Tag Archives: letter-materials-tech

Scientists at the National University of Singapore (NUS) have demonstrated a new way of controlling electrons by confining them in a device made out of atomically thin materials, and applying external electric and magnetic fields. This research, published on Dec. 23, 2015 in the prestigious scientific journal Nature, was led by Professor Antonio Castro Neto and his research team at the Centre for Advanced 2D Materials (CA2DM) of the NUS Faculty of Science.

Almost all modern technology like motors, light bulbs and semiconductor chips runs on electricity, harnessing the flow of electrons through devices. Explained Prof Castro Neto, “Not only are electrons small and fast, they naturally repel each other due to their electric charge. They obey the strange laws of quantum physics, making it difficult to control their motion directly.”

To control electron behaviour, many semiconductor materials require chemical doping, where small amounts of a foreign material are embedded in the material to either release or absorb electrons, creating a change in the electron concentration that can in turn be used to drive currents.

However, chemical doping has limitations as a research technique, since it causes irreversible chemical change in the material being studied. The foreign atoms embedded into the material also disrupt its natural ordering, often masking important electronic states of the pure material.

The NUS research team was able to replicate the effects of chemical doping in this study by using only external electric and magnetic fields applied to an atomically thin material, titanium diselenide (TiSe2), encapsulated with boron-nitride (hBN). The researchers were able to control the behaviour of the electrons accurately and reversibly, making measurements that had been theoretical up to now. The thinness of the two materials was crucial, confining the electrons within the material to a two-dimensional layer, over which the electric and magnetic fields had a strong, uniform effect.

Scientists at the National University of Singapore have demonstrated a new way of controlling electrons by confining them in a device made out of an atomically thin material and applying external electric and magnetic fields. Credit: National University of Singapore

“In particular, we could also drive the material into a state called superconductivity, in which electrons move throughout the material without any heat or energy loss,” Prof Castro Neto said.

Because they are atomically thin, two-dimensional superconducting materials would have advantages over traditional superconductors, in applications such as smaller, portable magnetic resonance imaging (MRI) machines.

One specific goal of the NUS research team is to develop high-temperature two-dimensional superconducting materials. Current materials require an extremely cold temperature of -270°C to function, ruling out exciting applications such as lossless electrical lines, levitating trains and MRI machines.

The technique, which took the researchers two years to develop, will enable new experiments that shine light on high-temperature superconductivity and other solid-state phenomena of interest. With a wide range of materials awaiting testing, electric field doping greatly widens the possibilities of solid-state science.

Engineers at MIT have devised a new technique for trapping hard-to-detect molecules, using forests of carbon nanotubes.

The team modified a simple microfluidic channel with an array of vertically aligned carbon nanotubes — rolled lattices of carbon atoms that resemble tiny tubes of chicken wire. The researchers had previously devised a method for standing carbon nanotubes on their ends, like trees in a forest. With this method, they created a three-dimensional array of permeable carbon nanotubes within a microfluidic device, through which fluid can flow.

Now, in a study published this week in the Journal of Microengineering and Nanotechnology, the researchers have given the nanotube array the ability to trap certain particles. To do this, the team coated the array, layer by layer, with polymers of alternating electric charge.

“You can think of each nanotube in the forest as being concentrically coated with different layers of polymer,” says Brian Wardle, professor of aeronautics and astronautics at MIT. “If you drew it in cross-section, it would be like rings on a tree.”

Depending on the number of layers deposited, the researchers can create thicker or thinner nanotubes and thereby tailor the porosity of the forest to capture larger or smaller particles of interest.

The nanotubes’ polymer coating may also be chemically manipulated to bind specific bioparticles flowing through the forest. To test this idea, the researchers applied an established technique to treat the surface of the nanotubes with antibodies that bind to prostate specific antigen (PSA), a common experimental target. The polymer-coated arrays captured 40 percent more antigens, compared with arrays lacking the polymer coating.

Wardle says the combination of carbon nanotubes and multilayer coatings may help finely tune microfluidic devices to capture extremely small and rare particles, such as certain viruses and proteins.

