Wagon-wheel pasta shape for better LED lights

<halloween tree lightsp> — One problem in developing more efficient organic LED light bulbs and displays for TVs and phones is that much of the light is polarized in one direction and thus trapped within the light-emitting diode, or LED. University of Utah physicists believe they have solved the problem by creating a new organic molecule that is shaped like rotelle -- wagon-wheel pasta -- rather than spaghetti.

The rotelle-shaped molecule -- known as a "pi-conjugated spoked-wheel macrocycle" -- acts the opposite of polarizing sunglasses, which screen out glare reflected off water and other surfaces and allow only direct sunlight to enter the eyes.

The new study showed wagon-wheel molecules emit light randomly in all directions -- a necessary feature for a more efficient OLED, or organic LED. Existing OLEDs now in some smart phones and TVs use spaghetti-shaped polymers -- chains of repeating molecular units -- that emit only polarized light.

"This work shows it is possible to scramble the polarization of light from OLEDs and thereby build displays where light doesn't get trapped inside the OLED," says University of Utah physicist John Lupton, lead author of a study of the spoked-wheel-shaped molecules published online Sunday, Sept. 29 in the journal Nature Chemistry.

"We made a molecule that is perfectly symmetrical, and that makes the light it generates perfectly random," he adds. "It can generate light more efficiently because it is scrambling the polarization. That holds promise for future OLEDs that would use less electricity and thus increase battery life for phones, and for OLED light bulbs that are more efficient and cheaper to operate."

Lupton emphasizes the study is basic science, and new OLEDs based on the rotelle-shaped molecules are "quite a way down the road."

He says OLEDs now are used in smart phones, particularly the Samsung Galaxy series; in pricey new super-thin TVs being introduced by Sony, Samsung, LG and others; and in lighting.

"OLEDs in smart phones have caught on because they are somewhat more efficient than conventional liquid-crystal displays like those used in the iPhone," he says. "That means longer battery life. Samsung has already demonstrated flexible, full-color OLED displays for future roll-up smart phones." Lupton says smart phones could produce light more efficiently using molecules that don't trap as much light.

The large rotelle-shaped molecules also can "catch" other molecules and thus would make effective biological sensors; they also have potential use in solar cells and switches, he adds.

The study was funded by the Volkswagen Foundation, the German Chemical Industry Fund, the David and Lucille Packard Foundation and the European Research Council.

Lupton is a research professor of physics and astronomy at the University of Utah and also on the faculty of the University of Regensburg, Germany. He conducted the study with Utah physics graduate student Alexander Thiessen; Sigurd Höger, Vikas Aggarwal, Alissa Idelson, Daniel Kalle and Stefan-S. Jester of the University of Bonn; and Dominik Würsch, Thomas Stangl, Florian Steiner and Jan Vogelsang of the University of Regensburg.

Freeing Trapped Light

While conventional LEDs use silicon semiconductors, OLEDs in some of the latest cell phones and TVs are made with "pi-conjugated polymers," which are plastic-like, organic semiconductors made of a chain of repeating molecular units.

"Conjugated polymers are a terrible mess," Lupton says. "They now make only mediocre OLEDs, although people like to claim the opposite."

For one thing, three-quarters of the light energy is in a state that normally is inaccessible -- a problem addressed by another recent University of Utah study of OLEDs. Lupton says his study deals with another problem, which exists even if the other problem is overcome: the polarization of light in pi-conjugated polymers that leads to the "trapping" or loss of up to 80 percent of the light generated.

"Light is an oscillating field like a wave, and a wave moves in a certain direction," Lupton says. "We call this direction of oscillation a polarization."

Because polymers are long molecules like spaghetti, when an electrical current is applied to a polymer, "the electrons can only flow in one direction and that generates the light waves," Lupton says. "Because those light waves only oscillate in one direction, the light can get trapped inside the OLED, which is a little bit like an optical fiber."

That, he adds, is why even with the latest OLED smart phones, "your battery is dead in two days because the display uses a lot of the electricity."

"The rotelle -- technically called oligomers -- are basically wrapped-up polymers," Lupton says. "They all have the same shape, but they do not emit polarized light because they are round. They generate waves that vibrate in all directions. The light doesn't have a fixed polarization; it doesn't vibrate in a fixed direction. It always can get out."

Lupton compares the ability of the wagon-wheel molecules to emit unpolarized light in all directions to what happens when a pencil is balanced perfectly on its tip and falls in a different, random direction each time.

Cooking up a Wagon Wheel-Shaped Molecule

The international team of physicists and chemists set out to make molecules that generate light waves in all directions rather than in a fixed direction. In the new study, they report how the created the spoked-wheel molecules, made images of them and did single-molecule experiments, including looking at photons, or light particles, emitted one at a time from a single molecule. In those experiments, they shined an ultraviolet light on the rotelle-shaped molecules to generate visible light photons.

