A new strategy of fabricating p-n junction in single crystalline Si nanowires, twisting


anewstrategy
Illustration of the relative formation energy as function of twist rate γ of doped Si nanowire for Sb and B dopants at different atomic sites. The strain-free and twisted Si nanowires are shown at the axial view. Credit: ©Science China Press

Can single crystalline materials be used for low dimensional p-n junction design? This is an open and long-standing problem. Microscopic simulations based on the generalized Bloch theorem show that in single crystalline Si nanowires, an axial twist can lead to the separation of p-type and n-type dopants along the nanowire radial dimension, and thus realizes the p-n junction. A bond orbital analysis reveals that this is due to the twist-induced inhomogeneous shear strain in the nanowire.

If a semiconductor crystal is doped with  dopants in one region and with  dopants in another region, a p-n junction configuration is formed. P-n junctions are fundamental building units of light emitting diodes, solar cells and other semiconductor transistors. P-n junctions in nano-structures are also expected to be the fundamental units of next generation nano-devices.

However, due to the strong attraction between them, n-type dopants and p-type dopants tend to form neutral pairs. As a result, the p-n junction fails. To prevent such attraction between n-type dopants and p-type dopants, heterostructures are introduced, where one  is doped with n-type dopants while the other is doped with p-type dopants, and the interface between two different semiconductor materials acts as an  between n-type dopants and p-type dopants.

Indeed, the usage of heterostructures stands for a paradigm for the material design of p-n junction. Recently, similar  configurations are also possible for nanowire heterostructures such as co-axial core-shell nanowires. However, there are several limitations in nanowire heterostructures. For example, the synthesis of core-shell nanowires usually involves a two-step process, which costs extra expense. Often the shell of the obtained nanowire heterostructure is polycrystalline. Such imperfection goes ill with the transports of carriers. Furthermore, the interface between the core and shell also introduces detrimental deep centers that largely hinder the device efficiency.

Can we make p-n junctions with single crystalline nanowires? Frankly, the answer will be “No” if one thinks the problem intuitively. Indeed, similar to the bulk, p-type dopants and n-type dopants in a codoped single crystalline nanowire also feel strong Coulomb attraction. Without an interface, how can we overcome such attraction? It requires an effective modulation/control of the spatial occupation sites, i.e., spatial distribution, of dopants. In fact, this is one of the long-standing and fundamental issues regarding doping in semiconductor.

From the point of view of materials engineering, this can be attributed to the failure of conventional approaches such as hydrostatic, biaxial and uniaxial stresses on the modulation of the spatial distribution of dopants. However, since all these mentioned distortions are uniform, can we employ some inhomogeneous ones, such as twisting? In fact, twisting of structures represents a focus of recent condensed matter physics research in low dimensions.

In a new paper published in National Science Review, a team of scientists from Beijing Normal University, the Chinese University of Hong Kong, and Beijing Computational Science Research Center present their theoretical advances of codoped Si nanowire under twisting. They employ both microscopic simulations based on the generalized Bloch theorem and analytical modeling based on the bond orbital theory to conduct the study and deliver the physics behind.

Interestingly, twisting has substantial impact on distribution of dopants in . From the displayed figure, in a twisted Si nanowire, a  of larger atomic size (Such as Sb) has a lower formation energy if it occupies an atomic site closer to nanowire surface; On the opposite, a dopant of smaller atomic size (Such as B) has a lower formation energy if it occupies an atomicsites around the nanowire core. According to their calculations, it is possible to separate n-type and p-type dopants in the codoped nanowire with proper choices of codoping pairs, e.g., B and Sb. A bond orbital analysis reveals that it is the twist-induced inhomogeneous shear strain along nanowire radial dimension that drives the effective modulation. These findings are fully supported by density-functional tight-binding based generalized Bloch theorem simulations.

This new strategy largely simplifies the manufacturing process and lowers the manufacturing costs. If the twisting is applied when the device is in working mode, the recombination of different types of dopants is largely suppressed. Even if the twisting is removed when the device is in working mode, due to the limited diffusion, the recombination is still difficult.

