This work was supported by NSF award number DMR-1610126.

1. Hot exfoliation of 2-D materials
NOTE: This procedure is done inside a glovebox.
<ol>
	<li>Tape Preparation
	<ol>
		<li>Cut a length of tape (see Table of Materials) that is &#8776;5-10 cm long and &#8776;2 cm wide. Place it, sticky side up, on the working area. Fold the ends of the tape for easier handling.</li>
		<li>Using tweezers, deposit the desired material approximately one-quarter of the way down the length of the tape by repeatedly pressing the material into the tape.</li>
		<li>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 1 cm2. Depending on the material, repeat this multiple times: 1-2 times for black phosphorus, or several times for graphite or hexagonal boron nitride.</li>
	</ol>
	</li>
	<li>Sample preparation
	<ol>
		<li>Using the desired method, such as a carbide-tipped scribe, cleave an oxidized silicon wafer or other desired substrate into chips suitable for the experiment, &#8804;1 cm wide. Clean the chips by sonicating for 2 min in acetone, followed by isopropanol (IPA), at relatively low power (we used 12 W). Blow the chips dry with N2.</li>
		<li>Using the prepared tape, firmly press the deposited material onto the substrate. Apply firm pressure with a thumb or gently press with tweezers so the material contacts the chip as much as possible</li>
		<li>Place the tape with substrate (substrate side down) on a hotplate at 120 &#176;C for 2 min.</li>
		<li>Allow the substrate to cool to RT and carefully remove it from the tape. Soak in acetone for 20 min to remove tape residue. Rinse with IPA for 30 s and dry the substrate with nitrogen. Depending on the material, further options for cleaning may be available, such as a forming gas anneal.</li>
	</ol>
	</li>
</ol>
2. Hermetic transfer cell construction, operation, and maintenance
<ol>
	<li>Construction
	<ol>
		<li>Construct the cell (Figure 2) out of the desired material (we used aluminum). It is 30 mm in diameter and 17.6 mm tall when closed. Fabrication drawings are available at&#160;http://churchill-lab.com/useful-things.</li>
		<li>Make the base 16.2 mm tall with a raised sample platform that is threaded with &#190; - 10 threads with a vent cut into the threads. Where the cap meets the base, make an inset for an O-ring (see Table of Materials).</li>
		<li>Make the cap 8.6 mm tall with matching female threads through the center.</li>
		<li>Recess the cap by 0.2 mm to accommodate a 24 mm diameter x 0.1 mm thick coverglass window (here, borosilicate glass).</li>
		<li>Apply a small amount of vacuum grease to the all sides of the O-ring and drop it into the base inset.</li>
		<li>Before affixing the window to the cap of the cell, clean the cap in acetone and IPA to remove any oil or debris left by the machining process.</li>
		<li>Attach the window to the cell cap using epoxy. Thoroughly mix the epoxy according to the manufacturer&#39;s specifications. Here, parts A and B are combined in a 1:1.8 ratio by weight.</li>
		<li>Apply a small amount of epoxy to the recessed area on the cap and spread it around as evenly as possible.</li>
		<li>Set a 0.1 mm thick, 24 mm diameter coverglass (borosilicate glass in this case) into the recess and gently press it into the epoxy. Ensure the window is level with the top of the cap and there that there are no bubbles in the epoxy.</li>
		<li>Wipe up any extra epoxy so that nothing protrudes from the surface of the cap. Allow the epoxy to cure for the manufacturer prescribed time at room temperature.</li>
	</ol>
	</li>
	<li>Operation 
	NOTE: This procedure is done inside a glovebox.
	<ol>
		<li>Using the desired method, affix a prepared sample to the cell base (double sided tape, glue, etc.). The cell is designed to accommodate samples up to 1 cm wide and 0.7 mm thick, including the adhesive.</li>
		<li>Firmly screw the cap onto the base. This makes a seal between the cap and base by compressing the O-ring. Make sure that the pressure inside the transfer cell does not exceed 3 mbar above the ambient pressure.</li>
		<li>Check that the sample sits just below the window. The sample can now be safely removed from the glovebox.</li>
	</ol>
	</li>
	<li>Window repair
	<ol>
		<li>Using tweezers, remove any broken glass that is not firmly affixed to the epoxy. Break up what glass remains (using a carbide-tipped scribe or other method) so that the epoxy beneath is exposed. 
		CAUTION: Wear gloves and eye protection when removing broken glass.</li>
		<li>Soak the cap in a 50:50 mix of acetone and trichloroethylene (TCE) for 1-2 h or until the epoxy softens and begins to separate from the cap. Rinse in IPA for 30 s.</li>
		<li>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.</li>
		<li>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.</li>
	</ol>
	</li>
</ol>
3. Example uses of the transfer cell
<ol>
	<li>Optical analysis
	<ol>
		<li>For flake imaging, place the transfer cell under the microscope. The cell can be used with any conventional microscope. When focusing, be careful not to crash the objective into the fragile widow.</li>
		<li>Proceed with desired method for finding material flakes.</li>
	</ol>
	</li>
	<li>Polarized Raman spectroscopy
	<ol>
		<li>For polarization-resolved Raman spectroscopy, align a laser spot to a flake of interest. In this case we use 633 nm wavelength and 50 &#181;W power and a 100x objective lens. For black phosphorus, low laser power is required to prevent damage to the flake.</li>
		<li>Using a half-wave plate, vary the polarization angle.</li>
	</ol>
	</li>
</ol>

