Using our Ocean system, scientists present new approaches to overcome the limitations in LPEM experiments by quantitatively mapping and controlling liquid layer thickness.

Original article by Hanglong Wu, Hao Su, Rick R. M. Joosten, Arthur D. A. Keizer, Laura S. van Hazendonk, Maarten J. M. Wirix, Joseph P. Patterson, Jozua Laven, Gijsbertus de With, Heiner Friedrich

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Liquid phase electron microscopy (LPEM) presents itself as a fundamental technique in the monitoring of nanoscale material processes in real-time. However, there are many challenges faced by the LPEM community with regard to quantifying and controlling liquid layer thickness to achieve laboratory-scale solution conditions and high-resolution imaging.

Using our dedicated LPEM solution, Ocean, Dr. Heiner Friedrich and his team at the Eindhoven University of Technology have paved the way for the better design of LPEM experiments and improved control of solution chemistry. Specifically, they present a simple low-dose method to quantitatively map the liquid layer thickness throughout the entire viewing area of any liquid cell with minimal beam effects even during repeated measurements. Moreover, they show how you can dynamically modulate liquid thickness by tuning the internal pressure in your liquid cell. Ultimately, this paper is a major step forward in the realization of LPEM experiment designs where bulk laboratory-scale solution conditions can be achieved yet at high resolutions. 

Mapping liquid layer thickness

Mapping the liquid layer thickness throughout the entire viewing area is crucial to avoid changes to the chemical environment but still provide the necessary information to 1) estimate which resolution can be reliably achieved, 2) model beam effects and 3) control the liquid thickness throughout the experiment.

Dr. Friedrich and his team use a simple low-dose method to map the liquid layer thickness throughout the entire viewing area. They did so by first acquiring two low-magnification, low-dose TEM images: one flat field image with no electron flux information, and another image with information on the number of electrons locally transmitting the viewing area. This information, combined with the silicon nitride (SiNₓ) membrane thickness, was then used to approximate the liquid layer thickness across the sample. The figure below shows in d) the intensity map of the viewing window area in a water-filled cell, and in e) the liquid layer thickness map calculated from the intensity map using the developed method. Shown in f) is the absolute liquid thickness map obtained from the same liquid cell in (d,e) using well-established STEM-EELS measurements.

Figure 1 - Schematic overview and mapping

Figure 1: a) to c) show the schematic overviews of the liquid cell. d) to f) show the resulting original intensity map, the calculated thickness map using the developed method and the absolute liquid thickness map obtained from the same liquid cell using STEM-EELS measurements, respectively.

The advantages of this method are that it can be adapted to any liquid and microscope which makes it a versatile tool for different experiments. This can be done by approximating the corresponding elastic mean free path (EMFP) from the chemical composition of the liquid and the acceleration voltage of the employed microscope.

Preparing the liquid cell

To prepare the liquid cell, the researchers employed a rather rare machine called the SciTEM (Scienion AG, a CELLINK company, Germany), which automates the liquid handling. The SciTEM is capable of patterning picoliter droplets of solutions onto the chip surface with a predefined array, and automatically loading the top chip to close the liquid cell. Ultimately, this means that accurate liquid volume control and reproducible liquid cell assembly can be achieved. The video below shows this process.

Movie 1: the automated preparation of the liquid cell

Controlling liquid thickness via internal pressure modulation

Because a pressure difference exists between the liquid and the microscope vacuum, the bending of the SiNₓ membrane windows typically occurs. This results in a spatially varying liquid layer thickness that makes it challenging to interpret LPEM results due to a locally varying achievable resolution and diffusion limitations. Therefore, to improve LPEM methodology, one needs to be able to accurately and dynamically control the liquid layer thickness, which has to be achieved by modulating the pressure inside the liquid cell.

Dr. Friedrich and his team show that with reproducible inward bulging of the window membranes, an ultra-thin liquid layer in the central window area for high-resolution imaging can indeed be realized. This is depicted very well in the figure below, showing the evolution of the meniscus in a liquid cell during evaporation. When evaporation occurs (2b), the flat gas-liquid interface gradually becomes curved, and the corresponding radius of curvature becomes smaller. At the same time, outward bulging is being reduced. Finally, as shown in 2d) and 2e), the inward bulging of the membrane is expected.

Figure 2 - Meniscus evolution with evaporation

Figure 2: Evolution of the meniscus in a liquid cell during evaporation

As shown in the in situ LPTEM video below, the liquid cell experienced outward bulging at the very beginning and within 3 minutes, nearly no bulging at all. Shortly after, you can see that an inward-bulged cell was obtained, entirely consistent with the model shown in Figure 2 above. Figure 3 demonstrates this, along with the corresponding intensity profile changes of the diagonal line across the window.

Movie 2: LPEM video showing outward to inward bulging in liquid cell

Figure 3 - Outward to inward bulging

Figure 3: Reversible transformation of a liquid cell containing water from outward to inward bulging via slow evaporation. Below are the intensity and thickness profiles of the bulging transition.

Rapid and dynamic control of liquid thickness

In addition, Dr. Friedrich and his team show that one can dynamically alter liquid thickness in a programmed fashion. They do this by independently controlling the inlet and outlet pressure on the fly via pressure pumps, and therefore the internal pressure inside the cell. Specifically, they show how pressure cycling gives rise to rapid dynamic control over liquid thickness, thus providing an additional adjustable parameter for experiment design. This dynamic control over the liquid layer thickness is depicted in the video below, where liquid thickness was continually mapped during the pressure cycling.

Movie 3: Rapid dynamic control of liquid layer thickness during external pressure cycles. The corresponding intensity and thickness maps are also shown.

Rapid dynamic control over liquid thickness is of key importance, in particular in bypass liquid cell systems, as it can help to overcome confinement problems such as diffusion limitations. In this way, the path can be paved towards achieving bulk solution conditions.

Novelty in findings

This paper contributes on two important fronts. Namely, it provides promising approaches to map and control liquid layer thickness in LPEM experiments with high accuracy and reliability. Using the above approaches, novel LPEM experimental designs with liquid thickness tailored to the requirement of the imaged process become imaginable. We are proud of the role that our LPEM solutions have played in making this research possible and strive to continue enabling groundbreaking research now and in the future.

Dr. Heiner Friedrich
“With this work on measuring and controlling liquid layer thickness using the Ocean system we hope to enable better science in the art of liquid phase electron microscopy.”
Dr. Heiner Friedrich
Assistant Professor |  Eindhoven University of Technology

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