 Explore further: Researchers decipher electrical conductivity in doped organic semiconductors

More information: Dong-Bo Zhang et al, Twist-Driven Separation of p-type and n-type Dopants in Single Crystalline Nanowires, National Science Review (2019). DOI: 10.1093/nsr/nwz014

 

EPFL and MIT Researchers Discover the ‘Holy Grail’ of Nanowire Production


Holy Grail Nanowire 5c6d75008f989

EPFL researchers have found a way to control and standardize the production of nanowires on silicon surfaces. Credit: Ecole Polytechnique Federale de Lausanne (EPFL)

Nanowires have the potential to revolutionize the technology around us. Measuring just 5-100 nanometers in diameter (a nanometer is a millionth of a millimeter), these tiny, needle-shaped crystalline structures can alter how electricity or light passes through them.

They can emit, concentrate and absorb light and could therefore be used to add optical functionalities to electronic chips. They could, for example, make it possible to generate lasers directly on  and to integrate single-photon emitters for coding purposes. They could even be applied in  to improve how sunlight is converted into electrical energy.

Up until now, it was impossible to reproduce the process of growing nanowires on silicon semiconductors – there was no way to repeatedly produce homogeneous nanowires in specific positions.

But researchers from EPFL’s Laboratory of Semiconductor Materials, run by Anna Fontcuberta i Morral, together with colleagues from MIT and the IOFFE Institute, have come up with a way of growing nanowire networks in a highly controlled and fully reproducible manner. The key was to understand what happens at the onset of nanowire growth, which goes against currently accepted theories. Their work has been published in Nature Communications.

“We think that this discovery will make it possible to realistically integrate a series of nanowires on silicon substrates,” says Fontcuberta i Morral. “Up to now, these nanowires had to be grown individually, and the process couldn’t be reproduced.”

The holy grail of nanowire production
Two different configurations of the droplet within the opening – hole fully filled and partially filled and bellow illustration of GaAs crystals forming a full ring or a step underneath the large and small gallium droplets. Credit: Ecole Polytechnique Federale de Lausanne (EPFL)

 

Getting the right ratio

The standard process for producing nanowires is to make  in  monoxide and fill them with a nanodrop of liquid gallium. This substance then solidifies when it comes into contact with arsenic. But with this process, the substance tends to harden at the corners of the nanoholes, which means that the angle at which the nanowires will grow can’t be predicted. The search was on for a way to produce homogeneous nanowires and control their position.

Research aimed at controlling the  has tended to focus on the diameter of the hole, but this approach has not paid off. Now EPFL researchers have shown that by altering the diameter-to-height ratio of the hole, they can perfectly control how the nanowires grow. At the right ratio, the substance will solidify in a ring around the edge of the hole, which prevents the nanowires from growing at a non-perpendicular angle. And the researchers’ process should work for all types of .

“It’s kind of like growing a plant. They need water and sunlight, but you have to get the quantities right,” says Fontcuberta i Morral.

This new production technique will be a boon for nanowire research, and further samples should soon be developed.

 Explore further: Nanowires have the power to revolutionize solar energy (w/ video)

More information: J. Vukajlovic-Plestina et al. Fundamental aspects to localize self-catalyzed III-V nanowires on silicon, Nature Communications (2019). DOI: 10.1038/s41467-019-08807-9

 

New Novel Nanowires open up new Possibilities in Nano-Electronics (Molecular Electronics)


Nanowires id46974_1Schematic representation of the folding and anchoring processes needed to obtain π-folded molecular junctions from a representative member of the foldamer family studied in this work. (© Nature) (click on image to enlarge)
The current demand for small-sized electronic devices is calling for fresh approaches in their design.A group of researchers at the Basque Excellence Research Center into Polymers (POLYMAT), the University of the Basque Country (UPV/EHU), the University of Barcelona, the Institute of Bioengineering of Barcelona (IBEC), and the University of Aveiro, and led by Aurelio Mateo-Alonso, the Ikerbasque research professor at POLYMAT, have developed a new suite of molecular wires or nanowires that are opening up new horizons in molecular electronics.The research is being published today in the prestigious journal Nature Communications (“High conductance values in π-folded molecular junctions”).

The growing demand for increasingly smaller electronic devices is prompting the need to produce circuits whose components are also as small as possible, and this is calling for fresh approaches in their design.

