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We report on the automatic alignment of a transmission electron microscope equipped with an orbital angular momentum sorter using a convolutional neural network. The neural network is able to control all relevant parameters of both the electron-optical setup of the microscope and the external voltage source of the sorter without input from the user. It can compensate for mechanical and optical misalignments of the sorter, in order to optimize its spectral resolution. The alignment is completed over a few frames and can be kept stable by making use of the fast fitting time of the neural network.

Background The implantable loop recorder (ILR) is a subcutaneous device used to diagnose heart rhythm disorders, allowing the detection of any arrhythmic episodes which are related to the onset of patient-reported symptoms. Although the importance of implantable cardiac monitors has been widely recognized, it is also true that due to their subcutaneous nature, they are inclined to detect false-positive episodes which generate several warnings requiring additional checks from the health care personnel. For this reason implanted cardiac devices monitoring has been recently implemented by the use of artificial intelligence (AI), which applies deep learning algorithms for speeding up the telemetry follow-up checks without affecting the accuracy. Considering the increased need of remote monitoring during pandemics, such achievement should reduce the overwork superimposed on healthcare providers, but to date it is not known to what extent this technology confers a quantitative gain. Our main purpose is to demonstrate how AI can enhance remote monitoring of patients with implantable cardiac monitors, reducing false positives and improving the accuracy of information received by cardiologist. Methods we included twenty-six patients who undergoing ILR implantation between May 2019 and April 2022. Mean age was 58 ± 28 years and there was a higher prevalence of male subjects (n = 20; 76,9%). Fifteen patients received ILR implemented by the use of AI, eleven patients received ILR with standard remote monitoring system. We did follow up from April 2022 to September 2022 with dedicated remote monitoring platforms. The main events detected by ILR were atrial fibrillation (AF), pauses > 3 sec, supraventricular and ventricular arrhythmias, patient's symptom. Results among AI-ILR a total of 25 events were recorded and 92% (n= 23) of these were correctly identified, meanwhile 4% (n= 1) event was considered as false positive. AI implementation showed a sensitivity of 93% (95% CI 0,7018 to 0,9966) and specificity of 90% (95% CI 0,5958 to 0,9949) Among the ILR not supported by AI a total of 26 events were registered, in 76,9% (n=20) of these there was a true correspondence between the event reported and the real event, while in 19,1% (n=5) there wasn't correspondence with a sensitivity of 91% (95% CI 0,6461 to 0,9957) and specificity of 64% (95% CI 0,3876 to 0,8366). Based on these data, AI improves the accuracy of diagnosis, reducing the incidence of false positives with an important saving time for health care providers. Conclusions with our analysis we support the hypothesis that AI algorithms can augment diagnostic capabilities and provides an important contribute to clinical decision-making. The implementation of ILR monitoring with AI is still at the beginning, but continuously further investigations will determine the real added value of these algorithm. The wider adoption of AI technology in cardiovascular medicine seems eventually inevitable in the digital world of 21st century healthcare.

Scanning transmission electron microscopy (STEM) has emerged as a uniquely powerful tool for structural and functional imaging of materials on the atomic level. Driven by advances in aberration correction, STEM now allows the routine imaging of structures with single-digit picometre-level precision for localization of atomic units. This Primer focuses on the opportunities emerging at the interface between STEM and machine learning (ML) methods. We review the primary STEM imaging methods, including structural imaging, electron energy loss spectroscopy and its momentum-resolved modalities and 4D-STEM. We discuss the quantification of STEM structural data as a necessary step towards meaningful ML applications and its analysis in terms of the relevant physics and chemistry. We show examples of the opportunities offered by structural STEM imaging in elucidating the chemistry and physics of complex materials and how the latter connect to first-principles and phase-field models to yield consistent interpretation of generative physics. We present the critical infrastructural needs for the broad adoption of ML methods in the STEM community, including the storage of data and metadata to allow the reproduction of experiments. Finally, we discuss the application of ML to automating experiments and novel scanning modes.

The orbital angular momentum (OAM) sorter is an electron optical device for the measurement of an electron's OAM. It is based on two phase elements, which are referred to as an ''unwrapper'' and a ''corrector'' and are located in Fourier conjugate planes. The simplest implementation of the sorter is based on electrostatic phase elements, such as a charged needle for the unwrapper and electrodes with alternating charges or potentials for the corrector. Here, we use a formal analogy between phase shifts introduced by charges and vertical currents to propose alternative designs for the sorter elements, which are based on phase shifts introduced by magnetic fields. We use this concept to provide a general guide for phase element design, which promises to provide improved reliability of phase control in electron optics.

