Showing posts with label Graphene. Show all posts
Showing posts with label Graphene. Show all posts

Monday, June 22, 2015

Penn State - Diode a few atoms thick shows surprising quantum effect

As publish by Penn State : A quantum mechanical transport phenomenon demonstrated for the first time in synthetic, atomically-thin layered material at room temperature could lead to novel nanoelectronic circuits and devices, according to researchers at Penn State and three other U.S. and international universities.


Atomic multilayer structure of van der Waals solids representing layering with a graphene substrate.



Current-voltage curves of single junction (green) van der Waals solid (no NDR) and multijunction (red, orange) van der Waals solids (NDR). Stacking and choice of materials determines the location and width of peak.

The quantum transport effect, called negative differential resistance (NDR), was observed when a voltage was applied to structures made of one-atom-thick layers of several layered materials known as van der Waals materials. The three-part structures consist of a base of graphene followed by atomic layers of either molybdenum disulfide (MoS2), molybdenum diselenide (MoSe2), or tungsten diselenide (WSe2).

NDR is a phenomenon in which the wave nature of electrons allows them to tunnel through any material with varying resistance. The potential of NDR lies in low voltage electronic circuits that could be operated at high frequency.

“Theory suggests that stacking two-dimensional layers of different materials one atop the other can lead to new materials with new phenomena,” said Joshua Robinson, a Penn State assistant professor of materials science and engineering whose student, Yu-Chuan Lin, is first author on a paper appearing online today, June 19, in the journal Nature Communications. The paper is titled “Atomically Thin Resonant Tunnel Diodes Built from Synthetic van der Waals Heterostructures.”

Achieving NDR in a resonant tunneling diode at room temperature requires nearly perfect interfaces, which are possible using direct growth techniques, in this case oxide vaporization of molybdenum oxide in the presence of sulfur vapor to make the MoS2 layer, and metal organic chemical vapor deposition to make the WSe2 and MoSe2.

Thursday, June 18, 2015

Graphene Benchmarked to TaN as Cu Diffusion Barrier for Ultimate Interconnect Scaling

Here is an interesting paper from VLSI2015 in Kyoto Japan (Symposia of VLSI Technology and Circuits ) from Stanford and Univ. of Wisconsin–Madison wrapping graphene around the Cu lines instead of tantalum nitride barriers for future scaled interconnects. It will be interesting to see more solid data once these become available. As you probably read this many times - graphene sucks as a future channel material for future CMOS since it does not have band gap and there are various attempts to solve this problem - we just have to use graphene for something and especially in semiconductor technology. However, in Cu interconnects you could´t care less about the lack of a bandgap and it would be sort of funny and maybe a bit unexpected if graphene were to be implemented in BEOL instead of FEOL.


H.-S. P. Wong, Professor of Electrical Engineering

“Graphene has been promised to benefit the electronics industry for a long time, and using it as a copper barrier is perhaps the first realization of this promise,” Wong said.

Check out this article in Stanford Engineering for more details and answers from the researchers: http://engineering.stanford.edu/news/stanford-engineers-find-simple-yet-clever-way-boost-chip-speeds


VLSI 2015 Abstract:

Cu Diffusion Barrier: Graphene Benchmarked to TaN for Ultimate Interconnect Scaling
L. Li*, X. Chen*, C.-H. Wang*, S. Lee*, J. Cao*, S. S. Roy**, M. S. Arnold** and H.-S. P. Wong*, *Stanford Univ. and **Univ. of Wisconsin–Madison, USA 

The advantages of graphene diffusion barrier are studied and benchmarked to the industry-standard barrier material TaN for the first time. Even when the wire width is scaled to 10 nm, the effective resistivity of the Cu interconnect is maintained near the intrinsic value of Cu using a 3 Å single layer graphene (SLG) barrier. In the time dependent dielectric breakdown (TDDB) test, 4 nm multi-layer graphene (MLG) gives 6.5X shorter mean time to fail (MTTF) than 4 nm TaN. However when the barrier thickness is reduced, 3 Å single-layer graphene (SLG) gives 3.3X longer MTTF than 2 nm TaN, showing that SLG has better scaling potential. The influences of graphene grain size and various transfer methods are presented for further improving the SLG barrier performance.

Tuesday, June 9, 2015

Graphene-based sensor capable of detecting cholera toxins for diagnosis of cancer

As recently reported by The Silicon Republic, researchers have developed a graphene-based sensor that is capable of detecting cholera toxins and providing earlier diagnosis of cancer and other diseases.

The sensor, known as a Surface Plasmon Resonance (SPR) sensor, is an established optical technique for medical diagnosis with high sensitivity and specificity, and can potentially be used for lab-on-a-chip sensors.

