Recently, Chang Liu’s group from the Department of Physics and Shenzhen Institute for Quantum Science and Engineering (SIQSE), Southern University of Science and Technology (SUSTech) had led to new progress in the field of metal-organic frameworks (MOFs). The results have been published in the applied physics journal Applied Physics Letters, entitled “Quantum-confinement-induced periodic surface states in two-dimensional metal-organic frameworks”. 

The discovery of topological insulators provides deeper understanding of the fundamental concepts in condensed matter physics. The surface or boundary states of topological insulators are protected from elastic backscattering and localization, and hence hold potential for technological applications in low-power-consumption electronic devices, spintronics and topological quantum computation. To date, all of the experimentally confirmed topological insulators are based on inorganic materials. In contrast, organic materials have the advantages of low cost, easy fabrication, and mechanical flexibility. At the same time, organic materials can own similar properties with conventional inorganic materials, such as organic superconductors, organic light-emitting diodes, organic solar cells and organic field effect transistors, and so on. In addition, the high tunability of organic materials through functionalizing the organic components will greatly increase their potential for technological applications. Considering these advantages of organic materials, researchers have predicted the possible existence of organic topological insulators and organic Chern insulators in 2D MOFs.

Figure: The electronic structures of single-layer Cu-T4PT MOFs grown on Cu(111). (a) A STM image of Cu-T4PT MOFs. (b) The structure model of the Cu-T4PT MOF on a Cu(111) surface. (c) ARPES band structure of Cu-T4PT MOFs with submonolayer coverage. (d) QPI patterns obtained by Fourier transform of the STM dI/dV maps over an area with both the Cu-T4PT MOF and bare Cu(111) substrate. (e) The band dispersions of Cu-T4PT MOFs grown on Cu(111) obtained by FT-STS and ARPES measurements.

There are many theoretical articles based on first-principles band structure calculations that predict a class of two-dimensional (2D) organic topological insulators made of single-layer metal-organic frameworks (MOFs), but their existence has not been confirmed by experiments so far. Chun-Sheng Zhou, senior research fellow in Chang Liu’s group, devotes to explore organic topological insulators experimentally. Combining the use of angle-resolved photoemission spectroscopy and scanning tunneling microscopy, Liu’s group reported the electronic structure of a single-layer Cu-coordinated 2,4,6-tri(4-pyridyl)-1,3,5-triazine (Cu-T4PT) MOF on a Cu(111) substrate grown by organic molecular beam epitaxy, and identified periodic surface states with the period of the Cu-T4PT reciprocal lattice. These periodic surface states, which have identical features to the Cu(111) Shockley surface states, can be attributed to the quantum confinement of the surface states of the underlying Cu(111) substrate by the network lattices of the Cu-T4PT MOF. Their work indicates that the surface states of the metal substrate can be tailored in a controlled manner using periodic nanostructures like surface-supported 2D MOFs, and will facilitate the engineering of electron wave functions in reduced dimensions for exploring the fundamental aspects of quantum physics.

The intrinsic bands and the possible topological properties of the Cu-T4PT MOF are not captured in the experiments. The lack of these bands may be attributed to the strong electronic coupling between the Cu-T4PT MOF and the Cu(111) substrates, which may modify and even suppress the intrinsic topological properties of the metal-organic lattices. In order to exploit organic topological materials predicted in MOFs, it is necessary to grow them on weak van der Waals interaction substrates in the future.

Chun-Sheng Zhou is the first author of the paper. Chang Liu is the only corresponding authors of the paper. The Department of Physics and the Institute for Quantum Science and Engineering of SUSTech is the leading affiliation of the paper. This work was supported by the National Natural Science Foundation of China (NSFC), the Guangdong Innovative and Entrepreneurial Research Team Program, the Shenzhen Key Laboratory, the Science, Technology and Innovation Commission of Shenzhen Municipality, and the National Key R&D Program of China. This paper was completed in cooperation with the Department of Physics and the Institute for Quantum Science and Engineering of SUSTech, School of Physics and Technology and Key Laboratory of Artificial Micro- and Nano-Structures of Ministry of Education of Wuhan University.

Paper Link: https://aip.scitation.org/doi/10.1063/5.0026372