The research leading to these results has received funding from the European Union Seventh Framework Program under grant agreement n&#176;604391 Graphene Flagship. C.B. acknowledges the German Research Foundation, DFG, under grant BA 4856/2-1.

1. Liquid Exfoliation &#8212; Preparation of Suitable Stock Dispersions
<ol>
	<li>Mount a metal cup underneath a sonotrode in an ice bath.</li>
	<li>Immerse 1.6 g of the TMD powder in 80 mL aqueous solution of sodium cholate (SC) surfactant (sodium cholate concentration, CSC= 6 g L-1) in the metal cup.</li>
	<li>Move the sonic tip to the bottom of the metal cup and then up by ~ 1 cm. Wrap aluminum foil around the sonic probe to avoid spillage.</li>
	<li>Sonicate the mixture under ice-cooling by probe sonication to avoid heating using a solid flathead tip (750 W processor) for 1 h at 60% amplitude (pulse of 6 s on and 2 s off).</li>
	<li>Centrifuge the dispersion at a centrifugation speed of 2,660 x g for 1.5 h. Discard the supernatant containing impurities and collect the sediment in 80 mL fresh surfactant solution (CSC= 2 g L-1). 
	NOTE: Use maximum filling heights in the centrifuge tubes of maximum 10 cm. Otherwise, increase the centrifugation time.</li>
	<li>Subject the dispersion to a second, longer sonication using the solid flathead tip for 5 h at 60% amplitude (pulse of 6 s on and 2 s off) under ice-cooling. Replace the ice bath every 2 h while pausing the sonication.</li>
</ol>
2. Nanosheet Size Selection by Liquid Cascade Centrifugation
NOTE: To select nanosheets by size, liquid cascade centrifugation with sequentially increasing centrifugal acceleration is applied (Figure 1). The following procedure is recommended as standard size selection of the cascade in the case of TMDs. For other materials, centrifugation speeds may need to be adjusted.
<ol>
	<li>Remove unexfoliated powder by centrifugation at 240 x g (1.5 krpm), 2 h. Discard the sediment.</li>
	<li>Centrifuge the supernatant at a higher centrifugal acceleration: 425 x g (2 krpm), 2 h. Collect the sediment in fresh surfactant at reduced volume (3-8 mL).</li>
	<li>Centrifuge the supernatant at even higher centrifugal acceleration: 950 x g (3 krpm), 2 h. Collect the sediment in fresh surfactant at reduced volume (3-8 mL).</li>
	<li>Repeat this procedure with the following centrifugal accelerations: 1,700 x g (4 krpm), 2,650 x g (5 krpm), 3,500 x g (6 krpm), 5,500 x g (7.5 krpm), and 9,750 x g (10 krpm).</li>
</ol>
3. Determination of Nanosheets Size and Thickness by Statistical Microscopy
NOTE: If spectroscopic metrics are already available, section 3 can be skipped or reduced, i.e., not carried out for every sample.
<ol>
	<li>Length: Transmission electron microscopy (TEM)
	<ol>
		<li>Deposition
		<ol>
			<li>Dilute the high concentration dispersions with water (to reduce the surfactant concentration) so that they are of light in color. Drop cast onto a grid (for example holey carbon, 400 mesh) placed on a filter membrane to wick away excess solvent.</li>
		</ol>
		</li>
		<li>Imaging
		<ol>
			<li>Record multiple images on different positions on the grid. Adjust the field of view depending on nanosheet size. For a comprehensive TEM imaging tutorial, see reference 18.</li>
		</ol>
		</li>
		<li>Statistical length analysis performed using ImageJ
		<ol>
			<li>Open the ImageJ software, select the relevant TEM image via the &#34;file&#34; menu and &#34;open&#34; the image. The image will open in a new window.</li>
			<li>Click the &#34;analyze&#34; tab. Select &#34;set scale&#34; from the drop down menu. A new window will open. Click &#34;remove scale&#34;, tick &#34;global&#34; and click &#34;ok&#34;.</li>
			<li>Select the &#34;line&#34; tool. Draw a line profile along the length of the scale bar of the TEM image.</li>
			<li>Click &#34;analyze&#34;. Select &#34;set scale&#34; from the drop down menu. Enter the length of the scale bar in nm into the &#34;known distance&#34; box and click &#34;ok&#34;. 
			NOTE: The distance of the line drawn on the scale bar is displayed in pixels.</li>
			<li>Select the &#34;line&#34; tool and measure the nanosheet length by drawing a line profile of the longest axis of the nanosheet.</li>
			<li>Press &#34;control+M&#34; to measure. A new box labelled &#34;results&#34; opens with the nanosheet length displayed in the &#34;length&#34; column.</li>
			<li>Repeat step 3.1.3.6 for all individually deposited nanosheets (not aggregated ones) in the image.</li>
			<li>When opening a new image, repeat steps 3.1.3.3- 3.1.3.7. Count the length of 150 nanosheets. 
			NOTE: All nanosheet length data is compiled in the &#34;results&#34; window and can be copied into other programs for further processing.</li>
		</ol>
		</li>
	</ol>
	</li>
	<li>Thickness: Atomic force microscopy (AFM)
	<ol>
		<li>Dilute the dispersion so that are almost transparent to the human eye (corresponding to extinction intensity of ideally ~0.2 per 1 cm pathlength at 400 nm). In the case of surfactant dispersions, dilute with water not surfactant.</li>
