Fabrication of Electrodes with Nano-Size Gap
Brian A. Higa
University of California, Irvine
Electrical Engineering
Mentor: Professor Peter J. Burke
Graduate Students: Lifeng Zheng and Sungmu Kang
University of California, Irvine
Department of Electrical Engineering and Computer Science
Engineering Gateway
Funded in part by the National Science Foundation
and the Undergraduate Research Opportunities Program
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Abstract:
As of now, the presence of intense study in nanotechnology suggests that the future will
need electronic devices that can measure the electrical properties of atoms and molecules.
Current fabrication methods of these devices require abstruse techniques and expensive
equipment. We propose a simple and inexpensive method of fabrication by electroplating metal
onto electrodes with a large gap (0.25 microns) in between them. We hypothesized that the
deposition rate can be controlled by monitoring the electronegativity of the electrodes and the
electrolyte concentration. We found that during a specific stage in metal deposition there was
rises in the conductance between electrodes by increments of 2e2/h. This stage shows the
quantization of conductance which can be interpreted as the first atoms closing the gap between
electrodes. With this result we conclude that metal deposition can be controlled which can be
used as an alternative fabrication method of electrodes with nano-size gap.
Key Terms:
Cation, electronegativity, electroplating, deplating, metal deposition rate, Ohm’s law,
quantization of conductance (QOC)
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Introduction:
The purpose of my research is to explore a method of fabricating electrodes with nanosize gap. The many disciplines of nanotechnology are being forestalled due to the lack of an
effective and inexpensive method to fabricate these devices at a large-scale. Current fabrication
methods require scanning probe microscopy, advanced lithography and special contacts. These
methods produce fragile devices at an exorbitant cost. The exorbitant cost results from the
method’s inefficiency to produce devices with punctilious gap sizes. They also need to be
produced in a remote and quiescent environment because even the slightest vibrations from an
iterant truck can stymie the fabrication process.
The pith of my method of fabrication involves starting out with electrodes with large gaps
of 100nm to 2um and allowing metal ions to adhere to the surface of the electrodes by
electroplating and metal deposition. As metal ions accrete to the surface it also shrinks the size of
the gap between the electrodes. This process is called metal deposition which is depositing metal
ions onto the surface of a metal. Electroplating is making the electrodes negatively charged in
respect to another metal so that the metal ions will be attracted to the electrodes and metal
deposition can occur. Fig. 1 shows two electrodes before and after electroplating.
Fig. 1: Left is before electroplating and right is after electroplating.
Notice that the gap between the electrodes decreases after electroplating.
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In comparison to current methods, my method is cheaper and
simpler because my method does not require expensive equipment nor
to be located in a remote and quiescent area. Controlling the metal
deposition rate, which is done by regulating the electronegativity of the
electrodes and how many metal ions are present, is not convoluted
and it is the cardinal reason why my method can produce accurate
Fig 2: Key before and
after electroplating
gap sizes efficiently. Another reason that adds to the simplicity of my method is that
electroplating is observable without the need of optical devices (Fig. 2).
These devices prove to be essential to measuring the electrical
properties of atoms and molecules and they can be implemented in
molecular electronics (Fig. 3). These devices can also be implemented
as electronic switches that can consume less power than the inimitable
metal-oxide-semiconductor-field-effect transistor (MOSFET). The
MOSFET (Fig. 4) is a three terminal device where a specific
Fig. 3: Molecular
Electronic Spectroscopy
terminal called the gate regulates the formation of an inversion
layer on the substrate that allows the conductance between the
other two terminals called the drain and source. The gate needs to
be kept at the required voltage to maintain the desired inversion layer.
Fig 4: MOSFET
Unlike the MOSFET my device only needs the required voltage for a
finite amount of time to obtain the desired conductance between the two electrodes.
