Step towards ‘holy grail’ of silicon photonics

Creation of first practical silicon-based laser has the potential to transform communications, healthcare and energy systems

A group of researchers from the UK, including academics from Cardiff University, has demonstrated the first practical laser that has been grown directly on a silicon substrate.

It is believed the breakthrough could lead to ultra-fast communication between computer chips and electronic systems and therefore transform a wide variety of sectors, from communications and healthcare to energy generation.

The EPSRC-funded UK group, led by Cardiff University and including researchers from UCL and the University of Sheffield, have presented their findings in the journal Nature Photonics.

Silicon is the most widely used material for the fabrication of electronic devices and is used to fabricate semiconductors, which are embedded into nearly every device and piece of technology that we use in our everyday lives, from smartphones and computers to satellite communications and GPS.

Electronic devices have continued to get quicker, more efficient and more complex, and have therefore placed an added demand on the underlining technology.

Researchers have found it increasingly difficult to meet these demands using conventional electrical interconnects between computer chips and systems, and have therefore turned to light as a potential ultra-fast connector.

Whilst it has been difficult to combine a semiconductor laser – the ideal source of light – with silicon, the UK group have now overcome these difficulties and successfully integrated a laser directly grown onto a silicon substrate for the very first time.

Professor Huiyun Liu, who led the growth activity, explained that the 1300 nm wavelength laser has been shown to operate at temperatures of up to 120°C and for up to 100,000 hours.

Professor Peter Smowton, from the School of Physics and Astronomy, said: “Realising electrically-pumped lasers based on Si substrates is a fundamental step towards silicon photonics.

“The precise outcomes of such a step are impossible to predict in their entirety, but it will clearly transform computing and the digital economy, revolutionise healthcare through patient monitoring, and provide a step-change in energy efficiency.

“Our breakthrough is perfectly timed as it forms the basis of one of the major strands of activity in Cardiff University’s Institute for Compound Semiconductors and the University’s joint venture with compound semiconductor specialists IQE.”

Professor Alwyn Seeds, Head of the Photonics Group at University College London, said: “The techniques that we have developed permit us to realise the Holy Grail of silicon photonics - an efficient and reliable electrically driven semiconductor laser directly integrated on a silicon substrate. Our future work will be aimed at integrating these lasers with waveguides and drive electronics leading to a comprehensive technology for the integration of photonics with silicon electronics"

Cardiff University

Silicon nanoparticle is a new candidate for an ultrafast all-optical transistor

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  Physicists from the Department of Nanophotonics and Metamaterials of ITMO University have experimentally demonstrated the feasibility of designing an optical analog of a transistor based on a single silicon nanoparticle. Because transistors are some of the most fundamental components of computing circuits, the results of the study have crucial importance for the development of optical computers, where transistors must be very small and ultrafast at the same time. The study was published in the scientific journal Nano Letters.

  The performance of modern computers, which use electrons as signal carriers, is largely limited by the time needed to trigger the transistor – usually around 0.1 - 1 nanoseconds (10−9 of a second). Next-generation optical computers, however, rely on photons to carry the useful signal, which heavily increases the amount of information passing through the transistor per second. For this reason, the creation of an ultrafast and compact all-optical transistor is considered to be instrumental in the development of optical computing. Such a nanodevice would enable scientists to control the propagation of an optical signal beam by means of an external control beam within several picoseconds (10−12 of a second).

  In the study, a group of Russian scientists from ITMO University, Lebedev Physical Institute and Academic University in Saint Petersburg put forward a completely new approach to design such optical transistors, having made a prototype using only one silicon nanoparticle.

  The scientists found that they can dramatically change the properties of a silicon nanoparticle by irradiating it with intense and ultrashort laser pulse. The laser thus acts as a control beam, providing ultrafast photoexcitation of dense and rapidly recombining electron-hole plasma whose presence changes the dielectric permittivity of silicon for a few picoseconds. This abrupt change in the optical properties of the nanoparticle opens the possibility to control the direction, in which incident light is scattered. For instance, the direction of nanoparticle scattering can be changed from backward to forward on picoseconds timescale, depending on the intensity of the incident control laser pulse. This concept of ultrafast switching is very promising for designing of all-optical transistor.

