Posts Tagged ‘nanowires’
Wednesday, October 2, 2013 @ 03:10 PM gHale
Computer modeling will help decide which of two competing materials will get a chance to shine as the nanoscale energy-harvesting technology of future solar panels — quantum dots or nanowires.
Computational chemistry models ended up used to predict the electronic and optical properties of three types of nanoscale (billionth of a meter) silicon structures with a potential application for solar energy collection: a quantum dot, one-dimensional chains of quantum dots and a nanowire, said North Dakota State University’s Andrei Kryjevski and his colleagues, Dimitri Kilin and Svetlana Kilina.
The ability to absorb light is much stronger in nanomaterials compared to those used in conventional semiconductors. Determining which form — quantum dots or nanowire — maximizes this advantage was the goal of the numerical experiment conducted by the three researchers.
“We used Density Functional Theory, a computational approach that allows us to predict electronic and optical properties that reflect how well the nanoparticles can absorb light, and how that effectiveness is affected by the interaction between quantum dots and the disorder in their structures,” Kryjevski said. “This way, we can predict how quantum dots, quantum dot chains and nanowires will behave in real life even before they are synthesized and their working properties experimentally checked.”
The simulations indicated that light absorption by silicon quantum dot chains significantly increases with increased interactions between the individual nanospheres in the chain.
They also found light absorption by quantum dot chains and nanowires depends strongly on how the structure aligns in relation to the direction of the photons striking it. Finally, the researchers learned the atomic structure disorder in the amorphous nanoparticles results in better light absorption at lower energies compared to crystalline-based nanomaterials.
“Based on our findings, we believe that putting the amorphous quantum dots in an array or merging them into a nanowire are the best assemblies for maximizing the efficiency of silicon nanomaterials to absorb light and transport charge throughout a photovoltaic system,” Kryjevski said. “However, our study is only a first step in a comprehensive computational investigation of the properties of semiconductor quantum dot assemblies.
“The next steps are to build more realistic models, such as larger quantum dots with their surfaces covered by organic ligands and simulate the processes that occur in actual solar cells,” he added.
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Wednesday, January 23, 2013 @ 06:01 PM gHale
Nanowires could pave the way for more efficient and cheaper solar cells.
“Our findings are the first to show that it really is possible to use nanowires to manufacture solar cells,” said Dr. Magnus Barnstorm, a researcher in semiconductor physics from Lund University in Sweden and the principal author of a paper on the subject.
Research on solar cell nanowires is on the rise globally. Until now the unattained dream figure was ten percent efficiency – but now Barnstorm said he and his team reached 13.8 percent efficiency.
The nanowires consist of the semiconductor material indium phosphate and work like antennae that absorb sunlight and generate power. The nanowires assemble on surfaces of one square millimeter that each house four million nanowires. A nanowire solar cell can produce an effect per active surface unit several times greater than today’s silicon cells.
nanowire solar cells have not yet made it beyond the laboratory, but the plan is the technology could work in large solar power plants in sunny regions such as the southwestern USA, southern Spain and Africa.
Lund researchers identified the ideal diameter of the nanowires and how to synthesize them. “The right size is essential for the nanowires to absorb as many photons as possible. If they are just a few tenths of a nanometer too small their function is significantly impaired,” Barnstorm said.
The silicon solar cells used to supply electricity for domestic use are relatively cheap, but inefficient because they are only able to utilize a limited part of the effect of the sunlight. The reason is that one single material can only absorb part of the spectrum of the light.
Research carried out alongside that on nanowire technology therefore aims to combine different types of semiconductor material to make efficient use of a broader part of the solar spectrum. The disadvantage of this is they become extremely expensive and can therefore only work in niche contexts, such as on satellites and military planes.
However, this is not the case with nanowires. Because of their small dimensions, the same sort of material combinations can come together with much less effort, which offers higher efficiency at a low cost. The process is also less complicated. The researchers said the nanowires can generate power at the same level as a thin film of the same material, even if they only cover around 10 percent of the surface rather than 100 percent.
Thursday, June 21, 2012 @ 06:06 PM gHale
The sun has plenty of energy to give, why not use as much of that power as possible – at least that is the thinking behind the new materials for a photovoltaic system.
