Scientists 'paint' the world's smallest Mona Lisa

Mini Lisa! Scientists have 'painted' a mini version of Leonardo da Vinci's masterpiece Mona Lisa on the world's smallest canvas - a surface one-third the width of a human hair.
Researchers at the Georgia Institute of Technology "painted" the world's most famous painting Mona Lisa on a substrate surface approximately 30 microns in width.
The creation, the "Mini Lisa," demonstrates a technique that could potentially be used to achieve nanomanufacturing of devices because the team was able to vary the surface concentration of molecules on such short-length scales.
The image was created with an atomic force microscope and a process called ThermoChemical NanoLithography (TCNL). Going pixel by pixel, researchers positioned a heated cantilever at the substrate surface to create a series of confined nanoscale chemical reactions.

By varying only the heat at each location, Keith Carroll controlled the number of new molecules that were created. The greater the heat, the greater the local concentration. More heat produced the lighter shades of gray, as seen on the Mini Lisa's forehead and hands.
Less heat produced the darker shades in her dress and hair seen when the molecular canvas is visualised using fluorescent dye. Each pixel is spaced by 125 nanometres.

"By tuning the temperature, our team manipulated chemical reactions to yield variations in the molecular concentrations on the nanoscale," said Jennifer Curtis, study's lead author.
"The spatial confinement of these reactions provides the precision required to generate complex chemical images like the Mini Lisa," said Curtis.

Production of chemical concentration gradients and variations on the sub-micrometre scale are difficult to achieve with other techniques, despite a wide range of applications the process could allow.
The Georgia Tech TCNL research collaboration produced chemical gradients of amine groups, but expects that the process could be extended for use with other materials.
"We envision TCNL will be capable of patterning gradients of other physical or chemical properties, such as conductivity of graphene," Curtis said.
"This technique should enable a wide range of previously inaccessible experiments and applications in fields as diverse as nanoelectronics, optoelectronics and bioengineering," said Curtis.
Another advantage, according to Curtis, is that atomic force microscopes are fairly common and the thermal control is relatively straightforward, making the approach accessible to both academic and industrial laboratories.

Because the technique provides high spatial resolutions at a speed faster than other existing methods, even with a single cantilever, Curtis is hopeful that TCNL will provide the option of nanoscale printing integrated with the fabrication of large quantities of surfaces or everyday materials whose dimensions are more than one billion times larger than the TCNL features themselves.