HAADF-STEM Images Research Articles

STEM beam channeling in BaSnO3/LaAlO3 perovskite bilayers and visualization of 2D misfit dislocation network

A study of the STEM probe channeling in a heterostructured crystalline bilayer specimens is presented here with a goal to guide STEM-based characterization of multilayer structures. STEM analysis of perovskite BaSnO3/LaAlO3 bilayers is performed and the dominating effects of beam channeling on HAADF- and LAADF-STEM are illustrated. To study the electron beam channeling through BaSnO3/LaAlO3 bilayers, probe intensity depth profiles are calculated, and the effects of probe defocus and atomic column alignment are discussed. Characteristics of the beam channeling are correlated to resulting ADF-STEM images, which is then tested by comparing focal series of plan-view HAADF-STEM images to those recorded experimentally. Additionally, discussions on how to visualize the misfit dislocation network at the BaSnO3/LaAlO3 interface using HAADF- and LAADF-STEM images are provided.

Exsolution of SrO during the Topochemical Conversion of LaSr3CoRuO8 to the Oxyhydride LaSr3CoRuO4H4

Reaction of then = 1 Ruddlesden-Popper oxide LaSr3CoRuO8 with CaH2 yields the oxyhydride phase LaSr3CoRuO4H4 via a topochemical anion exchange. Close inspection of the X-ray and neutron powder diffraction data in combination with HAADF-STEM images reveals that the nanoparticles of SrO are exsolved from the system during the reaction, with the change in cation stoichiometry accommodated by the inclusion ofn > 1 (Co/Ru)nOn+1H2n "perovskite" layers into the Ruddlesden-Popper stacking sequence. This novel pseudotopochemical process offers a new route for the formation of n > 1 Ruddlesden-Popper structured materials. Magnetization data are consistent with a LaSr3Co+Ru2+O4H4 (Co+, d8, S = 1; Ru2+, d6, S = 0) oxidation/spin state combination. Neutron diffraction and μ+SR data show no evidence for long-range magnetic order down to 2 K, suggesting the diamagnetic Ru2+ centers impede the Co-Co magnetic-exchange interactions.

Atomically dispersed cerium species in NMxCe1-xO2/Al2O3 (NM = Rh, Ru) catalysts

NMxCe1-xO2/Al2O3 (NM = Rh, Ru; 0 < x < 0.15) catalysts prepared by a microemulsion method contain, in addition to ceria nanoparticles, an atomically dispersed CeOx species that may be detected by high-resolution HAADF-STEM imaging. The CeOx species exist also in the samples subjected to high temperature treatment in oxidizing and reducing atmosphere, but their concentration increases rapidly after reduction above 700 °C. Generation of CeOx groups and their spreading over the alumina surface explains the effect of amorphization of ceria particles observed by XRD method.

Oxygen Reduction Investigation on Sputtered ZnO Layers with Nano‐granular Structure

AbstractZinc oxide (ZnO) films with nano‐granular structure were deposited by radio‐frequency magnetron sputtering. The absence of preferred orientation was verified by XRD. HAADF‐STEM images revealed a nanoporous structure with pore and grain sizes around 10 nm. The narrowness of the pores enabled to approximate that oxygen reduction takes place at the surface of the layers. During cathodic polarization, the surface of the electrodes became rougher due to the development of nodules on the surface. The ZnO electrodes exhibit a high activity towards oxygen reduction in KOH solution (pH=10). Rotating ring‐disk electrode measurements coupled with reaction modelling allowed to quantitatively demonstrate that the direct reduction prevailed with the indirect pathway being active as well.

Polytypism and Polysomatism in Mixed-Layer Chalcogenides: Characterization of PbBi4Te4S3 and Inferences for Ordered Phases in the Aleksite Series

