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Applications of Zinc Oxide Nanop


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									Design of Zinc Oxide
Tetrapod Devices

In fulfillment of the requirements of
ENMA490: Materials Design
Spring 2007
University of Maryland College Park
       From Left: Michael Figueroa, Abiodun Osho, ,Yilin Liu, Arthur M. Grace, Matthew Castille,
       Margaret Bennett, Patrick Stahl, Aron Cepler, Paul Pineau
       Not Pictured: Brian Smith

Special Thanks to Professor Gary Rubloff and Parag Banerjee for their guidance and technical support.

Thanks also to Susan Beatty, Dr. Tim Zhang, Laurent Henn-Lecordier Tom Loughran, Cytimmune, and
                                       Northrop Grumman.
           Work completed at Materials Teaching Lab, FabLab, LAMP Lab, and NISP Lab.
•   Motivation
•   Logistics
•   Timeline
•   Fabrication of Nanostructures
•   Different Device Concepts
•   Integration and Testing of Devices
•   Lessons Learned
•   Future Work
•   Summary
• The unique characteristics of ZnO nanomaterials
  (wide band gap, piezoelectric effect) lends them
  great potential for UV and pressure sensing
• Nanomaterials require precise structural and
  electrical characterization which are complicated
  by their size.
• In order to use ZnO nanostructures successfully
  in macro-scale devices, we must develop
  effective means to integrate nanostructures into
  a working device.
• The ENMA 490 class was divided into 5 different
  groups: Research, Management/Writing, Synthesis,
  Device Design, Integration/Application
   – Research: Use scholarly articles to understand properties of
     ZnO nanoparticles
   – Management/Writing: Guide all groups in the right direction;
     major writer of report and presentation
   – Synthesis: Making nanoparticles
   – Device design/fabrication: Work out physical problems with
     device (i.e. attaching nanoparticles to substrate)
   – Integration/Application: Testing of devices produced
• Group membership was not static; many members
  moved around to different groups as needed
2/12         2/26           3/12                  3/12               4/2                 4/9
First Nano   Made           Finish Mid term       Take SEM of        Made 6 PDMS         Made
structures   Electrode      presentation          First of first     Test First Device   Kapton
             mask           SEM of Gold           Finish Mid         Testing of 6        tape
                            Nano particles        term               PDMS device         device

                                          Spring Break

                4/16                   4/16
                Made PDMS              Test                  4/23
                covered                PDMS                  Made quantitative testing
                device                 devices               device, PVA, Kapton, PVA
 Synthesis of the Nanostructures
     Used Vapor Phase Transport Method

O2    Ar

                                         • Argon and Oxygen gas flow
                                         • Bubbles per minute controlled the rate
                          Glass Tube     • Inside the tube, solid reagents were
                                         placed in the boat.
     Gas Flow                            • In some runs, Si wafers with gold
                                         catalyst were placed downstream from
                                         the boat.
     Alumina Boat
            Processing Condition A: No Wafer
•   Solid Reagents: Zn metal powder
•   Gas Flow: Ar- 30 bubbles/min
                  O2- 10 bubbles/min
•   850 Celsius and held for 90 minutes.
•   Initially only Ar gas, then turn on O2 gas
    when system reached 410 degrees.
•   Results show a fluffy white product
    throughout much of the tube.
•   See Zinc Oxide Tetrapod Structures
    (ZOT) from ESEM images:

      Scale Bar: 20μm                            Scale Bar: 5μm
    Processing Condition B: Colloidal Gold
•Reasoning: Instead of Zn and O2 reacting        Si Wafer, Scale Bar: 1μm

in air (or on the glass tube/ceramic boat) to
form tetrapods, a catalyst-induced
nucleation may cause linear rods to grow.

•Cytimmune: 26 nm in 133 μg/ml H20

•Applied colloid to Si and GaN wafers.

•Placed Zn powder in boat and wafers
                                                GaN Wafer, Scale Bar: 1μm
downstream from boat. Same gas flow and
temperature conditions as Experiment A
             Processing Condition C:
        ZnO and Graphite with Colloidal Gold
•   ZnO and graphite source material
    instead of Zn powder.
•   Use 1:1 ratio of ZnO and graphite                                                 Scale Bar: 2 μm
    powder. Only use Argon for gas flow.
    Heat to 900 degrees and dwell for 90
•   Mechanism: ZnO powder is reduced
    by graphite to form Zn and CO vapor at
    high T (carbothermal reduction).

•   Zn vapor flows downstream to form
    alloy with Au colloid.
•   Vapor-Liquid-Solid growth: ZnO
    nanowires form, possibly from Zn and
    CO reaction.

