Nanobeam Electron Diffraction Research Articles

Two-dimensional Gaussian fitting for precise measurement of lattice constant deviation from a selected-area diffraction map.

Unlike X-ray diffraction or Raman techniques, which suffer from low spatial resolution, transmission electron microscopy can be used to obtain strain maps of nanoscaled materials and devices. Convergent-beam electron diffraction (CBED) and nanobeam electron diffraction (NBED) techniques detect the deviation of a lattice constant (i.e. an indicator of strain) within 0.01%; however, their use is restricted to beam-insensitive samples. Selected-area electron diffraction (SAED) does not have such limitations but has low spatial resolution and precision. The use of a spherical aberration corrector and a nanosized selected-area aperture improves the spatial resolution, but the precision is still low. In this study, a two-dimensional stage-scanning system is used to acquire arrays of diffraction patterns at different positions of the sample under fixed beam conditions. Data processing with iterative nonlinear least-squares fitting enabled the spot displacement for each point of the scan area to be measured with precision comparable to that of the CBED or NBED technique. The precise strain determination, in combination with the simplicity of the measurement process, makes the nanosized SAED technique competitive with other methods for strain mapping at nanoscale dimensions.

In situ nanobeam electron diffraction strain mapping of planar slip in stainless steel

Nanobeam electron diffraction strain mapping has been used to measure the strain evolution in stainless steel under in situ deformation. As the amount of deformation increases, the leading dislocation of a planar slip band leaves behind a residual strain in the form of a small lattice expansion. Dislocation analysis confirmed that the dislocations involved were <011> type. While the characteristic residual strain of planar slip has often been observed, it has never before been directly measured. Our results provide a view into the dynamic mechanisms of planar slip, and showcase the possibilities of multidimensional in situ imaging.

High precision strain mapping of topological insulator HgTe/CdTe

Topological insulators (TIs) represent a class of matter associated with unique electronic and spin properties held by chiral and massless Dirac fermions. The lattice mismatch between the HgTe layer and the CdTe substrate, which is the reason for the TI properties in HgTe/CdTe, is less than 0.4%. Measuring it necessitates an extremely sensitive technique, while a nanometric spatial resolution is mandatory to characterize the strain gradient across the layers. In this letter, we demonstrate the use of nano-beam precession electron diffraction to map the strain of the whole stack, thus experimentally addressing the strain distribution in such systems. Strain maps with a precision of 0.03% and a spatial resolution of 1.9 nm show good agreement with finite element simulations of the expected strain. Strain values and gradients are within the intervals predicted to yield the TI properties of the material.

Optimization of NBED simulations for disc-detection measurements

Nano-beam electron diffraction (NBED) is a method which can be applied to measure lattice strain and polarisation fields in strained layer heterostructures and transistors. To investigate precision, accuracy and spatial resolution of such measurements in dependence of properties of the specimen as well as electron optical parameters, simulations of NBED patterns are required which allow to predict the result of common disc-detection algorithms. In this paper we demonstrate by focusing on the detection of the central disc in crystalline silicon that such simulations require to take several experimental characteristics into account in order to obtain results which are comparable to those from experimental NBED patterns. These experimental characteristics are the background intensity, the presence of Poisson noise caused by electron statistics and blurring caused by inelastic scattering and by the transfer quality of the microscope camera. By means of these optimized simulations, different effects of specimen properties on disc detection – such as strain, surface morphology and compositional changes on the nanometer scale – are investigated and discussed in the context of misinterpretation in experimental NBED evaluations. It is shown that changes in surface morphology and chemical composition lead to measured shifts of the central disc in the NBED pattern of tens to hundreds of µrad. These shifts are of the same order of magnitude or even larger than shifts that could be caused by an electric polarisation field in the range of MV/cm.