Other applications for these devices include operating as a chemical detector. Due to the
fact that these devices are designed to measure the electrical properties of atoms and molecules
they can also be implemented to allow specific atoms and molecules to bind to the surface of the
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electrodes. Chemicals in an unknown solution (Fig. 5) can easily be
identified from observing if a specific atom or molecule is extant to
bridge the gap between the electrodes. These devices can also brook
astringent and acidic solutions which can broaden its utilization.
Fig 5: Chemical Spill
These devices are also applicable to serve as metal
collectors. They have a proclivity to allow metal ion to adhere
to their surface by electroplating and metal deposition. At high
metal deposition rates these devices can effectively collect
metal ions from aqueous and viscous solutions. This can
improve modern metal recycling techniques (Fig. 6) and prolong
Fig 6: Metal Recycling
the ineluctable depletion of raw metals and alloys.
The research of Krahne et al. gave me inspiration for my research by demonstrated how a
network of electrodes with nano-size gaps can be fabricated for use in integrated circuits. Their
method of fabricating is not by electroplating but by lithography. Although their work is not
apposite to mine, they showed me how these devices can be used in integrated circuits to study
the properties of nano-size objects. Since I am an electrical engineering major this really got me
interested in my research.
My Method:
My research incorporates the methods used in the research of Morpurgo et al. Their
research involves fabricating electrodes with nano-size gaps by electroplating using potassium
cyanaurate [KAu(CN)2] and a buffer (pH 10) composed of potassium bicarbonate (KHCO3) and
potassium hydroxide (KHO). In other words, the metal they used for electroplating is gold. My
method is to experiment to see if electroplating works for a different metal, copper. Although
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gold and copper have similar electric conductivity, they are quite disparate from each other. For
example, a gold atom is much bigger and heavier than a copper atom which means that the
Brownian motion of gold is slower than copper. This means that copper moves faster than gold
and could possibly mean electroplating is faster with copper than with gold. On the other hand,
copper has an inherent tendency to form copper oxide (CuO) very easily. This can create
problems when electroplating for a long period of time.
I also adopted methods from the research of Li et al. They used copper for electroplating,
but it wasn’t used for fabricating electrodes with nano-size gaps. It was used to show the
quantization of tunneling current which is not related to my research. What I learned from them
is the chemicals that they used for electroplating. They used 1mM copper sulfate (CuSO4) and
10mM sulfuric acid (H2SO4). I learned that H2SO4 prevents the formation of CuO and it is the
cardinal reason why they used it. This colored me to add an acid in my solutions.
The process that is the nexus of my research is electroplating. Electroplating is making
the electrodes negatively charged in respect to a metal so that the metal ions will be attracted to
the electrodes. This happens because metal ions are cations which are ions that are positively
charged due to having lost electrons by ionic bonding. This
is why metal ions have a propensity to be attracted to
Vd c
Metal (Cu Wire)
negatively charged objects. Electronegativity is how much
a metal is at a lower voltage than another metal.
Controlling the electronegativity of the electrodes is
accomplished by using a DC voltage supply (Fig. 7).
The positive pole is connected to a metal and the
negative pole is connected to an electrode.
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Electrodes
Fig. 7: DC voltage supply controls
electrodes’ electronegativity
In my experiments I have proven that there is a direct relationship between
electronegativity and metal deposition rate. Metal deposition rate is the average number of metal
ions being deposited on a surface of the electrodes per unit of time. At low electronegativity,
metal deposition is slow resulting in gradual and uniform electroplating. Complete closing of the
gap between electrodes occurs after several hours or longer. At high electronegativity, metal
deposition is fast resulting in capricious and non uniform electroplating. Complete closing of the
gap between electrodes occurs within seconds or minutes. Fig. 8 is a sample of electrodes after
electroplating at a low metal deposition rate and Fig. 9 is a sample of electrodes after
electroplating at a high metal deposition rate.