Indium gallium nitride can be a valuable future material for photovoltaic systems, researchers said.
Changing the concentration of indium allows researchers to tune the material’s response so it collects solar energy from a variety of wavelengths. The more variations designed into the system, the more of the solar spectrum the system can absorb, which can lead to increased solar cell efficiencies. There are limitations in silicon, today’s photovoltaic industry standard, as to how far the wavelength range can “see” and absorb.
As it always goes with solar energy, there is a problem: Indium gallium nitride, part of a family of materials called III-nitrides, typically grows on thin films of gallium nitride. Because gallium nitride atomic layers have different crystal lattice spacings from indium gallium nitride atomic layers, the mismatch leads to structural strain that limits the layer thickness and percentage of indium that can add in. Thus, increasing the percentage of indium broadens the solar spectrum that a system can collect, but it reduces the material’s ability to tolerate the strain.
But there is now a new twist. If researchers grow indium mixture on a phalanx of nanowires rather than on a flat surface, the small surface areas of the nanowires allow the indium shell layer to partially “relax” along each wire, easing strain, said Sandia National Laboratories scientists Jonathan Wierer Jr. and George Wang in a paper on the subject. This relaxation allowed the team to create a nanowire solar cell with indium percentages of roughly 33 percent, higher than any other reported attempt at creating III-nitride solar cells.
This initial attempt also lowered the absorption base energy from 2.4eV to 2.1 eV, the lowest of any III-nitride solar cell to date, and made a wider range of wavelengths available for power conversion. Power conversion efficiencies were low — only 0.3 percent compared to a standard commercial cell that hums along at about 15 percent — but the demonstration took place on imperfect nanowire-array templates. Refinements should lead to higher efficiencies and even lower energies.
Several unique techniques helped create the III-nitride nanowire array solar cell. A top-down fabrication process created the nanowire array by masking a gallium nitride (GaN) layer with a colloidal silica mask, followed by dry and wet etching. The resulting array consisted of nanowires with vertical sidewalls and of uniform height.
Next, shell layers containing the higher indium percentage of indium gallium nitride (InGaN) formed on the GaN nanowire template via metal organic chemical vapor deposition. Lastly, they were able to grow In0.02Ga0.98N in such a way that caused the nanowires to coalescence. This process produced a canopy layer at the top, facilitating simple planar processing and making the technology manufacturable.
The results, although modest, represent a promising path forward for III-nitride solar cell research, Wierer said. The nano-architecture not only enables higher indium proportion in the InGaN layers but also increased absorption via light scattering in the faceted InGaN canopy layer, as well as air voids that guide light within the nanowire array.
Thursday, March 8, 2012 @ 02:03 PM gHale
A forest consisting of tiny nanowire trees could cleanly capture solar energy without using fossil fuels and then harvest it for hydrogen fuel generation.
Nanowires, made from abundant natural materials like silicon and zinc oxide, also offer a cheap way to deliver hydrogen fuel on a mass scale, said electrical engineers at the University of California, San Diego.
“This is a clean way to generate clean fuel,” said Deli Wang, professor in the Department of Electrical and Computer Engineering at the UC San Diego Jacobs School of Engineering.
The trees’ vertical structure and branches are keys to capturing the maximum amount of solar energy, Wang said. That’s because the vertical structure of trees grabs and adsorbs light while flat surfaces simply reflect it, Wang said, adding it is also similar to retinal photoreceptor cells in the human eye. In images of Earth from space, light reflects off of flat surfaces such as the ocean or deserts, while forests appear darker.
Wang’s team has mimicked this structure in their “3D branched nanowire array” which uses a process called photoelectrochemical water-splitting to produce hydrogen gas. Water splitting refers to the process of separating water into oxygen and hydrogen in order to extract hydrogen gas to use as fuel. This process uses clean energy with no green-house gas byproduct. By comparison, the current conventional way of producing hydrogen relies on electricity from fossil fuels.
“Hydrogen is considered to be clean fuel compared to fossil fuel because there is no carbon emission, but the hydrogen currently used is not generated cleanly,” said Ke Sun, a PhD student in electrical engineering who led the project.
By harvesting more sunlight using the vertical nanotree structure, Wang’s team developed a way to produce more hydrogen fuel efficiently compared to planar counterparts.