		<li>Drop-cast on pre-heated wafers. For water-based dispersion, heat the wafer to ~170 &#176;C on a hot plate and deposit 10 &#181;L per 0.5 x 0.5 cm2 wafer.</li>
		<li>Rinse the wafers thoroughly with a minimum of 5 mL of water and 3 mL of 2-propanol to remove residual surfactant and other impurities.</li>
		<li>Scan and save multiple images across the sample with the AFM in tapping mode. For small nanosheets use a resolution of 512 lines per image and image sizes of maximum 2 x 2 &#181;m2. For samples containing larger nanosheets, increase the field of view to up to 8 x 8 &#181;m2. Use scan rates as appropriate (typically 0.4-0.7 Hz). Alternatively, scan larger areas at higher resolution.</li>
		<li>Thickness measurement using Gwyddion Software
		<ol>
			<li>Open the software and select the relevant AFM image via &#34;file&#34; and &#34;open&#34;. The image will open in a new window.</li>
			<li>Correct the background using the &#34;level data by mean plane subtraction&#34; &#34;align rows&#34; and &#34;correct horizontal scars&#34; in the &#34;Data Process&#34; section of the home menu. Apply the corrections, change the image color for better contrast by right-clicking on the legend and set z-plane to zero.</li>
			<li>Zoom in the region of choice (if convenient). Click on the &#34;crop&#34; tool in the home menu. Drag the cursor over the image to mark the region of choice. Press &#34;apply&#34;. Check the &#34;create new channel&#34; to open the selected region in a new window.</li>
			<li>Select &#34;extract profiles&#34; from the tools menu. A new window opens.</li>
			<li>Draw a line across the nanosheet. Write down the thickness in a table. In the case of non-homogeneously thick nanosheets, average the thickness across the nanosheet. Take extreme care to measure only individually deposited and non-aggregated nanosheets.</li>
			<li>Repeat 3.2.5.3-3.2.5.5 for all nanosheets on the image.</li>
			<li>Repeat 3.2.5.1-3.2.5.6 for all images recorded. Count minimum 150 nanosheets.</li>
		</ol>
		</li>
	</ol>
	</li>
	<li>Conversion of AFM thickness to layer number 
	NOTE: Apparent AFM heights from liquid exfoliated nanomaterials are usually overestimated due to the presence of residual solvent. In addition, accurate height measurements of inhomogeneous samples (such as nanomaterials deposited on substrates) using AFM are generally challenging due to contributions from effects such as capillary forces and adhesion which depend on the material and measurement parameters.19,20 To overcome these problems and to convert the apparent measured AFM thickness to the number of layers, a procedure termed step height analysis was developed as described in the following.12,13,16,21. Steps 3.3.1-3.3.4 can be skipped if the step height is known.
	<ol>
		<li>Open, correct and crop the AFM image as described in 3.2 to select a nanosheet with clearly discernible terraces.</li>
		<li>Measure the height across the nanosheet using the &#34;extract&#34; profile tool. 
		NOTE: Suitable profiles show discrete steps as the one in Figure 2B inset.
		<ol>
			<li>Record the height of these steps (i.e., the height difference from one terrace to the next on the nanosheet).</li>
		</ol>
		</li>
		<li>Count at least 70 of these steps.</li>
		<li>Plot the step height in ascending order (Figure 2C). 
		NOTE: Observe that for TMDs the apparent step height is always a multiple of ~1.9 nm.</li>
		<li>Divide the apparent AFM thickness (measured as described in section 3.2) by 1.9 nm to obtain the layer number. 
		NOTE: Other materials have other step height conversion factors requiring a different calibration.</li>
	</ol>
	</li>
</ol>
4. Determination of MoS2 and WS2 Size and Thickness Based on Extinction Spectra
<ol>
	<li>Spectra acquisition
	<ol>
		<li>Dilute the high concentration samples with the respective medium (here aqueous sodium cholate, 2 g L-1) to yield extinctions below 2 across the entire spectral range.</li>
		<li>Set the increments for the spectral acquisition to 0.5 nm in the instrument settings or use scan speed slow or medium.</li>
		<li>Choose the settings &#34;subtract baseline&#34; in the instrument settings. Place the cuvette containing the aqueous sodium cholate solution in the sample compartment of the spectrometer and run the measurement.</li>
		<li>Remove the cuvette with the sodium cholate solution from the spectrometer and empty it. Fill in the sample, place the sample in the sample compartment of the spectrometer and run a scan of the sample.</li>
	</ol>
	</li>
	<li>Length determination from intensity ratios
	<ol>
		<li>Option 1: Read-off the intensity at the A-exciton, ExtA (~660 nm for MoS2 and 620 nm for WS2) and the local minimum Extmin (345 nm for MoS2 and 295 nm for WS2). Divide the intensity at the A-exciton by the intensity at the local minimum to obtain the intensity ratio ExtA/Extmin.</li>
		<li>Determine the mean nanosheet length, &#60;L&#62; by using equation 1. 
		<img alt="Equation 1" src="/files/ftp_upload/54806/54806equation1.jpg" /> (Eq. 1) 
		where ExtA/Extmin is the intensity ratio of the extinction at the A-exciton (ExtA) and the local minimum (Extmin). 