Structures of Boron Loaded Turbostratic Graphene with Enhanced Performance in Solid Acid Fuel Cell

Solid acid fuel cells (SAFC) working at intermediate temperature (250°C) have many advantages compared to the lower temperature proton exchange membrane fuel cell (PEMFC), such as increased catalyst activity and are more resistant to CO poisoning.1, 2 The state-of-the-art cathode is comprised of porous protonic conductor coated by Pt.3, 4 There are two major roles of Pt in the cathode: (1) the oxygen reduction reaction (ORR) catalyst and (2) the electron conductor. To maintain a highly electron conductive network, a high fraction of Pt has to be loaded in ‘conventional’ SAFC electrodes. However, to generate wider application, the cost of the cathode must be reduced significantly. The function of Pt as electron conductor can be replaced by the catalyst support to achieve this reduction in loading. The catalyst support must have high electron conductivity, corrosion-resistance and high surface area. The carbon nanostructures are the common catalyst supports used in the fuel cells. In our previous studies, we demonstrated that multi-wall carbon nanotubes are much more stable during fuel cell testing compared to the single wall carbon structures.5 Indeed, pure single layer or few-layer carbon supports are well known to be particularly vulnerable to corrosion in the present of water. However, previous research showed that substitutional boron can lower the electron density of the reactive carbon, leading to a reduction in the rate of O2 chemisorption.6 Meanwhile, very small amounts of boron (~1%) have been demonstrated to enhance the conductivity of the single carbon structure.7In our recent studies, we synthesize the pure single wall carbon, carbon structures in hydrogen gas and with boron loading. Surprisingly, the boron loading induces the growth of few-layer graphenes (FLGs), which exhibit much better corrosion resistance than the single wall carbon and carbon synthesized in hydrogen gas which consists of multi-wall and single-wall carbon structure. In this research, we focus on the structural investigation of those FLGs. Characterization of the FLGs are performed by high resolution transmission electron microscopy (HRTEM) imaging, monochromated electron energy-loss spectroscopy (EELS), nano-beam electron diffraction (NBED), aberration corrected scanning transmission electron microscopy (STEM) imaging, X-ray photoelectron spectroscopy (XPS), nuclear magnetic resonance (NMR), Raman spectroscopy and X-ray diffraction (XRD). Representative STEM images in a low and high magnification are shown in Figure 1 (a)-(b). A significantly structure deviation compared to pure graphene/graphite is evident due to 1-3% boron loading. Those results indicate the few-layer graphene does not stack in a normal graphitic way in the c direction. The EELS, XPS and NMR show complicated electronic states of the boron in the carbon, implying boron does not have to be in the substitutional site. For example, Figure 1(c) show a representative EELS spectrum taken from a FLG. The featureless boron K-edge looks like to be taken from amorphous boron instead of substitutional boron. This result is also supported by the NBED and XRD, showing a non-flat features of the FLGs. Finally, we used the density functional theory (DFT) to explain the excellent oxygen resistance of FLG with this unique structure. Acknowledgments This work is supported by ARPA-E via cooperative agreement DE-AR0000499. The synthesis science is supported by BES-MSED. We thank JIAM and CNMS for microscopy use. Reference 1. K. Ishiyama, F. Kosaka, I. Shimada, Y. Oshima and J. Otomo, Journal of Power Sources, 2013, 225, 141-149. 2. A. B. Papandrew, R. W. Atkinson Iii, R. R. Unocic and T. A. Zawodzinski, Journal of Materials Chemistry A, 2015, 3, 3984-3987. 3. A. B. Papandrew, C. R. I. Chisholm, R. A. Elgammal, M. M. Özer and S. K. Zecevic, Chemistry of Materials, 2011, 23, 1659-1667. 4. A. B. Papandrew, C. R. I. Chisholm, S. K. Zecevic, G. M. Veith and T. A. Zawodzinski, Journal of The Electrochemical Society, 2013, 160, F175-F182. 5. A. B. Papandrew, R. A. Elgammal, M. Tian, W. D. Tennyson, C. M. Rouleau, A. A. Puretzky, G. M. Veith, D. B. Geohegan and T. A. Zawodzinski, Journal of Power Sources, 2017, 337, 145-151. 6. B. Yuan, W. Xing, Y. Hu, X. Mu, J. Wang, Q. Tai, G. Li, L. Liu, K. M. Liew and Y. Hu, Carbon, 2016, 101, 152-158. 7. M. Harada, T. Inagaki, S. Bandow and S. Iijima, Carbon, 2008, 46, 766-772. Figure 1

