麻豆淫院

(麻豆淫院Org.com) -- The energy bands of complex particles known as plasmarons have been seen for the first time by scientists working with graphene at the Advanced Light Source. Their discovery may hasten the day when these crystalline sheets of carbon just one atom thick can be used to build ultrafast computers and other electronic, photonic, and plasmonic devices on the nanoscale.

Scientists working at the Advanced Light Source (ALS) at the U.S. Department of Energy鈥檚 Lawrence Berkeley National Laboratory have discovered striking new details about the electronic structure of graphene, crystalline sheets of carbon just one atom thick. An international team led by Aaron Bostwick and Eli Rotenberg of the ALS found that composite particles called plasmarons play a vital role in determining graphene鈥檚 properties.

鈥淭he interesting properties of graphene are all collective phenomena,鈥� says Rotenberg, an ALS senior staff scientist responsible for the scientific program at ALS beamline 7, where the work was performed. 鈥淕raphene鈥檚 true electronic structure can鈥檛 be understood without understanding the many complex interactions of electrons with other particles.鈥�

The electric charge carriers in graphene are negative electrons and positive holes, which in turn are affected by plasmons鈥攄ensity oscillations that move like sound waves through the 鈥渓iquid鈥� of all the electrons in the material. A plasmaron is a composite particle, a charge carrier coupled with a plasmon.

鈥淎lthough plasmarons were proposed theoretically in the late 1960s, and indirect evidence of them has been found, our work is the first observation of their distinct energy bands in graphene, or indeed in any material,鈥� Rotenberg says.

Understanding the relationships among these three kinds of particles鈥攃harge carriers, plasmons, and plasmarons鈥攎ay hasten the day when graphene can be used for 鈥減lasmonics鈥� to build ultrafast computers鈥攑erhaps even room-temperature quantum computers鈥攑lus a wide range of other tools and applications.

Strange graphene gets stranger

鈥淕raphene has no band gap,鈥� says Bostwick, a research scientist on beamline 7.0.1 and lead author of the study. 鈥淥n the usual band-gap diagram of neutral graphene, the filled valence band and the empty conduction band are shown as two cones, which meet at their tips at a point called the Dirac crossing.鈥�

Graphene is unique in that electrons near the Dirac crossing move as if they have no mass, traveling at a significant fraction of the speed of light. Plasmons couple directly to these elementary charges. Their frequencies may reach 100 trillion cycles per second (100 terahertz, 100 THz)鈥攎uch higher than the frequency of conventional electronics in today鈥檚 computers, which typically operate at about a few billion cycles per second (a few gigahertz, GHz).

Plasmons can also be excited by photons, particles of light, from external sources. Photonics is the field that includes the control and use of light for information processing; plasmons can be directed through channels measured on the nanoscale (billionths of a meter), much smaller than in conventional photonic devices.

And since the density of graphene鈥檚 electric charge carriers can easily be influenced, it is straightforward to tune the electronic properties of graphene nanostructures. For these and other reasons, says Bostwick, 鈥済raphene is a promising candidate for much smaller, much faster devices鈥攏anoscale plasmonic devices that merge electronics and photonics.鈥�

The usual picture of graphene鈥檚 simple conical bands is not a complete description, however; instead it鈥檚 an idealized picture of 鈥渂are鈥� electrons. Not only do electrons (and holes) continually interact with each other and other entities, the traditional band-gap picture fails to predict the newly discovered plasmarons revealed by Bostwick and his collaborators.

The team reports their findings and discuss the implications in 鈥淥bservations of plasmarons in quasi-free-standing doped graphene,鈥� by Aaron Bostwick, Florian Speck, Thomas Seyller, Karsten Horn, Marco Polini, Reza Asgari, Allan H. MacDonald, and Eli Rotenberg, in the 21 May 2010 issue of Science, .

Graphene is most familiar as the individual layers that make up graphite, the pencil-lead form of carbon; what makes graphite soft and a good lubricant is that the single-atom layers readily slide over one another, their atoms strongly bonded in the plane but weakly bonded between planes. Since the 1980s, graphene sheets have been rolled-up into carbon nanotubes or closed buckyball spheroids. Theorists long doubted that single graphene sheets could exist unless stacked or closed in on themselves.

