Teacher Research at Caltech

The Yeh Lab group’s research activities at Caltech have been instrumental in studying semiconductors and making two-dimensional materials such as graphene, as highlighted on a BBC Horizons show.  

An emerging sub-field of semiconductor and two-dimensional research is that of Transition metal dichalcogenide (TDMC) monolayers. In particular, a monolayer of Tungsten disulfide, a TDMC, is believed to exhibit interesting semiconductor properties when exposed to circularly polarized light. My role in the Yeh Lab, as a visiting high school Physics Teacher intern,  for the Summer of 2017 has been to help research and set up a vacuum chamber to study Tungsten disulfide samples under circularly polarized light.

What makes semiconductors unique is that conductivity can be controlled by doping or changes in temperature. Higher temperatures or doping can bridge the energy gap between the valence and conduction bands; in other words, electrons can start moving from one side of the material to the other. Like graphene, Tungsten disulfide has a hexagonal, symmetric crystal structure. Monolayers of transition metal dichalcogenides in such a honeycomb structure have two valleys of energy. One valley can interact with another valley. Circularly polarized light is used to populate one valley versus another. This gives a degree of control over the population of electrons by polarized light.

The Yeh Lab Group prides itself on making in-house the materials and devices needed for research. For example, in order to study high temperature superconductors, the Yeh Group designed and built their own scanning tunneling microscope. When they began researching graphene, instead of buying vast quantities of graphene, they pioneered new ways of fabricating it. This research topic has been no different: Wei-hsiang Lin, a Caltech graduate student, has been busy fabricating Tungsten disulfide samples via chemical vapor deposition (CVD) using Tungsten oxide and sulfur powder.  

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Wei-hsiang Lin’s area for using PLD to form the TDMC samples

The first portion of my assignment was spent learning more about vacuum chambers and researching what to order to confine our sample into the chamber. One must determine how the electronic feeds should be attached, how many are necessary, which vacuum pump will be used, how many flanges and gaskets of each size must be purchased in order to prepare the vacuum chamber.

There were also a number of flanges and parts already in the lab that needed to be examined for possible use. After triple checking the details the order was set with Kurt J. Lesker. Following a sufficient amount of anti-seize lubricant and numerous nuts, washers, and bolts, we assembled the vacuum chamber that will hold the TDMC sample.

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The original vacuum chamber


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Fun in the lab


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The prepped vacuum chamber

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The second part of my assignment was spent researching how to set up the optics for our experiment and ordering the necessary equipment. Once the experiment is up and running we will be using a milliWatt broad spectrum light source that is directed into a monochromator to narrow down the light to specific wavelengths for testing. Ultimately we will be evaluating the giant wavelength range of 300 nm through 1800 nm. Following the monochromator, light will be refocused by a planoconvex lens. Next, light will pass through a linear polarizer and then a circular polarizer (quarter wave plate). Lastly, the light will be refocused by a biconvex lens into the vacuum chamber and onto a 1 mm by 1 mm area of the sample.  

Soon, we are excited to verify how tungsten disulfide responds to circularly polarized light.  Does our sample resonate at the exact same wavelengths as the first labs found? Why or why not?  What other unique properties are observed?  How can they be explained?  How is the Hall Effect observed?  What does this mean for the possible applications of semiconductors? How can the transfer of information from one valley to another be used in advanced electronics for communication?  Then, similar exciting experimentation will take place with graphene under circularly polarized light.

I love the sharp contrast of the high-energy, adolescent classroom to the quiet, calm of the lab.  I am grateful for getting to learn a different and new-to-me area of Physics during the summer.  Yes, I remember studying polarization and semiconductors in high school and as an undergraduate.  But it is completely different to set up an experiment from scratch, to be a part of groundbreaking research in these areas.  And it is just fun to get to work with your hands and build research equipment at a world leading research university.  Sometimes Science teachers can get bogged down with all the paperwork and meetings.  I am grateful to have had this fabulous opportunity during the summer to work on applied Science and to be re-energized in my love for Physics.  I look forward to meeting my new batch of students in a few short weeks to share my curiosity and joy for learning how the world works with them.

Building the future

At the start of the academic year, my high school Physics students want an easy lab with simple, clear-cut data.  They are satisfied with a clear-cut conclusion. Open-ended labs, especially those without cookbook procedures are at first daunting and intimidating.  Having to take time to troubleshoot a problem is a painful process for them, as it can be for many.  As the year progresses, they seem to grow more comfortable with their own exploration of Physics trends.

