Cs-corrected Scanning Transmission Electron Microscopy Research Articles

Strain distribution in the active region of InAs-based interband cascade laser

Energy-dispersive x-ray spectroscopy and high-angle annular dark-field in a Cs-corrected scanning transmission electron microscope are employed to characterize the atomic-scale strain distribution in the active region of the InAs-based interband cascade laser. For the first time, energy-dispersive x-ray spectroscopy is utilized for the quantitative calculation of the zero-strain region, by which the geometric phase analysis of high-angle annular dark-field imaging has been carried out. The strain distribution of the active region with high accuracy has been obtained. The analysis of the out-of-plane strain shows that the active region in the InAs-based interband cascade laser is strain-compensated, while a certain degree of elemental intermixing still exists in the active region. This detailed strain distribution can provide valuable insights into the optimization of the growth conditions for the active region such as growth temperature, V/III flux ratio, and growth process to minimize the elemental intermixing and obtain a better performance interface while maintaining the strain-compensated state.

A new high-pressure die casting Mg−Gd−Sm−Al alloy with high strength-ductility synergy

A high-pressure die casting (HPDC) Mg−5Gd−1.5Sm−0.7Al alloy was newly developed and exhibits outstanding strength-ductility synergy, with the yield strength and the tensile elongation to fracture being approximately 200 MPa and 8.5%, respectively. This alloy has two types of α-Mg grains: coarse α1-Mg ((46 ± 18) μm) and fine α2-Mg ((9.2 ± 2.3) μm) grains, and various Al-GS (GS = Gd and Sm) particles located at grain boundaries while clear solute-atom segregation near grain boundaries with limited or free intermetallic particles. Characterizations using Cs-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) indicate the crystal structures of Al-GS phases. After aging, dense β’ precipitates and chain-shaped β’’-like structures precipitated near grain boundaries while a high density of ultrafine β’’-(Mg,Al)3Sm precipitates and Al-GS clusters formed in grain center. Relatively fine grains, Al-GS primary particles, solute-atom segregation near grain boundaries, and/or multiple precipitates contribute to the high strength of the studied alloy, while the multi-scale α-Mg grains, variety of intermetallic particles but discontinuous skeleton, and the multi-typed precipitated lead to its satisfactory ductility.

High-performance of ZnO/TiO2 heterostructured thin-film photocatalyst fabricated via atomic layer deposition

Oxide semiconductor is one of the most promising candidates for photocatalysts due to its light absorption ability, electronic properties, and stability. It is used in various applications such as solar-energy conversion, CO2 reduction, and water splitting. In this research, ZnO, TiO2, and ZnO/TiO2 heterostructured thin films are fabricated via atomic layer deposition (ALD), and their photocatalytic performances are evaluated. The film thickness can be controlled using ALD, and surface reactions can easily occur in thin films owing to the short distances between the active sites and charge carriers. In addition, unlike a powder catalyst, the bilayer photocatalyst is fixed in the solution; therefore, it does not make the solution turbid or disturb the light penetration. Diethylzinc and titanium tetraisopropoxide are used as precursors for Zn and Ti, and the thin films are deposited on soda-lime glass substrates at 150 °C using H2O as the reactant gas. The photocatalytic activity and stability are evaluated through photodegradation tests using methylene blue aqueous solution. The ZnO single-substance thin film exhibits a high degradation rate, but its performance significantly decreases after three consecutive experiments. The TiO2 single-substance thin film exhibits a relatively low degradation rate, but high reusability, exhibiting characteristics opposite to that of ZnO. Therefore, a TiO2 thin film is coated on ZnO to leverage both these advantages. The thin films are heat-treated at 400 °C for 10 min after deposition in a vacuum atmosphere. The surface morphology, crystal structure, and electrical characteristics of the photocatalyst specimens are analyzed through high-resolution scanning electron microscopy, Cs-corrected scanning transmission electron microscopy, and x-ray diffraction analysis. Their photocatalytic performances under ultraviolet (UV) irradiation are measured through UV–visible spectroscopy. The heat-treated ZnO/TiO2 heterostructured thin film exhibits a photodegradation rate exceeding 80%, with little degeneration after three cycles, indicating enhanced photodegradation performance and stability.

Highly dispersed Pd-based pseudo-single atoms in zeolites for hydrogen generation and pollutant disposal.

