2D Materials
Fabrication:
- This step assumes that the chips have been cleaved and cleaned appropriately.
- Pull a daughter tape from a stored mother tape, if no mother tapes are currently being stored then follow this guide to prepare one.
- Place 2-5 SiO2 chips onto a glass slide and then place the slide on a hotplate for 1min @ 95˚C to remove water vapor from the surface of the SiO2.
- Remove the slide from the hot plate and set the daughter tape onto your chips carefully. You can do this while the chips are still hot or you can wait – we’re not really sure which is best. Note that if you ever leave the chips for longer than a minute, putting a petri dish lid or some other cover over them is best practice to avoid accumulation of dust particles on their surface (be mindful if you decide to use a cover on the hotplate as it will need to be heat resistant).
- Use a paintbrush with soft bristles to very gently apply the tape to the chips. Or don’t use a paintbrush and instead lightly press the tape with soft-tipped tweezers onto the chip – preferably where there is no bulk. One can also press the sides of the tape where there is an overhang off the chip – why 1/2″ chips are preferred. Oftentimes a combination of these three application techniques is chosen. This step is variable among all users and one must find the method that works for them. We do know that if you press too hard you will crush the crystal and therefore the exfoliation will be bad. However, if you don’t press enough then there won’t be any material exfoliated onto the chip surface – find the balance that works for you. Graphite is the only exception to this. When exfoliating graphite you can press as hard as you want.
- After waiting some amount of time (again this is variable but most believe waiting >5minutes is beneficial) peel the tape back from the chips very slowly (2-3~mins per chip).
- Use daughter tapes only once.
Since this step is very tedious, we tend to use an Arduino based exfoliation machine that peels the tape very slowly. Please ask a grad student if this is still operational.
Note: All acronyms of materials (ie: hBN = hexagonal boron nitride) are listed below in the section labeled Info on 2D materials. All temperatures are in Celsius.
- How to make PC
- How to make PDMS
- How to make a stamp
- Use the above guides to make a stamp. Note that there is most likely some leftover PC/PDMS from previous transfers – inquire with other students about this.
- Make sure you bake the stamp for 2 minutes at 175˚C once the stamp is completed to make the PC adhere to the PDMS.
Developing a plan in the form of a powerpoint is helpful – one can click the selection pane / name flakes / turn flakes on/off / adjust their transparency (in most ppt versions) to make visualizing the current transfer step easier.
To view a sample device plan go to the graphene network drive > Schaibley Lab > Wiki supplements and download the powerpoint labeled example of a device plan.pptx so that you can click around on the selection pane and transparency feature to see how useful these tools are.
Note the following terms to be used in this guide
Note the knobs on the micromanipulator
- Gently dust chip with N2.
- Place the chip onto the stage and turn on the vacuum system. Confirm it is holding the chip securely in place.
- Turn on camera and click “remote viewing session” on the canon software.
Using two microscope objectives during the transfer is good enough because the digital zoom on the canon software is available, so this gives you 4 zoom levels.
The best way to get good at transferring is to do many transfers. Beginner tip: start simple – try to pick up graphene with hBN, or make it a little more difficult and completely encapsulate a flake (could be graphene again) between two hBN pieces. Transferring with expendable flakes reduces beginner anxiety and allows one to gain confidence in the process.
- Next, get the stamp holder as parallel as possible with respect to the stage. This is done because when a steep angle was used (2˚~), TMDs, which are smaller, tended to slide around or change orientation which resulted in misaligned structures. The author of this section had best results when using a 0˚ angle, but you may want to try with a nonzero angle. For example, the Santa Barbara team suggests to use a 2-3˚ angle. Go to graphene lab server > Schaibley lab > Wiki supplements and click on Transfer Techniques (Santa Barbara).pptx for more information and resources. Place the digital angle meter onto the stage in the same orientation (along y-axis) that it will be placed onto the micromanipulator arm. Once the meter is on the stage, zero (tare) it. Now place it on the micromanipulator stamp holder (without stamp in it – to be more careful) and use the angle adjusting knob on the micromanipulator (see photo above) to get the meter close to 0˚.
- Load the stamp into the micromanipulator (PC/PDMS side down), there should be a small screwdriver around for this. Take care not to break the glass stamp.
- Move the micromanipulator by pushing on its base – you don’t want to tip it and ruin your stamp. Center the stamp under the light coming from the objective.
- Engage the magnetic clamp.
- Bring the stamp close to the surface.
- Turn on heater system and open labview program for the heater which will be different depending on which transfer station you will be working on. Set temp to 60˚C. Let the stage reach this temperature.
- Find a clean spot on the chip, away from the flake of interest (FOI).
- Quick note: For movements in the XY plane, one can use the stage XY wheel or the micromanipulator XY knobs; the micromanipulator will be more precise but sometimes you don’t need precision. However, to raise/lower the stamp (Z dimension), always use the micromanipulator.