            Tube setup from left: GaN, Si, Boat with mixed 1:1 ratio of ZnO and graphite
    Processing Condition D: Gold Coated Wafers
•   Motivation: a uniform 10 nm gold-
    coated Si wafer will induce better
    nucleation sites for nanorod growth
    than colloidal gold
•   Used ZnO and C powder with Ar and
    O2 gas
•   Heated to 900 Celsius
•   See rod-like structures, forming more
    material than with gold colloid
    processing conditions.

    Scale Bar: 10μm
                                            Conclusions from nanostructure synthesis:
                                            •Processing Condition A (without wafer)
                                            produced high yield of ZnO tetrapods
                                            •Methods employing wafers with gold
                                            colloid or coating were able to grow
                                            nanorods, but not in high enough yield to
                                            collect and use in devices.
                                            •All devices used ZOTs fabricated from
                                            processing condition A.
    Horizontal and Vertical Device Concepts
      Horizontal Device Top View               Vertical Device Cross Section

                              Figure A

                               Figure B

•   In Figure A, the unaligned            •   In both figures, the light blue area
    nanostructures are placed                 indicates nanomaterial, yellow
    between two electrodes.                   block show the polymer matrix,
                                              and dark blue shapes represent
                                              the electrodes
•   In Figure B, a pattern is used to
    induce alignment in rod-like
    nanomaterial.                         •   One concept involved use of a
                                              patterned catalyst (red) to grow
                                              vertically aligned nanorods.
    Horizontal Device Processing
                                                                                     Photoresist   Active Area


                                                               Contact Holes        Au Electrode

•   Multiple electrode variations were fabricated
•   Common features include a protective layer of photoresist to cover the electrodes and two holes in
    the layer to provide contact points
•   One version had trenches between the electrodes. If a polymer-dispersed method was used, the
    trenches were intended to keep the liquid dispersed nanomaterial in place.
•   Another version has one open field between the electrodes.
•   Two masks were used to create each electrode design
             Integrating Nanomaterial into
     • Langmuir-Blodgett and integration via PDMS fluidic flows
       were investigated. These techniques were considered
       too complex for our time frame.
     • Polymer matrix dispersions chosen
     • Various polymers were researched and utilized in
           Kapton                            PDMS                     PVA Glue

                                        Jung, et al. Nano Letters,
                                        Vol.,6, No. 3 413-418. 2006
                    Our First Device
                                             Scale Bar: 20μm
•   Two silver paint contacts on
    a microscope slide ~½ inch
•   A pile of ZOT was placed in
    between (see A).
•   More silver paint was
    applied at the contact
    regions (see B).

                                          An ESEM image of the Ag contact / ZOT interface.
                                                                     Scale Bar: 5μm


                                   A higher magnification of the Ag / ZOT interface. The arrow
                                   indicates a ZOT arm clearly embedded in the contact metal..
   Proof of Concept: Initial Device Testing

• The device was tested on
  the probe station in the
  Kim Building teaching lab.
• Force was applied by
  hand with a glass rod
  laying across the
• Light was applied with a
  lamp at the work station
• From both tests we saw
  electrical response and
  decided to investigate
  pressure applications of
  ZnO nanostructures
  Considerations: Pressure Device
• Before we integrated the ZOT into our microdevice we
  needed to test several approaches by making macrodevices
   – Kapton Tape
   – PDMS
   – Poly Vinyl Acetate Glue
• The qualitative results obtained from these macrodevices
  would allow us to select the best method of integration into
  our microdevices.
• Considerations we wanted to address in our macrodevice
   – ZOT connectivity in the given integration method
   – Noticeable electrical response to stimuli
   – Structural Stability of the device
   – Ease of integration into microdevice
     Zinc Oxide Tetrapods (ZOT) in PDMS
• For our pressure sensing device,
  we investigated two devices that
  used PDMS. The first device
  contained a mixture of PDMS and
  the ZOT powder

                                  The IV curve of 29 wt. % ZOT shows a lack of
                                  repeatability. It was determined through ESEM
                                  images that there was little connectivity between
                                  the dispersed ZOT at this wt.% High wt.% designs
                                  were too mechanically unstable to test.