Molecular electronics has sparked great interest because the manufacture of electronic circuits using molecules would entail a reduction in their size.
Nanowires are conducting wires on a molecular scale that carry the current inside these circuits. That is why the efficiency of these wires is crucially important.
In fact, one of the main novelties in this new suite of nanowires developed by the group led by Aurelio Mateo lies in their high efficiency, which constitutes a step forward in miniaturizing electronic circuits.
Source: University of the Basque Country

 

IBM Scientists: Quantum transport goes ballistic ~ ‘Q & A’ with NIST


 

IBM QT download

IBM scientists have shot an electron through an III-V semiconductor nanowire integrated on silicon for the first time (Nano Letters, “Ballistic One-Dimensional InAs Nanowire Cross-Junction Interconnects”). This achievement will form the basis for sophisticated quantum wire devices for future integrated circuits used in advanced powerful computational systems. (A Q&A with NIST)

NIST: The title of your paper is Ballistic one-dimensional InAs nanowire cross-junction interconnects. When I read “ballistic” rather large missiles come to mind, but here you are doing this at the nanoscale. Can you talk about the challenges this presents?
Johannes Gooth (JG): Yes, this is very similar, but of course at a much different scale. Electrons are fired from one contact electrode and fly through the nanowire without being scattered until they hit the opposed electrode. The nanowire acts as a perfect guide for electrons, such that the full quantum information of this electron (energy, momentum, spin) can be transferred without losses. 

Quantum Trnp goes Ballistic id46328_1IBM scientist Johannes Gooth is focused on nanoscale electronics and quantum physics.

 

We can now do this in cross-junctions, which allows us to build up electron pipe networks, where quantum information can perfectly be transmitted. The challenge is to fabricate a geometrically very well defined material with no scatterers inside on the nano scale. The template-assisted selective epitaxy or TASE process, which was developed here at the IBM Zurich Lab by my colleagues, makes this possible for the first time.
NIST: How does this research compare to other activities underway elsewhere?
JG: Most importantly, compared to optical and superconducting quantum applications the technique is scalable and compatible with standard electronics and CMOS processes.
NIST: What role do you see for quantum transport as we look to build a universal quantum computer?
JG: I see quantum transport as an essential piece. If you want to exercise the full power of quantum information technology, you need to connect everything ballistic: a quantum system that is fully ballistically (quantum) connected has an exponentially larger computational state space compared to classically connected systems.
Also, as stated above, the electronics are scalable. Moreover, combining our nanowire structures with superconductors allows for topological protected quantum computing, which enables fault tolerant computation. These are major advantages compared to other techniques.IBM QT II 51XVJRfhbNL._SX348_BO1,204,203,200_
NIST: How easily can this be manufactured using existing processes and what’s the next step?
JG: This is a major advantage of our technique because our devices are fully integrated into existing CMOS processes and technology.
NIST: What’s next for your research?
JG: The next steps will be the functionalization of the crosses, by means of attaching electronic quantum computational parts. We will start to build superconducting/nanowire hybrid devices for Majorana braiding, and attach quantum dots.
Source: NIST

 

Flexible, Bendable Digital Storage made Possible by Spray-on (nanoparticle inks) Memory


sprayonmemorDuke University researchers have developed a new ‘spray-on’ digital memory (upper left) that could be used to build programmable electronic devices on flexible materials like paper, plastic or fabric. To demonstrate a simple application of …more

USB flash drives are already common accessories in offices and college campuses. But thanks to the rise in printable electronics, digital storage devices like these may soon be everywhere—including on our groceries, pill bottles and even clothing.

Duke University researchers have brought us closer to a future of low-cost, flexible electronics by creating a new “spray-on” digital memory using only an aerosol jet printer and nanoparticle inks.

The device, which is analogous to a 4-bit flash drive, is the first fully-printed digital memory that would be suitable for practical use in simple electronics such as environmental sensors or RFID tags. And because it is jet-printed at relatively low temperatures, it could be used to build programmable electronic devices on bendable materials like paper, plastic or fabric.

“We have all of the parameters that would allow this to be used for a practical application, and we’ve even done our own little demonstration using LEDs,” said Duke graduate student Matthew Catenacci, who describes the device in a paper published online March 27 in the Journal of Electronic Materials.

At the core of the new device, which is about the size of a postage stamp, is a new copper-nanowire-based printable material that is capable of storing digital information.

“Memory is kind of an abstract thing, but essentially it is a series of ones and zeros which you can use to encode information,” said Benjamin Wiley, an associate professor of chemistry at Duke and an author on the paper.

Most flash drives encode information in series of silicon transistors, which can exist in a charged state, corresponding to a “one,” and an uncharged state, corresponding to a “zero,” Wiley said.

