Wei-Tse Chang

Method of electrochemical etching of tungsten tips with controllable profiles

We demonstrate a new and simple process to fabricate tungsten tips with good control of the tip profile. In this process, we use a commercial function generator without any electronic cutoff circuit or complex mechanical setup. The tip length can be varied from 160 μm to 10 mm, corresponding to an aspect ratio of 1.6-100. The radius of curvature of the tip apex can be controlled to a size <10 nm. Surface roughness and the taper angle can be controlled independently. Through control of the etching parameters, the tip length, the radius of curvature, surface roughness, and the taper angle can be controlled to suit different requirements of various applications. The possible etching mechanisms are also discussed.

Biprism electron interferometry with a single atom tip source

Experiments with electron or ion matter waves require a coherent, monochromatic and long-term stable source with high brightness. These requirements are best fulfilled by single atom tip (SAT) field emitters. The performance of an iridium covered W(111) SAT is demonstrated and analyzed for electrons in a biprism interferometer. Furthermore we characterize the emission of the SAT in a separate field electron and field ion microscope and compare it with other emitter types. A new method is presented to fabricate the electrostatic charged biprism wire that separates and combines the matter wave. In contrast to other biprism interferometers the source and the biprism size are well defined within a few nanometers. The setup has direct applications in ion interferometry and Aharonov-Bohm physics.

Correction of dephasing oscillations in matter-wave interferometry

Vibrations, electromagnetic oscillations, and temperature drifts are among the main reasons for dephasing in matter-wave interferometry. Sophisticated interferometry experiments, e.g., with ions or heavy molecules, often require integration times of several minutes due to the low source intensity or the high velocity selection. Here we present a scheme to suppress the influence of such dephasing mechanisms—especially in the low-frequency regime—by analyzing temporal and spatial particle correlations available in modern detectors. Such correlations can reveal interference properties that would otherwise be washed out due to dephasing by external oscillating signals. The method is shown experimentally in a biprism electron interferometer where a perturbing oscillation is artificially introduced by a periodically varying magnetic field. We provide a full theoretical description of the particle correlations where the perturbing frequency and amplitude can be revealed from the disturbed interferogram. The original spatial fringe pattern without the perturbation can thereby be restored. The technique can be applied to lower the general noise requirements in matter-wave interferometers. It allows for the optimization of electromagnetic shielding and decreases the efforts for vibrational or temperature stabilization.

Three-dimensional surface topography of graphene by divergent beam electron diffraction

There are only a handful of scanning techniques that can provide surface topography at nanometre resolution. At the same time, there are no methods that are capable of non-invasive imaging of the three-dimensional surface topography of a thin free-standing crystalline material. Here we propose a new technique—the divergent beam electron diffraction (DBED) and show that it can directly image the inhomogeneity in the atomic positions in a crystal. Such inhomogeneities are directly transformed into the intensity contrast in the first-order diffraction spots of DBED patterns and the intensity contrast linearly depends on the wavelength of the employed probing electrons. Three-dimensional displacement of atoms as small as 1 angstrom can be detected when imaged with low-energy electrons (50–250 eV). The main advantage of DBED is that it allows visualization of the three-dimensional surface topography and strain distribution at the nanometre scale in non-scanning mode, from a single shot diffraction experiment.

Low-kilovolt coherent electron diffractive imaging instrument based on a single-atom electron source

In this work, a transmission-type, low-kilovolt coherent electron diffractive imaging instrument was constructed. It comprised a single-atom field emitter, a triple-element electrostatic lens, a sample holder, and a retractable delay line detector to record the diffraction patterns at different positions behind the sample. It was designed to image materials thinner than 3 nm. The authors analyzed the asymmetric triple-element electrostatic lens for focusing the electron beams and achieved a focused beam spot of 87 nm on the sample plane at the electron energy of 2 kV. High-angle coherent diffraction patterns of a suspended graphene sample corresponding to (0.62 A)−1 were recorded. This work demonstrated the potential of coherent diffractive imaging of thin two-dimensional materials, biological molecules, and nano-objects at a voltage between 1 and 10 kV. The ultimate goal of this instrument is to achieve atomic resolution of these materials with high contrast and little radiation damage.

Coherent properties of a tunable low-energy electron-matter-wave source

A general challenge in various quantum experiments and applications is to develop suitable sources for coherent particles. In particular, recent progress in microscopy, interferometry, metrology, decoherence measurements and chip based applications rely on intensive, tunable, coherent sources for free low energy electron matter waves. In most cases, the electrons get field emitted from a metal nanotip where its radius and geometry towards a counter electrode determines the field distribution and the emission voltage. A higher emission is often connected to faster electrons with smaller de Broglie wavelengths, requiring larger pattern magnification after matter wave diffraction or interferometry. This can be prevented with a well-known setup consisting of two counter electrodes that allow independent setting of the beam intensity and velocity. However, it needs to be tested if the coherent properties of such a source are preserved after the acceleration and deceleration of the electrons. Here, we study the coherence of the beam in a biprism interferometer with a single atom tip electron field emitter if the particle velocity and wavelength varies after emission. With a Wien filter measurement and a contrast correlation analysis we demonstrate that the intensity of the source at a certain particle wavelength can be enhanced up to a factor of 33 without changing the transverse and longitudinal coherence of the electron beam. In addition, the energy width of the single atom tip emitter was measured to be 377 meV, corresponding to a longitudinal coherence length of 82 nm. The design has potential applications in interferometry, microscopy and sensor technology

