Low-energy Electron Point Source Microscope Research Articles

Mapping unoccupied electronic states of freestanding graphene by angle-resolved low-energy electron transmission

We report angle-resolved electron transmission measurements through freestanding graphene sheets in the energy range of 18 to 30 eV above the Fermi level. The measurements are carried out in a low-energy electron point source microscope, which allows simultaneously probing the transmission for a large angular range. The characteristics of low-energy electron transmission through graphene depend on its electronic structure above the vacuum level. The experimental technique described here allows mapping the unoccupied band structure of freestanding two-dimensional materials as a function of energy and probing angle, respectively in-plane momentum. Our experimental findings are consistent with theoretical predictions of a resonance in the band structure of graphene above the vacuum level [V. U. Nazarov, E. E. Krasovskii, and V. M. Silkin, Physical Review B 87, 041405 (2013)].

Nanoscale structuring of tungsten tip yields most coherent electron point-source

This report demonstrates the most spatially-coherent electron source ever reported. A coherence angle of 14.3 ± 0.5° was measured, indicating a virtual source size of 1.7 ± 0.6 Å using an extraction voltage of 89.5 V. The nanotips under study were crafted using a spatially-confined, field-assisted nitrogen etch which removes material from the periphery of the tip apex resulting in a sharp, tungsten–nitride stabilized, high-aspect ratio source. The coherence properties are deduced from holographic measurements in a low-energy electron point source microscope with a carbon nanotube bundle as sample. Using the virtual source size and emission current the brightness normalized to 100 kV is found to be 7.9 × 108 A sr−1 cm2.

On the difficulties in characterizing ZnO nanowires

The electrical properties of single ZnO nanowires grown by vapor phase transportwere investigated. While some samples were contacted by Ti/Au electrodes,another set of samples was investigated using a manipulator tip in a low energyelectron point-source microscope. The deduced resistivities range from 1 to103 Ωcm.Additionally, the resistivities of nanowires from multiple publicationswere brought together and compared to the values obtained from ourmeasurements. The overview of all data shows enormous differences(10−3–105 Ωcm) in the measured resistivities. In order to reveal the origin of the discrepancies, theinfluence of growth parameters, measuring methods, contact resistances, crystal structuresand ambient conditions are investigated and discussed in detail.

Preparation of Sub‐micrometer Copper Fibers via Electrospinning

Sub-micrometer length copper fibers are obtained via polymer supported electrospinning and subsequent thermal treatment. The copper nature of the fibers is confirmed by using energy- dispersive X-ray analysis and wide- angle X-ray analysis (see figure). The copper fibers show metallic conductivity as confirmed by conductance measurements in a low-energy electron point source microscope.

Observation of MWCNTs with low-energy electron point source microscope

The low-energy electron point source (LEEPS) microscope, which creates enlarged projection images with low-energy field emission electron beams, can be used to observe the projection image of nano-scale samples and to characterize the coherence of the field emission beam. In this paper we report the design and test operation performance of a home-made LEEPS microscope. Multi-walled carbon nanotubes (MWCNTs) synthesized by the CVD method were observed by LEEPS microscope using a conventional tungsten tip, and projection images with the magnification of up to 104 was obtained. The resolution of the acquired images is ~10 nm. A higher resolution and a larger magnification can be expected when the AC magnetic field inside the equipment is shielded and the vibration of the instrument reduced.

Field emission from individual B–C–N nanotube rope

The field-emission characteristics of individual ropes made of B–C–N nanotubes were measured in situ in a low-energy electron point source microscope. The tungsten field emission tip of the microscope was used as a movable electrode, approaching the rope, and acting as an anode during field-emission measurements. The atomic structure and chemical composition of the ropes were analyzed by high-resolution transmission electron microscopy and electron energy-loss spectroscopy. The tubes assembled within the ropes typically revealed open-tip ends, a small number of layers and zigzag chirality. We found that the field-emission properties of the B–C–N nanotube ropes are competitive with conventional C nanotubes, with the expected additional benefit that the B–C–N ropes exhibit higher environmental stability.

Low-energy electron point source microscope as a tool for transport measurements of free-standing nanometer-scale objects: Application to carbon nanotubes

We have developed a simple and reliable technique for two-terminal transport measurements of free-standing wire-like objects. The method is based on the low-energy electron point source microscope. The field emission tip of the microscope is used as a movable electrode to make a well-defined local electrical contact on a controlled place of a nanometer-size object. This allows transport measurements of the object to be conducted. The technique was applied to carbon nanotube ropes.

Low-energy electron point source microscope with position-sensitive electron energy analyzer

A low-energy electron point source microscope equipped with a position-sensitive energy analyzer is constructed. A nanometer-sized feature can be zoomed in and its energy-loss spectrum can be measured with a retarding field-type energy analyzer mounted in front of the imaging screen. The geometric and the electronic structures of carbon nanotubes are measured with the present system. Interference between the scattered and the transmitted electron beams through the carbon nanotubes is observed using an atomically sharp field emitter. The electron energy-loss spectrum shows two prominent peaks at ∼7 and 16–17 eV, which are identified as the π plasmon and (π+σ) surface-plasmon peaks. This result is consistent with the measurements of high-energy electron energy-loss spectroscopy as well as the theoretical calculation.

Electron holography of individual DNA molecules

High-contrast holograms of unstained DNA molecules, deposited on a dedicated sample holder fabricated by silicon micromachining techniques, have been obtained in the low-energy electron point-source microscope. Recording at television rates allows dynamic processes to be observed. Numerical reconstruction of the holograms reveals the structure of the molecules depicted as the magnitude of the electron object wave front.

Carbon nanotubes are coherent electron sources

The observation of electron interference effects provides direct evidence of the coherence of the electrons emitted from a carbon nanotube. To demonstrate this, the low-energy electron point source microscope has been used to mount an individual carbon nanotube onto a tungsten tip. In subsequent experiments, the electrons emitted from the nanotube were used to generate holograms. Comparison with a standard tungsten atomic point source emitter establishes a high degree of coherence for a nanotube emitter.