A.I. Akinwande

Massachusetts Institute of Technology, Cambridge, Massachusetts, United States

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Publications (99)63.97 Total impact

  • A. A. Patterson · A. I. Akinwande
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    ABSTRACT: For suitably small field emitters, the effects of quantum confinement at the emitter tip may have a significant impact on the emitter performance and total emitted current density (ECD). Since the geometry of a quantum system uniquely determines the magnitude and distribution of its energy levels, a framework for deriving ECD equations from cold field electron emitters of arbitrary geometry and dimensionality is developed. In the interest of obtaining semi-analytical ECD equations, the framework is recast in terms of plane wave solutions to the Schrödinger equation via the use of the Jeffreys-Wentzel-Kramers-Brillouin approximation. To demonstrate the framework's consistency with our previous work and its capabilities in treating emitters with non-planar geometries, ECD equations were derived for the normally unconfined cylindrical nanowire (CNW) and normally confined (NC) CNW emitter geometries. As a function of the emitter radius, the NC CNW emitter ECD profile displayed a strong dependence on the Fermi energy and had an average ECD that exceeded the Fowler-Nordheim equation for typical values of the Fermi energy due to closely spaced, singly degenerate energy levels (excluding electron spin), comparatively large electron supply values, and the lack of a transverse, zero-point energy. Such characteristics suggest that emitters with non-planar geometries may be ideal for emission from both an electron supply and electrostatics perspective.
    No preview · Article · May 2015 · Journal of Applied Physics
  • A. A. Patterson · A. I. Akinwande
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    ABSTRACT: Although the Fowler-Nordheim (FN) equation serves as the foundation of cold field emission theory, it may not be suitable for predicting the emitted current density (ECD) from emitters with a quantum-confined electron supply. This work presents an analytical framework for treating cold field emission from metals that includes the effects of a quantum-confined electron supply. Within the framework, quantum confinement in emitters is classified into transverse and normal quantum confinement based on the orientation of the confinement relative to the emission direction. The framework is used to generate equations predicting the ECD from rectangular and cylindrical emitter geometries comprised of electron supplies of reduced dimensionality. Transverse quantum confinement of the electron supply leads to a reduction in the total ECD as transverse emitter dimensions decrease and normal quantum confinement results in an oscillatory ECD as a function of the normal quantum well width. Incorporating a geometry-dependent field enhancement factor into the model reveals an optimal transverse well width for which quantum confinement of the electron supply and field enhancement equally affect the ECD and a maximum total ECD for the emitter geometry at a given applied field is obtained. As a result, the FN equation over-predicts the ECD from emitters with transverse dimensions under approximately 5 nm, and in those cases, geometry-specific ECD equations incorporating quantum-confinement effects should be employed instead.
    No preview · Article · Dec 2013 · Journal of Applied Physics
  • A. A. Fomani · A. I. Akinwande · L. F. Velásquez-García
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    ABSTRACT: We report propellantless neutralizers resilient to oxygen and low-vacuum environments based on arrays of Pt-coated, self-aligned, and gated Si field emitters. These devices emit currents in excess of 1 mA at bias voltages of less than 120 V, adequate for neutralizing the plume of a small spacecraft's electric propulsion system. The reported devices produce similar currents at fivefold less voltage and emitting area than state-of-the-art CNT neutralizers. Long-term (3 hours) continuous emission in a 1 μTorr oxygen partial pressure environment was demonstrated, confirming the compatibility of these neutralizers with low Earth orbit (LEO) conditions. A robust processing sequence was developed that could be employed for high-yield fabrication of large-area field emission neutralizers with active areas larger than 10 cm2 for current emission higher than 100 mA.
    No preview · Article · Dec 2013 · Journal of Physics Conference Series
  • A.A. Fomani · L.F. Velasquez-Garcia · A.I. Akinwande
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    ABSTRACT: Field ionization of helium is demonstrated at voltages as low as 150 V using large arrays of self-aligned gated tips. The operating voltages reported here are more than 3× lower than the best results reported in literature. These substantially reduced operating biases permit stable operation of the device at pressures as high as 1 Torr by avoiding arcing or plasma formation that can permanently damage the device. The fabricated arrays are composed of 320,000 nano-scale tips with radii below 5 nm. Ion current in nano-ampere-range, sufficient for most analytical instruments, can be produced with tip-to-gate biases below 200V. Long term (> 104 s) ionization of helium was accomplished at 1 Torr pressures.
