Analyzing the microstructure and related properties of 2D materials by transmission electron microscopy
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Two-dimensional materials such as transition metal dichalcogenide and graphene are of great interest due to their intriguing electronic and optical properties such as metal-insulator transition based on structural variation. Accordingly, detailed analyses of structural tunability with transmission electron microscopy have become increasingly important for understanding atomic configurations. This review presents a few analyses that can be applied to two-dimensional materials using transmission electron microscopy.
KeywordsTransmission electron microscopy 2D materials Transition metal dichalcogenide Graphene Van der Waals heterostructure
- 2D materials
Charge density wave
Fast Fourier transform
High-angle annular dark field
High-resolution transmission electron microscopy
Magic angle (1.1°)
Periodic lattice distortion
Selected area electron diffraction
Scanning transmission electron microscopy
Twisted bilayer graphene
Transmission electron microscopy
Transition metal dichalcogenides
Two-dimensional materials are considered as new candidates for future electronic and optical low-dimensional materials. In particular, the most spotlighted materials are graphene and transition metal dichalcogenides (TMD). TMDs are materials of type MX2, where M represents transition metal atoms such as Mo and W and X represents chalcogen atoms such as S, Se, and others. In contrast to graphene, TMDs have band gaps (Manzeli et al. 2017). The properties of graphene and TMDs’ can be tuned using varying growth conditions. Especially for TMDs, the band gap can be modified by decreasing the thickness from a few layers to a monolayer, by creating defects or implantations on the pristine surface and inducing electron-beam irradiation on the surface. For example, the most investigated material, MoS2, shows an indirect band gap in its bulk state, but when the layers are decreased into monolayers, the band gap changes into a direct band gap (He et al. 2014). This leads to better photoluminescence efficiency (He et al. 2014; Wang et al. 2015).
Both graphene and TMDs form van der Waals bonds, which is a weak force of attraction between the layers when they are stacked into few layers. In bilayer 2D materials, the stacking sequence modifies the crystal symmetry and equilibrium distance, which affects the physical properties of 2D materials, such as band gap, phonon vibration frequency, and superconductivity (He et al. 2014; Yan et al. 2015). Recent studies on the manipulation of the interlayer sequence twist angle showed electron tunneling in the heterostructure of 2D materials (Ribeiro-Palau et al. 2018). For example, graphene showed a strong coupling effect when the bilayers were twisted at the angle of 1.1°, which is called the magic angle (Cao et al. 2018). Likewise, the TMD heterostructure demonstrates angle sensitivity in the formation of interlayer excitons (Rivera et al. 2015; Ribeiro-Palau et al. 2018).
Other 2D materials such as 1 T-TaSe2 and TaS2 have a unique characteristic imparted by Fermi nesting, the charge density wave (CDW) (Johannes and Mazin 2008). The CDW is the periodic modulation of the charge density related to the periodic lattice distortion, which is observed as periodic structural changes in the atomic sites (Hossain et al. 2017). This lattice distortion occurs in two different forms, i.e., the commensurate and incommensurate structures; commensurate structures have electron density in the rational multiple of lattice distortion, whereas incommensurate structures have periodicities in the irrational number (Chen et al. 2016). At low temperatures, the property of CDW presents the superconductivity when it orders. Because of such distinct characteristics of CDW, much effort has been made to elucidate the underlying mechanism (Castro Neto 2001; Calandra 2015).
To understand the characteristics described, information on the atomic configuration of 2D materials is essential. Transmission electron microscopy (TEM) is the most commonly used technique for obtaining structural information. In particular, the Z contrast nature of high-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) imaging enables to discern each atom in the 2D layer. The well-known disadvantages of TEM, such as knock-on damage and resolution problems, have been overcome with the recent progress in aberration correction. Aberration-corrected TEM enables to image atomic configurations of 2D layered structures at low acceleration voltages, which significantly reduces knock-on damage. Moreover, the domain patterns that form between the van der Waals layers, such as moiré patterns, can be identified with TEM, and the CDW changes can also be depicted in the diffraction patterns.
This review presents the TEM observation of defects and implantation and structural changes of 2D layers. In addition, research regarding the microstructure of 2D materials with atomic resolution imaging and electrical property measurements is described.
Observation of defects, interstitial sites, and single atoms in 2D materials
TEM has long been used to observe individual atoms, defects, and interstitial sites. Atomic-resolution information on the crystal structure of 2D materials by TEM has gained immense importance because the band structure of the material significantly changes in response to the defects, number of layers, and atomic configurations (Wang et al. 2012; Wang et al. 2017; Elibol et al. 2018). For example, MoS2 ferromagnetism is enhanced with the fabrication of atomic vacancies with electrons or ion beam. In addition, semiconducting MoTe2 showed a new mid-gap state in the band gap induced by imbedded quantum dots and quantum walls (Lin et al. 2016; Elibol et al. 2018). The defects also affect the localization of spin-orbit coupling in graphene. The coupling effect modifies the spin transport characteristics of TMD and graphene layers (Garcia et al. 2018). Furthermore, in specific graphene edge structures, a new band gap and spin state are induced when an electric field is applied (Wang et al. 2017). Because of the tunability of the electrical property with structure, the fabrication of materials with defects, interstitial sites, vacancies, and edge construction has become important.
