Single atom vibration spectroscopy with chemical bonding sensitivity

On March 16, 2023, Zhou Wu’s research group at the University of Chinese Academy of Sciences published the research results entitled “Single-atom vibrational spectroscopy with chemical-bonding sensitivity” online in the journal Nature Materials.

In this study, the results of accurate systematic measurement of local lattice vibration signals at defects with different bonding configurations in monolayer graphene are reported by scanning transmission electron electron microscopy electron energy loss spectroscopy technology at single atomic scale, millielectron volt energy resolution, and the influence of bonding configuration and atomic mass of defective atoms on local characteristic lattice vibration spectrum signals is revealed for the first time in single-atom accuracy, and the measurement accuracy of single-atom-scale lattice vibration spectra in scanning transmission electron microscopy is improved to the level of chemical bonding sensitivity.

Professor Wu Zhou of the University of Chinese Academy of Sciences and Professor Sokrates T. Pantelides of Vanderbilt University are co-corresponding authors of the paper. Mingquan Xu, a 18th grade doctoral student at the University of Chinese Academy of Sciences, Deliang Bao, a postdoctoral fellow at Vanderbilt University, and Li Aowen, a 18th grade doctoral student at the University of Chinese Academy of Sciences, are the co-first authors of the paper. Key collaborators include Professor Shixuan Du and Professor Gang Su of the University of Chinese Academy of Sciences, and Stephen J. Pennycook, Distinguished Visiting Professor at the University of Chinese Academy of Sciences.

Phonons are an intrinsic excitation in condensed matter physics that describes the collective motion of periodic atoms, and its characteristics are affected by factors such as the lattice structure, chemical bonding, atomic mass, and lattice strain of the material. The lattice symmetry breaking caused by defects at the microscopic scale can change the corresponding phonon excitation, thereby affecting the physical properties of the material such as thermal conductivity, phase transition, and conductance, and further affect the superconductivity and other special properties of the material through electron-phonon coupling. Therefore, the development of atomic-scale vibration spectroscopy measurement technology and the continuous improvement of its measurement accuracy and sensitivity are of great significance for the study of the physical property regulation of quantum materials. With the development of electron monochromator technology, scanning transmission electron microscopy electron loss spectroscopy (STEM-EELS) can have both atomic-level spatial resolution and millielectron volt energy resolution, making it possible to detect local vibration signals of materials at the atomic scale.

Recently, the research team of Professor Zhou Wu of the School of Physical Sciences of the University of Chinese Academy of Sciences has pushed the sensitivity of atomic-scale vibration spectroscopy to a new limit by optimizing the spherical aberration correction scanning transmission electron microscope of the monochromator, improving the stability of the instrument and optimizing the setting of the electron optical path, realizing the single-atom vibration spectroscopy measurement with chemical bonding sensitivity, and revealing the influence of the bonding configuration and atomic mass of defective atoms on the local characteristic lattice vibration spectrum signal. This paper provides a direct experimental basis for understanding the influence of individual atomic defects on the vibration mode of the local lattice (Figure 1).

Using single-atom precision, millielectron volt-resolved scanning transmission electron microscopy electron loss spectroscopy, Zhou Wu’s research team carried out accurate and systematic measurements of the local lattice vibration signals of two stable types of silicon atomic point defects in the graphene lattice, namely the Si-C3 configuration in which silicon atoms replace one lattice carbon atom to form a three-coordinated bond, and the Si-C4 configuration that replaces two lattice carbon atoms to form a four-coordinated bond, and explored the sensitivity limit of single-atom vibration spectroscopy measurement technology in scanning transmission electron microscopy. The experimental results show that compared with the two types of point defects with the same impurity atom (Si) but different bonding configurations, Si-C4 has a stronger vibrational spectral signal than Si-C3 in the ~100 millielectron volt energy loss interval, indicating that the change of chemical bonding leads to a unique local vibration mode. By performing high-precision measurements of the vibrational spectrum signals of atoms around point defects at the single-atom scale, combined with theoretical computational simulations, the research team found that the low-energy vibration signal signature (P1) of carbon atoms around point defects changes with their distance from the central impurity Si atom, and the vibration spectrum signals on the corresponding carbon atoms in different point defect configurations are also significantly different (Figure 2). Combined with theoretical calculations, it is found that Si-C4 and Si-C3 reflect unique intrinsic vibration modes in different energy ranges due to their different local structural symmetry (Figure 3).

In addition, the research team also analyzed the local vibration signal of nitrogen atom defect (N-C3) with small mass difference compared to carbon atoms in graphene. As shown in Figure 4, unlike Si-C3, the N-C3 defect in graphene causes a significant redshift of the higher energy vibration signal (P2), and the vibration frequency of nitrogen atom and nearest neighbor carbon atom basically conforms to the law of simple harmonic oscillator model, indicating that the experimental technique can reveal the influence of small differences in atomic mass on local vibration frequency in single-atom accuracy.

Figure 1: Local vibrational spectroscopy signal measurement at silicon point defects of different configurations in the graphene lattice

Figure 2: Modulation of atomic-scale local vibration signals by chemical bonding at different silicon defects in graphene

Figure 3: Theoretical calculation of eigenmodes at Si-C4 and Si-C3 defects in graphene

Figure 4: Monatomic-scale vibrational spectroscopy at nitrogen defects in graphene

This work further improves the detection limit of electron energy loss spectroscopy in scanning transmission electron microscopy, reveals for the first time the modulation of chemical bonding and atomic mass on the local lattice vibration characteristics at the single atomic scale, and improves the measurement accuracy of single-atom scale lattice vibration spectroscopy in scanning transmission electron microscopy to a level that is sensitive to chemical bonding. This technology provides a new experimental means for directly measuring the modulation of the local lattice vibration mode of quantum materials by chemical bonding, exploring the regulation of quantum physical properties by lattice defects, and also providing help for understanding new physical phenomena induced by defects in graphene. The research work has been supported by the National Key R&D Program, the Beijing University Outstanding Young Scientist Program, the National Natural Science Foundation of China, the Pilot Project of the Chinese Academy of Sciences, and the Key Research Project of Frontier Science of the Chinese Academy of Sciences. (Source: Science Network)

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