The March of 2-D Materials Across the Flatlands
Graphene is not the only 2-D material out there but they aren’t all competitors, some are allies
In 2010, researchers at Ecole Polytechnique Federale de Lausanne’s (EPFL) in Switzerland started to experiment with molybdenum disulfide (MoS2)--which until then had been used primarily as a dry lubricant--to see if in its two-dimensional (2-D) form whether it had attractive electronic properties.
It indeed did have attractive electronic properties, and one that its 2-D cousin, graphene, lacked: an inherent bandgap. The lack of a band gap for graphene has been its Achilles Heel in computer logic applications since chips must be able to turn on and off the flow of electrons to create the “1” and “0” of binary logic.
Of course, there has been much research that has successfully engineered graphene so that it does have a band gap, but with MoS2 you had the high electron mobility of graphene and you could turn the flow of the electrons on and off without the need to engineer it.
Since that work with MoS2, the field of 2-D materials—sometimes referred to as the flatlands—has been growing with new materials being added to the mix fairly regularly. The early work with these materials has indicated that they all have different strengths and weaknesses. Some look promising for logic and memory chips while others look best suited for computing with light. And still others are best when combined with another 2-D material.
The List of 2-D Materials Grows
In the last few months, we have witnessed the growing emergence of three new 2-D materials: Tungsten diselenide, Rhenium disulfide and a third that doesn’t really have a name as of yet but belongs to a class of materials known as metal-organic frameworks (MOFs) in which metal ions are coordinated with rigid organic molecules to form a porous material that can be one-, two-, or three-dimensional. To add to this list, earlier this year, we got word that borophene may be joining the 2-D material club.
These 2-D materials have all been making the news in the past six months. However, a more complete list of the main 2-D materials would include: boron nitride, silicene and germanene. We should expect more introductions of even newer 2-D materials in the near future because there are a host of metal oxides that could lend themselves to a 2-D form. Some have calculated that are more than 100 of these layered materials that could be made into 2-D materials so surely more are on the way.
A Quick Rundown of Recent 2-D Material Developments
Back in March of this year, the latest issue of the journal Nature Nanotechnology looked as though it had been taken over by research on tungsten diselenide (WSe2) with three of the top four research papers being dedicated to this material.
Tungsten diselenide belongs to a larger group of transition metal dichalcogenides that also includes molybdenum disulfide (MoS2). This family consists of materials that combine one of 15 transition metals with one of three members of the chalcogen family: sulfur, selenium, or tellurium. So far, only a few of these transition metals have been experimented upon, so just as there are many metal oxides that could be potentially made into 2-D materials, we are likely to see many 2-D materials coming out of these transition metals.
Recent research has used tungsten diselenide for optoelectronic applications as well as the foundation for a transistor made entirely from 2-D materials.
In the field of optoelectronics, researchers at the University of Washington pursued applications of the material for a light emitting diode (LED). Another research group at the Vienna University of Technology focused on the material’s photovoltaic applications. And finally, researchers at the Massachusetts Institute of Technology (MIT) took a more open approach testing all of the optoelectronic applications for the material that would result from the way it can be switched from being a p-type to a n-type semiconductor.
The University of Washington team claim that with the 2-D version of tungsten diselenide they are capable of making the thinnest-known LED material with standard LED materials being as much as 10 to 20 times thicker.
The Vienna researchers also exploited the thinness of the tungsten diselenide to create a photovoltaic cell that is so thin that 95 percent of incident light passes through it, yet it’s still capable of capturing a tenth of the remaining 5 percent and convert into electricity. This could translate into windows that could serve as solar cells but still allow most of the light to pass through them.
First Transistor Made Entirely From 2-D Materials Using Tungsten Diselenide
One of the most significant developments in the last couple of months with tungsten diselenide has been the work done by researchers at Argonne National Laboratory in which WSe2 was used as the semiconducting layer of a transparent thin-film transistor.
The research, which was reported in the journal Nano Letters back in April, demonstrated how it was possible to make an entire transistor out 2-D materials using graphene for the electrodes, hexagonal boron nitride as the insulator and the tungsten diselenide for the semiconducting channel.
The transistors made entirely from 2-D materials will be only atoms thick, allowing them to be much smaller than their silicon-based competitors and facilitate a super-high density of pixel in next-generation displays.
Rhenium disulfide (ReS2) is a bit of twist on the usual characteristics of 2-D materials. It is in fact a 3-D material that behaves as though it were a 2-D material.
