Transmission Electron Microscopy Analysis Techniques Research Articles

Ca Solubility in a BiFeO3-Based System with a Secondary Bi2O3 Phase on a Nanoscale.

In BiFeO3 (BFO), Bi2O3 (BO) is a known secondary phase, which can appear under certain growth conditions. However, BO is not just an unwanted parasitic phase but can be used to create the super-tetragonal BFO phase in films on substrates, which would otherwise grow in the regular rhombohedral phase (R-phase). The super-tetragonal BFO phase has the advantage of a much larger ferroelectric polarization of 130–150 μC/cm2, which is around 1.5 times the value of the rhombohedral phase with 80–100 μC/cm2. Here, we report that the solubility of Ca, which is a common dopant of bismuth ferrite materials to tune their properties, is significantly lower in the secondary BO phase than in the observed R-phase BFO. Starting from the film growth, this leads to completely different Ca concentrations in the two phases. We show this with advanced analytical transmission electron microscopy techniques and confirm the experimental results with density functional theory (DFT) calculations. At the film’s fabrication temperature, caused by different solubilities, about 50 times higher Ca concentration is expected in the BFO phase than in the secondary one. Depending on the cooling rate after fabrication, this can further increase since a larger Ca concentration difference is expected at lower temperatures. When fabricating functional devices using Ca doping and the secondary BO phase, the difference in solubility must be considered because, depending on the ratio of the BO phase, the Ca concentration in the BFO phase can become much higher than intended. This can be critical for the intended device functionality because the Ca concentration strongly influences and modifies the BFO properties.

Direct Synthesis of ZIF-8 on Transmission Electron Microscopy Grids Allows Structure Analysis and 3D Reconstruction

The first example of layer‐by‐layer growth of a metal–organic framework (MOF) directly on transmission electron microscopy (TEM) grids is described. ZIF‐8 is deposited on thin amorphous carbon films and subjected to a structure analysis by (scanning) TEM ((S)TEM). This method serves as a two‐in‐one synthesis and TEM sample‐preparation technique and allows straightforward analysis of ZIF‐8 crystallites. Artifacts resulting from sample preparation are completely avoided by this approach. The morphological properties, crystal structure, and the chemical composition of the material are investigated with high spatial resolution by a variety of methods of (analytical) electron microscopy. Furthermore, the incorporation of metallic nanoparticles in ZIF‐8 by integrating a corresponding step into the layer‐by‐layer deposition process is examined. The formation of ZIF‐8 crystals on the film proceeds as under the absence of nanoparticle‐forming synthesis steps. However, the nanoparticles rather cover the supporting amorphous carbon film than being incorporated in the ZIF‐8 material. This information cannot be obtained from standard characterization techniques but requires the application of analytical (S)TEM techniques.

Direct Synthesis of ZIF‐8 on Transmission Electron Microscopy Grids Allows Structure Analysis and 3D Reconstruction

AbstractThe first example of layer‐by‐layer growth of a metal–organic framework (MOF) directly on transmission electron microscopy (TEM) grids is described. ZIF‐8 is deposited on thin amorphous carbon films and subjected to a structure analysis by (scanning) TEM ((S)TEM). This method serves as a two‐in‐one synthesis and TEM sample‐preparation technique and allows straightforward analysis of ZIF‐8 crystallites. Artifacts resulting from sample preparation are completely avoided by this approach. The morphological properties, crystal structure, and the chemical composition of the material are investigated with high spatial resolution by a variety of methods of (analytical) electron microscopy. Furthermore, the incorporation of metallic nanoparticles in ZIF‐8 by integrating a corresponding step into the layer‐by‐layer deposition process is examined. The formation of ZIF‐8 crystals on the film proceeds as under the absence of nanoparticle‐forming synthesis steps. However, the nanoparticles rather cover the supporting amorphous carbon film than being incorporated in the ZIF‐8 material. This information cannot be obtained from standard characterization techniques but requires the application of analytical (S)TEM techniques.

