Electron Energy Loss Spectroscopy Imaging Research Articles

Discovering invariant spatial features in electron energy loss spectroscopy images on the mesoscopic and atomic levels

Over the last two decades, Electron Energy Loss Spectroscopy (EELS) imaging with a scanning transmission electron microscope has emerged as a technique of choice for visualizing complex chemical, electronic, plasmonic, and phononic phenomena in complex materials and structures. The availability of the EELS data necessitates the development of methods to analyze multidimensional data sets with complex spatial and energy structures. Traditionally, the analysis of these data sets has been based on analysis of individual spectra, one at a time, whereas the spatial structure and correlations between individual spatial pixels containing the relevant information of the physics of underpinning processes have generally been ignored and analyzed only via the visualization as 2D maps. Here, we develop a machine learning-based approach and workflows for the analysis of spatial structures in 3D EELS data sets using a combination of dimensionality reduction and multichannel rotationally invariant variational autoencoders. This approach is illustrated for the analysis of both the plasmonic phenomena in a system of nanowires and in the core excitations in functional oxides using low loss and core-loss EELS, respectively. The code developed in this manuscript is open sourced and freely available and provided as a Jupyter notebook for the interested reader.

Air-based sputtering deposition of TiN(O)/a-TiNxOy multilayer films for enhanced mechanical properties and corrosion resistance

Extensive research has explored multilayer films for their distinctive mechanical and functional properties. This study employs an air-based sputtering technique, using air as a reactive gas under low vacuum conditions, to produce TiN(O)/a-TiNxOy multilayer films. By alternately adjusting the air/Ar flow ratios, the multilayer films with various modulation periods were created. The crystalline TiN(O)/amorphous TiNxOy multilayered structure was confirmed via cross-sectional microscopy and electron energy loss spectroscopy images and mappings. Hardness (H) and elastic modulus (E) initially increased, peaked, and then decreased with the modulation period, reaching a maximum hardness of 31.2 GPa and an elastic modulus of 343 GPa. The enhancements may be attributed to internal compressive stress, interface strain effects, and the inhibition of dislocation propagation throughout the films. Higher H/E ratios (0.09–0.11) and H3/E2 ratios (0.23–0.28 GPa) suggest enhanced resistance to elastic and plastic deformation, surpassing those of single-layered TiN(O). This enhancement is primarily attributed to the hard/soft multilayer design. Potentiodynamic polarization tests revealed improved corrosion resistance with decreasing modulation period, reaching optimal values Ecorr at −0.089 V and Icorr at 0.272 μA/cm2. This improvement is mainly ascribed to the multilayer design, which inhibits direct corrosion through grain boundaries. This air-based sputtering technique offers a facile approach for producing complex multilayer films, promising enhanced mechanical properties and corrosion resistance of coatings for various applications.

Direct Visualization of Homogeneous Chemical Distribution in Functional Polyradical Microspheres.

It is demonstrated that the postfunctionalization of solid polymeric microspheres can generate fully and throughout functionalized materials, contrary to the expectation that core-shell structures are generated. The full functionalization is illustrated on the example of photochemically generated microspheres, which are subsequently transformed into polyradical systems. Given the all-organic nature of the functionalized microspheres, characterization methods with high analytical sensitivity and spatial resolution are pioneered by directly visualizing the inner chemical distribution of the postfunctionalized microspheres based on characteristic electron energy loss signals in transmission electron microscopy (TEM). Specifically, ultrasonic ultramicrotomy is combined successfully with electron energy loss spectroscopy (EELS) and electron spectroscopicimaging (ESI) during TEM. These findings open a key avenue for analyzing all-organic low-contrast soft-matter material structures, while the specifically investigated system concomitantly holds promise as an all-radical solid-state functional material.

Ultrastructure of pathologic deposits and cellular inclusions

Intra- and extracellular depositions and inclusions occur in awide range of diseases with exogenous (e.g. infectious, environmental and toxic) or endogenous (e.g. genetic, inflammatory, neoplastic and degenerative) aetiology. The noxious agent and the pathogenesis influence the organ of manifestation, the subcellular localisation and the ultrastructural appearance of the depositions. Whereas some of the inclusions like pathogens, foreign material (e.g. asbestos) or microvilli have an almost pathognomonic morphology, other inclusions are present in lower amounts also under normal conditions (e.g. lipid vacuoles and glycogen). Therefore, the interpretation of ultrastructural findings makes acorrelation with the histological features and clinical constellation necessary. Auxiliary investigations by electron energy loss spectroscopy (EELS) or electron spectroscopic imaging (ESI) provide additional information about the chemical composition of the material and are therefore especially helpful for the identification of foreign substances. This review focuses on aselection of deposits and inclusions relevant to diagnostic pathology.

