We would like to make a summary of the characterization techniques of bulk and exfoliated 2D materials which can be employed in our lab. Let me start step by step:
1) When you pick a bulk crystal for exfoliation or functionalization, make sure you are confident in its composition.
How the bulk material can be characterized in our lab? The list is in a priority order:
- Crystal structure: XRD (X-ray Diffraction) allows you to confirm the crystallinity of the material and lattice structure. Software for analysis of diffractogram is called HighScore; you can ask Zdenek how to get it together with a database. You can learn how to navigate around HighScore by watching online tutorials (YT) or asking in the office 106, pretty much everyone there knows this software.
- Atomic vibrations: Raman spectroscopy (Raman) resolves structural information together with some polarization-dependent abilities of the investigated material. In B lab, we use a WiTec spectroscope, in G lab - a Renishaw. Both software allow for exporting TXT files, so you can treat the spectrum in the preferred way - in Origin or another. To get a license for Origin, ask Zdenek, and you can always take a look at the YouTube tutorial on it. Even though it is old, the software has not changed much: YT
- Surface analysis: SEM (Scanning Electron Microscopy) is a powerful tool to represent the 2D structure of bulk crystal layers. It shows you the topography and a general overlook of what the crystal looks like. We have two accessible SEMs: Maia and Lyra, both made by the Tescan company. This analysis required a bit of training, so it is better to join someone and start learning how to operate it safely. Then, you can contact Martin Vesely, who will arrange a training with you and set you an account to use a microscope. As well, there are YouTube tutorials even for the software of the microscope, so you can always refresh your knowledge: YT.
- Composition confirmation: EDX (Energy-Dispersive X-ray analysis) allows you to make sure that you have what you have. This detector is attached to the SEM, and so they are often performed together. This analysis does not lie; it is straightforward, but it has its limits; it can’t analyze very light elements (Li, Be) and does not show bonding. So it is a rustic confirmation of the elemental profile in the material and its stoichiometry.
- Chemical state: XPS (X-ray photoelectron spectroscopy) confirms the oxidation states of all the elements present within the material. Usually, we send a bulk crystal for XPS, paired with exfoliated/functionalized material, to serve as a reference. Here, you can take a look at the binding energies of any element, a very useful link.
XPS analysis and data manipulation can be performed in our lab by Jan Luxa (Karel), Vlastimil Mazánek (Vlasta), and Jan Plutnar (Pluto).
In summary, for fast characterizations, it is better to collect XRD and Raman spectra of the bulk, and ideally make EDX or ask Zdenek if that exact batch or crystals were investigated by other colleagues.
2) Imagine we have a crystal with a known composition. It means we can proceed further and exfoliate it or functionalize it. Exfoliated material can be analyzed by:
- Optical microscopy. Any optical microscope with a high enough magnification will show you the rough estimation of thickness based on the color of these flakes that they exhibit on the surface of SiO2. There is some smart physics behind this reflection phenomenon, which you can read about here: link
Obviously, optical analysis also allows for determining the shape and size of the exfoliated material. To treat these data statistically, I recommend using free software called ImageJ (link). There are also nice tutorials on it, and the program itself is user-friendly (for example, YT)
- Atomic vibrations: Raman spectroscopy will show you how the material changed upon exfoliation. Sometimes, the change in Raman of exfoliated material in comparison to bulk material is triggered by a thickness decrease, like in MoS2 (link). Sometimes, the surface can be oxidized, and the material can undergo a significant change, so collect your spectrum carefully and think, what could have happened with your material. Pay attention to the power of the laser you use; it can accidentally destroy a material if the power is too high.
- Thickness: Atomic Force Microscopy (AFM) is a tool for understanding the thickness of 2D flakes. I strongly advise not to perform this analysis until you are sure that there is no bulk in the sample. The AFM cantilever is tiny, so everything thicker than 1 μm can destroy it. We can do AFM in the G lab, the nanorobot lab, and in the B lab glovebox; suitable substrates are silicon and mica. Ask Kseniia Mosina or Jan Luxa (Karel) how to prepare a sample.
- Surface analysis: HRSEM (High Resolution Scanning Electron Microscopy) is an analysis done under a higher voltage in comparison to conventional SEM. Here, the special copper grids are required if you have a sample in the solvent. These grids you can get from Zdenek or Jan Luxa (Karel). NEVER ANALYSE MAGNETIC SAMPLES WITH HIGH RESOLUTION SEM!
- The composition confirmation tool (EDX) will allow you to confirm the stoichiometry of exfoliated material and trace the changes. As was mentioned, this analyzer is attached to the SEM, so prepare the sample accordingly.
- A transmission electron microscopy (TEM) can be performed in central labs of UCT. Please, take a look on the separate document dedicated to TEM sample preparation https://uach.vscht.cz/sofergroup-en.
- The chemical state confirmation tool (XPS) will show you how the bonding and oxidation states of the elements were changed upon delamination. Send the exfoliated sample to XPS only if the rest of the analysis is done, and you need to confirm the last details. Or if you are really confused about what is going on, and Zdenek or another experienced person recommends that you do so.
- Optical properties: UV-Visible Spectroscopy (UV-Vis) confirms the absorbance spectra in liquids, and the spectrofluorometer (PL-Horiba Fluorolog) confirms the emission spectra. Both instruments are in the G lab; ask Jiří Šturala to help you with them. Always try to be careful with the cuvette and with a sphere in UV-Vis. Here is the tutorial explaining UV-Vis setup and software: YT
- Electrical performance: conductivity (IV characteristics) is the most required characterization of materials if you claim any devices. Can be measured via a potentiostat (if you produce capacitors and batteries), and via a probe station (for transistors and other solid-state devices). Depending on your application, the right persons to help could be Tomáš Hartman (supercapacitors), Bing Wu (batteries), Aljoscha Söll and Kalyan Sarkar (transistors, etc).
For the instrument booking, use: https://teamup.com/ksj9puxc3cuk7k4nnd
For the potentiostat booking, use: https://teamup.com/ksbr3atse5o38v68ex
