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A novel lithographic technique to structure the chemical composition of nanocrystal films

Scientists of the Department of Nanochemistry at the Istituto Italiano di Tecnologia, in Genova have invented a lithographic approach that allows to locally modify the chemical composition of nanocrystal films, without affecting the film morphology. This method enables novel concepts to fabricate useful patterns for electronic circuits, light emitting pixels, photosensors and microchips.
In principle, this technique to create functionality in films by chemically modifying certain regions  recalls the process of exposure and development of a photographic film.
First, the nanocrystal film is exposed to an electron or X-ray beam, which modifies the structure of the organic ligands that coat the surface of the individual nanocrystals. Then the film is immersed in a chemical solution that modifies the nanocrystals not exposed to the beam, producing a negative pattern of the exposure that consists of nanocrystals of a new material, for example of Cu2Se instead of the CdSe. The X-ray exposures described in this report have successfully been conducted at the Deep X-ray Lithography beamline of the Elettra Sincrotrone in Trieste.

Figure 1. Sketch of the masked cation exchange process: (a) Cross section of a thin film consisting of NCs that are coated on their surface with organic surfactant molecules. (b) Selected regions of the NC film are exposed to an e-beam that cross-links the surfactant molecules. (c) Cation exchange occurs only in regions that had not been exposed to the e-beam. (d) Top view of possible patterns consisting of two chemically different NCs, for example, Cu2Se and CdSe, within the previously homogeneous film. The different optical and electrical properties of the two materials can be exploited to obtain patterned light emitting regions (square array on the left) or electrically conducting circuits (pads connected by a wire on the right).
 

The proof of principle was demonstrated using Cd-based chalcogenide nanocrystal films, like for example CdSe. CdSe nanocrystals have been intensively studied in the past decades and evolved as a model system for exploratory research. They are semiconductors with band gap in the visible and find applications in fluorescent labeling, LEDs, and lasers. Furthermore the cation exchange reaction from Cd to Cu is well established and can be performed at room temperature. Cu2Se is a semimetal, and nanocrystal films from this material show relatively high conductivity and are not fluorescent, i.e. have substantially different properties with respect to CdSe nanocrystal films.  Electron-beam and X-ray lithography was used for pattern exposure to provide the high energy beam needed to cross-link the surfactant layer on the nanocrystal surface.  Then a cation exchange reaction from CdSe to Cu2Se modified the chemical structure of the nanocrystals in the unexposed regions. With this approach patterns consisting of Cu2Se nanocrystals that function as electrical circuits within the CdSe nanocrystal film were obtained.

In conventional patterning techniques either material is deposited or removed from the layer that should be structured, like for example in standard electron beam lithography using PMMA resist, or imprint lithography just to name a few. One key novelty of the method described here is that although functional patterns can be generated by using high resolution lithography tools, the film morphology remains unaltered. This has considerable advantages for the fabrication of, for example, layered structures where the planarity of the surface needs to be conserved. Future developments of this method will be on enlarging the range of materials that can be structured, and to tailor suitable surface ligands for best resolution and performance. 
 

This research was conducted by the following research team:

Karol Miszta, Fanny Greullet, Sergio Marras, Mirko Prato, Andrea Toma, Milena Arciniegas, Liberato Manna, Roman Krahne, Istituto Italiano di Tecnologia, Genova, Italy


Contact person:
Roman Krahne: roman.krahne@iit.it
Liberato Manna: liberato.manna@iit.it


Reference

 K. Miszta, F. Greullet, S. Marras, M. Prato, A. Toma, M. Arciniegas, L. Manna,  R. Krahne, “Nanocrystal Film Patterning by Inhibiting Cation Exchange via Electron-Beam or Xray Lithography”, Nanoletters, 14,2116 (2014), DOI: 10.1021/nl500349j 

Last Updated on Wednesday, 02 July 2014 14:36