		NOTE: The equation holds for both MoS2 and WS2. However, its accuracy is limited especially for small nanosheets.</li>
		<li>Option 2: Determine the intensity ratio of the local maximum in the UV region of the spectrum, ExtMax-HE (270 nm for MoS2 and 235 nm for WS2) and the local minimum, Extmin (345 nm for MoS2 and 295 nm for WS2)</li>
		<li>Determine the mean nanosheet length, &#60;L&#62; by using equation 2. 
		<img alt="Equation 2" src="/files/ftp_upload/54806/54806equation2.jpg" /> (Eq. 2) 
		With Extmax-HE denoting the intensity at the local maximum at high energy (270 nm for MoS2 and 235 nm for WS2) and Extmin the extinction intensity at the local minimum (345 nm for MoS2 and 295 nm for WS2). 
		NOTE: Option 2 gives a more accurate measure of the lateral size. However, the high energy region may not be accessible in all solvents/surfactant.</li>
	</ol>
	</li>
	<li>Concentration
	<ol>
		<li>Record the extinction intensity relative to 1 cm pathlength at 345 nm for MoS2 and 235 nm for WS2, respectively. 
		NOTE: Divide the recorded measured extinction by the pathlength of the cuvette.</li>
		<li>Divide this intensity by the extinction coefficients of 68 Lg-1cm-1 at 345 nm for MoS2 and 47 Lg-1cm-1 at 235 nm for WS2 to obtain the nanosheet concentration in gL-1.</li>
	</ol>
	</li>
	<li>Thickness determination from A-exciton position
	<ol>
		<li>Calculate the second derivative of the spectrum.
		<ol>
			<li>Using the data analysis and graphing software (e.g., OriginPro), select the column containing the extinction intensity. Click on the &#34;analysis&#34; tab, select &#34;mathematics&#34; from the drop down menu and &#34;differentiate&#34;, &#34;open dialog&#34;. A new window will open. Set the derivative order to 2 and press ok.</li>
		</ol>
		</li>
		<li>Smooth the second derivative by Adjacent Averaging (~ 10-20 points per window in A-exciton region).
		<ol>
			<li>For example, using the data analysis and graphing software, plot the second derivative spectrum.
			<ol>
				<li>With the graphic window active, click on &#34;analysis&#34; and choose &#34;signal processing&#34;, then &#34;smooth&#34; then &#34;open dialog&#34; from the drop down menu. A new window will open.</li>
				<li>Choose &#34;Adjacent Averaging&#34; as smoothing method and set the points to 20.</li>
				<li>Plot the resultant smoothed spectrum which is displayed as new columns. If the noise is still high, repeat the smoothing. 
				NOTE: Usually, spectral smoothing is required to reduce the noise unless high integration times during the measurement are used. The appropriate smoothing is an important part of the data analysis and the appropriate smoothing method depends on the desired outcome. This particular smoothing method is only ideal to determine the mean peak position.13</li>
			</ol>
			</li>
		</ol>
		</li>
		<li>Read-off the peak position from the second derivative. This is the wavelength of the A-exciton, &#955;A. Alternatively, carry out the steps described in 4.4.4-4.4.7.</li>
		<li>Convert the x-axis from wavelength to energy using the relation: 
		E(eV) = 4.135E-6 * 2.997E8 / &#955;(nm)</li>
		<li>Fit the second derivative to the second derivative of a Lorentzian. 
		NOTE: A Lorentzian can be written as 
		<img alt="Equation 3" src="/files/ftp_upload/54806/54806equation3.jpg" /> (Eq. 3) 
		Where h is the height, E&#39;0 is the center and w is the FWHM. Differentiating twice with respect to E gives 
		<img alt="Equation 4" src="/files/ftp_upload/54806/54806equation4.jpg" /> (Eq. 4)
		<ol>
			<li>In the data analysis and graphing software, choose &#34;tools&#34; from the main menu and select &#34;fitting function builder&#34;. A new window will open.</li>
			<li>Select &#34;create a new function&#34;, click next.</li>
			<li>Leave the default settings, give the function a name and click next.</li>
			<li>Set &#34;h,E,w&#34; as parameters, click next.</li>
			<li>Enter &#34;(-8*h/w^2)*(1-3*(2*(E-x)/w)^2)/(1+(2*(E-x)/w)^2)^3&#34; as function body, click finish.</li>
			<li>Plot only the A-exciton region of the second derivative spectrum on the energy scale.</li>
			<li>With the graphics window active, click on the &#34;analysis&#34; tab. Choose &#34;fitting&#34;, &#34;nonlinear curve fit&#34;, &#34;open dialog&#34; from the drop down menu. A new window will open.</li>
			<li>Select &#34;user defined&#34; in category and choose the previously built function in the function box. In the tab &#34;parameters&#34; set initial values for w to 0.1, and E to 1.99 for WS2 and 1.85 for MoS2. Press &#34;fit&#34;</li>
		</ol>
		</li>
		<li>Record the energy E&#39;0, which is the energy associated with the A-exciton, E&#39;A.</li>
		<li>Determine the number of layers according to equations 5 (MoS2) and 6 (WS2). 
		<img alt="Equation 5" src="/files/ftp_upload/54806/54806equation5.jpg" /> (Eq. 5, MoS2) 
		<img alt="Equation 6" src="/files/ftp_upload/54806/54806equation6.jpg" /> (Eq. 6, WS2) 
		with &#955;A denoting the wavelength of the A-exciton and EA denoting the energy of the A-exciton.</li>
	</ol>
	</li>
</ol>

