Featured paper: Nanomembrane microtubes and hybrid materials for advanced electronic devices.

Some of the challenges of the next generation of smartphones, sensors and other electronic devices can be solved with the help of nanomembrane microtubes (sheets made from various materials with nanometric thicknesses and micrometric lateral dimensions, which can be self-rolled and form microtubes).

In the city of Campinas (São Paulo State), at the Brazilian Nanotechnology National Laboratory of the National Center for Research in Energy and Materials (LNNano / CNPEM), a group of scientists has developed expertise in processes for the synthesis, manufacture and characterization of nanomembranes and their applications. In papers published in January and February of this year in the journals Nano Letters and Nature Communications, the authors explore some nanomembrane potentials, mainly related to their flexibility and dimensions, to develop new memristors and transistors – two widely used electronic devices.

More precisely, the LNNano researchers show that nanomembrane microtubes can be key parts in the manufacture of promising components using organic and hybrid materials (organic-inorganic) such as memristors and transistors. Thus, the novelties proposed in the articles can contribute to the development of even smaller, flexible, portable, cheaper devices and equipment that combine better performance and more low energy consumption features.

In an interview with the B-MRS Newsletter, Carlos Cesar Bof Bufon, corresponding author of both articles, talks about the scientific, technological and social impact of the research and about the skills and infrastructure involved in carrying it out. Bufon is a researcher and head of the Devices Division at LNNano/CNPEM.

B-MRS Newsletter: The articles present advances, developed at LNNano, in technologies for electronic devices (transistors and memristors). In your opinion, what are the main technological and/or scientific contributions of these articles?

Carlos Cesar Bof Bufon: The work entitled “Ambipolar Resistive Switching in an Ultrathin Surface-Supported Metal–Organic Framework Vertical Heterojunction” published in Nano Letters (DOI: 10.1021/acs.nanolett.9b04355), reports the development of a new type of memristor using structures called Surface-Supported Metal-Organic Frameworks (SURMOFs). SURMOFs are hybrid structures composed of metallic ions connected by organic ligands. The structure is highly organized on surfaces covered with chemically active self-assembled monolayers (SAMs). There are more than 70 thousand types of metal-organic structures (MOFs) that can be prepared as SURMOFs to confer new features to electronic devices. In addition to the unique properties, its considerable mechanical flexibility and low cost arouse interest in the scientific community for applications. However, in the case of a hybrid material consisting of organic and inorganic elements, the integration of this class of nanomaterials becomes a technological challenge. To circumvent this limitation, this work presents a new architecture for the integration of these SURMOFs structures for the development of a new type of memristor. The proposed architecture is based on the upper electrical contact of SURMOF HKUST-1 ultrathin films using self-rolled metal nanomembranes (microtubes). The SURMOF films explored in this work have thicknesses of the order of 20 nanometers. This new architecture opens up promising paths in the area of electronic devices based on SURMOFs, as well as providing a technological platform for conducting studies not yet reported in the literature for this class of materials. For example, the investigation of how electrical charges are conducted in layers of ultrathin SURMOFs.

Photograph of a microchip fabricated using photolithography, and illustration of the electrical contact on the SURMOF performed by the strained metallic nanomembrane after the rolled-up process. The chip has 81 mm2 and contains 32 memristors.
Photograph of a microchip fabricated using photolithography, and illustration of the electrical contact on the SURMOF performed by the strained metallic nanomembrane after the rolled-up process. The chip has 81 mm2 and contains 32 memristors.

The work entitled “Edge-driven nanomembrane-based vertical organic transistors showing a multi-sensing capability” published in Nature Communications (doi: 10.1038/s41467-020-14661-x) presents the development of a vertical organic transistor platform. Transistors are the main components for electronic circuits and processors. Compared to inorganic transistors, the organic ones have the advantages of low-cost and easy fabrication, which makes them attractive for flexible electronic devices in several areas. A significant advantage of vertical transistors is their easy integration with light-emitting devices and photodetectors in the formation of integrated optoelectronic systems. The devices presented in this work were processed entirely using conventional microfabrication and photolithography techniques, which is an advantage of technological feasibility. One of the main differences of these new transistors is related to the use of self-rolled metallic nanomembranes as drain electrodes, thus promoting the formation of a smooth mechanical contact with the organic semiconductor layer. In this architecture, the organic semiconductor layer can reach thicknesses below 50 nanometers. The manufactured devices showed high current densities (~ 0.5 A / cm2) with low operating voltages (≤ 3 V). Based on our theoretical study, it was possible to predict an improvement in the structure of the transistor, resulting in projections of devices with current densities of up to 10 A / cm2. Such values, obtained at low operating voltages on a platform composed of active organic layers, highlights the potential of these devices to be integrated with flexible and portable electronic applications in the future. Besides, the new transistors were able to detect different levels of humidity and light, thanks to the device platform based on self-rolled nanomembranes. Therefore, the developed transistors also have great potential for advancing next-generation sensor technology.