‘This type of sensing platform offers a large variety for medical diagnostics, since it can be adapted to almost any type of disease markers’ — Prof Georg Duesberg, at the AMBER labs at Trinity’s School of Chemistry [One of the guys behind the Infineon /Qimonda carbon / high-k DRAM Trench Capacitor technology http://www.hes.ei.tum.de/fileadmin/w00bjl/www/uploads/Aichmayr_VLSI07_talk06.pdf]

Noncovalently Functionalized Monolayer Graphene for Sensitivity Enhancement of Surface Plasmon Resonance Immunosensors 

 Meenakshi Singh †‡, Michael Holzinger, Maryam Tabrizian, Sinéad Winters, Nina C. Berner, Serge Cosnier, and Georg S. Duesberg

J. Am. Chem. Soc., 2015, 137 (8), pp 2800–2803 DOI: 10.1021/ja511512m



Abstract

A highly efficient surface plasmon resonance (SPR) immunosensor is described using a functionalized single graphene layer on a thin gold film. The aim of this approach was two-fold: first, to amplify the SPR signal by growing graphene through chemical vapor deposition and, second, to control the immobilization of biotinylated cholera toxin antigen on copper coordinated nitrilotriacetic acid (NTA) using graphene as an ultrathin layer. The NTA groups were attached to graphene via pyrene derivatives implying π–π interactions. With this setup, an immunosensor for the specific antibody anticholera toxin with a detection limit of 4 pg mL–1 was obtained. In parallel, NTA polypyrrole films of different thicknesses were electrogenerated on the gold sensing platform where the optimal electropolymerization conditions were determined. For this optimized polypyrrole-NTA setup, the simple presence of a graphene layer between the gold and polymer film led to a significant increase of the SPR signal.

Sunday, June 7, 2015

Nanodiamond ball bearings wrapped in graphene create a virtually frictionless surface

A method that reduces friction between two surfaces to almost zero on macroscopic scales has been demonstrated by US researchers. The phenomenon combines nanodiamonds with sheets of graphene, which curl around the nanodiamonds to form ‘nanoscrolls’ that lubricate the two surfaces. As friction wastes so much energy in all sorts of mechanical devices this discovery has huge potential to save both energy and money.

Formation of the graphene scroll around nanodiamond at 300 K with sliding velocity of 40m/s in x-direction in a dry environment. The movie demonstrate the dynamic evolution of graphene patches from flat flakes to scrolls around the nano diamonds.

Just looking at the movie you want to go in there and test out how the system would react on a couple of ALD cycles of different materials.


Friday, June 5, 2015

Chemically converted graphene: scalable chemistries to enable processing and fabrication (Open Access)

Here is a very good and comprehensive Open Access review from University of Wollongong, Australia (1, 2) of the chemistries for development of aqueous and organic solvent graphene dispersions. 

The Fabrication of  graphene dispersions or composites is also reviewed and those are:
  • printing (inkjet and extrusion) 
  • spinning methods (wet)
In addition, their use for the preparation of a variety of polymer composites, materials useful for the fabrication of graphene based structures and devices is also reviewed.

To conclude - a good starting point for anybody who want to get started with graphene fabrication and applied research in the lab!

Chemically converted graphene: scalable chemistries to enable processing and fabrication (Open Access)

Sanjeev Gambhir, Rouhollah Jalili, David L Officer and Gordon G Wallace
Citation: NPG Asia Materials (2015) 7, e186; doi:10.1038/am.2015.47
Published online 5 June 2015





Steps involved in forming graphene composites or devices.


Abstract: Graphene, a nanocarbon with exceptional physical and electronic properties, has the potential to be utilized in a myriad of applications and devices. However, this will only be achieved if scalable, processable forms of graphene are developed along with ways to fabricate these forms into material structures and devices. In this review, we provide a comprehensive overview of the chemistries suitable for the development of aqueous and organic solvent graphene dispersions and their use for the preparation of a variety of polymer composites, materials useful for the fabrication of graphene-containing structures and devices. Fabrication of the processable graphene dispersions or composites by printing (inkjet and extrusion) or spinning methods (wet) is reviewed. The preparation and fabrication of liquid crystalline graphene oxide dispersions whose unique rheologies allow the creation of graphene-containing structures by a wide range of industrially scalable fabrication techniques such as spinning (wet and dry), printing (ink-jet and extrusion) and coating (spray and electrospray) is also reviewed.

