Name: liquid-cell transmission electron microscopy for tracking self-assembly of nanoparticles
Uploaded: 2017-10-16T13:00:00
Description: Drying a nanoparticle dispersion is a versatile way to create self-assembled structures of nanoparticles, but the mechanism of this process is not fully ...

We thank Prof. A. Paul Alivisatos at the University of California, Berkeley and Prof. Taeghwan Hyeon at Seoul National University for the helpful discussion. This work was supported by IBS-R006-D1. W.C.L. gratefully acknowledges support from the research fund of Hanyang University (HY-2015-N).

1. Synthesis of Nanoparticles
<ol>
	<li>Synthesis of platinum nanoparticles
	<ol>
		<li>Combine 17.75 mg of ammonium hexachloroplatinate(IV) ((NH4)2Pt(IV)Cl6), 3.72 mg of ammonium tetrachloroplatinate(II) ((NH4)2Pt(II)Cl4), 115.5 mg of tetramethylammonium bromide, 109 mg of poly(vinylpyrrolidone) (MW: 29,000), and 10 mL of ethylene glycol with a stir bar in a 100 mL 3-neck round bottom flask equipped with a rubber septum.</li>
		<li>Equi...

<ol>
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Platinum nanoparticles with a size of 7 nm were synthesized via the reduction of ammonium hexachloroplatinate (IV) and ammonium tetrachloroplatinate (II) using poly (vinylpyrrolidone) (PVP) as a ligand and ethylene glycol as a solvent and a reducing agent27. A ligand-exchange reaction with oleylamine was performed to disperse the particles in a hydrophobic solvent. Lead selenide nanoparticles were synthesized via the thermal decomposition of lead-oleate complexes using TOP-Se as a selenium source<...

The self-assembly of colloidal nanoparticles is of interest because it provides an opportunity to access collective physical properties of individual nanoparticles11. One of the most effective methods of self-assembly used in practical device-scale applications is self-organization of nanoparticles on a substrate through evaporation of a volatile solvent6,7,8,9,10,11. This solvent evaporation method is a nonequilibrium process, which is largely influenced by kinetic factors such as evaporation rate and changes in nanoparticle-substrate interactions. However, since it is difficult to estimate and control the kinetic factors, the mechanistic understanding of nanoparticle self-assembly by solvent evaporation is not fully mature. Although in situ X-ray scattering studies have provided ensemble-averaged information of the nonequilibrium nanoparticle self-assembly process12,13,14, this technique cannot determine the motion of individual nanoparticles, and their association with the overall trajectory cannot be easily accessed.
Liquid-cell TEM is an emerging tool for tracking the trajectory of individual nanoparticles, enabling us to understand the inhomogeneity of nanoparticle motions and their contribution to ensemble behaviors15,16,17,18,19,20,21,22,23,24,25,26. We have previously used liquid-cell TEM to track the motion of individual nanoparticles during solvent evaporation, showing that the movement of the solvent boundary is a major driving force for inducing nanoparticle self-assembly on a substrate18,19. Herein, we introduce experiments where we can observe the process of nanoparticle self-assembly using liquid-cell TEM. First, we provide protocols for the synthesis of platinum and lead selenide nanoparticles, before introducing the fabrication procedures of liquid-cells for TEM and how to load nanoparticles into the liquid-cell. As representative results, we show snapshot images from TEM movies of nanoparticle self-assembly driven by solvent drying. By tracking individual particles in these movies, we can understand the detailed mechanisms of solvent-drying-mediated self-assembly at a single nanoparticle level. During self-assembly, the platinum nanoparticles on the silicon nitride window mainly follow the movement of the evaporating solvent front because of the strong capillary forces acting on the thin solvent layer. Similar phenomena were also observed for other nanoparticles (lead selenide) and substrates (silicon), indicating that the capillary force of the solvent front is an important factor in particle migration near a substrate.

