Ultrasmall and Ultrathin: One Atom Thick Nanomaterials
Over the past few decades, our phones and computers have become increasingly powerful, allowing them to process more and more data and create incredibly impressive graphics. These increases in power and efficiency have been driven by the billions of tiny on-off switches, also known as transistors, which we have managed to fit onto a single piece of silicon by making transistors smaller and smaller. However, we are beginning to reach the physical limit of how small a transistor can be. Modern transistors are tens of nanometers long, which is around 1/1,000th the width of a human hair. For comparison, a Covid virus is about 100 nm in size, equivalent to around 10-20 cutting edge transistors.
The semiconductor industry describes the size of a transistor by the ‘node’ size. Cutting edge nodes are described as ‘14-nm’, ‘10'-nm’, and even ‘5-nm’. These sizes originally described the gate length of a transistor, which can be thought of as the transistor’s width. However, nowadays the names ascribed to node technologies are less physically meaningful. What we can say about them is that, given an atom of silicon is around 0.2 nm in size, current transistors are 10s of silicon atoms long. At these length scales, quantum effects start to become noticeable. For instance, tunneling of electrons across the transistor can occur, even in the off-state. This can prevent it from acting like a proper on-off switch, resulting in it inadvertently turning 0’s into 1’s and wreaking havoc on the software in your device. In order to get around this, companies have begun vertically stacking transistors on chips using what are known as chiplets. This presents many advantages over trying to continually shrink the size of transistors, but also presents a new potential challenge - making components thinner. This is where ultrathin materials could come into play.
Ultrathin materials are materials that are sub-nanometer in thickness. One especially promising class of ultrathin materials are what are known as layered materials. Layered materials are characterized as having weak interactions between layers, but strong interactions within each layer. Think of them like a stack of sticky notes where there is a small adhesive force holding the pieces together. Novel materials, such as graphene, phosphorene, and transition metal dichalcogenides all fall into this category.
These materials first gained popularity around 2004/2005 when it was discovered that the individual layers in these materials could be removed simply by peeling them apart with a piece of tape (we call this mechanical exfoliation). Using this method, graphene - which is a single atomic layer of carbon - was isolated in a stable form for the first time, leading to a Nobel prize. Since then, scientists have found these materials particularly interesting because their properties are confined in the vertical direction. They are therefore referred to as 2-dimensional. This vertical confinement leads to interesting quantum properties and has engendered an explosion of 2-D materials research over the past decade.
Today, layered materials are being studied across many scientific disciplines and show numerous potential use cases. For instance, these materials are able to withstand more strain than their bulk counterparts and are therefore being used to design flexible, wearable devices. They are also being used by physicists who are exploring the interesting quantum phenomena of these materials. One such phenomenon is an intriguing correlation between spin and momentum, providing possible opportunities for quantum computing applications.
These unique properties mean that these ultrathin materials could facilitate numerous novel devices. However, before they can replace silicon, they need to make the jump from the lab bench to the production line. Our modern world is currently powered by silicon because we have nearly perfected its production. Silicon is now purified to the 9N level, which means it is over %99.9999999 pure. Matching these standards in any other material at an industrial scale will take some time.
Once the processing techniques for these materials, such as chemical-vapor deposition and chemical exfoliation, become feasible on an industrial scale, silicon could be displaced in certain applications. This would open up a whole new world of innovation and even allow us to create the next generation of devices that change our world.