Bi-Pb-chalcogenides of the aleksite series represent homologous mixed-layer compounds derived from tetradymite (Bi2Te2S). Considering tetradymite as composed of five-atom (Bi2Te2S) layers, the named minerals of the aleksite homologous series, aleksite (PbBi2Te2S2) and saddlebackite, (Pb2Bi2Te2S3) have been considered as phases formed by regular stacking of longer seven- and nine-atom layers. High-angle annular dark-field scanning transmission electron microscope (HAADF-STEM) imaging of thinned foils prepared in-situ on domains deemed homogeneous from electron probe microanalysis, STEM energy-dispersive X-ray spectrometry (EDS) element mapping and fast Fourier transforms (FFTs) from the images offer new insights into these structures. The hitherto-unnamed phase, PbBi4Te4S3, previously interpreted as regular intergrowths of five- and seven-atom layers, is characterized in terms of regular repeats of five- and seven-atom layers over distances of at least 350 nm, defining the (57), or 24H polytype. Imaging of other domains in the same lamella with identical composition at the electron microprobe scale reveals the presence of two additional polytypes: (5559), or 48H; and (557.559) or 72H. Unit cell dimensions for all three polytypes of PbBi4Te4S3 can be both measured and predicted from the HAADF STEM imaging and FFTs. STEM EDS mapping of each polytype confirm the internal structure of each layer. Lead and S occur within the centre of the layers, i.e., Te–Bi–S–Pb–S–Bi–Te in the seven-atom layer, Te–Bi–S–Pb–S–Pb–S–Bi–Te in the nine-atom layer, and so on. Polytypism is an intrinsic feature of the aleksite series, with each named mineral or unnamed phase, except the simple five-atom layer, defined by several alternative stacking sequences of different length, readily explaining the differing c values given in previous work. Homology is defined in terms of layer width and the stacking arrangement of those layers. Coherent structures of the same composition need not only be built of layers of adjacent size (i.e., five- and seven-atom layers) but, as exemplified by the (5559) polytype, may also contain non-adjacent layers, a finding not anticipated in previous work. New polysomes remain to be discovered in nature and each potentially exists as multiple polytypes. The present study further emphasizes the utility of HAADF STEM imaging and atomic-scale STEM EDS element mapping as an optimal tool for tracking stacking sequences and characterising structures in mixed-layer compounds.

Facet-dependent anchoring of gold nanoparticles on TiO2 for CO oxidation

The interfacial perimeter of gold nanocatalysts is popularly viewed as the active sites for a number of chemical reactions, while the geometrical structure of the interface at atomic scale is less known. Here, TiO2-nanosheets and nanospindles were adapted to accommodate Au particles (∼2.2 nm), forming Au-TiO2{001} and Au-TiO2{101} interfaces. Upon calcination at 623 K in air, HAADF-STEM images evidenced that the Au particles on TiO2{101} enlarged to 3.1 nm and these on TiO2{001} remained unchanged, suggesting the stronger metal-support interaction on TiO2{001}. Au/TiO2{001} was more active for CO oxidation than Au/TiO2{101} system.

Palladium/Zeolite Low Temperature Passive NOx Adsorbers (PNA): Structure-Adsorption Property Relationships for Hydrothermally Aged PNA Materials

Zeolites with different framework structures (SSZ-13, ZSM-5, BEA) but similar Si/Al ratios (~ 12–15) and Pd loading (~ 1 wt%) were synthesized and evaluated as low-temperature passive NOx adsorbers (PNA). These materials exhibit high NOx adsorption efficiency with atomically dispersed Pd being the active adsorption site. Hydrothermal aging at 750 °C for 16 h in the presence of 10% water vapor in air resulted in the formation of PdO nanoparticles in all three samples as evidenced by high-energy XRD. Hydrothermal aging of the small-pore 1–3 wt% Pd/SSZ-13 (Si/Al = 6) materials, which contain ~ 100–90% atomically dispersed palladium ions, decreases its PNA performance only by ~ 10–20%, indicating agglomeration of only ~ 10–20% of atomically dispersed Pd into PdO. High-field solid state 27Al NMR studies on the fresh and aged samples reveal dealumination and significant changes in the distribution of Al (and thus, Bronsted acid) sites after hydrothermal aging. FTIR measurements with NO probe molecule and titration of Bronsted acid sites with nitrosyl (NO+) ions further corroborate the 27Al NMR data. Because framework aluminum atoms are the anchoring sites for atomically dispersed Pd ions, their elution from the framework causes the loss of active atomically dispersed Pd species. With the aid of HAADF-STEM imaging and synchrotron XRD studies, we further confirm and visualize the fate of these Pd species: they agglomerate into PdO nanoparticles on the external surface of zeolite. Consequently, these changes lead to the decrease in PNA performance of these materials after hydrothermal aging. The thus formed PdO agglomerates cannot be re-dispersed back to their ionic state due to the loss of framework Al T-sites and/or inherent stability of such large PdO particles. Our study demonstrates that, unlike in previous studies that found increased PNA performance upon HTA, high temperatures hydrothermal aging of PNA materials that contain atomically dispersed Pd initially results in a decrease in NOx storage efficiency due to the formation of PdO agglomerates. However, we also highlight the high hydrothermal stability of predominantly atomically dispersed 1–3 wt% Pd/SSZ-13 (Si/Al = 6), whose performance decreases only marginally after prolonged hydrothermal aging at 750 °C. This study shows that hydrothermally stable passive NOx materials can be prepared using small-pore SSZ-13 zeolite.