Spatiotemporal electron-beam shaping is a bold frontier of electron microscopy. Over the past decade, shaping methods evolved from static phase plates to low-speed electrostatic and magnetostatic displays. Recently, a swift change of paradigm utilizing light to control free electrons has emerged. Here, we experimentally demonstrate arbitrary transverse modulation of electron beams without complicated electron-optics elements or material nanostructures, but rather using shaped light beams. On-demand spatial modulation of electron wavepackets is obtained via inelastic interaction with transversely shaped ultrafast light fields controlled by an external spatial light modulator. We illustrate this method for the cases of Hermite-Gaussian and Laguerre-Gaussian modulation and discuss their use in enhancing microscope sensitivity. Our approach dramatically widens the range of patterns that can be imprinted on the electron profile and greatly facilitates tailored electron-beam shaping.

The key to optimizing spatial resolution in a state-of-the-art scanning transmission electron microscope is the ability to precisely measure and correct for electron optical aberrations of the probe-forming lenses. Several diagnostic methods for aberration measurement and correction with maximum precision and accuracy have been proposed, albeit often at the cost of relatively long acquisition times. Here, we illustrate how artificial intelligence can be used to provide near-real-time diagnosis of aberrations from individual Ronchigrams. The demonstrated speed of aberration measurement is important as microscope conditions can change rapidly, as well as for the operation of MEMS-based hardware correction elements that have less intrinsic stability than conventional electromagnetic lenses., Comment: 18 pages, 5 figures, 1 table

Beam shaping - the ability to engineer the phase and the amplitude of massive and massless particles - has long interested scientists working on communication, imaging and the foundations of quantum mechanics. In light optics, the shaping of electromagnetic waves (photons) can be achieved using techniques that include, but are not limited to, direct manipulation of the beam source (as in X-ray Free Electron Lasers (XFELs) and Synchrotrons), deformable mirrors, spatial light modulators, mode converters and holograms. The recent introduction of holographic masks for electrons provides new possibilities for electron beam shaping. Their fabrication has been made possible by advances in micrometric and nanometric device production using lithography and focused ion beam patterning. This article provides a tutorial on the generation, production and analysis of synthetic holograms for transmission electron microscopy. It begins with an introduction to synthetic holograms, outlining why they are useful for beam shaping to study material properties. It then focuses on the fabrication of the required devices from theoretical and experimental perspectives, with examples taken from both simulations and experimental results. Applications of synthetic electron holograms as aberration correctors, electron vortex generators and spatial mode sorters are then presented.

Transmission electron microscopes (TEM) achieve high resolution imaging by raster scanning a focused beam of electrons over the sample and measuring the transmission to form an image. While a TEM can achieve a much higher resolution than optical microscopes, they face challenges of damage to samples during the high energy processes involved. Here, we explore the possibility of applying computational ghost imaging techniques adapted from the optical regime to reduce the total, required illumination intensity. The technological lack of the equivalent high-resolution, optical spatial light modulator for electrons means that a different approach needs to be pursued. Using the optical equivalent, we show that a simple six-needle charged device to modulate the illuminating beam, alongside a novel reconstruction method to handle the resulting highly non-orthogonal patterns, is capable of producing images comparable in quality to a raster-scanned approach with much lower peak intensity.

In this paper we prove that any conformal transformation of a wave can be produced via a suitably arranged cascade of two, or at most four, discrete phase elements satisfying Laplace’s equation. Although this result is of general applicability, in the case of charged-matter waves it implies that such transformations can be exactly obtained by employing only electrostatic or magnetostatic phase elements. Furthermore, we illustrate how a basis for such generating phase elements is given by integer and fractional charge multipoles, proving that these transformations can be used to perform the efficient sorting of multipole-induced quantum states. This provides a fast, compact, and direct method to measure the strength and orientation of dipole systems and of astigmatism. It thus adds a further observable to the four whose spectrum can already be directly measured via spatial separation on the detector, i.e., position, momentum, energy, and orbital angular momentum. The results hold true in optics and for all kinds of charged-particle beams of sufficient coherence.

