Fabrication of Gate-tunable Graphene Devices for Scanning Tunneling Microscopy Studies with Coulomb Impurities

The overall goal of this procedure is to fabricate a back gated graphene field effect transistor decorated with ulam impurities for scanning, tunneling microscopy and spectroscopy studies. This is accomplished by first growing monolayer graphene on an electrochemically polished copper foil and transferring it onto a hexagonal boron nitride silicon dioxide substrate. The second step is to clean the graphene hexagonal boron nitride silicon dioxide hetero structure in an argonne hydrogen environment and evaporate a gold titanium contact pad onto graphene.

Next, the hetero structure is assembled into a gate tunable device by wire bonding it onto appropriate terminals on a scanning tunneling microscope sample holder. The final step is to calibrate the scanning tunneling microscope tip on a gold 1 1 1 surface and to evaporate cool a impurities onto graphene and an ultra high vacuum environment. Ultimately scanning tunneling microscopy is used to image the cool impurities on the gait T tuneable graphene device, and to investigate the spatially dependent electronic structure of graphene in the presence of charged impurities.

The main advantage of scanning tunnel microscopy over other methods such as angle resolve foal emission spectroscopy is that it has not only high energy resolution, but also has high spatial resolution as well. This method can test some fundamental predictions regarding the behavior of some relativistic charge carriers in response to a range of perturbations, such as the formation of atomic class resonances. When effective nuclear charge of a charge impurity exceeds a certain threshold, Generally individuals new to this method will struggle due to the number of seemingly trivial yet important details that are missing from the relevant literature.

Furthermore, interpretation of the necessary characterizations can become convoluted. For instance, interpretation of Raman spectroscopy can become complicated due to the presence of defects and impurity dopamines on graphene. To begin electrochemically polished a sheet of copper foil in order to expose the bare copper surface for graphene growth as described in the accompanying text protocol, place the quartz boat with electro polished copper foil inside the quartz tube using a long rod.

Push the boat into the center of the furnace and close the system with KF fittings. Pump down the system with a roughing pump and then purge the system with hydrogen gas. Close the furnace hood ramp up the temperature to 1050 degrees Celsius with 200 standard cubic centimeters per minute of hydrogen, and then hold the temperature there to ane the sample under the same gas flow for two hours.

Cool down to 1030 degrees Celsius with the same gas flow. Then grow graphene for 10 minutes with 40 SCCM of methane and 10 SCCM of hydrogen. As soon as the growth is over, open up the furnace hood to rapidly cool the sample down to room temperature while keeping the same gas flow.

Once the temperature is below 100 degrees Celsius, turn off the gas flow, close the metering valve and turn off the pump. Then vent the system with nitrogen gas by slowly opening the metering valve between the gas line and the gas cylinder. Take out the sample and cut the copper foil into pieces with the desired dimensions.

Next, perform mechanical exfoliation of hexagonal boron nitride onto a silicon dioxide chip as described in accompanying text protocol. Put one drop of PMMA onto the graphene copper graphene foil and spinco the foil at 3000 RPM for 30 seconds. Using an iron three chloride resistant spoon, float the spin coated copper foil on a solution of iron three chloride for one and a half minutes, and then transfer it to a bath of ultrapure water for five minutes.

Next, transfer the foil back to the aqueous iron three chloride for an additional minute and then into a fresh beaker of ultrapure water for five more minutes. After five minutes, transfer the foil back to the aqueous iron three chloride for 15 minutes, and then rinse the film in a fresh beaker of ultrapure water twice for five minutes each, followed by a rinse for 30 minutes. In fresh ultrapure water, fish out the PMMA graphene sample with the hexagonal boron nitride silicon dioxide chip.

Place the chip on a hot plate at 80 degrees Celsius for 10 minutes to remove water, and then increase the temperature to 180 degrees Celsius for 15 minutes. To relax the PMMA film. Next place the chip in chloro methane overnight to dissolve the PMA layer.

Once the PMMA is dissolved and kneel the sample in a mixture of argon and hydrogen gas, then evaporate a gold titanium contact pad onto the sample and wire bond the deposited gold titanium electrode to ground and the silicon bulk to a gate electrode on the scanning tunneling microscope sample holder For these steps, refer to accompanying text protocol. Sputter the gold 1 1 1 sample for five minutes with an argon ion beam accelerated to 500 volts. Then al the sample on the heater stage for five minutes at 375 degrees Celsius in an ultra high vacuum chamber to clean and flatten the gold surface.

Transfer the gold sample to the stage of the scanning tunneling microscope and approach the surface with the microscope tip. Apply 10 volt pulses on the STM tip until a gold 1 1 1 herringbone reconstruction is clearly visible. Then scan the gold 1 1 1 surface with a 40 nanometer by 40 nanometer frame to identify a clean and flat area.

If the surface has a high density of step edges, move to a new area for scanning. Gently crash the tip 0.4 to 1.0 nanometers into a clean region of the gold 1 1 1 surface. Then turn off the feedback and click take spectroscopy button.

To take a differential conductance spectrum, check the obtained differential conductance curve against a standard gold 1 1 1 differential conductance spectrum. Make sure the gold 1 1 1 surface state is present in the differential conductance curve and that the spectrum is absent of any anomalous feature. Once tip shape and differential conductance spectrum are optimized, wait 15 to 30 minutes.

If the STM tip is unstable, the spectrum will change during this time interval. Retake the spectrum at a different location. To confirm whether or not the STM tip is stable, transfer the graphene device to the scanning stage of a scanning tunneling microscope.

Use a long distance optical microscope to see the STM tip and the graphene device. After laterally aligning the tip and the hexagonal boron nitride flake of interest, approach the sample. Start scanning a two nanometer by two nanometer area.

Slowly enlarge the scan window and if a large impurity is encountered, move to a different area. Take a differential conductant spectrum and compare to the standard spectrum on a graphene hexagonal boron nitride substrate. If the spectrum is not comparable, recalibrate the tip on a gold 1 1 1 surface scan multiple areas to get a sense of how frequently large impurities that are over 100 TERs in height are encountered.

Based on these statistics deduce the cleanliness of the sample before the sample is assembled into a back gated device. The graphene surface is characterized by an optical microscope ramen spectroscopy and a FM.A good sample should appear clean, continuous, uniform, and mono layered under both optical microscope and a FM images. Moreover, a good sample should exhibit a minimal D peak intensity and less than a one to two ratio of G peak to 2D peak intensity ratio under ramen spectroscopy shown here is a waray pattern for the graphene hexagonal boron nitride substrate, which arises from a mismatch in the lattice Constance of graphene and hexagonal boron nitride.

Once the sample surface is examined, calcium ions are deposited onto graphene whose topography can be seen here While performing this procedure, it is important to take precaution while wire bonding. Otherwise, you'll fracture your dielectric layer and cause current leakage thus ruining your sample. After watching this video, you should have a good understanding of how to fabricate a gait tuneable graphing device for STM studies with cool impurities.

Furthermore, you should understand how to calibrate an STM tip and survey the device surface condition. Don't forget that working with flammable gases in extremely hot conditions can be very dangerous without proper vacuum seal during the CVD process and that precautions such as monitoring the vacuum pressure and examination of the court's tube during the process should always be taken.