Electrochemistry in Nanoelectronics _ Nanosensors by suchenfz

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									        Electrochemistry
in Nanoelectronics & Nanosensors


               N.J. Tao

       Arizona State University
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    Electrochemistry




                Nanoelectronics
Nanosensors
Electrochemical Nanofabrication

    • Electrodeposition & etching
    Electrodeposition: Then … and Now…
• Ancient origin. Romans soldered
  silver plates to articles of metals
  and in the 5th century iron
  weapons were coated with
  copper by dipping them in a
  copper solution. During the 18th
  century, plating of copper or
  brass with silver by fusion started
  in England.

• IBM announced in1997 a new
  advance in semiconductor
  process that entails replacing
  aluminum with copper. Cu has
  less "resistance" than Al.
      Local Probe Approach (STM & AFM)

The clusters can be dissolved by changing the sample potential and afterwards the
blank Au surface can be imaged again.




Array of 10 x 10 Cu clusters at           The same surface area after complete
Esubstrate = +10 mV vs. Cu/Cu2+.          dissolution of the clusters at Esubstrate =
                                          +300 mV.


                                                             Kolb et al, 1998
            Template Methods: Negative


                                  Nanowires




• The beginning: Possion used etched ion tracks in mica sheets as templates to
fabricate metal wires. P. E. Possion, Rev. Sci. Instrum. 41, 772 (1970).

• The templates: Ions tracks in mica or polycarbonate membranes, anodized
alumina, phase segregated copolymer films are the popular choices.
Building Block of Nanoelectronic Devices –
            Molecular Junctions

                                 Top: Scheme for preparing
                                 nanowire devices
                                 by:1)self-assembly of a
                                 MHDA monolayer, or
                                 2)layer-by-layer assembly
                                 of TiO2 /PSS multilayer
                                 film on the exposed tip of
                                 a bottom metal electrode,
                                 followed by electroless
                                 seeding and electroplating
                                 of a top metal electrode.




              Penn State Group
Building Block of Nanoelectronic Devices –
            CdSe Nanojunctions
   Au-CdSe-Au




        Ni-CdSe-Ni




                            • Graph of CdSe segment length vs the
                            number of cyclic voltammetric scans for 350-
                            nm diameter nanowires. Error bars show the
                            standard deviation in length.

      Penn State Group. J. Phys. Chem. 106, 7458(2002)
                     Positive Templates




• Positive template method uses wire-like nanostructures, such as DNA and
carbon nanotubes, as templates, and nanowires are formed on the outer surface
of the templates.
• Unlike negative templates, the diameters of the nanowires are not restricted by
the template sizes and can be controlled by adjusting the amount of materials
deposited on the templates.
            Carbon Nanotube Template


                                                                        Pt
                                               Au




• After Pt or Au deposition on SWNTs, the sample was annealed at 600°C in air
for 10 min, which leads to Pt/Au nanoparticles forming chain-like structures.
• In contrast to ordinary electroless deposition, no reducing agents are needed
for SWNTs.


                                             Dai et al JACS 124(31)9058, 2002.
DNA Template

        • The first step is to fix a DNA strand
        between two electrical contacts.
        • The DNA is then exposed to a solution
        containing Ag+ ions. The Ag+ ions bind to DNA
        and are then reduced by a basic
        hydroquinone solution to form Ag
        nanoparticles decorating along the DNA
        chain.
        • The nanoparticles are further ‘developed’
        into a nanowire using a photographic
        enhancement technique.




         Braun, E. et al. Nature 391, 775, 1998
         Graphite Step Edge Template

Step 1


                        Step 1: Electrodeposit Pd
                        nanowires.
Step 2
                        Step 2: Transfer the Pd wires to a
                        glass slide.

                        Step 3: Apply silver contacts.
Step 3




                                            Penner et al.
Graphite Step Edge Template




                          Penner et al.
    Applications
•   Nanoelectronics
•   Nanomechanics;
•   Optoelectronics;
•   Chemical and biosensors;
•   Catalysis;
•   Energy related;
•       - …..
   Electrochemistry




                 Nanoelectronics
Nanosensors
        “The 100 million MIPS to match human brain power
        arrive in home computers before 2030”

When will computer hardware match the human brain?
                                Hans Moravec
                              Robotics Institute
   MIPS – Million Instructions per Seconds
                          Carnegie Mellon University
                       Pittsburgh, PA 15213-3890, USA
                        Nanoelectronics
• Electronics based on new phenomena occurring
at Nano-scale
                        Single Electron Transistor


 Ballistic transport
                                                              Kondo effect
                              Electron tunneling



          Spintronics                      Molecular Electronics


                                   Localization
Single Electron Transistor (SET)
         What is a transistor (FET)?
                    0V                  Vg           Vd > 0


                               METAL          GATE                     Gate
                                   OXIDE LAYER
                    p+   or                          p+ or
                    n+   Si                          n+ Si


                                   n- or p-TYPE Si
                                                              Source                    Drain


                                         0V




                              Ec                                                   Ec
                              EF                                                   EF




                              Ev                                                   Ev

n-TYPE   p-TYPE    n-TYPE                            n-TYPE   p-TYPE      n-TYPE

         Off (0)                                                On (1)
                 Single Electron Transistor
                                     • Capacitor charging energy:

         Gate                               Q 2 Q 1e e 2
                                         E      
                                                
                                            2C        2C
                Dot                               e2
Source                       Drain       eV  E     ;       C ~ size
                                                  2C

                                     • To avoid thermal broadening:

                      e2/C                     e2
                                            E     k BT
                                               2C
    EF                                T=300 K (room temperature)

                                             C  1018 F
                                              C  2 0 d          (a sphere)
   What is Tunneling?




