As electronic devices shrink, we may be reaching the limit of how small they can be made. Samsung recently claimed to have made electronic circuits with transistors of length 7 nanometers (1 nanometer is a billionth of a meter). Would we able to go any smaller? Graphene—carbon in the form of a single sheet of atoms arranged in a hexagonal lattice—can make this possible, say scientists. In a recent study, a team from Indian Institute of Technology Bombay (IIT Bombay), in collaboration with Bhabha Atomic Research Center (BARC), Mumbai, have demonstrated a unique new way of making transistors and logic gates from graphene, by employing inorganic molecular dopants.
Semiconductors such as silicon are used to make electronic devices as the electrical conductivity of semiconductors can be controlled by adding dopants. Dominant charge carriers in the doped semiconductors could be positive or negative, depending on the dopants, making the material p-type or n-type. These two forms are the fundamental building blocks for all further electronic and digital devices, such as diodes and transistors. In contrast to silicon, it is extremely difficult to control the dominant charge carrier and conductivity in graphene. While p-type graphene can be easily made, its n-type counterpart has been extremely challenging to realize and operate at ambient conditions.
The current study is one of the first that achieves n-type graphene that functions under ambient conditions for more than 10 months without any degradation, and it is also the first time that an inorganic dopant was used to make n-type graphene. The researchers at IIT Bombay used a direct, one-step and precise technique that neither requires high-temperatures nor vacuum to operate. Additionally, the resulting n-type graphene transistors have demonstrated stable operation for more than 10 months, in spite of being exposed to high temperatures (250⁰ C) and humidity levels (Relative Humidity 95%). The n-type graphene reported current density 1000 times higher than those in earlier devices.
Conventionally, doping of silicon with inorganic dopants needs—in addition to a clean vacuum environment and precision apparatus—high temperature, a factor detrimental for graphene. Earlier, Scientists tried organic dopants, which could be easily applied like a coat or film, to make n-type graphene. However, the organic dopants are unstable and graphene quickly loses the n-doping. Oxygen and water have a high affinity towards graphene and are natural p-type dopants for it; hence, n-type graphene made with organic dopants would turn into p-type after few days as oxygen and water molecules from the air would replace the unstable dopants. It is from this perspective that the team decided to experiment with air-stable Lanthanide molecules for realising n-type graphene.
Lanthanides are a class of inorganic elements that are widely used as catalysts in the chemical industry. Such materials have been investigated for applications pertaining to data storage. “We were interested in studying the synergy between Lanthanide-based molecules with two-dimensional nanomaterials”, explains Prof Maheswaran of IIT Bombay, who synthesized the lanthanide complex used as a dopant. The researchers themselves made the required compounds in their laboratory, instead of using commercially available chemicals. Thus they were able to experiment with the various compositions and create a compound that had the desired properties.
Precision microinjection needles are widely used in biomedical applications. Prof C. Subramaniam and his team dispensed the dopant on specific areas of graphene using precision microinjection needles to create the n-type graphene transistors. Using these needles, the researchers could dispense the dopant on an area of 0.05 square millimetre on a 2 square millimetre graphene surface akin to painting a pinhead on a tennis court.
“The advantage of using an established technique is that it can be readily used without putting significant additional effort and it can easily be scaled for commercial production later; in fact, microneedle arrays are already commercially available”, comments Prof Subramaniam.
The research team conducted several experiments to understand the mechanism of binding between the lanthanide complexes and graphene. The lanthanide complexes used as dopants are synthesized by combining lanthanides with ligands—cyclic structures that consist of nitrogen donors and held together by carbon and hydrogen atoms. The lanthanide ion sits firmly into the cyclic cavity making the complex very stable. When in contact with graphene, the lanthanide complex warps the graphene structure making strong bonding possible, which combined with the inherent stability of the Lanthanide complex is the reason for the stability of n-type graphene. Theoretical analysis, carried out using the computational facility at IITB and BARC, helped the researchers understand the detailed mechanism of binding, and create the right molecule. “As the origin (of the behaviour) is now known, chemists can design complexes which could boost the stability/output to achieve the goal of realizing the potential applications proposed”, says Prof. G. Rajaraman of IIT Bombay, and a co-author of the study.
Inspired by the success and stability of the devices, the researchers created an inverter—the basic logic element in a digital electronic circuit—using the graphene transistor. They observed that the inverter could function at a voltage of 2 V, making it a good candidate for low-voltage, low-power electronics. They extensively tested these devices for 10 months in ambient conditions, without any protective packaging. They observed no degradation in the properties.
“This is a unique showcase of an interdisciplinary effort with electrical engineers and chemists working together to address a contemporary problem involving graphene transistors. This work paves way for the realization of basic circuit building blocks with the use of a novel doping methodology for graphene transistors” summarizes Prof. V. Ramgopal Rao, the Director of Indian Institute of Technology Delhi.
“We can visualise several applications of graphene transistors in digital electronics, radio frequency devices and spintronics devices. The proof-of-concept that we have established could be further developed into a prototype and put into commercial production with collaboration from industry”, says Prof Subramaniam. “We used lanthanum and cerium compounds for the current study. We have also synthesized a family of other lanthanide macrocyclic complexes (~10 different complexes) which are structurally similar to lanthanum and cerium complexes and plan to try each of these as a dopant to make n-type graphene”, concludes Prof Maheswaran.