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Another happy day in Sloan

There is no set manual for real scientific research, for uncharted territory. Exciting, new research has no “right” answer upon which to compare your data. And building your own, unique experimental set-up inherently requires much time to minimize new issues. It is interesting to me that when there is less guidance based on previous research, there is a larger possibility for great, new discoveries.

This summer I again retreated from the summer heat, plunging into the Caltech sub basements to further my understanding of the freshest research, efficient laboratory techniques, and culture in Physics research. The quiet hum of the air conditioner and lights marked an eerie contrast to the non-stop, bustling life of the classroom. It was an even more stark contrast to my 16-month-old daughter’s incessant joyful and curious exploration of the world.

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The SEM Chamber

My first project this summer focused on helping to get the SEM (Scanning Electron Microscope) up and running. Once the SEM is functional the first samples it will scan are solar cells comprised of graphene nanotubes. If grand scaled and mass produced, methane may be one source of the necessary carbon for graphene. What if we contained methane gases that are already problematically being released into our greenhouse-gas-ridden atmosphere and subsequently used them to make graphene solar cells? What a win-win solution to help the daunting problem of global climate change?

Helping to set up the SEM involved a variety of interesting tasks: I found the working distance from the SEM gun to the sample holder that would soon be loaded into the chamber. I researched Pirani gauge parts and later rubber pads to help with damping. I helped to install copper ConFlat flanges for making low pressure seals. We used sonification to clean parts used at the SEM lab. We found and installed a nitrogen (N2) line to flush out moisture in the SEM chamber. There were numerous rounds of baking out moisture that may have collected in the chamber in the years since this SEM was last in use.

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A tube scanner head

During “down time”, such as when the SEM chamber was being pumped down to less than one-part-per-billion pressure with multiple vacuum pumps, we directed our attention to two other projects. The first was making parts for the tube scanner head. Due to the possibility of burning out scanner heads in the alignment process when we first turn on the SEM gun, we needed to be prepared with alternative STM parts. This involved drilling, epoxying, baking, sanding, and soldering tiny pieces.  A diminutive coaxial cable with multiple insulating layers was stripped apart so that we could properly connect the gold conducting wire from within.

During the last week I focused my efforts by returning to an interferometer set up in the sub-basement of Sloan. Last summer, part of my time was spent learning about and setting up an interferometer system in order to measure the shift of a piezoelectric stack when particular voltages were applied. Once calibrated, these piezos will be used to control the motion of the tips in our lab’s STM (Scanning Tunneling Microscope). This summer was different because we had additional equipment from Thorlabs in order to move further along with the project.

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Overhead view of the interferometer set-up.

On the day of arrival of the much-needed parts, I felt like a child at Christmas. Ready, set, go. Racing against the impending end of the internship and start of the upcoming academic year, I worked to assemble our equipment.  

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LASER, function generator, amplifier.

This same procedure was completed roughly a decade ago by graduate students in our lab. Now, though, the remaining calibrated piezos have been used. In order to continue doing important STM measurements, new piezo stacks need to be calibrated.

A ray of red, coherent light from our LASER is directed to a beamsplitter. One arm of light is directed to a mirror and reflected back to the beamsplitter. Another arm of light is directed to a mirror fixed upon the piezoelectric stack. Depending on the applied voltage and particular piezo stacks, the orientation and magnitude of the shear varies. A signal generator and amplifier are connected to the opposite end of the piezoelectric stacks to carefully control the voltage signal applied to the piezos.  Once the beams are recombined at the beamsplitter, they should interfere.  An interference pattern should be detected on the oscilloscope.

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Confirmation that my oscilloscope was working properly

At first it was plain fun setting up the various parts, like fitting puzzle pieces with the various optics devices. The difficulty came later in troubleshooting. I had little issue with adjusting the set-up so that both beams from the LASER landed directly onto the photodetector. Getting a beautiful interference pattern was another case. Making sense of the output signal from the photodetector on the oscilloscope was also a process. Finding joy and benefit in the learning process as opposed to frustration in a trying time is an important lesson in life.  Of course it is inevitable that there will be difficulties in life. Can we grow from the learning opportunity as opposed to complaining about the struggle?

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What I at first thought was the interference pattern I had been hoping for… Not so fast.

The irony is that just like my students, I wanted an easy, beautiful interference pattern that could be interpreted on our oscilloscope. I had the opportunity to learn through trial and error and from additional research on interferometers. I look forward to hearing from the lab group about the progress that is made on this project during the academic year while I am in the classroom. I am grateful to IQIM and the Yeh Lab Group for allowing me to continue participating in this exciting program.