•A deep convolutional neural network is used to evaluate and correct the alignment of an orbital angular momentum sorter.•The convolutional neural network automatically analyses OAM spectra at a rate of 50 ms/spectrum, allowing for real time implementation.•The method is easily transferable to any programmable beam shaping technique. A convolutional neural network is used to align an orbital angular momentum sorter in a transmission electron microscope. The method is demonstrated using simulations and experiments. As a result of its accuracy and speed, it offers the possibility of real-time tuning of other electron optical devices and electron beam shaping configurations.

Electron energy loss spectroscopy is consolidating as a powerful tool to explore electronic (as well as vibrational) excitations of matter, including molecules. Performed in a scanning transmission electron microscope, this technique is based on inelastic scattering of fast electrons in a thin specimen. Very recently, new electron optics configuration have been introduced, opening the way to the analysis of the single components of orbital angular momentum of the outcoming electrons, that convey additional information on the spatial features of the investigated excitations: innovative double-dispersed spectroscopic experiments for metallic nanostructures have been therefore suggested. We propose here to extend this technology to probe molecular and supra-molecular systems, devising new kind of experiments: using state of the art quantum chemical methods to describe the molecular system in presence of an electron beam in a configuration that avoid molecular damage, we show that scattered electrons acquire the different azimuthal components of induced molecular transition potentials. Numerical simulations performed for systems of increasing size, point out that the conceived new technique can open up the possibility of probing the multipolar components and even the chirality of molecular transitions, superseding the usual optical spectroscopies for those cases that are problematic, such as dipole-forbidden transitions, at a very high spatial resolution., 23 pages, 5 figures

The control of structured waves has recently opened innovative scenarios in the perspective of radiation propagation, advanced imaging, and light-matter interaction. In information and communication technology, the spatial degrees of freedom offer a wider state space to carry many channels on the same frequency or increase the dimensionality of quantum protocols. However, spatial decomposition is much more arduous than polarization or frequency multiplexing, and very few practical examples exist. Among all, beams carrying orbital angular momentum gained a preeminent role, igniting a variety of methods and techniques to generate, tailor, and measure that property. In a more general insight into structured-phase beams, we introduce here a new family of wave fields having a multipole phase. These beams are devoid of phase singularities and described by two continuous spatial parameters which can be controlled in a practical and compact way via conformal optics. The outlined framework encompasses multiplexing, propagation, and demultiplexing as a whole for the first time, describing the evolution and transformation of wave fields in terms of conformal mappings. With its potentialities, versatility, and ease of implementation, this new paradigm introduces a novel playground for space division multiplexing, suggesting unconventional solutions for light processing and free-space communications.

Artificial nano-heterostructures (NHs) with controlled morphology, obtained by combining two or more components in several possible architectures, make them suitable for a wide range of applications. Here, we propose an oleate-based solvothermal approach to design silver-spinel ferrite flower-like NHs. Small oleate-coated silver nanoparticles were used as seeds for the growth of magnetic spinel ferrite (cobalt ferrite and spinel iron oxide) nanodomains on their surface. With the aim of producing homogeneous flower-like heterostructures, a careful study of the effect of the concentration of precursors, the reaction temperature, the presence of water, and the chemical nature of the spinel ferrite was carried out. The magnetic and optical properties of the NHs were also investigated. A heterogeneous growth of the spinel ferrite phase on the silver nanoparticles, through a possible defect-assisted mechanism, was suggested in the light of the high concentration of stacking faults (intrinsic and twins) in the silver seeds, revealed by Rietveld refinement of powder X-ray diffraction patterns and High-Resolution electron microscopy.

The orbital angular momentum (OAM) sorter is a new electron optical device for measuring an electron s OAM. It is based on two phase elements, which are referred to as the unwrapper and corrector and are placed in Fourier conjugate planes in an electron microscope. The most convenient implementation of this concept is based on the use of electrostatic phase elements, such as a charged needle as the unwrapper and a set of electrodes with alternating charges as the corrector. Here, we use simulations to assess the role of imperfections in such a device, in comparison to an ideal sorter. We show that the finite length of the needle and the boundary conditions introduce astigmatism, which leads to detrimental cross-talk in the OAM spectrum. We demonstrate that an improved setup comprising three charged needles can be used to compensate for this aberration, allowing measurements with a level of cross-talk in the OAM spectrum that is comparable to the ideal case.

