R. D. Ramsier

Electron-stimulated desorption: Principles and applications

Abstract The desorption of surface-bound species induced by electronic excitation mechanisms is a fascinating phenomenon of fundamental as well as technological importance. This article summarizes the wealth of knowledge that has been gained through studies of low-energy electron-stimulated desorption (ESD) of atoms, molecules, and molecular fragments from a variety of adsorbate/substrate systems. A survey of popular theoretical models often invoked to explain ESD phenomena is presented, followed by a brief discussion of the experimental aspects of a variety of ESD measurements. Specific literature examples are then highlighted to illustrate the usefulness of ESD techniques in elucidating the bonding geometry and surface dynamics of adsorbed species. Finally, a bibliography of published literature in the field is included as an appendix.

NO adsorption and thermal behavior on Pd surfaces. A detailed comparative study

The adsorption and thermal behavior of NO on `flat? Pd(111) and `stepped? Pd(112) surfaces has been investigated by temperature programmed desorption (TPD), high resolution electron energy loss spectroscopy (HREELS), and electron stimulated desorption ion angular distribution (ESDIAD) techniques. NO is shown to molecularly adsorb on both Pd(111) and Pd(112) in the temperature range 100?373 K. NO thermally desorbs predominantly molecularly from Pd(111) near 500 K with an activation energy and pre-exponential factor of desorption which strongly depend on the initial NO surface coverage. In contrast, NO decomposes substantially on Pd(112) upon heating, with relatively large amounts of N2 and N2O desorbing near 500 K, in addition to NO. The fractional amount of NO dissociation on Pd(112) during heating is observed to be a strong function of the initial NO surface coverage. HREELS results indicate that the thermal dissociation of NO on both Pd(111) and Pd(112) occurs upon annealing to 490 K, forming surface-bound O on both surfaces. Evidence for the formation of sub-surface O via NO thermal dissociation is found only on Pd(112), and is verified by dissociative O2 adsorption experiments. Both surface-bound O and sub-surface O dissolve into the Pd bulk upon annealing of both surfaces to 550 K. HREELS and ESDIAD data consistently indicate that NO preferentially adsorbs on the (111) terrace sites of Pd(112) at low coverages, filling the (001) step sites only at high coverage. This result was verified for adsorption temperatures in the range 100?373 K. In addition, the thermal dissociation of NO on Pd(112) is most prevalent at low coverages, where only terrace sites are occupied by NO. Thus, by direct comparison to NO/Pd(111), this study shows that the presence of steps on the Pd(112) surface enhances the thermal dissociation of NO, but that adsorption at the step sites is not the criterion for this decomposition.

Thermal dissociation of NO on Pd surfaces: The influence of step sites

The thermal behavior of NO on (flat) Pd(111) and (stepped) Pd(112) has been investigated by temperature programmed desorption (TPD), high resolution electron energy loss spectroscopy (HREELS), and Auger electron spectroscopy (AES) techniques. NO is shown to adsorb molecularly on both Pd(111) and Pd(112) in the temperature range 300–373 K. NO desorbs molecularly from Pd(111) near 500 K with evidence for slight NO dissociation. In contrast, on Pd(112), in addition to NO, relatively large amounts of N2(7x) and N2O(15x) are observed to desorb near 500 K, compared to Pd(111). This result indicates that the influence of the step sites on Pd(112) is to catalyze the decomposition of NO upon heating. This is a surprising result in light of the fact that NO molecules preferentially bind to terrace sites, instead of step sites, on Pd(112). HREELS measurements indicate the presence of small amounts of surface‐bound O (resulting from NO thermal decomposition) coadsorbed with NO on both Pd(111) and Pd(112) after NO ads...

Adsorption of Phosphorus Acids on Alumina.

Abstract Vibrational spectra of several phosphorus acids adsorbed on oxidized aluminum surfaces have been measured using inelastic electron tunneling spectroscopy. From analysis of the spectra, it is concluded that these acids chemisorb on alumina by a condensation reaction in which resonance stabilized structures are formed involving deprotonated hydroxilic and phosphoryl oxygen atoms. These structures are symmetric PO 2 groups in the case of phosphinic acid, whereas for phosphonic acids, they are symmetric PO 3 groups. In the spectrum of the hydration inhibitor N[CH 2 P(O)(OH) 2 ] 3 adsorbed on alumina, unexpected bands attributable to P-H motions appeared. It is therefore inferred that this material dissociates upon adsorption. The various phosphonic acids produced in this decomposition subsequently adsorb as symmetrical tridentate species.