(1) The Materials Node, The Australian National Fabrication Facility, Intelligent Polymer Research Institute, AIIM Facility, Innovation Campus, University of Wollongong, Wollongong, NSW, Australia
(2) ARC Centre of Excellence for Electromaterials Science, Intelligent Polymer Research Institute, AIIM Facility, Innovation Campus, University of Wollongong, Wollongong, NSW, Australia

Wednesday, June 3, 2015

Graphene oxide monolayers as atomically thin seeding layers for ALD of metal oxides

A international team of researchers from imec, MIT, BTU Cottbus and Samsung Advanced Institute of Technology have explored graphene oxide as an atomically-thin transferable seed layer for the atomic layer deposition (ALD) of dielectric materials on any substrate of choice!

Graphene oxide monolayers as atomically thin seeding layers for atomic layer deposition of metal oxides 

Amirhasan Nourbakhsh, Christoph Adelmann, Yi Song, Chang Seung Lee, Inge Asselberghs, Cedric Huyghebaert, Simone Brizzi, Massimo Tallarida, Dieter Schmeißer, Sven Van Elshocht, Marc Heyns, Jing Kong, Tomás Palacios and Stefan De Gendt 

Nanoscale, 2015, Advance Article DOI: 10.1039/C5NR01128K 
Published online 03 Jun 2015



Graphene oxide (GO) was explored as an atomically-thin transferable seed layer for the atomic layer deposition (ALD) of dielectric materials on any substrate of choice. This approach does not require specific chemical groups on the target surface to initiate ALD. This establishes GO as a unique interface which enables the growth of dielectric materials on a wide range of substrate materials and opens up numerous prospects for applications. In this work, a mild oxygen plasma treatment was used to oxidize graphene monolayers with well-controlled and tunable density of epoxide functional groups. This was confirmed by synchrotron-radiation photoelectron spectroscopy. In addition, density functional theory calculations were carried out on representative epoxidized graphene monolayer models to correlate the capacitive properties of GO with its electronic structure. Capacitance–voltage measurements showed that the capacitive behavior of Al2O3/GO depends on the oxidation level of GO. Finally, GO was successfully used as an ALD seed layer for the deposition of Al2O3 on chemically inert single layer graphene, resulting in high performance top-gated field-effect transistors.

Tuesday, June 2, 2015

Roll-to-Roll CVD manufacturing of graphene

New manufacturing process could take exotic material out of the lab and into commercial products

That could finally change with a new process described this week in the journal Scientific Reports by researchers at MIT and the University of Michigan. MIT mechanical engineering Associate Professor A. John Hart, the paper’s senior author, says the new roll-to-roll manufacturing process described by his team addresses the fact that for many proposed applications of graphene and other 2-D materials to be practical, “you’re going to need to make acres of it, repeatedly and in a cost-effective manner.”


Diagram of the roll-to-roll process (a) shows the arrangement of copper spools at each end of the processing tube, and how a ribbon of thin copper substrate is wound around the central tube. Cross-section view of the same setup (b) shows the gap between two tubes, where the chemical vapor deposition process occurs. Photos of the system being tested show (c) the overall system, with an arrow indicating the direction the ribbon is moving; (d) a closeup of the copper ribbon inside the apparatus, showing the holes where chemical vapor is injected; and (e) an overhead view of the copper foil passing through the system (MIT News).


The new process is an adaptation of a chemical vapor deposition method already used at MIT and elsewhere to make graphene — using a small vacuum chamber into which a vapor containing carbon reacts on a horizontal substrate, such as a copper foil. The new system uses a similar vapor chemistry, but the chamber is in the form of two concentric tubes, one inside the other, and the substrate is a thin ribbon of copper that slides smoothly over the inner tube.

Gases flow into the tubes and are released through precisely placed holes, allowing for the substrate to be exposed to two mixtures of gases sequentially. The first region is called an annealing region, used to prepare the surface of the substrate; the second region is the growth zone, where the graphene is formed on the ribbon. The chamber is heated to approximately 1,000 degrees Celsius to perform the reaction.

The researchers have designed and built a lab-scale version of the system, and found that when the ribbon is moved through at a rate of 25 millimeters (1 inch) per minute, a very uniform, high-quality single layer of graphene is created. When rolled 20 times faster, it still produces a coating, but the graphene is of lower quality, with more defects.

Saturday, May 30, 2015

Hydrophobic graphene coating could make power plants more efficient

Product Design & Development reports that a team of researchers at MIT has developed a way of coating condenser surfaces with a layer of graphene, just one atom thick, and found that this can improve the rate of heat transfer by a factor of four — and potentially even more than that, with further work. And unlike polymer coatings, the graphene coatings have proven to be highly durable in laboratory tests.


