Imperfections may improve graphene sensors

Tuesday, November 29, 2011

Although they found that graphene makes very good chemical sensors, researchers at the University of Illinois at Urbana-Champaign have discovered an unexpected "twist"?that the sensors are better when the graphene is "worse"?more imperfections improved performance.

"This is quite the opposite of what you would want for transistors, for example," explained Eric Pop, an assistant professor of electrical and computer engineering and a member of the interdisciplinary research team. "Finding that the less perfect they were, the better they worked, was counter intuitive at first."

The research group, which includes researchers from both chemical engineering and electrical engineering, and from a startup company, Dioxide Materials, reported their results in the November 23, 2011 issue of Advanced Materials.

"The objective of this work was to understand what limits the sensitivity of simple, two-terminal graphene chemiresistors, and to study this in the context of inexpensive devices easily manufactured by chemical vapor deposition (CVD)," stated lead authors Amin Salehi-Khojin and David Estrada.

The researchers found that the response of graphene chemiresistors depends on the types and geometry of their defects.

"Nearly-pristine graphene chemiresistors are less sensitive to analyte molecules because adsorbates bind to point defects, which have low resistance pathways around them," noted Salehi-Khojin, a research scientist at Dioxide Materials and post-doctoral research associate in the Department of Chemical and Biomolecular Engineering (ChemE) at Illinois. "As a result, adsorption at point defects only has a small effect on the overall resistance of the device. On the other hand, micrometer-sized line defects or continuous lines of point defects are different because no easy conduction paths exist around such defects, so the resistance change after adsorption is significant."

"This can lead to better and cheaper gas sensors for a variety of applications such as energy, homeland security and medical diagnostics" said Estrada who is a doctoral candidate in the Department of Electrical and Computer Engineering.

According to the authors, the two-dimensional nature of defective, CVD-grown graphene chemiresistors causes them to behave differently than carbon nanotube chemiresistors. This sensitivity is further improved by cutting the graphene into ribbons of width comparable to the line defect dimensions, or micrometers in this study.

"What we determined is that the gases we were sensing tend to bind to the defects," Pop said. "Surface defects in graphene are either point-, wrinkle-, or line-like. We found that the points do not matter very much and the lines are most likely where the sensing happens."

"The graphene ribbons with line defects appear to offer superior performance as graphene sensors," said ChemE professor emeritus and Dioxide Materials CEO Richard Masel. "Going forward, we think we may be able engineer the line defects to maximize the material's sensitivity. This novel approach should allow us to produce inexpensive and sensitive chemical sensors with the performance better than that of carbon nanotube sensors."

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University of Illinois College of Engineering: http://engineering.illinois.edu/

Thanks to University of Illinois College of Engineering for this article.

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Source: http://www.labspaces.net/115544/Imperfections_may_improve_graphene_sensors

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Group files war-crimes complaint against Calderon (AP)

MEXICO CITY ? Mexican activists lodged a war-crimes complaint Friday against President Felipe Calderon at the International Criminal Court, claiming his offensive against drug cartels has involved about 470 cases of human rights violations by the army or police.

Netzai Sandoval, a lawyer for the coalition behind the complaint, said Mexican drug lords have also committed crimes against humanity during the conflict, which has cost 35,000 to 40,000 lives since late 2006.

The complaint filed Friday at the court in the Netherlands also names Sinaloa cartel leader Joaquin "El Chapo" Guzman.

Calderon's administration has denied the accusations, saying it's an elected, democratic government fighting criminals and has established mechanisms to protect human rights.

On Friday, Mexico's Interior Department issued a statement saying "the public safety policy that has been implemented by no means constitutes an international crime." It said the government's actions "are aimed at stopping criminal organizations and protecting all citizens."

"The Mexican government is not at war, and there is no generalized or systematic attack against civilians, nor any government policy in that direction," the statement said.

Source: http://us.rd.yahoo.com/dailynews/rss/mexico/*http%3A//news.yahoo.com/s/ap/20111126/ap_on_re_la_am_ca/lt_mexico_international_court

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Grossman rallies Washington past Seattle 23-17

SEATTLE -- Rex Grossman found Anthony Armstrong for a 50-yard touchdown and the Washington Redskins rallied for 16 fourth-quarter points and stunned the Seattle Seahawks 23-17 on Sunday.

One play after committing a grounding penalty, Grossman stepped up in the pocket on third-and-19 and found Armstrong, who out maneuvered Seattle cornerback Brandon Browner for the pass in the corner of the end zone.

The touchdown pass was Grossman's second of the game and followed rookie Roy Helu's leaping 28-yard TD run that pulled the Redskins within 17-14.

Grossman finished 26 of 35 for 314 yards as Washington (4-7) snapped its six-game losing streak.

Marshawn Lynch rushed for 111 yards and caught a 20-yard touchdown pass for the Seahawks (4-7).