The application of in situ analytical transmission electron microscopy to the study of preferential intergranular oxidation in Alloy 600

In situ analytical transmission electron microscopy (TEM) can provide a unique perspective on dynamic reactions in a variety of environments, including liquids and gases. In this study, in situ analytical TEM techniques have been applied to examine the localised oxidation reactions that occur in a Ni-Cr-Fe alloy, Alloy 600, using a gas environmental cell at elevated temperatures. The initial stages of preferential intergranular oxidation, shown to be an important precursor phenomenon for intergranular stress corrosion cracking in pressurized water reactors (PWRs), have been successfully identified using the in situ approach. Furthermore, the detailed observations correspond to the ex situ results obtained from bulk specimens tested in hydrogenated steam and in high temperature PWR primary water. The excellent agreement between the in situ and ex situ oxidation studies demonstrates that this approach can be used to investigate the initial stages of preferential intergranular oxidation relevant to nuclear power systems.

Ex-Situ TEM Investigation on Additional Capacity of SnO2 Lithium-Ion Battery Anode Material

Having high energy densities, lithium ion rechargeable batteries are widely used for energy storage and still are the subject of intensive investigation. As anode materials, tin-based materials have long been considered as a promising candidate due to their twice larger theoretical specific capacity compared to commercially used graphite carbon material. Another anode candidates with extremely high electrochemical performances have been developed steadily resulting from intensive researches, but the interests on tin-based anode materials recently have been increased again since they also show great performance as an anode for next-generation sodium-ion batteries. Most of the various tin-based materials researched for high energy battery anodes can be subdivided into two groups, pure tin (Sn) metal based and tin oxide (SnOx) based compounds. Both anode materials are based on reversible alloying reaction mechanism forming LixSn alloys in lithium-ion batteries, but tin oxide undergoes additional reaction, i.e. conversion reaction, before alloying reaction occurs in the first charging process. Even though the conversion reaction, which indicates the reaction forming reduced tin metal and lithium oxide from tin oxide, is known as almost irreversible and it consumes plenty of lithium ions, it is normally considered as an advantage because the resulting lithium oxide is believed to act as mechanical buffer layers preventing from drastic volume expansion/contraction and therefore contributes capacity retention. However, there is still a lack of understanding on the reaction mechanism of tin oxide. In particular, the SnO2 anode usually shows additional specific capacity that greatly exceeds the theoretical value. On the basis of our experimental data, the extra capacity reaches 130% of its theoretical value on the first charging process. The extra capacity commonly found in transition metal oxide (TMO) anodes has also been a subject of great interest, and Hu et al. explained that reaction of lithium hydroxide from electrolyte decomposition mainly contributes to additional capacity in RuO2 anode (Nature Materials, 2013.12.). But it still does not give an explanation of SnO2 anode because they have different fundamental electrochemical reaction mechanisms that enable lithium-ion battery to be rechargeable; TMO anodes such as the RuO2 anode are operated by the reversible conversion reaction, whereas the SnO2 anode is operated by the reversible alloying reaction. Furthermore, the partial reversibility of conversion reaction has long been in debates on SnO2 anode materials. The partial reverse conversion reaction of the SnO2 anode has been commonly accepted and considered to contribute the extra capacity, but previous reports are mostly based on indirect experimental evidences. Here, we elucidated the origin of additional capacities and studied the possibility of partial reversibility of conversion reaction through ex situ transmission electron microscopy (TEM) research. We improved and optimized an ex situ TEM analysis technique to investigate electrochemical reactions inside the actual battery-operating system. Direct (dis)charging of SnO2 particles on TEM grid in a coin cell enabled us to track the same locations of nano-areas throughout the full reaction cycle and, therefore, all of the phase evolutions including solid electrolyte interphase (SEI) layers were identified. We additionally performed same experiments with pure Sn particles to draw an exact conclusion. Eliminating the conversion reaction step, pure Sn particles give us better understanding on the reaction mechanism of SnO2. Combining all of the experimental results as well as the aid of computational works, we found out that reactions of the Li2O phase contribute to the extra capacity and the reverse conversion reaction of SnO2 hardly occurs in the real battery system. We expect that this study will provide fundamental knowledge on behaviors of various tin-based materials in lithium-ion batteries.

