Radovan Urban

Low-energy electron point projection microscopy of suspended graphene, the ultimate 'microscope slide'

Point projection microscopy (PPM) is used to image suspended graphene by using low-energy electrons (100–205 eV). Because of the low energies used, the graphene is neither damaged nor contaminated by the electron beam for doses of the order of 107 electrons per nm2. The transparency of graphene is measured to be 74%, equivalent to electron transmission through a sheet twice as thick as the covalent radius of sp2-bonded carbon. Also observed is rippling in the structure of the suspended graphene, with a wavelength of approximately 26 nm. The interference of the electron beam due to diffraction off the edge of a graphene knife edge is observed and is used to calculate a virtual source size of 4.7±0.6 A for the electron emitter. It is demonstrated that graphene can serve as both the anode and the substrate in PPM, thereby avoiding distortions due to strong field gradients around nanoscale objects. Graphene can be used to image objects suspended on the sheet using PPM and, in the future, electron holography.

Tip apex shaping of gas field ion sources.

A procedure to control W(111) tip shape during etching to a single atom is described. It is demonstrated that the base of a single atom tip (SAT) can be shaped in order to alter the final operating voltage and emission opening angle of single atom tips for use as gas field ion sources or electron cold field emission sources. The operating voltages for single atom tips varied between 5 and 17kV during helium ion beam generation. The emission properties of SATs were evaluated by fitting SAT images and measuring the full width at half maximum (FWHM) of the helium ion images. The FWHM is related to the linear opening angle and was evaluated as a function of SAT operating voltage. The results show that a forward focussing effect is observed such that the spot size decreases faster than is expected solely from an acceleration effect, indicating an affect from the tip shape. These results have consequences in designing gas field ion sources where etching is used to prepare the emitter.

Creation and recovery of a W(111) single atom gas field ion source

Tungsten single atom tips have been prepared from a single crystal W(111) oriented wire using the chemical assisted field evaporation and etching method. Etching to a single atom tip occurs through a symmetric structure and leads to a predictable last atom unlike etching with polycrystalline tips. The single atom tip formation procedure is shown in an atom by atom removal process. Rebuilds of single atom tips occur on the same crystalline axis as the original tip such that ion emission emanates along a fixed direction for all tip rebuilds. This preparation method could be utilized and developed to prepare single atom tips for ion source development.

Field ion microscope evaluation of tungsten nanotip shape using He and Ne imaging gases

Field ion microscopy (FIM) using neon imaging gas was used to evaluate a W(111) nanotip shape during a nitrogen assisted etching and evaporation process. Using appropriate etching parameters a narrow ring of atoms centered about the tip axis appears in a helium generated image. Etching of tungsten atoms continues exclusively on the outside of this well-defined ring. By replacing helium imaging gas with neon, normally inaccessible crystal structure of a tip apex is revealed. Comparison of the original W(111) tip (before etching) and partly etched tip shows no atomic changes at the tip apex revealing extraordinarily spatially selective etching properties of the etching process. This observation is an important step towards a detailed understanding of the nitrogen assisted etching and evaporation process and will lead to better control over atomically defined tip shapes.

Gas field ion source current stability for trimer and single atom terminated W(111) tips

Tungsten W(111) oriented trimer-terminated tips as well as single atom tips, fabricated by a gas and field assisted etching and evaporation process, were investigated with a view to scanning ion microscopy and ion beam writing applications. In particular, ion current stability was studied for helium and neon imaging gases. Large ion current fluctuations from individual atomic sites were observed when a trimer-terminated tip was used for the creation of neon ion beam. However, neon ion current was stable when a single atom tip was employed. No such current oscillations were observed for either a trimer or a single atom tip when imaged with helium.

