Joel M. Vaughn

Atom-by-atom extraction using the scanning tunneling microscope tip-cluster interaction.

We investigate the atomistic details of a single atom-extraction process realized by using the scanning tunneling microscope tip-cluster interaction on a Ag(111) surface at 6 K. Single atoms are extracted from a silver cluster one atom at a time using small tunneling biases less than 35 mV. Combined total energy calculations and molecular dynamics simulations show a lowering of the atom-extraction barrier upon approaching the tip to the cluster. Thus, a mere tuning of the proximity between the tip and the cluster governs the extraction process. The atomic precision and reproducibility of this procedure are demonstrated by repeatedly extracting single atoms from a silver cluster on an atom-by-atom basis.

Scandium oxide coated polycrystalline tungsten studied using emission microscopy and photoelectron spectroscopy

Thermionic electron emission from 200 to 500 nm thick coatings of scandium oxide on tungsten foil have been examined in thermionic emission microscopy, spectroscopic photoelectron microcopy, synchrotron radiation and ultraviolet photoelectron spectroscopy (UPS). A clear dependence of the scandium oxide-W electron yield on the grain orientation of the polycrystalline tungsten is observed in thermionic emission and photoelectron emission.

In situ emission microscopy of scandium/scandium oxide and barium/barium oxide thin films on tungsten

The effect of a thin Sc/Sc Oxide base layer on Ba/Ba Oxide surface diffusion, adsorption and desorption on W is studied using photoelectron emission microcopy (PEEM) and thermionic emission microscopy (ThEEM).

17.3: Work function reduction by multilayer oxides :Thermionic electron emission microscopy of scandium oxide and barium oxide on tungsten

Many theories [1] of work function reduction on scandate cathodes rely on a monolayer surface dipole layer of Ba-Sc-O-W. The charge transfer between the surface atoms, in particular the electron transfer from the barium ion, is the primary agent of work function reduction. We have examined thick (200 nm) layers of reactively sputtered Sc2O3 and BaO on tungsten (W) foil. Thermionic electron emission microscopy (ThEEM) was used to image the surfaces during electron emission. Current vs. brightness temperature data were used to estimate the work function.

Model scandate cathodes investigated by thermionic emission microscopy

Model scandate thermionic cathodes have been prepared by rf reactive sputtering of scandium oxide and barium oxide onto tungsten foil. By preparing separate, 200 nm thick layers of these oxides, sputtered through different sized masks, the function of barium oxide and scandium oxide can be identified directly in the thermionic electron emission image. Scandium oxide is sputtered through a 100 micron square mask, with barium oxide sputtered through a 25 micron square mask. The location and deposition order of the oxide layers can be varied and observed directly in the image. Emission current is measured by a Faraday cup located at the center of the detector used to form the thermionic emission image. Based on these images, we show that the scandium oxide-barium oxide thermionic cathode operates due to a two-step work function reduction mechanism.

An in situ emission microscopy study of barium supply to the surface of scandium-coated porous tungsten

The transport of Ba to the surface of a porous tungsten disk packed with barium oxide powder is observed directly with emission microscopy. The W disk surface is patterned with sputter deposited scandium metal squares. Thermionic electron emission is observed from the Ba/Sc/W areas at temperatures below those required for thermionic electron emission from the non-Sc coated areas. The “dispensing” process is observed directly, in situ.

P1–20: A comparison of model thin film scandate cathode surfaces

Thin film model cathodes prepared as 200nm nominally thick sputter deposited coatings in many configurations of Sc, Sc2O3, Ba, BaO on W foil (0.05mm thick) were studied using photoelectron emission microcopy (PEEM) and thermionic emission microscopy (ThEEM) in a Bauer-Telieps style LEEM/PEEM. The sample structures are shown schematically in Figure 1 (listed in Table I). The samples were prepared (see Reference 1) by sputter depositing the material to a nominal thickness of 200nm on the indicated foil through a shadow mask with 100×100um squares. A second coating was applied in some cases with a shadow mask of 25×25µm squares. Because each sample is heated while being viewed in either ThEEM diffusion of the squares or changes in electron yield can be observed directly.

Atomistic Constructions by using Scanning Tunneling Microscope Tip

We demonstrate an atomic scale construction scheme, which is performed at an area as small as a few tens of nanometer square. In this atomic scale construction site, all the basic building blocks, single atoms, are extracted locally using a scanning-tunneling-microscope tip from the substrate. These extracted atoms are then precisely positioned on the surface to form desired structures. After the completion of the construction, the remaining debris are removed and the undesired holes near the construction site are filled with atoms/clusters to tidy up the area. This entire construction scheme closely resembles our real world construction process and can be considered as its atomic-scale analog.

Design and Construction of a UHV-LT-STM System for Atom Manipulation on MBE Grown Semiconductor Surfaces

An ultra-high-vacuum low-temperature scanning-tunneling-microscope (UHV-LT-STM) capable of single atom/molecule manipulation on molecular beam epitaxy (MBE) grown semiconductor surfaces has been designed and constructed. The STM scanner design is based on a modified Besoke-Beetle type and the thermal drift of the system is less than 0.1 nm/hr, which allows to perform I-V, dI/dV and vibrational tunneling spectroscopy measurements at single atom level. The sample holding stage is designed to have electrical contact at the surface layer of the sample. This permits tunneling into wide band-gap MBE grown semiconductor surfaces at low substrate temperatures. As a demonstration, the first low temperature STM image of GaN(0001) surface at 5 K and an atom-by-atom deposition process using vertical atom-manipulation on this are also presented.