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Surfactant controlled Cation Exchange Lithography - Towards Application

Periodic Reporting for period 1 - SCEL-TA (Surfactant controlled Cation Exchange Lithography - Towards Application)

Reporting period: 2015-09-01 to 2017-08-31

The goal of the project was to develop a new method for the production of electronic devices based on nanoparticles. The technique relied on the process of cation exchange, in which the cations of nanocrystals can be exchanged to a different chemical element. In general, this leads to a drastic change of the physical properties of the nanocrystals. For instance, CdSe nanoparticles show a strong fluorescence and exhibit a low conductivity. Cu2Se nanoparticles that have been obtained by exchanging the Cd with Cu, show no fluorescence and an increased conductivity.

The cation exchange, in general, is performed by providing a solution of cations to colloidal nanocrystals. When the conditions, e.g. temperature and the choice of cations and nanocrystals, are chosen correctly, the new cations diffuse towards and into the colloidal nanocrystals, where they replace the original cations, which then diffuse out of the nanocrystals.

In the framework of this project we investigated on the limits of this process in terms of applicability to different systems of nanoparticles and on potential applications. To this aim, we relied on an extension of the cation exchange. When performed on dry films of nanocrystals, we could selectively suppress the cation exchange by irradiating sections of the film with an electron beam. The electrons, or their deposited energy, induced a cross-linking of the surfactant molecules, i.e. the organic molecules surrounding the nanocrystals. Subsequently the substrates were immersed into a solution of cations. The cross-linked molecules then hindered the diffusion of the cations towards the nanocrystals and therefore suppressed the cation exchange, whereas the neighbouring regions that have not been exposed to the electron beam did undergo the cation exchange. Ultimately, this lead to substrates cover with at film composed of two different materials.

We performed this selective suppression of the cation exchange both on single nanocrystals and on continuous films of nanocrystals. Especially in the case of the continuous films we could demonstrate that this technique could be used for the production of a electronic devices.
In this project the following issues were successfully addressed:
- Stabilisation of the films. The cation exchange was performed by immersing the substrate into a solution of the new cations. In this process there was a risk of dissolving the particles, which therefore needed to be stabilized without inhibiting the cation exchange

- Determination of an optimal ligand system for the films. The inhibition of the cation exchange proceeds by the cross-linking of the ligands (aka surfactants) of the nanoparticles. These ligands therefore have an influence on the efficiency of the inhibition of the cation exchange. We could identify a ligand that minimises the energy required for the crosslinking and exhibited the best conductivity after the cation exchange was performed.

- Spatial resolution. The spatial resolution of the process on films was determined to be of the order of 20 nm.

- Fabrication of functional devices.
- Determination of the achievable spatial resolution of the cation exchange in films of nanoparticles.

- Fabrication of electronic devices
Optical micrograph of a test structure. The bright lines are of CdS, the dark areas are of Cu2S.