Fig 8: Low metal deposition rate
Fig 9: High metal deposition rate
Another factor that affects the metal deposition rate is the amount of metal ions available
for electroplating. Higher concentration of metal ions results in more metal ions available for
electroplating and faster metal deposition. Lower concentration of metal ions results in less metal
ions available for electroplating and slower metal deposition. However, unlike electronegativity
it is not pragmatic to easily adjust the metal ion concentration. It is also very untoward to know
the exact concentration because the solution is always evaporating and concentrations are never
completely uniform. Therefore adjusting the metal ion concentration to control the metal
deposition rate is not feasible.
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The goal for running experiments is to aver that my method can control the gap size
between electrodes. I can do this by determining a metal deposition rate that results in a constant
decreasing gap size rate. A constant decreasing gap size rate allows fabrication of electrodes with
precise gap sizes. This rate can be shown by
5
4
G (2e^2/h)
observing the quantization of conductance
(QOC) in the gap between the electrodes.
3
2
1
QOC is defined as conductance (G)
0
2
increasing by increments of 2e /h in
0
50
constant time intervals (∆t) (Fig 10). “e” is elementary charge
constant and “h” is Planck’s constant. The unit of conductance is
100
150
Time (second)
200
250
Fig 10: QOC
Y axis – Conductance (G)
X axis – Time (sec)
2e2/h because this is the
approximate conductance of an
atom. QOC is the stage in
electroplating when the very
first metal ions bridge the gap
between the electrodes. In
other words, what is observed
is one metal ion being
- Metal Ion
Quantization of Conductance
- Electrodes
deposited onto a surface of the
electrodes per ∆t (Fig. 11).
QOC is not observed at high
Fig 11: Going up to down then left to right. Electroplating
with gap size decreasing at a rate of 1atom per ∆t. The
region that is dark green is QOC which is what I observe.
conductance values because the
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bridge of metal ions between the electrodes has a large surface area (Fig.
12). This results in exponential and erratic increases in conductance.
Deplating is when the metal ions are repelled from the electrodes.
This is done by making the electrodes positively charged in respect to a
metal. This process results in reversing the effects of
electroplating. Unfortunately in my research, I was unable to
Fig. 12:
Top – low G values
Bottom – high G values
return a sample back to its intrinsic state after electroplating. An
abundant amount of metal ions do not come off the electrodes. This is something that I look
forward to working on in the future.
Methods and Materials:
The first step is to design the program on LabVIEW that will read the data from the lockin amplifier, analyze the data and record it on a spreadsheet file. The data read from the lock-in
amplifier is the AC voltage difference between two nodes. This data can be converted into
conductance by Ohm’s law: V = I * R (1). The program should also take data every second for a
given amount of time.
The second step is to prepare the solutions that I used in my
research. Before doing anything it is recommended to wear safety
gloves and glasses. I put into three containers 20uM CuSO4, 40uM
hydrochloric acid (HCl) and 200uM sodium hydroxide (NaOH)
(Fig. 13). CuSO4 is the electrolyte that contains metal ions for
metal deposition and it is directly proportional to the metal
Fig. 13: From left to right.
CuSO4, HCl and NaOH
deposition rate. HCl is an acid used to occlude the formation of CuO. CuO is not conductive and
sticks to the electrodes which engenders failure in an experiment. NaOH is for neutralizing the
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acid after I finish an experiment so that I can discard the solution in the sink. Next I get a beaker
and fill it with 2liters of water. I use a pipette to add the desired amount of CuSO4 and HCl into
the beaker. I prefer to add in discrete amounts of 5milliliters. That way each 5milliliters of
CuSO4 corresponds to adding 50uM CuSO4 into the beaker and each 5milliliters of HCl
corresponds to adding 100uM HCl into the beaker. I would advise that the beaker be placed in
area that is not susceptible to vibrations. My research is about measuring the conductance
between a very small gap and it is sensitive to noise and vibrations.