In this paper, we explore the properties of dynamical diffraction coefficients in orbital angular momentum resolved electron energy-loss magnetic chiral dichroism spectra, in a scanning transmission electron microscopy setup. We demonstrate that for basic zone axis geometries with fourfold or threefold symmetry the coefficients are constrained to have simplified forms. By exploiting these properties, we show how a dichroism spectrum accessible using this technique is only weakly dependent on sample thickness and, more generally, on dynamical diffraction effects. Our results indicate that in such cases it is possible to determine the orbital and spin components of atomic magnetic moments approximately from experimental spectra without the need for additional dynamical diffraction calculations.

How the Observation in a New Custom Basis Based on Orbital Angular Momentum Space Could Improve Our Dose Effective Reconstruction of the Protein Structure. - Filippo Troiani, Enzo Rotunno, Stefano Frabboni, Raimond Ravelli, Peter Peters, Ebrahim Karimi, Vincenzo Grillo

We consider the problem of discriminating macromolecular structures in an electron microscope, through a specific beam shaping technique. Our approach is based on maximizing the which-molecule information extracted from the state of each electron. To this aim, the optimal observables are derived within the framework of quantum state discrimination, which allows one to fully account from the quantum character of the probe. We simulate the implementation of such optimal observable on a generalized orbital angular momentum (OAM) sorter and benchmark its performance against the best known real space approach., Comment: 6 pages, 3 figures and 1 table

Electron energy-loss spectroscopy (EELS) is a powerful technique for measuring the energy state of electrons after scattering. It has had incredible success in measuring the atomic and chemical properties of materials, allowing chemical maps to be recorded with atomic spatial resolution. Energy information alone is often sufficient to obtain information about chemical species, plasmons or even phonon excitations, especially when using high-energy-resolution spectrometers and gun monochromators. However, it is sometimes desirable to add a second dispersion variable, either in the form of momentum and energy double-dispersion EELS, or by acquiring a spectrum for a given momentum transfer.

Electrons have played a significant role in the development of many fields of physics during the last century. The interest surrounding them mostly involved their wave-like features prescribed by the quantum theory. In particular, these features correctly predict the behaviour of electrons in various physical systems including atoms, molecules, solid-state materials, and even in free space. Ten years ago, new breakthroughs were made, arising from the new ability to bestow orbital angular momentum (OAM) to the wave function of electrons. This quantity, in conjunction with the electron's charge, results in an additional magnetic property. Owing to these features, OAM-carrying, or twisted, electrons can effectively interact with magnetic fields in unprecedented ways and have motivated materials scientists to find new methods for generating twisted electrons and measuring their OAM content. Here, we provide an overview of such techniques along with an introduction to the exciting dynamics of twisted electrons.

Colloidal magneto-plasmonic nanostructures are multifunctional materials with huge potential for applications in magnetism, optoelectronics, biomedicine and catalysis. Currently it is considered that their optical and magnetic properties are a combination of the modified properties associated with the material constituents. Herein we have investigated the morphological, magnetic and plasmonic properties of Au@magnetite core@shell heterostructured nanocrystals (HNCs) with eccentric topology. We shed light on their behavior as heat mediators for magnetic fluid hyperthermia, a promising approach to cancer therapy. A red-shift and damping of the plasmon resonance was observed, which correlated with the optical contribution and the dielectric screening of the asymmetrically distributed iron oxide shell. The magnetic properties of the Au@magnetite HNCs were investigated by comparison with those of the corresponding carved magnetite nanocrystals (NCs), obtained by selective etching of the Au domain with iodine. The iron oxide NCs featured higher magnetization and coercive field than their parent HNCs, which showed a superparamagnetic behavior instead. In addition, the carved NCs exhibited better hyperthermia performances than the HNCs, being the Specific Absorption Rate (SAR) of heat one order of magnitude higher. On the basis of the peculiar magnetic properties of the HNCs, we hypothesized that a minority wu¨stite phase was stabilized at the Au/iron-oxide interface, which could be eliminated upon oxidation to magnetite during the Au etching process. Our study opens a new scenario in the understanding of the physico-chemical behavior of this class of magneto-plasmonic heterostructures, whereby the asymmetric spatial distribution of the component materials, their complex multiphase composition and heterointerface structure determine their ultimate plasmonic, magnetic and hyperthermia properties.