    No preview · Conference Paper · Jan 2013
  • A.A. Patterson · A.I. Akinwande
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    ABSTRACT: This work presents a framework for treating cold field emission that includes the effects of quantum confinement. The framework is used to generate equations predicting the emitted current density (ECD) from quantum-confined emitters of various geometries. Quantum confinement of the emitter electron supply leads to a monotonic decrease in the total ECD as the emitter dimensions decrease and as a result, the Fowler-Nordheim equation over-predicts the ECD from quantum-confined emitters.
    No preview · Conference Paper · Jan 2013
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    E V Heubel · A I Akinwande · L F Velásquez-García
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    ABSTRACT: We report the design, fabrication, and preliminary characterization of a microfabricated retarding potential analyzer (RPA) that ensures unprecedented grid alignment accuracy. Through refined manufacturing methods, improved tolerances upon assembly serve to increase the signal to noise ratio (SNR). Furthermore, microfabrication permits smaller features overcoming sensor limitations previously barring RPAs from use in very dense plasmas such as those during reentry. Preliminary results show more than a twofold increase in signal strength compared to conventional RPAs. Finally, a new batch-fabricated RPA design that uses MEMS springs for grid assembly is demonstrated to help drive down cost while improving device reliability.
    Full-text · Article · Jan 2012
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    ABSTRACT: We report the first remote flight control of an insect using microfabricated flexible neuroprosthetic probes (FNPs) that directly interface with the animal's central nervous system. The FNPs have a novel split-ring design that incorporates the anatomical bi-cylinder structure of the nerve cord and allows for an efficient surgical process for implantation (Figure 1a). Additionally, we have integrated carbon nanotube (CNT)-Au nanocomposites into the FNPs to enhance the charge injection capability of the probe. The FNPs integrated with a wireless system are able to evoke multi-directional, graded abdominal motions in the moths thus altering their flight path.
    No preview · Conference Paper · Feb 2010
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    ABSTRACT: For the first time, we demonstrate control of organic thinfilm transistor's (OTFT) threshold voltage (V<sub>T</sub>) by modifying the gate work function. We present a near-room-temperature, fully lithographic process to fabricate integrated pentacene dual V<sub>T</sub> OTFTs suitable for large-area and flexible mixed signal circuits. Platinum and aluminum are used as the gate metals for the high V<sub>T</sub> (more depletion-like) and low V<sub>T</sub> (more enhancement-like) p-channel devices, respectively. The availability of a high V<sub>T</sub> device enables area-efficient zero-VGS current source loads. We demonstrate positive noise margin inverters which use pico Watts of power and a 3 V supply. Compared to a single V<sub>T</sub> implementation, the dual V<sub>T</sub> inverter occupies an area that is 30× smaller, and is 17× faster. These results show that p-channel only organic technologies can produce functional and low-power circuits without integrating a complementary device.
    Preview · Conference Paper · Jan 2010
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    ABSTRACT: We report the fabrication and experimental characterization of a novel low-cost carbon nanotube (CNT)-based electron impact ionizer (EII) with integrated gate for portable mass spectrometry applications. The device achieves low-voltage ionization using sparse forests of plasma-enhanced chemical vapor deposited (PECVD) CNTs field emitter tips, and a proximal gate with open apertures to facilitate electron transmission. The gate is integrated using a deep reactive ion etched (DRIE) spring-based high-voltage MEMS packaging technology. The device also includes a high aspect-ratio silicon structure (mufoam) that facilitates sparse CNT growth and limits the electron current per emitter. The devices were tested as field emitters in high vacuum (10<sup>-8</sup> Torr). Electron emission starts at a gate voltage of 110 V, and reaches a current of 9 uA at 250 V (2.25 mW) with more than 55% of the electrons transmitted through the gate apertures. The devices were also tested as electron impact ionizers using argon. The experimental data demonstrates that the CNT-EIIs can operate at mtorr-level pressures while delivering 60 nA of ion current at 250 V with about 1% ionization efficiency.
    Full-text · Conference Paper · Jul 2009
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    L F Velásquez-García · A I Akinwande