Composition analysis of coaxially grown InGaN multi quantum wells using scanning transmission electron microscopy

GaN nanotubes with coaxial InGaN quantum wells were analyzed by scanning transmission electron microscopy in order to determine their structural properties as well as the indium distribution across the InGaN quantum wells. For the latter, two process steps are necessary. First, a technique to prepare cross-sectional slices out of the nanotubes has been developed. Second, an existing scanning transmission electron microscopy analysis technique has been extended with respect to the special crystallographic orientation of this type of specimen. In particular, the shape of the nanotubes, their defect structure, and the incorporation of indium on different facets were investigated. The quantum wells preferentially grow on m-planes of the dodecagonally shaped nanotubes and on semipolar top facets while no significant indium signal was found on a-planes. An averaged indium concentration of 6% to 7% was found by scanning transmission electron microscopy analysis and could be confirmed by cathodoluminescence measurements.

Probing the Metal–Support Interaction in Carbon‐Supported Catalysts by using Electron Microscopy

AbstractCarbon is a unique and versatile element in nature that can form different architectures. Heteroatom (e.g., B, N, O, P, and S) modification has been proven to be an effective way to tailor the chemical and electrical properties of the carbon surface. Moreover, nanocarbons have been reported to be good supports for metal nanoparticles (NPs). It is essential to study the metal–support interaction between surface‐modified carbon and metal NPs to understand performance. Advanced analytical transmission electron microscopy techniques are powerful tools that can be used to probe the metal–carbon support interaction in carbon‐supported metal catalysts, as they can directly uncover local structures of the surface and the bulk of catalysts at the atomic scale and can also reveal chemical and electronic structural information that is related to their synthesis and catalytic properties significantly affecting their performance. Herein, several selected examples will be reviewed.

Specimen Preparation Method to Dual-Axis TEM Analysis Technique

We usually do Physical Failure Analysis (PFA) to visualize the failure in the semiconductor process. First, we find the site-specific target (or failed cell). And then, we should do de-processing which contains lapping and chemical etch. As the semiconductor gets more complex and multi-layer, we should do multi-de-processing steps. If the failure has the oxide-material, we cannot know the cause of failure. Because the oxide can be removed by Hydrogen Fluoride (HF) chemical, which means the failure will be gone during de-processing step. We should use Focused Ion Beam (FIB) which can make a thin sample to exclude chemical step. High-resolution analytical tools such as Transmission Electron Microscopy (TEM) have become indispensable in PFA. We make a Dual-Axis TEM (DAT) method for PFA. We make a Plan-view TEM (PTEM) sample with FIB first, we can find an exact location of failure through the first step. And then, we make cross-sectional TEM (XTEM) sample with a site-specific target in PTEM sample. Below summarizes the four major steps and its precautions;1. Making a Planar lamella by FIB2. Observing a PTEM specimen with TEM(Precaution : zone axis should be adjusted exactly)3. Making a cross-section lamella by FIB(Precaution : Protection layer should be deposited on both sides)4. Observing a XTEM specimen with TEMWe propose DAT method to improve the problem of artifact in de-processing step. The TEM result with DAT method is shown in figure 4. We can know the exact location of failure with observing a PTEM sample. We can know the cause of failure with observing a XTEM sample. Moreover, we can analysis the chemical and physical information through DAT method. This method is very effective and has a high success rate in analysis. REFERENCE [1] A.E.M. De Veirman, “3-Dimensional’ TEM silicon-device analysis by combining plan-view and FIB sample preparation”, Materials Science and Engineering: B, 102(1-3), pp. 63-69, 2003. [2] Jon C. Lee, David Su, J.H. Chuang, “A Novel Application of the FIB Lift-out Technique for 3-D TEM Analysis”, Microelectronics Reliability Asset, 41(9-11), pp. 1551-1556, 2001. Figure 1

Application of the analytical transmission electron microscopy techniques for detection, identification and visualization of localization of nanoparticles of titanium and cerium oxides in mammalian cells

