Category Archives: General

Inventors Synthesize Graphene with Lasers

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As featured in the LIA TODAY

By Liliana Caldero

Graphene – it’s the two-dimensional (2D) allotrope of carbon atoms that ignited the imaginations of researchers across the globe. Heralded as a ‘miracle material’, its potential seemed limitless and it was predicted to usher in the next generation of technology. Flexible, stronger than steel, transparent, lightweight, and an amazing conductor of heat and electricity, it was going to revolutionize everything from household electronics to biomedical nanotechnology.

 

THE PROBLEM

Yet, nearly eight years after Dr. Andre Geim and Dr. Konstantin Novoselov earned the Nobel Prize in Physics for first isolating graphene and identifying its properties, graphene has encountered barriers to moving out of the lab and into the marketplace. According to Prof. Dr. Aravinda Kar of the University of Central Florida’s Center for Research and Education in Optics and Lasers (CREOL), one of the most prominent barriers has been finding scalable manufacturing processes that can produce graphene of a quality and quantity ready for consumers and businesses.

Graphene is notoriously difficult to synthesize in large quantities at a consistent quality. Early methods of isolating graphene involved a slow and tedious mechanical exfoliation technique; the researchers would extract a thin layer of graphite from a graphite crystal using regular adhesive tape, continually reducing the graphite sample by sticking the tape together and pulling it apart until only a small, 2D section of carbon atoms with a honeycomb lattice remained. Graphene’s unique characteristics are only present when it is one, two, or three layers of atoms thick – any thicker and it becomes graphite, losing all of the exceptional properties of graphene. The tape exfoliation method, although useful for the lab, was not going to translate very well to an industrialized process.

 

SOLVENT-AIDED EXFOLIATION AND CVD

Two of the more promising and potentially scalable methods of producing graphene are solvent-aided exfoliation and chemical vapor deposition (CVD). In solvent-aided exfoliation, sonication is used to exfoliate graphene crystals which are then further separated in a solvent and later gathered into graphene monolayers.  Scientists at the National University of Singapore have identified a flocculation method that reduces the amount of solvent needed for their exfoliation process, which could yield graphene using far less solvent than was previously needed. Another method experiencing innovation is CVD, which uses thermal chemical reactions to ‘grow’ graphene on substrates of specific materials, typically copper or silicon. Recently, engineers at MIT developed a CVD process for producing graphene filtration membrane sheets at 5 cm per minute. One of the biggest issues with traditional CVD and exfoliation methods is the need to transfer graphene from its fabrication platform to a substrate. Lasers are going to change that.

 

THE MISSING PIECE – LASERS

Lasers may provide yet another avenue to the elusive mass production of graphene, with an eye toward innovating the semiconductor industry. In 2003, Kar, along with Dr. Islam Salama and Dr. Nathaniel Quick, realized that laser direct writing could be used to fabricate carbon-rich nanoribbons on a silicon carbide (SiC) wafer in a nitrogen rich environment. Although these ribbons were too thick to be considered graphene, Kar believed that with a few changes, this process could be reworked to synthesize graphene in situ on a large scale, very quickly. In 2013, Kar and Quick were issued a patent for a Laser Chemical Vapor Deposition (LCVD) method that could be scaled for mass production.

Their method involved a few simple components: a frequency doubled Nd:YAG (green) laser of 532 nm wavelength, methane (CH4) gas, a silicon substrate, and a vacuum chamber.

The 532 nm wavelength corresponds to a photon of energy 2.33 eV, so the energy of two photons is 4.66 eV, just within the range of the C-H bond energy (4.3-4.85 eV) in CH4. Focusing the laser beam to a high intensity can induce two-photon absorption at the focal plane, causing the decomposition of CH4 to release the hydrogen atoms and deposit carbon atoms only on the substrate. The laser heating of the silicon substrate is just low enough to avoid melting the silicon, while providing sufficient thermal and electromagnetic energies to assist the carbon-carbon bonds rearrange into graphene’s trademark hexagonal pattern.

An experimental set-up for multiphoton photolytic laser chemical vapor deposition (LCVD) of graphene from methane precursor. Image courtesy of Dr. Kar and Dr. Quick.

