Thin stripes of discotic liquid crystal can be used to build molecular nanowires, according to an international team. Their technique could be used to construct components for light-harvesting cells or low-cost biosensors.
As the name would suggest, discotic liquid crystals (DSCs) comprise disc-shaped molecular units. Their unique properties give them potential in solar energy conversion and molecular electronics. However, controlling the alignment of DSCs in a putative device has proven challenging and precluded widespread adoption by the liquid crystal industry. Leeds University's Richard Bushby and Stephen Evans and their colleagues at the University of Sheffield and at Seoul National University, Korea, hope to remedy that situation. [Evans et al., Adv Funct Mater, (2009) 20, 914-920; doi: 10.1002/adfm.200902140]
"DLC molecules have a tendency to stack on top of each other like a pile of coins," explains Evans. "But the difficulty comes in controlling the orientation of such columnar stacks with respect to the surface on which they lie. This is crucial for their future applications.” "Traditionally, scientists have tried to get DLCs to align simply by rubbing the surface they sit on with a cloth to create micro grooves." Unfortunately, this works only at the macroscopic level and so is imprecise.
The team has now developed a way of creating planar aligned columnar discotic liquid crystals (cDLCs) on surfaces by exploiting chemical patterning with microcontact printing of organothiol self-assembled monolayers (SAMs) on gold, or via deep ultraviolet patterning of organosilane SAMs on silicon. By adding and heating a droplet of liquid crystal they can spread it across the surface and form layers only where high-energy stripes are present, leaving low-energy regions bare. When the system is cooled from the isotropic into the hexagonal columnar phase polarizing microscopy reveals cDLCs aligned in a planar orientation.
The researchers have tested the approach with three triphenylene derivatives (HAT-6, HHTT, H7T) and a phthalocyanine derivative (8H_2 Pc). They found that H7T and HAT-6 both align perpendicular to the stripes, whereas HHTT and 8H_2 Pc both align parallel to the stripes.
"This relatively simple new method for creating planar aligned columnar phases of DLCs gives control over the azimuthal angle," the team explains. This is a prerequisite for constructing organic field-effect transistors using cDLCs, they add. Evans points out that, "Within the stripes we found molecules arranged into hemi-cylindrical columns each several microns long, which we believe to be the highest level of control over DLC alignment to date. We also found that the narrower the stripes, the better the ordered the columns."
David Bradley