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The authors have nothing to disclose.

Sample preparation
The samples described here are produced by tip sonication. Alternative exfoliation procedures can be used, but will lead to different concentrations, lateral sizes and degrees of exfoliation. Higher amplitudes and longer on pulses during the sonication should be avoided to prevent damaging of the sonicator. Similar results were obtained using 500 W processors. However, sonication time and amplitude has an impact on the nanosheet exfoliation and variations from this protocol may...

The possibility to produce and process graphene, related two-dimensional (2D) crystals in the liquid phase makes them promising materials for an ever growing range of applications as composite materials, sensors, in energy storage and conversion and flexible (opto) electronics.1-6 To exploit 2D nanomaterials within applications such as these will require inexpensive and reliable functional inks with on-demand lateral size and thickness of the nanoscale constituents, as well as controlled rheological and morphological properties amenable to industrial-scale printing/coating processes.7 In this regard, liquid phase exfoliation has become an important production technique giving access to a whole host of nanostructures in large quantities.6,8,9 This method involves the sonication or shearing of layered crystals in liquids. If the liquid is appropriately chosen (i.e., suitable solvents or surfactant) the nanosheets will be stabilized against reaggregation. Numerous applications and proof-of-principle devices have been demonstrated by such techniques.6 Probably the greatest strength of this strategy is its versatility, as numerous layered parent crystals can be exfoliated and processed in a similar way, providing access to a broad palette of materials which can be tailored to the desired application.
However, despite this recent progress, the resultant polydispersity that arises due to these liquid-phase production methods (in terms of nanosheet length and thickness) still presents a bottleneck in the realization of high performance devices. This is mostly because the development of novel and innovative size selection techniques has thus far required nanosheets length and thickness characterization using tedious statistical microscopy (atomic force microscopy, AFM and/or transmission electron microscopy, TEM).
Despite these challenges, several centrifugation techniques have been reported to achieve length and thickness sorting.6,10-13 The simplest scenario is homogeneous centrifugation, where the dispersion is centrifuged at a given centrifugal acceleration and the supernatant is decanted for analysis. The centrifugation speed sets the size cut-off, whereby the higher the speed, the smaller are the nanosheets in the supernatant. However, this technique suffers from two major disadvantages; firstly, when larger nanosheets are to be selected (i.e., the dispersion is centrifuged at low speeds and the supernatant is decanted) all smaller nanosheets will also remain in the sample. Secondly, regardless of the centrifugation speed, a significant proportion of the material tends to be wasted in the sediment.
An alternative strategy for size selection is density gradient (or isopycnic) centrifugation.11,14 In this case, the dispersion is injected into a centrifuge tube containing a density gradient medium. During ultracentrifugation (typically &#62; 200,000 x g), a density gradient is formed and the nanosheets move to the point in the centrifuge where their buoyant density (density including the stabilizer and solvent shell) matches the density of the gradient. Note that the nanomaterial can also move upward during this process (depending on where it was injected). In such a way, the nanosheets are effectively sorted by thickness rather than mass (opposed to homogeneous centrifugation). While this procedure offers a unique opportunity to sort nanosheets by thickness, it suffers from notable disadvantages. For example, the yields are very low and at present do not allow for the mass production of separated nanosheets. This is partly related to low contents of monolayers in stock dispersions after liquid-exfoliation and can potentially be improved by optimizing exfoliation procedures in the future. In addition, it is typically a time-consuming multi-step ultracentrifugation process involving multiple iterations to achieve efficient size selection. Furthermore, in the case of inorganic nanomaterials, it is restricted to polymer-stabilized dispersions to obtain the required buoyant densities and the gradient medium in the dispersion may interfere with further processing.
We have recently shown that a procedure we term liquid cascade centrifugation (LCC) offers an exciting alternative,13 as we will also detail in this manuscript. This is a multi-step procedure which is extremely versatile allowing various cascades to be designed according to the desired outcome. To demonstrate this process, a standard cascade is portrayed in Figure 1 and involves multiple centrifugation steps whereby each features a higher speed than the last. After each step, the sediment is retained and the supernatant is then used in the proceeding stage. As a result, each sediment contains nanosheets in a given size range which have been &#34;trapped&#34; between two centrifugations with different speeds; the lower one removing larger nanosheets into the previous sediment while the higher speed removes the smaller nanosheets into the supernatant. Critical to LCC, the resulting sediment can be redispersed completely by mild sonication in the respective medium, which in this case is aqueous sodium cholate H2O-SC (at SC concentrations as low as 0.1 g L-1). The result is dispersions with virtually any chosen concentration. Importantly, virtually no material is wasted in LCC, resulting in the collection of relatively large masses of size-selected nanosheets. As shown here, we have applied this procedure to a number of liquid-exfoliated nanosheets including MoS2 and WS2 as well as GaS,15 black phosphorus16 and graphene17 in both solvent and surfactant systems.
This unique centrifugation procedure enables the efficient size-selection of liquid exfoliated nanosheets and has subsequently enabled a significant advancement in terms of their size and thickness determination. In particular, through this approach we demonstrated previously that optical extinction (and absorbance) spectra of the nanosheets change systematically as function of both nanosheets lateral dimensions and nanosheets thickness. As we summarize here, this has allowed us to link the nanosheet spectral profile (specifically the intensity ratio at two positions of the extinction spectrum) to the mean nanosheet length as a result of nanosheet edge effects.12,13 Importantly, the same equation can be used to quantify the size of MoS2 and WS2. Furthermore, we show that the A-exciton position shifts towards lower wavelengths as a function of mean nanosheet thickness due to confinement effects. Even though exfoliation, as well as size selection and determination are in general rather robust procedures, the quantitative outcome depends on subtleties in the protocol. However, especially for newcomers to the field, it is difficult to judge which process parameters are most relevant. This comes down to the fact that experimental sections of research papers only provide a rough protocol, without discussing what outcome is to be expected when modifying the procedure or giving a rational behind the protocol. In this contribution, we intend to address this as well as provide a detailed guide and discussion to the production of liquid-exfoliated nanosheets of controlled size and to the accurate determination of size by either statistical microscopy or analysis of the extinction spectra. We are convinced that this will help to improve reproducibility and hope it will be a useful guide for other experimentalists in this research area.
<img alt="Figure 1" src="/files/ftp_upload/54806/54806fig1.jpg" /> 
Figure 1: Schematic of the size selection by liquid cascade centrifugation. Size-selected nanosheets are collected as sediments. Each sediment is collected or &#34;trapped&#34; between two centrifugation speeds (&#969;) starting from low speeds and going to higher ones from step to step. The sediment discarded after the first centrifugation contains unexfoliated layered crystallites while the supernatant discarded after the last centrifugation step contains extremely small nanosheets. Size-selected dispersions are prepared by re-dispersing the collected sediments in the same medium (here aqueous surfactant solution) at reduced volumes. Adapted with permission from 13. <a href="https://cloudflare-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/54806/54806fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

