Shaping and making carbon-based materials with chemical vapor deposition

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Carbon is one of the most abundant elements in the universe and can make a dazzling array of chemical compounds. However, when it comes to making thin films for semiconductors and optics, organic polymers like carbon nitride can be very tricky to work with. Dr Paolo Giusto at the Max Planck Institute of Colloids and Interfaces, Germany, has developed a new way of making carbon-based materials using chemical vapor deposition. This opens up many possibilities for moving away from toxic, expensive inorganic materials and using cheap, abundant carbon-based alternatives in their place.

Modern life is full of high-performance materials. From the silicon chips in all our electronic devices to lightweight polycarbonates used for the lenses in glasses, the remarkable properties of such high-performance materials have influenced every aspect of our daily lives.

Developing new materials is a highly active area of research. There is a constant demand for finding new materials that do not rely on rare, expensive or toxic chemical compounds, or offer new and exciting properties and performance.

Unlike synthesising new chemicals, coming up with new materials is not just about finding the right combination of atoms that produce a molecule with the desired properties. While the underlying chemical structure of a material is still incredibly important for determining its potential uses, finding the right techniques and methods for processing the final materials is also crucial for achieving the desired performance.

“Organic semiconductor-based devices can also be flexible and bendable, opening new possibilities of ‘wearable electronics’.”

One area of technology that has a particularly great demand for new materials is semiconductor design. Semiconductors are materials that allow for precise control of the flow of electrons, or charge, through them. As a result, they are at the heart of nearly all electronic devices – such as diodes, transistors and most modern electronics.

One of the most popular semiconductor materials used in high-tech devices is silicon, which is why the home of many of the largest technology companies bears the name ‘Silicon Valley’. The remarkable electronic properties of this element, particularly when doped with other materials, have completely transformed the world. Without silicon, modern computing would not exist, nor would it have been possible to develop the microchip – the device that made the miniaturisation of electronic circuits possible.

From left to right, carbon nitride thin film and thin films doped with increasing carbon contents, ambient light, on fused silica substrates.

From left to right, carbon nitride thin film and thin films doped with increasing carbon contents, UV light, on fused silica substrates.

Despite the widespread use and adoption of silicon-based semiconductors, there are many reasons for the strong interest in developing new semiconducting materials. Digital devices require very high-purity silicon, which requires expensive and energy-intensive processing of the raw materials. Other materials offer much higher electron transfer speeds, paving the way for faster computing and exotic properties – such as mechanical flexibility, which means devices can be bent.

Dr Paolo Giusto and his team of researchers at the Max Planck Institute of Colloids and Interfaces in Germany focus on the possibilities offered by alternative organic semiconductors. The research team has been finding ways to produce semiconducting materials from widely available chemical compounds that just might be the secret to a new generation of more sustainable optical and energy devices.

Carbon nitride

Dr Paolo Giusto has a particular interest in organic polymer semiconductor thin films. Organic polymer semiconductors differ from traditional, inorganic semiconductors like silicon, as they are based predominantly on light elements such as carbon and nitrogen. The strong interest worldwide in developing organic semiconductors comes from the vast natural abundance of carbon, which would open routes to making cheaper and more sustainable devices. Organic semiconductor-based devices can also be flexible and bendable, opening new possibilities of ‘wearable electronics’ such as biosensors or foldable screens.

Carbon nitride thin film deposited over quartz shaped flower, UV illumination.

Carbon nitride is one chemical compound that has caught the attention of the organic semiconductor community for some of its remarkable physical properties. Depending on the exact arrangement of the carbon nitride subunits and the presence of additional dopants, carbon nitride can form materials that are highly transparent to visible light. Carbon nitride can also appear as materials that are harder than Kevlar films – the material used to make bulletproof vests – or emit a very intense blue glow.

However, despite the myriad of applications that could make use of these properties, one of the barriers impeding the broader use of polymers like carbon nitride has been in the processing. Carbon nitride is poorly soluble and has low dispersibility, meaning many of the commonly used techniques to create even, smooth layers of the materials perform poorly. Creating useful devices relies on being able to control the thickness of layers precisely and ensure the layers are completely homogenous, so there are no local defects that could hinder its performance.

The structure of ideal carbon nitride (C₃N₄) – an organic polymer semiconductor.

