A new approach to two-dimensional materials could lead to novel design rules for microelectronics, membranes, and tissues. The same approach also bolsters to a scientific theory left unsupported by experimental evidence for more than a century. [Chen, J. et al., Science (2018) 362(6419), 1135; DOI: 10.1126/science.aau4146]
A collaboration between Pacific Northwest National Laboratory, the University of Washington, University of California Los Angeles, and others shows how some materials assemble on a surface a single row at a time. American scientist J. Willard Gibbs laid down predictions about how nucleation occurs and leads to such phenomenon in the 1870s, but scientists still argue over the details. Now, Washington graduate student Jiajun Chen, working at PNNL, has at last uncovered the underlying process using peptides. Collaborators at UCLA had been using peptides to drive nanomaterials to follow specific growth patterns. One that has a strong binding affinity for a molybdenum disulfide substrate was particularly intriguing. The team measured the self-assembly of the peptide on the surface using atomic force microscopy and compared the measurements with molecular dynamics simulations.
"It was complete serendipity," explains PNNL's James De Yoreo, who is Chen's supervisor. "We didn't expect the peptides to assemble into their own highly ordered structures." Nature, as ever, finds a way to minimize energy consumption in its processes. In attempting to unravel nucleation, Gibbs had predicted that if a material were to grow in a single dimension, row by row, in other words, there would be no insurmountable energy barrier to nucleation of the kind that allows water to freeze and materials to crystallize. The new study shows there are definitive instances in which Gibbs' theory applies even if there are other controversial examples where it apparently does not.
The researchers showed that even in the earliest stages of self-assembly, the peptides bound to the material one row at a time, with no energy barrier, just as Gibbs' theory predicts. This growth offers new clues as to how we might design novel 2D materials without pushing a system way beyond equilibrium and losing control. "In one dimension, the difficulty of getting things to form in an ordered structure goes away," De Yoreo explains. "Then you can operate right near equilibrium and still grow these structures without losing control of the system." It could change assembly pathways for those engineering microelectronics or even synthetic body tissues.
David Bradley blogs at Sciencebase Science Blog and tweets @sciencebase. You can see more of his macro and other photography via his website.