Preparation of Liquid-exfoliated Transition Metal Dichalcogenide Nanosheets with Controlled Size and Thickness: A State of the Art Protocol

The goal of this protocol is to produce two dimensional nanosheets stabalized in liquid with controlled lateral size and thickness from bulk crystals. We also demonstrate methods to characterize the morphology and quantitatively determine the nanosheet dimensions from extinction spectra. This method can help answer key questions in nanoscience, such as the influence of nanosheet dimensions on the properties of structures and composites containing these sheets exfoliated in liquid.

The main advantage of this technique is that it's applicable to many different true day materials and only requires commonplace laboratory equipment. In particular, the spectroscopic metrics allow rapid assessment of the dispersions that are produced. Demonstrating the procedure will be Farnia Rashvand, a PhD student, and Kevin Synnatschke, a Master's student from my laboratory.

To begin this procedure, mount a metal cup underneath a sonotrode in an ice bath. Immerse 0.6 grams of a transition metal dichalcogenide or TMD powder in 80 milliliters of an aqueous solution of sodium cholate surfactant in the metal cup. Move the solid flathead tip to the bottom of the metal cup and then raise it approximately one centimeter.

Wrap aluminum foil around the sonic probe to avoid spillage. To sonicate the mixture by probe sonication, set the amplitude to 60%and the pulsing to six seconds on, and two seconds off. Then switch on the sonicator and run the process for one hour.

Next, centrifuge the dispersion at 2, 660 times g for 1.5 hours. After discarding the supernatant, add 80 milliliters of fresh surfactant solution to the sediment and agitate the mixture. Then, transfer the mixture back to the metal cup.

Now, subject the dispersion to a second, longer sonication using the solid flathead tip for five hours at 60%amplitude under ice cooling. After every two hours, pause the sonication and replace the ice bath. Remove unexfoliated particles by centrifugation at 240 times g for two hours.

When finished, discard the sediment. Following this, centrifuge the supernatant at higher centrifugal acceleration. Collect the sediment in three to eight millileters of fresh surfactant solution.

Centrifuge the supernatant at an even higher centrifugal acceleration of 950 times g for two hours. Then collect the sediment in three to eight milliliters of fresh surfactant solution. For atomic force microscopy, dilute the dispersion so that it is almost transparent to the human eye.

Following this, heat a wafer to approximately 170 degrees Celsius on a hot plate. Then deposit the dispersion on the preheated wafer. Rinse the wafer thoroughly with a minimum of five milliliters of water and three milliliters of 2-Propanol to remove residual surfactant and other impurities.

Following this, load the sample in AFM instrument. Scan and save multiple images across the sample with the atomic force microscope in tapping mode. For samples containing larger nanosheets, increase the field of view up to eight by eight micrometers squared and use scan rates of 0.4 to 0.7 hertz.

To perform the thickness measurement, open the software and select the relevant AFM image via File and Open. Correct the background using the level data by mean plane subtraction, align rows, and correct horizontal scars in the data process section of the Home menu. Apply the corrections, change the image color for better contrast by right-clicking on the the legend and set the Z plane to zero.

Next, zoom into the region of choice by first clicking on the Crop tool in the Home menu. Then drag the cursor over the image to mark the region of choice and press Apply. Check the create new channel to open the selected region in a new window.

Select extract profiles from the Tools menu and draw a line across the nanosheet. After the window showing the thickness length profile opens, enter the thickness value in a table. Take the approximate median value of the thickness profile across the nanosheet, taking extreme care to measure only individually deposited and non-aggregated nanosheets.

For spectral acquisition, dilute the high concentration sample with the respective medium to yield extinctions below two across the entire spectral range. Set the increments for spectral acquisition to 0.5 nanometers in the instruments setting. Choose subtract baseline in the instruments settings.

After placing a cuvette containing the aqueous sodium cholate solution in the spectrometer, run the measurement. Following this, remove the cuvette from the spectrometer and empty it. Add the sample to the cuvette and place the cuvette in the spectrometer.

Then run the measurement. Using the data analysis and graphing software, select the column containing the extinction intensity. Click on the Analysis tab, select Mathematics from the dropdown menu, and click on Differentiate.

Then select Open Dialog. After the new window opens, set the derivative order to two and press OK.Select the column containing the derivative, click on Analysis and choose Signal Processing. Click on Smooth and then select Open Dialog from the dropdown menu.

Next, choose Adjacent Averaging as the smoothing method and set the points to 20. Plot the resultant smoothed spectrum, which is displayed as new columns. Finally, read off the peak position from the second derivative by placing the cursor in the center of the peak.

Liquid cascade centrifugation is used to sort liquid-exfoliated nanosheets by size and thickness as demonstrated for molybdenum and tungsten disulfide. A typical AFM image is shown here. And the nanosheet thickness is converted to layer number using step height analysis.

Statistical microscopic analysis yields length and number of layer histograms. The mean nanosheet length and layer number plotted as a function of central acceleration shows a similar trend for both materials. The length plotted as a function of nanosheet layer number confirms that smaller, thinner nanosheets are separated from larger, thicker ones.

Optical extinction spectra of molybdenum and tungsten disulfide with different mean nanosheet sizes and thicknesses are shown here. The corresponding fitted second derivatives of the A-exciton region illustrate well-defined peak shifts of the transition. Data for both materials collapses on the same curve if appropriate peak positions are chosen and means that the nanosheet size can be quantitatively linked to the nanosheet length via the same equations.

The A-exciton extinction coefficient is length dependent except at certain positions, so that the corresponding extinction coefficient can be used as measure for nanosheet concentration. The number of layers can be quantitatively related to the A-exciton peak position. Once mastered, large quantities of liquid-exfoliated nanosheet dispersions with controlled and known dimensions can be produced in only a couple of days.

This is enabled by the high throughput size determination based on the optical extinction spectra. Since the nanosheet size determines its properties, the size control is crucial. After the development of the metrics, this technique paved the way for researchers to explore them in a number of applications, from electrocatalysis to electronics to composite reinforcement.

After watching this video, you should have a good understanding of how to produce size selected nanosheet dispersions using liquid phase exfoliation and liquid cascade centrifugation procedures. Following the dispersion preparation, it's very important to perform basic characterization using statistical microscopy and optical extinction spectroscopy to confirm the nature of the dispersion constituents. Most excitingly, the resulting size selected nanosheet dispersions can then be subject to various liquid phase processing methods to produce thin films and other nanosheet containing structures for a wide range of potential applications.