Transmission of 3 keV Ne7+ ions through nanocapillaries etched in polymer foils: evidence for capillary guiding

Charging Effects in Inlet Capillaries

Glass or metal inlet capillaries are commonly used for flow restriction in atmospheric pressure ionization mass spectrometers. They exhibit a high ion transmission rate and stability at most operating conditions. However, transferring unipolar currents of ions through inlet capillaries can lead to sudden signal dropouts or drifts of the signal intensity, particularly when materials of different conductivity are in contact with the capillary duct. Molecular layers of water and other gases such as liquid chromatography solvents always form on the surfaces of inlet capillaries at atmospheric pressure ionization conditions. These surface layers play a major role in ion transmission and the occurrence of charging effects, as ions adsorb on the capillary walls as well, charging the walls to electric potentials of up to kilovolts and eventually leading to a hindrance of ion transport into or through the capillary duct. In this work, surface charging effects are reported in dependence on the capillary material, i.e., borosilicate glass, (reduced) lead silicate, quartz, and metal. Low electrical conductance materials show a more pronounced long-term signal drift (e.g., quartz), while higher electrical conductance materials lead to stable long-term behavior.

Guiding of keV ions between two insulating parallel plates

Experimental data are presented for low-energy singly charged ion transport between two insulating parallel plates. Using a beam intensity of approximately 20 pA, measurements of the incoming and transmitted beams provide quantitative temporal information about the charge deposited on the plates and the guiding probability. Using a smaller beam intensity (~ 1 pA) plate charging and discharging properties were studied as a function of time. These data imply that both the charge deposition and decay along the surface and through the bulk need to be modeled as acting independently. A further reduction of beam intensity to ~ 25 fA allowed temporal imaging studies of the positions and intensities of the guided beam plus two bypass beams to be performed. SIMION software was used to simulate trajectories of the guided and bypass beams, to provide information about the amount and location of deposited charge and, as a function of charge patch voltage, the probability of beam guiding and how much the bypass beams are deflected plus to provide information about the electric fields. An equivalent electric circuit model of the parallel plates, used to associate the deposited charge with the patch voltage implies that the deposited charge is distributed primarily on the inner surface of the plates, transverse to the beam direction, rather than being distributed throughout the entire plate.

Surface Channeling of Charged and Neutral Beams in Capillary Guides

In this review work, the passage of charged and neutral beams through dielectric capillary guides is described from a uniform point of view of beams channeling in capillaries. The motion of beams into the hollow channels formed by the inner walls of capillaries is mainly determined by multiple small-angle scattering (reflection) and can be described in the approximation of surface channeling. It is shown that the surface interaction potential in the case of micro- and nano-capillaries is actually conditioned by the curvature of the reflecting surface. After presenting the analysis of previously performed studies on X-rays propagation into capillaries, which is valid for thermal neutrons, too, the surface channeling formalism is also developed for charged particle beams, in particular, moving in curved cylindrical capillaries. Alternative theories explaining experimental results on the beams passage through capillaries are based on simple thermodynamic estimates, on various diffusion models, and on the results of direct numerical simulations as well. Our work is the first attempt to explain the effective guiding of a charged beam by a capillary from the general standpoint of quantum mechanics, which made it possible to analytically explore the interaction potential for surface channeling. It is established that, depending on the characteristics of a projectile and a dielectric forming the channel, the interaction potential can be either repulsive or attractive; the limiting values of the potential function for the corresponding cases are determined. It has been demonstrated that the surface channeling behaviour can help in explaining the efficient capillary guiding for radiations and beams.

Electron guiding in macroscopic borosilicate capillaries with large bending angles

This work presents experiments about the transmission of electrons with an energy of around 15 keV with beam currents up to 20 µA through macroscopic glass capillaries. A systematic study was conducted to experimentally investigate the transmission of electrons through borosilicate glass capillaries with curve angles of 90°, 180°, 270° and 360° for the first time. The focus of the work was to identify the conditions under which the injected electron current is transmitted through the capillary. It was also shown that the transmission process in the macroscopic capillaries can be optically observed by cathodoluminescence—the interaction of electrons with the capillary surface causes locally a blue glow. Different distinctive “glow states” were observed and are found to correlate with different states of electron transmission.

