Paulo C. Lozano

Energy properties of an EMI-Im ionic liquid ion source

The identity and the energy distributions of positive and negative ions electrostatically extracted from the liquid phase in an ionic liquid ion source (ILIS) are analysed with a time-of-flight mass spectrometer and a multi-grid retarding potential analyzer. Accurate energy measurements using ionic liquids in an externally wetted configuration are reported for the first time. Droplet-free beams are produced using the ionic liquid 1-ethyl-3-methylimidazolium bis(triflouromethylsulfonyl)amide (EMI-Im) in which the solvated ions (EMI-Im)nEMI+ and (EMI-Im)nIm− with n = 0,1,2 are observed. The small ion source size and the energy distribution widths and deficits of a few electronvolts are quite similar to those of liquid metal ion sources, confirming that ILIS can be used in applications requiring highly focusable beams, e.g. sub-micron ion lithography. Measurements also suggest that solvated ions with n ≥ 1 exhibit post-extraction fragmentation into lighter species at a rate increasing with their original degree of solvation. About 10% of the total beam current is carried away by metastable species that break up almost immediately after extraction while inside the emitter accelerating region.

Electrospray Propulsion Based on Emitters Microfabricated in Porous Metals

indicating that the thrust density can be increased with emitter miniaturization and clustering. A two-dimensional emitter array is constructed from three linear arrays and experiments show that the device produces an average current of around 1 �A per emitter at the highest voltages. Direct thrust measurements using a torsional balance show that the test fixture produces between 0.05 to 0:1 � N of thrust per emitter, as anticipated by theoretical estimates.

Mass Spectrometric Analysis of Colloid Thruster Ion Emission from Selected Propellants

Retarding potential, time-of-flight (TOF), and quadrupole mass spectrometric methods are applied to study the charged species emitted from a colloid thruster operated in a cone-jet mode at high-specific-impulse operating conditions. Measurements are conducted for two propellants, a 28.4 wt% NaI/formamide electrolyte solution and the ionic liquid 1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIM][BF4]). When operated at a volume flow rate of 2‐6 × × 10 −14 m 3 /s and a positive polarity, the electrolyte solution yields an average specific charge, q/m ,o f ∼11,000 C/kg with an ion/droplet current ratio of 9 to 1. Fully angularly integrated TOF measurements determine an average mass-to-charge ratio of 253 atomic mass units. The axial quadrupole mass spectrometric measurement identifies solvated sodium ions, Na + (HCONH2)n (n =1 ,..., 10), with a distribution peaked at n =4 .Stopping potential and retarding potential measurements suggest that most charged particles are produced with energies ∼200 V below the capillary bias potential. The inability to operate this propellant in a pure ion emission mode makes the thruster inefficient when operated in the ion-evaporation, high-specific-impulse conditions. The ionic liquid, [EMIM][BF4], is operated in a pure ion emission mode for both positive and negative polarity, yielding mean specific charges exceeding 250,000 C/kg. X([EMIM][BF4])n (n =0 , 1, 2) ions, where X = EMIM + or BF − ,a re identified. Contrary to the electrolyte solution, the energy analysis of the field-evaporated ions reveals that the ions are produced at the capillary potential with a narrower energy width of 50 V. For an acceleration voltage of 1500 V, a specific impulse of ∼4000 s at an efficiency exceeding 90% is recorded.

Monoenergetic source of kilodalton ions from Taylor cones of ionic liquids

The ionic liquid ion sources (ILISs) recently introduced by Lozano and Martinez Sanchez [J. Colloid Interface Sci. 282, 415 (2005)], based on electrochemically etched tungsten tips as emitters for Taylor cones of ionic liquids (ILs), have been tested with ionic liquids [A+B−] of increasing molecular weight and viscosity. These ILs have electrical conductivities well below 1S∕m and were previously thought to be unsuitable to operate in the purely ionic regime because their Taylor cones produce mostly charged drops from conventional capillary tube sources. Strikingly, all the ILs tried on ILIS form charged beams composed exclusively of small ions and cluster ions A+(AB)n or B−(AB)n, with abundances generally peaking at n=1. Particularly interesting are the positive and negative ion beams produced from the room temperature molten salts 1-methyl-3-pentylimidazolium tris(pentafluoroethyl) trifluorophosphate (C5MI–(C2F5)3PF3) and 1-ethyl-3-methylimidazolium bis(pentafluoroethyl) sulfonylimide (EMI–(C2F5SO3)2N)....

Emission measurements from planar arrays of porous ionic liquid ion sources

We present current emission measurements from arrays of 480 porous ionic liquid ion source emitters for applications in space propulsion and in the processing and analysis of materials. The emitters are fabricated with a top-down approach from 10 × 10 × 1 mm3 bulk porous nickel and are housed within a microetched silicon frame and stainless-steel aperture-matching extraction grids. Measurements of emitted and collected currents are reported using two ionic liquids (ILs), EMI-BF4 and EMI-Im. Total beam currents up to about 400 µA can be sustained through passive feeding with IL fed from the rear of the bulk porous substrate. These currents are in agreement with previous work where currents of about 1 µA were observed from single emitters. Ion transparencies as high as 0.95 are achieved with manually assembled grids. Current emission displays reasonable symmetry between polarities, although slight differences could yield to the accumulation of one type of ion in the source leading to electrochemical reactions despite voltage alternation. Strong current decays with time and evidence of surface contamination reinforce this hypothesis. Future work should address this issue through closed-loop control of charge emission.