        L        2m(U  E )
               2
T ~e                  
            Single Electron Transistor
                                              Coulomb
         Coulomb                              staircase
         blockade




          Gate


Source           Dot                   =

                       • Charge is quantized
                       • Electron Tunneling (R1, R2)
      Local Oxidation with a Thin Water Film

                         STM or AFM tip
                                           • Substrate Electrode:
                         Water layer
 5V
                         Ti oxide           M  nH 2O  MOn  2nH   2ne 
      Oxidation          Ti film (3nm)

                                           • Tip Electrode:
                       100 nm

                                            2ne  2nH 2O  nH 2  2nOH 

• The thin water film is extremely important in the practical resolution of the
fabricated structure. A controlled humidity is recommended.

                                              Snow et al. Science, 270, 1639-41 (1995).
                                              K. Matsumoto, Physica-B 227, 92-4 (1996).
  Room Temperature Single Electron Transistor




                                        ~ 30 nm metal
  K. Matsumoto et al.                    quantum dot


The island may seem to be big, but C,
determined by the junction cross
sectional area, is < 10-19F.
       Coulomb Blockade in Electrochemistry
               Au clusters




Shaowei Chen et al. Science 1998, 280, 2098.

              C60




            Echegoyen et al., 1998
Conductance Quantization
                         Classical conductance:
                                                  L
    L >> electron mean free path       D
    D>> lF, electron wavelength




                  L
          R
               D / 2 2




Conductance
                 1
                                       G (G0)
              G  ~ D2
                 R
     G changes continuously as D.
                                                      D
                         Conductance Quantization
L < electron mean free path           ballistic transport (no collisions).
D~ lF, electron wavelength            wave nature of electron important.




                                                                                    Quantized
                          Quantized
       Free                                                      Free
       Motion                                                    Motion

                  N=1                                                      N=2
                D=lF/2                                                    D=lF


                  N
     G  G0  Tn  NG0                                       5     lF ~ 1-3 Å
                 n 1                               G (G0)   4
     where, N=0, 1, 2, 3, …                                  3
     and G0 = 2e2/h=77mS                                     2
                                                             1
              R0=13 kW
                                                                                D
Quantum Confinement & Standing Waves
Conductance Quantization Metal Nanowires


  lF ~ 1-3 Å – must be atomically thin!
   l e ~ nm.

  Room temperature.


        How to fabricate such wires?
Electrochemical Fabrication


                               RE
                                                                       Li & Tao

 Bipotentiostat
                                    Electrolyte                        Appl. Phys. Lett.
                               CE
                                     Metal wire insulation
                                      Substrate
                                    substrate




           +                                                   +
                      +                                                +
+ +                                                       - -           - -
                      +                           -            -
                                                              -
+                         +                           -                 -
                  +        +                                       +     +
 +                                                         +
                                                                              tip


 Etching                                                  Deposition
                                 Etching                                                       Deposition




                               Etching (dissolution)                                             Deposition
Conductance (2e2/h)




                                                                 Conductance (2e2/h)
                      10
                                                                                   10
                       8
                                                                                       8
                      6
                                                                                       6
                      4
                                                                                       4
                      2                                                                2

                           0     1000 2000 3000 4000 5000 6000                             0   20 40 60 80 100 120 140 160
                                        Time (ms)                                                     Time (ms)
From Conductance Quantization to
      Quantum Tunneling
  Etching




    or
  Deposition


                  Li & Tao, Nanotechnology, 10, 221(1999).
                  Morpurgo et al., Appl. Phys. Lett., 13, 2082(1999).


  Large gap
  From Conductance Quantization to
        Quantum Tunneling
Etching                                                    Deposition



                             Tunneling!

                                                                    Large gap
                      4.5                          3
      Gap Width (A)




                      5.5                          2
                                                                       Tunneling




                                                        ln (I/nA)
                      6.5                          1                   current

                      7.5                          0

                      8.5                          -1      Ln(I) ~ width

                      9.5                          -2
                            Time (sec./division)
                      Stepwise Tunneling Current – Log scale

                4.5
                      a                  3                I ~ exp(-bL)   ln(I) ~ - L
                5.5                      2
                                                          • Stepwise ln(I) leads to
                6.5                      1
                                                           discrete change of s!
Gap Width (A)




                7.5                      0




                                              ln (I/nA)
                8.5                      -1
                                  10ms


                5.5   b                  2

                6.5                      1

                                         0
                                                                            Ds
                7.5

                8.5                      -1

                9.5                      -2               • Discrete Nature of Atom

								
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