Spray-on memory could enable bendable digital storage
Duke researchers demonstrated their new “spray-on” digital memory by programing a simple circuit to display four LED lights in different patterns. Credit: Duke University

 

 

 

 

 

The new material, made of silica-coated copper nanowires encased in a polymer matrix, encodes information not in states of charge but instead in states of resistance. By applying a small voltage, it can be switched between a state of high resistance, which stops electric current, and a state of low resistance, which allows current to flow.

And, unlike silicon, the nanowires and the polymer can be dissolved in methanol, creating a liquid that can be sprayed through the nozzle of a printer.

“We have developed a way to make the entire device printable from solution, which is what you would want if you wanted to apply it to fabrics, RFID tags, curved and flexible substrates, or substrates that can’t sustain high heat,” Wiley said.

To create the device, Catenacci first used commercially-available gold nanoparticle ink to print a series of gold electrodes onto a glass slide. He then printed the copper-nanowire memory material over the gold electrodes, and finally printed a second series of electrodes, this time in copper.

To demonstrate a simple application, Catenacci connected the device to a circuit containing four LED lights. “Since we have four bits, we could program sixteen different states,” Catenacci said, where each “state” corresponds to a specific pattern of lights. In a real-world application, each of these states could be programmed to correspond to a number, letter, or other display symbol.

Though other research groups have fabricated similar printable memory devices in recent years, this is the first to combine key properties that are necessary for practical use. The write speed, or time it takes to switch back and forth between , is around three microseconds, rivaling the speed of . Their tests indicate that written information may be retained for up to ten years, and the material can be re-written many times without degrading.

While these devices won’t be storing digital photos or music any time soon—their memory capacity is much too small for that—they may be useful in applications where low cost and flexibility are key, the researchers say.

“For example, right now RFID tags just encode a particular produce number, and they are typically used for recording inventory,” Wiley said. “But increasingly people also want to record what environment that product felt—such as, was this medicine always kept at the right temperature? One way these could be used would be to make a smarter RFID tags that could sense their environments and record the state over time.”

Explore further: Nanowire ‘inks’ enable paper-based printable electronics

More information: Matthew J. Catenacci et al, Fully Printed Memristors from Cu–SiO2 Core–Shell Nanowire Composites, Journal of Electronic Materials (2017). DOI: 10.1007/s11664-017-5445-5

 

Georgia Institute of Technology: New Low-Cost Technique Converts Bulk Alloys to Oxide Nanowires


git-nanowires-rd_1702_nanotech
Researchers have developed a new low-cost technique for converting bulk powders directly to oxide nanowires. Shown is a crucible in which an alloy of lithium and aluminum is being formed. Credit: Rob Felt, Georgia Tech

 

A simple technique for producing oxide nanowires directly from bulk materials could dramatically lower the cost of producing the one-dimensional (1D) nanostructures. That could open the door for a broad range of uses in lightweight structural composites, advanced sensors, electronic devices – and thermally-stable and strong battery membranes able to withstand temperatures of more than 1,000 degrees Celsius.

The technique uses a solvent reaction with a bimetallic alloy – in which one of the metals is reactive – to form bundles of nanowires (nanofibers) upon reactive metal dissolution. The process is conducted at ambient temperature and pressure without the use of catalysts, toxic chemicals or costly processes such as chemical vapor deposition. The produced nanowires can be used to improve the electrical, thermal and mechanical properties of functional materials and composites.

The research, which was reported this week in the journal Science, was supported by the National Science Foundation and California-based Sila Nanotechnologies. The process is believed to be the first to convert bulk powders to nanowires at ambient conditions.

“This technique could open the door for a range of synthesis opportunities to produce low-cost 1D nanomaterials in large quantities,” said Gleb Yushin, a professor in the School of Materials Science and Engineering at the Georgia Institute of Technology. “You can essentially put the bulk materials into a bucket, fill it with a suitable solvent and collect nanowires after a few hours, which is way simpler than how many of these structures are produced today.”

Yushin’s research team, which included former graduate students Danni Lei and James Benson, has produced oxide nanowires from lithium-magnesium and lithium-aluminum alloys using a variety of solvents, including simple alcohols. Production of nanowires from other materials is part of ongoing research that was not reported in the paper.

The dimensions of the nanowire structures can be controlled by varying the solvent and the processing conditions. The structures can be produced in diameters ranging from tens of nanometers up to microns.

“Minimization of the interfacial energy at the boundary of the chemical reaction front allows us to form small nuclei and then retain their diameter as the reaction proceeds, thus forming nanowires,” Yushin explained. “By controlling the volume changes, surface energy, reactivity and solubility of the reaction products, along with the temperature and pressure, we can tune conditions to produce nanowires of the dimensions we want.”

