Solution of Precise Control of Vacuum in Liquid Thickness Adjustment of Liquid Phase Transmission Electron Microscope

1. What is Liquid Phase Transmission Electron Microscope?

In recent years, liquid-phase transmission electron microscopy based on transmission electron microscopy, micro-nano processing, and thin-film manufacturing technologies have been used to build micro-experiment platforms at various nanoscale resolution scales and develop new nano-characterization techniques and related fields. We provide an effective way here. As shown in Figure 1, a standard liquid cell is a silicon microchip with two electronically transparent silicon nitride (SiN) film windows supported by an isolation material, between which a liquid sample is filled.

Theoretically, the thickness of liquid can be set by a spacer between microchips, but in actual observation, the microchip needs to be placed in an ultra-high vacuum environment of the transmission electron microscope, so that the pressure inside and outside of the membrane window is different. It will cause the membrane window to protrude and expand, causing the thickness of the liquid to change, and this change is often more than several times.

Therefore, unless bubbles are produced, this thickness variation will seriously affect the resolution of observation. In addition, pillars can be used to connect the top and bottom membrane windows to minimize expansion, but this kind of liquid pool with a fixed thickness cannot be loaded with different samples for observation, which is not universal and applicable.

It can be seen that the convex deformation and expansion of the liquid pool in the ultra-high vacuum environment of the transmission electron microscope is actually a usable characteristic. Through this expansion, liquid samples of different thicknesses can be reached under the condition of ensuring high resolution is more versatile and applicable to observing a variety of liquid samples. However, the premise that liquid thickness can be adjusted is that it can be accurately controlled.

Therefore, in order to reach the adjustable thickness of a liquid sample in the liquid phase electron microscope, it is necessary to try to precisely control the pressure difference between the inside and outside of the membrane window of the liquid pool. This article will introduce corresponding solutions for vacuum degree control inside the liquid pool. The high-precision control of vacuum degree will adopt the dynamic balance method, which can reach a control accuracy of ±1% under any vacuum degree within the range of 0.1~100kPa and can precisely adjustable and constantly control liquid sample thickness.

2. Solution

The solution described in this article also uses the above-mentioned dynamic balance method to control the vacuum degree of the liquid sample. The difference is that it is further refined, and the specific implementation plan and detailed description are given.

According to the vacuum degree control range corresponding to the adjustment of liquid thickness described in the above literature, we first determined that the vacuum degree control range that the solution needs to cover is 0.1~100kPa, which can basically satisfy all liquid samples under the liquid phase transmission electron microscope. Also, the need for thickness adjustment, while the vacuum degree control accuracy is required to be better than ±1%. The device is shown in Figure 2.


The vacuum degree control system for liquid sample thickness adjustment in the liquid phase electron microscope shown in Figure 4 mainly includes vacuum gauges, KaoLu's Proportional Pressure Regulator, vacuum pumps, vacuum pressure controllers, computers and their software, their respective functions and their details described as follows:

1. Vacuum gauge
   used to precisely measure the vacuum degree of liquid samples. The vacuum gauge adopts a thin-film capacitor vacuum gauge with high measurement accuracy. In order to meet the needs of full-scale vacuum measurement, two vacuum gauges with different ranges are equipped.
   
   
2. KaoLu's Proportional Pressure Regulator
   used to precisely adjust the intake and exhaust flow. KaoLu's Proportional Pressure Regulator is a high-speed needle valve driven by a stepping motor. It can precisely and quickly adjust the opening of the needle valve in less than 1s through an analog voltage signal of 0~10V to reach high-precision flow adjustment. It is very suitable for small sizes of vacuum control in the space. We equip two QKL series Proportional Pressure Regulators to adjust air intake and exhaust flow respectively to reach high-precision control of vacuum degree.
   
   
3. Vacuum pump
   It is used as a vacuum source. The vacuum pump used as a vacuum source generally adopts a low-pollution dry vacuum pump and reduces the impact of vibration and noise on the entire transmission electron microscope.
   
   
4. Vacuum pressure controller
   It is used to receive the measurement signal of the vacuum gauge, and automatically control the Proportional Pressure Regulator according to the set value of vacuum degree by PID so that the vacuum degree of the liquid sample can quickly reach the set value and keep it constant for a long time.
   
   For high vacuum control within the range of 0.1~1kPa, the controller needs to collect the vacuum gauge 1 signal with a range of 10Torr, and at the same time, fix the Proportional Pressure Regulator for the exhaust to a fully open state, the controller automatically adjusts the opening of Proportional Pressure Regulator for air intake.
   
   For low vacuum degree control in the range of 1~100kPa, the controller needs to collect the signal of vacuum gauge 2 with a range of 1000Torr, and at the same time fix the Proportional Pressure Regulator for the air intake at a certain opening state, and the controller controls Proportional Pressure Regulator for exhaust. The opening is automatically adjusted. In order to reach this kind of vacuum degree control in a wide range, it is equipped with an independent dual-channel KaoLu's QKL series high-precision Proportional Pressure Regulator. The two channels correspond to the signal acquisition of two vacuum gauges and form two independent channels. The closed-loop control loop automatically controls the vacuum degree in different ranges.
   
   
5. Computer and software
   The computer is used to communicate with the vacuum pressure controller, and the computer software can perform various parameter settings, operation control, digital display, graphic display, storage, and call of the vacuum pressure controller through the interface form. Although the vacuum pressure controller can be used alone to control the vacuum degree, it needs to be manually operated through the buttons on the controller, which is complicated to operate, but it is more intuitive and simple to operate the controller through computer software.

In order to meet the high-precision requirements of liquid thickness adjustment and control, the main technical indicators of the above key components are as follows:

1. Vacuum gauge
   Thin-film capacitance vacuum gauge, the measuring ranges are 10Torr and 1000Torr respectively, and the accuracy of any vacuum measurement value is 0.25%.
   
   
2. KaoLu's Proportional Pressure Regulator
   It is driven by a stepping motor, the control signal is an analog voltage or current signal, the whole response time from fully closed to fully open is less than 1s, the repeatability is better than ±0.1%, and the valve core has corrosion resistance.
   
   
3. Vacuum pressure controller
   24-bit AD, 16-bit DA, 0.01% minimum output percentage, PID parameters with self-tuning function, RS 485 communication, and standard MODBUS communication protocol, equipped with computer control software.

3. Conclusion

Liquid-phase transmission electron microscopy has become a basic technique for the real-time monitoring of nanomaterial processes in liquids. Due to the pressure difference between the liquid and high vacuum of the transmission electron microscope, the silicon nitride membrane window is usually bent, which can be adjusted by adjusting the liquid pool. Vacuum pressure is used to dynamically adjust the liquid thickness, resulting in an ultra-thin liquid layer in the central window region for high-resolution imaging.

Through the solution proposed in this article, an independent vacuum degree control device can be built for automatic adjustment and constant control of various thicknesses of microchip liquid samples in liquid transmission electron microscopy and can reach high control accuracy. In addition, the automatic control of high-precision vacuum inside the liquid pool also provides the possibility for dynamic change of liquid thickness according to the program, which is very beneficial to overcome the diffusion limitation and reach the bulk dissolution condition.

In summary, the solution provides a fundamental method for measuring and dynamically adjusting liquid thickness by using KaoLu's Proportional Pressure Regulator, enabling new experimental designs and better control of solution chemistry.

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