Atomically dispersed metal catalysts with excellent activity and stability are highly desired in heterogeneous catalysis. Herein, we synthesized zeolite-encaged Pd-based pseudo-single atoms via a facile and energy-efficient ligand-protected direct H2 reduction method. Cs-corrected scanning transmission electron microscopy, extended X-ray absorption, and pair distribution function measurements reveal that the metal species are close to atomic-level dispersion and completely confined within the intersectional channels of silicalite-1 (S-1) zeolite with the MFI framework. The Pd@S-1-H exhibits excellent activity and stability in methane combustion reactions with a complete combustion temperature of 390 °C, and no deactivation is observed even after 100 h on stream. The optimized bimetallic 0.8Pd0.2Ni(OH)2@S-1-H catalyst exhibits an excellent H2 generation rate from FA decomposition without any additives, affording a superhigh turnover frequency up to 9308 h-1 at 333 K, which represents the top activity among all of the best heterogeneous catalysts under similar conditions. Significantly, zeolite-encaged metal catalysts are first used for Cr(vi) reduction coupled with formic acid (FA) dehydrogenation and show a superhigh turnover number of 2980 mol(Cr2O72-) mol(Pd)-1 at 323 K, surpassing all of the previously reported catalysts. This work demonstrates that zeolite-encaged pseudo-single atom catalysts are promising in efficient hydrogen storage and pollutant disposal applications.

One-step hydrothermal synthesis of CuS/MoS2 composite for use as an electrochemical non-enzymatic glucose sensor

Early diagnosis may be crucial for the prevention of chronic diabetes mellitus. For that herein, we prepared a CuS/MoS2 composite for a non-enzymatic glucose sensor through a one-step hydrothermal method owing to the synergetic effect of CuS/MoS2. The surface morphology of CuS/MoS2 was studied by Field Emission Scanning Electron Microscopy (FESEM) and Cs-corrected Scanning Transmission Electron Microscopy (Cs-STEM). The crystallinity and surface composition of CuS/MoS2 were analyzed by X-ray Diffraction (XRD) and X-ray Photoelectron Spectroscopy (XPS) respectively. The working electrode was prepared from CuS/MoS2 electrocatalyst, and for that dispersed solution of electrocatalyst was used to fabricate the material-loaded glassy carbon electrode (GC). CuS/MoS2 composite shows the viability of electrocatalyst to oxidize glucose in an alkaline solution with sensitivity and detection limit of 252.71 μA mM−1 cm−2 and 1.52 μM respectively. The proposed glucose sensor showed reasonable stability and potential selectivity during electrochemical analysis. Accordingly, the CuS/MoS2 composite has potential as a viable material for glucose sensing in diluted human serum.

Synthetic control of correlated disorder in UiO-66 frameworks

Changing the perception of defects as imperfections in crystalline frameworks into correlated domains amenable to chemical control and targeted design might offer opportunities for the design of porous materials with superior performance or distinctive behavior in catalysis, separation, storage, or guest recognition. From a chemical standpoint, the establishment of synthetic protocols adapted to control the generation and growth of correlated disorder is crucial to consider defect engineering a practicable route towards adjusting framework function. By using UiO-66 as experimental platform, we systematically explored the framework chemical space of the corresponding defective materials. Periodic disorder arising from controlled generation and growth of missing cluster vacancies can be chemically controlled by the relative concentration of linker and modulator, which has been used to isolate a crystallographically pure “disordered” reo phase. Cs-corrected scanning transmission electron microscopy is used to proof the coexistence of correlated domains of missing linker and cluster vacancies, whose relative sizes are fixed by the linker concentration. The relative distribution of correlated disorder in the porosity and catalytic activity of the material reveals that, contrarily to the common belief, surpassing a certain defect concentration threshold can have a detrimental effect.

