Marian Mankos

Absolute magnetometry at nanometer transverse spatial resolution: Holography of thin cobalt films in a scanning transmission electron microscope

A method for the absolute measurement of magnetization at nanometer spatial resolution in magnetic thin films has been developed. A biprism placed in the illumination system of a scanning transmission electron microscope allows the operation of two distinct holography modes. The absolute mode displays a linear change in phase difference for regions of constant magnetization and thickness and the slope determines the magnitude of magnetization. The differential mode displays a constant value of phase difference in these regions allowing a simple and straightforward determination of domain wall profiles. Micromagnetic structure extracted from identical areas of thin Co films is compared using the new holography modes, differential phase constrast Lorentz microscopy and conventional Fresnel Lorentz microscopy in the same instrument.

Far-out-of-focus electron holography in a dedicated FEG STEM

Abstract Flexible operation of off-axis holography modes is achieved when a biprism is placed in the illumination system of a scanning transmission electron microscope, equipped with a field emission electron source. The separation of the two virtual sources created by the biprism can be varied by changing the voltage applied to the biprism or by simply changing the excitation of the condensor and/or objective lenses. Two distinct modes of holography are used to study the magnetic microstructure in thin magnetic films. In the absolute mode the phase difference changes linearly in regions of constant magnetization and thickness and the slope determines the magnitude of magnetization. In addition, this mode permits the determination of the mean inner potential of a solid of known geometry. In the differential mode the phase difference is constant in regions of constant magnetization, allowing a simple and straightforward determination of domain wall profiles. The contrast obtained in the holography modes is compared to well known contrast in the Fresnel and differential phase contrast modes of Lorentz microscopy. The combination of several scanning transmission electron microscopy based techniques presents a powerful tool for the investigation of magnetic microstructure.

A novel low energy electron microscope for DNA sequencing and surface analysis

Monochromatic, aberration-corrected, dual-beam low energy electron microscopy (MAD-LEEM) is a novel technique that is directed towards imaging nanostructures and surfaces with sub-nanometer resolution. The technique combines a monochromator, a mirror aberration corrector, an energy filter, and dual beam illumination in a single instrument. The monochromator reduces the energy spread of the illuminating electron beam, which significantly improves spectroscopic and spatial resolution. Simulation results predict that the novel aberration corrector design will eliminate the second rank chromatic and third and fifth order spherical aberrations, thereby improving the resolution into the sub-nanometer regime at landing energies as low as one hundred electron-Volts. The energy filter produces a beam that can extract detailed information about the chemical composition and local electronic states of non-periodic objects such as nanoparticles, interfaces, defects, and macromolecules. The dual flood illumination eliminates charging effects that are generated when a conventional LEEM is used to image insulating specimens. A potential application for MAD-LEEM is in DNA sequencing, which requires high resolution to distinguish the individual bases and high speed to reduce the cost. The MAD-LEEM approach images the DNA with low electron impact energies, which provides nucleobase contrast mechanisms without organometallic labels. Furthermore, the micron-size field of view when combined with imaging on the fly provides long read lengths, thereby reducing the demand on assembling the sequence. Experimental results from bulk specimens with immobilized single-base oligonucleotides demonstrate that base specific contrast is available with reflected, photo-emitted, and Auger electrons. Image contrast simulations of model rectangular features mimicking the individual nucleotides in a DNA strand have been developed to translate measurements of contrast on bulk DNA to the detectability of individual DNA bases in a sequence.

Absolute magnetometry of thin cobalt films and Co/Cu multilayer structures at nanometer spatial resolution

The absolute measurement of magnetization in thin magnetic specimens at nanometer spatial resolution has been made possible by implementing two off-axis holography modes in a scanning transmission electron microscope (STEM). The absolute mode of STEM holography displays a linear change in phase difference for regions with constant magnetization and the slope determines the absolute value of magnetization in the specimen. The differential mode of STEM holography displays a constant value of phase difference for regions with constant magnetization, which simplifies the identification of magnetic structures in the specimen and the determination of domain wall profiles. Taking into account the high spatial resolution of a STEM instrument, STEM holography provides a valuable tool for quantitative investigations of magnetic structures at the nanometer level. >