The theoretical possibility to use an electron microscope as a spin polarizer is studied. It turns out that a Bessel beam passing a standard magnetic objective lens is intrinsically spin polarized when post-selected on-axis. In the limit of infinitely small detectors, the spin polarisation tends to 100%. Increasing the detector size, the polarisation decreases rapidly, dropping below 10-4 for standard settings of medium voltage microscopes. For extremely low voltages, the Figure of Merit increases by two orders of magnitude, approaching that of existing Mott detectors. Our findings may lead to new desings of spin filters, an attractive option in view of its inherent combination with the electron microscope, especially at low voltage.

We propose a highly efficient atomically-resolved mode of electron magnetic chiral dichroism. This method exploits the recently introduced orbital angular momentum spectrometer to analyze the inelastically scattered electrons allowing for simultaneous dispersion in both energy and angular momentum. The technique offers several advantages over previous formulations of electron magnetic chiral dichroism as it requires much simpler experimental conditions in terms of specimen orientation and thickness. A novel simulation algorithm, based on the multislice description of the beam propagation, is used to anticipate the advantages of the new approach over current electron magnetic chiral dichroism implementations. Numerical calculations confirm an increased magnetic signal to noise ratio with in plane atomic resolution., 14 pages, 5 figures

Developing convenient and reliable synthetic methodologies for solution processable 2D layered ultrathin nanostructures with lateral size control is one of the major challenges for practical applications. In this study, a rational understanding a long-chain amphiphilic surfactant assisted non-hydrolytic synthesis that is able to generate dimension-controllable 2D-WS

Edge dislocations are frequently found in epitaxial BiFeO3 multiferroic thin films and are expected to exhibit distinctive and localized magnetoelectric properties. However, an exhaustive characterization of these dislocations at the atomic level has to date been largely overlooked. Here, we use a combination of scanning transmission electron microscopy techniques, atomistic simulations obtained from classical molecular dynamics calculations, and real-space multiple-scattering theory to explore the chemical properties and the bonding characteristics of the atoms located at and near the dislocation cores. We find that in addition to Bi, small amounts of Fe atoms are present in the BiFeO3 dislocation cores which result in uncompensated Fe spins along the dislocations and give rise to a magnetic signal. Our results suggest that edge dislocations in BiFeO3 films could be efficiently used for realizing BiFeO3-based magnetic devices.

In this paper the influence of 2s Bloch wave states on high angle annular dark field image contrast is studied quantitatively by Bloch wave and multislice simulations. We show that 2s states are excited beyond a critical Z value and cause a long period oscillation of the electron probe wave function that significantly influences the Z contrast. As a result, we find that the Z contrast for high Z alloys remains thickness dependent up to high sample thicknesses and this effect has to be considered for compositional analysis. In the special case of ordered alloys, the impact of 2s excitations on intensity provides a way to quantify long-range order phenomena in alloys with known composition.

Vortex-carrying matter waves, such as chiral electron beams, are of significant interest in both applied and fundamental science. Continuous wave electron vortex beams are commonly prepared via passive phase masks imprinting a transverse phase modulation on the electron's wave function. Here, we show that femtosecond chiral plasmonic near fields enable the generation and dynamic control on the ultrafast timescale of an electron vortex beam. The vortex structure of the resulting electron wavepacket is probed in both real and reciprocal space using ultrafast transmission electron microscopy. This method offers a high degree of scalability to small length scales and a highly efficient manipulation of the electron vorticity with attosecond precision. Besides the direct implications in the investigation of nanoscale ultrafast processes in which chirality plays a major role, we further discuss the perspectives of using this technique to shape the wave function of charged composite particles, such as protons, and how it can be used to probe their internal structure., 14 pages, 4 figures

Recently, a new device to measure the Orbital Angular Momentum (OAM) electronic spectrum after elastic/inelastic scattering in a transmission electron microscope has been introduced. We modified the theoretical framework needed to describe conventional low loss electron energy loss spectroscopy (EELS) experiments in transmission electron microscopes (TEM) to study surface plasmons in metallic nanostructures, to allow for an OAM post selection and devise new experiments for the analysis of these excitations in nanostructures. We found that unprecedented information on the symmetries and on the chirality of the plasmonic modes can be retrieved even with limited OAM and energy resolutions.

