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

Summary

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

Abstract

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.

Introduction

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 solvent^{6,7,8,9,10,11}. This solvent evaporation method is a nonequilibrium process, which is largely infl....

Protocol

1. Synthesis of Nanoparticles

1. Synthesis of platinum nanoparticles
   1. 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.
   2. Equip the flask with a reflux condenser and purge under vacuum. Stir the reaction mixture with a magnetic stir bar at 1,000 rpm.
   3. Heat the react....

Results

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.......

Discussion

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<.......

Materials

List of materials used in this article

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