Molecular Graphene


    

Researchers from Stanford University and the U.S. Department of Energy's SLAC National Accelerator Laboratory have created the first-ever system of "designer electrons" — exotic variants of ordinary electrons with tunable properties that may ultimately lead to new types of materials and devices. The handcrafted, honeycomb-shaped structures were inspired by graphene, a pure form of carbon widely heralded for its potential in future electronics.





Images (click for full size)

molecular graphene under strain Molecular Graphene. Precisely positioned carbon monoxide molecules (black) guide electrons (yellow-orange) into a nearly perfect honeycomb pattern called molecular graphene. Electrons in this structure have graphene-like properties; for example, unlike ordinary electrons, they have no mass and travel as if they are moving at the speed of light in a vacuum. To make this structure, scientists from Stanford and SLAC National Accelerator Laboratory used a scanning tunneling microscope to move individual carbon monoxide molecules into a hexagonal pattern on a perfectly smooth copper surface. The carbon monoxide repels the free-flowing electrons on the copper surface, forcing them into a graphene-like honeycomb pattern. Image credit: Hari Manoharan / Stanford University.

transformation into Dirac electrons Phantom Fields. A version of molecular graphene in which the electrons respond as if they're experiencing a very high magnetic field (red areas) when none is actually present. Scientists from Stanford and SLAC National Accelerator Laboratory calculated the positions where carbon atoms in graphene should be to make its electrons believe they were being exposed to a magnetic field of 60 Tesla, more than 30 percent higher than the strongest continuous magnetic field ever achieved on Earth. (A 1 Tesla magnetic field is about 20,000 times stronger than the Earth's.) The researchers then used a scanning tunneling microscope to place carbon monoxide molecules (black circles) at precisely those positions. The electrons responded by behaving exactly as expected — as if they were exposed to a real field, but no magnetic field was turned on in the laboratory. Image credit: Hari Manoharan / Stanford University.

Schrodinger meets Dirac (top view) Electrons Transform. Visualization showing how normal (massive) two-dimensional electrons (bottom) are transmuted into special (massless) "Dirac" electrons characteristic of graphene. The free electrons below appear as quantum waves. As these electrons travel upward into the molecular graphene lattice, they begin to change character and symmetry as they quantum mechanically interfere, eventually transforming into a new species (bright honeycomb region near top) characterized by zero mass and a quantum spin variable (pseudospin) denoting electron position on a honeycomb lattice. Image credit: Hari Manoharan / Stanford University.

    

Schrödinger Meets Dirac. Visualization depicting the transformation of an electron moving under the influence of the non-relativistic Schrödinger equation (upper planar quantum waves) into an electron moving under the prescription of the relativistic Dirac equation (lower honeycomb quantum waves). The light blue line shows a quasiclassical path of one such electron as it enters the molecular graphene lattice made of carbon monoxide molecules (black/red atoms) positioned individually by an STM tip (comprised of iridium atoms, dark blue). The path shows that the electron becomes trapped in synthetic chemical bonds that bind it to a honeycomb lattice and allow it to quantum mechanically tunnel between neighboring honeycomb sites, just like graphene. The underlying electron density in a honeycomb pattern (lower part of image, yellow-orange) is the quantum superposition formed from all such electron paths as they transmute into a new tunable species of massless Dirac fermions. Image credit: Hari Manoharan / Stanford University.

Designer Electrons. This graphic shows the effect that a specific pattern of carbon monoxide molecules (black/red) has on free-flowing electrons (orange/yellow) atop a copper surface. Ordinarily the electrons behave as simple plane waves (background). But the electrons are repelled by the carbon monoxide molecules, placed here in a hexagonal pattern. This forces the electrons into a honeycomb shape (foreground) mimicking the electronic structure of graphene, a pure form of carbon that has been widely heralded for its potential in future electronics. The molecules are precisely positioned with the tip of a scanning tunneling microscope (dark blue). Image credit: Hari Manoharan / Stanford University.

Molecular Graphene PNP Junction Device. Stretching or shrinking the bond lengths in molecular graphene corresponds to changing the concentrations of Dirac electrons present. This image shows three regions of alternating lattice spacing sandwiched together. The two regions on the ends contain Dirac "hole" particles (p-type regions), while the region in the center contains Dirac "electron" particles (n-type region). A p-n-p structure like this is of interest in graphene transistor applications. Image credit: Hari Manoharan / Stanford University.

Hari Manoharan Prof. Hari Manoharan.
TIFF Originals [Color | BW | CMYK]
Image credit: L. A. Cicero / Stanford News Service.

Manoharan Laboratory. View from ground floor. Image credit: Brad Plummer / SLAC.

Manoharan Laboratory. View of MOTA operations console area. Image credit: Brad Plummer / SLAC.


Videos (click to play)

Assembly VideoMolecular Graphene Assembly. Nanoscale assembly sequence showing carbon monoxide molecules (black circles) being moved one at a time by a scanning tunneling microscope into the hexagonal "molecular graphene" arrangement. The molecules repel the free-flowing electrons (yellow-orange) on the copper surface, forcing them into a honeycomb lattice. The video comprises 52 topographs acquired between manipulation steps.
Assembly.mov [QuickTime]

CO Manipulation VideoMolecular Manipulation. An individual carbon monoxide molecule (black/red) on the copper surface (yellow atoms) is positioned by the scanning tunneling microscope tip (comprised of iridium atoms, magenta). To manipulate the molecule, the quantum mechanical tunneling conditions are changed until the tip exerts an attractive force upon the molecule. The tip drags the molecule from its initial position to a desired final position. Once the molecule reaches its final position, the tunneling conditions are changed back to normal conditions suitable for imaging rather than manipulation. This process is repeated hundreds of times to create a molecular graphene lattice.
CO_Manipulation.mov [QuickTime]

Tunable Psuedomangetic Field VideoTunable Pseudomagnetic Field. This video shows the progression of different molecular graphene structures that produce "phantom" magnetic fields of 0, 15, 30, 45 and 60 Tesla. The strongest continuous magnetic field actually achieved on Earth is 45 Tesla. (A 1 Tesla magnetic field is about 20,000 times stronger than the Earth's.) Each particular arrangement of carbon monoxide molecules (black circles) on a copper surface causes the copper's surface electrons (yellow-orange) to behave as if they're experiencing a very high magnetic field, although none is actually present.
Tunable_Pseudofield.mov [QuickTime]