- Carefully touch down onto the surface and use epicpen to circle the touchdown (TD) spot. This is done to know where the stamp will touch first.
- Lift up and go back to FOI area.
- Now, you want to take care of which direction to make the stamp “flow” over the FOI, and therefore you need to take care where to put the TD spot in relation to the FOI (this is kind of an advanced tip, so just skip it if it’s too confusing right now).
- An example: Say you’re trying to pick up graphene underneath hBN, which is already on the stamp.
You have the least probability of tearing the graphene strip if you make the stamp “flow” in a certain direction.
The idea is that when the cooldown sequence occurs and the PC begins to “roll back” to its original position, that there are no competing forces pulling on the thin graphene. The vdW force between the hBN/graphene and then the force between the PC/graphene occur – one and then the other instead of both simultaneously which would increase the likelihood of ripping the graphene.
You also want to consider any dirt on your stamp/chip and try to avoid it. Remember these flakes are very small comparatively so if there is dirt nearby the flake a “tent” of PC might form and you may be unable to touch the flake of interest with the stamp. Gently dust the chip with N2 before transferring. The good news is that you only need a relatively small portion of your stamp to be pristine in order to build a nanostructure.
- Now that you’ve carefully chosen your touchdown area (with pc flow direction and dirt in mind), carefully bring your stamp close to the surface.
- Quick but IMPORTANT note: The closer to the surface the stamp is, the better optical image you get (after all, the camera is looking through the stamp to see the chip) and therefore all alignment and drawing should be done with stamp very close to surface of the chip. This author actually does the epicpen drawing after touching down and letting the stamp begin to flow, but it may take newer people more time to draw.
- Trace your flake of interest and landmarks/bulk with epicpen program. All 4 zoom levels should be marked with epic pen. One way to keep this organized is to use two different colors depending on which objective you’re looking through.
Note in the above picture that there are dotted and solid perimeters outlined. This author uses dotted perimeters as a signal to himself that this flake/bulk has been very carefully traced and the marks can be trusted more than the solidly outlined landmarks. Again, this whole process is a balance act. The more landmarks you outline the better (after all, some won’t pick up), but you don’t have to painstakingly trace each one. 2-3 with dotted perimeters will do (including the main flake on all 4 zoom levels), then quickly trace a couple more bulk pieces.
Another quick adam trick: one digital level =live at one time
[need picture of labview UI]
Information on Materials:
WSe2 is a transition metal dichalcogenide which, in monolayer form, has a direct band-gap of 1.7eV (~730nm). One can confirm the existence of a monolayer of WSe2 by collecting photoluminescence produced by electron/hole recombination with a spectrometer, because WSe2 consisting of >1 layers will have an indirect band-gap and the electron/hole recombination processes will not be predominantly optical.
Show a monolayer of WSe2. Also show its band structure and maybe a 3d cartoon
WSe2 PL spectrum here
WSe2 is a transition metal dichalcogenide which, in monolayer form, has a direct band-gap of 1.7eV (~730nm). One can confirm the existence of a monolayer of WSe2 by collecting photoluminescence produced by electron/hole recombination with a spectrometer, because WSe2 consisting of >1 layers will have an indirect band-gap and the electron/hole recombination processes will not be predominantly optical.
Show a monolayer of WSe2. Also show its band structure and maybe a 3d cartoon
WSe2 PL spectrum here
WSe2 is a transition metal dichalcogenide which, in monolayer form, has a direct band-gap of 1.7eV (~730nm).
WSe2 PL spectrum here, both RT and cold.
Also put band structure
WSe2 is a transition metal dichalcogenide which, in monolayer form, has a direct band-gap of 1.7eV (~730nm). One can confirm the existence of a monolayer of WSe2 by collecting photoluminescence produced by electron/hole recombination with a spectrometer, because WSe2 consisting of >1 layers will have an indirect band-gap and the electron/hole recombination processes will not be predominantly optical.
Show a monolayer of WSe2. Also show its band structure and maybe a 3d cartoon
WSe2 PL spectrum here
WSe2 is a transition metal dichalcogenide which, in monolayer form, has a direct band-gap of 1.7eV (~730nm). One can confirm the existence of a monolayer of WSe2 by collecting photoluminescence produced by electron/hole recombination with a spectrometer, because WSe2 consisting of >1 layers will have an indirect band-gap and the electron/hole recombination processes will not be predominantly optical.
Show a monolayer of WSe2. Also show its band structure and maybe a 3d cartoon
WSe2 PL spectrum here
FePS3 is a transition metal thiophosphate which exhibits out-of-plane, long range anti-ferromagnetic ordering below T_N=123K where T_N is called the Neel temperature. [1]
Here’s a slide from the following pdf that highlights exfoliation and characterization of FePS3 in the glovebox. [maybe enter some stuff about the crystal structure, such as a 3d cartoon]