Source: http://www.miamiherald.com/2011/11/27/2521072/grossman-rallies-washington-past.html

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2 key senators call for tough line with Pakistan

A Pakistani boy, bottom, shouts slogans along with other protestors during a rally to condemn NATO helicopters attacks on Pakistani troops, on the outskirts of Islamabad, Pakistan, Sunday, Nov. 27, 2011. Pakistan on Saturday accused NATO helicopters and fighter jets of firing on two army checkpoints in the country's northwest and killing 24 soldiers. Islamabad retaliated by closing the border crossings used by the international coalition to supply its troops in neighboring Afghanistan. (AP Photo/Muhammed Muheisen)

A Pakistani boy, bottom, shouts slogans along with other protestors during a rally to condemn NATO helicopters attacks on Pakistani troops, on the outskirts of Islamabad, Pakistan, Sunday, Nov. 27, 2011. Pakistan on Saturday accused NATO helicopters and fighter jets of firing on two army checkpoints in the country's northwest and killing 24 soldiers. Islamabad retaliated by closing the border crossings used by the international coalition to supply its troops in neighboring Afghanistan. (AP Photo/Muhammed Muheisen)

Pakistani protesters burn representation of U. S. flag to condemn NATO helicopters attacks on Pakistani troops, in Multan, Pakistan on Sunday, Nov 27, 2011. Pakistan on Saturday accused NATO helicopters and fighter jets of firing on two army checkpoints in the country's northwest and killing 24 soldiers. Islamabad retaliated by closing the border crossings used by the international coalition to supply its troops in neighboring Afghanistan. (AP Photo/Khalid Tanveer)

Afghanistan-bound containers carrying supplies for NATO forces parked as authorities close Chaman border in Pakistan on Sunday, Nov 27, 2011. Pakistan on Saturday accused NATO helicopters and fighter jets of firing on two army checkpoints in the country's northwest and killing 24 soldiers. Islamabad retaliated by closing the border crossings used by the international coalition to supply its troops in neighboring Afghanistan. (AP Photo/Arshad Butt)

Afghanistan-bound trucks carrying supplies for NATO forces parked as authorities close border at Torkham border in Pakistan on Sunday, Nov 27, 2011. Pakistan on Saturday accused NATO helicopters and fighter jets of firing on two army checkpoints in the country's northwest and killing 24 soldiers. Islamabad retaliated by closing the border crossings used by the international coalition to supply its troops in neighboring Afghanistan. (AP Photo/Qazi Rauf)

Afghanistan-bound containers carrying supplies for NATO forces parked as authorities close Chaman border in Pakistan on Sunday, Nov 27, 2011. Pakistan on Saturday accused NATO helicopters and fighter jets of firing on two army checkpoints in the country's northwest and killing 24 soldiers. Islamabad retaliated by closing the border crossings used by the international coalition to supply its troops in neighboring Afghanistan. (AP Photo/Shah Khalid)

WASHINGTON (AP) ? Senior lawmakers suggested Sunday that the U.S. take a harder line with Pakistan, after Islamabad retaliated for NATO's deadly misfire by closing parts of its border with Afghanistan and demanding the U.S. vacate a drone base.

The comments by Sens. Jon Kyl, an Arizona Republican, and Dick Durbin, an Illinois Democrat, show how strained Pakistan's relationship with the U.S., and Congress specifically, has become in recent months. Lawmakers approve billions of dollars in military and civilian aid for Pakistan with the expectation that its government will help target al-Qaida operatives and push Afghan militants toward peace talks.

"There's a lot of diplomacy that has to occur and it has to be tough diplomacy in the sense that they need to understand that our support for them financially is dependent upon their cooperation with us," said Kyl, the Senate's No. 2 Republican.

Durbin, the second-ranking Senate Democrat, said Pakistan's latest move is further evidence that the U.S. must end its military involvement in the region and bring troops home.

"As difficult as it is to fight our way thru this diplomatic morass between the incompetence and maybe corruption of Afghanistan and the complicity in parts of Pakistan, our soldiers are caught right in the middle of this at a time they are trying to bring peace to the region," Durbin said.

NATO says it is investigating its likely involvement in Saturday's attack, which killed 24 Pakistani troops along the Afghan border. Afghan officials say their soldiers called for help after being fired upon from the direction of Pakistani border posts.

Outraged by the attacks and claiming they were unprovoked, Islamabad swiftly closed its border to trucks delivering supplies to coalition troops in Afghanistan and demanded the U.S. vacate within 15 days a base used by American drones.

The blockade is guaranteed to frustrate Congress, already incensed that Pakistan never tipped off the U.S. to Osama bin Laden's hideout within its borders.

While calling for tougher diplomacy with Pakistan, Kyl said he would stop short of cutting off U.S. aid entirely to Pakistan. He said that severing ties in the past has only led to an increased influence of Islamic extremists among Pakistan's military ranks.

"It's very important to maintain the relationship for the long haul," he said, without offering more specifics on how that might be done.

Durbin suggested the U.S. back out from the region from a military standpoint.

"We've got to leave it to Afghan forces," he said.

Kyl and Durbin spoke on "Fox News Sunday."

Associated Press

Source: http://hosted2.ap.org/APDEFAULT/386c25518f464186bf7a2ac026580ce7/Article_2011-11-27-US-Pakistan/id-1b438ba84ce741ebb50588c5cd299a83

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