Single-Atoms Synthesized Via a Novel Method As the Active Site with Highly Selectivity Electrocatalytic Conversion of CO2 to Ethanol

Single-atom catalysts (SACs) are recently emerging as a new frontier in heterogenous catalysis for their effectiveness in promoting catalytic reactions. We report herein the study of carbon-supported single-atom catalysts synthesized by a novelty method exhibit distinctive selectivity for some special electrochemical reactions during the electrolysis process. The aberration-corrected HAADF-STEM images combined with X-ray absorption spectroscopy identified the presence of single atoms. Theoretical density functional theory simulations of the best performing catalytic activity and selectivity could be attributed to the existence of single atoms in the catalysts. Introduction CO2 is a cheap resource of carbon that can be used as raw materials for fuel synthesis using renewable energy In 2005, 36.2 billion metric tons of CO2 was emitted to the atmosphere from anthropogenic sources, [1] which is corresponding to an increase in global mean temperature of around 0.1oC. New carbon capture, sequestration and conversion technologies are widely pursued as potential solutions to mitigate the concerns. The electrochemical CO2 reduction reaction (CO2RR) can combine carbon capture storage with renewable energy to convert CO2 into fuels and chemical feedstocks. Which is extraordinary significance for industry and is highly competitive with water electrolysis. Therefore, efficient electrocatalysts are required to reduce the reaction overpotential and improve the faradaic efficiency and selectivity for high value chemical products further. [2] The earth-rare, high price and low selectivity of precious metals prevent further large-scale commercial applications. Furthermore, these precious metal catalysts exhibit desirable products at highly cathodic potential window with high overpotential, which causes lot of energy consuming problem in application. Recently, single-atom catalysts (SACs) are emerging as a new frontier in heterogenous catalysis for their effectiveness in promoting catalytic reactions. [3] The unique electronic structure and unsaturated coordination environments in single atom catalysts (SACs) have been proven to enhance catalytic activity and excellent selectivity in a variety of reactions. In this work, the catalyst was prepared by a proprietary method currently under patent examination. Results and Discussion We report herein the study of carbon-supported SACs synthesized by a novel method that exhibit distinctive selectivity in electrochemical synthesis. The aberration-corrected HAADF-STEM images combined with X-ray absorption spectroscopy identified the presence of single atoms. Theoretical density functional theory simulations of the best performing catalytic activity and selectivity could be attributed to the existence of single atoms in the catalysts. The prepared metal single atoms dispersed on substrate catalysts showed high faradaic efficiency and selectivity of CO2 to C2 products conversion with low onset potential. Fig. 1a shows linear sweep voltammetry (LSVs) over M/C-0.1 before and after chronoamperometry measurements at different potentials. CV curves (Figure 1b) also displayed obvious activity toward CO2RR in the potential range from 0.5 V to -1.5 V vs RHE, and the current density remained stable with no apparent decay before and after electrolysis in CO2 saturated KHCO3 solution. The FE reached to as high as ~91.8% for ethanol at a low potential of -0.6 V when the metal loading is 0.1 wt% on carbon support. Figure 1. Electrochemical CO2RR over M/C-0.1. (a) LSV in a potential range of 0 to -1.5 V vs RHE in Ar and CO2 saturated 0.1 M KHCO3. (Scan rate: 50 mV s-1). (b) CV in a potential range of 0.5 to -1.5 V vs RHE in Ar and CO2 saturated 0.1 M KHCO3. (Scan rate: 50 mV s-1, (c) FEs of key byproduct as the function of potential over the catalysts of M/C-0.1 Significance Understanding the role of metal single atoms on the CO2 electrochemical reduction reaction is fundamental for design of an appropriated catalyst for the large-scale application of CO2 RR that aims the production of high energy liquid chemicals.

