The recently discovered phenomenon of potential sputtering, i.e., the efficient removal of neutral and ionized target particles from certain insulator surfaces due to the potential rather than the kinetic energy of impinging slow highly charged ions, has now also been observed for stoichiometric SiO 2 surfaces. Using a sensitive quartz crystal microbalance technique, total sputter yields induced by Ar q1 ͑q # 14͒ and Xe q1 ͑q # 27͒ ions have been determined for LiF and SiO 2 surfaces. The primary mechanisms for potential sputtering (defect mediated sputtering) and its considerable practical relevance for highly charged ion-induced surface modification of insulators are discussed. [S0031-9007(97)03627-2]

A new form of potential sputtering has been found for impact of slow (#1500 eV) multiply charged Xe ions (charge states up to q 25) on MgO x . In contrast to alkali-halide or SiO 2 surfaces this mechanism requires the simultaneous presence of electronic excitation of the target material and of a kinetically formed collision cascade within the target in order to initiate the sputtering process. This kinetically assisted potential sputtering mechanism has been identified to be present for other insulating surfaces as well. DOI: 10.1103/PhysRevLett.86.3530 PACS numbers: 34.50.Dy, 79.20.Rf Ever since intense sources for slow, highly charged ions (HCI) have become available two decades ago, the possibility of exploiting the huge amount of potential energy stored in these projectiles for surface modification and nanofabrication has captured the imagination of researchers. Applications have been envisioned ranging from information storage via material processing to biotechnology. Compared to kinetic sputtering (i.e., sputtering of target atoms due to momentum transfer in a collision cascade), which unavoidably causes radiation damage in deeper layers, sputtering induced by the potential energy of slow highly charged ions [termed potential sputtering (PS)] holds great promise as a tool for more gentle nanostructuring. A profound understanding of the mechanisms responsible for conversion of projectile potential energy in PS processes is therefore highly desirable.PS phenomena have been reported by several groups for a variety of insulator target surfaces as, e.g., alkali [6], and hydrocarbon contaminated surfaces [7,8]. All investigations have in common that a dramatic increase of the total sputter yields, the secondary ion emission yields, or the size of single ion-induced defects with increasing projectile charge state has been observed. Depending on the surface material and/or the charge state and impact energy of the projectiles, several complementary models have been suggested to explain PS. The "Coulomb explosion" model [9,10] has long been favored, but with the exception of proton sputtering from hydrocarbon covered surfaces [8,11] has failed to provide even a semiquantitative interpretation of experimental data [12]. For GaAs a model to explain the observed high sputtering yields [5] was recently suggested, which involves structural instabilities arising from the destabilization of atomic bonds due to a high density of electronic excitation [13] produced during the neutralization and penetration of very highly charged ions with typically 500 keV where the kinetic energy exceeds the available potential energy.For slow medium charge-state projectile ions (q # 27) on alkali halides and SiO 2 the so-called "defect-mediated desorption" model has been most successful [12] in describing the experimental data [1,3,14]. This model requires a target material with strong electron-phonon coupling, where electronic excitations can be localized by forming lattice defects via self-trapping [e.g., "self-trapped excitons" (ST...

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Carbon nanotube pillar arrays (CPAs) for cold field emission applications were grown directly on polished 70∕30at.% NiCr alloy surfaces patterned by photolithography. A carbon nanotube (CNT) pillar is a localized, vertically aligned, and well-ordered group of multiwalled CNTs resulting from van der Waals forces within high-density CNT growth. The edge effect, in which the applied electric field is enhanced along the edge of each pillar, is primarily responsible for the excellent emission properties of CPAs. We achieved efficient emission with turn-on fields as low as 0.9V∕μm and stable current densities as high as 10mA∕cm2 at an applied macroscopic field of 5.7V∕μm. We investigated the effects of pillar aspect ratio, density, and spacing on CPA field emission and quantified the edge effect with respect to pillar aspect ratio through modeling. We also investigated the field emission stability and found substantial improvement with CPAs compared to continuous and patterned CNT films.

We report the effect of cathode structure on the field emission properties of individual carbon nanotubes. Experimental field emission data are obtained for two well-defined cathode structures: a multi-walled carbon nanotube (MWNT) attached to an etched Ni metal wire and a MWNT attached to a flat Ni-coated Si microstructure. We observed different macroscopic turn-on fields of 1.6 and 2.5 V µm−1, respectively, for the aforementioned experimental structures. This effect is investigated by detailed finite element analysis. We demonstrate that the geometry of the cathode structures significantly affects the microscopic tip field, leading to different turn-on voltages and field distributions for such individual MWNT emitters. Simulations show that changing the support geometry from a hemispherically capped shank to a cylindrical shank produces an increase in the macroscopic threshold field of 0.91 V µm−1. This effect is further investigated by varying the support radius from 0.5 to 30 µm for a cylindrically shaped support structure. The results show that such a variation in the radius of the support structure produces an increase in the macroscopic turn on field from 0.72 to 5.89 V µm−1. We also report quantitative evidence for the nonlinear relationship between the field enhancement factor as a function of support structure radius for nanostructures of three different aspect ratios.

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We introduce an innovative geometry carbon nanotube (CNT) field emitter array capable of achieving stable and high current densities. Arrays of toroid CNT pillars were grown directly on bulk metal alloy substrates and on patterned metal catalyst on silicon substrates. Compared to a solid CNT pillar array (CPA), this toroid CPA (tCPA) provides a larger edge area for achieving a higher stable current density of 50 mA/cm2 at an applied dc field of less than 8 V/μm. Electrostatic simulation data confirming the field enhancement at the inner and outer edges of the tCPA are also presented.

The emission of low-energy electrons caused by 4.5 keV Ne q1 ion impact on an Al surface was measured for incident charge states q 1 6. The electron spectra exhibit structures near 6.5 and 11 eV which are attributed to the creation of surface and bulk plasmons, respectively. The signature for bulk plasmon disappears for q 6. It is argued that, for the higher charge states, the bulk plasmons are selectively created below the surface where the resonance condition for plasmon-assisted electron capture is achieved. [S0031-9007(98)