For the first time, physicists have built reliable, efficient graphene nanomachines that can be fabricated on silicon chips. They could lead to even greater miniaturization.
The chances are that you own a microelectromechanical device—probably dozens of them. These devices fill the modern world. They make possible the accelerometers in smartphones, the microphones in laptops, and the micromirrors in digital projectors, to name just a few.
They are typically a few micrometers in size, tiny by any standards. But scientists and engineers want them even smaller—on the nanometer scale, if possible. At that size, these machines can work as simple switches in logic and memory devices, raising the prospect of more powerful and more efficient data-processing devices.
These micromachines are generally carved into silicon chips. But as they get smaller, silicon switches become less efficient because they leak current when they are off. A better option is a graphene switch, which is easy to carve on a nanometer scale and relatively straightforward to build into conventional silicon chips. Neither does it leak current when it is off.
But there is a problem. When graphene touches silicon, it tends to stick fast. Imagine a switch consisting of a flexible graphene bar that forms a circuit when the bar touches a silicon electrode. If the bar sticks to the electrode, it cannot be switched off again.
This problem is known as stiction. And despite significant financial investment in graphene research by governments all over the world, nobody has found a good way to solve it.
Enter Kulothungan Jothiramalingam at the Japan Advanced Institute of Science and Technology and colleagues, who have found a solution. Using it, they have created graphene-based nanoelectromechanical devices that can act as switches and even as logic gates.
Their method is straightforward. They coat a silicon chip with nanocrystalline graphene, which sticks fast to the surface. Then they cover this with a layer of hydrogen silsesquioxane, which acts as a resist and can be carved into various shapes. On top of this they place another layer of graphene.
The trick is to carve the top layer of graphene into a bar shape that is anchored at both ends by electrodes. Then they remove the hydrogen silsesquioxane layer underneath part of the graphene bar to leave it suspended above the graphene layer.
Bending this bar is simple. A potential difference between the layers creates a force that bends the bar to toward the chip. When it touches this lower surface, it forms a circuit, a process that can be exploited for logic and for data storage.
That’s the switch. And because the two surfaces that come into contact are both graphene, there is no stiction. Switching off the potential difference releases the bar, which springs back into its original position.
Jothiramalingam and co used this approach to build a variety of proof-of-principle nano-switches, including single switches and an array. They say the devices work well with low voltages of just 1.5 volts and that in the off state, there is very little current leakage because the graphene bars are well insulated from other conducting layers.
There are some challenges, though. For example, the shape and size of the graphene beam and its distance from the lower layer need to be optimized to achieve reliable switching. But this should be a straightforward engineering problem.
Once that is solved, more complex devices become possible. The team has designed a range of more complex switches including an AND logic gate and a three-terminal switch in which they place three layers of graphene on top of each other, separated by an insulating layer of hydrogen silsesquioxane.
That’s interesting work with the potential to make nanoelectromechanical devices even smaller, based on the promise of the wonder material that is graphene.
Ref: arxiv.org/abs/1901.07754 : Stacking of Nanocrystalline Graphene for Nano-ElectroMechanical (NEM) Actuator Applications