This work represents the results of the study on applicability of the modern methods of analytical transmission electron microscopy for detection, identification and visualization of localization of nanoparticles of titanium and cerium oxides in A549 cell, human lung adenocarcinoma cell line. A comparative analysis of images of the nanoparticles in the cells obtained in the bright field mode of transmission electron microscopy, under dark-field scanning transmission electron microscopy and high-angle annular dark field scanning transmission electron was performed. For identification of nanoparticles in the cells the analytical techniques, energy-dispersive X-ray spectroscopy and electron energy loss spectroscopy, were compared when used in the mode of obtaining energy spectrum from different particles and element mapping. It was shown that the method for electron tomography is applicable to confirm that nanoparticles are localized in the sample but not coated by contamination. The possibilities and fields of utilizing different techniques for analytical transmission electron microscopy for detection, visualization and identification of nanoparticles in the biological samples are discussed.

Exciton Quenching Due to Copper Diffusion Limits the Photocatalytic Activity of CdS/Cu2S Nanorod Heterostructures.

The formation of donor/acceptor junctions in hybrid nanomaterials is predicted to enhance photocatalytic activity as compared to single-component semiconductor systems. Specifically, nanomaterials containing a junction of n-type cadmium sulfide (CdS) and p-type copper sulfide (Cu2S) formed via cation exchange have been proposed as potential photocatalysts for reactions such as water splitting. Herein, we study the elemental distribution of Cu within these nanostructures using analytical transmission electron microscopy techniques. The resulting effects of this elemental distribution on photocatalytic activity and charge dynamics were further studied using a model photoreduction reaction and transient absorption spectroscopy. We find that copper diffusion in the hybrid nanostructure quenches the exciton lifetime and results in low photocatalytic activity; however, this effect can be partially mitigated via selective extraction. These results provide a deeper understanding of the physical processes within these hybrid nanostructures and will lead to more rational design of photocatalyst materials.

Magnetic anisotropy considerations in magnetic force microscopy studies of single superparamagnetic nanoparticles

In recent years, superparamagnetic nanoparticles (SPNs) have become increasingly important in applications ranging from solid state memory devices to biomedical diagnostic and therapeutic tools. However, detection and characterization of the small and unstable magnetic moment of an SPN at the single particle level remains a challenge. Further, depending on their physical shape, crystalline structure or orientation, SPNs may also possess magnetic anisotropy, which can govern the extent to which their magnetic moments can align with an externally applied magnetic field. Here, we demonstrate how we can exploit the magnetic anisotropy of SPNs to enable uniform, highly-sensitive detection of single SPNs using magnetic force microscopy (MFM) in ambient air. Superconducting quantum interference device magnetometry and analytical transmission electron microscopy techniques are utilized to characterize the collective magnetic behavior, morphology and composition of the SPNs. Our results show how the consideration of magnetic anisotropy can enhance the ability of MFM to detect single SPNs at ambient room temperature with high force sensitivity and spatial resolution.

Identification and quantification of reaction phases at Si3N4–Ti interfaces by using analytical transmission electron microscopy techniques

Silicon nitride (Si3N4) and titanium (Ti) were successfully bonded by using a capacitor discharge joining method. The resulting sample interfaces were characterized by scanning electron microscopy (SEM) and analytical transmission electron microscopy (TEM) techniques. SEM and chemical analyses by energy dispersive X-ray spectrometry (EDX) and wave length X-ray spectrometry (WDX) showed that if there is a reaction layer it is very small. Sample preparation from metal–ceramic joints for TEM by using conventional techniques is difficult. To overcome this problem, samples were prepared by using a focus ion beam (FIB) and investigated by TEM techniques. Analytical TEM techniques such as electron energy loss spectroscopy (EELS) revealed that Si3N4 interacted with Ti and reaction phases were formed at the interface. These phases are approximately 50nm thick Ti3N2 layer at the interface next to Si3N4 followed by continuous Ti6Si3N phase as a matrix containing Ti3N particles.

