Karina Ninni | FAPESP agency - Scientists from the Center for Advanced Research in Graphene, Nanomaterials and Nanotechnologies at Universidade Presbiteriana Mackenzie (MackGraphe), in collaboration with colleagues from Pennsylvania State University (United States) and other institutions, described in the journal Science Advances a method that improves the efficiency of transistors made of molybdenum disulfide (MoS2) - material already widely used as a solid lubricant, which has attracted the attention of the scientific community in recent years for its electronic and optical properties. The group applied gold atoms over a MoS2 transistor and, with that, managed to improve the electrical conductivity of the device. In addition, they realized that the gold atoms also improved their thermal conductivity.

“MoS2 is a transition metal dicalcogenide [TMD] formed by two sulfur atoms, which is a chalcogen, for each molybdenum atom, the transition metal. This material forms a two-dimensional crystal, just like graphene. Both natural MoS2 and graphite [from which graphene is obtained] are what we call lamellar materials, as they occur in sheets, making it possible to perform a delamination and reach a single layer. The interesting thing about this group is that several of them, including MoS2, are semiconductors ”, explains Christiano José Santiago de Matos, coordinator of a Thematic Project conducted at MackGraphe, with support from FAPESP.

As the researcher explains, in semiconductor materials it is easier to control electrical conductivity than in conductors, such as graphene, for example. “In semiconductors, such as silicon and MoS2, it is possible to control the flow of the current or not. These materials are very important for electronics, based on binary code 0 and 1; this is the ‘language’ of digital computers, ”he describes.

One of the strategies to control the conductivity of the semiconductor is doping: an atom of another material is added to the semiconductor, replacing an original atom, which is removed. But in two-dimensional or lamellar materials, this removal followed by subsequent replacement (that is, the “creation of defects” in the material too much) sometimes ends up hindering the conductivity, instead of improving it.

To avoid this effect, scientists doped the material without replacing the atoms, but adding others. “We use the concepts of an area of ​​chemistry called coordination chemistry in which, instead of taking atoms - of sulfur or molybdenum -, we apply gold atoms to the surface of the material. In this way, we make possible the occurrence of an interaction between the charges of MoS2 and gold, in which part of the semiconductor electrons ends up being partially trapped in the metal atoms. The gold atoms play a role in electron retention of the material, which then has an excess of positive charges, having a greater conductivity. Thus, we offer a possibility to improve the control of the device, causing little impact on its performance ”, says Matos.

According to him, there is a good chance that this type of technique, improved and sufficiently studied, will reach commercial electronic devices, since MoS2 and other lamellar materials have been widely studied, including for making transistors, which are the basic components. of electronic chips, responsible for memory, logical operations and communication flows in computers. “Our work represents a significant advance in the area, since the transistor is a basic unit for binary electronics. The first device made of graphene was precisely a transistor. Only graphene is a conductor. Therefore, the performance of a graphene transistor, compared to a silicon transistor, is much worse when it comes to the possibility of 'switching the current on and off'. Faced with this, of course, the scientists turned their eyes to other 2D materials that were semiconductors. MoS2 appeared as a great candidate and transistors made with it are being studied more and more ”, he says. The project received support from FAPESP for the acquisition of multi-user equipment, in addition to postdoctoral fellowships - in Brazil and abroad - awarded to Daniel Grasseschi, co-author of the article and currently a professor at the Federal University of Rio de Janeiro (UFRJ). The work also has the participation of scientists from Shinshu University, in Japan, the University of Virginia and Binghamton University, both in the United States.

Motivation
Matos says that several challenges have moved the work, including a question of basic chemistry. “This doping technique, specifically, had not yet been used in two-dimensional materials. There are several articles that demonstrate doping techniques, but most are based on substitution of atoms. And when you add something to the surface, they are much more complicated molecules. Working with isolated gold atoms is a rare thing, because they have a very high tendency to cluster; it is difficult to maintain an isolated gold atom. But we show that, once it is achieved, using the technique is relatively simple, and it has important applications. ” In addition to improving electrical conductivity, the presence of gold atoms had an effect on thermal conductivity. “Thermal dissipation is another problem with electronic devices. If the device generates heat that does not dissipate, it ends up damaged. The application of gold atoms resulted in an improvement that could be applied to increase the heat dissipation rate of 2D transistors based on this and other semiconductor TMDs ”, he explains. Another characteristic of semiconductors, which is the emission of light, has also undergone modifications. “The color, the frequency of the light emitted, is the result of material characteristics. By doping the semiconductor, we modified these characteristics. We did some studies with this modified material and realized that there is a difference in the emission of light: the set of frequencies available in the emitted light was different in the material added with the gold atoms. We are going to continue with optical studies ”, says Matos, emphasizing that the emission of light is yet another application of these materials, already used for this purpose in electronic equipment.

To test the conductivity of the transistors plus gold atoms, the scientists built ten devices. Then they used electron microscopy to visualize the interaction between the gold atoms and the sulfur atoms on the surface of the devices. “Much of the experimental MoS2 doping process, initial tests and electron microscopy images were made by Daniel Grasseschi, during his post-doctorate at Pennsylvania State University, with the participation of more than one group from there, under the baton of group of professor Mauricio Terrones. They built several transistors, they all worked well, and the conductivity was hardly affected. When he came back, we did optical and spectroscopic characterizations here at MackGraphe. The whole part of computer simulation was also carried out at Mackenzie, by professor Leandro Seixas, with contributions from professor Camila Maroneze ”, he reports. The scientists' intention now is to demonstrate that the technique works with other materials. “It is possible to change both the metal used and the TMD, but there are some challenges in using other metals and the reason is as follows: the coordination chemistry depends a lot on the number of electrons available in the last layer of the metal. Gold and silver, for example, have the same number of electrons in the last layer and are in the same column in the periodic table. So, if we exchange gold for silver, the chemical reaction happens in a very similar way. We demonstrated this in the article, because we did some tests with silver. Now, if we try to use metals from other columns in the table, we are dealing with a different number of electrons in the last layer, and then the chemistry changes. ”

The article, entitled Spontaneous chemical functionalization via coordination of Au single atoms on monolayer MoS2, can be read at advances.sciencemag.org/content/6/49/eabc9308.