<table>
	<tbody>
		<tr>
			<td>ammonium hexachloroplatinate (IV)</td>
			<td>Sigma-Aldrich</td>
			<td>204021</td>
			<td></td>
		</tr>
		<tr>
			<td>ammonium tetrachloroplatinate (II)</td>
			<td>Sigma-Aldrich</td>
			<td>206105</td>
			<td></td>
		</tr>
		<tr>
			<td>tetramethylammonium bromide, 98%</td>
			<td>Sigma-Aldrich</td>
			<td>195758</td>
			<td></td>
		</tr>
		<tr>
			<td>poly(vinylpyrrolidone) powder</td>
			<td>Sigma-Aldrich</td>
			<td>234257</td>
			<td>Mw ~29,000</td>
		</tr>
		<tr>
			<td>ethylene glycol, anhydrous, 99.8%</td>
			<td>Sigma-Aldrich</td>
			<td>324558</td>
			<td></td>
		</tr>
		<tr>
			<td>n-hexane, anhydrous, 95%</td>
			<td>Samchun Chem.</td>
			<td>H0114</td>
			<td></td>
		</tr>
		<tr>
			<td>ethanol, anhydrous, 99.5%</td>
			<td>Sigma-Aldrich</td>
			<td>459836</td>
			<td></td>
		</tr>
		<tr>
			<td>oleylamine, 70%</td>
			<td>Sigma-Aldrich</td>
			<td>O7805</td>
			<td>Technical grade</td>
		</tr>
		<tr>
			<td>lead(II) acetate trihydrate, 99.99%</td>
			<td>Sigma-Aldrich</td>
			<td>467863</td>
			<td></td>
		</tr>
		<tr>
			<td>oleic acid, 90%</td>
			<td>Sigma-Aldrich</td>
			<td>364525</td>
			<td>Technical grade</td>
		</tr>
		<tr>
			<td>diphenyl ether, 99%</td>
			<td>Sigma-Aldrich</td>
			<td>P24101</td>
			<td>ReagentPlus</td>
		</tr>
		<tr>
			<td>selenium powder, 99.99%</td>
			<td>Sigma-Aldrich</td>
			<td>229865</td>
			<td></td>
		</tr>
		<tr>
			<td>tri-n-octylphosphine, 97%</td>
			<td>Strem</td>
			<td>15-6655</td>
			<td>Air sensistive</td>
		</tr>
		<tr>
			<td>Toluene, anhydrous, 99.9%</td>
			<td>Samchun Chem.</td>
			<td>T2419</td>
			<td></td>
		</tr>
		<tr>
			<td>acetone 99.8%</td>
			<td>Daejung Chem.</td>
			<td>1009-2304</td>
			<td></td>
		</tr>
		<tr>
			<td>potassium hydroxide, 95%</td>
			<td>Samchun Chem.</td>
			<td>P0925</td>
			<td></td>
		</tr>
		<tr>
			<td>p-type silicon-on-insulator wafers</td>
			<td>Soitec</td>
			<td>Power-SOI</td>
			<td>for liquid cells with silicon windows</td>
		</tr>
		<tr>
			<td>tetramethylammonium hydroxide, 25% in H2O</td>
			<td>J.T.Baker</td>
			<td>02-002-109</td>
			<td></td>
		</tr>
		<tr>
			<td>AZ 5214 E</td>
			<td>AZ Electronic Materials</td>
			<td>AZ 5214 E</td>
			<td>Positive photorest</td>
		</tr>
		<tr>
			<td>AZ-327</td>
			<td>AZ Electronic Materials</td>
			<td>AZ-327</td>
			<td>AZ 5214 develper</td>
		</tr>
		<tr>
			<td>indium pellets 99.98-99.99%</td>
			<td>Kurt J. Lesker Company</td>
			<td>EVMIN40EXEB</td>
			<td>thermal evaporator target</td>
		</tr>
		<tr>
			<td>1,2-dichlorobenzene, &#62;99%</td>
			<td>TCI</td>
			<td>D1116</td>
			<td></td>
		</tr>
		<tr>
			<td>pentadecane, &#62;99%</td>
			<td>Sigma-Aldrich</td>
			<td>P3406</td>
			<td></td>
		</tr>
		<tr>
			<td>buffered oxide etch 7:1</td>
			<td>microchemicals</td>
			<td>BOE 7-1 VLSI</td>
			<td></td>
		</tr>
		<tr>
			<td>phosphoric acid, 85%</td>
			<td>Samchun Chem.</td>
			<td>P0449</td>
			<td></td>
		</tr>
	</tbody>
</table>

liquid-cell transmission electron microscopy for tracking self-assembly of nanoparticles

