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Building a Sustainable Future at the Nanoscale

Internationaliseringsstipendium | 27/05/2016

Our world has seen an enormous increase in new advanced nanomaterials, which has created new and improved products ranging from membranes over sensors to solar cells. Still, new technologies and innovative solutions are required to meet the demand of a future sustainable society. The next step in the development of such materials relies on the ability to control the formation of them at a molecular, or even atomic, level. The objective of the research project is to develop new types of linking chemistries compatible with molecular layer deposition (MLD), by utilising light-activated organic coupling reactions. MLD, a variant of atomic layer deposition (ALD), is an emerging technique used to deposit conformal organic coatings with molecular control. Compared to standard solution-based depositions, MLD is particularly interesting due to its environmentally friendly gas-phase nature and high precision. Currently the technique is limited by very few linking chemistries, which is why the full potential of MLD has yet to be realised. I expect to develop 1-3 new controllable MLD chemistries, which will open new avenues for creating thin organic nanocoatings applicable for e.g. nanoelectronics and membranes. I will also combine these new MLD-types with ALD to create new organic/inorganic hybrid materials, which has potential as catalyst materials, necessary to meet future sustainable energy demands.

Nanotechnology

The prefix nano comes from the Greek word ‘nanos’, meaning dwarf. Most organic molecules are on the order of 1 nanometer. The natural profusion of nanomaterials is today inspiring biologists, chemists, physicists, and engineers to join forces and develop a sustainable future.

A Need for Nanomaterial Formation with Molecular Control

How can we secure energy for a still growing population in a sustainable way? How can we ensure access to clean water? How can we continue the innovation of new technologies? These are three of the questions, which need an answer to meet the 17 UN Sustainable Development Goals aiming at sustainable development for people and planet by 2030. These are also just some of the tasks that nanotechnology and nanomaterials can help us solve in the near future. In this project, I seek to develop new ways to make organic nanomaterials, and in particular high quality nanoscale polymer films fabricated with molecular precision. This can be applied in the development of future nanomaterials, where molecular scale precision is necessary.

Nanomaterials today have to be engineered with control on a length scale corresponding to one millionth of a millimetre, or put differently, on the scale of small molecules or even just atoms. A sustainable technique used to build materials with nanoscale precision is atomic layer deposition (ALD), which enables the deposition of inorganic metals and metals oxide in a layer-by-layer fashion.1 ALD relies on the reaction between gas-phase molecules and solid substrates or particles. The deposition of only one molecular or atomic layer at each precursor dose provides exquisite control over film thickness, composition, and molecular conformality, and today the technique is widely used for example in the semiconductor industry. Molecular layer deposition (MLD) is the organic version of ALD, and it enables deposition of organic polymer films ranging from monolayers to tens or hundreds of nanometres by dosing bifunctional precursor molecules in the vapour phase (Figure 1).2 There is no need for solvents and catalysts, and this green aspect of MLD is very appealing compared to other ways of creating polymeric coatings. Additionally, this powerful technique offers exquisite control at the nanoscale, which is also a limitation of most other polymer deposition techniques.

Figure 1. Schematic illustration of the MLD process. After an initial surface treatment (a), one MLD cycle includes doses of two different bifunctional precursor molecules, (b) and (c). This cycle is repeated several times (d) to achieve a polymeric surface film with exquisite film thickness control.

Using Light to Activate New Reaction Types

In contrast to ALD, only few reaction types have been developed for MLD. It has so far limited the applications of this promising technique. At Stanford University, I work with a team of ALD and MLD experts. Here I use my grant from the Carlsberg Foundation to develop new ways to link gas phase organic molecules to surfaces for MLD. These reactions proceed in absence of solvent, pH cannot be varied, and catalysts cannot easily be employed. In short, tools usually used by an organic chemist are inapplicable. Therefore, only molecules with highly reactive groups have been used in MLD. I will overcome this shortcoming through the use of light as an activation source. One obvious advantage of this highly interesting and so far unexplored approach, is the direct input of energy. One mole of photons at 365 nm is energetically 130 times higher than the thermal energy available to activate a reaction at 25 °C and, otherwise, infeasible reactions become possible. Such photo-activated MLD also paves the way for C-C bond forming reactions, a key reaction type in organic chemistry, which otherwise has not been achieved by MLD. It may additionally be used to grow simple polymers like polyethylene (PE). Despite being the most widely produced plastic there are only very few demonstrations of surface bound PE, and it has never been achieved in a controlled fashion.

Understanding Light-Activated MLD

A key to success in MLD is to control the deposition of only one molecular layer at each precursor dose. The light-activated MLD is expected to proceed through the formation of very reactive radical species and the MLD process may therefore be harder to control than normal non-radical based MLD. I will therefore specifically investigate the growth mechanism of the light-activated MLD through x-ray and infrared (IR) spectroscopies. I will furthermore compare this very different class of material to the existing materials formed by MLD. The fundamental molecular understanding of the material is of great importance for the implementation e.g. in future nanoelectronics.

ALD and MLD

ALD was invented independently twice in the 1960s and 1970s. Semiconductor miniaturisation induced its industrial breakthrough in the early 2000s. This was also the start of MLD research, but few applicable linking chemistries have so far limited its application.

Combining ALD and MLD

MLD can also be combined with ALD to form organic-inorganic hybrid materials. Such materials are widely studied and particularly interesting as potential catalyst materials. I will use the light-activated MLD in combination with ALD to develop new nano hybrid materials. The molecular/atomic precision of MLD/ALD can here be utilised, for example, to tune the distance between active centres or the electronic environment surrounding them. Fine-tuning material properties at a molecular level are of great importance for developing efficient catalysts for renewable energy applications, and therefore also important for a greener future.

Figure 2. This is one of the home-build vacuum reactors for MLD we have available in the lab at Stanford University. Both solid and liquid precursors can be added at the line and dosed sequentially through an automated process.

From Polymer Chemistry of Chemical Engineering

I earned my PhD at Department of Chemistry, Aarhus University, where I worked on the development of thin functional polymeric coatings using controlled surface polymerisations.3 With the Carlsberg Foundation’s Internationalisation Fellowship, I was given a great opportunity to pursue new challenges and explore new ways to develop next generation nanomaterials. The Department of Chemical Engineering at Stanford University, in the heart of Silicon Valley and among the world’s leading tech companies, is a perfect setting. Here I can contribute with my background and knowledge and at the same time achieve new skills. These skills, including great knowledge about gas-phase chemistry and vacuum processes, and the experiences from working with colleagues in one of the world’s top universities will be valuable to bring back to Denmark, where I am looking forward to continue the hunt for nanomaterials for a green sustainable world.


References

(1) George, S. M. Chem. Rev. 2010, 110, 111–131.
(2) Loscutoff, P. W.; Zhou, H.; Clendenning, S. B.; Bent, S. F. ACS Nano 2010, 4 (1), 331–341.
(3) Lillethorup, M.; Shimizu, K.; Plumeré, N.; Pedersen, S. U.; Daasbjerg, K. Macromolecules 2014, 47, 5081–5088.