"We showed that every photon that comes out has a scrambled polarization, the polarization changes randomly from photon to photon," Lupton says.

The emitted light is blue-green, Lupton says, but images accompanying the paper -- taken with a scanning tunneling electron microscope -- show the rotelle- and spaghetti-shaped molecules with a false yellow-brown color to provide good contrast.

Each wagon-wheel molecule measures only six nanometers wide, which is large for a molecule but tiny compared with the 100,000 nanometer width of a human hair.

Using rotelle-shaped oligomers instead of spaghetti-shaped polymers, "in principle, we should be able to double the efficiency of getting the light out" -- although that remains to be proved, Lupton says.

"Even if we scramble the polarization, we're always going to have a bit of light trapped in the OLED," he says. "Those losses are now 80 percent, and we probably could get down to 50 or 60 percent."

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Journal Reference:

A. Vikas Aggarwal, Alexander Thiessen, Alissa Idelson, Daniel Kalle, Dominik Würsch, Thomas Stangl, Florian Steiner, Stefan-S. Jester, Jan Vogelsang, Sigurd Höger, John M. Lupton. Fluctuating exciton localization in giant π-conjugated spoked-wheel macrocycles. Nature Chemistry, 2013; DOI: 10.1038/nchem.1758
Source: Sciencedaily

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Making her first public appearance since separating from her husband Michael Douglas, Catherine Zeta Jones attended the groundbreaking of the Oriental Movie Metropolis in Qingdao, China on Sunday (September 22).

The "Chicago" actress looked stunning in a sleeveless red and white dress as she mingled among the other VIP's at the event.

The ceremony heralded the beginning of a $5 billion film studio in China, as well as announcing plans for a Chinese Film Festival.

Meanwhile, although they remain estranged, the English beauty and her actor husband have not filed for a legal separation or a divorce.

Enjoy the pictures of Catherine Zeta Jones at the groundbreaking ceremony for Oriental Movie Metropolis in Qingdao, China (September 22).

Source: Celebrity-gossip

NAIROBI, Kenya - Masked gunmen stormed into a fancy, crowded mall in Nairobi on Saturday and shot dead at least 39 people and wounded more than 150 in one of the most chilling terrorist attacks in East Africa since Al Qaeda blew up two American embassies in 1998.

Parents hurled their bodies over their children, people jumped into ventilation shafts to save themselves, and shoppers huddled behind the plastic mannequins of designer clothing stores as two squads of gunmen believed to be linked to a Somali terrorist group moved through the mall, shooting shoppers in the head. Hours later, the mall's gleaming floors were smeared with blood as police officers dashed through the corpse-strewn corridors, trying to find the assailants.

A standoff with the attackers, who were reported to be heavily armed and holding an unknown number of hostages, continued as the sun rose on Sunday.

The mall, called Westgate, is a symbol of Kenya's rising prosperity, an impressive five-story building where Kenyans can buy expensive cups of frozen yogurt and plates of sushi. On Saturdays, it is especially crowded, and American officials have long warned that Nairobi's malls were ripe targets for terrorists, especially Westgate, because a cafe on the ground floor, right off the street, is owned by Israelis.

Fred Ngoga Gateretse, an official with the African Union, was having coffee at that cafe around noon when he heard two deafening blasts. He cowered on the floor and watched eight gunmen with scarves twisted over their faces firing at shoppers and then up at Kenyan police officers who were shooting down from a balcony as panicked shoppers dashed for cover. "Believe me, these guys were good shooters," Mr. Gateretse said. "You could tell they were trained."

Several witnesses said the attackers had shouted for Muslims to run away while they picked off other shoppers, executing them one by one. The mall, one of Nairobi's most luxurious, with glass elevators and some of the most expensive shops in town, is also popular with expatriates. It has served as the place for a power lunch, to catch a movie, to bring children for ice cream.

Four Americans were believed to have been injured in the attack, American officials said, and none were reported killed. Secretary of State John Kerry, who called the attack "a heartbreaking reminder that there exists unspeakable evil in our world," said the wife of a local employee of the American government was among the dead. Two Canadians, one of them a diplomat based in Nairobi, and two French citizens were killed, their governments said.

A confidential United Nations security report on Saturday described the attack as "a complex, two-pronged assault" with two squads of gunmen dashing into the mall from different floors at the same time and opening fire.

The Shabab, an Islamist militant group based in Somalia, took responsibility for the attack, saying it was revenge for Kenya's military operations in Somalia, which began nearly two years ago. "Kenya will not get peace unless they pull their military out of Somalia," said Ali Mohamoud Rage, the Shabab's spokesman, in a radio address. The Shabab also sent out a barrage of buoyant Twitter messages, bragging about the prowess of their fighters before Twitter abruptly suspended the account late Saturday. Later, a new one was set up.