An uncoated copper condenser tube (top left) is shown next to a similar tube coated with graphene (top right). When exposed to water vapor at 100 degrees Celsius, the uncoated tube produces an inefficient water film (bottom left), while the coated shows the more desirable dropwise condensation (bottom right). Picture from www.pddnet.com - Courtesy of the researchers

The findings are reported in the journal Nano Letters by MIT graduate student Daniel Preston, professors Evelyn Wang and Jing Kong, and two others. The improvement in condenser heat transfer, which is just one step in the power-production cycle, could lead to an overall improvement in power plant efficiency of 2 to 3 percent based on figures from the Electric Power Research Institute, Preston says — enough to make a significant dent in global carbon emissions, since such plants represent the vast majority of the world’s electricity generation. “That translates into millions of dollars per power plant per year,” he explains.

Extremely thin hydrophobic coating is also obviously an open field for clever ALD solutions. Here is a recent report on conventional hydrophobic coating technologies from Vanderbilt University taking a    closer look at the US market.





The history of hydrophobic coating technologies

Saturday, May 16, 2015

Atomic Layer Deposition of Al2O3 on NF3-pre-treated graphene

Another great publication from ALD Lab Dresden, TU Dresden, Germany, and Marcel Junige and their and scientists at Linköping University of Technology, Sweden, using high resolution in-situ ellipsometer. This time these guys have grown Al2O3 on Graphene, which is very difficult unless you activate the inert grapheme surface. Marcel did this by a NF3 pre-treatment. The work was presented at SPIE micro technologies 2015 in Barcelona.

Atomic Layer Deposition of Al2O3 on NF3-pre-treated graphene

Marcel Junige, Tim Oddoy, Rositsa Yakimova, Vanya Darakchieva, Christian Wenger, Grzegorz Lupina, Matthias Albert, Johann W. Bartha
Conference: SPIE microtechnologies 2015 : Nanotechnology VII, At Barcelona, Spain, Volume: 9519



Optical Al2O3 layer thickness in progression over the ALD process time as observed by in-situ real-time Spectroscopic Ellipsometry, comparing the ALD of Al2O3 starting on a 100 nm thermally grown SiO2 reference versus an exfoliated graphene monolayer after 180 s NF3-pre-treatment.

Graphene has been considered for a variety of applications including novel nanoelectronics device concepts such as the recently reported Graphene Base Transistor (GBT). However, the deposition of ultra-thin films on top of graphene is still challenging: On the one hand, the deposition process must not damage or alter the pristine graphene monolayer; on the other hand, the finally deposited films have to provide appropriate functional properties regarding a specific application. In case of the GBT, a dielectric coating is desired which is both pin-hole free to prevent any short circuits and still thin enough (around 3-5 nm) to enable hot electron tunneling. Hence, the dielectric film closure on graphene needs to occur at an early stage of the deposition process. Atomic Layer Deposition (ALD) has been established as a physicochemical coating technique with excellent thickness control as well as unique conformality over complex three-dimensional-shaped substrates for the last decade. Especially the ALD of oxides has been extensively researched. Accordingly, an ALD process for Al2O3 yet exists that alternates the exposure of trimethylaluminum (TMA) and water (H2O) as the organometallic precursor and co-reactant of two corresponding self-terminating surface reactions, respectively. However, the ALD of Al2O3 has been reported to barely initiate on pristine graphene due to graphene’s lack of dangling bonds. A fluorine functionalization, using XeF2, has been found to provide additional nucleation sites resulting in conformal films without pinholes. Based on this literature finding, we studied the impact of pre-treatments by nitrogen trifluoride (NF3) on exfoliated as well as epitaxial graphene monolayers prior to the ALD of Al2O3. All experiments were conducted in vacuo; i. e. the pristine graphene samples were exposed to NF3 for 180 s in the same reactor immediately before applying 30 ALD cycles and the samples were transferred between the reactor and a surface analysis unit under high vacuum conditions. The ALD growth initiation was observed by in-situ real-time Spectroscopic Ellipsometry (irtSE) with a sampling rate of 1 Hz. The chemical surface composition before and after the ALD as well as the presence of graphene after the coating procedure were revealed by in-vacuo X-ray Photoelectron Spectroscopy (XPS). The morphology of the films was determined by Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM). The defect status was examined by Raman Spectroscopy before and after the coating procedure. Atomic Layer Deposition of Al2O3 on NF3-pre-treated graphene. Available from: https://www.researchgate.net/publication/276242739_Atomic_Layer_Deposition_of_Al2O3_on_NF3-pre-treated_graphene [accessed May 16, 2015].

Friday, May 15, 2015

Oak Ridge demonstrates first large-scale graphene fabrication

OAK RIDGE, Tenn., May 14, 2015 -- One of the barriers to using graphene at a commercial scale could be overcome using a method demonstrated by researchers at the Department of Energy's Oak Ridge National Laboratory.




ORNL's ultrastrong graphene features layers of graphene and polymers and is an effective conductor of electricity.