 Scanning electron microscopy image (artificially colored) of the vertical organic transistor based on rolled nanomembrane (yellow tones). The incident radiation and the water molecules (artificially placed) illustrate the multiple sensitivity characteristics of the new electronic device.

Scanning electron microscopy image (artificially colored) of the vertical organic transistor based on rolled nanomembrane (yellow tones). The incident radiation and the water molecules (artificially placed) illustrate the multiple sensitivity characteristics of the new electronic device.

B-MRS Newsletter: How could the results of the articles impact people’s daily lives (social impact)? Do you think that the proposed technologies could replace those currently used or create new applications? If so, what would be the advantages of these new technologies developed at LNNano? Would it still need many steps to bring the results of the articles to the market?

Carlos Cesar Bof Bufon: Memristor is considered one of the four fundamental electronic components. On a computer, it is capable of performing information processing and storage functions. In this work, the behavior of a memristor was observed in conditions of high relative humidity (between 90-70%). Analogous to a sponge, the water molecules present in the environment are absorbed by SURMOF nanopores. In a given electric field, these molecules facilitate the conduction of electrons within the material by changing its electrical resistance. The difference in electrical resistance can reach 1 million times using low operating voltages, less than 2 V. In a practical application, this difference can correspond to the binary states 0 and 1. The total fabrication of the device was carried out with conventional techniques photolithography, compatible with industrial scale production. The fact that it depends on the humidity for its operation does not limit its commercial application, as this can be easily bypassed through encapsulation processes, common in the electronic device industry.

The manufacture of organic transistors in vertical architecture using curled nanomembranes as drain electrodes allows the reduction of the region of interest to less than 50 nanometers (more than a thousand times less than the thickness of a hair). This implies a significant advance in the electronics industry, because the portability of electronic applications, such as smartphones, computers and televisions, depends on the reduction in the number and size of transistors. The mechanism of operation of the device reported in this work expands the current understanding of vertical organic transistors, showing that current densities can be improved by performing careful manipulations in the spatial structure of the intermediate metallic electrode (source). Besides, the preparation of transistor microchips was carried out using microfabrication techniques compatible with the industrial scale. Thus, these devices can be easily integrated into electronic applications with a few additional steps, such as encapsulating microchips, to prevent degradation of organic materials.

The two concepts of devices based on nanomembranes expand the possibilities for the use of hybrid materials in electronics. As much as there is a tendency to substitute one technology for another, the concepts developed in our works seek to open new frontiers and possibilities through the design of functional structures from their fundamental elements such as atoms and molecules.

All the technologies that we develop in the area of devices at LNNano/CNPEM are firmly grounded in two focus: the understanding of the fundamental concepts that govern the properties of the components, and their technological application. These two papers started fundamentally from scratch. In terms of technology readiness level (TRL), the devices have reached the functional validation of the components in the laboratory environment. Within the TRL scale, we achieved the fourth level out of a total of 9 – the latter being commercialization. It is also worth mentioning that the next level of development must involve the participation of the productive sector, which is who can accelerate the research towards the market.

B-MRS Newsletter: Briefly tell us the story of the works reported in the two articles: how and when the idea came up, whether it was necessary to gather very different skills, the infrastructure used, curiosities etc.