Surface Confined Hydrogenation of Graphene Nanoribbons.

On-surface synthesis with designer precursor molecules is considered an effective method for preparing graphene nanoribbons (GNRs) of well-defined widths and with tunable electronic properties. Recent reports have shown that the band gap of ribbons doped with heteroatoms (such as boron, nitrogen, and sulfur) remains unchanged in magnitude in most cases. Nevertheless, theory predicts that a tunable band gap may be engineered by hydrogenation, but experimental evidence for this is so far lacking. Herein, surface-confined hydrogenation studies of 7-armchair graphene nanoribbons (7-AGNRs) grown on Au(111) surfaces, in an ultrahigh vacuum environment, are reported. GNRs are first prepared, then hydrogenated by exposure to activated hydrogen atoms. High resolution electron energy loss spectroscopy (HREELS) and scanning tunneling microscopy (STM) images reveal a self-limited hydrogenation process. By means of a combination of bond-resolved scanning tunneling microscopy (BRSTM) imaging and tip-induced site-specific dehydrogenation, the hydrogenation mechanism is studied in detail, and density-functional theory (DFT) calculation methods are used to complement the experimental findings. In all cases, the results demonstrate the successful modification of the electronic properties of the GNR/Au(111) system by edge and basal-plane hydrogenation, and a mechanism for the hydrogenation process is proposed.

Unlocking the origin of compositional fluctuations in InGaN light emitting diodes

The accurate determination of the compositional fluctuations is pivotal in understanding their role in the reduction of efficiency in high indium content $In_{x}Ga_{1-x}N$ light-emitting diodes, the origin of which is still poorly understood. Here we have combined electron energy loss spectroscopy (EELS) imaging at sub-nanometer resolution with multiscale computational models to obtain a statistical distribution of the compositional fluctuations in $In_{x}Ga_{1-x}N$ quantum wells (QWs). Employing a multiscale computational model, we show the tendency of intrinsic compositional fluctuation in $In_{x}Ga_{1-x}N$ QWs at different Indium concentration and in the presence of strain. We have developed a systematic formalism based on the autonomous detection of compositional fluctuation in observed and simulated EELS maps. We have shown a direct comparison between the computationally predicted and experimentally observed compositional fluctuations. We have found that although a random alloy model captures the distribution of compositional fluctuations in relatively low In ($\sim$ 18%) content $In_{x}Ga_{1-x}N$ QWs, there exists a striking deviation from the model in higher In content ($\geq$ 24%) QWs. Our results highlight a distinct behavior in carrier localization driven by compositional fluctuations in the low and high In-content InGaN QWs, which would ultimately affect the performance of LEDs. Furthermore, our robust computational and atomic characterization method can be widely applied to study materials in which nanoscale compositional fluctuations play a significant role on the material performance.

Substrate-Selective Morphology of Cesium Iodide Clusters on Graphene.

Formation and characterization of low-dimensional nanostructures is crucial for controlling the properties of two-dimensional (2D) materials such as graphene. Here, we study the structure of low-dimensional adsorbates of cesium iodide (CsI) on free-standing graphene using aberration-corrected transmission electron microscopy at atomic resolution. CsI is deposited onto graphene as charged clusters by electrospray ion-beam deposition. The interaction with the electron beam forms two-dimensional CsI crystals only on bilayer graphene, while CsI clusters consisting of 4, 6, 7, and 8 ions are exclusively observed on single-layer graphene. Chemical characterization by electron energy-loss spectroscopy imaging and precise structural measurements evidence the possible influence of charge transfer on the structure formation of the CsI clusters and layers, leading to different distances of the Cs and I to the graphene.

