Unique properties of two-dimensional crystal bring potential for optoelectronics, solar cells, valleytronics

December 3, 2012

When the dry lubricant, molybdenum disulfide, is stripped down to a single layer of atoms, a tightly bound quasi-particle comprised of two electrons and a hole forms with unique spin and valley properties, researchers from Case Western Reserve University and colleagues discovered.

These charged quasi-particles, called negative trions, can be manipulated to change the light absorbed and emitted from this two-dimensional semi-conducting crystal, opening it to potential use in new and other that are controlled by light or designed to control light.

The discovery also opens up new opportunities to study what physicists call "many-body interactions"—in this case, the interactions among the charged particles—and a new concept of electronics called valleytronics.

The research is published in this week's online edition of .

"What we saw was something like a negative hydrogen ion–a positive charge bound to two ," said Jie Shan, an associate professor of physics at Case Western Reserve and senior author of the paper.

The trions formed in the molybdenum disulfide crystal when the researchers applied a bias voltage on the back gate of a device called . This was done at 10 degrees Kelvin, or -441 Fahrenheit. The introduced by the bias voltage were linked to electron-hole pairs generated by absorption of , resulting in negatively charged trions.

"The interactions between charged particles in three-dimensional bulk materials are usually screened by the presence of other charges in the material. Because this material has an atomic thickness, the interactions between the charges are much stronger than in the natural due the drastically reduced screening," Shan explained. "The quasi particles are stable. Adding energy comparable to or larger than that of room temperature to the is required to break away the extra negative charge."

Because the trions are charged, they can be controlled by a bias electric field, Shan said. "This is very basic science; it is early, but this may offer a unique opportunity to use controllable and directional transport of optical excitation in a material, which could be useful for applications such as photovoltaics and optoelectronics."

Shan worked with her graduate student, Kelinang He, and, from Columbia University, Kin Fai Mak, a postdoctoral researcher, Tony F. Heinz, professor of physics and electrical engineering, Gwan Hyoung Lee postdoctoral researcher and James Hone, professor of mechanical engineering. Changgu Lee, an assistant professor of mechanical engineering at Sungkyunkwan University also contributed to the research.

The discovery of the trions increases the potential that this material can be used for valleytronics, the researchers say. Earlier this year, Mak, He, Shan and Heinz, and other research groups found they could control what's known as valley polarization–essential to valleytronics—in a single layer of molybdenum disulfide.

Conventional electronics rely on the control of the charge through materials. Spintronics use the spin degree of freedom of the charge carriers. Valleytronics rely on another property.

Because of symmetry, crystals frequently have independent, degenerate valleys in their energy bands. This so-called "valley" (or momentum) degree of freedom has been proposed as something that could be manipulated for new classes of electronic devices.

Using polarized light, Shan and fellow researchers were able to nudge electrons into a desired valley.

"The trions also show some interesting valley properties," Shan said. "But their valley lifetime may be very different, because it involves the interaction of three particles."

The researchers are continuing to probe the properties in a series of experiments.

Explore further: Two graphene layers may be better than one

Related Stories

Two graphene layers may be better than one

April 27, 2011

(PhysOrg.com) -- Researchers at the National Institute of Standards and Technology have shown that the electronic properties of two layers of graphene vary on the nanometer scale. The surprising new results reveal that not ...

Researchers take a step toward valleytronics

April 27, 2011

Valley-based electronics, also known as valleytronics, is one step closer to reality. Two researchers at the Naval Research Laboratory (NRL) have shown that the valley degree of freedom in graphene can be polarized through ...

Bilayer graphene is another step toward graphene electronics

August 11, 2011

The Nobel Prize winning scientists Professor Andre Geim and Professor Kostya Novoselov have taken a huge step forward in studying the wonder material graphene and revealing its exciting electronic properties for future electronic ...

Recommended for you

Fusion reactors 'economically viable' say experts

October 2, 2015

Fusion reactors could become an economically viable means of generating electricity within a few decades, and policy makers should start planning to build them as a replacement for conventional nuclear power stations, according ...

Iron-gallium alloy shows promise as a power-generation device

September 29, 2015

An alloy first made nearly two decades ago by the U. S. Navy could provide an efficient new way to produce electricity. The material, dubbed Galfenol, consists of iron doped with the metal gallium. In new experiments, researchers ...

Invisibility cloak might enhance efficiency of solar cells

September 30, 2015

Success of the energy turnaround will depend decisively on the extended use of renewable energy sources. However, their efficiency partly is much smaller than that of conventional energy sources. The efficiency of commercially ...

Extending a battery's lifetime with heat

October 1, 2015

Don't go sticking your electronic devices in a toaster oven just yet, but for a longer-lasting battery, you might someday heat them up when not in use. Over time, the electrodes inside a rechargeable battery cell can grow ...


Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.