No, we’re not ready to quit graphene, but the field of super-exciting 2D materials is getting rather crowded these days. That’s good news for a laundry list of desirable items of the future, including solar energy, wind energy, energy storage, and electric vehicle batteries. Lighter-smaller-faster-cheaper is the name of the game, and the use of nontoxic, abundant materials is a plus.
With that in mind, let’s take a look at two new developments that could nudge conventional silicon semiconductors aside and spark a whole new clean technology revolution, if and when they finally go bounding out the laboratory door.
Graphene Rival #1: Molybdenum Disulfide
For those of you new to the topic, molybdenum disulfide belongs to an unusual family of semiconductors called TMDCs, which stands for transition metal dichalcogenides (molybdenum is a brittle metal often used to make steel alloys).
TMDCs have been called “ideal building blocks” for atomically thin electronic and catalytic devices — if only anybody could understand how they really work.
If you never heard of such a thing as 2D-TMDC before, join the club. They were only identified on an experimental basis a few years ago, in 2010 (graphene was discovered in 2004, for those of you keeping score at home).
Since 2010, researchers have been trying to match the performance in reality of 2D-TMDC materials with their performance in theory, without any luck.
The new breakthrough, from the Molecular Foundry at the Energy Department’s Lawrence Berkeley National Laboratory, unlocks one of the key mysteries of 2D-TMDC materials, at least as far as molybdenum disulfide goes.
Using a piece of equipment called a Campanile probe (more on that in a second), the team was able to identify something unexpected: an “energetically” disordered region at the edge of molybdenum disulfide crystals. Here’s the rundown from Berkeley Lab:
This disordered edge region, which has never been seen before, could be extremely important for any devices in which one wants to make electrical contacts. It might also prove critical to photocatalytic and nonlinear optical conversion applications.
As for the Campanile probe, that’s a microscope with a four-sided tip, two of which are coated with gold, all mounted on an optical fiber. Named after the clock tower at UC-Berkeley, the probe enabled the researchers to chart the complex activity at the edge of 2D-TMDC crystals.
This image shows the significant difference in resolution between the Campanile probe and conventional technology:
But wait, there’s more. In addition to identifying the disordered region for the first time, the new study also reveals that the edge of molybdenum disulfide consists of less sulfur than expected, which is a good thing:
Less sulfur means more free electrons are present in that edge region, which could lead to enhanced non-radiative recombination. Enhanced non-radiative recombination means that excitons created near a sulfur vacancy would live for a much shorter period of time.
Excitons, btw, are what enable semiconductors to do their job (excitons are couples consisting of an excited electron and a hole bound together).
For more on that you can find the Berkeley Lab study at Nature.com under the title, “Visualizing nanoscale excitonic relaxation properties of disordered edges and grain boundaries in monolayer molybdenum disulfide.”
Graphene Rival #2: Black Phosphorus
This next one comes to us from Pohang University of Science and Technology (POSTECH) in Korea, which this week reported that they figured out how to mod out black phosphorus into a “unique state of matter.”
Before we dig into that unique state of matter, the folks at POSTECH have provided us with a really good explanation for why the race is on for 2D alternatives to graphene.
Back in 2010, as they tell it, graphene was the only 2D game in town:
Graphene is extremely thin and has remarkable attributes. It is stronger than steel yet many times lighter, more conductive than copper and more flexible than rubber. All these properties combined make it a tremendous conductor of heat and electricity. A defect-free layer is also impermeable to all atoms and molecules. This amalgamation makes it a terrifically attractive material to apply to scientific developments in a wide variety of fields, such as electronics, aerospace and sports.
In its natural state, graphene has a band gap of zero, which means that it acts like a conductor. That’s great if you’re looking for a conductor. Weirdly, (and yes, graphene is weird) graphene also functions as a semiconductor under certain circumstances, but manipulating or “tuning” its band gap is difficult.
Meanwhile, like Berkeley Lab, POSTECH has gone off the graphene path. Black phosphorus is the stable form of phosphorus, and if you get some atom-thin layers of black phosphorus together you end up with something called phosphorene, which would be an allotrope of phosphorus (allotrope refers to elements that can take more than one physical form).
The POSTECH team took some phosphorene and transferred electrons to its surface, a process called doping. That enabled the team to manipulate the band gap, which they found could be done with relative ease.
Significantly, the team was able to achieve a band gap mimicking the unique behavior of graphene in its natural state. If the research progresses apace, phosphorene will be a serious challenger to the silicon semiconductor monopoly: a material that is as light and thin as graphene, but more easily tunable to fit different applications.
Image Credits: Top (rendering of Campanile probe scanning sample) via James Schuck, Berkeley Lab; middle (rotated; MoS2 flake captured by Campanile probe compared to scanning confocal microscopy) via Berkeley Lab; bottom (tunable band gap created in phosphorene) via Institute for Basic Science/Eurekalert.
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