Raman detection of hidden phonons assisted by atomic point defects in a two-dimensional semimetal

Hui Yuan,Yan Cao,Zhiqiang Mao,Shaojian Li,Jin Hu,Haigen Sun,Xieyu Zhou,Wei Ji,Minghu Pan,Yanglin Zhu,Zongyuan Zhang,Zhibin Shao,Qi Bian

doi:10.1038/s41699-019-0093-7

Abstract

Defects usually have an important role in tailoring various properties of two-dimensional (2D) materials. However, optical detection of defects, especially single-atom point defects, is very challenging in 2D layers. Here, we report our systematic studies on the Raman-activated defect vibrational modes in 2D semimetallic material by combining Raman spectroscopy, density functional theory (DFT) calculation and scanning tunneling microscopy (STM). We observed three common Raman-active vibrational modes located at 95 (A_{1g}^2), 228 (A_{1g}^1), and 304 cm−1 (B_{1g}^1) in ZrSiTe few-layers, consistent with our theoretical calculations. Moreover, a pronounced mode sitting at 131.7 cm−1 was found in the ZrSiTe monolayer. This mode fades out quickly in the bilayer (2L) and eventually disappears in 4L. The high-resolution STM images and DFT calculations suggest this mode to be an intralayer shear mode at the Brillouin zone boundary which is activated by atomic point defects, and STM-based inelastic tunneling spectrum further confirms the existence of such a defect mode. The appearance of such ‘forbidden’ modes in Raman spectra may pave an avenue for the optical characterization of single-atom point defects in metallic 2D layers.

Highlights

Topological nodal line semimetals (TNLSMs) ZrSiX (X = S, Se, and Te) have attracted extensive attention
For most 2D materials such as graphene, transition metal dichalchogenides (TMDs), and black phosphorus (BP), Raman spectroscopy provide a powerful tool to investigate the evolution of crystalline symmetry and interlayer interactions during a thinning down process[11,12,13,14,15,16,17,18,19,20,21]
Similar color codes have been found in TMDs such as MoS2, MoTe2 and WTe215,18,31,32, suggesting that the color of ZrSiTe on the SiO2/Si substrate is a good indicator of its thickness

Summary

INTRODUCTION

In TNLSMs, the conduction and valence bands cross at several points in k-space with forming a closed loop on the Fermi surface. These loops are protected by the nonsymmorphic symmetry[1,2,3], and the electronic states along the nodal lines are topologically nontrivial. Line defects, e.g., phonons at layer edges, can be detected in black phosphorus[24] and MoS225 by polarized Raman spectroscopy. It remains a huge challenge to quickly detect single-atom point defects using optical methods, especially in metallic, layered materials. The 131.7 cm−1 mode does not exist in the list of Raman-active modes predicted by the calculation of defect-free layers but can be understood as a combination of the intralayer shear modes at the Brillouin zone

RESULTS AND DISCUSSION

Yuan et al 3

METHODS

Theoretical methods