One of the attractive applications may be separator membranes for lithium-ion batteries, whose high power density has made them attractive for powering everything from consumer electronics to aircraft and motor vehicles. However, the polymer separation membranes used in these batteries cannot withstand the high temperatures generated by certain failure scenarios.

As result, commercial batteries may induce fires and explosions, if not designed very carefully and it’s extremely hard to avoid defects and errors consistently in tens of millions of devices.

Using low-cost paper-like membranes made of ceramic nanowires could help address those concerns because the structures are strong and thermally stable, while also being flexible – unlike many bulk ceramics. The material is also polar, meaning it would more thoroughly wetted by various battery electrolyte solutions.

“Overall, this is a better technology for batteries, but until now, ceramic nanowires have been too expensive to consider seriously,” Yushin said. “In the future, we can improve mechanical properties further and scale up synthesis, making the low-cost ceramic separator technology very attractive to battery designers.”

Fabrication of the nanowires begins with formation of alloys composed of one reactive and one non-reactive metal, such as lithium and aluminum (or magnesium and lithium). The alloy is then placed in a suitable solvent, which could include a range of alcohols, such as ethanol. The reactive metal (lithium) dissolves from the surface into the solvent, initially producing nuclei (nanoparticles) comprising aluminum.

Though bulk aluminum is not reactive with alcohol due to the formation of the passivation layer, the continuous dissolution of lithium prevents the passivation and allows gradual formation of aluminum alkoxide nanowires, which grow perpendicular to the surface of the particles starting from the nuclei until the particles are completely converted. The alkoxide nanowires can then be heated in open air to form aluminum oxide nanowires and may be formed into paper-like sheets.

The dissolved lithium can be recovered and reused. The dissolution process generates hydrogen gas, which could be captured and used to help fuel the heating step.

Though the process was studied first to make magnesium and aluminum oxide nanowires, Yushin believes it has a broad potential for making other materials. Future work will explore synthesis of new materials and their applications, and develop improved fundamental understanding of the process and predictive models to streamline experimental work.

The researchers have so far produced laboratory amounts of the nanowires, but Yushin believes that the process could be scaled up to produce industrial quantities. Though the ultimate cost will depend on many variables, he expects to see fabrication costs cut by several orders of magnitude over existing techniques.

“With this technique, you could potentially produce nanowires for a cost not much more than that of the raw materials,” he said. Beyond battery membranes, the nanowires could be useful in energy harvesting, catalyst supports, sensors, flexible electronic devices, lightweight structural composites, building materials, electrical and thermal insulation and cutting tools.

The new technique was discovered accidentally while Yushin’s students were attempting to create a new porous membrane material. Instead of the membrane they had hoped to fabricate, the process generated powders composed of elongated particles.

“Though the experiment didn’t produce what we were looking for, I wanted to see if we could learn something from it anyway,” said Yushin. Efforts to understand what had happened ultimately led to the new synthesis technique.

In addition to those already named, the research included Alexandre Magaskinski of Georgia Tech and Gene Berdichevsky of Sila Nanotechnologies.

Different aspects of this work were supported by NSF (grant 0954925) and Sila Nanotechnologies, Inc. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation. Gleb Yushin and Gene Berdichevsky are shareholders of Sila Nanotechnologies.

CITATION: Danni Lei, Jim Benson, Alexandre Magasinski, Gene Berdichevsky, Gleb Yushin, “Transformation of bulk alloys to oxide nanowires,” (Science, 2017).

Michigan Tech U: Understanding the physics of “Quantum Tunneling” for Ultra-Thin Nano-Wire Transistors


Nanowire 070716 probingquantA field effect transistor (FET) uses a gate bias to control electrical current in a channel between a source and drain, which produces an electrostatic field around the channel. Credit: Michigan Technological University

Nearly 1,000 times thinner than a human hair, nanowires can only be understood with quantum mechanics. Using quantum models, physicists from Michigan Technological University have figured out what drives the efficiency of a silicon-germanium (Si-Ge) core-shell nanowire transistor.

Core-Shell Nanowires

The study, published last week in Nano Letters, focuses on the quantum tunneling in a core-shell nanowire structure. Ranjit Pati, a professor of physics at Michigan Tech, led the work along with his graduate students Kamal Dhungana and Meghnath Jaishi.

