First Time: Chemical Reactions Imaged !
When Felix Fischer of the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) set out to develop nanostructures made of graphene using a new, controlled approach to chemical reactions, the first result was a surprise: spectacular images of individual carbon atoms and the bonds between them.
What the microscope showed the researchers, says Fischer, “was amazing.” The specific outcomes of the reaction were themselves unexpected, but the visual evidence was even more so. “Nobody has ever taken direct, single-bond-resolved images of individual molecules, right before and immediately after a complex organic reaction,” Fischer says.
The researchers report their results online in the May 30, 2013 edition of Science Express.
Graphene nanostructures can form the transistors, logic gates, and other elements of exquisitely tiny electronic devices, but to become practical they will have to be mass produced with atomic precision. Hit-or-miss, top-down techniques, such as exfoliating graphite or unzipping carbon nanotubes, can’t do the job.
Fischer and his colleagues set out to engineer graphene nanostructures from the bottom up, by converting linear chains of carbon atoms into extended hexagonal sheets (polyaromatic hydrocarbons), using a reaction originally discovered by UC Berkeley professor Robert Bergman. The first requirement was to perform the reactions under controlled conditions.
The single-atom tip of the noncontact atomic force microscope “feels” changes in the strength of electronic forces as it moves across the surface at a constant height. Resulting movements of the stylus are detected by a laser beam to compute images.
Fischer’s group collaborated with microscopy expert Crommie to devise the best possible view. The first attempt to track the reactions used a scanning tunneling microscope (STM), which senses electronic states when brought within a few billionths of a meter (nanometers) of the surface of the sample. But the image resolution of the tiny molecule and its products – each only about one nanometer across – wasn’t good enough to reliably identify the molecular structures.
The collaborators then turned to a technique called noncontact atomic force microscopy (nc-AFM), which probes the surface with a sharp tip. The tip is mechanically deflected by electronic forces very close to the sample, moving like a phonograph needle in a groove.
The single-atom moving finger of the nc-AFM could feel not only the individual atoms but the forces representing the bonds formed by the electrons shared between them. The resulting images bore a startling resemblance to diagrams from a textbook or on the blackboard, used to teach chemistry, except here no imagination is required.
The original reactant molecule, resting on a flat silver surface, is imaged both before and after the reaction, which occurs when the temperature exceeds 90 degrees Celsius. The two most common final products of the reaction are shown. The three-angstrom scale bars (an angstrom is a ten-billionth of a meter) indicate that both reactant and products are about a billionth of a meter across.
A chemical bond is not as simple a concept as it may appear, however. From the dozens of possibilities, the starting molecule’s reaction did not yield what had intuitively seemed to Fischer and his colleagues the most likely products. Instead, the reaction produced two different molecules. The flat silver surface had rendered the reaction visible but also shaped it in unexpected ways.
The nc-AFM microscopy provided striking visual confirmation of the mechanisms that underlie these synthetic organic chemical reactions, and the unexpected results reinforced the promise of this powerful new method for building advanced nanoscale electronic devices from the bottom up.