Current induced rapid phase transformation in Au/Sn reaction couple

Abstract The phase evolution as well as microstructural change of thin film Au/Sn reaction couple was investigated at room temperature with respect to 1.0 × 10 4 A/cm 2 of current stressing. Transmission electron microscopy (TEM) equipped with nano beam electron diffraction (NBED) and energy dispersive X-ray spectrometer (EDS) revealed the fully conversion of Sn layer into AuSn and Au 5 Sn IMCs after deposition. The rapid phase transformation of Au 5 Sn to Au and AuSn, regardless of direction of electron flow, was evidenced by TEM during short time of current stressing. The electron impingement induced lattice strain of Au 5 Sn higher than the offset yield limit was anticipated through in-situ XRD investigation. The lattice adjustment of Au 5 Sn to AuSn due to escape of Au from Au 5 Sn crystal was responsible for the phase transformation. The polarity migration of produced Au atoms from Au 5 Sn crystal to the anode gave rise to the numerous nano voids formation at the original Au 5 Sn/Au interface with forward electric current and dilution of Sn in Au layer with reverse current stressing.

Observation and understanding of anisotropic strain relaxation in selectively grown SiGe fin structures

The performance of heterogeneous 3D transistor structures critically depends on the composition and strain state of the buffer, channel and source/drain regions. In this paper we used an in-line high resolution x-ray diffraction (HRXRD) tool to study in detail the composition and strain in selectively grown SiGe/Ge fin structures with widths down to 20 nm. For this purpose we arranged fins of identical dimensions into larger arrays which were then analyzed using an x-ray beam several tens of micrometers in size. Asymmetric reciprocal space maps measured both parallel and perpendicular to the fins allowed us to extract the lattice parameters in all three spatial directions. Our results demonstrate an anisotropic in-plane strain state of the selectively grown SiGe buffer in case of narrower fins with significantly reduced relaxation in the direction along the fin. This observation was verified using nano-beam electron diffraction, and is explained based on the reduced probability for dislocation half-loops to evolve in trenches narrower than a few times the critical radius. Moreover, we introduce and discuss in detail a methodology for the determination of the composition in case of an anisotropic in-plane strain state which differs from the procedure commonly used for blanket layers. Our findings verify the importance of in-line HRXRD measurements for process development and monitoring as well as the fundamental study of relaxation and defect formation in confined volumes.

Quantitative crystallographic analysis of individual carbon nanofibers using high resolution transmission electron microscopy and electron diffraction

High resolution transmission electron microscopy (HRTEM) and electron diffraction allow for a thorough crystallographic characterization of the herringbone structure of carbon nanofibers (CNF). A newly developed method for an extended quantitative structural evaluation of single carbon nanofibers is applied. The method is based on quantitative analysis of radial and azimuthal profiles extracted from selected area electron diffraction (SAED) patterns, nano beam electron diffraction (NBED) patterns or power spectra obtained by Fourier transformation of HRTEM images. Precise quantification of structural parameters, in particular cone apex angle, interlayer spacing and undulation of graphene layers is carried out. For the first time a profound interpretation of CNF diffraction patterns is given, proving the rotational disorder of the turbostratic structure and suggesting a random rotation between successive graphene cones. A series of crystallographic analyses along the axis of a single CNF reveal a continuous increase of the interlayer spacing during CNF growth. Additionally, the different techniques HRTEM, SAED, and NBED are evaluated in terms of attainable structural information, precision and applicability to different diameters of CNFs.

