Nanocomposite based flexible ultrasensitive resistive gas sensor for chemical reactions studies – Nature Article

Nice Study

Abstract :

Room temperature operation, low detection limit and fast response time are highly desirable for a wide range of gas sensing applications. However, the available gas sensors suffer mainly from high temperature operation or external stimulation for response/recovery. Here, we report an ultrasensitive-flexible-silver-nanoparticle based nanocomposite resistive sensor for ammonia detection and established the sensing mechanism. We show that the nanocomposite can detect ammonia as low as 500 parts-per-trillion at room temperature in a minute time. Furthermore, the evolution of ammonia from different chemical reactions has been demonstrated using the nanocomposite sensor as an example. Our results demonstrate the proof-of-concept for the new detector to be used in several applications including homeland security, environmental pollution and leak detection in research laboratories and many others.

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Method to grow epitaxial nitride semiconductors on graphene

The US Naval Research Laboratory (NRL) in Washington DC has developed a method to grow epitaxial nitride semiconductors on graphene [Neeraj Nepal et al, Appl. Phys. Express, vol6, p061003, 2013]. The researchers hope that this could lead to high-speed current-switching applications using devices such as hot-electron transistors (HETs).

Present HETs use heavily doped semiconductor or metal base regions. Heavy doping hampers ballistic transport due to impurity and carrier-carrier scattering effects. Metal base regions suffer from electron reflection effects at the base-collector interface. The NRL team believes that using graphene as the base region, in conjunction with nitride semiconductors, could lead to devices with cut-off frequencies greater than 1THz (1000GHz).

Up to now, growth of nitride semiconductors on graphene has resulted in non-uniform GaN crystallites and not a continuous film.

The NRL method includes a functionalization step that produces “for the first time” nitride semiconductor layers of a quality similar to that obtained by traditional growth methods on conventional sapphire substrates. In fact, the crystal quality is achieved with thinner layers of less than 1μm compared with layers on other substrates.

The researchers comment: “These results support a successful demonstration of electronic-quality, heteroepitxial wurtzitic GaN on graphene that is currently unavailable and can improve the performance of present state-of-the-art devices such as HETs.”

The initial epitaxial graphene (EG) layer was prepared on 4° 4H-polytype silicon carbide (1.6mm x 1.6mm squares) using silicon sublimation. Silicon nitride was then applied using plasma-enhanced chemical vapor deposition (PECVD). The silicon nitride was patterned into discs of various diameters between 50μm and 500μm.

Figure 1

Figure 1: (a) Schematic of GaN/AlN/graphene/SiC layered structure, (b) atomic force microscope (AFM) image as-synthesized epitaxial graphene, (c) AFM image 1.2nm ALE AlN/graphene, (d) AFM image GaN/graphene, (e) scanning electron microscope (SEM) image pristine graphene, (f ) SEM image 1.2nm ALE AlN/graphene, (g) SEM image GaN on AlE AlN/graphene. (h) Al atoms replace F atoms, creating an AlN nucleation site on graphene resulting in proposed crystalline alignments.

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Carbon Nanotubes Increase Light Absorption in Thin-Film Solar Cells

UW-Madison_CNT_CellMaterials engineers at the University of Wisconsin-Madison (US) have developed the first thin-film solar cell that takes advantage of a previously unexploited property of carbon nanotubes — their ability to absorb light. Using nanotubes as the principle light absorbing material, the proof-of-concept device converts more than 75% of the light it absorbs into electricity.

Carbon nanotubes could be highly attractive candidates for the primary light-absorbing component in a solar cell because they are strong light absorbers in addition to having high charge transport mobility and solution-processability. Nanotubes are also more air stable than most other semiconducting conjugated carbon materials.

Other research groups have previously used carbon nanotubes in photovoltaic devices, but mostly as secondary materials such as electrode materials or charge-collection or charge-transport aids rather than for their ability to absorb light. “We estimate more than 60% of the current in our cell comes from the nanotube component,” says Michael Arnold, an assistant professor of materials science and engineering at UW-Madison. “We achieve these results using highly pure semiconducting carbon nanotubes with band gap in the near-infrared and very few metallic nanotube impurities and tuning the optical interference effects within the device.” The cell has a bilayer structure with a planar heterojunction between the carbon nanotubes and fullerene-C60. The carbon nanotubes absorb light, generating bound electron-hole pairs — or excitons — in the nanotube film. The electrons are extracted by the fullerene-C60 layer and transported to the silver top electrode, where they are extracted from the device.

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Semiconductor Manufacturing Using Silicon Carbide

CoorsTek has specially developed high-purity, full-density PureSiC CVD silicon carbide to address the challenging demands of silicon wafer processing. PureSiC CVD silicon carbide has a purity of over 99.9995% with zero porosity. These qualities enable it to maintain the cleanliness of semiconductor production processes. Moreover, PureSiC CVD silicon carbide exhibits high strength and stiffness and low thermal mass, a key design parameter. These characteristics make them suitable for fabricating thin, lightweight components. The high thermal shock resistance of PureSiC CVD silicon carbide improves ramp rates and extends component life in RTP processes.

PureSiC CVD silicon carbide withstands cleaning processes by exhibiting very high resistance to high temperature in-situ etching with gaseous HCl and concentrated HF/HNO3 wet cleans. CoorsTek supplies this material in high-resistivity (HR) and low-resistivity (LR) grades for applications demanding specific electrical properties, and in high and low transmissivity grades for applications requiring optical or infrared transmissivity. PureSiC CVD silicon carbide’s near-net shape capabilities facilitate the fabrication of complex geometries.

