Atomically thin materials like graphene and carbon nanotubes have the potential to provide significant benefits compared to today's electronics, like smaller features, lower operating voltages, and more efficient performance. So, even though we're struggling to figure out how to use them in bulk manufactured electronics, lots of organizations are spending money, brains, and time to work that out.

Note the phrasing above—potential. Since it's been incredibly hard to make transistors based on these materials, we aren't entirely sure how all of them will behave. A group of researchers from China's Peking University decided it was time to cut down on some of the uncertainty. The answer they came up with: transistors made with carbon nanotubes and graphene that perform so well they're pushing up against the fundamental limits set by Heisenberg's uncertainty principle.

We still can't necessarily make a chip full of these things, but their work does show it's worth the continued effort to try to figure out how.

The reason we can't make this sort of nanomaterial-based hardware in bulk comes down to the annoying problem of getting a carbon nanotube in the right place. They're difficult to grow, and we can't control whether the resulting nanotube acts as a metal or a semiconductor (or whether it's a p- or n-type semiconductor, for that matter). While we can grow them externally and select the kind of nanotube we want, it then gets very challenging to put them in the right place on the hardware.

The Beijing-based group avoided these issues entirely. They simply placed nanotubes at random on a silicon surface, determined their behavior, and then built circuits on top of the ones with the right properties. You can't build a computer this way, but it does allow you to test the properties of individual transistors.

Initially, the team built relatively standard transistors on top of the nanotube. Source and drain electrodes were provided by metal wiring, and a layer of hafnium oxide set between them acted as a gate that controlled the state of the transistor. The gate of the transistor was 10nm long, a size that chipmakers are currently struggling to reach using silicon. On some levels, the nanotube transistors worked pretty well, allowing a higher current through even at voltages that were half those used in recent Intel processors.

But the approach didn't scale down. When the researchers did a feature shrink to 5nm, the performance went up considerably in terms of the amount of current that went through when the transistor was switched on. That's because 5nm is smaller than the average distance it would take for an electron to bump into something when moving through the nanotube. The problem was that the transistor couldn't be controlled; electrons could simply tunnel between the source and drain even when the transistor was switched off.

The scale of the problem is largely a function of the size of the source and drain electrodes—you shrink those, and you shrink the amount of tunneling. So the team decided to go straight for the smallest electrodes possible: single-atom thick sheets of graphene.

To build these transistors, the researchers put down a small sheet of graphene and then used plasma etching to burn a 5nm gap in the sheet. The carbon nanotube was then placed across this gap, and a gate made of yttrium oxide and palladium was placed on top. This turned out to be extremely effective. The gate extended beyond the 5nm feature and so it had a larger area of contact with the carbon nanotube. In addition, it covered parts of the graphene sheets and influenced their ability to conduct charges as well.

The results were rather impressive. The authors estimate that Heisenberg's uncertainty principle limits the fastest switching speed of the transistor to 40 femtoseconds. The actual hardware switched in 43fs, about five times faster than 10nm silicon transistors. It still required very little voltage to switch and, once on, the nanotube was occupied by an average of only 1.3 electrons at a time. In other words, a single electron was enough to switch it between its on and off states. And the off state no longer suffered from the tunneling problems of the earlier iteration.

It can be hard to relate these numbers to things you'd actually care about, so let's run through them. The speed of switching is related to the clock speed of the processor—faster switching makes for faster processors. The amount of voltage required for it to operate is related to the power used by the processor, so lower voltage means lower power consumption. And resistance in the wiring is what produces a lot of the heat generated inside processors, so having the high conductance of a nanotube cuts down on this issue, as well.

So carbon nanotubes do actually perform like the wonder material we thought they would be. Now we just have to figure out how to build something with them.

Science, 2016. DOI: 10.1126/science.aaj1628 (About DOIs).