Extra-conventional strengthening mechanisms in non-recrystallized grains of an extruded Mg-Gd-Zr alloy

Mg-RE (RE = rare earth) based alloys generally exhibit outstanding mechanical properties. However, their high-strength seems to be unexplained using classic strengthening mechanisms in some cases. Herein, a Mg-13Gd-0.4Zr (wt%) alloy that was fabricated by a conventional differential thermal extrusion plus artificial aging treatment exhibits ultra-high yield strength over 510 MPa in both tension and compression. Characterizations using Cs-corrected scanning transmission electron microscopy (STEM) show two unusual microstructures in non-recrystallized grains as: a large density of basal stacking faults (SFs) and profuse distortion areas (DAs). Atomic-resolution STEM imaging indicates that basal SFs are consisted of two types of intrinsic SFs, namely I1 and I2, and DAs are self-assembled by 〈c〉 and 〈c + a〉 screw partials. Their strengthening mechanisms are analogous to grain boundary strengthening and dispersion strengthening, respectively, contributing satisfactory yield-strength increments of ∼46 MPa and ∼76 MPa, respectively. Moreover, DAs improved aging hardening by inducing novel clusters at DA-related boundaries, or increasing the number density of βH’ precipitate and promoting their distribution along a certain direction. This work supplements the strengthening mechanisms in traditional high-strength Mg-RE(-Zr) based alloys and provides novel insights in the development of ultra-high-strength Mg alloys.

High-resolution three-dimensional structural determination of unstained double-gyroid block copolymers through scanning transmission electron microscopy

Block copolymer-based multicomponent materials have garnered considerable attention because of tunable properties due to their various constituents. The use of electron tomography through transmission electron microscopy (TEM) for the three-dimensional (3D) imaging of stained block copolymers is an established approach for investigating structure-property relationships. Recently, scanning transmission electron microscopy (STEM) with an annular dark-field (ADF) detector has emerged as a method for the 3D structural analysis of unstained block copolymers. However, because of a lack of electron contrast, only a few low-resolution 3D reconstructions were reported for light elements. Herein, we report the first 3D structural analysis of a 200-nm-thick film composed of unstained double-gyroid block copolymers-polystyrene-b-poly(2-vinylpyridine) (PS-P2VP)-at a resolution of 8.6 nm through spherical aberration Cs-corrected STEM. At this resolution, P2VP molecules can be distinguished from PS molecules in z-contrast 3D reconstructions obtained both experimentally and theoretically. The 3D reconstructions revealed structural differences between stained and unstained specimens.

Ferroelectric solitons crafted in epitaxial bismuth ferrite superlattices

In ferroelectrics, complex interactions among various degrees of freedom enable the condensation of topologically protected polarization textures. Known as ferroelectric solitons, these particle-like structures represent a new class of materials with promise for beyond-CMOS technologies due to their ultrafine size and sensitivity to external stimuli. Such polarization textures have scarcely been demonstrated in multiferroics. Here, we present evidence for ferroelectric solitons in (BiFeO3)/(SrTiO3) superlattices. High-resolution piezoresponse force microscopy and Cs-corrected high-angle annular dark-field scanning transmission electron microscopy reveal a zoo of topologies, and polarization displacement mapping of planar specimens reveals center-convergent/divergent topological defects as small as 3 nm. Phase-field simulations verify that some of these structures can be classed as bimerons with a topological charge of ±1, and first-principles-based effective Hamiltonian computations show that the coexistence of such structures can lead to non-integer topological charges, a first observation in a BiFeO3-based system. Our results open new opportunities in multiferroic topotronics.

Interfacial characterization of InAs/GaSb superlattices grown by MOCVD at different growth temperatures

Strain-balanced InAs/GaSb superlattices (SLs) with GaAs-type interfaces (IFs) were grown by metalorganic chemical vapor deposition (MOCVD) on InAs substrates at different temperatures. High-angle annular dark-field (HAADF) imaging in a Cs-corrected scanning transmission electron microscope was utilized to analyze quantitatively the interfacial structure features of InAs/GaSb SLs. InAs-on-GaSb IFs were found to be more abrupt and strained than GaSb-on-InAs IFs. With increasing the growth temperature, the interfaces of strain-balanced InAs/GaSb SLs become narrower and contain more GaAs content. Formation of more GaAs in the IFs is considered to account for the improvement of SL interfacial quality by optimizing the specific growth conditions for the superlattices.