LASER DIRECT WRITING OF GRAPHENE

Kar believes this process could be adapted to add graphene directly onto any substrate. Using laser direct writing, a company could easily draw graphene circuits onto a board. For companies using a hybrid approach, the graphene could be deposited at precise points as interconnects. “You would have all the CAD/CAM capability you could want,” says Quick. Currently, green lasers are available at high output powers, 100 W in continuous wave mode from most large laser manufacturers, so adding this additional step to the manufacturing pipeline for semiconductors would be easy and inexpensive compared to other methods.

At 1.9 cm per second, or 45 inches per minute, this method of graphene production is fast and efficient. This LCVD method offers control over the number of graphene layers, whether one, two, or three are required.  This process also removes the need to manually place graphene onto its intended location, as it is synthesized precisely where it should be. It’s also worth mentioning that this process is conducive to minimal environmental impact, as the unreacted methane and hydrogen byproducts can be captured to be recycled and reused.

 

A LOOK AT THE FUTURE

Picture this: a template is placed over a substrate and a line-shaped laser beam sweeps over it briefly or a beam of large cross-sectional area illuminates the entire template in one shot; when the template is removed, an intricate graphene design has been printed onto a circuit board. That is the future that Kar says is possible, with the right equipment. He suggests that we need manufacturers to develop lasers producing line-shaped beams or large area beams with spatially uniform intensity profile to realize this vision cost-effectively. He emphasizes that a true line-shaped beam produced by a slab laser system or an array of optical fiber laser would be necessary, as shaping the beam synthetically by changing the shape of an aperture would result in too much lost energy. With this technology, graphene could easily be printed onto circuit boards immediately, only where it’s needed, saving in material costs and time.

Nearly 14 years after the excitement first began, researchers are still exploring the potential uses of graphene; from applications in microsupercapacitors to Organic LEDs in flexible displays to ultra-sensitive optical sensors, and even lightweight body armor, the possibilities are still as exciting as ever.

 

Acknowledgements

Prof. Dr. Aravinda Kar, University of Central Florida, CREOL

Dr. Nathaniel Quick, Executive Director of LIA

 

LEARN MORE

Laser Formation of Graphene: United States Patent 8617669. (N. Quick, A. Kar)
http://www.freepatentsonline.com/8617669.html

NUS-led research team develops cost effective technique for mass production of high-quality graphene – http://news.nus.edu.sg/press-releases/mass-production-graphene-slurry

MIT researchers develop scalable manufacturing process for graphene sheets – https://newatlas.com/mit-manufacturing-graphene-filtration-membranes/54274/

Arizona State Researchers Demonstrate White Lasers, Which Could Revolutionize Everyday Lighting

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Researchers at Arizona State University have created white laser light with an array of semiconductor lasers in a compact arrangement. The project helps pave the way toward better lighting and light-based wireless communication, as white lasers are more luminous and energy efficient than LEDs.

This schematic illustrates the novel nanosheet with three parallel segments created by the researchers, each supporting laser action in one of three elementary colors. The device is capable of lasing in any visible color, completely tunable from red, green to blue, or any color in between. When the total field is collected, a white color emerges.
Photo by: ASU/Nature Nanotechnology

ASU researchers created a novel nanosheet — a thin layer of semiconductor that measures roughly one-fifth of the thickness of human hair in size and roughly one-thousandth the thickness of human hair — with three parallel segments, each supporting laser action in one of three elementary colors. The device is capable of lasing in any visible color, completely tunable from red, green to blue or any color in between. When the total field is collected, a white color emerges.

Engineers in ASU’s Ira A. Fulton Schools of Engineering published their findings in the July 27 advance online publication of the journal Nature Nanotechnology. Cun-Zheng Ning, professor in the School of Electrical, Computer and Energy Engineering, wrote the paper, “A monolithic white laser,” with his doctoral students Fan Fan, Sunay Turkdogan, Zhicheng Liu and David Shelhammer.

