Fabricating functional nano-devices is an ultimate goal of nanotechnology. Atomic-scale modification and sculpting of materials can enable nano-machines with wide-varying application potential in biological (medical) and chemical (trace sensing) uses. In our most recent publication, together with Harvard University, the Lawrence Berkeley National Laboratory and FEI corporation, we demonstrate precise modification of graphene at the atomic scale.
In our paper “Direct Observation of a Long-Lived Single-Atom Catalyst Chiseling Atomic Structures in Graphene”, published in NanoLetters, we report the direct observation of single silicon atoms catalyzing a reaction on a graphene surface. The reaction removes carbon atoms in a controlled fashion, allowing for precise sculpting of nanopores in graphene. Nanopores in graphene hold great technological and scientific potential, and are already being considered for several uses, such as water filtration and DNA sequencing.
Figure: Nanopores in graphene, catalyzed by single silicon atoms and recorded by HTEM.
Known to humans for centuries, catalysts play an enabling role in many chemical processes that are important to the modern society. Recent advances in nanotechnology introduced nanocatalysts that enable the creation of novel nanostructures, such as carbon nanotubes and semiconductor nanowires. The characteristics of the resulting structures can be tuned by the structures of the corresponding nanocatalysts. For example, in the growth of semiconductor nanowires from metal nanoparticles, the diameter of the resultant nanowire is determined by the size of the catalytic nanoparticles.
Catalysis typically involves complex atomic-scale events that are hard to record, either because they are too fast or too small for the instrumentation used for the recording. We overcome these challenges by using high-resolution transmission electron microscopy (HRTEM) to record individual silicon atoms as they catalyze the graphene chiseling reaction. The products of the chiseling process are atomic-scale features including graphene pores and clean edges.
Video: Single silicon atom catalyzing a nanopore in graphene. More videos free of charge are available on the NanoLetters website.
The silicon atoms are naturally present impurities in the HRTEM chamber. The atoms freely drift along the graphene surface, until they come across an occasional atomic-scale defect in the sheet. The silicon atom then replaces a carbon atom in the chickenwire structure of graphene. A scientist starts the chiseling reaction by directing a focused electron beam to the defect site. The width of the pore starts from only a few angstroms, gradually increasing with the presence of silicon adatoms and under continuous electron irradiation. The pore size is controlled by stopping the irradiation when the desired size has been reached, as seen in the figure above.
These molecular-sized pores are excellent candidates for molecular detection applications, such as rapid DNA sequencing, because they can be tuned to match the size of a single DNA molecule (~10 Angstroms) for the sensitivity that is needed for single base recognition.
Apart from demonstrating the proof of principle, we found some interesting physics of the process, including the dynamics, stability and selectivity of the single-atom chiseling process. Our findings show that there are likely other pairs of atoms in nature, aside from silicon-graphene, that possess atomic chiseling ability.
The graphene used in this research is our standard high-quality CVD graphene, transferred onto a TEM grid.