Exfoliation and Analysis of Large-area, Air-Sensitive Two-Dimensional Materials

This protocol demonstrates methods for exfoliating large thin flakes of air-sensitive two-dimensional materials. And safely transporting them for analysis outside of a glovebox. Working inside a glovebox, prepare a length of tape that is about five to 10 centimeters long, and at least two centimeters wide.

Place it sticky side up on the working area. Fold up the ends to make it easier to handle. Using tweezers, deposit the desired material about 1/4 of the way down the length of the tape.

By repeatedly pressing it onto the tape. Further distribute the material by folding the tape in half, sticking it to itself and pulling it apart. So that the material covers an area of at least one square centimeter.

Start with a substrate cut into square chips less than one centimeter on a side. Using the prepared tape, firmly press the deposited material onto the substrate. Apply firm pressure with your thumb or gently press with tweezers.

So the material contacts the chip as much as possible. Place the tape and substrate with substrate side down on a hot plate at 120 degrees Celsius for two minutes. Allow the substrate to cool.

Then carefully remove it from the tape. Hot exfoliation will leave more tape residue than room temperature exfoliation. But most of the residue can be removed by soaking in acetone for 20 minutes.

Followed by 30 seconds in isopropyl alcohol. The transfer cell is made of a metal cap and base. It is 30 millimeterss wide, and when closed is only 17.6 millimeters tall.

The base has a raised sample platform that threads into the cap. This groove cut into the threads is a vent that prevents the cell window from breaking when the cap is screwed down. Note, where the cap meets the base there is an inset for an O-Ring.

And the cap is recessed to accommodate a thin cover glass window. An air tight seal is made by a Viton O-Ring seated in the base of the cell. Apply a small amount of vacuum grease to all sides of the O-Ring.

And drop it into place. Before fixing the window to the cap of the cell, clean the cap in acetone and isopropyl alcohol to remove any oil or debris left by the machining process. The window can now be attached to the cell cap using epoxy.

Thoroughly mix the epoxy according to the manufacturer specifications. In this case, parts A and B are combined in a one to 1.8 ratio by weight. Apply a small amount of epoxy to the recessed area on the cap, and spread it around as evenly as possible.

Carefully drop the glass window into the recess, and gently press it into the epoxy. Ensure the window is level with the top of the cap, and that there are no bubbles in the epoxy. Lastly, wipe up any extra epoxy, so that nothing protrudes from the surface of the cap.

Allow the epoxy to cure for the time prescribed by the manufacturer at room temperature. Using the desired method, affix a prepared sample to the cell base. Before closing the cell, the pressure in the glovebox needs to be less than three millibar above the ambient pressure.

Otherwise, the glass will break when it’s removed from the glovebox. Firmly screw the cap onto the base until the cap and base meet. Check that the sample sits just below the window.

The sample can now be safely removed from the glovebox for analysis. To fix a broken window, put on safety glasses and nitrile gloves, and remove any broken glass that is not firmly affixed to the epoxy. Break up what glass remains, so that the epoxy beneath is exposed.

Working in a fume hood, soak the cap in a 50/50 mixture of acetone and trichloroethylene for one to two hours. Until the epoxy softens and begins to separate from the cap. Remove the cap from the acetone, trichloroethylene mixture, and rinse with isopropyl alcohol.

Peel off any loose epoxy and scrape the remaining epoxy from the surface with a razor blade. Take care not to damage the surface of the cap. Repeat the previous step if necessary.

Scrub the recessed area with acetone until the surface is clean of any epoxy residue. The cell window can now be replaced following the aforementioned steps. The cell can be placed under a microscope to identify flakes.

When focusing, take care not to crash the objective into the window by starting above the focal point and moving the stage down. Exfoliated material can be clearly seen at five, 20 and 50 times magnification. Allowing for easy identification of thin flakes.

At higher magnifications, spherical aberration caused by the window significantly degrades image quality. Using our transfer cell, it is also possible to perform different types of optical measurements of air-sensitive two-dimensional materials. As a final example, we determine the crystal orientation of a black phosphorus sample using Polarization Resolved Raman Spectroscopy.

For Polarization Resolved Raman Spectroscopy align a laser spot to a flake of interest. In this case we use 633 nanometer wavelength, and 50 microwatt power. And a 100 times magnification Objective Lens.

For black phosphorus, low laser power is required to prevent damage to the flake. Raman Spectra are recorded as a function of polarization angle. Which is varied using a half-wave plate.

The goal of hot exfoliation is to produce many large flakes. Thereby increasing the probability of finding very thin flakes. For comparison, panels A and B show typical black phosphorus exfoliations at room temperature and at 120 degrees Celsius.

It is immediately clear that the flake coverage in panel B is many times that of panel A.Panel C shows the total area of exfoliated material on six different one square centimeter silicon chips, for both room temperature and hot exfoliation. Hot exfoliation results in six to ten times the amount of material being deposited on the chip. Using our transfer cell, the lifetime of air-sensitive two-dimensional materials can be greatly extended.

Samples that would degrade within minutes in air can last several hours. For example, panels A through C demonstrate that chromium triiodide stored outside the glovebox in the transfer cell, does not begin to show visible signs of degradation for up to 15 hours. Panel D demonstrates that this extremely air-sensitive material hydrates within seconds, when exposed to ambient atmosphere.

Finally, we used Raman Spectroscopy to determine the crystal orientation of a flake of black phosphorus preserved inside a transfer cell. With the laser spot aligned to the thick black phosphorus flake in the center of panel A, Raman Spectra are measured as a function of laser polarization from zero to 360 degrees. As shown in panel B.Three peaks typical of black phosphorus are observed at approximately 361, 438 and 466 wave numbers.

We see that the peak intensities modulate strongly with polarization angle. Panel C shows the integrated intensity of the A2G peak versus polarization angle. Which shows a maximum at 26.5 degrees.

Because this mode corresponds to in-plane vibrations along the armchair edge of black phosphorus, it is most intense for polarization parallel to the armchair direction. We therefor conclude that the armchair direction of this flake is oriented at 26.5 degrees with respect to the image in panel A.As compared to room temperature exfoliation, hot exfoliation produces higher quantities of large flakes. By preserving the inert atmosphere of a glovebox our hermetic transfer cell makes it possible to isolate, and optically characterize thin flakes of air-sensitive two-dimensional materials, without requiring analytical equipment to be housed inside the glovebox.