Now, using chemical vapor deposition, a team led by ORNL's Ivan Vlassiouk has fabricated polymer composites containing 2-inch-by-2-inch sheets of the one-atom thick hexagonally arranged carbon atoms.

The findings, reported in the journal Applied Materials & Interfaces, could help usher in a new era in flexible electronics and change the way this reinforcing material is viewed and ultimately used.

"Before our work, superb mechanical properties of graphene were shown at a micro scale," said Vlassiouk, a member of ORNL's Energy and Transportation Science Division. "We have extended this to a larger scale, which considerably extends the potential applications and market for grapheme."

Full Story here.

Friday, May 8, 2015

MIT Engineers Repair Graphene Water Filters by ALD

Here is an interesting story from Engineering.com on "How Engineers Repaired One-Atom Thick Graphene Filters"

(Image courtesy MIT News)

Graphene seems to be the next big thing in water filtration as scientists look to create ultrathin membranes to filter out contaminants. Only problem is, defects in the making of one-atom thick membranes are a common occurrence, causing leaks.


In a two-step process, engineers have successfully sealed leaks in graphene. First, the team fabricated graphene on a copper surface (top left) — a process that can create intrinsic defects in graphene, shown as cracks on the surface. After lifting the graphene and depositing it on a porous surface (top right), the transfer creates further holes and tears. In a first step (bottom left), the team used atomic layer deposition to deposit hafnium (in gray) to seal intrinsic cracks, then plugged the remaining holes (bottom left) with nylon (in red), via interfacial polymerization. (MIT News)

Hope is not lost though as engineers have found a way to repair the cracks and holes, filling them with a combination of chemical deposition and polymerization techniques.

The first of the two techniques used, addresses the smaller intrinsic defects. Using a process called “atomic layer disposition,” the team placed the graphene membrane in a vacuum chamber, pulsing in a hafnium containing chemical that normal does not interact with graphene.

In this scenario the chemical sticks to openings in the graphene, attracted to the area’s higher surface energy.

After several rounds of applied atomic layer deposition, the hafnium oxide successfully filled in the graphene’s nanometer-scale intrinsic defects.

This solution quickly unveiled a new issue, as the team realized it would require too much time to fill in the membranes larger defects. 
 

Monday, May 4, 2015

2D Molybdenum disulfide encapsulated between layers of boron nitride

Beautiful work of 2D material stacks for future electronics - layered stacks of molybdenum disulfide (MoS2) encapsulated in boron nitride (BN), with graphene overlapping the edge of the MoS2 to act as electrical contacts as Published by : Holly Evarts, "Two-Dimensional Semiconductor Comes Clean", Apr. 27, 2015 and in Nature Nanotechnology below.
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In 2013 James Hone, Wang Fong-Jen Professor of Mechanical Engineering at Columbia Engineering, and colleagues at Columbia demonstrated that they could dramatically improve the performance of graphene—highly conducting two-dimensional (2D) carbon—by encapsulating it in boron nitride (BN), an insulating material with a similar layered structure. In work published this week in the Advance Online Publication on Nature Nanotechnology’s website, researchers at Columbia Engineering, Harvard, Cornell, University of Minnesota, Yonsei University in Korea, Danish Technical University, and the Japanese National Institute of Materials Science have shown that the performance of another 2D material—molybdenum disulfide (MoS2)—can be similarly improved by BN-encapsulation.


Two-dimensional semiconductor comes clean 

Schematic cross-section view of atomic layer of molybdenum disulfide contacted by graphene, and encapsulated between layers of insulating hexagonal boron nitride. Credit: Gwan-Hyoung Lee/Columbia Engineering

Read more at: http://phys.org/news/2015-04-two-dimensional-semiconductor.html#jCp
Molybdenum disulfide encapsulated between layers of boron nitride (Image courtesy of Gwan-Hyoung Lee/Yonsei University).
Schematic cross-section view of atomic layer of molybdenum disulfide contacted by graphene, and encapsulated between layers of insulating hexagonal boron nitride. Credit: Gwan-Hyoung Lee/Columbia Engineering

Read more at: http://phys.org/news/2015-04-two-dimensional-semiconductor.html#jCp
Schematic cross-section view of atomic layer of molybdenum disulfide contacted by graphene, and encapsulated between layers of insulating hexagonal boron nitride. Credit: Gwan-Hyoung Lee/Columbia Engineering

Read more at: http://phys.org/news/2015-04-two-dimensional-semiconductor.html#jCp
Schematic cross-section view of atomic layer of molybdenum disulfide contacted by graphene, and encapsulated between layers of insulating hexagonal boron nitride. Credit: Gwan-Hyoung Lee/Columbia Engineering

Read more at: http://phys.org/news/2015-04-two-dimensional-semiconductor.html#jCp

Schematic cross-section view of atomic layer of molybdenum disulfide contacted by graphene, and encapsulated between layers of insulating hexagonal boron nitride. Credit: Gwan-Hyoung Lee/Columbia Engineering

“These findings provide a demonstration of how to study all 2D materials,” says Hone, leader of this new study and director of Columbia’s NSF-funded Materials Research Science and Engineering Center. “Our combination of BN and graphene electrodes is like a ‘socket’ into which we can place many other materials and study them in an extremely clean environment to understand their true properties and potential. This holds great promise for a broad range of applications including high-performance electronics, detection and emission of light, and chemical/bio-sensing.”