Carlos Cesar Bof Bufon: The works are the result of different stories, but were developed in the same research group sharing the idea of exploring the potential of nanomembranes in the area of functional devices. This theme is part of my Young Researcher project financed in 2016 by FAPESP. The idea of applying SURMOFs as a memristor, using nanomembranes as the top contact, was developed together with the post-doctorate Dr. Luiz Gustavo Simão Albano. Our research group started evaluating the feasibility of using SURMOFs as functional materials in devices about 3 years ago. Since then, the group has been continuously establishing the implementation of synthesis and integration routes for applications in the areas of electronic components. The work was carried out entirely at LNNano/CNPEM, and has a list of co-authors who actively contributed to making this work possible: Tatiana P. Vello (growth of SURMOFs), Davi HS de Camargo (manufacture of devices and illustrations), Ricardo ML da Silva (device manufacturing), Dr. Antonio CM Padilha (DFT simulations) and Prof. Dr. Adalberto Fazzio (DFT simulations).

The work with the transistor follows the same pattern as the memristor – a combination of functional materials and nanomembranes. About 15 years ago, I became interested in the potential of using nanostructured layers as fundamental blocks in vertical electronics. Once I was aware of the work of Prof. Ivo Humlegem, from UFPR (died 2018), in the area of vertical transistors, a series of advantages and challenges caught my attention. The idea of the work came by anticipating that an improvement of the performance of vertical transistors could be achieved with the use of a drain electrode based on nanomembrane combined with a patterned intermediate electrode (source) using photolithography. In May 2018, Dr. Ali Nawaz (a former student of Prof. Ivo!), started the execution of the project at LNNano/CNPEM. Being a complex project, the advanced infrastructures for processing and characterizing LNNano/CNPEM devices were essential. And like the memristor, the research depended on a set of varied expertise. During the project, Dr. Leandro Merces provided critical assistance in the investigation of the theoretical aspects of the devices, while collaborators Davi Camargo (specialist in microfabrication) and Denise de Andrade (graduate trainee at the State University of Ponta Grossa), provided the entire work the necessary technical support.

Main authors of the papers. From the left: Carlos Cesar Bof Bufon, Luíz Gustavo Simão Albano and Ali Nawaz.
Main authors of the papers. From the left: Carlos Cesar Bof Bufon, Luíz Gustavo Simão Albano and Ali Nawaz.


[Papers: Ambipolar Resistive Switching in an Ultrathin Surface-Supported Metal–Organic Framework Vertical Heterojunction. Luiz G. S. Albano, Tatiana P. Vello, Davi H. S. de Camargo, Ricardo M. L. da Silva, Antonio C. M. Padilha, Adalberto Fazzio, Carlos C. B. Bufon. Nano Lett. 2020, 20, 2, 1080-1088. https://doi.org/10.1021/acs.nanolett.9b04355 and Edge-driven nanomembrane-based vertical organic transistors showing a multi-sensing capability. Ali Nawaz, Leandro Merces, Denise M. de Andrade, Davi H. S. de Camargo & Carlos C. Bof Bufon. Nature Communications volume 11, Article number: 841 (2020). https://www.nature.com/articles/s41467-020-14661-x ].

Featured paper: Defect-free doped graphene for use in electronic devices.

Graphene-based products are already being used by manufacturers, from heat-dissipating helmets to antistatic packaging. However, this wonderful material, as it is often called, still has much to deliver to society. As it is two-dimensional, flexible and excellent conductor of electricity, among other properties, graphene can be the basis of a series of high-performance miniaturized electronic and optoelectronic devices. However, this requires producing, at an industrial scale, a graphene whose network of atoms is free of unwanted impurities, but which contains, besides the carbon inherent in the graphene, small amounts of other elements (doping) in order to control its electronic properties.

In a work totally carried out in Brazil, a scientific team has proposed a process that can help produce large-scale graphene that is suitable for electronic devices. “The process developed in our group allows us to improve and adjust the graphene properties, as well as the removal of contaminants from its surface,” said Professor Claudio Radtke (UFRGS), corresponding author of an article reporting the study, recently published in The Journal of Physical Chemistry C.

The authors of the paper, from the left: Henri Boudinov, Cláudio Radtke Gabriel Vieira Soares (all UFRGS Professors) and Guilherme Koszeniewski Rolim (postdoc at the graduate program on microelectronics at UFRGS).
The authors of the paper, from the left: Henri Boudinov, Cláudio Radtke Gabriel Vieira Soares (all UFRGS Professors) and Guilherme Koszeniewski Rolim (postdoc at the graduate program on microelectronics at UFRGS).

