Spintronics Goes Organic
Measuring spin in an organic LED is an important step toward a spintronic device.
In electronic circuits, transistors and memory devices process and store an electron’s charge. Manipulating another property of electrons, the quantum-mechanical phenomenon known as spin, could lead to faster, smaller, and more energy-efficient computers. University of Utah researchers have now taken a first step toward “spintronic” devices made from organic materials, which should be cheaper and easier to make than with materials used so far.
In a paper published in Nature Materials, the researchers outline a novel experiment that allowed them to measure the electron spins in an organic light-emitting diode (OLED). Using a magnetic field, they were able to control the material’s spin state, which also changed the electric current coming out of the device.
A practical spintronic device would have to use electric current to control and read spins. Even though the Utah researchers used a magnetic field to control the spin, their work demonstrates the possibility of spintronics in organic semiconductors, says Johan van Tol, who does spintronic research at the National High Magnetic Field Laboratory in Tallahassee, FL. “Manipulating spin has been done in other materials, but not in these kind of polymers,” he says.
Spintronic devices are most easily made from magnetic metals, and researchers have also reported advances in making them from conventional inorganic semiconductors such as silicon and gallium arsenide. But using organic semiconductors could have big advantages. “[Organic devices] are easy to make, easy to deposit and structure; it’s all very cheap,” says Christoph Boehme, an assistant professor of physics at Utah and a coauthor of the new paper. “You can deposit them on a flexible substrate, and you can deposit them with ink-jet printing.”
Electron spin can take on one of two directions: up and down. In conventional electronic circuits, current flowing through a transistor represents a bit with a value of 1, while absence of current signifies a bit with a value of 0. In a spintronic device, 1 and 0 are represented by an up or down spin.
To make a functional spintronic device, it’s important to measure spin direction accurately. Right now, electrons with aligned spins can be injected into a material, but determining whether those electrons maintain their spins is important if that spin is to be controlled in a device.
In their experiment, Boehme and his colleagues read the spin in a polymer OLED by measuring the current coming out of it. They attached electrodes to the device and bombarded it with a microwave pulse every 500 microseconds.
Boehme explains that spin can be thought of as a tiny bar magnet, pointing in a certain direction. In an LED, when the voltage is applied in a certain direction, negatively charged electrons and positively charged holes form pairs. Each pair subsequently decays, or loses some energy, emitting a photon. Because the electron and hole each have a specific spin, the electron-hole pair can take one of four spin states: up-up, up-down, down-up, and down-down. “Only one of these four can decay and produce light,” Boehme says. This means that OLEDs made from the polymer will probably not reach efficiencies higher than 25 percent, he adds.
At the same time, a particle’s spin direction can change. So among all the electron-hole pairs formed in the LED material, Boehme says, “one of those that cannot [emit light] can all of a sudden flip and turn into one of the four states that can produce light.” More light-emitting states increase the material’s light output, but since the electrons and holes get annihilated, the current decreases.
The microwave pulse changes the spins in the polymer OLED in a manner determined by the length and frequency of the pulse. The result is to alternately create more or fewer light-emitting states, decreasing and increasing the current. The greater the frequency of the microwave pulse, the faster the current increases and decreases.
“We have shown that when you coherently manipulate spins, when you turn them around from up to down and everything in between, you can see the imprint of spin motion on the current that you measure,” Boehme says.
The researchers believe that their work could also help improve OLEDs. Introducing impurities into the polymer materials could change the rate at which electrons in the material flip their spins, Boehme says. That could create more and more light-emitting states, increasing the efficiency of OLED over 25 percent and lead to brighter devices.