The movement of a single proton offers four different levels of conductance, researchers in Germany say


Source: © Nat. Nanotechnol.

Protons can switch between north-south and east-west configurations

Researchers in Germany claim to have created the world’s smallest molecular switch, relying on the movement of just a single proton. The switch could help scientists to develop molecular electronics for ultra-small computing devices. 

Molecules have long been proposed as alternative circuit components. In 1974, IBM researchers described how a single molecule could pass current in just one direction, thereby acting as a diode, and since then various other components, such as transistors and switches, have been put forward. In theory, single molecule electronics should be smaller than their silicon counterparts, and may require less power. 

Now, in a demonstration of the ultimate limits of molecular electronics, Wilhelm Auw?rter and colleagues at the Technical University of Munich have created what they say is the smallest switch to date. The switch is based on tetraphenyl-porphyrin, a ring-shaped molecule that has four bonding sites in the middle. Two of these sites can be occupied by a pair of protons at any one time, in either a north-south or an east-west configuration. At room temperature, these protons would dart between the two configurations on their own, but by cooling the porphyrin molecule to 6K, the researchers held the protons in just one configuration and then changed it at will, using the electric current supplied by the tip of a scanning tunnelling microscope (STM).  

Crucially, when the porphyrin molecule was adsorbed onto a silver surface, it adopted a curved, saddle shape, changing the conductance of the two proton configurations. In this way, Auw?rter’s group could switch the molecule’s conductance with the STM tip, simply by switching the protons between north-south and east-west. What’s more, when the researchers removed one of the protons with the STM, leaving one remaining, they could switch between four different conductance states - north, south, east and west. 

Saw-Wai Hla, an expert in single-molecule switches at Ohio University, US, thinks the demonstration is interesting because the molecule does not distort during the switching. ’Imagine a surface covered with a two-dimensional array of these molecular switches,’ he says. ’Since the conformation of the molecules does not change, the switching of the molecules will not necessarily perturb their neighbours.’ 

But David Carey, a nano-electronics expert at the University of Surrey, UK, points out that the demonstration, while interesting from a scientific point of view, will not necessarily be achievable in everyday conditions. ’Practically it will be difficult for it to find any real application, because the temperatures are so low and switching using an STM tip is very cumbersome.’ he says.  

Auw?rter admits his group’s demonstration doesn’t bring us ’too much closer’ to real-world molecular electronics. But, he adds, ’our research does not focus on the development of devices, but rather aims at an understanding of basic physical and chemical processes on the nanoscale. In this context, I’m quite amazed that it is possible to "move" an individual proton ... and to visualise the resulting effects.’ 

Jon Cartwright