  “Generally, researchers in this field are focused on designing nanoscale all-optical transistors by means of controlling the absorption of nanoparticles, which, in essence, is entirely logical. In high absorption mode, the light signal is absorbed by the nanoparticle and cannot pass through, while out of this mode the light is allowed to propagate past the nanoparticle. However, this method did not yield any decisive results,” explains Sergey Makarov, lead author of the study and senior researcher at the Department of Nanophotonics and Metamaterials.“Our idea is different in the sense that we control not the absorption properties of the nanoparticle, but rather its scattering diagram. Let’s say, the nanoparticle normally scatters almost all incident light in the backward direction, but once we irradiate it by a control pulse, it becomes reconfigured and starts scattering light forward.”

  The choice of silicon as a material for the optical transistor was not accidental. Creating an optical transistor requires the use of inexpensive materials appropriate for mass production and capable of changing their optical properties in several picoseconds (in the regime of dense electron-hole plasma) without getting overheated at the same time.

  “The time it takes us to deactivate our nanoparticle amounts to just several picoseconds, while to activate it we need no more than tens of femtoseconds (10−15 of a second). Now we already have experimental data that clearly indicates that a single silicon nanoparticle can indeed play the role of an all-optical transistor. Currently we are planning to conduct new experiments, where, along with a laser control beam, we will introduce a useful signal beam”, concludes Pavel Belov, coauthor of the paper and head of the Department of Nanophotonics and Metamaterials.

  ITMO University

  http://www.nanotechnologyworld.org/

New Paper-like Material Could Boost Electric Vehicle Batteries

Scanning electron microscope images of
(a) SiO2 nanofibers after drying, (b) SiO2
nanofibers under high magnification (c) silicon
nanofibers after etching, and (d) silicon nanofibers
under high magnification.

Researchers at the University of California, Riverside’s Bourns College of Engineering have developed a novel paper-like material for lithium-ion batteries. It has the potential to boost by several times the specific energy, or amount of energy that can be delivered per unit weight of the battery.

This paper-like material is composed of sponge-like silicon nanofibers more than 100 times thinner than human hair. It could be used in batteries for electric vehicles and personal electronics.

The findings were just published in a paper, “Towards Scalable Binderless Electrodes: Carbon Coated Silicon Nanofiber Paper via Mg Reduction of Electrospun SiO₂ Nanofibers,” in the journal Nature Scientific Reports. The authors were Mihri Ozkan, a professor of electrical and computer engineering, Cengiz S. Ozkan, a professor of mechanical engineering, and six of their graduate students: Zach Favors, Hamed Hosseini Bay, Zafer Mutlu, Kazi Ahmed, Robert Ionescu and Rachel Ye.

The nanofibers were produced using a technique known as electrospinning, whereby 20,000 to 40,000 volts are applied between a rotating drum and a nozzle, which emits a solution composed mainly of tetraethyl orthosilicate (TEOS), a chemical compound frequently used in the semiconductor industry. The nanofibers are then exposed to magnesium vapor to produce the sponge-like silicon fiber structure.

Conventionally produced lithium-ion battery anodes are made using copper foil coated with a mixture of graphite, a conductive additive, and a polymer binder. But, because the performance of graphite has been nearly tapped out, researchers are experimenting with other materials, such as silicon, which has a specific capacity, or electrical charge per unit weight of the battery, nearly 10 times higher than graphite.

(a) Schematic representation of the electrospinning
process and subsequent reduction process. Digital
photographs of (b) as-spun SiO2 nanofibers paper, (c)
etched silicon nanofiber paper, and (d) carbon-coated silicon
nanofiber paper as used in the lithium-ion half-cell
configuration.

The problem with silicon is that is suffers from significant volume expansion, which can quickly degrade the battery. The silicon nanofiber structure created in the Ozkan’s labs circumvents this issue and allows the battery to be cycled hundreds of times without significant degradation.

“Eliminating the need for metal current collectors and inactive polymer binders while switching to an energy dense material such as silicon will significantly boost the range capabilities of electric vehicles,” Favors said.

This technology also solves a problem that has plagued free-standing, or binderless, electrodes for years: scalability. Free-standing materials grown using chemical vapor deposition, such as carbon nanotubes or silicon nanowires, can only be produced in very small quantities (micrograms). However, Favors was able to produce several grams of silicon nanofibers at a time even at the lab scale.