The vertical branch structure also maximizes hydrogen gas output, Sun said. On the flat wide surface of a pot of boiling water, bubbles must become large to come to the surface. In the nanotree structure, very small gas bubbles of hydrogen can extract much faster. “Moreover, with this structure, we have enhanced, by at least 400,000 times, the surface area for chemical reactions,” Sun said.
In the long run, what Wang’s team is aiming for is even bigger: Artificial photosynthesis. In photosynthesis, as plants absorb sunlight they also collect carbon dioxide (CO2) and water from the atmosphere to create carbohydrates to fuel their own growth. Wang’s team hopes to mimic this process to also capture CO2 from the atmosphere, reducing carbon emissions, and convert it into hydrocarbon fuel.
“We are trying to mimic what the plant does to convert sunlight to energy,” Sun said. “We are hoping in the near future our ‘nanotree’ structure can eventually be part of an efficient device that functions like a real tree for photosynthesis.”
The team is also studying alternatives to zinc oxide, which absorbs the sun’s ultraviolet light, but has stability issues that affect the lifetime usage of the nanotree structure.
Wednesday, December 7, 2011 @ 11:12 AM gHale
Voltage increases of up to 25 percent in two barely separated nanowires should help designers of next generation phones, handheld computers, batteries and solar arrays.
“People have been working on nanowires for 20 years,” said Sandia National Laboratories Lead Researcher Mike Lilly. “At first, you study such wires individually or all together, but eventually you want a systematic way of studying the integration of nanowires into nanocircuitry. That’s what’s happening now. It’s important to know how nanowires interact with each other rather than with regular wires.”
Though the gallium-arsenide nanowire structures used by Lilly’s team are fragile, nanowires in general have very practical characteristics. They may crack less than their bigger cousins, they’re cheaper to produce and they offer better electronic control.
For years, the best available test method required researchers to put a charged piece of material called a gate between two nanowires on a single shelf. The gate, flooded with electrons, acted as a barrier: It maintained the integrity of the wires on either side of it by repelling any electrons attempting to escape across it. But the smallest wire separation allowed by the gate was 80 nanometers. Nanowires in future devices will pack together much more closely, so a much smaller gap was necessary for testing.
The current test design is simple. What Lilly and co-workers at McGill University in Montreal envisioned was to put the nanowires one above the other, rather than side by side, by separating them with a few atomic layers of extremely pure, home-grown crystal. This allowed them to test nanowires separated vertically by only 15 nanometers — about the distance next-generation devices will require. Because each wire sits on its own independent platform, electrical inputs varied by the researchers can independently feed and control the wires.
While applications for technical devices interest Lilly, it’s the characteristics of nanowires as a problem in one-dimensional (1-D) basic science that fascinates him.
A 1-D wire is not your common, thick-waisted, 3-D household wire, which allows current to move horizontally, vertically, and forward; nor is it your smaller, flattened micron-sized 2-D wires in typical electronic devices that allow electrons to move forward and across but not up and down. In 1-D wires, the electrons can only move in one direction: One behind the other.
“In the long run, our test device will allow us to probe how 1-D conductors are different from 2-D and 3-D conductors,” Lilly said. “They are expected to be very different, but there are relatively few experimental techniques that have been used to study the 1-D ground state.”
One reason for the difference is the Coulomb force, responsible for the Coulomb “drag” effect, regardless of whether the force hastens or retards currents. Operating between wires, the force is inversely proportional to the square of the distance; that is, in ordinary microelectronics, the force is practically unnoticeable, but at nanodistances, the force is large enough that electrons in one wire can “feel” the individual electrons moving in another placed nearby.
The drag means the first wire needs more energy because the Coulomb force creates, in effect, increased resistance. “The amount is very small,” said Lilly, “and we can’t measure it. What we can measure is the voltage of the other wire.”
There are no straightforward answers as to why the Coulomb force creates negative or positive drag, but it does. The force got its name from 18th century scientist Charles August Coulomb.
What researchers do know is “enough electrons get knocked along that they provide positive source at one wire end, negative at the other,” Lilly said. “A voltage builds up in the opposite direction to keep electrons in place,” thus increasing drag.