Drying a nanoparticle dispersion is a versatile way to create self-assembled structures of nanoparticles, but the mechanism of this process is not fully understood. We have traced the trajectories of individual nanoparticles using liquid-cell transmission electron microscopy (TEM) to investigate the mechanism of the assembly process. Herein, we present the protocols used for liquid-cell TEM studies of the self-assembly mechanism. First, we introduce the detailed synthetic protocols used to produce uniformly sized platinum and lead selenide nanoparticles. Next, we present the microfabrication processes used to produce liquid cells with silicon nitride or silicon windows and then describe the loading and imaging procedures of the liquid-cell TEM technique. Several notes are included to provide helpful tips for the entire process, including how to manage the fragile cell windows. The individual motions of nanoparticles tracked by liquid-cell TEM revealed that changes in the solvent boundaries caused by evaporation affected the self-assembly process of nanoparticles. The solvent boundaries drove nanoparticles to primarily form amorphous aggregates, followed by flattening of the aggregates to produce a 2-dimensional (2D) self-assembled structure. These behaviors are also observed for different nanoparticle types and different liquid-cell compositions.

The liquid-cell is composed of a top chip and a bottom chip, which are equipped with silicon nitride windows that are transparent to an electron beam with a thickness of 25 nm. The top chip has a reservoir for storing the sample solution and evaporated solvent. The chips are made via conventional microfabrication processing25. The masks used for the top and bottom chips are shown in Figure 1a and 1b, r...

Here we introduce experimental protocols for the real-time observation of a self-assembly process using liquid-cell transmission electron microscopy.

Liquid-cell Transmission Electron Microscopy for Tracking Self-assembly of Nanoparticles

Drying a nanoparticle dispersion is a versatile way to create self-assembled structures of nanoparticles, but the mechanism of this process is not fully ...