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    ABSTRACT: This paper describes the fabrication of large arrays (10(6) units in 1 cm(2)) of 100 µm tall, single-crystal silicon columns with submicron tip cross-sections. The columns are formed using thin film deposition and growth, reactive ion etching, and deep reactive ion etching. The columns can be either slightly tapered or have pencil-like morphology with nanoscaled tip diameter (41 nm). Conformal thin film coating was used to substantially and uniformly modify the porous structure and, thus, vary by orders of magnitude the fluid permeability of the structure. Gaps between the vertical pillars were varied between 9 µm and 50 nm. Isolated 45 nm diameter, 5 µm tall plasma enhanced chemical vapour deposited multi-walled carbon nanotubes (MWNTs) were grown on the top surface of the columns using a 7 nm thick evaporated Ni film as catalyst. Field emission characterization of the resulting structure was conducted and it is in agreement with scanning electron micrographs of the MWNTs.
    Preview · Article · Oct 2008 · Nanotechnology
  • L.F. Velasquez-Garcia · A.I. Akinwande
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    ABSTRACT: This paper reports a novel carbon nanotube (CNT)- based field ionizer that is part of a portable micro gas analyzer. The device uses a micro-fabricated 3D foam-like silicon structure (mufoam) to increase the neutral particle flux, resulting in higher ion current. The mufoam is fabricated using deep reactive ion etching (DRIE). The ionizer uses plasma enhanced chemical vapor deposited (PECVD) CNTs as field enhancers. The PECVD CNT growth interacts with the mufoam to produce a sparse array of isolated CNT clusters on top of the mufoam. Electrical tests of the device both as an electron field emitter and field ionizer are reported and discussed.
    No preview · Conference Paper · Feb 2008
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    ABSTRACT: A fully integrated MEMS planar electrospray array intended for space propulsion applications is reported. An extractor electrode, electrical insulation, liquid barriers and assembly clips are integrated into a single silicon and Pyrex component, fabricated with deep reactive ion etching (DRIE), laser micromachining and wafer bonding technology. An electrospray emitter head assembles by hand to the clips using a reversible high-precision hand-assembly method, allowing many emitters types to be tested with minimal fabrication effort. Externally-wetted emitters were formed using an alternation of DRIE and isotropic SF<sub>6</sub> plasma etching. A simple model is reported allowing the prediction of emitter geometry from mask geometry and etching steps. The fabricated thruster weighing 5 g was successfully fired with a 502 emitters array, and was shown to operate in the pure ion emission regime using the ionic liquid EMI-BF<sub>4</sub>. Starting voltages as low as 500 V were observed, though the device was tested up to 5 kV without damage.
    No preview · Conference Paper · Feb 2008
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    L.-Y. Chen · L.F. Velasquez-Garcia · X. Wang · K. Teo · A.I. Akinwande
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    ABSTRACT: We report micro-fabricated double-gated vertically aligned carbon nanofiber (CNF) arrays for ionization of gasses in low power portable mass spectrometers. The devices can be operated in one of two modes - electron impact ionization (EII ) or field ionization (FI). When operated as electron impact ionizer, power dissipation was reduced from >1 W typical of thermionic emission based electron impact ionizers to <100 mW. When operated as a field ionizer, the turn-on voltage for field ionization is reduced from 5-10 kV typical of ungated ionizers to 350 V.
    Full-text · Conference Paper · Jan 2008
  • L.F. Velasquez-Garcia · B. Adeoti · Y. Niu · A.I. Akinwande
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    ABSTRACT: We report the demonstration of the first dense (10<sup>6</sup> emitters / cm<sup>2</sup>), high current (10 mA) array of individually ballasted field emitters that use vertical ungated field effect transistors (FETs) as current limiters. Each silicon or carbon nanofiber (CNF) emitter is individually connected in series with a silicon pillar ungated FET or current limiter (Fig. 1). The ungated FET takes advantage of the saturation of carrier velocity in silicon to obtain current source-like behavior - required for uniform and high current operation with small power dissipation.
    No preview · Conference Paper · Jan 2008
  • L.-Y. Chen · L.F. Velasquez-Garcia · X. Wang · K. Cheung · K. Teo · A.I. Akinwande
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    ABSTRACT: In this work, we designed and fabricated two types of double-gated isolated vertically aligned carbon nanofiber field emission arrays (VACNF FEAs) to study how the tip position relative to the gate affects the device performance. In the first type, the tip is in-plane with the gate, and in the second type, the tip is 0.9 mum below the gate. To quantify the effectiveness of the two gates to affect the total emission current, the gate field factor (beta<sub>G</sub>) and the focus field factor (beta<sub>F</sub>) are examined for both types of VACNF FEAs experimentally and by simulation.
    No preview · Conference Paper · Aug 2007
  • L.F. Velasquez-Garcia · K. Cheung · A.I. Akinwande
