Imaging carbon nanoparticle agglomeration on MoS2 at a low dose rate
Gatan Instruments Used
The K3® IS camera delivers simultaneous low-dose imaging via real-time electron counting, fast continuous data capture, and a large field of view.
The model 652 is a furnace–type holder designed to directly observe microstructural phase changes, catalysis, nucleation, growth, and dissolution processes in 3 mm transmission electron microscope (TEM) samples at elevated temperatures.
MoS2 has many potential applications where void or pit formation plays a crucial role in determining material properties. In real-world synthesis, the nucleation and growth of pits may be affected by adventitious carbon (a thin layer of carbon usually present on materials exposed to air). In-situ TEM is a valuable technique for studying this adventitious carbon layer, its behavior at high temperatures, and its subsequent effects on the formation of pits in MoS2. Low-dose, low-voltage continuous imaging can be used alongside infrequent high-resolution imaging to observe these processes while minimizing the impact of the electron beam.
Materials and Methods
The MoS2 sample was imaged in a Titan ETEM at 80 kV to reduce beam damage. The sample was heated from room temperature up to 300 °C at 10 °C per min using a model 652 heating holder. In-situ TEM video was recorded with a K3 IS camera over the entire ramp, producing a counted video nearly 30 minutes long. The video captured a relatively large area (595 x 423 nm) with 1 Å pixels, enabling over 20 carbon nanoparticles to be observed simultaneously. The K3 IS camera, with its high detective quantum efficiency (DQE) and counted acquisition, enabled a low dose rate of just 8 e-/Å2/s to be used, further limiting beam damage. Over the entire video of over 8,000 frames resulted in a total dose of just 13,200 e-/Å2, comparable to just a few single atomic-resolution TEM images captured with a traditional TEM camera. For example, the OneView image in Figure 2c of  has a total dose of 3,200 e-/Å2 in a 0.32 s exposure.
The framerate of the original video saved to disk was 5 frames per second (fps), though the K3 IS camera captured and counted raw frames internally at 1,500 fps. The first 11 min of the video were unremarkable and not processed. The remaining dataset was 4,913 frames, each 5,760 x 4,092 pixels in size, for a total of 107 GB of data. After the acquisition, the data were binned by a factor of 4, drift corrected, cropped, and every 10 frames summed to produce a final video dataset. The final dataset was 492 frames, each 812 x 444 pixels in size, for a total of 790 MB of data in *.dm4 format. Gatan Microscopy Suite® (GMS) was used to process this data and to produce a *.mp4 video of this processed dataset, which was just 93 MB (a 99.9% reduction in data volume). A few frames from this video are shown in Figure 1. To download a full-resolution version of the video, please click here.
The 80 kV accelerating voltage and the low dose rate enabled by the K3 IS camera resulted in an in-situ video of MoS2 with relatively minor beam damage. By heating the sample using a Gatan heating holder up to 300 °C, it allowed the observation of the agglomeration of adventitious carbon into carbon nanoparticles. Processing over 100 GB of data with GMS resulted in a 93 MB *.mp4 video which can be seen in the referenced video in the paragraph above. The data show that most particles first appear around 170 – 180 °C. Other in-situ observations made on the same TEM in the presence of O2 gas showed that these particles were associated with the formation of pits in the MoS2 .
A special thanks to Stanford University, including Sangwook Park, Joonsuk Park, Taeho Roy Kim, Robert Sinclair, and Xiaolin Zheng.
 Park, S. et al. Effect of Adventitious Carbon on Pit Formation of Monolayer MoS2. Adv. Mater. 2003020 (2020) doi:10.1002/adma.202003020.