“There are smaller bioparticles that contain very rich amounts of information that we don’t currently have the ability to access in point-of-care [medical testing] devices like microfluidic chips,” says Wardle, who is a co-author on the paper. “Carbon nanotube arrays could actually be a platform that could target that size of bioparticle.”

The paper’s lead author is Allison Yost, a former graduate student who is currently an engineer at Accion Systems. Others on the paper include graduate student Setareh Shahsavari; postdoc Roberta Polak; School of Engineering Professor of Teaching Innovation Gareth McKinley; professor of materials science and engineering Michael Rubner, and Raymond A. And Helen E. St. Laurent Professor of Chemical Engineering Robert Cohen.

A porous forest

Carbon nanotubes have been a subject of intense scientific study, as they possess exceptional electrical, mechanical, and optical properties. While their use in microfluidics has not been well explored, Wardle says carbon nanotubes are an ideal platform because their properties may be manipulated to attract certain nanometer-sized molecules. Additionally, carbon nanotubes are 99 percent porous, meaning a nanotube is about 1 percent carbon and 99 percent air.

“Which is what you need,” Wardle says. “You need to flow quantities of fluid through this material to shed all the millions of particles you don’t want to find and grab the one you do want to find.”

What’s more, Wardle says, a three-dimensional forest of carbon nanotubes would provide much more surface area on which target molecules may interact, compared with the two-dimensional surfaces in conventional microfluidics.

“The capture efficiency would scale with surface area,” Wardle notes.

A versatile array

The team integrated a three-dimensional array of carbon nanotubes into a microfluidic device by using chemical vapor deposition and photolithography to grow and pattern carbon nanotubes onto silicon wafers. They then grouped the nanotubes into a cylinder-shaped forest, measuring about 50 micrometers tall and 1 millimeter wide, and centered the array within a 3 millimeter-wide, 7-millimeter long microfluidic channel.

The researchers coated the nanotubes in successive layers of alternately charged polymer solutions in order to create distinct, binding layers around each nanotube. To do so, they flowed each solution through the channel and found they were able to create a more uniform coating with a gap between the top of the nanotube forest and the roof of the channel. Such a gap allowed solutions to flow over, then down into the forest, coating each individual nanotube. In the absence of a gap, solutions simply flowed around the forest, coating only the outer nanotubes.

After coating the nanotube array in layers of polymer solution, the researchers demonstrated that the array could be primed to detect a given molecule, by treating it with antibodies that typically bind to prostate specific antigen (PSA). They pumped in a solution containing small amounts of PSA and found that the array captured the antigen effectively, throughout the forest, rather than just on the outer surface of a typical microfluidic element.

Wardle says that the nanotube array is extremely versatile, as the carbon nanotubes may be manipulated mechanically, electrically, and optically, while the polymer coatings may be chemically altered to capture a wide range of particles. He says an immediate target may be biomarkers called exosomes, which are less than 100 nanometers wide and can be important signals of a disease’s progression.

“Science is really picking up on how much information these particles contain, and they’re sort of everywhere, but really hard to find, even with large-scale equipment,” Wardle says. “This type of device actually has all the characteristics and functionality that would allow you to go after bioparticles like exosomes and things that really truly are nanometer scale.”

At last week’s IEEE International Electron Devices Meeting 2015, nano-electronics research center imec presented three novel aluminum gallium nitride (AlGaN)/ gallium nitride (GaN) stacks featuring optimized low dispersion buffer designs. Moreover, imec optimized the epitaxial p-GaN growth process on 200mm silicon wafers, achieving e-mode devices featuring beyond state-of-the-art high threshold voltage (Vt) and high drive current (Id).

To achieve a good, current-collapse-free device operation in AlGaN/GaN-on-Silicon (Si) devices, dispersion must be kept to a minimum. Trapped charges in the buffer between the GaN-based channel and the silicon substrate are known to be a critical factor in causing dispersion. Imec compared the impact of different types of buffers on dispersion and optimized three types: a classic step-graded buffer, a buffer with low-temperature AlN interlayers, and a super lattice buffer. These three types of buffers were optimized for low dispersion, leakage and breakdown voltage over a wide temperature range and bias conditions.