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Webinar Announcement – 12 July – X-ray Diffraction Techniques in Transmission Geometry b PANalytical y

PANalytical would like to invite you to attend our complimentary webinar on “X-ray Diffraction Techniques in Transmission Geometry.” This webinar will highlight the benefits of this geometry for challenging samples, with some side by side comparisons of data with traditional reflection geometry, and also talk about a host of applications where this geometry is essential, such as transmission texture, pair distribution function (PDF), small angle X-ray scattering (SAXS), and computed tomography (CT). Following the presentation, we will open the lines for a live Q & A session.

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Nanowires Grow Better on Graphene – from IEEE Spectrum

By Dexter Johnson
In an attempt to grow nanowires on a graphene substrate, researchers at the University of Illinois may have stumbled upon a new paradigm for epitaxy (the growth of crystals on a susbstrate).

Some believe that developing new manufacturing methods for nanoscale devices—like epitaxy—may be more crucial to meeting the demands of next generation chips than creating new materials, especially when feature sizes start falling below three nanometers. So, the Illinois researchers’ development of a new method of epitaxy may ultimately be more significant than creating a new material.

The research, which was published in the journal Nano Letters (“InxGa1–xAs Nanowire Growth on Graphene: van der Waals Epitaxy Induced Phase Segregation”), produced nanowires made from III-V compound semiconductors. Generally, III-V semiconductors like gallium arsenide don’t integrate well with silicon, but  recently  it was discovered that when these materials were brought down to the nanoscale that they were compatible.

Researchers have previously combined two of these semiconductors in gaseous form so that they deposit themselves on a graphene substrate (a process known as metalorganic chemical vapor deposition, or MOCVD) and self assemble into ordered crystalline form. However, the Illinois research marks the first time three of the semiconductors have been mixed together in this way.

The researchers discovered that something remarkable occurred when this third semiconductor was added to the mix. The materials began spontaneously to segregate into an indium arsenide (InAs) core with an InGaAs shell around the outside of the nanowire.

“This is unexpected,” says professor Xiuling Li, who led the research, in a press release. “A lot of devices require a core-shell architecture. Normally you grow the core in one growth condition and change conditions to grow the shell on the outside. This is spontaneous, done in one step. The other good thing is that since it’s a spontaneous segregation, it produces a perfect interface.”

This precise delineation between the core and the outside of the nanowire has to do with relationship between the atomic structure of the semiconductors and that of the graphene. The crystal structure of InAs has the same distance between its atoms as the carbon atoms in a sheet of graphene. As a result, the InAs fits in that space perfectly, leaving the gallium compound to form on the outside of that core.

The next step for the researchers will be to see if they can exploit their new manufacturing technique to create solar cells and other optoelectronic devices.

Image: Joshua D. Wood/University of Illinois at Urbana-Champaign


Nanotube Supply Glut Claims First Victim from IEEE Spectrum

By Dexter Johnson

Just three years after announcing a huge capacity increase to its multi-walled carbon nanotube (MWNT) production, Bayer Material Science has announced that it will completely close down its MWNT production to focus on its core business.

This is no surprise since there was a huge glut of product resulting in industry utilization rates that must have been in the single digits. This oversupplied market was the result of a MWNT capacity arms race that started in the mid-2000s. While this steep ramping up of production capacity reduced pricing from $700/kg in 2006 to below $100/kg in 2009—with some estimates putting the price at $50/kg as of last year—the problem seemed to be that no matter how cheap you made the stuff nobody was buying it because there were no applications for it. This resulted in stories, at once humorous and worrisome, of big chemical companies that had gotten themselves caught up in this arm race making desperate phone calls to laboratory researchers pitching application ideas for the material.

While some observers believed that this price cut would result in the applications being developed, most people recognized that this was a case of putting the cart before the horse, or “technology push” ahead of the preferable “market pull.”

This is not to say strategically it was wrong for a company like Bayer Material Science to build out capacity for a product that nobody seemed to want at that moment but may in the future. A company like Bayer can ramp up production with relatively little capital cost and manage to price everyone else out of the market. It was worth the risk.

However, hindsight makes it pretty clear that MWNTs applications were never really going to materialize as had been hoped. This became painfully clear when after a few years into production one of the target applications being touted for the material was the blades of large wind turbines. That announcement smacked of desperation.

Despite this, the story of MWNT capacity growth has been very instructive for how the so-called “nanotechnology industry” will shake out.

First, it’s clear that small operations that have found a way to produce a nanomaterial cheaply will have a difficult time competing with large chemical companies. This is not because they can’t produce the material more cheaply or at a better quality, but because they do not have the supply chain that well-established chemical companies have.

Second, you don’t want to be in the business of producing a nanomaterial that serves just to make some other product. You want to be making the final product. Many small start-ups no longer exist because they figured that they could just license their technology to a company that would make a product from their nanomaterial.

Bayer Material Science is in the position where it can just mothball its production without too much pain, but there may be some other companies that are less diversified for which that may not be an option. Sometimes when one domino falls the rest go in quick succession. So this should be an area to watch in the near future.

Image: Martin McCarthy/iStockphoto

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