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1.
Microsc Microanal ; : 1-9, 2022 Sep 09.
Article in English | MEDLINE | ID: mdl-36082682

ABSTRACT

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.

2.
J Synchrotron Radiat ; 28(Pt 5): 1343-1356, 2021 Sep 01.
Article in English | MEDLINE | ID: mdl-34475283

ABSTRACT

Imaging of biomolecules by ionizing radiation, such as electrons, causes radiation damage which introduces structural and compositional changes of the specimen. The total number of high-energy electrons per surface area that can be used for imaging in cryogenic electron microscopy (cryo-EM) is severely restricted due to radiation damage, resulting in low signal-to-noise ratios (SNR). High resolution details are dampened by the transfer function of the microscope and detector, and are the first to be lost as radiation damage alters the individual molecules which are presumed to be identical during averaging. As a consequence, radiation damage puts a limit on the particle size and sample heterogeneity with which electron microscopy (EM) can deal. Since a transmission EM (TEM) image is formed from the scattering process of the electron by the specimen interaction potential, radiation damage is inevitable. However, we can aim to maximize the information transfer for a given dose and increase the SNR by finding alternatives to the conventional phase-contrast cryo-EM techniques. Here some alternative transmission electron microscopy techniques are reviewed, including phase plate, multi-pass transmission electron microscopy, off-axis holography, ptychography and a quantum sorter. Their prospects for providing more or complementary structural information within the limited lifetime of the sample are discussed.


Subject(s)
Cryoelectron Microscopy/methods , Macromolecular Substances/ultrastructure , Electrons , Signal-To-Noise Ratio , Structure-Activity Relationship
3.
Opt Express ; 29(3): 4661-4671, 2021 Feb 01.
Article in English | MEDLINE | ID: mdl-33771037

ABSTRACT

We leverage the high spatial and energy resolution of monochromated aberration-corrected scanning transmission electron microscopy to study the hybridization of cyclic assemblies of plasmonic gold nanorods. Detailed experiments and simulations elucidate the hybridization of the coupled long-axis dipole modes into collective magnetic and electric dipole plasmon resonances. We resolve the magnetic dipole mode in these closed loop oligomers with electron energy loss spectroscopy and confirm the mode assignment with its characteristic spectrum image. The energy splitting of the magnetic mode and antibonding modes increases with the number of polygon edges (n). For the n=3-6 oligomers studied, optical simulations using normal incidence and s-polarized oblique incidence show the respective electric and magnetic modes' extinction efficiencies are maximized in the n=4 arrangement.

4.
Phys Rev Lett ; 126(9): 094802, 2021 Mar 05.
Article in English | MEDLINE | ID: mdl-33750150

ABSTRACT

The component of orbital angular momentum (OAM) in the propagation direction is one of the fundamental quantities of an electron wave function that describes its rotational symmetry and spatial chirality. Here, we demonstrate experimentally an electrostatic sorter that can be used to analyze the OAM states of electron beams in a transmission electron microscope. The device achieves postselection or sorting of OAM states after electron-material interactions, thereby allowing the study of new material properties such as the magnetic states of atoms. The required electron-optical configuration is achieved by using microelectromechanical systems technology and focused ion beam milling to control the electron phase electrostatically with a lateral resolution of 50 nm. An OAM resolution of 1.5ℏ is realized in tests on controlled electron vortex beams, with the perspective of reaching an optimal OAM resolution of 1ℏ in the near future.

5.
J Nanosci Nanotechnol ; 19(7): 4323-4325, 2019 Jul 01.
Article in English | MEDLINE | ID: mdl-30765013

ABSTRACT

Few-layer MoS2 flakes were synthesized on silica nanowires by using a standard chemical vapor deposition (CVD) process using Mo and S powders. The flakes on the silica nanowires were analyzed by transmission electron microscopy (TEM), confirming their few-layer nature, and their chemical composition was confirmed by X-ray microanalysis.

6.
Microsc Microanal ; 29(Supplement_1): 1870-1871, 2023 Jul 22.
Article in English | MEDLINE | ID: mdl-37613959
8.
Nanotechnology ; 26(40): 405704, 2015 Oct 09.
Article in English | MEDLINE | ID: mdl-26376605

ABSTRACT

Films of magnetic Ni@NiO core-shell nanoparticles (NPs, core diameter d ≅ 12 nm, nominal shell thickness variable between 0 and 6.5 nm) obtained with sequential layer deposition were investigated, to gain insight into the relationships between shell thickness/morphology, core-shell interface, and magnetic properties. Different values of NiO shell thickness t(s) could be obtained while keeping the Ni core size fixed, at variance with conventional oxidation procedures where the oxide shell is grown at the expense of the core. Chemical composition, morphology of the as-produced samples and structural features of the Ni/NiO interface were investigated with x-ray photoelectron spectroscopy and microscopy (scanning electron microscopy, transmission electron microscopy) techniques, and related with results from magnetic measurements obtained with a superconducting quantum interference device. The effect of the shell thickness on the magnetic properties could be studied. The exchange bias (EB) field H(bias) is small and almost constant for ts up to 1.6 nm; then it rapidly grows, with no sign of saturation. This behavior is clearly related to the morphology of the top NiO layer, and is mostly due to the thickness dependence of the NiO anisotropy constant. The ability to tune the EB effect by varying the thickness of the last NiO layer represents a step towards the rational design and synthesis of core-shell NPs with desired magnetic properties.

