Electronics are ubiquitous in our everyday lives—they are in the cars we drive, the microwaves that heat up our food, and the computers we use. This omnipresence is due to technology’s constant evolution. Currently, unbelievably complex technologies can be found in even common devices, such as our cell phones. The creation of materials that are elaborate in their complexity but simple in their design—and can thus be implemented into many technological devices—sits at the intersection of electrical engineering and materials science.
Judy Cha, Yale Professor of Mechanical Engineering and Materials Science, has led her lab in important work within these fields. Her team focuses on discovering new layered materials that can be used in electronics. Through manipulating their electrical properties, they seek to understand more about the materials themselves and how they can be used. The team hopes to elucidate which materials might be particularly ideal for a certain electronic device, as well as how the materials’ performance can be improved by altering electrical properties.
In the beginning of 2021, Cha’s group and its collaborators published two studies: one focusing on the mechanical properties of graphene with lithium between its layers, and the other on molecular doping—the addition of small molecules to materials to activate them for use in electronics.
2D materials are usually approximately one to three atoms thick. They come from layered materials that are exfoliated down to a single layer. The properties of 2D materials normally change when their layers are isolated. Graphite, commonly found in pencil lead, is a classic example of this: graphite consists of layers of graphene, and the individual graphene layers have properties that differ substantially from those of graphite as a whole. To harness such materials for device applications, it is important to understand these thickness-dependent changes.
Lithium intercalation into graphite—or, the insertion of lithium between layers of graphene that are held together by van der Waals (vdW) forces—is essential to create lithium-ion batteries, which power many of our modern electronics. Staging, or structural ordering, minimizes electrostatic repulsions within graphite’s crystal lattice, allowing lithium to order itself between the van der Waals gaps of graphene. Initially, lithium is randomly distributed throughout the graphite, but as the lithium concentration increases, there’s a phase transition where lithium moves laterally to form intercalated regions that are vertically separated by unintercalated regions. The kinetics of lithium moving between the sheets of graphene are directly related to how well the battery performs.
This staging process is well understood for bulk graphite, which is thick. But on a nanoscale, the way lithium and graphene are confined so closely together affects the overall structure. As lithium makes its way between vdW gaps, these gaps expand to accommodate the new atoms. However, anchoring graphene sheets by clamping the edges down changes the way lithium interacts with the graphene. This constrains how much the graphene is able to open, and it requires more work for lithium to squeeze into those gaps. The kinetics of lithium diffusion are also slowed down, since it becomes more difficult for lithium to move between the graphene sheets.
The effect of this mechanical strain sparked the interest of Cha’s group. To investigate it further, they used thin sheets of graphene—between four and fifteen layers thick—with gold electrodes on top that acted as a clamp. Although these electrodes were only one-hundred nanometers thick, each layer of graphene was even thinner: one-third of a nanometer. The difference in size allowed the gold to act as a source of pressure and hold down the ends of the graphene sheets.
But as they observed this configuration, the group realized that, while the edges of the graphene sheets stayed still, the center would expand freely, stretching in both the x and y directions and causing strain in the graphene. “Interestingly, what we found is this strain can delay the lithiation kinetics of graphene,” said Josh Pondick, a PhD candidate in Cha’s lab and one of the lead researchers for this experiment—lithiation referring to the process by which a lithium ion replaces hydrogen atoms.
The team looked at these properties in different thicknesses of graphene with in situ Raman spectroscopy, a method that provides molecular-level information about material surface structures based on light scattering. They found that, as the thickness of graphene increases, staging is delayed, requiring more electrochemical voltage to be induced.
A major obstacle in this study arose while dealing with lithium, since it is a rather flammable chemical. Lithium-ion batteries are typically assembled in glove boxes filled with an inert, unreactive gas. However, to experimentally monitor things like electrical properties, the lithium had to be taken out of the glovebox. “We spent quite a bit of time trying to devise device geometry and architecture that would allow us to contain all of these volatile chemicals inside a small pouch while we could still safely take it out and do the types of measurements that we wanted to do,” Cha said. But even with these tricky chemical properties under control, simply handling these materials was a challenge due to their small size. The flakes of graphene are on the order of ten to thirty microns—about the real-life size of a white blood cell. Thus, techniques in lithography, a special form of printing, were required to add the gold electrodes.
With the push for new kinds of batteries that use metals like magnesium and sodium rather than lithium, there are plans to see the results of these different kinds of atoms being intercalated in graphene. These atoms are bigger and will undoubtedly provide a larger mechanical strain. Given how strong graphene is, researchers are also considering looking into new materials—notably heterostructures that consist of multiple layers of different 2D materials. Adding intercalants could allow scientists to tune the electrical and chemical properties of these structures to be used in many different devices, from optoelectronic to logic devices.
Alongside the research on lithium-ion batteries and graphene, Cha’s group also published a study on molecular doping. An example of molecular doping is adding boron or phosphorus to silicon, which activates silicon so that it can be used in electronics. This is particularly useful for transistors, which are an important component of computers.
Adding impurity atoms to a 2D material like molybdenum sulfide (MoS2) is not as simple as doing so to a single layer of inactive atoms. The method used in this study involved sprinkling molecules on top of the 2D material. The molecules readily gave away electrons to the material, slightly altering its properties. For this particular study, a synthetic organic molecule known as DMAP-OED was added to MoS2. To evaluate the effect of this compound on the 2D material, the number of electrons donated from it to the material was investigated.
Finding the proper technique to do this proved to be an arduous process. Given the small size of the molecules, optical microscopes would be useless. Although electron microscopes have higher resolution, they would also be ineffective, since they would burn through all the molecules. Alternative spectroscopy techniques were also considered, but they were too crude to properly count the number of molecules. In the end, Cha’s group landed on atomic force microscopy. This method, which uses a sharp tip to scan a surface by interacting with it and tracing its topography, allows for high resolution of small objects.
Ultimately, Cha’s group found that DMAP-OED donates 0.63 to 1.26 electrons per molecule to MoS2—record molecular doping levels.
This work represents one of the first experiments of such a nature done with an organic electron donor. Thus, it will likely lead to the development of more organic electron donors beyond DMAP-OED for this purpose. Moving ahead, Nilay Hizari, Yale Professor of Chemistry and the collaborator involved in the making of DMAP-OED, looks forward to better understanding molecular doping levels in the context of other molecules and materials. “Now, there’s a whole range of small molecules we could try on [those] 2D materials to get some kind of desired property,” he said.
About the Author:
Catherine Zheng is a sophomore BME major in Pauli Murray College. In addition to writing for YSM, she’s involved in research and other organizations on campus like WGiCS.
Pondick, J. V., Yazdani, S., Yarali, M., Reed, S. N., Hynek, D. J., & Cha, J. J. (2021). The Effect of Mechanical Strain on Lithium Staging in Graphene. Advanced Electronic Materials, 7(3), 2000981. doi:10.1002/aelm.202000981
Yarali, M., Zhong, Y., Reed, S. N., Wang, J., Ulman, K. A., Charboneau, D. J., . . . Cha, J. J. (2020). Near‐Unity Molecular Doping Efficiency in Monolayer MoS 2. Advanced Electronic Materials, 7(2), 2000873. doi:10.1002/aelm.202000873
The author would like to thank Professors Judy Cha, Nilay Hizari, and Josh Pondick for their time and enthusiasm in sharing their research.