Printing nanodots with efficiency
Metal nanodot arrays with dot sizes of less than 100nm are attracting a lot of interest in applications ranging from solar panels and sensors to data storage.
Current methods of fabricating the arrays are complicated, expensive and give low throughput. Researchers at the Gwangju Institute of Science and Technology (GIST) in the Republic of Korea have found a new method that could overcome these drawbacks. Unlike existing fabrication methods, the new process is based on the dots being stamped onto a substrate using carbon nanopost (CNP) arrays tipped with the metal to be deposited. Unlike existing methods, it also works at room temperature.
The process used is contact printing, and consists of three stages. First the CNP arrays are formed by depositing pyrolytic carbons in the porous nanochannels of anodic aluminium oxide (AAO) matrices. The CNPs stand ‘proud’ of their AAO channels and are held tightly in them, allowing them to remain vertical and bear the pressure of printing.
The CNPs are then coated with a silane-based releasing agent to allow the metal to separate reliably from the CNP tips during printing. Metal is loaded onto the tips, which are coated with a layer of titanium to glue the nanodots to the substrate. The array is then turned upside down and stamped onto the substrate.
The researchers say the size and packing density of the CNPs, and hence of the nanodots, is readily controlled by tuning the pore dimensions of the AAO template. The team has fabricated CNP stamps of 33-79nm in AAO templates produced from sulphuric or oxalic acid electrolytes, which also produced pore densities of about 3x1010/cm and 1x1010/cm respectively.
They add that SEM images show that the CNPs exhibit a uniform size distribution of 50±6nm and a constant height of about 60nm from the AAO surface.
The technique should be applicable to any metal nanoparticle – including aluminium, copper, gold, nickel, silver and platinum – and it is suitable for substrates including aluminium foil, glass and silicon. In laboratory tests a CNP stamp has been re-used successfully three times, SEM images again showing that the geometry of the CNP arrays appears to remain intact.
The arrays are also said to show quite robust adhesion strength. The researchers sonicated a silver-printed substrate in an ethanol solvent for 30 minutes, but no destruction or detachment of the nanodots was observed.
Schematic illustrations of the entire process, (a) preparation of the CNP stamp, (b) loading of the metal layers on the pretreated CNP tips, (c) contact printing of the metal-loaded stamp onto the substrate, and (d) formation of controlled metal nanodot arrays
The technique remains in the lab for now though, as Professor Won Bae Kim from the GIST team explains. ‘For a practical implementation using the proposed stamping system, a more precise printing system will be needed together with various structured stamps, which are under study and development.
The researchers also say the nanodots can be printed over a large area, but here Professor Kim says, ‘We are not able to compare the fabrication area that our strategy can allow with other approaches, but we think the printed area can be large enough to apply the printed metal nanodots in a number of practical systems.’