Core-shell nanowires are like a much smaller version of electrical cable, where the core region of the cable is made up of different material than the shell region. In this case, the core is made from silicon and the shell is made from germanium. Both silicon and germanium are semiconducting materials. Being so thin, these semiconducting core-shell nanowires are considered one-dimensional materials that display unique physical properties.

The arrangements of atoms in these nanowires determine how the electrons traverse through them, Pati explains, adding that a more comprehensive understanding of the physics that drive these nanoscale transistors could lead to increased efficiency in electronic devices.

“The performance of a heterogeneous silicon-germanium nanowire transistor is much better than a homogeneous silicon nanowire,” Pati says. “In our study, we’ve unraveled the quantum phenomena responsible for its superior performance.”

Field Effect Transistors

Transistors power our digital world. And they used to be large—or at least large enough for people to see. With advances in nanotechnology and materials science, researchers have been able to minimize the size and maximize the numbers of transistors that can be assembled on a microchip.

The particular transistor that Pati has been working on is a field effect transistor (FET) made out of core-shell nanowires. It manipulates the in the nanowire channel using a gate bias. Simply put, a gate bias affects electric current in the channel like a valve controls water flow in a pipe. The gate bias produces an electrostatic field effect that induces a switching behavior in the channel current. Controlling this field can turn the device on or off, much like a light switch.

Probing quantum phenomena in tiny transistors
Quantum tunneling of electrons across germanium atoms in a core-shell nanowire transistor. The close-packed alignment of dumbbell-shaped pz-orbitals direct the physics of tunneling. Credit: Michigan Technological University

Several groups have successfully fabricated core-shell nanowire FETs and demonstrated their effectiveness over the transistors currently used in microprocessors. What Pati and his team looked at is the quantum physics driving their superior performance.

The electrical current between source and drain in a nanowire FET cannot be understood using classical physics. That’s because electrons do strange things at such a tiny scale.

“Imagine a fish being trapped inside a fish tank; if fish has enough energy, it could jump up over the wall,” Pati says. “Now imagine an electron in the tank: if it has enough energy, the electron could jump out—but even if it doesn’t have enough energy, the electron can tunnel through the side walls, so there is a finite probability that we would find an electron outside the tank.”

This is known as quantum tunneling. For Pati, catching the electron in action inside the nanowire transistors is the key to understanding their superior performance. He and his team used what is called a first-principles quantum transport approach to know what causes the electrons to tunnel efficiently in the core-shell nanowires.

The quantum tunneling of electrons—an atomic-scale game of hopscotch—is what enables the electrons to move through the nanowire materials connecting the source and drain. And the movement gets more specific than that: the electrons almost exclusively hop across the germanium shell but not through the silicon core. They do so through the aligned pz-orbitals of the germanium.nanowires-149_thumbnail_100

Simply put, these orbitals, which are dumbbell-shaped regions of high probability for finding an electron, are perfect landing pads for tunneling electrons. The specific alignment—color-coded in the diagram above—makes even easier. It’s like the difference between trying to burrow through a well with steel walls versus sand walls. The close-packed alignment of the pz-orbitals in the germanium shell enable electrons to tunnel from one atom to another, creating a much higher electrical current when switched on. In the case of homogeneous silicon nanowires, there is no close-packed alignment of the pz-orbitals, which explains why they are less effective FETs.

Nanowires in Electronics

There are many potential uses for nanowire FETs. Pati and his team write in their Nano Letters paper that they “expect that the electronic orbital level understanding gained in this study would prove useful for designing a new generation of core−shell nanowire FETs.”

Specifically, having a heterogeneous structure offers additional mobility control and superior performance over the current generation of transistors, in addition to compatibility with the existing silicon technology. The core-shell nanowire FETs could transform our future by making computers more powerful, phones and wearables smarter, cars more interconnected and electrical grids more efficient. The next step is simply taking a small quantum leap.

Explore further: Universal transistor serves as a basis to perform any logic function

More information: Kamal B. Dhungana et al. Unlocking the Origin of Superior Performance of a Si–Ge Core–Shell Nanowire Quantum Dot Field Effect Transistor, Nano Letters (2016). DOI: 10.1021/acs.nanolett.6b00359

 

Transparent, flexible supercapacitors pave the way for a multitude of applications


transparentf Super CapacitorsThe transparent, flexible supercapacitor prototype, based on single-walled carbon nanotube thin films, is shown during charging and discharging. Credit: Kanninen et al. ©2016 IOP Publishing

The standard appearance of today’s electronic devices as solid, black objects could one day change completely as researchers make electronic components that are transparent and flexible. Working toward this goal, researchers in a new study have developed transparent, flexible supercapacitors made of carbon nanotube films. The high-performance devices could one day be used to store energy for everything from wearable electronics to photovoltaics.