 29 wt. % ZOT suspended in PDMS
             PDMS Cover Design
• Pile of ZOT (similar to device
  one) covered with PDMS and
• When we reversed the device,
  unique characteristics were
  observed as shown in the
                                        ZOT powder covered with PDMS
• From this device we concluded
  that:                                                                 IV C urve P DMS C over on Z OT

   – When we reversed the                                                                        6.00E -09

                                                                                                 5.00E -09

     polarities, the results were not                                                            4.00E -09

                                                                                                 3.00E -09

                                        C urrent (Am ps )
                                                                                                 2.00E -09

   – There was a noticeable
                                                                                                 1.00E -09 0

                                                                                                0.00E + 00
                                                            -15         -10                -5                0            5              10      15

     response to pressure                                                                       -1.00E -09

                                                                                                -2.00E -09

   – These unique characteristics                                                               -3.00E -09
                                                                                                       Volta g e (v)

     were too complex for our                                     Tes t 1
                                                                  Tes t 3 (s ec ond revers al tes t)
                                                                                                                       Tes t 2 (R evers al)
                                                                                                                       Tes t 4 (40.996 grams )

                         Kapton Tape over Nanorods
• For our second device we decided to essentially bundle the
  nanostructures with Kapton tape
• From our tests we concluded that there is an electrical
  response with the application of pressure within the device
  but integration of the Kapton tape into a microdevice would
  be very difficult
                                   IV Curves for Kapton Tape



  Current (Amps)

                                                                   Applied Force Test 1
                             0.00E+00                              Applied Force Test 2
                   -6   -4    -2       0       2     4         6   No Force Applied



                                 Voltage (V)
                     PVA Adhesive and ZOT
                                                                   IV Curves for Glue Pressure Tests
• Neutral pH adhesive (polyvinyl
  acetate based; PVA) was                                        3.00E-07

  diluted with water
• Integration Methods:                                           1.50E-07

                                      Current (Amps)
   – dripping diluted glue onto                                  1.00E-07                              No Pressure
                                                                 5.00E-08                              First Pressure Test
      the powder in place                                        0.00E+00                              Second Pressure Test

   – mixing a slurry of glue,                          -6   -4   -2
                                                                           0       2     4       6

      water and ZOT and                                          -1.00E-07

      depositing this on the                                     -1.50E-07

      substrate.                                                     Voltage (V)

• From this we concluded:
   – The nanorods had an
      electrical response with the
      application of pressure
   – There was relative ease in
      making the device
   – That we had created a
      structurally stable structure
          Device Selection
• From the qualitative data we obtained from
  the macrodevices, we decided to further
  our venture into a microdevice using PVA
  glue as an integration staple because:
  – We felt it would be easiest to integrate
  – There was discernable electrical response
    (which allowed us to assume there was good
  – The structure of the device was stable
Quantifying Pressure Sensing
• In order to adequately access whether the conductance/resistance
  is changing with pressure we made a special platform that allowed
  us to apply quantifiable pressure on top of a device.
• What we expect is that the conductance should decrease with the
  application of pressure

    Apparatus created to apply quantifiable
    pressure on device. Rubber tip diameter is
      Pressure Testing: Glue Dispersed Powder
           on the Au Patterned Electrodes
                                 Pressure on Glue-Dispersed ZnO - Trial 1


                                                                                  Increasing Pressure


                                               0.00E+00                                             4.5kPa
           -4.5   -3.5    -2.5       -1.5          -0.5        0.5   1.5   2.5   3.5        4.5
                                               -1.00E-05                                            15kPa

                         Increasing Pressure


As the applied pressure increases, a noticeable decrease in the slope is shown.
                           Pressure Testing: Glue Dispersed Powder
                                on the Au Patterned Electrodes

                                                          Pressure on Glue-Dispersed ZnO



Resistance (ohms)

                                                                                            Increasing Pressure           3kN
                                                                                            Increasing Resistance         4.5kN
                                                             1.40E+05                                                     15kN


                    -4.5        -3.5      -2.5     -1.5       -0.5          0.5       1.5   2.5           3.5       4.5

                           Resistance varies with pressure between 1.5 and 3.5 V in both positive and negative
            Lessons Learned
• Working in a group of 10 is challenging:
  – Organization
  – Communication
  – Scheduling
• Even though BlackBoard is a good tool,
  individual lab notebooks would have been useful
• Competition for resources/equipment is fierce
• Planning and executing a multi-phased project
  requires technical foresight.
               Future Work
• Better apparatus for applying quantifiable
• Worth quantitatively investigating capacitance as
  a function of voltage
• Comparing electrical and piezoelectric response
  of nanorods versus tetrapods
• Modeling pressure versus resistance from data
  we obtain is necessary in order to:
  – Make a predictable device
  – Understand the nanomechanics of ZOT PVA
• Synthesized nanorods and tetrapods using vapor
  transport growth
• Used lithography to create two horizontal electrode
  designs on silicon wafers
• Integrated tetrapods into macroscale devices with:
   – Kapton Tape
   – PDMS
   – PVA Glue
• Observed I-V response to pressure and light stimuli in
• Quantified pressure effects on resistance on a wafer

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