What Methodology Best Fits 2D-Material Fabrication?

Philip Feng, an engineering professor at Case Western Reserve University, says the bandgap for MoS₂ depends on the number of its atomic layers. Bandgaps can also be continuously tuned by mechanical strain. MoS₂ has special optical properties in terms of transparency, absorption, and refractive indices that might lead to its use in optical sensors. With an ultra-high flexibility and fracture strain limit (~20% to 25%, similar to graphene), in addition to its piezoelectricity, MoS₂ should be a good candidate for electromechanical sensors.

Coleman said that among many types of 2D materials, black phosphorus (or phosphorene) seems to have captured people’s interest (see the table). It’s a single layer of phosphorus, and has a controllable bandgap of 1.5 eV for a monolayer to 0.59 eV for a five-layer sample.

A controllable bandgap, made possible by adding or removing layers of material, is important when designing electronics. A small gap would be desirable when building semiconductors and transistors. However, higher-power electronics, like some analog and radio-frequency devices, may demand a higher bandgap.

The bandgap is also direct, meaning it emits light. By changing the bandgap, engineers can change the color of the emitted light. Thus, it’s ideal for photodetectors and solar cells. When asked if black phosphorus could be used in quantum dots for televisions, Coleman said, “It is far too soon to make assumptions as to where black phosphorene will be adopted.”

Feng added, “Black phosphorus is also anisotropic in all electrical, optical, and mechanical properties. This means its properties can change based on its orientation along the x-axis or y-axis. With high electron mobility in both n- and p-type transistors, it could be used to make CMOS phosphorene devices.” This is also true for MoS₂.  

Black phosphorous does degrade in water and oxygen. This may not be a problem, though, because large quantities of it can be made with a liquid exfoliation process. In addition, experiments with a solvent of N-cyclohexyl-2-pyrrolidone (CHP) revealed prevention of degradation from water and oxygen. There are also methods to encapsulate phosphorene layers in protective coatings to prevent degradation.

Making 2D Materials

Many processes can be used to create 2D materials, four of which are discussed in this article: chemical vapor deposition, micromechanical cleavage, liquid exfoliation, and dry exfoliation. There are two general types of approaches—top-down and bottom-up methods. The top-down approach is like exfoliation, in which a bulk material gets separated into nanosheets. The bottom-up approach adds layers onto a substrate, or grows them from smaller atomic or molecular components.

Thin films can be grown using chemical vapor deposition (CVD). Various types of CVD processes are able to produce an array of materials, including 2D materials. The details of CVD can be rather complex, but the basic process involves gas-phase precursors deposited onto a substrate in a reaction chamber. CVD methods have been used for several decades to make thin films in the semiconductor industry.

Several important factors must be considered when selecting a type of CVD process to produce a material. They include cost and chemistry, along with desired qualities and properties of the resulting material.

Fortunately, for elemental materials like graphene, CVD growth should be more straightforward than materials with a more complex chemistry. Graphene can be grown using liquid, gas, and solid carbon-containing precursors. Two CVD processes that work well with making graphene are plasma-enhanced CVD and thermal CVD. Plasma-enhanced CVD is particularly beneficial because it’s also one of the least expensive methods. A range of pressures and substrates can be used when producing 2D materials in different growth processes and aid in moving them into a commercial market through mass production.

Qing Hua Wang, assistant professor of materials science at Arizona State University, mentioned that CVD of graphene on copper foil substrates is a promising technology used by companies like Samsung and IBM. Commercial applications that could arise from bottom-up-grown graphene include transparent conductors that exploit graphene’s broadband optical transparency, electrical conductivity, and mechanical properties. Other promising applications for top-down-exfoliated graphene include conductive inks and printable materials.

“The most promising methods for producing graphene at a scale appropriate for industrial applications would likely be CVD growth and liquid-phase exfoliation,” said Wang.

Mechanical cleavage, also known as micromechanical exfoliation, uses adhesive tape to pull apart layers of bulk graphite to obtain single atomic layers that are transferred to a supporting substrate. This method was utilized by Andre Geim and Konstantin Novoselov in their experimental work showing graphene’s unique properties (for which they won the Nobel Prize). This method produces very high-quality graphene, but has very low yield and cannot be scaled up for industrial applications.

In addition, electrostatic or electromagnetic forces can be used to split the bulk material into nanosheets. Such methods can be performed in air, a vacuum, or inert environments. These processes, including mechanical cleavage, are otherwise known as dry exfoliation.

On top of that, scientists developed a new process in which large amounts of nanosheets can be produced in a liquid-exfoliation process. It uses amide solvents such as N-methyl-2-pyrrolidone, N-cyclohexyl-2-pyrrolidone, and in some cases, isopropanol alcohol. The surface of the bulk material is weakened by the solvent, and then subjected to ultrasonic frequencies that displace or exfoliate nanometer-thick sheets from the bulk material. A centrifuge removes unexfoliated material.

Liquid exfoliation can make nanometer-thick sheets of black phosphorous. In early experiments, the standard was 1,000 rpm for 180 min to yield stable nanosheets of black phosphorous. These experiments also revealed that it was possible to change the centrifuge’s speed to produce layered sheets in different sizes. Liquid exfoliation could also be scaled up for mass production, according to Trinity College’s Coleman.

There’s a dry-transfer method that doesn’t generate nanosheets, but rather applies them to make electronic devices and mechanical sensors. It’s shown the ability to produce pristine MoS₂ field-effect transistors (FETs) without additional wet processes.

The process initially employs the conventional wafer-scale process to pattern the electrodes, trenches, and other features. Then it dry transfers the 2D crystals on top, with alignment, to form the devices. This does allow for post processes to be administered and, in the case of thermal annealing, can even improve device performance.

The dry-transfer process is clean, easy, and can produce high-quality devices. It works well for the fabrication of transistors, suspended transistors, and other nanoelectromechanical devices.

The goal in mass-producing 2D materials is to take advantage of unique physical properties found in these atomically thin materials, and to make them in a form that’s easy to process with existing industrial methods. A number of challenges still must be overcome in order to produce large, atomically uniform 2D layers, but the production of graphene for a commercial mass market seems inevitable.

Many of the mentioned processes have reel-to-reel capabilities, such as using a copper substrate for CVD growth. More manufacturing processes are geared toward mass production to reduce costs and make graphene more available. In that light, some of the techniques that work for multiple 2D materials may aid in bringing graphene to the market.

Market and Projections

 “The industry realizes that there are fundamental road blocks when working with silicon,” said Trinity’s Coleman. “You can only go so far with it until you need to find a new material. We are studying 2D materials to see if they might be the next step.”

According to Case Western Reserve’s Feng, “We may be able to learn from history—if graphene devices have already started to be commercialized for selected niche applications by now (2015), this is about 10 years after its discovery. By leveraging the infrastructures developed for graphene, the adoption of other 2D materials could potentially be faster, if the technology prototypes are ready. So, optimistically we can say 10 years or maybe be even sooner. The impact could be large, but it will likely happen gradually rather than abruptly.”

And Andy Goodwin, Commercial Director of Advanced Materials with Thomas Swan & Co. Ltd., said, “We developed our physical exfoliation route in collaboration with Coleman at AMBER Nanotechnology Centre, Trinity College, Dublin. The know-how which was developed is licensed exclusively to Thomas Swan & Co. Ltd.”