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The authors have nothing to disclose.

Hot exfoliation retains the ability of typical mechanical exfoliation to produce pristine thin flakes while also avoiding many downfalls of alternatives. Like typical mechanical exfoliation, this technique is not limited to a small subset of materials. Hot exfoliation can be applied to any material that can be exfoliated using room-temperature mechanical exfoliation as long as the material tolerates heating to 120 &#176;C for 2 min in an inert atmosphere. We also note that it has been shown20 that the heating time and temperature (above 100 &#176;C) do not make any noticeable difference in flake density. Along with increased contact, average flake size can also be improved by increasing the bond strength between the substrate and the flakes. One way to do this would be by treating the substrate with O2 plasma but this would also make the flakes hard or impossible to pick up for use in devices requiring heterostructure fabrication20. 
The transfer cell can be constructed from any suitable metal. We used aluminum because it is easy to machine but it should be noted that TCE (used to remove epoxy) is corrosive to aluminum when unstabilized, heated, or mixed with water. Stainless steel would be more durable and less reactive with TCE. However, we have not seen any corrosive effects using this method at RT. For imaging and analysis with high numerical aperture objectives, construction of the transfer cell is such that, when closed, the bottom of the window is 0.8 mm above the top of the base. With a 0.5 mm thick substrate and 0.1 mm thick adhesive, the sample sits 0.3 mm below the top of the transfer cell. This proximity allows for imaging and analysis with high magnification and relatively short working distance objectives. Exfoliated material can be clearly seen at 5, 20, 50 times magnification allowing for easy identification of thin flakes. At higher magnifications, spherical aberrations caused by the window significantly degrades the image quality. Provided that the sample substrate is less than 0.7 mm thick, there is no risk of over tightening the cell. When the cap is screwed down, excess gas is expelled through the vent in the threads. During construction, the precise location of the vent is not important, but it is important that it is not obstructed by the sample, vacuum grease or anything else. The vent prevents the fragile 0.1 mm thick window from breaking due to overpressure when the cap is screwed down. The window can only withstand pressure changes of a few mbar.
The coverglass window used for the transfer cells is made of borosilicate glass but for optical analysis at wavelengths other than visible to near-infrared, other window materials may be use. For the best imaging, care should be taken when installing the glass window. If not seated properly, the distance between the sample and window could be larger than expected. Especially for small working distance objectives, this could cause the objective to crash into and break the window. Also, some epoxies will cure faster at higher temperatures, but because metals and glass have different thermal expansion coefficients, the widow will deform after cooling back to room temperature. The epoxy should be cured at the same temperature at which it will be used (i.e., if the cell will be used at room temperature), the epoxy should also be cured at room temperature.