Directly revealing the structure-property correlation in Na+-doped cathode materials

The introduction of Na+ is considered as an effective way to improve the performance of Ni-rich cathode materials. However, the direct structure–property correlation for Na+ doped NCM-based cathode materials remain unclear, due to the difficulty of local and accurate structural characterization for light elements such as Li and Na. Moreover, there is the complexity of the modeling for the whole Li ion battery (LIB) system. To tackle the above-mentioned issues, we prepared Na+-doped LiNi0.6Co0.2Mn0.2O2 (Na-NCM622) material. The crystal structure change and the lattice distortion with picometers precision of the Na+-doped material is revealed by Cs-corrected scanning transmission electron microscopy (STEM). Density functional theory (DFT) and the recently proposed electrochemical model, i.e., modified Planck-Nernst-Poisson coupled Frumkin-Butler-Volmer (MPNP-FBV), has been applied to reveal correlations between the activation energy and the charge transfer resistance at multiscale. It is shown that Na+ doping can reduce the activation energy barrier from ΔG = 1.10 eV to 1.05 eV, resulting in a reduction of the interfacial resistance from 297 Ω to 134 Ω. Consequently, the Na-NCM622 cathode delivers a superior capacity retention of 90.8 % (159 mAh.g−1) after 100 cycles compared to the pristine NCM622 (67.5 %, 108 mAh. g−1). Our results demonstrate that the kinetics of Li+ diffusion and the electrochemical reaction can be enhanced by Na+ doping the cathode material.

Identifying polymorphic lattice positioning of copper and the effects on the thermoelectric performance of δ-LAST

N-type AgPbmSbTem+2 (abbreviated as LAST-m) has attracted much attention as an important branch of PbTe-based thermoelectric materials, mainly due to its complex microstructure and ultralow thermal conductivity. However, LAST-m met much difficulty in repeatability and reproducibility of thermoelectric performance largely due to the meta-stable feature of the component AgSbTe2. In this work, we replaced stoichiometric AgSbTe2 with non-stoichiometric Ag0.352Sb0.549Te, and alloyed it with PbTe to get thermodynamically stable and single-phase δ-LAST solid solutions. Further addition of extra Cu with excess dose results in the polymorphic lattice positioning of Cu atoms in the matrix of δ-LAST as revealed by Cs-corrected scanning transmission electron microscope (STEM), i.e., single Cu interstitial, Cu interstitial monolayer, Cu interstitial bilayer, Cu interstitial super-lattice, Cu interstitial intermediate compound and Cu2-xTe. It was argued that the polymorphic positioning of Cu atoms not only dynamically optimize the electron concentration at climbing temperatures but also effectively scatter heat-carrying phonons in a panoscopic approach, leading to an outstanding thermoelectric performance competitive with state-of-art LAST-m and other n-type PbTe-based thermoelectrics.

Study on corrosion resistance of artificially aged 7075 aluminium alloy by using Cs-corrected STEM

This research studied the mechanism of the corrosion resistance enhancement of artificially aged 7075 aluminium alloy using advanced Cs-corrected scanning transmission electron microscopy (STEM). The corrosion behaviors of artificially aged 7075 aluminium alloys in a 3.5 wt.% NaCl solution were investigated by impedance spectra, equivalent circuit analyses, polarization measurements and immersion tests. The results show that a longer aging treatment leads to better corrosion resistance, which can be attributed to the following microstructural features, as revealed by STEM. The Cu segregation at grain boundaries under over-aged conditions helps retard intergranular corrosion. The Mg(Zn, Cu)2 precipitates formed on the surfaces of Al18Mg3(Cr,Mn)2 dispersoids effectively insulate the dispersoids as cathodes in corrosion, from the Al matrix. This study demonstrates a potential strategy to design corrosion-resistant alloys achieved by proper alloying and subsequent aging.

Stabilizing the (003) Facet of Micron-Sized LiNi0.6Co0.2Mn0.2O2 Cathode Material Using Tungsten Oxide as an Exemplar

The structural stability of layered LiNi1-x-yCoxMnyO2 cathode materials is critical for guaranteeing their excellent electrochemical cycling performance, particularly at elevated temperatures. However, the notorious H2–H3 phase transition along with associated large changes in the c-axis or (003) facet is the fundamental origin of the anisotropic and abrupt change in the unit cell and the degradation of the cycling performance. In this study, we coat micron-sized LiNi0.6Co0.2Mn0.2O2 (NCM) with tungsten oxide via atomic layer deposition and investigate the atomic-to-microscopic structures in detail via advanced characterization techniques, such as Cs-corrected scanning transmission electron microscopy. The results reveal that coated tungsten oxide is predominately accumulated on the (003) facet of NCM, with the migration of a small amount of W6+ into this facet, resulting in a reduction of Ni3+ to Ni2+ and the formation of a rock-salt-like structure on the surface. The electrochemical cycling performance of tungsten-oxide-coated NCM is significantly improved, showing a capacity retention of 86.8% after 300 cycles at 55 °C, compared to only 69.4% for the bare NCM. Through further structural analysis, it is found that the initial tungsten-oxide-coating-induced (003) facet distortion effectively mitigates the expansion of the c-lattice during charge, as well as oxygen release from the lattice, resulting in a lowered strain in the cathode lattices and a crack in the cathode particles after prolonged cycling.