<table border="0" cellpadding="0" cellspacing="0" width="987">
	<colgroup>
		<col />
		<col />
		<col />
		<col />
	</colgroup>
	<tbody>
		<tr height="42">
			<td height="42" style="height:42px;width:233px;">Sodium cholate hydrate, from ox and/or sheep bile</td>
			<td style="width:105px;">Sigma Aldrich</td>
			<td style="width:127px;">C1254-100G</td>
			<td style="width:523px;">Surfactant used as stabilizer in the form of an aqueous solution (i.e. after dissolving the powder in millipore water)</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:233px;">MoS2 powder</td>
			<td style="width:105px;">Sigma Aldrich</td>
			<td style="width:127px;">69860-100G</td>
			<td style="width:523px;">Other distributors available, but exfoliation and outcome of size selection can vary</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:233px;">WS2, powder 2 um</td>
			<td style="width:105px;">Sigma Aldrich</td>
			<td style="width:127px;">243639-50G</td>
			<td style="width:523px;">Other distributors available, but exfoliation and outcome of size selection can vary</td>
		</tr>
		<tr height="84">
			<td height="84" style="height:84px;width:233px;">ImageJ Software</td>
			<td style="width:105px;">Developer: National Insitutes of Health</td>
			<td style="width:127px;">64-bit Java version 2.45 1.6.0_24</td>
			<td style="width:523px;">Image processing software used for TEM analysis, free download</td>
		</tr>
		<tr height="84">
			<td height="84" style="height:84px;width:233px;">Gwyddion Software</td>
			<td style="width:105px;">Developer: Czech Metrology Institute</td>
			<td style="width:127px;">64-bit Java version 2.45</td>
			<td style="width:523px;">Image processing software used for AFM analysis, free download</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:233px;">Origin Pro Software</td>
			<td style="width:105px;">OriginLab</td>
			<td style="width:127px;">Version 2016</td>
			<td style="width:523px;">Software used for data analysis such as differntiation and fitting of the extinction spectra</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:233px;">Centrifuge&#160;</td>
			<td style="width:105px;">HettichLab</td>
			<td style="width:127px;">Mikro 220R</td>
			<td style="width:523px;">any other benchtop centrifuge is suitable</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:233px;">Rotor 1</td>
			<td style="width:105px;">Hettich</td>
			<td style="width:127px;">Rotor 1016</td>
			<td style="width:523px;">for centrifugation &#60; 5000 x g</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:233px;">Rotor 2</td>
			<td style="width:105px;">Hettich</td>
			<td style="width:127px;">Rotor 1195-A</td>
			<td style="width:523px;">for centrifugation &#62; 5000 x g</td>
		</tr>
	</tbody>
</table>

preparation of liquid-exfoliated transition metal dichalcogenide nanosheets with controlled size and thickness: a state of the art protocol

We summarize recent advances in the production of liquid-exfoliated transition metal dichalcogenide (TMD) nanosheets with controlled size and thickness. Layered crystals of molybdenum disulphide (MoS2) and tungsten disulphide (WS2) are exfoliated in aqueous surfactant solution by sonication. This yields highly polydisperse mixtures containing nanosheets with broad size and thickness distributions. However, for most purposes, specific sizes (in terms of both lateral dimension and thickness) are required. For example, large and thin nanosheets are desired for (opto) electronic applications, while laterally small nanosheets are interesting for catalytic applications. Therefore, post-exfoliation size selection is an important step that we address here. We provide a detailed protocol on the efficient size selection in large quantities by liquid cascade centrifugation and the size and thickness quantification by statistical microscopic analysis (atomic force microscopy and transmission electron microscopy). The comparison of MoS2 and WS2 shows that both materials are size-selected in a similar way by the same procedure. Importantly, the dispersions of size-selected nanosheets show systematic changes in their optical extinction spectra with size due to edge and confinement effects. We show how these optical changes are related quantitatively to the nanosheets dimensions and describe how mean nanosheets length and layer number can be extracted reliably from the extinction spectra. The exfoliation and size selection protocol can be applied to a broad range of layered crystals as we have previously demonstrated for graphene, gallium sulphide (GaS) and black phosphorus.

Liquid cascade centrifugation (Figure 1) is a powerful technique to sort liquid-exfoliated nanosheets by size and thickness as illustrated in Figure 2 for both MoS2 and WS2. Nanosheet lateral sizes and thicknesses can be characterized by statistical TEM and AFM, respectively. A typical AFM image is shown in Figure 2A. The apparent nanosheet thickness is converted to layer number using step height analysis (Fi...

Watch this Scientific Journal Video about Preparation of Liquid-exfoliated Transition Metal Dichalcogenide Nanosheets with Controlled Size and Thickness: A State of the Art Protocol at JoVE.com

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

A protocol for the liquid exfoliation of layered materials to nanosheets, their size selection and size measurement by microscopic and spectroscopic techniques is presented.

与尺寸可控液体剥离的过渡金属二硫属化物纳米片和厚度的制备:艺术议定书的国家

We summarize recent advances in the production of liquid-exfoliated transition metal dichalcogenide (TMD) nanosheets with controlled size and thickness. ...

preparation-liquid-exfoliated-transition-metal-dichalcogenide

Research

JoVE Journal

Engineering

13.8K 次观看.  Ruprecht-Karls University Heidelberg. A protocol for the liquid exfoliation of layered materials to nanosheets, their size selection and size measurement by microscopic and spectroscopic techniques is presented.

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

Controlling the Size, Shape and Stability of Supramolecular Polymers in Water

For aqueous based supramolecular polymers, the simultaneous control over shape, size and stability is very difficult1. At the same time, the ability to do so is highly important in view of a number of applications in functional soft matter including electronics, biomedical engineering, and sensors. In the past, successful strategies to control the size and shape of supramolecular polymers typically focused on the use of templates2,3, end cappers4 or selective solvent techniques5. 
Here we disclose a strategy based on self-assembling discotic amphiphiles that leads to the control over stack length and shape of ordered, chiral columnar aggregates. By balancing electrostatic repulsive interactions on the hydrophilic rim and attractive non-covalent forces within the hydrophobic core of the polymerizing building block, we manage to create small and discrete spherical objects6,7. Increasing the salt concentration to screen the charges induces a sphere-to-rod transition. Intriguingly, this transition is expressed in an increase of cooperativity in the temperature-dependent self-assembly mechanism, and more stable aggregates are obtained. 
For our study we select a benzene-1,3,5-tricarboxamide (BTA) core connected to a hydrophilic metal chelate via a hydrophobic, fluorinated L-phenylalanine based spacer (Scheme 1). The metal chelate selected is a Gd(III)-DTPA complex that contains two overall remaining charges per complex and necessarily two counter ions. The one-dimensional growth of the aggregate is directed by &pi;-&pi; stacking and intermolecular hydrogen bonding. However, the electrostatic, repulsive forces that arise from the charges on the Gd(III)-DTPA complex start limiting the one-dimensional growth of the BTA-based discotic once a certain size is reached. At millimolar concentrations the formed aggregate has a spherical shape and a diameter of around 5 nm as inferred from 1H-NMR spectroscopy, small angle X-ray scattering, and cryogenic transmission electron microscopy (cryo-TEM). The strength of the electrostatic repulsive interactions between molecules can be reduced by increasing the salt concentration of the buffered solutions. This screening of the charges induces a transition from spherical aggregates into elongated rods with a length &gt; 25 nm. Cryo-TEM allows to visualise the changes in shape and size. In addition, CD spectroscopy permits to derive the mechanistic details of the self-assembly processes before and after the addition of salt. Importantly, the cooperativity -a key feature that dictates the physical properties of the produced supramolecular polymers- increases dramatically upon screening the electrostatic interactions. This increase in cooperativity results in a significant increase in the molecular weight of the formed supramolecular polymers in water.