Tuesday, September 27, 2011 @ 01:09 PM gHale
A new a technique that can organize copper atoms in water to form long, thin, non-clumped nanowires is now under development. This move could drive down costs for displaying information on items such as cell phones, iPads, and also improve solar cells.
These nanowires can then transform into transparent, conductive films which a manufacturer can then coat onto glass or plastic, said Ben Wiley, a Duke University chemist. The copper nanowire films have the same properties as those currently used in electronic devices and solar cells, but are less expensive to manufacture.
The films that currently connect pixels in electronic screens consist of indium tin oxide, or ITO. It is highly transparent, which transmits the information well. But the ITO film must come from a vapor in a process 1,000 times slower than newspaper printing, and, once the ITO is in the device, it easily cracks. Indium is also an expensive rare earth element, costing as much as $800 per kilogram.
These problems have driven worldwide efforts to find less expensive materials that can coat or print like ink at much faster speeds to make low-cost, transparent conducting films, Wiley said.
One alternative to an ITO film is to use inks containing silver nanowires. The first cell phone with a screen made from silver nanowires will be on the market this year. But silver, like indium, is still relatively expensive at $1400 per kilogram.
Copper, on the other hand, is a thousand times more abundant than indium or silver, and about 100 times less expensive, costing only $9 per kilogram.
Just last year Wiley and his graduate student Aaron Rathmell showed it was possible to form a layer of copper nanowires on glass to make a transparent conducting film.
But at that time, the performance of the film was not good enough for practical applications because the wires clumped together. The new way of growing the copper nanowires and coating them on glass surfaces eliminates the clumping problem, Wiley said.
They also created the new copper nanowires to maintain their conductivity and form when bent back and forth 1,000 times. In contrast, ITO films’ conduction and structure break after a few bends.
Wiley said the low-cost, high-performance, and flexibility of copper nanowires make them a natural choice for use in the next generation of displays and solar cells. He co-founded a company called NanoForge Corp. in 2010 to manufacture copper nanowires for commercial applications.
In early 2011, NanoForge received a $45,000 North Carolina IDEA grant for refinement and scale-up of the manufacturing process of copper nanowires, and it is now filling orders.
With continuing development, copper nanowires could be in screens and solar cells in the next few years, which could lead to lighter and more reliable displays and also to making solar energy more competitive with fossil fuels, Wiley said.
Thursday, September 30, 2010 @ 08:09 AM gHale
By using a novel “pyroelectric” method, it may soon be possible to power tiny devices using waste heat.
Using tiny structures called ferroelectric nanowires, they can rapidly generate an electrical current in response to any change in the ambient temperature, harvesting otherwise wasted energy from thermal fluctuations, according to a joint team of Ukrainian and American scientists.
“The second law of thermodynamics rules modern life: Through all kinds of industry, humans consistently produce an enormous amount of waste heat,” said lead researcher Anna Morozovska of the National Academy of Sciences of Ukraine. “However, the laws of thermodynamics do not exclude rescuing some of this energy by harvesting the thermal fluctuations to produce electricity.”
Pyroelectrictricity can play key role in consumer electronics, said Morozovska, and recovering this heat in the form of pyroelectric energy may bring about a new era of “tiny energy.”
Pyroelectric nanogenerators could be extremely useful for powering specific tasks in biological applications, medicine and nanotechnology, particularly in space because they perform well in low temperatures.
In an investigation of the pyroelectric properties of ferroelectric nanowires, the team analyzed how the pyroelectric coefficient corresponds to the radius of the wire and its coupling. They found the smaller the wire radius, the more the pyroelectric coefficient diverges until a critical radius at which the response changes to paraelectric (above the Curie temperature). This “size effect” could tune the phase transition temperatures in ferroelectric nanostructures, thus enabling a system with a large, tunable, pyroelectric response.
In theory, the use of rectifying contacts could enable the polarized ferroelectric nanowire to generate a giant, pyroelectric, direct current and voltage in response to temperature fluctuations which you could harvest and detect using a bolometric detector. Such a nanoscale device would not contain any moving parts and could be suitable for long-term operation in ambient applications such as in-vitro biological systems and outer space. The researchers calculate these little nanogenerators would have very high efficiency at low temperatures, decreasing at warmer temperatures.