TEM and HAADF STEM Imaging of Dislocation Loops in Irradiated GaAs

TEM and HAADF STEM Imaging of Dislocation Loops in Irradiated GaAs

Copper-Arsenic Nanoparticles in Hematite: Fingerprinting Fluid-Mineral Interaction

Metal nanoparticles (NP) in minerals are an emerging field of research. Development of advanced analytical techniques such as Z-contrast imaging and mapping using high-angle annular dark field scanning transmission electron microscopy (HAADF STEM) allows unparalleled insights at the nanoscale. Moreover, the technique provides a link between micron-scale textures and chemical patterns if the sample is extracted in situ from a location of petrogenetic interest. Here we use HAADF STEM imaging and energy-dispersive X-ray spectrometry (EDX) mapping/spot analysis on focused ion beam prepared foils to characterise atypical Cu-As-zoned and weave-twinned hematite from the Olympic Dam deposit, South Australia. We aim to determine the role of solid-solution versus the presence of discrete included NPs in the observed zoning and to understand Cu-As-enrichment processes. Relative to the grain surface, the Cu-As bands extend in depth as (sub)vertical trails of opposite orientation, with Si-bearing hematite NP inclusions on one side and coarser cavities (up to hundreds of nm) on the other. The latter host Cu and Cu-As NPs, contain mappable K, Cl, and C, and display internal voids with rounded morphologies. Aside from STEM-EDX mapping, the agglomeration of native copper NPs was also assessed by high-resolution imaging. Collectively, such characteristics, corroborated with the geometrical outlines and negative crystal shapes of the cavities, infer that these are opened fluid inclusions with NPs attached to inclusion walls. Hematite along the trails features distinct nanoscale domains with lattice defects (twins, 2-fold superstructuring) relative to hematite outside the trails, indicating this is a nanoprecipitate formed during replacement processes, i.e., coupled dissolution and reprecipitation reactions (CDRR). Transient porosity intrinsically developed during CDRR can trap fluids and metals. Needle-shaped and platelet Cu-As NPs are also observed along (sub)horizontal bands along which Si, Al and K is traceable along the margins. The same signature is depicted along nm-wide planes crosscutting at 60° and offsetting (012)-twins in weave-twinned hematite. High-resolution imaging shows linear and planar defects, kink deformation along the twin planes, misorientation and lattice dilation around duplexes of Si-Al-K-planes. Such defects are evidence of strain, induced during fluid percolation along channels that become wider and host sericite platelets, as well as Cl-K-bearing inclusions, comparable with those from the Cu-As-zoned hematite, although without metal NPs. The Cu-As-bands mapped in hematite correspond to discrete NPs formed during interaction with fluids that changed in composition from alkali-silicic to Cl- and metal-bearing brines, and to fluid rates that evolved from slow infiltration to erratic inflow controlled by fault-valve mechanism pumping. This explains the presence of Cu-As NPs hosted either along Si-Al-K-planes (fluid supersaturation), or in fluid inclusions (phase separation during depressurisation) as well as the common signatures observed in hematite with variable degrees of fluid-mineral interaction. The invoked fluids are typical of hydrolytic alteration and the fluid pumping mechanism is feasible via fault (re)activation. Using a nanoscale approach, we show that fluid-mineral interaction can be fingerprinted at the (atomic) scale at which element exchange occurs.

Origin of long afterglow in strontium aluminate phosphors: Atomic scale imaging of rare earth dopant clustering

Ceramic pigments emitting long afterglow have enormous potential for emerging lighting applications that consume zero energy from the power grid. One of the most efficient compounds is strontium aluminate, when co-doped with 2 rare-earth elements — an optically active emitter, such as Eu 2+ , and an auxiliary ion, such as Dy 3+ — and B. To date, spectrophotometric methods are commonly used to determine the material structure supporting long afterglow, yielding indirect evidence of energy transfer between the rare-earth co-dopants. Here, atomic resolution HAADF-STEM imaging is used to resolve columns of Sr sub-lattice sites in the (012)-projection of a Sr 4 Al 14 O 25 :Eu,Dy single crystal. Through quantitative STEM image simulations, heavy rare earth dopants are shown to incorporate substitutionally into Sr sites, causing an enhancement in image contrast over that of neighboring atomic columns by 125%. DFT structural simulations demonstrate that Eu 2+ and Dy 3+ would incorporate into adjacent Sr lattice sites along the [012], enabling energy transfer between them that we see as afterglow. With the help of atomic resolution HAADF-STEM imaging, we also provide direct experimental evidence of clustering of ionic point defects induced by B doping, leading to extremely long (>14 h) afterglow in strontium aluminate phosphors.