The accurate identification of atomic configurations of 2D materials is the main issue in material tuning. Thus, effort has been made to clearly analyze the point or line defects in structures by using TEM. However, radiation-induced damage remains one of the main limitations of high-resolution transmission electron microscopy (HRTEM) of 2D materials (Meyer et al. 2012). To solve this problem, Garcia et al. (2018) investigated the e-beam irradiation effects on the MoS2 layer at different kV levels. They calculated the energy transfer from the electron beam to the material and compared it with the displacement threshold energy of Mo and S. At 200 kV and 120 kV, structural damage was observed, whereas the sample was stable at 80 kV. This result indicates that an electron beam of 80 kV is optimum for reducing structural damage to MoS2. This acted as the basis for the TEM analysis of the possible stable state. The difficulty of immaculate sample preparation is another limitation of TEM imaging. Rooney et al. (2017) compared microstructures using various sample preparation methods to observe the relationship between sampling methods and defect creation between layers. They applied density functional theory (DFT) calculations to determine the interlayer distance in the presence/absence of defects and compared them with the intensity profiles of real TEM cross-sectional views. The layers were more distant than the calculated results of the pristine surface, proving that defects or layer distortions can occur during sample preparation. These results indicated that sample fabrication inside a glovebox was the least damaging method of sample fabrication.
Observation of structural distortion
The thin layer property of 2D materials enables to induce their structural deformation in the domain scale, and this is commonly observed using TEM.
-2D layer phase structural transformation
-periodic lattice distortion-induced modifications
Periodic lattice distortion (PLD) has been observed in selected area electron diffraction (SAED) (Ishiguro et al. 1991; Hovden et al. 2016). PLD modulates the nuclei site and CDW to the energetically stable state (Hovden et al. 2016). Modified domains can be observed in high-resolution imaging and diffraction patterns. Hovden et al. (2016) investigated the visibility of commensurate structure projection images of 1 T-TaS2 PLD in 65 layers by using HAADF imaging. Without the ordering of PLD in the z-axis direction, the superlattice could not be seen in the projection image. This visible ordering in the projection image supported the c-axis ordering of the CDW suggested in the free-energy calculations (McMillan 1975).
-in situ-based structural observation
Observation of the microstructure of 2D materials in combination with electric measurements
The alteration in the twist angle between the layers in a van der Waals structure is applied for electronic characteristic tuning in 2D materials (Ju et al. 2017; Cao et al. 2018; Ke et al. 2019; Yoo et al. 2019). The bilayer twist method in graphene shows charge carrier density tunability with the precisely adjusted interlayer angle (Li et al. 2010; Kim et al. 2016; Cao et al. 2018). Recently, a 2D superlattice structure, called the magic angle (1.1°) twisted bilayer graphene (MA-TBG), showed a flat band near zero Fermi energy, and the insulating property was revealed (Cao et al. 2018). Insulating states resulted from the competition of Coulomb energy and quantum-like energy and were consistent with a Mott-like insulating state. This state can be changed into a superconducting state with electron doping and at temperatures reaching 1.7 K (Cao et al. 2018). Moreover, the TBG superlattice showed a quasi-periodic moiré pattern, and the moiré potential trapped the interlayer excitons, which functioned as the charge carriers (Woods et al. 2014; Seyler et al. 2019).
Yoo et al. (2019) showed the atomic and electronic reconstruction at the van der Waals interface in twisted bilayer graphene. Twisted graphene bilayers changed from an incommensurate domain to a strong commensurate domain with the gradual change in the angle (Yoo et al. 2019). Similar phenomena were reported in different 2D van der Waals bilayers and proved by scanning probe microscopy and scanning tunneling microscopy, and the results showed surface reconstruction at the interface (Woods et al. 2014). Yoo et al. (2019) obtained the TEM dark-field image and diffraction pattern and compared the experimental result with the simulations in both reconstructed and unreconstructed states. The satellite peaks shown in the experiments were not observed in the unconstructed surface, which proves the atomic reconstruction in twisted bilayer graphene. In the commensurate state at the magic angle of 1.1°, the calculation of the unreconstructed state interlayer band structure of the twisted bilayer graphene did not comply with the real data. The experimental data showed better consistency with the reconstruction band structure. Moreover, the traverse displacement field applied in the moiré patterns proved the 1D channel formation due to atomic reconstruction (Yoo et al. 2019). These experimental results demonstrated surface reconstruction between the bilayers.