Various layered materials that can be exfoliated down to a single atomic layer have generated interest across a broad range of fields. However, investigation and application of many of these materials is complicated by the fact that they are unstable in air and quickly oxidize or hydrate when exposed. For example; black phosphorus is a semiconductor with a tunable direct band gap, high mobility and anisotropic optical and electrical properties1,2,3,4,5 but is unstable in air and will deteriorate in less than an hour6,7 due to interactions with oxygen8. CrI3 has been recently shown to exhibit two-dimensional ferromagnetism9,10,11 but, when exposed to air, it degrades almost instantly11.
Devices made from these materials can be protected from air by working in a glovebox and encapsulating them in a chemically inert material such as hexagonal boron nitride12,13. However, when developing these devices, it is often necessary to identify and analyze the flakes before encapsulation. This analysis requires either removing the sample from the inert environment of the glovebox or putting the analysis equipment in the glovebox. Removing the sample, even for a short time, risks damage via oxidation or hydration, while placing the necessary equipment inside a glovebox can be costly and cumbersome. To remedy this, we designed a hermetic transfer cell that safely encloses a sample, keeping it in an inert environment, so that it can be removed from the glovebox. While in the transfer cell, a sample sits 0.3 mm below a glass window to allow easy identification of flakes under a microscope as well as the use of optical analysis techniques such as photoluminescence or Raman spectroscopy.
Some two-dimensional materials, in addition to being air sensitive, are also difficult to exfoliate into thin flakes with the typical mechanical exfoliation method because a relatively high cleavage energy, relatively weak in-plane bonds, or both. Other methods, such as CVD growth14,15, liquid exfoliation16, or gold mediated exfoliation17,18 have been developed for producing thin layers but may result in less than pristine flakes and only work for certain materials. Although exfoliation of graphene at elevated temperatures has been known to produce large flakes for at least a decade19, this technique has been quantitatively characterized recently for both graphene and Bi2Sr2CaCu2Ox flakes20. Here, we demonstrate that hot exfoliation improves exfoliation yield also for black phosphorus, a material that is notoriously difficult to exfoliate. This technique, together with a hermetic transfer cell, facilitates the exfoliation and analysis of air sensitive, two-dimensional materials.

<table border="0" cellpadding="0" cellspacing="0" style="width:1000px;" width="1000">
	<tbody>
		<tr height="21">
			<td height="21" style="height:21px;width:345px;">Ablestik 286 epoxy</td>
			<td style="width:279px;">Loctite</td>
			<td style="width:205px;">256 6 OZ TUBE KIT</td>
			<td style="width:172px;">air-tight epoxy</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:345px;">Acetone</td>
			<td style="width:279px;">EDM Millipore Corporation</td>
			<td style="width:205px;">67-64-1</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:345px;">Circular coverglass, 24 mm dia, 0 thickness</td>
			<td style="width:279px;">Agar Scientific</td>
			<td style="width:205px;">AGL46R22-0</td>
			<td>window glass</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:345px;">Dicing tape</td>
			<td style="width:279px;">Ultron systems</td>
			<td style="width:205px;">1009R</td>
			<td>exfoliation tape</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:345px;">High-Vacuum grease</td>
			<td style="width:279px;">Dow Corning</td>
			<td style="width:205px;">1597418</td>
			<td>O-ring grease</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:345px;">Isopropanol</td>
			<td style="width:279px;">VWR Chemicals</td>
			<td style="width:205px;">BDH20880.400</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:345px;">Silicon wafer, 300 nm oxide</td>
			<td style="width:279px;">University Wafer</td>
			<td style="width:205px;">E0851.01</td>
			<td>flake substrate</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:345px;">Silicon wafer, 90 nm oxide</td>
			<td style="width:279px;">Nova Electronic Materials</td>
			<td style="width:205px;">HS39626-OX</td>
			<td>flake substrate</td>
		</tr>
	</tbody>
</table>

exfoliation and analysis of large-area, air-sensitive two-dimensional materials

We describe methods for producing and analyzing large, thin flakes of air-sensitive two-dimensional materials. Thin flakes of layered or van der Waals crystals are produced using mechanical exfoliation, in which layers are peeled off a bulk crystal using adhesive tape. This method produces high-quality flakes, but they are often small and can be hard to find, particularly for materials with relatively high cleavage energies such as black phosphorus. By heating the substrate and the tape, two-dimensional material adhesion to the substrate is promoted, and the flake yield can be increased by up to a factor of ten. After exfoliation, it is necessary to image or otherwise analyze these flakes but some two-dimensional materials are sensitive to oxygen or water and will degrade when exposed air. We have designed and tested a hermetic transfer cell to temporarily maintain the inert environment of a glovebox so that air-sensitive flakes can be imaged and analyzed with minimal degradation. The compact design of the transfer cell is such that optical analysis of sensitive materials can be performed outside of a glovebox without specialized equipment or modifications to existing equipment.