Quantitative study of the interfacial intermixing and segregation effects across the wetting layer of Ga(As,Sb)-capped InAs quantum dots

Quantitative chemical information from semiconductor nanostructures is of primary importance, in particular at interfaces. Using a combination of analytical transmission electron microscopy techniques, we are able to quantify the interfacial intermixing and surface segregation across the intricate non-common-atom wetting layer (WL) of Ga(As,Sb)-capped InAs quantum dots. We find: (i) the WL-on-GaAs(buffer) interface is abrupt and perfectly defined by sigmoidal functions, in analogy with two-dimensional epitaxial layers, suggesting that the interface formation process is similar in both cases; (ii) indium segregation is the prevailing mechanism (e.g., over antimony segregation), which eventually determines the composition profile across the GaAs(cap)-on-WL interface.

Microstructural differences between two Zr(C,N) coatings revealed by analytical transmission electron microscopy

The microstructures of two samples of a Zr(C,N) coating on steel, which unexpectedly differed in their tribological properties, were investigated by analytical transmission electron microscopy. The samples were produced by a cathodic arc evaporation process in two commercial coating devices under similar coating conditions with the exception of the number of Zr targets.The source of the differing tribological properties of the samples was detected by analytical transmission electron microscopy (TEM) methods energy-dispersive X-ray spectroscopy (EDX), energy filtering TEM (EFTEM), electron diffraction, high resolution electron microscopy, and high angel annular dark field. The TEM preparation and the results of the TEM investigations are shown in detail. The origin of the unexpected behavior was determined to be a nano-scale multilayer structure that existed only in the tribologically superior specimen. EDX and EFTEM investigations indicated enrichment in oxygen at the interface between coating and steel substrate in the tribologically inferior sample. Findings of the microstructural configuration were obtained by taking a closer look at the structure and comparing the results of the several analytical TEM techniques. This allows the allocation of the concentration fluctuations in N, C, and Zr to the two thickness fractions of the nano multilayers and a local correlation of the identified minority phase Zr3(C,N)4 to the higher N content in the narrower type of the multilayer fraction of the sample with the excellent tribological properties. The minority phase Zr3(C,N)4 is randomly distributed in the sample with the defective tribological properties.Coating conditions are not topic of this work, but after discussion of the TEM results, the fact that one of the coating devices worked with one Zr target and the other one with two, could be identified as cause for the formation of the nano multilayer structure in the sample with the superior tribological properties.

Experimental Evidence of Self-Limited Growth of Nanocrystals in Glass

Growth of nanocrystals precipitated in glasses with specific compositions can be effectively limited by diffusion barriers forming around crystallites. For the first time, we do experimentally prove this concept of self-limited growth on the nanoscale for a SiO(2)/Al(2)O(3)/Na(2)O/K(2)O/BaF(2) glass in which BaF(2) nanocrystals are formed. As shown by advanced analytical transmission electron microscopy techniques, the growth of these BaF(2) crystals, having great potential for photonic applications, is inherently limited by the formation of a ca. 1 nm wide SiO(2) shell.

In situ TEM of radiation effects in complex ceramics

In situ transmission electron microscopy (TEM) has been extensively applied to study radiation effects in a wide variety of materials, such as metals, ceramics and semiconductors and is an indispensable tool in obtaining a fundamental understanding of energetic beam-matter interactions, damage events, and materials' behavior under intense radiation environments. In this article, in situ TEM observations of radiation effects in complex ceramics (e.g., oxides, silicates, and phosphates) subjected to energetic ion and electron irradiations have been summarized with a focus on irradiation-induced microstructural evolution, changes in microchemistry, and the formation of nanostructures. New results for in situ TEM observation of radiation effects in pyrochlore, A(2)B(2)O(7), and zircon, ZrSiO(4), subjected to multiple beam irradiations are presented, and the effects of simultaneous irradiations of alpha-decay and beta-decay on the microstructural evolution of potential nuclear waste forms are discussed. Furthermore, in situ TEM results of radiation effects in a sodium borosilicate glass subjected to electron-beam exposure are introduced to highlight the important applications of advanced analytical TEM techniques, including Z-contrast imaging, energy filtered TEM (EFTEM), and electron energy loss spectroscopy (EELS), in studying radiation effects in materials microstructural evolution and microchemical changes. By combining ex situ TEM and advanced analytical TEM techniques with in situ TEM observations under energetic beam irradiations, one can obtain invaluable information on the phase stability and response behaviors of materials under a wide range of irradiation conditions.