Quantitative Imaging with Electron Probes of Cultured Erythroblasts Undergoing Erythropoiesis

The process of erythropoiesis whereby erythrocytes (red blood cells, RBCs) are produced in the body involves continuous regeneration of erythroblasts from hematopoietic stem cells, and their subsequent differentiation into enucleated RBCs. There has been much recent interest in ex vivo erythropoiesis as an approach to tissue engineering of blood, which is one of the reasons why we have explored the structural changes that occur in cultured human erythroblasts during their highly orchestrated differentiation. One important aspect of erythropoiesis is the enormous uptake of iron that is needed to fill the RBCs with hemoglobin, which in mammals constitutes 96% of red blood cells' dry content. We examined ex vivo erythroid cultures of primary human CD34+ cells, for accumulation of iron during 4 developmental stages: t1 = 0 days (hematopoietic progenitor cells), t2 = 7 days (erythroblast progenitor cells), t3 = 14 days (erythropoietin-dependent erythroblasts), and t4 = 21 days (nucleated erythroblasts and enucleated erythrocytes). To map and quantify the distribution of iron in differentiating erythroblasts, we initially acquired dark-field STEM images, which revealed bright particles in lysosomal vesicles located in proximity to mitochondria. Then we performed electron energy loss spectroscopic imaging which allowed us to map the distribution of iron atoms per unit area. The iron maps confirmed that the particles in the vesicles were ferritin cores, since on average they contained 2,430 ± 820 Fe atoms. The number of ferritin cores in the lysosomes was least for cells at t1 stage reaching the maximum at t2. Together with the biochemical analysis we found that the concentration of heme increased after t2 reaching the peak at t4, suggesting that the ferritin cores are serving the role of storage depots of Fe(III) used throughout heme production during differentiation.

Spatial and spectral dynamics in STEM hyperspectral imaging using random scan patterns

The evolution of the scanning modules for scanning transmission electron microscopes (STEM) allows now to generate arbitrary scan pathways, an approach currently explored to improve acquisition speed and to reduce electron dose effects. In this work, we present the implementation of a random scan operating mode in STEM achieved at the hardware level via a custom scan control module. A pre-defined pattern with fully shuffled raster order is used to sample the entire region of interest. Subsampled random sparse images can then be extracted at successive time frames, to which suitable image reconstruction techniques can be applied. With respect to the conventional raster scan mode, this method permits to limit dose accumulation effects, but also to decouple the spatial and temporal information in hyperspectral images. We provide some proofs of concept of the flexibility of the random scan operating mode, presenting examples of its applications in different spectro-microscopy contexts: atomically-resolved elemental maps with electron energy loss spectroscopy and nanoscale-cathodoluminescence spectrum images. By employing adapted post-processing tools, it is demonstrated that the method allows to precisely track and correct for sample instabilities and to follow spectral diffusion with a high spatial resolution.

In situ observation of hydride nucleation and selective growth in magnesium thin-films with environmental transmission electron microscopy

The comprehension of solute-induced phase transformations is crucial in a variety of research fields such as catalysis, memory switching or energy storage. We study solute-induced phase transformations on the model magnesium-hydrogen (Mg–H) system which provides high lattice expansion during the phase transformations. In situ precipitation and growth of MgH2 is analyzed in an environmental transmission electron microscope (ETEM), combining electron energy loss spectroscopy (EELS) and various imaging techniques. It is found that the Mg-hydride (MgH2) formation proceeds through the formation of nanocrystals that are separated by low-angle grain boundaries. This change in the microstructure is easily detectable in the ETEM and allows studying the growth of the hydride phase. The EELS results confirm the direct match between the nanocrystalline microstructure and the MgH2 phase. We attribute this microstructural change to large strains and stresses between the matrix and the MgH2 created during the transformation process. Half-spherical as well as finger-like regions of MgH2 is observed in the film. Combing the ETEM studies with finite element method simulations on the local stress distribution in the lamella suggests an influence of local stresses on the growth behavior. As the FEM simulations reveal, the local stress distribution depends on the shape of the hydrided region. This includes stresses that occur after the hydride nucleation and during the growth of the hydride. Hydrogen diffusion is suggested to be fast along the Pd/MgH2 interphase as well as along the high angle grain boundaries and less fast in low angle grain boundaries in MgH2, in contrast to the very slow diffusion in the MgH2 grains. This paper highlights the interdependency of the hydride growth and its self-created local stress fields in an in situ ETEM study. We consider these results not to be limited to the Mg–H system, but being of more general nature.

