Download Optimization of Electrochemical Wet Etching of Silver STM

Optimization of Electrochemical Wet Etching of Silver STM Tips
Anthony Davis
Hampton University
Wilson Ho PhD
University of California Irvine
Department of Physics and Astronomy
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Abstract
Various Scanning Tunneling Microscope (STM) experiments have insinuated that silver
molecules exhibit unique optical properties, which can be observed by coupling photons to
individual silver molecules that are electronically pulled from silver tips. One of the challenges
presented is creating these silver tips by electrochemical etching. The naturally soft and
malleable mechanic properties of silver make it difficult to manipulate the metal tip to create an
ideally sharp and conical apex. Thus is it meaningful to examine ways to etch silver tips, that
will reduce the amount of etching time, chemicals used, and reduce the chances of ruining the
apex beyond repair. Voltage is one of the parameters of etching silver tips; we have used
different levels of potential difference to see which have produced an ideal apex. We have also
tested various chemical recipes and monitored evaporation rates of these solutions to gain data
on how they affect the shape of the apex. Results have shown that by applying higher DC
voltage more expedient etching will occur. We have also found that the concentration of the
etch solution (ammonia) changes due to evaporation in particular time intervals which can
affect the shape of the apex. By collecting quantitative and qualitative data concerning apex
structure, etching solutions, and applied voltage the current etching process for silver tips will
be examined and improved. A more ample supply of silver tips that can be easily replenished
will serve as a significant assistance to those pursuing STM experimentation on individual
silver molecules.
Key Terms
STM- STM is an abbreviation that stands for Scanning Tunneling Microscope. The STM
microscope is different from optical microscopes because it instead of using a lens it uses a
scanning probe tip to detect the produce a 3-D map of a substrate for the viewer; a quantum
mechanical process called tunneling accomplishes this.
Tips- The scanning probe that is uses in STM microscopes is commonly referred to as the tip.
Tips are small conical shaped pieces of metal that have an end point that is nearly the size of a
single atom.
Etching- A process used to shape certain materials in different ways. Etching can be done with
both liquids and solids (wet and dry). Different etching methods are usually need for different
materials.
Introduction
As the relatively new fields of nanoscience and nanotechnology are becoming more pervasive,
and consequently more scientific research is being done on the atomic scale. Practical
applications of nanoscience include miniaturized sensors, use of atoms for electric circuit
components, antidotes to bio-toxins, and optimized material structures. In order to for these
extraordinary innovations to be created first a more thorough understanding of nanoscience
must be obtained. Individual molecules and atoms must be examined in order to obtain data on
their characteristic and properties. One of the key components used for this investigation is the
STM microscope. STMs are a part of a family of microscopes known as SPMs (Scanning
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Probe Microscopes). These powerful tool are able to investigate materials on the subnanometer scale, which is not possible with optical and electron microscopes.
The functionality of the STM microscope is fairly complex and involves a quantum mechanical
process known as tunneling. During this tunneling process the tip interrogates the substrate that
the spectator is attempting to view. The tip hovers over the specimen leaving only a mere
nanometer between the apex and the surface of the specimen. A small voltage is then applied
from the tip to the specimen, which is usually a potential difference in the milliVolt range.
When the voltage is applied and the tip is positioned close enough to the surface of the
substrate, electron become excited and can jump from the tip to the substrate, or vice versa.
This quantum mechanical movement of excited electrons is known as tunneling. In order for
the tunneling process to be successful STMs must maintain a constantly low current. A current
between a few picoAmperes and a few nanoAmperes is ideal for most STM experimentation
purposes. Tunneling current is dramatically contingent upon the distance between the apex of
the tip and the atom on the substrate closest to the tip. The numerical relationship between the
tip-substrate distance and tunneling current is exponentially inversely proportional. For
example, due to the laws of physics on the quantum scale, an increase in distance of a single
atomic diameter would result in a current drop by a factor of 1000. To control the distance
regulation, STMs are equip with a piezo-electric feedback system. The feedback electronics
and the piezo-electric electric element mutually mediate the tip-substrate distance, as the tip
scans the material, and automatically adjust the distance to ensure consistently low current.
Pictured in the image is a schematic diagram
of an STM with labeled major components.
A critical factor in making the
tunneling process successful is the shape and
type of tip used. An ideal STM tip should
have a conical shape that ends with an apex
of a single atom. Realistically, this ideal is
at best, extremely difficult to achieve using
current tip fabrication methods. However,
tips that come close to this ideal are
acceptable and are effective in yielding
Figure 1
atomic resolution of materials in STMs.