Optimizing disk registration algorithms for nanobeam electron diffraction strain mapping

Scanning nanobeam electron diffraction strain mapping is a technique by which the positions of diffracted disks sampled at the nanoscale over a crystalline sample can be used to reconstruct a strain map over a large area. However, it is important that the disk positions are measured accurately, as their positions relative to a reference are directly used to calculate strain. In this study, we compare several correlation methods using both simulated and experimental data in order to directly probe susceptibility to measurement error due to non-uniform diffracted disk illumination structure. We found that prefiltering the diffraction patterns with a Sobel filter before performing cross correlation or performing a square-root magnitude weighted phase correlation returned the best results when inner disk structure was present. We have tested these methods both on simulated datasets, and experimental data from unstrained silicon as well as a twin grain boundary in 304 stainless steel.

Orientation relationships of Mn0.75Ga0.25As crystallites on and within GaAs determined by scanning nano beam electron diffraction

Mn0.75Ga0.25As crystallites, partially embedded on and fully embedded within a single crystalline matrix of GaAs formed during metal organic vapor phase epitaxy (MOVPE) of Mn‐rich (Mn,Ga)As on (001) oriented GaAs wafers. Phase and orientation analysis of these crystallites were performed with scanning nano beam electron diffraction (SNBED). The investigation of plan‐view specimens using a liquid nitrogen cooling stage enabled the phase and orientation analysis of partially embedded ferromagnetic α‐phase particles. In all specimens the following two orientation relationships (O) between the α‐phase particles and the GaAs matrix were determined: urn:x-wiley:02321300:media:crat201600261:crat201600261-math-0001urn:x-wiley:02321300:media:crat201600261:crat201600261-math-0002The study of cross‐sectional specimens enabled the analysis of fully embedded crystallites. It could be unambiguously detected that a fully embedded crystallite has the structure of the atomically ordered monoclinic β´ phase. The β´ phase crystallite consists of two domains which are related in twin positions to each other. The orientation relations of the different particles are illustrated by color coded stereographic projections.

Direct measurement of chiral structure and transport in single- and multi-walled carbon nanotubes

Electrical devices based on suspended multi-wall carbon nanotubes were constructed and studied. The chiral structure of each shell in a particular nanotube was determined using nanobeam electron diffraction in a transmission electron microscope. The transport properties of the carbon nanotube were also measured. The nanotube device length was short enough that the transport was nearly ballistic, and multiple subbands contributed to the conductance. Thermal excitation of carriers significantly affected nanotube resistance at room temperature.

Environmental transmission electron microscopy investigations of Pt-Fe2O3 nanoparticles for nucleating carbon nanotubes

Elucidating the evolution of bimetallic catalyst for nucleating carbon nanotube has been challenging. In this work, acorn-like Pt-Fe2O3 nanoparticles are developed for the growth of single-walled carbon nanotubes (SWCNTs) by chemical vapor deposition. Using in situ environmental transmission electron microscopy, restructuring of the acorn-like Pt-Fe2O3 nanoparticles at reaction conditions is investigated. Upon heating to reaction temperature, ε-Fe2O3 is converted to β-Fe2O3, which can be subsequently reduced to metallic Fe once introducing CO. As Pt promotes the carburization of Fe, part of the metallic Fe reacts with active carbon atoms to form Fe2.5C instead of Fe3C, catalyzing the nucleation of carbon nanotubes. Nanobeam electron diffraction characterizations on SWCNTs grown under ambient pressure at 800 °C demonstrate that their chiral angle and diameter distributions are similar to those of SWCNTs grown on monometallic Fe. The results further indicate that the active components in both the catalysts, determining the chirality distribution of SWCNTs, are similar. In addition, Pt facilitates the reduction of Fe2O3, rendering SWCNT growth at a relatively low temperature of 700 °C. This work provides a profound understanding of the structural reconstruction in bimetallic catalyst, shedding more light on designing novel catalysts for the growth of SWCNTs.