In PLD, the commensurate and incommensurate structures are also related to the electrical resistivity; therefore, microstructural analysis combined with the electric measurements was conducted. For the functional use of 1 T-TaS2, a persisting and nearly commensurate structure throughout the temperature changes is necessary for better electric device conductivity. Tsen et al. (2015) measured the resistivity of 1 T-TaS2 of different thicknesses with changes in temperature. The resistivity-temperature curve of the thin sample showed the persistence of the nearly commensurate state. TEM diffraction peaks from 2 nm and 12 nm layers were compared and demonstrated the disappearance of commensurate peaks in the 2 nm sample. This result was consistent with the electronic measurements (Tsen et al. 2015). These experiments proved the importance of TEM imaging and diffraction in correlation with electrical property measurements to better understand the origin of material properties on an atomic scale.
Because the electronic and optical properties of 2D materials can be tuned with structural changes, previous investigations focused on microstructural analyses. TEM studies on 2D materials demonstrated the defects, interstitial atomic formations, and other structural scale distortions and atomic configurations. Beginning with low-voltage electron beams in which the 2D material sample remains stable, the progress of research has revealed many mechanisms in the formation of structural changes. Recent microstructural studies have assessed the structural distortion-induced electrical properties. These studies combined TEM analyses with other electrical measurements, e.g., conductivity measured using applied traverse field voltage and resistivity measured by voltage changes. These results have been supported by prior studies on 2D material electronic coupling and other unique electronic characteristics induced by microstructural changes.
Although progress has been made in this area, 2D material research continues to advance. Combined with technical developments in TEM, which have led to sub-Angstrom resolution analysis in low-voltage TEM, 2D material research will become a better understood field with the potential for exciting discoveries.
YYC performed data collection and analysis and wrote the entire manuscript. MK and HNH supervised the entire study process, including analysis. All authors read and approved the final manuscript.
This work was supported by the National Research Foundation of Korea (NRF) grants funded by the Ministry of Science and ICT (MSIT) [No. 2015R1A5A1037627].
The authors declare that they have no competing interests.
- L. Fei, S. Lei, W.B. Zhang, W. Lu, Z. Lin, C.H. Lam, Y. Chai, Y. Wang, Direct TEM observations of growth mechanisms of two-dimensional MoS 2 flakes. Nat. Commun. 7, 1–7 (2016)Google Scholar
- J. He, K. Hummer, C. Franchini, Stacking effects on the electronic and optical properties of bilayer transition metal dichalcogenides MoS 2, MoSe 2, WS 2, and WSe 2. Phys. Rev. B—Condens. Matter Mater. Phys. 89, 1–11 (2014)Google Scholar
- M.D. Johannes, I.I. Mazin, Fermi surface nesting and the origin of charge density waves in metals. Phys. Rev. B—Condens. Matter Mater. Phys. 77, 1–9 (2008)Google Scholar
- O.L. Krivanek, M.F. Chisholm, V. Nicolosi, T.J. Pennycook, G.J. Corbin, N. Dellby, M.F. Murfitt, C.S. Own, Z.S. Szilagyi, M.P. Oxley, S.T. Pantelides, S.J. Pennycook, Atom-by-atom structural and chemical analysis by annular dark-field electron microscopy. Nature 464, 571–574 (2010)CrossRefGoogle Scholar
- Y. Li, K.A.N. Duerloo, K. Wauson, E.J. Reed, Structural semiconductor-to-semimetal phase transition in two-dimensional materials induced by electrostatic gating. Nat. Commun. 7, 1–8 (2016)Google Scholar
- J.C. Meyer, F. Eder, S. Kurasch, V. Skakalova, J. Kotakoski, H.J. Park, S. Roth, A. Chuvilin, S. Eyhusen, G. Benner, A.V. Krasheninnikov, U. Kaiser, Accurate measurement of electron beam induced displacement cross sections for single-layer graphene. Phys. Rev. Lett. 108, 1–6 (2012)Google Scholar
- C.R. Woods, L. Britnell, A. Eckmann, R.S. Ma, J.C. Lu, H.M. Guo, X. Lin, G.L. Yu, Y. Cao, R.V. Gorbachev, A.V. Kretinin, J. Park, L.A. Ponomarenko, M.I. Katsnelson, Y.N. Gornostyrev, K. Watanabe, T. Taniguchi, C. Casiraghi, H.J. Gao, A.K. Geim, K.S. Novoselov, Commensurate-incommensurate transition in graphene on hexagonal boron nitride. Nat. Phys. 10, 451–456 (2014)CrossRefGoogle Scholar
- H. Yoo, R. Engelke, S. Carr, S. Fang, K. Zhang, P. Cazeaux, S.H. Sung, R. Hovden, A.W. Tsen, T. Taniguchi, K. Watanabe, G.C. Yi, M. Kim, M. Luskin, E.B. Tadmor, E. Kaxiras, P. Kim, Atomic and electronic reconstruction at the van der Waals interface in twisted bilayer graphene. Nat. Mater. 18, 448–453 (2019)CrossRefGoogle Scholar
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