The goal of this experiment is to determine and control the size, shape and stability of self-assembled discotic amphiphiles in water. For aqueous based supramolecular polymers such level of control is very difficult. We apply a strategy using both repulsive and attractive interactions. The experimental techniques applied to characterize this system are broadly applicable.

控制的大小，形状和稳定性超分子聚合物在水

For aqueous based supramolecular polymers, the simultaneous control over shape, size and stability is very difficult1. At the same time, the ability to do ...

科研

工程学

Synthesis of Phase-shift Nanoemulsions with Narrow Size Distributions for Acoustic Droplet Vaporization and Bubble-enhanced Ultrasound-mediated Ablation

High-intensity focused ultrasound (HIFU) is used clinically to thermally ablate tumors. To enhance localized heating and improve thermal ablation in tumors, lipid-coated perfluorocarbon droplets have been developed which can be vaporized by HIFU. The vasculature in many tumors is abnormally leaky due to their rapid growth, and nanoparticles are able to penetrate the fenestrations and passively accumulate within tumors. Thus, controlling the size of the droplets can result in better accumulation within tumors. In this report, the preparation of stable droplets in a phase-shift nanoemulsion (PSNE) with a narrow size distribution is described. PSNE were synthesized by sonicating a lipid solution in the presence of liquid perfluorocarbon. A narrow size distribution was obtained by extruding the PSNE multiple times using filters with pore sizes of 100 or 200 nm. The size distribution was measured over a 7-day period using dynamic light scattering. Polyacrylamide hydrogels containing PSNE were prepared for in vitro experiments. PSNE droplets in the hydrogels were vaporized with ultrasound and the resulting bubbles enhanced localized heating. Vaporized PSNE enables more rapid heating and also reduces the ultrasound intensity needed for thermal ablation. Thus, PSNE is expected to enhance thermal ablation in tumors, potentially improving therapeutic outcomes of HIFU-mediated thermal ablation treatments.

Phase-shift nanoemulsions (PSNE) can be vaporized using high intensity focused ultrasound to enhance localized heating and improve thermal ablation in tumors. In this report, the preparation of stable PSNE with a narrow size distribution is described. Furthermore, the impact of vaporized PSNE on ultrasound-mediated ablation is demonstrated in tissue-mimicking phantoms.

相移窄粒度分布的纳米乳液的合成声波液滴蒸发和泡增强的超声介导消融

High-intensity  focused ultrasound (HIFU) is used clinically to thermally ablate tumors. To  enhance localized heating and improve thermal ablation in ...

Fine-tuning the Size and Minimizing the Noise of Solid-state Nanopores

Solid-state nanopores have emerged as a versatile tool for the characterization of single biomolecules such as nucleic acids and proteins1. However, the creation of a nanopore in a thin insulating membrane remains challenging. Fabrication methods involving specialized focused electron beam systems can produce well-defined nanopores, but yield of reliable and low-noise nanopores in commercially available membranes remains low2,3 and size control is nontrivial4,5. Here, the application of high electric fields to fine-tune the size of the nanopore while ensuring optimal low-noise performance is demonstrated. These short pulses of high electric field are used to produce a pristine electrical signal and allow for enlarging of nanopores with subnanometer precision upon prolonged exposure. This method is performed in situ in an aqueous environment using standard laboratory equipment, improving the yield and reproducibility of solid-state nanopore fabrication.

A methodology for preparing solid-state nanopores in solution for biomolecular translocation experiments is presented. By applying short pulses of high electric fields, the nanopore diameter can be controllably enlarged with subnanometer precision and its electrical noise characteristics significantly improved. This procedure is performed in situ using standard laboratory equipment under experimental conditions.

微调尺寸和最小化固态纳米孔的噪音

Solid-state nanopores have emerged as a versatile tool for the characterization of single biomolecules such as nucleic acids and proteins1. However, the ...

Electron Channeling Contrast Imaging for Rapid III-V Heteroepitaxial Characterization

Misfit dislocations in heteroepitaxial layers of GaP grown on Si(001) substrates are characterized through use of electron channeling contrast imaging (ECCI) in a scanning electron microscope (SEM). ECCI allows for imaging of defects and crystallographic features under specific diffraction conditions, similar to that possible via plan-view transmission electron microscopy (PV-TEM). A particular advantage of the ECCI technique is that it requires little to no sample preparation, and indeed can use large area, as-produced samples, making it a considerably higher throughput characterization method than TEM. Similar to TEM, different diffraction conditions can be obtained with ECCI by tilting and rotating the sample in the SEM. This capability enables the selective imaging of specific defects, such as misfit dislocations at the GaP/Si interface, with high contrast levels, which are determined by the standard invisibility criteria. An example application of this technique is described wherein ECCI imaging is used to determine the critical thickness for dislocation nucleation for GaP-on-Si by imaging a range of samples with various GaP epilayer thicknesses. Examples of ECCI micrographs of additional defect types, including threading dislocations and a stacking fault, are provided as demonstration of its broad, TEM-like applicability. Ultimately, the combination of TEM-like capabilities &#8211; high spatial resolution and richness of microstructural data &#8211; with the convenience and speed of SEM, position ECCI as a powerful tool for the rapid characterization of crystalline materials.