Single-Atoms As the Active Site with High Selectivity for Electrochemical Application

Single-atom catalysts (SACs) are recently emerging as a new frontier in heterogenous catalysis for their effectiveness in promoting catalytic reactions. We report herein the study of carbon-supported SACs synthesized by a novel method that exhibit distinctive selectivity in electrochemical synthesis. The aberration-corrected HAADF-STEM images combined with X-ray absorption spectroscopy identified the presence of single atoms. Theoretical density functional theory simulations of the best performing catalytic activity and selectivity could be attributed to the existence of single atoms in the catalysts.

Determination of strain fields on two-dimensional images using the STC method

In this paper, a novel method called the Strain Tensor using Computational Fourier transform moiré (STC) method is proposed to obtain strain maps in a 2D lattice image such as an HRTEM or HAADF-STEM image. The method is a modification of the CFTM method, but it has two essential merits. Firstly, the method provides a 2 × 2 strain tensor using two independent Bragg peaks, and can hence be used to calculate both the axial and shear components, whereas the CFTM method can be used to calculate only the axial strain. Secondly, the STC method can provide more reliable results than the CFTM method by excluding the weak Bragg peaks of low signal/noise ratio on a digital diffractogram. In addition, the STC method is better than the GPA method because it does not require an unwrapping process. In this work, the STC method is demonstrated using a computer-generated image, showing that the method matches exactly with the CFTM method. Furthermore, the results for an experimental image reveal that the STC method can be an alternative method for strain analysis when the CFTM method cannot provide reliable results owing to weak Bragg peaks of low signal/noise ratio.

Some aspects of influence of the composition of volatile products and extracted material on grain swelling processes and volume changes of commercial coals of different rank

The course of pyrolysis process of the three industrial samples of lower, middle, and higher rank coals was studied using the TG/FT-IR technique and a laboratory unit for investigation of carbonization process by X-raying. The coal samples from the zones of plastic layer were studied by scanning electron microscopy and next extracted by a mixture of chloroform and methanol. The obtained extracts were analyzed by FT-IR and UV spectroscopies and STEM-HAADF technique.It was stated that the composition of volatile products of a lower rank coal (LRC) is characterized by the greatest relative content of saturated and unsaturated hydrocarbons in comparison with a middle rank coal (MRC) and a higher rank coal (HRC). However, a higher contribution ratio of water vapour is observed in the composition of volatile matter of HRC. It is suggested that low molecular products of destruction formed inside swelling grains of MRC can react with each other and form new compounds with a greater molecular mass. These can be macromolecules of heteroorganic compounds. The STEM-HAADF images showed that there are topological structures of various types in the material of extracts from the gas-saturated zone of coals of different rank. The presence of inorganic elements in the extracted material changes depending on coal rank. Heterorganic compounds of different shape and composition can catalyze the reactions taking place during thermal destruction of coals in a selective way. In MRC, these reactions will cause a decrease in pressure inside its swelling grains and without their substantial growth in volume. Such reactions do not take place in grains of HRC. Moreover, an increased contribution of water vapour formed during destruction can cause its adsorption inside grains and an increase in volume of swelling grains. This can be the cause for an increase in generation of dangerous coking pressure in the charge of studied HRC.

Influence of the crosstalk on the intensity of HAADF-STEM images of quaternary semiconductor materials.

The influence of the neighbouring atomic-columns in determining the composition at atomic column scale of quaternary semiconductor compounds, using simulated HAADF-STEM images is evaluated. The InAlAsSb alloy, a promising material in the photovoltaic field, is considered. We find that the so called 'crosstalk' effect plays an important role for the aimed compositional determination. The intensity transfer is larger from neighbouring atomic columns with higher average Z, and towards atomic columns with smaller Z. Our results show that in order to obtain precise information on the column composition, the HAADF-STEM intensities of both columns need to be taken into account simultaneously.