The next step is to make the circuit and to set up necessary
connections. I start by soldering metal contacts onto the electrodes using
indium. An effective way to prevent the contacts from breaking off is to
spread the indium across a large area (Fig. 14) because the strength of the
adhesion is directly proportional to the contact surface area. Then I hook
up the lock-in amplifier to the computer via a GPIB cable and I also take
Fig. 14: Soldered
contacts
note of the GPIB address of the amplifier so that the computer can read it. I set the frequency and
amplitude of the signal generator of the lock-in amplifier to 13Hz and 3mV respectively. Then I
connect it to input A of the lock-in amplifier. I also connect it to a
10kohm resistor. The resistor is connected to input B and to one of
the electrodes. The other electrode is grounded. Fig. 15 shows all
the connections made to the lock-in amplifier except the GPIB
cable. A simple method to check to see if the connections and
Fig. 15: Connections to
Lock-in Amplifier
LabVIEW program are correct is to connect a 13kohm resistor to
input B and observe the data recorded on the computer. It should read very close to 1 because
2e2/h is equal to the reciprocal of 13,000. Then I hook up a DC power supply and connect the
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positive pole to a copper metal and the negative pole to the grounded electrode. The copper
metal should be clean because if it is dirty it can introduce problems that can hinder the result of
an experiment. Then the electrodes and metal are submerged in the beaker containing known
concentrations of CuSO4 and HCl (Fig. 16). The beaker is covered with aluminum foil to
prevent contamination. After this the setup is complete
and it should look like Fig. 17.
Fig. 16: Beaker with
electrodes and copper metal
DC Supply
Lock-In Amplifier
B
Vd c
10 k
A
Copper Wire
Va c
Electrodes
Beaker
Fig. 17: Schematic of the experiment
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For deplating I reverse the DC voltage by switching the connections. The positive pole is
connected to the grounded electrode and the negative pole is connected to the copper metal.
Analyzing data from the spreadsheet file made from LabVIEW is done using the IGOR
program. This program can convert the spreadsheet file into limpid tables and graphs. It can also
do histograms which are very useful for compressing all of my results into one graph.
Results:
Before doing actual experiments to
progress in my research I ran experiments to
Conductance vs. HCl Concentration
0.30
come up with reasonable parameters for sundry
0.25
amplitudes for the signal generator and came up
G (2e^2/h)
variables. I tested different frequencies and
0.20
0.15
with 13Hz and 3mV. I learned that higher
0.10
frequencies and amplitudes results in little or no
0.05
metal deposition at the surface near the gap and
0
50
100
150
200
250
300
350
uM HCl
lower frequencies and amplitudes results in the
Fig. 18: Conductance vs. uM HCl
experiment being too sensitive to noise. I also have
done an experiment of the contribution HCl has to conductance (Fig.
18). I learned that too high a concentration of HCl results in very
high background conductance and therefore the experiment becomes
AC amplitude: 3mV
Frequency:
13Hz
HCl:
300uM
CuSO4: 50 or 100uM
very sensitive to noise. I came up with 300uM as a reasonable
concentration of HCl. Table 1 summarizes my setup parameters.
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Table 1: Setup
Parameters
Fig. 19 and 20 are QOC that I observed in my research. Note that Vdc means the DC
voltage which is the electronegativity strength.
12
11
10
9
8
G (2e^2/h)
7
6
5
4
3
2
1
0
1000
1100
1200
1300
1400
1500
Time (second)
Fig. 19: Sample D37 at 2Vdc and 50uM CuSO4
6
5
G (2e^2/h)
4
3
2
1
0
500
1000
1500
2000
Time (second)
Fig. 20: Sample D29 at 2Vdc and 100uM CuSO4
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Fig. 21 is a histogram of the occurrences of conductance for all the experiments in my
research. Notice that there are peaks in conductance near 1, 2, 3, 4, and 6. This supports that the
conductance of atoms is in units of 2e2/h. On the next page, there are images taken using a
scanning electron microscope (SEM). Fig. 22 is my devices before electroplating and Fig. 23 is
my devices after electroplating. The green circle that appear on the images is the approximate
size of 2000 atoms.