(Invited) Transistor Strain Measurement Using Electron Beam Techniques

Strain and the related stress in nanoelectronic devices play a critical role in semiconductor technologies. In general, strain arises from the lattice mismatch between dissimilar materials and thus strain is critical for the design and growth of epitaxial heterostructures in micro- and nano-electronics. In transistors, strained silicon channels provide a major boost to the performance of metal-oxide-semiconductor field-effect transistors (MOSFETs), e.g., the use of epitaxial strain of Si and Ge alloys in PMOS [1]. In the near term, the transistor technology is pushing towards the sub 10 nm technology node. At this length scale, resolving strain in the devices requires a resolution of sub nanometer with not only high resolution, also high accuracy and reliability. The use of novel architectures and alternative materials in the transistor technology also demands metrology capable of handling these structural and material changes. High resolution strain measurement can be performed using high energy electrons either in reciprocal space using convergent beam electron diffraction (CBED), scanning electron nanodiffraction (SEND), nanobeam electron diffraction (NBED) [2], or scanning precession electron diffraction (SPED) or in real space through direct imaging using high resolution electron microscopy (HREM), scanning transmission electron microscopy (STEM) [3] or dark-field electron holography (DFEH)[4]. The resolution and strain measurement sensitivities range from Å to several nm and 10-3 to 10-4respectively. Recent developments in aberration corrected STEM especially have improved the spatial resolution to sub- Å and the precision of measuring atomic column positions to picometers [3]. This talk will review high resolution and high sensitivity electron beam techniques for strain characterization in nanodevices and device materials. Examples include planar and a finfet transistors and III-V heterostructures. For finfet transistors, we demonstrate that NBED provides an effective approach for strain mapping (See figure below) and some of challenges will also be highlighted. In III-V heterostructures, we will describe atomic resolution Z-contrast imaging based methods for high resolution interfacial strain analysis and its applications for the investigation of interfacial intermixing and atomic layer-by-layer strain profiles in InAs/GaSb and InAs/InAsSb superlattices, targeted for middle- and long-wavelength IR detection.

Correlation of gate leakage and local strain distribution in GaN/AlGaN HEMT structures

GaN/AlGaN HEMT structures are observed to undergo a reversible, drastic change in the leakage current when covered with an additional polymer passivation layer. The polymer layer induces a stress on the HEMT structures, which initiates material migration processes and the formation of structural defects, influencing the electrical performance. Local strain measurements were performed in the semiconductor, at the critical HEMT gate electrode, to evaluate the impact of the stress on the Schottky gates. The strain distributions in the structures were measured with nanobeam electron diffraction from electron-transparent samples at cross sections and longitudinal sections at the positions of high leakage currents. A variation of the strain distribution underneath the gate electrode was detected in a cross-sectional sample. On the contrary, only minor differences in the strain values were measured in the longitudinal sections at different photoemission sites. Finally, localized metal interdiffusion was detected at the sites with the highest photoemission intensities.

Microstructural Characterization and Electrical Properties of Ti–GaSb Junctions

The microstructure of Ti–GaSb junctions in samples annealed at 300°C, 400°C, 500°C, and 600°C in N2 atmosphere has been characterized using transmission electron microscopy in combination with energy-dispersive spectrometry, nanobeam electron diffraction, and selected-area diffraction patterns. Only Ga3Ti phase formed at the Ti/GaSb interface at annealing temperature of 400°C. Upon increasing the temperature to 500°C, in addition to Ga3Ti phase, Sb from GaSb diffused toward Ti and accumulated at the interface to form a Sb-rich layer. Moreover, three phases, namely TiSb(Ga), Sb2Ti, and Ga3Ti, formed simultaneously at the Ti/GaSb interface when the annealing temperature was increased to 600°C, causing a significant increase in the sheet resistance, RS, of the Ti–GaSb alloy. These results indicate that the annealing temperature of the Ti/GaSb structure should be maintained below 500°C for successful formation of low-resistance metal Ti/GaSb contacts in GaSb-based p-type metal–oxide–semiconductor field-effect transistors.