9.
Nano Lett ; 13(12): 5900-6, 2013.
Article in English | MEDLINE | ID: mdl-24224918

ABSTRACT

It is demonstrated that boron-doped nanowires have predominantly long-term stable wurtzite phase while the majority of phosphorus-doped ones present diamond phase. A simplified model based on the different solubility of boron and phosphorus in gold is proposed to explain their diverse effectiveness in retaining the wurtzite phase. The wurtzite nanowires present a direct transition at the Γ point at approximately 1.5 eV while the diamond ones have a predominant emission around 1.1 eV. The aforementioned results are intriguing for innovative solar cell devices.


Subject(s)
Boron/chemistry , Nanowires/chemistry , Solar Energy , Gold/chemistry , Humans , Phosphorus/chemistry , Silicon/chemistry
10.
Nano Lett ; 12(3): 1509-15, 2012 Mar 14.
Article in English | MEDLINE | ID: mdl-22364321

ABSTRACT

The self-assembly of Ge(1)Sb(2)Te(4) nanowires (NWs) for phase change memories application was achieved by metal organic chemical vapor deposition, catalyzed by Au nanoislands in a narrow range of temperatures and deposition pressures. In the optimized conditions of 400 °C, 50 mbar, the NWs are Ge(1)Sb(2)Te(4) single hexagonal crystals. Phase change memory switching was reversibly induced by nanosecond current pulses through metal-contacted NWs with threshold voltage of about 1.35 V.


Subject(s)
Crystallization/methods , Gases/chemistry , Metals/chemistry , Nanostructures/chemistry , Nanostructures/ultrastructure , Organic Chemicals/chemistry , Macromolecular Substances/chemistry , Materials Testing , Molecular Conformation , Particle Size , Phase Transition , Surface Properties
11.
ACS Photonics ; 10(5): 1463-1472, 2023 May 17.
Article in English | MEDLINE | ID: mdl-37215321

ABSTRACT

Single-pixel imaging, originally developed in light optics, facilitates fast three-dimensional sample reconstruction as well as probing with light wavelengths undetectable by conventional multi-pixel detectors. However, the spatial resolution of optics-based single-pixel microscopy is limited by diffraction to hundreds of nanometers. Here, we propose an implementation of single-pixel imaging relying on attainable modifications of currently available ultrafast electron microscopes in which optically modulated electrons are used instead of photons to achieve subnanometer spatially and temporally resolved single-pixel imaging. We simulate electron beam profiles generated by interaction with the optical field produced by an externally programmable spatial light modulator and demonstrate the feasibility of the method by showing that the sample image and its temporal evolution can be reconstructed using realistic imperfect illumination patterns. Electron single-pixel imaging holds strong potential for application in low-dose probing of beam-sensitive biological and molecular samples, including rapid screening during in situ experiments.

12.
Ultramicroscopy ; 245: 113663, 2023 03.
Article in English | MEDLINE | ID: mdl-36566529

ABSTRACT

The key to optimizing spatial resolution in a state-of-the-art scanning transmission electron microscope is the ability to measure and correct for electron optical aberrations of the probe-forming lenses precisely. Several diagnostic methods for aberration measurement and correction 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 because microscope conditions can change rapidly. It is also important for the operation of MEMS-based hardware correction elements, which have less intrinsic stability than conventional electromagnetic lenses.


Subject(s)
Electrons , Lenses , Microscopy, Electron, Scanning Transmission/methods , Artificial Intelligence , Neural Networks, Computer
13.
ACS Photonics ; 9(10): 3215-3224, 2022 Oct 19.
Article in English | MEDLINE | ID: mdl-36281329

ABSTRACT

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.

14.
J Chem Theory Comput ; 17(4): 2364-2373, 2021 Apr 13.
Article in English | MEDLINE | ID: mdl-33646769

ABSTRACT

We devise a new kind of experiment that extends the technology of electron energy loss spectroscopy to probe (supra-)molecular systems: by using an electron beam in a configuration that avoids molecular damage and a very recently introduced electron optics setup for the analysis of the outcoming electrons, one can obtain information on the spatial features of the investigated excitations. Physical insight into the proposed experiment is provided by means of a simple but rigorous model to obtain the transition rate and selection rule. Numerical simulations of DNA G-quadruplexes and other biomolecular systems, based on time dependent density functional theory calculations, point out that the conceived new technique can probe 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.