The goal of exfoliating two-dimensional materials is to isolate atomically thin layers. During the exfoliation process, flakes separate from the bulk crystal, leaving behind flakes of varying thicknesses, with a small probability for some flakes to be monolayers. By increasing the density and size of all exfoliated flakes, hot exfoliation increases the density and lateral size of thin flakes. This is accomplished by increasing the material area that makes close contact with the substrate. While in contact, gasses trapped between the material and the substrate expand during heating and are pushed out from under flakes. The removal of trapped gas allows more of the material to come into close contact with the substrate, thus increasing the amount of exfoliated flakes (Figure 1A,B) as clearly explained in Ref 20. Exfoliations of black phosphorus were performed using the typical mechanical exfoliation and hot exfoliation technique on silicon chips with 90 nm thick SiO2. By measuring the total area of deposited material on a 1 cm x 1 cm silicon chip, it can be seen (Figure 1C) that hot exfoliation deposits 6-10 times more material. We note that in our experience other materials can be picked up from HF-cleaned substrates using polycarbonate following hot exfoliation, including graphene, hexagonal boron nitride, black phosphorus, MnPSe3, and WSe2. We used a 10:1 HF:water solution to clean the SiO2 substrates over a 15 s period. Note, 10% HF etches SiO2 at a rate of 23 nm/min21 , so this process etches our substrates by 6 nm.
We now consider the effectiveness of the hermetic transfer cell (Figure 3A) in maintaining an inert atmosphere when removed from a glovebox. CrI3 is particularly sensitive to hydration and degrades within seconds when exposed to air (Figure 3D). Inside a transfer cell, however, an exfoliated CrI3 sample remained unchanged for 15 hours (Figure 3B) and only began to show signs of degradation (blisters) after 24 hours (Figure 3C). While damage on a scale too small observe optically likely occurs on a shorter timescale, these results demonstrate that the hermetic transfer cell described here slows the sample degradation rate by at least three orders of magnitude (hours inside the cell compared with seconds outside).
To demonstrate use of the transfer cell for optical analysis of air-sensitive materials, we performed polarization-resolved Raman spectroscopy on a relatively thick (&#62; 50 nm) flake of black phosphorus (Figure 4A). The spectra were acquired using 50 &#181;W laser excitation at 632.8 nm with a 100x objective lens. A half-wave plate was used to rotate the polarization of the excitation beam. In Figure 4B, three Raman peaks can be observed in BP at around 466, 438 and 361&#8197;cm&#8722;1, corresponding to Ag2, B2g and Ag1 vibration modes respectively, regardless of the polarization, which agrees well with previous observations in bulk BP crystals for excitation and collection along the z-axis.5,22 The peak positions do not vary with polarization angle. However, the relative intensities of these three modes change significantly with incident light polarization. The vibration mode Ag2, which has the strongest intensity variation with the excitation laser polarization, as shown in Figure 4B,C, is associated with the atomic motion along the armchair direction. Therefore, as previously reported5, this vibration mode provides an effective method to determine the armchair direction of the BP crystal and hence the crystal orientation. In Figure 4C, the Raman intensity shows two maxima within one full rotation, located at 26.5&#176; and 206.5&#176; with respect to the X and Y axes defined in the microscope images, and we conclude that the armchair direction of the BP is oriented at 26.5&#176; for this flake. Similar optical spectroscopy methods can be used to determine crystal orientation and other properties, such as layer number or optical band gap, for other air-sensitive 2-D materials.
<img alt="Figure 1" class="xfigimg" src="/files/ftp_upload/58693/58693fig1.jpg" /> 
Figure 1: Distribution of material on an&#160;oxidized silicon chip. (A) Typical sample of black phosphorus exfoliated at room temperature. (B) Typical sample of black phosphorus exfoliated at 120 &#176;C. (C) Histogram of exfoliated black phosphorus area using room temperature(cold) and hot exfoliation. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/58693/58693fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 2" class="xfigimg" src="/files/ftp_upload/58693/58693fig2.jpg" /> 
Figure 2: Transfer cell. (A) Picture of a hermetic transfer cell showing separate cap and base. (B) Schematic drawing of transfer. A vent (green) is cut into the threads. Note that the bottom of the base is tapped and threaded for mounting. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/58693/58693fig2large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 3" class="xfigimg" src="/files/ftp_upload/58693/58693fig3.jpg" /> 
Figure 3: Transfer cell suppression of flake degradation. (A) Fresh CrI3 in a transfer cell (B) CrI3 in a cell after 15 h. (C) CrI3 in a cell after 24 h Hydration blisters can be seen at this point. (D) CrI3 in air after 24 h in transfer cell and 30 s in air. Large areas of hydrated CrI3 have collected at the edges of the flake. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/58693/58693fig3large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 4" class="xfigimg" src="/files/ftp_upload/58693/58693fig4.jpg" /> 
Figure 4: Crystalline orientation identification. (A) Optical micrograph of thick flakes of exfoliated BP. (B) Polarization-resolved Raman spectroscopy of a thick BP flake. (C) Polar plot of Raman intensity averaged over the spectral range in (B) as a function of linear excitation polarization angle (plot origin is zero intensity). The fit is a sine function plus a constant. The dashed line indicates the armchair direction.&#160;<a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/58693/58693fig4large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