The team acquired graphene samples produced by chemical vapor deposition (CVD) and transferred to silicon substrates. This technique is currently one of the most suitable for large-scale production of relatively large area graphene sheets, but it leaves residual impurities and generates defects in the graphene. To remove impurities, it is common to apply a heat treatment in an atmosphere of carbon dioxide (CO2), which is efficient in removing contaminants, but ends up generating new defects in the graphene sheet. The good news is that these defects can be neutralized (passivated).

While looking for strategies to passivate these defects, then PhD student Guilherme Koszeniewski Rolim found a scientific paper from 2011, which pointed to, through theoretical calculations, the possibility of using nitric oxide (NO) to passivate graphene defects with nitrogen atoms, while doping it to modulate its electronic properties (mainly transforming it into a semiconductor material, an essential condition for using graphene in electronic devices).

The team then decided to experimentally verify the theoretical prediction and, after performing the traditional treatment with CO2 at 500 °C, they applied a second heat treatment to the samples, this one in nitric oxide atmosphere and at different temperatures, from room temperature to 600 °C.

After the process, the researchers used various characterization techniques to check the results and gladly confirmed that nitrogen doping had taken place and that it had passivated the defects, thus improving the material’s electronic properties. However, the researchers also noted an unwanted effect of nitric oxide treatment: etching of graphene sheets at some points. After much scientific work, the team was able to determine the cause. During heating, there was a conversion of NO to NO2, which, as it is a much more reactive compound than the former, eventually oxidized the graphene.

However, the Brazilian team found a solution to this problem. The “eureka” moment occurred as the researchers were trying to determine the amount of nitrogen atoms that had been incorporated into graphene using a technique based on the analysis of nuclear reactions triggered by the effect of an ion beam on the graphene samples. In order to apply this technique, the team had to use an isotopically enriched nitric oxide in the heat treatment, which has a purity of 99.9999% instead of 99.9% of the gas previously used.

Illustrative scheme of the parameters to be controlled in the process proposed by the Brazilian team. Balancing gas purity and temperature ensures better graphene sheets for use in electronic devices.
Illustrative scheme of the parameters to be controlled in the process proposed by the Brazilian team. Balancing gas purity and temperature ensures better graphene sheets for use in electronic devices.

The analysis did not yield the expected results as it failed to quantify nitrogen, which was below the detection limit. However, the use of the enriched gas eventually brought great satisfaction to the team. Indeed, when the researchers compared the electronic properties of both sample types, they found that graphene treated with enriched gas always had superior properties. “Initially, such a result created much confusion in the interpretation of the results,” says Professor Radtke. “But after a few more experiments, it became one of the most important points of the article, highlighting the importance of gas purity during processing,” he adds. Specifically, the conclusion was that by properly controlling the temperature and purity of the gas during the treatment one can eliminate the problem of oxidative graphene degradation.

Thus, based on solid knowledge and scientific method, as well as some serendipity, the UFRGS team was able to develop a process of waste removal, defect neutralization and graphene doping, which improved the electronic properties of the material without producing deleterious side effects. Because it is a heat treatment in a gas atmosphere, a step that is now part of the industrial production of graphene, the process proposed by the Brazilian team could be easily applied in the production of graphene sheets for devices.

“The insertion of heteroatoms (such as nitrogen) into the graphene network without the degradation of its properties is especially important in the production of optoelectronic devices, high speed transistors, low power electronics and photovoltaic cells,” says Radtke, noting that manufacturing these graphene-based devices may be a reality in years to come. “The Graphene Flagship (European consortium of industries, universities and institutes) has announced the implementation of a pilot plant to integrate graphene at different production stages of devices as early as 2020,” comments the professor from UFRGS.

The study, which was funded by the Brazilian agencies CNPQ (mainly through INCTsNamitec and INES), Capes and Fapergs, was developed within the PhD in Microelectronics by Guilherme Koszeniewski Rolim, held at the UFRGS Graduate Program in Microelectronics and defended in 2018. The experimental work was carried out at the UFRGS Solid Surface and Interfaces Laboratory and the Brazilian National Synchrotron Light Laboratory.

[Paper: Chemical Doping and Etching of Graphene: Tuning the Effects of NO Annealing. G. K. Rolim, G. V. Soares, H. I. Boudinov, and C. Radtke. J. Phys. Chem. C,  2019, 123, 43, 26577-26582. https://doi.org/10.1021/acs.jpcc.9b02214.]