There are some tip structures that will cause problems in STMs, however. For example, a tip
that has a rounded apex instead of a “V” shaped apex will falter because it will be difficult for
electrons to jump from the tip to the material. This tips is not ideal because instead of having
one atom protruding from the tips further than the rest, there is rather a number of atoms
protruding at relatively close distances.
Recent research done by Dr. Wilson Ho and the Ho Group team has suggested that
silver tips are more efficient, in terms of photon coupling, than tungsten tips by a factor of
approximately 10. “The efficiency of photon coupling depends on the shape and elemental
composition of the STM tip.” (Wu, Ogawa, Ho). This photon-coupling is achieved when laser
illumination is incorporated into the STM. Photons will couple to the tunneling electrons as
they propagate from the tip to the specimen. Consequently, stronger electric fields are formed,
and more comprehensive data can be resolved from the output signals.
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Traditionally, tungsten metal is the material used to make STM tips because of its
strength, durability and high conductivity. Tungsten tips produce acceptably high resolution in
STM images, and their use has been quite common. The pervasive use of tungsten tips has
spawned several different means for etching the tips to resemble the ideal conically sharp shape
discussed earlier. There are several setups for electrochemically-etching tungsten tips both
manually in the lab and using automatic commercialized devices.
The idea of using silver STM tips is fairly new and the etching process is not as simple
as the etching process for tungsten tips. Silver is a relatively soft and significantly malleable
metal that bends without much force. These elemental properties are much more adversely
apparent when handling very thin rods of silver then thicker blocks of the metal. Tungsten is a
stronger and but brittle metal, it is more likely to snap or split than to bend like silver. Due to
the different chemical characteristic of the metals, different electrochemical etching processes
are used. The etching process for silver presents several challenges because of the natural
properties of silver. For example, the primary etching solution (ammonia), evaporates quickly
when left in an open beaker, this causes the concentration of the solution to change,
consequently negatively impacting the etch and tip shape. Also, when silver tips are fabricated
they must adhere to a certain conical shape to ensure stability within the STM. A silver tip that
is too slender is not desirable because the malleable property of the metal will cause it to vibrate
when in UVH (Ultra High Vacuum) inside the STM. This oscillating movement negatively
impacts tunneling.
Several parameters involved in the electrochemical etching of silver tips can be
investigated and optimized through experimentation. The voltage applied during etching is of
interest because is appears that higher voltage levels expedite etching. It is critical to also
examine the apex of the tips when etching at different voltages to confirm that voltage intensity
is not causing apex shape to be compromised for faster etching speeds. The electrolyte used for
etching, ammonium (NH4) is also of interest because the concentration of the solution can
impact the shape of the tip apex. The current flowing through the tip is yet another significant
parameter. By investigating and monitoring current flow though the tip while etching the
approximate etching time can be determined and the system can be automated using computer
software.
Ag Tip Fabrication Setups
The current fabrication procedure for etching
silver tips involves two different etching setups.
Each apparatus involves etching by means of
electrochemistry. The setup for the primary
etch uses a graphite block for the anode and the
tip for the cathode. A ring stand and alligator
clips are used to suspend the graphite block into
a beaker. A 250 milliliter glass beaker is used
to contain the ammonium, the electrolyte
Figure 2
solution used for etching. An analog DC power
supply supplies voltage to the system. A banana plug-alligator clip lead connects the positively
biased voltage to the clip suspending the tip. Likewise, another one of these leads connects the
negatively biased voltage from the source to the rod in which the tungsten block extends from.
The tungsten block should be partial submerged in the solution; however the metal alligator
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clips or prongs used to suspend the block should not be touching the solution. The tip is
positioned partially submerged in the solution also. In order to consistently submerge the same
amount of the tip inside the solution with each etch, a thin metal sheet is placed underneath the
beaker. After the apex of the tip makes contact with the liquid (before being submerged) the
metal plate can be placed underneath the beaker. This metal sheet is approximately 1/8 of an
inch and will directly correspond to the length of the tip that is etched. It is also important to
note that neither the tip or graphite block should be touching the glass beaker or each other
while etching is in progress. The primary setup for silver tip etching is pictured in the diagram
below.
Once the proper elements are arranged as shown a voltage between 15 and 27 voltage is
applied to the electrochemical circuit. For most etching purposes the ideal voltage to apply is
approximately 24 volts. When the DC voltage source is switched on small bubbles may appear
around the apex of the tip, numerous initially, but abating over time until the etching is
complete. As the process endures the apex of the tip will be corroded by the electrolyte until a
fraction of the tip breaks off complete. The tip will then be suspended over the liquid without
making contact; at this point the 1st etch is completed.