Watch this Scientific Journal Video about Exfoliation and Analysis of Large-area, Air-Sensitive Two-Dimensional Materials at JoVE.com

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

A method for exfoliating large thin flakes of air sensitive two-dimensional materials and safely transporting them for analysis outside of a glovebox is presented.

We describe methods for producing and analyzing large, thin flakes of air-sensitive two-dimensional materials. Thin flakes of layered or van der Waals ...

exfoliation-analysis-large-area-air-sensitive-two-dimensional

Nanyang Technological University

Research

JoVE Journal

Chemistry

11.4K Views.  University of Arkansas. A method for exfoliating large thin flakes of air sensitive two-dimensional materials and safely transporting them for analysis outside of a glovebox is presented. 

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

Seeded Synthesis of CdSe/CdS Rod and Tetrapod Nanocrystals

We demonstrate a method for the synthesis of multicomponent nanostructures consisting of CdS and CdSe with rod and tetrapod morphologies. A seeded synthesis strategy is used in which spherical seeds of CdSe are prepared first using a hot-injection technique. By controlling the crystal structure of the seed to be either wurtzite or zinc-blende, the subsequent hot-injection growth of CdS off of the seed results in either a rod-shaped or tetrapod-shaped nanocrystal, respectively. The phase and morphology of the synthesized nanocrystals are confirmed using X-ray diffraction and transmission electron microscopy, demonstrating that the nanocrystals are phase-pure and have a consistent morphology. The extinction coefficient and quantum yield of the synthesized nanocrystals are calculated using UV-Vis absorption spectroscopy and photoluminescence spectroscopy. The rods and tetrapods exhibit extinction coefficients and quantum yields that are higher than that of the bare seeds. This synthesis demonstrates the precise arrangement of materials that can be achieved at the nanoscale by using a seeded synthetic approach.

A protocol for the seeded synthesis of rod-shaped and tetrapod-shaped multicomponent nanostructures consisting of CdS and CdSe is presented.

We demonstrate a method for the synthesis of multicomponent nanostructures consisting of CdS and CdSe with rod and tetrapod morphologies. A seeded ...