The dance of the electrons

On the day I returned to the lab, Marcus Teague, a post-doc leader in the Yeh Group, was orienting two summer undergraduate interns. As he asked the students questions regarding superconductors and scanning tunneling microscopes (STM) I was happy with the amount of information I readily recalled from previous years. It was a good sign that I was ready to build on the content and skills I had already mastered. This may be expected of a graduate student at Caltech, but I am not a graduate student at Caltech. I am a High School physics teacher.

One of the first tasks assigned to me this summer, as I returned for my third year as an IQIM research intern, was to build and solder a connecting cord for the cryocooler in which we stored superconductors. That’s right – I have been working on cutting edge research involving high temperature superconductivity. But back to the soldering. This cord would communicate information, such as temperature and voltage, from the sensor. It had been years since my undergraduate soldering days at UCLA, but it turned out I was not so rusty. I was able to solder the tiny connecting pieces and show the undergraduate research students how to solder.

This summer I made additional tips for STM. Again, we chemically etched the tips using a Calcium Chloride solution in order to reach an ideal thickness of 1 atom across. We also employed the physical stretch and clip approach to make tips. Each person in the lab has their own philosophy on the best method to make tips. It’s amusing and interesting to compare the different techniques for attempting to make sharp, symmetric tips. Whether chemical or physical, the process to make sharp tips is tedious and time consuming, but imperative for a scan with good resolution.

I also worked with Kyle Chen again for certain projects, one of which involved making a low pass filter which would cut out signals above 2 GHz. We wrapped copper wire around a skewer in order to make a tiny solenoid. Care was taken to ensure the number of loops in one direction was matched in the other to avoid creating a net magnetic field. My first attempt was pathetic, but with each trial and error, I was able to construct decent solenoids. The completed solenoids were soldered to a SMA connector (like the one used for coaxial cable) which became one end of the RF filter. In order to connect the copper tube and SMA electrically, a silver epoxy is used. The solenoids were slid carefully into a small copper tube and then filled with the epoxy, a mixture of copper powder and Stycast A and B at a ratio of 100:28. Silver apoxy needs high temperature baking, above 100 degrees Celsius, in order to set, so the system was then baked in order to solidify our new low pass filter to be placed alongside the sample for STM testing.

This summer we also continued testing the YBCO superconducting samples I helped to make two summers ago with Professor Feng. After etching, the procedure to load the sample into the long, cylindrical, central tube required detailed planning. As with last year, the hood was flushed with Argon gas and then vacuum pumped in order to reduce contaminants. Using gloves in the hood, the sample is daintily set into place. Lifting the giant cylinder with the sample took four of us: two for lifting, one for holding it from the base within the hood and one person adjusting the gas levels.

Next we took the cylinder with the sample downstairs to the subbasement to be vacuum pumped and baked in order to expel all gaseous particles. Finally the central tube was loaded into the STM central chamber and the cooling process began, first with liquid nitrogen and then with liquid helium. Finally, it was time to scan the superconducting samples.

Last summer’s scans of the superconductor were in the 500 mV range. However, this summer’s scans are within 150 mV which will lead to much better resolution of the image and possibly valuable enough data to publish.

Each Friday Nai-Chung Yeh has a group meeting with everyone working in the lab. She thoughtfully discusses each persons’ progress, contributions, and questions from the past week. Her genuine curiosity and passion to discuss the best methods for experimentation are inspirational. She is expressive with her hands as she explains a concept, such as how she thinks the polymer doping is affecting the graphene samples. Each week she inevitably goes to the whiteboard and draws a picture of the hypothesized phenomenon observed in the data for that week. She gives insights from her vast wealth of background knowledge and suggests applicable equations, troubleshooting techniques, and information found in the current literature. It is fascinating to watch her warmly lead this group to a deeper understanding of the research topics at hand. I am moved by her work ethic and ability to balance oversight of the graphene projects, topological insulators, superconductors, and possible new solar cell technology. She supervises each sub-projects’ progress while writing papers, traveling the world over to present, as well as secure funding for research.

It is difficult to believe yet another year has passed. Again it is time to return to my classroom to meet my new 150 students, to get them fired up for learning about the exciting world of Physics. I am eager to share my learning experiences from the last three summers at Caltech.