Fig. 21: Conductance vs. Occurrence
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2000 Atoms
Fig. 22: SEM image of electrodes before electroplating
2000 Atoms
Fig. 23: SEM image of electrodes after electroplating
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Discussion:
My empirical data from my research proves that my method of fabricating electrodes
with nano-size gap is controllable. By observing QOC right after there is a connection between
the electrodes, it can be interpreted that the metal ions are being electroplated on the surface of
the electrodes at a rate of one atom per ∆t. By looking at Fig. 24 it can be shown that each jump
in the conductance results in the thickness of the connection between the electrodes to increase
by an atom. In other words, the metal deposition rate is one atom per ∆t. Since the metal
deposition rate only depends on the electronegativity and metal ion concentration it can be
concluded that this rate occurs throughout the entire experiment.
6
3 Atoms thick
G (2e^2/h)
5
4
3
2
1
2 Atoms thick
0
500
1000
1500
Time (second)
- Metal Ion
1 Atom thick
- Electrodes
Fig. 24: Each jump in conductance corresponds to the connection
between the electrodes increasing by an atom. By observing the QOC,
the connection thickness is increasing by 1atom per ∆t. Therefore the
metal deposition rate is controlled.
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2000
With more time I should be able to improve my results of conductance vs. occurrence in
Fig. 21. Specifically, I should be getting a peak at 5. I noticed in many of my experiments when
the gap size is very small that it sometimes skips some
jumps in conductance such as from 1 to 4 in Fig. 19 and
sometimes there are gradual increases in conductance
such as from 4 to 5 in Fig. 20. These problems can be
prevented my lowering the metal deposition rate. By
looking at my samples using a SEM I noticed that
electroplating is not uniform. The web-like impurities (Fig.
Fig. 25: SEM picture of
electrodes after electroplating
25) are chunks of metal ions that got deposited. I have tried
experiments with very low electrolyte concentration and low electronegativity, but it would take
a long time for the gap between electrodes to close. I also encountered new problems at low
metal deposition rate that required me to change my setup
parameters in Table 1. I needed to lower the AC
amplitude, frequency and HCl concentration. This made
my experiments very sensitive to noise and vibrations.
I wasn’t able to get deplating to work in my
experiments. As I said before, many metal ions do not
come off the surface of the electrodes when the DC voltage is
Fig. 26: SEM picture of
electrodes after deplating
reversed (Fig. 26). I believe that there might be impurities in the
solution that prevent metal ions from coming off the surface of the electrodes.
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Acknowledgements:
I would like to use this opportunity to thank the National Science
Foundation (NSF) for funding the IM-SURE program at UCI. I would like to
sincerely thank Said M. Shokair, Edward M. Olano, Sarah Martin, and the
rest of the UROP team for their meritorious efforts in making this edifying
experience possible. I would also like to give special thanks to John Porter for
arranging me to use the SEM provided by Carl Zeiss, Inc. As a final point, I would like
to express my gratitude to Professor Peter J. Burke, graduate students Lifeng Zheng
and Sungmu Kang, and the rest of the Burke research group.
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Work Cited
Krahne, R., Yacoby, A., Shtrikman, H., Bar-Joseph, I., Dadosh, T., and Sperling, J. “Fabrication
of nanoscale gaps in integrated circuits.” Applied Physics Letters 81.4 (2002): 730-732.
Li, C. Z., He, H., and Tao, N. J. “Quantized tunneling current in the metallic nanogaps formed by
electrodeposition and etching.” Applied Physics Letters 77.24 (2000): 3995-3997.
Morpurgo, A. F., Marcus, C. M., and Robinson, D. B. “Controlled fabrication of metallic
electrodes with atomic separation.” Applied Physics Letters 74.14 (1999): 2084-2086.
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