For the last few years, graphene has been all the rage, with repeated articles, discussions, and papers on the material’s amazing conductive properties and fundamental strengths. Unfortunately, all of that positive press has overshadowed a significant problem with using graphene in semiconductors — it’s nearly impossible to make the material stop conducting. Now there’s a new material gunning for graphene’s spot, black phosphorus. A recent manufacturing discovery could make black phosphorus easier to work with and spur adoption and research into the new material.
Phosphorene vs. graphene
Up until now, black phosphorus, also known as phosphorene, has been produced in the same fashion as graphene — by exfoliating sheets of the material using scotch tape. Unfortunately, this method has exactly the same problem as graphene — it’s nearly impossible to produce the material in volume. What sets phosphorene apart is a recent discovery that the material can be separated via ultrasonic waves.
The huge advantage of phosphorene over graphene, at least in theory, is that phosphorene has a natural band gap. This means that phosphorene doesn’t conduct electricity at every energy state. Phosphorene also maintains this bandgap in monolayers, few-layers, and bulk forms, which means that stacking it up in more than a single sheet doesn’t automatically destroy its direct bandgap properties.
The new method of exfoliating phosphorene is to place it in a liquid solvent and then bombard that solvent with acoustic waves in a process known as Liquid Phase Extraction (LPE). This creates stable thin sheets of the material that are markedly larger than traditional exfoliation methods and allow for further study. Even more importantly, this manufacturing process allows the team to “sort” sheets of varying size and separate them for specific study rather than lumping all of the nanosheet thicknesses together in an undifferentiated mass.
Material data on the black phosphorous sheets, including height, layers, and feature size
The caveat to this work, and phosphorene’s current Achilles’ heel, is that the material degrades upon contact with water and oxygen, including the atmosphere of Earth. The research team behind the LPE tests notes that this only occurs when the material is removed from its solvent.
This might not be a major problem going forward, if a way can be found to keep the material encapsulated, or if it can still be incorporated into hardware at the manufacturing stage. EUV (Extreme Ultraviolet Lithography) is done in near-vacuum conditions and under strict atmospheric controls. In theory, there could be opportunity to work with a material like black phosphorus without exposing it to water and oxygen at levels that would degrade its performance. The team notes that while the degradation does occur, it begins at the edges of the material and does not take place instantly — there’s time enough to incorporate the phosphorene into tests before it becomes a critical problem (8% of the phosphorene dissolved after three days).
If this degradation can be solved or managed, then phosphorene could be a breakthrough in technology, but after seeing both graphene and carbon nanotubes debut to such tremendous promise, only to run into problems at future dates, we’re going to hold back a bit of enthusiasm. Graphene’s production problems may or may not have been solved, but using the material remains a fundamental challenge