Experimental and DFT characterization of η′ nano-phase and its interfaces in Al[sbnd]Zn[sbnd]Mg[sbnd]Cu alloys

The structures and energetics of η′ nano-phase and its interfaces in a peak-aged AlZnMgCu alloy were thoroughly investigated, using the combination of aberration corrected HAADF-STEM imaging and first-principles calculations. The most feasible atomic structure of η′, along with the solute substitution in η′, were calculated and compared with the atom resolution Z-contrast images. The interface phase diagram of η′/Al was constructed as a function of the excess chemical potential of Zn, to determine the equilibrium interface structures. Solute segregation to these interfaces was further calculated, and the results were adopted to interpret the experimental Z-contrast images. Finally, all the bulk and interface results were integrated to predict the solute partition in the matrix and further its potential impacts on interface properties, and based on which, a new strategy was proposed for future optimal design of AlZnMgCu alloys.

A new efficient, highly dispersed, Pd nanoparticulate silica supported catalyst synthesized from an organometallic precursor. Study of the homogeneous vs. heterogeneous activity in the Suzuki-Miyaura reaction

A new Pd(0) catalyst supported on silica UVM-7 has been synthesized from the organometallic [Pd2{μ-(C6H4) PPh2}2(CH3CN)4](BF4)2 precursor, characterized by the high dispersion, activity, and small size of the palladium nanoclusters fixed on the silica surface. The catalyst was tested for the Suzuki-Miyaura (SM) reaction of different 4-substitutedphenyl halides with phenylboronic acid. The kinetic study concurs with most of the catalytic action was carried out by Pd species originated by the partial solubilization of Pd immobilized on mesoporous silica. The Schmidt’s analysis of differential selectivity (SADS) in several competitive SM reactions, together the STEM-HAADF and HRTEM images of the catalyst surface taken along several reuse cycles support the conclusions of the kinetic study. Furthermore, this hypothesis was critically investigated using different classic approaches to distinguish between homogeneous and heterogeneous catalysis as the three phases test, poisoning, and hot filtrate experiments.

Telluride-Based Atomically Thin Layers of Ternary Two-Dimensional Transition Metal Dichalcogenide Alloys

Alloying in two-dimensional (2D) transition metal dichalcogenides (TMDCs) has allowed band gap engineering and phase transformation, as well as modulation of electronic properties. However, most of the efforts have been focused on alloying between transition metal cations. Among those that emphasize alloying between chalcogenide anions, the sulfide–selenide combinations are popular with a few reports on selenide–telluride combinations. In this work, we show a facile chemical vapor deposition method to obtain stable alloying between selenide and telluride anions in monolayer MoSe2(1–x)Te2x alloy. These alloys retain the monolayer 2H symmetry and show good photoluminescence and band gap tunability in the near-infrared region. The nature and percentage of alloying is further confirmed and quantified via AFM, XPS, and HAADF-STEM imaging and polarized Raman spectroscopy. The stability of the two chalcogens in the monolayer 2H lattice is also consistent with thermodynamic phase mixing via DFT simulations. The w...

Atomic clusters in decagonal Al–Cr–Fe: hexagon, star and decagon

In this study, the atomic arrangement of the decagonal phase in a spark-plasma-sintered Al–Cr–Fe alloy has been studied by Cs-corrected high angle annular dark field scanning transmission electron microscopy (HAADF STEM) for the first time. We have found that three types of atomic clusters exist as basic structural units: star, flattened hexagon and decagon with an edge-length of 0.65 nm. These three types of units are linked to each other by sharing edges or inter-penetrating, with adjacent star clusters overlapping each other. The respective structural models of these structural units are proposed from the HAADF STEM images.

Ab Initio Structure Determination of Cu2- xTe Plasmonic Nanocrystals by Precession-Assisted Electron Diffraction Tomography and HAADF-STEM Imaging.

We investigated pseudo-cubic Cu2- xTe nanosheets using electron diffraction tomography and high-resolution HAADF-STEM imaging. The structure of this metastable nanomaterial, which has a strong localized surface plasmon resonance in the near-infrared region, was determined ab initio by 3D electron diffraction data recorded in low-dose nanobeam precession mode, using a new generation background-free single-electron detector. The presence of two different, crystallographically defined modulations creates a 3D connected vacancy channel system, which may account for the strong plasmonic response of this material. Moreover, a pervasive rotational twinning is observed for nanosheets as thin as 40 nm, resulting in a tetragonal pseudo-symmetry.