The second etch involves the use of an optical microscope, a push-button switch,
another DC voltage power supply, the electrolyte solution, and a film-loop apparatus. The
positive terminal the DC voltage source is attached to the switch which is attached to the clip
holder that secures the silver tip. Essentially the silver tip serves as the cathode. The anode is
the aluminum loop that is fastened at the end of rod
that extended from a push-pull finger trigger activated
mechanism. The amount of voltage applied is usually
between 5-12 volts. Show below is a graphic of the
entire setup as well as an image focused on the loop
which contains a thin film of electrolyte. Etching is
performed by moving the tip in an out of the loop,
while the loop contains a liquid film of solution. To
apply the liquid to the circular loop, the loop is
simply dipped into the a beaker containing some of
the electrolyte, ammonium, and extracted when a thin
film is visually present inside the loop. This film
inevitably breaks during etching; however it can be
replaced by repeating this process. It is important to
frequently rinse the solution in the electrolyte to
prevent a buildup of AgOH and to ensure that the
chemical reaction will effectively take place. After applying voltage, using the push-button
switch, and etching the tip for 30-50 seconds, the chemical reaction will terminate. The
solution will no longer etch the silver until the loop is replenished with electrolyte and voltage
is re-applied.
During this etching process, the tip and the electrolyte solution can be viewed underneath an
optical microscope. This success of this procedure is heavily based on experience because it is
a manual process that requires a meticulous eye and usage of two hands. Voltage is applied to
the system by the engaging the push-button switch with one hand. A push button is used
because voltage must be applied in pulses, and at different times. The other hand is used to
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operate the trigger mechanism that controls the loop which moves horizontally and applies
solution to the apex to shape it. By viewing the apex of the tip under a microscope it is possible
to see the progress being made. The 2nd etch is completed once the apex resembles a conical
shape with an intensely sharp point. Upon completion of this process the tip etching is
complete. Final preparations involve rinsing the apex with water (without touching it with any
utensils) and cutting the tip to removing unnecessary and excessive amounts of silver rod
extending from the apex.
Measurement & Acquiring Data
Voltage was the first parameter that was investigated to discern if there would be a significant
affect on time, and more the importantly apex shape after the etch. The primary objective was
to find an ideal voltage that could provide a time efficient etch without compromising apex
shape. Using the analog DC voltage
supply, the 1st etch was performed at 8
different voltage levels ranging from 9
volts to 30 volts (testing in voltage
increments of three). By repeating
this voltage testing for three trials,
sufficient data was acquired. These
voltage levels were tested using the
original electrolyte (ammonium) and a
new recipe which is discussed more in
detail later.
Figure 4
As stated earlier, in order to optimize
the process for electrochemical
etching the electrolyte solution must be stabilized. The electrolyte solution used for etching
silver tips is ammonium (NH4) which has a relatively low evaporation rate. During the primary
etching stage of etching silver tips the ammonium is poured into a beaker and the solution
evaporates quickly. This changes both the liquid level and the concentration of the solution.
This has a negative impact on the shape of the apex because the changing liquid level results in
a vertically uneven etch. Also, the changing concentration is undesirable because this causes a
wide range of variability in the electrochemical etch will does not offer a consistently ideal
apex shape. To correct this concern, hydrogen peroxide and ethane were added to the
ammonium solution. The volume ratio of each chemical for this particular recipe is 1:1:1. I
choose to add 66 ml of each chemical to make a 200 ml electrolyte solution that would be
consistent with the beaker size. The ammonium solution previously used during was replaced
by this new recipe and several trial etches were conducted. All other variables were held
constant, with the exception of DC voltage. Different voltages were applied used this new
electrolyte recipe to confirm that etching time was not being negatively impacted.
The final parameter examined was current flow through the silver tip during the primary
etch. It was predicted that the current would decrease over time considering the tip narrows
progressively while being etched. When voltage is held constant, a narrower tip would result
more resistance therefore reducing current according to Ohm’s Law (V=IR). When current no
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longer is flowing though the tip it serves as a direct indication that the etching is complete. The
benefit of this data is useful for not only analyzing the current trend over time, but also
automating the 1st etch. With use of LabVIEW® software in conjunction with circuitry to
measure the current going through the silver tip during etching both of these objectives could be
completed. I designed circuitry that incorporated use of a National Instruments® data aqusition
unit to serve as a voltmeter and measure the voltage across a single 100 ohm resistor or two 100
ohm resistors. Shown below is an image of the actual device and the schematics of the
circuitry.