Transmission electron microscopy study of the platinum germanide formation process in the Ge/Pt/Ge/SiO 2/Si structure

The mechanisms of the platinum germanide formation by RTA processes in the Ge/Pt/Ge/SiO 2/Si structure in the temperature range from 200 °C to 600 °C were investigated by means of transmission electron microscopy (TEM) techniques. The studies were focused on the observations of the layer microstructure and of Ge/Pt/Ge interfaces at an atomic scale by high resolution TEM (HRTEM), as well as on the identification of formed phases using analytical TEM and diffraction techniques. The formation of platinum germanides in the Ge/Pt/Ge/SiO 2/Si structure is visible at 200 °C. However, at this temperature only a part of Ge reacted with the whole Pt during annealing. The whole Ge layer reacted with the Pt layer at 300 °C. The formed Pt–Ge layer is not a mono-layer but consists of two platinum germanides layers. The results of the annealing of Ge/Pt/Ge/SiO 2/Si structure at higher temperatures are very similar to the results obtained at 300 °C. Various Pt–Ge phases have been found in the samples studied. The PtGe phase in the germanide layer of the sample annealed at 200 °C was found by means of TEM diffraction analysis and the energy-dispersive X-ray spectroscopy (EDXS) in scanning TEM. At 300 °C the diffraction pattern reveals the reflections corresponding to PtGe and Pt 2Ge 3 phases. Analytical TEM studies showed that the phase composition depends on annealing temperature and that the Ge content in the Pt–Ge phases increases with temperature.

TEM‐Based Metrology for HfO2 Layers and Nanotubes Formed in Anodic Aluminum Oxide Nanopore Structures

Nanotubes are fabricated by atomic layer deposition (ALD) into nanopore arrays created by anodic aluminum oxide (AAO). A transmission electron microscopy (TEM) methodology is developed and applied to quantify the ALD conformality in the nanopores (thickness as a function of depth), and the results are compared to existing models for ALD conformality. ALD HfO2 nanotubes formed in AAO templates are released by dissolution of the Al2O3, transferred to a grid, and imaged by TEM. An algorithm is devised to automate the quantification of nanotube wall thickness as a function of position along the central axis of the nanotube, by using a cylindrical model for the nanotube. Diffusion-limited depletion occurs in the lower portion of the nanotubes and is characterized by a linear slope of decreasing thickness. Experimentally recorded slopes match well with two simple models of ALD within nanopores presented in the literature. The TEM analysis technique provides a method for the rapid analysis of such nanostructures in general, and is also a means to efficiently quantify ALD profiles in nanostructures for a variety of nanodevice applications.

In Situ Growth of Mesoporous SnO2 on Multiwalled Carbon Nanotubes: A Novel Composite with Porous‐Tube Structure as Anode for Lithium Batteries

AbstractA novel mesoporous‐nanotube hybrid composite, namely mesoporous tin dioxide (SnO2) overlaying on the surface of multiwalled carbon nanotubes (MWCNTs), was prepared by a simple method that included in situ growth of mesoporous SnO2 on the surface of MWCNTs through hydrothermal method utilizing Cetyltrimethylammonium bromide (CTAB) as structure‐directing agents. Nitrogen adsorption–desorption, X‐ray diffraction and transmission electron microscopy analysis techniques were used to characterize the samples. It was observed that a thin layer tetragonal SnO2 with a disordered porous was embedded on the surface of MWCNTs, which resulted in the formation of a novel mesoporous‐nanotube hybrid composite. On the base of TEM analysis of products from controlled experiment, a possible mechanism was proposed to explain the formation of the mesoporous‐nanotube structure. The electrochemical properties of the samples as anode materials for lithium batteries were studied by cyclic voltammograms and Galvanostatic method. Results showed that the mesoporous‐tube hybrid composites displayed higher capacity and better cycle performance in comparison with the mesoporous tin dioxide. It was concluded that such a large improvement of electrochemical performance within the hybrid composites may in general be related to mesoporous‐tube structure that possess properties such as one‐dimensional hollow structure, high‐strength with flexibility, excellent electric conductivity and large surface area.