Two-dimensional (2D) materials created by “peeling’” atomically thin layers from bulk crystals are extremely stretchable, optically transparent, and can be combined with each other and with conventional electronics in entirely new ways. But these materials—in which all atoms are at the surface—are by their nature extremely sensitive to their environment, and their performance often falls far short of theoretical limits due to contamination and trapped charges in surrounding insulating layers. The BN-encapsulated graphene that Hone’s group produced last year has 50× improved electronic mobility—an important measure of electronic performance—and lower disorder that enables the study of rich new phenomena at low temperature and high magnetic fields.

“We wanted to see what we could do with MoS2—it’s the best-studied 2D semiconductor, and, unlike graphene, it can form a transistor that can be switched fully ‘off’, a property crucial for digital circuits,” notes Gwan-Hyoung Lee, co-lead author on the paper and assistant professor of materials science at Yonsei. In the past, MoS2 devices made on common insulating substrates such as silicon dioxide have shown mobility that falls below theoretical predictions, varies from sample to sample, and remains low upon cooling to low temperatures, all indications of a disordered material. Researchers have not known whether the disorder was due to the substrate, as in the case of graphene, or due to imperfections in the material itself.

In the new work, Hone’s team created heterostructures, or layered stacks, of MoS2 encapsulated in BN, with small flakes of graphene overlapping the edge of the MoS2 to act as electrical contacts. They found that the room-temperature mobility was improved by a factor of about 2, approaching the intrinsic limit. Upon cooling to low temperature, the mobility increased dramatically, reaching values 5-50× that those measured previously (depending on the number of atomic layers). As a further sign of low disorder, these high-mobility samples also showed strong oscillations in resistance with magnetic field, which had not been previously seen in any 2D semiconductor.

“This new device structure enables us to study quantum transport behavior in this material at low temperature for the first time,” added Columbia Engineering PhD student Xu Cui, the first author of the paper.

By analyzing the low-temperature resistance and quantum oscillations, the team was able to conclude that the main source of disorder remains contamination at the interfaces, indicating that further improvements are possible.

“This work motivates us to further improve our device assembly techniques, since we have not yet reached the intrinsic limit for this material,” Hone says. “With further progress, we hope to establish 2D semiconductors as a new family of electronic materials that rival the performance of conventional semiconductor heterostructures—but are created using scotch tape on a lab-bench instead of expensive high-vacuum systems.”

Multi-terminal transport measurements of MoS2 using a van der Waals heterostructure device platform
Xu Cui, Gwan-Hyoung Lee, Young Duck Kim, Ghidewon Arefe, Pinshane Y. Huang, Chul-Ho Lee, Daniel A. Chenet, Xian Zhang, Lei Wang, Fan Ye, Filippo Pizzocchero, Bjarke S. Jessen, Kenji Watanabe, Takashi Taniguchi, David A. Muller, Tony Low, Philip Kim & James Hone
Nature Nanotechnology(2015)doi:10.1038/nnano.2015.70
Schematic cross-section view of atomic layer of molybdenum disulfide contacted by graphene, and encapsulated between layers of insulating hexagonal boron nitride. Credit: Gwan-Hyoung Lee/Columbia Engineering

Read more at: http://phys.org/news/2015-04-two-dimensional-semiconductor.html#jCp
Schematic cross-section view of atomic layer of molybdenum disulfide contacted by graphene, and encapsulated between layers of insulating hexagonal boron nitride. Credit: Gwan-Hyoung Lee/Columbia Engineering

Read more at: http://phys.org/news/2015-04-two-dimensional-semiconductor.html#jCp
Schematic cross-section view of atomic layer of molybdenum disulfide contacted by graphene, and encapsulated between layers of insulating hexagonal boron nitride. Credit: Gwan-Hyoung Lee/Columbia Engineering

Read more at: http://phys.org/news/2015-04-two-dimensional-semiconductor.html#jCp
Schematic cross-section view of atomic layer of molybdenum disulfide contacted by graphene, and encapsulated between layers of insulating hexagonal boron nitride. Credit: Gwan-Hyoung Lee/Columbia Engineering