The researchers’ future work involves implementing the silicon nanofibers into a pouch cell format lithium-ion battery, which is a larger scale battery format that can be used in EVs and portable electronics.

The research is supported by Temiz Energy Technologies. The UC Riverside Office of Technology Commercialization has filed patents for inventions reported in the research paper.

http://ucrtoday.ucr.edu/27263

Electronics Based on a Two Dimensional Electron Gas

A new material could open the door to a new kind of electronics: researchers at the Vienna University of Technology have created a stable two-dimensional electron gas in strontium titanate.

Usually, microelectronic devices are made of silicon or similar semiconductors. Recently, the electronic properties of metal oxides have become quite interesting. These materials are more complex, yet offer a broader range of possibilities to tune their properties. An important breakthrough has now been achieved at the Vienna University of Technology: a two dimensional electron gas was created in strontium titanate. In a thin layer just below the surface electrons can move freely and occupy different quantum states.

Strontium titanate is not only a potential future alternative to standard semiconductors, it could also exhibit interesting phenomena, such as superconductivity, thermoelectricity or magnetic effects that do not occur in the materials that are used for today’s electronic devices.

The Surface Layer and the Inside

This project closely links theoretical calculations and experiments. Zhiming Wang from Professor Ulrike Diebold’s research team was the leading experimentalist; some of the experimental work was done at the synchrotron BESSY in Berlin. Zhicheng Zhong from Professor Karsten Held’s group studied the material in computer simulations.

Not all of the atoms of strontium titanate are arranged in the same pattern: if the material is cut at a certain angle, the atoms of the surface layer form a structure, which is different from the structure in the bulk of the material. “Inside, every titanium atom has six neighbouring oxygen atoms, whereas the titanium atoms at the surface are only connected to four oxygen atoms each”, says Ulrike Diebold. This is the reason for the remarkable chemical stability of the surface. Normally such materials are damaged if they come into contact with water or oxygen.

Migrating Oxygen Atoms

Something remarkable happens when the material is irradiated with high-energy electromagnetic waves: “The radiation can remove oxygen atoms from the surface”, Ulrike Diebold explains. Then other oxygen atoms from within the bulk of the material move up to the surface. Inside the material, an oxygen deficiency builds up, as well a surplus of electrons.

“These electrons, located in a two dimensional layer very close to the surface, can move freely. We call this an electron gas”, says Karsten Held. There has already been some evidence of two dimensional electron gases in similar materials, but until now the creation of a stable, durable electron gas at a surface has been impossible. The properties of the electrons in the gas can be finely tuned. Depending on the intensity of the radiation, the number of electrons varies. By adding different atoms, the electronic properties can also be changed.

“In solid state physics, the so-called band structure of a material is very important. It describes the relationship between the energy and the momentum of the electrons. The remarkable thing about our surface is that it shows completely different kinds of band structures, depending on the quantum state of the electron”, says Karsten Held.

The electron gas in the new material exhibits a multitude of different electronic structures. Some of them could very well be suitable for producing interesting magnetic effects or superconductivity. The promising properties of strontium titanate will now be further investigated. The researchers hope that, by applying external electric fields or by placing additional metal atoms on the surface, the new material could reveal a few more of its secrets.

Source: http://www.tuwien.ac.at/en/news/news_detail/article/8663/

Squeezing transistors really hard generates energy savings

Transistors, the workhorses of the electronics world, are plagued by leakage current. This results in unnecessary energy losses, which is why smartphones and laptops, for example, have to be recharged so often. 

Tom van Hemert and Ray Hueting of the University of Twente’s MESA+ Institute for Nanotechnology have shown that this leakage current can be radically reduced by “squeezing” the transistor with a piezoelectric material (which expands or contracts when an electrical charge is applied to it). Using this approach, they have smashed the theoretical limit for leakage current. Tom van Hemert will defend his PhD dissertation on 6 December.

If silicon is squeezed, this affects the freedom of movement of the electrons in this material. This can promote or restrict the flow of electrical current. Compare it to a garden hose. When you stand on it, less water comes out. But strangely enough, the flow of electrons in silicon actually increases when the material is compressed.