Kenya's president, Uhuru Kenyatta, called the terrorists cowards and said Kenya would remain "as brave and invincible as the lions on our coat of arms." He also sounded a somber note, pleading with Kenyans to give blood , and said he had lost "very close family members in this attack," though he did not specify further.

In addition to the 39 people killed, who included women and children, Mr. Kenyatta said, more than 150 were wounded. Government officials said the wounded ranged in age from 2 to 78.

By Saturday night, Kenyan commandos had cornered several of the assailants on the third floor of the mall, witnesses said. Western officials said they expected that the assailants would fight to the death, though the Kenyan news media reported that one wounded gunman had been captured and died in a hospital. Several witnesses also said one of the assailants was a woman.

Throughout the day, as the police cleared sections of the mall, terrified shoppers emerged with their hands up and collapsed in each other's arms.

Early Sunday, the government said that the mall's upper levels had been secured and the gunmen contained in one place, but that hostages remained in several locations.

Witnesses described attackers using AK-47 and G-3 assault rifles and throwing grenades.

Vivian Atieno, 26, who works on the first floor of the mall, described "intense shooting," starting around 11 a.m., before she escaped through a fire exit.

Haron Mwachia, 20, a cleaner at the mall, said he had survived by climbing over a wall. "I heard several gunshots and managed to run away," he said. "I've never seen anything like it."

Kenya serves as the economic engine of East Africa, and while it has been mostly spared the violence and turmoil of many of its neighbors, it has had other terrorist attacks. In 1998, Al Qaeda killed more than 200 people in an enormous truck bombing that nearly leveled the American Embassy in downtown Nairobi, while simultaneously attacking the American Embassy in Dar es Salaam, Tanzania. Islamist terrorists also struck an Israeli-owned hotel on Kenya's Indian Ocean coast in 2002 and fired missiles at an Israeli airliner.

More recently, the Shabab have put Kenya in its cross hairs, especially after Kenya sent thousands of troops into Somalia in 2011 to chase the Shabab away from its borders and then kept those troops there as part of a larger African Union mission to pacify Somalia. The Shabab have attacked churches in eastern Kenya, mosques in Nairobi and government outposts along the Kenya-Somalia border.

But this was the boldest attack yet. Within minutes, as the gunmen opened fire with assault rifles, Westgate was plunged into mayhem and carnage. People ran out screaming, and victims soaking in their own blood were wheeled out in shopping carts. Bodies were still sprawled on the mall's front steps hours afterward, and woozy shoppers continued to emerge from the stores where they had been hiding.

"This is such a shock," said Preeyam Sehmi, an artist, as she stumbled out of the mall, past a phalanx of Kenyan soldiers, after five hours of hiding. "Westgate was such a social place."

Ilana Stein, a spokeswoman for Israel's Ministry of Foreign Affairs, said the attack took place near the ArtCaffe, an Israeli-owned coffee shop and bakery popular with foreigners that is one of 80 businesses in the mall. Ms. Stein said that one Israeli had been lightly injured, that three others had escaped unharmed and that Israelis had not been specifically targeted. "This time, the story is not about Israel," Ms. Stein said.

As night fell, hours after the attack began, Kenyan police helicopters hovered overhead while soldiers in flak jackets and helmets jogged single file into the mall, faces grim, guns cocked. The flashing lights of ambulances lighted up the mall's facade. Gunshots continued to ring out well past dark, though the Kenyan authorities did not provide much information about what was happening inside the mall. Several Kenyan soldiers were later brought out grimacing from what appeared to be gunshot wounds.

Before its Twitter account was shut down, the Shabab sent out a message, saying the fighters in the mall would never give up.

"There will be no negotiations whatsoever at #Westgate," the message said.

The Shabab, who have pledged allegiance to Al Qaeda, used to control large parts of Somalia, imposing a harsh and often brutal version of Islam in their territory. They have beheaded civilians and buried teenage girls up to their necks in sand and stoned them to death. But in the past two years, the African Union forces, including the Kenyans, have pushed the Shabab out of most of their strongholds. The worry now, current and former American officials said Saturday, is that this attack could be the start of a comeback.

"I think this is just the beginning," said Rudy Atallah, the former director of African counterterrorism for the Pentagon. "An attack like this gives them the capability to recruit, it shows off their abilities, and it demonstrates to Al Qaeda central that they are not dead."

Jeffrey Gettleman reported from Nairobi, and Nicholas Kulish from Goma, Democratic Republic of Congo. Reporting was contributed by Reuben Kyama and Tyler Hicks from Nairobi; Jodi Rudoren from Jerusalem; Mark Mazzetti from Washington; and Mohamed Ibrahim from Mogadishu, Somalia.

This article has been revised to reflect the following correction:

Correction: September 21, 2013

Because of an editing error, an earlier version of this article misspelled the given name of the president of Kenya. He is Uhuru Kenyatta, not Uruhu.

Source: Nytimes