Read more at: http://phys.org/news/2015-04-two-dimensional-semiconductor.html#jCp
Schematic cross-section view of atomic layer of molybdenum disulfide contacted by graphene, and encapsulated between layers of insulating hexagonal boron nitride. Credit: Gwan-Hyoung Lee/Columbia Engineering

Read more at: http://phys.org/news/2015-04-two-dimensional-semiconductor.html#jCp



Figure 1c: Cross-sectional STEM image of the fabricated device. The zoom-in false-colour image clearly shows the ultra-sharp interfaces between different layers (graphene, 5L; MoS2, 3L;top hBN, 8nm; bottom hBN, 19 nm) [Figure and Abstract used with permission from Nature Publishing Group under License Number 3621820766388]

Atomically thin two-dimensional semiconductors such as MoS2 hold great promise for electrical, optical and mechanical devices and display novel physical phenomena. However, the electron mobility of mono- and few-layer MoS2 has so far been substantially below theoretically predicted limits, which has hampered efforts to observe its intrinsic quantum transport behaviours. Potential sources of disorder and scattering include defects such as sulphur vacancies in the MoS2 itself as well as extrinsic sources such as charged impurities and remote optical phonons from oxide dielectrics. To reduce extrinsic scattering, we have developed here a van der Waals heterostructure device platform where MoS2 layers are fully encapsulated within hexagonal boron nitride and electrically contacted in a multi-terminal geometry using gate-tunable graphene electrodes. Magneto-transport measurements show dramatic improvements in performance, including a record-high Hall mobility reaching 34,000 cm2 V–1 s–1 for six-layer MoS2 at low temperature, confirming that low-temperature performance in previous studies was limited by extrinsic interfacial impurities rather than bulk defects in the MoS2. We also observed Shubnikov–de Haas oscillations in high-mobility monolayer and few-layer MoS2. Modelling of potential scattering sources and quantum lifetime analysis indicate that a combination of short-range and long-range interfacial scattering limits the low-temperature mobility of MoS2.

Friday, May 1, 2015

The world smallest crak created by UCSD

Interesting work on making the smallest possible crack using graphene or so called nano gaps. In this case the use of single-layer graphene is used as a template for the formation of subnanometer plasmonic gaps using a scalable fabrication process called “nanoskiving.” The research was carried out by the University of California, San Diego (UCSD) and has been published in the journal Nano Letters.


Athermally photoreduced graphene oxides for three-dimensional holographic images
Aliaksandr V. Zaretski , Brandon C. Marin , Herad Moetazedi , Tyler J. Dill , Liban Jibril , Casey Kong , Andrea R. Tao , and Darren J. Lipomi
Nano Lett., 2015, 15 (1), pp 635–640, DOI: 10.1021/nl504121w

Abstract Image

This work demonstrates the use of single-layer graphene as a template for the formation of subnanometer plasmonic gaps using a scalable fabrication process called “nanoskiving.” These gaps are formed between parallel gold nanowires in a process that first produces three-layer thin films with the architecture gold/single-layer graphene/gold, and then sections the composite films with an ultramicrotome. The structures produced can be treated as two gold nanowires separated along their entire lengths by an atomically thin graphene nanoribbon. Oxygen plasma etches the sandwiched graphene to a finite depth; this action produces a subnanometer gap near the top surface of the junction between the wires that is capable of supporting highly confined optical fields. The confinement of light is confirmed by surface-enhanced Raman spectroscopy measurements, which indicate that the enhancement of the electric field arises from the junction between the gold nanowires. These experiments demonstrate nanoskiving as a unique and easy-to-implement fabrication technique that is capable of forming subnanometer plasmonic gaps between parallel metallic nanostructures over long, macroscopic distances. These structures could be valuable for fundamental investigations as well as applications in plasmonics and molecular electronics.



Figure text

Saturday, April 25, 2015

Chinese and US researchers dope & un-dope graphene FETs by ALD

Despite the tremendous world wide focus on the wonder material graphene, its pristine form can't be used in field-effect transistors (FETs) to replace current channel materials (Si, SiGe, III/V) between the source and drain suffer from the absence of a bandgap.

Here reseraches are seeking  to chemically modify or dope grapheneto open up a  band gap in the material. However, the carbon atoms in graphene are arranged in a two-dimensional sp2 hybridization surface, which makes it almost impossible to induce any chemical modification or doping without alteration of its idealized properties.

Finally, in order to form a super fast CMOS logic based on ultra fast graphene FETs (GFET) you need to be able to dope the GFETs in to NMOS and PMOS transistores and it has been proven very difficult to produce a stable n-type graphene transistors than its p-type counterpart.