Subject(s)
DNA/chemistry , Density Functional Theory , Electrons , Electron Transport , G-Quadruplexes , Spectroscopy, Electron Energy-Loss
15.
J Appl Crystallogr ; 53(Pt 3): 629-634, 2020 Jun 01.
Article in English | MEDLINE | ID: mdl-32684878

ABSTRACT

The mechanism of formation of residual strain in crystals with a damaged surface has been studied by transmission electron microscopy in GaAs wafers ground with sandpaper. The samples showed a dislocation network located near the sample surface penetrating to a depth of a few micrometres, comparable to the size of abrasive particles used for the treatment, and no other types of defects were observed. A simple model for the formation of a compressive strain induced by the dislocation network in the damaged layer is proposed, in satisfactory agreement with the measured strain. The strain is generated by the formation of dislocation half-loops at the crystal surface, having the same component of the Burgers vectors parallel to the surface of the crystal. This is equivalent to the insertion of extra half-planes from the crystal surface to the depth of the damaged zone. This model can be generalized for other crystal structures. An approximate calculation of the strain generated from the observed dislocation distribution in the sample agrees with the proposed model and permits the conclusion that this mechanism is in general sufficient to explain the observed compressive strain, without the need to consider other types of defects.

16.
ACS Appl Mater Interfaces ; 7(33): 18794-802, 2015 Aug 26.
Article in English | MEDLINE | ID: mdl-26259045

ABSTRACT

One of the main limitations to the application of gold nanorods (Au NRs) as surface-enhanced Raman scattering (SERS) probes for in situ monitoring of chemical processes is their instability in oxidative environments. Oxidation induces progressive anisotropic shortening of the NRs, which are eventually dissolved once this process has been completed. This paper compares two types of Au NRs, obtained through different routes and characterized by similar aspect ratios but different sizes. The key factors influencing the resistance of Au NRs to oxidation were systematically investigated, showing that the reduction of free bromide species and the increase of the particle size allowed the NRs to maintain their stability under harsh environments for several weeks. The most stable Au NRs were also demonstrated to be highly efficient SERS substrates in a series of Raman experiments involving molecular probes, treated under either oxidizing or nonoxidizing conditions, which simulate the oxidation of organic pollutants in water. These hallmarks make these "stainless" Au NRs attractive tools for ultrasensitive diagnostic under real working conditions.

17.
Sci Rep ; 4: 3603, 2014 Jan 08.
Article in English | MEDLINE | ID: mdl-24398782

ABSTRACT

Silicon, the mainstay semiconductor in microelectronic circuitry, is considered unsuitable for optoelectronic applications owing to its indirect electronic band gap, which limits its efficiency as a light emitter. Here we show the light emission properties of boron-doped wurtzite silicon nanowires measured by cathodoluminescence spectroscopy at room temperature. A visible emission, peaked above 1.5 eV, and a near infra-red emission at 0.8 eV correlate respectively to the direct transition at the Γ point and to the indirect band-gap of wurtzite silicon. We find additional intense emissions due to boron intra-gap states in the short wavelength infra-red range. We present the evolution of the light emission properties as function of the boron doping concentration and the growth temperature.

18.
Ultramicroscopy ; 125: 97-111, 2013 Feb.
Article in English | MEDLINE | ID: mdl-23265085

ABSTRACT

The software STEM_CELL, here presented, is a useful tool for (S) TEM simulation. In particular innovative solutions are presented in (1) the supercell manipulation and parameters setting (2) simulation execution through the modified Kirkland routines (3) simulation post-processing with extended output and comprehensive graphic tools (4) image contrast interpretation through a strain channeling equation accounting for strain effects in STEM-ADF.

19.
Nanoscale ; 5(4): 1557-63, 2013 Feb 21.
Article in English | MEDLINE | ID: mdl-23322217

ABSTRACT

Further improvement of phase change memory devices based on Ge-Sb-Te alloys imposes the reduction of the active cell dimensions to the nanoscale. We investigate the atomic arrangement of Ge(1)Sb(2)Te(4) and Ge(2)Sb(2)Te(5) nanowires. We identify the stacking sequence in each crystal structure by combining the direct observation by High Angle Annular Dark Field imaging and proper simulations. We find out that Ge and Sb atoms randomly share the same lattice sites, although this configuration is considered not stable according to the existing theoretical models elaborated for the bulk material.


Subject(s)
Antimony/chemistry , Crystallization/methods , Germanium/chemistry , Metal Nanoparticles/chemistry , Metal Nanoparticles/ultrastructure , Models, Chemical , Tellurium/chemistry , Computer Simulation , Macromolecular Substances/chemistry , Materials Testing , Models, Molecular , Molecular Conformation , Particle Size , Surface Properties
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