Unlike the much more studied hexagon-based 2D materials, pentagonal 2D materials tend to form puckered layers. Such a puckered pentagon-based structure presents low crystallographic symmetry, leading to an orthorhombic crystal structure with rectangular unit cells. The low crystallographic symmetry introduces large in-plane anisotropy and low thermal conductivity, making them promising candidate materials for future anisotropic electronics and thermoelectrics. Although pentagonal 2D materials have been predicted theoretically for many years, few of them have been studied experimentally. The lack of proper ways to synthesize and stabilize pentagonal 2D materials (particularly metastable ones) has largely hindered the exploration of this unique class of 2D materials.
In response to the above challenges, Prof. Youyong Li and Associate Prof. Yujin Ji from the Institute of Functional Nano and Soft Materials (FUNSOM) of Soochow University collaborated with Prof. Yong P. Chen’s group from Purdue University, have recently successfully synthesized a metastable pentagonal 2D material PdTe2 through symmetry-driven epitaxy. During the epitaxy, the substrate symmetry plays a crucial role. Due to the lattice matching with the substrate, the grown structure can be well controlled and stabilized. They demonstrated the synthesis of monolayer pentagonal PdTe2 by symmetry-driven epitaxy through direct tellurization of the Pd (100) surface. The successful growth of the monolayer pentagonal PdTe2 was confirmed by various structural and spectroscopic characterizations. Scanning tunneling microscopy (STM) measurements and simulations combined with low electron energy diffraction (LEED) analysis proved the atomic structure of the pentagonal PdTe2. X-ray photoelectron spectroscopy (XPS) verified the formation of PdTe2 and the monolayer thickness. Phonon dispersion was revealed by DFT calculations, and the corresponding lattice vibration modes were observed by high-resolution electron energy loss spectroscopy (HREELS). DFT calculations show that monolayer pentagonal PdTe2 is a semiconductor with an indirect band gap of 1.05 eV, which is consistent with the scanning tunneling spectroscopy (STS) results. At the same time, the valence bands of monolayer pentagonal PdTe2 were measured by angle-resolved photoelectron spectroscopy (ARPES). This direct synthesis method, as well as the comprehensive measurement of the atomic structure, phonon dispersion and electronic structure of monolayer pentagonal PdTe2, will greatly accelerate the development of the research field of pentagonal 2D materials. The relevant results have been published in the journal Nature Materials (DOI: 10.1038/s41563-024-01987-w).
Fig. Synthesis of monolayer hexagonal and pentagonal PdTe2
Link to paper：https://www.nature.com/articles/s41563-024-01987-w
Title：A metastable pentagonal 2D material synthesized by symmetry-driven epitaxy
Authors：Lina Liu#, Yujin Ji#, Marco Bianchi, Saban M. Hus, Zheshen Li, Richard Balog, Jill A. Miwa, Philip Hofmann, An-Ping Li, Dmitry Y. Zemlyanov*, Youyong Li* & Yong P. Chen*
Editor: Danting Xiang, Xin Du