Living systems such as the brain conduct electric signals via ions – charged atoms or molecules – rather than electrons. Like the living ones, artificial systems that run on ions could in some ways be more efficient than today’s electronic devices. Nanoionics is a new area of research in which ionic currents are conducted on the scale of nanometers, or billionths of a meter; and it may one day lead to innovative technologies. Weizmann Institute scientists have now made an important step toward the construction of artificial ionic circuits.
As reported recently in Nature Materials,
the Weizmann researchers have developed a method for building ion-conducting channels
with planned shapes and dimensions on the surface of a solid material. This method answers two technical challenges: The motion of ions through solids is rather sluggish at room temperature, and no previous method has been available for confining such motion to well-defined, predetermined paths.
To create ion-conducting channels on a solid surface, the Institute team – Dr. Jonathan Berson (then a Ph.D. student), Dr. Doron Burshtain and Dr. Assaf Zeira, Alexander Yoffe, Dr. Rivka Maoz and Prof. Jacob Sagiv, all of the Materials and Interfaces Department – started out with a self-assembled organic monolayer – that is, a one-molecule-thick layer of highly ordered organic material laid down on a silicon wafer by a process known as surface self-assembly. The scientists then “wrote” the desired ion-conducting channels on the top surface of the monolayer using an atomic force microscope. This microscope “feels out” a material’s surface with a fine-tip needle, but it can also chemically modify that surface by means of a small electric current flowing through the tip. Thus the researchers could modify selected sites on the monolayer surface, literally writing custom surface patterns of ion-conducting channels, with widths down to ten nanometers and lengths up to a hundred micrometers. (A micrometer is a tenth of a millimeter). Similarly, using special electric stamps instead of a sharp needle, the scientists could “print” larger, macroscopic ion-conducting channels covering surface areas of up to several square centimeters.
The researchers examined the resulting ion-conducting patterns using a combination of advanced tools: atomic force microscopy, surface-adapted infrared spectroscopy, X-ray photoelectron spectroscopy and scanning electron microscopy, along with sensitive electrical measurements. Driven by a small electric potential, ions of silver, titanium and other metals were found to effectively move along these channels.
The new method may one day be used in the manufacture of artificial nanoionic systems that could mimic biological ones, even if they do not necessarily employ the same mechanisms of action. And this, in turn, may in the future help create nanoionic devices for use in science and in day-to-day life that would offer significant advantages over some of today’s electronic technologies – just as, for example, the brain, a natural ionic system, still outperforms electronic computers at many tasks. Though ionic devices are not expected to be faster than electronic ones, they may require less energy for writing and erasing information. Most prominent among their potential applications are ion-based computer memory devices that promise to be much longer-lasting than the electronic ones existing today, a feature that is essential for safe long-term storage of information.
The researchers could modify selected sites on the monolayer surface, literally writing custom surface patterns of ion-conducting channels
In the more immediate future, the Weizmann Institute methodology offers new possibilities for exploring previously unknown properties of matter, as the properties of materials differ on the nanoscale compared with the macroscale. For example, the Weizmann scientists observed an unusually fast conduction of ions in their nanochannels – a finding that opens up a new direction of research.