The researchers, Kanninen et al., from institutions in Finland and Russia, have published a paper on the new supercapacitors in a recent issue of Nanotechnology.

In general, supercapacitors can store several times more charge in a given volume or mass than traditional capacitors, have faster charge and discharge rates, and are very stable. Over the past few years, researchers have begun working on making supercapacitors that are transparent and flexible due to their potential use in a wide variety of applications.

“Potential applications can be roughly divided into two categories: high-aesthetic-value products, such as activity bands and smart clothes, and inherently transparent end-uses, such as displays and windows,” coauthor Tanja Kallio, an associate professor at Aalto University who is currently a visiting professor at the Skolkovo Institute of Science and Technology, told Phys.org. “The latter include, for example, such future applications as smart windows for automobiles and aerospace vehicles, self-powered rolled-up displays, self-powered wearable optoelectronics, and electronic skin.”

The type of supercapacitor developed here, called an electrochemical double-layer capacitor, is based on high-surface-area carbon. One prime candidate for this material is single-walled carbon nanotubes due to their combination of many appealing properties, including a , high strength, high elasticity, and the ability to withstand extremely high currents, which is essential for fast charging and discharging.

The problem so far, however, has been that the carbon nanotubes must be prepared as in order to be used as electrodes in supercapacitors. Current techniques for preparing thin films have drawbacks, often resulting in defected nanotubes, limited conductivity, and other performance limitations.

In the new study, the researchers demonstrated a new method to fabricate thin films made of single-walled carbon nanotubes using a one-step aerosol synthesis method. When incorporated into a supercapacitor, the thin films exhibit the highest transparency to date (92%), the highest mass specific capacitance (178 F/g), and one of the highest area specific capacitances (552 µF/cm2) compared to other carbon-based, flexible, transparent supercapacitors. The films also have a high stability, as demonstrated by the fact that their capacitance does not degrade after 10,000 charging cycles.

With these advantages, the new device illustrates the continued improvement in the development of transparent, flexible supercapacitors. In the future, the researchers plan to further improve the energy density, flexibility, and durability, and also make the supercapacitors stretchable.

“One more important characteristic to be realized and urgently expected in future electronics is the stretchability of the conductive materials and assembled electronic components,” said coauthor Albert Nasibulin, a professor at the Skolkovo Institute of Science and Technology and an adjunct professor at Aalto University. “Together with Tanja, we are currently working on a new type of stretchable and transparent single-walled carbon nanotube supercapacitor. We are confident that one can create prototypes based on carbon nanotubes that might withstand 100% elongation with no performance degradation.”

Explore further: Researchers develop stretchable wire-shaped supercapacitor

More information: Kanninen et al. “Transparent and flexible high-performance supercapacitors based on single-walled carbon nanotube films.” Nanotechnology. DOI: 10.1088/0957-4484/27/23/235403

 

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World’s most efficient nanowire lasers: Benefit to Fiber Optics Communications


Perovskite Nano wires 160616141636_1_540x360
Perovskite-based nanowire lasers are the most efficient known. A topological image of a nanowire is shown (left insert). Room temperature emission images above the lasing threshold for two nanowires composed of different halides, iodide (red in center) and bromide (green on the right), are shown in top inserts.
Credit: Image courtesy of Xiaoyang Zhu, Columbia University

Known for their low cost, simple processing and high efficiency, perovskites are popular materials in solar panel research. Now, researchers demonstrated that nanowires made from lead halide perovskite are the most efficient nanowire lasers known.

Efficient nanowire lasers could benefit fiber optic communications, pollution characterization, and other applications. The challenge is getting the right material. These ultra-compact wires have a superior ability to emit light, can be tuned to emit different colors, and are relatively easy to synthesize. The development of these perovskite wires parallels the rapid development of the same materials for efficient solar cells.

Semiconductor nanowire lasers, due to their ultra-compact physical sizes, highly localized coherent output, and efficiency, are promising components for use in fully integrated nanoscale photonic and optoelectronic devices. Lasing requires a minimum (threshold) excitation density, below which little light is emitted.

