Looking at single atoms of hydrogen
As you probably know, graphene is a one-atom-thick sheet of carbon atoms packed in a dense two-dimensional honeycomb lattice. And it recently became very popular recently as a basis for future ultra-fast transistors. Now, according to Science News, U.S. researchers are using graphene to image individual hydrogen atoms via a standard transmission electron microscope (TEM) technology. Until now, heavy atoms, such as carbon, could be detected by electron microscopy. But the physicists from Berkeley, California, have shown it's possible to track the smallest atoms, hydrogen ones. But read more...
You can see on the left two transmission electron microscope (TEM) images. On the top is a "false-color TEM image of isolated hydrogen atoms (green) and an isolated carbon atom (red) on a graphene membrane." Here is a link to a larger version of this image. And below is a "false-color 3-D rendered TEM image of isolated hydrogen atoms (purple-tipped) and an isolated carbon atom (red-tipped) on a graphene membrane. The 'mountain ranges' in the fore- and background are amorphous carbon." Once again, here is a link to a larger version. (Credit: Zettl Research Group, Lawrence Berkeley National Laboratory and University of California at Berkeley)
This amazing work has been led by Alex Zettl, Professor of Physics specialized in condensed matter physics. He worked with the members of his research group who include Jannik Meyer and Çaglar Girit.
Now, let's look at the article of Science News, the weekly magazine of the Society for Science & the Public, Washington, DC, for more details about this discovery. "The researchers say that using graphene could enable scientists to understand the structure of molecules that have been difficult to image with other techniques capable of resolving single atoms, such as X-ray diffraction. Thanks to graphene's sturdiness, single-molecule motions and chemical reactions could be filmed as they happen, the team suggests. And the technique would be available to any lab that has a transmission electron microscope. Researchers would just have to use graphene as a petri dish. 'It opens up a whole new world for conventional-TEM users,' Zettl says."
Here is a second quote about the technology used. "A TEM scans microscopic objects with a thin beam of electrons, essentially running a current though the objects. Typically, researchers place samples on films of carbon. In the TEM, lighter elements tend to give lower contrast. Any atoms lighter than carbon -- itself one of the lightest elements -- are hard to see, even when the carbon films are just nanometers thick, or a few tens of atoms deep. The Berkeley team suspended graphene sheets across nanometer-sized holes in a conventional carbon sheet. The team reasoned that the uniform arrangement of atoms in graphene also makes it appear as a uniform shade of gray in TEM images, which is easy to remove from the data."
For more information, this research work has been published in the July 17, 2008 issue of Nature under the title "Imaging and dynamics of light atoms and molecules on graphene" (Volume 454, Number 7202, Pages 319-322). This project made the cover of the issue and is the subject of an Editor's summary, "Imaging atoms: 'Invisible' graphene brings electron microscopy to single carbons and hydrogens." Here is an excerpt. The researchers "show that atoms as small as carbon and even hydrogen adsorbed onto graphene can be imaged using standard TEM technology. Ultrathin graphene is an ideal support, either invisible or, if the lattice is resolved at high resolution, its contribution to the imaging signal is easily removed. This approach brings atomic resolution to biomolecules as well as to graphene itself."
Here is the end of the abstract of the report mentioned above. "Detecting an individual low-atomic-number atom, for example carbon or even hydrogen, is still extremely challenging, if not impossible, via conventional TEM owing to the very low contrast of light elements. Here we demonstrate a means to observe, by conventional TEM, even the smallest atoms and molecules: on a clean single-layer graphene membrane, adsorbates such as atomic hydrogen and carbon can be seen as if they were suspended in free space. We directly image such individual adatoms, along with carbon chains and vacancies, and investigate their dynamics in real time. These techniques open a way to reveal dynamics of more complex chemical reactions or identify the atomic-scale structure of unknown adsorbates. In addition, the study of atomic-scale defects in graphene may provide insights for nanoelectronic applications of this interesting material."
Here is a link to additional figures and tables provided by Nature. However, the pictures in this post have been picked from supplementary materials provided by the Berkeley researchers.
Sources: Davide Castelvecchi, Science News, July 16, 2008; Nature, July 17, 2008; and various websites
You'll find related stories by following the links below.