Simulations of Ion-Guiding Through Insulating Nanocapillaries of Varying Diameter: Interpretation of Experimental Results

The guiding of highly charged ions through a single nanocapillary is simulated in comparison with previous experiments performed with highly insulating polyethylene terephthalate (PET). The simulations are carried out using 3-keV Ne7+ ions injected into capillaries with diameters ranging from 100 nm to 400 nm. In the calculations, non-linear effects are applied to model the charge transport along the capillary surface and into the bulk depleting the deposited charges from the capillary walls. In addition to the surface carrier mobility, the non-linear effects are also implemented into the bulk conductivity. A method is presented to determine the parameters of the surface charge transport and the bulk conductivity by reproducing the oscillatory structure of the mean emission angle. A common set of charge depletion rates are determined with relatively high accuracy providing confidence in the present theoretical analysis. Significant differences in the oscillatory structures, experimentally observed, are explained by the calculations. Experimental and theoretical results of the guiding power for capillaries of different diameters are compared. Finally, dynamic non-linear effects on the surface and bulk relaxation rates are determined from the simulations.

Applications of Microbeams Produced by Tapered Glass Capillary Optics

Production of ion microbeams using tapered glass capillary optics was introduced more than 10 years ago. This technique has drawn attention in terms of both its peculiar transmission features and application to ion beam analysis. The transmission mechanism based on a self-organized charge-up process for keV-energy ions was observed for the first time in an experiment using a multitude of nanometer-sized capillaries in a polymer foil. The same mechanism can be seen for the transmission of keV ions through a single tapered glass capillary. The transmission experiments with keV ions showed a delayed transmission, focusing effects, guiding effects, and formation of microbeams. Experiments using MeV-energy ions always aim at applications of microbeam irradiation for material analysis, surface modification, cell surgery, and so on. In this article, the applications of MeV ion microbeams, including the fabrication method of the glass capillary, are reviewed, as well as the experimental and theoretical studies for the transmission mechanisms of keV/MeV ions.

Vacuum scanning capillary photoemission microscopy

We demonstrate the use of a conical capillary in a scanning probe microscopy for surface analysis. The probe can measure photoemission from a substrate by transmitting photoelectrons along the capillary as a function of probe position. The technique is demonstrated on a model substrate consisting of a gold reflecting layer on a compact disc which has been illuminated by an unfocused laser beam with a wavelength 400nm, from a femtosecond laser with a beam size of 4mm. A quartz capillary with a 2-µm aperture has been used in the experiments. The period of gold microstructure, shown to be 1.6µ, was measured by the conical probe operating in shear force mode. In shear force regime, the dielectric capillary has been used as a "classical" SPM tip, which provided images reflecting the surface topology. In a photoelectron regime photoelectrons passed through hollow tip and entered a detector. The spatial distribution of the recorded photoelectrons consisted of periodic mountain-valley strips, resembling the surface profile of the sample. Submicron spatial resolution has been achieved. This approach paves the way to study pulsed photodesorption of large organic molecular ions with high spatial and element resolution using the combination of a hollow-tip scanner with time-of-flight technique.

Transmission of Hundred-keV Protons through Insulating Nanocapillaries: Charge-patch-assisted Specular Reflections

In this work, we measured the time evolution of the transmission features of 10-100 keV protons transmitted through nanocapillaries in a polycarbonate (PC) membrane. After reaching equilibrium, transmitted particles with an incident energy of 100 keV were located around the direction along the incident beam but not along the capillary axis, indicating that the transport mechanism of the 100 keV ion was distinct from that of keV-energy ions. The simulation results indicated that charge-patch-assisted collective scatterings on the surface are the main transport mechanism for the hundred-keV ions in nanocapillaries. This scenario fills in the gap in the previous understanding of ion transmission in nanocapillaries from keV to MeV energies.