Two-dimensional transition-metal dichalcolgenides (2D-TMDs) are among the most intriguing materials for next-generation electronic and optoelectronic devices. Albeit still at the embryonic stage, building thin films by manipulating and stacking preformed 2D nanosheets is now emerging as a practical and cost-effective bottom-up paradigm to obtain excellent electrical properties over large areas. Herein, we exploit the ultrathin morphology and outstanding solution stability of 2D WS2 colloidal nanocrystals to make thin films of TMDs assembled on a millimetre scale by a layer-by-layer deposition approach. We found that a room-temperature surface treatment with a superacid, performed with the precise scope of removing the native insulating surfactants, promotes in-plane assembly of the colloidal WS2 nanoflakes into stacks parallel to the substrate, along with healing of sulphur vacancies in the lattice that are detrimental to electrical conductivity. The as-obtained 2D WS2 thin films, characterized by a smooth and compact morphology, feature a high planar conductivity of up to 1 μS, comparable to the values reported for epitaxially grown WS2 monolayers, and enable photocurrent generation upon light irradiation over a wide range of visible to near-infrared frequencies.

International audience; The origin of deflections of semiconductor nanowires (NWs) induced by an electron beam in scanning electron microscopy has been subject to different interpretations. Similarly, the subsequent clumping together of NWs into bundles is frequently observed, but no interpretation has yet been advanced. Here we present results on the bundling of NWs following the intentional bending by an electron beam. Furthermore, we extend the concept of lateral collapse, usually applied to fibrillar architectures mimicking the adhesiveness of natural surfaces (the so-called Gecko effect), to analyze the mechanism of the NW bundle formation. We demonstrate how the geometry of the NW arrays and the mechanical properties of the composing materials govern bundling and how these parameters should be taken into account in the design of NW arrays both for avoiding vertical misalignment when detrimental and for achieving patterning of NW arrays into nanoarchitectures.

The propagation of an electron beam in the presence of transverse magnetic fields possessing integer topological charges is presented. The spin-magnetic interaction introduces a nonuniform spin precession of the electrons that gains a space-variant geometrical phase in the transverse plane proportional to the field's topological charge, whose handedness depends on the input electron's spin state. A combination of our proposed device with an electron orbital angular momentum sorter can be utilized as a spin-filter of electron beams in a mid-energy range. We examine these two different configurations of a partial spin-filter generator numerically. The results of this analysis could prove useful in the design of an improved electron microscope. © 2013 Elsevier B.V.

Matter wave interferometry is becoming an increasingly important technique in quantum metrology. However, unlike its photonic counterpart, this technique relies on the interference of particles possessing a non-zero rest mass and an electric charge. Matter waves, thus, can experience alterations in their wave-like features while propagating through uniform fields to which a linear potential can be attributed. Here, we derive analytical expressions for structured matter waves subjected to linear potentials. We show that the center of mass of corresponding to these wavefunctions follows the classical parabolic trajectory attributed to this potential and also provide the additional phase profile acquired by the wave upon propagation. Furthermore, we find that these features are identical for any structured wave, thus significantly simplifying the action of quantum effects pertaining to this potential in applications relying on structured quantum waves., 11 pages, 6 figures

Vortices in electron beams can manifest several types of topological phenomena, such as the formation of exotic structures or interactions with topologically structured electromagnetic fields. For instance, the wavefunction of an electron beam can acquire a phase vortex upon propagating through a magnetic monopole, which, in practice, provides a convenient method for generating electron vortex beams. Here, we show how an electric field must be structured in order to achieve a similar effect. We find that, much as in the case of magnetic fields, closed but not exact electric fields can produce electron vortex beams. We proceed by fabricating a versatile near-obstruction-free device that is designed to approximately produce such fields and we systematically study their influence on incoming electron beams. With such a single device, electron vortex beams that are defined by a wide range of topological charges can be produced by means of a slight variation of an applied voltage. For this reason, this device is expected to be important in applications that rely on the sequential generation and manipulation of different types of electron vortices., Comment: 5 pages, 3 figures!

The cathodoluminescence (CL) depth profiling of the optical emissions from (0001) GaN/GaNbuffer/Al2O3 heterostructures has been performed at 77