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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 1 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 2 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. 3 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 4 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 5 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 6 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 7 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 9 10 11 12