ONLY PINCH WHEN NECESSARY

In modern microchips, every single transistor is continuously exposed to enormous pressures of up to 10,000 atmospheres. This pressure is sealed in during the manufacturing process, by surrounding the transistors with compressive materials. While this boosts the chip’s processing speed, the leakage current also increases. The use of piezoelectric material means that the transistors are only put under pressure when this is necessary. This can generate considerable savings in terms of energy consumption.

The electrical current passing through a transistor is conducted by a slice of silicon. In the new transistor, this is sandwiched between layers of piezoelectric material. As this material (shown in red) expands, the silicon (shown in blue) is compressed.

LIMIT SMASHED

The underlying concept was originally developed by Ray Hueting. In order to turn this into reality, Tom van Hemert had to find a way of linking theories of mechanical deformation with quantum-mechanical formulas describing the electrical behaviour of transistors. The calculations indicate that “garden hose transistors” are much better than conventional transistors at switching from off to on. According to the classical theoretical limit, a charge of at least 60 millivolts is needed to make a transistor conduct ten times more electricity. The piezoelectric transistor uses just 50 millivolts. As a result, either the leakage current can be reduced, or more current can be carried in the on-state. Either way, this will boost the performance of modern microchips, while - importantly - cutting their energy consumption.

The results of this research were recently published in a leading journal, Transactions on Electron Devices. On 6 December, Tom van Hemert hopes to be awarded a Phd for his dissertation, which is entitled “Tailoring strain in microelectronic devices”.

Source: http://www.utwente.nl/en/archive/2013/12/squeezing-transistors-really-hard-generates-energy-savings/

Major leap towards graphene for solar cells

Surprising result: Graphene retains its properties even when coated with silicon

Graphene has extreme conductivity and is completely transparent while being inexpensive and nontoxic. This makes it a perfect candidate material for transparent contact layers for use in solar cells to conduct electricity without reducing the amount of incoming light  - at least in theory. Whether or not this holds true in a real world setting is questionable as there is no such thing as "ideal" graphene - a free floating, flat honeycomb structure consisting of a single layer of carbon atoms: interactions with adjacent layers can change graphene's properties dramatically. Now, Dr. Marc Gluba and Prof. Dr. Norbert Nickel of the HZB Institute for Silicon Photovoltaics have shown that graphene retains its impressive set of properties when it is coated with a thin silicon film. These findings have paved the way for entirely new possibilities to use in thin-film photovoltaics.

"We examined how graphene's conductive properties change if it is incorporated into a stack of layers similar to a silicon based thin film solar cell and were surprised to find that these properties actually change very little," Marc Gluba explains.

To this end, they grew graphene on a thin copper sheet, next transferred it to a glass substrate, and finally coated it with a thin film of silicon. They examined two different versions that are commonly used in conventional silicon thin-film technologies: one sample contained an amorphous silicon layer, in which the silicon atoms are in a disordered state similar to a hardened molten glas; the other sample contained poly-crystalline silicon to help them observe the effects of a standard crystallization process on graphene's properties.

Even though the morphology of the top layer changed completely as a result of being heated to a temperature of several hundred degrees C, the graphene is still detectable.

"That's something we didn't expect to find, but our results demonstrate that graphene remains graphene even if it is coated with silicon," says Norbert Nickel. Their measurements of carrier mobility using the Hall-effect showed that the mobility of charge carriers within the embedded graphene layer is roughly 30 times greater than that of conventional zinc oxide based contact layers. Says Gluba: "Admittedly, it's been a real challenge connecting this thin contact layer, which is but one atomic layer thick, to external contacts. We're still having to work on that." Adds Nickel: "Our thin film technology colleagues are already pricking up their ears and wanting to incorporate it."

The researchers obtained their measurements on one square centimeter samples, although in practice it is feasible to coat much larger areas than that with graphene.

This work was recently published in Applied Physics Letters Vol. 103, 073102 (2013).
Authors: M. A. Gluba, D. Amkreutz, G. V. Troppenz, J. Rappich, and N. H. Nickel

Source: http://www.helmholtz-berlin.de/pubbin/news_seite?nid=13825&sprache=en&typoid=1

Which makes a better solar collector, quantum dot or nanowire

A trio of researchers at North Dakota State University, Fargo and the University of South Dakota have turned to computer modeling to help decide which of two competing materials should get its day in the sun as the nanoscale energy-harvesting technology of future solar panels - quantum dots or nanowires.