Low-voltage coherent electron microscopy based on a highly coherent electron source built from a nanoemitter

It has been a general trend to develop low-voltage electron microscopes due to their high imaging contrast of samples and low radiation damage. Atomic-lattice-resolved transmission electron microscopes with voltages as low as 15–40 kV have been demonstrated. However, achieving an atomic resolution at voltages lower than 10 kV is extremely difficult. An alternative approach is a coherent imaging or phase retrieval imaging, which requires a sufficiently coherent source, an adequately small illumination area on the sample, the detection of high-angle diffraction patterns with a sufficient signal-to-noise ratio, and an appropriate theoretical reconstruction algorithm. This study proposes several transmission-type schemes to achieve coherent imaging of thin materials (less than 5 nm thick) with atomic resolution at voltages lower than 10 kV. Experimental schemes of both lens-less and lens-containing designs and preliminary results based on a highly coherent single-atom electron source are presented. The image plate is designed to be retractable to record the transmission patterns at different positions along the beam propagation direction. In addition, the authors proposed reflection-type coherent electron imaging schemes as novel methods for characterizing surface atomic and electronic structures of materials. The ultimate goal is to achieve high-contrast and high-spatial-resolution imaging of thin materials, such as two-dimensional materials, or molecules, such as organic or biological molecules, under low-dose conditions.It has been a general trend to develop low-voltage electron microscopes due to their high imaging contrast of samples and low radiation damage. Atomic-lattice-resolved transmission electron microscopes with voltages as low as 15–40 kV have been demonstrated. However, achieving an atomic resolution at voltages lower than 10 kV is extremely difficult. An alternative approach is a coherent imaging or phase retrieval imaging, which requires a sufficiently coherent source, an adequately small illumination area on the sample, the detection of high-angle diffraction patterns with a sufficient signal-to-noise ratio, and an appropriate theoretical reconstruction algorithm. This study proposes several transmission-type schemes to achieve coherent imaging of thin materials (less than 5 nm thick) with atomic resolution at voltages lower than 10 kV. Experimental schemes of both lens-less and lens-containing designs and preliminary results based on a highly coherent single-atom electron source are presented. The image...

Visualizing the Spatial Distribution of Ripples in Graphene with Low-Energy Electron Diffractive Imaging

Graphene has received much attention owing to its outstanding electrical and mechanical properties. Ripples are an intrinsic feature of graphene sheets. Meyer et al. reported evidence of the intrinsic ripples in the form of broadened diffraction spots in reciprocal space [1]. However, real-space imaging of graphene rippling remains challenging. Such imaging may allow better understanding of origin and dynamics of the rippling. In this work, we demonstrate that graphene ripples can be visualized with an electron point projection microscopic/diffractive imaging instrument. Through numerical simulations, we showed that three-dimensional displacement of carbon atoms as small as 1 Å can be detected with electrons of low-energy (50-500 eV).

A nanoemitter based on a superconducting material

The coherence of an electron beam is crucial for the performance of electron microscopy, coherent diffractive imaging, holography, and many other advanced instrumentation methods that rely on the phase coherence of electron waves. Here we present a reliable method for preparing a niobium nanoemitter, which is thermally and chemically stable. The tip apex is a (100) facet with a lateral dimension of ∼1 nm, surrounded by four (310) facets. Adsorption of one monolayer of noble gas, particularly Xe, onto the nanoemitter greatly enhances the emission current and current stability. This electron source will probably possess both spatial and temporal coherence if the emitter is cooled below the superconducting temperature.

Low-Energy Electron Diffractive Imaging Based on a Single-Atom Electron Source

Imaging of the atomic structures of two-dimensional materials and organic materials is a challenge for current electron microscopes because of low imaging contrast and high radiation damage for highenergy electrons. It has been a general trend to develop electron microscopy of lower energies. Thanks to the progress in aberration-correction techniques, transmission electron microscopes with voltages down to 15-40 kV have recently been demonstrated. However, it becomes very difficult to achieve atomic resolution when the electron energy is reduced below 10 keV. An alternative approach is phase retrieval imaging, which requires a sufficiently coherent source, detection of high-angle diffracted patterns with a sufficient resolution, and a sufficiently small detection area on the sample. There is no need to fabricate high-quality lenses with a large numerical aperture. In this work, we propose several experimental schemes of low-energy electron coherent diffractive imaging (CDI). A great advantage of low energy electrons over high-energy electrons and x-ray is that the cross-sections of interaction with the atomic potentials are very large, so that the diffraction pattern has good signal-to-noise ratios even at high scattering angles and thus high-resolution images can be reconstructed.