A monochromatic, aberration-corrected, dual-beam low energy electron microscope

The monochromatic, aberration-corrected, dual-beam low energy electron microscope (MAD-LEEM) is a novel instrument aimed at imaging of nanostructures and surfaces at sub-nanometer resolution that includes a monochromator, aberration corrector and dual beam illumination. The monochromator reduces the energy spread of the illuminating electron beam, which significantly improves spectroscopic and spatial resolution. The aberration corrector utilizes an electron mirror with negative aberrations that can be used to compensate the aberrations of the LEEM objective lens for a range of electron energies. Dual flood illumination eliminates charging generated when a conventional LEEM is used to image insulating specimens. MAD-LEEM is designed for the purpose of imaging biological and insulating specimens, which are difficult to image with conventional LEEM, Low-Voltage SEM, and TEM instruments. The MAD-LEEM instrument can also be used as a general purpose LEEM with significantly improved resolution. The low impact energy of the electrons is critical for avoiding beam damage, as high energy electrons with keV kinetic energies used in SEMs and TEMs cause irreversible change to many specimens, in particular biological materials. A potential application for MAD-LEEM is in DNA sequencing, which demands imaging techniques that enable DNA sequencing at high resolution and speed, and at low cost. The key advantages of the MAD-LEEM approach for this application are the low electron impact energies, the long read lengths, and the absence of heavy-atom DNA labeling. Image contrast simulations of the detectability of individual nucleotides in a DNA strand have been developed in order to refine the optics blur and DNA base contrast requirements for this application.

Quantitative micromagnetics: electron holography of magnetic thin films and multilayers

A new method for the absolute measurement of magnetic microstructure at nanometer spatial resolution in magnetic materials has been developed by implementing off-axis electron holography in a scanning transmission electron microscope (STEM). The holography modes permit quantitative measurement of magnetic induction and magnetization, the determination of equimagnetization lines in domains and straightforward determination of domain wall and flux vortex profiles. STEM holography accompanied by the conventional techniques has been applied to the characterisation of thin magnetic films, magnetic multilayer structures and small magnetic particles. This combination of micromagnetic analysis techniques in one instrument provides a valuable tool for the investigation of magnetic microstructure and microscopic structure in magnetic materials with high sensitivity (/spl sim/10/sup -15/ emu) and at nm spatial resolution.

Absolute magnetometry of small particles using electron holography

A new method for the absolute measurement of magnetic microstructure at nanometer spatial resolution in magnetic materials has been developed by implementing off-axis electron holography in a scanning transmission electron microscope. The absolute spatial resolution of a few nanometers and absolute sensitivity to 10/sup -15/ emu are utilized to examine the magnetic structure in small particles.

Nucleotide-Specific Contrast for DNA Sequencing by Electron Spectroscopy.

DNA sequencing by imaging in an electron microscope is an approach that holds promise to deliver long reads with low error rates and without the need for amplification. Earlier work using transmission electron microscopes, which use high electron energies on the order of 100 keV, has shown that low contrast and radiation damage necessitates the use of heavy atom labeling of individual nucleotides, which increases the read error rates. Other prior work using scattering electrons with much lower energy has shown to suppress beam damage on DNA. Here we explore possibilities to increase contrast by employing two methods, X-ray photoelectron and Auger electron spectroscopy. Using bulk DNA samples with monomers of each base, both methods are shown to provide contrast mechanisms that can distinguish individual nucleotides without labels. Both spectroscopic techniques can be readily implemented in a low energy electron microscope, which may enable label-free DNA sequencing by direct imaging.

Novel electron monochromator for high resolution imaging and spectroscopy

A mirror electron monochromator has been developed for reducing the energy spread of commonly used high brightness electron sources from the characteristic range of 0.3–1u2009eV to values below 100u2009meV. The monochromator utilizes mirror optics and thereby exploits the symmetry inherent in reversing the electron trajectory to monochromatize the primary beam with a knife edge instead of a conventional slit. The performance of the key electron-optical components of the monochromator has been simulated. These components include the combination of an electron mirror with a magnetic prism array. In initial testing, the monochromator has demonstrated the ability to reduce the energy spread of a 5u2009keV electron beam that was generated by a Schottky electron emitter from its initial value of 0.65–0.75u2009eV to 180u2009meV with an exit beam current exceeding 1u2009nA and to 100u2009meV with an exit beam current of 50u2009pA. Here, the attainable energy resolution was found to be limited by the noise on the power supply. The monochromator ...

Scanning transmission electron microscopy of thin magnetic films

A new detection system, installed on a scanning transmission microscope, enables operation of the microscope in a variety of modes revealing microscopic and magnetic structure and dynamic magnetic phenomena at high spatial and temporal resolution. Experimental observations of magnetic systems including granular Co/Ag, Co and multilayer Fe/Mo thin films are discussed. >