The advance puts lasers a step closer to being a mainstream light source and potential replacement or alternative to light emitting diodes. Lasers are brighter, more energy efficient, and can potentially provide more accurate and vivid colors for displays like computer screens and televisions. Ning’s group has already shown that their structures could cover as much as 70 percent more colors than the current display industry standard.

Another important application could be in visible light communication in which the same room lighting systems could be used for illumination and communication. The technology under development is called Li-Fi for light-based wireless communication, as opposed to the more prevailing Wi-Fi using radio waves. Li-Fi could be more than 10 times faster than current Wi-Fi, and white laser Li-Fi could be 10 to 100 times faster than LED based Li-Fi under development.

“The concept of white lasers first seems counterintuitive because the light from a typical laser contains exactly one color, a specific wavelength of the electromagnetic spectrum, rather than a broad-range of different wavelengths,” Ning explained. “White light is typically viewed as a complete mixture of all of the wavelengths of the visible spectrum.”

Sandia National Labs in 2011 produced high-quality white light from four large lasers. The researchers showed that the human eye is as comfortable with white light generated by diode lasers as with that produced by LEDs, inspiring others to advance the technology.

But “those independent lasers cannot be used for room lighting or in displays,” Ning said. “A single tiny piece of semiconductor material emitting laser light in all colors or in white is desired.”

Semiconductors, usually a solid chemical element or compound arranged into crystals, are widely used for computer chips or for light generation in telecommunication systems. They are used to make lasers and LEDs because they can emit light of a specific color when a voltage is applied to them. The most preferred light emitting material for semiconductors is indium gallium nitride, though other materials such as cadmium sulfide and cadmium selenide also are used for emitting visible colors.

The main challenge lies in the way light emitting semiconductor materials are grown and how they emit light of different colors. Typically a semiconductor emits light of a single color — blue, green or red — that is determined by a unique atomic structure and energy bandgap.

The “lattice constant” represents the distance between the atoms. To produce all possible wavelengths in the visible spectral range you need several semiconductors of different lattice constants and energy bandgaps.

“Our goal is to achieve a single semiconductor piece capable of laser operation in the three fundamental lasing colors. The piece should be small enough so that people can perceive only one overall mixed color instead of three individual colors,” said Fan. — Sharon Keeler, chief media officer, Ira A. Fulton Schools of Engineering. Read more about the research here.

From dental and bone implants to restoration of engine parts, additive manufacturing changing lives and industries

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Two weeks ago today, the sixth-annual Laser Additive Manufacturing Workshop opened in Houston with a riveting keynote by GE Aviation’s Todd Rockstroh. Setting an attendance record, LAM 2014 closed the following day with a lively Q&A session on the qualities and varieties of AM alloys for powder-bed and powder-fed applications.

It’s always illuminating to go back to your notes a few days after an intensive conference and re-examine the “takeaways.” Several things stood out at LAM 2014: the increased presence of key industry players; the importance of data management to refine additive processes; the energetic efforts around the world to fund AM research and expand AM capabilities; and the work and personnel yet required to transform more exploratory concepts into established, repeatable — and commercially successful — parts and products.

Continue reading →

Single Walled Nano Tube

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By: Wei Xiong

Scientists and engineers over the world have thought for years that the next generation of smaller, more-efficient electronic and photonic devices could be based on the use of carbon nanotubes (CNTs), structures 10,000 times thinner than a human hair but with tremendous potential.

Although significant progress has been made in the carbon nanotube synthesis with various methods such as arc discharge, laser ablation, and chemical vapor deposition (CVD), there is still a big gap between high efficient carbon nanotube synthesis and the fabrication of CNT-based devices. The problem has always been finding a way to precisely integrate carbon nanotubes with other nano-scale structures.

Previous efforts in this area by other research groups tried to use advanced instrumentation to manipulate carbon nanotubes after growth. However, that approach is only good for research purposes but not suitable for large-scale fabrication of CNT-based devices because it is time consuming and expensive.

