Nanomaterials are often categorized based on number of dimensions of a material, which are outside the nanoscale (<100 nm) range. Accordingly, in one-dimensional nanomaterials (1D), one dimension is outside the nanoscale. This class includes nanofibers, nanotubes, nanorods, and nanowires. 1D materials are generally considered to be high‐capacity and stable electrode materials, due to their uniform structure, orientated electronic and ionic transport, and strong tolerance to stress change.
During the past decades, various 1D nanostructures with different size, structure, chemical composition and doping have been successfully synthesized and their applications in mechanical, electromechanical, electric and optoelectronic devices have been demonstrated.
To alleviate the severe energy problems we are facing nowadays, tremendous attention has been paid on harvesting clean and renewable energy from ambient energy sources. Given the enhanced piezoelectric effect and excellent mechanical properties, one-dimensional (1D) piezoelectric nanostructures have been regarded as the next-generation piezoelectric material.
Meanwhile, during the miniaturization of various functional devices, their high specific surface area, low energy consumption and easier integration also made them promising building blocks for future electronic devices, where the development of wireless and self-powered electronic devices are quite essential, especially in the field of sensing, medical science and wearable personal electronics.
1D materials are also promising for replacement of copper interconnects. Engineers at the University of California, Riverside, have demonstrated prototype devices made of an exotic 1D material that can conduct a current density 50 times greater than conventional copper interconnect technology.
Current density is the amount of electrical current per cross-sectional area at a given point. As transistors in integrated circuits become smaller and smaller, they need higher and higher current densities to perform at the desired level. Most conventional electrical conductors, such as copper, tend to break due to overheating or other factors at high current densities, presenting a barrier to creating increasingly small components.
The electronics industry needs alternatives to silicon and copper that can sustain extremely high current densities at sizes of just a few nanometers.
The advent of graphene resulted in a massive, worldwide effort directed at investigation of other two-dimensional, or 2D, layered materials that would meet the need for nanoscale electronic components that can sustain a high current density. While 2D materials consist of a single layer of atoms, 1D materials consist of individual chains of atoms weakly bound to one another, but their potential for electronics has not been as widely studied.
One-Dimensional Material Packs a Powerful Punch for Next Generation Electronics
One can think of 2D materials as thin slices of bread while 1D materials are like spaghetti. Compared to 1D materials, 2D materials seem huge.
A group of researchers led by Alexander A. Balandin, a distinguished professor of electrical and computer engineering in the Marlan and Rosemary Bourns College of Engineering at UC Riverside, discovered that zirconium tritelluride, or ZrTe3, nanoribbons have an exceptionally high current density that far exceeds that of any conventional metals like copper.
The new strategy undertaken by the UC Riverside team pushes research from two-dimensional to one-dimensional materials— an important advance for the future generation of electronics.
“Conventional metals are polycrystalline. They have grain boundaries and surface roughness, which scatter electrons,” Balandin said. “Quasi-one-dimensional materials such as ZrTe3 consist of single-crystal atomic chains in one direction. They do not have grain boundaries and often have atomically smooth surfaces after exfoliation. We attributed the exceptionally high current density in ZrTe3 to the single-crystal nature of quasi-1D materials.”
In principle, such quasi-1D materials could be grown directly into nanowires with a cross-section that corresponds to an individual atomic thread, or chain. In the present study the level of the current sustained by the ZrTe3 quantum wires was higher than reported for any metals or other 1D materials. It almost reaches the current density in carbon nanotubes and graphene.
Electronic devices depend on special wiring to carry information between different parts of a circuit or system. As developers miniaturize devices, their internal parts also must become smaller, and the interconnects that carry information between parts must become smallest of all. Depending on how they are configured, the ZrTe3 nanoribbons could be made into either nanometer-scale local interconnects or device channels for components of the tiniest devices.
The UC Riverside group’s experiments were conducted with nanoribbons that had been sliced from a pre-made sheet of material. Industrial applications need to grow nanoribbon directly on the wafer. This manufacturing process is already under development, and Balandin believes 1D nanomaterials hold possibilities for applications in future electronics.
“The most exciting thing about the quasi-1D materials is that they can be truly synthesized into the channels or interconnects with the ultimately small cross-section of one atomic thread— approximately one nanometer by one nanometer,” Balandin said.
The results of this investigation appear this month in IEEE Electron Device Letters [see A. Geremew, M. A. Bloodgood, E. Aytan, B. W. K. Woo, S. R. Corber, G. Liu, K. Bozhilov, T. T. Salguero, S. Rumyantsev, M. P. Rao, and A. A. Balandin, “Current Carrying Capacity of Quasi-1DZrTe3 van der Waals Nanoribbons,” IEEE, Electron. Device Lett., 39, 735 (2018).
Adane Geremew, the first author of the paper, is a Ph.D. student in Balandin’s group. Professor Tina Salguero, University of Georgia, synthesized the bulk materials, which were used for exfoliation of nanoribbons.
The research was supported by the Semiconductor Research Corporation and the National Science Foundation.
Researchers at IBM Research in Yorktown Heights, NY, have demonstrated a new way to convert electricity into light in nanowire-based light-emitting devices (LEDs). The nanowire LEDs could eventually be used for telecommunications and for faster communications between devices on microchips. The approach could also pave the way for a new type of bright, efficient display.
The researchers built an LED resembling a transistor that consists of an indium-nitride nanowire stretched between two electrodes on top of a silicon substrate. The nanowire is about 100 nanometers wide and spans a distance of less than 10 micrometers. When the researchers apply a current to the nanowire, it emits light. While nanowires that emit light have been made before, the new devices rely on different physical mechanisms that are simpler; as a result, the nanowire LED could be more efficient and have improved performance. What’s more, the device succeeds in emitting infrared light, which has been particularly difficult for nanowires to do, says Phaedon Avouris, one of the IBM researchers.