Electrical, structural, and chemical properties of semiconducting metal oxide nanofiber yarns

The electrical, structural, and chemical properties of twisted yarns of metal-oxide nanofibers, fabricated using a modified electrospinning technique, are investigated in this report. In particular, synthesized zinc oxide and nickel oxide yarns having diameters in the range of 4–40μm and lengths up to 10cm were characterized, whose constituent nanofibers had average diameters of 60–100nm. These yarns have one macroscopic dimension for handling while retaining some of the properties of nanofibers.

CO adsorption on stepped Pd (112): studies by thermal and electron stimulated desorption

Abstract The adsorption of CO on a stepped Pd(112) surface, Pd(S)[3(111) × (001)], at 300 K has been studied by temperature programmed desorption (TPD) , electron stimulated desorption ion angular distribution (ESDIAD) and low energy electron diffraction (LEED) methods. CO adsorbs molecularly on Pd (112) at 300 K, reversibly desorbing near 465 K at low coverages with first-order desorption kinetics. The activation energy and pre-exponential factor of CO thermal desorption are observed to decrease strongly with increasing CO coverage, exhibiting a logarithmic compensation effect. Evidence is presented that CO preferentially adsorbs at the step sites of Pd (112) at low coverages, and that both terrace and step sites are occupied at higher coverages. The cross-section for production of positive ions during electron bombardment of adsorbed CO depends strongly on the CO coverage. No excited-state neutral particles were detected at any CO coverage during ESD measurements. In addition, no evidence for long-range ordering of the CO adlayers on Pd (112) was observed.

Adsorbate–adsorbate repulsions—the coverage dependence of the adsorption structure of CO on Cu(110) as studied by electron‐stimulated desorption ion angular distribution

The coverage dependent orientation of CO adsorbed on a Cu(110) surface was studied by the electron‐stimulated desorption ion angular distribution (ESDIAD) technique. A neutral excited (CO*) species is imaged and in addition positive ions are measured. The adsorption temperature was varied between 32 K and 150 K. By applying the ESDIAD technique at a temperature below 80 K it was possible to decrease the beamwidths drastically, to determine the angular distributions better than ±0.5°, and to study the adsorption of CO chemisorbed and physisorbed on the surface. With increasing CO coverage we observe three distinct ESDIAD patterns. Starting from a normal beam pattern with an elliptical cross section with the major axis oriented in the 〈110〉 direction for coverages up to 0.2 monolayer (ML), a transformation of the ESDIAD pattern into a pattern of two separated beams is observed for a coverage of about 0.5 ML, indicating a tilting of the molecules in the 〈110〉 directions by ∼9°. With further increasing CO c...

Direct Observation of Chemical Bond Dynamics on Surfaces

The dynamics of chemisorbed species as they swing to-and-fro on their adsorption sites may be directly observed with electron-stimulated desorption. The observation of the thermal disorder in adsorbate chemical bond directions, through studies of the thermal excitation of librational modes, allows one to visualize the potential energy surfaces controlling the structure and dynamics of adsorbates on single crystal metal and semiconductor surfaces. This information may be useful in understanding surface diffusion as well as the spatial aspects of surface chemical reactions.

Bacterial adhesion to zirconium surfaces

Abstract Zirconium alloys are well-known for their resistance to harsh chemical environments. In the interest of assessing the potential of this class of materials for bio-implant applications, we need to determine if this environmental stability transfers to biological systems. We report on our use of viable counts and infrared spectroscopic techniques to monitor how well thermally processed Zircaloy-2 surfaces resist bacterial adhesion. Specifically, we test the propensity of Staphylococcus aureus, Staphylococcus epidermidis, Pseudomonas aeruginosa, and mixtures of all three to adhere to zirconium alloy surfaces. Our data and statistical analyses include stationary and shaken broth cultures and differently prepared surfaces, and we demonstrate differences due to the type of bacteria, the type of surface, and the exposure conditions.

Electrospinning Route for the Fabrication of P-n Junction Using Nanofiber Yarns

Electrospinning is a simple, versatile, and cost effective method for generating nanoscale fibers, wires, and tubes. Nanowires and nanotubes could be important building blocks for nanoscale electronics, optoelectronics, and sensors as they can function as miniaturized devices as well as electrical interconnects. We report on a simple method to fabricate free standing ceramic nanofiber heterostructures, which exhibit rectifying behavior of a p-n junction.Electrospinning is a simple, versatile, and cost effective method for generating nanoscale fibers, wires, and tubes. Nanowires and nanotubes could be important building blocks for nanoscale electronics, optoelectronics, and sensors as they can function as miniaturized devices as well as electrical interconnects. We report on a simple method to fabricate free standing ceramic nanofiber heterostructures, which exhibit rectifying behavior of a p-n junction.