In the presentation, we will demonstrate a pathway to a solution with little or no manipulation necessary. Our method involves the application of optical near-field effects in a laser-assisted CVD process, by which simultaneous growth of Single Walled Nano Tube (SWNT)-bridge arrays with precise location and orientation control can be achieved at a relatively low temperature. Our study shows that the localized thermal enhancement induced by optical near-field effects and an external electric field enabled the SWNT growth with precise control of growth sites and orientations. Furthermore, laser polarization also shows significant influence on the control of growth site for SWNTs.

Superior to the previous methods that manipulate carbon nanotubes one piece at a time with expensive instrumentation and tedious processes, the new method can make multiple self-aligned SWNT bridges in a one-step process. The laser-based growth process opens up a new route for controlled SWNT integration that can lead to applications in nano-devices such as biosensors, light emitters, photon sensors, molecular motors and memory cells.

Fig. 1 Illustration of laser-assisted chemical vapor deposition for self-aligned growth of a single-walled carbon nanotube bridge using optical near-field effects.

The above brief overview was extracted from its original abstract and paper presented at The International Congress on Applications of Lasers & Electro-Optics (ICALEO) in Orlando, FL. To order a copy of the complete proceedings from this conference click here

Characterization and Compositional Study of Fibre Laser Processed Engineering Ceramics

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By: P.P. Shukla and J. Lawrence

Wolfson School of Mechanical and Manufacturing Engineering, Loughborough University,
Leicestershire, LE11 3TU, United Kingdom

Ceramics have a wide usage in the engineering sector. Various applications in the aerospace and
automotive industry extensively use ceramics due to their desirable mechanical and thermal properties
in comparison to metal and metallic alloys. Commercial advantages offered by laser systems such as
high speeds, shorter processing times, accuracy, and deep penetrating treatment are attractive in
comparison with conventional processing techniques. Processing of engineering ceramics from
employing industrial lasers can be an asset to manufacturers by achieving reduced lead times,
production cost, faster delivery and achieving better tolerances. Laser processing can be a superior
technique applicable to engineering ceramics such as glazing for improving the surface finish,
aesthetics and enhancing the ceramics (component) functional life, providing that the effects of the
laser/ceramic material interaction are better understood.
Surface treatment by means of fibre laser radiation using various processing gas compositions was
conducted on cold isostatic pressed silicon nitride and zirconia engineering ceramics to observe
changes in their surface integrity, chemical composition and distribution of the heat affected zone. The
as received and fibre laser treated surface topography was measured in 3D using white-light
interferometry. Scanning electron microscopy was then used to observe the microstructural integrity
prior to and after the fibre laser treatment. Energy dispersive X-ray was employed thereafter to detect
the change in the chemical composition.
Laser surface treatment softened the treated zone specifically with silicon nitride and increased in
hardness within the interface and the respected untreated areas. The highest material removal for
silicon nitride ceramics was achieved using the oxygen gas. However, a change in the materials
topography resulted in the silicon nitride ceramic being much courser. Argon and nitrogen resulted in
producing the finest surface finish with a lower material removal in comparison with the sample treated
by oxygen. Although, nitrogen showed a significantly modified surface in comparison to the results of
other gases used. Investigation into thermal and microstructural analysis is further being undertaken for
both ceramics.
The surface topography of the fibre laser treated zirconia ceramics was considerably different to that of
the silicon nitride ceramics due to both materials comprising of different microstructure structures,
thermal properties and their effects during laser processing. The surface melting and solidification of
zirconia occurs at lower temperatures compared to silicon nitride ceramics. To create a sufficient
melted zone and to redistribute the surface; higher laser power is required which on the other hand, has
the tendency to produce cracks in to zirconia due to the thermal shocks induced. This was why zirconia
ceramic was processed using lower power opposed to silicon nitride for locally melting and
redistributing the melt zone. From this, the grain boundaries had began to connect and bind into each
other particularly using argon and oxygen. Using argon produced one of the finer surface finish and
lower material removal. It can be concluded that the fibre laser treatment using argon would be most
effective for surface treating and modification of zirconia based engineering ceramics

The above brief overview was extracted from its original abstract and paper presented at The International Congress on Applications of Lasers & Electro-Optics (ICALEO) in Orlando, FL. To order a copy of the complete proceedings from this conference click here

(Paper 204)