Typically, light in LEDs is produced by injecting both electrons and their positive counterparts, holes, into an active material, where they combine and emit light. With the new devices, the researchers only have to inject electrons; these cause electrons and holes to form locally, inside the nanowires. The mechanism could be more efficient because a single electron can be used to generate more than one electron-hole pair. What’s more, the researchers have demonstrated that the nanowires can produce more intense light emission than other LEDs.
The nanowires’ small size and compatibility with silicon make them attractive for integration on chips, says Eugene Fitzgerald, a professor of materials science and engineering at MIT. The nanowires also emit infrared light, which makes them ideal for fiber-optic telecommunications and for optical communications between devices on microchips that could help dramatically speed up computers.
The nanowire LEDs extend the range of colors that can be emitted from nitride-based materials, Fitzgerald says. Nitride materials are the basis of the blue lasers in high-definition DVD players, he says, and they have also been useful for emitting green light. If the nanowires can be tuned to emit red light, as seems likely, then red, green, and blue LEDs could all be created with variations of the same material, making it practical to manufacture them all on the same substrate. Eventually, it may be possible to arrange such LEDs into the pixels of full-color displays that are brighter, more efficient, and better looking than today’s flat-panel LCD displays, Fitzgerald says.
Not only did the wires emit infrared light, but they also showed a peculiar ability to emit more intense light as temperatures rose; ordinarily, at high temperatures light emission dims or stops. This could lead to LEDs that can withstand high temperatures, a property that could be useful for certain military applications, Avouris says.
The novel physical mechanisms underlying the indium-nitride nanowires’ ability to emit light might have wider implications for nanowire research. If the mechanism used here works in other materials, it could expand the number of materials that might be used to create LEDs, Fitzgerald says. That could make LEDs cheaper and give researchers far greater versatility in creating devices with improved performance.
New infrared camera goes multi-spectral
Researchers at McMaster University in Canada have developed a new type of multi-spectral camera from indium arsenide antimonide (InAsSb) nanowires that can indeed be directly integrated with existing Si technology. The device, which can be directly integrated with existing silicon sensors, will allow for low-cost, large-area detectors and cameras that might be employed in astronomy, automotive safety, surveillance, search and rescue, and defence applications, to name but a few.
There are two main IR technologies available today. The first is based on mercury-cadmium-telluride (MCT) and the second on group III-V elements of the periodic table (that is, indium gallium arsenide (InGaAs) and indium antimonide (InSb).
MCT is the only technology that can access all of the SWIR, MWIR and LWIR ranges and MCT-devices that dominate the market today. Imaging arrays based on MCT mostly make use of small-area and expensive CdZnTe substrates, however, which means they are mainly restricted to military applications. The fact that MCT detectors also contain mercury (Hg), which is toxic, makes them far from ideal. SWIR detectors based on group III-V materials, such as InGaAs grown on InP substrates or InSb on InSb, are obviously better in this respect, but they are also expensive to manufacture and restricted to small areas.
Lattice mismatch means IR device materials cannot be directly grown on Si
There is another big problem in that current technologies cannot be easily integrated with Si, something that is needed for image read-out and processing. “This is because IR device materials cannot be directly grown on Si due to lattice mismatch,” explains LaPierre. “To get around this problem, researchers generally grow them instead on expensive II-VI (for example, CdZnTe) or III-V (InP and InSb) substrates that can later be integrated with Si electronic circuitry using indium bump bonding or interconnects. These additional steps slow down the overall manufacturing process and make it costlier too.”
The problems do not end there. “Most current IR cameras cannot image over a wide range of wavelengths, but such a multi-colour capability would be very useful for advanced IR imaging systems,” he says. “Existing multi-spectral cameras are fabricated by growing layers with different material bandgaps on top of one another to absorb different regions of the infrared light spectrum. These bandgaps are generally restricted to lattice-matched materials, which restrict these cameras to only a few layers with specific absorption wavelengths.”
Multiple resonances in a single material system
The McMaster researchers have now found that semiconductor nanowire arrays support optical resonant modes thanks to an antenna effect. Indeed, they allow the nanowires to act as very effective waveguides that concentrate and absorb light over a length of just microns.
“At these resonant wavelengths, the nanowire arrays can absorb light much more than the equivalent thickness of a thin film thanks to this effect,” explains LaPierre. “This enables more efficient photodetection than existing technology and with relatively little material.”
And that is not all: “this resonant light absorption shows wavelength selectivity that can be tuned continuously across the visible and IR wavelengths by adjusting the nanowire diameter,” he tells nanotechweb.org. “Large diameter nanowires absorb long wavelengths of light while smaller diameter ones absorb short wavelengths. By tuning the diameter, we can select which wavelengths are optimally absorbed in a detector or a camera. This allows for multiple resonances in a single material system (with different nanowire diameters being placed on the same silicon substrate, or chip) and this principle can be exploited as a new concept for multi-spectral imaging with improved wavelength selection compared to existing detectors.”
Relieving” lattice mismatch
The researchers also succeeded in “relieving” the lattice mismatch when growing the nanowires on Si thanks to elastic relaxation near the nanowire surface. “This approach allows high lattice-mismatched InAsSb nanowires to be grown directly on Si sensors,” says LaPierre.
The team says that it will now be fabricating electrodes on the nanowire arrays to make a camera pixel that is sensitive to specific wavelengths. “This work is being performed in collaboration with Teledyne Dalsa and Lockheed Martin Canada,” he reveals.