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    ABSTRACT: Quadrupoles are mass filters composed of a set of four cylindrical rods in a symmetrical setup that are biased with a combination of DC and RF potentials. There has been an active interest in developing scaled-down quadrupole technology for over a decade. This has led to remarkable results in both miniaturized and MEMS-based quadrupoles. This paper reports the design, fabrication, and characterization of a second generation of MEMS out-of-plane quadrupoles mass filter, part of a portable mass spectrometer for gas sensing. The mass spectrometer is composed of a CNT-based ionizer array, a MEMS quadrupole mass filter, MEMS electrometer for charge sensing, and a MEMS displacement pump that provides the required vacuum level. Two key advantages of scaling down quadrupoles are device portability and the relaxation on the vacuum level required for operation. For a fixed rod aspect ratio the quadrupole weight roughly scales to the cube of the rod diameter. Also, scaled down quadrupoles can work at higher pressure provided the gas mean free-path is larger than the quadrupole length. We have experimentally demonstrated that higher working pressure, up to the mTorr level, is compatible with our CNT-based electron impact and field ionizers with no loss in ionizer performance, thus setting the maximum quadrupole length at 5 cm.
    No preview · Conference Paper · Aug 2007
  • L.F. Velasquez-Garcia · A.I. Akinwande
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    ABSTRACT: This paper reports the design, fabrication and characterization of a hand-assembled out-of-plane MEMS quadrupole mass filter. The device utilizes a 3D packaging technology that relies on mesoscale DRIE-patterned deflection springs for assembly. Microfabricated quadrupoles with 1.58 mm down to 0.56 mm diameter rods and 30 to 60 aspect ratios were built and tested. The quadrupoles were characterized in the first stability region at an RF frequency of 1.44 MHz, using a constant peak width sweeping method, obtaining a dynamic range of 650 amu and a half-peak width of 2 amu. Better performance is expected if a higher RF frequency is used.
    No preview · Conference Paper · Jul 2007
  • L.F. Velásquez-García · L.-Y. Chen · A.I. Akinwande
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    ABSTRACT: This presentation will introduce several MEMS/NEMS devices being developed in the Microsystems Technology Laboratories at MIT that are focused on scaling laboratory instruments to smaller dimensions. We will illustrate this trend with three examples that result in the reduction of power consumption as well as improvements in performance. These systems rely on massive parallel operation of the individual scaled-down elements and leverage micro- & nano-fabrication technologies to achieve improvement in their performance.
    No preview · Article · Jan 2007
  • L.F. Velásquez-García · L.-Y. Chen · A.I. Akinwande
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    ABSTRACT: This presentation will introduce several MEMS/NEMS devices being developed in the Micro-systems Technology Laboratories at MIT that are focused on scaling laboratory instruments to smaller dimensions. We will illustrate this trend with three examples that result in the reduction of power consumption as well as improvements in performance. These systems rely on massive parallel operation of the individual scaled-down elements and leverage micro- & nano-fabrication technologies to achieve improvement in their performance.
    No preview · Article · Jan 2007
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    ABSTRACT: We describe the fabrication and testing of an 1 cm by 1 cm microfabricated externally wetted electrospray thruster, which uses the ionic liquid EMI-BF4 as a propellant. In an electrospray thruster, an electrostatic field is used to extract ions or charged droplets from a liquid placed at the tip of a needle, and accelerate them. The novelty of our design is in the use of microfabricated ridges with regularly spaced tips as emitters, as well as the use of ceramic balls to position the extractor grid relative to the emitters and ensure electrical insulation. After describing the design of the thruster, we present the test firings we have done to date. In these tests, emission starts around 2 kV. To the best of our knowledge, this is the first time an externally wetted electrospray thruster has been fired with an integrated extractor electrode. Emission patterns gathered by firing into an aluminum plate show that the tips placed along the ridge allow emission to occur at multiple points along the ridge; without the tips, emission only occurs at the ends of the ridges.
    No preview · Conference Paper · Jul 2006

Publication Stats

642 Citations
63.97 Total Impact Points

Institutions

  • 2-2015
    • Massachusetts Institute of Technology
      • Department of Electrical Engineering and Computer Science
      Cambridge, Massachusetts, United States
  • 1989-1997
    • Honeywell
      Morristown, New Jersey, United States
  • 1990
    • Signal Processing Inc.
      Maryland, United States
  • 1989-1990
    • University of Minnesota Duluth
      • Department of Electrical Engineering
      Duluth, Minnesota, United States