Exfoliation of Egyptian Blue and Han Blue, Two Alkali Earth Copper Silicate-based Pigments

In a visualized example of the ancient past connecting with modern times, we describe the preparation and exfoliation of CaCuSi4O10 and BaCuSi4O10, the colored components of the historic Egyptian blue and Han blue pigments. The bulk forms of these materials are synthesized by both melt flux and solid-state routes, which provide some control over the crystallite size of the product. The melt flux process is time intensive, but it produces relatively large crystals at lower reaction temperatures. In comparison, the solid-state method is quicker yet requires higher reaction temperatures and yields smaller crystallites. Upon stirring in hot water, CaCuSi4O10 spontaneously exfoliates into monolayer nanosheets, which are characterized by TEM and PXRD. BaCuSi4O10 on the other hand requires ultrasonication in organic solvents to achieve exfoliation. Near infrared imaging illustrates that both the bulk and nanosheet forms of CaCuSi4O10 and BaCuSi4O10 are strong near infrared emitters. Aqueous CaCuSi4O10 and BaCuSi4O10 nanosheet dispersions are useful because they provide a new way to handle, characterize, and process these materials in colloidal form.

The preparation and exfoliation of CaCuSi4O10&#160;and BaCuSi4O10 are described. Upon stirring in hot water, CaCuSi4O10 spontaneously exfoliates into monolayers, whereas BaCuSi4O10&#160;requires ultrasonication in organic solvents. Near infrared (NIR) imaging illustrates the NIR emission properties of these materials, and aqueous dispersions of these nanomaterials are useful for solution processing.

In a visualized example of the ancient past connecting with modern times, we describe the preparation and exfoliation of CaCuSi4O10 and BaCuSi4O10, the ...

Self-assembly of Complex Two-dimensional Shapes from Single-stranded DNA Tiles

Current methods in DNA nano-architecture have successfully engineered a variety of 2D and 3D structures using principles of self-assembly. In this article, we describe detailed protocols on how to fabricate sophisticated 2D shapes through the self-assembly of uniquely addressable single-stranded DNA tiles which act as molecular pixels on a molecular canvas. Each single-stranded tile (SST) is a 42-nucleotide DNA strand composed of four concatenated modular domains which bind to four neighbors during self-assembly. The molecular canvas is a rectangle structure self-assembled from SSTs. A prescribed complex 2D shape is formed by selecting the constituent molecular pixels (SSTs) from a 310-pixel molecular canvas and then subjecting the corresponding strands to one-pot annealing. Due to the modular nature of the SST approach we demonstrate the scalability, versatility and robustness of this method. Compared with alternative methods, the SST method enables a wider selection of information polymers and sequences through the use of de novo designed and synthesized short DNA strands.

DNA tiling is an effective approach to make programmable nanostructures. We describe the protocols to construct complex two-dimensional shapes by the self-assembly of single-stranded DNA tiles.

Current methods in DNA nano-architecture have successfully engineered a variety of 2D and 3D structures using principles of self-assembly. In this ...

Supercritical Nitrogen Processing for the Purification of Reactive Porous Materials

Supercritical fluid extraction and drying methods are well established in numerous applications for the synthesis and processing of porous materials. Herein, nitrogen is presented as a novel supercritical drying fluid for specialized applications such as in the processing of reactive porous materials, where carbon dioxide and other fluids are not appropriate due to their higher chemical reactivity. Nitrogen exhibits similar physical properties in the near-critical region of its phase diagram as compared to carbon dioxide: a widely tunable density up to ~1 g ml-1, modest critical pressure (3.4 MPa), and small molecular diameter of ~3.6 &#197;. The key to achieving a high solvation power of nitrogen is to apply a processing temperature in the range of 80-150 K, where the density of nitrogen is an order of magnitude higher than at similar pressures near ambient temperature. The detailed solvation properties of nitrogen, and especially its selectivity, across a wide range of common target species of extraction still require further investigation. Herein we describe a protocol for the supercritical nitrogen processing of porous magnesium borohydride.

Nitrogen is an effective supercritical fluid for extraction or drying processes due to its small molecular size, high density in the near-liquid supercritical regime, and chemical inertness. We present a supercritical nitrogen drying protocol for the purification treatment of reactive, porous materials.

Supercritical fluid extraction and drying methods are well established in numerous applications for the synthesis and processing of porous materials. ...