A high “lasing threshold” not only makes critical technical advances difficult, but also imposes fundamental limits on laser performance due to the onset of other losses. In searching for an ideal material for nanowire lasing, researchers at Columbia University and the University of Wisconsin-Madison investigated a new class of hybrid organic-inorganic semiconductors, methyl ammonium lead halide perovskites (CH3NH3PbX3), which is emerging as a leading material for high-efficiency photovoltaic solar cells due to low cost, simple processing and high efficiencies.

The exceptional solar cell performance in these materials can be attributed to the long lifetimes of the carriers that move energy through the systems (electrons and holes) and carrier diffusion lengths.

These properties, along with other attributes such as high fluorescence yield and wavelength tunability, also make them ideal for lasing applications. Room temperature lasing in these nanowires was demonstrated with:

  • The lowest lasing thresholds and the highest quality factors reported to date
  • Near 100% quantum yield (ratio of the number of photons emitted to those absorbed)
  • Broad tunability of emissions covering the near infrared to visible wavelength region.

Specifically, the laser emission shifts from near infrared to blue with decreasing atomic number of the halides (X=I, Br, Cl) in the nanowires. These nanowires could advance applications in nanophotonics and optoelectronic devices. In particular, lasers that operate in the near infrared region could benefit fiber optic communications and advance pollution characterization from space.

This work was supported by the DOE Office of Science (Office of Basic Energy Sciences) and the National Science Foundation.


Story Source:

The above post is reprinted from materials provided byDepartment of Energy, Office of Science. Note: Materials may be edited for content and length.


Journal Reference:

  1. Haiming Zhu, Yongping Fu, Fei Meng, Xiaoxi Wu, Zizhou Gong, Qi Ding, Martin V. Gustafsson, M. Tuan Trinh, Song Jin, X-Y. Zhu. Lead halide perovskite nanowire lasers with low lasing thresholds and high quality factors. Nature Materials, 2015; 14 (6): 636 DOI: 10.1038/NMAT4271

Stanford University: Nanowire-Coated Cotton Cleans Water by Zapping Bacteria to Death: Application: Cheap, Abundant Material, Low-Voltage Nano-Water-Filter


nanofilt_wires_newsIllness-inducing bacteria, meet nano-engineered cotton–and a quick death. Researchers have created a new “filter” that zaps bacteria with electric fields to clean drinking water. They say their system may find use in developing countries since it requires only a small amount of voltage (a couple of car batteries, a stationary bike, or a solar panel could do the job) and cleans water an estimated 80,000 times faster than traditional devices.

Instead of trapping bacteria in small pores like many slow-going traditional filters, the cotton and silver nanowire combo uses small electric currents running through the nanowires to kill the bacteria outright. In a paper to appear in the journal Nano Letters researchers say that 20 volts and 2.5 inches worth of the material killed 98 percent of Escherichia coli in the water they tested in their lab setup.

The authors argue that the filter’s silver nanowires and carbon nanotubes are cheap; the small amount of silver required makes its expense “negligible,” coauthor Yi Cui says in a press release, and the group chose to use cotton because of its abundance.

They needed a foundation material that was “cheap, widely available and chemically and mechanically robust.” So they went with ordinary woven cotton fabric. “We got it at Wal-mart,” Cui said. [Stanford University]

They made the potent combination by dipping the cotton first in a “broth” containing carbon nanotubes and then the silver nanowires, allowing the structures to coat the cotton fibers. The scanning electron microscope image above shows the silver nanowires compared to the large cotton fibers (the red line is 10 microns long). The current running though the material, a few milliamperes, may be fatal for the bacteria but it would barely makes a human tingle.

[B]ecause the voltage is so low, it doesn’t require serious electricity generation. A person could generate the power from a stationary bike or a hand-cranked device. No pumping is required either. The force of gravity is enough to allow the water and its nasties to pass through the cotton and get zapped! [Discovery News]

Next, the group hopes to test the device on other microorganisms–perhaps those responsible for other waterborne illness such as cholera, typhoid and hepatitis. The researchers will also continue testing the filter to make sure only clean water comes out and not any nano-structures.Nano Filters hjgjgf

“So far, our evidence suggests that they don’t come off,” Cui told New Scientist. “It is an interesting academic study,” says nanoengineer Eric Hoek at the University of California, Los Angeles. He says proving that the potentially harmful CNTs [carbon nanotubes] do not leach into the water will be a key step in finding out if it is useful on a practical level. [New Scientist]

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Image: Yi Cui