May 2, 2013
AFM captures dynamics of photodissolution
Researchers from the University of Connecticut and Lehigh University in the US have teamed up to investigate the surface dynamics of photodissolution in silver-doped As2S3 chalcogenide (ChG) thin films. Such direct studies of surface dynamics are crucial to understanding and ultimately optimizing chalcogenide film applications such as photomasks, optoelectronics media and biochemical sensors.
Fabricated at Lehigh’s facility using thermal evaporation, the films are 400 nm thick with a 20 nm silver surface layer. To investigate the samples, the scientists used a combined inverted optics and AFM system at the University of Connecticut. A broadband light source with ~1 mW illumination intensity was used to optically excite the Ag/ChG interface, creating Ag+ ions, which then diffuse into the ChG layer. Masks were created to optically pattern the ChG and study edge effects of the photodissolution process.
The combined AFM/optics system allowed continuous observation of surface morphology, pre-, during and post-exposure. At the onset of photodissolution the surface roughness momentarily increases as a result of nano-faceting of the silver particles at energetically favourable locations on the silver and/or Ag-ChG interface. The roughness then drastically declines as silver further dissolves into the ChG layer. Both homogeneous and inhomogeneous reversibility were observed.
As shown in the movie, for the case of inhomogeneous reversibility the diffusion occurs in irregular 1–2 µm patches that eventually grow, network and “consume” the remaining undissolved areas. Once the illumination is shuttered off, the silver particles begin to return to the surface.
More complete and uniform dissolution and reversibility were also observed where silver particle dissolution progresses with an abrupt boundary between the smooth glass and silver coating. When the light is shuttered off, the nanoparticulate silver returns to the surface with uniform coverage and roughness values approximately equivalent to pre-illumination values.
Such findings warrant further investigations into the reversible mechanisms for silver photodissolution, especially for multiple cycles.
Full details can be found in the journal Nanotechnology.
About the author
Vincent Palumbo is a PhD candidate in the Department of Materials Science and Engineering at the University of Connecticut, US, working in the NanoMeasurements lab of Bryan D Huey. This group specializes in dynamic measurements with AFM, including high-speed property measurements and mapping. Palumbo’s current research focuses on the effects of elevated temperatures and shock loading on the microstructure and mechanical properties of high-strength low-alloy structural steels.