Fabrication of Large-area Free-standing Ultrathin Polymer Films

This procedure describes a method for the fabrication of large-area and ultrathin free-standing polymer films. Typically, ultrathin films are prepared using either sacrificial layers, which may damage the film or affect its mechanical properties, or they are made on freshly cleaved mica, a substrate that is difficult to scale. Further, the size of ultrathin film is typically limited to a few square millimeters. In this method, we modify a surface with a polyelectrolyte that alters the strength of adhesion between polymer and deposition substrate. The polyelectrolyte can be shown to remain on the wafer using spectroscopy, and a treated wafer can be used to produce multiple films, indicating that at best minimal amounts of the polyelectrolyte are added to the film. The process has thus far been shown to be limited in scalability only by the size of the coating equipment, and is expected to be readily scalable to industrial processes. In this study, the protocol for making the solutions, preparing the deposition surface, and producing the films is described.

We describe a method for the fabrication of large-area (up to 13 cm diameter) and ultrathin (as thin as 8 nm) polymer films. Instead of using a sacrificial interlayer to delaminate the film from its substrate, we use a self-limiting surface treatment suitable for arbitrarily large areas.

This procedure describes a method for the fabrication of large-area and ultrathin free-standing polymer films.  Typically, ultrathin films are prepared ...

Synthesis and Exfoliation of Discotic Zirconium Phosphates to Obtain Colloidal Liquid Crystals

Due to their abundance in natural clay and potential applications in advanced materials, discotic nanoparticles are of interest to scientists and engineers. Growth of such anisotropic nanocrystals through a simple chemical method is a challenging task. In this study, we fabricate discotic nanodisks of zirconium phosphate [Zr(HPO4)2&#183;H2O] as a model material using hydrothermal, reflux and microwave-assisted methods. Growth of crystals is controlled by duration time, temperature, and concentration of reacting species. The novelty of the adopted methods is that discotic crystals of size ranging from hundred nanometers to few micrometers can be obtained while keeping the polydispersity well within control. The layered discotic crystals are converted to monolayers by exfoliation with tetra-(n)-butyl ammonium hydroxide [(C4H9)4NOH, TBAOH]. Exfoliated disks show isotropic and nematic liquid crystal phases. Size and polydispersity of disk suspensions is highly important in deciding their phase behavior.

A two dimensional model material of discotic zirconium phosphate was developed. The inorganic crystal with lamellar structure was synthesized by hydrothermal, reflux, and microwave-assisted methods. On exfoliation with organic molecules, layered crystals can be converted to monolayers, and nematic liquid crystal phase was formed at sufficient concentration of monolayers.

Due to their abundance in natural clay and potential applications in advanced materials, discotic nanoparticles are of interest to scientists and ...

Facet-to-facet Linking of Shape-anisotropic Colloidal Cadmium Chalcogenide Nanostructures

Here, we describe a protocol that allows for shape-anisotropic cadmium chalcogenide nanocrystals (NCs), such as nanorods (NRs) and tetrapods (TPs), to be covalently and site-specifically linked via their end facets, resulting in polymer-like linear or branched chains. The linking procedure begins with a cation-exchange process in which the end facets of the cadmium chalcogenide NCs are first converted to silver chalcogenide. This is followed by the selective removal of ligands at their surface. This results in cadmium chalcogenide NCs with highly reactive silver chalcogenide end facets that spontaneously fuse upon contact with each other, thereby establishing an interparticle facet-to-facet attachment. Through the judicious choice of precursor concentrations, an extensive network of linked NCs can be produced. Structural characterization of the linked NCs is carried out via low- and high-resolution transmission electron microscopy (TEM), as well as energy-dispersive X-ray spectroscopy, which confirm the presence of silver chalcogenide domains between chains of cadmium chalcogenide NCs.

A protocol detailing how shape-anisotropic colloidal cadmium chalcogenide nanocrystals can be covalently linked via their end facets is presented here.

Here, we describe a protocol that allows for shape-anisotropic cadmium chalcogenide nanocrystals (NCs), such as nanorods (NRs) and tetrapods (TPs), to be ...