Direct Determination of Local Lattice Polarity in Crystals

M aterials with the wurtzite structure, such as CdSe quantum dots and rods, GaN-AlN quantum wells, and nanowires, play an increasingly important role in nanoscale science. For some, like the III-V nitrides, the existence of directional polarization is the driving force behind technical applications. Hexagonal crystals with the wurtzite (AB) structure exhibit physical and chemical properties that are different when they are parallel and antiparallel to the c axis. This difference is a result of particular stacking of A and B atomic layers, i.e. lattice polarity, along the E0001^ direction (Fig. 1A). For a long time, atomic-level determination of the local lattice polarity was not possible. Early methodswere based on differences in the surface properties of bulk crystals. Later, microscopic analysis relied on diffraction data recorded by conventional transmission electron microscopy (TEM) (1) or on a statistical analysis of scanning TEM (STEM) images (2). Unfortunately, these techniques cannot reveal local, atomic-level polarity in nanoscale structures, because they lack the required lateral spatial resolution or are limited to thick samples, where extensive analysis of dynamical diffraction is needed. Demonstration of polarity at the atomic level would allow for understanding and control of phenomena occurring in nanoscale objects, such as selective growth of PbSe with a rock-salt structure on the tips of wurtzite colloidal nanorods (3) and the bonding of zincblende CdTe cores with wurtzite CdTe/CdSe branches in nanocrystals (4). Recent developments in TEM analysis techniques (5) and aberration correction in the TEM (6) and in the STEM (7), where an electron probe smaller that 1 ) has been achieved (8, 9), potentially allow for atomic-level determination of local polarity. TEM analysis has been successful in improving the effective resolution from 1.7 ) (experimental) to E0.8 to 0.9 ) with computational analysis using many experimental images (5). We show here that annular dark field (ADF) imaging in an aberration-corrected STEM can provide direct quantitative discrimination during observation in real time without extensive analysis. Figure 1B shows an ADF image of a small area of wurtzite AlN in the E2110^ projection, recorded using a VG HB-501 dedicated STEM equipped with an aberration corrector at an energy of 120 keV. Operation at this low energy is important to minimize irradiation damage from direct electron impact. This instrument has demonstrated a 0.75 ) probe in systematic tests (8). ADF imaging in crystals uses large-angle elastic scattering to locate individual atoms and projected atomic columns, with good sensitivity to atomic number Z but with minor dependence on focusing conditions or specimen thickness. The challenge in this case is to image low-Z atoms, such as N, located in close proximity to very strongly scattering high-Z atoms. In the E2110^ orientation, the smallest projected spacing between Al and N columns is 1.1 ), requiring a very small electron probe for discrimination. In Fig. 1B, the N columns are seen as elongations of the column images alternately pointing diagonally up and down. This image represents the local polarity in AlN but also, for the first time, the identification of the single column of N atoms near the Al. In Fig. 1C, the visibility of N is enhanced by removal of the Al column signal. Because the column signal should be azimuthally symmetric, we measured the Al profile in the direction perpendicular to the line joining it with N (Fig. 1B, yellow line), and then subtracted to obtain the N profile. Fig. 1D shows an example of the use of this technique for a strained AlN/GaN/AlN quantum well, where we can easily pick out the polarity of the AlN substrate and cap. In the quantum well, the much stronger Ga scattering makes direct polarity determination there difficult, but establishing the lattice position and polarity in the AlN matrix allows comparisons of the positions of Ga columns with image calculations (Fig. 1D, inset) to obtain the polarity in the quantum well. This particular structure is unfaulted and carries the substrate polarity through the well to the capping material.