Graphene has long been seen as the future of computer processors and electronics. However, over the past couple of years, some remarkable two-dimension crystal materials have emerged. One new challenger is black phosphorus. This week, a Korean research team has figured out how to create a tunable band gap in the material, allowing it to be used as a semi-conductor, and (potentially) a superior replacement for silicon.
What does this mean for semiconductors, and the future of graphene? Let’s find out!
Like graphene, black phosphorus can be separated into one-atom-thick sheets. These sheet are known as phosphorene, but unlike graphene these layers act as an excellent semiconductor that can easily be turned on and off, hopefully substantially lowering the power requirements for a new generation of ultra-conductive transistors. Graphene is extremely conductive, but lacks a natural band gap, and this is where black phosphorus could step in.
Black phosphorus is a thermodynamically stable allotrope of the element, phosphorus. Stable at room temperature, black phosphorus is not a ‘naturally occurring’ substance and is only obtained by heating white phosphorus under extremely high pressure, some 12,000 atmospheres. The resulting black phosphorus crystals feature puckered honeycomb layers, with interlayer distance of 0.5 nanometers, another similar feature to graphene.
Once created, black phosphorus is difficult to manufacture in large quantities at the specified width. The traditional method, also applied to other two-dimensional materials, is that of mechanical exfoliation. In this painstakingly slow process researchers crush an amount of black phosphorus into a compressed powder, then use adhesive tape to slowly peel back layers until they create a film just a few layers thick. It is limited and limiting to both manufacturing and research.
Realizing just how restrictive this method is, Mark C. Hersam, a chemist at Northwestern University developed a new technique using solution chemistry to speed production. They place a crystal of black phosphorus and a solvent in the bottom of an ultrasonic tube, which uses a rapidly vibrating metal tip to agitate the liquid.
The resulting sonic action, combined with the solvent separates the black phosphorus into the required nanometer thick sheets, suspended within the liquid. Researchers can then spin-coat this ‘ink’ onto surfaces, creating a random distribution of thin black phosphorus flakes.
Whilst the ultrasonication technique produces a slightly larger yield, and is a quicker process, the random distribution is somewhat problematic. To create truly efficient transistors using black phosphorus researchers and engineers must be able to spin-coat the surfaces with much greater precision. This is the next goal for researchers.
A major advantage of black phosphorus appeal is its natural band gap. The band gap, or energy gap is what separates conductive materials from semiconductors. It works like this:
- Graphene is an excellent conductor, which is what makes it attractive for computer processors. Little resistance means little heat. Unfortunately, we don’t yet know how to switch it into a non-conductive state. Graphene transistors can’t turn off. While there may be ways to solve this problem, no-one’s cracked them yet.
- Black phosphorus is also an excellent conductor, but it also has an energy gap, meaning the amount of energy passing through the material can be switched between conducting and insulating. By doping black phosphorous, you can create traditional transistors easily. You can also tune it to produce really specific behaviors, allowing for exotic electronic circuits.
It is this wide-ranging band gap that fills materials scientists with excitement. This, combined with black phosphorus’ high photo-sensitivity could see the semiconductor used in everything from chemical detection to optical circuitry.
Black phosphorus is also referred to as a “direct-band” semiconductor. This is a rare property, meaning the material can effectively and efficiently convert electrical signals back to light, making it a prime candidate for on-chip optical communication. University of Minnesota Department of Electrical and Computer Engineering graduate student Nathan Youngblood, whose paper on black phosphorus featured in Nature Photonics believes:
“It is really exciting to think of a single material that can be used to send and receive data optically and is not limited to a specific substrate or wavelength. This could have huge potential for high-speed communication between CPU cores which is currently a bottleneck in the computing industry right now.”
A Silicon Replacement?
While Silicon Valley would need to be renamed, black phosphorus could be the material to take processor design to new heights. Ideally, Black Phosphorus will lower the operating voltage of transistors coated with the aforementioned ‘ink.’ This will lower the heat produced during usage, allowing processors to be clocked faster without overheating, a process that has largely stalled in favor of adding more cores. This would boost chip efficiency, and – most importantly – overall processing power.
Moore’s Law may well continue as planned!
It isn’t only transistors that could benefit from Black Phosphorus. Other applications within electronics include: solar panels, solar cells, batteries, switches, sensors, and more. But as with most wonder materials, working with, researching, and implementing atomic level materials will take time, so don’t expect an optoelectronic computer playing Minecraft any time soon.
Should We Be Excited?
Yes, of course. We are literally talking about the potential future of both computing and optical communication. We should, however, not rejoice and jump aboard a Black Phosphorus hype train, because it’ll be a long old journey with no definitive end in sight. Amazing materials like Black Phosphorus, like Graphene, like Molybdenum Disulphide are all set to change the future. Just not as quickly as we might like.
Are you excited by futuristic materials? Or is it all just a bunch of hype? Let us know what you think!