The use of electron channeling contrast imaging in a scanning electron microscope to characterize defects in III-V/Si heteroexpitaxial thin films is described. This method yields similar results to plan-view transmission electron microscopy, but in significantly less time due to lack of required sample preparation.

电子通道造影的快速III-V异质特征

Misfit dislocations in heteroepitaxial layers of GaP grown on Si(001) substrates are characterized through use of electron channeling contrast imaging ...

Indirect Fabrication of Lattice Metals with Thin Sections Using Centrifugal Casting

One of the typical methods to manufacture 3D lattice metals is the direct-metal additive manufacturing (AM) process such as Selective Laser Melting (SLM) and Electron Beam Melting (EBM). In spite of its potential processing capability, the direct AM method has several disadvantages such as high cost, poor surface finish of final products, limitation in material selection, high thermal stress, and anisotropic properties of parts. We propose a cost-effective method to manufacture 3D lattice metals. The objective of this study is to provide a detailed protocol on fabrication of 3D lattice metals having a complex shape and a thin wall thickness; e.g., octet truss made of Al and Cu alloys having a unit cell length of 5 mm and a cell wall thickness of 0.5 mm. An overall experimental procedure is divided into eight sections: (a) 3D printing of sacrificial patterns (b) melt-out of support materials (c) removal of residue of support materials (d) pattern assembly (e) investment (f) burn-out of sacrificial patterns (g) centrifugal casting (h) post-processing for final products. The suggested indirect AM technique provides the potential to manufacture ultra-lightweight lattice metals; e.g., lattice structures with Al alloys. It appears that the process parameters should be properly controlled depending on materials and lattice geometry, observing the final products of octet truss metals by the indirect AM technique.

An indirect additive manufacturing method combining a 3D printing of polymers with a centrifugal casting is outlined for manufacturing 3D octet truss metals (Al and Cu alloys) having a unit cell length of 5 mm with a wall thickness of 0.5 mm.

与超薄切片格金属制造间接使用离心铸造

One of the typical methods to manufacture 3D lattice metals is the direct-metal additive manufacturing (AM) process such as Selective Laser Melting (SLM) ...

Electrospray Deposition of Uniform Thickness Ge23Sb7S70 and As40S60 Chalcogenide Glass Films

Solution-based electrospray film deposition, which is compatible with continuous, roll-to-roll processing, is applied to chalcogenide glasses. Two chalcogenide compositions are demonstrated: Ge23Sb7S70 and As40S60, which have both been studied extensively for planar mid-infrared (mid-IR) microphotonic devices. In this approach, uniform thickness films are fabricated through the use of computer numerical controlled (CNC) motion. Chalcogenide glass (ChG) is written over the substrate by a single nozzle along a serpentine path. Films were subjected to a series of heat treatments between 100 &#176;C and 200 &#176;C under vacuum to drive off residual solvent and densify the films. Based on transmission Fourier transform infrared (FTIR) spectroscopy and surface roughness measurements, both compositions were found to be suitable for the fabrication of planar devices operating in the mid-IR region. Residual solvent removal was found to be much quicker for the As40S60 film as compared to Ge23Sb7S70. Based on the advantages of electrospray, direct printing of a gradient refractive index (GRIN) mid-IR transparent coating is envisioned, given the difference in refractive index of the two compositions in this study.

Electrospray Deposition of Uniform Thickness Ge23Sb7S70 and As40S60 Chalcogenide Glass Films

A method of uniform thickness solution-derived chalcogenide glass film deposition is demonstrated using computer numerical controlled motion of a single-nozzle electrospray.

厚度均匀葛电沉积<sub&gt; 23</sub&gt;锑<sub&gt; 7</sub&gt;取值<sub&gt; 70</sub&gt;和As<sub&gt; 40</sub&gt;取值<sub&gt; 60</sub&gt;硫系玻璃薄膜

Solution-based electrospray film deposition, which is compatible with continuous, roll-to-roll processing, is applied to chalcogenide glasses. Two ...

Hand Controlled Manipulation of Single Molecules via a Scanning Probe Microscope with a 3D Virtual Reality Interface

Considering organic molecules as the functional building blocks of future nanoscale technology, the question of how to arrange and assemble such building blocks in a bottom-up approach is still open. The scanning probe microscope (SPM) could be a tool of choice; however, SPM-based manipulation was until recently limited to two dimensions (2D). Binding the SPM tip to a molecule at a well-defined position opens an opportunity of controlled manipulation in 3D space. Unfortunately, 3D manipulation is largely incompatible with the typical 2D-paradigm of viewing and generating SPM data on a computer. For intuitive and efficient manipulation we therefore couple a low-temperature non-contact atomic force/scanning tunneling microscope (LT NC-AFM/STM) to a motion capture system and fully immersive virtual reality goggles. This setup permits &#34;hand controlled manipulation&#34; (HCM), in which the SPM tip is moved according to the motion of the experimenter&#39;s hand, while the tip trajectories as well as the response of the SPM junction are visualized in 3D. HCM paves the way to the development of complex manipulation protocols, potentially leading to a better fundamental understanding of nanoscale interactions acting between molecules on surfaces. Here we describe the setup and the steps needed to achieve successful hand-controlled molecular manipulation within the virtual reality environment.

We demonstrate the precise manipulation of individual organic molecules on a metal surface with the tip of a scanning probe microscope driven in 3D by the experimenter's hand using a motion capture system and fully immersive virtual reality goggles.

通过手扫描探针显微镜用三维虚拟现实接口控制单分子的操作

Considering organic molecules as the functional building blocks of future nanoscale technology, the question of how to arrange and assemble such building ...