Scattering of a ray of sunlight (white light) through a prism. BlackFarm/Shutterstock.com

Dr Giusto and his team have found an alternative method for depositing organic semiconductors like carbon nitride onto the substrates that form the basis of devices. Dr Giusto uses a technique called chemical vapor deposition, where the substrate is heated to high temperatures, and a highly controlled flow of gases is passed over it. As the gases pass over the hot substrate, they react to form a solid layer. The whole process of chemical vapor deposition can be carefully controlled by tuning gas mixtures, flow rates and the temperature of the substrate to produce very high-quality materials.

By using chemical vapour deposition, Dr Giusto has been able to make use of widely available, cheap chemical precursors to create thin films on a wide range of substrates, with thicknesses tuneable between a few nanometres to several hundreds of nanometres. These films are highly homogenous and can be deposited over flat or curved surfaces or even more complex shapes.

The Chemical Vapor Deposition system (planarTECH) is used to prepare highly homogeneous thin films.

From electronics to optics

By finding a highly versatile and reliable process for controlling the creation of organic semiconductor thin films, Dr Giusto has opened an exciting range of using organic polymer semiconductors. The films created using his techniques are very high quality and the level of control he has over the processing means the final device properties can be carefully tuned as required.

“One of the barriers impeding the wider use of polymers like carbon nitride has been in the processing.”

One property that goes beyond controlling the flow of electrons within the material is that Dr Giusto’s thin films have in addition a very high refractive index. The refractive index of a material is a measure of how much the material changes the speed of light when passing through the material, and the amount of light reflected and transmitted at the film interface. Fiber optic cables make use of materials with varying refractive indices to control the light path, facilitating information transfer at the speed of light.

A high refractive index means light is slowed down significantly in the material, resulting in a large perturbation of its path. Diamond has long held the honour of having one of the highest refractive indices amongst all the materials that are transparent to visible light. Some of Dr Giusto’s thin films have similarly large and controllable refractive indices, meaning that these organic semiconductors could be used in optical fibers or to develop new generations of energy devices, where currently inorganic semiconductors such as silicon are still having major shares.

What other organic polymers will you be exploring for semiconductor applications?

I am currently working on analogous carbon- and nitrogen-based materials, namely boron carbon nitrides. In these cases, the introduction of a lighter element, such as boron, allows to tune even further the optical properties, i.e. higher transparency in the visible range and higher thermal stability. Furthermore, I am investigating on how to combine these elements in different fashions and ratios, for different applications, from optics and optoelectronics to new generation of energy devices. In our group we aim to develop new strategies and solutions for a better, more sustainable tomorrow, using commonly available precursors for highly efficient materials.

References

Giusto, P., Cruz, D., Heil, T., Arazoe, H., Lova, P., Aida, T., Comoretto, D., Patrini, M. and Antonietti, M. (2020). Shine Bright Like a Diamond: New Light on an Old Polymeric Semiconductor. Advanced Materials, 32(10). Available at: https://doi.org/10.1002/adma.201908140
Giusto, P., Kumru, B., Zhang, J., Rothe, R. and Antonietti, M. (2020). Let a Hundred Polymers Bloom: Tunable Wetting of Photografted Polymer-Carbon Nitride Surfaces. Chemistry of Materials, 32(17), 7284–7291. Available at: https://doi.org/10.1021/acs.chemmater.0c01798
Giusto, P., Arazoe, H., Cruz, D., Lova, P., Heil, T., Aida, T. and Antonietti, M. (2020). Boron Carbon Nitride Thin Films: From Disordered to Ordered Conjugated Ternary Materials. Journal of the American Chemical Society. Available at: https://doi.org/10.1021/jacs.0c10945

DOI
10.26904/RF-135-1199311812

Research Objectives

Dr Giusto has developed an innovative method for the synthesis of a thin-film organic semiconductor.

Funding

Max Planck Society

Acknowledgements

Prof Markus Antonietti (Director of the Max Planck Institute of Colloids and Interfaces, Colloid Chemistry department)
Prof Maddalena Patrini (University of Pavia)
Prof Davide Comoretto (University of Genova)
Prof Takuzo Aida (University of Tokyo)
The research group of the Colloid Chemistry department at the Max Planck Institute of Colloids and Interfaces

Bio

Dr Paolo Giusto is currently a group leader at the Max Planck Institute of Colloids and Interfaces, Colloid Chemistry department. His main research focus lies on the preparation of organic semiconductor thin films for optics and energy applications via chemical vapor deposition. Besides research, he likes playing football and cooking.