Hydrogen Ion Beams from Nanostructured Gas Field Ion Sources

Hydrogen ion beams have been discussed as useful for scanning ion microscopy because of hydrogen’s low mass and low sputtering rates. Hydrogen ion beams have been reported from various nanotips including pure iridium tips and noble metal covered tungsten tips.[1, 2] However, hydrogen ion beams are known to occur as mixtures of H, H2 + and H3 + depending on the electric field strength.[3] There is some evidence that various tip orientations contribute differently to the ratios of the ions and also that site specific regions also affect the gas species but it has not been clearly determined. Understanding the relationship between tip shape and apex termination with specific hydrogen ion creation is important in order to prepare pure hydrogen ion beams of a single species. This would be beneficial to future applications related to hydrogen ion beam production using gas field ion sources.

Evaluating angular ion current density for atomically defined nanotips.

In this paper we investigate methods to characterize angular current density from atomically defined gas field ion sources. We show that the ion beam emitted from a single apex atom is described by a two-dimensional Gaussian profile. Owing to the Gaussian shape of the beam and the requirement to collect the majority of the ion current, fixed apertures have inhomogeneous illumination. Therefore, angular current density measurements through a fixed aperture record averaged angular current density. This makes comparison of data difficult as averaged angular current density depends on aperture size. For the same reasons, voltage normalization cannot be performed for fixed aperture measurements except for aperture sizes that are infinitely small. Consistent determination of angular current density and voltage normalization, however, can be achieved if the beam diameter as well as total ion current are known. In cases where beam profile cannot be directly imaged with a field ion microscope, the beam profile could be extracted from measurements taken at multiple acceleration voltages and/or with multiple aperture sizes.

Selective production of hydrogen ion species at atomically designed nanotips

Hydrogen scanning ion microscopy systems rely on nanotip gas field ion sources to generate the hydrogen ion beam. The exact structure of the nanotip and the applied electric field are shown to be important. It is demonstrated that hydrogen ion beams are found to occur as mixtures of H+, H2+ and H3+ depending on the electric field strength and the nanotip structure. Various nanotips were prepared, including single atom tips (SATs), trimers and other nano-structured tips to compare the contents of hydrogen ion beams. It was found that single atom tips produce primarily H2+ at low operating voltages, but as the voltage is increased, H3+ dominates. For the trimer case, H2+ becomes a significant species and equals the H3+ current but H3+ can be isolated at higher voltages. For the hexamer tip structure, H2+ almost completely dominates with little H3+ being produced. H+ is only observed in small quantities for all tip structures until a high voltage regime, where apex atom resolution is not observed. Comparisons W SATs and Ir SATs showed similar H3+/H2+ product ratios indicating the nanotip structure plays a key role in the catalytic formation of H3+. Temperature affects are also discussed and operating parameters for single species ion beams are discussed.

Single Atom Gas Field Ion Sources for Scanning Ion Microscopy

This chapter discusses fabrication and experimental evaluation of W(111) single atom tips (SATs) for gas field ion source applications. Firstly, a brief history of field ion microscopy (FIM) will be given since it will be heavily relied on throughout the text. We will discuss ion current generation in FIM and carry that knowledge over to fabricated SATs. Secondly, gas assisted etching and evaporation process will be discussed in detail. It will be shown that nanotip shape, and therefore SAT characteristics, can be controlled and modified to achieve desirable ion beam properties. Lastly, we will evaluate ion beam width as a function of tip voltage and temperature as examples of experimental efforts to better understand gas field ion source performance.

Reactive Gas Ion Beam Generation Using Single Atom W(111) Gas Field Ion Sources

The scanning ion microscopy is gaining momentum as it provides several key advantages over scanning electron microscopy: (i) enhanced depth of focus, (ii) improved surface and element sensitivity, (iii) better lateral resolution, and (iv) nanomachining and milling. It uses different ions to achieve these tasks ranging from inert gases like helium and neon for imaging and ion milling. Other gases such as argon, nitrogen, and oxygen have potential for further sputtering and etching. It is therefore crucial that gas field ion sources provide necessary robustness and stability for range of various gases.