On the dynamic response of externally wetted ionic liquid ion sources

The electrostatic extraction of nearly monochromatic solvated ions from externally wetted emitters is possible in the case of some ionic liquids or room temperature molten salts. These compact devices are similar to liquid metal ion sources but positive or negative ion beams can be obtained simply by selecting the appropriate polarity for the power supply. dc operation on a single polarity over relatively long periods of time induces electrochemical degradation of the liquid–metal system making voltage alternation at frequencies of the order of 1 Hz necessary to sustain chemical neutrality. This periodic interruption forces a non-steady state behaviour. In particular, the ion current onset is delayed from a square-shaped voltage input signal by a few milliseconds for tungsten metal emitters wetted with the ionic liquid 1-ethyl-3-methylimidazolium tetrafluoroborate, setting the upper bound for the alternation frequency at about 75 Hz. The dependences of these delays on absolute applied voltages are experimentally explored for this compound, suggesting that viscous drag is the main factor determining the dynamic response.

Development of ion sources from ionic liquids for microfabrication

In this article the authors present the potential of ionic liquid ion sources (ILISs) for direct microfabrication of silicon structures. The authors have developed a specific source geometry using the ionic liquid EMI-BF4 to obtain stable emission currents up to the 10 μA regime. ILIS (EMI-BF4) engraving properties were then investigated. The results and the chemical analysis of the patterned substrates suggest that reactive ion species can be generated from ILIS. This possibility is of major interest to allow decisive advances in the field of focused ion beam applications.

Effect of liquid properties on electrosprays from externally wetted ionic liquid ion sources

Ionic liquid ion sources (ILISs) are externally wetted and electrochemically etched and sharpened tungsten tips used as electrospraying sources for ionic liquids in a vacuum. They have recently shown an ability to operate as emitters of pure ion beams (no drops), even with ionic liquids of moderate surface tension (γ<40dyn∕cm) and electrical conductivity (K<1S∕m) that had in all prior reports (all based on conventional internally fed capillary tips) always operated in the mixed ion-drop regime. The present study uses time of flight mass spectrometry to analyze full ion beams emitted from ILISs for a diversity of ionic liquids with properties in the wide range 0.26<K(S∕m)<2.8, 39.3<γ(dyn∕cm)<48.6. Remarkably, all liquids tested achieve the purely ionic regime. The main effect of reducing electrical conductivity is a reduction of the emission current from 180to10nA.

Development and characterization of an iodine field emission ion source for focused ion beam applications

Emission of positive and negative ions is possible when a room-temperature molten salt, or ionic liquid, is exposed to a sufficiently high electric field. Ionic liquid ion sources (ILISs) have shown potential to be used in various focused ion beam (FIB) applications, since their operation and characteristics are similar to those of liquid metal ion sources, with the advantage that ILIS work at low temperatures in comparison and a large number of ionic liquids with many different compositions are available. In this article, the authors present results on the emission characteristics of negative ions extracted from an iodine-based ionic liquid using a time-of-flight mass spectrometer and a retarding potential analyzer. The ionic liquid 1-butyl-3-methylimidazolium iodide (BMI-I) is used as source media, producing a droplet free beam with multiple solvated ion species. Attention is given to BMI-I, in particular, due to the potential of creating a beam of pure and clustered I-ions, which are expected to improv...

The role of upstream distal electrodes in mitigating electrochemical degradation of ionic liquid ion sources

Ionic liquid ion sources produce molecular ions from micro-tip emitters wetted with room-temperature molten salts. When a single ion polarity is extracted, counterions accumulate and generate electrochemical reactions that limit the source lifetime. The dynamics of double layer formation are reviewed and distal electrode contacts are introduced to resolve detrimental electrochemical decomposition effects at the micro-tip apex. By having the emitter follow the ionic liquid potential, operation can be achieved for an extended period of time with no apparent degradation of the material, indicating that electrochemistry can be curtailed and isolated to the upstream distal electrode.Ionic liquid ion sources produce molecular ions from micro-tip emitters wetted with room-temperature molten salts. When a single ion polarity is extracted, counterions accumulate and generate electrochemical reactions that limit the source lifetime. The dynamics of double layer formation are reviewed and distal electrode contacts are introduced to resolve detrimental electrochemical decomposition effects at the micro-tip apex. By having the emitter follow the ionic liquid potential, operation can be achieved for an extended period of time with no apparent degradation of the material, indicating that electrochemistry can be curtailed and isolated to the upstream distal electrode.