 

Andrei Kryjevski and his colleagues, Dimitri Kilin and Svetlana Kilina, report in AIP Publishing's Journal of Renewable and Sustainable Energy that they used computational chemistry models 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. The ability to absorb light is substantially enhanced 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 made by Kryjevski, Kilin and Kilina indicated that light absorption by silicon quantum dot chains significantly increases with increased interactions between the individual nanospheres in the chain. They also found that light absorption by quantum dot chains and nanowires depends strongly on how the structure is aligned in relation to the direction of the photons striking it. Finally, the researchers learned that 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.

 

Source and top image: American Institute of Physics

Top image shows: Amorphous Silicon nanowire (yellow network) facilitates harvesting of solar energy in the form of a photon (wavy line). In the process of light absorption a pair of mobile charge carriers is created (red clouds depict an electron smeared in space, while the blue clouds visualize the so-called hole which is a positively charged carrier). The energy of their directed motion is then transformed into electricity. Electron and hole charge distributions are often located in different regions of space due to multiple structural defects in amorphous silicon nanowires.

 

For more read Energy Harvesting and Storage for Electronic Devices 2012-2022 

 

And attend Energy Harvesting & Storage USA 20-21 November 2013, Santa Clara, USA 

Source: http://www.energyharvestingjournal.com/articles/which-makes-a-better-solar-collector-quantum-dot-or-nanowire-00005858.asp?sessionid=1

Seeing through silicon

New microscopy technique allows scientists to visualize cells through the walls of silicon microfluidic devices.

Scientists at MIT and the University of Texas at Arlington (UTA) have developed a new type of microscopy that can image cells through a silicon wafer, allowing them to precisely measure the size and mechanical behavior of cells behind the wafer.

The new technology, which relies on near-infrared light, could help scientists learn more about diseased or infected cells as they flow through silicon microfluidic devices.

“This has the potential to merge research in cellular visualization with all the exciting things you can do on a silicon wafer,” says Ishan Barman, a former postdoc in MIT’s Laser Biomedical Research Center (LBRC) and one of the lead authors of a paper describing the technology in the Oct. 2 issue of the journal Scientific Reports.

Other lead authors of the paper are former MIT postdoc Narahara Chari Dingari and UTA graduate students Bipin Joshi and Nelson Cardenas. The senior author is Samarendra Mohanty, an assistant professor of physics at UTA. Other authors are former MIT postdoc Jaqueline Soares, currently an assistant professor at Federal University of Ouro Preto, Brazil, and Ramachandra Rao Dasari, associate director of the LBRC. 

Silicon is commonly used to build “lab-on-a-chip” microfluidic devices, which can sort and analyze cells based on their molecular properties, as well as microelectronics devices. Such devices have many potential applications in research and diagnostics, but they could be even more useful if scientists could image the cells inside the devices, says Barman, who is now an assistant professor of mechanical engineering at Johns Hopkins University. 

To achieve that, Barman and colleagues took advantage of the fact that silicon is transparent to infrared and near-infrared wavelengths of light. They adapted a microscopy technique known as quantitative phase imaging, which works by sending a laser beam through a sample, then splitting the beam into two. By recombining those two beams and comparing the information carried by each one, the researchers can determine the sample’s height and its refractive index — a measure of how much the material forces light to bend as it passes through.

Traditional quantitative phase imaging uses a helium neon laser, which produces visible light, but for the new system the researchers used a titanium sapphire laser that can be tuned to infrared and near-infrared wavelengths. For this study, the researchers found that light with a wavelength of 980 nanometers worked best. 

Using this system, the researchers measured changes in the height of red blood cells, with nanoscale sensitivity, through a silicon wafer similar to those used in most electronics labs.

As red blood cells flow through the body, they often have to squeeze through very narrow vessels. When these cells are infected with malaria, they lose this ability to deform, and form clogs in tiny vessels. The new microscopy technique could help scientists study how this happens, Dingari says; it could also be used to study the dynamics of the malformed blood cells that cause sickle cell anemia.