Microfluidic Dry-spinning and Characterization of Regenerated Silk Fibroin Fibers

The protocol demonstrates a method for mimicking the spinning process of silkworm. In the native spinning process, the contracting spinning duct enables the silk proteins to be compact and ordered by shearing and elongation forces. Here, a biomimetic microfluidic channel was designed to mimic the specific geometry of the spinning duct of the silkworm. Regenerated silk fibroin (RSF) spinning doped with high concentration, was extruded through the microchannel to dry-spin fibers at ambient temperature and pressure. In the post-treated process, the as-spun fibers were drawn and stored in ethanol aqueous solution. Synchrotron radiation wide-angle X-ray diffraction (SR-WAXD) technology was used to investigate the microstructure of single RSF fibers, which were fixed to a sample holder with the RSF fiber axis normal to the microbeam of the X-ray. The crystallinity, crystallite size, and crystalline orientation of the fiber were calculated from the WAXD data. The diffraction arcs near the equator of the two-dimensional WAXD pattern indicate that the post-treated RSF fiber has a high orientation degree.

A protocol for the microfluidic spinning and microstructure characterization of regenerated silk fibroin monofilament is presented.

The protocol demonstrates a method for mimicking the spinning process of silkworm. In the native spinning process, the contracting spinning duct enables ...

Synthesis and Characterization of Amphiphilic Gold Nanoparticles

Gold nanoparticles covered with a mixture of 1-octanethiol (OT) and 11-mercapto-1-undecane sulfonic acid (MUS) have been extensively studied because of their interactions with cell membranes, lipid bilayers, and viruses. The hydrophilic ligands make these particles colloidally stable in aqueous solutions and the combination with hydrophobic ligands creates an amphiphilic particle that can be loaded with hydrophobic drugs, fuse with the lipid membranes, and resist nonspecific protein adsorption. Many of these properties depend on nanoparticle size and the composition of the ligand shell. It is, therefore, crucial to have a reproducible synthetic method and reliable characterization techniques that allow the determination of nanoparticle properties and the ligand shell composition. Here, a one-phase chemical reduction, followed by a thorough purification to synthesize these nanoparticles with diameters below 5 nm, is presented. The ratio between the two ligands on the surface of the nanoparticle can be tuned through their stoichiometric ratio used during synthesis. We demonstrate how various routine techniques, such as transmission electron microscopy (TEM), nuclear magnetic resonance (NMR), thermogravimetric analysis (TGA), and ultraviolet-visible (UV-Vis) spectrometry, are combined to comprehensively characterize the physicochemical parameters of the nanoparticles.

Amphiphilic gold nanoparticles can be used in many biological applications. A protocol to synthesize gold nanoparticles coated by a binary mixture of ligands and a detailed characterization of these particles is presented.

Gold nanoparticles covered with a mixture of 1-octanethiol (OT) and 11-mercapto-1-undecane sulfonic acid (MUS) have been extensively studied because of ...

Analysis of Complex Molecules and Their Reactions on Surfaces by Means of Cluster-Induced Desorption/Ionization Mass Spectrometry

Desorption/Ionization Induced by Neutral SO2 Clusters (DINeC) is employed as a very soft and efficient desorption/ionization technique for mass spectrometry (MS) of complex molecules and their reactions on surfaces. DINeC is based on a beam of SO2 clusters impacting on the sample surface at low cluster energy. During cluster-surface impact, some of the surface molecules are desorbed and ionized via dissolvation in the impacting cluster; as a result of this dissolvation-mediated desorption mechanism, low cluster energy is sufficient and the desorption process is extremely soft. Both surface adsorbates and molecules of which the surface is composed of can be analyzed. Clear and fragmentation-free spectra from complex molecules such as peptides and proteins are obtained. DINeC does not require any special sample preparation, in particular no matrix has to be applied. The method yields quantitative information on the composition of the samples; molecules at a surface coverage as low as 0.1 % of a monolayer can be detected. Surface reactions such as H/D exchange or thermal decomposition can be observed in real-time and the kinetics of the reactions can be deduced. Using a pulsed nozzle for cluster beam generation, DINeC can be efficiently combined with ion trap mass spectrometry. The matrix-free and soft nature of the DINeC process in combination with the MSn capabilities of the ion trap allows for very detailed and unambiguous analysis of the chemical composition of complex organic samples and organic adsorbates on surfaces.

Neutral SO2 clusters of low kinetic energy (&#60; 0.8 eV/constituent) are used to desorb complex surface molecules such as peptides or lipids for further analysis by means of mass spectrometry using an ion trap mass spectrometer. No special sample preparation is required, and real-time observation of reactions is possible.