Orientational Transition in a Liquid Crystal Triggered by the Thermodynamic Growth of Interfacial Wetting Sheets

In liquid crystal (LC) physical chemistry, molecules near the surface play a great role in controlling bulk orientation. Thus far, mainly to achieve desired molecular orientation states in LC displays, the "static" surface property of LCs, so-called surface anchoring, has been intensively studied. As a rule of thumb, once the initial orientation of LCs is "locked" by specific surface treatments, such as rubbing or treatment with a specific alignment layer, it hardly changes with temperature. Here, we present a system exhibiting an orientational transition upon temperature variation, which conflicts with the consensus. Right on the transition, the bulk LC molecules experience the orientational rotation, with 90&#176; between the planar (P) orientation at high temperatures and the vertical (V) orientation at low temperatures in the first-order transitional manner. We have tracked thermodynamic surface anchoring behavior by means of polarizing optical microscopy (POM), dielectric spectroscopy (DS), high-resolution differential scanning calorimetry (HR-DSC), and grazing incidence X-ray diffraction (GI-XRD) and reached a plausible physical explanation: that the transition is triggered by a growth of surface wetting sheets, which impose the V orientation locally against the P orientation in the bulk. This landscape would provide a general link explaining how the equilibrium bulk orientation is affected by surface-localized orientation in many LC systems. In our characterization, POM and DS are advantageous by offering information on the spatial distribution of the orientation of LC molecules. HR-DSC gives information about the precise thermodynamic information on transitions, which cannot be addressed by conventional DSC instruments due to limited resolution. GI-XRD provides information on surface-specific molecular orientation and short-range orderings. The goal of this paper is to present a protocol for preparing a sample that exhibits the transition and to demonstrate how the thermodynamic structural variation, both in the bulk and on surfaces, can be analyzed through the abovementioned methods.

Here, we present a protocol to trigger an orientational transition of a liquid crystal in response to temperature. Methodologies are described for preparing a sample in order to observe the transition and the detailed transitional evolution.

由界面润湿片的热力学生长触发的液晶中的定向转变

In liquid crystal (LC) physical chemistry, molecules near the surface play a great role in controlling bulk orientation. Thus far, mainly to achieve ...

Preparation of Large-area Vertical 2D Crystal Hetero-structures Through the Sulfurization of Transition Metal Films for Device Fabrication

We have demonstrated that through the sulfurization of transition metal films such as molybdenum (Mo) and tungsten (W), large-area and uniform transition metal dichalcogenides (TMDs) MoS2 and WS2 can be prepared on sapphire substrates. By controlling the metal film thicknesses, good layer number controllability, down to a single layer of TMDs, can be obtained using this growth technique. Based on the results obtained from the Mo film sulfurized under the sulfur deficient condition, there are two mechanisms of (a) planar MoS2 growth and (b) Mo oxide segregation observed during the sulfurization procedure. When the background sulfur is sufficient, planar TMD growth is the dominant growth mechanism, which will result in a uniform MoS2 film after the sulfurization procedure. If the background sulfur is deficient, Mo oxide segregation will be the dominant growth mechanism at the initial stage of the sulfurization procedure. In this case, the sample with Mo oxide clusters covered with few-layer MoS2 will be obtained. After sequential Mo deposition/sulfurization and W deposition/sulfurization procedures, vertical WS2/MoS2 hetero-structures are established using this growth technique. Raman peaks corresponding to WS2 and MoS2, respectively, and the identical layer number of the hetero-structure with the summation of individual 2D materials have confirmed the successful establishment of the vertical 2D crystal hetero-structure. After transferring the WS2/MoS2 film onto a SiO2/Si substrate with pre-patterned source/drain electrodes, a bottom-gate transistor is fabricated. Compared with the transistor with only MoS2 channels, the higher drain currents of the device with the WS2/MoS2 hetero-structure have exhibited that with the introduction of 2D crystal hetero-structures, superior device performance can be obtained. The results have revealed the potential of this growth technique for the practical application of 2D crystals.

Through the sulfurization of pre-deposited transition metals, large-area and vertical 2D crystal hetero-structures can be fabricated. The film transferring and device fabrication procedures are also demonstrated in this report.

硫化过渡金属膜制备大面积垂直2D 晶体杂化结构

We have demonstrated that through the sulfurization of transition metal films such as molybdenum (Mo) and tungsten (W), large-area and uniform transition ...

Generation of Size-controlled Poly (ethylene Glycol) Diacrylate Droplets via Semi-3-Dimensional Flow Focusing Microfluidic Devices

Uniform and size-controllable poly (ethylene glycol) diacrylate (PEGDA) droplets could be produced via the flow focusing process in a microfluidic device. This paper proposes a semi-three-dimensional (semi-3D) flow-focusing microfluidic chip for droplet formation. The polydimethylsiloxane (PDMS) chip was fabricated using the multi-layer soft lithography method. Hexadecane containing surfactant was used as the continuous phase, and PEGDA with the ultraviolet (UV) photo-initiator was the dispersed phase. Surfactants allowed the local surface tension to drop and formed a more cusped tip which promoted breaking into tiny micro-droplets. As the pressure of dispersed phase was constant, the size of droplets became smaller with increasing continuous phase pressure before dispersed phase flow was broken off. As a result, droplets with size variation from 1 &#181;m to 80 &#181;m in diameter could be selectively achieved by changing the pressure ratio in two inlet channels, and the average coefficient of variation was estimated to be below 7%. Furthermore, droplets could turn into micro-beads by UV exposure for photo-polymerization. Conjugating biomolecules on such micro-beads surface have many potential applications in the fields of biology and chemistry.

Generation of Size-controlled Poly ethylene Glycol Diacrylate Droplets via Semi-3-Dimensional Flow Focusing Microfluidic Devices

Here, we present a protocol to illustrate the fabrication processes and verifying experiments of a semi-three-dimensional (semi-3D) flow-focusing microfluidic chip for droplet formation.

Semi-3-Dimensional 流聚焦微流控装置产生尺寸控制的聚乙二醇二丙烯酸酯液滴

Uniform and size-controllable poly (ethylene glycol) diacrylate (PEGDA) droplets could be produced via the flow focusing process in a microfluidic device. ...