The researchers also used their new system to monitor human embryonic kidney cells as pure water was added to their environment — a shock that forces the cells to absorb water and swell up. The researchers were able to measure how much the cells distended and calculate the change in their index of refraction. 

“Nobody has shown this kind of microscopy of cellular structures before through a silicon substrate,” Mohanty says.

“This is an exciting new direction that is likely to open up enormous opportunities for quantitative phase imaging,” says Gabriel Popescu, an assistant professor of electrical engineering and computer science at the University of Illinois at Urbana-Champaign who was not part of the research team. 

“The possibilities are endless: From micro- and nanofluidic devices to structured substrates, the devices could target applications ranging from molecular sensing to whole-cell characterization and drug screening in cell populations,” Popescu says.

Mohanty’s lab at UTA is now using the system to study how neurons grown on a silicon wafer communicate with each other.

In the Scientific Reports paper, the researchers used silicon wafers that were about 150 to 200 microns thick, but they have since shown that thicker silicon can be used if the wavelength of light is increased into the infrared range. The researchers are also working on modifying the system so that it can image in three dimensions, similar to a CT scan. 

The research was funded by the National Institute of Biomedical Imaging and Bioengineering and Nanoscope Technologies, LLC.

Source: http://web.mit.edu/newsoffice/2013/seeing-through-silicon-1002.html

Stanford Scientists Use DNA to Assemble a Transistor From Graphene

Graphene, a sheet of carbon atoms arrayed in a honeycomb pattern, could be a better semiconductor than silicon.

DNA is the blueprint for life. Could it also become the template for making a new generation of computer chips based not on silicon, but on an experimental material known as graphene?

That’s the theory behind a process that Stanford chemical engineering professor Zhenan Bao reveals in Nature Communications.

Bao and her co-authors, former post-doctoral fellows Anatoliy Sokolov and Fung Ling Yap, hope to solve a problem clouding the future of electronics: consumers expect silicon chips to continue getting smaller, faster and cheaper, but engineers fear that this virtuous cycle could grind to a halt.

Why has to do with how silicon chips work.

Everything starts with the notion of the semiconductor, a type of material that can be induced to either conduct or stop the flow of electricity. Silicon has long been the most popular semiconductor material used to make chips.

The basic working unit on a chip is the transistor. Transistors are tiny gates that switch electricity on or off, creating the zeroes and ones that run software.

To build more powerful chips, designers have done two things at the same time: they’ve shrunk transistors in size and also swung those gates open and shut faster and faster.

The net result of these actions has been to concentrate more electricity in a diminishing space. So far that has produced small, faster, cheaper chips. But at a certain point, heat and other forms of interference could disrupt the inner workings of silicon chips.

"We need a material that will let us build smaller transistors that operate faster using less power," Bao said.

Graphene has the physical and electrical properties to become a next-generation semiconductor material – if researchers can figure out how to mass-produce it.

To the right is a honeycomb of graphene atoms. To the left is a double strand of DNA. The white spheres represent copper ions integral to the chemical assembly process. The fire represents the heat that is an essential ingredient in the technique. (Anatoliy Sokolov) 

Graphene is a single layer of carbon atoms arranged in a honeycomb pattern. Visually it resembles chicken wire. Electrically this lattice of carbon atoms is an extremely efficient conductor.

Bao and other researchers believe that ribbons of graphene, laid side-by-side, could create semiconductor circuits. Given the material’s tiny dimensions and favorable electrical properties, graphene nano ribbons could create very fast chips that run on very low power, she said.

"However, as one might imagine, making something that is only one atom thick and 20 to 50 atoms wide is a significant challenge," said co-author Sokolov.

To handle this challenge, the Stanford team came up with the idea of using DNA as an assembly mechanism.

Physically, DNA strands are long and thin, and exist in roughly the same dimensions as the graphene ribbons that researchers wanted to assemble.

Chemically, DNA molecules contain carbon atoms, the material that forms graphene.

The real trick is how Bao and her team put DNA’s physical and chemical properties to work.

The researchers started with a tiny platter of silicon to provide a support (substrate) for their experimental transistor. They dipped the silicon platter into a solution of DNA derived from bacteria and used a known technique to comb the DNA strands into relatively straight lines.