A team of Chinese and US researchers [1, 2, 3, 4] have developed a simple method to produce n-type doping of graphene by using an ALD chamber. That is not all - the mechanism is reversible, meaning they can bring back graphene to p-type by a thermal anneal step. The main mechanism of n-type doping is driven by a surface charge transfer at graphene/redox interfaces during the ALD processing of Al2O3. Fantastic - Check out the details in the publication below!


Reversible n-Type Doping of Graphene by H2O-Based Atomic-Layer Deposition and Its Doping Mechanism
Li Zheng, Xinhong Cheng, Zhongjian Wang, Chao Xia, Duo Cao, Lingyan Shen, Qian Wang, Yuehui Yu, and Dashen Shen
J. Phys. Chem. C, 2015, 119 (11), pp 5995–6000
DOI: 10.1021/jp511562t





The pre-H2O treatment and Al2O3 film growth under a two-temperature-regime mode in an oxygen-deficient atomic layer deposition (ALD) chamber can induce n-type doping of graphene, with the enhancement of electron mobility and no defect introduction to graphene. The main mechanism of n-type doping is surface charge transfer at graphene/redox interfaces during the ALD procedure. More interestingly, this n-type doping of graphene is reversible and can be recovered by thermal annealing, similar to hydrogenated graphene (graphane). This technique utilizing pre-H2O treatment and an encapsulated layer of Al2O3 achieved in an oxygen-deficient ALD chamber provides a simple and novel route to fabricate n-type doping of graphene. (From grapfical abstract J. Phys. Chem. C, 2015, 119 (11), pp 5995–6000)

[1] State Key Laboratory of Functional Materials for Informatics
[2] Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, Shanghai 200050, China
[3] University of Chinese Academy of Sciences, Beijing 100049, China
[4] University of Alabama in Huntsville, Huntsville, Alabama 35899, United States
 

Sunday, April 5, 2015

ALD opens up for all-spin logic architectures


Researchers from France (CNRS/Thales), UK (University of Cambridge) and South Korea (University of Suwon) report on integration of low-cost, conformal, and versatile ALD Al2O3 dielectric in Ni–Al2O3–Co magnetic tunnel junctions (MTJs). According to the publication (below) in ACS Nano the spin-filtering effect of graphene is enhanced and shows the potential of ALD for spintronics with conformal, layer-by-layer control of tunnel barriers in magnetic tunnel junctions toward low-cost fabrication and down-scaling of tunnel resistances.

The research unveil the potential of ALD tunnel barriers for spintronics and MJTs. They conlude that "ALD has a high potential and may open new avenues for the development of scaled-up spin circuits, such as MRAMs and envisioned all-spin logic architectures, offering closer integration with conventional processes of the microelectronics industry."
The full version of the article below is also made available here by Department of Engineering, University of Cambridge.

Sub-nanometer Atomic Layer Deposition for Spintronics in Magnetic Tunnel Junctions Based on Graphene Spin-Filtering Membranes
Marie-Blandine Martin †, Bruno Dlubak †‡, Robert S. Weatherup ‡, Heejun Yang †§, Cyrile Deranlot , Karim Bouzehouane , Frédéric Petroff , Abdelmadjid Anane , Stephan Hofmann , John Robertson , Albert Fert , and Pierre Seneor *

† Unité Mixte de Physique CNRS/Thales, 91767 Palaiseau, France and University of Paris-Sud, 91405 Orsay, France
‡ Department of Engineering, University of Cambridge, Cambridge CB21PZ, United Kingdom
§ IBS Center for Integrated Nanostructure Physics (CINAP), Institute of Basic Science, Sungkyunkwan University, Suwon 440-746, South Korea
∥ Department of Energy Science, Sungkyunkwan University, Suwon 440-746, South Korea
ACS Nano, 2014, 8 (8), pp 7890–7895
 
 
Abstract Image
 
We report on the successful integration of low-cost, conformal, and versatile atomic layer deposited (ALD) dielectric in Ni–Al2O3–Co magnetic tunnel junctions (MTJs) where the Ni is coated with a spin-filtering graphene membrane. The ALD tunnel barriers, as thin as 0.6 nm, are grown layer-by-layer in a simple, low-vacuum, ozone-based process, which yields high-quality electron-transport barriers as revealed by tunneling characterization. Even under these relaxed conditions, including air exposure of the interfaces, a significant tunnel magnetoresistance is measured highlighting the robustness of the process. The spin-filtering effect of graphene is enhanced, leading to an almost fully inversed spin polarization for the Ni electrode of −42%. This unlocks the potential of ALD for spintronics with conformal, layer-by-layer control of tunnel barriers in magnetic tunnel junctions toward low-cost fabrication and down-scaling of tunnel resistances.