Scientists at Mackenzie Presbyterian University’s Graphene and Nanomaterials Research Center (MackGraphe) in São Paulo, Brazil, collaborating with colleagues at Pennsylvania State University in the United States and other institutions, have described in the journal Science Advances a method of improving the efficiency of transistors made of molybdenum disulfide (MoS2), a material already widely used as a solid lubricant. MoS2 has interested the scientific community in recent years because of its electronic and optical properties. The group applied gold atoms to a MoS2 transistor and enhanced its electrical and thermal conductivity.
“MoS2 is a transition metal dichalcogenide [TMD] comprising two atoms of sulfur, a chalcogen, for every atom of molybdenum, the transition metal. This material forms a two-dimensional crystal, like graphene. Both natural MoS2 and graphite [from which graphene is obtained] are what we call lamellar materials because they’re made up of thin layers. They can be delaminated to arrive at single layer. The interesting aspect of this group is that several of its members, including MoS2, are semiconductors,” said Christiano José Santiago de Matos, principal investigator for a Thematic Project conducted at MackGraphe and supported by FAPESP.
According to Matos, it is easier to control electrical conductivity in semiconductive materials than in conductors like graphene, for example. “In semiconductors, such as silicon or MoS2, you can control whether or not they pass an electric current. These materials are of paramount importance in electronics, which is based on a binary code of zeroes and ones, the ‘language’ of digital computers,” he said.
One way to control conductivity in a semiconductor is by doping it – removing an atom and adding an atom of another material – but in 2D or layered materials this can create “defects” in the material, and impair rather than enhance conductivity.
To avoid this effect, the scientists doped the material by adding atoms without removing any. “Using a concept from coordination chemistry, we added atoms of gold to the surface of the material instead of removing atoms of sulfur or molybdenum,” Matos explained. “This led to interaction between the charges in the MoS2 and the gold, so that some electrons in the semiconductor were partially trapped in the atoms of the metal. The gold atoms retained electrons, and the material was overloaded with positive charges, which increased its conductivity. The upshot was enhanced control of the device with hardly any effect on its performance.”
According to Matos, once the technique has been sufficiently studied and refined, it is likely to be used in commercial electronics manufacturing. MoS2 and other layered materials are widely studied for use in transistors, which are basic components of the chips responsible for memory, logical operations and communications in computers.
“Our work represents a significant advance in the field, as transistors are key components in binary electronics,” Matos said. “The first device made of graphene was a transistor. However, graphene is a conductor, so the performance of a graphene transistor compared to a silicon transistor is far worse in terms of the possibility of switching a current on and off. As a result, scientists have naturally begun looking for other 2D materials that are semiconductors. MoS2 has emerged as an excellent candidate. More and more research is being done into transistors made of this material.”
The project was supported by FAPESP via a grant for the purchase of multi-user equipment, and postdoctoral scholarships in Brazil and abroad awarded to Daniel Grasseschi, a co-author of the article and currently a professor at the Federal University of Rio de Janeiro (UFRJ). Scientists at Shinshu University in Japan and the University of Virginia and Binghamton University in the US also participated in the project.
According to Matos, the participants were motivated by several challenges, including a matter of basic chemistry. “This doping technique specifically hadn’t been used in 2D materials before,” he said. “Many articles describe doping techniques, but most involve substituting atoms, and when anything is added to the surface, it’s a much more complicated molecule. Working with isolated gold atoms is rare because they tend to cluster. It’s hard to keep a gold atom isolated. We showed that when you can do so, the technique is relatively easy to use. And it has important applications.”
As well as improving electrical conductivity, the presence of gold atoms also had a positive effect on thermal conductivity. “Heat dissipation is another problem in electronic devices. Heat generation without dissipation will damage a device. The application of gold atoms resulted in an enhancement that could be used to raise the heat dissipation rate in 2D transistors based on this and other semiconductive TMDs,” Matos said.
Another feature of semiconductors is light emission, which the scientists also changed. “Color, the frequency at which light is emitted, is the result of the material’s characteristics. When we doped the semiconductor, we changed these characteristics. We conducted a number of studies with this modified material, and found a difference in its light emission: the set of frequencies available in the emitted light was different in the material enriched with gold atoms. We continue to study the optics,” Matos said, stressing that light emission is one more application of these materials, which are already used for this purpose in electronic appliances.
To test the conductivity of transistors containing gold atoms, the researchers fabricated ten devices and used electron microscopy to examine the interaction between the gold and sulfur atoms on the surface. “Daniel Grasseschi did much of the experimental MoS2 doping process, as well as the initial tests and electron microscope imaging, during his postdoctoral fellowship at Pennsylvania State University, with more than one group there participating under the leadership of Professor Mauricio Terrones. They made several transistors, which all worked well, and conductivity wasn’t significantly affected. When he came back, we worked with him on optical and spectroscopic characterization at MackGraphe. The computer simulations were also performed here at Mackenzie by Professor Leandro Seixas, with contributions from Professor Camila Maroneze,” Matos said.
The scientists now plan to demonstrate that the technique works with other materials. “It’s possible to change both the material used and the TMD, but using other metals raises a few challenges because coordination chemistry depends a great deal on the number of electrons available in the last layer of the metal,” Matos explained. “Gold and silver, for example, have the same number of electrons in the top layer and are in the same column of the periodic table, so if we substitute silver for gold the chemical reaction is very similar. We show this in the article, which describes tests with silver. If we now use metals in other columns of the periodic table, we’ll be dealing with a different number of electrons in the top layer, and the chemistry will change as a result.”
The article “Spontaneous chemical functionalization via coordination of Au single atoms on monolayer MoS2” is at: https://advances.sciencemag.org/content/6/49/eabc9308.
Featured image: After proving that gold increases the efficiency of devices made of molybdenum disulfide, a solid lubricant and mooted ‘silicon substitute’, researchers at MackGraphe plan to test the technique on other materials (image: Leandro Seixas/MackGraphe)
Provided by FAPESP