Local strain measurements during in situ TEM deformation with nanobeam electron diffraction

Local strain measurements during in situ TEM deformation with nanobeam electron diffraction

Fe[sbnd]Ti[sbnd]O based catalyst for large-chiral-angle single-walled carbon nanotube growth

Catalyst selection is very crucial for controlled growth of single-walled carbon nanotubes (SWNTs). Here we introduce a well-designed FeTiO solid solution for SWNT growth with a high preference to large chiral angles. The FeTiO catalyst was prepared by combining Ti layer deposition onto premade Fe nanoparticles with subsequent high-temperature air calcination, which favours the formation of a homogeneous FeTiO solid solution. Using CO as the carbon feedstock, chemical vapour deposition growth of SWNTs at 800 °C was demonstrated on the FeTiO catalyst. Nanobeam electron diffraction characterization on a number of individual SWNTs revealed that more than 94% of SWNTs have chiral angles larger than 15°. In situ environmental transmission electron microscopy study was carried out to reveal the catalyst dynamics upon reduction. Our results identify that the phase segregation through reducing FeTiO catalyst leads to the formation of TiOx-supported small Fe nanoparticles for SWNT growth. The strong metal-support interactions induced by partial reduction of TiOx support promote the wettability of Fe nanoparticle, accounting for the preferential growth of large-chiral-angle SWNTs. This work opens a new avenue for chiral angle selective growth of SWNTs.

Study of crystallization behavior and kinetics of ZrO2-3Y2O3-xSrO (1≤x≤3) precursor powders using an isothermal process

Zirconium nitrate (Zr(NO3)4·xH2O), yttrium nitrate (Y(NO3)3·6H2O) and strontium nitrate (Sr(NO3)2) were used as the starting materials for synthesized ZrO2-3Y2O3-xSrO (1≤x≤3) precursor powders. The isothermal crystallization behavior and kinetics were investigated using an isothermal process. The characterization used X-ray diffraction (XRD), transmission electron microscopy (TEM), selected area electron diffraction (SAED), nano-beam electron diffraction (NBED) and high-resolution TEM (HRTEM). The XRD results show that all patterns only contain the single phase of tetragonal ZrO2 (t-ZrO2) for ZrO2-3Y2O3-xSrO precursor powders after calcination between 773 and 923K for various durations. When the ZrO2-3Y2O3-1SrO precursor powders were calcined at 923K, the crystallinity of t-ZrO2 increased from 29.98% to 81.64% with rose calcined time from 15min to 120min. When calcination time was held at 60min, the crystallinity of t-ZrO2 increased from 52.49% to 73.41% of ZrO2-3Y2O3-1SrO precursor powders with increasing calcination temperature from 773K to 923K. Moreover, when the precursor powders were maintained at the same calcination temperature and duration, the crystallinity of t-ZrO2 decreased with increasing addition of SrO. The activation energy and kinetics formula for t-ZrO2 crystallization of ZrO2-3Y2O3-xSrO precursor powders by an isothermal method were obtained.

Chemical and Structural Evolution of Layered Lithium-Transition Metal Oxide Cathode Material upon Cycling