Next, the DNA on the platter was exposed to a copper salt solution. The chemical properties of the solution allowed the copper ions to be absorbed into the DNA.

Next the platter was heated and bathed in methane gas, which contains carbon atoms. Once again chemical forces came into play to aid in the assembly process. The heat sparked a chemical reaction that freed some of the carbon atoms in the DNA and methane. These free carbon atoms quickly joined together to form stable honeycombs of graphene.

"The loose carbon atoms stayed close to where they broke free from the DNA strands, and so they formed ribbons that followed the structure of the DNA," Yap said.

So part one of the invention involved using DNA to assemble ribbons of carbon. But the researchers also wanted to show that these carbon ribbons could perform electronic tasks. So they made transistors on the ribbons.

"We demonstrated for the first time that you can use DNA to grow narrow ribbons and then make working transistors," Sokolov said.

The paper drew praise from UC Berkeley associate professor Ali Javey, an expert in the use of advanced materials and next-generation electronics.

"This technique is very unique and takes advantage of the use of DNA as an effective template for controlled growth of electronic materials,” Javey said. “In this regard the project addresses an important research need for the field."

Bao said the assembly process needs a lot of refinement. For instance, not all of the carbon atoms formed honeycombed ribbons a single atom thick. In some places they bunched up in irregular patterns, leading the researchers to label the material graphitic instead of graphene.

Even so, the process, about two years in the making, points toward a strategy for turning this carbon-based material from a curiosity into a serious contender to succeed silicon.

"Our DNA-based fabrication method is highly scalable, offers high resolution and low manufacturing cost," said co-author Yap. "All these advantages make the method very attractive for industrial adoption."

The experiment was supported in part by the National Science Foundation and the Stanford Global Climate and Energy Program.

Tom Abate is associate director of communications in the Stanford School of Engineering.

Source: http://engineering.stanford.edu/news/stanford-scientists-use-dna-assemble-transistor-graphene

Light scattering controlled by silicon nanoparticles

An experimental demonstration of light scattering controlled by silicon nanoparticles augurs well for the development of integrated optical circuits

Optical fibers are now delivering ultrafast internet connections to homes across the world. By replacing electronics-based technologies with architectures that process pulses of light, a similar leap in speed might also be possible for other forms of information handling. To realize this potential, scientists must first develop novel devices that are capable of controlling the flow of light at the nanometer scale. 

Such a device may now be within reach. Yuan Hsing Fu at the A*STAR Data Storage Institute and co‐workers have demonstrated a unique optical effect in nanoparticles that allows them to control the direction in which visible light scatters1. 

Miniaturization is key to the success of modern-day electronics: complicated circuitry must be made to fit into portable devices. Likewise, the hardware for processing optical signals must also be miniaturized. In this field, known as photonics, the design of optical components requires an entirely new approach. 

The effect demonstrated by Fu and co-workers reveals how nanoparticles can be used to scatter light controllably in the visible spectral range. The researchers first designed a method to measure the scattering, and then fired light at tiny spheres of silicon. When the beam hit a sphere, some scattered backward and some scattered forward. The researchers also showed that it is possible to control the ratio of movement in the two directions by changing the diameter of the nanosphere. 

Using silicon spheres with diameters of between 100 and 200 nanometers, the team observed that the amount of forward-scattered light varied from being roughly equal to the amount that was backward-scattered to being six times more intense. They also found that the effect could split the light according to wavelength: for example, nanoparticles of a particular size that backscattered predominantly green light also forward scattered mainly yellow radiation (see image). 

The researchers chose silicon over the more conventional choice of a metal such as gold because it reduces energy loss and can influence both the electric and magnetic components of light. The ‘preferential’ scattering of radiation arises because of the mutual interaction between the electric and magnetic resonances of the nanosphere. 

This effect is analogous to that of a radio-frequency antenna. “The experimental proof of such relatively simple nano-optical systems with both an electric and magnetic response in the optical spectral range could pave the way to scaling the optical nano-antenna concept down to a single nanoparticle,” says Fu. Optical nanoscale antennas could be useful for improving solar cells and might form a crucial building block for integrated optical circuits. 

Source: http://www.research.a-star.edu.sg/research/6743