Narrowing band gap chemically and physically: Conductive dense hydrocarbon

TL;DR

This study demonstrates that large polycyclic aromatic hydrocarbons like dicoronylene can undergo a pressure-induced transition from insulator to semi-metallic state at relatively low pressures, advancing the potential for hydrocarbon-based molecular metals.

Contribution

It reveals a pressure-induced insulator-to-semi-metal transition in dicoronylene, a large PAH, at much lower pressures than previously achieved, aided by increased π-electron count and chemical stability.

Findings

Dicoronylene becomes semi-metallic above 23 GPa.

Resistivity decreases by three orders of magnitude under pressure.

Structural and electronic changes confirmed by multiple spectroscopic and computational methods.

Abstract

Band gap energy of an organic molecule can be reduced by intermolecular interaction enhancement, and thus, certain polycyclic aromatic hydrocarbons (PAHs), which are insulators with wide band gaps, are expected to undergo insulator-metal transitions by simple compression. Such a pressure-induced electronic transition can be exploited to transform non-metallic organic materials into states featuring intriguing electronic characteristics such as high-temperature superconductivity. Numerous attempts have been made to metalize various small PAHs, but so far only pressure-induced amorphization well below the megabar region was observed. The wide band gap energy of the small PAHs and low chemical stability under simple compression are the bottlenecks. We have investigated the band gap energy evolution and the crystal structural compression of the large PAH molecules, where the band gap energy is significantly reduced by increasing the number of {\pi}-electrons and improved chemical stability with fully benzenoid molecular structure. Herein, we present a pressure-induced transition in dicoronylene, C48H20, an insulator at ambient conditions that transforms into a semi-metallic state above 23.0 GPa with a three-order-of-magnitude reduction in resistivity. In-situ UV-visible absorption, transport property measurement, Raman spectroscopy, X-ray diffraction and density functional theory calculations were performed to provide tentative explanations to the alterations in its electronic structure at high pressure. The discovery of an electronic transition at pressures well below the megabar is a promising step towards realization of a single component purely hydrocarbon molecular metal in the near future.