Imec also optimized the epitaxial p-GaN growth process demonstrating improved electrical performance of p-GaN HEMTs, achieving a beyond state-of-the-art combination of high threshold voltage, low on-resistance and high drive current (Vt >2V, RON = 7 Ω.mm and Id >0.4A/mm at 10V). The P-GaN HEMT results outperformed their MISHEMT counterparts.

Imec’s GaN-on-Si R&D program aims at bringing this technology towards industrialization. Imec’s offering includes a complete 200mm CMOS-compatible 200V GaN process line that features excellent specs on e-mode devices. Imec’s program allows partners early access to next-generation devices and power electronics processes, equipment and technologies, and speed up innovation at shared costs. Current R&D focuses on improving the performance and reliability of imec’s e-mode devices, while in parallel pushing the boundaries of the technology through innovation in substrate technology, higher levels of integration and exploration of novel device architectures.

“Imec’s presentations at the renowned IEDM meeting last week are a testament to the capabilities, sophistication, and maturity of our 200mm GaN-on-Silicon platform,” stated Rudi Cartuyvels, executive vice president of smart systems and energy technology at imec. “Building upon this success, we are now working with our GaN partners to implement and transfer specific device customizations. in parallel, we are exploring alternative substrate technologies to further push the boundaries of the GaN technology.”

Researchers from North Carolina State University have discovered a new phase of solid carbon, called Q-carbon, which is distinct from the known phases of graphite and diamond. They have also developed a technique for using Q-carbon to make diamond-related structures at room temperature and at ambient atmospheric pressure in air.

Phases are distinct forms of the same material. Graphite is one of the solid phases of carbon; diamond is another.

“We’ve now created a third solid phase of carbon,” says Jay Narayan, the John C. Fan Distinguished Chair Professor of Materials Science and Engineering at NC State and lead author of three papers describing the work. “The only place it may be found in the natural world would be possibly in the core of some planets.”

Q-carbon has some unusual characteristics. For one thing, it is ferromagnetic — which other solid forms of carbon are not.

“We didn’t even think that was possible,” Narayan says.

In addition, Q-carbon is harder than diamond, and glows when exposed to even low levels of energy.

“Q-carbon’s strength and low work-function — its willingness to release electrons — make it very promising for developing new electronic display technologies,” Narayan says.

But Q-carbon can also be used to create a variety of single-crystal diamond objects. To understand that, you have to understand the process for creating Q-carbon.

Researchers start with a substrate, such as such as sapphire, glass or a plastic polymer. The substrate is then coated with amorphous carbon — elemental carbon that, unlike graphite or diamond, does not have a regular, well-defined crystalline structure. The carbon is then hit with a single laser pulse lasting approximately 200 nanoseconds. During this pulse, the temperature of the carbon is raised to 4,000 Kelvin (or around 3,727 degrees Celsius) and then rapidly cooled. This operation takes place at one atmosphere — the same pressure as the surrounding air.

The end result is a film of Q-carbon, and researchers can control the process to make films between 20 nanometers and 500 nanometers thick.

By using different substrates and changing the duration of the laser pulse, the researchers can also control how quickly the carbon cools. By changing the rate of cooling, they are able to create diamond structures within the Q-carbon.

“We can create diamond nanoneedles or microneedles, nanodots, or large-area diamond films, with applications for drug delivery, industrial processes and for creating high-temperature switches and power electronics,” Narayan says. “These diamond objects have a single-crystalline structure, making them stronger than polycrystalline materials. And it is all done at room temperature and at ambient atmosphere – we’re basically using a laser like the ones used for laser eye surgery. So, not only does this allow us to develop new applications, but the process itself is relatively inexpensive.”

And, if researchers want to convert more of the Q-carbon to diamond, they can simply repeat the laser-pulse/cooling process.

If Q-carbon is harder than diamond, why would someone want to make diamond nanodots instead of Q-carbon ones? Because we still have a lot to learn about this new material.

“We can make Q-carbon films, and we’re learning its properties, but we are still in the early stages of understanding how to manipulate it,” Narayan says. “We know a lot about diamond, so we can make diamond nanodots. We don’t yet know how to make Q-carbon nanodots or microneedles. That’s something we’re working on.”

NC State has filed two provisional patents on the Q-carbon and diamond creation techniques.