in nanomaterials, even with a proper temporal resolution to capture the defect dynamics. The difference lies in the principles: the former STEM utilizes the electron–atom scattering of the fast-incident electron beam (30–300 keV) penetrating the sample to image atomic structures including defects both within a solid and on the surface; the SPM like scanning tunneling microscopy (STM) is based on the quantum tunneling effect of the valence electrons from the surface atoms of a solid sample. In the quantum limit system such as graphene-like 2D materials with only surface and no volume states, all atoms and defects appear as the surface, forming an ideal platform for the defect exploration using both techniques. Tremendous research examples on 2D materials system have demonstrated the versatility of both routes to identify atomic defects and the electronic states induced.
2.1. Principles of ADF-STEM and EELS in a TEM
Modern electron microscopy has developed into an era of aberration correction in electron optics. Owing to recent decade’s commercialization and improvement of probe aberration corrector, high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) has become a standard atomic resolution imaging mode to directly visualize the structure of the crystal samples. In HAADF imaging (Z-contrast imaging), the detector receives only electrons after a large-angle elastic scattering between incident electrons and sample atoms, as shown in Fig. 1(a). In this incoherent imaging, the intensity and contrast will not change drastically with the sample thickness and the defocus compared to high-resolution (HR) TEM and bright-field (BF) STEM, but obey an approximate elastic Rutherford scattering formalism ∝ Z2, where Z is the atomic number of the atomic column imaged. Heavy and light atoms will give very contrasting brightness in different columns in the imaging. Thus, HAADF-STEM imaging directly reflects the real atomic structure and resolves the species of the atoms in a compound material, even without any image simulation. A more accurate scaling relation of HAADF-STEM imaging can be utilized for the atom-by-atom structural and chemical analysis of light-element 2D materials. Along with the high-angle scattered signal, low-angle inelastic scattered electrons and transmitted electrons can be collected for annular dark/bright-field (ADF/ABF) imaging simultaneously with the HAADF imaging and spectroscopic analysis (Figs. 1(b)–1(d)). In the ADF-STEM and ABF-STEM imaging, electrons are partially coherent and the yielded contrast in the imaging will change with the sample thickness and defocus. Although the contrast does not directly indicate the atomic structure, they can be interpretable after a quantitative image simulation.
Figure 1. Electron scattering geometry, imaging and spectroscopy in a TEM. (a) Setup of STEM imaging and HAADF, BF detectors, and EELS spectrometer. (b), and (c) Typical ADF-STEM image and core-loss EELS of doped monolayer MoS2. Reproduced from Robertson et al. (2016) with permission. (d) Monochromated valence EELS of monolayer MoS2. Reproduced from Suenaga et al. (2015) with permission.
While imaging by annular detectors, the post-column spectrometer receives the low-angle scattered and transmitted electrons after an energy transfer to the target atom in the electron–sample interaction, to yield the electron energy loss spectroscopy (EELS). Through an inelastic scattering, the energy transfer from the fast-incident electron to the target atom will excite the core electron in the K and L shells, or the valence electrons forming the band structure, into unoccupied electronic states above the Fermi level of the sample. According to the Fermi’s golden rule, the scattering intensity, or cross section, mainly depends on the energy gap and the density of unoccupied states to accommodate the excited electron. This will give rise to the cross section of low loss being several orders of magnitude higher than that of core loss. Generally, low-loss signals with a large cross section are dependent on the electronic structure of the sample and behave as interband transition (Fig. 1(d)) and plasmon excitation related with the valence electrons of the specimen. In the high-loss regime of EEL spectrum, chemical species (Fig. 1(c)) and unoccupied electronic states of the target nanostructures can be quantitatively determined.
In recent years, instrumental performance has advanced to atom-by-atom spectroscopic analysis of the crystal specimen even with single-atom accuracy, higher sensitivity, and energy resolution. Owing to the special versatility in the light element analysis, the valence state of the target elements can be determined from a quantitative measurement of the chemical shift of the characteristic K, L and M edges in the core-loss EEL spectrum. Meanwhile, chemical environment and magnetic structure of the target atom can also be deduced from the energy loss near edge fine structure (ELNES) at the single-atom level. In defective crystals, ELNES can be employed to unveil the defect-induced nanophysics such as valence/spin states and coordination crystal fields of single transition metal dopants or other defects. Specifically, high-energy resolution EELS has been demonstrated as a versatile technique to measure bandgaps, plasmon excitations, or even the vibrational modes of various crystal materials.
2.2. Principles of STM and STS
STM/STS is a common atom probe used in electronic state imaging and analysis in surface science, based on the quantum tunneling effect between the sample surface and the STM tip. It is particularly suited for studying the defects in surfaces due to its high spatial resolution as well as the fact that electronic states introduced by defects will contribute to the tunneling current and thus be revealed by the difference in STM contrast between defects and the surrounding defect-free region. More importantly, by performing scanning tunneling spectroscopy measurements, one can derive the local density of states (LDOS) by taking the differential conductance spectra at the defects and its localization, thus providing direct evaluations of the electronic properties of the defects. Instead of repeating on the working principles of STM/S here, readers are referred to a few excellent monographs or book chapters on this celebrated technique [‘Methods of Experimental Physics, Vol. 27, Scanning tunneling microscopy’ Joseph A. Stroscio & William J. Kasiser (eds.), Academic Press 1993; ‘Introduction to Scanning Tunneling Microscopy (2nd ed.)’ C. Julian Chen, Oxford University Press 2008; ‘Scanning Probe Microscopy, Atomic Force Microscopy and Scanning Tunneling Microscopy’, Bert Voigtlaender, Springer 2015; ‘Scanning Probe Microscopy, Analytical Methods’ Roland Wiesendanger (ed.), Springer 1998; ‘Scanning Tunneling Microscopy and Its Application’ Chunli Bai, Springer 2000].
3. Atomic Defects in 2D Transition Metal Dichalcogenides
In the post-graphene era of 2D materials, the diverse layered transition metal dichalcogenides (TMDs) [4–7] are a large 2D family with unique structure, opto/electronic, and valleytronic properties, especially in valleytronics [8–11] and electronics [12–14] application. Among them, IV–VI group MX2(M = Mo, W; X = S, Se) has three types of crystal phases: 1T phase as a metal and 2H and 3R phases as semiconductors. The hexagonal MoS2/MoSe2 in the 2H phase has been extensively investigated in materials science and finds wide applications in industries [15] as lubricants and hydrodesulfurization catalysts.