The return of the superconducting high school teacher

Last summer, I was blessed with the opportunity to learn about the basics of high temperature superconductors in the Yeh Group under the tutelage of visiting Professor Feng. We formed superconducting samples using a process known as Pulse Laser Deposition. We began testing the properties of the samples using X-Ray Diffraction, AC Susceptibility, and SQUIDs (superconducting quantum interference devices). I brought my new-found knowledge of these laboratory techniques and processes back into the classroom during this past school year. I was able to answer questions about the formation, research, and applications of superconductors that I had been unable to address prior to this valuable experience.

This summer I returned to the IQIM Summer Research Institute to continue my exploration of superconductors and gain even deeper research experience. This time around I have accompanied Caltech second year graduate student Kyle Chen in testing samples using the Scanning Tunneling Microscope (STM), some of which I helped form using Pulse Laser Deposition with Professor Feng last summer. I have always been curious about how we can have atomic resolution. This has been my big chance to have hands-on experience working with STM that makes it possible!

The Scanning Tunneling Microscope was invented by the late Heinrich Rohrer and Gerd Binnig at IBM Research in Zurich, Switzerland in 1981. STM is able to scan the surface contours of substances using a sharp conductive tip. The electron tunneling current through the tip of the microscope is exponentially dependent on the distance (few Angstroms) to the substance surface. The changing currents at different locations can then be compiled to produce three dimensional images of the topography of the surface on the nano-scale. Or conversely the distance can be measured while the current is held constant. STM has a much higher resolution of images and avoids the problems of diffraction and spherical aberration from lenses. This level of control and precision through STM has enabled scientists to use tools with nanometer precision, allowing scientists even to manipulate atoms and their bonds. STM has been instrumental in forming the field of nanotechnology and the modern study of DNA, semiconductors, graphene, topological insulators, and much more! Just five years after building their first STM, Rohrer and Binning’s work rightfully earned them the 1986 Nobel Prize in Physics.

Descending into the Sloan basement, Kyle and I work to prepare and scan several high temperature superconducting (HTSC) Calcium Doped YBCO (\rm Y_{1-x} Ca_x Ba_2 Cu_3 O_{7-\delta}) samples in order better to understand the pairing mechanism that causes Cooper Pairs for superconductivity. In regular metals, the pairing mechanism via phonon lattice vibrations is fairly well understood by physicists. Meanwhile, the pairing mechanism for HTSC is still a mystery. We are also investigating how this pairing changes with doping, as well as how the magnetic field is channeled up vortices within HTSC.

One of our first tasks is to make probe tips for STM. Adding Calcium Chloride to de-ionized water, we are preparing a liquid conductive path to begin the chemical etching of the probe tip. Using a 10V battery, a wire bent into a ring is connected to the battery and placed in the Calcium Chloride solution. Then a thin platinum iridium wire, also connected to the voltage source, is placed at the center of the conductive ring. The circuit is complete and a current of about half an Ampere is used to erode uniformly the outer surface of the platinum iridium wire, forming a sharp tip. We examine the tip under a traditional microscope to scrutinize our work. Ideally, the tip is only one atom thick! If not, we are charged with re-etching until we reach a more suitable straight, uniform, sharp tip.   As we work to prepare the platinum iridium tips, a stoic picture of Neils Bohr looks down at our work with the appropriate adjacent quotation, ” When it comes to atoms, language can be used only as in poetry.  The poet, too, is not nearly so concerned with describing facts as with creating images. ”  After making two or three nearly perfect tips, we clean and store them in the tip case and proceed to the next step of preparation.

We are now ready to clean the sample to be tested. Bromine etching removes any oxidation or impurities that have formed on our sample, leaving a top bromine film layer. We remove the bromine-residue layer with ethanol and then plunge further into the (sub)basement to load the sample into the STM casing before oxidation begins again. The STM in the Yeh Lab was built by Professor Nai-Chang Yeh and her students eleven years ago. There are multiple layers of vacuum chambers and separate dewars, each with its own meticulous series of steps to prepare for STM testing. At the center is a long, central STM tube. Surrounding this is a large cylindrical dewar. On the perimeter is an exterior large vacuum chamber.

