The need to measure particle energies arises in many applications, from calibrating electron sources for electron guns in precision microscopes to determining the efficiency of space-based ion beam thrusters.  Retarding potential analyzers (RPAs) are capable of filtering particles based on their energy and have been used as early as the late 1950s and early 1960s for such purposes ^[1] .  However, these devices maintain limited application due to stringent dimensional constraints driven by plasma Debye length.  Cold dense plasmas require minute apertures and tight spacing tolerances between biasing grids that are difficult to enforce using conventional means.  We suggest microelectromechanical system (MEMS) batch-fabrication techniques in order to achieve unprecedented alignment accuracy of successive electrodes while incorporating the necessary micron-scale features.  Assembly to a precision of a few tens of microns has been demonstrated with a hybrid RPA (see Figure 1a) ^[2] .  Figure 1b shows the fully MEMS-fabricated sensor inspired by in-plane assembly of high-voltage devices, which will have tolerances on the order of 1μm ^[3] .

Augmenting the optical transparency of RPAs provides a more direct path for particles to the collector plate.  Signal strength is thus improved as the effective collection area is increased.  Preliminary results and comparisons between MEMS-fabricated electrodes and conventional stainless steel mesh have revealed an ameliorated signal quality.  Figure 2 shows a greater than two-fold improvement in peak signal strength with the micromachined grids over the conventional RPA ^[2] .  Currents captured by the various grids and simulations suggest the possibility of ion beam focusing and interception of ions prior to reaching the collector.  Alteration of the internal dynamics of the sensor provides a cleaner signal that may lead to a better interpretation of the measurements than with models that incorporated the stochastic behavior of charged species through randomly oriented electrode apertures.

1. W. C. Knudsen, “Evaluation and demonstration of the use of retarding potential analyzers for measuring several ionospheric quantities,” Journal of Geophysical Research, vol. 71, no. 19, pp. 4669–4678, Oct. 1966.
2. E. V. Heubel, A. I. Akinwande, and L. F. Velásquez-García, “MEMS-enabled retarding potential analyzers for hypersonic in-flight plasma diagnostics,” in Proceedings of the 15th Solid-State Sensors, Actuators, and Microsystems Workshop, Hilton Head Is., SC, June 2012, pp. 324–237.
3. B. Gassend, L. F. Velásquez-García, and A. I. Akinwande, “Precision in-plane hand assembly of bulk-microfabricated components for high-voltage MEMS arrays applications,” Journal of Microelectromechanical Systems, vol. 18, no. 2, pp. 332–346, Apr. 2009.

An electrospray emitter ionizes polar liquids using high electrostatic fields. The electric field produces suction on the free surface (meniscus) of an electrically conductive liquid, and the surface tension of the liquid tends to counteract the effect of the electrostatic suction. If the electric field is larger than a certain threshold, the meniscus snaps into a conic shape called a Taylor cone ^[1] (see Figure 1). A Taylor cone emits charged particles from its apex due to the high electrostatic fields present there; these particles can be ions, droplets, fibers, etc., depending on the working liquid and the emitter flowrate ^[2] . In particular, electrospray in cone-jet mode ^[3] creates near-monodispersed charged droplets that can be used for many applications including mass spectrometry ^[4] , etching ^[5] , and nanosatellite propulsion ^[6] . In this project we are exploring electrospray in cone-jet mode as a technology to create controlled nanoimprints on electrospun nanofiber mats with liquids such as fluorescent dye and nanoparticles solutions, as an alternative technology to nano-pipetting or ink jet printing. Using a shadow mask, we have shown imprints in close agreement with the dimensions of the mask aperture (see Figure 2). The long-term goal of the project is to investigate the design space of the technology to make low-cost and low false-positive biochemical detectors by exploring the multiplexing and scaling-down limits of cone-jet mode electrospray sources using batch micro- and nanofabrication ^[7] .

1. G. I. Taylor, “Disintegration of water drops in an electric field,” Proc. Royal Society of London A, 1964, vol. 280, pp. 383-397.
2. J. Fernandez de la Mora, “The fluid dynamics of Taylor cones,” Ann. Rev. of Fluid Mec., vol. 39, pp. 217-243, 2007.
3. J. Fernandez de la Mora, “The current emitted by highly conductive Taylor cones,” J. Fluid Mechanics, vol. 260, pp. 155-184, 1994.
4. J. B. Fenn, M. Mann, C. K. Meng, S. F. Wong, and C. M. Whitehouse, “Electrospray ionization for mass spectrometry of large biomolecules,” Science, vol. 246, no. 4926, pp. 64-71, 1989.
5. M. Gamero-Castaño and M. Mahadevan, “Sputtering of silicon by a beamlet of electrosprayed nanodroplets,” Appl. Surf. Sci., vol. 255, pp. 8556-8561, 2009.
6. L. F. Velásquez-García, A. I. Akinwande, and M. Martinez-Sanchez, “A planar array of micro-fabricated electrospray emitters for thruster applications,” J. of Microelectromech. Sys., vol. 15, no. 5, pp. 1272-1280, 2006.
7. B. Gassend, L. F. Velásquez-García, A. I. Akinwande, and M. Martinez-Sanchez, “A microfabricated planar electrospray array ionic liquid ion source with integrated extractor,” J. of Microelectromech. Sys., vol. 18, no. 3, pp. 679-694, 2009.