technique range from atomically perfect buried interconnects to single-electron transistors, high-density memories, light emission, semiconductor lasers, and tunnel diodes, along with the capability to engineer three-dimensional device structures. "This process has enabled us to understand the behaviour of nanoscale materials in unprecedented detail, and that knowledge can now be applied to other processes," said Hofmann.
A new, first-of-its-kind technique developed by Bobby Day (left) and Max Mankin, graduate students working in the lab of Charles Lieber, the Mark Hyman Jr. Professor of Chemistry, could have applications in areas ranging from consumer electronics to solar panels. “This is really a fundamental discovery,” Day said. “We’re still in the early stages, but we think there is a lot of room for discovery, both of fundamental properties of these structures as well as applications.” First described in 1870, Plateau-Rayleigh instability is normally associated with liquids, but researchers for years have recognized a similar phenomenon in nanowires. When heated to extreme temperatures, the wires transform from solid into a series of periodically spaced droplets. To create the new type of wire, Day and Mankin heated traditionally grown nanowires to just below that transformation point in a vacuum chamber, then pumped in silicon atoms, which spontaneously crystallize on the wire. Rather than form a uniform shell, the atoms grow into regularly spaced structures, similar to the droplets that appear when nanowires break down at high temperatures. Unlike with the droplets, though, the process can be tightly controlled. “By varying the temperature and pressure, we can exert some control over the size and spacing of these structures,” Day said. “What we found was if we change the conditions, we can ‘tune’ how these structures are built.” Along with duplicating the process in nanowires between 20 and 100 nanometers in diameter, researchers demonstrated the process using several combinations of materials, including silicon and germanium. In addition to being able to “tune” the distance between the lobes on nanowires, Mankin said tests showed they were also able to tune the cross-section of the wires. “We can tune the cross-section to produce more rounded or square-type wires,” Mankin said. “We were also able to produce wires with a platelet-like shape.” With those new structures, researchers found, came new properties for the wires. While Day and Mankin’s study focused on the wires’ ability to absorb different wavelengths of light, both said additional research is needed to explore other properties. “This paper is just one example,” Day said. “There are many other properties — including thermal conductance, electrical conductance, and magnetic properties — that depend on the wires’ diameter, and they still need to be explored.” Though it may take years to fully explore those additional properties, Day and Mankin said applications for the new wires could emerge in the near term. “Structures at this scale, because they are sub-wavelength in size, absorb light very efficiently,” Day explained. “They act almost like optical antennas, and funnel the light into them. Previous research has shown that different diameter wires absorb different wavelengths of light. For example, very small diameters absorb blue light well, and larger diameters absorb green light. What we showed is if you have this modulation along the structure … we can have the best of both worlds and absorb both wavelengths on the same structure.” The new wires’ unusual light-absorption abilities don’t end there, though. By shrinking the space between the crystalline structures, Day and Mankin discovered that the wires not only absorb light at specific wavelengths, they also absorb light from other parts of the spectrum. “It’s actually more than a simple additive effect,” Day said. “As you shrink the spacing down to distances smaller than about 400 nanometers, it creates what are called grating modes, and we see these huge absorption peaks in the infrared. What that means is that you could absorb the same amount of infrared light with these nanowires as you could with traditional silicon materials that are 100 times thicker.” “This is a powerful discovery because previously, if you wanted to use nanowires for photo-detection of green and blue light, you’d need two wires,” Mankin said. “Now we can shrink the amount of space a device might take up by having multiple functions in a single wire. We will be able to build smaller devices that still maintain high efficiency, and in some cases will take advantages of new properties that will emerge from this modulation that you don’t have in uniform-diameter wires.”
An array of helical elements absorbs radiation of a certain frequency while casting no shadow in light over a range of other frequencies. “These absorbers are completely transparent at non-operational frequencies”, concludes researcher Viktar Asadchy. While maintaining efficient absorption of light of the desired frequency, all conventional absorbers strongly interact with the radiation of other frequencies, reflecting it back and not letting it pass through. As a result, they create a reflected beam as well as a perceptible shadow behind and become detectable. The designed and tested structures are able to absorb and sense the light of one or several desired frequency spectra, while being invisible and undetectable at other frequencies. The research has proven that such an unparalleled operation can only be achieved with the use of structural inclusions whose electric and magnetic properties are strongly coupled. These functionalities can lead to a variety of unique applications for radio astronomy and stealth technology. They can also be very useful in everyday life. For example, they could be used in screens that can filter any cell phone signals and pass through Wi-Fi and other microwaves. “This research will also open new venues for general light control and enable novel devices such as flat lenses and light beam transformers”, explains Asadchy.