First we must load the newly etched YBCO sample and tip into the central STM tube. The inner tube currently lays across a work bench beneath desk lamps. We must transfer the tiny tip from the tip case to just above the sample. While loading the tip with an equally minuscule flathead screwdriver, it became quite clear to me that I could never be a surgeon! The superconducting sample is secured in place with a small cover plate and screw. A series of electronic tests for resistance and capacitance must be conducted to confirm that there are no shorts in the numerous circuits. Next we must vacuum pump the inner cylindrical tube holding the sample, tip, and circuitry until the pressure is 10^{-4} Bar. Then we “bake” the inner chamber, using a heater to expel any other gas, while the vacuum pump continues until we reach approximately 10^{-5} Bar. The heater is turned off and the vacuum continues to pump until we reach 10^{-6} Bar. This entire vacuum process takes approximately 15 hours…

During this span of time, I have the opportunity to observe the dark, cold STM room. The door, walls and ceiling are covered with black rubber and spongy padding to absorb vibration. The STM room is in the lowest level basement for the same reason. The vibration from human steps near the testing generates noise in the data, so every precaution is made to minimize noise. Giant cement blocks lay across the STM metal box to increase inertia and decrease noise. I ask Kyle what he usually does with this “down” time. We discuss the importance of reading equipment manuals to grasp a better understanding of the myriad of tools in the lab. He says he needs to continue reading the papers published by the Yeh Lab Group. In knowing what questions your research group has previously answered, one has a better understanding of the history and the direction of current work.

The next day, the vacuum-pumped inner chamber is loaded to the center of the STM dewar. We flush the surrounding chambers with nitrogen gas to extricate any moisture or impurities that may have entered since our last testing. Next we can set up the equipment for a liquid nitrogen transfer which lasts approximately 2 hours, depending on the transfer rate. As the liquid nitrogen is added to the system, we meticulously monitor the temperature of the STM system. It must reach 80 Kelvin before we again test the electronics. Eventually it is time to add the liquid helium. Since liquid helium is quite expensive, additional precautions are taken to ensure maximum efficiency for helium use. It is beautiful to watch the moisture in the air deposit in frost along the tubing connecting the nitrogen and helium tanks to the STM dewar. The stillness of the quiet basement as we wait for the transfer is calming. Again, we carefully monitor the temperature drop as it eventually reaches 4.2 Kelvin. For this research, STM must be cooled to this temperature because we must drop below the critical temperature of the sample in order to observe superconductivity. The lower the temperature, the more of the superconducting component manifests itself. Hence the spectrum will have higher resolution. Liquid nitrogen is first added because it can carry over 90% of the heat away due to its higher mass. Nitrogen is also significantly cheaper than liquid helium. The liquid helium is added later, because it is even cooler than liquid nitrogen.

After adding additional layers of rubber padding on top of the closed STM, we can move over to the computer that controls the STM tip. It takes approximately one hour for the tip to be slowly lowered within range for a tunneling current. Kyle examines the data from the approach to the surface. If all seems normal, we can begin the actual scan of the sample!
An important part of the lab work is trouble shooting. I have listed the ideal order of steps, but as with life, things do not always proceed as expected. I have grown in awe of the perseverance and ingenuity required for daily troubleshooting. The need to be meticulous in order to avoid error is astonishing. I love that some common household items can be a valuable tool in the lab. For example, copper scrubbers used in the kitchen serve as a simple conducting path around the inner STM chamber. Floss can be used to tie down the most delicate thin wires. I certainly have grown in my immense respect for the patience and brilliance required in real research.

I find irony in the quiet simplicity of recording and analyzing data, the stillness of carefully transferring liquid helium juxtaposed to the immense complexity and importance of this groundbreaking research. I appreciate the moments of simple quiet in the STM room, the fast paced group meetings where everyone chimes in on their progress, or the boisterous collaborative brainstorming to troubleshoot a new problem. The summer weeks in the Sloan basement have been a welcome retreat from the exciting, transformative, and exhausting year in the classroom. I am grateful for the opportunity to learn more about superconductors, quantum tunneling, vacuum pumps, sonicators, lab safety, and more. While I will not be bromine etching, chemically forming STM tips, or doing liquid helium transfers come September, I have a new-found love for the process of research that I will radiate to my students.

Superconductors in the Summer

As a little girl I would play school with the neighborhood children. Ever since fourth grade I knew I wanted to be a teacher in a classroom full of eager-to-learn nine-year olds, but it wasn’t until my freshman year of college that my plans changed. In Geology for Elementary Teachers, I remember thinking, “This material is great! I need to learn more!” My hunger for a deeper understanding of how the physical world works led me to reflect on what my favorite science in high school was: Physics. Not long after, I changed my major to Physics and I was on the path to becoming a high school Physics teacher. Fast forward a decade, and I have my dream job. I get to explore the exciting world of Physics all day with 150+ adolescents and I wouldn’t change that for the world.

At Caltech after an exciting day at the lab!


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