Nanopatterning by ion implantation through nanoporous alumina masks

The important problem of how to generate lateral order for ion implantation patterning of substrates is solved by using a nanoporous anodic alumina membrane as a mask. Co and Pt implantation is used at two implantation doses. In order to observe the achieved implantation zones free from artifacts, electron transparent thin nitride and oxide films are used as substrates, which allows the quality of pattern transfer from the mask to the thin film to be assessed by plan-view transmission electron microscopy. Characteristic density variations of implanted elements across projected pore-regions of the mask, such as ring and dome shapes, and corresponding variation of cluster size are discussed, and therefore the method also serves as a suitable test bed for ion beam focusing studies by cylindrical or conical pores.

Tailoring of keV-ion beams by image charge when transmitting through rhombic and rectangular shaped nanocapillaries

We report on an unexpected effect of tailoring transmission profiles of Ne(7+) ions through nanocapillaries of rhombic and rectangular cross sections in mica. We find that capillaries of rhombic cross sections produce rectangular shaped ion transmission profiles and, vice versa, that capillaries of rectangular geometry give a rhombic beam shape. This shaping effect only occurs for transmitted ions and is absent for the small fraction of neutralized particles. The experimental findings and simulations of the projectile trajectories give clear evidence that the observed effect is due to the image forces experienced by the transmitting ions. This novel beam shaping mechanism suggests applications for the guiding, focusing, and shaping of ion beams.

Nanochannel alignment analysis by scanning transmission ion microscopy

In this paper a study on the ion transmission ratio of a nanoporous alumina sample is presented. The sample was investigated by scanning transmission ion microscopy (STIM) with different beam sizes. The hexagonally close-packed Al(2)O(3) nanocapillary array, realized as a suspended membrane of 15 microm thickness, had pore diameters of approximately 215 nm and spacing of approximately 450 nm. When the proton beam size was limited to a single domain, a peak transmission ratio of 19% was observed as is expected from the geometry (approximately 19-20%). This result points out an almost perfectly parallel alignment of the capillaries within one domain. However, for larger beam scanning areas (sampling multiple domains) the transmission ratio was reduced to 5%. The STIM analysis over an area larger than the typical domain size revealed an overall capillary angular spread of approximately 2 degrees.

Electron guiding through insulating nanocapillaries

We simulate the electron transmission through insulating Mylar (polyethylene terephthalate, or PET) capillaries. We show that the mechanisms underlying the recently discovered electron guiding are fundamentally different from those for ion guiding. Quantum reflection and multiple near-forward scattering rather than the self-organized charge up are key to the transmission along the capillary axis irrespective of the angle of incidence. We find surprisingly good agreement with recent data. Our simulation suggests that electron guiding should also be observable for metallic capillaries.

Evidence of sequentially formed charge patches guiding ions through nanocapillaries

We investigated the time evolution of the dynamically shifting distribution of 7 keV Ne7+ ions guided through nanocapillaries in SiO2. We present evidence for a small number of charge patches, formed sequentially in the charging-up process, guiding the ions. We show that the charge patches are distributed along the whole length of the capillaries and that they are maintained in the equilibrium state of transmission. The interpretations are supported by model calculations.

Very fast hollow-atom decay processes in Xe30+-C60 collisions

In Xe30+-C60 collisions at low velocity (0.2 a.u.<v<0.4 a.u.), very fast electron ejection is observed not only for frontal collisions but also for near C60 cage collisions. In frontal collisions, the exit-charge distributions obtained with three projectile velocities are reproduced using a multistep exponential decay model. A mean Auger rate of about 0.4 a.u. is estimated for the decay of hollow atoms during the very short interaction time (<3 fs).