An electrodeposition approach to metal/metal oxide heterostructures for active hydrogen evolution catalysts in near-neutral electrolytes

Neutral water splitting is attractive for its use of non-corrosive and environmentally friendly electrolytes. However, catalyst development for hydrogen and oxygen evolution remains a challenge under neutral conditions. Here we report a simple electrodeposition and reductive annealing procedure to produce a highly active Ni-Co-Cr metal/metal oxide heterostructured catalyst directly on Ni foam. The resulting electrocatalyst for hydrogen evolution reaction (HER) requires only 198 mV of overpotential to reach 100 mA/cm2 in 1 M potassium phosphate (pH = 7.4) and can operate for at least two days without significant performance decay. Scanning transmission electron microscopy coupled with electron energy loss spectroscopy (STEM-EELS) imaging reveals a Ni-Co alloy core decorated with blended oxides layers of NiO, CoO and Cr2O3. The metal/metal oxide interfaces are suggested to be responsible for the high HER activity.

3D Imaging of Gap Plasmons in Vertically Coupled Nanoparticles by EELS Tomography.

Plasmonic gap modes provide the ultimate confinement of optical fields. Demanding high spatial resolution, the direct imaging of these modes was only recently achieved by electron energy loss spectroscopy (EELS) in a scanning transmission electron microscope (STEM). However, conventional 2D STEM-EELS is only sensitive to components of the photonic local density of states (LDOS) parallel to the electron trajectory. It is thus insensitive to specific gap modes, a restriction that was lifted with the introduction of tomographic 3D EELS imaging. Here, we show that by 3D EELS tomography the gap mode LDOS of a vertically stacked nanotriangle dimer can be fully imaged. Besides probing the complete mode spectrum, we demonstrate that the tomographic approach allows disentangling the signal contributions from the two nanotriangles that superimpose in a single measurement with a fixed electron trajectory. Generally, vertically coupled nanoparticles enable the tailoring of 3D plasmonic fields, and their full characterization will thus aid the development of complex nanophotonic devices.

Reconstruction of Randomly and Partially Sampled STEM Spectrum-Images

Scanning transmission electron microscopy (STEM) offers the ability to acquire several single- and multi-channels signals generated simultaneously for each probe position: cathodoluminescence (CL), electron energy loss spectroscopy (EELS) and high-angle annular dark-field imaging (HAADF). However, in some cases, sensitive samples (e.g., biological samples or molecular structures) may suffer from irradiation damages during acquisition. As a consequence, it is often necessary to reduce probe current or dwell time, which could significantly lower the signal-to-noise ratio. Another approach consists in acquiring only a subset of pixels randomly chosen in a given region of interest. This particular experimental set-up has been implemented on the STEM VG HB501 (LPS, Orsay, France), with the beam following a predetermined random path. The reconstruction of HAADF images is then straightforward with well known algorithms. However, when reconstructing the associated spectrum image, the high number of energy bands makes the problem computationally expensive.

Tomographic imaging of the photonic environment of plasmonic nanoparticles

The photonic local density of states (LDOS) governs the enhancement of light–matter interaction at the nanoscale, but despite its importance for nanophotonics and plasmonics experimental local density of states imaging remains extremely challenging. Here we introduce a tomography scheme based on electron microscopy that allows retrieval of the three-dimensional local density of states of plasmonic nanoparticles with nanometre spatial and sub-eV energy resolution. From conventional electron tomography experiments we obtain the three-dimensional morphology of the nanostructure, and use this information to compute an expansion basis for the photonic environment. The expansion coefficients are obtained through solution of an inverse problem using as input electron-energy loss spectroscopy images. We demonstrate the applicability of our scheme for silver nanocuboids and coupled nanodisks, and resolve local density of states enhancements with extreme sub-wavelength dimensions in hot spots located at roughness features or in gaps of coupled nanoparticles.

Atomic structure and chemistry of a[100] dislocation cores in La2/3Sr1/3MnO3 films

Oxide thin films with perovskite structures possess multifunctional properties, while defects in the films usually have significant influences on their physical properties. Here, the atomic structure and chemistry of a[100] dislocation cores in epitaxial La2/3Sr1/3MnO3 films were investigated by aberration-corrected scanning transmission electron microscopy combining with atomically resolved electron energy-loss spectroscopy imaging. The results demonstrated an edge dislocation terminated with Mn columns and significant nonstoichiometry at the dislocation core region. Quantitative analysis using core-loss spectrum indicates that La/Mn and O/Mn ratios are decreased at the dislocation core. Antisite defects with Mn ions at La-sites were directly determined at the dislocation cores with electron energy-loss spectroscopy. The structure of the dislocation core is discussed on the basis of high-angle annular dark-field imaging and electron energy loss spectroscopy results.

