Microfluidics for Transmission Electron Microscopy: Characterization of Copper Electrodeposition

This application presents the Liquid-phase Transmission Electron Microscopy (LPTEM) technique, which integrates liquid flow capabilities within microfabricated liquid cells providing the means to study different processes in solution with sub-nanometer spatial resolution and sub-microsecond temporal resolution. Using this technique, it is now possible to visually study topics ranging from material science to life science.

There are several key advantages of this technique, including:

  • Control over liquid environments in a micro or nanoscale space
  • The ability to study dynamic processes in the liquid phase with nanoscale spatial resolution
  • The ability to chemical composition in-situ
  • The ability to obtain electronic structure information

We kindly thank DENSsolutions for this collaboration, and for sharing the results obtained with their system. For information about DENSsolutions: www.denssolutions.com

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Introduction to Microfluidics for Transmission Electron Microscopy

An objective of electron microscopy is to observe the capacity of  liquid processes with spatial resolution of the nanoscale or higher.

Liquid-phase transmission electron microscopy (LPTEM) is a powerful in situ visualization technique for directly characterizing nanomaterials in the liquid state. This technique currently enables it to visually study a variety of topics, from life science to material science. Examples of measurements enabled by this technique are biomineralization processes, material changes during the cycling of batteries, the growth of metallic nanoparticles or structures in liquid, and electrochemical processes such as metal deposition.

However, existing LPTEM techniques frequently suffer from a lack of fluidic control and undesirable electron beam irradiation effects, which limits the use of analytical TEM approaches and produces unreplicable experiment results. To solve this problem, we use precision and control of microfluidics in transmission electron microscopy experiments.

Metallic or alloy nanostructures show benefits from increasing the contact area with the reaction medium, and accelerating mass/ion transportation in chemical or electrochemical reactions. Direct observation of formation processes as well as in-situ characterization of nanostructures to monitor the evolution of structural parameters are crucial to yield materials that deliver optimized performance.

In order to control this parameter evolution reliably and efficiently, DENSsolutions, in collaboration with Fluigent, implemented an LPTEM system employing pressure-based flow controllers and a dedicated Nano-Cell design. This system, which applies microfluidics to transmission electron microscopy experiments, ensures a constant and well-defined flow on the sample area, and enables full control of the liquid environment. As a result, the user is able to reliably measure the correlation between materials processing, structure, properties, and performance, while observing real-time dynamics in liquid as a function of either heat or biasing.

Materials and Methods for Transmission Electron Microscopy

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Figure 1. (a) Full setup of the system consisting of the Stream Nano-Cell (b) placed within the TEM Sample Holder (c), The LineUp microfluidic flow controllers consisting of two Flow EZ and a LINK (d), and a Flow Unit XS (e). The flow rate can be locally controlled using the Flow EZ display or by linking the Flow EZ to a computer and using OxyGEN software. 

Microscopy

Liquid cell
The Stream Nano-Cell is a an advanced flow cell for controlling liquid environment within the microscope.

Sample holder
The in situ TEM Sample Holder provides the platform for connecting the pressure-based flow controller to the Nano-Cell.

TEM
FEI Titan 80-300.

Liquid sample
CuSO4 (20 mM) and KH2PO4 (10 mM) mixed aqueous solution.

Partial results

I. Control of the liquid flow and thickness within the microfluidic chamber

The use of microfluidics for Transmission Electron Microscopy brings a high level of control and reproducibility to metal characterization experiments.

To demonstrate the high level of liquid control within the microfluidic chamber, the electrolyte was flowed into the window area and subsequently evacuated using the system described in the “Materials & Methods” part of the application note. 

HAADF-STEM image of the window area of a liquid Nano-Cell at different environmental condition
Figure 2. HAADF-STEM images of the window area of a liquid biasing Nano-Cell at different environmental conditions: (a) no liquid, (b) completely filled with liquid, (c) half evacuated and (d) no liquid.

II. Growth and morphology evolution of Cu dendrites

After flowing the electrolyte, the cyclic voltammetry technique was used to study the electrodeposition of Cu. To do so, the electrolyte was injected using a flow rate of 1,7 µL/min, and voltage ranging from -0,70 V to 0,30 V was applied to the Pt electrode. 

Three series of HAADF-STEM
Figure 3. Time series of HAADF-STEM images illustrating the growth and etching process of Cu dendrites on the Pt electrode by employing cyclic voltammetry scanning. In the experiments, incident electron flux was set ~50 e nm-2s-1, and the liquid flow rate was 1.7 μL/min.

III. Microstructure and chemical composition analysis

As mentioned above, by using microfluidics for transmission electron microscopy, it is possible to monitor and control the metal electrodeposition process with high precision. 

To analyze the microstructure and chemical composition, SAED analysis was performed. As a dry or thin liquid state is required for electron diffraction, the liquid is pushed out from the microfluidic chamber. After the electrodeposition, the diffraction rings for polycrystal were resolved. The indication of 220 diffraction means at least a lattice resolution of ~1.28 Å or better can be obtained in the liquid thickness of ~100 nm.

Finally, the chemical composition of the obtained depositions is directly analyzed in the liquid phase. EDX elemental analysis (point, line, or area analysis) is conveniently performed. Figure 4e shows the HAADF-STEM EDX elemental mapping, displaying the spatial distribution of the Cu dendrites and the Pt electrode in the electrolyte, thus confirming the great functioning of this analysis.

TEM images of PT electrode & Cu dendrites & corresponding SAED patterns

Figure 4. TEM images of (a) the Pt electrode and (c) the Cu dendrites on the electrode, and (b, d) the corresponding SAED patterns in the liquid phase. (e) HAADF-STEM EDX elemental mapping, showing the spatial distribution of the Cu dendrites and the Pt electrode in the electrolyte.

Conclusion

With the use of a transmission electron microscope (TEM), an advanced sample holder and flow cell, and precise pressure-based flow controls, we were able to demonstrate how to utilize a complete experimental setup to observe and characterize copper electrodeposition in liquid phase TEM. 

The main innovation of this design is the controlled flow, achieved through the use of microfluidics for Transmission Electron Microscopy. In fact, this design ensures that the liquid is forced to flow between the chips and through the imaging area, as this is the only route for the liquid to reach the output. The direct connection of the inlet tube to the microfluidic channel of the chip allows precise control of the flow direction and velocity. Consequently, the reagents are led directly to the imaging zone, and the cell can be flushed of reacted species. Finally, this configuration ensures that all liquid exiting has experienced the same environmental and temperature conditions.

References

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