Then in 2004 single graphene sheets were isolated, and graphene has since been used in many experiments. Graphene sheets suspended in vacuum don鈥檛 work for the kind of electronic studies that Bostwick and Rotenberg perform at ALS beamline 7.0.1. They use a technique known as angle-resolved photoemission spectroscopy (ARPES); for ARPES, the surface of the sample must be flat. Free-standing graphene is rarely flat; at best it resembles a crumpled bedsheet.

Using electrons to draw images of composite particles

鈥淥ne of the best ways to grow a flat sheet of graphene is by heating a crystal of silicon carbide,鈥� Rotenberg says, 鈥渁nd it happens that our German colleagues Thomas Seyller from the University of Erlangen and Karsten Horn from the Fritz Haber Institute in Berlin are experts at working with silicon carbide. As the silicon recedes from the surface it leaves a single carbon layer.鈥�

Using flat graphene made this way, the researchers hoped to study graphene鈥檚 intrinsic properties by ARPES. First a beam of soft x-rays from the ALS frees electrons from the graphene (photoemission). Then by measuring the direction (angle) and speed of the emitted electrons, the experiment recovers their energy and momentum; the spectrum of the cumulative emitted electrons is transmitted directly onto a two-dimensional detector.

The result is an image of the electronic bands created by the electrons themselves. In the case of graphene, the picture is x shaped, a cross-sectional cut through the two conical bands.

鈥淓ven in our initial experiments with graphene, we suspected that the ARPES distribution was not quite as simple as the two-cone, bare-electron model suggested,鈥� Rotenberg says. 鈥淎t low resolution there appeared to be a kink in the bands at the Dirac crossing.鈥� Because there really is no such thing as a bare electron, the researchers wondered if this fuzziness was caused by charge carriers emitting plasmons.

鈥淏ut theorists thought we should see even stronger effects,鈥� says Rotenberg, 鈥渁nd so we wondered if the substrate was influencing the physics. A single layer of carbon atoms resting on a silicon carbide substrate isn鈥檛 the same as free-standing graphene.鈥�

The silicon-carbide substrate could in principle weaken the interactions between charges in the graphene (on most substrates the electronic properties of graphene are disturbed, and the plasmonic effects can鈥檛 be observed). Therefore the team introduced hydrogen atoms that bonded to the underlying silicon carbide, isolating the graphene layer from the substrate and reducing its influence. Now the graphene film was flat enough to study with ARPES but sufficiently isolated to reveal its intrinsic interactions.

The images obtained by ARPES actually reflect the dynamics of the holes left behind after photoemission of the electrons. The lifetime and mass of excited holes are strongly subject to scattering from other excitations such as phonons (vibrations of the atoms in the crystal lattice), or by creating new electron-hole pairs.

鈥淚n the case of graphene, the electron can leave behind either an ordinary hole or a hole bound to a plasmon鈥攁 plasmaron,鈥� says Rotenberg.

Taken together, the interactions dramatically influenced the ARPES spectrum. When the researchers deposited potassium atoms atop the layer of carbon atoms to add extra electrons to the graphene, a detailed ARPES picture of the Dirac crossing region emerged. It revealed that the energy bands of graphene cross at three places, not one.

Ordinary holes have two conical bands that meet at a single point, just as in the bare-electron, non-interacting picture. But another pair of conical bands, the plasmaron bands, meets at a second, lower Dirac crossing. Between these crossings lies a ring where the hole and plasmaron bands cross.

鈥淏y their nature, plasmons couple strongly to photons, which promises new ways for manipulating light in nanostructures, giving rise to the field of plasmonics,鈥� Rotenberg says. 鈥淣ow we know that plasmons couple strongly to the charge carriers in graphene, which suggests that graphene may have an important role to play in the merging fields of electronics, photonics, and plasmonics on the nanoscale.鈥