Precipitation and crystallographic relationships of nanosized η/η’ precipitates at S-Al interface in Al-Zn-Mg-Cu alloy

Quench-induced S(Al 2 CuMg) phases are generally formed in Al-Zn-Mg-Cu alloys. The precipitation and relative crystallographic relationships of η-type (MgZn 2 ) precipitates at the interfaces between the quench-induced S phases and Al matrix were studied in an Al-Zn-Mg-Cu alloy by coupling Cs-corrected scanning transmission electron microscopy (STEM) with scanning precession electron diffraction (SPED) technology. It was found that Zn segregation occurred at S-Al interfaces due to higher lattice strain energy, which induced the heterogeneous nucleation of nanosized η’ precipitates at these interfaces. Crystallographic orientation relationships (ORs) between the S phases and η’/η precipitates were firstly reported and discussed in Al-Zn-Mg-Cu alloy. With the coarsening of η precipitates at S-Al interfaces, a new type of OR ([112] Al //[ 1 ¯ 2 1 ¯ 0] η , (11 1 ¯ ) Al //(10 1 ¯ 3) η ) between the interfacial η precipitates and Al matrix was identified, which was different from their non-interfacial counterparts in Al matrix reported to date.

Electron-beam induced damage process for Ca2Na2Nb5O16 nanosheets

Dielectric two-dimensional oxide nanosheets are attractive because of their thermal stability and high-k property. However, their atomic structure characterization has been limited since they are easily degraded by electron-beams. This study aimed to investigate the electron-beam induced damage mechanisms for exfoliated Ca2Na2Nb5O16 (CNNO) nanosheets. Knock-on damage dominantly occurred at high voltages, leaving short-range order in the final amorphous structure. On the other hand, a series of chemical reactions predominantly occurred at low voltages, resulting in random elemental loss and a fully disordered amorphous structure. This radiolysis was facilitated by insulated CNNO nanosheets that contained a large number of dangling bonds after the chemical solution process. The radiolysis damage kinetics was faster than knock-on damage and induced more elemental loss. Based on our understanding of the electron beam-induced degradation, atomic-scale imaging of the CNNO nanosheets was successfully performed using Cs-corrected scanning transmission electron microscopy at 300 keV with a decreased beam current. This result is of particular significance because understanding of electron-beam damage in exfoliated and insulating 2D oxide sheets could improve identification of their atomic structure using electron microscopy techniques and lead to a practical guide for further extensive characterization of doped elements and layered structures to improve their properties.

Direct Observation of Cu Clusters and Dislocation Loops by Cs-Corrected STEM in Fe-0.6wt%Cu Alloy Irradiated in BR2

The neutron irradiation of Fe-based fusion and fission reactor materials leads to an increase in ductile-to-brittle transition temperature with a decrease in upper shelf energy. It is well known that Cu content has a strong influence on the embrittlement phenomenon, as Cu-rich clusters (CRPs) are thought to be directly responsible for embrittlement. In contrast, mechanical property studies for steels with different Cu levels exhibit dominant matrix defects in the embrittlement of both low-Cu steels and high-Cu steels at high fluences. To determine the effects of dislocation loops and CRPs on radiation hardening in those steels, neutron irradiation was conducted on Fe-0.6wt%Cu alloy. The neutron irradiation was performed in BR2 at 290 °C up to a dose of 4.1 × 1024 n/m2. After irradiation, the microstructure was observed and analyzed by spherical aberration-corrected transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) combined with X-ray energy-dispersive spectroscopy, using a JEOL ARM200FC. This technique enabled simultaneous observation of ~10 nm CRPs and dislocation loops. Additional high-voltage electron irradiation was performed at room temperature, and the dislocation loops were identified as interstitial-type dislocation loops. Radiation-induced hardening due to neutron irradiation was estimated by measuring the size and density of the dislocation loops and the CRPs. These results suggest that simultaneous observation of dislocation loops and CRPs using the Cs-corrected STEM with EDS analysis is essential for the study of radiation-induced hardening in Fe-based alloys.