Unfortunately due to several time constraints that were experienced, I was unable to use the
automated data acquisition unit to obtain data from LabVIEW® concerning the current flow
though the silver tip during etching. However I was able to verify that the system was indeed
working by measuring the current through a single resistor (which is essentially an adequate
simulation of the tip) by using the data acquisition unit. Data that was tabulated concerning
current flow over time was found using a digital multimeter. The ammeter setting on the
multimeter was utilized and the device was incorporated into the primary etching setup by
being placed in series with the tip. Three trials were performed taking current readings at
voltage levels of 21, 24, and 30 volts respectively.
Results:
When measuring voltage and etching time the graph indicated a proportional relationship
between the two variables, as expected, more increased voltage yielded faster completion of
etching. A comparison is shown between etching tips with ammonium, and etching with the
ammonium, hydrogen peroxide, and ethane mixture.
Effect of Voltage on Etching Time When Using
Secondary Electrolyte
30
30
25
20
Time Trial 1
15
Time Trial 2
10
Time Trial 3
5
0
Etching Time (minutes)
Etching Time (minutes)
Effect of Voltage on Etching Time When Using
Primary Electrolyte
25
20
Time Trial 1
15
Time Trial 2
10
Time Trial 3
5
0
0
3
6
9 12 15 18 21 24 27 30 33
DC Voltage (Volts)
0
Figure 5
3
6
9
12 15 18 21 24 27 30 33
Voltage (volts)
Figure 6
It appears that the results are consistent with both the primary etching solution (NH4) and the
secondary solution (NH4:C2H:H2O2). The introduction of the secondary electrolyte shows
very minimal if not any affect on the etching times when voltage is changed when compared to
the primary solution. Also, according to the graphs the fastest etching took place when 30
volts and 33 volts were applied to the system, the slowest of course at 9 volts and 12 volts. As
a result of the voltage levels of 30 and 33 voltage producing such drastically high etching rates,
the tips etched at these voltages were imaged. Close inspection of the many apex shapes
showed that there was no correlation between the shape of the apex and the voltage levels
between 30V and 33V. For reasons uncertain, the apex shape proved to be ideally conically
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etched after some etches however unattractively blunt or awkwardly shaped after other etches at
these voltage levels. Some of the variation in these high-voltage etched tips is shown below.
When examining the tips etched during the next fastest set of voltage levels far less variation
was found in the apex shape. A significant majority of the apex shapes found were ideal for tip
appearance after the primary etch with the expectation of a very few minor outliers. Pictured
below are the most common apex shapes produced while performing the primary etch at 24
volts and 27 volts. These tips were viewed under an optical microscope at 20X magnification.
Based on the voltage
findings discussed
previously, current was
measured through the
etching system at three
particular levels of interest.
Three trials were completed
Figure 7
using 21V, 24V, and 27V
because these voltage levels
produce consistently acceptable apex shapes according to the above data. The current evidently
was proven to have an inversely proportional relationship with etching time. As illustrated in
the graphs, the current level declines gradually at the start of etching but more rapidly after
towards the etching implying exponential regression. It is also evident according to the data
that there is minimal difference between the current flow observed using the primary etching
solution and the secondary etching solution.
Current Flow Through Silver Tips While Etching
Using Secondary Electrolyte
Current Flow Through Silver Tips
While Etching Using Primary
Electrolyte
6
5
4
V=24V
3
V=27V
V=21V
2
Current (mA)
Current (mA)
6
5
4
V=24V
3
V=27V
2
V=21V
1
1
0
0
0
2
4
6
0
8 10 12 14 16 18 20 22
Time (minutes)
2
4
6
8
10 12
14 16
Time (minutes)
Figure 8
8
18 20
22
Figure 9
By monitoring the liquid levels of
the primary electrolyte and the
secondary electrolyte is was found
that the secondary electrolyte does
not evaporate for a prolonged
period of time. After several
etching trials it was confirmed that
when using the primary solution
for etching, the liquid level
declines as frequently as 20-30 mL
per hour. The secondary solution
yields a much more stable volume
decrease of only 10 mL per hour.
The final results contain
data concerning the secondary etch of silver tip. The procedure used to complete the secondary
etch is highly dependant on the shape of the apex given by the first etch. This data is almost
exclusive qualitative and reported in Figure 10. Several neutral differences are indicated in the
chart, progressive etching strategies, and also advantages and disadvantages that arise.
Discussion & Conclusion
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