Vertical challenge for organic semiconductors

Vertical growth of conducting tetraaniline crystals could lead to a new approach to efficient and powerful organic semiconductor technology thanks to work by Richard Kaner of the California NanoSystems Institute at the University of California Los Angeles and colleagues. The work could ultimately improve solar energy capture devices by allowing technologists to grow "light antennas", thin, pole-like devices on a substrate that could absorb light from all directions rather than being monodirectional as is the case with current smooth solar panels. [ACS Nano; DOI: 10.1021/acsnano.5b03465]

By working with a graphene substrate, the UCLA team was able to grow spikes of crystalline tetraaniline from solution. Vertical growth is rare and difficult to carry out with conventional inorganic semiconducting materials, such as silicon, although vertical growth with organic semiconductors had proven even tougher. The main motivation for vertical growth is that many long, thin crystals can be packed together on a surface. "Vertically oriented structures of single crystalline conductors and semiconductors are of great technological importance due to their directional charge carrier transport, high device density, and interesting optical properties," the team says.

"These crystals are analogous to organizing a table covered with scattered pencils into a pencil cup," explains Yue "Jessica" Wang, a former UCLA doctoral student who now is a postdoctoral scholar at Stanford University and was the first author on the ACS Nano paper. "The vertical orientation can save a great deal of space, and that could mean smaller, more efficient personal electronics in the near future." Kaner and Wang worked with Xiangfeng Duan, Yves Rubin, Adam Stieg, graduate students James Torres, Shan Jiang and Michael Yeung, and Santanu Chaudhuri of the Illinois Applied Research Institute at University of Illinois at Urbana–Champaign.

In preliminary experiments Kaner and his colleagues discovered that they could guide the tetraaniline solution to grow vertical crystals, and once this technique was understood, they next developed a one-step method for growing highly ordered, vertically aligned crystals for a variety of organic semiconductors using the same graphene substrate.

"The key was deciphering the interactions between organic semiconductors and graphene in various solvent environments," Wang explains. "Once we understood this complex mechanism, growing vertical organic crystals became simple."

Using graphene as the substrate had several advantages over conventional substrates, Kaner adds. "This technique enables us to pattern crystals wherever we want," he explains. "You could make electronic devices from these semiconductor crystals and grow them precisely in intricate patterns required for the device you want, such as thin-film transistors or light-emitting diodes." Tests on the tetraaniline system showed that charge carriers could travel most efficiently along the vertical interfacial stacking direction with a conductivity of 12.3 Siemens per centimetre in individual crystals, the team reports, which is the highest reported to date for an aniline oligomer.

"We are trying to grow vertical crystals on other layered compounds such as boron nitride and molybdenum disulfide to form different types of heterojunctions that can serve as the building blocks for various electronic devices," team member Jessica Wang told Materials Today.

David Bradley blogs at Sciencebase Science Blog and tweets @sciencebase, he is author of the bestselling science book "Deceived Wisdom".