Nanoscopic pores see the light

It takes a tool significantly more sensitive than a spanner or wrench to adjust the size of nanoscopic pores in fine filters, sensors, or diffraction gratings. Researchers in the US, however, believe they can make precise size adjustments of nanopores through use of a simple ultraviolet beam of light.

The precision is said to be so great that it could achieve the membrane-based separation of oxygen from nitrogen, a difference in size of 0.2 Angstroms, or 0.02 nanometres, said Jeff Brinker, senior scientist at Sandia National Laboratories.

‘Using light to change pore size is a kind of nanostructural engineering,’ said Brinker. ‘In addition to creating an overall pattern as achieved by conventional lithography, this kind of lithography also can help us define the internal structure of the films on the nanoscale.’

In effect, continued Brinker, the process creates a kind of tuneable zeolite – crystalline structures with tiny but unalterable pore sizes – enhancing the capability of membranes to separate molecules by size.

This is done by exposing the membrane — in this case, self-assembled thin-film silica that is photosensitive — to the proper amount of light.

Self-assembling photoacid molecules – that uniformly incorporate into a periodic nanostructure – are said to be the key to changes in pore size. A light shone on these molecules breaks them apart to form an acid that causes silica to solidify locally.

The amount of solidification, which necessarily shrinks pore sizes to create the denser material, is proportional to the amount of light shone on the membrane.

A further feature involves shining light through a lithographic mask that varies its intensity, producing so-called ‘grey-scale’ patterning, which theoretically allows for a broad continuous spatial variation of the materials’ structure and properties.

The same process also can be used to produce optical diffraction gratings made entirely of laser-damage-resistant silica.

Researchers used an evaporation-induced, self-assembly process to prepare and characterise photosensitive films.

The work incorporated molecular photoacid generators compartmentalised within a silica-surfactant mesophase.

‘While modifying pore sizes by small amounts so far seems to be completely controllable, we haven’t yet demonstrated control in going from huge to teeny pores,’ said Brinker. ‘We’re not sure what the dynamic range of the process is.’