Novel method for measurement of transistor gate length using energy-filtered transmission electron microscopy

As the feature size of devices continues to decrease, transmission electron microscopy (TEM) is becoming indispensable for measuring the critical dimension (CD) of structures. Semiconductors consist primarily of silicon-based materials such as silicon, silicon dioxide, and silicon nitride, and the electrons transmitted through a plan-view TEM sample provide diverse information about various overlapped silicon-based materials. This information is exceedingly complex, which makes it difficult to clarify the boundary to be measured. Therefore, we propose a simple measurement method using energy-filtered TEM (EF-TEM). A precise and effective measurement condition was obtained by determining the maximum value of the integrated area ratio of the electron energy loss spectrum at the boundary to be measured. This method employs an adjustable slit allowing only electrons with a certain energy range to pass. EF-TEM imaging showed a sharp transition at the boundary when the energy-filter’s passband centre was set at 90 eV, with a slit width of 40 eV. This was the optimum condition for the CD measurement of silicon-based materials involving silicon nitride. Electron energy loss spectroscopy (EELS) and EF-TEM images were used to verify this method, which makes it possible to measure the transistor gate length in a dynamic random access memory manufactured using 35 nm process technology. This method can be adapted to measure the CD of other non-silicon-based materials using the EELS area ratio of the boundary materials.

Detection of local chemical states of lithium and their spatial mapping by scanning transmission electron microscopy, electron energy-loss spectroscopy and hyperspectral image analysis.

Advancements in the field of renewable energy resources have led to a growing demand for the analysis of light elements at the nanometer scale. Detection of lithium is one of the key issues to be resolved for providing guiding principles for the synthesis of cathode active materials, and degradation analysis after repeated use of those materials. We have reviewed the different techniques currently used for the characterization of light elements such as high-resolution transmission electron microscopy, scanning transmission electron microscopy (STEM) and electron energy-loss spectroscopy (EELS). In the present study, we have introduced a methodology to detect lithium in solid materials, particularly for cathode active materials used in lithium-ion battery. The chemical states of lithium were isolated and analyzed from the overlapping multiple spectral profiles, using a suite of STEM, EELS and hyperspectral image analysis. The method was successfully applied in the chemical state analyses of hetero-phases near the surface and grain boundary regions of the active material particles formed by chemical reactions between the electrolyte and the active materials.

Extinction and Scattering Properties of High-Order Surface Plasmon Modes in Silver Nanoparticles Probed by Combined Spatially Resolved Electron Energy Loss Spectroscopy and Cathodoluminescence

We investigated the optical properties of localized surface plasmons with different orders in individual silver nanotriangles with different sizes by electron energy loss spectroscopy (EELS) and cathodoluminescence (CL) in the same scanning transmission electron microscope. EELS and CL spectral imaging within the same nanotriangles give information about the extinction and scattering from the nanostructures. As measured from both techniques, the first two order modes showed similar spatial distributions. However, the appearances of slightly different resonant energies were confirmed in both lower and higher order plasmon modes. For the first two lower order modes, the resonant energy in CL was blue-shifted compared to that in EELS, which can be understood in a simple damped oscillator picture. This confirms the experimental and theoretical observations recently made on dipolar plasmons in gold. It also extends it to the second-order modes. The next higher order modes exhibit an apparent inverted shift, wh...

Lithium Transport of Fast Battery Cycles in a LiMn2O4 Cathode Imaged By Operando Eels