The overall goal of this procedure is to use liquid-cell transmission electron microscropy to investigate the motion of nanoparticles in the solution phase in real time. This method can help answer key questions in the field of nanoscience, for example, about how nanoparticles form self-assembled structures during solvent drying. The main advantage of this technique is that it makes checking nanoparticle motion in real space and real time possible.The implications of this liquid cell technique is tends towards tracking individual motions of nanoparticles that are not shown by conventional methods. Though this method can provide insight into self-assembly of nanoparticles it can be also applied into other models such as oriented attachment of nanoparticles. To begin the procedure, place in a 100 milliliter three neck round bottom flask 17.75 milligrams of ammonium hexacholoroplatinate, 3.72 milligrams of ammonium tetrachloroplatinate and 115.5 milligrams of tetramethylammonium bromide.Add to the flask 109 milligrams of polyvinylpyrrolidone and 10 milliliters of anhydrous ethylene glycol. Equip the flask with a stir bar, a rubber septum and a reflux condenser. Start the stir motor and while stirring at 1000 rpm, degas the reaction flask under vacuum for one hour.Then, under a flow of argon, heat the reaction mixture to 180 degrees Celsius at 10 degrees per minute. Stir the mixture at 180 degrees Celsius for 20 minutes then allow the mixture to cool to room temperature. Transfer the cooled mixture to a 50 milliliter centrifuge tube.Add to the tube 30 milliliters of acetone to precipitate the platinum nanoparticles. Centrifuge the mixture at 2400 times G for 10 minutes. Discard the supernatant and disperse the precipitate in 10 milliliters of ethanol.Obtain a four inch 100 micron silicon wafer coated with about 25 nanometers of silicon nitride. Load photo resist using a spin coater. Then use photo resist to mount the ultra thin wafer on a 500 micron thick silicon wafer.Spin coat the wafer with 10 milliliters of positive photo resist at 3000 rpm for 30 seconds. Bake the wafer at 85 degrees Celsius for 60 seconds. Then cover the wafer with a chromium mask and expose the wafer to 365 nanometer light for 10 seconds.Immerse the wafer in 50 milliliters of the appropriate developer solution for 40 seconds and then in 50 milliliters of deionized water for one minute. Immerse the wafer in 50 milliliters of deionized water for one minute, then place the patterned wafer in a reactive ion etcher. Etch the exposed silicon nitride for one minute.Use a water bath to evenly heat a container of a 30 milligram per liter aqueous solution of potassium hydroxide to 85 degrees Celsius. Soak the ultra thin wafer in the hot potassium hydroxide for two hours to etch the exposed silicon. When the exposed silicon appears to have been completely etched away, carefully remove the wafer from the solution at an angle to avoid rupturing the silicon nitride window.Repeat this process with the second chromium mask to obtain the top and bottom chips. Use the third chromium mask to pattern indium spacers onto the bottom chip. Align the top and bottom chips and bond the chips together at 100 degrees Celsius.Transfer 20 microliters of the prepared nanoparticle dispersion to a five milliliter vial. Allow the solvent to evaporate under ambient conditions for 10 minutes. Inspect the liquid cell under an optical microscope to verify that the silicon nitride windows are intact.Then disperse the nanoparticles in a mixture of one milliliter of orthodichlorobenzene, 250 microliters of pentadecane and 10 microliters of oleylamine. With the liquid cell under the optical microscope use an injector with an ultra thin capillary to load 100 nanoliters of the dispersion into the liquid cell reservoirs. Use filter paper to absorb the excess dispersion outside the reservoirs.Allow the cell to sit in ambient air for 10 minutes to evaporate the orthodichlorobenzene. Then apply vacuum grease to one side of a two millimeter copper aperture grid with a 600 micron hole. Carefully cover the liquid cell with the grid, being careful to align the aperture with the liquid cell window.Mount the cell in a standard TEM holder and load the cell into the instrument. Acquire images in continuous image acquisition mode as the solvent dries. Use image processing software to calculate the radial distribution function for the particles in each acquired image.TEM images of a platinum nanoparticle suspension drying in a silicon nitride liquid cell showed the nanoparticles being pulled inward by the receding solvent front. This behavior was attributed to the strong capillary forces of the thin layer of solvent and the reduced free energy of the nanoparticles at the solvent interface. The nanoparticles initially formed amorphous multilayer agglomerates as they were drawn together.As the solvent dried, the agglomerates flattened into an ordered monolayer. This ordering is reflected in the radial distribution functions derived from the TEM images. The radial distribution function of the image taken after 90 seconds had a large peak at 8.3 nanometers.The oleylamine capped platinum nanoparticles are about 8.3 nanometers in diameter, suggesting that a significant number of particles were assembled as closely as possible. Once mastered, this technique can be done in two days if it is performed properly. Generally individuals who are new to this method may struggle because fabricating and working with the liquid cell require different levels of optimization for different nanoparticles or liquid cell compositions.While attempting this procedure, remember to protect the windows of the liquid cell from breaking. Following this procedure, other methods like applying voltages to the liquid cell can be performed to answer additional questions about the self-assembly of nanoparticles in the presence of external forces. After its development, this technique paved the way for researchers in the field of nanoscience to explore the assembly process of nanoparticles in the overall dry mechanism.After watching this video, you should have a good understanding of how to prepare liquid cells and measure the motions of nanoparticles in TEM experiment. Don't forget that working with KUH agent can be extremely hazardous. Precautions such as wearing safety glasses should always be taken while performing this experiment.

Research

JoVE Journal

Chemistry

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