In research, published in the journal Nature Communications, researchers at the U.S Department of Energy (DOE) Berkeley Lab discovered that rhenium disulfide, unlike transition metal dichalcogenides, such as molybdenum disulfide and tungsten diselenide, does not form strong interlayer bonds between the monolayers when in its bulk 3-D form. What this means is that when these other 2-D transition metals are layered they don’t behave like a monolayer 2-D material anymore. On the other hand, because rhenium disulfide doesn’t form these strong bonds between layers it maintains the characteristics of a 2-D monolayer.
In May, a team of researchers from Harvard University and again at MIT developed a 2-D material that when it is built up into a 3-D form still retains its attractive 2-D properties. The material is a combination of nickel and an organic compound called 2,3,6,7,10,11-hexaiminotriphenylene (HITP) and belongs to a class of materials known as metal organic frameworks (MOFs). MOFs are compounds in which metal ions are coordinated to rigid organic molecules to form a porous material that can be one-, two-, or three-dimensional.
In the joint research, the teams discovered that the material has an inherent band gap and self assembles, which promises much easier production routes than other 2-D materials. They also determined that it’s possible to tune the capabilities by simply adding or subtracting its constituent parts. In real applications, this could translate into the ability to tune the material for photovoltaics so that it could capture different wavelengths of light.
“Borophene” is a long-theorized 2-D version of the element boron. While it hasn’t yet been produced, researchers at Brown University did report making a precursor structure for the material that they claim proves the material could be produced.
The research confirmed the theory that borophene wouldn’t be able to form into the honeycomb-lattice pattern that graphene takes because it lacks one of carbon’s electrons. Instead boron has a triangular lattice and a hexagonal hole that would form in the middle of the sheet.
While the Brown researchers believe that borophene would maintain its monolayer characteristics if it could be synthesized in the real world, other researchers believe that the material is unstable and would immediately revert back to a multi-layer material.
The Materials That Fill Out the Flatlands
Borophene may not yet be a full-fledged member of the flatland community, but boron nitride (BN) certainly is.
Boron nitride is in fact a semiconductor, meaning it has a band gap. However, its band gap is so large that for all intents and purposes it’s an insulator.
For this reason, researchers have been focusing on combining boron nitride (semiconductor/insulator) with graphene (a conductor) to develop a hybrid material that could be applied in logic circuits. We’ve seen evidence of that in the work at Argonne Lab, where it was used as the insulator layer for a transparent thin film transistor.
However, in March this year, researchers at the University of California San Diego (UCSD) discovered that light can cause a ripple effect on hexagonal boron nitride that can be maintained long enough for the waves to be usable for practical applications. Among these applications are transmission of information in computer chips, better management of heat flow in nanoscale devices, or the creation higher resolution images than is possible with light.
The optical physics involved are quite complicated. However, Dimitri Basov, a professor of physics at UCSD explained it this way in a press release: "A wave on the surface of water is the closest analogy. You throw a stone and you launch concentric waves that move outward. This is similar. Atoms are moving. The triggering event is illumination with light."
Basov added: "You can bounce these waves off edges. You can bounce them off defects. You can play all sorts of cool tricks with them. And of course, you can design the wavelength and amplitude of these oscillations in a way that suits your purpose."
Silicene and Germanene
Silicene is the 2-D version of silicon and germanene is the 2-D version of germanium. Both silicon and germanium have the advantage in the 2-D arena of already being used extensively by the semiconductor industry.
When silicone was first introduced in 2010, it seemed the answer to all the problems presented by graphene. It had an intrinsic band gap and it wouldn’t require a retooling of the entire semiconductor industry that had formed itself around silicon for the last 50 years.
But at the beginning of this year, all of that promise appeared to be misplaced when researchers from the University of Twente in the Netherlands reported that silicene has what they termed “suicidal tendencies.” What they discovered was that as soon as you started to deposit silicon atoms on a layer of silicene the entire structure would revert back to its silicon crystal structure rather than maintain its 2-D honeycomb structure that allows such high electron mobility.
This latest Dutch research has thrown the future of silicene in electronics into question unless someone can find a way to stop that transformation.
Germanene had long been theoretical material until researchers last year at Ohio State University claimed to have succeeded at growing sufficient quantities of it to measure the material’s properties in detail. However, the history of germanium for electronics is long, dating back to the first transistors, which were made from germanium rather than silicon.
In the Ohio State research one of the properties that was revealed in germanene was that it possesses a “direct band gap” as opposed to the “indirect band gap” such is found in conventional silicon and germanium. What this means is that germanene can more easily absorb and emit light, making it attractive for optoelectronic applications. This could translate into photovoltaic cells that could be 100 times thinner than those made from silicon.