Understanding high-temperature cycling-induced crack evolution and associated atomic-scale structure in a Ni-rich LiNi0.8Co0.1Mn0.1O2 layered cathode material

X-ray nano-computed tomography (nano-CT) and deep learning combined with Cs-corrected scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy were employed to investigate the atomic to microscopic structural evolution of LiNi0.8Co0.1Mn0.1O2 (NCM) upon cycling at 55 °C. Two types of intergranular cracks were clearly distinguished by nano-CT for cycled cathode particles; denoted open and closed cracks depending on whether or not the cracks reach the surface of the NCM secondary particles. The volume of high-temperature cycling-induced cracks quantified by deep learning increased drastically, particularly for the open cracks, and this phenomenon was accompanied by rapid degradation of capacity retention. Further precise STEM analysis of the crack regions revealed that migration of transition metal (TM) ions to the Li layer forms a rocksalt-like structure, and the associated reduction of TM ions, e.g., Ni3+ to Ni2+, predominately occurred in the open crack regions in the presence of penetrated electrolyte, even for regions extending to the center of the secondary particle. In contrast, in the closed crack regions, no significant atomic-scale structure distortion and limited reduction of TM ions was observed. Thus, our results provide fundamental guidance for improving the electrochemical performance of Ni-rich layered cathode materials for cycling at elevated temperatures.

Precious metals can be green

In this chapter, a general view of metallic alloys nanoparticles from the atomic-scale view using advanced transmission electron microscopy techniques such as aberration-corrected electron microscopy (Cs-corrected) and high-resolution electron microscopy, is covered. It is well established that catalytic activity of metal nanoparticles is increased by roughness at the atomic scale. Indeed a flat smooth surface will be less active than its stepped counterpart. Metal nanoparticles have in most cases facets with different Miller indexes. However, the control of the shape and crystal structure is not easy to achieve. The possibility of passivizing the nanoparticle surface with organic molecules is a way to control the nanoparticle shape. Using separation techniques such as electrophoresis, it is possible to obtain a batch of particles with the same mass in average and hence the same number of atoms. Passivized particles still have enough free sites to allow catalytic activity. Then, it is possible by using aberration-corrected electron microscopy to obtain the structure and surface structure of nanoparticles. In this work, we will report those methods and demonstrate its application for mono- and multimetallic clusters. The surface of the metallic nanoparticles plays a crucial role in the catalytic activity and interaction between elements in the surface and in the core of the core-shell structures. In this way, the analysis of chemical composition and chemical bonding needs to be obtained using analytic techniques such as X-ray energy-dispersive spectroscopy and electron energy-loss spectroscopy. Herein, we also present quantitative analysis of particles with few atoms by atomic counting using a statistical method applied in Cs-corrected high-angle annular dark-field scanning transmission electron microscopy images.

Thickness-Dependent Polar Domain Evolution in Strained, Ultrathin PbTiO3 Films.

Ferroelectric ultrathin films have great potential in electronic devices and device miniaturization with the innovation of technology. In the process of product commercialization, understanding the domain evolution and topological properties of ferroelectrics is a prerequisite for high-density storage devices. In this work, a series of ultrathin PbTiO3 (PTO) films with varying thicknesses were deposited on cubic KTaO3 substrates by pulsed laser deposition and were researched by Cs-corrected scanning transmission electron microscopy (STEM), reciprocal space mapping (RSM), and piezoresponse force microscopy (PFM). RSM experiments indicate the existence of a/c domains and show that the lattice constant varies continuously, which is further confirmed by atomic-scale STEM imaging. Diffraction contrast analysis clarifies that with the decrease in PTO film thickness, the critical thickness for the formation of a/c domains could be missing. When the thickness of PTO films is less than 6 nm, the domain configurations in the ultrathin PTO films are the coexistence of a/c domains and bowl-like topological structures, where the latter ones were identified as convergent and divergent types of meron. In addition, abundant 90° charged domain walls in these ultrathin PTO films were identified. PFM studies reveal clear ferroelectric properties for these ultrathin PTO films. These results may shed light on further understanding the domain evolution and topological properties in ultrathin ferroelectric PTO films.