With the rapid development of electronic devices and hybrid-electric/electric vehicles (H/EVs), lithium-ion batteries have become the most promising electrical storage system due to its high volumetric and gravimetric energy density. However, the high energy density of lithium-ion batteries is difficult to maintain after extended cycles due to capacity fading, which can be attributed to multiple possible reasons including SEI formation,[1] electrolyte decomposition,[2] and structural changes in electrode materials.[3] These issues generate important concerns to the stability and lifetime of the battery. Layered lithium-transition metal oxides represent a major type of cathode materials that are widely used in the commercial market. However, these materials are suggested to suffer structural transformation during electrochemical cycling. Previous studies suggest the possible surface structural transitions from the layered structure to spinel[4], and/or rock-salt structures,[5] or possible further decomposition of the transformed phase.[6]A clear explanation of these surface phenomenon is still under debate. An in-depth investigation of the structural reconstruction is necessary to elucidate the degradation mechanisms of the layered cathode materials. In the present study, we investigated the chemical evolution and structural transformation of a prominent layered cathode material for lithium-ion batteries, LiNi1/3Mn1/3Co1/3O2 (NMC). The redox reaction of NMC cathode during charge-discharge process was analyzed in detail using high-resolution electron-energy loss spectroscopy (EELS) and 7Li magic-angle spinning (MAS) NMR. Our results suggest that the charge compensation of the NMC cathode during the delithiation-lithiation process is mainly achieved by the oxidation and reduction of Ni2+↔Ni4+, whereas Mn4+ and Co3+ remain mostly unchanged. This is consistent with the NMR findings, from which a trend to lower chemical shift upon lithium extraction is well-correlated with the oxidation change from paramagnetic Ni2+ to diamagnetic Ni4+. Furthermore, the electronic structure of the NMC cathode during initial charging process is found to be inhomogeneous from the particle surface to the bulk using spatially resolved STEM-EELS technique (Figure 1a-1d). It is revealed that the particle surface is at lower oxidation state compared with the bulk region, indicating that the surface evolution of NMC cathode material occurs during the initial cycle. This surface evolution is further analyzed at different numbers of cycles using atomic-resolution high angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) imaging combined with EELS and nano-beam electron diffraction (NBED). Figure 1e shows a HAADF-STEM image of the NMC particle after 50 cycles. It can be clearly seen from the image that the bulk of the cycled NMC particle shows identical layered structure symmetry, the NBED (Figure 1h) obtained from the bulk can be indexed to the R-3m space group. In contrast, the Li layers are occupied by transition metal ions at the particle surface, and this surface reconstruction layer thickens with extended cycles. When probing to the outer-most surface, the corresponding NBED can be indexed to an Fm-3m rock-salt structure, as shown in Figure 1f. The valence change of the transition metal cations is analyzed using STEM-EELS. The results indicate that the transition metal ions are reduced to divalent states at the surface. In addition, the TM:O ratio increases almost twice at the surface than the bulk. The results indicating that the surface layer is a MO-type rock-salt phase with small amount of residual Li ions and/or vacancies. In addition, a transition zone is observed (Figure 1e) between the bulk layered region and the surface rock-salt layer, where the Li sites are partially occupied by the transition metal ions (some are marked by the red arrows), and the diffraction spots corresponding to the alternating arrangement of Li-containing layers and transition metal layers (some are marked with arrows) become weaker (Figure 1g). 7Li NMR studies of the local Li-environments, following ten cycles and ending at the top of charge, indicate that the order of the TM layers remains unchanged in the bulk of the material. This highlights the importance of complementary studies that are sensitive to different length scales. Further data, including EELS and NMR of the NMC cathode at different cycling state and electrolyte effect will be discussed in the presentation. [1] K. Edström, T. Gustafsson, J. Thomas, Electrochim. Acta 2004, 50, 397. [2] L. Terborg, S. Weber, F. Blaske, et al. J. Power Sources 2013, 242, 832. [3] B. Xu, C.R. Fell, M. Chi, et al. Energy Environ. Sci. 2011, 4, 2223. [4] A. Boulineau, L. Simonin, J.F. Colin, et al. Chem. Mater. 2012, 24, 3558. [5] F. Lin, I. M. Markus, D. Nordlund, et al. Nat. Commun. 2014, 5, 3529. [6] J. Zheng, M. Gu, J. Xiao, et al. Nano Lett. 2013, 13, 3824. Figure 1

Direct Measurement of Polarization-Induced Fields in GaN/AlN by Nano-Beam Electron Diffraction

The built-in piezoelectric fields in group III-nitrides can act as road blocks on the way to maximizing the efficiency of opto-electronic devices. In order to overcome this limitation, a proper characterization of these fields is necessary. In this work nano-beam electron diffraction in scanning transmission electron microscopy mode has been used to simultaneously measure the strain state and the induced piezoelectric fields in a GaN/AlN multiple quantum well system.