Looking through a nanocrystal - Engineering defect-free patterns for advanced film study

Materials World magazine
,
1 Oct 2012

A team of US scientists has announced a method of making defect-free patterns of semiconductor nanocrystal films. The development could allow scientists to study a film’s fundamental properties.  

There are two common approaches to patterning films – micro-contact printing and inkjet printing – but neither enables them to be formed with nanoscale precision that is crack-free and electrically conductive. The technique, developed at MIT, uses electron-beam lithography and a lift-off process to solve those problems.

The method reported by the researchers relates to the preparation of lead sulphide (PbS) nanocrystal films, although the same approach holds for films of other types of nanocrystal. To pattern the films, silicon dioxide was used as a substrate, owing to its prevalence in semiconductor devices.   

In the first instance, a 100nm positive resist layer of polymethyl methacrylate (PMMA) is coated onto the substrate and then nanoscale trenches are patterned into it. The nanocrystals are prepared by high-temperature pyrolysis of Pb and S precursors in an oleic acid/octadecene mixture.   

The growth solution is then processed to remove remaining salts and byproducts, and to replace the native oleic acid capping molecule with a smaller molecule, n-butylamine – it is this exchange of capping ligand while the nanocrystals are still in solution that is critical for making films with a measurable current that are free of cracks, the researchers say. The PbS nanocrystals are then dropcast into the trenches.   

To lift off the film, the substrate is immersed in acetone to dissolve any remaining PMMA and leave a 50nm film of PMMA-defined nanocrystals on the substrate. It is then briefly sonicated in the acetone to ensure the film tears cleanly at the pattern boundaries.   

The scientists say the resulting films have patterns down to 30nm in size, are robust and show an electrical conductivity about 180 times that of unpatterned microscopic films – 17μS/cm against 0.09μS/cm. In addition, the thickness of the nanocrystal pattern can be tuned to emit and absorb a wide spectrum of light, and the substrate can be recycled if necessary.   

Lead author of the research paper, Dr Tamar Mentzel, says, ‘The ability to pattern the films with nanoscale resolution enables precise placement of the nanocrystals in devices for applications such as solar cells and nanoelectronic or nanophotonic circuits.   

‘For solar cells, a higher electrical conductivity may yield a more efficient device, and because the nanocrystals can be tuned they could enable a new kind of broad-spectrum solar cell.’   

Dr Mentzel does not know when these applications might become reality as her prime interest is in studying the films’ properties. ‘In these films, we are studying a new regime of charge transport, because we can study transport between nanocrystals unimpeded by cracking and may be able to learn more about the transport mechanism on a microscopic scale,’ she says. ‘This system has the potential to reveal new physics.’   

Aidan Quinn, Head of the Nanotechnology Group at the Tyndall National Institute in Cork, Ireland, comments, ‘The MIT team has developed a versatile technique to combine bottom-up approaches for synthesis and self-assembly of semiconductor nanocrystals, with top-down methods for patterning these nanocrystal films at length scales down to the 30nm. This work certainly opens up new routes for precise integration of nanostructures with top-down fabricated devices and offers a host of possibilities for hybrid nanodevices – from on-chip photodetectors to nanocrystal-based LEDs.’