Lithium ion batteries are required to be fast-chargeable. However, it has been reported that fast charge causes low reversible capacity and fast degradation of electrode materials [1]. Recently, in-situ observation of lithium ion batteries has been reported that phase transformation behaviour of cathode materials changed depending on charge rate. In-situ XRD studies showed that meta-stable phase appeared during fast charge, but not during slow charge in LiFePO4 cathode [2]. Our previous in-situ TEM studies on LiMn2O4 cathode showed that during fast charge-discharge two-phase reaction continued to the voltage range of solid-solution reaction [3]. Also, it was imaged that Li-excess tetragonal phase appeared at the interface between cathode and electrolyte during early stage of 4 V range [4]. These results suggest the different lithium transport mechanism of fast charge from that of slow charge. However, XRD and TEM methods monitor the structural change, not lithium ion itself. Direct Li imaging method would be very helpful to reveal the lithium transport behaviour during fast charge. Electron energy loss spectroscopy (EELS) is one of most suitable methods to detect lithium ion itself because Li-K edge peak due to the existence of lithium ions can be detected. Furthermore, recent quantitative analysis of EELS can visualize the lithium concentration change simply [5]. In this study, we performed operando EELS mapping for understanding the lithium transport by using our developed nano battery, which consists of LiMn2O4 nanowire cathode, ionic liquid electrolyte and Li4Ti5O12 anode. The nano battery was charged or discharged in TEM during operando EELS observation as loaded in our homemade in-situ TEM sample holder. Charge-discharge was performed by cyclic voltammetry of which scan speed was 0.55mV/s. EELS spectrum was recorded by Gatan Tridiem ER spectrometer (Gatan, Inc.) in aberration corrected TEM, R005 [6] which is equipped with cold field emission gun. The acceleration voltage was 200kV and the convergent semi-angle of electron probe was 23 mrad. The entrance aperture size was 60 mrad and the energy dispersion of 0.2eV/ch was used. The full width half maximum of zero loss peak was 1 eV. The pixel size of EELS images was around 9 x 9 nm2. The electron probe current was less than 5 pA in order to suppress electron beam damage. The dwell time per pixel was 0.2 s and the probe size was adjusted to pixel size. During fast battery cycles, we found that lithium concentration changed with time delay with the cell current change in both of charge and discharge. Furthermore, the lithium concentration was imaged to change from the area far from the cathode/electrolyte interface. The observed lithium concentration change cannot be explained by conventional diffusion model, but the lithium drift model that lithium moves due to electric potential gradient inside cathode caused by high charge rate. In addition, after the first battery cycle lithium concentration changed only slightly around the range of x~1 in LixMn2O4 near the interface. It is attributed that solid electrolyte interface formation at high voltage after charge makes the chemical potential changed locally as an interface effect. We believe that these findings open the new way to understand the phase transformation under non-equilibrium state such fast charge as well as the new strategy to design fast chargeable batteries. [1] S.S. Choi, H.S. Lim, J. Power Sources 2002, 111, 130-136. [2] H. Liu, F. C. Strobridge, O. J. Borkiewicz, K. M. Wiaderek, K. W. Chapman, P. J. Chupas, C. P. Grey, Science 2014, 344, 1252817 [3] S. Lee, Y. Oshima, E. Hosono, H. Zhou, K. Kim, H. M. Chang, R. Kanno, K. Takayanagi, ACS Nano, 2015, 9, 626 [4] S. Lee, Y. Oshima, E. Hosono, H. Zhou, K. Kim, H. M. Chang, R. Kanno, K. Takayanagi, J. Phys. Chem. C 2013, 117, 24236 [5] N. Taguchi, T. Akita, H. Sakaebe, K. Tatsumi, Z. Ogumi, J. Electrochem. Soc. 2013, 160, A2293 [6] H. Sawada, Y. Tanishiro, N. Ohashi, T. Tomita, F. Hosokawa, T. Kaneyama, Y. Kondo, K. Takayanagi, J. Electron. Microsc. 2009, 58, 357

Synthesis and characterization of composite membranes made of graphene and polymers of intrinsic microporosity

Polymers of intrinsic microporosity (PIMs) are a group of polymers with molecular sieve behaviour due to their rigid, contorted macromolecular backbones. They show great potential in organophilic pervaporation, solvent-resistant nanofiltration and gas and vapour separations. However, they are susceptible to physical ageing, leading to a reduction in permeability over time. An improvement in membrane permeability, control over diffusion selectivity and a reduction of the effect of physical ageing is expected by adding graphene as a nanofiller.Little is experimentally known about how the material disperses in the polymer. Here we used Raman spectroscopy, scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS) to study the composite membrane's structure. Our results show that both STEM and Raman spectroscopy are able to identify the presence of graphene-based material in the composite. We show that STEM, through medium angle annular dark field (MAADF) or EELS imaging, can be exploited to obtain information on the morphology and the thickness of the flakes. Our results indicate that there is strong re-agglomeration of initially exfoliated graphene in solution when forming the composite. This is expected to produce strong changes in the mechanical properties and the physical ageing of the membrane.