Organic Superconductors With Ultrafast Electrons Show Exceptional Properties Comparable to Graphene

Conducting two-dimensional polymers (C2Ps) have emerged as a new functional polymer material with unique chemical and physical properties. This is due to their extended pi-conjugation over the second dimension. However, C2Ps with fused aromatic linkages remain unexplored because they are hindered by strong stacking between adjacent layers.

Search for Unconventional Superconductivity

Graphene has attracted intense research interest because of its compelling properties. These include remarkably high thermal conductivity, high mechanical strengths, and high mobility of charge carriers. A study even confirmed that graphene superlattices exhibit unconventional superconductivity.

Known as the "dream material," graphene exhibits electron mobility 140 times faster than silicon and a strength 200 times greater than steel. However, graphene lacks a band gap, vital for regulating electrical current. This prevents graphene from being used as a semiconductor.

Scientists have been actively exploring different strategies to create a semiconductor that demonstrates graphene's exceptional properties. One promising approach is the development of conducting polymers.

Progress in polymer science, engineering, and industrialization of plastic products have triggered a paradigm shift to organic or polymeric materials with metallic properties, specifically electrical conductivity. These materials are expected to combine the benefits of lightweight and low-cost organic molecules and the mechanical flexibility of polymers with the high electrical conductivity of conventional metals.

Scientists have tried to develop conducting polymers with a fused aromatic backbone. They work by mimicking the chemical structure of graphene to attain exceptional properties. However, challenges still arise during synthesis because the interlayer stacking between growth intermediates hinders proper growth.

Development of Organic Semiconductors

In a recent study, a team of experts have developed conducting two-dimensional polymers that exhibit electron mobility comparable to graphene. The details of their research are described in the paper "Observation of ultrafast electrons in pendant-embedded conducting two-dimensional polymers."

Led by Dr. Yeonsang Lee from the Department of Chemistry at Pohang University of Science and Technology (POSTECH), the scientists utilized triazacoronene, a chemical compound with a chemical structure that resembles graphene. Lee and colleagues introduced bulky pendant functional groups to the periphery of triazacoronene.

The researchers introduced steric hindrance from the pendant groups, successfully suppressing the stacking of two-dimensional polymer intermediates during the polymerization of triazacoronene monomers. This resulted in increased solubility of the intermediates and facilitated the synthesis of two-dimensional polymers with a higher degree of polymerization and fewer defects. It also led to outstanding electrical conductivity after p-type doping.

After conducting magnetotransport measurements, the experiment revealed that coherent multi-carrier transport with finite n-type carriers demonstrates exceptionally high mobility and long phase coherence length. This is in contrast to hole-carrier transport, which has 25,000 times lower mobility at low temperatures.

According to the research team, they have achieved a breakthrough in addressing the challenge of low electron mobility, a significant issue in organic semiconductors. Their findings also confirmed that it is possible to control the conduction pathways for electrons and holes at the molecular level.